Ultrafast Interfacial Proton-Coupled Electron Transfer

In this review we have described recent experimental and theoretical research on ultrafast interfacial inner sphere PCET dynamics. What makes interfacial PCET processes unique is the presence of a strong potential gradient that imposes opposing forces on electrons and protons within a spatial region that is, at most, a few angstroms wide. Because of the strong but tunable gradients, whether a PCET process occurs through separate proton and electron transfer steps or through a concerted process can have a significant impact on the activation energies and overpotentials that are required for a chemical reaction. Considering that tantalizing photocatalytic processes, such as the decomposition of H2O into H2 and O2 and the reduction of CO2 into chemical fuels,11,154,155 have been demonstrated with band gap excitation of TiO2 surfaces, it is highly desirable to explore the reaction mechanisms and to devise chemical pathways that perform the multiple electron and proton transfer steps as close to the thermodynamic limit as possible. In addition to the complexity of describing chemical processes under strongly inhomogeneous potentials, it is also necessary to devise experimental methods that can probe ultrafast charge transfer processes at interfaces. Techniques such as time-resolved two-photon photoemission provide the means to probe interfacial charge transfer dynamics on time scales that range from a few femtoseconds to longer. The recently developed attosecond laser technology is promising for time resolving even faster processes,156 as attested by measurements of electron transport through a metal interface. 157 Electron spectroscopic methods, however, are limited to well-defined surfaces under UHV conditions where photoelectron energy and momentum can be related simply to the properties of the excited states of the system. It is also desirable to develop and employ other methods, such as X-ray scattering and surface nonlinear spectroscopy,46,158,159 which can provide chemical and dynamical contrast under more realistic chemical reaction conditions. Ultrafast interfacial PCET also presents substantial challenges for theory. In addition to the difficulty of describing the interfacial potential and, associated with it, the inhomogeneous screening by the fast and slow degrees of freedom of the system, it is also a challenge to treat the electron and proton degrees of freedom on an equal basis, as described elsewhere in this Special Issue.4 This is the case for PCET processes occurring under near equilibrium conditions, but challenges are particularly severe for excited state processes, such as would predominate under photocatalytic conditions. Nevertheless, several groups are paving the way to develop methods that are starting to provide valuable insights and powerful tools for investigating ultrafast interfacial PCET dynamics in photon mediated processes.118,134,138 The challenges are severe, but the rewards for learning how to harness PCET processes in photocatalysis based clean energy generation make the effort essential. © 2010 American Chemical Society.