Page 20 - Read Online

Page 16 of 21 Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23

Figure 10. (A) Principle of SECM setup. (B) Probe approach curves and (C) rate constants (K ) of BiVO -based photoanodes. (D)
eff
4
2
SECM image (100 × 100 µm ) of BiVO /Vo-FeNiOOH photoanode surface under light [38] .
4
TiO . As the UME approaches the substrate, the oxidation state (CDs ) generated is regenerated by reacting
+
2
with I in the electrolyte. Furthermore, the regeneration rate constant of CDs was achieved by fitting and
-
analyzing the approaching curves obtained by testing different concentrations of active species and light
intensities . SECM can also be used to further study the back-transfer behavior (electron participation
[101]
process) and dye regeneration (hole participation process) at the interface between the photoanode and
electrolyte for the effect of cocatalysts on the separation of electron-hole pairs. Yu et al. used SECM to study
the surface kinetics of a BiVO /NiFe-LDH composite photocatalyst . It was found that the rate constant
[102]
4
ratio of photogenerated surface holes and electrons reacting with the active species (k /k ) was five times
h+
e-
higher than that of BiVO . The results indicate that the NiFe-LDH cocatalyst can significantly inhibit
4
electron back transfer and ultimately reduce surface recombination .
[102]
CONCLUSIONS
In this review, the strategies of hole modulation to improve solar-to-energy conversion efficiency are
outlined. The modulation of holes facilitates the efficient separation of photogenerated carriers and
accelerates the subsequent oxidation reactions. The catalysts in photoanodes with different compositions
and structures show different reactivity. Hole modulation strategies, including hole sacrificial agents,
nanostructural modification, heterostructure construction and cocatalyst modification, are effective
methods to suppress electron-hole pair recombination and enhance PEC performance. The design of
catalysts to modulate holes is summarized and the application of low-cost carbon materials as cocatalysts in
hole modulation is highlighted. In addition, some kinetic techniques have also been summarized to analyze
the transport and transfer kinetics of photogenerated holes. Although some achievements have been made,
understanding how to acquire highly active PEC photoanodes via the rational design and regulation of fine
structures and interfaces remains a significant challenge. The effects of the compositions and structures of
the photoanode on the photogenerated charge migration and PEC properties need to be studied more
thoroughly.