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

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 O-2) 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 similar to 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 lambda = 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.