Electrochemical Solid-State Phase Transformations of Silver Nanoparticles

Adenosine triphosphate (ATP)-capped silver nanoparticles (ATP–Ag NPs) were synthesized by reduction of AgNO₃ with borohydride in water with ATP as a capping ligand. The NPs obtained were characterized using transmission electron microscopy (TEM), UV–vis absorption spectroscopy, X-ray diffraction, and energy-dispersive X-ray analysis. A typical preparation produced ATP–Ag NPs with diameters of 4.5 ± 1.1 nm containing ∼2800 Ag atoms and capped with 250 ATP capping ligands. The negatively charged ATP caps allow NP incorporation into layer-by-layer (LbL) films with poly­(diallyldimethylammonium) chloride at thiol-modified Au electrode surfaces. Cyclic voltammetry in a single-layer LbL film of NPs showed a chemically reversible oxidation of Ag NPs to silver halide NPs in aqueous halide solutions and to Ag₂O NPs in aqueous hydroxide solutions. TEM confirmed that this takes place via a redox-driven solid-state phase transformation. The charge for these nontopotactic phase transformations corresponded to a one-electron redox process per Ag atom in the NP, indicating complete oxidation and reduction of all Ag atoms in each NP during the electrochemical phase transformation.