Recent advances in tungsten oxide-based chromogenic materials: photochromism, electrochromism, and gasochromism

Abstract

As n-type and wide-bandgap semiconductor materials which are widely found in nature, tungsten oxides (WO_x) have attracted extensive attention because of their rich phase structures and unique sub-stoichiometric properties. Tungsten oxides have a good chromogenic response to optical, electrical, and gaseous stimuli, in which their phase changes with the change of temperature and ionic embeddedness, accompanied by significant changes in their optical properties. In addition, due to the presence of oxygen defects, the conductivity and adsorption capacity of tungsten oxides for surface substances are enhanced. These properties endow tungsten oxides with promising application potential in the optical and electronic device areas. This paper reviews the structural and optoelectrical properties of tungsten oxide-based chromogenic materials. Then we focus on the working mechanisms, performance indexes, and preparation methods of tungsten oxides in the field of intelligent chromogenic technology, including photochromism, electrochromism, and gasochromism of tungsten oxide-based chromogenic materials. Finally, a conclusion and outlook are provided, which may help to further advance the application of tungsten oxides in the field of smart chromogenic changes.

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Yaqi Zhang

Yaqi Zhang received her B.S. degree from Southwest University in 2021. Currently, she is pursuing her M.S. degree in the School of Chemistry and Materials, Nanjing University of Information Engineering and is an exchange student at the Department of Chemical Engineering, Tsinghua University, under the supervision of Prof. Jingfa Li and Prof. Rufan Zhang. Her main research interests include electrochromic materials and devices.

Yilin Ding

Yilin Ding received her PhD degree in Condensed Matter Physics from Beihang University, China (2023), under the guidance of Prof. Xungang Diao. Currently, she is conducting her postdoctoral work in the Department of Chemical Engineering, Tsinghua University. Her main research focuses on electrochromic materials and devices.

Jingfa Li

Jingfa Li received his PhD degree from the University of Science and Technology of China in 2012. After that, he moved overseas to the University of Wollongong in Australia and the National University of Singapore to pursue his postdoctoral research. He joined in Nanjing University of Information Science and Technology as a full-time professor in 2016. His research interests mainly focus on the design and synthesis of novel micro-/nanostructured materials for energy conversion and storage devices.

Rufan Zhang

Rufan Zhang recieved his PhD degree from Tsinghua University in 2014. After that, he worked as a postdoctoral researcher in the Department of Materials at Stanford University. He joined Tsinghua University in 2018 and was appointed as an associate professor in the Department of Chemical Engineering at Tsinghua University in 2019. His research interests mainly focus on carbon nanotubes and functional fibers, electrochromic materials and devices, structural chromogenic materials, radiation-cooled materials, and zinc–air battery catalysts.

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1. Introduction

In recent years, the fast-growing global energy consumption and carbon dioxide emission have posed serious challenges to the sustainable development of human society. Therefore, it is of significant importance to develop green, clean, and renewable energy technologies. Tremendous efforts have been made in recent years on solar cells,^9,10 fuel cells,¹⁴ photocatalytic materials,¹⁹ and photothermal management materials.^20,21 Among the most commonly used photothermal management materials, intelligent chromogenic materials have attracted wide attention because of their advantages in smart windows, building envelope materials and infrared camouflage.^22–25 The optical properties of intelligent chromogenic materials can be largely changed under external stimuli such as light, electricity, heat, pressure, etc. As abundant semiconductor materials found in nature, tungsten oxides (WO_x) have a chromogenic response to a wide range of external stimuli such as light, electricity, and gas, which are therefore considered as among the most popular inorganic intelligent chromogenic materials.

Tungsten oxides have various phase structures, which can be converted between each other with changes in temperature and the extent of ionic embedding, accompanied by significant changes in their optical properties, such as visible and infrared light transmittance, absorbance, reflectance, etc. Therefore, tungsten oxide materials have photochromic, electrochromic, gasochromic and thermochromic properties. A perfect WO₃ crystal is a cubic crystal composed of [WO₆] octahedra in a co-vertex mode and is the same as the ReO₃-type chalcogenide structure, which endows WO₃ with an excellent charge storage and transfer capability. In addition, WO_xx has unique sub-stoichiometric properties. Due to the presence of different degrees of oxygen defects, its electrical conductivity, adsorption capacity for surface substances, etc. are enhanced to different extents. In recent years, the complex multivalent properties and multi-defect structure of WO_x have shown great potential for applications in lithium batteries, supercapacitors, gas sensors, and smart windows (Fig. 1).^29–33

Fig. 1 Applications of WO_x in photochromic, electrochromic, and gasochromic materials and devices.

Herein, we briefly review the structural and sub-stoichiometric properties of WO_x materials and introduce the influence of these properties on their optoelectrical performance. Then we discuss the working mechanism of WO_x materials in intelligent chromogenic technology, including photochromism, electrochromism and gasochromism, evaluation indicators and the methods of performance enhancement.

2. Fundamental properties of WO_x

Materials are intricately linked between their structures and properties. The WO_x family is a series of materials with diverse structures and complex chemical states, a feature that makes the variation of its optical and electrical properties of great interest.

