Search

Significant advancements have been made in the performance of oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. State-of-the-art Pt-alloy catalysts show exceptionally high mass activity and recent research into ordered nanoparticles shows promising improvements in durability. The activity of platinum group metal-free (PGM-free) catalysts, which will be required as world-wide demand for fuel cell stacks increases, have also improved substantially, although significant durability issues persist. Accelerated materials discovery and development is required to keep pace with ever-changing market and performance demands, such as the shift in focus towards heavy duty transportation where highly durable, cost-effective catalysts are required. Scanning transmission electron microscopy (STEM) has long played an important role in assessing the quality and durability of electrocatalysts and electrode structures, and the increasing demands on catalyst performance require a corresponding improvement in both the resolution and quantitative precision of analytical methods. Fortunately, recent advances in high-speed cameras and spectrometers allow for an unprecedented view of catalyst structure and composition in two, and in some cases, three dimensions. The newly installed JEOL NEOARM atomic-resolution STEM at the Center for Nanophase Materials Sciences combines the latest generation spectrometers with new imaging modes such as electron ptychography. The ability to acquire multiple signals simultaneously at high data acquisition speeds and across a range of accelerating voltages (30-200 kV) allows for a fresh look at the atomic structure of both Pt-based and PGM-free catalysts. The simultaneous application of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to study the beam sensitive surfaces of sub-5nm Pt-alloy nanoparticles ( Acknowledgements Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), as part of the FC-PAD and ElectroCat Consortia, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.

After several decades of R&D, the thin film batteries using ORNL’s Lipon glassy electrolyte is still a promising technology for consumer electronics. These batteries also serve as model systems for other solid-state battery efforts, as the thin film batteries demonstrate exceptional stability with extended cycling and wide ranges of voltage and temperature. In light of recent thermodynamic analysis [1], we need to ask - Are we cheating thermodynamics? This presentation will briefly review the early work on Lipon-based batteries leading into recent efforts to advance the understanding of Lipon properties. Results for stability of Lipon with high voltage cathodes and Li metal will be highlighted. [1] William D. Richards, Lincoln J. Miara, Yan Wang, Jae Chul Kim, and Gerbrand Ceder, DOI: 10.1021/acs.chemmater.5b04082 Acknowledgement: This work is based on support from the US Department of Energy, Advanced Research Projects Agency for Energy (ARPA-E) IONICS program; and on earlier support from U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

In spite of different chemistries, many novel battery configurations, e.g., Li- air, Li-S, aqueous, and all-solid-state Li batteries, etc., share the same concept of using solid electrolyte materials to enable the use of lithium metal. An ideal solid electrolyte material must be highly ionically conductive and exhibit desirable stabilitywith metallic lithium. Over the past several decades, new solid electrolyte materials were developed that demonstrated high conductivity. However, the interfaces, including both internal grain boundaries within solid electrolytes and external interfaces between solid electrolytes and electrode, often show unexpected resistivity, degraded cyclability and vulnerability to dendrite propagation, presenting the current major limitations in realizing the practical application of solid electrolytes. Understanding what is happening at these interfaces at sufficient spatial, temporal and chemical resolutions are critical but challenging, due to spatial confinement and structural complications. Our research focuses on utilizing and developing advanced in situ and functional scanning transmission electron microscopy (STEM) to elucidate fundamental issues that lead to limited performance of interfaces in solid electrolytes, and to provide insights into the design and the engineering of high-performance interfaces. In this presentation, I will give several examples to demonstrate some of our recent studies in (i) elucidating the preferred dendrite-propagation at grain boundaries in the garnet-structured Al-Li7La3Zr2O12(LLZO); (ii) evaluating ion mobilities of H+and Li+ions individually in LLZO critical to its applications as a protection layer in aqueous batteries; and (iii) understanding the interfacial resistivity of LIPON with Li metal anode. Our results not only provide valuable insights into the understanding and the design of interfaces in Li-metal batteries, but also offer unique characterization methods that are generally applicable into the fundamental studies of solid-solid interfaces that involve mobile charge species and/or extremely chemically-active metallic lithium. Acknowledgement Research sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Microscopy performed as part of a user project at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE User Facility.

Solid-state electrolytes are intensively being investigated as a critical component in future generation Li-metal battery systems, including their uses as the primary electrolyte in all-solid-state batteries, and as the protection layer for Li-metal in Li-air, Li-S, or aqueous batteries. While several solid electrolyte materials were developed recently that demonstrated sufficient conductivity, unexpectedly high resistance and cycling instability from grain boundaries and interfaces are often observed. The practical applications of solid-state electrolytes cannot be realized without a thorough understanding of these phenomena. This talk will cover our recent efforts in elucidating the fundamental origin of (1) interfacial resistivity between solid electrolytes and Lithium metal, and (2) preferential dendrite growth along grain boundaries, by using in situ and atomic-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Interfacial phenomena among different solid electrolytes, e.g. Li(7−3×)AlxLa3Zr2O12 (LLAZO), Li1+xTi2–xAlx(PO4)3 (LATP) and LIPON, will be compared and potential strategies to mitigate interfacial challenges will be proposed.

