First-Principles Calculation of Synergistic (N, P)-Codoping Effects on

Jun 21, 2010 - The SEC Strategic Research Cluster and the Centre for Synthesis and Chemical Biology, Conway Institute of Biomolecular and Biomedical R...
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J. Phys. Chem. C 2010, 114, 11984–11990

First-Principles Calculation of Synergistic (N, P)-Codoping Effects on the Visible-Light Photocatalytic Activity of Anatase TiO2 Run Long and Niall J. English* The SEC Strategic Research Cluster and the Centre for Synthesis and Chemical Biology, Conway Institute of Biomolecular and Biomedical Research, School of Chemical and Bioprocess Engineering, UniVersity College Dublin, Belfield, Dublin 4, Ireland ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: May 28, 2010

The energetic and electronic properties of N- and/or P-doped bulk and anatase TiO2 (101) surfaces have been calculated based on first-principles density functional theory. For the bulk system, (N, P)-codoping of anatase TiO2 does not narrow the band gap much more than that of single N-doped anatase TiO2 at low concentration. An increasing P/N concentration ratio leads to more significant band-gap narrowing. For (N, P)-codoped surface systems, the band gap is narrowed slightly when both N and P act as substitutional dopants. However, upon N and P adsorption on the surface, the band gap narrows significantly even at low dopant concentrations. The calculated energy results support the viewpoint that incorporation of P into N-doped bulk TiO2 cannot promote further N introduction, whereas N and P can be doped on the pristine anatase (101) surface more easily vis-a`-vis single N- or P-monodoping. These results provide a reasonable explanation for recent experimental observations of different photocatalytic efficiencies in (N, P)-codoped and N- or P-monodoped anatase TiO2. 1. Introduction Titania has received intense attention as a promising material in the photocatalytic and photoelectrochemical field for many years.1,2 However, as a wide-band-gap semiconductor (3.20 eV for anatase), anatase TiO2 can absorb only ultraviolet irradiation, which amounts to ∼5% of solar energy. Further, its photoexcited electron-hole pairs tend to recombine relatively easily. To extend the optical absorption of TiO2-based materials to the visible-light region and to obtain the maximum amount of the energy from the solar spectrum, doping with metals and nonmetals3-10 has been used widely. Recently, codoping has attracted more and more attention, such as both theoretical work on (N, H)-, (N, W)-, (N + TM)-, (TM ) V, Cr, Nb, Mo)-, and (C + TM)-codoped TiO211-13 and experimental work on (Cr, Sb)-codoped TiO2,14 showing better photocatalytic efficiency with respect to single impurity doping. N-doped TiO2 is considered to be one of the most effective photocatalysts, and it has been investigated widely, both experimentally and theoretically.7 Recently, P-doped TiO2 has also attracted much more intense scrutiny.9,10 Lin et al. reported that P-cation-doped anatase TiO2 nanoparticles exhibit high photocatalytic activity with respect to pure samples under visible light;9 the impurity was shown to be in a pentavalent oxidation state (P5+), according to XPS, and in the form of Ti-O-P, as identified by FT-IR. Lin et al. claimed that higher photocatalytic activity in P-doped TiO2 originated from the large surface area and the crystallinity, rather than impurity energy levels in the band gap. Subsequently, Lin et al. prepared (N, P)-codoped TiO2, which was found to show better photocatalytic activity than that of single N- or P-doped TiO2 under visible-light irradiation10 with different P/N concentration ratios. However, there is no reported theoretical work focusing on the synergistic effects of (N, P) codoping on the mechanism of band-gap * To whom correspondence should be addressed. E-mail: niall.english@ ucd.ie.

