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| 2. |
- Conradson, Steven D., et al.
(författare)
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Closure of the Mott gap and formation of a superthermal metal in the Frohlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x
- 2017
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 96:12
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Tidskriftsartikel (refereegranskat)abstract
- Mixed valence O-doped UO2+x. and photoexcited UO2 containing transitory U3+ and U5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Frohlich-type, nonequilibrium, phononcoupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO2, U4O9, and U3O7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f states of the valence band by similar to 0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2-2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO2 therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5-10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived (> 3-10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating UO2 excited state. This heterogeneous, nonequilibrium, Frohlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.
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| 3. |
- Grespi, A., et al.
(författare)
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Probing the electrode-liquid interface using operando total-reflection X-ray absorption spectroscopy
- 2024
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Ingår i: Surface Science. - : Elsevier B.V.. - 0039-6028 .- 1879-2758. ; 748
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Tidskriftsartikel (refereegranskat)abstract
- Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H2SO4 was observed, showing a progression from two-dimensional Au1+/3+ to three-dimensional thick Au3+ surface oxide/hydroxide during OER.
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| 4. |
- Wang, Yang, et al.
(författare)
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A Tunable Multivariate Metal-Organic Framework as a Platform for Designing Photocatalysts
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:17, s. 6333-6338
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Tidskriftsartikel (refereegranskat)abstract
- Catalysts for photochemical reactions underlie many foundations in our lives, from natural light harvesting to modern energy storage and conversion, including processes such as water photolysis by TiO2. Recently, metal–organic frameworks (MOFs) have attracted large interest within the chemical research community, as their structural variety and tunability yield advantages in designing photocatalysts to address energy and environmental challenges. Here, we report a series of novel multivariate metal–organic frameworks (MTV-MOFs), denoted as MTV-MIL-100. They are constructed by linking aromatic carboxylates and AB2OX3 bimetallic clusters, which have ordered atomic arrangements. Synthesized through a solvent-assisted approach, these ordered and multivariate metal clusters offer an opportunity to enhance and fine-tune the electronic structures of the crystalline materials. Moreover, mass transport is improved by taking advantage of the high porosity of the MOF structure. Combining these key advantages, MTV-MIL-100(Ti,Co) exhibits a high photoactivity with a turnover frequency of 113.7 molH2 gcat.–1 min–1, a quantum efficiency of 4.25%, and a space time yield of 4.96 × 10–5 in the photocatalytic hydrolysis of ammonia borane. Bridging the fields of perovskites and MOFs, this work provides a novel platform for the design of highly active photocatalysts.
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