Effect of Compensated Codoping on the Photoelectrochemical Properties of Anatase TiO2 Photocatalyst

Published:July 06,2011

ARTICLE

https://www.sodocs.net/doc/c26644672.html,/JPCC

Effect of Compensated Codoping on the Photoelectrochemical Properties of Anatase TiO 2Photocatalyst

Xinguo Ma,?Ying Wu,?Yanhui Lu,?Jing Xu,?Yajun Wang,?and Yongfa Zhu*,?

?

Department of Chemistry,Tsinghua University,Beijing 100084,People's Republic of China ?

Department of Electronic Science and Technology,Huazhong University of Science and Technology,Wuhan 430074,People's Republic of China

1.INTRODUCTION

TiO 2has attracted extensive attention as a promising active semiconductor photocatalyst that degrades environmental pollutants 1,2or directly splits water 3under sunlight irradiation.However,as a wide band-gap semiconductor (e.g.,3.23eV for anatase),TiO 2can be only activated under ultraviolet-light irradiation,which accounts for a small proportion (less than 5%)of solar energy.It is desirable that the band gap of the semiconductor would be about 2.0eV for the e ?ective utilization of visible light.Therefore,great e ?orts were made to modify the band gap of TiO 2by various methods,4à12but with only marginal success.The most common one used for band-gap reduction is the incorporation of impurities,such as N,4à9C,10,11B,12and so on.However,their incorporations also lead to poor photoresponse because the partially occupied impurity bands can act as killers for photogenerated carriers.5In addition,the p-type doping remains di ?cult due to its fairly lower valence band (VB)edges measured with respect to a ?xed energy (e.g.,vacuum level),which is similar to that of ZnO,InN,etc.13,14

Recently,codoped TiO 2with nonmetals and transition metals (TMs)has become a rapidly growing ?eld of interest and has met expectations both theoretically 15à17and experimentally.18à21For example,the V/N-codoped TiO 2photocatalyst synthesized by Gu et al.18shows the enhancement of photocatalytic activity for the degradation of methylene blue under visible-light irradia-tion (λ>440nm)compared with those of V-monodoped and N-monodoped TiO 2.Another example is that the Ni/B-codoped TiO 2photocatalyst possesses superior photocatalytic activity to

that of the as-prepared monodoped TiO 2products.21These results indicate that codoped TiO 2with nonmetals and TMs may be a new promising second-generation photocatalyst because donor àacceptor codoping may suppress the recombination and yet maintain a reduced energy gap.15à19The concepts of compensated (CP)and noncompensated (NCP)were estab-lished as the powerful guiding principle in the future designing of photocatalysts and other functional materials.15,20For the for-mer,the electrons on the donor levels compensate the same amount of holes on the acceptor levels,so the systems still keep the semiconductor character.For the latter,the extra charge associated with the two dopants cannot be completely compensated by each other.Gai et al.15demonstrated a CP donor àacceptor codoping theory to explain the energy gap reduction and the suppression of recombination.On the con-trary,Zhu et al.20proposed an NCP theory as an explanation for the photoresponse mechanism because of the higher carrier density.Those theories remain to be argued.In previous codop-ing investigations,V/N-and Cr/C-codoped TiO 2show a narrowing of the band gap,but their impurity levels under the conduction band minimum (CBM)lower the reduction poten-tial of the CB edge,resulting in poor water splitting.15à17As discussed above,the present achievements are still far from the ideal goal.

Received:March 24,2011Revised:June 22,2011ABSTRACT:An e ?ective codoping approach is described to modify the photoelec-trochemical properties of anatase TiO 2by doping with nonmetal (N or C)and transition metal (Nb or Ta)impurities.Here,compensated and noncompensated codoped TiO 2systems are constructed with di ?erent proportions and dopant species,and then their dopant formation energies and electronic properties are calculated to study the stability and visible-light photoactivity by ?rst-principles density functional theory incorporating the LDA+U formalism,respectively.The calculated results demonstrate that the codoping with transition metals facilitates the enhancement of the concentration of p-type dopants (N and C)in the host lattice.Especially,both 1:2compensated Nb/C/Nb and Ta/C/Ta codopings not only reduce the energy gap to enhance the optical absorption and eliminate the local trapping to improve carrier mobility and conversion e ?ciency but also do not lower

the reduction potential of the conduction band edge.Our designated strategies of codoped anatase TiO 2simultaneously meet the criteria for water splitting.It should be pointed out that,to be successful,the proper proportion of transition metal and nonmetal impurities in the host lattice should be controlled so that reasonable photoelectrochemical properties can be

achieved.

