Novel BP/BiOBr S-scheme nano-heterojunction for enhanced visible-light photocatalytic tetracycline removal and oxygen evolution activity
Graphical abstract
Introduction
With the development of photocatalytic technology, photocatalytic degradation of pollutants, photocatalytic disinfection and water splitting have been attracting more and more attention (Wang et al., 2017a; Zhang et al., 2019; Li et al., 2017a; Dong et al., 2018a; Chen et al., 2019a; Cui et al., 2018). Due to the advantages of non-toxic, cheap, stable and reusable photocatalysts, photocatalytic treatment and degradation of pollutants have become a research hotspot in the field of environment. In addition, the operating conditions of the technology are facile to control and the oxidation ability is very strong. It can completely degrade the organic pollutants contained in water into water and carbon dioxide, etc, because the electrons and holes generated by photocatalyst after irradiation can react with oxygen and hydroxyl ions in water to produce strong oxidative active free radicals. Photocatalytic water splitting has also made great progress in recent years (Li et al., 2017b; Peng et al., 2018; Li et al., 2019a; Qi et al., 2017; Mei et al., 2019a; Li et al., 2019b; Zhong et al., 2019a, b), but it still faces great challenges (Qin et al., 2018; Dong et al., 2019a; Qi et al., 2019; Zhang et al., 2018a; Yang et al., 2016; Cai et al., 2019). From the perspective of thermodynamics and kinetics, the key step in water splitting is the oxygen evolution reaction (OER) on the anode. Compared with the two-electron hydrogen evolution process, the oxygen evolution process requires four-electron transfer, so it becomes the bottleneck of limiting the overall catalytic activity in the water splitting reaction (Wang et al., 2017b; Li et al., 2010; Liu et al., 2018; Jiang et al., 2019). It is very important to accumulate a large number of holes on the surface of the catalyst to increase the rate of oxidation, thereby reducing the overpotential required for the oxidation process.
Despite many significant advances have been made in the field of photocatalysis, the photocatalytic efficiency of photocatalysts is still low and far away from practical application. This is mainly due to the easy recombination of photoinduced electrons and holes in semiconductors and the low light-harvesting capability (Jiang et al., 2017a; Dong et al., 2019b; Li et al., 2019c; Deng et al., 2018). Since the two-dimensional (2D) material has a very high surface area-volume ratio and can effectively separate photoinduced electrons and holes, exfoliating the layered material into a few layer 2D material can significantly improve the photocatalytic activity (Luo et al., 2016; Wang et al., 2018a; Chen et al., 2019b; Liu et al., 2015). In addition, due to the high specific surface area of 2D materials, more pollutants can be touched on the surface of particles. As a heterogeneous photocatalyst, a larger contact area can increase the photocatalytic efficiency. In recent years, graphene-like 2D layered materials have attracted much attention due to their controllable chemical composition, abundant chemical valence states, high electrochemical active sites, unique crystal structure and electronic structure (Chen et al., 2019c; Ji et al., 2018; Jiang et al., 2018; Tian et al., 2018). Black phosphorus (BP) has recently emerged as an interesting 2D layered semiconductor with high charge-carrier mobility and widely tunable bandgap for photocatalysis (Liu et al., 2014; Zhang et al., 2014; Wang et al., 2015). BP is also a 2D material of single element, similar to graphite, whose atomic layers are stacked together by van der Waals force interaction. In a single layer, each phosphorus atom combines with three adjacent phosphorus atoms in the form of covalent bonds to form a folded honeycomb structure. BP exhibits high carrier mobility, excellent optical and optoelectronic properties, and high mechanical properties. Compared with bulk BP, nanocrystallized BP possesses larger specific surface area and higher electron mobility. Currently reported nano-BP includes zero-dimensional BP quantum dots and 2D BP nanosheets. At present, the main preparation methods of nano-BP are mechanical exfoliation, chemical vapor deposition, liquid phase exfoliation and solvothermal method (Hirsch and Hauke, 2018; Li et al., 2018; Wang et al., 2018b).
