Biogenic synthesis of photocatalytically active Ag/TiO2 and Au/TiO2 composites

In this work, spherical Ag and Au nanoparticles, primarily below 10 nm in diameter, were synthesised by contacting biological extracts from the vascular plant Citrus limon with aqueous Au3+ and Ag+ solutions at ambient temperature. The chemical constituents of the plant extract acted as reducing, capping and stabilising agents, and nanoparticle size and shape could be tuned by adjusting the pH of the extract suspension. Au nanoparticles synthesised at pH = 7 and Ag nanoparticles synthesised at pH = 11 were monodisperse and well-defined. Thin films of these Au and Ag nanoparticles were coated onto TiO2 by colloidal deposition and the resulting composites were tested as catalysts for the degradation of an organic dye. 1 wt% Au/TiO2 and Ag/TiO2 composites were at least as active as M/TiO2 composites prepared by reduction with NaBH4.