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Page 2 of 21 Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23

INTRODUCTION
Solar energy is clean, renewable, sustainable and abundant. Currently, solar energy conversion and storage
have become important options to solve the global energy shortage and environmental pollution
challenges . Semiconductor-based photocatalysis for splitting water into hydrogen is a renewable and
[1-3]
sustainable technology for direct solar-to-chemical energy conversion. However, the relatively low solar
[4-6]
energy conversion efficiency limits its practical applications . The main semiconductor photocatalytic
process can be divided into three steps: (i) the catalyst absorbs photons equal to or greater than the energy
of its bandgap width and generates electron-hole pairs by excitation; (ii) photogenerated electron-hole
separation and transfer to the semiconductor surface; (iii) transfer to the reactive species to undergo surface
reactions (surface complex or effective reactions). The solar energy conversion efficiency is determined by
these steps. To improve the efficiency of the photocatalytic process, it is necessary to effectively enhance the
separation and transportation of photogenerated charges and simultaneously make the charges migrate to
the surface and further initiate the surface reactions of the chemical compounds. During the photocatalytic
water splitting process, the hydrogen evolution reaction usually takes place simultaneously with the oxygen
evolution reaction (OER). It is noteworthy that the OER becomes the bottleneck of photocatalytic water
decomposition because it involves multiple proton- and electron-transfer steps.

The generation rate of holes depends on the illumination light intensity, light absorption and charge
separation of the photoelectrode, while the consumption rate of holes depends on the rate of surface charge
recombination and hole transfer into solution. However, the water oxidation reaction involving
photogenerated holes is much slower than the recombination process. Figure 1 shows a schematic
illustration of the processes of semiconductor-based photoelectrocatalytic water splitting and the associated
reaction timescales. The accumulated holes not only lead to an increase in the recombination rate of the
catalyst, resulting in a reduction in catalytic activity, but also oxidize the catalyst itself, leading to a decrease
in catalyst stability and deactivation. Since the discovery of water splitting catalyzed by a single-crystal TiO
2
photoelectrode in 1972, photoelectrochemical (PEC) catalysis based on semiconductor photoelectrodes has
[7-9]
attracted extensive attention for solar-to-energy conversion . PEC water splitting combines photocatalysis
and electrocatalysis, which can effectively promote charge separation and improve solar energy
[9]
utilization . Furthermore, a series of studies have been carried out on semiconductor materials used as
photocatalytic OER photoanodes, including transition metal oxides, hydroxides, nitrides and selenides,
metal-organic frameworks (MOFs) and non-metallic polymer semiconductor graphitic carbon nitrides
(g-C N ) [10,11] . However, due to the slow hole transfer and subsequent oxidation reactions, the kinetic
4
3
mismatch between the bulk charge carrier lifetime and the interfacial catalytic timescale results in high
electron-hole recombination rates, thus limiting the PEC performance. Furthermore, the water oxidation
reaction is also necessary as a counter reaction for electron-involved half reactions [12-14] , such as the CO and
2
NH reduction reactions. Therefore, intensive efforts need to be focused on the improvement of the half
3
OER regarding materials, systems, and so on. Understanding how to suppress the recombination of
electron-hole pairs and improve the subsequent oxidation reaction rate is the main research direction in the
PEC catalysis reaction. Hole modulation to accelerate the kinetics of photogenerated hole transfer and the
hole-involved surface oxidation reaction has significant potential for addressing these issues [15,16] .

Noble metal oxides, such as RuO and IrO , are the most commonly selected materials to modulate
2
2
photogenerated holes to enhance PEC performance . However, the high cost of noble metals is not
[17]
sustainable for practical implementation. Therefore, developing low-cost and efficient photogenerated hole
modulation candidates that can rapidly initiate oxidation half-reactions has become an urgent challenge in
this field. Regarding hole modulation, by designing and modifying the materials that match the valence
band energy level of the photoelectrode semiconductor, the photogenerated holes can be efficiently