Fig. 2: In-situ simultaneous mapping of activity, hole dynamics and defect composition in a model (photo)electrocatalyst.

(Top) Scanning a focussed 532 nm light source (∼1.3 μm FWHM; ∼8 MW cm⁻² intensity at sample) allows the photocurrent density under illumination to be measured locally at different regions in the sample. The spatial photocurrent density is then plotted at 1.25 V vs RHE (V_RHE). A dark CV is shown in the middle panel (in blue) for reference. (Middle) Scanning of the sample allows transient reflection microscopy kinetics (ΔR/R) to be obtained at each spatial location. The kinetics from each TRM experiment are used to obtain the time taken for the signal to decay to half its initial value (t_50%) as a function of bias and spatial location. (Bottom) Differential reflection spectrum as a function of bias (with respect to 0.5 V_RHE), shown for the region marked by the black box in the middle panel. Raman imaging (500 ± 10 nm spatial resolution) of a hematite photoelectrode measured using point scanning. The Raman spectrum of α-Fe₂O₃ is plotted in the bottom left with the various modes and their corresponding symmetries (A¹_g, E_g and E_u) marked. Image displays the spatial intensity of the 660 cm⁻¹ mode at open circuit potential. These measurements give spatial insight into the concentration of oxygen vacancies. Scale bars on all images in the figure are 1 μm.