Synthesis And Visible-light Photocatalytic Properties Of TiO₂ Nanotube Arrays Based Composite Photocatalyst

With the fast development of economy and the rapid growth of the population,the demands for pollution control are vital for the sustainable development of economy and society.Photocatalysis can effectively treat the waste by using the solar as the energy source,which has attracted more and more intensive attentions in pollution control in recent years.Among the reported photocatalytic semiconductor materials,nanosized TiO₂ has drawn intense interests due to its unique properties.TiO₂ nanotube arrays material?TNTAs?has been deemed as one of the most promising photocatalysts owing to its highly ordered structure,tunable morphology,large specific surface area and superior photocatalytic properties.However,the application of pure TNTAs has been seriously restricted due to the poor absorption of visible light and fast recombination of photo-generated electrons and holes.To face the above issues,TNTAs is modified by incorporating Au-Pt bimetallic quantum dots or constructing all-solid-state Z-scheme hybrid photocatalysts.The prepared photocatalysts have been well characterized and organic pollutants could be efficiently degraded by the visible-light photocatalytic process.Enhanced photocatalytic efficiencies have been observed and reaction mechanisms have also been proposed.Details are as follows:1.Preparation and visible-light photocatalytic properties of bimetal loaded Au-Pt/TNTAsTNTAs was prepared by anodic oxidation method by using the mixed solution of glycerol,deionized water and ammonium fluoride as the electrolyte.Then,bimetallic Au-Pt nanoparticles were successfully loaded on the TNTAs by a simple impregnation-photoreduction method.The bimetal loaded samples demonstrate enhanced photocatalytic activity under visible light in comparison with pure TNTAs and single metal loaded TNTAs.Bimetallic Au-Pt quantum dots were well dispersed on the surface of TNTAs with diameter of approximately 5 nm,when the sample was prepared by impregnating into mixed solution of chloroauric acid:chloroplatinic acid at a ratio of 2 mM:2 mM.This sample also possesses the highest photocatalytic efficiency for degradation of rhodamine B under visible light irradiation among all the prepared samples.The enhanced photocatalytic efficiency is attributed to the improved visible light absorption and charge separation efficiency.Schottky barrier may be formed at the interface between metals and TNTAs,and a route of charge transference is also proposed.2.In-situ immobilization of g-C₃N₄ quantum dot on TNTAs and visible light photo-degradation of Rh BAll-solid-state Z-scheme g-C₃N₄ quantum dots loaded TNTAs?g-C₃N₄/TNTAs?has been successfully prepared by a simply straightforward way by dissolving urea,the precursor of g-C₃N₄,into the electrolyte during the anodization.The g-C₃N₄quantum dots around 10 nm were homogeneously conglutinated on the walls of TNTAs nanotubes.The synthesized samples show good visible light absorption and prominent visible-light photocatalytic performance with good stability.The reaction intermediate was investigated by trapping experiments,which give evidences that an all-solid-state Z-scheme system is formed instead of a conventional heterojunction photocatalyst.The all-solid-state Z-scheme can not only successfully facilitate photogenerated electron-hole separation,but also maintain the redox ability of photogenerated electrons and holes.3.In-situ constructing g-C₃N₄ nanosheets/TNTAs and its visible-light photocatalytic propertiesThe g-C₃N₄ nanosheets were successfully prepared by chemical exfoliation of bulk g-C₃N₄,and the exfoliated g-C₃N₄ nanosheets were further ultrasonically smashed to facilitate their loading on the tube.Then,all-solid-state Z-scheme photocatalysis of g-C₃N₄ NSs loaded on TNTAs?g-C₃N₄ NSs/TNTAs?was prepared by simply dispersing pre-treated g-C₃N₄ NSs into the electrolyte during the anodization process of TNTAs.The formed samples display good visible light response and Rh B could be efficiently photo-degraded under visible light reached to76%within 300min with good stability.Moreover,colorless antibiotic agent tetracycline hydrochloride can be also photo-degrated without any by-products under visible irradiation.Z-scheme photocatalytic system is suggested to be formed and the improved photocatalytic activity was ascribed to the increased light absorption,spatial charge separation,and charge lifetime.