Effect of the partial replacement of Ni or Co by Cu on the electrocatalytic activity of the NiCo2O4 spinel oxide

Analysis of the NiCo2O4 spinel surface with Auger and X-ray photoelectron spectroscopy

“…The nickel 2p 3/2 and 2p 1/2 binding energies, at 854.9 and 872.9 eV, respectively, are also comparable to the octahedral Ni 2+ species found in NiO [20] and are in good agreement with the higher values reported in the literature for NiCo 2 O 4 [11,13,14]. Samples produced by thermal decomposition and by sol-gel methods yield comparable spectra.…”

“…However, the nickel 2p spectrum does not change signifi cantly during the surface reduction (Figure 9b), although the spectral features broaden slightly, with some of the broadening occurring to the higher binding energy side of the 2p 3/2 peak in the vicinity of the 856 eV peak. While the peak was previously associated with Ni(OH) 2 or NiOOH formation from ambient water adsorption during the synthesis of the material [11,13,14], this does not appear likely, at least in signifi cant amounts, under the present reaction conditions (UHV~3×10 −10 Torr at 750 K). Other possibilities include the oxidation of Ni 2+ to Ni 3+ [37], which is unusual given that the surface is being reduced under UHV but is not entirely impossible if the oxidation of nickel is compensated by an even larger number of co-balt cations reduced from Co 3+ to Co 2+ , or the migration of Ni 2+ to tetrahedral sites [38,39].…”

“…The surface composition of nickel cobaltite has also been studied, primarily by X-ray photoelectron spectroscopy (XPS) with a focus on the binding energies and peak shapes of the nickel and cobalt 2p core level photoemission features [10][11][12][13][14]. The reported results vary in part due to individual interpretations of the peak structure but potentially also due to differences in sample preparation and surface pretreatment.…”

“…Charge neutrality then dictates that the cobalt cations, presumably divided equally between tetrahedral sites and the remaining octahedral sites of the lattice, form one Co 2+ to compensate for every Ni 3+ with the remaining cobalt in the 3+ oxidation state. In other studies, the higher binding energy nickel peak was postulated to result not from an intrinsic nickel species but from surface hydroxylation and/or oxyhydroxide formation, perhaps accompanied by phase separation from the NiCo 2 O 4 remaining at the surface [11][12][13][14]. All of these studies have relied on curve-fi tting procedures to separate the overlapping, multi-peaked 2p XPS structure of the two metal oxidation states.…”

Solar absorptance of copper–cobalt oxide thin film coatings with nano-size, grain-like morphology: Optimization and synchrotron radiation XPS studies

“…3. b-d) and the quantification analysis are presented in Table 1. It is commonly recognized that the Cu 2p 3/2 photoelectron peak around of 933.3-934.0 eV with shake-up satellite is due to the CuO (or Cu 2+ ), while many researchers identified that the Cu 2p 3/2 photoelectron peak at around of 932.5-932.8 eV is from the tetrahedral Cu + with its counterpart peak from octahedral Cu + located below the tetrahedral one [15,[28][29][30][31][32][33][34][35][36]. From Table 1, the octahedral and tetrahedral Cu + as well as the octahedral and paramagnetic Cu 2+ oxidation states are detected with the tetrahedral Cu + being the most prominent.…”