CRYSTAL GROWTH & DESIGN
Synthesis of Bi2Se3 Nanosheets by Microwave Heating Using an Ionic Liquid Ya Jiang, Ying-Jie Zhu,* and Guo-Feng Cheng Biomaterials and Tissue Engineering Center and State Key Laboratory of High Performance Ceramics and Superfine Microstructure and Analysis and Testing Center for Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China
2006 VOL. 6, NO. 9 2174-2176
ReceiVed April 14, 2006; ReVised Manuscript ReceiVed June 19, 2006
ABSTRACT: Bi2Se3 nanosheets with thicknesses of 50-100 nm have been successfully synthesized by a simple microwave heating method using selenium powder, Bi(NO3)3‚5H2O, HNO3 aqueous solution, ethylenediamine or ethylene glycol, and an ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate. The effects of experimental parameters on the formation of the products were investigated. The products were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. Introduction Bi2Se3 is a narrow-gap layered semiconductor with the tetradymite structure. The study on the electronic structure of Bi2Se3 indicates that Bi2Se3 is suitable for application in optical and photosensitive devices.1-3 There have been reports on the synthesis of Bi2Se3 nanofilms,4,5 nanoparticles,6,7 heterostructured nanowires,8 nanobelts,9 and nanotubes10 by a variety of methods such as chemical deposition, solvothermal, sonochemical, template, and ultraviolet irradiation methods. The application of microwave heating in the synthesis of nanomaterials is a fast growing research area due to its advantages such as rapid volumetric heating, higher reaction rate and selectivity, and reduced reaction time compared to conventional heating methods. By microwave heating, we have already prepared Te nanorods and nanowires,11 oxides (ZnO, CuO),12,13 binary oxides (PbCrO4, Pb2CrO5),14 and sulfides (CdS, ZnS, Bi2S3, Sb2S3).15,16 Ionic liquids are good media for absorbing microwaves, and they can be used as surfactants. A2VAB3VIA binary compounds such as Bi2S3 and Sb2S3 nanostructures have been successfully synthesized by microwave heating in the presence of the ionic liquid. The ionic liquid played an important role in the control over the morphology of the product.16 Herein, we report a microwave-assisted method for synthesis of Bi2Se3 nanosheets using selenium powder, Bi(NO3)3‚5H2O, HNO3 aqueous solution, ethylenediamine or ethylene glycol, and an ionic liquid. Experimental Procedures Chemicals. Bi(NO3)3‚5H2O, Se powder, ethylenediamine (EDA), and ethylene glycol (EG) were purchased and used as received without further purification. The ionic liquid used was 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4). Synthesis. In a typical procedure for synthesizing Bi2Se3, Bi(NO3)3‚ 5H2O (0.041 mg) was dissolved in EDA (41 mL) or a mixed solvent of EDA (40.5 mL) and [BMIM]BF4 (0.5 mL) in a 100 mL flask at room temperature (solution A). Solution B was prepared by adding Se powder (0.01 g) in a mixture of 5 M HNO3 aqueous solution (0.5 mL) and EDA (0.5 mL). Solution A was microwave-heated to 110 °C, and solution B was rapidly added into solution A. The mixed solution was maintained at 110 °C for 20 min, then the microwave heating was terminated, and the solution was cooled to room temperature. A similar procedure was used for the synthesis of other samples, except that EG * Corresponding author. Fax: (+86) 215-241-3122. E-mail: y.j.zhu@ mail.sic.ac.cn.
Figure 1. XRD patterns of the samples synthesized by microwave heating: (a) the sample synthesized at 180 °C for 30 min in EG without using [BMIM]BF4; (b) sample 3; (c) sample 4; (d) sample 1 (* Se phase). was used instead of EDA, by microwave heating at 180 °C for 40 min. The product was separated by centrifugation, washed with absolute ethanol three times, and dried at 60 °C in a vacuum. Instruments and Characterization. The microwave oven used was a focused single-mode microwave synthesis system (Discover, CEM), which was equipped with magnetic stirring and a water-cooled condenser. Temperature was controlled by automatic adjustment of microwave power. X-ray powder diffraction (XRD) was performed with a Rigaku D/max 2550V X-ray diffractometer using graphite monochromatized high-intensity Cu KR radiation (λ ) 1.541 78 Å). Scanning electron microscopy (SEM) micrographs were taken with an Electron Probe microanalyzer (EPMA, JEOL JXA-8100). Transmission electron microscopy (TEM) micrographs and high-resolution TEM (HRTEM) were taken with a JEOL JEM-2100F field-emission transmission electron microscope using an accelerating voltage of 200 kV. UVVis spectrophotometer (Lambda 950, Perkin-Elmer) was used to record the ultraviolet-visible absorption spectra of the samples. The conditions and morphologies of some typical samples synthesized by microwave heating are listed in Table 1.
