Surface Conversion of ZnO Tetrapods Produces Pinhole-Free ZIF-8 Layers for Selective and Sensitive H2 Sensing Even in Pure Methane

As the necessary transition to a supply of renewable energy moves forward rapidly, hydrogen (H₂) becomes increasingly important as a green chemical energy carrier. The manifold applications associated with the use of hydrogen in the energy sector require sensor materials that can efficiently detect H₂ in small quantities and in gas mixtures. As a possible candidate, we here present a metal-organic framework (MOF, namely ZIF-8) functionalized metal-oxide gas sensor (MOS, namely ZnO). The gas sensor is based on single-crystalline tetrapodal ZnO (t-ZnO) microparticles, which are coated with a thin layer of ZIF-8 ([Zn(C₄H₅N₂)₂]) by a ZnO conversion reaction to obtain t-ZnO@ZIF-8 (core@shell) composites. The vapor-phase synthesis enables ZIF-8 thickness control as shown by powder X-ray diffraction, thermogravimetric analysis, and N₂ sorption measurements. Gas-sensing measurements of a single microrod of t-ZnO@ZIF-8 composite demonstrate the synergistic benefits of both MOS sensors and MOFs, resulting in an outstanding high selectivity, sensitivity (S ≅ 546), and response times (1-2 s) to 100 ppm H₂ in the air at a low operation temperature of 100 °C. Under these conditions, no response to acetone, n-butanol, methane, ethanol, ammonia, 2-propanol, and carbon dioxide was observed. Thereby, the sensor is able to reliably detect H₂ in mixtures with air and even methane, with the latter being highly important for determining the H₂ dilution level in natural gas pipelines, which is of great importance to the energy sector.