MORPHOLOGY CONTROLLED FABRICATION AND
APPLICATION OF COLLOIDAL FINE PARTICLES OF
ZINC COMPOUNDS
By:
NAILA ZUBAIR
NATIONAL CENTRE OF EXCELLENCE IN
PHYSICAL CHEMISTRY
UNIVERSITY OF PESHAWAR, PAKISTAN
December 2019
MORPHOLOGY CONTROLLED FABRICATION AND
APPLICATION OF COLLOIDAL FINE PARTICLES OF
ZINC COMPOUNDS
By:
NAILA ZUBAIR
A dissertation submitted in the partial fulfillment of the requirement of
the degree of Doctor of Philosophy in Physical Chemistry
NATIONAL CENTRE OF EXCELLENCE IN
PHYSICAL CHEMISTRY
UNIVERSITY OF PESHAWAR, PAKISTAN
December 2019
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL CHEMISTRY
UNIVERSITY OF PESHAWAR
DECEMBER 2019
It is recommended that the dissertation prepared by Miss Naila Zubair, entitled
“Morphology Controlled Fabrication and Application of Colloidal Fine Particles of
Zinc Compounds”, be accepted as fulfilling this part of the requirement for the degree
of Doctor of Philosophy in Physical Chemistry.
____________________________ ____________________________
(Prof. Dr. KHALIDA AKHTAR) (Prof. Dr. ABDUL NAEEM)
Research Supervisor Director
EXAMINATION SATISFACTORY
COMMITTEE ON FINAL EXAMINATION
___________________________ _________________________
External Examiner Internal Examiner
i
ACKNOWLEDGEMENTS
Foremost, I would like to express my deepest appreciation to my worthy
Supervisor, Prof. Dr. Khalida Akhtar for her valuable guidance, consistent
encouragement, constructive suggestions, patience and care. Without her dedicated
supervision and indispensable advices, this research work would not have been
possible.
I would like to express my sincere gratitude to Late Prof. Dr. Ikram Ul Haq,
for his persistence guidance, constant source of inspiration and motivation. He
continually and convincingly conveyed a spirit of adventure in regard to research.
His knowledge, expertise and cooperation were imperative to completion of my
research work.
I would like to thank Prof. Dr. Abdul Naeem, Director and Prof. Dr.
Muhammad Saleem Khan (Rtd) Ex. Director National Centre of Excellence in
Physical Chemistry, University of Peshawar, for their helpful suggestions during my
study and for all the facilities made available at the Centre. My appreciation also
extends to all my respected teachers for their constant encouragement and
cooperation.
I also acknowledge Higher Education Commission of Pakistan, for granting
me financial support to pursue my Ph.D. study.
I humbly extend special thanks to my family. Words cannot express how
grateful I am to my parents and siblings for their utmost love, unconditional support
and efforts which helped me attain this grace. Their prayers for me were what
sustained me thus far.
I am extremely thankful to all my lab fellows especially Ms. Hina Khalid, Mr.
Zia Ullah khan, Mr. Syed Sajjad Ali shah, Mr. Muhammad Gul, and my friends for
their assistance, collaborative attitude and moral support during my study.
Finally, thanks and appreciations are also extended to all staff members of the
Centre whose help was very valuable in this research.
NAILA ZUBAIR
ii
Dedicated To
My Parents
iii
LIST OF ABBREVIATIONS:
SEM (Scanning Electron Microscopy)
XRD (X-ray Diffractometry)
FT-IR (Fourier Transform Infrared Spectroscopy)
TG/DTA (Thermal Gravimetric/ Differential Thermal Analysis)
BET (Brunauer –Emmett–Teller)
PZC (Point of zero charge)
ZnO (Zinc oxide)
ZP (Zinc phosphate)
NPs (Nanoparticles)
HMT (Hexamethylenetetramine)
RT (Room temperature)
SMOs (Semiconducting metal oxides)
LPG (Liquid petroleum gas)
ZnO-AP (Zinc oxide, as-prepared)
ZnO-Cal (Zinc oxide, calcined)
ZnO-Com (Zinc oxide, commercial)
S. aureus (Staphylococcus aureus)
S. mutans (Streptococcuss mutans)
E. coli (Escherichia coli)
P. aeruginosa (Pseudomonas aeruginosa)
Enterobactor (Enterobactor cloacae)
iv
ABSTRACT
Zinc compounds nanostructures with controlled morphological features were
synthesized in aqueous solutions through simple and economical route without using
any type of surfactant or template. The resulting powders were subjected to SEM
analysis which revealed that morphology of the prepared powders was strongly
dependent upon the applied experimental conditions like pH, reaction time, reaction
temperature and reactants composition. As such synthesis conditions were optimized
out in a systematic manner to obtain nanostructures of uniform morphological
characteristics. Various morphologies ranging from nanorods, nano/microspheres,
nanoellipsoids, nano/micro flowers, cubes, sea urchin like hierarchical microspheres
composed of nanoneedles and hexagonal nanorods, sea shells and trigonal pyramidal
shape were synthesized. Selected batches of the prepared powders were also
investigated by XRD, FT-IR, TG/DTA and BET surface area analysis. Test samples
of as-prepared powders were subjected to calcination under controlled heat treatment.
XRD results illustrated the crystalline nature of the as-prepared and calcined powders.
Various crystallographic parameters i.e., crystallite sizes, lattice constants, x-ray
density and specific surface area were calculated from XRD results.
Selected samples were then employed for the fabrication of room temperature
gas sensor of industrial importance as well as for the application of effective
antibacterial agent. The gas sensing behavior of selected batches of zinc oxide (Z1cal–
Z4cal) and zinc phosphate (ZP1–ZP3) nanostructures were evaluated in specially
designed gas sensor setup. The sensor response was evaluated towards ammonia,
acetone and ethanol vapors. The effect of operating temperature, gas concentration
and nanostructure morphology on the sensing performance of the desired systems
were studied. Sensors based on the synthesized samples showed superior and
v
reproducible performance with high selectivity and stability towards 1 ppm ammonia
at room temperature (29 oC). This was attributed to the unique morphology and
remarkable uniformity in shape and size of the synthesized nanostructures. For
instance, ZP1 sensor showed highest room temperature gas sensing response of 89%
with response recovery time 31/12 s, towards 5 ppm ammonia. This can be attributed
to highly porous and hierarchical surface characteristics of synthesized powders.
Moreover, the lowest detection limit investigated was <1ppm, which demonstrated
excellent ammonia sensing characteristics of the synthesized nanostructures. In
addition, plausible reaction mechanisms for gas sensing of ZnO and ZP sensors were
studied. The superior gas response with excellent reproducibility was due to novel
hierarchical surface characteristic, considerable uniformity in shape and size and high
specific surface area of synthesized structures.
It is mentioned that up to our knowledge, no literature report is available
concerning the gas sensing properties of zinc phosphate micro/nanostructures.
Because of the excellent gas sensing performance, the studied samples could be
employed as promising candidates for developing highly sensitive and selective room
temperature ammonia gas sensor.
Furthermore, selected ZnO powders (Z1cal –Z4cal) and commercial ZnO were
then employed for in-vitro evaluation of antibacterial activity against various
pathogenic bacteria (Staphylococcus aureus, Streptococcuss mutans, Escherichia coli,
Pseudomonas aeruginosa and Enterobactor cloacae) of clinical importance. The
synthesized nanostructures were found to exhibit a promising anti-bacterial activity by
producing inhibition zones to the tested bacterial strains. Z4cal exhibited highest
antibacterial activity compared to other ZnO samples (Z1cal –Z3cal) due to high surface
area (95.20 m2/g) of its hierarchical porous structure.
vi
In addition, concentration dependent antibacterial study unfolded that size of
the inhibition zones increased from ~28 mm to 32 mm with increasing ZnO
concentration. However, ZnO-Com showed no antibacterial response in the employed
concentration range. Moreover, the synthesized nanostructures significantly enhanced
the antibacterial activity of ciprofloxacin, a standard antibiotic when employed in
combination. The present study suggests that the application of synthesized ZnO
nanostructures as antibacterial agent in biomedical sides may be effective at inhibiting
certain pathogenic bacteria.
Key Words: Zinc compounds, Zinc oxide, Monodispersed, Controlled morphologies,
Hierarchical structures, Gas sensor, Sensor response, Response time, Recovery time,
antibacterial activity, Pathogenic bacteria.
vii
LIST OF FIGURES
FIGURE CAPTION PAGE
Figure 1 The hexagonal wurtzite crystal structure of ZnO. 2
Figure 2 Schematics showing the interaction between SMO’s NPs and
reducing gas (NH3).
4
Figure 3 SEM images of asprepared ZnO nanostuctures obtained at 80
oC for various reaction times, a) 30 min, b) 15 min, c) & d) 45
min, e) & f) 1 h.
32
Figure 4 Schematics showing change in nanostructure morphology
with change in reaction time.
33
Figure 5 SEM analysis of ZnO nanostructures precipitated from zinc
salt and ammonia gas at 30 oC after time interval a) 5 min, b)
10 min, c) 15 min, d) 20 min, e) 25 min and f) 30 min.
36
Figure 6 Schematics illustrating the effect of aging time on the growth
pattern of particles depicted in Figure 5f.
37
Figure 7 ZnO nanostrucrures synthesized from aqueous solution of
zinc salt and ammonia gas after reflux heating for a)10 min,
b) 20 min and c) 30 min.
39
Figure 8 Schematics illustrating the effect of refluxing time on
particles growth depicted in Figure 7a–c.
39
Figure 9 SEM images of nanostructures precipitated from aqueous
solutions containing zinc nitrate (1.5–5 mol.L-1
) and
ammonium hydroxide (1–15%) heated for 15 min. at: a) 90
oC, b) 80
oC, c) 60
oC, d) 50
oC, e) 40
oC, f) 30
oC.
42
Figure 10 SEM images of zinc oxalate nanostructures prepared in 15
min from zinc salt and oxalic acid in ratio, a) 1:1 at 30 oC , b)
1:1.5 at 30 oC , c) 1: 2 at 30
oC, d) 1:3 at 30
oC, e) 1:3 at 40
oC.
44
Figure 11 SEM images of zinc oxalate microstructures prepared from
zinc salt and oxalic acid in 1:1 ratio at 30 o
C in time period, a)
30 min, b) 1h.
46
Figure 12 SEM images of zinc oxalate nanostructures prepared in 15 47
viii
min from zinc salt and oxalic acid in 3:1 ratio at 40 oC.
Figure 13 Schematic illustration of effect of precursors composition (a–
d), reaction temperature (d–e), order of addition (e–f) and
reaction time (a–g–h) on morphology of zinc oxalate
nanostructures.
49
Figure 14 a) Zinc phosphate hierarchical microspheres produced by
heating aqueous solutions of zinc salt and diammonium
hydrogen phosphate at 80 oC, b) 10 min, c) 20 min and d) 30
min.
50
Figure 15 a) Zinc phosphate hierarchical microspheres produced by
heating aqueous solutions of zinc salt and diammonium
hydrogen phosphate at 90 oC, b) 10 min, c) 20 min and d) 30
min.
52
Figure 16 SEM images of as prepared nanoflowers synthesized by
heating reactant solution used for particles (shown in Figure
14a) in the absence of diammonium hydrogen phosphate at 80
oC for; a) 30 min, b) high magnification image of a, c) 1 h, d)
high magnification image of c.
55
Figure 17 Schematics showing the effect of synthesis parameters on
morphology of aspreapred nanostructures synthesized from
reactant mixture containing zinc salt, diammonium hydrogen
phosphate and ammonia; a) at 80 oC for 30 min, b) 90
oC for
30 min, c) zinc salt and ammonia at 80 oC for 30 min, d) zinc
salt and ammonia at 80 oC for 1h.
56
Figure 18 SEM images showing the effect of synthesis temperature on
particle size and morphology of zinc phosphate
nanostructures synthesized from zinc nitrate and diammonium
hydrogen phosphate (1: 4) at; a & b) 40 oC, c & d) 50
oC, e &
f) 60 oC, g) 70
oC and h) 80
oC.
57
Figure 19 Zinc phosphate nanostructures achieved after reversing the
order of addition of reactants for particles shown in Figure
18h, a & b) after 30 min, c & d) overnight ageing in mother
liquor at room temperature.
59
ix
Figure 20 XRD patterns of the selected asprepared powders, Z1–Z4. 62
Figure 21 FT-IR spectra of selected asprepared powders, Z1–Z4. 64
Figure 22 TG/DTA plots of selected asprepared zinc compounds, a) Z1–
Z3, b) Z4.
67
Figure 23 a) α vs Temperature curve for the asprepared Z4 particles, b)
& c) straight lines for the corresponding step-I and step-II in
Figure 22b.
70
Figure 24 SEM images of ZnO particles calcined at; a–c) 750 oC; Z1cal–
Z3cal, d) 450 oC; Z4cal.
73
Figure 25 XRD diffractograms of the calcined ZnO particles, Z1cal–
Z4cal.
74
Figure 26 FT-IR spectra of the calcined ZnO particles, Z1cal–Z4cal. 77
Figure 27 XRD diffractograms of the asprepared zinc phosphate
powders.
79
Figure 28 FT-IR spectra of selected asprepared zinc phosphate powders
(ZP1–ZP3).
82
Figure 29 TG/DTA curves of selected zinc phosphate powders (ZP1–
ZP3).
84
Figure 30 SEM micrographs of Zn3(PO4)2 obtained after heat treatment
of ZP1–ZP3 (SEM, Fig 10a, 11a &14c), a & b) ZP1cal, c & d)
ZP2cal and e) ZP3cal.
87
Figure 31 XRD spectra of the calcined zinc phosphate powders (ZP1cal–
ZP3cal).
89
Figure 32 FT-IR spectra of the calcined zinc phosphate Zn3(PO4)2
(ZP1cal–ZP3cal).
90
Figure 33 BET plots for selected ZnO samples. 92
Figure 34 BET plots for the selected asprepared zinc phosphate samples
(ZP1–ZP3).
93
Figure 35 Block diagram of gas sensor setup. 95
Figure 36 Electrical resistance as a function of temperature of the
fabricated gas sensors (Z1cal–Z4cal & ZP1–ZP3).
96
Figure 37 Ln (σ) versus 1/Tk plots along with the corresponding
activation energies for Z1cal–Z4cal sensors.
99
x
Figure 38 a) Dynamic resistance response curves of the ZnO sensors. b)
First cycle of resistance curves shown in (a) for response (%)
calculation.
102
Figure 39 a) Dynamic resistance response curves of zinc phosphate
based sensors, b) First cycle of resistance curves shown in (a)
for response (%) calculation.
103
Figure 40 Schematics showing the interaction between ammonia gas
and the surface of zinc phosphate sensor (ZP1–ZP3).
106
Figure 41 Response of Z1cal–Z4cal based sensors at different operating
temperatures towards ammonia vapors (5 ppm).
111
Figure 42 Dynamic resistance curves of ZnO based sensors towards
different ammonia concentrations.
113
Figure 43 Dynamic resistance curves of zinc phosphate based sensors
towards different ammonia concentrations.
114
Figure 44 Bar graph showing response of ZnO sensors towards different
ammonia concentrations.
115
Figure 45 Bar graph showing response of zinc phosphate sensors
towards different ammonia concentrations.
116
Figure 46 Stability in response of ZnO sensors towards 25 ppm
ammonia.
119
Figure 47 Stability in response of zinc phosphate sensors towards 25
ppm ammonia.
120
Figure 48 FT-IR spectra of ZnO sensor materials after exposure to
ammonia followed by flushing with dry air.
121
Figure 49 FT-IR spectra of zinc phosphate based sensors (ZP1–ZP3)
after exposure to ammonia gas followed by flushing with dry
air.
122
Figure 50 Bar graph showing selective response of different sensors
towards 1 ppm ammonia, acetone and ethanol vapors.
124
Figure 51 Selectivity of Z4Cal towards the same concentrations of: a)
ammonia, b) acetone, c) ethanol vapors (1ppm) at room
temperature.
125
Figure 52 Selectivity of zinc phosphate based sensors towards 1 ppm 126
xi
ammonia, acetone and ethanol vapors at room temperature.
Figure 53 Point of zero charge (PZC) of the selected ZnO samples. 128
Figure 54 Antibacterial activity of the selected ZnO and positive control
against various pathogenic bacterial strains.
130
Figure 55 Antibacterial activity of ZnO-Com against various pathogenic
bacterial strains.
130
Figure 56 Antibacterial activity of the selected ZnO samples and
ciprofloxacin against various pathogenic bacterial strains.
131
Figure 57 Antibacterial effect of the selected samples Z2cal–Z4cal)
combined with ciprofloxacin in 1:1 ratio.
135
Figure 58 Schematics showing the interaction of ciprofloxacin and ZnO
nanostructure complex with bacterial cell membrane.
137
Figure 59 Schematics showing the interaction of ZnO nanostructures
with the bacterial cell.
140
xii
LIST OF TABLES
TABLE CAPTION PAGE
Table 1 Wavenumber positions at which the chemical groups on the
selected asprepared solids absorb IR radiations.
65
Table 2 Thermal weight losses and corresponding activation energies
estimated for asprepared Z4 sample.
71
Table 3 Illustration of various crystallographic parameters estimated out
from XRD patterns of calcined ZnO samples (Z1cal–Z4cal).
75
Table 4 Illustration of various crystallographic parameters estimated out
from XRD patterns of selected zinc phosphate samples (ZP1–
ZP3).
81
Table 5 Wavenumber positions at which the chemical groups on the
selected asprepared solids absorb IR radiations.
82
Table 6 Temperatures and corresponding weight losses estimated for the
asprepared ZP1–ZP3 samples.
85
Table 7 Activation energies, gas response and response/recovery time of
the fabricated ZnO based sensors towards the detection of
ammonia gas.
100
Table 8 Comparison of sensor response of the fabricated sensors with
the previously reported ZnO based ammonia gas sensors.
109
Table 9 Comparison of antibacterial activity of the present work with
the reported literature.
134
xiii
TABLE OF CONTENTS
CHAPTER CONTENTS PAGE
CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW 1
1.1 Introduction 1
1.2 Gas Sensing Properties 2
1.2.1 Gas Response 5
1.2.2 Response/Recovery Time 5
1.2.3 Selectivity and Stability 5
1.2.4 Concentration of Test Gas 6
1.2.5 Working Temperature 7
1.2.6 Types of Test Gas 8
1.3 Zinc Phosphate as Gas Sensor 13
1.4 Antibacterial Activity of ZnO 14
1.4.1 Mechanism of Antibacterial Activity 17
1.4.2 Particle Size and Concentration 18
1.4.3 Particle Morphology 19
1.5 Aim and Objectives 21
CHAPTER 2 EXPERIMENTAL 22
2.1 Materials 22
2.2 Synthesis of Zinc Compounds 22
2.2.1 Synthesis of Zinc Oxide (ZnO) 22
2.2.2 Synthesis of Zinc Oxalate 23
2.2.3 Synthesis of Zinc Phosphate 23
2.2.4 Calcination 23
2.3 Characterization of Zinc Compounds 24
2.3.1 Scanning Electron Microscopy (SEM) 24
2.3.2 X-ray Diffractometry (XRD) 24
2.3.3 Fourier Transform Infrared Spectrometry (FT-IR) 24
2.3.4 Thermogravimetric /Differential Thermal Analysis
(TG/DTA)
24
2.3.5 Surface Area Analysis 25
2.3.6 Point of Zero Charge (PZC) 25
xiv
2.4 Gas Sensing Properties 26
2.5 Antibacterial Activity 26
CHAPTER 3 RESULTS AND DISCUSSION 28
3.1 SEM Analysis of Zinc Compounds 28
3.1.1 Synthesis of ZnO Nanostructures using HMT 29
3.1.2 Synthesis of ZnO Nanostructures using Ammonia 31
3.1.3 Synthesis of Zinc Oxalate 41
3.1.4 Synthesis of Zinc Phosphate Nanostructures 48
3.2 XRD Analysis of the As-prepared ZnO and Zinc Oxalate 61
3.3 FT-IR Analysis of ZnO and Zinc Oxalate 63
3.4 Thermal Analysis of ZnO and Zinc Oxalate 66
3.4.1 Calcination 69
3.5 XRD Analysis of Asprepared Zinc Phosphate 78
3.6 FT-IR Analysis of Asprepared Zinc Phosphate 80
3.7 Thermal Analysis of Asprepared Zinc Phosphate 83
3.7.1 Calcination 86
3.8 Surface Area Analysis 91
3.9 Gas Sensing Properties of ZnO and ZP Sensors 94
3.9.1 Semiconducting Properties 94
3.9.2 Gas Sensing Properties 101
3.9.3 Ammonia Sensing Mechanism 105
3.9.4 Response/Recovery Time 107
3.9.5 Effect of Temperature on Sensor Response 110
3.9.6 Effect of Ammonia Gas Concentration 112
3.9.7 Gas Sensor Stability and Reproducibility 118
3.9.8 Selectivity 123
3.10 Antibacterial Activity of ZnO 127
3.10.1 Point of Zero Charge (PZC) 127
3.10.2 Antibacterial Effect of ZnO and Ciprofloxacin
Combination
133
3.10.3 Mechanism of Antibacterial Action 138
Conclusions 141
xv
Future goals 143
References 144
1
CHAPTER 1
INTRODUCTION AND LITERATURE REVIEW
1.1. Introduction
Monodispersed fine particles of zinc compounds have emerged the foremost
multifunctional materials due to remarkable performance in several high-tech
applications, such as gas sensors, catalysis, cosmetics, food preservation, and
nanomedicines. In particular, zinc oxide (ZnO) being the most interesting and
versatile zinc compound is of prime importance due to its unique properties, like good
electrical conductivity, enhanced UV protection, biocompatibility and excellent
antimicrobial activity [1–3]. All these physical, chemical and optical properties like
photocatalytic, electrical, gas sensing, and antimicrobial activity strongly depend upon
the structure, particle size and morphology of ZnO nanostructures [4–9].
