Abstract
Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in ...properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called
h’-WO
3
, built of (WO
6
)
6
tunnel cavities.
h’-WO
3
is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze
h’-H
0.07
WO
3
. Gentle heating results in
h’-WO
3
with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.
Abstract
Nanoshells made of a silica core and a gold shell possess an optical response that is sensitive to nanometer-scale variations in shell thickness. The exponential red shift of the plasmon ...resonance with decreasing shell thickness makes ultrathin nanoshells (less than 10 nm) particularly interesting for broad and tuneable ranges of optical properties. Nanoshells are generally synthesised by coating gold onto seed-covered silica particles, producing continuous shells with a lower limit of 15 nm, due to an inhomogeneous droplet formation on the silica surface during the seed regrowth. In this paper, we investigate the effects of three variations of the synthesis protocol to favour ultrathin nanoshells: seed density, polymer additives and microwave treatment. We first maximised gold seed density around the silica core, but surprisingly its effect is limited. However, we found that the addition of polyvinylpyrrolidone during the shell synthesis leads to higher homogeneity and a thinner shell and that a post-synthetic thermal treatment using microwaves can further smooth the particle surface. This study brings new insights into the synthesis of metallic nanoshells, pushing the limits of ultrathin shell synthesis.
An aqueous synthetic route at 95 °C is developed to reach selectively three scarcely reported vanadium oxyhydroxides. Häggite V2O3(OH)2, Duttonite VO(OH)2, and Gain’s hydrate V2O4(H2O)2 are ...obtained as nanowires, nanorods, and nanoribbons, with sizes 1 order of magnitude smaller than previously reported. X-ray absorption spectroscopy provides evidence that vanadium in these phases is V+IV. Combined with FTIR, XRD, and electron microscopy, it yields the first insights into formation mechanisms, especially for Häggite and Gain’s hydrate. This study opens the way for further investigations of the properties of novel V+IV (oxyhydr)oxides nanostructures.
This works presents a new method for the production of colloidal suspensions of Zn2SiO4:Mn (ZSM) luminescent nanoparticles. The process consists in achieving a high temperature solid state reaction ...among silica nanoparticles homogenously dispersed within a ZnO matrix in large excess. Highly crystalline, well dispersed nanoparticles are recovered as a colloidal suspension after dissolution of the ZnO matrix in slightly acidic water. Yellow emitting β-Zn2SiO4:Mn or green emitting α- Zn2SiO4:Mn are obtained depending on the reaction temperature (either 800 or 900 °C) and the Mn content. Zn2–2x SiO4:Mn x particles with x up to 3% can be obtained with a maximum relative emission efficiency of about 16% as compared to a bulk ZSM phosphor reference. The principle of this new synthetic route opens a new field of investigation for the production of colloidal dispersion of refractory compounds with a high crystallinity.
We describe a new approach to making ultrathin Ag nanoshells with a higher level of extinction in the infrared than in the visible. The combination of near-infrared active ultrathin nanoshells with ...their isotropic optical properties is of interest for energy-saving applications. For such applications, the morphology must be precisely controlled, since the optical response is sensitive to nanometer-scale variations. To achieve this precision, we use a multi-step, reproducible, colloidal chemical synthesis. It includes the reduction of Tollens' reactant onto Sn
-sensitized silica particles, followed by silver-nitrate reduction by formaldehyde and ammonia. The smooth shells are about 10 nm thick, on average, and have different morphologies: continuous, percolated, and patchy, depending on the quantity of the silver nitrate used. The shell-formation mechanism, studied by optical spectroscopy and high-resolution microscopy, seems to consist of two steps: the formation of very thin and flat patches, followed by their guided regrowth around the silica particle, which is favored by a high reaction rate. The optical and thermal properties of the core-shell particles, embedded in a transparent poly(vinylpyrrolidone) film on a glass substrate, were also investigated. We found that the Ag-nanoshell films can convert 30% of the power of incident near-infrared light into heat, making them very suitable in window glazing for radiative screening from solar light.
Thermal degradation of blue phosphor BAM:Eu2+ under air is investigated using XPS, XRD, EPR and photoluminescence (PL) in order to analyze the loss of intensity in terms of modifications of the ...dopant distribution in the crystal cell and between crystal bulk and surface. This study reveals the key role played by oxidation-driven 2D diffusion of europium and barium, that results in important concentration gradients and makes degradation strongly dependent on both microstructure and cooling speed. Also based upon cation mobility, a possible regeneration effect is evidenced at moderate temperature. Flux treatments, known to increase the intensity of photoluminescence, also appear to reduce thermal degradation by lowering specific surface; they also enhance the regeneration process by forming extended single-crystal domains conducive to cation diffusion.
•High stake but still controversial thermal degradation of blue phosphor BAM:Eu2+ is explained.•A new multi-scale structural model is proposed, based on a wide spectrum of analytical methods.•Long-range transport of Ba2+ and Eu2+/Eu3+ ions and concentration gradients are evidenced.•Also based upon cation diffusion, an inedite regeneration process is proposed.
•Effects of three fluoride fluxes on structural and spectroscopic properties of BAM:Eu2+ phosphor are studied.•Flux recrystallization results in modifications of the dopant's distribution between ...host sites.•Photoluminescence is strongly enhanced by LiF, in particular in the domain of excitation of near-UV LEDs.
In the aim of understanding the relations between the spectroscopic properties of blue phosphor BaMgAl10O17:Eu2+ and its crystal structure, microstructure and chemical composition, three fluoride fluxes (LiF, MgF2 and NH4F) have been added before final thermal treatment and their effect analyzed. LiF changes the polycrystalline and spherical grains into hexagonal platelets without significant cations exchange and improves the luminescence. NH4F and MgF2 only have a limited effect, the former because of its low point of decomposition, the latter because of the formation of spurious MgAl2O4 that reduces the luminescence. In a general way, re-crystallization results in variations of the dopant distribution between the three host sites of the structure. Chromaticity is only faintly affected, but important variations of the absorption and excitation spectra are observed, allowing a strong increase of photoluminescence intensity under near-UV excitation.
Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in ...properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h'-WO
, built of (WO
)
tunnel cavities. h'-WO
is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h'-H
WO
. Gentle heating results in h'-WO
with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.
A route towards carbon-MoO2 core-shell spheres has been developed, through hydrothermal decomposition of ascorbic acid combined with precipitation of MoO2 nanoparticles. In this one-pot and green ...process, carbon spheres originating from ascorbic acid act as seeds for the in situ deposition of a corona made of 30 nm molybdenum dioxide particles. The as-obtained hierarchical nanostructured carbon-MoO2 core-shell spheres exhibit an ideal combination of electrical conductivity and lithium reactivity for Li-ion battery electrodes. This nanocomposite offers the opportunity to master the collector-active material and active material-electrolyte interfaces. Direct transfer ``from the beaker to the battery'' without any additives nor thermal treatment yields storage capacity values of ca. 600 mA h g(-1) at C/5 rate with excellent stability that challenges state-of-the-art molybdenum oxide-based batteries.
We present a new process for the synthesis of colloidal europium-doped yttrium vanadate with a particle diameter of about 10 nm. Nanocrystals are produced by precipitation of citrate complexes of ...rare-earth salts with sodium orthovanadate. NMR and IR studies show that the interaction between citrate ligands and lanthanide ions limits the growth of particles and ensures the stability of the colloidal solutions through electrostatic and steric repulsions. The optimized process leads to stable and highly concentrated transparent colloidal solutions in water (up to 400 g·L-1).
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