Photocatalysts with adsorption property for dye-contaminated water
purification
Luhong Zhang
Bachelor’s degree in Science
A thesis submitted for the degree of Doctor of Philosophy at
The University of Queensland in 2017
School of Chemical Engineering
II
Abstract
Dye-polluted water discharged from textile, dyeing, carpet manufacturing, pulp, and paper
industries must be properly treated before it is discharged to the environment because the
dyes are hazardous to human beings, also can cause problems to the ecosystem.
Adsorption and photocatalytic degradation of dyes are promising methods for the treatment
of dye-polluted wastewater. Zinc (Zn) aluminium (Al) layered double hydroxide (ZnAl-LDH)
materials, and layered double oxides (LDOs) that have semiconductor properties and
anionic exchange properties can be used for the removal of anionic dyes. Correspondingly,
graphitic carbon nitride (g-C3N4) is a fascinating metal-free semiconductor with synergistic
adsorption property due to its polymeric π-conjugated structure, which is a promising
photocatalyst in environmental remediation. Therefore, this thesis focuses on developing
novel photocatalysts, which is activated under UV (ultraviolet) and/or visible light irradiation
with adsorption property for dyes removal from wastewater. The studies include exploring
the optimal experimental conditions for the synthesis of LDOs with the best performance in
adsorption and photocatalytic degradation, modifying g-C3N4 with carbon black for tuning its
adsorption selectivity and improving the photocatalytic activity, and constructing novel
inorganic-organic heterogeneous semiconductor composites (ZnAl-LDH@C3N4) with
desired adsorption and photodegradation performance toward both cationic and anionic
dyes. The relationship between structures, physicochemical properties of photocatalysts
and dye removal performances has also been well studied in the thesis.
The first part of the experimental chapters focuses on the zinc aluminium layered double
hydroxide based photocatalysts. ZnAl-layered double oxide composites (LDOs) were
developed to remove anionic dye Orange II sodium salt (OrgII). Synthetic parameters
including the molar ratio of Zn to Al, and calcination temperature were adjusted to study the
optimal synthetic condition. The relationship between the structural features and the
adsorption properties and photocatalytic activity of LDOs was thoroughly investigated.
The second part of the experimental chapter focuses on the modification of g-C3N4. Series
of carbon black (CB) modified g-C3N4 samples with efficient adsorption and photocatalytic
activity were prepared by heating the mixtures of CB and urea. The adsorption and
photocatalytic activities were evaluated by removing both cationic dye methylene blue (MB)
and anionic dye OrgII. CB modified g-C3N4 exhibited higher photocatalytic activities than
pristine g-C3N4. CB worked as both dopant and reactional site during the polycondensation
III
of urea, which was supposed to increase the crystallisation and condensation degree of g-
C3N4. The existing of CB in the g-C3N4 matrix not only enhanced the light absorption for g-
C3N4 but also quenched the recombination of charge carriers. Therefore, modified g-C3N4
samples exhibited improved performance for dyes’ removal.
The third part of the experimental chapters focuses on the composite between inorganic
photocatalyst ZnO-LDH and organic semiconductor g-C3N4. A ZnO-layered double
hydroxide@graphitic carbon nitride composite (ZnO-LDH@C3N4) was synthesised via co-
precipitation method with solvothermal treatment. The ZnO-LDH@C3N4 composite
displayed superior performance in both adsorption and photocatalytic degradation of the MB
and OrgII, comparing to ZnO-LDH and g-C3N4 individually. For OrgII, ZnO-LDH@C3N4
showed higher adsorption capacity with three synergetic steps including electrostatic and
π-π conjugation adsorption followed by ion exchange. For MB, ZnO-LDH@C3N4 exhibited
substantial adsorption and high photocatalytic degradation rates under UV and visible light
irradiation. The enhanced performance in photocatalytic degradation of MB was induced by
the high separation efficiency of photogenerated charges, attributing to the novel inorganic-
organic heterogeneous structure of this composite.
Sum up, the thesis presents the key findings in this work and raises the perspective for the
future research direction.
IV
Declaration by author
This thesis is composed of my original work, and contains no material previously published
or written by another person except where due reference has been made in the text. I have
clearly stated the contribution by others to jointly-authored works that I have included in my
thesis.
I have clearly stated the contribution of others to my thesis as a whole, including statistical
assistance, survey design, data analysis, significant technical procedures, professional
editorial advice, and any other original research work used or reported in my thesis. The
content of my thesis is the result of work I have carried out since the commencement of my
research higher degree candidature and does not include a substantial part of work that has
been submitted to qualify for the award of any other degree or diploma in any university or
other tertiary institution. I have clearly stated which parts of my thesis, if any, have been
submitted to qualify for another award.
I acknowledge that an electronic copy of my thesis must be lodged with the University Library
and, subject to the policy and procedures of The University of Queensland, the thesis be
made available for research and study in accordance with the Copyright Act 1968 unless a
period of embargo has been approved by the Dean of the Graduate School.
I acknowledge that copyright of all material contained in my thesis resides with the copyright
holder(s) of that material. Where appropriate I have obtained copyright permission from the
copyright holder to reproduce material in this thesis.
V
Publications during candidature
Peer-reviewed papers:
1. Uptake and degradation of Orange II by zinc aluminium layered double oxides
L. Zhang, Z. Xiong, L. Li, R. Burt, X.S. Zhao, Journal of Colloid and Interface Science 469
(2016) 224.
2. ZnO-Layered Double Hydroxide@Graphitic Carbon Nitride Composite for
Consecutive Adsorption and Photodegradation of Dyes under UV and Visible Lights
L. Zhang, L. Li, X. Sun, P. Liu, D. Yang and X. Zhao, Materials 9 (2016) 927.
3. One-Step Synthesis of Metal@Titania Core–Shell Materials for Visible-Light
Photocatalysis and Catalytic Reduction Reaction
Z. Xiong, L. Zhang, X.S. Zhao, Chemistry – A European Journal 20 (2014) 14715.
4. Synthesis of TiO2 with controllable ratio of anatase to rutile
Z. Xiong, H. Wu, L. Zhang, Y. Gu, X.S. Zhao, Journal of Materials Chemistry A 2 (2014)
9291.
1. Hierarchical Nanoheterostructures: Layered Double Hydroxide-based Photocatalysts
L. Zhang, Z. Xiong, G. Zhao, Chapter 13 in Green Photo-active Nanomaterials: Sustainable
Energy and Environmental Remediation; The Royal Society of Chemistry, 2016, p 309.
VI
Publications included in this thesis
Hierarchical Nanoheterostructures: Layered Double Hydroxide-based Photocatalysts
L. Zhang, Z. Xiong, G. Zhao
Chapter 13 in Green Photo-active Nanomaterials: Sustainable Energy and Environmental
Remediation, The Royal Society of Chemistry: 2016; pp 309-338.
Publication citation – incorporated in Chapter 2
Contributor Statement of contribution
Luhong Zhang (Candidate) Wrote the book chapter (100%)
Edited the book chapter (70%)
Zhigang Xiong Edited the book chapter (25%)
Xiusong (George) Zhao Edited the book chapter (5%)
Provided revision suggestion (100%)
Uptake and degradation of Orange II by zinc aluminium layered double oxides
L. Zhang, Z. Xiong, L. Li, R. Burt, X.S. Zhao
Journal of Colloid and Interface Science, 2016, 469, 224-230.
Publication citation – incorporated as Chapter 4
Contributor Statement of contribution
Luhong Zhang (Candidate) Designed experiments (100%)
Wrote the paper (90%)
Edited the paper (50%)
Zhigang Xiong Wrote the paper (10%)
Edited the paper (10%)
Li Li Edited the paper (30%)
Ryan Burt Edited the paper (10%)
Xiusong (George) Zhao Provided revision suggestion (100%)
VII
ZnO-layered double hydroxide@graphitic carbon nitride composite for consecutive
adsorption and photodegradation of dyes under UV and visible lights
Luhong Zhang, Li Li, Xiaoming Sun, Peng Liu, Dongfang Yang and Xiusong Zhao
Materials 2016, 9 (11), 927.
Publication citation – incorporated as Chapter 6
Contributor Statement of contribution
Luhong Zhang (Candidate) Designed experiments (100%)
Wrote the paper (90%)
Edited the paper (50%)
Li Li Wrote the paper (10%)
Edited the paper (40%)
Xiaoming Sun Edited the paper (5%)
Xiusong (George) Zhao Edited the paper (5%)
Provided revision suggestion (100%)
VIII
Contributions by others to the thesis
Ms Xiaoming Sun helped with TEM characterization in work described in Chapter 4.
Mr Peng Liu helped with SEM characterization in work described in Chapter 4.
Miss Dongfang Yang helped with HRTEM and SEAD characterization in work described in
Chapter 5.
Mr Hao Lu helped with the EIS test in work described in Chapter 5 and 6.
Miss Qinglan Zhao helped with the FT-IR in work described in Chapter 5.
Mr Rohit Gaddam helped with the HRTEM and EDX characterization in work described in
Chapter 6.
Statement of parts of the thesis submitted to qualify for the award of another degree
None.
IX
Acknowledgements
Firstly, I would like to give my sincere thanks to my respectable supervisors, Professor
George Zhao, Dr Zhigang Xiong and Dr Li Li for giving me guidance, encouragement and
support throughout the whole course of my PhD research. I am grateful to receive the critical
feedbacks, inspirational suggestions from Professor George Zhao. He encouraged me that
writing research paper should be like writing an interesting story, which enlightened me in
writing scientific papers holding with full of interest. I am also really appreciated the help
from Dr Xiong. He taught me the key essentials in doing experiments, gave me a clear guide
to experimental design, lab safety and paper writing. I have benefited a lot from discussion
with him. Dr Li is an unforgettable advisor in my life. She was always available to talk with
me about the problems I encountered in research and daily life. For my work, she always
revised my manuscripts in detail and made suggestions to my research patiently. She was
also willing to share her experiences with me. I am appreciated to her genuine
encouragement and wise advice.
Secondly, I will never forget to give my thanks to my coworkers and friends in our research
group including Professor Xu, Dr Cynthia Lin, Dr Hao Wu, Mr Binghui Xu, Miss Yi Gu, Mr
Bo Wang, and my friend Yanfeng Wen for their valuable suggestions in doing research. I
would like to thank Ms Xiaoming Sun, Mr Peng Liu, Miss Dongfang Yang, Mr Hao Lu, Miss
Qinglan Zhao and Miss Shengnan Wang for their help with my samples’ characterizations
including TEM, SETM, FT-IR, etc. I would also like to thank Mr Ashok Nanjundan for his lab
administration and management.
I am also grateful to the four years’ financial support from the China Scholarship Council
(CSC) and the top-up scholarship from the School of Chemical Engineering, University of
Queensland.
Importantly, I would also like to thank my parents for their encouragements and sensible
suggestions, directions and generous support in my life and study career. Finally, I am
especially grateful to my husband Juncai Que and my son Jayden and my little angel
Jasmine. Without their love and support, I don’t think I can persist to the end of my PhD
track. All in all, I am sincerely grateful to all the people who have helped me in my life and
work.
X
Keywords
layered double hydroxide, zinc oxide, carbon nitride, semiconductor, adsorption,
photocatalyst, photocatalytic degradation
Australian and New Zealand Standard Research Classifications (ANZSRC)
ANZSRC code: 091205, Functional Material, 40%
ANZSRC code: 091202, Composite and Hybrid Materials, 40%
ANZSRC code: 100701, Environmental Nanotechnology, 20%
Fields of Research (FoR) Classification
FoR code: 0904, Chemical Engineering, 100%
XI
Table of contents
Abstract ............................................................................................................................... II
Declaration by author ....................................................................................................... IV
Publications during candidature ...................................................................................... V
Publications included in this thesis ................................................................................ VI
Acknowledgements ........................................................................................................... IX
Table of contents ............................................................................................................... XI
List of Figures ................................................................................................................. XIV
List of Tables .................................................................................................................. XIX
List of Schemes ............................................................................................................... XX
List of Abbreviations ....................................................................................................... XX
1 Introduction .................................................................................................................. 1
1.1 Background ............................................................................................................. 1
1.2 Research aim and scope of the project ................................................................... 4
1.3 Thesis structure ....................................................................................................... 5
1.4 References .............................................................................................................. 6
2 Literature Review ......................................................................................................... 8
2.1 Layered double hydroxides ..................................................................................... 8
2.2 Layered Double Hydroxide-Based Photocatalysts ................................................ 10
2.2.1 Structure of LDH ............................................................................................. 10
2.2.2 Methods for preparing LDH materials ............................................................. 15
2.2.3 Modification of LDH ........................................................................................ 18
2.2.4 Photocatalytic Applications of LDH ................................................................. 26
2.3 Graphitic carbon nitride ......................................................................................... 34
2.4 Graphitic carbon nitride-based photocatalysts for water purification..................... 35
2.4.1 Structure and morphology of g-C3N4 .............................................................. 35
2.4.2 Synthesis methods ......................................................................................... 38
2.4.3 Functionalization of g-C3N4 for wastewater treatment .................................... 39
2.4.4 Synergistic adsorption and photocatalysis ..................................................... 55
2.5 Summary and future perspectives ........................................................................ 58
2.6 Reference .............................................................................................................. 61
3 Research Methodology ............................................................................................. 81
3.1 Introduction ........................................................................................................... 81
3.2 Materials synthesis ................................................................................................ 81
3.2.1 Chemicals and reagents ................................................................................. 81
3.2.2 Synthesis of ZnO-LDH .................................................................................... 81
XII
3.2.3 Synthesis of (x)LDO samples ......................................................................... 82
3.2.4 Synthesis of ZnO−r(x)LDH ............................................................................. 82
3.2.5 Synthesis of g-C3N4 nanosheets .................................................................... 82
3.2.6 Synthesis of carbon black modified g-C3N4 samples ..................................... 82
3.2.7 Synthesis of ZnO-LDH@C3N4 composite ....................................................... 83
3.3 Adsorption measurement ...................................................................................... 83
3.4 Photocatalytic evaluation ...................................................................................... 83
3.5 Materials characterizations ................................................................................... 83
3.5.1 X-ray diffraction .............................................................................................. 83
3.5.2 Fourier transform infrared spectra .................................................................. 84
3.5.3 X-ray photoelectron spectroscopy .................................................................. 84
3.5.4 Scanning electron microscopy ........................................................................ 84
3.5.5 Transmission electron microscopy ................................................................. 84
3.5.6 Nitrogen adsorption-desorption ...................................................................... 84
3.5.7 Thermogravimetric-differential scanning calorimetry ...................................... 84
3.5.8 Diffuse reflectance spectra ............................................................................. 85
3.5.9 Photoluminescence spectra ........................................................................... 85
3.5.10 Electrochemical impedance spectroscopy .................................................. 85
3.5.11 Zeta potential ............................................................................................... 85
3.6 References ............................................................................................................ 85
4 Uptake and degradation of Orange II by zinc aluminium layered double oxides 86
4.1 Introduction ........................................................................................................... 86
4.2 Adsorption and photodegradation experiment details ........................................... 87
4.3 Results and discussion ......................................................................................... 88
4.3.1 The feature structure of ZnO-LDH and ZnO-rLDH composites ...................... 88
4.3.2 Adsorption of OrgII on LDO composites ......................................................... 95
4.3.3 Proposed uptake mechanism ......................................................................... 98
4.3.4 Photocatalytic performance .......................................................................... 100
4.3.5 The DRS and bandgap of ZnO-rLDH composites ........................................ 103
4.4 Conclusions ......................................................................................................... 104
4.5 References .......................................................................................................... 105
5 Carbon black modified g-C3N4 as adsorptive photocatalysts for decontamination
of dyes under visible light ............................................................................................. 108
5.1 Introduction ......................................................................................................... 108
5.2 Adsorption and photodegradation experiment details ......................................... 109
5.3 Results and discussion ....................................................................................... 110
5.3.1 Characterization ........................................................................................... 110
XIII
5.3.2 Adsorption and photocatalytic activities of samples ..................................... 124
5.4 Conclusion .......................................................................................................... 128
5.5 References .......................................................................................................... 129
6 ZnO-layered double hydroxide@graphitic carbon nitride composite for
consecutive adsorption and photodegradation of dyes under UV and visible lights
132
6.1 Introduction ......................................................................................................... 132
6.2 Experimental details ............................................................................................ 133
6.2.1 Adsorption and photodegradation ................................................................ 133
6.2.2 Intermediate species of photocatalytic degradation ..................................... 134
6.3 Results and discussion ....................................................................................... 134
6.3.1 Characterization of samples ......................................................................... 134
6.3.2 Adsorption and photocatalytic activity of ZnO-LDH@C3N4 composite ......... 146
6.4 Conclusion .......................................................................................................... 158
6.5 References .......................................................................................................... 158
7 Conclusions and recommendations ...................................................................... 163
7.1 Conclusions ......................................................................................................... 163
7.2 Recommendations .............................................................................................. 164
XIV
List of Figures
Figure 1-1 Schematic illustration of photocatalysis. ............................................................. 4
Figure 2-1 Schematic Representation is for comparing the crystal structure of brucite (A)
and LDH (B). Reproduced from ref.11 ................................................................................... 9
Figure 2-2 XRD patterns of (a) Zn2Al-LDH, (b) Zn3Al- LDH, and (c) Zn4Al-LDH.Reprinted
with permission from ref.26 .................................................................................................. 11
Figure 2-3 FT-IR Spectra of (a) ZnAl-LDH, (b) ZnAlSn-LDH and (c) ZnSn-LDH. Reprinted
with permission from ref.27 .................................................................................................. 12
Figure 2-4 SEM micrograph of the as-prepared CoAl-CO3-LDH. Reprinted with permission
from ref.29 ........................................................................................................................... 13
Figure 2-5 Particle size distribution of Mg2Al-Cl-LDH samples collected with the photon
correlation spectroscopy (PCS). Left: (X) Coprecipitated and stirred for 10 min at room
temperature, with two peaks at 320 and 2300nm; (Y) sample X aged at 50 °C for 16 h, with
two broad peaks at 220 and 955 nm; (Z) 100 oC Hydrothermal treatment for 16, with one
sharp peak at 114 nm. Right: Dispersion of Mg2Al-Cl-LDH aggregates with heating
duration during the hydrothermal treatment at 100 °C, where the distribution curves were
obtained with PCS. Reproduced from ref.36 ....................................................................... 14
Figure 2-6 SEM images of plate LDH sample. Inset is the photographic image of 2 cm × 2
cm Al plate fabricated with ZnAl-LDH assembles. Reproduced from ref.39 ........................ 16
Figure 2-7 (a) ZnO nanorod/ZnAl-LDH; (b) ZnO nanotube/ZnAl-LDH; (c) ZnO film/ ZnAl-
LDH. Reproduced from ref.44 .............................................................................................. 18
Figure 2-8 SEM and TEM images of MgFe-LDH microspheres with different inner
architecture: A, D) solid, B, E) yolk-shell, C, F) hollow, and G) EDX mapping results of a
single LDH hollow microsphere. Reprinted from ref.56 ....................................................... 20
Figure 2-9 The simulated structure of the composite SDS-LDHs/TiO2. Reproduced from
ref.65 .................................................................................................................................... 21
Figure 2-10 SEM images of the as-obtained F-MMO sample: (a) an overall view of the F-
MMO microspheres; (b) a single F-MMO microsphere; (c) the nanoflakes of an F-MMO
microsphere; (d) a portion of an F-MMO nanoflake with hexagonal ZnO nanoplatelets
embedded on its surface (inset: the cross section of the F-MMO nanoflake). Reprinted
permission from ref.69 ......................................................................................................... 23
Figure 2-11 UV–vis diffuse reflectance spectra of ZnAl: LDH nanostructures and their
nanostructures produced by calcination at various temperatures. Reproduced from ref.88 26
XV
Figure 2-12 Schematic diagram illustrating the principle of semiconductor photocatalysis.90
........................................................................................................................................... 27
Figure 2-13 Proposal ligand-to-metal charge-transfer mechanism, initially responsible for
the degradation of phenol; (A) inner-sphere complexation occurring on a Brönsted basic
site, and (B) outer-sphere complexation occurring on a Lewis basic site. Reprinted from
ref.73 .................................................................................................................................... 29
Figure 2-14 Photodegradation on (a) MB and (b) MO/MB mixture monitored as the
normalised concentration vs. irradiation time.98 ................................................................. 30
Figure 2-15 Schematic diagram of the process of electrostatic self-assembly between the
negatively charged CG monolayer and the positively charged ZnAl-LDH nanosheets.99 .. 31
Figure 2-16 (a) Photocatalytic activity of the catalyst for MB decolorization; (b)
Photocatalytic activity of the catalyst for OG decolorization; (c) PL spectra of rCG/LDO and
the pristine LDO and (d) UV-vis diffuse reflectance spectra of rCG/LDO and the pristine
LDO.99 ................................................................................................................................ 32
Figure 2-17 Schematic of monolayer graphitic carbon nitride. N, C and H atoms are
represented by light blue, white, and small yellow balls, respectively. The melem unit is
marked by the white circle.125 ............................................................................................. 36
Figure 2-18 (a) XRD pattern of g-C3N4. b, c) High-resolution XPS spectra of C1s (b) and
N1s (c) of g-C3N4.124 ........................................................................................................... 37
Figure 2-19 Rich morphologies of the g-C3N4 family with dimensions ranging from bulk to
quantum dots.128 ................................................................................................................. 37
Figure 2-20 (a) Effect of different scavengers on the RhB degradation in the presence of
CdS QDs/npg-C3N4, and (b) schematic of photogenerated charge transfer in the CdS
QDs/npg-C3N4 system under visible light.154 ...................................................................... 45
Figure 2-21 The chemical structure of PANI. ..................................................................... 46
Figure 2-22 Schematic of the separation and transfer of photo-generated charge carriers in
the PANI–g-C3N4 system under visible light irradiation. ..................................................... 47
Figure 2-23 (a) and (b) are the idealized motifs of graphene and g-C3N4 sheet respectively.
(c) The brief procedure of preparing rGO-intercalated g-C3N4. .......................................... 49
Figure 2-24 Typical TEM images of (a) pg-C3N4, (b) GR, (c) Au/pg-C3N4/GR and HRTEM
(d) image of Au/pg-C3N4/GR composite. ............................................................................ 52
Figure 2-25 Possible photocatalytic mechanism of the ternary hybrid composites. ........... 53
Figure 2-26 Proposed mechanism of charge separation and photocatalytic activity over
ZnIn-MMO/g-C3N4 photocatalyst under visible light irradiation. ........................................ 55
XVI
Figure 2-27 Schematic diagram for the photocatalytic degradation of dye molecules over
the g-C3N4 sample. The synergy between electrostatic adsorption and photocatalysis
under visible light facilitates the decomposition of dye molecules. .................................... 56
Figure 2-28 Band structures of powder g-C3N4 and C3N4/GOA: (a) XPS valence band
spectra; (b) band structure diagram. .................................................................................. 58
Figure 4-1 (A) XRD patterns of (a) ZnO−(3)LDH, (b) (3)LDO prepared at 400 oC, and
ZnO−r(3)LDH prepared by rehydration of (3)LDO with the calcination temperatures of (c)
400 oC, (d) 500 oC, (e) 600 oC, (f) 700 oC. The XRD patterns of standard LDH, ZnO, and
ZnAl2O4 are also included. (B) XRD patterns of (g) ZnO−r(1)LDH, (h) ZnO−r(2)LDH, (i)
ZnO−r(3)LDH, (j) ZnO−r(4)LDH, and (k) ZnO−r(5)LDH samples that are produced from
LDO calcined at 600 oC. ..................................................................................................... 89
Figure 4-2 XRD patterns of ZnO−r(3)LDH gained from 800 oC. ........................................ 90
Figure 4-3 XRD patterns of ZnO−r(x)LDH (from 400 oC) and ZnO: a, b, c, d and e
represent ZnO, ZnO−r(2)LDH ZnO−r(3)LDH ZnO−r(4)LDH and ZnO−r(5)LDH respectively.
........................................................................................................................................... 92
Figure 4-4 SEM (left) and TEM (right) images for (A/a) ZnO−(2)LDH, (B/b) (2)LDO, (C/c)
ZnO−r(2)LDH and (D/d) ZnO−r(5)LDH. .............................................................................. 93
Figure 4-5 N2 adsorption/desorption of isotherms of (a) ZnO−(2)LDH, (b) (2)LDO and
(c)ZnO−r(2)LDH. ................................................................................................................ 94
Figure 4-6 The adsorption dynamics of OrgII on (2)LDO (600 oC). The insert is the
adsorption capacity as a function of calcination temperatures for preparing LDOs. .......... 96
Figure 4-7 Adsorption isotherms of OrgII on (a) ZnO−(2)LDH, (b) (2)LDO, (c) (1)LDO, (d)
(3)LDO, (e) (4)LDO and (f) (5)LDO. (the LDOs were calcined at 600 oC. The dashed lines
are Langmuir isotherm simulations). .................................................................................. 97
Figure 4-8 XRD patterns for (a) (2)LDO, (b) ZnO−r(2)LDH,(c) (2)LDO after adsorption of
OrgII and (d) (2)LDO after adsorption of SDS .................................................................... 97
Figure 4-9 FT-IR spectra of (a) OrgII, (b) (2)LDO after adsorption of OrgII, (c) ZnO-
r(2)LDH, (d) (2)LDO, (e) ZnO-(2)LDH. ............................................................................. 100
Figure 4-10 (A) Comparison of OrgII photodegradation in water under UV-light irradiation
on (a) (1)LDO, (b) (2)LDO, (c) (3)LDO, (d) (4)LDO, (e) (5)LDO, (f) TiO2 and (g) ZnO
respectively; (B) The change of absorption spectra for OrgII solution during the
photodegradation process over (2)LDO under UV-light irradiation; (C) UV-visible
reflectance spectra of (a) ZnO−r(1)LDH, (b) ZnO−r(2)LDH, (c) ZnO−r(3)LDH, (d)
ZnO−r(4)LDH, and (e) ZnO−r(5)LDH samples; (D) Bandgaps for ZnO−(2)LDH,
XVII
ZnO−r(2)LDH, ZnO−r(5)LDH, TiO2 (P25) and commercial ZnO represented by a, b, c and
d individually. .................................................................................................................... 102
Figure 4-11 The pseudo-first-order kinetic fitting lines for the photocatalytic profiles of OrgII
decomposition over on (a) (1)LDO, (b) (2)LDO, (c) (3)LDO, (d) (4)LDO, (e) (5)LDO, (f) TiO2
and (g) ZnO respectively. ................................................................................................. 102
Figure 5-1 XRD patterns of g-C3N4, 200(U+CB), 600(U+CB), 1000(U+CB), 1500(U+CB)
and 3000(U+CB) samples. ............................................................................................... 111
Figure 5-2 FT-IR spectra for pristine g-C3N4 and 1500(U+CB). ....................................... 111
Figure 5-3 FT-IR spectra for 3000(U+CB), 1500(U+CB), 1000(U+CB), 600(U+CB),
200(U+CB) and CB. ......................................................................................................... 112
Figure 5-4 XPS images of the pristine (a) g-C3N4, (b) 3000(U+CB) and (c)1500(U+CB)
sample. (A) The survey spectra; (B) The high-resolution C 1s XPS spectra; (C) The high-
resolution N 1s XPS spectra; (D) The high-resolution O 1s XPS spectra. ....................... 113
Figure 5-5 The deconvolution of high-resolution XPS spectra for C 1s (A), N 1s (B) and O
1s (C) for sample 1500(U+CB). ........................................................................................ 115
Figure 5-6 (a) and (b) are TEM images for 1500(U+CB) and pristine g-C3N4 respectively;
(c) and (d) are HRTEM images with inset SAED patterns for 1500(U+CB) and pristine g-
C3N4 respectively. ............................................................................................................. 116
Figure 5-7 N2 adsorption/desorption of isotherms of pristine g-C3N4, 200(U+CB),
600(U+CB) 1000(U+CB), 1500(U+CB) and 3000(U+CB). ............................................... 118
Figure 5-8 Thermogravimetric curves of pristine g-C3N4, 200(U+CB), 600(U+CB)
1000(U+CB), 1500(U+CB) and 3000(U+CB) under air at a heating rate of 10 oC∙min-1. . 118
Figure 5-9 DSC profiles for g-C3N4 samples and CB. ...................................................... 120
Figure 5-10 (A) UV-vis diffuse reflectance spectra of the photocatalysts: g-C3N4,
200(U+CB), 600(U+CB) 1000(U+CB), 1500(U+CB) and 3000(U+CB). (B) the estimated
band gap energies of g-C3N4, 200(U+CB), 600(U+CB) 1000(U+CB), 1500(U+CB) and
3000(U+CB) with corresponding tangent lines ................................................................. 121
Figure 5-11 Photoluminescence spectra of g-C3N4, 200(U+CB), 600(U+CB) 1000(U+CB),
1500(U+CB) and 3000(U+CB). ........................................................................................ 123
Figure 5-12 Electrochemical impedance spectroscopy of g-C3N4, CB and 1500(U+CB). 123
Figure 5-13 Comparison of MB (A) and OrgII (B) adsorption and photodegradation in water
under visible light over pristine g-C3N4 and n(U+CB), with n equal to 3000, 1500, 1000,
600 and 200 respectively. ................................................................................................ 124
XVIII
Figure 5-14 First-order kinetic plots for the photodegradation of MB (A) and OrgII (B) over
g-C3N4 samples. .............................................................................................................. 125
Figure 5-15 Temporal UV-vis absorption spectral changes during the adsorption and
photocatalytic degradation of MB (A) and OrgII (B) in aqueous solution in the presence of
3000(U+CB). .................................................................................................................... 126
Figure 5-16 Band structures of pristine g-C3N4 and 3000(U+CB): (A) XPS valence band
spectra; (B) band structure diagram. ................................................................................ 127
Figure 6-1 XRD patterns of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4. ............................. 135
Figure 6-2 XPS images of the g-C3N4 and ZnO-LDH@C3N4 composite. (A) Survey
spectrum for g-C3N4; (B) High-resolution N 1s XPS spectrum of g-C3N4; (C) High-
resolution C 1s XPS spectrum of g-C3N4; (D) Survey spectrum for the ZnO-LDH@C3N4
composite; (E–I) are high-resolution XPS spectra for N 1s, C 1s, Zn 2p, O 1s and Al 2p of
the ZnO-LDH@C3N4 composite respectively. .................................................................. 137
Figure 6-3 XPS spectra of ZnO-LDH. (a) Survey spectrum for ZnO-LDH; (b) High-
resolution C 1s XPS spectrum of ZnO-LDH; (c) High-resolution XPS spectrum for Zn 2p of
ZnO-LDH; (d) High-resolution XPS spectrum for O 1s of ZnO-LDH. (e) High-resolution
XPS spectrum for Al 2p of ZnO-LDH. ............................................................................... 138
Figure 6-4 Weight loss of ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 composite determined
by TGA. ............................................................................................................................ 140
Figure 6-5 Zeta potential of g-C3N4, ZnO-LDH, and ZnO-LDH@C3N4 aqueous
suspensions. .................................................................................................................... 140
Figure 6-6 (A) TEM image for g-C3N4; (B) TEM image for ZnO-LDH@C3N4; (C) SEM
image for ZnO-LDH@C3N4; (D) SEM image for ZnO-LDH. ............................................. 142
Figure 6-7 (a) TEM image for bulk g-C3N4, (b) SEM image for g-C3N4, (c) TEM image for
ZnO-LDH. ......................................................................................................................... 143
Figure 6-8 (A) HRTEM image for the composite ZnO-LDH@C3N4; (B) EDX spectrum of
ZnO-LDH@C3N4. .............................................................................................................. 144
Figure 6-9 The elemental mapping for ZnO-LDH@C3N4. ................................................ 145
Figure 6-10 N2 adsorption/desorption of isotherms of (a) g-C3N4, (b) ZnO-LDH, (c) ZnO-
LDH@C3N4. ...................................................................................................................... 145
Figure 6-11 (A) The adsorption dynamic of ZnO-LDH@C3N4 in OrgII adsorption. The insert
is adsorption capacity comparison among g-C3N4, ZnO-LDH@C3N4 and ZnO-LDH; (B) FT-
IR spectra of (a) g-C3N4, (b) ZnO-LDH, (c) ZnO-LDH@C3N4, (d) OrgII, (e) ZnO-LDH@C3N4
after saturated adsorption with OrgII. ............................................................................... 147
XIX
Figure 6-12 The adsorption dynamics of g-C3N4 (A) and ZnO-LDH (B) in OrgII adsorption.
