Semiconductor-metal nanocomposites.: Photoinduced fusion and photocatalysis of gold-capped TiO2 (TiO2/Gold) nanoparticles

Semiconductor/metal composite nanoparticles have been synthesized by chemically reducing HAuCl4 on the surface of preformed TiO2 nanoparticles. These gold-capped TiO2 nanoparticles (particle diameter 10-40 nm) were stable in acidic (pH 2-4) aqueous solutions. At low [TiO2]:[Au] ratio (<1:1), these composite nanoparticles are bigger in size (20-40 nm) and readily undergo aggregation. The TiO2/Au nanoparticles with TiO2:Au ratio >1:10 were relatively stable to 532 nm laser irradiation. On the other hand 532 nm laser pulse excitation of aggregated nanoparticles ([TiO2]:[Au] less than or equal to 1:1) led to morphological changes as they fused to produce large-size particles. Laser-induced melting/fusion was confirmed from the growth in the particle size as well as the disappearance of the aggregation absorption band. The process of melting and fusion occurred with an apparent rate constant of 1.25 x 10(9) s(-1). The role of the gold layer in promoting the photocatalytic charge transfer has been probed using thiocyanate oxidation at the semiconductor interface. More than 40%; enhancement in the oxidation efficiency is seen with TiO2/Au nanoparticles capped with low concentration of the noble metal.