Microwave irradiation induced UIO-66-NH2 anchored on graphene with high activity for photocatalytic reduction of CO2

Carbon dioxide (CO2), as a kind of the greenhouse gas, was highly desirable to be converted into fuels by using solar energy. Herein, a novel microwave-induced synthesis route was explored for the in-situ growth and assemble of highly dispersed UIO-66-NH2 nanocrystals onto graphene (GR) with the formation of highly active photocatalyst for converting CO2. The as-obtained UIO-66-NH2/GR hybrid exhibited both high activity and selectivity in the photocatalytic reduction of CO2 to formic acid under visible-light irradiation (λ > 410 nm, 300 W Xe lamp). The photo-reduction efficiency of CO2 for UIO-66-NH2/GR was about 11 times of that for the pure UIO-66-NH2, and 2 times of as that for the UIO-66-NH2/GR sample obtained via traditional hydrothermal synthesis. The proposed microwave-assisted synthesis route may produce lots of "super hot spots" (SHS) on the surface of GR. These SHS not only resulted in small UIO-66-NH2 nanocrystals with a high dispersion onto the surface of GR, but also can greatly improve the interaction between UIO-66-NH2 and GR. Such highly dispersed UIO-66-NH2 ultrafine nanocrystals can allow more active surface for both trapping CO2 and enhancing the light absorption capability to generate photogenerated electrons from UIO-66-NH2 frameworks for reducing CO2 molecules. The strong UIO-66-NH2/GR interaction can effectively facilitate the photoelectron-hole separation and inhibit the leaching of UIO-66-NH2 from GR, contributing a high CO2 photo-reduction activity and excellent recyclability. Such work supplied a novel route for constructing strong interaction between MOFs and graphene with the aim at efficiently reducing CO2 under visible-light irradiation. It could also be explored for other applications, including N2 reduction, water-splitting, and solar cells.