Quantum molecular dynamics study of water on TiO2(110) surface

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.

Surface chemistry of TiO2 connecting thermal catalysis and photocatalysis

Chemical reactions catalyzed under heterogeneous conditions have recently expanded rapidly from traditional thermal catalysis to photocatalysis due to the rising concerns about sustainable development of energy and the environment. Adsorption of reactants on catalyst surfaces, subsequent surface reactions, and desorption of products from catalyst surfaces occur in both thermal catalysis and photocatalysis. TiO₂ catalysts are widely used in thermal catalytic and photocatalytic reactions. Herein we review recent progress in surface chemistry, thermal catalysis and photocatalysis of TiO₂ model catalysts from single crystals to nanocrystals with the aim of examining if the surface chemistry of TiO₂ can bridge the fundamental understanding between thermal catalysis and photocatalysis. Following a brief introduction, the structures of major facets exposed on TiO₂ catalysts, including surface reconstructions and defects, as well as the electronic structure and charge properties, are firstly summarized; then the recent progress in adsorption, thermal chemistry and photochemistry of small molecules on TiO₂ single crystals and nanocrystals is comprehensively reviewed, focusing on manifesting the structure-(photo)activity relations and the commonalities/differences between thermal catalysis and photocatalysis; and finally concluding remarks and perspectives are given.

Modeling and Simulations in Photoelectrochemical Water Oxidation: From Single Level to Multiscale Modeling

This review summarizes recent developments, challenges, and strategies in the field of modeling and simulations of photoelectrochemical (PEC) water oxidation. We focus on water splitting by metal-oxide semiconductors and discuss topics such as theoretical calculations of light absorption, band gap/band edge, charge transport, and electrochemical reactions at the electrode-electrolyte interface. In particular, we review the mechanisms of the oxygen evolution reaction, strategies to lower overpotential, and computational methods applied to PEC systems with particular focus on multiscale modeling. The current challenges in modeling PEC interfaces and their processes are summarized. At the end, we propose a new multiscale modeling approach to simulate the PEC interface under conditions most similar to those of experiments. This approach will contribute to identifying the limitations at PEC interfaces. Its generic nature allows its application to a number of electrochemical systems.

Correlation of the depletion layer with the Helmholtz layer in the anatase TiO2–H2O interface via molecular dynamics simulations

By correlating the Helmholtz layer with the depletion layer, the depletion layer widths of anatase TiO₂ surfaces (001), (100), and (101) are calculated.

Cysteine on TiO2(110): a theoretical study by reactive dynamics and photoemission spectra simulation

Owing to the importance of bioinorganic interface properties for the biocompatibility of implants and for biosensing technology, it has become indispensable to gain understanding of their crucial structure-property relations at the atomistic level. Motivated by this fact, we use cysteine amino acid on perfect and defective TiO2(110) surfaces as model systems and study adsorption by means of classical all-atom reactive molecular dynamics and ab initio O 1s, N 1s, and S 2p photoemission spectra (XPS) simulations of the most relevant adsorbate structures. By analysis of the dynamics results and a detailed comparison with spectra recently collected for this adsorbate, we obtain conclusions of both general and particular character. It is shown that the interaction of cysteine with the TiO2(110) surface has multipoint character involving the carboxylic group as well as the amino and sulfur groups. The proton-transfer reactivity of cysteine is enhanced by the presence of the surface, and different forms of cysteines are confirmed to be present in the adsorbate. A general conclusion is that reactive force field dynamics combined with selected spectroscopy provides a viable path to understanding bioinorganic surfaces with ramifications for the design of such surfaces for future technological applications.

Controlled (110) and (101) crystallographic plane growth of single crystalline rutile TiO2 nanorods by facile low cost chemical methods

A new approach has been employed to grow large rutile TiO₂ nanorods (NRs) with a high number density by low cost chemical methods.

Adsorption of arginine-glycine-aspartate tripeptide onto negatively charged rutile (110) mediated by cations: the effect of surface hydroxylation

Classical molecular dynamics (MD) simulations were employed to investigate the adsorption behaviors of arginine-glycine-aspartate (RGD) tripeptide onto the negatively charged hydroxylated/nonhydroxylated rutile (110) surfaces, mediated by biologically important cations (Na+ or Ca2+). The simulation results indicate that the inherent nature of the cation determines its binding strength, thereby regulating the adsorption geometry of the peptide. The sparse hydroxyl groups on the nonhydroxylated rutile diminish the probability of H-bond formation between RGD and the surface, resulting in an early desorption of the peptide even with a mediating Na+ ion. In contrast, the negatively charged aspartate (Asp) side chain is bridged to the negatively charged hydroxylated rutile by an inner-sphere Na+ ion, in coordination with the Asp-rutile hydrogen bonds at the anchoring sites. The inner- and outer-sphere Ca2+ ions are demonstrated to be capable of "trapping" RGD on both hydroxylated and nonhydroxylated rutile, in the absence of hydrogen bonds with the surface. The strongly bound inner-sphere mediating Ca2+ ion exerts a "gluing" effect on the Asp side chain, producing a tightly packed RGD-rutile complex, whereas a less localized distribution density of the outer-sphere mediating Ca2+ ion results in a higher mobility of the Asp side chain. The intramolecular interaction is suggested to facilitate the structural stability of RGD adsorbed on the negative rutile in a "horseshoe" configuration.