Molecular behavior of water in TiO2 nano-slits with varying coverages of carbon: a molecular dynamics simulation study

It is well known that titanium dioxide (TiO2) is biocompatible and environmentally friendly. Consequently, TiO2 is widely applied in many fields, such as implant materials, photocatalysis, pigments, cosmetic additives, etc. Mesoporous TiO2 finds many industrial applications, because of its high surface area and stable structure. However, the strong interaction between TiO2 and water molecules sometimes limits its application to solution environments. Our previous computational work showed that changes to the surface chemistry of TiO2 can affect the hydrogen bond network of water molecules on the TiO2 surface, and so influence the diffusion of water in the slits. Thus, a carbon-modified TiO2 surface could be an alternative way to avoid this limitation. In this work, a slit pore model with a modified TiO2 surface (pore widths 1.2 nm, 1.6 nm and 2.0 nm) with varying carbon coverages (0%, 7%, 47%, 53%, 93% and 100%) was presented. Molecular dynamics (MD) simulations were then performed to investigate the sorption and diffusion of water in these slits. Simulation results showed that the interfacial water molecules on bare TiO2 regions were little affected by the neighboring carbon, and they have the same properties as those on bare TiO2 surfaces. However, the diffusion of water molecules in the center of the slit was enhanced on increase of carbon coverage, because the carbon layer broke the hydrogen bond network between the interfacial water molecules and those on the bare TiO2 surface. It was found that in the slits (>1.2 nm) fully covered by carbon the diffusion coefficients of water are larger than that of bulk water. Moreover, large pore sizes caused an increase in the mobility of water molecules in carbon-modified TiO2, in agreement with previous experimental work.