To design a“visible-light photocatalyst”using the codoping method for water splitting,one needs to identify impurities that (i)are soluble in the host to reduce the energy gap and en-hance optical absorption,(ii)do not lower the conduction band minimum energy level,and(iii)are able to shed the photoexcited electrons transferring to the CBM of the host.Exhibiting high4d and5d atomic orbital energies,Nb and Ta are more likely to transfer their excess electrons to the CBM of the host than V having a much lower3d atomic orbital energy.22,23In addition,N and C have higher atomic p orbital energies than O to form acceptor impurity levels above the VB maximum(VBM),which reduces a phototransition energy.6,7,11Therefore,Nb and Ta are predicted to be the best choices as donors,and N and C are the best choices as acceptors.

Up to now,there is no corresponding systemic theoretical report about the comparison of photoelectrochemical properties of CP and NCP codoped TiO2.Thus,we speci?cally constructed CP and NCP codoping systems with di?erent dopant pairs. Here,Nb/C-and Ta/C-codoped(1:1)TiO2are chosen as NCP models.Nb/N-and Ta/N-codoped(1:1)and Nb/C/Nb-and Ta/C/Ta-codoped(1:2)TiO2are chosen as CP models.In this work,we presented the calculated results of the formation energies and electronic properties of both CP and NCP codop-ing systems with di?erent dopant pairs using?rst-principles density functional theory(DFT)incorporating the LDA+U formalism.Our strategies can overcome the di?culties of some previous schemes and may provide some guidance for improving the photoelectrochemical activity of anatase TiO2by the codop-ing approach.

2.METHODS

All of the spin-polarized DFT calculations were performed using a development version of the SIESTA code.24A double-ζbasis set with additional orbitals of polarization was employed. The TroullieràMartins scheme25was taken for the norm-con-serving pseudopotentials.A grid cuto?of150Ry and a MonkhorstàPack26k-points mesh of5?5?5were used for

geometry optimization and electronic properties calculations. The doped systems were constructed from a relaxed2?2?1 48-atom supercell of anatase TiO2,with the atomic concentra-tion of impurities comparable to experiments.4,5,7,19,20Standard local density approximation(LDA)and the generalized gradient approximation(GGA)functionals fail to give reasonably accu-rate results for their band characteristics and especially band-gap values,due to the self-interaction error inherent to such functionals.27à29The LDA+U approach aims to correct for this by adding an orbital-dependent term to the LDA potential. However,when the correction is only applied on transition metal d orbitals,the band-gap value is still underestimated compared to the experimental one,even at high U values.30à32 Recently,a few theoretical studies on transition-metal oxides discussed the e?ect of the U parameter on the p electrons(U p)of oxygen in addition to the d electrons(U d)of the transition metal.31à34

A U(Ti d)value of4.2eV was applied to the Ti3d orbitals in the present work.This value has been previously shown to provide a description of O vacancies at the TiO2(110)surface that is in good agreement with the available spectroscopic data.35à37If the U is only incorporated for the Ti3d states, the band-gap energies are increased with increasing U(Ti d) values from0to15eV,from a value of2.15eV within the conventional LDA to3.17eV within LDA+U d;however,the band-gap energies are still underestimated compared to the experimental value of3.23eV(see inset of Figure1).To determine U(O p),we used the?tting procedure of Lany and Zunger.38For the exact density functional,the energy change of the system is linear when part of the electrons are added or removed.39For addition of an electron to a hole state,this condition requires that

Een ht1TàEen hT?e ien hTe1T

where E(n h+1)àE(n h)is the electron addition energy E ad, that is,the di?erence in the energy between the system with a self-trapped hole present(the+1charge state)and the neutral system,where both are calculated at the optimized geometry for the hole system,and e i(n h)is the eigenvalue of the hole state relative to the VBM.Figure1shows the variation in E ad and e i(n h)for the+1charge state of stoichiometric anatase TiO2 with U(O p)(here,4.2eV is taken for U(Ti d)).It is obvious that eq1is satis?ed for U(O p)=4.02eV,giving the correct splitting between occupied and unoccupied O2p states.The result is smaller than U(O p)=5.25eV of ref

40calculated using the projector augmented-wave method.

Figure1.Electron addition energy,E ad,and the eigenvalue e i(n h)of the hole state of the+1charge state of anatase TiO2as a function of the U(O p)parameter calculated within the LDA+U d+U p approach.The solid arrow shows the value of U(O p)for which eq1is satis?ed.The U parameter corresponding to the experimental band gap is indicated by the dashed arrow.In addition,the band gap E g as a function of the U(Ti d)parameter within the LDA+

U d approach is shown in the inset.

Figure2.Calculated binding energies(in eV)of the X TiàY O com-plexes in TiO2with LDA(red)and LDA+U(green),where X=Nb,Ta and Y=N,C.

