N Codoped TiO2 Anatase (101) Surface

Article pubs.acs.org/Langmuir

Synergistic Effects in La/N Codoped TiO2 Anatase (101) Surface Correlated with Enhanced Visible-Light Photocatalytic Activity Liming Sun,† Xian Zhao,‡ Xiufeng Cheng,‡ Honggang Sun,‡ Yanlu, Li,‡ Pan Li,‡ and Weiliu Fan*,† †

Department of Chemistry and Chemical Engineering and ‡State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China ABSTRACT: The interaction between implanted La, substitutional N, and an oxygen vacancy at TiO2 anatase (101) surface has been investigated by means of first-principles density function theory calculations to investigate the origin of enhanced visible-light photocatalytic activity of La/N-codoped anatase observed in experiments. Our calculations suggest that both the adsorptive and substitutional La-doped TiO2 anatase (101) surfaces are probably defective configurations in experiments. The h-Cave-adsorbed La doping decreases the formation energy for the substitutional N implantation and vice versa, while the charge compensation effects do not take effect between the adsorptive La and substitutional N dopants, resulting in some partially occupied states in the band gap acting as traps of the photoexcited electrons. The Ti5c-substituted La doping decreases the energy required for the substitutional N implantation, and the substitutional La and N codoping promotes the formation of an oxygen vacancy, which migrates from the Osb‑3c site at the inner layer toward the surface Ob site. For the substitutional La/N-codoped (Ti5c_O3c‑down) surface, the charge compensation between the substitutional La and substitutional N leads to the formation of two isolated occupied Ns−O π* impurity levels in the gap, while the excitation energy from the higher impurity level to the CBM decreases by about 0.89 eV. After further considering an oxygen vacancy on the Ti5c_O3c‑down surface, the two electrons on the double donor levels (Ob vacancy) passivate the same amount of holes on the acceptor levels (substitutional La and N), forming the acceptor−donor−acceptor compensation pair, which provides a reasonable mechanism for the enhanced visible-light photocatalytic activity of La/N codoped TiO2 anatase. This knowledge may aid the further design and construction of new effective visible-light photocatalysts.

1. INTRODUCTION TiO2 anatase has been investigated as a promising photocatalytic semiconductor for decades due to its high photocatalytic activity, resistance to photocorrosion, photostability, low cost, and nontoxicity.1−4 Unfortunately, the photocatalytic activity of TiO2 anatase is restricted to the ultraviolet light region due to its large band gap (Eg = 3.2 eV), which carries approximately 4% of the solar radiation at the Earth’s surface. Another drawback that hindered the actual application of anatase is its fast recombination rate of photogenerated electron−hole pairs, that is, the low quantum efficiency. In recent years, the main strategy toward achieving activity under visible light is nonmetal doping, such as B, C, N, F, and S.5−14 In particular, N has been thought as the most effective substitution doping nonmetal element, since the work of Asahi et al.6 Some modified methods also have been developed to slow down the recombination rate of electron−hole pairs, such as surface chelating, surface derivatization, and rare-earth metal ions doping.15−20 Lanthanum oxide was also reported as an effective catalyst for the catalytic oxidation of different hydrocarbons and the removal of organic compounds derived from wastewater.17 Recently, both Cong et al.21 and Zhang et al.22 have reported that the N and La codoped TiO2 anatase could greatly improve the photocatalytic activity in visible-light irradiation, which could be ascribed to the synergistic effects of the N and La codoping; that is, the N doping narrows the band © 2012 American Chemical Society

gap of TiO2 anatase and enhances the utilization efficiency of visible light, while the La doping accelerates the separation of photogenerated electrons and holes. However, two ambiguous problems still exist in the La/Ncodoped TiO2 anatase system. First, it is commonly believed that La3+ ions do not enter the matrix of TiO2 anatase but rather form lanthanum oxides, which are uniformly adsorbed on the surface of TiO2 anatase, due to the radius of La3+ ion (0.115 nm) being too large to replace the Ti4+ ion (0.068 nm) in the TiO2 anatase matrix.23 Nevertheless, a recent study reported an absence of a red-shift for La3+ doping attributed to the La3+ replacing Ti4+ ions in the TiO2 lattice,24 while most other literature is in agreement with La3+ being able to red-shift the adsorption edges to longer wavelengths.25,26 Thus, confusions exist in the formation mechanism of lanthanum oxides as to whether the La3+ ions replace the Ti4+ ions at TiO2 anatase surface or adsorb on the surface bonding with surface O atoms to form lanthanum oxides. Second, Cong et al.21 reported that the N impurities substitute O atoms in the TiO2 anatase lattice, while the results of FTIR analysis by Zhang et al.22 indicated that some substitutional N dopants in the N/Lacodoped TiO2 anatase sample forming the NOx species. So, Received: January 21, 2012 Revised: March 5, 2012 Published: March 8, 2012 5882

dx.doi.org/10.1021/la3003178 | Langmuir 2012, 28, 5882−5891

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Figure 1. General view of the TiO2 anatase (101) surface. (a) Labeling of the nine La adsorption sites: parts a1−a9 correspond to Top-Ob, TopO3c‑up, Top-O3c‑down, Ob-Ob, h-Cave, v-Cave, Top-Os3c‑up, Top-Os3c‑down, and interstitial, respectively. (b) Labeling for the substitutional O sites: O1 (Ob), O2 (O3c‑up), O3 (O3c‑down), O4 (Ob‑3c), O5 (Osb‑3c), O6 (Os3c‑up), and O7 (Os3c‑down). (c) Labeling for the substitutional Ti sites: Ti1 (Ti5c), Ti2 (Ti6c), Ti3 (Tis6c‑up), and Ti4 (Tis6c‑down). The gray, red, and light blue spheres represent titanium, oxygen, and lanthanum, respectively.