Zinc oxide is also considered an excellent material for gas sensing due to the
direct bandgap (3.37 eV), good semiconducting property and high thermal and
chemical stability [10–11]. ZnO exists in three types of crystal structures that are rock
salt, zinc blende, and hexagonal wurtzite. The rock salt structure can only be observed
at quite high pressures of ~10 GPa and zinc blende form can be made stable by
developing ZnO over substrates of the cubic lattice arrangement, whereas hexagonal
wurtzite form is thermodynamically the most stable crystal phase at room temperature
(Figure 1). The zinc cations, Zn+2
and oxygen anions, O2-
are tetrahedrally coordinated
through SP3
covalent bonding and are alternately arranged along the c-axis [12]. The
whole structure has no central symmetry. The unit cell is comprised of two lattice
parameters i.e., a (3.296 Å) and c (5.2065 Å) [13–14]. However, these values of
lattice parameters may change due to variations in growth conditions and doping of
ZnO [15]. The top face is terminated by Zn2+
ions while at the bottom face O2-
ions
2
are terminated and thus make both the faces polar and are designated as basal polar
planes. For instance, the wurtzite crystal unit has two polar and two non-polar faces
which differ in their growth rates [16]. The different growth rates of these polar and
non-polar planes are the main factors that are responsible for variation in surface
morphology of ZnO nanostructures [17].
Figure 1. The hexagonal wurtzite crystal structure of ZnO.
Various ZnO nanostructures can thus be employed to fabricate excellent gas
sensors due to their greater surface area, higher charge carriers transport rate and non-
toxicity. ZnO has been employed as a promising gas-sensitive candidate for the
detection of various flammable and hazardous gases like H2, NO, H2S and NH3 as
well as volatile organic compounds including, acetone and ethanol [18]. On exposure,
the test gas adsorbs over the sensor surface and a specific kind of surface state appears
on the ZnO nanostructures which would have obvious impacts on their electrical
conductance.
1.2. Gas Sensing Properties
Since semiconducting metal oxides (SMOs) based gas sensors are devices that
display changes in their electrical resistance on the exposure of the target gas. In
3
general, the basic mechanism regarding the gas sensing of the SMOs gas sensor
involves surface controlled reactions between the test gas and the adsorbed oxygen
over the sensor surface. Mostly, the SMOs sensors recognize the test gases through
the oxidation-reduction reaction (redox) between the test gas molecules and surface
adsorbed oxygen ions species [19]. Generally, in the air ambient at a temperature
below 100 oC, oxygen is chemisorbed over the sensor surface as O
- and O
2-, by
trapping the electrons nearby the surface (Eqn. 1) thus generating depletion layer
inside the conduction band and increases the sensor resistance [20–21]. At higher
temperatures above 100 oC, the adsorbed oxygen ions dissociate into O⁻ and O
2⁻
depending on the electrons trapped from the conduction band through Eqn. 2 & 3.
Whereas upon the exposure of a reducing gas like NH3, there is co-adsorption of
target ammonia molecules and their interactions with the adsorbed oxygen ions
resulting in oxidation reaction at the surface thereby release the trapped electrons of
the conduction band. This phenomenon leads to a decrease in the sensor’s resistance.
The reaction between surface adsorbed oxygen ions and the target gas molecule is
presented in Eqn.4.
O2(gas) + e− → O2
−(ads) (1)
O2−
(ads) + e− → 2O
− (ads) (2)
O−
(ads) + 2e− → O
2− (ads) (3)
4NH3 + 3O2−
(ads) → 2N2 + 6H2O + 3 e−
(4)
The generalized gas sensing mechanism and the corresponding
increase/decrease in resistance of n-type SMOs is illustrated through schematics in
Figure 2. For n-type semiconducting materials (as ZnO and SnO2) the sensor
resistance decreases on exposure to reducing gases like NH3 and H2S, while increases
4
for p-type materials (as CuO). In contrast, the effect on exposure to oxidizing gases is
reversed as compared to reducing gases.
Figure 2. Schematics showing the interaction between SMO’s NPs and reducing gas
(NH3).
Actually, the reaction among the surface adsorbed oxygen species and the
target reducing gases plays a key role in the gas sensing performance of
semiconducting materials. By applying the nanostructured materials, gas sensing
characteristics could be improved. Recently, SMOs nanostructures in a number of
morphologies like nanowires, nanorods, nanobelts, tetrapods nano/micro flowers have
been employed for gas sensing applications and have explored that gas sensing
characteristics of these SMOs based sensors significantly depend upon the
nanostructure morphology of the materials. Interestingly, the gas sensing response of
SMOs can be amended by decreasing the nanostructure size. Investigation of gas
5
sensing properties of In2O3 of different grain sizes revealed that the sensor response
increased sharply with the decrease in grain diameter [22].
In addition, other important parameters of SMOs based gas sensors, including
sensor stability, selectivity, working temperature, response/recovery times and
concentration of the test gas, are discussed below.
1.2.1. Gas Response
The response (S) of a gas sensor can be defined as the ratio of the resistance
change caused by the target gas to the initial resistance of the sensor [20, 23–24]. For
reducing gases the gas response is defined as S= (Ra–Rg)×100/Ra and for oxidizing
gases, it is defined as S= (Rg–Ra)×100/Ra. Where, Ra and Rg represent the resistance
of the gas sensor in air and the test gas, respectively.
1.2.2. Response/Recovery Time
In fact, a gas sensor with rapid response/recovery times is desirable for real-
time usage in their practical applications. As such, response/recovery times are also
the basic parameters that are used to determine the performance of a gas sensor. It is
mentioned that the response time is referred to the time taken by the sensor to attain
90% of its total resistance change when exposed to target gas, while the recovery time
is the time taken by the sensor to regain 90% of the recovery from the maximum
impact of the target gas.
1.2.3. Selectivity and Stability
Likewise, selectivity is another challenge encountered by gas sensors in their
practical applications. A gas detecting study demonstrated that the ZnO microrods
based sensor indicated high sensitivity and good selectivity towards the detection of
liquid petroleum gas (LPG) in comparison to H2S, H2, ethanol, and ammonia [25].
Likewise, ZnO NPs subjected towards different gases showed a good response
6
towards ammonia gas at 150 oC. Similarly, Mn-doped ZnO exhibited a good response
value of 28.48 towards the ammonia gas as compared to other target gases [26].
Besides, the gas sensors designed for market usage have to guarantee stability
in operation. It means that sensors should display a stable and reproducible response
towards a certain gas for a definite time period. In fact, there are some factors which
lead to the instability in sensor response [19], such as (i) structural variation i.e.,
difference in grain size and morphology, (ii) design errors, (iii) variation in
surrounding environment and (iv) phase changes of the sensor material, etc. To
overcome the mentioned problems, following points should be considered during
fabrication of sensors i.e., (i) use of the sensor materials of high chemical & thermal
stability, (ii) control on the composition and morphological features of the sensing
materials and (iii) application of specific techniques for pretreatment of the sensor
surfaces.
1.2.4. Concentration of Test Gas
The response of a gas sensor material depends upon the concentration of the
test gases as greater the concentration of the test gas greater is the sensor response. It
is because when the test gas concentration is increased, a greater number of gas
molecules are available for interaction with the sensor material which then increases
the sensor response. ZnO and aluminum-doped ZnO have been exposed to various
concentrations of ammonia in the range of 5–500 ppm. It has been demonstrated that
sensor response increased with the increasing concentration of ammonia gas and also
observed higher response for aluminum-doped ZnO than pure ZnO nanorods, on
exposure to 100 ppm ammonia [27]. Li and coworkers [23] adopted nanocrystalline
ZnO film for the fabrication of an ammonia gas sensor. They examined the sensor
performance under the effect of ammonia concentration (50-600 ppm) as well as
7
operating temperature (150-300 oC. Their findings showed the maximum sensing
response value of 57.5% under the exposure of 600 ppm ammonia with
response/recovery times 160/660 s at 150 oC. While in the ambiance 50 ppm
ammonia, the response recorded was around 18% with the corresponding
‘response/recovery times’ of 660/600 s respectively.
1.2.5. Working Temperature
It has been reported in the literature that the gas response of metal oxide-based
sensors is generally affected by the ambient temperature [28–29]. As Zeng et al. [30]
reported a temperature of 350 oC as the optimal working temperature for pure ZnO
nanostructures based ammonia sensor. Fan et al. [31] obtained the highest ethanol
sensing response of ZnO as 34.5% at 250 oC optimum working temperature and found
lower sensor response below and above the optimum working temperature. Similarly,
Zhang et al. [32] also observed the lower response value below and above the
optimum temperature. They reported 300 oC as optimum temperature for acetone
sensing and observed the corresponding highest response as 41.
It has been demonstrated that lower response value at a lower temperature was
due to the fact that the thermal energy was not enough to support the interaction of the
adsorbed oxygen ions with the target acetone molecules. However as the temperature
increased to a certain value, the gas response reached its maximum value and that
temperature was described as the optimum for ZnO based acetone sensor. Further
increase in temperature beyond the optimum value again led to a decrease in the
sensor response because at high-temperature desorption of the adsorbed oxygen ions
take place.
8
1.2.6. Types of Test Gas
So far, researchers have investigated different types of gaseous species of
environmental concern. In particular, ammonia gas is of great commercial importance
and is broadly used in many common industrial processes, food processing, and
fertilizers [33]. It is known that ammonia is a very toxic pollutant. According to the
Occupational Safety and Health Administration (OSHA) the specified threshold limit
value of ammonia gas at the workplace is about 50 ppm. Ammonia adversely affects
both human and animal life. Many countries have embraced standard prerequisites
that ammonia level must not surpass 20 ppm of lower detection zones [34–36].
Ammonia leakage is always a danger in these processes and needs to be detected early
on, before the onset of an accident. To address this concern, efforts have been made in
the development of efficient, durable, and stable ammonia gas sensors [37–38].
However, the equipment currently available for controlling ammonia level works at
elevated temperatures and requires high upkeep which motivates researchers to
develop high-performance ammonia gas sensors that possibly work at relatively lower
temperatures.
Besides, volatile organic compounds (VOC’s) especially acetone and ethanol
are widely used both at laboratory scale as well as in industries. For instance, acetone
is a common industrial solvent and is also generally accepted as one of the
biomarkers, used for noninvasive diagnosis of type-1 diabetes [39]. Similarly, ethanol
has also a myriad of uses in industrial productions, as biofuel and in medicines as a
disinfectant, analgesic, and sedative, etc. However, due to high volatile and
flammable nature at room temperature, their exposures increase to the human body
and can cause several adverse health effects [40–41]. Therefore, it is also essential to
develop efficient and highly sensitive sensors that could detect a trace amount of
9
VOCs in residential as well as in industrial environments. However, it has generally
been reported that a number of parameters, such as operating temperature and
interfering gases, affect the performance and sensitivity of the sensor. The sensor
operation temperature is a matter of great concern [42]. To meet various detection
requirements, researchers have been using different techniques to control material
synthesis and morphology and thus investigated gas sensors with upgraded sensing
performance. To date, pure ZnO nanostructures based sensitive and reliable NH3 gas
sensors have been reported, however, they require high working temperatures
(generally > 250 oC) for activating the NH3 adsorption and desorption processes,
which also results in rather high energy consumption as well as limits their practical
application in a low-temperature environment. Therefore, a reliable and sensitive
room temperature detection of NH3 is still challenging.
Chen et al. [37] investigated ammonia gas sensing properties of the ZnO
nanorods based sensors. The effect of temperature of sensor response illustrated the
optimum working temperature as about 300 oC. Measurement of the sensor response
determined a maximum value of 81.6 in the ambient of 1000 ppm ammonia at the
mentioned temperature. The lower detection limit of the sensor was determined as 10
ppm.
Senthil and Anandhan [43] synthesized nanocrystalline ZnO with fibrous
structures through the sol-gel method accompanied by the electrospinning technique.
They investigated the influence of calcination temperature on structural properties of
ZnO nanofibers and found that grain size and crystallinity were enhanced with the
increasing calcination temperature. The sensitivity of the nanofibers towards the
detection of ammonia gas was also tested and was noticed to increase with increasing
the ammonia concentration (40–100 ppm). Similarly, sensitivity vs. temperature plot
10
revealed 160 o
C as the optimal working temperature for 50 ppm ammonia. Maximum
sensitivity was experienced to 100 ppm ammonia at a temperature greater than 250
oC.
Ozutok et al. [44] examined the ammonia gas sensing properties of the ZnO
and aluminum (Al) doped ZnO towards 75 ppm NH3. They measured gas sensing at
different operating temperatures i.e., 50-210 oC and different ammonia
concentrations. They observed~33% response for Al-ZnO and ~5% for pure ZnO at
190 oC.
Ammonia sensing performance of pure ZnO and Ag/ZnO composite have
been analyzed under the influence of operating temperature and 150 oC was observed
as the optimum working temperature [21]. Pure ZnO showed a good response
compared to Ag/ZnO at 150 oC for 100 ppm of ammonia concentration. The obtained
results also showed that the response value for Ag/ZnO increased from 0.7% at 10
ppm ammonia to 29.5% at 100 ppm of ammonia.
Rawal [45] measured the gas sensing behavior of ZnO and SnO2 nanoparticles
for ammonia gas sensing at 100 oC. They found a response of 3.96% and 4.53% for
ZnO and SnO2 with the corresponding response/recovery times of 38/156 s and 31/66
s in the ammonia environment at the concentration of 46 ppm.
Notably, some researchers have made worthy attempts to produce ZnO based
ammonia gas sensors at room temperature [46–47]. Like, ZnO nanorods,
functionalized with gold (Au) particles and doped with manganese were used in the
gas sensors that could detect flammable gases. It was observed that Au loaded ZnO
sensor and Mn-doped ZnO showed higher response towards NH3 gas as compared to
pure ZnO sensor, at room temperature [18, 26].
11
Ang et al. [42] fabricated a gas sensor based on ZnO nanorods and reported
that the prepared sensor displayed a good response of 8% towards ammonia at 500
ppm concentration. The sensor recovery was found slow which demonstrated that
ammonia could not be desorbed easily from the sensor surface. However, when the
working temperature was amplified to 150 oC, the response enhanced from 8% to
60% and the sensor recovery time also shortened. The authors attributed the room
temperature high sensor response to the small particle size and large surface to
volume ratio.
Kuo and his coworkers [48] demonstrated ammonia sensing of the poly(3-
hexylthiophene): ZnO hybrid film produced by using a spin coating process. They
proposed that the devised sensor composed of hybrid ZnO film had better sensing
response to ammonia gas than pure poly(3-hexylthiophene) or pure ZnO film in the
examined ammonia concentrations. Moreover, their sensing device could attain a
maximum response of about 37% against 5 ppm of ammonia.
Zhang et al. [49] examined the ammonia sensing response of pure ZnO,
MoS2/PDDA and MoS2/ZnO film-based sensors at room temperature. They obtained
response values about 5.97%, 19.63% and 23.96% for pure ZnO, MoS2/PDDA and
MoS2/ZnO sensors towards 5 ppm ammonia, respectively. Their work also showed
maximum response value (46.2%) for the MoS2/ZnO sensor with response/recovery
time 10/11 s on exposure to 50 ppm ammonia compared to pure ZnO sensor (~10%)
with response/recovery time 7/19 s. Their work reported the MoS2/ZnO sensor as a
promising candidate for ammonia sensing.
Similarly, aluminum-doped ZnO/CuO and yttrium doped ZnO showed a good
response of about 54% compared to pure ZnO with around 23% response towards 500
ppm of ammonia gas [50–51].
12
Besides, measurements of the electrical and NH3 sensing properties of the
nanorods ZnO synthesized over sapphire substrates determined the response of
22.6%, 1.4% and 4.1% towards 100 ppm NH3 at room temperature for different
surface activated ZnO samples [52]. As such the sensing performance is greatly
affected by the chemical composition, structural features, like particle’s size and
surface morphology of the sensor material, concentration as well as working
temperature of the target gases, etc.
So far, many researchers have made efforts to improve various properties of
ZnO to enhance its gas sensing properties. Therefore, nanotechnology experts have
focused on the synthesis of ZnO nanoparticles in various uniform morphological
features for the development of highly efficient and economical gas sensors because
of the growing concern about environmental protection, industrial safety, and
increasing market demand.
For instance, 1D nanostructure-based gas sensors are rather more superior in
their performance than thin-film based sensors because of the greater surface area
[53]. Similarly, the ZnO nanostructures based gas sensor has been fabricated for room
temperature detection of H2S gas [54]. However, ZnO based sensors have experienced
certain drawbacks; for example, poor selectivity, high working temperature, lower
response, and longer response recovery times which are the basic problems
encountered by SMOs sensors [49]. So far, findings suggest that particle shape, size,
and uniformity, among other variables, play a vital role in controlling the properties of
the ZnO based gas sensors. As such, we believe that there exists ample room for
further research in this important area, especially in tailoring the performance and
sensitivity of the ZnO based gas sensors through the use of ZnO powders composed
of particle systems of different morphological features and chemical compositions.
13
ZnO particles have been prepared through various routes, such as thermal evaporation
process [55], chemical vapor deposition [28], electrophoretic deposition method [56],
homogeneous precipitation method [57], and refluxing route [58]. Therefore,
researchers made the synthesis of ZnO in different morphological features for
fabrication highly sensitive and selective gas sensors that could work at room
temperature with a stable and reproducible response.
1.3. Zinc Phosphate as Gas Sensor
In addition, zinc phosphate is also one of the important zinc compounds which
have found application in the commercial, scientific, and industrial as well as health
sectors. Zinc phosphate is extensively used as dental cement and anticorrosive
pigment due to its superb properties like low solubility, nontoxicity and biocompatible
nature [59–60]. Zinc phosphate likewise has applications as a drug carrier in the
biomedical field [61–62]. Besides, zinc phosphate has been investigated for its
catalytic properties. In this regard, various synthesis routes have been employed for
obtaining zinc phosphate nanoparticles in various morphological structures which can
further extend its application field. For instance, various capping agents and
templates like disodium phosphate, cetyltrimethylammonium bromide (CTAB) and
yeasts have been employed for the fabrication of zinc phosphate nanoparticles [59,
63–64]. The solid-state route has been employed for the synthesis of zinc phosphate
nanoparticles with nearly spherical morphology [65]. However, it required a
surfactant and longer fabrication time of about 24 h. Oleic acid has also been reported
essential to obtain monodispersed zinc phosphate microspheres through the
solvothermal process in a sealed autoclave, at 180 oC for 24 hours. While without the
use of oleic acid, amorphous agglomerated nanoparticles were obtained as well as the
synthesis process is too long [66].
14
In order to obtain monodispersed particles, ultrasonic technology has also
been adapted in combination with triton x-100 for the fabrication of zinc phosphate
nanocrystals in recent years [60]. However, the obtained product was composed of
agglomerated particles. In the same way, without using any surfactant different
morphologies of micron size zinc phosphate have also been obtained with the aging
time of 2h which revealed that pH of the reactants mixture, reactant concentration,
and reaction temperature corresponded to the morphology of synthesized product [63,
67]. In addition, zinc phosphate nanosheets, nanoplates, and microsphere have also
been synthesized [59, 68–69], however, the reported microstructures synthesized in a
reaction time of 5 days were irregular in shape and size. It has been reported that the
synthesis of micro/nanostructures with controlled morphologies is very important for
exploring morphology dependent properties and their performance in various
applications. Therefore, in the present work, it was also focused on synthesizing zinc
phosphate micro/nanostructures with controlled morphological features for the
fabrication of efficient gas sensors that could work at room temperature. To the best
of our knowledge, gas sensing properties of zinc phosphate have not been reported so
far.
1.4. Antibacterial Activity of ZnO
ZnO offers remarkable benefits in biomedical and clinical sites due to
significant antibacterial activities over broad-spectrum pathogenic bacteria and has
been revealed as a promising candidate for orthopedic and dental implant coating [3].
Recently, bacterial infective diseases have become serious health problems that have
attracted the general population consideration worldwide as human wellbeing risk and
reach out to economic as well as social complexities. Increased outbursts and diseases
of pathogenic bacterial species, the emergence of bacterial mutation and antibiotic
15
resistance, the absence of appropriate immunization in underdeveloped nations and
hospital-acquired infections are worldwide peril to humans, especially children [70].
