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A facile and green synthetic approach based on deep eutectic solvents toward synthesis of CZTS nanoparticles
Author’s Accepted Manuscript A facile and Green Synthetic Approach Based on Deep Eutectic Solvents toward Synthesis of CZTS Nanoparticles M. Karimi, M.J. Eshraghi, V. Jahangir www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30226-9 http://dx.doi.org/10.1016/j.matlet.2016.02.065 MLBLUE20354
To appear in: Materials Letters Received date: 2 January 2016 Revised date: 13 February 2016 Accepted date: 15 February 2016 Cite this article as: M. Karimi, M.J. Eshraghi and V. Jahangir, A facile and Green Synthetic Approach Based on Deep Eutectic Solvents toward Synthesis of CZTS Nanoparticles, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.065 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 galley proof before it is published in its final citable 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.
A facile and Green Synthetic Approach Based on Deep Eutectic Solvents toward Synthesis of CZTS Nanoparticles *
M. Karimi , M.J. Eshraghi, V. Jahangir Department of Semiconductors, Materials and Energy Research Center, Karaj, Iran. *
Corresponding Author. Tel: +98 26 3628 0035; fax: +98 263 62018888. Email address: [email protected] (M. Karimi).
Abstract High efficiency, abundant, low-cost materials, easy and inexpensive method of fabrication, and long-term stability of photovoltaic devices based on CZTS nano-absorbent has made them the dominant technology for the next generation of solar cells. In this study, a facile, low-cost, and sustainable method was developed for synthesis of CZTS nanoparticles using deep eutectic solvents (DESs) to act as both solvent and template. CZTS nanoparticles were synthesized in choline chloride:urea DES in the presence of metal chloride precursors and thiourea as sulfur source. The results on characterization of synthesized nanoparticles revealed the synthesis of kesterite CZTS nanoparticles having average crystal size of 6.5 nm, combined spherical-platelet morphology with particle diameter of 20-25 nm, elemental composition corresponding to Cu2ZnSnS4, and band gap of about 1.6 eV. The present study is worthy of attention in view of providing a simple, fast and sustainable synthesis of CZTS nanoparticles. Keywords: CZTS; Nanoparticles; Semiconductors; Green synthesis; Deep eutectic solvent. 1. Introduction Copper–zinc–tin chalcogenide (Cu2ZnSnS4, CZTS) has recently attracted considerable attention in the field of photovoltaics owing to its nontoxicity, low cost, and earth abundance [1, 2]. CZTS is a p-type semiconductor with high absorption coefficient of 104 cm-1 in the wavelength range from visible to near infrared of the solar spectrum and direct band gap of 1.5 eV. This make CZTS an attractive material as the absorber layer of thin-film solar cells compared to its rare and costly analogous CdTe and Cu(In,Ga)Se2 (CIGS) [1, 3, 4]. There are many methods developed for the synthesis of CZTS nanoparticles including hot injection solution synthesis [5], solvent mediated thermolysis process [6], solvothermal-hydrothermal synthesis [7, 8], and high temperature polycondensation route [9]. The quaternary chemical composition of CZTS is
1
one of the main challenging factors in its synthesis. Furthermore, synthesis of CZTS nanoparticles with favorable composition, morphology and particle size is problematic using existing methods and need to use harmful solvents, capping agents and surfactants under harsh conditions [10]. Therefore, green synthesis has remained as a major challenge for production of CZTS nanoparticles. Herein, as first report in the field, we developed an eco-friendly and facile method based on choline chloride-urea deep eutectic solvent (DES) for green synthesis of CZTS nanoparticles. DESs are a new generation of ionic liquids (ILs) that possess many outstanding physicochemical properties such as high conductivity, polarity, thermal stability, and negligible vapor pressure [11-13]. They are classified in the category of green solvents due to their bio-degradability, environmental friendliness, low toxicity, low price, and ease of production on large scale [13-15]. In this work, an attempt was made to use DESs as both solvent and template in synthesis of CZTS nanoparticles. The crystal phase, elemental composition, morphology-particle size, and optical properties of the nanoparticles were investigated by XRD, Raman spectroscopy, EDS, FESEM, and UV-Vis absorption spectroscopy. 