Morphology and Optical Measurements of Nanostructured In 2 O 3 : SnO 2 Nanoparticle

Ultra-fine and uniform ITO nanopowders can be prepared by very simple Combustion method. In this paper, effects of Indium oxide with Sn doping on crystallinity, band gap values by UV studies and morphological studies by SEM and TEM analysis of nanopowders are reported. Powder mixtures of In2O3:SnO2 of 90:10 compositions are prepared and calculated grain size from X-ray diffraction measurements. The free electron absorption is determined from spectral transmission and reflection measurements.


Introduction
In 2 O 3 :SnO 2 (also called indium tin oxide or ITO) is a highly degenerate n-type wide gap semiconductor that is produced by doping Sn atoms in In 2 O 3 .Since ITO powders have a high transmittance in the visible range and a high conductivity simultaneously, they are widely used in a variety of electronic and optoelectronic fields.In Combustion process In 2 O 3 :SnO 2 powders show an interesting and technologically important combination of properties: they have luminous transmittance, high infrared reflectance, good electrical conductivity, excellent substrate adherence, hardness, and chemical inertness.Composite powders have been patented for use as agrochemicals and herbicides.Parent et al. [1] used extended X-ray absorption fine structure (EXAFS) investigations and found that the In atomic environment was modified by the Sn doping.Even at low tin concentrations the first oxygen polyhedron and the metallic In-In co-ordination shells were disordered.This disorder increases with the amount of tin inserted.Tin doped indium oxide is extensively studied as a base material for its gas sensor applications.It is oxygen deficient and therefore is an n type semiconductor with a wide band gap (3.53 eV) [2].The effect of crystallite size on the optical properties has been widely studied [3].Nanocrystalline powders have attracted considerable interest because of their high transmittance.Here, we have presented the optical properties of 90:10 nanocrystalline ITO powders [4].

Physical Characterization
The visible transmission was recorded using Hitachi S3400N Spectrophotometer.The calcined powders were further characterized by particle size analysis (Autosizer IIC Malvern) and powder X-ray diffraction (XRD) (Xray diffractometer (XRD) with monochromation CuK α target (1.5406Å) at a scan rate of 2˚C/min).Unit cell parameter was calculated from the observed "d"-spacing, which was accurately measured with the help of silicon as an internal standard.Particle size and morphology of the synthesized powders were further evaluated with the help of a transmission electron microscope TEM (JEOL JEM 200CX).Scanning electron microscope (SEM) and Energy dispersive X-ray analysis (EDAX) work on the calcined powders were performed under Scanning Electron Microscope Cambridge 53400N and elemental analysis was studied by EDAX Make "thermo software Norton systems".The asprepared powders were pelletised at a pressure of 500 kg/m 2 .

Experimental
The Combustion method is a useful technique that has been shown to be good preparation route for ITO nanopowders [5].It is based on exothermic and usually very rapid chemical reaction between metal nitrates as an oxidizer and an organic fuel [6], such as Urea, glycine and so on.
A key feature of the method is that heat is required to maintain the chemical reaction supplied from the reaction itself that is not from an external source but from an internal one.Therefore to achieve an optimized combustion reaction condition, many chemical reaction parameters must be considered [7].Among known fuels we used the urea which had the versatility of combustion synthesis methods by showing successful preparation of a large number of well crystallized multicomponent oxides [8].The raw materials used in this study are In(NO 3 ) 2 •2H 2 O, Sn(NO 3 ) 2 •3H 2 O, these raw materials were dissolved in distilled water and mixed in a appropriate ratio to form a tin nitrate solution.Then urea (CO(NH 2 ) 2 ) was added to this solution.The amount of urea was fixed at 1.0 mol [9].Amount of urea was calculated based on total valence of the oxidizing and reducing agents for maximum release of energy during combustion.Oxidant/fuel ratio of the system was adjusted by adding nitric acid and ammonium hydroxide; and the ratio was kept at unity.The solution was heated under constant stirring at a temperature of about 60˚C in a Pyrex vessel on a hot plate.Then the concentration of the solution slowly became higher.The resulting translucent solution was heated on a hot plate (at about 100˚C) until it turned into a viscous solution.The solution boils upon heating and undergoes dehydration accompanied by foam.The foam then ignites by itself due to persistent heating giving a voluminous and fluffy product of combustion.The combustion product was subsequently characterized as single phase nanocrystals of ITO.The resulting ashes were then fired at a temperature higher than 350˚C until complete decomposition of the residues was achieved.Yellow ashes obtained after combustion were then collected for structural characterization and other morphological studies.The system was homogeneous during the whole process and no precipitation was observed.The entire processing steps are illustrated in Figure 1.The final mixtures are heated for two different temperatures of 300˚C and 500˚C.

