First-principles Study Of The Surface Of Metal And Non-metal Co-doped Titania

As a typical wide bandgap semiconductor,anatase TiO2 can only show response to the UV light,whereas it cannot show photocatalytic activity under visible light region,thus resulting in huge waste of solar energy resources.Besides,the photo-excited electron-hole pairs tend to recombine easily,which leads to the low photo-quantum efficiency.To address these issues,modification by doping,as an effective method,has been widely investigated.Most catalytic reactions just happen on the surfaces of catalytic materials.Therefore,doping on surface has become an advanced research focus in the photocatalysis field,thus improving the surface electronic structures effectively and obtaining the unique TiO2 surface matching with the visible light.To explore the doping modification mechanism of anatase TiO2,thus further improving the photocatalytic performances of anatase TiO2,we systematically study the electronic structures and optical properties for Fe/C doped TiO2(101)surface,using density functional calculations.Besides,we also investigate the effect of different doping methods and positions on the performances of TiO2(101)surface.The conventional density functional theory is an effective research tool,but the LDA and GGA always underestimate the bandgap.The DFT+U method might overcome this well-known shortcoming,but they need a special parameter U,whose value is uncertain,because this parameter relates to the chemical environment.Therefore,we adopt the Heyd-Scuseria-Emzerhof(HSE06)hybrid functional with the revised 6-31G basis set to calculate the electronic structures,especially the bandgap,using Gaussian-type orbitals(GTO)implemented in the Gaussian 09 programs,without introducing any empirical parameters.We find that C mono-doping leads to some structural distortions on the TiO2(101)surface.The structural distortion induced by C interstitial doping is much greatest than that of C substitutional doping.The doping surface displays higher stability under reduction conditions.Furthermore,the doping surface shows the highest stability when C substitution for the bridging oxygen O2c.The C substitution doping introduces the deep impurity states appearing in the middle of band gap,which are easy to become the recombination centers of photo-excited electron-hole pairs.The C interstitial doping introduces the shallow impurity states and shows response to visible light,thus improving the photocatalytic activity of TiO2(101)surface under visible light region.Fe mono-doping TiO2(101)surface displays higher stability under oxidation conditions.The doping surface of Fe substitution for Ti5c is the easiest to be synthesized.For Fe substitution doping surface,the bandgap nearly doesn't change.The impurity states locate in the middle of the band gap.They belong to the deep impurity energy levels,so Fe substitution doping couldn't improve the photocatalytic activity of TiO2(101)surface.After Fe interstitial doping,the impurity states appear at the bottom of the conduction bands and at the top of the valence bands simultaneously,and couple with them,thus narrowing the bandgap and resulting in much bigger red-shift of absorption edge.This not only enhances the visible light absorption,but also promotes the separation of photo-excited electron-hole pairs.Therefore,Fe interstitial doping can enhance the photocatalytic activity of TiO2(101)surface under visible light region.The Fe/C co-doping surface displays higher stability under oxidation conditions.The Fe5cC2c possesses the lowest formation energy,suggesting this doping surface is the easiest to be synthesized.Due to the synergistic effects,the impurity states appear at the bottom of the conduction bands and at the top of the valence bands simultaneously,leading to the optimal electronic structures compared to the mono-doping systems.In the co-doping surfaces Fe5cC2c and Fe6cC2c,the impurity states couple with the conduction bands and valence bands,respectively.This not only narrows the bandgap,resulting in obvious red-shift of absorption edge,but also effectively inhibits the recombination of photo-excited electron-hole pairs,thus greatly enhancing the photocatalytic activity of TiO2(101)surface under visible light region.