Facile Fabrication of Hierarchical Mesoporous Co₃O₄/In₂O₃ p–n Heterojunctions for Formaldehyde Gas Sensing

In this work, three-dimensional hierarchical mesoporous Co3O4/In2O3 p–n heterojunctions with high sensitivity, selectivity, and stability were successfully constructed. The response of the optimized hierarchical mesoporous Co3O4/In2O3 p–n heterojunction reached 41 at 50 ppm formaldehyde (160 °C), 9.5 times higher compared with pure In2O3 (4.3), and showed good selectivity to formaldehyde over typical interfering gases of ethanol, xylene, toluene, methanol, and acetone. Moreover, the response of the Co3O4/In2O3 p–n heterojunction was maintained at around 40 during the 30-day test. The greatly improved sensing performance of the Co3O4/In2O3 heterojunction could be attributed to its hierarchical structure with vast mesopores and the formation of numerous closely contacted Co3O4–In2O3 p–n heterojunctions, which offers abundant active sites and tunes the electronic and geometric structures, facilitating the adsorption of oxygen and formaldehyde gases, the formation of more active oxygen species, and the reactions between active oxygen species and formaldehyde gases.