Double‐Phase Ga‐Doped In2O3 Nanospheres and Their Self‐Assembled Monolayer Film for Ultrasensitive HCHO MEMS Gas Sensors

In the sensing field, the electronic structure of sensing materials has a great influence on the properties of the sensors. Here, by Ga doping pure rhombohedral In2O3 (h‐In2O3), the double‐phase In2O3 (cubic/rhombohedral In2O3, c/h‐In2O3) porous nanospheres are obtained. And then Micro Electromechanical System (MEMS) gas sensors based on monolayer film are further fabricated by self‐assembling the above nanospheres. The 5% Ga‐doped In2O3 sensors exhibit excellent HCHO sensing performance with a high‐response (110.6@100 ppm), rapid response/recovery time (5.2/18.4 s) and low limit of detection (50 ppb) at an operating temperature of 180 °C. The 5% Ga‐doped In2O3 sensors also show high consistency (fluctuations of only 8.3%). Besides, a handheld device is developed to enable real‐time monitoring and early warning of indoor HCHO at ppb‐level. Based on experimental results and DFT theoretical calculation, the enhanced sensing mechanism is revealed, which is correlated with the optimization of electronic band structure by Ga doping and the appearance of double‐phase heterostructures caused by Ga doping. Therefore, the relationship between electronic structure and gas sensing properties has also been established. This work significantly introduces a novel approach for the mass production of MEMS gas sensors, ensuring high sensitivity, repeatability and consistency.