The frontier of tungsten oxide nanostructures in electronic applications

Abstract

Electrochromic (EC) glazing has garnered significant attention recently as a crucial solution for enhancing energy efficiency in future construction and automotive sectors. EC glazing could significantly reduce the energy usage of buildings compared to traditional blinds and glazing. Despite their commercial availability, several challenges remain, including issues with switching time, leakage of electrolytes, production costs, etc. Consequently, these areas demand more attention and further studies. Among inorganic-based EC materials, tungsten oxide nanostructures are essential due to its outstanding advantages such as low voltage demand, high coloration coefficient, large optical modulation range, and stability. This review will summarize the principal design and mechanism of EC device fabrication. It will highlight the current gaps in understanding the mechanism of EC theory, discuss the progress in material development for EC glazing, including various solutions for improving EC materials, and finally, introduce the latest advancements in photo-EC devices that integrate photovoltaic and EC technologies.

Keywords: Electronic materials; Materials chemistry.

Graphical abstract

Figure 1

Applications of electrochromic devices (A) Design of electrochromic window. Reprinted with permission ref. Copyright 2013, Macmillan Publishers. (B) Smart switchable window applied in Boeing aircraft produced by SmartTintW. (C) Photographs of the electrochromic lens. Reprinted with permission from ref. Copyright 2015, American Chemical Society. (D) Automatic dimming mirror based on electrochromism produced by Gentex. Printable and flexible electrochromic displays designed by (E) Prelonic Technologies and (F) Siemens. Copyright©2023 Elsevier B.V. or its licensors or contributors. ScienceDirect is a registered trademark of Elsevier B.V.

Figure 2

Schematic of the electrochromic device Electrons flow through an external circuit into the electrochromic material, while ions flow through the electrolyte to compensate the electronic charge. Reprinted with permission from ref. Copyright 2014, Royal Society of Chemistry.

Figure 3

Schematic illustration of the formation of WO₃ nanocubes via hydrothermal synthesis Reprinted with permission from ref. Copyright © The Authors, published by EDP Sciences, 2021.

Figure 4

TEM images of tungsten oxide particles TEM images (top) and corresponding high-resolution atomic images (bottom) of tungsten oxide particles: (A), (B) sample ES, monoclinic W₁₈O₄₉; (C), (D) sample MS, hexagonal WO₃; and (E), (F) sample WS, monoclinic WO₃. Reprinted with permission from ref. Copyright © 2023 The American Ceramic Society, all rights reserved.

Figure 5

The EC properties of WO_x films prepared by DC magnetron sputtering (A1 and A2) Capacity of inserted and extracted charge, (B) coloration efficiency, and (C) optical modulation range for ∼300 nm-thick films of W oxide prepared by sputtering at the shown values of O₂/Ar gas flow ratio in the sputter plasma Γ, total pressure of the sputter plasma ptot, and sputtering power Ps. Data were taken for different values of Ps during continuous voltammetric cycling in an electrolyte of LiClO₄ in PC at a voltage scan rate of 20 mV/s. Measured data are indicated by symbols which are joined by straight lines for clarity. Data for Ps = 200 W were shown in Figure 4 and but are included here to allow easy assessment on the role of Ps. Reprinted with permission from ref. Copyright © 2023 Elsevier B.V. or its licensors or contributors.

Figure 6

The XRD pattern of the solvothermally synthesized WO_3-x powder (A) XRD pattern (black line) of the synthesized WO_3-x powder. Red lines corresponding to XRD pattern of the W₁₈O₄₉ (jcpds No. 64–0773) – the insert shows the dark blue color of the synthesized WO_3-x solution in ethanol (∼10 mg mL − 1), (B) XRD pattern of the WO_3-x films annealed at 130°C, 250°C, 350°C, and 450°C, and TEM/HR-TEM (insert) images of (C) the near WO_3-x powder and (D) the 250 W sample (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.). Reprinted with permission from ref. Copyright©2022 The Author(s). Published by Elsevier B.V.

Figure 7

Schematic illustration of the fabrication and electrochromic property of the Ti-doped hierarchically mesoporous silica microspheres/tungsten oxide (THMS/WO₃) hybrid films Reprinted with permission from ref. Copyright©1996–2023 MDPI (Basel, Switzerland) unless otherwise stated.

Figure 8

Conventional procedure for reporting switching times (A) Square-wave potential steps with fixed pulse lengths. (B) Corresponding transmittance evolution at a fixed wavelength. (C) Switching time at an arbitrarily chosen percentage of a full switch (90% in this case), whether for bleaching or coloration. Reprinted with permission from ref. Copyright© 2018 Elsevier B.V. All rights reserved.

Figure 9

Perovskite solar cell (left) harvests solar energy to drive ECD (electrochromic devices)/ECS (electrochromic supercapacitors) to different colored states under different light intensities, and the stored energy of ECS (in deep colored state) can light a red LED (right) Reprinted with permission from ref.Copyright© 2023 Springer Nature Limited.

Figure 10

A metal oxide-based smart supercapacitor electrode by embedding a nanoscale metallic nanofilament array (NFA) (A) Schematic showing the formation of a conducting nanofilament array (NFA) across a WO₃ electrode film via an electroforming process. (B) Resistance-switching current–voltage (I–V) curves of the WO₃ film showing bi-stable resistance states. (C) The resistance distribution of the low- and high-resistance states extracted at a 0.1 V read voltage. XPS spectra of (D) the pristine WO₃ and (E) NFA-embedded WO₃ films. (F) Cross-sectional high-resolution TEM image of the NFA-embedded WO₃ film and (G) a magnified view showing the Moiré fringes. (H) Conductive-AFM image of the NFA-embedded WO₃ electrode measured with a compliance current of 10 nA. Reprinted with permission from ref. Copyright© Royal Society of Chemistry 2023.

Figure 11

Schematic illustration of the synthesis of the 3D WO_3-x NWNs/FTO electrode Copyright © 2023 Springer Nature Limited. Reprinted with permission from ref. Copyright© 2023 Springer Nature Limited.