Chaper-3 : Synthesis Techniques and Morphological...

Chaper-3 : Synthesis Techniques and Morphological Studies

3.0 Introduction

The performance of materials depends on their properties. The properties in tern

depend on the atomic structure, composition, microstructure, defects and interfaces which

are controlled by thermodynamics and kinetics of the synthesis. A current paradigm of

synthesizing and processing of advanced materials emphasizes the tailored assembly of

atoms and particles from the atomic or molecular scale to the macroscopic scale. Intrinsic

and extrinsic bulk (microcrystalline) semiconductors are generally synthesized using high

temperature thermal diffusion, molecular deposition techniques like chemical vapour

deposition, atomic layer epitaxy, gas phase deposition techniques, vacuum evaporation

etc. Since the last three decades, researches have reported number of synthesis techniques

for the preparation of various nanocrystals [1-4]. Nanostructured materials can be made

by attrition of parent coarse- grained materials using the top-down approach from

microscale to the nanoscale, or conversely, by assembly of atoms or molecules using the

bottom–up approach. The control of arrangement of atoms from the nanoscale to the

macroscale is indeed the strength of materials chemistry. Chemical reactions for material

synthesis can be carried out in the solid, liquid, or gaseous states. The more conventional

solid-state synthesis approach is to bring the solid precursors (such as metal oxides or

carbonates) into close contact by grinding and mixing, and subsequently to the heat

treatment at high temperatures to facilitate diffusion of atoms or ions in the host material

by chemical reaction. The diffusion of atoms depends on the temperature of the reaction

and grain boundary contacts. The transport across grain boundary is also affected by

impurities and defects located there. The mixing and grinding steps are usually repeated

throughout the heat cycle, and generally involve a great deal of effort to mix materials at

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the nanoscale and also to prepare fresh surfaces for further reactions. For systems that do

not contain means to inhabit grain growth (such as grain growth inhibitors and

immiscible composites), grain growth at elevated temperature reaction leads to solids

with large grain size.

Compared to solid- state synthesis, diffusion of matter in the liquid or gas phase is

typically and advantageously many orders of magnitude larger than in the solid phase,

thus the synthesis of nanostructured materials can be achieved at lower temperatures.

Low reaction temperatures also discourage detrimental grain growth. Many materials can

be synthesized in aqueous or non aqueous solutions. But the wet-chemical synthesis of

extrinsic semiconductor nanomaterials faces new problems that are not encountered in

bulk materials. The synthesis parameter such as temperature, pH of the solution, reactants

concentration and reaction time should be ideally correlated with factors such as

supersaturation, nucleation and growth rates, surface energy and diffusion coefficients, in

order to ensure the reproducibility of reactions.

During the last few decades, various synthesis methods have been reported for

preparation of intrinsic semiconductor nanocrystals [5-6] and the methods of preparation

of doped nanoparticles are still evolving [7-8]. Generally, the doping of nanoparticles is

more difficult as compared to bulk crystals because at nanoscale quantum–confined

systems containing only few tenth of atoms or molecules [9]. At nanoscale doping is

carried out by low temperature processes, mostly by wet–chemical synthesis routes. The

doping through wet-chemistry invites some new difficulties, for example, dopant ions

used in the reaction may preferably precipitate as a separate stable phase prior to the

incorporation into the host lattice leading to very low or no doping at all. Further, even

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dopant ions incorporated into the lattice, may tend to diffuse onto the nanoparticle

surface to the surrounding matrix because the impurity ions are always only a few lattice-

constant away from the surface. Thus the preparation of effectively doped semiconductor

nanocrystals and their applications in the nanotechnology frontiers still remain a

challenging task. This chapter includes different methods of preparation of effectively

doped semiconductor nanocrystals. This chapter presents different methods of

preparation of doped ZnO nanoparticals, nanorods and nanobelts used in the present

investigations and various characterization techniques employed to study their chemical

and physical properties.

3.1 Synthetic Approaches for Nanomaterial Preparation

There are two general approaches for the synthesis of nanomaterials :

a) Top- down techniques

b) Bottom–up techniques.

Before discussing the synthesis processes adopted in the present investigations, a

brief review of some of the well known Top-down and Bottom-up methods of synthesis

are discussed in the following sections:

3.1.1 Top-down Method

Top-down approach involves the breaking down of the bulk material into nano-

sized structures or particles. Top-down synthesis techniques are extension of those that

have been used for producing micron sized particles. Top-down approaches are inherently

simpler and depend either on removal or division of bulk material or on miniaturization

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of bulk fabrication processes to produce the desired structure with appropriate properties.

The biggest problem with the top-down approach is the imperfection of surface structure.

