Effect of Capping Agent on the Morphology , Size and Optical Properties of In 2 O 3 Nanoparticles

The Indium Oxide (In2O3) nanoparticles were synthesized through Acacia gum mediated method with the surfactants CTAB (Cetyl Trimethyl Ammonium Bromide) and SDBS (Sodium Docecyl Benzene Sulfonate). The characterization of the synthesized In2O3 nanoparticles was carried out by XRD, FTIR, RAMAN, TEM, SEM, EDAX, UV-Vis and PL techniques. TG-DTA analysis was performed to know the calcination temperature of In2O3 nanoparticles. XRD analysis confirmed the crystalline nature of the synthesized In2O3 nanoparticles. The morphology and chemical composition were characterized by TEM, SEM and EDAX respectively. It was observed that morphology and size of synthesized nanoparticles measured by TEM and SEM analysis were dependent on the type of capping agent (surfactant) used. Raman and UV-Vis spectral analysis confirmed that the band gap value of CTAB capped In2O3 particles were larger than the SDBS capped In2O3 particles. FTIR analysis indicated that the bands were stretched in In2O3 particles capped by SDBS than by CTAB. From the photoluminescence studies (PL technique), a blue shift in the emission peaks of CTAB and SDBS capped In2O3 particles was observed that indicates larger optical band gap than the bulk.


Introduction
Indium Oxide (In 2 O 3 ) is an important n-type semiconductor.It has a wide band gap of approximately 3.6eV, shows high transparency in the visible region and excellent electrical conductivity 1 .Semiconductor nanomaterials with a wide band gap have potential applications in nonlinear optics and optoelectronics 2 .It has fascinating properties such as strong interaction between certain poisonous gas molecules and its surfaces 3 .These properties make In 2 O 3 , a remarkable material for a variety of applications such as solar cells 4 , Liquid Crystal Displays 5 , Architectural Glasses 6 , Gas Sensors 7 , Flat Panel Display 8 , and in Photo-catalytic conversions 9 .To widen the technological applications of In 2 O 3 , investigations were made to synthesize them in different forms such as nanotubes, nanobelts, nano wires and nanoparticles.The properties of nanomaterials not only depend on the composition of the material but also depend on its size and shape which are influenced by the method of synthesis.
In 2 O 3 nanoparticles have been synthesized by several methods such as sol-gel method 10 , pulse laser deposition 11 , thermal decomposition 12 , thermal hydrolysis 13 , microemulsion method 14 , spray pyrolysis 15 , mechanical chemical processing method 16 , hybrid induction and laser heating (HILH) method 17 , non-aqueous synthesis 18 , and hydrothermal synthesis 19 .Among various synthesis methods, simple and cost effective route that use cheap, non-toxic and eco-friendly materials is the area of interest.One such simple method is gum mediated technique.Besides this, surfactants play an essential role in controlling morphology of nanostructure.It is because of their soft -template effect, ability to modify the chemical kinetics and simple maneuverability.
In the present work, In 2 O 3 nanocrystals with size 16-17nm have been successfully synthesized through simpler and easiest gum mediated method using CTAB (Cetyl Trimethyl Ammonium Bromide) and SDBS (Sodium Docecyl Benzene Sulfonate) as capping agents.It is reported that capping assisted synthesis yielded very fine nanoparticles than noncapping mediated routes 20 .

