Structure, Dynamics, and Phase Behavior of Water in TiO2 Nanopores
Mesoporous titania is a highly studied material due to its energy and environment-related applications, which depend on its tailored surface and electronic properties. Understanding the behavior of water in titania pores is a central issue for practical purposes in photocatalysis, solar cells, bone implants, or optical sensors. In particular, the mechanisms of capillary condensation of water in titania mesopores and the organization and mobility of water as a function of pore filling fraction are not yet known. In this work, molecular dynamics simulations of water confined in TiO2-rutile pores of diameters 1.3, 2.8, and 5.1 nm were carried out at various water contents. Water density and diffusion coefficients were obtained as a function of the distance from the surface. The proximity to the interface affects density and diffusivity within a distance of around 10 Å from the walls, beyond which all properties tend to converge. The densities of the confined liquid in the 2.8 and the 5.1 nm pores decrease, respectively, 7% and 4% with respect to bulk water. This decrease causes the water translational mobility in the center of the 2.8 nm pore to be appreciably larger than in bulk. Capillary condensation takes place in equilibrium for a filling of 71% in the 2.8 nm pore and in conditions of high supersaturation in the 5.1 nm pore, at a filling of 65%. In the former case, the surface density increases uniformly with filling until condensation, whereas in the larger nanopore, a cluster of water molecules develops on a localized spot on the surface for fillings just below the transition. No phase transition is detected in the smaller pore. For all the systems studied, the first monolayer of water is strongly immobilized on the interface, thus reducing the accessible or effective diameter of the pore by around 0.6 nm. As a consequence, the behavior of water in these pores turns out to be comparable to its behavior in less hydrophilic pores of smaller size.