| 1. |
- Bouri, Maria, et al.
(author)
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Surface Chemistry of Perovskite Oxynitride Photocatalysts: A Computational Perspective
- 2021
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In: Chimia. - : Swiss Chemical Society. - 0009-4293 .- 2673-2424. ; 75:3, s. 202-207
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Journal article (peer-reviewed)abstract
- Perovskite oxynitrides are an established class of photocatalyst materials for water splitting. Previous computational studies have primarily focused on their bulk properties and have drawn relevant conclusions on their light absorption and charge transport properties. The actual catalytic conversions, however, occur on their surfaces and a detailed knowledge of the atomic-scale structure and processes on oxynitride surfaces is indispensable to further improve these materials. In this contribution, we summarize recent progress made in the understanding of perovskite oxynitride surfaces, highlight key processes that set these materials apart from their pure oxide counterparts and discuss challenges and possible future directions for research on oxynitrides.
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| 2. |
- Edge, Jonathan M., et al.
(author)
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Quantum Critical Origin of the Superconducting Dome in SrTiO3
- 2015
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 115:24
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Journal article (peer-reviewed)abstract
- We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped SrTiO3 using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing O-18 isotope substitution, a scenario that is experimentally verifiable. Our model is applicable to other quantum paraelectrics, such as KTaO3.
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| 3. |
- Flores, Eibar, et al.
(author)
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Cation Ordering and Redox Chemistry of Layered Ni-Rich LixNi1-2yCoyMnyO2 : An Operando Raman Spectroscopy Study
- 2020
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In: Chemistry of Materials. - : AMER CHEMICAL SOC. - 0897-4756 .- 1520-5002. ; 32:1, s. 186-194
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Journal article (peer-reviewed)abstract
- Although layered transition metal oxides are state-of-the-art cathode active materials for Li-ion batteries, many fundamental aspects of their operation are poorly understood, in particular, how the local lattice structure and transition metal composition influence their electrochemical activity. In this work, the local structure and redox activity of Ni-rich LixNi1-2yCoyMnyO2 (y = 0.1, 0.2, and 0.33, abbreviated as NCM811, NCM622, and NCM111, respectively) Li-ion cathodes are characterized under standard and overcharge operating conditions with a recently developed operando Raman spectroscopy methodology. Supported by DFT phonon calculations and advanced data analysis methods, we demonstrate that the Raman spectra of NCMs entail spectroscopic signatures of cation ordering phenomena, sequential oxidation/reduction of nickel, and participation of bulk lattice oxygen in the charge-compensation process at a low state of lithiation (SOL). Our methodology enables monitoring such processes during cycling and offers the potential for investigating the mechanisms by which certain strategies (i.e., doping, surface coatings, etc.) ameliorate electrochemical performance.
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| 4. |
- Flores, Eibar, et al.
(author)
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Elucidation of LixNi0.8Co0.15Al0.05O2 Redox Chemistry by Operando Raman Spectroscopy
- 2018
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 30:14, s. 4694-4703
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Journal article (peer-reviewed)abstract
- The local structure evolution of LixNi0.8Co0.15Al0.05O2 (NCA) is linked to its electrochemical response during cycling (and overcharge) by operando Raman spectroscopy with findings supported by complementary techniques, such as online electrochemical mass spectrometry (OEMS) and density functional theory (DFT) phonon calculations. The vibrational motion of lattice oxygens is observed to be highly dependent on the local LixMO2 lattice environment, e.g. M—O bonding strength/length and state of lithiation x. All vibrational modes generally harden upon delithiation due to M—O bond character (ionic → covalent) evolution (disregarding an early bond softening due to Li+ vacancy formation) and evidence the important influence of the local structural lattice configuration on the electrochemical response of NCA. Although the intensities of all Raman active bands generally increase upon delithiation, a major inflection point at x = 0.2 marks the onset of a partly irreversible fundamental transition within NCA that is most likely related to electron removal from MO bonding states and partial oxidation of oxygen sublattice, which is also indicated by the observed concomitant O2 release from the particle surface. Operando Raman spectroscopy with higher time resolution provides unique possibilities for detailed studies of how chemical parameters (Li+ vacancy formation, transition metal cation concentration, and lattice doping, etc.) may govern the onset and nature of processes (such as bond character evolution and stability) that define the performance of the LixMO2 class of oxides. The further insights thus gained can be exploited to guide the development of next-generation layered cathodes for Li-ion batteries operating stably at higher voltages and capacities.