Improved methods for the flux growth of single crystals of the important battery material LiFePO4 have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li1–2xFex)FePO4 partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO4 stoichiometry. The LiFePO4 defects are shown to be sarcopside-like (2 Li+ → Fe2+ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters o...

Mn-doped SrTiO3 shows promising magnetic and electrical properties, but the doping mechanism remains unclear. In this research Mn4+ is found to substitute Ti in bulk SrTiO3, but Mn2+ segregates inside grain boundaries at both Sr and interstitial sites. Mn interstitial doping has never been reported, but is found possible with the formation of Sr vacancies. This finding is significantly different from the amphoteric doping of Mn2+ substituting Sr and Mn4+ substituting Ti sites, therefore leads to different understanding on the defect mediated electrical and magnetic properties of transition metal-doped perovskites.

This article describes the solution-phase synthesis of 4 nm gold nanoparticles with 0.7 atom-thick, 1.9 atom-thick, and 3.8 atom-thick layers of Pd on their surfaces. These well-defined core–shell nanoparticles were deposited on a silica support, calcined, and reduced at 300 °C to create alloyed nanoparticles containing 10.9, 20.2, and 28.5% Pd (w/w). Monometallic Pd nanoparticles sintered during calcination at 300 °C, but no sintering was observed for AuPd nanoparticles. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of adsorbed CO suggests that Au donates d electron density to Pd in the core–shell and alloy structures and confirms the presence of Au and Pd atoms on the surface of the nanoparticles after calcination and reduction. The properties of the AuPd alloy catalysts were tested in the vapor-phase conversion of α-limonene to p-cymene. AuPd nanoparticles containing 20% or more Pd per particle produced p-cymene yields greater than 80%, equivalent to conventional Pd catalysts prepared ...

Precise placement of twin boundaries and stacking faults promises new opportunities to fundamentally manipulate the optical, electrical, and thermal properties of semiconductor nanowires. Here we report on the appearance of consecutive twin boundaries in Si nanowires and show that sidewall morphology governs their spacing. Detailed electron microscopy analysis reveals that thin {111} sidewall facets, which elongate following the first twin boundary (TB1), are responsible for deforming the triple-phase line and favoring the formation of the second twin boundary (TB2). While multiple, geometrically correlated defect planes are known in group III-V nanowires, our findings show that this behavior is also possible in group IV materials.

Palladium wavy nanowires with an ultrathin diameter of 2 nm are synthesized using the polyol method without the involvement of any template. The success of this synthesis relies on the use of a suitable precursor that could be reduced instantaneously to generate a large number of small Pd nanoparticles. Due to a quick depletion of precursor, the small nanoparticles were unable to grow in size through atomic addition. In the case of low surface charges and high surface energies, these small nanoparticles were forced to coalesce into ultrathin nanowires with a wavy morphology via an attachment mechanism. Thanks to the unique structure and involvement of twin defects, the as-obtained Pd ultrathin nanowires show a catalytic current density of 2.5 times higher than the conventional Pd/C catalyst towards formic acid oxidation. This work not only offers a powerful route to the synthesis of nanowires through attachment-based growth but also opens the door to the rational design and fabrication of novel metal nanostructures with enhanced properties.

Highly strained BiFeO3 films transition into a true tetragonal state at 430 °C but remain polar to much higher temperatures (∼800 °C). Piezoelectric switching is only possible up to 300 °C, i.e., at temperatures for which strain stabilizes the stripe-like coexistence of multiple polymorphs.

Size, shape, nuclearity: Aberration-corrected scanning transmission electron microscopy was used to determine the 3D structures of MgO-supported Os3, Os4, Os5, and Os10 clusters, which have structures nearly matching those of osmium carbonyl compounds with known crystal structures. The samples are among the best-defined supported catalysts.

Dynamic structural changes during the first electrochemical charge and discharge cycle in the Li-excess layered oxide compound, Li[Li1/5Ni1/5Mn3/5]O2, are studied with synchrotron X-ray diffraction (SXRD), aberration corrected scanning transmission electron microscopy (a-S/TEM), and electron energy loss spectroscopy (EELS). At different states of charge, we carefully examined the crystal structures and electronic structures within the bulk and have found that increased microstrain is accompanied with the cation migration and a second phase formation which occurs during the first cycle voltage plateau as well as into the beginning of the discharge cycle. The evidence indicates that the oxygen vacancy formation and activation may facilitate cation migration and results in the formation of a second phase. The EELS results reveal a Mn valence change from 4+ to 3+ upon oxygen vacancy formation and recovers back to 4+ at the discharge. The oxygen vacancy formation and activation at the partially delithiated sta...

Grain boundaries (GBs) in complex oxides such as perovskites have been shown to readily accommodate nonstoichiometry changing the electrostatic potential at the boundary plane and effectively controlling material properties such as capacitance, magnetoresistance and superconductivity. Understanding and quantifying exactly how variations in atomic scale nonstoichiometry at the boundary plane extend to the practical mesoscale operating length of the system is therefore critical for improving the overall properties. Bicrystals of SrTiO3 were fabricated to provide the model GB model structures that are analysed in this paper. We show that statistical analysis of aberration-corrected scanning transmission electron microscope images acquired from a large area of GB is an effective routine to understanding the variation in boundary structure that occurs to accommodate nonstoichiometry. In the case of the SrTiO3 22.6° ∑13 (510)/[100] GB analysed here, the symmetric atomic structures observed from a micron-long GB...

