Eggshell‐Assisted Preparation of Mixed Nanoparticles Simultaneously Encapsulated in Carbon Microcubes for Reversible Lithium Storage

Nanoparticle‐based electrodes often suffer from poor electrical properties due to high interparticle resistance, as well as low Coulombic efficiency attributed to large surface area induced parasitic reactions. In order to address this issue, a strategy of encapsulating two kinds of nanoparticles of both metal oxide and metallic nanoparticles is attempted, simultaneously, in microscale carbon cubic shells for highly reversible lithium storage. The unique structure is synthesized by simultaneous reactions of (1) decomposition of crystalline Co₂(OH)₃Cl microparticle precursor, synthesized in unique eggshell reactor systems, into nanoparticles, (2) partial reduction of CoO into metallic Co by eggshell membrane, (3) carbon coating by chemical vapor deposition facilitated by presence of catalytic Co with carbon released from the eggshell membrane, and (4) microscale carbon shell formed using the Co₂(OH)₃Cl particles as microtemplates. The carbon shells can prevent the encapsulated mixed nanoparticles from direct contact with electrolyte and reduce undesirable parasitic reactions, and accommodate volumetric variation during cycling. The introduction of metallic Co nanoparticles can reduce interparticle resistance. When evaluated for lithium storage, the unique structures of CoO–Co@C demonstrate superior electrochemical performances in terms of electrode stability and rate performance, as compared to that of pure CoO.     

Combustion-formed nanoparticles

The processes by which carbonaceous nanoparticles are produced from combustion of liquid and gaseous fuels are reviewed. The focus of the paper is on the formation and properties of nanoparticles in laboratory laminar, premixed and diffusion flames and on the most popular methods of sampling and detection of these particles. Particle chemical nature is analyzed from data obtained by several measurement techniques. Measurements characterizing nanoparticles in the exhausts of practical combustion systems such as engines and commercial burners are also reported. Two classes of carbonaceous material are mainly formed in combustion: nanoparticles with sizes in the range 1-5 nm, and soot particles, with sizes from 10 to 100 nm. Nanoparticles show unique chemical composition and morphology; they maintain molecular characteristics in terms of chemical reactivity, but at the same time exhibit transport and surface related phenomena typical of particles. The emission of these particles contributes to atmospheric pollution and constitutes a serious health concern. A simplified modeling analysis is used to show how the growth of aromatics and the chemical nature of the particles depend on temperature and radical concentration distributions encountered in flames.     

Nanometric Antimony Powder Synthesis by Activated Alkaline Hydride Reduction of Antimony Pentachloride

A novel chemical reduction method using an activated alkaline hydride (LiH or NaH-t-BuONa) in tetrahydrofuran solvent has been applied to antimony salt reduction. X-ray diffraction and transmission electron microscopy studies have been carried out to characterize the morphology and structure of the materials. Alkali hydride nature influence has been proved. In both cases the process allows to prepare antimony particles in nanometer range from few nanometers to about 20nm which could be used as anodic materials for lithium–ion batteries. With lithium hydride well-crystallized particles inclined to agglomeration were observed whereas finely dispersed amorphous particles were pointing out after activated sodium hydride reduction.     

Chemical Reactivities of Magnesium Nanopowders

The chemical reactivities of two forms of superfine magnesium powders differing in particle size are compared with commercial magnesium powder for nitridation, hydrogenation and Grignard reactions. The chemical reactivities of these magnesium powders are approximately proportional to their specific surface areas or their reciprocal particle diameters. Since the magnesium nanopowders give a specific surface area around three orders of magnitude larger than that of the commercial ones, they can undergo reactions under mild conditions at which the extent of reaction of the commercial magnesium powder is too small to be measured.     

Composition dependence on dispersion of Au/Cu bimetallic nanoparticles in nylon 11 thin films

Effect of composition on the dispersion of Au_xCu_1-x bimetallic nanoparticles into nylon 11 matrix has been investigated. TEM, EDX, and XPS depth profiling were used for characterizing the changes in the composition of the bimetallic particles and in the depth distribution of the particles in the nylon 11 layer caused by heat treatment in N₂ atmosphere. The island-like bimetallic particles were found to be formed on the nylon 11 surface before heat treatment. The results of XPS depth profiling revealed that, by the heat treatment, the Au_xCu_1-x bimetallic particles with x? 0.55 were not dispersed into the nylon 11 layer while those with x≥ 0.70 were homogeneously dispersed in the films, indicating the existence of critical composition for penetration of the bimetallic particles. By comparing the composition and structure of the bimetallic particles, the cause of these finding is discussed in terms of surface free energy of the particles. Received 29 November 2000     

Particle size dependence of the superconductivity and ferromagnetism in YBCO nanoparticles

