Chongmin Wang

Uranium in Framboidal Pyrite from a Naturally Bioreduced Alluvial Sediment

Samples of a naturally bioreduced, U-contaminated alluvial sediment were characterized with various microscopic and spectroscopic techniques and wet chemical extraction methods. The objective was to investigate U association and interaction with minerals of the sediment. Bioreduced sediment comprises approximately 10% of an alluvial aquifer adjacent to the Colorado River, in Rifle, CO, that was the site of a former U milling operation. Past and ongoing research has demonstrated that bioreduced sediment is elevated in solid-associated U, total organic carbon, and acid-volatile sulfide, and depleted in bioavailable Fe(III) confirming that sulfate and Fe(III) reduction have occurred naturally in the sediment. SEM/EDS analyses demonstrated that framboidal pyrites (FeS(2)) of different sizes ( approximately 10-20 microm in diameter), and of various microcrystal morphology, degree of surface weathering, and internal porosity were abundant in the <53 microm fraction (silt + clay) of the sediment and absent in adjacent sediments that were not bioreduced. SEM-EMPA, XRF, EXAFS, and XANES measurements showed elevated U was present in framboidal pyrite as both U(VI) and U(IV). This result indicates that U may be sequestered in situ under conditions of microbially driven sulfate reduction and pyrite formation. Conversely, such pyrites in alluvial sediments provide a long-term source of U under conditions of slow oxidation, contributing to the persistence of U of some U plumes. These results may also help in developing remedial measures for U-contaminated aquifers.

Competitive reduction of pertechnetate (99TcO4-) by dissimilatory metal reducing bacteria and biogenic Fe(II).

The fate of pertechnetate ((99)Tc(VII)O(4)(-)) during bioreduction was investigated in the presence of 2-line ferrihydrite (Fh) and various dissimilatory metal reducing bacteria (DMRB) (Geobacter, Anaeromyxobacter, Shewanella) in comparison with TcO(4)(-) bioreduction in the absence of Fh. In the presence of Fh, Tc was present primarily as a fine-grained Tc(IV)/Fe precipitate that was distinct from the Tc(IV)O(2)·nH(2)O solids produced by direct biological Tc(VII) reduction. Aqueous Tc concentrations (<0.2 μm) in the bioreduced Fh suspensions (1.7 to 3.2 × 10(-9) mol L(-1)) were over 1 order of magnitude lower than when TcO(4)(-) was biologically reduced in the absence of Fh (4.0 × 10(-8) to 1.0 × 10(-7) mol L(-1)). EXAFS analyses of the bioreduced Fh-Tc products were consistent with variable chain length Tc-O octahedra bonded to Fe-O octahedra associated with the surface of the residual or secondary Fe(III) oxide. In contrast, biogenic TcO(2)·nH(2)O had significantly more Tc-Tc second neighbors and a distinct long-range order consistent with small particle polymers of TcO(2). In Fe-rich subsurface sediments, the reduction of Tc(VII) by Fe(II) may predominate over direct microbial pathways, potentially leading to lower concentrations of aqueous (99)Tc(IV).

Complete Decomposition of Li2CO3 in Li–O2 Batteries Using Ir/B4C as Noncarbon-Based Oxygen Electrode

Instability of carbon-based oxygen electrodes and incomplete decomposition of Li2CO3 during charge process are critical barriers for rechargeable Li-O2 batteries. Here we report the complete decomposition of Li2CO3 in Li-O2 batteries using the ultrafine iridium-decorated boron carbide (Ir/B4C) nanocomposite as a noncarbon based oxygen electrode. The systematic investigation on charging the Li2CO3 preloaded Ir/B4C electrode in an ether-based electrolyte demonstrates that the Ir/B4C electrode can decompose Li2CO3 with an efficiency close to 100% at a voltage below 4.37 V. In contrast, the bare B4C without Ir electrocatalyst can only decompose 4.7% of the preloaded Li2CO3. Theoretical analysis indicates that the high efficiency decomposition of Li2CO3 can be attributed to the synergistic effects of Ir and B4C. Ir has a high affinity for oxygen species, which could lower the energy barrier for electrochemical oxidation of Li2CO3. B4C exhibits much higher chemical and electrochemical stability than carbon-based electrodes and high catalytic activity for Li-O2 reactions. A Li-O2 battery using Ir/B4C as the oxygen electrode material shows highly enhanced cycling stability than those using the bare B4C oxygen electrode. Further development of these stable oxygen-electrodes could accelerate practical applications of Li-O2 batteries.

