12145
2007, 111, 12145-12148 Published on Web 07/27/2007
Synthesis, Growth Mechanism, and Work Function at Highly Oriented {001} Surfaces of Bismuth Sulfide Microbelts Yu Zhao, Xi Zhu, Yanyan Huang, Sunxi Wang, Jinlong Yang, and Yi Xie* Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, P. R. China ReceiVed: April 21, 2007; In Final Form: July 11, 2007
We demonstrate in this communication a facile hydrothermal route to fabricate Bi2S3 microbelts in large scale with unique highly oriented {001} surfaces. The growth mechanism follows two-stage crystallization with initial bubble-template growth and a subsequent anisotropic growth process, which is obviously different from previous reports for fabricating 1D micro/nanostructures. Because the work function is an important parameter for the surface and applied to the investigation of many surface phenomena, the work function of Bi2S3 at the highly oriented {001} surfaces is calculated using a density functional theory based on the planewave method, implemented in the Vienna ab initio simulation package. The high work function of 4.93 eV at the highly oriented {001} surfaces of Bi2S3 microbelts contributes to the potential application for anode materials and understanding the field-emission characteristics and photoelectrochemical behaviors.
The synthesis of functional materials with highly oriented surfaces, which has been proven to have enhanced physical properties,1 is a hot topic these days. Recently, experimental and theoretical studies have shown the surface-structuredependent reactivity of nanocrystallites;2 the highly oriented surface plays an important role in all cases, and the synthesis of functional materials with highly orientated surfaces in high yields is a key step for industrial application. Materials with belt-like structures have attracted much attention for their aspect ratios and unique orientated surfaces.3 Therefore, the synthesis of belt-like structure materials opens up new avenues for building expected surface structures, which may enhance their electronic, magnetic, optical, or catalytic properties. A number of materials with belt-like structures made of metals, metal oxides, and polymers have been prepared in the past few decades.4 Particularly, considerable effort has been devoted to the chalcogenides semiconductors because of their promising physical properties and potential applications.5 Bismuth sulfide with a lamellar structure6 is an excellent photoconductive semiconductor and has potential applications in variant fields.7 Previous research reports are focused mainly on the synthesis methods and various different morphologies of Bi2S3, such as nanobelts, nanorods, nanoflowers, and so forth, are synthesized via many routes.8 Among those, belts represent a special geometrical shape, in contrast to the other 1D/3D micro/ nanostructures, and have stimulated extensive research interest.9 Therefore, the controlled synthesis of Bi2S3 with a belt-like structure and the theoretical investigation of the relationship between the atomic structure and electronic properties of Bi2S3 surfaces is desirable. Because the work function is one of the * Corresponding author. Address: Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China. Fax: (+) 86-551-3603987. E-mail:
[email protected].
10.1021/jp073081v CCC: $37.00
most important basic physical quantities needed to determine the electronic structure that depends on the surface structure and affects the photoelectrochemical behaviors,10 the investigation of work function on the oriented surface of Bi2S3 is significantly important, for both experimental and theoretical studies. The relationship between its microscopic structure of the surface and photoelectrochemical behaviors is also important. Herein, we demonstrate a facile hydrothermal route to fabricate Bi2S3 microbelts with unique highly oriented {001} surfaces. Thanks to their favorable aspect ratios and the uniformity of the surface structure compared with other micro/ nanostructures, the Bi2S3 microbelts represent a special geometrical shape and show an attraction to investigate their chemical and physical behaviors. Because the work function is an important parameter for the surface and applied to the investigation of many surface phenomena, we offer the first opportunity to theoretically investigate the work function at the oriented Bi2S3 {001} surface. The calculated work function at the highly oriented {001} surfaces of Bi2S3 microbelts is as high as 4.93 eV, which make it extremely intriguing to investigate their potential applications in anode materials and photoelectron devices. In a typical procedure, 1.0 mmol bismuth nitrate (Bi(NO3)3‚ 5H2O, A.G.) and 1.0 mmol ethylenediamine tetraacetic acid (EDTA, A. G.) were dissolved into 40 mL 2 mol‚L-1 nitric acid and stirred for 30 min to give a suspended solution. Then 25% (mass ratio) ammonia hydrate was added dropwise into the suspended solution until the suspended solution turned out to be transparent. After that, 1.5 mmol potassium thiocyanate (KSCN, A. G.) and 0.5 mL polyethylene glycol 1000 (PEG 1000) were added and stirred for another 10 min. Finally, the pH value of the transparent light yellow solution was adjusted to 3-4 by 2 mol‚L-1 nitric acid and transferred into a Teflonlined stainless-steel autoclave with a capacity of 50 mL and © 2007 American Chemical Society
12146 J. Phys. Chem. C, Vol. 111, No. 33, 2007
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Figure 2. (a-f) FESEM images of the samples collected at different time intervals of 20, 40, 60, 120, 180, and 240 min, respectively. Figure 1. (a) Typical FESEM image of Bi2S3 microbelts. (b) Magnified FESEM image of a single Bi2S3 microbelt with natural curving morphology. The thickness can be estimated to be about 30-50 nm (inset). (c) XRD pattern of the as-prepared Bi2S3 microbelt. (d) HRTEM image associated with the SAED pattern (inset) of an individual Bi2S3 microbelt.
