PhD Thesis
SYNTHESIS, CHARACTERIZATION AND APPLICATION OF NICKEL NANOPARTICLES
NAZAR HUSSAIN KALWAR
A THESIS SUBMITTED TOWARDS THE PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF DOCTOR OF PHILOSOPHY
IN ANALYTICAL CHEMISTRY
National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro - PAKISTAN
2013
Certificate
This is to certify that Mr. NAZAR HUSSAIN KALWAR has carried out his research
work on the topic “SYNTHESIS, CHARACTERIZATION AND APPLICATION OF
NICKEL NANOPARTICLES” under our supervision at the laboratories of the National
Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, Pakistan, and
the Interface Analysis Centre, University of Bristol, Bristol, England with support from the
International Research Support Initiative Programme (IRSIP) of the Higher Education
Commission, Islamabad, Pakistan to pursue part of the Ph.D. research work abroad. The work
reported in this thesis is original and distinct. His dissertation is worthy of presentation to the
University of Sindh, Jamshoro, Pakistan for the award of degree of Doctor of Philosophy in
Analytical Chemistry.
Prof. Dr. Sirajuddin Supervisor National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, Pakistan
Prof. Dr. Syed Tufail Hussain Sherazi Co-Supervisor National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, Pakistan
DEDICATION
This Endeavor Is Dedicated To My Affectionate Supervisors, My
Beloved Parents and Friends Whose Prayers, Encouragements and
Co-Operation Have Enabled Me to Achieve the Honor of the
Highest Seat of Learning. I also Dedicate This Work With Love
To My Sister Miss Saher Kalwar And My Niece Maria Kalwar
Who Always Pray for My Success and Bright Future.
i
Summary of Contents
Contents i
List of Tables v
List of Figures v
Abbreviations viii
Acknowledgements x
Abstract xiii
Contents
Synthesis, Characterization and Application of
Nickel Nanoparticles
Chapter – 01 1-16
INTRODUCTION
1.1 Aims and Objectives of Present Work 1
1.2 What is Nanotechnology 1
1.3 Significance of the Nanoscale 2
1.4 History of Nanotechnology 3
1.5 Nanoscale Structures and Functions 5
1.5.1 Zero-Dimensional Nanomaterials 7
1.5.2 Manipulation of Zero-Dimensional Nanostructures into
Arrays 9
1.5.3 One-Dimensional Nanostructures 10
1.6 Some Specific Types of Nanoscale Structures and their Uses 11
1.7 Nanocomposites Production Methods 12
1.7.1 Synthesis of Metallic Nanoparticles 14
1.7.2 Influences of Reduction Reagents 15
1.7.3 Influence of Stabilizing Agents 15
1.8 Characterization Techniques 16
ii
Chapter – 02 17-40
LITERATURE REVIEW
2.1 Nanoscience and Nanotechnology 17
2.2 Methods for Synthesis of Zero Dimensional Nickel
Nanostructures or Spherical Nanoparticles 18
2.3 Methods for Preparation of Other Metal Nanostructures with
Zero Dimensions 30
2.4 Methods for Preparation of One Dimensional Nanoscale Nickel
Composites 35
2.5 Methods for Synthesis of Multi-Dimensional Nanostructures 37
2.6 Applications of Nanoparticles/Nanostructures 39
Chapter – 03 41-59
EXPERIMENTAL
3.1 Synthesis of L-Cysteine Derived Nickel Nanoparticles
(Ni NPs) in Ethylene Glycol 41
3.1.1 Cleaning of Glassware 41
3.1.2 Chemical Reagents used during Synthesis of L-Cysteine
Capped Ni NPs 41
3.1.3 Preparation of Standard Stock Solutions 42
3.1.3.1 Stock Solution of Nickel (II) Chloride 42
3.1.3.2 Stock Solution of L-Cysteine 42
3.1.3.3 Stock Solutions of NaOH and Na2CO3 42
3.1.3.4 Stock Solutions of 4-Nitrophenol and NaBH4 42
3.1.4 Procedure for Synthesis of L-Cysteine Capped Ni NPs 43
3.1.5 Optimization Studies for Formation L-Cysteine Capped
Ni NPs 43
3.1.6 Instrumentation Used to Characterize L-Cysteine Capped
Ni NPs 43
3.1.7 Characterizations of L-Cysteine Capped Ni NPs 44
3.1.7.1 Sample Preparation for UV-Vis Spectroscopy 44
3.1.7.2 Sample Preparation for XRD and FTIR 44
3.1.7.3 Sample Preparation for SEM 45
3.1.8 Procedure for Catalytic Reduction of 4-Nitrophenol 45
iii
3.1.9 Procedure for Catalytic Reduction of Cr(VI) ions 46
3.2 Fabrication of L-Methionine Capped Ni NWs 47
3.2.1 Materials and Chemicals Used to Fabricate L-Methionine
Capped Ni NWs 47
3.2.2 Preparation of Standard Stock Solutions to Fabricate
L-Methionine Capped Ni NWs 47
3.2.3 Procedure for Synthesis of L-Methionine Capped Ni NWs 48
3.2.4 Procedure for Catalytic Test of Ni NWs and Acetone
Formation 48
3.2.5 Characterization of L-Methionine Capped Ni NWs 49
3.2.5.1 Instrumentation Used for Characterization of
L-Methionine Capped Ni NWs 49
3.2.5.2 Sample preparation for SEM Imaging and FTIR Studies 49
3.2.5.3 Sample Preparation for XRD Analysis of L-Methionine
Capped Ni NWs 50
3.3 Synthesis of Triton X-100 Stabilized Ni NPs 50
3.3.1 Materials and chemicals used for synthesis of TX-100
stabilized Ni NPs 50
3.3.2 Instrumentations Used for Characterizations Studies of TX-
100 Stabilized Ni NPs 50
3.3.3 Procedure for Synthesis of TX-100 Stabilized Ni NPs 51
3.3.4 Catalytic Test for Degradation of Dyes 51
3.4 Preparation of L-Threonine Capped Ni NPs 52
3.4.1 Chemicals and Reagents Used for L-Threonine Capped
Ni NPs 52
3.4.2 Preparation of Stock Solutions for L-Threonine Capped Ni
NPs 52
3.4.3 Procedure for Fabrication of L-Threonine Capped Ni NPs 52
3.4.4 Nanoparticles Separation by Centrifugation 53
3.4.5 Characterization Studies for L-Threonine Capped Ni NPs 53
3.4.5.1 Sample Preparation for FTIR Studies of L-Threonine
Capped Ni NPs 54
3.4.5.2 Sample Preparation for AFM Studies of L-Threonine
Capped Ni NPs 54
3.4.5.3 Sample preparation for TEM Studies of L-Threonine
Capped Ni NPs 54
3.4.6 Catalytic Test for Reduction of CR Dye 54
iv
Chapter – 04 56-106
RESULTS AND DISCUSSION
4.1 Results and Discussion for Formation of L-Cysteine Derived Ni NPs in Ethylene Glycol
56
4.1.1 Characterization Studies of L-Cysteine Derived Ni NPs 56
4.1.1.1 UV-Vis Spectroscopy for L-Cysteine Derived
Ni NPs 57
4.1.1.2 TEM Analysis of L-Cysteine Derived Ni NPs 59
4.1.1.3 FTIR Spectroscopy of L-Cysteine Derived Ni NPs 60
4.1.2 Application of Ni NPs as Catalyst for Reduction of 4-NPh 61
4.1.3 Reuse of Regenerated Ni NPs as Catalyst 64
4.1.4 Catalytic Activity of Ni NPs for Reduction of Cr (VI) ions 65
4.2 Results and Discussion for L-Methionine Capped Ni NWs 69
4.2.1 Characterization of L-Methionine Capped Ni NWs 69
4.2.1.1 UV-Vis Spectrometry of L-Methionine Capped Ni NWs 69
4.2.1.2 FTIR Spectroscopy of L-Methionine Capped Ni NWs 74
4.2.1.3 SEM Analysis of L-Methionine Capped Ni NWs 77
4.2.1.4 XRD Analysis of L-Methionine Capped Ni NWs 78
4.2.2 Application of L-Methionine Capped Ni NWs as Catalyst 80
4.3 Results and Discussion for TX-100 Derived Ni NSs 84
4.3.1 Characterization Studies of TX-100 Derived Ni NPs 84
4.3.1.1 UV-Vis Spectroscopy of TX-100 Derived Ni NSs 84
4.3.1.2 FTIR Spectroscopy of TX-100 Derived Ni NSs 87
4.3.1.3 SEM Analysis of TX-100 Derived Ni NSs 88
4.3.1.4 XRD Analysis of TX-100 Derived Ni NSs 90
4.3.2 Degradation of Dyes Catalyzed by TX-100 Derived Ni NSs 91
4.4 Results and Discussion for L-Threonine Capped Ni NPs 94
4.4.1 Characterization Studies of L-Threonine Derived Ni NPs 94
4.4.1.1 UV-Vis Spectroscopy for Preparation of L-
Threonine Capped Ni NPs 94
4.4.1.2 FTIR Spectroscopy Characterization for L-Threonine
Capped Ni NPs 96
4.4.1.3 AFM Analysis of L-Threonine Capped Ni NPs 97
4.4.1.4 TEM Analysis of L-Threonine Capped Ni NPs 99
4.4.2 Application of L-Threonine Capped Ni NPs as Catalyst 101
4.4.2.1 Chemical Properties of Congo Red 101
v
4.4.2.2 Catalytic Activity of L-Threonine Capped Ni NPs 102
4.4.2.3 Recovery and Reuse of Ni NPs Catalyst 104
Chapter – 05 106-118
CONCLUSIONS
Conclusions 106
Recommendations 109
Societal Implications 110
References 111
LIST OF TABLES
Table 1.1 Nanostructured materials and their usual applications in
nanoscience and nanotechnology 12
Table 1.2 The use of bottom-up and top-down techniques in
manufacturing 14
LIST OF FIGURES
Figure 4.1.1 UV-Vis spectra of l-cysteine derived Ni NPs, (a) increasing
Ni (II) ions / l-cysteine ratio as (1) 1:2, (2) 1:3 and (3) 1:6, (b) Ni
NPs as (1) fresh solution, (2) solution after one week and (3) after
two weeks.
58
Figure 4.1.2 TEM images of freshly formed l-cysteine Ni NPs by
microwave irradiation in ethylene glycol, a) low resolution, scale
bar = 100nm, b) high resolution, scale bar = 50nm. 59
Figure 4.1.3 FTIR spectra of (a) pure l-cysteine and (b) newly synthesized
Ni NPs. 60
Figure 4.1.4 Scheme for the synthesis of spherical Ni NPs. 61
Figure 4.1.5 UV-Vis spectra of (1) 10 µM aqueous solution of 4-NPh and
(2) 10 µM 4-NPh in the presence of 0.15 M NaBH4 62
Figure 4.1.6 UV-Vis spectra (1), of aqueous solution of 10 µM 4-NPh; (2),
10 µM 4-NPh with 0.15 M NaBH4 (3), reduction of 10 µM 4-NPh
with 0.1 mg and (4) reduction of 10 µM 4-NPh with 0.2 mg Ni
NPs.
62
Figure 4.1.7 UV-Vis spectra (1), of 10 µM aqueous solution of 4-NPh; (2),
10 µM 4-NPh with 0.15 M NaBH4 and (3, 4), reduction of 10 µM
4-NPh with 0.1 mg and 0.2 mg Ni powder respectively. 63
vi
Figure 4.1.8 Histograms showing the % conversion of 10 μM of 4-NPh in
the presence of 0.15 M NaBH4 solutions by fresh and four times
regenerated and reused Ni NPs. 64
Figure 4.1.9 Reduction of 10 ppm Cr(VI) ions in water, 1, without
addition of NaBH4 a peak at 352 nm, 2, after addition of 0.1 M
NaBH4 (fresh solution) and 3, same as 2 but after 15 minutes. 65
Figure 4.1.10 Reduction of 10 ppm Cr(VI) ions in water with 0.1M
NaBH4 using 0.5 mg Ni powder, with spectral changes recorded
after 5 min interval up to total time of 15 minutes. 67
Figure 4.1.11 Reduction of 10 ppm Cr(VI) ions in water (same as Figure
4.1.10) but using Ni NPs instead of Ni powder and spectra
recorded after 60 sec each showing complete reduction of Cr(VI)
ions in a very short time.
67
Figure 4.2.1 UV-Vis spectra recorded for varying concentration of Ni2+
ions in the presence of 0.018 M hydrazine (0.2 ml, 0.3 ml and 0.4
ml of 0.01 M) left side peaks from below to above and untreated
sample of Ni solution on right.
70
Figure 4.2.2 UV-Vis spectra for varying concentration of hydrazine
monohydrate (0.6 ml, 1.2 ml and 1.8 ml of 0.1 M) with increasing
absorbance and shift in λmax from right to left. 71
Figure 4.2.3 UV-Vis spectra recorded for varying concentration of l-
methionine (0.4 ml, 0.8 ml and 1.2 ml of 0.01 M) increasing
absorbance and again shift in λmax from right to left. 71
Figure 4.2.4 UV-Vis spectra recorded for (a) pH study with variation of
pH 3-11 from right to left with shift in λmax from 400 nm to 360 nm
and increase in absorbance and (b) time study of l-meth-Ni NWs
samples showing stable absorption band at 361 nm after 1 hour
until many days.
72
Figure 4.2.5 FTIR spectra of (a) standard l-methionine and (b) Ni NWs
functionalized with l-methionine. 74
Figure 4.2.6 High resolution SEM images obtained from formation of Ni
NWs with (a) 1:2 (b) 1:4 and (c) 1:6 Ni/l-methionine molar ratios. 78
Figure 4.2.7 XRD patterns recorded from Ni NWs with varying molar
ratios of (a) 1:2, (b) 1:4 and (c) 1:6 of Ni/l-methionine. 79
Figure 4.2.8 UV-Vis spectra for catalytic oxidation of IPA to acetone (a)
varying concentration of NaBH4 in the range 0.001-0.01 M at fixed
concentration of IPA (0.125 M) in the absence of Ni NWs, (b) varying
concentration of IPA in the range 0.025-0.125 M at fixed amount of
Ni NWs (0.1 mg) and 0.005 M NaBH4, (c) at fixed concentration of
0.125 M IPA with varying amounts 0.1-0.5 mg Ni NWs and 0.005 M
NaBH4 and (d) calibration curve for 0.01-0.12 M concentration of
acetone.
81
vii
Figure 4.3.1 UV-Vis spectral changes recorded for optimization of
reducing agent showing blue shift in λmax from 391 nm to 366 nm
with increase in absorbance at various concentrations. 85
Figure 4.3.2 UV-Vis spectral changes recorded for optimization of
surfactant (TX-100) at various concentrations showing red shift in
λmax from 382 nm to 393 nm with increase in absorbance. 86
Figure 4.3.3 Selected UV-Vis spectra from optimization of pH study
where we observed blue shift in λmax from 391 nm to 357 nm with
increase in absorbance. 87
Figure 4.3.4 FTIR spectra of (a) standard TX-100 solution recorded using
ATR and (b) TX-100 stabilized Ni NSs at pH 9.4. 88
Figure 4.3.5 SEM images of TX-100 stabilized Ni NSs obtained at pH 9.4. 89
Figure 4.3.6 XRD pattern of TX-100 stabilized Ni NSs obtained at pH 9.4. 90
Figure 4.3.7 UV-Vis spectral analysis for catalytic reduction/degradation
of 0.02 mM RB carried out in 4.0 ml deionized water with 10 mM
NaBH4 in the presence of a fixed amount of TX-100 stabilized Ni
NSs (0.2 mg) obtained at pH 9.4.
91
Figure 4.3.8 UV-Vis spectral analysis for catalytic reduction/degradation
of 0.02 mM MB carried out in 4.0 ml deionized water with 10 mM
NaBH4 in the presence of a fixed amount of TX-100 stabilized Ni
NSs (0.2 mg) obtained at pH 9.4.
92
Figure 4.3.9 UV-Vis spectral analysis for catalytic reduction/degradation
of 0.02 mM EB carried out in 4.0 ml deionized water with 10 mM
NaBH4 in the presence of a fixed amount of TX-100 stabilized Ni
NSs (0.2 mg) obtained at pH 9.4.
93
Figure 4.3.10 UV-Vis spectral analysis for catalytic reduction/degradation
of 0.02 mM ECBT carried out in 4.0 ml deionized water with 10
mM NaBH4 in the presence of a fixed amount of TX-100 stabilized
Ni NSs (0.2 mg) obtained at pH 9.4.
93
Figure 4.4.1 UV-Vis spectra of freshly prepared samples of Ni NPs
obtained in an aqueous environment by variation of the reducing
agent concentration. 95
Figure 4.4.2 FTIR spectra of (a) standard l-threonine and (b) sample of
powdered Ni NPs obtained after separation by centrifugation
method. 97
Figure 4.4.3 AFM images of well dispersed l-threonine capped Ni NPs, (a)
at high magnification and (b) low magnification. 98
Figure 4.4.4 Schematic diagram of the formed Ni NPs capped with l-
threonine molecules and plot of size distribution - AFM and TEM
micrographs were used for size calculations. 99
Figure 4.4.5 TEM image of l-threonine capped Ni NPs mounted on a
carbon coated Cu grid. 100
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Figure 4.4.6 (A) Linear molecular structure of CR dye, its (B) sulfonazo
form and (C) chinone form produced during the course of reaction. 102
Figure 4.4.7 UV-Vis spectra obtained from (a) Congo red dye (20 μM) in
pure Milli-Q water and (b) catalytic degradation of dye (20 μM)
with NaBH4 (0.1 M) in the presence of Ni NPs (0.1 mg) deposited
on glass cover slips.
103
Figure 4.4.8 % conversion of 4-NPh to 4-APh by freshly prepared and
four times regenerated Ni NPs used as catalyst. 105
List of Abbreviations
4-NPh 4-Nitrophenol
AFM Atomic Force Microscopy
AP Ammonium Perchlorate
CdSe Cadmium Selenide
CHN Carbon, Hydrogen, Nitrogen
CR Congo Red
CTAB Cetyltrimethyl Ammonium Bromide
DDA Dodecylamine
DMF Dimethyl Form Amide
DNA Deoxyribonucleic Acid
DSC Differential Scanning Calorimeter
EB Eosin-B
ECBT Ereochrome Black-t
EDS Electron Dispersion Spectroscopy
FCC Face Centered Cubic
FTIR Fourier Transform Infrared
HD-DVD High-Density Digital Versatile Disc
HPLC High Performance Liquid Chromatography
IPA Isopropyl Alcohol
LBL Layer-By-Layer
LEDs Light Emitting Diodes
MB Methylene Blue
MWCNT Multiwall Carbon Nanotubes
Ni NPs Nickel Nanoparticles
Ni NSs Nickel Nanostructures
Ni NWs Nickel Nanowires
PAA Poly(Acrylic Acid)
PVP Polyvinyl Pyrolidone
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RB Rose Bengal
SDS Sodium Dodecylsulphate
SEM Scanning Electron Microscopy
SPR Surface Plasmon Resonance
TEM Transmission Electron Microscopy
THF Tetrahydrofuran
XPD X-Ray Photon Dispersion
XRD X-Ray Diffraction
x
ACKNOWLEDGEMENTS
Praise be to Allah, Lord of the Worlds, The Beneficent, The Merciful, Who is
the entire and ultimate source of every knowledge and wisdom. We are fortunate to
be bestowed with the incessant and eternal blessings of Almighty Allah and divine
guidance and teachings of the Holy Prophet (Sallallaho Alaihe Wasallam) that makes
us capable to accomplish some assignments and jobs.
I am highly indebted to my kind and industrious, supervisor Prof. Dr.
Sirajuddin and kind hearted Co-supervisor Prof. Dr. Syed Tufail Hussain Sherazi
for their genius and innovative ideas and untiring efforts for my appreciation
throughout the research work that enabled me to complete my task productively.
I would significantly extend my gratefulness to compassionate and ever
encouraging Director Prof. Dr. Muhammad Iqbal Bhanger, NCEAC University of
Sindh Jamshoro, for his sincere recommendations, suggestions and his efforts
towards enrichment of the research centre with required equipments in exigency, and
the resulting impetus in our research work.
Thanks to both HEC, Islamabad, Pakistan (for awarding me a scholarship for
six months of research abroad) and thanks to the University of Bristol, Bristol,
United Kingdom (for facilitating me with the favorable environment in this regard).
I am highly thankful to the commendable supervision of Dr. Keith Richard
Hallam and the Director, Interface Analysis Centre, Dr. Thomas Bligh Scott, at the
University of Bristol, England. My truly acknowledgements are for both of them for
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facilitating and guiding me exploring these research findings. And special thanks, to
Dr. Sean Davis and Dr. Loren Picco for their precious help with electron
microscopy studies.
Further, I would extend my thanks to highly adept and supportive teachers,
especially respected Prof. Tasneem Gul Kazi and Prof. Shahabdin Memon. All of
them always welcomed and helped me whenever I needed – sharing their proficient
skills and ideas – hence maintaining a professional and formal environment in the
Centre.
Some of my friends who always served for me as sources of inspiration and
motivation, exemplary friends and colleagues include Dr. Abdul Rauf Khaskheli,
Dr. Aftab Ahmed Kandhro, Dr. Sarfraz Ahmed Mahesar, Dr. Munawer Saeed
Qureshi, Dr. Abdul Niaz, Dr. Syed Afzal Shah, Dr. Imam Bakhsh Solangi,
Dr. Raja Adil Sarfraz and Dr. Muhammad Bilal Araen. I would like to thank my
fellows, especially Mr. Muhammad Younis Talpur, Mr. Aijaz Ahmed Bhutto,
Mr. Muhammad Ali Mallah, Mr. Razium Ali Soomro and Mr. Asif Ali Jamali –
without them this project would have been a most uphill task.
It would be unfair not to mention the names of a couple of my colleagues
very special, professional and unforgettable, Mr. Zulfiqar Ali Tagar, Miss Syed
Sara Hassan, Miss Yasmin Junejo, Miss Fozia Tabasum Minhas and Miss Saba
Naz. I always found them hard working, straight forward and paradigms of academic
excellence.
Administrative staffs are of vital importance, especially Pir Ziauddin – I
extend my cordial thanks because of his unforgettable services to the Centre and to
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the research scholars – and other efficient administrative staff members Mr.
Munawar Soomro, Mr. Muddassir Arain and Mr. Akhtar Ali Vighio.
And finally, I offer my heartiest gratitude to all my family members,
especially, Father, Mother, Sisters and Brothers for their cooperation, moral and
financial support, patience, trust and prayers for my health and success.
NAZAR HUSSAIN KALWAR
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Abstract
A new facile and effortless method has been introduced for the fabrication of
l-cysteine capped nickel nanoparticles (Ni NPs) in an organic solvent (ethylene
glycol) under microwave irradiation with the aim to employ such nanoparticles as
catalysts in remediation/reduction of pollutants for environmental or analytical
purposes. Appropriate amounts of NaOH, Na2CO3 and l-cysteine were significant for
the formation of Ni NPs. The surface interaction of l-cysteine with Ni NPs was
monitored by UV-Vis spectrometry and Fourier transform infrared (FTIR)
spectroscopy while size and shape of as-synthesized Ni NPs were judged by
transmission electron microscopy (TEM). These studies confirmed the interaction of
biomolecules with the surface of Ni NPs via the -S- linkage to form spherical
Ni NPs. The Ni NPs were recovered and reused four times for the reduction of fresh
4-nitrophenol (4-NPh) with 100-98% efficiency that exhibit negligible catalytic
poisoning with excellent economic output. Further these Ni NPs were also used to
investigate their catalytic efficiency to reduce hexavalent chromium i.e. Cr(VI) to
trivalent chromium i.e. Cr(III) in aqueous system. We observed complete reduction
of Cr(VI) in only five minutes by the use of 0.5 mg quantity of l-cysteine derived Ni
NPs as catalysts.
Synthesis of nickel nanowires (Ni NWs) by a simple chemical approach and
their use as highly active and recyclable catalysts for conversion of isopropyl alcohol
(IPA) to acetone by the transfer hydrogenation reaction was carried out in an
aqueous medium. The Ni NWs were obtained by reducing Ni2+
ions with hydrazine
xiv
monohydrate as the reducing agent and capped by l-methionine (amino acid)
molecules. The basic pH, high concentration of reducing agent and higher molar
ratio of Ni/l-methionine were necessary for synthesis of Ni NWs. UV-Vis
spectroscopy, FTIR spectroscopy and scanning electron microscopy (SEM) were
used for characterization of Ni NWs. The catalytic test was performed in the
presence of the rich hydrogen source NaBH4, which helps in the conversion of IPA
to acetone. The effects of concentration of IPA, concentration of NaBH4, reaction
time and amount of Ni NWs were monitored to investigate the efficiency of
catalysts.
