Rapid Microwave-Enhanced Solvothermal Process for Synthesis of

Jun 29, 2010 - Chih-Chung Wu,† Ching-Yeh Shiau,*,† Delele Worku Ayele,† Wei-Nien Su,†. Ming-Yao Cheng,† Chiu-Yen Chiu,‡ and Bing-Joe Hwang...
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Chem. Mater. 2010, 22, 4185–4190 4185 DOI:10.1021/cm1006263

Rapid Microwave-Enhanced Solvothermal Process for Synthesis of CuInSe2 Particles and Its Morphologic Manipulation Chih-Chung Wu,† Ching-Yeh Shiau,*,† Delele Worku Ayele,† Wei-Nien Su,† Ming-Yao Cheng,† Chiu-Yen Chiu,‡ and Bing-Joe Hwang*,†,§ †

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan, Republic of China, ‡Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan, Republic of China, and §National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China Received March 2, 2010. Revised Manuscript Received June 7, 2010

CuInSe2 nanoparticles are prepared using a microwave-assisted solvothermal method with ethylenediamine as a solvent. The morphology of the as-prepared CuInSe2 nanoparticles has been successful manipulated by controlling the morphology of selenium seeds. We achieved this through a systematic treatment of selenium powders before the addition of indium and copper precursors. This pretreatment of selenium powders, under microwave heating, converts the morphologies of selenium seed from spherical to nanorod. As a result, this nanorod morphology of selenium seeds are responsible for the formation of crystalline CuInSe2 particles with rodlike morphology, while the crystalline CuInSe2 particles with platelike morphology are obtained without pretreatment of selenium powders. The XRD data and SEM images show that pure chalcopyrite CuInSe2 particles with good crystallinity are obtained with the pretreatment of selenium and ethylenediamine mixture in the microwave heating for 30 min and followed by further microwave-enhanced reaction with indium and copper precursors for 30 min. We proposed the reaction mechanism based on XRD and SEM data, which includes the intermediate products. The Raman spectrum of the chalcopyrite CuInSe2 nanoparticle shows an intense peak at 175 cm-1 corresponds to the A1 phonon mode of tetragonal CuInSe2 chalcopyrite. The compositions of the products consistent very well with their nominal value are confirmed by energy-dispersive X-ray analysis. 1. Introduction In recent years, the synthesis of semiconductors nanoparticles has been of interest to researchers, because of their outstanding physical or chemical properties.1 Semiconductor materials of ternary chalcogenide compounds, which belong to the I-III-VI2 family, have been studied extensively, because of their tunable electronic and optical characteristics.2-7 The chalcopyrite CuInSe2 compound is a candidate as a promising material for solar cell applications, because of its high absorption coefficient, promise of high performance, low cost, suitable band gap, good radiation stability, and easy conversion of *Authors to whom correspondence should be addressed. E-mails: bjh@ mail.ntust.edu.tw, [email protected].

(1) Reithmaier, J. P.; Sek, G.; Loffler, A.; Hofmann, C.; Kuhn, S.; Reitzenstein, S.; Keldysh, L. V.; Kulakovskii, V. D.; Reinecke, T. L.; Forchel, A. Nature 2004, 432, 197–200. (2) Axtell, E. A.; Liao, J.-H.; Pikramenou, Z.; Park, Y.; Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115, 12191–12192. (3) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. J. Phys. Chem. B 2004, 108, 12429–12435. (4) Hu, J.; Lu, Q.; Tang, K.; Qian, Y.; Zhou, G.; Liu, X. Chem. Commun. 1999, 1093–1094. (5) Tian, L.; Elim, H. I.; Ji, W.; Vittal, J. J. Chem. Commun. 2006, 4276–4278. (6) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. J. Am. Chem. Soc. 2008, 130, 16770–16777. (7) Allen, P. M.; Bawendi, M. G. J. Am. Chem. Soc. 2008, 130, 9240– 9241. r 2010 American Chemical Society

