Discoloration Tungsten Trioxide 2/3
Tungsten trioxide photochromic properties (Photochromism) refers to a compound A when exposed to light of a wavelength of λ1, can be generated with different structures and spectroscopic properties of the product B by specific chemical side, and at a wavelength λ2 of light or heat Under the action, B and compound A reversible phenomenon generated. The basic features of this process are: A, B can under certain conditions stable and significantly different color parallax; change between A, B is reversible. Tungsten trioxide as a transition metal oxide, is an excellent photochromic material, the study found it will turn blue under UV irradiation. In 1975, it was suggested that double charge injection extraction model, the current model has been widely used to explain the photochromic phenomenon. This model indicates that: after WO3 by ultraviolet radiation, the valence band electrons are excited into the conduction band, leaving holes in the valence band, the photo-generated electrons are W (Ⅵ) capture, generate W (Ⅴ), while the optical hole oxidation reducing substances inside or surface film, such as water, generating H +, injected inside the film, with the reduction of oxides generated blue tungsten bronze HxWO3. Blue is the result of W (Ⅴ) in the valence band electrons to W (Ⅶ) conduction band transitions.
Recent research concerning the photochromic properties of WO3 mostly powder and film-based. Sodium tungstate as raw materials, the use of hydrothermal synthesis, in a wide pH range (0.5-4.5), is prepared with high stability, particle size smaller metastable WO3 superfine powder. Wherein after at pH of about 0.5 tungsten bronze synthesized sample was 350W high pressure mercury lamp 10min, color from light blue to dark blue. When the samples were heated at 95 ℃ 30min, dark blue powder experience is oxidized by oxygen in the air, back to the primary colors, showed good reversibility. XPS test confirmed by a hexagonal WO3 price changes in a small amount of tungsten (W5 ) exist to give discoloration is W5 and W6 into each other produce.
Sodium tungstate tungsten source, adjust the pH value of hydrochloric acid, formaldehyde, organic induced hydrothermal synthesis to obtain better dispersion of nanometer WO3 powders. In 3W ultraviolet light, not add formaldehyde WO3 powders color value of less than 2, and formaldehyde-induced WO3 nano powder color value of 20, it was found formaldehyde induced significantly improved photochromic properties WO3 powders. The results are formaldehyde-induced synthesis of nanosized WO3 powders smaller than ultraviolet wavelengths, quantum size effect significantly, WO3 excited by light electrons from the valence band to the conduction band transitions easier; larger surface area have higher reactivity and excitation light is strong absorptive capacity. When the pH value is small, the same multi-tungstate ion photochromic effect, so from this point to start, choose a water-soluble polymer of polyacrylic acid (PAA) as a carrier and a film, adding an appropriate amount of glycerin as a humectant, prepared Na2WO4-PAA-Glycerol composite film. After the film is affected by the sun light, color began to change color after 10s, 10min after the change to the deepest, then faded into the dark or light blue to colorless, less than 2min, showing good speed and discoloration discoloration corresponding reversibility.
Since the aqueous film, which will help the electron donor carboxylate, small molecule oxidation products, oxygen diffusion, thus discoloration and fading speed are faster. For the experimental results can be obtained from the discoloration mechanism W (Ⅴ) and W (Ⅵ) reversible changes. This is consistent with the double charge injection extraction model. As can be seen, WO3 ultrafine photochromic body also more demanding conditions (350W high pressure mercury lamp), and the effect of nano-sized powders and Na2WO4-PAA-Glycerol composite film better. This is due to the process of the photochromic substance migration (e.g. electronic transition, the electron donor, oxidation products, and oxidant diffusion) easier.
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