Chapter 1 INTRODUCTION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/7914/6/06_chapter...

Chapter 1

INTRODUCTION

CHAPTER.1: INTRODUCTION

2

1. Introduction

Zinc oxide (ZnO) is a II-VI compound semiconductor having wide band gap of 3.37

eV. It has large exciton binding energy of 60 meV at room temperature. It shows

piezoelectricity. It is nontoxic material which is cheaply available. Variety of

nanostructures of ZnO can be grown. Due to these properties, ZnO is a material of huge

technological importance. It has applications in optoelectronic devices such as Solar cell

[1], Optical wave guide [2], Light emitting diodes (LED) [3] and transparent thin film

transistor [4]. It has been applied in surface acoustic wave (SAW) devices [5].

Nanostructures of ZnO has been used as nanogenerators [6] for harvesting energy in

nanoscale.

All these interesting properties and applications of ZnO have attracted us for its study

in thin film form as well as its nanostructures.

1.1 Crystal structure of Zinc oxide

Zinc oxide naturally crystallizes in Wurtzite structure which belong to the space group

P63mc. The Wurtzite structure is a hexagonal lattice in which each Zn2+ ion is

tetrahedrally bonded to four O2- ions and vice–versa; this is shown in figure 1. In this

structure the Zn terminated face (0001) and O terminated face (0001�) are the polar faces

while the non-polar faces are (112�0) and (101�0) which contain equal number of Zinc

and Oxygen atoms. The plane perpendicular to the c-axis are called basal planes.

Thus there is a polar symmetry along the hexagonal axis. This gives rise to

piezoelectricity in ZnO and also plays key role in its crystal growth.

The tetrahedral coordination of ZnO indicates the presence of sp3 hybridized covalent

bonding, but the strong ionic character of the Zn-O bond, makes ZnO behave like both

covalent and ionic compound. The lattice parameters of hexagonal unit cell are

a = 3.2495Å and c = 5.2069Å [7].


3

Fig.1.1 Hexagonal structure of Zinc Oxide (ZnO) showing the polar faces

(perpendicular to c-axis) and non-polar faces (parallel to c-axis) [8].


4

1.2 Defects and Impurities in Zinc oxide

Zinc oxide crystal has native point defect which greatly affects its optical and electrical

properties. These defects create electronic states in the band gap which influence its

optical emission properties.The as grown ZnO crystal has always found to be n-type .It

has been shown theoretically that both Oxygen vacancy VO and Zinc interstitial ZnI

have high formation energies in n-type ZnO and they are deep level donors[9].Thus it is

considered that neither VO nor ZnI exists in measurable quantity. Van de Walle has

proposed that hydrogen H is a dominant background donor in ZnO, that were exposed

to H during growth [10]. Group III elements Al, Ga and In are donor impurities to ZnO

that can substitute Zn upto concentration greater than 1020 cm-3.

The search for high conductivity p-type ZnO still remains an active area of research.

It has been predicted theoretically that Li substitituted Zn, LiZn and Na substituted Zn

NaZn creates shallow acceptor levels, but neither produces high-conductivity p-type

ZnO[11]. N,P, As and Sb have been used as acceptors to produce n-type ZnO[12],

where it is reported that Zn-vacancy in ZnO acts as defect- type acceptor.

1.3 Band structure of Zinc Oxide

The electronic band diagram for wurtzite ZnO is shown in figure1.2 [13]. The diagram

indicates that both the valance band edge maxima and conduction band edge minima

occur at k = 0 showing ZnO is a direct band gap semiconductor. It has been found

experimentally that due to spin–orbit interaction and crystal-field splitting the valance

band in ZnO is split up into three subbands A, B and C, as illustrated in in figure 1.2.

The A and C subbands of valance band are known to posses Γ7 symmetry and the

middle subband B possess Γ9 symmetry[13].At 4.2 K the energy band gap of ZnO is

3.43eV .


5

Fig.1.2 Band diagram of ZnO showing the splitting of valance band into three subbands

A, B, and C due to Crystal field splitting and spin orbit coupling [13].


6

1.4 Properties of Zinc Oxide

Table 1.1 Properties of ZnO compared to In2O3, SnO2 and Si [14]

Parameter Unit ZnO In2O3 SnO2 Si

Eg eV 3.4 (direct) 3.6(direct) 3.6 (direct) 1.12(indirect)

Lattice Hexagonal Cubic Tetragonal Cubic

structure Wurtzite Bixbyite Rutile Diamond

a, c nm 0.325, 0.5207

1.012 0.474,0.319 0.5431

Density gcm-3 5.67 7.12 6.99 2.33

Thermal conductivity

, κ

Wm-1K-1 69║, 60 ┴ 98║, 55┴ 150

Thermal expansion coefficient

at RT, α

10-6/K 2.92║, 4.75┴ 6.7 3.7║, 4.0┴ 2.59

e33,e31,e15

d33,d31,d15

k33,k31,k15

C m-2 10-12C

N-1

1.32, -0.57, -0.48

11.7, -5.43, -11.3

0.47, 0.18, 0.2

ε (0) 8.75║, 7.8┴ 8.9 9.58║, 13.5┴

ε (∞) 3.75║, 3.70┴ 4.6 4.17║, 3.78┴

Tm °C 1975 1910 1620 1410


7

Table 1.1 gives the list of important properties of ZnO that has been compared with

other Transparent conductors viz. Indium Oxide In2O3, Tin Oxide SnO2 and Silicon Si