......................................................................................................................................... 148
Figure 6-13 FT-IR spectra of ZnO-LDH@C3N4, ZnO-LDH@C3N4 after OrgII adsorption in
1h and ZnO-LDH@C3N4 after OrgII adsorption in 24h. .................................................... 150
Figure 6-14 (a) Comparison of MB adsorption and photodegradation in water under UV-
light over ZnO, ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 respectively; (b) Comparison of
MB adsorption and photodegradation in water under visible-light over ZnO, ZnO-LDH, g-
C3N4 and ZnO-LDH@C3N4 respectively; (c) Kinetic fit for the degradation of MB with the
ZnO, ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 respectively under visible light; (d) UV-vis
diffuse reflectance spectra of the photocatalysts with corresponding tangent lines; (e)
Photoluminescence spectra of g-C3N4, ZnO, ZnO-LDH and ZnO-LDH@C3N4; (f)
Electrochemical impedance spectroscopy of g-C3N4, ZnO, ZnO-LDH and ZnO-LDH@C3N4
composite. ........................................................................................................................ 152
Figure 6-15 The cycling runs of ZnO-LDH@C3N4 in the photodegradation of MB under UV
irradiation. ......................................................................................................................... 154
Figure 6-16 (A) The experimental data and the fitting plots of photogenerated carriers
trapping in the photodegradation of MB by ZnO-LDH@C3N4 under UV-light irradiation; (B)
The experimental data and the fitting plots of photogenerated carriers trapping in the
photodegradation of MB by ZnO-LDH@C3N4 under visible-light irradiation. ................... 156
Figure 6-17 Schematic illustration of the mechanism of uptake of anionic dye OrgII and the
charge separation and photocatalytic activity of the ZnO-LDH@C3N4 under UV- and
visible-light irradiation, respectively. ................................................................................. 157
List of Tables
Table 1-1 Advantage and disadvantage of the current methods of dye removal from
industrial effluents.1 .............................................................................................................. 1
Table 2-1 Ionic radii of some cations with a coordinate number of 6. Reproduced from
ref.21 .................................................................................................................................... 10
Table 4-1 Properties of ZnO and LDH in samples. ............................................................ 91
Table 4-2 Langmuir and Freundlich isotherms parameters of OrgII uptake on (x)LDO ..... 98
Table 4-3 Apparent reaction rates of photocatalysts. ....................................................... 103
Table 5-1 BET surface area, pore volume, bandgap and the interplanar distance for
samples and the respective pseudo-first-order rate constant for MB and OrgII
photodegradation over photocatalysts. ............................................................................ 115
XX
Table 6-1 Zeta potential, BET surface area, pore volume, crystallite size and bandgap for
the samples and the pseudo-first-order rate constant for MB photocatalytic degradation
over different photocatalysts. ........................................................................................... 139
List of Schemes
Scheme 2-1 Microscopic mechanism of the reaction paths of polymeric carbon nitride.150
........................................................................................................................................... 38
Scheme 2-2 Photodegradation pathways of MO and RhB over g-C3N4. ........................... 42
Scheme 2-3 Photocatalytic degradation mechanism for the heterostructured ternary
nanocomposite. .................................................................................................................. 50
Scheme 6-1 Schematic representation of the synthesis process of the ZnO-LDH@C3N4
composite. ........................................................................................................................ 132
List of Abbreviations
AC activated carbon
AOPs advanced oxidative processes
BET Brunauer, Emmett, and Teller theory of gas adsorption
BJH Barrett-Joyner-Halenda analysis
BC biochar
BQ p-benzoquinone
CB carbon black
CLDH calcinated layered double hydroxide
CG carboxyl graphene
CDs carbon dots
CBM conduction band minimum
CDs carbon dots
CR Congo Red
CN carbon nitride
CNB B-modified graphitic carbon nitride
DRS diffuse reflectance spectra
EPR electron paramagnetic resonance
EDX energy dispersive X-ray spectroscopy
EDTA-2Na ethylenediaminetetraacetic acid disodium salt
EIS electrochemical impedance spectroscopy
FT-IR Fourier transform infrared
GO graphene oxide
XXI
g-C3N4 graphitic carbon nitride
HCH hexachlorocyclohexane
HMTA hexamethylenetetramine
HMT hexamethylenetetramine
HRTEM high-resolution transmission electron microscopy
HOMO highest occupied molecular orbital
IPCE incident photon to current conversion efficiency
IPET interparticle electron transfer
IPA isopropyl alcohol
LBL layer-by-layer
LDH layered double hydroxide
LDO layered double oxide
LSE light scattering electrophoresis
LMCT ligand-to-metal charge-transfer
LUMO lowest unoccupied molecular orbital
NFs nanofibers
NPs nanoparticles
MMO mixed metal oxide
MB methylene blue
OG orange G
OrgII Orange II sodium salt
PL photoluminescence
PCS photon correlation spectroscopy
POM polyoxometalates
PANI polyaniline
QCE quantum confinement effect
RLDH rehydrated (and/or) reconstructed layered double hydroxide
RhG rhodamine 6G
RHE reversible hydrogen electrode
ROS reactive oxygen species
RGO reduced graphene oxide
SAED selected area electron diffraction
SEM scanning electron microscopy
SDS sodium dodecyl sulphate
XXII
SPR surface plasmon resonance
TEM transmission electron microscopy
TG-DSC thermogravimetric-differential scanning calorimetry analysis
THF tetrahydrofuran
TEOA triethanolamine
t-BuOH tertbutyl alcohol
UV ultraviolet
UV-vis DRS ultraviolet-visible light diffuse reflectance spectroscopy
VB valence band
VBM valence band maximum
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
1
1 Introduction
1.1 Background
Water is not only fundamental to our life and health but also essential for all life on the earth
without any substitutes. Recently, a growing population with increasing standard of human
beings’ living, food production and industrialisation has been putting much pressure on the
water resource. Pollutions and contaminations have been turning the limited fresh water into
wastewater. Although saving water is a matter for easing water problems, water is still limited
globally; therefore, wastewater treatment seems increasingly important when more and
more sewage is discharged from industries and households.
Manufacturers including the textile industry utilise thousands of types of dyes and pigments,
generating a large amount of wastewater containing dyes, which must be treated before
released. Currently, there are mainly three means to treat dye-polluted water: the chemical,
physical and biological methods. The advantages and disadvantages of the methods are
compared in Table 1-1.1
The most popular Fenton process generates hydroxyl radicals (•OH) to decompose organic
compounds into water and carbon dioxide. However, the traditional Fenton reaction requires
large amounts of chemical reagents and suffers from the generation of excess sludge and
strict pH requirement (pH~3). The leaching problems and the potential secondary
contamination still obstinately exist in any metal-based homogeneous and heterogeneous
systems in the advanced oxidative processes (AOPs).2
The adsorption technology is also a popular tertiary treatment method for dye containing
effluents because of its variety, relatively high efficiency and portability. Various kinds of
adsorbents are available, such as activated carbon (AC), silica gel, activated alumina,
zeolites and molecular sieves, clays, and polymeric resins.3 One of the key issues of the
adsorption technology is the regeneration of adsorbents, which requires high energy input
and often creates secondary environmental problems (e.g. CO2 emitted during incineration).
Table 1-1 Advantage and disadvantage of the current methods of dye removal from industrial
effluents.1
Methods Advantages Disadvantages
2
Chemical
methods
Fenton reagent Effective decolourisation of
both soluble and insoluble
dyes
Sludge generation
Ozonation Applied in gaseous state: no
alteration of volume Short half-life (20 min)
Photochemical No sludge production Formation of by-products
NaOCl Initiates and accelerates azo
bond cleavage
Release of aromatic
amines
Cucurbituril Good sorption capacity for
various dyes High cost
Electrochemical
destruction
Breakdown compounds are
non-hazardous High cost of electricity
Physical
methods
Activated
carbon
Good removal of wide variety
of dyes Very expensive
Peat Good adsorbent due to
cellular structure
Specific surface areas for
adsorption are lower than
activated carbon
Wood chips Good sorption capacity for
acid dyes
Requires long retention
times
Silica gel Effective for basic dye
removal
Side reactions prevent
commercial application
Membrane
filtration Removes all types of dyes
Concentrated sludge
production
Ion exchange Regeneration: no adsorbent
loss Not effective for all dyes
Irradiation Effective oxidation at lab
scale
Requires a lot of
dissolved O2
Electrokinetic
coagulation Economically feasible High sludge production
Biological
methods
Decolourisation
by white-rot
fungi
Economically feasible
Require a fermentation
process, incomplete
decomposed
3
Adsorption by
living/dead
microbial
biomass
High effect adsorption of toxic
dyes
Early saturation can be
problem
Anaerobic
textile-dye
bioremediation
systems
Production of biogas Additional carbon and
hydrogen are required
Lots of interest has been devoted to semiconductor photocatalysts due to their potential
applications in environmental purification and solar energy transformation since the initial
work of Fujishima and Honda using TiO2 for hydrogen generation by water photoelectrolysis
in 1972.4 Since then on, the semiconductor photocatalysis for photocatalytic degradation5,
sterilisation,6 water splitting (hydrogen and/or oxygen generation),7 conversions of CO2 to
methonal8 (or other gases reduction such as NOx reduction9) has been studied extensively.
Figure 1-1 schematically illustrated the photocatalytic process upon sunlight irradiation.
Under illumination, an electron in the valence band absorbs light energy which is higher than
the bandgap energy of the semiconductor will jump to the conduction band, leaving behind
a hole. The electron can move to the solid surface to react with dissolved oxygen to form
reactive oxygen species such as superoxide anions (•O2-). On the other hand, the hole can
also move to the surface of the solid to react with water then form hydroxyl radicals (•OH).
Both radicals (•O2- and •OH) and holes are oxidative that can oxidise organic pollutants.
Unlike traditional adsorption technology and AOPs, photocatalytic degradation of organic
dyes over semiconductors offers greater potential for the complete elimination of toxic
chemicals in water with a predominant advantage i.e. recycle and sustainability.
Among all the semiconductors, both the layered double hydroxide based photocatalysts and
the graphitic carbon nitride (g-C3N4) are promising materials for wastewater treatment,
considering their unique 2D structure, high abundance in nature, environmental friendliness
and synergistic adsorption property.
4
Figure 1-1 Schematic illustration of photocatalysis.
1.2 Research aim and scope of the project
This project aims at developing a series of heterogeneous photocatalysts based on layer-
structured semiconductors for photodegradation of aqueous dyes which are accompanied
by synergistic adsorption. Two types of semiconductors with layered structure were chosen
to be the starting materials; that is zinc aluminium layered double hydroxide with brucite
structure and graphitic carbon nitride (g-C3N4) with graphite-type structure.
The research objectives of this project are to:
(1) Explore the optimal synthesis conditions for the zinc aluminium layered double oxide
composites with adjusting the synthetic parameters including the molar ratio of Zn
and Al, calcination temperature.
(2) Investigate the modification of graphitic carbon nitride with carbon black.
(3) Construct inorganic/organic semiconductor composites with ZnO-LDH layered
semiconductors and the g-C3N4 nanosheets.
(4) Study the structures and physicochemical properties of the composites.
(5) Evaluate the adsorption kinetic and equilibria of dyes over photocatalysts.
(6) Evaluate the photocatalytic degradation properties of photocatalysts under UV and/or
visible light irradiation.
(7) Understand the relationship between structure, physicochemical properties and the
adsorption, photodegradation performance.
5
1.3 Thesis structure
Chapter 1. Introduction
This chapter introduces the background of the thesis and outlines the research goal and
scope.
Chapter 2. Literature review
This chapter presents the review on LDH and g-C3N4 based photocatalysts, respectively.
Chapter 3. Methodology
This chapter describes the experimental design and the main synthesis methods and then
outlines all the characterization analysis techniques used in this research finally, introduces
the evaluation on the removal of dyes in contaminated water for materials.
Chapter 4. Uptake and degradation of Orange II by zinc aluminium layered double
oxides
This chapter is based on the investigation on the adsorption, and photocatalytic
decomposition of OrgII over zinc aluminium layered double oxide composites (LDOs) under
UV light irradiation. The effect of synthetic parameters including the molar ratio of Zn to Al
and the calcination temperatures on adsorption and photocatalytic activity was investigated.
The relation between the structural features of the ZnO-rLDH composites (rehydrated
layered double hydroxide) and the adsorption properties and photocatalytic activity of LDOs
was also studied.
Chapter 5. Carbon black modified g-C3N4 as adsorptive photocatalysts for
decontamination of dyes under visible light
This chapter is based on the investigation on photodegradation of dyes over the carbon
black (CB) modified g-C3N4 under visible light irradiation. The effects of varying CB amount
on the structure, physicochemical properties of g-C3N4 were well investigated. The
adsorption and photocatalytic degradation of both anionic dye OrgII and cationic dye MB
over CB modified g-C3N4 samples were comprehensively evaluated. The relations between
the structure features of CB modified g-C3N4, and the adsorption properties and
photocatalytic activity were also thoroughly analysed.
6
Chapter 6. ZnO-layered double hydroxide@graphitic carbon nitride composite for
consecutive adsorption and photodegradation of dyes under UV and visible lights
This chapter is based on the investigation on the adsorption and photocatalytic properties
of ZnO-LDH@C3N4 composite towards dyes under UV and visible light irradiation
respectively. Adsorption performance on anionic dye OrgII was studied. Adsorption and
degradation performances on cationic dye MB under UV and visible light were also
investigated. The adsorption and photocatalytic mechanism were consequently proposed
based on the experimental result.
Chapter 7. Conclusion and recommendation for future work
This chapter summarises the overall conclusion and presents the recommendation for
future work.
1.4 References
1. Robinson, T.; McMullan, G.; Marchant, R.; Nigam, P., Remediation of dyes in textile
effluent: a critical review on current treatment technologies with a proposed alternative.
Bioresource Technol 2001, 77 (3), 247-255.
2. Ai, B.; Duan, X. G.; Sun, H. Q.; Qiu, X.; Wang, S. B., Metal-free graphene-carbon
nitride hybrids for photodegradation of organic pollutants in water. Catalysis Today 2015,
258, 668-675.
3. Jovančić, P.; Radetić, M., Advanced sorbent materials for treatment of wastewaters.
In Emerging Contaminants from Industrial and Municipal Waste, Springer: 2008, pp 239-
264.
4. Fujishima, A.; Honda, K., ELECTROCHEMICAL PHOTOLYSIS OF WATER AT A
SEMICONDUCTOR ELECTRODE. Nature 1972, 238 (5358), 37-38.
5. Zhao, H. X.; Chen, S.; Quan, X.; Yu, H. T.; Zhao, H. M., Integration of microfiltration
and visible-light-driven photocatalysis on g-C3N4 nanosheet/reduced graphene oxide
membrane for enhanced water treatment. APPL CATAL B-ENVIRON 2016, 194, 134-140.
6. Yokomizo, Y.; Krishnamurthy, S.; Kamat, P. V., Photoinduced electron charge and
discharge of graphene–ZnO nanoparticle assembly. Catal Today 2013, 199 (0), 36-41.
7. Mou, Z.; Yin, S.; Zhu, M.; Du, Y.; Wang, X.; Yang, P.; Zheng, J.; Lu, C.,
RuO2/TiSi2/graphene composite for enhanced photocatalytic hydrogen generation under
visible light irradiation. Phys Chem Chem Phys 2013, 15 (8), 2793-2799.
7
8. Hsu, H.-C.; Shown, I.; Wei, H.-Y.; Chang, Y.-C.; Du, H.-Y.; Lin, Y.-G.; Tseng, C.-A.;
Wang, C.-H.; Chen, L.-C.; Lin, Y.-C., Graphene oxide as a promising photocatalyst for CO2
to methanol conversion. Nanoscale 2013, 5 (1), 262-268.
9. Heo, I.; Kim, M. K.; Sung, S.; Nam, I.-S.; Cho, B. K.; Olson, K. L.; Li, W., Combination
of Photocatalysis and HC/SCR for Improved Activity and Durability of DeNOx Catalysts.
Environ Sci Technol 2013, 47 (8), 3657–3664.
8
2 Literature Review
2.1 Layered double hydroxides
Layered double hydroxides (LDHs), also known as anionic or hydrotalcite-like clays,
constitute a class of layered materials consisting of positively charged brucite-like layers and
the interlayer exchangeable anions. The LDH can be represented by the general formula
[M2+1-x M3+
x(OH)2]q+ [An-q/n·mH2O], where M2+ and M3+ are divalent and trivalent metal cations
such as Mg2+, Ca2+, Mn2+, Fe2+, Ni2+, Cu2+ or Zn2+ and Al3+, Mn3+, Fe3+, Co3+ or Ni3+,
respectively.1 An- is an interlayer exchangeable anion such as CO32-, SO4
2- and NO3-, Cl-
and OH-, S2- et al.
The first natural LDH mineral [Mg6Al2(OH)6]CO3∙4H2O was discovered in Sweden around
1842, but the layered structure of LDHs wasn't ascertained until 1969. With the swift
progress of science and technology, the research on LDHs is developed rapidly after the
1990s. The structure characteristic, properties and applications of LDHs have blossomed to
the uttermost in recent decades.2
An advantage of LDHs is that they can be prepared in large quantities in a reliable and
reproducible manner by precipitation of aqueous solutions of the corresponding metal salts
by increasing the solution pH. Moreover, the atomic ratio between the divalent and trivalent
metal ions of the LDHs can be varied in a wide range without altering the layered structures.
By selectively changing the composition of divalent and trivalent metal ions, all the LDHs
make up a rare class of claylike materials with flexible two-dimensional platelet structures,
which could be further exfoliated into functional unilamellar nanosheets.3 In addition, the
charge-balancing anions located in the interlayers can be easily replaced via ion exchange
reactions. By virtue of this property, the incorporation of a wide variety of anions including
simple inorganic anions (e.g. CO32- NO3
-),4 organic anions (e.g. benzoate, succinate)5 and
complex biomolecules (e.g. DNA)6 into the interlayer has been realised. Such an anion
exchange property also endows LDHs the remarkable shape-selective properties that can
be used for the separation of isometric drug molecules.4-6
The positively charged layers in LDHs are stocked together with anionic ions in the
interlayers. The charged layers can be exfoliated out through swelling in delaminating
agents such as formamide and butanol.7-9 The thickness of an exfoliated LDH nanosheet is
9
around 0.5–1 nm that is in the range of molecular dimensions, while the lateral dimension is
ranged from hundreds of nanometers to micrometer.7 The quantum size effect10 and the
increased active sites on the surface of LDHs will remarkably broaden their potential
applications.9 The basic structure and crystal characters of LDHs are shown in Figure 2-1.11
Figure 2-1 Schematic Representation is for comparing the crystal structure of brucite (A) and LDH
(B). Reproduced from ref.11
Due to the large interlayer spaces and the significant number of exchangeable anions, LDHs
have high surface area and high anion exchange capacity, having the potential to be good
ion-exchangers and adsorbents. They (and their derivatives such as surfactant modified
LDH and calcined LDH) have, indeed, been reported as effective adsorbents for the removal
of contaminants12 such as azo dyes, triphosphate and arsenate from aqueous solutions.13-
16 The versatility of LDHs in eliminating different contaminants enables them to be viable
alternatives for environmental remediation.14, 17 Recently, LDHs and their derivatives
working as photocatalytic materials have received increasing attentions and have
demonstrated excellent performance in photocatalytic water treatment or
photoelectrochemical processes.18 It is noticed there are many reviews on LDH and its
derivatives in past years.3, 7, 9, 19
This section aims to give an overview of the most important procedures developed in this
remarkably investigated field of the research. It will highlight the synthetic potential, always
taking into account the previously mentioned synthetic challenges. The potential
applications of LDH and LDH-based heterostructures in photocatalytic water treatment, CO2
10
conversion and water splitting, etc. will also be presented while discussing the various LDH
materials.
2.2 Layered Double Hydroxide-Based Photocatalysts
2.2.1 Structure of LDH
LDH is a class of anionic clay consisting of various anions and metal hydroxide layers. The
hydroxide layer is like the brucite layer Mg(OH)2 in which each Mg2+ ion is surrounded by
six OH- ions in an octahedral arrangement. The partial replacement of the divalent cations
in brucite by trivalent ions leads to positively charged LDH layers, which are balanced by
the intercalation of interlayer anions such as CO32-, NO3
-, Cl- etc. The identities of the di-
and trivalent cations (M2+ and M3+ respectively) and the interlayer anion (An-) together with
the mole ratio between M2+ and M3+ may be varied over a wide range, giving rise to a large
class of isostructural materials.20 There are a number of combinations of divalent and
trivalent cations for LDH production, and the layered structures can be formed as long as
the radii in the octahedral coordination of M2+ and M3+ are close to those of Mg2+ and Al3+.21
The ionic radii of some cations incorporated in LDH materials are shown in Table 2-1.
Table 2-1 Ionic radii of some cations with a coordinate number of 6. Reproduced from ref.21
M2+ Radius/ nm M3+ Radius/ nm
Fe
Co
Ni
Mg
Cu
Zn
Mn
Pd
Cd
Ca
Ti4+
Sn4+
0.061
0.065
0.069
0.072
0.073
0.074
0.083
0.086
0.095
0.100
0.061
0.069
Al
Co
Fe
Mn
Ga
Rh
Ru
Cr
V
Y
La
Zr4+
0.054
0.055
0.055
0.058
0.062
0.067
0.068
0.069
0.074
0.090
0.013
0.072
11
The structure of LDH can be characterised by many techniques. Among them, X-ray
diffraction (XRD) is the most widely used method to characterise the interlayer spacing and
the thickness of LDH materials. Generally, the diffraction peaks of LDH in XRD patterns
locate in 2Ө range of 5 ~ 60o ascribing to the characteristic diffraction peaks of the
rhombohedral phase (Figure 2-2).22-23 The peaks will shift depending on the LDH
compositions. Based on the (003) diffraction peak, the interlayer basal spacing of LDH can
be calculated according to Bragg's law equation: nλ=2dsinӨ, where n is the order number
of diffraction, λ is the incident wavelength of radiation ray, Ө is the angle diffraction occurs,
and d is the distance of the spacing.
The cell parameters of LDH can also be calculated using the following equations:24
a=2×d(110)
c=3×d(003)(or 6×d(006))
And the crystallite size of the LDH materials can be determined using the Scherrer equation
in its simplified version (lattice distortions are neglected), whereas L, k, λ and β represent
the mean size of the ordered (crystalline) domains, a dimensionless shape factor, X-ray
wavelength and the line broadening at half the maximum intensity (FWHM) respectively.25
L=(λk)/(βcosӨ)
Figure 2-2 XRD patterns of (a) Zn2Al-LDH, (b) Zn3Al- LDH, and (c) Zn4Al-LDH.Reprinted with
permission from ref.26
12
LDH is also characterised by Fourier transform infrared spectroscopy (FT-IR) technique to
identify the anions especially organic anions located in the interlayers. Typically, LDH
materials have a strong and broadband centred around 3400 cm-1 attributing to the OH-
stretching vibrations of the hydroxyl groups in the layers and interlayer water molecules
(Figure 2-3), and the water deformation band located at around 1635 cm-1. The vibration at
around 1350 cm-1 indicates the presence of inorganic CO32-or NO3
- in the interlayers.27
Figure 2-3 FT-IR Spectra of (a) ZnAl-LDH, (b) ZnAlSn-LDH and (c) ZnSn-LDH. Reprinted with
permission from ref.27
13
It is known that a crystallised LDH normally has hexagonal flakes composed of octahedral
units of metal hydroxide layers. By utilising ammonia-releasing hydrolysis agents such as
urea28-29 or hexamethylenetetramine (HMT) together with the hydrothermal treatment, highly
crystallised CoAl-LDH with distinct hexagonal shape (Figure 2-4) could be readily
synthesised with the lateral diameter ranging from 10 µm to 80 µm. The thickness of the
flake is about several hundred nanometers due to the stacking of LDH flakes.
Figure 2-4 SEM micrograph of the as-prepared CoAl-CO3-LDH. Reprinted with permission from
ref.29
The nature of anions including the types and orientation of anions13 in the LDH interlayers
could affect the anion exchange behaviors30 and the adsorption kinetics of LDH.31-32 It’s
revealed that stacking products can be derived from different interlayer contents or different
orientation of the same anionic species compensating for host layer charge in LDH
materials.33 Moreover, the stacking is not only influenced by the anionic species but also
influenced by the M2+/M3+ ratio, the metallic composition ratio in starting brucite influences
the staging phenomena of LDH. Sasaki et al.34 discussed the correlation between staging
product and metallic composition ratio. They discovered that for transition metals LDH when
a stoichiometric amount of iodine was used, Co2+2/3Fe2+
1/3(OH)2 was completely transformed
14
into a pure LDH phase with a different formula: Co2+1-xFe2+
x(OH)2 + 0.5xI2 →Co2+1-
xFe3+x(OH)2Ix.
The layer structure order and the interlayer species order are changed with the hydrothermal
treatment time, and temperatures.35 Gray and coworkers36 fabricated Mg2Al-Cl-LDH with a
fast co-precipitation process followed by controlled hydrothermal treatment and analysed in
detail the influence of hydrothermal treatment temperature and duration time on the particle
size. The suspension of freshly precipitated Mg2Al-Cl-LDH consisted of a bimodal particle
size distribution with diameters of 320 and 2300 nm, respectively. After ageing at 50 °C
overnight, the LDH aggregates decreased in size to 220-955 nm. However, the particle size
of LDHs fabricated by hydrothermal treatment at 100 °C for 16 hrs after co-precipitation had
a narrow distribution with a diameter of 114 nm (Figure 2-5). It is also noted that 2 h
treatment at 100 °C was not long enough to de-aggregate all aggregates into individual LDH
crystallites, while the 144 h treatment produced larger LDH crystallites. The results indicate
that hydrothermal treatment is effective in de-aggregating LDHs to form nanosheets.
Figure 2-5 Particle size distribution of Mg2Al-Cl-LDH samples collected with the photon correlation
15
spectroscopy (PCS). Left: (X) Coprecipitated and stirred for 10 min at room temperature, with two
peaks at 320 and 2300nm; (Y) sample X aged at 50 °C for 16 h, with two broad peaks at 220 and
955 nm; (Z) 100 oC Hydrothermal treatment for 16, with one sharp peak at 114 nm. Right:
Dispersion of Mg2Al-Cl-LDH aggregates with heating duration during the hydrothermal treatment at
100 °C, where the distribution curves were obtained with PCS. Reproduced from ref.36
2.2.2 Methods for preparing LDH materials
The conventional method for synthesising LDH materials is the direct coprecipitation
method. At a constant pH, M2+ and M3+ in their mixed ions saline solution is precipitated to
form double hydroxide sediments by adding equal stoichiometric amount of aqueous
alkaline solution containing NaOH and Na2CO3 (or KOH and K2CO3).37 Mixed metal
hydroxides hydrate is dominant in the synthesis products for the simple coprecipitation step,
with further ageing treatment, LDH can be gradually formed and is mainly in an aggregation
state with the sizes of 1-10 μm, which contains hundreds and thousands of sheet-like LDHs
nanocrystallites in each aggregate.38 In general, hydrothermal treatment is the most
common process for the formation of hexagonal shape LDH plates. In this process, all
reagents are put into a sealed autoclave and then hydrothermally treated at a low
temperature at 100 to 200 oC for a period of time. The advantage of this method is that the
reaction conditions are controllable and always generate highly crystallised LDH products.34
With pure Al or Zn plates as a template, ZnAl-LDH crystallises on the substrate is a
promising phase transition method to fabricate high-crystallized ZnAl-LDH.39 In this method,
a certain proportion of Zn and Al ions with hexamethylenetetramine (HMTA) as buffer were
prepared and adjusted to pH 7.5 by adding ammonia into the solution. The resulting solution
severed as the bulk solution was then transferred to a hydrothermal bottle with a clean Al or
Zn plate suspended horizontally in the solution held by a Teflon container. After
hydrothermal treatment, ZnAl-LDH gradually grew with its ab plane perpendicular to the Al
or Zn substrate i.e. on substrate plate vertically (c-axis parallel to the substrate) (Figure 2-
6), and a proposal tentative mechanism for the formation of ZnAl-LDH platelets on Al plate
is also shown in the following reaction equations:39
C6H12N4 + 6H2O 4NH3 + 6HCHO
NH3 + H2O NH4
+ + OH-
2Al + 2OH- + 6H2O 2Al(OH)4- + 3H2
16
Al(OH)4- Al(OH)3 + OH-
Zn2+ + 2OH- Zn(OH)2 + 2OH- Zn(OH)42-
Zn2+ + 4NH3 Zn(NH3)42+
Zn(OH)2 + NH3 Zn(NH3)42+ + 2OH-
Al(OH)3/Al(OH)4- + Zn(OH)4
2-/Zn(NH3)42+ + OH- +NO3
- +H2O ZnAl-LDH
The crystallised ZnAl-LDH has been demonstrated to be effective photocatalyst for the
decomposition of Congo red in aqueous medium and could also serve as anode materials
for Li-ion batteries.39-42
Figure 2-6 SEM images of plate LDH sample. Inset is the photographic image of 2 cm × 2 cm Al
plate fabricated with ZnAl-LDH assembles. Reproduced from ref.39
. Reproduced Copyright (2012).
17
Sasaki and coworkers,7, 32 reported the formation of LDH with the transition metals served
as the host layer composition instead of the limited M2+-Al3+ pattern. The CoFe-LDHs were
synthesised via oxidative intercalation reaction using an excess amount of iodine as the
oxidising agent. This synthesis method is called the topochemical oxidative intercalation
method and has recently been developed to realise micrometer-sized crystals consisting of
various transition metal ions such as Co2+-Fe3+, Co2+-Co3+, Ni2+-Co3+. In this method, mono-
or bimetallic brucite-like hydroxides including Co2+(OH)2, Co2+2/3Fe2+
1/3(OH)2, and
Co2+xNi2+
1-x(OH)2 were firstly precipitate via the hexamethylenetetramine (HMT) hydrolysis,
which was transformed to Co2+-Fe3+, Co2+-Co3+, Ni2+(Co2+)-Co3+ LDHs through partial
oxidation of divalent transition metal ions (Fe2+, Co2+) to a trivalent state by employing
halogens (iodine, bromine) in organic solvents, such as chloroform or acetonitrile. At the
same time, halogen anions (I-, Br-) were intercalated between the layers. The original
morphology and size of brucite-like crystals were well maintained during the process,
namely a topotactic conversion. The halide-intercalating transition metal LDHs, after anion-
exchange into other anionic forms (e.g. CO32-, NO3
-, ClO4-), further exfoliated in formamide
to produce LDH nanosheets.
As LDH could restore its original structure after a two-step annealing and reconstruction
procedures, such property is often used as an alternative method for the preparation of
LDHs with different anionic ions in the interlayers.43 In addition, there are several reports on
the LDH fabrication which combines the coprecipitation process with the microwave
hydrothermal treatment.33 A series of ZnAl-LDH hexagonal particles were synthesised by
using the microwave irradiation method. With the assistance of microwave irradiation,
secondary growth led to the formation of ZnO nanorod on ZnAl-LDH substrate shown in
Figure 2-7, and then to ZnO nanotube/ZnAl-LDH heterostructures with further ageing. Two-
dimensional ZnO film/ZnAl-LDH sandwich-like heterostructures were also formed during
secondary growth in a citrate anion-containing solution.44
18
Figure 2-7 (a) ZnO nanorod/ZnAl-LDH; (b) ZnO nanotube/ZnAl-LDH; (c) ZnO film/ ZnAl-LDH.
Reproduced from ref.44
2.2.3 Modification of LDH
2.2.3.1 Modification of LDH structures
A diverse combination of M2+-M3+ cations in LDH host sheets has been extensively
pursued, and can be routinely attained through a convenient coprecipitation of
corresponding di- and trivalent metal salts under an alkaline condition, which produces a
large family of M2+-M3+ LDHs. However, the research interest in LDH materials has been
traditionally driven and dominated by M2+-Al3+ category, particularly Mg2+-Al3+,2 partly
because hydrotalcite Mg6Al2(OH)16(CO3)•4H2O is a widely known anionic clay found in
nature. Moreover, the amphoteric feature of Al3+ plays a very favourable role in promoting
the precipitation and crystallisation of M2+-Al3+ LDH. Besides Al3+, other trivalent transition
metal ions including Fe3+, Cr3+, Mn3+, Ga3+, V3+ and Ni3+,45 and some tetravalent metal ions
(such as Ti4+,46 and Zr4+ ions47) have also been used to prepare LDHs materials.
LDHs are well-known catalyst supports (e.g. Mg-Al hydrotalcite),48 they have also been used
as catalysts for different applications. By modifying the nature and mole ratio of cations in
the LDH platelets, the properties of LDHs have been optimised. Silva et al.49 prepared a
novel series of ZnM-LDHs (M = Cr, Ti, Ce) at different Zn/M atomic ratio and tested them for
the visible light photocatalytic oxygen generation. They found that the most active material
was ZnCr-LDH and the efficiency of these chromium LDHs for oxygen generation increases
asymptotically with the Cr content. In addition to binary metal elements, up to 30% of Al3+ in
Al-containing LDHs could be substituted by other tervalent or tetravalent ions to form a new
ternary LDH without changing the layered structures, a lot of multi-element LDHs such as
19
NiMgAl-LDH, ZnMgAl-LDH and MgCrAl-LDH have been successfully prepared.12, 24, 50 By
incorporating various metal elements (M = Ni, Zn, Cu, Co, etc.) into MgAl-LDHs, Beltramini
and coworkers51 revealed that CuAlMg-LDH materials are active in catalytic oxidation of
glycerol. The selectivity remarkably increased over Cu-containing LDHs compared with
other metal incorporated LDH materials. Valente and coworkers17 prepared MgZnAl-LDH
with varying amounts of Zn, which showed that introducing a small amount of Zn in MgAl-
LDH modified the band gap energy and the adsorption capacities of these materials
substantially, thus enhanced the photocatalytic degradation of 2,4-dichlorophenoxiacetic
acid and phenol. The degradation rate for 4-chlorophenol was even faster in the presence
of MgZnAl-LDH than that in the presence of Degussa P25. Other ternary LDHs such as
ZnAlFe-LDH and rare earth elements doped LDHs such as Eu-doped ZnAl-LDH have also
been prepared with excellent photocatalytic activities,52 or photoluminescent properties.53-54
Changing the nature of the interlayer anions is another method to modify the properties of
LDHs. Recently, Satpathy and coworkers55 fabricated molybdate/tungstate intercalated LDH
using nitrate intercalated ZnY-LDH as precursors through ion exchange process. The
composite material demonstrated high reactivity towards the degradation of rhodamine 6G
(RhG) upon visible-light irradiation with excellent stability. The electron paramagnetic
resonance (EPR) measurement revealed that the absorption in the visible region was
attributed to the metal-to-metal charge transfer (MMCT) excitation of oxo-bridged bimetallic
linkage of Zn−O−Y in ZnY-LDH. The intercalation of MoO42-/WO4
2- ions enhanced the
harvesting of visible light, lowered the bandgap of ZnY-LDH, and increased the surface
areas by broadening the interlayer space where the photodegradation reaction occurred.55
Apart from changing the LDH compositions, the modification of LDH morphology provides
an alternative solution for extending the potential application of LDH materials. Shao et al56
demonstrated that using sodium dodecyl sulfonate (SDS) surfactant as the template, the
MgFe-LDHs were fabricated in microspheres structure instead of being a layer-by-layer
structure with the tunable interior. By controlling the concentration of SDS, the structure of
the LDHs could be tailored from hierarchical flower-like solid spheres to yolk-shell and then
to hollow spheres with the shell thickness of around 85 nm, close to the lateral size of LDH
nanoflakes (Figure 2-8). The hollow LDH microspheres showed high performance in
electrocatalytic oxidation of ethanol in an alkaline medium.
