Gasochromic Hydrogen Sensors based on Pd-functionalized WO₃ film: A Brief Review

1. INTRODUCTION

Hydrogen (H₂) is widely recognized as a critical energy source owing to its high gravimetric energy density, high energy conversion efficiency, and zero-carbon-emission capability [1-3]. However, H₂ is highly flammable with a wide explosive range (4-75 vol% in air) and an extremely low ignition energy (~0.02 mJ), which poses significant safety challenges [4]. In addition, being colorless, odorless, and tasteless hinders leakage detection by human senses, leading to an increased demand for highly sensitive, selective, and fast-responding H₂ sensors [5,6].

Over the past decades, various types of gas sensors, including surface acoustic wave [7], electrochemical [8], catalytic combustion [9], fiber-optic [10], and chemiresistive sensors [11,12], have been widely explored for H₂ detection. Despite their high sensitivity and fast responses, they require external electronics for signal processing and display, which increases operational costs and may limit their long-term applicability in industrial environments, including exposed pipelines and remote installations. In contrast, gasochromic sensors enable intuitive and real-time H₂ detection through visible color changes of the sensing material, without needing additional electronic components. This sensing mechanism reduces system complexity and improves cost-effectiveness, enabling scalable and reliable deployment in industrial H₂ monitoring environments.

Gasochromic sensors typically comprise two major components: a metal catalyst (e.g., Pd [13-16], Pt [17-20], and Pd-Pt alloy [21,22]) and a sensing material (e.g., WO₃ [23-26], MoO₃ [27,28], or V₂O₅ [29]). Fig. 1 presents the distribution of metal catalysts and sensing materials, obtained from a Web of Science search using the keywords "gasochromic sensors" and "hydrogen.". Among the various metal catalysts, Pd has been the most extensively studied (54.5% of publications), owing to its exceptional ability to dissociate H₂ molecules and strong affinity for H₂ adsorption. These characteristics facilitate the efficient diffusion of atomic hydrogen toward the sensing layer, thereby enhancing gas response and reaction kinetics. The sensing material, WO₃ has been widely investigated, accounting for 64.5% of related publications. This is primarily due to its excellent reversibility, high sensitivity to H₂, and distinct color change characteristics. These properties stem from its flexible lattice structure and the presence of oxygen vacancies that facilitate efficient charge transport [30]. Therefore, WO₃ is regarded as a highly suitable material for gasochromic H₂ sensing applications.

Fig. 1.

Studies on gasochromic H2 sensors materials. (a) Metal catalysts and (b) sensing materials for gasochromic H2 sensors (internet search of Web of Knowledge on March 25, 2025). Others in the right graph includes NiO, Na2W6O19, CuS, V2O5-MoO3, and NiO/TiO2.

In this review, we discuss three gasochromic mechanism models: 1) double injection of ions, 2) generation of oxygen vacancies, and 3) localized water molecules. Furthermore, Pd-functionalized WO₃-based gasochromic sensors are reviewed, highlighting various structural and functional modifications aimed at enhancing coloration performance and rapid response/recovery times. This paper comprehensively reviews recent advances, focusing on the critical challenges to be addressed in future work, and offers valuable insights on their commercial applications.

2. GASOCHROMIC MECHANISM

2.1 Double injection of ions model

The "double injection of ions" model, initially developed to explain the electrochromism in WO₃, has also been widely adopted to describe gasochromism. In electrochromic systems, coloration is driven by the simultaneous insertion of protons (H⁺) and electrons into the WO₃ lattice under an external electric field [31]. Gasochromic systems operate on the same principle, with surface catalysts replacing the electric field.

Using Raman spectroscopy analysis (Fig. 2), Lee et al. demonstrated that the gasochromic process is consistent with the double injection model [32]. During coloration, H₂ is adsorbed on the catalyst surface and dissociated into atomic hydrogen. These atoms migrate into the WO₃ lattice, where they are injected as H⁺ and electrons. The resulting charge transfer at the interface reduces W⁶⁺ to W⁵⁺, leading to the formation of a blue-colored tungsten bronze phase (H_xWO₃) [33]. This coloration can be described by the following equation:

Fig. 2.

Schematic illustration of double injection of ions model.

$\[ x{H}^{+}+x{e}^{-}+{WO}_3\to H_x{WO}_3 \]$

(1)

During the bleaching process, O₂ molecules dissociate on the catalyst surface and diffuse into the WO₃ lattice, where they oxidize W⁵⁺ back to W⁶⁺. This oxidation regenerates the transparent phase of WO₃.

2.2 Generation of oxygen vacancies model

Georg et al. proposed an alternative mechanism to the conventional double injection model for explaining gasochromic coloration in porous WO₃ films (Fig. 3) [34]. In this model, atomic hydrogen dissociated on the catalyst surface diffuses into the grain boundaries and reacts to form H₂O, thereby creating oxygen vacancies. These vacancies alter the electronic structure of WO₃ by locally reducing W⁶⁺ to W⁵⁺, leading to a visible color change without the formation of a tungsten bronze phase.

