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Colloidal synthesis of indium nanoparticles by sodium reduction method
Materials Letters 59 (2005) 1032 – 1036 www.elsevier.com/locate/matlet
Colloidal synthesis of indium nanoparticles by sodium reduction method P.K. Khannaa,b, Ki-Won Juna,*, Ki Bum Honga, Jin-Ook Baega, R.C. Chikatea, B.K. Dasb a
Micro-Chemical Technology Laboratory, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, South Korea b Centre for Materials for Electronics Technology (C-MET), Panchwati, Off Pashan Road, Pune-411 008, India Received 2 July 2004; accepted 12 November 2004 Available online 18 December 2004
Abstract Nanocrystalline indium particles are prepared by direct reaction of sodium metal with anhydrous indium trichloride in N,NVdimethylformamide (In-1) or n-trioctylphosphine (In-2 and In-3) as a solvent at a temperature between 120 and 360 8C under the atmosphere of argon. The product was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Visible spectroscopy and thermogravimetric analysis (TGA). XRD patterns of In-1 exhibit broad peaks with particle diameter of about 15 nm while In-2 and In-3 particles have bigger particle size of about 50 nm reflecting the effect of solvents and reaction temperatures. Absorption spectroscopy measurements reveal the solvent dependence of surface plasmon resonance with the sharp absorption peaks at about 290 nm for In-1 in toluene and 260 nm for In-3 in dichloromethane. D 2004 Elsevier B.V. All rights reserved. Keywords: Indium nanoparticles; Chemical reduction; n-Trioctylphosphine; Absorption spectroscopy; XRD; SEM; TGA/DTG
1. Introduction Metal nanoparticles have attracted a great deal of attention because of their diverse applications and technological importance particularly from the electronics point of view. Although, much of the focus has been centered on nanoparticles of noble and transition metal, there are very few reports on the nanoparticles of other metals including indium [1–5]. In general, indium metal particles have been applied as wear resistant, catalyst and bearings in highspeed aircraft engines [6]. The nanoparticles of indium have successfully been explored for their potential applications in electronics (for single electron transistor), bio-nanotechnology (as tags for detection of DNA hybridization) [7–10], as printing building blocks in nanoxerography [11] and as starting material for convenient synthesis of InP using phosphide ions [12]. More recently, it has been observed
* Corresponding author. Tel.: +82 42 860 7671; fax: +82 42 860 7388. E-mail address: [email protected] (K.-W. Jun). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.050
that the indium nanoparticles serve as a growth promoter for the III–V semiconductor nanorods [13] while they also function as catalyst in various organic reactions like reduction of allyl bromide, conversion of aldehydes and ketones to esters and benzoylation of naphthalene [9,10,14]. Also, indium is an important film lubricating materials [15]. While there are several methods employed for the preparation of metal nanoparticles [16–19], solution methods are still considered to be the most versatile and convenient method. Indium nanoparticles have previously been synthesized by vapor phase deposition method [16], electrochemical reduction [17] and chemical reduction of salts [18]. It is observed that the sonochemical formation of indium phosphide proceeds via in-situ formation of indium nanoparticles due to reduction of indium chloride by potassium borohydride [19]. Recently, indium nanoparticles have been synthesized from its bulk metal chunk in paraffin oil, although, the final product was found to be a mixture of indium oxide and indium nanoparticles [15]. Such impurities, if present in single electron transistors, adversely affect and alter the performance of single electron transistors and
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
therefore it is of utmost importance to develop a process for the production of nanoparticles of indium with high purity or with minimum impurity levels. The solution methods offer such a process involving solvents functioning either as reducing or the dispersing agents and/or for the control of the particle growth during the synthesis. Mostly, narrow size distribution of particles has been effectively achieved via surface capping of the particles by solvents utilizing either the metal salts or the organometallic compounds as the starting materials [13]. Since the nanoparticles of indium phosphide via sodium phosphide process are contaminated with indium impurity, it is quite logical to investigate and identify the mechanism of formation of indium metal nanoparticles by reduction of indium salt by sodium. The present attempts are offshoot to the synthesis of indium phosphide by use of indium chloride and sodium phosphide [20]. In this communication, we report the synthesis of indium nanoparticles by sodium reduction method.
