Facile Strategy for Efficient Charge Separation and High Photoactivity of Mixed-Linker MOFs

Two new sets of UiO-Zr metal-organic framework (MOF) bearing mixed linkers BDC-(SCH3)(2) and BDC-(SOCH3)(2) that have different band gaps and edges were prepared through post oxidation and direct methods, namely, UiO-66-(SCH3)(2)-xh (x = 4, 9, 12 oxidation hours) and UiO-66-(SOCH3)(x)(SCH3)(2-x) (x = 0, 0.4, 0.6, 2), respectively. These composites with stoichiometric components were fully characterized via proton nuclear magnetic resonance (H-1 NMR) spectroscopy, powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectra, Brunauer-Emmett-Teller (BET), photo electrochemical measurements, and femtosecond transient absorption (fs-TA) spectroscopy. The structure, electronic property, and photoresponsive and catalytic ability as the functions of the molar ratio of linkers and the synthetic protocol were first investigated. The mixed-linker UiO-66-(SCH3)(2)-xh and UiO-66-(SOCH3)(x)(SCH3)(2-x) exhibited improved performances as compared to the UiO-66-(SCH3)(2) and UiO-66-(SOCH3)(2) possessing neat linkers only. Their photo response and catalytic activity varied with different linker ratios. For UiO-66-(SCH3)(2)-xh, the performance increased with the increasing linker BDC-(SOCH3)(2) ratio upon oxidation but reached the highest as the BDC-(SOCH3)(2) being of 24.4% in UiO-66-(SCH3)(2)-9h. In comparison, the best photocurrent (80.74 uA/cm(-2)) and the highest H-2 generation rate (2018.8 mu mol g(-1) h(-1)) (lambda > 400 nm) in UiO-66-(SCH3)(2)-9h are about twice those of UiO-66-(SOCH3)(0.4)(SCH3)(1.6) obtained by direct synthesis, although the linker BDC-(SOCH3)(2) ratio of those two composites is almost the same (24.4% vs 23.9%). Recorded shorter lifetime and higher charge separation efficiency of the former than those of the latter suggest the postsynthetic protocol as the efficient method for achieving the mixed-liner-MOF-based photocatalyst with high performance. A new type-II tailored homojunction is proposed in these mixed-linker MOFs for their efficient charge separation and improved activity.