Room-temperature synthesis from molecular precursors and photocatalytic activities of ultralong Sb₂S₃ nanowires

Abstract

Starting from molecular precursors antimonyO-ethyldithiocarbonate (ethyl xanthate, (C₂H₅OCS₂)₃Sb), ultralong Sb₂S₃ nanowires with a diameter in the range of 5–10 nm were easily synthesized at room temperature by employing ethylenediamine both as solvent and a bidentate ligand. By varying crystallization duration, the shape of Sb₂S₃ particles evolved from belts, tubes and finally to nanowires through the well-known rolling process. This synthetic route was workable for the preparation of other chalcogenides such as ZnS and PbS particles with well-defined nanostructures. The photocatalytic experiment of the as-prepared Sb₂S₃ nanowires under visible light indicated that the photodegradation ratio of methyl orange in aqueous solution was up to 95% after 25 min of irradiation, which was much better than that of its bulk counterparts under the same conditions. High efficiency resulted from the broad spectrum response and high surface area nature of the as-synthesized Sb₂S₃ nanowires.

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Introduction

In the late 1990s, the field of semiconductor nanowires underwent a significant expansion and became one of the most active research areas within the nanoscience community such as nanowire electronics, nanowire photonics, nanowires for energy conversion and storage, and interfacing nanowires with living cells.¹ In addition, the high surface-area nature of the nanowires often leads to a flexible mesh having a porous and open framework, which could greatly benefit the surface catalytic reaction as well as gas evolution. Due to their potential application, controllable growth of semiconductor nanowires has been a crucial issue for nanoscale science. The methods for making semiconductor nanowires are commonly placed into two categories, i.e. the bottom-up and top-down approaches.^2–5 The solution phase method of the bottom-up has been widely used due to its mild synthetic conditions.

Antimony sulfide (Sb₂S₃), an important member of the chalcogenides, has received significant attention for potential applications in solar energy conversion, thermoelectric cooling technologies, fast ion conductors and optoelectronics in the IR region. Various approaches have been employed for the preparation of one-dimensional (1D) Sb₂S₃ nanoparticles, including hydrothermal or solvothermal method,^6–8 wet chemical route,^9,10 microwave irradiation and sonochemical methods,^11,12 VLS growth technique,¹³ and so on. Specifically, the synthesis of chalcogenide nanocrystals through solution phase thermolysis of molecular (i.e. single-source) precursors provided a potential route to tune the size and shape of the products.^14–16 For example, Qian et al. reported the synthesis of Sb₂S₃ rod bundles via the thermal decomposition of an antimony(III) diethyldithiocarbamate complex at 290 °C.¹⁷ Dispersed Sb₂S₃ and Bi₂S₃ nanorods were prepared from single-source precursors (xanthate) in our previous work.^18,19 Different from the literature and our previous reports, herein, we easily synthesized ultralong Sb₂S₃ nanowires by the decomposition of antimonyO-ethyldithiocarbonate in ethylene diamine (EDA) solution and ageing for 10 days at room temperature. The present route is simple, reproducible and does not need any surfactants or templates.

It is well-known that the photocatalytic activity is closely interrelated to the size, morphology, and structure of the photocatalysts. Nanoscale semiconductor particles possess a higher surface area-to-volume ratio than their bulk counterparts, and thus can greatly increase the density of active sites available for adsorption and catalysis. Furthermore, the size-dependent band gap allows tuning of the electron–hole red–ox potentials to achieve selective photochemical reactions. Finally, the reduced dimensions of the nanocatalysts are expected to allow the photogenerated charges to readily migrate on the catalyst surface, thus reducing the probability of undesired bulk recombination.²⁰Sb₂S₃, in a nanostructured form, has recently been reported to be a high-efficiency visible-light responsive photocatalyst for the degradation of methyl orange aqueous solution.^21,22 However, to the best of our knowledge, Sb₂S₃ nanowires, with a higher surface-to-volume ratio, has been rarely reported as photocatalyst. In this paper, we demonstrated a high photocatalytic activity of ultralong Sb₂S₃ nanowires for methyl orange (MO) degradation under visible-light irradiation.

Results and discussion

Fig. 1 shows TEM images of the as-synthesized Sb₂S₃. The products contain large amounts of nanowires with a diameter in the range of 5–10 nm (shown in the circled area in the inset of Fig. 1b) and length up to tens of micrometres. Most are compactly parallel-arranged to form nanowire-based bundles except for some scattered distribution, owing to one-dimensional antimony sulfide having a strong preference for attachment to one another in the vertical direction to the growth direction.⁹

Fig. 1 (a) TEM and (b) high-magnification TEM (the circled area of the inset showing a single nanowire) images of the as-prepared Sb₂S₃ nanowires.