from microscopic mechanism, and aid the further design and construction of new effective visible-light photocatalysts.

what form in which the N dopants exactly exist in the La/Ncodoped TiO2 anatase system is worth studying. All in all, the detailed mechanism of the improved photocatalytic activity of TiO2 in visible-light irradiation by La/N codoping has not been clear yet, and further research is necessary. Around the problems mentioned above, we have studied the structural, energetic, and electronic properties of the TiO2 anatase (101) surface under conditions of the La doping, the substitutional N doping, and the presence of an oxygen vacancy to obtain microscopic insight to the detailed mechanism in the enhanced visible-light photocatalytic activity of La/N-codoped anatase TiO2 using periodic density functional theory (DFT) based methods. The choice for the anatase polymorph of TiO2 comes from the fact that the anatase is the most abundant phase of the TiO2 catalyst and the activity of anatase in photocatalytic process is found to be much higher than that of rutile.27 Likewise, the choice of the (101) surface is justified because it is the thermodynamically most stable low-index surface of anatase.28 Thus, the TiO2 anatase (101) surface is expected to play a relevant role in the activation of photochemical processes. In the present paper, the study is focused on: (i) the La implantation by adsorption and substitution, (ii) the Laad−Ns, Laad−Ns-vacancy, Las−Ns, and Las−Ns-vacancy interactions, and (iii) the capability of these species to improve the visiblelight absorption and the photocatalytic behavior of the TiO2 anatase (101) surface. The results provide a possible of acceptor−donor−acceptor mechanism in implementing an efficient visible-light activity of TiO2 anatase by the synergistic effects of substitutional La/N codoping and oxygen vacancies. It is expected that this knowledge would provide useful information for a greater understanding of the enhanced visiblelight photocatalytic activity of La/N-codoped TiO2 anatase

2. COMPUTATIONAL DETAILS All of the spin-polarized DFT calculations were performed using plane-wave pseudopotential method, as implemented in the Cambridge Sequential Total Energy Package (CASTEP) code.29 The general gradient approximation (GGA)30 with PW9131 functional and ultrasoft pseudopotential32 were used to describe the exchangecorrelation effects and electron−ion interactions, respectively. The valence states that were explicitly included in the calculation were 3s, 3p, 3d, and 4s for Ti atoms, 2s and 2p for O and N atoms, together with 5s, 5p, 5d, and 6s states for La atoms, while the core electrons were kept at frozen states. The wave functions were expanded into a basis set of plane waves with a kinetic energy cutoff of 300 eV. The geometrical optimizations was implemented at the Γ point for all the surface structures,33 while the Monkhorst−Pack grid34 with 2 × 3 × 1 k-point in the Brillouin zone of the TiO2 anatase (101) surface systems was used for electronic properties calculations to achieve the accurate density of the electronic states.33,35,36 The self-consistent convergence accuracy was set at 2.0 × 10−5 eV/atom, the convergence criterion for the force between atoms was 5 × 10−2 eV/Å, and the maximum displacement was 2 × 10−3 Å. The optimized lattice parameters were found to be a = 3.799 Å and c = 9.756 Å for TiO2 anatase, in good agreement with experimental values (a = 3.782 Å, c = 9.502 Å)37 and other theoretical calculated results,35 indicating that our computational approach is reasonable. The TiO2 anatase (101) surface was simulated using a slab model with 2 × 2 surface unit cell periodicity and four TiO2 layers, containing 96 atoms. The slab model was separated by a vacuum spacing of 12 Å to wipe out the interaction between periodic images, and the depth of the supercell was 24.96 Å. The two bottom TiO2 layers of the slab were maintained in their bulk parameters during geometry optimization in order to simulate the presence of the bulk underneath, and the upper two layers were fully relaxed in our calculations. The implantation of La atoms in the structure was performed on both adsorptive and substitutional positions, and the implantation was 5883


