Insight Into The Transfer Mechanisms Of Photogenerated Carriers For Heterojunction Photocatalysts

Developing heterojunction photocatalysts with high activity via composition of two or more semiconductors is a main research direction to construct novel photocatalysts.Two kinds of heterojunction photocatalysts were chosen as research objects(one is Ti O2(A)and Zn O(B),and the CB and VB are close to each other;the other is Ti O2(A)and WO3(B),Ti O2 has more negative CB and WO3 has more more positive VB).The project will reveal the transfer mechanisms of the photogenerated carriers in the heterojunction structure and expound the electroconductibility of heterojunction photocatalysts with different components by studying the influences of the contents of A and B on the photocatalytic reduction/oxidation performance of A/B and B/A heterojunction photocatalysts,respectively.The mechanisms and the natural laws for the effect of electroconductibility on the transfer of the photogenerated carriers and the photocatalytic performance will be revealed by means of systematic investigations of the electroconductibility of the heterojunction photocatalysts with different components,and subsequent theoretical calculations and the correlation analysis..A series of Zn O/Ti O2 composites with different main components(Ti O2 or Zn O)were synthesized by electrostatic self-assembly.When Ti O2 is the main part of Zn O/Ti O2 heterojunction photocatalysts(Zn O/Ti O2),its photocatalytic activity decreases rapidly with the increase of the amount of Zn O.Through a series of tests found that the conductivity of the sample has changed,so the reason for that may be due to the relative p-n junction(p-Zn O/n-Ti O2)generated between Zn O and Ti O2.The direction of electron and hole migration in the p-n junction is opposite to the direction of migration of photoelectrons and holes in VB and CB.It suppressed the seperation of photo-generated electrons and holes.However,when the main part of the heterojunction photocatalyst is Zn O(Ti O2/Zn O),the photocatalytic activity of the sample increases with the amount of Ti O2 up to 5%(95% Zn O/Ti O2).The reason may be that the migration direction of electrons and holes in p-n(p-Ti O2/n-Zn O)is thesame as the migration direction of photo-excited electrons and holes in VB and CB between two semiconductors.It accelerates the separation of photogenerated electrons and holes.As a result,the photocatalytic performance of the heterojunction photocatalyst increases.A series of WO3/Ti O2 composites with different principal components(Ti O2 or WO3)were synthesized by in situ-calcination.Coupling a photocatalytst with other photocatalyst is one of the most widely used strategies to realize effective transfer of the photogenerated carriers.In the paper,a series of WO3/Ti O2 composites with different weight ratios were prepared.And the WO3/Ti O2 composites were characterized in detail.The result showed that no matter what the primary part of WO3/Ti O2 composites is Ti O2 or WO3,the photocatalytic activities of WO3/Ti O2 are much higher than that of pure Ti O2 or WO3.The reason may be attributed to the formation of a relatively p-n junction between WO3 and Ti O2(p-WO3/n-Ti O2).The transfer of the photogenerated charge carriers adopts a Z-scheme system in the WO3/Ti O2 heterojunctions.Under the effect of the internal electric field,because the migration directions of electrons and holes in the relative p-n junction are opposite to the transfer directions of the photogenerated electrons and holes in the CB and VB of WO3 and Ti O2,the photogenerated electrons in the conduction band(CB)of n-Ti O2 could not migrate to conduction band of p-WO3 and the photogenerated holes in the valence band(VB)of p-WO3 could not move to valence band of n-Ti O2.Thus,the fast combination is achieved between the photoexcited electrons in the CB of WO3 and photoexcited holes in the VB of Ti O2.The accumulated photogenerated electrons in the CB of Ti O2 with more negative potential can reduce O2 to ·O2-.And the photogenerated holes in the VB of WO3 with more positive potential may oxidate H2O(OH-)into ·OH radicals.The photocatalytic activities of the WO3/Ti O2 heterojunctions are significantly promoted.This work not only reveals the transfer mechanisms of photogenerated carriers and internal natural behavior of heterojunction photocatalysts,but also guides the design and structure of composite photocatalysts,and thus has theoretical and practical significance.