Formation of Metastable Water Chains on Anatase TiO₂(101)

Anatase TiO₂ is indispensable material for energy-harvesting applications and catalysis. In this study, we employ scanning tunneling microscopy and study water adsorption on most stable TiO₂(101) surface of anatase. We demonstrate that at very low temperatures (80 K) water has the transient mobility that allows it to move on the surface and form extended chains. In contrast with many other oxides, these water chains are only metastable in nature. Adsorption at higher temperatures, where sustained diffusion is observed (190 K), leads to isolated water monomers in accord with prior literature. We speculate that the observed low-temperature mobility is a result of adsorption in a long-lived precursor state with a low diffusion barrier

Diffusion and Photon-Stimulated Desorption of CO on TiO₂(110)

Thermal diffusion of CO adsorbed on rutile TiO₂(110) was studied in the 20–110 K range using photon-stimulated desorption (PSD), temperature-programmed desorption (TPD), and scanning tunneling microscopy. During UV irradiation, CO desorbs from certain photoactive sites (e.g., oxygen vacancies). This phenomenon was exploited to study CO thermal diffusion in three steps: first, empty these sites during a first irradiation cycle, then replenish them with CO during annealing, and finally probe the active site occupancy in the second PSD cycle. The PSD and TPD experiments show that the CO diffusion rate correlates with the CO adsorption energystronger binding corresponds to slower diffusion. Increasing the CO coverage from 0.06 to 0.44 monolayer (ML) or hydroxylation of the surface decreases the CO binding and increases the CO diffusion rate. Relative to the reduced surface, the CO adsorption energy increases and the diffusion decreases on the oxidized surface. The CO diffusion kinetics can be modeled satisfactorily as an Arrhenius process with a “normal” prefactor (i.e., ν = 10¹² s^–1) and a Gaussian distribution of activation energies where the peak of the distribution is ∼0.26 eV and the full width at half-maximum (fwhm) is ∼0.1 eV at the lowest coverage. The observations are consistent with a significant electrostatic component of the CO binding energy on the TiO₂(110) surface which is affected by changes in the surface dipole and dipole–dipole interactions

Light Makes a Surface Banana-Bond Split: Photodesorption of Molecular Hydrogen from RuO₂(110)

The coordination of H₂ to a metal center via polarization of its σ bond electron density, known as a Kubas complex, is the means by which H₂ chemisorbs at Ru⁴⁺ sites on the rutile RuO₂(110) surface. This distortion of electron density off an interatomic axis is often described as a ‘banana-bond.’ We show that the Ru–H₂ banana-bond can be destabilized and split using visible light. Photodesorption of H₂ (or D₂) is evident by mass spectrometry and scanning tunneling microscopy. From time-dependent density functional theory, the key optical excitation splitting the Ru–H₂ complex involves an interband transition in RuO₂ which effectively diminishes its Lewis acidity, thereby weakening the Kubas complex. Such excitations are not expected to affect adsorbates on RuO₂ given its metallic properties. Therefore, this common thermal cocatalyst employed in photocatalysis is, itself, photoactive