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limited to the first two TiO2 layers in the slab, which were fully relaxed. The adsorption of La was performed by addition of one La atom to the TiO2 anatase (101) surface, and the substitutional La doping was carried out by replacement of one lattice Ti with one La impurity. In Figure 1a, there were nine possibly adsorptive sites labeled Top-Ob, Top-O3c‑up, Top-O3c‑down, Ob-Ob, h-Cave (horizontal-Cave), v-Cave (vertical-Cave), Top-Os3c‑up, Top-Os3c‑down, and interstitial. As shown in Figure 1b, the “s” prefix was used to denote subsurface sites and the label “nc” corresponded to the number of Ti atoms bound to the O atoms. Tis6c‑up atoms were outward relaxed 6-fold subsurface Ti atoms, while Tis6c‑down atoms were relaxed inward. The substitutional N sites are also shown in Figure 1b. The substitutional N doping was modeled by replacing one O atom with one N impurity. To identify the nonequivalent substitutional implantation sites, we used the nomenclature similar to that used for Ti atoms. The label “b” was used to denote bridging O atoms. So, the seven substitutional N sites were Ob, O3c‑up, O3c‑down, Ob‑3c, Osb‑3c, Os3c‑up, and Os3c‑down. After La/N codoping, an oxygen vacancy had been created by removing one lattice O atom from one of the first two TiO2 layers; we identified the vacancy positions following the same labeling that of the substitutional N sites. For the La/N-codoping surfaces with an oxygen vacancy, the position of the La (adsorptive and substitutional) was indicated at first, followed by the substitutional N impurity, and finally the vacancy position. For instance, Ti5c_O3c‑down_O3c‑down meant that the La dopant substituted the surface Ti5c atom, the O3c‑down was substituted by the N impurity, and the vacancy had been created in an O3c‑down site.

Table 1. Adsorption Energies (eV) of La Atoms Deposited Different Positions on the Stoichiometric TiO2 Anatase (101) Surface positions

adsorption energies (eV)

coordination number

Top-Ob Top-O3c‑up Top-O3c‑down Ob-Ob h-Cave v-Cave Top-Os3c‑up Top-Os3c‑down interstitial

1.20 −1.21 −1.54 −1.72 −1.87 0.72 1.11 −1.15 1.16

1 3 2 4 5 5 6 6 6

Figure 2. Optimized partial geometrical configurations for the most stable structure of TiLaxO2 and Ti1−xLaxO2 systems: (a) h-Caveadsorbed and (b) Ti5c-substituted TiO2 anatase (101) surfaces. The gray, red, light blue, and blue spheres represent titanium, oxygen, lanthanum, and nitrogen, respectively.

3. RESULTS AND DISCUSSION 3.1. Mono La Implantation on the TiO2 Anatase (101) Surface. In this section, we consider two different ways for the La impurities to implant on the TiO2 anatase (101) surface, that is, adsorption and substitution of a lattice Ti atom by a La impurity, with various possible nonequivalent adsorptive and substitutional La impurity arrangements in the first two layers. The changes of the local microstructures and the electronic structures aroused by the La doping are also discussed. Our main concern is to establish whether the adsorptive La bonding with surface O atoms is favored with respect to the La impurity replacing one lattice Ti on the TiO2 anatase (101) surface. A. La Implantation by Adsorption. The energy necessary to adsorb the La impurity on the TiO2 anatase (101) surface can be represented according to the following formula:

the most stable structure with the La adsorption energy of −1.87 eV. The lengths of the five resulting La−O distances in this configuration are 2.329, 2.371, 2.403, 2.646, and 2.715 Å. B. La Implantation by Substitution. To compare the relative stability of the substitutional La-doped systems, the formation energies Ef of the substitutional La-doped systems were estimated according to E f = E(Ti1 − xLaxO2) − E(TiO2) − μLa + μ Ti

where E(Ti1−xLaxO2) is the total energy of surface supercell containing the substitutional La impurity and μTi represents the chemical potential of the Ti atom calculated from bulk metal titanium. It should be mentioned that we do not consider the dependence on the oxygen chemical potential in this work. There are two main reasons to only considering the O-rich conditions. One is that most of the photocatalysts are prepared by thermal decomposition or calculations under the condition of open atmosphere. The other is that the comparison of formation energies at different sites of the same supercell is much more meaningful than that for calculations performed with different setups. In Table 2, the calculated and relative formation energies of the different substitutional La-doped TiO2 anatase (101) surfaces are reported. Since the absolute formation energies for the surface Las-doped species (Ti5c and Ti6c) differ from that for the subsurface species (Tis6c‑up and Tis6c‑down) by more than 1 eV, the La impurities prefer to substitute the superficial Ti atoms at the TiO2 anatase (101) surface. In particular, the case of the La substituting superficial Ti5c atom is the most probable occurring defective configuration in experiments with a La-substituted formation energy of 2.48 eV. Given this, we shall only discuss the details of the structural and electronic properties for the most stable Ti5c-substituted case.