XRD
Structural studies by XRD showed the presence of dominant β-phase with a minor quantity of α-phase.The atomic arrangements in the grain boundary seem to be somewhat different from regular periodic arrangement whereas inside the grain there is a good periodic arrangement of atoms.nm for the prominent of peak (222) (400), ( 440) and (622) orientation.From the figures the analysis of crystal struc-he solubi tend mainly to gather near the Sn ion on In 2 sites and can form strongly bound O-Sn-O units.This occupation mechanism does not produce additional free electrons.The increase in conductivity is only small.Larger doping concentrations cause a further oxygen reception.Apart from the O-Sn 2 -O units O-Sn 1 -O arrangements can also occur.This effects a further distortion of the bix-byte structure.The microstrain increases and the domain size decreases which is shown in Figures 2(b)-(d).The grain size strain values are shown in Table 1.
ture revealed that the detected peak of the nanoparticle was corresponded with that of crystallized ITO.That is very intense peak was found at the three most important peaks of In 2 O 3 namely (222), (400) and (440) reflections.The diffraction patterns are well matched with TEM results shown in Figure 3. EDAX analysis confirmed the prepared nanoparticles were fine indium tin oxide powdered and there was no impurity in it as shown in Figure 5.
ITO materials retain the cubic structure upto t lity limit of the SnO 2 in In 2 O 3 [11] at 90:10 doping concentrations when the temperature increases the lattice constant also increases rapidly.Then the tin ions occupy In 1 and In 2 sites shown in Figure 1(b).The stoichiometric balance can be maintained only if additional oxygen is inserted into the lattice.These oxygen atoms can fill the empty tetracheal sites in the bixbyite structure.They

UV Studies
It was reported that, the model developed by Franck and Kostlin [12] for tin insertion into In 2 O 3 results from an analysis of the carrier concentration and additional measurements of the lattice constants dependent on the doping  oncentration.Optical analysis comprises two kinds of c interstitial oxygen, one of which is loosely bound to tin, while the other forms a strongly bound Sn 2 O 4 complex.At low doping concentration (10% Sn) here we found decreasing lattice constants and therefore concluded a dominance of the loosely bound tin-oxygen defects.Optical absorbance and transmission (T) spectra Figures 6(a)-(c) were measured using a near-infrared to UV double-beam spectrometer.Optical absorption coefficients (α) were calculated by correcting the reflection using a formula α , where E g is the optical band gap (Tauc n s a constant.Using the model with n = 2 proposed by Tauc, E g was estimated by linearly extrapolating the plot of gap) and i hv  vs hv and finding the intersection with the abscissa auc Plot) [13,14] shown in Figures 7(a)-(c) and 8.The band gap for different temperatures are shown in Table 2.

Sensor Analysis
The addition of a small amount of doping elements to SnO 2 powders has been known to be effective in modifying their optical and electrical properties [16].In particular, in the case of gas sensor applications, a gene strategy for tailoring mate e selective sensor ddition of dopants during sensor fabrication [17].Hence, there is a growing interest in doping SnO 2 with different ral rials for th response involves the modification of the surface by the a element ent study, we o ease in grain siz rease in temperatur of the samp epared is of the order nm.In this works, grai effects on the sensitiv gas sensor devices have reported shown in Figure 9.
But the powders are highly resistive.Hence, In 2 O 3 :SnO 2 dopings can be used in areas where only a moderate electronic conductivity is required.

Conclusion
The experimental studies revealed that the sensitivity is enhanced when the grain size is decreased.Also Xu et al. [18] have observed that, by reducing the particle size (~6 nm), high gas sensitivity and short response times can be achieved.In our research the study of indium-doped SnO 2 powders, it has been concluded that uniform, homogenous and highly transparent powders with small grain size 9 nm can be prepared by indium doping on tin oxide.

Figure 2 .Figure 3 .
Figure 2. (a) XRD analysis fo (d) Micro strain.r two different temperatures; (b) Grain Size; (c) Lattice Constant; ) leading to an interconnected ance SEM photograph ITO ( spherical structure with good mechanical strength.The nano-structural changes taking place during combustion process leads to a decrease in pore size and pore diameter.The nano-structural changes are accompanied by the associated internal stresses in the membrane.Increasing the annealing temperature with respect time can lead to local shifts in the interconnected grains, so as to reduce the internal stresses.It is well documented that the surface of nanostructure materials.The reaction temperature is one among them.We obtained surface spherical morphologies of ITO asprepared, 300˚C and 500˚C using ITO powder as source material.Figures4(a)-(c) show Scanning Electron Microscopy (SEM) images of spherical-shaped ITO nanostructures at (100˚C, 300˚C and 500˚C) different temperatures[15].morphology has a significant impact on the perform

Figure 8 .
Figure 8. Grain size and bandgap values of ITO.

igure 9 .
Sensitivity with respect to time of as prepared sample.