For example, nanowires made by lithography are not smooth and may contain a lot of

impurities and structural defects on its surface. Examples of such techniques are high-energy

wet ball milling, electron beam lithography, atomic force manipulation, gas-phase

condensation, aerosol spray, etc.

Fig. 3.1 Block diagram representing various nanomaterial synthesis strategies

Synthetic Approach for Nanomaterials

Top-Down Method

Bottom-Up Method

1. E- Beam Lithography.2. Atomic Force Manipulation.

3. Aerosol Spray. 4. Gas Phase Condensation etc.

1. Reverse-Micelle Route.2. Sol-Gel Synthesis.3. Collodial precipitation.4. Hydrothermal synthesis.5. Electron Deposition.6. Template Assisted Sol-gel.7. Organo Metallic Reaction Route etc.

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Figure: 3.2 Schematic representation of ‘bottom-up’ and top-down’ synthesis processes

3.1.2 Bottom-up Method

The alternative approach, which has the potential of creating less waste and

hence the more economical, is the ‘bottom- up’. Bottom-up approach refers to the build

up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by-

cluster. Many of these techniques are still under development or are just beginning to be

used for commercial production of nanopowders. Oraganometallic chemical route,

revere-micelle route, sol-gel synthesis, colloidal precipitation, hydrothermal synthesis,

template assisted sol-gel, electrodeposition etc, are some of the well- known bottom–up

techniques reported for the preparation of luminescent nanoparticals. Norris et. al [10]

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used organometallic reaction route for the synthesis of Mn doped ZnO nanoparticles.

Huang et.al [11] reported the synthesis and optical properties of one dimensional ZnO

nanostructures. Yang et.al [12] made ZnO nanowire arrays with ultraviolet laser. Various

other researchers [13-16] reported on single-crystal nanobelts, nanorings and bicrystalline

ZnO nanowires. Doping in semiconductor with selective elements offers an effective

approach for the electrical, optical and magnetic properties, which is crucial for practical

applications [16-17]. The nanoparticles preparation and their doping are carried out by

co-precipitation reaction of organometallic reagents at relatively high temperature

(3000C) under controlled environmental conditions. This route has the advantage of

making nanoparticles with better crystallites; however, the laborious process, high cost,

rare availability and often toxic nature of the reagents etc. make this method largely not

acceptable. Pileni et.al [18-19] used reverse-micelle route for nanoparticles synthesis.

During this process the particles are precipitated within a size restricted water pool of the

water–in–oil ternary micelle system (Water/AOT/ Heptanes; AOT=bis (2-ethyhexyl)

sodium sulfosuccinate). The size of the water pool is controlled by concentration ratio

W= [H2O] /[AOT]. This method has been widely used for the synthesis of monodispersed

particles of ZnO:Mn, ZnS:Mn, etc. In the case of doped nanoparticles, this method has

some difficulties due to the unavoidable use of high concentrations of the surfactant

medium, which hinders the doping as well as particle separation process. The sol-gel

synthesis is another important method used for making nanophosphors. Preparation of

Y2O3:Eu3+,Mn2+, activated silica xerogels, etc., are some of the recent reports based on

sol-gel synthesis[20-21] .

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3.2 Synthesis of ZnO Nanophosphors

ZnO is considered as an important nanomaterial for optical and electronic devices

especially for low voltage and short wavelength electro-optics devices such as light

emitting diodes and laser diodes. ZnO nanomaterials have unique feature due to versatile

optical properties like wide bandgap, high exciton binding energy (60 meV) and less

power needed for threshold pumping in lasers [21-22] etc. Congkang et.al [23] have

reported the synthesis of ZnO nanobelts via a vapor phase transport of a powder

mixture of Zn, BiI3 and MnCl2.H2O at room temperature. These belts have widths of

40-150 nm and length of tens of microns [23]. They had also reported that

photoluminescence spectra peak has a blue shift of 18 meV as compared to

photolumniscence spectra peak of Bi doped ZnO nanowires at 10K. Different

nanostructures of ZnO including nanowires, nanobelts, nanobridges, nanonails and

nanoribbons have been synthesized by thermal evaporation of oxide powder [24-25]. Gao

et.al [26] reported the synthesis ZnO nanobelts by thermal evaporation, and Mn2+ion

implantation with 30 keV on the nanobelts [26]. Transmission electron microscope and

photoluminescence investigations show highly defective material directly after the

implantation process. Manoj et.al [27] reported the effect of Mg and Cd doping on the

band-gap of ZnO. The fabrication of doped nanostructures was carried out via solution

route. Many other researches [28-29] have opted various bottom-up and top-down

techniques to construct the different semiconductor nanostructures. As nanobelts of ZnO

have a rectangular cross-section with well defined geometry, the nanobelts may be ideal

systems for dimensionally confined transport phenomenon and fabrication of functional

nanodevices based on individual nanobelt [30]. Qiongrong et.al [31] reported the

synthesis of ZnO nanophosphors with diameter of 7-50 nm under oxygen atmosphere.

