CHAPTER- 3

SYNTHESIS AND CHARACTERIZATION OF PURE AND TIN-DOPED ZnO NANOCRYSTALLINE

(BULK) MATERIALS 3.0 INTRODUCTION

Recently, zinc oxide (ZnO) has attracted much attention within the scientific

community as a ‘future material’. This is however, somewhat of a misnomer, as ZnO has

been widely studied since 1935, with much of our current industry and day-to-day lives

critically reliant upon this compound. The renewed interest in this material has arisen out

of the development of growth technologies for the fabrication of high quality single

crystals and epitaxial thin films/layers, allowing for the realization of ZnO-based

electronic and optoelectronic devices [1]. ZnO, a II-VI semiconductor with a direct wide

band gap of 3.35 eV at room temperature and large exciton binding energy of 60 meV

[2], is one of the most promising materials for the fabrication of optoelectronics devices

[3] operating in the blue and ultraviolet (UV) regions and gas sensing applications. It has

a wide range of technological applications including transparent conducting electrodes

for solar cells, flat panel displays, surface acoustic devices, chemical and biological

sensors and UV lasers. Controlled synthesis of semiconductor nanostructures in terms of

size and shape has strong motivation to researchers because their properties can be

controlled by shape and size. Novel applications can be investigated and are dependent of

their structural properties. As the morphology of nano-materials is one of the key factors

that affect their properties. ZnO is a versatile functional nanomaterial with novel

morphologies. It has a rich family of nanostructures [4] as shown in figure 3.1 and 3.2,

such as nanotubes, nanowires, nanorods, nanobelts, nanocables, nanosheets,

nanotetrapods, nanomultipods, nanoflowers, nanoneedles, shuttle-like, combs-like,

nanorings and nanoribbons which can be fabricated by different techniques.

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Figure 3.1 A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders. Most of the structures presented can be produced with 100% purity.

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Figure. 3.2 Seamless single-crystal nanorings of ZnO. (a) Structure model of ZnO, showing the ±(0001) polar surfaces. (b-e) Proposed growth process and corresponding experimental results showing the initiation and formation of the single-crystal nanoring via the self-coiling of a polar nanobelt. The nanoring is initiated by folding a nanobelt into a loop with overlapped ends as a result of long-range electrostatic interactions among the polar charges; the short-range chemical bonding stabilizes the coiled ring structure; and the spontaneous self-coiling of the nanobelt is driven by minimization of the energy contributed by polar charges, surface area, and elastic deformation. (f) SEM images of the as-synthesized, single-crystal ZnO nanoring. (g) The ‘slinky’ growth model of the nanoring. (h) The charge model of an α-helix protein, in analogy to the charge model of the nanobelt during the self-coiling process.

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During the past few years, attention has also been focused on the research

field of one-dimensional (1D) nanostructure materials, such as nanowires and nanorods,

because of their fundamental importance and the wide range of potential applications for

nanodevices [5-6]. In the present investigation, ZnO nanocrystals were synthesized using

co-precipitation method to realize the size controllable growth of ZnO. A number of

reaction conditions for example solvents, precursors, acidity and basicity were used to

synthesize ZnO nanocrystal with difference morphologies. The effects of the reaction

conditions on the final products were systematically investigated. Important parameters

related to the physical properties of ZnO are tabulated in Table 3.1. It should be noted

that still there exists uncertainty in some of these values like hole mobility, thermal

conductivity etc.

3.1 GENERAL ASPECTS OF ZINC OXIDE AND ITS APPLICATIONS

3.1.1 CRYSTAL STRUCTURE OF ZINC OXIDE (ZO)

Most of the group II–VI binary compound semiconductors crystallize in

either cubic zinc blende or hexagonal wurtzite (Wz) structure where each anion is

surrounded by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral

coordination is typical of sp3 covalent bonding nature, but these materials also have a

substantial ionic character that tends to increase the bandgap beyond the one expected

from the covalent bonding. ZnO is a II–VI compound semiconductor whose ionicity

resides at the borderline between the covalent and ionic semiconductors. The crystal

structures shared by ZnO are wurtzite (B4), zinc blende(B3), and rocksalt (or Rochelle

salt) (B1) as schematically shown in Figure 3.3. B1, B3, and B4 denote the

Strukturbericht designations for the three phases. Under ambient conditions, the

thermodynamically stable phase is that of wurtzite symmetry. The zinc blende ZnO

structure can be stabilized only by growth on cubic substrates, and the rocksalt or

Rochelle salt (NaCl) structure may be obtained at relatively high pressures, as in the case

of GaN. At ambient pressure and temperature, ZnO crystallizes in the wurtzite (B4 type)

structure, which has a hexagonal unit cell with two lattice parameters a and c in the ratio

of c/a= (8/3)1/2 =1.633 (in an ideal wurtzite structure) and belongs to the space group C46v

in the Schoenflies notation and P63mc in the Hermann–Mauguin notation. symmetry. A

schematic representation of the wurtzitic ZnO structure is shown in figure 3.4.

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Table 1.1 Different parameters of physical properties of ZnO Materials.

PROPERTIES VALUES

Molecular formula ZnO

Molar mass 81.408 g/mol

Appearance White solid

Odor Odorless

Stable phase Wurtzite

Density 5.606 g/cm3

Melting point 1950oC (decomposes)

Boiling point 1950oC (decomposes)

Solubility in water 0.16mg/100ml (30oC)

Lattice parameters

a=b

c

a/c

3.249 Å

5.206 Å

1.602

Lattice Hexagonal

Space Group P63mc

Band gap 3.3 eV (direct)

Refractive Index 2.008-2.029

Thermal conductivity 1-1.2 W cm - 1 K – 1

Exciton binding Energy 60 meV

Electron effective mass 0.24

Electron Hall Mobility 200 cm2/Vs

Hole effective mass 0.59

Hole Hall Mobility 5-50 cm2/Vs

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Figure 3.3 Stick-and-ball representation of ZnO crystal structures:(a) cubic rocksalt

(B1), (b) cubic zinc blende (B3), and (c) hexagonal wurtzite (B4). Shaded gray and

black spheres denote Zn and O atoms, respectively.

Figure 3.4 Schematic representation of a wurtzitic ZnO structure with lattice constants a in

the basal plane and c in the basal direction, u parameter, which is expressed as the bond

length or the nearest-neighbor distance b divided by c (0.375 in ideal crystal), a and b

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(109.47 in ideal crystal) bond angles, and three types of second-nearest-neighbor distances

b’1, b’2, and b’3.

The structure is composed of two interpenetrating hexagonal close packed (hcp)

sub-lattices, each of which consists of one type of atom displaced with respect to each

other along the threefold c-axis by the amount of u=3/8=0.375 (in an ideal wurtzite

structure) in fractional coordinates. The internal parameter u is defined as the length of

the bond parallel to the c-axis (anion–cation bond length or the nearest-neighbor distance)

divided by the c lattice parameter. The basal plane lattice parameter (the edge length of

the basal plane hexagon) is universally depicted by a; the axial lattice parameter (unit cell

height), perpendicular to the basal plane, is universally described by c.

