Dynamical and structural properties of adsorbed water molecules at the TiO2 anatase-(1 0 1) surface: Importance of interfacial hydrogen-bond rearrangements

Water Structure in the First Layers on TiO2: A Key Factor for Boosting Solar-Driven Water-Splitting Performances

The water hydrogen-bonded network is strongly perturbed in the first layers in contact with the semiconductor surface. Even though this aspect influences the outer-sphere electron transfer, it was not recognized that it is a crucial factor impacting the solar-driven water-splitting performances. To fill this gap, we have selected two TiO2 anatase samples (with and without B-doping), and by extensive experimental and computational investigations, we have demonstrated that the remarkable 5-fold increase in water-splitting photoactivity of the B-doped sample cannot be ascribed to effects typically associated to enhanced photocatalytic properties, such as band gap, heterojunctions, crystal facets, and other aspects. Studying these samples by combining FTIR measurements under controlled humidity with first-principles simulations sheds light on the role and nature of the first-layer water structure in contact with the photocatalyst surfaces. It turns out that the doping hampers the percolation of tetrahedrally coordinated water molecules while enhancing the population of topological H-bond defects forming approximately linear H-bonded chains. This work unveils how doping the semiconductor surface affects the local electric field, determining the water splitting rate by influencing the H-bond topologies in the first water layers. This evidence opens new prospects for designing efficient photocatalysts for water splitting.

Nonequilibrium Ab Initio Molecular Dynamics Simulation of Water Splitting at Fe2O3-Hematite/Water Interfaces in an External Electric Field

In the exploration of the optimal material for achieving the photoelectrochemical dissociation of water into hydrogen, hematite (α-Fe2O3) emerges as a highly promising candidate for proof-of-concept demonstrations. Recent studies suggest that the concurrent application of external electric fields could enhance the photoelectrochemical (PEC) process. To delve into this, we conducted nonequilibrium ab initio molecular dynamics (NE-AIMD) simulations in this study, focusing on hematite-water interfaces at room temperature under progressively stronger electric fields. Our findings reveal intriguing evidence of water molecule adsorption and dissociation, as evidenced by an analysis of the structural properties of the hydrated layered surface of the hematite-water interface. Additionally, we scrutinized intermolecular structures using radial distribution functions (RDFs) to explore the interaction between the hematite slab and water. Notably, the presence of a Grotthuss hopping mechanism became apparent as the electric field strength increased. A comprehensive discussion based on intramolecular geometry highlighted aspects such as hydrogen-bond lengths, H-bond angles, average H-bond numbers, and the observed correlation existing among the hydrogen-bond strength, bond-dissociation energy, and H-bond lifetime. Furthermore, we assessed the impact of electric fields on the librational, bending, and stretching modes of hydrogen atoms in water by calculating the vibrational density of states (VDOS). This analysis revealed distinct field effects for the three characteristic band modes, both in the bulk region and at the hematite-water interface. We also evaluated the charge density of active elements at the aqueous hematite surface, delving into field-induced electronic charge-density variations through the Hirshfeld charge density analysis of atomic elements. Throughout this work, we drew clear distinctions between parallel and antiparallel field alignments at the hematite-water interface, aiming to elucidate crucial differences in local behavior for each surface direction of the hematite-water interface.

Interactions of limonene and carvone on titanium dioxide surfaces

Limonene, a monoterpene, found in cleaning products and air fresheners can interact with a variety of surfaces in indoor environments. An oxidation product of limonene, carvone, has been reported to cause contact allergens. In this study, we have investigated the interactions of limonene and carvone with TiO₂, a component of paint and self-cleaning surfaces, at 297 ± 1 K with FTIR spectroscopy and force field-based molecular dynamics and ab initio simulations. The IR absorption spectra and computational methods show that limonene forms π-hydrogen bonds with the surface O-H groups on the TiO₂ surface and that carvone adsorbs on the TiO₂ surface through a variety of molecular interactions including through carbonyl oxygen atoms with Ti⁴⁺ surface atoms, O-H hydrogen bonding (carbonyl O⋯HO) and π-hydrogen bonds with surface O-H groups. Furthermore, we investigated the effects of relative humidity (RH) on the adsorption of limonene and carvone on the TiO₂ surface. The spectroscopic results show that the adsorbed limonene can be completely displaced by water at a relative humidity of ca. 50% RH (∼2 MLs of water) and that 25% of carvone is displaced at ca. 67% RH, which agrees with the calculated free energies of adsorption which show carvone more strongly adsorbs on the surface relative to limonene and thus would be harder to displace from the surface. Overall, this study shows how a monoterpene and its oxidation product interact with TiO₂ and the impact of relative humidity on these interactions.

Observation of Molecular Hydrogen Produced from Bridging Hydroxyls on Anatase TiO2(101)

Anatase TiO₂ is used extensively in a wide range of catalytic and photocatalytic processes and is a promising catalyst for hydrogen production. Here, we show that molecular hydrogen was produced from bridging hydroxyls (HO_b) on the (101) surface of single-crystal anatase (TiO₂(101)). This stands in contrast to rutile TiO₂(110), where HO_b pairs react to form H₂O. Electron bombardment at 30 K produced bridging oxygen vacancies in the surface. Deuterated bridging hydroxyls (DO_b) were subsequently formed via dissociation of adsorbed D₂O and confirmed by infrared reflection-absorption spectroscopy. During temperature-programmed desorption (TPD) spectroscopy, D₂ desorption was observed at 520 K. Density functional theory calculations show that both H₂ and H₂O production from HO_b are endothermic at 0 K on TiO₂(101), but H₂ (H₂O) desorption is entropically driven above 230 K (800 K). The calculated activation barrier for H₂ desorption is 1.40 eV, which is similar to the desorption energy obtained from analysis of the D₂ TPD spectra. The H₂ desorption likely proceeds in two steps: H atom diffusion on the surface and then recombination.