We demonstrate the benefit of homemade nanopowder precursors on the electrochromism of V2O5 films deposited by the “Doctor Blade” method. Using the polyol process, nanostructured V2O5 powder were ...synthesized. Orthorhombic V2O5 thin films deposited from as-synthesized powder exhibit good cycling stability associated with significant reflectance modulation in both lithium- and sodium-based electrolytes. The orange to green reversible color change appears well suitable for display application. To conclude, the electrochromic performances of complete devices using WO3 as complementary electrode and 0.3M Lithium Bis(Trifluoromethanesulfonyl)Imide LiTFSI in BMITFSI plastified with polymethylmetacrylate (PMMA) membrane electrolyte are reported.
•V2O5 nanopowders synthesized by the polyol process.•Preparation of V2O5 thin films by the “Doctor Blade” method.•Colorful electrochromism in Li- and Na-based electrolytes.•WO3/V2O5-based EC device for display application.
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.
The oxygen substoichiometry of as-prepared WO3 (tungsten trioxide) nano-powders (NPs) from a polyol process was tuned via peptization of the NPs in aqueous solutions of different Cr2O72− oxidizing ...concentrations. The as-synthesized materials have been characterized by X-ray scattering (XRD and PDF), transmission electron microscopy (TEM), X-ray photoelectron (XPS), and UV-VIS photochromic activity. The local atomic structure of WO3-X was investigated using total scattering atomic pair distribution function (PDF) analysis based on X-ray total scattering data collected on powder at ambient conditions. The PDF analysis confirms that the crystal structure of all studied samples can be described in terms of very small crystallites with a P21/n space-group monoclinic framework but with anomalous unit cell distortion parameters, indicating that small crystallite sizes resulted in a larger monoclinic distortion (as measured by the beta angle of the unit cell). The nanometer dimension of the crystallites combined as well as the oxygen-tungsten stoichiometric ratio control, are key features for optimized photochromic properties. Moreover, we present the fabrication of a WO3-x thin film based UV photosensor, which was carried out on silica glass substrates via the dip-coating method. The obtained films exhibited a UV photoresponse and photoelectric characteristics at 5 V bias voltages able to detect very low UV doses, inferior to 10 W/m2. The photo-detection measurements prove the usability of our device as a UV photodetector with a good responsivity of 0.37 A/W and external quantum efficiency of more than 100% even at a very low power density (9.2 W/m2) of UV illumination.
•WO3-x nano-powders prepared from a polyol process with tunable oxygen sub-stoichiometry.•Local atomic structure of WO3-X determined thanks to coupled X-ray and PDF analyses.•Optimized colors and photochromic properties obtained thanks to oxygen substoichiometry control.•Best WO3-x thin films with a UV photoelectric response able to quantify micro UV dose.
ZnO/MoO3 powder mixture exhibits a huge photochromic effect in comparison with the corresponding single oxides. The coloring efficiency of such combined material after UV-light irradiation was ...studied in terms of intensity, kinetics, and ZnO/MoO3 powder ratio. Additionally, the incidence of the pretreatment step of the ZnO and MoO3 powders under different atmospheres (air, Ar or Ar/H2 flow) was analyzed. The huge photochromic effect discovered herein was interpreted as the creation of “self-closed Schottky barrier” at the solid/solid interfaces between the two oxides, associated with the full redox reaction which can be pictured by the equation ZnO1−ε + MoO3 → ZnO + MoO3−ε. Remarkable optical contrast between virgin and color states as well as self-bleaching in dark allowing the reversibility of the photochromism is emphasized. From this first discovery, deeper characterization of the self-bleaching process shows that the photochromic mechanism is complex with a bleaching efficiency (possibility to come back to the virgin material optical properties without any deterioration) and a bleaching kinetics, which are both dependent on the coloring irradiation time. This demonstrates that the oxygen exchange through the Schottky interface proceeds in at least two convoluted steps: an anionic surface exchange allowing a reversibility of the redox reaction followed by bulk diffusion of the exchanged anions which are then definitively trapped. An emergent “negative photochromism effect” (i.e., photochromism associated with a self-bleaching instead of a darkening under irradiation) is observed after a long irradiation time.
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.
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.
A thorough investigation by X-ray diffraction, UV–vis and luminescence spectroscopy is carried out to demonstrate how the chromium content of alumina matrices impacts the temperature of the γ→α ...irreversible phase transition. The Cr3+ contents influence slightly the phase transition temperature but control the brightness of the powders. Nice colorimetric contrasts from green to pink are observed between the two allotropic forms. Furthermore, drastic changes of the spectral distribution and of the intensity of luminescence are observed, thus allowing to use this pigment as a both thermochromic and luminescent thermal sensor. Additional measurements at low temperature revealed that the Cr3+ emission of the γ-Al2O3 matrix is constituted by a large band. A configurational diagram schematic approach suggested for the first time that this emission is due to spin-allowed 4T2→4A2 transition on this largely investigated γ-Al2O3 compound.
WO
3−
x
(oxygen deficient tungsten oxide) thin films and WO
3−
x
/Ta
2
O
5
(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 WO
3−
x
thin films and the WO
3−
x
/Ta
2
O
5
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).
WO
3−
x
(oxygen deficient tungsten oxide) thin films and WO
3−
x
/Ta
2
O
5
(tantalum oxide) composite films exhibit a huge photochromic effect with 60% near-infrared transmission modulation.
•Gram scale synthesis of pure monoclinic VO2(M1) powder with high crystallinity.•Innovative sintering process combining SPS and induction annealing to prepare smooth VO2 ceramics.•Remarkable ...thermochromic behavior investigated by resistance measurements.
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In this paper, a pyrolysis method is used to synthesize high quality VO2 powder. Black carbon and V2O5 powder are mixed and annealed at 650 °C or 800 °C under argon flow to initiate a carbo-reduction reaction. The 650 °C VO2 powder presents submicron particle size with an amorphous fraction estimated at 11.5%. With the increase of the annealing temperature up to 800 °C, grains become larger than the micron while the crystallinity goes up to 98.5% (i.e. amorphous fraction drops to 1.5%). This one-step process allows the preparation of quantities up to 10 g of VO2 powder and should be easily transferable to large-scale production. An innovative sintering process is then adopted to produce high quality thermochromic VO2 ceramics from the as-prepared VO2 powder. First, SPS process is used to obtain high-density VO2 pellet. Then, the VO2 pellet is post-annealed in a homemade induction furnace at 1300 °C under a controlled atmosphere to allow grain growth by coalescence and improve pellet surface quality. According to a surface topography analysis, the mean amplitude roughness parameter (Ra) of the obtained VO2 ceramic is found to be 2.93 ± 0.01 nm. A large resistance change by three orders of magnitude along the phase transition is observed, which makes the as-prepared smooth VO2 ceramic a promising candidate for various applications requiring optical and/or electronic switching behavior depending on temperature.
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