Characterization and Optical Properties of the Single Crystalline SnS Nanowire Arrays
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The SnS nanowire arrays have been successfully synthesized by the template-assisted pulsed electrochemical deposition in the porous anodized aluminum oxide template. The investigation results showed that the as-synthesized nanowires are single crystalline structures and they have a highly preferential orientation. The ordered SnS nanowire arrays are uniform with a diameter of 50 nm and a length up to several tens of micrometers. The synthesized SnS nanowires exhibit strong absorption in visible and near-infrared spectral region and the direct energy gapEgof SnS nanowires is 1.59 eV.
KeywordsSnS nanowires Pulse electrodeposition Optical properties
Semiconductor nanostructures have been attracting worldwide attention due to their exceptional electrical, optical, and magnetic properties, and their potential applications in nanoscale electronics, photonics, and functional materials as well [1–3]. Among them, tin sulfide (SnS) has sparked intensive interest for its semiconducting and optical properties. SnS, as one of the important IV–VI group semiconductors, exhibits both the p- and n-type conduction , has an energy band gap of about 1.3 eV . Normally, SnS is composed of double layers of tightly bound Sn–S atoms and the bonding between layers are extremely weak Van der Vaals forces, which has an orthorhombic structure . Additionally, SnS has the advantage of its constituent elements being abundant in nature and not posing any health and environmental hazards. Therefore, SnS has a big potential to be used as solar absorber in a thin film solar cell and near-infrared detector [4, 5], as photovoltaic materials , and as a holographic recording medium . Therefore, single crystalline SnS nanowires reported in this paper are expected to offer enhanced properties. Therefore, it is important to investigate practical synthesis routes for novel SnS nanostructures, especially in single crystalline.
Crystalline tin sulfides have been prepared by a variety of methods, such as direct vapor transport method , stoichiometric composition technique , physical vapor transport method , and Bridgman–Stockbarger technique . In recent years, thin films of SnS have been investigated widely due to their applications in photovoltaic and photoelectrochemical solar cells. SnS thin films have been prepared by spray pyrolytic deposition , electrochemical deposition [4, 5], chemical vapor deposition [14, 15], and chemical bath deposition . To our knowledge, preparation of novel wire-like SnS nanostructures has been reported sparsely. Panda et al.  has reported surfactant-assisted synthesis of SnS nanowires grown on tin foils and SnS nanorods were reported by Biswas et al. . We had used the anodic aluminum oxide (AAO) template synthesized from some metal sulfide nanowire arrays [19, 20] and in this paper, we have presented single crystalline SnS nanowires prepared by template-assisted electrochemical deposition.
A three-electrode cell was used for pulse electrochemical deposition: a saturated calomel electrode (SCE) as the reference electrode, an AAO template with aluminum substrate as the working electrode (cathode), and a platinum sheet as the contrary electrode (anode). The deposition area was about 1 × 2 cm2. An aqueous bath containing 30 mM SnCl2 and 100 mM Na2S2O3 was used. The pH value of the solution was around 1.8 before deposition. The temperature of the solution was kept at 10 °C. The potential applied to the cathode was pulsed-form, its “on” potential Von was 10 V and “off” potential Voff was 0 V, both “on” time and “off” time were 10 s in all the voltage conditions. More details of the experiment can be seen in the Refs. [4, 20, 21]. Deposition period was 5 min. After deposition experiment, the deposited sample was washed softly in pure water, and naturally dried in air. All the chemicals used were analytical grade reagents and the water used was deionized distilled water.
The phase purity of as-synthesized product was examined by X-ray diffraction (XRD) using Rigaku Rint-2000 diffractometer with monochromatized CuKαradiation (λ = 0.15405 nm). The nano/microstructure of the SnS product was further observed by transmission electron microscope (TEM) and field-emission scanning electron microscope (FESEM) with an energy dispersive spectrometer (EDS) analysis attachment, which were performed on a Hitachi Model H-800 (200 kV) and a field-emission microscope (S-4800, 15 kV), respectively. The high resolution transmission electron microscope (HRTEM) image and the corresponding selected area electron diffraction (SAED) pattern were taken by a JEOL-2010 TEM with an accelerating voltage of 200 kV. UV–VIS–NIR absorption spectra were measured at room temperature with a Cary 5000 UV–VIS–NIR spectrometer.
Results and Discussion
Energy dispersive spectrometer (EDS) analysis reveals that the product is composed of stannum and sulfur, and the ratio of the S atom and Sn atom is 1:0.985, which just accords with the stoichiometric ratio of SnS.
where, A is a constant and n characterizes the transition process. We can see n = 2 and 2/3 for direct allowed and forbidden transitions, respectively, and n = 1/2 and 1/3 for indirect allowed and forbidden transitions, respectively.
The low-toxicity SnS nanowire arrays have been successfully synthesized using the template-assisted pulsed electrochemical deposition in the AAO template. The XRD pattern indicates that the nanowires are composed of SnS phase and have a highly preferential (101) orientation. The sample obtained in our experiment forms a stable orthorhombic superstructure. The TEM images show that the diameter of the SnS nanowires is about 50 nm and length up to several tens of micrometers. The SAED shows that the product is single crystalline structure. EDS result indicates that the ratio of S atom and Sn atom in our samples is 1:0.985, which just accords with the stoichiometric ratio of SnS. The synthesized SnS nanowires exhibit strong absorption in the visible and near-infrared spectral region. The direct energy gapEgof the SnS nanowires has been calculated as 1.59 eV, and this experimental optical band gap value is the evidence for the quantum confinement of the SnS nanowires.
This work was partially supported by the National Outstanding Youth Science Foundation of China (No. 50825101), and the National Natural Science Foundation of China (No. 50671087). The correspondence author (D. L. Peng) acknowledges the Minjiang Chair Professorship Program released by Fujian Province of P.R. China for financial support.