75
Chapter 3
Synthesis and Characterization of ZnO
Nanoparticles
“This chapter describes the synthesis of nanocrystalline ZnO
particles using sol-gel route and solid state reaction method.
Section–A describes the synthesis and characterization of ZnO
nanoparticles via sol-gel route and Section–B describes the synthesis
and characterization of ZnO nanoparticles via solid state reaction
method. In Section-C the effect of pH variation on the sol-gel
synthesized ZnO nanoparticles is studied. The sol-gel synthesized
ZnO nanoparticles are used in the fabrication of QDSSCs as
described in later chapters.”
77
3.1 Introduction
ZnO is a member of II-VI semiconducting compounds and occurs naturally as the
mineral zincite. It is a hexagonal wurtzite type crystal exhibiting anisotropy. ZnO is a
well known n-type semiconductor having piezoelectric, dielectric properties with a wide
direct bandgap of 3.37 eV at room temperature (300 K) and a large exciton binding
energy of 60 meV, which is 2.4 times the effective thermal energy (KBT=25meV) at
room temperature. ZnO is considered a good candidate for TCO electrodes in solar cells
because it is transparent to the visible light (>80%) [1]. It is also considered a prime
candidate for UV and blue light emitting devices such as blue LED and LASERs due to
its large exciton binding energy [2]. Due to large exciton binding energy, the excitons
remain dominant in optical processes even at room temperature. Due to its vast industrial
applications such as electro-photography, electroluminescence phosphorus, pigment in
paints, flux in ceramic glazes, filler for rubber products, coatings for paper, sunscreens,
medicines and cosmetics, ZnO is attracting considerable attention in powder as well as
thin film form. Its resistance to radiation damages also makes it useful for space
applications. The fabrication of ZnO nanostructures have attracted intensive research
interests [3-4] as these materials have found uses as TCO [5-9]. Since ZnO is the hardest
of the II-VI semiconductors due to the higher melting point of 2248 K and large cohesive
energy of 1.89 eV, its performance is not degraded as easily as the other compounds
through the appearance of defects. Since Zinc, the main constituent is cheap, non-toxic
and abundant, ZnO has become commercially viable. Some of the important properties of
ZnO are listed in table 3.1.
Earlier TiO2 was used extensively in solar cell applications [10, 11]. But now a
days, in place of TiO2, ZnO is extensively used. The bandgap of ZnO is almost same as
that of TiO2 and the electron mobility and electron diffusion coefficient of ZnO showed
78
much higher values than TiO2, that would be favourable for electron transport with
reduced recombination loss when used in QDSSCs. To understand this issue, QDSSC
technology based on ZnO has been explored extensively. Although the conversion
efficiencies of 0.4–5.8% obtained for ZnO are much lower than that of 14% for TiO2,
ZnO is still thought of as a distinguished alternative to TiO2 due to its ease of
crystallization and anisotropic growth. These properties allow ZnO to be produced in a
wide variety of nanostructures. Thus fabrication of ZnO based QDSSC presents unique
properties for electronics, optics, or photocatalysis [12-15].
Table 3.1 Important properties of ZnO
Properties Values
Crystal structure Rock salt, Zinc blende and Wurtzite
Bandgap (eV) 3.37 at room temperature
Electron Mobility (cm2Vs
-1) 2.5-300 (Bulk ZnO), 1000 (Single nanowire)
Exciton Binding Energy 60 meV
Density 5.606 g/cm3
Refractive Index 2.0041
Electron Effective Mass (me) 0.26
Relative Dielectric Constant 8.5
Melting point 1975 oC
Boiling point 2360 oC
Electron Diffusion Coefficient 5.2 cm2s
-1 (Bulk ZnO),
1.7 × 10-7
cm2s
-1 (Particulate Film)
79
In particular, recent studies on ZnO nanostructure based QDSSCs have delivered
many new concepts, leading to a better understanding of photoelectrochemical based
energy conversion. This, in turn, would speed up the development of QDSSCs that are
associated with TiO2. Moreover, these ZnO nanomaterials can be synthesized through
simple chemical methods with the wide range of structural evolution; fabricating
QDSSCs with ZnO nanostructured materials will be advisable and reliable in place of
TiO2, whose structural controllability is not easy in a conversional chemical synthetic
route. The production of these structures can be achieved through sol–gel synthesis,
hydrothermal, physical or chemical vapour deposition, low-temperature aqueous growth,
chemical bath deposition and electrochemical deposition etc. This chapter focuses on the
synthesis of ZnO nanoparticles using sol-gel method and solid state reaction method.
