3.3. Impurity levels of 3d transition metals in BiOBr forbidden band
The TDOSs of BiOBr and Ti-, V-, Fe-, Cr-, Co-, Ni-, Cu-doped BiOBr were plotted in Figure 5 within the range of -6 to 5 eV. It is found that the electronic structural features of pure BiOBr remain intact except for the appearance of IELs, which mainly originate from TMs 3d electron states, within the BiOBr forbidden band. In general, the present of IELs could provide more photoexcited charge carriers under the condition of less absorption energy than pure BiOBr[36], leading to a repression in the recombination rates of photogeneratede --h + pairs and an improvement in the optical absorption ability. Besides, except for Ni-doped BiOBr system, the band gaps of other TMs doped BiOBr increase to the different extents, and the shift of relative positions for VBM and CBM occurs, leading insuperably to the change of redox potentials of BiOBr. Surprisingly, all the doping systems still retain an indirect-band-gap advantage of BiOBr, suppressing the recombination probability of photoinduced e --h + pairs.
The calculated band gaps of Ti-, V-, Fe-, Cr-, Co-, Ni-, Cu-doped BiOBr are 2.206, 2.203, 2.197, 2.193, 2.195, 2.171, 2.189 eV, respectively. The increased band gaps contribute to the improvement of redox capability and the enhancement of separation efficiency for photogenerated e--h+ pairs, however, the band gap of Ni-BiOBr system is only narrowed slightly. In addition, the VBMs of V-, Fe-, Co-, Ni-, Cu-doped BiOBr shift to the more negative direction, locating at -0.406, -0.448, -0.351, -0.176, -0.270 eV, respectively, revealing that the oxidation abilities of these doped-BiOBr systems exhibited a significant enhancement after the introduction of V, Fe, Co, Ni, Cu atoms, the reasons are why that such reported as-prepared photocatalysts demonstrate excellent photocatalytic activity in terms of degrading organic pollutants[13, 14, 37, 38].
The PDOSs of Ti-, V-, Cr-, Fe-, Co-, Ni-, Cu atoms in such doping BiOBr are calculated and shown in Figure 6 (a-g). For Ti-, V-, Fe-doped BiOBr, the PDOSs of Ti, V, Fe atoms show the similar 3d electron states distribution. Ti 3d states mainly contribute to CBM and CB with bandwidth of 1.37~4.05 eV, as well as a small contribution for VB, nevertheless, there is an IEL (at 1.641 eV within the forbidden band), which originating from Ti 3d down-spin state forms, exhibiting ann-type semiconductor property, which should help photoexcitede- transfer to more reactive sites of catalyst under light irradiation. Doping V into BiOBr crystal will induce two IELs located at -0.116 and 1.797 eV, originating from V 3d down-spin states. For Fe-doped BiOBr system, two obvious IELs located at 0.966 and 1.468 eV are from the contribution of Fe 3d down-spin states. Meanwhile, Fe 3d down-spin states have higher density and localized distribution than Ti and V. As everyone knows, appropriate depth of IELs will play an important role in the transition process of proton, so the IELs in the Fe-doped BiOBr system should act as a springboard for electron transitions from the VB to CB, and then provide more photoexcitede- to CBM and more h+to VBM, improving the separation of photoexcited charge carriers effectively. Experimentally, a novel Fe3+-doped BiOBr magnetic Janus micromotors, which has excellent solar Fenton catalytic activity, was reported by Liu and co-workers[39]. More worthy of reference for the theoretical explains of such previous experiments, novel super paramagnetic BiOBr/Fe3O4 was fabricated via an in-situ growth method that can easily achieve the norfloxacin (NOR) recovery from solution[40].
