J. Phys. Chem. C 2007, 111, 12279-12283
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Photoswitchable Semiconductor Bismuth Sulfide (Bi2S3) Nanowires and Their Self-Supported Nanowire Arrays Haifeng Bao,†,‡ Xiaoqiang Cui,†,‡ Chang Ming Li,*,†,‡ Ye Gan,†,‡ Jun Zhang,†,‡ and Jun Guo§ School of Chemical and Biomedical Engineering, Center for AdVanced Bionanosystems, and School of Materials Science and Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457 ReceiVed: May 8, 2007; In Final Form: June 20, 2007
Self-supported Bi2S3 nanowire arrays with sizes up to several millimeters were prepared by a facile hydrothermal method. In our work, the oriented nanowire arrays were supported on a self-generated nanowire networked substrate. The as-prepared Bi2S3 nanowire exhibited nonlinear current-voltage (I-V) characteristics and excellent photoresponse. It is suggested that the rectifying behavior comes from the Schottky contact between the Bi2S3 nanowire and the Au electrodes. As the light source was switched on and off, the nanowire could be reversibly switched between low and high conductivity, indicating its potential applications in optoelectronic nanodevices.
1. Introduction Over the past decade, one-dimensional (1D) semiconductor nanomaterials have been the focus of scientific research due to their unique chemical/physical properties and potential applications in nanodevices.1-6 Nanorods,7 nanowires,8,9 nanotubes, and even nanobelts5,10 have been successfully synthesized, which offer great opportunities to investigate their novel electronic and optical properties for deep fundamental insights into nanoscience. More recently, much effort has been made to assemble functional 1D nanomaterials into two-dimensional (2D) and three-dimensional (3D) well-organized superstructures,3,9,11-17 which are considered to be a substantial step toward the fabrication of nanodevices.14 Bismuth sulfide (Bi2S3) is a semiconductor with a direct band gap of 1.3 eV and has potential applications in photovoltaic converters18-21 and thermoelectric-cooling technologies based on the Peltier effect.22 Recently, Qian et al. have reported the synthesis of Bi2S3 nanoribbons using the solvothermal process.23 Komarneni has prepared snowflake-like Bi2S3 nanorods24 and nanowires25 by a biomolecule-assisted method. The synthesis of Bi2S3 nanorods via a microwave-assisted ionic liquid approach26 and solvothermal,27,28 hydrothermal,29 and sonochemical methods30 has also been reported. However, most of the 1D Bi2S3 nanomaterials are randomly arranged in a powder form and the assembly of such 1D nanomaterials is still a great challenge. There are few reports on Bi2S3 arrays with an electrochemical fabrication, which requires porous anodic aluminum oxide (Al2O3) as template and supporting substrate.31,32 However, it is tedious and costive to fabricate the template. In this work, a simple and more controllable method to assemble nanowires into arrays is reported. In this paper, a one-step hydrothermal method is used to synthesize Bi2S3 nanowires, and novel self-supported nanowire arrays with up to millimeter sizes are obtained by simply controlling the concentration of reactant without any template and substrate. * Corresponding author. Telephone: +65 67904485. Fax: +65 67911761. E-mail:
[email protected]. † School of Chemical and Biomedical Engineering. ‡ Center for Advanced Bionanosystems. § School of Materials Science and Engineering.
Figure 1. XRD pattern of as-synthesized Bi2S3 products.
The organic molecule mercaptosuccinic acid (MSA) is employed as the sulfur source and plays an important role in the formation of Bi2S3 nanowire arrays. It is of course quite essential to investigate the electrical transport properties of an individual nanowire to explore the practical application of its nanowire arrays. In our work, the detailed investigation of the electrical transport properties of an individual nanowire was performed by homemade Au microchannel electrode pads in open air. The current-voltage (I-V) characteristics of the as-prepared Bi2S3 nanowire exhibit a unique rectifying behavior and the photoswitching response is fast and reversible under on/off light exposure conditions. In comparison to other works on the electrical transport of Bi2S3 nanowire,29,33,34 the photoconductivity behavior of our Bi2S3 nanowire exhibits great improvements in both good stability and quick photoresponse. These improvements can be attributed to the good crystallinity of the nanowires. Further investigation on the application of array devices is under way in our group.