3.RESULTS AND DISCUSSION

3.1.Doped Configuration and Geometry Structure.The

lattice parameters of a relaxed cell for pure anatase TiO 2are found to be a =3.789?and c =9.550?within the spin-polarized LDA+U approach,in good agreement with experimental and other theoretical results.41,42The calculated Ti àO bond lengths are 1.954and 1.997?,respectively.The doped systems are constructed from a relaxed 2?2?148-atom supercell of anatase TiO 2.Other theoretical results have identified that the Ti and O sites are preferentially substituted by the TMs and nonmetal atoms,respectively.15à17,20To see if the defect pairs can form,we calculated the binding energy of defect pairs in the host lattice.The binding energy E b is defined as 15,43

E b ?E T eX Ti T

tE T eY O TàE T eX Ti tY O TàE T eTi O 2T

e2T

where E T is the total energy of the system calculated with the same supercell.A positive E b indicates that the defect pairs tend to bind to each other when both are present in the sample.The calculated bind energies E b of several defect pairs within the LDA and LDA+U approaches are positive,as shown in Figure 2,indicating that the defects pairs are stable with respect to the isolated impurities in the host lattice.The large binding energy can be understood to form the defect complex.15,43The donor levels donate one or two electrons to one of the acceptors,forming the neutral and charge defect pairs,which results in a large Coulomb interation between donor and acceptor impu-rities (e.g.,between Nb Ti 1+and C 02à

).As the electronegativity decreases from N to C,the Coulomb binding energy decreases.To further determine the stable codoped con ?gurations,we constructed several nearest-neighbor codoping systems and then calculated their total energies.Two O sites and three Ti sites are labeled with numbers 1à5,as shown in Figure 3.For Nb/C-,Nb/N-,and Ta/N-codoped TiO 2,the (1,3)con ?gurations have lower total energies than the (2,3)con ?gurations (the di ?er-ences are 0.089,0.043,and 0.049eV,respectively).On contrary,for Ta/C-codoped TiO 2,the total energy of the (2,3)con ?g-uration is 0.015eV lower than that of the (1,3)con ?guration.In 1:2codoped case,the (5,1,3)con ?guration is 0.063eV energetically favorable than the (4,1,3)con ?guration for Nb/C/Nb-codoped TiO 2,whereas the (4,1,3)con ?guration is favorable by 0.008eV than the (5,1,3)con ?guration for Ta/C/Ta-codoped TiO 2.In succedent work,we only focus on the con ?gurations of the lowest energy for 1:1and 1:2codoped systems.In every case,the geometrical optimization is always

made and the convergence is assured when the forces on atoms are less than 0.05eV/?,based upon which the electronic structures are calculated.

In the Nb Ti con ?guration,the internuclear distances to the coordinating oxygens are r Nb àO =1.982?(+1.4%)on the xy plane and r Nb àO =2.010?(+0.6%)along the z axis.The numbers in parentheses show the changes relative to the lattice parameters of the pure TiO 2calculated by us.The extension of the chemical bonds can been rationalized in light of the ionic radius of the two,that is,r =0.61?for Ti 4+and r =0.64?for Nb 5+.44Experimental X-ray absorption spectroscopy reported previously suggests that the Nb atoms have the charge state of Nb 5+and are located in distorted octahedral sites.45However,the position of the Nb Ti did not change remarkably from the position of the replaced Ti atom.46In the Ta Ti case,the e ?ect of Ta doping on the geometry is nearly identical to that of Nb doping,due to the large space in the TiO 6octahedra.In the N O con ?guration,the Ti àN bond lengths,1.956and 2.041?,are slightly longer than the original Ti àO ones,1.954and 1.997?,respectively.The structural changes for replacing an O with a C atom are more noticeable than ones for replacing an O with a N atom.

For the Nb/N-and Ta/N-codoped TiO 2,the lattice distortion is signi ?cantly moderated by the compensation between the opposite actions induced by codoping.The phenomenon is also reported in ref 47.The lattice distortion of Nb/C-codoped TiO 2is larger than that of Nb/N-codoped TiO 2due to NCP codoping with the à1charge state.In the Nb/C/Nb-codoped con ?gura-tion,the two Nb àC bonds have the same length (1.987?),which is slight longer than the original Ti àO bond.It is obvious that 1:2CP codoping systems su ?er less stress and strain.To further investigate the e ?ect of doping on the structure stability,in the next section,the dopant formation energies are calculated under O-rich and Ti-rich conditions.

