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

► We have prepared Ag, Ni and AgNi NPs by derived seed-mediated growth method. ► The combined use of optical, structural and chemical characterization techniques allows to determine the presence of core–shell structures. ► The oxidation states of Ag and Ni at the outer layers of the NPs have been studied by XPS. ► Ag NPs are purely metallic with a fcc structure. ► Ni NPs are formed by Ni core@NiO+Ni(OH)2 shell structure. ► Ag core@NiO+Ni(OH)2 shell structure is determined for AgNi NPs, with oxidized silver atoms at the interface. The chemical structure of silver, nickel and bimetallic silver–nickel nanoparticles, i.e. Ag, Ni and AgNi NPs, with sizes ≤35nm, 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 600nm 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 375nm with respect to the SPR of Ag NPs, located at 405nm, reflecting the character of the oxidized nickel shell.