Adjusting Nitrogen Doping Level in Titanium Dioxide by Codoping

Article pubs.acs.org/JPCC

Adjusting Nitrogen Doping Level in Titanium Dioxide by Codoping with Tungsten: Properties and Band Structure of the Resulting Materials Jonathan Z. Bloh,†,¶ Andrea Folli,†,‡ and Donald E. Macphee*,† †

Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, United Kingdom Danish Technological Institute, Gregersensvej 4, 2630 Taastrup, Denmark

‡

ABSTRACT: For a systematic study of the material, samples of W/N-codoped titania with different concentrations of the dopants were prepared. The physicochemical properties and in particular their band structure were subsequently evaluated experimentally to elucidate the effect of W-doping, N-doping, and W/N-codoping on the band structure of TiO2. For this purpose, a combined approach of optical spectroscopy and electrochemical impedance spectroscopy was chosen. The doped samples featured both a reduced band gap and a positively shifted conduction band edge. Both conduction band edge and band gap followed a linear dependence on the nitrogen content. They also demonstrate visible light absorption capability, which is associated with interstitial nitrogen doping. Tungsten doping did not influence the band structure of TiO2 directly. It did, however, facilitate nitrogen uptake and stabilize it at higher temperatures. These higher nitrogen doping levels then in turn reduced the band gap and lowered the conduction band edge. Codoping with tungsten therefore offers an excellent way to precisely adjust the nitrogen content and correspondingly the conduction band position of titanium dioxide.

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INTRODUCTION

One of the best-known visible light photocatalysts is nitrogen-doped TiO2, which has been first reported as NOxdoped TiO2 as early as 1986 by Sato18 and has since been investigated intensively as a promising visible light active photocatalyst.19−24 Spectroscopic investigations revealed that there are two kinds of nitrogen doping, substitutional and interstitial. In the former, nitrogen replaces an oxygen in the titania lattice, becoming NO′ in the Kröger−Vink notation. Interstitially doped nitrogen, however, occupies an interstitial space and associates with an oxygen atom, becoming a formal [NO]′O. It seems that sol−gel synthetic routes mainly yield interstitially bound nitrogen, while annealing under ammonia flow at high temperatures or oxidation of TiN mostly leads to substitutional doping. However, both species may also coexist in the same material and as a result of the synthesis conditions, there are usually other nondoped nitrogen species present in these materials, such as adsorbed NO.25−32 This makes it exceptionally difficult to attribute observed effects to one species or another and to quantitatively determine the amount of nitrogen doping, since most methods cannot distinguish between the individual species. Apart from the appearance of a mid-gap state located about 0.7 eV above the valence band, little is known about the effects of nitrogen doping on the band structure of TiO2.20,33 In many

Since the discovery of photoinduced water-splitting on titanium dioxide electrodes in 1969 there has been a growing interest in the field of photocatalysis.1 Semiconductor photocatalysis has been proposed and investigated for, and applied in a variety of applications, most of them based on the ability to oxidize and mineralize virtually all organic and inorganic pollutants.2−6 Recently, titanium dioxide based photocatalysts have even been explored for the large-scale application as a concrete supplement or finish for the removal of nitrogen oxides from the atmosphere.7−11 However, most applications for photocatalysts are designed to use sunlight as their primary irradiation source, which only contains about 5% of UV light under ideal conditions and even less in extreme latitudes and/or in the darker seasons.12 This greatly limits the achieved activity to the point where it is often no longer viable, since ordinary titanium dioxide can only utilize UV light due to its wide band gap of 3.0 to 3.2 eV. Also, some applications focus on the use of indoor lighting, which usually does not contain ultraviolet light at all. For this reason, a lot of effort has been devoted to the development of visible light active photocatalysts in recent years resulting in the discovery of several such materials.13 The majority of those are based on modified or doped titanium dioxide, utilizing the advantageous properties of titania as a material, most noticeably abundance and hence low cost, nontoxicity, and high chemical stability.3,14−17 © XXXX American Chemical Society