The study also describes synthesis of highly active and ordered structures of
nickel nanocatalysts by a green and economically viable approach. The study reveals
efficient catalytic activity for the degradation of a number of toxic and lethal organic
dyes such as Eosin-B (EB), Rose bengal (RB), Ereochrome black-T (ECBT) and
Methylene blue (MB). The stable colloidal dispersions of ordered nickel
nanostructures (Ni NSs) arrays were prepared via a modified hydrazine reduction
route with unique and controllable morphologies in a lyotropic liquid crystalline
medium using a nonionic surfactant (Triton X-100). Characterization studies and
optimization of various parameters for preparation of these nanoscale nickel
structures, surface binding interactions, size and morphologies of the fabricated
Ni NSs were carried out using UV-Vis spectroscopy, FTIR spectroscopy, X-ray
diffraction (XRD) and SEM analysis.
We introduced a simple and primitive seed-mediated growth approach for
fabrication of well dispersed l-threonine derived nickel nanoparticles (Ni NPs) using
xv
nickel chloride as the precursor in an aqueous medium via a modified borohydride
reduction method. L-threonine molecules served to tune the nanoscale composites.
Appropriate amounts of NaOH/HCl were added to adjust the pH range of the
solution to the range 2.6-11.3, however basic pH 8.5 was found to favor the
formation of spherical shapes and achieve well dispersed Ni NPs as shown in TEM
micrographs. Freshly prepared Ni NPs covered mean nanoscale dimensions of 5.06
nm for bigger nanospheres and 1.68 nm of smaller NPs, determined from atomic
force microscopy (AFM) and TEM data. Microscopy studies reveal that bigger
Ni NPs consist of small individual nano-composites with fine crystal structures. The
nanoparticles thus prepared were exploited to check their catalytic activity. Congo
red (CR) dye was used as a model reagent to monitor catalytic degradation.
Experiments highlighted no or very little reduction of dye in the absence of Ni NPs.
Conversely the addition of only 0.2 mg of nano-catalysts (Ni NPs) produced 100%
conversion/degradation efficiency within a fraction of a minute; the present study
also showed recovery and reuse of the same catalysts which performed with no loss
of activity even after several cycles of reuse.
1
Chapter 1
INTRODUCTION
This chapter includes the basic nomenclatures of nanotechnology, concepts and
descriptions of the nanosized regime structures. Additionally, a brief conversation is aimed
to understand the types of nanoscale structures, methods of fabrication and specific
applications of different nanomaterials. In addition, a thorough introduction to the working
principles and importance of analytical techniques used for surface and materials analysis,
namely UV-Visible spectroscopy, XRD, FTIR, TEM and AFM is also included.
1.1 Aims and Objectives of the Present Work
Use of environmental friendly and less expensive materials for fabrication and
encapsulation of Ni NPs.
Investigation of interactions between NPs and used stabilizing/capping materials.
Control of particle size at lower level to get enhanced catalytic properties for the Ni
NPs.
Characterization of size and morphology of the nanoparticles.
Application of Ni NPs in the field of catalysis.
1.2 What is Nanotechnology
Nanotechnology is defined as, the study of science and technology that deals with
engineering and manipulation of materials and devices having at least one dimension of 1-
100 nm on the length scale. Nanotechnology does not merely work with materials at the
nanoscale, but also covers the research and development of architectures of systems,
2
devices and materials that possess novel properties and functions because of their nanoscale
components and dimensions*.
Nanotechnology has also been described as, The study to produce, design,
characterize and use structures, systems and devices by controlling size and shape at the
nanoscale level**
. This fascinating technology has produced a variety of materials with one,
two and three dimensions in the nanoscale range. The science of nanomaterials and their
use underpins vast areas of modern technology. Hence, it is subjected that nanotechnology
is a highly interdisciplinary field by its nature. Nanoscientists and technologists have
strongly agreed that nanotechnology focuses on functional materials of the vast area of
laboratory/industry use, where chemists produce new nanoscale materials, physicists
explore their electronic and photonic properties and engineers plan to use them in
appropriate devices and circuits.
1.3 Significance of the Nanoscale
A nanometer (nm) is 10-9
or one billionth part of a meter. In comparison, the average
width of a human hair is 100,000 nm. Normal human blood cells range in diameter between
2000 nm and 5000 nm; a deoxyribonucleic acid (DNA) strand is 2.5 nm in diameter; and
ten hydrogen atoms in a line span 1 nm. Nanotechnology holds out the promise to
manipulate functions and structures within cells. This fascinating new technology has the
ability to understand and organize matter at the nanometer level, to closely simulate and
control life and matter at their most fundamental levels.
*The National Nanotechnology Initiative at Five Years: Assessment and Recommendations of the National Nanotechnology Advisory
Panel, President’s Council of Advisors on Science and Technology, Washington D.C., May 2005, p. 7. **
Nanoscience and Nanotechnologies: Opportunities and Uncertainties, Royal Society and The Royal Academy of Engineering, UK, July
2004, p. 5.
3
1.4 History of Nanotechnology
Nanoscience and nanotechnology have blossomed over the past three decades and
their significance, and that of miniaturization in general, has grown in almost all scientific
areas, such as biomedical, chemical, computing, electrical/optical sensing, electronics and
mechanics. The advances in nanoscience predominantly depend on the ability to fabricate
nanodevices of various materials having smaller sizes and different shapes, such as
spherical nanoparticles, nanowires, nanorods and nanoprisms as well as efficiently
assembled complex architectures of these nanostructures. Nowadays, the synthesis of
nanoparticles/structures is a fairly established phenomenon but materials of submicron or
nanosized dimensions have been manipulated for far longer than might be imagined.
Evanoff et al., (2005) reviewed that he first well recognized example of silver and gold
nanoparticles is the “Roman Lycurgus Cup” that dates from around fourth century AD, a
cup made up of a very special type of bronze glass, a dichroic glass, with colored lines on
the outer walls. (Barber et al., 1999; Evanoff et al., 2005) described - according to a study
commissioned by the British Museum that exhibits the cup (though it is currently on loan to
the Art Institute of Chicago through until August 2013), this extraordinary glass under
reflected light appears green but becomes a glowing translucent red color when light is
shone through it, due to presence of tiny amounts of colloidal silver and gold nanoparticles
on the walls of the cup. Although this particular property of the nanoparticles was
unintentional, such nanoparticles were later used to make ruby red, gold and lemon-yellow
stained glass with very small, silver nanoparticles (Evanoff et al., 2005).
Evanoff et al., (2005) described that the first distinguished scientific study of the
optical properties of metal nanoparticles was conducted in the 1850s by Michael Faraday.
Inspired by the capacity of extremely thin gold films to transmit green light, Faraday
4
investigated metallic substances capable of interactions with light, though their dimensions
might decrease to as small as invisible sizes. He prepared ruby red colloidal gold
suspensions by various methods and proposed that these suspensions react with acids in
similar way to bulk gold, but that the sizes of the suspended particles were too small to be
seen by his microscope. Faraday also mentioned that color of the colloidal gold particles
was apparently changing with varying of the amounts of the agents inducing aggregation;
these successful protocols, termed aggregation assays, are presently pursued by many
nanotechnologists. Some of the colloidal suspensions studied by Faraday are still displayed
at London’s Royal Institution, Evanoff et al., (2005).
The subsequent noteworthy triumph came in 1908 from Mie (1908), who produced
his now seminal work on the extinction of light by nanosized metal spheres. Kerker (1969)
in a historical talk, however, discussed that many scientists prior to Mie reported similar
studies which remained obscure to members of the modern scientific community.
Mie, who held the title “Professor Extraordinarius” at the University of Greifswald,
made the first moves to experimental search of colloidal metal nanoparticles and convinced
Walter Steubing to compose the study for his doctoral thesis (Lilienfeld, 1991). Mie is
renowned for his 1908 paper, in which he mathematically described the optical responses
of Au suspensions that previously Steubing and others like Faraday and Zsigmondy had
studied (Mie, 1908; Evanoff et al., 2005).
Initiating from Maxwell’s macroscopic electromagnetism equation (Evanoff et al.,
2005), Mie for the very first time, studied how colloidal suspensions evolve spectra as a
function of particle size. He discovered the extinction coefficients, absorption cross-
sections and scattering efficiencies of Au NPs and clarified that how the spectra of gold
suspensions develop as function of nanoparticle’s size. His calculations also provided
5
information to draw scattering diagrams for various particle sizes, illustrations describing
the electronic and magnetic fields of dipole, quadrupole, sextupole and octupole resonance
components. Shankland, (2005) described that Mei proved by that optical resonances
created by suspensions of the noble-metal nanoparticles result from collective oscillations
by the conduction electrons also called plasmons. Shankland (2005) illustrated how
precisely Mie calculated these optical resonances, especially considering how recent the
concept of an electron was at the time.
In a historic lecture, Faraday (1857) explored a number of procedures to synthesize
gold suspensions; however, the Turkevich (1951) method is the most popular. On the other
hand, the Lee-Meisel (1982) method is frequently followed for the preparation of silver
nanoparticles. This is a modification of the Turkevich method, but - unlike the Turkevich
method - is excellent at assembling metal nanoparticles in broad size ranges.
One of the efficient means of synthesis of precisely small silver nanoparticles is
referred to as the Creighton (1979) method, which can also be adopted to prepare other
metal nanoparticles like Pt, Pd, Cu, Ni, etc. (Scott et al., 2003). Ni and Cu suspensions are
not stable as these metal particles are readily oxidized; therefore, these metals require
efficient stabilizing agents to prevent surface oxidation (Sinha et al., 1999; Hou et al.,
2003).
1.5 Nanoscale Structures and Functions
A general query raised by almost everyone new to nanoscience is “Why are
nanomaterials so special?” The main advantage of the nanosized regime is that
nanomaterials exhibit large surface area/volume ratios. Consequently, the surfaces of
nanomaterials encompass very high surface reactivity and are ideal for catalytic and sensor
6
applications (Torres, 2003). In addition, the systematic organization of nanomaterials
featuring biosystem-like proteins are able to create materials that can contribute as artificial
components to cure illnesses, fight against diseases, viruses and superficial weaknesses
(e.g. artificial muscles) (Torres, 2003). Further, the ability to vary fundamental properties
of nanomaterials, such as optical, magnetic, melting point and hardness, relative to the
equivalent bulk materials without any modification in chemical composition is another key
benefit of the nanosized regime (Torres, 2003).
Nanotechnologists and chemists have largely focused on “bottom-up” approaches to
the fabrication of nanomaterials that offer self-assembly of nanoscale species. In principle,
parallel efforts have been made to produce nanoscale devices and building blocks thorough
“top-down” approaches by materials engineers via advanced etching, lithographic and
ablation techniques (Torres, 2003). For this, one may consider nanoscale entities as being
“mesoatomic” or “mesomolecular”, i.e. aggregates of smaller atomic/molecular subunits.
There are two principal types of nanosized regime building blocks that are used for
nanodevice synthesis and application: (1) 0D (e.g. nanocrystals, nanoparticles,
nanoclusters); and (2) 1D (e.g. nanowires, nanotubes, nanofibers). Torres (2003) also
highlighted that these nanoarchitectures can be directly incorporated into existing
devices/materials to generate advanced properties, so-called incremental nanotechnology.
Furthermore, the self-assembly of these nanoscale entities into 2D and 3D building blocks
may yield entirely new objects and functionalities this is called evolutionary
nanotechnology.
7
1.5.1 Zero-Dimensional Nanomaterials
Comprehensive and healthy discussions in the literature summarize that a
“zero-dimensional” (0D) structure is the most primitive composition to use as building
blocks for nanomaterials design and manufacture. Such materials possess sizes <100 nm
and are comprised of nanoparticles, nanocrystals and nanoclusters (Mori et al., 2005).
The term nanoparticles is commonly used to cover all 0D nanosized regime
building blocks (regardless of size and shape), or those which are amorphous and attain
relatively irregular shapes (Mori et al., 2005). In addition, nanoparticles are defined as 0D
nanostructures with amorphous or semi-crystalline morphology that possess larger than
10 nm dimensions and fairly large (>15%) size distributions.
Conversely, Mori et al., (2005) said that the semi-crystalline or amorphous
nanostructures with sizes smaller than 10 nm (i.e. 1-10 nm) and comparatively narrower
size distribution are appropriately termed as nanoclusters. This difference is an
uncomplicated extension of the term “cluster” that is normally used in
organometalic/inorganic chemistry to signify small molecular cages with fixed size.
Similar to bulk materials, aggregations of nanocrystaline nanostructural subunits are
well expressed as nanopowders, Mori et al., (2005).
It is of importance here to distinguish between nanoparticles/nanoclusters and usual
colloids, which date back to the early 1860s. As we all are aware, the term colloid is used to
express solid/liquid or solid/gas suspensions like paints, milk, butter, smog and smoke, etc.
Though both types of substances possess sizes within nanoregime, the principle
difference is control over morphology and composition of the matter. As many workers
have identified, stabilizing agents must be employed to avoid agglomeration of nanosized
metal structures into larger sized powders (Lyshevski et al., 2002). In addition,
8
polydispersed organic polymers and other ionic species may be used for colloids, in which
they may adhere with the colloid’s surface. Differences in the nature of stabilizing agents
result in large differences in overall morphology and other properties of colloids
(Lyshevski et al., 2002). In order for nanostructures to be obtained by “bottom-up” design,
the fabrication and resultant properties should necessarily be reproducible. This is
accomplished effortlessly by encapsulating agents of known structures that do not react
with surfaces but which, however, deactivate entrained nanosized regime organization
(e.g. dendrimers, surfactants and polyoxoanions).
So far, this discussion has focused on nomenclature for amorphous 0D
nanostructures (Mori et al., 2004). Similar to those of the bulk materials any nanoscale
object that is crystalline should be referred to as a nanocrystal. The term is engaged for
substances which are singly crystalline; if only regions of crystallinity are exhibited by a
particle, subsequently, it is better illustrated nanocluster or nanoparticles supported by its
dimensions. The crystallinity of any nanoscale materials is efficiently determined by TEM
especially equipped with parallel electron diffraction.
Mori et al., (2004) described semiconductor nanocrystals as quantum dots. In
principle, the architectures of these nanoscale structures lie in the range 1-30 nm across.
These quantum dots have recently found applications as light emitting diodes (LEDs),
lasers and sensors. Moreover, new high-density discs, such as the HD-DVD or blu-ray
DVD formats, are manufactured from quantum dots. High efficiency solar cells and optical
computing devices are examples of durable nanostructures of such materials.
In principal, the term nanocluster should be used to express 0D nanostructures of
homogeneous size distribution (Mori et al., 2004; Williams, et al., 2005). Conversely those
9
nanostructures which exhibit relatively larger size distribution should be described as
nanoparticles (Huang et al., 2004; Schmid et al., 2004).
Nanocrystals should be categorized by the existence of well-ordered lattice arrays
of constituent subunits, as exemplified by single nanocrystals of cadmium selenide
(Scher et al., 2003; Rotello et al., 2004). In contrast to nanocrystal, a nanosized regime
structure is characterized as nanopowder when it is composed of microscopic grains, each
consisting of nanoscale amorphous units (Leconte et al., 2006; Rao et al., 2004).
Material structures/devices with intermediate sizes between the nano- and micro-
regimes are better expressed as submicron. However the bulk powder scale is generally
referred to as 200 μm and above (Wolf et al., 2004; Williams et al., 2005).
1.5.2 Manipulation of Zero-Dimensional Nanostructures into Arrays
In the elucidation of how 0D nanostructures can be created and stabilized in different
media and functionalized, it is of great importance to explain how these nanosized regime
structures are engineered to align into further complex arrays. The “bottom-up” approach to
design and manipulate nanostructures operates via fixed placement of individual nano
objects to build specific functional devices (Hammond et al., 2004).
The current synthetic methodology involves establishing materials with high control
over device properties through fixed arrangements of individual “mesoatomic” or
“mesomolecular” nanoscale entities to produce aggregates of these smaller
atomic/molecular units. In addition, methodologies have been introduced to furnish
organized nanostructures, e.g. nanoclusters/nanoparticles comprised of small molecules
around the surface. Decher et al., (1991) suggested that stabilizing/encapsulating agents are
employed to prevent rapid agglomeration of 0D nanostructures. It is worthwhile noting that
10
the stabilizing agents also offer an effective handle to bind nanoscale structures to
particular surfaces. It is also believed that the spacing between adjacent nanoscale objects
may be predicted and orderly manipulated (Hong et al., 1992; Hammond, 2004), if
stabilizing agents of well defined sizes are used to encapsulate these nanostructures.
Nevertheless, 2D matrices of nanostructures are most often obtained by this
approach; similarly, nanostructures may also be aligned to fabricate 1D chains or wires. A
popular methodology followed to achieve 3D arrays, such as layered nanostructured thin
films, is the layer-by-layer (LBL) self-assembly process pioneered by Decher during the
early 1990s (Decher et al., 1991; Decher et al., 1992) and recently reported on by
Hammond (2004).
1.5.3 One-Dimensional Nanostructures
Another class of nanosized regime building blocks is referred to as 1D nanoscale
architectures, which are reserved for those structures that possess nanoscale dimensions
equivalent in all but one direction. As discussed earlier, 0D structures contain
length = width, whereas 1D structures have length > width. Commonly used terms for 1D
nanostructures are nanorods, nanowires, nanotubes and nanofibers, which have become
confused in the literature by their use for different nanostructures without suitable
explanations.
With analogous bulk materials (including no “nano” in the names), there remains no
ambiguity in applying these terms. However, a common requirement with the “nano”
descriptors is that the diameters of the particles/structures must remain in the nanosized
regime (i.e. 1-100 nm range), whereas the lengths are more often of the micro (or larger)
11
size regime. A nanotube is a 1D structure that has a hollow core inside, while the other
three nano-architectures are solid throughout.
The term nanofiber is reserved for amorphous and usually nonconductive 1D
nanostructures that are composed of polymers and other non-graphitized carbonaceous
structures. Conversely, a nanowire is characterized as a 1D nanostructure that is crystalline
having either metallic or semiconducting electrical properties.
A nanorod is designated as a crystalline 1D nanostructure that contains an overall
length approximately equivalent to its width (i.e. both diameters should be less than
100 nm). Seemingly, the crystalline nanorods acquire overall shapes as needle-like bulk
crystals. In addition, the term “nanocrystal” may also be more appropriately utilized for
these nanostructures or more feasibly “rod-like nanocrystals”.
These structures - nanofibers, nanotubes and nanowires - can define an interwoven
array; nanorods exhibit a complete linear morphology. In fact nanorods possess the ability
to stack onto each other and yield interesting 2D and 3D architectures, though not so simple
to create with “spaghetti-like” morphs of some 1D nanostructure materials (Wolf, 2004;
Williams et al., 2005).
1.6 Some Specific Types of Nanoscale Structures and their Uses
It is widely accepted that small size is not the only requirement for nanoparticle
synthesis. For any practical appliances, the processing conditions are needed to be
controlled so that the resulting nanoparticles acquire the following characteristics:
(i) identical particle size, i.e. monosized or of uniform size distribution; (ii) identical
morphology or shape; (iii) identical crystal structure and chemical composition (surface and
12
core composition should remain same); and (iv) must remain mono-dispersed or
individually dispersed, i.e. no agglomeration.
Table 1.1 Nanostructured materials and their usual applications in nanoscience and
nanotechnology
Nanostructure materials Applications
Nanoparticles/Nanoclusters Catalysis
Quatum dots Sensors
Nano films/Utra thin layers Adhesives and coatings
Nanostructure interfaces and layers Paints, dyes and cosmetics
DNA or any biological structures Medicines/Drug delivery systems
Nanocomposites hybrid materials Materials ceramics
Nanoactuators Optics
Nano/Meso porous materials Information storage
Nanowires Magnetic, electrical, optical devices and switches
and electronics (quantum computers)
Nanotubes Separation technologies
Nanoreactors Energy (batteries, fuel cells, solar cells)
1.7 Nanocomposites Production Methods
Methods for the fabrication of nanosized regime structures can be grouped into two
main categories: the manipulation of molecular components through self-assembly, a
widely seen process adopted by nature that is expressed as the bottom-up approach
(construction of small things into larger objects); and the opposite, the top-down approach,
where larger objects are cut down to smaller products, e.g. apparatus used for modern
13
semiconductor manufacture (Drexler, 1992). The bottom-up approach is considered to be
preferable, while modern technology strives to manipulate molecular scale materials
employing the top-down approach, for example writing patterns using a scanning tunneling
microscope as the pen and xenon atoms as ink (Eigler et al., 1990).
However, there are a wide number of techniques that are used to create
nanostructures materials with different shapes, sizes, degrees of quality, speed and cost.
These approaches responsible for manufacture of nanoscale building blocks fall under the
two philosophies of “bottom-up”, and “top-down”. A table showing some of these types of
approaches covering the techniques to achieve nanosized regime objects is given below
(Bezryadin et al., 1997; Jones et al., 1995).
14
Table 1.2 The use of bottom-up and top-down techniques in manufacturing
Bottom-up Top-down
Chemical
synthesis Self-assembly
Positional
assembly Lithography
Cutting, etching,
grinding
Particles
Molecules
Crystals
Films
Tubes
Experimental
atomic or
molecular
devices
Electronic devices
Chip masks
Precision engineered
surfaces
Cosmetics
Fuel
additives
Displays
Quantumwell lasers
Computer chips
MEMS
High quality optical
mirrors
1.7.1 Synthesis of Metallic Nanoparticles
Reduction of metal complexes in dilute solutions is the common synthetic route for
metal colloidal dispersions and a large number of experimental procedures have been
introduced to initiate and control the fabrication of metal nanostructures by reduction
reactions (Henglein, 1989; Schmid, 1992; Schmid, 1994; Schon et al., 1995; Wang, 1998).
Authors have argued that fabrication of homogeneously-sized metal nanoparticles is carried
out in most cases in combination with low solute concentrations and polymeric monolayers
adhered onto the growing nanostructure surfaces (Henglein, 1989; Wang, 1998). In the
construction of metal nanoparticles, particularly the metal colloidal dispersions, a number
of precursors, reducing agents, other chemical reagents, and methods have been explored to
promote and control the nanoparticle synthesis as well as the initial nucleation and
subsequent growth patterning of nanostructures/architectures.
Precursors include metal complexes, metal salts, such as NiCl2.6H2O, HAuCl4,
H2PtCl6, RhCl3, and PdCl2, and elemental metals like Ni, Cu, Co, Zn, Pd, Rh, Ag, Au, Pt,
etc.
15
1.7.2 Influences of Reduction Reagents
The size and size distribution of metal nanoparticles or metallic colloids
significantly depend on the types of reducing agents employed for their synthesis. In
principle, fast reaction rates are induced by strong reducing agents and support the
fabrication of smaller nanoscale structures. Conversely, weak reducing agents promote
slow reaction rates and favor the formation of comparatively larger particles. Moreover, a
slow reaction rate may result in either narrower or wider size distribution; if slow reaction
results in continuous formation of secondary or new nuclei a wide size distribution would
be achieved.
Reducing agents include sodium borohydride, hydroxylamine hydrochloride,
sodium citrate, citric acid, phosphorus, carbon monoxide, formaldehyde, hydrogen,
aqueous methanol, hydrazine hydroxide, etc.
1.7.3 Influences of Stabilizing Agents
If no further nucleation or secondary nucleation continues, only limited growth will
occur due to the slow reaction, which is generally hindered by using a variety of capping
agents/materials; the growth of nucleation can be controlled by available stabilizing
reagents, resulting in narrower size distributions.
Capping or stabilizing agents include surfactants (e.g. cetyltrimethyl ammonium
bromide (CTAB), sodium dodecylsulphate (SDS) and TX-100, etc), polymers
(e.g. polyvinyl alcohol (PVA), polyvinyl pyrolidone (PVP) and sodium polyacrylate, etc.),
other low molecular weight compounds, such as amino acids, drugs and other molecules.
16
1.8 Characterization Techniques
The characterization of nanosized regime materials/structures - the determination of
morphology, size, distribution, and chemical characteristics – is a vital part of industrial
processes. This serves two wide-ranging purposes: quality control; and a contribution
towards the research and development of novel materials, processes and products. Modern
age technologists and industrialists believe that a nanotechnology “breakthrough” has
occurred in the techniques used to examine and explore the properties and processes of
nanosized scale materials. Sophisticated tools, such as AFM, TEM, and STEM, are capable
of surface and interfacial characterizations of nanoscale materials, promising observation
and analysis of nanoscale building blocks. In belief, this generates greater understanding of
the relationships between structure and material properties and provides control over the
design of materials and size distribution of nanoscale objects.
17
Chapter 2
LITERATURE REVIEW
2.1 Nanoscience and Nanotechnology
Nanotechnology continues to allow scientists and technologists to discover new
materials and benefits to society, but it is also highly important issue to consider potential
negative impacts of nanoscale products on health and environment. It has thus become
highly important to investigate how to minimize hazardous impacts introduced by new
materials. Therefore, nanotechnologists are facing challenges to understand and
discriminate the hazards of nanoscience (Hutchison et al., 2008).