maintained at 220 °C for 10 h. After cooling, the as-prepared shiny-gray precipitate was collected and washed with distilled water and absolute ethanol several times and dried in a vacuum at 60 °C for 6 h. The as-prepared product was checked in sequence with field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F), X-ray powder diffraction (XRD, Philips X’Pert Pro Super diffractometer with Cu KR radiation (λ ) 1.54178 Å)), and high-resolution transmission electron microscopy (HRTEM, JEOL-2010) associated with selectedarea electron diffraction (SAED). Figure 1a shows the representative FESEM image of Bi2S3 microbelts with widths ranging from 0.8 to 2 µm and lengths up to hundreds of micrometers. A magnified single microbelt is shown in Figure 1b. The natural curving morphology indicates the belt-like structure. The thickness of a typical microbelt is 30-50 nm as shown in the inset of Figure 1b, which reveals the large ratio of surface area to thickness. Figure 1c shows the XRD pattern of the as-prepared Bi2S3 belts; all of the peaks can be indexed as the orthorhombic lattice (space group Pbnm) of Bi2S3 with cell constants of a ) 11.13 Å, b ) 11.26 Å, and c ) 3.96 Å, which are consistent with the values given in the standard card (JCPDS 17-0320). No peaks of any other impurities were detected, indicating the high purity of the product. The significantly intensified (200) and (060) diffraction peaks compared with the characteristic (130) diffraction peak of Bi2S3 reveal that there is a bias of orientations of the {200} and {060} crystallographic plane. This phenomenon is consistent with the HRTEM image shown in Figure 1d. The HRTEM image of an individual Bi2S3 belt is taken with the electron beam perpendicular to the wide surface of the microbelt. The examined region is perfectly free of dislocation and distortion; the lattice spacings of 5.6 and 3.5 Å correspond to the separations between two (200) and (130) planes, respectively. The belt shows a preferred [010] orientation, which is different from the previous reports of Bi2S3 1D structures.8 The wide surface can be indexed as {001} surface. In view of surface-structure-dependent reactivity, the highly oriented surfaces could be of great importance for both theoretical investigations and technological applications. The angle between (200) and (130) planes is 71.8°, which is consistent with the SAED pattern shown in the inset of Figure 1d. The sharp SAED pattern spots indicate that the belt is single-crystalline. The bias of orientation of {100} and
SCHEME 1: Illustration of the Formation Process of Bi2S3 Microbelts.
{010} surfaces explains the intensified (200) and (060) peaks in the XRD pattern. To investigate the growth process of the Bi2S3 belts, we carried out time-dependent experiments during which samples were collected at different time intervals from the reaction mixture once a precipitate appeared in the clear solution. Figure 2 shows the morphology evolution of Bi2S3 by means of collecting 6 samples during the reaction that had proceeded for 20, 40, 60, 120, 180 and 240 min, respectively. After aging the solution at 220 °C for 20 min, a dark-gray sparse precipitate was found (Figure 2a). As the reaction time was increased to 40 min, particles with diameters of about 5 µm were found (Figure 2b). After the reaction had proceeded for 60 min, these particles turned out to be flower-like (Figure 2c). With a prolonged reaction time of 120 min, the petals grew further and spread out in radial way (Figure 2d). The petals kept growing in radial way, and the belt-like structure formed (Figure 2e and f). On the basis of the FESEM observation of the intermediate products, the whole formation process of the Bi2S3 belt can be illustrated in Scheme 1. At the beginning of the reaction, Bi2S3 particles with diameters of about 200 nm and CO2 bubbles (the decomposed product of KSCN) formed (step a). The freshly crystalline particles are unstable because of the high surface energy, and they tend to aggregate around the gas/liquid interface of CO2 and water, driven by the minimization of interfacial energy11 (step b). Because crystalline Bi2S3 is in the intrinsically anisotropic orthorhombic phase, Bi2S3 crystal growth tends to obtain a match between the symmetry of the crystals and the uniaxial geometry of one-dimensional species.12 The coming process (step c) of these particles results in the formation of Bi2S3 flowers. Such a process is consistent with previous reports involving a fast nucleation of amorphous primary particles followed by a slow aggregation and crystallization of primary particles.13 With prolonged reaction time,
Letters the continuously formed Bi2S3 particles served as the seeds for the growth of petals from flower-like to belt-like (step d). This morphology transformation process of Bi2S3 from flower-like to belt-like is different from the previous report for building Bi2S3 1D structures. Actually, the whole formation process involves two independent processes: the first process is bubbletemplate growth (steps a and b), during which the concentrations of Bi2S3 particles and the CO2 bubbles are relatively high and the Bi2S3 colloids tend to aggregate around the gas/liquid interface (see the Supporting Information). The second process is an anisotropic growth process (steps c and d), during which the concentration of reactants is low and the crystallization occurs according to the intrinsically anisotropic orthorhombic phase of Bi2S3. Additionally, because the Bi2S3 crystal has a lamellar structure,6 the neighbor layers are connected via weaker van der Waals interactions and the microbelts with broad surface tends to crack and fall off from the mother body to form individual ones. The overall reaction can be formulated as follows:
