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Exploiting basic principles to control the selectivity of the vapor phase catalytic oxidative cross-coupling of primary alcohols over npAu catalysts
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First-Principles Search for Efficient Activators for LaI3
First-principles calculations were performed using density functional theory with Hubbard corrections or hybrid exchange-correlation functionals, as well as the GW approximation, to identify activator dopants that could maximize the light yield of LaI3, a potentially very bright scintillator. The dopants considered in this work included a series of lanthanide ions (Ce, Pr, Nd, Eu, Gd, and Tb) and several ns2 ions (Tl, Pb, Sb, and Bi). Based on both ground-state calculations and constrained DFT calculations to simulate excited states, the trivalent lanthanide dopants were shown not to constitute an improvement over Ce3+, for which experimental data exist, as they showed occupied 4f states below the valence band maximum (VBM) and 5d states above the conduction band maximum (CBM). In contrast, the only divalent lanthanide considered, Eu2+, displayed occupied 4f states within the band gap of the host, but its 5d states were calculated to be above the CBM. Eu2+ could nonetheless be exploited as an activator by increasing the band gap energy slightly through substitution of iodide ions by bromide ions. Similar results were obtained for Tl+. Finally, Bi3+ and Sb3+ were predicted to be efficient activators in LaI3 by virtue of having unoccupied p states below the CBM, which can serve as electron traps and can combine with holes at the VBM to form localized emission centers. Comparison with empirical relationships of the relative positions of the energy levels of rare-earth dopants and implications for efficient doping schemes and scintillation mechanisms in LaI3 are also discussed.
Application of lean manufacturing principles to improve a conceptual 238Pu supply process
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First-principles study of the Pd–Si system and Pd(0 0 1)/SiC(0 0 1) hetero-structure
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Equation of state for technetium from X-ray diffraction and first-principle calculations
Abstract not provided
Design principles and field performance of a solar spectral irradiance meter
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Revealing the Conversion Mechanism of Transition Metal Oxide Electrodes during Lithiation from First-Principles
Transition metal oxides such as Co3O4 and NiO are of significant interest as conversion anode materials for lithium-ion batteries (LIBs), due to their remarkably high theoretical capacities and low cost. While many previous experiments have found that the charge/discharge reactions of Co3O4 and NiO can be highly reversible, detailed information about the mechanisms of these reactions, such as the origin of the voltage hysteresis (>1.0 V) between the charge/discharge cycles, is still poorly understood. In this work, we develop and utilize a new computational mechanistic approach that helps elucidate the hysteresis and nonequilibrium reaction pathways associated with these conversion materials. We apply this methodology to investigate a variety of lithiation reaction pathways of Co3O4 and NiO by systematically exploring the energetics of a large number of equilibrium and nonequilibrium LixCo3O4 (0 ≤ x ≤ 8) and LixNiO (0 ≤ x ≤ 2) structural configurations using first-principles calculations. The overall value of the voltages from our nonequilibrium pathway is in much better agreement with experimental lithiation than the calculated equilibrium voltage while the overall value of the latter reasonably agrees with experimental delithiation. Hence, we propose the charge and discharge processes proceed through equilibrium and nonequilibrium reaction paths, respectively, which contribute significantly to the experimentally observed voltage hysteresis in Co3O4 and NiO. Additionally, we find a low-energy, lithiated intermediate phase (Li3Co3O4) with an oxygen framework equal to that of the initial Co3O4 spinel phase. This intermediate phase represents the capacity threshold below which limited volume expansion and better reversibility can be realized and above which reactions lead to structural degradation and huge expansion.