narrowing, to the best of our knowledge. Given the widespread interest in N and P doping, it would be highly desirable to elucidate the microscopic mechanisms of N codoping with P, by either experimental or theoretical means. It is hoped that this knowledge would aid the further design and construction of new effective visible-light photocatalysts. The present study focuses on the energetic and electronic structure of the N- and/or P-doped anatase TiO2 using density functional theory (DFT) calculations. This paper attempts to elucidate the origin of the synergistic effects of (N, P) codoping on the mechanism of band-gap reduction of TiO2. We considered N- and/or P-doped bulk and (101) surface systems of anatase. The calculated results show that (N, P)-doped bulk TiO2 does not exhibit an obvious reduction in the band gap at low dopant concentrations. However, an increase in the P/N concentration ratio will narrow the band gap significantly. On the other hand, (N, P)-codoping at the anatase (101) surface displays synergistic effects when one of either N or P is present as a substitutional dopant and the other acts as an adsorptive dopant. The dopants’ formation and adsorption energies are also discussed. To best of our knowledge, this is the first theoretical explanation to rationalize the gap narrowing mechanism and the substitutional and adsorptive roles of (N, P) doping in bulk- and surface-state anatase. 2. Computational Methods and Models All of the spin-polarized calculations were performed using the projector augmented wave (PAW) pseudopotentials as implemented in the VASP code.15,16 The Perdew-BurkeErnzerhof parametrization17 of the generalized gradient approximation18 was adopted for the exchange-correlation potential. The electron wave function was expanded in plane waves up to a cutoff energy of 400 eV, and Monkhorst-Pack k-point meshes19 of 2 × 2 × 2 and 2 × 2 × 1 were used for the bulk and surface systems, respectively. Geometry optimization and electronic property cal-

10.1021/jp100802r  2010 American Chemical Society Published on Web 06/21/2010

(N, P)-Codoping Effects on Anatase TiO2

J. Phys. Chem. C, Vol. 114, No. 27, 2010 11985 of P following N incorporation into the O site or on-top adsorption of N following P incorporation into the Ti site. To compare the relative stability of the P-doped systems, the formation energies Ef of the substitutionally doped systems (either for O or for Ti) were estimated according to

Ef ) E(doped) - E(pure) - µN - µP + µO + µTi

(1) for interstitial N- and P-doped systems Figure 1. Top view of the anatase (101) surface. The light spheres and the red spheres represent the Ti and O atoms, respectively. The labeling of the different O and Ti atomic positions is shown.

culations were carried out, using the block Davidson scheme20,21 for geometry optimization. The cell and atomic relaxations were carried out until the residual forces were below 0.01 eV/Å. The electronic structure calculations were conducted using the GGA + U method22 in conjunction with the GGA geometry, which can lead to a good description for the TiO2, as shown in previous work on N/Ta-codoped and Cr-doped TiO2.23 The DFT + U approach introduces an on-site correction in order to describe systems with localized d and f electrons, which can produce better band gaps in comparison with experimental results. Here, effective on-site Coulombic interactions U (U ) U′ - J) for Ti 3d were used to obtain the correct band gap. U′ and J represent the energy cost of adding an extra electron at a particular site and the screened exchange energy, respectively. A series of U and J values up to U ) 6.3 eV (U ) 4.5 5.0, 5.5, and 6.3 eV) and J ) 1 eV (0, 0.5, and 1.0 eV) were used. It was found that the band gap was 3.14 eV for a bulk system with U ) 6.3 eV and J ) 1.0 eV and was only weakly dependent on the various J values. This agrees well with the experimental value of 3.2 eV, somewhat larger than the U value used in previous work.24 The optimized lattice parameters were a ) 3.800 Å and c ) 9.483 Å, in good agreement with experimental and other theoretical results.25,26 Both of these results indicate that our computational approach is reasonable. The bulk doped systems were constructed from the relaxed (2 × 2 × 2) 96-atom anatase TiO2 supercell. Although experiment confirmed that N and P atoms are incorporated as anions and cations, replacing O and Ti ions, respectively,10 a variety of positions of N, P atoms in the TiO2 lattice were considered, such as substitutional N at the O site (N@O), interstitial N (Nin), substitutional P at the Ti site (P@Ti), O site (P@O), and interstitial P (Pin). For codoped systems, P locates either at Ti or at O and N locates either at O or at interstitial sites, such as NP@OTi, NinP@O, and NinP@Ti. Finally, we find that an O atom replacement by an N atom and a Ti atom replacement by a P atom (NP@OTi) was energetically favorable. In particular, the formation of a N-P atomic pair in (N, P)-codoped TiO2 is energetically favorable by 0.36 eV visa`-vis other substitutional N-P configurations. This is supported by XRD and XPS measurements of the formation of O-N-P linkages in the experiment of Lin et al.10 The anatase (3 × 2) (101) surface was modeled by a periodically replicated slab containing 126 atoms. We used slabs of four TiO2 layers (12 atomic layers) separated by a vacuum of 10 Å. The top view of this system is shown in Figure 1. The depth of the supercell was 21.4 Å. The atoms in the bottom layer were fixed to their bulk positions during geometry optimization in order to simulate the presence of the bulk underneath. Adsorptive doping was created by on-top adsorption

