Accelerating Palladium Nanowire H2 Sensors Using Engineered Nanofiltration.

The oxygen, O2, in air interferes with the detection of H2 by palladium (Pd)-based H2 sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air - relative to N2 or Ar. Here, we describe the preparation of H2 sensors in which a nanofiltration layer consisting of a Zn metal-organic-framework (MOF) is assembled onto Pd NWs. Polyhedron particles of Zn based zeolite imidazole framework (ZIF-8) were synthesized on lithographically patterned Pd NWs, leading to the creation of ZIF-8/Pd NWs bi-layered H2 sensors. The ZIF-8 filter with many micropores (0.34 nm for gas diffusion) allows the predominent penetration of hydrogen molecule with kinetic diameter of 0.289 nm, while relatively larger gas molecules including oxygen (0.345 nm) and nitrogen (0.364 nm) in air, are effectively screened resulting in superior hydrogen sensing properties. Very importantly, the Pd NWs filtered by ZIF-8 membrane (Pd NWs@ZIF-8) exhibited dramatically accelerated H2 response (7 sec to 1% of H2) and recovery (10 sec to 1% of H2) at room temperature in air, compared to pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H2). The ZIF-8 filter reduced the H2 response amplitude of Pd NWs@ZIF-8 slightly (R/R0 = 3.5% to 1% of H2 versus 5.9% for Pd NWs), and the Pd NWs@ZIF-8 showed 20-fold faster recovery and response speed than pristine Pd NWs. These outstanding results, which are mainly attributed by the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, ranks highest in H2 sensing speed among room-temperature Pd based H2 sensors.