Quantum Confinement and Decoherence Effect on Excited Electron Transfer at the Semiconductor–Molecule Interface: A First-Principles Dynamics Study
Excited electron dynamics at the semiconductor–molecule interface play a vital role in many solar energy conversion applications and beyond. First-principles dynamics simulation is used to study how excited electron dynamics in a type-II semiconductor–molecule interface is affected when the surface of a bulk semiconductor is replaced with that of a quantum dot. In particular, the interface between the hydrogen-terminated silicon(111) surface and a simple organic molecule is investigated. Quantum confinement was found to significantly slow down the excited electron relaxation and the interfacial transfer of the excited electron. Additionally, the role of decoherence was examined. The decoherence affects the dynamics much more significantly for the interface with the quantum dot, while the bulk surface case is affected only marginally by it. Finally, compared to the excited electron relaxation within the quantum dot, the decoherence effect was found to slow down the interfacial transfer from the quantum dot to the molecule by more than an order of magnitude, exhibiting a significant quantum Zeno effect.
First-Principles Predictions of Near-Edge X-ray Absorption Fine Structure Spectra of Semiconducting Polymers
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Machine Learning Accelerated First-Principles Study of the Hydrodeoxygenation of Propanoic Acid
The complex reaction network of catalytic biomass conversions often involves hundreds of surface intermediates and thousands of reaction steps, greatly hindering the rational design of metal catalysts for these conversions. Here, we present a framework of machine learning (ML)-accelerated first-principles studies for the hydrodeoxygenation (HDO) of propanoic acid over transition metal surfaces. The microkinetic model (MKM) is initially parametrized by ML-predicted energies and iteratively improved by identifying the rate-determining species and steps (RDS), computing their energies by density functional theory (DFT), and reparameterizing the MKM until all the RDS are computed by DFT. The Gaussian process (GP) model performs significantly better than the linear ridge regression model for predicting both the adsorption free energies and transition state free energies. Parameterized with energies from the GP model, only 5–20% of the full reaction network has to be computed by DFT for the MKM to possess DFT-level accuracy for the TOF and dominant reaction pathway. While the linear ridge regression model performs worse than the GP model, its performance is greatly improved when only transition states are predicted by the regression model and adsorption energies are computed by DFT. Overall, we find that a high accuracy in adsorption free energies is more important for a reliable MKM than a high accuracy in TS free energies. Lastly, based on the GP model with GOH and GCHCHCO as catalyst descriptors, we build two-dimensional volcano plots in activity and selectivity that can help design promising alloy catalysts for HDO reactions of organic acids.
First Principles Simulations of the Electrochemical Lithiation and Delithiation of Faceted Crystalline Silicon
Silicon is of significant interest as a next-generation anode material for lithium-ion batteries due to its extremely high capacity. The reaction of lithium with crystalline silicon is known to present a rich range of phenomena, including electrochemical solid state amorphization, crystallization at full lithiation of a Li15Si4 phase, hysteresis in the first lithiation–delithiation cycle, and highly anisotropic lithiation in crystalline samples. Very little is known about these processes at an atomistic level, however. To provide fundamental insights into these issues, we develop and apply a first principles, history-dependent, lithium insertion and removal algorithm to model the process of lithiation and subsequent delithiation of crystalline Si. The simulations give a realistic atomistic picture of lithiation demonstrating, for the first time, the amorphization process and hinting at the formation of the Li15Si4 phase. Voltages obtained from the simulations show that lithiation of the (110) surface is thermodynamically more favorable than lithiation of the (100) or (111) surfaces, providing an explanation for the drastic lithiation anisotropy seen in experiments on Si micro- and nanostructures. Analysis of the delithiation and relithiation processes also provides insights into the underlying physics of the lithiation–delithiation hysteresis, thus providing firm conceptual foundations for future design of improved Si-based anodes for Li ion battery applications.
A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics
Conjugated polymers with a one-dimensional p-orbital overlap exhibit optoelectronic anisotropy. Their unique anisotropic properties can be fully realized in device applications only when the conjugated chains are aligned. Here, we report a molecular design principle of conjugated polymers to achieve concentration-regulated chain planarization, self-assembly, liquid-crystal-like good mobility and non-interdigitated side chains. As a consequence of these intra- and intermolecular attributes, chain alignment along an applied flow field occurs. This liquid-crystalline conjugated polymer was realized by incorporating intramolecular sulphur–fluorine interactions and bulky side chains linked to a tetrahedral carbon having a large form factor. By optimizing the polymer concentration and the flow field, we could achieve a high dichroic ratio of 16.67 in emission from conducting conjugated polymer films. Two-dimensional grazing-incidence X-ray diffraction was performed to analyse a well-defined conjugated polymer alignment. Thin-film transistors built on highly aligned conjugated polymer films showed more than three orders of magnitude faster carrier mobility along the conjugated polymer alignment direction than the perpendicular direction.