Sheeparamatti et al., (2007) explained that nanotechnology is the field of atomic or
molecular manipulation of substances resulting from controlled synthesis of materials
having at least one dimension in the nanoscale size range, i.e. 1-100 nm. This fascinating
technology is not new to nature as it has been carrying out molecular manipulation since
the very origins of life on Earth to create its systems such as plants and animals. After
intensive observation and maximising understanding of the elementary design principles of
nature, one can acquire inspiration to construct nanoscale materials. This interesting field of
science offers the study of nanoscale devices and to align these with nature.
We have recently investigated simple, inexpensive and green experimental
procedures, which are described in detail in the following chapters, involving interesting
areas of present nanotechnology to study fabrication methods of Ni NPs, Ni NWs,
nanorods, and nanostructures that possess at least one dimension in the nanoscale range.
18
Nanowires have proved to be immense important devices, worthy of study, due to their
potential application in magnetic, electronic, mechanical and optical devices (Zhou, et al.,
2009).
2.2 Methods for Synthesis of Zero Dimensional Nickel Nanostructures or Spherical Nanoparticles
Many works have been performed to develop electrochemical and optical
biosensors; one such approach was introduced for determination of bisphenol A using
Ni NPs immobilization platforms. The response of Ni NPs sensors was proved to be highly
sensitive and fast amperometric detection was exhibited compared with analytical
performances of the iron-oxide or gold nanoparticles based electrochemical sensors. The
results proposed that Ni NPs can successfully be employed to develop biosensors for
electrochemical determination of biologically benign molecules or their derivatives
(Nguyen et al., 2010).
Microwave assisted methods have achieved relative importance to synthesize metal
nanoparticles with controlled size. Dongsheng et al., (2006) fabricated dodecylamine
(DDA) and polyvinyl pyrrolidone (PVP) protected nickel nanoparticles in the presence of
ethylene glycol as both the solvent and reducing agent. The metal concentration and DDA
to PVP ratio were observed to be responsible for control of nanoparticle size and size
distribution. The size and morphology of Ni NPs synthesized by microwave assisted
method were characterized by TEM.
Mandal et al., (2001) argued that calculations based on Mei Theory showed good
agreement with experimentally recorded UV-Visible spectra of synthesized Ni NPs.
The fabrication of flower like self assembled, uniform microstructures of PVP
stabilized Ni NPs in ethylene glycol using microwave irradiation method has been reported
19
(Wei et al., 2008). These NPs were obtained by the reduction of nickel chloride using
hydrazine monohydrate as the reducing agent and PVP as the capping material. A small
amount of Na2CO3 was found to be essential to produce monomorphic Ni NPs with
uniform structures. However, an appropriate concentration of hydrazine and a little NaOH
were also favorable to form nickel nanoflowers of small size and narrow distributions.
TEM and XRD characterization of such interesting Ni nanoflowers showed manifestation
of self-organized assemblies resulting from huge amount of smaller primary NPs of
average diameter of approx 6.3 nm (Wei et al., 2008).
Preparation of Ni NPs suspensions by controlled hydroxylation of Ni2+
ions with
complexion via citrate anions has been reported (Grardin et al., 2005). The work showed
similar behavior to be exhibited by intercalated nickel complex colloids even in different
compositional architectures upon reduction. It was observed that metal nickel
nanocomposite particles presented relatively similar average sizes, or ~5 nm diameter, with
well dispersed homomorphism on metal alloy matrixes.
Wenjea et al., (2002) constructed nanometer-size structures of oxidized nickel
particles which were fabricated in air at temperatures. It was demonstrated that oxide
formation started at 300°C and weight gained was increased by lengthening the isothermal
time.
Kan et al., (2001) prepared highly pure nanoscale nickel colloids via a hydrazine
reduction method with the use of palladium as the nucleating agent. It was shown that pure
Ni NPs were obtained above 80°C, whereas nanosized nickel hydroxide precipitates were
obtained at lower reaction temperatures. The factors affecting size, shape and stability of
the colloids were also checked, out of which the impact of poly(acrylic acid) (PAA)
20
addition after the preparation of nanoscale nickel colloids was prominent to stabilize the
nickel colloidal suspension.
Szu et al., (2003) fabricated Ni NPs at 60°C by hydrazine reduction in ethylene
glycol without using any stabilizing agent. However, a small amount of NaOH was found
to be helpful in preparation of pure Ni NPs, although it was not needed to carry out reaction
in an inert atmosphere. Formation of pure fcc Ni NPs was observed by analysis of
HR-TEM, XRD and EDS patterns. The TEM studies showed a decrease in mean diameter
of NPs by increasing the ratio of hydrazine to nickel salt before reaching a constant when
[N2H5OH]/[NiCl2] > 12. The Ni NPs thus formed were magnetically collected and
redistributed in ethylene glycol without aggregation. The magnetic measurements of
formed Ni NPs with mean diameter of 9.2 nm indicated superparamagnetic behavior with
22 emu/g saturation magnetization, 6.4 emu/g remnants magnetization and 0.1 Oe
coerectivity. In addition, the magnetization behaviors were enhanced at decreased
temperatures due to reduced thermal energy, such that the indicated magnetic properties
replicated the nanoparticle nature.
Sungil et al., (2005) used a pulsed laser ablation technique to prepare Ni NPs. The
UV-Visible absorbances promised metallic nickel properties but TEM showed an
inhomogeneous size distribution of these Ni NPs. The catalytic properties of these Ni NPs
were examined. They were found to be highly active catalysts for many organic reactions
when compared to larger size nickel particles.
Boudjahem et al., (2004) immobilized nickel nanocatalysts/particles on low surface
area silica beds by reducing nickel acetate (a precursor) with hydrazine monohydrate in an
aqueous solution. The characterization of these substances were carried out using atomic
absorption, SEM, TEM, and XRD. Surface areas were measured using BET analysis, TPD
21
and H2 chemisorption techniques; hydrogen generating capacity was measured by gas
phase hydrogenation of benzene.
Teyeb et al., (2005) discussed how tetraphenyldibismuthine [Bi2Ph4] and
bis(cyclooctadiene)nickel (0) [Ni(COD)2] simultaneously decompose at a reflux
temperature to produce Ni/Bi alloy nanoparticles in tetrahydrofuran (THF) with an average
size of 8-10nm. The as-synthesized Ni/Bi alloy NPs, when characterized by SQUID
measurements, showed superparamagnetic behaviours above 45K and antiferromagnetic
dipolar interactions between the NPs. TEM and XRD confirmed the hexagonal structures of
Ni/Bi magnetic NPs. XRD study further confirmed hexagonal crystallization of Ni/Bi
particles at 600°C. They also obtained a black shiny powder of well separated face centered
cubic (fcc) nickel particles with relatively small sizes, 4-5 nm, broad dipersity and similar
morphology by the decomposition of Ni(COD)2 using H2 in anisole and low molecular
weight PVP (K 30). The structural morphology of the products was characterized by high
resolution TEM as earlier reported by Teyeb, (1999).
Antoninho et al., (2004) synthesized metal nanoparticles by combination of Ni and
Al/Mg. High pore volumes and large surface areas were obtained using a modified
polymeric precursor technique. The authors demonstrated that the material composition and
pyrolysis temperature had a large effect on the achieved surface area of the products. They
demonstrated that the method yielded smaller sized Ni NPs, in the range 2.3-39.8 nm, using
H2 chemisorption, with promising catalytic properties. They characterized the amount of
residual carbon black after pyrolysis using carbon hydrogen and nitrogen (CHN) elemental
analysis, and argued that carbon might be essential component in the formation of Ni NPs
with effective catalytic potential.
22
Herbert et al., (2008) produced colloidal Ni NPs within the size range 4-16 nm.
They synthesized these with narrow size distribution, tunable sizes and characterized their
structural morphologies. According to their exploration, high temperatures were significant
for reduction of the nickel salt to obtain these improved, highly ordered, spherical Ni NPs.
The nanoparticle atomic ordering was increased by annealing at high temperature in an
organic solvent; XRD results identified the fcc structures.
Houa, (2005) in another approach fabricated uniform monodispersed Ni NPs by a
facile reduction route using nickel acetylacetonate [Ni(aceac)2], sodium borohydride or
superhydride, trioctyl phsphenoxide (TOPO) and hexadecyl amine (HDA). He suggested
that semiconductor nanocrystals can be synthesized with controlled size and growth by a
combination of TOPO and HAD. Careful optimization of the TOPO and HDA ratio in the
nickel colloidal suspension lead to the formation of Ni NPs with readily tunable size in the
range 3-11 nm. Selected area electron diffraction (SAED) and powder XRD revealed cubic
structures, TEM determined the narrow size distribution, and SQUID analysis measured the
magnetic properties of the as-synthesized Ni NPs. The method was introduced for
preparation of Ni NPs, but can be extended to produce a variety of high quality metal alloy
NPs.
Dayong et al., (2008) introduced chemical reduction methods for fabrication of PVP
capped nanoscale nickel particles with different morphologies. SEM, TEM, and SAED
characterization showed different morphologies of Ni NPs resulted from using different
amounts of PVP - thus, it has been shown to be a structure-directing agent. It was said that
nanoscale nickel crystallites aggregated to form different morphological patterns depending
on the concentration of PVP. Furthermore, the number and size of the Ni NPs increased
with aging of the suspension. The authors summarized that nucleation, aggregation, and
23
crystal growth were influenced by addition of different quantities of PVP resulting in the
formation of nanoscale nickel particles with various morphologies.
Zhi et al., (2010) described the synthesis of pure Ni NPs with spherical shapes and
controllable sizes by a simple nontoxic chemical reduction route. Reduction of nickel
chloride was carried out using hydrazine to form Ni NPs at normal laboratory conditions
without the help of extra protective agents or an inert gas environment for protection of the
particles from oxidation. SEM and XRD were used to characterize the effect of
concentration of precursor salt properties of Ni NPs.
Giselle et al., (2007) reported the synthesis of Ni NPs capped with
poly (N-vinylpyrrolidone) (PVP) by a modified polyol process using nickel chloride as the
precursor and NaBH4 as the reducing agent; these NPs were obtained using a range of
Ni/PVP ratios. XRD and TEM characterization showed well dispersed fcc Ni NPs with
3.8 nm diameters. Effective interactions between oxygen atoms of the carboxyl group of
PVP with surface of Ni NPs were observed from the data provided by FTIR spectroscopy
and X-ray photoelectron spectroscopy (XPS). Magnetic measurements demonstrated
dipolar magnetic coupling between nanoparticles resulting in single-domain non-ideal
superparamagnetic behavior.
Ya et al., (1999) reported fabrication of Ni NPs by rapid expansion of nickel
chloride in ethanol at room temperature using NaBH4 in the presence of
dimethylformaldehyde (DMF) and PVP solutions as protective agents. TEM measurements
showed that the as-prepared PVP-Ni NPs were highly stable in DMF/ethanol mixed
solutions.
Andrej et al., (1999) introduced a chemical reduction process to fabricate
submicrometer sized nickel powders from nickel salts by reducing with hydrazine hydrate
24
in non-aqueous solutions. The reduction was carried out at high temperatures using di- and
tri-ethanolamine, ethylene glycol and paraffin oil as reaction media to offer the appropriate
basic environment. The characteristics of the nanomaterials, namely size and purity of
particles, size distribution, and structural morphologies, were found to be highly dependent
on the reaction conditions. The sizes of the nanoscale nickel particles could be varied by
replacing heterogeneous nucleation with homogeneous through the addition of trace
amounts of platinum; 99.8% pure Ni NPs were obtained by this method.
Guan et al., (2007) described how while multiwall carbon nanotubes (MWCNT)
have been frequently used for deposition of Ni particles, their new method
electrocrystallized Ni NPs on 4-nitroaniline through radical monolayers grafted onto
MWCNT by molecular level design. FESEM, XPS, and XRD were used to characterize the
structures of the Ni/NA/MWCNT, demonstrating homogeneous electrodeposition of
Ni NPs on the surfaces of the MWCNT. The products were found to be efficient catalysts
for electrochemical oxidation of ethanol in alkaline medium.
Zhijie et al., (2009) synthesized metal oxide supported Ni NPs by a modified
electroless nickel plating route. UV-Visible spectroscopy, SEM, TEM, and H2
chemisorption were used to monitor the mechanism and process of electroless plating. The
surface areas of metal oxides, activity and change in dynamic metal loading (Ag) were also
studied. The dispersion of Ni NPs was found to be led by interface reactions between the
plating solution and metal oxide or the used active metal. The change of Ni NPs dispersion
was mainly affected by the acidity of the metal oxide and the Ag loading. The authors also
proposed that the size and distribution of the Ni NPs could be managed by varying the
solvent medium from water to ethylene glycol and using microwaves or ultrasonic waves.
25
Synthesis of porous Ni NPs with reticular structures using PVP as a soft template
has been carried out by a polyol process (Joon et al., 2009). By the addition of PVP,
agglomerates of nickel particles were changed into an interconnected particle network. The
change in particle surface morphology was dictated by the concentration of nickel, reaction
temperature and degree of PVP polymerization. Reticular structures were formed at a
certain nickel concentration and the porosity of the powders increased with increasing PVP
content.
Amarajothi et al., (2008) introduced a simple and less expensive approach to
produce Ni NPs in K10-montmorillonite clay by chemical reduction at a moderate
temperature. UV-Visible spectroscopy, EDX, XRD, and HRTEM were used to characterize
the as-obtained Ni NPs in the clay inter-lamellar spaces. These nanoscale particles showed
excellent catalytic properties for hydrogenation of unsaturated organic molecules and
promised eco-friendly performance as reducing agents for alkenes and alkynes.
Zhijie et al., (2009) described a modified nickel plating method to manipulate
Ni NPs supported on metal oxide surfaces. The effects of metal loading, surface area, and
activity of metal oxide were investigated to examine the process and mechanism of
electroless plating. Further characterization was carried out by UV-Visible spectroscopy,
SEM, TEM, and hydrogen chemisorption which identified that the nanoparticle dispersion
was mainly dependent on interface reactions between the plating solution and active metal
or metal oxide. In addition, the use of ultrasonic or microwaves and varying liquid media,
such as ethylene glycol in place of water, was shown to affect dispersion and size of the
Ni NPs.
Jung et al., (2006) reported the synthesis of homogeneous, fine size nickel powders
by reduction of nickel hydrazine complexes in an aqueous medium. Pure [Ni(N2H4)3]Cl2
26
complexes were formed with a Ni2+
/hydrazine ratio equal to 4.5, whereas mixtures of
Ni(N2H4)2Cl2, [Ni(N2H4)3]Cl2, and [Ni(NH3)6]Cl2 complexes were prepared when the ratio
was less than 4.5. FTIR, SEM, and XRD results identified that reduction of Ni2+
ions to Ni0
particles was initiated by the formation of nickel hydroxide followed by reduction of
hydroxide products through ligand exchange reactions. The surface roughness of the
nanoscale nickel particles was significantly improved by increasing the amount of
hydrazine hydrate due to catalytically decomposed excessive hydrazine on their surfaces.
The size of these particles was controlled by adjusting the reaction temperature and
Ni2+
/hydrazine molar ratio.
Kudlash et al., (2008) described a process to produce Ni NPs by a one step synthetic
procedure via interface reduction of nickel oleate with sodium borohydride without any
protective agent in an aqueous medium at room temperature. The Ni NPs formed black
colloidal dispersions. The size and structural characterization of these particles were carried
out using FTIR, SEM, TEM, and XRD. Well-dispersed crystalline nickel nanoparticles
mixed with some nickel boride phase were prepared in these colloidal solutions, with their
average size being between 2-6 nm. IR results showed that particle stability was achieved
by encapsulation of nanoparticles with surfactants.
Yanping et al., (2005) introduced a solid-state borohydride reduction procedure to
synthesize NiO nanoparticles using nickel acetate as the precursor and tween 80 as the
dispersant. The as-obtained products were then characterized by FTIR, TGA, DTA, and
XRD; the size and morphology of the NiO NPs were also confirmed by TEM. Catalytic
applications of these NiO NPs were checked by thermal decomposition of ammonium
perchlorate (AP) through TGA and DTA. The efficiency of the NiO nanocatalysts was
superior to that of the bulk NiO catalysts; only a 2% addition of NiO NPs to AP lowered
27
the decomposition temperature by 93°C and enhanced the heat of decomposition from 590
to 1490 J/g.
Corrie et al., (2002) carried out an alkoxide based fabrication of NiO and CuO
nanocrystals by the use of the corresponding metal salts, water and ethanol. NiO crystallites
of 3-5 nm and CuO crystallites of 7-9 nm were formed. The aggregation of crystallites
resulted in the formation of larger nanospheres which possessed pores and tunnels, as
explored by TEM and Brunaur-Emmet-Teller studies. This resulted in the manufacture of
very small nanocrystals with highly enhanced surface areas, up to 375 m2/g for NiO and
135 m2/g for CuO nanoparticles.
Kensuke et al., (2008) produced a fine chemical surface modification protocol to
synthesize nanosized nickel composites which can be used to fabricate particles with
controlled size and the spacing between as-prepared embedded Ni NPs. They demonstrated
manipulation of a variety of structural forms of Ni NPs by changes of the polymer matrix.
SEM, TEM, and FTIR studies were used to explore the morphology and interaction of the
polymer matrix and nanospheres.
Stepanov et al., 2004 described novel nanocomposites structures of diamagnetic
matrix based nickel nanoparticles produced by a high vacuum laser driven universal cluster
ablation method. The magnetic properties and shapes of the products were monitored by
ferromagnetic resonance (FMR) measurements and TEM. An out of plane magnetic
anisotropy was revealed by FMR spectral analysis which showed that there was strong
interaction between as-prepared particles in the composite formation. FMR studies also
suggested the formation of granular nanospheres; their spherical shapes, with 3.2 nm
average diameter, were confirmed by TEM data.
28
2-3nm metallic nickel particles with 10-15 wt% Ni/PVP were prepared in a stable
colloidal suspension by reduction/decomposition of bis(cyclooctadiene)nickel in a reaction
with polyvinylpyrrolidone using dichloromethane (Dominique et al., 1997). These products
formed bridged and linear geometries and adsorbed CO in both structural forms which
provided ordered surfaces of colloidal nickel particles.
Yingwen et al., (2004) synthesized Ni NPs with different sizes at room temperature
in an alcohol-water system through an autocatalytic reduction process. Characterizations
were performed by XRD and TEM analysis.
Yan et al., (2006) deduced an improved chemical reduction route for synthesis of
Ni NPs which were then used to catalyze hydrogenation of p-nitrophenol to produce
p-aminophenol. The Ni NPs were found to be highly efficient catalysts for reduction of
nitroaromatic compounds to their amino counterparts.
Ghani et al., (2004) studied the reducibility of use of nickel acetate and hydrazine in
an aqueous solution in the presence of silica at moderate temperature (80°C) and high pH
(10-12) for the preparation of metallic nickel particles. The products thus obtained were
characterized by thermal desorption, XRD, and TEM which showed nickel acetate alone
experienced only 45% reduction to make a metal film composed of metal precipitates or at
the liquid gas interface with 120 nm average size. In contrast, in the presence of silica
100% reduction of nickel acetate was achieved and deposition of nickel as a thin film on
the silica surface with 25 nm average particle size. It was demonstrated that each particle
was made up of very tiny crystallites of 3 nm mean diameter and it was observed that
increasing the hydrazine content or decreasing the nickel acetate decreased made the
particle size smaller. The nucleation rate and metal-support interactions were claimed to be
29
important to determine the size and morphology of the silica supported nanoscale metal
particles.
Mandal et al., (2001) discussed how greener approaches have been developed to
prepare and design nanoscale materials and how cross-linking of metal nanoparticles by
amino acid molecules in solutions has been extensively studied. However, it is still of
concern to examine the interaction of amino acid molecules such as cysteine with the
surfaces of nanoparticles as it is widely used for the surface encapsulation of silver and
gold nanoparticles prepared in colloidal suspensions.
Deepti et al., (2009) prepared oleic acid capped Ni NPs in the presence of sodium
dodecylsulfate (SDS) by a wet chemical method. They observed that the nature and shape
of the NPs were critically governed by the concentration of capping agent and surfactant.
At optimized concentrations of ocleic acid and SDS, highly ordered structures of
hexagonally close-packed Ni NPs with monodispersed architectures were formed, as shown
by TEM images. The addition of oleic acid led to stable dispersions of Ni NPs; the particles
were also examined to possess magnetic properties.
An attempt has been made to fabricate Ni NPs by reducing functionalized reverse
micelles of Ni(AOT)2/Na(AOT) with NaBH4 in an air or inert atmosphere (Legrand et al.,
2002). XPS studies showed a mixture of Ni2B and pure Ni NPs from the inert atmospheres,
while Ni-B particles were formed in air. These samples showed different magnetic
properties relating to the effect of inert or air atmospheres confirming X-ray photon
dispersion (XPD) results.
Pu et al., (2004) produced complete hollow nanospheres of nickel silica composites
with average diameters of 650 nm and controlled shell thickness by a coating process. The
materials were characterized by FESEM, TEM, and XRD. A large BET surface area of
30
288 m2/g exhibited by the particles explained the high catalytic activity and selectivity of
hydrogenation reactions in acetone. A hydrothermal method was applied to synthesize
β-Ni(OH)2 nanoscale particles with different shapes, nanoparticles, hexagonal nanosheets,
and irregular nanosheets. The EDS, FTIR, and XRD confirmed brucite-type structures of
β-Ni(OH)2. It was summarized that size, morphology, and growth mechanism of the
various nanostructures was highly dependent on the amount of additives and the pH of the
sample solution.
2.3 Methods for Preparation of Other Metal Nanostructures with Zero Dimensions
Yua et al., (2008) successfully fabricated nanoscale metal core shell (M0Cs)
structures of carbon coated cobalt and nickel (Co/Ni) particles using maize-derived starch
and metal nitrates as the carbon source and metal precursors respectively. These were
characterized by SEM, TEM, and XRD for evaluation of size and shapes. Further
characterization for measurement of magnetic properties was undertaken using vibrating
sample magnetometry (VSM). The effects of different metal precursors were investigated
for directing the size and shape of M0Cs materials. The results illustrated that Co
0Cs
materials formed graphitic carbon shells and fcc-Co cores, with core diameters 20-35 nm,
whereas the Ni M0Cs materials were composed of amorphous carbon shells and fcc-Ni core
having diameters in the range 30-50 nm. Magnetic measurements of the M0Cs materials at
room temperature demonstrated superparamagnetic properties. An idea was developed that
the helical structure of starch led to the formation of the core/shell structures of M0Cs
materials and a mechanism was proposed to explain the growth patterning of as-prepared
M0Cs materials.
31
Bas et al., (2011) synthesized nickel and cobalt nanoparticles and their stabilities
were studied with a view to use as catalysts for hydrogenation reactions. A number of
techniques were used to characterize these particles, including X-ray photoelectron
spectroscopy (XPS), temperature programmed reduction (TPR), HR-TEM, etc. Quasi
in situ HR-TEM was used to monitor the formation of active phases by transformation of
oxidic precursors, a technique proved to be excellent for visualization of nanoscale metal
particles. The authors argued that closer investigations revealed that the reduction
temperature is critical for preparation of nickel catalysts whereas heating time during the
reaction was more crucial for synthesis of cobalt nanocatalysts.
Manova et al., (2007) developed sol-gel methods to prepare nickel-ferrite or
nickel-silica nanocomposites and characterized them using a number of techniques,
including XRD, IR, TPR with hydrogen, and magnetic measurements. These
nanocomposites were then utilized for catalytic decomposition of methanol to produce
methane and carbon monoxide and substantial differences in their catalytic efficiency and
selectivity were, observed depending on several factors like ferrite/silica ratio, phase
transformation of samples in the ,reaction mixture and annealing temperature.
Kyung et al., (2002) used a variety of combinations of metals, e.g. Pt, Pt/Ni (1:1 &
3:1), Pt/Ru/Ni (5:4:1 & 6:3.5:0.5), and Pt/Ru (1:1) to synthesize alloy nanoparticles and
examined their electrocatalytic efficiency for methanol oxidation using sulfuric acid
solution as the medium. The nanoparticle alloys of Pt/Ni, Pt/Ru, and Pt/Ni/Ru showed
excellent catalytic behavior in comparison to their bulk forms. It was also seen that the
addition of Ni increased the catalytic activity of the alloy nanoparticles even more because
Ni played the role of enhancing electroxidative signals during the electrochemical
32
measurements, as confirmed by cyclic voltametery; chromatography, XRD, XRPS, and
TEM were used to characterize and examine the alloy nanoparticle properties.
Approaches for the formation of metallic Cu/Sn alloy nanowires capped with Ni
using a sequential electrodeposition method and nanoporous alumina templates for Cu/Sn
bronze nanoparticles were introduced by Bentley et al., (2004) to prepare librated
nanowires. Optical microscopy was used to monitor the behavior of suspensions. Magnetic
fields were applied to observe orientation and spinning of NWs acting as nano stirrer bars.
