J. Phys. Chem. C 2008, 112, 7653–7664
7653
Nitrogen/Hydrogen Codoping of Anatase: A DFT Study Salvy P. Russo,† Ian E. Grey,*,‡ and Nicholas C. Wilson‡ Applied Physics, School of Applied Sciences, RMIT UniVersity, Melbourne 3001, Australia, and CSIRO Minerals, BayView AVenue, Clayton, Victoria 3169, Australia ReceiVed: NoVember 28, 2007; ReVised Manuscript ReceiVed: March 5, 2008
The stability of nitrogen incorporation at protonated titanium vacancy sites, VTi+nH, n ) 0-4, and interstitial sites in anatase under oxygen-rich growth conditions has been investigated using density functional theory quantum mechanical modeling. The 0 K DFT energy results were corrected using thermodynamic free energy data to obtain defect formation energies, Ed, at a typical sol-gel calcination temperature of 700 K. Doping of sol-gel anatase was simulated using a 2 × 2 × 1 defected anatase supercell as host, in which one Ti atom was replaced by 4 H atoms, giving [H4]Ti15O32. A number of different nitrogen configurations were found to be stable at VTi sites in the defected anatase, having negative or small positive Ed values at 700 K. These included nitrogen bonded to one, two and three framework oxygen atoms and NH bonded to two framework oxygens, HNO2. Maximum stability due to codoping with H was obtained with one H atom per VTi, although models with up to 3 codoped H atoms gave negative Ed values. An interesting observation was that extra stability was obtained in models where the structure relaxed to give bonding between the N at the VTi site and one of the surrounding Ti atoms, with a Ti-N distance of ∼2 Å. Electronic structure calculations showed that the 2p orbitals of N at a VTi site mix with both O 2p states at the top of the valence band and with Ti 3d states at the bottom of the conduction band. A small amount (∼0.1 eV) of band gap narrowing occurs for N doping at VTi. 1. Introduction Honda1
that TiO2 could be used The report by Fujishima and as the photoanode in a photoelectrochemical cell to split water into hydrogen and oxygen was made at about the time of the first oil crisis in the early 1970s. Their work raised great hopes of an inexpensive route to large-scale hydrogen production using solar energy, thereby reducing the world’s dependence on fossil fuels. However, despite intensive research over more than three decades, the hopes have not been realized. A key factor in limiting commercial development is the low efficiency of the photoelectrochemical route. The photoanode needs to be nontoxic, have a high stability against photocorrosion and be capable of efficient charge separation. Wide band gap oxides like TiO2 meet these requirements, but because of their high band gap energy they can only utilize a small fraction of the solar radiation at the ultraviolet end of the spectrum. Initial efforts to increase the solar acceptance of TiO2 focused on doping with different metals, particularly transition elements.2–6 However, although dopants like cobalt and chromium produce absorption in the visible region, the absorption is due to d electron transitions rather than excitation across the band gap. These localized states created within the band gap have a low associated absorption coefficient, and they can act as charge recombination centers. Thus not only is the extra photocurrent due to visible light absorption very small, but the high performance in the UV obtained for undoped samples can be compromised.7 Decreasing the band gap energy by metal atom doping to lower the energy of the conduction band has another disadvantage. For wide band gap semiconductors like TiO2, the electron * Corresponding author. † RMIT University. ‡ CSIRO Minerals.
