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.