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Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization
Accepted Manuscript Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization S. Sugan, K. Baskar, R. Dhanasekaran PII:
S1567-1739(14)00252-1
DOI:
10.1016/j.cap.2014.08.011
Reference:
CAP 3710
To appear in:
Current Applied Physics
Received Date: 30 April 2014 Revised Date:
16 July 2014
Accepted Date: 18 August 2014
Please cite this article as: S. Sugan, K. Baskar, R. Dhanasekaran, Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization, Current Applied Physics (2014), doi: 10.1016/j.cap.2014.08.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization S. Sugan*, K. Baskar, R. Dhanasekaran
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Crystal Growth Centre, Anna University, Chennai - 600 025, India. *E-mail: [email protected], Phone No. +9144-22358317.
SC
Abstract
CuInS2, CuInSe2 and CuInTe2 nanocubes of chalcopyrite structure have been
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successfully synthesized by hydrothermal process using deionized water as solvent at 180°C for 20 hr. The crystallinity, compositional, morphological and optical properties of the synthesized samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Raman and photoluminescence (PL) spectra analyses. The Raman spectra of the synthesized CuInS2,
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CuInSe2 and CuInTe2 samples show the dominant A1 modes at 293, 172 and 121 cm-1 respectively. The possible chemical reaction and mechanism of nanocubes formation were
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discussed. The emission wavelength of as synthesized CuInS2, CuInSe2 and CuInTe2 samples were blue shifted at 746 nm (1.66 eV), 863 nm (1.43 eV) and 859 nm (1.44 eV) respectively.
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Keywords: Semiconductors; Chemical synthesis; Electron microscopy; X-ray techniques
1. Introduction
Copper indium sulfide (CuInS2), copper indium selenide (CuInSe2) and copper
indium telluride (CuInTe2) I-III-VI2 semiconductor materials are of much interest due to their high optical absorption coefficient, good photostability and promising application in photovoltaic and optoelectronic devices [1]. The band gaps of CuInS2, CuInSe2 and CuInTe2 around 1.53, 1.04 and 1.0 eV at room temperature are very close to the optimum for solar 1
ACCEPTED MANUSCRIPT energy conversion. For this reason, CuInX2 (X= S, Se and Te) are considered to be attractive for solar cell applications [2]. In recent times, quantum dot based solar cells have gained much interest due to the possibility of replacing the thin film devices. The main advantage of quantum dots is that it can be tuned to absorb specific wavelength from visible to IR region
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by changing the particle size [3]. Several methods have been reported on the deposition of CuInS2, CuInSe2 and CuInTe2 thin films and nanoparticles such as magnetron sputtering, chemical bath deposition, flash evaporation, spray pyrolysis as well as thermolysis,
SC
photolysis, microwave synthesis, microwave irradiation, sol-gel, wet chemical, solvothermal and hydrothermal methods. However, the above approaches required complicated facility,
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high temperature and some of them use toxic reagents like organometallic compounds and hydrogen sulfide (H2S). To avoid these drawbacks solvothermal methods have been widely studied for the synthesis of CuInS2 and CuInSe2 particles at much lower temperature [4, 5]. The required ethylenediamine solvent was costly and easily evaporated [6]. To solve this
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problem, the hydrothermal method was used to synthesis CuInS2, CuInSe2 and CuInTe2 nanocubes at temperature of 180°C for 20 hr, to get excellent reproducibility and high yield of product [7-9]. Deionized water was used as the solvent in the hydrothermal method and it
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was more environmental friendly rather than ethylenediamine [4].
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A facile route to the synthesis of CuInS2 nanoparticles have been reported by Dutta et al [10]. Rapid synthesis and size control of CuInS2 semiconductor nanoparticles have been prepared by Gardner et al [11] using microwave irradiation. Grisaru et al reported the preparation of the CuInTe2 and CuInSe2 nanoparticles using microwave-assisted polyol method [12]. Harichandran et al have synthesized the CuAlS2 nanorods using convenient wet chemical method [13]. The structural, morphology and optical properties of CuAlS2 thin films deposited by spray pyrolysis method have been reported by Caglar et al [14]. Kim et al have made the chalcogen-based thin film transistor using CuInSe2 photo-active layer [15]. 2
ACCEPTED MANUSCRIPT Solvothermal synthesis and characterization of chalcopyrite CuInSe2 nanoparticles have been studied by Chen et al [16]. Synthesizing wurtzite CuInS2 nanoplates at low temperature has been analyzed by Guo et al [17]. Chen et al have prepared the solvothermal synthesis and characterization
of
copper
indium
diselenide
microflowers
[18].
