Size and Morphology Adjustment of PVP-Stabilized Silver and Gold Nanocrystals Synthesized by Hydrodynamic Assisted Self-Assembly

A novel process, i.e., ultrasound-assisted membrane reduction, was introduced and applied to synthesize Ag and Au nanocrystals. High-resolution transmission electronic microscopy and UV-vis spectra were adopted to observe and study the resulting colloids. The nearly spherical Ag and Au nanoparticles with mean diameters of 5.8 and 6.8 nm, respectively, were synthesized with the novel method. The silver nitrates/hydrogen tetrachloroaurate hydrate, sodium borohydride, and poly(N-vinyl-2-pyrrolidone) (PVP) were used as metal precursor, reductant, and stabilizer, respectively. The reverse mixing of silver nitrate and reductant resulted in larger Ag nanoparticles in the range of 5-24 nm. The addition of ethylenediaminetetraacetic acid (EDTA), which may serve as the second protecting agent together with PVP, could reduce the Au particle size to a mean value of 2.6 nm. By varying the NaBH4 injection rate in the process, a portion of the primary An crystallites could be self-assembled to form decahedral Au nanocrystals with a diameter of ca. 24 nm. The two-dimensional networked gold nanowires with mean diameters of 5-10 nm could be fabricated by regulating the acidity of the reaction system to pH 10 with aqueous ammonia.