The purity and composition of the as-prepared sample are analysed by energy dispersive X-ray analysis (EDX). No obvious impurities are detected in the EDX spectrum of Sb₂S₃ (Fig. 2a). Quantification of the EDX peaks gives the atomic ratio of Sb : S as approximately 2 : 3, which is consistent with the stoichiometric composition of Sb₂S₃. The copper and carbon signals come from the supporting TEM Cu grid.

Fig. 2 (a) EDX spectrum of the as-prepared Sb₂S₃ nanowires. (b) Raman spectra of the Sb₂S₃ nanowires obtained after ageing for 5 and 10 days.

The Raman spectra of Sb₂S₃ nanowires prepared by ageing for 5 and 10 days are shown in Fig. 2b. They reveal two main bands with maxima near 130 and 290 cm⁻¹, corresponding to two main structural units: SbS₃ pyramids and Sb–Sb bonds in S₂Sb–SbS₂ structural units.²³ The presence of broad peaks suggests the formation of poorly crystallized products as compared with the crystalline or amorphous form of Sb₂S₃.^23,24 The peaks are relatively broad for the sample aged for 5 days compared to that of 10 days, which is in good agreement with the XRD results discussed below.

The XPS spectra were obtained to identify the surface composition of the as-prepared Sb₂S₃ nanowires. No obvious impurities were detected in the XPS survey spectrum of Sb₂S₃ (Fig. 3a) except for some C and N peaks which may come from the ligand EDA, as discussed below. The high-resolution spectra of Sb₂S₃ are shown in Fig. 3b and c. The two peaks at 528.6 eV and 538.3 eV corresponds to the Sb 3d^5/2 and 3d^3/2, respectively. The high-resolution spectra of S 2p show two overlapped peaks for S 2p^3/2 and S 2p^1/2 at 160.6 eV and 162.1 eV, respectively. All of the observed binding energy values for Sb 3d and S 2p coincide with the reported data.¹⁰ Quantification of the XPS peaks gives the molar ratio of the products as 0.7 : 1.0 for Sb : S, which are nearly consistent with the given formula for the as-prepared products.

Fig. 3 XPS spectra of the as-prepared Sb₂S₃ nanowires: (a) survey scan, (b) Sb (3d) and (c) S (2p) core.

A time-dependent experiment has been conducted to track the formation of the Sb₂S₃ nanowires. When (C₂H₅OCS₂)₃Sb was added into EDA solution, the clear solution became brown and cloudy in several minutes, indicating that the decomposition reaction of (C₂H₅OCS₂)₃Sb occurred and Sb₂S₃ was generated. When the mixture was stirred for 6 h, belt-like products are observed, as shown in Fig. 4a. If the ageing time lasted for 4 days, predominantly wire-like products were obtained (Fig. 4b). The higher-magnification TEM image shows that some tubular structures with distinguishably darker walls are present (Fig. 4c). The bundle-like Sb₂S₃ nanowires began to appear when the crystallization time was increased to 5 days (Fig. 4d). This observation implies that the Sb₂S₃ nanowires might be formed by the rolling process of the nanoribbons, which has been proposed as the usual mechanism involved in the formation of nanowires from lamellar structures.²⁵ That is to say, as the crystallization time is prolonged, the dangling bonds of the lamella or nanoribbon edges may be eliminated by the intrachain interaction (covalent bonds) in the structure of Sb₂S₃, forming more stable nanowires.²⁶

Fig. 4 TEM images of Sb₂S₃ nanoparticles prepared at varied ageing times: (a) 6 h; (b) 4 days; (c) a higher-magnification image of (b); (d) 5 days.

XRD patterns of the Sb₂S₃ nanowires obtained by ageing for 10 days are shown in Fig. 5b. All peaks may be indexed to an orthorhombic phase Sb₂S₃ (JCPDS files, no. 06-0474). The intensity of the (hk1) reflection peaks is enhanced, while that of the (hk0) planes such as (130) and (120) is absent or weakened, suggesting that the ultralong Sb₂S₃ nanowires might be preferentially elongated according to the [100] direction.^7,8 The peak at 2 θ = 21.1° is likely to come from some impurities, and further experiments are under way to identify what the impurities are. The shape of the diffraction peaks indicate that the product obtained by ageing for 10 days is relatively well-crystallized to that prepared by 5 days of ageing. These are in good agreement with the electron diffraction (ED) patterns (Fig. 6a and b), which reveal clearer Debye–Scherrer rings due to the polycrystalline atomically more ordered structure for the sample with 10 days of ageing than that with 5 days of ageing.

