Intertwining Roles of Silver Ions, Surfactants, and Reducing Agents in Gold Nanorod Overgrowth: Pathway Switch between Silver Underpotential Deposition and Gold–Silver Codeposition

The past two decades have witnessed great success achieved in the geometry-controlled synthesis of metallic nanoparticles using the seed-mediated nanocrystal growth method. Detailed mechanistic understanding of the synergy among multiple key structure-directing agents in the nanocrystal growth solutions, however, has long been lagging behind the development and optimization of the synthetic protocols. Here we investigate the foreign ion- and surfactant-coguided overgrowth of single-crystalline Au nanorods as a model system to elucidate the intertwining roles of Ag⁺ foreign ions, surface-capping surfactants, and reducing agents that underpin the intriguing structural evolution of Au nanocrystals. The geometry-controlled nanorod overgrowth involves two distinct underlying pathways, Ag underpotential deposition and Au–Ag electroless codeposition, which are interswitchable upon maneuvering the interplay of the Ag⁺ ions, surfactants, and reducing agents. The pathway switch governs the geometric and compositional evolution of nanorods during their overgrowth, allowing the cylindrical Au nanorods to selectively transform into a series of anisotropic nanostructures with interesting geometric, compositional, and plasmonic characteristics. The insights gained from this work shed light on the mechanistic complexity of geometry-controlled nanocrystal growth and may guide the development of new synthetic approaches to metallic nanostructures with increasing architectural complexity, further enhancing our capabilities of fine-tuning the optical, electronic, and catalytic properties of the nanoparticles

Pressure-Induced Metathesis Reaction To Sequester Cs

We report here a pressure-driven metathesis reaction where Ag-exchanged natrolite (Ag₁₆Al₁₆Si₂₄O₈₀·16H₂O, Ag-NAT) is pressurized in an aqueous CsI solution, resulting in the exchange of Ag⁺ by Cs⁺ in the natrolite framework forming Cs₁₆Al₁₆Si₂₄O₈₀·16H₂O (Cs-NAT-I) and, above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the initial cation exchange, the precipitation of AgI occurs. Additional pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to a pollucite-related, highly dense, and water-free triclinic phase with nominal composition CsAlSi₂O₆. At ambient temperature after pressure release, the Cs remains sequestered in a now monoclinic pollucite phase at close to 40 wt % and a favorably low Cs leaching rate under back-exchange conditions. This process thus efficiently combines the pressure-driven separation of Cs and I at ambient temperature with the subsequent sequestration of Cs under moderate pressures and temperatures in its preferred waste form suitable for long-term storage at ambient conditions. The zeolite pollucite CsAlSi₂O₆·H₂O has been identified as a potential host material for nuclear waste remediation of anthropogenic ¹³⁷Cs due to its chemical and thermal stability, low leaching rate, and the large amount of Cs it can contain. The new water-free pollucite phase we characterize during our process will not display radiolysis of water during longterm storage while maintaining the Cs content and low leaching rate