Photocatalytic Activity and Fluorescence of Gold/Zinc Oxide Nanoparticles Formed by Dithiol Linking

Monolayer-protected gold nanoparticles (AuNPs) with average diameters of 2-4 nm have been covalently attached to zinc oxide nanorods using dithiol ligands. Electron microscopy and Raman spectroscopy show that ozone treatment or annealing at 300 or 450 °C increases the average diameter of the AuNPs to 6, 8, and 14 (±1) nm, respectively, and decomposes the organic layers to various degrees. These treatments locate the AuNPs closer to the nanorods. Heating and subsequent ozone exposure changes the color of the as-prepared nanocomposite powder from blue to purple due to oxidation of the outer layer of the AuNPs, and heating to 300 °C changes it to pink due to oxygen desorption. ZnO nanorods have a bimodal photoluminescence spectrum that consists of an ultraviolet excitonic peak and a visible, surface defect-related peak. Ozone treatment and annealing of the nanocomposite decreases the intensities of both peaks due to quenching by the AuNPs, but the visible peak is affected less. The photocatalytic efficiency of the nanocomposites toward oxidative degradation of rhodamine B has been measured and follows the order 300 °C > 450 °C > ozone treated ≈ as-prepared ≈ bare ZnO. The greater efficiency of the annealed samples likely arises from decreased electron-hole pair recombination rates.