Synthesis Of Two-dimensional MOF Nanosheets And Their Application For Preparing Single Atom Catalysts For Photochemical CO₂ Reduction

Single-atom catalysts(SACs)have attracted condidrable attention due to their unique electronic structures and maximum atom utilization.This paper describes a ligand replacement approach,exfoliating a 3D layer-pillared MOFs into 2D ultrathin MOF nanosheets.The resulting MOF nanosheets were further used as precursors mixed with urea to in situ synthesize a SACs based on carbon nitride(g-C₃N₄).The performance of the obtained SACs for photocatalytic CO₂ reduction was explored.The main contents of the paper are summarized as follows:(1)We developed a ligand replacement approach,firstly realizing the general production of 2D ultrathin Co-MOF nanosheets from 3D layer-pillared Co-MOFs.A remarkable color changed from turbid orange solution to pink clear solution after exfoliated and a typical Tyndall effect when a green laser passes through the solution were observed.The infrared spectra(IR)of Co-MOF nanosheets shows that a new peak at 1043 cm^-1 corresponding to the-S=O stretching vibration of DMSO appear in contrast to that of bulky Co-MOF.Thermogravimetric analyses(TG)demonstrate the slightly decreased thermal stability of the Co-MOF nanosheets compared with the bulky Co-MOF,which can be attributed to the easy loss of the terminal DMSO than the pillared bridge 4,4'-bpy.Scanning electron microscope(SEM)results show morphology changed from bulky to thin after exfoliated,and the atomic force microscopy(AFM)analyses clearly showed that the Co-MOF nanosheets have uniform thickness of?1.6 nm,about three times thicker than the single layer MOF nanosheet simulated from the crystal structure.All above results evidence that 2D ultrathin Co-MOF nanosheets have been successfully prepared.(2)The as-prepared 2D ultrathin Co-MOF nanosheets were used as cobalt resources mixed with urea for in situ synthesizing g-C₃N₄ to obtain a Co(?)SACs.The results of PXRD,TEM,HRTEM,HAADF-STEM and X-ray absorption spectrum confirm the isolated dispersion of Co atoms on g-C₃N₄.The SACs were further used as catalysts for photocatalytic CO₂ reduction.The result shows that in CH₃CN/H₂O/TEOA system(v/v/v=3:1:1),the CO generation rate is 471?mol·g^-1·h^-1,superior than all reported g-C₃N₄-based photocatalysts.