10.1021/jp073504t CCC: $37.00 © 2007 American Chemical Society Published on Web 07/31/2007
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Figure 2. FESEM images of the three samples with 90 (A), 180 (B), and 360 (C-F) mg of MSA added in the crude solution. (A) is the powder form nanowires, (B) is the film adhering on the inside wall of Teflon autoclave. For the self-supported Bi2S3 nanowire arrays, (C) and (D) are the top view of nanowire arrays with different magnifications. (E) is the side view of the array; (F) shows the network-like substrate of the array.
2. Experimental Section Synthesis. All starting materials were obtained from commercial suppliers and were used without further purification. The water used was purified through a Millipore system. In a typical synthesis, 186 mg (0.5 mmol) of ethylenediaminetetraacetic acid disodium salt (EDTA-Na) was dissolved in 50 mL of purified water, followed by addition of 194 mg (0.4 mmol) of Bi(NO3)3‚5H2O. The mixture was ultrasonicated
until the white deposition hydrolyzed from Bi(NO3)3 was dissolved and the solution color changed to clear. Then 90 mg (0.6 mmol) of mercaptosuccinic acid (MSA) was added in with stirring. The solution above was finally transferred into a 50 mL Teflon-lined autoclave with a stainless steel shell. The autoclave was heated at 180 °C for 16 h and then cooled to room temperature gradually. Three samples were obtained by adding different amounts (90, 180, 360 mg) of mercaptosuccinic
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Figure 3. Low magnification SEM images from the top view (A) and the back view (B) of as-prepared self-supported Bi2S3 nanowire arrays transferred on carbon black paste belt.
acid (MSA) with stirring. The main products were collected, washed with distilled water three to four times, and air-dried for characterization. Sample Preparations for Characterization. The sample for X-ray diffraction (XRD) was supported on a glass slide. For the field-emission scanning electron microscopy (FESEM) characterization, the products solution was transferred onto a carbon black paste belt and dried at room temperature. For electrical transport measurements, the precipitations of nanowire arrays were suspended in water and treated ultrasonically for about 2 min. The number of nanowires across the Au electrode pair was determined by SEM. In the photoconductivity experiments, the green laser was induced on the nanodevices by an optical fiber (QMMF-RIVIS-365/400-0.73). Instrumentation. XRD was carried out using a Bruker AXS X-ray diffractmeter. The high-resolution transmission electron microscopy (HRTEM) images and the corresponding selectedarea electron diffraction (SAED) patterns were taken with a JEM 2100 (JEOL, Japan) electron microscope operating at 200 kV. The FESEM images were obtained with a JEOL Field Emission Electron Microscope JSM-6700F at an accelerating voltage of 5 kV. The I-V curves of Bi2S3 nanowire devices were measured using a semiconductor parameter analyzer Agilent E5270B in air condition. In photoconductivity experiments, the green laser and the visible light source were produced from a 100 mW LD pumped green (532 nm) laser manufactured by Fengyuan Optoelectronics Co. Ltd. and a GE Quartzline lamp, respectively. 3. Results and Discussion The obtained products were confirmed to be Bi2S3 by powder XRD. As Figure 1 shows, all peaks can be indexed to orthorhombic Bi2S3 with calculated lattice constants a ) 11.121, b ) 11.263, and c ) 3.973 comparable to the values of JCPDS Card 75-1306, and no impurity is detected in this pattern, indicating that the products have high purity. In the experiments, it was observed by the naked eye that the products of these three samples had different forms due to the amount of MSA added in the crude solution. The detailed morphology and structure of the three products were determined by FESEM. When 90 mg of MSA was added in, the product was a powder form of nanowires with widths of 100-400 nm and ultralong lengths of tens of micrometers (Figure 2A). When 180 mg of MSA was added in, the main product was a film adhered on the inside wall of the Teflon autoclave, which could
