20948
J. Phys. Chem. B 2005, 109, 20948-20952
New Insights into the Origin of Visible Light Photocatalytic Activity of Nitrogen-Doped and Oxygen-Deficient Anatase TiO2 Zheshuai Lin,†,‡ Alexander Orlov,§ Richard M. Lambert,§ and Michael C. Payne*,† Department of Physics, CaVendish Laboratory, Cambridge UniVersity, Madingley Road, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Cambridge UniVersity, Lensfield Road, Cambridge, CB2 1EW, United Kingdom, and Beijing Center for Crystal R&D, Technical Institute of Physics and Chemistry, CAS, Beijing 100080, P. R. China
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ReceiVed: June 29, 2005; In Final Form: August 23, 2005
The spin-polarized plane-wave pseudopotential method, based on density-functional theory, has been used to calculate the electronic band structures and the optical absorption spectra of nitrogen-doped and oxygendeficient anatase TiO2. The calculated results are in good agreement with our experimental measurements. These ab initio calculations reveal that the optical absorption of nitrogen-doped TiO2 in the visible light region is primarily located between 400 and 500 nm, while that of oxygen-deficient TiO2 is mainly above 500 nm. These results have important implications for the understanding and further development of photocatalytic materials that are active under visible light.
I. Introduction With the rapid development of semiconductor-based photocatalysis, studies of surface processes on titanium dioxide have significant value. Despite the numerous papers published since the discovery of photoinduced decomposition of water on TiO2 electrodes in 1972,1 many questions remain. TiO2 is nontoxic, relatively inexpensive, and an extremely promising photocatalyst for environmental applications. However, the photocatalytic efficiency of TiO2 under visible light is very low due to its large band gap of 3.0-3.2 eV. Many attempts have been made to improve the photocatalytic performance of TiO2 under visible light irradiation, such as transition metal doping2-4 or surface modifications,5-7 but few have proved successful. Recently, it was found that doping of TiO2 with nonmetallic elements results in an improvement in photocatalytic activity in the visible light region.8-13 In particular, nitrogen doping into substitutional sites of anatase TiO2 to form p-type photocatalysts (TiO2-xNx) produced materials which are active for methylene blue and 2-propanol decomposition under visible light.10,14,15 Various mechanisms have been proposed to account for the visible light activity of N-doped TiO2. Sato synthesized NOxdoped TiO2 and suggested that the visible light photocatalytic sensitization of the resulting product was due to NOx impurities.8 However, Noda et al., who also synthesized nitrogen-doped TiO2 catalysts, concluded that the visible light activity was due to oxygen vacancies.9 More recently, other groups have produced visible light active TiO2 catalysts using a variety of procedures;16-19 they concluded that visible light photoactivity was due to oxygen vacancies which give rise to donor states located below the conduction bands, while substitutional nitrogen acted as an inhibitor for electron-hole recombination. On the other hand, Asahi et al. took the view that visible light sensitivity of nitrogen-doped TiO2 was due to substitutional nitrogen atoms.10 * To whom correspondence should be addressed. E-mail address:
[email protected]. † Department of Physics, Cambridge University. ‡ Technical Institute of Physics and Chemistry. § Department of Chemistry, Cambridge University.
On the basis of spin-restricted local density approximation (LDA) calculations, they proposed that N 2p acceptor states contribute to the band gap narrowing by mixing with O 2p states. By means of spin-polarized GGA calculations Valentin et al.20 investigated the N-doping of both rutile and anatase forms of TiO2: they found that nitrogen causes a significant change in the absorption spectra of the TiO2 and showed that the N 2p orbitals are localized above the top of the O 2p valence bands, even for relatively large values of doping (∼9.3 atom %)20 in broad agreement with experimental studies.14, 21-23 From the viewpoint of photocatalysis, it is the anatase form of TiO2 that is most important.24-26 Accordingly, to evaluate the separate contributions of oxygen vacancies and nitrogen doping in anatase, we present the results of spin-polarized density functional-theory (DFT) calculations that have been used to calculate the electronic band structures and optical absorption spectra that arise for a range of concentrations of (i) substitutional nitrogen and (ii) oxygen vacancies in anatase TiO2. Our results show that absorption below 500 nm is mainly due to nitrogen states located above the valence bands, whereas absorption above 500 nm is mainly caused by oxygen vacancies. These findings compare favorably with our own experimental data and enable conclusions to be drawn about the nature of the practical catalyst. II. Calculation and Experimental Methods A. Calculation Method. The plane-wave pseudopotential method27,28 has been used to optimize crystal geometries, to obtain the corresponding electronic band structures, and to simulate the optical absorption spectra. In these calculations, the energy cutoff was chosen as 500 eV. Ultrasoft pseudopotentials29 were used with the 1s, 2s, and 2p electrons for Ti, and the 1s electrons for oxygen and nitrogen treated as core electrons. The generalized gradient approximation (GGA) with the PBE exchange correlation functional30 was adopted. Nitrogen doping or the presence of oxygen vacancies was modeled by replacing or removing one oxygen atom in anatase TiO2 cells, thus forming the TiO2-xNx or TiO2-x compounds.
