Reaction of NH3 with Titania - American Chemical Society

7 Nov 2006 - Reaction of NH3 with Titania: N-Doping of the Oxide and TiN Formation ... The thermal nitridation of titania powder (anatase and P25) by ...
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J. Phys. Chem. C 2007, 111, 1366-1372

Reaction of NH3 with Titania: N-Doping of the Oxide and TiN Formation Haiyan Chen,† Akira Nambu,† Wen Wen,† Jesus Graciani,†,§ Zhong Zhong,‡ Jonathan C. Hanson,† Etsuko Fujita,† and Jose A. Rodriguez*,† Chemistry Department and National Synchrotron Light Source, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: September 19, 2006; In Final Form: NoVember 7, 2006

The thermal nitridation of titania powder (anatase and P25) by reaction with ammonia has been investigated by in-situ reaction studies and ex-situ characterizations using X-ray photoelectron spectroscopy (XPS), nearedge X-ray absorption fine structure (NEXAFS), and X-ray diffraction (XRD). At temperatures below 750 °C, the formation of TiN from the reaction of NH3 with the titania sample was negligible. Above 750 °C, in-situ reaction studies using XRD revealed a smooth phase transition from anatase or P25 to cubic TiN. On the basis of comprehensive characterizations, the crystalline structure of nitrogen-doped TiO2 is in general similar to that of TiO2. Incorporation of nitrogen into the interstitial sites of TiO2 anatase is supported by Rietveld refinement of XRD data. Interstitial nitrogen may form N2-like species bound to either oxygen vacancies or the cavity-framework atoms with various degrees of bond strength. N 1s XPS and N K-edge NEXAFS spectra support the idea that nitrogen present in titania as atoms may combine to form N2 molecules evolving into the gas phase upon heating to elevated temperatures, a behavior that agrees with the results of DFT calculations which show that nitrogen embedded in TiO2 is unstable energetically and spontaneously forms trapped or gaseous N2.

1. Introduction Nitrogen-doped metal oxides, including nitrogen-doped titania, have been demonstrated to absorb visible light and could be used to produce hydrogen from solar energy-driven water splitting.1-4 Ammonia, as a nitrogen source in a myriad of nitrogen-doping processes, has been extensively employed in the syntheses of doped powder materials by thermal,5-7 mechanochemical,8-10 hydrothermal,11 and sol-gel methods under ambient or supercritical conditions.12,13 In the presence of ammonia, nitrogen-doped TiO2 thin films have been made by atomic layer deposition14 and thermal nitridation.15 In fact, a well-known catalyst for solar hydrogen production, the RuO2 loaded GaN/ZnO solid solution, was prepared by thermal annealing of oxide mixtures in ammonia.2-4 Other nitrogen sources, such as amines, molecular nitrogen, and TiN, have also been utilized. However, nitrogen-doping of titania by alkyl or aromatic amines16-18 often requires heat treatment of the wet synthesis products to obtain photocatalytic activity. Doping by molecular nitrogen under thermal conditions is rarely seen,1,19 and an ionization source is frequently involved in the process for doped film fabrication.15,20-23 An alternative to titania nitridation is TiN oxidation, which was also used in the preparation of nitrogen-doped TiO224 and the optimization of the visible light photocatalytic activity of a TiO2-TiN mixture by partial oxidation of the TiN.25 Therefore, a detailed investigation of the thermal reaction between ammonia and titania should shed light on the mechanisms of related nitrogen-doping processes. * To whom correspondence should be addressed. Phone: (+1) 631-3442246. Fax: (+1) 631-344-5815. E-mail: [email protected]. † Chemistry Department. ‡ National Synchrotron Light Source. § Present address: Departamento de Quı´mica Fı´sica, Universidad de Sevilla, E-41012 Sevilla, Spain.