potential at the conduction band minimum is very close to the redox potential required for hydrogen evolution. Any lowering of the conduction band will mean that an external bias has to be applied to drive the hydrogen evolution reaction, and this will decrease the efficiency. In contrast, the electron potential at the top of the valence band in oxide semiconductors is considerably more positive than that needed for water oxidation, even allowing for a high overpotential for oxygen evolution.8 There is thus considerable scope for improving the efficiency of photocatalysts like TiO2 by raising the valence band to narrow the band gap thereby increasing the solar acceptance. This approach requires the incorporation of nonmetals like N, S, C that have orbitals of appropriate energy to mix with the oxygen orbitals. Chandra Babu and Srivasta9 appear to be the first researchers to report experiments aimed specifically at raising the energy of the valence band in TiO2. In 1988 they oxidized single crystal TiS2 to give S-doped TiO2 and reported photocurrents eight times higher than obtained for undoped TiO2. Two years earlier Sato10 had noted that the enhanced spectral sensitization of heated titanium hydroxide was due to the incorporation of NOx from NH4OH used to precipitate the hydroxide. However, it was not until 2001 that a seminal paper by Asahi and co-workers11 provided a theoretical framework for understanding the role of nonmetal dopants in TiO2. The authors applied first principles quantum mechanical (QM) calculations to determine the electron density of states (DOS) for anatase-form TiO2 with substitutional replacement of oxygen by F, N, C, S and P as well as interstitial doping by N. Their results showed that substitutional N was the most effective dopant to achieve band gap narrowing, by the mixing of N p states with the O 2p states which raised the energy at the top of the valence band. The authors complemented their theoretical studies with the synthesis and characterization of N-doped TiO2. The yellow films and
10.1021/jp711282u CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
7654 J. Phys. Chem. C, Vol. 112, No. 20, 2008 powders thus obtained showed absorption and catalytic activity under visible light. From X-ray photoelectron spectroscopy (XPS) measurement of the N 1s binding energy, the authors concluded that nitride ion, as in TiN, with a binding energy of 396 eV, was responsible for the visible light photoactivity. XPS peaks observed at 400 and 402 eV were attributed to molecularly chemisorbed N2, consistent with the QM calculations for interstitial nitrogen. The great interest engendered by the Asahi et al.11 publication has led to dozens of publications on N-doped TiO2 (anatase and rutile forms) in recent years. The deeper understanding of the photocatalytic behavior that would be expected from such an intensive onslaught has been obscured by the often conflicting results and interpretations presented. This is due in part to the wide variety of procedures used for doping, including sol-gel methods, reactive sputtering, heating in an NH3 atmosphere, oxidation of TiN and ion implantation, which give rise to different forms of nitrogen incorporation. The problems associated with characterizing the bonding configuration and valence state of the nitrogen dopant have been reviewed recently by a number of researchers.12–15 The characterization ambiguities are compounded for high surface area samples prepared by sol-gel methods due to atmospheric contaminants interfering with the interpretation of near-surface probe data such as that obtained by XPS.14 These complications can be avoided by using a first-principles QM computational approach to calculate the most stable forms of nitrogen incorporation and the associated electron DOS. As mentioned above, this approach was used by Asahi et al.,11 but the authors did not present defect formation energies, Ed, which are needed to determine the relative stabilities of different models. Density functional calculations of Ed have subsequently been reported by Di Valentin et al.16–18 for N-doping of anatase for models involving substitution at an oxygen site (NS-O), interstitial nitrogen bonded to a