As such, the development of novel and effective antibacterial agents against drug-
resistant pathogenic bacteria particularly Pseudomonas aeruginosa (P. aeruginosa),
Escherichia coli (E. coli), Enterobacter cloacae (Enterobacter), Streptococcus
mutans (S. mutans) and Staphylococcus aureus (S. aureus), etc. has become the
utmost demand.
Since, antibacterial activity is one of the present hot research programs, which
has attracted considerable interest in nanomedicine to fulfill the drug delivery
requirements, to minimize antibiotic concentration and to control drug-resistant
pathogenic bacteria. According to the American Heritage Medical Dictionary,
antibacterial activity is known as the action through which bacterial growth is
inhibited or destroyed. In other words, it can also be illustrated as a function of the
surface area the microorganisms are in contact with [71]. Likewise, antibacterial
agents are described as specific concentration drugs proficient to selectively inhibit or
damage the bacterial cells and are not damaging to host cells. These antibacterial
compounds/drugs act just like chemotherapeutic agents for prevention and medication
of the bacterial infections.
In this respect, it has been reported that ZnO could be effectively used as an
antibacterial agent because of its biocompatible nature and high purity. In addition, it
shows no resistance against antibiotics [26, 72]. Zinc is an essential element required
for body health and ZnO nanoparticles likewise show good biocompatibility to the
human body. Recently, ZnO has been listed as generally biosafe material by the U.S.
food and drug administration, FDA [73].
16
Being, biosafe material ZnO is also used as an antibacterial agent in the food
packaging industry, against various foodborne diseases. ZnO NPs are properly
incorporated in the packaging materials, where they interact with pathogenic bacteria
on the food surface, inhibiting the growth of bacterial cells and finally cause bacterial
death [74].
Investigations disclosed the ZnO NPs as peculiarly non-toxic towards human
cells. The toxicity perspective revealed that ZnO nanoparticles exhibit selective
toxicity towards bacteria with no harmful effects on human cells [72]. This
recognition demanded the use of ZnO NPs as an antimicrobial agent, poisonous to
microorganisms while holds great biocompatibility to human cells [75].
The possible mechanisms regarding the antibacterial action of nanomaterials
are generally ascribed to the greater surface area [76] and unique physiochemical
properties. Investigations of antibacterial nanomaterials, for the most ZnO NPs would
improve the exploration of nanostructured materials and the possible mechanisms
effective behind the antibacterial action of nanomaterials.
ZnO has been used as an antibacterial agent in both nano and microscale
formulations. A comparative study for examining antibacterial activity of CuO, ZnO,
and Fe2O3 revealed that among the three metal oxide, ZnO showed greatest inhibitory
effects against various bacterial species in the order of ZnO > CuO > Fe2O3, and
explored that ZnO nanoparticles possess the potential to be used as antibacterial agent
against various pathogenic bacteria [77].
Though various methods are in use for detecting the antibacterial activity
however, the agar diffusion method has been officially standardized by American
Type Culture Collection (ATCC) and is, therefore, most frequently used. For instance,
the antibacterial properties of ZnO NPs through the well diffusion method has been
17
studied against E.coli and S. aureus with inhibition zones of ~5 mm at the
concentration of 10 µg mL [78].
1.4.1. Mechanism of Antibacterial Activity
However, various mechanisms have been proposed for the antibacterial action
of ZnO NPs, yet the particular mechanism responsible for ZnO antibacterial action is
not totally enlightened and still disputable. Various distinctive mechanisms proposed
for the antibacterial action of ZnO NPs include direct contact with bacterial cell wall
destroying the bacterial cell integrity, production of reactive oxygen species (ROS)
and internalization of ZnO NPs into bacterial cells [79–81].
Xie et al. [82] examined the antibacterial action of commercially available
ZnO nanoparticles against Campylobacter jejuni. They examined the growth
inhibition under a range of ZnO concentration (0–0.10 mg/mL) and determined the
minimum inhibitory concentration (MIC) value of ZnO as to be 0.025 and 0.05
mg/mL. SEM analysis showed significant morphological changes in bacterial cells
after the exposure of ZnO NPs at the concentration of 0.5 mg/mL for ~16 hours.
Based on the observed results, the authors proposed that bacterial inactivation
involved the direct interaction of ZnO with the cell surfaces, which modified the
membrane permeability where the nanoparticles entered and induced the oxidative
stress inside the cells. It subsequently inhibited cell growth and resulted in cell
death.
A survey on the antibacterial activity of ZnO micro flowers disclosed the
direct damage of bacterial cell membrane and cytoplasm on the incorporation of ZnO
micro flowers [71]. It has been suggested that the level of toxicity also varies in
different media, as the dissolved Zn2+
species may change as per the medium
composition [83].
18
The potential usage of ZnO-NPs for the antimicrobial activity coupled with a
number of variables affecting the activity has been studied [84]. Fundamentally, by
improving various factors like ZnO powder concentration, particle size, and
morphology, surface modification and illumination by UV light, etc., incredible
antibacterial outcomes could be acquired.
In addition, improved antibacterial activity is also ascribed to the surface
deformities like abrasive edges present over the rough surface of ZnO [75, 84].
Successful application of ZnO-NPs as antibacterial agents can be accomplished by
controlling impurities, surface charges and particle morphology by the systematic and
careful tuning of the experimental conditions.
1.4.2. Particle Size and Concentration
It is generally believed that NPs size and concentration play a vital role in
determining the antibacterial activity. Several studies revealed a direct correlation
between antibacterial activity and NPs concentration. Likewise, the antibacterial
activity is also size-dependent. Smaller the particle size greater will be the
antibacterial activity.
Sumathi et al. [85] investigated the microstructural and antibacterial properties
of the synthesized product against S. aureus. They explored that small-sized particles
employed at 100 µg/mL concentrations showed excellent antibacterial activity.
Though, their synthesized powders show no uniformity in particle microstructures,
which is of prime importance for obtaining reproducible results.
They also demonstrated that antibacterial activity strongly dependent upon the
type of reagents used during the synthesis as well as the particle size of ceramic
nanopowders. Inspection of particle size on antibacterial activity against the E. coli
19
and S. aureus unfolded that smaller particle size led to enhanced antibacterial action
[80, 86–87].
Similarly, Thomas et al. [89] prepared ZnO using sodium dodecyl sulfate as a
stabilizing agent. They evaluated the antibacterial activity of both bulk and
nanoparticle over a broad spectrum of bacterial species and observed that nanoparticle
produced better results than bulk ZnO. Another research group [90] broadly assessed
the size-dependent antibacterial action of ZnO on various gram-positive and gram-
negative bacterial strains.
Furthermore, it has been demonstrated that smaller particle size produced a
higher concentration of oxygen species especially H2O2 and therefore greatly
enhanced the antibacterial activity. However, it has been explored that the generation
of ROS took place in response to UV light illumination [75].
In addition, Zhang et al. [91] investigated the antibacterial activity by utilizing
ZnO nanofluids. Their outcomes demonstrated bacteriostatic activity towards E. coli,
which enhanced at a reduced particle size and increased concentration of NPs.
Besides, the authors carried out SEM investigations to analyze the morphological
changes and demonstrated that ZnO-NPs directly interacted with the cell wall of E.
coli, bringing considerable damage which then disintegrated the cell membrane.
Besides the particle size, the concentration of ZnO NPs has also been
described to influence the antibacterial action significantly, against both Gram-
positive as well as Gram-negative bacterial strains [92–93].
1.4.3. Particle Morphology
The effect of particle morphology on materials properties has attracted the
current research consideration [94]. Numerous investigations have detailed that the
toxic nature is altogether influenced by different morphologies of ZnO NPs [95–97].
20
Reddy et al. [98] demonstrated that ZnO NPs showed selectivity in their toxicity to
various bacterial strains and human T lymphocytes and suggested that ZnO NPs may
prove useful antimicrobial agents in nanomedicine at the selective dosing range and
by controlling NPs shape.
Subsequently, the ZnO nanostructure morphology can be tailored by
optimization of synthesis parameters, for example, pH of the medium, synthesis
temperature, reaction time, shape directive agents and reactants composition, etc.
[99].
Since surface morphology of ZnO nanostructures can affect their bacterial cell
internalization mechanism, for example, wires and rods shaped structures penetrate
the cell of the microorganism effortlessly than other shaped nanostructures [100].
Similarly, flowers like structures revealed greater antibacterial action towards E. coli
and S. aureus compared to spherical and rods shape ZnO structures [96].
Elkady et al. [101] employed ZnO nanotubes for evaluation of antibacterial
activity against various pathogenic bacteria because of the greater surface area (17.8
m2/g). ZnO nanotubes showed profound antibacterial action by producing inhibition
zones of about 22, 24, 18 and 15 mm size at a minimum concentration of 0.938 mg/
mL against E. coli, P. aeruginosa, S. aureus and B. subtilis, respectively.
In light of the above-mentioned review, it has been amply demonstrated that
the performance of all NPs-based gadgets is known to strongly depend upon the size,
morphology, and uniformity of the nanoparticles. As such, main efforts have been
made to synthesize NPs with controlled morphological growth. Most of the research
groups succeeded in obtaining nanoparticles of zinc compounds, comprised of
uniform morphological features. However, they either used longer synthesis routes
with elevated temperatures, stabilizing agents, or sophisticated instrumentations
21
during the synthesis procedures, which in actual practice are the major hindrance to
apply those synthesis routes. They did explore that uniformity in structural and
morphological features of nanoparticles, plays an imperative role in all powder-based
applications. Therefore, development of simple, economically feasible and
environmentally benign routes for the synthesis of zinc compounds with substantial
uniformity, excellent control over particle’s morphology with narrow particle size
distribution is of prime importance and still challenging in the field of nanoscience.
1.5. Aim and Objectives
The current study was thus aimed to synthesize monodispersed fine particles
of zinc oxide and zinc phosphate with controlled morphological features which were
then effectively used in the fabrication of efficient gas sensors of industrial
importance. It is added that up to our knowledge, no literature report is available
concerning the gas sensing properties of zinc phosphate. Furthermore, uniform
particle systems of zinc oxide were also analyzed for antibacterial activity against
various pathogenic bacterial strains of clinical importance.
22
CHAPTER 2
EXPERIMENTAL
2.1. Materials
Zinc nitrate, ammonium hydroxide, ammonium dihydrogen phosphate, oxalic
acid hexamethylenetetramine were purchased from reputed firms. The bacteriological
grade nutrient agar and nutrient broth were also purchased for performing
antibacterial activity. Experiments were conducted using Pyrex glassware. Distilled
water was used for making all sorts of solutions.
2.2. Synthesis of Zinc Compounds
Monodispersed fine particles of various zinc compounds i.e., zinc oxide, zinc
oxalate, and zinc phosphate were synthesized by heating aqueous solutions,
containing various compositions of reactants through controlled precipitation method
and reflux method. For this purpose, aqueous solutions containing varying
composition of zinc nitrate in the presence of hexamethylenetetramine (HMT)/
aqueous and gaseous ammonium hydroxide (NH4OH)/ oxalic acid (C2H2O4) or
ammonium dihydrogen phosphate (NH4H2PO4) were aged at different temperatures
for predetermined time periods.
2.2.1. Synthesis of Zinc Oxide (ZnO)
ZnO fine particles were synthesized directly from heating reactant solutions
containing zinc nitrate (1.5–5 molL-1
) and ammonium hydroxide (25%) at various
constant temperatures (30–90 oC) for predetermined time by using controlled
precipitation method. In some cases, aqueous solutions of zinc nitrate were purged
with ammonia gas at the flow rate 110 cm3min
-1, rather than using aqueous
ammonium hydroxide under the same reaction conditions.
23
Similarly, ZnO particles were also prepared using the same amounts of zinc
nitrate and HMT (mixed in 1:1 ratio) instead of ammonium hydroxide under reflux
boiling for different time intervals.
2.2.2. Synthesis of Zinc Oxalate
In addition, zinc oxalate particles were synthesized by mixing aqueous
solutions of zinc nitrate and oxalic acid (1:2) and heated at a constant temperature for
15 min using controlled precipitation method. Experimental parameters were carefully
tuned to get fine particles with uniform and well-defined morphologies.
2.2.3. Synthesis of Zinc Phosphate
Zinc phosphate particles with various novel morphological features were
synthesized at 40–80 oC
from an aqueous solution containing zinc nitrate and
diammonium hydrogen phosphate as precursor material by employing controlled
precipitation method. Aqueous ammonia was used for pH adjustment. Precipitated
particles were isolated from mother liquors using micropore membrane filters,
extensively washed with distilled water and dried.
2.2.4. Calcination
As mentioned earlier, it was of interest to employ oxide and phosphate forms
of synthesized zinc compounds for the evaluation of gas sensing properties as well as
antibacterial activity. For this purpose known amounts of the selected as-prepared
powders were calcined at the desired temperature in a programmable furnace
(Nebertherm, M7/11) in the air ambient for 2 h. The heating rate was set at 5 oC /min,
considered good for maintaining the integrity of the particles of the asprepared
powders. After calcination, the powder samples were allowed to cool inside the
furnace. Calcined powders were then kept in a desiccator for characterization and
application purposes.
24
2.3. Characterization of Zinc Compounds
2.3.1. Scanning Electron Microscopy (SEM)
To analyze particle morphology, synthesized powders were inspected with a
scanning electron microscope (SEM; JEOL, JSM-5910). For this purpose, a small
amount of the selected batches were applied through double stick carbon tape over
aluminum stubs. Prepared sample stubs were then coated with a platinum layer for
~40 s inside the auto-fine coater (JEOL, JFC-1600). Finally, coated samples were then
shifted to the evacuated chamber of the SEM for analysis of particle morphological
features.
2.3.2. X-ray Diffractometry (XRD)
For crystallinity and phase identification, selected zinc compounds samples
were subjected to x-ray diffractometric analysis (XRD; JEOL, JDX-3532). All
samples were examined in the ‘2θ’ range of 10 to 80˚ with step angle 0.1˚/s. For peaks
identification and estimation of other crystallographic parameters of the tested powder
samples, CMPR and JDX-3500 software were used.
2.3.3. Fourier Transform Infrared Spectroscopy (FT-IR)
For the determination of composition and functional groups of the synthesized
products, FT-IR spectroscopic analysis was carried out using FT-IR (Shimadzu; IR
Prestige-21 & FT-IR 8400S). For this purpose selected powder samples were evenly
mixed with potassium bromide (KBr), placed in the sample holder and scanned in the
wavenumber range of 400–4000 Cm-1
.
2.3.4. Thermogravimetric /Differential Thermal Analysis (TG/DTA)
For analysis of thermal stability and to examine the thermally triggered
reactions of synthesized products, a simultaneous TG/DTA analyzer (Perkin Elmer,
Diamond series) was used. Desired powder samples were subjected to heating in the
25
range from room temperature (RT) up to 800 oC in the presence of a controlled flow
of air. The heating rate was kept constant at 5 oC min
-1.
2.3.5. Surface Area Analysis
For measurement of the Brunauer–Emmett–Teller (BET) surface area of the
selected zinc compounds, the surface area analyzer (Quantachrome; NOVA 2200e,
USA) was employed using nitrogen (N2) sorption at the temperature of 77 K. For this
purpose, known quantity of the selected samples was taken in specially designed
quartz tube attached to the degassing part of the said instrument. Degassing was done
at 90 oC for about 3 h to remove the adsorbed moisture and other volatile impurities
present in the samples. The degassed samples were then shifted to the analysis station
for measurement of specific surface area.
2.3.6. Point of Zero Charge (PZC)
In addition, to perform antibacterial activity it was prior to control the surface
charge of the desired particles. For this purpose, PZC values of the selected ZnO
systems were estimated through the established method [102]. To accomplish this, a
known amount of selected ZnO powders (0.1g) were added into 0.01 M solution of
NaNO3 and sonicated for about 2 h. Then 50 mL of each of the dispersion was
transferred to Pyrex flasks systematically. For adjustment of pH of prepared
dispersions in the range of 3–12, 0.1 N NaOH/ HCl were used. Prepared dispersions
were subjected to agitation for about 24 h. After equilibrating, pH of each suspension
was recorded precisely as initial pH (pHi). After that 1 g solid NaNO3 was added to
each of the dispersion and was agitated again for the duration of 24 h. Then final pH
(pHf) was noted. Finally, PZC values of selected ZnO samples were estimated from
the corresponding plots of pH difference (∆pH) versus the initial pH values.
26
2.4. Gas Sensing Properties
Slurries of the selected particles of ZnO and zinc phosphate were applied
through screen printing method on sensor plate (1 cm × 1cm alumina plate carrying
interdigitated electrodes of gold). To increase the adhesion of the printed film with the
electrodes as well as to improve its mechanical stability, the sensor plates were heated
at ~250 oC for about 1h. After cooling to room temperature, sensors were placed in a
specially designed gas sensing chamber and connected with the data collection
system. Fabricated sensor plates were then exposed to test gases like ammonia (NH3),
ethanol (C2H5OH), and acetone (CH3)2CO) and change in the electrical resistance of
the sensor was recorded continuously as a function of exposure time, the
concentration of the test gas and working temperature. The obtained data was then
employed for calculation of the sensor response as well as response/recovery times.
Moreover, observed gas sensing responses of fabricated sensors were then compared
with literature.
2.5. Antibacterial Activity
The antibacterial activity was assessed against clinical strains of both Gram-
positive and Gram-negative bacteria (S. aureus, S. mutans, E.coli, P.aeruginosa, and
Enterobactor cloacae), using agar well diffusion method. Bacterial strains were
collected from Khyber Teaching Hospital KPK, Pakistan. Fresh culture of each test
microorganism at the concentration of 7.5 Χ 106 CFU/50 µL was spread over Muller-
Hinton agar plates having wells of ~8 mm diameter. Then 20 µL of the desired
particle suspensions were added into wells at three different concentrations (5 µg/20
µL, 10 µg/20 µL, 15 µg/20 µL). Ciprofloxacin was used as a positive control. The
plates were incubated overnight at ~37 oC. Antibacterial activity was examined by
measuring zones of inhibition against the test microorganisms.
27
In addition, the effect of particle morphology and powder concentration on
antibacterial activity of desired samples was also evaluated. Furthermore, desired
samples were also employed in combination with ciprofloxacin (1:1) and tested for
the effects on antibacterial activity of ciprofloxacin. All experiments were performed
in sets of three and then the average values were estimated out.
28
CHAPTER 3
RESULTS AND DISCUSSION
This work describes the synthesis of zinc oxide, zinc oxalate and zinc
phosphate fine particles with novel morphologies, using controlled precipitation and
reflux boiling methods. Selected batches of as-prepared powders were calcined at
elevated temperatures. Both the as-prepared and calcined products were characterized
by different characterization techniques.
Selected batches of as-prepared and calcined ZnO powders were employed for
the evaluation of gas sensing properties. In addition, selected batches of as-prepared
zinc phosphate samples were also investigated for gas sensing properties. Effect of
type of analyte gas, operating temperature, gas concentration and particle morphology
on gas sensing properties were evaluated. Furthermore, selected batches of ZnO were
also assessed for antibacterial activity against both gram-positive and gram-negative
bacteria i.e., S. aureus, S. mutans, E. coli, Enterobactor, P. aeruginosa.
3.1. SEM Analysis of Zinc Compounds
The incentive for controlled synthesis of nanomaterials arises from the fact
that various properties of nanomaterials strongly depend upon their particle size,
shape, structure (hollow versus solid interiors) and composition, which in turn are
strongly dependent upon the growth conditions like reactants concentration, growth
time, temperature and pH of the reactant solution [103]. Therefore, precise control
over the aforementioned parameters unlocks the possibilities of designing a number of
nanostructures with desired performances, essential for a particular application.
Below is given a detailed discussion of the morphological tweaking of various
nanostructures of zinc compounds, fabricated under different growth conditions as
well as in the presence of a different precipitating agents.
29
3.1.1. Synthesis of ZnO Nanostructures using HMT
ZnO nanostructures of uniform and tunable architectures were produced from
heating aqueous solutions of zinc nitrate and HMT under reflux conditions for various
time intervals. It is generally believed that in the course of precipitation, hexamine
plays an imperative role in both the nucleation and growth process of ZnO
nanostructures. First, it acts as a template for the formation of ZnO nuclei and
secondly, it passivates the ZnO particles surface against excessive growth.
For instance, in the reaction medium, hexamine is thermally decomposed by
water and provides a controlled supply of ammonia, which acts as a Bronsted-Lowery
base and provides ample hydroxyl ions (OH-) by reacting with water (Eqn. 5–6).
From reaction 7, it is suggested that ammonia serves as the primary agent for the
production of ZnO nanostructures. Ammonia is consumed by providing hydroxyl ions
for the formation of growth units of ZnO. Which is in fact, the phenomenon of
attraction of divalent Zn2+
cation and hydroxyl anions OH-
to form zinc hydroxide
complex, [Zn (OH)4]2-
(Eqn. 8) that ultimately leads to ZnO formation through
condensation reaction (Eqn. 9) [104].
