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Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23 Page 13 of 21

[89]
spectroscopy . A range of carrier lifetimes can be obtained by the stepwise addition of electron and hole
i2+
sacrificial agents (N /H A) [Figure 8A and B]. The final electron and hole transfer rates were obtained by
2
fitting the obtained data, indicating a calculated electron transfer rate from InP/ZnS-S QDs to Ni of
2+
-1
8 -1
7.64 × 10 s ·mM and a hole transfer rate from InP/ZnS-S QDs to H A of 1.25 × 10 s ·mM
-1
8 -1
2
[Figure 8C and D] . Fan et al. tested the static photoluminescence quenching and trap-state
[89]
photoluminescence decay curves of CdSe and CdSe with surface S (CdSe-nS) QDs with the sacrificial agent
2-
isopropyl alcohol . After the TRPL measurements of CdSe QDs with different molar ratios of S ions, it
2-
[90]
was found that the corresponding hole transfer rate increased with increasing molar ratios of S ions and
2-
was saturated at n = 75. The TRPL measurements showed that the S ligands were beneficial for hole
2-
transfer .
[90]
Transient photoelectric technology (TPV)
TPV is also an effective technique for determining interfacial charge transfer kinetics and photoelectron
extraction in composites. In contrast to TAS and TRPL, TPV is a characterization technology that excites
the catalysts in a photoelectrode by an optical signal and measures the dynamic process of the electrical
signal inside the composite materials. Zhang et al. reported SnO overlay-coated BiVO (SnO /BiVO ) and
2-x
2-x
4
4
investigated its hole migration via TPV . The photoanode provided positive TPV signals, meaning that the
[58]
surface migration behavior of photoinduced holes occurred. Compared with pure BiVO , the SnO /BiVO
2-x
4
4
photoanode had the highest TPV signal intensity, indicating that the coated SnO overlay facilitated the
2-x
photoinduced holes to reach the photoanode surface. In addition, the prolonged delay time of the
photovoltage signal for both the SnO /BiVO and SnO /BiVO photoanodes indicated the existence of
2
4
4
2-x
long-lived holes as a result of the reduction in surface recombination . Kang et al. reported a metal-free
[58]
[62]
dual-function photo-assisted catalyst of PANI/CDs for overall water splitting . It was found that the
addition of CDs reduced the recombination rate of photogenerated carriers. The number of photogenerated
electrons consumed in the catalytic reaction could be calculated by integrating the tested TPV curves. It was
found that the photocurrent intensity of PANI/CDs in 5 vol.‰ KOH (pH = 8.5)/ACN (v/v) was much
higher than that in ACN solution, which may be due to the participation of photogenerated holes in the
OER, resulting in increasing photocurrent intensity .
[76]
Electrochemical impedance spectroscopy (EIS)
Different from the above photophysical methods, electrochemical methods can directly record information,
such as the photocurrent and photocurrent-potential (i-V) curves of PEC reactions. Photocurrent responses
can reveal the separation efficiency of photogenerated electron-hole pairs. Among the electrochemical
methods, EIS, through transient measurements, can be employed to investigate interfacial processes, such as
redox reactions at the interfaces and the migration of electroactive species [91,92] . EIS measurements are based
on the perturbation signals with a set of sine voltages or current signals at different frequencies. Currently,
EIS analysis is a relatively well-established method for investigating PEC systems .
[93]
Abbas et al. studied the hole transfer pathway in a TiO photoanode sensitized by Au nanoclusters using
2
EIS . Through the analysis of the Nyquist plots obtained as a function of applied potentials under light, the
[94]
possible charge transfer mechanism in the photoelectrode can be achieved. From the low-frequency
semicircle in the Nyquist plots at low bias potentials, the hole transfer process can be analyzed. The EIS
studies confirmed that when Au (SG) and Au (SG) were employed, hole transfer occurred through the
15
13
14
18
HOMO level of the nanoclusters and when Au (SG) was used, the hole transfer proceeded with the
18
25
surface traps in TiO . In EIS, the charge transfer resistance (R ) of the photoanode is sensitive to surface
[94]
2
ct
processes and can be used to analyze the transfer of photogenerated holes. Kolay et al. investigated the
charge recombination rate in a CdS-sensitized TiO photoanode by modifying CDs through EIS and found
2
that the R has a strong dependence on illumination but a low dependence on voltage . This implies that
[95]
ct