PNNL

Abstract

Hydrated TiO2 nanocrystals play a central role in photocatalysis and biomass conversion, yet the molecular-level structure of water on anatase (101) remains actively debated. Here, we synthesize anatase nanocrystals exposing >90% of the (101) facet with minimal surface contamination, and we combine diffuse-reflectance infrared spectroscopy (DRIFTS) with first-principles calculations to establish a consistent thermodynamic and vibrational picture of adsorbed water up to 1.0 monolayer. Density functional theory is used to compute the adsorption energetics, harmonic and anharmonic vibrational spectra, and the free-energy phase diagram. The calculations show that the stoichiometric (101) surface favors a full monolayer of molecularly adsorbed H2O at temperatures (298–355 K), with dissociated hydroxyls becoming favorable only at elevated temperature and pressure. Projection analyses of the computed normal modes enable rigorous assignment of the ?(OH) region and clarify long-standing ambiguities in peak interpretation. Comparison with DRIFTS spectra reveals that a commonly reported feature near 3640 cm?¹ corresponds to a red-shifted terminal OH, not a water symmetric stretch. The integrated thermodynamic and spectroscopic analysis provides a refined framework for identifying molecular and dissociated water on anatase TiO2(101), and establishes reference points for interpreting water-mediated surface chemistry in oxide-catalyzed transformations.