Novel BP/BiOBr S-scheme nano-heterojunction for enhanced visible-light photocatalytic tetracycline removal and oxygen evolution activity

https://doi.org/10.1016/j.jhazmat.2019.121690Get rights and content

Highlights

  • BiOBr with higher Fermi level were self-assembled on the surface of BP.

  • Strong Psingle bondOsingle bondBi bonds were formed between BiOBr and BP.

  • TC degradation and O2 evolution of Sol-10BP/BiOBr was 7.8 and 7.0 times than that of pure BiOBr.

  • OH, H2O2 and radical dotO2 played important roles in mineralization of TC and O2 evolution.

  • Boosting photocatalytic performance is due to the formation of S-scheme nano-heterojunction.

Abstract

Designing heterojunction photocatalysts with strong interfacial interaction and matched band structure is an effective way to reduce the recombination of photogenerated carriers. Herein, the exfoliated black phosphorus (BP) nanosheets were coupled with BiOBr nanosheets having higher Fermi level, and thereby it constructed a novel layered BP/BiOBr nano-heterojunction with chemically bonding, larger contact interface and unique band structures. BiOBr nanosheets were self-assembled on the surface of BP nanosheets by a facile liquid-phase ultrasound combined with solvothermal method. The photocatalytic performance for tetracycline (TC) degradation, oxygen evolution and H2O2 production rate of Sol-10BP/BiOBr was 7.8, 7.0 and 2.6 times than that of pure BiOBr, respectively. The in-situ generated H2O2 and radical dotOH became the main active species of mineralization and decomposition of TC. The novel S-scheme two-dimensional BP/BiOBr nano-heterojunction for boosting spatial charge separation retained the useful holes-electrons with higher redox ability, which was very beneficial for producing more radical dotOH, H2O2 and O2, and the photocatalytic activity was greatly improved.

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).

References (68)

  • S. Dong et al.

    Crystal structure and photocatalytic properties of perovskite MSn(OH)6 (M = Cu and Zn) composites with d10-d10 configuration

    Appl. Surf. Sci.

    (2019)
  • X. Dong et al.

    Visible-light-induced charge transfer pathway and photocatalysis mechanism on Bi semimetal@defective BiOBr hierarchical microspheres

    J. Catal.

    (2018)
  • H. Dong et al.

    Synthesis of g-C3N4 by different precursors under burning explosion effect and its photocatalytic degradation for tylosin

    Appl. Catal. B: Environ.

    (2018)
  • G. Drewelow et al.

    Work function investigations of Al-doped ZnO for band-alignment in electronic and optoelectronic applications

    Appl. Surf. Sci.

    (2019)
  • J. Fu et al.

    Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst

    Appl. Catal. B: Environ.

    (2019)
  • T. Hu et al.

    One-pot synthesis of step-scheme Bi2S3/porous g-C3N4 heterostructure for enhanced photocatalytic performance

    Mater. Lett.

    (2019)
  • M. Ji et al.

    Graphene-like boron nitride induced accelerated charge transfer for boosting the photocatalytic behavior of Bi4O5I2 towards bisphenol a removal

    Chem. Eng. J.

    (2018)
  • X. Jia et al.

    One pot milling route to fabricate step-scheme AgI/I-BiOAc photocatalyst: energy band structure optimized by the formation of solid solution

    Appl. Surf. Sci.

    (2019)
  • Y. Jiang et al.

    Enhancement of photocatalytic hydrogen evolution activity of porous oxygen doped g-C3N4 with nitrogen defects induced by changing electron transition

    Appl. Catal. B: Environ.

    (2019)
  • Z. Jiang et al.

    Enhanced photocatalytic CO2 reduction via the synergistic effect between Ag and activated carbon in TiO2/AC-Ag ternary composite

    Chem. Eng. J.

    (2018)
  • Z. Jiang et al.

    Photocatalytic reduction of CO2 to methanol by three-dimensional hollow structures of Bi2WO6 quantum dots

    Appl. Catal. B: Environ.

    (2017)
  • D. Jiang et al.

    Modified g-C3N4/TiO2 nanosheets/ZnO ternary facet coupled heterojunction for photocatalytic degradation of p-toluenesulfonic acid (p-TSA) under visible light

    Phys. E

    (2017)
  • X. Li et al.

    Hydrothermal synthesized novel nanoporous g-C3N4/MnTiO3 heterojunction with direct Z-scheme mechanism

    Electrochim. Acta

    (2017)
  • Y. Li et al.

    Oriented ZnmIn2Sm+3@In2S3 heterojunction with hierarchical structure for efficient photocatalytic hydrogen evolution

    Appl. Catal. B: Environ.

    (2019)
  • X. Li et al.

    Recent advances in 3D g-C3N4 composite photocatalysts for photocatalytic water splitting, degradation of pollutants and CO2 reduction

    J. Alloys Compd.

    (2019)
  • X. Li et al.

    Novel g-C3N4/h′ZnTiO3-a′TiO2 direct Z-scheme heterojunction with significantly enhanced visible-light photocatalytic activity

    J. Alloys Compd.

    (2019)
  • X. Li et al.

    Defect-assisted surface modification enhances the visible light photocatalytic performance of g-C3N4@C-TiO2 direct Z-scheme heterojunctions

    Chin. J. Catal.

    (2019)
  • Q. Liu et al.

    3D reduced graphene oxide aerogel-mediated Z-scheme photocatalytic system for highly efficient solar-driven water oxidation and removal of antibiotics

    Appl. Catal. B: Environ.

    (2018)
  • E. Liu et al.

    A facile strategy to fabricate plasmonic Cu modified TiO2 nano-flower films for photocatalytic reduction of CO2 to methanol

    Mater. Res. Bull.

    (2015)
  • F. Mei et al.

    Construction of Ag SPR-promoted step-scheme porous g-C3N4/Ag3VO4 heterojunction for improving photocatalytic activity

    Appl. Surf. Sci.

    (2019)
  • S. Peng et al.

    In situ loading of Ni2P on Cd0.5Zn0.5S with red phosphorus for enhanced visible light photocatalytic H2 evolution

    Appl. Surf. Sci.

    (2018)
  • K. Qi et al.

    A review on TiO2-based Z-scheme photocatalysts

    Chin. J. Catal.

    (2017)
  • K. Qi et al.

    Electroless plating Ni-P cocatalyst decorated g-C3N4 with enhanced photocatalytic water splitting for H2 generation

    Appl. Surf. Sci.

    (2019)
  • Y. Qin et al.

    Persistent free radicals in carbon-based materials on transformation of refractory organic contaminants (ROCs) in water: a critical review

    Water Res.

    (2018)
  • Cited by (436)

    View all citing articles on Scopus
    View full text