Received: July 21, 2014 Revised: August 14, 2014

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dx.doi.org/10.1021/jp507264g | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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tungstic acid (Hopkin & Williams) was dissolved in 10 mL of warm deionized water and subsequently added to the solution. After thorough stirring for at least 4 h, the solution was filtered, washed several times with deionized water, and then dried at 60 °C for 4 h. The dry powders were ground in an agate mortar and then transferred into a crucible for calcination. The samples were calcined at either 400 or 600 °C for 4 h and ground again afterward. X-ray Diffraction Analysis. X-ray diffraction patterns were recorded in the range of 10 to 100° 2θ on a Siemens D5000 diffractometer in Bragg−Brentano geometry with Cu Kα1,2 radiation. The resulting patterns were subsequently used for Rietveld refinement, using the software PowderCell 2.4. For the refinement anatase, rutile and brookite as well as hexagonal, triclinic, and monoclinic tungsten trioxide were taken into account to calculate the phase composition. Additionally, size, strain, and lattice parameters of the individual phases were determined. UV−Vis Diffuse Reflectance Spectroscopy. UV−vis diffuse reflectance spectra were collected on a StellarNet EPP2000 spectrophotometer with barium sulfate as reference in the range of 200 to 800 nm. The resulting reflectance spectra were transformed into apparent absorption spectra by using the Kubelka−Munk function F (R∞) = (1 − R∞)2/2R∞.64 Those absorption spectra were subsequently used to determine the optical band gap of the materials by constructing Tauc plots by plotting (F(R∞)hv)1/2 against hν.65,66 The indirect optical band gap was obtained by extrapolating the linear part of this plot to the x-axis (energy). Electrochemical Impedance Spectroscopy. The electrochemical impedance spectroscopy (EIS) measurements require the fabrication of electrodes from the semiconducting material prior to the experiment. This was accomplished by coating our samples by electrophoretic deposition on a slide of conductive glass (fluorine-doped tin oxide, FTO). Square slides (25 × 25 mm2) of FTO (Sigma-Aldrich, 7Ω/□) were ultrasonically cleaned in acetone and water and then dried at ambient conditions. A coating solution was prepared by ultrasonically dispersing 100 mg of the sample in 100 mL of a 0.1 mmol L−1 ammonia solution. The FTO glass slides were coated by using electrophoretic deposition, in a cell where the FTO glass was used as anode and a platinum mesh as cathode. The deposition was performed at a potential of +15 V for 10 min. After deposition, the coated electrode was annealed at 400 °C for 1 h. The EIS measurements were performed in a 3-electrode setup with the coated FTO-Glass as a working electrode, a coiled platinum wire as a counter electrode, and an Ag/AgCl reference electrode (3 mol L−1 NaCl, +205 mV vs SHE). Those electrodes were fixed in a Zahner PECC-1 measurement cell in such a way that only the coated part of the FTO-Glass was in contact with the 0.1 mol L−1 sulfuric acid electrolyte, with a circular contact area of 1.77 cm2. Impedance spectra were collected with use of a Zahner IM6e potentiostat in the potential range +1.30 to 0.0 V vs SHE with 25 mV steps. Each spectrum (i.e., at a fixed potential) was recorded in a frequency range of 1 × 105 to 1 × 10−1 Hz with 11 steps per decade and an amplitude of 10 mV. The spectra were subsequently fitted to a Randles circuit (used as an equivalent circuit to represent a simple electrode/ electrolyte interface, Figure 1) to derive the values for the space charge layer capacitance (CSC).67 In the equivalent circuit, a constant phase element (CPE) replaces the ordinary double layer capacitance (CSC) to simulate an uneven surface of the