n/p-carrier type.8-12 In past years, solar cells based on CuInSe2 (CIS) have been reported with an efficiency of ∼17%.13,14 Since the device properties of CuInSe2-based solar cells are known to be critically influenced by their stoichiometric composition, defect chemistry, impurities, and structures, which are strongly dependent on methods and conditions of preparation,15-17 developing a facile method to prepare ternary chalcopyrite materials with suitable morphologies is vital for further applications. Different preparation methods have been established to prepare CuInSe2, such as coevaporation,18 decomposition (8) Yi-Han, Y.; Yit-Tsong, C. J. Phys. Chem. B 2006, 110, 17370– 17374. (9) Cahen, D.; Gilet, J.-M.; Schmitz, C.; Chernyak, L.; Gartsman, K.; Jakubowicz, A. Science 1992, 258, 271–274. (10) Guillen, C.; Herrero, J. Sol. Energy Mater. Sol. Cells 1996, 43, 47–57. (11) Guillen, C.; Herrero, J. Sol. Energy Mater. Sol. Cells 2002, 73, 141– 149. (12) Guillen, C.; Herrero, J. Sol. Energy Mater. 1992, 23, 31. (13) Zhang, L.; Liang, J.; Peng, S.; Shi, Y.; Chen, J. Mater. Chem. Phys. 2007, 106, 296–300. (14) Yang, L.-C.; Xiao, H. Z.; Shafarman, W. N.; Birkmire, R. W. Sol. Energy Mater. Sol. Cells 1995, 36, 445. (15) Grisaru, H.; Palchik, O.; Gedanken, A. Inorg. Chem. 2003, 42, 7148–7155. (16) Li, B.; Xie, Y.; Huang, J.; Qian, Y. Adv. Mater. 1999, 11, 1456. (17) Xiao, J.; Xie, Y.; Xiong, Y.; Tang, R.; Qian, Y. J. Mater. Chem. 2001, 11, 1417–1420. (18) Stolt, L.; Hedstrom, J.; Kessler, J.; Ruckh, M.; Velthaus, K.-O.; Schock, H.-W. Appl. Phys. Lett. 1993, 62, 597.

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of organometallic precursors,19 metal reactions with H2Se,20 electrodeposition,21 solid-state reaction involving pure metal powders by microwave heating,22,23 and some other techniques.24,25 Many approaches involve aqueous solution26,27 and the classical techniques of colloidal chemistry. 28,29 In general, these techniques usually require either a high processing temperatures or special devices, and some of them even use toxic agents such as H2Se or organometallic compounds. Recently, solution-based methods have been developed for the preparation of CuInSe2 nanoparticles at much lower temperature.8,28 The solvothermal method has emerged as a powerful tool for the controlled synthesis of nanostructures/microstructures, using organic amine as a solvent.16,17,30,31 The solvothermal method is similar to the hydrothermal method, except that organic solvents are used instead of water. This method can effectively prevent the products from oxidation and has been used to synthesize a variety of nonoxides.32 In a solvothermal route,33 the solvent plays an important role in the formation of the chalcopyrite CuInSe2 compound. Ethylenediamine was selected as the solvent, because of its special properties, such as strong basic capacity, strong chelation capability, and its ability to act as an absorber of the excess heat released in the reaction. Alkylamine can greatly enhance solubility, diffusion, and crystallization but still provide reaction conditions mild enough to enable molecular building blocks to participate in the following formation of the solid-state phase.16,17 Although a solvothermal method plays an important role for the synthesis of CuInSe2 particles, as mentioned previously, its longer reaction time and difficulty in controlling the morphology of the particles in particular should be optimized. Recently, preparation of nanoparticles based on microwave heating is found to be a good technique in (19) McAleese, J.; O’Brien, P.; Otway, D. J. Mater. Res. Soc. Symp. Proc. 1998, 485, 157. (20) Alberts, V.; Schon, J. H.; Bucher, E. J. Mater. Sci.: Mater. Electron. 1999, 10, 469. (21) Sudo, Y.; Endo, S.; Irie, T. Jpn. J. Appl. Phys. 1993, 32, 1562. (22) Schumann, B.; Tempel, T.; Kuhn, G. Sol. Cells 1986, 16, 43. (23) Landry, C. C.; Barron, A. R. Science 1993, 260, 1653. (24) Shirakata, S.; Murakami, T.; Kariya, T.; Isomura, S. Jpn. J. Appl. Phys., Part 1 1996, 35, 191. (25) Park, J.-Y.; Park, J. P.; Hwang, C. H.; Kim, J.; Choi, M. H.; Ok, K. M.; Kwak, H.-Y.; Shim, I.-W. Bull. Korean Chem. Soc. 2009, 30, 2713. (26) Kaelin, M.; Rudmann, D.; Kurdesau, F.; Meyer, T.; Zogg, H.; Tiwari, A. N. Thin Solid Films 2003, 58-62, 431–432. (27) Bensebaa, F.; Aouadou, A. PCT International Patent Application WO/2008/104087 A1, 2008. (28) Guo, Q.; Kim, S. J.; Kar, M.; Shafarman, W. N.; Birkmire, R. W.; Stach, E. A.; Agrawal, R.; Hillhouse, H. W. Nano Lett. 2008, 8, 2982–2987. (29) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 47. (30) Koo, B.; Patel, R. N.; Korgel, B. A. J. Am. Chem. Soc. 2009, 131, 3134–3135. (31) Jiang, Y.; Wu, Y.; Mo, X.; Yu, W.; Xie, Y.; Qian, Y. Inorg. Chem. 2000, 39, 2964–2965. (32) Qian, Y. Adv. Mater. 1999, 11, 1101. (33) Lu, W.-L.; Fu, Y.-S.; Tseng, B.-H. J. Phys. Chem. Solids 2008, 69, 637–640. (34) Palchik, O.; Kerner, R.; Zhu, J.; Gedanken, A. J. Solid State Chem. 2000, 154, 530. (35) Zhu, J.; Palchik, O.; Chen, S. G.; Gedanken, A. J. Phys. Chem. B 2000, 104, 7344.

