Acceleration of Photoinduced Electron Transfer by Modulating Electronegativity of Substituents in Stable Zr-Metal-Organic Frameworks to Boost Photocatalytic CO 2 Reduction.

Photoreduction of CO 2 with water into chemical feedstocks of fuels provides a green way to help solve both the energy crisis and carbon emission issues. Metal-organic frameworks (MOFs) show great potential for CO 2 photoreduction. However, poor water stability and sluggish charge transfer could limit their application. Herein, three water-stable MOFs functionalized with electron-donating methyl groups and/or electron-withdrawing trifluoromethyl groups are obtained for the CO 2 photoreduction. Compared with UiO-67- o -CF 3 -CH 3 and UiO-67- o -(CF 3 ) 2 , UiO-67- o -(CH 3 ) 2 achieves excellent performance with an average CO generation rate of 178.0 μmol g -1 h -1 without using any organic solvent or sacrificial reagent. The superior photocatalytic activity of UiO-67- o -(CH 3 ) 2 is attributed to the fact that compared with trifluoromethyl groups, methyl groups could not only elevate CO 2 adsorption capacity and reduction potential but also promote photoinduced charge separation and migration. These are evidenced by gas physisorption, photoluminescence, time-resolved photoluminescence, electrochemical impedance spectroscopy, transient photocurrent characteristics, and density functional theory calculations. The possible working mechanisms of electron-donating methyl groups are also proposed. Moreover, UiO-67- o -(CH 3 ) 2 demonstrates excellent reusability for the CO 2 reduction. Based on these results, it could be affirmed that the strategy of modulating substituent electronegativity could provide guidance for designing highly efficient photocatalysts.