cases, a reduction in band gap upon nitrogen doping was reported, but it was not investigated whether this was due to a change in the position of the conduction band, valence band, or both. There are only a very limited number of publications with experimental data on the band positions of nitrogen-doped TiO2 and those results do not offer a definite conclusion.34−39 Due to the difficulty in obtaining experimental data for the band positions, many publications focus on computational models instead and present simulated band structures. However, the vast majority of those calculations focus only on substitutional nitrogen, despite the fact that most synthetic routes lead to interstitial nitrogen.40−47 Also, those calculations are ambiguous and sometimes contradict each other. So even almost three decades after nitrogen-doped titania was first reported, it is still not definitely known whether or not the band edges of TiO2 are changed by nitrogen doping and in what way, let alone supported by experimental evidence. While being a promising candidate as a visible light active photocatalyst, nitrogen-doped TiO2 also has several drawbacks. Mainly, charge compensation demands either an increase in oxygen vacancies or decrease in charge carrier density upon introduction of the p-dopant, which in turn reduces the overall efficiency of the catalyst.36,48−51 Also, the doped materials are sensitive to higher temperatures and the amount of nitrogen that can be introduced to titanium dioxide seems to be quite limited.52 More recently, codoping was explored as a way to further increase the photocatalytic activity of the materials and to minimize crystallographic defects by better charge compensation.32,53−63 To balance the excess negative charge of the doped nitrogen species, the codopant has to have excess positive charge. This can be achieved by anion doping with halogens or cation doping with any metal ions that have a charge of +5 or more. Of the codoping systems studied, the W/ N-system has repeatedly been reported as a promising candidate for visible light photocatalysis.61−63 However, even though the materials were often thoroughly characterized and were found to exhibit a synergistic effect that could not be explained by a single dopant alone, little is known about the effects of the individual dopants in the matrix and how they influence one another. In particular, similar to nitrogen-doped titania, there is no information concerning actual experimentally determined band structures of W/N-codoped TiO2. In this study, we offer a systematic study of the physicochemical properties and band structure of tungstendoped, nitrogen-doped, and tungsten−nitrogen-codoped titanium dioxide. The band structures reported herein are derived from experimental evidence, using a combination of electrochemical impedance and optical spectroscopy. To the best of our knowledge, this is the first time such a comprehensive study on the band structure of a codoped TiO2 has been reported.

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EXPERIMENTAL SECTION Sample Preparation. For a typical preparation of the sample catalysts, 10 mL of titanium isopropoxide (≥97%, Sigma-Aldrich) was dissolved in 10 mL of anhydrous ethanol. After the solution was thoroughly mixed, 5 mL of deionized water (18 MΩ cm) was slowly added to the solution. The resulting white precipitate redissolved upon further stirring. In the next step, 20 mL of a pH 10 ammonia/ammonium chloride buffer (5% ammonia, Sigma-Aldrich) for the nitrogencontaining samples or 20 mL of deionized water for the nitrogen-free samples was added to the solution. Finally, the desired amount of ammonium tungstate (BDH Chemicals) or B



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(CSC)−2 =

k T⎞ 2 ⎛ ⎜U − Ufb − B ⎟ ϵϵ0end ⎝ e ⎠

(1)

Additionally, the donor density nd was calculated from the slope of the linear part by using a value of 41 for the dielectric constant of anatase.72 As the donor density was always found to be higher (≥1 × 1020 cm−3) than the effective density of states in the conduction band (2.42 × 1019 cm−3 assuming a value of 1.0me for the electron effective mass73,74) in all cases, the energy difference between the Fermi level and the conduction band edge is very small (50 nm) crystallites. The tungsten content, however, does not appear to have any effect on the particle size. Samples calcined at 600 °C generally exhibit a larger particle size as expected due to the particulate growth at higher temperatures. Except for the tungsten-only doped sample with a particle size of 15.5 nm, they are all in the range of 30.0 to 43.9 and therefore about 50% larger than at 400 °C. Again there is no observable trend in the particle size with increasing tungsten content. The lattice parameters, as obtained by the refinement, were only slightly changed by the codoping with tungsten and nitrogen. Starting from pristine anatase with a = 3.782 Å and c = 9.480 Å, which is well within the range of previously reported values,75,76 the c parameter first increases with nitrogen addition and then gradually decreases with higher tungsten content. Reduction of the c parameter is observable upon W addition exclusively. The a parameter, however, remains largely unchanged with values ranging from 3.782 to 3.799 Å without D



