1. INTRODUCTION
In the past few decades, increasing concerns had been paid for energy crisis and environmental pollution issues, and photocatalysis technology, as a promising route for renewable energy development and environmental governance, has drawn more and more attentions[1]. BiOBr, was deemed as the most promising photocatalyst due to the special structure-dependent photocatalytic performance, excellent chemical stability and environmental-friendly quality[2], it possessed a lamellar structure, which can offer enough space for polarizing the corresponding atoms and orbitals and provide self-built internal electric field (IEF), and consequently, obtaining that the induced dipole could accelerate efficiently the separation and migration of photoinduced electron-hole (e- -h +) pairs, improving the photocatalytic performance[3-5]. More importantly, BiOBr shows an appropriate indirect band gap, implies that photogenerated e- must cross a certaink- space to be emitted to CB, further to prevent the recombination rates of photoexcitede- -h +pairs[5, 6]. Nevertheless, there exist low quantum efficiency and fast photoexcitede- -h +pairs recombination rates as the “bottleneck” for satisfying with the requirement of practical applications[7, 8]. Therefore, it is rather critical to explore novel strategies to improve the photocatalytic performance of BiOBr.
In recent years, domestic and foreign scientists based on the basic principles of semiconductor photocatalysis, from three aspects of expanding light absorption, improving carrier separation efficiency and enhancing surface interaction to strengthen the photocatalytic reaction[9], and among the normal modification strategies, the construction of doping systems[10-15] is one of the most representative ways to enhance visible-light response and improve photocatalytic activity of BiOBr. TMs doping may induce potential electron traps to prolong the lifetime of photoexcited carriers and introduce magnetic feature, as well as change energy band structure to increase light absorption ability and adjust the redox potentials, thus TMs-doped BiOBr used to obtain high photocatalytic activity[16]. For instances, Wang et al. adopted double self-assembly method to successfully synthesize Ti-doped BiOBr photocatalyst with more superior photocatalytic activity than undoped BiOBr in terms of degrading RhB, attributing to the synergetic effect of larger specific surface area, unique microspheres structure and Ti doping[10]. Jiang et al. reported a Ti doping and Ag decorating BiOBr microsphere with excellent photocatalytic activity and durability[11]. For Fe-doped BiOBr photocatalysts, Fe ions played key role for doping effect in the self-assembly process of hollow microspheres, revealing high photocatalytic and electrochemical performance[13]. Recently, Guo et al. utilized experimental and DFT methods to investigate Zn-doped BiOBr system, and theoretical calculations reveal that there still maintain the advantages of indirect band gap after the introduction of Zn, and Zn 3d states with deeper energy levels could induce negligible effect on VB and CB of Zn-doped BiOBr system[15]. Furthermore, DFT calculations were adopted to evaluate the influence of Co doping on the electronic structure of BiOBr by Xia et al., revealing that there is an additional energy level introduced into the forbidden band of Co-doped BiOBr system[17]. Based on above-mentioned results, it could be inferred that the doping of 3d TMs should affect the electronic and optical properties as well as macro-performances of BiOBr to a certain extent.
However, there are few reports about exploring a series of 3d TMs doped modulation electronic structures to influence the micro- and macro- properties as well as both correlations and redox potentials of BiOBr. Motivated by the previous experimental and theoretical achievements[15-18], we investigate carefully and systematically the electronic structure of 3d TMs-doped BiOBr using DFT+U calculations. Given our theoretical findings, it is to clarify that how the electronic structure influence visible light absorption and provide possible explanations for previous experimental observations, more importantly, also could provide significative guidance to synthesize BiOBr-based materials with highly photocatalytic activity.