Ef ) E(doped) - E(pure) - µN(µP)

(2)

for substitutional N- and interstitial P-codoped systems

Ef ) E(doped) - E(pure) - µN - µP + µTi

(3)

The following was used to calculate the adsorption energy Ead for adsorptive doping on the surface

Ead ) E(doped) - E(pure) - µN + µO - µP

(4)

Ead ) E(doped) - E(pure) - µP + µTi - µN

(5)

or

in which E(doped) is the total energy of the bulk and surface supercell containing the P impurity, E(pure) denotes the total energy of pure bulk and surface systems, respectively, whereas µN, µP, µO, and µTi represent the chemical potentials of the N, P, O, and Ti atoms, respectively. Here, the adsorption energy is defined in the same way as the formation energy. The larger the adsorption energy, the more difficult adsorptive doping is at the surface. It is expected that thermodynamic solubility is directly related to the value of the formation or adsorption energy. It should be noted that the formation energy depends on growth conditions, which may be either O- or Ti-rich.25 µO and µTi obey the relationship µTi + µO ) µ(TiO2). Under Tirich growth conditions, µTi is assumed to be the energy of one metal ) and µO was calculated by the atom in bulk Ti (µTi ) µTi above formula. Under O-rich growth conditions, µO was estimated by consideration of the O2 molecule (i.e., µO ) µ(O2)/ 2) and the chemical potential of Ti was taken again as that of one atom in bulk Ti. The chemical potential µN was calculated by consideration of one N2 molecule (i.e., µN ) µ(N2)/2) and the chemical potential µP was calculated from the formula µP ) 1/4[(P4O10) - 5µ(O2)]. 3. Results and Discussion N- and/or P-Doped Bulk Anatase TiO2. In this section, we investigated a variety of possible configurations of N, P locating at different positions in the TiO2 lattice. Our first main aim is to establish whether addition of P enhances the ease of N incorporation into bulk TiO2 relative to the incorporation of a single N impurity. Our second main concern is to investigate whether addition of P into N-doped TiO2 can narrow further the band gap vis-a`-vis single N-doped TiO2. The calculated formation energies are summarized in Table 1. This suggests that (1) N prefers to occupy the O site under Ti-rich growth conditions or locate at an interstitial site, (2) P substitutes for the Ti atom preferentially under O-rich conditions, (3) both N

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TABLE 1: Formation Energy Ef (eV) for N-, P-, and (N, P)-Doped Bulk TiO2 N@O Nin P@Ti P@O Pin NP@OTi NinP@O NinP@Ti

