First - Principles Study On The Interaction Between Surface And Water Of Graphene - Zinc Oxide Composites

Graphene-based semiconductor composite materials have received significant attention in recent years owing to their extensive applications in many fields such as solar cells, photocatalysis, and gas sensors. Particularly in the light catalysis, because most photocatalytic reactions occur in aqueous solution or humid environment, and water molecules are directly or indirectly involved in the photocatalytic reaction, it is very important to research water-solid surface interaction at an atomic level to reveal its intrinsic chemical mechanisms, In this paper, we focus on the adsorption behavior of water molecules on the surfaces of Graphene-ZnO (G-ZnO) composites. A series of adsorption model systems are constructed with ZnO (0001) surface and G (001) surface and the Dmol3 and CASTEP packages based on density functional theory (DFT) have been utilized to optimize the geometries of these systems. The energy, total charge density, density charge difference, Mulliken population, band sturcture, density of state, and work function have been calculated and analyzed to investigate the influence of adsorption sites, coverage and zinc oxide slice thickness on the adsorption behavior of water molecules on G-ZnO surface. The main conclusions are as follows:Adsorption research of single water molecule on ZnO surface of G-ZnO composite indicated that the optimum adsorption site is on the top position of zinc atoms with hydrogen bond interactions between water molecule and two oxygen atoms in surface of ZnO, with the adsorption energy of-49.82kJ/mol. The optimum adsorption site of water molecule adsorbed on Graphene surface of G-ZnO composite is the center (hole) position of six-membered carbon ring with the adsorption energy of-16.49kJ/mol. It can be seen that the adsorption of water molecule on ZnO surface is much more stable than adsorption on Graphene surface. With the increase of ZnO layers, the adsorption energy of water molecule on ZnO surface of G-ZnO composite is gradually increased, but no obvious changes appear in pure ZnO. The van der waals interactions between zinc oxide and graphene as well as the charge transfer from zinc oxide to graphene gradually increased with the increase of ZnO layers. The charge transfer between graphene and ZnO surfaces not only enhance their stability and adsoption activity for water molecures, but also promote the effective separation of electron-hole, which is beneficial to enhance photocatalysis.Adsorption research on water molecule clusters (H2O)n (n=1-9) on single layer of ZnO in G-ZnO composite materials with low water-coverage ((?)<1 ML, ML: Monolayer) shows that, as the number of water molecules namely surface coverage increased, the adsorption energy of water molecule clusters on G-ZnO surface and the hydrogen bond energy between water molecules all gradually increased but the average adsorption energy does not exhibit a synergistic effect, while average hydrogen bond energy perform a synergistic effect due to the increase of water molecules. This synergistic effect renders a competition between the hydrogen bonds of water molecules and adsorption of water molecules on G-ZnO surface. In addition, with the number of water molecules increased, the charge transfer and electron hole separation trend between zinc oxide and graphene interface of composite materials was much obvious but did not have a significant impact on the energy band structure. The adsorption of water molecules has an effect on the work function of G-ZnO surface. With the increase of the water molecules, the work fuctions of G-ZnO composite slightly increase, but still lower than the pure zinc oxide system.Comprehensive considering the effect zinc oxide layers and the surface coverage of water molecule clusters on adsorption behavior of G-ZnO composite surface, further research on G-nZnO-mH2O (n=1～-3, m=1-6) systems have been carried out. It indicated that the adsorption energy of water molecule clusters on G-ZnO surface increase with the increase of both water-coverage and zinc oxide layers. As the water-coverage is low, the average adsorption energy increases significantly with the increase of zinc oxide layers. While at he high water-coverage, the increase trend of average adsorption energy is not obvious. Increasing the number of zinc oxide layers and surface water-coverage are effective in promoting the electron-hole separation between zinc oxide and graphene phase interface, indicating that G-ZnO composite under the actual aqueous environment will keep its excellent electron conduction properties and photocatalytic activity, In addition, the adsoption thermodynamic research of water molecule on a single layer of G-ZnO composite surface manifested that the adsorption process was a spontaneous process, and the adsorption process was an exothermic process with entropy control at high temperature. The adsorption enethalpies of H2O at a coverage of 2.41 H2O/nm2 is-108.01kJ/mol at 298.15K, which is basically consistent with the experimental results.