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nitrogen species present alongside the active one responsible for the visible light absorption, e.g. surface-adsorbed nitrogen oxide species. Therefore, this number should only be taken as an upper limit for the amount of nitrogen doping, since the real value is likely only a fraction of it. To pin down the band edges on an absolute potential scale, information about the conduction band edge potential is needed in addition to the band gap value. There are several techniques available to measure the conduction band edge of semiconducting materials, in this work we chose electrochemical impedance spectroscopy (EIS) with Mott−Schottky plots, a complex procedure involving several subsequent steps. Details on the procedure and a comparison with other techniques can be found in several excellent reviews on the subject.77−80 The first step involves measuring the impedance response of the semiconductor in dependence of the applied potential to calculate the space charge layer capacitance. After calculating the capacitance of the space charge layer by fitting the experimental data to a simulated equivalent circuit, the inversed square of the capacitance normalized for the contact area was plotted against the potential at which it was determined. This so-called Mott−Schottky plot has a linear regime that follows the Mott−Schottky relation given in eq 1 and through its intercept with the x-axis the position of the conduction band edge can eventually be derived. From the example shown in Figure 3, the Mott−Schottky plots indeed

Figure 4. Experimentally determined band edges of the samples calcined at 400 °C. Displayed are the conduction band edge (blue bars), the valence band edge (black bars), and the mid-band gap edge (green lines).

mV to −0.12 V vs RHE. This effect is more pronounced when tungsten and nitrogen are both present. In the codoped samples the conduction band is changed further and reaches the value of +0.06 vs RHE in the case of WN-9.1-400. Although this phenomenon is observable in the codoped samples, no apparent dependency on the W content was found, suggesting that the conduction band shift is mainly dependent on the amount of N. Taking into account the respective values of the band gap, the position of the valence band was also calculated. This was situated at a potential of +3.07 to +3.18 V vs RHE with no apparent dependency on either nitrogen or tungsten content. Unfortunately, the conduction band edge of both the W-9.1-400 and WN-16.7-400 samples could not be determined due to large interference in the frequency region of the double layer capacitance, likely caused by the tungsten trioxide present in those two materials. As illustrated in Figure 5, a different behavior was observed for the samples calcined at 600 °C. The pure titania sample (W0-600), consisting mainly of rutile at this calcination temperature, displays a conduction band edge of −0.06 V vs RHE, which is typical of a rutile TiO2.81 Both the sample with only

Figure 3. Exemplary Mott−Schottky plot of one of the electrodes coated with the WN-0-400 sample. The space charge layer capacitance was derived by using the pseudocapacitance of the CPE in the equivalent circuit shown in the upper left corner. The individual data points (black circles) were extrapolated to the x-axis by using linear regression (red line).

yielded a linear regime that could be extrapolated to the x-axis by using linear regression. The thus calculated conduction band edges are an average of at least two measurements which were performed on separately prepared electrodes. The uncertainties given for the conduction band include both the error of the individual measurement as well as the variance between the separately prepared electrodes, combined according to the laws of error propagation. As seen in Figure 4, the conduction band edge of pure titania doped with neither nitrogen nor tungsten and calcined at 400 °C (W-0-400) was determined at −0.20 V vs RHE, in agreement with previously reported values for anatase.77,81 The addition of solely nitrogen (WN-0-400) already has a noticeable impact, shifting the conduction band edge by 84

Figure 5. Experimentally determined band edges of the samples calcined at 600 °C. Displayed are the conduction band edge (blue bars), the valence band edge (black bars), and the mid-band gap edge (green lines). E



