Green synthesis of wurtzite copper zinc tin sulfide nanocones for improved solar photovoltaic utilization
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Cu2ZnSnS4 (CZTS) is considered to be one of the most promising light absorbing materials for low-cost and high-efficiency thin-film solar cells. It is composed of earth abundant, non-toxic elements. In the present study, wurtzite CZTS nanocone has been synthesized by a green chemistry route. The nanocones have been characterized for its optical, structural and microstructural properties using UV–Vis spectrophotometer, X-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy. Optical absorption result shows a band gap of 1.42 eV. XRD and Raman results show wurtzite structure and TEM studies reveal the nanocone structure of the grown material. Growing vertically aligned nanocone structure having smaller diameter shall help in enhancing the light absorption in broader range which shall enhance the efficiency of solar cell. This study is a step in this direction.
KeywordsCu2ZnSnS4 Wurtzite Nanocone Green synthesis
There are several reports on the synthesis of CZTS nanocrystals by hot-injection method (Guo et al. 2010, 2009; Riha et al. 2009). In general, CZTS nanocrystals are mostly in the kesterite or stannite phase (Shavel et al. 2010). It has been reported by Regulacio et al. that in colloidal nanocrystal synthesis, the presence of organic surfactants or capping ligands is known to strongly influence the crystallographic phase, morphology and growth of the nanocrystals (Regulacio et al. 2012). Thus, the formation of metastable phases can be induced by proper selection of surfactants. CZTS nanocrystals having wurtzite (WZ) phase has also been synthesized by various methods such as noninjection (Regulacio et al. 2012), hot-injection (Singh et al. 2012) and hydrothermal method (Jiang et al. 2012). These processes require inert gas protection (nitrogen or argon), stepwise heating and relatively high temperature. Herein, we provide a facile noninjection synthetic route for preparing monodisperse anisotropic CZTS nanocones that adopt a WZ-type crystal structure. The noninjection or “heating up” approach for the formation of colloidal nanocrystals is better in terms of synthetic reproducibility, convenience in manipulating, and suitable for large-scale production compared to the hot-injection method. Optical properties of the grown nanocones were studied using UV–VIS spectrophotometer (Model: Shimadzu UV–VIS 1800). Structural properties were studied using powder XRD (Philips X’pert pro X-ray diffractometer) and Transmission electron microscopy (TEM, F30 S-twin 300 kV) was used to analyze the size and shape of the nanocrystals.
For the synthesis of wurtzite CZTS nanocones, copper dithiocarbamates [Cu(dedtc)2] (36.0 mg, 0.1 mmol), zinc dithiocarbamates [Zn(dedtc)2] (18.1 mg, 0.05 mmol), tin dithiocarbamates [Sn(dedtc)4] (35.6 mg, 0.05 mmol), dodecanethiol (5 ml) and trioctylamine (5 ml) were taken in a 50-ml three-neck flask and the reaction mixture was degassed at 100 °C for 20–30 min. The clear yellow solution formed was heated to 250 °C under Ar atmosphere while stirring it vigorously. A change in color from yellow to topaz to dark brown was observed at around 220–240 °C. The stirring of resulting mixture was continued for 30 min while maintaining the 250 °C temperature. Then, it was placed in a H2O bath to allow the mixture to cool to 40 °C. Methanol was added to precipitate the nanocrystals. The nanocrystals are centrifuged. The solid obtained was washed thoroughly with methanol, and finally dissolved in chloroform.
Results and discussion
CZTS nanocones have been synthesized through a facile greener and inexpensive route, which involves the surfactant-assisted thermolysis of metal dithiocarbamates. Characterization using XRD, Raman shift and TEM confirm the phase purity of the CZTS nanocones. XRD and Raman shift measurements show that the CZTS nanocones have wurtzite structure. From optical absorption data, the band gap of the WZ-type CZTS nanocones is estimated to be 1.42 eV, which is optimal for solar cell applications. TEM studies show elongated nanocone structure with one end being wider (~13–15 nm) than the other (~5–8 nm). This nanocone structure should help in enhancing the efficiency of solar cell.
V.N. Singh acknowledges Department of Science and Technology (DST) India for support through the fast track project for young scientist by sanction order no. SR/FTP/PS-124/2011 dated February, 2012. Authors are thankful to MNRE-India (sanction no. 31/29/2010-11/PVSE) for the financial support. The authors are grateful to CSIR-India for TAP-SUN program. OPS and NM are thankful to UGC for SRFships.
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