Preparation Of GC₃N₄-based Z-scheme Heterojunction Supported By Dual Promoters And Its Photoreduction Performance For CO₂

Constructing artificial Z-scheme heterojunction systems for photocatalytic CO2 conversion is one of the effective tactics to alleviate the energy crisis and global warming.The reductive semiconductor graphitic carbon nitride(g-C3N4)has a negative conduction band bottom energy level and benign visible light absorption.The Z-scheme heterojunctions based on g-C3N4 is a research hotspot in the field of photoreduction CO2.However,the g-C3N4-based Z-scheme heterojunction still exhibit low efficiency of charge separation and lack catalytic active sites both are important for photocatalytic activity.Introducing cocatalyst on g-C3N4-based Z-scheme heterojunction surface is a viable way to solve the above problems,but the key is to achieve precise loading on the surface of the redox components.To address the issues,BiVO4/g-C3N4 Z-scheme heterogeneitie is used as the model in this work.By introducing spatially separated subnanometer CoOx clusters and ionic liquid(IL)on the surface of BiVO4 and g-C3N4 can significantly promotes the Z-scheme charge transfer and separation.The photocatalytic reduction CO2 activity of the g-C3N4-based Z-scheme heterojunction is significantly enhanced.In addition,the mechanisms of charge transfer and involved in the CO2 conversion process are also revealed.The paper consists mainly of the following two sections:(Ⅰ)Modification of BiVO4/g-C3N4 Z-scheme heterojunction photocatalyst by CoOx clusters.Firstly,the acid-treated g-C3N4 nanosheets integrates with BiVO4 to construct the Z-scheme heterojunction,and then subnanometer CoOx clusters are oriented to BiVO4 surface by the photogenerated hole-assisted oxidation strategy.In this way,we can obtained the BiVO4/g-C3N4 Z-scheme heterojunction photocatalyst anchored with CoOx clusters.The CoOx-BiVO4/g-C3N4 heterojunction optimized the load of BiVO4 and CoOx shows the higher CO production rate(39.7 μmol g-1 h-1).The catalyst exhibits a 10-fold enhancement in CO conversion over g-C3N4.Results from in-situ irradiation XPS,Kelvin probe force microscopy and electrochemical tests indicate that the key of the enhancement CO2 reduction activity is that CoOx effectively captures the photogenerated holes of BiVO4,significantly facilitating Z-scheme charge transfer and separation,and accelerating the water oxidation dynamics.(Ⅱ)Modification of CoOx-BiVO4/g-C3N4 Z-scheme heterojunction photocatalyst by ionic liquid.Based on the CoOx-BiVO4/g-C3N4 Z-scheme heterojunction,the 1-ethyl-3-methy-limidazolium bis(trifluoromethylsulfonyl)imide([Emim]NTf2)is controllably decorated on g-C3N4 by impregnation and the following H-bond induced assembly procedure.The CoOx-BiVO4/g-C3N4-IL with the optimal IL loadings gives the highest CO production rate(83.5 μmol g-1 h-1).The catalyst exhibits a 20-fold enhancement in CO conversion over g-C3N4 while no H2 was detected.Results from in-situ transient absorption spectroscopy and electrochemical tests indicate that the boosted CO2 conversion is attributed to CoOx modification that significantly prolongs lifetime of photogenerated electrons on g-C3N4,and that the spatially separated CoOx and IL respectively promotes the kinetic processes of holes-H2O and electrons-CO2 reduction.In addition,the intermediates of the photocatalytic reduction CO2 reaction process were investigated by in-situ DRIFTS,and then a possible CO2 conversion pathway on the fabricated CoOx-BVO/CN-IL heterojunction is proposed.This paper provides a feasible idea for the construction of efficient g-C3N4-based Z-scheme heterojunction for photocatalytic reduction CO2 and diversifies a reference for in-situ technology to reveal the charge transfer and catalytic mechanism.