ORIGINAL PAPER Cetyltrimethylammonium bromide

Chemical Papers 68 (8) 1079–1086 (2014) DOI: 10.2478/s11696-014-0543-9

ORIGINAL PAPER

Cetyltrimethylammonium bromide- and ethylene glycol-assisted preparation of mono-dispersed indium oxide nanoparticles using hydrothermal method Selvakumar Dhanasingh, b Dharmaraj Nallasamy*, c Saravanan Padmanapan, a Vinod Chidambar Padaki a Defence b Inorganic

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a,b

Bioengineering and Electromedical Laboratory, 560 093 Bangalore, India

& Nanomaterials Research Laboratory, Department of Chemistry, Bharathiar University, 641 046 Coimbatore, India c Defence

Metallurgical Research Laboratory, 500 058 Hyderabad, India

Received 13 August 2013; Revised 16 November 2013; Accepted 21 December 2013

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The influence of cetyltrimethylammonium bromide and ethylene glycol on the size and dispersion of indium oxide nanoparticles prepared under hydrothermal conditions was investigated. The precursor compound, indium hydroxide, obtained by the hydrothermal method in the absence as well as the presence of cetyltrimethylammonium bromide, was converted to indium oxide by sintering at 400 ◦C. The formation of nanoscale indium oxide upon sintering was ascertained by the characteristic infrared adsorption bands and X-ray diffraction patterns of indium oxide. Transmission electron microscopy and band gap values confirmed that the cetyltrimethylammonium bromide facilitated the formation of indium oxide nanoparticles smaller in size and narrower in distribution than those prepared without the assistance of cetyltrimethylammonium bromide. c 2014 Institute of Chemistry, Slovak Academy of Sciences

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Keywords: indium oxide, hydrothermal, cetyltrimethylammonium bromide, nanoparticles

Introduction

Indium oxide (In2 O3 ) is an important n-type semiconductor with a wide band gap of approximately 3.6 eV that exhibits high transparency in the visible region and excellent electrical conductivity (Gopchandran et al., 1997; Steffes et al., 2001; Tanaka & Esaka, 2001). Nanostructured In2 O3 has been reported as having applications in the field of optoelectronic devices such as solar cells, liquid crystal display (Niederberger et al., 2006; Pinna et al., 2005; Eranna et al., 2004), architectural glasses (Shigesato et al., 1992), gas sensors (Takada et al., 1993), flat-panel display (Granqvist, 1993; Hamburg & Granqvist, 1986) and in photocatalytic conversions (Lei et al., 2006). Of the wet chemical methods, the hydrothermal route has been shown to pro*Corresponding author, e-mail: [email protected]

duce well-crystallised metal oxides under moderate reaction conditions, i.e. low temperature and short reaction time (Hayashi & Hakuta, 2010). Nanostructured In2 O3 with various morphologies has been prepared by controlling the synthetic parameters of the hydrothermal route such as precursors, base, reaction time, temperature and surfactant (Elouali et al., 2012; Souza et al., 2009; Xu et al., 2013; Rumyantseva et al., 2009; Li et al., 2010a; Tseng & Tseng, 2009; Guo et al., 2011). Porous cube-like nanocrystalline In2 O3 particles have been fabricated by laser ablationreflux process (Huang & Yeh, 2008). As a modified hydrothermal route, ethylene glycol-mediated synthesis was previously attempted for the size-controlled preparation of metal oxides, in which the higher boiling point of ethylene glycol rendered it convenient to perform the synthetic manipulations at relatively

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S. Dhanasingh et al./Chemical Papers 68 (8) 1079–1086 (2014)