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tungsten (W-9.1-600) and that with only nitrogen (WN-0-600) show a noticeably lower conduction band edge with −0.17 and −0.15 V vs RHE, respectively, which can be attributed to their lower rutile content of 2% and 4%, respectively. The samples with very low tungsten content (0.05 to 0.15 atom %) also display a conduction band edge of −0.18 to −0.20 V vs RHE, which is more in line with an anatase polymorph (−0.20 V vs RHE). With further increased tungsten content, however, the conduction band edge gradually increases until finally reaching a potential of +0.03 V vs RHE at the highest studied tungsten doping ratio, 16.7 atom % (WN-16.7-600). When taking a closer look at the valence band edge, resulting from the conduction band edge and the band gap, there appears to be a slight change in relation to the tungsten content, as well. While the pure titania (W-0-600) displays a valence band edge of +2.95 V vs RHE, this potential gradually increases with increasing tungsten content, reaching up to 3.16 V vs RHE (WN-4.8-600). In addition to the band edges, all of the samples with nitrogen doping also display a mid-band gap energy state, which is located at a potential of +2.32 to +2.51 V vs RHE, or 0.70 to 0.85 eV above the valence band edge, assuming the optical transition happens from the energy state to the conduction band as described previously.20,22,82,83 In addition to the conduction band edge, the impedance spectroscopy also yielded data for the donor density of the materials. The donor density was relatively unchanged throughout the different samples, yielding values ranging from 1.03 × 1020 to 3.49 × 1020 cm−3 without any observable trend in calcination temperature or tungsten or nitrogen content.

Figure 6. Change in the lattice parameter c of nitrogen-doped anatase titanium dioxide upon codoping with tungsten. Displayed are both samples calcined at 400 (open circles) and 600 °C (filled circles) and a linear regression as a red line.

elongated and the parameter c is shortened with increasing tungsten content, leading to less stress along the equatorial direction and a more uniform octahedral shape. The so modified anatase crystals are therefore less stressed and energetically more favorable. This may account for the added stability against conversion to rutile at elevated temperatures since the relative energy gain by transforming to rutile is much lower. The observed changes in lattice parameters are also in accordance with data provided by Štengl et al.,84 who also observed a decrease in c and an increase in a, but attributed it to tungsten doping alone. However, since they were using ammonium hydroxide in the synthesis they are likely to have formed a W/N-codoped material as well. The observed absorption band in the visible light region (400 to 550 nm) is most likely to be caused by nitrogen doping, as already observed by many other authors. Through EPR spectroscopy (as reported elsewhere85), we were able to confirm nitrogen doping and identify the species as interstitial nitrogen ([NO]′O) as described by Giamello and co-workers.20,22,29,33,83,86 Moreover, this absorption band exclusively appears when nitrogen species were present in the synthesis and is completely absent without. So this absorption feature is in no way connected to the tungsten addition, since even adding 9.1 atom % tungsten does not induce any visible light absorption without using any nitrogen-containing reagents. This is in contradiction to several previous publications reporting visible light absorption for tungsten-doped titania.75,84,87−90 However, in all of those reports, a nitrogenspecies-containing reagent was employed in the synthesis, e.g. ammonium tungstate or ammonium hydroxide. Given the observation in this work as to how easily titania is doped with nitrogen if tungsten is also present, we hypothesize that those synthesis actually yielded W/N-codoped materials as well. Consequently, the observed visible light absorption is a result of unintentional, undetected nitrogen (co)doping rather than tungsten doping. Adding tungsten does, however, seem to increase the level of nitrogen doping when both are present in the synthesis. Figure 7 displays the amount of nitrogen doping abstracted by the visible absorption at 450 nm in relation to the nominal tungsten content for the samples calcined at 600 °C. The nitrogen doping increases significantly with increasing tungsten content, displaying a saturation curve-like behavior. This is a clear