(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3 (5)
NH3 + H2O ↔ NH4+ + OH
- (6)
Zn2+
+ 4NH3 ↔ [Zn (NH3)4]2+
(7)
[Zn (NH3)4]2+
+ 4OH- → [Zn(OH)4]
2- + 4NH3 (8)
Zn2+
+ 4OH-
→ [Zn (OH)4]2-
→ ZnO + H2O + 2OH-
(9)
Different ZnO nanostructures (1D ellipsoidal rods, 3D flowers and
microspheres) were obtained from heating reactant solutions of zinc nitrate and HMT
by careful and systematic tailoring of the experimental conditions. SEM images in
Figure 3 show the interesting morphological evolution of the ZnO nanostructures in
30
sequence, obtained from homogenous precipitation of aqueous solutions of zinc
nitrate and HMT (1:1) under reflux conditions. From SEM images, it can be shown
that powders obtained after 30 min reaction time at 80 oC (Figure 3a) are comprised
of nanorods having a diameter of about 150-200nm and length of about 1 or less than
1µm. The SEM image also depicts a few numbers of ellipsoidal nanorods in the same
powder sample (Figure 3a). The appearance of such asymmetric particle distribution
might be due to the simultaneous occurrence of both the nucleation and growth stages.
For monodisperse nanostructures formation, a high rate of nucleation in a short time
followed by a preliminary fast growth is necessary which rapidly drops the
concentration level below the supersaturation value and thus secondary nucleation is
avoided [105].
To understand, how two different morphologies appear in a single ZnO
system, time-dependent trials were performed. For this purpose, reaction time was
first lessened to 15 min, which resulted in a burst of nucleation and produced a large
number of primary particles followed by subsequent growth into uniform
monodispersed ellipsoidal nanorods in a relatively short time period as can be seen in
Figure 3b. Each ellipsoidal nanorod appears to have about 250 nm ellipsoidal tips
while its length goes up to about 3–4 µm. Since, it has been found that particle
morphology of ZnO significantly depended upon the reaction time and temperature
[106]. When precipitation reaction occurred for longer time intervals (45 min) at 80
oC, ZnO flower-like microstructures of about 4–5 µm emerged (Figure 3c). It can be
shown from high magnification SEM image (Figure 3d) that each microflower is in
fact a partially developed microsphere which is composed of a random network of
short hexagonal nanorods having no distinct origin or center. It is then suggested that
the individual hexagonal nanorods (with length ranges 1–2 µm and width about 200
31
nm) formed simultaneously, and self-assembled into a network during the growth
process. When the reaction continued for 1 h, it seems that the flower-like structures
hold enough surface energy and thus serves to provide nucleation sites for further
attachment of nanorods and eventually developed into well-defined 3D microspheres
like structures (Figure 3e & 3f).
Despite other reaction parameters, refluxing time also plays a crucial role in
the formation and stability of ZnO nanostructures [106]. As refluxing time exceeds
the optimum, the small ZnO nanoparticles collided to form a giant structure due to
aggregation. These unique microsphere structures possess a higher surface area than
similar size other ZnO structures because of the random growth of apparently flexible
nanorods, as obvious from the SEM images. ZnO nanorods with large particle size
distribution have been synthesized elsewhere from HMT and zinc nitrate while
methenamine has been used as a surfactant at 90 ˚C [107].
SEM study of the prepared particles depicted in Figure 3 indicated that various
nanorod morphologies did not emerge concurrently. Ellipsoidal nanorods displayed in
Figure 3b formed when the reaction time was 15 min. However, as the reaction time
was prolonged to 30 min hexagonal nanorods formed, which then self-assembled to
form flowers of nanorods and finally converged into 3D microspheres like structures
with increasing growth time to 1h. Schematics regarding the change in particle
morphology with reaction time are given in Figure 4.
3.1.2. Synthesis of ZnO Nanostructures using Ammonia
By using ammonia as a precipitant, ZnO nanostructures in different surface
morphologies from reacting aqueous solutions of zinc nitrate and ammonia at various
temperatures (30–90 oC) for various time intervals (5–30 min) were synthesized.
32
Figure 3. SEM images of asprepared ZnO nanostructures obtained at 80 oC for
various reaction times, a) 30 min, b) 15 min, c & d) 45 min, e & f) 1 h.
33
Figure 4. Schematics showing change in nanostructure morphology with a change in
reaction time.
34
In this process, both aqueous and gaseous ammonia was used as the
precipitating agent and its effect on the particle's morphological features was
investigated. In all cases, ZnO powders in the reactant mixtures were formed from the
precipitation reactions that essentially emerged in the aqueous medium. The
mechanism of ZnO formation through the reaction of ammonia with dissolved Zn2+
ions can be summarized through the chain of reactions represented by Eqn. 6–9) [2,
104, 108].
It is well known that crystal formation is generally controlled both by the
nucleation and growth rates, which in turn are preceded by the induction time. All
over the induction period, ammonia provides hydroxyl ions that are used in generating
and subsequent development of the primary growth units [Zn (NH3)4]2+
and
[Zn(OH)4]2-
[109]. In our designed experimental conditions, at pH greater than pHPZC
of ZnO (pHPZC~9.3), Zn2+
cations form mainly [Zn(OH)4]2-
with a small quantity of
[Zn (NH3)4]2+
complex. When zinc nitrate solution was purged with ammonia gas for
30 min with a 110 cm3.min
-1 flow rate at 30
oC temperature, fairly uniform
hierarchical 3D ZnO nanoflowers were obtained (Figure 4).
It was of interest to know about the growth mechanism of these appealing
flower like nanostructures. Since it has been investigated that reaction time plays a
substantial role during the production and growth of nanoparticles. To understand
how reaction time affects the particle morphology, the same experiment was repeated
and the precipitated powders were isolated after every 5 min of the reaction and
analyzed for morphological characteristics. Figure 5a–f shows the whole scenario of
the successive growth stages of hierarchical flower-like ZnO structures as a function
of reaction time (5–30 min). This process of flower formation clearly illustrated the
growth process, consisting of a fast nucleation stage followed by slow aggregation.
35
As can be seen from the SEM image in Figure 5a, initially uniform and well-
distributed 2D nanopetals were produced. When the reaction time exceeded upto10
min, the primarily obtained nanopetals self-assembled in a specific manner (Figure
5b). In this way these nanopetals followed a common crystallographic orientation and
thus reduced their surface energy [110].
It indicated that as the induction time is increased beyond 5 min, the more
growth units i.e., [Zn(OH)4]2-
were generated due to Coulomb's electrostatic forces of
interaction according to equation 8 and thus more ZnO nuclei were produced. As the
growth time prolonged further after nuclei formation, it caused sheet defects on the
surfaces of newly formed ZnO nuclei, thereby increasing the radii of newly formed
ZnO crystals as well as enlarged the phase boundaries of these crystals to the extent
they touched one another. As such, the base or core point of the flower-like structure
was established (Figure 5c). Once this basic structure was created, further growth in
radial direction sprouted with exceeding reaction time (Figure 5d–e) and developed
into fully grown hierarchical flower-like structures in 30 min which closely resemble
with French marigold flower, given in the inset of Figure 5f. For instance, a schematic
diagram illustrating the growth mechanism of hierarchical flower-like structures is
also given in Figure 6.
Besides the reaction time, other parameters like pH of the reactant solution,
and reaction temperature also affects the particle’s morphological features. In this
regard, another set of experiments was carried out, in which pH of the reactant
solution containing zinc nitrate and aqueous ammonia was kept below the point of
zero charge of ZnO and heated for 10–30 min under reflux conditions.
36
Figure 5. SEM analysis of ZnO nanostructures precipitated from zinc salt and
ammonia gas at 30 oC after time interval a) 5 min, b) 10 min, c) 15 min, d) 20 min, e)
25 min and f) 30 min.
37
Figure 6. Schematics illustrating the effect of aging time on the growth pattern of
particles depicted in Figure 5f.
38
Figure 7 illustrates a fascinating morphological evolution under the effect of
refluxing time. It is obvious from the SEM image (Figure 7a) that the powders
obtained after 10 min refluxing time were composed of colloidal nanospheroids with
excellent uniformity in particle shape and size. When the refluxing time was
prolonged to 20 min, a considerable increase in particle size was observed with a
slight change in particle morphology from spherical to oval shape with tapering ends
(Figure 7b). On further heating for 30 min, the particles attained the shape of
monodispersed ellipsoidal nanorods as shown in Figure 7c.
Moreover, schematic illustrating the effect of refluxing time on particle
growth is shown in Figure 8. To account for these monodispersed nanoparticle
formations, it was believed that when pH of the reactant mixture was kept below the
point of zero charge, the ZnO nanoparticles were probably formed through the
reaction 8 and therefore inhibited the expected homocoagulation during their growth
stage due to highly positive charges at their surfaces and therefore stayed smaller in
size. According to Baruah and Dutta [103], low pH favored high nucleation rate and
the presence of less OH- ions in the medium produced fewer growth units which thus
slowed down the growth rate. As a result, monodispersed fine particles of smaller size
were formed.
Furthermore, the transformation of nanospheroids into aesthetic nanorods
clues that system energy in addition to refluxing time was also responsible for
controlling the particle morphological features. It has been proposed that initially
when the reaction time was 10 min, the energy of the precipitated powders was low
but as the reaction time was prolonged to 20 and then 30 min, the precipitated
powders obtained sufficient energy, which caused the morphological transformation.
39
Figure 7. ZnO nanostructures synthesized from an aqueous solution of zinc salt and
ammonia gas after reflux heating for a) 10 min, b) 20 min and c) 30 min.
Figure 8. Schematics illustrating the effect of refluxing time on particle growth
depicted in Figure 7a–c.
40
It is added that these smaller size ellipsoidal ZnO nanorods (Figure 7c) are
reported for the first time which were formed through ammonia without the assistance
of any type of surfactant or templates. Though ZnO nano/micro flowers have been
reported, however, the researchers either used higher temperatures with time-
demanding experimental procedures or they employed shaped directing additives like
CETAB, monoethanolamine (MEA) and benzoic acid, etc. [111–113]. Similarly,
Elkady et al. [101] employed various types of surfactants (CTAB, PEG, PVA, and
PVP) to construct ZnO with different particle morphologies.
Likewise in another set of experiments, aqueous ammonia was employed
rather than gaseous ammonia to study the effect of ammonia state on particle
morphology if any. Figure 9 shows the SEM images of synthesized ZnO fine particles
obtained from an optimized amount of aqueous ammonia and zinc nitrate solution
within 15 minutes, at 30–90 oC temperature. From the SEM images, it can be seen
that interesting and uniform morphologies appeared as the reaction temperature varied
in the range 30–90 oC. Figure 9a shows that powders precipitated at 90
oC are
comprised of monodispersed ellipsoidal nanostructures. Since, the total reaction rate
increases with increasing reaction temperature. Accordingly, high reaction
temperature favors nucleation, while low temperature favors growth in the case of wet
chemical synthesis of nanoparticles. In other words, at high temperatures, the
nucleation rate constant k1 would be larger, while the growth rate constant k2 would
get relatively smaller [114]. Therefore, it could be concluded that high temperature
would result in the production of smaller size particles as compared to a low
temperature which would favor the formation of relatively larger size particles. It can
be assumed that the nanoparticles could grow even better and larger at lower synthesis
temperature because lower temperature favors growth. As such, it was observed that
41
the size of ellipsoidal nanostructures increased when the precipitation reaction was
carried out at 80 oC, which can be examined from the SEM image shown in Figure 9b.
The nanostructures obtained at 60 oC are 3D, well-dispersed flowers (Figure 9c). It
indicated that the reaction temperature not only controls the particle size but also the
growth orientation of the nanocrystals. Figure 9d shows that the size of nanoflowers
increased at 50 oC and continued to grow larger in size at 40
oC as given in Figure 9e.
Further decrease in reaction temperature (30 oC), favored the orientation of
hierarchical flower-like nanostructures, as can be seen from Figure 9f. It has been
reported that initially the nanocrystals nuclei formed in the reactant solution which
then grew up into nanopetals by Ostwald ripening. Due to the intrinsic anisotropic
property of hexagonal crystalline structure, these nanopetals radially arranged and
grew up along the c-axis to decrease the surface energy during the process and
developed into 3D hierarchical flower-like nanostructures [115]. Such hierarchical
flower-like ZnO structures have been reported previously by Meng et al. [116] at a
relatively higher temperature of 95 oC and longer reaction time of 6 h as compared to
15 min of aging at 30 oC in the present work.
3.1.3. Synthesis of Zinc Oxalate
In addition, zinc oxalate fine particles were prepared by conducting a set of
experiments in which aqueous solutions containing zinc nitrate and oxalic acid (1:1)
were allowed to age for 15 min at 30 oC. In all cases, the precipitation solids were
formed as a result of the following basic reaction [117].
Zn (NO3)2.6H2O + H2C2O4 → ZnC2O4.2H2O↓ + 2HNO3 + 4H2O (10)
SEM analysis of the obtained powder revealed that the morphology of these
particles significantly depended upon the composition of the reactant mixtures.
42
Figure 9. SEM images of nanostructures precipitated from aqueous solutions
containing zinc nitrate (1.5–5 mol.L-1
) and ammonium hydroxide (1–15%) heated for
15 min at: a) 90 oC, b) 80
oC, c) 60
oC, d) 50
oC, e) 40
oC, f) 30
oC.
43
SEM micrograph in Figure 10a depicts that the powder sample is composed of
about cube shaped particles. However, it can be observed that almost all the cubes
were broken which demonstrated the incomplete growth of the precipitated particles.
In order to account for the appearance of broken cubes, it was assumed that it
may likely be either due to insufficient amount of oxalic acid (the precipitant) or zinc
nitrate in the starting reactant mixture or due to short aging time which was essential
for particles builds up. To prove the above statement, experiments were conducted in
which the amount of oxalic acid was increased to double in the reactant solution while
all other reaction parameters remained constant. It was wonderful to see that cubes
grew enough in size (Figure 10b) as compared to the previous one (Figure 10a).
However, these cubes possessed pits on the two opposite corners. Therefore, with
further increase in oxalic acid concentration in the reactant mixture, the pit size
decreased considerably (Figure 10c), and finally, these pits disappeared from the
particle's surface (Figure10d).
It indicated that the presence of a relatively large amount of oxalic acid in the
reaction mixture made possible the formation of full-grown cubes and is a vital factor
for controlling the morphology of zinc oxalate particles [118].
Since, preliminary trials indicated that for a certain precipitation reaction, an
increase in temperature resulted in shortening of the induction time [118]. In this
regard, the reactant solution of the same composition was allowed to age at 40 o
C,
under the same conditions. Ensuing powders were inspected with the SEM image in
Figure 10e which shows that the obtained particles were smaller in size as compared
to particles prepared from the same reactant solution at 30 o
C (Figure 10d) while
maintaining the same cubic morphology.
44
Figure 10. SEM images of zinc oxalate nanostructures prepared in 15 min from zinc
salt and oxalic acid in ratio, a) 1:1 at 30 oC, b) 1:1.5 at 30
oC, c) 1: 2 at 30
oC, d) 1:3 at
30 oC, e) 1:3 at 40
oC.
45
It is indicated that shortening of induction time with increasing temperature
produced primary particles which subsequently grew in a rather shorter time period,
most likely because of the endothermic nature of the precipitation process [118].
In order to account for this aesthetic cubic morphology evolution, it is
believed that primary particles were originated in cubic shape followed by subsequent
three-dimensional growth on their plane faces into larger cubes by coagulation
process. On the other side when the aging time for the particles shown in Figure 10a
was increased to 30 min, the powders precipitated were composed of relatively large
hexagonal structures of rough surfaces with few cubic particles as depicted from SEM
image in Figure 11a. It seems that initial cubic particles started three-dimensional
growths into hexagonal structures by accumulating extra material around cubic cores,
as the reaction time increased. SEM image illustrated that hexagonal structure stayed
like a suspended substrate for further deposition of extra material, resulting from the
ongoing precipitation reaction. Finally, these hexagonal structures developed into 3D
flower-like structures composed of a relatively large number of nanopetals, when
precipitation reaction was continued for 1h (Figure 11b).
It was of interest to see the effect of zinc nitrate concentration if any on the
morphology of zinc oxalate particles. As such another precipitation reaction was
conducted under the same experimental conditions of type Figure 10e particles,
except that the composition of the starting reactant solution was changed by inverting
the amount of zinc salt and oxalic acid i.e., 3:1 at the same temperature of 40 oC. SEM
study of the isolated powders revealed a fascinating morphological change, as is clear
from Figure 12. The particles emerged as uniform needle shape rods assembled
around the axis to form nice flower-like structures. The following observations
demonstrated that the particle morphology of zinc oxalate precursors sensitively
46
Figure 11. SEM images of zinc oxalate microstructures prepared from zinc salt and
oxalic acid in 1:1 ratio at 30 oC in the time period, a) 30 min, b) 1h.
47
Figure 12 . SEM images of zinc oxalate nanostructures prepared in 15 min from zinc
salt and oxalic acid in a 3:1 ratio at 40 oC.
48
depended on both the composition of starting reactants (oxalic acid and zinc nitrate)
in solution and the aging time of the precipitation process. Furthermore, the combined
schematics of Figure 10–12 showing the change in particle morphology with reaction
conditions is given in Figure 13.
3.1.4. Synthesis of Zinc Phosphate Nanostructures
Fine powders of zinc phosphate were prepared by reacting aqueous zinc
nitrate and diammonium hydrogen phosphate in the molar ratio of (1:2) at a constant
temperature of 80 oC for 30 min. The initial pH of the reactant solution was kept at 9
using ammonia solution (25%). In the reactant solution, diammonium hydrogen
phosphate ionizes to yield NH4+
and HPO42˗
ions, as shown through reaction (11). It is
generally known that HPO42˗
preferentially ionizes (Ka = 2.2 × 10˗13
) instead of
hydrolyzing (Kh = 1.6 × 10˗7
). The phosphate ions (HPO42˗)
remain stable in alkaline
solution (pH > 7) due to the availability of a large amount of OH- ions in solution.
Therefore, the following chain of reactions (Eqn. 11–13) occurs in solution during the
precipitation of zinc phosphate [67, 119].
(NH4)2HPO4 → 2NH4 +
+ HPO4 2-
(11)
HPO4 2-
+ H2O → H2PO4 - + OH
- (12)
6Zn2+
+ 4HPO4 2-
+ 4OH- → 2Zn3(PO4)2 . 4H2O (13)
SEM analysis of the precipitated powder revealed an interesting morphology
of the particles having uniform hierarchical microspheres with a diameter of about
9µm each (Figure 14a). SEM image depicts that each hierarchical microsphere is
further composed of a high density of hexagonal nanorods, radially oriented to form
microspheres with narrow particle size distribution. SEM images of time-dependent
morphological evolution at different growth stages of the stated microspheres (10–30
min) are given in Figure 14b–d. Initially, loose microspheres composed of hexagonal
49
Figure 13. Schematic illustration of the effect of precursor’s composition (a–d),
reaction temperature (d–e), the order of addition (e–f) and reaction time (a–g–h) on
the morphology of zinc oxalate nanostructures.
50
Figure 14. a) Zinc phosphate hierarchical microspheres produced by heating aqueous
solutions of zinc salt, ammonia and diammonium hydrogen phosphate at 80 oC, b) 10
min, c) 20 min, d) 30 min.
51
nanorods were observed at a reaction time of 10 min. With the increase in growth
time, these nanorods moved closer to finely packed microspheres. It was observed
that the nanorods originated from core points and grew uniformly in a radial direction.
It might be due to the fact that first the nucleation of nanocrystals occurred and then
hexagonal nanorods sprout during the growth stage and grew uniformly in all
directions from the nucleation sites along the surface of initially formed nanocrystals.
In fact, the increased reaction temperature was directly responsible for supersaturation
in solution, thus affected the crystal's nucleation, growth kinetics and essentially the
preferential orientation of the product [120].
In another set of experiments, the reaction temperature was increased to 90 oC
while keeping the other reaction parameters constant. SEM images showed
hierarchical urchins like microspheres (Figure 15) and each hierarchical microsphere
could be characterized as a solid structure contained an array of radially grown fine
nanoneedles on its surface just like sea urchins. To study the growth mechanism of
these hierarchical structures, a time-dependent set of experiments (10–30 min) was
conducted under identical conditions in a controlled manner. At the initial stage, the
reaction time was adjusted to 10 min and uniform solid spheres having small spines
like protuberances at their surfaces were formed (Figure 15b). It seems that primarily
nanocrystals were produced which tend to aggregate together forming solid
microspheres. These microspheres then provide energetically favorable sites for
further adsorption of reactive molecules from the solution. Thus, enormous spines like
protuberances can be seen along their surfaces. It is clear from the SEM image that
these spines sprout radially over the entire surface of the solid microspheres (Figure
15b). When the reaction time was prolonged to 20 min, the spines like protuberances
grew in size and continued to develop into nanoneedles (Figure 15c).