3.1.1 Crystal and Surface Structure of ZnO
At ambient pressure and temperature, ZnO crystallizes in the wurtzite structure, as
shown in figure 3.1. This is a hexagonal lattice, belonging to the space group P63mc with
lattice parameters a = 0.3296 and c = 0.52065 nm and is characterized by two
interconnecting sublattices of Zn2+
and O2−
, such that each Zn ion is surrounded by
tetrahedra of O ions, and vice-versa [16]. This tetrahedral coordination gives rise to polar
symmetry along the hexagonal axis. This polarity is responsible for a number of the
properties of ZnO including its piezoelectricity and spontaneous polarization, and is also
a key factor in crystal growth, etching and defect generation.
The four most common face terminations of wurtzite ZnO are the polar Zn
terminated (0001) and O terminated (0001) faces (c-axis oriented), and the non-polar
(1120) (a-axis) and (1010) faces both of which contain an equal number of Zn and O
atoms [17]. The polar faces are known to possess different chemical and physical
properties, and the O-terminated face possesses a slightly different electronic structure
80
compared to the other three faces. Additionally, the polar surfaces and the (1010) surface
are found to be stable, however the (1120) face is less stable and generally has a higher
level of surface roughness than its counterparts [18]. The (0001) plane is also basal. Apart
from causing the inherent polarity in the ZnO crystal, the tetrahedral coordination of this
compound is also a common indicator of sp3 covalent bonding. However, the Zn–O bond
also possesses very strong ionic character, and thus ZnO lies on the borderline between
being classed as a covalent and ionic compound, with an ionicity of fi = 0.616 on the
Phillips ionicity scale [19].
Fig. 3.1 ZnO crystallizes in the wurtzite structure
3.1.2 Comparison of ZnO with its Chief Competitors
ZnO was one of the first semiconductors to be prepared in rather pure form after
Si and germanium (Ge). It was extensively characterized as early as the 1950’s and
81
1960’s due to its promising piezoelectric/acoustoelectric properties. Wide bandgap
semiconductors have gained much attention during last few decades because of their
possible uses in optoelectronic devices in the short wavelength and UV region of the
electromagnetic spectrum. These semiconductors such as ZnO, ZnSe, ZnS, and GaN have
shown similar properties with their crystal structures and bandgaps. Some of the
important properties of these wide bandgap semiconductors are summarized in table 3.2.
Initially, ZnSe based devices and the GaN based technologies obtained large
improvements such as blue and UV LED and injection laser. ZnSe has produced some
defect levels under high current drive. No doubt, GaN is considered to be the best
candidate for the optoelectronic devices. However, ZnO has great advantages for LEDs
and LASER diodes over the currently used semiconductors. Recently, it has been
introduced that ZnO as II–VI semiconductor is promising for various technological
applications, especially for optoelectronic short wavelength light emitting devices due to
its wide and direct bandgap. The most important advantage is the high exciton binding
energy giving rise to efficient exitonic emission at room temperature. Since ZnO and GaN
have almost identical lattice parameters and the same hexagonal wurtzite structure, ZnO
can be satisfactorily used as lattice matched substrate in GaN based devices or vice versa.
ZnO has excellent radiation hardness among all other semiconductors. This property
supplies the uses of ZnO based devices in space applications and high energy radiation
environments. Bandgap of ZnO energy can be varied from 3.3 up to 4.5 eV with alloying
process. Hence it can be used as an active layer in the doubly confined hetero structured
LEDs and quantum well lasers. These unique nanostructures unambiguously demonstrate
that ZnO is probably the richest family of nanostructures among all materials, both in
structure and properties [20-22].