Magnetic properties of superconducting yttrium barium copper oxide (YBa₂Cu₃O_7-δ) nanoparticles (31–43 nm) prepared by a chemical route have been studied. These nanoparticles have been found to clearly exhibit ferromagnetism at room temperature while superconducting transition is observed at lower temperatures. The low temperature hysteresis loops show evidence suggesting the presence of a large paramagnetic contribution in addition to the superconducting contributions from the particles. Bulk YBCO obtained by pelletizing and heating the same nanoparticles at a high temperature, displays the usual superconducting characteristics and gives no trace of ferromagnetism down to 10 K. The superconducting transition temperature of the nanoparticles is lower than for the bulk YBCO and there is a trend of decreasing T _c with smaller size of the particles. In contrast the ferromagnetic moment increases with decreasing particle size. The development of ferromagnetism is attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. The simultaneous decrease of superconducting T _c and the increase of ferromagnetism with decreasing size considered as being reflective of the increased role of finite size and surface defects that weaken the superconductivity and enhance the ferromagnetism. Possible coexistence of surface ferromagnetism and bulk superconductivity at lower temperatures is discussed.     

Room‐Temperature Synthesis of High Surface Area Anatase TiO2 Exhibiting a Complete Lithium Insertion Solid Solution

The synthesis of highly divided anatase TiO₂ nanoparticles displaying 300 m² g^?1 surface area is achieved by following a two‐step synthetic process at room temperature. The particles exhibit a needle‐like morphology composed of self‐assembled 4 nm nanoparticles. The crystallization process from amorphous TiO₂.1.6H₂O to oriented aggregation of anatase TiO₂ proceeds according to a slow solid dehydration process taking place in a large range of pH in deionized water (1 < pH < 12) or alternatively when including a low amount of NH₄F_(aq) in solution. Driven by their high surface area enhancing the chemical/electrochemical reactivity, it is reported in the case of the anatase TiO₂ that a modification in the lithium insertion mechanism is no longer attributable to a two‐phase reaction between the two‐end members Li_εTiO₂ and Li_0.5±αTiO₂ when downsizing the particle size, but instead through a complete solid solution all along the composition range.     

Electrical resistivity of alkali metals

The temperature variations of the electrical resistivity of alkali metals lithium, sodium, potassium, rubidium and caesium are calculated in the free electron approximation and using Krebs's model for the phonon spectrum. The double average over the Fermi surface is evaluated by an improved method due toBailyn and the separation between normal and Umklapp processes is affected in a more satisfactory manner. The results of the calculations are compared with the experimental data obtained at different temperatures. The theoretical resistivity curves for sodium, potassium, rubidium and caesium show satisfactory agreement with experiment, but not in the case of lithium.     

Decomposition of lithium amide and imide films on nickel

Thin films of lithium hydride, lithium amide and lithium imide were grown from lithium and ammonia under controlled conditions in an ultra high vacuum chamber. By making thin films instead of bulk or powder samples, it was possible to study the stability and the release of hydrogen without influence of transport phenomena. Surprisingly, lithium amide and lithium imide were seen to decompose at higher temperatures than lithium hydride. Furthermore, it was seen that hydrogen reversibly could be removed by heat treatment and subsequently refilled.     

Improvement of photocatalytic behavior of chemical-vapor-synthesized TiO2 nanopowders by post-heat treatment

Anatase-type TiO₂ nanopowders less than 10 nm in average diameter were synthesized by a chemical vapor synthesis method. The TiO₂ nanopowders showed very poor photocatalytic properties, in spite of their large surface area. With subsequent heat treatment of the TiO₂ powders, their photocatalytic properties determined by measuring the degradation of 2-propanol were improved at temperatures up to 600 °C and then diminished along with formation of a rutile phase. This improvement in the photocatalytic properties of TiO₂ nanopowders was attributed to both a morphology change and a change in the electronic surface characteristics of TiO₂ particles during heat treatment.     

Nanoscale Iron Particles for Environmental Remediation: An Overview

Nanoscale iron particles represent a new generation of environmental remediation technologies that could provide cost-effective solutions to some of the most challenging environmental cleanup problems. Nanoscale iron particles have large surface areas and high surface reactivity. Equally important, they provide enormous flexibility for in situ applications. Research has shown that nanoscale iron particles are very effective for the transformation and detoxification of a wide variety of common environmental contaminants, such as chlorinated organic solvents, organochlorine pesticides, and PCBs. Modified iron nanoparticles, such as catalyzed and supported nanoparticles have been synthesized to further enhance the speed and efficiency of remediation. In this paper, recent developments in both laboratory and pilot studies are assessed, including: (1) synthesis of nanoscale iron particles (10–100nm, >99.5% Fe) from common precursors such as Fe(II) and Fe(III); (2) reactivity of the nanoparticles towards contaminants in soil and water over extended periods of time (e.g., weeks); (3) field tests validating the injection of nanoparticles into aquifer, and (4) in situ reactions of the nanoparticles in the subsurface.     

