First-principles Study On Phase Transition Of Polymorphous TiO₂and Electronic Structure Of The Doped-TiO₂Photocatalyst

In this paper, the phase transition of the polymorphous TiO2and the electronic structure of the doped-TiO2photocatalyst were studied by using first-principles calculations based on the density functional theory (DFT) and projector augmented wave (PAW) pseudo-potentials (PP) method. All calculations were carried out using the Vienna Ab initio Simulation Package (VASP).By calculating the equilibrium total-energy of the seven polymorphous phases with rutile (RT), anatase (AT), brookite (BT), Columbite (CL), Baddeleyite (BD), Cotunnite (CT), Flourite (FL) structure at ground state, the lattice stability sequence of the polymorphous TiO2were obtained, i.e., AT>BT>CL>RT>BD>CT>FL. The calculated structural parameters and internal parameters obtained by optimizing method are in good agreement with experimental and other calculated values within1%. The bulk moduli obtained by fitting the plot of total-energy vs. volume (E-V) using third-order Birch-Murnaghan equation of state. The bulk moduli are172GPa、191GPa、209GPa、208GPa、163GPa、188GPa、263GPa for the AT, BT,CL, RT, BD, CT and FL, respectively. The transition pressure on ground state with the transition AT to RT is4GPa, while with the transition from BT to RT is3GPa, CL to RT is2GPa, CL to BD is6GPa, BD to CT is40GPa, CT to FL is5GPa.By calculating the band structure and DOS, we obtained the electronic structure of the polymorphic TiO2. The result show that RT and BT are direct-band-gap semiconductors with band-gap value1.66eV and2.08eV, while AT is an indirect-band-gap semiconductor with a band-gap value1.93eV. The conduction band (CB) near the Fermi-level is composed by Ti-3d and the valence band (VB) is composed by O-2p, which also agrees with experiments’ result.In the transition metals M (where M=Co, Ni, Ag, Pt and Zn) doped system M1Ti15O32with rutile structure and nonmetallic elements A (whereA=C, N, S, P, B) Ti16O31A1in rutile-TiO2, the electronic structurecalculations suggest that except Zn, most doping elements can alter theband structure considerably, which will thereby affect the opticalabsorption and photocatalytic properties of TiO2under visible lightirradiation.The band-gap is mainly controlled by the interaction between the dorbital of the doping transition metals and the Ti-3d and/or O-2p, and thebest choice is Ni, while the interaction between the p orbital of the dopingnonmetallic element and O-2p is the mainly factor, and the best choice isC. Furthermore, the band structure changed from direct band gap toindirect band gap when doping Pt, B in rutile TiO2, which decreases thephotocatalytic properties. This character is the first report.The dilute magnetic semiconductor of the Co-doping TiO2is a hottopic in information materials. We calculated the occupying configurationand its magnetic ordering behaviors of the Co2Ti14O32system. Thecalculated results show that when the two Co atoms is aligning to closeeach other along the c axis. The total energies are the most negative bothfor the ferromagnetic state and antiferromagnetic state, meanwhile thetotal energy of the ferromagnetic state is more negative than theantiferromagnetic state. So the stable lattice configuration isaccompanied by strong ferromagnetic state, and Co-3d is the reason offerromagnetism under room temperature, which affords the opportunity toexplore the multifunction materials with photocatalystic properties anddilute magnetic semiconductive properties.