10.1021/jp053547e CCC: $30.25 © 2005 American Chemical Society Published on Web 10/15/2005
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J. Phys. Chem. B, Vol. 109, No. 44, 2005 20949
We considered 24, 48, and 96 atom supercells, with corresponding x values of 0.125, 0.062, and 0.031, respectively. The Monkhorst-Pack k-point sampling31 is set as 3 × 5 × 2, 3 × 3 × 2, and 1 × 2 × 1 in the respective supercells. The initial geometry configurations were optimized by the Broyden, Fletcher, Goldfarb, and Shannon minimizer32 for spin polarized systems with different initial numbers of spin-up and spin-down electrons. The spin occupation numbers were then optimized during the electronic iterations. The energy change, maximum force, maximum stress, and maximum displacement tolerances in the optimization are set as 10-5 eV/atom, 0.03 eV/Å, 0.05 GPa, and 0.001 Å, respectively. After obtaining the optimal geometry, the electronic band structures and the densities of state (DOS) of N-doped and oxygen deficient TiO2 were calculated. On the basis of these detailed electronic structures, the transition rates between occupied and unoccupied states caused by the interaction with photons were determined. The imaginary part of the dielectric constant 2 can be described as a joint density of states between the valence and conduction bands, weighted by the appropriate matrix elements:33
2(pω) )
2e2π Ω0
|〈ψck|uˆ ‚r|ψVk〉|2δ(Eck - Eck - pω) ∑ k,V,c
Figure 1. Electronic band structure along high-symmetry directions in stoichiometric TiO2 crystal.
(1)
where Ω is the volume of the elementary cell, V and c represent the valence and conduction bands, respectively, k represents the k point, ω is the frequency of the incident light, and uˆ is the vector defining the polarization of the electric field of the incident light, which is averaged over all spatial directions in the polycrystalline case. The absorption curves can be obtained easily from the imaginary part of the dielectric constant. It is well-known that the band gap calculated by DFT is smaller than that obtained from experiments. This error is due to the discontinuity of exchange-correlation energy. Therefore, a scissors operator34,35 was introduced to shift all the conduction levels to agree with the measured value of the band gap. B. Experimental Methods. Following a procedure described by Sakthivel et al.,22 nitrogen-doped titania samples were prepared by hydrolysis of titanium tetrachloride with ammonium bicarbonate followed by calcinations in air at 400 °C for 4 h, to produce a nominal doping level of about 3.5% N. Powder XRD patterns were obtained on a Philips PW1710 instrument employing Cu KR radiation. XPS measurements were performed in a VG ADES 400 electron spectrometer system operated at a base pressure of 1 × 10-10 Torr. XP spectra were obtained by using Mg KR radiation and a fixed analyzer pass energy of 50 eV. Diffuse reflectance UV-visible spectra were recorded on a Lambda 12 Perkin-Elmer UV/vis spectrometer, using a Labsphere RSA-PE-20 diffuse reflectance and transmittance accessory. III. Results and Discussion A. Stoichiometric TiO2. For the optimized stoichiometric anatase TiO2, the lattice parameters were a ) 3.7845 Å and c ) 9.7153 Å, and the internal parameter was u ) 0.2059 (u ) dap/c, where dap is the apical Ti-O bond length). These parameters were in good agreement with experimental values36 as well as with the spin-restricted full-potential linearized augmented plane wave (FLAPW)37 and orthogonalized linear combination of atomic orbitals (OLCAO)38 calculations. Mulliken population analysis39 revealed that the electronic charges on Ti and O ions were +1.33 and -0.66, respectively, implying strong covalent bonding between Ti and O in the TiO2 crystal.
Figure 2. Partial density of states plots calculated for different levels of N doping in the TiO2 crystal. The PDOS projected on Ti, O, and N atoms are represented by solid, dash, and dash-dot lines, respectively. The straight dash lines indicate valence band maximum (VBM), which is located at the top of O 2p valence states.
