Chemiresistive CO2 sensing is attractive due to low cost and ease of chip-level integration. Our previous studies (Yong Xia, 2021) showed the well-developed ZnO material fabricated by in-situ annealing exhibited good CO2 sensing performance. Here, we have expanded on those studies, including CO2 cyclic tests under both dry air and N2 background whereby a much higher response to CO2 in N2 background was observed. Detailed density functional theory calculations were conducted to understand the behavior. The results indicated nitrogen doping is mainly responsible for the observed response. In the presence of pre-adsorbed O2, N-doped ZnO can no longer interact with CO2, which agrees well with the observation of higher response in N2 background. Furthermore, density of states analysis showed N sp2 hybridized orbital and N 2p orbital of the N dopant mixed with sp2 hybridized orbital of C atom and 2p orbitals of C/O atoms in CO2 to form σ and π bonds, respectively. However, they mixed with O 2s/2p orbitals of O atom in O2 when pre-adsorbed O2 was present, hindering CO2 interaction with N-doped ZnO, and resulting in limited response in air. The illustrated mechanism does not only further the understanding of metal oxide-based CO2 sensing, but also guide the design of new functional materials for CO2 sensing or capture.
Keywords: In-situ annealing; N-doped ZnO; CO2 sensing; Mechanism study; DFT calculation