Plasmonic enhancement of visible-light water splitting with Au–TiO2 composite aerogels

We demonstrate plasmonic enhancement of visible-light-driven splitting of water at three-dimensionally (3D) networked gold–titania (Au–TiO2) aerogels. The sol–gel-derived ultraporous composite nanoarchitecture, which contains 1 to 8.5 wt% Au nanoparticles and titania in the anatase form, retains the high surface area and mesoporosity of unmodified TiO2 aerogels and maintains stable dispersion of the ∼5 nm Au guests. A broad surface plasmon resonance (SPR) feature centered at ∼550 nm is present for the Au–TiO2 aerogels, but not Au-free TiO2 aerogels, and spans a wide range of the visible spectrum. Gold-derived SPR in Au–TiO2 aerogels cast as films on transparent electrodes drives photoelectrochemical oxidation of aqueous hydroxide and extends the photocatalytic activity of TiO2 from the ultraviolet region to visible wavelengths exceeding 700 nm. Films of Au–TiO2 aerogels in which Au nanoparticles are deposited on pre-formed TiO2 aerogels by a deposition–precipitation method (DP Au/TiO2) also photoelectrochemically oxidize aqueous hydroxide, but less efficiently than 3D Au–TiO2, despite having an essentially identical Au nanoparticle weight fraction and size distribution. For example, 3D Au–TiO2 containing 1 wt% Au is as active as DP Au/TiO2 with 4 wt% Au. The higher photocatalytic activity of 3D Au–TiO2 derives only in part from its ability to retain the surface area and porosity of unmodified TiO2 aerogel. The magnitude of improvement indicates that in the 3D arrangement either a more accessible photoelectrochemical reaction interphase (three-phase boundary) exists or more efficient conversion of excited surface plasmons into charge carriers occurs, thereby amplifying reactivity over DP Au/TiO2. The difference in photocatalytic efficiency between the two forms of Au–TiO2 demonstrates the importance of defining the structure of Au‖TiO2 interfaces within catalytic Au–TiO2 nanoarchitectures.