Crystal Size Dependence of the Photo-Electrochemical Water Oxidation on Nanoparticulate CaTiO3

Nanocrystalline CaTiO3 materials with controlled particle size were prepared using spray-freezing/freeze-drying approach utilizing gelatine as a structure-directing agent. The resulting materials show characteristic particle size between 19 and 60 nm. The shape of the nanocrystals changes from cube-like single crystal containing particles into less regular isometric particles. Prepared materials as identified by X-ray diffraction analysis are formed by orthorhombic perovskite with small admixture of cubic phase. The ratio of both perovskite phases is independent of the particle size or prevailing crystal shape. All prepared materials show n-semiconducting character with band gap of ca 3.6 eV. They also show photo-electrochemical activity in water oxidation in acid media if a bias greater than 400 mV with respect to the flat band potential is applied. The specific photo-electrochemical activity decreases with increasing specific surface area. This behavior is attributed to increased probability of the electron transfer at the illuminated CaTiO3 surface facilitated by the surface states. The CaTiO3 materials also generate significant amount of ozone upon illumination in oxygen saturated solutions. The tendency to form ozone increases with increasing particle size suggesting that the ozone formation is hindered on materials with large number of low dimensionality states (crystal edges and vertices).