Reduction by the End Groups of Poly(vinyl pyrrolidone): A New and Versatile Route to the Kinetically Controlled Synthesis of Ag Triangular Nanoplates

Polymers are widely used in the chemical synthesis of colloidal nanocrystals, and their roles are generally documented as steric stabilizers or capping agents. In particular, poly(vinyl pyrrolidone) (PVP) has received special attention because of its high chemical stability, nontoxicity, and excellent solubility in many polar solvents. Although the repeating unit of PVP has been extensively investigated for its coordination capability, the end groups of PVP remain largely unexplored in terms of functionality and reactivity. For commercially available PVP, their ends are terminated with the hydroxyl (–OH) group because of the involvement of water as a polymerization medium and the presence of hydrogen peroxide. Like long-chain alcohols, we suspect that such polymers can serve as a new class of reductants, whose mild reducing power is desired for kinetically controlled synthesis of metal nanocrystals. As established for a number of systems, kinetic control provides a simple and versatile route to the synthesis of metal nanocrystals with well-defined shapes. For a face-centered cubic (fcc) noble metal, the thermodynamically favorable shapes are truncated nanocubes and multiple twinned particles (MTPs). When metal atoms are generated at a sufficiently high rate, the final product will have no choice but to take the thermodynamically favored shapes. As the reduction becomes substantially slower, however, the nucleation and growth will be turned into kinetic control and the final product can take a range of shapes that deviate from the thermodynamic ones. Here we demonstrate for the first time that the reduction kinetics of AgNO3 by the hydroxyl end group of PVP can be maneuvered in at least two different ways to produce Ag triangular nanoplates in high yields. Silver nanostructures have especially been of interest because of their unique surface-plasmonic features, which have enabled their use as optical labels, active substrates for surface-enhanced Raman scattering (SERS), near-field optical probes, and contrast agents for biomedical imaging. Like many other systems, shape control has received considerable attention for silver, because in many cases it allows one to tune the properties for various applications with a greater versatility than can be achieved otherwise. Most recently, particular emphasis has been placed on triangular nanoplates, as metal nanostructures with sharp corners and edges are capable of generating maximum electromagnetic-field enhancement and thus make these nanoparticles attractive as substrates for SERS detection or other spectroscopic techniques. Although several research groups have developed diverse methods to generate triangular and circular nanoplates of Ag in a number of different solvents, the ability to control and fine-tune the shape of Ag nanostructures has been modestly successful. It still remains a grand challenge to produce Ag triangular nanoplates with controllable size in bulk quantities and with high yield via a facile and clean method. In this communication, Ag nanoplates between 50 and 350 nm in size were synthesized by heating an aqueous solution of AgNO3 and PVP in a capped vial, in which the reduction kinetics and growth were significantly altered towards the thin-plate morphology. Figure 1a–e shows scanning electron microscopy (SEM) images of samples taken at different stages from a synthesis where the molar ratio of PVP (weight-average molecular weight Mw = 29 000 g mol , calculated in terms of the repeating unit) to AgNO3 was 30. As shown in Figure S1 in the Supporting Information, the sample contained 80 % circular nanoplates 50 nm in size and 20 % spherical nanoparticles 20 nm in size at synthesis time t = 20 min. At t = 40 min (Fig. 1a), the average size of the circular nanoplates and spherical nanoparticles grew to 60 and 25 nm, respectively. The inset in Figure 1a shows an SEM image of tilted circular nanoplates, revealing their thickness as ca. 5 nm. As the reaction proceeded, the circular shape of nanoplates gradually C O M M U N IC A IO N S