Metallic Cobalt to Spinel Co₃O₄Electronic Structure Evolution by Near-Ambient Pressure Photoelectron Spectroscopy

In the present study, valence band (VB) and core level photoelectron spectroscopy was carried out to investigate the electronic structural changes from polycrystalline Co to spinel Co₃O₄, via CoO at near ambient pressures (NAP; ∼0.1 mbar). O₂–Co and H₂–CoO_<i>x</i> gas–solid oxidative and reductive interactions, respectively, have been explored with UV photons (He–I) or low kinetic energy electrons (≤16 eV) under NAP conditions. Typical VB features of Co metal, CoO, Co₃O₄, and a mixed phase between any two adjacent features were observed and well corroborated with core level changes. Very significant and characteristic changes were observed with Co 3d features in the VB for each stage from Co oxidation to Co₃O₄ as well as Co₃O₄ reduction to CoO. Co₃O₄ and CoO can be reversibly obtained by alternating the conditions between 0.1 mbar of H₂ at 650 K and 0.1 mbar of O₂ at 400 K, respectively. A meaningful correlation is observed between the changes in work function with cation oxidation state; small changes in the stoichiometry can strongly influence the shift in Fermi level and changes in work function under NAP conditions. Reversible work function changes are observed between oxidation and reduction conditions. While the O 2p derived feature for CoO_<i>x</i> was observed at a constant BE (∼5 eV) throughout the redox conditions, the Co 3d band and molecular oxygen or hydrogen vibration feature shifts significantly underscoring the physicochemical changes, such as charge transfer energy and hence changes in satellite intensity. The peak close to <i>E</i>_F originated from the 3d⁶<u>L</u> final state of the octahedral Co³⁺ 3d band of Co₃O<sub>4