In Situ Construction And Modification Of Fe₂O₃/Fe₃O₄ Heterojunction For Photoelectrochemical Water Splitting

Energy plays the role of the lifeblood of the country’s economy,and the development process of society is carried out with energy as the carrier.In addition,the massive burning of fossil fuels has also caused serious environmental pollution and even formed an irreparable ecological environment.Hydrogen energy is widely known as a new type of clean and non-polluting energy.Compared with traditional hydrogen production methods,photoelectrocatalystic water splitting for hydrogen production has attracted extensive attention of researchers owing to the utilization of a small amount of external energy and inexhaustible solar energy.A series of advantages such as high efficiency,stability and low cost make it a research hotspot.As an excellent semiconductor photocatalyst,Fe₂O₃ is widely favored by scientific researchers.Its main advantages are that it has a suitable forbidden band width（2.1e V）,excellent light absorption ability,easy availability,stable chemical properties at room temperature,and the most important,an extremely high theoretical photocurrent density(12.6m AHcm^-2).However,the theoretical value is always far from the actual one,which makes it impossible to be used commercially.The main disadvantages of this material are:（1）poor electrical conductivity;（2）short hole diffusion length（2–4 nm）;（3）slow water oxidation kinetics.In order to realize the wider application of the material,the modification and optimization of the material are particularly important.In this paper,Fe₂O₃/Fe₃O₄heterojunctions were constructed in situ by a simple two-step annealing method（that is,annealed in air and then in argon）,and their supported cocatalysts were further modified to increase the carrier concentration,inhibit the recombination of photogenerated charges,and optimize a series of problems such as short hole diffusion length and slow water oxidation kinetics.（1）The pure iron foil（99.9%）was ground and polished,the oxide layer on the surface was removed and ultrasonically cleaned,Fe₂O₃ nanotubes were prepared by anodizing with a DC regulated power supply,and then annealed in two steps,the sample was recorded as NTAs-Air（400）-Ar（X）,where X represents the annealing temperature.Comparative samples NTAs-Ar（500）and NTAs-Ar（500）were prepared,and the phases of the samples were determined by XRD.The results show that there are two phases of Fe₂O₃ and Fe₃O₄ in the samples prepared by the two-step annealing method.The XRD results show that the content of Fe₃O₄ formed by annealing in an argon atmosphere at different temperatures is different.According to the SEM results,a large amount of Fe₃O₄ is formed.This results in a severe lattice mismatch with Fe₂O₃,resulting in the collapse of the nanotube morphology.The XPS results of Fe element show that.A characteristic peak of Fe²⁺in Fe₃O₄ appeared at 709.6 e V,which proved the formation of Fe₃O₄.While the sample NTAs-Air（500）annealed only in air showed pure Fe₂O₃ phase,and the sample NTAs-Ar（500）annealed only in argon showed pure Fe₃O₄ phase.（2）A series of photoelectrochemical tests and total water splitting performance tests were performed on all prepared samples,and all test results were carried out under an applied bias voltage of 1.6 Vνs.RHE.The test results show that NTAs-Air（400）-Ar（500）has the highest photocurrent density(2.5 m AHcm^-2),which is improved compared with the pure Fe₂O₃sample（NTAs-Air（500））3.3 times,and its onset position was reduced to 0.556 Vνs.RHE.The research shows that the content of Fe₃O₄ has a great influence on the performance,showing a trend that the more Fe₃O₄ content,the stronger the performance.Absorbs light,thereby degrading performance.The results of water splitting test showed that the hydrogen and oxygen production rates of the best sample reached 23.84 and 13.18μmol·cm^-2·h^-1.（3）The NTAs-Air（400）-Ar（500）samples were continuously loaded with the cocatalyst Co-Pi,and the composite materials with different deposition times were finally successfully prepared with different deposition times as variables.The test results of IT,LSV and EIS show that the sample reaches the highest photocurrent density(5.2 m AHcm^-2 at 1.6 Vνs.RHE)after400 s deposition,which is 2 higher than that of the previous best-performing sample.times,and more importantly,its starting point is as low as 0.417 Vνs.RHE.The carrier concentration is an order of magnitude higher than the samples in the previous chapter.The faster interfacial reaction rate enables the hydrogen and oxygen production rates to reach 44.78 and 23.64μmol·cm^-2·h^-1,which are similar to the hydrogen production rates of the best-performing samples in the previous chapter.than about twice as high.In addition,after 5 cycles of testing for a total of 20 h,the sample still showed good performance,proving the stability of the sample performance.