Materials and methods
Indium III Acetylacetonate (99.99% purity, Sigma Aldrich), Acacia gum (from Dawaa Saas in local market of Hyderabad), CTAB (Sigma Aldrich) and SDBS (Sigma Aldrich) were used as starting materials as purchased without further purification for the synthesis of In 2 O 3 nanoparticles.
1g of Indium III Acetylacetonate, 0.25g of Acacia gum and 0.25g of CTAB are mixed thoroughly by using mortar and pestle for the synthesis of CTAB capped In 2 O 3 nanoparticles.In a similar manner SDBS capped In 2 O 3 nanoparticles were synthesized by mixing 1g of Indium III Acetylacetonate , 0.25g of SDBS and 0.25g of Acacia gum in mortar and pestle.
Prepared Indium oxide particles capped by the two surfactants were subjected to TG-DTA analysis (Thermo gravimetric-differential thermal analysis) to know the calcination temperature.X-ray diffraction patterns were recorded using X-ray diffractometer with Cuk α (λ=1.5406A o ) in the range of 20-80 o (2θ) to know the crystal phase.An energy-dispersive X-ray Spectrometer (EDX) equipped with the Scanning Electron Microscopy was used to determine the morphology and sample composition.Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM) and Selected Area Electron Diffraction (SAED) on a JEOLJEM microscope operated at 200kV were used to know the surface morphology and crystallite size.FTIR spectra, Raman spectra, UV-vis absorption spectrum and the Photo Luminescence spectrum were recorded to know the band gap in the prepared nanoparticles capped by different capping agents.

TG-DTA Analysis (Thermo gravimetric-Differential Thermal Analysis)
There is an endothermic peak at 191.9 o C for CTAB and 190 o C for SDBS corresponding to the evaporation of OH as shown in Figure 1.The endothermic peak at 279 o C for CTAB and 269.4 0 C for SDBS indicates the heat released by decomposition of organic substance in CTAB and SDBS.This is due to the existence of organic solvent, CO and OH desorption.A small exothermic peak at 720 o C for CTAB and 730 o C for SDBS was observed.At these temperatures, CTAB and SDBS are considered to enter into In 2 O 3 crystal lattice respectively.The final mass loss in the TGA group was 4% and 6% for CTAB and SDBS respectively.The calcination temperature can be taken as 730 o C for the prepared In 2 O 3 particles.Hence, nano sized yellow powder of In 2 O 3 is obtained by calcining the prepared material at 730 o C for both CTAB & SDBS surfactants for 2 hours in air.

XRD Analysis
The XRD patterns of In 2 O 3 nanoparticles capped with CTAB and SDBS were shown in Figure 2. The major diffraction peaks (2 2 2), (4 0 0 ), (4 4 0) and ( 6 There is no impurity peak found indicating the formation of pure sample.The XRD peaks of In 2 O 3 were sharp in case of SDBS capping than that of CTAB capping which indicates good crystallinity of nanoparticles.The average crystallite size of the prepared In 2 O 3 nanoparticles capped by CTAB and SDBS was calculated by using Scherrer's equation 21 and was estimated as 16.6 nm and 17.4 nm respectively.The lattice parameters calculated using maximum intensity peak (222) plane were found to be 10.095Å for CTAB and 10.023Å for SDBS capped In 2 O 3 nanoparticles respectively.The values were reported in Table 1 and were found to be in good agreement with the literature 22 .

EDAX and SEM Analysis
The surface morphology of the respective CTAB and SDBS capped In 2 O 3 nanoparticles were analysed using SEM, and the images are presented in Figure 3.The images show that the CTAB capped In 2 O 3 NPs were spherical in shape and SDBS capped In 2 O 3 NPs were like spherical clusters with porous structure.The morphology of In 2 O 3 nanoparticles prepared in the present study show a well patterned distribution of nanocrystalline grains with porous nature.This spherical and porous nature renders them suitable for gas sensing applications.EDAX patterns of prepared In 2 O 3 nanoparticles stabilized by Acacia gum extract obtained from CTAB and SDBS capping agents are shown in Figure 3.
Strong signals from In and O atoms are recorded.From the EDAX data (table 2), it is clear that there is no reference to other phases.This confirms that the CTAB and SDBS capped In 2 O 3 samples contains pure In and O phases as evident from Table 2.
There is loss in atomic % of In in CTAB.It may be due to the presence of traces of (Na, S and Br) which might have occurred in the process of calcination.The atomic ratio of In and O was found to be 2:3 which is nearer to the stoichiometry ratio of In 2 O 3 .The discrepancy in the stoichiometry may be due to the presence of oxygen vacancies.These vacancies may be created during the calcination in the static atmosphere i.e. box-type muffle furnace.

