Docheon Ahn

Surface-Stress-Induced Mott Transition and Nature of Associated Spatial Phase Transition in Single Crystalline VO2 Nanowires

We demonstrate that the Mott metal-insulator transition (MIT) in single crystalline VO(2) nanowires is strongly mediated by surface stress as a consequence of the high surface area to volume ratio of individual nanowires. Further, we show that the stress-induced antiferromagnetic Mott insulating phase is critical in controlling the spatial extent and distribution of the insulating monoclinic and metallic rutile phases as well as the electrical characteristics of the Mott transition. This affords an understanding of the relationship between the structural phase transition and the Mott MIT.

Enhanced High-Rate Performance of Li4Ti5O12 Nanoparticles for Rechargeable Li-Ion Batteries

Li 4 Ti 5 O 12 was successfully synthesized by solvothermal techniques using cost-effective precursors in polyol medium. The x-ray diffraction (XRD) pattern of the sample (LTO-500) was clearly indexed to the spinel shaped Li 4 Ti 5 O 12 and in order to accurately determine the lattice parameters, synchrotron powder XRD pattern was fitted by the whole-pattern profile matching method using the model space group, Fd3m. The particle size, morphology, and crystallinity of LTO-500 were identified using field-emission scanning electron microscopy and transmission electron microscopy. The electrochemical performance of the sample revealed fairly high initial discharge/charge specific capacities of 230 and 179 mAh/g, respectively, and exhibited highly improved rate performances at C-rates as high as 30 and 60 C, when compared to Li 4 Ti 5 O 12 by the solid-state reaction method. This was attributed to the achievement of small particle sizes in nanoscale dimensions, a reasonably narrow particle size distribution and, hence, shorter diffusion paths combined with larger contact area at the electrode/electrolyte interface.

Anomalous Jahn–Teller behavior in a manganese-based mixed-phosphate cathode for sodium ion batteries

We report a 3.8 V manganese-based mixed-phosphate cathode material for applications in sodium rechargeable batteries; i.e., Na4Mn3(PO4)2(P2O7). This material exhibits a largest Mn2+/Mn3+ redox potential of 3.84 V vs. Na+/Na yet reported for a manganese-based cathode, together with the largest energy density of 416 W h kg−1. We describe first-principles calculations and experimental results which show that three-dimensional Na diffusion pathways with low-activation-energy barriers enable the rapid sodium insertion and extraction at various states of charge of the Na4−xMn3(PO4)2(P2O7) electrode (where x = 0, 1, 3). Furthermore, we show that the sodium ion mobility in this crystal structure is not decreased by the structural changes induced by Jahn–Teller distortion (Mn3+), in contrast to most manganese-based electrodes, rather it is increased due to distortion, which opens up sodium diffusion channels. This feature stabilizes the material, providing high cycle stability and high power performance for sodium rechargeable batteries. The high voltage, large energy density, cycle stability and the use of low-cost Mn give Na4Mn3(PO4)2(P2O7) significant potential for applications as a cathode material for large-scale Na-ion batteries.

Pyro-Synthesis of Functional Nanocrystals

Despite nanomaterials with unique properties playing a vital role in scientific and technological advancements of various fields including chemical and electrochemical applications, the scope for exploration of nano-scale applications is still wide open. The intimate correlation between material properties and synthesis in combination with the urgency to enhance the empirical understanding of nanomaterials demand the evolution of new strategies to promising materials. Herein we introduce a rapid pyro-synthesis that produces highly crystalline functional nanomaterials under reaction times of a few seconds in open-air conditions. The versatile technique may facilitate the development of a variety of nanomaterials and, in particular, carbon-coated metal phosphates with appreciable physico-chemical properties benefiting energy storage applications. The present strategy may present opportunities to develop “design rules” not only to produce nanomaterials for various applications but also to realize cost-effective and simple nanomaterial production beyond lab-scale limitations.

Gas hydrate inhibition by perturbation of liquid water structure

Natural gas hydrates are icy crystalline materials that contain hydrocarbons, which are the primary energy source for this civilization. The abundance of naturally occurring gas hydrates leads to a growing interest in exploitation. Despite their potential as energy resources and in industrial applications, there is insufficient understanding of hydrate kinetics, which hinders the utilization of these invaluable resources. Perturbation of liquid water structure by solutes has been proposed to be a key process in hydrate inhibition, but this hypothesis remains unproven. Here, we report the direct observation of the perturbation of the liquid water structure induced by amino acids using polarized Raman spectroscopy, and its influence on gas hydrate nucleation and growth kinetics. Amino acids with hydrophilic and/or electrically charged side chains disrupted the water structure and thus provided effective hydrate inhibition. The strong correlation between the extent of perturbation by amino acids and their inhibition performance constitutes convincing evidence for the perturbation inhibition mechanism. The present findings bring the practical applications of gas hydrates significantly closer, and provide a new perspective on the freezing and melting phenomena of naturally occurring gas hydrates.

