With the current energy crisis, H2 production through the water-splitting reaction has drawn attention recently. In this thesis, I studied the structural (geometry) and electronic properties (vertical detachment energy and electron affinity) of ZnO monomers and dimers using density functional theory. ZnO is a metal oxide with a 3.37 eV band gap and can be a commercially cheaper photocatalyst in hydrogen (H2) production. The B3LYP/DGDZVP2 pair was selected after investigating different pairs of exchange functionals and basis sets to study the hydration, hydrolysis, and water-splitting reaction. The singlet-triplet energy gaps of small (ZnO)n clusters (n=1-6) of different sizes were compared and the (ZnO)3 cluster was selected as an optimal cluster size to study the water-splitting reaction. A detailed study of water-splitting reaction pathways in the gas phase showed that oxygen is produced after hydrogen and the rate-determining step is the formation of the Zn-H bond. Graphene and graphene oxide (GO) based metal oxides play an important role as substrates for the photocatalytic reaction. The π conjugation structure of GO shows greater electron mobility and may enhance the photocatalytic performance of ZnO by increasing the electron-hole separation. In this work, I studied the impact of graphene and GO on (ZnO)3 in hydration and hydrolysis reaction using 2 water molecules and in producing H2 and O2 as products of water splitting reaction in the gas phase. I used 5 different GO models anchoring carboxyl, hydroxyl, and epoxy functional groups on a graphene layer to study the hydration and hydrolysis reaction with two water molecules. The (ZnO)3 anchored on GO model 1 was used to study the water-splitting reaction pathway
