Chemical and Electronic Modulation via Atomic Layer Deposition of NiO on Porous In2O3 Films to Boost NO2 Detection

To achieve high sensitivity under low-temperature operation is currently a challenge for metal oxide semiconductor gas sensors. In this work, a unique NiO-functionalized macroporous In2O3 thin film is designed by atomic layer deposition (ALD), which demonstrates great potential in electronic sensors for detecting NO2 at low temperature. This strategy allows for efficient engineering of the oxygen vacancy concentration and the formation of p–n heterojunctions in the hybrid In2O3/NiO thin films, which has been found to greatly impact the surface chemical and electrical properties of the sensing films. The sensor based on the optimized In2O3/NiO films exhibits a very high response of 532.2 to 10 ppm NO2, which is 26 times higher than that of the In2O3, at a relatively low operating temperature of 145 °C. In addition, an ultralow detection limit of ca. 6.9 ppb has been obtained, which surpasses most reports based on metal oxide sensors. Mechanistic investigations disclose that the improved sensor properties are resultant from the paramount surface active sites and high carrier concentration enabled by the oxygen vacancies, while excessive NiO ALD leads to a decreased sensor response due to the formed p–n heterojunctions.