Investigation of the Photochromism of WO3, TiO2, and Composite WO3-TiO2 Nanoparticles.

We report the synthesis of WO3, TiO2, and TiO2-WO3 nanoparticles by a polyol route, with the objective of studying the influence of the preparation method on their photochromic properties. By combining transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance experiments, we show that low W6+ concentrations and high ripening temperatures allow the preparation of WO3 nanoparticles with high photochromic efficiency. WO3-TiO2 nanocomposites (NCs) prepared by the introduction of a TiO2+ solution in a WO3 nanoparticle suspension exhibit a strong coloring photochromism, which is attributed to the TiO2 coating of the WO3 nanoparticles as it involves the formation of W-O-Ti oxo-bonds in place of W5+-νO defects. Especially, after an oxidative treatment in order to obtain an initial pale-yellow material, such WO3-TiO2 NCs exhibit a fully reversible photochromism with a large contrast between the colored and bleached state. They could therefore be incorporated in hybrid smart films for solar control on building window glasses. On the other hand, while the WO3-TiO2 NCs are functionalized with DPA (n-dodecyl phosphonic acid), the as-prepared nanocomposites exhibit exacerbated coloring contrast but with a nearly nonreversible photochromism (very limited bleaching), which makes them good candidates for the fabrication of smart UV-sensor devices that can indicate the cumulative UV dose which is received.

Synthesis and Characterization of Micro/Nanoscale VO2(M) through Vanadylethylene Glycolate Decomposition.

Thanks to a homemade dynamic vacuum system, fully crystallized VO2 (M) is successfully synthesized in a merged step of vanadyl ethylene glycolate (VEG) decomposition and crystallization of VO2 at high temperatures (>500 °C). During the whole process, vanadium valence (+4) is well maintained, and VEG microstructure plays an important role in the end-product size and shape. Finally, the suggested route appears well suitable for the mass production of VO2 nanoparticles.

NaMoO2: a Layered Oxide with Molybdenum Clusters.

NaMoO2 was synthesized as a layered oxide from the reaction between the layered oxide Na2/3MoO2 and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO2 distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of "diamond-like" clusters of molybdenum were evidenced in the [MoO2] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d3 electronic configuration. Molybdenum-molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO2 presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo-Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO2 was diamagnetic at 0 K. A sodium battery was built using NaMoO2 as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO2, indicating that this compound is one of the many phases existing in the NaxMoO2 system.

Two-Step Synthesis of VO2 (M) with Tuned Crystallinity.

Highly crystallized monoclinic vanadium dioxide, VO2 (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO2 powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO2 powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO2 (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO2 particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO2, calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, V2O3 and V2O5 with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.

Near the Ferric Pseudobrookite Composition (Fe2TiO5).

Because of a very low thermodynamic stability, obtaining a pure monophasic compound of ferric pseudobrookite is quite difficult to achieve. Indeed, the low reticular energy of this phase leads easily to its decomposition and the occurrence of the secondary phases: hematite (Fe2O3) and/or rutile (TiO2). Samples with global composition Fe2-xTi1+xO5 (x = 0, 0.05, and 0.10) have been synthesized by the Pechini route and, thereafter, thermally treated at different temperatures. The concentrations of Fe2O3 and TiO2 secondary phases were accurately determined and correlated with the target compositions and the synthesis parameters, especially the thermal treatment temperature. As revealed by Mössbauer spectroscopy, all iron ions are at the III+ oxidation state. Thus, the formation of hematite or rutile as a secondary phase may be related to the occurrence of cationic vacancies within the pseudobrookite structure, with the amount of vacancies depending on the annealing temperature. In light of the presented results, it appears unreasonable to propose a "fixed" binary phase diagram for such a complex system. Furthermore, the occurrence of cationic vacancies induces a coloration change (darkening), preventing any industrial use of this reddish-brown pseudobrookite as a ceramic pigment.

Dual lithium insertion and conversion mechanisms in a titanium-based mixed-anion nanocomposite.

