| Photocatalysis has been studied and developed widely to solve the issues of environmental pollution and energy shortage.However,it is still unpractical to realize the large-scale applications based on the existing photocatalytic materials because of the low photocatalytic efficiency.Therefore,extensive researches have been conducted,aiming at improving the catalytic efficiency of photocatalysts.An efficient photocatalytic reaction is facilitated synergistically by the wide spectral absorption,fast transfer of carriers and sufficient redox capacity.The light absorption of some photocatalysts have been extended into visible,or even near-infrared,regions by appropriately modifying traditional photocatalytic materials as well as actively exploring new materials with excellent characteristics.Therefore,current researches turn to focus more on the effects of transfer and separation of carriers on the photocatalytic activities.During the process of electrons and holes from producing to participating in the surface reactions,the internal or surface recombination will consume lots of effective carriers,resulting in only a few electrons or holes being involved in the surface photocatalytic reactions ultimately,then the overall photocatalytic efficiency being severely limited.So,for the improvement of the overall quantum efficiency,it is particularly important to reduce the loss of photo-induced electrons and holes by lowering the corresponding internal and surface recombination probability.The electric field that can drive the faster transfer of electrons and holes would be induced by constructing hetero-junction systems,depositing metal particles and controlling synthesis of photocatalysts with specific surfaces.Nevertheless,the localization of the electric field in surface or interface region limits the promotion only at the surface or interface of materials,and has little effect on the internal transfer of carriers.Considering that only the electrons or holes,surviving from the internal recombination,have opportunities to reach the surface and further participate in the reactions,decreasing the recombination of photo-induced electrons and holes is a prerequisite for realizing an efficient photocatalytic reaction.In view of the limited effect of local electric field on the transfer of carriers,applying polar materials into the field of photocatalysis produces obvious advantages in achieving high quantum efficiency.In polar material,there is a built-in electric field throughout the whole material,which can effectively promote the transfer of electrons and holes in the material,thus shortening the time required for the transfer of carriers from interior to surface and contributing to the high separation efficiency of carriers.Additionally,a shortened transport distance for carriers can be obtained directly by reducing the dimensions of materials to two or even one dimension,which also provides a new effective way to promote the separation of electrons and holes.In this dissertation,we systematically study the transfer related characteristics and advantages of carriers in K3B6O10X(X=Br,Cl),Ag6Si2O7 and 2D Bi2WO6 systems based on the first-principles calculations and summarize the effects of built-in electric field,distribution as well as intrinsic transfer properties of carriers on the recombination of electrons and holes.And on this basis,the active surfaces in K3B6OioBr and Ag6Si2O7 are predicted,and the origin of the superior photocatalytic activities in monolayer and double-layer Bi2WO6 nanosheets are disclosed.This dissertation contains six chapters.In the first chapter,we introduce the basic principles of semiconductor photocatalysis,the recent progress of photocatalytic materials and the main contents of this dissertation.In the second chapter,we briefly introduce the theoretical fundamentals and the related software package of first-principles calculations.In the third chapter,we discuss the effect of the synergism between built-in electric field and intrinsic transfer characteristics of carriers on the transfer of electrons and holes in nonlinear optical materials,K3B6O10X(X=Br,Cl).In the fourth chapter,we study the contribution of the effective separation of carriers to the excellent photocatalytic performance in Ag6Si2O7 induced by the built-in electric field and multiple Ag-O units.In the fifth chapter,we uncover the possible origin of the excellent photocatalytic activities in 2D Bi2WO6 systems by comparatively investigating the corresponding geometrical structures and electronic properties of monolayer,double-layer and bulk Bi2WO6.In the sixth chapter,we summarize the main results of this dissertation,and prospect the further developments of photocatalysts.The main contents and conclusions are listed as follows:(1)We investigate the synergistic effect between built-in electric field and effective mass of carriers on the transfer of electrons and holes in K3B6O10X(X=Br,Cl),in order to reveal the origin of the outstanding photocataytic performances observed in K3B6O10Br and search a new idea for the improvement of photocatalytic efficiency in K3B6O10Br.Based on our systematic studies,isotropic effective mass with small values is obtained for electrons,while the effective mass of holes shows anisotropy with the smallest effective mass along[001]direction,in the same direction as built-in electric field.Thus the electric field within this system can promote the transfer of photo-induced carriers along[001]direction synergistically with their smallest effective mass,making the K3B6O10Br nanosheets exposing {001}facets with the highest photocatalytic activity.The same synergistic effect also exists in K3B6O10Cl system,contributing to the effective separation of carriers and the enhanced activity of {001} surface.These results may provide some theoretical guidance for the design and synthesis of nonlinear optical materials with higher photocatalytic activity in experiment.(2)As a new type of Ag based photocatalytic material,Ag6Si2O7 exhibits better photocatalytic activity compared with the common photocatalysts Ag2O and Ag3PO4.To get a profound understanding about the advantages of Ag6Si2O7 in photocatalytic applications,we study the geometrical structures and electronic properties of Ag2O,Ag3PO4 and Ag6Si2O7 comparatively using first-principles calculations.In contrast to the cases in Ag2O and Ag3PO4,the existence of the multiple Ag-O units and built-in electric field in Ag6Si2O7 lead to the separated distribution and transfer of electrons and holes,reducing their recombination rate and contributing to the higher photocatalytic activity.In addition,the accumulated holes at the most stable(100)surface are also favorable to the surface photo-oxidation reaction.Our results provide a deep insight for the effect and related mechanism of inner polarization and multiple Ag-O units on the photocatalytic activity of Ag6Si2O7,and indicate that compared with Ag2O and Ag3PO4,the better transfer and separation ability of electrons and holes in Ag6Si2O7 account for its superior photocatalytic activity.We expect these results can give some help for the design and development of other high-efficient photocatalytic materials.(3)Reducing the dimension of layered Bi-based photocatalysts to atomic thickness provides a promising way to improve the photocatalytic activities.We investigate the structural and electronic properties of monolayer,double-layer and bulk Bi2WO6 systems comparatively to disclose the corresponding origin of different photocatalytic performances.Our results show that compared with bulk Bi2WO6,the monolayer and double-layer systems induce different structure reconstructions,which results in small band gap and obvious downward shift of VBM in monolayer Bi2WO6,but upward shift of CBM in double-layer Bi2WO6.Thus the absorption can be still maintained in visible-light region in monolayer Bi2WO6 system.And the combination of monolayer and double-layer structures is expected to be promising for both the oxidation and reducing reactions.Meanwhile,in both monolayer and double-layer systems,the efficient separation of carriers can be anticipated based on the surface distribution of holes and the fast transfer of electrons in the middle layers.Consequently,compared with bulk counterpart,the effective transfer of carriers and enhanced redox capacity contribute to the superior photocatalytic activities in monolayer and double-layer Bi2WO6 systems.Our results not only provide a deep insight for understanding the better photocatalytic activity of 2D Bi2WO6,but also conduce to further improve the photocatalytic activity of Bi2WO6,as well as explore other non-van der Waals layered photocatalysts with atomic thickness. |
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