In-situ synthesized N-doped ZnO for enhanced CO₂ sensing: Experiments and DFT calculations

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.