2. Experimental Indium chloride (99.8%), N,NV-dimethylformamide (DMF) and anhydrous n-trioctylphosphine (TOP) were purchased from Aldrich Chemical Co. and were stored under argon. Sodium metal was obtained locally and was stored in liquid paraffin. All manipulations were carried out under the atmosphere of argon gas using standard Schlenk type apparatus. Indium chloride (2.21 g) and sodium metal (0.70 g) were mixed in about 50 mL of N,NV-dimethylformamide (DMF) and then stirred at a temperature between 120 and 150 8C (In-1) for 4–5 h. When n-octylphosphine (TOP) was used as solvent, the reaction was carried out two different temperatures: (i) at b150 8C (In-2); (ii) at ~360 8C (In-3) with constant stirring for 0.5–2 h. The grayish
1033
suspension thus obtained after centrifugation was carefully washed with ethanol and water to remove any unreacted sodium and other by-product (NaCl). The final product was isolated from toluene via centrifugation in N80% yield. UV/ VIS absorption spectra were measured in toluene, dichloromethane or the solvent of reaction by UV-2501 PC Shimadzu spectrophotometer. X-ray diffraction (XRD) measurements were carried out on Rigaku D/MAX IIIB X-ray diffractometer by dispersing the powder on vaseline for adhesion. Scanning electron microscopic (SEM) and energy dispersive X-ray spectroscopic (EDS) analyses were performed on Philips XL30S FEG microscope/EDAX Phoenix spectrometer. The TG/DTG analysis was done on Setaram TG 92-12 at a temperature rate of 10 8C/min with oxygen as carrier gas.
3. Results and discussion The reaction of sodium metal with indium chloride in DMF at a temperature in between 120 and 150 8C produced a dark brown coloured product (In-1) within an hour due to the reduction of indium (III) to indium (0) as given in Eq. (1). DMF was selected for the present study because it is good solvent for reactions with sodium and acts as a good dispersing medium. Also, it is found to be a good particles growth terminator for the synthesis of nanoparticles of metals and semiconductors [21,22]. InCl3 þ 3NaYInð0Þ þ 3NaCl
ð1Þ
The sole aim of carrying out the same reaction in TOP stems from the fact that such reaction would generate organically capped nanoparticles of indium thus controlling the particle size. Such a beneficial property of TOP as a capping agent along with n-trioctylphosphine oxide (TOPO)
Fig. 1. Absorption spectra of (a) In-1 in toluene (b) In-3 in CH2Cl2.
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P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
Fig. 2. Effect of reaction time on particle growth for In-3.
has already been explored for various semiconductors [23,24]. For manipulating the shape and morphology of nanocrystals, variety of high boiling solvents that are liquid at room temperature have been frequently utilized [25]. It is now well established that the formation nanoparticle of larger particle diameter within nanometer regime takes place at higher reaction temperatures by selecting appropriate solvents for the synthesis. In the present studies also, such pronounced effects of solvent and temperature are indeed observed. In-2 and In-3 are larger in size by about 35 nm as compared to In-1. Such a difference may be originating from the fact that at higher temperatures, the homogeneous mixing of molten indium chloride and sodium modulates the appropriate reaction conditions necessary for obtaining organically capped indium particles of bigger sizes. Since DMF is water miscible and hygroscopic in nature, the removal of by-product such as NaCl was little difficult. However, no such difficulty was encountered when TOP was used as a solvent. Centrifugation of reaction mixture of In-1 with toluene or hexane resulted in the formation of residue which after washing it with ethanol and water followed by re-dissolution in toluene produced gray suspension. Further centrifugation of this suspension in toluene gave silvery gray residue with pale toluene solution containing small amount indium particles. The absorption spectra of this filtrate showed a characteristic band at 290 nm (Fig. 1a) due to the surface plasmon resonance in nanoparticles. On the other hand, when the progress of the reaction with time for In-3 was monitored, it was observed (Fig. 2) that the intensity of this band (290 nm) decreases with increase in reaction time thus demonstrating the effect of reaction time on the particle growth. Shift of about 30 nm for this absorption for In-3 in dichloromethane (Fig. 1b) probably
indicates the formation of larger sized indium particles thus corroborating with the earlier report [15]. Inspection of the XRD patterns of In-1, In-2 and In-3 (Fig. 3) reveals the characteristic indium peaks at a scattering angles (2h) of 32.96, 36.32, 39.16, 54.48, 56.60 and 63.24, originating from 101 002, 110, 112, 200 and 103 crystal planes of tetragonal phase of indium. Along with these peaks, additional set of bands of very weak intensity was also observed at (2h) values of 30.60, 35.49, 51.10 and 60.60 corresponding to 22, 400, 440 and 622 crystal planes of In2O3. There also appeared one more peak at lesser 2h value (22.538) probably originating from In(OH)3 [25]. By comparing the indium scattering patterns, it is significant to
Fig. 3. Powder X-ray diffraction patterns of (a) In-1 (b) In-2 and (c) In-3.