2.1 Crystallography and sub-chemometrics

As a common member of the WO_x family, the WO₃ crystal is a cubic crystal composed of [WO₆] octahedra in a co-vertex manner, which is shown in Fig. 2a. In other WO₃ crystal forms, it can be seen as an assembly of structural units obtained by rotating or twisting on the basis of an ideal WO₃ crystal structure. As the temperature increases, WO₃ undergoes a phase transition, which varies sequentially from ε-WO₃ (monoclinic phase, below −43 °C) to δ-WO₃ (triclinic phase, −43–17 °C), γ-WO₃ (monoclinic phase, 17–330 °C), β-WO₃ (orthorhombic phase, 330–740 °C), and α-WO₃ (tetragonal phase, above 740 °C) (Fig. 2b–f).^4,34 Among various types of WO₃, γ-WO₃ is the most common stable phase at room temperature. Besides the above-mentioned crystal types, h-WO₃ (hexagonal phase) is also a relatively stable crystal type of WO₃ (Fig. 2g).⁴ However, it cannot be obtained from other crystalline structures by heat treatment but can only be prepared by dehydration of WO₃ with water of crystallization. Such specific preparation conditions result in the presence of many hexagonal and triangular pores in h-WO₃, which facilitates the transport of ions inside the structure and also provides active space for redox reactions. Compared with crystalline tungsten oxides, amorphous tungsten oxides have disordered bond lengths and bond angles, and there are a large number of dangling bonds and various structural distortions and defects in their distorted structures with a more active electronic structure.³⁵ Higher atomic disorder and interstitial spacing of amorphous phase tungsten oxide can serve as a fast channel for ion diffusion, exhibiting excellent properties and performance of tungsten oxides-based electrochromic devices.³⁶ In general, amorphous tungsten oxides have enhanced optical modulation and coloring efficiency over bulk crystalline films due to their relatively loose structure, yet their stability is limited.

Fig. 2 (a) Ideal WO₃ crystal. (b) Monoclinic WO₃. (c) Triclinic WO₃. (d) Monoclinic WO₃ (17–330 °C). (e) Orthorhombic WO₃. (f) Tetragonal WO₃. (g) Hexagonal WO₃. Reproduced with permission.⁴ Copyright 2019, John Wiley and Sons. (h) Sub-stoichiometric WO_3−x. Reproduced with permission.¹² Copyright 2021, Springer Nature.

Based on the content of oxygen vacancies in the lattice, WO_x is classified as stoichiometric WO_x and sub-stoichiometric WO_x. In 1989, Glember and Saurr first found the sub-stoichiometric properties of WO_x.^37,92 Since then, a large number of WO_x materials with sub-stoichiometric ratios have been continuously reported, in which the value of x is between 2 and 3, which represents the concentration of oxygen vacancies. Thus, a series of sub-stoichiometric WO_x materials (including W₃₂O₈₄, W₃O₈, W₁₈O₄₉, W₁₇O₄₇, W₅O₁₄, W₂₀O₅₈, etc.) exist between the two stable tungsten oxides, WO₃ and WO₂ (Fig. 2h).¹² The crystal structure of sub-stoichiometric WO_x is similar to that of stoichiometric WO₃, also composed of co-rimmed [WO₆] octahedra, with the difference that the ordered oxygen defect surface acts as its crystal shear (CS) surface to split it.³⁸ WO_2.87 and higher concentrations of oxygen vacancies in WO_x result in the formation of pentagonal columns (PCs), leading to the appearance of new crystalline phases.¹² Taking the advantage of the structural features of the [WO₆] octahedral edges, the WO_x lattice can withstand a considerable degree of oxygen deficiency, and part of W⁶⁺ atoms are reduced to W⁵⁺. The absence of lattice oxygen atoms affects the energy gap and free electron density of WO_x. With the increase of oxygen vacancy concentration, the metallic properties of WO_x are revealed. Therefore, diverse and interesting physicochemical properties can be tapped by simply modulating the stoichiometric ratio of WO_x.

2.2 Optical and electrical properties

WO_x is an n-type and wide-bandgap semiconductor material with an indirect bandgap between the valence and conduction bands, where the valence band consists mainly of O 2p orbitals and the conduction band consists mainly of W 5d orbitals.³⁷ W 5d orbital occupancy is strongly correlated with lattice distortion, thus the change in crystal phase has a significant effect on the band gap of WO_x. The bandgap of bulk phase γ-WO₃ is usually in the range of 2.6–2.8 eV. The bandgap varies with the degree of distortion of the crystal structure, therefore amorphous WO_x with large lattice distortion has a wide bandgap of 3.4 eV. Apart from the crystalline phase structure, the size of the WO_x material also affects its band gap. Due to the quantum confinement effect, when the size of WO_x is close to or smaller than its exciton Bohr radius (3 nm), its bandgap becomes wider as the particle size decreases. In contrast, when the crystal size is larger than the exciton Bohr radius, the effect of size on the bandgap diminishes.³⁹

Standard stoichiometric WO₃ powder is yellow, while sub-stoichiometric WO_x is variable in color. For instance, WO_2.9 is bluish-purple, WO_2.72 is purplish-red, and WO₂ is brown. The light absorption range of WO_x covers the entire solar spectrum, i.e., the ultraviolet, visible, and near-infrared bands, and the whole process involves electron interbond leaps, polariton leaps, and plasmon resonance (LSPR).^40–42 Absorption in the visible region may be related to the level of oxygen defects, a process that can be elucidated in terms of the polariton jump mechanism. The injected electrons are first trapped by the W 5d orbitals, and the lattice around them undergoes polarization to produce polaritons. Incident photons can induce mutual leaps of polaritons from two neighboring W sites, which is thought to be the reason for the blue color of WO_x materials when stimulated by light, gases, and electric fields.³⁷

