Dodecanethiol-protected copper/silver bimetallic nanoclusters and their surface properties.

Dodecanethiol-protected copper/silver bimetallic nanoclusters were prepared by a liquid-phase method using different copper to silver feed ratios. The morphology and size of the prepared nanoclusters were analyzed with X-ray diffraction (XRD) and transmission electron microscopy (TEM), while their spectroscopic and surface properties were characterized by infrared (IR) and X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and 13C cross-polarization magic angle spinning NMR (13C CPMAS NMR). TEM analysis indicated that all the bimetallic clusters prepared are approximately 4-6 nm in size. On the other hand, the results of XRD, XPS, and Fourier transform infrared (FTIR) spectroscopy suggested that the surfaces of the alloy nanoclusters are mostly enriched with the less noble metal copper atoms. This surface enrichment of copper may be attributed to a galvanic exchange process during preparation, and the extent of enrichment is directly related to the copper feed ratio used. Interestingly, DSC studies showed two melting transitions in some of these alloy samples, suggesting different packing behavior of the dodecanethiol chains onto the heterogeneously intercalated silver- and copper-rich surfaces.