3.2.Formation Energy.In thermodynamic equilibrium,the concentration c of point defects is given by the expression c =N site N config exp(àE f /kT ).48Here,E f is the defect formation energy,and a lower E f means a higher defect concentration c ,which directly determines the physical and chemical behavior of TiO 2.In other words,defects with lower formation energies are more likely to form.To explore the possibility and stability of doping and optimal growth conditions,the calculations of the dopant formation energies have been performed,according to the expression 14à17,49

E f ?E T edoped TàE T eundoped Tàn NM μNM àn M μM

tn O μO tn Ti μTi

e3T

where E T (doped)and E T (undoped)are the total energies of the codoping systems containing all the impurities,of the pure host supercell,respectively.μNM and μM are the chemical potentials of the nonmetal elements and metal elements,respectively;μO -(μTi )is that of O(Ti).n NM and n M are the numbers of the host atoms substituted by nonmetal atoms and metal atoms,respec-tively.The formation energy depends on growth conditions,which may be varied from Ti-to O-rich.For TiO 2,μO and μTi satisfy the following relationship:2μO +μTi =μ(TiO 2).Here,gas O 2and N 2,graphite,hcp bulk metal Ti,bcc bulk metal Nb,and bcc bulk metal Ta are used to determine the chemical potentials:μO =μ(O 2)/2,μN =μ(N 2)/2,μC =μ(graphite)/4,μTi =μTi metal ,μTa =μTa metal ,and μNb =μNb metal

.To study the relation between the defect formation energies

and the chemical potential of O,the

Figure 3.2?2?1supercell structure of anatase TiO 2.Gray and red spheres represent the Ti and O atoms,respectively.Roman numbers labeled on the O and Ti atoms are used to identify the doped con ?guration.

formation enthalpyΔH f[TiO2]is expressed asΔH f[TiO2]=

ΔμTi+ΔμO=(μTiàμTi[bulk])+2(μOàμO[O

2]

).According to

our previous study,49ΔH f[TiO2]=à10.3eV;thus,à5.15eV e ΔμO e0.In other words,ΔμO=0eV indicates under extreme O-rich conditions andΔμO=à5.15eV indicates under extreme

T-rich conditions.

The calculated formation energies for the substitutions of O by C are9.02and3.87eV under O-rich and Ti-rich conditions, respectively,which are larger than that of N-monodoped TiO2 (5.18and0.03eV).This indicates that C monodoping is relatively more di?cult than N monodoping.Although N O and C O induce some acceptor levels in the band gap,the previous studies show that p-type doping remains di?cult due to the low VBM and strong self-compensation.13,15,43For the Nb/N-and Ta/N-codoped TiO2,the formation energies are aboutà9.16 andà6.67eV under O-rich conditions,respectively,which are much less than that by doping with N alone.The reduced formation energies by codoping indicate that the concentration of the N impurity in the host lattice can be greatly enhanced in the presence of Nb or Ta doping.It suggests that TM/nonmetal codopings are facilitated by the electrostatic attraction of the two dopants with opposite charge states.13A similar phenomenon is also reported in ref50.

The calculated formation energies of codoping are listed in Table1.There are several features:(1)Under O-rich conditions, the formation energies of these codoped con?gurations are less than those under Ti-rich conditions.It shows that all these codoped systems are more likely to be formed under O-rich conditions.(2)All of the codoped TiO2with Nb have lower dopant formation energies than those of codoped TiO2with Ta, partly because of the larger radius of the Ta atom.15,44(3)Under O-rich conditions,the1:2CP codoped systems are more energetically favorable than1:1NCP codoped systems.It can be explained that the con?gurations codoped with another Nb (Ta)make the doping easier by symmetrizing and balancing the stress on the C atoms.As a result,Nb/C/Nb-and Ta/C/Ta-codoped systems can be formed easily under O-rich conditions. In principle,the codoping approach can overcome the p-type doping bottleneck,and it is easy to control the dopant concen-tration under di?erent growth conditions.13,15,50Especially,the dopant formation energies of CP systems are lower than that of NCP systems,which indicates that the CP codoping greatly facilitates the enhancement of the concentration of p-type dopants in the host lattice.