ZnO nanobelts have been synthesized by various researchers at very high temperature

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13500C by thermal evaporation of ZnO powders [32-33]. However in present

investigation very simple and low temperature fabrication method for long length

nanobelts, nanorods and nanoparticles of intrinsic and extrinsic ZnO have been utilized.

Recently large number of studies on ZnO nanocrystals are going on because of their

various applications in different fields such as opto-electronic industry, field emission

displays, field effect transistors, plasma display panels, fluorescent lamps etc. Intrinsic

and extrinsic ZnO nanostructures seem to be good candidates for next era smart

applications like gas sensors, noble composite sensors for pH of the solution and

humidity measurement, plasma resonance bio-sensors and nanophosphors for field

emitting devices. ZnO act as a piezoelectric device because of asymmetries in its

crystalline structure. In particular, with a single crystal ZnO, the oxygen end of its

molecule always stays attached to the substrate, while the zinc end always stick up

perpendicular to the surface. Put under stress, piezoelectric materials produce an electric

current or respond to electric field by changing shape. This allows the ZnO nanoring to

function as a small scale pressure and force sensors. The other important application of

ZnO nanoparticle could be injectable pressure sensor that could monitor blood pressure at

the specific sights in real time. The nanoring could also serve as tiny fluid pumps for lab-

on-a-chip-systems. The ZnO nanoscale piezoelectric rings can be employed as bio-

sensors for blood pressure in MEMS through some suitable fabrication techniques. ZnO

nanostructures such as a nanobelts and nanoparticle grown by chemical precipitation

technique can act as a gas sensor. The particle size and order of crystallinity of ZnO

nanostructures can be studied by X-ray diffraction techniques where, the pore size can be

obtained from SEM studies. From SEM investigation it can be easily concluded that there

is formation of clusters of particles through the surface of nanobelts. The pores play an

important role in gas adsorption and hence influence the sensitivity of the nano structure.

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The gas adsorption takes place leading to change in its resistance. Resistance of ZnO

nanostructure can be calculated in the presence of test gas at different temperature and

concentration of the test gas. Moreover grain size also plays an important role to

determine the gas sensitivity. A well-known fact is that with decreasing grain size, the

gas sensitivity decreases, the specific chemical, surface and nanostructures properties of

ZnO make it potential candidates especially for catalysis and gas sensing applications,

where the exposing surface of the particle to target gas is very important [34].

ZnO nanomaterials are very well known active medium for laser oscillations as it

requires low threshold power for optical pumping [35-36]. Whenever a semiconductor is

irradiated with suitable radiation, there are trapping levels at different depth within the

forbidden gap of the material leading to transition from the various traps. Selective

excitation of the levels can make the ZnO as the best suitable laser medium. Several

workers have observed random laser action with coherent feedback in semiconductor

powders [37-38]. When the scattering mean free path becomes equal to or less than the

wavelength, light may return to scatter from which it was scattered, and thereby forming

closed loop path. If amplification in the loop exceeds the loss, laser oscillation could

occur which serves as a laser resonator. Scattering merely increases path length of light in

the gain region, but can not provide coherent feedback which is essential for laser action.

Microcrystalline grain boundaries can provide optical feedback to promote amplified

spontaneous emission [37-39].

Due to large number of potential applications of ZnO nanostructures, during these

days various researchers [40-46] have been working on synthesis and characterization of

different nanostructures of pure and doped ZnO.

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Present chapter deals with fabrication and morphological characterization long length

nanobelts, nanorods and nanoparticles of intrinsic and extrinsic ZnO synthesized using

wet chemical precipitation method.

3.3 Synthesis Techinque of ZnO Nanostructures

All the nanophosphor samples reported in the present thesis have been prepared

by colloidal precipitation method at room temperature [47]. As compared to all the other

referred methods [45-47] the colloidal chemical precipitation technique has been found to

have a number of advantages including easy process ability at ambient conditions,

possibility of doping of different kinds of impurities even at room temperature, good

control over the chemistry of co-doping particularly when different impurities are

incorporated simultaneously in the host lattice, easiness of surface capping with a variety

of reagents (organic as well as inorganic) etc. The details of different steps involved in

the synthesis process of nanophosphors of different morphologies (nanobelts, nanorods,

nanocrystals) are given below. General schematic representation of various ZnO

nanostructures synthesis has been shown in Fig. 3.3.