2. Material and methods 2.1. Preparation of deep eutectic solvent Deep eutectic solvent were prepared according to method described in Ref. [16]. Briefly, choline chloride (ChCl) and urea (U) were mixed in a 1:2 molar ratio, and then stirred and heated at 100 ℃ for 1 h to form a homogenous liquid. The resulting DES was marked as CCU. 2.2. Synthesis of CZTS nanoparticles In a typical experiment toward synthesis of CZTS nanoparticles, 0.41 g ZnCl2, 0.68 g SnCl2.2H2O, and 1.02 g CuCl2.2H2O respectively were dissolved in 50 ml of CCU deep eutectic solvent. Then 1.8 g thiourea as sulfur source was added to the solution under vigorous stirring and heating at 180 ℃. The resulting dark green solution was kept under mentioned conditions for 12 h. The dark precipitate formed after the time was filtered, washed several times with ethanol and deionized water and then dried at 60 ℃ for 8 h. 2.3. Characterization of CZTS nanoparticles The crystal structure, chemical composition, morphology, and optical properties of as-synthesized CZTS nanoparticles were characterized by X-ray diffraction (XRD, Siemens D-500 diffractometer), Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 400), Raman spectroscopy (Bruker,
2
Senterra), energy dispersive X-ray spectroscopy (EDS, Bruker, Quantax 200), field-emission scanning electron microscopy (FESEM, Tescan Mira 3 LMU), and UV-vis absorption spectroscopy (PerkinElmer Lambda 14). 3. Results and discussion Fig.1a shows XRD pattern of as-synthesized CZTS nanoparticles. The diffraction pattern consists of four major peaks located at 2θ = 28.5°, 33.0°, 47.4°, and 56.2° related to planes (112), (200), (220), and (312) that can be assigned to the kesterite crystal structure of CZTS (JCPDS 26-0575) [2, 3]. The wide breadth of main peak at 2θ=28.5° indicates the nanocrystalline nature of as-synthesized CZTS. The crystallite size of CZTS particles is calculated using Debye-Scherrer formula [8, 9]. Accordingly, the average crystallite size of as-synthesized CZTS nanoparticles calculated to be 6.5 nm. No other diffraction peaks related to possible impurities such as CuS, Cu2S or CuSnS3 is found in the XRD pattern, suggesting the high purity of as-synthesized CZTS particles. Since the XRD patterns related to ZnS (JCPDS 65-1691) and Cu2SnS3 (JCPDS 27-0198) fully coincide with pattern of kesterite CZTS, therefore, it is necessary to use Raman analysis for further verification of CZTS [7, 10]. As seen in Fig.1b, Raman spectrum of as-synthesized CZTS particles represents one intense Raman shift at 338 cm-1 and a weak shift at 287 cm-1 which are well matched with kesterite CZTS. No Raman shift attributed to ZnS (at 275 and 325 cm-1) and Cu2SnS3 (at 303, 318 and 351 cm-1) can be observed emphasizing the absence of ZnS and Cu2SnS3 phases in the synthesized sample [7, 17]. Fig. 2 shows EDS pattern and 2D-projected elemental maps of 2-of as-synthesized CZTS nanoparticles. Semi-quantitative analysis using EDS give the atomic ratios of 23.43:12.08:11.91:46.55 = 1.97:1.01: 1:3.91 for the elements Cu, Zn, Sn and S that are in good agreement with empirical formula of Cu2ZnSnS4. The ratios of Zn/Sn, Cu/ (Zn+Sn) and S/(Cu+Zn +Sn) were calculated to be 1.01, 0.98 and 0.98, respectively. Based on results, the as-synthesized CZTS shows a low degree of sulfur deficiency which is also referred in some literature [2, 10]. Some traces of gold (Au) and oxygen (O) can be seen in EDS pattern that are attributed to the gold coating of samples and also the oxygen presented in the microscope chamber. There is no sign of the other elements in EDS pattern revealing the high purity of as-synthesized CZTS nanoparticles. Distribution of elements in as-synthesized nanoparticles can be assessed by SEM-EDS mapping. 2D-projected elemental maps shown in Fig. 2 clearly demonstrate the uniform distribution of Cu, Zn, Sn and S in as-synthesized CZTS nanoparticles.
