Exploring surface science and restructuring in reactive atmospheres of colloidally prepared bimetallic CuNi and CuCo nanoparticles on SiO2in situ using ambient pressure X-ray photoelectron spectroscopy

Abstract

Bimetallic nanoparticles (∼11 nm diameter) of CuNi and CuCo were prepared by a new synthetic route and the 1 : 1 atomic ratio of their constituent elements confirmed using STEM-EDS at a single particle level. These nanoparticles, supported on the native oxide layer of a silicon wafer, were studied in situ in a series of reactive gas atmospheres (H₂ → CO or CO/H₂ → O₂ → H₂) using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Despite the deliberate similarity of nickel and cobalt with respect to copper, their restructuring behaviour is different. CuNi nanoparticles were found to surface segregate nickel in H₂, but copper in O₂ reversibly, while CuCo nanoparticles were found to surface segregate copper irreversibly when exposed to O₂, such that the surface remains copper rich when re-exposed to H₂. Both systems also restructure in opposition to the behaviour predicted by the surface free energies and enthalpies of oxide formation of the elements from which they are comprised – factors that are seen to control restructuring and surface segregation in many similar systems.