Perovskite-like NaTaO3 photocatalysts, synthesized by sol-gel and solid-state methods, were loaded with NiO co-catalyst to enhance water splitting activity under UV illumination. Activity increased significantly with NiO loading and reached a maximum at 3 and 0.7 wt%, respectively, for the sol-gel and solid-state synthesized NaTaO3. Beyond this point, photocatalytic activity decreased with further loading. Analysis using X-ray diffraction, high-resolution transmission electron microscopy, and diffuse reflectance spectroscopy shows that the interdiffusion of Na+ and Ni2+ cations created a solid-solution transition zone on the outer sphere of NaTaO3. For NiO contents less than 3 wt%, no NiO clusters appeared on the NaTaO3 surface, and the reduction/oxidation pretreatment did not enhance photocatalytic activity. The high activity resulting from a low NiO loading suggests that the interdiffusion of cations heavily doped the p-type NiO and n-type NaTaO3. reducing the depletion widths and facilitating charge transfers through the interface barrier. (C) 2010 Elsevier Inc. All rights reserved.