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 TM-E 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.