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
1035
Fig. 4. SEM of In-3.
note that In-2 and In-3 have better crystalline nature (Fig. 3b and c) as compared to In-1 (Fig. 3a) reflecting the solvent and temperature dependency. The effect of reaction temperature on the crystalline nature is found to be more pronounced for In-3 as evidenced from the sharpness of the peak at 32.968 (Fig. 3b and c). The presence of In2O3 and In(OH)3 is very less for In-3 as compared to In-1 and In-2 suggesting that at high temperatures, the capping of bigger sized indium particles with TOP hinders the formation of indium droplets at the surface there by reducing the probability of the surface oxidation process. However, the concentrations of such surface impurities in In-1 and In-2 seem to be insignificant (lower than 2% in terms of XRD peak intensity). The particle size of In-1 was found to be about 15 nm in diameter whereas larger sized crystals (about 50 nm) were obtained for In-2 and In-3 in the calculation
using Scherrer formula. Such a difference stems from the fact that these compounds were prepared in two different solvents. Surprisingly, the effect of temperature on particle size was not observed, since In-2 and In-3 did not differ significantly in their sizes. SEM images of the powder (Fig. 4) revealed that the particles are spherical in nature with most of them in the form of individual balls with particle size b50 nm. Although formation of agglomerates is observed, the majority of the spots are non-coagulated particles within the microgram levels. The energy dispersive spectroscopy (EDS) analysis of In-1 indicates the presence of indium along with small amounts of carbon, nitrogen and oxygen. From the intensity ratio assessment of EDS spectra, the carbon, nitrogen, oxygen and indium contents were found to be 0.56%,
Fig. 5. TGA/DTG curve for In-3.
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P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
0.96%, 2.2% and 96.22% by weight, respectively. The presence of C, N and O may be due to organics around the surface of the particles because of the usage of high boiling solvents such as DMF and TOP for the synthesis. The thermal analysis of indium particles was conducted to understand the effect of temperature on the stability of the particles as well as the weight loss due to presence of organics around the particles. TGA/DTG profile of In-1 (Fig. 5) revealed that the weight loss mainly takes place below 300 8C and there is total loss of about 2.5 % in two steps. The first step loss (~1%) may be due to the adsorbed water where as the second step loss (~1.5%) may be due to removal of organic matter associated with the metal particles. In spite of the fact that the present synthesis is carried in high boiling solvents, the negligible weight loss is observed as compared to the indium nanoparticles synthesized in paraffin oil (wt. lossN50%) [15] which certainly advocates for the nearly solvent-free nature of indium nanoparticles. After removal of the capped organic materials, slow weight gain is seen in TGA curve after 350 8C possibly due to the oxidation of indium to its oxide [15].
4. Conclusion The present study illustrates one of the most convenient and significant methods for the synthesis of indium metal through the reduction of indium chloride with sodium in high boiling solvents. The nanoparticles by this chemical method not only have better crystalline nature but also uniform particle size distribution. The absorption phenomenon in indium nanoparticles, due to surface plasmon resonance is found to be solvent dependent. The current method would provide a simple and cheap way to produce the nanoparticles of indium metal, which has great commercial potential.