52
Figure 15. a) Zinc phosphate hierarchical microspheres produced by heating aqueous
solutions of zinc salt and diammonium hydrogen phosphate at 90 oC, b) 10 min, c) 20
min, d) 30 min.
53
It was quite interesting that the size of solid microspheres remained unaffected
during the growth of nanoneedles at their surfaces. As the reaction time was further
increased to 30 min, the nanoneedles developed and fully covered exterior of the
microspheres which led to uniform hierarchical urchin-like microspheres (Figure
15d). The synthesized hierarchical microspheres resemble the surface morphology of
sea urchin, given in the inset of Figure 15a. The later microspheres (Figure 15d) are
significantly different from the former (Figure 14d) in the shape of nanorods on their
surfaces.
Shen et al. [121] obtained urchin-like Co3O4 over the Indium Tin oxide (ITO)
glass substrate through a hydrothermal reaction at 95 oC for 24h. They probed that the
formation of Co3O4 urchins was controlled significantly by the reaction time and
described that the formation of Co3O4 was composed of two stages: formation of
microspheres and then radial growth of nanofibers over these microspheres.
Yao et al. [122] investigated the growth process of urchin-like titania
microspheres in relation to hydrothermal time, temperature and hydrogen peroxide
concentration and obtained fully developed urchin-like microspheres at 150 oC with a
growth time of 4h. However, they employed commercially available titania powder to
prepare urchin-like microspheres.
In the present work, we followed one-step preparation route to obtain such
novel microstructures in an aqueous medium without using any type of templates or
surfactants, as reported earlier for obtaining urchin-like architectures [122–124]. It is
worth to mention that the obtained zinc phosphate hierarchical microstructures are
reported for the first time. Moreover, uniform nanoneedles/nanorods bring on high
surface area to the synthesized microspheres, which categorize them for their potential
applications in gas sensors, anticorrosive pigments, dentistry, and photocatalysis, etc.
54
Furthermore, when the reactant solution of the same composition and pH (~9
using ammonia) as employed for the preparation of urchin-like zinc phosphate
microstructures (see Figure 14a) was taken without the addition of diammonium
hydrogen phosphate and heated at the same temperature (80 oC) for 30 min, an
interesting morphological change occurred (Figure 16).
The SEM analysis revealed that the precipitated particles were well-dispersed
nanoflowers, which increased in size upon prolonging the reaction time to 1h. It
appears that diammonium hydrogen phosphate not only provides PO43-
but also
influences the preferred orientation of primarily formed nanocrystals. Moreover, to
illustrate the effect of synthesis parameters on the morphology of asprepared
structures (SEM; Figure 14–16), the combined schematics are given in Figure 17,
which clearly demonstrates the effect of reaction temperature, reactant composition
and aging time on particle's morphology.
Since, the morphology of the as-prepared particles could be tailored by
changing the experimental parameters like pH, reaction temperature, aging time, etc.
[2, 59, 125]. Kumar et al. [126] showed that temperature is one of the vital factors that
precisely control the particle size, shape, and chemical composition. Therefore, the
effect of reaction temperature (40–80 oC) on particle size and morphology of the as
precipitated phosphate particles under the captioned conditions was studied and were
analyzed through SEM. The inspection of SEM micrographs (Figure 18) revealed that
the obtained powders were comprised of monodispersed particles with unique
morphological features. To the best of our knowledge, such types of morphologies
have not been reported so far. At 40 oC, turban-like (inset of Figure 18a)
microstructures with considerable uniformity were formed. The high magnification
image in Figure 18b revealed that the surface of these particles is rather rough.
55
Figure 16. SEM images of as prepared nanoflowers synthesized by heating reactant
solution used for particles (shown in Figure14a) in the absence of diammonium
hydrogen phosphate at 80 oC for; a) 30 min, b) high magnification image of a, c) 1 h,
d) high magnification image of c.
56
Figure 17. Schematics showing the effect of synthesis parameters on morphology of
aspreapred nanostructures synthesized from reactant mixture containing zinc salt,
diammonium hydrogen phosphate, and ammonia; a) at 80 oC for 30 min, b) 90
oC for
30 min, c) zinc salt and ammonia at 80 oC for 30 min, d) zinc salt and ammonia at 80
oC for 1h.
57
Figure 18. SEM images showing the effect of synthesis temperature on particle size
and morphology of zinc phosphate nanostructures synthesized from zinc nitrate and
diammonium hydrogen phosphate (1: 4) at; a & b) 40 oC, c & d) 50
oC, e & f) 60
oC,
g) 70 oC and h) 80
oC.
58
Close observation of the SEM explored that burst of nuclei were formed
simultaneously which then self-assembled due to high surface energy and formed nice
turban-like structures. As the reaction temperature was increased to 50 oC, the
products transformed interestingly from turban shape to the one having sea shell-like
appearance (Figure 18c & d). Precipitated particles obtained at 60 oC were comprised
of self-assembled trigonal shape nanoplates (Figure 18e & f) which finally
transformed into trigonal pyramidal structures at 70 and 80 oC shown in Figure 18g &
h respectively.
The only change observed was the surface smoothness at high temperature
(Figure 18h). This interesting transition in particle morphology with the increase in
reaction temperature might be due to the change in crystal growth patterns and
orientations of the synthesized particles. As reported earlier by Pang and Bao [127]
that reaction temperature affected the crystallographic behavior of the hydroxyapatite
particles. Under certain reaction condition i.e., pH, reaction time and temperature, etc.
the microstructure of the initially formed nuclei determined the growth orientation of
the zinc phosphate nanocrystals. In the ambient conditions, the crystal interface
energy might be very high and thus caused the layer upon layer adsorption of crystals.
It is expected that crystals tended to adjust their shapes to be in a stable state and
reduced their interfacial energy to a minimum. As such the free interface energy of
crystals was constantly decreased, and thereby established a dynamic equilibrium
between the crystal's growth and reactant solution [128].
In another experiment, the order of addition of the reactants was changed
while keeping other reaction conditions the same as in the formation of particles
shown in Figure 18h. The resulting powders revealed to comprise of uniform
nanospheres displayed in Figures 19a & b.
59
Figure 19. Zinc phosphate nanostructures achieved after reversing the order of
addition of reactants for particles shown in Figure 18h, a & b) after 30 min, c & d)
overnight aging in mother liquor at room temperature.
60
It indicated that the order of the addition of the reactants can also affect the
crystal growth rate and therefore changes the preferred orientation. Some portion of
the same dispersion was left unfiltered and kept at room temperature for overnight. It
was then filtered, dried and analyzed by SEM imaging. Figure 19c & d shows the
morphological variation of the synthesized particles from nanospheres (Figure 19a–b)
to hierarchical nanoellipsoids, indicating that particle morphology of zinc phosphate
also significantly depended upon the aging time.
It seems that the leftover unreacted ionic species present in the precipitation
medium significantly affected the growth of primarily formed nanospheres (Figure
19b) in either by aggregation or surface precipitation process. It is believed that the
growth of three-dimensional hierarchical nanoellipsoids (Figure 19c–d) appeared to
consist of two aspects: first was the formation of nanospheres and secondly, the
nanospheres self-assembled to form multilayer ellipsoidal nanostructures. Researchers
investigated that oleic acid and triton-x played an imperative role in the preparation of
microspheres by reducing the agglomeration of zinc phosphate particles [60, 66].
Similarly, another research group established that cetyltrimethylammonium bromide
(CTAB) assisted in the self-assembling process of zinc phosphate nanosheets into
hierarchical structures [59].
The present work employed a very simple, time effective and low-temperature
aqueous synthesis route for obtaining uniform fine particles of zinc compounds
without adding any type of surfactants. Though ZnO nano/micro flowers have been
reported, however, the researchers either used higher temperatures with time requiring
experimental procedures or they employed shaped directing additives like CTAB,
monoethanolamine (MEA) and benzoic acid, etc. [112–113, 129]. Similarly, various
61
other types of surfactants (CTAB, PEG, PVA, and PVP) have also been employed to
construct ZnO particles with different particle morphologies [101].
Since, particle shape plays a vital role in all the powder-based hi-tech
processes. Therefore, it is believed that the products obtained in the present work may
prove to be very useful systems of colloidal substrates for various types of
applications in biomedical, gas sensors, catalytic/adsorption processes, etc.
It is further added that the nano/microstructures shown in Figure 3b
(nanorods), 7b (nanospheres), 9c (nanoflowers) and 10e (cubes) of zinc compounds
were selected for further characterizations and named as Z1, Z2, Z3, and Z4,
respectively. Similarly, hierarchical structures shown in Figures 14a, 15a and 19c of
zinc phosphate compounds were employed for further analysis and termed as ZP1,
ZP2, and ZP3, respectively.
3.2. XRD Analysis of the Asprepared ZnO and Zinc Oxalate
To determine the phase composition and crystallinity, selected batches of the
asprepared Zn-compound powder samples were subjected to XRD analysis. Figure 20
shows the XRD patterns of the samples in which the diffraction peak intensity was
recorded over the 2θ range of 10–80˚. The presence of intense diffraction peaks
confirmed the crystalline nature of the test powders. The acquired diffraction patterns
for Z1, Z2 and Z3 samples in Figure 20 matched well with the standard ICDD 50664
and index to wurtzite hexagonal phase of ZnO with characteristic reflection lines,
identified as (100), (002), (101), (102), (110), (103) and (200), respectively. The
presence of similar diffraction peaks with different intensities in XRD patterns for Z1,
Z2 and Z3 can be attributed to the different crystallographic arrangement which
supported the evolution of different particle morphologies as described above in SEM
analysis.
62
10 20 30 40 50 60 70 80
(-804)(412)(130)(023)(221)(002)
(021)(-402)(002)
(-202)
(200)(103)(110)(102)
(101)
(002)
Z4: ZnC2O
4.2H2O
Z3: ZnO
Z2: ZnO
Inte
nsi
ty (
a.u
.)
2 (degree)
Z1: ZnO(100)
Figure 20. XRD patterns of the selected asprepared powders, Z1–Z4.
63
For instance, the relative intensities of the peaks (100) and (101) are different
for Z1–Z3, which implied to the preferred orientations of the crystals, occurred during
the growth stage. Similarly, the XRD patterns for the Z4 sample in Figure 20
coincided well with the standard XRD patterns of ICDD card no. 251029 as zinc
oxalate dihydrate (ZnC2O4.2H2O). The observed results of XRD patterns showed
good consistency with the XRD results reported in the literature [130–131]. As can be
seen in Figure 20, no extra peaks regarding the impurities or other crystalline phases
were detected in the obtained XRD patterns, which confirmed the synthesized
powders as pure crystalline ZnO and ZnC2O4.2H2O.
3.3. FT-IR Analysis of ZnO and Zinc Oxalate
To further confirm the composition and purity of the synthesized powders,
selected batches were analyzed by FT-IR spectroscopy and the results are depicted in
Figure 21. The spectra are composed of several absorption bands, which appeared due
to different vibrational modes of FT-IR detectable various chemical groups present on
the surfaces of test particles (see Table 1). The broad absorption bands centered at a
wavenumber of about 3469 cm-1
were due to the moisture adsorbed over the surface
of the synthesized powders and can be attributed to O–H stretching modes [143–145].
Similarly, the bands at 1600–1400 cm-1
were due to the bending vibrations of O–H
[134–135]. Similarly, the FT-IR spectrum for the Z4 sample manifested the presence
of strong absorption band at 1367, 1315 cm-1
and relatively small band at 821 cm-1
which were ascribed to the particular O–C–O stretching modes and C=C–O bending
modes of zinc oxalate [132, 135]. The absorption bands appeared in the wavenumber
region at about 1085 and 892 cm-1
were indicative of the stretching vibrational modes
of nitrate (NO-3
) group, possibly inherited by ZnO particles from the zinc nitrate
solution during the precipitation process [136].
64
4000 3500 3000 2500 2000 1500 1000 500
Z4
Z3
Z2
21471907
8211315
13671647
541-4121055
14261641
Tra
nsm
itta
nce (
%)
Wavenumber (cm-1)
3469
892Z1
Figure 21. FT-IR spectra of selected asprepared powders, Z1–Z4.
65
Table 1. Wavenumber positions at which the chemical groups on the selected
asprepared solids absorb IR radiations.
Band Position (cm-1
)
Group
Species
Vibration
Mode
References Z1 Z2
Z3 Z4
3469 3469 3469 3479 O–H Stretching
modes
[132–134]
1641
1426
1640
1426
1529
1407
1447 O–H Bending modes [134–135]
----- ----- ----- 1367
1315
O–C–O Stretching
modes
[135]
1055 892 895 ------ NO3- Stretching
modes
[2, 136]
----- ----- ----- 821 C=C–O Bending modes [132, 135]
521–412 544–412 544–412 612–416 Zn–O Stretching
modes
[133–135]
66
The occurrence of strong absorption bands at 541–412 represented the
characteristic stretching modes of Zn–O bond, specifically confirmed the asprepared
Z1, Z2, and Z3 powders as ZnO [133–135]. The acquired FT-IR spectra for Z1, Z2,
and Z3 samples thus supported Eqn. 7-9, for identifying the synthesized powders as
ZnO, while the FT-IR spectrum for Z4 supported Eqn. 10. FT-IR results of ZnO and
zinc oxalate are also in good agreement with those reported elsewhere [132, 135].
3.4. Thermal Analysis of ZnO and Zinc Oxalate
In order to explore the influence of temperature on the properties of the
selected batches of asprepared powder samples, the latter were heated from room
temperature (RT) to 800 oC in the TG/DTA analyzer and the obtained thermal profiles
are displayed in Figure 22. This figure showed that the tested samples lost weights in
the temperature range of 30–400 oC and beyond 400
oC, weights of the tested samples
remained stable with no distinct loss. An insight in the TGA curves plotted in Figure
22a illustrated that weight loss was nearly the same for all the three asprepared ZnO
particle systems which took place immediately once heating started and continued to ͠
400 oC. The total weight loss observed was about 6.87% in the temperature range of
30–400 oC, which was due to the loss of surface adsorbed water molecules [137–138].
After 400 o
C, the weight of the tested samples stayed nearly unaffected with the
increase in temperature. This observation reflected the fact that the tested materials
possessed only the surface adsorbed water and no other volatile components which
could be thermally decomposed upon heating to 800 oC. The experimentally observed
weight loss (6.87%) calculated from TGA curves was in good agreement to
theoretically calculated weight loss (6.864%), shown through the following equation
(Eqn. 14).
ZnO · 1/3H2O → ZnO + 1/3H2O ↑ (theoretical wt. loss = 6.864 %) (14)
67
100 200 300 400 500 600 700 800
76
80
84
88
92
96
100
100 200 300 400 500 600 700 800
-60
-50
-40
-30
-20
-10
0
10
20
Hea
t fl
ow (
V)
Temperature (oC)
Z3
Z2
Z1
DTA curves
Temperature (oC)
Weig
ht
(%)
Z3
Z2
Z1
TGA curvesa
100 200 300 400 500 600 700 800
40
50
60
70
80
90
100
TGA
b
DTA
Z4
-40
-20
0
20
40
60
80
100
Weig
ht
(%)
Temperature (oC)
Hea
t fl
ow
(
V)
Figure 22. TG/DTA plots of selected asprepared zinc compounds, a) Z1–Z3, b) Z4.
68
In addition, the corresponding DTA curves recorded simultaneously with the
TGA data over the same temperature range (RT–800 oC) are given in inset of Figure
22a, respectively. The DTA curves showed no prominent peak which could be
associated with exo/endothermic nature of thermal weight loss occurred during the
TGA experiment for asprepared ZnO samples (Z1-Z3). This indicated that the
association of the water molecules with the ZnO nanostructures was purely physical
in nature. Other researchers also observed such type of behavior during the heat
treatment of the ZnO particles, synthesized through other routes [117].
Besides, the TG/DTA thermogram of zinc oxalate (Z4) was also recorded
(Figure 22b) which showed two distinct weight loss steps between 30–405 oC. A
weight loss of about 19.12 % was noted in the temperature range of 100–151 oC (step-
I). The observed weight loss reflected the evaporation of water from the crystal lattice
of ZnC2O4.2H2O and converted to anhydrous zinc oxalate (ZnC2O4). In step-II, a huge
amount (48.89%) as compared to the first step (19.12%) was lost at around 329–405
oC which corresponded to the thermal decomposition of the oxalate group to ZnO
with the removal of CO and CO2. These steps for the as prepared zinc oxalate
dihydrate (Z4) can be represented by the following chemical equations (Eqn. 15–16).
ZnC2O4.2H2O → ZnC2O4 + 2H2O ↑ (theoretical wt. loss = 19.04%) (15)
ZnC2O4 → ZnO + CO2 ↑ + CO ↑ (theoretical wt. loss = 47.05%) (16)
The experimentally calculated weight losses in two steps were nearly 19.12% and
48.89% (indicated in Figure 22b) and are very close to theoretically estimated values
from equation 15 & 16, respectively.
In addition, the appearance of two peaks in the corresponding DTA curve
(Figure 22b) shows the endothermic nature of both weight loss steps.
69
Lucilha et al. [10] also observed similar behavior for the thermal
decomposition of ZnC2O4.2H2O to ZnO at 400 oC.
Using the TGA data (Figure 22b), the activation energies associated with the
observed weight losses for the as prepared ZnC2O4.2H2O powders during the two
steps (pointed in the same Figure 22b) were estimated through a well-known Coats
Redfern equation (Eqn. 17) [124].
ln[- ln(1 - α/T 2)] = -Ea/RT - ln[(AR/β E)(1 - (2RT/E))] (17)
Where,
β is the heating rate,
ln[(AR/β E)(1 - (2RT/E))] is constant term,
R is the universal gas constant,
T is the temperature
“α” the fractional decomposed mass of the original material.
By plotting the TGA data in terms of ln[–ln(1-α/T 2)] against 1/T for the two
weight loss steps in Figure 23, activation energies were calculated by using Eqn. 17
and are listed in Table 2. As can be observed from this table, the activation energy for
step-I was less (0.55 kJ/mol ) compared to 1.573 kJ/mol for step-II, which indicated
that the thermally induced weight loss in step-I was due to the desorption of surface
water while step-II was associated with phase change in the solid matrix from ZnC2O4
to ZnO.
3.4.1 Calcination
Based on the information about the thermal response of the ZnO (Z1, Z2, Z3)
and ZnC2O4.2H2O (Z4) asprepared powder samples towards the temperature (TGA
data in Figure 22), another experiment was performed.
70
400 600 800 1000-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
II
Temperature (K)
I
a
0.002694 0.002697 0.002700 0.002703 0.002706
2.4691
2.4692
2.4693
2.4694
2.4695
2.4696
2.4697
2.4698
1/T (K
-1)
Original data
Linear fit
Slope = -66.18
Slope = -Ea/R
Ea = 0.55 KJ.mol-1
SD = 1.106E -4
ln[-
ln(1
-)/
T2]
b
0.001596 0.001599 0.001602 0.0016052.5880
2.5885
2.5890
2.5895
2.5900
2.5905
1/T (K
-1)
ln[-
ln(1
-)/
T2]
Slope = -189.16
Slope = -Ea/R
Ea = 1.573 KJ.mol-1
SD= 3.04E -4
Original data
Linear fit
c
Figure 23. a) α vs Temperature curve for asprepared Z4 particles, b) & c) straight
lines for the corresponding step-I and step-II in Figure 22b.
71
Table 2. Thermal weight losses and corresponding activation energies estimated for
the asprepared Z4 sample.
Steps
Temperature
(oC)
Weight loss (%)
Ea (kJmol-1
)
Experimental Theoretical
Step-I 100–151 19.12 19.04 0.55
Step-II 329–405 48.89 47.05 1.573
72
For this purpose, rather larger amounts of the selected batches of ZnO (Z1-Z3)
and ZnC2O4.2H2O (Z4) powders were calcined in a furnace under controlled heating
rate of 5 oC /min at 750
oC and 450
oC, respectively.
SEM images of the calcined particles, Z1cal, Z2cal, Z3cal, Z4cal, respectively in
(Figure 24a–d) revealed that the thermal weight losses slightly affected the surface
morphology of the end product (Z1cal-Z4cal). As can be seen from Figure 24d that
calcinations transformed the solid cubes of ZnC2O4.2H2O (Z4) into porous cubes of
ZnO (Z4cal) by thermal decomposition of oxalate group which then followed by
elimination of CO and CO2 through Eqn.16. It seems that the elimination of crystal
water and thermally decomposed material like CO and CO2 produced pores in the
primary formed solid cubes.
Furthermore, calcined powders (Z1cal–Z4cal) were then subjected to XRD
analysis (Figure 25) to confirm the composition and any phase changes that occurred
as a result of high-temperature calcination. The diffraction data of the major peaks of
all XRD spectra were employed for the estimation of the crystallite sizes of calcined
particles. Debye Scherrer formula (Eqn. 18) was used for the calculation of crystallite
sizes [139–140].