[14] .From the table we observe that oxide semiconductors viz. In2O3, SnO2 and ZnO

are wide band gap semiconductors with band gap greater than 3.3 eV. This causes

transparency for wavelength greater than 360 nm and thus they are used as transparent

conductors. Of the semiconductors listed only ZnO exhibits piezoelectricity with

piezoelectric stress and strain coefficients ei s and di s respectively. The

electromechanical coupling factors are ki s. ZnO has linear thermal expansion

coefficient α║, along the a-axis 2.92 X 10-6/K, which is lower than In2O3 and SnO2 and

slightly higher than that of silicon. Along the c-axis, linear thermal expansion

coefficient, α┴ of ZnO is 4.75 X 10-6/K which is higher, than the value along a-axis. All

the semiconductors listed have high melting point Tm in the range 1400 °C – 2000 °C.

1.5 Zinc Oxide thin films

Thin films of Zinc Oxide can be prepared by various techniques. These are Sputtering

[15], Chemical Vapour Deposition (CVD) [16], Laser ablation [17], Sol-gel process [18,

19], Spray pyrolysis [20].

We have used the Sol-gel process of ZnO film synthesis for our study.

Sol-gel process of film preparation has following advantages:

(i) Controllability of composition

(ii) Simplicity in processing

(iii) Cost effectiveness.

1.5.1 Method of preparation of ZnO thin film by sol-gel process

In this process we have used Zinc acetate dihydrate Zn(CH3COO)2.2H2O (ZAD) as the

precursor material for ZnO film preparation. For the sol preparation, 10% solution of

Zinc acetate dihydrate was prepared in boiling isopropanol at 84 °C which is stirred

over a magnetic stirrer. The colour of the solution turns milky after 15 minutes of

stirring and heating.Then 6-10 drops of Diethanolamine is added to the solution by a 5

ml dropper and is kept for heating and stirring. Diethanolamine acts as sol stabilizer.The

solution now turns transparent, while its heating and stirring was continued for further


8

30 minutes. Then the solution under stirring was allowed to cool to room temperature.

This prepared sol is used for spin coating the substrate.

We have used spin coating technique of film preparation in our study. In this method of

coating the cleaned substrate is fixed to the rotating stage using double sided tape. We

then take the sol in a 5 ml dropper and put six drops at the centre of the substrate and

allow it to rotate at 3000 rpm for 20 seconds. The rpm and rotation duration can be

varied depending on the experiment. The wet coated film on the substrate is called

Xerogel. This film is then allowed to dry in a griller oven at 100 °C and then annealed

at 450°C for 1 hour in air in furnace. Drying at 100°C is required before annealing at

high temperature, to avoid the cracking of the solid film. The annealing temperature of

the film can be varied depending on the experiment. In this way multiple coating of the

substrate is done to get the workable thickness of the film. Figure 1.3 shows the

summary of ZnO film preparation by sol-gel process using the flow chart.


9

6-10 drops of Diethanolamine

10% solution of

Zn(CH3COO)2.2H2O in isopropanol

Solution turns milky

Transparent solution

Spin coating of sol

on substrate

Heating of Xerogel film at 100°C for 10 min.

Annealing at 450°C for 1 hour

ZnO thin film

Multiple coating

Boiling + Stirring at 84°C

Fig.1.3 Flowchart of ZnO film preparation by sol-gel process


10

1.5.2 Physics and Chemistry of Sol-gel method of ZnO film

preparation

A detail review on sol–gel processes of ZnO film preparation has been reported by

Lamia Znaidi [21]. In the sol-gel process a molecular precursor in a homogeneous

solution undergoes following successive transformations:

(i) hydrolysis of the molecular precursor;

(ii) polymerization via successive bimolecular additions of ions, forming oxo-,

hydroxyl, or aquabridges;

(iii) condensation by dehydration;

(iv) nucleation;

(v) growth [22,23].

Depending on the nature of the molecular precursors, two sol–gel routes are currently

used: metal alkoxides in organic solvents or metal salts in aqueous solutions [24]. The

main methods of ZnO preparation is intermediate between the two sol–gel methods

since they use metal salts in alcoholic solutions.

ZnO films are obtained starting from inorganic salts such as nitrates, chlorides,

perchlorates – or organic salts like acetates and acetylacetonates, dissolved in alcoholic

media. In such media the process involves two steps. The first one consists of in situ

formation of alkoxide or alkoxy-complexes. In the second step, these complexes

undergo transformation through hydrolysis and polymerization to lead to the oxide.

1.5.2.1 Precursors

Metal salts are generally used as precursors due to low cost, facility of use and

commercial availability. The metal salts can be both inorganic and organic. Inorganic

salts like nitrates are often used, as precursors for sol–gel ZnO-based materials, even

though their main drawback is the inclusion or difficult removal of anionic species in

the final product [25, 26]. Using zinc acetate as a precursor, the acetate groups, as

contaminants of the gel, decompose under annealing producing volatile by-products

[26]. Bahnemann et. al. [27] synthesized transparent colloidal suspensions of zinc oxide

in water, 2-propanol, acetonitrile, and using different zinc salts. They have reported that

the anion, in zinc salt, is critical for the preparation of transparent and stable ZnO

colloids.