Ead = E(TiLaxO2) − E(TiO2) − μLa

in which E(TiLaxO2) is the total energy of the TiO2 anatase (101) surface with the adsorptive La impurity, E(TiO2) denotes total energy of the pure surface system, and μLa represents the chemical potential of the La atom calculated from bulk metal lanthanum. If we compare the adsorption energies for different adsorptive La sites reported in Table 1, we can see that the superficial adsorption of La impurity is an exothermic process, except the Top-Ob adsorptive site, while the process of La adsorbing on the subsurface is generally endothermic, indicating that the adsorption of La atom is favored on the surface layer of TiO2 anatase (101) surface instead of the subsurface. The relative lower value in adsorption energy means the preference of La adsorption on the surface. From the adsorption energies for different superficial adsorptive positions, we find that the more negative adsorption energy is in relation to the higher La coordination number with respect to surface O atoms. In particular, the case of La adsorbing on the h-Cave site (Figure 2a), bonding with five surface O atoms, is 5884


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attributed to the incomplete cancellation of the self-interaction of the pure GGA exchange/correlation functionals.39 In the case of TiO2, DFT hybrid functionals including exchange (B3LYP) and the DFT+U (Hubbard parameter) approximation have been shown to correctly estimate the electronic structures.40−42 However, as we will show in the following, this inaccurate description of the band gap does not affect the qualitative description of the electronic interaction between the defects, because only the relative positions of the occupied states and empty states need be taken into account. As shown in Figure 3b, the band gap of the h-Cave-adsorbed TiO2 anatase (101) surface narrows to about 1.71 eV. Some states of the CBM pass through the highest occupied level, which mainly originate from the Ti 3d and have little La 5d contribution, indicating that the adsorptive La impurity acts as a donor doping in this system. For the Ti5c-substituted structure (Figure 3c), the band gap has a slight increase to about 2.00 eV. One unoccupied impurity level for the spin-down channel, composed by the O 2p states, appears above the highest occupied level, showing an acceptor character, due to the substitutional La impurity has one less valence electron than the lattice Ti. As our calculations shown, a red-shift in the adsorption spectra of TiO2 is dependent on the type of La doping; that is, the adsorptive La doping is able to red-shift the adsorption edges of TiO2 to lower regions, while the substitutional La doping cannot raise a red-shift in the adsorption spectra. This result explains the experimental dispute about whether the La doping is able to red-shift the adsorption edges of TiO2 to longer wavelengths or not.24−26 In this section, the geometrical and electronic structures of the most stable Laad- and Las-doped TiO2 anatase (101) surfaces have been discussed. The results show that both the superficial adsorptive and substituted La-doped TiO2 anatase (101) surfaces, forming the La pentacoordinated structure with

Table 2. Formation Energies (eV) for Substitutional La Implantation into the Stoichiometric TiO2 Anatase (101) Surface positions

formation energies (eV)

relative energies

Ti5c Ti6c Tis6c‑up Tis6c‑dowm

2.48 3.18 4.48 4.34

0.00 0.70 2.00 1.86

For the Ti5c-substituted case (Figure 2b), the lengths of five La−O bonds formed by the substitutional La and adjacent five O atoms are 2.267, 2.302, 2.317, 2.455, and 2.492 Å, resulting in significant structural distortion in comparison to the original Ti−O bond lengths (1.837, 1.975, 1.988, 1.784, and 2.039 Å), due to the larger ionic radius of La3+ ion vis-à-vis Ti4+ ion. The stretched La−O bonds extrude the substitutional La atom out to the TiO2 anatase (101) surface, which will be better to realize the adsorption of organic pollutants on the La-doped surface. The reason is that La can form complexes with various Lewis bases through interaction of functional groups with the forbit of La.38 C. Electronic Structures. To investigate the effects of adsorptive and substitutional La-doped motifs on electronic structures and photocatalytic activity, the band structures and projected density of states (PDOS) of h-Cave-adsorbed and Ti5c-substituted systems are calculated and shown in Figure 3b and c. For comparison, the band structures and PDOS of pure TiO2 anatase (101) surface are also shown in Figure 3a. In the pure TiO2 anatase (101) surface (Figure 3a), the valence band maximum (VBM) is dominated by the O 2p states whereas the conduction band minimum (CBM) mainly consists of Ti 3d states resulting in a GGA calculated band gap of 1.94 eV, which is, as usual, underestimated by DFT-GGA. This underestimation of the gap is well-known and mainly

Figure 3. Band structures and projected density of states (PDOS) for pure TiO2 and the most stable structure of TiLaxO2 and Ti1−xLaxO2 systems: (a) TiO2, (b) h-Cave-adsorbed, and (c) Ti5c-substituted TiO2 anatase (101) surfaces. (a′) displays the Brillouin zone containing the spatial positions of G, F, Q, and Z k-points for TiO2 anatase (101) surface. The blue and red lines represent the spin up and down channels, respectively. The dashed lines represent the highest occupied level. 5885


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in particular, the h-Cave_Ob‑3c surface is the most stable configuration with the substitutional N formation energy of 2.55 eV. Optimized partial geometrical structure of the h-Cave_Ob‑3c system is shown in Figure 4a. The adsorptive La impurity forms

surface O atoms, are probably occurring in defective configurations in experiments. The calculated electronic structures indicate that though the adsorptive La implantation can raise a red-shift in the adsorption spectra of TiO2, some partially occupied states pass through the highest occupied level, which may act as traps of the photoexcited electrons and thus reduce the photogenerated current. 3.2. The TiLaxO2−yNy and TiLaxO2−y−zNy Anatase (101) Surfaces. On the basis of above calculations and analyses, the defective configuration of La implantation by superficial adsorption will probably occur in experiments, while the superficial adsorptive La narrows the band gap of TiO2 anatase (101) surface accompanied by the introduction of some partially occupied states in the gap. In this section, to passivate the partially occupied states in the premise of narrowing the band gap of TiO2, the interplay between adsorptive La, substitutional N, and an oxygen vacancy on the TiO2 anatase (101) surface is considered. We select the h-Cave-adsorbed TiO2 anatase (101) surface, which is the most stable structure of the adsorptive La-doped TiO2 anatase (101) surface, as the starting model. The formation energies of the substitutional N doping on the TiLaxO2 anatase (101) surface and an oxygen vacancy on the TiLaxO2−yNy anatase (101) surface are calculated according to the two following formulates, respectively.