be exfoliated from the Teflon substrate by flowing water. As shown in Figure 2B, the film consists of only randomly oriented nanowires with diameters in the range of 60-300 nm and lengths of up to 10 µm. With an increase of the MSA amount to 360 mg, the main product was flakes floating in the water, which turned out to be an oriented nanowires array. The nanowires were oriented in a roughly perpendicular fashion and were arranged in large arrays, as shown in Figure 2C,D. As can be seen from the side view, fairly well-aligned nanowires, typically 60-300 nm wide and up to 10 µm long, grew on a self-generated substrate (Figure 2E). The structure of the selfgenerated substrate is nanowires networked with many nodes on the surface, as shown in Figure 2F. In the network, tens of nanowires with widths of 60-300 nm are connected together and stretched from each node. The result shows that the nanowire array has a thickness of about 10 µm. It is believed that only the formation of a nanowired network could support the nanowire array with sizes up to several millimeters (Figure 3). It is very interesting that the nanowire alignment on the top of the arrays is much better than that on the substrate, indicating that the nanowire alignment is caused by the space-limited growth. The results reveal that the flake has a novel structure of oriented Bi2S3 nanowire arrays grown on a nanowire network substrate. In our experiments, the organic molecule mercaptosuccinic acid (MSA) acts as the sulfur source. Upon hydrothermal heating, MSA is attacked by the strong nucleophilic O atoms of H2O molecules, leading to s slow release of S2- anion. The newly formed S2- reacts with bismuth ion to produce Bi2S3. This is similar to the biomolecule-assisted method reported by Komarneni.24 It was observed that the three samples were all nanowires, but the arrangements of the nanowires were quite different due to the amount of MSA added in the precursor solution, demonstrating that the molar ratio between MSA and Bi(NO3)3 determined the morphology and structure of the product. There were only powder and film adhered on the inside wall of the Teflon autoclave for Bi2S3 nanowires obtained with lower MSA concentration during the preparation. With increasing MSA concentration, the main product became nanowire arrays floating in the water demonstrated above. In view of the smooth network-like surface and the millimeter-scale size of the nanowire arrays (Figure 3), it is most likely that the floating arrays initially form on the surface of the Teflon substrate, and then exfoliate by the water flowing at high temperature and
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Figure 4. (A) TEM image of a Bi2S3 nanowire and the corresponding SAED pattern (inset) taken along the [010] zone axis. (B) HRTEM image of the Bi2S3 nanowire. The lattice planes with spacings of 0.55 and 0.37 nm correspond to the d spacings of the (200) and (101) planes, respectively, of orthorhombic phase Bi2S3. The angle between the two planes is 70.4°.
Figure 5. Typical rectifying I-V characteristics of single Bi2S3 nanowire devices in air environment at room temperature. (A) and (B) were measured under dark condition and upon green laser exposure, respectively. Inset a of (A) shows the SEM image of a single Bi2S3 nanowire across the pair Au electrodes; the scale bar is 5 µm. Inset b is the schematic view of the Au-Bi2S3 semiconductor-Au structure.
pressure. Considering most nanowire arrays are aligned on heterogeneous substrates, such as silicon wafers13,15 and metal surfaces,16,17 the oriented self-supported Bi2S3 nanowire arrays are an exciting addition to a fast-growing family of oriented nanowires and nanorods, which can provide a new method for fabrication of oriented nanostructures on self-generated homogeneous substrates. The crystallinity of an individual nanowire was analyzed by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Figure 4A is a TEM image of a Bi2S3 nanowire, and the inset is the corresponding SAED pattern, which reveals that the nanowire is single crystalline. The high-resolution TEM shows in Figure 4B illustates that the nanowire has lattice planes with spacings of 0.55 and 0.37 nm, corresponding to the d spacings of the (200) and (101) planes, respectively, of orthorhombic phase Bi2S3. The angle between the two planes is 70.4°. Further studies of the SAED pattern and HRTEM image demonstrate that the nanowires grow nearly exclusively in the [001] direction. The orientation of the lattice planes found in the HRTEM images is also consistent with the above XRD pattern, which exhibits an increased relative (200) peak intensity compared to the standard pattern for bulk Bi2S3.