Acknowledgement The financial support from bR&D Program for NT-IT Fusion Strategy of Advanced TechnologiesQ is gratefully acknowledged. One of us (PKK) thanks Department of Information Technology, Govt. of India for grant of leave.
References [1] Y. Singh, J.R.N. Javier, S.H. Ehrman, M.H. Magnusson, K. Deppert, J. Aerosol Sci. 33 (2002) 1309. [2] C.T. Black, C.B Murray, R.L. Sandstrom, S.H. Sun, Science 290 (2000) 1131. [3] S.A. Majetich, Y. Jin, Science 284 (1999) 470. [4] D.I. Gittins, D. Bethell, D.J. Schiffrin, R.J. Nichols, Nature 408 (2000) 67. [5] S.A. Maier, M.L. Brongersma, P.G. Kik, S. Meltzer, A.A.G. Requicha, H.A. Atwater, Adv. Mater. (Weinh., Ger) 13 (2001) 1501. [6] J.A. Slatter, Indium and Indium Compounds, 4th ed., Kirk-Othmer Encyclopedia of Chemical Technology, vol. 14, John Wiley, New York, 1995, p. 155. [7] T. Junno, M.H. Magnusson, S.B. Carlsspn, K. Deppert, J.O. Malm, M. Montelius, L. Samuelson, Microelectron. Eng. 47 (1999) 179. [8] J. Wang, G. Liu, Q. Zhu, Anal. Chem. 75 (2003) 6218. [9] K. Takai, Y. Ikawa, Org. Lett. 4 (2004) 1727. [10] K. Bang, K. Lee, Y.K. Park, P.H. Lee, Bull. Korean Chem. Soc. 23 (2002) 1272. [11] C.R. Barry, N.Z. Lwin, W. Zheng, H.O. Jacobs, Appl. Phys. Lett. 83 (2003) 5527. [12] J.M. Nedeljkovic, O.I. Micic, S.P. Ahrenkiel, A. Miedaner, A.J. Nozik, J. Am. Chem. Soc. 126 (2004) 2632. [13] S.H. Kan, A. Aharoni, T. Mokari, U. Banin, Faraday Discuss. 125 (2004) 23. [14] N.H. Heo, S.W. Jung, S.W. Park, M. Park, W.T. Lim, J. Phys. Chem., B 104 (2000) 8372. [15] Y. Zhao, Z. Zhang, H. Dang, J. Phys. Chem., B 107 (2003) 7574. [16] Y. Zhang, H. Ago, J. Liu, M. Yumura, K. Uchida, S. Ohshima, S. Iijima, J. Zhu, X. Zhang, J. Cryst. Growth 264 (2004) 363. [17] I. Rodriguez-Sanchez, M.C. Blanco, M.A. Lopez-Quintela, J. Phys. Chem., B 104 (2000) 9683. [18] S. Ayyappan, R.S. Gopalan, G.N. Subbanna, C.N.R. Rao, J. Mater. Res. 12 (1997) 398. [19] B. Li, Y. Xie, J. Huang, Y. Liu, Y. Qian, Ultrason. Sonochem. 8 (2001) 331. [20] P.K. Khanna, M.S. Eum, K.W. Jun, J.O. Baeg, S. Seok, Mater. Lett. 57 (2003) 4617. [21] P.K. Khanna, R. Gokhale, V.V.V.S. Subbarao, Mater. Lett. 57 (2003) 2489. [22] P.K. Khanna, C.P. Morley, R.M. Gorte, R. Gokhale, V.V.V.S. Subbarao, C.V.V. Satyanarayana, Mater. Chem. Phys. 83 (2003) 323. [23] J.S. Steckel, S. Coe-Sullivan, V. Bulovic, M.G. Bawendi, Adv. Mater. 15 (2003) 1862. [24] S.P. Ahrenkiel, O.I. Micic, A. Miedaner, C.J. Curtis, J.M. Nedeljkovic, A.J. Nozik, Nano Lett. 3 (2003) 833. [25] S.G. Chen, C.H. Li, W.H. Xiong, L.M. Liu, H. Wang, Mater. Lett. 58 (2004) 294.
