Photoelectorochemical and Photocatalytic Aspects of Semiconductor-Metal Nanocomposites

Semiconductor and metal nanoparticles display unique size dependent photophysical, electrochemical, photocatalytic and optical properties. The presence of metal facilitates electron transfer from photoexcited semiconductor to the surroundings and decreases recombination rate between the electrons-holes in the semiconductor.

The focus of this work has been the synthesis and characterization of novel semiconductor – metal composites in films and solutions. Four areas of importance in these nanocomposites have been addressed: Photoelectrochemical properties of composite films, Fermi-level equilibration between semiconductor-metal, dynamic nature of semiconductor-metal interface and role of metal in promoting photocatalytic activity of the semiconductor.

Photoelectrochemical performance Deposition of metals on semiconductor films leads to increased light to energy conversion efficiency and demonstrates beneficial photoelectrochemical properties such as higher photocurrent and photovoltage compared to plain semiconductor. A TiO2 – Au film shows ~25% energy efficiency compared to <5 % for the semiconductor alone. The photocurrent using a TiO2-Au film is 3-5 times higher than a plain TiO2 film.

Fermi level equilibration Photoexcited semiconductor in contact with the metal undergoes charge equilibration with metal which alters the energetics of the composite by shifting the Fermi level to more negative potentials. Furthermore, a size dependent negative shift in the Fermi level is also observed in a TiO2 – Au composite. Greater negative shifts in Fermi level occurs with smaller size Au nanoparticles.

Dynamic nature of interfacial process The semiconductor-metal interface is dynamic in nature and demonstrates photomediated chemical transformations. For example, metal nanoparticles undergo change in oxidation state when a TiO2-Au composite is photoexcited in the presence of oxygen. Such transformations diminish the beneficial photoelectrochemical properties of a TiO2-metal film following prolonged photoillumination.

Photocatalytic properties Colloidal suspension of TiO2-Au Nanocomposites demonstrates greater photocatalytic activity than TiO2 alone. Presence of a metal- metal ion mixture enhances the catalytic activity of the composite semiconductor. Phototoillumination of TiO2 containing an optimal ratio of Au(0)/Au(III) lead to a ~ 40 % enhancement in the photocatalytic oxidation efficiency of thiocyanate – a probe specie – compared to TiO2 alone. Similarly, the reduction of fullerene is catalyzed in the presence of Au nanoparticles of different sizes.