Solvothermal Growth of Single-Crystal Bismuth Sulfide Nanorods using Bismuth Particles as Source Material Feng Wei, Jie Zhang, Li Wang, and Zhi-Kun Zhang* Key Laboratory of Nanostructured Materials, Qingdao UniVersity of Science & Technology, Zhengzhou Road 53, Qingdao 266042, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1942-1944
ReceiVed September 6, 2005; ReVised Manuscript ReceiVed February 22, 2006
ABSTRACT: Bismuth sulfide (Bi2S3) nanorods have been successfully synthesized by a solvothermal process using bismuth (Bi) particles and Na2S2O3 as source materials. The X-ray powder diffraction patterns show that the as-obtained products belong to the orthorhombic phase. The morphology transformation from Bi particles to Bi2S3 nanorods during the solvothermal process has been characterized by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that the Bi2S3 nanorods about 60 nm in diameter and 1-2 µm in length can be acquired by following the anisotropic growth. The effect of the solvent system on the quality of the nanorods was analyzed. It was found that using ethanol as solvent promotes the synthesis of pure and uniform product. Introduction As a semiconducting material with a direct band gap ranging from 1.3 to 1.7 eV, bismuth(III) sulfide (Bi2S3) has many potential applications in the fabrication of optoelectronic and thermoelectric cooler devices.1,2 Recently, one-dimensional (1D) Bi2S3 nanostructures, such as nanotubes, nanorods, and nanowires, have attracted considerable interest for fabrications of nanodevices.3-5 Various methods have been used to synthesis 1D Bi2S3 nanostructures, such as evaporation routes,6 microwave irradiation,7 thermal decomposition,8 sonochemical methods,9 hydrothermal methods,10,11 thermolysis methods,12 biomoleculeassisted pathways,13 and so on. In these methods, bismuth complexes such as Bi(S2CNEt2)3 are widely used as precursors to synthesize 1D Bi2S3 nanowires or nanorods.8,14 However, to our knowledge, there have been few literature reports on the large-scale synthesis of single-crystal Bi2S3 nanorods using Bi particles as the bismuth source. Herein, we propose a solvothermal approach using Bi particles as the source material to synthesize Bi2S3 nanorods. In our experiments, the structures and morphology transformations from Bi nanoparticles to Bi2S3 nanorods have been investigated. It is observed that the aspect ratio and length of the nanorods which gradually grow from the surface of Bi nanoparticles are significantly affected by the reaction time. Consequently, by controlling the reaction time, predominantly single-crystal Bi2S3 nanorods with different lengths can be successfully synthesized under mild conditions. To acquire products with better purity and quality, different solvents were studied in our experiment. A possible growth mechanism of the uniform nanorods and the roles of ethanol and ethylenediamine are discussed. Experimental Section Metal Bi particles (purity >99.5%, provided by Key Laboratory of Nanostructured Materials) with an average diameter of ∼250 nm that were prepared by thermal evaporation methods under an ambient of Ar/H2 atmosphere were used as source materials. Chemically pure Na2S2O3‚5H2O and analytically pure hydrochloric acid (14 M) were used without further purification. In a typical procedure, bismuth powder (0.52 g, 2.5 mmol) and hydrochloric acid (5 mL, 0.07 mol) were mixed into 15 mL of ethanol. * To whom correspondence should be addressed. Tel/fax: +86-5328402-2869. E-mail:
[email protected].
Figure 1. XRD patterns of bismuth particles (a) and bismuth sulfide nanorods (b). Then the mixture was pretreated with ultrasonication for 15 min before 15 mL of an ethanolic solution of Na2S2O3 (0.5 M, 7.5 mmol) was added. After 5 mL of ethylenediamine (EDA) was added to adjust the pH value to ∼13, the mixture above was finally transferred into a 50 mL Teflon-lined autoclave. Thus, the solvent system we selected is EtOH/EDA/H2O with a volume ratio of 6:1:1 as the reaction medium. The autoclave was maintained at 140 °C for different periods of time and then cooled to room temperature naturally. The black product was carefully separated, washed in a centrifuge with distilled water and ethanol several times, and dried under vacuum at 60 °C for 4 h. The sample was characterized on a D/MAX-500 X-ray powder diffractometer with Cu KR radiation (λ ) 1.5418 Å). The morphologies and sizes of the final product were determined by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) carried out on a JEM-2000EX transmission electron microscope. The samples for TEM imaging were prepared by placing a drop of the
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Figure 3. (A) TEM image of Bi2S3 nanorods synthesized at 140 °C for 16 h. (B) TEM image of a single Bi2S3 nanorod. (C) ED pattern of the Bi2S3 nanorod shown in B.