So far, research efforts dedicated solely to mechanistic studies of these important reactions are rare. The main focus has been on finding a set of reaction conditions that yield photochemically active products through massive experimental searches. Abundant in the literature are various characterizations and evaluations of the activities of the optimized products. Among the characterization methods, surface sensitive techniques are more popular than bulk sensitive techniques. For example, regarding the chemical states of nitrogen in doped titania, X-ray photoelectron spectroscopy (XPS) has been dominantly employed and the N 1s spectra have been of prominent interest. Under UHV conditions, fairly comparable results have been generated between different groups. However, for samples synthesized at ambient pressure or under aqueous conditions, the N 1s peak position and the nitrogen content vary considerably.26 Mostly on the basis of the N 1s peak positions, by comparison to binding energies of known compounds plus educated speculations, nitrogen is envisioned to be in the configuration of N-TiO,27 Ti-N-O (oxynitride),28 NO,29 N-,30,31 N3-,32 NH,15 or adsorbed NH3.33,34 Despite many models proposed on various experimental and theoretical bases, the electronic properties of doped titania, the bonding configuration, and the oxidation states of nitrogen in the lattice are yet to be unambiguously determined. One of the debated fundamental issues is whether the nitrogen is accommodated by the titania lattice substitutionally or interstitially. Substitutional doping has been favored by some groups, but recent density functional theory (DFT) calculations32,35,36 are in favor of both substitutional and interstitial nitrogen doping, with interstitial doping more favorable in energy especially for surfaces with low concentration of oxygen vacancies.32 In this paper, in-situ reaction studies were performed to study the fundamental issues discussed above. In addition to oxidation state sensitive XPS, bonding geometry sensitive near-edge X-ray

10.1021/jp066137e CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

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absorption fine structure (NEXAFS) and bulk structure sensitive X-ray diffraction (XRD) techniques were employed to extract critical local and global bonding information. During the reaction of ammonia with titania, the phase transformation was monitored in a time-resolved manner by XRD, and the gas-phase composition was followed by a quadrupole mass spectrometer. Representative in-situ reaction products were separately synthesized and probed with XPS, NEXAFS, and XRD techniques. Collectively, these data demonstrate the lack of an ordered intermediate phase in the whole reaction course and point to a kinetically controlled interstitial incorporation mechanism. 2. Experimental Section The reactions of ammonia and titania were performed under ambient pressure in two schemes to accommodate the characterization techniques employed. The samples used for ex-situ XRD, XPS, and NEXAFS characterization were made by thermal annealing of TiO2 anatase (Aldrich, 99.9%) powder in pure NH3 (Praxair, 99.999%) flow in a quartz tube reactor heated by a temperature-programmed tubular furnace (Lindberg/Blue M). In this paper, only four samples are selected as representatives for discussion. The samples were prepared by ramping to a temperature, holding at the temperature for a certain amount of time, and then cooling to room temperature, all under an atmosphere of NH3. For example, the denotations of 600_1, 750_0, and 750_1 (temperature_time) represent samples heated to 600 °C and held for 1 h, heated to 750 °C and cooled immediately, and heated to 750 °C and held for 1 h, respectively. The sample 750_1_air was obtained by exposing the 750_1 sample to air at ∼600 °C for 5 min and was chosen to represent the oxidation of TiN. For in-situ XRD experiments, TiO2 anatase and P25 (a commercial mixture of 75% of anatase and 25% rutile) samples were loaded into a sapphire capillary (i.d. 0.7 mm) cell specifically designed for time-resolved XRD monitoring. An amount of 5% NH3 in He (Praxair) was admitted into the cell through a flow system, and the capillary was heated by a resistive heater wrapped around the capillary. During the oxidation process, a TiN-TiO2 mixture made from the partial oxidation of TiN was heated in 5% O2 in He flow. The flow rate was 10 mL/min for both nitridation and oxidation. The temperature was monitored with a 0.1-mm type K thermocouple inside the capillary and was program controlled using an temperature controller (CN2011R, Omega). In the reaction studies, the temperature was ramped to 450 °C from room temperature in 20 min, then to 850 or 810 °C at a ramp rate of ∼2 K/min, and held at final temperature until the completion of the phase transformation. The gas phase was constantly monitored by a quadrupole residual gas analyzer RGA-100 (Stanford Research Systems). The time-resolved X-ray diffraction experiments were carried out on beamlines X7B (λ ) 0.922 Å) and X17B1 (λ ) 0.165 Å) of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Two-dimensional diffraction patterns were collected with a Mar345 image plate detector and were integrated using the FIT2D code (Hammersley/ESRF, 1987-2005). In-situ diffraction patterns were collected during nitridation and oxidation at ∼2 min intervals. The Rietveld refinement was performed using the GSAS (general structure analysis system) code developed by A. C. Larson and R. B. Von Dreele at Los Alamos National Laboratory (Report LAUR 86-784, 2004). XPS experiments were performed in a standard UHV chamber (base pressure 3 × 10-9 Torr) equipped with a 100 mm

