Structure, Dynamics, and Phase Behavior of Water in TiO2 Nanopores

ESTEFANIA GONZALEZ SOLVEYRA; EZEQUIEL DE LA LLAVE; VALERIA MOLINERO; GALO J. A. A. SOLER ILLIA; DAMIAN A SCHERLIS

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Lugar: Washington; Año: 2013 vol. 117 p. 3330 - 3342

1932-7447

Mesoporous titania is a highly studied materialdue to its energy and environment-related applications, whichdepend on its tailored surface and electronic properties.Understanding the behavior of water in titania pores is acentral issue for practical purposes in photocatalysis, solar cells,bone implants, or optical sensors. In particular, themechanisms of capillary condensation of water in titaniamesopores and the organization and mobility of water as afunction of pore filling fraction are not yet known. In this work, molecular dynamics simulations of water confined in TiO2-rutilepores of diameters 1.3, 2.8, and 5.1 nm were carried out at various water contents. Water density and diffusion coefficients wereobtained as a function of the distance from the surface. The proximity to the interface affects density and diffusivity within adistance of around 10 Å from the walls, beyond which all properties tend to converge. The densities of the confined liquid in the2.8 and the 5.1 nm pores decrease, respectively, 7% and 4% with respect to bulk water. This decrease causes the watertranslational mobility in the center of the 2.8 nm pore to be appreciably larger than in bulk. Capillary condensation takes place inequilibrium 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 of65%. In the former case, the surface density increases uniformly with filling until condensation, whereas in the larger nanopore, acluster of water molecules develops on a localized spot on the surface for fillings just below the transition. No phase transition isdetected 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 thesepores turns out to be comparable to its behavior in less hydrophilic pores of smaller size.