Each sub-lattice includes four atoms per unit cell, and every atom of one kind

(group II atom) is surrounded by four atoms of the other kind (group VI), or vice versa,

which are coordinated at the edges of a tetrahedron. The crystallographic vectors of

wurtzite are a’=a(1/2, 31/2/2, 0); b’= a(1/2, -31/2/2, 0) and c’=a(0,0, c/a). In Cartesian

coordinates, the basis atoms are (0, 0, 0), (0, 0, uc), a(1/2, 31/2/6, c/2a) and a(1/2, 31/2/6,

[u+1/2]c/a). Also this hexagonal lattice is characterized by two interconnecting

sublattices of Zn2+ and O2−, such that each Zn ion is surrounded by tetrahedra of O ions,

and vice-versa. This tetrahedral coordination gives rise to polar symmetry along the

hexagonal axis. This polarity is responsible for a number of the properties of ZnO,

including its piezoelectricity and spontaneous polarization, and is also a key factor in

crystal growth, etching and defect generation. The four most common face terminations

of wurtzite ZnO are the polar Zn terminated (0001) and O terminated (0001) faces (c-axis

oriented), and the non-polar (1120) (a-axis) and (1010) faces which both contain an equal

number of Zn and O atoms. The polar faces are known to posses different chemical and

physical properties, and the O-terminated face possess a slightly different electronic

structure to the other three faces [7]. Additionally, the polar surfaces and the (1010)

surface are found to be stable, however the (1120) face is less stable and generally has a

higher level of surface roughness than its counterparts.

The (0001) plane is basal. Aside from causing the inherent polarity in the ZnO

crystal, the tetrahedral coordination of this compound is also a common indicator of sp3

covalent bonding. However, the Zn-O bond also possesses very strong ionic character

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and thus ZnO lies on the borderline between being classed as a covalent and ionic

compound, with an iconicity of fi = 0.616 on the Phillips iconicity scale [8]. The lattice

parameters of the hexagonal unit cell are a = 3.2495 Å and c = 5.2069 Å, and the density

is 5.605 gcm-3[9].

In an ideals wurtzite crystal, the axial ratio c/a and the u parameter (which is a measure

of the amount by which each atom is displaced with respect to the next along the c-axis)

are correlated by the relationship u.c/a = (3/8)1/2, where c/a = (8/3)1/2 and u= 3/8 for an

ideal crystal. ZnO crystal deviate from this ideal arrangement by changing both of these

values. This deviation occurs such that the tetrahedral distance is kept roughly constant in

the lattice. Experimentally, for wurtzite ZnO, the real values of u and c/a were

determined in the range u = 0.3817-0.3856 and c/a = 1.593-1.6035[10-12].

In addition to the wurtzite phase, ZnO is also known to crystallize in the cubic

zincblende and rocksalt (NaCl) structures, which are illustrated in figure 3.2. Sphalerite

structure, which is known as the zincblende structure, is stable only by the growth on

cubic structure [13-15], whilst the rocksalt structure is a high-pressure metastable phase

forming at ~ 10GPa, and cannot be epitaxially stabilized [16]. Theoretical calculations

indicate that a fourth phase, cubic cesium chloride, may be possible at extremely high

temperatures, however, this phase has yet to be experimentally observed [17]. The

wurtzite structure (Figure 3.1) differs from sphalerite structure in being derived from an

expanded hcp anion (O2-) array rather than a ccp array, but as in sphalerite the cations

(Zn2+) occupy one type of tetrahedral hole. This structure has (4, 4)-coordination as same

as sphalerite structure. The local symmetries of cations and anions are identical towards

their nearest neighbours in wurtzite and sphalerite but differ at second - nearest

neighbours.

3.1.2 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

[18].Thus it is considered that neither VO nor ZnI exists in measurable quantity. Van de

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Walle has proposed that hydrogen H is a dominant background donor in ZnO that were

exposed to H during growth [19]. 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 [20]. N,P, As

and Sb have been used as acceptors to produce n-type ZnO [21], where it is reported that

Zn-vacancy in ZnO acts as defect- type acceptor.

3.1.3 ELECTRONIC STRUCTURE/PROPERTIES OF ZINC OXIDE

ZnO has a relatively large direct band gap of ~3.3 eV at room temperature;

therefore, pure ZnO is colorless and transparent. The electronic band structure of ZnO

has been calculated by a number of groups [22–28]. The results of a band structure

calculation using the Local Density Approx- imation (LDA) and incorporating atomic

self-interaction corrected pseudopotentials (SIC-PP) to accurately account for the Zn 3d

electrons, is shown in figure 3.5 [28]. The band structure is shown along high symmetry

lines in the hexagonal Brillouin zone. Both the valence band maxima and the lowest

conduction band minima occur at the point k = 0 indicating that ZnO is a direct band

gap semiconductor. The bottom 10 bands (occurring around −9 eV) correspond to Zn 3d

levels. The next 6 bands from −5 eV to 0 eV correspond to O 2p bonding states. The first

two conduction band states are strongly Zn localized and correspond to empty Zn 3s

levels. The higher conduction bands (not illustrated here) are free-electron-like. The O 2s

bands (also not illustrated here) associated with core-like energy states, occur around −20

eV. The band gap as determined from this calculation is 3.77 eV. This correlates

reasonably well with the experimental value of 3.4 eV, and is much closer than the value

obtained from standard LDA calculations, which tend to underestimate the band gap by ∼3 eV due to its failure in accurately modeling the Zn 3d electrons.

In addition to calculations for the band structure of bulk ZnO, Ivanov and

Pollmann have also carried out an extensive study on the electronic structure of the

surfaces of wurtzite ZnO [27]. Using the empirical tight-binding method (ETBM) to

determine a Hamiltonian for the bulk states, the scattering theoretical method was applied

to determine the nature of the surface states. The calculated data was found to be in very

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good agreement with experimental data obtained from electron energy loss spectroscopy

(EELS) and ultra- violet photoelectron spectroscopy (UPS).

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Figure 3.5 The LDA band structure of bulk wurtzite ZnO calculated using dominant atomic

self-interaction-corrected pseudopotentials (SIC-PP). This method is much more efficient at

treating the d –bands than the standard LDA method. [Reprinted with permission from D.

Vogel, P. Krüger and J. Pollmann, Phys.Rev. B 52, R14316 (1995). Copyright 1995 by the

American Physical Society]

Figure 3.6 shows the wave-vector-resolved local density of states (LDOSs) on the

first three layers of the (0001)-Zn (left panel) and (0001)-O (right panel) surfaces, for the,

M and K points of the surface Brillouin zone. The bulk LDOS (calculated using the

ETBM) is given by the dashed lines. Surface induced positive changes to the LDOS are

shown as hatched. No surface states are present in the band gap, the Zn surface shows an

increase in back bonds (denoted by B in figure 3.6) and anti-back bonds (denoted by A)

surface states, while the O face simply shows an increase in P resonances and states. This

result suggests that the Zn face possesses more covalent character, arising from the Zn

4s–O 2p states, whilst the O face is more ionic. Experimentally, the ZnO valence band

splits into three band states, A, B and C by spin-orbit and crystal-field splitting. This

splitting is schematically illustrated in figure 3.7. The A and C subbands are known to

posses Г7 symmetry, whilst the middle band, B, has Г9 symmetry [29]. The band gap

has temperature dependence up to 300 K given by the relationship:

Eg(T)= Eg (T=0)5.05× 10−4 T 2

900−T …………………….

