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Letter
Cite This: J. Phys. Chem. Lett. 2019, 10, 6672−6678

pubs.acs.org/JPCL

Versatile Nature of Oxygen Vacancies in Bismuth Vanadate Bulk and
(001) Surface
Franziska Simone Hegner,† Daniel Forrer,‡ José Ramón Galán-Mascarós,†,¶ Núria López,*,†
and Annabella Selloni*,§

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†

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Avinguda Paisos
Catalans 16, 43007 Tarragona, Spain
‡
ICMATE-CNR and INSTM, Via F. Marzolo 1, 35131 Padua, Italy
¶
ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
§
Princeton University, Department of Chemistry, New Jersey 08544, United States
S
* Supporting Information

ABSTRACT: Bismuth vanadate (BiVO4) has emerged as one of the most promising
photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the
photoelectrochemical efficiency, yet their electronic nature and contribution to n-type
conductivity are still under debate. Using first-principles calculations, we show that oxygen
vacancies in BiVO4 have two distinguishable geometric configurations characterized by
either undercoordinated, reduced VIVO3 and BiIIO7 subunits or a VIV−O−VIV/V bridge
(split vacancy), quenching the oxygen vacancy site. While both configurations have similar
energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split
oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V2O7 unit allows for
partial electron delocalization throughout the near-surface region, consistent with recent
experimental observations indicating that BiVO4(001) is an electron-rich surface.

O

BiVO4 are related to increasing the number of accessible
surface sites, thus favoring adsorption of oxidation reaction
intermediates, decreasing the band gap, and enhancing
interfacial charge transfer to the electrolyte.24,25 However, by
forming electron−hole recombination centers and hindering
charge-separation, O’s can also have a detrimental effect on
the PEC activity.23,25,26 Despite their important impact on the
behavior of BiVO4, conclusive studies on oxygen vacancies are
still limited and controversial.14,27 Although reduced V sites on
the BiVO4 surface were directly observed and attributed to
surface O’s,28 to our knowledge an unambiguous experimental determination of their nature and location with respect
to the surface has not yet been reported.
While several theoretical studies of vacancy formation in the
bulk suggest that O’s generate shallow intragap states,21,22,29
more recent work indicates that the O-induced electronic
states lie deep in the band gap, where they cannot contribute
significantly to n-type conductivity27,30 but may act as
polaronic carriers.14,15 Only few reports have considered
14,25,31
It was suggested that
O ’s on the surface of BiVO4.
those O’s have a beneficial influence on water oxidation on
the (001) surface because of better adsorption of reaction

ver the last two decades, bismuth vanadate, BiVO4, has
become one of the most important photoanode
materials for light-driven water splitting.1−4 Among the class
of n-type metal oxide semiconductors, it has demonstrated the
highest performance in photoelectrochemical (PEC) water
oxidation to date.4,5 In addition to its suitable band gap (2.4−
2.5 eV) and valence band edge position (∼2.8 V vs RHE), it
also shows superior electron−hole separation and hole
mobility.2,3 Notwithstanding, its efficiency is hindered by fast
surface recombination6,7 and low electron mobility.8,9 The
latter is typically ascribed to local lattice distortions forming
small polarons, which limit electronic conduction.9−15
Various experimental studies have reported that the PEC
performance of BiVO4 is improved by the formation of oxygen
vacancies ( O’s).16−18 The effect of O’s is typically ascribed
to enhancing n-type conductivity and therefore electronic
transport, as in many metal oxide semiconductors.18−22
Removal of an oxygen atom leaves two excess electrons,
which may be either mobile or (partially) bound to the
vacancy. In the latter case, the metal atoms (V and Bi)
surrounding the vacancy are expected to be reduced. For being
effective n-type dopants, the electronic states created by O’s
have to lie close to the conduction band minimum (CBM),
from where they can be thermally excited and act as shallow
donors. Otherwise, if deep, localized intragap states are formed,
electrons are trapped and hardly participate in electron
conduction.23 Other possible roles of oxygen vacancies in
© 2019 American Chemical Society