Fig. 3.

Schematic illustration of generation of oxygen vacancies model.

During the bleaching process, oxygen species generated on the catalyst surface diffuse back into the WO₃ lattice and recombine with the previously formed oxygen vacancies, restoring the original transparent state.

2.3 Localized water molecules model

In contrast to conventional models that focus on tungsten reduction, Luo et al. proposed the "localized water molecules" model, highlighting the structural and electronic role of water in WO₃ (Fig. 4) [35]. Similar to the generation of oxygen vacancies model, atomic hydrogen dissociated on the catalyst surface diffuses into the WO₃ lattice and reacts with lattice oxygen to form H₂O and oxygen vacancies.

Fig. 4.

Schematic illustration of localized water molecules model.

However, the generated H₂O remains localized near the original lattice oxygen sites, including structural distortion and generating electronic defect states. These localized species, combined with oxygen vacancies, contribute to the observed coloration through defect-state transitions within the bandgap. This process can be described by the following equation:

$\[ {WO}_3+x{H}_2\to {WO}_{3-x}\cdot x{H}_{2}O \]$

(2)

The bleaching process is suggested that oxygen atoms from dissociated O₂ occupy the vacancies and destabilize the localized H₂O molecules, which are subsequently decomposed and released.

According to the "localized water molecules" mechanism, H₂O molecules can remain locally confined within the WO₃ lattice and are released during the bleaching process. This water-mediated mechanism can be considered a key factor regarding the reversibility and reaction kinetics of the gasochromic reaction. Some studies have reported the degradation of gasochromic response under humid conditions, thereby highlighting the importance of developing humidity-resistant systems [19].

3. DESIGN OF SENSING MATERIAL

3.1 Design of Rapid-Response Gasochromic Sensors

The development of nanostructures with geometric advantages—including effective gas diffusion and a large surface area—is a key strategy for achieving rapid coloring/bleaching dynamics. Accordingly, several studies have investigated the design of WO₃-based gasochromic sensors in various forms, such as porous thin films [36-38], nanoparticles [16,25,39,40], and nanostructures [41-43].

Cho et al. proposed a novel strategy for accelerated gasochromic-H₂ sensing by inducing morphological changes [44]. Amorphous WO₃ nanorods (NRs) were fabricated by electron-beam evaporation combined with a glancing angle deposition (GLAD) method, which controlled porosity and density by adjusting the deposition angle (Fig. 5 (a)). Pd nanoparticles (NPs) were subsequently decorated onto the WO₃ surface through a solution-based redox process (Fig. 5 (b)). As shown in Fig. 5 (c-d), the as-deposited WO₃ NRs turned pale yellow, indicating successful functionalization with Pd NPs. To investigate the morphological trend, the WO₃ NRs were fabricated with various deposition angle (0^o, 70^o, 75^o, 80^o, and 85^o) (Fig. 5 (e-i)). As the deposition angle increased, the WO₃ nanorods became more porous and sparsely distributed, which facilitates gas diffusion. However, this structural change also led to a reduction in the effective sensing area, highlighting a trade-off between gas diffusion efficiency and reactive surface area.

Fig. 5.

Fabrication and gasochromic response of Pd–decorated WO3 nanorods. Schematic illustrations of (a) GLAD method using electron-beam evaporation and (b) Pd decoration. Photographs of the WO3 NRs deposited on the transparent substrate (c) before and (d) after Pd decoration. (e-i) Top-view SEM images of the Pd-decorated WO3 NRs deposited at 0°, 70°, 75°, 80°, and 85°. Enlarged response curves for (k) coloring and (l) bleaching kinetics of Pd-decorated WO3 NRs. Reprinted with permission from Ref. [44], Copyright (2024) Small, Under the terms of the Creative Commons CC BY 4.0 license.

To evaluate the morphological effects, all samples were measured to 5% H₂ (Ar balanced) at room temperature, exhibiting typical response curves with reversible characteristics upon H₂ removal (Fig. 5 (j)). The enlarged transmittance curves indicate the coloring and bleaching behavior as a function of deposition angle (Fig. 5 (k-l)). The response time and recovery time (t_res and t_rec, time required to achieve 90% coloring and bleaching, respectively) tended to decrease with increasing angle.

