Nanogold Foundry Involving High-Shear-Mediated Photocontact Electrification in Water

Controlling the size and morphology of gold nanoparticles occurs in the absence of added reducing agents or other excipients such as surfactants, on UV irradiation (λ 254 nm) of aqueous auric acid (H[AuCl₄]) in a thin film of liquid in a vortex fluidic device (VFD) within a rapidly rotating tilted quartz tube. This involves contact electrification (CE), which occurs at the solid-liquid interface with the oxidation of water photoinduced, forming the hydroxyl radical, OH^•. In air, the redox couple is reduction of ³O₂ to the superoxide radical anion, O₂ ^-•, which then reduces Au³⁺ to elemental gold, as does other reactive oxygen species present, competing with CE reduction of Au³⁺. The resulting nanogold structures effectively mold the different high-shear topological fluid flows in the VFD, being isolated as ultrathin 2D sheets, prisms, hierarchical structures comprising nanoparticles embedded within these sheets, and rosette and tubular structures, depending on the VFD processing parameters and the concentration of auric acid. Processing under a nitrogen atmosphere while UV irradiated affords mainly 2D gold through the above reduction of Au³⁺. The findings establish a paradigm for VFD processing in water involving photo-CE, generating hydrogen peroxide and hydrogen gas with the surfaces of the gold nanoparticles pristine.