(476e) Role of Poly(N-vinylpyrrolidone) and Its Influence on Ag+ Reduction Kinetics during Formation of Ag Nanostructures

Poly(N-vinylpyrrolidone) (PVP) is ubiquitously used in shape-controlled polyol synthesis of metal nanoparticles (NPs). Among the various systems using PVP, polyol synthesis of Ag nanocubes (NCs) has emerged as one of the most robust systems, where PVP is considered as the structure-directing agent, and glycolaldehyde (GA), an oxidation product of ethylene glycol (EG) as the reducing agent for Ag⁺ at elevated temperature. While several reports indicate molecular weight (M_w) and monomer concentration (C_m) of PVP impact the final shape of NPs, a universal agreement on the role of PVP is thus far lacking. Recent experimental studies from our group indicate the differential heat of adsorption of PVP to (100) versus (111) facets is too small to predict a cube over an octahedron by thermodynamic considerations. We further demonstrate chloride (Cl^¯) added as HCl is the structure-directing agent in the synthesis with about half monolayer of Cl present on the Ag NCs. However, it has been observed PVP can impact the final shape of Ag NPs as its C_m or M_w is changed, suggesting PVP may influence the rate of reduction leading to kinetically-preferred shapes even in the presence of Cl^¯. We demonstrate PVP acts both as the dominant reducing agent, where the end groups are responsible for influencing Ag⁺ reduction kinetics, and the pyrrolidone ring is unique for stabilizing the Ag NPs. We constructed an experimental phase diagram for the formation of Ag NCs by varying the PVP C_m and M_w at constant temperature and Cl^¯ concentration. Between the upper boundary (related to the role of PVP in Ag⁺ reduction) and the lower boundary (related to a required minimum amount of PVP for stabilization) of the phase diagram, any combination of PVP C_m and M_w, can yield uniform Ag NCs. Ag nanowires formed above the high boundary were found to lack the Cl^¯ monolayer necessary for shape control in cubes. Optical studies of Ag⁺ reduction at different PVP C_m and M_w show direct dependence of Ag⁺ reduction on the molar ratio of PVP/Ag and a decreased rate with higher M_w at constant C_m. These combined results suggest PVP rather than EG or GA plays a dominant role in the reduction of Ag⁺. Most of the GA (boiling point lower than the synthesis temperature) formed in situ from EG oxidation, is lost during the initial 20 h of Ag NC synthesis. Colorimetric test with 2,4-DNPH confirmed concentration of GA is much lower than the total Ag⁺ present and the concentration varies very slightly over the period of 20 hours. Although, the PVP end-group/Ag⁺ ratio is also well below stoichiometry, we suspect Ag⁺ reduces auto-catalytically after the initial reduction by PVP end-groups. In order to further investigate the complex mechanism of Ag⁺ reduction by PVP, we synthesized end-groups specific PVP polymers with a precise control over the average M_w. Experimental results indicate end groups are indeed responsible for influencing Ag⁺ reduction kinetics; PVP with –OH end groups generate kinetically-preferred Ag NPs (nanorods) lacking adsorbed Cl^- while those with –CHO end groups form Ag NCs covered with a half monolayer Cl^-. This work provides key insights into understanding the role of PVP in influencing reduction kinetics of Ag⁺, and thereby the shape of Ag NPs during polyol synthesis.