Acetylacetone-Directed Controllable Synthesis of Bi2S3

Jul 7, 2009 - Synopsis. Uniform Bi2S3 nanocrystals with novel three-dimensional superstructures, including microspheres and microflowers in the ...
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DOI: 10.1021/cg801405e

Acetylacetone-Directed Controllable Synthesis of Bi2S3 Nanostructures with Tunable Morphology

2009, Vol. 9 3862–3867

Hongyang Zhou, Shenglin Xiong, Lingzhi Wei, Baojuan Xi, Yongchun Zhu, and Yitai Qian* Department of Chemistry and Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui, 230026, China Received December 26, 2008; Revised Manuscript Received June 9, 2009

ABSTRACT: Uniform 5 μm Bi2S3 microspheres and 8 μm microflowers were solvothermally synthesized in acetylacetone solution through thermolysis of the Bi3þ-dithizone complex without any templates or surfactants. Bi2S3 microspheres composed of nanorods with a diameter of 20-40 nm were synthesized at 180 °C for 12 h. In similar conditions at 240 °C for 3 days, microflowers composed of nanowires with lengths up to several micrometers and diameter of 20-40 nm were obtained. Fieldemission scanning electron microscopy (FESEM) showed in the initial stage in the formation process that smooth spherical cores were observed, then on the surface of the cores nanoparticles appeared, and finally nanorods or nanowires grew out and microspheres and microflowers formed. Electrochemical experiments using Bi2S3 in a lithium ion battery indicated that the first discharge capacity of Bi2S3 microflowers could reach about 148 mA h g-1.

1. Introduction Bismuth sulfide, Bi2S3, with a lamellar structure and direct bandgap of 1.3-1.7 eV is a candidate in the field of photoelectricity, sensors, and thermoelectricity, since the bandgap could be tuned by different dimensions and morphologies of the nanomaterials.1 Bi2S3 nanoparticles have also been found to have new applications as imaging agents in X-ray computed tomograpy.2 Bi2S3 has been synthesized by a variety of methods, such as sonochemical techniques,3a electrochemical deposition,3b,3c organometallic complex decomposition,3d,3e and chemical vapor deposition (CVD).3f,3g Hydrothermal routes have been applied to synthesize Bi2S3 nanotubes4 and nanoribbons.5 Solvothermal routes have also been developed to prepare Bi2S3 nanorod bundles6 and nanorods.7 Recently, three-dimensional (3D) Bi2S3 nanostructures have been obtained. Bi2S3 snowflake-like microstructures were hydrothermally synthesized using glutathione.8 Uniform Bi2S3 flowers were prepared in a large scale by the template effect of the ionic liquid solution.9 Bi2S3 microflowers composed by nanorod bundles were hydrothermally synthesized using L-cysteine as both the sulfur source and the directing molecule.10 A hollow microscale organization of Bi2S3 nanorods was formed through a polyethylene glycol (PEG)-assisted approach in ethylene glycol solution.11 Urchin-like patterns consisting of Bi2S3 nanorods were achieved through a refluxing process in ethylene glycol solution without any templates or surfactants.12 In this paper, uniform 5 μm Bi2S3 microspheres composed of nanorods and 8 μm microflowers composed of nanowires were synthesized by the thermolysis of Bi3þ-dithizone complex in the acetylacetone solution using dithizone as the sulfur source without any templates or surfactants. Field-emission scanning electron microscopy (FESEM) images showed the formation process of microspheres and microflowers: in the initial stage smooth spherical cores were observed, then on the *To whom correspondence should be addressed. E-mail: ytqian@ustc. edu.cn. Tel: 86-551-3607234. Fax: 86-551-3607402. pubs.acs.org/crystal

