Synthesis of Coral-Like, Straw-Tied-Like, and Flower-Like Antimony Sulfides by a Facile Wet-Chemical Method

1 Division of Chemistry, School of Science, University of Phayao, Phayao 56000, Thailand 2Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand


Introduction
Sb 2 S 3 is an orthorhombic V-VI semiconductor with 1.78-2.50eV direct band gap, covering the visible and near IR range of solar spectrum [1].It has a potential application for solar cells and thermoelectric and optoelectronic devices [1,2].A number of processes were used to synthesize antimony sulfide with different morphologies: single-crystal Sb 2 S 3 nanotubes via EDTA-assisted hydrothermal route [1], nanocrystalline Sb 2 S 3 by microwave-assisted synthesis [2], Sb 2 S 3 peanutshaped superstructures [3], rod-like Sb 2 S 3 dendrites [4] and Sb 2 S 3 nanorods [5] by hydrothermal reaction, double sheaflike Sb 2 S 3 by copolymer-assisted hydrothermal synthesis [6], Sb 2 S 3 nanowires [7] and Sb 2 S 3 nanoribbons [8] by solvothermal route, Sb 2 S 3 nanowires by PEG-assisted solvothermal process [9], and orthorhombic Sb 2 S 3 twin flowers in the solutions containing CTAB by a cyclic microwave radiation [10].Crystal structures, crystalline degree, different phases, purities, defects, and others can play a role in the properties of materials, such as strength and corrosion resistance, including electrical and thermal conductivities.Previously, Sb 2 S 3 with different morphologies was successfully synthesized by different methods: straw-tied-like architectures by a one-pot hydrothermal method both with and without CTAB as an additive [11], hydrothermal synthesis [12], a novel precursorsolvothermal-pyrolysis route [13], and a refluxing polyol process of the solution containing PVP as a surfactant [14], including flower-like Sb 2 S 3 by a refluxing polyol process of the solution containing PVP as a surfactant [14] and a refluxing method of the solution containing PEG400 as a surfactant [15].The above methods require high temperature and pressure, different additives, and complicated equipment.Thus, the purpose of the present research is to synthesize Sb 2 S 3 with different morphologies by a facile wet-chemical method without using a template.This method is novel, very simple, and inexpensive.The influences of the experimental parameters on the structures and morphologies were also investigated.

Experiment
A wet-chemical route was used in this research of which 0.002 mol SbCl 3 (assay: 99%, Sigma-Aldrich) and 0.003 mol