Synthesis
and
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characterization of polycrystalline CuInS2 thin films for solar cell devices at low temperature processing conditions have been synthesized by Park et al [19]. Previous reports on hydrothermal synthesis of CuInSe2 nanoparticles use acetic acid as a solvent at temperature
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of 200°C and time duration of 12 hr [4]. There are no reports available in the literature about the comparative studies on the CuInS2, CuInSe2 and CuInTe2 nanocubes using hydrothermal
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synthesis. In the present investigation a detailed study on structural, morphological and optical characteristics of CuInS2, CuInSe2 and CuInTe2 nanocubes synthesized by hydrothermal method has been reported. The above synthesized materials could be used for extensive potential applications in photovoltaic devices.
2.1 Materials synthesis
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2. Materials and methods
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Analytical grade of copper (II) chloride, indium (In), sulfur (S), selenium (Se) and
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tellurium (Te) were used as source materials and deionized water was taken as a solvent. In a typical procedure 1:1:2 mole ratio of CuCl2, In and S were added to 80 ml of deionized water and the solution was stirred for 1 hr. Then the resultant solution was transferred to a stainless steel autoclave of 100 ml capacity with 80 % of the volume filled. Then the sealed autoclave was kept in the resistive heating furnace at 180°C for 20 hr and allowed to cool to room temperature. After synthesis process, the precipitate in the autoclave was separated by filtration and then washed with deionized water to remove residual impurities such as S and
3
ACCEPTED MANUSCRIPT Cl. A dark colored CuInS2 powder was obtained after drying at 100°C for 5 hr. The same procedure was followed for the synthesis of CuInSe2 and CuInTe2 samples. CuInS2, CuInSe2 and CuInTe2 nanocubes were formed in hydrothermal synthesis. The
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possible chemical reaction for the formation of CuInS2, CuInSe2 and CuInTe2 nanocubes is given below CuCl 2 (H) + In (H) + 2X (H) → CuInX 2(s) + Cl 2 (g)
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(1)
Here, the symbol ‘H’ refers to deionized water and X = S, Se and Te. Under the high
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pressure, copper (II) chloride (CuCl2), indium (In) and X (sulfur, selenium and tellurium) reacts with deionized water to form the final products of CuInS2, CuInSe2 and CuInTe2 nanocubes [12]. 2.2 Characterizations
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The XRD patterns were recorded using X’pert Pro XRD (PANalytical). The surface morphologies of the synthesized samples were studied using Scanning Electron Microscope
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(Carl Zeiss MA15 / EVO 18) and Transmission Electron Microscope (FEI 300 kV). Elemental analysis of the samples was carried out using Energy Dispersive X-ray (INCA
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Energy 250). Raman vibrational modes of as synthesized samples were obtained using Microraman Spectrometer with 633 nm He-Ne laser (Horiba HR 800). Photoluminescence spectra were recorded using an excitation wavelength of 244 nm.
4
ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Structural and elemental analysis Fig. 1 shows the powder XRD patterns of as synthesized CuInS2, CuInSe2 and
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CuInTe2 nanocubes. The peaks observed for CuInS2, CuInSe2 and CuInTe2 nanocubes were compared and indexed with JCPDS card No. 15-0681, JCPDS card No. 89-5646 and JCPDS card No. 65-0354 respectively. All the diffraction peaks were assigned and there were no
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secondary phases of CuInS2, CuInSe2 and CuInTe2 indicating the formation of tetragonal structure. Fig. 2 shows the EDX spectra of CuInS2, CuInSe2 and CuInTe2 nanocubes. The
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result confirms that no elements other than Cu, In, S, Se and Te were present in the synthesized samples which show their purity and the estimated composition values are tabulated in Table 1. It is evident from the EDX and X-ray diffraction that the materials synthesized are of stoichiometric composition without any additional phases or impurities.
