Efficient synthesis of plate-like crystalline hydrated tungsten trioxide thin films with highly improved electrochromic performancew

In recent years, nanostructured materials have attracted much attention due to their large surface area and size dependent properties, which offer great advantages in various applications. Tungsten trioxide (WO3), a well-known transition metal oxide, has evoked wide interest owing to its distinctive physical and chemical properties that make it suitable for extensive applications in electrochromic (EC) devices, gas sensors, photocatalytic systems and photoelectrochemical devices. EC devices made of WO3 films have been intensively studied during the last few years, because of their promising applications in energy-efficient windows that can control the solar light transmission indoors through reversible color changes, contributing therefore not only significantly to energy saving in buildings (as a result of curtailing of air conditioning) but also comfortable esthetics. WO3 can display colorless and blue color by alternately applying suitable positive and negative electrical voltages, which induces the cations such as proton (H) and lithium (Li) to insert into or extract from the host material at the same time. Compared with the amorphous structure, crystallineWO3 demonstrates better durability due to the denser structure and slower dissolution rate in electrolytes, especially in acidic ones where a faster ion diffusion kinetics can be achieved than in the Li system. Nanostructured crystallineWO3 with a large specific area and a porous structure is expected to significantly improve the coloration/bleaching response and coloration efficiency by reducing the ion diffusion path length and resistance. In this work, we report the development of plate-like nanostructured WO3 (hydrate) films via an efficient and facile crystal-seed-assisted hydrothermal approach with sodium sulfate (Na2SO4) used as the capping agent. The film shows a fast switching response (tc90% = 4.3 s and tb90% = 1.4 s) and a highly improved coloration efficiency (112.7 cm C ). In a typical procedure, a crystal-seed-coated FTO substrate and a peroxopolytungstic acid precursor containing Na2SO4 were hydrothermally treated at 180 1C for 2 h (see the experimental section in ESIw). Fig. 1a shows the X-ray powder diffraction (XRD) patterns of the bare FTO substrate and the as-prepared thin films grown with and without Na2SO4. The XRD pattern of the substrate can be clearly indexed to tin oxide (JCPDF 70-4176). The hydrothermally grown films have the same crystalline structure since all diffraction peaks appear at the same position, and no other impurity peaks are observed. The sharp peaks indicate the good crystalline quality of the as-fabricated films. All peaks can be well indexed to the orthorhombic phase of