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].