3
Fig. 3 displays FESEM micrographs of as-synthesized CZTS nanoparticles. From figure, the assynthesized CZTS is composed of spherical particles with an average diameter of 20 nm and plate-like particles with an average thickness of 25 nm and diameter of 140 nm. The as-synthesized CZTS show relatively wide size dispersion. Regarding the effect of size dispersion on optical properties, recent studies have recently shown that the value of optical constants such as absorption coefficient (α), extinction coefficient (ɛ), and refractive index (n) at energies far above the band gap bears more practical significance. The modeling by the Maxwell‒Garnett (MG) theory gives optical constants which are in good agreement with the experimental data at short wavelengths. This demonstrates that the effect of quantum confinement on optical constants is negligible at short wavelengths. Hence, the values of optical constants are not affected by the particle size dispersion at energies above the band gap that enhances the accuracy of the results [18]. So we have neglected the effect of particle size dispersion on CZTS optical properties in our experiments. UV-vis absorption spectrum of as-synthesized CZTS nanoparticles is shown in Fig.4. CZTS nanoparticles show high absorption in the visible region indicating their high potential for application in solar cells. As plotted in the inset of Fig.4, the band gap (Eg) of CZTS nanoparticles is determined using Tauc's equation via extrapolating straight line of the plot of (αhυ)2 against hυ (α: absorbance coefficient, h: Planck's constant and ν: frequency) [4, 19]. The band gap of as-synthesized CZTS nanoparticles according to aforementioned method is calculated to be 1.6 eV that is close to the optimum band gap for photovoltaic solar conversion [5, 20]. Based on DFT calculations, the band gap of CZTS originates from the valence band determined by Cu-3d and first conduction band derived by a linear combination of Sn-5s and S-3p states [21]. The Eg value obtained in this case is higher than the values reported for particulate CZTS (1.45 to 1.51 eV) [6, 8, 20]. This observation can be explained in terms of crystal size effect in smaller nanoparticles resulted in decrease of the band gap. Furthermore, Eg value of 1.6 eV obtained for assynthesized CZTS nanoparticles is lower than theoretical value that can be attributed to crystal defects caused by non-stoichiometric amounts of ions [22]. According to theoretical calculations, CZTS nanocrystals show a recombination between electrons and holes in the quantum well arising from some defect complexes such as (ZnCu+CuZn). These defects induce local band gap energy decrease in CZTS [23]. There is a significant difference between calculated Eg value of as-synthesized CZTS nanoparticles
4
and Eg values of 3.7 eV and 0.93 eV reported for ZnS and Cu2SnS3 providing another evidence on absence of these phases in CZTS nanoparticles [3, 22]. 4. Conclusions CZTS nanoparticles were successfully synthesized by a novel, low-cost, and green method using choline chloride:urea DES as green solvent/template, thiourea as sulfur source and metal chloride reagents. Characterization of synthesized nanoparticles by XRD, Raman spectroscopy, EDS, FESEM, and UV-Vis absorption spectroscopy revealed the synthesis of kesterite CZTS nanocrystallites with elemental composition matched with Cu2ZnSnS4, combined spherical-platelet morphology with particle size in the range of 20-25 nm, and band gap of 1.6 eV. As-synthesized CZTS nanoparticles showed lower band gap than theoretical value that can be attributed to crystal defects that induce local band gap energy decrease. As first attempt to synthesize CZTS nanoparticles in DESs, the present work may open a new door for green and economical synthesis of CZTS and related nanomaterials. Acknowledgment The financial support from Materials & Energy Research center (grant no. 421392014) is greatly acknowledged. References [1] M. Zhou, Y. Gong, J. Xu, G. Fang, Q. Xu, J. Dong, J. Alloy. Compd. 574 (2013) 272–277. [2] Y-L. Zhou, W-H. Zhou, Y-F. Du, M. Li, S-X Wu, Mater. Lett. 65 (2011) 1535–1537. [3] J. Zhou, L. You, S. Li, Y. Yang, Mater. Lett. 81 (2012) 248–250. [4] Y. Zhao, W-H. Zhou, J. Jiao, Z-J. Zhou, S-X. Wu, Mater. Lett. 96 (2013) 174–176. [5] M. Wei, Q. Du, D. Wang, W. Liu, G. Jiang, C. Zhu, Mater. Lett. 79 (2012) 177–179. [6] M. Gusain, P. Rawat, R. Nagarajan, Mater. Lett. 133 (2014) 220–223. [7] V. Tiing Tiong, Y. Zhang, J. Bella, H. Wang, Cryst. Eng. Comm. 16 (2014) 4306–4313. [8] X. Li, X. Qian, Y-Q. Cao, Z-Y. Cao, X-J. Liu, L. Zhu, A-D. Li, W-C. Liu, D. Wu, Mater. Lett. 150 (2015) 12–15. [9] O. Zaberca, F. Oftinger, J.Y. Chane-Ching, L. Datas, A. Lafond, P. Puech, A. Balocchi, D. Lagarde, X. Marie, Nanotech. 23 (2012) 1-11. [10] C. Zou, L. Zhang, D. Lin, Y. Yang, Q. Li, X. Xu, X. Chen, S. Huang, Cryst. Eng. Comm. 13 (2011) 3310–3313.