single oxygen (Nint) and substitutional nitrogens coupled with an oxygen vacancy (2NS-O+VO). The authors presented their Ed results as a function of varying oxygen potential,18 showing that the Nint model was favored under oxidizing conditions whereas the other two models became more stable with increasing extent of reduction, with the substitutional model further stabilized by the presence of oxygen vacancies. The extension of these studies to N-doping at the anatase (101) surface gave broadly the same conclusions.19 The extra stability conferred by the coexistence of oxygen vacancies was confirmed by DFT studies undertaken by Nambu et al.20 These authors showed that oxygen vacancies stabilize the N dopant with respect to N2 gas formation. Yang et al.21,22 applied DFT calculations to nitrogen doping in both rutile21 and anatase22 at several concentrations of the dopant. In the case of anatase, they reported Ed values under both oxidizing and reducing conditions, giving agreement with Di Valentin et al.18 that the NS-O model is markedly more stable under strongly reducing conditions (TiO2 coexisting with metallic Ti). However, such conditions are not relevant to most doping experiments, especially sol-gel syntheses. An aspect of nitrogen doping that has received little attention is the possible stabilizing role of hydrogen. Okada et al.23 reported that coirradiation of TiO2 with H and N ions of 0.2 keV improved its visible-light photocatalytic activity. Diwald et al.12 increased the visible-light photoactivity of TiO2 (rutile) crystals by heating them in NH3, and interpreted the N 1s XPS peak at 399.6 eV as due to a form of N bound to H. They proposed that the enhanced photoactivity was due to a codoping effect between N and H. The only QM study on N/H codoping
Russo et al. is the recent work of Mi et al.24 in which NH substitution for oxygen in anatase was modeled. Another aspect that has been neglected in QM modeling of N doping of TiO2 is the possible role of titanium vacancies, VTi, as dopant sites. Such sites are stable native defects in both anatase25 and rutile26 forms of TiO2, and they occur in high concentrations in sol-gel prepared TiO2.27–29 We have recently shown29 from QM modeling studies that VTi sites are stabilized by the incorporation of protons, analogous to the Ruetschi defects in MnO2,30 and that B dopant atoms are stabilized at VTi by protons under oxygen-rich growth conditions.31 In this paper we extend these studies to consider models for the incorporation of N at VTi sites in the presence of different concentrations of protons. 2. Experimental Section 2.1. Computation Details. Quantum mechanical total energy calculations were performed using DFT within the framework of the VASP code,32 and the generalized-gradient approximation using the exchange-correlation functional of PerdewBurke-Ernzerhof.33 We also used the PAW pseudopotentials provided with VASP,34 with a planewave cutoff energy of 400 eV. k-space integrations were performed using a Monkhorst-Pack sampling scheme,35 with a 7 × 7 × 7 k-point mesh. One N atom and 0 to 4 H atoms were included per 2 × 2 × 1 anatase supercell. Geometry relaxations were performed using the RMM-DIIS scheme.36 A multistep relaxation procedure was used to progressively relax the dopant models. The relaxation alternated between atom relaxation of the dopant(s) and surrounding atoms over progressively larger regions, and cell parameter relaxation. The final step of the relaxation process allowed all atoms, the cell volume and cell shape to relax. Relaxations were also performed on the different phases and components involved in the chemical potential reservoirs for the dopant elements. The latter included crystalline TiO2 and Ti, and molecular O2, N2, H2, H2O and NH3. The defect formation energies, Ed, at 0 K and 0 bar pressure were then calculated from
Ed ) Edefect - Ehost + nTiµTi - nDµD
(1)
