A Hollow Assembly and Its Three-Dimensional Network Formation of Single-Crystalline Co₃O₄ Nanoparticles for Ultrasensitive Formaldehyde Gas Sensors

The detection of formaldehyde at a very low concentration is a significant research topic, due to its detrimental impact on human health. In the present study, we fabricated a hierarchical structure by the rational assembly of single-crystalline Co 3 O 4 nanoparticles. A hollow morphology using sacrificial ZnO spheres could form a three-dimensional conducting network in a solid state. The resulting structure was selectively active for formaldehyde sensing, and the detection limit was 50 ppb, which was nearly close to the record-high value among the other semiconducting materials. Such superior properties were attributed to the regular, hierarchically assembled structures with a small crystalline domain size, a thin hollow morphology with a large surface area, and a three-dimensional conductive network with a narrow diameter. We believe that this hierarchical assembly can show great potential as a platform for improving human health through the monitoring of indoor environments.