Received: August 31, 2019
Accepted: October 12, 2019
Published: October 12, 2019
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loc
Figure 1. (a) Pristine bulk 2 × 2 × 2 supercell of BiVO4 as used in the simulations (viewed from the [010] direction). (b) O : Localized oxygen
split
a IV
a II
a IV
b IV/V
vacancy forming undercoordinated V O3 and Bi O7. (b) O : Split vacancy between V and V
that results in a V2O7 dimer with a
shared (or split) oxygen, labeled Os. Reported distances were computed with the PBEsol functional. Classical oxidation states of the relevant atoms
are indicated. Color code: Bi, V, and O are green, yellow, and red, respectively (visualized with VESTA39).

intermediates and improved charge transfer on intrinsic24 and
W-doped25 BiVO4.
Herein, we used first-principles density functional theory
(DFT) calculations to study the role of O’s in the bulk and on
the most stable (001) surface of BiVO4. Our results show that
O removal leads to different possible geometric arrangements
with distinct electron structures, as shown in Figure 1.
DFT calculations were performed with the Quantum
Espresso (QE) software package,32,33 as described in the
Supporting Information. The vacancy formation energies EO
f
were referenced to the chemical potential of oxygen in the gas
phase (Supporting Information, section S1.2). It was recently
found that the used DFT functional has a large impact on the
positions of O-induced energy levels and hence their
contribution to conductivity.15,30 In order to address such
dependency of the electronic structure, we used functionals of
different levels of accuracy: GGA (PBEsol), GGA+U (PBEsol
+U with U = 2.5 eV25,34 (Supporting Information, section
S1.3)), and hybrid functionals (PBE0-10*/PBE0-10**) with
an optimized fraction (10%) of exact exchange (Supporting
Information, section S1.4).35 For the hybrids (PBE0-10*/
PBE0-10**), single-point calculations were carried out on
structures, from prior optimization with PBEsol (PBE0-10*)
or PBEsol+U (PBE0-10**). All structures have been uploaded
to the ioChemBD36 database and can be accessed under the
DOI 10.19061/iochem-bd-1-133.37
The photocatalytically active polymorph of bismuth
vanadate exhibits a monoclinic scheelite phase, ms-BiVO4
(ms is henceforth omitted).1,3 It contains BiO8 units and
VO4 tetrahedra, which are linked by bridging oxygens (Bi−O−
V) and stacked along the main [001] axis of the unit cell with
interplanar distances of 2.896 ± 0.002 Å, as shown in Figure
1a.38

