Hydrothermal Synthesis of Sb 2 S 3 Nanorods Using Iodine via Redox Mechanism

1 Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51664, Iran 2 WCU Nano Research Center, School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea 3 Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran 14115-175, Iran 4 Center for Research Facilities, Yeungnam University, Gyongsan 712-749, Republic of Korea


Introduction
Antimony sulfide, a layer-structured direct-band-gap semiconductor with orthorhombic crystal structure, is an important semiconductor with high photosensitivity and high thermoelectric power [1]. In the past few years, main-group metal chalcogenides such as A 2 B 3 (where A = As, Sb, Bi and B = S, Se, Te) as significant semiconductors have received ever-increasing attention. Due to its good photoconductivity, Sb 2 S 3 has received significant attention for potential application in solar energy conversion [2]. It has also been used in switching devices [3], thermoelectric cooling technologies, optoelectronics in the IR region [4,5], microwave devices [6], and television cameras [7]. Sb 2 S 3 exists in two forms: orange amorphous phase and black orthorhombic modification with a ribbon-like polymeric structure along the [001] direction as building blocks [8]. Each Sb atom and each S atom are bonded to three atoms of the opposite kind within the ribbon-like polymeric structure, forming interlocking SbS 3 and SSb 3 pyramids. Consequently, amorphous Sb 2 S 3 tends to crystallize into one-dimensional shape to support the stronger intrachain covalent bonds over the relatively weak secondary interchain interaction, during the period of crystallization and lattice arrangement, as what is found in chain-structured trigonal selenium [9]. Over the past two decades, many methods have been employed to prepare Sb 2 S 3 including thermal decomposition [10], solvothermal reaction [11,12], microwave irritation [13], vacuum evaporation [2], and other chemical reaction approaches. Besides an elemental reaction, Sb 2 S 3 can be prepared by chemical routes, such as sodium thiosulfate and thioacetamide, ammonium sulfide, and thiourea, as well as with complex agents in aqueous or nonaqueous solution. Li et al. [14] have reported a hydrothermal growth of Sb 2 S 3 nanorods without the existence of catalysts or templates. In recent years, the solvothermal method has been applied to synthesize Sb 2 S 3 2 Journal of Nanomaterials  nanoparticles, nanorods, and microtubular Sb 2 S 3 crystals. Polygonal bulk tubular Sb 2 E 3 (E = S, Se) crystals and stibnite nanorods were prepared via the solvothermal route by Zheng et al. [15] and Qian et al. [16], respectively. However, in these methods, the reaction temperature was usually high and the products were usually impure. Therefore, the development of facile, mild, and effective methods for creating novel architectures based on nanorods/submicrometer-sized rods or nanoparticles still remains a great challenge. Recently, there has been a strong trend towards the application of solution chemical synthesis techniques to materials preparation, in which the particle size and distribution, phase homogeneity, and morphology of materials could be well controlled [17]. In this study, Sb 2 S 3 nanorods were prepared via hydrothermal method by using antimony, sulfur, and iodine in elemental form as raw materials. This is a new route for the preparation of Sb 2 S 3 nanomaterials. Elemental iodine is an oxidizing irritant and acts as an initiator material in the reaction of elemental antimony and sulfur. Without iodine, no reaction is occurred. Using oxidation reagent like iodine as an initiator of redox reaction to prepare Sb 2 S 3 is reported for the first time.

Experimental
All the reagents were of analytical grade and were used without further purification. In a typical procedure, 2 mmoL Sb, 3 mmoL S, and 1 mmoL I 2 were added to 50 mL distilled water and stirred well for 20 min at room temperature. Then, the mixture was transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 180 • C for 24 h, and cooled at room temperature, naturally. The black precipitate was filtered and washed with dilute chloride acid and water. Yields for the products were 95%. Finally, the obtained sample was dried at room temperature and used for characterization. The best conditions for this reaction are pH 12, temperature 180 • C, and time of reaction 24 h.Under other conditions, some impurity is seen in XRD patterns and EDS related to unreacted raw elements or formation of antimony oxides.The crystal structure of the product was characterized by X-ray diffraction (XRD D500 Simens) with CuKα radiation (λ = 1.5418Å).The morphology of materials were examined by a scanning electron microscope SEM (Hitachi S-4200).The HRTEM image and SAED pattern were recorded by a Cs-corrected high-resolution TEM (JEM-2200FS, JEOL) operated at 200 kV. The TEM sample was prepared by using an FIB (Helios Nanolab, FEI). Elemental analysis was carried out using a linked ISIS-300, Oxford EDS (energy dispersion spectroscopy detector). In order to further confirm the chemical compositions of these nanomaterials, elemental composition analysis was performed by EDXS. Figure 2 shows a typical EDXA spectrum recorded on single crystals, whose peaks are assigned to Sb and S. The atom ratio of Sb and S are 2 : 3 according to EDXA. This data indicates that we have obtained pure Sb 2 S 3 single crystals.

