ZnO Colloid Crystal Facet-Type Determines both Au Photodeposition and Photocatalytic Activity

Abstract

This study investigated the crystal facet-dependence of the photochemical deposition of Au onto four differently shaped ZnO colloids; hexagonal-based nanocones, nanorods, nanobullets, and nanoplates. The different ZnO nanoparticle (NP) shapes were approximately the same size and synthesized without the use of strong-binding capping agents. Direct photoreduction of AuCl2 - onto the ZnO NPs by UV illumination at 370 nm proved to be an effective approach to produce Au-ZnO NP hybrids, where the active Au(I) precursor was photogenerated from AuCl4 - using the same UV light source. Electrospray ionization mass spectrometry confirmed that preirradiation of ethanolic HAuCl4 solutions with a 370 nm UV-diode transforms the AuCl4 - to AuCl2 -, a metastable species that is stable in a range of solvents. The solvent system used, the irradiation exposure time, and the dissolved oxygen content in the solvent were modified to generate changes in the pattern of Au NP photodeposition onto the different ZnO shapes. The relative surface area of exposed high-energy facets of the ZnO NPs were observed to have a dramatic effect on the energy barrier to Au NP nucleation on different ZnO surfaces, where the facet-dependent activity was established to be (0001) > (101 1) ≈ (0001) > (101 0). For ZnO nanoplates, typically 4-6 Au NPs deposited where approximately 50% attached to the {0001} ZnO facets and 50% to the {1010} ZnO facets. For ZnO nanocone hybrids, 1 Au NP deposited per particle, with approximately 30% depositing on the {0001} and 70% on the {1010} facet. On ZnO nanobullets, 8 Au NPs deposited, with a distribution of 8% on the {0001} facet, 75% on {1010}, and 17% on the {1011} facets. For the ZnO nanorod samples, 4 Au NPs were deposited per rod, with 20% attached to {0001} facets in the sample and 80% fixed to the {1010} facets. The different Au NP deposition distributions on different ZnO shapes caused major changes to their photocatalytic activity, as tested by degrading an organic dye, toluidine blue, in aqueous conditions. The Au-ZnO hybrid NP photoactivities were greater than pure, ZnO NP photoactivities, attributed to better separation of charge and a reduced electron-hole recombination rate. Small molecule, free-radical scavengers were added to control samples to confirm the mechanism of dye degradation, which was found to be by hydroxyl radicals generated through oxidative reaction pathways.