Polyoxometalate-Assisted Galvanic Replacement Synthesis of Silver Hierarchical Dendritic Structures

A simple organic-free replacement process was realized between an aqueous silver nitrate solution and an aluminum wafer, in the presence of a polyoxometalate (POM), [PW12O40](3-) (PW12). The same protocol was used with a carbon-coated copper grid and a silicon wafer. The POM was photochemically reduced and used as both a reductant and a stabilizer of silver nanoparticles, Ag@POM, in the homogeneous phase. The presence of the POM and Ag@POM colloids was found to modulate the reaction kinetics, and the POM serves as a kind of soft template, maintaining the anisotropic conditions and directing the Ag nanoparticles in the solution to the seeds. Multiple morphologies were obtained, including nanoplates decorated at their periphery with fractal structures, dendrites, and nanospheres. The objects were studied by FESEM, TEM, XRD, XPS, and EDX. Through the variation of the silver salt to POM ratio and the initial concentrations of reactants, in addition to time-dependent observations, it has been possible to distinguish a kinetically dominated route resulting in nanoplates and the nonequilibrium and anisotropic growth yielding fractal and dendritic structures. Altogether, these observations have suggested a global growth mechanism. SERS study of Rhodhamine B on the dendrites gives particularly sharp results.