The Controllable Preparation And Photocatalytic Overall Water Splitting Performance Of COF/MOFs-Based Heterojunctions

Solar-driven overall water splitting is considered an ideal way to generate renewable hydrogen energy,however,developing efficient photocatalysts for the production of hydrogen（H₂）and oxygen（O₂）without using any sacrificial agents remains a significant challenge.Metal organic frameworks（MOFs）and covalent organic frameworks（COFs）have broad application potential in the field of photocatalytic hydrogen or oxygen evolution due to their adjustable pore structure,strong light absorption ability,and abundant active sites.However,the research on photocatalytic overall water splitting in pure water is still in its infancy.The main reason is the lack of sufficient oxidation and reduction capacity,and the slow kinetics and reverse reaction make it difficult to overall water splitting.In response to the above issues,a series of high-performance heterojunction photocatalysts have been designed and synthesized based on MOFs and COFs,which are used for photocatalytic complete water decomposition and to explore alternative reactions for oxygen production.The specific works are as follows:（1）The core shell NH₂-MIL-68（In）@NH₂-MIL-53（Fe）heterojunction was prepared using a strategy of thermodynamic control of metal ion exchange,and was used in the study of photocatalytic overall water splitting.The shell thickness of NH₂-MIL-53（Fe）can be effectively regulated by adjusting the concentration of Fe³⁺.It benefits from the synergistic effect between the built-in electric field generated by the stress-induced polarization potential in the asymmetric amino functionalized MOFs on carrier separation and enrichment of active site,the efficiency of carrier transport and utilization has been greatly improved.Under the coupling effect of ultrasound and visible light,the optimal H₂-MIL-68（In）@NH₂-MIL-53（Fe）（In@Fe 1）obtained excellent photocatalytic H₂(200.14μmol·h^-1·g^-1)and O₂(100.09μmol·h^-1·g^-1)yields.This work provides a unique idea for the efficient piezo-photocatalytic synergistic systems of MOFs materials.（2）MOF derived Co@NC and Co-Pi are used as electron and hole transfer media for Zn_0.5Cd_0.5S,respectively,to achieve directional separation strategy and effective separation of photoexcited charge carriers,greatly improving photocatalytic activity.Moreover,the improved specific surface area can also provide more abundant active sites.The optimization Co@NC/Zn_0.5Cd_0.5S/Co-Pi sample possesses the yields of H₂ and H₂O₂ up to 613.8 and 592.7μmol/h/g in pure water,which are 13.2 and 14.4 times higher than Zn_0.5Cd_0.5S,respectively.The optimal sample exhibits an apparent quantum efficiency of 2.73%at 420 nm and exhibits excellent photocatalytic stability（>90 h）.This work illustrates the importance of charge carrier separation to the photocatalytic performance lays the foundation for designing efficient`photocatalyst systems for overall water splitting.（3）The controllable preparation and piezo-photocatalytic overall water splitting performance of Z-scheme BiFeO₃@TpPa-1-COF heterojunctions.For the first time,this work combined COFs and amino functionalized piezoelectric material（BiFeO₃）by covalent linkages to form Z-scheme core@shell heterostructure with precisely adjustable shell thickness,demonstrating excellent photocatalytic total water splitting performance.Benefiting from the synergistic effect between the polarized electric field and photo-generated charges,as well as the precise adjustment of shell thickness,the carrier separation and utilization efficiency is greatly improved.The optimal BiFeO₃@TpPa-1-COF photocatalyst revealed H₂ and O₂production rates of 1416.4 and 708.2μmol·h^-1·g^-1 under the excitation of ultrasonication coupled with light irradiation,which is the best performance among various piezo-and COF-based photocatalysts.This provides a new sight for the development and practical application of COFs based high-efficiency photocatalyst for overall water splitting.（4）The controllable preparation and photocatalytic overall water splitting performance of COF/MOFs-basedα-Fe₂O₃/TpPa-1-COF/Fe P-PC heterojunctions.MIL-88B（Fe）derivedα-Fe₂O₃ and Fe P-PC are used as dual cocatalysts to achieve spatial separation of photogenerated electron hole pairs in TpPa-1-COF.Porousα-Fe₂O₃ and Fe P-PC as skeleton support materials effectively prevent the aggregation of TpPa-1-COF during the preparation and photocatalytic process.Meanwhile,the construction of Z-scheme heterostructures and the connection of covalent bonds in the interface ofα-Fe₂O₃/TpPa-1-COF/Fe P-PC heterostructures effectively facilitate the separation and transport of charge.Consequently,the optimalα-Fe₂O₃/TpPa-1-COF/Fe P-PC exhibits very competitive photocatalytic H₂(97.45μmol·h^-1·g^-1)and O₂(48.45μmol·h^-1·g^-1)performance.This work opens up a new way for designing COF-based photocatalysts with high photocatalytic water decomposition efficiency.（5）The ternary NH₂-Fe-MOF/TpPa-1-COF/NH₂-Co-MOF photocatalyst was prepared by covalent bonding of COF and MOF heterojunction using a simple solvothermal method,which was used for photocatalytic overall water splitting performance,and simultaneously photocatalytic hydrogen production and degradation of methylene blue（MB）pollutants.Among them,NH₂-Fe-MOF enriches electrons to become hydrogen evolution sites,while NH₂-Co-MOF enriches holes to become oxidation active site,inhibiting the recombination of electron hole pairs,greatly improving the efficiency of carrier separation and utilization.The optimized NH₂-Fe-MOF/TpPa-1-COF/NH₂-Co-MOF has highly competitive photocatalytic overall water splitting(H₂ evolution rate of 152.5μmol·h^-1·g^-1,O₂ evolution rate of 76.3μmol·h^-1·g^-1)and photocatalytic degradation（degradation rate within 1 h:96.3%）simultaneous hydrogen production(H₂ evolution rate:6725μmol·h^-1·g^-1)efficiency.This study provides new insights for the design photocatalysts of oxidation semi reaction substitution reactions for wastewater treatment while producing hydrogen.