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 synth esis of nonaqueous colloidal gold particles in a highly polar and viscous m edium, formamide in the presence of poly(vinylpyrrolidone). The effective p hotoreduction of AuCl4- in the nonaqueous system is attributed to the highe r 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 dime thylformamide because AuCl4- cannot be dissociated in dimethylformamide, ha ving no amino group but an aldehyde group as in formamide. Under similar co nditions, uniform gold particles were prepared much more easily in formamid e than in water, and the photoreduction of AuCl4- was also carried out much more completely in formamide than in water. A Coulomb staircase was clearl y observed at room temperature on the nonaqueously grown uniform gold parti cles 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 effect ive to produce other uniform transition-metal particles such as silver, pal ladium, and platinum.