2D double-layer-tube-shaped structure Bi2S3/ZnS heterojunction with enhanced photocatalytic activities
Introduction
In recent years, semiconductor material has attracted comprehensive interest due to the photo-degradation of environmental pollutants and production of hydrogen from water under light irradiation. However, many semiconductors can be excited only under ultraviolet light irradiation because of its relatively wide band gap and inefficient quantum yield, which hinders its further application in industrial process [1], [2], [3], [4], [5], [6], [7]. Up to date, a variety of strategies have been employed to improve the photocatalytic efficiencies of photocatalysts in the visible range by different modified methods, such as doped with nonmetal/metal element [8], [9], [10], [11], [12], [13]. Unfortunately, the methods used are not completely controlled. Moreover, these dopants may become recombination centers between photogenerated electrons and holes [14], [15], [16]. Evidently, the expected methods are somewhat limited. Therefore, to explore more efficient photocatalyst with visible light responsiveness and thermal stability is urgent and indispensable.
Recently, some novel semiconductor heterojunctions have been designed and fabricated by coupling a narrow band gap semiconductor with metal and/or other semiconductors [17], [18], [19], [20], [21], [22], [23], [24]. Moreover, the construction of semiconductor heterojunctions has proved to be an effective approach due to its perfect effectiveness in decreasing the recombination rate of photogenerated electron and holes, and thus improving the photocatalytic activity [25], [26], [27], [28], [29], [30]. During the past few years, many important findings have been reported on the fabrication of semiconductor heterojunctions such as, AgBr–Ag–Bi2WO6 three component nanojunction [31], Bi2WO6–TiO2 hierarchical heterostructure [32], SnO2–TiO2 heterostructured photocatalysts [33], CuO|CuBi2O4 heterojunction [34], and so on. Despite a large number of encouraging versatile heterojunction photocatalysts being available, there are still a few promising developments to designing and fabricating the semiconductor heterojunction photocatalysts, while providing some stimulating perspectives on the future developments.
In recent years, as an important photosensitive semiconductor, ZnS has attracted increasing interest because of the photodecomposition of environmental pollutants and generation of hydrogen from water under light irradiation [35], [36], [37]. Unfortunately, there are three main drawbacks, which limit the application of ZnS photocatalyst. Firstly, ZnS can be excited only under ultraviolet light irradiation due to its relatively wide band gap size of approximately 3.66 eV. Secondly, the rapid recombination of photogenerated electrons–holes pairs and inefficient quantum yield seriously limits the light energy-conversion efficiency. Lastly, ZnS exhibits lower chemical stability, which is oxidated by oxygen in the air [38], [39], [40]. Therefore, to improve these drawbacks, especially, to broaden the range of visible light responsiveness and enhance the separation of photogenerated carriers are important in enlarging the efficiency photocatalytic application. Bi2S3 is another attractive and promising semiconductor with a small size of approximately 1.3 eV [41], [42], [43]. It is well-known that Bi2S3 has a conduction band position suitable for photocatalytic reaction. Moreover, Bi2S3 could cause red-shift of the photoabsorption wavelength, produce nonlinear optical responsiveness and enhance the oxidation and reduction capacity of other semiconductor, thus it has been used as coating on other semiconductor for visible light absorption and to improve the photochemical stability of the semiconductor, respectively. Especially, due to the photoabsorption property up to wavelength more than 800 nm, and thus Bi2S3 has been widely used as sensitizer [44], [45]. Furthermore, the conduction band of Bi2S3 possesses smaller electropositive than the corresponding ZnS, and the valence band have more electronegative than that of ZnS. To address the above severe issue, the formation of Bi2S3/ZnS heterojunction with the efficiency separation and transfer of photogenerated carriers is more possible, and well-fabricated Bi2S3/ZnS heterojunction could availably restrict the recombination of photogenerated carriers and effectively enhance the quantum yield. However, the synthesis and application of Bi2S3/ZnS was limited to Wu [46] research. They synthesized Bi2S3/ZnS heterostructure microspheres by situ cation-exchange method, and the result of the photocatalytic experiment was shown that the Bi2S3/ZnS composite exhibited much higher photocatalytic activities than that of pure ZnS microspheres. Thus, the corresponding relationship among loaded content to structure property of Bi2S3/ZnS composite need further discuss, and the photocatalytic application of Bi2S3/ZnS composite needs further explore.
Therefore, to further enrich versatile photocatalytic material with visible light catalytic efficiency, we design and fabricate a Bi2S3/ZnS composite by coupling ZnS with a narrow band gap Bi2S3 semiconductor. Nevertheless, to the best of our knowledge, 2D double-layer-tube-shaped structure Bi2S3/ZnS composite has never been constructed. Afterwards, as-prepared Bi2S3/ZnS heterojunctions were characterized by XRD, SEM, UV–vis DRS and EDS. Phenol and thiophene was chosen as model pollutants to evaluate the photocatalytic activity of the samples under visible light irradiation. The photocatalytic mechanism of Bi2S3/ZnS composite was also discussed based on the calculated energy positions of Bi2S3 and ZnS.
Section snippets
Preparation
All the reagents were analytical grade. The typical preparation process of Bi2S3/ZnS sample is showed in the following way.
The phase purity of Bi2S3/ZnS composite
The XRD patterns of the Bi2S3/ZnS composite are shows in Fig. 1. From Fig. 1, the XRD patterns of ZnS, Bi2S3, 1.0 wt% Bi2S3/ZnS, 4.0 wt% Bi2S3/ZnS, 8.0 wt% Bi2S3/ZnS and 10.0 wt% Bi2S3/ZnS composite are presented. The diffraction peaks at 2θ of 16.8°, 24.9°, 28.9°, 43.2°, 47.7° and 56.5°, can be perfectly indexed to orthorhombic Bi2S3 (JCPDS no. 84-0279) and hexagonal ZnS(JCPDS no. 89-2942), corresponding to the indices of (013), (121), (025), (151), (204) planes of orthorhombic Bi2S3 and
Conclusion
Bi2S3/ZnS composite with different morphologies were prepared by the facile synthesis method. The corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property of Bi2S3/ZnS by UV–vis DRS, XRD, SEM, and EDS. The results shown that Bi2S3 loaded could evidently change the crystallinity of ZnS, enhance the optical absorption ability for visible light of ZnS, and improve the morphologies and microstructure of ZnS. The photocatalytic results
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant no. 21406188), the Industorial Public Relation Project of Department of Science & Technology of Shaanxi (Grant no. 2014K10-04), and the Project of Shaanxi Youth Science and Technology Star (Grant no. 2015KJXX-38).
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