3.1. Electronic properties of pure BiOBr
In order to confirm the influence of electronic structure on visible-light response of BiOBr within and without the doping of 3d TMs (TMs=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), the relaxed atomic structure with 3×2×2 BiOBr supercell is deemed as the basic research subject. The obtained lattice constants after geometry optimization are a = b = 3.909 Å, c = 8.678 Å, in accordance with the previously reported literatures[24, 28]. The electronic band structure of BiOBr supercell is projected in the Brillion zone, as shown in Figure 2(a). Generally, the zero of energy axis is set as Fermi level (Ef ).
The calculated band gap (Eg), the distance between valence-band maximum (VBM) and conduction-band minimum (CBM), of 3×2×2 BiOBr supercell is 2.173 eV using DFT+U method, smaller than the experimental value 2.6~2.9 eV[29, 30], which is caused by the well-known inherent shortcomings of GGA-PBE functional[31]. Since the carrier effective mass and mobility are directly related to the band wave curvature, it can be seen that the electronic energy levels at CBM of BiOBr are high dispersive, revealing that the smaller effective mass and higher mobility of photoinduced e - in CBM. Besides, VBM is the between G and F point, yet CBM is located at G point, which belong to the various locations in the Brillouin zone and exhibit the indirect bandgap characteristic of BiOBr with special bandgap transition pathway that photogeneratede - will be transited from VB to CB through a certain k -space and leave h + on VB, which should inhabit the recombination rates of photogeneratede --h + pairs and improve the photocatalytic performance effectively[5, 6].
Figure 2(b) illustrates the calculated total density of states (TDOS) and projected densities of states (PDOSs) of BiOBr. Calculations indicate that the VBM of BiOBr is mainly determined by Br 4p and O 2p states, whereas CBM is mainly determined by Bi 6p states and a small contribution of Bi 6s electron states, therefore, pure BiOBr might belong to the p-to-p (O 2p and Br 4p to Bi 6p) charge-transfer type. Our analysis results of electronic structure are in good agreement with the previous published results using DFT approach, revealing that the adopted calculation method and obtained results should be reasonable and credible[28, 32].
3.2. No impurity levels of 3d transition metals in BiOBr forbidden band
As shown in Figure 3(a), compared with TDOS spectra of Sc, Mn, Zn-doped BiOBr and pure BiOBr, the calculated forbidden band widths of Sc-, Mn- and Zn-doped BiOBr are 2.200 eV, 2.105 eV, 2.112 eV, respectively, very closer to the bandgap (2.173 eV) of BiOBr, and the introduction of Sc, Mn or Zn atom has no effect on indirect-band-gap characteristic of BiOBr. Additionally, there is no IELs appearing in the forbidden band of Sc-, Mn-, Zn-doped BiOBr. Meanwhile, the PDOSs of Sc-, Mn- and Zn-doped BiOBr are shown in Figure 3 (b-d). It is found that Mn 3d up-spin states mainly contribute to VBM and occupy VB with a bandwidth of 5.54 eV, which will trap photoexcited h +, enhancing the separation efficiency of photogenerated carriers in the Mn-doped BiOBr system to a certain extent[16]. Meanwhile, the previous experimental work indicated that the introduction of Mn could bring about oxygen vacancies to obtain more active sites on the BiOBr surface[33], the reasons why is that both of Mn-doped BiOBr[12] and BiOBr-Mn3O4 nanoheterojunction photocatalysts[34] exhibited the higher photocatalytic performance than pure BiOBr.
For Sc-, Zn-doped BiOBr, 3d up and down spin states of Sc and Zn atoms in the energy band are symmetric, indicating that the total magnetic moment of BiOBr system is still zero after the introduction of Sc or Zn atom, associated with their relatively stable outermost electron configurations of Sc 3d14s2 and Zn 3d104s2. Furthermore, the introduction of Sc results in the bandgap increase from 2.173 to 2.200 eV, indicating that required photon energy to excitee - transition of Sc-doped BiOBr system, higher than BiOBr. Similarly, for Zn-doped BiOBr system, there is no IELs observed within the forbidden band, because e -on closed 3d orbitals of Zn atom is not interactive to offer electronic states to band gap[15]. Obviously, there is a significant variability in the electron contribution of Zn 3d states compared with Sc and Mn 3d states. Zn 3d up- and down-spin states contribute to deeper energy levels in VB, lower than VBM, locating at the range of -3~-6 eV with much better oxidizability, consistent with reported experimental findings of Zn-doped BiOBr system by Guo et al.[15].
To further comprehend the doping effects of 3d TMs on the optical properties of BiOBr, the optical absorption spectrums for BiOBr and Sc, Mn, Zn-doped BiOBr are shown in Figure 4. Pure BiOBr shows a wide light response range with an absorption edge at 448 nm, corresponding to a band gap value of 2.9 eV, whereas there is no obvious effects in case of Sc- or Zn-doped BiOBr. Therefore, we believe that Sc or Zn doping only affect the electronic localization in the region of VB or CB, has negligible influence on the optical properties of BiOBr. However, for Mn-doped BiOBr system, the absorption intensity between 450 and 800 nm has an obvious enhancement compared with pure BiOBr, and the visible-light response is enhanced, which may be attributed to the contribution of Mn 3d states to VB. The above calculation results are in good accordance with the pervious published references[12, 15, 35].