Insights into Selective Mechanism of NiO-TiO₂ Heterojunction to H₂ and CO

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.