where Edefect and Ehost are the calculated total energies of the defect supercell and the host (TiO2) supercell, nTi and nD are the number of Ti atoms removed and the number of dopant atoms added and µTi and µD are the chemical potentials of Ti and the dopants in their respective reservoirs, based on calculated 0 K total energies of the reservoirs. All results are reported in relation to molar quantities. For the electronic density of states (EDOS) spectra we also used a Monkhorst-Pack k-point sampling of 7 × 7 × 7; however, we tested whether this mesh density was sufficient by also calculating selected runs using a 12 × 12 × 12 mesh and found no difference in the EDOS except for a slight difference in the tail at the conduction band maximum. Points in the EDOS spectrum were calculated at intervals of 33 meV. 2.2. Chemical Potential Reservoirs. Previous QM calculation for N-doped TiO2 have used nitrogen gas as the chemical reservoir for N with µN ) (1/2)µ(N2).18,19,21 However, nitrogen atoms are very strongly bound in N2 and high energy techniques such as reactive sputtering are required when using nitrogen gas as a dopant source. Using N2 as the N reservoir gives large positive calculated defect formation energies (>4 eV)18 which are not compatible with measured N-dopant concentrations of typically up to 2 wt % N.10,11,37,38 More commonly, NHx species, including ammonia gas, ammonium ion, amines and amides,
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are used as a nitrogen source for synthesis of N-doped TiO2. In QM modeling relevant to such experiments, NH3 is a more realistic chemical reservoir for nitrogen. We have used H2O as the reservoir for H atoms and TiO2 as the reservoir for Ti. Within the stability fields of the bulk reservoir phases the following chemical potential relations hold:
µTi + 2µO ) µTiO2
(2)
2µH + µO ) µH2O
(3)
µN + 3µH ) µNH3
(4)
In N-doping experiments involving heating TiO2 in NH3 or heat treating sol-gel products, the temperature is typically in the range 600 to 900 K. At such temperatures, and under the catalytic influence of the TiO2 surface, there is likely to be at least partial equilibrium between the TiO2 and the gaseous species. Consequently the chemical potentials given in eqs 2, 3 and 4 are interdependent. Our study will focus on the so-called oxygen-rich synthesis condition,39 where the chemical potential for oxygen is related to the chemical potential for gaseous O2 at 1 bar. Using this potential and the chemical potentials for the bulk species, the chemical potentials for Ti and for dopants H and N can be calculated using eqs 2–4. The defect formation energies calculated using eq 1 are for 0 K and 0 bar, constant volume conditions. In order for the results to be relevant to the doping conditions, they need to be corrected for the effect of temperature and for the relevant gaseous atmosphere. As we have previously described,31 and following Reuter and Scheffler,39 the chemical potentials can be corrected to temperature T and to a standard pressure p° of 1 bar, µ(T,p°), by adding the increase in the Gibb’s free energy per mole from 0 K to temperature T, ∆G(∆T,1) from tabulated thermodynamic data.40 In the case of gaseous components, the correction to the experimental partial pressure, p, is obtained using an additional term of the type RT ln(p/p°). For oxygen the complete expression for µO (T,p) is
µO(T,p) ) (1/2)µO2) (1/2)[EO2(0,1) + ∆GO2(∆T,1) +RT ln pO2] (5) where EO2 is the QM total energy for O2. In addition to correcting the chemical potential terms in eq 1 to the experimental temperature and gas pressures, the total energy terms Edefect and Ehost should strictly be converted to free energies, G(T,p), so that the resulting calculated defect formation energies are applicable to the experimental conditions. G(T,p) is related to the DFT total energy, E(0,0), by expression 6,
G(T, p) ) E(0, 0) + Fconf + Fvib + pV Fconf
Fvib
(6)
where and are the configurational and vibrational Helmholtz free energies at temperature T. A simple dimensional analysis analogous to that given by Reuter and Scheffler39 shows that the pV term will be of the order of tens of meV at the 1 bar total pressure used in this study, and so its contribution can be ignored. For the case of B-doped TiO2 we have recently reported the evaluation of Fvib in the harmonic approximation using ab initio phonon DOS calculations.31 It was found that Fconf and Fvib were of opposite sign and of similar magnitude for the defect model. The net contribution to G(T,p) and thus to Ed from the defect and host values of Fconf and Fvib was only 0.3 eV at 700 K.31 Based on these results it is likely that neglecting the configurational and vibrational free energy terms
