KAUSAR BINTI HARUN

FIRST-PRINCIPLES CALCULATIONS ON SOL-

GEL ZINC OXIDE NANOPARTICLES

OPTOELECTRONIC PROPERTIES

KAUSAR BINTI HARUN

UNIVERSITI SAINS MALAYSIA

2018

FIRST-PRINCIPLES CALCULATIONS ON SOL-GEL ZINC OXIDE

NANOPARTICLES OPTOELECTRONIC PROPERTIES

by

KAUSAR BINTI HARUN

Thesis submitted in fulfillment of the

requirements for the degree of

Master of Science

February 2018

ii

ACKNOWLEDGEMENT

Alhamdulillah, with all Allah’s will, I manage to complete my research and this

thesis. A million thank dedicated to my supervisor, Associate Professor Dr. Ahmad

Azmin Mohamad for his endless motivation, guidance, and patient in minoring my

work. A sincere appreciation also goes to my co-supervisor, Professor Dr. Zainal

Arifin Ahmad for being very helpful in giving suggestions and comments for my

research.

I thank my dear colleagues from Battery Research Group for their supporting

ideas and critics. I must thank the three great people from UiTM Shah Alam, Dr.

Mohamad Fariz, Dr. Muhamad Kamil and Mr. Abdul Wafi for a valuable knowledge

sharing on first-principles calculation. This appreciation also deserved by all

lecturers, technical, and administrative staffs of School of Materials and Mineral

Resources Engineering. Their help in all areas has contributed to the success of my

research.

I would also acknowledge the funding bodies, Universiti Sains Malaysia (USM) for

awarding USM Fellowship Scheme. Finally, my hearties appreciation goes to my

husband Dr. Mohd Amin, my kids Yusuf and Maryam and my entire family for their

endless love and support. May Allah blessed and reunited us in His paradise, In Shaa

Allah.

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TABLE OF CONTENTS

ACKNOWLEDGEMENT ii

TABLE OF CONTENTS iii

LIST OF TABLES vii

LIST OF FIGURES viii

LIST OF ABBREVIATIONS xiv

LIST OF SYMBOLS xvi

LIST OF CHEMICAL FORMULA xviii

ABSTRAK xix

ABSTRACT xx

CHAPTER ONE: INTRODUCTION

1.1 Study background

1.2 Problem statement

1.3 Objectives of study

1.4 Thesis outlines and significance of study

1

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4

5

CHAPTER TWO: LITERATURE REVIEW

2.1 Introduction

2.2 Overview of ZnO as a semiconductor

2.3 Synthesis route of ZnO nanoparticles: Sol-gel method

2.4 Physical characterization of sol-gel synthesized ZnO

2.4.1 Sol-gel synthesized ZnO: Thermal properties

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2.4.2 Sol-gel synthesized ZnO: Phase and structural properties

2.4.3 Sol-gel synthesized ZnO: Microstructural and morphological

properties

2.4.4 Sol-gel synthesized ZnO: Optical properties

2.5 Background and development of first-principles calculation

2.6 Density functional theory

2.6.1 The Hohenberg-Kohn Theorems

2.6.2 Kohn-Sham method

2.7 Exchange-correlation functional

2.7.1 Local density approximation

2.7.2 Generalized gradient approximation

2.8 Hubbard-U scheme

2.9 First-principles calculation: Structure modeling of ZnO

2.10 First-principles calculation: Geometrical optimization of ZnO

structure

2.11 First-principles calculation: The electronic properties of ZnO

2.12 First-principles calculation: The optical properties of ZnO

2.13 Combined studies on first-principles and experimental

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CHAPTER THREE: METHODOLOGY

3.1 Introduction

3.2 Experimental materials and apparatus

3.3 Synthesis of ZnO nanoparticles by sol-gel storage method

3.4 Characterization of experimentally-grown ZnO nanoparticles

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3.4.1 Thermal analysis

3.4.2 Phase and structural analysis

3.4.3 Morphological analysis

3.4.4 Optical properties evaluation

3.5 First principles calculation of ZnO nanoparticles

3.5.1 ZnO structure modelling

3.5.2 Geometrical optimization

3.5.3 Energy calculation for electronic properties

3.5.4 First-principles calculation for defective ZnO

3.5.5 Optical properties calculation

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CHAPTER FOUR: RESULTS AND DISCUSSIONS

