In this study, β-In2S3nanowires were first synthesized by sulfurizing the pure Indium (In) nanowires in an AAO membrane. As FE-SEM results, β-In2S3nanowires are highly ordered, arranged tightly corresponding to the high porosity of the AAO membrane used. The diameter of the β-In2S3nanowires is about 60 nm with the length of about 6–8 μm. Moreover, the aspect ratio of β-In2S3nanowires is up to 117. An EDS analysis revealed the β-In2S3nanowires with an atomic ratio of nearly S/In = 1.5. X-ray diffraction and corresponding selected area electron diffraction patterns demonstrated that the β-In2S3nanowire is tetragonal polycrystalline. The direct band gap energy (Eg) is 2.40 eV from the optical measurement, and it is reasonable with literature.
KeywordsNanomaterials In2S3 Nanowire AAO
Nowadays, one-dimensional (1-D) nanomaterials (nanorods, nanowires, nanobelts and nanotubes) 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 . Various methods such as chemical vapor deposition (CVD), laser ablation, thermal evaporation, hydrothermal process, and anodic aluminum oxide (AAO) membrane-assisted synthesis method have been employed to prepare 1-D nanomaterials. AAO membrane-based assembling has been widely applied in recent years to produce nanowires with extremely long length and high aspect ratios, and it also provides a simple, rapid and cheap way for fabricating nanowires as aligned arrays [4, 5].
Indium sulphide (In2S3) is an III-VI compound material, and it exists in three phases at normal pressure: α (cubic), β (tetragonal) and γ (trigonal) . β-In2S3, an ordered-defect superstructure of the defect-spinel α-In2S3, is stable below 420 °C , whereas γ-In2S3 is only stable above 754 °C, unless by adding As or Sb . β-In2S3 crystal is known an n-type semiconductor with the band gap energy of 2.0 eV . Furthermore, its photoconductive properties make it a promising candidate for photovoltaic applications such as solar cells . Solar cell devices prepared by using β-In2S3 as a buffer layer show 16.4% conversion efficiency, which is very close to that of the standard CdS buffer layer .
A variety of methods have been developed into synthesized In2S3 powder, thin films, and nanofibers. Powder of β-In2S3 was synthesized using indium chloride (InCl3) and thioacetamide (CH3CSNH2) as precursors via the sonochemical route . β-In2S3 thin films were prepared using the CSP technique, and the spray solutions were mixtures of indium chloride (InCl3) and thiourea (CS(NH2)2) . In addition, β-In2S3 nanofibers were prepared using indium chloride and thiourea as source precursors by hydrothermal method with AAO membrane at 150 °C for 15 h. . However, β-In2S3 nanowires were fabricated within AAO template synthesis, using the electrodeposition and sulfurizing methods that are not yet reported.
To fabricate the β-In2S3nanowires, an effective and economical technique-anodic alumina oxide (AAO) membrane-assisted method was utilized in this study. The microstructure and optical properties of β-In2S3nanowires are discussed.
Porous anodic aluminum oxide (AAO) membrane with average channel diameter of 60 nm and thickness of 25 μ m was fabricated by two-step anodization process as described previously [4, 5]. First, high purity aluminum sheet (99.9995%) was anodized in the oxalic acid solution under constant voltage 40 V for several hours. Subsequently, the anodized Al sheet was put into H3PO4 solution to completely remove the alumina layer. The AAO membrane can be fabricated by repeating the anodization process under the same conditions of the first step anodization. After the anodization, the AAO membrane was obtained by etching away the underlying aluminum substrates with HgCl2 solution. The transparent AAO membrane was immersed in H3PO4 solution to widen the nanochannels. Finally, the diameter of the nanochannel was about 60 nm.
In order to prepare for pure Indium (In) nanowires, a layer of Pt was sputtered onto one side of the membrane acting as the working electrode in a standard two-electrode electrochemical cell. Pure Indium (In) was electrodeposited inside the nanochannels of the AAO membrane under constant voltage, using an electrolyte containing In(SO4)3, H3BO3,and distilled water. The AAO membrane with pure Indium (In) nanowires was washed with distilled water and air dried, put into a glass tube and together with sulfur powder (99.99%). 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 held at this temperature for 10 h to completely sulfurize the pure Indium (In) nanowires. It is expected that S atoms would react with the In to form β-In2S3.