(3.1)

These properties, combined with the lattice dynamics of ZnO give rise to interesting

optical properties. Advantages associated with a large band gap include higher

breakdown voltages, ability to sustain large electric fields, lower electronic noise, and

high-temperature and high-power operation. The bandgap of ZnO can further be tuned

from ~3–4 eV by its alloying with magnesium oxide or cadmium oxide. Most ZnO has n-

type character, even in the absence of intentional doping. Nonstoichiometry is typically

the origin of n-type character, but the subject remains controversial. An alternative

explanation has been proposed, based on theoretical calculations, that unintentional

substitutional hydrogen impurities are responsible. Controllable n-type doping is easily

achieved by substituting Zn with group-III elements such as Al, Ga, In or by substituting

oxygen with group-VII elements chlorine or iodine.

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Reliable p-type doping of ZnO remains difficult. This problem originates from low

solubility of p-type dopants and their compensation by abundant n-type impurities. This

problem is observed with GaN and ZnSe. Measurement of p-type in "intrinsically" n-type

material is complicated by the inhomogeneity of samples. Current limitations to p-doping

do not limit electronic and optoelectronic applications of ZnO, which usually require

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Figure 3.6 Wave-vector-resolved LDOS’s on the first three layers of the (0001)-Zn (left

panel) and (0001¯)-O (right panel) surfaces. The bulk LDOS is given by the dashed lines

and surface induced positive changes to the LDOS are shown as hatched. The letters A, B, P

and S represent anti-back bonds, back bonds, P resonances and S resonances respectively.

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Figure 3.7 Schematic diagram representing the crystal-field and spin-orbit splitting of the

valence band of ZnO into 3 subband states A, B and C at 4.2 K

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junctions of n-type and p-type material. Known p-type dopants include group-I elements

Li, Na, K; group-V elements N, P and As; as well as copper and silver. However, many

of these form deep acceptors and do not produce significant p-type conduction at room

temperature.

3.1.4 MECHANICAL AND THERMAL PROPERTIES

The mechanical properties of materials involve various concepts such as hardness,

stiffness, and piezoelectric constants, Young’s and bulk modulus, and yield strength. The

solids are deformed under the effect of external forces and the deformation is described

by the physical quantity strain. The internal mechanical force system that resists the

deformation and tends to return the solid to its undeformed initial state is described by the

physical quantity stress. Within the elastic limit, where a complete recoverability from

strain is achieved with removal of stress, stress σ is proportional to strain ε . ZnO is a

relatively soft material with approximate hardness of 4.5 on the Mohs scale. Its elastic

constants are smaller than those of relevant III-V semiconductors, such as GaN. The high

heat capacity and heat conductivity, low thermal expansion and high melting temperature

of ZnO are beneficial for ceramics. Among the tetrahedrally bonded semiconductors, it

has been stated that ZnO has the highest piezoelectric tensor or at least one comparable to

that of GaN and AlN. This property makes it a technologically important material for

many piezoelectrical applications, which require a large electromechanical coupling.

3.1.5 ELECTRICAL PROPERTIES OF ZnO

As a direct and large bandgap material, ZnO is attracting much attention for a

variety of electronic and optoelectronic applications. Advantages associated with a large

bandgap include high-temperature and high-power operation, lower noise generation,

higher breakdown voltages, and ability to sustain large electric fields. The electron

transport in semiconductors can be considered for low and high electric fields.

(i). At sufficiently low electric fields, the energy gained by the electrons from the

applied electric field is small compared to the thermal energy of electrons and therefore

the energy distribution of electrons is unaffected by such a low electric field. Because the

scattering rates determining the electron mobility depend on the electron distribution law

function, electron mobility remains independent of the applied electric field, and Ohm’s

law is obeyed.

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(ii). When the electric field is increased to a point where the energy gained by

electrons from the external field is no longer negligible compared to the thermal energy

of the electron, the electron distribution function changes significantly from its

equilibrium value. These electrons become hot electrons characterized by an electron

temperature larger than the lattice temperature.

Furthermore, as the dimensions of the device are decreased to submicron range,

transient transport occurs when there is minimal or no energy loss to the lattice during a

short and critical period of time, such as during transport under the gate of a field effect

transistor or through the base of bipolar transistor. The transient transport is characterized

by the onset of ballistic or velocity overshoot phenomenon. Because the electron drift

velocity is higher than its steady-state value, one can design a device operating at

frequencies exceeding those expected from linear scaling of dimensions.

3.1.6 APPLICATIONS OF ZnO NANOMATERIALS

The applications of zinc oxide powder are numerous, and the principal ones are

summarized below. Most applications exploit the reactivity of the oxide as a precursor to

other zinc compounds. For material science applications, zinc oxide has high refractive

index, good thermal, binding, antibacterial and UV-protection properties. Consequently,

it is added into various materials and products, including plastics, ceramics, glass,

cement, rubber, lubricants, paints, ointments, adhesive, sealants, pigments, foods,

batteries, ferrites, fire retardants, etc.

Electronics

ZnO has wide direct band gap (3.37 eV or 375 nm at room temperature).

Therefore, its most common potential applications are in laser diodes and light emitting

diodes (LEDs). Some optoelectronic applications of ZnO overlap with that of GaN,

which has a similar bandgap (~3.4 eV at room temperature). Compared to GaN, ZnO has

a larger exciton binding energy (~60 meV, 2.4 times of the room temperature thermal

energy), which results in bright room-temperature emission from ZnO. Other properties

of ZnO favorable for electronic applications include its stability to high-energy radiation

and to wet chemical etching. Radiation resistance makes ZnO a suitable candidate for

space applications. The pointed tips of ZnO nanorods result in a strong enhancement of

an electric field. Therefore, they can be used as field emitters. Aluminium-doped ZnO

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layers are used as transparent electrodes. The constituents Zn and Al are much cheaper

and less poisonous compared to the generally used indium tin oxide (ITO). One

application which has begun to be commercially available is the use of ZnO as the front

contact for solar cells or of liquid crystal displays. Transparent thin-film transistors

(TTFT) can be produced with ZnO.

As field-effect transistors, they even may not need a p–n junction, thus avoiding

the ptype doping problem of ZnO. Some of the field-effect transistors even use ZnO

nanorods as conducting channels.

Spintronics

ZnO has also been considered for spintronics applications: if doped with 1-10% of

magnetic ions (Mn, Fe, Co, V, etc.), ZnO could become ferromagnetic, even at room

temperature. Such room temperature ferromagnetism in ZnO:Mn has been observed, but

it is not clear yet whether it originates from the matrix itself or from Mn-containing

precipitates.

Piezoelectricity

The piezoelectricity in textile fibers coated in ZnO have been shown capable of "self-

powering nanosystems" with everyday mechanical stress generated by wind or body

movements. In 2008 the Center for Nanostructure Characterization at the Georgia

Institute of Technology reported producing an electricity generating device (called

flexible charge pump generator) delivering alternating current by stretching and releasing

zinc oxide wires. This mini-generator creates an oscillating voltage up to 45 millivolts,

converting close to seven percent of the applied mechanical energy into electricity. The

researchers used wires having the lengths of 0.2-0.3 mm and diameters of three to five

micrometers, but the device could be scaled down to nanometer size.