It was also demonstrated that magnetized nickel strips could be used to trap these
segmented nanowires, elaborating the possible manipulation and positioning of metallic
nanowires precapped with magnetic ends.
A number of metal oxides, such as α-Al2O3 and CeO2, have been used to deposit
Ni NPs structures (Chettibi et al., 2009). The precursor is first adsorbed onto a surface and
then irradiated with γ-rays. UV-visible, SEM/EDS, XRD, and H2-TPR were used for
characterization at various stages. The homogeneity and high reducibility of the catalyst
were characterized. The gamma irradiation brought about the facile reduction of nickel with
strong oxide interactions highlighted the catalytic properties of Ni NPs, which also direct
the formation of well-dispersed NPs. Ni/CeO2 exhibited higher efficiency than Ni/Al2O3 in
a test for hydrogenation of benzene.
A simple method for preparation of surface modified glassy carbon electrodes uses
immobilized NiO NPs and water soluble dyes (Abdollah, et al., 2009). An extremely thin
film of the NPs was created on to the electrode surfaces. The results revealed high loading
capability of NiO NPs and enhanced facilitation for electron transfer. Excellent
electrocatalytic activity was observed for reduction of hydrogen peroxide. The developed
modified electrodes were found to be advantageous in many respects compared to the
33
unmodified electrodes, for instance in terms of reproducibility, electrocatalytic activity,
ease of preparation, and stability during hydrogen peroxide reduction.
Exciting research has been done to elaborate on the formation of horse radish
peroxidase-nickel oxide nanoparticles immobilized on glassy carbon electrodes (HRP/NiO
NPs/GCE) (Ali et al., 2009). The NiO NPs were first mounted on GCE and then
horseradish peroxidase was immobilized on the surface of the NPs. SEM and AFM were
used to determine the size and morphology of the products. The HRP provided
electrochemical redox performance by direct electron transfer between nanoparticles and
protein pertaining to HRP (Fe(III) to Fe(II)) with a formal potential of -55.5 mV
vs. Ag/AgCl and 141.5 mV vs. NHE using 50 mM phosphate buffer solution (PBS). The
rate constants of heterogeneous electron transfer coefficient and anodic charge transfer (α)
were 0.75 s-1
and 0.42 s-1
respectively. In addition, hydrogen peroxide reduction and
biocatalytic activity of HRP/NiO NPs/GCE were studied.
Dharmaraj et al., (2006) prepared well-dispersed uniform nickel oxide nanoparticles
with cubic structures by mixing poly(vinylacetate) (PVAc) and nickel acetate. Particles
were formed in the size range 40-50 nm at 723K. UV-Visible spectroscopy, FT-IR, XRD,
SEM, and TEM were used for characterization.
NiO hierarchical architectures have been formed with controlled sizes and
morphologies by a solvothermal method combined with calcination (Xuefeng et al., 2008).
Initially, many hollow R-Ni(OH)2 hierarchical structures assembled from R-Ni(OH)2
nanosheets were fabricated. Tuning the nucleation rate overwhelmingly determined the
morphology. Low nucleation rate led to hollow R-Ni(OH)2 tubes whereas spherical hollow
R-Ni(OH)2 were formed at high nucleation rates. Hollow NiO hierarchical structures, with
morphologies unmodified from those of the precursor, resulted from simple calcination.
34
These hollow NiO nanoarchitectures promised high photocatalytic efficiencies for
remediation of acid red 1 pollutant; external magnetic fields could be used to easily recover
the catalyst, generating the potential for use in environmental clean-up.
Nickel sulfide (NiS) nanocatalysts have been produced, to react with H2S/H2 gas
mixtures, using graphitized carbon supports (Richard et al., 2002). A number of metal
nanocomposite structures, e.g. NiS, Ni3S2, Ni7S6 and Ni9S8, were selectively synthesized
using different concentrations of H2S and optimizing parameters such as reaction
temperature and time. The increase in average particle size by conversion of Ni NPs to
NiS NPs was monitored. The structural morphologies and phases of the nanostructures
analyzed by HRTEM, EDX, and PXRD agreed with previously performed bulk sample
experiments at 300°C.
Mandal et al., (2001) reported that cysteine nanoparticles accomplished
encapsulation of silver nanoparticles (Ag NPs) surface via thiolet bonds. The stabilization
of colloidal silver nanoparticles resulted from electrostatic interactions by ionization of the
carboxylic acid group of amino acid molecules. They examined aggregation of
nanoparticles by aging the colloidal suspensions, caused by hydrogen bond formation
between cysteine molecules on the surface of adjacent cysteine capped silver nanoparticles.
Aggregation of these particles was found to be reversible upon heating the colloids above
60°C. The authors monitored the aggregation process and heat-induced dispersal of silver
nanoparticles by laser light scattering, UV-Visible spectroscopy, and TEM.
Mi-Ran Kimand et al., (2009) introduced a new methodology to load mono- and
bi-metallic nanoparticles on nanoscale ZnO surfaces. Products were characterised using
many techniques such as XRD, HR-TEM, TEM-EDS ,and ICP-AES. These confirmed the
formation of mixed metallic nanostructures, like Pd-Ag, Pd-Ni, and ZnO-supported Pd, in a
35
4/1, v/v-% methanol/water systemat normal temperatures with γ-irradiation. The catalytic
behavior of synthesized nanoparticles was examined in some organic reactions which
showed Pd-Ag/ZnO products were highly efficient catalysts.
2.4 Methods for Preparation of One Dimensional Nanoscale Nickel Composites
Rheem et al., (2007) prepared Ni NWs with different sizes via electrodeposition and
argued that these Ni NWs showed paramagnetic behavior. Ni electrodes of bridged
microstructures were prepared at temperatures from 10-300 K.
The effects of temperature and surface to volume ratio on mechanical properties of
Ni NWs have been investigated, including use of molecular dynamics simulations
(Setoodeh et al., 2008).
Zhou et al., (2009) formed polycrystalline Ni NWs capped with NiO shells.
Differential scanning calorimetry (DSC) and the Kissinger equation were used to
investigate thermal stability while the influence of NiO layer thickness and grain size were
also examined.
Xueliang et al., (2006) introduced a new method to synthesize NiO nanostructure
arrays from nickel dimethylglyoximate. The characterization of the products was carried
out by UV-Visible spectroscopy, XRD, SAED, and TEM.
The template synthesis technique is a most common route for preparation of
nanowires, which first became popular a decade ago (Bentley et al., 2005). Nanoporous
polycarbonate or alumina membranes are used as templates for nanowire growth.
Electrodeposition is frequently used to assemble one or more nanoscale metal particles into
the membrane pores and the metal nanowires thus formed are recovered by removing the
membrane chemically. The template synthesis technique is employed to fabricate nanosized
36
superconductors, semiconductor oxides, gaint magnetoresistance materials, and magnetic
storage devices. Ni NWs have gained considerable importance to researchers and
nanotechnologists owing to their use in magnetic storage devices - it has been argued that
nanoscale nickel wires can be used to amplify magnetic storage density. However, it is
accepted that the ability to control alignment of nanowires in devices is a common problem.
Nasser et al., (2009) introduced an electrospinning technique to synthesize nickel
nanofibers (Ni NFs) with smooth surfaces. PVA and nickel acetate tetrahydrate sol-gel
were electrospun. The formed nanofibers were dried and calcined in an inert atmosphere at
700°C. Characterization of these products confirmed the fabrication of pure Ni NFs with an
average width size of 120 nm. Greatly enhanced coerectivities were observed, assuring
unique magnetic properties. It is suggested that the safety, ease of assembly and simplicity
of the formation of these Ni NFs supersedes common procedures to prepare one
dimensional magnetic nanostructures.
Zhenguo et al., (2009) developed a method to make prickly nickel chains and sea
urchin-like Ni NPs by chemical reduction at low temperature. The crystal growth
orientations were kinetically controlled and found to be independent of external magnetic
fields and other stabilizing agents. The reaction components and suitably tuned process
conditions were customized to control branch lengths and overall product morphologies. It
was demonstrated that Ni nanostructures produced by this method were ferromagnetic in
nature and were obtained with different morphologies by a facile and effective strategy of
variables.
Rod-like nanocrystalline nickel sulfide particles were synthesized by a solvothermal
method using ethylene diamine and hydrazine hydrate as solvent and reducing agent
respectively (Ning et al., 2002). Pure millerite nickel sulfide spherical particles were
37
synthesized with aqueous ammonia with small size - and hence large surface areas -
favorable for catalytic application.
Bin et al., (2006) fabricated nanoscale thread-based porous sponge-like biomolecule
capped Ni3S2 nanostructures directly on Ni foils by a simple experimental procedure. The
electrochemical hydrogen storage performance of these structures was and these materials
were found to be good candidates to be hydrogen storage media. These nanostructures and
their nanoporous morphs exhibited a two charging-plateaux phenomenon, signifying two
independent steps in the charging processes. The authors further clarified the influence of
morphology of nanomaterials on their hydrogen storage capacity.
2.5 Methods for Synthesis of Multi-Dimensional Nanostructures
Zhiyong et al., (2004) explained the use of surface tension supported self assembly
of mechanically stable aggregations of metallic nanorods with average diameters of
200 nm. The formation occurred by minimizing liquid layer interfacial tension through
precipitating a hydrophobic polymerizable adhesive onto the surface of the nanorods. After
self-assembling, the adhesive was found to polymerize and bring about stable aggregates. It
has been shown that either open 2D networks or closed 3D bundles were formed depending
on chemical functionalization or patterning of the nanorods.
Li et al., (1999) prepared ultra-fine nickel powders and crystalline nickel nano-films
by reducing NiSO4 with hydrazine hydrate via chemical reduction. The experiments were
carried out to check the effect of pH and temperature. The formation of nickel powders
were favored by pH > 10 and 85°C temperature. Further addition of AgNO3 increased the
reaction rate and crystalline nickel films were produced by the addition of appropriate
38
amounts of sodium dodecyl sulfate (SDS,) which may also alter the size and aggregated
morphs of the ultra fine powders.
Dong et al., (2002) prepared Ni NPs by reducing nickel chloride with hydrazine
monohydrate and stabilizing with cationic surfactant CTAB-TC12AB. The use of small
amounts of NaOH and acetone were essential to produce pure Ni NPs at elevated
temperatures. It was argued that reaction was carried out in an inert atmosphere and XRD
results revealed that the manifested particles were pure crystalline fcc structures with mean
diameters 10-36 nm. The size of the as-obtained particles was dependant on the ratio of
reducing agent to precursor salt. These Ni NPs were superparamagnetic at small sizes; the
magnetic measurements showed saturation magnetization (32 emu/g), remnant
magnetization (5.0 emu/g) and coercivity (40 emu/g) values. Hence, all characterization
techniques proved the nanoparticulate nature of the products.
Zhen et al., (2004) explained that single crystalline nanosheets of β-nickel
hydroxide (β-Ni(OH)2) were successfully fabricated by hydrothermal chemical reduction at
200°C using aqueous ammonia as both the complexing agent and alkaline medium with
nickel acetate as the source salt. This method proved to be better for large scale production
of nanosheet powders with very low cost and a simple procedure. Thermal decomposition
at 400°C for 2 hours was employed for synthesis of single crystalline NiO nanosheets using
β-Ni(OH)2 as the precursor. Characterization of nanosheets for thermal stabilization was
investigated using DSC and TGA while the morphology and size of the products was
examined by TEM and XRD.
The formation of flower like nickel particles with ~1µm diameter was carried out by
a simple chemical reduction method using hexa decyltrimethyl ammonium bromide as a
soft template (Zhou et al., 2008). SEM and HR-TEM analysis of the nanoflower structures
39
revealed their architectures to be composed of particles with 250 nm average diameters,
which were again made up of small grains 20-30 nm in size.
Joon et al., (2009) formed porous nickel powders with rectangular structures via a
modified template process through addition of PVP. This also transformed agglomerated
nickel particles into an arrangement of attractive networks. The surfaces of the metal
nanoparticles altered from prickly to smooth with increase in temperature. At a fixed
concentration, rectangular nickel nanostructures (Ni NSs) were formed above which neck
regions of the particles became thick. Increasing PVP concentration to a particular level,
the porosity of the nanostructures increased to a maximum.
Yong et al., (2003) fabricated ordered nickel nanoporous films by a full replicated
process using an alumina membrane. Nanofilms with high aspect ratios approaching 360:1
were formed with narrow size distribution of nanopore diameters and ordered nanopore
array structures. Unlike other nanofilms produced by conventional electroless deposition
methods, these nanoporous nickel films showed ferromagnetic behavior with none of the
obvious magnetic anisotropy shown by typical ferromagnetic nanofilms.
2.6 Applications of Nanoparticles/Nanostructures
The commercialization of this recently emerged technology has found enormous
applications in many fields, including mechanics, electronics, magnetics, chemistry,
enhanced fabrics, physics, biomedical, and drug targeting. In spite of extensive emphasis
on the commercialization of nanotechnology by the funding agencies, research institutes,
and industries, its entrance into the college and high school chemistry syllabus has been
very slow. There is meager opportunity for undergrad chemistry students to study
40
nanoscale materials and to understand how these nanocomposite materials can be employed
for future use in different technologies (Tanase, et al., 2001).
Abbas et al., (2010) used nanoparticles for remediation of pollutants, an increased
area of research interest where removal of hazardous organic/inorganic species, like dyes,
toxic metal ions, and poisonous organic molecules and ions, has been give tremendous
attention. The efficiency of maghemite nanoparticles for adsorption/ remediation of CR dye
and its desorption has been investigated. Surfactant free microemulsions of maghemite
nanoparticles were prepared by a co-precipitation method. XRD and SEM were used to
determine size and structure. The popular adsorption isotherm models of Langmuir and
Freindlich were used to evaluate the adsorption capacity; they proved to be excellent
materials for removal of CR from aqueous media.
Dadong et al., (2009) explored the effect of positively charged
tetraheptylammonium capped Ni NPs for functionalized uptake of quercetin drug into
hepato cellular carcinoma cells (SMMC-7721). This was investigated by use of
electrochemical characterization and microscopy. It was observed that Ni NPs signify
effective improvement for the permeability of cancer cell membranes to remarkably
enhance the accretion of quercetin on carcinoma cells; quercitin and Ni NPs have
compatible synergistic effects on inhibiting propagation of cancer cells.
41
Chapter 3
EXPERIMENTAL
In this chapter, attention will be focused on materials and methods used for fabrication
of Ni NPs through solvothermal approaches. Experimental procedures will be introduced,
such as a modified microwave assisted reduction route to synthesize l-cysteine capped
Ni NP, synthesis of surfactant stabilized Ni NPs by a hydrazine reduction route and
production of borohydride reduced Ni NPs capped with different amino acid molecules. For
new nanosized regime structures fabricated through chemical reduction methods, we
illustrate in detail the solution synthesis of Ni NPs to demonstrate the fundamental
experimental conditions. However, the basic principles are pertinent more generally to the
synthesis of nanoscale nickel materials with minimal modifications or new approaches.
3.1 Synthesis of L-Cysteine Derived Nickel Nanoparticles (Ni NPs) in Ethylene Glycol
3.1.1 Cleaning of Glassware
Glassware was cleaned and washed with detergent powder solutions of tap water.
Then it was rinsed several times with Milli-Q water and acetone to remove any inorganic
and organic impurities. All the glassware was then put into an oven at 100°C for 1 hour in
order to dry properly and cooled to room temperature before use.
3.1.2 Chemical Reagents Used During Synthesis of L-Cysteine Capped Ni NPs
Analytical grade, nickel (II) chloride (NiCl2.6H2O), sodium borohydride (NaBH4),
sodium carbonate (Na2CO3), acetone (CH3COCH3), and methanol (CH3OH) were
42
purchased from E. Merck (Darmstadt), l-cysteine (C3H7NO2S), ethylene glycol
(CH2OHCH2OH) and sodium hydroxide (NaOH) were purchased from Fluka Chemika.
The 4-nitrophenol was acquired from Fisher Scientific laboratory suppliers. All chemical
reagents were used as received.
3.1.3 Preparation of Standard Stock Solutions
3.1.3.1 Stock Solution of Nickel (II) Chloride
The NiCl2.H2O solution containing Ni (II) ions was prepared from its analytical
grade salt by dissolving the appropriate quantity in 100 ml of Milli-Q water in a volumetric
flask.
3.1.3.2 Stock Solution of L-Cysteine
The stock solution of 0.05 M l-cysteine was prepared from analytical grade reagent
in ethylene glycol. The volumetric flask containing the solution thus prepared was put in a
sonicator bath to ensure complete dissolution of the reagent.
3.1.3.3 Stock Solutions of NaOH and Na2CO3
The sodium carbonate and sodium hydroxide solutions of 0.1 M and 0.01 M
concentration respectively were prepared in Milli-Q water by dissolving the desired
quantity of each. Samples of Ni NPs were then prepared by taking a certain quantity of
these solutions.
3.1.3.4 Stock Solutions of 4-Nitrophenol and NaBH4
The aqueous solutions of 0.25 M 4-nitrophenol and 0.1 M sodium borohydride were
prepared in 100 ml volumetric flasks using Milli-Q water.
43
3.1.4 Procedure for Synthesis of L-Cysteine Capped Ni NPs
In a typical synthesis of nickel nanoparticles, a clean conical flask of 25 ml capacity
was used. The NPs were synthesized by taking 1ml stock solution of nickel chloride, to
which 0.3 ml sodium carbonate, 0.1 ml of sodium hydroxide solution, and 0.6 ml stock
solution of l-cysteine were added sequentially. The mixture was then diluted to 6.0 ml (the
final volume) by adding 4.0 ml ethylene glycol. The mixed solution was then put into a
domestic microwave oven (Glanz, 900 W) for 60 sec (the color of the solution turned from
clear/transparent to brown then dark black as observed after each interval of 10 sec) and the
black colloid was formed. The hot black colloidal dispersion was then removed carefully
from inside the microwave oven with a pair of tongs and cooled quickly in an ice water
bath. As soon as the sample reached room temperature, it was put into a 1 cm quartz cuvet
to record the UV-Vis spectrum. The above scheme was followed repeatedly for preparation
of each sample during optimization of various parameters as well as sample preparation for
different characterization techniques.
3.1.5 Optimization Studies for Formation L-Cysteine Capped Ni NPs
Optimization studies of various parameters were carried out, covering such as
concentration of precursor salt, catalyst, reducing and capping agent, pH of solution,
heating time, temperature of the reaction, and stability of the as-prepared Ni NPs.
3.1.6 Instrumentation Used to Characterize L-Cysteine Capped Ni NPs
UV-Vis spectra of Ni NPs solutions were recorded with a Perkin-Elmer Lambda 2
spectrometer. A Jeol scanning electron microscope (SEM), model JSM 6380A, was used
for imaging of dried drops of the Ni NPs solutions on microscope glass cover slips after
coating with gold layer for 5 minutes in a DC ion sputterer model, JFC-1500. A Bruker D-8
44
Advance powder diffractometer was used to record patterns from crystalline Ni NPs
samples. DSC thermograms were recorded using a Mettler Toledo DSC822. FTIR spectra
were recorded with a Nicolet 5700 FT-IR.
3.1.7 Characterizations of L-Cysteine Capped Ni NPs
A number of techniques were used to characterize l-cysteine capped Ni NPs in
ethylene glycol obtained by the modified hydrazine reduction route. Procedures for
preparation of Ni NPs samples for characterization on different techniques are described
below.
3.1.7.1 Sample Preparation for UV-Vis Spectroscopy
A freshly prepared solution of l-cysteine capped Ni NPs would be put into a
domestic microwave oven (Glanz, 900 W) for 60 sec (the color of solution would turn from
clear/transparent to brown then dark black) to form the black colloid. The hot black
colloidal dispersion would then be removed carefully from inside the microwave oven with
a pair of tongs. It would then be cooled quickly in ice water bath and as soon as the sample
reached room temperature UV-Vis spectra would be recorded. The optical spectral analysis
of these colloids was performed using 1 cm quartz cuvets and a Perkin-Elmer Lambda 2
UV-Vis spectrometer.
3.1.7.2 Sample Preparation for XRD and FTIR
Sample preparation for XRD, FTIR and TG-DSC analysis was carried out as
follows. The black colloidal solution of Ni NPs was put into petri dishes and heated in a
temperature controlled electric water bath set at 100°C to bring about solvent evaporation.
Once the solvent was evaporated, the products were washed with Milli-Q water followed
45
by acetone and methanol for several cycles, to remove any uncapped l-cysteine molecules
and other reagents. These products were then carefully dried in an oven at 100°C for
10 minutes; black particles on the glass were then scraped off with a clean glass slide.
As-prepared samples were collected into a small sample bottle before they were analyzed.
X-ray powder diffraction (XRD) measurements of as-prepared samples of l-cysteine capped
Ni NPs for crystalline phase analysis were performed on a Bruker D8 Advance X-ray
diffractometer employing Cu-Kα radiation (50 mA and 40 kV) over a 2θ range from 20° to
90°. Infrared spectra were recorded using a Nicolet 5700 FT-IR spectrometer.
3.1.7.3 Sample Preparation for SEM
SEM analysis to monitor size and shape of as-prepared cyst-Ni NPs was carried out
by a drop casting method. The sample for SEM was prepared by putting a small quantity of
black colloidal solution of l-cysteine capped Ni NPs prepared in ethylene glycol onto a
cleaned glass cover slip. For solvent removal the cover slip was carefully put onto a
temperature controlled electric hot plate already set at 100°C.
3.1.8 Procedure for Catalytic Reduction of 4-Nitrophenol
The newly synthesized l-cysteine capped Ni NPs were used to catalyze the
reduction of environmental and industrial toxic aromatic nitro compounds. Initially, 0.1 ml
of (1.0x10-6
M) aqueous 4-nitrophenol solution was put into a quartz cuvet. To this, 1.0 ml
of aqueous NaBH4 (1.0x10-3
M) solution was added. The as-prepared sample was then
submitted to a UV-Vis spectrophotometer to record spectra. Optimization of various
parameters, like concentration of 4-nitrophenol solution, concentration of NaBH4, quantity
of NPs composite, pH of solution, and reaction time with and without NPs, was carried out.
The catalytic performance of l-cysteine capped Ni NPs was tested by reduction of
46
4-nitrophenol with an excess quantity of NaBH4 alone and in the presence of newly
prepared Ni NPs. For this, NPs were deposited on glass cover slips by the drop casting
method followed by solvent evaporation on a hotplate at 140°C. Cover slips were weighed
before and after NPs deposition, with the difference between the two values giving the total
mass of NPs deposited on each. To vary NPs quantities, different volumes of sample
solution were placed on the cover slips and the solvent evaporated.
3.1.9 Procedure for Catalytic Reduction of Cr (VI) Ions
In a typical experiment, 0.4 ml of 100 ppm Cr (VI) solution was put into a quartz
cell and diluted to 4.0 ml by adding 3.6 ml of Milli-Q water for recording the UV-Vis
spectra. This solution was prepared to check the absorption profile of Cr (VI) species in the
aqueous medium and showed a prominent absorption peak at 352 nm. Similarly, 4.0 ml of
solution was prepared by taking 10 ppm Cr (VI) ions and 0.1 M NaBH4. The UV-Vis
spectroscopy results showed a red shift in λmax from 352 nm to 372 nm and resulted in a
stable absorption peak, but demonstrating only 6% reduction of Cr (VI) ions after 15
minutes. Another sample with similar concentrations of the reactants was monitored
through UV-Vis spectroscopy in the presence of 0.5 mg of bulk nickel powder as catalyst;
this gave 23% reduction within 15 minutes. Finally, we carried out an experiment for
catalytic reduction of Cr (VI) that proceeded to 99% reduction in a very short time (i.e.
within 5 minutes) by adding 0.5 mg of nanosized nickel particles (as catalysts deposited on
glass cover slips), to a similar sample solution.
47
3.2 Fabrication of L-Methionine Capped Ni NWs
This study involved fabrication of nickel nanowires in an aqueous system by the use
of a strong reducing agent and l-methionine molecules as capping/stabilizing agents
through a new route, as described below. A comprehensive description of the material used
for preparation of Ni NWs and the experimental conditions employed during synthesis are
here described.
3.2.1 Materials and Chemicals Used to Fabricate L-Methionine Capped Ni NWs
Nickel (II) chloride (NiCl2. 6H2O, 97%), hydrazine hydrate (N2H4. H2O, 80%),
sodium hydroxide pellets (NaOH, 99%), sodium borohydride (NaBH4, 98%), isopropyl
alcohol (CH3CH2CHOH, 99%), acetone (CH3COCH3, 97%), and hydrochloric acid
(HCl, 37%) were purchased from E. Merck, and l-methionine (C5H11NO2S) from Fluka
Chemicals. All the chemicals were analytical grade reagents and used as-received without
further purification. The solutions of these reagents, e.g. the precursor metal salt, reducing
agent, and capping agent, were all freshly prepared in Milli-Q water for each experiment
during optimization and characterization studies.