Results and Discussion
The morphology of as-prepared Sb 2 S 3 at 180 • C and 24 h was examined by SEM indicating the length of nanorods up to 4 μm and 50-140 nm as diameter (Figure 3). Figure 4(a) shows TEM image of as-prepared Sb 2 S 3 nanorods. Also, the typical HRTEM image recorded from the same nanorods is shown in Figure 4(b). The crystal lattice fringes are clearly observed, and average distance between the neighboring fringes is 0.79 nm, corresponding to the [110] plane lattice distance of orthorhombic-structured Sb 2 S 3 , which suggests that Sb 2 S 3 nanorods grow along the  direction. The SAED pattern of the nanorods indicates its single-crystal nature and long axis .
To explain the synthesis process, possible chemical reaction involved in the synthesis of Sb 2 S 3 could be assigned to iodine and antimony standard electrode potential values. Considering the values of standard electrode potentials of Sb 3+ /Sb (E 0 = 0.20 V) and I 2 /I − (E 0 = 0.54 V), the oxidation reaction between Sb and I 2 is possible In terms of electrochemistry, since difference of cathodic and anodic standard electrode potentials values is positive, this redox reaction can occur. In aqueous solution, Iodine and I − form complex of I 3 − which dissolve in water and makes a yellowish solution. The existence of I − was examined by the formation of red precipitate of Hg 2 I 2 Disproportion of sulfur in this solution is another possibility.
Besides the nature of sulfur, the temperature and pressure of autoclave help to disproportion sulfur Because the precipitate of Sb 2 S 3 has a great stability (Ksp = 1.7 × 10 −97 ), the black precipitate of Sb 2 S 3 is formed as soon With regard to oxygen standard electrode potential, as long as difference of cathodic and anodic standard electrode potential values is negative, getting electron from it in order to form S 2− is impossible S + 2e S 2− E 0 = 0.14 V During the precipitation of Sb 2 S 3 , the conditional electrode potential equals E 0 = E 0 + 0.06 Pksp, and therefore a reaction of Sb 3+ and S 2− with high rate rather than primary rate is done. As Sb 2 S 3 is a narrow band gap semiconductor (Eg is 1.7 ev for bulk), with decreasing diameter to nanoscale, novel optical properties may be observed [18]. The photoluminescence (PL) spectrum of synthesized antimony sulfide, shown in Figure 5, has an excitation peak at 348 nm ( Figure 5(a)), and the emission peak can be observed at 450, 500 nm ( Figure 5(b)). The UV/Vis spectrum (prepared by dispersion of Sb 2 S 3 products in ethanol) shows an absorption band at 215 nm with band gap around 2.50 ev which indicates a blue-shift phenomenon, as commonly observed for nanomaterials ( Figure 6).
Most of the materials have different structural defects that create defect energy levels between band gaps of material. These defects result in difference between the UV absorption and PL excitation spectra.

Conclusion
In summary, a redox reaction approach in hydrothermal condition has been developed to prepare Sb 2 S 3 nanorods with high yield at 180 • C and 24 h. The length of nanorods is up to 4 μm, and their diameter is around 50-140 nm. Using iodine as an initiator of oxidation-reduction reaction is reported for the first time. The formation mechanism of Sb 2 S 3 based on redox reaction is proposed. In the current process, I 2 plays an important role in the formation of Sb 2 S 3 nano materials, and other oxidizing agents can be worthwhile for preparing nanostructures in the future. As a common feature for nanomaterials, a blue shift was observed in the case of optical absorption.