| 1. |
- Baeumer, Christoph, et al.
(author)
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Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis
- 2021
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In: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 20:5, s. 674-682
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Journal article (peer-reviewed)abstract
- Structure–activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.
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| 2. |
- Duchon, Tomas, et al.
(author)
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Establishing structure-sensitivity of ceria reducibility : real-time observations of surface hydrogen interactions
- 2020
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In: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 8:11, s. 5501-5507
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Journal article (peer-reviewed)abstract
- The first Layer of atoms on an oxide cataLyst provides the first sites for adsorption of reactants and the Last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria cataLyst with different surface terminations under an H2 environment to unequivocally establish the effect of the Last Layer of atoms on surface reduction. (111) and (100) terminated epitaxiaL isLands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of isLands, more pronounced oxygen release, and overall higher reducibility were observed for (100) isLands compared to (111) ones. The findings are in agreement with coordination -Limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.
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| 3. |
- Kettner, Miroslav, et al.
(author)
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Anion-mediated electronic effects in reducible oxides : Tuning the valence band of ceria via fluorine doping
- 2019
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 151:4
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Journal article (peer-reviewed)abstract
- Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F--Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.
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