We demonstrate synthesis of a new type of heterostructures that comprise two-dimensional (2D) nanosheets (NSs) epitaxially grown at one-dimensional (1D) nanowires (NWs). The synthesis involves materials with a graphite-like layered structure in which covalently bonded layers are held by weak van der Waals forces. GeS was used as a prototype material in this work. The synthesis also involves a seeded-growth process, where GeS NWs are grown first as seeds followed by a seeded growth of NSs at the pre-grown NWs. We observe that exposing the pre-grown NWs to air prior to the seeded growth is critical for the formation of NSs to yield NS-NW heterostructures. Our experimental results suggest that this might be due to a mild oxidation at the NW surface caused by the air exposure, which could subsequently facilitate the nucleation of NSs at the NWs. It also suggests that the surface oxidation needs to be controlled in a proper range in order to achieve optimized NS growths. We believe that this synthetic strategy may generally apply to the growth of NS-NW heterostructures of other layered chalcogenide materials. NS-NW heterostructures provide capabilities to monolithically integrate the functionality of 1D NWs and 2D NSs into a 3D space. It holds great potential in applications that request complex nanomaterials with multiple functionality, high surface area, and efficient charge transport, such as energy storage, chemical sensing, solar energy conversion, and 3D electric and photonic devices.

Self-organized amorphous TiO 2 nanotube arrays (NTAs) were successfully fabricated on both Ti foil and porous Ti foam through electrochemical anodization techniques. The starting Ti foams were fabricated using ARCAM's Electron Beam Melting (EBM) technology. The TiO 2 NTAs on Ti foam were used as anodes in lithium ion batteries; they exhibited high capacities of 103 μAh cm −2 at 10 μA cm −2 and 83 μAh cm −2 at 500 μA cm −2 , which are two to three times higher than those achieved on the standard Ti foil, which is around 40 μAh cm −2 at 10 μA cm −2 and 24 μAh cm −2 at 500 μA cm −2 , respectively. This improvement is mainly attributed to higher surface area of the Ti foam and higher porosity of the nanotube arrays layer grown on the Ti foam. In addition, a Na-ion half-cell composed of these NTAs anodes and Na metal as the counter electrode showed a self-improved specific capacity upon cycling at 10 μA cm −2 . These results indicate that TiO 2 NTAs grown on Ti porous foam are promising electrodes for Li-ion or Na-ion rechargeable batteries.

High voltage Li-ion cathodes push the limits of stability for both cathode and electrolyte. Here subnanometer coatings of an amorphous thin film electrolyte (Lipon) improved the room temperature and 60 C cycling stability of a LiMn1.5Ni0.5O4 spinel cathode when charged to 4.9V with a standard LiPF6 carbonate electrolyte. The cathodes delivered superior C-rate performances up to a 5C discharge, when compared to the uncoated cathodes. Enhanced performance extended for at least 100 cycles. Electrochemical impedance spectroscopy indicates that Lipon slows the increase of interface resistance. Thicker 1-3nm Lipon coatings are sufficiently insulating as to block electronic transport to the cathode particles. Thick coatings also slow Mn dissolution. Results suggest that Lipon may act to scavenge impurities or block active sites that promote electrolyte decomposition. While greatly improved by the Lipon coating, this cathode is not sufficiently stable for long cycle life applications. Further work is needed to assess if and what surface coatings will ultimately stabilize the high voltage cathodes. Comments include insight from other studies of Lipon coated cathodes and directions for future research.

Monodisperse Ni/Pd core/shell nanoparticles (NPs) have been synthesized by sequential reduction of nickel(II) acetate and palladium(II) bromide in oleylamine (OAm) and trioctylphosphine (TOP). The Ni/Pd NPs have a narrow size distribution with a mean particle size of 10 nm and a standard deviation of 5% with respect to the particle diameter. Mechanistic studies showed that the presence of TOP was essential to control the reductive decomposition of Ni-TOP and Pd-TOP, and the formation of Ni/Pd core/shell NPs. Using the current synthetic protocol, the composition of the Ni/Pd within the core/shell structure can be readily tuned by simply controlling the initial molar ratio of the Ni and Pd salts. The as-synthesized Ni/Pd core/shell NPs were supported on graphene (G) and used as catalyst in Suzuki-Miyaura cross-coupling reactions. Among three different kinds of Ni/Pd NPs tested, the Ni/Pd (Ni/Pd = 3/2) NPs were found to be the most active catalyst for the Suzuki-Miyaura cross-coupling of arylboronic acids with aryl iodides, bromides and even chlorides in a dimethylformamide/water mixture by using K2CO3 as a base at 110 °C. The G-Ni/Pd was also stable and reusable, providing 98% conversion after the 5th catalytic run without showing any noticeable Ni/Pd composition change. The G-Ni/Pd structure reported in this paper combines both the efficiency of a homogeneous catalyst and the durability of a heterogeneous catalyst, and is promising catalyst candidate for various Pd-based catalytic applications. Open image in new window

