Analysis of X-ray Absorption Spectra of Some Nickel Oxycompounds Using Theoretical Standards

Mansour, A. N.; Melendres, C. A.

doi:10.1021/jp9619853

Cited by 111 publications

(96 citation statements)

References 54 publications

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“…3b), which show a clear energy shift of the edge of +2.5(3) eV and some subtle differences at higher energy [18]. The sign of the energy shift is fully consistent with an electron-rich Ni1 site [19,20] and is uniquely determined from the data. In addition, the absolute scaling of the peak intensities ensures an accurate and reliable determination of the magnitude of the edge shift.…”

Section: ′′supporting

confidence: 58%

Direct Observation of Charge Order in Triangular MetallicAgNiO2by Single-Crystal Resonant X-Ray Scattering

et al. 2011

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We report resonant X-ray scattering measurements on a single crystal of the orbitally-degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below TS = 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K-edge. The empirically extracted K-edge shift between the crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ∼ 1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide bandstructure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.PACS numbers: 75.25. Dk, 78.70.Ck, 71.45.Lr Since the discovery of the Verwey transition in 1939 [1], electronic ordering in oxides has been extensively studied, as more evidence progressively emerged of its importance in underpinning phenomena such as metalinsulator transitions, colossal magnetoresistance and possibly high-temperature superconductivity. Orbital ordering (OO) in particular, whereby orbital degeneracy is lifted either by a spontaneous lattice distortion driven by the (cooperative) Jahn-Teller (JT) effect [2], or by similar orbital physics [3], has become a cornerstone of our understanding of oxides. This was hitherto considered ubiquitous in both band and Mott insulators containing "JT active" ions. For this reason, the recent proposal [4] of a radically different type of electronic ordering in the weakly metallic 2H-AgNiO 2 came initially as a surprise. In this scenario, orbital degeneracy at JT-active low-spin Ni 3+ would be lifted through charge disproportionation and charge ordering (CO) rather than OO, in sharp contrast with the closely related (but insulating) nickelate NaNiO 2 [5], a rather conventional JT system. Although CO was observed before, e.g. at the simultaneous metal-insulator and magnetic ordering transition in rare-earth perovskite nickelates [6,7], elucidating the physics of 2H-AgNiO 2 is crucial in establishing a pure CO (decoupled from magnetic ordering [8]) as an alternative paradigm to the Jahn-Teller mechanism in "weakly" metallic systems close to the Mott transition [4,9], which remain metallic in the CO phase. Although the original clue for CO was from the magnetic structure below T N = 19.7 K, the primary indication was structural, and relied on an accurate neutron refinement of an oxygen breathing mode around the different Ni sites (Fig. 1b) in the √ 3a 0 × √ 3a 0 × c supercell structure at T < T S = 365 K. The pattern of charges ("honeycomb" CO -see Fig. 1a) and the amount of charge (formal valence) on each site can be estimated by analysing the Ni-O bond lengths through the so-called Bond Valence Sum (BVS) method [10], which yields Ni1 2.42+ and Ni2,3 3.07+ , see Fig. 1b), values that are fairly...

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Section: ′′supporting

confidence: 58%

Direct Observation of Charge Order in Triangular MetallicAgNiO2by Single-Crystal Resonant X-Ray Scattering

et al. 2011

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“…The peaks at 8351, 8365, and 8398 eV in β-Ni(OH) 2 /rGO were consistent with those of reported β-Ni(OH) 2 . [ 33 ] The shoulder at 8336 eV in Ni-NiO/NrGO indicates the presence of metallic Ni species. Meanwhile, the EXAFS for β-Ni(OH) 2 /rGO is also very different to those of Ni-NiO/N-rGO (Figure 3 f).…”

