Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

Valkeapää, Markus; Katsumata, Yuki; Asako, Isao; Motohashi, Teruki; Chan, Ting Shan; Liu, Ru‐Shi; Chen, J.M.; Yamauchi, H.; Karppinen, Maarit

doi:10.1016/j.jssc.2007.03.001

Cited by 12 publications

(8 citation statements)

References 42 publications

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“…Regardless of the applied treatment time (7 days was selected as the standard procedure for all WOCs discussed below), the degree of delithiation remained constant around Li 0.4 CoO 2 for HT and LT phases ( Table S6 in the Supporting Information), which is in line with the reported lithium content of 0.49 with bromine as oxidant and the degree of delithiation as a function of oxidation potential. 21 Both HT-and LT-Li 0.4 CoO 2 maintained their basic structural motifs after delithiation while exhibiting the expected lattice constant changes. 22 LT-Li 0.4 CoO 2 undergoes contraction along b and c, whereas HT-Li 0.4 CoO 2 displays shortened a and b axes along with an expansion along the c axis due to decreasing electrostatic attraction between the {CoO 6 } layers (Table S1 in the Supporting Information).…”

Section: ■ Results and Discussionmentioning

confidence: 93%

Correlations among Structure, Electronic Properties, and Photochemical Water Oxidation: A Case Study on Lithium Cobalt Oxides

et al. 2015

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Construction of {M 4 O 4 } motifs is an effective design paradigm for molecular polyoxometalate-and oxidebased water oxidation catalysts (WOCs). However, the mechanisms beneath this bioinspired design strategy remain a topic of intense debate. The two modifications of LiCoO 2 with spinel-type and layer structures are exceptionally versatile model systems to explore the correlations among structure, electronic properties, and photochemical water oxidation. The electronic properties of both LiCoO 2 modifications are tunable through delithiation while the basic structural frameworks are maintained. This provides a unique opportunity to assign the respective influence of structures and electronic properties on the water oxidation properties. While spinel-type LiCoO 2 with {Co 4 O 4 } cubane motifs is active for photochemical water oxidation, the layered modification without cuboidal structural elements is nearly inactive. Here, we demonstrate that the water oxidation performance of both modifications can be significantly improved through chemical delithiation. A wide range of analytical methods were applied to investigate the transition of electronic properties upon delithiation, and a direct correlation between enhanced hole mobility and improved water oxidation activity was established. The difference in water oxidation activities between the two structural modifications was further linked to the role of {Co 4 O 4 } cubane motifs in constructing 3D Co−O−Co networks with expanded hole transfer paths. Thus, the promoting effects of both delithiation and {Co 4 O 4 } cubane motifs on water oxidation can be consistently explained by enhanced hole mobility.Conversion of solar energy into chemical fuels by artificial photosynthesis could provide a sustainable and flexible solution to current energy and climate problems. 1 To date, one of the most demanding steps toward solar water splitting remains the development of efficient and economic water oxidation catalysts (WOCs). 2 Even after decades of synthetic and mechanistic studies, the complex multistep processes of water oxidation with heterogeneous catalysts are not completely understood. 3 Mechanistic insight requires highly sophisticated in situ techniques which are currently being developed. 4 In a search for general WOC design concepts, cuboidal {M 4 O 4 } motifs (M = Co, Mn) inspired by the {CaMn 4 O 5 } oxygen evolving cluster (OEC) in photosystem II 5 have been established as an effective design paradigm 6 for molecular-, 7 oxide-, 8 and polyoxometalate-based 9 WOCs. Nevertheless, the precise impact of {M 4 O 4 } cubane motifs on the oxygen evolution performance remains a topic of extensive debate, 10 with the amorphous Co-Pi electrocatalysts 11 as a typical example. The absence of long-range order renders structure− activity relations of amorphous Co-Pi type WOCs rather difficult to clarify, 12 so that the precise mechanistic role of the proposed {Co 4 O 4 } building blocks remains under investigation. 13 While many water oxidation active oxides, such as Co 3 O...

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Section: ■ Results and Discussionmentioning

confidence: 93%

Correlations among Structure, Electronic Properties, and Photochemical Water Oxidation: A Case Study on Lithium Cobalt Oxides

et al. 2015

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“…The need of the current study is based on the accompanying facts and answers obtained in earlier works of cobalt electronic structure studies. From recent concise accounts of Co Ledge XAS spectra of various systems (van Elp et al, 1991;van Schooneveld et al, 2012;Miedema et al, 2011;He et al, 2013;Iablokov et al, 2012;Zheng et al, 2011;Herranz et al, 2009;Alayoglu et al, 2011;Tuxen et al, 2013;Morales et al, 2004;Lin et al, 2010;Knupfer et al, 2006;Karvonen et al, 2010;Uchimoto et al, 2001;Valkeapä ä et al, 2007;Milewska et al, 2014;Tamenori, 2013;Kikas et al, 1999;Kumagai et al, 2008;Bazin et al, 2000) it is evident that a good reference data set is often needed to compare the changes in the spectral characteristics of these compounds for different applications. For example, it has been contemplated that the Co K-edge XAS spectrum of CoO nanocrystals and bulk single crystals showed a spectrum sensitive to an octahedral field (van Elp et al, 1991).…”