The electronic band structures of stoichiometric anatase TiO2 are plotted in Figure 1. Our calculations showed that anatase TiO2 is an indirect gap crystal. This is different from the FLAPW calculation,37 but is consistent with the OLCAO calculations38 and experimental measurements.40 Moreover, the direct gap at G is 0.41 eV larger than the indirect band gap. The calculated band gap is 2.14 eV, which is significantly smaller than that determined experimentally (3.2 eV),40 which is typical in DFT. The Partial Densities of States (PDOS) are shown in Figure 2a. The Ti 3s and 3p states are located at much lower energies at about -56 and -33 eV (not shown), respectively, and do not participate in the bonding between Ti and O. The peak in the PDOS located at -17 eV belongs to the 2s orbitals of the O atoms. The upper valence bands show strong hybridization (σ bonding) between O 2p and Ti 3d orbitals with a bandwidth of about 5.0 eV. It is important to note that the top of the valence bands is dominated
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Lin et al.
Figure 3. PDOS for spin-up (sv) and spin-down (sV) electrons in the nitrogen atom (x ) 0.0625). N 2s and 2p levels are shown with dash and solid lines, respectively. The straight dash lines indicate the top of O 2p valence levels.
by the O 2p orbitals. On the other hand, the conduction bands contain significant contribution from the Ti 3d orbitals. The bands are split into two parts, t2g (4 eV) orbitals, due to crystal field effects. There is some contribution from the O 2p states to the conduction bands in the form of antibonding interactions with the Ti 3d bands. The conduction bands above 8 eV, which do not have a direct impact on the optical properties of TiO2 in the visible light region, have mainly Ti 4s and 4p character. In summary, the calculated electronic structures described in this work are consistent with the results from other theoretical methods.37,38 On the basis of the electronic band structure and eq 1, the optical absorption spectra of pure polycrystalline anatase TiO2 (x ) 0) between 300 and 700 nm were calculated. The results are shown in Figure 4a. For these calculations, the scissors operator applied was 1.06 eV, accounting for the difference between the experimental band gap (3.20 eV) and the calculated band gap (2.14 eV). This curve for pure anatase TiO2 will be used as a benchmark for comparing the results from N-doped and oxygen-deficient TiO2 calculations. B. Nitrogen-Doped Anatase TiO2. To obtain the correct optimal geometry and electronic band structures of N-doped TiO2, we performed spin-polarized calculations with one more spin-up electron than spin-down (as the initial spin number). This approach is based on the fact that an electron is effectively removed and a hole formed when a nitrogen atom replaces an oxygen atom in the unit cell. The spin-polarized total energy was indeed about 0.2 eV lower than that of a spin-restricted calculation for the same configuration. Mulliken population analysis shows that, independent of its concentration in the crystal, the charge on the N ion was about -0.58, which was close to the charge on the original oxygen atom in this site (-0.66). This means that the N impurity acts as a deep electron trap in the material: the hole is bound to the N ion. Parts b, c, and d of Figure 2 show the PDOS for various levels of N doping in TiO2. It is clear from these plots that the nitrogen-substitutional doping does not significantly affect the energy levels of the oxygen and titanium states as compared to stoichiometric TiO2. The N 2s states are located at about -12 eV and the N 2p orbitals make small contributions to the valence and conduction bands. It is important to note the existence of the N 2p orbitals in the band gap. These form the acceptor states and their strength decreases with decreasing nitrogen concentration. Furthermore, the acceptor states have a contribution from the spin-down electrons on nitrogen atoms, as illustrated by the spin PDOS of nitrogen atoms in 6.2% N-doped TiO2 (Figure
Figure 4. (a) The optical absorption curves calculated for various N concentrations in the polycrystalline TiO2: (I) undoped TiO2; (II) 12.5% nitrogen doped; (III) 6.2% nitrogen doped; and (IV) 3.1% nitrogen doped. (b) Experimental diffuse reflectance UV-visible spectra of (I) undoped anatase sample and (II) 3.4 atom % nitrogen-doped TiO2 samples.
3). Our calculations reveal that even for high nitrogen concentration (x ) 0.125), the N 2p states are still localized, lying slightly above the top of the O 2p valence band. This conclusion is in contrast to the spin-restricted FLAPW calculations10 but is consistent with spin-polarized calculations20 and several experimental results.14,21-23 We also found that in the case of at least 20% nitrogen-doped TiO2 the N 2p states would mix with the O 2p valence band, facilitating the transfer of photoexcited carriers to reactive sites at the catalyst surface within their lifetime.10 In practice, however, very high nitrogen doping might result in the formation of TiN,15 which is metallic and would not be transparent in the visible region. Another possible scenario at high levels of nitrogen doping is the introduction of a significant number of defects, which would serve as recombination centers for holes and electrons and would therefore decrease a quantum yield of the photocatalytic reactions.14 Therefore, low concentrations of nitrogen doping are likely to be of more significant practical interest than high levels of N-doping. Optical absorption spectra were calculated for the various levels of nitrogen doping and the results are shown in Figure 4a. The same scissors operator correction of 1.06 eV was applied as in the case of TiO2. This choice is justified by the fact that screening properties of TiO2-xNx and TiO2 are very similar.10 For the relatively high nitrogen concentration (12.5%), the large contribution to the electron transitions from the acceptor states results in optical absorption in the entire visible light region. The optical absorbance curves for the 6.2% and 3.1% nitrogendoped TiO2 are very similar except for the shoulder in the region
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J. Phys. Chem. B, Vol. 109, No. 44, 2005 20951
Figure 6. Calculated optical absorption spectra of polycrystalline TiO2 with different O vacancy contents: (I) updoped TiO2; (II) 12.5% oxygen vacancies; (III) 6.2% oxygen vacancies; and (IV) 3.1% oxygen vacancies. Figure 5. Partial density of states plots with the concentration of oxygen vacancies in the TiO2 crystal. The PDOS projected on O and Ti atoms are shown with solid and dash lines, respectively. The straight dash lines indicate VBM.
