Jafar F. Al-Sharab

Structure and Electrochemistry of Carbon-Metal Fluoride Nanocomposites Fabricated by Solid-State Redox Conversion Reaction

Utilizing a solid-state redox-driven conversion reaction enabled by mechanochemistry, conductive C:FeF 3 nanocomposites were fabricated from insulative CF 1 :FeF 2 precursors. All reactions were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The latter provided insights to the progression of the CF and C phases and the metal fluoride during the course of the reaction. Such nanocomposites resulted in a four order of magnitude increase in electrical conductivity and enabled excellent specific capacity approaching 500 mAh/g vs. Li with good reversibility, although at slow rates. Utilizing the theoretical basis of the technique, other couples were examined to experimentally isolate the oxidative power of CF 1 . In the process, we have also shown that a composite of CF 1 :CrF 2 can be easily converted to C:CrF 3 . The resulting nanocomposite exhibited a specific capacity of 682 mAh/g at an average voltage of approximately 1.9 V. The technique is also a powerful method for the fabrication of single phase metal fluoride solid solutions, as demonstrated with the fabrication of Cr 0 . 5 Fe 0 . 5 F 3 .

Catalytic reduction of p-nitrophenol over precious metals/highly ordered mesoporous silica

Precious metals, Au, Pt, and Pd, were successfully deposited on highly ordered mesoporous SBA-15. Two different reduction routes were employed to deposit precious metals: (1) under H2 and (2) using sodium citrate. Samples prepared using sodium citrate exhibit a uniform particle size of 10 nm while samples synthesized under H2 show high dispersion with a particle size of 8 and 20 nm for Pt and Pd, respectively. The surface area and pore volume of the mesoporous SBA-15 were significantly reduced due to the impregnation of the precious metals. Detailed TEM and XPS analyses reveal a uniform distribution of particles with a metallic valence state and no evidence of metallic oxides. The prepared catalysts were used to reduce p-nitrophenol (PNP) into p-aminophenol (PAP) where two trends were observed. The catalytic reduction efficiency for PNP reduction using the catalyst prepared with sodium citrate as a reducing agent is in the order of Au > Pt > Pd with a rate constant of 3.24 × 10−1 s−1 for Au/SBA-15. On the other hand, the catalyst prepared with H2 as a reducing agent showed a reverse trend Pd > Pt > Au with a rate constant of 7.15 × 10−1 s−1 for Pd/SBA-15. The highest catalyst efficiency was observed for the case of Pd/SBA-15 synthesized via the H2 route with a rate constant of 7.15 × 10−1 s−1. Also the reaction rate of Pd/SBA-15 synthesized via the H2 route was 2.2 times higher than that of Au/SBA-15 prepared using the sodium citrate route.

Investigation of the Lithiation and Delithiation Conversion Mechanisms of Bismuth Fluoride Nanocomposites

and LiF during lithiation and the reformation of BiF3 during delithiation. It has been shown that only the high-pressure tysonite phase of BiF3 reforms during the oxidation sweep and that no bismuth fluoride compound with an oxidation state of the bismuth lower than 3 is formed as intermediate during the lithiation or delithiation reactions. Finally, it has been demonstrated that the different plateaus or pseudo plateaus observed on the lithiation and delithiation voltage profiles stem from polarization changes brought about by the dramatic structural changes occurring in the nanocomposite upon cycling. A model, based on the variation of the electronic and ionic transport mechanisms as a function of the state of completion of the conversion and reconversion reactions, is proposed to explain those polarization changes.

Edge-Plane-Rich Nitrogen-Doped Carbon Nanoneedles and Efficient Metal-Free Electrocatalysts†

CNN news: N-doped carbon nanoneedles (CNNs) are synthesized by self-assembling core-shell nanostructures and nanoreactors around cellulose nanoneedles, and subsequent graphitization. The resulting graphitic nanoneedles (see picture) have well-organized graphitic multi-layers and large proportions of N-doped edge planes. The materials serve as efficient metal-free electrocatalysts for hydrazine oxidation.

EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries

A new type of positive electrode for Li-ion batteries has been developed recently based on FeF3/C and FeF2/C nanocomposites. The microstructural and redox evolution during discharge and recharge processes was followed by electron energy loss spectroscopy (EELS) to determine the valence state of Fe by measuring the Fe L3 line energy shift and from Fe L3/L2 line intensity ratios. In addition, transition metal fluorides were found to be electron beam sensitive, and the effect of beam exposure on EELS spectra was also investigated. The EELS results indicate that for both FeF3/C and FeF2/C nanocomposite systems, a complete reduction of iron to FeO is observed upon discharge to 1.5 V with the formation of a finer FeO/LiF subnanocomposite ( approximately 7 nm). Upon complete recharging to 4.5 V, EELS data reveal a reoxidation process to a Fe2+ state with the formation of a carbon metal fluoride nanocomposite related to the FeF2 structure.

A polyaniline based ultrasensitive potentiometric immunosensor for cardiac troponin complex detection.