range from 20◦ to 70◦ and the crystallite size was estimated by the line-broadening method. Scanning electron microscopic (SEM) images of the samples were recorded on a JEOL JSM-6490L scanning electron microscope (Japan). Transmission electron microscopic (TEM) images were taken using a JEOL JEN 2010 operated at 200 kV accelerating voltage using a copper grid dipped in ethanol containing dispersed nanoparticles. A stock solution of indium nitrate (0.08 mol L−1 ) was prepared by dissolving indium metal in nitric acid and ultrapure water. To 12.5 mL of the indium nitrate solution, 2.5 mL of ethylene glycol was added under stirring. Next, 0.2804 g of HMT (mole ratio between indium and HMT was 1 : 2) dissolved in 15 mL of de-ionised water was added drop-wise at a rate of 0.25 mL min−1 under vigorous stirring. In the case of the CTAB-assisted method, 3.0 g of CTAB was dissolved in the aqueous solution of indium nitrate/ethylene glycol as prepared above prior to adding HMT. The reaction mixture was stirred for a further 5 h and the resulting translucent solution was autoclaved in a Teflon -lined stainless steel autoclave, with an inner volume of 60 mL, in a thermostated oven at 180 ◦C for 8 h. The In(OH)3 thus formed as a white precipitate was filtered, washed repeatedly with water followed by absolute ethanol and dried at 110 ◦C. Finally, the In(OH)3 was calcined at 400 ◦C for 3 h under atmospheric conditions to afford bright yellow In2 O3 .

Experimental

Souza and Muccillo (2006) demonstrated that the use of indium nitrate resulted in smaller-sized In2 O3 nanoparticles than when using indium chloride as the starting material. Under hydrothermal conditions, HMT decomposes into formaldehyde, ammonia and OH− species (Blažzević et al., 1979). In this process, the uniform formation of hydroxide ions throughout the reaction medium favours the nucleation of nanocrystallites of [In(OH)x ]n− with controlled size and shape. The reaction occurring in the hydrothermal process could be formulated as follows:

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higher temperatures (Jiang et al., 2004, 2005; Lee et al., 1999; Kakihana et al., 1999; Ho et al., 2005). Nanorods and microsphere-like indium hydroxide (In(OH)3 ) have been prepared by the cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/1propanol microemulsion-mediated hydrothermal process by changing the pH values of the microemulsion (Yang et al., 2006). In(OH)3 with various microand nanomorphologies have been reported using a similar microemulsion route by varying the experimental parameters such as concentration of reactants and reaction temperature (Yin et al., 2009). Tao et al. (2010) prepared one- dimensional nanostructures of In(OH)3 by refluxing solutions of indium nitrate and CTAB in an ethanol/water solvent system. The above studies established that the cationic surfactant, CTAB, stabilises the surface around the In(OH)3 nuclei. The hydrophobically modified In(OH)3 surfaces lead to the formation of admicelles preventing the interaction among the In(OH)3 nuclei and thus controlling the growth of nanoparticles. As a result, CTABassisted synthesis yielded smaller nanoparticles than non-CTAB-mediated routes (Li et al., 2010b). The present study sought to investigate the combined influence of CTAB and ethylene glycol on the formation of In2 O3 nanoparticles under hydrothermal reaction conditions using hexamethylenetetramine (HMT) as the source of hydroxide ions. The thermal decomposition of In(OH)3 to In2 O3 and their optical, photoluminescence and morphological characteristics were investigated.

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Indium metal (Strem Chemicals, USA), HMT (Alfa Aesar, UK), ethylene glycol (Sigma–Aldrich, USA), CTAB (Sigma–Aldrich) and nitric acid (Merck, Germany) were used as received without further purification. The thermal decomposition behaviour of the In(OH)3 precursor was studied by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) using universal V2.5H TA instruments (TA Instruments, UK) under a flow of synthetic air (50 mL min−1 ) at a heating rate of 5 ◦C min−1 . Fourier transform infrared spectroscopic (FT-IR) spectra using KBr pellets were recorded on a Perkin–Elmer 1000 FT-IR (USA) spectrophotometer in the range of 4000– 400 cm−1 . The UV-VIS absorption spectra of the samples were recorded using a JASCO–UV VIS spectrophotometer (Japan) applying a quartz cell of 1 cm optical length at ambient temperature. The photoluminescence spectra (PL) of the samples were recorded on a JASCO spectrofluorometer with a 310 nm excitation line using a 450 W xenon lamp as the excitation source. The powder X-ray diffraction (XRD) patterns of the samples were recorded using a Panalytical X-ray diffractometer (The Netherlands) (CuKα radiation, λ = 1.54 ˚ A) employing a scan rate of 0.02◦ s−1 in a 2θ

Results and discussion

C6 H12 N4 + 6H2 O → 6HCHO + 4NH3

(1)

− NH3 + 2OH− → NH+ 4 + OH

(2)