(18)
Where, Dp is the crystallite size of the particles, k is the Scherrer constant
(0.9), λ is the wavelength of X-rays used (0.154 nm) and β is full width at half
maximum of major diffraction peaks at the Bragg angle (θ). The calculated average
crystallite sizes for calcined ZnO samples are given in Table 3, which ranged as
25.30, 25.36, 25.37 and 25.42 nm for Z1cal, Z2cal, Z3cal, and Z4cal samples,
respectively.
73
Figure 24. SEM images of ZnO particles calcined at; a–c) 750 oC; Z1cal–Z3cal, d) 450
oC; Z4cal.
74
10 20 30 40 50 60 70 80
Z1cal
: ZnO
Z2cal: ZnO
Z3cal
: ZnO
2 (degree)
Inte
nsi
ty (
a.u
.)
Z4cal: ZnO
Figure 25. XRD diffractograms of calcined ZnO particles, Z1cal–Z4cal.
75
Table 3. Illustration of various crystallographic parameters estimated out from XRD patterns
of calcined ZnO samples (Z1cal–Z4cal).
Sample
Code
2θ (˚) FWHM
(˚)
Crystallite
size
Dp (nm)
Lattice constants (Å) Unit cell
volume
Vcell (Å)3
X-ray
Density
Dx
(g/cm3)
Specific
surface
area
SSA
(m2/g)
a c
Z1cal
31.80
34.51
36.30
0.31
0.32
0.32
25.30
3.246
5.207
47.51
5.69
41.68
Z2cal
31.80
34.51
36.30
0.32
0.32
0.32
25.36
3.251
5.208
47.65
5.671
41.72
Z3cal
31.80
34.52
36.30
0.33
0.32
0.32
25.37
3.248
5.205
47.68
5.665
41.74
Z4cal
31.80
34.51
36.30
0.32
0.32
0.33
25.42
3.246
5.209
47.53
5.651
41.78
76
In addition, other crystallographic parameters like lattice constants, unit cell volume,
X-ray density and specific surface area were also calculated from the obtained XRD
patterns using Eqn. 19–23 and illustrated in Table 3.
(19)
(20)
(21)
(22)
(23)
Where a, b, c are lattice constants, Vcell is volume of unit cell, Dx is X-rays
density, n is number of formula units per unit cell, M is molecular weight of sample
analyzed, N is Avogadro’s number, SSA is the specific surface area of the sample.
The specific surface area calculated out from crystallographic parameters
through Eqn. 23 is 41.78 m2
/g for the Z4cal sample which is greater than the other
ZnO samples (Z1cal–Z3cal). It is because the specific surface area is inversely related
to the X-rays density and crystallite size according to Eqn. 23 (Table 3).
FT-IR analysis of the calcined powder samples (Z1cal–Z4cal) was also
performed to determine the composition of final products. Figure 26 shows the FT-IR
spectral profile of the final products after calcination.
It can be observed from the spectra that peaks due to stretching and bending
vibrations of O–H groups (3469, 3479, 1641–1407 cm-1
, Table 1) have almost
vanished. Similarly, nitrate (NO-3
) group decomposed during high-temperature
calcination and therefore characteristic nitrate peaks (892, 893 & 1055 cm-1
, Table 1)
got nearly disappeared in the calcined powders [2, 136].
77
4000 3500 3000 2500 2000 1500 1000 500
Z3cal
Z2cal
Z1cal
541-412
Wavenumber (cm
-1)
Tra
nsm
itta
nce
(%
)
Z4cal
Figure 26. FT-IR spectra of calcined ZnO particles, Z1cal–Z4cal.
78
As can be seen from Figure 26, high-temperature calcination of the Z4 sample
decomposed the initial zinc oxalate dihydrate to pure ZnO (Z4cal) with the removal of
all hydroxyl and carbonyl groups [132–135]. It was also found that the strong
absorption band characteristic of Zn–O fall in the same wavenumber region (541–412
cm-1
) irrespective of the synthesis route i.e., either obtained directly through HMT and
ammonia (Z1cal–Z3cal) or obtained indirectly through the calcination of ZnC2O4.2H2O
precursor (Z4cal).
The FT-IR analysis of calcined samples, therefore, endorsed the
aforementioned TGA and XRD results and confirmed the four calcined products
(Z1cal–Z4cal) as pure ZnO.
3.5. XRD Analysis of Asprepared Zinc Phosphate
In order to determine the phase purity and composition of the asprepared zinc
phosphate samples (ZP1, ZP2& ZP3), XRD patterns were recorded in a 2θ range of
10–70 degree (Figure 27). The presence of narrow and intense peaks in the XRD
patterns evidenced the formation of well crystalline materials.
The observed diffraction peaks matched well with the typical diffraction
patterns of Zn3 (PO4)2.4H2O with the orthorhombic crystal structure (ICDD No. 33-
1474) [59]. No other phases regarding impurities were detected in XRD patterns,
indicating the purity of the synthesized zinc phosphate. XRD patterns for sample ZP2
is composed of more intense peaks compared to ZP1 and ZP3.
For instance, the obtained results also matched with the XRD patterns in the
same 2θ range (5–70˚), reported elsewhere for Zn3(PO4)2.4H2O [59–60] which
exclusively confirmed the prepared products as zinc phosphate tetrahydrate also
called hopeite.
79
10 20 30 40 50 60 70
ZP3
ZP2
Zn3(PO)
4.4H
2O
2 (degree)
Inte
nsi
ty (
a.u
.)
ZP1
Figure 27. XRD diffractograms of asprepared zinc phosphate powders.
80
Furthermore, the crystallite size of the same samples (ZP1–ZP3) was
calculated from strong diffraction peaks of their respective XRD pattern (Figure 27)
using Debye Scherrer formula (Eqn. 18). Also, other crystallographic parameters were
also calculated from the same XRD signatures, using equations 21–25 and the
obtained values are given in Table 4.
(24)
(25)
The average crystallite sizes estimated out to be 16.50, 30.15 and 37.01 nm for
ZP1, ZP2, and ZP3, respectively. XRD analysis indicated that variation in the growth
conditions not only changed the crystallite size but also the preferred orientation of
crystals which resulted in different morphologies of the synthesized products.
3.6. FT-IR Analysis of Asprepared Zinc Phosphate
Figure 28 shows the FT-IR spectra of asprepared fine powders of ZP1, ZP2,
and ZP3. The obtained spectral profiles are composed of absorption bands associated
with the characteristic vibrations of H2O and PO43-
, which are consistent with the
characteristic FT-IR spectra of zinc phosphate reported by other research groups [66,
128, 142] confirming the synthesized product as pure zinc phosphate tetrahydrate.
The FT-IR results are also in good agreement with Eqn.13 as well as with the XRD
results, shown in Figure 27.
The broad absorption bands centered at wavenumber 3273 cm-1
and relatively
sharp peaks at 1442 cm-1
and 1628 cm-1
are ascribed to the characteristics stretching
and bending vibrations of O–H [128, 142], respectively indicating the presence of
crystal water in the analyzed samples. The absorption peaks appeared in the range
1270 cm-1
to 940 cm-1
and 614–400 cm-1
are assigned to the particular stretching and
bending vibrations of PO43-
group [98, 151-152] (Table 5).
81
Table 4. Illustration of various crystallographic parameters estimated out from XRD
patterns of selected zinc phosphate samples (ZP1–ZP3).
Sample
Code
2θ (˚) FWHM
(˚)
Crystallite
size
Dp (nm)
Lattice constants (Å) Unit
cell
volume
V (Å)3
X-ray
Density
Dx
(g.cm -3
)
Specific
surface
area
SSA
(m2/g
-1)
a B c
ZP1
14.04
20.16
27.89
0.51
0.44
0.54
16.50
10.608
18.313
5.02
975.21
3.121
116.51
ZP2
11.45
20.0
28.4
0.27
0.25
0.29
30.15
10.603
18.301
5.02
974.11
3.125
63.68
ZP3 9.75
19.45
0.22
0.22
37.01
10.598
18.298
5.03
975.43
3.121
51.94
82
4000 3500 3000 2500 2000 1500 1000 500
1677
940
ZP3
ZP2
614-4001033
12701442
1628
3273
Wavenumber (cm-1)
Tra
nsm
itta
nce
(%
) ZP1
Figure 28. FT-IR spectra of selected asprepared zinc phosphate powders (ZP1–ZP3).
Table 5. Wavenumber positions at which the chemical groups on the selected
asprepared solids absorb IR radiations.
Band Position (cm-1
) Group
Species
Vibration
Mode
References
ZP1 ZP2 ZP3
3273 3273 3273 O–H Stretching
modes
[66,128, 142]
1677, 1442, 1628, 1442, 1628, 1442, O–H Bending
modes
[128, 142]
1033 1277, 1033,
940
1277, 1033,
940
PO43-
Stretching
modes
[60, 128, 142]
614–400 614–400 614–400 PO43-
Bending
modes
[60, 128, 142]
83
The existence of these prominent bands in the observed scanned FT-IR profile
(Figure 28) demonstrate the formation of pure zinc phosphate tetrahydrate,
Zn3(PO4)2.4H2O as no peaks were noted for any impurity [60, 66].
3.7. Thermal Analysis of Asprepared Zinc Phosphate
For thermal analysis, the as-prepared powders of selected zinc phosphate
samples (ZP1, ZP2 & ZP3) were subjected to heating from RT to 800 oC in
thermogravimetric analyzer with the ramp rate of 5 o
C min-1
. The obtained data were
plotted as TG/DTA curves and shown in Figure 29. The inspection of thermal profiles
in Figure 29 illustrated three stages of weight loss in the temperature range of 30–650
oC for ZP1-ZP3. The corresponding DTA curves show sharp endothermic peaks
which manifested the specific temperature at which the weight loss occurred (Table
6). It has been reported that weight loss during the three stages for all samples can be
attributed to the evaporation of the water of crystallization of the as-prepared zinc
phosphate, Zn3(PO4)2·4H2O. The experimentally and theoretically calculated weight
losses depicted in Table 6, showed slight variations which may be due to the
hygroscopic nature of the samples [143].
The thermograms become stable beyond 500 oC and 650
oC for ZP1–ZP2 and
ZP3 respectively which indicated that there was no further weight loss after thermal
dehydration of Zn3(PO4)2 ·4H2O and the final product was anhydrous zinc phosphate,
Zn3(PO4)2. The three steps of weight loss during thermal dehydration of zinc
phosphate can be represented through the following reactions [144].
Zn3(PO4)2·4H2O → Zn3(PO4)2·2H2O + 2H2O (26)
Zn3(PO4)2·2H2O → Zn3(PO4)2·H2O + 1H2O (27)
Zn3(PO4)2·H2O → Zn3(PO4)2 + H2O (28)
84
100 200 300 400 500 600 700 800
80
85
90
95
100 ZP1
DTA
Heat
flow
(
V)
Temperature (oC)
Weig
ht
(%)
TGA
T1
T2
T3 -25
-20
-15
-10
-5
0
5
10
100 200 300 400 500 600 700 800
80
85
90
95
100
Temperature (oC)
Weig
ht
(%)
-30
-25
-20
-15
-10
-5
0
5
10
TGA
DTA
ZP2
Heat
flow
(
V)
T1
T2
T3
100 200 300 400 500 600 700 80070
75
80
85
90
95
100
105
T3
T2
DTA
TGA
ZP3
Temperature (oC)
Weig
ht
(%)
T1
-60
-50
-40
-30
-20
-10
0
H
eat
flow
(
V)
Figure 29. TG/DTA curves of selected zinc phosphate powders (ZP1–ZP3).
85
Table 6. Temperatures and corresponding weight losses estimated for the asprepared
ZP1–ZP3 samples.
Samples Temperature
(˚C)
Weight loss
(%)
T1 T2 T3 Experimental Theoretical
ZP1 55 195 474 19.4
15.72 ZP2 58 185 444 20.5
ZP3 42 294 360 25.0
86
It is mentioned that other research groups have also reported the formation of
anhydrous zinc phosphate from thermal dehydration of zinc phosphate tetrahydrate
[128, 143].
Moreover, following the TGA study it can be established that besides the
difference in morphology and the synthesis route employed, the thermal behavior of
zinc phosphate materials is nearly the same to those reported in the literature [143,
145–146].
3.7.1. Calcination
Following the TG/DTA analysis (Figure 29), known amounts of the selected
as-prepared zinc phosphate powder i.e., ZP1, ZP2, and ZP3 were subjected to
calcination in a furnace at 650 oC for 2h. To assess the morphological changes of the
resulted calcined powders (Zn3(PO4)2), SEM analysis was carried out and the obtained
images are displayed in Figure 30.
Inspection of SEM images revealed significant changes in microstructures due
to the elimination of crystal water and similarly temperature nonresistant materials
like surface nanoneedles and nanorods of hierarchical microstructures in case of
ZP1cal (SEM; Figure 30a & b) and ZP2cal (SEM; Figure 30 c & d). Apart, the SEM
image in Figure 30e indicated morphological transformation in the case of ZP3cal and
the hierarchical structures disintegrated into their primary nanospheres of a rather
small size (Figure 19a). Thus, it is concluded that the asprepared hydrated zinc
phosphate lost their original microstructure integrity and converted to anhydrous zinc
phosphate as a result of thermally triggered reactions (26–28) which caused the
elimination of water of crystallization and loss of few percents of water molecules
which were attained by the material due to hygroscopic nature.
87
Figure 30. SEM micrographs of Zn3(PO4)2 obtained after heat treatment of ZP1–ZP3
(SEM, Fig 10a, 11a &14c), a & b) ZP1cal, c & d) ZP2cal and e) ZP3cal.
88
Similar results regarding the non-uniform morphology of Zn3(PO4)2 powders
in the form of heterogeneous aggregates of particles of varied sizes have been
reported from thermal dehydration of Zn3(PO4)2 ·4H2O [146].
In addition, to determine the phase composition of the product obtained from
thermal dehydration of zinc phosphate tetrahydrate at 650 o
C, XRD diffraction
patterns were recorded in the 2θ range of 10–70 ⁰ (Figure 31). The diffraction patterns
matched well with the standard XRD spectrum of JCPDS-291390 [119] and
confirmed the formation of anhydrous zinc phosphate, Zn3(PO4)2 with a monoclinic
phase which clearly supported the thermal dehydration reactions, mentioned in Eqn.
27–29. The average crystallite sizes were estimated out to be 16.5, 20.9 and 22.4 nm
for ZP1cal, ZP2cal, and ZP3cal, respectively.
Thermal changes in zinc phosphate solids (Figure 30) as a result of heat
treatment of the as-prepared materials were further supported by the FT-IR analysis.
The FT-IR spectral profiles (Figure 32) of the calcined products (ZP1cal –ZP3cal)
revealed that in comparison with the FT-IR spectra of the asprepared Zn3(PO4)2
·4H2O (FT-IR, Figure 28), the intensity of the broad absorption bands (3273 cm-1
and
1667–1442 cm-1
) (Table 5) corresponding to O–H stretching and bending vibrations
were reduced obviously due to the loss of water molecules from the crystal structure.
In contrast, the absorption peaks assigned to the particular stretching and bending
vibrations of PO43-
group became more prominent in the spectral profiles (Figure 32)
of the calcined samples (ZP1cal–ZP3cal). The observed changes in the spectrum of the
Zn3(PO4)2 (ZPcal) were due to the controlled heat treatment at 650 oC and subsequent
formation of pure anhydrous zinc phosphate, Zn3(PO4)2.
89
15 20 25 30 35 40 45 50 55 60 65 70 75 80
ZP1cal : D = 16.5 nm
In
ten
sit
y (
a.u
.)
2 (degree)
ZP2cal : D = 20.9 nm
ZP3cal : D = 22.4 nm
Figure 31. XRD spectra of calcined zinc phosphate powders (ZP1cal–ZP3cal).
90
4000 3500 3000 2500 2000 1500 1000 5000
34
68
102
0
34
68
102
28
56
84
112
ZP1cal
PO3-
4
Wavenumber (cm-1)
Tran
sm
itta
nce (
%)
ZP2cal
ZP3cal
Figure 32. FT-IR spectra of calcined zinc phosphate Zn3(PO4)2 (ZP1cal–ZP3cal).
91
It is to be mentioned that calcined powders of ZnO (SEM; Figure 24) and as
asprepared zinc phosphate ZP1, ZP2 and ZP3 (SEM, Figure 14a, 15a & 19c) were
selected for gas sensing application. While calcined ZnO (SEM; Figure24) was also
tested for its antibacterial activity. It is added that no literature report is available
regarding the gas sensing properties of zinc phosphate. However, before employing
for application purposes, the mentioned samples were also analyzed for surface area
determination.
3.8. Surface Area Analysis
As described earlier that various properties of nanomaterials such as gas
sensing properties, photocatalysis, antimicrobial activity and drug delivery
characteristics, etc. strongly depend on their crystal structure, particle size,
morphology, and surface area [4–9]. As such, all high tech applications require
nanomaterials of high surface area to obtain improved properties. In this regard,
samples selected for application were employed for surface area analysis.
The surface area was determined using the BET (Brunauer, Emmet and Teller)
method. The selected samples (Z1cal–Z4cal and ZP1–ZP3) were subjected to BET
characterization. Figures 33 & 34 illustrate the N2 adsorption-desorption isotherms of
these samples [147].
The BET specific surface area for calcined ZnO samples calculated to be
54.74, 62.18, 67.41 and 95.20 m2/g for Z1cal, Z2cal, Z3cal, and Z4cal, respectively. The
relatively greater surface area was obtained for Z4cal which was attributed to the
highly porous cubic structure of Z4cal particles. Similarly, the surface area determined
for the asprepared zinc phosphate samples were 137.46, 104.5, and 96.15 m2
/g for
ZP1, ZP2, and ZP3, respectively.
92
0.0 0.1 0.2 0.3 0.4 0.5
0
10
20
30
40
50
Relative Pressure (P/Po)
Volu
me A
dso
rb
ed
(cm
3/g
)
Z1cal = 54.74 m2/g
Z2cal = 62.18m2/g
Z3cal= 67.41m2/g
Z4cal = 95.20 m2/g
ZnO-Com = 30.03 m2/g
Figure 33. BET plots for selected ZnO samples.
93
0.0 0.1 0.2 0.3 0.4 0.5
0
10
20
30
40
Relative Pressure (P/Po)
Volu
me A
dso
rb
ed
(cm
3/g
) ZP1 = 137.46 m2/g
ZP2 = 104.5 m2/g
ZP3 = 96.15 m2/g
Figure 34. BET plots for selected asprepared zinc phosphate samples (ZP1–ZP3).
94
The high surface area of zinc phosphate samples can be attributed to the
hierarchical structures of the material particles. The BET surface area values showed
good consistency with the corresponding SEM results of the mentioned powder
samples (Figure 14a, 15a, 19c and Figure 24).
3.9. Gas Sensing Properties of ZnO and ZP Sensors
Considering the significant role of semiconducting ZnO in gas sensing,
selected batches of ZnO were employed in the fabrication of highly sensitive gas
sensors to study their sensing performance towards the detection of various test gases.
Slurries of the selected nanostructures (Z1cal–Z4cal & ZP1–ZP3) were applied in the
form of film through screen printing method on sensor plate (1 cm × 1cm alumina
plate carrying interdigitated electrodes of gold). The thickness of the sensor film was
determined using the weight difference method by the following working formula, t =
m/ρA [148–149]. Where, t is the thickness of the film, m is mass of the deposited
film, A is the area of the sensor plate and ρ is the density of the sensor material
calculated from the formula (ρ= (1.6609×M×n/a2c/ √¾) [148–149]. Where M is the
molecular weight, n represents the number of formula units of ZnO (n=2), 'a' and 'c'
are lattice parameters of a unit cell of ZnO. Gas sensing properties of the fabricated
sensors (Z1cal–Z4cal & ZP1–ZP3) were monitored in a self-designed gas sensor setup
which can be illustrated through the block diagram given in Figure 35.
3.9.1. Semiconducting Properties
Semiconducting properties of the prepared sensors (Z1cal–Z4cal & ZP1–ZP3)
were monitored continuously in terms of the electrical resistance against increasing
operating temperature (29 oC to 250
oC) in the presence of pure dry air. Figure 36
shows variation in the electrical resistance of the selected sensors as a function of
temperature.
95
Figure 35. Block diagram of gas sensor setup.
96
50 100 150 200 250
0
50
100
150
200
Temperature (oC)
Resi
san
ce (
)
Z4cal
Z3cal
Z2cal
Z1cal
ZP3
ZP2
ZP1
Figure 36. Electrical resistance as a function of the temperature of fabricated sensors
(Z1cal–Z4cal & ZP1–ZP3).
97
The obtained curves showed sharp and well-defined fall in resistance of the
sensors in the narrow temperature range of 76–100 oC, which then followed a leveled
off region extended from about 140 oC to 250
oC.
The attained thermally triggered changes in the electrical property of the
sensor material may be ascribed to the semiconducting nature of the fabricated
sensors based on ZnO particles (Z1cal–Z4cal), which started conduction on elevated
temperatures. Since, it is believed that upon heating the semiconducting materials,
electrons from the valence band are shifted to the conduction band and hence make
them conductors. For instance, a sharp fall in the sensor resistance was due to the
acquisition of pretty sufficient thermal energy by the charge carriers i.e., electrons, to
endure their existence in the conduction band.
