Sophie Cassaignon

Size tailoring of TiO2 anatase nanoparticles in aqueous medium and synthesis of nanocomposites. Characterization by Raman spectroscopy

Nanoparticles of anatase with mean size in the range 5–10 nm were prepared by precipitation of TiCl4 in aqueous medium in the range 2 ≤ pH ≤ 6. Hydroxylation of TiCl4 at room temperature leads instantaneously to an amorphous titanium oxyhydroxide phase which crystallizes as anatase upon aging at 60 °C in suspension. Small amounts of brookite or rutile are concurrently obtained depending on the acidity. The size of anatase particles was characterized by X-ray diffraction, electron microscopy and Raman spectroscopy. The latter was also used to determine the particle size and to characterize the crystallinity of particles through the phonon confinement effect. The particle size, dependent on the acidity, is closely related to the electrostatic surface charge density of particles. The size variation was interpreted as resulting from a lowering of the interfacial tension due to the protonation of particle surface groups. Composite materials were synthesized by polymerisation of silica in aqueous sols of anatase. The dispersed anatase nanoparticles are stable against the transformation to rutile up to 1000 °C.

Comparison of optical and electrochemical properties of anatase and brookite TiO2 synthesized by the sol–gel method

Three polymorphs of TiO2, rutile, anatase and brookite, are well known. Each variety has its own physical properties, bandgap, surface states, etc. For photovoltaic applications, anatase is the phase most used. Up to now, the third polymorph of TiO2, brookite, could not be obtained by soft chemistry methods. We have now synthesized nanometric particles of pure brookite by ‘chimie douce’ in aqueous solution. For the first time, this phase will be characterized in view of its possible application in dye-sensitized cells. We discuss the potential of brookite as a good candidate for photovoltaic devices in comparison to other phases, in terms of its morphology, bandgap and electrochemical properties in water and acetonitrile before sensitization.

Size tailoring of oxide nanoparticles by precipitation in aqueous medium. A semi-quantitative modelling

Chemistry in aqueous solution is an easy and versatile method to form nanosized metal oxide particles. Considering our previous results on magnetite Fe3O4, anatase TiO2, brucite Mg(OH)2, and boehmite γ-AlOOH, we show that the strict control of the physicochemical conditions of the precipitation, essentially the acidity and ionic strength in the absence of complexing species, enables the tailoring of the particle size in the range 2–15 nm and, in some cases, of their morphology. We show that the variations in size and/or shape are tightly related to the variation of the electrostatic surface charge density of the particles, which induces a variation of the oxide-solution interfacial tension, and, consequently, a decrease of the surface energy. Such an effect enables the control of the surface area of the system. A semi-quantitative model is presented, which accounts for the effects observed for particles isotropic or anisotropic in shape.

Water-mediated structuring of bone apatite

It is well known that organic molecules from the vertebrate extracellular matrix of calcifying tissues are essential in structuring the apatite mineral. Here, we show that water also plays a structuring role. By using solid-state nuclear magnetic resonance, wide-angle X-ray scattering and cryogenic transmission electron microscopy to characterize the structure and organization of crystalline and biomimetic apatite nanoparticles as well as intact bone samples, we demonstrate that water orients apatite crystals through an amorphous calcium phosphate-like layer that coats the crystalline core of bone apatite. This disordered layer is reminiscent of those found around the crystalline core of calcified biominerals in various natural composite materials in vivo. This work provides an extended local model of bone biomineralization.

A Core–Corona Hierarchical Manganese Oxide and its Formation by an Aqueous Soft Chemistry Mechanism

One of the main challenges that still needs to be overcome in the design of nanotextured materials is the synthesis of uniform complex architectures. Indeed, many properties are known to be greatly modified by the size and shape of nanostructures. Other than size and shape tailoring, however, control of the ordering between nanostructures and the resulting texture is still difficult. Synthetic routes that make use of organic solvents and templates (i.e., surfactants) often require subsequent purification procedures which significantly increase production costs. The development of environmentally friendly, low-cost, and template-free synthetic methods that produce complex architectures is therefore key to enhancing both the control of the properties and the viability of such materials. In this context, porous manganese oxide materials are attracting great interest due to their applicability in domains such as ion-exchange, catalysis, and energy storage in Li batteries and supercapacitors. Indeed, layered birnessitelike manganese oxides (LMO) are particularly relevant due to their lamellar structure, which contain layers of MnO6 octahedra between which different species can be intercalated (see Figure S1 in the Supporting Information). However, the design of ordered LMO architectures remains a significant challenge as their synthesis usually takes place in an aqueous medium by sol–gel or precipitation methods, both of which result in fast and uncontrolled solid growth that hinders the synthesis of well-ordered nanostructures. Herein we present a low-temperature aqueous precipitation of potassium-intercalated LMO with a peculiar hierarchical core–corona architecture in the absence of both a template and an organic medium. Particle formation takes place in an easy “one-pot” process involving two distinct precipitation kinetic stages. The synthesis of similar inorganic/ inorganic core–corona morphologies generally requires two steps for core formation and shell growth, and there are very few reports concerning one-pot procedures that lead to fully inorganic core–shell particles. Furthermore, these methods are generally limited to metal–oxide structures. The approach presented herein, which uses in situ seeding to control the solid growth, significantly broadens the range of strategies available for the elaboration of hierarchical inorganic structures and can be extended to the design of new functional nanostructured materials, by taking advantage of the unique oxide properties, in areas such as catalysis, energy harnessing, and information storage. The synthesis of birnessite (see the Experimental Section and the Supporting Information) is based on the redox reaction between MnSO4 and an excess of KMnO4 [4f, 5b] (total Mn concentration of 0.2 molL ) in water according to Equation (1). Mixing the acidic (pH 2) solutions of the