4.1 Introduction

4.2 Synthesis of ZnO from sol-gel method

4.3 Characterization of sol-gel synthesized ZnO

4.3.1 Thermal analysis

4.3.2 Qualitative phase and structural analysis

4.3.3 Quantitative phase and structural analysis

4.3.4 Proposed growth mechanism

4.3.5 Morphological analysis

4.3.6 Absorption and energy band gap evaluation

4.3.7 Luminescence properties of ZnO

4.4 First-principles calculation of ZnO

4.5 Structure modeling of perfect ZnO unit cell

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4.5.1 Optimization and energy calculation of perfect ZnO unit cell

4.5.2 Electronic density of state of perfect ZnO unit cell

4.6 Structure modeling of defective ZnO

4.6.1 Energy and electronic calculation of defective ZnO

4.7 Optical properties of ZnO

4.8 Summary

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CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

5.2 Recommendations for future work

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107

REFERENCES 109

LIST OF PUBLICATIONS

APPENDICES

vii

LIST OF TABLES

Page

Table 2.1 List of precursors, solvents and additives used during sol-gel

synthesis of ZnO nanoparticles for various applications from

literature

10

Table 2.2 Summary on the characterization techniques of sol-gel

synthesized ZnO

23

Table 2.3 The optimized lattice constant and energy band gap obtained

from various approximations

34

Table 2.4 The convergence, input criteria and calculated energy band

gap obtained from first-principles calculation of ZnO system

36

Table 4.1 Refined structural parameters of ZnO nanoparticles aged at

various time and the corresponding agreement indices. The

standard ICSD (98-010-6787) is presented for comparison and

relative deviation is indicated in bracket

69

Table 4.2 Variation of particle size measured from FESEM and

HRTEM images

76

Table 4.3 The calculated lattice parameter using conventional functional

with relative deviation from experimental approach. The band

gap obtained from the UV-VIS plot is compared with

calculated energy band gap

88

Table 4.4 The calculated lattice parameter using corrected Hubbard-U

method with relative deviation from experimental approach.

The band gap obtained from the UV-VIS plot is compared

with calculated energy band gap

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LIST OF FIGURES

Page

Figure 2.1 The unit cell of ZnO (a) wurtzite, (b) zinc-blende and

(c) rock salt structure. The grey sphere represent Zn

atom and red sphere represent O atom [32]

7

Figure 2.2 (a) Basic TGA curve of dry ZnO gel derived from zinc

acetate dihydrate and methanol (adapted from Ref. [64])

and (b) TGA measurement of ZnO-DMLT nanoparticles

with the TGA curve of pure DMLT serving as reference

to ensure the full decomposition of DMLT (adapted

from Ref. [65])

12

Figure 2.3 (a) The comparison of XRD patterns between bulk and

nanosize ZnO, clear broadening observed in nanosized

ZnO (adapted from Ref. [67]), (b) basic XRD patterns

of as-prepared ZnO nanoparticles synthesized from zinc

acetate, ethanol and lithium hydroxide (adapted from

Ref. [56]) and (c) the diffraction patterns of ZnO

nanoparticles with the effect of precipitation and

washing on nanoparticle purity (adapted from Ref. [69])

14

Figure 2.4 Rietveld refinement on ZnO which the observed (red

dots) profiles represent the experimental data and the

calculated data (black line) represent the modelled fit

curve (adapted from Ref. [73])

16

Figure 2.5 FESEM images of ZnO powder synthesized using

different procedures (a) sol–gel storage and (b) sol–gel

centrifugation (adapted from Ref. [48]), (c)

microstructural images using TEM for EDTA-capped

ZnO, and (d) HRTEM image of EDTA-capped ZnO

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(adapted from Ref. [83])

Figure 2.6 (a) The UV-Vis absorption spectra of the obtained ZnO

quantum dot sols prepared by different solvent: A-

methanol, B-ethanol and C-hexanol (adapted from Ref.

[85]) and (b) the UV–Vis-IR absorption spectrum of

ZnO nanoparticles for different annealing temperatures.