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comparison with other methods, because of its various advantages.34-39 Among many, some of the advantages of the microwave-assisted method reactions are (a) the reactions are conducted at normal atmospheric pressure and at the boiling temperature of the solvent, (b) nanoparticles with a diameter of a few nanometers can be prepared,34-36 (c) shorter reaction times are required,40,41 (d) usually the products show a very low level of impurities, (e) very simple reagents can be used as precursors, and (f ) there are excellent reproducibility and high yields of products.40,41 Therefore, by considering both the advantage of solvothermal and microwave-assisted methods, herein, we report on the microwave-assisted solvothermal synthesis of CuInSe2 nanoparticles in ethylenediamine using CuCl2 3 2H2O, InCl3, and selenium as reagents without organometallic precursor or other toxic precursors. Recent reports show that the shape, size, and compositions of CuInSe2 nanoparticles were controlled in various ways.28,30,40,41 Moreover, using bulk selenium in ethylenediamine, the preparation of one-dimensional selenium structures (nanorods and nanowires) was reported. By choosing suitable precipitating solvents and adjusting the selenium concentration, the morphology of selenium nanoparticles has been manipulated.42 In this paper, we demonstrate a successful manipulation of the morphology of CuInSe2 particles by controlling the morphology of selenium seeds via systematic treatment of selenium powder prior to the addition of copper and indium precursors. In addition, the composition and structural characterization of CuInSe2 particles are presented. 2. Experimental Section Chemicals. All reactants and solvents were used without further purification. Selenium powder (99.70%, 99%, extra pure) were all purchased from Acros. Ethanol (CH3CH2OH, 95%) and copper(II) chloride dihydrate (CuCl2 3 2H2O, 95%) were purchased from Acros and JANSSEN CHIMICA, respectively. Characterization. The as-prepared nanoparticles were characterized by different instruments. X-ray diffraction (XRD) measurements (Rigaku Model Dmax-B, Japan) were recorded using a Cu KR radiation source that was operated at 40 kV and 100 mA. The X-ray diffractogram was obtained at a scan rate of 0.05 deg s-1 for 2θ values between 20° and 80°. The particle images were obtained using a scanning electron microscopy (36) Palchik, O.; Kerner, R.; Gedanken, A.; Weiss, A. M.; Slifkin, M. A.; Palchik, V. J. Mater. Chem. 2001, 11, 874. (37) Grisaru, H.; Palchik, O.; Gedanken, A.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Rosenfeld Hacohen, Y. Inorg. Chem. 2001, 40, 4814. (38) Grisaru, H.; Palchik, O.; Gedanken, A.; Palchik, V.; Slifkin, M. A.; Weiss, A. M. J. Mater. Chem. 2002, 12, 339. (39) Hwang, B.-J.; Yu, T.-H.; Cheng, M.-Y.; Santhanam, R. J. Mater. Chem. 2009, 19, 4536–4544. (40) Bensebaa, F.; Durand, C.; Aouadou, A.; Scoles, L.; Du, X.; Wang, D.; Le Page, Y. J. Nanopart. Res. DOI 10.1007/s11051-009-9752-5. (41) Tang, J.; Hinds, S.; Kelley, S. O.; Sargent, E. H. Chem. Mater. 2008, 20, 6906–6910. (42) Yang, Z.; Cingarapu, S.; Klabunde, K. J. Chem. Lett. 2009, 38, 252.

