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OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE NANOPARTICLES SYNTHESIZED BY DIFFERENT SOL-GEL ROUTES MOHAMMED ABDULSALAM ALRAHABI UNIVERSITI TEKNOLOGI MALAYSIA

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Page 1: OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE ...eprints.utm.my/id/eprint/48549/1/MohammedAbdulsalamAlrahabiMFS2014.pdfNanopartikel zink oksida merupakan bahan yang baik untuk pelbagai

OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE

NANOPARTICLES SYNTHESIZED BY DIFFERENT SOL-GEL ROUTES

MOHAMMED ABDULSALAM ALRAHABI

UNIVERSITI TEKNOLOGI MALAYSIA

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OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE

NANOPARTICLES SYNTHESIZED BY DIFFERENT SOL-GEL ROUTES

MOHAMMED ABDULSALAM ALRAHABI

A dissertation submitted in fulfilment of the

requirements for the award of the degree of

Master of Science (Physics)

Faculty of Science

Universiti Teknologi Malaysia

JUNE 2014

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To my beloved mother and father

To my dear wife and son Osamah

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ACKNOWLEDGEMENT

Alhamdulillahirabbulalamin, all praise to Allah Almighty for fulfilling

everything I required for finishing this dissertation. It has been amazing experience

throughout the periods I studied at Universiti Teknologi Malaysia (UTM) and it

would not has been so without peoples who have been involving contributing in

many ways to this work. Therefore, I want to acknowledge them as to express my

heartfelt gratitude to them.

In particular, I wish to express my sincere appreciation to my supervisor,

Prof. Dr. Zulkafli Othaman, for encouragement, critics and friendship. I would have

never been able to complete this work without his guidance, comments and advices.

I am also indebted to Ministry of Higher Education in Yemen for funding my

master study also special thanks to Universiti Teknologi Malaysia UTM for their

assistance in supplying the relevant literatures as well as my fellow postgraduate

students should also be recognised for their support. My sincere appreciation also

extends to all my colleagues and others who have provided assistance at various

occasions. Their views and tips are useful indeed. Unfortunately, it is not possible to

list all of them in this limited space. I am grateful to all my family members and

friends.

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ABSTRACT

Transformation into nanoscale from bulk structures involves an outstanding

modification in the properties of materials. It gives an opportunity to get a novel

behaviour such as size dependent in structural and optical properties. Zinc oxide

nanoparticle is a promising material for many applications especially optical devices.

There are many methods to synthesis these nanoparticles. In this work ZnO

nanoparticles are synthesized by sol-gel method using different routes with different

stirring temperatures. The structural, morphological and optical properties of ZnO

nanoparticles are studied. The result of the XRD shows that the average particle size

of ZnO particles decreases with increasing stirring temperature. By using

Williamson-Hall formula to calculate the average size of ZnO nanoparticles it

reveals that the Zinc Acetate precursor gives smaller nanoparticles than Zinc Nitrate

(at 70 o C ~22 and ~50 nm respectively). FESEM images display the semi-spherical

shaped of the synthesized nanoparticles and emphasise that the nanoparticles

synthesized by Zinc Acetate are finer and more homogenous than those synthesized

with Zinc Nitrate. Furthermore, the photoluminescence result shows that the band

energy of the zinc oxide nanoparticles synthesized by Zinc Nitrate is 3.02 eV while

for those synthesized with Zinc Acetate is 3.10 eV. The excellent features of the

result suggest that this work may constitute a basis for the tuneable synthesis of ZnO

nanoparticles suitable for optoelectronics devices.

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ABSTRAK

Transformasi kepada skala nano dari struktur berskala besar melibatkan

pengubahsuaian luar biasa (ketara) di dalam sifat-sifat suatu bahan. Ia memberi

peluang untuk memperolehi kelakuan novel seperti kebergantungan saiz di dalam

sifat struktur dan optik. Nanopartikel zink oksida merupakan bahan yang baik untuk

pelbagai aplikasi terutama peranti optik. Terdapat pelbagai kaedah boleh digunakan

dalam proses sintesis nanopartikel. Di dalam kajian ini, nanopartikel zink oksida