Structural and morphological control of manganese oxide nanoparticles upon soft aqueous precipitation through MnO4−/Mn2+ reaction

A low-temperature (60 or 95 °C) “one-pot” procedure for the aqueous precipitation of manganese oxide nanoparticles is developed through the MnO4−/Mn2+ reaction. Characterization of the particles is carried out using elemental analysis, mean oxidation state determination, powder XRD, FESEM, TEM and electron diffraction. Many synthesis parameters are investigated, such as acidity, reactant ratio, concentration and nature of the counter-cation, temperature and aging time. Speciation diagrams are drawn for the synthesis of five different pure phases (spinel-type Mn3O4, layered birnessite-type δ-MnO2, tunnel-based frameworks cryptomelane-type α-MnO2, γ-MnO2 and pyrolusite β-MnO2). The influence of synthetic parameters is rationalized and reaction pathways toward various structures and morphologies are discussed.

Brookite TiO2 Nanoparticle Films for Dye-Sensitized Solar Cells

Brookite TiO(2) nanoparticles have been synthesized at low temperature by a soft solution growth method and have been used as building blocks to prepare pure brookite nanoparticle porous films. The film brookite structure was confirmed by XRD and Raman spectroscopy. By spectrophotometry, it was shown that the films had a direct band gap of 3.4 eV. After sensitization by the N719 dye, efficient cells have been produced. A best overall conversion efficiency of 5.97%, without a scattering layer, was found for the larger TiO(2) starting nanoparticles. The cell open-circuit voltage was improved compared with that of anatase cells and a lower electron diffusion coefficient was found in the photoanodes made of smaller brookite particles. Lanthanum-doped brookite nanoparticle films were also studied. They showed a marked decreased in the amount of dye loading, and hence, the solar cells had a reduced current density that was not compensated for by the increased open-circuit voltage of the cells.

Vanadium Oxide: From Gels to Nanotubes

Vanadium oxide nanotubes (VOx-NT) have been prepared by mixing hexadecylamine with V2O5·nH2O gels. This procedure was followed by an hydrothermal treatment (150–180°C, 2–7 days) which leads to a large quantity of VOx-NT. SEM and XRD analysis have been used to optimize the temperature and reaction time required for production of VOx-Nt and morphology of the nanotubes investigated by TEM.

Selective heterogeneous oriented attachment of manganese oxide nanorods in water: toward 3D nanoarchitectures

MnOOH and MnO2 polymorphs are synthesized using a reaction between Mn2+and MnO4− in water at low temperature (95 °C). Nanoparticles are characterized using powder XRD, TEM and electron diffraction. Depending on the acidity of the aging medium, γ-MnO2 compounds are shown to be obtained as nanorods (final pH = 2.0) or hollow nanocones (3.6 ≤ final pH ≤ 4.5). The external faces of the cones originate from heterogeneous oriented attachment of α-MnOOH nanorods on early cones. The selective attachment between (−110)γ–MnO2 and (101)α–MnOOH faces is discussed by taking into account the surface charge of the primary particles and the reactivity of the Mn–OHx surface groups. This study underlines the role of the electrostatic repulsions and the surface reactivity in the oriented attachment mechanism in water for oxides.

A sustainable aqueous route to highly stable suspensions of monodispersed nano ruthenia

Highly stable suspensions of monodispersed ruthenia nanoparticles have been prepared via a sustainable aqueous oxidative pathway. The nanoparticles (2 nm) have been thoroughly characterized by TEM, XRD, XPS, MS-TGA and thermodiffraction. The addition of hydrogen peroxide in the RuCl3 solution provokes a fast oxidation of Ru(III) ions into Ru(IV). This increases the rate of the hydrolysis/condensation reactions and further promotes the nucleation over the growth of the particles. The very high stability conditions of the colloidal suspension have been studied. This aqueous one-step process, which uses no organic solvent or toxic pollutant additive, is quick and produces calibrated ruthenia nanoparticles in high yields. It presents a green alternative to the preparation and use of ruthenia. As examples, two applications are presented. In the first, RuO2 coatings have been tested for their electrical capacitance. In the second, RuO2/TiO2 catalysts, prepared from the controlled deposition of ruthenia nanoparticles on TiO2 particles, have been proven to be highly effective for the production of methane from CO2.