Hydrogen production by water splitting using photocatalysts under the solar irradiation is one of the artificial photosynthetic process to obtain sustainable hydrogen energy from water using photon energy. The solar energy conversion efficiency of powder photocatalysts has reached to ca. 0.3%, which is equivalent to that of general agricultural plants. For water splitting photocatalysts, surface modification with co-catalysts frequently plays as one of the important roles. In this manuscript, we discuss the function of surfaces of cocatalysts on semiconductor photocatalysts, which accept photogenerated carriers from the semiconductor and catalyze hydrogen or oxygen evolution reaction.