Jaafar Ghanbaja

Electroreduction of graphite in LiClO4-ethylene carbonate electrolyte. Characterization of the passivating layer by transmission electron microscopy and Fourier-transform infrared spectroscopy

Abstract Electrochemical intercalation of unsolvated lithium into pitch carbon fibres P100 and natural graphite UF4 has been carried out in LiClO4-ethylene carbonate electrolyte. The reversible electrochemical capacity for a current equal to 7 μA/mg is 260 mAh/g for P100 carbon fibres and about 350 mAh/g for UF4 graphite, respectively. During the first discharge (reduction) an electrochemical capacity greater than the theoretical value (372 mAh/g) corresponding to LiC6 is obtained. This excess of capacity can be related to the formation of a passivating layer on the carbon surface. Analysis of this layer by means of transmission electron microscopy (electron diffraction, electron energy loss spectroscopy, and imaging) and Fourier-transform infrared spectroscopy has shown that this layer is composed of lithium carbonate Li2CO3 and alkylcarbonates of lithium ROCO2Li. Formation of Li2CO3 occurs at potentials in the 1−0.8 V range versus Li + Li , and formation of lithium alkylcarbonates then follows at potentials below 0.8 V. We then attributed the voltage plateau at 0.9 V versus Li + Li observed in the electrochemical waves to the reduction of ethylene carbonate into Li2CO3. Transmission electron spectroscopy revealed the presence of lithium chloride in the electrolyte which appears as small rods.

Water-Based Route to Colloidal Mn-Doped ZnSe and Core/Shell ZnSe/ZnS Quantum Dots

Relatively monodisperse and highly luminescent Mn(2+)-doped zinc blende ZnSe nanocrystals were synthesized in aqueous solution at 100 °C using the nucleation-doping strategy. The effects of the experimental conditions and of the ligand on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of molar ratio of precursors and heating time on the optical properties of ZnSe:Mn nanocrystals. Using 3-mercaptopropionic acid as capping ligand afforded 3.1 nm wide ZnSe:Mn quantum dots (QDs) with very low surface defect density and which exhibited the Mn(2+)-related orange luminescence. The post-preparative introduction of a ZnS shell at the surface of the Mn(2+)-doped ZnSe QDs improved their photoluminescence properties, resulting in stronger emission. A 2.5-fold increase in photoluminescence quantum yield (from 3.5 to 9%) and of Mn(2+) ion emission lifetime (from 0.62 to 1.39 ms) have been observed after surface passivation. The size and the structure of these QDs were also corroborated by using transmission electron microscopy, energy dispersive spectroscopy, and X-ray powder diffraction.

One-Pot Noninjection Route to CdS Quantum Dots via Hydrothermal Synthesis

Water-dispersible CdS quantum dots (QDs) emitting from 510 to 650 nm were synthesized in a simple one-pot noninjection hydrothermal route using cadmium chloride, thiourea, and 3-mercaptopropionic acid (MPA) as starting materials. All these chemicals were loaded at room temperature in a Teflon sealed tube and the reaction mixture heated at 100 °C. The effects of CdCl(2)/thiourea/MPA feed molar ratios, pH, and concentrations of precursors affecting the growth of the CdS QDs, was monitored via the temporal evolution of the optical properties of the CdS nanocrystals. High concentration of precursors and high MPA/Cd feed molar ratios were found to lead to an increase in the diameter of the resulting CdS nanocrystals and of the trap state emission of the dots. The combination of moderate pH value, low concentration of precursors and slow growth rate plays the crucial role in the good optical properties of the obtained CdS nanocrystals. The highest photoluminescence achieved for CdS@MPA QDs of average size 3.5 nm was 20%. As prepared colloids show rather narrow particle size distribution, although all reactants were mixed at room temperature. CdS@MPA QDs were characterized by UV-vis and photoluminescence spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectrometry and MALDI TOF mass spectrometry. This noninjection one-pot approach features easy handling and large-scale production with excellent synthetic reproducibility. Surface passivation of CdS@MPA cores by a wider bandgap material, ZnS, led to enhanced luminescence intensity. CdS@MPA and CdS/ZnS@MPA QDs exhibit high photochemical stability and hold a good potential to be applied in optoelectronic devices and biological applications.

First direct evidence of size-dependent structural transition in nanosized nickel particles

Abstract Finely dispersed Ni powders were obtained after chemical reduction of Ni(II) salts by activated sodium hydride in organic solvents. This process allows one to prepare particles in the nanometre range. The particles with sizes smaller than 4nm were found to be hcp whereas larger particles were fcc. To the best of our knowledge, this is the first observation that unsupported Ni powder may crystallize in the hcp system and that the phase stability of this metallic element is particle size dependent.

