Surface composition and electronic properties of Co-Cu mixed oxalates: A detailed XPS analysis

[Display omitted] •First comprehensive XPS study of binary oxalates CoxCu1-xC2O4·nH2O (x = 0.33, 0.5 and 0.75)•Co0.75Cu0.25C2O4 has a Co oxalate based solid solution structure.•Co0.75Cu0.25C2O4 has the highest degrees of covalency of Co-O and ionicity of Cu–O bonds.•The valence band of Co0.33Cu0.67 oxalate exhibits an enhanced density of Co 3d states.•X-ray exposure of CoCu oxalate changes the Cu1+ content and the Me–O bonding strength. X-ray photoelectron spectroscopy and X-ray diffraction were employed to characterize the structure, surface composition, atoms’ chemical state and electronic structure of bimetallic Co-Cu oxalates with a nominal composition of Co0.75Cu0.25C2O4, Co0.5Cu0.5C2O4 and Co0.33Cu0.67C2O4. High-resolution core-level spectra of the components, X-ray induced Cu L3VV Auger spectra and valence band spectra are presented for these oxalates along with a detailed analysis of a number of spectral parameters in comparison with those for monometallic Co and Cu oxalates. A consistent variation of different spectral parameters is considered in terms of changes in the strength of Me–O bonding. Due to charge transfer processes, Co-rich Co0.75Cu0.25C2O4 (Co/Cu = 3) oxalate displays the highest degree of covalency of Co–O bonds and ionicity of Cu–O bonds compared to CoC2O4, CuC2O4 and the other Co-Cu oxalates. The role of Cu1+ ions either initially present in Cu-containing oxalates or progressively accumulated during their X-ray irradiation is discussed in terms of strengthening/weakening of interatomic Me–O bonds.