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

Abstract

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.

Fig. 1. The image sequences of the patterned-illumination time-resolved phase microscope (PI-PM) in different solvents.

The images correspond to the refractive index changes after photo-excitation by a UV pump light for SrTiO₃:Rh/ITO/BiVO₄:Mo (STOR/ITO/BVOM) Z-scheme water-splitting sheet measured by the PI-PM method (a in ACN, b in water). The light intensity patterns of the pump light are shown at the bottom of each image sequence, and the selected regions for the cluster analysis are indicated in red squares. The result of the cluster analysis for area No. 1 is shown in Figs. 4–6, and the others are presented in Supplementary Figs. 4–8).

Fig. 2. Average refractive index change responses.

a SrTiO₃:Rh/ITO/BiVO₄:Mo, b BiVO₄:Mo only, and c SrTiO₃:Rh only until 100 μs in acetonitrile (ACN) and water obtained from the image sequence obtained by the patterned-illumination time-resolved phase microscope. (red: in ACN, blue: in water) The response was obtained from the signal amplitude at the light irradiated regions. The response in methanol (MeOH) is also shown for c STOR only (green: in MeOH).

Fig. 3. Schematic drawing of the entire charge dynamics.

This scheme corresponds to the charge transfer diagram in SrTiO₃:Rh/ITO/BiVO₄:Mo (STOR/ITO/BVOM) in acetonitrile (ACN). Step 1 includes intrinsic recombination inside materials and charge transfer between two materials. Step 2 indicates the decay of surface-trapped charge carriers in ACN, which are utilized for water-splitting reactions with water outside. Step 3 includes the hole trapping to Rh^3+/4+ state in STOR and the slower recombination. Surface-trapped carriers cannot be transferred to the solution side in ACN, where all processes of charge dynamics are completed in the system.

Fig. 4. Image sequence and clustering analysis with mediator.

a An image sequence of the refractive index change for SrTiO₃:Rh/ITO/BiVO₄:Mo (STOR/ITO/BVOM) in acetonitrile (ACN) in a square region (20 × 50 μm) corresponding to no. 1 in Fig. 1, bottom on the order from nanoseconds to microseconds. The scale bar corresponds to 20 µm. b The categorized mapping of the charge carrier responses of a. An outlier positioned far from all categories was colored in black (#0). c A microscopic image in the same area as in a. d The averaged responses for each category in b are shown. (red: category 1, blue: category 2, green: category 3).

Fig. 5. Image sequence and clustering analysis without mediator.

a An image sequence of the refractive index response for SrTiO₃:Rh/BiVO₄:Mo STOR/BVOM in acetonitrile (ACN) in a square region (18 × 50 μm) corresponding to no.1 in Fig. 1c on the order from nanoseconds to microseconds. The scale bar corresponds to 20 µm. b The categorized mapping of the charge carrier responses of a. An outlier positioned far from all categories was colored in black (#0). c A microscopic image in the same area as a. d The averaged responses for each category in b are shown. (red: category 1, blue: category 2, green: category 3).

Fig. 6. Image sequence and clustering analysis in water.

a An image sequence of the refractive index response for SrTiO₃:Rh/ITOBiVO₄:Mo (STOR/ITO/BVOM) in water in a square region (20 × 50 μm) corresponding to no. 1 on the order of nanoseconds to microseconds. The location of the sample was exactly the same as Fig. 5a. The scale bar corresponds to 20 µm. b The categorized mapping of the charge carrier responses of a. An outlier positioned far from all categories was colored in black (#0) c A microscopic image in the same area as a. d The averaged responses for each category in b are shown. (blue: category 1, green: category 2).