D. Billaud

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.

Electrochemical synthesis of binary graphite-lithium intercalation compounds

Abstract Electrochemical intercalation of lithium has been carried out using a cathode of pyrographite in a LiClO 4 -ethylene carbonate (EC) electrolyte. Charge and discharge curves present slopes and plateaus related to the existence of pure phases and biphasic systems respectively. Binary graphite-lithium intercalation compounds of stages I, II, III and IV were isolated and identified based on their (00 l ) X-ray diffraction diagrams. Our experimental conditions allow the obtention of compounds which do not contain coinserted solvent molecules, even for low stage materials.

Electrochemical insertion of sodium in pitch-based carbon fibres in comparison with graphite in NaClO4–ethylene carbonate electrolyte

The electrochemical intercalation of sodium ions into pitch carbon fibres and natural graphite UF4 was studied using an electrolyte composed of ethylene carbonate as the solvent and NaClO4 as the salt. The reversible electrochemical capacity for a current equal to 7 μA mg−1 is negligible for graphite, a little bit higher for P100 fibres (28 mAh g−1) and reaches 55 mAh g−1 for P75 fibres. The sodium uptake can be increased by grinding our materials: the reversible capacity is then around 60 and 83 mAh g−1 for ground P100 and P75 carbon fibres, respectively. During the first reduction, a great part of the current is used for the formation of a passivating layer on the carbon surface. Analysis of this layer by transmission electron microscopy (TEM) has shown that it is composed of sodium carbonate Na2CO3 and alkylcarbonates ROCO2Na. Formation of Na2CO3 occurs at potentials in the 1–0.8 V range versus Na+/Na, and formation of sodium alkylcarbonates then follows at potentials below 0.8 V. TEM also revealed the presence of sodium chloride, attributed to NaClO4 reduction.

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.

New results about the sodium-graphite system

Abstract For a long time, it was accepted that the intercalation of sodium in graphite was possible only in the presence of impurities; the best compound obtained at high temperature was described by Asher; it belongs to the eighth stage with a formula NaC64. Based on new considerations, we tried to prepare lower stages. It is possible to obtain small amounts of stage four mixed with unreacted graphite. Pure sixth and seventh stages were prepared. The 00l reflections analysis allow the determination of the interplanar distance between two carbon layers surrounding a sodium plane: 4.52 A. A study of the influence of the temperature of preparation shows that the lowest stages are obtained at very low temperature, apparently in the absence of impurities.

Synthesis and characterization of new organic metals formed by interaction of FeCl3 with polyacetylene (CH)x and poly(para)phenylene (C6H4)x

Abstract The reaction of FeCl 3 dissolved in dry nitromethane with polyacetylene, (CH) x , results in the formation of p-type conducting polymers ( σ = 780 ohm −1 cm −1 ). IR spectra of (CH) x lightly doped with FeCl 3 exhibit the formation of two new bands characteristic of other p-type dopants of (CH) x . Mossbauer spectroscopy shows that the anion formed in the reaction is a high spin Fe II complex. The doping causes significant change in (CH) x interchain distances as evidenced by X-ray diffraction. Similar reaction occurs between poly(para)phenylene, (C 6 H 4 ) x and FeCl 3 causing the increase of the conductivity of compressed poly(para)phenylene powder to metallic regime. The reaction mechanism is more complex than in the case of (CH) x since Mossbauer spectroscopy shows the existence of two types of Fe II iron ions.

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.

Polyindole and poly(5-cyanoindole) : electrochemical and FT-IR spectroscopic comparative studies

Abstract Indole (C 8 NH 7 ) and 5-cyanoindole (C 9 N 2 H 6 ) can be electropolymerized in the presence of ClO 4 − anions. The electrochemical behaviour of both polymers is discussed. FT-IR studies of their doped and undoped forms agree with a 2,3 type structure of regular, statistical or random conformation

Electrochemical oxidation of (CH)x in sulphuric acid

Abstract Polyacetylene reacts with pure and water- or nitromethane-diluted sulfuric acid. Two reversible doping steps have been evidenced by cyclic voltammetry techniques. A preliminary EPR study effected on heavily-doped samples shows the metallic character of these materials.

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.