In order to enhance the photocatalytic efficiency of BP, various modification strategies have been studied, including semiconductor anion and cation doping, surface coating, construction of heterostructure, etc. Among them, the construction of heterojunction photocatalysts with efficient light-harvesting and fast charge transfer is one of the widely used methods to enhance the photocatalytic properties. The reason for this is that the heterojunction is conducive to the transfer and spatial separation of photoinduced electron-hole pairs between two semiconductors, to reduce their combination, and to enhance the photocatalytic efficiency of the system. Wang and his co-workers (Zheng et al., 2018) developed the binary BP/g-C3N4 heterostructure photocatalyst, which exhibited excellent visible-light photocatalytic activity in the generation of ·O2− and H2O2. (Zhu et al. (2017, 2018) constructed BP/g-C3N4 and BP/BiVO4 heterojunctions, respectively. They exhibited excellent photocatalytic performance in H2 evolution from visible to near-infrared region and pure-water splitting in the visible-light region. Nevertheless, as a reduction type semiconductor, it is more suitable for BP to construct heterojunctions by selecting a semiconductor with strong oxidation or lower valence band position to match its energy band structure. As a highly anisotropic layered semiconductor, BiOBr possesses excellent photocatalytic activity (Ai et al., 2015; Wang et al., 2018c; Zhang et al., 2018b). While, it has a lower valence band position and stronger oxidation ability. In addition, BiOBr prepared by different methods is thin layers or spherical structures consisting of thin layers with a thickness of only a few nanometers, which is very useful for promoting the transfer and separation of electron-hole pairs (Song et al., 2019; Di et al., 2016; Dong et al., 2018b). However, up to now, there is no report on BP/BiOBr heterojunction. We found that the energy levels of BP and BiOBr are matched, which is suitable for highly efficient separation of photogenerated charge carriers. Therefore, for the first time, we tried to construct layered BP/BiOBr nano-heterojunction for photocatalytic application.
It is well known that the combination of a reduction and an oxidation semiconductor generally forms a conventional Ⅱ-type heterojunction. However, more and more studies have pointed out that a more reasonable explanation is the direct Z-scheme heterojunction mechanism (Zhang et al., 2019; Li et al., 2017a; Qi et al., 2017; Mei et al., 2019a; Li et al., 2019b, c; Zhu et al., 2018; Li et al., 2019d; Li et al., 2016; Di et al., 2019). Recently, on the basis of direct Z-scheme heterojunction mechanism, Yu et al. (Fu et al., 2019) proposed a new concept of S-scheme heterojunction. The S-scheme heterojunction photocatalyst is mainly composed of two n-type semiconductor photocatalysts i.e., PS I and PS II, and they represent oxidation and reduction photocatalyst, respectively. The transfer of photogenerated electrons in S-scheme heterojunction is more like “Step”. The driving force of charge carrier transfer mainly comes from the internal electric field. In S-scheme heterojunction, the relatively useless electrons in the CB of PS I and the relatively useless holes in the VB of PS II are recombined at the interface. In contrast, the useful holes in the VB of PS I and the useful electrons in the CB of PS II are retained due to the presence of the internal electric field. Therefore, in order to construct the S-scheme heterojunction, the VB energy level of the useless holes should be as close as possible to the CB energy level of the other semiconductor where the useless electrons are located.
In this study, in order to obtain a unique layered BP/BiOBr nano-heterojunction with intimate interfacial contact, BiOBr nanosheets were self-assembled on a few layers of BP surface by liquid-phase ultrasound combined with solvothermal treatment to construct the layered nano-heterojunction. The layered BP/BiOBr nano-heterojunction with intimate interfacial contact creates favorable conditions for the rapid transfer and separation of photoinduced electron-hole pairs and the capture of visible-light. The photocatalytic activities of the BP/BiOBr nano-heterojunction for tetracycline (TC) removal and oxygen evolution under visible-light were investigated. Furthermore, the possible photocatalytic mechanism of the composite photocatalyst was discussed. This study proposes an effective idea for the rational fabrication of the layered BP/BiOBr composites for potential photocatalytic applications.
Section snippets
The structure and morphology of BP/BiOBr nanosheets
In order to improve the effective spatial separation of the photo-generated charge of the semiconductor and improve its quantum yield and photocatalytic activity, BiOBr and BP with a large difference in energy level were combined for the first time to obtain BiOBr/BP nano-heterojunction. The BiOBr/BP nano-heterojunction was prepared by facile two-step self-assembly process (Scheme 1). Firstly, BP/BiOBr composites were prepared by ultrasonically mixing separately prepared BiOBr and BP in NMP
Conclusions
In summary, a novel layered BP/BiOBr nano-heterojunction photocatalyst with unique nanoscale heterostructure, band structures and chemically bonding interface was successfully fabricated by a facile liquid-phase ultrasound combined with solvothermal method. The unique Sol-10BP/BiOBr nano-heterojunction yielded enhanced photocatalytic performance, including high apparent reaction rate constant for TC degradation (0.021 min−1), superior O2 evolution rate (89.5 μmol g−1 h−1), and outstanding H2O2
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No.51962023, 51772140), the Natural Science Foundation of Jiangxi Province, China (Grant No.20192ACBL21047, 20171ACB21033), the Scientific Research Foundation of Jiangxi Provincial Education Department, China (Grant no. GJJ170578).
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