3.3.1 Synthesis of ZnO Nanobelts

Synthesis of ZnO long length nanobelts was carried out using chemical like zinc

acetate, absolute ethanol and lithium hydroxide. Different analytical grade chemicals

have been purchased from M/s. Sd-fine chemicals Ltd, India. Alcohols are commonly

used because the solvent act as reagent. However the solvent does not participate in the

reaction forming ZnO from 0.1M zinc acetate. Zn2+ precursor dissolved in absolute

ethanol was refluxed for 3 hours under constant magnetic stirring at 800C. Further two

roots were opted for obtaining nanobelts from precursor which are as follows:

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1. In the first method, the precursor obtained was mixed with 0.1M LiOH prepared in

100ml triply deionized water. Precipitates were formed immediately and separated out

using centrifugal machine (3000 rpm) at room temperature. Finally, Precipitates were

dried in vacuum oven at 800C for 12 hours.

2. In the second method, 0.14M LiOH prepared in 100ml triply deionized water was

added drop by drop with constant speed of one drop per sec to the refluxed Zn2+ .

Immediately precipitates were formed, which were kept at 40C for few hours to stabilize

the growth process and then the precipitates, were separated out using centrifugal

machine (3000 rpm) at -100C. Precipitates were then dried in vacuum oven at 800C for 12

hours.

The difference in first and second method is that growth of nanobelts is controlled

with the increasing molar concentration of LiOH from 0.1M to 0.14M. By doing so the

near neutral clusters are formed and the pH value of solution comes to be nearly 8.

3.3.2 Synthesis of ZnO Nanorods

The precursor used for the synthesis of ZnO nanorods was zinc acetate. In the

typical synthesis process Zn2+ precursor is dissolved in 100 ml of absolute ethanol and

refluxed for 1 hour at 800C. 2ml of 0.07M triethanolamine (TEA) was added to the Zn2+

precursor. In another beaker 0.14M of LiOH is prepared in 100 ml of deionised water,

then later solution was added in the pervious solution drop by drop with constant speed of

one drop per second. Soon after the addition of LiOH, white colour precipitates of ZnO

were produced. The precipitates were centrifuged at 5000rpm.

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The precipitates were washed several times with deionised water to remove any

by product and TEA. Then the washed product was dried in vacuum oven at 800C for 12

hour. The final product was kept in vacuum desicator till further characterization.

When the product was centrifuged at room temperature then the temperature of product

increase which destroyed the basic structure of nanorods, which means not well defined

nanorods were observed. On the other hand when the product was centrifuged at low

temperature -100C then very well defined nanorods was observed as shown in Fig 3.10

TEA played crucial part in the formation of nanorods. TEA diffused layer into the ZnO

crystal and produced an intermediate compound, without the intermediate compound

there would be no nanorods. The intermediate compound was crystal zinc acetate covered

by TEA.

3.3.3 Preparation of ZnO Nanocrystals

Different synthesis methods viz. reverse micelle, homogeneous precipitation and

colloidal precipitation, etc. have been carried- out to prepare the doped nanocrystalline

ZnO phosphors. By comparing properties of the materials obtained from different routes,

colloidial precipitation was found better for producing efficiently luminescent

nanophosphors in terms of process simplicity, effectiveness of doping, and higher yield,

etc. Synthesis of ZnO nanocrystals was carried out using AR grade chemicals like zinc

acetate, absolute ethanol and lithium hydroxide. All were purchased from M/s Sd-fine

chemicals Ltd., India. Alcohol was commonly used because the solvent acts as reagent.

However, the solvent does not participates in the reaction forming ZnO from zinc

acetate .This is initially based on the method given by Spanhel et.al [40]. 21.95gm zinc

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acetate is dissolved in 1000ml of absolute ethanol to get 0.1M solution. Also in another

beaker 5.864 gm of LiOH was dissolved in 1000 ml of deionized water. In typical

synthesis method 250 ml 0.1M zinc acetate solution was refluxed under distillation and

magnetic stirring for 3 hour at 800C. The above prepared solutions were mixed under

stochiometric concentration with and without addition of capping agent. In the first

method condensate was separated out. To the remaining hygroscopic product 100 ml of

0.14 LiOH solution was mixed drop wise under vigorous stirring which causes the

formation of ZnO.

