The Direction Of Photogenerated Charge Carriers Transfer In M_xO_y-TiO₂

In order to know the mechanism of M_xO_y/TiO₂ composite photocatalyst in photocatalysis field,pure TiO₂ powder were prepared with hydrothermal method,which then tranformed into TiO₂ film via coating method.M_xO_y/TiO₂ were prepared by combing TiO₂ film with another metal oxide film by sol-gel method.A series of charactertzation tools including UV-vis,XRD,XPS were used to characterize these photocatalysts.With the irradiation of UV-light,photocatalytic deposition of silver was employed as a probe reaction to obtain the photocatalytic mechanism of M_xO_y/TiO₂ composite photocatalyst.In this process,transfer direction of election was confirmed according to the scanning electron microscope elements distribution diagram.Semiconductor physics could be applied to explain the tranfer direction of election in our study.The results were showed as follows:?1?UV-vis diffuse reflectance spectra shows that the main crystal form of prepared-TiO₂film is appeared as anatase.Combination of TiO₂ with a lot of metal oxides including Fe₂O₃,CuO,ZnO,Cr₂O₃ and In₂O₃ makes absorption boundary wavelength of TiO₂ occur red-shift and increase visible light absorption of TiO₂.Among these metal oxides,Fe₂O₃/TiO₂,CuO/TiO₂ and Cr₂O₃/TiO₂ composite show obvious absorption peak in visible region while ZnO/TiO₂ composite shows obvious absorption peak in UV region.However,In₂O₃/TiO₂composite dosen't show obvious absorption peak either in visible region or UV region.These phenomenon mentioned above results form two reasons:one is relative to absorption of metal oxides,the other is relative to depressed band-gap energy of TiO₂ in visible-light absorption.The latter reason attributes to the separation of semiconductor electron and hole facilitate by new transition band via combination of semiconductors.?2?XRD analysis results manifest that diffraction peak of TiO₂ film prepared by hydrothermal and coating method is obvious,owing to good grain size and crystal shape of anatase TiO₂.What's more,metal oxide display different grain size and crystal shape result from the compatibility between method using to prepare metal oxide film and metal oxide,which shows as follows:the diffraction peaks CuO and ZnO are obvious,both oxides have good grain size and crystal shape,however,the diffraction peaks of other metal oxides aren't obvious perhaps because of their constituent particles have small crystal shape,bad grain size,homogeneous dispersion and amorphous structure.To sum up,using sol-gel method,some metal oxides show bad grain size.?3?XPS analysis results indicate that the binding energy of O 1s and Ti 2p in pure TiO₂ is458.3eV and 530.1eV,respectively.When combining with different metal oxides,all of the bindng energy shift to forward direction,which probably results from the reduction of outer electron density and weaker shielding effect due to the formation of Ti-O-M band in composite photocatalysts.What's more,three peaks of O 1s are fiited in the whole composite photocatalysts,which belong to Ti-O,M-O and-OH,show the sample we prepared is M_xO_y/TiO₂ composite photocatalysts.?4?Scanning electron microscope elements distribution diagram illustrate that the deposition site of silver locates on TiO₂ film in Fe₂O₃/TiO₂ and CuO/TiO₂ composites while it locates on M_xO_y film in ZnO/TiO₂,Cr₂O₃/TiO₂ and In₂O₃/TiO₂ composites.In brief,silver deposits on the film with higher Fermi level.Taking Fe₂O₃/TiO₂ as an example,first of all,a n-n heterojunction is formed between Fe₂O₃ and TiO₂.Then a built-in electron field with the direction from TiO₂ to Fe₂O₃ is formed by the transformation of electron from TiO₂ with higher Fermi level to Fe₂O₃ with lower Fermi level while hole transfers opposite.Finally,with the irradiation of light,photogenic electron-hole pairs generates in semiconductors and then transfer from Fe₂O₃ to TiO₂.Therefore,with the irradiation of light,photogenic electrons transfer to TiO₂ with higher Fermi level.