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Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23 Page 9 of 21

Figure 5. (A) Schematic illustration of depositing NiFe hydroxide overlays on an Fe O nanowire photoanode. (B and C) Lattice-resolved
2 3
[57]
TEM images of a NiFe hydroxide-coated Fe O nanowire photoanode .
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Carbon-based materials
Carbon-based materials have been widely investigated as cocatalysts for PEC catalysis due to their
[59]
[60]
abundance, low cost and variety . Carbon-based nanomaterials, such as graphene , carbon nanotubes
[62]
[61]
(CNTs) and carbon dots (CDs) , offer multiple benefits in PEC systems. For example, they not only
facilitate light harvesting but also act as channels for charge transport, thereby facilitating enhanced PEC
activity due to synergistic effects and electrical coupling . The high charge-carrier mobility of graphene
[63]
and CNTs makes them promising candidates for high-mobility charge. However, the reports on those
materials are mainly for electron modulation and rarely for hole modulation. Zhao et al. fabricated a WO
3
[64]
nanosheet array/silane/graphene oxide (WO NS/silane/GO)-based photoanode . On the photoanode, the
3
silane molecules act as hole collection sites and drive the fast transfer of the collected holes to the GO. As a
hole transfer channel, GO can immediately promote holes to participate in chemical reactions at the
electrode-electrolyte interface, resulting in stable and continuous charge carrier separation and transfer. The
-2
results showed that the photocurrent of the WO NS/silane/GO electrode (1.25 mA·cm at 1.23 V vs. a
3
[64]
-2
RHE) can be enhanced by a factor of 1.8 compared to the pristine WO NS (0.69 mA·cm ) electrode .
3
Compared to the above-mentioned carbon materials, CDs exhibit unique PEC properties, including
photoluminescence and photoinduced electron transport and storage, and have been widely used as
photoelectrocatalytic cocatalysts [65,66] . A large number of studies have reported CDs as cocatalysts for
electron regulation. In recent years, some works have also reported the application of CDs in hole
regulation. Liang et al. reported a CD-modified Co O /In O composite photocatalyst for an efficient photo-
3
3
4
2
driven CO reduction reaction . CDs can be obtained by the electrolysis of graphite rods. The preparation
[67]
2
process of CDs is simple and compared with the noble metal Ru, the economic cost of CDs is negligible
[Figure 6A]. Through analysis by transient photoelectric technology, it was found that the CDs in the
composite not only participated in the electron transfer process but also in the hole transfer process