Characterising local environments in high energy density Li-ion battery cathodes: a combined NMR and first principles study of LiFe x Co 1-x PO 4
Olivine-type LiCoPO4 (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co2+/Co3+ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO4 (LFP). The co-substituted LiFexCo1-xPO4 olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using 31P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual M–O–P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFexCo1-xPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1-xPO4 have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d6 Fe2+ as compared with d7 Co2+ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the Co–O–P pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP 31P shifts. This behaviour contrasts with that observed previously in LiFexMn1-xPO4, where all 31P shifts lay within the LiMnPO4–LFP range. Although there are 24 distinct local P environments in LiFexCo1-xPO4, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically-cycled samples, which would otherwise be challenging to interpret.
First-principles calculations of oxidation potentials of electrolytes in lithium–sulfur batteries and their variations with changes in environment
In this work, oxidation potentials of electrolyte molecules in Li–sulfur (Li/S) batteries and their variations in various solvent environments are investigated using first-principles calculations in order to understand oxidative decomposition reactions of electrolytes for cathode passivation. Electrolyte solvents, Li salts, and various additives in Li/S batteries along with some Li-ion battery additives are studied. Oxidation potentials of isolated electrolyte molecules are found to be out of the operating range of typical Li/S batteries. The complexation of electrolyte molecules with Li + , salt anion, salt, S8, and pyrene alters oxidation potentials compared to those of the isolated systems. The salt anion lowers oxidation potentials of electrolyte molecules by at least 4.7% while the complexes with Li + have higher oxidation potentials than the isolated molecules by at least 10.4%. S8 and pyrene, used as model compounds for sulfur and sulfur/ carbon composite cathode materials, also affect oxidation potentials of electrolyte molecules, but their influence is negligible and the oxidation trends differ from those of the Li + and salt anion. Although complexations change the oxidation potentials of electrolyte molecules, they are still higher than the operating voltage range of Li/S batteries, which indicates that oxidation of the studied electrolytes in Li/S batteries is not expected under ambient conditions.
Design principles for the energy level tuning in donor/acceptor conjugated polymers
Illustration of the charge transfer associated with the development of HOMO and LUMO levels in conjugated polymers and that the electron donating and withdrawing powers depend on the collective of atoms that carry the frontier orbitals involved in the charge transfer process.
Alkaline earth ion exchange study in pure silica LTA zeolites using periodic first-principles calculations
Experiments show that Zeolites, such as Linde Type A (LTA), are promising materials for radioactive decontamination processes. In this work, the thermodynamic properties associated with alkaline earth ion (Ca2+, Sr2+ and Ba2+) exchange in pure silica LTA was studied using periodic first-principles calculations. The adsorption energies of alkaline earth ions compared with that of Na+ were investigated. The driving force for the alkaline earth exchange and related isotherms were calculated, and analyzed as a function of the electron chemical potential. The results demonstrate that Na+ in a pure silica LTA can completely be removed by Ba2+ and almost completely be removed by Sr2+ from a stream, but cannot be effectively exchanged by Ca2+. We also showed that electron-donating dopants should suppress the Sr2+ and Ba2+ ion exchange, and that there is a substantial preference for incorporating Ba2+ over Sr2+. Lastly, the substantial difference between the adsorption energies of the ions in an assumed vacuum and those computed in water suggests that a solvation model is needed for accurate representation of ion adsorption in an LTA zeolite.
Cryogenic systems for proof of the principle experiment of coherent electron cooling at RHIC
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