The Stability And Photoelectric Nature Of Anatase TiO₂（001）: Effect Of Surface Reconstructions

At present,the world is facing the problems of energy shortage and environmental pollution.It isa promising way that solar energy can be converted into chemical energy by using inexpensive and readily available semiconductor materials.Titanium dioxide（Ti O₂）is effective in degrading pollutants and catalyzing hydrogen production from water because of its cheap and easy availability,non-toxic and stable,efficient reaction and appropriate band edge positions.The studies had inferred that anatase Ti O₂（001）surface（A-Ti O₂（001））are the most active Ti O₂ surfaces,theoretically.However,experimental studies recently have shown that the A-Ti O₂（001）is highly inert.To make any further progress,it is important to clarify surface structures and to get deeper insight of the relationship between surface structure and activity.The ab initio evolutionary algorithm were used to perform a search for the surface structure of A-Ti O₂（001）.Meanwhile,the thermodynamic stability,photoelectric nature and surface reactivity of the A-Ti O₂（001）have been systematically investigated by the first-principles calculation.It provides theoretical guidance for experimental design and synthesis of efficient photocatalysts.The main research contents are as follows:1.The defective and reconstructed structures of A-Ti O₂（001）with the existence potential areinvestigated by theoretical calculations,and their thermodynamic stability is also systematically analyzed.The structures of A-Ti O₂（001）with different periods and different Ti-O ratios are searched,and thus the several low-energy stable structures are also obtained.The results of calculated surface energy reveal the two new and stable reconstructions have lower surface energy among the obtained structures.And the regions of their stability under realistic conditions were simulated.The analysis of surface atomic configuration,defect formation energy and surface stress reveal that the predicted structures have lower formation energy and also release significant surface stress compared to clean surface.Therefore,the predicted structures may be also obtained experimentally.Meanwhile,it is found that the simulated STM images of“Ti_int-（1×4）”and“Ti_int-（1×3）”are the same as the experimental STM image.The results provide a possible atomic configurations for experimental structure.In the work,it not only enriches the structure of A-Ti O₂（001）,but also provides theoretical guidance for the experimental synthesis of prepared of A-Ti O₂（001）.2.The effect of surface reconstruction on the electronic structure,optical properties and surface activity of A-Ti O₂（001）are investigated by theoretical calculations.In terms of electrical properties,it is found that the reconstrcution cause the surface to induce new electronic states within the bandgap and shift the Fermi level from near the valence band maximum to near the conduction band minimum,and reduce the work function,indicating that the surface reconstruction may enhances the electronic conductivity,light absorption and the ability of electrons to be injected into surface.Based on the calculated band edge positions of surface,it is revealed that oxidation ability of oxygen-rich structures and reduction ability of oxygen-poor structures are enhanced compared to the clean surface,which will expand the application of Ti O₂ surfaces in catalysis.The theoretical analysis of the effective mass of electrons,electron concentration and static dielectric constant show that the partial oxygen-poor structures have the high conductivity and their electrons are hardly affected by charged defects or impurities,implying that their structures have the potential to become an efficient visible photocatalyst.Meanwhile,the optical properties of surface are also analyzed,the calculated real part of dielectric function shows that reconstructions have stronger carrier transition ability in the visible region than the clean surface,and the imaginary part of dielectric function and the absorption spectra show that the reconstructions can achieve visible light absorption,and the partial oxygen-poor reconstructions can absorb sunlight over nearly all the visible region.To further evaluate relative chemical reactivity of the reconstructions,we also study adsorption and dissociation adsorption of H₂O and O₂molecules on the partial oxygen-poor reconstructions and clean surface for comparison.It is found that the dissociation barriers for the dissociation of H₂O on the partial oxygen-poor reconstructions is lower,and O₂undergoes spontaneous dissociation on the predicted Ti_int-deficient surface and froms the stable"add-molecule model".In the work,the photoelectric nature of the reconstructions is investigated,and it is shown that the partial oxygen-poor reconstructions have the potential to become excellent photocatalysts,which provides new ideas for experimental design and synthesis of photocatalysts.