3.2.2 Preparation of Standard Stock Solutions to Fabricate L-Methionine Capped Ni NWs
Stock solutions of 0.01 M nickel (II) chloride, 0.1 M hydrazine monohydrate and
0.01 M l-methionine were prepared by dissolving appropriate amounts of the respective
compounds in Milli-Q water. Solutions of 1 M NaOH and 3M HCl were also prepared to
adjust the pH from 3-11. The stock solutions of 0.2 M sodium borohydride, 1 M isopropyl
alcohol and 1.6 M acetone were similarly prepared by dissolving desired quantities of each
in Milli-Q water.
48
3.2.3 Procedure for Synthesis of L-Methionine Capped Ni NWs
In a typical synthesis procedure, 0.2 ml of 0.01 M Ni solution was placed into a
10 ml volumetric flask and 1.8 ml of 0.1 M hydrazine hydrate solution was added followed
by mixing 1.2 ml of 0.01 M l-methionine and Milli-Q water to dilute up to the mark.
During the course of reduction, the color of sample solution prepared was observed to
change immediately from grass green to sky blue, depending on the molar ratio of Ni,
hydrazine and l-methionine. The as-prepared sample solution was then put forward for
recording the UV-Vis spectra to confirm the fabrication of nickel nanowires. The proposed
mechanism for reduction of Ni2+
to Ni0 with hydrazine is as below.
2Ni2+
+ N2H4 + 4OH- 2Ni + N2↑ + 4H2O
3.2.4 Procedure for Catalytic Test of Ni NWs - Acetone Formation
The dehydrogenation of IPA was conducted by heterogeneous catalysis at normal
laboratory conditions under three different experimental setups, including: (i) varying
concentration of NaBH4 at fixed concentration of IPA in absence of Ni NWs; (ii) varying
concentration of IPA at fixed amount of Ni NWs; and (iii) at fixed concentration of IPA
with varying amounts of Ni NWs and NaBH4. The optimized concentration of IPA
(0.125 M), amount of Ni NWs (0.5 mg) and NaBH4 concentration (0.005 M) were found to
be best for complete catalytic dehydrogenation. In the case of testing the catalytic
performance, the Ni NWs were immobilized by adding a 20 µl sample solution on glass
cover slips and dried under nitrogen. These cover slips were weighed before and after
immobilization of Ni NWs to determine the amount of deposited Ni NWs. These cover
slips were put inside the solution containing appropriate amounts of IPA and NaBH4 in a 4
49
ml capacity quartz cell and the UV-Vis spectra were recorded. Similar studies were
undertaken for other experimental setups. The UV-Vis spectra were also recorded to get a
calibration curve for varying concentration of acetone. The amount of acetone formed in
each catalytic experiment was determined from linear regression of the calibration plot.
3.2.5 Characterization of L-Methionine Capped Ni NWs
3.2.5.1 Instrumentation Used for Characterization of L-Methionine Capped Ni NWs
All UV-Vis spectroscopy studies were conducted using a Perkin Elmer double beam
Lambda 35 spectrophotometer equipped with cells of 1.0 cm path length. FTIR spectral
studies of Ni NWs and l-methionine were performed using a Nicolet 5700 after
incorporating the dried samples into solid state KBr discs. The SEM imaging of Ni NWs
was carried out with a Jeol JSM 6380 analytical scanning electron microscope (ASEM).
3.2.5.2 Sample Preparation for SEM Imaging and FTIR Studies
10 µl volumes of colloidal dispersions of l-methionine encapsulated Ni NWs were
placed on glass cover slips, dried at 100°C on a hot plate and processed for SEM imaging
after carbon coating for 5 minutes. For FTIR studies, such samples were prepared in bulk in
large petri dishes and dried on a hot water bath set to 100°C. The product was washed
several times with Milli-Q water and methanol to remove any unreacted or uncoordinated
amino acid molecules, dried for 15 minutes in an oven at 100°C and cooled to room
temperature. The products were then collected and processed for FT-IR spectroscopy.
50
3.2.5.3 Sample Preparation for XRD Analysis of L-Methionine Capped Ni NWs
Required volumes of colloidal dispersions of l-methionine encapsulated Ni NWs
were placed in glass petri dishes and dried on a hot water bath at 100°C. The products were
washed several times with Milli-Q water and methanol to remove any unreacted or
uncoordinated amino acid molecules, dried for 15 minutes in an oven at 100°C and cooled
to room temperature. The products were then collected and processed for XRD studies.
3.3 Synthesis of Triton X-100 Stabilized Ni NPs
3.3.1 Materials and Chemicals Used For Synthesis of TX-100 Stabilized Ni NPs
Nickel (II) chloride hexahydrate (97%), hydrazine monohydrate (99%),
Triton X-100 (100%, the stabilizing agent), pellets of sodium hydroxide (99%),
hydrochloric acid (37%), and sodium borohydride (98%) were all purchased from
E. Merck. Due to possible oxidation of reagents, fresh solutions of chosen concentrations
were prepared in Milli-Q water for each experiment. All these reagents were analytical
grade and used without additional purification.
3.3.2 Instrumentations Used for Characterizations Studies of TX-100 Stabilized Ni NPs
Optimization studies of various parameters were carried out by recording UV-Vis
spectra with a Shimadzu UV-160 digital spectrophotometer (Kyoto, Japan) and 1 cm quartz
cuvets. A Nicolet 5700 FT-IR spectrophotometer was used to record infrared spectra. A
Jeol model JSM 6380A SEM was used to image the nanostructures.
51
3.3.3 Procedure for Synthesis of Triton X-100 Stabilized Ni NPs
A typical experiment was performed by mixing NiCl2.6H2O (0.5 ml, 0.033 M),
NaOH (0.3 ml, 0.1 M), N2H4.H2O (1.0 ml, 0.2 M), and Triton X-100 (0.5 ml, 0.5 M) at
room temperature, the solution being finally diluted to 10 ml with Milli-Q water. Addition
of reducing agent lead to the appearance of a light blue color after a few minutes as the
reaction proceeded, which became gradually became deep with increasing quantities of
reducing agent.
3.3.4 Catalytic Test for Degradation of Dyes
The investigation of catalytic efficiency of newly synthesized Ni NSs for degradation
of a number of organic dyes has been carried out; fresh samples of Ni NSs were prepared
and deposited on glass cover slips each time. For efficient adhesion of Ni NSs onto glass
surfaces the catalyst deposits were thermally treated on hotplates at 150°C for 10 minutes.
For measurements, the supported catalyst of suitable quantity was placed into a
quartz cell and a cuvet to monitor the course of reaction by in-situ UV-Vis spectroscopy. A
typical catalytic test of Ni NSs for degradation of organic dyes was carried out under
normal laboratory conditions. The organic dyes, such as eosin-b (EB), rose bengal (RB),
ereochrome black-t (ECBT), and methylene blue (MB), were selected as target compounds
to check the catalytic efficiency. All experiments were performed in an aqueous medium
using 20 µM concentration of respective dyes, 0.1 M NaBH4 (reducing agent) and 0.1 mg
quantity of Ni NSs (catalyst).
We observed that in the absence of Ni NSs there was no reduction/degradation of MB
and RB dyes, though EB and ECBT were degraded to small extent. On the other hand, all
dyes were completely degraded in a very short time when treated with NaBH4 in the
52
presence of 0.1 mg Ni NSs. For example, complete degradation of RB dye was seen within
20 sec and EB degraded within 30 sec. Other dyes, such as ECBT and MB, were degraded
within 40 sec. UV-Vis spectra were recorded every 10 sec during the course of reaction.
These Ni NSs were reused five times to confirm the catalytic efficiency of the
fabricated nanocatalysts and we observed the complete reduction/degradation of 20 µM
dyes every time.
3.4 Preparation of L-Threonine Capped Ni NPs
3.4.1 Chemicals and Reagents Used for L-Threonine Capped Ni NPs
Nanoparticle synthesis was carried out using NiCl2.6H2O (97%), purchased from
E. Merck, l-threonine (99%) of Fluka Chemicals, and NaBH4 (98%), NaOH (98%), HCl
(37%), and CR dye (99%) from Sigma-Aldrich, which were all analytical grade and used
without further purification. Milli-Q water was used throughout the experiments.
3.4.2 Preparation of Stock Solutions for L-Threonine Capped Ni NPs
Stock solutions of 0.1 M NiCl2.6H2O, 0.1 M l-threonine amino acid and 0.5 M
NaBH4 were prepared in 100 ml volumetric flasks using the required quantities of each and
Milli-Q water. 1 M NaOH and 1 M HCl were also prepared to adjust the pH (2.6-11.3) of
freshly prepared sample solutions of Ni NPs in aqueous media. CR dye was dissolved in
Milli-Q water to make 1 mM concentration stock solutions and facilitate experiments to
explore the catalytic efficiency of Ni NPs for reduction degradation of dye.
3.4.3 Procedure for Fabrication of L-Threonine Capped Ni NPs
In a typical experiment, a 10 ml sample of Ni NPs in Milli-Q water was prepared by
mixing 0.1 ml of (0.1 M) NiCl2.6H2O and 0.5 ml of (0.5 M) NaBH4 under gentle stirring.
53
The solution turned dark brown in color during addition of the reducing agent. 1.2 ml of
0.05 M l-threonine was added to the solution immediately after the NaBH4 and the pH
value of the solution was adjusted to 8.5 using the required amounts of 1 M NaOH. The
mixture was left for 10-15 minutes to ensure completion of the reduction and encapsulation
of nickel ions to form neutral nanoscale objects. Milli-Q water was used throughout the
experiments and as-prepared samples were observed to be stable for several hours as shown
by UV-Vis spectra.
3.4.4 Nanoparticles Separation by Centrifugation
As-prepared l-threonine capped Ni NPs in aqueous medium were separated by high
speed centrifugation using a Kubota 7780 high-speed refrigerated centrifuge operated at
22,000 rpm. The settled nanoscale products at the bottom of the centrifuge tubes were then
separated by decanting off the solvent and washed several times with Milli-Q water
followed by thorough washing with acetone. These Ni NPs were gained by density gradient
ultracentrifugation from an aqueous medium, re-dispersed in ethanol by sonication on an
ultrasonic device and collected in separate glass bottles; they remained stable for long times
as depicted in the UV-Vis spectroscopy results.
3.4.5 Characterization Studies for L-Threonine Capped Ni NPs
UV-Vis spectroscopy measurements during experiments to fabricate Ni NPs
samples were performed with a double beam Perkin Elmer Lambda 35 spectrophotometer
equipped with a cell of 1.0 cm path length. FTIR spectra of standard l-threonine and that of
Ni NPs capped with l-threonine molecules were recorded using a Nicolet 5700 through KBr
discs of solid state amino acid standard and nano-powders. AFM image analysis was
carried out with a Dimension 3100 microscope plus Nanoscope IV controller (Veeco
54
Instruments). Transmission electron micrographs were taken at 120 kV and magnifications
up to x500k using a Jeol JEM 1200 EX MKI.
3.4.5.1 Sample Preparation for FTIR Studies of L-Threonine Capped Ni NPs
Solid state Ni NPs capped with l-threonine were separated from colloidal
dispersions by putting 500 ml quantities of these products into petri dishes and drying by
making use of a hot water bath set at 1000°C. Further drying took place in an oven for
30 minutes to ensure complete evaporation of residual solvent and water molecules.
As-obtained Ni NPs samples were collected through scratching the dried NPs samples and
processed to prepare KBr discs for FTIR spectroscopy.
3.4.5.2 Sample Preparation for AFM Studies of L-Threonine Capped Ni NPs
10 µl volumes of colloidal l-threonine capped Ni NPs dispersions in ethanol were
placed onto glass cover slips and vacuum dried in a desiccator followed by air drying for
30 minutes to ensure binding of Ni NPs with the glass surface and to allow escape of the
solvent.
3.4.5.3 Sample Preparation for TEM Studies of L-Threonine Capped Ni NPs
Small quantities of colloidal dispersion of l-threonine capped Ni NPs in ethanol
were mounted on carbon coated copper grids (300 mesh, EM Science) by a dip casting
method and vacuum dried in a desiccator for 30 minutes to make sure that the water/ethanol
solvent had been removed.
3.4.6 Catalytic Test for Reduction of CR Dye
The catalytic activity of newly synthesized l-threonine capped Ni NPs was
investigated for reduction degradation of CR dye by using 20 µM concentrations of dye and
55
100 mM NaBH4 in aqueous solution within quartz cells and using UV-Vis spectroscopy.
Nanocatalysts were first deposited on glass cover slips and dried by heating to get fine
attachment of these Ni NPs with the surface of glass. Optimizations of various parameters,
like molar concentration of CR dye, reducing agent (NaBH4), loading quantity of the
catalyst (Ni NPs), reaction time, and yield of reduction at fixed NPs quantity, were carried
out.
56
Chapter 4
RESULTS AND DISCUSSIONS
This chapter describes the details of research investigations carried out for fabrication
of nanosized regime nickel particles/structures, which have been synthesized by a variety of
approaches with total or significant modifications. Here, we also describe the optimization
studies of nickel nanoparticle synthesis, characterization and application. In previous
chapters, we have explained the experimental conditions and materials as well as different
procedures followed to design and fabricate nickel nanoparticles in the laboratory and their
use in different purposes. In addition, here we expand upon detailed descriptions of
experimental set up and optimization conditions as well as the use of various techniques for
characterization of nanoscale nickel particles and their utilization for different
environmental and analytical purposes.
4.1 Results and Discussion for Formation of L-Cysteine Derived Ni NPs in Ethylene Glycol
In this study, we explored the formation of l-cysteine capped Ni NPs by a simple
economical and highly reproducible experimental procedure. The new and facile route is
introduced to synthesize stable black colloidal dispersions of Ni NPs in ethylene glycol
(i.e. solvent and complementary reducing agent). These Ni NPs were utilized in catalytic
reduction reactions.
4.1.1 Characterization of L-Cysteine Derived Ni NPs
A number of analytical techniques, including UV-Vis spectroscopy, FTIR
57
spectrometry, and TEM, were used to characterize l-cysteine derived Ni NPs. The
extent of initial fabrication of NPs was estimated from the UV-Vis spectral analysis,
while the surface binding interactions of the l-cysteine molecules with nanoparticles
were analyzed with FTIR analysis. Sizes and shapes of the as-prepared Ni NPs,
along with size distributions, were monitored by TEM imaging.
4.1.1.1 UV-Vis Spectroscopy for L-Cysteine Derived Ni NPs
Various parameters, such as concentration of nickel chloride, l-cysteine and NaOH
solution, pH, and heating time of the reaction mixture were studied and optimized. The
most stable and small sized (blue shifted) Ni NPs were synthesized by taking appropriate
concentrations of the various salt solutions. UV-Vis spectra representing the Ni NPs in
fresh, one week old and two weeks old samples under optimized conditions are shown in
Figure 4.1.1 below.
58
300 350 400 450 500 5500.0
0.2
0.4
0.6
0.8
1.0
32
1
Ab
s (
a.u
.)
Wavelength (nm)
a
300 350 400 450 500 5500.0
0.2
0.4
0.6
0.8
1.0
3
21
Ab
s (
a.u
.)
Wavelength (nm)
b
Figure 4.1.1 UV-Vis spectra of l-cysteine derived Ni NPs, (a) increasing Ni (II) ions /
l-cysteine ratio as (1) 1:2, (2) 1:3 and (3) 1:6, (b) Ni NPs as (1) fresh solution, (2) solution
after one week and (3) after two weeks.
No change in wavelength and absorption reveals the high stability of as synthesized
Ni NPs. There was no change in color and wavelength even after several weeks. The
spectra show broader absorption bands due to some contribution from the relatively few
larger spherical nanoparticles, as reported previously (Zhong et al., 2005). UV-Vis spectra
of Ni NPs prepared by taking different Ni(II) to l-cysteine molar ratio, such as 1:2, 1:3 and
59
1:6 showed small increases in absorbance and a change in λmax from a red shifted value of
392 nm to blue shifted 386 nm with increasing l-cysteine. So an increase in concentration
of the biomolecule (l-cysteine) as capping agent increases the formation of metallic Ni NPs
along with reduction in size, as has previously been observed in the case of gold NPs
(Zhong et al., (2005). This also has been observed previously and explained by the concept
of amino acid molecules preventing the formation of oxide or hydroxides formation
(Kalwar et al., 2011).
4.1.1.2 TEM Analysis of L-Cysteine Derived Ni NPs
TEM images of sample Ni NPs prepared with the 1:6 molar ratio of Ni to l-cysteine
are shown in Figures 4.1.2(a-b).
Figure 4.1.2 TEM images of freshly formed l-cysteine Ni NPs by microwave irradiation in
ethylene glycol, a) low resolution, scale bar = 100nm, b) high resolution, scale bar = 50nm.
These figures reveal that most Ni NPs particles are spherical and smaller than 10
nm size. However there is a minor contribution from larger Ni NPs as well. Observations of
some of the larger Ni NPs reveal that they have been formed by aggregation of smaller Ni
NPs. Such aggregation results from intra molecular interactions and peptide bond formation
60
between l-cysteine molecules bound to the surface of Ni NPs. In depth observation reveals
that the smaller and larger Ni NPs are in general spherical and well distributed. The mean
size of the Ni NPs was estimated at 7.5 nm, with an overall range of 2-31 nm, while the
most abundant particles were 4 nm in size. These rough surfaced Ni nanospheres are
responsible for excellent catalytic activity.
4.1.1.3 FTIR Spectroscopy of L-Cysteine Derived Ni NPs
FTIR spectra of l-cysteine and l-cysteine derived Ni NPs are shown in Figure 4.1.3,
which clearly depict the characteristic signals for structure elucidation and for the
characterizations of surface binding interaction of amino acid molecules with Ni NPs.
3500 3000 2500 2000 1500 1000 5000.0
0.5
1.0
1.5
2.0
b
Ab
s (
a.u
)
Wavenumber (cm-1)
2580
2110
a
Figure 4.1.3 FTIR spectra of (a) pure l-cysteine and (b) newly synthesized Ni NPs.
The absorption band at 2551 cm-1
(in the range, 2600–2550 cm-1
) is characteristic
for S-H stretching (Zhong et al., 2005).In the light of discussion we propose a schematic
diagram for the formation of Ni nanospheres, as illustrated in Figure 4.1.3.
61
Figure 4.1.4 Schematic diagram for the synthesis of spherical Ni NPs.
4.1.2 Application of Ni NPs as Catalyst for Reduction of 4-Nitrophenol
Sharma et al., (2007) used 4-nitrophenol (4-NPh) as a model compound to examine
the role of NPs as catalysts for the reduction of aromatic nitro compounds to their
corresponding amine derivative in the presence of NaBH4. We tested Ni NPs similarly; the
results are illustrated in Figure 4.1.5 which presents various spectra related to the reduction
of 4-NPh to 4- aminophenol (4-APh).
Figure 4.1.5 represents the formation of 4-nitrophenolate ions produced by treating
the solution of 4-nitrophenol with NaBH4. Schematically, 4-NPh is converted to
4-nitrophenolate with the removal of a proton in the presence of NaBH4 and UV light. The
peaks at 317 nm and 400 nm are characteristic absorption signals of the 4-NPh molecule
and 4-nitrophenolate ions in aqueous media in the absence (317 nm) and presence (400 nm)
of NaBH4 respectively (Sharma et al., 2007).
62
200 300 400 500 6000.0
0.5
1.0
1.5
2.0
1
2
Ab
s (
a.u
.)
Wavelength (nm)
Figure 4.1.5 UV-Vis spectra of (1) 10 µM aqueous solution of 4-NPh and (2) 10 µM 4-
NPh in the presence of 0.15 M NaBH4
200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
NiNPs
4
3
2
1
Ab
s (
a.u
.)
Wavelength (nm)
Figure 4.1.6 UV-Vis spectra (1), of aqueous solution of 10 µM 4-NPh; (2), 10 µM 4-NPh
with 0.15 M NaBH4 (3), reduction of 10 µM 4-NPh with 0.1 mg and (4) reduction of 10
µM 4-NPh with 0.2 mg Ni NPs.
The 4-nitrophenolate ions are rapidly converted into 4-APh in the presence of Ni
NPs as a result of rapid hydrogen transfer from NaBH4, generating a peak at 300 nm.
Figure 4.1.6 illustrates the reduction of 4-nitrophenolate ions to 4-APh using Ni NPs in the
presence of NaBH4.
63
200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
4
3
2
Ni (powder)
1
Ab
s (
a.u
.)
Wavelength (nm)
Figure 4.1.7 UV-Vis spectra (1), of 10 µM aqueous solution of 4-NPh; (2), 10 µM 4-NPh
with 0.15 M NaBH4 and (3, 4), reduction of 10 µM 4-NPh with 0.1 mg and 0.2 mg Ni
powder respectively.
Each spectrum was recorded after 40 sec at an scan rate, 960 nm min-1
in all cases
of catalytic reduction of 4-NPh. Figure 4.1.7 represents the reduction of 4-nitrophenol by
using Ni (powder) in the presence of the same concentration of NaBH4 and in similar times
(40 sec) as for the Ni NPs which is presented in Figure 4.1.6. Mandlimath et al., (2011)
reported complete reduction of 4-NPh in 1080 sec by NiO. Comparison between spectra
reveals that the reduction of 4-NPh by 0.1 mg and 0.2 mg Ni NPs is 67.5% and 100%
respectively, while the same amounts of bulk nickel gives 9.6% and 25.5% reduction
respectively, of 4-NPh to 4-APh. This study thus clearly differentiates the two types of
catalysts based on variation in size. Rostovshchikova et al., (2005) described studies that
showed how l-cysteine capped Ni NPs play the role of highly active catalysts for the
reduction of 4-NPh to 4-APh. It has also been reported that the nature of support and
density of NPs are also crucial for catalytic behavior, as observed in case of Cu, Ni and Pd
NPs (Rostovshchikova et al., 2005).
64
4.1.3 Reuse of Regenerated Ni NPs as Catalyst
Figure 4.1.8 shows the % conversion of 4-NPh to 4-APh by freshly prepared and 4-
times regenerated Ni NPs used as catalyst. The results show excellent catalytic performance
by the regenerated and reused Ni NPs and elaborates that the process is highly economized
due to high efficiency of the catalyst with negligible poisoning by the solution
environment.
0 1 2 3 4 5 60
20
40
60
80
100
99.4 98.199.1 98.5
% C
on
ve
rsio
n
No. of Cycles
100%
Figure 4.1.8 Histograms showing the % conversion of 10 µM of 4-NPh in the presence of
0.15 M NaBH4 solutions by fresh and four times regenerated and reused Ni NPs.
Furthermore the recovery and reuse of catalyst provides greener synthesis of 4-APh
(with no intermediate formation) for the purpose of its industrial utilization in the
manufacture of analgesics, antipyretics, etc. and hence its subsequent removal from the
aquatic environment.
65
4.1.4 Catalytic Activity of Ni NPs for Reduction of Cr (VI) ions
The investigation for catalytic behavior of l-cysteine derived Ni NPs was monitored
by UV-Vis spectroscopy. During this method potassium dichromate (K2Cr2O7) was used as
a model compound to check the efficiency of nanaocatalysts and sodium borohydride as the
reducing agent. In this connection a number of parameters were studied like concentration
of potassium dichromate, concentration of sodium borohydride, quantity of Ni NPs, and
reaction time. In a typical experiment a 10 ppm Cr(VI) ion solution was treated with 0.1 M
NaBH4 in the presence and absence of Ni powder and Ni NPs in quartz cells and spectra
recorded in the range of 300-500 nm. The results are presented in Figure 4.1.9.
300 350 400 450 5000.0
0.4
0.8
1.2
1.6
1
Ab
s (
a.u
.)
Wavelength (nm)
1- Without NaBH4
2- With NaBH4 fresh
3- With NaBH4 15 minutes
Figure 4.1.9 Reduction of 10 ppm Cr(VI) ions in water, 1, without addition of NaBH4 a
peak at 352 nm, 2, after addition of 0.1 M NaBH4 (fresh solution) and 3, same as 2 but after
15 minutes.
This shows a broad and clear absorption peak for Cr(VI) ions in an aqueous
medium with a characteristic λmax value of 352 nm, which on addition of NaBH4 shift to
longer wavelength, with the enhanced absorption peak at λmax = 372 nm, this change in
absorption profile was observed after addition of NaBH4. We observed that the pH of the
solution was 6.9 before and 9.7 after addition of NaBH4; this pH change caused a change in
66
the absorption maximum for the Cr(VI) moieties as, shown in Figure 4.1.9. Further, we
observed only a very small decrease in the absorbance at 372 nm, due to the reduction of
Cr(VI) ions produced by NaBH4 alone in the aqueous medium without the addition of any
catalyst. Catalytic reduction of Cr(VI) to Cr(III) is occurred through excited electrons
initiated by hydride ions in presence of UV-Vis irradiation. Catalytic reduction of Cr(VI) is
illustrated in the equations (4) and (5) below. The results for nanoparticle-catalyzed
reduction of Cr(VI) with strong reducing agent (i.e. NaBH4) under UV-Vis irradiation
presented in Figure 4.1.9 would be expected to follow the same reaction pathway.