telah disintesis melalui kaedah sol-gel dengan menggunakan bahan kimia) berbeza

dengan suhu kacau yang berbeza. Sifat struktur, morfologi dan optik terhadap

nanopartikel zink oksida dikaji. Hasil pencirian XRD menunjukkan bahawa saiz

purata partikel ZnO berkurangan apabila suhu kacau ditambah. Dengan

menggunakan formula Williamson-Hall bagi kiraan saiz purata nanopartikel ZnO,

hasil menunjukkan bahawa pelopor zink acetat memberikan saiz nanopartikel lebih

kecil berbanding pelopor zink nitrat (pada suhu 70º iaitu 22 dan 50 nm). Imej

FESEM menunjukkan bahawa nanopartikel ZnO yang terbentuk adalah semi-sfera

serta menunjukkan permukaan nanopartikel yang disintesis menerusi zink acetat

lebih halus dan sekata berbanding zink nitrat. Tambahan lagi, hasil pencirian

kefotopendarcahayaan (fotoluminasi) menunjukkan bahawa jurang jalur tenaga bagi

sampel nanopartikel ZnO bagi zink nitrat adalah 3.02 eV manakala bagi zink acetat

adalah 3.10 eV. Hasil terbaik pencirian yang diperolehi dicadangkan bahawa kajian

ini mampu memberikan asas kepada sintesis nanopartikel ZnO sesuai bagi aplikasi

peranti optoelektronik.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATION xiv

LIST OF SYMBOLS xvi

1 INTRODUCTION 1

1.1 Background of the Study 1

1.2 Problem Statement 2

1.3 Research Objectives 3

1.4 Scope and Significance of Study 4

2 LITERATURE REVIEW 5

2.1 Introduction 5

2.2 Nanotechnology 7

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2.2.1 Properties of Nanomaterials 7

2.2.2 Quantum Confinement Effect 8

2.3 Crystal Structure of ZnO 10

2.3.1 Energy Band Structure 11

2.3.2 The Exciton 13

2.3.3 Defects 15

2.4 Optical Properties 16

2.4.1 Optical Properties of Semiconductor 16

2.4.2 Optical Properties of ZnO 17

2.4.3 Nonlinear Optics 18

2.5 ZnO Nanostructures Application 19

2.6 Sol-Gel Method 19

3 METHODLOGY 21

3.1 Introduction 21

3.2 Experimental Set-up 23

3.3 Experimental Design 23

3.4 Analysis Method 24

3.4.1 X-ray Diffraction 24

3.4.2 Field Emission Scanning Electron Microscope 27

(FESEM)

3.4.3 Photoluminescence Spectroscopy 29

4 RESULTS AND DISUSSION 31

4.1 The Effect of Stirring Temperature on ZnO 31

nanoparticles

4.1.1 Structural Properties 31

4.1.2 Morphological Properties 35

4.1.3 Optical Properties 37

4.2 Comparative Study of ZnO nanoparticles Synthesized 38

by Two Different Routes

4.2.1 Structural Properties 38

4.2.2 Morphological Properties 41

4.2.3 Optical Properties 42

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5 CONCLUSION AND RECOMMENDATIONS 46

5.1 Conclusion 46

5.2 Recommendation 47

REFFERENCES 48

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LIST OF TABLES

TABLE NO. TITLE PAGE

Table 4.1 The diffraction angle (2θ), the FWHM (β) and the size of

nanoparticles for the synthesized ZnO with different

stirring temperatures calculated from XRD results.

34

Table 4.2 The diffraction angle (2θ) and the FWHM (β) of the peak (100)

that appears in XRD for ZnO nanoparticles synthesized by

different routes with different stirring temperatures.

39

Table 4.3 The particle size and strain of ZnO nanoparticles

synthesized by different sol-gel routes calculated from

Williamson-Hall plot.

40

Table 4.4 The calculated band energy for ZnO nanoparticles

synthesized by different sol-gel routes taken from

photluminescence spectrum.

44

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

Figure ‎2.1 Schematic representations of the energy levels in bulk

material and the splitting in the energy levels in

nanomaterials due to the quantum confinement effect, the

band gap increases by decreasing the size of

nanoparticles.

9

Figure ‎2.2 Stick-and-ball representation of ZnO crystal structures:

(a) cubic rocksalt, (b) cubic zinc blende and (c) hexagonal

wurtzite. Shaded gray and black spheres denote Zn and O

atoms, respectively.

11

Figure ‎2.3 A schematic diagram representing the direct gap energy of

ZnO.

12

Figure 2.4 A graphical representation of an exciton energy level in

comparison to the valence and conductive band within a

semiconductor.

14

Figure 3.1 Flow chart of ZnO nanoparticles synthesized by different

sol-gel routes.

23

Figure 3.2 (a) A basic features of X-ray diffractometer. (b) X-ray

diffraction and bragg’s law.

25

Figure 3.3 The X-ray diffractometer Bruker D8 Advance that used to

characterize the samples, located at Ibnu Sina Institute-

UTM.

27

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Figure 3.4 (a) Schematic diagram of an FESEM set-up. (b) The

(FESEM) that used to observe the samples surfaces JSM-

6701F, located at Ibnu Sina Institute-UTM.

