XPS study of silver, nickel and bimetallic silver–nickel nanoparticles prepared by seed-mediated growth

The chemical structure of silver, nickel and bimetallic silver–nickel nanoparticles, i.e. Ag, Ni and AgNi NPs, with sizes ≤35 nm, obtained by derived seed-mediated growth method on transparent and conductive indium tin oxide (ITO) substrates, has been studied by a comparative X-ray photoelectron spectroscopy (XPS) analysis of Ag 3d, Ni 2p and O1s core levels in combination with X-ray diffraction and optical absorption spectroscopy in the visible range. XPS indicates that the surface of Ag NPs is not oxidized, while Ni NPs are clearly oxidized to nickel oxide and hydroxide. Absorptions at 384 and 600 nm in Ni optical spectrum are consistent with the presence of nickel in oxidized state; however the presence of metallic Ni 2p signal in Ni XPS spectrum indicates that a metallic nickel core is still present. In the case of bimetallic AgNi NPs, the XPS results are consistent with the presence of metallic silver core surrounded by NiO + Ni(OH)2 shell. XPS spectra also show the presence of Ag2O at the interface between the Ag metallic core and the oxidized nickel shell. XRD patterns of AgNi and Ag NPs show the typical fcc structure of metallic silver, confirming the presence of Ag metallic core in AgNi NPs. The surface plasmon resonance peak (SPR) of AgNi NPs shows a blue shift to 375 nm with respect to the SPR of Ag NPs, located at 405 nm, reflecting the character of the oxidized nickel shell.