In recent years, near-infrared absorption of sub-stoichiometric WO_x has also received extensive attention. Sub-stoichiometric WO_x has oxygen vacancies that increase its carrier density. At higher concentrations of oxygen vacancies, the absorption of WO_x in the near-infrared (NIR) band is greatly increased, which is usually attributed to the generation of the LSPR effect.^43,44 LSPR is usually found in noble metal nanoparticles, which absorb photon energy strongly when the incident photon frequency matches the overall vibrational frequency of the carriers, which results in a strong resonance absorption peak in the spectrum. Sub-stoichiometric WO_x is distinguished from other semiconductor oxides due to its rather high carrier density, which is also necessary for the strong LSPR effect.^8,45–48 The tunable plasmon resonance of sub-stoichiometric WO_x gives it a strong absorption in the near-infrared region, and therefore has a great potential for application in the field of building energy efficiency.

3. Photothermal management applications

Owing to its unique physicochemical properties, WO_x offers great potential for its application in optical and electrical fields. This section focuses on the working mechanism, performance indexes, and preparation methods of WO_x in intelligent chromogenic technologies, including photochromism, electrochromism, and gasochromism.

3.1 Photochromism

Photochromism is one of important branch of photochemical and photophysical applications which is a reversible process triggered by light absorption and other external stimuli (e.g., light avoidance, thermal stimuli, etc.). As shown in Fig. 3a, a chemical reaction occurs when A is irradiated with light to produce product B. B is then able to be reconverted to A after overcoming an energy barrier or a photothermal stimulus, with a distinct difference in its absorption spectrum and a change in color.¹ Photostimulation has the advantages of being clean and contactless, and can be adjusted remotely and precisely (wavelength, intensity, coverage area, duration, etc.). Therefore, photochromic materials have a promising application in the fields of data storage,^49,50 information displays,¹¹ sensors,²⁶ erasable rewritable paper,⁵ and smart windows.^51–53

Fig. 3 (a) Photochromic process. Reproduced with permission.¹ Copyright 2011, Royal Society of Chemistry. (b) Illustration of the photochemical reaction mechanism in the WO₃/PVA/PAA hybrid system. (c) Time evolution and color holographic reconstruction of the first-order diffraction efficiency in the recorded configurations at 457 nm, 473 nm and 532 nm for WO₃ and WO₃/PVA/PAA films. Reproduced with permission.¹³ Copyright 2022, Royal Society of Chemistry. (d) Transient photovoltaic and photocurrent responses of pristine WO₃, pristine Bi₂WO₆ and WO₃/Bi₂WO₆. (e) Schematic insertion of Bi atoms on the [WO₆] octahedral framework, WO₃/Bi₂WO₆ photogenerated carrier separation and the photocatalytic color change mechanism. Reproduced with permission.²⁶ Copyright 2023, Elsevier.

WO_x, as the most representative inorganic photochromic material, has received much attention. Although the photochromism of WO_x has been studied for several decades, the mechanism of photochromism is still not well defined. Researchers have proposed many different models to explain the photochromic phenomenon on the basis of their respective experiments. Here, we give a brief overview of the photochromic mechanism of WO₃. Under UV irradiation, WO₃ produces photogenerated electrons and holes (eqn (1)). The photogenerated holes decompose the adsorbed water of the material, thereby generating protons, which participate in water decomposition together with the photogenerated electrons (eqn (2)). The photogenerated electrons and protons then reduce some of the W⁶⁺ ions in the WO₃ to W⁵⁺, forming a blue tungsten bronze at the same time (eqn (3)). The tungsten bronze is oxidized by oxygen in the air (eqn (4)), a process known as photochromic bleaching.

H₂O + 2h⁺ → 2H⁺ + O (2)

WO₃ + xH⁺ + xe⁻ → H_xW_1−x⁶⁺W_x⁵⁺O₃ (3)

Excitation wavelength, coloring time, bleaching conditions, bleaching time, and the degree of color changes are key parameters for evaluating photochromic performance. The excitation wavelength is the maximum absorption wavelength that can trigger the photochromic behavior, and a larger excitation wavelength not only improves the solar photoelectricity utilization, but also generates more photogenerated electrons. The relationship between the band gap (E_g) and the excitation wavelength (λ) of semiconductor materials is shown as follows:

where h is the Planck constant (6.63 × 10⁻³⁴ J s) and c is the speed of light (2.998 × 10⁸ m s⁻¹ in vacuum).⁵⁴ Hence, reducing the band gap by modifying the material to redshift the excitation wavelength is one of the ways to effectively improve the photochromic properties of the material. In a recent study, Zhang et al. embedded photochromic WO₃ nanoparticles in a polyvinyl alcohol/polyacrylic acid two-component polymer (PVA/PAA).¹³ The embedding of PVA/PAA resulted in a significant redshift in the absorption spectrum of the film, with the edge of the absorption band stretching from 400 nm to 550 nm, which increased its photosensitivity and sunlight availability. With homogeneous transparency and suitable density, the WO₃/PVA/PAA nanocomposite film is capable of plasmonic diffusion and electron transfer, which achieves an efficient photochromic reaction under broadband excitation (Fig. 3b and c). Tuning the band gap of WO₃ by elemental doping to improve its photochromic properties has been a common approach. Akoth Okwako et al. successfully synthesized Al–Pt co-doped WO₃ thin films on the surface of soda lime glass using a low-cost spin-coating and screen-printing method.⁵⁵ The band gaps of the Al-doped and Al–Pt co-doped WO₃ films were 1.88 eV and 1.95 eV, respectively, which were greatly reduced compared with the optical band gap of WO₃ (2.36 eV). Optical tests indicated that the optical transmittance of the device assembled from this photochromic thin film assembly decreased by 43.61% in 10 min, and more than 85% of the initial transmittance could be recovered by bleaching in the dark for 2 h.

