2.1. Preparation of WO₃ Thin Films

Electrochromic WO₃ films were fabricated on 6.25 cm² glass and FTO-coated glass substrates (7 Ω∕square, 735 159, Aldrich) by DC reactive magnetron sputtering, using a 4-inch metal tungsten metal target (Dasom RMS Co. Ltd.; 99.99%). The substrates were ultrasonically cleaned in acetone, methanol, and deionized water for 10 min each, followed by flowing nitrogen gas. In the sputtering chamber, tungsten atoms from the metal target were ejected by argon plasma sputtering and reacted with oxygen gas flowing in the chamber to form WO₃, which was then deposited onto FTO-coated glass substrates. The base pressure in the chamber was reduced to 10⁻⁶ mTorr before deposition. Kapton tape was used to mask the FTO films for the cathode connection. The temperature and pressure were set to 25°C and 5 mTorr, respectively. The target was presputtered for 10 min to eliminate the contamination in the argon environment. Following this, WO₃ was deposited for 500 s to achieve a film thickness of 100 nm, using a DC power of 150 W with an argon flow rate of 20 sccm and an oxygen flow rate of 5 sccm. The substrate was rotated at five rotations per minute during the deposition to ensure uniform film coverage.

2.2. Characterization of WO₃ Thin Films

The structural properties of the WO₃ films were determined by X-ray diffraction (XRD, Hypix3000) over a 2θ range from 10° to 70°. The transmittance spectra of the films were obtained using a UV–vis–NIR spectrophotometer (Shimadzu, UV-2600). In situ transmittance measurements for specific wavelengths were performed in kinetic mode, while spectrum mode was used for the UV, visible, and IR broad-spectrum transmittance measurements. Scanning electron microscopy (JSM 7800F) is used to obtain surface morphology and cross-section images. The elemental compositions were recorded using the energy-dispersive X-ray analysis system (EDX, Oxford Instruments). The chemical states of WO₃ films were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI). The film thickness, growth rate, and optical constants are determined by spectroscopic ellipsometry (JA Woollam Alpha-SE). The electrochromic properties of WO₃ film were recorded by an electrochemical workstation (WonA tech, Zive SP2). The WO₃ films were tested in 0.01 M aqueous HCl electrolyte, wherein the film was the working electrode, and the platinum gauze was the counter electrode. Cyclic voltammetry (CV) measurements were conducted in the range from −3 to 1 V with a scan rate of 100 mV/s.

3.1. Growth of WO₃ at Room Temperature

WO₃ thin films were grown using a physical vapor deposition process, specifically reactive DC magnetron sputtering, as depicted in Figure 1a. Tungsten can exist in multiple oxidation states, with W⁶⁺ being the most stable and promoting oxide formation. A high DC voltage was applied to ionize argon gas, creating a plasma. This plasma consists of accelerated Ar⁺ ions confined around the tungsten target by a magnetic field. The high-energy collisions between the argon ions and the target release tungsten atoms, which then travel to the substrate, forming the WO₃ film. After oxygen gas is introduced into the system, O₂ molecules are ionized to O²⁻ species. These ions react with the tungsten atoms (W⁶⁺) in the vacuum chamber, resulting in the spontaneous formation of WO₃ at room temperature [10]. Due to the higher ionization energy of the sputtering plasma, the injected oxygen gas turned into ionized species, which then collided with the tungsten species of higher kinetic energy [11]. This energetic interaction between the sputtered tungsten and ionized oxygen resulted in the spontaneous formation of WO₃ on the substrate surface, leading to thin film growth. Room-temperature deposition makes this process suitable for substrates such as PET and other polymer sheets, enabling flexible applications. Lower temperatures also result in more amorphous WO₃, which enhances the film’s coloration properties [12]. Our findings show that a gas mixture of 5 sccm O₂ and 20 sccm Ar produces the WO₃ phase, as confirmed by XRD and SEM measurements in the following sections.

