Electronic Structures And Optical Properties Studies Of Doped TiO₂from Frist Principles Calculations

Anatase TiO2has several excellent properties, such as nontoxic, relativelyinexpensive, physical and chemical stability, and high photocatalytic activity.Therefore, TiO2is considered as a kind of ideal semiconductor photocatalyst, and iswidely applied in environmental protection and renewable energy. Unfortunately,anatase TiO2has a wide band gap (about3.2eV), which only responds to UV lightirradiation accounting for only a small fraction of solar light, while visible lightoccupying most fraction of solar light can’t be utilized. In order to use solar energyeffectively, we studied the visible light response of several kinds of ions doped TiO2based on the density functional theory.First, we studied the geometry, electronic structures, and optical properties of N-Bcodoped TiO2. Three codoped structures were chosen, including N s B i, N s Bs, andN i Bi(s means substitution and i means interstitial). Geometry optimization wasperformed by GGA method. Further, we calculated the electronic structures andoptical properties of N-B codoped TiO2using GGA+U method. It was shown that theband gap states were observed in the three codoped structures, however, the visibleoptical absorption only appeared in N s B i structure. In addition, the N s Bistructure wassensitive to the distance of dopants, and a decrease of the distance between N and Batoms induces a decrease of the absorption. The visible optical transition in N s Bistructure was attributed to the N2p-Ti3d transition.Second, we investigated the electronic structures and optical properties of C-doped,N-doped, and C-N codoped TiO2. Band structure of pure TiO2was calculated usingGGA and GGA+U methods, and the results showed that the band gap of pure TiO2could be corrected effectively. It was shown that the band gaps of C-doped, N-doped,and C-N codoped TiO2were reduced compared with pure TiO2. Optical propertiesresults showed that the band edges of the three doped systems shifted to the longwaveregion, and the optical absorptions were all observed in450-800nm. Moreover, theabsorption intensity of C-N codoped TiO2was larger than that of C-doped andN-doped TiO2, which mean that C and N have a synergy in the C-N codoped TiO2.According to the calculated results, we concluded that the visible optical transition inthe C-N codoped TiO2was attributed to the transition of N and C2p-Ti3d states. Finally, we investigated the electronic structures and optical properties of Cu, Ag,and Au-doped TiO2. Cu doping could produce some electronic states near the top ofvalence band of TiO2, and Ag and Au doping also produced band gap states. Theincorporation of the three dopants could reduce the band gap of TiO2, and also,produced visible optical absorption. The visible absorption intensities of Cu andAg-doped TiO2were larger than that of Au-doped TiO2. We gave the visible opticaltransition mechanisms of three doped systems. For the Cu-doped TiO2, the visibleoptical transition corresponded to the Cu3d-Ti3d states transition. The Ag dopingcaused the visible optical transition between middle Ag4d and Ti3d states, while theAu doping caused the visible optical transition between O2p and middle Au5d states.