3.3.Electronic Properties.To compare modifications in the electronic structure of different doped systems,the density of states(DOS)and the projected DOS(PDOS)are calculated. Our previous results show that the VB edge is mainly contributed by O2p states,whereas the CB edge consists mainly of Ti3d states.51For N-monodoped TiO2,it is clear from Figure4a that two isolated N2p states are localized just0.30eV above the top of the VB of the host TiO2because N is less electronegative than O. The upshift of the VBM is only0.04eV without altering the CBM,and thus,the energy gap is about2.89eV.Therefore,it is reasonable to consider that the N-derived localized energy state above the top of the VB(as indicated in Figure4a)cannot be expected to significantly influence the optical absorption property.51In addition,N doping creates a partially occupied impurity band in the band gap of the host TiO2due to deficient an electron on N2p levels.It has been proved that the reduction of Ti4+to Ti3+occurs in N-doped TiO2due to the charge imbalance between O2àions and N3àions.52Figure4b shows that three isolated C2p states(two occupied and one empty)as deep acceptors appear in the band gap for C-monodoped TiO2, with less mixing with that of O2p,which gives rise to the large red shift in the absorption edge.In other words,the electrons in the VB can be excited to localized impurity states in the band gap and subsequently to the CB under visible-light irradiation.However, the empty C2p states can act as traps for excited electrons,which can promote recombination rates of electronàhole pairs.5 The DOS of Nb-monodoped TiO2calculated with standard DFT show that no new states are found in the fundamental band gap.Instead,the Nb4d states are distributed throughout the CB and the excess charge occupies the bottom of the CB.The results disagree with experimental PES data that show a defect state in the band gap well separated from the CB,with a height proportional to the Nb concentration.53Our calculated DOS with the LDA+U approach are in good agreement with experi-ment,showing a narrow peak in the band gap(1.4eV above the top of the VB),as shown in Figure4c.Figure4d shows that the DOS of Ta-monodoped TiO2calculated with the LDA+U approach is nearly identical to that of Nb-monodoped TiO2,a gap state1.5eV above the top of the VB.Morgan et al.presented the projection of the charge associated with the gap state,which shows occupation of the Ti3d xy on one of the nearest-neighbor cation sites,with no contribution from the Ta5d state.54

Table1.Formation Energies E f for Codoped TiO2(eV) under Ti-Rich or O-Rich Conditions within the LDA+U Approach.Δr is the Displacement of Nb(Ta)Atoms from the Position of an Original Ti Atom in the TiO6Octahedron impurity Ti-rich O-richΔr

C 3.879.02

N0.03 5.18

Nb/Cà0.03à5.180.488

Nb/Nà4.00à9.160.263

Nb/C/Nbà3.56à19.010.448/0.435 Ta/C 2.51à2.640.331

Ta/Nà1.52

à6.670.265

Ta/C/Ta 1.79à13.660.460/0.457

Figure4.Total DOS and corresponding PDOS of the dopant in the

TiO2lattice:(a)N-doped TiO2,(b)C-doped TiO2,(c)Nb-doped

TiO2,and(d)Ta-doped TiO2.Here,we only showed spin-up DOS.The

insets in(a)and(b)show the enlarged two N2p states and three C2p

states,which are indicated by dashed arrows.

For Nb/N-and Ta/N-codoped TiO 2,no isolated energy states appear in the band gap,as shown in Figures 5b and 6b.In other words,the Nb(or Ta)-derived impurity bands vanish from the band gap.The Fermi level lies above the N 2p states,suggesting its full occupation and rendering these systems less likely to form electron àhole recombination centers.However,the energy gap reductions for Nb/N-and Ta/N-codoped TiO 2are 0.1and 0.3eV (corresponding to the extension of the absorption edge about 12.5and 40.1nm),respectively,far less for the requirement of red shifting the absorption edge into the optimal visible-light region.Although no recombination center has been formed,the Nb/N-and Ta/N-codoped systems have a limited ability in visible-light photocatalytic activity.

In Nb/C (Ta/C)codoping con ?gurations,an O atom and a Ti atom are replaced by a C atom and a Nb (Ta)atom,respectively.It is well known that C has two fewer valence electrons than O,and Nb (Ta)has one more valence electron than Ti.Therefore,both 1:1Nb/C and Ta/C codopings are noncompensated.Two distinct features are observed.First,it is clear in Figure 5g that hybridized C 2p and Nb 4d states are located in the band gap,leaving the CBM almost unchanged and thus reducing the energy gap by 1.50eV.

Second,the analysis of the PDOS indicates that

Figure 5.DOS (left)for (a)pure TiO 2,(b)Nb/N-doped TiO 2,(c)Nb/C-doped TiO 2,and (d)Nb/C/Nb-doped TiO 2.The corresponding PDOS are shown

on the right in (e)à(h).

Figure 6.DOS (left)for (a)pure TiO 2,(b)Ta/N-doped TiO 2,(c)Ta/C-doped TiO 2,and (d)Ta/C/Ta-doped TiO 2.The corresponding PDOS are shown on the right in (e)à(h).

the codoping brings about more distortion of anatase TiO 2.The distorted crystal ?eld splits the energy band of the hybridized C 2p and Nb 4d,and the Fermi level locates between the spin-up and spin-down states.So is Ta/C-codoped TiO 2.The partially occupied impurity states can easily trap the carriers and lead to the reduction in carrier mobility and conversion e ?ciency,5,51which is a drawback for the application of monodoped and NCP codoped TiO 2in the photoelectrochemical conversion of solar energy.Thus,although these NCP codoped systems have a signi ?cant reduction of the phototransition energy,they are not so suitable for enhancing the photocatalytic activity in the visible-light region.This view does not con ?ict with that of ref 47.Pan and co-workers think that NCP CrO codoping improves the carrier mobility of GaN because the e ?ect of local trapping is eliminated relative to that of Cr monodoping.