The inset is Tauc’s plot showing the Eg spectrum for

different temperatures (adapted from Ref. [12])

20

Figure 2.7 Comparison of luminescence emission between bulk,

nanocrystal and quantum dots ZnO [90]

21

Figure 2.8 (a) The 1×1×1 unit cell of ZnO, (b) 2×3×4 ZnO

supercell with Cd as foreign atom, (c) 2×2×4 ZnO

supercell with Cd as foreign atom and (d) 1×3×3 ZnO

supercell with Cd as foreign atom [112]

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Figure 2.9 The band gap variation along with lattice parameter for

(a) at fixed Up,O = 0 eV with varied Ud,Zn and (b) at

fixed Ud,Zn = 10 eV with varied Up,O [32]

35

Figure 2.10 The calculated band structure using different XC

functional (a) LDA with red circle show underestimated

band gap (adapted from Ref. [125], (b) GGA and

GGA+U and (c) the enlarged energy gap (adapted from

Ref. [32])

37

Figure 2.11 The calculated DOS of ZnO (a) combined DOS using

LDA (adapted from Ref. [101]), (b) schematic

representation of individual and total DOS by GGA

(adapted from Ref. [128]), (c) spin-up and spin-down

DOS of ZnO with oxygen vacancy by GGA and (d)

40

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spin-up and spin-down DOS of ZnO with oxygen

vacancy by GGA+U (adapted from Ref. [129])

Figure 2.12 (a) The imaginary part of dielectric function of ZnO

calculated by LDA+U (adapted from Ref.[97]) and (b)

the calculated absorption of ZnO by LDA, GGA and

GGA-EV (adapted from Ref. [128])

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Figure 3.1 Flowchart of the project starting with ZnO synthesis

followed by first-principles calculation

46

Figure 3.2 Synthesis of ZnO nanoparticles (a) initial clear solution

of Zn(CH3COO)2.2H2O dissolved in CH3OH, (b) milky-

white slurry after titration with 1 M NaOH and (c)

sedimentation of ZnO after 48 h aging

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Figure 3.3 Build crystal window for space group insertion

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Figure 3.4 Selection for lattice parameter insertion (a and c for

ZnO)

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Figure 3.5 (a) Adding atomic coordination of Zn atom and (b)

adding atomic coordination of O atom

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Figure 3.6 The CASTEP calculation window for geometrical

optimization task with specified XC functional

54

Figure 3.7 (a) The convergence tolerance in optimization process

and (b) the specified energy cutoff and k-point basis set

55

Figure 3.8 The CASTEP calculation window for optimization with

Hubbard-U correction scheme. The red arrow indicated

the activation of Hubbard-U in the calculation

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Figure 3.9 The selection of Coulomb energy U for (a) Zn-3d and

(b) O-2p state

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Figure 4.1 Synthesis of ZnO nanoparticles by the sol-gel method;

(a) fresh pasty-like product, (b) gel obtained after 48 h

aging, (c) xerogel produced after drying, (d) grounded

powder before calcination and (e) grounded powder

after calcination

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Figure 4.2 The mass-loss traces from TGA curves of as-

synthesized ZnO at different aging time

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Figure 4.3 The TGA curve for precursor zinc acetate dihydrate,

Zn(CH3COO)2.2H2O heated in normal air from –

at min-1

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Figure 4.4 The diffraction patterns of a as-synthesized nO

and nO after calcination at

65

Figure 4.5 Rietveld-refinement profiles of ZnO phase synthesized

by sol-gel method aged at various time. Iobs refers to

the intensity of observed data (experimental data) and

Icalc refers to intensity of calculated model. The

difference plot (blue line) was obtained from Iobs –

Icalc which represent the residue between observed and

calculated model

67

Figure 4.6 The schematic of ZnO growth from sol-gel method at

different aging time

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Figure 4.7 The FESEM images of ZnO synthesized at different

aging time (a) 0, (b) 6, (c) 12, (d) 24, (e) 36 and (f) 48 h

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at magnification 30 000×

Figure 4.8 The HRTEM images of ZnO synthesized at various

aging time at magnification 97 000 × and the scale bar

for each images are 50 nm

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Figure 4.9 Plot of particle size of ZnO measured by FESEM and

HRTEM

78

Figure 4.10 The absorbance of ZnO nanoparticles obtained from

UV-VIS spectroscopy: (a) aged at 0 and 6 h and (b)

aged at 12, 24, 36, and 48 h

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Figure 4.11 Tauc plots of ZnO at synthesized at different aging time:

