Using a synthesis route based on carbon as the reducing agent mixed with Nb-doped V2O5 nanopowders, highly crystallized Nb-doped VO2 (M) particles, with tunable content of niobium (Nb), are ...successfully prepared. The morphology and the crystalline structure are first studied in regard to synthesis parameters and chemical composition for various thermal treatment temperatures/durations and niobium doping concentrations. The second part of this paper is devoted to the control, in Nb-doped VO2, of the phase transition temperature from monoclinic to rutile (M → R transition) versus the niobium doping concentration associated with thermochromic properties and different property changes characterized by differential scanning calorimetry and magnetic and electrical measurements. The niobium introduction allows the decrease of the thermochromic transition from 68 °C down to around room temperature for an effective Nb concentration of approximatively 6 mol % for the V0.94Nb0.06O2±δ composition. A significant result is the decrease of both the hysteresis width as well as the phase transition amplitude supported by latent heat, magnetic behavior, and electrical transport versus niobium concentration. These phenomena are correlated to the breaking of the V–V pairing occurring in the monoclinic form when niobium ions are introduced. Finally, a deep interpretation of the Nb-doped VO2 magnetic behavior is performed considering the impact of niobium concentration and crystallite sizes on the intensity of the Curie magnetic residue plus Van Vleck susceptibility in the low temperature form (M) and on the intensity of the Pauli magnetism of the high temperature form (R).
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.
New inorganic pigments with intense and saturated coloration have been prepared by a solid-state route and exhibit a large color scale from magenta to yellow. Indeed, yellow and magenta are two of ...the three subtractive model’s colors with wide application in printing or displays as e-book readers. To develop yellow and magenta hue, we focused on cobalt- and nickel-based orthophosphates thanks to the chemical stability, low density, low price, and easy preparation of such a pigment class. All of these orthophosphates crystallize with the well-known olivine-type structure (orthorhombic Pnma space group) where transition metals are stabilized in a distorted octahedral site. This paper deals with the optical absorption properties of various orthophosphates, the correlations with structural features, and their colorimetric parameters (in L*a*b* color space). The LiCo1–x Mg x PO4 series show near-magenta color with tunable luminosity, while the LiNiPO4 compound exhibits a frank yellow coloration. Co2+ (4T1) and Ni2+ (4A2) chromophore ions occupy a more or less distorted octahedral site, leading to tuning of the intensity of the d–d electronic transitions in the visible and near-IR ranges and providing a subtractive color scale; i.e., a LiCo1–x Ni x PO4 solid solution possesses a very rich panel of colors between the two yellow and magenta extremes. It is worth noting that the crystal-field splitting and B Racah parameter have been estimated in a first approximation on the basis of the Tanabe–Sugano diagram and lead to the conclusion of a slightly higher crystal-field splitting of around 0.9 eV for Ni2+ ions and similar β covalent parameters, despite the same crystallographic sites of both of these transition metals.
WO3−x (oxygen deficient tungsten oxide) thin films and WO3−x/Ta2O5 (tantalum oxide) composite films exhibit a huge photochromic effect with 60% near-infrared transmission modulation, in comparison to ...results in the literature. The colouring efficiency of such films results from both the stoichiometry of the particles (oxygen/tungsten deficient ratio) and the quality of the films (films are thin, homogeneous, and constituted of nanoparticles of about 5 nm in diameter and so with low scattering). Additionally, the high colouring efficiency obtained with a low fluence ultraviolet (UV) lamp is followed by a nearly complete bleaching phenomenon after a few hours in the dark. Beyond the attractiveness of our films toward multiple applications, and especially as films able to regulate the solar flow through smart windows, the optical properties and the colouring and bleaching kinetics were deeply investigated. For both the WO3−x thin films and the WO3−x/Ta2O5 composite films, colouring under irradiation and bleaching in the dark are phenomena that both proceed with a complex kinetics combining ultra-fast processes (from charge transfer, with a characteristic time of about one minute) and slower processes (linked to ionic diffusion, with a characteristic time of about one hour or even longer).
Al-doped ZnO powder was synthesized via the Pechini route with a doping rate varying from 1 to 4
mol.%. A solubility limit has been estimated under 0.3
mol.% of Al using X-ray diffraction ...refinements. The incorporation of aluminium into the ZnO lattice was investigated by
27Al NMR, which suggests an extremely low amount of Al in a distribution of sites in ZnO. In order to assess the impact of such a low dopant amount, diffuse reflection experiments were performed for a wavelength range from 200 to 2500
nm. If the effect of doping was negligible for samples prepared at 850
°C, annealing at 1200
°C clearly reveals enhanced IR absorption properties for the doped samples, which are similar whatever be the nominal Al content.
Comparison of optical properties of pure ZnO and AZO for different temperatures and atmospheres.