Although the band gap can be modified to enhance light absorption, the complexation of photogenerated electrons and holes needs to be taken into account to obtain a better photochemical energy conversion efficiency. Suppressing photogenerated electron–hole pair composites, improving the carrier lifetime, increasing the number of photogenerated electrons in the system, and utilizing these carriers to initiate photochemical reactions lead to higher chemical reaction rates and photochemical energy–chemical energy conversion efficiencies. The construction of heterojunctions is one of the commonly used strategies to inhibit photogenerated electron–hole pair complexation. Dong et al. synthesized WO₃/Bi₂WO₆ heterostructure nanoparticles with efficient sunlight-responsive photochromism by introducing Bi atoms into the [WO₆] framework to form a [Bi₂O₂] component (Fig. 3d).²⁶ The WO₃/Bi₂WO₆-type II heterostructure ensured efficient photoexcited electron transfer from Bi₂WO₆ to WO₃, and the abundant [WO₆] units served as fast sites for trapping and consuming photogenerated electrons, effectively promoting the separation of photogenerated electrons and holes (Fig. 3e). In addition, compounding with organics or noble metals (Au, Ag, etc.) is also a common solution. Precious metals are deposited on the surface of semiconductors, forming a Schottky barrier at the interface between the two and facilitating the separation of photogenerated carriers.^56,57 As organics are electron donors and act as hole sacrificers to eliminate photogenerated holes in semiconductors, the compounding of photogenerated electrons and holes in the semiconductors themselves is prevented.^58,59 Si et al. introduced fluor-silane and designed a WO₃-based photochromic material with superamphiphobicity, low adhesion, heat and chemical resistance, as well as UV and abrasion resistance (Fig. 4a).⁵ Fluor-silane acted as an electron donor to promote rapid electron transfer and proton insertion, which significantly improved the photochromic properties of WO₃. Subsequently, they verified the photochromic performance of the material and its surface antifouling and self-cleaning ability by UV repetitive writing erasure, as illustrated in Fig. 4b.

Fig. 4 (a) Schematic illustration of the F-WO₃/ZnO superamphiphobic powder surface adapting to harsh environmental conditions. (b) Process flow diagram for ultraviolet (UV) mask writing and thermal erasure. Reproduced with permission.⁵ Copyright 2024, Elsevier. (c) Schematic demonstration of the photochromic process in WO₃·0.33H₂O hybrid nanostructures. (d) Schematic photochromic mechanism diagrams of different WO₃·0.33H₂O hybrid nanostructures. Reproduced with permission.¹⁶ Copyright 2022, Elsevier. (e) HR-TEM pattern of the WO₃ nanofiber. (f) UV-Diffused reflectance spectra of the WO₃ nanofiber. (g) Number of photochromic reruns of the WO₃ nanofiber. Reproduced with permission.²⁷ Copyright 2022, Springer Nature.

The photochromic behavior depends on the electronic energy band structure and charge transfer process, which are affected by the physical and chemical properties of the photochromic materials. G. Deonikar et al. introduced plasma ions (Cu and Al) and non-plasma ions (Zn and Sm) to modulate tungsten oxide hydride (WO₃·0.33H₂O) nanostructures in order to enhance their reversible photochromic ability (Fig. 4c).¹⁶ Based on the double insertion-extraction model of ions and electrons, they explained the effect of different metal ions on the photochromic mechanism of WO₃ (Fig. 4d). They found that the insertion of hot electrons generated by plasma metals (Cu and Al) into the WO₃·0.33H₂O nanostructures inhibited the complexation of photogenerated electrons and holes, enhanced light absorption, and improved the photochromic ability of the hybrid materials.