Electrochemically assisted generation of highly ordered azide-functionalized mesoporous silica for oriented hybrid films.

One key challenge in inorganic mesoporous films is the development of oriented mesostructures with vertical channels, and even more challenging is their functionalization while maintaining accessible the selected surface groups. Combining the electrochemically assisted deposition of ordered and oriented azide-functionalized mesoporous silica with alkyne-azide click chemistry enables such nanostructured and vertically aligned hybrid films to be obtained with significant amounts of active organic functional groups, as illustrated for ferrocene and pyridine functions. A good level of mesostructural order was obtained, namely up to 40% of organosilane in the starting sol. The method could be applied to a wide variety of functional groups, thus offering numerous new opportunities for applications in various fields.

Lithium insertion into new graphite–antimony composites

Metal-based composites are under investigation as possible negative-electrode materials in lithium-ion batteries. In this paper, we present a new composite material constituted of antimony particles dispersed on graphite. The antimony–graphite compound is prepared by antimony pentachloride reduction with KC8 in tetrahydrofuran. The high reversible capacity of 420 mAh g−1 and the good stability suggest that the association of antimony with graphite allows not only to improve reversible capacity but also to prevent the metal from particle pulverisation generally occurring during lithium alloying.

Sources, nature, and fate of heavy metal-bearing particles in the sewer system.

A preliminary insight into metal cycling within the urban sewer was obtained by determining both the heavy metal concentrations (Cu, Zn, Pb, Cd, Ni, Cr) in sewage and sediments, and the nature of metal-bearing particles using TEM-EDX, SEM-EDX and XRD. Particles collected from tap water, sump-pit deposits, and washbasin siphons, were also examined to trace back the origin of some mineral species. The results show that the total levels in Cu, Pb, Zn, Ni, and Cr in sewage are similar to that reported in the literature, thus suggesting that a time-averaged heavy metal fingerprint of domestic sewage can be defined for most developed cities at the urban catchment scale. Household activities represent the main source of Zn and Pb, the water supply system is a significant source of Cu, and in our case, groundwater infiltration in the sewer system provides a supplementary source of Ni and Cd. Concentrations in heavy metals were much higher in sewer sediments than in sewage suspended solids, the enrichment being due to the preferential settling of metal-bearing particles of high density and/or the precipitation of neoformed mineral phases. TEM and SEM-EDX analyses indicated that suspended solids, biofilms, and sewer sediments contained similar heavy metal-bearing particles including alloys and metal fragments, oxidized metals and sulfides. Copper fragments, metal carbonates (Cu, Zn, Pb), and oxidized soldering materials are released from the erosion of domestic plumbing, whereas the precipitation of sulfides and the sulfurization of metal phases occur primarily within the household connections to the sewer trunk. Close examination of sulfide phases also revealed in most cases a complex growth history recorded in the texture of particles, which likely reflects changes in physicochemical conditions associated with successive resuspension and settling of particles within the sewer system.

A new synthesis of ultrafine nanometre-sized bismuth particles

A new synthesis of Bi(0) nanoparticles is reported. A low temperature solution phase reduction of BiCl3 with t-BuONa activated sodium hydride at 65 °C has been successfully used to prepare large quantities of colloidal Bi(0) nanoparticles with a diameter in the range 1.8–3.0 nm. The resulting Bi nanoparticles were characterized using transmission electron microscopy, XPS analysis and x-ray powder diffraction.

New halogenated additives to propylene carbonate-based electrolytes for lithium-ion batteries

Lithium cannot be electrointercalated into graphite in an electrolyte containing propylene carbonate (PC) as the only solvent species. In order to improve the cycleability of graphite electrodes in the presence of PC two methods were used: use of solvent mixtures containing PC and halogen-substituted solvent molecules (α-bromo-γ-butyrolactone and methyl chloroformate); impregnation of the graphite electrode by halogenated solvents prior to cycling in PC-based electrolytes. It appears that the reversible capacity is increased by ∼10% when such halogenated solvent molecules are used. Moreover, the cycleability is dependent on the nature of lithium salt, the concentration of halogen solvent and the specific current.

Influence of high temperature treatments on single-walled carbon nanotubes structure, morphology and surface properties

Single-walled carbon nanotubes produced by the MER Corporation have been treated in an argon atmosphere at temperatures between 1400 and 2400°C. The modifications resulting from the heat treatment have been followed by adsorption volumetry and TEM measurements. 1600°C appears as a turning point for the high temperature treatments. Heating nanotubes up to 1600°C essentially results in the removal of metal particles. At higher temperatures the changes in morphology are predominant, and single-walled nanotubes packed in bundles are transformed into multi-walled nanotubes.