2Bi3+ + 3SCN- + 6H2O f Bi2S3 + 3CO2 + 3NH4+ Because the work function is an important parameter for the surface and applied to the investigation of many surface phenomena, such as contamination, adsorption, passive films, and tribological processes,14 we offer the first opportunity to investigate the work function on Bi2S3 {001} surfaces. The work function is defined as the energy required to move an electron from deep within the bulk to a point far away from the surface. In electronic structure codes, surfaces can be modeled as slabs having an infinite extension in the surface plane and a finite thickness orthogonal to it. The work function is then obtained as the difference of the energy of an electron at infinity minus the Fermi energy: EV - EF. The calculation of the work function is nowadays done routinely in density functional calculations, and the agreement between theoretical and experimental data is usually very good, see for example, ref 15. Knowledge of the electrostatic potential, which is determined by the charge density, and of the Fermi energy, is necessary to compute EV - EF. If the exact functional was known, and if numerical noise was neglected, then the charge density, the position of the Fermi energy, and thus the work function would be obtained exactly.16 In our study, work function calculation was carried out using a density functional theory17-based plane-wave method, implemented in the Vienna ab initio simulation package.18 The generalized gradient approximation proposed by Perdew and Wang was used with spin unpolarization included.19 To treat the ion cores, the standard potentials of the projector augmented wave20 type supplied with VASP were used. We choose the 2 × 2 18-layer (∼33.2 Å) slabs including 180 atoms to calculate the work function; all of the layers are fully relaxed, and the slabs have a 20.0 Å vacuum layer as showed in the inset of Figure 3. The calculated work function is shown in Figure 3. The calculated work function of Bi2S3 microbelts at {001} surfaces is 4.93 eV. In comparison, the work function at {100} and {010} surfaces is also calculated: 5.11 eV for {100} surfaces and 5.13 eV for {010} surfaces, respectively. This work function is larger than that of graphite, which is typically around 4.34 eV,21 demonstrating the potential application as competitive candidates for field emitters. Notably, the uniformity of surface structure of Bi2S3 microbelts is advantageous for field emitters with high aspect ratios.22 The high work function contributes to the potential application for anode materials and understanding the field-emission characteristics and photoelectrochemical
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Figure 3. Calculated work function of the {001} surface of Bi2S3 belts. The inset shows the 2 × 2 18-layer slabs in the work-function calculation.
behaviors. Meanwhile, the surface catalysis ability and surface adsorption are also worth noting. In summary, Bi2S3 microbelts with highly oriented {001} surfaces have been synthesized in large scale via a facile hydrothermal route. The microbelts on average are 30-50 nm in thickness, 0.8-2 µm in width, and hundreds of micrometers in length. On the basis of the observation of intermediate products in different growth moments, a two-stage crystallization growth mechanism with an initial bubble-template growth process and a subsequent anisotropic growth process is proposed. This growth mechanism is novel and different from the previous reports for fabricating the Bi2S3 1D structure. This reaction system would be expected to extend to the synthesis of other lamellar-structured materials with favorable 1D structures. The work function at the highly oriented {001} surfaces of Bi2S3 microbelts shows a high work function of 4.93 eV, which reveals the potential applications for anode materials and helps us understand the field-emission characteristics and photoelectrochemical behaviors. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (no. 20621061) and the state key project of fundamental research for nanomaterials and nanostructures (2005CB623601). Supporting Information Available: FESEM images and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Linsebigler, A.; Rusu, C.; Yates, J. T. J. Am. Chem. Soc. 1996, 118, 5284. (b) Tsai, M. H.; Hass, K. C. Phys. ReV. B 1995, 51, 14616. (c) Yang, S. W.; Gao, L. J. Am. Chem. Soc. 2006, 128, 9330. (2) (a) Sayle, D. C.; Maicaneanu, S. A.; Watson, G. W. J. Am. Chem. Soc. 2002, 124, 11429. (b) Skorodumova, N. V.; Baudin, M.; Hermansson, K. Phys. ReV. B 2004, 69, 07540. (3) (a) Zhao, M. H.; Wang, Z. L.; Mao, S. Nano Lett. 2004, 4, 587. (b) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (c) Wang, W. Z.; Zeng, B. Q.; Yang, J.; Poudel, B.; Huang, J. Y.; Naughton, M. J.; Ren, Z. F. AdV. Mater. 2006, 18, 3275. (4) (a) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (b) Tanaka, T.; Fukuda, K.; Ebina, Y.; Takada, K.; Sasaki, T. AdV. Mater. 2004, 16, 872. (c) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126, 5851. (d) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092.
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