Cr2O72-
+ 14H+ + 6e
-• 2Cr
3+ + 7H2O ---------- (4)
2H2O + 4h+ ---- O2 + 4H
+ ------------------------ (5)
The time profiles demonstrated in Figure 4.1.11 for remediation of Cr(VI) ions from
an aqueous solution catalyzed by l-cysteine derived Ni NPs present a clear picture of high
catalytic activity of the nanosized nickel catalysts as compared to that of reduction
performed without catalyst and in the presence of metallic nickel powder, Figure 4.1.10.
The UV-Vis spectra show marked differences in catalytic efficiencies between the metallic
nickel powder and nanocatalyst. Specific surface area, crystallinity degree, and crystalline
phases have generally been believed to be the factors important for controlling activity of
catalysts (Kera et al., 2003). The nanocatalysts possess large specific surface area which
implies high adsorption capacities for the analytes (Fang et al., 2006). These are, in
principle, responsible for the rapid and efficient catalytic reduction of Cr(VI) in the present
study.
67
300 350 400 450 5000.0
0.4
0.8
1.2
1.6
Ab
s (
a.u
.)
Wavelength (nm)
Fresh
05 minutes
10 minutes
15 minutes
Figure 4.1.10 Reduction of 10 ppm Cr(VI) ions in water with 0.1M NaBH4 using 0.5 mg
Ni powder, with spectral changes recorded after 5 min interval up to total time of 15
minutes.
300 350 400 450 500
0.0
0.4
0.8
1.2
1.6
Ab
s (
a.u
.)
Wavelength (nm)
Fresh
60 sec
120 sec
180 sec
240 sec
300 sec
Figure 4.1.11 Reduction of 10 ppm Cr(VI) ions in water using Ni NPs with spectral
changes recorded after 60 sec showing complete reduction of Cr(VI) ions in a very short
time.
There was only a 6% reduction of Cr(VI) ions (in the absence of any catalyst) after
15 minutes, as calculated from UV-Vis spectra absorbance results of reduction of Cr(VI)
ions after addition of NaBH4 alone. We also performed experiments to reduce Cr(VI) ions
with NaBH4 in the presence of Ni powder as catalyst, which achieved 23% reduction within
68
15 minutes, as shown in Figure 4.1.10. Similar experiments were also carried out to check
the catalytic activity of newly synthesized l-cysteine capped Ni NPs. For this we used the
same concentrations of the model reagent and reducing agent, just with nickel nano catalyst
instead of nickel powder, which achieved 99% reduction of Cr(VI) ions in a very short
time, as shown in Figure 4.1.11. A schematic for reduction of Cr(VI) is presented below.
Scheme 2.
In this case a maximum reduction of 99% was recorded for Cr (VI) ions. These
results display a clear picture of the effects of the difference in size between Ni powder and
Ni NPs and how that influences their use as reduction catalysts. The reduction of Cr (VI)
has not previously been studied by using Ni NPs but Pd NPs (Nevskaya et al., 2005) and
Fe2O3 stabilized Fe0 NPs (Omole et al., 2007) have been used as efficient catalysts for this
purpose. The present findings show that 99% reduction of Cr(VI) species in 5 minutes
using only 0.2 mg Ni NPs as catalysts. Whereas the reports already published did not show
such prompt reduction to that extent.
69
4.2 Results and Discussion for Formation of L-Methionine Capped Ni NWs
This study demonstrates the synthesis of Ni NWs via a simple seed mediated
approach. Highly stable l-methionine capped Ni NWs were fabricated in an aqueous
medium using nickel chloride salt and hydrazine monohydrate as the reducing agent.
4.2.1 Characterization of L-Methionine Capped Ni NWs
Since the aim of our research was to explore the application of newly synthesized
nanoscale nickel structures encapsulated with l-methionine molecules, the results obtained
from the catalytic transfer hydrogenation of isopropyl alcohol (IPA) to acetone
demonstrated that nanoscale nickel structures produced through chemical reduction
methods proved highly efficient catalysts for acetone formation.
The method introduced exploits the ease of fabrication of nickel nanowires
(Ni NWs), their catalytic efficiency, tolerance to the environmental toxicants, and total
transfer hydrogenation of IPA to acetone. Here we describe in detail the experimental
conditions, optimization of different parameters during synthesis of Ni NWs and unique
means for preparation of acetone.
4.2.1.1 UV-Vis Spectroscopy of L-Methionine Capped Ni NWs
According to Li et al., (2006), a UV-Vis absorption band around 300-350 nm
results from Ni NPs solutiona and is attributed to the surface plasmon resonance (SPR)
band of Ni0 particles as shown in Figures 4.2.1 to 4.2.3. An absorption peak at slightly
longer wavelength (i.e. in the range 360-400 nm) has been observed in our experiments that
is due to formation of Ni NWs with varying sizes.
70
300 350 400 450 500
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Untreated
0.1 ml Ni ions sol
0.1 ml Ni ions sol
0.2 ml Ni ions solA
bs
(a
.u.)
Wavelength (nm)
0.3 ml Ni ions sol
Treated with reducing agent
Figure 4.2.1 UV-Vis spectra recorded for varying concentration of Ni2+
ions in the
presence of 0.018 M hydrazine (0.2 ml, 0.3 ml and 0.4 ml of 0.01 M) left side peaks from
below to above and untreated sample of Ni solution on right.
UV-Vis spectrometric studies were carried out to record absorption profiles resulted
from different quantities of Ni (II) ions (0.2-0.4 ml of 0.01M) in the sample solution, as
shown in Figure 4.2.1. A further optimization study for the concentration of hydrazine
hydrate (0.6-1.8 ml of 0.1 M) solution is shown in Figure 4.2.2 and the effects of
concentration of l-methionine - the capping agent (0.4-1.2 ml of 0.01 M) solution - are
shown in Figure 4.2.3.
The UV-Vis spectra identified that 0.2 ml of 0.01 M Ni solution, 1.8 ml of 0.1 M
hydrazine hydrate and 1.2 ml of 0.01 M l-methionine were optimum volumes for synthesis
of Ni NWs, as mentioned in the experimental given in chapter 3 section 3.2.3. A blue shift
in λmax and gradual increase in absorbance resulted from increase of hydrazine hydrate and
l-methionine concentrations; an increase in absorbance with increasing concentration of Ni
(II) ions was observed, as shown in Figure 4.2.1.
71
300 350 400 450 500
0.02
0.04
0.06
0.08
1.8 ml hydarzine
1.2 ml hydarzine
Ab
s (
a.u
.)
Wavelength (nm)
0.6 ml hydarzine
Varriation of reducing agent
Figure 4.2.2 UV-Vis spectra for varying concentration of hydrazine monohydrate (0.6 ml,
1.2 ml and 1.8 ml of 0.1 M) with increasing absorbance and shift in λmax from right to left.
300 350 400 450 500
0.02
0.04
0.06
0.08
0.4 ml
1.2 ml
Ab
s (
a.u
.)
Wavelength (nm)
0.8 ml
Varriation of capping agent
Figure 4.2.3 UV-Vis spectra recorded for varying concentration of l-methionine (0.4 ml,
0.8 ml and 1.2 ml of 0.01 M) increasing absorbance and again shift in λmax from right to
left.
72
We also observed that increasing the concentration of reducing agent (0.6-1.8 ml of
0.1 M) resulted in a shift in λmax from 380 nm to the lower wavelength of 369 nm, as shown
in Figure 4.2.2, which is believed to be evidence in favor of the formation of smaller
nanostructures.
300 350 400 450 500
0.00
0.03
0.06
0.09
0.12A
bs
(a
.u.)
Wavelength (nm)
(pH 3)
(pH 4)
(pH 5)
(pH 6)
(pH 7)
(pH 8)
(pH 9)
(pH 10)
(pH 11)
a
300 350 400 450 500
0.00
0.03
0.06
0.09
0.12
Ab
s (
a.u
.)
Wavelength (nm)
(a)
(b)
(c)
(d)
(e)
(f )
(g)
(h)
b
Figure 4.2.4 UV-Vis spectra recorded for (a) pH study with variation of pH 3-11 from right
to left with shift in λmax from 400 nm to 360 nm and increase in absorbance and (b) time
study of l-meth-Ni NWs samples showing stable absorption band at 361 nm after 1 hour
until many days.
73
It is interesting to note that a further blue shift in wavelength was generated from
369 nm to 361 nm as shown in Figure 4.2.3, when different concentrations (0.4-1.2 ml of
0.01 M) of l-methionine were used to analyze the effect of capping agent on the fabrication
of Ni NWs with controlled size. UV-Vis spectra were also recorded to study the effects of
change in pH in the range of 3-11, as presented in Figure 4.2.4(a). The effect of time on
fabrication of Ni NWs and their stability in the aqueous medium (showing gradual increase
in absorbance at 361nm) is depicted in Figure 4.2.4(b). The UV-Vis spectra showed
prominent peaks in the wavelength range of 360 nm to 400 nm obtained from samples with
pH range 3-11, as shown in Figure 4.2.4(a). The spectra recorded from neutral to basic pH
7-11 samples absorbed at lower wavelength, in the range 360-370 nm, while entirely
different behavior was observed in those of highly acidic pH samples, which showed peaks
near 400 nm wavelength. UV-Vis spectra were also recorded to examine the stability of
Ni NWs in solution and also to observe the effect of time on growth of nanostructures.
Analysis started with the fresh solution, after regular interval of 10 minutes each for a total
of two hours, then for a further three hours in 1 hour steps, and on to a total of six days
even; as elaborated in Figure 4.2.4(b). The spectra show that formation of Ni NWs was
complete after one hour and also that these nanowires were stable for many days in the
solution.
Similar results were obtained by repetition of the experiment for synthesis of
Ni NWs at pH 9, confirming the analogous growth pattern of nanostructures. The broad and
clear SPR band at a λmax = 361 nm was selected after optimization studies of all the
above-mentioned parameters to a characteristic indicator of the presence of elongated
Ni NWs. UV-Vis spectroscopy provided information about functionalizations of Ni NWs
with l-methionine, accomplished by electrostatic interaction via thio and amino groups; the
74
carboxyl group does not approach the surface of Ni NWs, possibly due to the dielectric
medium. These nanowires interact via transient forces influenced by electron lone pairs of
the sulfur and nitrogen atoms present in l-methionine molecules, as evidenced in the FTIR
spectral studies described in the following section.
4.2.1.2 FTIR Spectroscopy of L-Methionine Capped Ni NWs
FTIR spectroscopy allows interpretation of the surface binding of l-methionine to
the nanostructures. Explorations of structural features of amino acids (i.e. building blocks
of protein) are facilitated by the observation of the more common and specific bands, as
shown in Figure 4.2.2.
3500 3000 2500 2000 1500 1000 5000.0
0.5
1.0
1.5
2.0
b
Ab
s (
a.u
.)
Wavenumber (cm-1)
2580
2110
a
Figure 4.2.5 FTIR spectra of (a) standard l-methionine and (b) Ni NWs functionalized with
l-methionine.
A prominent and broad band of medium intensity in the range 3400-3000 cm-1
has
elsewhere been assigned to NH stretching (Ramachandran, et al., 2006). However, NH
stretching has also been reported at 3431cm-1
(Lee, et al., 2007). This is in accordance with
75
our work where the same band was seen at 3430 cm-1
. Another band at 3211 cm-1
in the
FTIR spectra, though seen at 3280 cm-1
in our results, is due to mixed overlap of OH and
NH stretching vibrations (Lee et al., 2007), as depicted in Figure 4.2.5(b). A less intense
and slightly broader band at 2580 cm-1
was present in l-methionine. Immediate
disappearance of this band in the FTIR spectra of Ni NWs corresponds to the transition
moment, indicating disposition of functional moieties in the molecule. The diagnostic IR
band at 2094 cm-1
for CO stretching has been reported (Ramachandran et al., 2006), it was
appeared 2110 cm-1
in the present findings. This change in position of FTIR spectral bands
is due to difference in metal legand interactions.
The variation in IR-spectral bands induced by l-methionine encapsulation over the
surfaces of Ni NWs can be compared with the literature, for instance with reports on
ZnS:Mn nanocrystals capped with amino acids, where strong peaks around 2950 cm-1
and
1600 cm-1
were assigned to Zn coordinated by the amino group (Lee et al., 2007). In our
work, similar spectral bands have been seen at 2920 cm-1
and 1580 cm-1
in both
l-methionine and l-methionine capped Ni NWs, as shown in Figure 4.2.5(a-b). This finding
supports the evidence for surface binding of newly fabricated Ni NWs with l-methionine
via -NH2 and not via the -COO group. The evidence is further strengthened by the presence
of a clear and well resolved IR band at 2580 cm-1
resulting from l-methionine powder,
which appeared at slightly longer wavelength, i.e. 2621 cm-1
, in a report by Ramachandran
et al., (2006). This is attributed to the existence of intramolecular hydrogen bonding at
carboxyl and amino groups as (N─H…..
O); this prominent band was completely diminished
from the spectra of Ni NWs as a result of surface binding of l-methionine via amino
linkage. Another interesting fact of important concern is that the presence of this band in
l-methionine spectra also proves the existence of a zwiterionic system in the amino acid;
76
whereas the same band completely disappearing in Ni NWs reveals that the zwiterionic
system does not exist after encapsulation of the newly fabricated Ni NWs with amino acid
molecules.
On the other hand, many vibrational modes resulting from side chains of
l-methionine remained unaltered, for instance bands around 1587 cm-1
and 1415 cm-1
arising from the presence of the COO- group, as reported by Koleva et al., (2007). The
same bands were also observed by others at 1581 cm-1
and 1414 cm-1
(Ramachandran,
et al., 2006). In our work, these bands appeared at 1580 cm-1
and 1410 cm-1
from both
l-methionine and Ni NWs without any change or further shift, strong evidence of free
carboxyl groups in both l-methionine and Ni NWs samples.
Another diagnostic IR-spectral band at 1320 ± 20 cm-1
explores typically the
presence of the SCH3 fragment in the molecule (Lee et al., 2007; Koleva et al., 2007).
Previously, IR-spectral studies of l-methionine and selenomethionine have been carried out
which also demonstrated that only few alterations in peak positions occurred involving the
binding interactions of l-methionine with selenium (Roeges et al., 1993). They reported that
a bending vibration at 1345 cm-1
due to S-C-H has been observed at the start of substitution
with selenium which has to be consistent in the range 1311-1318 cm-1
, resulting from
possible up shift of 36 cm-1
because of surface binding of l-methionine via S-CH3 linkage
(Roeges et al., 1993). In our studies, this band appeared at 1350 cm-1
in both l-methionine
and Ni NWs.
The present studies showed l-methionine functionalized Ni NWs were strongly
capped with l-methionine via thionyl and amino linkages and the COO- group was utilized
by Na+ ions to form the sodium salt of l-methionine (i.e. due to basic pH of the medium) on
the surfaces of Ni NWs. FTIR spectroscopy studies confirmed significant structural
77
changes in the spectral frequencies of pure l-methionine powder and Ni NWs encapsulated
with amino acid molecules. These results obtained experimentally from IR analysis
corresponding to the origin of different functional groups and their characteristic IR
spectral frequencies revealed successful fabrication of l-methionine capped Ni NWs.
4.2.1.3 SEM Analysis of L-Methionine Capped Ni NWs
This study investigated the changes in size and morphological properties of newly
synthesized l-methionine capped Ni NWs. SEM images were recorded for different
Ni/hydrazine/l-methionine molar ratio to monitor the effects on morphology and size of
nanowires; Ni NWs with different size and morphology were fabricated from different
molar ratios of Ni/l-methionine, as shown in Figure 4.2.6(a-c), corresponding to 1:2, 1:4
and 1:6 respectively.
The brighter areas observed in the SEM images illustrate the presence of newly
formed Ni NWs on surface of the glass cover slip. Small sized Ni NWs resulted from
higher molar ratios of Ni/l-methionine, as evidenced by the SEM images shown in
Figure 4.2.6(c). The NWs in each case are double strands extended in parallel with closed
ends in most cases. The wires show branched perturbations as well. The most common and
widely accepted factors for size control are the concentrations of the precursor salt and
capping agent (Janardhanan et al., 2009). Relatively large Ni NWs, with mean length of
9.8 µm and width of 70 nm for single strands, resulted from a 1:2 ratio of Ni/methionine as
shown in Figure 4.2.4 (a). This suggests that the Ni(II) ions reduced to Ni0 particles; these
then coangulate and form bigger aggregates. Figure 4.2.4(b) confirms that relatively small
nanowires resulted from the encapsulation of Ni0
particles with l-methionine molecules
using a 1:4 ratio of Ni/l-methionine. The average length and width in this case are 3.6 µm
78
and 85 nm respectively. The Ni NWs with mean length of 6.8 µm and width of 55 nm were
obtained from a 1:6 ratio of the Ni/l-methionine, as shown in Figure 4.2.4(c).
Figure 4.2.6 High resolution SEM images obtained from formation of Ni NWs with (a) 1:2
(b) 1:4 and (c) 1:6 Ni/l-methionine molar ratios.
This study reveals that the concentration of precursor salt and capping material are
responsible for size and morphology of nanostructure materials which is in good agreement
with work carried out by Mallat et al., (2004).
4.2.1.4 XRD Analysis of L-Methionine Capped Ni NWs
XRD analysis of the l-methionine capped Ni NWs confirmed the metallic and
crystalline nature and supported estimation of the sizes (widths) of these Ni NWs. The
results presented in Figure 4.2.7 show XRD diffraction patterns of the nanoscale nickel
powders obtained from 1:2, 1:4, and 1:6 molar ratios of Ni/l-methionine. The results
79
illustrate that pure Ni face centered cubic (fcc) structures with (1 1 1) and (2 0 0) planes
were formed in the case of Ni NWs samples obtained from 1:2 and 1:4 molar ratio of
Ni/l-methionine (Figure 4.2.7(a and b)), whereas only (1 1 1) planes were formed in a
sample obtained from a 1:6 molar ratio of Ni/l-methionine (Figure 4.2.7(c)).
Figure 4.2.7 XRD patterns recorded from Ni NWs with varying molar ratios of (a) 1:2,
(b) 1:4 and (c) 1:6 of Ni/l-methionine.
However, the possibility of formation of Ni(OH)2 and NiO, as given in Fig. 4(a and
b), cannot be completely ruled out [7,23]. It is clearly seen in Figure 4.2.7(c) (products
formed with 1:6 molar ratio) that weak peaks due to Ni(OH)2 or NiO were absent. That
indicates that the addition of more l-methionine molecules (capping agent) protects Ni
NWs from oxidation.
These results are in good agreement with those cited in the literature (Liu, et al.,
2009; Wang, et al., 2010).The Scherrer formula was used to determine the sizes of the
80
nanoscale nickel objects (Sirajuddin, et al., 2010).
L = K λ / β cosθ
Where K is a constant (0.94), λ has the value of X-ray wavelength (CuKα =
1.54184Ǻ), β represents the true half-peak width, and θ is the half diffraction angle of the
centroid of the peak in degrees. The average size (widths) of Ni NWs calculated by the
Scherrer formula for the samples of Ni NWs fabricated with 1:2, 1:4 and 1:6 molar ratio of
Ni/l-methionine were 64 nm, 85 nm and 51 nm respectively. These sizes are very close to
those results arising from SEM image analysis, which further confirmed the validity of the
XRD results.
4.2.2 Application of L-Methionine Capped Ni NWs as Catalyst
Catalytic transfer hydrogenation reactions are highly important and advantageous
for many reasons, particularly for large scale synthesis of many chemical compounds
(Mallat et al., 2004). Studies have been carried out for conversion of IPA to acetone in the
presence of oxidizing agents like di-oxygen which help to remove two hydrogen atoms, one
from the -OH group and the other from the secondary carbon atom (C-H) of the IPA
molecules using Pt catalyst (Nicolettilb et al., 1989).
We used NaBH4 as a hydrogen-rich source with and without l-methionine capped
Ni NWs to release molecular hydrogen on the surface of highly active sites of newly
synthesized Ni NWs. UV-Vis spectra obtained from experiments to thoroughly investigate
the parameters which influence rates of catalytic transfer hydrogenation reactions are
depicted in Figure 4.2.8.
81
Figure 4.2.8 UV-Vis spectra for catalytic oxidation of IPA to acetone (a) varying concentration of
NaBH4 in the range 0.001-0.01 M at fixed concentration of IPA (0.125 M) in the absence of Ni
NWs, (b) varying concentration of IPA in the range 0.025-0.125 M at fixed amount of Ni NWs (0.1
mg) and 0.005 M NaBH4, (c) at fixed concentration of 0.125 M IPA with varying amounts 0.1-0.5
mg Ni NWs and 0.005 M NaBH4 and (d) calibration curve for 0.01-0.12 M concentration of
acetone.
In our study, controlled experiments were conducted to determine the effect of
NaBH4 concentration on catalytic transfer hydrogenation of IPA. No change was observed
by the addition of different concentrations of NaBH4 to constant amounts of IPA for its
conversion to acetone, as shown in Figure 4.2.8(a). The spectra were recorded periodically
for 20 minutes but there was no conversion of IPA to acetone. The UV-Vis spectra show a
characteristic peak for acetone at 265 nm, as reported elsewhere Li, et al., (2006). We
82
observed a similar absorption band at ~255 nm when the reaction was observed after the
addition of different amounts of IPA (in the range 0.025-0.125 M) followed by the addition
of identical amounts of Ni NWs (0.1 mg) and fixed concentration of NaBH4 (0.005 M), as
illustrated in Figure 4.2.8(b). A plot of IPA concentration against absorbance is given in the
inset of Figure 4.2.8(b). An increase in absorbance of acetone formation (at 255 nm) with
increase in IPA concentration after addition of fixed amount of Ni NWs in the presence of
(0.005 M) NaBH4 elaborates the catalytic performance of Ni NWs.
The use of a small concentration of NaBH4 (0.005 M) is necessary to facilitate the
transformation as it provide BH4- ions into the aqueous medium, which help to remove the
hydrogen from the OH group of IPA molecules to carry out the catalytic transfer
hydrogenation reaction. Consequently, the rate of transfer hydrogenation (i.e. oxidation in
this case) of IPA to acetone is regarded as independent of NaBH4 concentration. Progress of
the reaction for fixed amounts of IPA was also monitored using different quantities of Ni
NWs (0.1-0.5 mg) immobilized on glass cover slips.
The UV-Vis spectroscopy results showed an increase in absorption of acetone with
increasing amount of Ni NWs, as shown in Figure 4.2.8(c); the inset of Figure 4.2.8(c)
shows of the mass of Ni NWs versus absorbance, showing the extent of acetone formation.
This demonstrates that quantity of Ni NWs favored to enhance the extent of reaction by
providing large surface areas available for the hydrogen transfer reaction i.e. formation of
acetone from isopropyl alcohol. The reaction was completed within 60 sec at 300 K under
UV-Vis irradiation.
The proposed step wise mechanism for the above reaction is demonstrated below,
related to the similar reaction carried out in presence of supported Cu catalysts (Han et al.,
2001).
83
(CH3)2CHOH + Ni NWs → (CH3)2CHO–Ni NWs + H–Ni NWs (1)
(CH3)2CHOH–Ni NWs + NiNWs → (CH3)2CO–Ni NWs + H–Ni (2)
(CH3)2CO–Ni NWs → (CH3)2CO + Ni NWs (3)
2H–Ni NWs → H2 + Ni NWs (4)
The reproducibility of the preparation of l-methionine capped Ni NWs immobilized
on glass and activity of these nanowires was investigated with another batch of such
reactants and the rate was found to be in good agreement with that first observed. The
Ni NWs in the reaction mixture led to the oxidation of IPA because they presented large
surface areas hence the use of Ni NWs facilitated the catalytic transfer hydrogenation.
UV-Vis spectra representing calibration of varying concentrations of acetone were
also recorded to ascertain the extent of dehydrogenation reaction, as shown in
Figure 4.2.8(d). Comparing with the calibration plot we observed no residual concentration
of IPA when using only 0.5 mg Ni NWs against 0.125 M IPA in a 4 ml volume of reaction
mixture as similar absorbance was seen as for 0.12 M acetone.
In constrast to this, UV-Vis spectroscopy observations showed that IPA was not
converted to acetone merely on treatment with NaBH4 or by the use of Ni NWs alone.
Conversely, we have shown how the addition of only a very small amount Ni NWs is
responsible for significant oxidation of IPA to acetone by the transfer hydrogenation
mechanism. Improved results were obtained by optimization of various parameters and it is
premeditated that the ease of fabrication and the catalytic performance of our synthesized
nanowires may find highly important applications in the future.
