Showing papers by "Miaofang Chi published in 2017"

Crystal Structural Effect of AuCu Alloy Nanoparticles on Catalytic CO Oxidation

Abstract: Controlling the physical and chemical properties of alloy nanoparticles (NPs) is an important approach to optimize NP catalysis. Unlike other tuning knobs, such as size, shape, and composition, crystal structure has received limited attention and not been well understood for its role in catalysis. This deficiency is mainly due to the difficulty in synthesis and fine-tuning of the NPs’ crystal structure. Here, Exemplifying by AuCu alloy NPs with face centered cubic (fcc) and face centered tetragonal (fct) structure, we demonstrate a remarkable difference in phase segregation and catalytic performance depending on the crystal structure. During the thermal treatment in air, the Cu component in fcc-AuCu alloy NPs segregates more easily onto the alloy surface as compared to that in fct-AuCu alloy NPs. As a result, after annealing at 250 °C in air for 1 h, the fcc- and fct-AuCu alloy NPs are phase transferred into Au/CuO and AuCu/CuO core/shell structures, respectively. More importantly, this variation in heter...

Interfaces in Heterogeneous Catalysts: Advancing Mechanistic Understanding through Atomic-Scale Measurements.

Abstract: ConspectusDeveloping novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic a...

Unrivaled combination of surface area and pore volume in micelle-templated carbon for supercapacitor energy storage

Abstract: We created Immense Surface Area Carbons (ISACs) by a novel heat treatment that stabilized the micelle structure in a biological based precursor prior to high temperature combined activation – pyrolysis. While displaying a morphology akin to that of commercial activated carbon, ISACs contain an unparalleled combination of electrochemically active surface area and pore volume (up to 4051 m2 g−1, total pore volume 2.60 cm3 g−1, 76% small mesopores). The carbons also possess the benefit of being quite pure (combined O and N: 2.6–4.1 at%), thus allowing for a capacitive response that is primarily EDLC. Tested at commercial mass loadings (∼10 mg cm−2) ISACs demonstrate exceptional specific capacitance values throughout the entire relevant current density regime, with superior rate capability primarily due to the large fraction of mesopores. In the optimized ISAC, the specific capacitance (Cg) is 540 F g−1 at 0.2 A g−1, 409 F g−1 at 1 A g−1 and 226 F g−1 at a very high current density of 300 A g−1 (∼0.15 second charge time). At intermediate and high currents, such capacitance values have not been previously reported for any carbon. Tested with a stable 1.8 V window in a 1 M Li2SO4 electrolyte, a symmetric supercapacitor cell yields a flat energy–power profile that is fully competitive with those of organic electrolyte systems: 29 W h kg−1 at 442 W kg−1 and 17 W h kg−1 at 3940 W kg−1. The cyclability of symmetric ISAC cells is also exceptional due to the minimization of faradaic reactions on the carbon surface, with 80% capacitance retention over 100 000 cycles in 1 M Li2SO4 and 75 000 cycles in 6 M KOH.

Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts

Abstract: Precious metals have broad applications in modern industry and renewable energy technologies. The high cost and limited availability of these materials, however, have caused a grand challenge for sustainability. Here, we report on the plating of a precious metal on nonprecious metal nanoparticles for the development of sustainable electrocatalysts. Cobalt/platinum core/shell (denoted as Co@Pt) nanoparticles were synthesized via seed-mediated growth. The Co seeds were first synthesized by thermal decomposition of cobalt carbonyl, and the Pt shell was overgrown in situ by adding platinum acetylacetonate (Pt(acac)2). The galvanic replacement reaction between Co and the Pt precursor was successfully suppressed by taking advantage of CO (generated from the decomposition of cobalt carbonyl) as the stabilizing ligand and/or reducing agent. The obtained Co@Pt nanoparticles were further found to exhibit enhanced catalytic activity for the oxygen reduction reaction (ORR).