20
Figure 2-8 SEM and TEM images of MgFe-LDH microspheres with different inner architecture: A,
D) solid, B, E) yolk-shell, C, F) hollow, and G) EDX mapping results of a single LDH hollow
microsphere. Reprinted from ref.56
2.2.3.2 Coupling with other materials
Since the discovery of the ultraviolet (UV) light-induced photoelectrochemical water
splitting on TiO2 surface in 1972,57 the research on photocatalysis based on metal oxide
photocatalysts and their derivatives has never ceased.58-59 Considering the high adsorption
capacity and the diversity of LDHs, the synergistic effect derived from the combination
between LDH and TiO2 or other photocatalysts has been extensively investigated.23, 60-61 It
has been reported that by distributing TiO6 units in an MTi-LDH (M = Ni, Zn, Mg), the
composite materials displayed enhanced photocatalytic activity with a hydrogen production
rate of 31.4 µmolh-1 as well as excellent recyclable performance.61 The structural and
morphological studies revealed that a high dispersion of TiO6 octahedra in the LDH matrix
was obtained by the formation of an M2+-O-Ti network, rather different from the aggregation
state of TiO6 in the inorganic layered material K2Ti4O9. Such a dispersion strategy for TiO6
units within a 2D inorganic matrix decreased its bandgap to 2.1 eV and significantly
depressed the electron-hole recombination process, which accounts for its superior water
21
splitting behaviour. Lu et al.62 reported that by selectively reconstructing a Cu2+, Mg2+, Al3+,
Ti4+-containing LDH precursor through calcination and rehydration process, anatase TiO2
nanoparticles were found to be homogeneously distributed on the surface of the selectively
reconstructed CuMgAl-LDH support. The composite showed superior photocatalytic
properties to the single TiO2 phase and the physical mixture of TiO2 and LDH due to the
presence of TiO2/LDH heterojunction, which contributed efficient spatial separation between
the photogenerated electrons and holes. The aggregation of TiO2 nanoparticles was also
suppressed by the 2D LDH framework.63 By depositing TiO2 on sodium dodecyl sulphate
(SDS) intercalated LDH, Huang and coworkers64 prepared the composite SDS-LDHs/TiO2
(Figure 2-9) which combined the high adsorption properties of SDS-LDH and the
photocatalytic activity of TiO2. The enrichment of dimethyl phthalate (DMP) onto the organic
LDHs composite and the external hydroxyl groups generated by TiO2 produced a synergistic
effect leading to significantly enhanced degradation of DMP. The optimum amount of the
LDH powders as a carrier for the enhancement of the photocatalytic reactivity of SDS-
LDHs/TiO2 was estimated to be the mass ratio of 1:1.
Figure 2-9 The simulated structure of the composite SDS-LDHs/TiO2. Reproduced from ref.65
22
Another metal oxide ZnO, a semiconductor similar to TiO2 photocatalyst,66-68 has also been
coupled with LDHs. The deposition of ZnO with desired crystalline and nanostructure can
be obtained through etching ZnAl-LDH precursors or reconstruction of calcinated LDH
materials. As shown in Figure 2-10, ZnO with a higher percentage of exposed (0001) facets
was embedded in a hierarchical flower-like matrix using the in situ topotactic transformations
of an LDH precursor. The ZnO embedded material, also named flower-like mixed metal
oxides (F-MMO), showed enhanced photocatalytic activity under visible light irradiation
when compared with other ZnO nanorods and ZnO nanoplates with few exposed (0001)
facets.69 It should be noted that although F-MMO was used as a photocatalyst, the active
component should be a combination of ZnO and the reconstructed LDH,69-70 as the MMO
would easily reconstruct to its original LDH structure when dispersing in an aqueous solution
due to the memory effect of LDH.71 Dvininov et al.72 embedded the MgAl-LDH particles in
SnO2 domains by simple impregnation of the hydrotalcite with tin precursor followed by
hydrolysis and calcination, which improved the photocatalytic activity of SnO2 used for the
degradation of methylene blue (MB). The inactive MgAl-LDH particles in contact with SnO2
act as a barrier layer for charge recombination and electron trap sites leading to a better
charge separation. The combination of magnetic Fe3O4 nanoparticles and ZnCr-LDH not
only improved the photocatalytic degradation of MB and methyl orange (MO) but also
enhanced the separation and re-dispersion performance of LDH in aqueous solution.22 The
photocatalytic performance of CeO2 deposited on MgAl-LDH is even far superior to Degussa
P25 photocatalyst for the degradation of phenol and 4-chlorophenol under UV irradiation.73
23
Figure 2-10 SEM images of the as-obtained F-MMO sample: (a) an overall view of the F-MMO
microspheres; (b) a single F-MMO microsphere; (c) the nanoflakes of an F-MMO microsphere; (d)
a portion of an F-MMO nanoflake with hexagonal ZnO nanoplatelets embedded on its surface
(inset: the cross section of the F-MMO nanoflake). Reprinted permission from ref.69
With the layer-by-layer accumulation technique, CdS nanoparticles were densely
immobilised on LDH sheets without aggregation into larger particles.74 The “quantum size
effect” arising from monodispersed CdS broadened the bandgap of CdS. Thus the
absorbance and photoluminescence intensity of immobilised CdS particles were enlarged
in LDH/CdS. LDH/CdS multilayers deposited on an F-doped SnO2 (FTO) electrode behaved
as an n-type semiconductor photoelectrode in an acetonitrile solution. Vectorial
photoinduced electron transfer and energy transfer along the band gap gradient from the
interface between the LDH/CdS film and the electrolyte solution to the collecting FTO
electrode enhanced the photocurrent generation in LDH/CdS multilayer films. By reacting
Zn, Cd, Al-containing LDH with H2S,75 ZnxCd1-xS nanoparticles have been successfully
loaded onto LDH surface. The ordered arrangement on an atomic level of the metal cations
within the LDH layers led to a homogeneous array structure of the ZnxCd1-xS solid, which
24
suppressed the recombination of photoexcited electrons and holes, leading to improving
photocatalytic activity. Besides, the absorption edge of ZnxCd1-xS was monotonically shifted
to the visible light region as the dissolved amount of Zn2+ ions into the CdS lattice decreased.
The photocatalytic performance of the ZnxCd1-xS increased with decreasing Zn mole fraction
and reached a maximum for a Zn mole fraction of 0.20.76
2.2.3.3 LDH derivatives
Annealing treatment is the most common method to obtain LDH derivatives. The thermal
stability of LDHs varies with their different constitutes.77 In general, the thermal
decomposition of LDH takes place in three steps: it starts with the removal of water
molecules from surface, edge and interlayer space, then the elimination of hydroxyl groups
from the brucite-like layers which is called dehydroxylation, followed by or overlapped with
the decomposition of interlayer anions.78 The first derivative of annealed LDH is layered
double oxide (LDO), which is the primary product from the first two decomposition steps.49,
68, 79-80 LDO has a memory effect which enables it to reform the LDHs structure after
contacting with water and anions.71 Increasing the calcination temperatures leads to the
decomposition of LDO compounds with the formation of metastable phase at temperature
300−600 oC. The metastable mixed metal oxides (MMOs) normally have a porous structure
and high thermal stability.79 Calcination of LDHs above 600°C is generally known to
crystallise and yield AO and spinel-type oxides, AB2O4, which can resist water from LDHs
structure reforming.23, 35 These derived MMO materials have been reported to display
outstanding magnetic, catalytic and textual properties.80-82
There are a large number of studies concerning the preparation of multi-cationic oxides or
spinel-type oxides through thermal decomposition of LDHs,23 for example, zinc aluminium
mixed metal oxide (ZnAl-MMO) nanostructures with prominent optical properties could be
obtained by calcinating ZnAl-LDH nanoplates in air at 400-800oC.80, 83 Calcination of LDHs
has been an alternative to the traditional chemical and physical methods for the fabrication
of a wide variety of MMO nanocomposite materials.26 Due to the hierarchical structure along
with a high specific surface area and wide pore size distribution, MMO derived from LDHs
has been under research as new type of highly efficient photocatalysts.84 MMO of calcined
LDH even has been studied in the fabrication of antireflection (AR) and antifogging coating
materials which can be applied in a variety of transparent materials including eyeglasses,
periscopes, swimming goggles and lenses in endoscopic surgery.32, 85
25
Duan and coworkers84 fabricated a polycrystal ZnAl-MMO framework from calcination of the
LDH. Ahmed et al.86-87 reported that after high-temperature annealing, the ZnAl-NO3-LDH
turned into ZnO and ZnAl2O4 in which the band gap of ZnO and the oxygen vacancies in
ZnO and ZnAl2O4 increased with the calcination temperatures. Cho et al.88 also used LDH
as a precursor to prepare ZnO and ZnAl2O4 mixture. In this work, LDH was initially
intercalated by terephthalate anions. After undergoing calcination in nitriding gas, the
terephthalate collapsed down into C, leading to the production of MMO with C and N evenly
distributed on the surface. The porous MMO demonstrated excellent performance in water
oxidation reaction, which was much higher than pure MMO and exhibited a 2.5-fold
enhancement in the photocurrent density (0.053 mA) at 1.23 V vs. Reversible Hydrogen
Electrode (RHE) as compared to N-doped MMO nanostructured photoanodes. Although the
incident photon to current conversion efficiency (IPCE) of MMOs over the entire UV-visible
region was below 5%, the IPCE curve traced almost the same as their UV-vis absorbance
curves, indicating that all the photons (even with the longest wavelength) absorbed by the
visible light absorbing sites created by the C and N co-doping were contributed to the water
oxidation.
As ZnAl-LDH nanostructures did not absorb UV and visible light (Figure 2-11),88
nanostructures prepared by calcination of LDH at 450 oC for 2 h in air demonstrated high
absorbance in the UV region. When the calcination temperature increased from 450 oC to
650 oC, the UV absorbance spectrum of the nanostructures gradually shifted toward longer
wavelengths. Above 750 oC, the calcined MMO nanostructures showed strong absorption
in the near ultraviolet region, which is the characteristic absorption of ZnO and ZnAl2O4.80
The ZnO/ZnAl2O4 nanocomposites derived from the ZnAl-LDH precursors had superior
photocatalytic performances to either single phase ZnO or similar ZnO/ZnAl2O4 samples
prepared by chemical coprecipitation or physical mixing method. The heterojunction
nanostructure and the strong coupling between ZnO and ZnAl2O4 phase were proposed to
contribute the efficient spatial separation between the photo-generated electrons and holes,
which concomitantly improve the photocatalytic activities.26
26
Figure 2-11 UV–vis diffuse reflectance spectra of ZnAl: LDH nanostructures and their
nanostructures produced by calcination at various temperatures. Reproduced from ref.88
The spinel structure MM2O4 such as ZnFe2O4, ZnAl2O4 derived from calcined LDH
precursors have also been reported with good photocatalytic activity.20 By dissolving a
mixture of ZnO and ZnFe2O4 gained from the calcination of ZnFe-LDH at 500 oC in aqueous
NaOH, pure ZnFe2O4 could be obtained with the substantially increased surface area and
pore volume. This kind of ZnFe2O4 showed higher photocatalytic activity in the degradation
of phenol than the corresponding ZnO and ZnFe2O4 mixture.20 The MMO particles Zn1-
xMgxO, prepared by a polymer-based calcination-purification method,81 effectively tuned the
bandgap of ZnO in the metastable solid solution state with the photoluminescence changed
from 2.12eV to 2.32eV. The enhanced photoluminescence in the visible region was due to
the incorporation of magnesium ions on zinc ion lattice sites.
2.2.4 Photocatalytic Applications of LDH
2.2.4.1 Basic principle of photocatalysis
In a solid material, the electrons occupy energy bands because of the extended bonding
network. In a semiconductor, the highest occupied and lowest unoccupied energy bands are
separated by a bandgap, Eg, a region devoid of energy levels. Activation of a semiconductor
photocatalyst is achieved through the absorption of a photon which results in the promotion
27
of electron, e- from the valence band into the conduction band with the concomitant
generation of a hole, h+, in the valence band.89
Figure 2-12 Schematic diagram illustrating the principle of semiconductor photocatalysis.90
The basic principle of semiconductor photocatalysis relies on the formation of an electron-
hole pair upon the absorption of a photon with energy equal to or greater than the
semiconductor band gap. The generated electron-hole pairs are highly reactive hence
involved in reductive and oxidative reactions on the surface of the semiconductor
surface immediately. Figure 2-12 shows a schematic diagram illustrating the general
principle of a photocatalysis process. In the photodegradation process, organic pollutants
are easily targeted by the photocatalytic generated radicals and holes.90 For an effective
photocatalysis process, the electron and hole should be transferred and this process must
compete effectively with the major deactivation route of electron-hole recombination.89, 91-92
2.2.4.2 LDH’s application in photocatalytic degradation of organic pollutants
Metal oxide nanostructures such as zinc oxide and titanium dioxide (TiO2) are useful for
photocatalysis, photoelectrochemical and photovoltaic processes. The energy levels for the
conduction and valence bands and the electron affinity of ZnO are similar to those of TiO2.
However, both ZnO and TiO2 are wide bandgap metal oxides, and they are only
photocatalytically active under ultraviolet irradiation. Although band gap narrowing can be
achieved by either elevating the valence band maximum (VBM) or lowering the conduction
band minimum (CBM) via doping or other techniques,93 the ultrafine particles are difficult to
separate from the reaction systems owing to their small particles sizes and the formation of
the milky dispersion.88 For other photocatalysts, such as CdS, photocorrosion is seemingly
28
unavoidable during the photocatalytic processes.94 As for LDH, an ample class of material
with a stable structure and positively charged framework has prospective application in
photocatalysis areas, which can be a huge supplement for the metal oxide photocatalysts.
There are so many reports about the application of LDH and its derivatives for the
photocatalytic degradation of non-biodegradable dyes and recalcitrant organic pollutants
such as phenol and chlorophenols.23, 55 Valente et al.73 studied the photodegrading
capabilities of CeO2/MgAl-LDH composite under UV irradiation, and discovered that the
degradation of phenol was superior to those obtained with benchmark Degussa P25 TiO2
photocatalyst under the same experimental conditions. They proposed that degradation of
the contaminants may be taking place by two different mechanisms: ligand-to-metal charge-
transfer (LMCT) and UV-generated electron–hole pairs depending on the solution pH and
the pKa values of the contaminants. If the solution pH < pKa, the acidic proton of the phenol
substrate interacts with a surface basic group or is physisorbed via hydrogen bonding, and
a charge-transfer complex is formed at defect sites near the CeO2/LDH interface (Figure 2-
13). Upon photon excitation, an electron could be transferred directly from the highest
occupied molecular orbital (HOMO) of the ligand (phenol or 4-chlorophenol) to the catalyst,
most likely to the 4f band of CeO2. Then the electron was transferred to a suitable electron
acceptor, O2, to form radical species O2•-, which in turn mineralized the contaminant phenol
into small molecules or even into CO2 and H2O. However, if solution pH > pKa, the
interaction of deprotonated phenol and the surface basic sites are hindered by electrostatic
repulsion, the LMCT is also prohibited. Photocatalysis takes place by the better-known
mechanism of UV-driven electron excitation from the valence band to the conduction band
of CeO2, followed by conversion of the electron–hole pair to oxidising radical species (mainly
O2•- and •OH) that in turn degrade the contaminant.
29
Figure 2-13 Proposal ligand-to-metal charge-transfer mechanism, initially responsible for the
degradation of phenol; (A) inner-sphere complexation occurring on a Brönsted basic site, and (B)
outer-sphere complexation occurring on a Lewis basic site. Reprinted from ref.73
Chen et al.23 synthesised highly active photo-responsive material, titanium-containing MMO
nanocomposites, from calcination of a NiTi-LDH and investigated their activities in degrading
MB under UV and visible light irradiation. The materials had a high surface area and a broad
absorption range from visible to ultraviolet wavelength, enabling the photodegradation of
MB to occur under UV and visible-light irradiation. Pode and coworkers60 used ZnAl-LDH as
a photocatalyst for the degradation of p-chlorophenol by advanced oxidation processes
under UV light irradiation. The degradation proceeded in a photo-Fenton method instead of
traditional heterogeneous photocatalysis with the addition of H2O2 and ferrous ions at the
beginning of the photocatalytic reaction. Paratungstate combined with LDHs has also been
reported to have high photocatalytic activities.95 Paratungstate, a class of polyoxometalates
(POM), has similar features with the semiconductor.96 By irradiating a Mg12Al6(OH)36-
(W7O24)∙4H2O suspension in the near UV area, trace aqueous organochlorine pesticide,
hexachlorocyclohexane (HCH), was totally degraded and mineralized into CO2 and HCl.97
30
The disappearance of trace HCH was well fitted with Langmuir–Hinshelwood first-order
kinetics and the photogeneration of •OH radicals was responsible for the degradation.
Zn/Al/W(Mn) mixed oxides produced by calcination of the POM-containing LDH precursors
in air at 600–700 oC showed great photocatalytic activity in degradation of aqueous HCH
solutions.97
Xu and coworkers synthesised hierarchical ZnO/ZnAl2O4 derived from ZnAl-LDH, which
showed high activities in photocatalytic degradation of both anionic dye methylene orange
(MO) and cationic dye MB, superior to ZnO photocatalyst alone.98 It is noted that the
degradation of MO over ZnO/ZnAl2O4 is faster than that of MB (Figure 2-14). The
enhancement may be attributed to the stronger coupling interaction between MO and the
interface of ZnO and ZnAl2O4, as the adsorption of MO on ZnO/ZnAl2O4 is around 20.5%,
much higher than 12.0% of MB. The homogeneously dispersed ZnAl2O4 phase in the
network of ZnO also greatly contributed to the enhanced separation of charge-hole pairs
and the subsequent photoactivities.
Figure 2-14 Photodegradation on (a) MB and (b) MO/MB mixture monitored as the normalised
concentration vs. irradiation time.98
Zhu and coworkers99 reported the synthesis of self-assembled carboxyl graphene (CG) and
ZnAl-LDH and its derivative product, reduced CG/ZnAl-layered double oxide nanohybrid
(rCG/ZnAl-LDO) prepared by by high vacuum calcination of the CG/LDH nanohybrid at 700
oC, and tested their photocatalytic performance in degrading cationic MB and anionic dye
orange G (OG) under visible light irradiation. The exfoliated LDH and CG solutions were
prepared by using formamide as the solvent and mixed for electrostatically driven self-
31
assembly, and the formation of a layer-by-layer ordered nanohybrid (See Figure 2-15). The
photocatalytic activity of the CG/LDH nanohybrid was not satisfactory probably because of
the low photocatalytic activity of ZnAl-LDH. However, after calcination, rCG/LDO showed
excellent photocatalytic properties. The degradation efficiency of MB by the rCG/LDO
photocatalyst after 120 min was ~100%, whereas efficiencies of about 57% and 39% were
achieved by ZnAl-LDO and photolysis, respectively (Figure 2-16a). In the system of OG, as
rCG/LDO had OG adsorption capacity, the degradation efficiency of OG by rCG/LDO after
90 min was 100%, much higher than that of pristine ZnAl-LDO (Figure 2-16b). The
photoluminescence spectra (PL) intensity of ZnAl-LDO appeared to be obviously weaker
after hybridization with rCG, suggesting the reduction of electron-hole recombination (Figure
2-16c). Moreover, the rCG/LDO showed a significant adsorption ability in the visible light
region (Figure 2-16d), contrary to the absorption edges observed in pristine ZnAl-LDO.
Therefore, the hybridization of LDO with rCG plays a critical role in the photocatalytic
reaction under visible light. The photocatalytic properties of the rCG/LDO nanohybrid
exhibited only a 10% decrease for MB decolorization and 8% decrease for OG
decolorization after 3 cycles of reuse. The excellent photostability was attributed to the
effective physical contact and strong electronic coupling between graphene and ZnAl-LDO.
A mechanism was also proposed to illustrate the enhanced charge separation in ZnAl-LDO
composed of both ZnO and ZnAl2O4 phases and between ZnAl-LDO and rCG, benefitted
the spatial separation of electron-hole pairs and the production of hydroxyl radicals for dye
degradations.
Figure 2-15 Schematic diagram of the process of electrostatic self-assembly between the
negatively charged CG monolayer and the positively charged ZnAl-LDH nanosheets.99
32
Figure 2-16 (a) Photocatalytic activity of the catalyst for MB decolorization; (b) Photocatalytic
activity of the catalyst for OG decolorization; (c) PL spectra of rCG/LDO and the pristine LDO and
(d) UV-vis diffuse reflectance spectra of rCG/LDO and the pristine LDO.99
2.2.4.3 LDH’s application in photocatalytic reduction of carbon dioxide
Photocatalytic conversion of CO2 is attractive from the viewpoint of sustainable energy
production and greenhouse gas reduction. CO2 can be adsorbed on a solid base, let alone
LDH, whose anion layers are constantly CO32- intercalation. LDHs with an M2+/M3+ (M2+=
Mg2+, Zn2+, Ni2+; M3+=Al3+, Ga3+, In3+) ratio of 3 showed activity for the photocatalytic
conversion of CO2 to CO in water.100 A copper-modified LDH was also reported to show
activity for the photocatalytic conversion of CO2 to methanol in the presence of H2 gas,101
and the methanol selectivity (26 mol%) obtained using ZnCuAl-LDH catalysts was improved
to 68 mol% using ZnCuGa-LDH catalysts. In the interlayer space of LDH photocatalysts,
CO2 was suggested for the reaction with the hydroxy group bound to the Cu sites to form a
hydrogen carbonate intermediate. Under UV–visible light, the Cu ions in the cationic layer
facilitated charge separation utilising the reduction-oxidation (redox) of CuII/CuI. Hydrogen
carbonate species were gradually reduced to formic acid, formaldehyde, and finally to
33
methanol utilising the trapped photogenerated electrons as CuI ions. Therefore, the
interlayer space of these LDH photocatalysts served as an active pocket for the reduction
of CO2 to methanol. An increase in the available reaction space would lead to enhanced
photocatalytic activity. By substituting the interlayer CO32- anions to [Cu(OH)4]2-, the
methanol formation rates using ZnGa-LDH and ZnCuGa-LDH increased by a factor of 5.9
and 2.9, respectively.102 The hydroxy groups that were bound to Cu sites were important,
and the effects of the interlayer [Cu(OH)4]2- were greater than those of the in-layer octahedral
Cu sites because of its steric availability (accessibility) and semiconductivity (Eg values of
3.0–4.2 eV).
2.2.4.4 LDH’s application in photocatalytic water splitting
The search for suitable semiconductors as photocatalysts for the splitting of water into
hydrogen by using solar energy is one of the primary missions of material science, which
has received considerable attention because of the increasing number of global energy
crises. It has been reported that MTi–LDHs (M=Ni, Zn, Mg) synthesised by the co-
precipitation method showed remarkable photocatalytic performance for water splitting into
hydrogen.61 The H2 generation rate was 31.4 μmol/h with 0.133 vol% lactic acid as a
sacrificial electron donor, which was 18 times higher than that of K2Ti4O9. Meng et al.103
reported a 3D Ce(III) polymer LDH which is photocatalytically active in H2 evolution under
UV irradiation. Silva et al. synthesised a series of ZnTi-, ZnCe-, and ZnCr-LDHs at different
Zn/metal atomic ratio (from 4:2 to 4:0.25) and tested them for the visible light photocatalytic
oxygen generation. 49 The ZnCr-LDH with two absorption bands in the visible region at λmax
of 410 and 570 nm was found to be highly active, the quantum yields for oxygen generation
were 60.9% and 12.2% at 410 and 570 nm, respectively. Besides, the efficiencies of the
chromium layered double oxides for oxygen generation increased asymptotically with the Cr
content. The overall efficiency of ZnCr-LDH for visible light oxygen generation was found to
be 1.6 times higher than that of WO3 under the same conditions. It has also been reported
that hybridising ZnCr-LDH nanoplate with graphene nanosheets or TiO2 could dramatically
increase their performance in photocatalytic O2 generation.104, 8
2.2.4.5 Other applications
Photoelectrode made from NiFeTi-LDH showed good photovoltaic effect.105 Moreover, by
intercalating I3-/I- into the LDH interlayer, the composite material can be potentially applied
for dye-sensitized solar cells (DSSC) as a solid electrolyte.106 Intimately mixed metal oxides
34
derived from calcined ZnTi-LDH precursor could serve as semiconductors for DSSC
assembly.68 The photovoltaic activity of mixed oxides derived from the calcination of ZnTi-
LDH is high with the efficiency parameters Voc=0.63 V and Jsc= 2.18 mA cm-2, which are as
good as an analogous photovoltaic cell prepared using TiO2.
In addition, being as sorbents and photocatalysts, LDHs also have been studied as catalysts
and catalyst supporters in many areas.107-110 LDHs are of particular interest in therapeutic
and pharmaceutical applications due to their low toxicity compared to other inorganic
nanoparticles.111 LDH nanosheets, like their bulk form, are attractive candidates as anion
exchangers and can be used as drug delivery capsules or systems for controlled release of
therapeutic, electroactive and photoactive materials.7, 112-114 LDH can be used as
polymerization supporter in organic synthesis.35, 115-118 Modification of LDH with organic
molecules has been investigated extensively for the purpose of using as a nanofiller for
polymer matrix to improve flame retardancy and mechanical properties.25, 119-122 Certain kind
of LDHs such as CoAl- and NiAl-LDH can be used as electrode materials for supercapacitor
due to their high activity for the faradaic redox reaction, facilitating the energy conversion
and storage.34 Langhals et al.123 prepared MgAl- and ZnAl-LDHs with chromophore
molecules in the interlayers. The composite materials exhibited the prime requisites of
pigments such as brilliant colours and insolubility, which can be used for colouring cement
and polymers.
2.3 Graphitic carbon nitride
Graphitic carbon nitride, generally known as g-C3N4, normally is in the form of 2D sheets
consisting of tri-s-triazines interconnected via tertiary amine.124-125 g-C3N4 has a graphite-
like layered structure with strong C–N covalent bonding in the in-plane direction and weak
van der Waals forces between the C–N layers. Different from graphene which is typical zero-
bandgap, the g-C3N4 is a typical polymeric semiconductor with a sp2 π-conjugated
system.126 The introduction of g-C3N4 into the field of heterogeneous catalysis was in 2006,
which means the development of covalent organic semiconductor is still in its babyhood.124,
127
Recently, graphitic carbon nitride has emerged as an appealing metal-free material and
received enormous attention in a wide range of research.127-129 Owing to its combination of
multiple physicochemical properties, g-C3N4 has demonstrated promising application in
35
numerous fields, including photocatalysis,130-134 energy conversion and storage e.g. fuel
cell126 and solar cells,135-138 and heterogeneous catalysis, etc. 139-141 Generally, g-C3N4 is
easily prepared which has controllable bandgaps, suitable band positions, and high thermal
and chemical stability. One of the most important characteristics of g-C3N4 is its simple
compose. Carbon and nitrogen, two earth-abundant elements, are the only component for
g-C3N4, which promises its low-cost preparation and sustainable usage.126 The real atomic
ratio of carbon to nitrogen in graphitic carbon nitride is smaller or larger than 0.75 in
stoichiometric g-C3N4 due to the incomplete condensation of NH/NH2 groups during the
pyrolysis of precursors. g-C3N4 is one of the most stable allotropes among various carbon
nitride under ambient condition, which is also thermally stable even in the air up to 600 oC.
Due to the stable C–N bonding in its aromatic heterocycles and the van der Waals
interactions between layers, g-C3N4 is chemically stable in most solvent such as water,
alcohols, N, N-dimethylformamide (DMF), tetrahydrofuran (THF), diethyl ether, and toluene,
as well as glacial acetic acid and 0.1 M NaOH aqueous solution.124
g-C3N4 has attracted great research interest largely because of its relatively small bandgap
of 2.7 eV and suitable band edges for visible light absorption to around 460 nm, which
determines its potential to be an effective photocatalyst.128, 142 The application of g-C3N4
regarding photocatalysis was firstly reported by Wang et al. as a metal-free photocatalyst
for producing hydrogen from splitting water under visible light in 2009.143 Due to its typical
semiconductor property, recently, g-C3N4 is regarded as a kind of promising photocatalyst
for the water splitting and degradation of organic pollutants under visible light irradiation.144
The following overview focuses on the introduction of the composition, structure and
morphology characters of g-C3N4, the functionalization of g-C3N4 in improving the
photocatalytic property and the application of g-C3N4 based photocatalysts in water
purification. Also, the synergistic adsorption effect from photodegradation of pollutants over
g-C3N4 based photocatalysts is mentioned. Finally, a promising perspective for g-C3N4 in
the field of energy conversion and environmental remediation is expected.
2.4 Graphitic carbon nitride-based photocatalysts for water
purification
2.4.1 Structure and morphology of g-C3N4
36
g-C3N4 has a similarly layered structure to graphite with weak van der Waals forces
between layers but a completely different planar structure. The tri-s-triazine (heptazine)
rings, relevant to the hypothetical aggregate melem in structure, is shown to be energetically
favoured structure for g-C3N4.129 (Figure 2-17) In contrast, to the planar pure covalent
bonding of graphite, the planar bonding of g-C3N4 is partially due to hydrogen bonding
between strands of polymeric melem units with NH/NH2 groups.125
Figure 2-17 Schematic of monolayer graphitic carbon nitride. N, C and H atoms are represented
by light blue, white, and small yellow balls, respectively. The melem unit is marked by the white
circle.125
XRD pattern (Figure 2-18a) for g-C3N4 gives two typical diffraction peaks at around 13o and
27o, which are respectively due to the in-plane structural packing motif and periodic stacking
of layer along the c-axis.125 For graphitic materials, the peak at 13o can be indexed as the
(002) peak characteristic for interlayer stacking of aromatic systems, and the peak at 27o
can be indexed as the (100) peak that corresponds to the interplanar separation.124
37
Figure 2-18 (a) XRD pattern of g-C3N4. b, c) High-resolution XPS spectra of C1s (b) and N1s (c) of
g-C3N4.124
X-ray photoelectron spectroscopy (XPS) measurements are used to investigate the status
of carbon (Figure 2-18b) and nitrogen elements (Figure 2-18c) in g-C3N4, including sp2-
bonded carbon in C–C (ca. 284.6 eV) and N–C=N (ca. 288.1 eV), the sp2-bonded nitrogen
in C–N=C (ca. 398.7 eV), the nitrogen in tertiary N–(C)3 groups (ca. 400.3 eV), and the
presence of amino groups (C–N–H, ca. 401.4 eV) caused by imperfect polymerization.124
Sometimes, a weak peak at 404.4 eV was reported in literature, which could be ascribed to
the charging effects145-146 or the π-excitation.147-148
Different morphologies of g-C3N4 bring different properties. Herein, Figure 2-19 is for the rich
morphologies of the g-C3N4 family with dimensions ranging from bulk, mesoporous g-C3N4
to nanosheets and thin films of g-C3N4, then to one-dimensional g-C3N4 nanowires and even
to quantum dots.
Figure 2-19 Rich morphologies of the g-C3N4 family with dimensions ranging from bulk to quantum
dots.128
38
2.4.2 Synthesis methods
g-C3N4 can be synthesised by directly heating low-cost ingredients like melamine149,
cyanamide, dicyandiamide and urea,148 etc. Based on experimental studies and ab initio
calculation,150 a microscopic mechanism of the reaction paths of generating g-C3N4 from
urea or thiourea is shown in Scheme 2-1. Upon heating under a closed air atmosphere, urea
first decomposes to ammonia and isocyanic acid and then is converted into some
intermediates, such as cyanuric acid, ammelide and ammeline. Cyanuric acid further turns
into melamine, which can condense to form melem. Melem undergoes polymerization to
melon and further to extended polymers.151 Finally, polymeric g-C3N4 is generated from the
pyrolysis of precursors.
Scheme 2-1 Microscopic mechanism of the reaction paths of polymeric carbon nitride.150
Yan reported that g-C3N4 powder could be synthesised by directly heating melamine in a
semi-closed system using a two-step heat treatment method. Melamine was first heated at
500 oC (heating rate: 20 oC /min) for 2 h, followed by further deamination treatment at a
higher temperature (550 oC) for 2 h.149 Liu and coworkers148 used urea as the single-source
39
molecular precursor via pyrolysis from 400 oC to 550 oC under ambient pressure without
additive assistance to obtain g-C3N4. The obtained g-C3N4 as photocatalyst exhibited stable
adsorption and photocatalytic activity for decomposing organic dyes. g-C3N4 was also
gained from the thermal condensation method reported by Xie and coworkers.152 In detail,
the precursor urea in a crucible with a cover was heated to 600 oC at a heating rate of 5
oC/min in a tube furnace for 4h in air under ambient pressure.152 g-C3N4 was gained after
washing with distilled water and absolute ethanol to remove residual alkaline species
adsorbed on the sample surface. One stable polymer built from tri-s-triazines units derived
from the condensation of urea was named as melon. The formation of melon (g-C3N4) from
urea is a condensation process involving the surface hydroxyl groups of the substrate. It can
be conveniently divided into three stages: (1) thermal decomposition of urea into isocyanic
acid and ammonia at 320-400 oC (2) melamine formation by the reaction of isocyanic acid
(and/or cyanamide) with surface OH groups (3) melamine undergoes polycondensation to
form melem, melam and finally g-C3N4.153
The template-induced method is the most commonly used method of synthesising porous
structure for g-C3N4. In a typical synthetic procedure, 5.0 g melamine and 2.5 g Triton X-100
were added to 100 mL distilled water, and then the mixture was heated in an oil bath at 100
oC with stirring for 1 h under refluxing. Then 2 mL concentrated sulfuric acid (98 wt.%) was
added to the solution dropwise while the white precipitate gradually formed, and the mixture
was stirred at 100 oC for another hour. After naturally cooling down to room temperature,
the precipitate was filtrated and washed several times with distilled water to remove Triton
X-100, then dried in an oven at 80 oC overnight. The obtained sample was put into an
alumina crucible with a cover and then heated to 500 oC in a muffle furnace for 2 h at a
heating rate of 2 oC min−1, and further heat treatment was set at 580 oC for another 2 h.154
As for the g-C3N4 base composites, two types of synthetic methods are mainly employed for
the synthesis of g-C3N4 base composites. The first one is mixing the substance with a carbon
nitride precursor following by the thermal condensation at desired temperature; the second
one is post-treatment of substance with as-formed g-C3N4 by deposition or simply mixing.128
2.4.3 Functionalization of g-C3N4 for wastewater treatment
40
2.4.3.1 Texture Modification
g-C3N4 with controlled structures and morphologies has attracted substantial attention,
which can be categorised into six types: mesoporous, nanosheets, nanorods, nanotubes,
nanofibers and nanospheres.128 Lin and coworkers144 reported a versatile and scalable
mixed solvent method to prepare monolayer g-C3N4 nanosheets. g-C3N4 nanosheets
exhibited distinctive physicochemical properties and unique electronic structures, such as
high surface area, lower surface defects, stronger reduction ability of the photogenerated
electrons, increased photoelectric response, and promoted the charge carrier migration and
separation. Furthermore, it showed greatly improved activities compared with its counterpart
bulk g-C3N4, with higher efficiency in photocatalytic oxidation of both benzyl alcohol and RhB
under visible light irradiation. Within 70 min of light irradiation, RhB is degraded almost
completely in g-C3N4 nanosheets system, on the contrary, only 50% RhB was decomposed
in bulk g-C3N4 system.
