Computational Study of HO Adsorption, Hydrolysis, and Water Splitting on (ZnO) Nanoclusters Deposited on Graphene and Graphene Oxides.

Graphene and graphene oxide (GO)-based metal oxides could play an important role in using metal oxide like zinc oxide (ZnO) as photocatalysts to split water. The π conjugation structure of GO shows greater electron mobility and could enhance the photocatalytic performance of the bare ZnO catalyst by increasing the electron-hole separation. In this work, we use density functional theory (DFT) with the B3LYP exchange functional and DGDZVP2 basis set to study the impact of adsorbing (ZnO) nanoparticles on graphene and four different GO models (GO1, GO2, GO4, and GO5) on the hydration and hydrolysis of water that precedes water splitting to produce H and O atoms in the gas phase and compare them with our previous studies on the bare catalyst in the absence of the substrate. The potential energy curves and activation energies are similar, but the triplet states are lower in energy than the singlet states in contrast to the bare (ZnO) catalyst. We extend our calculations to water splitting from the hydrolyzed (ZnO) on GO1 (GO1-(ZnO)). The triplet state energy remains lower than the singlet state energy, and hydrogen production precedes the formation of oxygen, but there is no energy inter-crossing during the formation of O that occurs in the absence of a GO1 substrate. Although the hydrolysis reaction pathway follows similar steps in both the bare and GO1-(ZnO), water splitting with (ZnO) absorbed on the GO1 substrate skips two steps as it proceeds toward the production of the second H. The production of two hydrogen molecules precedes oxygen formation during water splitting, and the first Zn-H bond formation step is the rate-determining step. The ZnO trimer deposited on GO systems could be potentially attractive nanocatalysts for water splitting.