A nontoxic and low-cost hydrothermal route for synthesis of hierarchical Cu2ZnSnS4 particles
© Xia et al.; licensee Springer. 2014
Received: 8 January 2014
Accepted: 18 April 2014
Published: 4 May 2014
We explore a facile and nontoxic hydrothermal route for synthesis of a Cu2ZnSnS4 nanocrystalline material by using l-cysteine as the sulfur source and ethylenediaminetetraacetic acid (EDTA) as the complexing agent. The effects of the amount of EDTA, the mole ratio of the three metal ions, and the hydrothermal temperature and time on the phase composition of the obtained product have been systematically investigated. The addition of EDTA and an excessive dose of ZnCl2 in the hydrothermal reaction system favor the generation of kesterite Cu2ZnSnS4. Pure kesterite Cu2ZnSnS4 has been synthesized at 180°C for 12 h from the reaction system containing 2 mmol of EDTA at 2:2:1 of Cu/Zn/Sn. It is confirmed by Raman spectroscopy that those binary and ternary phases are absent in the kesterite Cu2ZnSnS4 product. The kesterite Cu2ZnSnS4 material synthesized by the hydrothermal process consists of flower-like particles with 250 to 400 nm in size. It is revealed that the flower-like particles are assembled from single-crystal Cu2ZnSnS4 nanoflakes with ca. 20 nm in size. The band gap of the Cu2ZnSnS4 nanocrystalline material is estimated to be 1.55 eV. The films fabricated from the hierarchical Cu2ZnSnS4 particles exhibit fast photocurrent responses under intermittent visible-light irradiation, implying that they show potentials for use in solar cells and photocatalysis.
The quaternary Cu2ZnSnS4 (CZTS) compound, derived from CuInS2 by replacing In(III) with Zn(II) and Sn(IV), has the advantages of optimum direct band gap (around 1.5 eV) for use in single-junction solar cells, abundance of the constituent elements, and high absorption coefficient (>104 cm-1) [1–5]. Thus, increasing attention has been paid on CZTS materials in recent years [6–10]. Low-cost solar cells based on CZTS films as absorber layers have achieved an increasing conversion efficiency [11–15]. CZTS nanocrystalline materials have been found to show potentials for use in negative electrodes for lithium ion batteries  and counter electrodes for high-efficiency dye-sensitized solar cells [17–19] and as novel photocatalysts for hydrogen production . Obviously, an environment-friendly and low-cost synthesis route for the large-scale production of CZTS in high quality is an essential prerequisite for its applications in all those fields.
At present, the routes for synthesis of CZTS nanocrystalline materials can be subsumed under two broad categories: the hot-injection method [12, 21–23] and the solvothermal process [13, 18, 24–26]. Although the hot-injection method can be used to synthesize CZTS nanocrystals with narrow size distribution, this method suffers from several shortcomings such as the need of expensive raw materials with high levels of toxicity, complicated processes, and high reaction temperatures (above 250°C). In contrast with the hot-injection method, the solvothermal process, which usually produces hierarchical CZTS particles by one-pot reaction, possesses the advantages of simple process and relative cheap raw materials. Furthermore, it has been found that hierarchical particles can provide a large surface area along with the functions of generating light scattering and favoring electron transport, as compared with nanocrystals . Up to now, anhydrous ethylenediamine [24, 26], the mixture of ethylenediamine and water [27–29], ethylene glycol [13, 18], triethylene glycol , and dimethyl formamide (DMF)  have been used as a solvent for the solvothermal method, respectively. In contrast with those organic solvents, water is much cheaper and more environment-friendly. Undoubtedly, if water is used to replace these organic solvents, a hydrothermal route will be developed, which is more desirable for the environment-friendly and low-cost synthesis of CZTS nanocrystalline materials. However, few investigations on synthesis of CZTS nanocrystalline materials by the hydrothermal method have been reported, except the hydrothermal reactions with Na2S  or thiourea  as the sulfur source. Note that selecting a suitable sulfur source is important for exploring a green hydrothermal process for preparing CZTS nanocrystalline materials. It has been reported that H2S is usually generated as a toxic and corrosive intermediate product from the reaction systems containing sulfur, Na2S, or thiourea as the sulfur source . Different from those sulfur sources, l-cysteine has been used to prepare metal sulfide nanomaterials without the generation of H2S as a by-product . Thus, in the current work, by the aid of ethylenediaminetetraacetic acid (EDTA) as a complexing agent, a low-cost and nontoxic hydrothermal route for synthesis of CZTS has been developed by using water as the solvent and l-cysteine as the sulfur source. The effects of the amount of EDTA, the mole ratio of the three metal ions, and the hydrothermal temperature and time on the phase composition of the obtained samples have been systematically investigated. The phase composition of the obtained CZTS sample has been further confirmed by Raman spectrometry. The microstructure and morphology of the pure CZTS sample have been characterized, and its optical absorption property has been examined. Moreover, the prepared CZTS nanocrystalline material has been employed to fabricate films, and the photoelectrochemical property of the obtained films has been evaluated.