82
Table 3.2 Comparison of ZnO with its competing materials
Wide bandgap
semiconductor
Crystal
structure
Lattice
parameter
(Å)
Eg
(eV
at
RT)
Melting
temperature
(K)
Exciton
binding
energy
(MeV)
Dielectric
Constant
a b ɛo
ZnO Wurtzite 3.25 5.20 3.37 2248 60 8.75 3.72
GaN Wurtzite 31.9 51.6 3.4 1973 21 9.5 5.15
ZnSe Zin blende 5.67 - 2.7 1790 20 7.1 5.3
ZnS Wurtzite 3.83 6.26 3.7 2103 36 9.6 5.7
83
3.2 Synthesis of ZnO Nanoparticles using Sol-Gel Method
Zinc acetate dihydrate (Zn(CH3COO)2.2H2O) was used as zinc precursor. ZnO
nanoparticles were prepared by dissolving 0.2M zinc acetate dihydrate in methanol at
room temperature and then mixing this solution ultrasonically at 25oC for 2h. Clear and
transparent sol with no precipitate and turbidity was obtained. 0.02 M of NaOH (0.1N
NaOH) was then added to the sol and stirred ultrasonically for 60 min. The sol was kept
undisturbed till white precipitates settled down at the bottom of sol. After precipitation,
the precipitates were filtered and washed with excess methanol to remove starting
material. Precipitates were dried at 80 οC for 15 min on hot plate. Precipitates were then
annealed at 400 οC for 30 min [23]. The flowchart for the synthesis of ZnO nanoparticles
using sol-gel method is shown in figure 3.2.
3.3 Synthesis of ZnO Nanoparticles using Solid State Reaction Method
In solid-state reaction method, 0.2M of Zinc acetate dihydrate in methanol was
first ground for by mortar pestle for 10 min and then mixed with 0.02M of NaOH. After
the above mixture was ground for 30 min, the product was washed many times with
deionized water. After that the product was again washed with methanol to remove the
by-products. The final product was then filtered using micron filter paper and dried into
solid powder at 80ºC for 15 min on hot plate. After that the powder was annealed at
400ºC for 30 min [24]. The flowchart for the synthesis of ZnO nanoparticles using solid
state reactionmethod is shown in figure 3.3.
84
Fig. 3.2 Flow chart for sol-gel synthesis of ZnO nanoparticles
85
Fig. 3.3 Synthesis of ZnO nanoparticles using solid state reaction method
86
Section – A
Characterization of ZnO Nanoparticles Synthesized using Sol-Gel Method
3.4 Structural and Morphological Properties
3.4.1 XRD Analysis
The XRD pattern of the nanoparticles obtained by sol-gel route is shown in
figure 3.4. The nanoparticles showed crystalline nature with 2θ peaks lying at 31.750o
<100>, 34.440o <002>, 36.252
o <101>, 47.543
o <102>, 56.555
o <110>, 62.870
o <103>,
66.388o
<200>, 67.917o
<112>, 69.057o
<201>, 72.610o
<004>, 76.95o
<202>, 81.405o
<104>, and 89.630o <203>. The preferred orientation corresponding to the plane <101> is
also observed. These peak positions coincide with JCPDS card no. 36-1451 for ZnO
powder.
20 30 40 50 60 70 80 90
(10
4)
(20
2)
(00
4)(2
01
)(1
12
)(2
00
)
(10
3)
(11
0)
(10
2)
(10
1)
(00
2)
(10
0)
Inte
ns
ity
(a
.u.)
2 (degree)
Fig. 3.4 XRD pattern of ZnO nanoparticles synthesized using sol-gel method
87
Crystallite size was obtained by Debye-Scherrer formula [25] given by equation (3.1).
θosβ
λ0.94D
c (3.1)
Where D is the crystallite size, 0.94 is the particle shape factor which depends on the
shape of the particles, λ is the CuKα radiations (1.54 Å), β is the full width at half
maximum (FWHM) of the selected diffraction peak corresponding to <101> plane and θ
is the Bragg angle obtained from 2θ value corresponding to maximum intensity peak in
XRD pattern. The crystallite size obtained was 23.59 nm [26].
3.4.2 TEM Analysis
TEM images of sol-gel synthesized ZnO are shown in figure 3.5. Clear hexagonal
structures can be seen in the TEM image. Selected area electron diffraction (SAED)
pattern is shown in figure 3.5(b). Bright and well aligned diffraction rings clearly
indicates that the ZnO nanoparticles are crystalline in nature. Three bright fringes were
observed in SAED which correspond to <100>, <002> and <101> planes of pure wurtzite
hexagonal structure of ZnO. Hexagonal structures can be seen in the figure 3.5(c) having
particle size ~ 24 nm.