Fig. 5 XRD patterns of the Sb₂S₃ nanowires prepared after ageing for (a) 5 days and (b) 10 days.

Fig. 6 ED patterns of the Sb₂S₃ nanowires prepared after ageing for (a) 5 days and (b) 10 days.

According to Bashouti's report,¹⁵ the precursor molecules decomposed to form Sb₂S₃ upon a nucleophilic attack by the EDA solvent molecules, as shown schematically in Scheme 1.

Scheme 1 The formation of Sb₂S₃ from xanthate in EDA solution.

The solvent EDA plays an important role in the formation and growth of Sb₂S₃ nanowires. When using ethanolamine (EA) as solvent, the decomposition reaction of (C₂H₅OCS₂)₃Sb occurred quickly. However, the yield of Sb₂S₃ was low and the length of the Sb₂S₃ nanowires was short (Fig. 7a). If ethylene glycol (EG), dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF) was used as solvent, no decomposition reaction took place in two days. As a bidentate ligand with two nitrogen donors, a stronger coordination interaction of EDA with Sb³⁺ may benefit the decomposition of xanthate, control the growth rate of different crystal faces and thus result in the formation of ultralong Sb₂S₃ nanowires. This synthetic route is also workable for the preparation of the other chalcogenides such as ZnS and PbS nanoparticles with well-defined nanostructures (Fig. 7b).

Fig. 7 TEM images of (a) Sb₂S₃ nanowires and (b) ZnS nanobelts and PbS cubes (inset) prepared by ageing for 5 days in EA and EDA solution, respectively.

Fourier transform infrared (FTIR) studies of the as-prepared Sb₂S₃ also provide preliminary proof for the intermolecular bonding (Fig. 8). In comparison with the IR spectra of pure (C₂H₅OCSS)₃Sb (Fig. 8a), the frequencies at 1190, 1105 and 1020 cm⁻¹ attributed to the characteristic absorption for ν (C=S), ν (C–O–R), ν (C–S) of C₂H₅OCS₂⁻group disappeared after reacting for 2 h (Fig. 8c), suggesting that the xanthate decomposed completely at room temperature in EDA solution. While, ν (C–N), ν (C–C), ν (CH₂) and ν (NH₂) signals at 1160, 1485 and 1608–1597 cm⁻¹ in the IR data of free EDA (Fig. 8b) shift to 1400, 1518 and 1620 cm⁻¹ in the case of Sb₂S₃ nanowires (Fig. 8c and d), respectively, which further demonstrates that the nitrogen atoms of the amine groups coordinated to the metal atom.

Fig. 8 FTIR spectra of (a) (C₂H₅OCSS)₃Sb, (b) EDA, (c) and (d) Sb₂S₃ prepared by ageing for 2 h and 10 days, respectively.

Furthermore, Sb₂S₃ belongs to the orthorhombic system and forms a quasi-layered structure, which in itself has a strong preference for anisotropic growth, to form a 1D structure under certain conditions.⁶

The influence of pH and the concentration of the solution on the formation of Sb₂S₃ were investigated. The yield of Sb₂S₃ nanoparticles increased if some HCl was added into the solution, indicating that the decomposition rate of xanthate increased with decreasing pH, analogous to Sun's report.²⁷ Nevertheless, the final morphology and structure of the products were the same. If the amount of the precursors (Sb xanthate) was decreased to 1.0 mmol in 20 mL of solution, the mixture sometimes changed color between clear yellow and turbid brown during ageing, suggesting that the decomposition reaction of xanthate might be reversible in EDA or EA solution at room temperature, and so the reaction rate increased with increasing concentration of the precursors.

The optical properties of the Sb₂S₃ nanowires have been studied by UV-vis reflectance spectroscopy. As shown in Fig. 9a, it can be found that Sb₂S₃ shows a strong photo-absorption property in the visible-light region and the absorption edge is as long as 750 nm. For a direct-gap semiconductor, the band gap of the as-prepared Sb₂S₃ nanowires is estimated at 1.63 eV using the formula [F(R)E]² = A(E − E_g), where E and E_g are the photon energy and optical band gap energy, respectively, and A is the characteristic constant of semiconductors.²¹

Fig. 9 (a) UV-vis diffuse reflectance spectrum of the as-prepared Sb₂S₃ nanowires and its optical band gap energy E_g (inset); (b) UV-vis spectral changes of MO in aqueous Sb₂S₃ dispersions at various irradiation times.

