Assessing and Quantifying Thermodynamically Concomitant Degradation during Oxygen Evolution from Water on SrTiO3

The oxygen evolution reaction (OER) from water, while more stable on transition metal oxide surfaces than others, has nonetheless proved to be concomitant with charge-induced surface degradation. Since heterogeneous and nanostructured electrodes are often used and with a large excitation area, the degradation can be difficult to quantify. Here, we utilize single crystalline SrTiO3, highly efficient photoexcitation of the OER, and a focused laser to spatially define the degradation. A repetitive, ultrafast laser pulse above the band gap energy is employed, which allows for highly varied exposure of the surface using different scan methods. It also connects the work to the OER and its time-resolved mechanisms. By characterizing the degradation using optical spectroscopy and electron microscopy, the material dissolution constitutes an upper bound of 6% of the charge passed in a pH 13 electrolyte, while for pH 7, it reaches 23%; the pH dependence is anticorrelated with the ultrafast population of trapped charge. Although a minority component, the remarkable consistency of the 6% upper bound in the pH 13 electrolyte across a large range of linearly increasing degradation volumes and changing electrode composition defines a dominant lattice dissolution reaction as thermodynamically concomitant with the OER. Along with the pH dependence, the elemental composition of the degraded layer quantified by energy-dispersive and photoelectron and absorption X-ray spectroscopy suggests the relevance of certain chemical cation redeposition reactions. Altogether, using spatially and temporally defined photoexcitation of a crystalline surface provides a means to quantify semiconducting transition metal oxide degradation during the OER and constricts its mechanisms.