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Vanadium dioxide is the key thermochromic compound for the next generation of smart windows. Monoclinic vanadium dioxide with tuned crystallinity is synthesized by a two-step thermal treatment: ...thermolysis of vanadyl ethylene glycolate and postannealing of the poorly crystallized VO2 powder. The proportion of crystallized VO2, calculated with the Fullprof program, increases from 44% to 79% and 100% with increasing annealing temperature up to 700 °C. Besides, V2O3 and V2O5 with particular microstructures are also successfully prepared using this method.
Our e-connected society is eager to develop devices with tunable colors. Electrochromic materials, able to modify their optical properties under an applied voltage, offer a smart solution. In the ...present study, we have successfully synthesized two vanadium oxide powders from a polyol-mediated synthesis, and powder suspensions were coated on glass/ITO substrates by doctor blading. The electrochemical and optical properties of the V x O y films are investigated by cyclic voltammetry (CV) coupled with in situ UV–visible spectroscopy. Both V2O5 and V2O3 films exhibit reasonably good cycling stability, significant reflectance modulation, high optical contrast, and good memory effects revealing the unknown EC properties of V2O3. The similar green ⇄ blue ⇄ orange reversible color changes for both vanadium oxides appear suitable for display application. Then, the evolution of the vanadium cation oxidation states and of the structure of V2O5 and V2O3 upon cycling are analyzed by ex situ XPS and ex situ XRD (at grazing incident angle). This work highlights a robust and novel scenario upon cycling, nearly the same whatever the raw film composition that shows, for each cycle, the crystallization of V2O5 upon oxidation, followed by amorphization upon reduction.
In this study, vanadium sesquioxide (V2O3), dioxide (VO2), and pentoxide (V2O5) were all synthesized from a single polyol route through the precipitation of an intermediate precursor: vanadium ...ethylene glycolate (VEG). Various annealing treatments of the VEG precursor, under controlled atmosphere and temperature, led to the successful synthesis of the three pure oxides, with sub-micrometer crystallite size. To the best of our knowledge, the synthesis of the three oxides V2O5, VO2, and V2O3 from a single polyol batch has never been reported in the literature. In a second part of the study, the potentialities brought about by the successful preparation of sub-micrometer V2O5, VO2, and V2O3 are illustrated by the characterization of the electrochromic properties of V2O5 films, a discussion about the metal to insulator transition of VO2 on the basis of in situ measurements versus temperature of its electrical and optical properties, and the characterization of the magnetic transition of V2O3 powder from SQUID measurements. For the latter compound, the influence of the crystallite size on the magnetic properties is discussed.
Two sets of Ga-doped ZnO powders were synthesized via solid-state and Pechini routes with a substitution rate varying from 0 to 4 mol %. The gallium solubility limit is strongly dependent on the ...synthesis history. Indeed, a low temperature annealing allows incorporating about 1.5 mol % (X-ray diffraction (XRD), inductive coupled plasma spectroscopy (ICP), optical properties) whereas under 0.1% of dopant is introduced after thermal treatment at high temperature: 1500 °C (from XRD and pellets conductivity). The incorporation of gallium leads to an anisotropic distortion of the zincite crystal lattice (a and c parameters increase and decrease, respectively, versus the Ga content leading to a decrease of the c/a ratio) which can be explained from the valence bond model. XRD analysis, chemical titration by ICP, and conductivity measurements (on pellets obtained at high temperature) allow determining accurately the maximum Ga content in the zincite. The optical properties (IR absorption efficiency) linked to electron carriers are directly correlated to the gallium rate introduced in ZnO oxide; nevertheless, the non linear correlation between these two parameters tends to show that the concentration of charge carriers in the system is not equal to the amount of Ga3+ atoms inserted per ZnO volume unit. A saturation regime is observed and was here explained once again on the basis of the valence band model by the increase of inhibiting p type defects with the increase of (n-type donors) Ga3+ concentration.
Transition metal oxides (TMOs) have attracted considerable attention due to their variety of chromogenic properties. Among them, vanadium pentoxide (V2O5) has gained significant interest in respect ...of multichromism associated with orange, green and blue colors. Herein, we report a simple and easy method for the fabrication of Mo doped V2O5 thick films, leading to improved cyclability. Molybdenum doped vanadium pentoxide powders were synthesized from one single polyol route through the precipitation of an intermediate precursor: molybdenum doped vanadium ethyleneglycolate (Mo doped VEG). The as-synthesized Mo-doped V2O5 exhibits improved electrochromic performance in terms of capacity, cycling stability, and color contrast compared to single-component V2O5 in lithium as well as sodium based electrolyte. The improvement in EC performances lies in films of higher porosity as well as higher diffusion coefficients. To conclude, an electrochromic device combining Mo-V2O5 to WO3.2H2O, via a PMMA-lithium based electrolyte membrane exhibits simultaneously reversible color change from yellow to green for Mo-V2O5 and from blue to yellow white for WO3.2H2O with a cycling stability up to 10 000 cycles.
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