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Page 6 of 21 Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23

Figure 3. (A-D) Morphological and structural characterization, (E) linear sweep voltammograms and (F) half-cell applied bias photon-
to-current efficiency results of BiVO /N:NiFeO photoanodes [18] .
4
x
photogenerated holes and higher separation efficiency for the photogenerated carriers, thus promoting the
photocatalytic removal of oxygen and photocatalytic pollutants .
[39]
In addition to oxygen vacancies, the introduction of other vacancies can also prolong and migrate the
[33]
photogenerated carriers to the surface to participate in the reactions . Zhao et al. introduced S vacancies to
a CdS photoanode surface through H O etching and adjusted the vacancy concentration by controlling the
2
2
[40]
etching time . The photocurrent density of the CdS nanorods etched by H O for 35 s reached
2
2
3.09 mA·cm , which was 6.5 times higher than for CdS . Ma et al. prepared a WO overlayer with dual
-2
[40]
3
[41]
oxygen and tungsten vacancies on a WO photoanode by a solution-based process . The obtained
3
mesoporous WO achieved a cathodic shift of the onset potential and enhanced photocurrent for the
3
OER . Although significant progress has been made in improving PEC performance by introducing
[41]
defects, many issues remain unresolved. For example, surface oxygen vacancies favor the final performance
of water-splitting photoanodes because they improve charge separation by narrowing the space-charge
layer. In general, however, bulk oxygen vacancies are detrimental because they enhance recombination
kinetics, thereby reducing the PEC performance. In addition, the concentration of oxygen vacancies also has
an effect on the performance. To improve the PEC performance, the optimization of defect density is
[42]
necessary . However, similar to the heteroatom doping engineering discussed above, control of the defect
concentration and distribution in photoanodes remains a challenge.