The flexibility of the structure, as well as the versatile
chemistry of the tetrahedral VO4 subunits, allows for the
formation of two distinct types of oxygen vacancies:14,40,41 a
loc
lattice-centered or localized vacancy
O , leaving undera IV
a II
coordinated, reduced V O3 and Bi O7 (Figure 1b), and a
split
bridged or split vacancy, O , where one O atom (Os) is at
the bond-center of a Va−Os−Vb bridge (Figure 1c).42,43 The
latter is created when the neighboring VbO4 orthovanadate
from the adjacent layer rotates and moves toward the
undercoordinated VaO3, so that its oxygen can coordinate to
IV/V
the vacant site, forming V2 O7 pyrovanadate and reestablishing the 8-fold coordination sphere of Bi and its
oxidation state of III. This quenches the vacancy and leads to a
concomitant (partial) electron delocalization as discussed later
in the text and in more detail in the Supporting Information
(section S2.2). In such a distortion, the Va−Os distance
decreases from 2.732 Å (2.882 Å) to 2.038 Å (2.152 Å), while
the Vb−Os distance increases by only about 0.039 Å (0.031 Å)
with PBEsol (PBEsol+U) (see Table S2 for the changes in
bond length). Condensation of VO4 orthovanadates to
produce polyoxovanadates is well-known in solution41 and
has been identified also in solids between the crystalline layers
at the V2O5(001) surface, reducing the cost of O formation.44
split
However, to the best of our knowledge, O formation in
BiVO4 and its impact on the electronic structure has not yet
been investigated.
For bulk BiVO4 our calculations yield similar formation
split
loc
energies EO for both O and O vacancies. The differences
f
between them lie in a range of 0.2 eV with a transition barrier
≲0.2 eV depending on the employed DFT functional (see
Figures 2c and 3b and Table S3). We therefore expect both
types of vacancies to be present in synthesized BiVO4, because
during the calcination process at elevated temperatures
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split
loc
(typically greater than 400 °C)2−4
may
O
O and
interconvert. At higher n-type doping densities, the split
split
vacancy O may be favored, as its greater number of bonds
allows for better stabilization of additional electrons.42
We now turn to the (001) surface, the thermodynamically
most stable surface of BiVO4,34,45 where O’s are suggested to
form quite easily.25 The (001) surface is cut parallel to the Bi−
V layers such that the tetrahedral coordination of the VO4
tetrahedra is preserved and only 2 weaker Bi−O bonds are
broken, hence forming surface BiO6.45 We have introduced
oxygen vacancies in different surface and subsurface layers and
calculated their formation energies EO as shown in Figure 2
f
and Table S4.
Upon relaxation the surface Bi−V plane moves inward,
decreasing the interplanar distance in the subsurface by 0.16 Å
without undergoing reconstruction.45,46 This compression
leads to a higher density of O atoms in the subsurface,
which in turn facilitates the removal of oxygen. This decreases
the vacancy formation energy in the subsurface, i.e. in positions
2 and 3, as indicated in Figure 2. In contrast, the interlayer
spacing in the subsubsurface increases, thus destabilizing
oxygen vacancies at positions 4 and 5. A correlation of surface
relaxation and oxygen vacancy formation energies is given in
the Supporting Information (Table S5 and Figure S5). More
importantly, the interplanar compression in the subsurface
largely favors the formation a V−O−V bridge between surface
and subsurface vanadiums upon O removal. The resulting
split
O configuration in the subsurface is stabilized by more than
1 eV, independent of the employed DFT functional (see
orange-shaded area in Figure 2c), and most likely to be
predominant. In other words, the (001) surface represents an
split
energetic sink toward the formation of subsurface O . This
finding is consistent with a recent report by Rossell et al. using
local electron energy-loss spectroscopy, with which they
observed reduced V sites on the BiVO4 surface.28 The
accumulation of O vacancies close to the surface was also
suggested by a recent photoemission study.47 The authors
found that treating BiVO4 with oxygen plasma shifts the Fermi
level downward and increases the workfunction, which could
be due to reducing the number of O’s near the surface. Our
computed workfunctions show a similar behavior (Supporting
Information, section S4.2.1). The presence of oxygen vacancies
in the subsurface gives workfunctions of around 5.0−5.3 and
split
loc
4.8−5.0 eV for O and O , respectively, while the pristine,
defect-free (001) surface possesses a workfunction of 6.2−6.4
eV (see Table S7).
split
When removing O in the subsurface and forming O , the
surface V rotates, so that it builds a V−O−V bridge, by
pointing one of its V−O bonds up, perpendicular to the
split
surface, as shown in Figure 2b. This further facilitates O
formation in contrast to the bulk where the VO4 unit needs to
move to fill the empty coordination site at the adjacent VO3.
Climbing image nudged elastic band (CI-NEB) calculations
(Figure 3) show that in the bulk there is an energy barrier of
∼0.2 eV (∼0.1 eV) for PBEsol (PBEsol+U) to be surmounted
split
loc
when going from O to O , whereas in the subsurface this
transition is quasi-barrierless (0.0 eV for PBEsol, 0.1 eV for
PBEsol+U).
For the discussion of the electronic structure of the different
types of vacancies we focus on hybrid functional results, which

