2D metal-organic framework derived co-loading of Co₃O₄ and PdO nanocatalysts on In₂O₃ hollow spheres for tailored design of high-performance breath acetone sensors

Highly dispersed Co₃O₄ nanoclusters encapsulating PdO nanoparticles were loaded on In₂O₃ hollow spheres to design high-performance breath acetone sensors. Nanolayers of two-dimensional (2D) metal-organic frameworks (MOFs), pure and Pd-intercalated leaf-like cobalt zeolitic-imidazolate frameworks (Co-ZIF-L), were uniformly coated (thickness: approximately 10 nm) on the surface of the In₂O₃ spheres by controlling the growth and self-assembly of 2D Co-ZIF-L on In₂O₃, which were converted into pure or Co₃O₄ nanoclusters (size: 10 nm) encapsulating PdO nanoparticles (size: approximately 4 nm) by thermal annealing. The gas response, selectivity, and optimal sensing temperature could be tuned by loading different quantities and configurations of the Co₃O₄ or Co₃O₄/PdO nanocatalysts. The In₂O₃ sensors co-loaded with Co₃O₄/PdO exhibited ultra-high responses (ratio of resistances in air and gas) to 5 ppm of acetone (145.9) as well as high selectivity over the interference of other biomarker gases at 225 °C, even in high humidity conditions (80% relative humidity), thereby demonstrating the promising potential for monitoring diabetes and the ketogenic diet. This unprecedented acetone sensing performance can be explained by the electronic sensitization due to the formation of p(Co₃O₄)-n(In₂O₃) heterojunction and the chemical sensitization due to the synergistic catalytic effect of Co₃O₄ and PdO. Ultrathin 2D-MOFs incorporating metallic nanoparticles provide a promising template for co-loading two different nanocatalysts in a highly dispersed and well-mixed configuration that can be used to establish diverse catalyst-oxide hetero-nanostructures for various functional applications, including high-performance gas sensors.