We report a quantitative understanding of the reduction kinetics responsible for the formation of Pd-Pt bimetallic nanocrystals with two distinctive structures. The syntheses involve the use of KBr to manipulate the reaction kinetics by influencing the redox potentials of metal precursor ions via ligand exchange. In the absence of KBr, the ratio between the initial reduction rates of PdCl4(2-) and PtCl4(2-) was about 10.0, leading to the formation of Pd@Pt octahedra with a core-shell structure. In the presence of 63 mM KBr, the products became Pd-Pt alloy nanocrystals. In this case, the ratio between the initial reduction rates of the two precursors dropped to 2.4 because of ligand exchange and, thus, the formation of PdBr4(2-) and PtBr4(2-). The alloy nanocrystals took a cubic shape owing to the selective capping effect of Br(-) ions toward the {100} facets. Relative to the alloy nanocubes, the Pd@Pt core-shell octahedra showed substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). Specifically, the specific (1.51 mA cm(-2)) and mass (1.05 A mg(-1) Pt) activities of the core-shell octahedra were enhanced by about four- and three-fold relative to the alloy nanocubes (0.39 mA cm(-2) and 0.34 A mg(-1) Pt, respectively). Even after 20000 cycles of accelerated durability test, the core-shell octahedra still exhibited a mass activity of 0.68 A mg(-1) Pt, twice that of a pristine commercial Pt/C catalyst.

Nanocages have received considerable attention in recent years for catalytic applications owing to their high utilization efficiency of atoms and well-defined facets. Here we report, for the first time, the synthesis of Ru cubic nanocages with ultrathin walls, in which the atoms are crystallized in a face-centered cubic (fcc) rather than hexagonal close-packed (hcp) structure. The key to the success of this synthesis is to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temperature and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, we obtain Ru nanocages with an average wall thickness of 1.1 nm or about six atomic layers. Most importantly, the Ru nanocages adopt an fcc crystal structure rather than the hcp structure observed in bulk Ru. The synthesis has been successfully applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent density functional theory calculations indicate that these unique fcc-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp Ru(0001), on the basis of strengthened binding of atomic N and substantially reduced activation energies for N2 dissociation, which is the rate-determining step for ammonia synthesis on hcp Ru catalysts.

In this work, we investigated the feasibility of five quaternary (aprotic) and four tertiary (protic) ammonium ionic liquids (ILs) with an identical organophosphate anion as lubricant anti-wear additives. Viscosity, oil solubility, thermal stability and corrosivity of the candidate ILs were characterized and correlated to the molecular structure. The protic group exhibits higher oil solubility than the aprotic group, and longer alkyl chains seem to provide better oil solubility and higher thermal stability. Selected ILs were applied as oil additives in steel–cast iron tribological tests and demonstrated promising anti-scuffing and anti-wear functionality. The thickness, nanostructure, and composition of the tribofilm formed by the best performing IL were revealed by surface characterization for mechanistic understanding of the tribochemical interactions between the IL and metal surface. Results provide fundamental insights of the correlations among the molecular structure, physiochemical properties and lubricating performance for ammonium-phosphate ILs.

Solid electrolytes can simultaneously overcome two of the most formidable challenges facing current state-of-art Li-ion batteries: the severe safety issues and insufficient energy densities. On the other hand, two major limitations of solid electrolytes must be overcome before they can be implemented for practical use: the relatively slow ionic conductivity and their limited interfacial stability and conductivity with electrodes. Since ion transport in solids and their interfaces is based on a hopping mechanism from atomic sites to nearby vacancy sites or suitable interstitial sites, a prerequisite to overcome these challenges is a thorough understanding of the material’s behavior at the microscopic and/or the atomic level. Transmission electron microscopy is a powerful tool that provides the spatial resolution required. Here, we review recent electron microscopy investigations towards understanding the ion transport behavior in solid electrolyte materials and their interfaces. Specifically, three aspects of recent microscopy research will be highlighted: the influence of individual grain microstructure on ion conductivity, the contribution of grain boundaries, and the behavior of solid electrolyte/electrode interfaces. Based on these results, the perspectives for future research will be discussed.

Electron microscopy is undergoing a transition; from the model of producing only a few micrographs, through the current state where many images and spectra can be digitally recorded, to a new mode where very large volumes of data (movies, ptychographic and multi-dimensional series) can be rapidly obtained. Here, we discuss the application of so-called “big-data” methods to high dimensional microscopy data, using unsupervised multivariate statistical techniques, in order to explore salient image features in a specific example of BiFeO3 domains. Remarkably, k-means clustering reveals domain differentiation despite the fact that the algorithm is purely statistical in nature and does not require any prior information regarding the material, any coexisting phases, or any differentiating structures. While this is a somewhat trivial case, this example signifies the extraction of useful physical and structural information without any prior bias regarding the sample or the instrumental modality. Further interpretation of these types of results may still require human intervention. However, the open nature of this algorithm and its wide availability, enable broad collaborations and exploratory work necessary to enable efficient data analysis in electron microscopy.