Colloidal synthesis of indium nanoparticles by sodium reduction method P.K. Khannaa,b, Ki-Won Juna,*, Ki Bum Honga, Jin-Ook Baega, R.C. Chikatea, B.K. Dasb a
Micro-Chemical Technology Laboratory, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, South Korea b Centre for Materials for Electronics Technology (C-MET), Panchwati, Off Pashan Road, Pune-411 008, India Received 2 July 2004; accepted 12 November 2004 Available online 18 December 2004
Abstract Nanocrystalline indium particles are prepared by direct reaction of sodium metal with anhydrous indium trichloride in N,NVdimethylformamide (In-1) or n-trioctylphosphine (In-2 and In-3) as a solvent at a temperature between 120 and 360 8C under the atmosphere of argon. The product was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Visible spectroscopy and thermogravimetric analysis (TGA). XRD patterns of In-1 exhibit broad peaks with particle diameter of about 15 nm while In-2 and In-3 particles have bigger particle size of about 50 nm reflecting the effect of solvents and reaction temperatures. Absorption spectroscopy measurements reveal the solvent dependence of surface plasmon resonance with the sharp absorption peaks at about 290 nm for In-1 in toluene and 260 nm for In-3 in dichloromethane. D 2004 Elsevier B.V. All rights reserved. Keywords: Indium nanoparticles; Chemical reduction; n-Trioctylphosphine; Absorption spectroscopy; XRD; SEM; TGA/DTG
1. Introduction Metal nanoparticles have attracted a great deal of attention because of their diverse applications and technological importance particularly from the electronics point of view. Although, much of the focus has been centered on nanoparticles of noble and transition metal, there are very few reports on the nanoparticles of other metals including indium [1–5]. In general, indium metal particles have been applied as wear resistant, catalyst and bearings in highspeed aircraft engines [6]. The nanoparticles of indium have successfully been explored for their potential applications in electronics (for single electron transistor), bio-nanotechnology (as tags for detection of DNA hybridization) [7–10], as printing building blocks in nanoxerography [11] and as starting material for convenient synthesis of InP using phosphide ions [12]. More recently, it has been observed
* Corresponding author. Tel.: +82 42 860 7671; fax: +82 42 860 7388. E-mail address: [email protected] (K.-W. Jun). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.050
that the indium nanoparticles serve as a growth promoter for the III–V semiconductor nanorods [13] while they also function as catalyst in various organic reactions like reduction of allyl bromide, conversion of aldehydes and ketones to esters and benzoylation of naphthalene [9,10,14]. Also, indium is an important film lubricating materials [15]. While there are several methods employed for the preparation of metal nanoparticles [16–19], solution methods are still considered to be the most versatile and convenient method. Indium nanoparticles have previously been synthesized by vapor phase deposition method [16], electrochemical reduction [17] and chemical reduction of salts [18]. It is observed that the sonochemical formation of indium phosphide proceeds via in-situ formation of indium nanoparticles due to reduction of indium chloride by potassium borohydride [19]. Recently, indium nanoparticles have been synthesized from its bulk metal chunk in paraffin oil, although, the final product was found to be a mixture of indium oxide and indium nanoparticles [15]. Such impurities, if present in single electron transistors, adversely affect and alter the performance of single electron transistors and
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
therefore it is of utmost importance to develop a process for the production of nanoparticles of indium with high purity or with minimum impurity levels. The solution methods offer such a process involving solvents functioning either as reducing or the dispersing agents and/or for the control of the particle growth during the synthesis. Mostly, narrow size distribution of particles has been effectively achieved via surface capping of the particles by solvents utilizing either the metal salts or the organometallic compounds as the starting materials [13]. Since the nanoparticles of indium phosphide via sodium phosphide process are contaminated with indium impurity, it is quite logical to investigate and identify the mechanism of formation of indium metal nanoparticles by reduction of indium salt by sodium. The present attempts are offshoot to the synthesis of indium phosphide by use of indium chloride and sodium phosphide [20]. In this communication, we report the synthesis of indium nanoparticles by sodium reduction method.
