A facile chemical conversion synthesis of Sb2S3 nanotubes and the visible light-driven photocatalytic activities
© Shuai and Shen; licensee Springer. 2012
Received: 24 December 2011
Accepted: 26 March 2012
Published: 26 March 2012
We report a simple chemical conversion and cation exchange technique to realize the synthesis of Sb2S3 nanotubes at a low temperature of 90°C. The successful chemical conversion from ZnS nanotubes to Sb2S3 ones benefits from the large difference in solubility between ZnS and Sb2S3. The as-grown Sb2S3 nanotubes have been transformed from a weak crystallization to a polycrystalline structure via successive annealing. In addition to the detailed structural, morphological, and optical investigation of the yielded Sb2S3 nanotubes before and after annealing, we have shown high photocatalytic activities of Sb2S3 nanotubes for methyl orange degradation under visible light irradiation. This approach offers an effective control of the composition and structure of Sb2S3 nanomaterials, facilitates the production at a relatively low reaction temperature without the need of organics, templates, or crystal seeds, and can be extended to the synthesis of hollow structures with various compositions and shapes for unique properties.
KeywordsNanotubes Chemical transformation Cation exchange Growth mechanism Optical and photocatalytic properties
Since the discovery of carbon nanotubes in 1991 , extensive research has been carried out on one-dimensional (1D) tubular nanostructures, owing to their unique size-dependent properties and remarkable potential applications in electronics, optoelectronics, catalysis, biotechnology, separation, and so on [2–7]. However, the preparation of nanotubes is relatively difficult, and fewer synthetic techniques have been developed compared to those for other 1D nanostructures, such as nanorods and nanowires [8–10]. So far, different types of nanotubes have been prepared by various approaches including vapor-liquid-solid, chemical vapor deposition, template-directed synthesis, and low-dimensional sacrificial precursors [11–14]. Nevertheless, these strategies often require high temperature, special conditions, and tedious procedures, and most of them are complicated and uncontrollable. Therefore, development of a facile, versatile, and effective synthetic pathway to prepare 1D nanotubes is very important and quite necessary. In particular, it is highly desirable to control and manipulate the chemical compositions and structures of nanotubes.
In fact, chemical conversion and cation exchange have been demonstrated as powerful tools to convert the chemical compositions of nanostructures without destroying the original morphology [15, 16]. Our previous studies on the transformation of composition in the core/shell microspheres (from ZnO/ZnS to ZnO/Ag2S and ZnO/CuS)  and in the hollow microspheres, as well as nanotubes (from ZnS to other various metal sulfides) [18, 19], have indicated the significance of chemical conversion and cation exchange. Compared to other strategies, the chemical conversion and cation exchange have the following advantages: (1) reactions can take place in a solution under mild conditions (low growth temperature, without any special equipments or templates); (2) this approach is a typical one-step process, which needs no tedious procedures or further purification of the products; (3) the products can be produced on a large scale; and (4) this strategy can be developed as a general method to fabricate functional semiconductor hollow structures with various compositions and shapes for unique properties, which is quite important with respect to technical applications.
As an important V-VI group binary chalcogenide, antimony trisulfide (Sb2S3) with an energy bandgap varying between 1.5 and 2.2 eV has attracted particular attention, owing to its good photovoltaic properties, high thermoelectric power , broad spectrum response, and suitable valence band position . This material has been applied in various areas such as television cameras with photoconducting targets, thermoelectric cooling devices, electronic and optoelectronic devices, solar energy conversion, and visible light-responsive photocatalysis [20–26]. It has been demonstrated that the properties of antimony trisulfide are determined predominantly by their crystal structure, size, and morphology. Therefore, the synthesis of Sb2S3 materials with well-controlled size and shape is of great significance for their applications. Up to date, a variety of 1D nanostructures of Sb2S3 such as nanorods [27–30], nanowires , microtubes [32, 33], and nanoribbons  have already been synthesized by various methods. Nevertheless, little has been devoted to the development of a general and low-cost synthetic method to fabricate Sb2S3 nanotubes without using any templates or crystal seeds. Although as-grown Sb2S3 presents in general an amorphous structure, it can be transformed in the polycrystalline phase by successive annealing . Considering the technical importance of this material, fabrication of Sb2S3 with some inspired structures such as a tubular structure by a convenient and efficient method has always been a great interest.
In this paper, we have realized the first synthesis of Sb2S3 nanotubes by conversion from ZnS nanotubes via chemical conversion and cation exchange at a low temperature of 90°C. The key point of the method is to utilize the large difference in solubility between ZnS and Sb2S3 for the effective transformation. Structural, morphological, and optical changes have been observed in these samples after annealing at different temperatures in an argon atmosphere. We have further shown high photocatalytic activities of Sb2S3 nanotubes for methyl orange (MO) degradation under visible light irradiation, due to the large specific surface area and good crystallinity [36, 37]. The present technique is very convenient and efficient, free of any organics, templates, or crystal seeds, and has been demonstrated to control and manipulate effectively the chemical compositions and structures of nanotubes.
