Fast responding and highly reversible gasochromic H2 sensor using Pd-decorated amorphous WO3 thin films

• Pd-decorated WO 3 for hydrogen detection at room temperature are fabricated on PET substrates. • The Pd-decorated WO 3 sensor exhibits super rapid response and recovery reaction. • Coloring and bleaching mechanism at room temperature has been demonstrated. For the safe use of H 2 gas, an eco-friendly energy source in the spotlight but highly explosive at the same time, the need for gas sensors for early H 2 detection has been highly increasing. Although promising H 2 sensors have been developed with different sensor principles, most of them accompany bulky and large measurement systems which limit their further application to everyday life. In this study, we propose gasochromic H 2 sensors based on Pd nanoparticle-decorated amorphous WO 3 film (Pd-aWO 3 ) that can visualize H 2 detection by the rapid color change of the film with high reversibility using dissociated H + ions. In specific, extremely fast H + diffusion coefficient through amorphous WO 3 film prepared by electron beam evaporation enables rapid and highly reversible operation that could not be achieved by previously reported crystalline WO 3 film. As a result, Pd-aWO 3 could exhibit a transmittance change of 57% and super rapid recovery time (16 s), which is the fastest compared to previously reported gasochromic H 2 sensors to the best of the authors' knowledge. Moreover, in-situ surface analysis using ambient pressure X-ray photoelectron spectroscopy revealed detailed coloring and beaching mechanisms. This study can provide promising perspectives to future advances in gasochromic H 2 sensors toward everyday life application.