The construction of p-n heterojunctions has become a widely adopted strategy for achieving the selective detection of reducing gases, including H2 and CO. Nevertheless, the elucidation of the gas selectivity mechanism at the nanoscale remains elusive. First-principle calculations provide an attractive avenue for comprehending the influence of coordination structures on gas-sensitive selectivity, thereby unveiling the structure-activity relationship of p-n heterojunction sites. In this study, we investigate the selective adsorption behavior of H2 and CO on a NiO-TiO2 heterojunction using density functional theory. The results of d-band center analysis confirm that the NiO-TiO2 heterojunction with adsorbed oxygen significantly enhances the adsorption stability of reducing gases. Intriguingly, our calculations reveal that H2 has a higher affinity for adsorbed oxygen on the heterojunction surface compared to that of CO, corresponding to a lower H2 adsorption energy. Density of states (DOS) results indicate that the NiO-TiO2 heterojunction, with preadsorbed oxygen, exhibits ultrahigh selectivity with an n-type gas-sensitive response to H2, effectively eliminating the cross-sensitivity observed with CO, as confirmed by gas-sensitive characterization research. The sensing mechanism of the NiO-TiO2 heterojunction's selective detection of H2 without interference from CO can be visually explained by electron transfer and potential barrier changes, paving the way for future developments in novel, selective gas-sensitive materials.
Keywords: NiO-TiO2 heterojunction; charge transfer; first-principle calculations; gas sensors; reducing gases; selectivity mechanism.