Sensors

Zinc oxide nanorod sensors are devices detecting changes in electrical current

passing through zinc oxide nanowires due to adsorption of gas molecules. Selectivity to

hydrogen gas was achieved by sputtering Pd clusters on the nanorod surface. The

addition of Pd appears to be effective in the catalytic dissociation of hydrogen molecules

into atomic hydrogen, increasing the sensitivity of the sensor device. The sensor detects

hydrogen concentrations down to 10 parts per million at room temperature, whereas there

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is no response to oxygen. ZnO has high biocompatibility and fast electron transfer

kinetics. Such features advocate the use of this material as a biomimic membrane to

immobilize and modify biomolecules.

Cigarette Filters

Zinc oxide is a constituent of cigarette filters for removal of selected components

from tobacco smoke. A filter consisting of charcoal impregnated with zinc oxide and iron

oxide removes significant amounts of HCN and H2S from tobacco smoke without

affecting its flavor.

Rubbers Manufacture

About 50% of ZnO use is in rubber industry. Zinc oxide activates vulcanization,

which otherwise may not occur at all. Zinc oxide and stearic acid are ingredients in the

commercial manufacture of rubber goods. A mixture of these two compounds allows a

quicker and more controllable rubber cure. ZnO is also an important additive to the

rubber of car tyres. Vulcanization catalysts are derived from zinc oxide, and it

considerably improves the thermal conductivity, which is crucial to dissipate the heat

produced by the deformation when the tyre rolls. ZnO additive also protect rubber from

fungi (see medical applications) and UV light.

Concrete Industry

Zinc oxide is widely used for concrete manufacturing. Addition of ZnO improves the

processing time and the resistance of concrete against water.

Medical

Zinc oxide as a mixture with about 0.5% iron (III) oxide (Fe2O3) is called calamine

and is used in calamine lotion. There are also two minerals, zincite and hemimorphite,

which have been called calamine historically. When mixed with eugenol, a chelate, zinc

oxide eugenol is formed which has restorative and prosthodontic applications in dentistry.

Reflecting the basic properties of ZnO, fine particles of the oxide have deodorizing and

antibacterial action and for that reason are added into various materials including cotton

fabric, rubber, food packaging, etc. Enhanced antibacterial action of fine particles

compared to bulk material is not intrinsic to ZnO and is observed for other materials,

such as silver. Zinc oxide is a component of barrier cream used in nappy rash or diaper

rash. It is also a component in tape (called "zinc oxide tape") used by athletes as a

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bandage to prevent soft tissue damage during workouts.

Food Additive

Zinc oxide is added to many food products, e.g., breakfast cereals, as a source of

zinc, a necessary nutrient. (Other cereals may contain zinc sulfate for the same purpose.)

Some prepackaged foods also include trace amounts of ZnO even if it is not intended as a

nutrient.

Pigment

Zinc white is used as a pigment in paints and is more opaque than lithopone, but

less opaque than titanium dioxide. It is also used in coatings for paper. Chinese white is a

special grade of zinc white used in artists' pigments. Because it reflects both UVA and

UVB rays of ultraviolet light, zinc oxide can be used in ointments, creams, and lotions to

protect against sunburn and other damage to the skin caused by ultraviolet light. It is the

broadest spectrum UVA and UVB absorber that is approved for use as a sunscreen by the

FDA, and is completely photo stable. It is also a main ingredient of mineral makeup.

Coatings

Paints containing zinc oxide powder have long been utilized as anticorrosive

coatings for various metals. They are especially effective for galvanized Iron. The latter

is difficult to protect because its reactivity with organic coatings leads to brittleness and

lack of adhesion. Zinc oxide paints however, retain their flexibility and adherence on

such surfaces for many years. ZnO highly n-type doped with Al, Ga or nitrogen is

transparent and conductive (transparency ~90%, lowest resistivity ~10−4Ωcm). ZnO: Al

coatings are being used for energy-saving or heat-protecting windows. The coating lets

the visible part of the spectrum in but either reflects the infrared (IR) radiation back into

the room (energy saving) or does not let the IR radiation into the room (heat protection),

depending on which side of the window has the coating. Various plastics, such as poly

(ethylene-naphthalate) (PEN), can be protected by applying zinc oxide coating. The

coating reduces the diffusion of oxygen with PEN. Zinc oxide layers can also be used on

polycarbonate (PC) in outdoor applications. The coating protects PC form solar radiation

and decreases the oxidation rate and photo-yellowing of PC.

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3.2 LITERATURE SURVEY Over the past decade, tremendous efforts have been made to synthesize nanoscaled

or microscaled ZnO crystals. Up to now, ZnO nanostructures with various sizes and

morphologies have been successfully synthesized and reported in the literature [4].

Besides, as is well known, impurity doping in semiconductors with selective elements

greatly affects the basic physical properties, such as the electrical, optical, and magnetic

properties, which are crucial for their practical application, and the doping effect has

attracted extraordinary attention. Recently, various doped ZnO nanostructures with

different elements (e.g., Al, As, In, Sn, Mg, and Sb) have been achieved [12-17].

Preetam Singh et al. [30] reported the preparation of ZnO nanopowder by

ultrasonic mist chemical vapor deposition (UM–CVD) system. This is a promising

method for large area deposition at low temperature inspite of being simple, inexpensive

and safe. The high temperature X-ray diffraction (XRD) of the powder showed prominent

(100), (002) and (101) reflections among which (101) are of highest intensity. With

increase in temperature, a systematic shift in peak positions towards lower 2θ values has

been observed, which may be due to change in lattice parameters. Temperature

dependence of lattice constants under vacuum shows linear increase in their values. The

synthesized powder exhibited the estimated direct bandgap (Eg) of 3 43 eV. ⋅ It has been reported that annealing temperature in the range of 400-650 0C,

affects the crystallography, particle size and thermo-power of bulk ZnO [31]. The small

change in lattice constants of a and c (lattice constants a =b=3.2469 Å increase to 3.2488

Å, c=5.2049 Å slightly decrease to 5.2031 Å). The Zn-O bond length was related with

ZnO unit cell views of the direction approximately parallel to O2-and Zn2+. The powder of

bulk ZnO exhibited good distribution of particles after being annealed below 600 0C and

covered with the nanoparticles, while other portions retained the smooth morphology.

The particle sizes increased from 73.50 to 79.67 nm with increase in annealing

temperatures from 400 oC to 650 0C. The bulk ZnO has highest thermo power of -92.99

µVK-1 at room temperature for annealing temperature of 550 0C and indicating that the

behavior of the n-type thermoelectric material.

S. Suwanboon et al. [32] reported the synthesis of nanocrystalline ZnO powder

by precipitation method by using zinc acetate dihydrate and PVP as starting materials.

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The indexing of the XRD pattern of calcined powders in air at 600 0C for 1 hour, reveals

the hexagonal structure with the smallest crystallite size of ~ 44.76 nm, and lattice

parameters a and c of 0.3249 and 0.5204 nm, respectively at 3x10-4 M PVP. The SEM

images showed that the morphology has been changed from plate-like to small rod shape

when adding PVP to solutions and the morphology has tended to be monosized at higher

PVP concentration. The smallest grain sizes of ZnO powders were ~ 130 nm at 3x10 -4 M

PVP. The optical band gap of ZnO powders in this study varied between 3.218-3.229 eV.