In addition to the structural design of WO_x photochromic materials, the modulation of morphology and size is also an important part to be considered in order to improve their photochromic properties, especially the light modulation ability and bleaching efficiency. Ejeromedoghene et al. reported a WO₃ nanofiber synthesized by non-electrostatic spinning under moderate conditions with particle diameters in the range of 200 nm (Fig. 4e).²⁷ This WO₃ fiber rapidly changed its visual color from white to blue with excellent reflectance (72%), reproducibility and stability after exposure to UV light for up to 7 s, as shown in Fig. 4f and g. Zhu et al. prepared extremely small-sized WO₃ quantum dots (WQDs) and WQDs-polyvinyl alcohol (PVA) transparent films by the casting method (Fig. 5a and b).⁶ The WQDs are monodisperse near-spherical crystals with an average diameter of 1.2 nm.⁶⁰ Owing to the size effect of WQDs and the provision of protons by polyol, the composite films exhibited a fast light response (<60 s) and a large optical modulation amplitude (>90%) before and after coloration (Fig. 5c and d). However, their bleaching time was more than 8 h. Recently, Meng et al. reported a method for the in situ growth of WO₃ nanoparticles in PMMA matrix and obtained highly dispersed small-sized WO₃ nanoparticles in composite films simply and efficiently by utilizing the spatial confinement effect of PMMA chains (Fig. 5e).¹⁸ This method allowed the preparation of photochromic films with high luminescence transparency (transmittance T_lum = 91%) and scalability (30 × 350 cm²) at low cost. The high modulation of visible light (ΔT_lum = 73%) and solar heat (modulated solar transmittance ΔT_sol = 73% and modulated solar heat gain coefficient ΔSHGC = 0.5) by the films improved indoor daylight comfort and energy efficiency (Fig. 5f and g). Furthermore, they introduced Cu ions into this film to accelerate its bleaching process (20 min). This photochromic film, with its excellent light modulation capability, short bleaching time and ease of large-area preparation, offered an attractive strategy for achieving more energy-efficient buildings and carbon neutrality.

Fig. 5 (a) SEM image of the PVA WQD/PVA film. (b) Digital photographs of colored and bleached WQD-PVA films (25 cm × 30 cm). (c) Variation of the transmittance of the WQD-PVA film with UV irradiation time and its transmittance change at different times in the dark. (d) Real-time data of the outdoor experiment of the WQD-PVA film. Reproduced with permission.⁶ Copyright 2022, Elsevier. (e) Schematic diagram of the preparation process of Cu-W-PC films. (f) Transmission spectra of Cu-W-PC films to standardized AM1.5 solar spectra (green shading) under coloring and bleaching conditions (6 h of outdoor sunlight). (g) Photographs of 30 × 40 cm² Cu-W-PC films prepared by the blade coating method in the bleached (left) and colored (right) states. Reproduced with permission.¹⁸ Copyright 2023, John Wiley and Sons.

3.2 Electrochromism

Electrochromism is the phenomenon in which a material reversibly changes its color or optical properties (absorbance, transmittance, reflectance, etc.) through redox reactions in the presence of an applied voltage or current.^61–63 In 1961, the concept of “electrochromism” was first proposed by Platt.⁶⁴ Subsequently, Deb discovered the electrochromic phenomenon of WO₃ for the first time,⁶⁵ and proposed the electrochromic principle of the color-centered theory on the basis of this research, which became an important milestone in the history of the development of electrochromism.⁶⁶ As the most classic inorganic electrochromic material, WO_x has been widely used in the fields of infrared stealth,^67–69 flexible electronic devices,⁷⁰ and smart windows.^2,21,71,72

Regarding the electrochromic mechanism of WO₃, the F-color core model and the dual ion implantation model are mainly discussed and accepted at present. The F-color center model was first proposed by Deb in 1973,⁷³ aiming to explain the electrochromic mechanism of amorphous WO₃ at that time. He suggested that amorphous WO₃ has an ionic crystal structure with oxygen vacancies in localized states, and that when a negative voltage is applied, electrons enter the oxygen vacancies and are trapped by them to form the F-color center, and the electrochromic film changes from colorless to blue. When a positive voltage is applied, the electrons are removed from the oxygen vacancies and the F-color center disappears, and the electrochromic film changes from blue to colorless. The dual ion implantation model, also known as the Faughnan model, is currently the most accepted mechanism model for electrochromism in the field since it explains not only the phenomenon of WO₃ electrochromism, but also other common electrochromic materials.⁷⁴ When a negative voltage is applied, the electrons enter into the interior of the material, and the cations enter at the same time and undergo valence transitions to produce the color change. When the opposite voltage is applied, the electrons and ions are simultaneously withdrawn, causing the color to return to its initial state.

There are mainly six performance parameters for evaluating electrochromic materials and devices. The first is the contrast ratio (CR), which reflects the degree of color changes of a material or device during the coloring or bleaching process, and is the basic index for evaluating the effect of color change. The second is optical density (OD), which describes the transmittance of the material or device at a constant wavelength, and is defined as:

where T is a constant wavelength at the transmittance. The third is coloration efficiency (CE), which refers to the unit area of power consumption caused by the absorbance of the degree of color change, defined as:where ΔA refers to the absorbance color change at a certain wavelength and Q refers to the power consumption per unit area. The fourth is the switching time (t), which refers to the time required to switch between the coloring and bleaching states. The switching time is related with many factors, such as the ionic conductivity of the electrolyte, the ionic diffusion rate, the thickness and morphology of the film layer, the magnitude of the applied voltage, etc. The fifth is the cycle life, which can be characterized by the number of cycles of repeating the coloring and bleaching processes.⁷⁴ In addition, the optical memory performance is one of the important indicators for evaluating electrochromic materials or devices. Optical memory performance, also called color stability, refers to the ability of optical properties (transmittance, reflectance, etc.) maintaining stability without external voltage.

