First-principles Investigation Of C/Al-doping And C+Sl-codoping TiO₂（101） Surface

Titanium dioxide（TiO₂）has attracted considerable attention as a fascinating photocatalytic material because of its low cost,nontoxicity and long-term stability to corrosion.Unfortunately,TiO₂ has a serious drawback in efficiency: it can absorb only the ultraviolet light,which amounts 4% of solar energy,due to its large intrinsic band gap（3.2 eV for anatase）.Therefore,many efforts have been made to enhance the utilization of TiO₂ in the visible spectra region,among which the doping was extensively studied and demonstrated to be an effective way.And from the photocatalytic mechanism of TiO₂,we know that the TiO₂ photocatalytic reaction mainly occurs on the surface particles,namely the surface properties of TiO₂ have greater influence on the photocatalytic performance than bulk TiO₂.Hence,in the present work,the thermodynamically stable low-index surface of anatase TiO₂（101）was adopted,and the effects of C/Al-monodoping and C+Al-codoping on the structural,electronic and optical properties of anatase TiO₂（101）surface have been systematically investigated on the basis of density functional theory.For the system of C-TiO₂（101）,the investigation of three possible carbon doping species in TiO₂（101）surface,substitutional C at O（CO）or Ti site（CTi）and interstitial C doping（Cin）,was carried out.For C-doped TiO₂ with CTi or Cin,the crystal structure distorts strongly,and a planar CO3 unit is observed,leading to the decrease of band gap,which thereby causes the red shift of absorption edge.For C-doped TiO₂ with CO,the crystal structure does not exhibit obvious distortion.The intrinsic band gap does not change.But some new impurity states are introduced in the band gap,which leads to the red shift of absorption edge.In the three different configurations,the amount of redshift of the case of CO is the largest and the absorption capacity of visible light is the most excellent.For Al-TiO₂（101）,the crystal structure has no significant distortion.The results of electronic and optical property indicate that the incorporation of Al leads to the red shift of absorption edge,which results from the formation of impurity states rather than the band gap narrowing.And the system of Al-doped surface is more conducive to enhancing the absorption efficiency of visible light region of TiO₂ than doped bulk TiO₂.For C+Al-codoped TiO₂（101）surface,the crystal structure exhibits small distortion.The C and Al codoping does not change the band gap,but introduces three impurity levels formed by the hybridization between O 2p,C 2p and Ti 3d orbital,are responsible for the red shift of absorption edge.From the results of optical properties,we can find that the incorporation of nonmetal element C into TiO₂（101）surface can be more effective than Al-doped and C+Al-codoped TiO₂（101）surface to extend the range of spectral response and improve the photocatalytic performance of TiO₂ in the visible spectra region.