Charge carrier mapping for Z-scheme photocatalytic water-splitting sheet via categorization of microscopic time-resolved image sequences

Photocatalytic water splitting system using particulate semiconductor materials is a promising strategy for converting solar energy into hydrogen and oxygen. In particular, visible-light-driven ‘Z-scheme’ printable photocatalyst sheets are cost-effective and scalable. However, little is known about the fundamental photophysical processes, which are key to explaining and promoting the photoactivity. Here, we applied the pattern-illumination time-resolved phase microscopy for a photocatalyst sheet composed of Mo-doped BiVO4 and Rh-doped SrTiO3 with indium tin oxide as the electron mediator to investigate photo-generated charge carrier dynamics. Using this method, we successfully observed the position- and structure-dependent charge carrier behavior and visualized the active/inactive sites in the sheets under the light irradiation via the time sequence images and the clustering analysis. This combination methodology could provide the material/synthesis optimization methods for the maximum performance of the photocatalyst sheets.

S3 STOR/BVOM system in ACN (inert solvent) and MeOH. The result clearly showed that the slow rising component until 10 µs in STOR/BVOM in ACN disappeared in MeOH, and instead, the fasterrising response until 1 µs showed up, which was similar to that of STOR/ITO/BVOM in ACN. It is supposed that MeOH worked to reduce the Rh 4+ state, instead that the electrons in BVOM were used to reduce it with an ITO mediator. These facts verified that the ITO mediator worked to prevent the increase in the Rh 4+ states in STOR and proceed the water-splitting reaction efficiently. Supplementary

Preparation of photocatalyst sheets
Printed photocatalyst sheets were prepared as follows. Photocatalysts used, Rh-doped SrTiO3

Phenomenological kinetic analyses for charge carrier dynamics in Z scheme materials
Supplementary where ℎ, , ℎ, , and , , correspond to the number of charge carriers for the bandgap states and the trapped hole and electron states, respectively. The kinetic parameters are described in Supplementary Fig. S12. Since the electrons and holes involved in the intrinsic recombination cannot be distinguished in our measurement, they were represented by the total number of charge carriers and approximated with a single kinetic parameter, although the actual recombination should be bimolecular-type recombination. In STOR,   is the number of charges trapped in the Rh states. The kinetic parameters are described in Supplementary Fig. S12.
In acetonitrile, the water-splitting reactions are blocked, and the second terms in Eq. (2) and (6) can be neglected. When BVOM and STOR are not combined to make a Z scheme material, or the charge transfer between two materials is inefficient without the ITO mediator, the charge compensation is hindered, and the second terms in Eq. (3) and (5) can be ignored.
In the simulation, the intrinsic recombination in BVOM and STOR was approximated with a single exponential decay. Actually, they cannot be described by a simple exponential function, and they consist of multiple exponential functions and are sometimes described by a stretched exponential function. The functional form can be recognized in Fig. 2(a) and (b) in water. However, they cannot be included in the analysis, and they were approximated as Eq. (1) and (4).
For the analysis, the rate for the intrinsic recombination for BVOM and STOR was estimated by using Fig. 2 a and b, and they were 3.5 and 1.0 (/s cm -3 ), respectively. Then, Eq.(1)-(7) were solved simultaneously, and one of the simulated results is shown in Supplementary Fig.S13. The parameters were manually adjusted to match the observed responses peaked around 1 s for water-splitting and 10 s for the Rh state formation and decay. The charge carriers for water-splitting (electrons in STOR and holes in BVOM) were reproduced with a peaked around 1 s, and the inactive state for watersplitting (Rh states) was reproduced with a peaked around 10 s. The final refractive index response is made up of electron and hole responses of BVOM and STOR and the Rh state in STOR, and there are many parameters to adjust for reproducing the real signal, and we could not decide the appropriate combinations of parameters. However, we could confirm the effect of the charge compensation between BVOM and STOR via the charge mediator, ITO, by adjusting . As shown in Supplementary Fig. S13. It clearly shows that the amplitude of the Rh states was much reduced by increasing the charge transfer rate. This simulation strongly supports that the ITO mediator suppressed the charge trapping at the Rh states. units]