A polystyrene (PS) thin film containing palladium (Pd) was formed on a WO₃ thin film formed by photo irradiation; as a result, a PS(Pd)/WO₃ thin film with a laminated structure was produced. Under a nitrogen atmosphere, 1.196 g of WCl₆ (Sigma-Aldrich) was dissolved in 6 ml of dehydrated ethanol to prepare a 0.5M solution. This solution was spin-coated (3000 rpm for 30 s) on a PET film (30 mm × 30 mm; thickness: 100 μm; Toyobo Co. A7300). The spin coating was performed inside a glove box with humidity kept at about 50% by a humidifier. Subsequently, the WO₃ precursor film formed on PET was irradiated with ultraviolet rays (wavelength: 254 nm and 185 nm; light intensity: 12 mw/cm²) at 100˚C for 20 minutes. This thin-film-formation process was repeated three times to increase the thickness of the WO₃ thin film. Pd (CH₃COO)₂ (Kanto Chemical Co.) was added to a 10 wt% PS (Wako Pure Chemical Co.: Degree of polymerization 2000) toluene solution to prepare a solution containing 1 wt% of Pd. This solution was spin-coated on the WO₃ thin film (at 3000 rpm for 30 s) and then dried at 100˚C for 10 minutes.

Moreover, a PS(Pd)/WO₃ laminated thin film was formed on a PET film, on which a Ti/Pt electrode was formed, by the same method described above, and that film was used as a sample for measuring change in resistance during exposure to hydrogen.

The PET film coated with the PS(Pd)/WO₃ laminated film was cut into a 8 × 24-mm, which was then placed in a 1-cm cell for spectrophotometer measurement, and the upper part of a cell was covered with polydimethylsiloxane (PDMS).

Two injection needles were inserted into the cell, and hydrogen (of a predetermined concentration) was introduced through one of the needles and discharged from the other. The inside of the cell was filled with a hydrogen atmosphere at constant concentration. As for the hydrogen, a nitrogen-hydrogen mixed gas (hydrogen concentration: 0.1%, 0.25%, 0.5%, 1.0%, and 4%; Japan Fine Products) was used. Change in transmittance of the thin film when hydrogen gas was introduced was measured by spectrophotometer (Shimadzu UV3600). Dry air was used for discharging the hydrogen gas, and a three-way cock was used to switch between hydrogen gas and dry air.

With respect to the resistance change at the time of hydrogen exposure, the resistance change of the PS(Pd)/WO₃ laminated thin film formed on the PET film (on which the Ti/Pt comb-shaped electrode was formed) was tracked by using a resistivity meter.

The chemical structure of the WO₃ film was investigated by measuring its infrared absorption spectrum (Shimadzu FT-IR8400S). The surface of the thin film was observed by optical microscope (Kyowariken ME-LUX2) or digital microscope (Keyence VHX-500F) as well as by scanning electron microscope (SEM: Carl Zeiss ULTRA55). The cross-sectional structure of the PS(Pd)/WO₃ film was observed by transmission electron microscope (TEM: HITACHI HF-2000). The samples for these observations were fabricated by FIB (focused ion beam) processing.

