Hervé Martinez

InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description

Advanced (1)H, (13)C, and (31)P solution- and solid-state NMR studies combined with XPS were used to probe, at the molecular scale, the composition (of the core, the shell, and the interface) and the surface chemistry of InP/ZnS core/shell quantum dots prepared via a non-coordinating solvent strategy. The interface between the mismatched InP and ZnS phases is composed of an amorphous mixed oxide phase incorporating InPO(x) (with x = 3 and predominantly 4), In(2)O(3), and InO(y)(OH)(3-2y) (y = 0, 1). Thanks to the analysis of the underlying reaction mechanisms, we demonstrate that the oxidation of the upper part of the InP core is the consequence of oxidative conditions brought by decarboxylative coupling reactions (ketonization). These reactions occur during both the core preparation and the coating process, but according to different mechanisms.

Acid–base properties of Mg–Ni–Al mixed oxides using LDH as precursors

The thermal behavior of a series of Mg/Ni/Al hydrotalcites with (Mg + Ni)/Al = 2 and different Mg/Ni ratios has been studied (by means of X-ray diffraction (XRD), thermogravimetry and X-ray photoelectron spectroscopy (XPS), as well as the acid-base properties of the mixed oxides resulting from their calcination at 723 K (by means of adsorption microcalorimetry). Up to 500 K, only the physisorbed and interlayer water is released, causing a small loss of crystallinity. The decomposition of carbonate anions begins above 500 K, but this loss does not destroy the LDH structure which remains up to temperatures between 600 and 650 K. Above, the structure collapses and dehydroxylation occurs, leading to the formation of mixed oxides with large surface areas. At 723 K, a temperature usually chosen for the catalytic reactions, about 30% of the initial carbonates are still detected and poison a part of the strongest basic sites. The decarbonation is practically completed at 923 K. When the stability of the carbonates increases with the nickel content, the stability of the hydroxides increases, on the contrary, with the magnesium content. The NH3 adsorption data indicate an increase of the concentration and strength of acid sites with the nickel content. The compound SO2 seems to be more efficient than CO2 for the analysis of the basicity, because of its higher acidity and the presence of remaining carbonates in the mixed oxides. The highest concentration of basic sites is observed for the Mg/Ni/Al sample with low magnesium and high nickel contents. This gives evidence of the synergy effect of these metals favoring the basic properties and the role of MgO as promoter. Above 723 K, the concentration of the basic sites of mixed oxides decreases, which can be related to a more complete dehydroxylation.

XPS investigation of surface reactivity of electrode materials: effect of the transition metal.

The role of the transition metal nature and Al2O3 coating on the surface reactivity of LiCoO2 and LiNi(1/3)Mn(1/3)Co(1/3)O2 (NMC) materials were studied by coupling chemisorption of gaseous probes molecules and X-ray photoelectron (XPS) spectroscopy. The XPS analyses have put in evidence the low reactivity of the LiMO2 materials toward basic gaseous probe (NH3). The reactivity toward SO2 gaseous probe is much larger (roughly more than 10 times) and strongly influenced by the nature of metal. Only one adsorption mode (redox process producing adsorbed sulfate species) was observed at the LiCoO2 surface, while NMC materials exhibit sulfate and sulfite species at the surface. On the basis of XPS analysis of bare materials and previous theoretical work, we propose that the acid-base adsorption mode involving the Ni(2+) cation is responsible for the sulfite species on the NMC surface. After Al2O3 coating, the surface reactivity was clearly decreasing for both LiCoO2 and NMC materials. In addition, for LiCoO2, the coating modifies the surface reactivity with the identification of both sulfate and sulfite species. This result is in line with a change in the adsorption mode from redox toward acid-base after Al/Co substitution. In the case of NMC materials, the coating induced a decrease of the sulfite species content at the surface. This phenomenon can be related to the cation mixing effect in the NMC.

Heterogeneous sulfoxidation of thioethers by hydrogen peroxide over layered double hydroxides as catalysts

A new method for mild oxidation of thioethers with hydrogen peroxide in heterogeneous catalysis is described. The layered double hydroxides (or hydrotalcite-like materials) act as basic catalysts for the sulfoxidation reaction with hydrogen peroxide in the presence of acetonitrile. The influence of the nature of thioether, the type of catalyst, the reaction temperature and reaction time on the catalytic activity and selectivity in this reaction has been investigated. A mechanism of the sulfoxidation reaction is proposed.

Effect of the nanoparticle synthesis method on dendronized iron oxides as MRI contrast agents

Aqueous suspensions of dendronized iron oxide nanoparticles (NPs) have been obtained after functionalization, with two types of dendrons, of NPs synthesized either by coprecipitation (leading to naked NPs in water) or by thermal decomposition (NPs in situ coated by oleic acid in an organic solvent). Different grafting strategies have been optimized depending on the NPs synthetic method. The size distribution, the colloidal stability in isoosmolar media, the surface complex nature as well as the preliminary biokinetic studies performed with optical imaging, and the contrast enhancement properties evaluated through in vitro and in vivo MRI experiments, have been compared as a function of the nature of both dendrons and NPs. All functionalized NPs displayed good colloidal stability in water, however the ones bearing a peripheral carboxylic acid function gave the best results in isoosmolar media. Whereas the grafting rates were similar, the nature of the surface complex depended on the NPs synthetic method. The in vitro contrast enhancement properties were better than commercial products, with a better performance of the NPs synthesized by coprecipitation. On the other hand, the NPs synthesized by thermal decomposition were more efficient in vivo. Furthermore, they both displayed good biodistribution with renal and hepatobiliary elimination pathways and no consistent RES uptake.