As displayed in Figure 6(d-f), the PDOSs of Cr, Co, Ni atoms exhibit the similar distribution characteristics of electronic states. Their 3d up- and down-spin electronic states mainly contribute to VB and IELs, and surprisingly, with their outermost electron configurations gradual increase, the distances between the IELs (3d down-spin states) and VBM are reduced from 2.193 to 0.397 eV, and the localization of 3d down-spin states gradually weakens, it should be because of the high occupancy of d-orbitals can lower the energy of d electron states, thus moving the IELs away from CB and contributing to the VBM, which maybe lead to a redshift of optical absorption edge.
For the PDOSs of Cu atoms plotted in Figure 6(g), Due to the stable outermost electron configuration, Cu 3d states show the symmetric up- and down-spin. There exists an IEL located at 0.308 eV, dominating by Cu 3d electron states. The emergence of new IEL with the stronger density slightly above VBM can make the electronic transition from the original one-step excitation to two-step process, consequently, reducing photocatalytic threshold and extending the light absorption range.
We adopted electric dipole approximation to calculate optical properties. The probability of a linear transition between occupied and unoccupied states excited by photons is determined by the electronic structure[42]. Figure 7 shows the calculated absorption spectrum of BiOBr and Ti-, V-, Fe-, Cr-, Co-, Ni-, Cu-doped BiOBr. Due to inter-band absorbance, pure BiOBr has an absorption band edge at 448 nm and a wide optical response range. After the TMs atom incorporated into BiOBr, the optical absorption band edges exhibit the redshift to different extents, in particular, Fe-, Co-, Ni-, Cu-doped BiOBr have obvious tail that can expand absorption in the visible-light region of 400-800 nm[43], surprisingly, Liu reported that Fe3+-doped BiOBr exhibited an obvious shift toward long wavelength region compared to pure BiOBr experimentally, in agree with our theoretical results[39]. Firstly, the narrower band gap facilitates electronic transitions by absorbing low-energy photons, enhancing light harvesting capacity, such as Ni-doped BiOBr catalysts. IELs, which appear in the bandgap, then could act as footstep in the electronic transition, causing the electronic transition from original one-step excitation to two-step process, or even multi-step excitation, resulting in a lower threshold for photoexcitation between the VBM and CBM, owing to higher oxidation states of the 3d TMs ions required for charge compensation[44]. In addition, Cu with localized surface plasmon resonance (LSPR) could effectively improve the collection and conversion of light energy. These are the three main reasons why those optical properties are improved after the incorporation of 3d TMs into BiOBr crystal. Therefore, it can be speculated that if V-, Cr-, Ni-, Cu-doped BiOBr photocatalyst are prepared experimentally, it will be obtained such similar enhanced visible light response and performances.
When additional atoms are introduced into crystal lattice, the association between outer-shell orbitals of these atoms and energy band electronic states induces IELs, and regulate the electronic structures. In order to clearly illustrate the effect of modulation of the electronic structure on the light response and redox potential after the introduction of the 3d TMs, Figure 8 shows the electronic energy level distribution position and light absorption condition of 3d TMs-doped and pure BiOBr. On the one hand, the VBM shifts to more negative direction than pure BiOBr when V, Fe, Co, Ni, Cu atoms are inserted into BiOBr crystal lattice, and consequently, obtaining more excellent oxidation ability, the analysis results of the Fe- and Co-doped BiOBr catalysts are good consistent with previous reports in the literature[7, 16]. On the other hand, due to the change in the electronic structure via 3d TMs doping, the Mn-, Fe-, Co-, Ni-, Cu-doped BiOBr obtain strong visible-light absorption, and the absorption band edge of Ti-, V-, Cr-doped BiOBr exhibit a redshift.