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[11] A. Pandey, S. Pandey, J. Phys. Chem. B 118 (2014) 14652-14661. [12] C. Florindo, F.S. Oliveira, L.P.N. Rebelo, A.M. Fernandes, I.M. Marrucho, ACS Sustainable Chem. Eng. 2 (2014) 2416−2425. [13] T.D. Ho, C. Zhang, L.W. Hantao, J.L. Anderson, Anal. Chem. 86 (2014) 262–285. [14] F. Cardellini, M. Tiecco, R. Germani, G. Cardinali, L. Corte, L. Roscini, N. Spreti, RSC Adv. 4 (2014) 55990–56002. [15] Q. Zhang, K. De Oliveira Vigier, S. Royer, F. Jerome, Chem. Soc. Rev. 7 (2012)7108-7146. [16] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun. 1 (2003) 70– 71. [17] K. Woo, Y. Kim, J. Moon, Energy Environ. Sci. 5 (2012) 5340-5345. [18] I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J.C. Martins, F. Vanhaecke, A. Vantomme, C. Delerue, G. Allan, Z. Hens, ACS Nano 3 (2009) 3023‒3030. [19] Y. Sun, K. Zong, H. Zheng, H. Wang, J. Liu, H. Yan, M. Zhu, Mater. Lett. 92 (2013) 195–197. [20] Y. Gao, H. Yang, Y. Zhang, J. Li, H. Zhao, J. Feng, J. Sun, Z. Zheng, RSC Adv. 4 (2014) 17667– 17670. [21] A.H. Reshak, K. Nouneh, I.V. Kityk, J. Bila, Int. J. Electrochem. Sci. 9 (2014) 955–974. [22] J. Wang, P. Zhang, X. Song, L. Gao, RSC Adv. 5 (2015) 1220–1226. [23] Y.E. Romanyuk, L.P. Marushko, L.V. Piskach, I.V. Kityk, A.O. Fedorchuk, V.I. Pekhnyoe, O.V. Parasyuk, Cryst. Eng. Comm. 15 (2013) 4838–4843.
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Figure captions: Fig. 1. XRD pattern (a) and Raman spectrum (b) of CZTS nanoparticles. Fig. 2. 2D-elemental maps along with EDS pattern of CZTS nanoparticles. Fig. 3. FESEM micrographs of CZTS nanoparticles. Fig. 4. Optical absorption spectrum of CZTS nanoparticles with inset showing Tauc plot.
Highlights:
CZTS nanoparticles were synthesized in choline chloride:urea deep eutectic solvent:
As-synthesized CZTS were crystalized in kesterite phase with crystal diameter of 6.5 nm;
XRD-Raman spectroscopy revealed the synthesis of pure CZTS phase;
CZTS nanoparticles showed an optical band gap of about 1.6 eV.
7
Fig. 1
Fig. 2
8
Fig. 3
Fig. 4
9
PII: DOI: Reference:
S0167-577X(16)30226-9 http://dx.doi.org/10.1016/j.matlet.2016.02.065 MLBLUE20354
To appear in: Materials Letters Received date: 2 January 2016 Revised date: 13 February 2016 Accepted date: 15 February 2016 Cite this article as: M. Karimi, M.J. Eshraghi and V. Jahangir, A facile and Green Synthetic Approach Based on Deep Eutectic Solvents toward Synthesis of CZTS Nanoparticles, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.065 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 galley proof before it is published in its final citable 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.
A facile and Green Synthetic Approach Based on Deep Eutectic Solvents toward Synthesis of CZTS Nanoparticles *
M. Karimi , M.J. Eshraghi, V. Jahangir Department of Semiconductors, Materials and Energy Research Center, Karaj, Iran. *
Corresponding Author. Tel: +98 26 3628 0035; fax: +98 263 62018888. Email address: [email protected] (M. Karimi).
