Tuning Surface States as Performance Parameters for Water Oxidation on Metal Oxide Photoanodes

During the past decades, photoelectrochemical strategies have attracted increasing interest for the direct conversion of solar energy into sustainable chemical fuels. Generally, surface states generated from the water oxidation process by photogenerated holes are recognized as a key parameter for tuning the performance of photoanodes in water splitting setups, but their behavior and precise chemical nature remains to be explored for many prominent materials.<br/>This applies specifically for hematite Fe₂O₃ photoanodes, for which we recently investigated fundamental questions associated with its two different types of surface states (referred to as S1 and S2) and their interaction.[1] While S1 has long been identified as an iron-oxo species through operando spectroscopy and related rate law investigations, the precise nature and behavior of S2 with lower oxidative energy has been controversially discussed. Photoanodes with hematite nanorods were prepared via published protocols and the dynamic interaction of their surface states was evaluated concerning the influence of oxidative potentials, illumination intensity, and electrolyte pH. We tentatively assigned S2 to an iron-peroxo intermediate of the water oxidation process via photoelectrochemical impedance spectroscopy (PEIS) measurements, which were complemented with transient photocurrent spectroscopy (TPS) results. Our assignment was based on the following key observations: Potential-dependent rate law analyses revealed a transition of 1^st order reaction kinetics for surface holes with low densities to 3^rd order kinetics at higher hole density for higher applied potentials (1.3 V vs. RHE). At lower potentials (0.9 V vs. RHE) a transition from zero to 3^rd order kinetics was observed. Further, the lifetime of S2 was rather long with &gt;3 min, and it was generated from the accumulation of shorter-lived S1 states. In strongly alkaline media, S1 states became more mobile to be transformed into S2 states which displayed 3^rd order reaction kinetics. Near the point of zero charge, a unique three-stage transformation of the reaction mechanism was unraveled. The quick migration and reaction of oxo species under intense illumination or surface deprotonation conditions can give rise to longer-lived peroxo species and higher reaction orders. This model paves new ways to optimize photoanode performance through tuning the accumulation and migration of oxo species.<br/>In a parallel study, we enhanced the efficiency of bismuth vanadate BiVO₄ photoanodes through introduction of a reduced catalytic layer on TiO₂ protected BiVO₄.[2] The surface of the resulting R-TiO₂@BiVO₄ photoanodes was enriched in oxygen vacancies and their performance was clearly superior to pristine BiVO₄. Our PEIS studies revealed the presence of two surface states on BiVO₄ photoanodes (S1 around 0.45 V and S2 near the water oxidation potential around 1.05 V), while R-TiO₂@BiVO₄ displayed only S2 with S1 being passivated. Moreover, the hole densities in S2 were notably increased for R-TiO₂@BiVO₄ as the accumulation of intermediates was promoted with rapid water oxidation kinetics, which identifies S2 as reaction centers related to oxygen vacancies. Our R-TiO₂ surface coating strategy was furthermore investigated with a wide range of analytical methods, showing that the excellent water oxidation activity of R-TiO₂@BiVO₄ photoanodes arises from a simultaneous enhancement of S2 reaction centers and suppression of adverse VO₂+/VO²⁺ recombination processes. Therefore, our surface engineering strategy not only stabilizes BiVO₄ photoanodes but also highlights the essential role of surface states in their performance optimization.<br/><br/>[1] J. Li, W. Wan, C. A. Triana, H. Chen, Y. Zhao, C. K. Mavrokefalos, G. R. Patzke, <i>Nat. Commun.</i> <b>2021</b>, <i>12</i>, 255.<br/>[2] H. Chen, J. Li, W. Yang, S. E. Balaghi, C. A. Triana, C. K. Mavrokefalos, G. R. Patzke, <i>ACS Catal.</i><b> 2021</b>, <i>11</i>, 7637.