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3.2 Morphological (SEM and TEM) analysis
Fig. 3 shows the SEM images of as synthesized CuInS2, CuInSe2 and CuInTe2
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samples. From the SEM images, it is clearly observed that the samples consist of cube like morphologies. Fig. 4 shows the typical TEM and selected area electron diffraction (SAED)
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pattern of CuInS2, CuInSe2 and CuInTe2 nanocubes respectively. From the TEM images average size of the CuInS2, CuInSe2 and CuInTe2 cubes were calculated approximately as 88 nm, 106 nm and 228 nm respectively. The SAED pattern of prepared CuInS2 nanocubes was shown in the Fig. 4. The diffraction rings consist of many discrete spots and it’s the hkl values (112), (224) and (312) were well matched with JCPDS value (15-0681). It confirms the tetragonal polycrystalline CuInS2. In CuInSe2 and CuInTe2 nanocubes, the hkl values of the diffraction patterns were (202), (213) and (200), (220), (321), (420) which were in good agreement with the respective JCPDS values (89-5646) of CuInSe2 and JCPDS values (655
ACCEPTED MANUSCRIPT 0354) of CuInTe2. The orientation of planes in CuInTe2 was well ordered in comparison with CuInS2 and CuInSe2. 3.3 Raman spectra analysis
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Raman spectra of CuInS2, CuInSe2 and CuInTe2 nanocubes synthesized at 180°C for 20 hr are presented in Fig. 5. The spectra show the vibration bands in the range of 50-450 cm1
. The CuInS2 sample can be divided into three vibration peaks located around 293, 269 and
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233 cm-1. The dominant peak at 293 cm-1 can be assigned to A1 mode. The other peaks of 269 and 233 cm-1 may be attributed to E (LO/TO) and B2 (LO/TO) modes [20, 21]. Similarly, the
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Raman spectra of CuInSe2 show A1 mode at around 175 cm-1, usually observed in the I–III– VI2 chalcopyrite compounds. A1 band is the most intense band in the spectra of chalcopyrite type compounds [22]. In the present case, the dominant peaks at 172 cm-1 is attributed to A1 modes of chalcopyrite CuInSe2. The A1 mode of CuInSe2 results due to motion of Se atom, in
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the perpendicular direction to the c-axis, and the Cu and In atoms remains at rest [23]. The next intensity peak which appeared at 196 cm-1 is due to E mode for CuInSe2. In CuInTe2 nanocubes revealed two peaks at 121 and 174 cm-1. The dominant intensity peak appears at
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121 cm-1 and has been assigned to the A1 mode. The A1 mode of CuInTe2 due to the motion of tellurium atoms with the cations at rest and is the most intense mode among those expected
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for the chalcopyrite structure [24].The peak appeared at 174 cm-1 is due to E or B2 (LO) mode for CuInTe2.
3.4 Photoluminescence (PL) studies Fig. 6 shows the PL spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes. The spectra were recorded at room temperature with an excitation wavelength of 244 nm. In CuInS2 sample, the emission peaks appear at 746, 927 and 1081 nm. The emission peak at 746 nm corresponds to energy gap of 1.66 eV, which is larger than the reported value 6
ACCEPTED MANUSCRIPT of 1.55 eV [25]. The emission peak at 927 nm corresponds to energy gap of 1.33 eV, is slightly less than that reported value of 1.55 eV for single crystal but it is closely matched with reported value for thin films [26]. The emission peak at 1081 nm corresponds to energy gap of 1.14 eV, which may be due to defect. The broad emission peak of CuInSe2 sample
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centered at 863 nm corresponding energy gap of 1.43 eV, which is larger than the reported value of 1.04 eV for bulk α–CIS [27]. For CuInTe2 sample, a stronger emission peak appears at 859 nm accompanied with weak emission peak at 980 nm and the corresponding energy
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gap value is 1.44 and 1.26 eV, which is larger than the reported value of 1.02 eV for bulk CuInTe2 [28, 29]. Sridharan et al [30] reported a relative blue shift in the absorption spectrum
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of PbTe nanorods with an average diameter of 100 nm and the length of 600 nm sizes is caused from quantum size effect arising from the tips. They mentioned that the effective mass approximation estimates the excitonic Bohr radius of PbS to be about 200 nm, so that the quantum size effects should be felt even in the micrometer sized crystallites. Kungumadevi et
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al [31] observed that the optical absorption spectrum of PbTe micro-needles (diameter varies from 90 to 130 nm and the lengths are 1-2 µm) shows large blue shift (1.26 eV) with respect to those of the bulk (0.32 eV) due to quantum confinement of charge carriers, which is
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consistent with the blue shift of the band emission peak in the PL spectrum. Chang et al [32]
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have reported the surface bond contraction and the increase in the surface-to-volume ratio with reducing particle size creates the change of the band features of nanometric semiconductors and corresponding properties such as the blue shift in the PL spectra. The similar blue shift from the PL spectra of CuInTe2 with the size range of 80-200 nm (SEM images) and attributed that it is caused from size effect in the nanocubes [33]. Moreover, the crystallite size of the CuInTe2 nanocubes calculated from the XRD results about 65 nm and this may be also influence in the energy gap enhancement of nanocubes [34]. Hence, the above results suggested that the size effect (grain size and crystallite size) could be the reason
7
ACCEPTED MANUSCRIPT for increase in the optical band gap energy (1.44 eV and 1.26 eV) of CuInTe2 nanocubes as compared to the bulk (1.02 eV). 4. Conclusions
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In summary, CuInS2, CuInSe2 and CuInTe2 nanocubes have been synthesized through hydrothermal method. XRD pattern confirms the chalcopyrite (tetragonal) structure of synthesized samples. SEM and TEM analyses show that the synthesized samples were
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composed of cubes with size of nanometers. The dominant Raman scattering vibration has been attributed to the A1 mode. The emission wavelength and corresponding energy gaps of
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synthesized samples were obtained from PL spectra. Acknowledgement
The authors are thankful to the Council of Scientific and Industrial Research (CSIR) New
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Delhi, India for financial support. The authors are grateful to Mr. C. Vijaya Bhaskar, Department of English, Anna University, Chennai for carrying out English language
References
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correction.