Synthesis of ZnS nanotubes
The preparation details for ZnS nanotubes can be found in our recently published papers . Briefly, ZnO nanowires were first prepared by a hydrothermal process. As a typical synthesis process, 0.2 g ZnCl2 and 20.0 g Na2CO3 were added into a 50-mL Telfon-lined stainless steel autoclave and filled with distilled water up to 90% of its volume. After vigorous stirring for 30 min, the autoclave was maintained at 140°C for 12 h, followed by cooling down naturally to room temperature. The synthesis of ZnO nanowires could be realized after the product was washed and dried. Subsequently, the as-prepared ZnO nanowires on substrates (silicon or glass slides) were transferred to a Pyrex glass bottle containing 40 mL of 0.2 M thioacetamide (TAA). The sealed bottle was then heated to 90°C for 9 h in a conventional laboratory oven to synthesize ZnS nanotubes. The final products on the substrates were washed repeatedly with deionized water and then dried at 60°C before being used for the next step in the reaction and further characterization.
Synthesis of Sb2S3 nanotubes
The synthesis of Sb2S3 nanotubes was realized by transferring the silicon or glass slides with ZnS nanotubes on them to a Pyrex glass bottle containing 150 mM C8H4K2O12Sb2 and 70 mM tartaric acid. During the reaction process, the solution temperature was kept at 90°C. The final products on the substrates were washed thoroughly using deionized water to remove any co-precipitated salts and then dried at air at 60°C. For better crystal quality, the as-prepared Sb2S3 nanotubes were annealed in an argon atmosphere.
Morphological and structural characterization
The morphology and structure of the samples were characterized using field-emission scanning electron microscopy (FE-SEM; Philips XL30FEG, FEI Co., Hillsboro, OR, USA) with an accelerating voltage of 5 kV and a high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100 F, JEOL Ltd., Akishima, Tokyo, Japan). Selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) microanalysis were also performed during the TEM and SEM observations. X-ray diffraction (XRD) was carried out on a diffractometer (D/max-2200/PC, Rigaku Corporation, Tokyo, Japan) equipped with a high intensity Cu Kα radiation (λ = 1.5418 Å). Raman spectra were measured at room temperature on a Jobin Yvon LabRAM HR 800 UV micro-Raman/PL system (HORIBA Jobin Yvon Inc., Edison, NJ, USA)at a backscattering configuration under the excitation of a He-Cd laser (325.0 nm) for ZnS nanotubes and a Ar+ laser (514.5 nm) for Sb2S3 nanotubes.
Photocatalytic activity measurements
The photocatalytic activities under visible light were monitored through the photodegradation of MO. Visible light irradiation was carried out using a 500-W Xe lamp with a 420-nm UV cutoff filter, which was surrounded by a quartz jacket to allow for water cooling. Photocatalyst powder (30 mg) was added into 80 mL of aqueous MO (20 mg L-1) solution and magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium before visible light illumination. The absorbance of the corresponding target organics was monitored by measuring with a UV-vis spectrophotometer (PerkinElmer Lambda 950, PerkinElmer, Waltham, MA, USA).
Results and discussions
The corresponding EDX spectra in Figures 2a',b',c',d' give clear evidence for the FE-SEM observation of the samples obtained through various reaction times. From Figure 2a', we can observe the successful incorporation of the Sb element into the ZnS nanotubes in the compositional information. The signal of Si originates from the substrate. With the increase of the reaction time, the Sb/Zn stoichiometric ratio becomes higher and higher due to the fact that more and more Zn atoms were replaced by Sb atoms with the reaction processing, as shown in Figures 2b',c'. Further chemical reaction will yield pure Sb2S3 nanotubes, which can be unambiguously confirmed by the EDX spectrum in Figure 2d'. There are only Sb, S, and Si elements without any Zn element.