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Figure 1. SEM images of (a) commercial selenium powder at 1000 magnification and (b and c) selenium pretreated in ethylenediamine at 20 000 magnification.

(SEM) system (JEOL Model JEM-6500F FESEM, Tokyo, Japan). The elemental compositions of the particle were performed by energy-dispersive X-ray spectroscopy (EDX elemental analysis, Model JSM 6500). The crystalline structure was also determined by Raman spectroscopy using argon-ion laser light with a wavelength of 532 nm (Kaiser Optical Systems, Inc.). Microwave Synthesis. Microwave synthesis was performed in a single-mode CEM Discover System operating at 300 W, 2.45 GHz. The solution containing precursors was sealed and heated in 220 W at 180 °C for 5, 10, and 30 min. The reaction was rapidly cooled using high-pressure air, following the termination of the reaction. Synthesis of CuInSe2 Nanocrystal. The CuInSe2 nanoparticles were prepared by the microwave-enhanced solvothermal method in two approaches. The first approach was without pretreatment of the selenium powder and ethylenediamine mixture under microwave heating. For a typical synthesis, 0.259 mmol of selenium, 0.129 mmol of InCl3, and 0.126 mmol of CuCl2 3 2H2O were added to 4 mL of ethylenediamine and mixed together. The mixture was stirred and sonicated for 10 min, respectively, to form a homogeneous solution. Next, the solution were sealed and heated in 220 W of heat at 180 °C for 5, 10, and 30 min. The second approach involves the pretreatment of the selenium powder and ethylenediamine mixture in microwave heating before the addition of the indium and copper precursor. This method is analogous to the first approach, with the exception that the pretreatment of selenium powder in ethylenediamine was performed in a microwave vessel at 180 °C for 30 min prior to the addition of copper and indium precursors. The separation of nanocrystals was achieved by filtration with deionized water and ethanol as a washing agent. This process was repeated several times with deionized water and ethanol to ensure reagent-free particles. Finally, the product was dried in a vacuum oven at 80 °C for 6 h.

2. Results and Discussion To control the morphology of selenium seeds, selenium powder in ethylenediamine was treated under microwave heating in 220 W at 180 °C for different times. Ethylenediamine is an excellent template for the formation of onedimensional materials.16 Figure 1 shows the SEM images of selenium powder before (Figure 1a) and after microwave treatment in ethylenediamine for 3 min (Figure 1b) and 30 min (Figure 1c). The images in Figure 1 indicate one-dimensional growth for trigonal selenium seeds after the pretreatment of selenium powder in ethylenediamine for 30 min in particular. Consequently, the morphology of selenium seeds was manipulated successfully before the addition of copper and indium precursors.

Figure 2. SEM images of CuInSe2 particles synthesized by microwaveassisted solvothermal method (a-c) without pretreatment and (d-f) with pretreatment of selenium powder at different reaction times.