Recent development of electrochemical technologies for renewable energy conversion and storage has enlightened the importance of tailoring the structures of catalytic materials at the nanoscale. In particular, the design and synthesis of nanocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have attracted great attention and been extensively studied. Remarkable progress has been made in improving the catalytic activity and stability of ORR catalysts by controlling and tuning the particle size, shape, composition and surface structures, as well as building up sophisticated composite nanostructures in core/shell and nanoporous configurations. Here a brief review is provided for the recent development of Pt-based nanocatalysts for the ORR. Instead of providing a complete list of the great amount of work reported in this topic, our focus is placed on fundamental understanding of the structure-property relationships of platinum-based nanomaterials, in particular alloy nanoparticles, in the electrochemical environment. The discussion is guided by correlations between well-defined extended surfaces and practical high-surface-area catalysts. Conclusions are made on challenges that remain and potential implications on other catalytic systems.

Supported triosmium clusters, formed from Os3(CO)12 on MgO, were treated in helium at 548 K for 2 h, causing fragmentation of the cluster frame and the formation of mononuclear osmium dicarbonyls. The cluster breakup and the resultant fragmented species were characterized by infrared and X-ray absorption spectroscopies, and the fragmented species were imaged by scanning transmission electron microscopy. The spectra identify the surface osmium complexes as Os(CO)2{Osupport}n (n = 3 or 4) (where the braces denote support surface atoms). The images show site-isolated Os atoms in mononuclear osmium species on MgO. The intensity analysis on the images of the MgO(110) face showed that the Os atoms were located atop Mg columns. This information led to a model of the Os(CO)2 on MgO(110), with the distances approximated as those determined by EXAFS spectroscopy, which are an average over the whole MgO surface; the results imply that these complexes were located at Mg vacancies.

Mesoporous carbon–Cr 2 O 3 (M-C–Cr 2 O 3 ) composite was prepared by co-assembly of in situ formed phenolic resin, chromium precursor, and Pluronic block copolymer under acidic conditions, followed by carbonization at 750 °C under Argon. The TEM results confirmed that the Cr 2 O 3 nanoparticles, ranging from 10 to 20 nm, were well dispersed in the matrix of mesoporous carbon. The composite exhibited an initial reversible capacity of 710 mAh g −1 and good cycling stability, which is mainly due to the synergic effects of carbons within the composites, i.e. confining the crystal growth of Cr 2 O 3 during the high temperature treatment step and buffering the volume change of Cr 2 O 3 during the cycling step. This composite material is a promising anode material for lithium ion batteries.

An SPR biosensor was developed by employing highly stable Au-protected Ag nanoplates (NP) as enhancers (see picture). Superior performance was achieved by depositing a thin and uniform coating of Au on the Ag surface while minimizing disruptive galvanic replacement and retaining the strong surface plasmon resonance (SPR) of the silver nanoplates.

In situ X-ray diffraction patterns were collected using a laboratory X-ray diffractometer during the first electrochemical charge/discharge cycle of the layered lithium excess compound Li[Li0.2Ni0.2Mn0.6]O2 in the family of Li[NixLi1/3 − 2x/3Mn2/3 − x/3]O2 (x = 1/5). Dynamic changes in peak positions, lattice parameters, and microstrain help to explain the lithium de-intercalation mechanism in this class of materials. Strong anisotropy is observed in the shifts of the lattice parameters during the first cycle. The in situ electrochemical measurement shows dynamically changing strain during the first electrochemical cycle that is explained by known lithium and transition metal (TM) migration mechanisms.

Silicon nanowires (NWs) have been reported as a promising anode that demonstrated high capacity without pulverization during cycling, however, they present some technical issues that remain to be solved. The high aspect ratio of the NWs and their small contact areas with the current collector cause high electrical resistance, which results in inefficient electron transport. The nano-size interface between a NW and the substrate experiences high shear stress during lithiation, causing the wire to separate from the current collector. In addition, most reported methods for producing silicon NWs involve high-temperature processing and require catalysts that later become contaminants. This study developed a new self-aligned Cu–Si core–shell NW array using a low-temperature, catalyst-free process to address the issues described. The silicon shell is amorphous as synthesized and accommodates Li-ions without phase transformation. The copper core functions as a built-in current collector to provide very short (nm) electron transport pathways as well as backbone to improve mechanical strength. Initial electrochemical evaluation has demonstrated good capacity retention and high Coulombic efficiency for this new anode material in a half-cell configuration. No wire fracture or core–shell separation was observed after cycling. However, electrolyte decomposition products largely covered the top surface of the NW array, restricting electrolyte access and causing capacity reduction at high charging rates.

Synthesized two-dimensional GaSe/MoSe2 misfit heterostructures form p-n junctions with a gate-tunable photovoltaic response., Two-dimensional (2D) heterostructures hold the promise for future atomically thin electronics and optoelectronics because of their diverse functionalities. Although heterostructures consisting of different 2D materials with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) epitaxy, constructing heterostructures from layered semiconductors with large lattice misfits remains challenging. We report the growth of 2D GaSe/MoSe2 heterostructures with a large lattice misfit using two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientation between the two layers, forming a periodic superlattice. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe2 monolayer domains in lateral GaSe/MoSe2 heterostructures, GaSe monolayers are found to overgrow MoSe2 during CVD, forming a stripe of vertically stacked vdW heterostructures at the crystal interface. Such vertically stacked vdW GaSe/MoSe2 heterostructures are shown to form p-n junctions with effective transport and separation of photogenerated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These GaSe/MoSe2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.

