Accelerating Palladium Nanowire H Sensors Using Engineered Nanofiltration

The oxygen, O, in air interferes with the detection of H by palladium (Pd)-based H sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air-relative to N or Ar. Here, we describe the preparation of H 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 NW bilayered H sensors. The ZIF-8 filter has many micropores (0.34 nm for gas diffusion) which allows for the predominant penetration of hydrogen molecules with a kinetic diameter of 0.289 nm, whereas 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) reduced the H response amplitude slightly (ΔR/R = 3.5% to 1% of H versus 5.9% for Pd NWs) and showed 20-fold faster recovery (7 s to 1% of H) and response (10 s to 1% of H) speed compared to that of pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H). These outstanding results, which are mainly attributed to the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, rank highest in H sensing speed among room-temperature Pd-based H sensors.