Of numerous environmental problems, the air pollution caused by toxic nitrogen dioxide has become more and more serious. Its timing detection is of utmost importance but challenging. It is known that NO2 sensors fabricated by zinc oxide-derived materials suffer some issues. And thus synthetic strategies such as doping and combining have been developed to improve sensitive performance and modify operation condition. However, the relevant sensing reaction mechanism still remains unclear; moreover, the experimentally structural characterizations on sensitive materials (SMs) and reaction intermediates are rather difficult at the atomic level, which turn much worse for the doped and combined SMs. In the work, density functional theory calculations have been exploited to examine copper-doped zinc oxide (marked as ZOC) and its composites ZOC/CN and ZOC/Gr. CN and Gr are short for 2D materials, graphitic carbon nitride (g-C3N4) and graphene, respectively. The local structures have been accessible for these SMs and their intermediates along the reaction pathway. It is shown that ZOC bears the Cu-Zn heterobimetallic adsorption active sites, which hold NO2 via Cu/Zn-O dative bonds. The introduction of copper increases metal contribution to high-lying occupied molecular orbitals (MOs). Major NO2→ZOC donation and minor back-donation interactions have been recognized by charge decomposition analyses in terms of fragment MOs. Consequently, its adsorption free energy towards NO2 is strengthened by 0.27 eV relative to pristine ZnO. These well interpret the experimental findings that the copper-doped zinc oxide has much faster response time. Further combination with g-C3N4 enhances the NO2-sensing performance, which turns out to be the best SM candidate. Exemplarily, ZOC/CN shows the most negative NO2 adsorption energy (-0.31 eV), very small uphill energy for the rate-determining step (0.27 eV) and modest formation energy of nitrate (-1.06 eV). With these, the SM not only responses NO2 rapidly but also desorbs nitrate at mild