2. Experimental Indium chloride (99.8%), N,NV-dimethylformamide (DMF) and anhydrous n-trioctylphosphine (TOP) were purchased from Aldrich Chemical Co. and were stored under argon. Sodium metal was obtained locally and was stored in liquid paraffin. All manipulations were carried out under the atmosphere of argon gas using standard Schlenk type apparatus. Indium chloride (2.21 g) and sodium metal (0.70 g) were mixed in about 50 mL of N,NV-dimethylformamide (DMF) and then stirred at a temperature between 120 and 150 8C (In-1) for 4–5 h. When n-octylphosphine (TOP) was used as solvent, the reaction was carried out two different temperatures: (i) at b150 8C (In-2); (ii) at ~360 8C (In-3) with constant stirring for 0.5–2 h. The grayish
1033
suspension thus obtained after centrifugation was carefully washed with ethanol and water to remove any unreacted sodium and other by-product (NaCl). The final product was isolated from toluene via centrifugation in N80% yield. UV/ VIS absorption spectra were measured in toluene, dichloromethane or the solvent of reaction by UV-2501 PC Shimadzu spectrophotometer. X-ray diffraction (XRD) measurements were carried out on Rigaku D/MAX IIIB X-ray diffractometer by dispersing the powder on vaseline for adhesion. Scanning electron microscopic (SEM) and energy dispersive X-ray spectroscopic (EDS) analyses were performed on Philips XL30S FEG microscope/EDAX Phoenix spectrometer. The TG/DTG analysis was done on Setaram TG 92-12 at a temperature rate of 10 8C/min with oxygen as carrier gas.
3. Results and discussion The reaction of sodium metal with indium chloride in DMF at a temperature in between 120 and 150 8C produced a dark brown coloured product (In-1) within an hour due to the reduction of indium (III) to indium (0) as given in Eq. (1). DMF was selected for the present study because it is good solvent for reactions with sodium and acts as a good dispersing medium. Also, it is found to be a good particles growth terminator for the synthesis of nanoparticles of metals and semiconductors [21,22]. InCl3 þ 3NaYInð0Þ þ 3NaCl
ð1Þ
The sole aim of carrying out the same reaction in TOP stems from the fact that such reaction would generate organically capped nanoparticles of indium thus controlling the particle size. Such a beneficial property of TOP as a capping agent along with n-trioctylphosphine oxide (TOPO)
Fig. 1. Absorption spectra of (a) In-1 in toluene (b) In-3 in CH2Cl2.
1034
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
Fig. 2. Effect of reaction time on particle growth for In-3.
has already been explored for various semiconductors [23,24]. For manipulating the shape and morphology of nanocrystals, variety of high boiling solvents that are liquid at room temperature have been frequently utilized [25]. It is now well established that the formation nanoparticle of larger particle diameter within nanometer regime takes place at higher reaction temperatures by selecting appropriate solvents for the synthesis. In the present studies also, such pronounced effects of solvent and temperature are indeed observed. In-2 and In-3 are larger in size by about 35 nm as compared to In-1. Such a difference may be originating from the fact that at higher temperatures, the homogeneous mixing of molten indium chloride and sodium modulates the appropriate reaction conditions necessary for obtaining organically capped indium particles of bigger sizes. Since DMF is water miscible and hygroscopic in nature, the removal of by-product such as NaCl was little difficult. However, no such difficulty was encountered when TOP was used as a solvent. Centrifugation of reaction mixture of In-1 with toluene or hexane resulted in the formation of residue which after washing it with ethanol and water followed by re-dissolution in toluene produced gray suspension. Further centrifugation of this suspension in toluene gave silvery gray residue with pale toluene solution containing small amount indium particles. The absorption spectra of this filtrate showed a characteristic band at 290 nm (Fig. 1a) due to the surface plasmon resonance in nanoparticles. On the other hand, when the progress of the reaction with time for In-3 was monitored, it was observed (Fig. 2) that the intensity of this band (290 nm) decreases with increase in reaction time thus demonstrating the effect of reaction time on the particle growth. Shift of about 30 nm for this absorption for In-3 in dichloromethane (Fig. 1b) probably
indicates the formation of larger sized indium particles thus corroborating with the earlier report [15]. Inspection of the XRD patterns of In-1, In-2 and In-3 (Fig. 3) reveals the characteristic indium peaks at a scattering angles (2h) of 32.96, 36.32, 39.16, 54.48, 56.60 and 63.24, originating from 101 002, 110, 112, 200 and 103 crystal planes of tetragonal phase of indium. Along with these peaks, additional set of bands of very weak intensity was also observed at (2h) values of 30.60, 35.49, 51.10 and 60.60 corresponding to 22, 400, 440 and 622 crystal planes of In2O3. There also appeared one more peak at lesser 2h value (22.538) probably originating from In(OH)3 [25]. By comparing the indium scattering patterns, it is significant to
Fig. 3. Powder X-ray diffraction patterns of (a) In-1 (b) In-2 and (c) In-3.