11

The use of zinc perchlorate instead of zinc acetate yields a turbid suspension; i.e.,

coagulation of the particles takes place, the acetate acts as stabilizer of the colloidal sol.

Also, the experiments with ZnCl2 or Zn(NO3)2 reveal a faster coagulation than in the

case of Zn(ClO4)2 following the initial formation of a clear colloidal suspension.

1.5.2.2 Solvents

The solvent in the sol-gel method of ZnO film preparation must have a relatively high

dielectric constant so that it can dissolve the inorganic salts [22, 28, 29 ]. Most alcohols

are dipolar, amphiprotic solvents with a dielectric constant that depends on the chain

length [30]. Alcohols with low carbon number, up to 4, are mostly used as solvents such

as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methoxyethanol. Among

all the monoalcohols, the most used are the ethanol and 2-propanol. Hosono et. al. [28],

studied the chemical reactions of zinc acetate dihydrate to ZnO preparation using

different types of solvents, i.e. methanol, ethanol, and 2-methoxyethanol.

Zinc acetate dihydrate (ZAD) was more soluble in methanol than in ethanol or 2-

methoxyethanol according to dielectric constants of these alcohols. The reflux time

necessary for the formation of ZnO increases with the solutions in order, MeOH (12 h)

<<EtOH (48 h) < 2-ME (72 h). Also, the XRD analysis of particles, obtained from the

three alcoholic solutions of ZAD revealed, after refluxing, the formation of intermediate

product as Zn5(OH)8(Ac)2·2H2O called layered hydroxide zinc acetate (LHZA). This

complex (LHZA) was also observed by Meulenkamp [31] using ethanol as a solvent

and by Fujihara et. al. [32] and Wang et. al. [33] using methanol.

Finally these complexes undergo hydrolysis and inorganic polymerization leading to the

formation of sols consisting of zinc oxide nanoparticles.

1.5.2.3 Additives

Additives are chemical species having at least one functional group. They act as basic or

acid and/or chelating agent. Alkali metal hydroxides, carboxylic acids, alkanolamines,

alkylamines, acetylacetone and polyalcohols are used for this purpose. They facilitate

the zinc salt dissolution in alcoholic media. ZAD has a limited solubility in alcohols

like ethanol and 2-propanol in the absence of other agents or heating. The agents,

like (mono- to tri-) ethanolamines or lactic acid help in complete dissolution and

formation of a stable sol [34]. Furthermore, the additives play the role of chelating and

stabilizing ligands, which avoid the rapid precipitation of zinc hydroxide and allow


12

stable dispersions to be formed. The amino groups and/or the hydroxyl groups of

alkanolamines coordinate the metal atoms of alkoxides,thus improving the solubility

and stability against hydrolysis of the alkoxides [35]. The addition of alkanolamine in

zinc acetate provides a clear solution. MEA a bidentate ligand, coordinate with zinc

atoms; one way by acting as a chelating ligand and the other way so as to bridge two

zinc atoms [35]. Inorganic bases, lithium or sodium hydroxide is used to form stable

dispersions of colloids.

MEA acts as a complexing agent, which retards the Zn2+ condensation; however, its

presence also increases the pH, which promote the formation of ZnO. The acetate group

plays a very relevant role, by complexing Zn2+ in competition with the MEA. The

complex chemical relationships of the main species are indicated in figure1.4 The

three nucleophilic species (MEA, HO− and CH3COO−) compete for the Zn2+ Lewis acid

center: attack of an HO− group leads to the formation of small zinc-oxo-acetate

oligomers, which are expected to be formed in the initial stage, from gradual forced

hydrolysis of Zn-MEA or Zn-OCOCH3 soluble complexes during aging. The

progressive condensation of the hydrolyzed moieties gives rise to colloids or

precipitates. This gives rise to stable acetate-capped colloidal nanometric particles in

dilute solutions.


13

Fig.1.4 Chemical equilibria taking place in the initial solutions followed by Hydrolysis

and condensation on heating which results in Soluble or colloidal condensed moieties

that can be deposited as film precursors; [36].


14

1.5.2.4 Growth mechanisms

Spanhel and Anderson [37] have reported that there are two possible ways of describing

the growth of ZnO crystals; by Ostwald ripening and by aggregation. In this growth

process, as soon as the smallest stable molecular clusters are formed, they rapidly

combine to give the next most stable aggregate. The primary aggregates then further

rapidly combine to give the next most stable secondary aggregate and so on.