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[1] J.F. Guillemoles, L. Kronik, D. Cahen, U. Rau, A. Jasenek, H.W. Schock, J. Phys. Chem B. 104 (2000) 4849. [2] G. Wei, S.R. Forrest, Nano. Lett. 7 (2007) 218. [3] R.P. Raffaelle, S.L. Castro, A.F. Hepp, S.G. Bailey, Progress in Photovoltaics. 10 (2002) 433. [4] Jang Bo Shim, Chang Gyoun Kim, Dong Ju Jeon, Taek-Mo Chung, Ki- Seok An, Sun Suk Lee, Jong Sun Lim, Seok Jong Jeong, Bo Keun Park, Young Kuk Lee, J. Phys. Chem. Solids. 74 (2013) 867. 8
ACCEPTED MANUSCRIPT [5] W. L. Lu, Y. S. Fu, B. H. Tseng, J. Phys. Chem. Solids. 69 (2008) 637. [6] C. H. Wu, F. S. Chen, S. H. Lin, C. H. Lu, J. Alloys. Compd. 509 (2011) 5783. [7] Chia- Chien Chang, Hsin- Lin Wu, Chun- Hong Kuo, Michael H. Huang, Chem. Mater. 20 (2008) 7570.
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[8] Wojciech L. Suchanek, Richard E. Riman, Advances in Science and Technology. 45 (2006) 184.
[9] K. Byrappa, T. Adschiri, Progress in Crystal Growth and Characterization of Materials.
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[10] Dimple P. Dutta, Garima Sharma, Mater. Lett. 60 (2006) 2395.
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[11] Gardner Joseph S, Shurdha Endrit, Lau Lisa D, Wang Chongmin, Rodriguez Rene G, Pak Joshua J, J. Nanopart Res. 10 (2008) 633.
[12] Haviv Grisaru, Oleg Palchik, Aharon Gedanken, Valery Palchik, Michael A. Slifkin, Arie M. Weiss, Inorg. Chem. 42 (2003) 7148.
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[13] G. Harichandran, N.P. Lalla, Mater. Lett. 62 (2008) 1267.
[14] Mujdat Caglar, Saliha Ilican, Yasemin Caglar, Optics Communications. 281 (2008) 1615.
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1326.
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[15] Kyung- Am Kim, Ki-Bong Song, JunHo Kim, Kyuman Cho, Curr. Appl. Phys. 9 (2009)
[16] Huiyu Chen, Seong- Man Yu, Dong- Wook Shin, Ji- Beom Yoo, Nanoscale Res Lett. 5 (2010) 217.
[17] Jianyong Guo, Gang Chang, Wei Zhang, Xiong Liu, Taosheng Zhou, Yunbin He, Mater.Lett. 123 (2014) 169. [18] Huiyu Chen, Dan Nie, Chunju Xu, Guizhe Zhao, Yaqing Liu, Mater.Lett. 106 (2013) 79. [19] Mi Sun Park, Seung Yeol Han, Eun Jin Bae, Tae Jin Lee, Chih Hung Chang, Si Ok Ryu, Curr. Appl. Phys. 10 (2010) S379.
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ACCEPTED MANUSCRIPT [20] H. Matsushita, S. Endo, T. Irie, Jpn. J. Appl. Phys. 31 (1992) 18. [21] W.H. Koschel, M. Bettini, Phys. Stat. Sol B. 72 (1975) 729. [22] E. P. Zaretskaya, V. F. Gremenok, V. Riede, W. Schmitz, K. Bente, V. B. Zalesski, O. V. Ermakov, J. Phys. Chem. Solids. 64 (2003) 1989.