TEM , HRTEM & SAED
The morphology and the structure of the In 2 O 3 nanoparticles were investigated by TEM.From the TEM images shown  in Figure 4, it was found that the In

Raman Spectra Analysis
The acquired Raman spectra of CTAB and SDBS capped In 2 O 3 nanoparticles also provides the evidence for the cubic In 2 O 3 nanoparticles .The observed Raman peaks from the figure 5 at 160, 304, 626, 989 cm -1 for CTAB capped and 213, 404, 600, 1371 cm -1 for SDBS capped In 2 O 3 Nanoparticles are ascribed to the phonon vibration modes of cubic In 2 O 3 nanoparticles.The values are in good agreement with those reported in the literature 23,24 .Raman spectra related to the pure vibrational modes and the peaks at 160 cm -1 for CTAB and 213cm -1 for SDBS capping agrees with the reported value in literature 25,26 .It is found that the vibrational modes of SDBS capped are finer than that of CTAB capped In 2 O 3 nanoparticles.

UV and Tauc's Plot
The UV-Vis absoption spectra of the as-prepared In 2 O 3 nanoparticles dispersed in Ethylene Glycol are shown in Figure 6(a).The prepared nanoparticles with the capping agents CTAB showed a band edge at 288nm whereas those prepared with the capping of SDBS showed the band at 292nm.This fact is due to the excitonic transition of the valence Materials Research

FTIR Analysis
The bands around 2925cm -1 , 2858cm -1 and 1416cm -1 can be ascribed to the C-H vibration of the organics.The band at 1050cm -1 is attributed to the absorption of C-O vibration, while the absorptions around 500 cm -1 are due to In-O vibrations 28 .The peak at 1568cm -1 appeared on the IR spectrum is due to C-O vibrations from the Acetylacetone species 29 .Based on the experiments, it can be reported that the acetylacetone species coordinated to In 3+ cations on the particle surface could be substituted by the capping agents during the biosynthesis process.The bands of CTAB & SDBS Capped In 2 O 3 nanoparticles are in good agreement with the literature values and are stretched in SDBS than in CTAB capped as shown in Figure 7 & Table 4.

PL Analysis
Photo Luminescence emission was mainly attributed to the presence of vacancies or defects.Vacancies may be Indium or Oxygen vacancies, while the defects may be interestial Indium or anti-site Oxygen 30 .Vacancies present in the materials induce the formation of new energy levels in the bandgap and as a result emissions will arise from their trap levels.While exciting the sample, emissions occur due to radiative recombination of a photo excited hole with an electron and the emission peaks are commonly referred to as deep level or trap state emissions due to oxygen vacancies 31 .The luminescent property of the prepared In 2 O 3 cubic crystal capped by CTAB and STBS, calcined at 650 0C was analyzed by exciting the sample at 310nm wavelength of Xenon lamp. Figure 8 shows the room temperature Photo Luminescence (PL) Spectra of In 2 O 3 samples capped with CTAB and SDBS.The spectra of CTAB capped In 2 O 3 NPs exhibited a strong emission with its maximum at 342nm and a few broad peaks centered at 400nm and 425nm.Whereas, in the PL spectra of SDBS capped In 2 O 3 NPs, the maximum was observed at 326nm and a very few broad peaks were centered at 400nm and 430nm.The maximum absorption observed at 326nm and 342nm may be accounted for by deep-level emissions due to amorphous In 2 O 3 , and very few broad peaks at around 400nm to 425nm may be due to Indium interstitials and Oxygen vacancies (Impurities) 32

Conclusion
The In 2 O 3 nanoparticles were prepared using two capping agents (CTAB and SDBS).They exhibited different particle size and morphology, which were studied from XRD, 6 2) observed in the figure are indexed to the cubic bixbyite structure of In 2 O 3 as evidenced from JCPDS No.06-0416.