SEM images of the as-prepared CuInSe2 particles are shown in Figure 2, in which platelike morphologies (Figures 2a-c) and rodlike morphologies (Figures 2d-f) of CuInSe2 particles were synthesized by two different approaches for 5, 15, and 30 min of reaction, respectively. The morphologies of CuInSe2 particles have been controlled with careful manipulation of the morphologies of the intermediate products.43,44 The formation of rodlike CuInSe2 particles was attributed to the one-dimensional morphology of selenium seeds that resulted from the pretreatment of the selenium powder, whereas the platelike CuInSe2 particles were produced without any pretreatment of selenium powder. The dissolution-precipitation takes place during the course of formation of the rodlike selenium seeds via pretreatment of the powder. It suggests (43) Exstrom, C. L.; Darveau, S. A.; Martinez-Skinner, A. L.; Ingersoll, M; Olejnicek, J.; Mirasano, A.; Haussler, A. T. IEEE Photovoltaic Specialists Conference; IEEE Electron Devices Society; Piscataway, NJ, 2008. (ISBN 978-1-4244-1641-7.) (44) Xu, J.; Lee, C.-S.; Tang, Y.-B.; Chen, X.; Chen, Z.-H.; Zhang, W.-J. ACS Nano 2010, 4, 1845–1850.

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Figure 3. XRD spectra of CuInSe2 particles synthesized by microwave-assisted solvothermal method (a) without pretreatment and (b) with pretreatment of selenium powder in ethylenediamine for 30 min with 5, 15, and 30 min of reaction time.

that the selenium with a rodlike morphology is more stable in ethylenediamine solution under this condition. On other hand, in the absence of pretreatment of selenium powder, the formation of hexagonal CuSe intermediates was confirmed via EDX and XRD analyses of the as-prepared CuInSe2 particles. The CuSe nanowire bundles have been used as a selfsacrifical template for making bundles of tetragonal chalcopyrite CuInSe2 nanowires by reacting with InCl3 under solvothermal condition.44 In our synthesis approach, the CuSe species with hexagonal structure is the most stable intermediate formed without pretreatment of selenium powder which is supposed to be crystalline seeds for platelike morphology of CuInSe2 particles. In both synthesis approaches, the formation of CuInSe2 nanoparticles can be summarized as follows. When selenium is dispersed into ethylenediamine, its reactivity increases greatly.45 In other words, a nucleophilic attack by the amines of ethylenediamine can activate elemental selenium to form reactive Se2- ions, similar to how elemental sulfur activates and converts to S2- ions via amine or hydroxides (eq 1).45,46 Once these reactive selenium species (Se2- ions) are formed, they can react with both InCl3 and Cu2þ simultaneously in a solution to form In2Se3 and CuSe, respectively, as seen in the proposed mechanism (see eqs 2 and 3). As the temperature and the reaction time increases, indium selenide dissolves in ethylenediamine and reacts with Se2- ions to form InSe2-, as eq 4.16 Meanwhile, ethylenediamine, as a reducing solvent, reduces the Cu2þ to Cuþ, because of the electron transfer reaction, and chelates Cuþ to form a relatively stable [Cu(en)2]þ as the reaction proceeds (eq 5).16,17 Once copper complex ([Cu(en)2]þ) is formed, its formation can effectively prevent the formation of binary copper chalcogenide,17 since a two five-membered ring chelated structure of [Cu(en)2]þ complex has been considered to be the most stable in energy among the possible isometric structure.16 At the same time, under the

influence of reducing and chelating effects of ethylenediamine, the dissolution of CuSe occurs and is facilitated by the microwave heating. As a result, the formation of a binary CuSe intermediate can be effectively deterred and controlled. Finally, [Cu(en)2]þ reacts with (InSe2)- to produce CuInSe2, as shown in eq 6.16 To further understand the formation mechanism of CuInSe2 particles without pretreatment and with pretreatment of selenium powder in ethylenediamine under microwave heating, we have investigated the samples obtained at different reaction times using the SEM and XRD techniques. Figure 3 shows the XRD pattern of CuInSe2 particles prepared without pretreatment (Figure 3a) and with pretreatment of selenium powder, followed by different microwave reaction times (Figure 3b). In both approaches, at shorter reaction times, many diffraction peaks appeared in the XRD patterns, which can be attributed to CuSe and the unreacted selenium, along with the characteristic peaks of CuInSe2 particles. There are no such peaks at longer reaction times (30 min), but only characteristic peaks of CuInSe2 nanoparticles are observed, indicating the formation of a pure chalcopyrite phase with an intense peak at 2θ ≈ 26.6°, which is oriented along the (112) direction. Although the intensity of this peak increases with reaction time, it is higher for CuInSe2 particles obtained with a pretreatment of selenium powder than that without pretreatment. It suggests that the CuInSe2 particles with higher crystalline were obtained via the pretreatment of selenium powder. The absence of Se XRD peaks as the reaction progresses shows that all of the selenium has been reacted. Only in the absence of pretreatment, a characteristic peak of the (400) phase47,48 for InSe appeared at 2θ ≈ 21.6° at a reaction time of 15 min. No such peak is detected for a sample prepared at longer reaction times. The InSe compound appears at a reaction time of 15 min and disappears afterward, indicating the competitive formation of InSe and CuInSe2 compounds at 15 min and