According to the experimental observation described above, the whole process can be described as follows: Once the obtained ZnS nanotubes were transferred into C8H4K2O12Sb2 solution, cation exchange began at the interfaces between the ZnS nanotube surfaces and solution. With the increase in the reaction time, Zn2+ was gradually substituted by Sb3+, resulting in the synthesis of Sb2S3 nanotubes. The driving force for the cation exchange is provided by the large difference in solubility between ZnS and Sb2S3 (solubility product constant (Ksp) of ZnS is 2.93 × 10-25, whereas Ksp of Sb2S3 is 1.5 × 10-93) . The above conversion mechanism reveals that the ZnS nanotubes can act as both reactants and templates during the cation-exchange process. Therefore, a general, facile, and economic method has been proposed and realized to synthesize Sb2S3 nanotubes, and this strategy can control and manipulate effectively the chemical compositions and structures of nanotubes. Furthermore, we can extend this chemical conversion approach to the synthesis of other metal sulfide nanotubes under the condition that those yielded metal sulfides have lower Ksp values than ZnS. In fact, it is because of the large Ksp in ZnS that we choose ZnS nanotubes as the reactants and templates to synthesize various metal sulfide nanotubes, like Ag2S, CuS, PbS, Bi2S3, and Sb2S3 nanotubes in the present paper. It is a convenient one-pot method without using any organics, templates, or crystal seeds and has great potential in industrialized high-volume production.
HRTEM observation can give deep insight into the structural features of the Sb2S3 nanotubes before and after annealing. Figure 4e is a representative HRTEM image taken on the edge of the obtained Sb2S3 16-h nanotube before annealing (Figure 4a). The lattice fringes are highly disordered and ambiguous, revealing that the un-annealed Sb2S3 16-h nanotubes have poor crystallization . The corresponding SAED pattern of the nanotube (inset of Figure 4e) exhibits weak ring diffractions, indicating slight crystallization. Figure 4f presents a HRTEM image recorded from a certain Sb2S3 16-h nanotube after annealing at 400°C (Figure 4d); only the polycrystalline nature of Sb2S3 nanotubes can be observed. The clearly observed crystal lattice fringes demonstrate that the nanotubes are highly crystallized and free from dislocation and stacking faults . The corresponding SAED pattern shown in the inset of Figure 4f having characteristic ring diffractions also confirms the polycrystalline feature of the nanotubes after annealing.
To confirm the transition from a weak crystallization to a polycrystalline structure, Raman spectra have also been measured at different annealing temperatures. The results are summarized in Figure 5b. For Sb2S3 16-h nanotubes before annealing, the spectrum is very broad, indicating poor crystallinity . The sample after annealing in argon at 200°C for 1 h presents similar spectrum to the un-annealed sample but with a single peak. At higher temperatures of 250°C and 400°C, several sharp peaks appear, which correspond to the Raman spectra of crystalline Sb2S3 (stibnite structure) [24, 42]. The band centered at 170 cm-1 can be assigned to the vibration of Sb-Sb bonds in S2Sb-SbS2 structural units . The presence of peaks at 189 and 252 cm-1 suggests the formation of a good crystalline product . The peak at 279 cm-1 is in accordance with the symmetric vibrations of SbS3 pyramidal units having C3v symmetry [44, 45], and the peak at 450 cm-1, with the S-S vibrations  or the symmetric stretching of the Sb-S-S-Sb structural units . These results agree well with the XRD observation in Figure 5a. Our Sb2S3 nanotubes will yield a poor morphology and crystal quality when annealed above 400°C, which can be attributed to a sulfur deficiency as a consequence of sulfur loss during the high-temperature annealing without sulfur vapor .
In summary, Sb2S3 nanotubes have been successfully synthesized by chemical conversion and cation exchange at a low temperature of 90°C. The conversion mechanism of the Sb2S3 nanotubes from ZnS nanotubes is due to the large difference in solubility between ZnS and Sb2S3. Samples have been annealed at different temperatures in the range of 200°C to 400°C in an argon atmosphere. The morphological, structural, and optical characteristics of the yielded Sb2S3 nanotubes before and after annealing were characterized by SEM, TEM, XRD, and Raman spectra in detail. It is revealed that the synthesized Sb2S3 nanotubes can be transformed from a weak crystallization to a polycrystalline structure through the successive annealing treatment. Furthermore, the Sb2S3 nanotubes exhibit high photocatalytic activities for MO degradation under visible light irradiation as a result of large specific surface area and good crystallinity. The present strategy is a very convenient and efficient method to control and manipulate effectively the chemical composition and structure of nanomaterials. Although the present work focuses on Sb2S3 nanotubes, other metal sulfide hollow structures are also expected to be realized based on ZnS hollow structures with the corresponding shapes as the precursors during the chemical conversion process. We have therefore expected that the general and economic technique of material synthesis demonstrated in this article can be used in a broad range of applications to fabricate innovative micro- and nanostructured semiconductor materials with different compositions and geometries having unique properties.
energy dispersive X-ray
field-emission scanning electron microscopy
high-resolution transmission electron microscopy
selected area electron diffraction
This work was supported by the National Major Basic Research Project of 2012CB934302 and the Natural Science Foundation of China under contracts of 11074169 and 11174202.
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