(a)
(c)
(b)
Fig. 3.5 (a) TEM image (b) SAED (c) Hexagonal structure of the sol-gel synthesized ZnO nanoparticles
88
3.4.3 SEM Analysis
SEM images of the ZnO nanoparticles prepared via sol-gel route are shown in
figure 3.6. Clear nanostructures can be seen having grain size of ~ 70 nm. The crystallite
size as observed from TEM in this case is ~ 23 nm. This shows that one grain in sol-gel
derived nanoparticles is approximately equal to three crystallites. So it is clear that the
nanoparticles seen by SEM image consist of a number of crystallites which are seen by
TEM image.
Fig. 3.6 SEM of ZnO nanoparticle synthesized via sol-gel route
3.5 Optical Properties
3.5.1 UV-Visible Absorbance Analysis
The absorbance curve of the sol-gel derived nanoparticles in the visible region is
shown in figure 3.7(a). The graph shows that ZnO does not absorb light in the visible
region. This result is in accordance with the bandgap value of the bulk ZnO (3.37 eV)
according to which ZnO absorbs only a small portion of UV range. Bandgap is calculated
89
using Tauc’s plot (Fig. 3.7 b) which comes out to be 3.23 eV. Tauc’s equation is given by
equation (3.2) [27].
ngEhνAαhν (3.2)
Where α is the absorption coefficient, hυ is the photon energy, A is the constant, Eg is the
bandgap of the sample. The value of n is ½ or 2 depending upon whether the transition
from valence band to conduction band is direct or indirect. The value is ½ in case of
direct transition and 2 in case of indirect transition. Since ZnO has a direct band structure,
the value of n is ½ in this case. So the equation (3.2) takes the form of equation (3.3).
gEhνB2αhν (3.3)
In the above equation, B is a constant related effective masses of charge carriers
associated with valence and conduction bands. Intersection of the slope of (αhυ)2 Vs hυ
curve provides bandgap energy of the samples. According to the experimentally
calculated bandgap, the synthesized ZnO nanoparticles should absorb light below 383 nm
and absorbance graph is in agreement with this.
400 500 600 700 800
Ab
so
rban
ce (
a.u
.)
Wavelength (nm)
3.0 3.1 3.2 3.3 3.4 3.50.0
3.0x10-19
6.0x10-19
9.0x10-19
1.2x10-18
(h
)2 (
cm
-1e
V)2
h (eV)
(a) (b)
Fig.3.7 (a) Absorbance and (b) Tauc’s plot of sol-gel derived nanoparicles in visible range
(a)
90
3.5.2 Photoluminescence Analysis
PL spectra of the ZnO nanoparticles synthesized using sol-gel method is shown in
figure 3.8. The first peak in PL spectra corresponds to band to band transition and the
spectrum between 420–500 nm is showing blue luminescence. As can be seen from the
PL spectrum of sol-gel derived nanoparticles, the high intensity peak is observed at
388.6 nm. If we calculate the bandgap value from this wavelength, it comes out to be
3.2eV. The bandgap calculated using PL spectra is approximately same as the one
calculated using Tauc’s plot.
400 450 500 550 600
Inte
ns
ity
(a
.u.)
Wavelength (nm)
Fig. 3.8 Photoluminescence peak of ZnO nanoparticle synthesized using sol-gel route
91
Section – B
Characterization of ZnO Nanoparticles using Solid State Reaction Method
3.6 Structural and Morphological Properties
3.6.1 XRD Analysis
The XRD patterns of the ZnO nanoparticles synthesized using solid state reaction
method is shown in figure 3.9. The nanoparticles showed crystalline nature with 2θ peaks
lying at 31.750o <100>, 34.440
o <002>, 36.252
o <101>, 47.543
o <102>, 56.555
o <110>,
62.870o
<103>, 66.388o
<200>, 67.917o
<112>, 69.057o
<201>, 72.610o
<004>, 76.95o
<202>, 81.405o <104>, and 89.630
o <203>. The preferred orientation corresponding to the
plane <101> is also observed in this case also. These peak positions coincide with JCPDS
card no. 36-1451 for ZnO powder. Crystallite size was calculated by Debye-Scherrer’s
formula (eq. 3.1) and was obtained to be 37 nm.
20 30 40 50 60 70 80 90
(10
4)
(20
2)
(00
4)(2
01
)(11
2)
(20
0)
(10
3)
(11
0)
(10
2)
(10
1)
(00
2)
(10
0)
Inte
ns
ity
(a
.u.)