Meulenkamp [38] suggested that the particle growth in colloidal systems can take place

in two ways. The first one corresponds to Ostwald ripening: large particles grow at the

expense of smaller particles, which have a higher solubility according to the so-called

Ostwald–Freundlich equation. The second way describes growth by the addition of

reactive precursors available in solution to already existing particles. The rate of particle

growth is governed by the concentration of precursors or dissolved species and their

reactivity, which depends on the number of particle surface atoms, and the solution

composition. Tokumoto et. al. [39] reported that the formation of ZnO colloidal

particles in an alcoholic solvent consists of two stages. During the early stage of phase

transformation, small oligomers are continuously formed. At advanced stages, the

aggregation of the oligomers leads to crystalline wurtzite, the primary colloidal

particles. The primary particles then aggregate and form the secondary colloidal

particles. The growth of the colloidal particles is a stepped, discontinuous, process

which indicates that the predominant mechanism of aggregation is heterogeneous

coagulation. This mechanism of growth leads to a hierarchical structure.

1.5.2.5 Nucleation and growth

It has been reported by Spanhel [40] that at least four different “primary particles or

clusters” serving as initiators of the ZnO colloid growth, from ZAD as precursor, could

be identified. The nature of these “primary particles or clusters” depends strongly on the

synthesis conditions chosen viz. initial salt concentration, the temperature and time of

the thermal treatment, the nature of alcohol solvent as well as the humidity, storage and

analysis conditions. Among these primary particles, are Zn5(OH)8(Ac)2·2H2O and

Zn4O(Ac)6.In other respects, Meulenkamp [38], prepared ZnO nanoparticles by addition

of LiOH to an ethanolic zinc acetate solution. He showed that control of the particle size


15

was improved by the influence of temperature, water, and reaction products during the

aging of ZnO sols. Water and acetate accelerates the particle growth. Hu et. al. [41]

have synthesized ZnO nanoparticles by precipitation from zinc acetate in a series of n-

alkanols from ethanol to 1-hexanol as a function of temperature.

The kinetics of nucleation and growth are expected to be strongly dependent on the

properties of the solvent. For the shorter chain length alcohols, ethanol and 1-propanol,

nucleation and growth are retarded compared to longer chain length alcohols, from 1-

butanol to 1-hexanol, where nucleation and growth are fast. The particles size increases

with increasing temperature for all solvents and increases with alkanol chain length. In

fact, during the growth of colloidal ZnO nanoparticles, the alcohols not only provide the

medium for the reactions, but also act as ligands to help to control the morphology and

particle size of ZnO.

1.5.2.6 Film formation

In sol-gel method, ZnO films are prepared by dip- or spin-coating of substrate from sols

freshly prepared or aged at room temperature or around 60 ◦C. The heat treatment of the

deposited films is carried out in two steps.

For the first step, a pre-heat treatment (40–500◦C) is applied during a short time for

solvent evaporation and organic compounds removal [42-46]. The second step, a post-

heat treatment is employed in order to obtain a well-crystallized films and the final

decomposition of organic by-products varying from 250 to 900 ◦C, according to the

substrate nature.

1.6 Zinc Oxide one dimensional nanostructures

One-dimensional (1-D) ZnO nanowires/rods have been widely studied for its

technological applications. C.Y. Lu et. al.[47] have developed ZnO nanowire UV

sensor. Z.L.Wang et. al.[48] have developed nano power generators using ZnO

nanowires.


16

The methods for synthesizing ZnO nanorods are Chemical Vapour Deposition (CVD)

[49], metal organic CVD [50], thermal evaporation [51], the template method [52], and

electrochemical deposition [53]. However, these methods involve strictly controlled

synthesis environment, complicated procedures and expensive instruments.

In our work we have used two simple, cost effective and rapid synthesis methods for

fabricating ZnO nanorods:

(i) The hydrothermal method [54]

(ii) Thermal decomposition of Zinc acetate dihydrate in air [55].

1.6.1 Hydrothermal process

The first publication on hydrothermal synthesis of ceramics dated from the middle of

the 19th century [56]. At the beginning, the geologists tried to simulate in the laboratory

natural hydrothermal phenomena occurring in the Earth’s crust. The development of

pressure engineering vessel in the 19th century determined the enhancing of researches

in the field of hydrothermal synthesis in Germany, France, Italy and Switzerland. In the

20th century the main centres for studying and development of the hydrothermal

techniques were in USA, Russia and Japan. In the 20th century hydrothermal synthesis

was clearly identified as an important technology for material synthesis mainly for

single crystal growth [56]. Due to the severe conditions required for the single crystal

growth in the hydrothermal conditions (supercritical conditions), the commercialization

was very difficult. In the recent years the interest in commercialization of the

hydrothermal methods enhanced in part due to the possibilities to obtain a large family

of materials under mild conditions (temperature <350ºC, pressure < 100 MPa) [56].

Riman [56] defined hydrothermal synthesis as a process that utilizes single or

heterogeneous phase reactions at temperatures > 25ºC and elevated pressures > 100 kPa

to crystallize ceramic materials directly from solutions. The pressure is the 1vapor

pressure above the solution at the hydrothermal parameters namely temperature,

composition and concentration of the precursor solutions. Some additives are used:

mineralizing agent (organic / inorganic) to control pH and to promote solubility and

other agents (organic / inorganic) to control the morphology or to promote the particle

dispersion. A significant number of powders and films can be obtained in hydrothermal


17

conditions at temperatures in the range 25-200ºC and pressures <1.5 MPa. This

hydrothermal synthesis breakthrough made it more interesting for the industry.