will not change the relative stabilities of different models for N-doping. Thus in the presentation of the defect formation energies, Ed(T,p) will be calculated using expression 1, where the chemical potential terms are corrected for temperature and pressure but the Edefect - Ehost terms are from the 0 K total energy calculations. 3. Structural Models Anatase has tetragonal symmetry, space group I41/amd, with a ) 3.784 Å, c ) 9.514 Å.41 The structure is usually presented in projection down the short a/b axes. However this representation obscures the fact that the structure of anatase is based on closest-packed anion layers, a feature that it has in common with other TiO2 polymorphs including brookite, TiO2-II (high pressure form), and rutile.28 Anatase has cubic stacking of the anion layers, whereas rutile and TiO2-II have hexagonal stacking and brookite has mixed cubic/hexagonal stacking. Anatase, brookite and TiO2-II have identical zigzag chains of octahedra between pairs of closest-packed oxygen layers, while rutile has linear chains. The closest packed representation thus allows easy structural comparison between different polymorphs. The closest-packed anion layers in anatase are parallel to {112}anat. Because of the face-centered cubic symmetry of the anion packing, there are four such equivalent layers. A representation of the structure viewed normal to the closestpacked layers is shown in Figure 1(a). The zigzag chains of octahedra are separated by channels of empty octahedral sites (interstitial sites). One such interstitial site is shown by the dotted lines in Figure 1(a). A QM relaxation was conducted on a model with nitrogen located within an interstitial site, for comparison with previously published work.17,18 A feature of anatase prepared using sol-gel methods is the presence of relatively high concentrations of titanium vacancies, VTi, that are stabilized by the incorporation of protons.28,29 In sol-gel preparations involving ammonium ions, amines or amides, there is the possibility for incorporation of NHx or oxidized NOx at the VTi sites during the calcination step used to crystallize anatase. The six oxygen atoms associated with the VTi sites have their coordination with Ti reduced from 3 (in the bulk) to 2 and so their formal (Pauli) valence sum is only (2 × 4)/6 ) 1.33. These undersaturated anions represent likely sites for coordination of positively charged dopants. Whereas, in pure anatase, the oxygen atoms are all crystallographically equivalent, when a Ti atom is removed to create a VTi site the six oxygen atoms surrounding the site divide into two nonequivalent sets. Two oxygen atoms participate in corner sharing between two octahedra and have an approximately linear Ti-O-Ti configuration whereas the other 4 oxygen atoms participate in edge-sharing between two octahedra. Examples of oxygen atoms involved in corner-sharing and edge-sharing are labeled Oc and Oe respectively in Figure 1(b). We have conducted QM relaxations on models for incorporation of N bonded to either Oc or Oe, with starting N-O distances of 1.1 Å. In both cases the influence of protons on the defect stability was studied by varying the number of H atoms coordinating to the other Oc and Oe anions, from zero to four. We will first consider the calculation of defect formation energies for doping pure anatase, where the energy associated with removing a titanium atom to create a VTi site is taken into account. This will be followed by calculations relating to sol-gel anatase, where protonated VTi sites have already been created during the sol-gel process.
7656 J. Phys. Chem. C, Vol. 112, No. 20, 2008
Russo et al.
Figure 1. Models for N incorporation in anatase. The polyhedral models show two successive layers of octahedra viewed normal to the closest packed anion layers. (a) Interstitial model. An octahedral interstitial site is shown by the dashed lines. (b) Alternative locations for N, bonded to Oe or Oc at a VTi site. The VTi site is outlined by the dashed lines.
