Site-Selective Ion Intercalation Controls Spectral Response in Electrochromic Hexagonal Tungsten Oxide Nanocrystals

Abstract

The electrochromism of tungsten oxide occurs through electrochemical reduction, providing active solar control to smart windows, but control over the spectral response is limited and largely empirical. To determine how specific chemical changes result in the optical absorption processes responsible for coloration, we pair structurally well-defined electrochromic nanocrystals (NCs) with cations of varying ionic radii to limit intercalation into specific crystallographic sites. The localized surface plasmon absorption of hexagonal cesium-doped tungsten oxide (Cs:WO3) NCs is enhanced, and new absorption features appear, depending on how the cations intercalate into various interstitial voids. These differences were rationalized using X-ray photoelectron and Raman spectroscopies to reveal the chemical and structural changes leading to the observed variations in the electrochemically induced absorption spectra. Smaller cations, Na+ and Li+, can access additional interstitial sites, leading to a secondary, polaronic mechanism of electrochromism, accounting for enhanced visible light absorption.

Dual-Band Electrochromism in Hydrous Tungsten Oxide

“…These electrolytes were selected to compare the influence of cation insertion on the electrochromic response. Due to its small ionic radius, Li + (radius = 76 pm) can participate in charge compensation at the surface and within the bulk via insertion reactions, whereas TBA + (radius = 494 pm) can only participate at the surface. , Electrochemical charge compensation with WO 3 ·H 2 O and WO 3 can be expressed as WO 3 · normalH 2 normalO ; WO 3 + x normalM + + x normale − ↔ normalM x WO 3 · normalH 2 normalO ; WO 3 false( 0 ≤ x ≤ 1 false) where x is the number of electrons and M + is the electrolyte cation (here, M + = Li + or TBA + ). , For reversible electrochromism of tungsten oxides, x is generally limited to a maximum of 1 (e.g., when all W 6+ is reduced to W 5+ ). Chronoamperometry was used to poise the film electrodes at varying states of charge, and optical spectra were collected from the visible (VIS) to the infrared (IR) (350–2000 nm) to assess the electrochromic response of each material.…”

“…Surface plasmonic charge can be compensated via ion adsorption in the electric double layer and does not induce significant structural distortions in the material. The structure of TMOs plays an integral role in determining the nature of the electrochromic response. − …”

Metal–Support Interactions in Pt-WO3 Heterostructures: Role of WO3 Polymorphism

“…The relatively large channels of the h ′-WO 3 hexagonal structure (5.1 Å) should enable hosting various cations, like in the h -WO 3 framework. ,, This characteristic makes them potential candidates in applications based on ion intercalation/deintercalation processes, such as electrochromic devices where ion intercalation leads to tungsten reduction and then optical absorption modification. The h ′-WO 3 structure can only be obtained as nanostructures, which sets favorable conditions for ion exchange, as suggested by shortened intercalation paths and superior tunnel accessibility that enhance coloration efficiency and switching speed, in comparison with bulk materials. ,, Modifying the nature of cations inside the structural channels can also modify the electronic properties of tungsten bronzes, as exemplified by the catalytic properties of tungsten bronzes for soot oxidation, which depends on the alkali cation modifying the electron donor behavior, and by the plasmonic properties of bronzes with the h -WO 3 structure …”

“…1,11,14 of cations inside the structural channels can also modify the electronic properties of tungsten bronzes, as exemplified by the catalytic properties of tungsten bronzes for soot oxidation, which depends on the alkali cation modifying the electron donor behavior, 10 and by the plasmonic properties of bronzes with the h-WO 3 structure. 15 Beyond the properties of bronzes, their conception also enables the design of further new materials. Indeed, cationic exchange enables the incorporation of additional transition metals in WO 3 frameworks.…”

2D-Layered Structure WS2 Nanosheets with Improved Electrochromism for Organic-Based Device

“…In recent decades, Electrochromic device (ECD) has earned a lot of attention [1][2][3] due to its reversible optical regulation properties (transmittance, absorbance or reflectance) at the small potential and broad prospect of application, such as smart window, 4,5 electronic ink, 6,7 energy storage battery, 8,9 and military camouflage. 10,11 Traditional ECD is usually sandwich structure, including electrochromic layer (redox active material), ion conducting layer (electrolyte), and ion storage layer (support the redox activity of electrochromic layer). 12 In fact, at least one layer has to undergo the color change under voltage drive to obtain the overall color control of the device.…”