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[23] H. Chen, S. M. Yu, D. W. Shin, J. B. Yoo, Nanoscale Res. Lett. 5 (2010) 217. [24] M. R. Ananthan, Bhaskar Chandra Mohanty, S. Kasiviwanathan, Semicond. Sci. Technol. 24 (2009) 075019.
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[25] I.A. Aksenov, N.A. Sobolev, V.A. Sheraukhov, Phys. Status Solidi A. 123 (1991) K171. [26] M. Gossala, H. E. Mahnke, H. Metzner, Thin Solid Films. 361-362 (2000) 56.
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[27] Q. Guo, S.J. Kim, M. Kar, W.N. Shafarman, R.W. Birkmire, E.A. Stach, R. Agrawal, H.W. Hillhouse, Nano Lett. 8 (2008) 2982.
[28] C. Rincon, S.M. Wasim, G. Marin, Mater. Lett. 36 (1998) 245. [29] Sungwoo Kim, Meejae Kang, Seajin Kim, Jin- Hyuk Heo, Jun Hong Noh, Sang
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Hyuk Im, Sang II Seok, Sang- Wook Kim, ACS Nano. 7 (2013) 4756. [30] K. Sridharan, V. Tamilselvan, D. Yuvaraj, K. Narasimha Rao, R. Philip, Opt. Mater. 34 (2012) 639.
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[31] L. Kungumadevi, R. Sathyamoorthy, Mater. Res. Bullet. 48 (2013) 1891. [32] Chang Q Sun, T. P Chen, B. K Tay, S. Li, H. Huang, Y. B Zhang, L. K Pan, S. P Lau, X.
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W. Sun, J. Phys. D: Appl. Phys. 34 (2001) 3470. [33] N. Zhao, L. Qi, Adv. Mater. 18 (2006) 359. [34] Yu Chang, Joong Jiat Teo, Hua Chun Zeng, Langmuir, 21 (2005) 1074.
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ACCEPTED MANUSCRIPT
Table caption
Samples
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Table 1 Compositional analysis of CuInS2, CuInSe2 and CuInTe2 nanocubes
Composition, At (%) Cu
In
S
Se
Te
CuInS2
24.17
24.23
51.60
--
--
CuInSe2
24.03
25.75
--
50.22
--
CuInTe2
25.94
25.81
--
--
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48.25
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Figure captions Fig. 1 XRD patterns of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 2 EDX spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes Fig. 3 SEM images of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
Fig. 4 TEM and SAED patterns of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 5 Raman spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 6 PL spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 4
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Fig. 5
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Fig. 6
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ACCEPTED MANUSCRIPT Highlights CuInS2,
CuInSe2
and
CuInTe2
nanocubes
were
synthesized
through
hydrothermal method
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The reaction was carried out at 180°C SEM and TEM images shows the cube like morphologies for all synthesized samples
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The optical properties of nanocubes have been studied by Raman and PL spectra
S1567-1739(14)00252-1
DOI:
10.1016/j.cap.2014.08.011
Reference:
CAP 3710
To appear in:
Current Applied Physics
Received Date: 30 April 2014 Revised Date:
16 July 2014
Accepted Date: 18 August 2014
Please cite this article as: S. Sugan, K. Baskar, R. Dhanasekaran, Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization, Current Applied Physics (2014), doi: 10.1016/j.cap.2014.08.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization S. Sugan*, K. Baskar, R. Dhanasekaran
RI PT
Crystal Growth Centre, Anna University, Chennai - 600 025, India. *E-mail: [email protected], Phone No. +9144-22358317.
SC
Abstract
CuInS2, CuInSe2 and CuInTe2 nanocubes of chalcopyrite structure have been
M AN U
successfully synthesized by hydrothermal process using deionized water as solvent at 180°C for 20 hr. The crystallinity, compositional, morphological and optical properties of the synthesized samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Raman and photoluminescence (PL) spectra analyses. The Raman spectra of the synthesized CuInS2,
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CuInSe2 and CuInTe2 samples show the dominant A1 modes at 293, 172 and 121 cm-1 respectively. The possible chemical reaction and mechanism of nanocubes formation were
EP
discussed. The emission wavelength of as synthesized CuInS2, CuInSe2 and CuInTe2 samples were blue shifted at 746 nm (1.66 eV), 863 nm (1.43 eV) and 859 nm (1.44 eV) respectively.
