Article pubs.acs.org/JPCC
Photoenergy Conversion in p‑Type Cu2ZnSnS4 Nanorods and n‑Type Metal Oxide Composites Masahiro Miyauchi,*,†,‡ Takumi Hanayama,† Daiki Atarashi,† and Etsuo Sakai† †
Department of Metallurgy and Ceramic Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Single-crystalline Cu2ZnSnS4 (CZTS) nanorods with a kesterite crystal phase were synthesized using a facile solvothermal reaction. Thin-film electrodes for p-type CZTS nanorods and n-type metal oxide semiconductor composites were fabricated on mesoporous n-type metal oxide electrodes by introducing the electrodes into a solvothermal condition. The composites exhibited high photocurrent and strong photocatalytic oxidative activity for the decomposition of organic compounds.
1. INTRODUCTION Thin-film solar cells based on CuInxGa1−xSe2 (CIGS) or CdTe have attracted considerable recent attention, owing to their suitable band gap for solar energy conversion and large absorption coefficient. However, CIGS and CdTe consist of toxic elements, including Se, Cd, and Te, and rare elements, such as In and Ga. A more promising photovoltaic materials for the construction of low-cost thin-film-type solar cells is Cu2ZnSnS4 (CZTS) because it is composed of nontoxic and inexpensive elements.1 CZTS has a crystal structure similar to that of CIGS, which adopts a chalcopyrite kesterite structure2 and exhibits p-type semiconductor behavior with direct optical transition.3 The band gap of CZTS ranges between 1.2 and 1.6 eV,2,4 which is close to the optimum value for solar light absorption. Thin films of CZTS have been fabricated by vaporphase deposition processes, including sputtering,5 metal sulfurization,6 and thermal evaporation.7 In addition to these vacuum processes, the wet-chemical synthesis of CZTS nanoparticles is also of interest because this approach is applicable for the coating of large areas at low cost. Solutionbased synthesis of CZTS nanocrystals has also been reported by several groups. 8−12 Furthermore, the formation of nanostructures such as nanosheets, nanorods, and nanotubes is critical for improving the effective charge separation and carrier mobility of thin films. Notably, sheet-like CZTS particles have been synthesized by a solution-based method,13 and flower-like CZTS particles have also been formed.14 In addition to these unique structures, CZTS nanowires have been fabricated using nanoporous anodic alumina hard templates.15 Although CZTS nanorods with wurtzite crystal structure have also been synthesized using a solution-based method, 1D CZTS nanorods with chalcopyrite kesterite crystal structure have yet to be synthesized by wet-chemical synthesis in the absence of hard or soft templates. © 2012 American Chemical Society
Herein, we synthesized single-crystalline CZTS nanorods by a facile solvothermal method without the use of template materials. The generated CZTS nanorods could easily be combined with n-type metal oxide semiconductors, such as TiO2 and WO3, on a transparent electroconductive substrate. Notably, the CZTS nanorods exhibited p-type behavior, and the composites of CZTS nanorods and n-type metal oxides resulted in superior photocurrent and photocatalytic oxidative reactions for gaseous organic compounds under visible-light irradiation.
2. EXPERIMENTAL SECTION Synthesis of CZTS Nanorods. CZTS nanorods were synthesized by a solvothermal process using copper(II) acetylacetonate, zinc(II) acetate, tin(IV) chloride, and sulfur as starting materials. Because controlling the amount of sulfur is necessary to avoid the formation of metal oxides such as SnO or CuO, the atomic ratio of Cu/Zn/Sn/S among the starting materials was set to 2:1:1:12, in which the amount of sulfur was three times higher than that for the stoichiometric chemical composition of CZTS (2:1:1:4). The starting materials were dissolved into oleylamine solvent in a Teflon-lined autoclave and stirred for 30 min at room temperature. The autoclave was heated at 180 °C for incubation times of 2, 10, and 48 h, cooled to room temperature, and the formed brown aggregates were then centrifuged and thoroughly washed with ethanol until all of the unreacted ions were removed. Washed aggregates were dried overnight at 80 °C in air to obtain powder samples. Fabrication of Thin Films of CZTS and TiO 2 Composites. Mesoporous TiO2-coated electrodes were Received: August 9, 2012 Revised: October 15, 2012 Published: October 22, 2012 23945
dx.doi.org/10.1021/jp307949n | J. Phys. Chem. C 2012, 116, 23945−23950
The Journal of Physical Chemistry C
Article
