Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films

Coating a polymeric membrane for gas separation is a feasible approach to fabricate gas sensors with selectivity. In this study, poly(methyl methacrylate)-(PMMA-)membrane-coated palladium (Pd) nanoparticle (NP) films were fabricated for high-performance hydrogen (H₂) gas sensing by carrying out gas-phase cluster deposition and PMMA spin coating. No changes were induced by the PMMA spin coating in the electrical transport and H₂-sensing mechanisms of the Pd NP films. Measurements of H₂ sensing demonstrated that the devices were capable of detecting H₂ gas within the concentration range 0-10% at room temperature and showed high selectivity to H₂ due to the filtration effect of the PMMA membrane layer. Despite the presence of the PMMA matrix, the lower detection limit of the sensor is less than 50 ppm. A series of PMMA membrane layers with different thicknesses were spin coated onto the surface of Pd NP films for the selective filtration of H₂. It was found that the device sensing kinetics were strongly affected by the thickness of the PMMA layer, with the devices with thicker PMMA membrane layers showing a slower response to H₂ gas. Three mechanisms slowing down the sensing kinetics of the devices were demonstrated to be present: diffusion of H₂ gas in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA matrix. The retardation effect caused by these three mechanisms on the sensing kinetics relied on the phase region of Pd hydride during the sensing reaction. Two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were proposed to compensate for retardation of the sensing response.