What factors control the size and shape of silver nanoparticles in the citrate ion reduction method?

Citrate ion, a commonly used reductant in metal colloid synthesis, undergoes strong surface interaction with silver nanocrystallites. The slow crystal growth observed as a result of the interaction between the silver surface and the citrate ion makes this reduction process unique compared to other chemical and radiolytic synthetic methods. Addition of citrate ions to preformed silver colloids (Ag-capped SiO2) results in the complexation of citrate with silver colloids. The difference absorption spectra of SiO2-Ag colloids in the presence of citrate ions show an increase in the absorption at 410 nm with increase in concentration of citrate. The apparent association constant as determined from these absorption changes is 220 M-1. Pulse-radiolysis studies show that citrate ions complex with the silver seeds and influence the particle growth. For example, one of the primary intermediates, Ag-2(+) produced in the radiolytic reduction of silver ions, readily interacts with citrate to form a complex absorbing in the 410-nm region. The complex, [Ag-2(+)-citrate], undergoes slower transformations compared to uncomplexed Ag-2(+). This slow transformation of the citrate complex eventually leads to the formation of larger clusters of silver. Steady-state and pulse-radiolysis experiments provide evidence for the multiple roles of citrate ions as a reductant, complexant, and stabilizer that collectively dictate the size and shape of silver nanocrystallites.