Bimetallic Nickel–Cobalt Nanosized Layers Supported on Polar ZnO Surfaces: Metal–Support Interaction and Alloy Effects Studied by Synchrotron Radiation X-ray Photoelectron Spectroscopy

Abstract

The interaction of ultrathin bimetallic Ni–Co layers (0.25 and 1.5 nm) supported on polar (0001)­Zn–ZnO and (0001̅)­O–ZnO substrates was investigated by synchrotron-based photoelectron spectroscopy (PES) under ultrahigh vacuum (UHV) and O₂ environments. Monometallic Ni and Co layers were also characterized to highlight the influence of Ni–Co synergetic effects on the metal–support interaction. At room temperature, cobalt is partially oxidized, while nickel is metallic. The effect of ZnO surface termination is minor, while the influence of surface hydroxyl groups is discussed. Annealing at 773 K in UHV promotes oxidation of monometallic Ni and Co layers but has little influence on bimetallic Ni–Co. In addition, significant agglomeration of the Ni–Co overlayer is observed, with a parallel increase in the surface Co concentration. Agglomeration of Ni–Co is more pronounced on O-terminated ZnO. Upon annealing in 1 × 10^–6 mbar of O₂, both Ni and Co readily oxidize and redisperse over the ZnO substrate. Moreover, cobalt tends to segregate over nickel, creating a concentration gradient between the two alloy constituents (probably a core–shell-like structure). Overall, our results indicate that the interaction at the Ni–Co/ZnO interface is influenced by the synergetic effects between the two metals and to a lesser extent by the substrate termination. Taking into account the substantial progress made in the synthesis of ZnO nanostructures and surfaces, this study can assist in the effort toward improved ZnO-based catalysts with tailored properties