A novel C3N4-CDot composite photocatalyst was very recently shown to be highly efficient and very stable in water splitting by solar radiation without using any sacrificial reagent (J. Liu, et al., Science, 2015, 347(6225), 970). This photocatalyst utilizes a two-electron/two-step process in which the production of H2O2 and H2 is photocatalyzed by using C3N4 in the first step and H2O2 is decomposed by using CDots in the second step. The present work is a study on the generality of this approach by application of a C3N4/MnO2 catalyst. This new catalyst indeed splits water by a two step process in a stable way, without any sacrificial agent. It was however found that though the absorbance of the new catalyst in the visible range of 500-600 nm is much larger than that of the C3N4-CDot catalyst, its water splitting efficiency is much lower. These findings add insight into and assist in the further optimization of this new class of photocatalysts to meet the requirements of commercial water splitting systems.