Ti-rich

O-rich

0.69 5.04 11.82 11.86 10.04 10.84 14.06 17.53

5.69 5.04 1.82 16.86 10.04 5.84 19.06 7.53

and P at the substitutional sites are energetically favorable for other configurations even if N locates at the interstitial site and P substitutes for either O or Ti sites, and (4) the incorporation of P does not promote the incorporation of N into bulk TiO2 under both O- and Ti-rich growth conditions. However, N and P both locate at the substitutional sites possessing the lowest formation energy of 5.84 eV among codoped systems under O-rich growth conditions. At the same time, O-rich growth conditions are closest to the experimental environment in conjunction with the sol-gel method for the synthesis of titania. Hence, we will select this configuration to discuss its electronic structure with (N, P) codoping compared to N or P monodoping. Furthermore, in the NP@OTi case, the electrons on the donor levels passivate the same amount of holes in the acceptor levels, so the system still retains its semiconductor character. To check the energy term results, a larger 108-atom supercell was used to calculate the formation energies for N@O and P@Ti, and the values were 5.67 eV (0.67) and 1.80 eV (11.80) under O-rich (Ti-rich) growth conditions; these differences are only 0.02 eV from the 96-atom supercell. This suggests that our energy results are reliable. The calculated band gap of undoped TiO2 was 3.14 eV, as shown in Figure 2a, which agrees with the experimental value of 3.20 eV due to partial correction by DFT+U of the “bandgap problem”. The calculated band structures for monodoping and codoping cases are displayed in Figure 2b-f. The dashed lines represent the Fermi level EF. Here, we have aligned the valence band maximum of the defective cell with the valence band maximum27 of the pure TiO2 bulk state. For N-doped TiO2 (N@O), the isolated state is located above the valence band maximum (VBM) and the resultant band gap is 3.0 eV, as shown in Figure 2b, which is 0.14 eV smaller than that of undoped TiO2 (3.14 eV). For P-doped TiO2 (P@Ti), several impurity states appear in the forbidden gap, one located above the VBM and other located below the conduction band minimum (CBM), as shown in Figure 2c. It is expected that these states are from O 2p orbitals due to lattice distortion, whereas Ti 3d orbitals (Ti3+) form due to Ti4+ transfer to Ti3+ through gaining one more electron from the P dopant. In this case, the host band gap expanded slightly to 3.24 eV. However, the photon transition energy should be around 2.82 eV measured from the states above the VBM to the state above EF. For (N, P)-codoped TiO2, namely, NP@OTi, no impurity states located below the CBM and one more impurity energy level (N orbital) appeared above the VBM compared with the P-doped case. Here, the N dopant should trap one excess electron donated from P so as to cancel the Ti3+ orbitals,28 as displayed in Figure 2d. The host band gap is about 3.10 eV and narrowed only by 0.04 eV vis-a`-vis the undoped case, while 0.10 eV is expanded compared with the N-doped case. However, the band gap should be much smaller than N-doped TiO2 if measured from the occupied states above the VBM to CBM, which can possibly explain why (N,

Long and English P)-codoped TiO2 has better photocatalytic activity than N-doped TiO2 reported in experiment.10 To examine the influence of different P/N concentration ratios on the (N, P)-codoped TiO2 electronic structure, we created two other configurations with one and two more P atoms than the NP@OTi case (namely, NP2@OTi2 and NP3@OTi3), despite higher formation energies, so that we obtain P/N ratios of 1, 0.5, and 0.33. For the NP2@OTi2 case, continuum-like impurity bands above the host VBM and an isolated energy level located above the continuum-like band are formed, along with Ti3+ orbitals below CBM, as shown in Figure 2e. The photon transition energy should be about 2.4 eV from the isolated energy level to the lowest unoccupied molecular orbital (LUMO). For the NP3@OTi3 case, the feature of the band structure is different with the NP2@OTi2 case, as shown in Figure 2f. In particular, the photon transition energy from the highest occupied molecular orbital (HOMO) to the LUMO is about 1.1 eV and from the states above the host VBM to LUMO is about 2.3 eV. Obviously, additional P concentration in N-doped TiO2 would modify the band gap significantly with a certain high P/N ratio. This conclusion is consistent with the experiment reported by Lin et al.10 On the other hand, one should note that photocatalytic efficiency is a complex phenomenon, and experimental results are influenced by many factors, such as, inter alia, the crystalline size, diffusion time scales of photoexcited electron transfer from bulk to surface, and by electron-hole recombination rates. This implies that band-gap narrowing alone is not a sufficient factor to lead to high photocatalytic efficiency. As reported by experiment,10 studies of (N, P)-codoped TiO2 with either large (PNT004) or low (PNT005) amounts of P were both found to exhibit lower visible-light photocatalytic efficiency than the P-monodoped case (PT001). It is quite possible that the influence of other factors, such as those mentioned above, counteract band-gap narrowing. To further understand the origin of the band-gap narrowing due to N and/or P (co)doping, the density of states (DOS) and projected density of states (PDOS) are plotted in Figure 3. For comparison, the DOS and PDOS of undoped TiO2 are also shown in Figure 3a,a′. It is readily apparent that a conduction band arises mainly from Ti 3d states and that the O 2p states dominate the valence band. For N-doped TiO2 (cf. Figure 3b,b′), the isolated unoccupied N 2p states above the VBM can be seen clearly. This serves to extend the adsorption edge to the visible-light region. However, on the other hand, N-doping may also act as electron traps so as to depress the photocatalytic activity. The electronic structure description for N-doped systems can be found in refs 12 and 13. For P-doped TiO2 (cf. Figure 3c,c′), the impurity states lying above the VBM do not originate from the P 3s and 3p orbitals but from the O 2p orbitals due to lattice distortion. At the same time, the P-induced Ti3+ 3d orbitals are composed of states below the host CBM, meaning that electrons can be excited between these impurity states as well as the host band. For (N, P)-codoped TiO2, namely, NP@OTi (cf. Figure 3d,d′), the PDOS shows that N 2p states locate above the host VBM as well as mixing well with O 2p orbitals. At the same time, Ti 3d states dominate the host CBM while leaving P 3s and P 3p orbitals residing deeper in the conduction band. For the NP2@OTi2 case, the conduction band tail originates mostly from the Ti 3d orbital but not from P 3s and P 3p states, as shown in Figure 3e,e′. The gap narrowing is very large with respect to the pure case, as measured from occupied N 2p and Ti 3d states to unoccupied Ti 3d orbitals. For the NP3@OTi3 case (cf. Figure 3f,f′), the N 2p orbital hybridizes well with the O 2p orbital to form a continuum band.