Figure 4. Optimized partial geometrical configurations for the most stable structure of TiLaxO2−yNy and TiLaxO2−y−zNy systems: (a) hCave_Ob‑3c and (b) h-Cave_Ob‑3c_Osb‑3c TiO2 anatase (101) surfaces. The gray, red, light blue, and blue spheres represent titanium, oxygen, lanthanum, and nitrogen, respectively.

penta-coordinated structure with surface O atoms, and the resulting distances of five La−O distances are 2.316, 2.436, 2.446, 2.611, and 2.672 Å, respectively. The substitutional N replaces the lattice Ob‑3c atom to form three N−Ti bonds (1.807, 1.921, and 1.967 Å). It can be found that the whole structure has little changes compared with the h-Cave-adsorbed TiO2 anatase (101) surface, indicating that the interaction between the adsorptive La doping and substitutional N implantation in the structure is almost negligible. B. Oxygen Vacancies on the TiLaxO2−yNy Anatase (101) Surface. On the basis of the above calculations, we consider different oxygen vacancies in the first two layers, taking the hCave_Ob‑3c surface as the starting structure. Table 4 gives the

E f = E(Ti1 − xLaxO2 − y Ny) − E(TiLaxO2) − μN + μO

and E fv = E(TiLaxO2 − y − z Ny) − E(TiLaxO2 − y Ny) + μO

where E(TiLaxO2−yNy) is the energy of the substitutional N on the TiLaxO2 anatase (101) surface, E(TiLaxO2−y−zNy) represents the energy of the TiLaxO2−yNy surface with an oxygen vacancy, and μN and μO are the energies of half a N2 and O2 molecule, respectively. A. Substitutional N Implantation on the TiLaxO2 Anatase (101) Surface. The formation energies of substitutional N implantation obtained that correspond to different substitutional N sites are reported in Table 3. For the bare TiO2

Table 4. Formation Energies (eV) for O Vacancies at the TiO2, TiLaxO2−yNy, and Ti1‑xLaxO2−yNy Anatse (101) Surfaces formation energies (eV)

Table 3. Formation Energies (eV) for Substitutional N Implantation into the TiO2, TiLaxO2, and Ti1−xLaxO2 Anatse (101) Surfaces formation energies (eV) positions

TiO2

TiLaxO2

Ti1−xLaxO2

Ob O3c‑up O3c‑down Ob‑3c Osb‑3c Os3c‑up Os3c‑down

5.56 5.47 5.27 5.28 5.23 5.32 5.26

2.83 2.61 2.73 2.55 2.74 2.83 2.77

3.91 3.36 2.86 3.55 3.44 3.53 3.66

Vo positions

TiO2

TiLaxO2−yNy

Ti1‑xLaxO2−yNy

Ob O3c‑up O3c‑down Ob‑3c Osb‑3c Os3c‑up Os3c‑down

5.05 6.46 5.97 5.71 4.91 5.84 5.76

5.74 5.78 6.14 5.71 5.67 5.75 5.75

2.32 2.28 2.25 2.56 2.83 3.70 3.92

formation energies for different oxygen vacancies considered. As shown, the preferred oxygen vacancy is at the Osb‑3c site on the undoped TiO2 anatase (101) surface with the formation energy of 4.91 eV. After introducing the adsorptive La and substitutional N on TiO2 anatase (101) surface, the Osb‑3c vacancy is still the most stable oxygen vacancy defect, whose formation energy increases to 5.67 eV compared with the undoped surface. That is to say, the h-Cave_Ob‑3c_Osb‑3c surface is the most stable configuration for the TiLaxO2−yNy anatase (101) surface with an oxygen vacancy. Figure 4b shows partial geometric configuration of the hCave_Ob‑3c_Osb‑3c anatase (101) surface. The structure of the h-Cave_Ob‑3c_Osb‑3c surface is almost same as the h-Caveadsorbed and h-Cave_Ob‑3c surfaces, indicating that the

anatase (101) surface, the N impurity replacing the lattice Osb‑3c is the most stable structure with the formation energy of 5.23 eV, in agreement with previous studies on the substitutional N doping on TiO2 anatase, which have shown that N prefers substitutional subsurface positions rather than surface sites, specifically, the Osb‑3c site.43,44 From Table 3, we can see that the adsorptive La doping has stimulative effects on the substitutional N implantation on TiO2 anatase (101) surface; 5886