Precipitates were separated out using centrifugal machine (7000 rpm) at room

temperature. The sample was then dried in vacuum oven at 800C for 12 hour. In the

second method similarly above mentioned synthesis process was performed except the

addition of 25 ml of 1% solution of polyvinyalpyrrolidon (PVP), the capping agent was

added to avoid agglomeration of grown nanoparticles. Condensate was separated out. To

the remaining hygroscopic product 100 ml of 0.14M LiOH solution was mixed drop wise

with vigorous stirring. Precipitates were separated out using centrifugal machine at (7000

rpm) at low temperature [-100C]. The sample is then dried in vacuum oven at 800C for 12

hours. This process is shown below

Zn(CH3COO)2 .2H2O + CH3CH2OH Nanocrystals of ZnO

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1. Refluxing with distillation 3hr.

2. PVP

3.Centrifugation and Vacuum Drying


Fig 3.3 Block diagram for synthesis of various nanostructures

3.4 XRD Studies of ZnO Phosphors

3.4.1 XRD Study of ZnO Nanobelts and Nanorods

X-Ray diffraction (XRD) data for structural characterization of various prepared

samples of ZnO were collected on the X-ray diffractometer (PW1710) using Cu-Kα

(λ =1.54A0). X-ray diffraction pattern of various synthesized phosphors are helpful in

studying the crystalline structure and phase priority as well as crystallite size for

nanophosphros. The room temperature XRD patterns of synthesized ZnO phosphors

followed by various synthesis routes are shown below:

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Fig. 3.4 XRD pattern for ZnO nanobelts

ZnO normally forms the hexagonal (Wurtzite) structure with lattice constant

(a=b=0.32 nm and c= 0.52 nm). Each Zn atom is tetrahedraly co-ordinated to four O

atoms, where the Zn d- electron hybridized with O p- electrons : layers occupied by Zn

atoms alternate with layer occupied by O atoms [48-49]. The structural characterization

of the sample was carried out by powder X-ray diffraction method performed on a Philips

X’ Pert pro system by using Cu-Kα radiations (λ =1.54 Å).

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(100

)(0

02)

(101

)

(102

) (110

)

(103

)

(112

)(2

01)

(200

)


Table 3.1 Bragg’s peaks for hexagonal ZnO nanophosphor (nanobelt) obtained in

recorded XRD pattern :

Plane Angle(2θ) [ degree] d-value [A0 ]100 31.725 2.8182002 34.340 2.6093101 36.200 2.4794102 47.495 1.9128110 56.570 1.6256103 62.820 1.4781200 66.360 1.4075112 67.910 1.3791201 69.025 1.3595

80

Inte

nsity

(ar

b. u

nits

)


Fig. 3.5 XRD pattern for ZnO nanorods

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2θ in degree


Table 3.2 Bragg’s peak of hexagonal ZnO nanorods in XRD

Plane Angle2θ[in degree] d-value [A0]100 31.520 2.7182002 34.240 2.5993101 35.980 2.4394102 47.290 1.8124110 56.570 1.6956103 62.720 1.5782200 65.450 1.4975112 67.230 1.3590201 69.025 1.3395

The XRD results showed that the as prepared product has a single hexagonal

phase . The high c-axis orientation of crystalline structure along (101) plane has been

observed. A high degree of crystal orientation reduces the probability of the scattering of

the carriers at the grain boundary.

The recorded XRD pattern confirmed that nanobelts are high crystalline in

nature. The corresponding X-ray diffraction peak for (100), (002) and (101) planes

confirm the formation of wurtzite structure of ZnO. XRD pattern for ZnO nanorods is

almost similar to the nanobelts. Compared the XRD patterns recorted for ZnO nanobelts

and nanorods as shown in Figs. 3.4 and 3.5 show that both the morphologies of ZnO

nanostructures have almost similar diffraction pattern.

The average crystallite size has been calculated from the recorded XRD pattern

using well known Scherrer equation[50 ].

D = 0.9 λ/ β λ cosθ (3.1)

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λ is the X-ray wavelength used, β full width of diffraction peak at half maximum

intensity and θ is Bragg’s angle in degree. The average crystallite size comes out to be 30

nm for nanobelts.

3.4.2 XRD study of ZnO nanocrystals

X-ray diffraction peak profiles of ZnO nanopowder capped with PVP and without

PVP are shown in Fig 3.6 and 3.7. The diffraction peaks were recorded in the 2θ range

from 300 to 700 Cu-Kα (λ =1.54A0 ) radiation with a graphite monochromator in the

diffracted beam. Comparison of the recorded XRD patterns with standard JCPDS data

base file 36-1451 shows that synthesized ZnO crystallite is Wurtzite in phase.

Crystallite size was estimated by using Scherrer`s equation [50].