It can be noticed from Figure 36 that temperature-dependent variation in
resistance leveled off at about 140 oC, pointing to the fact that the transition process of
electrons from the valence band to the conduction band of the sensors approached
saturation limit in the temperature range of 76–150 oC. It is believed that during the
heating phase, ZnO undergoes a reduction reaction and subsequently generates an
oxygen vacancy (V₀) inside the solid matrix (Eqn. 29).
ZnO → Zn + ½ O2 + V₀ (29)
V₀ → V₀2+
+ 2e (30)
This oxygen vacancy (V₀) then immediately gets ionized and releasing two
electrons in the conduction band of solid (Eqn. 30). As such the transition of these
free electrons to the conduction band caused an observed decrease in the resistance of
sensors. It was also noted that the Z4cal sensor started conduction earlier at 76 oC
while Z3cal, Z2cal and Z1cal sensors showed conduction at 88 oC, 93
oC, and Z1cal 100
oC, respectively. It has been reported that better gas sensing response of the sensor
98
material was due to the large quantity of V₀ and greater surface area to volume ratio
[150–151]. On the other side, the electrical resistance of the zinc phosphate based
sensors (ZP1–ZP3) showed no variation in resistance with a change in temperature as
can be seen in Figure 36. This showed that unlike ZnO sensors (Z1cal–Z4cal), zinc
phosphate sensors do not exhibit semiconducting properties.
To calculate activation energy for the observed variation in resistance of Z1cal–
Z4cal sensors with the increase in ambient temperature, the linear form of the
Arrhenius equation (Eqn. 31) was employed [152–153].
Ln(σ) = Ln(σo) - ΔEa/kT (31)
Where,
σ is electrical conductance (siemens),
σo is a constant factor,
∆Ea is the activation energy (eV),
K is Boltzmann constant (8.617 × 10-5
eV. K-1
) and
T is the operating temperature (K).
Figure 37 shows the corresponding plots of lnσ vs 1/T. The obtained linear
behavior demonstrated that within the employed temperature range, thermionic
emission played an imperative role in the transport of charge carriers [154].
Activation energies estimated out from the slopes of the corresponding plots
are given in Table 7. The activation energy obtained for Z4cal based sensor was
relatively smaller i.e., 0.900 eV as compared to 0.903, 0.905 and 0.914 eV for Z3cal,
Z2cal and Z1cal based sensors, respectively. It is worth to mention that the activation
energy observed in this study for Z1cal–Z4cal based sensors was smaller than that
reported by other research groups [155] for the ZnO sensor.
99
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8-4
-3
-2
-1
0
1
Slope = -10615.09
Ea = 0.905
Original data
Linear fit
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8
-4
-3
-2
-1
0
1 Z4calZ3
cal
Original data
Linear fit
2.1 2.2 2.3 2.4 2.5 2.6 2.7-4
-3
-2
-1
0
Slope = -10536.13
Ea = 0.914
Slope = -10475.37
Ea =0.903
Original data
Linear fit
Z1cal
2.2 2.3 2.4 2.5 2.6 2.7
-4
-3
-2
-1
0
Z2cal
Slope = -10451.67
Ea = 0.900
Original data
Linear fit
Ln
(1/TK)×1000 (1/TK)×1000
Ln
Figure 37. Ln (σ) versus 1/Tk plots along with corresponding activation energies for
Z1cal–Z4cal sensors.
100
Table 7. Activation energies, gas response and response/recovery time of ZnO sensors
towards the detection of ammonia gas.
Sample
code
Response (%) for
Ammonia Concentration
(ppm)
Response
time (s)
Recovery
time (s)
Pore
radius
Activation
energy
(eV)
Optimum
Tempera-
ture (˚C)
Maximum
response
(%)
1 2 5 25 100
Z1cal 58 60 72 84 92 11 10 15.90 0.914 130 80
Z2cal 61 67 73 90 95 9 10 15.98 0.905 120 85
Z3cal 66.8 72.4 75.4 92 98 9 9 16.23 0.903 120 88
Z4cal 70 76 85 98 99 9 8 16.37 0.900 109 91
ZP1 74 79 89 97 99 31 12 15.84 No
response
No
response
No
response
ZP2 65 71 86.5 94 96 35 15 13.91 No
response
No
response
No
response
ZP3 61 65 78 89 94 37 17 12.88 No
response
No
response
No
response
101
The relatively higher value of activation energy obtained for the Z1cal sensor
(0.914 eV) compared to other ZnO sensors (Z2cal–Z4cal) could be attributed to the
smaller crystallite size of the same sensor particles. In fact, smaller crystallite sizes
lead to scattering of charge carriers at the grain boundaries, which in turn hamper
their mobility inside the microstructure [156].
As such the smaller crystallite size resulted in higher bandgap value which,
therefore, needed relatively greater activation energy for the electronic transition from
the valence band to the conduction band.
3.9.2. Gas Sensing Properties
It was found in the preliminary experiments that the semiconducting nature of
the fabricated sensors (Z1cal–Z4cal, ZP1–ZP3) significantly depended upon the
microstructure of these samples. Therefore it was of interest to study the room
temperature gas sensing properties of the fabricated sensors in order to observe the
effect of microstructure on room temperature sensing behavior.
As such, the dynamic response of the selected sensors (Z1cal –Z4cal, ZP1–ZP3)
was measured towards 5ppm of ammonia over seven repeated cycles at room
temperature under the same experimental conditions (Figure 38–39). The recorded
response patterns show that the exposed sensors responded in the form of a decrease
in electrical resistance upon exposure to ammonia gas and immediately retained their
original resistance values as ammonia gas was switched to the stream of dry air.
It can also be examined from the resistance bands in Figure 38a & 39a that the
dynamic response of all the sensors is quite stable with no measurable drift.
102
0 200 400 600 800 1000 1200
0
50
100
150
2000
50
100
150
200
0
50
100
150
2000
50
100
150
200
Time (s)
Res
ista
nce
()
Z1cal
Z2cal
Z3cal
Z4cal
a
0 20 40 60 80 100 120 140 160 180
0
20
40
60
80
100
Res
pon
se (
%)
Time (s)
Z4cal
Z3cal
Z2cal
Z1cal
b
Figure 38. Dynamic resistance response curves of the ZnO sensors. b) The first cycle
of resistance curves shown in (a) for response (%) calculation.
103
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0
40
80
120
160
200
Time (s)
Resis
tan
ce (
M
)
ZP3 ZP2 ZP1 a
0 50 100 150 200 250 300
0
20
40
60
80
100
Resp
on
se (
%)
Time (s)
ZP3
ZP2
ZP1
b
Figure 39. a) Dynamic resistance response curves of zinc phosphate based
sensors, b) First cycle of resistance curves shown in ‘a)’ for response (%)
calculation.
104
The electrical resistance data in Figure 38a & 39a was then employed to
calculate the corresponding sensor response values for each of the selected sensors by
using the relation in Eqn. 32 [20, 36, 153].
S (%) = [Ra- Rg / Ra] × 100 (32)
Where, S is the sensor response (%), Ra is the sensor resistance in the air (Ω) and Rg is
the sensor resistance in the presence of test gas (Ω).
The corresponding response (%) curves are plotted in Figure 38b & 39b,
respectively. The calculated sensor response values along with response/recovery time
are also given in Table 7.
Inspection of Figure 38–39 reveals that the maximum response value (89%)
was observed for the ZP1 sensor while the minimum response value was obtained for
the Z1cal sensor (72 %). Observation of the room temperature ammonia sensing
response unfold that the selected sensors showed a response in the order
ZP1>ZP2>ZP3>Z4cal>Z3cal> Z2cal>Z1cal.
It is to mention that room temperature gas sensing of the selected sensors
(Z1cal –Z4cal, ZP1–ZP3) belongs to surface controlled phenomenon. It has been known
that gas sensing properties of the nanostructures significantly depend upon the
nanostructure size, morphology, adsorbed oxygen quantity and surface area of the
material which affects the adsorption rate of the target gas [157–158].
Greater the surface area of the sensor material, the stronger is the interaction
between the target gas molecules and the sensor surface and hence greater is the
sensor response. In fact, due to high surface area (Figure 33–34), the hierarchical and
porous cubic structures of the synthesized sensor materials i.e., Z4cal & ZP1–ZP3 are
responsible for the increased diffusion of ammonia gas molecules by offering more
surface active sites for interaction, which led to rather highest sensing response of
105
ZP1–ZP3 and Z4cal sensors at room temperature. For instance, due to the high specific
surface area of ZnO/Graphene oxide composite based sensor showed an improved
sensing response at 280 oC than pure ZnO [159].
3.9.3. Ammonia Sensing Mechanism
The aforementioned investigation of semiconducting properties (section
3.9.1.) explored that no change was observed in the electrical resistance of the zinc
phosphate sensors (ZP1–ZP3) in the applied temperature range (29–250 oC). It
indicated that there is no involvement of the conduction band electrons in the room
temperature ammonia sensing phenomenon.
Therefore, in the framework of the obtained results showing the behavior of
selected sensors towards ammonia exposure, the following mechanism is proposed for
the interaction of ammonia gas with the sensor surface. It is believed that at room
temperature, the water molecules present on the sensor surface act as Bronsted acid
sites while the incoming ammonia molecules act as Bronsted base and undergo the
following acid-base reaction [28, 153].
H2O + NH3 NH4+ + OH (33)
NH4+ NH3 + H
+ (34)
Schematics of the interaction of ammonia gas with sensors surface is shown in Figure
40. The NH4+ ions that resulted from reaction 33, hopped amongst the adjacent
Bronsted acid sites, which created a temporary conducting film over the sensor
surface and consequently caused a decrease in the electrical resistance of the sensor.
When ammonia was substituted with the stream of pure dry air, the NH4+ ions
decomposed through Eqn. 34 and desorption of NH3 took place recovering the sensor
back to its initial resistance value.
106
Figure 40. Schematics showing the interaction between ammonia gas and the surface
of the zinc phosphate sensor (ZP1–ZP3).
107
It showed that the ammonia adsorption on the sensor surface was reversible in
nature and suggested that the bonds involved between ammonia molecules and the
sensor surface were weak physical forces, which were easily broken away when a
stream of dry air was passed through.
3.9.4. Response/Recovery Time
In addition, response/recovery time is also one of the basic parameters used to
determine the sensor performance and is important for their practical use. In fact, a
gas sensor with rapid response/recovery time is desirable for real-time usage in their
practical applications. Therefore, response/recovery times of the studied sensors
(Z1cal–Z4cal & ZP1–ZP3) were also calculated (Table 7) using the first band of the
dynamic resistance response patterns given in Figure 38a & 39a.
Among the studied sensors, Z4cal based sensor showed relatively fast
response/recovery time of about 9 and 8 s, respectively. The quick response/recovery
time of the sensor was attributed to the highly porous structure of the sensor material
which would have enabled fast and easier diffusion of ammonia gas to grain
boundaries and consequently displayed quicker response.
On the other side, zinc phosphate sensors took a relatively longer
response/recovery time (Table 7). Among these sensors, ZP1 exhibited a relatively
quick response to ammonia (31s) compared to ZP2 (35s) and ZP3 (37 s) sensors. This
can be attributed to the hierarchical structure of ZP1 which thus provided greater
surface active sites, making the ammonia gas diffusion across the grain boundaries
easier for the above-mentioned surface reaction (Eqn. 33).
In contrast, considering the recovery times, the ZP1 sensor took 12 s, while
ZP2 and ZP3 sensors showed a bit longer recovery times of 15 and 17 s, respectively.
108
It has been observed that ammonia sensing response of zinc phosphate based
sensors (ZP1–ZP3) were comparable with response values of ZnO based sensors
(Z1cal–Z4cal), however, the response/recovery time of zinc phosphate sensors was
longer compared to ZnO sensors. It might be due to the morphological difference of
the test sensor materials. Since the target gas molecules adsorbed not only on the
surface of the hierarchical structures of the zinc phosphate sensor material but also
diffused through the fine porous channels of each particle. The diffusion through
porous materials is commonly described as either ordinary, surface or Knudsen
diffusion [160].
In the case of porous materials, consisting of very fine particles it is the
Knudsen diffusion that contributes to the high sensing performance and longer
response/ recovery of the zinc phosphate sensors (ZP1–ZP3). The Knudsen diffusion
coefficient (DK) can be expressed as (Eqn.35).
DK = 9700 × r (T/M)1/2
(35)
Where r is the pore radius, T is the working temperature and M is the
molecular weight of the test gas. Pore radii of selected sensor materials observed from
BET surface analysis are given in Table 7. According to Eqn.35, for constant film
thickness, a large pore radius directly increases the DK value and hence would enhance
the sensor response [160].
Moreover, considering the ammonia sensing properties of ZnO based sensors,
Table 8, shows the comparison of ammonia sensing properties of the as-fabricated
sensors with those of the earlier reported ZnO based ammonia sensors. As can be seen
(Table 8), room temperature ammonia sensing performance of the fabricated sensors
in the present work is superior with the highest response (85%) and fast
response/recovery time (9/8 s) than the previously reported sensing devices.
109
Table 8. Comparison of sensor response of fabricated sensor with previously reported
ZnO based ammonia gas sensors.
Sensor material Ammonia
concentration
(ppm)
Operating
temperature
(oC)
Sensor
response
(%)
Response/
Recovery
time (s)
Refere
nces Composition Morphology
ZnO Spherical
nanoparticles
50 150 18 660/600 [23]
ZnO Irregular
nanostructures
25
25
RT
150
3
16
49/19
--
[29]
ZnO Hexagonal
cylinder shape
50 RT 1.2 -- [161]
ZnO Nearly
spherical
100 RT 23 122/104 [52]
ZnO Hexagonal 46 100 3.96 38/156 [46]
ZnO Nanorods 500 RT
150
8
~60
-- [48]
Z1cal Ellipsoidal
nanorods
5 RT 72 11/10 Present
work
Z2cal Nanoellipsoids 5 RT 73 9/10 Present
work
Z3cal Nanoflowers 5 RT 75.4 9/9 Present
work
Z4cal Hierarchical
porous cubes
5 RT 85 9/8 Present
work
*RT=room temperature
110
Besides, Chen et al. [37] investigated ZnO based ammonia sensing and
recorded maximum sensor response of 81.6 at 300 oC towards 1000 ppm ammonia
and reported 10 ppm as the lower detection limit. While Ponnusamy and
Madanagurusamy [162] studied room temperature (30 oC) detection of ammonia and
reported ~92 and ~111 s response/recovery times for the lowest ammonia
concentration of 5 ppm. In addition, Venkatesh et al. [29] obtained 10 % response to
100 ppm ammonia with 49 and 14 s response recovery times at room temperature.
Similarly, Andre et al. [38] reported 4.5% for pure ZnO and 17% for poly (styrene
sulfonate) loaded ZnO with 51 s and 160 s response/ recovery time to 100 ppm
ammonia at room temperature.
3.9.5. Effect of Temperature on Sensor Response
Since, it is generally stated that the gas response of semiconducting metal
oxide sensors is affected by the working temperature [28, 53, 153], As mentioned
above that zinc phosphate sensors showed no response to variation in working
temperature, therefore it was of interest to evaluate the ammonia sensing behavior of
the ZnO based sensors (Z1cal–Z4cal) towards the effect of operating temperature. For
this purpose, the sensing experiments were conducted in the presence of a continuous
flow of 5ppm of ammonia gas and the temperature of the sensing chamber was
increased in a controlled manner from room temperature to 250 oC.
The obtained response versus working temperature plots are given in Figure
41. It can be noted from Figure 41 that all the ZnO based sensors (Z1cal–Z4cal)
exhibited different responses at different working temperatures. For instance, the Z4cal
sensor displayed an excellent response of 91% at the optimum temperature of 109 oC
in comparison to other ZnO sensors in the applied temperature range.
111
0 50 100 150 200 250
0
20
40
60
80
100
Temperature (oC)
Sen
sor r
esp
on
se (
%)
Z4cal
Z3cal
Z2cal
Z1cal
Figure 41. The response of Z1cal–Z4cal based sensors at different operating
temperatures towards ammonia vapors (5 ppm).
112
In addition, maximum response values of all ZnO sensors with their
corresponding optimum temperatures are given in Table 7. It is interesting to see from
Figure 41, that the response values of all the sensors were lower than maximum
response values at below and above their respective optimum temperatures.
This strange behavior of the sensor response with the operating temperature
was controlled by the atmospheric oxygen adsorption/desorption phenomenon at the
surface of the sensor film. In the low-temperature range, more oxygen molecules were
adsorbed in the form of O2⁻ and O
⁻ ions. Thus providing more active sites for reducing
gases to react with and therefore, released electrons into the conduction band. As a
result, sensor response increased with increasing operating temperature up to the
optimum limit. Since oxygen adsorption on the film surface was an exothermic
process [163].
Therefore, at a temperature above the optimum value, desorption of oxygen
adsorbate anions started, thereby decreasing the number of active sites for interaction
with reducing gases and favored a decrease in response with further increase in
operating temperature. The excellent ammonia sensing response of ~84–98%
achieved in the present study for the fabricated sensors (Z1cal–Z4cal) at such low
working temperatures has not been reported yet (see Table 8). The outstanding sensor
responses of self-fabricated ZnO sensors were attributed to the unique morphologies
as well as remarkable uniformity in the size and shape of the synthesized
nanostructures.
3.9.6. Effect of Ammonia Gas Concentration
Furthermore, the effect of ammonia concentration on the response of selected
sensors (Z1cal–Z4cal & ZP1–ZP3) was also studied in the concentration range of 1 to
100 ppm at room temperature as depicted in Figure 42–45.
113
0
59
118
177
0
58
116
174
0
58
116
174
0
60
120
180
Z1cal
Z2cal
Z3cal
Resi
stan
ce (
M
)
1
Ammonia concentration (ppm)
Z4cal
100 25 5 2
Figure 42. Dynamic resistance curves of ZnO based sensors towards different
ammonia concentrations.
114
0
59
118
177
0
59
118
177
0
59
118
177
R
esi
sta
nce (
M
)
Ammonia concentration (ppm)
ZP1
ZP2
ZP3
1 100 25 5 2
Figure 43. Dynamic resistance curves of zinc phosphate based sensors towards
different ammonia concentrations.
115
0
20
40
60
80
100
1 1002552
Resp
on
se (
%)
Ammonia Concentration (ppm)
Z4cal
Z3cal
Z2cal
Z1cal
Figure 44. Bar graph showing the response of ZnO sensors towards different
ammonia concentrations.
116
0
20
40
60
80
100
1002552
Ammonia concentration (ppm)
Resp
on
se (
%)
ZP3
ZP2
ZP1
1
Figure 45. Bar graph showing the response of zinc phosphate sensors towards
different ammonia concentrations.
117
The obtained dynamic response bands for all sensors in Figure 42–43 showed
that the sensor resistance decreased linearly with the increase in ammonia gas
concentration and reached to saturation point at about 100 ppm. The response (%)
values calculated for each sensor were represented in the bar graph (Figure 44–45). It
can be noted that the response of the studied sensors enhanced significantly with the
increase in ammonia concentration (Table 7).
On exposure of low concentration of ammonia gas towards the selected
sensors of fixed surface area, there was a lower coverage of gas molecules over the
surface than larger surface coverage at high gas concentration. Beyond 25 ppm of
ammonia concentration, there was a gradual increase in the surface gas interaction
and the response of the sensor became nearly constant at around 100 ppm. For
instance, the response of the ZP1 sensor amplified from ~74% to 99% with an
increase in ammonia concentration from 1 to 100 ppm (Table 7).
This showed that at higher concentrations, the greater number of ammonia
molecules adsorbed at the surface of the sensor film and thus hoping of the NH4+ ions
became more effective and resulted in the enhancement of sensor response [153]. The
leveling off of the response values at around 100 ppm concentration pointed out
obviously to the saturation limit of the sensor surface with the adsorbed ammonia
molecules. It can further be noted from Figure 44–45 that the studied sensors can
detect the concentration of ammonia gas even less than 1 ppm.
It is believed that measured detection of ammonia gas by fabricated sensors in
such a wide range of concentration at room temperature makes it evident that the
mentioned sensors possess great potential for the application in industrial
environments, where personnel is exposed to ammonia gas during their operational
processes.
118
3.9.7. Gas Sensor Stability and Reproducibility
Stability is another important characteristic that reflects the sensor
performance at long term use. In many cases, parts of the sensor film wear away after
long use, which leads to reduce the sensor response. It is a common drawback
experienced in the case of oxide-based sensors.
Therefore, in the present study stability in the response of the selected sensors
(Z1cal–Z4cal & ZP1–ZP3) was measured over time at room temperature. The response
of fabricated sensors towards 25 ppm ammonia was measured on 15th
, 30th
, 45th
and
60th
day after their first exposure to ammonia gas at room temperature. Figure 46–47
indicated that the response values of the sensors remained the same with no detectable
change after 30 days, confirming the stability of synthesized sensor materials. One
can easily see from Figure 46–47 that even on the 60th
day, the sensor materials
responded about 98% of their initial response values, confirming the excellent
stability in the response of the sensor materials.