The zinc oxide whiskers were synthesized by Fan xi-mei et al. [33] using the

equilibrium gas expanding method at the temperature of 700 0C with metallic zinc as the

main raw material without any catalysts. The results showed that the as-grown samples

are composed of uniform tetrapod-like ZnO whiskers. The length and diameter of the

arms of the tetrapod-like ZnO whiskers increase obviously with the increase of the

growth time. The strong single ultraviolet (UV) emission centering 385−391 nm without

any accompanying deep-level emission is observed in the room temperature

photoluminescence (PL) spectra of the whiskers. The intensity of UV emission increases

markedly with the increase of growth time.

Mansi Dhingra et al. [34] s reported the preparation of ZnO nanoparticles,

by the sol–gel method and the powder pressed in the form of pellets were used for gas

sensing. The hybrid Zinc oxide/polyaniline (ZnO/PANI) structure was obtained by the

addition of PANI on the surface of ZnO. The UV–VIS absorption of the modified pellets

showed band edge at 363 nm corresponding to ZnO, while a change in the absorption

peaks for PANI was observed. The possible interaction between Zn2+ of ZnO and NH-

group of PANI was confirmed using Raman spectroscopy studies. The results reveal that

the hybrid structures exhibit much higher sensitivity to NH3 gas at room temperature than

pure ZnO, which is sensitive to NH3 gas at higher temperatures. This enhancement has

been attributed due to the creation of active sites on the ZnO surface due to the presence

of PANI.

Y. H. Shin et al. [35] have grown the high-quality single-crystalline ZnO by

using chemical vapor transport and the photoluminescence (PL) measurements were

performed on as-grown, hydrogenated and hydro-genated and annealed n-type ZnO bulk

samples in order to investigate the origins of their yellow and green emission bands.

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After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the PL

spectrum at room temperature, the peak intensity at 2.43 eV was unchanged and the

intensity of the emission peak at 3.27 eV was strongly increased. These results indicate

that the yellow band emission is due to oxygen vacancies because the emission peak at

2.10 eV disappears when these vacancies are passivated by hydrogen atoms. The

emission peak at 2.43 eV originates from complexes between oxygen vacancies and other

crystal defects. The peak at 3.274 eV is related to shallow donor impurities due to

hydrogen donors.

The quantum dots structure of ZnO prepared by wet chemical method has been

reported by H. Zhou et al. [36]. By annealing treatment at 150 0C –500 0C, the effect of

the change in the structure of the dots on their luminescence properties has also been

studied. The surface of the as-prepared dots is passivated by a thin layer of Zn(OH) 2 ,

thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. The weak excitonic

transition of ZnO quantum dots is strongly correlated with the presence of the surface

shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.

The synthesis and characterization of n-type ZnO nanomaterial and its

application as temperature sensor has been reported Richa Srivastava et al. [37]. The ZnO

nanomaterial has been synthesized by flash heating the oxalate at 450 0C for 15 min. The

oxalate produced by a conventional co-precipitation method is pressed in the form of

pellet and then it is used as a sensing element. The variations in resistance of sensing

pellet at different temperatures were recorded. The relative resistance was decreased

linearly with increasing temperatures over the range, 120 0C – 260 0C. The activation

energy of ZnO calculated from Arrhenius plot was found 1.12 eV. Temperature response

in terms of the relative variation, ΔR, of sensor resistance to a given temperature was

measured. Scanning electron micrograph of the sensing element has been studied. Pellet

of the ZnO is comprised of nanorods of varying diameters and different lengths. Diameter

of ZnO nanorods varies from 75 to 300 nm. X-ray diffraction pattern of the sensing

element reveal their nano-crystalline nature. Optical characterization of the sensing

material was carried out by UV-visible spectrophotometer. By UV-Vis spectra, the

estimated value of band gap of ZnO was found 4.7 eV.

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3.3 GENERAL METHODS OF SAMPLE PREPARATION (IN BULK) Bulk zinc oxide (ZnO) nanostructures have been synthesized using various

approaches such as electrodepositions, oxidation process, chemical reduction, vacuum

evaporation, hydrothermal method, vapour transport and already reported in literature.

Some of these techniques have been discussed and described briefly below.

3.3.1 ELECTRODEPOSITION

Electrochemical deposition is a very powerful technique for achieving

uniform and large area synthesis of ZnO nanostructures, because it exerts a strong

external driving force to make the reactions take place, even if they are non-spontaneous.

The growth of ZnO nanostructures can occur on a general substrate, flat or curved,

without any seeds, as long as the substrate is conductive. Also, under such an external

electric field, better nanowire alignment and stronger adhesion to the substrate have been

observed. The ZnO nanowire growth was observed at only the cathode of a D.C. power

source, and at both electrodes for an A.C. power source. Most importantly,

electrodeposition has been shown to be an effective way of doping ZnO nanowires by

adding different ingredients into the reaction solution. For electrodeposition, a standard

three-electrode setup is typically used, with a saturated Ag/AgCl electrode as the

reference electrode and Pt as the counter-electrode. The anode, where growth usually

takes place, is placed parallel to the cathode in the deposition solution. The electrical bias

throughout the reaction system is controlled by a constant voltage source to maintain a

constant driving force to the reaction, or by a constant current source to keep a constant

reaction rate. Sheng Xu et al. used a ZnCl2 and KCl mixed solution electrolyte to grow

vertically aligned ZnO nanowire arrays on a SnO2 glass substrate. During the growth, O2

was continuously bubbled through the solution in order to keep a relatively high level of

O2 dissolved in the solution, which was necessary for the growth of high quality ZnO

nanowires. Reduction of O2 at the cathode provides a source of OH−, which is required to

coordinate with Zn2+ and then undergo dehydration to form ZnO. It was found that the

dimensions of ZnO nanowires could be controlled from 25 to 80 nm by the varying the

ZnCl2 concentration.

3.3.2 OXIDATION PROCESS

The simplest way to obtain ZnO consists in oxidizing a zinc sheet in an atmosphere

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containing oxygen. Obviously it necessary to choose the temperature and the oxygen

partial pressure of the oxidation in the region of stability of ZnO. Experimentally it was

found that copper, like almost all metals, oxidizes in the presence of oxygen gas in the

form of a uniform film of oxide. Two regimes are usually observed. The first one is

when zinc is oxidized at pressure below the ZnO dissociation pressure in this case a

single layer of ZnO is formed over the zinc.

3.3.3 CHEMICAL REDUCTION

In contrast to microemulsion systems, nanoparticles are synthesized in one

phase in which the metal salt was initially dissolved. This method is a simple one-pot

solution-phase method for synthesis of a variety of metal nanoparticles, including zinc

nanocrystals. Zinc salt and reducing agent are injected in the same solvent in the presence

of a stabilizer. The reaction temperature and additives are the factors affecting on the

shape of zinc nanoparticles.

3.3.4 VACUUM VAPOR DEPOSITION (VVD)

The Zn and Cd nanowires were prepared by evaporating Zn grains and Cd

grains onto an Si substrate under vacuum without any catalyst. Commercial grains

(Beijing Chemical Factory) of Zn or Cd had their oxide layer removed with dilute HCl

solution, washed with ethanol, and dried under vacuum. The substrate was cleaned with

ethanol and dried in air as well. The cleaned substrate was placed with the Zn or Cd

grains in a pyrex glass tube, which was subsequently evacuated at room temperature, to a

level of the order of 10−2 Torr. For deposition of Zn nanowires, the glass tube was heated

rapidly in a tube furnace from room temperature to 300 0C at a temperature increasing

rate of 14 0C min−1 and then to 350 0C at a rate of 10 0C min−1 and this temperature was

maintained for 20 min. For deposition of Cd nanowires, the raised temperature program

is from room temperature to 250 0C at a temperature increasing rate of 12 0C min−1 and

then to 300 0C at a rate of 10 0C min−1 and this temperature was maintained for 20 min.