Some advantages of the hydrothermal synthesis are presented below:

(i) Hydrothermal synthesis is an environmentally friendly procedure due to the fact that

it takes place at lower temperatures and pressures closely to the living conditions on

Earth. Other processes require higher temperatures and higher/lower pressures and

therefore they are considered environmentally stressed as shown in figure 1.5 [57]. Low

reactions temperatures avoid problems related to the volatilization of components and

stress induced defects;

(ii) The rate and uniformity of nucleation, growth and aging can be controlled;

(iii) Powders, fibers, single crystals, monolithic bodies, coatings on metals, polymers,

and ceramics can be prepared;

(iv) The costs for energy, instrumentation and precursors are lower. According to [57] a

large quantity of energy is necessary to create melt, vapor, gas, plasma comparing to the

formation of an aqueous solution at the same temperature. The time and energy

consuming is lower for the hydrothermal processes due to the fact that mixing and

milling steps are not necessary.

(v) Hydrothermal processes can be combined with electrochemical, mechanical,

microwave techniques. Hydrothermal synthesis is a soft solution processing (SSP) [57]

which allows in situ fabrication of shaped, sized, oriented ceramic materials without

firing, sintering or melting steps. The increasing interest in hydrothermal synthesis is

illustrated by the growing of number of scientific papers.


18

Fig.1.5 Pressure dependence on temperature for different processing routes [57].


19

In the last years hydrothermal methods were intensively studied and were applied to

obtain nanomaterials: powders, thin/thick films, nanorods, fibers, nanotubes. Some of

the recent examples are presented below: nanotubes arrays of barium titanate (BT) and

barium strontium titanate (BST) were synthesized under hydrothermal conditions taking

oxidized titania nanotubes as templates [58]; submicron, spherical Ba0.75Sr0.25TiO3

powders were prepared by microwave-hydrothermal route using potassium titanyl

oxalate, barium and strontium titanates as precursors; the mineralizing agent was

potassium hydroxide [59]; barium titanate powders were obtained by hydrothermal

route starting from barium hydroxide and anatase [60]; thin films based on BST were

grown on titanium electrodes in aqueous solutions by hydrothermal-electrochemical

method starting from barium and strontium hydroxides deposited on anodized titanium

[61]; thin or thick ferroelectric films (PZT for example) were obtained by

electrophoretic deposition using a trifunctional additive [62]; PZT thin films were

obtained by a hydrothermal method [63, 64]; Fe2O3 nanoparticles were synthesized in

hydrothermal conditions in aqueous organic microemulsion under mild conditions [65];

alkali-metal titanates [66] and single crystalline spinal cobalt ferrite nanorods [67] were

also synthesized in hydrothermal conditions; indium hydroxide nanocubes (around

70nm length) have been prepared by a hydrothermal synthesis [68]. In this thesis also

ZnO nanorods has been synthesized by this hydrothermal technique.

1.6.2 Chemistry of hydrothermal synthesis of ZnO nanorods

The growth of ZnO nanorods by hydrothermal process follows certain chemical

reactions that have been established [69]. Zinc nitrate hexahydrate salt provides Zn2+

ions required for building up ZnO nanowires. Water molecules in the solution provide

O2− ions. The hexamethylenetetramine (HMTA) during the ZnO nanowire growth, act

as a weak base, which would slowly hydrolyze in the water solution and gradually

produce OH−. This is important in the synthesis process because, if the

hexamethylenetetramine hydrolyzes very fast and produces a lot of OH− in a short

period of time, the Zn2+ ions in solution would precipitate out very quickly due to the

high pH, which would have little contribution to the ZnO nanowire oriented growth,


20

and eventually results in fast consumption of the nutrient and prohibits further growth

of ZnO nanowires.

(CH2)6N4 + 6H2O ↔ 4NH3 + 6HCHO (1.1)

NH3 + H2O ↔ NH3·H2O (1.2)

NH3·H2O ↔ NH4+ + OH− (1.3)

Zn2+ + 2OH− ↔ Zn(OH)2 (1.4)

Zn(OH)2 ↔ ZnO +H2O (1.5)

The growth process of ZnO nanowires can be controlled through the above listed

chemical reactions. All the five reactions are in equilibrium and can be controlled by

adjusting the reaction parameters, such as precursor concentration, growth temperature

and growth time, in order to push the reaction equilibrium forward or backward. In

general, precursor concentration determines the nanowire density. The growth time and

temperature control the ZnO nanowire morphology and aspect ratio.

1.6.3 Works on ZnO nanorods by hydrothermal process

Hydrothermal solution synthesis [70-71] and electrochemical deposition in porous

membranes were investigated to produce oriented ZnO nanorods and tubes respectively.

Although oriented nanowires and nanorods have attracted wide attention, the direct

fabrication of large arrays of complex nanostructures with controlled crystalline

morphology; orientation and surface architectures remains a significant challenge.

Zhengrong et. al.[72] reported a low-temperature, environmentally benign, solution

based approach for the preparation of complex and oriented ZnO nanostructures and the

systematic modification of their crystal morphology. Using controlled seeded growth

and citrate anions that selectivity adsorb on ZnO basal planes as the structures directing

agents, they prepared large arrays of oriented ZnO nanorods with controlled aspect

ratios, complex film morphologies made of oriented nanocolumns and nanoplates and

complex bilayers showing in situ column-to-rod morphological transitions.