Alloying has shown enormous potential for tailoring the atomic and electronic structures, and improving the performance of catalytic materials. Systematic studies of alloy catalysts are, however, often compromised by inhomogeneous distribution of alloying components. Here we introduce a general approach for the synthesis of monodispersed and highly homogeneous Pt-bimetallic alloy nanocatalysts. Pt{sub 3}M (where M = Fe, Ni, or Co) nanoparticles were prepared by an organic solvothermal method and then supported on high surface area carbon. These catalysts attained a homogeneous distribution of elements, as demonstrated by atomic-scale elemental analysis using scanning transmission electron microscopy. They also exhibited high catalytic activities for the oxygen reduction reaction (ORR), with improvement factors of 2-3 versus conventional Pt/carbon catalysts. The measured ORR catalytic activities for Pt{sub 3}M nanocatalysts validated the volcano curve established on extended surfaces, with Pt{sub 3}Co being the most active alloy.

PdPt alloy nanoparticles (NPs) are promising catalysts for various chemical reactions because of the presence of powerful catalytic components of Pt and Pd on the surface of one nanostructure. In this paper, we report a facile synthesis of polyhedral PdPt alloy NPs via coreduction of Pd(acac)2 (acac = acetylacetonate) and Pt(acac)2 with morpholine borane in oleylamine at 90 and 180 °C. In the synthesis, the molar ratio of the two metal precursors added in the reaction mixture was carried over to the final PdPt NP product, and compositions of the PdPt NPs were readily tuned from Pd88Pt12 to Pd34Pt66. These PdPt NPs show the composition-dependent catalytic activity for methanol oxidation, with NPs in 40–60 atomic % Pt exhibiting the superior activity and durability.

Catalysts for the dehydrogenation of light alkanes were prepared by dispersing Pt on the surface of a calcined hydrotalcite-like support containing indium, Mg(In)(Al)O. Upon reduction in H2 at temperatures above 673 K, bimetallic particles of PtIn are observed by TEM, which have an average diameter of 1 nm. Analysis of Pt LIII-edge extended X-ray absorption fine structure (EXAFS) data shows that the In content of the bimetallic particles increases with increasing bulk In/Pt ratio and reduction temperature. Pt LIII-edge X-ray absorption near edge structure (XANES) indicates that an increasing donation of electronic charge from In to Pt occurs with increasing In content in the PtIn particles. The activity and selectivity of the Pt/Mg(In)(Al)O catalysts for ethane and propane dehydrogenation reactions are strongly dependent on the bulk In/Pt ratio. For both reactants, maximum activity was achieved for a bulk In/Pt ratio of 0.48, and at this In/Pt ratio, the selectivity to alkene was nearly 100%. Coke deposition was observed after catalyst use for either ethane or propane dehydrogenation, and it was observed that the alloying of Pt with In greatly reduced the amount of coke deposited. Characterization of the deposit by Raman spectroscopy indicates that the coke is present as highly disordered graphite particles

For high-voltage cycling of rechargeable Li batteries, a nano-scale amorphous Li-ion conductor, lithium phosphorus oxynitride (Lipon), has been coated on surfaces of LiCoO2 particles by combining a RF-magnetron sputtering technique and mechanical agitation of LiCoO2 powders. LiCoO2 particles coated with 0.36 wt% (∼1 nm thick) of the amorphous Lipon, retain 90% of their original capacity compared to non-coated cathode materials that retain only 65% of their original capacity after more than 40 cycles in the 3.0–4.4 V range with a standard carbonate electrolyte. The reason for the better high-voltage cycling behavior is attributed to reduction in the side reactions that cause increase of the cell resistance during cycling. Further, Lipon coated particles are not damaged, whereas uncoated particles are badly cracked after cycling. Extending the charge of Lipon-coated LiCoO2 to higher voltage enhances the specific capacity, but more importantly the Lipon-coated material is also more stable and tolerant of high voltage excursions. A drawback of Lipon coating, particularly as thicker films are applied to cathode powders, is the increased electronic resistance that reduces the power performance.

Since the idea of using ionic liquids (ILs) as lubricants was raised in 2001, many studies have been conducted in this area and results have demonstrated superior lubricating performance for a variety of ILs. It is widely believed that a protective tribo-boundary film is formed on the contact area by tribochemical reactions between the metal surface and the IL during the wear process and, as a result, reduces friction and wear. However, the study of this critical boundary film in the literature has been limited to two-dimensional topography examination and chemical analysis from the top surface. This study demonstrates a multi-technique three-dimensional approach to characterize the boundary films on IL-lubricated metallic surfaces. The complementary characterizations at the top surface, cross-section, and different layers of the boundary film provide direct measurement of the film thickness, visualization of the nanostructure, and analysis of the composition change. The boundary films observed on different alloys are substantially distinct from both physical and chemical perspectives. The measured mean film thicknesses for cast iron, steel, and aluminum worn surfaces are 300, 60, and 200 nm, respectively. The boundary films on ferrous alloys are dominated by amorphous phase mixing with well-dispersed very fine (a few nm) nanocrystals, while the film on aluminum contains many larger size (tens of nm) metallic particles in a less organized manner.

