Radiation Induced Synthesis of In 2 O 3 Nanoparticles-Part 1 : Synthesis of In 2 O 3 Nanoparticles by Sol-gel Method Using Un-irradiated and γ-Irradiated Indium Acetate

Pure phase of In2O3 nanoparticles were synthesized by sol-gel method in non-aqueous medium using un-irradiated and γ-irradiated indium acetate precursors. The as-prepared In2O3 nanoparticles were characterized by XRD, FT-IR, SEM, TEM and TG techniques. The results showed that preirradiation to indium acetate with 102 KGy γ-ray absorbed dose has significant effects on the shape and size of the as-synthesized In2O3 nanoparticles. In case of un-irradiated precursor, the SEM images show agglomerated and disordered spherical nanoparticles. In γ-irradiated case the particles showed coral-like structure. The as-synthesized In2O3 nanoparticles prepared using γ-irradiated precursor showed higher thermal stability compared with those prepared using un-irradiated one. The results were discussed in view of the role of γ-irradiation on the morphology and thermal behavior of In2O3 nanoparticles.


Introduction
In 2 O 3 is an important n-type semi-conductor with a wide band gap of potential use in optoelectronic devices and gas sensing applications 1-3 . In 2 O 3 has two crystalline phases: Cubic In 2 O 3 (c-In 2 O 3 ) and hexagonal In 2 O 3 (h-In 2 O 3 ) with cubic phase as favorable phase for industrial applications 4,5 . Several routes such as chemical vapor deposition (CVD) 6 , sol-gel 7 , solution-phase growth 8 and solid state decomposition 9 have been employed for preparation of In 2 O 3 nanoparticles. Pre-irradiation of the precursors with high energy charged particles (electrons, ions) as well as X-rays and γ-rays can aid in the synthesis of nanoparticles by sol-gel and by solid state thermal decomposition methods. Whereas the un-irradiated precursors are commonly used, the control provided by the broad range of dose and dose rate might offer advantages in the fine-tuning of the size and morphology of the nanoparticles [10][11][12][13] . Irradiation effects on the preparation of In 2 O 3 nanoparticles were scanty investigated and to the best of our knowledge only one publication dealing with radiation induced synthesis of In 2 O 3 nanoparticles is reported 14 .
In the present work, we started a comprehensive study on the radiation-induced synthesis of In 2 O 3 nanoparticles by both sol-gel and solid-state thermal decomposition methods. Several influencing factors including variation of γ-ray absorbed dose, addition of surfactants, and variation of the used indium precursors were thoroughly investigated.
In our first submission, we started with un-irradiated and γ-irradiated indium acetate with 10 2 KGy γ-ray absorbed dose as precursors for preparation of In 2 O 3 nanoparticles by sol-gel method.

Synthesis of In 2 O 3 nanoparticles
All the chemicals used in this study were of analytical grade, purchased from Sigma-Aldrich and used without further purification 0.1 mole of both un-irradiated and 10 2 KGy absorbed dose of γ-irradiated anhydrous indium acetate (InAc) were included in two separate round flasks at room temperature. 0.3 mole of anhydrous benzyl alcohol was added dropwise to each flask with good stirring until white gels were formed. 100 mg of sodium dodecyl sulphate were added to each gel with stirring for ≈5 min. The two gels were subjected to thermal treatments at different calcination temperatures for different time intervals. At each calcination stage, the gel was investigated using FT-IR and TG techniques to follow the formation of In 2 O 3 nanoparticles as final product.