In3+ + 3OH− → In(OH)3

(3)

The co-presence of ethylene glycol functions as a bi-dentate coordinating ligand and also as spacer molecule that restricts the interaction and growth of nanocrystallites (Marques et al., 2010). In the case of the CTAB-assisted reaction, the hydrophilic head of CTAB molecules attaches to [In(OH)x ]n− nuclei due to electrostatic attraction. Fig. 1a shows the thermal decomposition pattern of In(OH)3 prepared in the ab-

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Fig. 2. FT-IR spectra of In(OH)3 without CTAB (a), In(OH)3 with CTAB (b) and In2 O3 nanoparticles calcined at 400 ◦C (c).

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Fig. 1. TGA/DTA curves of In(OH)3 prepared without CTAB (a) and TGA/DTA curves of In(OH)3 prepared with CTAB (b).

sence of CTAB. The mass loss which occurred up to 150 ◦C (Fig. 1a, Region 1) was due to the removal of surface-adsorbed water molecules. Another significant mass loss (16 %) was noted in the range from 150 ◦C to 300 ◦C (Fig. 1a, Region 2). According to the cocondensation reaction, the theoretical mass loss for the conversion of In(OH)3 to In2 O3 was calculated as 16.3 %. Hence, the mass loss which occurred in Region 2 of TGA is attributed to the thermal conversion of In(OH)3 to In2 O3 . The minor mass loss of approximately 0.70 % observed between 300 ◦C to 400 ◦C (Fig. 1a, Region 3) is attributed to the decomposition of ethylene glycol molecules, which are adsorbed onto In(OH)3 .The endothermic peak occurring in the DTA curve at 260 ◦C can be attributed to the conversion of In(OH)3 to In2 O3 . The exothermic peak observed at 300 ◦C occurred due to the burning of ethylene glycol molecules. Fig. 1b shows the thermal decomposition pattern of In(OH)3 prepared in the presence of

CTAB. The TGA/DTA trace exhibited a similar pattern to that in Fig. 1a. The exothermic peak which occurred at 308 ◦C may be due to the combined decomposition of the ethylene glycol and CTAB adsorbed onto In(OH)3 . Hence, the corresponding mass loss of approximately 0.90 % observed from 300 ◦C to 400 ◦C can be accounted for as the adsorbed ethylene glycol and CTAB (Tao et al., 2010). The final mass loss corresponding to the decomposition of organic molecules from the indium hydroxide samples prepared using ethylene glycol without CTAB was found to be 0.70 % (corresponding to 0.113 mM of ethylene glycol per g of In(OH)3 ) and for the sample of indium hydroxide prepared with the assistance of both ethylene glycol and CTAB was calculated to be 0.90 % (corresponding to 0.113 mM of ethylene glycol and 0.006 mM of CTAB per g of In(OH)3 ), respectively. Figs. 2a–2c display the FT-IR spectra of In(OH)3 and In2 O3 samples. Figs. 2a and 2b show the IR spectrum of In(OH)3 prepared without and with CTAB. The absorption bands which appeared at approximately 3380 cm−1 are assigned to the stretching mode of OH groups of adsorbed water. The band at 3235 cm−1 is attributed to the stretching vibrations of OH groups from In(OH)3 . The OH deformation bands appeared at 1156 cm−1 (sharp), 1096 cm−1 (shoulder) and 1067 cm−1 (shoulder) as reported by Yang et al. (2011). The bands observed at 782 cm−1 , 499 cm−1 and 410 cm−1 were assigned to the In—OH stretching vibration modes. In addition, both the spectra registered two bands at 1465 cm−1 and 1385 cm−1 , respectively, due to the anti-symmetric and symmetric bending vibrations of the CH2 groups of organic molecules adsorbed onto In(OH)3 . However, these absorptions are rather weak, suggesting the very low concentration of adsorbed organic species, as identified by thermal studies. The IR spectrum of In2 O3 nanoparticles prepared after calcination of the precursor, indium hydroxide,

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Fig. 4. Plot of (αhν)2 vs. hν of In(OH)3 without CTAB (a) and with CTAB (b). The linear solid lines show the extrapolation of the linear region of the plots on x-axis.

Fig. 3. UV-VIS spectra of In2 O3 without CTAB (a) and with CTAB (b).