Figure 1 :Figure 2 :
Figure 1: TG-DTA curves of thermal decompostion of In 2 O 3 capped with (a) CTAB (b) SDBS at a heating rate of 10oC/min in static air 2 O 3 particles capped by CTAB were like spherical balls and that capped by SDBS were like spherical clusters.Their average sizes were found to be 15-20nm capped by CTAB and 20-25nm capped by SDBS respectively which are in good agreement with those values calculated by XRD analysis.The corresponding selected-area electron diffraction (SAED) patterns (Figure 4) of CTAB and SDBS capped In 2 O 3 samples show spotty ring patterns without any additional diffraction spots and rings of second phases, revealing their crystalline cubic structure.The interplanar spacing d hkl from SAED patterns for CTAB and SDBS capped In 2 O 3 nanoparticles was in good agreement with the values in the standard data (JCPDS: 06-0416) as summarized in Table 3.The high-resolution TEM images confirm that the synthesized particles are crystalline in nature.The nanoparticles are clearly well identified and no effective aggregation of bulk particles is observed.This fact indicates the effective capping of CTAB and SDBS on the surface of nanoparticles.

Figure 3 :
Figure 3: EDAX and SEM images of In 2 O 3 samples capped with (a) CTAB (b) SDBS

Figure 6 (
b) and fig.6(c) represents the Tauc's plot for In 2 O 3 nanoparticles obtained from optical absoption data by plotting (αhν) 2 vs hν (photo energy) where, α, h and ν are the absorption coefficient, Planck's constant and photo frequency respectively.The inflections of the plots afforded band gap values of 3.844eV for CTAB and 3.857eV for SDBS capped In 2 O 3 nanoparticles respectively.These values are higher than the bulk band gap values of In 2 O 3 .The band gap value of CTAB capped In 2 O 3 is larger than the SDBS capped In 2 O 3 nanoparticles.Hence, the stronger ionic interaction of CTAB than that of SDBS with In 2 O 3 has resulted in smaller size nanoparticles.
Photo Luminescence emission was mainly attributed to the presence of vacancies or defects.Vacancies may be Indium or Oxygen vacancies, while the defects may be interestial Indium or anti-site Oxygen30 .Vacancies present in the materials induce the formation of new energy levels in the bandgap and as a result emissions will arise from their trap levels.While exciting the sample, emissions occur due to radiative recombination of a photo excited hole with an electron and the emission peaks are commonly referred to as deep level or trap state emissions due to oxygen vacancies31 .The luminescent property of the prepared In 2 O 3 cubic crystal capped by CTAB and STBS, calcined at 650 0C was analyzed by exciting the sample at 310nm wavelength of Xenon lamp.Figure8shows the room temperature Photo Luminescence (PL) Spectra of In 2 O 3 samples capped with CTAB and SDBS.The spectra of CTAB capped In 2 O 3 NPs exhibited a strong emission with its maximum at 342nm and a few broad peaks centered at 400nm and 425nm.Whereas, in the PL spectra of SDBS capped In 2 O 3 NPs, the maximum was observed at 326nm and a very few broad peaks were centered at 400nm and 430nm.The maximum absorption observed at 326nm and 342nm may be accounted for by deep-level emissions due to amorphous In 2 O 3 , and very few broad peaks at around 400nm to 425nm may be due to Indium interstitials and Oxygen vacancies (Impurities)32 .A blue shift observed in the emission peaks of CTAB and SDBS capped In 2 O 3 nanoparticles indicates the presence of In 2 O 3 NPs only.

Figure 6 :Figure 7 :
Figure 6: Room temperature Optical Absorbance Spectra of In 2 O 3 samples capped with (a) UV-Vis absoption spectra of In 2 O 3 samples capped with CTAB and SDBS and (b) CTAB and (c) SDBS

Figure 8 :
Figure 8: Room temperature PL Spectra of In 2 O 3 samples capped with (a) CTAB (b) SDBS calcined at 650 o C SEM and TEM techniques.The average particle size of the nanoparticles determined by TEM and XRD analyses was almost same and was found to be 16nm with CTAB surfactant and 17nm with SDBS surfactant.The particles

Table 1 :
Average Particle size and Cubic Lattice parameter of In 2 O 3 samples capped with CTAB and SDBS from XRD, TEM analysis

Table 2 :
EDAX data of In 2 O 3 samples capped with CTAB and SDBS

Table 3 :
Interplanar Spacings of In 2 O 3 samples capped with CTAB and SDBS from SAED patterns of Fig.4 with standard JCPDS(06-0416)data