Section: Introductionmentioning

confidence: 99%

“…To understand this, a model standard data set is required in order to acquire data necessary to understand spectral characteristics in correlation with the site symmetry of the material of interest. Similarly, these reference data sets can be helpful further to better understand the results obtained from changes in the oxidation state of the cobalt atom in a metallo-organic complex in response to oxygen binding (van Schooneveld et al, 2012;Miedema et al, 2011), catalytic action of cobalt metallic and bimetallic nanoparticles (He et al, 2013;Iablokov et al, 2012;Herranz et al, 2009;Alayoglu et al, 2011), hightemperature catalysis during the Fischer-Tropsch reaction (Tuxen et al, 2013) and electrochemical lithiation and delithiation of cobalt-based layered alkali compounds (Morales et al, 2004;Lin et al, 2010;Knupfer et al, 2006;Karvonen et al, 2010;Uchimoto et al, 2001;Valkeapä ä et al, 2007;Milewska et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Influence of crystal structure, ligand environment and morphology on CoL-edge XAS spectral characteristics in cobalt compounds

et al. 2015

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The electronic structure of a material plays an important role in its functionality for different applications which can be probed using synchrotron-based spectroscopy techniques. Here, various cobalt-based compounds, differing in crystal structure, ligands surrounding the central metal ion and morphology, have been studied by soft X-ray absorption spectroscopy (XAS) at the Co L-edge in order to measure the effect of these parameters on the electronic structure. A careful qualitative analysis of the spectral branching ratio and relative intensities of the L3 and L2 peaks provide useful insight into the electronic properties of compounds such as CoO/Co(OH)2, CoCl2.6H2O/CoF2.4H2O, CoCl2/CoF2, Co3O4 (bulk/nano/micro). For further detailed analysis of the XAS spectra, quantitative analysis has been performed by fitting the spectral profile with simulated spectra for a number of cobalt compounds using crystal field atomic multiplet calculations.

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“…27,28 Therefore, peak A 1 in the CoO 2 spectrum stems from the transitions to hybridized O 2p and Co 4+ a 1g states, while no such peak is observed in CoO spectrum. The peak A 2 in the CoO spectrum is characteristic of a Co 3+ oxidation state due to transitions to hybridized O 2p-Co 3+ e g states, 29 which indicates the presence of Co 3+ ions in CoO layers. Finally, the first peak of the main O K-edge ͑peak B͒ has been shown to originate from transitions to hybridized O 2p-Ca 4sp orbitals, 30,31 while peak C has been attributed to the transitions to the hybridized O 2p-Co 4sp band.…”

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confidence: 99%

Direct measurement of charge transfer in thermoelectricCa3Co4O9

2008

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The misfit-layered cobalt oxide Ca 3 Co 4 O 9 exhibits outstanding physical properties including high thermoelectric power, low thermal conductivity, low resistivity, and high thermal stability. We utilize atomicresolution Z-contrast imaging in conjunction with electron energy-loss spectroscopy ͑EELS͒ in an aberrationcorrected scanning transmission electron microscope ͑STEM͒ to characterize the local atomic and electronic structure of Ca 3 Co 4 O 9 . We will show that the position of the O atoms in the CoO 2 layers can be directly imaged, and that the CoO columns in the rocksalt layer exhibit a strong modulation in the ͑010͒ direction. Further, we measure the local Co valence and find significant hole transfer from the rocksalt CoO to the hexagonal CoO 2 layers. Our results are confirmed by self-consistent multiple-scattering calculations and we conclude that this hole transfer increases the mobile hole concentration and breaks the electron-hole symmetry in the CoO 2 layers, thereby enabling the high thermoelectric power in the strongly correlated CoO 2 subsystem.

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Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

Cited by 12 publications

References 42 publications

Correlations among Structure, Electronic Properties, and Photochemical Water Oxidation: A Case Study on Lithium Cobalt Oxides

Correlations among Structure, Electronic Properties, and Photochemical Water Oxidation: A Case Study on Lithium Cobalt Oxides

Influence of crystal structure, ligand environment and morphology on CoL-edge XAS spectral characteristics in cobalt compounds

Direct measurement of charge transfer in thermoelectricCa3Co4O9

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