The crystallite size was ~ 50 nm for the sample synthesized without PVP where

as crystallite size ~ 10 nm for PVP modified has been observed. In case of PVP capped

sample broadening of diffraction peaks takes place due to lesser no planes at the surface.

The intensity of peak (002) is high as compared to other peaks showed that more oxygen

atom has entered into the ZnO film lattice.

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Fig. 3.6 XRD pattern of without PVP capped ZnO nanocrystals

Table 3.3 Bragg's Peak of hexagonal ZnO nanocrystals obtained in XRD pattern :

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Plane Angle(2θ) [in degree] d-value[A0 ]

100 31.485 2.8391

002 34.170 2.6219

101 36.000 2.4927

102 47.245 1.9223

110 56.280 1.6333

103 62.665 1.4813

100 66.170 1.4323

112 67.675 1.3833

201 68.800 1.3634

Fig 3.7 XRD pattern of PVP capped ZnO nanocrystals

Table 3.4 Bragg’s Peaks to hexagonal ZnO nanocrystals obtained :

Plane Angle(2θ) [in degree] d-value [A0 ]

100 32.725 2.6184

002 35.440 2.4073

101 36.200 2.1794

102 47.495 1.7124

85

2θ in degree

Inte

nsity

(arb

. uni

ts)


110 56.820 1.4254

3.5 Particle Size Measurement for ZnO Nanophosphors

If average crystallite size of ZnO nanoparticle is equivalent to twice of exciton

Bohr radius , quantum confinement effect is expected to occur within these nanocrystals,

exhibiting an enlargement of the optical bandgap. Increase in energy bandgap was

calculated using effective mass model. The formula used to calculate band-gap of ZnO

nano crystal is given below:

2 2 2

2

1.82 4g

o r

eE Em R Rπ

µ π ε= + −h

(3.2)

Where E is band gap of nanocrystal, Eg is the bandgap of bulk material. First

term in the relation corresponds to band gap (Eg = 3.37eV) of bulk crystal, second term is

due to quantum confinement effects and third term is related with coulomb interaction. R

is the radius of the nanoparticles which can be obtained from the XRD pattern or TEM

studies. mo is the rest mass of electron and µ is the reduced mass of the electron hole

pair. Bandgap of ZnO nanocrystal equal to 3.4 eV is calculated using effective mass

model (Eqn.3.2).

3.6 Morphological Characterization

3.6.1 Characterization of ZnO nanostructures by Scanning electron microscopic

[SEM]

Scanning electron microscope (SEM) images of ZnO nanobelts were obtained

using JSM-6100 type microscope. It is very much clear from SEM images of synthesised

samples depend on the reaction conditions. Figs 3.8 (a & b) and 3.9 (a,b,c& d) shows

nanobelts of deformed morphologies as well as defined morphology corresponding to

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synthesis methods 1 and 2, details of which have been already described in sections 3.3.1

and 3.3.2. The nanobelts obtained are of ~ 2mm in length. Refluxing of precursor

contains zinc acetates and ethanol for long time results in longer nanobelts of ZnO.

Addition of a catalyst stops isotropic agglomeration of the particle, While on the other

hand anisotropic agglomeration occur resulting in nanowire or nanobelts. The

corresponding XRD pattern also shows enhancement of (002) peak intensity, indicating

preferential growth of nanobelts along c-axis direction. SEM image at 500X

magnification indicates homogenous growth of nanobelts over large surface area as

shown in Fig 3.9. SEM image of ZnO exhibits smoother topography and long length

morphology. Scanning electron microscopy has been used to reveal the morphological

details of fabricated ZnO. The recorded micrographs revealed the uniform, very fine

structure of nanobelts which helps in trapping more light to improve solar cell efficiency

Fig 3.8 (a&b) shows the deformed morphology of the nanobelts on the other hand

Fig.3.9 (a,b,c&d) shows the well defined morphology of the nanobelts.

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Fig. 3.8 (a & b) SEM images of ZnO nanobelts by method 3.3.1 (Ist method)

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Fig. 3.9 (a,b,c & d) SEM images of ZnO nanobelts by method 3.3.1 (2nd method)

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3.6.2 Characterization Using TEM

The general TEM images of the synthesized product are as shown in Fig.3.10

(a&b), recorded TEM image shows the rods are typically 10 nm thick and 100 nm in

diameter. The morphology of ZnO nanorods can be described by considering the

hexagonal close packing of oxygen and zinc atoms in the unit cell. Essentially, in ZnO

the occupancy of four of the eight tetrahedral sites of hexagonal close packed oxygen

arrays controls the morphology. Therefore the availability of Zn2+ and O2- ions in the

solution controls the growth process as well as morphology. In the present work

optimization of TEA concentration in the Zn2+ solution have essentially controlled the

availability of metal ions in the solution so as to obtained robust pyramidal nanorods.