Herein, we report novel gold catalysts made by dispersing Au@Fe2O3 core–shell structures on solid supports. In the synthesis of Au@Fe2O3 core–shell structures, dodecanethiol-capped gold nanoparticles were used as the seed and Fe(CO)5 was used as the precursor to Fe2O3 shell. The Au@Fe2O3 core–shell particles were deposited onto SiO2 support to obtain Au@Fe2O3/SiO2 catalysts that were highly active for low-temperature CO oxidation. The catalytic activity was even higher than that of Au/SiO2 or Au/Fe2O3 prepared by colloidal deposition with comparable gold loadings. The influences of thermal pretreatment, shell thickness, and different supports (e.g., SiO2, TiO2, C, Fe2O3) were investigated, and relevant characterization using TG/DTG, XRD, TEM, HAADF, and EDX was conducted.

Quantitative analysis of point defects is demonstrated through the use of multivariate statistical analysis. This analysis consists of principal component analysis for dimensional estimation and reduction, followed by independent component analysis to obtain physically meaningful, statistically independent factor images. Results from these analyses are presented in the form of factor images and scores. Factor images show characteristic intensity variations corresponding to physical structure changes, while scores relate how much those variations are present in the original data. The application of this technique is demonstrated on a set of experimental images of dislocation cores along a low-angle tilt grain boundary in strontium titanate. A relationship between chemical composition and lattice strain is highlighted in the analysis results, with picometer-scale shifts in several columns measurable from compositional changes in a separate column.

Monodisperse and homogeneous Pt{sub x}Ni{sub 1-x} alloy nanoparticles of various compositions are synthesized via an organic solution approach in order to reveal the correlation between surface chemistry and their electrocatalytic properties. Atomic-level microscopic analysis of the compositional profile and modeling of nanoparticle structure are combined to follow the dependence of Ni dissolution on the initial alloy composition and formation of the Pt-skeleton nanostructures. The developed approach and acquired knowledge about surface structure-property correlation can be further generalized and applied towards the design of advanced functional nanomaterials.

We demonstrate an unusual electrochemical reaction of sulfur with lithium upon encapsulation in narrow-diameter (subnanometer) single-walled carbon nanotubes (SWNTs). Our study provides mechanistic insight on the synergistic effects of sulfur confinement and Li+ ion solvation properties that culminate in a new mechanism of these sub-nanoscale-enabled reactions (which cannot be solely attributed to the lithiation-delithiation of conventional sulfur). Two types of SWNTs with distinct diameters, produced by electric arc (EA-SWNTs, average diameter 1.55 nm) or high-pressure carbon monoxide (HiPco-SWNTs, average diameter 1.0 nm), are investigated with two comparable electrolyte systems based on tetraethylene glycol dimethyl ether (TEGDME) and 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5). Electrochemical analyses indicate that a conventional solution-phase Li-S reaction occurs in EA-SWNTs, which can be attributed to the smaller solvated [Li(TEGDME)]+ and [Li(15-crown-5)]+ ions within the EA-SWNT diameter. In stark contrast, the Li-S confined in narrower diameter HiPco-SWNTs exhibits unusual electrochemical behavior that can be attributed to a solid-state reaction enabled by the smaller HiPco-SWNT diameter compared to the size of solvated Li+ ions. Our results of the electrochemical analyses are corroborated and supported with various spectroscopic analyses including operando Raman, X-ray photoelectron spectroscopy, and first-principles calculations from density functional theory. Taken together, our findings demonstrate that the controlled solid-state lithiation-delithiation of sulfur and an enhanced electrochemical reactivity can be achieved by sub-nanoscale encapsulation and one-dimensional confinement in narrow-diameter SWNTs. Fil: Fu, Chengyin. University Of California Riverside; Estados Unidos Fil: Oviedo, María Belén. University Of California Riverside; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Zhu, Yihan. Zhejiang University Of Technology; China Fil: von Wald Cresce, Arthur. U. S. Army Research Laboratory; Estados Unidos Fil: Xu, Kang. U. S. Army Research Laboratory; Estados Unidos Fil: Li, Guanghui. University Of California Riverside; Estados Unidos Fil: Itkis, Mikhail E.. University Of California Riverside; Estados Unidos Fil: Haddon, Robert C.. University Of California Riverside; Estados Unidos Fil: Chi, Miaofang. Oak Ridge National Laboratory; Estados Unidos Fil: Han, Yu. King Abdullah University Of Science And Technology; Arabia Saudita Fil: Wong, Bryan M.. University Of California Riverside; Estados Unidos Fil: Guo, Juchen. University Of California Riverside; Estados Unidos

A novel approach is described for preparing Ga-promoted Pt particles for the dehydrogenation of light alkanes to alkenes. The modifying element, Ga, was introduced by transference from the support, a calcined Mg(Ga)(Al)O hydrotalcite-like material. Pt nanoparticles were dispersed onto the calcined Mg(Ga)(Al)O starting from an organometallic precursor, followed by reduction. The formation of PtGa alloy particles is dependent on reduction temperature. Reduction at 723 K produces mainly metallic Pt particles. The average diameter of the Pt nanoparticles increased from 1.4 nm to 2.2 nm with increasing Ga content, and decreasing Al content of the support, demonstrating the importance of support Al atoms in stabilizing the dispersion of Pt. After reduction at 773–873 K, PtGa alloys were observed. It is proposed that at high reduction temperatures, H atoms formed on the surface of the metal particles spill over onto the support where they reduce Ga3+ cation to atomic Ga, which then interacts with the supported Pt to form PtGa alloys. The activity, selectivity and stability of Pt/Mg(Ga)(Al)O catalysts for ethane and propane dehydrogenation are described in the second part of this study (G. Siddiqi, P. Sun, V. Galvita, A.T. Bell, Journal of catalysis (2010), doi:10.1016/j.jcat.2010.06.016 [40] ).