4. Results and Discussion 4.1. 0 K Relaxations with Pure Anatase as Host. The 0 K defect formation energy for the interstitial model for Nincorporation into pure anatase is reported in Table 1, together
with relaxed cell parameters and the N-O distance. The energy obtained using N2 gas as the reservoir for N is shown in parentheses, for comparison with previous studies.17,18 We obtain a dopant formation energy of +4.7 eV, compared with +4.2
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TABLE 1: Results from QM Relaxations at 0 K with Ti16O32 as Host for N/H Doping relaxed cell parameters model
Ed (eV)
a
b
7.575
Anatase 7.575
9.498
2.01(4.70)
7.544
Interstitial 7.584
9.623
#1 #2 #3 #4
0 1 2 3
H H H H
2.54 1.46 1.46 1.07
VTi Models with Monodentate NO 7.553 7.644 9.475 7.589 7.630 9.509 7.597 7.633 9.549 7.573 7.582 9.566
#5
3H
2.12
7.569
#6
4H
1.15
#7 #8
0H 1H
-0.33 -1.12
VTi Models with Bidentate NO2 7.574 7.579 9.566 7.564 7.575 9.556
#9
1H
-0.50
7.622
7.621
9.481
#10
1H
-0.76
7.543
7.582
9.613
#11
2H
0.13
7.574
7.572
9.491
#12
2H
0.19
7.619
7.598
9.566
#13
3H
1.24
7.604
7.620
9.561
#14
4H
3.26
7.670
7.633
9.502
#15
2H
0.09
VTi Models with Bidentate NO2 and NH 7.527 7.574 9.630
#16
3H
0.55
7.547
#17
0H
-0.19
7.695
9.613
VTi Model with Monodentate NO and NH 7.572 7.592 9.578
7.588
9.563
VTi Model with Tridentate NO3 7.563 7.621 9.462
eV reported.18 When NH3 is used as the reservoir, the calculated formation energy for interstitial N is considerably lower, 2.0 eV at 0 K. For the relaxed interstitial model the nitrogen is bonded to a single oxygen atom at a N-O distance of 1.34 Å, compared with 1.36 Å obtained by Di Valentin et al.17 As previously reported,17 the NO group is oriented perpendicular to the join between two Ti atoms, Ti(1) and Ti(2) in Figure 1, and interacts with them through π-bonding. The Ti-N and Ti-O distances are 2.11 (×2) Å and 2.01 (×2) Å respectively in the relaxed structure. For the models involving doping at a VTi site, the different coordination combinations of N + nH to the two Oc and four Oe anions of the VTi site, coupled with the range of possible orientations of the O-N and O-H bonds, allow for a large number of geometric configurations. The possibility of the relaxation terminating within a local (higher energy) minimum is high in these circumstances. A number of relaxations were repeated with different relative locations of the N and H atoms to try to reduce the chance of local minima occurring. In addition, some relaxed models were rerelaxed after removing one or two H atoms, to try to isolate the influence of H atoms on the energy from other influences. The 0 K defect formation energies, relaxed unit cell parameters and dopant bond distances and O-N-O angles for
bond lengths (Å), O-N-O angles (deg)
c
N-O 1.34 N-Oc 1.21 N-Oc 1.21, O-H 0.98 N-Oc 1.26, O-H 0.99, 1.00 N-Oe 1.26, N-Ti 2.19 O-H 1.00, 0.99, 1.03 N-Oc 1.29, O-H 0.99 (×2), 1.03 N-Oe 1.31, N-H 1.06, N-Ti 1.98 O-H 0.98, 1.000, 1.01 N-Oe 1.22, 1.23, Oe-N-Oe 126.2 N-Oe 1.25, 1.29, Oe-N-Oe 115.7 O-H 1.00, N-Ti 2.37 N-Oe 1.26, N-Oc 1.31, Oe-N-Oc O-H 0.99 N-Oe 1.25, 1.28, Oe-N-Oe 115.6 O-H 1.02 N-Oe 1.33, 1.33, Oe-N-Oe 115.9 O-H 1.00, 1.00 N-Oe 1.32, N-Oc 1.36, Oe-N-Oc O-H 0.99, 1.037 N-Oe 1.32, N-Oc 1.36, Oe-N-Oc O-H 1.00, 1.00, 1.01 N-Oe 1.32, N-Oc 1.38, Oe-N-Oc O-H 0.98, 0.98, 0.99, 1.01
108.7
107.0 105.9 105.9
N-Oe 1.29, 1.32, Oe-N-Oe 121.4 O-H 1.05, N-H 1.05 N-Oe 1.39, 1.42, Oe-N-Oe 114.1 O-H 0.99, 1.00, N-H 1.03 N-Oe 1.29, 1.33, N-Oc 1.38 Oe-N-Oe 119.1, 119.9, Oe-N-Oc 106.2
different models for N/H codoping at a VTi site are reported in Table 1. The relaxations were conducted in triclinic superlattices (no assumed symmetry), and the unit cell angles deviated from 90° in some runs. Generally the deviations were small (