Effect of Surface Structure of TiO2 Nanoparticles on CO2 Adsorption and SO2 Resistance

Abstract: The effect of surface structure of TiO2 nanocrystals on the structure, amount, and strength of adsorbed CO2 and resistance to SO2 was investigated using in situ IR spectroscopy and mass spectrometric techniques along with first-principles density functional theory (DFT) calculations. TiO2 nanoshapes, including rods {(010) + (101) + (001)}, disks {(001) + (101)}, and truncated octahedra {(101) + (001)}, were used to represent different TiO2 structures. Upon CO2 adsorption, carboxylates and carbonates (bridged, monodentate) are formed on TiO2 rods and disks, whereas only bidentate and monodentate carbonates are formed on TiO2 truncated octahedra. In general, the order of thermal stability of the adsorbed CO2 species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates. TiO2 rods and disks adsorb CO2 more strongly than TiO2 truncated octahedra, which is explained by the larger number of low coordinated surface oxygen and oxygen vacancies on the rods and disks th...

Facile Synthesis of Ru-Based Octahedral Nanocages with Ultrathin Walls in a Face-Centered Cubic Structure

Abstract: Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temperature were all instrumental to the formation of Pd@Ru core–shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five atomic layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent dens...

Understanding the Thermal Stability of Palladium–Platinum Core–Shell Nanocrystals by In Situ Transmission Electron Microscopy and Density Functional Theory

Abstract: Core–shell nanocrystals offer many advantages for heterogeneous catalysis, including precise control over both the surface structure and composition, as well as reduction in loading for rare and costly metals. Although many catalytic processes are operated at elevated temperatures, the adverse impacts of heating on the shape and structure of core–shell nanocrystals are yet to be understood. In this work, we used ex situ heating experiments to demonstrate that Pd@Pt4L core–shell nanoscale cubes and octahedra are promising for catalytic applications at temperatures up to 400 °C. We also used in situ transmission electron microscopy to monitor the thermal stability of the core–shell nanocrystals in real time. Our results demonstrate a facet dependence for the thermal stability in terms of shape and composition. Specifically, the cubes enclosed by {100} facets readily deform shape at a temperature 300 °C lower than that of the octahedral counterparts enclosed by {111} facets. A reversed trend is observed for ...

In situ TEM observation of the electrochemical lithiation of N-doped anatase TiO2 nanotubes as anodes for lithium-ion batteries

Abstract: Due to their high specific capacity and negligible volume expansion during cycling, anatase titanium dioxide (a-TiO2) nanotubes have been considered as a prime candidate for anodes in lithium-ion batteries. However, their rate capability for electrochemical cycling is limited by the low electronic conductivity of a-TiO2 nanotubes. Here, we show that a desirable amount of nitrogen doping can significantly enhance the electronic conductivity in a-TiO2 nanotubes, resulting in improvements in both the capacity stability and the rate capability at fast charge–discharge rates. Electron energy loss spectroscopy revealed a high doping concentration of nitrogen (∼5%) by substituting for oxygen ions in a-TiO2 nanotubes. The lithiation mechanism of N-doped a-TiO2 nanotubes was further investigated using in situ transmission electron microscopy, where a three-step lithiation mechanism was revealed. Lithium ions initially intercalate into the a-TiO2 lattice structure. Further insertion of lithium ions triggers a phase transformation from a-TiO2 to orthorhombic Li0.5TiO2 and finally to polycrystalline tetragonal LiTiO2. Our results reveal that nitrogen doping significantly facilitates lithiation in TiO2 through enhanced electronic conductivity, while the structural and chemical evolutions during the lithiation process remain similar to those of undoped TiO2.

Nonequilibrium Synthesis of TiO2 Nanoparticle "Building Blocks" for Crystal Growth by Sequential Attachment in Pulsed Laser Deposition.

