Correlated single-molecule reaction imaging and photocurrent measurements reveal underlying rate processes in photoelectrochemical water splitting

Understanding reaction processes at the semiconductor/electrolyte interface is critical to developing (photo)electrochemical energy conversion systems. For the water oxidation reaction at a photoanode in a solar water splitting cell, there are multiple reaction steps that produce surface adsorbed reaction intermediates, such as OH• and H₂O₂. Little is known about how the reaction intermediate surface concentrations evolve during the water oxidation reaction. Here we use single-molecule fluorescence microscopy to track the time-dependent surface concentration of Ti-OH• species on TiO₂ photoanodes under chopped light illumination. We correlate the reaction intermediate surface concentration with the net rate of photoelectrochemical water oxidation (i.e., the photocurrent). The reaction intermediate dynamics follow different time scales in its temporal evolution from the photocurrent dynamics. By fitting the temporal evolutions of Ti-OH• species and photocurrent over a range of light on/off conditions and applied potentials, we observed that the rate constants for interfacial hole transfer and O-O bond formation depend on the applied potential under illumination. The variation in the rate constants could be attributed to the presence of surface states and/or a change in the chemical nature of Ti-OH• species on the photoanode surface. Our findings provide insight into water oxidation kinetics under intermittent solar irradiation conditions.