Research Interests:
Research Interests:
The work function is one of the most fundamental surface properties of a material, and understanding and controlling its value is of central importance for manipulating electron flow in applications ranging from high power vacuum... more
The work function is one of the most fundamental surface properties of a material, and understanding and controlling its value is of central importance for manipulating electron flow in applications ranging from high power vacuum electronics to oxide electronics and solar cells. Recent computational studies using Density Functional Theory (DFT) have demonstrated that DFT-calculated work function values for metals tend to agree well (within about 0.3 eV on average) with experimental values. However, a detailed validation of DFT-calculated work functions for oxide materials has not been conducted and is challenging due to the complex dipole structures that can occur on oxide surfaces. In this work, we have focused our investigation on the widely studied perovskite SrTiO3 as a case study example. We find that DFT can accurately predict the work function values of clean and reconstructed SrTiO3 surfaces vs experiment at about the same level of accuracy as metals when direct comparisons ...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests: Materials Science, Electrochemistry, Thin Film, Oxide, Electrode, and 2 moreCathode and Heterojunction(Cathode and Heterojunction)
(Cathode and Heterojunction)
Electrocatalysts play an important role in catalyzing the kinetics for oxygen reduction and oxygen evolution reactions for many air-based energy storage and conversion devices, such as metal-air batteries and fuel cells. Although noble... more
Electrocatalysts play an important role in catalyzing the kinetics for oxygen reduction and oxygen evolution reactions for many air-based energy storage and conversion devices, such as metal-air batteries and fuel cells. Although noble metals have been extensively used as electrocatalysts, their limited natural abundance and high costs have motivated the search for more cost-effective catalysts. Oxides are suitable candidates since they are relatively inexpensive and have shown reasonably high activity for various electrochemical reactions. However, a lack of fundamental understanding of the reaction mechanisms has been a major hurdle toward improving electrocatalytic activity. Detailed studies of the oxide surface atomic structure and chemistry (e.g., cation migration) can provide much needed insights for the design of highly efficient and stable oxide electrocatalysts. In this Account, we focus on recent advances in characterizing strontium (Sr) cation segregation and enrichment n...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The properties of (Mg,Fe)SiO3 perovskite at lower mantle conditions are still not well understood, and particular attention has recently been given to determining the Fe spin state. A major challenge in spin states studies is... more
The properties of (Mg,Fe)SiO3 perovskite at lower mantle conditions are still not well understood, and particular attention has recently been given to determining the Fe spin state. A major challenge in spin states studies is interpretation of Mössbauer spectra to determine the electronic structure of iron under extreme conditions. In this paper ab initio methods are used to predict quadrupole splitting values of high‐, intermediate‐ and low‐spin Fe2+ and Fe3+ in perovskite, as a function of pressure and composition. The calculations in (Mg0.75Fe0.25)SiO3 yield quadrupole splitting values in the range of 0.7–1.7 mm/s for all spin and valence states except high‐spin Fe2+, which has two possible quadrupole splittings, 2.3 and 3.3 mm/s. The unexpected multiple quadrupole splitting values for high‐spin Fe2+ are explained in terms of small changes in local structure and d‐orbital occupations. The computed results are applied to interpret existing perovskite Mössbauer data for iron's ...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The prediction of crystal structure is a key outstanding problem in materials science and one that is fundamental to computational materials design. We argue that by combining the predictive accuracy of quantum mechanics with data mining... more
The prediction of crystal structure is a key outstanding problem in materials science and one that is fundamental to computational materials design. We argue that by combining the predictive accuracy of quantum mechanics with data mining tools to extract knowledge from a large body of historical experimental or computational results, this problem can be successfully addressed.
Research Interests:
Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle... more
Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe(2+) in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe(3+)/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe(3+) in bridgmanite combined with the strong partitioning of Fe(2+) into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant cha...
Research Interests:
Metal fluoride and oxides can store multiple lithium-ions through conversion chemistry to enable high energy-density lithium-ion batteries. However, their practical applications have been hindered by an unusually large voltage hysteresis... more
Metal fluoride and oxides can store multiple lithium-ions through conversion chemistry to enable high energy-density lithium-ion batteries. However, their practical applications have been hindered by an unusually large voltage hysteresis between charge and discharge voltage-profiles and the consequent low energy efficiency (< 80%). The physical origins of such hysteresis are rarely studied and poorly understood. Here we employ in situ X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), density-functional-theory (DFT) calculations, and galvanostatic intermittent titration technique (GITT) to first correlate the voltage profile of iron fluoride (FeF3), a representative conversion electrode material, with evolution and spatial distribution of intermediate phases in the electrode. The results reveal that, contrary to conventional belief, the phase evolution in the electrode is symmetrical during discharge and charge. However, the spatial evolution of the elec...