Section: Resultsmentioning

confidence: 99%

Metal (Ni, Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

et al. 2015

Adv Funct Materials

502

253

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5799wileyonlinelibrary.com issues associated with energy security and environmental pollution. [1][2][3][4][5] Oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are the most crucial electrochemical reactions to realize energy storage and conversion in these technologies. Although Pt-, Ir-, and Ru-based materials exhibit the highest activity for these electrochemical reactions, these precious-metal catalysts cannot be largely used for these clean energy because of their scarcity on earth and high cost. [ 6,7 ] Therefore, high-active, low-cost, and durable precious-metalfree catalysts from earth-abundant elements have been attracted considerable attention since last decade. [8][9][10][11] Among studied nonprecious metal catalysts, [ 12 ] nickel and cobalt are earth-abundant, low-cost, and environment-friendly materials that have widely been explored as electrocatalysts for the oxygen or hydrogen reactions in energy conversion and storage devices. [13][14][15] However, the pure cobalt and nickel oxides usually show insuffi cient electrical conductivity and low reactive surface areas, resulting in limited kinetics during these electrochemical reactions such as ORR, OER. [ 16 ] Oppositely, standalone metallic Ni or Co has good electrical conductivity, but is less active than Pt, because the formation energies of Ni-H or Co-H is lower than that of Pt-H for the HER. [ 17 ] Furthermore, combining graphene with metal or metal oxide is an effective way to improve catalytic activities due to the high surface area and excellent electrical conductivity of graphene, [18][19][20] thereby increasing number of active sites and promoting the charge transfer in electrodes. [21][22][23] For example, Co 3 O 4 /graphene, [ 24 ] Ni/graphene fi lm, [ 25 ] and NiO/rGO [ 26 ] composite catalysts have been explored showing enhanced catalytic activity, relative to single metals or metal oxides. Based on previous studies on the nickel or cobalt electrocatalysts, in this work, we synthesized a new family catalyst including Co-CoO/N-rGO and Ni-NiO/N-rGO via a pyrolysis of graphene oxide-supported cobalt and nickel salts, respectively. The possible synergetic effect among transition metals, metal oxides, and graphene was systematically studied, making them simultaneously highly active for the OER, ORR, or HER. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional ElectrocatalystsXien Liu , Wen Liu , Minseong Ko , Minjoon Park , Min Gyu Kim , Pilgun Oh , Sujong Chae , Suhyeon Park , Anix Casimir , Gang Wu , * and

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“…A linear background was first fitted to the pre-edge region and subtracted before the XANES spectra were normalized to one at energy 8360 eV. The XANES spectrum of sample 1 resembled that of the Ni foil but showed a maximum typical for oxides indicating that nickel was in metallic and oxidized forms (Mansour & Melendres, 1998). Spectra of samples 2-4 showed smoother features, which indicated that Ni was in weakly ordered nanoparticles.…”

Section: Resultsmentioning

confidence: 99%

Structure of nickel nanoparticles in a microcrystalline cellulose matrix studied using anomalous small-angle X-ray scattering

et al. 2007

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Nickel nanoparticles were synthesized by adding aqueous nickel salt into a microcrystalline cellulose matrix. The NiII ions were reduced with either sodium borohydride, NaBH, or potassium hypophosphite, KHPO, in water or aqueous NH medium. The mass fraction of Ni in the samples was between 3.7 and 8.9%. X‐ray absorption spectra at the Ni K‐edge showed that Ni was partially oxidized only in a sample reduced with NaBH. Wide‐angle X‐ray scattering results showed that nickel was in nanocrystalline or amorphous form in the samples. Upon heating fcc Ni, hcp Ni, NiO, NiP and other Ni–P phases formed depending on the reduction parameters. Using anomalous small‐angle X‐ray scattering the nanometre‐scale particle size distributions of the Ni particles were determined. A large fraction of particles less than 15 nm in size were observed in the samples reduced in aqueous ammonium compared with the samples reduced in water. Particles reduced in aqueous ammonium had a large ferromagnetic component.

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Analysis of X-ray Absorption Spectra of Some Nickel Oxycompounds Using Theoretical Standards

Cited by 111 publications

References 54 publications

Direct Observation of Charge Order in Triangular MetallicAgNiO2by Single-Crystal Resonant X-Ray Scattering

Direct Observation of Charge Order in Triangular MetallicAgNiO2by Single-Crystal Resonant X-Ray Scattering

Metal (Ni, Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

Structure of nickel nanoparticles in a microcrystalline cellulose matrix studied using anomalous small-angle X-ray scattering

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