Michel Mermoux

FTIR and 13C NMR study of graphite oxide

From FTIR and 13C NMR studies, we determined that graphite oxide contains ether bridges, hydroxyl groups, and sp2 coordinated carbons. 13C NMR studies of the compound at various degrees of oxidation enable us to propose a structure for the local arrangement of these structural components. This local structure would lead to an ideal graphite oxide with the composition C8O2(OH)2. We suggest that the limit composition C8O1.7(OH)1.7 observed in graphite oxide prepared from natural Madagascar graphite is due to structural defects of the pristine graphite.

Analysis of the structure of multi‐component carbon films by resonant Raman scattering

Diamond and carbonaceous films grown in a microwave assisted plasma reactor have been characterized by x‐ray diffraction, scanning electron microscopy, and Raman spectroscopy. This study is mainly focused on the identification of the different carbonaceous compounds which can coexist with the diamond depending on the synthesis parameters. Selective etching reactions and the excitation wavelength dependence of Raman cross sections of the different carbon species reveal that the broad and poorly structured Raman spectra in the 1100–1700 cm−1 region contain six components that arise from three different carbonaceous species: (i) one species is graphitic and is identified from the components at about 1350 and 1590 cm−1 which are resonantly enhanced as compared to the other components when the excitation energy is lowered from 3 to 2 eV. (ii) The second species is associated with the two broad bands at 1350 and 1550 cm−1 and is attributed to amorphous diamond‐like carbon (a‐C:H); both bands are resonantly enha...

Carboxylated nanodiamonds are neither cytotoxic nor genotoxic on liver, kidney, intestine and lung human cell lines

Abstract Although nanodiamonds (NDs) appear as one of the most promising nanocarbon materials available so far for biomedical applications, their risk for human health remains unknown. Our work was aimed at defining the cytotoxicity and genotoxicity of two sets of commercial carboxylated NDs with diameters below 20 and 100 nm, on six human cell lines chosen as representative of potential target organs: HepG2 and Hep3B (liver), Caki-1 and Hek-293 (kidney), HT29 (intestine) and A549 (lung). Cytotoxicity of NDs was assessed by measuring cell impedance (xCELLigence® system) and cell survival/death by flow cytometry while genotoxicity was assessed by γ-H2Ax foci detection, which is considered the most sensitive technique for studying DNA double-strand breaks. To validate and check the sensitivity of the techniques, aminated polystyrene nanobeads were used as positive control in all assays. Cell incorporation of NDs was also studied by flow cytometry and luminescent N–V center photoluminescence (confirmed by Raman microscopy), to ensure that nanoparticles entered the cells. Overall, we show that NDs effectively entered the cells but NDs do not induce any significant cytotoxic or genotoxic effects on the six cell lines up to an exposure dose of 250 µg/mL. Taken together these results strongly support the huge potential of NDs for human nanomedicine but also their potential as negative control in nanotoxicology studies.

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)2O3 initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.

Effect of hydrogen on the orientation of carbon layers in deposits from the carbon monoxide disproportionation reaction over Co/Al2O3 catalysts

The solid product of CO disproportionation over well-calibrated Co/Al2O3 catalysts was studied by HRTEM, Raman spectroscopy and X-ray diffraction. In the case of pure Co or of a mixture of CO and CO2, the disproportionation reaction leads to the formation of carbon nanotubes where graphene layers are coaxial cylinders. When H2 is added to CO or to the CO/CO2 mixture, the production rate of carbon is higher. The solid products are made of carbon filaments whose overall structure is quasi nanotubular. In this case, the carbon layer display is conical rather than cylindrical. The reticular distance between the (00l) layers seems slightly higher for the carbon cylinders than for the cones. In both cases, Raman spectra indicate a high degree of disorder in the graphene layers.

