Evidence of charge-transfer ferromagnetism in transparent diluted magnetic oxide nanocrystals: switching the mechanism of magnetic interactions.

We report the experimental evidence of a new form of room-temperature ferromagnetism in high surface area nanocrystalline manganese-doped In2O3, prepared from colloidal nanocrystals as building blocks. The nanocrystal structure (bixbyite or corundum) and assembly were controlled by their size, and the type and concentration of dopant precursors. The existence of substitutional paramagnetic Mn dopant ions in mixed valence states (Mn(2+) and Mn(3+)) was confirmed and quantified by different spectroscopic methods, including X-ray absorption and magnetic circular dichroism. The presence of different oxidation states is the basis of ferromagnetism induced by Stoner splitting of the local density of states associated with extended structural defects, due to charge transfer from the Mn dopants. The extent of this charge transfer can be controlled by the relationship between the electronic structures of the nanocrystal host lattice and dopant ions, rendering a higher magnetic moment in bixbyite relative to corundum Mn-doped In2O3. Charge-transfer ferromagnetism assumes no essential role of dopant as a carrier of the magnetic moment, which was directly confirmed by X-ray magnetic circular dichroism, as an element-specific probe of the origin of ferromagnetism. At doping concentrations approaching the percolation limit, charge-transfer ferromagnetism can switch to a double exchange mechanism, given the mixed oxidation states of Mn dopants. The results of this work enable the investigations of the new mechanisms of magnetic ordering in solid state and contribute to the design of new unconventional magnetic and multifunctional materials.

Stability and tunability of O/W nanoemulsions prepared by phase inversion composition.

We report on an analysis of the parameters that control both the stability and tunability of O/W nanoemulsions prepared by the phase inversion composition (PIC). These nanoemulsions are prepared with Tween 80 and Span 80, two nonionic surfactants, that can be mixed to adjust the hydrophilic lipophilic balance (HLB). We used a process mixture design method, which combines mixture and process design with phase diagrams, to describe the cross-link between parameters like composition, temperature of preparation, and HLB. Nanoemulsions, stable for several days, are obtained by this method, and they remain unchanged even at high concentration. We have identified the different critical distances of interactions that control the degree of freedom in the motion of the oil droplets. The diameter of these oil droplets could be adjusted between 50 and 300 nm. Different parameters, among them the temperature of preparation, the surfactant over oil ratio (S/O), and the HLB, allow control the final size of the nanoemulsions. As these parameters can exhibit opposite effects on the oil droplet size, the process mixture design method allowed us to illustrate these cross-interactions.

Varying TiO2 macroscopic fiber morphologies toward tuning their photocatalytic properties.

In a context of volatile organic compound photodecomposition, we have addressed TiO2-based macroscoscpic fiber generation. We have extruded hybrid sols of amorphous titania nanoparticles, latex nanoparticles, and nonionic surfactant (Tergitol) as structure-directing agents into a poly(vinyl alcohol) (PVA) solution bearing salts acting as a flocculating medium. The resulting nanocomposite TiO2/latex/PVA macroscopic fibers were thermally treated in air to open porosity by organic removal while generating the photocatalytically active anatase phase of TiO2 along with residual brookite. Considering the synthetic paths, we have varied both the diameter of the latex particles as well as their concentration within the starting sol. These parameters allow tuning both the voids created through the applied thermal treatment and the fiber final diameters. For gas-phase photocatalysis, we have shown that the fiber diameters, mesoscopic roughness, and macroscopic topological defects represent indeed important morphological parameters acting cooperatively toward both acetone degradation and its mineralization processes. Particularly, triggering the fiber morphological characteristics, we have increased their efficiency toward acetone degradation of around 550% when compared with previous work.

Synthesis of monolithic meso-macroporous silica and carbon with tunable pore size.

Monolithic porous silica and carbon structures have been obtained by the synthesis of silica inside the aqueous phase of a sponge-like Swollen Liquid Crystal, and the parallel preparation of carbon replica.

Nanocrystalline iron oxide synthesised within Hierarchical Porous Silica prepared by nanoemulsion templating.

PIC (Phase Inversion Composition) O-W nanoemulsions was used as a template for the synthesis of Hierarchical Porous Silica (HPS), and the oil phase of the nanoemulsion was used as a nanoreactor for the preparation of magnetic gamma-Fe(2)O(3) nanoparticles, confined within the silica matrix.

Differential scanning calorimetry study of the structure of water confined within AOT lamellar mesophases.

We combined thermal analysis with FT-IR measurements to solve the structure of the water network confined within lamellar bilayers of AOT surfactant. Unlike previous FT-IR analyses that allowed us only to point out spectroscopic molecular components characterized by their local connectivity, differential scanning calorimetry (DSC) allowed us to identify homogeneous thermal populations listed as low, medium and high temperature. HT water was identified with bulk water, and LT water, as hydration water closely linked with surfactants heads. MT water was assigned to a disrupted network of water corresponding to a layer of 0.5 nm modified by the effect of confinement created by the vicinity of the surfactant bilayer. It appears from this study that the actual extent of confinement influence is lower than expected, at least for lamellar structures. Each thermal population was finally described by a different distribution of the spectroscopic components identified in parallel by FT-IR.

Existence and stability of new nanoreactors: highly swollen hexagonal liquid crystals.

We report the preparation of direct hexagonal liquid crystals, constituted of oil-swollen cylinders arranged on a triangular lattice in water. The volume ratio of oil over water, rho can be as large as 3.8. From the lattice parameter measured by small-angle X-ray scattering, we show that all the oil is indeed incorporated into the cylinders, thus allowing the diameter of the cylinders to be controlled over one decade range, provided that the ionic strength of the aqueous medium and rho are varied concomitantly. These hexagonal swollen liquid crystals (SLCs) have been first reported with sodium dodecyl sulfate as anionic surfactant, cyclohexane as solvent, 1-pentanol as co-surfactant, and sodium chloride as salt (Ramos, L.; Fabre, P. Langmuir 1997, 13, 13). The stability of these liquid crystals is investigated when the pH of the aqueous medium or the chemical nature of the components (salt and surfactant) is changed. We demonstrate that the range of stability is quite extended, rendering swollen hexagonal phases potentially useful for the fabrication of nanomaterials. As illustrations, we finally show that gelation of inorganic particles in the continuous aqueous medium of a SLC and polymerization within the oil-swollen cylinders of a SLC can be conducted without disrupting the hexagonal order of the system.