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DISCUSSION The addition of both nitrogen and tungsten seems to stabilize the anatase polymorph and suppress rutile formation at higher temperatures. While the pristine titania shows a rutile content of 72% after being calcined at 600 °C, the addition of nitrogen lowers this to 4% and the addition of 9.1 atom % tungsten to 2%. If both nitrogen and tungsten are present, there is a synergistic effect, completely suppressing rutile formation at 600 °C at tungsten ratios above 0.15 atom %. The refinement of the X-ray diffraction patterns revealed a slight increase in the a parameter and a more pronounced decrease in the c parameter with increasing tungsten content. Further analysis of this lattice parameter reveals a good linear correlation of c with the tungsten content (9.513 ± 0.002 Å − 0.0039 ± 0.0003 Å per atom % W). This correlation is shown in Figure 6 and is a good indication that the tungsten atoms are actually doped into the TiO2 lattice. The a parameter, however, does actually show a better correlation with the light absorption at 450 nm, proportional to the nitrogen-doping level. However, the overall change is so small and the correlation not definite enough to draw any conclusions from it. In the anatase and rutile polymorphs, there are two oxygen atoms in each octahedron with a longer Ti−O bonding length than the remaining four oxygen atoms. The distance for the longer bonding length is almost the same for both polymorphs, 1.9799 and 1.9796 Å for anatase and rutile, respectively.76 The shorter bonding length, however, is noticeably shorter in anatase, measuring 1.9338 Å instead of 1.9486 Å for rutile.76 Due to these shorter bonds, the octahedra in anatase are compressed and distorted in the equatorial direction. In the W/ N-codoped samples, however, the lattice parameter a is slightly F



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determined by EIS, was observed to be rather constant with little variation throughout the samples. Similarly, in nitrogen-doped titanium dioxide, the nitrogen’s excess negative charge has to be compensated. Even though we do not assume a substitutional nitrogen doping, replacing O2− by N3−, interstitial nitrogen associated with a lattice oxygen ([NO]O′ , typical of sol−gel synthetic routes and confirmed by EPR measurement85) also yields excess negative charge. The charge compensation can happen either through free holes (h•), effectively reducing the donor density or even turning the material p-type in the extreme case, or by creating additional 91 oxygen vacancies (V•• An example of these mechanisms, O ). considering ammonia as the nitrogen source, is expressed by the following mass-charge-balance equations: 2NH3 +

Figure 7. Amount of doped nitrogen, given by the absorption at 450 nm, in relation to the nominal tungsten content for the samples calcined at 600 °C. The red line is a visual guide to illustrate the saturation curve-like behavior.

1TiO2 3 O2 ⎯⎯⎯⎯⎯→ Ti ×Ti + 2[NO]′O + 3H 2O + 2h• 2 2TiO

2 × 2NH3 + O2 ⎯⎯⎯⎯⎯→ 2Ti ×Ti + 2[NO]′O + OO + V •• O

+ 3H 2O

3TiO2

(5)

In the codoped system, however, both dopant’s excess charge can also be compensated by each other:

indication that adding tungsten to titania actually facilitates nitrogen doping and stabilizes it at higher temperatures as well. When the samples are calcined at only 400 °C, the behavior is different. Even though it is quite obvious that the nitrogen doping is enhanced by codoping with tungsten, as well, a clear trend as observed in the case of the samples calcined at 600 °C could not be obtained. The amount of nitrogen doping under these conditions seems to be more dependent on other experimental parameters which were not well controlled in this case such as the amount of ammonia gas formed during the sintering process. Nonetheless, even minute amounts of tungsten seem to vastly facilitate nitrogen uptake at 400 °C: only 0.1 atom % of tungsten increases the nitrogen-doping level more than 6-fold as compared to a tungsten-free sample. Higher tungsten additions, however, do not further increase the nitrogen-doping level suggesting that saturation is already reached at 0.1 atom % of tungsten. The facilitated nitrogen uptake and its stabilization at higher temperatures by codoping with tungsten can be explained by charge compensation. W6+ is replacing Ti4+ ions in the TiO2 lattice (W•• Ti in the Krö ger−Vink notation commonly used for crystallographic defects), hence the excess of positive charge has to be compensated by a corresponding negative one. This is usually done by either ionic, eq 2, or electronic, eq 3, compensation: ′′′′ × 2WO3 ⎯⎯⎯⎯⎯→ 2W •• Ti + V Ti + 6OO

(4)

TiO

2 WO3 + O2 + 2NH3 ⎯⎯⎯⎯→ W •• Ti + 2[NO]′O + 3H 2O

(6)

