Following these two steps, the production of H 2 and O 2 occurs through water splitting even in the absence of light. It is the mechanisms of these reactions on graphene and on functionalized GOs that are studied in this paper, which are of importance in themselves, and also as a prelude to understanding what occurs when the system is solvated further with more water molecules beyond the bare stoichiometric minimum required 1, which is two for each ZnO trimer and also in the absence of light. This would require modifications to include time to correspond to different light frequencies. To implement the restricted case in the absence of light, we need to be specific about the computational method and the form of the catalyst. We use density functional theory (DFT) with the same basis set and exchange functional that was used in our earlier study of the catalytic activity of bare (ZnO) 3 nanoclusters. As before, the triplet state was approximated as the excited singlet state of spin one S 1 . We find that the energies of triplet states S 1 are lower than the energies of ground states of spin zero S 0 unlike the energy of the bare (ZnO) 3 nanoclusters. We also calculate the energies of absorption and hydrolysis of H 2 O on the ZnO trimers in the singlet and excited triplet states to understand the thermodynamics of adsorption and hydrolysis in each state and construct the corresponding potential energy surfaces (PES) to gain insight into the mechanism of the two processes, and the energy gaps between the singlet and triplet states at different stages of adsorption and hydrolysis. Following this we discuss the reaction pathways for the formation of hydrogen and oxygen on two of the zinc oxide nanocatalysts (ZnO) 3 deposited on graphene oxides, GO1, and compare them with what occurs with the bare catalyst that we studied earlier. To the best of our knowledge, this study of different combinations of (ZnO nanocluster-GO) systems has not been reported in the literature.