Notably, Pd-decorated WO₃ NRs deposited at 80^o exhibited superior performance with the fastest response time (14 s) and recovery time (1 s). Meanwhile, the transmittance variation gradually decreased with increasing deposition angle, which can be attributed to the reduction in the coloring region of WO₃. To clearly investigate the optimal structure, the coloring (R_c) and bleaching rate (R_b) were calculated using the following equations:

$\[ R_c =\frac{\mathrm{\Delta }T}{t_{res}} \]$

(3)

$\[ R_b =\frac{\mathrm{\Delta }T}{t_{rec}} \]$

(4)

The Pd-decorated WO₃ NRs deposited at 80^o exhibited exceptional coloring and bleaching rates (approximately 1.7 and 23.5, respectively), confirming 80^o as the optimal deposition angle. These results highlight the structural advantages of Pd-decorated WO₃ NRs gained by controlling the glancing angle, which enhances the response and recovery kinetics in H₂ detection.

3.2 Transmittance Modulation Induced by Thermal Treatment

The structural and optical properties of WO₃ play a critical role in sensing performance, such as the transmittance variation, response time, and recovery time. Thermal treatment is an effective approach for enhancing these characteristics, as it induces crystallization of amorphous WO₃ films, promotes morphological evolution, and reduces the optical band gap [17,45,46].

Jang et al. fabricated amorphous WO₃ thin films with a Pd catalyst using magnetron sputtering, followed by thermal treatment at various temperatures (RT, 473 K, 573 K, and 673 K) (Fig. 6 (a-d)) [47]. As thermal treatment temperature increased, the WO₃ films were crystallized into the orthorhombic phase and formed columnar structures. When exposed to 200-1000 ppm H₂ (Ar balanced), the WO₃ film annealed at 673 K exhibited the highest performance with enhanced ΔT and rapid coloring/bleaching (Fig. 6 (e-g)). These results can be attributed to crystallization, morphological change, and a reduced optical band gap.

Fig. 6.

SEM analysis and gasochromic response as a function of thermal treatment temperature. (a) RT, (b) 473 K, (c) 573 K, and (d) 673 K. Time-dependent transmittance responses of Pd-decorated WO3 thin films with different thermal treatment temperature: (e) 473 K, (f) 573 K, and (g) 673 K. Reprinted with permission from Ref. [47], Copyright (2023) Materials, Under the terms of the Creative Commons CC BY 4.0 license.

Crystallization improves the atomic alignment, thereby facilitating electron transport by forming effective conduction pathways and enhancing charge mobility across grain boundaries. The formation of nanostructures facilitates H₂ diffusion within the sensing layer, resulting in enhanced response and recovery kinetics. After thermal treatment, the WO₃'s optical band gap decreased from approximately 3.07 eV to 2.53 eV, promoting the more efficient absorption of lower-energy photons. This red shift at the absorption edge enhances gas sensing capability under visible light by facilitating electron excitation, leading to improved transmittance variation and faster response/recovery dynamics. However, thermal treatment may not improve gas sensing performance under all conditions. Cho et al. reported that crystallized WO₃ films' response and recovery behavior were slower than that of amorphous WO₃ films, since the grain boundaries hindered H⁺ diffusion.

Therefore, the gasochromic performance is influenced by a balance between the structural advantages—such as enhanced H₂ diffusion and improved charge transport—and the drawbacks associated with proton migration impeded by grain boundaries.

3.3 Role of Secondary Metal Oxides for Enhanced Stability

For long-term reliability, gasochromic sensors must offer highly stable sensing performance. Several studies have incorporated SiO₂ into WO₃ matrices by pre-hydrolysis for robust frameworks with suitable rheological properties [48-51]. This strategy improves the gasochromic film's mechanical and thermal stability thereby enhancing reliability.

Li et al. demonstrated the crucial role of SiO₂ in Pd-loaded WO₃–SiO₂ composite films fabricated via a sol-gel dip-coating technique. Morphological stability was assessed through atomic force microscopy (AFM) analysis on both WO₃–SiO₂ and pure WO₃ films in the as-deposited state and after 30 coloring–bleaching cycles (Fig. 7 (a-c)). Upon repeated cycling, the WO₃–SiO₂ film maintained its porous structure owing to the presence of the SiO₂ framework, whereas the pure WO₃ film exhibited distinct collapse and densification. Consequently, the coloring and bleaching kinetics revealed a significant difference between the two samples after 30 cycles.

Fig. 7.

AFM analysis and gasochromic response. (a) AFM images of WO3–SiO2 and WO3 films in the as-prepared state and after 30 coloring-bleaching cycles. Coloring and bleaching kinetics of WO3-SiO2 and WO3 films in (b) the as-prepared and (c) after 30 coloring-bleaching cycles and (d) annealed at 50, 150, and 300oC. (e) Cyclic responses of WO3–SiO2 film. Reprinted with permission from Ref.[50], Copyright (2011) American Chemical Society.

