Hierarchical TiO₂:Cu₂O Nanostructures for Gas/Vapor Sensing and CO₂ Sequestration

We investigate the effect of high-surface-area self-assembled TiO₂:Cu₂O nanostructures for CO₂ and relative humidity gravimetric detection using polyethylenimine (PEI), 1-ethyl-3-methylimidazolium (EMIM), and polyacrylamide (PAAm). Introduction of hierarchical TiO₂:Cu₂O nanostructures on the surface of quartz crystal microbalance sensors is found to significantly improve sensitivity to CO₂ and to H₂O vapor. The response of EMIM to CO₂ increases fivefold for 100 nm-thick TiO₂:Cu₂O as compared to gold. At ambient CO₂ concentrations, the hierarchical assembly operates as a sensor with excellent reversibility, while at higher pressures, the CO₂ desorption rate decreases, suggesting possible application for CO₂ sequestration under these conditions. The gravimetric response of PEI to CO₂ increases by a factor of 3 upon introduction of a 50 nm TiO₂:Cu₂O layer. The PAAm gravimetric response to water vapor also increases by a factor of 3 and displays improved reversibility with the addition of 50 nm TiO₂:Cu₂O structures. We found that TiO₂:Cu₂O can be used to lower the detection limits for CO₂ sensing with EMIM and PEI and lower the detection limits for H₂O sensing with PAAm by over a factor of 2. Coarse-grained and all-atom molecular dynamics simulations indicate the dissociative character of ionic liquid assembly on TiO₂:Cu₂O interfaces and different distributions of CO₂ and H₂O molecules on bare and ionic liquid-coated surfaces, confirming experimental observations. Overall, our results show high potential of hierarchical assemblies of TiO₂:Cu₂O/room temperature ionic liquid and polymer films for sensors and CO₂ sequestration.