The ordered tin disulfide (SnS2) nanowire arrays were first fabricated by sulfurizing the Sn nanowires, which are embedded in the nanochannels of anodic aluminum oxide (AAO) template. SnS2nanowire arrays are highly ordered and highly dense. X-ray diffraction (XRD) and corresponding selected area electron diffraction (SAED) patterns demonstrate the SnS2nanowire is hexagonal polycrystalline. The study of UV/Visible/NIR absorption shows the SnS2nanowire is a wide-band semiconductor with three band gap energies (3.3, 4.4, and 5.8 eV).
KeywordsNanomaterials SnS2 Nanowire AAO Template
In recent years, one-dimensional (1-D) nanostructural materials have been attractive due to their physical and chemical properties. These nanostructures in particular show results in electronics , magnetic , optics, etc., that have great potential applications in the next generation of nanodevices . Anodic aluminum oxide (AAO) template-based assembling has been widely applied in recent years to produce nanowires with extremely long length and high aspect ratio, and it also provides a simple, rapid, and cheap way for fabricating nanowires as aligned arrays .
Tin disulfide (SnS2) is an n-type semiconductor with hexagonal cadmium iodide (Cdl2) structure. It is composed of sheets of tin atoms sandwiched between two close-packed sheets of sulfur atoms . Single crystal and polycrystalline films of SnS2 have shown optical band gaps in the range of 2.12–2.44 eV . As an important member of the IV–VI group semiconductors, SnS2 is a possible choice in solar cells and optoelectronic devices . A variety of methods have been developed into synthesized SnS2 nanoparticles, films, and crystals. SnS2 nanoparticles were synthesized through the microware plasma process either by the reaction between chloride or carbonyl of tin and H2S . The films of SnS2 were prepared by chemical deposition from an acidic medium solution containing Sn4+ ions . Single crystals of SnS2 have been grown by the vapor phase . In addition, SnS2 nanocrystallites with hexagon flake shape were synthesized by a hydrothermal reaction between SnCl4 · 5H2O and (NH2)2CS . SnS2 nanobelts were produced from SnCl2 · 2H2O and Na2S by a thioglycollic acid (TGA) assisted hydrothermal method . However, SnS2 nanowires are fabricated within AAO template synthesis, using the electrodeposition and sulfurizing methods that are not yet reported.
For this purpose, we report a new synthesis method for semiconductor SnS2nanowires. Pure metal Sn is electrodeposited in the nanochannels of the AAO template. Sn nanowires are sulfurized in the S atmosphere to form SnS2nanowires. According to past research, our first successful fabrication attempt was sulfurizing the Sn nanowires that were electrodeposited in the AAO template and studying its properties.
Preparation of AAO Template
The AAO template used in our experiment was prepared by a two-step anodization process as described previously . Briefly, high purity aluminum sheet (99.9995%) was first anodized at constant voltage in the sulfuric acid solution for 3 h. After anodization, the anodized Al sheet was put into an acid to completely remove the porous layer. Then, the AAO template can be fabricated by repeating the anodization process under the same conditions of the first step anodization. The AAO template was obtained by etching away the underlying aluminum substrates with a mercuric chloride solution. The transparent AAO template was immersed in a phosphoric acid solution to widen the nanochannels. After this process, the diameter of the nanochannel was about 40 nm.
Preparation of SnS2Nanowires
In order to prepare Sn nanowires, a platinum (Pt) film was deposited by vacuum evaporation onto one surface of the AAO template to provide a conductive contact. The Sn nanowires were electrodeposited in the pore of the nanochannels of AAO template under constant voltage, using an electrolyte containing SnSO4and distilled water. After washing with distilled water and air drying, the AAO template with Sn nanowires was put into a glass tube with the pure S powder together. The glass tube was evacuated by using a pump, and it was placed into the furnace. The samples were then heated from room temperature (heating rate: 5 °C/min) to 500 °C and kept at this temperature for 10 h to completely sulfurize the Sn nanowires. It is expected that S atoms would react with the metal Sn to form SnS2. After the reaction was terminated, the furnace was naturally cooled down to room temperature and SnS2nanowires were completely formed after sulfurization.
Characterization of SnS2Nanowires
The morphology and microstructure of the as-prepared SnS2nanowire arrays were characterized by field emission scanning electron microscopy/energy dispersive spectrometer (FE-SEM/EDS, HITACHI S-4800). The identification of the crystallization and phase structure were analyzed by X-ray diffraction (XRD, SHIMADZU XRD-6000) utilizing Cu Kα radiation. More details about the microstructure of the SnS2nanowires were investigated by the high-resolution transmission electron microscopy/corresponding selected area electron diffraction (HR-TEM/SAED, JEOL JEM-2010). For HR-TEM and SAED analysis, the SnS2nanowires were dispersed in ethanol and vibrated for few minutes. Then, a few drops of the resulting suspension were dripped onto a copper grid. For optical analysis, the AAO template was dissolved by NaOH solution at room temperature and was washed with distilled water to expose freely nanowires of SnS2. After the SnS2nanowires are absolutely dispersed in distilled water using a supersonic disperser, the absorption spectra of the SnS2nanowires were measured on an UV/Visible/NIR spectrophotometer (HITACHI U-3501).
Results and Discussion
Morphology of AAO Template and SnS2Nanowires
Crystal Structures of SnS2Nanowires
Optical Properties of SnS2Nanowires
SnS2nanowires arrays have been successfully fabricated within template synthesis by using the electrodeposition and sulfurizing methods. The results show SnS2nanowires have high wire packing densities with uniform wire diameters and lengths of about 40 nm and 3–5 μm, respectively. In XRD results, after sulfuring the Sn nanowire at 500 °C for 10 h, the sulfured nanowires have SnS2phase and a preferred orientation (011). The analysis of the HR-TEM/SAED reveals the SnS2nanowire is polycrystalline. The SnS2nanowires show three band gap energies (3.3, 4.4, and 5.8 eV) and exhibit a linear relationship at 3.9–4.75, 5.4–6.1, and 6.2–6.35 eV, respectively asm = 1/2. The absorption spectra of nanowires contain three spectral intervals with the shapes typical for direct allowed interband transitions with the effective bandgaps.
The research was supported by the National Science Council of R.O·C. under grant No. NSC-96-2122-M-035-003-MY2.