Understanding water/solid interactions is a critical scientific challenge with important implications for a variety of fundamental and applied processes. Here we study the interactions of water with RuO2, which has a wide range of applications in photocatalytic water splitting, heterogeneous catalysis, and electrochemistry. We prepared stoichiometric (s-), reduced (r-) and oxidized (o-) RuO2(110) surfaces and studied water adsorption, dissociation, and diffusion using time-lapsed scanning tunneling microscopy and density functional theory calculations. Our studies on s-RuO2(110) show that water monomers become mobile above ~240 K and form dimers which are immobile below 273 K. More importantly, we find that the mobile water dimers dissociate readily to form Ru-bound H3O2 and hydroxyl species (HOb) on Ob rows, (H2O)2 + Ob → H3O2 + HOb. The onset for diffusion of H3O2 on s-RuO2(110) is observed at ~273 K, indicating a significantly higher diffusion barrier than that for water monomers. The experimentally determined diffusion barriers are in agreement with those obtained from the DFT calculations. The observed behavior is compared and contrasted with that observed for water on isostructural rutile TiO2(110) where both molecularly-bound monomers and dimers are in equilibrium with their deprotonated configurations. In contrast with TiO2(110), the larger separation of Ru atoms induces the segmentation of water chains at high water coverages. On slightly oxidized o-RuO2(110), water molecules react with oxygen adatoms (Oa’s) on Ru rows and form pairs of Ru-bound terminal hydroxyl (HOt) groups which can reversibly dissociate back to a water molecule and Oa, H2O + Oa 2HOt. Along- and across-row diffusion of isolated water molecules is tracked at room temperature on both slightly, and heavily oxidized o-RuO2(110) by following the position of hydroxyl pairs. On r-RuO2(110), we find that water molecules readily dissociate at bridging oxygen vacancies (VO’s) and form HOb groups, H2O + VO + Ob → 2HOb. The mechanism of along- and across-row diffusion of the bridging hydroxyl protons is also studied at room temperature. The atomically-detailed, quantitative assessment of binding and diffusion of the surface species formed upon water adsorption on RuO2(110) represent a critical step in achieving fundamental level understanding of the role RuO2 plays as H2 and O2 evolution co-catalysts in photocatalytic water splitting reactions.

Imaging water reactions with reduced, stoichiometric, and oxidized RuO2(110) surfaces

251st National Meeting (2016)

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