PVP is ubiquitously used in shape-controlled synthesis of Ag nanocubes (NCs). While previous studies considered PVP as the structure-directing agent, recent reports demonstrate Cl-, added as HCl, is the dominant structure-directing agent. However, PVP influences the final shape of Ag NPs beyond simple stabilization since changes in monomer concentration (Cm) or molecular weight (Mw) impact shape. This suggests the concentration and potentially the identity of end groups impact the final shape. We demonstrate PVP acts as the dominant reducing agent, where the end groups influence the rate of Ag+ reduction leading to kinetically preferred shapes even in the presence of Cl-. We constructed an experimental phase diagram for the formation of Ag NCs by varying the PVP Cm and Mw at constant [H+] and [Cl-]. Between the upper (related to the role of PVP in Ag+ reduction) and lower (related to a required minimum amount of PVP for stabilization) boundary of the phase diagram, any combination of PVP Cm and Mw can yield Ag NCs. Optical studies show direct dependence of Ag+ reduction to the molar ratio of PVP/Ag and a decreased rate with higher Mw at constant Cm. These results suggest PVP dominates Ag+ reduction and the reducing power is enhanced with increased PVP end-groups concentration. Although the end-group/Ag+ ratio is well below stoichiometric, we demonstrate PVP end-groups induce the reduction (nucleation) of Ag+, followed by an autocatalytic reduction at a rate commensurate with AgCl dissolution. The impact of PVP end-groups is further demonstrated by synthesizing Ag NCs using end-group specific PVP synthesized with precise control over the average Mw. Experimental results reveal PVP with –OH end groups generate kinetically-preferred Ag nanorods while those with –CHO end groups form thermodynamically preferred Ag NCs. Overall this work provides key insights into understanding the role of PVP in influencing Ag+ reduction kinetics and begins to shed light on why PVP is an effective polymer is shape-controlled growth.

Poly(N-vinylpyrrolidone) end-groups influence shape-control of Ag nanocubes through Ag+ reduction kinetics

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.