Water incorporation in oxides: A moving boundary problem

We previously reported a peculiar non-monotonic water incorporation kinetics in a model oxide system of Fe-doped SrTiO3 [Angew. Chem. Int. Ed. 46 (2007) 8992]. In-situ optical absorption spectroscopy unambiguously indicated that the oxide was first strongly reduced and then oxidized to the final equilibrium by the decoupled water transport, in which hydrogen fast diffuses into the oxide and oxygen lags behind. Electronic carriers constitute the ambipolar diffusion couples for the respective transports. Here we performed a defect-chemical thermodynamic analysis with the frozen-in oxygen vacancies, which successfully represent the overshooting state caused by the sluggish oxygen diffusion. The quantitative investigation of the spatially resolved optical absorption images based on the thermodynamic calculation revealed a hitherto unknown novel kinetic aspect. The absorption effects cannot be explained by the simple superposition of hydrogenation and oxygenation as assumed in the integral analysis. The diffusion-controlled hydrogenation appears to proceed in the region narrowed by the moving oxidation fronts. The oxidation front in the oxide, which is presently equivalent to the hydration front, not the sample surface in contact with water vapor, behaves as the source planes for water incorporation. The hydrogen and oxygen transport are not completely decoupled in this sense but constitute a correlated moving boundary diffusion problem. The process can be quantitatively described by modeling of the diffusion coefficients which are exponentially increasing with water concentration for hydrogenation and oxygenation, respectively. The overall oxidation process of the later stage is non-trivial but apparently surface reaction controlled.