Preparation Of Bi₄O₇-based Semiconductor Composite Materials And Study On Their Visible Light Photocatalytic Properties

With the rapid development of society,the intractable organic pollutants in the wastewater from the pharmaceutical,printing,textile and other industries have caused a great threat to human health.As a green and efficient technology,photocatalysis is considered to be a promising method to solve the problem of water pollution.Among the various catalysts that have been reported,Ti O₂ has been applied in the field of photocatalysis due to its advantages such as non-toxicity and good chemical stability.However,the wide band gap of Ti O₂ makes it only active under ultraviolet light that accounts for 5%of sunlight,which greatly limits its application range.In recent years,bismuth-based semiconductors have attracted great attention in the field of photocatalysis owing to their non-polluting,low biotoxicity,low cost and other advantages.Among all bismuth-based compounds,binary bismuth oxides have broad development prospects in practical applications due to their simple structure and facile synthesis.Bismuth ions exist in oxides in trivalent or pentavalent forms.In Bi³⁺-containing oxides,the mixed valence band of O 2p and Bi 6s² helps to form a narrow band gap and makes bismuth oxide have visible light response activity.The 6s empty orbital can further reduce the band gap of Bi⁵⁺-containing bismuth oxide.However,for single-phase photocatalysts,the rapid recombination of photogenerated carriers caused by the narrow band gap greatly inhibits their photocatalytic activity.Among various methods to solve this problem,composites with heterojunctions are widely used for photocatalytic degradation of pollutants due to the interface charge transfer effect.In this paper,theδ-Bi₂O₃/Bi₄O₇ heterojunction composite material was prepared by introducing the stableδ-Bi₂O₃ into the Bi₄O₇ precursor by the hydrothermal method.SEM and TEM were used to observe that the rod-shaped Bi₄O₇ grew on the sphericalδ-Bi₂O₃ to form a special morphology,and the BET test proved that the specific surface area of the composite increased.Under simulated visible light irradiation,theδ-Bi₂O_3/Bi₄O₇ composite exhibits highly effective photocatalytic degradation activity for Rh B and phenol.The degradation rate constants are 0.07275 min^-1 and 0.01042 min^-1,respectively,which are much higher than the single phaseδ-Bi₂O₃ and Bi₄O₇.The theoretical analysis and the results of trapping agent experiments confirm that the main active species in the photocatalytic system are holes and superoxide radicals.Therefore,the enhancement of the photocatalytic activity of theδ-Bi₂O_3/Bi₄O₇ composite is attributed to the increase in active sites caused by the increase in specific surface area and the heterojunction formed at the interface to enhance the migration and separation efficiency of photogenerated carriers.Furthermore,we hope to make full use of the coexistence of Bi³⁺and Bi⁵⁺in the mixed-valence bismuth oxide,and introduce Bi₂O₄ with good visible light response range and charge transfer characteristics into the system to construct composites.Bi₂O₄/Bi₄O₇ composites were successfully prepared by the one-pot hydrothermal method using Na Bi O₃·2H₂O and KBi O₃ as precursors.The phase compositions of the samples were characterized by XRD,and the proportion of the two phases in the composite was determined by the Rietveld structure refinement.Under simulated visible light irradiation,the apparent rate constants of Bi₂O₄/Bi₄O₇ heterojunction composites for the degradation of Rh B and phenol are 2.91 and 2.34 times that of the single-phase Bi₄O₇,respectively.The results of the trapping agent experiments show that the main active species in the photocatalytic system are holes and superoxide radicals.Thus,the formation of heterojunctions at the interface of the composite enhances the efficiency of photo-generated carriers’migration and separation,leading to increasing the number of active species produced in the photocatalytic system,and then results in the improvement of the photocatalytic performance of the Bi₂O₄/Bi₄O₇composite.