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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