[Synthesis and absorption spectra properties of Au-Ag alloy nanoparticles using gallic acid as reductant].

Gold-silver alloy nanoparticles with a homogeneous size distribution and good stability were synthesized in aqueous solution by one-step reduction with gallic acid as reductant in the absence of other stabilizers for the first time. The absorption spectra of as prepared gold-silver alloy nanoparticles under different reaction temperatures and with various gold-silver mole ratios were studied. The absorption spectra exhibit only one single peak with the maximum wavelength located between the absorption peaks of Au nanoparticles and Ag nanoparticles, however, the absorption spectrum of the physical mixtures of Au nanoparticles and Ag nanoparticles exhibits two absorption peaks coming from their corresponding monometallic metal nanoparticles, suggesting primarily the formation of the alloy nanoparticles. The maximal absorption peaks of alloy nanoparticles showed gradually a blue-shift with the rising of reaction temperature companying with the increase in absorbance and the narrowing of peak shape. The maximum absorption wavelengths of alloy nanoparticles showed a red-shift with increasing Au/Ag molar ratio in a linear fashion, further indicating the formation of alloy nanoparticles. HRTEM images showed that the as-prepared alloy nanoparticles with Ag/Au molar ratio of 1:1 are close to sphere in shape, homogeneous in size distribution and without capping phenomenon. EDX analysis suggested the present of Au and Ag in individual nanoparticles, and the Au/Ag mole ratio value of a sample with a nominal element composition was found to be close to the nominal value.