28

Figure 3.5 (a) Diagram of PL experiment set-up. (b) Luminescence

Spectrophotometer LS55 (Perkin Elmer) located at

Material Characterization Laboratory at Physics

department-UTM.

29

Figure 4.1 XRD of ZnO nanoparticles with different stirring

temperatures 50, 70, 90, 110 and 130 o C.

32

Figure 4.2 Standard JCPDS card of bulk ZnO with hexagonal

structure.

33

Figure 4.3 Plot between particles size (d) and temperature, which

show that d decreases when stirring temperature is

increased.

34

Figure 4.4 Image of FESEM confirm the semi-spherical shape of the

synthesized ZnO nanoparticles.

36

Figure 4.5 FESEM images for ZnO nanoparticles synthesized by

different temperature: a) 70o C, b) 90

o C.

36

Figure 4.6 Photoluminescence spectrum of ZnO nanoparticles

synthesized with different temperatures.

37

Figure 4.7 XRD of ZnO nanoparticles using different routes at 90 oC

stirring temperature.

38

Figure 4.8 Williamson-Hall plot for ZnO nanoparticless for different

routes. The strain is extracted from the slope and the

crystalline size is extracted from the y-intercept of the fit.

40

Figure 4.9 FESEM images shows the ZnO nanoparticles synthesized

at the same temperature using different precursor (a) Zinc

Nitrate, (b) Zinc Acetate.

41

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Figure 4.10 FESEM images with different magnifications for ZnO

nanoparticles synthesized by different precursor at the

same temperature: (a) Zinc Nitrate (10,000X, 25,000X

and 50,000X), (b) Zinc Acetate (10,000X, 25,000X and

50,000X).

42

Figure 4.11 Photoluminescence of ZnO nanoparticles synthesized by

sol-gel with different routes: (a) Zinc Nitrate precursor.

(b) Zinc Acetate precursor it shows the head peak occurs

at 400 nm and 410 nm respectively, as well as the big

shoulder peak for both around 445 nm.

43

Figure 4.12 Schematic representation of the electron-hole

recombination (EHR) energy 3.10 and 3.02 eV for ZnO

nanoparticles synthesized by Zinc Nitrate and Zinc

Acetate respectively and Position of the defect level 2.79

and 2.41 eV attributed to Oxygen vacancies (VO) in

singlet (SO) and triplet (T1) states respectively. Finally the

Zinc interstitial (Zni) which caused the defect level at 2.18

eV.

45

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LIST OF ABBREVIATION

1D - One Dimensional

2D - Two Dimensional

3D - Three Dimensional

Å - Angstrom (=10-10

m)

arb. u. - Arbitrary Unit

C.B - Conduction Band

DC - Direct Current

DOS - Density of State

EHR - Electron-Hole Recombination

eV - Electron Volt

FESEM - Field Emission Scanning Electron Microscopy

FWHM - Full Width Half Maximum

GaAs - Gallium Arsenide

Ge - Germanium

IR - Infra Red

JCPDS - Joint Committee on Powder Diffraction Standard

nm - Nanometer

NPs - Nanoparticles

LED - Light Emitting Diodes

Oi - Oxygen Interstitial

PL - Photoluminescence

QD - Quantum Dot

Rad. - Radian

Si - Silicon

So - Singlet State

T - Temperature

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T1 - Triplet State

UV - Ultra-Violet

V.B - Valence Band

Vo - Oxygen Vacancy

XRD - X-Ray Diffraction

W-H - Williamson-Hall

Zni - Zinc Interstitial

ZnO - Zinc Oxide

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LIST OF SYMBOLS

A/V - Surface to Volume Ratio

d - Size of Particles

E - Energy

Eg - Band Gap Energy

ξ - Electric Field

k - Shape Factor

β - Full Width Half Maximum

λ - Wavelength

θ - Angle of Diffraction

ε - Strain of Particles

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CHAPTER 1

INTRODUCTION

1.1 Background of the Study

Recently design, characterization, production, and application of materials,

which involves the manipulation of matter at the smallest scale (from 1 to 100 nm),

have been widely used as current meaning of nanotechnology rather than just

materials. Three distinct aspects can be considered for evolution of nanotechnology:

indirect, direct, and conceptual. The advanced miniaturization of obtainable

technologies, which open new areas of application for those technologies, can be

explained by indirect aspect. Direct refers to the application of novel nanoscale

artifacts to improve the performance of presented process and materials or for

completely novel purposes. Finally, there is a conceptual aspect of nanotechnology,

in which all materials and process considered from molecular or even atomic

viewpoint especially in living system and biology. Now a few areas of technology

are exempt from the advantages of nanotechnology. The information and

communication systems such as novel semiconductor and optoelectronic device,

environment (filtration), energy (reduction of energy, consumption increasing, the

efficiency of energy production nuclear accident cleanup and waste storage), heavy

industry (aerospace and catalysis), and consumer goods are some applications of

nanotechnology (Capper et al., 2011; Jagadish et al., 2011).

