Fast Responding and Highly Reversible Gasochromic H₂ Sensor Using Pd-Decorated Amorphous WO₃ Thin Films

Hydrogen (H2) has been widely used in various industrial fields such as ammonia synthesis, semiconductor manufacturing process, and glass manufacturing process. Recently, rather than as industrial resources, H2 has been regarded more as eco-friendly future energy resources for auto-mobiles, fuel cells, or batteries, following global trends of escaping from the use of fossil fuels, the main reasons for global warming. For the safe use of H2 gas, an eco-friendly energy source in the spotlight, the need for sensors operating at room temperature is increasing. Recent studies have proposed gas sensors utilizing color change, called gasochromic sensors for the detection of target gas molecules. Without any external input or equipment, distinctive color changes of sensor materials can provide the highly intuitive experience of target gas detection by perception through naked human eyes and overcome the limitations in current gas sensor technologies of high power consumption. Pd and WO3 interactions have been widely adopted for the gasochromic H2 sensor application, but most of them suffer from incomplete recovery or very slow recovery kinetics. To improve recovery characteristics, several efforts have been suggested including high operating temperature, forming nanostructures, changing recovery gases to higher O2 concentration or vacuum condition, or utilizing hybrid materials. In this study, we propose highly reversible and rapid visual gasochromic H2 sensors for concentrations below 4% using Pd nanoparticle-decorated amorphous WO3 film (Pd-aWO3) and an ambient-pressure X-ray photoemission spectroscopy (AP-XPS) analysis was performed to identify the gasochromic reaction between H2 and Pd-aWO3. The gasochromic performance can be improved by Pd decoration that has great catalytic effect to H2 and an amorphous structure of WO3 film that has a large hydrogen ion (H+) diffusion coefficient. Amorphous WO3 film was deposited by electron beam evaporation on the flexible polyethylene terephthalate (PET) film to be applied to various environments, and Pd nanoparticles used as a catalyst to decompose H2 molecules were synthesized by chemical reduction reaction via K2PdCl4 precursor. Pd-aWO3 was fabricated with different WO3 film thicknesses (100, 250, 550, and 800nm) and K2PdCl4 concentrations (75, 150, 225, and 300μM) to optimize sensing properties. At an optimized condition of Pd-aWO3, the sensor device exhibited excellent gasochromic performance upon exposure to H2 with a transmittance change of 57%, super rapid recovery time (16s), great reversibility, and extremely low detection limit (0.002%). Also, the bent sensor shows almost same transmittance change, response and recovery rate with a flat sensor. We strongly believe that this study can provide a new perspective on the safe use of H2 in various industries based on rapid recovery and excellent sensitivity.