Figure 1. In-situ XRD patterns of the nitridation of TiO2 anatase (top) and P25 (bottom) by 5% NH3 in helium at a flow rate of 10 mL/min. X-ray wavelength λ ) 0.922 Å. In our annotations, R and A stand for TiO2 rutile and anatase phases, respectively. For clarity of presentation, the intense A (101) peaks are truncated.

hemispherical electron analyzer (Scienta, SES 100).32 Mg KR radiation (hν ) 1253.6 eV) was used to acquire the core level spectra (O 1s, N 1s, Ti 2p). The powder samples were smeared on a carbon tape, and the binding energy is referenced to the C 1s peak at 284.6 eV from the carbon tape. Nitrogen K-edge, oxygen K-edge, and Ti L-edge NEXAFS spectra were taken at the U7A NIST/DOW end station of the NSLS.37 The partial electron yield (PEY) signal was collected using a channeltron electron multiplier with an adjustable entrance grid bias (EGB) of -150 V. The incident photon energy resolution was 0.2 eV. The powder samples were smeared onto Cu tapes and mounted onto a stainless steel sample holder inside a UHV chamber through a sample load lock system. A low-energy electron flood gun was used to compensate for charging effects. 3. Results and Discussion 3.1. In-Situ XRD Reaction Studies. Presented in Figure 1 are in-situ XRD patterns taken during the reaction of TiO2 anatase and P25 with NH3. The TiO2 anatase structure is maintained even after the appearance of a cubic TiN phase around 750 °C, and the peak width broadening is not significant before the TiN formation, meaning there is no drastic disruption of the anatase lattice during the embedding of nitrogen. A complete conversion to TiN phase is achieved after the nitridation process is held at 850 °C for ∼6 h, indicating that the phase transformation from TiO2 to TiN is kinetically controlled. The phase transformation is progressive without additional phases observed, ruling out the possible formation of an ordered oxynitride.38 The reaction of P25 with ammonia shows a similar trend, but the phase transformation is faster due primarily to the significantly smaller particle size of P25 used in the experiment (∼15 nm for P25 versus ∼5 µm for anatase). The reaction between the rutile phase and ammonia is slower than that with the anatase phase. The temperature and time dependence of the TiO2 anatase transformation to TiN is depicted in Figure 2. The normalized peak areas of A (101) and TiN (200) experience no variation from room temperature to 450 °C. From 450 to 770 °C, the A (101) peak decreases but no TiN (200) peak intensity increase is observed. Apparently, in this stage, TiO2 is not transformed to TiN. XRD powder pattern refinement shows a small distortion

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Figure 2. Temperature dependence of the transformation of TiO2 anatase to TiN. The crosses (related to the right axis) represent the temperature profile. In the initial step, the sample was ramped from 25 to 450 °C in 20 min. The normalized peak areas in the figure are for the TiO2 anatase (101) (hollow circles) and TiN (200) features (solid triangles).

of the TiO2 cell, and this may account for the decrease in intensity of the A (101) peak. When the temperature is higher than 770 °C, the reaction of ammonia with TiO2 transforms anatase to cubic TiN. The reaction proceeds to completion at 850 °C with time, showing that this transformation is kinetically limited. During the reaction of TiO2 anatase and ammonia, the composition of the gas phase in the reaction cell was continuously monitored and the evolution of H2, and N2 was detected when the temperature was higher than 650 °C (data not shown). Production of H2 and N2 suggests the pyrolysis of ammonia under the experimental conditions. Therefore, a wide range of molecular or radical nitrogen-containing intermediates of ammonia pyrolysis, such as N, NH, NH2, and N2Hx (x ) 1-4),39,40 may also react with TiO2 to produce N-doped titania. 3.2. Ex-Situ Characterizations. To better understand the reaction of TiO2 and ammonia at various stages, several synthesized nitrogen-doped titania samples were characterized by XRD, XPS, and NEXAFS at bulk and surface levels to uncover the geometric and electronic information essential to establish the nitrogen doping mechanism. 3.2.1. X-ray Diffraction Phase Specification. The samples synthesized by thermal nitridation of TiO2 anatase at different conditions have colors that range from yellow, to dark blue, and to black, apparently with different optical absorption ability. For samples that are not black, only the anatase phase can be seen in the corresponding XRD patterns (see Figure 3). However, for the black sample 750_1, XRD shows a mixture of TiO2 anatase and cubic TiN phases. Therefore, samples 600_1 and 750_0 can be treated as the products from the nitrogendoping process and the sample 750_1 as the result of a TiN formation process. The oxidation product 750_1_air is in the anatase crystal structure, showing a complete transformation from the cubic TiN to the anatase phase. In contrast to another report,24 we observed TiO2 anatase instead of the TiO2 rutile phase, consistent with our in-situ measurements where there is a smooth conversion between the anatase TiO2 and cubic TiN. In general, these data are consistent with the results of DFT calculations which show a low stability of nitrogen atoms in N-doped TiO2 or TiONx compounds.32 These systems prefer segregation into TiO2/TiN phases, and only a small amount of N (e5%) can be incorporated into the titania lattice. 3.2.2. X-ray Photoelectron Spectroscopy Measurements. The chemical states of the incorporated nitrogen were characterized

Figure 3. X-ray diffraction (XRD) patterns of selected nitrogen-doped titania anatase samples together with TiO2 anatase (TiO2_A) and TiN standards. The peak intensities are not normalized. The samples were prepared by exposure of anatase to pure ammonia at the indicated temperatures.

Figure 4. N 1s XPS spectra of selected nitrogen-doped titania anatase samples. These samples were prepared as described in Figure 3.

by XPS, and the N 1s core-level spectra of selected samples are presented in Figure 4. By binding energies, the N 1s peaks can be divided into two groups, one with binding energy at 396.1 eV and another at 400.2 eV. On the basis of our previous experimental and theoretical work on the interactions of nitrogen with TiO2 single crystal,32,41 we assign the peak at 400.2 eV to nitrogen species bound to various surface oxygen sites (NOlike species) and the N 1s peak at 396.1 eV to TiN-like nitrogen species in the lattice. With this assignment, it is interesting to note that the 750_0 sample shows no TiN crystalline phase with the presence of TiN-like nitrogen in XPS. This suggests that the incorporated nitrogen atoms remain in the oxide structure and do not segregate into ordered structures of TiN. Whether the nitrogen is bonded to titanium or is just inside the lattice could not be distinguished by XPS. In the order from 750_0 to

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Figure 6. NEXAFS spectra for the N K-edge of selected samples and a TiN reference. These samples were prepared as described in Figure 3.

Figure 5. Ti 2p XPS spectra of selected nitrogen-doped titania anatase samples (top panel) and the curve fitting of the Ti 2p XPS for the 750_1 sample (bottom panel). The corresponding N 1s spectra are displayed in Figure 4. These samples were prepared as described in Figure 3.

750_1, the amount of N presented in the samples increases and a TiN phase is detected by XRD (see Figure 3). Upon the exposure of the 750_1 sample to oxygen at 600 °C, the N 1s peak shifts to a position centered around 400 eV, suggesting the displacing of N inside the lattice by oxygen. The displaced N eventually forms N2 that desorbs, or it can remain in the sample bonded to oxygen (as revealed by the N 1s XPS peak at ∼400 eV BE).32 The influence of nitrogen incorporation on the state of titanium was probed through the Ti 2p spectra displayed in the top panel of Figure 5. These spectra can be divided into two groups. The spectra of the first group (600_1, 750_0, and 750_1_air) have a Ti 2p3/2 binding energy of 458.5 eV and a split of 5.7 eV between the doublets, the same as the spectrum of Ti4+ in pure TiO2 powder.42 Upon the incorporation of nitrogen into TiO2 anatase lattice, the Ti 2p spectra of this group show no significant amount of reduced titanium species before the formation of a TiN phase. This result suggests that for the nitrogen doping process, there is no significant amount of net electron transfer to titanium from the vacancy formation. DFT calculations show that the adsorbed and interstitial N atoms have a tendency to bond to oxygen vacancy sites and accept the electrons.32 For 750_1, a TiO2-TiN mixture, the spectra can be fitted with three doublets (bottom panel in Figure 5), containing Ti4+, Ti3+, and titanium species with oxidation states between 3+ and 4+.32 According to the XRD characterization, Ti4+ can be correlated to TiO2 and Ti3+ to TiN, and the other species could be titanium in a distorted lattice located between the TiO2 and TiN phases. 3.2.3. Near-Edge X-ray Absorption Fine Structure Analysis. The N K-edge, O K-edge, and Ti L-edge features are very

sensitive to the local symmetry and ligand coordination, providing information on crystal structure, oxidation states, and surface composition.43 The probing depth of electron yield in NEXAFS is limited by the escape depth of Auger electrons, which was calculated to be ∼10 Å for emissions corresponding to these edges with a -150 V bias applied. In the nitrogen K-edge spectrum of TiN (dashed trace) in Figure 6, the first two features below 403 eV are assigned to unoccupied N 2p states hybridized with Ti 3d orbitals in TiN, and the features between 403 and 415 eV to unoccupied N 2p states hybridized with Ti 4sp orbitals.44,45 From the apparent spectral similarities between the sample 750_1 and 750_0 to TiN, it is clear that the chemical environment around nitrogen in these samples is similar to that in TiN. This is in agreement with XPS measurements on these two samples but is not consistent with the XRD data of sample 750_0. XRD, as a bulk technique, could not detect phases without long-range order. The N K-edge spectra of the light colored samples are not the same as that of TiN. From the spectrum of sample 600_1, the doublet at lower energy could still be discerned. However, the features at higher energy are rather flat, again indicating a lack of long-range order. For the oxidized sample, the doublets are further disrupted compared to the sample 750_1, implying the local bonding environment of this sample is different. Since nitrogen-doped titania synthesized under conditions similar to those of our 600_1 and 750_1_air have been claimed to be photocatalytically active in the literature,7,25 the origin of the photoactivity may be related to more than one type of incorporated nitrogen species. In the O K-edge spectrum of TiO2 shown in Figure 7, the first doublet, located at 531 and 534 eV, corresponds to an electron transfer from the O 1s orbital to covalently mixed states derived from the O 2p and Ti 3d states, t2g and eg of TiO2.43 The features between 537 and 550 eV are due to the covalent mixing of O 2p and Ti 4sp states and are sensitive to longrange order. The main features of the spectra of doped TiO2 are similar to that of TiO2 but not as sharp, possibly due to minor deviations from the anatase. However, the doublets of the TiO2-TiN mixture merge into unresolved broad features (750_1 sample), signaling the appearance of reduced species. These reduced species disappear after oxidation in air (750_1_air sample). The Ti L-edge spectrum of TiO2 anatase contains two sets of doublets (see Figure 8), Ti L3 at lower energy and Ti L2 at

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Figure 7. NEXAFS spectra for the O K-edge of selected samples and a TiO2 reference. These samples were prepared as described in Figure 3.

Figure 8. NEXAFS spectra for the Ti L-edge of selected samples, a TiN reference, and a TiO2 anatase reference. These samples were prepared as described in Figure 3.

higher energy. The two doublets originate primarily from an electron transfer from the Ti 2p3/2 and Ti 2p1/2 levels to Ti 3d orbitals, which split into t2g and eg orbitals in the octahedral symmetry.46 The similarities between the L-edge spectra of nitrogen-doped TiO2, 600_1 or 750_0, and TiO2 anatase indicate that the coordination environment of titanium atoms in the doped nitrogen is not very different from that of anatase. This is in accordance with the results from O K-edge spectra. On the other hand, the spectrum of the TiO2-TiN mixture (750_1) bears the features of TiN, again pointing to the presence of reduced Ti atoms that at the end vanish upon oxidation at 750 °C in air. 3.3. Interstitial Nitrogen Incorporation. On the basis of the characterizations of nitrogen-doped titania discussed above, the anatase structure is preserved with minor distortions and the titanium is predominantly in +4 states. In principle, nitrogen could be either at the substitutional positions of the lattice or embedded in the interstitial positions.32 With this as a starting point, the incorporation of nitrogen atoms into the anatase lattice was explored with least-square refinement of XRD profiles combined with difference Fourier analysis. This analysis provides a means for locating atoms not included in the model, in our case, locating nitrogen atoms in the TiO2 anatase, by subtracting the calculated electron density of the model (Fc) from the observed electron density (Fo). The peak positions in a difference synthesis show possible locations of atoms added to the model. To explore how nitrogen atoms “squeeze” into the lattice, the difference Fourier map of sample 750_0 was constructed and is presented in the top panel of Figure 9. From this map, two important features can be observed:

Figure 9. Contour plot of difference (Fo - Fc) Fourier map (top panel) computed on the TiO2 anatase (100) plane using the XRD profile of sample 750_0. The X-ray wavelength is 0.922 Å. Displaced oxygen atoms are marked by a red circle, and incorporated nitrogen atoms, by blue circles. In the map, the assumed atomic positions are indicated by + and the peak maximum is around 0.2 e/Å3. The stick model shown in the bottom panel is a cross section parallel to the (100) surface. The Ti atoms are in gray, and O atoms, in red. Blue circles represent nitrogen atoms in the cavities.

(1) Electron density peaks located slightly away from the oxygen lattice positions along the Ti-O bond in the direction of the c-axis. (2) electron density peaks found inside the lattice cavities around titanium atoms, at the interstitial positions. The first feature suggests the elongation of the Ti-O bond along the c-axis, and the second can be viewed as nitrogen atoms at interstitial positions. When a nitrogen atom with the fractional coordinates extracted from the difference Fourier synthesis was added to the TiO2 anatase structure and a Rietveld refinement was performed with this new model, the refinement did converge and yielded a total occupancy of nitrogen around 0.3 in a cell with 4 Ti atoms and 8 O atoms. This refinement picture shows that nitrogen atoms enter the lattice cavities. This refinement could not be used to determine if there is substitutional nitrogen doping.

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J. Phys. Chem. C, Vol. 111, No. 3, 2007 1371 TABLE 1: Internuclear Distances from Rietveld Refinement of XRD Patterns for N-TiO2 Anatase Samples and from DFT Calculations of N-Doped TiO2 Rutile32,a sample

dTi-O (Å)

TiO2_anatase TiN N-TiO2 DFT-N2

1.939, 1.976

a

Figure 10. Contour plot of a difference (Fo - Fc) Fourier map computed for the TiO2 anatase (100) plane using an XRD pattern of nitrogen-doped TiO2 synthesized during the in-situ oxidation of a mixture of TiO2 and TiN. The X-ray wavelength is 0.165 Å, and the maximum electron density is ∼1.2 e/Å3.

Above we found evidence for nitrogen interstitials formed during the reaction studies of titania nitridation. On the other hand, we can also look at the newly formed TiO2 created during the oxidation of TiN. For this study we measured a series of powder patterns during the oxidation of a mixture of TiO2 and TiN. This in-situ study was performed with a short wavelength (λ ) 0.165 Å) at X17B1 of the NSLS, and consequently, many more diffraction peaks were obtained. This allows for refinement of the amount of substitution at oxygen sites and for a more reliable detection of interstitials. The most interesting data set is the powder pattern obtained just after the TiN phase is no longer present. The refinement of this data set do not show any clear evidence for substitution at oxygen sites, but this result could not be used to exclude the substitution. However, the difference map shown in Figure 10 exhibits significant electron density (∼1.2 e/Å3) at interstitial sites, which is evidence for interstitial nitrogen doping. Interestingly, the nitrogen atoms in this sample appear to be in pairs, in accordance with the assignments of doped nitrogen in TiO2 to molecular nitrogen by a NEXAFS study.45 The electron-yield N K-edge spectra in Figure 6 do not show features of N2 species since they only probe the surface and subsurface region. From this region, DFT calculations shows that N2 is expected to evolve into the gas phase due to a weak adsorption bond.32 However, according to DFT, N2 could be trapped in the bulk of TiO2, and we detected it in bulk sensitive fluorescence-yield N K-edge NEXAFS spectra of samples equivalent to sample 750_0. In Figure 10, the distance between two nitrogen atoms is ∼1.559 Å, much larger than the N-N distance in free gas-phase N2 (1.10 Å) but close to N2 trapped inside the channels of rutile (1.527 Å, DFT calculations).32 In fact, a large N-N distance up to 1.548 Å47 has been observed for N2-metal complexes.48 In this complex, the NtN triple bond is weakened by the partial occupation of the antibonding π levels.47 Listed in Table 1 are the interatomic distances obtained from Rietveld refinement using a nitrogen-added anatase model constructed with the coordinates found from Fourier difference syntheses together with the DFT calculated distances for an interstitial N2 bound inside the rutile channels. In the table, N-TiO2 stands for the sample whose Fourier difference map is shown in Figure 10. The Ti-N is found to be 2.035 Å, smaller than the Ti-N distance (2.118 Å) in cubic TiN but comparable

1.953, 1.976

dTi-N (Å)

dN-N (Å)

dN-O (Å)

2.118 2.035 2.106

2.996 1.559 1.527

1.551 1.393

DFT-N2 denotes interstitial N2 bound inside the rutile channels.32

to the results of the DFT calculation for interstitial N2 in rutile channels (2.106 Å).32 The N-O distance is 1.551 Å, much larger than the NO triple bond length (1.15 Å),49 excluding the presence of this molecule in the lattice. This bond length is slightly larger than the single N-O bond length in ON-OH (1.46 Å)49 and the N-O bond length from the DFT calculations (1.3932 and 1.36 Å36), indicating a weak bond of N (or N2) to an oxygen atom of the lattice framework. The influence of nitrogen doping on the Ti-O distance is trivial. The Ti-O distances of N-TiO2 are 1.994 and 1.950 Å, only less than 1% longer than those of TiO2 anatase (1.976 and 1.939 Å). Incorporation of nitrogen into the TiO2 lattice involves at least two key factors: the geometric and electronic accommodation of nitrogen. From the geometric point of view, entities small in size have a higher chance to “squeeze” into the lattices at high temperatures. At high temperatures, oxygen vacancy production on a single-crystal TiO2(110) surface is known to happen at ∼580 °C in vacuum,50 and the presence of nitrogen in TiO2 facilitates the formation of oxygen vacancies.51 Associated with each oxygen vacancy are two localized electrons. In the nitrogen doping process, one of these electrons moves to the nitrogen to produce a closed shell in the atom.32 4. Conclusion Both in-situ and ex-situ XRD investigations point to a phase separation between TiO2 and cubic TiN during the reaction of ammonia with anatase or P25 powders. For nitrogen-doped TiO2, which has no TiN phase, two types of nitrogen species with binding energies of 396.1 and 400.2 eV were observed by X-ray photoelectron spectroscopy (XPS), but only Ti4+ was identified. The nitrogen species can be assigned to substitutional N on a TiN-like configuration and N bound to oxygen sites. NEXAFS studies show that the local bonding environment of oxygen and titanium atoms in nitrogen-doped TiO2 is similar to that in undoped TiO2. Rietveld refinement of XRD data support interstitial incorporation of N2 into the TiO2 anatase, with Ti-N and N-N bond distances of 2.035 and 1.559 Å, respectively. Acknowledgment. The research carried out at the Chemistry Department of Brookhaven National Laboratory was funded by the U.S. Department of Energy, Division of Chemical Sciences (Contract No. DE-AC02-98CH10086). The NSLS is supported by the Divisions of Materials and Chemical Sciences of DOE. Financial support from the Spanish Ministerio de Ciencia y Tecnologia (Grant MAT2005-01872) and the Junta de Andalucı´a (FQM-132) is appreciated by J.G. References and Notes (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286.

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