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Zinc oxide (ZnO) is a chemical compound which commonly grows in a white

powder form that is insoluble in water. This powder is mostly used as an additive

into numerous materials and products such as ceramics, plastics, glass etc. Although

ZnO exists as a mineral zincite in the Earth crust, most of the commercially usage is

produced synthetically. Zinc and oxygen belong to the second and sixth groups in

the periodic table respectively (Morkoç et al., 2008).

ZnO nanoparticles have been used extensively as the preferable

semiconductor because of their potential applications, as piezoelectric transducers,

optical waveguides, acousto-optic media, surface acoustic wave devices, conductive

gas sensors, transparent conductive electrodes, solar cell windows, and varistors for

about two decade (Kim et al., 2005; Look et al., 2004; Schwenzer et al., 2006;

Shaoqiang et al., 2005). ZnO is a II–VI semiconductor which has wide-band gap of

~3.37 eV and large exciton binding energy of 60meV (Yamamoto et al., 2001)

therefore, it is now known as a promising candidate for blue and ultraviolet light-

emitting diodes (Goyal and Kachhwaha, 2012), laser diodes, and LEDs for general

illuminations such as traffic signals, backlight for liquid-crystal display (LCD),

mobile phone, and automotive lighting. The large exciton binding energy allows

excitonic absorption and recombination even at room temperature, which makes this

material interesting for optoelectronic devices (Schwenzer et al., 2006; Yamamoto et

al., 2001).

1.2 Problem Statement

Controlled synthesis and fabrication of semiconductor nanostructure by

simplicities and cost effectiveness method is challenging for electronic,

optoelectronic and sensor application. Research shows that, through metal oxide

semiconductor nanoparticles, the ZnO semiconductor is a preferred engineering

material for its advantages including its wide bandgap (3.37 eV). ZnO presents itself

as one of the potential and important material for the fabrication of light emitting

diodes, laser diodes, optical switches and so on (Jagadish et al., 2011; Takahashi et

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al., 2007; Zhang et al., 2012). Moreover, the high exciton binding energy (60 meV)

makes it as an appropriate choice for the room temperature ultraviolet laser diodes

(Lim et al., 2006).

Remarkably, a variety of techniques has been used to control the size of ZnO

particles in the nanometer range. Experimental observation requires further

verification and surface modification require further studies. The results generated

from sol-gel method need to be compared with other techniques. However it can be

observed from the previous studies that to attain ZnO nanostructure usually catalyst

or additives with a high temperature and/or low pressure is required (Umar et al.,

2008).

Consequently, there is a need to improve an effective and simple method to

synthesis these types of nanoparticles in a large quantity with a high quality at low-

temperature without using catalyst or additives. The easy and economic nature of our

method is suitable for the fabrication of varieties of other nanostructures to

understand the fundamental physics of nanoscale structure under different synthesis

condition is also important.

1.3 Research Objectives

The aims of this research will focus on the following points:

To determine the structural properties of ZnO nanoparticles synthesized by

two different sol-gel routes with different stirring temperatures using X-Ray

diffraction (XRD).

To investigate the surface morphology of the nanoparticles by Field Emission

Scanning Electron Microscopy (FESEM).

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To characterize the optical behaviour using photoluminescence spectroscopy.

1.4 Scope and Significance of Study

Sol-gel method with two different routes is employed to synthesis ZnO

nanoparticles. X-ray diffraction and Field emission scanning electron microscopy

(FESEM) are used to characterize the structural properties and surface morphology

of the samples. The effect of synthesis parameters on optical behaviour are studied

by photoluminescence (PL) spectroscopy.

Nanostructuring of semiconductors is a novel means of developing new

electronic and optoelectronic devices. In particular, the discovery of room-

temperature visible photoluminescence (PL) from ZnO nanostructures has stimulated

much interest in these particular kinds of nanostructures and in small semiconductor

particles. Easy and economic fabrication technique would be developed. The

instrumentation for large-scale fabrication has social economic impact. The

fundamental physics behind the synthesis would be understood.

ZnO nanoparticles can be exploited extensively for the use in optoelectronic

devices such as high bright full color light emitting diodes (LED), high quality flat

panel displays, high efficiency blue laser and low threshold optical pumped laser

applications at room temperature (Wang et al., 2003).

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