(a) aged at 0 and 6 h and (b) aged at 12, 24, 36 and 48 h

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Figure 4.12 Photoluminescence spectra of ZnO at different aging

time

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Figure 4.13 The ZnO structure drawn on Material Studio Visualizer

(a) the unit cell of ZnO and (b) tetrahedral coordination

of Zn and O atom

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Figure 4.14 Variation of energy band gap and lattice parameter

calculated using different functionals (a) LDA+U, (b)

GGA-PBE+U and (c) GGA-PBESol+U. In all

calculation, the Ud,Zn value was fixed to 10 eV and Up,O

was varied accordingly

91

Figure 4.15 Calculated energy band structure of synthesized ZnO

using different functionals (a) LDA, GGA-PBE and

GGA-PBESol and (b) LDA+U, GGA-PBE+U and

GGA-PBESol+U. The right figure demonstrate enlarged

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energy band structure focussing at highest valence band

and lowest conduction band along G-G path

Figure 4.16 Comparison of density of state calculated from (a)

GGA-PBE functional and (b) GGA-PBE+U functional

with Ud,Zn = 10 eV and Up,O = 6.1 eV. The grey dotted

line was the Fermi level located at 0 eV

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Figure 4.17 The ZnO structure drawn on Material Studio Visualizer

with (a) unit cell, (b) 3 × 3 × 2 supercell structures and

(c) the top view from (002) plane consisting oxygen

vacancy at the dotted circle

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Figure 4.18 The calculated band gap of 3 × 3 × 2 ZnO structure

containing oxygen vacancy using GGA-PBE+U

100

Figure 4.19 (a) The calculated density of state of ZnO structure

containing oxygen vacancy and (b) schematic view of

the band structure

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Figure 4.20 The calculated dielectric function of ZnO based on

GGA-PBE+U approximation

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Figure 4.21 The calculated absorption of ZnO based on GGA-