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Keywords: Semiconductors; Chemical synthesis; Electron microscopy; X-ray techniques
1. Introduction
Copper indium sulfide (CuInS2), copper indium selenide (CuInSe2) and copper
indium telluride (CuInTe2) I-III-VI2 semiconductor materials are of much interest due to their high optical absorption coefficient, good photostability and promising application in photovoltaic and optoelectronic devices [1]. The band gaps of CuInS2, CuInSe2 and CuInTe2 around 1.53, 1.04 and 1.0 eV at room temperature are very close to the optimum for solar 1
ACCEPTED MANUSCRIPT energy conversion. For this reason, CuInX2 (X= S, Se and Te) are considered to be attractive for solar cell applications [2]. In recent times, quantum dot based solar cells have gained much interest due to the possibility of replacing the thin film devices. The main advantage of quantum dots is that it can be tuned to absorb specific wavelength from visible to IR region
RI PT
by changing the particle size [3]. Several methods have been reported on the deposition of CuInS2, CuInSe2 and CuInTe2 thin films and nanoparticles such as magnetron sputtering, chemical bath deposition, flash evaporation, spray pyrolysis as well as thermolysis,
SC
photolysis, microwave synthesis, microwave irradiation, sol-gel, wet chemical, solvothermal and hydrothermal methods. However, the above approaches required complicated facility,
M AN U
high temperature and some of them use toxic reagents like organometallic compounds and hydrogen sulfide (H2S). To avoid these drawbacks solvothermal methods have been widely studied for the synthesis of CuInS2 and CuInSe2 particles at much lower temperature [4, 5]. The required ethylenediamine solvent was costly and easily evaporated [6]. To solve this
TE D
problem, the hydrothermal method was used to synthesis CuInS2, CuInSe2 and CuInTe2 nanocubes at temperature of 180°C for 20 hr, to get excellent reproducibility and high yield of product [7-9]. Deionized water was used as the solvent in the hydrothermal method and it
EP
was more environmental friendly rather than ethylenediamine [4].
AC C
A facile route to the synthesis of CuInS2 nanoparticles have been reported by Dutta et al [10]. Rapid synthesis and size control of CuInS2 semiconductor nanoparticles have been prepared by Gardner et al [11] using microwave irradiation. Grisaru et al reported the preparation of the CuInTe2 and CuInSe2 nanoparticles using microwave-assisted polyol method [12]. Harichandran et al have synthesized the CuAlS2 nanorods using convenient wet chemical method [13]. The structural, morphology and optical properties of CuAlS2 thin films deposited by spray pyrolysis method have been reported by Caglar et al [14]. Kim et al have made the chalcogen-based thin film transistor using CuInSe2 photo-active layer [15]. 2
ACCEPTED MANUSCRIPT Solvothermal synthesis and characterization of chalcopyrite CuInSe2 nanoparticles have been studied by Chen et al [16]. Synthesizing wurtzite CuInS2 nanoplates at low temperature has been analyzed by Guo et al [17]. Chen et al have prepared the solvothermal synthesis and characterization
of
copper
indium
diselenide
microflowers
[18].
Synthesis
and
RI PT
characterization of polycrystalline CuInS2 thin films for solar cell devices at low temperature processing conditions have been synthesized by Park et al [19]. Previous reports on hydrothermal synthesis of CuInSe2 nanoparticles use acetic acid as a solvent at temperature
SC
of 200°C and time duration of 12 hr [4]. There are no reports available in the literature about the comparative studies on the CuInS2, CuInSe2 and CuInTe2 nanocubes using hydrothermal
M AN U
synthesis. In the present investigation a detailed study on structural, morphological and optical characteristics of CuInS2, CuInSe2 and CuInTe2 nanocubes synthesized by hydrothermal method has been reported. The above synthesized materials could be used for extensive potential applications in photovoltaic devices.