Research Interests:
Research Interests:
Research Interests:
Recent studies have shown that high pressure ( P ) induces the metallization of the Fe 2+ –O bonding, the destruction of magnetic ordering in Fe, and the high-spin (HS) to low-spin (LS) transition of Fe in silicate and oxide phases at the... more
Recent studies have shown that high pressure ( P ) induces the metallization of the Fe 2+ –O bonding, the destruction of magnetic ordering in Fe, and the high-spin (HS) to low-spin (LS) transition of Fe in silicate and oxide phases at the deep planetary interiors. Hematite (Fe 2 O 3 ) is an important magnetic carrier mineral for deciphering planetary magnetism and a proxy for Fe in the planetary interiors. Here, we present synchrotron Mössbauer spectroscopy and X-ray diffraction combined with ab initio calculations for Fe 2 O 3 revealing the destruction of magnetic ordering at the hematite → Rh 2 O 3 -II type (RhII) transition at 70 GPa and 300 K, and then the revival of magnetic ordering at the RhII → postperovskite (PPv) transition after laser heating at 73 GPa. At the latter transition, at least half of Fe 3+ ions transform from LS to HS and Fe 2 O 3 changes from a semiconductor to a metal. This result demonstrates that some magnetic carrier minerals may experience a complex sequ...
Research Interests: Chemistry, Magnetic Recording, High Pressure, Mössbauer Spectroscopy, Medicine, and 15 moreMultidisciplinary, Physical sciences, Planets, Phase transition, Electronic Structure, Hematite, Pressure, Magnetics, Electrical Conductivity, Silicate, Ab Initio Calculation, Electric Conductivity, Ferric Compounds, Low Velocity Zone, and Electromagnetic coupling
Research Interests:
Research Interests:
Research Interests:
Research Interests: Materials Science, Semiconductors, Transmission Electron Microscopy, Nanowire, Medicine, and 11 moreMultidisciplinary, Zinc Oxide, Nanotubes, Thermal conduction in Nanomaterials, Particle Size, Antimony, Molecular Conformation, Materials Testing, Equipment Design, Equipment Failure Analysis, and Electric Conductivity
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests: Engineering, Physics, Chemistry, Technology, Adsorption, and 3 moreCHEMICAL SCIENCES, Dimer, and Monolayer
Research Interests:
Research Interests: Evolutionary Biology, Physics, Materials Science, Plasma Physics, Materials Chemistry, and 15 moreCell Biology, Infectious Diseases, Space Science, Chemical Physics, Medicine, Doping, Adsorption, DFT, Density Functional Theory, Physical sciences, Surface Reconstruction, Dipole, CHEMICAL SCIENCES, IDB, and Vacancy Defect
ABSTRACTDespite its importance as a cathode material in primary alkaline batteries, the structure of γ-MnO2 is still not well determined. Different authors have suggested that a number of different polymorphs, as well as highly disordered... more
ABSTRACTDespite its importance as a cathode material in primary alkaline batteries, the structure of γ-MnO2 is still not well determined. Different authors have suggested that a number of different polymorphs, as well as highly disordered phases, may be present in γ-MnO2. The origin of this structural complexity remains largely unexplained. In this paper we use first principles methods to explore the energetics of the MnO2 system. We find a number of low-energy polymorphs with similar energies, suggesting that relatively small changes in the energetics might influence the stable phases. Using nonzero-temperature models we demonstrate that thermal disorder is not the cause of structural disorder in these materials. However, we then show that point (Ruetschi) defects, even in surprisingly low concentrations, have a dramatic effect on the phase stability. We propose that Ruetschi defects may be the key to some of the structural complexity in γ-MnO2, and that any realistic structural st...
Research Interests:
Research Interests:
Fast oxygen transport materials are necessary for a range of technologies, including efficient and cost-effective solid oxide fuel cells, gas separation membranes, oxygen sensors, chemical looping devices, and memristors. Strain is often... more
Fast oxygen transport materials are necessary for a range of technologies, including efficient and cost-effective solid oxide fuel cells, gas separation membranes, oxygen sensors, chemical looping devices, and memristors. Strain is often proposed as a method to enhance the performance of oxygen transport materials, but the magnitude of its effect and its underlying mechanisms are not well-understood, particularly in the widely-used perovskite-structured oxygen conductors. This work reports on an ab initio prediction of strain effects on migration energetics for nine perovskite systems of the form LaBO3, where B = [Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga]. Biaxial strain, as might be easily produced in epitaxial systems, is predicted to lead to approximately linear changes in migration energy. We find that tensile biaxial strain reduces the oxygen vacancy migration barrier across the systems studied by an average of 66 meV per percent strain for a single selected hop, with a low of 36 and ...