shows excellent electron mobility due to its π conjugated structure, which should make it an appropriate substrate material that can accept electrons for exciton separation. However, the hydrophobic property of graphene makes it unsuitable for water-splitting reactions. On the other-hand "suitably functionalized" graphene oxide(GO)s are hydrophilic and have tunable properties and multiple functions that can make them useful substrates for photocatalysts to split water. By "functionalized" we mean different chemical groups attached to graphene to form different GOs. Certain metal oxide-decorated carbon materials including GO have also attracted researchers as they exhibit better stability, durability, and electrocatalytic and photocatalytic properties. Since TiO 2 is the most widely studied photocatalyst, systems combining GO and TiO 2 systems have become an important field of study that probes changes in photocatalytic efficiencies of different combinations of GO-TiO 2 . The π electrons on suitable GOs can bond with Ti atoms in TiO 2 to fine-tune their window of light absorption. The GO-TiO 2 systems can also form a heterojunction between p-type GO and n-type TiO 2 , which operates as a separator of photogenerated electron-hole pairs. ZnO is also a semiconductor with a wide band gap (about 3.37 eV at room temperature) and a wide range of potential applications due to its attractive properties like optical, photoelectric 31 and gas sensing. Photocatalytic reduction of carbon dioxide in the presence of ZnO semiconductor powder suspended in a water catalyst has been studied experimentally and it has been reported to be a more efficient photocatalyst than SrTiO 3 , TiO 2 and WO 3 in the Fischer-Tropsch synthesis of methanol and methane from CO 2 and H 2 O. ZnO nanoclusters are also known to be highly active photocatalysts and are commercially cheaper than TiO 2 . 36 Similar to TiO 2 , ZnO can also form a p-n heterojunction with a suitable GO for visible light absorption, that could be a better photocatalyst for water splitting than some others that are currently in use. Previous computational studies of ZnO(10 10) and (10 20) sheets in water using QM/MD or neural networks reveal spontaneous partial dissociation of water on the surfaces with proton transfer between adsorbed or bulk water to surface oxygen atom of the ZnO layer. Motivated by these reports in the literature, and our own study of bare (ZnO) 3 nanoclusters, our ultimate goal is to investigate the mechanism of H 2 and O 2 production via the water splitting reaction 1 using the same (ZnO) 3 nanoclusters but now deposited on graphene or suitably functionalized graphene oxides (GOs) anchored on graphene that could be better and cheaper than the TiO 2 catalyst. An essential step, prior to water splitting in the gas phase, is the adsorption of a water molecule on the zinc atom of the photocatalyst followed by hydrolysis that involves simultaneous dissociation of the adsorbed water and proton transfer to a neighboring oxygen atom on the ZnO ring. This hydrolytic process is repeated with a second water molecule that has been studied recently.