Photochemical synthesis in formamide and room-temperature Coulomb staircase behavior of size-controlled cold nanoparticles

An effective photochemical route is reported here for size-controlled synthesis of nonaqueous colloidal gold particles in a highly polar and viscous medium, formamide in the presence of poly(vinylpyrrolidone). The effective photoreduction of AuCl4- in the nonaqueous system is attributed to the higher degree of dissociation of AuCl4- in formamide and the direct reduction by the photogenerated free radicals from the coordinated formamide molecules. Evident photochemical formation of gold particles was not achieved in dimethylformamide because AuCl4- cannot be dissociated in dimethylformamide, having no amino group but an aldehyde group as in formamide. Under similar conditions, uniform gold particles were prepared much more easily in formamide than in water, and the photoreduction of AuCl4- was also carried out much more completely in formamide than in water. A Coulomb staircase was clearly observed at room temperature on the nonaqueously grown uniform gold particles of 12 nm, but this behavior did not occur on the aqueously grown gold particles of similar size. This new photochemical route is also very effective to produce other uniform transition-metal particles such as silver, palladium, and platinum.