In Situ Quantification of Surface Intermediates and Correlation to Discharge Products on Hematite Photoanodes using a Combined Scanning Electrochemical Microscopy Approach

Hematite is a promising photoanode for solar driven water splitting. Elucidating its surface chemical pathways is key to improving its performance. Here, we use redox titrations in the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM) to quantitatively probe in situ the reactivity and time evolution of surface species formed on hematite during photo assisted water oxidation. Using SI-SECM, two distinct populations of oxidizing surface species were resolved with measured k_(si) of 316 m^3/(mol·s) and 2 m^3/(mol·s) for the more and less reactive species, respectively. While the surface coverage of both species was found to increase as a function of applied bias, the rate constants did not change appreciably, suggesting that the mechanism of water oxidation is independent of bias potential. In the absence of applied potential, both populations exhibit decay that is well described by second order kinetics, with k_d values of 1.2 × 10^5 ± 0.2 × 10^5 and 6.3 × 10^3 ± 0.9 × 10^3 m^2/(mol·s) for the fast and slow reacting adsorbates, respectively. Using transient substrate generation/tip collection mode, we detected the evolution of as much as 1.0 μmol/m^2 of H_2O_2 during this decay process, which correlates with the coverage observed by one of the titrated species. By deconvoluting the reactivity of multiple adsorbed reactants, these experiments demonstrate how SI-SECM enables direct observation of multiple adsorbates and reaction pathways on operating photoelectrodes