In the ethanol/water mixed system, the concentration of the suspension g-C3N4 nanosheets
gradually increased with decreasing the amount of organic solvent, and then with the further
increase in the water content, a gradual decrease in the concentration was observed. So,
when the volume ratio of water reached 75%, the dispersion of g-C3N4 nanosheets reached
its maximum concentration and "milky" dispersion was obtained with as high as 3 mg/mL
concentration. The BET surface area of the g-C3N4 nanosheets was estimated about 59.4
m2g−1, which was about 5 times larger than that of bulk C3N4 (12.5 m2g−1).
A nanowires type g-C3N4 was successfully synthesised by Xie and coworker from binary
mixture precursors.155 In their work, a mixture of cyanuric chloride and melamine precursor,
was dispersed in a solvothermal reaction and then followed by a subsequent calcination
step. The obtained novel nanowire product had a diameter of 10–20 nm and a length of
several hundreds of nanometers, while the nanofibers revealed fibrous nanostructures of
randomly dispersed fibres with an average diameter of ~15 nm. The 1D g-C3N4 showed
enhanced adsorption and photocatalytic activity compared with bulk g-C3N4, which was due
to the increase in the surface area providing more reactive sites for interactions between g-
C3N4 and reactants. Moreover, the charge carrier mobility was improved in the 1D g-C3N4
structure because of the advantage in anisotropic structure. The DRS result indicated that
the absorption edges of obtained g-C3N4 nanowires and nanofibers showed a remarkable
shift to longer wavelengths. Moreover, the bandgap was calculated to be 1.52 eV and 1.61
41
eV which were narrower than the bulk g-C3N4 2.62 eV. The reason for the large difference
in the DRS of 1D g-C3N4 and bulk g-C3N4 was ascribed to the high mobility in the hot fluid
which improved the inter-planar packing toward J-type aggregates of heptazine building
block. When the solvothermal processing improved the π-electron delocalization in the
conjugated system, the intrinsic optical and electronic properties of the 1D g-C3N4 are greatly
modified.
2.4.3.2 Modification with foreign elements
Foreign elements such as boron, carbon and nitrogen can be introduced into the structural
framework of graphite, leading to tunable electric properties from highly conductive
graphene to semiconducting boron carbonitride (BCN) and C3N4.146
The introduction of boron into structural framework of g-C3N4 has been reported with
narrower band gap which absorbed more visible light.146 Wang and coworkers reported
graphitic carbon nitride (CN) modified with B (CNB) which was fabricated via a facile
calcination process using cheap, sustainable and available sodium tetraphenyl boron and
urea as precursors.131 Yan and coworkers146 reported that boron doped g-C3N4 could
promote photodegradation of RhB. Compared with g-C3N4, boron doping improved the dye
adsorption and the light absorption of the catalyst. The B 1s spectrum in XPS data indicated
that boron atoms had been introduced into the g-C3N4 frame successfully. The authors also
pointed out the adsorption property of g-C3N4 before the photoreaction. 5-12.5% of RhB
before the illumination was adsorbed on the surface of g-C3N4 obtained at 520 oC to 600 oC.
Moreover, they also indicated that increase in the heating temperature could improve the
adsorption of RhB on g-C3N4. Indeed, increasing the heating temperature will induce the
structure defects due to the decomposition of g-C3N4 at high temperature. The structure
defects may cause a strong adsorption of RhB on the catalyst and therefore accelerate the
photodegradation process. The boron-doped sample obtained at 580 oC possessed the
highest degrading rate of RhB which was 1.5 times faster than RhB photodegrading over
the pristine g-C3N4 prepared at the same temperature. The photodegradation activity of
semiconductor photocatalyst is associated with oxidation ability of photogenerated holes in
the valence band and reduction ability of photogenerated electrons in conduction band. The
active species are different in degradation of RhB and MO systems over B-doped g-C3N4.
Moreover, the different pathways for photodegradation of RhB and MO are given in Scheme
2-2. The active species trapping experiments demonstrated that the photogenerated hole is
42
main oxidation species for the degrading RhB. However, the superoxide and hydroxyl
radicals are the main effective oxidation species for the degrading of MO. Density functional
theory calculations suggested that the visible light response of g-C3N4 originates from the
electron transfer from the valence band populated by N2p orbitals to the conduction band
formed by C2p orbitals. The XPS results indicated the boron is incorporated into the N sites
by the form of 2C-NB or C-NB2 in g-C3N4, which suggested that the small decrease in band
gap by boron doping originated from an increase in the top of the valence band of g-C3N4.
That means that the oxidation ability of photogenerated hole was slightly decreased in the
B-doped g-C3N4. However, in all, two advantaged factors, the improved adsorption ability
for B-doped g-C3N4, and an increased light absorption due to the slight decrease of 0.04 eV
bandgap overcame the decrease in photo-oxidation ability. Therefore the B-doped g-C3N4
presented the enhanced activity for photodegrading RhB.
Scheme 2-2 Photodegradation pathways of MO and RhB over g-C3N4.
After Kang et al. reported that carbon nanodot modified carbon nitride (C3N4) exhibited
impressive performance for photocatalytic solar water splitting, studies on carbon dots
modification on g-C3N4 have been emerging thereupon.156-157 A carbon dots (CDs)
decorated g-C3N4 photocatalyst was synthesised via a facile impregnation-thermal method
by Zhang and coworkers.158 g-C3N4/CDs composite (with loading 0.5 wt.% carbon dot)
resulted in 3.7 times faster reaction rate for phenol photodegradation than pristine g-C3N4.
For the pristine g-C3N4, the weak van der Waals interactions between adjacent CN layers in
g-C3N4 impeded charge transfer and separation, and thus imposed severe limitations on the
photocatalytic performance. For the g-C3N4/CDs composite sample, CDs adhered to the g-
C3N4 surface, which may arise from π-π stacking interactions. Considering the similar π-
43
conjugated structure of g-C3N4 and CDs, the combination of these two materials was
expected to deliver high photocatalytic performance. Li et al. reported the carbon dots
modified g-C3N4 hybrid via direct calcination of the mixture of C-dots and dicyandiamide,
where C-dots was obtained from the combustion of soot of an alcohol burners. The C-dots
modified g-C3N4 hybrid exhibited superior photocatalytic activity in dye removing and
hydrogen generation compared with pure g-C3N4.159
2.4.3.3 Hybridization
Considering combining with other semiconductors has been demonstrated an effective
way to inhibit the recombination of photogenerated electron-hole pairs, it is supposed that
introducing another semiconductor to the g-C3N4 can improve the photocatalytic activity of
the catalyst.154, 160
2.4.3.3.1 g-C3N4/inorganic semiconductor heterojunction
The semiconductor heterojunction has been an effective architecture to enhance
photocatalytic activity by promoting photogenerated charge separation.131 The highest
occupied molecular orbital for g-C3N4 has been reported to be -1.3 eV versus a normal
hydrogen electrode, and it is more negative than the conduction band of conventional wide
bandgap semiconductors such as TiO2 and ZnO.161 This would promote forming a
heterostructure with wide bandgap semiconductors and to extend their visible light
response. Moreover, the combination could not only improve the separation efficiency of
electron-hole pairs but could also broaden the absorption band toward the visible region.
Therefore, it is no doubt for the trial to build heterojunction between g-C3N4 with other
inorganic semiconductors. After immobilisation on the g-C3N4, the problem of agglomeration
of inorganic semiconductor e.g. TiO2 and ZnO have been reported being solved, with further
enhancement in the photocatalytic activities.152, 154, 162
Yin and coworkers160 synthesised SnO2/g-C3N4 nanocomposites via facile ultrasonic-
assisting deposition method. The SnO2/g-C3N4 composite with the mass ratio of g-C3N4
equalled 70 wt.% exhibited the highest photocatalytic activity in MO degradation. Di et al.
reported sphere-like g-C3N4/BiOI photocatalyst exhibited higher photocatalytic activity in the
photodegradation of dyes than pure BiOI, which contributed to improved electron-hole
separation and broadened absorption in the spectrum.163 Jiang et al.145 and Zhou et al.164
both reported the heterojunction between g-C3N4 and TiO2, which showed effective in water
44
pollution treatment, hydrogen production and an efficient photoreduction of CO2 to CO
respectively. Song and coworkers synthesised a g-C3N4 sensitised NaNbO3 substrated II-
type heterojunction, which not only exhibited narrower bandgap compared with NaNbO3 but
also displayed excellent photocatalytic activity for dyes and tetracycline degradation under
visible-light irradiation.165 Lan and coworkers reported that the Zn-In mixed metal oxide/g-
C3N4 (ZnIn-MMO/g-C3N4) nanohybrids showed stronger absorption in the visible region than
the pristine ZnIn-MMO, with exhibiting enhanced photodegradation activity for RhB under
visible light irradiation in comparison with pure g-C3N4 and ZnIn-MMO.166 In addition, inter-
electron transfer between g-C3N4 and ZnO heterogeneous junction was also reported and
realized for photocatalytic degradation of pollutants.162, 167-169 Liu and coworkers154
synthesized CdS quantum dots coupled nanoporous graphitic carbon nitride (CdS QDs/npg-
C3N4 composites), which not only exhibited the extension in optical absorption of visible light
up to 600 nm and enhancement in photocatalytic activity for photodegradation of RhB than
each of the components but also showed good catalytic stability. The photodegradation
efficiency on RhB can be as high as 88.2%, which is higher than that of pure npg-C3N4
(47.6%) and CdS QDs (46.5%). It was reported that the coupling between CdS and g-C3N4
could mutually suppress the drawbacks of each other simultaneously. The composite
between g-C3N4 and CdS facilitated the separation of photogenerated charges in g-C3N4,
so the recombination of charges was effectively suppressed. With the high surface area and
rolled and curled edges of npg-C3N4, the aggregation of CdS QDs were successfully
inhibited. Further the self-oxidized of S2- in CdS by photogenerated holes was successfully
prevented by the effective holes' transfer from CdS to g-C3N4. The photocatalytic
degradation mechanism of RhB by CdS QDs/npg-C3N4 composites was also investigated in
their work. By introducing 10 mM isopropyl alcohol (IPA), triethanolamine (TEOA), and p-
benzoquinone (BQ) three effective scavengers, the reactive species known as hydroxyl
radicals, holes and superoxide radicals can be targeted respectively.
45
Figure 2-20 (a) Effect of different scavengers on the RhB degradation in the presence of CdS
QDs/npg-C3N4, and (b) schematic of photogenerated charge transfer in the CdS QDs/npg-C3N4
system under visible light.154
As shown in Figure 2-20a, the photocatalytic degradation was remarkably suppressed by
adding TEOA or BQ, which indicated that the holes and superoxides radicals were the main
oxidative species in the photocatalytic process. Based on the photocatalytic experimental
results, a schematic mechanism was proposed and illustrated in Figure 2-20b.
Photogenerated holes transfer into the HOMO of g-C3N4, and the electrons transfer into the
CB of CdS QDs simultaneously. Therefore, the recombination of photogenerated charge
pairs can be effectively inhibited, and the charge separation efficiency can be enhanced.
Thus, high photocatalytic efficiency for CdS QDs/npg-C3N4 is reasonable.
2.4.3.3.2 g-C3N4/metal junction
Various metal nanoparticles such as Fe,170 Pt,171 and Ag172-173 have been immobilised
over g-C3N4 and used for photocatalytic decontamination of aqueous organic pollutants
under UV and visible light irradiation. The functional organic-metal hybrid materials were
supposed to exhibit modified electronic properties. Thereby, better performance in pollutant
eliminations was reasonable. The incorporation of noble metals into g-C3N4 have been
proven to be effective for the construction of highly efficient composite photocatalytic
systems.174-181 Yang et al. reported an in situ fabricated Ag/C3N4 composites with improved
photodegradation of RhB compared with pristine g-C3N4 samples.175 The highest
photodegradation performance of 85% was achieved for the Ag/D-C3N4 (in which
dicyandiamide served as the precursor for the g-C3N4). Contrary, only 50% RhB was
degraded in the presence of M-C3N4 sample (in which melamine worked as the precursor
46
for pyrolysis of g-C3N4), while around 60% RhB was removed when D-C3N4 was employed
as the photocatalyst. The Ag component in the Ag/C3N4 composite was crucial for the
enhancement both in light-harvesting ability and photocatalytic activity, which could be
ascribed to its unique surface plasmonic effects of Ag. Holes and radicals trapping
experiments implied that photo-induced active holes and superoxide radicals were
predominant under visible light irradiation and made major contributions to improving
photocatalytic performance.
2.4.3.3.3 g-C3N4/polymer nanocomposites
Polyaniline (PANI) is one of the most intensively studied conducting polymers due to its
unique electron and hole transporting properties, ease of synthesis and good chemical
stability.182 The chemical structure of PANI polymer is shown in Figure 2-21.183
Figure 2-21 The chemical structure of PANI.
Composites containing PANI and other semiconductor photocatalysts such as TiO2 have
been synthesised and demonstrated higher photocatalytic activity of degradation toward
dyes.184-185 In recent years, the combination of the g-C3N4 and PANI has also got great
attentions from researchers.183, 186-187 Liu et al183 firstly reported the PANI–g-C3N4 composite
photocatalyst. In their work, the PANI–g-C3N4 composite was synthesised by in situ oxidative
deposition polymerization of aniline monomer in the presence of g-C3N4 powder. The work
was mainly focused on the visible light driven photodegradation of MB. The PANI–g-C3N4
showed significantly enhanced photocatalytic performance than pure g-C3N4 and TiO2
(P25). Under the same experimental condition, the PANI–g-C3N4 composite sample showed
the highest activity with an MB degrading rate of 92.8%. However, the pure PANI sample
showed almost no photodegradation. Moreover, only 41.2% MB can be photodegraded by
pure g-C3N4 under visible light in 120 min. The MB degradation over P25 and N–TiO2 was
also carried out as a comparison to the composite photocatalysts. The experimental results
revealed that the photocatalytic activities of all the PANI–g-C3N4 samples were higher than
those of N–TiO2 and P25 photocatalysts.
47
A proposed photocatalytic mechanism is shown in Figure 2-22. The improved photocatalytic
activity was supposed from the promotion of electron–hole separation, which was caused
by the synergistic effect between PANI and g-C3N4. The polymeric semiconductor g-C3N4 in
the composite photocatalyst absorbs photons then excites electron and hole pairs when the
system is irradiated with visible light. The PANI also absorbs photons to induce π–π*
transition, transporting the excited electrons to the π*-orbital. The band gap of PANI is about
2.76 eV; the π*-orbital and π-orbital edge potentials are determined at -2.14 eV and +0.62
eV. The CB and VB potentials of g-C3N4 are at -1.13 eV and +1.57 eV, respectively. Thus,
the excited state electrons produced by PANI are injected into the CB of the g-C3N4.
Subsequently, simultaneous holes on the VB of g-C3N4 migrate to the π-orbital of PANI
because of the enjoined electric fields of the two materials. The photo-excited electrons are
effectively collected by g-C3N4, and the holes by PANI. Therefore, the efficient electron-hole
separation leads to significant enhancement of photocatalytic MB degradation in the PANI–
g-C3N4 composite system.
Figure 2-22 Schematic of the separation and transfer of photo-generated charge carriers in the
PANI–g-C3N4 system under visible light irradiation.
Besides PANI, there are other polymers composited with g-C3N4.188 Li and coworkers
reported the composite between carbon nitride (CN) and B-modified graphitic carbon nitride
(CNB), the CN-CNB-25 semiconductor with a suitable CNB content exhibited the highest
visible light activity. Its degradation ratio for methyl orange and phenol was more than twice
that of CN and CNB.131 Wu et al189 reported Polypyrrole(Ppy) and polythiophene(Ptp)
48
modified g-C3N4 nanocomposites, which were prepared from a simple sonochemical
approach. The polymer modification can decrease the band gap which is beneficial for
visible-light absorption. Instead of acting as a visible light sensitizer, the introduction of
polymers facilitated the separation and transport of photogenerated carriers. Therefore, a
stronger interaction between polymers and g-C3N4 promised a higher electron-hole
separation rate, which resulted in better photocatalytic activity and stability with showing
higher RhB photodegradation performance under visible light irradiation than pristine g-
C3N4.
2.4.3.3.4 g-C3N4 /graphene composites
Graphene which is one of the various carbon allotropes was found to be an electron
collector and transporter. This character can be utilised into boosting the performance of
various energy conversion and storage devices such as photovoltaic conversion devices,
supercapacitors, batteries, catalyst et al. Hybrid g-C3N4 and graphene should be a great
strategy to improve the optical absorption and utilisation of visible light region for g-C3N4.
Moreover, the mobility and separation rate of charge carriers with predictable improvement
in photocatalytic activity were also imaginable for g-C3N4 and graphene composite. Several
studies were carried out to modify g-C3N4 with graphene or reduced graphene oxide (rGO)
obtaining improvement in the photodegradation of pollutants and wastewater treatment.132,
190-193 Ye and coworkers used graphene as a dopant for semiconductor g-C3N4 in tuning its
band structure.137 The band structure of carbon nitride was modulated after doping with
reduced graphene, which could associate with g-C3N4 via π-π stacking interaction.
Following Figure 2-23 is the graph abstract for the synthesis of the composite. They found
that rGO could profoundly influence the band structure of host semiconductors via a strong
π-π electronic interaction, which was seldom reported.
49
Figure 2-23 (a) and (b) are the idealized motifs of graphene and g-C3N4 sheet respectively. (c) The
brief procedure of preparing rGO-intercalated g-C3N4.
By simply intercalating with different concentrations of rGO of different oxygen defects, the
flat band potential of g-C3N4 shifted greatly, while the conduction band (CB) and valence
band (VB) edge changed little. Thus, the band structure of g-C3N4 was well modulated.
Consequently, a significant increase in either anodic or cathodic photocurrent of g-C3N4 after
doping was observed, measured in a photoelectrochemical cell.
2.4.3.3.5 g-C3N4 based ternary photocatalysts
Silver nanoparticles (NPs) decorated porous TiO2 nanofibers (NFs) were interpolated on
large g-C3N4 sheets via using a simple and efficient electrospinning calcination method by
Kim and coworkers.161 The large surface area for g-C3N4 sheets provided the formation of
ternary heterostructures among the NPs, NFs and sheets, and encouraged electron transfer
between them. Therefore, the prepared composite showed excellent photocatalytic activity
with visible light when compared with pristine TiO2, g-C3N4 and single-component modified
Ag/TiO2 and TiO2/g-C3N4 composites, as measured by the degradation of different organic
dyes.
Jo and Selvam reported a novel ternary nanocomposite consisting of ZnO, g-C3N4, and
graphene oxide (GO) that provided enhanced photocatalytic performance and stability.194
50
Firstly, ZnO nanospheres were dispersed in the porous g-C3N4, resulting in shifted
absorption edge for the binary composite. Lately, GO was loaded in the ZnO–g-C3N4
composite, resulting in increased absorption in the visible range and improved charge
separation efficiency. Successful hybridization of the ternary nanocomposite was confirmed
by drastic quenching of fluorescence and broader visible light absorption. The optimal
content of g-C3N4 in the ZnO–g-C3N4 composite was 50%, which exhibited the effective
hybridization between ZnO and g-C3N4, and high photocatalytic efficiency. However, the
ZnO–g-C3N4/GO ternary nanocomposites showed performance that was two times greater
than ZnO–g-C3N4, exhibiting 99.5% photodegradation efficiency after just 15 min of light
irradiation. The combined heterojunction and synergistic effects of this composite accounted
for the improved photocatalytic activity. A synergistic mechanism for the heterostructured
ternary photocatalyst is illustrated in Scheme 2-3. The Fermi level of GO, the conduction
band edge (CBE) of ZnO and the CBE of g-C3N4 are ca. -0.08, ca. -0.5 and ca. -1.3 eV
versus NHE, respectively. Thus, g-C3N4 absorbed light and transferred electrons from its VB
to its CB. The excited electrons then migrated to the CB of ZnO, while the positive holes in
the VB of ZnO were transferred to the VB of g-C3N4, reducing recombination of charge
carriers. With the presence of GO, the photoinduced electrons in the CB of ZnO migrated to
GO, attributing to the lower work function of GO. Therefore, the GO served as an acceptor
of electrons and suppressed charge recombination effectively. The charge carriers
subsequently migrated to the composite surface, producing hydroxyl and superoxide
radicals with water and dissolved oxygen, respectively, which can then proceed to oxidise
the dye MB.
Scheme 2-3 Photocatalytic degradation mechanism for the heterostructured ternary
nanocomposite.
51
The investigation on g-C3N4 based ternary photocatalysts with magnetically recyclable
property is an attractive trial. Many attempts have been paid about preparation of some
magnetically recyclable g-C3N4 based photocatalysts.195-196 Mitra and Aziz synthesised a
magnetically separable ternary g-C3N4/Fe3O4/BiOI nanocomposites, which can be used in
the effective photodegradation of RhB under visible light irradiation.197 The results revealed
that weight percent of BiOI has a considerable effect on photodegradation of rhodamine B.
Among the prepared samples, the g-C3N4/Fe3O4/BiOI (20%) nanocomposite had the best
photocatalytic activity. The activity of this nanocomposite was about 10, 22, and 21 folds
higher than that of the g-C3N4 sample in degradation of RhB, methylene blue, and methyl
orange under the visible-light irradiation. The excellent activity of the magnetic
nanocomposite was attributed to more harvesting of the visible-light irradiation and efficiently
separation of the electron-hole pairs. Moreover, the g-C3N4/Fe3O4/BiOI nanocomposites can
be magnetically separated after five successive cycles.
Novel ZnIn2S4–g-C3N4/BiVO4 nanorod-based ternary nanocomposite photocatalysts with
enhanced visible light absorption were synthesised by Wan-Kuen Jo and Thillai Sivakumar
Natarajan.198 Ternary ZnIn2S4–g-C3N4/BiVO4 nanocomposites exhibited superior visible
light photocatalytic decomposition efficiency when used for the degradation of Congo Red
(CR) dye and metronidazole (MTZ) pharmaceutical, as well as excellent stability and
reusability. Kinetic studies showed that the degradation followed pseudo-first-order kinetics
and that the ternary photocatalysts could be reused up to three times with excellent stability.
The enhanced visible light absorption, high surface area, high adsorption capacity, Z-
scheme charge carrier transfer, and increased lifetime of photo-produced electron-hole pairs
were responsible for the increased visible light photocatalytic decomposition efficiency.
Au-loaded porous graphitic C3N4/graphene layered composite (Au/pg-C3N4/GR) reported by
Wang and coworkers was fabricated by a facile sonication-photo-deposition technique.199
The photocatalytic performance of the as-prepared Au/pg-C3N4/GR composite was
evaluated by degradation of methylene blue (MB) and ciprofloxacin (CIP) as representative
dye pollutant and antibiotic pollutant under visible light irradiation, respectively. The
degradation rates of MB and CIP over the Au/pg-C3N4/GR photocatalyst were 4.34 and 3.05
times higher that of porous g-C3N4 (pg-C3N4), respectively, and even 7.42 and 6.09 times
higher than that of pure g-C3N4, respectively. The composite also exhibited advantageous
52
adsorption activity, attributing to its porous structure. The improvement in photocatalytic
efficiency can be ascribed to the cooperation among the ternary system. The surface
plasmon resonance (SPR) effect of Au and electron acceptor role of graphene, which would
improve the visible light harvesting ability, facilitate photogenerated charge carrier
separation, as well as create more active reaction sites, and synergistically contribute to the
enhancement of photocatalytic activity. TEM and HRTEM were used to investigate the
detailed morphology of and microstructure of Au/pg-C3N4/GR photocatalyst. As indicated in
Figure 2-24a, pg-C3N4 possesses a special structure consisting of small flat sheets with
wrinkles and irregular shape, and a typical porous morphology of the sample is also
exhibited. The TEM image of GR (Figure 2-24b) displays that it has a two-dimensional
structure consisting of sheets with micrometre long wrinkles. Figure 2-24c shows a typical
TEM image of Au/pg-C3N4/GR composite. It is observed that pg-C3N4 sheets were
immobilised on the surfaces of GR sheets with most of the Au nanoparticles dispersed on
the surfaces of pg-C3N4 and the other portion deposited on the GR surfaces. Such special
sheet-on-sheet structure displayed a distinguished and close layered connection between
pg-C3N4 and graphene. Furthermore, the interplanar distances of 0.234 nm measured out
in the HRTEM image (Figure 2-24d) can be indexed to the lattice spacing of the Au (111)
plane. All the above observations suggested that the heterostructured Au/pg-C3N4/GR was
indeed formed.
Figure 2-24 Typical TEM images of (a) pg-C3N4, (b) GR, (c) Au/pg-C3N4/GR and HRTEM (d) image
of Au/pg-C3N4/GR composite.
53
Jiang and coworkers reported a ternary nano-heterojunction with TiO2-In2O3 decorated
porous graphitic carbon nitride.152 The composite TiO2–In2O3@g-C3N4 ternary composites
exhibit the highest RhB degradation rate, which was 6.6 times than that of pure g-C3N4. The
enhanced activities were mainly attributed to the interfacial transfer of photogenerated
electrons and holes among TiO2, In2O3 and g-C3N4, leading to the effective charge
separation on these semiconductors. In Figure 2-25, the LUMO and HOMO levels of the g-
C3N4 are located at -1.2 eV and +1.5 eV, the CB and the VB positions of In2O3 are about -
0.6 eV and +2.2 eV, the CB and the VB positions for TiO2 are about -0.3 eV and +2.9 eV.
So, both the conductive level and valence level lower than the previous one. So, the excited
electrons on the LUMO of g-C3N4 and CB of In2O3 can inject into the CB of TiO2, and the
holes on the VB of In2O3 can transfer to the LUMO of g-C3N4, which prolong the lifetime of
photoexcited carriers and separate the charges effectively.
Figure 2-25 Possible photocatalytic mechanism of the ternary hybrid composites.
2.4.3.3.6 Composite and Hybrid between carbon nitride and LDH
Because of its layered structure character, coupling g-C3N4 with layered semiconductors
should be an effective strategy for improving the photocatalytic properties.200 In this regard,
LDH can be considered to be an ideal candidate for coupling with g-C3N4 owing to their
resemblance of the similar layered materials. It is predictable that combining LDH with g-
C3N4 results in inorganic/organic hybrid composites that would exhibit the following
advantages: firstly, coupling two layered semiconductors to form a composite semiconductor
system facilitates the separation of the photogenerated electron-hole pairs; secondly, the
layered structure with considerable thermodynamic stability, including mixed valency metal
54
ions (divalent, trivalent) and good exposure of catalytically active sites, can participate in the
process due to the dispersion of g-C3N4 over the surface of LDH.201
However, there is limited research on the composition between LDH and g-C3N4.166, 201-204
There is even fewer research on the composition between LDH and g-C3N4 which is used
in water purification.
A novel g-C3N4/NiFe-LDH (CNLDH) composite photocatalyst was successfully synthesised
by Parida and coworkers.201 The synergistic effect between NiFe-LDH and g-C3N4 was
realised, and the photocatalytic activity of the g-C3N4/NiFe-LDH composite was remarkably
enhanced. The charge transfer between LDH and g-C3N4 were investigated by using PL.
The PL signal became significantly quenched after coupling of LDH with g-C3N4, which
indicating a notable depression of the electron-hole recombination implying a high
photocatalytic activity. Consequently, the exfoliated g-C3N4 dispersed over the brucite layer
of LDH evenly in this composite, resulting in strong coupling between them and strong
transfer of charge carriers happening on the intersurface. Nevertheless, instead of
investigating the water purification effect, the photocatalytic activity of CNLDH composite
was investigated towards H2 and O2 evolution.
Li and coworkers hybrid Zn-In mixed metal oxides (ZnIn-MMO) with g-C3N4 via a facile
thermal decomposition method with Zn-In layered double hydroxide (ZnIn-LDH) and
melamine mixture as precursors.166 MMO is the derivative from LDH, and the ZnIn-MMO/g-
C3N4 nanocomposite exhibited strong absorption in the visible light region with enhanced
photodegradation activity for RhB in comparisons with pure g-C3N4 and ZnIn-MMO. The
unique heterostructure of the ternary semiconductor coupling system which was composed
of g-C3N4, In2O3 and ZnO in the composites, facilitated efficient transportation and
separation of the photogenerated electron-hole pairs. Therefore, more reactive oxygen
species continuously generated could act on the degradation of the dye RhB.
Figure 2-26 shows the separation and transport of photogenerated electron-hole pairs at the
g-C3N4 and ZnIn-MMO interface. g-C3N4 and amorphous In2O3 formed can be excited under
visible light irradiation and generate excited electrons in the CB due to their narrow bandgap.
Because of the lower CB location of ZnO, the excited electron can transfer from the CB of
g-C3N4 and In2O3 into the CB of ZnO, at the same time, the generated hole can transfer from
55
the VB of In2O3 to the higher VB of g-C3N4. Therefore, the above charge transfer may reduce
the recombination of the photogenerated electrons and holes of and significantly increase
the photocatalytic activity of ZnIn-MMO/ g-C3N4 photocatalysts.
Figure 2-26 Proposed mechanism of charge separation and photocatalytic activity over ZnIn-
MMO/g-C3N4 photocatalyst under visible light irradiation.
2.4.4 Synergistic adsorption and photocatalysis
For the utilisation of photocatalyst in removal dyes, normally before irradiation, the mixture
of dye and photocatalyst solution was stirred in the dark for some time to obtain the
equilibrium adsorption. g-C3N4 and g-C3N4 based photocatalysts exhibited substantial
adsorption effect before photocatalysis.155, 190 Unlike the conventional adsorptive carbon
materials which have saturated adsorption capacity, g-C3N4 based adsorbents retain a
sustaining decontamination capability due to the photocatalytic degradation of adsorbed
organic pollutants under irradiation.205
The synergic effect of adsorption and photocatalysis of g-C3N4 was reported by Qi and
coworkers.206 In their work; experiments revealed that it was the photocatalysis that played
the main role in the complete decomposition of MB and MO. The adsorption was the basis
and premise of photocatalysis, and the synergic effects between adsorption and
photocatalysis facilitated the decomposition of dye molecules. The possible photocatalytic
process for dye discoloration over g-C3N4 is illustrated in Figure 2-27. The synergy between
adsorption and photocatalysis governed the quick decomposition of dye molecules. Firstly,
large amounts of dye molecules were adsorbed on or near g-C3N4 surface via the
electrostatic attraction, then the sorption equilibrium was established, finally under the
irradiation by visible light, photogenerated electrons and holes were consequently produced
56
with further reaction with oxygen and hydroxyl respectively to generate superoxide radicals
and hydroxyl radical, finally decomposed dyes completely.
Figure 2-27 Schematic diagram for the photocatalytic degradation of dye molecules over the g-
C3N4 sample. The synergy between electrostatic adsorption and photocatalysis under visible light
facilitates the decomposition of dye molecules.
Besides, g-C3N4 was also reported to be utilised in modifying biochar (BC) into an adsorptive
and photocatalytic composite.205 Aside from the intrinsic adsorptive property of carbon
materials, the BC@C3N4 composite had a light-responsive self-regeneration capability in
decontamination of aqueous pollutants. In their work, the pristine carbon nitride showed the
very weak capability of uptaking dyes compared with BC@C3N4 and BC, which meant the
dominating adsorption governor was BC in BC@C3N4 composite. The surface area and
electrical property should be the dominated factors for the adsorption capacity toward dyes.
Three kinds of dyes were chosen as models in this report. Only MB was massively adsorbed
on the composites, which can be due to the negatively charged surface of the composites
favouring the adsorption of cationic dyes via electrostatic attraction. After incorporation of g-
C3N4, BC@C3N4 composites showed well-defined absorption edges in DRS spectra,
indicating their capability of utilising UV and visible-light for photocatalytic processes.
Decolourization of MB was realised for saturated BC@C3N4 composites with further
irradiation under visible light, demonstrating the incorporation of g-C3N4 endowed the
composite with an additional photocatalytic activity for a sustaining clean-up of pollutants.
A higher porosity and surface area would enable greater adsorption densities of the
reactants and thereby favour photocatalytic reactions. Therefore, metal organic frameworks
57
(MOFs) having porous structure character with the high specific surface area was used for
compositing with g-C3N4.207 This kind of MOF/CN heterojunction nanocomposites have been
proved with enhanced photocatalytic degradation effect because of firstly enhanced
reactant/catalyst interaction via adsorption and secondly inhibited electron-hole
recombination. A homogeneous well-mixed structure was the key factor in improving
photodegradation effect. The MOF/CN were prepared via in situ synthesis method.
Primarily, g-C3N4 (CN) was exfoliated into 2D nanosheets named CNNS then an intimate
contact between the MOF crystals and the CNNSs was realised via the ‘in situ’ growth of
MOF crystalline, which would lead to high separation of charge carriers resulting greater
performance of photodegradation.