To eliminate the partially occupied impurity states,the 1:2con ?gurations (i.e.,Nb/C/Nb and Ta/C/Ta codoping)are constructed and then their DOS and PDOS are calculated.Obviously,hybridized states composed of C 2p orbitals and Nb 4d or Ta 5d orbitals are formed.In particular,the hybridized states locate mainly at the VB edge,while leaving the CBM almost unchanged (only 0.1eV lower than that of pure TiO 2),thus reducing the energy gap by about 1.17eV for Nb/C/Nb-codoped TiO 2and 1.25eV for Ta/C/Ta-codoped TiO 2,respec-tively,as shown in Figures 5h and 6h.As C acts as a double acceptor and Nb (Ta)acts as a single donor,codoping with one acceptor and two donors,respectively,can compensate for the charges of the impurities from each other.In other words,two electrons move from the partially occupied Nb 4d (Ta 5d)orbitals to the partially occupied C 2p orbitals.The Fermi level lies above the hybridized states,and full occupied C 2p states appear.As discussed above,both 1:2Nb/C/Nb and Ta/C/Ta codopings are compensated.For Nb/C/Nb-codoped TiO 2,Nb 4d states may be responsible for the lowering of the energy levels of C 2p states,bringing the C 2p states much closer to the VB and,therefore,enhancing the mixing of C 2p and O 2p states in the VB.In this case,continuum states above the VB edge are formed rather than isolated states,which is favorable toward enhancing the lifetimes of photoexcited carriers.55It is similar to the phenomenon appearing in N/H-codoped TiO 2.56For the Ta/C/Ta-codoped TiO 2,Ta 5d states contribute less in the lowering of the C 2p states and the con ?guration is less balanced both geometrically and electronically,so the isolated states are formed.However,the CP systems keep the semiconductor character,which subsequently promotes the separation of elec-tron àhole pairs excited under visible-light irradiation.These results indicate that the 1:2CP systems guarantee a signi ?cant

photocatalytic activity in the visible-light region by narrowing the energy gap to nearly 2eV and suppress the recombination of electron àhole pairs.Figure 7is a schematic illustration of the band structure of Nb/C/Nb-codoped anatase TiO 2.

For a further understanding of the transfer mechanism of the electrons,we plotted the charge density di ?erences of codoped and pure TiO 2.Figure 8c shows that,for the Nb/N-codoped situation,electrons move from Nb 4d orbitals to N 2p orbitals,and shallow states appear.For Nb/C-codoped TiO 2,the elec-trons around Nb atom are so nonlocalized that partially occupied C 2p states appear,as shown in Figure 8b.What ’s more,we have con ?rmed that an electric dipole between Nb and C atoms is more than one between Nb and N.Figure 8d shows that electrons from both sides of Nb atoms move to C atoms and ?ll the C 2p states for the Nb/C/Nb-codoped system.In fact,Ta/C/Ta codoping has similar charge density di ?erences to Nb/C/Nb codoping.The Nb àC bond with a short distance indicates that two NbO 5N octahedrons are strongly distorted,in which Nb 5+ions are displaced by 0.448and 0.435?from the position of an original Ti atom in the TiO 6octahedron,as shown in Table 1.On the basis of the displacement of two Nb 5+ions,there are two dipole moments of 8.747and 8.493D (D =debye)generated inside the octahedral coordination.The dipole mo-ment causes an internal local polarization ?eld that promotes the separation of photoexcited holes and electrons,which plays an important role in photocatalysis.57à59

4.CONCLUSIONS

We have proposed the use of the codoping concept to improve the material property limitation that must be addressed before it can be considered for photoelectrochemical water splitting.On the basis of the codoping TiO 2with Nb (Ta)and C,the following conclusions are considered:(1)Nonmetal àmetal defect pairs in the host TiO 2tend to bind to each other,and the formation energy of an anion doping is greatly reduced by codoping with the cation,including 1:1and 1:2codoped TiO 2systems,which facilitate the enhancement of the concentration of p-type dopants.(2)The hybridized states of Nb 4d (Ta 5d)and C 2p orbitals above the top of the VB for these codopings should be responsible for the red shift of the optical absorption edge.(3)Codopings almost do not change the CBM and thus do not a ?ect the reducibility of electrons in the conduction band.(4)The CP codoping can eliminate the local trapping,which improves the carrier mobility.It is demonstrated theoretically

that Nb/C/Nb-and Ta/C/Ta-codopings can be candidates for

Figure 7.Schematic illustration of the band

structure of Nb/C/Nb-codoped anatase TiO 2.

