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Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23 Page 5 of 21

edges of some semiconductors are commonly more positive than the thermodynamic potential for the OER,
leading to a high oxidation potential for holes in the valence band. The modulation of holes is a feasible
method to enhance photocatalytic performance. Researchers have tuned the electronic structure of the
semiconductor by introducing heteroatoms, as well as introducing defect states to adjust the atomic
structure to increase the charge separation efficiency and further carry out the surface reaction.

Heteroatom doping
The introduction of impurity states through heteroatom doping with, for example, metal cations, S, C, N
and O in semiconductors provides the opportunity of trapping photogenerated electrons or holes, so that
the photogenerated electrons and holes will be located in different regions, respectively [29,30] . This strategy
can reduce the overlap of photogenerated electrons and holes and improve charge transport and
separation [31,32] . Zhang et al. introduced non-metallic N atoms into NiFeO catalysts and used them as
x
cocatalysts to modify a BiVO photoanode, in which the O sites in NiFeO and BiVO were partially
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x
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replaced by low-electronegativity N atoms, leading to their electronic reconfiguration . The morphological
[18]
and structural characterization of the BiVO /N:NiFeO photoanodes with a rough flocculent structure is
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x
shown in Figure 3A-D. The weak electron-attracting capacity of N atoms leads to electron enrichment on Fe
and Ni. The electron injection from Ni atoms into the V sites in the BiVO lattice is beneficial for improving
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the stability of oxygen evolution, while the Fe sites can effectively attract holes to promote the PEC activity.
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The final obtained BiVO /N:NiFeO exhibited an excellent photocurrent density of 6.4 mA·cm at 1.23 V
x
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(vs. a reversible hydrogen electrode (RHE)) under light [Figure 3E]. The half-cell applied bias photon-to-
current efficiency of the BiVO /N:NiFeO photoanode (1.9% at 0.73 V vs. a RHE) was also much higher than
x
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that of BiVO /NiFeO (1.1% at 0.8 V vs. a RHE) and pristine BiVO (0.29% at 0.96 V vs. a RHE), as shown in
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x
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Figure 3F .
[18]
Meng et al. synthesized two-dimensional (2D) atomically thin Zn In S nanosheet arrays and achieved
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[33]
oxygen doping and Zn and S vacancies at the surface through low-temperature heat treatment . The
excessive surface defect states were then passivated by an ultrathin Al O layer. The obtained photoelectrode
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showed remarkably enhanced PEC OER performance due to the fast electron-hole pair separation and
[33]
prolonged lifetime of the carriers . However, improper chemical doping may alter the crystal form of the
semiconductor and reduce the lifetime of holes, thereby increasing the kinetic challenge of driving
catalysis [10,34] . In addition, the control of the concentration and distribution of heteroatom doping on the
surface of the photoanode catalysts remains challenging.
Defect engineering
Defect engineering can efficiently modulate the electronic structure and surface properties of a catalyst to
reduce the reaction energy barrier for PEC performance. Oxygen vacancies are the most common
defects . Any treatment that changes the chemical environment of the catalyst (temperature annealing,
[35]
treatment under different atmospheres during growth, laser irradiation, and so on) can lead to the
formation of these oxygen defect states [36,37] . Zhang et al. fabricated a BiVO catalyst modified with a
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FeNiOOH cocatalyst rich in oxygen vacancies (BiVO /Vo-FeNiOOH) through a simple and economical
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[38]
NaBH reduction method . The introduced oxygen vacancies accelerate hole transfer and promote
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efficient electron-hole pair separation, leading to a negative shift in the starting potential and OER
acceleration. The achieved photocurrent of the BiVO /Vo-FeNiOOH catalyst was more than four times that
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of pure BiVO due to the introduction of oxygen vacancies in the system . Li et al. successfully prepared
[38]
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Bi O I microspheres rich in oxygen vacancies by a solvothermal method assisted by an ionic liquid .
[39]
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Compared with Bi O I with fewer oxygen vacancies after annealing, the valence band maximum position of
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Bi O I with rich oxygen vacancies shifted upward, which resulted in better photooxidation ability for the
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