Additionally, it has been shown both theoretically and experimentally that in the codoped system, both dopants preferentially occupy neighboring positions with several hundred lower energy of formation.62,85 For a complete compensation by this mechanism, two nitrogen dopants are needed for each tungsten dopant. Even though the exact amount of nitrogen ([NO]O′ ) doping could not be determined, the total nitrogen content of the samples (5 ± 2 atom %) should provide an upper limit. Given the lower formation energy of the nitrogen dopant in the codoped system it is reasonable to assume that charge compensation by complementary doping is the predominant mechanism (eq 6) and favored in contrast to compensating each with additional vacancies (eqs 2 and 5).62 In this case, it becomes clear that at least for the samples with higher tungsten content not all of the tungsten dopants can be compensated by nitrogen doping since the amount of tungsten greatly exceeds the amount of nitrogen. So at these higher concentrations, compensation by nitrogen doping is insufficient and there have to be additional titanium vacancies as well.62,92 Considering this mechanism, it is not surprising that doping with either nitrogen or tungsten increases the likelihood for the other since a charge compensated codoping system needs no other crystal defects to compensate. Also, this mechanism can be utilized to precisely adjust the level of nitrogen doping by controlling the tungsten doping level, especially at higher temperatures. Since it is well-known that the effects of nitrogen doping on the electronic structure of anatase and rutile are quite different,93−95 we will only evaluate those samples which consist exclusively of anatase in the following section. Samples with a small fraction of rutile or tungsten trioxide are ignored in the analysis since we cannot exclude that these minor phase contaminations might have an influence on the analysis of the anatase phase. As a single exception, our undoped reference sample (W-0-400) is allowed into the analysis even though it contains approximately 30% of brookite since its electronic

(2)

1 O2 (3) 2 In the case of the ionic compensation, the positive charge in excess is compensated by titanium vacancies (VTi ⁗) which have four negative charges. Consequently, there would have to be one titanium vacancy for every two tungsten ions. There is experimental evidence in EXAFS measurements of TiO2/WO3 solid solutions that the W−Ti coordination number is lower than 4, indicating a high number of cation vacancies and thus that tungsten charge compensation is achieved by titanium vacancies.75 Also, an electronic compensation, i.e. by excess electrons (e′), would lead to a noticeable increase in the donor density of the materials. The donor density, however, as 1TiO2

× WO3 ⎯⎯⎯⎯⎯→ W •• Ti + 2OO + 2e′ +

G



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inadvertently attributed the conduction band edge shift to tungsten doping instead. Similarly, the band gap changes were found to be a function of the nitrogen content as well. As seen in Figure 9, there is an

properties match exactly with reported values for pure anatase in the literature.3,77,81 There are some changes to the band structure of TiO2 upon codoping with tungsten and nitrogen. Most prominently, there is the well-known mid-band gap level induced by nitrogen doping. This extra energy level appears exclusively when ammonia is present during the synthesis and the edge of this level is situated 0.7 to 0.9 eV above the valence band. This is in excellent agreement with the 0.73 eV suggested by Di Valentin et al.20,33 for interstitial nitrogen doping, whereas the energy level of substitutional nitrogen would appear only 0.14 eV above the valence band edge. This can be taken as further confirmation that the main nitrogen species in these materials is indeed interstitial nitrogen. The second change in the band structure is the position of the conduction band edge. Most of the codoped samples exhibit a positive shift in the conduction band edge, up to as much as 0.26 V compared to pristine TiO2. Even though the results of the samples calcined at 600 °C suggest a dependence of the conduction band shift on the tungsten content, this observation is not mirrored by the samples calcined at 400 °C. Additionally, even the sample solely doped with nitrogen (WN0-400) has a reduced conduction band edge suggesting an involvement of the nitrogen dopant. Figure 8 shows a plot of the conduction band edge versus the absorption at 450 nm as a measure for the nitrogen content.