84
4.3 Results and Discussions for TX-100 Derived Ni NSs
Formations of mixed nickel nanostructures (Ni NSs), sheets/foils, cubes, and
spheres with controllable morphologies have been created using a nonionic surfactant
(Triton X-100) as the stabilizing agent under normal laboratory conditions in an aqueous
solution.
4.3.1 Characterization Studies of TX-100 Derived Ni NSs
The size and structural characterization of newly synthesized Ni NPs stabilized with
surfactant molecules was carried out using various analytical techniques.
2Ni2+
+ N2H4 + 4OH- 2Ni + N2↑ + 4H2O
The nanosized Ni NSs have been prepared by using hydrazine as reducing agent,
mechanism for reduction as given above, which is similar as cited in literature (Kalwar et
al., 2011; Bai et al., 2008).
4.3.1.1 UV-Vis Spectroscopy of TX-100 Derived Ni NSs
The nanometer size regime of newly synthesized nickel structures was
spectrophotometrically monitored via a characteristic absorption profile in the range
350-400 nm (Figure4.3.1) in fine agreement with already reported work (Kalwar et al.,
2011). Depending on the nature of sample/solution, Ni particles generated an absorption
edge in the UV-Vis spectral range 374-422 nm, corresponding to the surface plasmon
resonance (SPR) band of nanosized Ni particles after the reduction of Ni (II) ions (Nouneha
et al., 2011; Amekura et al., 2004). The optimization studies for reducing agent
concentration, surfactant concentration and pH were carried out to check their effect on the
peak shape. Further helping estimation of structure/morphology and size of nanoscale metal
particles was carried out by UV-Vis spectroscopy. The fabrication of Ni NSs with
85
controlled morphologies by the modified hydrazine reduction route brought about the
appearance of a faint blue color in the sample solution and blue shifts in λmax from 393 nm
to 357 nm with gradual increase in absorbance with increase in the reducing agent, as
shown in Figure 4.3.1.
300 350 400 450 5000.00
0.05
0.10
0.15
0.20
1
2
3
4
5Ab
s (
a.u
.)
Wavelength (nm)
optimization of reducing agent
Figure 4.3.1 UV-Vis spectral changes recorded for optimization of reducing agent showing
blue shift in λmax from 391 nm to 366 nm with increase in absorbance at various
concentrations.
The photoevolution of Ni particles suggests that the change in nature and profile of
UV-Vis spectra is due to complete reduction of Ni(II) ions in the surfactant solution. The
effect of time on the stability of Ni NPs was studied by aging the solution for many days.
We observed that surfactant molecules impart stability to the colloidal dispersions/clusters
and also control over morphology and size of the crowded nanoparticles/structures, as
described other workers (Comparelli et al., 2005; Liu et al., 2008; Sau et al., 2001). The
development of the spectral profile of TX-100 stabilized Ni NSs recorded after increases in
concentration of surfactant (stabilizing/capping) are most different in the present study
from that of previous results (Kalwar et al., 2011), as here gradual increases in
86
concentration of the stabilizing/capping agent generated red shifts in λmax from 382 nm to
393 nm, as shown in Figure 4.3.2, due to formation of larger micelles networks of
surfactant molecules in the adopted wet chemical method.
300 350 400 450 5000.00
0.02
0.04
0.06
0.08
0.10
Ab
s (
a.u
)
Wavelength (nm)
4
3
2
1
optimization of TX-100
Figure 4.3.2 UV-Vis spectral changes recorded for optimization of surfactant (TX-100) at
various concentrations showing red shift in λmax from 382 nm to 393 nm with increase in
absorbance.
Denkova et al., (2008) illustrated that formation of larger micelle networks at high
concentrations might be due to the presence of attractive intermicellar interactions which
lead to enhanced micellar growth, and potential agglomeration of micelles to larger
spherical particles. In principle, for practical increase in surfactant concentrations, the
reliance of red shift in λmax (from 382 nm to 393 nm) on the concentration is linear and can
be credited to an increase of micellar size. The study shows that UV-Vis spectra recorded
from different pH values of the samples/solutions exhibited distinctive absorption maxima
that explore the formation of nanoscale nickel structures with different sizes and shapes.
87
300 350 400 450 5000.00
0.02
0.04
0.06
0.08
0.10
pH 10
pH 8
pH 4
Ab
s (
a.u
.)
Wavelength (nm)
slected spectra for pH study
Figure 4.3.3 Selected UV-Vis spectra from optimization of pH study where we observed
blue shift in λmax from 391 nm to 357 nm with increase in absorbance.
Analysis of a selection of samples at different pH was carried out to see the effect of
pH on the sizes and shapes of Ni NSs. The lowest observed λmax value was from pH 9.4.
Further studies were carried out using FTIR spectroscopy, SEM and XRD for structural
elucidation and rationalization of the interactions of surfactant molecules with particles and
to confirm size and morphologies of the TX-100 stabilized Ni NSs.
4.3.1.2 FTIR Spectroscopy of TX-100 Derived Ni NSs
FTIR spectra were recorded to examine the interaction of TX-100 molecules with
the surfaces of Ni NSs. These results showed that the extended complexity and
functionality of the nanoscale systems are predictable from efficient linkages employed by
the OH group of TX-100 molecules and Ni particles obtained in lyotropic liquid crystalline
medium. The study showed that a very strong absorption band at 2870 cm-1
in connection
with a weak signal at 3480 cm-1
appeared in the FTIR spectra of absolute TX-100
88
(standard/pure) due to the presence of the free OH group in the surfactant molecules as in
Figure 4.3.2(a-b).
4000 3500 3000 2500 2000 1500 10000.0
0.5
1.0
1.5
2.0
2870
b
Ab
s (
a.u
.)
Wavelength (cm-1)
a3480
3260
Figure 4.3.4 FTIR spectra of (a) standard TX-100 solution recorded using ATR and
(b) TX-100 stabilized Ni NSs at pH 9.4.
On the other hand, TX-100 derived Ni nanostructures with nanosheet and/or
nanosphere morphology brought about the appearance of a weak but broad signal at 3260
cm-1
at the optimized pH 9.4. The basic pH generated the band at 3260 cm-1
due to
relatively large concentrations of OH moieties in the products.
4.3.1.3 SEM Analysis of TX-100 Derived Ni NSs
Fabrication of nanoscale materials with ordered and controlled structures has been
given intensive attention due to their fascinating properties; the performance of such
materials is mainly dependent on size and morphology (Qi et al., 2010). We have
synthesized Ni NSs with controlled morphologies via a hydrazine reduction route as
explained in chapter 3 in experimental section 3.3.3, as shown in SEM images, Figure
89
4.3.5.
Figure 4.3.5 SEM images of TX-100 stabilized Ni NSs obtained at pH 9.4.
The SEM images show that Ni NSs sample prepared at pH 9.4 consisted of mixed
structures of hexagonal nanosheets with smooth surfaces and well dispersed spherical
nanoparticles with rough surfaces but uniform size. Thus, we have demonstrated that mixed
structures of fine nickel nanosheets (Ni NSs) and rough surfaced nickel nanoparticles
(Ni NPs) are clearly seen at pH 9.4. The method is introduced for synthesis of mixed
nanostructures containing highly ordered assemblies of 2D nanosheets/foils with smooth
surfaces and thickness in the range of 24-240 nm, mean thickness = 72 nm, the width of
these nanosheets is found to be in the range 0.9-7.0 μm with an average of 3.1 μm, and the
size of the spherical nickel nanoparticles is seen to range from 8-300 nm, with the observed
90
average size of 45 nm at pH 9.4. These Ni NSs were used as heterogeneous catalysts for the
reduction/degradation of a number of organic dyes and found to be highly active catalysts
and recoverable and reusable several times.
4.3.1.4 XRD Analysis of TX-100 Derived Ni NSs
The phase composition of crystal structures of these products were also analyzed by
X-ray diffraction. Figure 4.3.4 shows the XRD pattern of powdered Ni NSs.
20 30 40 50 60 70 80
200
400
600
800
1000
1200
1400
111
010
100
111
Co
un
ts
2 theta (degrees)
TX-100- Ni NSs
200
Figure 4.3.6 XRD pattern of TX-100 stabilized Ni NSs obtained at pH 9.4.
Moreover, the characteristic peaks (111) and (200) of products are in strong
correlation with other formed nanoscale Ni structures (Zhang et al., 2006; Wang et al.,
2008). We obtained mixed crystal structures with typical diffraction patterns indicating the
formation of fcc Ni NPs with additional peaks for Ni based nanosheets/foils, as observed
also by others (Zhang et al., 2006), which are confirmed by surface morphology SEM
imaging.
91
4.3.2 Degradation of Dyes Catalyzed by TX-100 Derived Ni NSs
We performed experiments for dye degradation/reduction with NaBH4 in the
presence and absence of nanoscale nickel catalysts to investigate the catalytic activity of
Ni NSs. The rates of degradation for several organic dyes, such as eosin-b (EB), rose
bengal (RB), ereochrome black-t (ECBT), and methylene blue (MB), were monitored under
UV-Vis irradiation as model reactions, as shown in Figures 4.3.7 to 4.3.11.
450 500 550 600 6500.0
0.5
1.0
1.5
Ab
s (
a.u
.)
Wavelength (nm)
Fresh
10 Sec
20 Sec
30 Sec
40 Sec
Rose B Degradation
Figure 4.3.7 UV-Vis spectral analysis for catalytic reduction/degradation of 0.02 mM RB
carried out in 4.0 ml deionized water with 10 mM NaBH4 in the presence of a fixed amount
of TX-100 stabilized Ni NSs (0.2 mg) obtained at pH 9.4.
Dyes were chosen because they produce different color shades in degraded and
undegraded forms. The study show that newly fabricated Ni NSs are excellent catalysts.
Progress of the degradation of each dye was examined by decrease in absorbance at their
respective λmax value in the UV-Vis spectra. Complete degradation of methylene blue is
shown in Figure 4.3.8 below; similar studies have also been reported by Sundip et al.,
(2007).
92
550 600 650 700 7500.00
0.75
1.50
2.25
3.00
Ab
s (
a.u
.)
Wavelength (nm)
Fresh
10 sec
20 sec
30 sec
40 sec
Degradation of M Blue
Figure 4.3.8 UV-VisVis spectral analysis for catalytic reduction/degradation of 0.02 mM
MB carried out in 4.0 ml deionized water with 10 mM NaBH4 in the presence of a fixed
amount of TX-100 stabilized Ni NSs (0.2 mg) obtained at pH 9.4.
The results revealed that RB (as depicted in Figure 4.3.7), MB (as depicted in
Figure 4.3.8), EB (as depicted in Figure 4.3.9) and ECBT (as depicted in Figure 4.3.10)
were all reduced in a very short time when nanocatalysts were employed. However, these
dyes were not reduced without the use of catalyst; MB and ECBT showed little decrease in
absorbance with time.
93
450 500 550 6000.0
0.5
1.0
1.5
Ab
s (
a.u
.)
Wavelength (nm)
fresh
10 sec
20 sec
30 sec
40 sec
Degradation of Eosin B
Figure 4.3.9 UV-Vis spectral analysis for catalytic reduction/degradation of 0.02 mM EB
carried out in 4.0 ml deionized water with 10 mM NaBH4 in the presence of a fixed amount
of TX-100 stabilized Ni NSs (0.2 mg) obtained at pH 9.4.
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
Ab
s (
a.u
.)
Wavelength (nm)
fresh
10 sec
20 sec
30 sec
40 sec
Degradation of ECBT
Figure 4.3.10 UV-Vis spectral analysis for catalytic reduction/degradation of 0.02 mM
ECBT carried out in 4.0 ml deionized water with 10 mM NaBH4 in the presence of a fixed
amount of TX-100 stabilized Ni NSs (0.2 mg) obtained at pH 9.4.
The reduction/degradation reactions of the dyes (i.e. RB, EB, MB, and ECBT) and
94
their mixture were employed with TX-100-stabilized Ni NSs/NPs in an aqueous solution;
progress of reaction was monitored by UV-Vis spectroscopy. Other works have already
been done to determine mass effect and catalytic rate for dye degradation using different
sized palladium nanoparticles (Sudip et al., 2007) and gold nanoparticles (Sau et al., 2001)
prepared in TX-100. However, we observed that newly fabricated TX-100 stabilized Ni
NSs catalyzed dye reduction to the 100% efficiency in presence of NaBH4 in a very small
time i.e. 10-40 sec; furthermore, the same catalysts could be recovered and reused several
times.
4.4 Results and Discussion for L-Threonine Capped Ni NPs
We have introduced a method for fabrication of well dispersed spherical Ni NPs
consisting of amino acid layers on the nanoparticles surface. Evidences for the fabrication
of Ni NPs are obtained from various characterization techniques.
4.4.1 Characterization Studies of L-Threonine Derived Ni NPs
Characterization studies included the use of UV-Vis spectroscopy that helped in
monitoring the initial optimization of different parameters for synthesis of Ni NPs. FTIR
spectral characterizations were carried out to investigate interactions of the l-threonine
molecules with particle surfaces. TEM and AFM images were recorded to observe the
sizes, morphologies, and arrangements of atoms to form nanoscale materials/structures.
4.4.1.1 UV-Vis Spectroscopy for Preparation of L-Threonine Capped Ni NPs
UV-Vis spectra (Figure 4.4.1) recorded after variation of l-threonine concentrations
showed blue shift in wavelength with increasing concentration that is strong evidence for
manufacture of smaller nanoparticles at high capping agent concentrations. The use of
95
NaBH4 prior to the capping agent has been carried out, employing reduction of nickel ions
into neutral nickel atoms (elemental forms) which is confirmed by the change in color of
solution from light green to black, blue shift in wavelength with increasing concentration as
shown in Figure 4.4.1.
300 350 400 450 5000.00
0.02
0.04
0.06
5
4
3
2
Ab
s (
a.u
.)
Wavelength (nm)
1
Varriation of NaBH4
1) 0.1 ml
2) 0.2 ml
3) 0.3 ml
4) 0.4 ml
5) 0.5 ml
Figure 4.4.1 UV-Vis spectra of freshly prepared samples of Ni NPs obtained in an aqueous
environment by variation of the reducing agent concentration.
The reduced nickel ions form atomic nickel which then undergoes a process of
nucleation to produce strongly bound homoatomic aggregates of tiny black particles. These
aggregates were formed in the absence of any capping agent and continued to form bigger
agglomerates the in solution. Moreover, the addition of varying concentrations of l-
threonine amino acid molecules immediately after the addition of sodium borohydride to
nickel chloride solution resulted in the formation of relatively stable colloidal dispersions
inside the test tubes which were black in color and remained clear solution for long times.
The l-threonine molecules employed natural physisorption on the nanocomposite
96
surfaces through amino groups, but not chemical functionality attachment, which
subsequently helped the growth patterning of these nanocomposites into spherical shapes.
The obtained colloidal dispersions of Ni NPs in aqueous environments were then separated
by high speed centrifugation, re-dispersed into ethanol, and found to be highly stable for
several months in organic solution/medium; however, freshly fabricated Ni NPs were
comparatively unstable in aqueous medium.
The physisorption of l-threonine molecules onto the particle surfaces showed strong
dependence on the solution medium, which might be a result of the zwitterionic property of
the amino acid molecules, i.e. their tendency to form doubly-charged ions. This is
necessarily the case in favor of smaller non-covalently binding l-threonine molecules with
the surface of Ni NPs.
4.4.1.2 FTIR Spectroscopy Characterization for L-Threonine Capped Ni NPs
FTIR spectra, as shown in Figure 4.4.2(a-b), were recorded for l-threonine and
Ni NPs capped with l-threonine molecules. Attempts were then made to investigate the
surface binding interactions of amino acid molecules with nanocomposite structures. These
studies confirm the surface binding of nanoscale nickel composites via amino linkages.
FTIR spectra shown above (Figure 4.4.2) support the evidence for nanofabricated
architectures derived from UV-Vis spectroscopic studies and AFM micrographs taken on
glass substrates as shown in Figure 4.4.3 of l-threonine capped Ni NPs. FTIR spectra of
pure l-threonine given in Figure 4.4.2(a) illustrate a small representative zwitterionic band
at 2050 cm-1
that is absent in the FTIR spectra of l-threonine capped Ni NPs shown in
Figure 4.3.2(b).
97
4000 3500 3000 2500 2000 1500 1000 5000.0
0.5
1.0
1.5
2.0
2.5
3.0
3400
3170
3170
1390
1630
a
Ab
s (
a.u
.)
Wavelength (cm-1)
b
2050
Figure 4.4.2 FTIR spectra of (a) standard l-threonine and (b) sample of powdered Ni NPs
obtained after separation by centrifugation method.
IR spectra of nickel nanocomposites showed two broad and strong signals at
1630 cm-1
and 1390 cm-1
which can be assigned to the hydroxyl group (Gao et al., 2004;
Hobart 1986). Similar peaks have been seen at 1651 cm-1
and 1382 cm-1
(Janardhanan
et al., 2009). The complex spectral bands below 1500 cm-1
are assigned to C-H bending
vibrations resulting from methylene and methyl groups and those of the weak IR signals in
range 1200-1050 cm-1
can be assigned to C-N stretching due to the presence of the amino
group (Singla et al., 2007). Hence it is also clear from the FTIR spectra (Figure 4.4.2)
recorded for standard l-threonine and Ni NPs that amino group interact with surface of Ni
NPs. Two broad FTIR bands above 3000 cm-1
correspond to the presence of O-H (3400 cm-
1) and N-H (3170 cm
-1) stretching frequencies.
4.4.1.3 AFM Analysis of L-Threonine Capped Ni NPs
AFM micrographs present highly distributed Ni NPs grown after capping with
98
l-threonine molecules on glass cover slip, as shown in Figure 4.4.3(a-b). One of the most
useful aspects of atomic force microscopy is its ability to measure quantitative special
dimensions of a variety of surface features like small particles deposited on substrate or
etch pits. Thus, size homogeneity and nanoscale dimensions of the as-prepared Ni NPs
were also examined through AFM and TEM data (which were found in a very good
agreement), as depicted in Figure 4.4.4.
Figure 4.4.3 AFM images of well dispersed l-threonine capped Ni NPs, (a) at high
magnification and (b) low magnification.
99
AFM analysis reveals that the products correspond to well dispersed nanoscale
nickel particles.
Figure 4.4.4 Schematic diagram of the formed Ni NPs capped with l-threonine molecules
and plot of size distribution - AFM and TEM micrographs were used for size calculations.
Each amino acid molecule individually attached at the surface of nanocomposites
and final structures followed similar shapes, which have been given on right upper side of
the Figure 4.4.4. We assume that these nanosized particles are containing structures formed
by encapsulation of l-threonine molecules by surface binding through transient interactions.
The colloidal dispersions of l-threonine capped Ni NPs were collected in stoppered glass
bottles and a 10 µl sample was put on a cover slip to dry, giving well-attached nanoparticles
which were then processed for AFM analysis.
4.4.1.4 TEM image Analysis L-Threonine Capped Ni NPs
High resolution TEM is employed to examine the size and shape of nanoscale
objects, well known to impact structure and chemistry of the nanomaterials (Tanase et al.,
2001; Wang et al., 2000). Samples of powdered Ni NPs ultrasonically dispersed in ethanol
100
and then mounted on carbon coated copper grids were prepared for TEM image analysis
(Figure 4.4.5), showing the nanoscale nickel structures which are known to have a tendency
to connect with each other through their magnetic dipole interaction (Li et al., 2006).
Therefore, it is very difficult to prepare isolated magnetic nanoparticles as a result of the
particles undergoing agglomeration easily (Frid et al., 2007).
Figure 4.4.5 TEM image of l-threonine capped Ni NPs mounted on a carbon coated Cu
grid.
In this current study, we propose that surface atoms of nickel particles may
coordinate with nitrogen atoms of the l-threonine molecules which can then regulate the
formation of one dimensional assembly of reduced nickel atoms. Thus, a satisfactorily
characterized method has been introduced for fabrication of reproducible and well-
distributed Ni NPs. The reasons and driving forces to obtain homogeneously distributed Ni
NPs arise from the use of l-threonine molecules which experience specific interactions and
101
investigate desired sites to connect and enter into processing for strong encapsulation via
the amino group to form colloidal dispersions of Ni NPs.
4.4.2 Application of of L-Threonine Capped Ni NPs as Catalyst
Small sized spherical Ni NPs capped with l-threonine molecules were employed to
see the catalytic efficiency in reduction/degradation of congo red dye. We used the dye as a
model compound because it provides different color shades in degraded and undegraded
forms in aqueous media. This makes the study and observations more facile. We observed
extra ordinary active behaviors for the catalytic efficiency of our nanoparticles which are
discussed in detail below.
4.4.2.1 Chemical Properties of Congo Red
Congo red (C32H22N6O6S2, MW-696.7), depicted in Figure 4.4.6, is a linear
symmetrical molecule. It possesses a hydrophobic center made up of two phenyl rings
bound together via diazo (-N=N-) linkages with two charged terminal naphthalene moieties
which contain sulfonic acid (-HSO3) and amino (-NH2) groups. The CR dye molecule is
~21Å in diameter which, on dissociation via diazo linkages, produce chinone and
sulphonazo forms (Frid et al., 2007); these are presented in Figure 4.4.6(b-c).
102
N
N
N
N
SS
NH2 H2N
O
OHOOHO
O
A.
Terminator-Connector-Linker-Connector-Terminator
21 A
N
N
S
NH2
OO
O
HN
N
S
NH3
OHO
O
B. C.
Sulphonazo form-red Chinone form -blue
Figure 4.4.6 (A) Linear molecular structure of CR dye, its (B) sulfonazo form and (C)
chinone form produced during the course of reaction.
4.4.2.2 Catalytic Activity of L-Threonine Capped Ni NPs
UV-Vis spectroscopy results given in Figure 4.4.7(a-b) were carried out to check
the performance of freshly prepared Ni NPs for the catalytic reduction/degradation of CR
dye. We observed high catalytic efficiency of the l-threonine capped Ni NPs fabricated by
the newly introduced rapid and inexpensive synthetic route.
103
200 300 400 500 600 7000.00
0.15
0.30
0.45
0.60
0.75
a
238
348Ab
s
Wavelength (nm)
498
200 300 400 500 600 7000.00
0.15
0.30
0.45
0.60
0.75 250
348
498
Ab
s (
a.u
.)
Wavelength (nm)
fresh
10 sec
20 sec
30 sec
40 sec
50 sec
278
b
Figure 4.4.7 UV-Vis spectra obtained from (a) Congo red dye (20 µM) in pure Milli-Q
water and (b) catalytic degradation of dye (20 µM) with NaBH4 (0.1 M) in the presence of
Ni NPs (0.1 mg) deposited on glass cover slips.
Figure 4.4.7(a) presents the absorption profile of CR dye in Milli-Q water. It is easily seen
in the spectra that peaks such as those at 498 nm, 348 nm, and 238 nm, were stable and did
not undergo structural transformations initially under UV-Vis irradiation alone, even after
104
one hour irradiation time. However, we did observe decoloration/degradation of dye by
addition of sodium borohydride (a reducing agent) in the presence of Ni NPs catalysts. The
degradation profile revealed by UV-Vis spectroscopy given in Figure 4.4.7(b) suggests
complete degradation in a very short time. The dye degradation was characterized by steady
decreases of absorption, at 498 nm and 348 nm, which are believed to be due to attacked by
electrons on the nitrogen double bonds (Sladewski et al., 2006).
In contrast to the gradual decrease in absorption at 498 nm and 348 nm, completely
inverse behavior at 278 nm and 249 nm (a regular increase) was observed, which is
attributed to the formation of smaller structural units. This characteristic behavior of CR
dye was commenced by the addition of NaBH4 in the presence of Ni NPs catalysts. UV-Vis
spectra also showed an isobestic point in the spectral profile at around 300 nm which is a
clear indication for the degradation of large dye molecules to smaller fragments - low
molecular weight compounds.
4.4.2.3 Recovery and Reuse of Ni NPs Catalyst
The newly devised protocol to fabricate and use l-threonine capped Ni NPs for the
catalytic reduction degradation of congo red dye allows for simple and effortless recovery
to reuse these nanoparticles for four times. A simple bar graph presenting the clear picture
of high catalytic efficiency of l-threonine capped Ni NPs after several uses is shown in
Figure 4.4.8.
105
1 2 3 4 50
20
40
60
80
100
98.598.899.399.6
% C
on
ve
rsio
n
No. of Cycles
100
Figure 4.4.8 % conversion of 4-NPh to 4-APh by freshly prepared and four times
regenerated Ni NPs used as catalyst
Ni NPs used for catalytic application were deposited on glass cover slips which were
easily recovered after use and washed with sufficient quantity of deionized water followed
by drying on a hot plate at 100°C (for 10 minutes) to remove water vapors. We observed
negligible/no loss of activity for reduction reaction.