Published on Web 07/07/2009

surface of the cores nanoparticles appeared, finally nanorods or nanowires grew out and microspheres and microflowers formed. Electrochemical experiments of Bi2S3 in a lithium ion battery indicated that the first discharge capacity of Bi2S3 microflowers can reach about 148 mA h g-1. 2. Experimental Section In a typical experiment, all the reagents were analytic grade, purchased from Shanghai Chemistry Company and used without any further purification. In a typical procedure, 0.315 g of (1.0 mmol) anhydrous BiCl3 white powders and 0.768 g (3.0 mmol) of purpleblack dithizone powders were added into a 50 mL Teflon-lined stainless steal autoclave. Then 40 mL acetclyacetone was transferred into the autoclave. The color of the solution soon turned from red to green indicating the complexing of Bi3þ ion to dithizone. Green powder was collected for UV-Vis absorption (see Figure S4, Supporting Information) comparing it with the existing literature.13b The autoclave was sealed and maintained at 180 °C for 12 h. When the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The product was transferred into a 100 mL glass beaker and precipitated by acetone for about 12 h. The final black solid product was centrifuged, washed with absolute alcohol and distilled water repeatedly at least three times to rinse impurity, and finally dried at 80 °C under a vacuum. After that, the product was collected, in preparation for analysis and characterization. Powder X-ray diffraction (XRD) measurements were performed on a Philips X’Pert MPD ProX-ray diffractometer, with graphite monochromatized high-intensity Cu KR radiation at 40 kV and with a 30 mA flux at a scanning rate of 0.066° s -1. FESEM was progressed on A JEOL JSM-6300F scanning electron microscope. High-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were taken on a JEOL 2010 high-resolution TEM performed at 200 kV. X-ray (EDX) spectrum was taken with a JEOL-2010 transmission electron microscope with an accelerating voltage of 200 kV. The UV-Vis absorption was recorded on a UV-Vis spectrophotometer Specord 200 (Analytic Jena AG) absorption diode array spectrometer using 1 cm quartz curets. The electrode laminate for the electrochemical testing was prepared by casting a slurry consisting of active material powders (84 wt %), acetylene black (8 wt %), and poly (vinylidene fluoride) (PVDF; 8 wt %) dispersed in 1-methyl-2-pyrrolidinone (NMP) onto an aluminum foil. The laminates were then dried at 70 °C for 1 h. The Bi2S3/Li coin r 2009 American Chemical Society

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Figure 1. (a) XRD pattern and (b) the corresponding EDS spectrum of the product prepared at 180 °C by the solvothermal route. cells (2032 size) were made with 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC; 1:1 w/w) was the electrolyte. The cells were tested on a multichannel battery cycler (Shenzhen Neware Co. Ltd.) and subjected to charge-discharge cycles at 0.032 mA/cm2 between 0.0 and 3.0 V (vs Li metal).

3. Result and Discussion 3.1. Structure and Morphologies. Figure 1a shows the typical powder XRD pattern of the as-synthesized sample and reveals that the product is composed of Bi2S3 with lattice constants a=11.13 A˚, b=11.24 A˚ and c=3.97 A˚, which are consistent with those of bulk orthorhombic Bi2S3 (JCPDS 75-1306). No other diffraction peaks were detected. Energy dispersive X-ray (EDX) analysis was employed to analyze the chemical composition. As presented in Figure 1b, only the elements of bismuth and sulfur were detected, and the atom ratio of Bi/S is 1:1.47, indicating that S is slightly in shortfall. The peaks of Cu and C elements in the EDX spectrum come from the TEM copper grid. The microstructure of the product was investigated through FESEM and transmission electron microscopy (TEM). Figure 2a shows its panoramic image, revealing that the product is composed of large-scale Bi2S3 urchin-like microspheres with an average diameter of about 5 μm. High-magnification FESEM images of Figure 2b-d indicate that these microspheres are constructed of nanorods with an average diameter of 20-40 nm. Figure 3a shows the TEM image of a portion of an individual Bi2S3 urchin-like sphere. The HRTEM image

