Facile one-pot synthesis of polytypic CuGaS2 nanoplates
© Liu et al.; licensee Springer. 2013
Received: 31 October 2013
Accepted: 3 December 2013
Published: 13 December 2013
CuGaS2 (CGS) nanoplates were successfully synthesized by one-pot thermolysis of a mixture solution of CuCl, GaCl3, and 1-dodecanethiol in noncoordinating solvent 1-octadecene. Their morphology, crystalline phase, and composition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), respectively. Crystalline structure analysis showed that the as-prepared CGS nanoplates were polytypic, in which the wurtzite phase was interfaced with zincblende domains. The growth process of CGS nanoplates was investigated. It was found that copper sulfide nanoplates were firstly formed and then the as-formed copper sulfide nanoplates gradually transformed to CGS nanoplates with proceeding of the reaction. The optical absorption of the as-synthesized CGS nanoplates was also measured and the direct optical bandgap was determined to be 2.24 eV.
KeywordsCuGaS2 Polytypism Nanoplates Thermolysis
I-III-VI2 semiconductor nanocrystals have received much research interest in recent years because they have low toxicity, high absorption coefficient, narrow bandgap, and tunable emission wavelength in the red to near-infrared region and have shown great potential in many fields such as low-cost solar cells, bio-imaging, light-emitting diodes, and visible-light photocatalyst [1–6]. These compounds have two different metal ions, complex structures, and flexible compositions, so it is a formidable challenge to synthesize their nanomaterials in a controlled manner [7–11].
As a member of the I-III-VI2 compounds, CuGaS2 (CGS) has a direct bandgap of approximately 2.49 eV for the bulk, and can be applied in green-light emission as well as in visible-light-induced photocatalysis [12, 13]. Generally, CGS crystallizes in tetragonal chalcopyrite phase at room temperature, and corresponding nanocrystals were previously synthesized by hydrothermal and solvothermal methods [14–16]. However, the products obtained using these methods are mostly in the form of large crystallites with a board size distribution. Recently, CGS nanocrystals with well-defined sizes and shapes, including quantum dots, tadpole-like nanocrystals, nanorods, and nanoplates, were prepared by several research groups [17–21]. For instance, Tung et al. synthesized chalcopyrite CGS nanorods by irradiating the precursor solution with intense X-rays . In particular, several research groups have synthesized CGS nanocrystals with metastable wurtzite structure which is a cation-disordered phase [18–21]. Wang et al. reported tadpole-like CGS nanocrystals with wurtzite phase by a hot-injection approach . Xiao et al. prepared wurtzite CGS nanorods by the reaction of copper(I) acetate, gallium(III) acetylacetonate, and 1-dodecanethiol (DT) in the solvent 1-octadecene at elevated temperature . However, two-dimensional CGS nanocrystals such as nanoplates are less reported up to now, despite the fact that Kluge et al. obtained CGS nanoplates by bulk thermolysis of complex single-source precursors .
In this work, we present a facile one-pot method to synthesize CGS nanoplates, wherein the mixed solution of CuCl, GaCl3, and 1-dodecanethiol was thermally decomposed in non-coordinating solvent 1-octadecene at elevated temperature. The crystal phase of the as-prepared CGS nanoplates was revealed to be wurtzite-zincblende polytypism. Their growth process and optical absorption were also investigated.
CuCl, DT, toluene, and anhydrous ethanol were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China); GaCl3 (99.999%) was purchased from Alfa Aesar (Wardhill, MA, USA); 1-octadecene (ODE, 90%) was purchased from Aldrich (St. Louis, MO, USA). All the reagents were used as received without any further purification.
Synthesis of CuGaS2 nanoplates
In a typical synthesis, 0.25 mmol CuCl, 0.25 mmol GaCl3, 0.5 mL DT, and 5 mL ODE were loaded into a 50-mL three-neck flask in a glovebox. The flask was then attached to a Schlenk line. Prior to heating, the mixture system was cycled between vacuum and nitrogen three times, heated to 90°C and then was vacuumed for 10 min. The flask was then filled with nitrogen and heated to 270°C at a rate of 12°C · min-1 with magnetic stirring. After the reaction was allowed to proceed for 40 min, the reaction flask was naturally cooled to room temperature. The resulting CuGaS2 nanocrystals were collected by centrifugation and were washed thoroughly with toluene and ethanol. Finally, the purified nanocrystals were dried under vacuum for characterization.
The samples were characterized by powder X-ray diffraction (XRD) on a Philips X'pert X-ray diffractometer (Amsterdam, The Netherlands) equipped with Cu Kα radiation (λ =1.5418 Å). Transmission electron microscope (TEM) images were taken with a Hitachi H-7650 microscope at an acceleration voltage of 100 kV. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 microscope (Akishima-shi, Japan). The scanning electron microscopy (SEM) images were taken using a Zeiss Supra 40 field emission scanning electron microscope (Oberkochen, Germany) operated at 5 kV. X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKII X-ray photoelectron spectrometer (VG Scienta, Newburyport, MA, USA). The UV–vis absorption spectra were recorded on a Solid Spec-3700 spectrophotometer.
Results and discussion
In summary, we have developed a facile one-pot method to synthesize CuGaS2 nanoplates, wherein the mixed solution of CuCl, GaCl3, and n-dodecanethiol was thermally decomposed in non-coordinating solvent 1-octadecene at elevated temperature. The as-synthesized CuGaS2 nanoplates adopt a unique crystal structure of wurtzite-zincblende polytypism. In the growth process of CuGaS2 nanoplates, copper sulfides firstly formed, and then the as-formed copper sulfides were gradually phase-transformed to CGS nanoplates with proceeding of the reaction. The optical bandgap energy of the nanoplates is estimated to be approximately 2.24 eV. Our results will aid in the application of two-dimensional CuGaS2 nanoplates and the synthesis of other multicomponent sulfide nanomaterials.
This work was supported by the National Natural Science Foundation of China (No. 91022033, No. 21171158), and National Basic Research Program of China (2010CB934700).
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