immersed in oleylamine solvent in a Teflon-lined reactor under the same experimental conditions used for the powder synthesis. A TiO2 paste was printed on a transparent electroconductive glass substrate coated with fluorine-doped tin oxide and was subsequently sintered at 500 °C in air for 2 h. The thickness of the TiO2 layer was 2 μm. The screen printing paste was composed of TiO2 (Degussa, P-25; particle size: 200−500 nm), ethyl cellulose (as a binder), and α-terpineol (as a solvent). Characterization. The crystal structure of CZTS nanorods was analyzed by X-ray powder diffraction (XRD) with Cu Kα X-rays (MXP3 II; Bruker AXS). The morphologies of CZTS nanorods were observed using a transmission electron microscope (TEM; HF-2000; Hitachi) equipped with an energy dispersive X-ray (EDX) spectroscope. The optical properties of powders were analyzed by measuring UV−visible (UV−vis) spectra in diffuse-reflectance mode using a spectrophotometer (V-660; Jasco) with BaSO4 as the background. The chemical composition of powder samples was analyzed by inductively coupled plasma (ICP) analysis using an atomic emission spectrometer (Prodigy ICP; Leeman Laboratories). Evaluation of Photoenergy Conversion. To investigate the photoelectrochemical properties, we sealed CZTS-modified TiO2 electrodes in a sandwich cell containing a 30 μm spacer using Pt-coated FTO glass as a counter electrode. The CZTSmodified TiO2 electrode had a total area of 0.25 cm2. An electrolyte solution, which contained the I− and I3‑ redox couple, was used to fill the gap between the working and counter electrodes of the sandwich cell. Short-circuit photocurrent was observed under irradiation from an AM 1.5 solar simulator. To test photocatalytic activity, acetaldehyde was selected as the model organic compound. CZTS powder was combined with commercial WO3 powder (Pure Chemical, particle size: 100−500 nm) by mechanical mixing. After mixing, 0.1 g of the powder composite was uniformly dispersed on a circular glass dish, and the sample was pretreated for 12 h using blue LED. The sample was then mounted on a cylindrical glass air-filled static reactor (500 mL total volume) equipped with a quartz window. An O2 (20%)−N2 mixture adjusted to a relative humidity of 50% was used to fill the reaction vessel. Gas-phase acetaldehyde was then introduced into the reactor using a syringe until the concentration reached ∼450 ppmv. Before illumination, the catalyst was kept in the dark for 3 h to ensure the establishment of an adsorption−desorption equilibrium between the catalyst and acetaldehyde. The wavelength of visible-light in the blue LED source ranged from 400 to 530 nm. The intensity of visible light, which was measured using a spectro-radiometer (USR-40D; Ushio), was 20 mW/cm2. The concentration of acetaldehyde and the generation of CO2 were measured using a micro gas chromatograph (3000 Micro GC; Inficon) equipped with a thermal conductivity detector (TCD). Photocatalytic oxidation activities for CZTS/WO3, pure WO3, and pure CZTS were evaluated. Because our experiment was conducted under light-limited conditions as opposed to lightrich conditions, the observed photocatalytic activity strongly depended on the efficiency of electron−hole charge separation, rather than the adsorbability of acetaldehyde molecules to the catalyst surface.
Figure 1. XRD patterns for CZTS powder prepared with reaction times of 2 (a), 10 (b), and 48 h (c).
reaction time (2 h), CuS peaks were observed as an impurity. Under the solvothermal conditions used here, CZTS crystals were formed through the dissolution and reprecipitation of the gel-like starting material. Because the solubility of copper acetylacetonate is higher than that of the zinc or tin precursors, CuS was formed when the solvothermal reactor was cooled to room temperature during the initial stage of the reaction. When the reaction time exceeded 10 h, all diffraction peaks were assigned to CZTS with a kesterite crystal structure. The halfwidth of the diffraction peaks became sharper with increasing reaction times, indicating the higher crystallinity of the CZTS nanorods. Aggregates of CZTS were examined by TEM, which revealed the presence of numerous rod-like structures (Figure 2a). We also captured TEM images for CZTS aggregates formed with shorter reaction times of 2 and 10 h (Supporting Information; Figure S1) and observed that the number of CZTS nanorods increased with increasing time of solvothermal treatment. We have statistically counted the numbers of CZTS nanorods per
Figure 2. TEM (a) and HR-TEM images (b) of CZTS nanorods prepared with a reaction time of 48 h. (c) Electron diffraction pattern for the CZTS nanorod shown in panel b. (d) EDX analysis results for points (i) and (ii) in panel a.
3. RESULTS AND DISCUSSION Figure 1 shows X-ray diffraction patterns for the CZTS nanorods at each examined reaction time. For the shortest 23946
dx.doi.org/10.1021/jp307949n | J. Phys. Chem. C 2012, 116, 23945−23950
The Journal of Physical Chemistry C
Article
mainly from the hybridization of S and Cu states, whereas the low conduction band is mainly from the hybridization of S-s, p and Sn-s orbitals.17 The estimated band gap of the sample synthesized with the shortest reaction time (2 h) was larger than that of the other two sample types because the former contained CuS, which has a band gap of 2.2 eV,18 as an impurity. Consistent with this finding, the sample synthesized with a 48 h reaction time exhibited the narrowest band gap among the examined samples. Khare et al.12 reported that the quantum confinement effect was observed for CZTS aggregates with sizes smaller than 10 nm. Here the sample synthesized with the shortest reaction time (2 h) contained aggregates of amorphous CZTS nanoparticles with sizes of
