Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P-to-N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement

Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal–organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO2 (n-SnO2) sensing layer with hollow polyhedron structures, obtained from p–n transition of MOF-templated p-type Co3O4 (p-Co3O4) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co3O4 clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co3O4 islands) on the porous and hollow polyhedron SnO2 structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p–n junctions, the GRR-treated SnO2 structures exhibited 21.9-fold higher acetone response (Rair/Rgas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co3O4 hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p–n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route.