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

Figure 1. Schematic illustration of the processes of photoinduced charge separation, recombination, transfer and the final catalytic
reaction in semiconductor-based photoelectrocatalytic water splitting and the associated reaction timescales.

extracted and the recombination of photogenerated electron-hole pairs can be suppressed. Catalytic
regulation to ensure that the holes in the valence band have sufficient potential to oxidize the target
molecule is an alternative method [18,19] . Thus, the effective separation and transport of photogenerated
carriers on the surface can be improved and the charges can effectively participate in the surface reaction,
which can effectively improve the PEC efficiency. In 2019, Sun et al. reviewed the development of
photogenerated hole modulation in PEC catalysis for solar fuel production, with a focus on surface
[15]
polarization strategies . So far, the recent advancements in PEC catalysts have yet to be reviewed clearly.
Compared to electron modulation, limited research work has been contributed to the field of hole
modulation.

This review focuses on the hole modulation of semiconductor-based photoelectrocatalysts in the last five
years. The studies of modulation strategies and dynamics form two main sections. The PEC performance is
sensitive to the electrode surface structure. In this review, the modulation strategies section covers the
recent advances in the design strategies of photoelectrocatalysts with excellent PEC performance through
hole modulation, including hole sacrificial agents, nanostructural modification, heterostructure
construction and cocatalyst modification [Figure 2]. In the cocatalyst modification section, recent progress
on low-cost carbon-based materials is highlighted. The analysis of the transport process of photogenerated
holes on the surface is helpful for the design and synthesis of efficient photoelectrocatalysts. Therefore, this
review also emphasizes various techniques for studying hole modulation dynamics, such as transient
absorption spectroscopy, time-resolved photoluminescence spectroscopy, transient photovoltage and
scanning electrochemical microscopy. Moreover, relevant conclusions and outlooks are proposed. This
review will trigger the design and construction of efficient photoelectrocatalysts.

HOLE MODULATION STRATEGIES IN PHOTOELECTROCATALYSIS
To suppress photogenerated electron-hole recombination and accelerate hole-to-solution transfer, methods
such as introducing hole sacrificial agents, nanostructural modification, heterostructure construction and