Hydrothermal Synthesis Of Al₂O₃-based Heterojunctions And Their Photocatalytic Activities

Al₂O₃ is widely used as a support in the fields of chemical engineering and material because of its low price,good stability and easy controllability of structure.Our group previous researches have confirmed that amorphous Al₂O₃ possesses photocatalytic activity,which is ascribed to its abundant surface defects and the band gap has been shorten because of the unsaturated coordination number of Al-O.Nevertheless,the recombination rate of photoinduced electron-hole pairs of Al₂O₃ is still fast and the utilization efficiency towards sunlight is too low.In order to promote the separation efficiency of photoinduced e--h+ pairs and the utilization of sunlight,alumina has been produced by hydrothermal method firstly in this work.Subsequently,a series of amorphous Al₂O₃-based heterojunction photocatalysis nanomaterials have been synthesized so as to improve its photocatalytic ability.Then the as-prepared samples were characterized by X-ray diffraction?XRD?,Transmission electron microscopy?TEM?,Scanning electron microscopy?SEM?,UV-Vis diffuse reflectance?DRS?,X-ray spectroscopy?XPS?in order to investigate the relationships among photocatalytic activity and the heterojunction structure,morphology,pore structure,surface area and so on.Firstly,a hydrothermal synthesis approach has been employed to prepare amorphous Al₂O₃ using Al?NO₃?₃·9H₂O as the aluminium source and NaOH,NH₄OH and urea as the precipitants.In this way,the amorphous Al₂O₃ catalysts have abundant surface defect and large surface areas.Phenol was taken as a model pollutant to evaluate the photocatalytic activity of amorphous Al₂O₃ under simulated sunlight irradiation.The results indicate that NaOH-Al₂O₃ exhibits the highest photocatalytic activity.The increased photocatalytic activity of amorphous Al₂O₃ could be attributed to the abundant surface defect,which can effectively absorb UV light and suppress the recombination of photoinduced electron-hole pairs.Moreover,the Al₂O₃/g-C₃N₄ heterojunctions were constructed by an in situ hydrothermal route for photocatalytic water splitting and the degradation of MO,Then,the photocatalytic mechanism of heterojunctions was explored.Experiments showed that a structure modification function of Al₂O₃ for g-C₃N₄ is found in the hydrothermal process,and this effect lead to higher surface area.When Al₂O₃ was combined with g-C₃N₄,the heterojunctions show absorbance in the visible light region.Due to the defect band,Al₂O₃ can accept free electron and produce hydrogen,Thus the Al₂O₃/g-C₃N₄ heterojunctions was applied to hydrogen production in the visible light,and its efficiency is much higher than single g-C₃N₄.Finally,the ternary g-C₃N₄/Al₂O₃/ZnO heterojunctions were prepared by step-by-step precipitation method based on the features that amorphous Al₂O₃ can transfer photoinduced carrier and has unordered atomic configuration.The lattice matching of amorphous materials is proposed for the first time.The photocatalytic activity was investigated by degrading Methylene blue in the visible light.The experimental results showed that the amorphous Al₂O₃ acted as mediated material for matching the lattice of g-C₃N₄ and ZnO.Meanwhile,amorphous Al₂O₃ could be the bridge of photoinduced carrier to promote electron mobility.Thus,ternary g-C₃N₄/Al₂O₃/ZnO heterojunction exhibits cascade electron transfer route and superior molecular oxygen activation performance.As a result,the effect of visible light is enhanced.