To investigate further the mechanical and thermal stability, WO₃–SiO₂ and pure WO₃ films were annealed at 450^oC. In top-view SEM images, the WO₃–SiO₂ film maintained its porous structure without noticeable densification or collapse, while the pure WO₃ film exhibited a dense and flat surface. The gasochromic performance was subsequently evaluated at different annealing temperatures (50, 150, and 300^oC) for 4% H₂. As Fig. 7 (d) shows, all the samples' performances degraded as the annealing temperature increased. Interestingly, the WO₃–SiO₂ film somewhat maintained its sensing properties compared to the pure WO₃ film. Furthermore, WO₃–SiO₂ film revealed highly stable and reliable performance after 72 cyclic responses (Fig. 7 (e)).

3.4 Dual-Response Sensors for Wide-Range H₂ Detection

Gasochromic sensors face several challenges, including gas detection at low concentrations and the precise quantification of gas levels. To overcome these limitations, chemiresistive gas sensors have been integrated into dual-response H₂ sensors, owing to their high sensitivity, quantitative detection capability, simple structure, and low fabrication cost [52-55].

While chemiresistive sensors prov ide quantitative electrical signals with fast response/recovery times, they often require a power supply and external circuitry [56]. In contrast, gasochromic sensors offer passive, power-free operation with a visible color change, which facilitates intuitive detection without instrumentation. Therefore, the integration of both mechanisms in dual-response sensors enables complementary advantages in sensitivity, readability, and applicability across a wide concentration range.

Kim et al. introduced a hybrid Pd/WO₃ film onto ITO-patterned transparent glass via DC magnetron sputtering, followed by Pd decoration using electron beam evaporation. Herein, the gasochromic responses were calculated by the real color difference (ΔE) using the following equation:

$\[ \mathrm{\Delta }E = \sqrt{{\left(L^2 - L^1\right)}^2+ {\left(a^2 - a^1\right)}^2 + {\left(b - b^1\right)}^2} \]$

(5)

where L, a, and b represent the transitions from light to dark, red to green, and yellow to blue between the initial (1) and final (2) states, respectively. To evaluate the gasochromic responses, Pd/WO₃ film were measured across a wide range of H₂ concentrations from 200 ppm to 10,000 ppm (Air balanced) at room temperature (Fig. 8 (a)). Upon exposure to 10,000 ppm H₂, a distinct color change was observed with ΔE value of 18.4. At lower H₂ concentrations (<400 ppm), the Pd/WO₃ film exhibited poor responses (<4), as a ΔE of 4 is typically regarded as the threshold for human visual perception [57].

Fig. 8.

Gasochromic and chemiresistive response of the Pd/WO3 film. (a) Gasochromic and (b) chemiresistive response curves of the Pd/WO3 film across a wide range of H2 concentrations from 200 ppm to 10,000 ppm at room temperature and 300oC, respectively.

To improve the H₂ detection capability at low concentrations, the chemiresistive responses were measured across a wide range of H₂ concentrations from 200 ppm to 10,000 ppm at 300^oC and calculated using the following equations:

$\[ Relative resistance change =\frac{\left|\mathrm{\Delta }R\right|}{R_0}\times 100 \left(\%\right) \]$

(6)

where R_s and R₀ represent the resistance after (R_s), and before (R₀) exposure to H₂ gas and ΔR represents the resistance change (R_s−R₀). As shown in Fig. 8 (b), Pd/WO₃ film indicated the typical resistance changes of an n-type metal oxide semiconductor upon exposure to H₂. The chemiresistive response decreases linearly with an increasing H₂ concentration and is clearly observable even at low concentrations (<400 ppm). These results demonstrate that the dual-mode Pd/WO₃ sensor has improved H₂ detection capability compared to gasochromic sensors at low concentrations.

4. CONCLUSIONS

This review highlights the gasochromic mechanism and recent advances of Pd-functionalized WO₃ film. Key approaches were introduced, such as a nanostructure design, thermal treatment, incorporation of secondary metal oxides, and dual-response hybrid sensors. These advancements have notably improved response/recovery kinetics, optical modulation, and cyclic stability. The optimization of gasochromic performance can be attributed to three main factors: (1) engineering the density and porosity of the sensing material; (2) the operating temperature during thermal treatment; and (3) the appropriate ratio between secondary oxides and sensing material. These factors have significantly improved the gas sensing properties, including response/recovery kinetics, transmittance variation, and long-term cycling stability.

To facilitate the practical deployment of gasochromic H₂ sensors, we suggest several specific directions for future research. First, gasochromic performance should be evaluated under air-balanced H₂ to replicate typical leakage conditions in ambient air. Second, the enhancement of humidity tolerance through surface modification is critical for ensuring reliable performance in humid environments. Third, continuous cycling tests are required to assess the long-term durability of gas sensing properties.

Acknowledgments

This research was supported by the National Research Foundation (NRF) funded by the Korean government (MSIT)(No. RS-2021-NR061721).

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