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obtained with CTAB at 400 ◦C is presented in Fig. 2c. This spectrum did not register the bands at 782 cm−1 , 499 cm−1 and 410 cm−1 due to In—OH but new bands were observed at approximately 560 cm−1 , 535 cm−1 , 450 cm−1 and 450 cm−1 due to In—O—In and O— In—O vibrations (Ho & Yen, 2006). In addition, the non-appearance of bands at 1465 cm−1 and 1385 cm−1 confirmed the complete removal of adsorbed ethylene glycol and CTAB during the calcinations process. The UV-VIS absorption spectra of the as-prepared In2 O3 nanoparticles dispersed in ethanol are shown in Figs. 3a and 3b. The nanoparticles prepared without the assistance of CTAB showed a broad absorption edge at 311 nm, whereas the nanoparticles prepared in the presence of CTAB registered the band at 307 nm, due to the excitonic transition of the valence band electrons to the conduction band. These excitonic peak values are in good agreement with previous reports of In2 O3 thin film and In2 O3 nanoclusters (Kundu & Biswas, 2005; Zhou et al., 1999). The blue shift in the absorption edge of nanoparticles prepared through the CTAB-assisted route reveals their smaller particle size than when formed by the nonCTAB route. Figs. 4a and 4b show the Tauc plots for In(OH)3 obtained from optical absorption data by plotting (αhν)2 vs. photon energy (hν), where α, h and ν are the absorption coefficient, Plank’s constant and photon frequency, respectively (Davis & Mott, 1970). The inflection of the plots afforded band gap values of 5.46 eV and 5.66 eV prepared with and without the addition of CTAB, respectively. These values are higher than the bulk band gap value of In(OH)3 , which is 5.15 eV (Avivi et al., 2000). Similarly, Figs. 5a and 5b show the band gap values of the In2 O3 nanoparticles. The band gap values obtained at 3.78 eV and 4.15 eV are for In2 O3 nanoparticles prepared with and without the addition of CTAB, respectively, which are higher than the values reported for bulk In2 O3 .

Fig. 5. Plot of (αhν)2 vs. hν of In2 O3 without CTAB (a) and with CTAB (b). The linear dashed lines show the extrapolation of the linear region of the plots on x-axis.

The inflexions appearing at lower energies can be attributed to the presence of larger crystals (Kundu & Biswas, 2005). The band gap value of In2 O3 nanoparticles and In(OH)3 prepared in the presence of CTAB was larger than for the same materials prepared without CTAB. In the case of the CTAB-assisted method, the nanocrystallites of [In(OH)x ]n− form ionic pairs with CTA+ (Tao et al., 2010) and the surfactant ions attached to the surface of the [In(OH)x ]n− function as a growth stabiliser. Hence, the stronger ionic interaction of CTA+ with [In(OH)x ]n− resulted in smaller nanoparticles of In(OH)3 and In2 O3 due to the better surface passivation of nanocrystallites than with the ethylene glycol molecules. A schematic representation of the formation of In(OH)3 with and without CTAB in the presence of ethylene glycol and water is given in Fig. 6. The ambient temperature PL emission spectra of the In2 O3 nanoparticles recorded with an excitation at 310 nm are presented in Figs. 7a–7b. It has been

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Fig. 6. Capping of In(OH)3 nanocrystallites by ethylene glycol only (a) and by both ethylene glycol and CTAB (b).

