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]-.

The seeding approach for preparation of gold nanoflowers in which 25 nm gold nanoparticles were used as the seeds and a mixture of HAuCl4 and hydroxylamine as growth solution was investigated systematically. It is revealed that the formation and stability of the nanoflowers were affected greatly by the intraparticle ripening induced by the chlorine ions that existed in the reaction system. In this seeding approach, hydroxylamine promoted the rapid reduction of HAuCl4 and thus rapid formation of small Au particles with a diameter around 3 nm in the growth solution. The attachment of the small particles on the seed surface contributed to the growth of the nanoflowers. The branch length of the nanoflowers increased with the increased pH of the growth solution due to the suppressed ripening at higher pH. The stability of the nanoflowers can be improved by increasing the pH of the storing solution and/or removal of the chlorine ions.

Triangular silver nanoprisms were synthesized by stepwise reduction of silver nitrate with sodium borohydride (NaBH4) and trisodium citrate. In this approach, first small spherical silver nanoparticles were prepared by the rapid reduction of the precursor with NaBH4 at ice-bath temperature. After being heated to 70 °C, further reduction of the precursor contributed to the formation of additional small spherical silver nanoparticles attributed to the catalysis effect of the silver particles formed at the low temperature. The residual precursor after the formation of the small spherical silver nanoparticles is necessary to promote the dissolution of the small nanoparticles and their transformation into the triangular nanoprisms. After consumption of most of the precursor, the nanoprisms became more uniform in shape and size driven by the Ostwald ripening. The triangular nanoprisms prepared by such an approach are expected to be potentially useful in biolabeling since the citrate ligand on the nanoprism surface is ready to be replaced by biomolecules.