Zhou and coworkers synthesised a versatile g-C3N4/graphene oxide macroscopic aerogel
(C3N4/GOA) photocatalyst via a one-step cryodesiccation route.193 The obtained aerogel
exhibited both an excellent adsorption capacity for oil, organic solvents, and dyes, and an
enhanced visible-light photocatalytic activity toward the degradation of dyes and the
oxidation of NO. Such a fascinating multifunctional photocatalyst integrated 2D lamellar
powders into the 3D macroscopic structure, which promoted the potential commercial
application of photocatalysis. The enhanced visible-light photocatalytic performance was
mainly ascribed to the electronic interfacial interaction between g-C3N4 and GO as well as
effective charge separation and transfer. The photocatalytic activity of C3N4/GOA in aqueous
solution was evaluated in the photodegradation of RhB. However, due to the excellent
adsorption ability of C3N4/GOA, it was difficult to distinguish photocatalytic degradation from
adsorption. In fact, within 1 h, nearly 85% of RhB had been adsorbed by C3N4/GOA in
darkness, and the adsorption process continued after the irritation with light. The C3N4/GOA
also exhibited good adsorption on MB, and the degree of adsorption by C3N4/GOA was
much greater than that by GOA or g-C3N4, which could be ascribed to the high surface area
of C3N4/GOA (the surface areas of C3N4/GOA and g-C3N4 were calculated to be 385.2 and
78.3 m2g-1, respectively.) Interestingly, adsorption capacities about 50 to 90 times the weight
of C3N4/GOA can be achieved for other organic solvents and oils including phenixin, lube,
toluene, n-hexane and alcohol. Methyl orange (MO) was finally chosen as the model organic
contaminant, and a net degradation percentage of 73% (which subtracted the contribution
from adsorption) was yielded by C3N4/GOA within 5 h without any assistance of external
forces. And it proved that C3N4/GOA could also be a good macroscopic photocatalyst in
aqueous solution. The XPS valence band (VB) spectra of powder g-C3N4 and C3N4/GOA
58
are shown in Figure 2-28a. The VB maximum of C3N4/GOA is shifted from 1.57 to 1.97 eV
compared to the pristine C3N4 powder, suggesting a stronger oxidative power of the
photogenerated holes over C3N4/GOA. Considering that the band gaps of C3N4/GOA and
powder g-C3N4 are 2.58 and 2.74 eV, the conduction band (CB) minima could occur at -0.61
and -1.17 eV, respectively. Therefore, the band structures of C3N4/GOA and powder C3N4
are shown in Figure 2-28b. The potential energy of the VB holes from both C3N4/GOA and
powder g-C3N4 are lower than that of OH-/•OH (1.99 eV), whereas the potential energy of
the CB electrons is more negative than that of O2/•O2- (-0.28 eV). Therefore, the holes
cannot directly oxidise OH- into •OH, but the electrons have enough energy to promote the
formation of •O2-. Therefore, the main reactive species in the photocatalytic reactions for
these samples are •O2- radicals and photogenerated holes.
Figure 2-28 Band structures of powder g-C3N4 and C3N4/GOA: (a) XPS valence band spectra; (b)
band structure diagram.
Consequently, the interfacial interaction between GO and g-C3N4 can narrow the bandgap
of C3N4/GOA to improve the light absorption and result in a 0.4 eV downshift of the VB
maximum. Moreover, due to the high conductivity of GO, C3N4/GOA revealed an effective
separation of photogenerated electron-hole pairs and rapid interfacial charge transfer. So,
the photocatalytic reaction which was mainly governed by •O2- radicals and photogenerated
holes were significantly enhanced.
2.5 Summary and future perspectives
LDHs and derivatives have been used in photocatalysis for a wide range. In this chapter,
the most comment LDH synthesis methods, and the application of LDH and its derivatives
59
such as MMO for photocatalysis have been reviewed. As the exfoliation technique of LDH
nanosheets has been more and more mature,9, 19, 208 together with the unusual structural
features, that is, ultimate two-dimensional anisotropy and the novel physical and chemical
properties due to an extremely small thickness of around 1 nm, it is anticipated that ultrathin
and even transparent LDH single nanosheet will find its way in photocatalysis. Furthermore,
these nanosheets are useful as a building block for the fabrication of a wide variety of
functional nanostructured materials, such as the layer-by-layer assembly of nanosheets with
appropriately charged counterparts through a wet process has advancing application in the
near future.19
Duan and coworkers209 used LDHs as substrate, graphene nanosheets were fabricated in
the interlayer of LDH via thermal hydrogen reducing methyl methacrylate (MMA)
polymerization. Wu et al16 made magnetite-graphene-LDH composites which were used in
waste water treatment; there is more and more information about the combination between
graphene and LDH. All these forecast the trend that LDH combined with graphene i.e. the
fabrication of LDH/carbon nanocomposites.210 This combination is a novel method for the
development of novel multifunctional nanocomposites based on the existing nanomaterials.
It should be noticed that most of the properties of nanocarbons and LDHs are
complementary. Therefore, the combination of nanocarbons and LDHs into hierarchical
nanocomposites is a promising method to integrate their distinguishing properties together:
nanocarbons can provide good electrical conductivity and high mechanical strength, and
LDHs can provide good chemical reactivity. LDHs/carbon is versatile in energy storage such
as supercapacitors and Li-ion batteries, catalysis i.e. severed as a catalyst in materials
fabrications, drug delivery, and environmental protection. It is anticipated that the research
on exfoliated LDH nanosheets and/or LDH-based composites combined with other
photocatalysts will break new ground in LDH’s photocatalytic applications in the near future.
In addition, a brief overview on the polymer semiconductor g-C3N4 is presented. In this
perspective, the structure and morphology characters, synthetic strategies and
functionalization of g-C3N4 based photocatalysts used in water purification were introduced.
Based on the above abundant review of researchers' work on LDH and g-C3N4
photocatalysts, it's supposed to be a promising perspective for their practical application in
wastewater treatment. The hybridization of nanosheet LDHs with other layered
60
semiconductors produced binary even multi hybrids, providing a powerful method for further
enhancing the photocatalytic efficiency. However, despite of promising results reported so
far, the works are still in initial stage and further developments are particularly needed.
Specifically, the photocatalysis still suffers from the relatively low efficiency and stability of
the hybrid composites, which is still far from the industrial requirements. Moreover,
developing wastewater treatment technologies based on LDH photocatalysts is an
interesting but difficult job, because the generation and disposal of the used composite
photocatalysts is still a big challenge. The LDH/magnetic oxide composites should be a good
solution to solve the above problems because magnetic materials can be easily separated
using an external magnet.211 Alternatively, synthesis of LDH based photocatalysts on a
basal plate is a good strategy to cope the disposal problem. Tasks like increasing the stability
of the hybrid photocatalysts, controlling the particle size of semiconductors on the g-C3N4
substrate are still challenging for researchers recently. Furthermore, some key issues that
account for the high photocatalytic activity, i.e. optical absorption, electronic band structure,
and interfacial charge transfer across the g-C3N4-based heterojunction nanocomposites,
should be exhaustively investigated to gain theoretical insights by means of first-principles
DFT calculations.212 Finally, I think joint efforts from both chemists and industrial engineers,
are mandatory to deliver the prospects of LDH based photocatalysts and g-C3N4-based
nanostructures into practical wastewater treatment.
As can be seen from the review, there is already considerable progress on the
functionalization of g-C3N4 used in water purification. However, the composition between g-
C3N4 and LDH derivate is still in its infancy. In this regard, future research focused on the
combination of g-C3N4 and inorganic semiconductors is believed to catch more attentions
from researchers, especially for the composite between g-C3N4 and LDH. Moreover, to
reach a more fundamental understanding, a thorough study of the physicochemical
properties of g-C3N4 is needed. Moreover, the correlations between the g-C3N4 structures
and the photocatalytic activity also need a deeper and more comprehensive understanding.
Without a doubt, in a word, there will be intensified interest toward the g-C3N4 based
photocatalyst being used in green chemistry in the coming future.
61
2.6 Reference
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3 Research Methodology
3.1 Introduction
After reading relevant literature, appropriated experimental designs schemes were
proposed. The overview of the research in this thesis was composed of three independent
work. Firstly, it was the study on the inorganic semiconductor ZnAl-LDH based
photocatalysts. Secondly, the work was centred on the study of the modification of polymeric
semiconductor g-C3N4 with CB. Finally, the work was focused on the hybrid between
inorganic and organic semiconductors i.e. the synthesis of heterogeneous photocatalyst
ZnO-LDH@C3N4 composite. The PhD research work was following the flow of three main
steps. Primarily, the exploration of the synthesis methods. Several experimental methods
were used for the synthesis of materials in this research, including co-precipitation,
solvothermal and pyrolysis. Afterwards, materials were used to investigate the photocatalytic
and adsorption activity toward dye solutions. In this research, OrgII and MB were chosen as
the representatives for anionic and cationic organic dyes, respectively. Herein, the summary
description of the evaluation was provided, whereas the elaborate operation processes were
presented in following chapters. The last and but not least, in order to understand the
properties of catalysts comprehensively, a variety of characterization techniques were
needed. Therefore, all relevant characterization instruments and parameters were listed in
this chapter. The major aim of this chapter is to describe the synthesis procedures and
summarise the evaluation process then list the characterization techniques used in this
work.
3.2 Materials synthesis
3.2.1 Chemicals and reagents
All the chemicals used in this work such as urea, carbon black, Zn(NO3)2∙6H2O,
Al(NO3)3∙9H2O, NaOH, ZnO, methylene blue (C16H18ClN3S) and Orange II sodium salt
(C16H11N2NaO4S) were purchased from Sigma-Aldrich with analytical grade and used
without further purification. Milli-Q water (ultrapure laboratory grade water) was utilised in all
experiments.
3.2.2 Synthesis of ZnO-LDH
82
ZnO−LDH composites were prepared via a co-precipitation method as reported in
reference 1. Typically, NaOH aqueous solution (30 mL) was dropwise added to the mixed
Zn(NO3)2 and Al(NO3)3 aqueous solutions (50 mL) at the molar ratios of Zn2+ / Al3+ in the
range of 1~5 under constant stirring. Then, the mixtures were aged at 60 oC for 24 h.
Centrifugation was used to collect the ZnO−LDH solids; these were then washed with
deionized water by centrifugation three times. Finally, the ZnO−LDH was dried at 70 oC for
12 h. The samples are denoted as ZnO−(x)LDH, where x indicates the molar ratio of Zn2+ to
Al3+ in the initial solution. It should be noted that due to the formation of ZnO phase, the ratio
of Zn2+/Al3+ in the layers of LDH was always lower than the initial ratio of Zn2+/Al3+ in the
solution.
3.2.3 Synthesis of (x)LDO samples
The (x)LDO samples were prepared from corresponding ZnO−(x)LDH. In detail,
ZnO−(x)LDH samples were calcined at 400, 500, 600, 700 and 800 oC for two hours with a
heating rate of 10 oC/min, respectively. These calcined samples are denoted as (x)LDO,
where x also indicates the molar ratio of Zn2+ to Al3+ in the initial solution.
3.2.4 Synthesis of ZnO−r(x)LDH
The (x)LDO solids were dispersed in water with constant stirring for 24h to reconstruct the
layered lamellar structure. Then samples were filtered out and dried at 70 oC for 12 h. These
rehydrated LDH samples are denoted as ZnO−r(x)LDH.
3.2.5 Synthesis of g-C3N4 nanosheets
The g-C3N4 used in this study was prepared by calcining urea at 550 oC for 3h in an alumina
crucible according to the literature.2 In brief, a given amount of urea was dried at 80 oC for
24 h, then calcined at 550 oC for 3 h. The heating rate of calcination was 2 oC/min. After
cooling, the yellowish powder was collected and washed with 10% HNO3 solution and Milli-
Q water three times by centrifugation.
3.2.6 Synthesis of carbon black modified g-C3N4 samples
The carbon modified g-C3N4 samples in this work were prepared by calcining the mixture
of urea and carbon black at 550 oC for 3h in an alumina crucible. In brief, a series of different
ratios of urea and carbon black mixture were dried at 80 oC for 24 h independently, then
83
calcined at 550 oC for 3 h. The heating rate of calcination was 2 oC/min. After cooling, the
dark grey powders were collected and washed with 10% HNO3 solution and Milli-Q water
three times by centrifugation. For abbreviation, carbon black modified g-C3N4 was donated
as n(U+CB), where n was the initial weight ratio of urea to carbon black.
3.2.7 Synthesis of ZnO-LDH@C3N4 composite
The preparation of ZnO-LDH@C3N4 composite is via a facile solvothermal method. First,
0.42 g of g-C3N4 and 2.80 g of NaOH were added to 40 mL of ethylene glycol (EG) under
stirring. Second, 5.95g of Zn(NO3)2∙6H2O and 3.75 g of Al(NO3)3∙9H2O (with a molar ratio of
Zn/Al = 2:1) were added to another 40 mL of EG. Then, these two EG suspensions were
combined under stirring. After 0.5 h, the suspension was transferred to a 100 mL Teflon-
lined stainless steel autoclave for solvothermal treatment at 120 °C for 24 h. Finally, the
solids were collected and washed with ethanol two times and Milli-Q water three times by
centrifugation and dried in an oven at 80 °C for 24 h. The yield of the solids was 2.88 g.
3.3 Adsorption measurement
The adsorption of dyes was carried out at ambient temperature. In a typical test, a certain
amount of catalyst was suspended in a certain volume of specific dye aqueous solution,
followed by constant mechanical stirring for allocated time under the dark conditions until
approaching sorption equilibrium. The residual concentration of dyes in the aliquot was
analysed using a Shimadzu UV-2600 UV–Vis spectrophotometer.
3.4 Photocatalytic evaluation
The photocatalytic degradation of dyes was carried out in an open thermostatic
photoreactor. After saturated adsorption, the mixture was irradiated with a 200 W mercury
lamp, applying an optical cut-off filter (λ>420 nm) to remove UV light when necessary. At
interval times, 5 mL of aliquots were extracted using a syringe and filtered through a
membrane (0.45 µm). Each time the concentration of dye in the solutions was analysed
using a Shimadzu UV-2600 UV–Vis spectrophotometer.
3.5 Materials characterizations
3.5.1 X-ray diffraction
84
XRD X–Ray diffraction (XRD) patterns were collected on a Shimadzu diffractometer (XRD–
6000, Tokyo, Japan), operating in the reflection mode with Cu Kα radiation at a scanning
rate of 0.02° per second with 2θ ranging from 5° to 80°.
3.5.2 Fourier transform infrared spectra
Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu IRAffinity-1 over
a wavenumber range of 4000-1000 cm-1 and 4000-600 cm-1.
3.5.3 X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis
ULTRA X-ray photoelectron spectrometer with a 165 mm hemispherical electron energy
analyser and monochromatic Al Kα X-ray source (1486.6 eV) at 225 W (15 kV, 15 mA) with
a charge neutralizer.
3.5.4 Scanning electron microscopy
The scanning electron microscopy (SEM) images were taken using the JEOL JSM-6460LA
and the Hitachi SU3500.
3.5.5 Transmission electron microscopy
The transmission electron microscopy (TEM) images were taken using the JEOL JEM-
1010 TEM. The high-resolution TEM (HRTEM) images and Energy Dispersive X-Ray (EDX)
spectroscopy analysis were conducted on a transmission electron microscope (JEOL JEM-
2100 S/TEM) equipped with an energy-dispersive X-ray analyser. The selected area
electron diffraction (SAED) was done using an analytical transmission electron microscope
(The Philips Tecnai F20 FEG-S/TEM).
3.5.6 Nitrogen adsorption-desorption
The specific surface areas were calculated using the Brunauer-Emmette-Teller (BET)
method, and the pore volumes were calculated using the Barrett-Joyner-Halenda (BJH)
method from the adsorption isotherms of N2 measured at 77 K on a Micromeritics TriStar II
3020.
3.5.7 Thermogravimetric-differential scanning calorimetry
85
The thermogravimetric-differential scanning calorimetry analysis (TG-DSC) was performed
on a Shimadzu DTG-60A analyser, in detail, 5~10 mg of dry sample was loaded in a Pt
crucible without a lid and scanned at a rate of 2 or 10 °C·min−1.
3.5.8 Diffuse reflectance spectra
Diffuse reflectance spectra (DRS) were recorded on a Shimadzu UV-2600 spectrometer
equipped with an integrating sphere ISR-3100 using BaSO4 as the reference.
3.5.9 Photoluminescence spectra
Photoluminescence (PL) spectra were measured on a Fluorescence Spectrometer (FLS
920, Edinburgh Instruments).
3.5.10 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) were measured in 6 M KOH solution with
sinusoidal ac perturbation of 5 mV over a frequency range from 0.1 to 1 ×106 Hz.
3.5.11 Zeta potential
Light Scattering Electrophoresis (LSE) in Nanosizer Nano ZS, MALVERN Instrument was
used to analyse the zeta potentials.
3.6 References
1. Goh, K. H.; Lim, T. T.; Dong, Z., Application of layered double hydroxides for removal
of oxyanions: A review. Water Res. 2008, 42 (6-7), 1343-1368.
2. Liu, J.; Zhang, T.; Wang, Z.; Graham, D.; Chen, W., Simple pyrolysis of urea into
graphitic carbon nitride with recyclable adsorption and photocatalytic activity. J MATER
CHEM 2011, 21 (38), 14398.
86
4 Uptake and degradation of Orange II by zinc
aluminium layered double oxides
4.1 Introduction
Layered double hydroxides (LDHs), a family of anionic clay, have attracted considerable
interest in adsorption, catalysis, and biomedical studies recently.1-4 LDHs consist of cationic
brucite-like layers and interlayer anions, with the general chemical composition M2+1-
xM3+x(OH)2(An-
x/n)•yH2O. M2+ and M3+ are typically divalent and trivalent metal cations
respectively. An- is an anion, and x is the molar ratio of the trivalent to the total cations.5 The
anions in the interlayer of LDHs can be exchanged with other anions via an ion exchange
mechanism.1, 5 Upon calcination, LDH can decompose into layered double oxides (LDO),
which are composed of mixed metal oxides. When an LDO is in contact with an aqueous
solution with anions, it can regenerate the layered structure of LDH (i.e., rLDH).6 During this
rehydration process, water is absorbed to reform the hydroxyl layers, and anions and water
are intercalated into the interlayer of LDH. Due to this property LDOs have been utilised as
adsorbents for the removal of anionic pollutants from water.7-9
Recently, the technique of photocatalysis has shown great potential for wastewater
treatment due to its low cost, sustainable technology under mild reaction conditions, and an
energy-saving property.10-14 Various semiconductor photocatalysts have been developed,
such as TiO2, ZnO, and Fe2O3, CdS, GaP, ZnS. Among them, zinc oxide (ZnO) has received
much interest as a photocatalyst, owing to its wide energy bandgap (3.37 eV) and high
photocatalytic efficiency.15-17 Recent studies reported that ZnO supported on LDH showed
improved photocatalytic activities.18-21 Cho et al.20 prepared ZnO/ZnAl-LDH with various
morphologies using microwave irradiation method. Duan et al.19 developed well-aligned
ZnO@CoNi-LDH core-shell hierarchical nanoarrays via an electrosynthesis method for
enhanced photoelectrochemical water splitting. Wu et al.18 deposited ZnO nanoparticles on
the surface of MgAl-LDH microspheres by a two-step co-precipitation process, and the
heterojunction catalysts showed high photocatalytic activity towards the degradation of
phenol under UV irradiation. The common point for these reports is that the formation
processes of ZnO and LDH are independent, occurring in different steps. Thus, based on
the literature findings, this work aimed to combine the “memory effect” and anion exchange
properties of ZnAl-LDH and the photocatalytic properties of ZnO to develop high-efficiency
87
ZnO and ZnAl-LDH composite adsorbents, which are supposed to be photocatalytically
regenerable. Specifically, the purpose of the work in this chapter was to develop ZnAl-
layered double oxide composites (LDO) and then research their utilizing in removal of
organic dyes in water by adsorption and photocatalysis, detailed in exploring the optimal
synthesis conditions for LDO, investing the adsorption and photocatlysis effect on removing
dye OrgII, studying the relations between structural characteristics and removal properties.
In this work, a facile approach was used to synthesise LDO composites by a co-precipitation
method and following calcination treatment. The LDO removal efficiency for organic dyes
under UV light irradiation was investigated. It was found that removal of organic dyes on
LDO composites involved consecutive adsorption of organic dyes in solution to fabricate
organic dye-intercalated rehydrated ZnO−LDH composites, and then degradation of
nonbiodegradable dye pollutants on these rehydrated ZnO−LDH composites under UV light
irradiation. The result showed that the optimal (2)LDO, prepared at the Zn/Al molar ratio of
2, had better adsorption and photocatalytic performance compared with commercial TiO2
and ZnO. The adsorption capacity of (2)LDO for OrgII can reach as high as 800.8 mg/g,
which is attributed to the rehydration property of LDO, known as the “memory effect” of LDH.
Further, the photocatalytic decomposition of OrgII can reach 74.3% after 100 mins UV
illumination. The relationship between the adsorption property of LDO and the structural
features of rehydrated LDH, which has not been reported in the literature yet, was also
investigated.
4.2 Adsorption and photodegradation experiment details
The adsorption of OrgII was carried out at ambient temperature. Suspensions containing
80 mL of OrgII water solution (0.21 mM) and 0.010 g of a solid photocatalyst were sonicated
for 5 min, followed by constant mechanical stirring for 24 h under the dark conditions to allow
sorption equilibrium. The residual concentration of OrgII in the aliquot was analysed using a
Shimadzu UV-2600 UV–Vis spectrophotometer. The photocatalytic degradation was carried
out in an open thermostatic photoreactor. After saturated adsorption, the mixture was
irradiated with a 200 W mercury lamp. At interval times, 5 mL of aliquots were extracted
using a syringe and filtered through a membrane (0.45 µm). Each time the concentration of
OrgII in the solutions was analysed using a Shimadzu UV-2600 UV–Vis spectrophotometer.
The solution pH was kept at ~7.5 during the experimental process.
88
4.3 Results and discussion
4.3.1 The feature structure of ZnO-LDH and ZnO-rLDH composites
The crystalline phases of ZnO−LDH and rehydrated ZnO−rLDH were determined by XRD.
As shown in Figure 4-1A, the as-synthesized ZnO−(3)LDH (a) exhibited broad diffraction
peaks at 11.6o and 23.3o, which can be indexed to (003) and (006) planes of ZnAl−LDH
(JCPDS no. 48−1021). ZnO−(3)LDH (a) also exhibited peaks at 31.8o, 34.4o, and 36.3o,
which were attributed to (100), (002) and (101) planes of ZnO (JCPDS no. 05−0664),
respectively. The peaks were in good accordance with other values in literature.22-23 The
XRD pattern of ZnO−(3)LDH indicated both ZnO and LDH phases were simultaneously
generated during the co-precipitation process. The characteristic peaks of ZnAl−LDH
disappeared after being calcined at 400 oC, suggesting the complete destruction of lamellar
structures, and formation of LDO. When redispersing the calcined LDO samples in water
narrower and sharper diffraction peaks at 11.6o and 23.3o were observed than in the
ZnO−(3)LDH, indicating reconstruction of LDH with improved crystallinity. With the increase
in the calcination temperature from 400 oC to 600 oC, it was found that the XRD peaks of
the rehydrated ZnO−rLDH composites (c-e) became sharper and narrower, indicating the
calcination temperature may influence the crystal size of ZnO−rLDH. Furthermore, the new
XRD peaks were observed at 30.6 o, 36.1o, and 58.4 o after calcination at 700 oC, suggesting
the formation of ZnAl2O4 (JCPDS no. 01−1146), which was also in good agreement with the
literature.24 Small peaks of LDH were observed, indicating only partial reconstruction to LDH
after calcination at 700 oC. The crystal size of the samples was calculated using Scherrer’s
equation and listed in Table 4-1. As shown in Table 4-1, the crystal size of ZnO gradually
increased with the increase of calcination temperature from 3.30 nm in 400 oC to 10.84 nm
in 800 oC. The crystal size of LDH component in ZnO−rLDH gradually increased with the
increasing calcination temperature from 58.76 to 89.63 nm from 400 to 600 oC, and then
decreased to 56.56 nm when the calcination temperature reached 700 oC. When the
calcination temperature further increased to 800 oC no LDH phase was detected (Figure 4-
2), suggesting that LDO cannot reconstruct to ZnO−r(3)LDH at this condition. The effect of
calcination temperature on the size and composites of ZnO−r(3)LDH composites were also
observed in other ZnO−r(x)LDH composites. The greatest LDH average crystal size for the
ZnO−r(3)LDH samples was obtained at 600 oC. Based on the calculation from Bragg’s law,
the d(003) of all ZnO−rLDH is similar and around 0.76 nm which infers that the interlayer
distances are similar for all ZnO−r(x)LDH. Considering that the interlayer distance is
89
governed by the amount and nature of interlayer anion, its charge density and orientation in
the galleries, and the electrostatic attraction with the cationic sheets,22 LDH’s crystal size in
c direction increases with more interlayers. It is believed that the increased number of
ZnO−r(3)LDH interlayers obtained at 600 oC could contribute to its higher adsorptive
capacity for the uptake of carbonate ions and water molecules. To test this, all ZnO−LDH
samples were calcined at 600 oC, before being redispersed to restore the layered lamellar
structures in water.
10 20 30 40 50 60 70 80
(10
1)
(002)
(100)
(00
6)
Two-theta (o)
Inte
ns
ity
f
e
d
c
b
a
LDH
ZnO
ZnAl2O
4
(00
3)
(A)
10 20 30 40 50 60 70 80
Two-theta (
o)
Inte
ns
ity
k
j
i
h
g
(B)
Figure 4-1 (A) XRD patterns of (a) ZnO−(3)LDH, (b) (3)LDO prepared at 400 oC, and
ZnO−r(3)LDH prepared by rehydration of (3)LDO with the calcination temperatures of (c) 400 oC,
(d) 500 oC, (e) 600 oC, (f) 700 oC. The XRD patterns of standard LDH, ZnO, and ZnAl2O4 are also
included. (B) XRD patterns of (g) ZnO−r(1)LDH, (h) ZnO−r(2)LDH, (i) ZnO−r(3)LDH, (j)
ZnO−r(4)LDH, and (k) ZnO−r(5)LDH samples that are produced from LDO calcined at 600 oC.
90
10 20 30 40 50 60 70 80
Two-theta (o)
In
ten
sit
y
Figure 4-2 XRD patterns of ZnO−r(3)LDH gained from 800 oC.
The effect of the molar ratio of Zn2+ to Al3+ on the size and composition of ZnO−rLDH after
calcination at 600 oC and rehydration was investigated. The molar ratio of Zn2+ to Al3+ was
adjusted in the range of 1:1~5:1. This property can be advantageously used to tune the layer
charge density, the inherent ion exchange capacity, and the interlamellar anion
concentration.1, 25 As shown in Figure 4-1B, by tuning the molar ratio of Zn2+ to Al3+ from 1
to 5, all ZnO−rLDH composite showed high crystallinity of LDH and ZnO. The intensity of
XRD peaks for LDH increased with the increase of Zn2+/Al3+ ratio from 1 to 2 and decreased
with further increase in Zn2+ ion initial amount, indicating the highest proportion of LDH is in
ZnO−r(2)LDH. As listed in Table 4-1, ZnO−r(1)LDH and ZnO−r(2)LDH composites
demonstrated a larger crystal size in c direction compared to ZnO−r(3)LDH (Table 4-1). A
similar result was also observed for these samples calcined at 400 oC (Figure 4-3). The
results suggest that increasing the amount of initial Al3+ ions can remarkably increase the
proportion and the crystal size of LDH in ZnO−r(x)LDH. This agrees with later results of the
adsorptive capacity of LDO in the uptake of OrgII. The highest LDH intensity peaks and the
biggest average crystal size of ZnO−r(2)LDH suggest its strongest adsorption of OrgII when
(2)LDO is used as a start material for the uptake of OrgII from the water.
91
Table 4-1 Properties of ZnO and LDH in samples.
Samples
ZnO
D(002)a
(Å)
LDH
average crystallite
size in c directionb
(nm)
ZnO−r(3)LDH(400)
ZnO−r(3)LDH(500)
ZnO−r(3)LDH(600)
ZnO−r(3)LDH(700)
ZnO−r(3)LDH(800)
ZnO−r(2)LDH(400)
ZnO−r(3)LDH(400)
ZnO−r(4)LDH(400)
ZnO−r(5)LDH(400)
ZnO−r(1)LDH(600)
ZnO−r(2)LDH(600)
ZnO−r(3)LDH(600)
ZnO−r(4)LDH(600)
ZnO−r(5)LDH(600)
c-ZnO
33.03
38.99
46.63
68.56
108.4
21.83
33.03
39.95
46.06
46.49
46.64
46.63
47.40
62.71
158.5
58.76
73.69
89.63
56.56
0
63.34
58.76
39.10
29.75
116.1
117.0
89.63
78.60
73.33
0
ZnO−r(x)LDH(y): x represents the initial ZnAl ratios; y means the treatment temperatures;
c-ZnO: commercial ZnO;
Da: ZnO Crystallite size: calculated using Scherrer’s equation: Size=K λ/ [FW(S)•cos (θ)] Size (Å); K
is constant=1 here; λ is the X-ray wavelength =1.5406 Å. FW(s) is the FWHM of sample at θ, normally
low θ is chosen into calculation;
b value calculated from the value of the FWHM of the (003) and (006) diffraction peaks from the
Scherrer equation.
92
10 20 30 40 50 60 70 80
e
d
c
b
*
***
(110)
(101)
(002)
oo o
o
(003)
Inte
ns
ity
(a
.u.)
Two-theta(0)
(006) (015)
ZnAl-LDH ZnO
(100)
a
Figure 4-3 XRD patterns of ZnO−r(x)LDH (from 400 oC) and ZnO: a, b, c, d and e represent ZnO,
ZnO−r(2)LDH ZnO−r(3)LDH ZnO−r(4)LDH and ZnO−r(5)LDH respectively.
The morphologies of ZnO−(2)LDH, (2)LDO and ZnO−r(x)LDH were analysed by SEM and
TEM (Figure 4-4). As-synthesized ZnO−(2)LDH (Figure 4-4A and Figure 4-4a) presented
the typical platelike morphology of LDH with a hexagonal and lamellar structure. After
calcined at 600 oC, the formed LDO in Figure 4-4B and Figure 4-4b displayed as aggregated
particles with a hexagonal structure and with the crystal size in the range of 50~100 nm. The
TEM images of ZnO−(2)LDH and ZnO−r(2)LDH samples after ultrasonication shown in
Figure 4-4a and Figure 4-4c revealed the stacking character of layered particles.
Interestingly, ZnO−r(2)LDH and ZnO−r(5)LDH after reconstruction exhibited rough and
dotted surfaces, as shown in Figure 4-4C and Figure 4-4D and TEM images in Figure 4-4c
and Figure 4-4d, which further demonstrated that small nanoparticles were well distributed
on the surface of LDH plates. These results suggest that ZnO nanoparticles were prevented
from aggregation and distributed evenly in ZnO−rLDH as LDH served as a substrate during
the reconstruction in aqueous solution. The size of ZnO nanoparticles was around 5~10 nm
(Figure 4-4c and Figure 4-4d), which would be beneficial for photocatalysis. Also,
93
ZnO−r(5)LDH had a higher crystal size of ZnO compared with that of ZnO−r(2)LDH, which
corresponded well to the XRD data (Table 4-1).
Figure 4-4 SEM (left) and TEM (right) images for (A/a) ZnO−(2)LDH, (B/b) (2)LDO, (C/c)
ZnO−r(2)LDH and (D/d) ZnO−r(5)LDH.
94
The structural and textural properties of ZnO−(2)LDH, (2)LDO and ZnO−r(2)LDH was
studied by N2 adsorption-desorption isotherms at 77 K. Figure 4-5 shows that all of the
samples displayed the type IV nitrogen adsorption isotherm with H3-hysteresis loops for the
desorption isotherm. The specific characteristic of this type of isotherm is its hysteresis loop,
which is associated with capillary condensation taking place in mesopores. According to the
IUPAC classification, hysteresis loops are classified into four types. Isotherms with Type H3
loops that do not level off at relative pressures close to the saturation vapour pressure
suggest that these composites were comprised of the aggregates (loose assemblages) with
platelike morphology and slitlike pores.26
0.0 0.2 0.4 0.6 0.8 1.0
0
20
40
60
80
100
120
140
160
Relative Pressure (P/P0)
Volu
me a
dso
rbed
(cm
3g
-1)
a
b
c
Figure 4-5 N2 adsorption/desorption of isotherms of (a) ZnO−(2)LDH, (b) (2)LDO and
(c)ZnO−r(2)LDH.
The initial plateau part of the isotherm is attributed to monolayer adsorption, which followed
by Langmuir adsorption isotherm. While at high p/po values, adsorption isotherms are no
longer the plant forms, which suggest that the physic-sorption of N2 is taking place between
the aggregates of platelets particles and accounts for the lamellar morphology of the
materials. The specific surface areas are calculated using Multipoint BET method. The BET
95
specific surface areas for ZnO−(2)LDH, (2)LDO and ZnO−r(2)LDH are 47.5 m2/g, 39.4 m2/g,
and 49.6 m2/g respectively. Thermal treatment decreases the surface area, as particle
aggregation happens in the calcination process. After rehydration, metal oxides
reconstructed back to layered structure thus the surface area for ZnO−r(2)LDH is close to
ZnO−(2)LDH. Moreover, ZnO particles on the surface of ZnO−r(2)LDH increase the surface
area a little bit.
4.3.2 Adsorption of OrgII on LDO composites
The adsorption kinetic of OrgII for (2)LDO obtained from 600 oC in Figure 4-6 shows that
the concentration of OrgII reduced significantly from 100% to 38% in the first 100 min,
followed by a moderate decrease 18% in the period of 100-400 min, and finally slowly
reduced to 5% till the sorption equilibrium was established. Based on the adsorption profiles,
a three-step adsorption of OrgII on (2)LDO is proposed herein. Firstly, OrgII molecules are
quickly adsorbed on the surface of (2)LDO by electrostatic attraction. Then, large adsorption
of OrgII dyes on (2)LDO occurs in the second step, relating to the reconstruction of (2)LDO
to ZnO−(2)LDH. Specifically, OrgII dyes intercalate into the interlayer of the formed
ZnO−(2)LDH during the rehydrated process of LDH. Finally, the slow adsorption step of
OrgII on ZnO−(2)LDH composite is through the anionic exchange of ZnO−(2)LDH. From the
adsorption profile of (2)LDO, it was found that the sorption equilibrium of OrgII on (2)LDO
requires 24 h. Therefore, based on the sorption equilibrium time for (2)LDO, the following
adsorptions were allocated 24 h to ensure saturated adsorption. Then, (2)LDO prepared
from different calcined temperatures were used as the start materials in OrgII adsorption.
The inserted figure in Figure 4-6 shows that adsorption amounts of OrgII are 426.4, 676.1,
800.8, 230.5 and 11.3 mg/g for (2)LDO composites from 400, 500, 600, 700 and 800 oC
treatment respectively. The adsorption results demonstrated that the adsorption capacity of
(2)LDO increased with increasing calcination temperature, maximising at 600 oC with 800.8
mg/g, and then reduced following a further increase in temperature. The adsorption property
was consistent with the LDH content and crystal size of ZnO−r(2)LDHs in the XRD results.
This result suggests that the featured structures of LDO and ZnO−LDH composites play an
important role in OrgII adsorption.