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established that the bulk- and micron-sized In2 O3 cannot emit light under ambient temperature conditions (Ohhata et al., 1979; Avivi et al., 2000). However, nanostructured and thin films of In2 O3 have exhibited strong PL emissions in both the UV and visible regions (Zhao et al., 2004; Seo et al., 2003; Zhou et al., 1999; Peng et al., 2002; Guha et al., 2004). The PL spectra exhibited a strong emission with its maximum at 354 nm (3.51 eV) and a few broad peaks centred at 379 nm, 395 nm, 422 nm, 450 nm and 468 nm. The PL emission peaks in UV regions are associated with nearband emission and the peaks observed above 400 nm may be accounted for by deep-level emissions due to amorphous In2 O3 , indium interstitials and oxygen vacancies as reported previously (Kumar et al., 2008; Zhang et al., 2007). The oxygen vacancies form deep defect donors and cause the formation of new energy levels within the band gaps of In2 O3 . Hence, the PL emission of In2 O3 nanoparticles resulting from the radioactive recombination of an electron-occupying oxygen vacancies with a photon-excited hole can be compared to the PL mechanism of ZnO and SnO2 semiconductors. A blue shift in the emission peak for CTABmediated nanoparticles is associated with their larger optical band gap due to their smaller particle size than nanoparticles prepared without CTAB. Figs. 8a and 8b show the powder XRD pattern of In(OH)3 . All the diffraction peaks of In(OH)3 accorded well with that of a body-centred cubic phase as per JCPDS card No. 85-1338. The XRD patterns of In2 O3 nanoparticles prepared with and without CTAB after calcination at 400 ◦C are shown in Figs. 8c and 8d. In the case of In2 O3 , all the detectable reflection peaks matched with the bulk cubic bixbyite-

Fig. 7. Ambient temperature photoluminescence spectra of In2 O3 without CTAB (a) and with CTAB (b).

type reflections according to JCPDS No. 0606416. The absence of any diffraction peak from In(OH)3 further confirms the complete conversion of In(OH)3 to In2 O3 during the calcination process. The XRD peaks were broader than in the bulk In2 O3 , which is a general, size-dependent phenomenon of nanoparticles. The crystallite sizes were estimated from linebroadening using the Debye–Scherrer equation (Cullity & Stock, 2001). The average particle sizes were approximately 25 nm and 12 nm for the nanoparticles prepared with and without CTAB, respectively. The surface morphology and particle size of the respective In2 O3 nanoparticles prepared with and without CTAB were analysed using SEM images and are presented in Figs. 9a and 9b. The images show that

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Fig. 8. XRD patterns of: In(OH)3 without CTAB (a), with CTAB (b); In2 O3 prepared without CTAB (c) and with CTAB (d).

Fig. 9. SEM images of In2 O3 without CTAB (a) and with CTAB (b).

the In2 O3 nanoparticles were of cubic morphology with porous structure. However, the CTAB-mediated process yielded In2 O3 nanoparticles of a smaller size than by using the non-CTAB-mediated method. The cubic morphologies of the In2 O3 nanoparticles prepared in the present study are similar to the solvothermal routes reported previously (Yang et al., 2010;

Fig. 10. TEM and SAED images of In2 O3 without CTAB (a) and with CTAB (b).

Liu et al., 2008, 2011). The TEM micrographs of In2 O3 nanoparticles along with their respective selected area electron diffraction (SAED) patterns are shown in Figs. 10a and 10b, which confirm that both the methods produced particles in the nanometre regime and are agglomerated as observed in SEM images. Nanoparticles prepared without CTAB were heterogeneous in shape and polydispersed, with sizes ranging mostly from 10–50 nm with a few plate-like particles in the range of 50–75 nm. On the other hand, the image of the CTAB-mediated nanoparticle showed almost monodispersed particles with sizes between 10 nm and 25 nm. The SAED pattern of both the samples revealed that the particles were well-crystallised. The bright spots in the patterns correspond to the (222) reflection plane with additional planes due to (440), (440) and (622) reflections of In2 O3 .

Conclusions The preparation of In2 O3 nanoparticles in the presence of ethylene glycol was undertaken using the hydrothermal method. The effect of CTAB as a surfactant on the size control, monodispersity and surface morphology of indium oxide nanoparticles was


investigated. The band gap values calculated from the optical spectral data indicated that the CTABmediated method afforded indium oxide nanoparticles of a smaller size. From the powder-XRD and TEM analysis, the average sizes of the nanocrystalline In2 O3 were found to be approximately 25 nm and 12 nm for the two different preparation methods, with and without the addition of CTAB, respectively. In addition, the TEM images confirmed that the surfactantassisted process favoured the formation of monodispersed indium oxide nanoparticles.

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Acknowledgements. The authors would like to express their gratitude to Dr. N. S. Kumar, Joint Director, DEBEL, Bangalore, India, for fruitful suggestions and Dr. K. Kadirvelu, Joint Director, DRDO-BU CLS, Bharathiar University, Coimbatore, India, for the XRD and SEM facilities provided for this work.

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