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

Abstract

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.