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Figure 2. Band structures for (a) pure TiO2, (b) N@O, (c) P@Ti, (d) NP@OTi, (e) NP2@OTi2, and (f) NP3@OTi3. The top of the valence band of pure anatase is taken as the reference level. The blue dashed lines represent the Fermi level EF.

Figure 3. (A) DOS and (B) PDOS for (a) pure TiO2, (b) N@O, (c) P@Ti, (d) NP@OTi, (e) NP2@OTi2, and (f) NP3@OTi3. The top of the valence band of pure anatase is taken as the reference level.

For both NP2@OTi2 and NP3@OTi3 cases, it is possible that the introduction of more P atoms into N-doped TiO2 leads to larger lattice distortion and induces strong hybridization between N 2p and O 2p states, thereby forming a continuum of O 2p-N 2p states. On the other hand, the conduction band minimum also originates from Ti 3d states, whereas P 3s and P 3p orbitals lie in the deeper energy region. It is evident that an isolated occupied Ti 3d level is located in the forbidden gap because strong lattice distortions cause the O 2p-N 2p-Ti 3d hybrid state to become more significant, which induce these states to reside in the middle of the gap region. It is expected that these states are responsible for the larger gap narrowing than those of lower P/N-concentration ratio codoping.

Substitutional (N, P)-Codoped Anatase (101) Surface. Besides the N/P-codoped bulk system, we will discuss the effect of N and/or P doping on surface electronic properties. In this section, we have two main goals. The first is to investigate further whether the addition of P into the surface promotes N incorporation or not. The second is to investigate whether codoping of N with P would serve to enhance band-gap narrowing or not. Therefore, we have considered various N and P doping motifs at anatase (101) surface configurations, namely, substitution and adsorption. Substitution was limited to the top layer of the supercell based on our calculated energy results and reported experimental facts with the formation of O-N-P linkages.10 For example, the lowest formation energy of a (N,

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Long and English

TABLE 2: Formation Energy Ef (eV) for N-, P-, and (N, P)-Doped Anatase (101) Surfaces O2c O3c-down O3c-up Ti5c Ti6c O2cTi5c O2cTi6c O3c-downTi5c O3c-downTi6c O3c-upTi5c O3c-upTi6c