96
0 200 400 600 800 1000 1200 1400
0
20
40
60
80
100
400 500 600 700 8000
100
200
300
400
500
600
700
800
Adsorp
tion c
apacity (m
g/g
)
Temperature ( oC)
(C
/Co)%
Adsorption time (min)
Figure 4-6 The adsorption dynamics of OrgII on (2)LDO (600 oC). The insert is the adsorption
capacity as a function of calcination temperatures for preparing LDOs.
The effect of the state of LDH and the molar ratio of Zn2+ to Al3+ on the adsorption of OrgII
for LDO composites was also investigated. Figure 4-7 shows the adsorption isotherms for
ZnO−(2)LDH and (1)LDO, (2)LDO, (3)LDO, (4)LDO, (5)LDO composites gained from 600
oC treatment. As shown in Figure 4-7, the adsorption types of these composites fit well with
Langmuir isotherm models. The adsorption property of (2)LDO derived from ZnO−(2)LDH
composite after calcination at 600oC in Figure 4-7A was significantly higher than that of
ZnO−(2)LDH (800.8mg/g vs. 196.4mg/g) due to the reconstruction of ZnO−r(2)LDH during
LDO’s rehydration process and intercalation of OrgII dyes into the interlayer of
ZnO−r(2)LDH. As shown in Figure 4-8, the typical (003) and (006) peaks of ZnO−r(2)LDH
after adsorption of OrgII and SDS (SDS stands for Sodium dodecyl sulfate) existed and
shifted to the lower angles, indicating the d spacings of LDH increased from 0.33 nm to 0.69
nm and 0.83 nm respectively. Thus, these XRD results confirmed the reconstruction of ZnO-
r(2)LDHs and intercalation of OrgII and SDS into the interlayer of ZnO−r(2)LDH. The
intensity of (003) and (006) peaks was weaker for ZnO−r(2)LDH when OrgII or SDS was
adsorbed, contributing to the low crystallinity of ZnO−r(2)LDH after uptake of dye during
rehydration process.
97
0.0 0.5 1.0 1.5 2.0 2.50
200
400
600
800b
qe(m
g/g
)
Ce(mM/L)
a
(A)
0.0 0.5 1.0 1.5 2.0 2.5
0
200
400
600
800
f
e
d
c
qe (
mg
/g)
Ce(mM/L)
b
(B)
Figure 4-7 Adsorption isotherms of OrgII on (a) ZnO−(2)LDH, (b) (2)LDO, (c) (1)LDO, (d) (3)LDO,
(e) (4)LDO and (f) (5)LDO. (the LDOs were calcined at 600 oC. The dashed lines are Langmuir
isotherm simulations).
10 20 30 40 50 60 70 80
d
c
b
Inte
ns
ity
(a
.u.)
Two theta(0)
a
Figure 4-8 XRD patterns for (a) (2)LDO, (b) ZnO−r(2)LDH,(c) (2)LDO after adsorption of OrgII and
(d) (2)LDO after adsorption of SDS
98
The adsorption isotherms of OrgII over (x)LDO are shown in Figure 4-7B. Langmuir and
Freundlich isotherm models were used to simulate the adsorption of OrgII on (x)LDO, the
parameters of these models are listed in Table 4-2. The maximum equilibrium adsorption
capacities (qm) obtained from Langmuir isotherm simulation are 636.9, 819.7, 330.0, 243.3
and 168.4 mg/g for (1)LDO, (2)LDO, (3)LDO, (4)LDO and (5)LDO respectively. These are
closer to the experimental results than those obtained from Freundlich isotherm simulation.
Furthermore, the adsorption process of OrgII on (x)LDO fits better using the Langmuir
isotherm model, with a higher value of regression coefficient R2 than in Freundlich model.
As presented in Figure 4-7B, (2)LDO showed the best adsorption capacity among all the
samples. The adsorption capacity changed with Zn2+/Al3+ratios, which was consistent with
the content and the crystal size of LDH in ZnO−r(x)LDH. Therefore, the increased size and
proportion of LDH in ZnO−r(x)LDH resulted in the increased adsorption quantity of OrgII on
corresponding (x)LDO.
Table 4-2 Langmuir and Freundlich isotherms parameters of OrgII uptake on (x)LDO
Samples
Langmuir isotherm model Freundlich isotherm model
qm (mg/g) KL (L/mg)
R2
KF (L/mg) 1/n R2
(1)LDO
(2)LDO
(3)LDO
(4)LDO
(5)LDO
636.9 98.4 0.999
819.7 49.2 0.999
330.0 39.6 0.995
243.3 48.1 0.999
168.4 169.7 0.998
593.3 0.160 0.991
794.1 0.198 0.499
312.6 0.140 0.868
267.2 0.199 0.834
182.8 0.125 0.855
4.3.3 Proposed uptake mechanism
Based on the above results and discussion, it is proposed that the adsorption mechanism
of LDO composites occurs by three synergistic steps: surface adsorption, reconstruction of
calcined LDH precursors by the memory effect, and interlayer anion exchange, as shown in
following reaction equations:22, 27
ZnO+H2O↔Zn(OH)2
Al2O3+3H2O↔2Al(OH)2++2OH-
CO2+H2O↔CO32-+2H+
Zn(OH)2+q[Al(OH)2]++An-→[Zn1-x Alx(OH)2]q+[An-q/n·mH2O]
99
Zn(OH)2+[Al(OH)2]++qOH-→[Zn1-x Alx(OH)2]q+[OH-q·mH2O]
Zn(OH)2+[Al(OH)2]++q/2CO32-→[Zn1-x Alx(OH)2]q+[ CO3
2-q/2·mH2O]
[Zn1-x Alx(OH)2]q+[OH-q·mH2O]+ q/nAn-→[Zn1-x Alx(OH)2]q+[An-
q/n·mH2O]+qOH-
[Zn1-x Alx(OH)2]q+[ CO32-
q/2·mH2O]+q/nAn-→[Zn1-x Alx(OH)2]q+[An-q/n·mH2O]+ q/2 CO3
2-
For LDH, the adsorption process of ionic molecules in the solution is the surface adsorption
and ionic exchange process, which depends on the charge-balancing anions in the
interlayer, and the charge density of the LDH. Typically, carbonate-containing LDH shows
very low anion exchange capacity because of the high affinity between carbonates with the
positively charged sheets.22 With LDH upon calcination at the temperatures used, mixed
metal oxides (LDO) from LDH were obtained. After the calcined LDO samples were soaked
in the aqueous solution, they could be reconstructed to LDH through a rehydration process.
In this process, water molecules serve as a hydroxyl resource, and the anions in the solution
serve as interlayer counterparts. The XRD results in Figure 4-1A confirmed metal oxides in
the LDO composites after calcination. When LDO contacts with water, partial metal oxides
combine with water and generate metal hydroxides. When part of Zn2+ in Zn(OH)2 is
substituted by Al3+, the hexagonal metal hydroxide layers change to become positively
charged, and also receiving an offset from anionic counterparts. In dye solution, OrgII can
be vastly adsorbed during this reconstruction process via intercalation and surface
adsorption. In detail, the anions of OrgII intercalate into the interlayer of LDH with the SO3-
groups connecting with metal ions and hydroxyl from two layers of LDH through electrostatic
attraction and hydrogen bonding.28 Meanwhile, LDO is rehydrated back to LDH, and
intercalating highly affiliated OH- and CO32- anions. Finally, slow exchange adsorption
happens where OrgII is adsorbed via exchanging with interlayered OH- and CO32-. Part of
ZnO remains during the rehydration process, which was confirmed by detected ZnO peaks
in ZnO−rLDH XRD (Figure 4-1). In Figure 4-9 the FT-IR characteristic peaks disappear and
then reappear from ZnO−LDH to LDO and then ZnO−rLDH. It also demonstrates the
rehydration character of LDH with the functional group adsorption peaks of OrgII exhibited,
confirming the OrgII was adsorbed and intercalated into the layers of ZnO−rLDH during the
rehydration process.
100
4000 3500 3000 2500 2000 1500 1000
e
d
c
b
Inte
nsi
ty
Wavenumber (cm-1
)
a
Figure 4-9 FT-IR spectra of (a) OrgII, (b) (2)LDO after adsorption of OrgII, (c) ZnO-r(2)LDH, (d)
(2)LDO, (e) ZnO-(2)LDH.
Peaks at 3500 cm-1 around are the infrared absorption from hydroxyl stretching vibration,
which represents the existing of -OH and H2O. Peaks at 3500 cm-1 around disappear from
ZnO-(2)LDH to (2)LDO, which indicates OH- and H2O have been removed after thermal
treatment. And then with dispersion in water, (2)LDO has rehydrated back to its layered
structure and reappearing 3500 cm-1 absorption peak in ZnO-r(2)LDH. The FT-IR spectra of
OrgII and the (2)LDO after adsorption of OrgII are shown in Figure 4-9 (a) and (b). The
obvious peak at 3500 cm-1 reveals that (2)LDO has reconstructed into the layered structure
of LDH after OrgII adsorption. A band at 1600cm-1 is attributed to the absorption of C-C
aromatic stretch. And the bands around 1000-1250 cm-1 corresponds to -SO3- vibrations
from OrgII. The similar absorption band in 1000-1500 cm-1 indicates that OrgII has
successfully intercalated in the LDH after adsorption.
4.3.4 Photocatalytic performance
After reaching the adsorption equilibrium, the photocatalytic performance of LDO and
ZnO−rLDH composites was investigated. Figure 4-10A shows the photocatalytic
performance of LDOs, ZnO and TiO2 (P25) in 100 min UV irradiation upon OrgII degradation
after sorption equilibrium. As seen in Figure 4-10A, 9.9% and 30.4% of OrgII were removed
101
on commercial ZnO and P25 respectively. Except for (2)LDO, photocatalytic activities of
LDOs were similar to those of ZnO and P25. The (2)LDO sample showed 74.3% of OrgII
decomposition over 100 mins, which was better than the other samples. The photocatalytic
profiles of OrgII decomposition fit well with a pseudo-first-order kinetic equation,29 ln(C/Co)=-
k•t, where C and Co are the reactant concentration at time t=t and t=0 respectively; k and t
are the apparent reaction rate and time respectively (Fitting lines were presented in Figure
4-11). Compared with ZnO and TiO2 (P25), (2)LDO exhibited higher photocatalytic activity
with higher reaction rate, 1.3×10-2 (Table 4-3). Figure 4-10B is the typical absorption spectra
of OrgII in aqueous solution with the presence of (2)LDO under UV irradiation at different
periods of time. The major absorption peak of OrgII at 485 nm decreases gradually along
with the photocatalytic process. As commonly known, photocatalytic activity mainly depends
on three factors including phase structure, adsorption property, and the separation efficiency
of photogenerated electrons and holes.30 The high photocatalytic activity of (2)LDO might
be due to the larger adsorption capacity of (2)LDO, the small particle size, and high
dispersion of ZnO on rehydrated LDH surface, which is facile for intimate contact between
dye and photocatalyst, and easy transfer and separation of charges.
0 20 40 60 80 1000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1g
f
C/C
' o
irradiation time(min)
a
b
c
d
e
(A)
200 300 400 500 600 700 800
Ab
sorb
an
ce(a
.u)
wavelength (nm)
UV irradiation
0 min
20 min
40 min
60 min
80 min
100 min
(B)
102
200 300 400 500 600 700
b
c
e
d
Ab
sorb
an
ce (
a.u
)
Wavelength (nm)
a
(C)
2.50 2.75 3.00 3.25 3.50 3.75 4.00
(D)
e
d
cb
(h)2
(eV
)2
Band gap energy (eV)
a
Figure 4-10 (A) Comparison of OrgII photodegradation in water under UV-light irradiation on (a)
(1)LDO, (b) (2)LDO, (c) (3)LDO, (d) (4)LDO, (e) (5)LDO, (f) TiO2 and (g) ZnO respectively; (B) The
change of absorption spectra for OrgII solution during the photodegradation process over (2)LDO
under UV-light irradiation; (C) UV-visible reflectance spectra of (a) ZnO−r(1)LDH, (b) ZnO−r(2)LDH,
(c) ZnO−r(3)LDH, (d) ZnO−r(4)LDH, and (e) ZnO−r(5)LDH samples; (D) Bandgaps for
ZnO−(2)LDH, ZnO−r(2)LDH, ZnO−r(5)LDH, TiO2 (P25) and commercial ZnO represented by a, b,
c and d individually.
0 20 40 60 80 100
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0 gda
b
ce
ln(C
/Co)
Time (min)
f
Figure 4-11 The pseudo-first-order kinetic fitting lines for the photocatalytic profiles of OrgII
decomposition over on (a) (1)LDO, (b) (2)LDO, (c) (3)LDO, (d) (4)LDO, (e) (5)LDO, (f) TiO2 and (g)
ZnO respectively.
103
Table 4-3 Apparent reaction rates of photocatalysts.
Samples
(1)LDO
(2)LDO (3)LDO (4)LDO (5)LDO TiO2
(P25)
ZnO
Apparent
reaction
rate
k(min-1)
2.0×10-3
1.3×10-2
2.2×10-3
0.9×10-3
2.5×10-3
3.6×10-3
1.1×10-3
4.3.5 The DRS and bandgap of ZnO-rLDH composites
Because the photocatalytic reaction was carried out after reconstruction and adsorption of
LDO in aqueous solution, the diffuse reflectance spectra of samples were tested on
ZnO−rLDHs instead of LDOs. The DRS of ZnO−rLDH samples with the molar ratio of
Zn2+/Al3+ from 1-5 are shown in Figure 4-10C. All the ZnO−rLDH samples presented an
intensive absorption in UV region, which should be caused by the presence of ZnO. The
absorption edge shifted to longer wavelength when the initial Zn/Al ratio increased from 1 to
5.
Figure 4-10D shows the bandgap energy for ZnO−(2)LDH, ZnO−r(2)LDH, ZnO−r(5)LDH,
TiO2 (P25) and commercial ZnO. The bandgap was obtained from the plot of the Tauc
equation, which was transformed from the Uv-vis DRS data: (αhυ)2 = B(hυ-Eg), where α is
the absorption coefficient; hυ is the incident photon energy; and B is constant that is
dependent on the transition probability; the index 2 is a theoretical value that depends on
the transition type of energy, here 2 represents the direct allowed transition bandgap for
semiconductors ZnO and TiO2. As shown in Figure 4-10D, ZnO−(2)LDH did not exhibit
semiconductor properties, even though there was the presence of ZnO, due to the low
crystallisation of ZnO in ZnO−(2)LDH. The bandgap of ZnO−r(2)LDH was similar to that of
ZnO and TiO2 (P25), with a value of 3.20 eV. The bandgap of ZnO−r(5)LDH was 3.05eV,
which was narrower than that of ZnO−r(2)LDH, ZnO and TiO2 (P25). For ZnO−r(x)LDH, the
strong coupling between ZnO and LDHs resulted in the narrowing bandgap of ZnO, as also
reported in the literature.31-32 The strong coupling between ZnO and LDH, and the slightly
narrowed down bandgap may lead to the enhancement of light harvesting, and charge
transfer and separation, which contributed to the better photocatalytic performance of LDO
for OrgII removal. After adsorption, ZnO−rLDH exhibited typical semiconductor character
during the following outlined photocatalytic degradation process. When the solution is
104
irradiated, electron–hole (e-−h+) pairs are generated on the photocatalyst. With LDH serving
as a substrate for supporting and coupling with ZnO, charges are separated effectively. Then
super oxygen-anion free radicals are produced through the reduction of oxygen by electrons.
Simultaneously, hydroxyl radicals are produced via oxidation of water or hydroxide ions by
holes. Such reactive oxygen species (ROS) degrade OrgII into CO2 and H2O water
completely.
4.4 Conclusions
Adsorption and photocatalytic tests of the LDO composites for the degradation of the OrgII
dye revealed extremely high synergic adsorption and photocatalytic activity in this research.
LDO showed excellent photocatalytic activity after reconstruction and saturated adsorption
of OrgII. Because of the good stability and “memory effect”, LDO exhibited strong adsorption
of OrgII during its rehydration process. ZnO dispersed on the LDH after rehydration exhibited
superior photocatalytic activity for decomposing OrgII under UV irradiation. The Zn2+/Al3+
molar ratio and calcination temperature were shown to affect the adsorption and
photocatalytic activity. LDO synthesised at 600 °C showed the optimal adsorption and
photocatalytic performance, with an 800.8 mg/g adsorption capacity, and 74.3% OrgII
decomposition after 100 mins of UV illumination. An adsorption mechanism that includes
three synergistic steps was proposed for LDO. The ZnO−r(2)LDH exhibited the largest
crystal size and the highest proportion of LDH, and also highly dispersed ZnO on rehydrated
LDH. The adsorption and photocatalytic properties of LDO have been reported being related
to the structural characters of LDO.7, 33-34 However, the correlation between adsorption
photocatalytic properties of LDOs and the structural characteristics of ZnO−rLDHs has been
discovered and illuminated in this work. This unique correlation between properties and
characteristic structure of rehydrated LDH could provide new physical and chemical insight
into the engineering of the microarchitecture and texture of LDH and its derivates, enhancing
their application in environmental remediation. There is remaining adsorbed OrgII in
ZnO−r(2)LDH after UV irradiation, although the photocatalytic efficient of ZnO−r(2)LDH can
reach 74.3%. Therefore, improving the photocatalytic property and tuning the bandgap into
being responsible in the visible light range should be a main future study to fulfil the
application of recyclable visible-light photocatalyst in large-scale wastewater treatment.
105
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108
5 Carbon black modified g-C3N4 as adsorptive
photocatalysts for decontamination of dyes
under visible light
5.1 Introduction
The efficiency of bulk g-C3N4 in the visible light is rather low because it is hindered by the
marginal absorption of visible light and grain boundary effects.1 Chemical doping is an
effective strategy to modify the electronic structure of g-C3N4 and improve the performances.
Doping of g-C3N4 with S, F, C, or B via substituting for lattice atoms has been applied to
modify its texture and electronic structure for improving the photocatalytic performance.2
The externally doped atoms either substitute for the lattice atoms or exist in the in-planar
caves of g-C3N4.2 For layered g-C3N4, intercalation provides the materials with promising
properties, such as improved chemical and thermal stability, increased charge carrier
separation and transportation, prolonged charge carriers’ lifetime, enlarged specific surface
area and adequate adsorption sites.3-4 Given that g-C3N4 has a layered structure with
interlayer galleries that could allow doping of heteroatom within the interlayer, it is a feasible
route to synthesize an intercalated g-C3N4 compound to achieve enhanced photocatalytic
performance.2 Carbon which can be in various allotropes, such as graphite and graphene,
was found to be an electron collector and transporter. This character can be utilized into
boosting performance of various energy conversion and storage device such as photovoltaic
conversion devices, supercapacitors, batteries, catalyst et al.1 By taking advantage of
carbon’s unique electron transport ability, carbon modification of g-C3N4 can accelerate
electron transfer from the photocatalyst to the liquid-solid interface therefore increase the
separation of electron and hole pairs, further improvement in the photocatalytic activity can
be realized.5
Kang et al. reported that carbon nanodot modified carbon nitride (C3N4) exhibited impressive
performance for photocatalytic solar water splitting. The quantum efficiency could reach as
high as 16% for visible light wavelength r=420+-20 nm. Such carbon nanodot embedded
C3N4 nanocomposite performed as an excellent photocatalyst which can fulfil the overall
water splitting requirement.6 Tang et al. reported that the synergistic effect on the spectral
and electronic coupling of g-C3N4 nanosheets and carbon quantum dots (CQDs) contributed
the large promotion of the photocatalytic activity of g-C3N4 in the UV-visible light region.
109
They firstly reported a CQDs coupled g-C3N4 with photocatalytic activity for H2 production in
the NIR region.7 Very recently, Li et al. reported the carbon dots modified g-C3N4 hybrid via
direct calcination of the mixture of C-dots and dicyandiamide, wherein C-dots was obtained
from the combustion of soot of alcohol burners.8 The C-dots modified g-C3N4 hybrid
exhibited superior photocatalytic activity in dye removing and hydrogen generation
compared with pure g-C3N4. Although the procedure for fabrication of C-dots was relatively
simpler than the methods from the work of Kang 6 and Tang 7 et al, to keep the dispersion
of the C-dots the treatment process were still complex with four steps including hydrothermal
treatment, centrifugation and being dispersed in ammonia solution. A carbon dots (CDs)
decorated graphitic carbon nitride (g-C3N4) photocatalyst was synthesised via a facile
impregnation-thermal method by Zhang and coworkers.9 g-C3N4/CDs composite (with
loading 0.5 wt% carbon dots) resulted in a 3.7 times faster reaction rate for phenol
photodegradation than pristine g-C3N4. As we know, carbon black (CB) is a form of
paracrystalline carbon that has a high surface-area-to-volume ratio, which is also a good
conductor of electricity. CB has been used in various applications for electronics. However,
there is seldom report on the g-C3N4 modification with CB. The conductivity improved by the
cooperation between g-C3N4 and elements such as Na and K was also reported by Hou and
coworkers.10 Therefore, it is predictable that intercalated CB will benefit the transfer and
separation of charge carriers in the g-C3N4 matrix.
Herein, series of CB modified g-C3N4 were firstly prepared by simply heating the mixture of
urea and CB in this chapter. The aim of the work was an attempt in modification of g-C3N4
with carbon in a facile route, achieving in structural modification and physicochemical
characteristic improvement. Ultimate aim of the modification is to advance the utilizing of g-
C3N4 in dye removal. For abbreviation, carbon black modified g-C3N4 was donated as
n(U+CB), where n was the initial weight ratio of urea and CB. After the modification with CB,
the crystallisation and condensation degrees of g-C3N4 was increased. The light absorption
property and charge carriers’ transfer and separation have been improved. Furthermore, the
visible light photocatalytic activity of CB modified g-C3N4 toward MB and OrgII degradation
also increased compared with pristine g-C3N4.
5.2 Adsorption and photodegradation experiment details
The adsorption of the dye pollutants was measured using a batch mode. For the adsorption
of OrgII over the solid samples (including 3000(U+CB), 1500(U+CB), 1000(U+CB),
110
600(U+CB), 200(U+CB) and pristine g-C3N4), 10 mg of a solid sample was added to 50 mL
of an OrgII solution (the concentration was 10 mg/L) in the dark under stirring. Every 20
minutes’ interval, 5 mL of aliquots were extracted using a syringe and filtered through a
membrane (0.45 µm) then to be tested the concentration via UV-Vis spectrophotometer.
After the adsorption equilibrium, the photocatalytic degradation of OrgII was undertaken in
an open thermostatic photoreactor under visible light irradiation. The light resource was
obtained from a 200 W mercury lamp. Visible light irradiation was operated via adding a 400
nm cutoff filter on the mercury lamp. At given time intervals during irradiation, 5 mL of
aliquots were extracted using a syringe and filtered through a membrane. The concentration
of OrgII in the solutions was analysed using a Shimadzu UV-2600 UV-Vis
spectrophotometer.
For the adsorption and photocatalytic degradation of MB, the initial concentration of MB was
the same as OrgII, and the test processes were the same as the mentioned above.
5.3 Results and discussion
5.3.1 Characterization
5.3.1.1 XRD
Figure 5-1 shows the XRD patterns for the g-C3N4 samples. Peaks at 13 o around are
corresponding to the (100) diffraction peaks, which indicate the in-plane repeated units.
Peaks at 27o around can be indexed to the diffraction plane (002), which represent the
periodic graphitic stacking of conjugated system with an interlayer distance around 0.3nm.11-
12 Both peaks become sharper, and the intensities become stronger when the n increased
from 200 to 1500, and then keep in 3000. Therefore, the crystallisation of g-C3N4 was
improved after the modification with CB. A small quantity of dopant CB benefited the
crystallisation degree of g-C3N4, further increase in the CB amount impaired the
crystallisation instead. Moreover, both peaks for (100) and (002) shift to the higher degree
for CB doped g-C3N4 samples compared with pristine g-C3N4, meaning narrower interlayer
distances. The interlayer distance of aromatic units from the (002) peak was calculated to
be 0.322, 0.314, 0.314, 0.315, 0.316 and 0.317 nm for pristine g-C3N4, 3000(U+CB),
1500(U+CB), 1000(U+CB) 600(U+CB) and 200(U+CB) respectively. The shift of (100) and
(002) peaks towards a lower degree for g-C3N4 causing by doping have been reported in
the literature.13 Most of the reported dopants substituted the atoms in the C-N cycles,
therefore, increased the interlayer distance due to the larger atomic radius.2 Herein, the
111
narrower interlayer distance was supposed to be caused by the covalent bonding between
interlayer dopant carbon atoms and the elements on the aromatic rings, which can also be
demonstrated by the FT-IR spectra.
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2 Theta (0)
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
g-C3N4
Inte
nsi
ty (
a.u
.)
Figure 5-1 XRD patterns of g-C3N4, 200(U+CB), 600(U+CB), 1000(U+CB), 1500(U+CB) and
3000(U+CB) samples.
5.3.1.2 FT-IR
4000 3500 3000 2500 2000 1500 1000 500
g-C3N4
1500(U+CB)
wavenumber (cm-1
)
Inte
nsi
ty (
a.u
.)
Figure 5-2 FT-IR spectra for pristine g-C3N4 and 1500(U+CB).
112
Figure 5-2 exhibits the FT-IR spectra for pristine g-C3N4 and 1500(U+CB). The comparisons
of all the n(U+CB) samples are given in Figure 5-3. n(U+CB) samples had the similar FT-IR
absorption patterns. All the g-C3N4 samples had the typical absorption band at 1200 – 1650
cm-1 region, corresponding to characteristic stretching modes of C-N heterocycles.14
Another strong absorption bands at 800 cm were also found in all the g-C3N4 samples,
attributing to the ring sextant out-of-plane bending vibrations of triazine units.15 Moreover,
a broad band located in the range from 3000 to 3500 cm-1 corresponded to the stretching
vibration modes of N-H from the terminal of the aromatic rings.16 The FT-IR results were
consistent with the literature, but with more branches absorption in the range from 1200 –
1650 cm-1, implying that dopant carbon has built a covalent bond with the g-C3N4 aromatic
rings. As seen, the FT-IR absorption intensities for all the vibrations from the bonds in
sample 1500(U+CB) are much stronger than in pristine g-C3N4. The stronger absorption
intensities implied more bonding in samples, indicating higher crystallisation degree for g-
C3N4. Therefore, the FT-IR results well confirmed the speculation from the XRD patterns.
4000 3500 3000 2500 2000 1500 1000 500
CB
200(U+CB)
600(U+CB)
1000(U+CB)
1500(U+CB)
Inte
nsi
ty (
a.u
.)
wavenumber (cm-1
)
3000(U+CB)
Figure 5-3 FT-IR spectra for 3000(U+CB), 1500(U+CB), 1000(U+CB), 600(U+CB), 200(U+CB) and
CB.
113
5.3.1.3 XPS
0 200 400 600 800
c
ba
(A)
C 42.64 43.15 43.39
N 55.71 55.09 55.43
O 1.65 1.75 1.19
O1s
C1s
N1s
Binding energy (eV)
Inte
nsi
ty (
a.u
)
At %
g-C3N4 3000(U+CB) 1500(U+CB)
282 284 286 288 290 292 294 296 298
c
b
C1s
Binding energy (eV)
Inte
nsi
ty (
a.u
)
a
(B)
396 398 400 402 404 406
c
b
a
(C)
Inte
nsi
ty (
a.u
)
Binding energy (eV)
N1s
524 526 528 530 532 534 536 538 540
c
b
a(D)
O1s
Binding energy (eV)
Inte
nsi
ty (
a.u
)
Figure 5-4 XPS images of the pristine (a) g-C3N4, (b) 3000(U+CB) and (c)1500(U+CB) sample. (A)
The survey spectra; (B) The high-resolution C 1s XPS spectra; (C) The high-resolution N 1s XPS
spectra; (D) The high-resolution O 1s XPS spectra.
XPS spectra were used to determine the valence state and chemical environment of the
constituents in samples. The pristine g-C3N4 and CB modified g-C3N4 samples consisted of
mainly C and N elements with a small amount of oxygen shown in survey spectra Figure 5-
4A. Based on XPS data, the atomic ratio of C vs. N was 0.783 for both 3000(U+CB) and
1500(U+CB). The atomic ratio for C vs. N in pristine g-C3N4 was 0.76, which was closer to
theoretical value 0.75. The higher atomic concentration of C in CB modified g-C3N4
suggested the successful doping of C in 3000(U+CB) and 1500(U+CB). The atomic
concentrations of O were determined to be 1.19%, 1.75% and 1.65% for 1500(U+CB),
3000(U+CB) and pristine g-C3N4 respectively. The small amount of oxygen in g-C3N4
samples may be caused by the adsorbed H2O on samples surface and the incomplete
polymerization of urea.17-18 Figure 5-4B, C and D are the high-resolution spectra for C, N
and O respectively. In Figure 5-4B, g-C3N4 samples had two bonding states of carbon
114
species, which were evidenced with the C1s binding energies of around 285 and 289 eV. In
Figure 5-4C, the main peaks of N1s binding energies were located around in 399 and 401
eV for all samples. The high-resolution C1s XPS spectrum for sample 1500(U+CB) given in
Figure 5-5A was deconvoluted into three peaks at 289.0, 286.2, and 284.6 eV,
corresponding to sp2-bonded carbon (N–C=N), C–O and graphitic carbon (C–C),
respectively. Figure 5-5B shows the peak deconvolution results of N 1s XPS spectrum for
1500(U+CB). Three peaks at 399.2, 400.7 and 401.6 eV were ascribed to sp2 hybridised
aromatic N bonded to carbon atoms (C=N–C), the tertiary N bonded to carbon atoms in the
form of N–(C)3 and N–H side groups, respectively. Both the binding energies of sp2-bonded
C and N shifted to higher energies for 3000(U+CB) and 1500(U+CB) compared with pristine
g-C3N4, with the highest intensities for 3000(U+CB). It was also the same for the O 1s
binding energy in Figure 5-4D. The highest intensity of the binding indicated the highest
degree of the polymerization. Both the bonding energies of sp2-bonded C and N from CB
modified g-C3N4 samples were relatively higher than pristine g-C3N4 and reported g-C3N4.19
This could be attributed to the incorporation of intercalated C atoms and the CN
heterocycles. C atoms existing in g-C3N4 interlayer bonded with adjacent C and N atoms,
which increased the bonding energy in the in-plane heptazine units. Such bonding force
from the graphitic interlayer was supposed to drag the g-C3N4 layers and made them into
more compact layers. This hypothesis from XPS result confirms the shrink of crystal lattice
space for g-C3N4 from the XRD (Table 5-1). The shifted XRD and XPS patterns were also
reported in other elemental modifications for g-C3N4.2, 13
115
280 282 284 286 288 290 292 294 296 298
C-OC-C
N=C-N
Inte
nsi
ty (
a.u
.)
Binding energy (eV)
C1s(A)
396 398 400 402 404 406
-excitations
N-H
(C)3-N
C=N-C
N1s
Binding energy (eV)
Inte
nsi
ty (
a.u
.)
(B)
526 528 530 532 534 536 538
O1s
Binding energy (eV)
Inte
nsi
ty (
a.u
.)
(C)
Figure 5-5 The deconvolution of high-resolution XPS spectra for C 1s (A), N 1s (B) and O 1s (C)
for sample 1500(U+CB).
Table 5-1 BET surface area, pore volume, bandgap and the interplanar distance for samples and
the respective pseudo-first-order rate constant for MB and OrgII photodegradation over
photocatalysts.
Samples SBET
(m2g-1)
Pore
volume
(cm3g-1)
Bandgap
(eV)
K(MB)
(10-3 min-1)
K(OrgII)
(10-3 min-1)
d(002)
(nm)
g-C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
128.6
77.2
54.9
61.7
69.4
87.2
0.598
0.341
0.152
0.165
0.170
0.350
2.89
2.95
2.89
2.86
2.82
2.63
1.07
3.05
2.94
2.36
2.24
1.97
5.34
6.07
2.25
0.861
0.693
0.125
0.322
0.314
0.314
0.315
0.316
0.317
116
5.3.1.4 TEM and SAED
Figure 5-6 (a) and (b) are TEM images for 1500(U+CB) and pristine g-C3N4 respectively; (c) and
(d) are HRTEM images with inset SAED patterns for 1500(U+CB) and pristine g-C3N4 respectively.
The morphologies of 1500(U+CB) and pristine g-C3N4 are presented in Figure 5-6a and
Figure 5-6b, respectively. As shown in Figure 5-6a and Figure 5-6b, both TEM images
exhibit the typical layered platelet-like surface morphology with wrinkling edges.
1500(U+CB) is with more compacted and intact flakes, on the contrary, pristine for g-C3N4
is with irregular interstitial pores at the edge of its flakes. The porous structure of pristine g-
C3N4 was generated due to the releasing NH3 and CO2 H2O etc. gases during the pyrolysis
of urea. For 1500(U+CB), CB in the mixture was assumed to exert its absorption property,
accommodating generated gases during the heating process. Moreover, it served as the
condensation sites for the generated intermediate gases. Therefore, 1500(U+CB) is more
117
intact and compact than pristine g-C3N4, corresponding to the N2 adsorption-desorption
results, with SBET 128.6 m2g-1 for pristine g-C3N4, higher than 54.9 m2g-1 for 1500 (U+CB)
listed in Table 5-1.
The crystalline characters for both samples were also characterised by SAED patterns,
which are insets in Figure 5-6c and Figure 5-6d. Both SAED patterns show a full outer
diffraction ring which can be indexed as the (002) reflection of g-C3N4 and an inner weak
diffraction ring which can be indexed as the refection of the (100) plane from g-C3N4,
indicating that both g-C3N4 samples consist of extremely small crystallites with random
orientation. The SAED patterns are consistent with the XRD data.
5.3.1.5 Nitrogen adsorption-desorption analysis
Figure 5-7 shows the nitrogen adsorption-desorption isotherm of g-C3N4 and the series of
n(U+CB) samples. The adsorption-desorption isotherms for all samples are type IV (BDDT
Classification), suggesting the presence of mesopores (2–50 nm).20 The hysteresis loops
are type H3 (IUPAC classification), indicating the formation of slit-shaped pores from the
aggregates of plate-like particles.21-22 Regarding the overall view, the BET surface areas
and pore volumes were less for n(U+CB) samples compared with pristine g-C3N4.
