Dynamics of the Photogenerated Hole at the Rutile TiO2(110)/Water Interface: A Nonadiabatic Simulation Study

Hydrogen production in photoelectrochemical cells constitutes an important avenue toward carbon-free fuel. The most convenient process for hydrogen production is the splitting of water molecules, which necessitates a catalytic reaction involving a semiconductor. Here, we introduce a framework for the study of photocatalyzed reactions on semiconductor surfaces based on time-dependent density functional theory that explicitly accounts for the evolution of electronically excited states. Within this framework, we investigate the possibility of hole-mediated splitting of molecularly adsorbed water on a representative metal oxide surfacethe rutile TiO2(110). We find that oxidative dehydrogenation of water is possible in synergy with thermal effects at temperatures between 60 and 100 K only when defects like Ti interstitials are present in the subsurface region. This study presents a general computational strategy for describing photoexcited semiconductor/adsorbate interfaces and also demonstrates that the occurrence of water dissociation on the rutile TiO2(110) surface depends sensitively on the local atomic environment and external parameters such as temperature. ■ INTRODUCTION Light-assisted hydrogen production in photoelectrochemical cells (PECs) constitutes an avenue toward solar energy conversion for the production of carbon-free fuel. Upon illumination the oxygen evolution reaction (water oxidation reaction) occurs on the anode electrode, which can be described by the overall reaction + * → + + 2H O 4h O 4H 2 2 with hydrogen being evolved on the cathode. The conversion efficiency of the PEC depends critically on the catalytic performance of the electrodes. Nanostructured devices based on titanium dioxide (TiO2) are promising candidates for wide use in photon-induced reactions because of this material’s photochemical stability, nontoxicity, and natural abudance. TiO2 is a semiconductor metal oxide with an optical band gap of ∼3.2 eV in its bulk rutile form and band gap edges that straddle the water redox potentials; that is, the involved chemical reactions become thermodynamically accessible upon photon absorption. The catalytic activity of TiO2 can be enhanced by cocatalysts or by metal dopants or defects due to ambient contamination during catalyst preparation. The effect of such structural modifications on the thermochemistry at the surface/adsorbate interface has been discussed extensively in the literature, but a comprehensive atomistic description of dynamic processes, such as the transport of the photogenerated charge carriers in the surface, is lacking even for pure TiO2. Photon-induced water dissociation on the rutile TiO2(110) surface has recently been reported in the scanning tunneling microscope study of Tan et al., which demonstrated a very low rate (few events per hour) O−H bond-breaking in water upon irradiation with ultraviolet (UV) light under ultrahigh-vacuum conditions at 80 K. However, according to density functional theory (DFT) calculations reported by Patel et al., the highest occupied molecular orbitals of molecular water are 1.40 eV below the valence band maximum (VBM) of the surface, which raises the question whether the reported dissociation of water is light-driven. The measured properties of real materials depend on the preparation methods and conditions, which makes the identification of universal structure−property relationships a challenging task and hinders a systematic approach to catalyst optimization. Atomistic modeling and simulation of surface/ adsorbate interfaces can provide insights into the microscopic physicochemical processes that control catalysis, an inherently atomic-scale phenomenon, as well as interfacial charge-carrier transfer. Previous theoretical studies of photon-mediated catalysis on rutile TiO2 surfaces used Received: August 24, 2014 Revised: November 3, 2014 Published: November 6, 2014 Article