Results and Discussion Figure 1a,b shows XRD patterns of samples synthesized by microwave heating at 180 °C for 30 and 40 min, respectively,
10.1021/cg060219a CCC: $33.50 © 2006 American Chemical Society Published on Web 07/29/2006
Synthesis of Bi2Se3 Nanosheets
Crystal Growth & Design, Vol. 6, No. 9, 2006 2175
Table 1. Typical Samples Synthesized by Microwave Heating under Different Conditions sample no.
reaction system
temp (°C)
time (min)
morphology
1
Se (0.01 g) + 5 M HNO3 (aq, 0.5 mL) + Bi(NO3)3‚5H2O (0.041 g) + EDA (41.5 mL) Se (0.01 g) + 5 M HNO3 (aq, 0.5 mL) + [BMIM]BF4 (0.5 mL) + Bi(NO3)3‚5H2O (0.041 g) + EDA (41 mL) Se (0.01 g) + 5 M HNO3 (aq, 0.5 mL) + Bi(NO3)3‚5H2O (0.041 g) + EG (41.5 mL) Se (0.01 g) + 5 M HNO3 (aq, 0.5 mL) + [BMIM]BF4 (0.5 mL) + Bi(NO3)3‚5H2O (0.041 g) + EG (41 mL)
110
20
nanosheet and nanoparticle
110
20
irregular nanosheet
180
40
irregular nanosheet
180
40
hexagonal nanosheet
2 3 4
Figure 2. (a) SEM micrograph of sample 1 synthesized by microwave heating without using [BMIM]BF4; (b) TEM micrograph of sample 1; (c) SEM micrograph of sample 2 synthesized by microwave heating in the presence of [BMIM]BF4sthe inset is an enlarged SEM micrograph showing curved nanosheets; (d) TEM micrograph of sample 2; (e) the UV-vis absorption spectrum of sample 2; (f) the (Rhν)2-hν curve of sample 2.
in EG without using [BMIM]BF4. Elemental Se was present in both samples. However, a single phase of crystalline Bi2Se3 with a hexagonal structure (JCPDS 33-0214) was obtained for sample 4 synthesized by microwave heating at 180 °C for 40 min in EG in the presence of [BMIM]BF4. Figure 1d shows the XRD pattern of sample 1 synthesized by microwave heating at 110 °C for 20 min without using [BMIM]BF4. It indicates that sample 1 consisted of a single phase of crystalline Bi2Se3 with a hexagonal structure (JCPDS 33-0214). The morphologies of the samples were investigated by SEM and TEM. Figure 2a,b shows the SEM and TEM micrographs of Bi2Se3 (sample 1) synthesized under microwave heating at 110 °C for 20 min without using the ionic liquid [BMIM]BF4. One can see mixed morphologies of nanoparticles and nanosheets. The standing nanosheets looked like rods in the SEM micrograph. Wang et al.6 had synthesized Bi2Se3 nanoparticles using BiCl3, Se powder, and NaI as reagents and EDA as the solvent by a solvothermal method. However, no nanosheets were obtained. In our experiment, irregular nanosheets with thick-
nesses of 50-100 nm formed. To obtain exclusively Bi2Se3 nanosheets instead of particles, we tried to use the ionic liquid [BMIM]BF4. Figure 2c,d shows the SEM and TEM micrographs of sample 2 synthesized under the same conditions as sample 1 except that [BMIM]BF4 was used. One can see that nanosheets were obtained exclusively. The enlarged SEM micrograph (inset of Figure 2c) and TEM micrograph (Figure 2d) show curved nanosheets. This implies that [BMIM]BF4 played an important role in the formation of Bi2Se3 nanosheets, and it favored the formation of sheet-like morphology. The UV-vis absorption spectrum of sample 2 is shown in Figure 2e. The absorption onset was observed at near 780 nm, and the absorption band gap, Eg, was determined by the equation (Rhν)n ) B(hν - Eg), in which R is the absorption coefficient, hν is the photo energy, and B is a constant. For the direct band gap semiconductor Bi2Se3, n is 2. Figure 2f shows the (Rhν)nhν curve for sample 2, from which the band gap, Eg, of Bi2Se3 nanosheets was estimated to be 1.57 eV. The Eg value of sample 2 was significantly larger than that of bulk material (Eg ≈ 0.25 eV),17 and it was in good agreement with the reported value (1.5 eV).18 Similarly, EG was used as the solvent instead of EDA. Figure 3a,b shows the