The electrochemical reaction of lithium with a vacancy-containing titanium hydroxyfluoride was studied. On the basis of pair distribution function analysis, NMR, and X-ray photoelectron spectroscopy, we propose that the material undergoes partitioning upon initial discharge to form a nanostructured composite containing crystalline Li(x)TiO(2), surrounded by a Ti(0) and LiF layer. The Ti(0) is reoxidized upon reversible charging to an amorphous TiF(3) phase via a conversion reaction. The crystalline Li(x)TiO(2) is involved in an insertion reaction. The resulting composite electrode, Ti(0)-LiF/Li(x)TiO(2) ⇔ TiF(3)/ Li(y)TiO(2), allows reaction of more than one Li per Ti, providing a route to higher capacities while improving the energy efficiency compared to pure conversion chemistries.

Doubling of the Phase Transition Temperature of VO2 by Fe Doping.

Vanadium dioxide (VO2) undergoes a fully reversible first-order metal-insulator transition from the M1 monoclinic phase (P21/c) to a high-temperature tetragonal phase (P42/mnm) at around 68 °C. Modulation of the phase transition of VO2 by chemical doping is of fundamental and technological interest. Here, we report the synthesis of highly crystallized Fe-doped VO2 powders by a carbo-thermal reduction process. The impact of Fe doping on the structural and phase transition of VO2 is studied. The as-prepared Fe-doped VO2 samples crystallize in the M2 monoclinic form (C2/m), which is linked to segregation of the doping ions in the V2 zigzag chains. A large increase in the transition temperature to 134 °C is observed, which does correspond to a breakthrough in VO2-type thermochromic materials.

Carbon-reduction as an easy route for the synthesis of VO2 (M1) and further Al, Ti doping.

A low-cost and facile method to synthesize highly crystallized VO2 (M1) particles is proposed, using carbon black as the reducing agent mixed with V2O5 nanopowders comparing two types of vacuum systems for the thermal activation. In a sealed vacuum system, CO gas is generated in the first reductive step, and continues to reduce the new born VO2, until all the V (+4) is reduced to V (+3), resulting in V2O3 formation at 1000 °C. In contrast, in a dynamic vacuum system, CO gas is ejected through pumping as soon as it is generated, leading to the formation of pure VO2 (M1) at high temperatures (i.e. in the range 700 °C ≤ T ≤ 1000 °C). The evolution of the carbon content, determined by CHNS, of each sample versus the synthesis conditions, namely temperature and type of vacuum system, confirms that the transformation of V (+5) into V (+4) or V (+3) can be controlled. The characterization of the morphologies and crystal structures of two synthesized VO2 (M1) at 700 °C and 1000 °C shows the possibility to tune the crystallite size from 1.8 to more than 5 µm, with a uniform size distribution and highly crystallized powders. High purity VO2 (M1) leads to strong physical properties illustrated by a high latent energy (∼55 J g-1) during the phase transition obtained from DSC as well as high resistivity changes. In addition, with this method, dopants such as Ti4+ or Al3+ can be successfully introduced into VO2 (M1) thanks to the preparation of Al or Ti-doped nano-V2O5 by co-precipitation in polyol medium before carbon-reduction.

Geometric considerations of the monoclinic-rutile structural transition in VO2.

The mechanism of the displacive phase transition in VO2 near the transition temperature is discussed in terms of a geometrical approach, combining simple calculations based on the Brown's band valence model and in situ X-ray diffraction experimental results. Considering that the structural origin is well linked to the electrostatic potential optimization as in a Peierls model, our geometrical calculations and experimental studies are in agreement and suggest that VO2 phase transition is the consequence of very short atomic shifts mainly associated to a decrease of the 2nd sphere coulombic interactions. Hence, at a given temperature, the allotropic form (monoclinic versus rutile form) offering the largest unit-cell volume is stabilized over the lower unit-cell volume allotropic, while the transition occurs at the intercept of the unit cell variation versus temperature of the two forms, which exhibit significantly different thermal expansion coefficients.

Toward Simplified Electrochromic Devices Using Silver as Counter Electrode Material.