Silica is one of the most widely used catalyst supports for metal nanocatalysts. Although the sintering of metal nanoparticles on various silica supports has been extensively studied, the restructuring of silica supports and its effect on supported metal nanoparticles have been seldom investigated. In this paper, silica-supported gold catalysts were used as a model system to probe the interplay of silica supports and metal nanoparticles under high-temperature treatment conditions. Gold was loaded onto mesoporous SiO 2 (SBA-15) using Au(en) 2 Cl 3 as the precursor in the presence of aqueous NaOH (pH ∼ 10). The influence of high-temperature treatment on the textural and structural changes of SBA-15 and Au/SBA-15 was studied by X-ray diffraction (XRD), N 2 adsorption–desorption, and transmission electron microscopy (TEM). Control experiments were conducted using an amorphous SiO 2 (Cab-O-Sil) as the support. It was found that SBA-15 undergoes significant phase transformation to crystalline cristobalite upon high-temperature treatment, resulting in the dramatic decrease in surface area. More interestingly, the crystallization of SiO 2 leads to the encapsulation of gold nanoparticles inside the SiO 2 matrix. This conclusion was proven by aqua regia leaching, EDX, and SEM/TEM experiments. Gold nanoparticles can also be encapsulated into the SiO 2 matrix when using Cab-O-Sil as the support, but the process takes place under much higher temperatures. The encapsulation of gold nanoparticles can be mitigated by coating Au/SBA-15 with amorphous Al 2 O 3 or by coating SBA-15 with Al 2 O 3 before loading gold. Our findings shed new light on the deactivation of supported gold catalysts under high-temperature conditions.

Dodecanethiol-protected gold nanoparticles were deposited onto fumed SiO2 (Cab-O-Sil) via colloidal deposition. The catalyst was treated with a strongly oxidative KMnO4 or K2MnO4 solution. Low-temperature conversion in catalytic CO oxidation increased dramatically following the oxidative treatment and subsequent thermal activation at 300 °C to burn off residual organic species. On the other hand, the treatment with Fenton’s reagent did not lead to any positive effect. The influences of the average sizes of pre-synthesized gold particles (1.8, 2.1, 3.9, 9.9 nm) and the choice of different supports (SiO2, TiO2, C) were investigated, and relevant characterization using TG/DTG, XRD, TEM, EDX, and HAADF was conducted. The catalyst stability as a function of time on stream was also surveyed. This work establishes the beneficial effect of treating dodecanethiol-capped gold catalysts by KMnO4 or K2MnO4. Dodecanethiol-capped gold nanoparticles were deposited onto SiO2, TiO2, or carbon support, and the resulting catalysts were treated by aqueous KMnO4 or K2MnO4 followed by treatment in O2–He at 300 °C. The catalytic activity in low-temperature CO oxidation was increased due to the creation of Au–MnO x interfaces.

Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1 and occurs at, or above, 1000 K, making them unfavorable especially for intermediate temperature applications. In this report, the Zintl-Klemm rule is demonstrated to be valid for Ni, Cu, and Zn transition metal substitution in the framework of type I clathrates and offers many degrees of freedom for material modification, design, and optimization. The cross-substitution of framework elements introduces ionized impurities and lattice defects into these materials, which optimize the scattering of charge carriers by the substitution-induced ionized impurities and the scattering of heat-carrying lattice phonons by point defects, respectively, leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the bandgap of these materials can be tuned between 0.1 and 0.5 eV by adjusting the cross-substitution ratio of framework elements, making it possible to design clathrates with excellent thermoelectric properties between 500 and 1000 K.

The minor phase accumulated at intergranular regions in ceramics is usually associated with the global doping level of additives; however, such simplification often undermines the local variation in liquid composition that leads to the emergence of precipitation. In this paper the correlation between precipitation and intergranular liquid was investigated in a TiO2-doped Al2O3 ceramic system. The discrepancy between XRD and SEM in detection of the intergranular Al2TiO5 phase was rationalized by their difference in sensitivity that corresponds to a change from the casual to general mode of local liquid variation. Analytical TEM in addition revealed Ti-rich nanoprecipitates aligning along grain boundary (GB). The two intergranular phases were associated with separate processes of intergranular liquid, as clarified via a representative case that exhibited the extra grain growth from annealing. The nanoprecipitates were created from a binary liquid in local equilibration with segregation to GB, which served as transporting routes to form ternary liquid at the multigrain junctions to initiate the main intergranular phase Al2TiO5.