Construction And Photoelectrochemical Water Splitting Performance Of Hematite-based Nanorod Arrays

From the beginning of the first Industrial revolution,coal,oil,natural gas and other traditional fossil energy has been the main source of energy for human life.With the continuous development of the world economy and the prosperity and stability of social life,people have gradually realized the seriousness of excessive use of fossil energy.As a new type of secondary energy with abundant sources,high energy density and green environmental protection,hydrogen energy is an ideal substitute for fossil energy.Among the many hydrogen production methods,photochemistry’s（PEC）splitting water to produce hydrogen is the most promising scheme.In the water decomposition reaction,the formation of O₂ involves the transfer of four electrons,which is the speed determination step.Therefore,the design and preparation of efficient and stable photoanode is the key to achieve efficient PEC water decomposition hydrogen production.Hematite（α-Fe₂O₃）is considered to be one of the most potential photoanode materials because of its suitable band gap width（≈2.1e V）,excellent light stability,environmental friendliness,and large natural abundance.However,the defects of α-Fe₂O₃ such as low conductivity,short carrier lifetime and slow water oxidation kinetics limit its further application in PEC water oxidation.Therefore,the necessary modification ofα-Fe₂O₃is required.Among many modification methods,supported cocatalyst can significantly accelerate the water oxidation kinetics ofα-Fe₂O₃,promote the separation and transfer of photogenerated holes,and is an effective means to construct efficient composite photoanodes.Based on this,this paper designed and prepared an efficient co-catalyst based onα-Fe₂O₃ nanorods array,obtained a composite photoanode with excellent PEC properties,and studied its charge behavior and water oxidation mechanism.The main research contents are as follows:（1）Study on the application of Co-V bimetallic oxides to improve the water-splitting performance of hematite nanorods arrays:First,α-Fe₂O₃ nanorods arrays were prepared on FTO substrate by hydrothermal method,and then Co VO_x/α-Fe₂O₃ composite photoanodes were constructed by impregnation annealing method.The optimized Co VO_x/α-Fe₂O₃composite photoanode exhibits an excellent photocurrent density of 1.17 m A cm^-2 at 1.23V_RHE,which is 102%higher than that of the originalα-Fe₂O₃ photoanode,and exhibits excellent stability under continuous lighting for 5 h.The results show that Co VO_xexists in an ultra-thin amorphous form and is uniformly covered on the surface of theα-Fe₂O₃nanorod array,which is able to maintain its good nanorod morphology.At the same time,Co VO_x can not only effectively extract holes to accelerate the water oxidation reaction,but also passivate excessive surface states,thus inhibiting surface charge recombination.The electron effect between Co-V bimetals can promote the electron-hole transfer and improve the charge transfer efficiency.（2）Research on Co-Fe bimetallic phosphate used to improve the water decomposition performance of hematite nanorods array:Co Fe PO_x/α-Fe₂O₃ composite photoanode was successfully constructed on theα-Fe₂O₃ nanorods array synthesized by hydrothermal method by impregnation,adsorption and annealing.The photocurrent density of Co Fe PO_x/α-Fe₂O₃ composite photoanode increased from 0.17 m A cm^-2 to 0.58 m A cm^-2 at1.23 V_RHE,showing excellent PEC activity and good long-term stability.A series of characterization and test results showed that Co Fe PO_x was uniformly distributed as amorphous nanoparticles onα-Fe₂O₃ nanorods array.Abundant Co sites provide a large number of active sites for water oxidation reaction,which can effectively accelerate the water oxidation kinetics.The electron coupling of Co-Fe bimetal provides the power for charge transfer,and phosphate provides the channel for charge transfer,reduces the surface charge transfer resistance,promotes the electron-hole separation and transfer,and also enhances the hydrophilicity of the composite photoanode,reduces the overpotential of the water oxidation reaction,and is conducive to the water oxidation reaction.