Figure 47 also shows that the response of all the sensors was quite stable
which indicated that interaction between the sensor surface and ammonia gas is
physical in nature. It is further added that reproducibility in the performance of the
sensors was also confirmed through FT-IR analysis of the selected sensor materials
after exposure to 25 ppm ammonia gas and then flushed with pure dry air.
The material after the sensing experiments was analyzed by FT-IR
spectroscopy and the obtained spectra (Figure 48–49) indicated no peak concerning
the ammonia chemisorption over the sensor surface and supported the proposed
mechanism for ammonia sensing.
119
75
80
85
90
95
100
Days
Resp
on
se (
%)
Z4cal
Z3cal
Z2cal
Z1cal
604530151
Figure 46. Stability in the response of ZnO based sensors towards 25 ppm
ammonia.
120
75
80
85
90
95
100
6045301 15
Resp
on
se (
%)
Days
ZP3
ZP2
ZP1
Figure 47. Stability in the response of zinc phosphate based sensors towards 25 ppm
ammonia.
121
4000 3500 3000 2500 2000 1500 1000 500
0
20
40
60
80
100
120
Wavenumber (cm-1)
Tran
sm
itta
nce (
%)
Z4cal
Z3cal
Z2cal
Z1cal
Figure 48. FT-IR spectra of ZnO sensor materials after exposure to ammonia
followed by flushing with dry air.
122
4000 3500 3000 2500 2000 1500 1000 500
1677
940
ZP3
ZP2
614-4001033
1270
14421628
3273
Wavenumber (cm-1)
Tra
nsm
itta
nce (
%)
ZP1
Figure 49. FT-IR spectra of zinc phosphate based sensors (ZP1–ZP3) after exposure
to ammonia gas followed by flushing with dry air.
123
The FT-IR spectra of the sensing material before (FT-IR, Figures 26 & 28)
and after (Figure 48 & 49) the sensing process were comparable which indicating
superior response with excellent reproducibility and long term stability.
3.9.8. Selectivity
Selectivity in the performance of the gas sensor is also another very important
parameter associated with its practical usage. The gas response of the selected sensors
(Z4cal & ZP1–ZP3) towards the same concentration of ammonia, acetone, and ethanol
were determined at room temperature for investigation of selectivity of the mentioned
sensors. The obtained results are illustrated in Figure 50–52.
It can be noticed that the studied sensors displayed the highest response
towards ammonia. While to other volatile compounds (VOCs) such as acetone and
ethanol, the sensor responses were poor.
Furthermore, Figure 50–52 explores the dynamic resistance response curves of
Z4cal & ZP1–ZP3 sensors towards 1 ppm ammonia which shows stable and
reproducible performance with no drift in the observed response after repeated cycles.
It clearly demonstrated that the fabricated sensors exhibited excellent selectivity and
high sensitivity towards ammonia and could be employed as an excellent candidate
for room temperature detection of ammonia gas in actual practice.
All these facts i.e., high sensor response, fast response/recovery time,
excellent reproducibility and high stability, etc. permit the use of synthesized powders
for room temperature detection of ammonia gas. As such, it is concluded that the
sensors fabricated in the present work possess a promising potential for application in
highly sensitive and selective room temperature ammonia gas detection.
124
0
20
40
60
80
100
EthanolAcetone
Resp
on
se (
%)
Ammonia
Z4cal
ZP1
ZP2
ZP3
Figure 50. Bar graph showing a selective response of different sensors
towards 1ppm ammonia, acetone and ethanol vapors.
125
Figure 51. The selectivity of Z4Cal towards the same concentrations of: a)
ammonia, b) acetone, c) ethanol vapors (1ppm) at room temperature.
0 200 400 600 800 1000 1200 1400 1600 1800
50
150
100
200
Gas outGas in
Time (s)
Res
ista
nce
(M
)
a b c
0
126
0 200 400 600 800 1000 1200 1400 1600 1800
0
40
80
120
160
200
Time (s)
Resis
tan
ce (
M
)
ZP3 (Ammonia)
ZP2 (Ammonia)
ZP1 (Ammonia)
ZP3 (Acetone)
ZP2 (Acetone)
ZP1 (Acetone)
ZP3 (Ethanol)
ZP2 (Ethanol)
ZP1 (Ethanol)
Figure 52. The selectivity of zinc phosphate based sensors towards 1 ppm ammonia,
acetone, and ethanol vapors at room temperature.
127
3.10. Antibacterial Activity of ZnO
3.10.1. Point of Zero Charge (PZC)
Before performing the antibacterial activity of the desired ZnO samples, it was
necessary to find out their PZC values because it plays a major role in deciding the pH
of ZnO dispersions to be employed as antibacterial agents. For this purpose PZC of
the selected ZnO samples (Z1cal–Z4cal) were determined through salt addition method,
discussed prior in section 2.3.
The obtained results are plotted in the form of curves of ΔpH versus pHi, in
Figure 53. Where, ΔpH stands for the difference between initial (pHi) and final pH
(pHf) values, while pHi and pHf correspond to pH values of ZnO dispersions before
and after the addition of NaNO3 during the process of pH measurements.
As can be noticed from Figure 53, all curves intersected the 0 value of ΔpH at
pHi around 9.3. This pH value was designated as PZC and coincided well with the
reported PZC values for ZnO [164–165]. In fact, at PZC value, the dispersed ZnO
nanostructures carry net zero charge at their surfaces. However, below and above this
PZC, the dispersed nanostructures possess net positive and net negative surface
charges, respectively.
In addition, a small difference in ΔpH values can be observed on either side
of the PZC values in the plotted curves. These variations in ΔpH values for Z1cal–Z4cal
samples referred to the difference in surface charge density of ZnO nanostructures,
regardless of the sign of their charges. From PZC measurements it can be concluded
that PZC of the employed ZnO samples was dependent upon the sample composition,
whereas the net surface charge at the given pH was dependent upon the shape and size
of the synthesized ZnO nanostructures.
128
4 6 8 10 12-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
p
H
(pHi)
Z4cal
Z3cal
Z2cal
Z1cal
Figure 53. Point of zero charge (PZC) of the selected ZnO samples.
129
Antibacterial activity of the selected ZnO samples (Z1, Z2, Z3 and Z4 and
ZnO-Com) was investigated against various pathogenic bacteria, including both
Gram-positive bacterial strains (S. mutans and S. aureus) and Gram-negative bacterial
strains (E.coli, P.aeruginosa, and Enterobactor cloacae). Agar well diffusion method
was used and three different concentrations (5 µg/20 µL, 10 µg/20 µL and 15 µg/20
µL) of test ZnO sample powder and positive control were employed.
The presence of inhibition zones after 24 h of incubation indicated the
bactericidal property of the tested samples (Figure 54). On the other side, ZnO-com
showed no antibacterial property in the employed concentration range, shown in
Figure 55. The measured data is also illustrated in the bar graph, given in Figure 56.
Figure 56 reveals that the tested ZnO samples were potentially effective to
suppress the growth of test microorganisms to variable potency, which can be
examined clearly from the size of produced inhibition zones. It was in fact due to; a)
the type of test bacteria, b) the concentration of particles and c) morphology of the
particles. Investigations revealed that different physiochemical properties like particle
size, shape, surface charge, crystal structure, agglomeration state as well as solubility
can affect the toxicity of ZnO NPs [166]. Since, it has been reported that the presence
of uneven texture, rough surface corners, and edges of ZnO nanoparticles resulted in
more effective abrasiveness compared to bulk ZnO and therefore, contributed to
severe mechanical damage to the bacterial cell membrane [167].
From Figure 56, it can be seen that the antibacterial activity of the synthesized
ZnO powders falls in the order Z4cal >Z3cal >Z2cal >Z1cal. The greatest antibacterial
activity of Z4cal nanostructures was ascribed to their porous structures which provided
more active sites for interaction with the bacterial surface.
130
Figure 54. Antibacterial activity of the selected ZnO and positive control against
various pathogenic bacterial strains.
Figure 55. Antibacterial activity of ZnO-com against various pathogenic bacterial
strains.
131
Figure 56. Antibacterial activity of the selected ZnO samples and ciprofloxacin
against various pathogenic bacterial strains.
12.6
16.8
21.0
25.2
18.4
23.0
27.6
32.2
19.2
24.0
28.8
33.6
5 g/20L
Enterobactor cloacae
P. aeruginaosa
StandardZ4cal
Z3calZ2
cal
10g/20L
E. coli
S. aureus
Zo
ne
of
inh
ibit
ion
(m
m)
S. mutans
Z1cal
15g/20L
132
Therefore, it is believed that the greater surface area of the Z4cal
nanostructures (95.20 m2/g) is responsible for the efficient antibacterial activity by
providing better contact with the test microorganisms [168]. Similarly, morphology
dependent antibacterial study explored that spherical Nps showed more antibacterial
property against gram-negative strains as compared to nanosheets. [169].
Since, the antibacterial efficacy enhanced with the decrease in particle size. It
has been demonstrated that the size of inhibition zones of ZnO NPs increased
significantly from 10 to 13 mm for S. aureus and 10 to 17 mm for E. coli with the
decreasing size of ZnO NPs [89]. The enhanced bioactivity of smaller particles can be
attributed to their respective higher surface area to volume ratio [75, 170].
Furthermore, it can also be examined from Figure 56 that the diameters of
inhibition zones increased with increasing concentration of ZnO powders. For
instance, the antibacterial activity of the selected samples has been enhanced by
producing maximum inhibition zones at the concentration of 15 µg/20 µL. For
instance, the size of inhibition zone produced by Z4cal nanostructures was ~28 mm
against S. aureus at the concentration of 10µg/20 µL, which was greater than 24.5 mm
inhibition zone observed at the same ZnO concentration [171]. Similarly, it has been
reported that nanoparticle concentration seemed to be more effective in enhancing the
antibacterial activity as compared to the particle size [80].
In addition, Figure 56 also shows that bactericidal activity was more toward
Gram-positive than Gram-negative bacteria. It was because the antibacterial activity
of NPs is also dependent upon the composition of specific bacterial cells. Likewise,
certain researchers believe that the structure of the bacterial cell can also affect the
inhibitory action of NPs. For instance, due to the presence of a single membrane cell
wall, Gram-positive bacteria were more susceptible to the antimicrobial action of ZnO
133
than Gram-negative bacterial strains [172]. Similarly, the thickness of the cell wall
and certain other components of Gram-negative bacteria also affected the
antimicrobial function of ZnO NPs.
A comparison of the obtained results with the antibacterial activity of ZnO
reported earlier (Table 9) revealed that the synthesized ZnO samples hold promising
antibacterial properties and can be employed as effective antibacterial agents.
3.10.2. Antibacterial Effect of ZnO and Ciprofloxacin Combination
A comparative study revealed that the combination of ZnO nanoparticles with
ciprofloxacin showed better antibacterial activity compared to penicillin G,
amoxicillin and nitrofurantoin [173]. Likewise, Sharma and coworkers [176]
conducted a study on the synergistic activity of pure and doped ZnO nanoparticles in
combination with several antibiotics i.e., ampicillin, amphotericin B, ciprofloxacin
and fluconazole against various pathogenic microorganisms. The better activity was
reported for ciprofloxacin and nanoparticles combination, compared to other
antibiotics and ZnO nanoparticles combination.
In this regard, a set of experiments was performed in which a combination of
the selected ZnO powders and ciprofloxacin in a 1:1 ratio at the lowest concentration
(5 µg/20 µL) were evaluated for antibacterial activity against the selected bacterial
strains. It was found that ZnO nanostructures enhanced the antibacterial activity of
ciprofloxacin significantly in the order Z4cal > Z3cal > Z2cal.
The enhanced antibacterial effect can be examined from the size of the
observed inhibition zones, shown in Figure 57. The measured values of the enhanced
antibacterial effect are also illustrated through bar graph. It is indicated that
antibacterial property enhanced effectively for ciprofloxacin and ZnO nanoparticles
combinations.
134
Table 9. Comparison of antibacterial activity of the present work with the reported
literature.
Samples Concentration Zone of inhibition (mm) References
S. aureus E. coli
ZnO 15 mg/mL 29 22 [87]
ZnO 10 mg/mL 13 17 [74]
ZnO 500 µg/50 µL 2.67 3.67 [174]
ZnO 100 µg/mL 21 17 [175]
ZnO 125 µg/mL 22 22 [176]
ZnO 1 mg/mL 20.3 18.6 [177]
ZnO 10 µg/mL 5 5 [88]
ZnO (Z4cal) 15 µg/mL 31 29 Present work
135
Figure 57. Antibacterial effect of the selected samples Z2cal–Z4cal) combined with
ciprofloxacin in a 1:1 ratio.
0
10
20
30
40
50
60
70 5 g/20L
Enterobactor cloacae
P. aeruginaosa
E. coli
S. aureus
S. mutans
En
ha
ncem
en
t o
f a
nti
ba
cte
ria
l a
cti
vit
y (
%)
Z2cal
Z3cal
Z4cal
136
The greater enhancement in the case of Z4cal, compared to other samples
(Z2cal, Z3cal) could be attributed to the highly porous nature of Z4cal nanostructures
which provided the greater surface area of 95.20 m2/g for interaction with bacterial
cells, compared to 62.18 m2/g and 67.41 m
2/g for Z2cal, Z3cal, respectively. For
instance, an increase in antibacterial effect against E.coli and S. aureus for Z4cal
(54.2% and 50%) in the present study was much greater as compared to 22.5% and
10.53% increase in inhibition zones observed earlier for the same bacterial strains
[173].
The increased antibacterial efficacy of positive control ciprofloxacin in the
presence of ZnO nanostructures can be attributed to the efflux of ciprofloxacin from
bacterial cells due to the interference of ZnO nanostructures with the pumping activity
of NorA protein in cell membrane and binding reaction between the ciprofloxacin and
ZnO, thus stabilizing the ciprofloxacin–ZnO nanostructures combination [177–178].
Schematics regarding ZnO & ciprofloxacin interaction with the bacterial cell are
shown in Figure 58.
The interaction of ciprofloxacin with complex forming metal atoms such as
Cu (II), Co (II) and Ni (II) has been characterized by x-ray and spectroscopic analysis
which disclosed that ciprofloxacin possesses the ability to form complexes with metal
ions [179–180]. Since the ciprofloxacin, a broad-spectrum antibiotic is a member of
fluoroquinolone. It is considered that the nitrogen atoms present in the quinolone ring
of ciprofloxacin may interact with the hydroxylated surface of ZnO nanostructures
and thus stabilizing the ciprofloxacin–ZnO nanostructures combination through a
network of ionic interactions [181]. Similarly, Patel et al. [166] indicated the
formation of ciprofloxacin-cobalt (II) complex and reported that ciprofloxacin
interacted effectively with DNA in the presence of chelating agent cobalt.
137
Figure 58. Schematics showing the interaction of ciprofloxacin and ZnO
nanostructure complex with the bacterial cell membrane.
138
3.10.3. Mechanism of Antibacterial Action
The mechanism of the antibacterial activity of ZnO NPs is not well understood
so far. The antibacterial action is due to either one or a combination of following
proposed mechanisms; (1) production of reactive oxygen species (ROS), (2) toxic
ions release, (3) direct interaction of ZnO particles damage to cell membrane caused
through adhesion of particles with bacterial surface; penetration through the cell
membrane [182]. However, the generation of reactive oxygen species takes place
under the effect of UV light illumination of nanoparticles [82, 183].
It has been also reported that the antibacterial activity of ZnO synthesized in
diethylene glycol and described that biocidal effects of ZnO nanoparticles on E. coli
cells resulted through cellular internalization [79].
It is considered that the antibacterial action of ZnO through a well diffusion
method is possibly due to the disruption of the cell membrane by direct interaction of
ZnO particles with the bacterial surface. The direct contact might be due to the stress
stimuli initiated by particle shape, size and surface charge of particle which resulted in
electrostatic interaction between the ZnO particle and the surface of the bacterial cell.
Such type of surface interaction was also confirmed by Zhang and coworkers [80]
through the electrochemical measurements.
It is believed that the antibacterial action of different ZnO nanostructures in
the present case might be possibly due to the physical interactions of ZnO
nanostructures with the target bacterial cells. It is known at biological pH i.e., under
the physiological conditions the bacterial cell surfaces are negatively charged because
of the ionization of the amino, phosphate and carboxyl groups. While the employed
ZnO nanostructures were positively charged at the mentioned pH [184–185].
139
The PZC measurements shown in Figure 53 depicted that ZnO nanostructures
were positively charged at pH below the pHPZC of ZnO (pHPZC~9.2). The isoelectric
point 9–10 indicated that ZnO particles have a strong positive charge on their
surfaces, under the physiological conditions [186]. It is believed that the greater the
difference between the zeta potential signs of the bacterial surface and inorganic
materials stronger will be the electrostatic attractions between the two surfaces [184].
The strong electrostatic attractions between the opposite charges of the
bacterial cells and ZnO nanostructures are thus responsible for the physical interaction
between them. Based on such a mechanistic approach, it has been proposed that due
to strong electrostatic attractions, the ZnO nanostructures were accumulated on the
outer surface of bacterial cells and neutralized the surface potentials of the later as can
be seen schematically in Figure 59.
This resulted in increased surface tension and depolarization of the cell
membrane which led to alter the cell morphology, membrane textures, increased
membrane permeability, cellular internalization, membrane disruption, and finally
leakage of the intracellular fluids and components thus causing the bacterial cell death
[167, 187]. Figure 59 shows the possible schematics for the interaction of ZnO
nanostructures with generalized bacterial cells. As physical interactions played an
imperative role in bactericidal action, these were the surface modifications of ZnO
nanostructures which led to an enhanced interaction with bacterial cell walls and
increased cell permeability. In addition, it is believed that the presence of uneven
texture, rough surface corners and edges of ZnO nanoparticles resulted in more
effective abrasiveness and therefore, contributed to severe mechanical damage to the
bacterial cell membrane [185].
140
Figure 59. Schematics showing the interaction of ZnO nanostructures with the
bacterial cell.
141
Antibacterial properties of two different shaped NPs unfolded the fact that
spherical shaped Nps showed more antibacterial property against gram-negative
strains whereas sheet-shaped NPs were more active towards positive bacterial strains
[169].
Conclusions
The employed simple and time-effective approach for the production of
uniform and novel nanostructures of crystalline zinc compounds without using
any kind of template or shape directing agents proved to be very successful
technique.
SEM analysis revealed that particle morphology of zinc compounds could be
controlled to the desired extent by tuning the applied experimental parameters,
such as the composition of the reactant mixtures, temperature, and reaction
time, etc.
FT-IR analysis revealed that particle morphology of zinc compounds affected
their spectral profiles.
Z4cal sensor showed good semiconducting properties amongst ZnO based
sensors (Z1cal– Z4cal) because of the smaller activation energy value 0.900 eV.
Fabricated sensors based on ZnO and zinc phosphate nanostructures showed
superior and reproducible performance with good stability towards the
detection of ammonia gas due to uniform particle morphology and high
surface area of the synthesized material.
Among the fabricated sensors, ZP1showed the highest response of 89% with
shortest response recovery times of 31/12 s towards 5 ppm ammonia at room
temperature. While the response of other sensors was in the order ZP2>
ZP3>Z4cal> Z3cal>Z2cal>Z1cal.
142
The effect of ammonia gas concentration on sensor response revealed the
detection limit of fabricated sensors to be less than 1ppm.
Selected ZnO samples (Z1cal–Z4cal) of uniform particle morphology
demonstrated promising antibacterial activity by producing inhibition zones to
all tested bacterial strains. In contrast, ZnO-com showed no antibacterial
response.
The antibacterial activity revealed to be strongly dependent upon the
nanostructures morphology and powder concentration.
Concentration-dependent antibacterial study unfolded that the size of the
inhibition zones increased from ~28 mm to 32 mm with increasing ZnO
concentration from 5 µg/ 20 µL to15 µg/ 20 µL).
The higher antibacterial response of Z4cal samples could be ascribed to their
porous structure and greater surface area of 95.20 m2/g.
The synthesized ZnO nanostructures effectively enhanced the antibacterial
activity of the standard antibiotic ciprofloxacin. A total of ~50%, 50%,
54.17%, 29.17% and 65% increase in inhibition zones was observed in the
presence of Z4cal nanostructures with ciprofloxacin against S. mutans, S.
aureus, E. coli, P. aeruginosa and Enterobactor cloacae, respectively.
The antibacterial activity was more towards Gram-positive than Gram-
negative bacteria.
The antibacterial activity of the synthesized ZnO powders was more compared
to the reported literature which suggested that the synthesized ZnO
nanostructures possess the potential to be used as antibacterial agents at
inhibiting drug-resistant pathogenic bacteria.
143
Future goals
To extend the methodologies employed in this research work for large scale
synthesis of zinc compounds nanostructures.
To study the effect of various types of dopants on the gas sensing and
antibacterial properties of the synthesized nanostructures.
To study in detail the mechanism responsible for zinc phosphate-based
sensors.
144
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