Novel design of electrochromic devices (ECDs) known for their ability to modify optical properties under an applied voltage, based on a minimization of the number of layers is reported. The use of a metallic electrode, playing the role of both the conductive layer and the counter electrode, allows us to simplify the assembly of a commonly five-layer battery-type device to four-layer ECD. Further minimization of the number of layers is achieved using a conductive and electrochromic material. The novelty of the device configuration is illustrated using poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials as EC layer, lithium-based ionic liquid as electrolyte, and Ag as counter electrode. Such a four- or three-layer ECD deposited on paper substrate switches from light to deep blue in a narrow 0.7 V voltage window. Preliminary investigations of the mechanism indicate traces of Ag on the PEDOT layer upon cycling. Finally, the printed ECD is successfully activated using a mobile phone.

LiSc(BH4)4: a novel salt of Li+ and discrete Sc(BH4)4(-) complex anions.

LiSc(BH4)4 has been prepared by ball milling of LiBH4 and ScCl3. Vibrational spectroscopy indicates the presence of discrete Sc(BH4)4(-) ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH4(-) groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 A and c = 12.034 A (space group P-42c). The local structure of the Sc(BH4)4(-) complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.

Structural transformations of the La2-xPrxNiO4+δ system probed by high-resolution synchrotron and neutron powder diffraction.

Compositions in the La2-xPrxNiO4+δ series offer an attractive balance of chemical stability and electrochemical performance for use as cathode materials in solid oxide fuel cells (SOFCs). A detailed crystallographic study of this system has been performed, combining both high resolution synchrotron and neutron powder diffraction data, in order to investigate structural details of the series as a function of composition, temperature and oxygen over-stoichiometry. The monoclinic structure (space group F2/m) of ambient temperature Pr-rich compositions for 1.0 < x ≤ 2.0 is discussed in terms of octahedra tilt arrangements and possible long-range structural modulations. In situ synchrotron diffraction experiments and TEM are employed to examine the role of temperature and interstitial oxygen on these structural distortions. With increasing La substitution, a region of mixed monoclinic and tetragonal phases is described for 0.5 ≤ x ≤ 1.0. La-Rich compositions are found to be single phase tetragonal (F4/mmm for 0 < x < 0.5) or orthorhombic (Fmmm for x = 0). Possible origins and electrochemical property consequences of the refined structural trends are considered.

Probing Co- and Fe-doped LaMO3 (M = Ga, Al) perovskites as thermal sensors.

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O3 perovskite via the Pechini process aimed to achieve a color change induced by temperature and associated with spin crossover (SCO). In Fe-doped samples, iron was shown to be in the high-spin state, whereas SCO from the low-spin to the high-spin configuration was detected in Co-doped compounds when the temperature increased. Fe-doped compounds clearly adopted the high-spin configuration even down to 4 K on the basis of Mössbauer spectroscopic analysis. The original SCO phenomenon in the Co-doped compounds LaGa1-xCoxO3 (0 < x < 0.1) was evidenced and discussed on the basis of in situ X-ray diffraction analysis and UV-vis spectroscopy. This SCO is progressive as a function of temperature and occurs over a broad range of temperatures between roughly 300 °C and 600 °C. The determination of a crystal field strength of about 2 eV and a Racah parameter B of about 500 cm-1 for Co3+ (3d6) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa1-xCoxO3 compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO3 in which Co3+ is stabilized in the LS state shows that electronic effects with the creation of M-O covalent bonds are predominant and contribute to the stabilization of a high crystal field around Co3+ (LS) although its ionic radius is smaller in comparison with that of Ga3+.

Spray-Drying to Get Spin-Crossover Materials.

Spin-crossover (SCO) triazole-based coordination polymers can be synthesized by micelle techniques, which almost always lead to rod-shaped nanoparticles. In order to notably reach new morphologies, we explore here the potentiality of the spray-drying (SD) method to get SCO materials. Three SCO coordination polymers and a mononuclear complex are investigated. In all cases, the SD method obtains particles definitely showing SCO. The features of the latter are yet always different from those of the referenced materials, in the sense that SCO is more gradual and incomplete, in adequacy with the poor crystallinity of the powders obtained by SD. In the case of coordination polymers, the particles are preferentially spherical. Indications of possible polymorphism and/or new materials induced by the use of the SD method are evidenced. In the case of the mononuclear complex, the SD method has allowed reproducing, in a quick and easy way, the well-known bulk compound. This exploratory work demonstrates the relevance of the concept and opens the way to a systematic scrutiny of all the experimental parameters to tune the size, morphology, and properties of the SD-synthesized SCO particles.