Studies of 1T TiS2 by STM, AFM and XPS: The mechanism of hydrolysis in air

Scanning tunnelling microscopy, atomic force microscopy and X-ray photoelectron spectroscopy were used to study the surface of TiS2, a layered material. STM and AFM images in air and at room temperature revealed the trigonal symmetry of the lattice. After a few minutes exposure to air, we have shown by AFM and XPS the growth of a thin layer of TiO2 on the surface.

Highly conformal electrodeposition of copolymer electrolytes into titania nanotubes for 3D Li-ion batteries

The highly conformal electrodeposition of a copolymer electrolyte (PMMA-PEO) into self-organized titania nanotubes (TiO2nt) is reported. The morphological analysis carried out by scanning electron microscopy and transmission electron microscopy evidenced the formation of a 3D nanostructure consisting of a copolymer-embedded TiO2nt. The thickness of the copolymer layer can be accurately controlled by monitoring the electropolymerization parameters. X-ray photoelectron spectroscopy measurements confirmed that bis(trifluoromethanesulfone)imide salt was successfully incorporated into the copolymer electrolyte during the deposition process. These results are crucial to fabricate a 3D Li-ion power source at the micrometer scale using TiO2nt as the negative electrode.

Percolation network of organo-modified layered double hydroxide platelets into polystyrene showing enhanced rheological and dielectric behavior

A hybrid organic inorganic layered double hydroxide incorporating 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) is characterized by means of XRD, FTIR, and XPS. The in situ polymerization is scrutinized by 13C CPMAS as well as by a set of XRD experiments with varying temperature. It is found that the in situ polymerization is complete at 200 °C, and the hybrid framework sustains temperatures as high as 350 °C. Direct incorporation of poly(AMPS) is reported and the resulting hybrid LDH phases studied. Subsequently, all generated hybrid platelets are used as organo-modified 2D-type filler dispersed into polystyrene (PS). An immiscible PS composite structure with salient gel-like viscoelastic properties is obtained after bulk polymerization. In the low-frequency region, the typical Newtonian flow behaviour of PS is found to change progressively against filler loading into a shear-thinning behaviour evidenced by a pseudo-plateau in the elastic and loss modulus curves and associated with a shift of the glass transition temperature of PS to higher temperature. It is interpreted by hybrid LDH platelet domains presenting a large interface with the polymer, thus having the effect of restricting the plastic deformation by obstructing polymer chain motion. Such dispersed hybrid LDH tactoids forming a three-dimensional percolated network are indirectly evidenced by the enhancement of the dielectric properties illustrated by an increase in bulk dc conductivity of about one order at room temperature and in the dissipation factor. The study shows that hybrid LDH assembly is of relevance in topical applications regarding mechanical reinforcement as well as electrostatic energy dissipation.

Electrochemical Fabrication and Properties of Highly Ordered Fe-Doped TiO2 Nanotubes

Highly-ordered Fe-doped TiO(2) nanotubes (TiO(2)nts) were fabricated by anodization of co-sputtered Ti-Fe thin films in a glycerol electrolyte containing NH(4)F. The as-sputtered Ti-Fe thin films correspond to a solid solution of Ti and Fe according to X-ray diffraction. The Fe-doped TiO(2)nts were studied in terms of composition, morphology and structure. The characterization included scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, UV/Vis spectroscopy, X-ray photoelectron spectroscopy and Mott-Schottky analysis. As a result of the Fe doping, an indirect bandgap of 3.0 eV was estimated using Taucs plot, and this substantial red-shift extends its photoresponse to visible light. From the Mott-Schottky analysis, the flat-band potential (E(fb)) and the charge carrier concentration (N(D)) were determined to be -0.95 V vs Ag/AgCl and 5.0×10(19) cm(-3) respectively for the Fe-doped TiO(2)nts, whilst for the undoped TiO(2)nts, E(fb) of -0.85 V vs Ag/AgCl and N(D) of 6.5×10(19) cm(-3) were obtained.

Electronic structure of intercalated metal disulfides (Ag13TiS2 and Fe13TiS2) studied by XPS and theoretical calculations

Abstract The effect of metal intercalation (silver and iron) into lT-Cdl 2 -type TiS 2 layered crystals, expressed as M x TiS 2 , has been studied by X-ray photoelectron spectroscopy (XPS) and self-consistent electronic calculations (augmented sphere wave method). The spectra are found to depend strongly on the guest metals, and we have shown how the XPS valence bands are modified by the ‘host-guest’ interactions by using calculated densities of states. In the first approximation, the chemical shift of the core peaks are correlated with Mulliken population analysis.