(45) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. J. Mater. Chem. 2002, 12, 2755– 2761. (46) Malike, M. A.; O’Brien, P. Adv. Mater. 1999, 11, 1441.

(47) Revanprasadu, N.; Malik, M. A.; Carstens, J.; O’Brien, P. J. Mater. Chem. 1999, 9, 2885–2888. (48) Lu, W.-L.; Fu, Y.-S; Tseng, B.-H. J. Phys. Chem. Solids 2008, 69, 637–640.

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Table 1. EDX Analyses of Synthesized CuInSe2 Particles and the Grain Size at Different Reaction Times Elemental Analyses Without Pretreatment (Platelike)

With Pretreatment (Rodlike)

reaction time (min)

Cu (at. %)

In (at. %)

Se (at. %)

grain size (nm)

Cu (at. %)

In (at. %)

Se (at. %)

grain size (nm)

5 15 30

35.25 42.90 23.33

17.37 13.67 22.06

47.38 43.43 54.62

16.9 20.6 26.6

3.48 15.50 26.66

7.42 20.91 20.07

89.11 63.59 53.27

21.4 31.6 45.1

Figure 4. Raman spectra of CuInSe2 particles synthesized by microwave-assisted solvothermal method (a) without pretreatment and (b) with pretreatment of a selenium powder and ethylenediamine mixture.

then pure CuInSe2 particles are formed by consuming this (400) phase.48 For the case of pretreatment with selenium powder, the absence of any InSe peaks in all reaction times shows that the pretreatment condition may be disfavored for the formation of the (400) phase of InSe. The XRD spectra also show that the starting crystalline phase of the CuInSe2 particles prepared with the pretreatment of selenium powders are different from the one without pretreatment. In the case of without pretreatment, the initial XRD pattern showed only small signals of CuInSe2 particles. However, the CuInSe2 nanoparticles became more crystalline as the reaction progresses. On the other hand, with a 30-min pretreatment of the selenium and ethylenediamine mixture, a strong XRD peak for CuInSe2 was present even only after 5 min of reaction time, suggesting a more dominant presence of the (112) phase of CuInSe2 and less impurity. Furthermore, after 15 min of reaction time, pure CuInSe2 was almost obtained. This suggested that the pretreatment of selenium powders is a favorable condition for the formation of pure crystalline CuInSe2 particles. Furthermore, using triethylenetetramine (trien) as a solvent, the early formation of solid Cu2-xSe intermediate and its reaction with soluble indium species to form CuInSe2 particles over a period of 24 h at reflux temperature have been investigated by Raman spectroscopy.43 However, in both of our approaches, there is no any characteristic peaks of Cu2-xSe phase in XRD data shown in Figure 3. The aforementioned XRD results are supported by the SEM images. The SEM image in Figures 2a and d show the morphologies of impurities along with the morphologies of CuInSe2 particles in the early stage of the reaction. The morphologies of impurities disappeared gradually with reaction time, as seen in Figures 2b and e. However, compared to the pretreatment condition, the approach without pretreatment resulted in

more impurities. For a reaction time of 30 min, pure rodlike CuInSe2 particles were obtained with pretreatment of the selenium powder, whereas platelike CuInSe2 particles were produced without pretreatment. Accordingly, the proposed mechanism involved in this solvothermal reaction for the growth of CuInSe2 nanoparticles can be summarized by the following equations:16,17,44,49 Se þ 2en f SeðenÞx