Atomic Resolution Structural and Chemical Imaging Revealing the Sequential Migration of Ni, Co, and Mn upon the Battery Cycling of Layered Cathode

Layered lithium transition metal oxides (LTMO) are promising candidate cathode materials for next-generation high-energy density lithium ion battery. The challenge for using this category of cathode is the capacity and voltage fading, which is believed to be associated with the layered structure disordering, a process that is initiated from the surface or solid-electrolyte interface and facilitated by transition metal (TM) reduction and oxygen vacancy formation. However, the atomic level dynamic mechanism of such a layered structure disordering is still not fully clear. In this work, utilizing atomic resolution electron energy loss spectroscopy (EELS), we map, for the first time at atomic scale, the spatial evolution of Ni, Co and Mn in a cycled LiNi1/3Mn1/3Co1/3O2 layered cathode. In combination with atomic level structural imaging, we discovered the direct correlation of TM ions migration behavior with lattice disordering, featuring the residing of TM ions in the tetrahedral site and a sequential migration of Ni, Co, and Mn upon the increased lattice disordering of the layered structure. This work highlights that Ni ions, though acting as the dominant redox species in many LTMO, are labile to migrate to cause lattice disordering upon battery cycling, while the Mn ions are more stable as compared with Ni and Co and can act as pillar to stabilize layered structure. Direct visualization of the behavior of TM ions during the battery cycling provides insight for designing of cathode with high structural stability and correspondingly a superior performance.

Effect of the Ligand Shell Composition on the Dispersibility and Transport of Gold Nanocrystals in Near-Critical Solvents

The development of more efficient and environmentally benign methods for the synthesis and manipulation of nanomaterials has been a major focus of research among the scientific community. Supercritical (ScFs) and near-critical fluids (NcFs) offer numerous advantages over conventional solvents for these purposes. Among them, ScFs and NcFs offer dramatic reductions in the volume of organic waste typically generated during advanced material processes with the feasibility of changing a number of physicochemical properties by discrete variations in solvent pressure or temperature. In this work, we study the dispersibility of gold nanocrystals with a 3.7 nm core size stabilized by different ligand shells in NcF ethane and propane over a wide range of densities by fine-tuning the pressure of these fluids. Dispersibility vs density plots are obtained by following the variation in the surface plasmon resonance (SPR) absorption spectra of the nanoparticles. To understand the results obtained in this study, three models are briefly discussed: the total interaction theory, the sedimentation coefficient equation, and the Chrastil method. The dispersibility and behavior of the nanocrystals with variations in fluid density are strongly dependent on the surface chemistry of the nanocrystal and the solvent employed. A correlation between measured dispersibility values and calculated sedimentation coefficients was observed in both compressed solvents. In addition, we successfully applied the Chrastil equation to predict and describe the dispersibility of gold nanocrystals with different shells as a function of density, determining that the reason for the high stabilities of some of the nanocrystal dispersions is the strong solvent-nanocrystal interactions. While NcF propane showed higher nanocrystal dispersibilities, using NcF ethane led to improved tunability of nanoparticle dispersions formed in the pressure range studied. Therefore, with a judicious selection of the fluid, NcFs seem to offer a remarkable advantage over conventional solvents for manipulation of nanomaterials, which could be applied to transport, purification, and separation of nanocrystals.

Excess Li-ion storage on reconstructed surfaces of nanocrystals to boost battery performance

Because of their enhanced kinetic properties, nanocrystallites have received much attention as potential electrode materials for energy storage. However, because of the large specific surface areas of nanocrystallites, they usually suffer from decreased energy density, cycling stability, and effective electrode capacity. In this work, we report a size-dependent excess capacity beyond theoretical value (170 mA h g-1) by introducing extra lithium storage at the reconstructed surface in nanosized LiFePO4 (LFP) cathode materials (186 and 207 mA h g-1 in samples with mean particle sizes of 83 and 42 nm, respectively). Moreover, this LFP composite also shows excellent cycling stability and high rate performance. Our multimodal experimental characterizations and ab initio calculations reveal that the surface extra lithium storage is mainly attributed to the charge passivation of Fe by the surface C-O-Fe bonds, which can enhance binding energy for surface lithium by compensating surface Fe truncated symmetry to create two types of extra positions for Li-ion storage at the reconstructed surfaces. Such surface reconstruction nanotechnology for excess Li-ion storage makes full use of the large specific surface area of the nanocrystallites, which can maintain the fast Li-ion transport and greatly enhance the capacity. This discovery and nanotechnology can be used for the design of high-capacity and efficient lithium ion batteries.