PBE+U approximation

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LIST OF ABBREVIATIONS

Abbreviation

Description

CASTEP Cambridge Serial Total Energy Package

CB Conduction band

CBM Conduction band minimum

DFT Density functional density

DOS Density of state

FESEM Field emission scanning electron microscope

GGA Generalized gradient approximation

GGA-PBE Generalized gradient approximation with Perdew-Burke-

Ernzerhof scheme

GGA-PBE+U Generalized gradient approximation with Perdew-Burke-

Ernzerhof and Hubbard-U correction scheme

GGA-PBESol Generalized gradient approximation with Perdew-Burke-

Ernzerhof scheme for solid

GGA-PBESol+U Generalized gradient approximation with Perdew-Burke-

Ernzerhof scheme for solid and Hubbard-U correction

scheme

GOF Goodness of fit

ICSD Inorganic Crystal Structure Database

LDA Local density approximation

xv

LDA+U Local density approximation with Hubbard-U correction

scheme

PDOS Partial density of state

PL Photoluminescence

SGC Sol-gel centrifuge

SGS Sol-gel storage

SIC Self-interaction correction

TGA Thermal gravimetric analysis

TEM Transmission electron microscope

UV Ultra violet

UV-VIS Ultra violet-visible

VB Valence band

VBM Valence band maximum

XC Exchange-correlation

XRD X-ray diffraction

3D Three dimension

xvi

LIST OF SYMBOLS

Symbols

% Percentage

° Degree

°C Degree Celcius

°C min-1

Degree Celcius per minute

Ψ Wavefunction

ε1 Real part of dielectric function

ε2 Imaginary part of dielectric function

ω Photon frequency

∇ Gradient of electron density

λ Wavelength

Å Angstrom

α Absorption coefficient

a Lattice parameter in x-axis

c Lattice parameter in z-axis

e Charge of electron

ehom

Homogenous electron gas

m Mass of ion

n(r) Particle density at point r

h Planks’s constant

h Hour

j Jam

xvii

r Coordinates of electron

v Frequency of light

Eg Energy band gap

EHF Hartree-Fock energy

Ĥ Hamiltanion operator

M Mass of electron

P1 Momenta of ion

P2 Momenta of electron

R Coordinate of ion

R Residual factor

Rexp Expected profile residual

Rwp Weighted profile residual

U Coulomb repulsion energy

Z Charge of ion

GPa Giga Pascal

nm Nano meter

eV Electron Volt

meV Mili electron Volt

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LIST OF CHEMICAL FORMULA

CH3OH Methanol

H2O Water

NaOH Sodium hydroxide

Zn(CH3COO)2.2H2O Zinc acetate dihydrate

Zn(OH)2 Zinc hydroxide

Zn(OH)42-

Zincate ion

OH- Hydroxide ion

Zn2+

Zinc ion

O2-

Oxygen ion

ZnO Zinc oxide

TiO2 Titanium oxide

CdS Cadmium sulfide

CdSe Cadmium selenide

SnO2 Tin oxide

xix

PENGIRAAN PRINSIP-PERTAMA TERHADAP CIRI-CIRI

OPTOELEKTRONIK PARTIKEL NANO SOL-GEL ZINK OKSIDA

ABSTRAK

Diagnostik berkesan antara eksperimentasi dan pengiraan teori adalah perlu untuk

memastikan sinergi antara kedua pendekatan. Kajian ini menggunakan input struktur

daripada experimentasi ke dalam rangka kerja teori. Permulaannya, partikel nano

ZnO telah disintesis melalui kaedah sol-gel pada waktu penuaan berbeza. Analisa

fasa dan struktur mengesahkan penghasilan struktur ZnO wurtzit heksagon dengsn

sampel dituakan selama 36 j menunjukkan penghabluran tertinggi dan memberikan

visual tepat terbaik dalam analisa Rietveld. Pemerhatian morfologi menunjukkan

penghasilan partikel nano sfera yang seragam pada masa penuaan melebihi 6 j

manakala variasi yang kecil direkodkan pada jurang jalur tenaga antara 3.08 – 3.12

eV. Jalur kependarkilauan menunjukkan pelepasan hijau kerana kekosongan oksigen.

Di dalam pengiraan prinsip pertama, sel unit ZnO dibina berdasarkan parameter

struktur daripada analisa Rietveld bagi menghubungkan kajian eksperimental.

Beberapa fungsi penukaran-korelasi termasuk LDA, GGA-PBE, GGA-PBESol,

LDA+U, GGA-PBE+U dan GGA-PBESol+U. Fungsi GGA-PBE+U (Ud,Zn = 10 eV

dan Up,O = 6.1 eV) menunjukkan sisihan kekisi terendah dan berjaya mengulang

jurang jalur tenaga eksperimentasi. Struktur ZnO super sel bersama kekosongan

oksigen menunjukkan kedudukan kecacatan lebih nyah-setempat dan berada pada

1.90 eV dari atas jalur konduksi. Posisi ini menepati tenaga pembebasan foton (2.06

eV) seperti terlihat di spektrum kependarkilauan. Dapatan ini bermanfaat dalam

rekabentuk anod sel solar bagi meningkatkan penyerapan cahaya nampak.

xx

FIRST-PRINCIPLES CALCULATIONS ON SOL-GEL ZINC OXIDE

NANOPARTICLES OPTOELECTRONIC PROPERTIES

ABSTRACT

An efficient diagnostic between experimental and theoretical calculation is

essential to ensure the synergy between these two approaches. This study made

attempt to use structural input from experimental in the theoretical framework.

Initially, ZnO nanoparticles were synthesized by sol-gel method at different aging

time. The phase and structural analyses confirmed the formation of hexagonal

wurtzite ZnO structure at which sample aged at 36 h showed highest crystallinity and

gave the best visual fit in Rietveld analysis. Morphological observation revealed

spherical nanoparticles were formed at aging time higher than 6 h while only small

variation in energy band gap recorded between 3.08 – 3.12 eV. The

photoluminescence spectra revealed a green emission due to oxygen vacany. In first-

principles calculation, the ZnO unit cell was built based on structural parameter from

Rietveld analysis in order to provide a bridge with experimental study. Several

exchange-correlation functional including LDA, GGA-PBE, GGA-PBESol, LDA+U,

GGA-PBE+U and GGA-PBESol+U were tested. The GGA-PBE+U (Ud,Zn = 10 eV

and Up,O = 6.1 eV) showed lowest lattice deviation and successfully reproduced the

experimental band gap. ZnO supercell structure with oxygen vacancy showed that

defect state were more delocalized and appeared at 1.90 eV from top of conduction

band. This position was close to the photon energy released due to recombination of

electron (2.06 eV) as observed in luminescence spectra. The results are beneficial in

designing photoanode material in solar cell that will enhance visible light absorption.