Ti-rich

O-rich

0.98 0.88 0.83 11.93 12.88 10.66 12.66 10.07 10.28 11.63 10.42

5.98 5.88 5.83 1.93 2.88 5.66 7.66 5.07 5.28 6.63 5.42

P) substitutional surface configuration is about 0.2 eV lower than the most stable structure when both N and P located at the subsurface host atomic sites. Furthermore, the formation energies are always high if both N and P locate in the interstitial site. There are three different O atom sites on the surface. For instance, the “O2c” position is a site that bridges two Ti atoms on the surface; “O3c-up” is an outward-relaxed 3-fold surface site, whereas “O3c-down” is relaxed inward. There are two different kinds of Ti atom sites at the surface, that is, 5- and 6-fold, denoted by Ti5c and Ti6c, respectively. Here, the N atom was substituted for the O and the P atom was substituted for the Ti atom. The calculated formation energies are summarized in Table 2. These results suggest that (1) the possibility of N occupying the O3c-down or O3c-up site is nearly the same due to similar formation energies, (2) P favors location at the Ti5c site, and (3) the formation energy of the most stable (N, P)-codoped configurations, “O3c-upTi5c”, is smaller than the equivalent for single N doping, indicating that the addition of P at N-doped TiO2 surfaces will promote N incorporation further. Although the formation energy of the O3c-upTi5c case, 5.07 eV, is slightly smaller than that of the NP@OTi case, 5.84 eV, the large 126atom supercell for the surface was used with respect to the bulk 96-atom supercell. Therefore, P and N codoping at the pristine surface of anatase is not more energetically feasible than that in the bulk system. Here, a larger 162-atom anatase (101) slab

was used to check the energy results. The calculated formation energies were 6.05 (1.05) eV and 1.99 (11.99) eV under O-rich (T-rich) growth conditions: this constitutes a small difference 0.17 eV vis-a`-vis the 126-atom slab. This suggests that our energy results are reasonable. To investigate the effect of (N, P)-codoping on the electronic structures of the anatase (101) surface, the calculated band structures for the most stable configurations, O3c-up, O3c-down, Ti5c, and O3c-upTi5c for N-, P-, and (N, P)-doped systems, respectively, are plotted in Figure 4. The calculated band gap is about 2.84 eV using the GGA+U method for the pure anatase (101) surface (cf. Figure 4a). For the N-doped TiO2 O3c-up and O3c-down cases, the impurity states locate just above the host VBM and lead to band gaps of 2.66 and 2.74 eV, respectively. The gap narrowing is only about 0.18 and 0.1 eV vis-a`-vis the undoped anatase (101) surface, respectively, as shown in Figure 4b,c. For the P-doped case Ti5c, the feature is similar with P-doped bulk TiO2 P@Ti (cf. Figure 2c). Here, the P-induced unoccupied Ti3+ states locate essentially in the middle of the forbidden gap, which can serve either to benefit electron transfer or to act as electron traps. The band gap is about 2.82 eV, giving rise to only a 0.02 eV narrowing compared with the undoped surface, as displayed in Figure 4d. For the (N, P)-codoped O3c-upTi5c case, N traps one more electron from P and forms a continuum band above the VBM. The photon transition energy measured from this state to the CBM is 2.79 eV. The gap narrowing is also only 0.05 eV, as shown in Figure 4e. To further explore the origin of gap narrowing owing to N and/or P doping, the DOS and PDOS have been plotted in Figure 5. Due to the low N concentration, the DOS and PDOS for N-doping (cf. Figure 5b,b′,c,c′) show that the unoccupied N 2p acceptor levels move to a lower-energy re´gime. For the Ti5c system (cf. Figure 5d,d′),the DOS and PDOS show that P impurity levels do not contribute to the VBM and CBM, but P induced one transition of Ti4+ to Ti3+ so that Ti 3d orbitals reside in the middle of the forbidden gap. For the O3c-upTi5c case, the N 2p-O 2p hybridized continuum band is responsible for the gap narrowing, as displayed in Figure 5e. However, the gap narrowing is relatively small in this case. It appears that

Figure 4. Band structures for the (a) pure anatase (101) surface and (b) O3c-up, (c) O3c-down, (d) Ti5c, and (e) O3c-downTi5c configurations. The top of the valence band of pure anatase is taken as the reference level. The blue dashed lines represent the Fermi level EF.

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Figure 5. (A) DOS and (B) PDOS for the (a) pure anatase (101) surface and (b) O3c-up, (c) O3c-down, (d) Ti5c, and (e) O3c-downTi5c configurations. The top of the valence band of the pure anatase (101) surface is taken as the reference level.

Figure 6. Geometrical structures for 10 adsorption configurations. The light spheres and the red spheres represent the Ti and O atoms, respectively. The blue sphere is the N atom, whereas the pink one denotes the P atom.