An ultrasensitive immunosensor based on potentiometric ELISA for the detection of a cardiac biomarker, troponin I-T-C (Tn I-T-C) complex, was developed. The sensor fabrication involves typical sandwich ELISA procedures, while the final signal readout was achieved using open circuit potentiometry (OCP). Glassy carbon (GC) working electrodes were first coated with emulsion-polymerized polyaniline/dinonylnaphthalenesulfonic acid (PANI/DNNSA) and the coated surface was utilized as a transducer layer on which sandwich ELISA incubation steps were performed. An enzymatic reaction between o-phenylenediamine (OPD) and hydrogen peroxide (H2O2) was catalyzed by horseradish peroxidase (HRP) labeled on the secondary antibodies. The polymer transducer charged state was mediated through electron (e(-)) and charge transfers between the transducer and charged species generated by the same enzymatic reaction. Such a change in the polymer transducer led to potential variations against an Ag/AgCl reference electrode as a function of Tn I-T-C complex concentration during incubations. The sequence of OPD and H2O2 additions, electrochemical properties of the PANI/DNNSA layer and non-specific binding prevention were all crucial factors for the assay performance. Under optimized conditions, the assay has a low limit of detection (LOD) (< 5 pg/mL or 56 fM), a wide dynamic range (> 6 orders of magnitude), high repeatability (coefficient of variance < 8% for all concentrations higher than 5 pg/mL) and a short detection time (< 10 min).

Flame synthesis of aligned tungsten oxide nanowires

Aligned single-crystal WO2.9 nanowires are grown directly from tungsten substrates at high rates using a flame synthesis method. The nanowires have diameters of 20–50nm, lengths >10μm, coverage density of 109–1010cm−2, and growth rates >1μm∕min. Growth occurs by the vapor-solid mechanism, with local gas-phase temperature (∼1720K) and chemical species (O2, H2O, and H2) strategically specified at the substrate for self-synthesis. Advantages of this synthesis method are reduced processing times, absence of necessity for substrate pretreatment or catalysts, scalability for large-area surface coverage, high purity and yield of oriented nanowires, and continuous processing conditions.

Reductive Deprotection of Monolayer Protected Nanoclusters: An Efficient Route to Supported Ultrasmall Au Nanocatalysts for Selective Oxidation

Bulk gold has long been considered too inert to be a catalyst until the discovery that Au nanoparticles (AuNPs) supported on metal oxides such as TiO 2 , CeO 2 and Fe 2 O 3 could be very active for CO oxidation. [ 1 ] Supported Au and other NPs have now been successfully shown to catalyze various chemical reactions. [ 2 ] AuNP-catalyzed oxidation reactions, in particular, have attracted special attention because the reactions can lead to a range of important value-added oxygenated chemical products and pharmaceuticals, and also because oxidation (or epoxidation) of various alkenes, arenes and alcohols are proven to be effectively catalyzed by AuNPs. [ 3 ]

Stoichiometric, Morphological, and Electrochemical Impact of the Phase Stability of Li x CoO2

The effect of stoichiometry, heat-treatment, and resulting bulk and surface properties on the electrochemical cycling stability of native Li x CoO 2 under a 35% depth-of-discharge protocol was investigated. The materials were fabricated from mixtures of Li 2 CO 3 and Co 3 O 4 with Li/Co ratios spanning from 0.95 to 1.20. The single-phase stoichiometric sample exhibited the highest electrochemical performance under the applied protocol. A combination of X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and thermogravimetric (TGA) analyses revealed residual Co 3 O 4 and Li 2 CO 3 existed in the materials fabricated from all the nonstoichiometric mixtures with Li/Co 1, respectively. The use of TGA was found to be by far the most effective and sensitive tool for the detection and quantification of Li 2 CO 3 . Using various surface characterization techniques, we showed at least part of the residual Li 2 CO 3 phase forms a layer at the surface of the LiCoO 2 particles fabricated from lithium excess mixtures. The Li 1 + x CoO 2 samples, which cycle poorly at room temperature and exhibit bulk crystallographic properties that differ from stoichiometric LiCoO 2 , were found to cycle well after Li 2 CO 3 removal. The surface phase Li 2 CO 3 is the root of the poor room temperature cycling for the overstoichiometric Li 1 + x CoO 2 samples.

Combined Flame and Electrodeposition Synthesis of Energetic Coaxial Tungsten-Oxide/Aluminum Nanowire Arrays

A nanostructured thermite composite comprising an array of tungsten-oxide (WO2.9) nanowires (diameters of 20-50 nm and lengths of >10 μm) coated with single-crystal aluminum (thickness of ~16 nm) has been fabricated. The method involves combined flame synthesis of tungsten-oxide nanowires and ionic-liquid electrodeposition of aluminum. The geometry not only presents an avenue to tailor heat-release characteristics due to anisotropic arrangement of fuel and oxidizer but also eliminates or minimizes the presence of an interfacial Al2O3 passivation layer. Upon ignition, the energetic nanocomposite exhibits strong exothermicity, thereby being useful for fundamental study of aluminothermic reactions as well as enhancing combustion characteristics.