Considering the similar TEM between 1500(U+CB) and pristine g-C3N4, we proposed they
have the similar particle size, so the BET results hinted that modified g-C3N4 should be more
compact than pristine g-C3N4, which demonstrated that CB modification could improve the
condensation degree of urea. The BET surface areas and pore volumes (listed in Table 5-
1) decreased when increasing the amount of carbon black (SBET were 128.6, 77.2 and 54.9
m2g-1 for g-C3N4, 3000(U+CB) and 1500(U+CB) respectively; pore volumes were 0.598,
0.341 and 0.152 cm3g-1 for g-C3N4, 3000(U+CB) and 1500(U+CB) respectively). Further
increase of CB, the BET surface area and pore volume gradually increased instead. (SBET
were 61.7, 69.4 and 87.2 m2g-1 for 1000(U+CB), 600(U+CB) and 200(U+CB) respectively;
pore volumes were 0.165, 0.170 and 0.350 cm3g-1 for 1000(U+CB), 600(U+CB) and
200(U+CB) respectively.) Small dopant amount was supposed to improve the
polymerization. Further increase of CB could contribute to the higher surface area of
n(U+CB) due to the excess porous CB. (the SBET and pore volume for CB is 237.1m2g-1 and
0.370 cm3g-1 respectively.)
118
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
350
400
Volu
me a
dso
rb
ed
(cm
3g
-1)
Relative pressure (P/PO)
g-C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
Figure 5-7 N2 adsorption/desorption of isotherms of pristine g-C3N4, 200(U+CB), 600(U+CB)
1000(U+CB), 1500(U+CB) and 3000(U+CB).
5.3.1.6 Thermostability analysis
Figure 5-8 Thermogravimetric curves of pristine g-C3N4, 200(U+CB), 600(U+CB) 1000(U+CB),
1500(U+CB) and 3000(U+CB) under air at a heating rate of 10 oC∙min-1.
200 300 400 500 600 700 800
0
20
40
60
80
100
Temperature (oC)
W
eig
ht
(%)
CB
g-C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
119
The thermal stabilities of as prepared g-C3N4 samples were evaluated by thermogravimetric-
differential scanning calorimetry analysis (TG-DSC). In Figure 5-8, CB modified g-C3N4
samples were found to be stable at temperatures up to 550°C, suggesting a stable
construction of heptazine-base units in the materials. The slight mass loss below 500 °C
was mainly due to the volatilization of absorbed H2O and other substances on the surface
of the polymers.23 Note that the thermal performance of pristine g-C3N4 was somewhat
different from those n(U+CB) samples. Gradual mass loss of the polymer was first detected
when the temperature was elevated to 550 °C, resulting from the loss of surface hydroxyl
groups and absorbed water molecules, which was followed by the decomposition of defects
and edge functional groups such as uncondensed amine functional group and the edge
cyano-group of polymer g-C3N4.21 The temperature of thermal decomposition and complete
weight loss of 600(U+CB), 1000(U+CB), 1500(U+CB) and 3000(U+CB) were determined at
around 620 °C, which was 15 °C lower than that of pristine g-C3N4. However, the complete
weight loss for 200(U+CB) was at 655 °C, exhibiting a hunch in the weight loss slope at a
temperature above 600 °C, which could be ascribed to the high content of CB. Therefore,
the thermogravimetric analysis indicates that urea can get a high degree of
polycondensation after the mixture with CB. And less uncondensed branches and functional
group are presented in the CB modified g-C3N4.
Figure 5-9 illustrates DSC curves of g-C3N4 samples. The DSC curves show that
decomposition of the as-prepared g-C3N4 occurred at around 600 oC. Moreover, no
endothermal peaks appeared for all samples, meaning no deammonation process was
observed during the heating of g-C3N4 samples i.e. all the as prepared g-C3N4 were highly
polymerised.14, 24 The exothermic peaks for CB modified g-C3N4 samples shifted to a little
bit of lower temperature compared with pristine g-C3N4, echoing the TG curves.
120
Figure 5-9 DSC profiles for g-C3N4 samples and CB.
5.3.1.7 The DRS analysis
The optical absorption spectra were used to investigate the effect of CB modification on
the electronic structure of g-C3N4. UV-vis diffuse reflectance absorption spectra (DRS) of
the obtained g-C3N4 products are presented in Figure 5-10. As can be seen, CB modified g-
C3N4 and pristine g-C3N4 show typical absorption patterns of semiconductors with
absorption edges at~445 nm, indicating that these products can absorb solar energy in the
blue-light region.18 The incorporation of CB into the g-C3N4 matrix led to an increase in the
UV-vis absorption over the entire wavelength range. Moreover, absorption tail intensity was
increased along with the increasing amount of CB. (Figure 5-10A)
300 350 400 450 500 550 600 650 700 750 800
Temperature(oC)
H
eat
flow
(m
W)
Exo.
CB
g-C3N4
3000
1500
1000
600
200
121
200 300 400 500 600 700 800
0.0
0.5
1.0
1.5
g-C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
Ab
sorb
an
ce (
a.u
)
Wavelength (nm)
200(U+CB)
(A)
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
h (eV)
(h
)2 (
eV)2
g-C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
(B)
Figure 5-10 (A) UV-vis diffuse reflectance spectra of the photocatalysts: g-C3N4, 200(U+CB),
600(U+CB) 1000(U+CB), 1500(U+CB) and 3000(U+CB). (B) the estimated band gap energies of
g-C3N4, 200(U+CB), 600(U+CB) 1000(U+CB), 1500(U+CB) and 3000(U+CB) with corresponding
tangent lines
The band gap energies for all the samples can be calculated using the Tauc equation:
αhʋ=A(hʋ-Eg)n where α, ʋ, A and Eg are the absorption coefficient, light frequency,
proportionality constant, and bandgap energy, respectively. In the equation, n decides the
features of the transitions in a semiconductor i.e. n=1/2 for direct transitions, and n=2 for the
indirect transition. In this case, the optical absorption of the g-C3N4 samples is directly
122
allowed. The bandgap energies for all the photocatalyst are in the region from 2.60 to 3.00
eV as shown in Figure 5-10B and listed in Table 5-1. Low concentration of CB in modified
g-C3N4 widen the bandgap of g-C3N4, which could probably due to the increase in the valent
band (VB) of g-C3N4 attributing to the increase bonding energy in in-plane units. The higher
CB concentration, the narrower is the bandgap, which is most likely due to the effect of C
addition leading to the formation of more sub-band states in the band gap.6
5.3.1.8 PL spectra analysis
The photoluminescence (PL) spectra of photocatalysts are presented in Figure 5-11. The
PL spectra are useful to reveal the migration, transfer and separation efficiency of the
photogenerated charge carriers.25 A faster recombination of electron-hole results in a more
intense PL spectrum.26 All the samples show only one kind of luminescence peak in the
spectrum. The emission peak of the samples is broad and centred at around 440 nm. This
strong peak at 450 nm for pristine g-C3N4 is attributed to the band-band PL phenomenon
with the energy of light close to the bandgap energy of g-C3N4 as the bandgap energy of the
g-C3N4 in this investigation is observed to be around 2.9 eV. This near band edge emission
can be attributed to the direct recombination of excitons through an exciton-exciton collision
process.17 The intensity of the emission peak appreciably decreased when g-C3N4 modified
with a small concentration of CB, such as sample 3000(U+CB) and 1500(U+CB). With
further increase in the concentration of CB, the intensity of the emission peak decreased
gradually instead. A comparative study of the overall intensity of all samples indicates that
a substantial inhibition of charge carriers’ recombination happened in the samples
3000(U+CB) and 1500(U+CB). Low intensities of PL for 1000(U+CB), 600(U+CB) and
200(U+CB) should be ascribed to the absorption of incident light by CB. The photocatalytic
activity is related to the concentration of free charge carriers during the photocatalytic
process. Therefore, sample 3000(U+CB) and 1500(U+CB) are relatively effective for the
separation of charge carriers, indicating the potential higher photocatalytic activity compared
to the other samples.
123
420 440 460 480 500 520 540 560 580 600
Inte
nsi
ty (
a.u
.)
Wavelength (nm)
C3N4
3000(U+CB)
1500(U+CB)
1000(U+CB)
600(U+CB)
200(U+CB)
Figure 5-11 Photoluminescence spectra of g-C3N4, 200(U+CB), 600(U+CB) 1000(U+CB),
1500(U+CB) and 3000(U+CB).
5.3.1.9 The EIS analysis
0 10 20 30 40
0
50
100
150
200
1500(U+CB)
g-C3N
4
CB
Z''
(oh
m)
Z'(ohm)
Figure 5-12 Electrochemical impedance spectroscopy of g-C3N4, CB and 1500(U+CB).
In addition, EIS measurement was carried out to investigate the charge transfer resistance
and the separation efficiency of the photoinduced charge carriers. Figure 5-12 demonstrates
the diameter of the Nyquist semicircle for the 1500(U+CB), pristine g-C3N4 and CB
electrodes. The Nyquist semicircle for 1500(U+CB) is located between those for pristine g-
124
C3N4 and CB. A small arc radius implies a high efficiency of charge transfer and
separation.11, 27 Therefore, this result is consistent with PL spectra, indicating that after
modification with CB the interface of the g-C3N4 sample has an enhanced separation and
transfer efficiency of photoinduced electron–hole pairs, which is favourable condition for
improving the photocatalytic activity.
5.3.2 Adsorption and photocatalytic activities of samples
5.3.2.1 Performance on cationic dye MB and anionic dye OrgII
-40 0 40 80 120 160 200 240
20
40
60
80
100
C/C
o (
%)
Time (min)
3000 600
1500 200
1000 g-C3N4
(A)
-40 0 40 80 120 160 200 240
0
20
40
60
80
100
Time (min)
C/C
o (
%)
3000 600
1500 200
1000 g-C3N4
(B)
Figure 5-13 Comparison of MB (A) and OrgII (B) adsorption and photodegradation in water under
visible light over pristine g-C3N4 and n(U+CB), with n equal to 3000, 1500, 1000, 600 and 200
respectively.
The photocatalytic activities of the g-C3N4 samples were evaluated by monitoring the
degradation of MB and OrgII solution under visible light irradiation respectively (wavelength
≥400 nm). The adsorption equilibrium established in one hour before undergoing the
photocatalytic test. After modification with CB, the adsorption property of g-C3N4 was
increased toward cationic dye MB (Figure 5-13A), while the adsorption of anionic dye OrgII
(Figure 5-13B) was weaken compared with pristine g-C3N4. The MB removal rates were
59.7%, 63.3%, 58.5%, 54.8%, 70.2% and 26.5% over 3000(U+CB), 1500(U+CB),
1000(U+CB), 600(U+CB), 200(U+CB) and pristine g-C3N4 respectively. The OrgII removal
rates were 80.1%, 50.1%, 28.0%, 28.0%, 29.2% and 87.8% over 3000(U+CB), 1500(U+CB),
1000(U+CB), 600(U+CB), 200(U+CB) and pristine g-C3N4 respectively. The pristine g-C3N4
has the highest SBET and pore volume, but the poorest adsorption toward MB and the highest
adsorption toward OrgII. The 200(U+CB) has the close SBET and pore volume with
125
3000(U+CB), but with much higher adsorption toward MB and OrgII. Due to the positive
charge character of pristine g-C3N4,21 the inferior adsorption toward cationic dye MB can be
imaginable. Moreover, based on the adsorption, more negative charge for n(U+CB)
compared with pristine g-C3N4 can also be speculated. The adsorption property toward dyes
should depend more on the surface charge character instead of surface area and pore
volume. Exceptionally, 200(U+CB) exhibited substantial adsorption to both MB and OrgII,
which can be attributed to the over amount of existing CB in the g-C3N4 matrix.
The photodegradation experimental data were fitted by a first-order kinetic model as
expressed by the equation: ln(C/C0) = −k·t, with the value of the rate constant k giving an
indication of the activity of the composites photocatalyst (Listed in Table 5-1).28 The
photoactivity has been improved obviously for degradation of MB over n(U+CB), with the
highest k value 3.05×10-3 min-1 for 3000(U+CB). (Figure 5-14A) Although pristine g-C3N4
still exhibited superior photoactivity toward degrading OrgII, 3000(U+CB) suppressed it with
the highest k value 6.07×10-3 min-1. (Figure 5-14B)
0 50 100 150 200 250
-1.0
-0.5
0.0
ln
(C
/Co)
Time (min)
Fitting Curves
200
600
1000
1500
3000
g-C3N4
(A)
0 50 100 150 200 250
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
ln
(C
/Co
)
Time (min)
Fitting Curves
200
600
1000
1500
3000
g-C3N4
(B)
Figure 5-14 First-order kinetic plots for the photodegradation of MB (A) and OrgII (B) over g-C3N4
samples.
The UV-Vis absorption spectra of MB solution and OrgII solution after different periods of
the adsorption and photocatalytic decomposition are illustrated in Figure 5-15, from which it
can be observed that the main absorption peaks at 665 nm and 480 nm for MB and OrgII
respectively weakened after 1 h adsorption and 4 h visible-light irradiation, implying that
dyes were decomposed into water and carbon dioxide gradually.
126
200 300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
irritation (h)
Ab
sorb
an
ce (
a.u
.)
Wavelength (nm)
1 h adsorption
1
2
3
4
(A)
200 300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1
2
3
4
irritation (h)
1 h adsorption
(B)
Wavelength (nm)
Ab
sorb
an
ce (
a.u
.)
Figure 5-15 Temporal UV-vis absorption spectral changes during the adsorption and photocatalytic
degradation of MB (A) and OrgII (B) in aqueous solution in the presence of 3000(U+CB).
127
5.3.2.2 Proposed mechanism under visible light irradiation
0 1 2 3 4 5 6 7 8 9 10
b
Inte
nsi
ty (
a.u
.)
Binding energy (eV)
a
(A)
Figure 5-16 Band structures of pristine g-C3N4 and 3000(U+CB): (A) XPS valence band spectra;
(B) band structure diagram.
It is accepted that •OH, •O2- and photogenerated holes are the main reactive species in
photocatalytic reactions, which are determined by the surface redox potential of the
photocatalyst.29 The XPS valence band (VB) spectra of pristine g-C3N4 and 3000(U+CB) are
shown in Figure 5-16A. The VB maximum of pristine g-C3N4 and 3000(U+CB) is 1.45 and
1.78 eV respectively. The higher VB value suggests a stronger oxidation of the
photogenerated holes over 3000(U+CB). Considering that the band gaps of pristine g-C3N4
and powder 3000(U+CB) are 2.89 and 2.95 eV (Figure 5-10B), the conduction band (CB)
128
minimum could locate at -1.44 and -1.17 eV, respectively. Therefore, the band structures of
pristine g-C3N4 and 3000(U+CB) are shown in Figure 5-16B. The potential energy of the VB
for both pristine g-C3N4 and 3000(U+CB) are lower than that of OH-/•OH (1.99 eV), whereas
the potential energy of the CB is more negative than that of O2/•O2- (-0.28 eV). Therefore,
the photogenerated holes cannot directly oxidise OH- into hydroxyl radicals •OH, but the
electrons can reduce the O2 into superoxide radicals •O2-. Hence, the main reactive species
in the photocatalytic reactions for these samples are •O2- and photogenerated holes.30
Based on the above discussion, CB in the g-C3N4 matrix could not only play a key role in
improving the transfer of the free charge carriers and reducing the recombination of
electrons and holes but also broaden the bandgap of g-C3N4 via covalent bonding with g-
C3N4 matrix resulting in the increase in the VB of g-C3N4 and the improvement photocatalytic
oxidation property. This explains the reason why 3000(U+CB) exhibited stronger
photodegradation of dyes compared with pristine g-C3N4.
5.4 Conclusion
In this report, we modified g-C3N4 with CB via pyrolysis the mixture of urea and CB. After
the modification, the crystallisation and condensation degrees of g-C3N4 were increased.
CB modified g-C3N4 samples exhibited more compact layers with a decrease in the interlayer
distance, which was ascribed to the covalent bonding between carbon atoms and C-N
cycles. The existing of carbon atoms in the g-C3N4 matrix not only enhanced the light
absorption for g-C3N4 but also quenched the recombination of charge carriers. As a result,
the bandgap structure of g-C3N4 was tuned, and the electron-hole separation was improved.
Therefore, modified g-C3N4 samples exhibited improved performance for dyes’ removal.
200(U+CB) exhibited 70.2% removal rate for MB which is higher than 26.5% from the
pristine g-C3N4. Although, pristine g-C3N4 had the highest removal rate for OrgII with 87.8%,
the 3000(U+CB) shown the most superior photocatalytic activity with the highest kinetic rate
constant k in both degradation of MB and OrgII. In addition, the photocatalytic mechanism
was proposed to explain the experimental results. This CB modified g-C3N4 with facile
preparation steps, and unique physicochemical structure features should have a promising
application in energy transition and environmental remediation.
129
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9. Zhang, H.; Zhao, L.; Geng, F.; Guo, L.-H.; Wan, B.; Yang, Y., Carbon dots decorated
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12. Liu, G.; Niu, P.; Sun, C. H.; Smith, S. C.; Chen, Z. G.; Lu, G. Q.; Cheng, H. M., Unique
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by Directly Heating Melamine. Langmuir 2009, 25 (17), 10397-10401.
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and H2 evolution. APPL. CATAL. B-ENVIRON. 2015, 170–171, 195-205.
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efficient hydrogen production under visible light. J. Mater. Chem. A 2013, 1 (26), 7816-7824.
18. Dong, F.; Zhao, Z.; Xiong, T.; Ni, Z.; Zhang, W.; Sun, Y.; Ho, W.-K., In Situ
Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light
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20. Kruk, M.; Jaroniec, M., Gas Adsorption Characterization of Ordered
Organic−Inorganic Nanocomposite Materials. Chemistry of Materials 2001, 13 (10), 3169-
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21. Zhang, L.; Li, L.; Sun, X.; Liu, P.; Yang, D.; Zhao, X., ZnO-Layered Double
Hydroxide@Graphitic Carbon Nitride Composite for Consecutive Adsorption and
Photodegradation of Dyes under UV and Visible Lights. Materials 2016, 9 (11), 927.
22. Zhang, L.; Xiong, Z.; Li, L.; Burt, R.; Zhao, X. S., Uptake and degradation of Orange
II by zinc aluminum layered double oxides. J Colloid Interf Sci 2016, 469, 224-230.
23. Chen, Y.; Wang, B.; Lin, S.; Zhang, Y.; Wang, X., Activation of n → π* Transitions in
Two-Dimensional Conjugated Polymers for Visible Light Photocatalysis. J. Phys. Chem. C
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24. Ai, B.; Duan, X. G.; Sun, H. Q.; Qiu, X.; Wang, S. B., Metal-free graphene-carbon
nitride hybrids for photodegradation of organic pollutants in water. Catalysis Today 2015,
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Wei, Y., Facile in situ synthesis of graphitic carbon nitride (g-C3N4)-N-TiO2 heterojunction
as an efficient photocatalyst for the selective photoreduction of CO2 to CO. APPL CATAL
B-ENVIRON 2014, 158, 20-29.
26. Xie, M.; Wei, W.; Jiang, Z. F.; Xu, Y. G.; Xie, J. M., Carbon nitride
nanowires/nanofibers: A novel template-free synthesis from a cyanuric chloride-melamine
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27. Kumar, S.; Baruah, A.; Tonda, S.; Kumar, B.; Shanker, V.; Sreedhar, B., Cost-
effective and eco-friendly synthesis of novel and stable N-doped ZnO/g-C3N4 core-shell
nanoplates with excellent visible-light responsive photocatalysis. Nanoscale 2014, 6 (9),
4830-4842.
28. Sun, J. X.; Yuan, Y. P.; Qiu, L. G.; Jiang, X.; Xie, A. J.; Shen, Y. H.; Zhu, J. F.,
Fabrication of composite photocatalyst g-C3N4-ZnO and enhancement of photocatalytic
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macroscopic aerogel visible-light photocatalyst. J MATER CHEM 2016, 4 (20), 7823-7829.
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132
6 ZnO-layered double hydroxide@graphitic
carbon nitride composite for consecutive
adsorption and photodegradation of dyes under
UV and visible lights
6.1 Introduction
Several commonly used dyes are toxic and mutagenic for aquatic organism even
carcinogenic for humans.1 Adsorption and photodegradation are the common methods that
can be used to eliminate the dyes in water. Recently, layered double hydroxides (LDHs), a
family of anionic clay, have attracted much attention in the removal of organic dyes from
wastewater due to their unique properties such as high ionic exchange capacity, tunable
particle size, large surface area and various composition as well as good stability.2-4 LDHs
and the layered double oxide (LDO) by thermal treatment of LDH have shown to display
high adsorption capacity for organic dyes such as Orange II (OrgII).5-7 Removal of the
adsorbed organic dyes in the solid adsorbent is an essential step if the technology is to be
used in practice. Photocatalytic degradation is a sustainable and green approach to
removing the adsorbed dyes. However, these LDHs and their derivatives are only
photocatalytically active under UN light.6 In addition, these LDH-based materials showed
little adsorption towards cationic dyes and didn’t show good photocatalytic performance
under visible lights.
Scheme 6-1 Schematic representation of the synthesis process of the ZnO-LDH@C3N4 composite.
133
Graphitic carbon nitride (g-C3N4), a polymeric material with the electron-rich property, has
been shown to be a promising visible light photocatalyst due to the suitable bandgap (2.7
eV), good stability, ease of preparation and the feature of environmentally friendly.8-11
However, g-C3N4 is limited by its low quantum efficiency and high recombination rate of
excited charges.8, 12 To improve its photocatalytic properties, g-C3N4 has been combined
with inorganic semiconductor materials to develop composite photocatalysts with improved
properties.13-15 Di et al. reported that a sphere-like g-C3N4/BiOI composite exhibited a higher
photocatalytic activity in the photodegradation of dyes than pure BiOI, which was due to the
enhanced electron-hole separation and broadened light absorption range.16 Jiang et al.17
and Zhou et al.18 both reported the heterojunction between g-C3N4 and TiO2, which showed
an effect in water pollution treatment, hydrogen production and an efficient photoreduction
of CO2 to CO respectively. Song and coworkers synthesized a g-C3N4(CN) sensitized
NaNbO3(NN) substrated II-type heterojunction, which not only exhibited narrower bandgap
compared with NN but also displayed excellent photocatalytic activity for dyes and
tetracycline degradation under visible-light irradiation.19 Lan et al reported that the Zn-In
mixed metal oxide/g-C3N4 (ZnIn-MMO/g-C3N4) nanohybrids showed stronger absorption in
the visible region than the ZnIn-MMO pristine, with exhibiting enhanced photodegradation
activity for Rhodamine B under visible light irradiation in comparison with pure g-C3N4 and
ZnIn-MMO.12 In addition, inter-electron transfer between g-C3N4 and ZnO heterogeneous
junction was also reported and realized for photocatalytic degradation of pollutants.20-23
Thus, the introduction of g-C3N4 to ZnO-LDH composite may tune the bandgap of ZnO-LDH
composite and facilitate the separation of excited charges to improve the photocatalytic
performance. Therefore, the aim of the work in this chapter was to constructing an novel
inorganic/organic semiconductor composites ZnO-LDH@C3N4, which was supposed to
make up the limitation from each other.
In this chapter, a simple method for the design and preparation of a ZnO-LDH@C3N4
composite (Scheme 6-1) with improved adsorption and photocatalytic properties towards
both anionic and cationic dyes in water was reported.
6.2 Experimental details
6.2.1 Adsorption and photodegradation
134
The adsorption of the dye pollutants was measured using a batch mode. For the adsorption
of OrgII over the solid samples (including ZnO-LDH, ZnO-LDH@C3N4, and C3N4), 5 mg of a
solid sample was added to 100 mL of an OrgII solution (the concentration was 50 mg/L) in
the dark under stirring for 24 h to achieve sorption equilibrium. For the adsorption of MB,
100 mg of a solid sample was added to 100 mL MB solution (the concentration was 10 mg/L)
in the dark under stirring for 1 h to achieve the adsorption equilibrium. After a given time
interval, 5 mL of the aliquot was taken and filtrated through a membrane (0.45 µm). The
OrgII and MB concentrations were analysed using a Shimadzu UV-2600 UV–Vis
spectrophotometer.
After achieving the adsorption equilibrium, the photocatalytic degradations of MB on the
adsorbents were undertaken in an open thermostatic photoreactor under UV light and visible
light, respectively. The UV-vis light resource was obtained from a 200 W mercury lamp.
Visible light irradiation was operated via adding a 450 nm cutoff filter on the mercury lamp.
At given time intervals during irradiation, 5 mL of aliquots were extracted using a syringe
and filtered through a membrane (0.45 µm). The concentration of MB in the solutions was
analysed using a Shimadzu UV-2600 UV-Vis spectrophotometer.
6.2.2 Intermediate species of photocatalytic degradation
The species generated in the photocatalytic system were analysed using tertbutyl alcohol
(t-BuOH) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na). In detail, t-BuOH
or EDTA-2Na (1 mmol) was mixed with the MB solutions before adding ZnO-LDH@C3N4
composite, respectively. Then the photocatalytic degradation of MB was performed in the
thermostatic photoreactor under UV or visible light irradiation with a similar process. The
active species generated in the photocatalytic process could be detected through trapping
by t-BuOH and EDTA-2Na.23
6.3 Results and discussion
6.3.1 Characterization of samples
The XRD patterns of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4 are shown in Figure 6-1. Two
diffraction peaks at 13.4o and 28.4o two theta can be seen from the g-C3N4 sample,
attributing to the diffractions of the (100) and (002) planes of g-C3N4. The (100) peak of g-
C3N4 represents the heptazine unit with an interplanar separation of 0.66 nm and (002) peak
135
represents the graphitic-like layer with an interlayer distance of 0.31 nm.24 The XRD pattern
of ZnO-LDH showed the presence of both ZnAl-LDH and ZnO.6 The strong peaks at 31.2o,
33.8o, and 35.6o two theta found in ZnO-LDH sample are indexed to the diffraction of the
(100), (002) and (101) planes of ZnO (JCPDS no. 05-0664), indicating the formation of ZnO
in the sample.25-26 The diffraction peaks at 8.6o and 12.1o are ascribed to the diffractions of
the (003) and (006) planes of ZnAl-LDH in the ZnO-LDH sample. The interlayer distance of
ZnAl-LDH in the ZnO-LDH sample d(003) was 1.03 nm, which is larger than that of
conventional carbonate intercalated LDH (0.73 nm), resulting from the intercalation of EG
during solvothermal treatment.27-29 The ZnO-LDH@C3N4 sample exhibited an XRD pattern
similar to that of the ZnO-LDH sample. No characteristic diffraction peaks of g-C3N4 can be
seen, probably due to the relatively weak intensity of the XRD peaks of g-C3N4.
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
*ZnO o
*
*
**
*
**
*
***
*
*
oo
oo
oo
o
ZnO-LDH@C3N
4
ZnO-LDH
Inte
nsi
ty (
a.u
)
2 Theta (o
)
g-C3N4
o*
LDH
Figure 6-1 XRD patterns of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4.
Figure 6-2A shows the XPS survey spectrum of the g-C3N4 sample. It can be seen that the
g-C3N4 sample synthesised in this work consisted of mainly C and N elements (the atomic
concentration: 42.6 % for C and 55.7 % for N) with a small amount of oxygen (1.65 %).
Based on XPS data, the molar ratio of C to N is 0.76, closer to 0.75, suggesting the formula
of g-C3N4. The small amount of oxygen in g-C3N4 may be caused by incomplete
polymerization of urea. Figure 6-2B shows the peak deconvolution results of N 1s XPS
spectrum of g-C3N4. Three peaks at 399.1, 400.4 and 401.6 eV, attributing to sp2 hybridised
aromatic N bonded to carbon atoms (C=N–C), the tertiary N bonded to carbon atoms in the
form of N–(C)3 and N–H side groups, respectively, can be seen.30-31 A weak peak at 404.5
136
eV ascribing to the π-excitations is also seen.32 The high-resolution C 1s XPS spectrum of
(Figure 6-2C) was deconvoluted into three peaks at 288.3, 286.4, and 284.8 eV,
corresponding to sp2-bonded carbon (N–C=N), C–O and graphitic carbon (C–C),
respectively.31-32 The graphitic carbon C–C at 284.8 eV can usually be observed in carbon
nitrides.16, 33 The XPS survey scan of ZnO-LDH@C3N4 composite is shown in Figure 6-2D.
The presence of O, C, N, Zn and Al in the ZnO-LDH@C3N4 sample is clearly seen. The
atomic concentrations for O/C/N/Zn/Al are 41.59/36.52/4.93/6.67/6.80 (%) from the survey
of XPS, which represent the superficial atomic ratios. The high-resolution XPS spectra of N
1s, C 1s, Zn 2p, O 1s, and Al 2p are displayed in Figure 6-2E to 2I, respectively. The binding
energies for N 1s and C 1s of ZnO-LDH@C3N4 in Figure 6-2E and Figure 6-2F showed
similar deconvolution peaks as pristine g-C3N4 but with lower energies compared to g-C3N4,
suggesting the strong electrostatic interaction between ZnO-LDH and g-C3N4.34-35 The
strong peaks of C–C and C–O were due to the existence EG and CO32- in ZnO-LDH@C3N4.
The binding energy values of Zn 2p3/2 and Zn 2p5/2 were fitted with 1022 and 1045 eV in
Figure 6-2G. The high-resolution XPS spectra for O 1s and Al 2p in ZnO-LDH@C3N4
composites were also fitted in Figure 6-2H and Figure 6-2I respectively.
0 200 400 600 800
O1s
N1s
Inte
nsi
ty(a
.u)
Binding energy (eV)
C1s
At%
C1s 42.64
N1s 55.71
O1s 1.65
(A)
394 396 398 400 402 404 406 408
C=N-C
(C)3-N
Binding energy (eV)
Inte
nsi
ty(a
.u)
N1s
excitations
N-H
(B)
280 282 284 286 288 290 292 294
C-CC-O
N=C-N
Binding energy (eV)
Inte
nsi
ty(a
.u)
C1s
(C)
0 150 300 450 600 750 90010501200
O 1s
Zn 2p
N 1
sC 1
s
Al
2p
Binding energy (eV)
Inte
nsi
ty (
a.u
)
(D)
392 394 396 398 400 402 404 406
(E)
N-H
(C)3
-N
C=N-C
N 1s
Binding energy (eV)
Inte
nsi
ty (
a.u
)
279 282 285 288 291 294 297
N-C=N
C-C
C1s
Inte
nsi
ty (
a.u
)
Binding energy (eV)
C-O
(F)
137
1020 1030 1040 1050
2p5/2
Zn 2p
2p3/2
Binding energy (eV)
Inte
nsi
ty (
a.u
)
(G)
526 528 530 532 534 536
O 1s
Binding energy (eV)
Inte
nsi
ty (
a.u
)
(H)
68 70 72 74 76 78 80
(I)
Al 2p
Binding energy (eV)
Inte
nsi
ty (
a.u
)
Figure 6-2 XPS images of the g-C3N4 and ZnO-LDH@C3N4 composite. (A) Survey spectrum for g-
C3N4; (B) High-resolution N 1s XPS spectrum of g-C3N4; (C) High-resolution C 1s XPS spectrum of
g-C3N4; (D) Survey spectrum for the ZnO-LDH@C3N4 composite; (E–I) are high-resolution XPS
spectra for N 1s, C 1s, Zn 2p, O 1s and Al 2p of the ZnO-LDH@C3N4 composite respectively.
The XPS spectra for ZnO-LDH are illustrated in Figure 6-3. The binding energy of C 1s in
ZnO-LDH only displayed one peak, and no N peaks were detected in ZnO-LDH. Thus, the
XPS spectra confirm that layered g-C3N4 was loaded on ZnO-LDH successfully.
138
0 200 400 600 800 1000 1200Binding energy (eV)
Al
2p
C 1s
O 1s
Zn 2p
Inte
nsi
ty(a
.u)
(a)
276 280 284 288 292
C 1s
Binding energy (eV)
Inte
nsi
ty(a
.u)
(b)
1020 1026 1032 1038 1044 1050
2p5/2
2p3/2
Zn 2p
Binding energy (eV)
In
ten
sity
(a.u
)
(c)
524 528 532 536
O 1s
Binding energy (eV)
Inte
nsi
ty(a
.u)
(d)
66 68 70 72 74 76 78 80
(e)
Al 2p
Binding energy (eV)
Inte
nsi
ty(a
.u)
Figure 6-3 XPS spectra of ZnO-LDH. (a) Survey spectrum for ZnO-LDH; (b) High-resolution C 1s
XPS spectrum of ZnO-LDH; (c) High-resolution XPS spectrum for Zn 2p of ZnO-LDH; (d) High-
resolution XPS spectrum for O 1s of ZnO-LDH. (e) High-resolution XPS spectrum for Al 2p of ZnO-
LDH.
139
The surface charges of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4 were measured by LSE. As
listed in Table 6-1, the g-C3N4 possessed a positive charge of 20.2 mV attributed to the –
NH2/NH functional groups at the heptazine rings generated from the incomplete
polymerization of g-C3N4.36 The uncondensed amine functional group and the edge cyano-
group bring positive charge characters for g-C3N4.37-38 During the synthetic process with
ultrasonic treatment, g-C3N4 was unfolded and mixed well with NaOH in EG solution, and
the pH of the mixture was changed to 13. The zeta potential of the g-C3N4 was changed into
-23.1 mV, attributing to the hydroxyl groups’ adsorption on the g-C3N4 surface. Therefore,
after adding Zn and Al ions, hydroxides generated and anchored on g-C3N4. Further
solvothermal treatment would reduce the aggregation of the ZnO-LDH@C3N4 composite.
After the hybridization of g-C3N4 and ZnO-LDH (the weight ratio of g-C3N4 in ZnO-
LDH@C3N4 is 14.6 wt%, calculated from experimental parameter and confirmed by TG-DTA
Figure 6-4), the zeta potential of the composite was shifted to 32.9 mV because of the
dominant content of ZnO-LDH (30.4 mV), as illustrated in Figure 6-5.
Table 6-1 Zeta potential, BET surface area, pore volume, crystallite size and bandgap for the
samples and the pseudo-first-order rate constant for MB photocatalytic degradation over different
photocatalysts.