Results and Discussion
Figure 1 shows XRD spectra of the products synthesized under different temperatures and lengths of time.At different temperatures of 180, 160, 140, and 120 ∘ C for 60 min, the products were specified as orthorhombic Sb 2 S 3 (JCPDS number 06-0474) [16] with pure crystal.To save energy consumption, the temperature was reduced in series of steps from 180 to 120 ∘ C. The results show that the intensity of XRD spectra became lowered and the crystalline degree was lessened.At 180 ∘ C for different lengths of time in EG, XRD spectra became sharper and narrower by increasing the lengths of time from 30 to 60 min, and the crystalline degree was improved in sequence.The orange colloidal complexes were the intermediate products, which were subsequently transformed into Sb 2 S 3 black precipitates by heating at high temperatures: Orange colloidal complexes high temperature heating in oven SEM images (Figures 2 and 3) show Sb 2 S 3 products synthesized at different temperatures and lengths of time.Their surfaces were smooth and clean.Increasing in the temperatures and lengths of time has the influence on the change in morphology of the products.For those synthesized at different temperatures, the as-produced Sb 2 S 3 was straw-tied-like architecture at 120 ∘ C for 60 min.Then, the temperature was increased to 140 ∘ C with the length of time kept constant for 60 min; the product transformed into corallike architecture.Finally, the products gradually transformed into nanostructured flowers at 160 and 180 ∘ C within 60 min.Moreover, the length of time also has the influence on the morphology formation of Sb 2 S 3 .SEM images of the products at 180 ∘ C for different lengths of time were compared.The Sb 2 S 3 product, synthesized at 180 ∘ C for 30 min, was straw-tied-like architecture with a number of rods split at their ends.The extent of opening or splitting was influenced by the temperatures and lengths of time.As time passed, nanorods formed, grew, and split at their ends [17,18].Hence, the flower-like architecture began to form at 180 ∘ C for 45 min and constantly formed at 180 ∘ C for 60 min.At this stage, the as-synthesized product became complete flowers.The present phenomenon is similar to the report of Zhu et al. [15] who stated that flower-like structured Sb 2 S 3 could be transformed into nanorods by increasing in the length of time from 1 to 2 h during 85 ∘ C refluxing of the solutions containing PEG400.Previously, orthorhombic Sb 2 S 3 twin flowers, composed of single crystalline square nanorod petals, were successfully synthesized in the solutions containing CTAB as a template and splitting agent by a cyclic microwave radiation [10], including the flower-like Sb 2 S 3 by refluxing polyol process of the solution containing PVP as a surfactant [14] and by refluxing method of the solution containing PEG400 as a surfactant [15].But for the present research, Sb 2 S 3 coral-like and flower-like architectures were successfully synthesized by a facile wet-chemical method without the use of any template and splitting agent.TEM and HRTEM images, SAED, and simulated patterns [19] of Sb 2 S 3 synthesized in EG at 180 ∘ C for 60 min are shown in Figure 4. TEM and HRTEM images show the flowerlike architecture which consisted of nanorods.SAED and simulated patterns of the synthesized Sb 2 S 3 also show that each flower was composed of Sb 2 S 3 nanorods, growing along the [001] direction (along the c-axis)-in accordance with the growth direction characterized by Yang et al. [2], Wang et al. [11], Ota et al. [17], and Hu et al. [20].The (110) planes along the growth direction were also detected, showing that the nanorod was single crystal.In addition, the interpreted SAED pattern, composed of bright spots in lattice array, was specified that each nanorod was single crystal, corresponding to orthorhombic Sb 2 S 3 [16].To justify the presence of Sb 2 S 3 single crystal, its pattern was simulated [19] and appears as systematic spots-in good accordance with the above interpretation.Figure 5 shows the EDX spectroscopy measurement of Sb 2 S 3 synthesized at 180 ∘ C for 60 min.It shows the characteristic peaks of only S (K 1,2 at 2.31 keV) and Sb (L  at 3.61 keV and L 1 at 3.84 keV), suggesting that the product contains only antimony and sulfur elements.Quantitative EDX analysis shows that the atomic ratio of Sb : S was 40.85 : 59.15, close to 2 : 3, confirming that the product was really crystalline Sb 2 S 3 .It should be noted that Cu of the stub was also detected at 0.93 keV (L  ) and 8.04 keV (K 1,2 ).
An optical absorption experiment has been carried out using a UV-vis absorption spectrometer, which provides an effective method for explaining some features concerning the band structure of Sb 2 S 3 nanorods.Figure 6 shows the (ℎ]) 2 and (ℎ]) plot for the direct allowed transition, where , ℎ, and ] are the total absorption coefficient, Planck constant, and photonic frequency, respectively [1,15].In this research, the photonic wave attenuated through the solid.The change of photonic absorption was controlled by two photonic energy ranges.When the photonic energy is higher than the energy gap, absorption was linearly increased with the increasing of photonic energy.But for the photonic energy with less than the energy gap, the absorption became different from linearity which was influenced by the absorption relating to defect levels between the valence and conduction bands of the product.Its band gap, determined by extrapolation of the linear curve to zero absorbance, corresponds to 1.52 eVin accordance with 1.52 eV band gap of Sb 2 S 3 nanorods determined by Zhu et al. [15].It should be noted that this value is less than the 2.43 eV of the as-deposited thin film [21] and 1.95 eV thin film deposited on the 300 K substrate [22], which were lessened by high-temperature annealing [21,22].This value is blue shift relative to 1.28 eV energy gap of Bi 2 S 3 [23].Generally, different morphologies and crystalline degrees can play a role in the properties of products, such as energy gap and photoluminescence (PL) [6,[24][25][26][27][28].The value of the band-gap energy for Sb 2 S 3 nanorods is close to the optimum value for the photovoltaic conversion, solar energy converters, and optical nanodevices [15].

Conclusions
The coral-like, straw-tied-like, and flower-like Sb 2 S 3 products were successfully synthesized by a facile wet-chemical method in EG.The phase was detected by XRD and SAED.SEM and TEM revealed the transformation of the assynthesized straw-tied-like at 120 ∘ C into flower-like at 180 ∘ C. The flowers were good crystalline nanorods with 1.52 eV direct band gap.

Figure 5 :
Figure 5: EDX spectrum of Sb 2 S 3 synthesized by wet-chemical method at 180 ∘ C for 60 min.