Emergence and Cooperativity during Anisotropic Nanostructure Synthesis

With the support of the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Rioux of the Pennsylvania State University will investigate the role of emergence and cooperativity in the synthesis of anisotropic colloidal nanostructures. Utilizing the synthesis of Ag nanocubes (NCs) by the polyol method as a prototypical example of a complex nanostructure synthesis, Professor Rioux and their group will demonstrate the impact of polymer structure and halide acid identity on the emergence of shape control, which requires cooperativity between solid Ag halide dissolution and the reduction of Ag ions to form Ag NC structures. The team will demonstrate the ubiquitous nature of emergent phenomena and cooperative behavior of reducing agents and halide ions utilizing the synthesis of small Au nanorods (NRs). In this colloidal synthesis, cooperative behavior between constituents dictates nucleation and growth events, in a manner similar to that proposed for Ag NC synthesis. The research aims to advance the understanding of the complex reaction networks associated with colloidal nanostructure formation. The research will provide the information needed to synthesize nanostructures with specific properties to enable precise function. A postdoctoral scholar and undergraduate students will be trained in colloidal synthesis with a specific emphasis on mechanistic evaluation of anisotropic growth. Trainees will present their results via publications and presentations to the scientific community. The postdoctoral scholar will participate in a pilot program with the Materials Characterization Laboratory at Penn. State to develop rigor and reproducibility standards in nanochemistry. The size and shape of nanostructures impact their specific function, but apriori control of such properties is challenging due to a lack of understanding the governing physico-chemical interactions within complex reaction networks leading to shape control. Ag NCs or other shapes will be synthesized utilizing different lactam polymers and hydrohalic acids (HX, X = Cl, Br, and I). Changes in the lactam ring size and alkyl amide polymers will be utilized to examine the influence of structure and polymer-solvent interactions on the rate of Ag ion autocatalytic reduction. Kinetic-thermodynamic coupling between AgX dissolution, where the identity of X controls solubility, and autocatalytic Ag ion reduction by lactam polymer ends is required for shape control. Deviation from balanced dissolution-reduction leads to low selectivity to cubes. Slow dissolution of AgX or lactam polymer structure promoting fast Ag+ reduction leads to rapid Ag+ reduction leading to the formation of Ag structures with large aspect ratios. They will employ a new synthetic approach, called ‘perturbative synthesis’ to probe the mechanism of nanostructure growth through the introduction of new reagent(s) at varying extents of reaction. This approach will enable the ranking of the kinetics of nucleation to growth events at different reaction conditions. Professor Rioux and their team's research aims to develop a complete mechanistic understanding of Ag NC and Au NRs syntheses to propose a unified mechanism inclusive of emergent and cooperative features. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.