Citrate-Free Synthesis of Silver Nanoplates and the Mechanistic Study

We report a citrate-free synthesis of Ag nanoplates with an edge length of 50 nm that involved the reduction of AgNO3 by poly(vinyl pyrrolidone) (PVP) in ethanol at 80 °C under a solvothermal condition. Within a period of 4 h, greater than 99% of the initially added AgNO3 could be converted into Ag nanoplates with excellent stability. To understand this remarkably simple and efficient process, we systematically investigated the roles played by various reaction parameters, which include the type of precursor, reducing powers of PVP and ethanol, molar ratio of PVP to AgNO3, solvent, involvement of O2, and effects of pressure and temperature. Our results suggest a plausible mechanism that involves (i) fast reduction of AgNO3 to generate Ag multiple twinned particles (MTPs) via a thermodynamically controlled process, (ii) kinetically controlled formation of plate-like seeds and their further growth into small nanoplates in the presence of Ag+ ions at a low concentration, and (iii) complete transfer of Ag atoms from the MPTs to nanoplates via O2-mediated Ostwald ripening. We demonstrated that the molar ratio of PVP to AgNO3 in ethanol plays an essential role in controlling the reduction rate for the formation of MTPs and plate-like seeds under the solvothermal condition, transformation kinetics, and final morphology taken by the Ag nanoplates. In particular, when the reaction temperatures were above the boiling point of ethanol, the pressure induced by a solvothermal process accelerated the oxidative etching of Ag MTPs to facilitate their complete conversion into nanoplates. The mechanistic insight could serve as a guideline to optimize the experimental parameters of a solvothermal synthesis to control the reduction kinetics and thus the formation of metallic nanocrystals with controlled shapes and in high yields and large quantities.