Characterization
X-ray powder diffraction patterns (XRD) were recorded on siemens JDX-3530 X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ= 1.54178 Å). FT-IR measurements were recorded as KBr pellets in the range of 4400-400 cm -1 on Perkin-Elemer FT-IR spectrophotometer. TEM images were captured on JOEL JEM 2010 TEM. SEM images were recorded using HITACHIS-4800 SEM. TG measurements were recorded on Perkin-Elemer TG A7 thermogravimetric analyzer in the range of 30-1000 °C. The sample weight was 10.0 ± 0.1 mg with heating rate of 10 °C/min. For irradiation, samples were encapsulated under vacuum in glass vials and exposed to successively increasing doses of radiation at constant intensity using a Co-60 γ-ray Cell 220 (Nordion INT-INC, Ontario, Canada). The exposure dose was attenuated to be 10 4 KGy/h in order to save irradiation time. The source was calibrated against a Fricke ferrous sulphate dosimeter and the absorbed doses in the irradiated samples were calculated by applying appropriate corrections on the basis of photon mass attenuation and the energy absorption coefficients for the sample and the dosimeter solutions. After irradiation the samples were stored at room temperature for 24 h before analysis. Figure 1 shows typical TG curves of the gel prepared using un-irradiated InAc precursor. Decomposition of the gel proceeds through three major steps. The first decomposition step was in the range of 50-110 °C, the second weight loss step was in the range of 520-270 °C and the third weight loss was in the range of 280-350 °C. The total experimental weight loss percentage (≈75%) was in good agreement with the percentage (74%) expected theoretically assuming release of three benzyl alcohol molecules, with the formation of In2O3 as final solid residue. The decomposition of the gel prepared using γ-irradiated InAc precursor proceeds by the same thermal behaviour as with the un-irradiated one. Figure 2 shows FT-IR of the gels formed using unirradiated and γ-irradiated InAc precursors before and after thermal oxidation treatments at 200 and at 400 °C calcinations temperatures for 5 hours respectively. The Figure 2 shows systematic decrease in the intensity of the absorption bands characteristic for benzyl alcohol molecules. Complete annealing of the gel and formation of In 2 O 3 nanoparticles was achieved by calcination at 400 °C for 5 hours as shown in Figure 2.The figure show the characteristic bands attributed to In-O phonon mode of vibration of cubic In 2 O 3 nanoparticles in the range of 400-800 cm -1 [15] . The chemisorbed H 2 O and CO 2 molecules on the surface of In 2 O 3 could be easily detected by recording the bands appeared in the range of 3400-3500 cm -1 and 1700-1600 attributed to νOH and νC=O respectively.