Synthesis of CZTS
CuCl2 · 2H2O, ZnCl2, SnCl2 · 2H2O, l-cysteine, and EDTA were of analytical grade and used as received without further purification. In a typical synthesis, 2 mmol CuCl2 · 2H2O, 2 mmol of ZnCl2, 1 mmol of SnCl2 · 2H2O, 4 mmol of l-cysteine, and 0 to 3 mmol of EDTA were dispersed in 20 ml of deionized water for 5 min under constant stirring, and then the obtained solution was transferred to an acid digestion bomb (50 ml). The hydrothermal synthesis was conducted at 170°C to 190°C for 6 to 16 h in an electric oven. After synthesis, the bomb was cooled down naturally to room temperature. The final product was filtrated and washed with 30% and 80% ethanol, followed by drying at 60°C in a vacuum oven. Moreover, in order to investigate the mole ratio of the three metal ions (Cu/Zn/Sn) in the reaction system on the phase composition of the obtained product, three samples were synthesized at 2:1:1, 2:2:1, and 2:3:1 of Cu/Zn/Sn, respectively.
Powder X-ray diffraction (PXRD) patterns of samples were performed on a Bruker D8 ADVANCE diffraction system (Bruker AXS GmbH, Karlsruhe, Germany) using Cu Kα radiation (λ = 1.5406 Å), operated at 40 kV and 40 mA with a step size of 0.02°. The morphology of the pure CZTS sample was observed by using a scanning electron microscope (SEM, Nova Nano 430, FEI, Holland). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by using a JEOL JEM-2100 F field emission electron microscope (JEOL Ltd., Akishima, Tokyo, Japan). The Raman spectrum of the sample was recorded on a microscopic Raman spectrometer (LabRAM Aramis, Horiba Jobin Yvon Inc., Edison, NJ, USA). The diffuse reflectance spectrum (DRS) of the CZTS sample was obtained by using a Shimadzu U-3010 spectrophotometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) equipped with an integrating sphere assembly.
The prepared CZTS sample was used to fabricate films as follows: 0.05 g of the sample was mixed with ethanol followed by ultrasound. The obtained CZTS ‘ink’ was then coated onto indium-tin (ITO) oxide glass by spin coating for several times, followed by drying at 120°C for 1 h. Photoelectrochemical measurements were conducted on the obtained CZTS films. Photocurrents were measured on an electrochemical analyzer (CorrTest CS350, CorrTest Instrument Co., Wuhan, China) in a standard three-electrode system by using the prepared CZTS film as the working electrode, a Pt flake as the counter electrode, and Ag/AgCl as the reference electrode. A 300-W Xe lamp served as a light source, and 0.5 M Na2SO4 solution was used as the electrolyte.
Results and discussion
Effects of hydrothermal reaction conditions
Amount of EDTA
Mole ratio of three metal ions
Effect of hydrothermal temperature
Effect of reaction time
Microstructure, morphology, and optical absorption property
Photoelectrochemical property of CZTS films
The reaction conditions including the amount of EDTA, the mole ratio of the three metal ions, and the hydrothermal temperature and time have an important effect on the phase composition of the obtained product. A suitable amount of EDTA is needed for synthesis of pure kesterite CZTS by the hydrothermal process with l-cysteine as the sulfur source. An excessive dose of ZnCl2 (double the stoichiometric ratio of Zn in CZTS) in the reaction system favors the production of kesterite CZTS. Pure kesterite CZTS can be produced by the hydrothermal process at 180°C for no less than 12 h. It is confirmed that those binary and ternary phases are absent in the kesterite CZTS product. The kesterite CZTS material synthesized by the hydrothermal process consists of flower-like particles with 250 to 400 nm in size. The particles are assembled from the single-crystal CZTS nanoflakes with ca. 20 nm in size. The band gap of the CZTS material is estimated to be 1.55 eV. The CZTS films fabricated from the flower-like CZTS particles exhibit fast photocurrent responses, making them show potentials for use in solar cells and photocatalysis.
This work is supported by the National Natural Science Foundation of China (60976053 and 21276088).
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