2.1 Materials synthesis
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2. Materials and methods
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Analytical grade of copper (II) chloride, indium (In), sulfur (S), selenium (Se) and
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tellurium (Te) were used as source materials and deionized water was taken as a solvent. In a typical procedure 1:1:2 mole ratio of CuCl2, In and S were added to 80 ml of deionized water and the solution was stirred for 1 hr. Then the resultant solution was transferred to a stainless steel autoclave of 100 ml capacity with 80 % of the volume filled. Then the sealed autoclave was kept in the resistive heating furnace at 180°C for 20 hr and allowed to cool to room temperature. After synthesis process, the precipitate in the autoclave was separated by filtration and then washed with deionized water to remove residual impurities such as S and
3
ACCEPTED MANUSCRIPT Cl. A dark colored CuInS2 powder was obtained after drying at 100°C for 5 hr. The same procedure was followed for the synthesis of CuInSe2 and CuInTe2 samples. CuInS2, CuInSe2 and CuInTe2 nanocubes were formed in hydrothermal synthesis. The
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possible chemical reaction for the formation of CuInS2, CuInSe2 and CuInTe2 nanocubes is given below CuCl 2 (H) + In (H) + 2X (H) → CuInX 2(s) + Cl 2 (g)
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Here, the symbol ‘H’ refers to deionized water and X = S, Se and Te. Under the high
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pressure, copper (II) chloride (CuCl2), indium (In) and X (sulfur, selenium and tellurium) reacts with deionized water to form the final products of CuInS2, CuInSe2 and CuInTe2 nanocubes [12]. 2.2 Characterizations
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The XRD patterns were recorded using X’pert Pro XRD (PANalytical). The surface morphologies of the synthesized samples were studied using Scanning Electron Microscope
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(Carl Zeiss MA15 / EVO 18) and Transmission Electron Microscope (FEI 300 kV). Elemental analysis of the samples was carried out using Energy Dispersive X-ray (INCA
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Energy 250). Raman vibrational modes of as synthesized samples were obtained using Microraman Spectrometer with 633 nm He-Ne laser (Horiba HR 800). Photoluminescence spectra were recorded using an excitation wavelength of 244 nm.
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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Structural and elemental analysis Fig. 1 shows the powder XRD patterns of as synthesized CuInS2, CuInSe2 and
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CuInTe2 nanocubes. The peaks observed for CuInS2, CuInSe2 and CuInTe2 nanocubes were compared and indexed with JCPDS card No. 15-0681, JCPDS card No. 89-5646 and JCPDS card No. 65-0354 respectively. All the diffraction peaks were assigned and there were no
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secondary phases of CuInS2, CuInSe2 and CuInTe2 indicating the formation of tetragonal structure. Fig. 2 shows the EDX spectra of CuInS2, CuInSe2 and CuInTe2 nanocubes. The
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result confirms that no elements other than Cu, In, S, Se and Te were present in the synthesized samples which show their purity and the estimated composition values are tabulated in Table 1. It is evident from the EDX and X-ray diffraction that the materials synthesized are of stoichiometric composition without any additional phases or impurities.
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3.2 Morphological (SEM and TEM) analysis
Fig. 3 shows the SEM images of as synthesized CuInS2, CuInSe2 and CuInTe2
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samples. From the SEM images, it is clearly observed that the samples consist of cube like morphologies. Fig. 4 shows the typical TEM and selected area electron diffraction (SAED)
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pattern of CuInS2, CuInSe2 and CuInTe2 nanocubes respectively. From the TEM images average size of the CuInS2, CuInSe2 and CuInTe2 cubes were calculated approximately as 88 nm, 106 nm and 228 nm respectively. The SAED pattern of prepared CuInS2 nanocubes was shown in the Fig. 4. The diffraction rings consist of many discrete spots and it’s the hkl values (112), (224) and (312) were well matched with JCPDS value (15-0681). It confirms the tetragonal polycrystalline CuInS2. In CuInSe2 and CuInTe2 nanocubes, the hkl values of the diffraction patterns were (202), (213) and (200), (220), (321), (420) which were in good agreement with the respective JCPDS values (89-5646) of CuInSe2 and JCPDS values (655
ACCEPTED MANUSCRIPT 0354) of CuInTe2. The orientation of planes in CuInTe2 was well ordered in comparison with CuInS2 and CuInSe2. 3.3 Raman spectra analysis
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Raman spectra of CuInS2, CuInSe2 and CuInTe2 nanocubes synthesized at 180°C for 20 hr are presented in Fig. 5. The spectra show the vibration bands in the range of 50-450 cm1
. The CuInS2 sample can be divided into three vibration peaks located around 293, 269 and
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233 cm-1. The dominant peak at 293 cm-1 can be assigned to A1 mode. The other peaks of 269 and 233 cm-1 may be attributed to E (LO/TO) and B2 (LO/TO) modes [20, 21]. Similarly, the
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Raman spectra of CuInSe2 show A1 mode at around 175 cm-1, usually observed in the I–III– VI2 chalcopyrite compounds. A1 band is the most intense band in the spectra of chalcopyrite type compounds [22]. In the present case, the dominant peaks at 172 cm-1 is attributed to A1 modes of chalcopyrite CuInSe2. The A1 mode of CuInSe2 results due to motion of Se atom, in
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the perpendicular direction to the c-axis, and the Cu and In atoms remains at rest [23]. The next intensity peak which appeared at 196 cm-1 is due to E mode for CuInSe2. In CuInTe2 nanocubes revealed two peaks at 121 and 174 cm-1. The dominant intensity peak appears at
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121 cm-1 and has been assigned to the A1 mode. The A1 mode of CuInTe2 due to the motion of tellurium atoms with the cations at rest and is the most intense mode among those expected
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for the chalcopyrite structure [24].The peak appeared at 174 cm-1 is due to E or B2 (LO) mode for CuInTe2.
