Polyvinylpyrrolidone (PVP) is ubiquitously used in the synthesis of colloidal transition metal nanoparticles (NPs). Several reports indicate molecular weight (Mw) and monomer concentration (Cm) of PVP impact the final shape of NPs, a general consensus on the role of PVP during the synthesis of Ag NCs using polyol method is thus far lacking. The dominant hypothesis in the literature is the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets; however, there is no unified agreement on the exact shape control mechanism in Ag NPs growth as both Ag cubes and octahedron can be synthesized in the presence of PVP. This observation suggests the role of PVP is significantly more complex than simply a structure-directing agent. We measured adsorption isotherms of PVP on different-shaped Ag NPs to determine the thermodynamics of PVP adsorption to Ag(100) and Ag(111) surfaces and found the preferential binding of PVP is not responsible for shape control in Ag nanocube synthesis. Addition of Cl- during synthesis is critical for the formation of Ag nanocubes. In sufficient [Cl-], cubes form exclusively. However, it has been observed PVP impacts the final shape as its Cm or Mw is changed suggesting PVP may influence the rate of reduction leading to kinetically-preferred shapes even in the presence of Cl-. Optical studies of Ag+ reduction at different PVP Cm and Mw shows faster reduction rate at higher Cm for the same Mw of PVP, and at lower Mw for the same Cm, suggesting the end groups of PVP rather than EG plays a dominant role in the reduction of Ag+. We constructed a constant temperature phase diagram for the formation of Ag NCs by varying the PVP Cm and Mw. Between the high and the low boundaries of the phase diagram, any combination of PVP Cm and Mw, can yield uniform Ag NCs. The lower boundary is related to nanocube stability, while the upper boundary is related to the role of PVP in Ag+ reduction. In order to determine the functional (end)group in PVP responsible for Ag+ reduction, we synthesized PVP with modifiable end-groups and controlled Mw for a systematic comparative study of Ag NC synthesis. Experiments suggest end groups are responsible for Ag+ reduction kinetics; PVP with –OH end groups are more potent reducing agent than –CHO groups. This work provides key insights into understanding the role of PVP in controlling reduction kinetics of Ag+ and thereby influencing the shape of Ag NPs during polyol synthesis.

Complex role of PVP in shape-controlled growth of Ag nanocubes: Stucture directing agent, stabilizer, reducing agent and more

2019 Spring National Meeting

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