Abstract High efficiency, abundant, low-cost materials, easy and inexpensive method of fabrication, and long-term stability of photovoltaic devices based on CZTS nano-absorbent has made them the dominant technology for the next generation of solar cells. In this study, a facile, low-cost, and sustainable method was developed for synthesis of CZTS nanoparticles using deep eutectic solvents (DESs) to act as both solvent and template. CZTS nanoparticles were synthesized in choline chloride:urea DES in the presence of metal chloride precursors and thiourea as sulfur source. The results on characterization of synthesized nanoparticles revealed the synthesis of kesterite CZTS nanoparticles having average crystal size of 6.5 nm, combined spherical-platelet morphology with particle diameter of 20-25 nm, elemental composition corresponding to Cu2ZnSnS4, and band gap of about 1.6 eV. The present study is worthy of attention in view of providing a simple, fast and sustainable synthesis of CZTS nanoparticles. Keywords: CZTS; Nanoparticles; Semiconductors; Green synthesis; Deep eutectic solvent. 1. Introduction Copper–zinc–tin chalcogenide (Cu2ZnSnS4, CZTS) has recently attracted considerable attention in the field of photovoltaics owing to its nontoxicity, low cost, and earth abundance [1, 2]. CZTS is a p-type semiconductor with high absorption coefficient of 104 cm-1 in the wavelength range from visible to near infrared of the solar spectrum and direct band gap of 1.5 eV. This make CZTS an attractive material as the absorber layer of thin-film solar cells compared to its rare and costly analogous CdTe and Cu(In,Ga)Se2 (CIGS) [1, 3, 4]. There are many methods developed for the synthesis of CZTS nanoparticles including hot injection solution synthesis [5], solvent mediated thermolysis process [6], solvothermal-hydrothermal synthesis [7, 8], and high temperature polycondensation route [9]. The quaternary chemical composition of CZTS is
1
one of the main challenging factors in its synthesis. Furthermore, synthesis of CZTS nanoparticles with favorable composition, morphology and particle size is problematic using existing methods and need to use harmful solvents, capping agents and surfactants under harsh conditions [10]. Therefore, green synthesis has remained as a major challenge for production of CZTS nanoparticles. Herein, as first report in the field, we developed an eco-friendly and facile method based on choline chloride-urea deep eutectic solvent (DES) for green synthesis of CZTS nanoparticles. DESs are a new generation of ionic liquids (ILs) that possess many outstanding physicochemical properties such as high conductivity, polarity, thermal stability, and negligible vapor pressure [11-13]. They are classified in the category of green solvents due to their bio-degradability, environmental friendliness, low toxicity, low price, and ease of production on large scale [13-15]. In this work, an attempt was made to use DESs as both solvent and template in synthesis of CZTS nanoparticles. The crystal phase, elemental composition, morphology-particle size, and optical properties of the nanoparticles were investigated by XRD, Raman spectroscopy, EDS, FESEM, and UV-Vis absorption spectroscopy. 2. Material and methods 2.1. Preparation of deep eutectic solvent Deep eutectic solvent were prepared according to method described in Ref. [16]. Briefly, choline chloride (ChCl) and urea (U) were mixed in a 1:2 molar ratio, and then stirred and heated at 100 ℃ for 1 h to form a homogenous liquid. The resulting DES was marked as CCU. 2.2. Synthesis of CZTS nanoparticles In a typical experiment toward synthesis of CZTS nanoparticles, 0.41 g ZnCl2, 0.68 g SnCl2.2H2O, and 1.02 g CuCl2.2H2O respectively were dissolved in 50 ml of CCU deep eutectic solvent. Then 1.8 g thiourea as sulfur source was added to the solution under vigorous stirring and heating at 180 ℃. The resulting dark green solution was kept under mentioned conditions for 12 h. The dark precipitate formed after the time was filtered, washed several times with ethanol and deionized water and then dried at 60 ℃ for 8 h. 2.3. Characterization of CZTS nanoparticles The crystal structure, chemical composition, morphology, and optical properties of as-synthesized CZTS nanoparticles were characterized by X-ray diffraction (XRD, Siemens D-500 diffractometer), Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 400), Raman spectroscopy (Bruker,