3.4 Photoluminescence (PL) studies Fig. 6 shows the PL spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes. The spectra were recorded at room temperature with an excitation wavelength of 244 nm. In CuInS2 sample, the emission peaks appear at 746, 927 and 1081 nm. The emission peak at 746 nm corresponds to energy gap of 1.66 eV, which is larger than the reported value 6
ACCEPTED MANUSCRIPT of 1.55 eV [25]. The emission peak at 927 nm corresponds to energy gap of 1.33 eV, is slightly less than that reported value of 1.55 eV for single crystal but it is closely matched with reported value for thin films [26]. The emission peak at 1081 nm corresponds to energy gap of 1.14 eV, which may be due to defect. The broad emission peak of CuInSe2 sample
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centered at 863 nm corresponding energy gap of 1.43 eV, which is larger than the reported value of 1.04 eV for bulk α–CIS [27]. For CuInTe2 sample, a stronger emission peak appears at 859 nm accompanied with weak emission peak at 980 nm and the corresponding energy
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gap value is 1.44 and 1.26 eV, which is larger than the reported value of 1.02 eV for bulk CuInTe2 [28, 29]. Sridharan et al [30] reported a relative blue shift in the absorption spectrum
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of PbTe nanorods with an average diameter of 100 nm and the length of 600 nm sizes is caused from quantum size effect arising from the tips. They mentioned that the effective mass approximation estimates the excitonic Bohr radius of PbS to be about 200 nm, so that the quantum size effects should be felt even in the micrometer sized crystallites. Kungumadevi et
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al [31] observed that the optical absorption spectrum of PbTe micro-needles (diameter varies from 90 to 130 nm and the lengths are 1-2 µm) shows large blue shift (1.26 eV) with respect to those of the bulk (0.32 eV) due to quantum confinement of charge carriers, which is
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consistent with the blue shift of the band emission peak in the PL spectrum. Chang et al [32]
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have reported the surface bond contraction and the increase in the surface-to-volume ratio with reducing particle size creates the change of the band features of nanometric semiconductors and corresponding properties such as the blue shift in the PL spectra. The similar blue shift from the PL spectra of CuInTe2 with the size range of 80-200 nm (SEM images) and attributed that it is caused from size effect in the nanocubes [33]. Moreover, the crystallite size of the CuInTe2 nanocubes calculated from the XRD results about 65 nm and this may be also influence in the energy gap enhancement of nanocubes [34]. Hence, the above results suggested that the size effect (grain size and crystallite size) could be the reason
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In summary, CuInS2, CuInSe2 and CuInTe2 nanocubes have been synthesized through hydrothermal method. XRD pattern confirms the chalcopyrite (tetragonal) structure of synthesized samples. SEM and TEM analyses show that the synthesized samples were
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composed of cubes with size of nanometers. The dominant Raman scattering vibration has been attributed to the A1 mode. The emission wavelength and corresponding energy gaps of
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synthesized samples were obtained from PL spectra. Acknowledgement
The authors are thankful to the Council of Scientific and Industrial Research (CSIR) New
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Delhi, India for financial support. The authors are grateful to Mr. C. Vijaya Bhaskar, Department of English, Anna University, Chennai for carrying out English language
References
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Table caption
Samples
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Table 1 Compositional analysis of CuInS2, CuInSe2 and CuInTe2 nanocubes
Composition, At (%) Cu
In
S
Se
Te
CuInS2
24.17
24.23
51.60
--
--
CuInSe2
24.03
25.75
--
50.22
--
CuInTe2
25.94
25.81
--
--
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48.25
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Figure captions Fig. 1 XRD patterns of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 2 EDX spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes Fig. 3 SEM images of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
Fig. 4 TEM and SAED patterns of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 5 Raman spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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Fig. 6 PL spectra of as synthesized CuInS2, CuInSe2 and CuInTe2 nanocubes
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ACCEPTED MANUSCRIPT Highlights CuInS2,
CuInSe2
and
CuInTe2
nanocubes
were
synthesized
through
hydrothermal method
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The reaction was carried out at 180°C SEM and TEM images shows the cube like morphologies for all synthesized samples
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The optical properties of nanocubes have been studied by Raman and PL spectra