Solvothermal Synthesis of Ternary Sulfides of Sb2 − xBi x S3(x = 0.4, 1) with 3D Flower-Like Architectures
Flower-like nanostructures of Sb2 − xBi x S3(x = 0.4, 1.0) were successfully prepared using both antimony diethyldithiocarbamate [Sb(DDTC)3] and bismuth diethyldithiocarbamate [Bi(DDTC)3] as precursors under solvothermal conditions at 180 °C. The prepared Sb2 − xBi x S3 with flower-like 3D architectures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The flower-like architectures, with an average diameter of ~4 μm, were composed of single-crystalline nanorods with orthorhombic structures. The optical absorption properties of the Sb2 − xBi x S3 nanostructures were investigated by UV–Visible spectroscopy, and the results indicate that the Sb2 − xBi x S3 compounds are semiconducting with direct band gaps of 1.32 and 1.30 eV for x = 0.4 and 1.0, respectively. On the basis of the experimental results, a possible growth mechanism for the flower-like Sb2 − xBi x S3 nanostructures is suggested.
KeywordsNanostructures Semiconductor Ternary sulfide Solvothermal Optical properties
Semiconductor nanocrystals have attracted much attention in the past few decades [1, 2]. Among them, binary chalcogenide semiconductors of the A2VE3VI type (A=Sb, Bi; E=S, Se, Te) have aroused great interest due to their potential and practical applications in thermoelectric and optoelectronic devices. For example, bismuth sulfide (Bi2S3), which crystallizes in the orthorhombic system, is a direct band gap semiconductor with E.g. = 1.3 eV and can be applied in photovoltaic converters  and thermoelectric cooling technologies based on the Peltier effect . At the same time, Bi2S3 nanocrystalline films have been found to significantly alter the performance of photochemical cells due to quantum size effects . Moreover, antimony sulfide (Sb2S3), which is isostructural to Bi2S3, shows interesting high photosensitivity and high thermoelectric power , and its direct band gap of 1.5–2.50 eV covers the visible and near infrared range of the solar spectrum [7–9]. As a result, Sb2S3 has wide applications in solar energy conversion, thermoelectric cooling technologies, television cameras, microwave devices, switching devices, rechargeable storage cells, and optoelectronics in the infrared (IR) region [10–13].
The band gap of a material determines its applicability as an optoelectronic material; therefore, tailoring of the band gap is very helpful. A usual approach to adjust the band gap is to synthesize materials on the nanoscale to take advantage of the quantum confinement effect. However, due to the low Bohr radius of most materials, the method is often far from effective. As an alternative, the band gap can also be tailored by adjusting the composition of materials. It is well known that in doped compound semiconductors, in contrast to undoped ones, the impurity states play a special role in the electronic energy structures and transition probabilities . For doped nanocrystalline semiconductor compounds, confinement effects in the energy states also produce unusual physical and optical behavior. Recently, several research groups have reported the effects of the composition on the quantum efficiency of Zn1 − xMn x S and Cd x Zn1 − xS nanoparticles [15–18]. In this paper, for the first time, we report the synthesis and band gap of Bi-doped Sb2S3 ternary sulfides, Sb2 − xBi x S3 (x = 0.4, 1.0) with flower-like nanostructures, prepared by a facile solvothermal method.
All the chemical reagents used in our experiments were of analytical grade and were used without further purification. The molecular precursors, antimony and bismuth diethyldithiocarbamate, Sb(DDTC)3 and Bi(DDTC)3, were prepared as follows: 0.01 mol of SbCl3[or Bi(NO3)3] and 0.02 mol of (C2H5)2NCS2Na·3H2O were dissolved in 100 mL of distilled water, respectively. Then, the two solutions were mixed by stirring in a 500-mL beaker. The resulting white precipitates were filtered, washed with distilled water, and dried in air at 60 °C.
In a typical procedure for synthesizing Sb2 − xBi x S3, the molecular precursors of Sb(DDTC)3 and Bi(DDTC)3 in the appropriate ratios (1 mmol in all) were put into a Teflon-lined stainless steel autoclave (30 mL capacity) to which 20 mL of ethylene glycol was added. The autoclave was sealed and maintained at 180 °C for 12 h; then it was allowed to cool to room temperature naturally. The as-formed black precipitates were separated by centrifugation, washed with ethanol and distilled water several times, and dried at 60 °C for 3 h.
The phase of the as-synthesized products was characterized using X-ray diffraction (XRD, Shimadzu XRD-6000) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 4º min −1. The X-ray tubes were operated with electric current of 30 mA and voltage of 40 kV. The composition, morphology, and sizes of the products were examined by field emission scanning electron microscopy (FESEM; JSM-7001), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM; JEOL-2100). Samples for TEM were prepared by dropping the products on a carbon-coated copper grid after ultrasonic dispersion in absolute ethanol. The band gap energy of the products was determined from the onset of the absorbance spectra of the samples on a UV–Visible (UV–Vis) spectrophotometer with near IR (NIR) capability (Shimadzu UV-4100).
Results and Discussion
In our previous study, Bi(DDTC)3 and Sb(DDTC)3 have been used as single-source molecular precursors for the syntheses of Bi2S3 and Sb2S3 nanomaterials, respectively. Considering the highly similar crystal structures of Bi2S3 and Sb2S3, we herein synthesized ternary sulfides Sb2 − xBi x S3 by using both Bi(DDTC)3 and Sb(DDTC)3 as precursors in a one-pot reaction. Based on the experimental observations, we infer that the formation process of the flower-like Sb2 − xBi x S3 nanostructures can be divided into three steps: First, under the solvothermal action, the precursors of Bi(DDTC)3 and Sb(DDTC)3 were decomposed and produced Sb2 − xBi x S3, which would form Sb2 − xBi x S3 crystal nuclei when the degree of supersaturation of the Sb2 − xBi x S3 reached a certain critical point. Secondly, these crystal nuclei grew and/or aggregated into a bigger core, which was thermodynamically favorable due to the decrease in the surface energy. Finally, the as-formed cores may serve as the substrates for epitaxial growth of the Sb2 − xBi x S3 nanorods. As a result, the flower-like architecture with Sb2 − xBi x S3 nanorods on its surface was formed. To check the proposed mechanism, we have done several parallel experiments with shorter reaction time of 10 and 6 h with the other synthetic conditions remaining unchanged. It was found that with the decrease of the reaction time, there were more separated nanorods in the products. This result is consistent with the formation mechanism of the Sb2 − xBi x S3 flowers.
In summary, we have developed a facile and mild solvothermal method for the large-scale preparation of ternary sulfide Sb2 − xBi x S3(x = 0.4, 1.0) flower-like nanostructures. The possible formation mechanism of the flower-like Sb2 − xBi x S3 is suggested. The optical properties of the Sb2 − xBi x S3 products were evaluated by UV–Vis spectroscopy at ambient temperature. The results indicate that the Sb2 − xBi x S3 compounds are semiconducting with direct band gaps of 1.32 and 1.30 eV for x = 0.4 and 1.0, respectively. This method can probably be extended to the fabrication of other ternary sulfide semiconductors nanostructures with various morphologies and functions.
We are grateful for financial support from the Natural Science Foundation of Jiangsu Province (No. BK2009196) and the National Natural Science Foundation of China (No. 20875039).