Fig.3.11(a&b) shows the TEM images recorded for synthesized ZnO

nanocrystals which shows the average particle size to be 50 nm. Fig 3.12 shows the

spherical particles having average particle size 10 nm. Nanaocrystals of ZnO were

characterized using transmission electron microscopy (TEM). TEM images of the

samples were obtained using JEOL-JEM 2000 Ex-type microscope. TEM images of

synthesized ZnO followed by various synthesized routes are shown in Figs 3.10, 3.11 and

3.12. The practicle size calculated using Scherer formula from XRD pattern matches with

particle size calculated using TEM images.

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(a) (b)

(a) (b)


Fig. 3.10 (a & b) TEM images of ZnO nanorods

Fig. 3.11 (a &b) TEM images of ZnO nanocrystals without PVP modified sample

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Fig. 3.12 TEM image of ZnO nanocrystals with PVP modified sample

3.7 Results and Discussion

The morphological characterizations of the prepared ZnO nanobelts were

examined by XRD, TEM and SEM studies. High purity nanobelts of ZnO having length

in the range of few millimeters have been synthesized in the laboratory. SEM

investigations show beautiful results of ZnO nanobelts indicating the ribbon-like structure

having rectangular cross–section of these synthesized nanobelts. To the best of

knowledge, the long length nanobelts mentioned in the literature are of 0.7 nm length

[38]. ZnO nanobelt shown in Fig.3.9 is of 2.0 mm length, which is four times longer than

that reported in literature. Refluxing of precursor containing zinc acetate and ethanol for

long time results in longer nanobelts of ZnO. Addition of a catalyst stops isotropic

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agglomeration of the particle instead anisotropic agglomeration occurs resulting in

nanowires or nanobelts [38]. Typical XRD pattern of ZnO nanobelts is shown in Fig. 3.4.

Lattice constant (a=b=0.32nm and c=052nm) and diffraction peaks corresponding to the

planes<100>,<002> and <101> obtained from X-Ray diffraction data confirm the

hexagon wurtzite crystal structure of zinc oxide (consistent with JCPDS data File )

nanobelts.

The crystal growth of ZnO nanostructure includes two steps: (i) nucleation

(vaporization) and (ii) growth (condensation). The shape of nanostructure (such as

rectangular cross-section, flower like, tubes etc.) is determined mainly by the synthesis

route followed, which again depends upon potential for hydrogen (pH) value of the

reactant products. After the nucleation process, pH values of a reaction determine the

number of active sites. The second step involved in the formation of nanostructure is

termed as growth which further depends on the number of active sites, more is the

number of available active sites, much larger will be the growth of nanostructure. The

hydroxide ions in an alkaline medium will be more if the pH value is greater than 7.0, the

quantities Zn(OH2) and the growth unit ([Zn(OH4)]2-, contribute respectively to

nucleation and growth of ZnO. It is well known that the pH value comes to be 12.0 when

NaOH is used as mineralize resulting in a larger quantity of zinc hydroxide and a smaller

quantity of growth unit ([Zn(OH)4]2- . Thus, during this process the quantity of

corresponding ZnO nuclei is larger but there is not enough growth unit [Zn (OH)4]2- to

grow ZnO nuclei. Moreover, there are no active sites around the circumference of ZnO

nuclei due to the limitations of increasing temperature of the reaction. Therefore, due to

anisotropic growth habit of ZnO tip like particles of ZnO will be obtained at low

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temperature. However if LiOH is used instead of NaOH as a mineralize and when the pH

value of reaction is set to be 8.0 the following reaction will takes place :

Zn(OH)2 + 2OH- [Zn(OH)4 ]2-

This indicates that Zn(OH)2 not only contributes to ZnO nuclei as mentioned

above, but also transform to [Zn(OH)4 ]2- as to increase the growth of ZnO. Furthermore,

in the reaction of high temperature, active sites can generate around ZnO nuclei, so that

ZnO can preferentially be grown on the active sites. Refluxing of the ethanol, stops

isotropic agglomeration, instead anisotropic agglomeration takes place, which results in

the formation of quasi one dimensional nanobelts having rectangular cross section.

However, in this case, the number of growth units [Zn(OH)4 ]2- is still not enough to

make ZnO nanorods which are grown from the circumference of ZnO particles. But, the

flower-like ZnO nanostructure can be obtained if the pH value of the solution is increased

to 13.5,which is due to reason that a smaller quantity of Zn(OH)2 and a larger quantity

of growth unit [Zn(OH)4 ]2- are obtained. The more schematic and elaborative diagram

involving the mechanism process for synthesis of various nanostructures is as shown

below in Fig. 3.13.

