TiO2 has been studied extensively as a promising photo-catalyst because it is abundant, cheap and stable under reaction conditions. TiO2 absorbs ultraviolet (UV) light, creating electron/hole (e/h) pairs, which can perform redox half-reactions for a range of photochemical processes. However, UV accounts for only a small portion of the solar spectrum. Shifting the band gap of TiO2 to lower energy (e.g., by adding various dopants) allows for absorption in the visible spectrum, which represents ~50% of the solar spectrum. The result is a material that shows photo-catalytic activity upon illumination with visible light. However, the absorption efficiency of the doped materials in the visible region is inherently low, which leads to low visible photo-reaction rates.
In this contribution we show that the photocatalytic activity of visible light active nitrogen-doped TiO2 can be significantly enhanced by the addition of optically active Ag nanostructures of targeted size and shape to form composite metal/semiconductor photocatalysts. The observed activity enhancement is attributed to the radiative decay of Ag surface plasmon resonance (SPR) states, increasing the steady-state concentration of e/h pairs in N-TiO2, thereby increasing the reaction rate. The ability to predictably tune the Ag SPR by controlling particle shape and size allows for the rational design of optimized metal/semiconductor composite photocatalysts and photo-electro-catalysts wherein the overlap of the metal nanostructure SPR, semiconductor band gap, and energy of the light source is maximized.