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Figure 5. Band structures and projected density of states (PDOS) for the most stale structure of TiLaxO2−yNy and TiLaxO2‑y‑zNy systems: (a) hCave_Ob‑3c and (b) h-Cave_Ob‑3c_Osb‑3c TiO2 anatase (101) surfaces. The blue and red lines represent the spin up and down channels, respectively. The dashed lines represent the highest occupied level.

substituted TiO2 anatase (101) surface as the starting model to research the interplay both between substitutional La and substitutional N and between substitutional La, substitutional N, and an oxygen vacancy. Our main aim is to investigate whether the substitutional La/N codoping or the substitutional La/N codoping with an oxygen vacancy can passivate the partially occupied states in the mono La-doped systems by the charge compensation mechanism. The formation energies of substitutional N implantation on the Ti1−xLaxO2 anatase (101) surface and an oxygen vacancy on the Ti1−xLaxO2−yNy surface are calculated according to the two following formulates, respectively.

adsorptive La, substitutional N and an oxygen vacancy do not affect each other in the structure. The adsorptive La impurity still bonds with five surface O atoms to form a pentacoordinated structure, and the five La−O bond lengths are 2.329, 2.431, 2.255, 2.645, and 2.653 Å. C. Electronic Structures. To investigate the effects of adsorptive La, substitutional N, and an oxygen vacancy on the electronic structures and photocatalytic activity of TiO2, the band structures and PDOS of e h-Cave_Ob‑3c and hCave_Ob‑3c_Osb‑3c TiO2 anatase (101) surfaces are calculated and shown in Figure 5. For the h-Cave_Ob‑3c surface (Figure 5a), the highest occupied level is pinned in the bottom of the CB, which is mainly contributed to the Ti 3d states, and the host band gap narrows to about 1.77 eV. The VBM of the h-Cave_Ob‑3c system is composed by the N 2p states, due to the higher orbital energy of neutral 2p orbital than O 2p orbital energy. For the h-Cave_Ob‑3c_Osb‑3c system (Figure 5b), some levels of the CBM, mainly constituted by the Ti 3d, pass through the highest occupied level, and the VBM is also contributed to the N 2p states, resulting in decreasing the band gap to about 1.78 eV, which is similar to the band structure of h-Cave_Ob‑3c TiO2 anatase (101) surface. While, two localized Ti 3d states lie below the CBM of the h-Cave_Ob‑3c_Osb‑3c surface, which are associated to an oxygen vacancy causing the reduction of Ti4+ to Ti3+ ions. Though the band gap has been narrowed by about 0.17 eV in both the h-Cave_Ob‑3c and h-Cave_Ob‑3c_Osb‑3c TiO2 anatase (101) surfaces, some partially occupied levels of the CBM still locate in the gap. Thus, the charge compensation does not take place either between the adsorptive La and substitutional N or between the adsorptive La, substitutional N, and an oxygen vacancy. From the above analysis, two main standpoints can be obtained. First, the adsorptive La doping promotes the substitutional N implantation, while the adsorptive La and substitutional N codoping increases the energy required for the formation of an oxygen vacancy. Second, the charge compensation does not take place either between the adsorptive La and substitutional N or between the adsorptive La, substitutional N, and an oxygen vacancy, resulting in some partially occupied states still in the band gap. 3.3. The Ti1−xLaxO2−yNy and Ti1−xLaxO2−y−zNy Anatase (101) Surfaces. As the above results show, the interactions either between adsorptive La and substitutional N or between the adsorptive La, substitutional N, and an oxygen vacancy on TiO2 anatase (101) surface both fail to passivate the partially occupied levels in the gap, due to the absence of charge compensation. Thus, in this section, we consider the Ti5c-

E f = E(Ti1 − xLaxO2 − y Ny) − E(Ti1 − xLaxO2) − μN + μO

and E fv = E(Ti1 − xLaxO2 − y − z Ny) − E(Ti1 − xLaxO2 − y Ny) + μO

where E(Ti1−xLaxO2−yNy) represents the energy of the Ti1−xLaO2 anatase (101) surface containing a substitutional N impurity and E(Ti1−xLaxO2−y−zNy) is the energy of the system with an oxygen vacancy on the Ti1−xLaxO2−yNy anatase (101) surface. a. Substitutional N Implantation on the Ti1−xLaxO2 Anatase (101) Surface. Geometric Structures. As Table 3 shows, the substitutional La implantation on TiO2 anatase (101) surface promotes the N impurity replacing a lattice O atom. In particular, theTi5c_O3c‑down becomes the most stable configuration and the formation energy for the N substituting lattice O3c‑down atom decreases to 2.86 eV. As the optimized partial geometrical structure of Ti5c_O3c‑down system shows (Figure 6a), due to the larger ionic radius of La3+ than Ti4+, the substitutional La3+ pushes the adjacent Ob to the position close

Figure 6. Optimized partial geometrical structures for the most stable structure of Ti1−xLaxO2−yNy and Ti1−xLaxO2−y−zNy systems: (a) Ti5c_O3c‑down and (b) Ti5c_O3c‑down_O3c‑down TiO2 anatase (101) surfaces. The gray, red, light blue, and blue spheres represent titanium, oxygen, lanthanum, and nitrogen, respectively. 5887


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to the substitutional N impurity, resulting in the formation of Ns-O species with the N−O bond length of 1.441 Å. To understand the charge redistribution induced by substitutional La and N codoping, we calculate electron density and different density maps for the Ti5c_O3c‑down anatase (101) surface, shown in Figure 7a and a′. The total electron density

Figure 9. Schematic representation of the Ns−O molecular orbital rearrangement in the Ti5c_O3c‑down Ti1−xLaxO2−yNy anatase (101) surface.