106
Chapter 5
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
The objective of my study was to synthesize nickel nanoparticles using different
reducing and capping agents by new simple, inexpensive and green approaches. Synthesis
of l-cysteine capped Ni NPs was carried out by a simple and inexpensive approach using a
modified microwave irradiation method. These nanoparticles were used to monitor
reduction of 4-nitrophenol (i.e. an organic pollutant) that reached total reduction (100%) of
the target compound into 4-aminophenol in a very short time - the reaction was followed
using UV-Visible spectroscopy and completion observed after just 40 sec. The recovery
and reuse of Ni NPs with negligible poisoning made the task further viable in terms of cost
effectiveness and environmental friendliness. So this study uncovered a simpler way to
synthesize Ni NPs with controllable size and highly efficient catalytic application for the
immediate formation of useful industrial products and control of environmental pollution
against the mentioned species. This work is extendable to the reduction of other phenols as
well as compounds like dyes, nitrates and so on.
Another study was conducted to demonstrate that l-cysteine capped Ni NPs are excellent
catalysts for complete remediation of hexavalent chromium ions from aqueous
environments. We observed 99% reduction of Cr(VI) ions in a very short time, representing
a four times increased extent of reduction in about one-third of the time compared with
conventional nickel powder catalyst. These nanosized catalysts were prepared in ethylene
107
glycol by microwave assisted reduction of nickel chloride with hydrazine hydrate and used
as heterogeneous catalysts for reduction of Cr(VI) ions by depositing on glass cover slips.
We observed that l-cysteine capped Ni NPs have not previously been used for similar
reduction/remediation studies of hexavalent chromium species and also believe that the
current study should open new doors for their further application in similar and other
experiments of environmental importance.
A method for the fabrication of l-methionine capped Ni NWs has been introduced.
In view of our study, it is concluded that the size of Ni NWs can be controlled by careful
adjustment of parameters including pH, precursor salt concentration and capping agent
concentration. It is also emphasized that synthesized l-methionine capped Ni NWs were
responsible for efficient transfer hydrogenation reactions during the conversion of IPA to
acetone. Thus the formed Ni NWs are green catalysts with the ability to be recovered and
are recommended for other dehydrogenation reactions of low molecular weight organic
compounds under normal laboratory conditions. This work represents a simple, effortless
and inexpensive approach for the fabrication of Ni nanostructures and their use in oxidative
catalytic process.
A protocol for synthesis of ordered nickel nanostructure arrays with unique
morphologies has been described. We have presented a modified hydrazine reduction route
for fabrication of stable colloidal dispersions of nickel nanostructures in a lyotropic liquid
crystalline medium using Triton X-100 nonionic surfactant. The method is introduced for
synthesis of mixed nanostructures containing highly ordered assemblies of 2D nanosheets
with absolutely smooth surfaces and sheet thicknesses in the range of 24-240 nm (average
thickness is 72 nm). The diameter of these nanosheets (NSs) is found to be in the range
0.9-7.0 μm (average diameter of 3.1 μm). The size of spherical nickel nanoparticles is in the
108
range 8-300 nm (observed average size of 45 nm). The extended complexity and
functionality of the nanoscale systems are predictable from efficient linkages between the
OH group of TX-100 molecules and Ni particle surfaces in a lyotropic liquid crystalline
medium. Different parameters, such as concentration of Ni, hydrazine hydrate and TX-100,
were optimized using UV-Vis spectrometry. The effects of temperature, pH, and stability of
Ni NPs were studied by aging the solution for many days. The characterization studies
included scanning electron microscopy (SEM), X-ray diffractometery (XRD), and Fourier
transform infra red (FTIR) spectroscopy. These NSs were used for the reduction of organic
dyes and found to be highly active catalysts.
We have synthesized l-threonine capped Ni NPs and used them for the
remediation/reduction of an organic dye (i.e. Congo red). It is suggested that the simplicity,
cost effectiveness and safety of the present nanoparticle fabrication route supersedes
numerous procedures to manipulate well dispersed Ni NPs with spherical shapes. Freshly
prepared Ni NPs deposited on glass surfaces produced significantly high yields for catalytic
performance. We observed complete degradation of 20 µM dye with 0.1M NaBH4 within
50 seconds during the course of reaction under UV-Visible irradiation in the presence of
nanoparticles, and it was found to be easy to recover and reuse the catalyst material several
times without loss of activity.
109
Recommendations
The present studies show that a wide range of fascinating nickel nanostructures have
been formed, such as nanorods, nanowires, nanospheres, hexagonal nanosheets,
roughened surface nanoparticles, and multi lamellar nanoflowers. These can find
broad application on a commercial scale.
We also observed the effectiveness of nickel based nanomaterials for catalytic
reduction/oxidation of a number of chemical compounds with different nature and
origins, such as low molecular weight organic compounds, inorganic metal ions,
organic dyes, pesticides, and nitrophenols. Thus, these nanocatalysts can be
commercialized and used for remediation of hazardous species from aqueous
environments.
The results show that catalytic performance of these nanosized materials is far more
aggressive than for the equivalent bulk materials due to greater surface area
available. That may save cost and time in many folds.
We also conclude that nanostructures provide pathways for the remediation of toxic
chemicals from aqueous environments; these nanoscale nickel materials were used
as heterogeneous catalysts and recovered easily.
A common theme in nanotechnology is the recycling of nanomaterial catalysts.
Therefore, protocols have been devised that we have applied and which we have
found to generate results with good reproducibility, precision. and problem-solving
ability.
It is clear that new simple wet chemical reduction methods produced attractive
nanoarchitectures which may find striking commercial applications, as they also
110
provide the ways to easily modify the nanomaterial morphologies, with economic
and societal benefits.
Nickel nanorods may find an important role in a number of novel applications.
However, to develop such systems their precise properties need to be known.
Societal Implications
Societal aspects (implications) of nanotechnology consist of a broad family of
highly integrated components and forces that merge with technology to form our
civilization, business, academic, and other social institutions.
Technology has always been one of the primary drivers of change. However, this
change may be beneficial, detrimental, or somewhere in between.
Societal implications in turn have the capacity to alter any technology.
We have and intend to continue to cover a wide variety of catalytic applications of
nanoscale nickel materials.
The newly synthesized Ni NPs are both numerous and diverse and encompass the
legal, ethical, cultural, medical, and environmental concerns.
Almost all sectors, including electronics, telecommunications, healthcare,
biotechnology, agriculture, transportation and computing, are expected to be
benefited by the newly developed methodologies.
Results show that the catalytic performance of these nanosized materials is far more
aggressive than for the equivalent bulk materials due to the greater surface area
available, opening up possibilities for both cost and time savings in many
applications.
111
References
Afkhami, A.; Moosavi, R. J. Hazard. Mater. 174 (2010) 398.
Aiken, J. D.; Finke, R. G. J. Mol. Catal. A: Chem. 145, (1999) 1.
Akamatsu, K.; Adachi, S.; Tsuruoka, T.; Ikeda, S.; Tomita, S.; Nawafune, H. Chem. Mater.
20 (2008) 3042.
Amekura, H.; Takeda, Y.; Kishimoto, N. Nucl. Instrum. Methods Phys. Res. B 222 (2004)
96.
An, Z.; Pan, S.; Zhang, J. J. Phys. Chem. C, 113 (2009) 1346.
Andrievskii, R. A.; Glezer, A. M. Phys. Metals Metallogr. 88 (1999) 45.
Anne, K. B.; Jeremy, S. T.; Arthur, B. E.; Wendy, C. C. Nano Lett. 4 (2004) 487.
Bai, L.; Yuan, F.; Tang, Q. Mater. Lett. 62 (2008) 2267.
Barakat, N. A. M.; Kim, B.; Kim, H. Y. J. Phys. Chem. C, 113 (2009) 531.
Barber, D. J.; Freestone, I. C.; Archaeometry, 32 (1990) 33.
Becker, J. A.; Schafer, R.; Festag, R.; Ruland, W.; Wendorff, J. H.; Pebler, J.;
Quaiser, S. A.; Helbig, W.; Reetz, M. T. J. Chem. Phys. 103 (1995) 2520.
Bentley, A. K.; Trethewey, J. S.; Ellis, A. B.; Crone, W. C. Nano Lett. 4 (2004) 487.
Bezryadin, A.; Dekker, C.; Schmid, G. Appl. Phys. Lett. 71 (1997) 1273.
Boudjahem, A. G.; Monteverdi, S.; Mercy, M.; Bettahar, M. M. J. Catal. 221 (2004) 325.
Boudjahem, G.; Monteverdi, S.; Mercy, M.; Bettahar, M. M. Langmuir 20 (2004) 208.
Carnes, C. L.; Stipp, J.; Klabunde, K. J.; Bonevich, J. Langmuir 18 (2002) 1352.
Caro, D.; Bradley, J. S. Langmuir 13 (1997) 3067.
Chen, D. H.; Hsieh, C. H. J. Mater. Chem. 12 (2002) 2412.
Chen, N.; Zhang, W.; Yu, W.; Qian,Y. Mater. Lett. 55 (2002) 230.
112
Chettibi, S.; Benguedouar, Y.; Keghouche, N. Physics Procedia 2 (2009) 707.
Choi, J. H.; Kim, D. H.; Yoon, J. C.; Kim, Y. J. J. Alloys Compd. 487 (2009) 8.
Chou, K. S.; Huang, K. C. J. Nanopart. Res. 3 (2001) 127.
Comparelli, R.; Fanizza, E.; Curri, M. L.; Cozzoli, P. D.; Mascolo, G. Agostiano, A. Appl.
Catal. B: Environ. 60 (2005) 1.
Couto, G. G.; Kleinb, J. J.; Schreiner, W. H.; Mosca, D. H.; Oliveira, A. J. A.;
Zarbina, A. J. G. J. Colloid Interface Sci. 311 (2007) 461.
Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc. Farad. Trans 75 (1979)
790.
Decher, G.; Hong, J. D. Makromol. Chem. Macromol. Symp. 46 (1991) 321.
Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 210 (1992) 831.
Degen, A.; Matek, J. Nano Structured Materials 12 (1999) 225.
Denkova, P. S.; Lokeren, L. V.; Verbruggen, I.; Willem, R. J. Phys. Chem. B 112 (2008)
10935.
Dhakshinamoorthy, A.; Pitchumani, K. Tetrahedron Lett. 49 (2008) 1818.
Dharmaraj, N.; Prabu, P.; Nagarajan, S.; Kim, C. H.; Park, J. H.; Kim, H.Y. Mater. Sci.
Eng., B 128 (2006) 111.
Dongsheng, L.; Sridhar, K. J. Am. Ceram. Soc. 89 (2006) 1510.
Du, Y.; Chen, H.; Chen, R.; Xu, N. Chem. Eng. J. 125 (2006) 9.
Duan, Y.; Li, J. Mater. Chem. Phys. 87 (2004) 452.
Eigler, D. M.; Schweizer, E. K. Nature 344 (1990) 524.
Evanoff, D. D.; Chumanov, G. Chem Phys Chem. 6 (2005) 1221.
Fang, J.; Zheng, Z.; Zhaoyi, X.; Shourong, Z.; Zhaobing, G.; Liqiang, C. Journal of
Hazardous Materials B, 134 (2006) 94.
113
Frid, P.; Anisimov, S. V.; Popovic, N. Brain Res. Rev. 53 (2007) 135.
Gao, X. Y.; Wang, S. Y.; Li, J.; Zheng, Y. X.; Zhang, R. J.; Zhou, P.; Yang, Y. M.;
Chen, L. Y. Thin Solid Films, 438 (2004) 455.
Grardin, C.; Kostadinova, D.; Sanson, N.; Coq, B.; Tichit, D. Chem. Mater. 17 (2005)
6473.
Gu, Z.; Chen, Y.; Gracias, D. H. Langmuir, 20 (2004) 11308.
Guo, D.; Wu, C.; Li, J.; Guo, A.; Li, Q.; Jiang, H.; Chen, B.; Wang, X. Nanoscale Res. Lett.
4 (2009) 1395.
Hammond, P. T. Adv. Mater. 16 (2004) 1271.
Han, Y.; Shen, J.; Chen, Y. App. Catal. A Gen. 205 (2001) 79.
Henglein, A. Chem. Rev. 89 (1989) 1861.
Hou, Y.; Gao, S. J. Mater. Chem. 13 (2003) 1510.
Houa, Y.; Kondohb, H.; Ohtab, T.; Gao, S. Appl. Surf. Sci. 241 (2005) 218.
Huang, J.; Kunitake, T.; Onoue, S. Chem. Commun. (2004) 1008.
Hussain, M. S.; Haque. K. M. A. J. Sci. Res. 2 (2010) 313.
James, E. H. ACS Nano, 2 (2008) 395.
Janardhanan, R.; Karuppaiah, M.; Hebalkar, N.; Rao, T. N. Polyhedron 28 (2009) 2522.
Jin, G. P.; Ding, Y. F.; Zheng, P. P. J. Power Sources, 166 (2007) 80.
Jin, P.; Chen, Q.; Hao, L.; Tian, R.; Zhang, L.; Wang, L. J. Phys. Chem. B, 108 (2004)
6311.
Kalwar, N. H.; Sirajuddin; Sherazi, S. T. H.; Abro, M, I.; Tagar, Z. A.; Hassan, S. S.;
Junejo, Y.; Khattak, M. I. Appl. Catal. A 400 (2011) 215.
Kera, Y.; Kominami, H.; Murakami, S.; Ohtani, B. Photocatalysis: Science and
Technology, Springer, (2003) 29.
114
Kerker, M. Appl. Opt. 30 (1991) 4699.
Kevin, C. H.; Gemma, C. S.; John, P. H. Anal. Chem. 78 (2006) 416.
Kima, S.; Yooa, B. K.; Chuna, K.; Kanga, W.; Choob, J.; Gongc, M. S.; Joo, S. W. J. Mol.
Catal. A: Chem. 226 (2005) 231.
Kimand, M. R.; Choi, S. H. J. Nanomaterials (2009) 7.
Koleva, B. B. Vibrat. Spectro. 44 (2007) 30.
Kudlash, A. N.; Vorobyova, S. A.; Lesnikovich, A. I. J. Phys. Chem. Solids 69 (2008)
1652.
Leconte, Y. ;Maskrot, H.; Herlin, B. N.; Porterat, D.; Reynaud, C.; Gierlotka, S.;
Swiderska, S. A. Vicens, J. J. Phys.Chem. B. 110 (2006) 158.
Lee, J. H.; Kim, Y. A.; Kim, K.; Huh, Y. D.; Hyun, J. W.; Kim, H. S.; Noh, S. J.; Hwang,
C. S. Bull. Korean Chem. Soc. 28 (2007) 1091.
Lee, P. C.; Meisel, D. J. Phys. Chem. 86 (1982) 3391.
Legrand, J.; Taleb, A.; Gota, S.; Guittet, M. J.; Petit, C. Langmuir 18 (2002) 4131.
Lei, Y.; Chim, W. K.; Zhang, Z.; Zhou, T.; Zhang, L.; Meng, G.; Phillipp, F. Chem. Phys.
Lett. 380 (2003) 313.
Li, D.; Komarneniw, S. J. Am. Ceram. Soc. 89 (2006) 1510.
Li, X.; Zhang, X.; Li, Z.; Qian. Y. Solid State Commun. 137 (2006) 581.
Li, Y. D.; Li, C. W.; Wang, H. R.; Li, L. Q.; Qian, Y. T. Mater. Chem. Phys. 59 (1999) 88.
Liang, Z. H.; Zhu, Y. J.; Hu, X. L. J. Phys. Chem. B 108 (2004) 3488.
Lieser, K. H. Angew. Chem. Int. Ed. Engl. 8 (1969) 188.
Lilienfeld, P. Appl. Opt. 30 (1991) 4696.
Liu, D.; Ren, S.; Wu, H.; Zhang, Q.; Wen, L. J. Mater. Sci. 43 (2008) 1974.
115
Ma, Z.; Han, H. Coll. Surf. A: Physicochem. Eng. Aspects 317 (2008) 229.
Mallat, T.; Baiker, A. Chem. Rev. 104 (2004) 3037.
Mandlimath, T. R.; Gopal, B. J. Mol. Catal. A: Chem. 350 (2011) 9.
Manova, E.; Tsoncheva, T.; Paneva, D.; Rehspringer, J. L.; Tenchev, K.; Mitov, I.; Petrov,
L. Applied Catalysis A: General 317 (2007) 34.
Mer, V. K. Ind. Eng. Chem. Res. 44 (1952) 1270.
Mie, G, Ann. Physik IV 25 (1908) 377.
Mie, G. Ann. Physik IV 1908, 25, 377; an English translation of this paper was provided by
the British Library.
Mohammadi, A.; Moghaddam, A. B.; Kazemzad, M.; Dinarvand, R.; Badraghi, J. Mater.
Sci. Eng., C 29 (2009) 1752.
Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 126 (2004)
10657.
Moser, J.; Gratzel, M. J. Am. Chem. Soc. 106 (1984) 6557.
Nevskaya, S. M.; Nikolaev, S. A.; Loktev, E. S. Catal. Today, 105 (2005) 344.
Nguyen, T.; Luke, G. Nano Today, 5 (2010) 213.
Nicolettilb, J. W.; Whitesides, G. M. J. Phys. Chem. 93 (1989) 759.
Noorbakhsha, A.; Salimia, A. Electrochim. Acta 54 (2009) 6312.
Nouneha, K.; Oyamab, M.; Diazc, R.; Abd-Lefdild, M.; Kityke, I. V.; Bousminaa, M. J.
Alloys Compd. 509 (2011) 5882.
Omole, M. A.; K’Owino, I. O.; Sadik, O. A. Appl. Catal. B: Envir. 76 (2007) 158.
Park, J. W.; Chae, E. H.; Kim, S. H.; Lee, J. H.; Kim, J. W.; Yoon, S. M.; Choi, J. Y.
Mater. Chem. Phys. 97 (2006) 371.
116
Park, K. W.; Choi, J. H.; Kwon, B. K.; Lee, S. A.; Sung, Y. E.; Ha, H. Y.; Hong, S. A.;
Kim, H.; Wieckowski, A. J. Phys. Chem. B 106 (2002) 1869.
Qi, L. Coord. Chem. Rev. 254 (2010) 1054.
Ramachandran, E.; Natarajan, S. Cryst. Res. Technol. 41 (2006) 411.
Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 116 (1994) 7401.
Rheem, Y.; Yoo1, B. Y.; Beyermann, W. P.; Myung, N. V. Nanotechnology 18 (2007) 6.
Rostovshchikova, T. N.; Smirnov, V. V.; Gurevich, S. A.; Kozhevin, V. M.; Yavsin, D. A.;
Nevskaya, S. M.; Nikolaev, S. A.; Lokteva, E. S. Catal. Today 105 (2005) 344.
Saikat, M.; Anand, G.; Neeta, L.; Rajesh, G.; Vivek, G.; Murali, S. Langmuir 17 (2001)
6262.
Sastry, M.; Patil, V.; Mayya, K. S.; Paranjape, D. V.; Singh, P.; Sainkar, S. R. Thin Solid
Films 324 (1998) 239.
Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 105 (2001) 9266.
Scher, E. C.; Manna, L.; Alivisatos, A. P., Philos. Trans. R. Soc. Lond. A. 361, (2003) 241.
Schmid, G. Chem. Rev. 92 (1992) 1709.
Schon, G.; Simon, U. Colloid Polym Sci. 273 (1995) 101.
Scott, R. W. J.; Ye, H.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 15 (2003) 3873.
Setoodeh, A.R.; Attariani, H.; Khosrownejad, M. Computational Mater. Sci. 44 (2008) 378.
Shankland, R. S. “Electron”, Encyclopedia Americana, Grolier Online, Feb. 1, 2005,
http://ea.grolier.com.
Sharma, N. C.; Sahi, S. V.; Nath, S.; Parsons, J. G.; Torresdey, J. L. G.; Pal, T. Environ.
Sci. Tech. 41 (2007) 5137.
Sheeparamatti, B. G.; Sheeparamatti, R. B.; Kadadevaramath, J. S. IETE Technical
Nanotechnology: Inspiration from Nature. Review, 24 (2007) 5.
117
Sidhaye, D. S.; Bala, T.; Srikanth, S.; Srinath, H.; Poddar, P.; Sastry, M.; Prasad, B. L. V.
J. Phys. Chem. 113 (2009) 3426.
Singla, M. L.; Negi, A.; Mahajan, V.;Singh, K. C.; Jain, D. V. S. Appl. Catal. A. 323 (2007)
51.
Sinha, A.; Das, S. K.; Kumar, T. V.; Rao, V.; Ramachandrarao1, P. J. Mater. Synth.
Proces. 7 (1999) 373.
Sladewski, T. E.; Shafer, A. S.; Hoag, C. M. Spectrochim Acta, Part A. 65 (2006) 985.
Song, X.; Gao, L. J. Phys. Chem. C 112 (2008) 15299.
Stepanov, A. L.; Khaibullin, R. I.; Rameev, B. Z.; Reinholdt, A.; Kreibig, U. Technical
Phy. Lett. 30 (2004) 151.
Sudip, N.; Snigdhamayee, P.; Sudipa, P.; Soumen, B.; Tarasankar, P. Current Sci. 92
(2007) 786.
Sun, D.; Zhang, J.; Ren, H.; Cui, Z.; Sun, D. J. Phys. Chem. 114 (2010) 12110.
Sun, Y. P.; Rollins, H. W.; Guduru, R. Chem. Mater. 11 (1999) 7.
Tanase, M.; Bauer, L. A.; Hultgren, A.; Silevitch, D. M.; Sun, L.; Reich, D. H.;
Searson, P. C.; Meyer, G. J. Nano Lett. 1 (2001) 155.
Teyeb, O. E.; Catherine, A.; Bruno, C.; Etienne, S.; Marc, V.; Marc, R.; Jean, M. B. Chem.
Mater. 11 (1999) 526.
Teyeb, O. E.; John, H. T.; Arvind, K.; Marc, R.; Wenhua, G.; Weidenthaler, C.;
Kenton, H. W. Chem. Mater. 17 (2005) 4750.
Tilley, R. D.; Jefferson, D. A. J. Phys. Chem. B, 106 (2002) 10895.
Tseng, W. J.; Hsu, C. K.; Chi, C. C.; Teng, K. H. Mater. Lett. 52 (2002) 313.
Turkevich, J. Stevenson, P. C. Hillier, J. Faraday Soc. 11 (1951) 55.
118
Valentinia, A.; Carrenob, N. L.V.; Probsta, L. F. D.; Leite, E. R.; Longo, E. Microporous
and Mesoporous Mater. 68 (2004) 151.
Vogelaar, B. M.; Langeveld, A. D.; Kooyman, P. J.; Lok, C. M.; Bonne, R. L.C.;
Moulijn, J. A. Catalysis Today 163 (2011) 20.
Wang, H.; Kou, X.; Zhang, J. Li, J. Bull. Mater. Sci. 31 (2008) 97.
Wang, Y.; Zhu, J.; Yang, X.; Lu, L.; Wang. X. Thermochimica Acta, 437 (2005) 106.
Wang, Z. L. Adv. Mater. 10 (1998) 13.
Wang, Z. L. J. Phys. Chem. B. 104 (2000) 1153.
Wei, X.; Kong, Y. L.; Hanfan, L.; Tao, H.; Chuntao, S.; Yanxi, Z. Mater. Lett. 62 (2008)
2571.
Williams, G. L.; Vohs, J. K.; Brege, J. J.; Fahlman, B. D. J. Chem. Ed. 82 (2005) 771.
Winnischofer, H.; Rocha, T. C. R.; Nunes, W. C.; Socolovsky, L. M.; Knobel, M.;
Zanchet, D. ACS Nano 2 (2008) 1313.
Wu, S. H.; Chen, D. H. J. Colloid Interface Sci. 259 (2003) 282.
Wu, Z. G.; Munoz, M.; Montero, O. Adv. Powder Technol. 21 (2010) 165.
Wu, Z.; Ge, S.; Zhang, M.; Li,W.; Tao, K. J. Colloid Interface Sci 330 (2009) 359.
Yoon, M.; Kim,Y.; Kim,Y. M.; Volkov,V.; Song, H. J.; Park, Y. J.; Park, I. W. Mater.
Chem. Phys. 91 (2005) 104.
Yua, C.; Qiu, J. S. Chem. Eng. Res. Des. 86 (2008) 904.
Zhang, B.; Ye, X.; Dai, W.; Hou, W.; Xie, Y. Chem. Eur. J. 12 (2006) 2337.
Zhong, Z.; Subramanian, A. S.; Highfield, J.; Carpenter, K.; Gedanken, A. Chem. A Eur. J.
11 (2005) 1473.
Zhou, W.; Guo, L.; He, L.; Chen, C. P. Phys. Stat. Sol. 205 (2008) 1109.
Zhou, Z. F.; Zhou, Y. C.; Pan, Y.; Lei, W. X.; Xu, C. F. Scripta Materialia, 60 (2009) 512.