Figure 2. (A) SEM image of Bi2S3 nanorods at low magnification. (B) SEM image of Bi2S3 nanorods at high magnification. (C) EDXA spectrum of the Bi2S3 nanorods. sample suspended in anhydrous ethanol on a silicon wafer. Field emission scanning electron microscope (FE-SEM) images of the product and energy-dispersive X-ray analysis (EDXA) images were taken on a JEOL JSM-6700F field emission scanning electron microscope. For SEM imaging, the samples were sputtered with thin layers of Pt.
Result and Discussion The XRD patterns of the Bi nanoparticles prepared by evaporation and the Bi2S3 nanorods prepared by solvothermal methods are shown in Figure 1. All of the peaks of the Bi particles shown in Figure 1a can be indexed as hexagonal phase (JCPDS Card No. 85-1329) with lattice constants of a ) 4.546 Å and c ) 11.86 Å. No other impure peaks are observed. Figure 1b shows the XRD pattern of the as-obtained Bi2S3 via
solvothermal treatment at 140 °C for 16 h. The diffraction peaks can be indexed to the orthorhombic phase (JCPDS Card No. 65-2435) with lattice constants of a ) 11.11 Å, b ) 11.25 Å, and c ) 3.970 Å. The size and morphology of the nanorods synthesized via solvothermal processes at 140 °C for 16 h have been analyzed by SEM, shown in Figure 2. The low-magnification SEM image (Figure 2A) displays uniform Bi2S3 nanorods in high yield. The Bi2S3 nanorod image at high magnification given in Figure 2B reveals that the nanorods have an average diameter of ca. 70 nm and an aspect ratio of at least 15. As shown in Figure 2C, the EDXA spectrum of the products indicates that as-acquired nanorods are made up of Bi and S, and the average atomic ratio of Bi to S is 38:62, which is nearly 2:3. The Si signal in the spectrum is attributed to the silicon wafer used as the substrate. Further characterization of Bi2S3 nanorods has been achieved by TEM. Figure 3A shows that the morphology and size of nanorods obtained by TEM are similar to those observed by SEM. The ED pattern given in Figure 3C and the TEM image of a single nanorod (shown in Figure 3B) demonstrate that the nanorod is single crystal in nature and grows along the [002] direction. To investigate the growth process of the Bi2S3 nanorods, a series of experiments were performed at times of 4, 8, and 12 h at 140 °C. The results show that with an increase in the reaction time, the length and diameter of nanorods gradually grow. Before the reaction, the spherical Bi particles used are about 250 nm in diameter (shown in Figure 4A). When the reaction time is maintained for 4 h, thin nanorods less than 20 nm in diameter can be obtained. As shown in Figure 4B, a certain amount of nanoparticles with a few nanorods at the
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To investigate the roles of the solvent on the structures and morphologies of nanorods, different solvent media were used in our experiments under the same reaction conditions (temperature, time, and pH value). First, the role of EDA was studied. To keep an constant pH value, we used an NaOH aqueous solution to replace EDA. The results show that Bi2S3 nanorods also can be obtained without EDA. Unlike the results of the synthesis of CdS nanorods, EDA has no obvious effect on the formation of 1D Bi2S3 nanorods.15 Therefore, EDA mainly acted as a pH value adjustor in our experiments. In addition, ethanol was substituted with water or diethanolamine to investigate the effect on the morphologies of the products. It was found that nanorods also can be obtained using other solvent systems, but without uniform and pure products. We feel the reason for this is that ethanol possesses better liquid-solid adsorption with bismuth particles than do other solvents. Bismuth particles could provide more homogeneous release of the Bi source to form Bi2S3 nanorods. Conclusion
Figure 4. TEM images of Bi particles (A) and Bi2S3 nanorods synthesized at 140 °C for 4 h (B), 8 h (C), and 12 h (D), respectively.
outside is observed by TEM. When the reaction time is prolonged to 8 h, the length and diameter of the nanorods grow further. Figure 4C shows more rodlike structures in the product, while some small particles still exist. After 12 h of reaction time, large-scale homogeneous nanorods can be obtained. Figure 4D reveals a typical nestlike aggregation composed of Bi2S3 nanorods without any residual Bi particles. According to the results, the chemical reaction we employed in the synthesis process can be described as follows. Before the solvothermal process, S is formed as the starting reactant in the solvothermal process.
