AVS2012 Session EN+SS-FrM: Photocatalysis and Solar Fuels

Molecular and dissociative forms of adsorbed methanol were prepared on the rutile TiO₂(110) surface to study their relative photocatalytic activity for hole-mediated oxidation. Molecular methanol is the dominant surface species on the vacuum-annealed TiO₂(110) surface in ultrahigh vacuum (UHV). Coadsorption of methanol with oxygen results in ~ 20% of the adsorbed methanol decomposing to methoxy and OH. Subsequent heating of the surface to 350 K leaves a surface with only adsorbed methoxy groups. Using temperature-programmed desorption, we show that adsorbed methoxy is at least an order of magnitude more reactive than molecularly adsorbed methanol for hole-mediated photooxidation. Methoxy photodecomposes through cleavage of a C-H bond forming adsorbed formaldehyde and a surface OH group. These results suggest that methoxy, and not molecular methanol, is the effective hole scavenger in photochemical reactions of methanol on TiO₂. Same reactions were also studied with water coadsorption.

Photoelectrochemical (PEC) water splitting devices hold great promise for harvesting solar energy, however, existing electrodes suffer from either stability or efficiency limitations. Owing to its low production cost, environmental compatibility, and remarkable stability, TiO₂ has been widely investigated as a PEC electrode since 1972. However, the solar-to-fuel conversion efficiency of TiO₂ PEC electrodes is still much lower than the theoretical value. This is partially due to the dilemma of short minority carrier diffusion length and long optical absorption length, as well as the low electron mobility. Nanostructured conductive scaffolds show promise to solve this challenge by decoupling light absorption and charge carrier diffusion while enhancing conductivity. In this research, we synthesized TiO₂ PEC electrodes on a conductive scaffold comprised of antimony doped tin oxide particles (ATO-particle film). These structures, which are a cost effective alternative to semiconductor supported TiO₂ electrodes, yielded a photocurrent density of 0.58 mA/cm². This is approximately 3× the corresponding current density for planar TiO₂ PEC electrodes on FTO glass. Our results have shown that the porosity of ATO-particle film has limited the further efficiency improvement, which can be addressed by optimizing particle size, thickness, and assembly strategy for ATO-particle films. Owing to its transparency in a wide range of wavelengths, the ATO-particle scaffold also has great potential to boost the efficiency of devices using other narrow bandgap PEC materials, e. g. Fe₂O₃.

Solar water splitting to produce hydrogen represents a potential approach to satisfy the global energy demand in a sustainable manner. Recently it has been reported that the incorporation of plasmonic nanoparticles into semiconductor architectures offers a potential route to increase the efficiency of photoelectrochemical water splitting due to the unique optical properties of plasmonic nanomaterials. We investigated the energetics and dynamics of electron flow in Au-TiO₂ heterostructures following excitation of the Au nanoparticles surface plasmon resonance with visible light. Our results show that the incorporation of Au nanoparticles into wide band gap semiconductors has promise for use as visible light sensitizers. Moreover, we have studied the role of the hole scavenger methanol in the plasmon-mediated charge transfer process in order to ascertain the nature of possible thermodynamic or kinetic limitations involved in this process. These results demonstrate that the excited-state lifetime of these hot electrons in the TiO₂ conduction band is dramatically extended relative to direct band gap excitation within the semiconductor itself, suggesting a possible strategy for improving the efficiency of photocatalytic reactions.

Catalytic reduction of carbon dioxide into fuels would provide an ideal storage medium for intermittent renewable energy sources. Copper and copper oxides electro-catalysts have been found to be capable of producing significant quantities of hydrocarbons or alcohols from CO₂ in aqueous solutions. Selectivity to methanol is speculated to be due to Cu(I) species in electrochemical systems; however, these pathways have not been experimentally verified. Here, the third-generation charge optimized many body (COMB3) potentials, which are proven to be successful to characterize different types of bonding in the heterogeneous systems, are employed to investigate the atomic scale mechanisms associated with catalytic reactions on Cu surfaces and clusters supported on metal oxide surfaces. In particular, the reaction free energies of selected CHO molecules on the Cu (211) surface are investigated and validated with density functional theory calculations. The electrochemical systems are simulated with room temperature, low-energy (5 or 10 eV) deposition of CO₂ or CO₂+H₂O on the Cu (211) surface and Cu cluster interface with the ZnO (101-1) surface. The results suggest that the higher incident energy and the presence of water molecules facilitate CO₂ dissociation. The charge state of the Cu cluster and the charge transfer process are predicted to play significant roles in the selectivity of the catalysts. In particular, the Cu(I) species at the Cu/ZnO interface are predicted to be preferable sites for CO₂ reduction and dissociation, which is consistent with experimental observations. This work was supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001058.