This study investigated how to control the rate of photoreduction of metastable AuCl2(-) at the solid-solution interface of large ZnO nanoparticles (NPs) (50-100 nm size). Band-gap photoexcitation of electronic charge in ZnO by 370 nm UV light yielded Au NP deposition and the formation of ZnO-Au NP hybrids. Au NP growth was observed to be nonepitaxial, and the patterns of Au photodeposition onto ZnO NPs observed by high-resolution transmission electron microscopy were consistent with reduction of AuCl2(-) at ZnO facet edges and corner sites. Au NP photodeposition was effective in the presence of labile oleylamine ligands attached to the ZnO surface; however, when a strong-binding dodecanethiol ligand coated the surface, photodeposition was quenched. Rates of interfacial electron transfer at the ZnO-solution interface were adjusted by changing the solvent, and these rates were observed to strongly depend on the solvent's permittivity (ε) and viscosity. From measurements of electron transfer from ZnO to the organic dye toluidine blue at the ZnO-solution interface, it was confirmed that low ε solvent mixtures (ε ≈ 9.5) possessed markedly higher rates of photocatalytic interfacial electron transfer (∼3.2 × 10(4) electrons·particle(-1)·s(-1)) compared to solvent mixtures with high ε (ε = 29.9, ∼1.9 × 10(4) electrons·particle(-1)·s(-1)). Dissolved oxygen content in the solvent and the exposure time of ZnO to band-gap, near-UV photoexcitation were also identified as factors that strongly affected Au photodeposition behavior. Production of Au clusters was favored under conditions that caused electron accumulation in the ZnO-Au NP hybrid. Under conditions where electron discharge was rapid (such as in low ε solvents), AuCl2(-) precursor ions photoreduced at ZnO surfaces in less than 5 s, leading to deposition of several small, isolated ∼6 nm Au NP on the ZnO host instead.

Metal-semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au-ZnO particles (Pt-Au-ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au-ZnO particles (Au@Au-ZnO). An epitaxial connection at the Au-ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on-off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation properties of Pt-Au-ZnO hybrids compared to Au-ZnO. In contrast, Au-ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au-ZnO (Au, ∼5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi-level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au-ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au-ZnO. Pt-Au-ZnO hybrids showed better photodegradation and mineralization properties compared to both Au-ZnO and Au@Au-ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt-Au-ZnO, Au@Au-ZnO, and Au-ZnO were 84, 66, and 39%, respectively. In the interest of effective metal-semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired.

The self-assembling behavior and microscopic structure of zinc oxide nanoparticle Langmuir-Blodgett monolayer films were investigated for the case of zinc oxide nanoparticles coated with a hydrophobic layer of dodecanethiol. Evolution of nanoparticle film structure as a function of surface pressure (π) at the air-water interface was monitored in situ using Brewster's angle microscopy, where it was determined that π = 16 mN/m produced near-defect-free monolayer films. Transmission electron micrographs of drop-cast and Langmuir-Schaefer deposited films of the dodecanethiol-coated zinc oxide nanoparticles revealed that the nanoparticle preparation method yielded a microscopic structure that consisted of one-dimensional rodlike assemblies of nanoparticles with typical dimensions of 25 × 400 nm, encased in the organic dodecanethiol layer. These nanoparticle-containing rodlike micelles were aligned into ordered arrangements of parallel rods using the Langmuir-Blodgett technique.

This work reports the effect of seed nanoparticle size and concentration effects on heterogeneous crystal nucleation and growth in colloidal suspensions. We examined these effects in the Au nanoparticle-seeded growth of Au-ZnO hetero-nanocrystals under synthesis conditions that generate hexagonal, cone-shaped ZnO nanocrystals. It was observed that small (~ 4 nm) Au seed nanoparticles form one-to-one Au-ZnO hetero dimers and that Au nanoparticle seeds of this size can also act as crystallization 'catalysts' that readily promote the nucleation and growth of ZnO nanocrystals. Larger seed nanoparticles (~9 nm, ~ 11 nm) provided multiple, stable ZnOnucleation sites, generating multi-crystalline hetero trimers, tetramers and oligomers. ABSTRACTThis work reports the effect of seed nanoparticle size and concentration effects on heterogeneous crystal nucleation and growth in colloidal suspensions. We examined these effects in the Au nanoparticle-seeded growth of Au-ZnO hetero-nanocrystals under synthesis conditions that generate hexagonal, cone-shaped ZnO nanocrystals. It was observed that small (~ 4 nm) Au seed nanoparticles form one-to-one Au-ZnO hetero dimers and that Au nanoparticle seeds of this size can also act as crystallization 'catalysts' that readily promote the nucleation and growth of ZnO nanocrystals. Larger seed nanoparticles (~9 nm, ~ 11 nm) provided multiple, stable ZnOnucleation sites, generating multi-crystalline hetero trimers, tetramers and oligomers.

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) > (101̅1) ≈ (0001̅) > (101̅0). 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.