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| 5. |
- Flores, Eibar, et al.
(author)
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Operando Monitoring the Insulator-Metal Transition of LiCoO2
- 2021
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In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:19, s. 22540-22548
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Journal article (peer-reviewed)abstract
- LiCoO2 (LCO) is one of the most-widely used cathode active materials for Li–ion batteries. Even though the material undergoes an electronic two-phase transition upon Li–ion cell charging, LCO exhibits competitive performance in terms of rate capability. Herein the insulator–metal transition of LCO is investigated by operando Raman spectroscopy complemented with DFT calculations and a developed sampling volume model. We confirm the presence of a Mott insulator α-phase at dilute Li-vacancy concentrations (x > 0.87, x in LixCoO2), which gradually transitions to primarily a metallic β-phase as x approaches 0.75. In addition, we find that the charge–discharge intensity trends of LCO Raman-active bands exhibit a characteristic hysteresis, which, unexpectedly, narrows at higher cycling rates. When comparing these trends to our numerical model of laser penetration into a spatially heterogeneous particle we provide compelling evidence that the insulator–metal transition of LCO follows a two-phase route at very low cycling rates, which is suppressed in favor of a solid-solution route at rates above 20 mA/gLCO (∼C/10). The observations explain why LCO exhibits competitive rate capabilities despite being observed to undergo an intuitively slow two-phase transition route: a kinetically faster solid-solution transition route becomes available when the active material is cycled at rates >C/10. Operando Raman spectroscopy combined with sample volume modeling and DFT calculations is shown to provide unique insights into fundamental processes governing the performance of state-of-the-art cathode materials for Li–ion batteries.
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| 6. |
- Ninova, Silviya, et al.
(author)
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Suitability of Cu-substituted beta-Mn2V2O7 and Mn-substituted beta-Cu2V2O7 for photocatalytic water-splitting
- 2020
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In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 153:8
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Journal article (peer-reviewed)abstract
- The pyrovanadates beta -Mn2V2O7 and beta -Cu2V2O7 were previously investigated as photoanode materials for water splitting. Neither of them, however, was found to be sufficiently active. In this work, we predict the properties of these two structurally similar pyrovanadates upon Cu/Mn substitution in their corresponding lattices via density functional theory calculations to explore the suitability of their band structure for water splitting and to assess their ease of synthesis. We predict that a concentration of up to 20% Cu and Mn into beta -Mn2V2O7 and beta -Cu2V2O7, respectively, leads to a narrowing of the bandgap, which, in the former case, is experimentally confirmed by UV-vis spectroscopy. Calculations in the intermediate composition range, however, yield nearly constant bandgaps. Moreover, we predict the materials with higher substitution levels to be increasingly difficult to synthesize, implying that low substitution levels are most relevant in terms of bandgaps and ease of synthesis.
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| 7. |
- Stuer, Michael, et al.
(author)
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Transparent polycrystalline alumina using spark plasma sintering : effect of Mg, Y and La doping
- 2010
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In: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 30:6, s. 1335-1343
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Journal article (peer-reviewed)abstract
- Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on thegrain size and residual porosity which need to be controlled for real inline transmittances (RIT), that are high enough for possible applications.To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping areinvestigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75–450 ppm) as well as theSPS sintering pressure and temperature on the RIT and grain size of PCA.Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640 nm) for 0.8mm thick samples for almost alldoping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450 ppm. These results are significantly better thanpreviously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under variousdoping and sintering conditions.
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