ð1Þ

2InCl3 þ 3Se2 - f In2 Se3 þ 6Cl-

ð2Þ

Cu2þ þ Se2 - f CuSe

ð3Þ

In2 Se3 þ Se2 - T 2ðInSe2 Þ-

ð4Þ

Cuþ þ 2en T ½CuðenÞ2 þ

ð5Þ

ðInSe2 Þ- þ ½CuðenÞ2 þ f CuInSe2 þ 2en

ð6Þ

Therefore, the key factor of the shape control was concluded to be the pretreatment of selenium powder in ethylenediamine before reaction with copper and indium precursors in this work. In addition, the traditional solvothermal method requires ∼48 h to obtain pure chalcopyrite crystalline of CuInSe2 particles.8 The proposed synthesis method has been proven capable of reducing the reaction to ∼30 min. The EDX elemental analyses of CuInSe2 particles at different reaction times are summarized in Table 1. The EDX elemental analysis data is well-supported by the XRD data and is in good agreement with the proposed formula of CuInSe2 particles. As the reaction time increases, the atomic ratio of Cu, In, and Se tends to approach a pure crystalline structure proportion of 1:1:2 for CuInSe2 particles. At a given reaction time, the pretreatment of selenium powder (49) Su, H.; Xie, Y.; Li, B.; Qian, Y. Mater. Res. Bull. 2000, 35, 465–469.

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resulted larger grain sizes and higher crystalline CuInSe2 particles than those formed in the absence of pretreatment. Raman Spectra of CuInSe2 Particles. The symmetry of the chalcopyrite crystal decomposes the optical vibrational modes into 15 normal modes, among which the Ramanscattering intensity of the nonpolar A1 mode is much stronger than that of any other mode.50 In Figure 4, the very strong signal measured at 175 cm-1 for CuInSe2 particles corresponds to the A1 optical phonon mode in tetragonal CuInSe2 crystal.50-52 The A1 mode in CuInSe2 results from the motion of the Se atom, and the Cu and In atoms remain at rest.50-52 The full width at half-maximum (fwhm) values are ∼9.5 and ∼9.2 cm-1 for CuInSe2 particles. These values are smaller than the typical (fwhm) values of ∼12-13 cm-1 for CuInSe2 thin films.8,53 The feature could be attributed to a more-uniform size distribution and higher crystallinity in the single-crystalline CuInSe2 particles,53 particularly for CuInSe2 particles synthesized with the pretreatment of selenium powder in ethylenediamine via the microwave-assisted solvothermal route. 3. Conclusions Pure CuInSe2 particles of high crystallinity have been successfully synthesized by the microwave-assisted solvothermal method for a short reaction time of 30 (50) Matsushita, H.; Endo, S.; Irie, T. Jpn. J. Appl. Phys. 1992, 31, 18. (51) Roy, S.; Guha, P.; Kundu, S. N.; Hanzawa, H.; Chaudhuri, S.; Pal, A. K. Mater. Chem. Phys. 2002, 73, 24–30. (52) Chen, H.; Yu, S.-M.; Shin, D.-W. Nanoscale Res. Lett. 2010, 5, 217–223. (53) Das, K.; Panda, S. K.; Chaudhuri, S. Appl. Surf. Sci. 2007, 253, 5166–5172.

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min, which showed great improvement in reduction of the reaction time, energy consumption, and reproducibility, compared with a traditional heating method. The products obtained via shape control formation mechanisms were characterized by different analytical techniques such as XRD, SEM, Raman scattering, and EDX elemental analysis. As the results have shown, onedimensional growth of selenium seeds due to the pretreatment of selenium powders in ethylenediamine was responsible for the formation of the rodlike morphology of CuInSe2 particles. In contrast, the formation of a hexagonal CuSe intermediate without pretreatment was closely related to the platelike morphology. The key factor of the shape control was concluded to be the pretreatment of selenium powder in ethylenediamine before reaction with copper and indium precursors in this work. This new synthetic strategy provides encouraging evidence for the researchers to control the morphology and particle distribution of CuInSe2 particles by systematic manipulation of the morphology of selenium seeds. The microwave-assisted solvothermal method developed in this work can be easily extended to other chalcopyrite systems with tunable morphology. Acknowledgment. The authors gratefully acknowledge the financial support from the National Science Council of Taiwan (NSC-97-2120-M-011-001 and NSC-97-2221-E011-075-MY3), and the National Taiwan University of Science (NTUST) and Technology and Industrial Technology Research Institute (ITRI).