2 (degree)
Fig. 3.9 XRD pattern of ZnO nanoparticles synthesized using solid state reaction method
92
3.6.2 TEM Analysis
TEM image and SAED pattern of the ZnO nanoparticles synthesized using solid
state reaction method are shown in figure 3.10 (a) and 3.10 (b) respectively. SAED
pattern of the nanoparticles indicates that the ZnO nanoparticles prepared using solid state
reaction method are crystalline in nature. However the diffraction rings in this case are
not properly aligned as in the case of sol-gel derived nanoparticles. No clear hexagonal
structures can be seen in the TEM image. Nanoparticles obtained in this case are adhering
to one another. Agglomeration of nanoparticles is more in this case than the former one.
As can be seen from the TEM image that the average particle size is ~ 37 nm which is in
agreement with the crystallite size obtained from XRD.
(a) (b)
Fig. 3.10 (a) TEM image (b) SAED of ZnO nanoparticles synthesized using solid state reaction method
3.6.3 SEM Analysis
SEM image of ZnO nanoparticles prepared by solid state reaction method is
shown in figure 3.11. Grain size in this case is ~ 200 nm. Crystallite size as seen from
TEM image is ~ 37 nm in this case. This shows that one grain in solid state reaction
93
derived nanoparticles consists of approximately five crystallites. XRD results are
confirmed by the combined study of these SEM and TEM images.
Fig. 3.11 SEM of ZnO nanoparticles synthesized via solid state reaction method
3.7 Optical Properties
3.7.1 UV-Visible Absorbance Analysis
The absorbance curve of the solid state reaction synthesized nanoparticles in the
visible region is shown in figure 3.12 (a). Tauc’s plot is shown in Figure 3.12 (b). The
bandgap comes out to be 3.15 eV from the Tauc’s plot. The bandgap values validates our
crystallite size results according to which smaller crystallite size should have larger
bandgap (23.59 nm, 3.23 eV for sol-gel derived nanoparticles) and large crystallite size
should have smaller bandgap (37.34 nm, 3.15 eV for solid state reaction derived
nanoparticles).
94
2.5 3.0 3.5 4.00.00
2.50x10-19
5.00x10-19
7.50x10-19
(h
)2 (
cm
-1e
V)2
h (eV)
400 500 600 700 800
Ab
so
rba
nc
e (
a.u
.)
Wavelength (nm)
(a) (b)
Fig. 3.12 (a) Absorbance and (b) Tauc’s plot of ZnO nanoparticles synthesized using solid state reaction method
3.5.2 Photoluminescence Analysis
PL spectrum of the ZnO nanoparticles synthesized by solid state reaction method
is shown in figure 3.13. ZnO exhibits a strong luminescence around 391.5 nm, which can
be attributed to bound exciton emission [26]. The first peak in PL spectra corresponds to
band to band transition and the spectrum between 420-500 nm is showing blue
luminescence. ZnO nanoparticles prepared using solid state reaction method show high
luminescence than sol-gel derived nanoparticles. The PL intensity peak in case of solid
state reaction synthesized nanoparticles is observed at 391.5 nm. From this value,
bandgap comes out to be 3.16 eV. The bandgap energies calculated using PL spectra are
approximately same as the ones calculated using Tauc’s plot.
400 450 500 550 600
Inte
nsit
y (a
.u.)
Wavelength (nm)
Fig. 3.13 Photoluminescence peak of ZnO nanoparticle synthesized using solid state reaction metod
(a)
95
Section - C
Effect of pH Variation on Properties of ZnO Nanoparticles
In the previous sections, ZnO nanoparticles were synthesized and characterized by
two different methods: (i) Sol-gel route and (ii) Solid state reaction method. The ZnO
nanoparticles prepared using sol-gel route had smaller crystallite size (~ 24 nm) as
compared to the one prepared by solid state reaction method (~ 37 nm).
As compared to other techniques sol-gel technique has advantages such as
simplicity, low cost, excellent homogeneity as well as purity of the product, relatively
low processing temperatures such as room temperature [27]. The other advantages of
sol-gel process are that the composition of the sol can be controlled and doping can
be achieved easily. Sol-gel processing has been found to be an economical,
convenient and non- vacuum method to synthesize homogenous and high quality
nanoparticles as we l l a s f i lms on large scale and on different types and shapes of
substrates. This technique is especially useful for growth of ZnO nanoparticle and
films, since zinc belongs to the group of elements that form polymeric hydroxides
easily, a fundamental requirement for sol-gel chemistry. The main factors affecting the
sol-gel der ived nanopart ic le propert ie s are sol concentration, sol’s chemical
equilibrium, pH value of the sol, time, temperature and order-time-temperature of
reagent mixing. The stepwise methodology of synthesis of ZnO nanoparticles with pH
variation is shown in figure 3.14. The only difference in this case is the variation in pH
value (7 to 12) of the sol using different concentration of NaOH.