For deposition of Cd nanowires, the raised temperature program is from room

temperature to 250 0C at a temperature increasing rate of 12 0C min−1 and then to 300 0C

at a rate of 10 0C min−1 and this temperature was maintained for 20 min.

3.3.5 HYDRTHERMAL METHOD

The ZnO nanoparticles by hydrothermal method, in this method the stock

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solutions of Zn(CH3COO)2.2H2O (0.1 M) was prepared in 50ml methanol under stirring.

To this stock solution 25ml of NaOH (varying from 0.2 M to 0.5 M) solution prepared in

methanol was added under continuous stirring in order to get the pH value of reactants

between 8 and 11. These solutions was transferred into teflon lined sealed stainless steel

autoclaves and maintained at various temperature in the range of 100 – 200 0C for 6 and

12 hrs under autogenous pressure. It was then allowed to cool naturally to room

temperature. After the reaction was complete, the resulting white solid products were

washed with methanol, filterednand then dried in air in a laboratory oven at 60 0C.

3.3.6 VAPOR TRANSPORT

A method which produces very high quality bulk ZnO wafers is based on

vapor transport. In this method, the reaction takes place in a nearly closed horizontal

tube. Pure ZnO powder used as the ZnO source is placed at the hot end of the tube which

is kept at about 1150 0C. The material is transported to the cooler end of the tube,

maintained at about 1100 0C, by using H2 as a carrier gas. A carrier gas is necessary

because the vapor pressures of O and Zn are quite low over ZnO at these temperatures.

The likely reaction in the hot zone is ZnO(s) +H2 (g) →Zn (g) +H2O (g). At the cooler

end, ZnO is formed by the reverse reaction, assisted by a single-crystal seed. To maintain

the proper stoichiometry, a small amount of water vapor is added. Growth time of 150–

175 h provided 2-inch.-diameter crystals of about 1 cm in thickness. Vapor transport

using chlorine and carbon as transporting agents has been used to achieve ZnO crystal

growth at moderate temperature of 950–1000 0C.

3.3.7 MELT GROWTH

Another method for producing bulk ZnO is that of melt growth. The melt method

is based on a pressurized induction melting apparatus. The melt is contained in a cooled

crucible. Zinc oxide powder is used as the starting material. The heat source used during

the melting operation is radio frequency (r.f.) energy, induction heating. The r.f. energy

produces joule heating until the ZnO is molten at about 1900 0C. Once the molten state is

attained, the crucible is slowly lowered away from the heated zone to allow

crystallization of the melt.

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3.4 PRESENT INVESTIGATION The present study focuses on the synthesis of pure and Sn-doped ZnO

nanopowders by co-precipitation method and the effect of reaction temperatures,

concentration of the precursors and time of growth on its properties. This synthesis

method of doped ZnO powders has many advantages such as (1) powders with

nanometer- size can be obtained by this method, (2) the reaction is carried out under

moderate conditions, (3) powders with different morphologies by adjusting the reaction

conditions and (4) the as-prepared powders have different properties from that of the

bulk.

3.4.1 SYNTHESIS/PREPARATION OF PURE AND Sn-DOPED ZINC OXIDE

In the present investigation the pure and Sn-doped ZnO nanomaterials in

bulk have been synthesized by co-precipitation method. All the chemical reagents used in

present investigations were of analytical grade and used without any further purification.

In a typical procedure, 0.1 M solution of Zinc acetate dehydrate (Merck purity > 98 %)

[Zn(CH3COO2)•2H2O] was first dissolved in methanol and double distilled water in the

volume ratio 3:1, respectively. Further appropriate wt. % of Tin chloride pentahydrate

(SnCl4•5H2O, Sigma- Aldrich purity > 98 % ) was added into starting solution for tin

doping with continuous stirring until a homogeneous solution with pH value of reactants

between 8 and 10 was obtained. A few drops of acetic acid were added to improve the

clarity of solution. The Sn/Zn ratio was kept 0, 5, 10 and 15 wt. %. The 1 M Sodium

hydroxide (NaOH, Merck purity > 97 %) solution was dissolved in base precursor

solution. The white precipitates were obtained and were then vigorously stirred at room

temperature for 5 hour. This white precipitate was washed with double distilled water,

filtered and dried at 300 0C for 10 hours in oven. The dried powder was thoroughly mixed

and ground for at least two hours, then shaped into pellets (10 mm dia & 2mm thick) and

finally sintered at 700 0C for 12 hours.

3.4.2 STRUCTURAL STUDIES3.4.2 (a) Phase Identification and Determination of Lattice Parameters

The phase identification and lattice parameters of ZnO have been investigated over many

decades. The lattice parameters of a semiconductor usually depend on the following

factors: (i) free electron concentration acting via deformation potential of a conduction

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band minimum occupied by these electrons, (ii) concentration of foreign atoms and

defects and their difference of ionic radii with respect to the substituted matrix ion, (iii)

external strains (e.g., those induced by substrate), and (iv) temperature. The lattice

parameters of any crystalline material are commonly and most accurately measured by

high resolution X-ray diffraction (HRXRD) using the Bond method [31] for a set of

symmetrical and asymmetrical reflections.

For phase identification/gross structural characterization of the as synthesized zinc

oxide material (in form of pellet), the most appropriate technique i.e. X-ray diffraction

was employed. The X-ray diffraction was carried out through wide-angle Philips (X’ Pert

PRO, Model PW 3040, at Indian Institute of Technology Kanpur (IIT-K)) powder

diffractometer having CuKα radiation. The pellets were mounted by a cello tape on the

specimen holder with the X-ray beam incident on the flat smooth surface of the pellet.

Proper care was taken in mounting the sample, so that any error due to misorientation

may not creep in the measurements. The diffractometer was first calibrated by monitoring

theta (2θ) values from a standard silicon sample. The diffractograms were recorded with

scan speed 20/minute (0.030/s) and step of (0.20) in range of 20-800. A large number of

specimens, synthesized under different conditions were explored through XRD. Lattice

constant and crystal structure have been usually measured by X–ray powder diffraction

(XRD) using Cu Kα radiation in θ/2θ mode [38]. Figure 3.8 shows the representative

powder x-ray diffraction patterns of pure ZnO and Sn-doped ZnO with various Sn doping

concentration (5,10 & 15 wt.%). The presence of reflections such as (100), (002), (101),

(102), (110), (103), (200), (112), (201) and (202) has been detected with considerable

intensities. Quantitative analysis of powder x-ray diffraction patterns revealed that the as

synthesized doped and undoped ZnO powder consists of pure single phase crystalline

hexagonal wurtzite phase of Zinc oxide (JCPDS card no. 89 -1379) which belongs to the

space group P63mc. No other reflection peaks from impurities, such as other oxides of Sn

or Zn are detected, indicating high purity of the product of Sn-doped ZnO. The lattice

parameters of Sn-doped ZnO have been calculated using high angle XRD lines such as

(200), (112) and (201) shown in Figure 3.8. The variation of calculated lattice parameters

of Sn-doped ZnO with dopant ratio of [Sn]/[Zn] equal to 0, 5, 10, 15 by wt.% are shown

in Figure 3.9. A small decrease in the a-lattice parameter of the hexagonal unit cell has

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Figure 3.8 The x-ray diffraction (XRD) patterns of nanocrystalline pure and Sn doped

ZnO powders prepared by co-precipitation method.