Figure 7. Total electron density maps (left) and electron difference density maps (right) for the most stable structure of Ti1−xLaxO2−yNy and Ti1−xLaxO2−y−zNy systems: (a), (a′) Ti5c_O3c‑down and (b), (b′) Ti5c_O3c‑down_O3c‑down TiO2 anatase (101) surfaces.

ion, and this Ob− ion bonds with the substitutional N2‑ ion, forming the Ns−O species, to achieve the charge compensation between the substitutional La and N dopants. As Figure 9 shows, energies of the occupied σ and π states are lower than the O 2p valence band and the energy of the unoccupied σ* states is higher than that of Ti 3d states, so the existence of them cannot be reflected in the Figure 8a, which only shows the VBM and CBM of band structure. The two occupied π* states, whose energies lie in the band gap, appear as the impurity levels. From the above calculations, we can find the substitutional La promotes the N impurity replacing one lattice O atom on the TiO2 anatase (101) surface, and the charge compensation mechanism in the Ti5c_O3c‑down surface takes effect to passivate the partially occupied states in the band gap, though the substitutional La and N are dopants of electron deficiency. However, the excitation energy from the higher occupied impurity level to the CBM decreases by about 0.89 eV in the Ti5c_O3c‑down anatase (101) surface, while the adsorption edge of La/N codoped anatase TiO2 obtained by experiments only red-shifts by 0.17−0.27 eV.22 So, another more reasonable compensation mechanism of the improved photocatalytic activity of TiO2 in visible light irradiation by La/N codoping may exist.

map in part (a) of Figure 7 shows the substitutional N impurity bonding with the adjacent Ob atom is major covalent-like bonding interactions by common electron clouds and there are some charges transferring between the substitutional N impurity and lattice Ob atom, which confirms the formation of Ns−O species. b. Electronic Structures. The band structure and PDOS of Ti5c_O3c‑down anatase (101) surface are calculated and shown in Figure 8a. From Figure 8a, there are no partially occupied states in the band gap. Besides, we can see that the host band gap of the Ti5c_O3c‑down system increases to 1.97 eV, and two isolated occupied Ns−O π* states lie in the gap, resulting in the transition energy from the higher occupied impurity state to the CBM decreasing to about 1.05 eV. Though both the substitutional La and N dopants have one less valence electron than the lattice Ti and O atoms, the charge compensation effect still takes place between them, since no partially occupied states locate in the gap. In order to more clearly expound the charge compensation mechanism in the Ti5c_O3c‑down system, the diagram of the Ns−O molecular orbital rearrangement is given in Figure 9. The substitutional La having one less valence electron than the lattice Ti leads to the formation of the Ob−

Figure 8. Band structures and projected density of states (PDOS) for the most stable structure of Ti1−xLaxO2−yNy and Ti1−xLaxO2−y−zNy systems: (a) Ti5c_O3c‑down and (b) Ti5c_O3c‑down_O3c‑down TiO2 anatase (101) surfaces. The blue and red lines represent the spin up and down channels, respectively. The dashed lines represent the highest occupied level. 5888