Results and Discussion
XRD patterns of the as-prepared samples for both unirradiated and γ-irradiated InAc with 10 2 KGy calcined at 200 °C for 5 hours show diffraction lines indexed as mixed phase of cubic and hexagonal In 2 O 3 for both un-irradiated and γ-irradiated InAc 16 .
The diffraction lines for the samples calcined at 400 °C for 5 hours are all indexed to pure body centered cubic (bcc) structure of In 2 O 3 (JCPDS Card No. 06-0416) as shown in Figure 3. No others impurity peaks were detected in the pattern which reveal the high phase purity of the sample. In addition, the EDS analyses also indicate only indium and oxygen as constituent's elements. The broadening of the obtained lines confirm nanocrystalline nature of the phase. However, in case of γ-irradiation precursor, the XRD pattern shows more broadening in the width of diffraction lines. This broadening leads to a decrease in the lattice dimensions due to the radiation damage induced by γ-irradiation in the host lattice of InAc precursor. This behavior was confirmed by calculating the particle size of In 2 O 3 nanoparticles by using of the Deby-Sherrer formula 17 .
where λ is the wavelength (in Angstrom), β is the broadening of the diffraction peaks (in radians) or full width at half maximum (FWHM), θ B is the Bragg diffraction angle. The calculated average particle size of In 2 O 3 nanoparticles was estimated to be in the ranges of (10-20) and (10)(11)(12)(13)(14)(15) nm in case of un-irradiated and γ-irradiated precursors respectively.
SEM images of In 2 O 3 nanoparticles obtained by using of un-irradiated and γ-irradiated InAc precursors are shown in Figure 4. Both images display the nanocrystalline nature of the as-prepared powders, with significant differences in the shape and size of the as-prepared In 2 O 3 nanoparticles as a result of γ-irradiation. In case of un-irradiated precursor, the SEM images show agglomerated and disordered spherical nanoparticles. In γ-irradiated case the particles showed coral-like structure. TEM images of the as-prepared In 2 O 3 nanoparticles showed also significant changes in the morphology of the particles prepared using un-irradiated and γ-irradiated precursors. In case of γ-irradiation, in contrast to the agglomerated cubic In 2 O 3 obtained using un-irradiated precursors, the image obtained for γ-irradiated precursor showed blocks of agglomerated nanocubes (primary of nanocrystallites ) with big holes between these blocks These nanocubes exhibit different orientation compared with those prepared by un-irradiated precursor. The nanocubes in case of ɣ irradiated precursor are preferentially oriented along 222 planes as was confirmed by XRD pattern of In 2 O 3 obtained using γ-irradiated precursor. The average particle size (≈10 nm) was estimated to be quite smaller than the value calculated using un-irradiated precursor (≈15 nm).
Thermal stability of In 2 O 3 nanoparticles were examined to illustrate the role of γ-irradiation on the thermal stability of the as-prepared In 2 O 3 nanoparticles. In general, the In 2 O 3 nanoparticles prepared using γ-irradiated InAc precursor showed higher thermal stability compared with those prepared using un-irradiated InAc. The weight loss in the temperature range of 50-600 °C was attributed to release of adsorbed water and volatile carbonate from the surface of In 2 O 3 nanoparticles. Starting by 650 °C the TG curve for In 2 O 3 nanoparticles prepared by un-irradiated InAC precursor show continuous weight loss due to degradation process. No detectable weight loss was recorded in the TG curves for In 2 O 3 nanoparticles of γ-irradiated InAc up to 800 °C the results are shown in Figure 5.The stability of In 2 O 3 nanoparticles prepared using un-irradiated precursor up to 800 °C clearly indicate that these particles can act as molten salts in the temperature range of 600 to 800 °C.This is the first report of the behavior of In 2 O 3 as molten salt in the temperature range of 600 to 800 °C. Figure 6 shows the RT-PL spectra of the as-synthesized In 2 O 3 nanoparticles. While exciting the sample with 266 nm, a strong emission at 383 nm and a weak emission at 650 nm

Role of irradiation
Upon irradiation with Co-60 γ-ray source, the photoelectric effect is the main mode of interaction of γ-ray with indium atom. In this way, stable molecules (M) are converted into solvated electrons (e) and high reactive free radicals (M + ).In solid samples, the radiation effect is dominated by direct ionization of the material whereas for  aqueous liquids the reaction with radical species such as OHor solvated electrons is the dominant mechanism for damage to a solute. The solvated electrons are trapped in the interstitials and vacancies positions in the host lattice of indium acetate unit cell and not allowed to interact with anhydrous benzyl alcohol moiety. These crystal defects could lead to point defect (anion Frenkel defect where O 2is the interstitial ion), defect clusters and extended defects. As a result of these events, impurity and nonstoichiomertric centers are created. Accordingly some kinds of modifications either in the morphology of the as-synthesized nanoparticles or in the course of thermal decomposition behavior of γ-irradiated materials may be generated 19 . From crystallographic point of view, when unit cell is exposed to γ-irradiation , the atoms may be disturbed from the equilibrium positions. Accordingly, the scattering action changes so that the structure factor becomes different. The lattice parameters was calculated from 222 plane by using of the formula + + =

Conclusion
In 2 O 3 nanoparticles were prepared by sol-gel method in non-aqueous medium using anhydrous benzyl alcohol and indium acetate as precursor. Pre-irradiation of indium acetate with 10 2 KGy absorbed γ-ray dose led to significant changes in the morphology and thermal properties of the assynthesized In 2 O 3 nanoparticles. Further studies concerning the effects of varying γ-ray dose on the preparation of In 2 O 3 nanoparticles and their applications as sensors are in progress in our laboratory.