Samples
Zeta
Potential
1 (mV)
SBET
(m2g−1)
Pore
Volume
(cm3g−1)
Crystallite
Size 2 (nm)
Bandgap
3 (eV)
k 4
(min−1)
g-C3N4 40.7 128.6 0.598 - 2.72 0.185
ZnO-LDH 30.4 113.7 0.261 5.2 3.08 0.0378
ZnO-
LDH@C3N4
32.9 152.5 0.298 3.4 3.06 0.487
ZnO - - - 16.1 3.20 0.0775
Note: 1 The zeta potential of each sample was measured by testing the suspension with the initial
pH value. The initial pH values for g-C3N4, ZnO-LDH, and ZnO-LDH@C3N4 aqueous suspensions
are 3.29, 7.17 and 6.22 respectively. 2 ZnO crystallite size: calculated using Scherrer’s equation:
Size = K λ/[FW(s)·cos (θ)]; K is constant = 1 here; λ is the X-ray wavelength = 1.5406 Å. FW(s) is
the Full Width at Half Maximum (FWHM) of the sample at θ, here (002) is chosen for the calculation.
3 The bandgap of the samples was calculated according to the equation Eg = 1240/λ. λ is the
absorption edge from UV-vis diffuse reflection spectroscopy. 4 k is the pseudo-first-order rate
constant for all the photocatalysts in processing MB photodegradation under visible-light irradiation.
140
0 100 200 300 400 500 600 7000
10
20
30
40
50
60
70
80
90
100
Weig
ht
(%)
Temperature (oC)
ZnO-LDH
C3N4
ZnO-LDH@C3N4
14.30%
33.76%
Figure 6-4 Weight loss of ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 composite determined by TGA.
There is a slow weight loss before 500 oC for g-C3N4. The first stage with 17.0 wt% mass
loss was caused by the loss of surface hydroxyl groups and absorbed water molecules,
which was followed by the decomposition of defects and edge functional groups such as
uncondensed amine functional group and the edge cyano-group of polymer g-C3N4. Then
there is a sharp weight loss during 500-600 oC, which indicates the disintegration of the
whole g-C3N4 polymer. Thus, the crossing point of the two tangents of the two weight loss
platforms is supposed to be the weight loss point of g-C3N4. The weight loss rate for ZnO-
LDH and ZnO-LDH@C3N4 are 14.3% and 33.8% respectively. The difference is 19.5%,
which is more than the experimental result 14.6 wt% for the content of g-C3N4. The larger
difference should be ascribed to the larger relative amount of LDH in ZnO-LDH@C3N4, which
is confirmed by the XRD patterns.
0 20 40 60 80 100
0
50000
100000
150000
200000
250000
Ta
ble
co
un
t
Zeta potential (mV)
ZnO-LDH@C3N4
g-C3N4
ZnO-LDH
Figure 6-5 Zeta potential of g-C3N4, ZnO-LDH, and ZnO-LDH@C3N4 aqueous suspensions.
141
The zeta potentials of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4 were measured at room
temperature. Typically, each sample powder (20 mg) was dispersed in 20 ml of MQ water
by ultrasonication for 30 min. The initial pH values for g-C3N4, ZnO-LDH, and ZnO-
LDH@C3N4 aqueous suspensions are 3.29, 7.17 and 6.22 respectively. When the pH value
for g-C3N4 aqueous suspension is adjusted into 13.89, the zeta potential is -23.1 mV.
The morphologies of g-C3N4, ZnO-LDH, and ZnO-LDH@C3N4 composite are presented in
Figure 6-6. As shown in Figure 6-6A, TEM of g-C3N4 exhibits a flake-like morphology with
irregular interstitial pores at the edge of the flake. The porous structure was generated due
to releasing NH3 and CO2 gas during the thermal treatment of urea. The surface area of g-
C3N4 was 128.6 m2g-1, much higher than the literature reports (Table 6-1).36, 39 Such a high
surface area of g-C3N4 might result from the irregular interstitial pore in g-C3N4. As shown in
Figure 6-7a, bulk g-C3N4 exhibits overlapped wrinkles and the randomly aggregated g-C3N4
sheets. (The SEM of bulk g-C3N4 is given in Figure 6-7b, which also reveals the layered
structure.) After hybridising ZnO-LDH with C3N4, TEM image in Figure 6-6B shows the
aggregates of the ZnO-LDH@C3N4 composite. Different from pristine ZnO-LDH (Figure 6-
7c), ZnO-LDH nanoparticles grew on the g-C3N4 sheets in ZnO-LDH@C3N4 composite,
suggesting the good affinity between ZnO-LDH and g-C3N4. Figure 6-6C and Figure 6-6D
are the SEM images of the ZnO-LDH@C3N4 and ZnO-LDH composite, respectively. As seen
in Figure 6-6C, ZnO nanoparticles were evenly distributed on the surface of LDH in ZnO-
LDH@C3N4 composite, which was similar to the characteristic of ZnO-LDH in Figure 6-6D.
Different from ZnO-LDH, a grey veil can be distinguished from the SEM of ZnO-LDH@C3N4,
which represents a covering layer of g-C3N4 on ZnO-LDH. The average particle sizes of ZnO
in ZnO-LDH@C3N4 and ZnO-LDH are 3 nm and 5 nm respectively, which are coincident
with the ZnO crystal sizes (Table 6-1) calculated from XRD. In the ZnO-LDH@C3N4
composite, the highly-exfoliated g-C3N4 wrapped up ZnO-LDH particles tightly, which not
only provided an intimate contact and cooperation between g-C3N4 and ZnO-LDH but also
increased the surface area of the ZnO-LDH@C3N4 photocatalyst (152.5 m2g-1).
142
Figure 6-6 (A) TEM image for g-C3N4; (B) TEM image for ZnO-LDH@C3N4; (C) SEM image for
ZnO-LDH@C3N4; (D) SEM image for ZnO-LDH.
143
Figure 6-7 (a) TEM image for bulk g-C3N4, (b) SEM image for g-C3N4, (c) TEM image for ZnO-
LDH.
144
The HRTEM image and the EDX for the ZnO-LDH@C3N4 are provided in Figure 6-8. The
lattice spaces for the (100) and (101) planes of ZnO were measured to be 0.282 nm and
0.253 nm, in good agreement with d(100) and d(101), determined from XRD pattern (0.286 nm
and 0.252 nm). The EDX analysis shows the presence of Zn, Al, O, C, and N with a Zn/Al
atomic ratio of ~1, matching well with the ratio from XPS.
Figure 6-8 (A) HRTEM image for the composite ZnO-LDH@C3N4; (B) EDX spectrum of ZnO-
LDH@C3N4.
0 1 2 3 4 5 6 7 8 9 100
5000
10000
15000
20000
Zn
Zn
AlZn
O
Inte
ns
ity
(c
ou
nt)
Energy(keV)
C
B
145
The elemental mapping of ZnO-LDH@C3N4 is presented in Figure 6-9, exhibiting a uniform
distribution of Zn, Al, O, C, and N in the composite.
Figure 6-9 The elemental mapping for ZnO-LDH@C3N4.
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
350
400
Vo
lum
e a
dso
rb
ed
(cm
3g
-1)
Relative Pressure (P/P0)
a
b
c
Figure 6-10 N2 adsorption/desorption of isotherms of (a) g-C3N4, (b) ZnO-LDH, (c) ZnO-
LDH@C3N4.
146
The textural structure of g-C3N4, ZnO-LDH and ZnO-LDH@C3N4 studied by N2 adsorption-
desorption isotherm at 77 K are shown in Figure 6-10 and listed in Table 6-1. ZnO-
LDH@C3N4 shows the highest surface area compared to g-C3N4 and ZnO-LDH, which
facilitates the high contact between photocatalyst and dyes. Moreover, the intimate two-
dimensional nanojunction between ZnO-LDH and g-C3N4 favours the photogenerated
charge carriers’ transfer between ZnO-LDH and g-C3N4, which may be a key factor for
photocatalytic activities of the ZnO-LDH@C3N4 photocatalyst.
6.3.2 Adsorption and photocatalytic activity of ZnO-LDH@C3N4
composite
6.3.2.1 Adsorption performance on anionic dye OrgII
Figure 6-11A exhibits the adsorption dynamics of OrgII on ZnO-LDH@C3N4. We did four
times parallel experiments, so the error bars were also given. The concentration of OrgII
firstly reduced from 100% to 85% in 1 h, followed by a moderate decrease in 23% during 9
h, and then slowed down gradually to 55% after 24 h. Thus, the equilibrium contact time was
24 h. Based on the adsorption profile, the adsorption of OrgII on ZnO-LDH@C3N4 may
involve two steps. Firstly, OrgII molecules were quickly adsorbed on the surface of ZnO-
LDH@C3N4 by electrostatic adsorption and π-π conjugation adsorption. Then the massive
adsorption of OrgII dyes on ZnO-LDH@C3N4 happened through anionic exchange between
the interlayered EG and OrgII, which gradually slowed down due to the establishment of the
charge balance. The inserted graph is the adsorption capacities of ZnO-LDH@C3N4, g-C3N4,
and ZnO-LDH. ZnO-LDH@C3N4 showed the highest OrgII adsorption amount with 431.4
mg/g compared to ZnO-LDH and g-C3N4 with 418.4 and 91.6 mg/g, respectively. This
enhancement in adsorption was ascribed to the increased contact between dyes and
catalyst derived from π-π conjugation and electrostatic interaction with g-C3N4 after the
combination between g-C3N4 and ZnO-LDH.
147
-2 0 2 4 6 8 10 20 22 24 26
50
55
60
65
70
75
80
85
90
95
100
105
110
C3N4 ZnO-LDH@C3N4 ZnO-LDH0
100
200
300
400
500
Ad
sorp
tion
cap
aci
ty (
mg/g
)
C/C
0(%
)
Time (h)
(A)
4000 3500 3000 2500 2000 1500 1000
c
b
a
e
Tra
nsm
ita
nce
(a.u
.)
Wavenumber(cm-1
)
d
(B)
Figure 6-11 (A) The adsorption dynamic of ZnO-LDH@C3N4 in OrgII adsorption. The insert is
adsorption capacity comparison among g-C3N4, ZnO-LDH@C3N4 and ZnO-LDH; (B) FT-IR spectra
of (a) g-C3N4, (b) ZnO-LDH, (c) ZnO-LDH@C3N4, (d) OrgII, (e) ZnO-LDH@C3N4 after saturated
adsorption with OrgII.
The adsorption dynamics of OrgII on g-C3N4 and ZnO-LDH are presented in Figure 6-12.
For g-C3N4, the absorption equilibrium of OrgII was established in 5 min, indicating the low
148
adsorption of OrgII on g-C3N4. The adsorption of OrgII on g-C3N4 was ascribed to the quick
π-π conjugation and electrostatic adsorption. ZnO-LDH exhibited a similar adsorption kinetic
profile as ZnO-LDH@C3N4, suggesting that the adsorption of OrgII on ZnO-LDH@C3N4 was
mainly attributed to ZnO-LDH.
-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65
80
82
84
86
88
90
92
94
96
98
100
C/C
0(%
)
Time (min)
(A)
-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26
50
55
60
65
70
75
80
85
90
95
100
105
110
C/C
0(%
)
Time (h)
(B)
Figure 6-12 The adsorption dynamics of g-C3N4 (A) and ZnO-LDH (B) in OrgII adsorption.
FT-IR can be used to investigate the different situations and types of anionic functional
groups between the layers of composites as reported in the literature.30, 40 Therefore, FT-IR
was employed to determine the interaction between g-C3N4 and ZnO-LDH in the ZnO-
LDH@C3N4 composite and its adsorption character. For all the samples, the broad
absorption bands located in the range from 3600 to 3000 cm-1 are originated from stretching
vibration of O-H, C-H, and N-H. These bonds are derived from the hydroxyl group of LDH
layers, interlayer EG molecules (O-H and C-H) and uncondensed amino groups in g-C3N4.35,
149
41 The FT-IR spectrum of pristine g-C3N4 (Figure 6-11Ba) shows the featured distinctive
stretch modes of aromatic CN heterocycles at 1200-1700 cm-1 accompanied by the
breathing mode of the bending vibration of heptazine rings at 800 cm-1, which is
corresponding to the reported values.17 For ZnO-LDH (Figure 6-11Bb), the characteristic
absorption bands of the intercalated molecules like EG are observed at 1500-1800 cm-1 and
900-1200 cm-1. Furthermore, the band at 650 cm-1 can be attributed to the Zn-O-H and Al-
O-H stretching vibration in ZnO-LDH. For the composite ZnO-LDH@C3N4 (Figure 6-11Bc),
a series of peaks observed from 1650 to 1300 cm-1 were attributed to the typical stretching
modes of CN heterocycles and C=N stretching bonds, which were derived from g-C3N4 in
this composite. It can be clearly seen that the main characteristic peaks for ZnO-LDH and
g-C3N4 appear in ZnO-LDH@C3N4, suggesting the existing of both ZnO-LDH and g-C3N4 in
the as-prepared composite. The FT-IR spectrum of OrgII (Figure 6-11Bd) exhibits a sharp
peak at 1500 cm-1 which can be attributed to the absorption of C=C aromatic stretch.
Moreover, several strong absorption bands around 1000-1250 cm-1 correspond to -SO3-
vibrations in OrgII molecules. Furthermore, the bands around 700-850 cm-1 are ascribed to
the aromatic C-H out-of-plane bending absorption from OrgII. After adsorption of OrgII, the
peaks at 810 cm-1 assigned to the bending vibration of heptazine rings became sharper and
changed into multi-peaks for ZnO-LDH@C3N4 (Figure 6-11Be), which could be induced by
the molecular cooperation between adsorbed OrgII and g-C3N4.32 The similar strong
absorption bands with OrgII in 1000-1500 cm-1 and 700-850 cm-1 for ZnO-LDH@C3N4
(Figure 6-11Be) indicates that OrgII has successfully intercalated into the LDH layers after
adsorption. The successful intercalation of OrgII into LDH was also demonstrated by the
shifting XRD pattern in our previous report.6
150
Figure 6-13 FT-IR spectra of ZnO-LDH@C3N4, ZnO-LDH@C3N4 after OrgII adsorption in 1h and
ZnO-LDH@C3N4 after OrgII adsorption in 24h.
FT-IR spectra of ZnO-LDH@C3N4 samples before and after OrgII adsorption in 1 h and 24
h were shown in Figure 6-13. The change patterns of FT-IR spectra along with the
adsorption time were presented. The peaks at 810 cm-1 assigned to the bending vibration
of heptazine rings became sharper along with the adsorption time and changed into multi-
peaks after saturated adsorption of OrgII, which may be induced by the molecular
cooperation between adsorbed OrgII and g-C3N4
6.3.2.2 Adsorption performance on cationic dye MB
As shown in Figure 6-14a, adsorption equilibrium of MB on the ZnO-LDH@C3N4 composite
was established in 20 min, and the adsorption capacity of MB on the ZnO-LDH@C3N4
composite was 8.0 mg/g, which was much lower than the adsorption capacity for OrgII
(431.4 mg/g). The lower adsorption capacity for MB was caused by the positive charges of
MB and ZnO-LDH@C3N4. Different from adsorption of anionic OrgII, MB dye was adsorbed
on ZnO-LDH@C3N4 mainly via the π-π conjugation adsorption instead of electrostatic
attraction and ion-exchanged intercalation adsorption. The π-π conjugation adsorption was
mainly contributed to the electron-rich properties of g-C3N4. In addition, the hydrophobicity
of C3N4 may also help to MB adsorption on ZnO-LDH@C3N4. Though the adsorption
3900 3600 3300 3000 2700 2400 2100 1800 1500 1200 900
ZnO-LDH@C3N
4
Tra
nsm
itta
nce
(a.u
.)
Wavenumber (cm-1
)
after 24h adsorption of OrgII
after 1h adsorption of OrgII
151
capacity of MB was low, 80% of MB in solution was absorbed on the ZnO-LDH@C3N4
composite. Compared with g-C3N4, ZnO-LDH@C3N4 composite showed better MB
adsorption, ascribing to high surface area of the composite and the uniform g-C3N4
distribution on ZnO-LDH as well as intimate contact between MB and g-C3N4. The π-π
conjugation adsorption was achieved from the exposure of CN heterocycles on g-C3N4.
Polymeric g-C3N4 is hydrophobic, which tends to agglomerate in aqueous solution. After
hybridising with ZnO-LDH, g-C3N4 was unfolded in ZnO-LDH@C3N4 composite with more
exposure of CN heterocycles. The hydrophilic property of ZnO-LDH@C3N4 promised the
intimate contact between catalyst and MB. Therefore, the adsorption amount of MB was
superior for ZnO-LDH@C3N4 composite. In contrast, there was neglected adsorption of MB
for ZnO and ZnO-LDH.
0 20 40 60 80 100 120
0
20
40
60
80
100 (a)
C/C
o(%
)
Time (min)
ZnO-LDH@C3N
4
C3N
4
ZnO-LDH
ZnO
UV light
0 40 80 120 160 200 240 280 320-10
0
10
20
30
40
50
60
70
80
90
100
110
C/C
o(%
)
Time (min)
ZnO-LDH@C3N
4
C3N
4
ZnO-LDH
ZnO
visible light
(b)
0 60 120 180 240-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln(C
/Co)
Time(min)
ZnO-LDH@C3N
4
C3N
4
ZnO-LDH
ZnO
fitting curves
(c)
360 380 400 420 440 460
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
C3N4
ZnO
ZnO-LDH
ZnO-LDH@C3N4
Ab
s (a
.u)
Wavelength (nm)
(d)
152
390 420 450 480 510 540
Wavelength (nm)
C3N4
ZnO
ZnO-LDH
ZnO-LDH@C3N4
(e)
Inte
nsi
ty (
a.u
.)
-10 0 10 20 30 40 50 60 70 80
0
100
200
300
400
500
Z''
(oh
m)
Z'(ohm)
C3N
4
ZnO
ZnO-LDH
ZnO-LDH@C3N
4
(f)
Figure 6-14 (a) Comparison of MB adsorption and photodegradation in water under UV-light over
ZnO, ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 respectively; (b) Comparison of MB adsorption and
photodegradation in water under visible-light over ZnO, ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4
respectively; (c) Kinetic fit for the degradation of MB with the ZnO, ZnO-LDH, g-C3N4 and ZnO-
LDH@C3N4 respectively under visible light; (d) UV-vis diffuse reflectance spectra of the
photocatalysts with corresponding tangent lines; (e) Photoluminescence spectra of g-C3N4, ZnO,
ZnO-LDH and ZnO-LDH@C3N4; (f) Electrochemical impedance spectroscopy of g-C3N4, ZnO,
ZnO-LDH and ZnO-LDH@C3N4 composite.
6.3.2.3 Photocatalytic degradation performance on MB over ZnO-LDH@C3N4 under
UV and visible light
After saturated adsorption of MB, the photocatalytic degradation of MB dyes on commercial
ZnO, ZnO-LDH, g-C3N4, and ZnO-LDH@C3N4 was undertaken under UV and visible light
irradiation. As shown in Figure 6-14a, ZnO-LDH@C3N4 composite removed all the MB in
the water after UV irradiation for 1 h. 55.0% of MB was removed by g-C3N4, and 21.0% of
MB was removed by ZnO-LDH. ZnO-LDH@C3N4 composite showed better photocatalytic
performance compared to C3N4 and ZnO-LDH. However, the commercial ZnO also removed
100% of MB in the water in 20 min under UV irradiation. Interestingly, as shown in Figure 6-
14b, 100% of MB was also removed by the ZnO-LDH@C3N4 composite in 4 h under visible
light irradiation; on the contrary, only 27.2% of MB could be degraded on commercial ZnO
photocatalyst. The low photocatalytic activity of commercial ZnO under the visible light was
ascribed to its wide bandgap, which cannot be excited upon visible light irradiation. The
153
removal rate of MB over ZnO-LDH and g-C3N4 under visible light irradiation was 15.3% and
64.1% respectively. The better photocatalytic performance of ZnO-LDH@C3N4 composite
under UV or visible light was ascribed to its narrower bandgap, high surface area and
increased contact surface area between MB and the composite. As mentioned in MB
adsorption, a large amount of MB was absorbed on the surface of ZnO-LDH@C3N4; the
intimate contact between MB and photocatalyst shortened the mass transfer, which would
lead to the dramatic improvement in catalytic performance. Moreover, a combination of ZnO-
LDH and g-C3N4 reduced the bandgap of the composite, which increased the photon
utilisation efficiency. Most importantly, the heterojunction between ZnO-LDH and g-C3N4
could facilitate the transfer of excited photoelectrons during the photocatalytic process,
which increased the photocatalytic effect fundamentally. Moreover, after the reaction, the
colour of the sediment changed back to the original yellowish (the colour after adsorption of
MB was blue), suggesting MB has been fully decomposed.
The photocatalytic degradation kinetics of MB on the photocatalysts were investigated and
shown in Figure 6-14c. The photocatalytic profile of MB on ZnO-LDH@C3N4 followed
pseudo-first-order kinetics plot by the equation:
ln(C/Co) = -k∙t
Where k is the pseudo-first-order rate constant, Co and C are the MB concentration in
solution at times 0 and t, respectively.
After fitting, k of commercial ZnO, ZnO-LDH, g-C3N4 and ZnO-LDH@C3N4 in Table 6-1 were
0.0775, 0.0378, 0.185, and 0.487 min-1, respectively. The value of k gives an indication of
the activity of the photocatalyst.42 ZnO-LDH@C3N4 had the highest rate constant among all
the photocatalysts under visible light irradiation, almost 7 times as high as ZnO. The highest
rate constant of ZnO-LDH@C3N4 further demonstrated the better photocatalytic
performance of ZnO-LDH@C3N4 than that of commercial ZnO. The reusability of ZnO-
LDH@C3N4 was further tested. As shown in Figure 6-15, the remove rate can still be 90%
at the fifth cycle.
154
0 1 2 3 4 5
0
20
40
60
80
100
Deg
ra
da
tio
n p
ercen
tag
e (
%)
Recycle time
Figure 6-15 The cycling runs of ZnO-LDH@C3N4 in the photodegradation of MB under UV
irradiation.
UV-vis diffuse reflection spectroscopy (DRS) was used to test the light harvesting ability of
ZnO, g-C3N4, ZnO-LDH and ZnO-LDH@C3N4 samples. As shown in Figure 6-14d, sharp
absorption edges for all samples were in the range of 380 ~ 450 nm. The bandgap can be
inferred from the UV-vis absorption measurements. The bandgaps of ZnO, g-C3N4, ZnO-
LDH and ZnO-LDH@C3N4 were calculated to be 3.20, 2.72, 3.08 and 3.06 eV respectively.
With the existence of g-C3N4, we expected that the bandgap of ZnO-LDH@C3N4 would be
tuned to the lower bandgap. However, the bandgap of ZnO-LDH@C3N4 was 3.06 eV, slightly
lower than that of ZnO-LDH (3.08 eV) instead of being close to that of g-C3N4 (2.72 eV). No
change in the bandgap of the ZnO-LDH@C3N4 composite may be due to the low content of
g-C3N4. The other reason should be that during the solvothermal treatment process, the g-
C3N4 was unfolded and covered on the ZnO-LDH. Therefore, the strong quantum
confinement effect (QCE) derived from the highly extended g-C3N4 increased the bandgap
of g-C3N4 simultaneously.10 PL spectroscopy was used to investigate the separation
efficiency of photoexcited electron-hole pairs.14, 24 All the samples were excited at 360 nm,
and the emission spectra were recorded in a range between 380 and 550 nm. As shown in
Figure 6-14e, the g-C3N4 had the highest emission peak at 470 nm. The emission peak at
155
470 nm was ascribed to the band-band PL phenomenon with the energy of light
approximately equal to the bandgap energy of g-C3N4. This high intensive emission was
attributed to the direct recombination of excitons.43 Compared to g-C3N4, the emission
intensity of ZnO-LDH@C3N4 was much lower, suggesting that their e-h+ pair recombination
rate was much lower. The strong separation of charge carriers resulted in the potential
higher photocatalytic activity for ZnO-LDH@C3N4. The charge transport process occurs in
photocatalyst under dark condition, which directly reflects its capacity to shuttle and convey
charge carriers to the targeted reactive sites.24 So, to deep understand the charge transport
behaviour of ZnO-LDH@C3N4 in the absence of light excitation, EIS measurements were
carried out under dark condition. Figure 6-14f displayed the EIS Nyquist plots of all the
samples. As known, the arc radius on the EIS Nyquist plot reflects the reaction rate on the
surface of the electrode. The smaller arc radius, the more effective separation of
photogenerated electron-hole pairs, the higher efficiency of charge immigration across the
electrode-electrolyte interface.20-21, 44 Among all the samples, ZnO-LDH@C3N4 showed the
smallest diameter for arc radius, suggesting its lowest resistance for interfacial charge
transfer from the electrode to electrolyte molecules. Therefore, EIS measurements were
consistent with PL data, demonstrating that the ZnO-LDH@C3N4 had lower resistance than
other samples and made the separation and immigration of photogenerated charges more
efficient, indicating the high photocatalytic activity for ZnO-LDH@C3N4.
6.3.2.4 Proposed mechanism under UV and visible light irradiations
As known, detection of the main oxidative species in the photocatalytic process is a
significant tool to reveal the photocatalytic mechanism. The active species generated during
the photocatalytic process can be detected through trapping by tertbutyl alcohol (t-BuOH)
and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na).23 t-BuOH can be the
scavenger for radicals like hydroxyl and superoxide; EDTA-2Na is the scavenger for holes.
As shown in Figure 6-16A, in the ZnO-LDH@C3N4 system, the MB concentration decreased
dramatically upon UV irradiation without adding trapping chemicals. However, the addition
of t-BuOH only resulted in a small change in the photocatalytic degradation of MB. On the
contrary, the photocatalytic activity of ZnO-LDH@C3N4 was greatly suppressed by the
addition of EDTA-2Na. The experiment results indicated that holes were the main oxidative
species when the photocatalyst was under UV irradiation. When the photocatalytic reaction
was under visible light irradiation, the situation was the reverse. As shown in Figure 6-16B,
the addition of t-BuOH suppressed the photocatalytic degradation compared to the addition
156
of scavenger EDTA-2Na. The experiment results suggested that the radicals were the main
oxidative species when photocatalytic degradation processed under visible irradiation.
0 20 40 60
-2.0
-1.6
-1.2
-0.8
-0.4
0.0
ln(C
/Co
)
Time (min)
+ 1m mol EDTA-2Na
+ 1m mol tBuOH
ZnO-LDH@C3N4
Fit curves
(A)
0 60 120 180 240
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln(C
/Co
)
Time (min)
+1m mol EDTA-2Na
+1m mol tBuOH
ZnO-LDH@C3N4
Fit curves
(B)
Figure 6-16 (A) The experimental data and the fitting plots of photogenerated carriers trapping in
the photodegradation of MB by ZnO-LDH@C3N4 under UV-light irradiation; (B) The experimental
data and the fitting plots of photogenerated carriers trapping in the photodegradation of MB by
ZnO-LDH@C3N4 under visible-light irradiation.
Through the adsorption and photocatalytic performance results, we proposed the following
adsorption and photocatalytic mechanism shown in Figure 6-17. As shown in Figure 6-17,
ZnO-LDH@C3N4 composites mixed with the OrgII solution and then OrgII was absorbed on
the surface of ZnO-LDH@C3N4 composites via electrostatic interaction and π-π conjugation
adsorption and further intercalated into ZnO-LDH via layered adsorption including ion
exchange. For cationic MB dye, MB was adsorbed on the surface of ZnO-LDH@C3N4 by π-
π conjugation, firstly. Upon UV irradiation, ZnO-LDH could be excited and produce
157
photogenerated electron-hole pairs. Since the valence band (VB) position of ZnO-LDH is
lower than the highest occupied molecular orbital (HOMO) of g-C3N4, the photogenerated
holes on ZnO-LDH could directly transfer to g-C3N4.23 g-C3N4 is relatively stable with holding
holes. The g-C3N4 with holes would accept electrons from MB degradation and then return
to the ground state. Upon visible light irradiation, g-C3N4 instead of ZnO-LDH absorbed
visible light to induce π-π* transition, transporting the excited-state electrons from HOMO to
the lowest unoccupied molecular orbital (LUMO). The LUMO potential of g-C3N4 is more
negative than the conduction band (CB) edge of ZnO-LDH, due to the comparable energy
difference between the CB of ZnO-LDH and g-C3N4, there is a strong thermodynamic driving
force for electron transfer from excited g-C3N4 to ZnO-LDH.45 The electrons would
subsequently transfer to the surface of ZnO-LDH@C3N4 to react with water and oxygen with
generating superoxide and hydroxyl radicals. The radicals can subsequently oxidise the MB
into CO2 and H2O.
Figure 6-17 Schematic illustration of the mechanism of uptake of anionic dye OrgII and the charge
separation and photocatalytic activity of the ZnO-LDH@C3N4 under UV- and visible-light irradiation,
respectively.
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6.4 Conclusion
The ZnO-LDH@C3N4 composite was synthesised via the facile solvothermal method in this
work. The introduction of g-C3N4 on ZnO-LDH significantly improved the adsorption and
photocatalytic activities of ZnO-LDH@C3N4. For OrgII, ZnO-LDH@C3N4 showed higher
adsorption capacity with three synergetic steps including electrostatic and π-π conjugation
adsorption followed by ion exchange. Besides, ZnO-LDH@C3N4 exhibited substantial
adsorption of cationic MB dye and high photocatalytic activities in MB removal under UV
and visible light. Moreover, ZnO-LDH@C3N4 showed stable photocatalytic activities for MB
removal in five cycles, and 90% of MB was removed at the fifth cycle. The enhanced
performance in photocatalytic degradation of MB under UV and visible light irradiation were
induced by the high separation efficiency of photogenerated charges. This work
demonstrated that an attractive ternary hybridization of LDH, ZnO and g-C3N4 could launch
out the newly research on highly active photocatalytic adsorbent for the environmental and
energetic application.
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7 Conclusions and recommendations
7.1 Conclusions
This research in this thesis focused on developing heterogeneous photocatalysts which
were based on layer-structured semiconductors for photodegradation of dyes with
accompanied synergistic adsorption. Initially, the research works on the ZnAl LDH based
photocatalysts, and the g-C3N4 modification was carried out respectively. Then,
inorganic/organic semiconductor composites were constructed between ZnO-LDH and g-
C3N4. During this research, the synthesis methods and conditions were explored, structures
and physicochemical properties of the composites were well studied, and the adsorption
and photocatalytic degradation activities were completely evaluated. Finally, the mechanism
and the relationship between structure, physicochemical properties and the adsorption,
photodegradation performance were also tried to be explained. The main works have been
done can be concluded as following:
(i) LDO composites exhibited strong adsorption of OrgII during its rehydration
process. ZnO dispersed on the LDH after rehydration exhibited superior
photocatalytic activity for decomposing OrgII under UV irradiation.
The Zn2+/Al3+ molar ratio and calcination temperature affected the adsorption and
photocatalytic activity of LDO composites. LDO synthesised at 600 °C with the
Zn2+/Al3+ molar ratio equalled 2 showed the optimal adsorption and photocatalytic
performance.
An adsorption mechanism including three synergistic steps was proposed.
Moreover, the correlation between adsorption photocatalytic properties of LDOs
and the structural characteristics of ZnO−rLDHs was explored and illuminated in
this work.
(ii) g-C3N4 was modified with CB via pyrolysis the mixture of urea and CB. The
crystallisation and condensation degrees of g-C3N4 was increased after
modification.
164
CB modified g-C3N4 samples exhibited more compact layers with a decrease in
the interlayer distance ascribing to the covalent bonding between carbon atoms
and C-N cycles.
The carbon atoms in the g-C3N4 matrix not only enhanced the light absorption but
also quenched the recombination of charge carriers. Consequently, the bandgap
structure of g-C3N4 was tuned after CB modification and the electron-hole
separation was improved. The modification improved the dyes’ removal
performance for g-C3N4.
(iii) An attractive ternary hybrid of LDH, ZnO and g-C3N4 was synthesised via the
facile solvothermal method. ZnO-LDH@C3N4 exhibited significant improvement in
the adsorption and photocatalytic activities.
ZnO-LDH@C3N4 showed high adsorption capacity with three synergetic steps
including electrostatic and π-π conjugation adsorption followed by ion exchange
toward OrgII. ZnO-LDH@C3N4 exhibited substantial adsorption of cationic MB dye
and high photocatalytic activities in MB removal under UV and visible light.
The enhanced performance in photocatalytic degradation of MB under UV and
visible light irradiation were induced by the high separation efficiency of
photogenerated charges.
7.2 Recommendations
The works on the assembly between LDH and g-C3N4 are still very limited. Further deep
investigations are deserved for the composite materials. Therefore, several aspects for
future work have been recommended and listed as following:
(i) Varying the hybrid ratios between ZnO-LDH and g-C3N4. Although the composite
ZnO-LDH@g-C3N4 exhibited significant improvement in the adsorption and
photocatalytic degradation, the influence of ratios on the activities remains
unknown. Investigating the optimal ratio between ZnO-LDH and g-C3N4 can help
to understand the cooperation between inorganic and organic photocatalysts
more deeply.
165
(ii) Exfoliating the CB modified g-C3N4. CB modified g-C3N4 samples in this research
have more compacted structures than pristine g-C3N4. After exfoliation, the
surface areas will be increased, which can largely increase the catalytic contact.
Moreover, the quantum confinement effect after exfoliation can also bring
unexpected optical and electronic properties for g-C3N4.
(iii) Assembling inorganic and organic composite photocatalyst by layer-by-layer
(LBL) method. LDH can be exfoliated in the organic solvents such as formamide
and butanol. g-C3N4 can also be exfoliated in solvents or via ultrasonic treatment.
So, it is quite possible to assemble LDH and g-C3N4 in a facile routeing. A
sufficient contact after LBL method is supposed to bring particular chemical,
electrical and optical properties.
(iv) A hybrid between ZnO-LDH and CB modified g-C3N4. The synergistic effect
derived from heterostructure and doping is expected when try to assemble ZnO-
LDH and CB modified g-C3N4. It is imaginable that the assembly of ZnO-LDH and
CB modified g-C3N4 with LBL method will lead to a robust photocatalyst.
(v) Extending the assembly to other LDH and g-C3N4. The works on the inorganic-
organic heterostructured photocatalyst between LDH and g-C3N4 should not be
limited to zinc and aluminium based LDH. Such as ZnCr-LDH, which was reported
with narrower bandgap can be assembled with g-C3N4 expecting high effects in
photodegradation and photocatalytic water splitting.
(vi) Introducing the High-Performance Liquid Chromatography (HPLC) to detect the
intermediates in the photocatalytic process, also the Total Organic Carbon (TOC)
analysis and the chemical oxygen demand (COD) to evaluate the mineralisation
degree. The content of total sulphur in the solution after reaction is supposed to
be measured by inductively coupled plasma (ICP) emission spectroscopy.