96
Fig. 3.14 Flow chart for sol-gel synthesis of ZnO nanoparticles with pH variation
97
3.8 Structural and Morphological Properties
3.8.1 XRD Analysis
The XRD pattern of the ZnO nanoparticles with varying pH is shown in
figure 3.15. Intensity peaks are showing that nanoparticles are highly crystalline in nature.
The preferred orientation corresponding to the plane <101> is observed in all the samples.
These peak positions coincide with JCPDS card no. 36-1451 for ZnO powder.
20 30 40 50 60 70 80 90
(112)(103)
Inte
ns
ity
(a
.u.)
2 (degree)
7 PH
(100)
(110) 8 PH
(102)9 PH
10 PH
11 PH
(101)
(002)
12 PH
Fig. 3.15 XRD pattern of ZnO nanoparticles for pH ranging from 7 to 12
Crystallite size (D) in the orientation <101> was calculated by Debye-Scherrer’s
formula given by equation (3.1). Crystallite size of the prepared samples increased from
28 nm to 34 nm with increasing pH value from 7 to 11. At 11 pH, the size was maximum
(34 nm). After that, on increasing pH to 12, crystallite size decreased to 32 nm. When the
98
concentration of OH- ions i.e. pH is low, ZnO nanoparticles do not grow in size due to the
lack of Zn(OH)2 formation in the sol [28, 29]. Since sol with pH < 7 would not have
sufficient OH- concentration, no formulation of nanoparticles has been observed. For
pH > 7 formulation of nanoparticles is observed. The reason for decrease in size after a
certain pH level (pH 11 in present work) is that precipitates start to dissolve due to higher
reaction rates. When ZnO reacts with OH- , the dissolution of OH
- occurs. Figure 3.16
shows the variation in FWHM of the dominating 2θ peak <101> with increasing pH. It
can be clearly seen that FWHM decreases on increasing pH. The decrease in FWHM with
increase in pH implies the increase in crystallite size. After 11 pH, FWHM increases
thereby decreasing the crystallite size decreases.
7 8 9 10 11 12
0.26
0.27
0.28
0.29
0.30
0.31
0.32 (a)
FW
HM
pH Value
7 8 9 10 11 1227
28
29
30
31
32
33
34
35
(b)
Cry
sta
llit
e S
ize (
nm
)
pH Value
Fig. 3.16 Variation of (a) FWHM and (b) crystallite size with pH
As the pH of the sol is increased, the intensity value of the dominant 2θ peak also
increases (Fig. 3.17). This implies that the number of crystallites in the orientation <101>
is also increasing with increase in pH.
99
7 8 9 10 11 12
1800
2000
2200
2400
2600
Inte
ns
ity (
a.u
.)
pH Value
Fig. 3.17 Variation in intensity with increasing pH
3.8.2 TEM Analysis
TEM images confirm the results obtained by XRD (Fig. 3.18). As the pH is
increased from 7 to 11 the particle size increases from 28 to 34 nm. At 11 pH, the size
was maximum 34 nm. After that, on increasing pH to 12, crystallite size decreased to 32
nm. Hexagonal structures of the nanoparticles can be seen in the TEM images. This range
of size (28-34 nm) is suitable for making front wide bandgap semiconductor electrode in
QDSSC.
3.9 Optical Properties
3.9.1 UV-Visible Absorbance Analysis
The optical absorption spectra of ZnO nanoparticles in the visible region are
shown in figure 3.19. The graph shows that ZnO does not absorb light in the visible
region which means that prepared ZnO samples are transparent to visible light. Bandgap
values are calculated using Tauc’s plot as explained earlier by equations (3.2) and (3.3).
100
Fig. 3.18 TEM images of ZnO nanoparticles wih increasingt pH value 7-12 (a-f)
300 350 400 450 500
0.0
0.4
0.8
1.2
1.6
Ab
so
rpti
on
(a.u
.)
