Constructing Hetero-interface Of α-Fe₂O₃/Fe-MOFs For Efficient Photoelectrochemical Water Oxidation

Solar-powered photoelectrochemical decomposition of water using semiconductor photoelectrodes is a sustainable and environmentally friendly way to convert renewable energy.Hematite（α-Fe₂O₃）,one of the most suitable candidate photoanode materials,has been widely studied owing to its characteristics of resistance to photocorrosion,biocompatibility,and natural abundance.However,limitations such as low intrinsic conductivity,short hole diffusion distance and slow water oxidation kinetics severely hinder the solar-to-electric conversion performance of PEC devices.Hence,it is of great significance to construct suchα-Fe₂O₃-based photoanodes with high-efficiency photoelectrochemical water oxidation performance.In this thesis,a nanostructured heterojunction system by constructing the overlayer onα-Fe₂O₃ was proposed to boost the PEC water splitting performance of hematite.The research contents are listed as follows:（1）α-Fe₂O₃ microcluster film was covered with one layer of iron-based metal-organic framework material（MIL-101）by a one-drop method to obtain the Fe₂O₃/MIL-101.Fe₂O₃@MIL-101 heterogeneous film with a tight interface was prepared via chemical vapor deposition,in which Fe₂O₃ microcluster serves as a self-sacrificed template for in-situ growth of the dense MIL-101 layer.The morphology,structure,and photoelectrochemical properties of the prepared photoanodes were studied.Combined with the Mott-Schottky curve and electrochemical impedance spectroscopy,the mechanism of improvement in the photoelectrochemical performance of photoanodes was analyzed.The results show that the matched energy band positions between MIL-101 and Fe₂O₃ constituted a typeⅡheterojunction,which could improve the charge separation efficiency in bulk.Compared with that of pure Fe₂O₃ microclusters,the photocurrent density increased by 2.8 times to 11.5μA·cm^-2（1.23 V vs.RHE）.The interfacial properties of heterogeneous films were optimized owing to the CVD synthesis method.The close interface could lower the charge transfer barrier,reducing charge transfer impedance of Fe₂O₃@MIL-101 from 6745Ω（Fe₂O₃）to 3438Ω（Fe₂O₃@MIL-101）.The significantly decreased fluorescence emission intensity indicated improved carrier transfer efficiency.MIL-101 with developed pore structure and abundant catalytic sites enlarged the specific surface area of the photoanode to 71.9 m²·g^-1,which was beneficial to the contact between the electrode and electrolyte,promoted mass transfer and provided active sites for surface reactions.The photocurrent density reached 14.6μA·cm^-2（1.23 V vs.RHE）,which is 5 times that of the pure Fe₂O₃.（2）Fe₂O₃ nanorod arrays were synthesized by hydrothermal,followed by CVD for decorating MIL-101 on the top to obtain Fe₂O₃@MIL-101 nanorod arrays.The morphology,structure and photoelectrochemical properties of the prepared photoanode were systematically characterized.By combining high-resolution transmission electron microscopy,transient/steady-state fluorescence spectroscopy and electron paramagnetic resonance,the effects of morphological structure on charge transfer and photoelectrochemical properties of photoanode were explored,the relationship between the thickness of the MIL-101 layer and the PEC performance of the Fe₂O₃@MIL-101was revealed,and the photoelectrochemical water oxidation mechanism of the system was analyzed.The results show that the photocurrent density of Fe₂O₃ nanorod array is increased by 100 times to 0.4 mA·cm^-2,and exciton lifetime is 4.6 times longer than microcluster film,which indicated that the nanorod arrays have an advantage of charge transport ability.The built-in electric field of the typeⅡheterojunction constructed by Fe₂O₃nanorod array and MIL-101 layer could also drive charge separation.MIL-101 could passivate surface defects,prolong the exciton life and increase the electrochemically active surface area.By comparing the PEC performance exhibited by photoanodes covered with MIL-101 layers of different thicknesses,it could be seen that the 6-nm porous MIL-101 layer facilitated the diffusion of holes from the inside to the surface of photoanode,and can,displaying high photoelectric conversion efficiency and excellent potential for photoelectrochemical water oxidation.The charge transfer impedance of the obtained Fe₂O₃@MIL-101 nanorod arrays was reduced to 1577Ω,and the photocurrent density achieved 1 mA·cm^-2（1.3 V vs.RHE）,which is 2.5 times that of the Fe₂O₃ nanorod array.