Coordination-assisted Core-shell Photoanode: Design,preparation And Application For Water Oxidation

Photoelectrocatalytical（PEC）water splitting has been considered as a promising method for hydrogen evolution.Compared with Hydrogen Evolution Reaction（HER）,Oxygen Evolution Reaction（OER）is a sluggish four-electron process,which is regarded as the rate determining step for PEC water splitting.Therefore,the design and preparation of efficient and stable photoanodes and the exploration of OER mechanism have become one of the focuses in PEC water splitting.Although researchers have developed a variety of n-type semiconductors for PEC water oxidation,single n-type semiconductors generally have limitations such as narrow light absorption range,weak hydrophilicity,low carrier separation efficiency,sluggish surface water oxidation kinetics and poor stability.Therefore,the development of universal strategies to effectively enhance the light absorption,hydrophilicity,carrier separation,water oxidation kinetics,stability of n-type semiconductor photoanodes has been widely concerned by researchers.The construction of core-shell photoanode is expected to overcome the above limitations.However,due to lattice mismatch and lack of interaction between guest material and n-type semiconductor photoanode,the preparation of uniform and stable core-shell photoanode is a great challenge.In this paper,a coordination-assisted self-assembly strategy is proposed to construct core-shell photoanode.In material design and preparation:Caffeic Acid（CA）was used as coordination agent for the first time to construct the core-shell Ov-BiVO₄@Ni Fe-MOFs photoanode.Moreover,Ti:Fe₂O₃ was conformally coated with conductive MOFs（c MOFs）based on the coordination ability of CA.Furthermore,the core-shell BiVO₄@metal phthalocyanine（BiVO₄@M-Pc）photoanode was constructed using Pyrazine（Pz）as coordination agent for the first time.In terms of structure–activity study:X-ray absorption spectroscopy（XAS）and theoretical calculation were applied to explore the effects of CA coordination on the electronic structures variation of O_V-BiVO₄ and Ti:Fe₂O₃ as well as the chemical environment variation of Ni Fe-MOFs in core-shell Ov-BiVO₄@Ni Fe-MOFs;Meanwhile,the phase transformation behavior of core-shell photoanodes during PEC water oxidation were studied based on quasi-operando characterization.The specific research contents are as follows:（1）The core-shell Ov-BiVO₄@Ni Fe-MOFs was constructed based on the strong coordination between catechol of CA and oxygen-vacancy-rich bismuth vanadate（O_V-BiVO₄）and the coordination between carboxyl groups in CA and Ni²⁺and Fe³⁺in the precursors of Ni Fe-MOFs.Core-shell Ov-BiVO₄@Fe-MOFs and Ov-BiVO₄@Co Fe-MOFs were prepared by regulating the metal ion species of MOFs precursor.XAS shows that the core-shell Ov-BiVO₄@Ni Fe-MOFs has shorter bond length and smaller coordination number in Ni–O and Fe–O compared with bulk Ni Fe-MOFs.DFT results show that the water oxidation overpotential of Ov-BiVO₄@Ni Fe-MOFs（0.83 e V）is significantly lower than that of BiVO₄（2.10 e V）.The PEC water oxidation activity test indicates that the current density of Ov-BiVO₄@Co Fe-MOFs is 5.3±0.15 m A cm^-2 at 1.23 V_RHE under simulated solar illumination,and the current density still maintained above 95%after 10 h of reaction.The quasi-operando Raman test shows that during PEC water oxidation,the Ni Fe-MOFs in the core-shell Ov-BiVO₄@Ni Fe-MOFs undergoes phase transformation and generates Ni OOH,thus promoting the OER kinetic process.（2）Core-shell BiVO₄@Zn Co Fe-Pc photoanode was constructed based on Pz axial coordination for the first time.A series of core-shell BiVO₄@Fe-Pc,BiVO₄@Co-Pc,BiVO₄@Ni-Pc,BiVO₄@Zn-Pc and BiVO₄@Co Fe-Pc photoanodes were further prepared by regulating the types of metal ions in M-Pc precursor.The axial coordination of Pz significantly enhances the carrier separation efficiency of the core-shell photoanode BiVO₄@Zn Co Fe-Pc.The hydrophilicity of BiVO₄@Zn Co Fe-Pc was further enhanced by O₂ plasma etching.BiVO₄@Zn Co Fe-Pc（Pz）-O₂exhibits outstanding PEC water oxidation current density of 5.7±0.1 m A cm^-2,under simulated solar illumination at 1.23 V_RHE,and the current density still maintained more than 95%after 20 h of reaction.（3）Core-shell photoanode Ti:Fe₂O₃@Co Fe-c MOFs was constructed based on the coordination of CA.Moreover,core-shell Ti:Fe₂O₃@Fe-c MOFs,Ti:Fe₂O₃@Co-c MOFs and Ti:Fe₂O₃@Ni-c MOFs were further prepared by regulating the metal ion species of MOFs precursor.XAS indicates that the surface coordination of CA resulted in shorter Fe–O and increased electron cloud density in Ti:Fe₂O₃.After coating c MOFs,the electrons are transferred from c MOFs to Ti:Fe₂O₃ via CA,resulting in the generation of high energy surface state,thereby increasing its capacity for holes storage.The synthesized Ti:Fe₂O₃@Co Fe-c MOFs exhibits outstanding photocurrent density of 3.3 m A cm^-2 under the simulated solar illumination at 1.23 V_RHE,and the current density still maintained above 90% after 10 h of reaction.