Figure 8.Di ?erence between charge density and the superposition of atomic densities for the (100)plane:(a)pure TiO 2,(b)Nb/C-doped TiO 2,(c)Nb/N-doped TiO 2,(d)Nb/C/Nb-doped TiO 2.

considerable enhancement of the photocatalytic activity of anatase TiO2in the visible-light region.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:zhuyf@https://www.sodocs.net/doc/c26644672.html,.Tel:(+86)10-6278-3586. Fax:(+86)10-6278-7601.

’ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China(20925725and50972070),the National Postdoctoral Science Foundation of China(20100480254), and the National Basic Research Program of China (2007CB613303).The authors gratefully acknowledge Pro-fessor Huijun Liu for valuable discussions on this topic.

’REFERENCES

(1)Linsebigler,A.L.;Lu,G.;Yates,J.T.Chem.Rev.1995,95,735.

(2)Hadjiivanov,K.;Klissurski,D.G.Chem.Soc.Rev.1996,25,61.

(3)Wang,P.;Zakeeruddin,S.M.;Moser,J.E.;Nazeeruddin,M.K.; Sekiguchi,T.;Gr€a tzel,M.Nat.Mater.2003,2,402.

(4)Asahi,R.;Morikawa,T.;Ohwaki,T.;Aoki,K.;Taga,Y.Science 2001,293,269.

(5)Irie,H.;Watanabe,Y.;Hashimoto,K.J.Phys.Chem.B2003, 107,5483.

(6)Valentin,C.D.;Pacchioni,G.;Selloni,A.Phys.Rev.B2004, 70,085116.

(7)Yang,K.S.;Dai,Y.;Huang,B.B.J.Phys.Chem.C2007,111, 12086.

(8)Yang,K.S.;Dai,Y.;Huang,B.B.;Han,S.H.J.Phys.Chem.B 2006,110,24011.

(9)Finazzi,E.;Valentin,C.D.;Selloni,A.;Pacchioni,G.J.Phys. Chem.C2007,111,9275.

(10)Khan,S.U.M.;Al-Shahry,M.;Ingler,W.B.Science2002, 297,2243.

(11)Valentin,C.D.;Pacchioni,G.;Selloni,A.Chem.Mater.2005, 17,6656.

(12)Yang,K.S.;Dai,Y.;Huang,B.B.Phys.Rev.B2007,76,195201.

(13)Wei,https://www.sodocs.net/doc/c26644672.html,put.Mater.Sci.2004,30,337.

(14)Yan,Y.F.;Wei,S.H.Phys.Status Solidi B2008,245,641.

(15)Gai,Y.Q.;Li,J.B.;Li,S.S.;Xia,J.B.;Wei,S.H.Phys.Rev.Lett. 2009,102,036402.

(16)Zhao,Z.Y.;Liu,Q.J.Catal.Lett.2008,124,111.

(17)Yin,W.J.;Tang,H.W.;Wei,S.H.;Al-Jassim,M.M.;Turner,J.; Yan,Y.F.Phys.Rev.B2010,82,045106.

(18)Gu,D.E.;Yang,B.C.;Hu,https://www.sodocs.net/doc/c26644672.html,mun.2008,9,1472.

(19)Liu,J.W.;Han,R.;Zhao,Y.;Wang,H.T.;Lu,W.J.;Yu,T.F.; Zhang,Y.X.J.Phys.Chem.C2011,115,4507.

(20)Zhu,W.;Qiu,X.;Iancu,V.;Chen,X.;Pan,H.;Wang,W.; Dimitrijevic,N.M.;Rajh,T.;Meyer,H.M.;Paranthaman,M.P.;Stocks, G.M.;Weitering,H.H.;Gu,B.;Eres,G.;Zhang,Z.Phys.Rev.Lett.2009, 103,226401.

(21)Huang,Y.;Ho,W.K.;Ai,Z.H.;Song,X.;Zhang,L.Z.;Lee, S.C.Appl.Catal.,B2009,89,398.

(22)Osorio-Guill e n,J.;Lany,S.;Zunger,A.Phys.Rev.Lett.2008, 100,036601.

(23)Lany,S.;Zunger,A.Phys.Rev.B2005,72,035215.

(24)Soler,J.M.;Artacho,E.;Gale,J.D.;García,A.;Junquera,J.; Ordej o n,P.;S a nchez-Portal,D.J.Phys.:Condens.Matter2002,11,2745.