The photocatalytic activities of the as-prepared Sb₂S₃ nanowires are evaluated by decomposition of MO in aqueous solution under visible-light irradiation. The degradation course has been monitored by recording UV-vis absorbance spectra and following the decay of the main dye absorption peak at 464 nm. Fig. 9b shows that the as-prepared Sb₂S₃ nanowires were able to catalyze a complete decoloration after 25 min of irradiation, which is comparable to the degradation ratio of Sb₂S₃ nanorods determined by Sun's group.²¹ While, the degradation ratio of bulk Sb₂S₃ particles is about 7% (Fig. 10a), revealing that the photocatalytic activity is relative to particle size and surface area of the catalysts. In addition, the photocatalytic performance drops gradually for the Sb₂S₃ nanoparticles obtained by various ageing duration from 5 d to 6 h (Fig. 10), suggesting that the high crystallinity of a photocatalyst can effectively increase its photocatalytic activity.²¹

Fig. 10 Photocatalytic properties of different catalysts: (a) bulk Sb₂S₃ particles; Sb₂S₃ nanoparticles prepared after ageing for (b) 6 h, (c) 24 h, (d) 5 days, and (e) 10 days.

Experimental

Syntheses

The experiments proceeded at room temperature and under atmospheric conditions. All chemicals were used as received without further purification. In a typical procedure, 1.1 g (0.005 mol) of SbCl₃ was added into 60 mL of distilled water with stirring, and then about 4 mL of concentrated hydrochloric acid (HCl, 36%) was added dropwise to prevent the hydrolysis of SbCl₃ and a homogeneous solution was obtained. 2.4 g (0.015 mol) of CH₃CH₂OCS₂K was added into the above solution under agitation. Immediately, an orange precipitation ((C₂H₅OCS₂)₃Sb) was formed, and was collected after filtration, washed with deionized water several times and dried in air. Then, 0.8 g (about 1.6 mmol) of (C₂H₅OCS₂)₃Sb was dissolved in 20 mL of EDA, and the solution changed from clear yellow to turbid brown within several minutes. After the mixture was stirred for 10 days, brown Sb₂S₃ precipitates were collected by filtration, washed several times with water and absolute ethanol, and dried spontaneously in air.

Characterizations

Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and the corresponding selected area electron diffraction (SAED) were carried out on a JEM-2100 microscope (JEOL) equipped with a METEK (PV 97-56700 ME) X-ray energy dispersive spectrometer. The specimens of TEM and HRTEM measurements were prepared via spreading a droplet of ethanol suspension onto a carbon-coated copper grid and allowing to dry in air. Raman spectra were collected on a RENISHAW InVia Raman microscope. The X-ray photoelectron spectra (XPS) were recorded on a PHI QUANTERA II X-ray photoelectron spectrometer, using Al Kα radiation as the exciting source. The results obtained in the XPS analysis were corrected by referencing the C 1s line to 284.6 eV. The X-ray diffraction patterns (XRD) were recorded on a Bruker D8 advanced X-ray diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å) in the range of 10–70°. The infrared spectroscopy on KBr pellets was performed on a Bruker Vector 22 spectrophotometer in the 4000–400 cm⁻¹ region. The diffuse reflection spectra were measured on a Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere, using BaSO₄ as a reference.

Photocatalytic activity test

The photocatalytic reaction was conducted in a reactor equipped with a 500 W Xe lamp and a 420 nm UV cut-off filter. A 200 mg quantity of photocatalyst was suspended in 200 mL of 10 mg L⁻¹ MO aqueous solution. The suspension was ultrasonicated for 15 min and magnetically stirred in the dark for 2 h to ensure the establishment of an adsorption–desorption equilibrium of the dye on the catalyst surface. Then, the suspension was stirred and illuminated by visible light. At given time intervals, 5 mL of the suspension was taken out from the reactor and centrifuged to remove the photocatalyst particles. The concentration of MO during degradation was determined on a UV-vis spectrometer and estimated from the absorbance of the absorption maximum at 464 nm.

Conclusions

In summary, we have used a room temperature crystallization process to produce Sb₂S₃ nanowires with a diameter in the range of 5–10 nm and length up to tens of micrometres from molecular precursors (xanthate) in EDA solution. This synthetic route is also workable for the preparation of belt-like ZnS and cubic PbS nanostructures. The as-prepared Sb₂S₃ nanowires with a direct band gap of 1.63 eV show high photocatalytic activity of MO degradation under visible-light irradiation. This work not only provides a facile route to synthesize chalcogenides with well-defined nanostructures at room temperature in a solution-phase system, but also gives insight into understanding the decomposition behavior of molecular precursors in various solvents.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China and NUST Research Funding (2010GJPY042).

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