亚纳米厚的氧化铝包裹层可以显著提高负载型金纳米颗粒催化剂的热稳定性

负载型金纳米颗粒催化剂在许多催化反应中展现出非常好的催化活性,但是金纳米颗粒在高温等反应条件下容易烧结团聚,极大地限制了金催化剂的应用。利用原子层沉积技术在Au/TiO₂催化剂表面分别精确沉积了一层超薄的二氧化钛和氧化铝包裹层,并对比研究了包裹层对金纳米颗粒的热稳定性影响。原位红外漫反射CO吸附和x-射线光电子能谱数据证实了氧化物包裹层的存在。发现亚纳米厚的氧化铝包裹层能够在600 C完全避免金纳米颗粒的团聚;相反,二氧化钛包裹层对金纳米颗粒稳定性的提高没有明显效果。通过CO氧化探针反应的活性测试,发现随着煅烧温度的升高氧化铝包裹的Au/TiO₂ 催化剂的活性逐渐提高,表明高温处理可以促进被包裹金原子的暴露并表现出催化活性。提供了提高金纳米颗粒稳定性的有效方法,为拓展金催化剂在条件苛刻的反应中的应用奠定了技术基础.     

Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction

Pt-transition metal alloy catalysts with an active Pt surface have exceptional properties for use in oxygen electro-reduction reactions in fuel cells. Herein, we report the simple synthesis of dealloyed PtCu catalysts and their catalytic performance in oxygen reduction. The dealloyed PtCu catalysts consisted of a Pt-enriched shell with a Pt–Cu alloy core and were synthesized through a chemical co-reduction process followed by thermal annealing and chemical dealloying. During synthesis, thermal annealing leads to a high degree of formation of PtCu alloy particles (e.g., PtCu or PtCu₃), and chemical dealloying causes selective dissolution of unstable Cu species from the surface layers of the PtCu alloy particles, resulting in a PtCu alloy@Pt-enriched surface core–shell configuration. Our PtCu₃/C catalyst exhibits a great improvement in the oxygen reduction reaction with a mass activity of 0.501 A/mg_Pt, which is 2.24 times greater than that of a commercial Pt catalyst. In this article, the synthesis details, characteristics and performance improvements in ORR of chemically dealloyed PtCu catalysts are systemically explained.     

Thermal reactivity of three lithiated carbonaceous materials

The thermal stabilities of hard carbon spherule (HCS), artificial graphite (AG), and natural graphite (NG) were investigated by thermo-gravimetric differential scanning calorimetry (TG-DSC). After lithiation, AG shows the lowest onset exothermic temperature. However, all there materials exhibit similar onset temperatures for thermal reactions after ten cycles. It is obvious that the thermal behaviors of solid electrolyte interphase (SEI) film for HCS and AG change gradually with the electrochemical cycling. In contrast, the thermal stability of the surface film on NG is maintained during repeated lithium ion insertion/extraction. Because of their different Li storage behaviors, their thermal reactivities with electrolyte are quite different from each other. Especially for HCS, it shows several successive and different exothermic peaks at the 1st and 11th lithiated states, while both AG and NG display similar thermal reactivity before and after repeated cycles. In summary, it is found that thermal properties of SEI layer and lithium in lithiated carbonaceous materials for all three samples have different impacts on the whole thermal behaviors of electrode.     

Fabrication and characterization of thermo-sensitive magnetic polymer composite nanoparticles

Novel dual-functional nanospheres composed of Fe₃O₄ nanoparticles embedded in a thermo-sensitive polymer were synthesized by emulsifier-free emulsion polymerization. The Fe₃O₄ nanoparticles were prepared by chemical precipitation. The surface of these particles was modified by oleic acid to achieve stability against agglomeration. These stable particles were then polymerized using N-isopropylacrylamide as the main monomer, divinylbenzene as the crosslinker and potassium persulfate as the initiator. The nanospheres were characterized by Fourier-transform infrared spectrum, transmission electron microscopy, thermogravimetric analysis, vibrating sample magnetometer and dynamic light scattering. The results show that the lower critical solution temperature of thermo-sensitive magnetic immunomicrospheres was between 40 and 45 °C.     