Figure 3.9 The variation of lattice parameters, calculated from X-ray diffraction data of

the pure and Sn-doped ZnO nanocrystalline materials.

113

been observed with increasing Sn content. This may possibly occur due to the strong

covalent bonding in a-b plane and difference in ionic radii of Zn+2(0.74 Å) and Sn+4(0.72

Å) ions. However, the c-lattice parameter increases due to the weaker covalent bonding

along c-axis (since the c-parameter (4.592 Å) is larger than a-parameter (3.407 Å)) in a-c

plane. It is also perceptible from the XRD peak (101) shown in Figure 3.10, that the

undoped as well as doped Zinc oxide powder grows along the orientation of (101) with

different crystallite size.

3.4.2(b) Grain Size Determination

The crystallite size was calculated from x-ray diffraction data using the Debye-Scherrer

formula;

Dhkl = 0.9λ/β cosθ …………………………………… (3.2)

where λ is the x-ray wavelength (1.5418 Å for CuKα), ϴ is the Bragg angle and β is the

full width at half its maximum intensity (FWHM) of the most intense diffraction peak

(101).The calculated crystallite size of doped ZnO as a function of Sn doping

concentrations is shown in table 3.2. From the table it is observed that the crystallite size

decreases with increasing Sn concentration in ZnO. The minimum crystallite size of 292

Å is found for 15 wt% Sn-doped ZnO. This is due to the lesser ionic radius of Sn+4 (RSn+4

3.4.2 (c) Surface Morphological Studies

The scanning electron microscope is one of the most useful and versatile instruments for

the investigation of surface topography, microstructutral features, etc. The principle

involved in imaging is to make use of the scattered secondary electrons when a finely

focused electron beam impinges on the surface of the specimen. The electrons are

produced by a thermal emission source, such as heated tungsten filament, or by using

field emission cathode. To create SEM image, the incident electron beam is scanned in a

raster pattern across the sample surface. Secondary electrons are produced due to the

interaction of the primary electron beam. The emitted electrons are detected at each

position in the scanned area by an electron detector. Intensity of the emitted electron

signal is displayed as brightness on a cathode ray tube. There are two modes of imaging:

one is by using Secondary Electrons and the other is by using Backscattering Electrons.

Secondary electron imaging provides high resolution imaging of fine surface morphology

and for this, the samples must be electrically conductive. ZEISS (at IIT Kanpur) scanning

114

Figure 3.10 XRD patterns of (101) plane of nanocrystalline pure and Sn doped ZnO

powders prepared by co-precipitation method.

Table 3.2 The variation of bandgap of the Pure and Sn-doped ZnO

powder with different Sn concentration.

115

Sn/Zn dopant

concentration in ZnO

Average crystallite

size D( Å)

0 wt% 307

5 wt% 304

10 wt. % 302

15 wt.% 292

electron Microscope was used for recording surface image of ZnO pellets in the present

work. A very thin layer of gold was coated over the ZnO samples to obtain conductivity

without significantly affecting surface morphology. The surface morphological

examination with Field emission Scanning Electron Microscopy (FESEM) shown in

figure 3.11 revealed the fact that the particles (group of grains) are closely packed and

pores/voids between the grains are very few and pores/voids between the grains are

decreases with increasing the Sn dopent concentration as shown in figure 3.11 (b) to 3.10

(d). It may also be noted that the particle sizes observed by FESEM (figure 4a-d) are

higher as compared with that calculated from the XRD data. This is due the fact that the

XRD technique provide the average mean crystallite size of grains/crystallites (single

crystals) while FESEM shows the particles which are agglomeration of many grains. The

XRD and FESEM data can be reconciled by the fact that smaller primary particles have a

large surface free energy and would, therefore, tend to agglomeration faster and grow

into larger grains.

3.4.2 (d) Structural / Microstructural Characterization Explored Through

Transmission Electron Microscope (TEM)

Like the other physical properties of solids e.g. mechanical, electrical, magnetic, thermal

properties, surface reactivity etc., the ‘microstructural changes’ are found to significantly

affect the physical properties too. The term ‘Microstructure’ refers to the assemblages of

lattice defects in solids, which are commonly, classified as point, line, surface and

volume defects. For developing new semiconductor oxides with improved properties, an

understanding of microstructure-properties correlation is of fundamental importance.

The structural features of semiconducting metal oxides (SMO’s) are especially important

since the semiconducting properties are known to be crucially dependent on the

microstructure, phase composition, phase transformation, order-disorder transition, etc.

The X-ray and neutron diffraction techniques are widely used to examine the average i.e.

gross structural features of semiconducting oxide and other related phases. However,

these techniques are not useful in gauging the microstructural features of the materials. In

the case of new semiconducting oxide materials, there are several features, which need

yet another technique that can probe the material with regard to the local structures (up to

about 50-100 Å). Transmission Electron Microscopy is capable of characterizing the

116

Figure 3.11 Field Emission Scanning Electron Microscopic (FESEM) images

of (a) pure ZnO and (b) 5% Sn-doped ZnO pellets, sintered at 7000C.

117

Figure 3.10 Field Emission Scanning Electron Microscopic (FESEM) images

of (c) 10% Sn doped ZnO and (d) 15% Sn doped ZnO pellets, sintered at 7000C.

118

local structural characteristics for example local phases and can also provide information

about the chemical composition of the local regions. This technique has been extensively

applied to the study of all semiconducting oxide materials.

In present investigations, we have used the transmission electron microscopic technique

(Tecnai 20 G2 TEM, at CSR, Indore) widely to investigate the microstructural

characteristics of the pure and doped ZnO nanomaterials. In this chapter, the results

obtained on the Sn-doped ZnO have been described and discussed.

Specimen Preparation Techniques

In order to observe fine high-resolution images, it is necessary to prepare thin

samples without introducing contamination or defects. For this purpose, it is important to

select an appropriate specimen preparation method for each material, and to find an

optimum condition for each method. There are various specimen preparation techniques

for high-resolution transmission electron microscopy listed below.

I. Crushing

II. Electropolishing

III. Ultramicrotomy

IV. Ion Milling

V. Focused Ion Beam (FIB)

VI. Vacuum Evaporation

Out of these techniques crushing method is used in present investigation. In this

technique, a specimen is usually crushed with an agate mortar and agate pestle. The

flakes, which are obtained, are suspended in organic solvent such as methanol (CH3OH),

and dispersed with by stirring with a glass stick. Finally, the solvent containing the

specimen flakes is dripped onto a microgrid (carbon coated and 200 mesh) on a filter

paper. Since this is the simplest method, and it is also possible to find thin regions of a

few nanometers thickness with little contamination on the surface, it is quite useful for

high-resolution electron microscopy. However, since grain boundaries are rather fragile,

it is usually difficult to observe them in specimens prepared by this method.

