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Page 8 of 21 Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23

Figure 4. Schematic diagram of conjugated π electrons in a MOF (UiO-67) inducing an internal electric field to promote the charge
transfer, followed by the Z-scheme mechanism [25] .

abundant and low-cost cocatalysts is indispensable.

Transition metal-based catalysts
To replace high-cost precious metals and their oxides for OER cocatalysts, many transition metal-based
catalysts have been investigated extensively for improving PEC kinetics, including transition metal oxides
(e.g., Co O , CoO and NiO ), transition metal hydroxides (e.g., NiOOH , CoOOH and FeOOH )
[50]
[49]
[52]
[53]
[51]
[54]
3
x
4
and transition metal phosphates (e.g., Fe-Pi and Co-Pi ). However, despite these impressive
[55]
[56]
achievements, the role of the cocatalysts in photoanodes is still under debate. Traditional cocatalysts as OER
electrocatalysts increase the oxidation rate of water by reducing the activation energy of the four-electron
oxidation process, which is the rate-determining step in the water splitting process. Some researchers have
reported that the role of the cocatalyst is to rapidly trap the hole, thereby reducing recombination at the
semiconductor surface. Li et al. demonstrated a citrate-assisted deposition method, in which Ni-Fe
hydroxide was overlaid on a Fe O nanowire photoanode [Figure 5A] . As shown by TEM images
[57]
3
2
[Figure 5B and C], the Fe O sample was coated with a Ni-Fe hydroxide layer (~5 nm thick). In the
3
2
photoanode, Ni-Fe hydroxide as the OER catalyst is beneficial for hole migration and surface passivation
and reduces electron-hole recombination on the Fe O surface. Therefore, the obtained composite
3
2
photoanode exhibited a turn-on potential as low as 0.53 V vs. a RHE. Compared to that of the pristine Fe O
3
2
photoanode, the turn-on potential had a large cathodic shift of 300 mV . In addition, by loading a
[57]
cocatalyst, the water oxidation kinetics can also be regulated to tune the product. Zhang et al. tuned the
surface hole oxidation reaction kinetics of BiVO by modifying it with SnO rich in oxygen vacancies (SnO 2-x
4
2
/BiVO ) . In contrast to pure BiVO , the modified BiVO photoanode showed that in the process of water
[58]
4
4
4
oxidation, the complete reaction of H O and O evolution was transformed into H O evolution and OH
2
2
2
2
2
radicals (OH·), accompanied by the inhibition of H O decomposition through the hole re-oxidation
2
2
process. The SnO /BiVO photoanode achieved an average FE of 86% for the release of H O , thereby
2
2
2-x
4
[58]
enabling highly selective water oxidation for producing H O .
2
2