Ming Wang et. al. [73] investigated the photoluminescence of ZnO nanorod arrays on

Si substrate. They found strong UV emission and a broad weak green emission. They

further investigated the origin of green emission by varying the post treated conditions.


21

Sea-Fue Wang et. al. [74] studied the effect of preparation conditions on the growth

rate, morphology and crystallinity of ZnO nanorod arrays on seeded substrates. In

particular they found that largest growth rate for [Zn(NO3)2]/[C6H12N4] ratio

=1.Doubling of growth rate and enhancement in crystallinity of ZnO nanorods with

mechanical stirring of the solution. They also found the linear increase of radial and

axial dimensions of ZnO nanorods with increase in reaction time up to 4 hours.

G. Kenanakis et. al. [75] prepared c-axis oriented ZnO nanorod arrays on seeded glass

and si substrates. They found preparation of thin ZnO seed layer is crucial for c-axis

orientation. They also found that aqueous solution growth on bare substrates leads to the

formation of flower like nanostructures, consisting of randomly oriented nanorods.The

grown nanorods were highly transparant in the visible region while the complex

nanostructures do not show high transmittance. N. Shakti et. al. [76] have prepared ZnO

nanorods by hydrothermal process and found ultra violet emission at room temperature,

the electrical property of nanorods showed rectifying behaviour.

1.6.4 Chemistry and growth mechanism of ZnO nanorods prepared by

thermal decomposition of Zinc acetate dihydrate

The thermal decomposition of zinc acetate dihydrate have been studied by Thermo

Gravimetric–Differential Scanning Calorimetry to understand its thermal stability and

decomposition temperature [55] .In the start of the process zinc acetate dehydrate

undergoes thermal dehydration and becomes anhydrous zinc acetate. Further

decomposition of anhydrous zinc acetate in the temperature region of 150–280◦C,

causes the formation of Zn4O(CH3CO2)6, which finally decomposes into ZnO.The

gaseous products generated during the thermal process, are water (H2O), carbon dioxide

(CO2), acetone ((CH3)2CO) and acetic acid (CH3COOH), respectively. These products

reached their highest concentration at about 270◦C, as the reaction equation indicates

(1.6)–(1.9). As the temperature increased, the ZnO nanowires were formed by the

following chemical reactions:

Zn(CH3COO)2.2H2O −heat→ Zn(CH3COO)2 + 2H2O ↑ (1.6)

4Zn(CH3COO)2 + 2H2O −heat→ Zn4O(CH3COO)6 + 2CH3COOH ↑ (1.7)


22

Zn4O(CH3COO)6 + 3H2O −heat→ 4ZnO + 6CH3COOH ↑ (1.8)

Zn4O(CH3COO)6 −heat→ 4ZnO + 3CH3COCH3 ↑ + 3CO2 ↑ (1.9)

Thus zinc acetate dihydrate thermal process can be considered a process of dehydration,

vaporization/decomposition, and ZnO formation. Therefore, in the ZnO nanowire

synthesis experiment, the temperature was set and maintained at 300 ◦C and for about 3

hours for a complete decomposition of zinc acetate dihydrate.

The growth of ZnO nanorods via thermal decomposition of zinc acetate dehydrate is

considered to follow Vapour-solid (VS) growth mechanism.

In the VS growth process, oxide vapor generated from the solid oxide material in a

high-temperature region is transported and directly deposited onto a substrate in a lower

temperature region. Surface defects or dislocations of the substrate provide favorable

nucleation sites for the oxide vapor. The vapor condenses on these sites, forming seeds

for a continuous deposition of oxide vapor.

In our work, we have synthesized ZnO film by sol-gel spin coating process [77] and

investigated quantum confinement effect in them. Li and Al doping of ZnO films were

performed and their structural, optical and electrical properties were studied.we have

also synthesized ZnO nanorods both by hydrothermal process and thermal

decomposition of Zinc acetate dehydrate. We have applied ZnO nanorods array

prepared by hydrothermal process as vibration sensor, utilizing its piezoelectric

property.


23

References

[1] E. Fortunato, A. Goncalves, A. Marques, A. Viana, H. Aguas, L. Pereira, I. Ferreira,

P. Vilarinho, R. Martins, Surf. Coat. Technol., 180 (2004) 20.

[2] P.Yu, Z.K. Tang, G.K.L.Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa,

J. Cryst. Growth, 184-185 (1998) 601.

[3] A. Tsukazaki, M. Kubota, A. Ohtomo, T. Onuma, K. Ohtani, H. Ohno, S.F.

Chichibu, M. Kawasaki, Jpn. J. Appl. Phys., 44 (2005) L643.

[4] B. J. Norris, J. Anderson, J. F. Wager, D. A. Keszler, J. Phys. D: Appl. Phys., 36

(2003) L105. [5] F.S. Hickernell, J. Appl. Phys., 44 (1973) 1061.

[6] Z. L. Wang, J. Song, Science, 312(5771) (2006) 242.

[7] Chennupati Jagadish & Stephen J. Pearton, Zinc oxide Bulk, Thin Films and

Nanostructures, China: Elsevier (2007).

