Gold Nanoparticle Formation from Photochemical Reduction of Au3+ by Continuous Excitation in Colloidal Solutions. A Proposed Molecular Mechanism

Abstract

A photochemical reduction of Au³⁺ with continuous 250−400 nm excitation is studied in ethylene glycol, and poly(vinylpyrrolidone) (PVP) is used as a capping material. After the absorption of Au³⁺ disappears, excitation is stopped. The surface plasmon absorption of gold as well as the thermal reappearance of the Au³⁺ absorption are found to increase as a function of time. The rates of these changes are studied as a function of the mole fraction of ethylene glycol in water. Experimental results show that a small amount of ethylene glycol increases the formation of gold nanoparticles and decreases the reformation of the Au³⁺ absorption after irradiation. Increasing the glycol concentration first increases the rate of formation of gold nanoparticles to a maximum at a mole fraction 0.40. As the glycol concentration is further increased, the rate of formation of the gold nanoparticles and the rate of re-formation of Au³⁺ decrease. A mechanism is proposed that involves the reduction of the excited Au³⁺ to Au²⁺ by ethylene glycol. This is followed by the disproportionation of Au²⁺ to Au³⁺ and Au¹⁺. Both the reduction of Au¹⁺ by ethylene glycol and its disproportionation lead to the formation of Au⁰, which upon nucleation and growth form Au nanoparticles.