Stress-induced domain dynamics and phase transitions in epitaxially grown VO₂ nanowires.

We demonstrate that surface stresses in epitaxially grown VO₂ nanowires (NWs) have a strong effect on the appearance and stability of intermediate insulating M₂ phases, as well as the spatial distribution of insulating and metallic domains during structural phase transitions. During the transition from an insulating M1 phase to a metallic R phase, the coexistence of insulating M₁ and M₂ phases with the absence of a metallic R phase was observed at atmospheric pressure. In addition, we show that, for a VO₂ NW without the presence of an epitaxial interface, surface stresses dominantly lead to spatially inhomogeneous phase transitions between insulating and metallic phases. In contrast, for a VO₂ NW with the presence of an epitaxial interface, the strong epitaxial interface interaction leads to additional stresses resulting in uniformly alternating insulating and metallic domains along the NW length.

Energy Transfer Between Activators at Different Crystallographic Sites

Several divalent europium-doped alkali-earth silicon oxide or nitride (AESON) phosphors, which are of great use in white light-emitting diodes, exhibit a two-peak emission. We examined the two-peak emission of Sr 2 SiO 4 :Eu 2+ , which is known as a representative AESON phosphor. The two-peak emission is closely associated with the energy transfer between two different Eu 2+ sites. The two-peak emission in light-emitting diode phosphors is a very important issue from a practical point of view because it directly relates to the control of color tuning. By analyzing the time-resolved photoluminescence spectra, we identified the energy transfer taking place between Eu 2+ activators located at two different crystallographic sites in the Sr 2 SiO 4 structure. A rate-equation model involving various energy-transfer routes among inhomogeneously distributed activators in the crystalline host was developed and used to give relevant interpretation to color chromaticity.

Inhibition of methane and natural gas hydrate formation by altering the structure of water with amino acids.

Natural gas hydrates are solid hydrogen-bonded water crystals containing small molecular gases. The amount of natural gas stored as hydrates in permafrost and ocean sediments is twice that of all other fossil fuels combined. However, hydrate blockages also hinder oil/gas pipeline transportation, and, despite their huge potential as energy sources, our insufficient understanding of hydrates has limited their extraction. Here, we report how the presence of amino acids in water induces changes in its structure and thus interrupts the formation of methane and natural gas hydrates. The perturbation of the structure of water by amino acids and the resulting selective inhibition of hydrate cage formation were observed directly. A strong correlation was found between the inhibition efficiencies of amino acids and their physicochemical properties, which demonstrates the importance of their direct interactions with water and the resulting dissolution environment. The inhibition of methane and natural gas hydrate formation by amino acids has the potential to be highly beneficial in practical applications such as hydrate exploitation, oil/gas transportation, and flow assurance. Further, the interactions between amino acids and water are essential to the equilibria and dynamics of many physical, chemical, biological, and environmental processes.

Surface Structures and Electrochemical Activities of Pt Overlayers on Ir Nanoparticles

Pt overlayers were deposited on carbon-supported Ir nanoparticles with various coverages. Structural and electrochemical characterizations were performed using transmission electron microscopy (TEM), X-ray diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), cyclic voltammetry (CV), CO stripping voltammetry, and N2O reduction. The surface of Ir nanoparticles was covered with Pt overlayers with thickness varying from the submonolayer scale to more than two monolayers. Surface analyses such as CV and CO stripping voltammetry indicated that the Pt overlayers were uniformly deposited on the Ir nanoparticles, and the resultant Pt overlayers exhibited gradual changes in surface characteristics toward the Pt surface as the surface coverage increased. The distinct CO stripping characteristics and the enhanced Pt utilization affected electrocatalytic activities for methanol oxidation. The electrochemical stability of the Pt overlayer was compared with a commercial carbon-supported Pt catalyst by conducting a potential cycling experiment.

Iron Oxide Photoelectrode with Multidimensional Architecture for Highly Efficient Photoelectrochemical Water Splitting

Nanostructured metal oxide semiconductors have shown outstanding performances in photoelectrochemical (PEC) water splitting, but limitations in light harvesting and charge collection have necessitated further advances in photoelectrode design. Herein, we propose anodized Fe foams (AFFs) with multidimensional nano/micro-architectures as a highly efficient photoelectrode for PEC water splitting. Fe foams fabricated by freeze-casting and sintering were electrochemically anodized and directly used as photoanodes. We verified the superiority of our design concept by achieving an unprecedented photocurrent density in PEC water splitting over 5 mA cm-2 before the dark current onset, which originated from the large surface area and low electrical resistance of the AFFs. A photocurrent of over 6.8 mA cm-2 and an accordingly high incident photon-to-current efficiency of over 50 % at 400 nm were achieved with incorporation of Co oxygen evolution catalysts. In addition, research opportunities for further advances by structual and compositional modifications are discussed, which can resolve the low fill factoring behavior and improve the overall performance.