Researchers have proposed various methods to enhance the performance of WO_x electrochromic materials, for instance, control of the structure and morphology, doping and compounding with other inorganic materials, and compounding with organics. Different synthesis approaches can yield WO_x with different nanostructures. Huang et al. found that the concentration of precursors plays an essential role in morphology control.⁸ The morphology of WO_3−x (0 < x < 1) nanostructures gradually changed from nanowires (NWs), nanoclusters (NCs), to nanoflowers (NFs) with the increase of precursor concentration (Fig. 6a and b) and the color of the as-prepared WO_3−x nanostructure solutions gradually changed from light blue to dark blue, which indicated the increase of light absorption. It was also found that the lattice-stripe spacing of the WO_3−x nanostructures widened with increasing solution concentration. The weaker long-range ordering and lower crystallinity of WO_3−x NFs compared to WO_3−x NWs and WO_3−x NCs suggest that WO_3−x NFs are more conducive to the insertion/extraction of electrolyte ions in the electrochromic process. As shown in Fig. 6c, the WO_3−x NFs have the best optical modulation ability of 62.98% over WO_3−x NWs (52.06%) and WO_3−x NCs (61.01%) in the VIS and NIR regions, showing the importance of nanostructures. Fig. 6d shows the bright, cold and dark modulation modes of the WO_3−x NF electrochromic film, which demonstrates its potential application electrochromic smart windows. Du et al. prepared novel WO_3−x hydrate nanosheets by the template method, and the fabricated electrochromic films exhibited excellent electrochemical and electrochromic properties (Fig. 6e and g).¹¹

Fig. 6 (a) Synthesis process of WO_3−x with different structures and morphologies. (b) SEM images of WO_3−x NWs, WO_3−x NCs, and WO_3−x NFs. (c) Optical transmittance spectra of WO_3−x NWs, WO_3−x NCs and WO_3−x NFs in 1.0 M LiClO₄/PC electrolyte at −1.5 and 1.0 V. (d) Illustration of dual-band electrochromic smart windows in bright, cool, and dark modes. Reproduced with permission.⁸ Copyright 2023, Springer Nature. (e) TEM images of the WO_3−x hydrate nanosheets. (f) Transmittance spectra in the bleached and colored states of the H-WO_3−x film. (g) In situ transmittance change curve between the colored state and the bleached state of the H-WO_3−x film in a cycle. Reproduced with permission.¹¹ Copyright 2022, Elsevier.

The crystalline phase structure of WO_x thin films has a significant effect on their electrochemical properties. Crystalline films prepared by conventional evaporation and magnetron sputtering have better cycling stability, yet they tend to be dense, which is not conducive to the diffusion of ions and will affect their coloring efficiency and response time.^75,76 It is generally recognized that amorphous films have a looser structure and more holes, which facilitates the diffusion of ions in them, and provides greater color contrast and speed of color change. Zhang et al. developed a fast-switching dual-band electrochromic smart window based on a single-component amorphous porous WO₃ (AP-WO₃) cathode (Fig. 7a and b).³ The amorphous and porous structure of WO₃ not only significantly improves the ion transport, but also provides a large surface area for Li⁺ adsorption, resulting in tunable surface plasmon resonance in the near-infrared range (Fig. 7c and d). As a result, the single-component AP-WO₃ films can independently and effectively modulate the near-infrared and visible transmittance through three different modes with high optical modulation and a fast switching speed, but their stability is slightly poor (Fig. 7e), which is also a common problem of amorphous WO₃ electrochromic films.^35,77,78 Pham et al. prepared WO₃ porous films on indium tin oxide (ITO) substrates using a simple three-pulse electrodeposition method.⁷⁹ These porous films outperformed the corresponding dense films formed by continuous electrodeposition in terms of long-term stability, with insignificant changes after 10 000 cycles. This research result will inspire more researchers to improve the preparation of electrodeposited films, reduce the preparation cost of electrochromic films, and promote the industrialization of electrochromic devices.

Fig. 7 (a) Surface and cross-section SEM images of AP-WO₃ thin films. (b) Transmittance spectra of AP-WO₃ thin films before and after coloring. (c) Cyclic stability test of AP-WO₃ thin films. (d) Transmittance spectra of the AP-WO₃ electrochromic device in bright, cold, and dark modes and (e) corresponding digital photographs. Reproduced with permission.³ Copyright 2022, John Wiley and Sons. (f) SEM images of Nb₁₈W₁₆O₉₃ thin films. (g) Transmission spectra and (h) coloring efficiencies of WO₃, Nb₁₈W₁₆O₉₃, and Nb₂O₅ films in initial, colored, and bleached states. (i) Cycling stability test of the Nb₁₈W₁₆O₉₃ film. (j) Schematic diagram of the electrochromic energy storage device assembled with the Nb₁₈W₁₆O₉₃ film (green) as an electrochromic layer. Reproduced with permission.¹⁵ Copyright 2022, John Wiley and Sons.