SEM and TEM micrographs of sample 3 synthesized by microwave heating at 180 °C for 40 min using EG as the solvent in the absence of [BMIM]BF4, from which one can see irregularly shaped Bi2Se3 nanosheets. However, hexagonally shaped Bi2Se3 nanosheets were obtained in the presence of [BMIM]BF4 (sample 4, Figure 3c,d), implying that [BMIM]BF4 facilitated the formation of symmetrically hexagonal morphology of nanosheets. The inset of Figure 3c is an enlarged SEM micrograph showing a hexagonal morphology of an individual nanosheet. Yang et al.18 synthesized Bi2Se3 nanorods using Bi(NO3)3‚5H2O and Se powder in EG by a solvothermal method at 200 °C for 12 h. Harpeness and Gedanken19 synthesized Bi2Se3 planar and leaf-like aggregate nanostructures using BiONO3 and Se powder in EG by a microwave heating method. No hexagonal nanosheets were observed. Figure 3e shows the HRTEM micrograph of a single hexagonal Bi2Se3 nanosheet in sample 4, which provides more detailed structural information on the nanosheet. The HRTEM shows that the nanosheet was structurally single crystalline and that the lattice structure was uniform. Its corresponding fast Fourier transform (FFT) pattern can be indexed to singlecrystalline Bi2Se3 nanosheet along the [001] zone axis, as presented in the inset of Figure 3e. The UV-vis absorption spectrum of sample 4 is shown in Figure 3f. The absorption onset was observed at near 776 nm, and the absorption band gap, Eg, was estimated to be 1.58 eV based on the (Rhν)n-hν curve (inset of Figure 3f), which was similar to that of sample 2. The crystal structure of Bi2Se3 can be represented as a stack of hexagonally arranged atomic planes, and five atomic planes are stacked as one period. The much weaker Se-Se interaction between periods leads to an easy cleavage of the (001) planes, which contributes to the formation of sheet-like Bi2Se3 crys-
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surfactant that adsorbed on specific crystal planes of Bi2Se3. In the reaction, the low reduction potential of Bi3+ (Bi3+ + 3e) Bi, 0.308 V) made it easy for it to be reduced from Bi3+ to Bi0 by EG; then Se and Bi reacted to form Bi2Se3.18,19 The detailed formation mechanism of Bi2Se3 nanosheets in the presence of [BMIM]BF4 under microwave heating needs to be further investigated. Conclusion The microwave-assisted ionic liquid method has been successfully used for the synthesis of hexagonally shaped Bi2Se3 nanosheets. The ionic liquid ([BMIM]BF4) plays an important role in the morphology of Bi2Se3. When EDA was used as the solvent, both nanoparticles and irregularly shaped nanosheets were obtained without using [BMIM]BF4, while exclusively irregularly shaped nanosheets were formed in the presence of [BMIM]BF4. When using EG as the solvent, irregularly shaped nanosheets could be obtained in the absence of [BMIM]BF4, while hexagonally shaped nanosheets were prepared in the presence of [BMIM]BF4. The band gap, Eg, of Bi2Se3 nanosheets significantly blue-shifted compared with that of the bulk material. We expect that the microwave-assisted ionic liquid method may also be extended to synthesize nanostructures of other A2VAB3VIA binary compounds.
Figure 3. (a) SEM micrograph of sample 3 synthesized by microwave heating without using [BMIM]BF4; (b) TEM micrograph of sample 3; (c) SEM micrograph of sample 4 synthesized by microwave heating in the presence of [BMIM]BF4sthe inset is an enlarged SEM micrograph showing a hexagonal nanosheet; (d) TEM micrograph of sample 4; (e) HRTEM micrograph of a hexagonal nanosheet in sample 4sthe inset is the corresponding fast Fourier transform (FFT) pattern; (f) the UV-vis absorption spectrum of sample 4sthe inset is the (Rhν)2-hν curve of sample 4.
