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Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23 Page 7 of 21

Construction of heterostructures
In addition to the nanostructural modification of photoanodes, a highly conductive and active
semiconductor can also be used directly on the photoanode surface to form a heterojunction
photoelectrode. The construction of heterojunctions can effectively separate the photoelectron holes by
satisfying the appropriate energy level positions between the two semiconductors (p-n/n-n) [43,44] . For low-
energy light-induced holes, Z-type nanocomposites have been successfully constructed by coupling suitable
band gap semiconductors . Photoanodes with Z-type electronic transfer result in enhanced light-
[45]
harvesting properties and charge separation. For a g-C N /semiconductor composite, the photogenerated
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electrons of the semiconductor transfer to the valence band of g-C N and then recombine with the
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photogenerated holes derived from g-C N . Therefore, the photogenerated electrons on g-C N and the
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photogenerated holes on modified oxides have strong reducing and oxidizing properties. This can lead to
charge separation and improve the PEC activity of the catalyst. MOFs with a large surface area are also used
in PEC systems. The construction of MOF-based heterojunctions can increase the internal electric field by
the conjugated π electrons in the linkers [Figure 4]. Wang et al. fabricated Z-scheme heterostructures of
TiO nanorods (NRs) coated by MOFs and obtained UiO-66@TiO and UiO-67@TiO photoanodes.
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Compared with pristine TiO , UiO-66@TiO and UiO-67@TiO showed enhanced photocurrent density in
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the PEC water oxidation process .
[25]
For a type-II heterojunction, the semiconductor photoanode will be combined with a semiconductor that
has a relatively low valence band position. The photogenerated electrons in the conduction band of the
modified semiconductor will be transferred to the photoanode. The photogenerated holes in the
semiconductor anode will be transferred to the modified semiconductor and further induce the transfer of
holes. Different type-II heterojunction photoanodes, such as WO /BiVO , ZnO/BiVO , ZnO/Fe O ,
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TiO /ZnO, and so on, have been reported with remarkable PEC performance. Maity et al. fabricated a one-
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dimensional n-ZnO/p-ZnCo O nanoheterojunction photoanode . In this type-II heterojunction
[46]
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photoanode, the ZnCo O surface overlayer passivated the surface states of ZnO nanorods, thereby
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significantly reducing the recombination of photogenerated electron-hole pairs. The generated holes from
the ZnO nanorods can migrate rapidly to the surface of ZnCo O and initiate the OER. Compared with the
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pristine ZnO photoanode, the n-ZnO/p-ZnCo O nanoheterojunction photoanode achieved a significant
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[46]
increase in photocurrent density . N-type semiconductors with suitable band edges for water oxidation
have also been coupled with photoanodes to improve the PEC water splitting performance. Ho et al.
constructed an epitaxial Fe TiO /ZnO nanodendrite heterojunction array photoanode. Due to the
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decoupled light harvesting and hole transport paths, the photocurrent density was greatly improved .
[47]
Type-II heterojunctions are widely used for hole modulation. The construction of the heterojunction
reduces the chance of photogenerated carrier recombination, thereby improving the energy conversion
efficiency of PEC catalysts. In addition to these, in order to fully reveal and exploit the advantages of
heterostructures, more in-depth fundamental research, especially on the understanding of interfacial
properties, is required.
Loading of cocatalysts
Coupling semiconductor photoanodes with good electrocatalytic OER cocatalysts is a common strategy to
enhance the charge transfer efficiency from the semiconductor to the electrolyte and improve the oxidation
[48]
kinetics . The addition of OER catalysts can significantly inhibit the surface recombination of
photogenerated charge carriers and reduce the accumulation of holes on the electrode surface. Moreover,
some OER cocatalysts have been reported to passivate the photoanode surface to prevent corrosion of the
photoanode, thereby improving its stability under operating conditions. Noble metal oxides, such as RuO
2
and IrO , are the most commonly used hole transfer cocatalysts, which can effectively reduce the
2
[17]
overpotential of oxidation reactions . However, due to the high cost of these catalysts, the development of