Crystal-Defect-Induced Longer Lifetime of Excited States in a Metal-Organic Framework Photocatalyst to Enhance Visible-Light-Mediated CO 2 Reduction.

We report the structural defects in Zr-metal-organic framework (MOFs) for achieving highly efficient CO 2 reduction under visible light irradiation. A series of defective Zr-MOF- X ( X = 160, 240, 320, or 400) are synthesized by acid-regulated defect engineering. Compared to pristine defect-free Zr-MOF (NNU-28), N 2 uptake increases for Zr-MOF- X synthesized with the HAc modulator, producing a larger pore space and Brunauer-Emmett-Teller surface area. The pore size distribution demonstrates that defective Zr-MOF- X exhibits mesoporous structures. Electrochemistry tests show that defective Zr-MOF- X possesses a more negative reduction potential and a higher photocurrent responsive signal than that of pristine NNU-28. Consequently, the defective samples exhibit a significantly higher efficiency in the photoreduction of CO 2 to formate. Transient absorption spectroscopies manifest that structural defects modulate the excited-state behivior of Zr-MOF- X and improve the photogenerated charge separation of Zr-MOF- X . Furthermore, electron paramagnetic resonance and in-suit X-ray photoelectron spectroscopy provide additional evidence of the high photocatalytic performance exhibited by defective Zr-MOF- X . Results demonstrate that structural defects in Zr-MOF- X also improve the charge transfer, producing abundant Zr(III) catalytically active sites, exhibiting a slower decay process than defect-free Zr-MOF. The long-lifetime Zr(III) species in defective Zr-MOF- X are fully exposed to a high-concentration CO 2 atmosphere, thereby enhancing the photocatalytic efficiency of CO 2 reduction.