between 400 and 550 nm, with the shoulder for the 6.2% doped sample being significantly more pronounced than that for the 3.1% doped sample. Therefore, we expect that for even lower nitrogen concentration the optical absorbance spectra in N-doped TiO2 in the visible region would have an almost identical shape: the optical absorption would mainly occur in the region of 400-500 nm with absorbance decreasing with decreased nitrogen concentration. C. Oxygen-Deficient TiO2. For the calculations of oxygendeficient TiO2, two more spin-up electrons than spin-down are chosen as the initial spin state. Mulliken population analysis showed that the excess electrons were redistributed among the nearest neighbor Ti atoms around the oxygen vacancy site. Moreover, the total energy difference between spin-polarized and spin-restricted calculations in oxygen-deficient anatase TiO2 was smaller than that in reduced rutile.41,42 This can be explained by the weaker electronic repulsion in anatase compared to rutile since the volume of the anatase cell is about 9% larger than that of rutile. Figure 5 shows the theoretical PDOS calculated for oxygendeficient anatase. The donor states are mainly formed from Ti 3d orbitals, which pull the valence band maximum (VBM) closer to the conduction states. In contrast to the case of the reduced rutile phase, the donor states in oxygen-deficient anatase are comprised of almost equal numbers of spin-up and spin-down electrons. The donor states are about 0.15-0.30 eV below the conduction band edge, the value correlating with the concentration of oxygen vacancies. Published experimental results indicate that oxygen vacancy states are located 0.75 eV below the conduction band edge.43 We have chosen appropriate scissors operators (described above) to calculate the polycrystalline optical absorbance spectra of oxygen-deficient TiO2. The results are shown in Figure 6. The relatively large number of 3d Ti donor states that are present in the TiO2 with 12.5% oxygen vacancies results in strong optical absorption across the entire visible light region. The TiO2 with the lower oxygen vacancy concentrations (6.2% and 3.1%) do not show significant absorption in the region between 400 and 500 nm. D. Implications for Visible Light Active Photocatalysts. The results described above have significant implications for an understanding of photocatalytic behavior. A key factor that
impedes interpretation of the experimental data has been confusion about the roles, if any, played by interstitial N and oxygen vacancies, since most preparation methods can introduce both types of defect. Even experiments with well-defined model systems do not eliminate these difficulties, due to differences in depth distribution of the two species.44 Our theoretical results are therefore of substantial value: they clearly show that oxygen vacancies and nitrogen doping contribute to different regions of the absorbance spectrum, thus providing important guidance for the synthesis of visible light photoactive materials. A comparison of the calculated spectra with our experimentally observed absorption spectra for pure anatase and 3.4% N-doped anatase is instructive (Figure 4a,b). The structure and chemical composition of these samples were determined by XRD and XPS (see the Supporting Information), respectively, and the corresponding absorption spectra measured by UVvis spectroscopy are shown in Figure 4b. There is good agreement between the observed spectrum (3.4% N) and the calculated absorption spectrum for 3.1% N. Moreover, comparison with Figure 4a indicates that our N-doped sample does not contain a significant concentration of oxygen vacancies: there is no red shift of the band edge and negligible absorption at >600 nm. Interestingly, we have recently demonstrated that N-doped materials characterized by spectra of this type are very efficient visible light photocatalysts for the destruction of the important pollutant MTBE.45 It is noteworthy that our calculated spectrum for 3.1% N is also in good agreement with other experimental results obtained for nitrogen concentrations of 2.3%15 and 0.50%.14 (In passing, we note that the measured spectra for doped and undoped samples exhibit a minor difference in the band edge at ∼400 nm (Figure 4b). In part, this might be due to a difference in the particle size of the undoped and doped materials (1 and
