Research Of Photocatalytic Performance Of Titanium Dioxide And Electronic Properties Of Silicene

In this thesis, by using the first-principles method within the framework of the densityfunctional theory, we have concentrated on studying some TiO2and silicene based advancedmaterials with the purpose to tailor their electronic properties toward the potentialapplications in the high-performance photo-electronics. The electronic structure and opticalproperties of transition metal and B codoped rutile TiO2, the adsorption and decompositionbehavior of water molecule on the anatase TiO2(100) surface without and with transitionmetal modification, and the band structure engineering of silicene sheet have been carefullyexamined in our studies.First, we have carried out a detailed density functional theory study on improving theoptical properties in visible light region of rutile TiO2by codoping Mn+B, Tc+B, or Re+B. Forthe codoped rutile TiO2, we have calculated the light absorption spectrum and found theobvious red shifts of the optical absorption edges, which in turn improve the light absorptionproperties in the visible light range. The band structure of the Mn+B codoped rutile TiO2changes from the direct band structure to the indirect band structure, while the Tc+B and Re+Bcodoped materials still hold the direct band structure characters. In the calculated lightabsorption spectrum, the first peaks are respectively red shifted to1.30,0.65, and0.95eV forthe Mn+B, Tc+B, and Re+B codoped TiO2, which can be associated with the electron excitationfrom the highest fully occupied impurity states to the lowest empty d orbitals of transitionmetals.Secondly, we have studied the interaction between water molecule with anatase TiO2(100)surface and effects on this interaction by doping transition metal. The results show that theZr-doped TiO2(100) surface gains the best performance, which have the lowest activationenergy barrier of about0.28eV. Also, the reaction is exothermic, which may help the thewater decomposition.Finally, by applying both the first-principles method and the tight-binding method, theelectronic properties of silicene superlattices are studied in detail. These studies provide a newmethodology for band structure engineering of the silicene-based sheet materials and couldbenefit the further studies toward their applications in the high-performance nanoelectrics, such as the usage in the field of solar energy conversion. Both the inversion symmetry breaking andthe degenerate perturbation could open bandgaps of silicene superlattices. The width of thebandgap could also be further tuned. Referring to the primitive unit cell of silicene sheet crystal,for the pseudo silicene superlatitces, the twofold Dirac points K and K′could be folded to the Γpoint resulting in the fourfold degeneracy when the conditions of both M and N are integermultiple of3of the hexagonal supercell (M,N) and the Q=3q (q is integer) of the orthogonalsupercell [P,Q] are satisfied. Then, the bandgap could be opened by introducing degenerateperturbation no matter whether the inversion symmetry remains at the same time. For the othercases, the Dirac points still remain two-fold degenerate at symmetric K and K′points inBrillouin zone, for which the perturbation preserving inversion symmetry does not affect thegapless nature. However, the gap could be opened by breaking the inversion symmetry throughdestroying the equivalence between the sublattices of silicene.