Revealing the correlation between gas selectivity and semiconductor energy band structure derived from off-stoichiometric spinel CdGa2O4

Herein, the spinel CdGa2O4 and off-stoichiometric spinel CdGa2O4 (CdO-CdGa2O4) with 3D inverse opal spherical structure have been synthesized by the combination of ultrasonic spray pyrolysis technology and self-assembled hard-template method. The CdGa2O4, with band gap of 4.14 eV and baseline resistance of Mohm-level, exhibited excellent selectivity and high response to 1 ppm formaldehyde. On the other hand, the CdGa2O4 composites (Cd:Ga atom ratio = 1.2:2), with band gap of 2.46 eV and baseline resistance of kohm-level, exhibited best sensing performance to 1 ppm NO2 with negligibly low cross-responses (R-air/R-gas < 3.5) to any VOCs. Due to the semiconductor band gap is narrowed, and the conducting channel depending on the agglomeration of surface loaded CdO nanocrystals, the gas selectivity has been changed and the baseline resistance of off-stoichiometric spinel CdGa2O4 was reduced by many orders of magnitude. Meanwhile, the improvement of gas response can be attributed to ordered bimodal-porous structure, and the n-n heterojunctions induced decrease in work function and enhancement in chemisorbed oxygen. Accordingly, the proposed correlation between gas selectivity change and band gap modulation can open new route for solving the shortcoming of poor selectivity of metal oxide semiconductor gas sensor.