Figure 2. (a) Pristine BiVO4(001) surface (viewed from [010]
direction), showing the positions of the O atoms that are removed to
loc
split
form localized vacancies O . (b) Structure of a split vacancy O in
the subsurface. (c) O formation energies EO as a function of position
f
with respect to the (001) surface. Values computed with different
functionals are reported. The orange-shaded area highlights the
split
energetic sink for O in the subsurface (between positions 2 and 3).
loc
split
O
The bulk Ef ( O and O ) is shown as dark shaded area on the
right. Color code: Bi, V, and O are green, yellow, and red,
respectively.

loc
split
Figure 3. (a) Transition from localized ( O ) to split ( O ) oxygen
vacancy (b) in the bulk and (c) in the (001) subsurface of BiVO4.
The energy profile connecting the two vacancy structures was
obtained from NEB calculations with PBEsol and PBEsol+U
functionals. ΔE on the ordinate represents the relative energy with
respect to the localized vacancy.

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Figure 4. PBE0-10 results of chemical structures with relevant bond lengths and (formal) oxidation states, atom-projected density of states (aligned
against the VBM; filled colors: occupied electronic states, empty colors: nonoccupied states; middle), and integrated local density of states (blue,
loc
isovalue 0.0015 per electron) of the vacancy-induced electronic state(s) for (a) O , (b) Osplit*, and (c) Osplit** in the (001) subsurface (viewed
from the side).

Table 1. Relevant Distances d(Va−Os) and d(Vb−Os);
Vacancy Formation Energies EO; Position of Trap States
f
with Respect to the Conduction Band Minimum (CBM);
ECBM − Etrap (Singly or Doubly (d) Occupied); Bader
Charges (q);48 and Magnetizations μ on Va, Vb, and Bi for
the pristine (001) Surface and the Three O Vacancy
split
loc
split
Configurations O , O *, and O ** in the Subsurface
(at Position 2) Calculated with PBE0-10(*/**)a

typically provide a good description of the electronic
properties of BiVO4.12−14,35 A detailed discussion on the
functional dependency is given in the Supporting Information
(section 5). For the subsurface O’s, we found three
distinguishable configurations, while for the bulk even more
electronic states with similar energies may be accessed.14
Those additional bulk states are further discussed in the
Supporting Information, while herein we focus on the (001)
subsurface. Figure 4 presents the geometric arrangements
(including formal oxidation states) on the left, the projected
densities of states (DOS) in the middle, and the spatial
distributions of the vacancy-induced intragap state(s) on the
loc
right for the three configurations O (Figure 4a), Osplit*
split
(Figure 4b), and O ** (Figure 4c). The relevant structural
and electronic parameters for the subsurface are summarized in
Table 1.
loc
Localized O s (Figure 4a) form vacancy-trapped electronic
14
states: the excess electrons are trapped at the vacancy site,
undercoordinated VO3 and BiO7 centers. Therefore, the
adjacent Va gets reduced from +V to +IV, and Bi from +III
to +II, assuming classical oxidation states (Tables 1 and S8).
The nonbonding electron pair induces a deep, doubly
occupied singlet state 1.0−1.2 eV below the CBM. This
mixed electronic state is composed mainly of the Va t2 set (with
decreasing contributions: dyz, dxy, dxz) and Bi px and pz orbitals,
as shown in Figure 4a in the middle and on the right.
split
When forming the split vacancy O in the subsurface
(Figure 4b, c), the two excess electrons are not trapped
anymore as a bound pair but split onto different V centers,
while Bi no longer bears additional electrons as its 6-fold
surface coordination sphere is mostly re-established. We found

pristine
surface
d(V −O ) (Å)
d(Vb−Os) (Å)
d(Bia−Os) (Å)
EO (eV)
f
ECBM − Etrap (eV)
q(Va)/|e|
q(Vb)/|e|
q(Bi)/|e|
μ(Va)/μB
μ(Vb)/μB
μ(Bi)/μB
a

a

s

2.894
1.762
4.348

2.20
2.23
2.08
0.00
0.00
0.00

loc
O

2.224
1.766
3.767
3.98
1.22 (d)
1.89
2.22
1.56
0.00
0.00
0.00

split
O *

1.783
1.946
2.295
3.08
1.30, 0.35
2.18
1.99
2.00
0.02
0.64
0.00

split
O **

1.945
1.905
2.259
3.04
1.36, 1.00
2.09
1.99
2.01
0.67
0.66
0.00

Os is the O atom coordinated to subsurface Vb in pristine and
configurations.

