Electrochromic Behavior Originating from the W⁶⁺/W⁵⁺ Redox in Aurivillius-type Tungsten-Based Layered Perovskites

Abstract

Simple oxide materials, typically, WO3, have been conventionally employed for electrochromic (EC) materials because of their high coloration efficiency; however, it is quite difficult to realize multiple coloration because they involve redox reactions due to single ions. On the other hand, multiple oxides are expected to show various colors when applying different voltages due to the diverse structures and combinations of ions; however, multiple oxide-type EC materials are still in the research stage, and the discovery of further EC materials is necessary. Toward the development of multiple oxide-type EC materials, tungsten-containing layered perovskites have been synthesized, and their optical properties have been evaluated. X-ray diffraction and X-ray absorption fine structure analyses revealed that the discovered tungsten-based layered perovskites Bi2Na0.5La0.5TiWO9 (BNLTW) and Bi2LaTi1.5W0.5O9 (BLTW) have an orthorhombic phase with an Aurivillius-type layered perovskite structure. EC devices fabricated with three kinds of perovskites, including well-known Aurivillius-type Bi2W2O9 (BWO), have no absorption in the visible-light region when no voltage is applied, while they show absorption over the whole visible-light spectrum to black when a voltage of +4.5 V is applied. Furthermore, with an applied voltage of −4.5, the transmittance recovered to the same level as the initial state, meaning the EC function is reversible. In this reaction, only tungsten in the perovskite framework acted as a redox-active species (W6+/W5+ redox) without the redox of the other metal ions. From the electrochemical analysis of the EC materials using cyclic voltammetry, redox peaks could be observed at −0.2 to 0.4 V for reduction and +0.1 to +0.3 V for oxidation. Interestingly, the redox potentials are linearly related to the W content in the perovskite unit, indicating that the redox potentials can be tuned by controlling the chemical formula. The coloration efficiency of the BNLTW EC device was the best at 37.1 cm2/C in the prepared perovskite-based EC device, which is comparable to that of a typical WO3 EC material.

Temperature-Dependent Electrochromic Devices for Energy-Saving Dual-Mode Displays

“…The peak intensity of the −OH bonds is increased in the colored EC electrode due to the valance reduction of W cations, indicating that more oxygen vacancies are produced after coloration. It is also consistent with the XPS results of relevant EC studies. , …”

“…It is also consistent with the XPS results of relevant EC studies. 36,37 Performances and Kinetics of a Temperature-Dependent EC Device. Taking advantage of the good conductivity of the 1 M PVA/EG−ZnCl 2 organohydrogel between −40 °C and 40 °C, we explored the feasibility of fabricating the EC devices with a wide temperature tolerance.…”

3 nm-Wide WO3–x Nanorods with Abundant Oxygen Vacancies as Substrates for High-Sensitivity SERS Detection

“…In most cases, the V o and W 5+ exist simultaneously due to the fact that one electron tends to escape from the initial V o and is then trapped by a nearby W 6+ ion, leading to a W 5+ ion with an occupied localized 5d orbit . The W 5+ was also identified in the XPS spectra . As revealed in Figure S6, the W 4f XPS spectrum of the WO 3– x showed two main peaks that can be deconvoluted into two pairs of doublets: the main peaks at 35.4 eV (W 4f 7/2 ) and 37.4 eV (W 4f 5/2 ) can be assigned to the W 6+ doublet, while the W 5+ doublet appears as shoulder peaks at 34.4 eV (W 4f 7/2 ) and 36.4 eV (W 4f 5/2 ).…”

“…48 The W 5+ was also identified in the XPS spectra. 49 As revealed in Figure S6, the W 4f XPS spectrum of the WO 3−x showed two main peaks that can be deconvoluted into two pairs of doublets: the main peaks at 35.4 eV (W 4f 7/2 ) and 37.4 eV (W 4f 5/2 ) can be assigned to the W 6+ doublet, while the W 5+ doublet appears as shoulder peaks at 34.4 eV (W 4f 7/2 ) and 36.4 eV (W 4f 5/2 ). The relative contents of the two forms of the W element with different valence states were also calculated using CasaXPS software.…”

Self‐Powered and Light‐Adaptable Stretchable Electrochromic Display

“…These values are in the reasonable range when compared with the reported literature. [38] Moreover, the open circuit potential between the WO 3 electrode and Zn electrode (Figure 5c) was tested to reveal the chemical potential variation of the WO 3 electrode with ion-insertion/ extraction during the self-coloring/bleaching process, [39] which was associated with the optical properties of the WO 3 electrode inside in Figure 5c. When the two electrodes are connected/ disconnected, the WO 3 undergoes the self-coloring/bleaching process with decreasing/increasing reflectance.…”