The Zn ions on the top of molecule is positively charged while the bottom oxygen

ions is negatively charged, the resultant is electrostatic polarization along its surface

causing a perpendicular dipole-moment here. The electric field of the dipole creates the

dipole-moment, but when neutralized by spiraling the material achieves a lower energy

state. Using such individual nanosprings, nanosized biosensors can be made.

Piezoeleectric semiconductors are natural resonators when stimulated physically: a

piezoelectric material will naturally oscillate at a known frequency.

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Fig. 3.13 Formation of ZnO nanostructures

Therefore its surface is treated to attract, for example, a protein from a cancer cell,

and then even a single molecule of that protein could be detected with one of its

nanosprings. If one can place a single molecule on the surface , change in its resonance

frequency can be detected and by determining the frequency , one can tell what molecule

is placed on the surface. With this technique cancer cell can be detected. Due to

compatibility of ZnO with human body it is best suited material for cancer cell detection.

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Micro-Electro-Mechanical System (MEMS) is the integration of mechanical components,

sensors, actuators and electronic components on single chip. The ZnO can be employed

as bio-sensors in MEMS by some suitable fabrication technique.

ZnO nanomaterials are very attractive for field emission display (FEDs) which

operate with low voltage and high current density. Advantage includes high resolution

screens, enhanced lifetime at high current density and excellent thermal and mechanical

stabilities. Many researches [5-8] have studied luminescence of nanophosphors in an

attempt to improve the properties through increasing light scattering centers using surface

modification, reducing defects density by post annealing at high temperature and

identifying new phosphors material. The study of nanophosphors is useful not only for

achieving high luminescence performance, but also in understanding the fundamental

properties of the nanomaterials.

3.7.1 ZnO Nanocrystals

Different sized nanocrystals of ZnO phosphors have been synthesized using

different techniques. Their optical and morphologically studies have been done by using

optical absorptions spectroscopy (UV-Visible), X-Ray diffraction analysis (XRD image)

scanning electron microscopic (SEM) and transmission electron microscopic (TEM)

image. XRD pattern confirms hexagon, wurrtzile crystal structure of the ZnO. Lattice

constant and diffraction peaks obtained from X-ray diffraction data are consistent with

JCPDS data file no. (36-1451). The scanning electron microscopic image of ZnO tells

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that the formation of nanobelts of ~ 2mm in length by changing the concentration of

LiOH as shown in Fig 3.9. The transmission electon microscopic image of nanorodes,

nanoparticles and nanobelts. The aspect ratio for the nanorods in the Fig. 3.10 (a) is of 10

and in the Fig. 3.10 (b) it is 15 , shows a drastic transmission of nanoparticles into

nanorods of average diameter 10 nm and length 100 nm. This is due to Zn2+ have led to

fast anisotropic growth of ZnO along the c-axis the [002] direction.

The TEM images in the Figs.3.11(a& b) and 3.12 shows that the particle size of

50 nm and 10 nm are obtained, This difference in particle size is due to the difference of

the capping agent which help in formation of a small size particle spherical in shape. The

XRD pattern for the ZnO nanoparticle and one dimension nanorods are shown in Fig

3.5,3.6, and3.7, the X-ray diffraction peak for a well crystalline hexagonal Wurtzite ZnO

structure. In comparison the diffraction peaks for the nanoparticles are broadened than

those of nanorods. In addition nanorods spectrum shows a sharp diffraction peaks. This is

constituent with one dimension nanorods formation along c-axis [52-53]. The TEM

image tells that the particle size for the ZnO nanocrystal is of 10 nm and was confirmed

by Scherer formula (~ 10 nm) using XRD data of ZnO nanocrystals.

3.8 Conclusions

High purity nanobelts , nanorods and nanoparticles of ZnO have been synthesized

in the laboratory by controlling the reaction time and the concentration of LiOH type

applied method is simple and reproductive for variety of nanostructure on a potentially

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large synthetic scale. The nanoparticles are driven to be arranged in linear fashion to form

one dimension naorods. Structural analysis shows that typical diffraction peaks for

nanoparticles, naorods and nanobelts of well crystalline hexagonal ( Wurtzite ) structure

of ZnO. The nanostructure of ZnO synthesized using various techniques are characterized

using XRD, SEM, TEM. Scanning electron microscope and transmission electron

microscope image of ZnO indicate the size of various ZnO crystals. The size obtained

from TEM image is agreement well with Scherrer formula.

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