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
1035
Fig. 4. SEM of In-3.
note that In-2 and In-3 have better crystalline nature (Fig. 3b and c) as compared to In-1 (Fig. 3a) reflecting the solvent and temperature dependency. The effect of reaction temperature on the crystalline nature is found to be more pronounced for In-3 as evidenced from the sharpness of the peak at 32.968 (Fig. 3b and c). The presence of In2O3 and In(OH)3 is very less for In-3 as compared to In-1 and In-2 suggesting that at high temperatures, the capping of bigger sized indium particles with TOP hinders the formation of indium droplets at the surface there by reducing the probability of the surface oxidation process. However, the concentrations of such surface impurities in In-1 and In-2 seem to be insignificant (lower than 2% in terms of XRD peak intensity). The particle size of In-1 was found to be about 15 nm in diameter whereas larger sized crystals (about 50 nm) were obtained for In-2 and In-3 in the calculation
using Scherrer formula. Such a difference stems from the fact that these compounds were prepared in two different solvents. Surprisingly, the effect of temperature on particle size was not observed, since In-2 and In-3 did not differ significantly in their sizes. SEM images of the powder (Fig. 4) revealed that the particles are spherical in nature with most of them in the form of individual balls with particle size b50 nm. Although formation of agglomerates is observed, the majority of the spots are non-coagulated particles within the microgram levels. The energy dispersive spectroscopy (EDS) analysis of In-1 indicates the presence of indium along with small amounts of carbon, nitrogen and oxygen. From the intensity ratio assessment of EDS spectra, the carbon, nitrogen, oxygen and indium contents were found to be 0.56%,
Fig. 5. TGA/DTG curve for In-3.
1036
P.K. Khanna et al. / Materials Letters 59 (2005) 1032–1036
0.96%, 2.2% and 96.22% by weight, respectively. The presence of C, N and O may be due to organics around the surface of the particles because of the usage of high boiling solvents such as DMF and TOP for the synthesis. The thermal analysis of indium particles was conducted to understand the effect of temperature on the stability of the particles as well as the weight loss due to presence of organics around the particles. TGA/DTG profile of In-1 (Fig. 5) revealed that the weight loss mainly takes place below 300 8C and there is total loss of about 2.5 % in two steps. The first step loss (~1%) may be due to the adsorbed water where as the second step loss (~1.5%) may be due to removal of organic matter associated with the metal particles. In spite of the fact that the present synthesis is carried in high boiling solvents, the negligible weight loss is observed as compared to the indium nanoparticles synthesized in paraffin oil (wt. lossN50%) [15] which certainly advocates for the nearly solvent-free nature of indium nanoparticles. After removal of the capped organic materials, slow weight gain is seen in TGA curve after 350 8C possibly due to the oxidation of indium to its oxide [15].
4. Conclusion The present study illustrates one of the most convenient and significant methods for the synthesis of indium metal through the reduction of indium chloride with sodium in high boiling solvents. The nanoparticles by this chemical method not only have better crystalline nature but also uniform particle size distribution. The absorption phenomenon in indium nanoparticles, due to surface plasmon resonance is found to be solvent dependent. The current method would provide a simple and cheap way to produce the nanoparticles of indium metal, which has great commercial potential.
Acknowledgement The financial support from bR&D Program for NT-IT Fusion Strategy of Advanced TechnologiesQ is gratefully acknowledged. One of us (PKK) thanks Department of Information Technology, Govt. of India for grant of leave.
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