Cation distribution and vacancies in nickel cobaltite

Abstract

Samples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles.Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are 2 only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state.3

Structural Changes of Spinel MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) Electrocatalysts during the Oxygen Evolution Reaction Investigated by In Situ X-ray Absorption Spectroscopy

“…The second peak at ∼2.5 Å represents the M–M distances between two B sites (abbreviated as M B –M B ), while the third peak at ∼3.5 Å represents the M–M distances between two A sites (abbreviated as M A –M A ) or between the A and B sites (abbreviated as M A –M B ). The EXAFS FT data for the Co K-edge of all samples are consistent with previously reported FT data of the cation-substituted Co 3 O 4 . ,,− Similar trends in the shape of FTs are also observed in the same R -space at the M K-edge (M = Mn, Fe, Ni, and Zn). In fact, the FT at the Ni K-edge (Figure S7f) has a major peak at ∼2.5 Å and a weak contribution at ∼3.0 Å, whereas the FT at the Zn K-edge (Figure S7h) has only a major peak at ∼3.0 Å.…”

“…48 Similarly, the edge position at the Ni K-edge of the NiCo 2 O 4 powder sample lies between those of standard NiO and NiO 2 (average oxidation state of NiO 2 is about +3.3) (Figure S5d). It is noteworthy that the presence of a pre-edge peak at the Ni K-edge indicates that Ni ions occupy both tetrahedral and octahedral sites, 49 ruling out the possibility of Ni ions only existing in the octahedral site in the spinel structure. 50 However, for the edge position at the Zn K-edge, the ZnCo 2 O 4 powder sample exhibits the same position as the standard ZnO (Figure S5e), suggesting the oxidation state of +2.…”

Oxidative Thermal Conversion of Hydrothermal Derived Precursors toward the Mixed-Metal Cobaltite Spinel Oxides (ZnCo2O4 and NiCo2O4): In-Situ Investigation by Synchrotron-Radiation XRD and XAS Techniques

“…TRXAS spectra in Figure 5 demonstrate the transformation progress of the local structure of cobalt and substituents in each precursor and/or the obtained products. The K-edge positions of cobalt and nickel (Figure 5a,c,d) slightly shift towards higher photon energy upon heating to the elevating temperatures, suggesting a higher contribution of trivalent species, i.e., Co 3+ and Ni 3+ , in the resulting spinel oxide compounds similar to previous reports [1,[4][5][6]46]. However, due to the lack of an adequate energy calibration curve to examine nickel valency state precisely, all nickel cations in the obtained NiCo 2 O 4 will be presumed as Ni 2+ hereafter.…”

“…Cobaltite spinel oxide (CoCo 2 O 4 , or Co 3 O 4 ) has gained great interest as a material with high potential for energy storage, magnetic, and chemical catalytic applications [1][2][3]. The distribution of Co 2+ and Co 3+ ions over the tetrahedral (T d ) and octahedral (O h ) spinel interstices is a key factor to deliver remarkable material properties for specific applications [2,[4][5][6][7]. Moreover, the cobaltite spinel framework can accommodate other first-row transition metals (such as nickel, copper, and zinc) as the substituents, leading to a variety of mixed-metal spinel derivatives [8][9][10][11][12].…”

Development of Highly Efficient and Durable Three-Dimensional Octahedron NiCo2O4 Spinel Nanoparticles toward the Selective Oxidation of Styrene

“…EXAFS analyses on NiCo 2 O 4 were reported on the basis of its spinel structure. − Ideally, Ni occupies apparently in the octahedral sites, and Co occupies in both octahedral as well as tetrahedral sites. However, actual distribution of cationic sites is rather complicated depending on the sample preparation. , In the present study, according to the simple treatment in previous report, only Co backscatters are included in the Co K-edge analysis and Ni backscatters are included in the Ni K-edge analysis because of the resemblance in Co and Ni backscattering amplitudes (Figure ). Table lists the EXAFS data and structural characteristics of the spinel NiCo 2 O 4 .…”