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(Figure 3b) and relevant SAED pattern (inset in Figure 3b) were recorded on a nanorod in Figure 3a to provide additional insight into the structure of orthorhombic Bi2S3. The clear lattice fringes with a d-spacing of 0.39 nm are consistent with that of the (001) planes of orthorhombic Bi2S3 and are parallel to the nanowire axis. The clear lattice fringes with a d-spacing of 0.50 nm are consistent with that of the (120) planes of orthorhombic Bi2S3. The HRTEM and SAED analyses both demonstrate that the Bi2S3 nanorod grows along the [001] direction. As for the synthesis of Bi2S3 nanostructures, reaction temperature plays an important role in determining the morphology of products. When the temperature was 240 °C, Bi2S3 microflowers rather than urchin-like spheres were obtained. Figure 2e,f is the FESEM images of the sample prepared at 240 °C for 3 days, indicating the products transform into microflowers. Figure 2f is a typical FESEM image of a single microflower composed by nanowires with a length up to several micrometers. 3.2. The Influence of Solvent. In the experiments, acetylacetone played an important role. Figure 4 presents the FESEM images of products when acetylacetone was substituted by the common solvents of ethanol, ethylenediamine, acetone, and ethylene glycol. When the solvent is ethanol, the product is Bi2S3 disordered nanorod bundles as shown in Figure 4a,b. When acetone was introduced, the product is the mixture of nanoparticles and few microflowers composed of nanorods (Figure 4c). As ethylenediamine was employed, the product was nanoflakes (Figure 4d). It is noticeable to point out the product was full of morphologies like dendrite, coral, and bamboo shoot when ethylene glycol was introduced, as Figure 4e,f shows. The sizes of Bi2S3 range from a few micrometers to several hundred nanometers. The formation and shape evolution of Bi2S3 architectural structures in different solvents are presented in Scheme 2. 3.3. Influence of the Reaction Temperature on the Architectures. A series of experiments were made to study the influence of the reaction temperature and time on the architectures of products. It is surprising that the products are all similar urchin-like spheres in the temperature range of 120-180 °C with the reaction time varied. For example, the product obtained at 180 °C for 0.5 h (Figure 5a) is similar to that obtained at 120 °C for 6 h (Figure 5d). However, when the temperature was higher than the typical reaction temperature of 180 °C and increased to 240 °C, the result is different. The XRD patterns of the products obtained at different temperatures and for different reaction times are shown in Figure S1, Supporting Information. For example, Figure S1(I a-c) is the XRD patterns of products obtained at 120 °C for 6, 12, and 36 h, which indicate that orthorhombic Bi2S3 can be observed within 6 h and then the products gradually crystallized with a prolonged time. The similar results are also investigated for the products obtained at 140, 180, and 240 °C with a prolonged reaction time. The FESEM images of products influenced by both the reaction temperature and time are shown in Figure 5. As Figure 5a shows, the product consists of microspheres with a smooth surface after 6 h at 120 °C, and some microspheres are in the construction of nanoparticles. As the reaction time was prolonged to 12 h (Figure 5b), nanorods on the surface of spheres appear. Then they transformed into roundish microspheres in the construction of nanorods with an

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Figure 2. (a-d) FESEM images of the sample prepared at 180 °C for 12 h; (e, f) FESEM images of the sample prepared at 240 °C for 3 days.

Figure 3. (a) TEM image of a portion of an individual Bi2S3 nanostructure and an individual Bi2S3 nanorod (inset); (b) the HRTEM and SAED (inset) patterns of a single nanonod recorded on the Bi2S3 nanorod inset in panel a.

average diameter of 20-40 nm. Even though the reaction temperature was lowered as low as 120 °C, the products were

similar to the typical result at 180 °C, when the reaction time was prolonged to 36 h (see Figure 5c).

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Figure 4. FESEM images of the samples synthesized when acetylacetone was substituted by other solvents: (a, b) ethanol; (c) acetone; (d) ethylenediamine; (e, f) ethylene glycol.

Moreover, the time-dependent morphology evolution process of the product at 140 °C was also investigated (see Figure S2, Supporting Information). The images of the typical samples synthesized at 180 °C at the different stages were also shown. As Figure 5d shows, the product consists of microspheres with a smooth surface after 0.5 h reaction at 180 °C and some microspheres are in the construction of nanoparticles. As the reaction time is prolonged to 2 h, nanorods on the surface of spheres appear. In contrast, the product was more nanoparticles (Figure 5e). Then they transformed into the typical roundish morphology of 5 μm microspheres in the construction of nanorods with an average diameter of 20-40 nm (Figure 5f). It is concluded that at least from 120 to 180 °C, only by tuning the reaction times, the results are very similar. This synthetic route might be meaningful and potentially used for the future large-scale industrialized production at a relatively low temperature. The time-dependent experiments of the products obtained at 240 °C are shown in Figure 5 (g-i). As 0.5 h (Figure 5g), the product was microspheres like that at 180 °C. When the reaction time was prolonged to 6 h (Figure 5h), round-spheres disappeared, become loosened, and began transforming into