Na2S2O3 + 2HCl ) 4SV + 2NaCl + SO2v + H2O During the solvothermal process, according to the experiments above, it could be noted that bismuth particles are gradually dissolved in the solvothermal process to provide a Bi source to the surroundings. Then, S reacts with Bi to produce Bi2S3:
2Bi + 3S f Bi2S3 During the process of solvothermal treatment, 1D Bi2S3 crystals are finally formed by anisotropic growth. The influence of the pH value of the solvent on the final product was investigated. Comparison experiments were performed in pH 1 and 13 solutions, respectively. The results indicate that Bi2S3 cannot be obtained in a strongly acidic solution in our experiments. As a result, we use EDA to adjust the pH value to acquire the products.
In summary, we have demonstrated a simple solvothermal route to the synthesis of large-scale single-crystal Bi2S3 nanorods in an EtOH/EDA/H2O solvent system using Bi particles as source materials. The uniform nanorods are of orthorhombic phase and are single crystal in nature. The results show that, in the ethanol solvent medium, Bi particles could provide a bismuth source for homogeneous nuclear growth, followed by the anisotropic growth of Bi2S3. It is considered that this method could provide a tunable route to acquire uniform and predominantly single-crystal Bi2S3 nanorods by controlling the synthesis conditions. References (1) Stella, M. H. Science 2002, 295, 767. (2) Desai, J. D.; Lokhande, J. D. Mater. Chem. Phys. 1995, 41, 98. (3) Peng, X. S.; Meng, G. W.; Zhang, J.; Zhao, L. X.; Wang, X. F.; Wang, Y. W.; Zhang, L. D. J. Phys. D: Appl. Phys. 2001, 34, 3224. (4) Liu, Z.; Peng, S.; Xie, Q.; Hu, Z.; Yang, Y.; Zhang, S.; Qian, Y. AdV. Mater. 2003, 15, 936. (5) Schricker, A. D.; Sigman, M. B., Jr.; Korgel, B. A. Nanotechnology 2005, 16, s508. (6) Ye, C.; Meng, G.; Jiang, Z.; Wang, Y.; Wang, G.; Zhang, L. J. Am. Chem. Soc. 2002, 124, 15180. (7) Liao, X. H.; Wang, H.; Zhu, J. J.; Chen, H. Y. Mater. Res. Bull. 2001, 36, 2339. (8) Monteiro, O. C.; Nogueira, H. I. S.; Trindade, T.; Motevalli, M. Chem. Mater. 2001, 13, 2103. (9) Wang, H.; Zhu, J. J.; Zhu, J. M.; Chen, H. Y. J. Phys. Chem. B 2002, 106, 3848. (10) Zhang, H.; Yang, D.; Li, S.; Ji, Y.; Ma, X.; Que, D. Nanotechnology 2004, 15, 1122. (11) Xie, G.; Qiao, Z. P.; Zeng, M. H.; Chen, X. M.; Gao, S. L. Cryst. Growth Des. 2004, 4, 513. (12) Sigman, M. B., Jr.; Korgel, B. A. Chem. Mater. 2005, 17, 1655. (13) Lu, Q.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54. (14) Gupta, A.; Sharma, R. K.; Bohra, R.; Jain, V. K.; Drake, J. E.; Hursthouse, M. B.; Light, M. E. J. Organomet. Chem. 2003, 678, 122. (15) (a) Yu, S. H.; Yang, J.; Wu, Y. S.; Han, Z. H.; Lu, J.; Xie, Y.; Qian, Y. T. J. Mater. Chem. 1998, 8, 1949. (b) Yang, J.; Zeng, J. H.; Yu, S. H.; Yang, L.; Zhang, Y. H.; Qian, Y. T. Chem. Mater. 2000, 12, 3259.
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