Preparation Of MOFs/TiO₂ Heterojunction Photoelectrode And Performance Study On Photoelectrochemical Water Splitting

As an efficient energy conversion technology,photoelectrochemical（PEC）water splitting could provide a sustainable and environmental protection way for the conversion of solar-hydrogen energy,so it plays an important role in dealing with energy shortage and environmental pollution.As an important unit in the PEC water splitting system,the photoelectrode determines the overall efficiency of PEC water splitting.Therefore,it is the key to further improve the PEC water splitting efficiency by the reasonable design and construction of high-performance photoelectrode.Titanium dioxide（TiO₂）is considered to be one of the most potential photoelectrode candidate material in the field of hydrogen production by PEC water splitting due to its many excellent properties,but its PEC performance has been severely limited by three aspects,such as the undesirable light response ability,the high recombination rate of photogenerated electron-hole pairs and sluggish water oxidation kinetics.Based on the limitation of pure TiO₂,three feasible strategies are put forward in this study,which could significantly improve the PEC performance.Through a series of characterization analysis and performance tests,the structure-activity relationship between photoelectrode heterostructure and PEC performance is systematically studied,and the corresponding charge transfer mechanism is further proposed.The specific work is as follows:（1）In this work,the precursor FeOOH was first in-situ etched with H₃BTC,then the FeOOH was successfully converted into MIL-100（Fe）to modify TiO₂photoelectrode,thus a new kind of MIL-100（Fe）/TiO₂ heterojunction photoelectrode was constructed.By means of SEM,XRD,FTIR,TEM and XPS,the micro-morphology and phase structure of MIL-100（Fe）/TiO₂ heterojunction photoelectrode were characterized.The as-fabricated MIL-100（Fe）/TiO₂heterojunction could effectively inhibit the recombination of photogenerated electron-hole pairs,which is helpful for the rapid separation and transfer of charges.Meanwhile,MIL-100（Fe）acted as the oxygen evolution reaction（OER）catalyst,the Fe sites of unsaturated coordination exposed on the surface of MIL-100（Fe）structure are conducive to promoting the water oxidation kinetics on the surface of the photoelectrode.（2）In order to further explore the mechanism of metal-organic frameworks（MOFs）with bimetallic atom as the center on TiO₂ photoelectrode,we used NiFe-MOF as OER catalyst to modify TiO₂ photoelectrode,thus we successfully prepared the NiFe-MOF/TiO₂ heterojunction photoelectrode.We deeply analyzed the the influence of NiFe-MOF centered with bimetallic atom on the performance of TiO₂photoelectrode,and investigated the charge transfer mechanism of NiFe-MOF/TiO₂photoelectrode.The results suggest that Ni as well as Fe sites on NiFe-MOF play a synergistic effect in the process of PEC water oxidation,which could significantly enhance the PEC water oxidation performance of the photoelectrode.（3）To solve the problem of weak photoresponse ability and poor conductivity of TiO₂ photoanode,we fabricated a ternary Ag/NH₂-MIL-125/TiO₂ heterojunction photoelectrode.In this work,the NH₂-MIL-125/TiO₂ photoelectrode was firstly prepared by twice hydrothermal method,and then the silver nanoparticles（Ag NPs）were uniformly deposited on the surface of NH₂-MIL-125/TiO₂ photoelectrode by an immersion method using the reducibility of amino group in NH₂-MIL-125 structure,and finally the Ag/NH₂-MIL-125/TiO₂ heterojunction photoelectrode was obtained.We deeply elaborated the role of NH₂-MIL-125 and Ag NPs in PEC water splitting system,and further discussed the charge transfer mechanism of Ag/NH₂-MIL-125/TiO₂ photoelectrode for PEC water splitting.The results suggest that the synergistic effect between NH₂-MIL-125/TiO₂ and Ag NPs could significantly improve the absorption ability of TiO₂ photoelectrode for visible light,while the presence of Ag NPs could further enhance the overall conductivity of heterojunction photoelectrode.