In order to explore furthermore the structural/ microstructural of the pure

and Sn-doped ZnO nanomaterial, the transmission electron microscopic (TEM) technique

has been employed in both the imaging and diffraction modes. The TEM investigations

119

reveals that the as synthesized bulk Sn-doped ZnO with increasing Sn-dopant

concentration from 0 to 15% have many nanostructures of different shapes e.g.

nanospheres (≈50 nm) and nanorods (≈100 to 200 nm). Typical transmission electron

micrograph of a nanospheres & nanorods are shown in figures 3.11 (a, c, e, g) and their

corresponding selected area electron diffraction (SAED) patterns are shown in figures

3.11 (b, d, f, h) respectively. The SAED patterns (circles) from nanorods and

nanospheres have been indexed to a face hexagonal wurtzite system with lattice

parameter a=b=0.3249 nm and c=0.5206 nm. These tallies quit well the lattice parameter

of ZnO showing that the Sn occupy the Zn-sites in the lattice structure. It also seems that

these nanoshperes and nanorods having the dimension of 70-150 nm, are evolved due to

the agglomeration of the very small nanoparticles.

3.4.3 OPTICAL PROPERTIES

Zinc oxide is generally transparent to visible light but strongly absorbs ultra violet light

below 3655 A. The absorption is typically stronger than other white pigments. The

optical absorption spectra of the as synthesized pure and Sn-doped ZnO powders with

various Sn-dopant concentration (0 to 15 wt %) as a function of wavelength were

recorded using dual beam ultraviolet–visible (UV–VIS-NIR) spectrometer (Cary 50) in

the wavelength range between 200 and 800 nm at room temperature at IIT-Kanpur, India

and are shown in figure 3.12. It is evident clearly from the figure 3.12 that the as

synthesized nanomaterials have low absorbance in the visible/near infrared region while

the absorbance is high in the ultraviolet region. The bandgap of pure and Sn-doped ZnO

powder have been determined by ultraviolet (UV) absorption spectra. The results are in

good agreement with the reports by other investigators [39-41]. The absorption

coefficient ‘α’ was found to follow the Tauc relation [42-44];

α = Ao(h- Eg)n / h ..………….(3.3)

where Ao is a constant which is related to the effective masses associated with the bands

and Eg is the bandgap energy, h is the photon energy, α is the absorption coefficient, n

is a constant which is 1/2 for direct bandgap material and n is 2 for indirect bandgap

material. As our material was direct bandgap, So we put in equation (ii), n= ½;

α = Ao(h- Eg)1/2 / h, ……………(3.4)

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Figure 3.11 (a) Transmission Electron micrographs (TEM) of pure zinc oxide

showing the direct images of nanoparticles and (b) their corresponding selected

area electron diffraction (SAED) patterns.

Figure 3.11 (c) & (d) Transmission Electron micrographs (TEM) of 5% Sn-

doped zinc oxide showing the direct images of nanoparticles and their

corresponding selected area electron diffraction (SAED) patterns.

121

Figure 3.11 (e) & (f) Transmission Electron micrographs (TEM) of 10% Sn-

doped zinc oxide showing the direct images of nanoparticles and their

corresponding selected area electron diffraction (SAED) patterns.

Figure 3.11 (g) Transmission Electron micrographs (TEM) of 15% Sn-doped zinc

oxide showing the direct images of nanoparticles and (h) their corresponding

selected area electron diffraction (SAED) patterns.

122

Figure 3.12 The Optical Absorbance spectra of pure and Sn-doped ZnO powders

123

The extrapolation of straight line to (αh)2 = 0 axis gives the value of energy

of bandgap. Plots of (αh)2 vs. the photon energy h for powder of varying Sn-doping

concentrations are shown in figure 3.13. Linearity of the plots indicates that the material is of direct bandgap nature.

Extrapolation of linear portion of the graph to the energy axis at α= 0 gives the optical

bandgap of about 3.35, 3.39, 3.49 & 3.42, at pure and 5 wt.% , 10 wt.% and 15 wt.% Sn-

doped ZnO powder respectively shown in table 3.3. From it is clear that the optical band

gap increased or shifted to higher energy (blue shift) with increasing Sn doping

concentration. This blue-shift behavior can explain by the modification of the band

structure, i.e., narrowing of both the valence and conduction bands. The change in band

gap can be attributed due to the Burstein-Moss band gap widening and band gap

narrowing due to electron-electron and electron-impurity scattering [45].

3.4.4 ELECTRICAL PROPERTIES The fundamental study of the electrical properties of ZnO nanostructures

is crucial for developing their future applications in nanoelectronics. In order to study the

effect of Sn-doping on the conductivity as well as the conduction mechanism in the Zinc

oxide semiconductor, the electrical resistivity of all pelletized pure and tin doped zinc

oxide with different Sn contents were measured by collinear four probe method at room

temperature and it is graphically reported as a function of the dopant (Sn) concentration

in Figure 3.14. Pure ZnO has a very high resistivity of the order of 1.27x103 Ω cm. It is

remarked that the resistivity of Sn-doped ZnO (SZO) decreased considerably as the tin

concentration increased, with the sample containing 10 wt % Sn, showing the lowest

resistivity of 2.86x101 Ω cm. However, with further increase of the dopant (Sn)

concentration from 15 wt%, the resistivity started to increase significantly. Reports in the

literature indicate that the most widely accepted explanation for this effect is that tin play

the role of an effective donor in ZnO layers, when a small amount of Sn is introduced in

the precursor solution of ZnO. It can be further explained by the substitution/introduction

of Sn4+ into the Zn2+ sites, generating free electrons.

3.4.5 CONCLUSIONS

124

The pure and tin doped zinc oxide were prepared by co-precipitation method.

The analysis of x-ray diffraction patterns revealed that as synthesised doped and undoped

Figure 3.13 Evolution of the (αhυ)2 vs. hυ curves of pure and Sn-doped ZnO powders

prepared from 0.1 M Zn(CH3COO)2.2H2O.

Table 3.3 The variation of Average crystallite size of the Pure and Sn-

doped ZnO powder with different Sn concentration.

125

Sn/Zn dopant

concentration in ZnO

Bandgap (eV)

0 wt% 3.3500

5 wt% 3.3955

10 wt. % 3.4948

15 wt.% 3.4246

ZnO materials are pure crystalline hexagonal wurtzite phase of Zinc oxide. However, a

small decrease in the lattice parameters has been observed with increasing Sn content.

This possibly occurs due to the difference in ionic radii of Zn+2(0.74 Å) and Sn+4(0.72 Å)

ions. Surface morphological examination with FESEM revealed the fact that the grains

are closely packed and pores between the grains are very few. The formation of ZnO

nanoparticles / nanorods were also confirmed by transmission electron microscopy

(TEM) and selected area electron diffraction (SAED) studies. The average particle size

have been found to be about 70-150 nm. The optical bandgap of Sn-doped ZnO

nanomaterials were obtained from optical absorption spectra by UV-Vis absorption

spectroscopy. Upon increasing the Sn dopent concentration the optical bandgaps of the

ZnO increases from 3.35 to 3.42 eV. The electrical resistivity first decreased with the

increase of tin concentration, this is due to the partial substitution of divalent Zn2+

ions with tetravalent Sn4+ ions, generating more free electrons for conduction.

126

Figure 3.13 The variation of electrical resistivity of the pure and Sn-doped ZnO

nanocrystalline materials as a function of Sn dopant concentration.

127

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