microflowers. Ultralong nanowires were also found. Some nanowires are as long as 10 micrometers. The product is also mixed with small balls. The balls are as large as about half a micrometer (see Figure S3, Supporting Information). When the reaction time was prolonged for 3 days (Figure 5i), the balls disappeared, and the product was completely composed of 8 μm microflowers in the construction of nanowires. 3.4. Formation Process. It was observed that the red color in solution faded, indicating the Bi3þ-dithizone complex decomposed completely to form a mass of Bi2S3 nuclei. Spherical cores appeared at the initial stage. On the basis of the above-mentioned experimental results, the spherical cores might be responsible for the similar products of urchinlike spheres shaping over the big temperature range. The incipient Bi2S3 nuclei immediately grew along the [001] direction into nanorods. As a result, the possible forming process of Bi2S3 microspheres and microflowers (see Scheme 1 at Figure 6) is as follows: the Bi3þ dithizone complex formed, the complex was decomposed to form Bi2S3 nuclei with the red color in solution fading, and the spherical cores formed by the nuclei in the orientation-directed effect of acetylacetone; by prolonging the

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Figure 5. FESEM images of the samples synthesized at different reaction conditions: (a) 120 °C, 6 h. (b) 120 °C, 12 h. (c) 120 °C, 36 h. (d) 180 °C, 0.5 h. (e) 180 °C, 2.0 h. (f) 180 °C, 12 h. (g) 240 °C, 0.5 h. (h) 240 °C, 6 h. (i) 240 °C, 3 days.

Scheme 1. Formation and Shape Evolution of 3D Bi2S3 Architectural Structures with Different Times and Temperatures, in Acetylacetone Solvent

reaction time, hierarchical spheres (microspheres and microflowers) were gradually formed. The formation process of microspheres and microflowers: in the initial stage smooth spherical cores were observed (Figure 5a,d,g) on the surface of the cores nanoparticles appeared (Figure 5b,e,h) finally nanorods or nanowires grew out along the [001] direction and microspheres and microflowers formed (Figure 5c,f,i). 3.5. The Study of the Performance of Bi2S3 Microflowers in Lithium Ion Battery. Here, the performance of the microflowers prepared in acetylacetone at 240 °C for 3 days (a) in a lithium ion battery was studied. As is reported, the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into the

Scheme 2. Formation and Shape Evolution of Bi2S3 Architectural Structures in Other Solvents

interlayer, the tunnels, and the holes in the crystal structure.14 Bi2S3 with a lamellar microstructure is expected to exhibit excellent electrochemical properties in the lithium ion battery. Figure 6 shows the (Figure 6a) first discharge curves with a cutoff potential of 0.0 V at a current density of 0.032 mA/cm2. The discharge plateau corresponding to 1.95 and 1.60 V appeared in the first discharge process. The first discharge capacity of orthorhombic Bi2S3 microflowers can reach about 148 mA h g-1. Comparatively, the performance of the nanorod prepared in ethanol at 180 °C (Figure 6b) and the dendrite prepared in ethylene glycol at 180 °C (Figure 6c) were also tested at the same condition. The discharge curves (Figure 6b,c) present even better performance, and the first discharge capacity reached as high as 770 and 810 mA h g-1. The Bi2S3 electrochemical property may be related with the morphology transforming from 1D

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References

Figure 6. The first discharge curve of the Bi2S3 (a) prepared in acetylacetone solvent at 240 °C for 3 days; (b) ethanol, 180 °C for 12 h; (c) ethylene glycol 180 °C for 12 h, at the current density of 0.32 mA cm-2.

nanostructure (nanorods, dendrite) to 3D (microflowers). Further work is still underway. 4. Conclusion In summary, a solvothermal method was developed to fabricate large-scale Bi2S3 nanostructures, and acetylacetone was introduced in this synthesis route. The product was composed of uniform urchin-like microspheres composed of nanorods and microflowers composed of nanowires. The growth process of microspheres and microflowers was observed. An electrochemical test was also introduced to investigate the performance of Bi2S3 in a lithium ion battery. Acknowledgment. This work was supported by the 973 Project of China (Number 2005CB623601). Supporting Information Available: XRD patterns of the products at different stages in every reaction temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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