Optical Trapping of Nanocrystals at Oil/Water Interfaces: Implications for Photocatalysis

Optical trapping of inorganic nanocrystals at oil/water interfaces was investigated under a loose focus condition. The target nanocrystals were octahedral gold nanoparticles (OGPs, 70 nm in size), rice-shaped (10 nm × 20 nm) ZnS-AgInS₂ (ZAIS) nanoparticles, and octahedral titania (TiO₂) nanoparticles (OTPs, 100 nm). While we were unable to trap these nanoparticles in homogeneous liquids (n-hexane or water), they were successfully trapped at an oil/water interface. We also investigated trapping for a binary system at an interface, where OGPs and ZAIS nanoparticles were dispersed in water (lower layer) and n-hexane (upper layer), respectively. We observed the formation of a complex (hybrid) of an OGP and ZAIS particle trapped at the oil/water interface. Interestingly, the complex particle kept rotating during trapping even under irradiation of linearly polarized light. This phenomenon suggested the spontaneous breakdown of rotational symmetry in the optically coupled nanoparticles. This interfacial trapping technique was applied to a photocatalytic reaction. We observed the generation of Au nanoparticles from OTPs trapped at an interface under UV light irradiation. Liquid/liquid interfaces can strongly assist optical trapping and induce characteristic chemical behavior.