Compounding and doping of WO_x with other inorganic materials or elements is one of the most commonly used means to enhance its electrochromic properties. Most of the doping and compounding processes of WO_x were based on transition metals, especially focusing on transition metal oxides with electrochromic properties, such as Mo,⁸⁰ Ti,^81,82etc. Cai et al. synthesized a Nb₁₈W₁₆O₉₃ nanomaterial with superstructural motifs and prepared homogeneous self-supported electrochromic thin films on transparent conductive substrates (Fig. 7f–i),¹⁵ which showed large optical modulation (93% at 633 nm and 89% at 1200 nm), high color rendering efficiency (105.6 cm² C⁻¹), excellent multiplication capability and long-term electrochemical stability (6000 cycles). Based on this high-performance electrochromic film, they further assembled a multifunctional electrochromic energy storage device to realize the combination of dynamic dimming and energy storage applications (Fig. 7j). Such devices can not only manage solar thermal radiation entering a building and protect personal privacy, but also deposit electrical energy into it. The electrochromic films prepared in this work are low-cost and scalable, providing a new material option for electrochromic smart windows. Zhao et al. have constructed one-dimensional WO₃@PB arrays with porous core–shell structures.⁷ As shown in Fig. 8a and b, the modification of WO₃ enables the film to have dual-band modulation capability at low voltage. Moreover, the porous one-dimensional structure of PB@WO₃ provides sufficient space for the lattice expansion of PB during the EC process, which improves the stability of PB (Fig. 8c). Owing to the formation of a heterostructure between PB and WO₃, the electron transfer is accelerated and the electronic interactions are enhanced, which further improves its electrochemical activity and stability.

Fig. 8 (a) Digital photos of the PB@WO₃ film electrode at −0.2, 0.6, and 1.2 V. (b) UV–vis–NIR transmittance spectra of PB@WO₃ under different voltages of −1.0, −0.2, 0.6, and 1.2 V. (c) Schematic illustration of the EC mechanism in PB@WO₃. Reproduced with permission.⁷ Copyright 2023, American Chemical Society. (d) Schematic illustration of the preparation and structure of oxygen vacancy-modulated a-WO_3−x–O_v thin films. (e) HRTEM image of the a-WO_3−x–O_v film. (f) Transmission spectrum of the a-WO_3−x–O_v thin film. (g) Corresponding digital photographs of the a-WO_3−x–O_v electrochromic device at different applied voltages. Reproduced with permission.² Copyright 2023, Royal Society of Chemistry.

Moreover, as a new doping means, oxygen vacancy doping significantly improves the electrochromic properties of WO_x, especially the modulation ability in the near-infrared band. Chen et al. reported a novel oxygen vacancy-modulated amorphous tungsten oxide (a-WO_3−x–O_v, 0 < x < 1), as shown in Fig. 8d and e.² It was found that the introduction of oxygen vacancies not only enabled this film to modulate the near-infrared transmittance independently through the LSPR effect, but also provided more sites for ion diffusion and binding, which resulted in the optimum conductivity performance (Fig. 8f). They assembled these films into devices with effective energy storage performance and significant energy savings, which can greatly reduce the energy consumption of air conditioning and lighting in buildings, as shown in Fig. 8g. Additionally, as demonstrated in Fig. 9a, it can selectively control visible and NIR transmittance in three different modes: bright mode (+0.8 V, NIR and visible transparency), cold mode (0.2 V, visible transparency, mostly NIR opacity), and dark mode (0.8 V, NIR and visible opacity). Among the non-metallic element doping N⁸³ and phosphorus⁸⁴ doped WO₃ films have also been reported. It is worth mentioning that due to the superior electrochromic properties of WO_x itself, the doping of W atoms into various electrochromic oxides also leads to a better enhancement in their properties.⁸⁵ Wang et al. fabricated single tungsten atom (W)-modified hydrangea-like porous V₂O₅ nanoflowers (HLP-W/V₂O₅) with a high loading content of atomic W (13.7 wt%) using a low-temperature solvothermal method (Fig. 9b and c).¹⁷ The ion diffusion and electronic conductivity of HLP-W/V₂O₅ were greatly improved with the assistance of W atom modification. Below 1.2 V, HLP-W/V₂O₅ exhibited three reversible color transformations (yellow, blue, and brown) and excellent cycling stability (10 000 cycles without significant decay), as demonstrated in Fig. 9d–f.

Fig. 9 (a) Application scenarios of the a-WO_3−x–O_v electrochromic device in bright, cool, and dark operation modes. Reproduced with permission.² Copyright 2023, Royal Society of Chemistry. (b) SEM images of HLP-W/V₂O₅. (c) AC HAADF-STEM image of HLP-W/V₂O₅. (d) Prototype of the panda fabricated based on HLP-W/V₂O₅ and photographs at different colored states. (e) Structural schematic diagram of the solid EC device and photographs of the solid EC device at different color states. (f) Cycling stability of HLP-V₂O₅ and HLP-W/V₂O₅ with multi-potential steps between −0.6 and 1 V for 79.4 h. Reproduced with permission.¹⁷ Copyright 2023, Elsevier. (g) Transmittance of PANI-WO₃ thin films at different applied voltages. Reproduced with permission.²⁸ Copyright 2021, Elsevier.

As a typical representative of inorganic electrochromic materials, WO_x possesses excellent chemical stability, but it also has the drawbacks of singular color change and a slow response speed, thus compositing with organic materials is a reliable method to realize its fast response and multiple color change. Nguyen et al. reported an electrochromic hybrid film based on polyaniline and WO₃, which achieved multiple color changes (green, blue, violet and dark blue) in a single film and has ultra-short switching times (1.5 s for each of the coloring and bleaching processes) and cycle lifetimes (Fig. 9g).²⁸ Xue et al. constructed an electrolyte-free, integrated electrochromic device using WO₃ sols doped with carbon nanodots (CDs).⁸⁶ Unlike conventional WO₃ that exhibit color transitions between transparent and blue, the device enabled reversible color changes between yellow and green due to the intrinsic color of the carbon dots. Their study provided a new idea for the composite of WO_x and organic materials.