Wavelength (nm)
pH7
pH8
pH9
pH10
pH11
pH12
3.1 3.2 3.3 3.4 3.50.0
3.0x10-19
6.0x10-19
9.0x10-19
1.2x10-18
(h
)2 (
cm
-1eV
)2
h (eV)
pH7
pH8
pH9
pH10
pH11
pH12
(a) (b)
Fig. 3.19 (a) Absorption spectra and (b) Tauc’s plot of ZnO nanoparticles
101
The variation in bandgap energy with increasing crsytallite size (pH value) is
shown in figure 3.20. Bandgap decreases from 3.25 to 3.21 eV with increase in crsytallite
size from 28 to 34 nm [30].
28 30 32 343.20
3.22
3.24
3.26
Ban
dg
ap
en
erg
y (
eV
)
Crystallite size (nm)
7 8 9 10 11 123.20
3.21
3.22
3.23
3.24
3.25
3.26
Ban
dg
ap
en
erg
y (
eV
)
pH value
(a) (b)
Fig. 3.20 Variation in bandgap with (a) crystallite size and (b) pH value
3.10 Growth Mechanism of ZnO Nanoparticles
The growth of ZnO from zinc acetate dihydrate precursor using sol-gel process
generally undergoes four stages which are –
1. Solvation
2. Hydrolysis
3. Polymerization and
4. Finally transformation into ZnO
The zinc acetate dihydrate precursor was first solvated in methanol, and then hydrolyzed,
regarded as removal of the intercalated acetate ions and results in a colloidal-gel of zinc
hydroxide (Eq. 3.4). The size and activity of solvent have obvious influence on the
reacting progress and product. Methanol has smaller size and a more active –OH and
–OCH3 groups. Methanol can react more easily to form a polymer precursor with a higher
102
polymerization degree, which is required to convert sol into gel [31]. These zinc
hydroxide splits into Zn2+
cation and OH- anion according to reactions (Eq. 3.5) and
followed by polymerization of hydroxyl complex to form “Zn-O-Zn” bridges and finally
transformed into ZnO (Eq. 3.6) [30].
Zn (CH3COO)2 . 2H2O + 2NaOH Zn (OH)2 + 2CH3COONa + 2H2O (3.4)
Zn(OH)2 + 2H2O Zn(OH)42+
+ 2H2+ (3.5)
Zn(OH)42+
ZnO + H2O + 2OH- (3.6)
When the concentration of OH- i.e. pH is low, the growth of ZnO particle does not
proceed because of the lack of Zn(OH)2 formation in the solution. Therefore in sol gel
technique, there is a threshold pH level above which the nanostructure may be formed. In
this study, the growth of ZnO nanoparticles in zinc acetate solution was observed from a
solution having pH of 7. A solution with a pH < 7 would have insufficient OH-
concentration to form ZnO. Since pH controls the rate of ZnO formation, it affects the
size and their way of combination to get stable state. As the freshly formed nuclei in the
solution are unstable, it has a tendency to grow into larger particles. The largest crystallite
size was observed when the pH of the solution was 11. Further increase in the
concentration of OH- from this point, reduced the crystallite size of ZnO. This is
presumed to be because of the dissolution of ZnO via back reaction of equation (3.6).
When ZnO reacts with OH-, the dissolution of ZnO occurs [32]. The decrease in
crystallite size above 11 pH level is the evidence of the acceleration of ZnO dissolution
during competitive ZnO formation.
103
3.11 Summary
ZnO nanoparticles were synthesized via sol-gel route and solid state reaction
method. Under the same growth conditions, the crystallite size was ~24 nm in case of sol-
gel method and ~37 nm in case of solid state reaction method. Crystallinity was better in
case of sol-gel method. Because of ease of preparation and control over crystallite size in
sol-gel synthesis, the effect of pH variation on sol-gel synthesized ZnO nanoparticles was
studied. The size of the ZnO nanoparticles increased with increasing pH. After a certain
pH level, the size started decreasing. Minimum and maximum crystallite size was ~27 nm
and ~32 nm for pH 7 and 11 respectively. A blue shift in the bandgap was observed with
decrease in the size of the ZnO nanoparticles. The growth mechanism of ZnO has been
discussed in terms of solvation, hydrolysis and polymerization. Aggregation has been
found to be dominant growth mechanism of ZnO nanoparticles.
104
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