Pb nanowire formation on Al/lead zirconate titanate surfaces in high-pressure hydrogen

Piezoelectric systems are well known to degrade in hydrogen because of various mechanisms including loss of polarization, Pb migration into the electrode, and surface blistering. Understanding damage mechanisms is crucial for potential high-pressure applications such as injectors for hydrogen-fueled vehicles. In this paper, we report on a previously unreported form of high-pressure hydrogen damage resulting from growth of surface Pb nanowires from an Al electrode on lead zirconate titanate. Wires were observed with roughly 80 nm diameter and with length that varied between 5 and 100 μm. Microscopy to characterize the nanowires and ion-scattering measurements to quantify concurrent Pb surface migration and hydrogen absorption effects are described.

Tuning Li-Ion Diffusion in α-LiMn1–xFexPO4 Nanocrystals by Antisite Defects and Embedded β-Phase for Advanced Li-Ion Batteries

Olivine-structured LiMn1-xFexPO4 has become a promising candidate for cathode materials owing to its higher working voltage of 4.1 V and thus larger energy density than that of LiFePO4, which has been used for electric vehicles batteries with the advantage of high safety but disadvantage of low energy density due to its lower working voltage of 3.4 V. One drawback of LiMn1-xFexPO4 electrode is its relatively low electronic and Li-ionic conductivity with Li-ion one-dimensional diffusion. Herein, olivine-structured α-LiMn0.5Fe0.5PO4 nanocrystals were synthesized with optimized Li-ion diffusion channels in LiMn1-xFexPO4 nanocrystals by inducing high concentrations of Fe2+-Li+ antisite defects, which showed impressive capacity improvements of approaching 162, 127, 73, and 55 mAh g-1 at 0.1, 10, 50, and 100 C, respectively, and a long-term cycling stability of maintaining about 74% capacity after 1000 cycles at 10 C. By using high-resolution transmission electron microscopy imaging and joint refinement of hard X-ray and neutron powder diffraction patterns, we revealed that the extraordinary high-rate performance could be achieved by suppressing the formation of electrochemically inactive phase (β-LiMn1-xFexPO4, which is first reported in this work) embedded in α-LiMn0.5Fe0.5PO4. Because of the coherent orientation relationship between β- and α-phases, the β-phase embedded would impede the Li+ diffusion along the [100] and/or [001] directions that was activated by the high density of Fe2+-Li+ antisite (4.24%) in α-phase. Thus, by optimizing concentrations of Fe2+-Li+ antisite defects and suppressing β-phase-embedded olivine structure, Li-ion diffusion properties in LiMn1-xFexPO4 nanocrystals can be tuned by generating new Li+ tunneling. These findings may provide insights into the design and generation of other advanced electrode materials with improved rate performance.

Creation and Ordering of Oxygen Vacancies at WO3−δ and Perovskite Interfaces

Changes in the structure and composition resulting from oxygen deficiency can strongly impact the physical and chemical properties of transition-metal oxides, which may lead to new functionalities for novel electronic devices. Oxygen vacancies (VO) can be readily formed to accommodate the lattice mismatch during epitaxial thin film growth. In this paper, the effects of substrate strain and oxidizing power on the creation and distribution of VO in WO3-δ thin films are investigated in detail. An 18O2 isotope-labeled time-of-flight secondary-ion mass spectrometry study reveals that WO3-δ films grown on SrTiO3 substrates display a significantly larger oxygen vacancy gradient along the growth direction compared to those grown on LaAlO3 substrates. This result is corroborated by scanning transmission electron microscopy imaging, which reveals a large number of defects close to the interface to accommodate interfacial tensile strain, leading to the ordering of VO and the formation of semi-aligned Magnéli phases. The strain is gradually released and a tetragonal phase with much better crystallinity is observed at the film/vacuum interface. The changes in the structure resulting from oxygen defect creation are shown to have a direct impact on the electronic and optical properties of the films.

Real-time Observation of Sintering Process of Carbon Supported Platinum Nanoparticles in Oxygen and Water through Environment TEM

Sintering of Pt electrocatalyst largely determines the life-time of PEM fuel cells. While extensive efforts have been made to increase the durability and explore the sintering mechanism of carbon supported Pt NPs, the real behaviors of Pt catalysts in working ambient of PEM fuel cells remain elusive. It has long been recognized that deactivation of the Pt-based electrocatalyst is directly related to the coarsening of the Pt nanoparticles under working conditions of PEMFC. In situ TEM is inherently an effective and straightforward technique to visualize the sintering process and reveal the mechanisms of the sintering process of Pt NPs.