The morphologies of the as-prepared AAO membrane and the β-In2S3nanowires were analyzed by field emission scanning electron microscopy/energy dispersive spectrometer (FE-SEM/EDS, HITACHI S-4800). The crystal structure of the nanowires was examined by X-ray diffraction (XRD, SHIMADZU XRD-6000) utilizing Cu Kα radiation. More details about the microstructure of the β-In2S3nanowires 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 β-In2S3nanowires 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 membrane was dissolved by NaOH solution at room temperature and washed with distilled water to expose freely nanowires of β-In2S3. After the β-In2S3nanowires are absolutely dispersed in distilled water using a supersonic disperser, the absorption spectra of the β-In2S3nanowires were measured on an UV/Visible/NIR spectrophotometer (HITACHI U-3501).
Results and Discussion
In which the S atoms would react with In atoms at high temperatures to form β-In2S3nanowires. The completeness of reaction is mainly dominated by time and temperature, long periods and high temperatures of the sulfurization process were needed to prepare the fine crystalline β-In2S3nanowires. We observed that the β-In2S3nanowires were completely formed when the sulfurization time reached to 10 h and the temperatures up to 500 °C.
where hv is the photon energy, A is the optical transition dependent constant, Eg is the optical band gap, and m is a constant that depends on the types of transitions involved. The values of m = 1/2, 3/2, 2, and 3 are for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. This equation gives band gap (Eg), when straight portion of (αhν)1/m against hν plot is extrapolated to the point α = 0. The analysis of the absorption spectra obtained for our samples shows that the spectral variation of the absorption coefficient, within the fundamental absorption region can be fitted by Eq. 2. However, m = 3/2, 2, and 3, the band gap energies were found to be a negative number, which is not reasonable in physics. The inset in Fig. 4 shows the (αhν)2 against hν plot. Relationship fitting to the absorption spectra of β-In2S3 as m = 1/2, which is the allowed direct transition for these nanowires. In this inset figure, we observed that the curve has a very good straight line fit from 2.7 to 2.9 eV. This result indicates that the optical energy gap is direct transition. We also observed that the curve has been breakaway this line range when the photon energy is above 2.9 eV. The band gap energy (Eg) of β-In2S3 nanowires with diameter of about 60 nm is estimated to be 2.40 eV as m = 1/2 for extrapolation. In literature , powder of β-In2S3 was synthesized using InCl3 and CH3CSNH2 as precursors followed by annealing in Ar atmosphere in the temperature range 573–1,123 K. The grain size of powder varied in the range 0.40–1.48 μm for different annealing temperatures. The band gap energy of β-In2S3 powder for various grain sizes was between 2.12 and 2.14 eV. Moreover, thin films of In2S3 were prepared from In2S3 powder using vacuum evaporation with resistively heated graphite crucible . The average grain sizes of In2S3 thin film were around 0.22 μm. The optical band gaps of In2S3 thin film for different annealing temperatures and times were between 1.75 and 2.19 eV. β-In2S3 films were prepared by rapid heating of metallic Indium (In) films in H2S atmosphere . The grain sizes of the β-In2S3 films were in the range 22–30 nm. The films exhibit the higher band gap of 2.58 eV. Consequently, our band gap energy of β-In2S3 nanowires with 60 nm is between the largest band gap energy with grain sizes of 22–30 nm (2.58 eV) and the smallest band gap energy with grain sizes around 0.22 μm (1.75 eV). Our band gap energy of β-In2S3 nanowires with 60 nm is acceptable with literature [19–21].
In summary, we have presented a simple, inexpensive, and reasonable method to fabricate β-In2S3nanowires. The β-In2S3nanowires were first fabricated by sulfurizing the pure Indium (In) nanowires at 500 °C for 10 h, which is embedded in an AAO membrane. β-In2S3nanowires have high wire packing densities with uniform wire diameters and lengths of about 60 nm and 6–8 μm, respectively. The analysis of the HR-TEM/SAED revealed that the β-In2S3nanowire is polycrystalline. The β-In2S3nanowires exhibited a linear relationship at 2.7–2.9 eV asm = 1/2, indicating that the direct band gap energy is 2.40 eV.
The research was supported by the National Science Council of R.O·C. under grant No. NSC-96-2122-M-035-003-MY2.