2
Senterra), energy dispersive X-ray spectroscopy (EDS, Bruker, Quantax 200), field-emission scanning electron microscopy (FESEM, Tescan Mira 3 LMU), and UV-vis absorption spectroscopy (PerkinElmer Lambda 14). 3. Results and discussion Fig.1a shows XRD pattern of as-synthesized CZTS nanoparticles. The diffraction pattern consists of four major peaks located at 2θ = 28.5°, 33.0°, 47.4°, and 56.2° related to planes (112), (200), (220), and (312) that can be assigned to the kesterite crystal structure of CZTS (JCPDS 26-0575) [2, 3]. The wide breadth of main peak at 2θ=28.5° indicates the nanocrystalline nature of as-synthesized CZTS. The crystallite size of CZTS particles is calculated using Debye-Scherrer formula [8, 9]. Accordingly, the average crystallite size of as-synthesized CZTS nanoparticles calculated to be 6.5 nm. No other diffraction peaks related to possible impurities such as CuS, Cu2S or CuSnS3 is found in the XRD pattern, suggesting the high purity of as-synthesized CZTS particles. Since the XRD patterns related to ZnS (JCPDS 65-1691) and Cu2SnS3 (JCPDS 27-0198) fully coincide with pattern of kesterite CZTS, therefore, it is necessary to use Raman analysis for further verification of CZTS [7, 10]. As seen in Fig.1b, Raman spectrum of as-synthesized CZTS particles represents one intense Raman shift at 338 cm-1 and a weak shift at 287 cm-1 which are well matched with kesterite CZTS. No Raman shift attributed to ZnS (at 275 and 325 cm-1) and Cu2SnS3 (at 303, 318 and 351 cm-1) can be observed emphasizing the absence of ZnS and Cu2SnS3 phases in the synthesized sample [7, 17]. Fig. 2 shows EDS pattern and 2D-projected elemental maps of 2-of as-synthesized CZTS nanoparticles. Semi-quantitative analysis using EDS give the atomic ratios of 23.43:12.08:11.91:46.55 = 1.97:1.01: 1:3.91 for the elements Cu, Zn, Sn and S that are in good agreement with empirical formula of Cu2ZnSnS4. The ratios of Zn/Sn, Cu/ (Zn+Sn) and S/(Cu+Zn +Sn) were calculated to be 1.01, 0.98 and 0.98, respectively. Based on results, the as-synthesized CZTS shows a low degree of sulfur deficiency which is also referred in some literature [2, 10]. Some traces of gold (Au) and oxygen (O) can be seen in EDS pattern that are attributed to the gold coating of samples and also the oxygen presented in the microscope chamber. There is no sign of the other elements in EDS pattern revealing the high purity of as-synthesized CZTS nanoparticles. Distribution of elements in as-synthesized nanoparticles can be assessed by SEM-EDS mapping. 2D-projected elemental maps shown in Fig. 2 clearly demonstrate the uniform distribution of Cu, Zn, Sn and S in as-synthesized CZTS nanoparticles.
3
Fig. 3 displays FESEM micrographs of as-synthesized CZTS nanoparticles. From figure, the assynthesized CZTS is composed of spherical particles with an average diameter of 20 nm and plate-like particles with an average thickness of 25 nm and diameter of 140 nm. The as-synthesized CZTS show relatively wide size dispersion. Regarding the effect of size dispersion on optical properties, recent studies have recently shown that the value of optical constants such as absorption coefficient (α), extinction coefficient (ɛ), and refractive index (n) at energies far above the band gap bears more practical significance. The modeling by the Maxwell‒Garnett (MG) theory gives optical constants which are in good agreement with the experimental data at short wavelengths. This demonstrates that the effect of quantum confinement on optical constants is negligible at short wavelengths. Hence, the values of optical constants are not affected by the particle size dispersion at energies above the band gap that enhances the accuracy of the results [18]. So we have neglected the effect of particle size dispersion on CZTS optical properties in our experiments. UV-vis absorption spectrum of as-synthesized CZTS nanoparticles is shown in Fig.4. CZTS nanoparticles show high absorption in the visible region indicating their high potential for application in solar cells. As plotted in the inset of Fig.4, the band gap (Eg) of CZTS nanoparticles is determined using