As a potentially low-cost, high-efficiency solar energy storage solution, solar water splitting faces great challenges. One of the issues is the poor catalytic activity and low stability. Applications of co-catalysts have been shown effective to correct the deficiency by promoting desired chemical reactions so as to minimize charge recombination at the surface and to reduce parasitic corrosion reactions. The detailed behaviors of the light absorber/catalyst interface, however, remain poorly understood. Here we present our recent research in this area. We show that the application of the co-catalysts may greatly influence the charge separation behaviors of the photoelectrode. Detailed thermodynamic and kinetic measurements support our understanding. Furthermore, we show that the photoelectrode substrate also exerts great influences on the co-catalyst behaviors. A strong interaction between the photoelectrode and the catalyst can be beneficial. The combined system may also serve as a new platform to understand heterogeneous catalysis such as water oxidation at a level previously inaccessible. The knowledge generated by our work will likely contribute significantly to the development of solar water splitting technology for a future powered by renewable energies.