[8] Lambert K. van Vugt PhD thesis 2007 Optical properties of semiconducting

nanowires www.phys.uu.nl/~vugt

[9] A.F.Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev.B, 61(2000)

15019.

[10] C.G. Van de Walle, Phys. Rev. Lett., 85(2000) 1012.

[11] X.S.Wang, Z.C.Wu, J.F. Webb, Z.G. Liu, Appl. Phys. A, 77(2003) 561.

[12] Chennupati Jagadish, & Stephen J. Pearton, Zinc oxide Bulk, Thin Films and

Nanostructures, China: Elsevier (2007). (Chapter 2).

[13] B. K. Meyer, H. Alves, D. M. Hofmann, W. Kriegseis, D. Forster,

F. Bertram, J. Christen, A. Hoffmann, M. Straßburg, M. Dworzak, U. Haboeck,

A. V. Rodina, Phys. Status Solidi B, 241 (2) (2004) 231.

[14] Klaus Ellmer, Andreas Klein, Bernd Rech, Transparent Conductive Zinc oxide,

Berlin: Springer (2008).

[15] A. Moustaghfir, E.Tomasella, S.Ben Amor, M. Jcquet, J. Cellier, T.Sauvage, Surf.

Coat. Technol., 174-175 (2003)193.

[16] K. Haga, M. Kamidaira, Y. Kashiwaba, T.Sekiguchi, H. Watanabe, J. Cryst.

Growth, 214 (2000) 77.


24

[17] K.L. Narasimhan, S.P. Pai, V.R. Palkar, R. Pinto, Thin Solid Films, 295 (1997)

104.

[18] Dinguha Bao, Haoshuang Gu, Anxiang Kuang, Thin Solid Films, 132 (1998) 37.

[19] Hongxia Li, Jiyang Wang, Hong Liu, Huaijin Zhang, Xia Li, J. Cryst. Growth , 275

(2005) e943.

[20] F.D.Paraguay, W.L. Estrada, D.R.N. Acosta, E. Andrade, M. Mikiyoshida, Thin

Solid Films, 350 (1999) 192.

[21] Lamia Znaidi, Mater. Sci. Engg. B., 174 (2010) 18.

[22] M.Z.-C. Hu, E.A. Payzant,C.H. Byers, J. Colloid Interface Sci., 222 (2000) 20.

[23] A.C. Pierre, Introduction to Sol–Gel Processing, Kluwer Academic Publishers,

Boston/Dordrecht/London (1998) 91.

[24] (a) J. Livage, D. Ganguli, Sol. Energy Mater. Sol. Cells , 68 (2001) 365.

(b) J. Livage, Curr. Opin. Solid State Mater. Sci., 2 (1997) 132.

[25] M. Guglielmi, G. Carturan, J. Non-Cryst. Solids, 100 (1988) 16.

[26] L. Armelao, M. Fabrizio, S. Gialanella, F. Zordan, Thin Solid Films , 394 (2001)

90.

[27] D.W. Bahnemann, C. Kormann, M.R. Hoffmann, J. Phys. Chem.,91 (1987) 3789.

[28] E. Hosono, S. Fujihara, T. Kimura, H. Imai, J. Sol–Gel Sci. Technol.,29 (2004) 71.

[29] D. Sun, M. Wong, L. Sun, Y. Li, N. Miyatake, H.J. Sue, J. Sol–Gel Sci. Technol.,

43, (2007) 237.

[30] Z. Hu, G. Oskam, P.C. Searson, J. Colloid Interface Sci.,263 (2003) 454.

[31] E.A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566.

[32] S. Fujihara, E. Hosono, T. Kimura, J. Sol–Gel Sci. Technol., 31 (2004) 165.

[33] H. Wang, C. Xie, D. Zeng, J. Cryst. Growth, 277 (2005) 372.

[34] S. Chakrabarti, D. Ganguli, S. Chaudhuri, Mater. Lett. 58 (2004) 3952.

[35] M. Ohyama, H. Kozuka, T. Yoko, S. Sakka, J. Ceram. Soc. Jpn. 104 (1996)

296.

[36] L. Znaidi, G.J.A.A. Soler Illia, S. Benyahia, C. Sanchez, A.V. Kanaev, Thin Solid

Films 428 (2003) 257.

[37] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826.

[38] E.A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566.


25

[39] M.S. Tokumoto, S.H. Pulcinelli, C.V. Santilli, A.F. Craievich, J. Non-Cryst. Solids

247 (1999) 176.

[40] L. Spanhel, J. Sol–Gel Sci. Technol. 39 (2006) 7.

[41] Z. Hu, G. Oskam, P.C. Searson, J. Colloid Interface Sci. 263 (2003) 454.

[42] T. Nagase, T. Ooie, J. Sakakibara, Thin Solid Films 357 (1999) 151.

[43] Y. Kokubun, H. Kimura, S. Nakagomi, Jpn. J. Appl. Phys. 42 (2003) L904.

[44] J.H. Lee, K.-H. Ko, B.-O. Park, J. Cryst. Growth 247 (2003) 119.