3.4. Charge carriers separation efficiency and structural stability of 3d TMs-dopedBiOBr
To further investigate charge separation efficiency, the effective masses of photogenerated charge carriers of 3d TMs-doped BiOBr were calculated based on our obtained results of electronic structure. All the calculations about the effective mass of e - andh + according to the following equation:
\(m^{*}\ =\pm\frac{\hslash}{d^{2}E/dk^{2}}\) (1)
where m * is the effective mass of photoexcited charge carriers,\(\hslash=h/2\pi\), h is the planck constant and\(d^{2}E/dk^{2}\) is the coefficient of quadratic term in quadratic fit of \(E(k)\) curves for band edge[45]. Meanwhile, the relative ratio (D ) of effective masses can be evaluated via an equation: \(D=\frac{m_{h}^{*}}{m_{e}^{*}}\), wheremh * and me * represent the effective mass of h+ and e- , respectively[42]. In general, the higher values ofD imply a lower recombination probability of photogeneratede --h + pairs, and the smaller effective masses of photoinduced h+ or e- suggest that carriers have the higher delocalization and mobility. The calculated effective mass of h+ and e-as well asD of 3d TMs-doped BiOBr are illustrated in Table 1. It is easy to find that the D value of Mn-, Ni-, Zn-doped BiOBr is higher than other systems, confirming that the recombination rates of photogeneratede --h + pairs decrease after the introduction of Mn, Ni, Zn atoms, consistent with the analysis results from the PDOS of Mn-doped BiOBr system. Surprisingly, for Fe-doped BiOBr system, the effective mass of charge carriers diminishes slightly, indicating the greater mobility of photogeneratede - and h + than pure BiOBr, then the carriers will reach to surface reaction sites within the lifetime easily, which is why that Fe-doped BiOBr samples had superior highly-efficient photocatalytic performance[19].
It is well known that the structure stability is a significant parameter for evaluating the performance of photocatalytic material. Formation energies are calculated to judge thermodynamic stability of doping systems and ensure the most suitable doping site, importantly, theoretical calculated results can guide the preparation of structurally stable photocatalysts in the experiment[46]. That being the case, we calculated\(\text{\ E}_{\text{form}}\) E form of 3d TMs-doped BiOBr by the following equation:
E form=E TM-BiOBr-(E BiOBr+E TME Bi) (2)
where E BiOBr, E TM-BiOBr,E TM and E Bi represent the total energy of BiOBr, 3d TMs-doped BiOBr, an isolated 3d TMs and Bi atom, respectively[17]. TheEg andE form of 3d TMs-doped BiOBr were tabulated in Table 2.
The formation energies of Ni-, Cu-, Zn- doped BiOBr are 0.781, 3.174 and 4.199 eV, respectively, indicating that it is an endothermic reaction when Ni, Cu, Zn atoms replace Bi atom into BiOBr lattice. In contrast, the formation energies of Sc-, Ti-, V-, Cr-, Mn-, Fe-, Co-doped BiOBr are -4.721, -4.187, -3.707, -3.794, -3.182, -1.993, -0.013‬ eV, respectively, implying the structure stability of Sc, Ti, V, Cr, Mn, Fe, Co-doped BiOBr catalysts, some systems have been reported by the research group[10, 12-14]. According to calculated results of such formation energies, we can also intuitively understand the priority order of 3d TM atoms substituting Bi atom into BiOBr crystal lattice: Sc>Ti>Cr>V>Mn>Fe>Co>Ni>Cu>Zn, associated with their atomic radius. Finally, in order to clarify the relationship between the forbidden band width and structure stability of 3d TMs-doped BiOBr, we plotted their relevance diagram in Figure 9.
In order to intuitively reveal the doping effect of 3d TMs on photo response, structural stability and recombination probability of photoinduced carriers of BiOBr, based on the calculation results, a 3D scatter plot has been drawn in Figure 10. The three coordinates after the 3d TMs represent their priority order in inhibit charge carrier recombination, structural stability and light response, respectively. For example, the coordinates of the Mn-doped system are (2, 5, 2), indicating that the introduction of Mn atom could inhibit the recombination of photogeneratede- -h + pairs effectively and enhance visible light absorption significantly, while the thermodynamic stability of such system is not outstanding. Our findings should provide theoretical guide for experimenters to design novel photocatalytic material with extensive application prospects.