The WO₃ thin film was used as an element for detecting hydrogen, and the “gasochromism” property of the thin film was used. The detection principle is shown schematically in Figure 1. Although WO₃ is colorless and transparent, tungsten bronze (HxWO₃) is formed by incorporating H⁺ ions into the WO₃ and thereby color it blue. By incorporating H⁺ in the structure of WO₃, the valence of W changes to a mixed valence state composed of W⁶⁺ and W⁵⁺. At that time, absorption based on intervalence charge-transfer transitions appears extensively in range from the visible-light region (around 450 nm) to the near-infrared-light region (around 2000 nm) [19]. The thin film turns blue due to absorption of light in the visible-light region. At that time, electrons are conducted between the W⁵⁺ and W⁶⁺ states by hopping conduction, so that the electrical characteristics of the WO₃ thin film change, and its insulating property changes to conductive. Pd was used as a catalyst for dissociating hydrogen molecules into hydrogen atoms. The catalytic effect of Pd and Pt is well known as so-called spillover effect [20]. The hydrogen atoms dissociated on the Pd are induced into the WO₃ film by the spillover effect, and tungsten bronze is generated. On the contrary, in the absence of hydrogen, the Pd promotes the reaction between the Hx of HxWO₃ and atmospheric oxygen, so it plays a role as a catalyst for generating H₂O and WO₃. Since this reaction is a reversible according to the presence or absence of hydrogen, hydrogen can be detected by the coloring of the WO₃ thin film. Since this reversible reaction occurs at room temperature and selectively reacts with only hydrogen molecules, it is possible to develop a high-sensitivity hydrogen sensor that operates at room temperature.

The sol-gel method was used for producing the WO₃ thin film. As for this method, a precursor of an inorganic polymer is generated by chemical reactions (hydrolysis and polycondensation) in a solution, and the precursor is applied to a substrate and heat-treated to form an inorganic thin film. The general sol-gel method requires heat treatment of several hundred degrees in order to cure and

densify the thin film [18]; however, as for the sol-gel method used in this study, photo irradiation was used for curing the thin film. This method is effective for forming an inorganic thin film at low temperature, and it can form an inorganic thin film on an organic-resin substrate (which has no heat resistance). It is reported that the sol-gel method using photo-irradiation gives various kinds of metal oxide films with good quality at low temperature [21] [22] [23] [24].

As the WO₃ sol-gel solution, tungsten alkoxide (produced by dissolving WCl₆ in ethanol) was used as the starting material. It has already been reported that WCl₆ produces tungsten alkoxide in alcohol [12]. W(OC₂H₅)₆ reacts with H₂O present in the solution or in the atmosphere to form a network structure composed of -O-W-O- bond by hydrolysis and polycondensation, resulting in the WO₃ precursor.

The sol-gel solution (serving as a WO₃ precursor) is applied to a PET-film (thickness: 100 μm) substrate, and it is irradiated with 254-nm (ultraviolet) light by using a low-pressure mercury lamp. The 254-nm light is almost coincident with the absorption wavelength of the metal-ligand charge-transfer absorption band of tungsten alkoxide W(OC₂H₅)₆, which is the starting material for forming WO₃. Since W-OC₂H₅ bond is selectively broken to yield W-OH species and form W-O-W bond by polycondensation, the production of inorganic polymer is promoted efficiently. After the sol-gel derived solution is spin-coated on a PET, the ozone and active oxygen generated in the atmosphere by the 254-nm ultraviolet irradiation penetrate into the film and promote the formation of the WO₃ thin film. At that time, if the substrate is heated to about 100˚C, the diffusion of active oxygen into the film is accelerated, and a better-quality WO₃ film is obtained.

An infrared-absorption spectrum of the WO₃ film prepared by the above-described method is shown in Figure 2. The spectra of thin films prepared at room temperature, 50˚C, and 100˚C under photo-irradiation are also shown, and for the sake of comparison, the spectra of thin films dried at room temperature and heated at 300˚C are also shown. The peak of stretching vibration of the generated WO₃-derived W-O-W bond appears around 650 - 820 cm⁻¹. Although in the dried product, absorption based on C-H stretching vibration of the alkoxy group is observed at around 2800 cm⁻¹, these absorptions were not observed in the case of the films irradiated at room temperature, 50˚C, and 100˚C, and strong absorption by W-O-W bond was observed around 650 - 820 cm⁻¹, suggesting the formation of WO₃. Furthermore, absorption by W-O-W bond became stronger with increasing temperature during photo-irradiation. The spectral pattern of the film photo-irradiated at 100˚C is similar to that of the film heat-treated at 300˚C, indicating that the structure of the photo-irradiated film at 100˚C is similar to that of the 300˚C-heat-treated film. It has been reported that a sol-gel film heat-treated at low temperature (about 100˚C) shows absorption attributed to the W=O bond around 1000 cm⁻¹ and disappears with increasing heat-treatment temperature to form a W-O-W network structure [14]. However, in the present study, such absorption was not observed in the case of a film produced by photo-irradiation. This finding suggests that WO₃ is generated at a lower temperature by photo-irradiation.