(25)Troullier,N.;Martins,J.L.Phys.Rev.B1991,43,1993.

(26)Monkhorst,H.J.;Pack,J.D.Phys.Rev.B1976,13,5188.

(27)Kohn,W.;Sham,L.J.Phys.Rev.1965,140,A1133.

(28)Perdew,J.P.;Burke,K.;Ernzerhof,M.Phys.Rev.Lett.1996, 77,3865.

(29)Kilic),C).;Zunger,A.Phys.Rev.Lett.2002,88,095501.

(30)Walsh,A.;DaSilva,J.L.F.;Wei,S.H.Phys.Rev.Lett.2008, 100,256401.

(31)Lathiotakis,N.N.;Andriotis,A.N.;Menon,M.Phys.Rev.B 2008,78,193311.

(32)Park,S.G.;Magyari-K€o pe,B.;Nishi,Y.Phys.Rev.B2010, 82,115109.

(33)Sheetz,R.M.;Ponomareva,I.;Richter,E.;Andriotis,A.N.; Menon,M.Phys.Rev.B2009,80,195314.

(34)Ma,X.G.;Lu,B.;Li,D.;Shi,R.;Pan,C.S.;Zhu,Y.F.J.Phys. Chem.C2011,115,4680.

(35)Henrich,V.E.;Dresselhaus,G.;Zeiger,H.J.Phys.Rev.Lett. 1976,36,1335.

(36)G€o pel,W.;Anderson,A.;Frankel,D.;Jaehnig,M.;Phillips,K.; Sch€a fer,J.A.;Rocker,G.Surf.Sci.1984,139,333.

(37)Morgan,B.J.;Watson,G.W.Surf.Sci.2007,601,5034.

(38)Lany,S.;Zunger,A.Phys.Rev.B2009,80,085202.

(39)Mori-S a nchez,P.;Cohen,A.J.;Yang,W.Phys.Rev.Lett.2008, 100,146401.

(40)Morgan,B.J.;Watson,G.W.Phys.Rev.B2009,80,233102.

(41)Burdett,J.K.;Hughbanks,T.;Miller,G.J.;Richardson,J.W.; Smith,J.V.J.Am.Chem.Soc.1987,109,3639.

(42)Lazzeri,M.;Vittadini,A.;Selloni,A.Phys.Rev.B2001,63, 155409.

(43)Wang,D.;Zou,Y.H.;Wen,S.C.;Fan,D.Y.Appl.Phys.Lett. 2009,95,012106.

(44)Shannon,R.D.Acta Crystallogr.,Sect.A1976,32,751.

(45)Sacerdoti,M.;Dalconi,M.C.;Carotta,M.C.;Cavicchi,B.; Ferroni,M.;Colonna,S.;Di Vona,M.L.J.Solid State Chem.2004, 177,1781.

(46)Liu,X.D.;Jiang,E.Y.;Li,Z.Q.;Song,Q.G.Appl.Phys.Lett. 2008,92,252104.

(47)Pan,H.;Gu,B.H.;Eres,G.;Zhang,Z.Y.J.Chem.Phys.2010, 132,104501.

(48)Van de Walle,C.G.;Neugebauer,J.J.Appl.Phys.2004, 95,3851.

(49)Ma,X.G.;Jiang,J.J.;Liang,P.Acta Phys.Sin.2008,57,3120.

(50)Ahn,K.-S.;Yan,Y.F.;Shet,S.;Deutsch,T.;Turner,J.; Al-Jassim,M.Appl.Phys.Lett.2007,91,231909.

(51)Ma,X.G.;Miao,L.;Bie,S.W.;Jiang,J.J.Solid State Commun. 2010,150,689.

(52)Nakano,Y.;Morikawa,T.;Ohwaki,T.;Taga,Y.Appl.Phys.Lett. 2005,86,132104.

(53)Morris,D.;Dou,Y.;Rebane,J.;Mitchell,C.E.J.;Egdell,R.G.; Law,D.S.L.;Vittadini,A.;Casarin,M.Phys.Rev.B2000,61,13445.

(54)Morgan,B.J.;Scanlon,D.O.;Watson,G.W.J.Mater.Chem. 2009,19,5175.

(55)Tang,J.;Ye,J.Chem.Phys.Lett.2005,410,104.

(56)Mi,L.;Xu,P.;Shen,H.;Wang,P.N.Appl.Phys.Lett.2007, 90,171909.

(57)Sato,J.;Kobayashi,H.;Inoue,Y.J.Phys.Chem.B2003,107,7970.

(58)Long,M.;Cai,W.;Wang,Z.;Liu,G.Chem.Phys.Lett.2006, 420,71.

(59)Inoue,Y.Energy Environ.Sci.2009,2,364.