In-situ Raman investigation of diamond films during growth and etching processes

Abstract Diamond films were prepared in a microwave plasma-assisted chemical vapour deposition (CVD) reactor. The deposition parameters were deliberately optimized to obtain mixtures of diamond and different carbonaceous compounds. We present the results of an in-situ Raman study of the growth of such films and of their etching under different atmospheres. At least six Raman bands in the range 1100–1600 cm−1 were observed; their relative intensity variation with the elaboration parameters, with the excitation wavelength and with the etching conditions led to the conclusion that at least three different carbonaceous forms are present in CVD diamond films. During the growth process, one first obtains the diamond phase along with a diamond-like compound; the disordered graphitic form appears only in a second stage.

Oxygen and Water Vapour Oxidation of 15Cr Ferritic Stainless Steels with Different Silicon Contents

The life time of stabilized ferritic stainless steels used as car exhaust parts (manifolds, tubes) can be largely reduced in the presence of water vapour, by breakaway oxidation when big iron-containing oxides nodules rapidly grow in place of the thin chromiarich protective scale. This phenomenon occurs after a certain incubation time and was shown to be influenced by the composition of the stainless steel, particularly by its silicon content. In this paper, a detailed photoelectrochemical characterization of the oxide scales prior to breakaway is presented in function of the chromium content. The silicon-rich oxide formed in contact with the steel during thermal oxidation at 900°C is shown to exhibit a different PEC signature when formed in oxygen or in water vapour (150 mbar).

Analysis of the fine structure of the Raman line and of X-ray reflection profiles for textured CVD diamond films

Abstract Textured diamond films prepared by CVD were studied by X-ray measurements and Raman spectroscopy in the high dispersive mode in order to get structural information. A 〈100〉 fibre texture is observed for the films prepared with a 2% methane concentration and a low substrate temperature. For all the 2% CH4 samples the diffraction spectra reveal an important broadening of the 111 reflection and two additional bands at about 0.207 and 0.217 nm attributed to stacking faults along the 〈111〉 direction. Domains of hexagonal symmetry can then locally be formed with the defect ordering. The recording of a 1322 cm−1 Raman line generally attributed to the lonsdaleite or hexagonal diamond polytypes supports this assumption. This 〈100〉 fibre texture is recorded for samples obtained with a high methane concentration (2%) related to a fast growth rate, which may explain these stacking faults along the 〈111〉 direction.

Micro-Raman scattering from undoped and phosphorous-doped (111) homoepitaxial diamond films: Stress imaging of cracks

We report postgrowth micro-Raman stress imaging of cracks in (111) homoepitaxial diamond films. Undoped and phosphorous-doped diamond thin films grown by microwave plasma-enhanced chemical-vapor deposition on Ib (111)-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy. For comparison purposes, a film grown on a (100) Ib substrate was also examined. Thanks to the confocal optics, the Raman signal arising from the epilayer could be discriminated from that arising from the substrate. As was already reported, the (111) films exhibited substantial tensile stress, indicated by a downshift in the Raman peak and by spontaneous cracking in films grown thicker than 5μm. The sixfold symmetry of the cracks supported that the films were homoepitaxial. A high compressive stress was also detected at the substrate near surface, and a partial stress relaxation was observed to occur in the vicinity of the cracks. Possible origins of the high tensile stress observed in the (111) homoepitaxial...

In situ Raman spectroscopy during diamond growth in a microwave plasma reactor

An experimental set‐up designed for in situ Raman analysis during the growth of diamond films in a microwave plasma reactor is described. A gated multichannel detection synchronized with a pulsed YAG laser is used to discriminate the Raman signals from the plasma emission. The in situ detection of a diamond film during its growth on a single crystal of alumina substrate is presented. The detectivity of the method has been estimated to be about a few tens of μg/cm2 for an acquisition time of 800 s. Peak shifts are interpreted in terms of temperature and stress dependences. It is shown that the diamond in the first stages of deposition is free of stress, then when grains come into contact compressive stresses are observed, when the film thickness reaches about 1 μm stresses are relaxed.