The structure, appearance, and hydrogen reactivity of the flexible hydrogen sensor are shown in Figure 3. A WO₃ thin film was formed on a PET film (film thickness: 100 μm), and a polystyrene (PS) thin film containing Pd was formed. The upper PS film functions as both a support layer of Pd (the catalyst) and a protective layer of the WO₃ thin film (the sensor layer). The PET film is flexible because its thickness is only 100 μm. Furthermore, the adhesion strength of the PS(Pd)/WO₃ film to PET was high, so the film did not peel off during a tape test

performed according to JIS standards (JIS K5600: Japan Industrial Standard). The film immediately turned blue in 100% hydrogen and showed high reactivity to hydrogen. Moreover, as soon as the film was re-exposed to air, the color disappeared, and the film returned to its original state.

SEM images of the surface state of the film are shown in Figure 4. The surface state of the WO₃ film (without PS formed as an upper layer) was porous with many cracks. In the case of the WO₃ film in which PS (Pd) was formed as an upper layer, cracks are reduced, and penetration of PS (Pd) into the WO₃ film is confirmed. Cross-sectional TEM observation images and the results of elemental analysis by EDX are shown in Figure 5. It can be seen that a WO₃ layer (thickness: about 600 nm), connected in the form of particles, is formed on the substrate, and a PS (Pd) layer (thickness: about 90 nm) is formed on top of the WO₃ layer. As for the WO₃ layer, since light and dark areas are observed, it can be said that this layer is in a porous state. As for the distribution of elements, although tungsten is observed to correspond to the WO₃ layer, since palladium and carbon

are also observed in the WO₃ layer, it is suggested that the PS(Pd) solution penetrates into the WO₃ layer. Since the specific surface area of the WO₃ layer is increased by the porous state, it is considered that the abundance of reaction sites with hydrogen is increased, so the film exhibits higher reactivity to hydrogen. It is assumed that since the PS solution containing Pd penetrates into the porous WO₃ layer, the Pd catalyst (exhibiting the spillover effect) is also dispersed in the WO₃ layer, and the reactivity between hydrogen atoms, which are generated form hydrogen molecules by Pd catalyst, and WO₃ is enhanced. In addition, PS plays a role of adhesion with the WO₃ particles and PET film, so it is considered to contribute to improving the strength of the hydrogen-sensor film. When the adhesive strength of the sensor film in regard to the PET film was examined by a tape-peeling test based on JIS standards (JIS K5600), it showed good adhesion of 100/100 (remaining number/cut number). Since PS has water repellency, it also has the effect of preventing deterioration of the WO₃ film due to water vapor and the like.

The reactivity to hydrogen of the PS(Pd)/WO₃ thin film appears in the change of its transmittance curve; accordingly, by tracking this change, it is possible to quantitatively evaluate hydrogen concentration. Change in the transmittance curve (in the 400 - 2000 nm region) of the PS(Pd)/WO₃ thin film when exposed to 100% hydrogen is shown in Figure 6. Under hydrogen exposure, the film instantly turned blue, and the transmittance from the visible-light region to the near-infrared region was greatly reduced due to the mechanism as shown in 3.1, indicating that by incorporating H⁺ in WO₃ thin layer, the valence of W changed to a mixed valence state composed W⁶⁺ and W⁵⁺. In particular, transmittance

was the lowest near 1000 nm, at which point the amount of change in transmittance was large. And transmittance decreased drastically—from 90% to 8%—by hydrogen exposure. When hydrogen was removed, the transmittance curve returned to its original shape, thereby confirming the reversibility of transmittance.