loc
O

split
two distinct O configurations with equivalent energies,
depending on whether the input geometry was taken from
PBEsol (PBE0-10*) or PBEsol+U (PBE0-10**). In analogy to
split
split
the calculation method, these are labeled O * and O **,
respectively. We note here that the electronic configuration
obtained with a semilocal GGA functional (PBEsol) is similar

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crystal direction and accumulation of charges at the (001)
surface.51,52
To further understand the implications of the different
subsurface O states, we also analyzed the layer-resolved DOS
(Figures S7 and S8).53 We see that the contribution of the
surface layer to the valence band maximum is slightly reduced
split
when creating O , as compared to the pristine surface and
loc
O . The surface layer does not significantly contribute to the
CBM, which aids photooxidation because photoexcited
electrons tend to go to the bulk and not to the surface. A
more detailed discussion is given in the Supporting
Information (section S4.2.2).
Taken together, we suggest that oxygen vacancies in BiVO4
do increase conductivity. In the bulk, O removal allows for a
reversible formation of V−O−V bridges, which increases both
electronic and ionic conduction: Partial electron delocalization
split
in O leads to polaronic14,15 or delocalized charged states at
the bottom of the conduction band (Figure S6), which
promote electron transport. Further, the moderately low
transition barriers for consecutive forming (≲0.2 eV) and
breaking (≲0.4 eV) of V2O7 from neighboring VO3 and VO4
(Figure 3b) units may enable ionic conduction through oxygen
migration (Supporting Information, section S5.5).40 Considsplit
ering the (001) surface, O ’s accumulate in the subsurface
where they are highly stabilized and formed without barrier.
The large energy barrier (∼1 eV; Figure 3c) to the breaking of
V2O7 in the subsurface acts as a trap for oxygen vacancies. As
such, electrons transfer from the surface plane (Va) to the
subsurface and sub-subsurface where they partially delocalize,
leading to enhanced electronic conduction parallel to the
(001) facet.51
In conclusion, two different types of oxygen vacancies were
found in BiVO4: one forming a vacancy-trapped charge state
with the two surplus electrons paired and localized at the
vacancy site, the other quenching the vacancy by bridging V−
O−V and concomitantly delocalizing one electron. While in
the bulk both structures are almost isoenergetic and may
coexist at thermal equilibrium, the (001) subsurface presents a
large energetic sink for the creation of V−O−V units. This
may enhance electronic conduction parallel to the (001)
surface. We investigated the impact of vacancies on the
electronic properties of BiVO4 for the first time, not only in
bulk but also on the most relevant surface, and show that the
defective surface is a very promising candidate to explain the
photochemical properties of BiVO4.