Preparation of Reinforced Anisometric Patchy Supraparticles for Self-Propulsion

The preparation of fumed silica-based anisometric supraparticles with well-defined catalytically active patches suitable for self-propulsion is presented here. These sub-millimeter-sized particles can self-propel as they contain Pt-covered magnetite (Fe₃O₄) nanoparticles, where the Pt can decompose catalytically a “fuel” like H₂O₂ and thereby propel the supraparticles. By their magnetic properties, the catalytically active nanoparticles can be concentrated in patches on the supraparticle surface. The goal is to obtain robust supraparticles with well-defined patchiness and long-time stability during self-propulsion through evaporation-induced self-assembly (EISA) on a superhydrophobic surface. The latter is a major issue as oxygen evolution can lead to the disintegration of the supraparticles. Therefore, enhanced mechanical stability is sought using a number of different additives, where the best results are obtained by incorporating polystyrene microspheres followed by heat treatment or reinforcement with microfibrillated cellulose (MFC) and sodium trisilicate (Na₂SiO₃). The detailed internal structure of the different types of particles is investigated by confocal micro-X-ray fluorescence spectroscopy (CMXRF), which allows for precisely locating the catalytic Fe₃O₄@Pt nanoparticles within the supraparticles with a resolution in the µm range. The insights on the supraparticle structure, together with their long-time stability, allow fabricating optimized patchy supraparticles for potential applications in propulsion-enhanced catalysis.     

Artificial solid electrolyte interphase based on polyacrylonitrile for homogenous and dendrite-free deposition of lithium metal

High chemical reactivity, large volume changes, and uncontrollable lithium dendrite growth have always been the key problems of lithium metal anodes.Coating has been demonstrated as an effective strategy to protect the lithium metal.In this work, the effects of polyacrylonitrile(PAN)-based coatings on electrodeposited lithium have been studied.Our results show that a PAN coating layer provides uniform and dendrite-free lithium deposition as well as better cycling performance with carbonate electrolyte.Notably, heat treatment of the PAN coating layer promotes the formation of larger deposit particle size and higher coulombic efficiency(85%).The compact coating layer of heat-treated PAN with a large Young modulus(82.7 GPa) may provide stable protection for the active lithium.Improved homogeneity of morphology and mechanical properties of heat-treated PAN contribute to the larger deposit particles.This work provides new feasibility to optimize the polymer coating through rational modification of polymers.     

Thermal processing and native oxidation of silicon nanoparticles

In this study, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) were used to investigate in-air oxidation of silicon nanoparticles ca. 11 nm in diameter. Particle samples were prepared first by extracting them from an RF plasma synthesis reactor, and then heating them in an inert carrier gas stream. The resulting particles had varying surface hydrogen coverages and relative amounts of SiH _x (x = 1, 2, and 3), depending on the temperature to which they had been heated. The particles were allowed to oxidize in-air for several weeks. FTIR, XPS, and EELS analyses that were performed during this period clearly establish that adsorbed hydrogen retards oxidation, although in complex ways. In particular, particles that have been heated to intermediate hydrogen coverages oxidize more slowly in air than do freshly generated particles that have a much higher hydrogen content. In addition, the loss of surface hydride species at high processing temperatures results in fast initial oxidation and the formation of a self-limiting oxide layer. Analogous measurements made on deuterium-covered particles show broadly similar behavior; i.e., that oxidation is the slowest at some intermediate coverage of adsorbed deuterium.     

Behaviour of heat treated Co-Cr2O3 electrodeposited surfaces under tribological conditions

Surface mechanical properties could be altered if a coating with a second phase of fine tough particles is given. There is increasing demand for the wear resistant composite coatings to minimize the surface damage at elevated temperatures also. Cobalt depositions with Cr₂O₃ in the matrix have been studied for their surface performance at higher temperatures. Effect of heat treatment was studied from the view point of operating performance against abrasive wear. Surface microhardness and material loss due to rubbing abrasion after heat treatment in the range of 100 to 650 °C for a constant time are reported. The composite containing 14% by volume of Cr₂O₃ in Co film heat treated at 550 to 650 °C was found to exhibit high abrasive resistance due to the formation of a glass like layer. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Channai, India, Nov. 28–30, 2003.     

Surface modification of permalloy (Ni<Subscript>80</Subscript>Fe<Subscript>20</Subscript>) nanoparticles for biomedical applications

We report a simple and novel method for surface biofunctionalization onto recently reported Ni₈₀Fe₂₀ permalloy nanoparticles (~71 nm) and the immobilization of a model protein, IgG from human serum. The strategy of protein immobilization involved attachment of histidine-tagged streptavidin to the Ni₈₀Fe₂₀ nanoparticles via a non-covalent ligand binding followed by biotinylated human IgG binding on the nanoparticle surface using the specific high affinity avidin–biotin interaction. The biofunctionalization of Ni₈₀Fe₂₀ permalloy nanoparticles was confirmed by Fourier Transform InfraRed (FTIR) spectroscopy and protein denaturing gel electrophoresis (lithium dodecyl sulfate-polyacrylamide gel electrophoresis, LDS-PAGE). This protocol for surface functionalization of the novel nanometer-sized Ni₈₀Fe₂₀ permalloy particles with biological molecules could open diverse applications in disease diagnostics and drug delivery.