Improvement of Hydrogen Sensing Performances of Sol-Gel Derived Platinum Doped Tungsten Trioxide Gasochromic Films

Abstract

Introduction Various hydrogen sensitive gasochromic materials, such as V2O5 [1], MoO3 [2], and Y-Mg [3], have been reported. Among them, Catalyst-loaded WO3 film has been intensively studied to apply to smart windows or optic hydrogen sensors. Although there are many fabrication methods of WO3, sol-gel method [4, 5] is relatively cost-effective, does not need a large-scale apparatus, can be operated easily, and makes it possible to fabricate metal oxide film on complex shape surface. However, the gasochromic performance and its long-term stability sensitively depends on fabrication conditions. In this study, the effect of calcination temperature on crystalline structure, chemical state and hydrogen response of Pt-loaded WO3 (Pt/WO3) fabricated by sol-gel method were evaluated. Then, degradation behaviors were tested under various conditions. Principle In the presence of hydrogen, Pt/WO3 is reduced into tungsten bronze through reactions (1) and (2). Hydrogen molecules dissociate on the surface of catalyst into hydrogen adatoms even at room temperature. Consequently, they react with WO3 through spill-over mechanism. In the recovery process, tungsten bronze is oxidized into initial WO3 by oxygen gas in air following reaction (3). With exposure to hydrogen gas, it is well known that WO3 turns into dark blue color. In this study, transmission loss is focused as a tool for evaluating the hydrogen sensing properties of tested materials. Method WO3 precursor solution was prepared by ion-exchange method. This solution was added to the Pt containing solution which was composed of H2PtCl6, ethanol, and surfactant to make molar ratio of Pt:W about 1:5. After mixing, Pt/WO3 precursor solution was spin-coated onto the quartz glass substrates at 500 rpm for 300 seconds. The film was dried for over 1 hour. Then, the films were calcined for 1 hour in air. Calcination temperature was set in the range from 250°C to 500°C. X-ray diffraction measurements of the hydrogen sensing films were performed with CuKα radiation. The accelerating voltage and current were 40 kV and 45 mA, respectively. Fig. 1 represents the experimental set-up of transmitted light intensity measurement. The fabricated Pt/WO3 film was set into a homemade gas chamber. The one end of fiber cable was coupled with 1.3 µm LED light source. The light transmitting through the film was monitored by the optical power meter. 4 vol.% H2 gas balanced with N2 gas was used as test gas in order to evaluate hydrogen sensitivity of the film. Air as recovery gas was fed into the chamber by using an air compressor. Results and Conclusions The results of XRD measurement of Pt/WO3 films calcined at various temperature are shown in Fig. 2. The broad peak around 21° appears in all results comes from the quartz glass substrate. A sharp peak appeared at 24.0° in the film calcined at 500°C. This peak indicates formation of crystalline WO3. This peak became smaller for film calcined at 400°C and disappeared below 350°C. It indicates that crystal structure of WO3 films calcined below 350°C were amorphous. On the other hand, any peak related to Pt did not appeared. It is surmised that Pt exists in the form of microcrystalline and is well dispersed in the WO3 film. Fig. 3 represents the optical transmittance changes of the fabricated Pt/WO3 films calcined at various temperature with exposure to hydrogen containing gas. The transmitted light intensity sensitively responded to hydrogen gas and immediately reached steady states. The recovery reaction by air readily proceeded. Although the sensitivity increased with increasing calcination temperature, the response speed became slower. It would be closely related to amount of Pt° formed as a result of decomposition reaction of H2PtCl6. In order to evaluate long-term stability of Pt/WO3 film in severe condition, the effect of sodium addition to Pt/WO3 film was tested. In this experiment, Pt/WO3 coated substrate covered with small NaCl salt particles was calcined at 500°C in air at 500°C for 1 hours. Fig. 4 represents the result of hydrogen sensing test of Na-doped Pt/WO3 film. No variation in optical transmittance was observed. XPS studies suggested that catalyst-poisoning compounds generated and therefore gasochromic property of the film was considerably degraded.