3.2. Structural, Optical, Physical, and Chemical Properties of WO₃ Thin Film

The electrochromic performance of WO₃ thin films strongly depends on their oxidation state, as this determines the material’s ability to intercalate and de-intercalate ions, leading to changes in optical properties. This process is primarily governed by the reduction and oxidation of tungsten, which modulates the material’s electronic structure and optical absorption. The XPS spectra of deposited WO₃ films are collected to investigate the W element’s oxidation state and the film’s stoichiometry. Figure 1g,h shows the survey spectra of WO₃ film and the high-resolution binding energy spectra of W 4f, respectively. Figure 1h presents the 7/2 and 5/2 spin–orbit doublet for the W 4f valence state. The binding energies of W 4f_7/2 and W 4f_5/2 levels are observed at around 35.6 and 37.8 eV, which correspond to the W⁶⁺ and W⁵⁺ oxidation states, respectively [19]. Controlling the oxidation state during ion intercalation and extraction is critical for optimizing the electrochromic contrast, efficiency, and durability of WO₃ thin films. Figure 1i shows the deconvoluted O 1s spectra, which confirms the presence of oxygen vacancies in the present WO₃ film. The substoichiometric of the WO₃ films can be roughly estimated through the O/W atomic ratios calculated from the area covered by XPS peaks O 1s and W 4f, as summarized in Table 1.

3.3. Optical Modulations by Electrochromic Window

3.4. Electrochromic Performance of WO₃ Thin Film

The electrochromic properties of WO₃ films were analyzed using an electrochemical workstation and UV–visible spectrophotometer. To investigate the electrochemical reaction process, CV curves were measured in a two-electrode electrochemical cell system in 0.01 M HCl solution. Figure 3a shows the cyclic voltammogram, where the voltage is swept from −3 to 1 V at a scan rate of 100 mV s⁻¹. The redox reaction refers to the insertion/extraction of H⁺ ions and electrons in the WO₃ matrix, with the transmittance changing between blue and transparent (Equation 9).

For response time analysis, a square wave potential of −3 and 1 V was applied to the working electrode in the electrolyte using the chronoamperometry technique. In situ transmittance measurements at 550 nm wavelength are done using kinetic mode. Figure 3d depicts the response time for the transparency variation with a coloration time of 10 s and a bleaching time of 24 s. This response speed is faster than previously reported works using concentrated and highly reactive electrolytes (Table 2).

The intercalation of ions into the WO₃ lattice is responsible for the electrochromic spectral change. This process occurs due to ionic transportation under applied bias, leading to electrochromic performance. In this case, intercalation is faster than de-intercalation. We applied a higher potential (3 V) for coloration, while a lower potential (1 V) was used for bleaching. Additionally, the higher concentration of H⁺ ions in the electrolyte solution enhances spontaneous interaction with the WO₃ lattice, facilitating faster coloration. Conversely, the reverse process may take more time for ions to diffuse back into the electrolyte. Another possible reason is that the H⁺ ions intercalated into the lattice may bond to the electrochromic material, altering its structure. This could slow down the extraction of ions from the material.

3.5. Working Mechanism of Electrochromic WO₃ Thin Film

(Transparent) (bias applied) (translucent/opaque).

Here, M⁺ = H⁺ ions, and the quantity x becomes the stoichiometric parameter of the product and can vary between 0 and 1. Typically, WO₃ films consist of W⁶⁺ and W⁴⁺ states. During the intercalation of ions, W⁶⁺ states reduce to W⁵⁺. The optical absorption in the colored film is caused by the transition between different tungsten oxidation states [24].

The pristine WO₃ film is highly transparent in the visible spectrum. As confirmed by optical characteristics, the reactive sputtering-grown WO₃ film possesses an optical bandgap value of 2.64 eV. This optical bandgap, along with the stoichiometric film preparation, provides WO₃ with higher transmittance in the visible and IR wavelength regions. During electrochromic operation, protonation occurs in the coloration states, inducing intraband energy levels that lead to the absorption of visible and IR wavelengths. In the bleaching state, as H⁺ ions diffuse back into the electrolyte from the WO₃ film, the film resumes its highly transparent state. The highly reversible operation of H⁺ ion diffusion during coloration and bleaching states allows for tunable transmittance states through electrochromic operation.

Figure 4b,c shows a cross-sectional SEM image and the elemental distribution of tungsten, oxygen, and tin in the colored state, respectively, confirming no change in thickness but a variation in the elemental distribution due to H⁺ intercalation. Additionally, the stoichiometric fraction shifted, computed using the EDX mapping (Supporting Information 1: Figure S3). Before and after coloring, the O:W ratio increased from 2.90 to 2.92 by atomic percentage (O = 74.5 and W = 25.5). It emphasizes the electrochromic behaviors underlying oxidation state variation speculation. Figure 4d illustrates the experimental setup for the coloration of WO₃ in HCl. The active area of WO₃ is 1 cm², and the bleached and colored states are shown. Supporting Information 2: Video S1 demonstrates the coloring and decoloring (Supporting information 2: Video S1).