Abstract: Nonequilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gas-phase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally and spatially resolved gated-intensified charge couple device (ICCD) imaging and ion probe measurements are employed as in situ diagnostics to understand and control the plume expansion conditions for the synthesis of nearly pure fluxes of ultrasmall (∼3 nm) amorphous TiO2 nanoparticles in background gases and their selective delivery to substrates. These amorphous nanoparticles assemble into loose, mesoporous assemblies on substrates at room temperature but dynamically crystallize by sequential particle attachment at higher substrate temperatures to grow nanostructures with different phases and morphologies. Molecular dynamics calculations are used to simulate and understand the crystallization dynamics. This work demonstrates that nonequil...

Self-Assembled Framework Formed During Lithiation of SnS2 Nanoplates Revealed by in Situ Electron Microscopy

Abstract: ConspectusLithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g–1; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lith...

New promising lithium malonatoborate salts for high voltage lithium ion batteries

Abstract: Three new lithium salts, lithium difluoro-2-methyl-2-fluoromalonatoborate (LiDFMFMB), lithium difluoro-2-ethyl-2-fluoromalonatoborate (LiDFEFMB), and lithium difluoro-2-propyl-2-fluoromalonatoborate (LiDFPFMB), have been synthesized and evaluated for application in lithium ion batteries. These new salts are soluble in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1 : 2 by wt) and 1.0 M salt solutions can be easily prepared. The ionic conductivities of these new salts are close to those of LiBF4 and LiPF6. Cyclic voltammograms reveal that these new salt based electrolytes can passivate both natural graphite and high voltage spinel LiNi0.5Mn1.5O4 (LNMO) to form effective solid electrolyte interphases (SEIs). In addition, these new salt-based electrolytes exhibit good cycling stability with high coulombic efficiencies in both LiNi0.5Mn1.5O4 and graphite based half-cells and full cells.

Improving superconductivity in BaFe 2 As 2 -based crystals by cobalt clustering and electronic uniformity

Abstract: Quantum materials such as antiferromagnets or superconductors are complex in that chemical, electronic, and spin phenomena at atomic scales can manifest in their collective properties. Although there are some clues for designing such materials, they remain mainly unpredictable. In this work, we find that enhancement of transition temperatures in BaFe2As2-based crystals are caused by removing local-lattice strain and electronic-structure disorder by thermal annealing. While annealing improves Neel-ordering temperature in BaFe2As2 crystal (T N = 132 K to 136 K) by improving in-plane electronic defects and reducing overall a-lattice parameter, it increases superconducting-ordering temperature in optimally cobalt-doped BaFe2As2 crystal (T c = 23 to 25 K) by precipitating-out the cobalt dopants and giving larger overall a-lattice parameter. While annealing improves local chemical and electronic uniformity resulting in higher T N in the parent, it promotes nanoscale phase separation in the superconductor resulting in lower disparity and strong superconducting band gaps in the dominant crystal regions, which lead to both higher overall T c and critical-current-density, J c .

Novel Acid Catalysts from Waste‐Tire‐Derived Carbon: Application in Waste–to‐Biofuel Conversion

Abstract: Many inexpensive biofuel feedstocks, including those containing free fatty acids (FFAs) in high concentrations, are typically disposed of as waste due to our inability to efficiently convert them into usable biofuels. Here we demonstrate that carbon derived from waste tires could be functionalized with sulfonic acid (-SO3H) to effectively catalyze the esterification of oleic acid or a mixture of fatty acids to usable biofuels. Waste tires were converted to hard carbon, then functionalized with catalytically active -SO3H groups on the surface through an environmentally benign process that involved the sequential treatment with L-cysteine, dithiothreitol, and H2O2. When benchmarked against the same waste-tire derived carbon material treated with concentrated sulfuric acid at 150 °C, similar catalytic activity was observed. Both catalysts could also effectively convert oleic acid or a mixture of fatty acids and soybean oil to usable biofuels at 65 °C and 1 atm without leaching of the catalytic sites.

Dimensionality Effects in FeGe 2 Nanowires: Enhanced Anisotropic Magnetization and Anomalous Electrical Transport

Abstract: We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.

Integrating Novel Microscopy into Battery Research: From Atomic Resolution to In Situ and Functional Imaging

Abstract: 1. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA 2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 3. School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing, China 4. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA