Atomic N Modified Rutile TiO2(110) Surface Layer with Significant

Publication Date (Web): December 26, 2013. Copyright © 2013 American ... *S. J. Wang: telephone, (+65)-68748184; e-mail, [email protected]...
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Atomic N Modified Rutile TiO2(110) Surface Layer with Significant Visible Light Photoactivity Junguang Tao,*,† M. Yang,‡ J. W. Chai,† J. S. Pan,† Y. P. Feng,‡ and S. J. Wang*,† †

Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

‡

S Supporting Information *

ABSTRACT: It has been recently emphasized that the surface of a photocatalyst plays crucial roles on its photocatalytic performance. By nitridizing the top layer of the rutile TiO2(110) surface using reactive atomic N flux, we report direct experimental evidence of significantly enhanced photocatalytic activity under both ultraviolet (UV) and visible light irradiations. The visible light activity of a nitridized surface is found to be comparable to the UV light activity of the pristine surface. On the basis of X-ray photoemission spectroscopy (XPS) measurements and densityfunctional theory (DFT) calculations, top surface N doping efficiently narrows the local band gap, ∼2.0 eV, which accounts for the visible light activity. Under visible light excitation, nearly all free charges contribute to the photocatalytic reactions. The improvement of photocatalytic activity is attributed to the N 2p add-on shoulder at the valence band maximum (VBM) as well as the strong exchange-splitting at the surface that settles the N 2pz states inside the conduction band of the TiO2 matrix, which avails efficient charge transfer.

1. INTRODUCTION Owing to its chemical stability, nontoxicity, and low cost, TiO2 has been recognized as an attractive photocatalyst for air and water purification, self-cleaning/sterilizing, and solar energy conversion.1−3 However, because of its intrinsic large band gap (>3.0 eV), the photoactivity of TiO2 is limited to ultraviolet (UV) light and thus is very inefficient for solar light harvesting. In recent decades, great efforts have been devoted to enhance its visible light absorption as well as photocatalytic performance.4−7 As a simple approach, doping TiO2 with impurity ions has been early proposed and investigated,4,6,8,9 including both the cation and anion doping. For instance, Asahi et al.6 first reported that N-doped TiO2 (N-TiO2) leads to enhancement of optical absorption as well as visible light activity up to 550 nm. These were attributed to the mixing of N 2p and O 2p states at the top of the valence band and prolonged lifetime of photoexcited carriers. This finding sparked a huge hope for band gap narrowing by doping with N. However, different results are reported in the subsequent studies, with some support band gap narrowing,10−12 whereas some others indicate negligible band gap narrowing at low doping levels and the visible light activity was ascribed to the localized N 2p states within the band gap.4,13,14 In the N-TiO2 samples, the Nderived states act as stepping sites to facilitate excitation of electron to conduction band under irradiation of visible light. But localized midgap states decrease the oxidation power of photogenerated holes in comparison with that of pristine TiO2.15,16 At early stages, great efforts have been devoted to the preparation of N-TiO2 powders and films by using various synthesis routes, such as magnetron sputtering,17 high energies ion bombardments,16 the sol−gel approach,18 etc. Although the © XXXX American Chemical Society

samples exhibit some visible light activity, their photon performance under UV irradiation is generally decreased due to enhanced charge recombination sites.15,18 The recombination sites are strongly correlated to the structural defects generated during synthesizing processes. In other words, the photocatalytic performance actually gets worse regardless of the increased visible light absorption. By far, it is still challenging to establish unambiguous cause-and-effect relationships for samples with heterogeneous distribution of doping species, multiple orientations of the crystal structure, and defective assembles of nanostructures. To go beyond these impasses, a structurally perfect N-TiO2 with precisely controlled dopant location is desirable, which can retain the quantum yield and be beneficial to establish the cause-and-effect relationship of the charge transport behavior. Recently, considerable efforts have been devoted to the surface (including surface defects) engineering5,19,20 and the successes opened a new avenue in the quest of visible light photocatalysis. In contrast, the majority of early strategies are to incorporate N into the bulk materials.4,6,18 Although there are a few studies on surface doping,16,21−23 the N was incorporated using high energy N2+ ions that bombard the surface structure22 and the photocatalytic performance were not tested. On the other hand, various nitrogen-containing molecules such as N2,6 NH3,4,24 N2H4,24 NH4Cl,24 HNO3,25 NH4OH,26 etc. have been used as chemical nitridation agents; atomic N has never been used by other groups to the best of our knowledge. The success Received: September 2, 2013 Revised: December 25, 2013

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of rutile TiO2(110) surface is about 1.4 eV, which are in good agreement with previous studies.32

of this approach to nitridize the TiO2 surface has been recently demonstrated by our group.27 In view of the advantages of this method, such as the high reactivity of the atomic N source, precisely engineering the surface layers only, and thermal reaction with mild structure destruction and thus negligible structure defects, it is of importance and urgent to prepare excellent visible light active N doped rutile TiO2(110) via this method. In the present research, the photoactivity of atomic N modified rutile TiO2(110) surface has been examined. Its high visible light activity was demonstrated as comparable to UV light activity. In contrast to previous studies that the gains in visible light activity come at the expense of UV activity,4,28 we showed that this atomic N modified rutile TiO2 surface exhibits enhanced UV light activity as well.

3. RESULTS AND DISCUSSION We employed X-ray photoelectron spectroscopy (XPS) to access the charge state and surface N concentration. After nitridation at 400 °C using an atomic N source, a well-defined N 1s peak is observed (Figure 1a). The sharp N 1s peak with

2. EXPERIMENTAL AND THEORETICAL METHODS Atomic nitrogen flux was generated with an Oxford Applied Research atomic nitrogen source (HD25) mounted in the sample preparation chamber. Before loading into the chamber, the TiO2 substrates were cleaned by ultrasonication in acetone, isopropyl alcohol (IPA), and deionized (DI) water in sequence. To avoid the effect of additional bulk defects, the substrates were only heated to ∼400 °C in a vacuum chamber before nitridizing. During the nitridizing process, the samples were kept at an elevated temperature of ∼400 °C unless specified otherwise. The nitrogen partial pressure is 4 × 10−5 mbar, and the plasma power is 450 W. The N doping concentration is controlled by nitridizing time, for example, the 7% sample is treated for 45 min. Double-sided polished rutile TiO2(110) samples were supplied from CrysTec. The sample size is 10 mm × 10 mm × 0.5 mm. After nitrogen doping, the samples were transferred in situ to the XPS analysis chamber where high resolution core level and valence band spectra were recorded for electronic structure analysis. The nitridized samples remain the same color as the pristine ones. The XPS measurements were performed using VG ESCALAB 220i-XL instrument equipped with a monochromatic Al Kα (1486.7 eV) X-ray source. All spectra were recorded in the constant pass energy mode with pass energy of 20 eV and step width of 0.1 eV. The calculations were carried out by using density-functional theory (DFT) based Vienna ab initio simulation package (VASP) with the Perdew−Burke−Ernzerhof (PBE) approximation for the exchange−correlation functional29,30 and the frozen-core all-electron projector-augmented wave (PAW)31 method for electron-ion interaction. The cutoff energy for the expansion of plane-wave basis was set to 500 eV. The effective on-site Coulomb correction (U − J = 5.0 eV) has been applied to d orbital electrons in Ti atoms. The first Brillouin zone was sampled by Monkhorst−Pack method generated k-point meshes, in which 8 × 8 × 12, 8 × 4 × 1, and 2 × 2 × 1 were used for bulk rutile TiO2, the rutile TiO2(110) surface, and the N-doped rutile TiO2(110) surface, respectively. The most stable rutile TiO2(110) surface is oxygen terminated, and the thickness is five layers (each layer contains three atomic layers with a central TiO layer and two bridging O layers). To avoid artificial dipole potential, the top and bottom surface layers are identical. A vacuum layer of ∼15 Å was applied along the TiO2(110) surfaces to minimize the coulomb interaction between two neighbor surfaces. The electronic convergence was set to 10−6 eV, and the force on each atom for all structures was optimized smaller than 0.01 eV/Å. On the basis of these parameters, the lattice constant of rutile TiO2 bulk is calculated to be 4.67 and 2.97 Å for a and c, respectively, and the band gap

Figure 1. XPS spectra of pristine (black open squares) and N-TiO2 (red open circles) for (a) N 1s, (b) Ti 2p, (c) O 1s, and (d) valence band (VB). The blue line in (a) is the Shirley background, and the red one is the fitting curve. In (b), the fittings for Ti peaks attributed to Ti−O6 and Ti−O5N are shown in the inset. In (d), the change of the VBM is emphasized in the inset.

the full-width-at-half-maximum (fwhm) of 1.02 eV, as compared to 0.96 eV for O 1s in a pristine rutile TiO2(110) single crystal surface, indicates a single chemical environment for the N dopant. The binding energy (BE) is found to be 396.6 eV, which corresponds to the N−Ti−O bond and suggests substitutional replacement of O by N (Ns) in the TiO2 host lattice.4,15,33,34 There is no feature at ∼400 eV (Ti−O−N bond), thus the existence of interstitial N species (Ni) and titanium oxynitrides can be ruled out.9,35 As seen in Figure 1b for the Ti 2p spectra, a lower BE shoulder appears at ΔBE = 0.95 eV after nitridation. This feature originates from those Ti atoms having mixing bounds with O and N ions in the octahedrons rather than Ti3+ induced by surface oxygen vacancies (VO’s) or Ti interstitials (Tiint’s) that would appear at ΔBE = ∼2.0 eV.21,36,37 This assignment is also supported by the absence of band gap states (BGS) in valence band spectra (Figure 1c,d) and serves as further evidence for substitutional doping. Our observation is in contrast to previous results that suggest the incorporation of N favors the formation of VO’s.17,25 This contradiction is strongly correlated to the sample synthetic routes. First, on single crystalline rutile TiO2(110) surface, VO’s only form at high temperature, >∼580 °C,4 which is much higher than our sample preparation temperature. Second, the atomic N flux has a very low kinetic energy; therefore, the formation of VO’s lacks of dynamics drive. On the basis of integrated peak intensities of N 1s and O 1s and the probing depth of XPS, the surface nitrogen concentration is estimated to be 7% (Supporting Information). We also notice a small B

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other methods and the 11% doped sample is still more reactive than the pristine TiO2(110) surface (Supporting Information). It will be evidenced below that high local doping concentration is desirable for the improved photocatalytic activity under both UV and visible light irradiation. Nevertheless, these results ensure the beneficial effect of our strategy. The visible light photoactivity was examined under illumination with wavelength >413 nm. As shown in Table 1, the visible light activity for the 7% doped sample, N(7), is nearly equivalent to that of pristine TiO2 under UV light irradiation. As pointed out before,40 selfdoping by defects that produces Ti3+ species in TiO2 may also lead to visible light absorption. To assess this effect that may affect our judgment, a moderate reduced TiO2 surface (r-TiO2) was prepared by high temperature vacuum annealing. However, under our experimental conditions, this r-TiO2 exhibits negligible visible light activity, which emphasizes the roles of N introduced states for the visible light activity. Although the 3d states of Ti3+ below conduction band (CB) contribute visible light absorption, they act as recombination centers leading to the decrease in photocatalytic activity. In comparison, a sample was prepared via room temperature (RT) reaction. As shown in Figure 3a by XPS measurement,

(∼0.1 eV) BE shift to lower values for both O 1s and Ti 2p, which could be due to p-type doping of N that lowers the Fermi level (EF), which is also reported before.4 Interestingly, the valence band is broadened and new states associated with N 2p located on top of O 2p lift the valence band maximum (VBM) edge by ∼1.0 eV toward EF (Figure 1d). This suggests a reduced surface band gap of 2.0 eV. Considering the crosssection of XPS as compared to that of ultraviolet photoemission spectroscopy (UPS), the N 2p states observed here is more pronounced than other surface N doping methods.21 As reported before, the substitutional N species serve as contributors to enhanced visible light absorption and photocatalytic activity.6,17,38 The photocatalytic activities of our Nrutile TiO2(110) surfaces are examined by monitoring the degradation of methyl orange (MO) and the results are summarized in Table 1. As an example, the evolution of MO Table 1. Summary of Photoactivity for the As-Received Pristine TiO2(110) Surface (a-TiO2) and N-Doped TiO2 [N(x), Where x Represents the Surface N Percentage] and the N(7) Sample with Additional TiO2 Capping Layers [NT(y nm), Where y Indicates the Thickness of the TiO2 Capping Layer] photoactivity (×10−3 min−1·W−1·cm2) samples

UV + vis

vis light

pristine TiO2 N(7) NT(4 nm) NT(8 nm) NT(16 nm)

4.50 17.00 18.00 14.50 11.06

0.00 4.03 3.20 3.65 0.00

absorption spectra as a function of irradiation time for N(7) sample is given in Figure 2a and the comparison of N-treated TiO2 with pristine TiO2 is plotted in Figure 2b. In contrast to previous results where a loss of UV activity was found for NTiO2 due to the rapid electron and hole recombination rate introduced by the impurity level,4,6,15,18,36 all our N-TiO2 surfaces exhibit enhanced UV photoactivity, with the maximum improvement of 3.8 times at the surface N concentration of 7% (Table 1 and Figure 2). In fact, we also observed a decrease of photoactivity at higher N concentration (∼11%), which is attributed to increased surface trapped electron by additionally induced VO’s or Tiint’s at higher doping level.39 However, it needs to be emphasized that the optimal doping concentration of 7% obtained here is already at higher level achievable by

Figure 3. (a) N 1s XPS spectra of N doping at RT (red curve) and 400 °C (blue curve). (b) Ti 2p for untreated surface (black curve) and for doping at RT (red curve) and 400 °C (blue curve). Valence band spectra and the zoom-in view near VBM are given in (c) and (d).

Figure 2. Photodegradation of methyl orange: spectral evolution for N(7) (a) and the peak intensity as a function of irradiation time for selected samples (b). C

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down N 2p orbitals are well separated. This splitting can be better seen in the band structure plot (lower panel of Figure 5).

the N 1s peak is observed at 400.1 eV and is broader than higher temperature samples, indicating different and multiple chemical environments. As reported before, this peak position has been attributed to Ni or molecularly absorbed N.9 Theses results indicate substitutional N doping is an endothermic process, which agrees with previous DFT calculations.13 In Figure 3b, the Ti 2p spectrum remains identical to that of the untreated surface, indicating weak bonding of N with Ti. Therefore, the N atoms are suggested to mainly bond to surface O atoms. In terms of the valence electronic structure, RT surface treatment expands the VBM to even lower BE side as shown in Figure 3c,d. However, the examination of visible light photocatalytic activity for this sample exhibits negligible effect. Therefore, we conclude that substitutional doping that incorporates the N atoms into TiO2 lattice sites is essential for visible light photoactivity, in which the charge transport and separation is availed via Ti−N bond in a structure perfection lattice. On the other hand, the stability of the samples is also tested. The photoactivity of the samples remains the same after kept in air for three months. To understand the origin of enhanced photoactivity for this N doped rutile TiO2(110) surface, we calculated the related density of states (DOS) and energy band structure using density-functional theory (DFT). On the basis of our experimental observations, we focused our calculations on the surface band gap modification and the states that N dopants introduced. We studied surface substitutional N doping cases in which one or two bridging O atoms on the first surface layer were substituted by N atoms for 1N (3.1%) and 2N (6.3%) doping cases, respectively, and additional in-plane O atom was substituted by N for 3N (9.4%) doping case. With incorporation of N atoms into TiO2, new electronic states are formed that significantly contribute to the formation of add-on shoulder on the edge of VBM and the midgap energy levels, as seen in Figure 4a. The contribution of N 2p to these states increases with an increase in N concentration. In all doping cases, the spin-up N 2p states are strongly overlapped with O 2p and they are located at VBM. On the contrary, the spin-

Figure 5. Spin polarized band structure for 1N-TiO2 (a), 2N-TiO2 (b), and 3N-TiO2 (c), respectively. The upper panels are for spin-up bands; the lower ones, for spin-down. The insets are zoom-in views of N bands near the EF.

This splitting has been observed previously.41 As shown in Figure 4b for the N projected DOS (PDOS) in the case of 3N doping, the N 2px orbitals are located just on top of VBM, whereas the interaction between N and Ti atoms pushes the N 2py to ∼0.20 eV below the EF. However, both the N 2px and N 2py states lie below the EF. The experimentally observed upshift of VBM and surface band gap narrowing are mainly attributed to the hybridization of N 2px states and O 2p at VBM. On the other hand, the 2py states may introduce some localized in-gap states. Moreover, the strong exchange-splitting pushes the spindown N 2pz states into the CB of TiO2. This has not been reported in previous N doping studies.11,14,41 However, we notice that this electronic feature only occurs on the top surface N dopants, whereas for the in-plane N dopant both the spin-up and spin-down orbitals are confined at the edge of the VBM (Figure S3 in Supporting Information). This solves the puzzles between our findings and earlier reports. In their studies,11,41 the N’s are introduced into bulk positions. Even for the surface modeling case,14 the detailed difference between the top layer and in-plane doping was not examined. On the other hand, our results provide new insights on the sensitive relationship of the electronic structures with the dopant position and affirm the importance of the surface layer modification that received increasing attentions recently.5,19,20 The strong upward shift of 2pz orbitals may be related to the broken symmetry at the surface along z direction. This energy level feature will prompt efficient charge excitation on the same atoms and therefore leads to higher quantum yield. Moreover, with increasing N concentration, the states on top of VBM gradually becomes broader (Figure 4a), which renders local band formation at higher doping level as supported by the appearance of dispersion in the band structure (Figure 5). Existing studies suggest formation of VO’s upon N doping.11,14 However, the current work is simplified without forming VO’s on the basis of the facts that (1) our experimental methods disfavor the formation of VO’s as stated above and (2) the formation of VO’s is favored in terms of system energy but it has little effect on N

Figure 4. (a) DOS evolution with increasing N doping concentration. (b) Projected DOS of the N atom for a 3N-doped rutile TiO2(110) surface (3N-TiO2). (c) 2-dimensional (2D) charge density of 3NTiO2. The vertical dashed lines in (a) and (b) represent the position of EF. D

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electronic states,4 with the latter being the main focus of the current study. Contrary to DOS plots, electron density plots give a clearer picture of the electron distributions. At lower N concentration, only localized states are observed. When the doping concentration is increased, the coupling between the dopants and surrounding Ti atoms becomes stronger, which results in more delocalized electronic states. As shown in Figure 4c for the highest doping case, the Ti−N bond is much more covalent than the Ti−O bond. As a result, the electrons around the dopant spread further away to its neighboring Ti atoms and exhibit delocalization nature, thus favoring the charge separation. Usually, high doping will generate pronounced structure distortion that degrades the electron−hole separation. However, as suggested by the simulation results, the substitutional replacement of surface bridging O’s can remain structure perfection even at the 9.4% doping level, which is achievable by our method. The samples therefore combine beneficial effects of both extraordinary electronic structure at high doping level and superior charge separation efficiency with structure perfection. The high activity for surface modified samples implies the importance of surface charging trapping sites as compared to the bulk sites.19 Although the surface N doping increases the photoactivity dramatically, it does not show noticeable change of optical absorption (Figure S4, Supporting Information). This is not surprising because only a small portion of the surface has been modified, which is not detectable by the UV−vis spectrometer. This, however, signifies the importance of this strategy. In the N-TiO2 surface layer, the photogenerated electron can be captured by the π-conjugated structure of N via a percolation mechanism. Hence, the N sites act as the electron acceptors in the TiO2, effectively suppressing the charge recombination and leaving more holes at Ti sites to form reactive species that promote the degradation of dyes. Therefore, enhanced photocatalytic activity is observed. On the other hand, based on the results presented in Table 1, a conclusion can be drawn that the photocatalytic activity is wavelength dependent. To interpret this, a schematic drawing of the energy diagram is shown in Figure 6. For the sample under UV irradiation, all the excitations of (1)−(4) will be facilitated, whereas only (2)−(4) are possible under visible light. Therefore, from the data in Table 1, the visible light activities for (2)−(4) are ∼2.85 times lower than that under UV light. Therefore, the wavelength dependence of the external reaction efficiency (η) was found to be η(vis)/η(UV) = 0.35,

which is in good agreement with the value of 0.3 for the charge separation efficiency ratio, Φ(vis)/Φ(UV).42 This implies that nearly all free charges generated by visible light excitation contribute to the photocatalytic reactions. The wavelength dependence of external reaction efficiency is reasonable because the final states of the electrons induced by visible light (Ti 3d ← N 2p transition) are different from that induced by UV excitation (Ti 3d ← O 2p and Ti 3d ← N 2p transition). Therefore, different redox potentials are generated by photons with different energies. The efficiency observed in the present work is twice that of the previous results6,15 probably due to the fact that our modification is confined at the surface. Moreover, on the basis of XPS measurement and DFT calculations, broad states are formed at the VBM, which benefits enhancing the lifetime of the photoexcited carriers.43 The formation of broad states makes the diffusion of electrons more easily inside the lattice, which retards the corresponding recombination. A photoreaction generally takes place in water environment. Under this circumstance, photogenerated electron−hole pairs can react with H2O to form reactive oxygen species (ROS). Typically, the photogenerated electron combine with adsorbed oxygen to form superoxide anion (O2), and the holes react with OH− to form hydroxyl radicals, (OH•). Subsequently, a chain reaction is activated to decompose the pollutants into small molecules. The majority of electrons/holes are trapped at bulk trapping sites and recombine there with release of heat. Therefore, to have higher activity, not only the materials should absorb more photons but also the amount of surface trapped electron/hole should increase, competing their recombination in the bulk. As evidenced above, surface nitridation can improve the photoactivity under visible light irradiation. However, the ability that the electron−hole pairs generated at N sites to migrate to the surface is also important. To measure this, we need to be able to “isolate” the electron−hole pairs generated at N sites from the intrinsic TiO2 lattice sites. For this purpose, additional TiO2 capping layers were grown on top of the N modified surface. Therefore, the N-TiO2 layers were buried underneath the pure TiO2 layers. When the thickness of TiO2 layer is controlled, the electron/holes diffusion property can be gauged by probing those only from the N sites. Because the N(7) sample possesses the highest photoactivity, we chose it in the following experiments. As shown in Table 1, the high activity of N-TiO2 remains until 16 nm [ NT(16 nm) ] under UV and 8 nm [ NT(8 nm) ] under visible light irradiation, respectively. In other words, the charges generated at N sites can diffuse ∼16/8 nm (UV/vis) before being consumed by recombination centers. The value of hole diffusion length obtained here is in the same order as predicted by other methods44 and for hydroxyl radicals.45 Therefore, to achieve the best modified results, only the top ∼10 nm of the surface needs to be engineered. At the same time, our results suggest that the carrier diffusion dynamics may also exhibit different behavior under UV and visible light irradiation.

4. CONCLUSIONS In conclusion, with an atomic N source, large surface doping concentration can be realized on the rutile TiO2(110) surface with remaining structure perfection. A narrowed surface band gap is obtained on the basis of both experimental measurements and DFT calculations. In the end, both UV and visible light photocatalytic activities are enhanced dramatically. By isolating the signal of the holes generated at N sites from other TiO2 lattice sites, we have shown that the charges can diffuse on

Figure 6. Schematic drawing of energy levels and the proposed processes of electron excitation in the N-TiO2 samples. E