1

CHAPTER ONE

INTRODUCTION

1.1 Study background

The 21st century has marked a tremendous research work focusing on potential

clean and renewable energy technology. The new generation of solar cell known as

dye-sensitize solar cell (DSSC) is an example of energy device that actively studied.

In DSSC, the photoanode consist of a metal oxide semiconductor plays important

role that contributes to overall efficiency. It serves as a scaffold that supports the dye

molecules and transferring electrons [1]. Zinc oxide (ZnO) has become a potential

photoanode material pertaining to its unique and comparable properties from its

former counterpart.

ZnO is a II-VI semiconductor with a wide energy band gap (3.3 eV) and high

electron mobility with magnitude larger than anatase TiO2 μTiO2 = 0.1-4 cm2 Vs

-1,

μZnO = 200-300 cm2 Vs

-1) [2]. To date, issue on the incapability of ZnO to fully utilize

visible light due to its wide band gap has limited its potential use especially in solar cell.

Several attempts have been conducted such as the introduction of a doping element

and monitoring the native defects [3, 4]. These work in return involved number of

experiments before the ideal properties can be achieved.

Pure ZnO nanoparticles can be obtained through several synthesis routes such as

solid state reaction [5], hydrothermal [6] and sol-gel methods [7-9]. Notably, the sol–

gel method has been favoured for the synthesis of nO ecause it can take place at a

2

lower temperature , involves simple starting materials, and produces nO

with excellent chemical homogeneity. The synthesis condition including solution pH

[8, 10], type of starting materials [11], and pre- and post-heat treatment [12] are found to

give impact on properties of sol-gel derived ZnO.

Meanwhile, the current practice used first-principles calculations based on the

density functional theory (DFT) to study the properties of ZnO. DFT has become the

preferred computational method due to the simplicity of the software and its ability

to calculate the ground state properties with predictive accuracy. The principles of

DFT are based on two theorems pioneered by Hohenberg-Kohn [13] and Kohn-Sham

[14] that simplify the complexity of the many-body Schrodinger equation. By

considering the electron density instead of many-body wave function, DFT has made

the computational work much easier to be solved [15].

A number of theoretical studies have been conducted to simulate the

optoelectronic properties ZnO [16-19]. Based on this method, fast and accurate

results have been achieved, along with reduced trial and error, as often happens in

experimental work. However, calculations based on the DFT are sensitive as the

varying of unnecessary parameters may lead to unphysical and misinterpreted results.

1.2 Problem Statement

Previous studies have shown that intensive investigations on the properties of

ZnO have been carried out by means of experimental and theoretical methods.

Hence, it is necessary to verify these two approaches to ensure the synergy of each

3

work and in return leading to a significant improvement. One intriguing approach is

to integrate the theoretical calculation with the input from the experimental result.

This approach used lattice parameters and atomic coordination obtained from

experimentation to build the ZnO crystal structure in theoretical framework.

However, the reported studies had used lattice inputs from random literature during

the structure modelling stage [20, 21]. This strategy successfully created a ZnO

model, but it did not offer a close representation of experimentally-grown ZnO. As a

result, no bridging is attained and the calculated optoelectronic properties are merely

belong another system.

To obtain an exact crystal structure is a challenging task. The refined diffracted

profile from X-ray diffraction analysis offered structural information that is close

representation to the synthesized version. Therefore, a well synthesized ZnO must be

produced with a controlled parameter and carefully characterized.

In the sol-gel method, several processes involved such as hydrolysis,

condensation, nucleation and aging. The growth of ZnO mainly occurred during

aging [22] and if the gel is freely aged over time, the formation of ZnO nanoparticles

could be investigated. Previous literature has noted that stabilized ZnO can be

obtained after short-time aging lasting 0–36 h [23], 48 h [7, 8], and even after a

month [24]. The range of aging time is rather very wide and may lead to difficulties

when the optimum aging is to be chosen for practical consideration. Hence, aging

time must be carefully examined to allow complete formation of ZnO.