split
to O * and the configuration calculated with PBEsol+U is
split
split
split
similar to O **. Both O * and O ** present a lower
intragap state 1.30−1.35 eV below the CBM mainly localized
on the bridge forming subsurface Vb, which is therefore
reduced from +V to +IV. When forming the Va−Os−Vb bridge,
charge transfers from the surface Va to the subsurface Vb where
it becomes stabilized. This is not the case for the bulk, where
Va and Vb are equivalent (Supporting Information, section
S4.1). The resulting electronic state is to the largest extent
composed of the Vb dz2 orbital, which hybridizes with dxy and
dx2−y2 as the subsurface V loses its ideal tetrahedral symmetry,
and has some contribution from adjacent oxygen atoms.
split
The difference of the two configurations ( O */**) is
split
given by the nature of the upper intragap state. For O * this
state is 0.3 eV below the CBM and localized on different
subsurface and subsubsurface V atoms, forming polaron-like
states of dz2 symmetry, similar to those observed in the bulk.14
split
For O ** the upper intragap state is much deeper inside the
gap, 1.0 eV below the CBM, and is localized on the bridgeforming, reduced surface Va IV with mixed d-orbital contributions. Isoenergetic singlet and triplet states were obtained for
the split vacancy configurations. This suggests that the
separated electrons interact so weakly that their spin-pairing
energy is negligible.
The reason for the critical dependence of the electronic
structure on the geometry may be explained by the variations
in the relevant V−O bond distances, which are given in Figure
4 and Table 1. Every time the excess electrons of the O
vacancy are accepted by a V atom, they are placed in the t2
(and e) orbital sets, which derive from tetrahedral splitting in
the VO4 environment and which have a slightly antibonding
character with respect to the V−O bond.49 The shorter the V−
O distance, the larger the antibonding character, and therefore
the higher the energy cost to populate the 3d orbitals of the
split
respective V atom. In the O * geometry the Va−Os bond
length is only 1.783 Å and Va does not bear a significant
amount of excess electron density, whereas the longer Vb−Os
distance (1.946 Å) allows for electron localization at Vb. For
split
the symmetric O **, both V−Os lengths are larger than 1.9
Å and the excess electron is accepted by both bridging V sites
Va and Vb. It may also be argued vice versa, i.e. electron
localization at V atoms leads to longer V−O bond lengths
because of populating antibonding states.
split
split
Because the O * and O ** electronic states depend
crucially on the bond distances, which constantly vary because
of thermal motion at room temperature, it is reasonable to
assume that both electronic configurations coexist in a thermal
equilibrium under experimental conditions in the BiVO4
photoanode.50 This means the second higher-energy electron
is either localized on surface Va or delocalized in the (subsplit
)subsurface. Similar to the latter O *, a two-dimensionally
delocalized state has recently been reported for the WO3
surface and is expected to be favorable for water oxidation
catalysis.50 It may lead to enhanced electronic conduction
parallel to the (001) surface and therefore contribute favorably
to n-type conduction. Anisotropic conduction within the Bi−V
planes was indeed experimentally measured.9 The favored
stabilization of O in the subsurface and the two-dimensionally
extended charge delocalization could also explain experimentally observed preferential charge separation along the [001]

■

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpclett.9b02552.
Computational methods, geometrical and electronic
structures, vacancy formation energies, relevant energies
(such as band gaps, trap state energies, and work
functions), and a detailed functional comparison (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: nlopez@iciq.es.
*E-mail: aselloni@princeton.edu.
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Franziska Simone Hegner: 0000-0002-7645-4418
Daniel Forrer: 0000-0002-1969-3842
José Ramón Galán-Mascarós: 0000-0001-7983-9762
Núria López: 0000-0001-9150-5941
Annabella Selloni: 0000-0001-5896-3158
Notes

The authors declare no competing financial interest.
All structures can be found at the ioChemBD database under
the DOI 10.19061/iochem-bd-1-133.37

■

ACKNOWLEDGMENTS
The authors thank the groups of Prof. S. Gimenez and Prof. J.
Durrant, in particular S. Selim, for fruitful discussions about
oxygen vacancy defects. This work was funded by the
European Union’s Horizon 2020 project A-LEAF (Grant
Agreement No. 732840); the Spanish Ministerio de Ciencia,
Innovación y Universidades (RTI2018-101394-BI00); and the
“LaCaixa” -Severo Ochoa International Programme of Ph.D.
Scholarships for F.S.H.’s predoctoral grant. The authors further
thank the Barcelona Supercomputaing Centre (BSC-RES) for
generous computational resources. A. S. acknowledges the
support of DoE-BES Division of Chemical Sciences, Geosciences and Biosciences under Award DE-SC0007347.

■

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