Figure 4f summarizes the charge versus time profile of various colored WO₃ electrodes from the transient current profiles. The constant b can be estimated from the plot of ln (1 − (Q/Q_t)) versus t, as shown in Figure 4g. According to this analysis, we estimated b values of 0.2714 and 0.2976 for WO₃ devices protonated at −1.7 and −2.5 V, respectively. The calculated diffusion coefficient values for WO₃ devices range from 3.72 × 10⁻¹⁰ cm² s⁻¹ to 3.30 × 10⁻¹⁰ cm² s⁻¹, which are comparable to Li-doped WO₃ devices [27] and iridium oxide-based thin films [28], and significantly faster than solid-state electrochromic devices [25].

3.6. Stability and Durability of Electrochromic WO₃ Device

To assess the stability of the WO_₃ electrochromic device, we conducted CV tests, measuring the current during coloration and bleaching cycles by sweeping the potential from 1 to −2.5 V at 200 mV s⁻¹. Over 1000 cycles, the device maintained consistent current density–voltage characteristics, as shown in Figure 5a–e. The current density for the first coloration state was −2.7 mA cm⁻² at −2 V vs. Pt, remaining around −3.2 mA cm⁻² after 1000 cycles. For bleaching cycles, the current density ranged from 2 to 2.9 mA cm⁻², demonstrating outstanding stability, as summarized in Figure 5f.

Thin films grown by reactive sputtering methods achieve a dense nature due to the higher kinetic energy from ionized argon and oxygen species [11], hydrothermal, sol–gel, spray, hot injection, or doctor blade methods which require additional thermal processing. Reactive sputtering enables WO_₃ thin film growth at room temperature, exhibiting exceptional stability.

Environmental stability is a critical factor in developing real-world applications for electrochromic devices, especially for use in houses and vehicles [29]. To validate the durability of the WO₃ electrochromic device, we integrated a Peltier module to facilitate testing of electrochromic cycles across a working temperature range from 0 to 50°C. For this examination, we performed CV with a total of 50 coloration and bleaching cycles. The temperature of the setup and device was monitored using a thermal camera, while the temperature was controlled by regulating the power supplied to the Peltier module.

Figure 5g–k presents the CV (I–V) plots of the WO₃ electrochromic device across the temperature range from 5 to 50°C. The results demonstrate consistent coloration and bleaching states throughout the operational temperature range, making it suitable for domestic applications. As the device temperature increases, ionic conductivity also increases [30], leading to a corresponding rise in current density for both coloration and bleaching states.

Figure 5l illustrates the current density as a function of the operating temperature of the WO₃ electrochromic device. We observed that the current density value corresponding to the coloration states (−2 V vs. Pt) was −3 mA cm⁻² at 0°C, which increased to −4.27 mA cm⁻² at an operating temperature of 50°C. Similarly, the current density corresponding to the bleaching state (0 V vs. Pt) was 2.3 mA cm⁻² at 0°C, increasing to 2.8 mA cm⁻² at an operating temperature of 50°C. This performance by the WO₃ electrochromic device across a broad range of operating temperatures underscores its durability, which is essential for reliability in real-world applications.

In order to compare the performance of solid-state WO₃ electrochromic performances, the features of WO₃ are summarized in Table 2 [31–42]. Most previous works employed Li⁺ electrolytes in concentrated form, whereas the solid-state WO₃ can be operated in the diluted HCl environment free from Li dependency, which is less environmentally harmful. H⁺ ions, being the smallest, enable faster and more efficient coloration. Additionally, unlike other studies that involve high-temperature processes, the solid-state WO₃ method requires no heat treatment, allowing for a wider range of substrates, including flexible ones like PET. Furthermore, the comparison of our WO_₃-based electrochromic device with other electrochromic materials, including Ti-doped WO₃, PANI, TiO_2-x, Nb₁₂O₂₉, Nd-doped TiO₂, and NiO summarize stability cycles and operational temperature in Table 2, highlights the unique strengths of our WO₃ film, developed by reactive sputtering at room temperature, indicating an efficient, fast, and durable electrochromic device. This suggests that higher possibility to deploy electrochromic windows for versatile applications with suitable processes.