3.3 Gasochromism

The gasochromism effect refers to the interaction of a material with a target gas, which causes changes in the optical properties of the material, such as transparency, reflectivity, and color changes. Typically, the material returns to its initial state after isolation of the target gas, and the process is reversible.⁸⁷ The target gas may be different for different materials. As for WO₃, the target gas is H₂. Therefore, the gasochromism of WO₃ can also be called hydrochromic response.

Since the gasochromism of WO₃ was studied later than its electrochromism, there are relatively limited studies on its gasochromic mechanism. In this review, we choose the commonly used double-injection model to explain the H₂ gasochromic mechanism of WO₃. Compared with electrochromism, the coloration process of WO₃ under an H₂ atmosphere is much more complicated. During the coloration process, H₂ molecules are adsorbed on the catalyst surface and dissociate into H atoms, which further diffuse along the surface and finally inject into the WO₃ in the form of H⁺ and electrons, which is known as the so-called double injection. There is only one charge transfer reaction in the double injection process, where an electron from the H atom is transferred into WO₃, reducing the W⁶⁺ ion to W⁵⁺. In the bleaching process, O₂ adsorbed on the surface dissociates and transfers to form chemically active O atoms or O²⁻, which attracts and combines with the H atoms to form an H₂O molecule, and then the H₂O is desorbed, thus causing discoloration of the film.⁸⁸ The above coloring and bleaching processes can be described by eqn (8) and (9):

It can be analyzed from the WO₃ hydrochromic mechanism that the discoloration rate of WO₃ is mainly related to the adsorption of H₂, the diffusion of H atoms in WO₃, and the chemical reaction between H atoms and WO₃. On the other hand, the adsorption and diffusion of H₂ are closely related to the distribution of the catalyst as well as the composition and structure of the material. Except for the effect of temperature, it is difficult to accelerate the chemical reaction between H atoms and WO₃ by other methods. Hence, it can be designed from the morphological structure and composition in order to improve the hydrochromic performance of WO₃.

Foroushan et al. prepared three WO₃ nanofiber meshes by electrostatic spinning.⁸⁹ WO₃ nanofiber meshes had good sensitivity to hydrogen gas (in argon) down to 2% at room temperature. Upon exposure to hydrogen, the almost colorless nanofibers turned blue in less than 2 min. Liu et al. synthesized sub-stabilized hexagonal WO₃ with three typical morphologies using a hydrothermal method, which exhibited excellent hydrochromic properties.⁹⁰ Moreover, they pointed out that the correlation between the morphology and the hydrochromic properties was mainly attributed to the growth difference of the preferred crystalline surface of WO₃. The evaluation of the morphology of WO₃ nanowires, microspheres and nanorods exhibited a decrease in the aspect ratio, which corresponds to the microscopically preferred growth orientation changing from (002) to (100). Increasing the exposure ratio of oxygen ions on the c-axis by morphological modulation can improve the injection process of WO₃ and promote the coloring process. This is helpful to deepen the understanding of the morphology effect of WO₃ in the H₂ gas chromatography process and to provide theoretical guidance for the preparation of highly sensitive WO₃ gas chromatography materials. A detailed study of the hydrochromic properties of WO₃ at different operating temperatures, solvation conditions (low, medium, high) and doping concentrations has been carried out by Nisha et al. and the effect of precious metal (Pt, Pd) doping on the hydrochromic properties has been discussed.⁹¹ The results show that the hydrochromic response increases significantly with increasing working temperature, and the Pt-doped samples exhibit better hydrochromic response than the Pd-doped samples. A low detection limit of 0.01% was observed for the samples at temperatures between 50 °C and 100 °C, and they did not show any cross-sensitivity to other gases. It is a positive contribution to the development of hydrogen sensors based on WO₃. More and further research is needed to be put into the gasochromism of WO₃, which would be a significant advancement forward in gas sensing.

4. Summary and outlook

This review focuses on the functional mechanisms, performance indexes, properties, and applications of WO_x in photochromism, electrochromism and gasochromism. WO_x materials have been widely investigated in recent years due to their tunnel-like structure, sub-stoichiometric properties, and polycrystalline phase. However, there are still many challenges for WO_x, such as the structural design, stability improvement, large-scale and low-cost preparation, etc. Much effort should be made for the development of WO_x in intelligent chromogenic technology. It is crucial to improve the long-term stability of WO_x materials to realize their practical applications. There is an urgent need to combine nanostructure design and multiple synthesis methods to construct a series of WO_x materials with long-term stable recycling. Since WO_x-based photothermal management devices typically have a multilayer structure and interactions between the layers, an appropriate device structural design is critical. In the case of electrochromic devices, the selection of different substrates, transparent conductive layers, and electrolytes has a significant impact on the performance and sustainability of the devices. In addition, large-scale preparation of high-performance WO_x films at a reasonable cost is crucial. Currently, electrodeposition, electrostatic spinning, sol–gel, etc. are the main approaches for the preparation of WO_x films. However, their cost and scale are not suitable for practical production. Hence, it is essential to improve the traditional preparation techniques and investigate new preparation techniques to promote its industrial development and application.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Tsinghua-Toyota Joint Research Fund, the National Key Research and Development Program of China (Grant No. 2020YFC2201103 and 2020YFA0210702), the Jiangsu Specially Appointed Professor program and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX24_1486).

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