tals.1,2,20 Cui et al.10 reported that curved Bi2Se3 nanosheets were synthesized at 180 °C for 24 h, and hexagonal nanosheets were formed at a higher temperature (210 °C) for 24 h. In our method, the utilization of microwave heating leads to the fast synthesis of Bi2Se3 nanosheets, and shorter time was needed to obtain Bi2Se3 by microwave heating. When EDA was used as the solvent under microwave heating, a black product appeared in a few minutes after the mixing of the reagents. But the morphologies were not uniform. In the presence of [BMIM]BF4, irregularly shaped nanosheets were obtained. When EG was used as the solvent instead of EDA, the reaction could not proceed at room temperature. At the temperature of 180 °C by conventional heating, no product was obtained after 2 h heating, a gray product could be observed after 4 h heating, and the color darkened gradually as time proceeded. The ionic liquid [BMIM]BF4 played an important role in the formation of Bi2Se3 nanosheets. [BMIM]BF4 is a good medium for absorbing microwaves, leading to a fast heating rate.11 In the EDA synthetic system, nanoparticles as well as nanosheets were formed without using [BMIM]BF4. However, nanosheets were formed exclusively in the presence of [BMIM]BF4. In the EG reaction system, irregularly shaped nanosheets were prepared in the absence of [BMIM]BF4, and hexagonally shaped nanosheets were obtained using [BMIM]BF4. The formation mechanism of Bi2Se3 nanosheets is obviously different in the EDA and EG reaction system. We propose that [BMIM]BF4 acted as a
Acknowledgment. Financial support from the National Natural Science Foundation of China (Grant 50472014) and Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We also thank the Fund for Innovation Research from Shanghai Institute of Ceramics, Chinese Academy of Sciences. References (1) Larson, P.; Greanya, V. A.; Tonjes, W. C.; Liu, R.; Mahanti, S. D.; Olson, C. G. Phys. ReV. B 2002, 65, No. 085108. (2) Urazhdin, S.; Bilc, D.; Tessmer, S. H.; Mahanti, S. D.; Kyratsi, T.; Kanatzidis, M. G. Phys. ReV. B 2002, 66, No. 161306. (3) Woollam, J. A.; Beale, H. A.; Spain, I. L. ReV. Sci. Instrum. 1973, 44, 434. (4) Waters, J.; Crouch, D.; Raftery, J.; O’Brien, P. Chem. Mater. 2004, 16, 3289. (5) Pejova, B.; Grozdanov, I.; Tanusevski, A. Mater. Chem. Phys. 2004, 83, 245. (6) Wang, W. Z.; Geng, Y.; Qiang, Y. T.; Xie, Y.; Liu, X. M. Mater. Res. Bull. 1999, 34, 131. (7) Qiu, X. F.; Zhu, J. J.; Pu, L.; Shi, Y.; Zheng, Y. D.; Chen, H. Y. Inorg. Chem. Commun. 2004, 7, 319. (8) Qiu, X. F.; Burda, C.; Fu, R. L.; Pu, L.; Chen, H. Y.; Zhu, J. J. J. Am. Chem. Soc. 2004, 126, 16276. (9) Cui, H. M.; Liu, H.; Wang, J. Y.; Li, X.; Han, F.; Boughton, R. I. J. Cryst. Growth 2004, 271, 456. (10) Cui, H. M.; Liu, H.; Li, X.; Wang, J. Y.; Han, F.; Zhang, X. D.; Boughton, R. I. J. Solid State Chem. 2004, 177, 4001. (11) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410; Angew. Chem. 2004, 116, 1434. (12) Wang, W. W.; Zhu, Y. J. Inorg. Chem. Commun. 2004, 7, 1003. (13) Liang, Z. H.; Zhu, Y. J. Chem. Lett. 2004, 33, 1314. (14) Wang, W. W.; Zhu, Y. J. Cryst. Growth Des. 2005, 5, 505. (15) Jiang, Y.; Zhu, Y. J. Chem. Lett. 2004, 33, 1390. (16) Jiang, Y.; Zhu, Y. J. J. Phys. Chem. B 2005, 109, 4361. (17) Mishra, S. K.; Satpathy, S.; Jepsen, O. J. Phys.: Condens. Mater. 1997, 9, 461. (18) Yang, X. H.; Wang, X.; Zhang, Z. D. J. Cryst. Growth 2005, 276, 566. (19) Harpeness, R.; Gedanken, A. New J. Chem. 2003, 27, 1191. (20) Urazhdin, S.; Bilc, D.; Mahanti, S. D.; Tessmer, S. H.; Kyratsi, T.; Kanatzidis, M. G. Phys. ReV. B 2004, 69, No. 085313.
CG060219A