Change in transmittance at 1000 nm, at which the largest change in transmittance occurs, when hydrogen concentration is reduced from 4% (the lower explosion limit) to 1%, 0.5%, 0.25%, and finally to 0.1%, is shown in Figure 7. For any of the hydrogen concentrations, transmittance changed greatly when the hydrogen was introduced, and when the hydrogen was removed, it immediately returned to its original state (Figure 7(a)). The ultimate transmittance differs according to the hydrogen concentration, and the higher the hydrogen concentration, the larger the change in transmittance. Taking a log-log plot of the varying transmittance and hydrogen concentration reveals a clear linear relationship (Figure 7(b)). This result indicates that hydrogen concentration can be quantified by measuring the change in transmittance due to hydrogen exposure. The cycling characteristics of sensor performance due to hydrogen exposure are shown in Figure 8. This figure shows that the change in transmittance under hydrogen concentration of 4% and the recovery to the initial transmittance when the film is re-exposed to air are highly reproducible.

A PS(Pd)/WO₃ laminated film was formed on a PET film on which a Ti/Pt comb-shaped electrode was formed, and the change in the resistance of WO₃ thin film on the PET film when exposed to hydrogen was measured. The appearance of the sensor and the results of the measurements are shown in Figure 9 and Figure 10, respectively. Cross-sectional SEM images are also shown in Figure 9.

Porous WO₃ layer with about 500 - 550 nm film thickness was observed on Ti/Pt electrode (Figure 9(a)) and PET (Figure 9(b)). Outermost layer of Pt is protective layer which was prepared when forming cross-sectional SEM sample for observation. When hydrogen was introduced, the resistance decreased significantly, and after 0.2 - 0.5 seconds, the value decreased from a state close to insulation resistance (55 MΩ) to 12 - 33 Ω due to hopping conduction of electron between W⁶⁺and W⁵⁺ by forming the mixed valence state of W⁶⁺ and W⁵⁺. And when the hydrogen was removed and air was introduced, the resistance state immediately returned to that close to the original insulation resistance. The log-log plot of the changed resistance (values after 300 s) when hydrogen at concentration of 4% to 0.1% was introduced against hydrogen concentration shows a good linear relationship. This result indicates that hydrogen concentration can be quantified from the change in resistance of the sensor.

As described above, the transmittance and resistance of the developed hydrogen sensor instantly change (in terms of color) even at low concentration of hydrogen, and the sensor is extremely sensitive to hydrogen. By incorporating both changes in transmittance and resistance due to hydrogen exposure into the sensor, it is possible to detect low-concentration hydrogen with high reliability. It is also possible to develop a flexible hydrogen sensor because the PS(Pd)/WO₃ laminated film can be formed on a flexible PET film at low temperature by the method (modified sol-gel using photo-irradiation) used in this study. By developing the compact flexible sensor which is integrated with the areas measuring transmittance change and resistivity change on the same PET film, hydrogen gas sensor with high accuracy and reliability would be possible.

Since the hydrogen sensor formed on the PET film is flexible, it can be fitted in the form of a film sensor at sites with complicated shapes. The results of an experiment in which hole was formed in a cylindrical cardboard pipe (simulating real piping), the film sensor was installed on the pipe, and hydrogen was flowed into the cylindrical pipe are shown in Figure 11. It is clear from these results

that the film sensor is gradually colored blue due to hydrogen leaking from the hole in the simulation pipe. The developed film sensor can thus be considered useful for visually detecting hydrogen leakage from piping of a hydrogen tank. A flexible hydrogen sensor has a high degree of freedom with respect to the location where it can be installed; therefore, it can be installed on complex shapes such as hydrogen tanks, and that capability will help to prevent explosions due to hydrogen leakage.