Thermal/Photo Conversion of Silver 4-Nitrobenzoate to Nitro/Amine-Terminated Silver Nanoparticles

We have investigated the thermal and photochemical characteristics of silver 4-nitrobenzoate (Ag-4NBA) using various analytical tools, such as, X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy. The XRD pattern was composed of a series of peaks that could be indexed to (0k0) reflections of a layered structure. Upon heating the sample, structural changes took place particularly at two specific temperatures. The binding state of the carboxylate group changed from bridging to unidentate at ∼380 K. A second dramatic structural change occurred at ∼590 K, producing 4NBA-capped Ag nanoparticles. The latter transition temperature for Ag-4NBA is about 80 K higher than that for silver alkanecarboxylates, and such enhanced thermal stability of Ag-4NBA must be associated with the conjugation of the carboxylate group to the aromatic ring. On the other hand, Ag-4NBA was readily subjected to photolysis even by visible radiation. Under irradiation by a He/Ne laser at 632.8 nm, 4-NBA-capped Ag nanoparticles were produced from Ag-4NBA. Upon irradiation by an Ar⁺ laser at 514.5 nm, nitro-to-amine group conversion took place consecutively, finally producing 4-aminobenzoate-capped Ag nanoparticles. Accordingly, the Raman spectrum of Ag-4NBA taken using the 514.5-nm line as the excitation source became identical to the surface-enhanced Raman scattering (SERS) spectrum of 4-NBA adsorbed on Ag.