Facile Fabrication\nof Hierarchical Mesoporous Co<sub>3</sub>O<sub>4</sub>/In<sub>2</sub>O<sub>3</sub> p–n Heterojunctions\nfor Formaldehyde Gas Sensing

In\nthis work, three-dimensional hierarchical mesoporous Co<sub>3</sub>O<sub>4</sub>/In<sub>2</sub>O<sub>3</sub> p–n heterojunctions\nwith high sensitivity, selectivity, and stability were successfully\nconstructed. The response of the optimized hierarchical mesoporous\nCo<sub>3</sub>O<sub>4</sub>/In<sub>2</sub>O<sub>3</sub> p–n\nheterojunction reached 41 at 50 ppm formaldehyde (160 °C), 9.5\ntimes higher compared with pure In<sub>2</sub>O<sub>3</sub> (4.3),\nand showed good selectivity to formaldehyde over typical interfering\ngases of ethanol, xylene, toluene, methanol, and acetone. Moreover,\nthe response of the Co<sub>3</sub>O<sub>4</sub>/In<sub>2</sub>O<sub>3</sub> p–n heterojunction was maintained at around 40 during\nthe 30-day test. The greatly improved sensing performance of the Co<sub>3</sub>O<sub>4</sub>/In<sub>2</sub>O<sub>3</sub> heterojunction could\nbe attributed to its hierarchical structure with vast mesopores and\nthe formation of numerous closely contacted Co<sub>3</sub>O<sub>4</sub>–In<sub>2</sub>O<sub>3</sub> p–n heterojunctions, which\noffers abundant active sites and tunes the electronic and geometric\nstructures, facilitating the adsorption of oxygen and formaldehyde\ngases, the formation of more active oxygen species, and the reactions\nbetween active oxygen species and formaldehyde gases.