The I-V curves of nanodevices based on single or several Bi2S3 nanowires were measured. To fabricate such nanodevices, Bi2S3 nanowires were first dispersed ultrasonically in distilled water and then spin-coated on a substrate which consisted of 3 × 3 or 2 × 2 pairs of predefined Au electrodes. The Au electrodes were patterned on a silicon wafer with a ∼100 nm SiO2 layer and had a channel length of 4 µm. Inset a, an SEM image, of Figure 5 shows the individual nanowire across the paired Au electrodes. The I-V characteristic of the nanodevices fabricated by this process displays a rectifying behavior at room temperature in air environment as shown in Figure 5A. The I-V measurement shows a nonlinear characteristic. The current increases rapidly when the bias voltage is more than 0.5 V. This phenomenon is similar to the I-V characteristic of the metal-semiconductor-metal structure, which can be explained by two back-to-back Schottky diodes. In this situation, the nanowire-metal contact can be considered as a Schottky diode (Figure 5, inset b). On the occasion that the electron energy is high enough to overcome the Schottky barrier height at the AuBi2S3 interface, the electrons could transport through the semiconductor nanowire. No matter how the voltage is polarized, one of the two Schottky contacts must be reverse biased when
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Figure 6. Reversible switching of a Bi2S3 nanowire between low and high conductivity states when the green laser (A) and the quartz lamp (B) were turned on and off. The bias on the nanowire is 3 V.
a bias voltage is applied to the electrodes. Under the high reverse-biased electric field condition, impact ionization may occur and induce a large amount of electrons, and the energetic electrons can overcome the energy barrier or tunnel through the barrier, resulting in a rapid increase in current. Similar rectifying behavior of a ZnO nanobelt has been reported by Wang et al.35 It is worth noting that the measurements are performed under open-air condition, indicating that the nanowires have better stability than that of reported Bi2S3 nanowires that are performed under vacuum27,30 or in a nitrogen environment.31 The dark current and the photoresponse of such nanodevices in our work were measured by illumination with a laser and a visible light source. The I-V characteristics under laser exposure also displayed a rectifying behavior like the dark condition (Figure 5B). Dark conductivity of ∼1.2 × 10-3 S/cm was obtained from calculation of the experimental data for this nanowire. The conductivity of the nanowire rapidly increased by ∼15 times upon exposure to the laser (λmax ) 532 nm). Similar changes in photoconductivity were also observed when a quartz lamp was used. In these cases, the energy from the light excited the electrons in the semiconductor Bi2S3 from the valence band into the conduction band, increasing the charge carrier concentration via direct electron-hole pair creation and thus enhancing the conductivity of the nanowire. Figure 6shows the photoresponse as a function of time when the green laser and the quartz lamp were switched on and off, which indicated that Bi2S3 nanowires could be reversibly switched for many times between low and high conductivity. The rise and decay times of the nanowire switches are less than 1 s, exhibiting faster photoresponse than the results reported by Korgel et al.31 The photoconductivity characteristics suggest that Bi2S3 nanowires are good candidates for optoelectronic switches.6 4. Conclusion In summary, a novel millimeter-sized Bi2S3 nanowire array was spontaneously formed via a facile hydrothermal approach. Such an approach does not require any template or surfactant to control the orientation. This may provide a new method for the growth of oriented nanowires on a self-generated substrate. The photoswitchable conductivity of individual Bi2S3 nanowires was studied, indicating possible applications in optoelectronic nanodevices. The fabrication and electrical transport properties of devices based on Bi2S3 nanowire arrays are under further investigation. References and Notes (1) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455.
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