Tauc's equation via extrapolating straight line of the plot of (αhυ)2 against hυ (α: absorbance coefficient, h: Planck's constant and ν: frequency) [4, 19]. The band gap of as-synthesized CZTS nanoparticles according to aforementioned method is calculated to be 1.6 eV that is close to the optimum band gap for photovoltaic solar conversion [5, 20]. Based on DFT calculations, the band gap of CZTS originates from the valence band determined by Cu-3d and first conduction band derived by a linear combination of Sn-5s and S-3p states [21]. The Eg value obtained in this case is higher than the values reported for particulate CZTS (1.45 to 1.51 eV) [6, 8, 20]. This observation can be explained in terms of crystal size effect in smaller nanoparticles resulted in decrease of the band gap. Furthermore, Eg value of 1.6 eV obtained for assynthesized CZTS nanoparticles is lower than theoretical value that can be attributed to crystal defects caused by non-stoichiometric amounts of ions [22]. According to theoretical calculations, CZTS nanocrystals show a recombination between electrons and holes in the quantum well arising from some defect complexes such as (ZnCu+CuZn). These defects induce local band gap energy decrease in CZTS [23]. There is a significant difference between calculated Eg value of as-synthesized CZTS nanoparticles
4
and Eg values of 3.7 eV and 0.93 eV reported for ZnS and Cu2SnS3 providing another evidence on absence of these phases in CZTS nanoparticles [3, 22]. 4. Conclusions CZTS nanoparticles were successfully synthesized by a novel, low-cost, and green method using choline chloride:urea DES as green solvent/template, thiourea as sulfur source and metal chloride reagents. Characterization of synthesized nanoparticles by XRD, Raman spectroscopy, EDS, FESEM, and UV-Vis absorption spectroscopy revealed the synthesis of kesterite CZTS nanocrystallites with elemental composition matched with Cu2ZnSnS4, combined spherical-platelet morphology with particle size in the range of 20-25 nm, and band gap of 1.6 eV. As-synthesized CZTS nanoparticles showed lower band gap than theoretical value that can be attributed to crystal defects that induce local band gap energy decrease. As first attempt to synthesize CZTS nanoparticles in DESs, the present work may open a new door for green and economical synthesis of CZTS and related nanomaterials. Acknowledgment The financial support from Materials & Energy Research center (grant no. 421392014) is greatly acknowledged. References [1] M. Zhou, Y. Gong, J. Xu, G. Fang, Q. Xu, J. Dong, J. Alloy. Compd. 574 (2013) 272–277. [2] Y-L. Zhou, W-H. Zhou, Y-F. Du, M. Li, S-X Wu, Mater. Lett. 65 (2011) 1535–1537. [3] J. Zhou, L. You, S. Li, Y. Yang, Mater. Lett. 81 (2012) 248–250. [4] Y. Zhao, W-H. Zhou, J. Jiao, Z-J. Zhou, S-X. Wu, Mater. Lett. 96 (2013) 174–176. [5] M. Wei, Q. Du, D. Wang, W. Liu, G. Jiang, C. Zhu, Mater. Lett. 79 (2012) 177–179. [6] M. Gusain, P. Rawat, R. Nagarajan, Mater. Lett. 133 (2014) 220–223. [7] V. Tiing Tiong, Y. Zhang, J. Bella, H. Wang, Cryst. Eng. Comm. 16 (2014) 4306–4313. [8] X. Li, X. Qian, Y-Q. Cao, Z-Y. Cao, X-J. Liu, L. Zhu, A-D. Li, W-C. Liu, D. Wu, Mater. Lett. 150 (2015) 12–15. [9] O. Zaberca, F. Oftinger, J.Y. Chane-Ching, L. Datas, A. Lafond, P. Puech, A. Balocchi, D. Lagarde, X. Marie, Nanotech. 23 (2012) 1-11. [10] C. Zou, L. Zhang, D. Lin, Y. Yang, Q. Li, X. Xu, X. Chen, S. Huang, Cryst. Eng. Comm. 13 (2011) 3310–3313.
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Figure captions: Fig. 1. XRD pattern (a) and Raman spectrum (b) of CZTS nanoparticles. Fig. 2. 2D-elemental maps along with EDS pattern of CZTS nanoparticles. Fig. 3. FESEM micrographs of CZTS nanoparticles. Fig. 4. Optical absorption spectrum of CZTS nanoparticles with inset showing Tauc plot.
Highlights:
CZTS nanoparticles were synthesized in choline chloride:urea deep eutectic solvent:
As-synthesized CZTS were crystalized in kesterite phase with crystal diameter of 6.5 nm;
XRD-Raman spectroscopy revealed the synthesis of pure CZTS phase;
CZTS nanoparticles showed an optical band gap of about 1.6 eV.
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Fig. 1
Fig. 2
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Fig. 3
Fig. 4
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