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B. Oxygen Vacancies on the Ti1−xLaxO2−yNy Anatase (101) Surface. Since only the compensation effect between substitutional La and N impurity could not act as a satisfactory mechanism in the improved photocatalytic activity of La/N codoped TiO2 anatase, we will further study the role of an oxygen vacancy in this system to provide a more reasonable explanation for the enhanced visible-light photocatalytic activity of La/N codoped TiO2 anatase reported by different expeiments.21,22 To this aim, we have taken the Ti5c_O3c‑down anatase (101) surface as the starting surface model to consider all kinds of possible oxygen vacancies within the first two layers. a. Geometric Structures. As shown in Table 4, we find that the substitutional La and N codoping can facilitate the formation of an oxygen vacancy on the TiO2 anatase (101) surface, and the O3c‑down oxygen vacancy is the preferred oxygen vacancy on the Ti5c_O3c‑down anatase (101) surface with the formation energy of 2.25 eV. It is noteworthy that if an oxygen vacancy is created at the O3c‑down position on the Ti5c_O3c‑down anatase (101) surface, the Ob atom connected with the substitutional La moves spontaneously to fill this vacancy, leaving a surface Ob defect, as the partial geometric structure of Ti 5c _O 3c‑down _O 3c‑down (Figure 6b) shows. So, in the Ti5c_O3c‑down_O3c‑down case, the Ns−O species existing in the Ti5c_O3c‑down anatase (101) surface disappear, and the substitutional La coordination number decreases to 4. The lengths of these four corresponding La−O bonds are 2.288, 2.290, 2.295, and 2.359 Å. b. Electronic Structures. The band structure and PDOS of Ti5c_O3c‑down_O3c‑down anatase (101) surface are calculated and shown in Figure 8b. In this case, no partially or isolated occupied energy states appear in the band gap, and the host band gap is narrowed by 0.17 eV vis-à-vis the pure TiO2 anatase (101) surface, which is agreement with the magnitude of redshift of adsorption edge in experiments (0.17−0.27 eV).22 In Figure 8b, no isolated N 2p states are observed and the N 2p states are slightly above the VB of TiO2 anatase, which sufficiently mixed with the O 2p states in the VB. To further understand the origin of the absence of partially and isolated occupied states in the gap, the electron density and different density maps for the Ti5c_O3c‑down_O3c‑down anatase (101) surface are depicted in Figure 7b and b′. From Figure 7b′, we find that the substitutional La captures electrons from the Ob vacancy, compared with the Ti5c_O3c‑down anatase (101) surface (Figure 7a′). As both the substitutional La and N have one less valence electron than the lattice Ti and O, the substitutional La and N impurities on the Ti and O sites act as two single acceptors, which accept two electrons left by the removed bridging oxygen. The bridging oxygen vacancy (double donor) may contribute to the lowering of the energy levels of the substitutional N (acceptor), bridging the N 2p states closer to the VB. Therefore, the isolated states disappear and an occupied continuum is formed as demonstrated in our computation. The above discussions clearly indicate that the substitutional La and N codoping not only facilitates the formation of an oxygen vacancy, but also makes the oxygen vacancy migrate from the Osb‑3c sites at the inner layer toward the surface Ob positions. Moreover, the two electrons on the double donor levels (bridging oxygen vacancy) passivate the same amount of holes on the acceptor levels (substitutional La and N), forming the acceptor−donor−acceptor compensation mechanism, to eliminate the partially occupied states in the gap. Since the band gap of Ti5c_O3c‑down_O3c‑down anatase (101) surface

decreases by 0.17 eV, which is in line with the red-shift of adsorption edge reported in experiments,22 the acceptor− donor−acceptor compensation mechanism is a more reasonable explanation for the enhanced visible-light photocatalytic activity of La/N-codoped TiO2.

4. CONCLUSIONS The structural and electronic properties of TiO2 anatase (101) surface under conditions of the La doping, substitutional N implantation and the presence of an oxygen vacancy have been studied by means of first-principles DFT calculations. Both the superficial adsorptive and substitutional La-doped TiO2 anatase (101) surfaces are probably defective configurations in experiments, forming the pentacoordinated structures with surface O atoms. Whether the La doping can lead to red-shift the adsorption edge depends on the forms of La doping; that is, the adsorptive La implantation will raise a red-shift in the adsorption spectra of anatase, while the substitutional La doping could not red-shift the adsorption edge. However, for both the h-Cave-adsorbed and Ti5c-substituted surfaces, some partially occupied states locate in the gap, which may act as traps of the photoexcited electrons and thus reduce the photogenerated current. For the TiLaxO2−yNy and TiLaxO2−y−zNy systems, the preimplanted adsorptive La decreases the formation energy for the substitutional N implantation, while the pre-codoped adsorptive La and substitutional N increases the energy required for the formation of an oxygen vacancy. Besides, the compensation effects do not take place either between the adsorptive La and substitutional N or between the adsorptive La, substitutional N, and an oxygen vacancy, so some partially occupied states still lie in the gap. In the Ti1−xLaxO2−yNy system, the substitutional La implantation decreases the formation energy for the substitutional N doping, and the compensation effect takes place between the substitutional La and N, though the substitutional La and N have one less valence electron than the lattice Ti and O. However, in the Ti5c_O3c‑down case, the excitation energy from the higher occupied Ns−O π* impurity level to the CBM decreases by about 0.89 eV, which is too large to match with the experimental value (0.17−0.27 eV). When we consider the effects of an oxygen vacancy on the Ti5c_O3c‑down surface, we find that the substitutional La and N codoping not only facilitates the formation of an oxygen vacancy, but also makes the oxygen vacancy migrate from the Osb‑3c sites at the inner layer toward the surface O b sites. Moveover, in the Ti5c_O3c‑down_O3c‑down case, the two electrons on the double donor levels (bridging oxygen vacancy) passivate the same amount of holes on the acceptor levels (substitutional La and N), forming the acceptor−donor−acceptor compensation pair, which leads to form an occupied continuum of states and narrow the band gap by 0.17 eV, in line with the experimental value. Therefore, the acceptor−donor−acceptor compensation pair formed by the substitutional La, N, and a bridging oxygen vacancy is a more reasonable mechanism for the enhanced visible-light photocatalytic activity of La/N codoped TiO2 anatase. We hope that our calculations would provide useful information for the further design and construction of new effective visible-light photocatalysts.

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*Telephone: 86-531-88366330. Fax: 86-531-88365174. E-mail: [email protected]. 5889


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 91022034, 51172127, and 21173131), Excellent Youth Foundation of Shandong Scientific Committee (Grant No. JQ201015), and Youth Scientist (Doctoral) Foundation of Shandong Province of China (Grant No. BS2009CL038).

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