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(14) Graciani, J.; Alvarez, L. J.; Rodriguez, J. A.; Sanz, J. F. N Doping of Rutile TiO2(110) Surface. A Theoretical DFT Study. J. Phys. Chem. C 2008, 112, 2624−2631. (15) Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2‑xNx Powders. J. Phys. Chem. B 2003, 107, 5483−5486. (16) Batzill, M.; Morales, E. H.; Diebold, U. Influence of Nitrogen Doping on the Defect Formation and Surface Properties of TiO2 Rutile and Anatase. Phys. Rev. Lett. 2006, 96, 026103/1−4. (17) Nakano, Y.; Morikawa, T.; Ohwaki, T.; Taga, Y. Deep-level Optical Spectroscopy Investigation of N-doped TiO2 Films. Appl. Phys. Lett. 2005, 86, 132104/1−3. (18) Sakthivel, S.; Janczarek, M.; Kisch, H. Visible Light Activity and Photoelectrochemical Properties of Nitrogen-Doped TiO2. J. Phys. Chem. B 2004, 108, 19384−19387. (19) Kong, M.; Li, Y.; Chen, X.; Tian, T.; Fang, P.; Zheng, F.; Zhao, X. Tuning the Relative Concentration Ratio of Bulk Defects to Surface Defects in TiO2 Nanocrystals Leads to High Photocatalytic Efficiency. J. Am. Chem. Soc. 2011, 133, 16414−16417. (20) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (21) Batzill, M.; Morales, E. H.; Diebold, U. Surface studies of nitrogen implanted TiO2. Chem. Phys. 2007, 339, 36−43. (22) Nambu, A.; Graciani, J.; Rodriguez, J. A.; Wu, Q.; Fujita, E.; Sanz, J. F. N Doping of TiO2(110): Photoemission and DensityFunctional Studies. J. Chem. Phys. 2006, 125, 094706/1−8. (23) Diwald, O.; Thompson, T. L.; Goralski, E. G.; Walck, S. D.; Yates, J. T. The Effect of Nitrogen Ion Implantation on the Photoactivity of TiO2 Rutile Single Crystals. J. Phys. Chem. B 2003, 108, 52−57. (24) Di Valentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Paganini, M. C.; Giamello, E. N-doped TiO2: Theory and experiment. Chem. Phys. 2007, 339, 44−56. (25) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. Origin of Photoactivity of Nitrogen-Doped Titanium Dioxide under Visible Light. J. Am. Chem. Soc. 2006, 128, 15666−15671. (26) Yin, Y.; Zhang, W.; Chen, S.; Yu, S. Theoretical and Experimental Study on the Electronic Structure and Optical Absorption Properties of Nitrogen-doped Nanometer TiO2. Mater. Chem. Phys. 2009, 113, 982−985. (27) Chai, J. W.; Yang, M.; Chen, Q.; Pan, J. S.; Zhang, Z.; Feng, Y. P.; Wang, S. J. Effects of Nitrogen Incorporation on the Electronic Structure of Rutile-TiO2. J. Appl. Phys. 2011, 109, 023707/1−4. (28) Kurtoglu, M. E.; Longenbach, T.; Sohlberg, K.; Gogotsi, Y. Strong Coupling of Cr and N in Cr−N-doped TiO2 and Its Effect on Photocatalytic Activity. J. Phys. Chem. C 2011, 115, 17392−17399. (29) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558−561. (30) Kresse, G.; Hafner, J. Ab initio molecular dynamics for openshell transition metals. Phys. Rev. B 1993, 48, 13115−13118. (31) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (32) Hameeuw, K. J.; Cantele, G.; Ninno, D.; Trani, F.; Iadonisi, G. The Rutile TiO2(110) Surface: Obtaining Converged Structural Properties From First-Principles Calculations. J. Chem. Phys. 2006, 124, 024708/1−8. (33) Yu, Y. P.; Liu, W.; Wu, S. X.; Li, S. W. Impact of Nitrogen Doping on Electrical Conduction in Anatase TiO2 Thin Films. J. Phys. Chem. C 2012, 116, 19625−19629. (34) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Enhanced Nitrogen Doping in TiO2 Nanoparticles. Nano Lett. 2003, 3, 1049−1051. (35) Chen, X.; Burda, C. The Electronic Origin of the Visible-Light Absorption Properties of C-, N- and S-Doped TiO2 Nanomaterials. J. Am. Chem. Soc. 2008, 130, 5018−5019. (36) Ohsawa, T.; Lyubinetsky, I.; Du, Y.; Henderson, M. A.; Shutthanandan, V.; Chambers, S. A. Crystallographic Dependence of

the order of 10 nm before they recombine. We demonstrate a way to precisely control the location and chemical environment of N dopants, which is critical in terms of improving visible light activity and also provides more insight into the photoactivity for N-TiO2. Our results open a new venue to improve visible light photoactivity with N doping.

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ASSOCIATED CONTENT

S Supporting Information *

More experimental details and DFT calculation results. Angle resolved N 1s XPS spectra, projected DOS of N-TiO2, and UV−vis transmittance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Junguang Tao: telephone, (+65)-68745207; e-mail, taoj@ imre.a-star.edu.sg. *S. J. Wang: telephone, (+65)-68748184; e-mail, sj-wang@ imre.a-star.edu.sg. Notes

The authors declare no competing financial interest.

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REFERENCES

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