Size control of gold nanocrystals in citrate reduction: the third role of citrate.

Growth kinetics and temporal size/shape evolution of gold nanocrystals by citrate reduction in boiling water were studied systematically and quantitatively. Results reveal that the size variation and overall reaction mechanism were mostly determined by the solution pH that was in turn controlled by the concentration of sodium citrate (Na3Ct) in the traditional Frens's synthesis. This conclusion was further confirmed by the reactions with variable pH but fixed concentrations of the two reactants, HAuCl4 and Na3Ct. Two substantially different reaction pathways were identified, with the switching point at pH = 6.2-6.5. The first pathway is for the low pH range and consists of three overlapping steps: nucleation, random attachment to polycrystalline nanowires, and smoothing of the nanowires via intra-particle ripening to dots. The second pathway that occurred above the pH switching point is consistent with the commonly known nucleation-growth route. Using the second pathway, we demonstrated a new synthetic route for the synthesis of nearly monodisperse gold nanocrystals in the size range from 20 to 40 nm by simply varying the solution pH with fixed concentrations of HAuCl4 and Na3Ct. The switching of the reaction pathways is likely due to the integration nature of water as a reaction medium. In the citrate reduction, the solution pH was varied by changing the initial HAuCl4/Na3Ct ratio. Consequently, when pH was higher than about 6.2, the very reactive [AuCl3(OH)]- would be converted to less reactive [AuCl2(OH)2]- and [AuCl(OH)3]-.