[45] R. Castanedo-Pérez, O. Jiménez-Sandoval, S. Jiménez-Sandoval, J. Márquez-

Marín, A. Mendoza-Galván, G. Torres-Delgado, A. Maldonado-Alvarez, J. Vac.

Sci. Technol. A 17 (1999) 1811.

[46] Y. Natsume, H. Sakata, Mater. Chem. Phys. 78 (2002) 170.

[47] C.Y. Lu, S.P. Chang, S.J.Chang,T.J.Hsueh, C.L.Hsu, Y.Z.Chiou, I.C.Chen,

Semicond. Sci.Technol. 24(7), (2009) 075005.

[48] Z.L.Wang, J.H.Song, Science 312, (2006) 242.

[49] J.J.Wu, S.C.Liu, J. Phys. Chem. B 106, (2002) 9546.

[50] J.Y.Park, D.J. Lee, Y.S.Yun, J.H.Moon, B.T.Lee, S.S.Kim, J. Cryst. Growth, 276,

(2005) 158.

[51] Y.S. Zhang, L.S. wang, X.H. Liu, Y.J.Yan, C.Q. Chen, J. Zhu, J. Phys. Chem.B

109, (2005) 13091 .

[52] J.S. Jie, G.Z. Wang, Q.T. Wang, Y.M. Chen, X.H. Han, X.P. wang, J.G. Hou, J.

Phys. Chem. B 108, (2004) 11976.

[53] M.J. Zheng, L.D. Zhang, G.H. Li, W.Z. Shen, Chem. Phys. Lett. 363, (2002) 123.

[54] H.Q. Le, S.J. Chua, K.P. Loh, E.A. Fitzgerald, Y.W. Koh, Nanotechnology 17,

(2006) 483.

[55] C. C. Lin, Y.Y. Li, Mat. Chem. Phys. 113, (2009) 334.

[56] R.E.Riman, Ann. Chim. Sci. Mat., 27(6) (2002)16.

[57] M.Yoshimura,W.Suchanek, Solid State Ionics, 98 (1997)197.

[58] Jianlig Zhao, Xiaohui Wang, Renzheng Chen and Longtu Li, Mater. Lett., 59 (18)

(2005) 2329.

[59] S.B.Deshpande, Y.B.Khollam, S.V.Bhoraskar, S.K.Date, S.R. Sainkar, H.S.Potdar,

Mater. Lett., 59 (2005) 293.

[60] Lai Qi, Burtrand I. Lee, Prerak Badheka, Dang-Hyok Yoon, William D.Samuels,


26

Gregory J.Exarhos, J.Eur. Ceram.Soc, 24 (2004) 3553.

[61] Seema Agarwal, G.L. Sharma, Sens. Actuators, B ,85 (2002) 205.

[62] Patent US 2001002681

[63] Patent EP 1020937

[64] Patent EP 1039559

[65] S.Giri, S.Samanta, S.Maji, S.Ganguli and A.Bhaumik, J.Magnetism and Magnetic

Mat., 285 (2004) 296.

[66] Dong-Seok Seo, Jong-Kook Lee, Hwan Kim, J.Cryst. Growth, 229 (1-4) (2001)

248.

[67] G.B.Ji, S.L.Tang, S.K.Ren, F.M.Zhang, B.X.Gu and Y.W.Du, J.Cryst. Growth, 270

(2004), 156.

[68] Hongliang Zhu, Yong Wang, Naiyan Wang, Ye Li, Jun Yang, Mater. Lett. , 58

(2004), 2631.

[69] Zhong Lin Wang, Nanogenerators for Self-powered Devices and Systems, Georgia

Tech., U.S.A., June (2011).

[70] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lingdquist, Chem. Mater 13 (2001)

4395.

[71] L.Vayssieres, K.A Keis, S.E Lingdquist, A. Hagfeldt, J.Phys. Chem. B 105

(2001) 3350.

[72] Zhengrong R. Tian, James A. Voigt, Jun Liu, Bonnie Mckenzie, Matthew J.

Mcdermott, Mark A. Rodriguez, Hiromi Konishi and Huifang Xu, Nat. Mater., 2

(2003) 821.

[73] Ming Wang, Chang-Hui Ye, Ye Zhang, Guo-Min Hua, Hui-Xin Wang, Ming-

Guang Kong, Li-De Zhang, J.Cryst. Growth, 291(2006) 334.

[74] Sea-Fue Wang, Tseung-Yuen Tseng, Yuh-Ruey Wang, Chun-Yun Wang, His-

Chuan Lu, Wen-Lin Shih, Int. J. Appl. Ceram. Technol., 5 (5) (2008) 419.

[75] G. Kenanakis, D. Vernardou, E. Koudoumas, N. Katsarakis, , J.Cryst. Growth,

311(23) (2009) 4799.

[76] Nanda Shakti, Sunita Kumari, P.S. Gupta, J. Ovon. Res., 7(3) (2011) 51.

[77] Nanda Shakti, P.S. Gupta, Appl. Phys. Res., 2(1) (2010)19.

Date post:	26-Mar-2018
Category:	Documents
Upload:	dangdiep
View:	221 times
Download:	6 times

Chapter 1 INTRODUCTION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/7914/6/06_chapter...

Documents