Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO2 and Au–TiO2

• Published in: Journal of physical chemistry. C Vol. 117; no. 29; pp. 15035 - 15049
• Main Authors: Panayotov, Dimitar A, DeSario, Paul A, Pietron, Jeremy J, Brintlinger, Todd H, Szymczak, Lindsey C, Rolison, Debra R, Morris, John R
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 25.07.2013
• Subjects: Charge carriers: generation, recombination, lifetime, trapping, mean free paths; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport phenomena in thin films and low-dimensional structures; Exact sciences and technology; Materials science; Nanoscale materials and structures: fabrication and characterization; Other topics in nanoscale materials and structures; Physical radiation effects, radiation damage; Physics; Structure of solids and liquids; crystallography; Surface and interface electron states; Surface states, band structure, electron density of states; Porous materials; Electron hole pair; Nanostructures; Surface plasmons; High vacuum; Nanoparticles; Wide band; Visible radiation; Illumination; Formates; Absorption spectra; Electron density; Methanol; Photochemistry; Conduction bands; Radiation effects; Electronic density of states; Excited states; Mesoporosity; Charge carrier trapping; Lifetime; Ultraviolet radiation; Electronic structure; Trapped electrons; Electron state; Resonance; Nanostructured materials; Photooxidation; Hole traps; Energy transfer
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp312583w

Comparison of methanol photochemistry at three-dimensionally (3D) networked aerogels of TiO2 or Au–TiO2 reveals that incorporated Au nanoparticles strongly sensitize the oxide nanoarchitecture to visible light. Methanol dissociatively adsorbs at the surfaces of TiO2 and Au–TiO2 aerogels under dark, high-vacuum conditions. Upon irradiation of either ultraporous material with broadband UV light under anaerobic conditions, adsorbed methoxy groups act as hole-traps and extend conduction-band and shallow-trapped electron lifetimes. A higher excited-state electron density arises for UV-irradiated TiO2 aerogel relative to commercial nanoparticulate TiO2, indicating that 3D networked TiO2 more efficiently separates electron–hole pairs. Upon excitation with narrow-band visible light centered at 550 nm, long-lived excited-state electrons are evident on CH3OH-exposed Au–TiO2 aerogelsbut not on identically dosed TiO2 aerogelsverifying that incorporated Au nanoparticles sensitize the networked oxide to visible light. Under aerobic conditions (20 Torr O2) and broadband UV illumination, surface-sited formates accumulate as adsorbed methoxy groups oxidize, at similar rates, on Au–TiO2 and TiO2 aerogels. Moving to excitation wavelengths longer than ∼400 nm (i.e., the low-energy range of UV light) dramatically decreases methoxy photoconversion for methanol-saturated TiO2 aerogel, while Au–TiO2 aerogel remains highly active for methanol photooxidation. The wavelength dependence of formate production on Au–TiO2 tracks the absorbance spectrum for this material, which peaks at λ = 550 nm due to resonance with the surface plasmon in the Au particles. The photooxidation rate for Au–TiO2 aerogel at 550 nm is comparable to that for TiO2 aerogel under broadband UV illumination, indicating efficient energy transfer from Au to TiO2 in the 3D mesoporous nanoarchitecture.