(N, P) codoping cannot effectively narrow the band gap of the anatase (101) surface with respect to N or P monodoping at this P/N concentration ratio when both N and P are in the presence of substitutional dopants. Adsorptive (N, P)-Doped Anatase (101) Surface. To find other possible reasons to rationalize the observed high photocatalytic activity of (N, P)-codoped TiO2, we have considered adsorptive codoping at the surface. Here, we also considered a series of configurations with either N or P locating at the subsurface or N or P adsorption at surface O or Ti atomic sites, respectively. The lowest calculated formation energies were 0.44 eV (NO83PTi42, P adsorption) and 0.66 eV (NO84PTi41, N adsorption) higher than the most stable configurations for N and P adsorptive codoping at the surface, respectively. Ten different adsorptive surface doping configurations were considered: (1) the P atom binds directly to the N atom, while the N atom locates at the surface O atom site, (2) the P atom binds to the surface O atom, while the N atom locates at the surface O atom site, (3) the N atom binds to the P atom while the P atom locates at the surface Ti atom site, and (4) the N atom binds to the O atom while the P atom locates at the surface Ti atom site. The geometrical structures are shown in Figure 6. First, “NO 2cPN”, “NO 3c-downPN”, “NO 3c-upPN”, “NO 2cPO”, NO 3c-downPO”, and “NO 3c-upPO” have the same atomic composition (NO83PTi42), so we selected the “NO 3c-downPN” configuration,

TABLE 3: Adsorption Energy Ead (eV) for (N, P)-Doped Anatase (101) Surfaces NO 3c-downPN PTi 5cNO

Ti-rich

O-rich

7.82 15.28

12.82 5.28

which has the lowest total energy (lower than others by 0.7-2.5 eV). On the other hand, as “PTi 5cNP”, “PTi 6cNP”, “PTi 5cNO”, and “PTi 6cNO” has the same atomic composition (NO84PTi41), PTi 5cNO has the lowest total energy (lower than others above 1 eV) to calculate the electronic structure. The adsorption energies of “NO 3c-downPN” and “PTi 5cNO” are summarized in Table 3, which shows that PTi5cNO has a formation energy of 5.28 eV under O-rich growth conditions as well; it is also the sol-gel method to synthesize TiO2. Here, we only investigate its electronic properties. The calculated band structures of the PTi 5cNO case are plotted in Figure 7a, which shows that there are some occupied and unoccupied impurity states located in the forbidden gap that contribute significantly to electron transfer as well as photocatalytic activity. The gap narrowing is about 1.5 eV, which is similar to the value calculated from the experimental adsorption edge (>600 nm).10 To examine the origin of the impurity states and gap narrowing, the corresponding DOS and PDOS are shown in Figure 7b,c. Electrons can transfer between occupied 2p donor states and unoccupied N

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Long and English while the other is an adsorptive dopant, then band-gap narrowing may be induced significantly. Acknowledgment. This work was supported by the Foundation of Irish Research Council for Science, Engineering and Technology (IRCSET), and the support of Science Foundation Ireland and the Irish Centre for High End Computation in the provision of computational resources is acknowledged. References and Notes

Figure 7. (a) Band structures, (b) DOS, and (c) PDOS for the PTi 5cNO configuration. The top of the valence band of pure anatase is taken as the reference level. The blue dashed lines represent the Fermi level EF.

2p states, which is responsible for the large band-gap narrowing. Therefore, this indicates that adsorption of N/P can also lead to large gap narrowing even at this low N/P concentration. Comparing N- and/or P-doped bulk- and (101) surface-state anatase, these observations indicate that P/N adsorption at the surface can narrow the band gap more significantly than N and P monodoping in the bulk and at the surface at low dopant concentrations. For bulk systems, an increasing P/N concentration ratio can enhance the extent of band gap narrowing. 4. Conclusions We have calculated the electronic properties of N- and/or P-doped bulk anatase TiO2 and the anatase (101) surface by means of density functional theory calculations. The calculated energy results indicate that incorporation of P into bulk and surface titania does not promote further incorporation of N. The electronic structures show that codoping of N with P into bulk titania cannot help to narrow the band gap with respect to the single N- or P-doped titania. However, increases in the P/N concentration ratio will lead to large gap narrowing. On the other hand, if either N or P is present at a substitutional site

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