Figure 9. Measured optical band gaps of all samples calcined at 400 (open circles) and 600 °C (filled circles) that do not contain rutile or tungsten trioxide phases in dependence of their nitrogen content approximated by their optical absorption at 450 nm. The red line represents a weighted linear regression with 99% confidence band marked in slashed blue lines.

excellent fit for a linear dependency as well, with an intercept of 3.25 ± 0.01 eV and a slope of −0.78 ± 0.07 eV per absorption at 450 nm. The slope is slightly smaller than for the conduction band edge, leading to a very slight change in the valence band edge with nitrogen content as well, since it is obtained purely mathematically from both conduction band and band gap. However, since this change is vanishingly small with a considerable uncertainty (0.19 ± 0.20 V per absorption at 450 nm), the valence band edge is likely to be constant and independent of both nitrogen and tungsten doping. Furthermore, these findings are consistent with results obtained by other groups who reported a slight reduction in the band gap in nitrogen-doped titania caused by a positive shift in the conduction band edge as well but did not study enough samples to observe the linear dependency.36,38,39,96−99 In contrast to the present investigation Sakthivel et al. and Beranek et al. used the photovoltage method to determine the conduction band edge, proving that the positive shift in the conduction band is not an experimental artifact of the impedance spectroscopy and can be reproduced by an independent method.38,39,97,98 It should be mentioned that the proposed conduction band shift has yet to be predicted by any of the numerous theoretical calculations of the band structure of nitrogen-doped titania and is thus in apparent contradiction with theoretical predictions.33,34,40,41,45,47,100−102 However, the band gap narrowing is experimentally observed in virtually every report about nitrogen-doped TiO226−28,36−39,103−110 and cannot easily be dismissed as an experimental error or artifact given that the Tauc-plot method used to evaluate the band gap is well established and quite precise. That said there are only two possibilities to accomplish such a band gap narrowing, either by a positive shift in the conduction band edge or by a negative shift in the valence band. The latter has often been proposed as a consequence of nitrogen states in the band gap mixing with the valence band.19,41 However, current consensus is that the

Figure 8. Measured conduction band edge of all samples calcined at 400 (open circles) and 600 °C (filled circles) that do not contain rutile or tungsten trioxide phases in dependence of their nitrogen content approximated by their optical absorption at 450 nm. The red line represents a weighted linear regression with 99% confidence band marked in slashed blue lines.

This plot reveals a good linear correlation of both parameters and weighted linear regression yields an intercept of −0.17 ± 0.01 V vs RHE with a slope of 0.97 ± 0.13 V per absorption at 450 nm. This suggests that the shift in the conduction band position is caused by the nitrogen doping rather than the tungsten doping. This is further emphasized by the fact that the nitrogen-free but tungsten-doped sample (W-9.1-600) does not show a shift in the conduction band at all. In contrast to these observations there is a study by Zhang et al. that reports a conduction band shift of up to 0.21 eV upon doping with tungsten.88 However, considering that they used a nitrogencontaining species (tetramethylammonium hydroxide) and given the results obtained here, it is likely that they unintentionally formed W/N-codoped titania as well and H



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nitrogen-associated states within the band gap are distinct and do not hybridize with the valence band.86,94,100,111 There is only a single report where the valence band of N-doped titania was studied experimentally by X-ray photoelectron spectroscopy and therein no shift in the valence band was observed.37 This only leaves the conduction band shift as a plausible cause for the observed band gap narrowing, a theory supported by experimental evidence in both the present and other studies.36,38,39,96 The reason why theoretical calculations have yet to reproduce this effect probably originates in the complexity of the nitrogen doping itself. The fact that we have good reason to assume multiple nitrogen species present in our samples and that theoretical calculations usually assume only a single type of dopant may account for the different results of theory and experiment. The appearance of the positively shifted conduction band is counterintuitive since the conduction band of anatase is made up of empty Ti 3d orbitals and the additional nitrogen orbitals are thought to appear near the valence band so one would naturally expect a valence band shift upon nitrogen doping. A possible explanation could be an increase in oxygen vacancies which usually manifest as additional states slightly below the conduction band edge and could lead to a reduced band gap.40,50,51,91 However, while an increase in oxygen vacancies is expected for nitrogen doping via charge compensation, eq 5, it should not appear in the compensated codoped system. Another possibility would be that the interstitially doped nitrogen alters the polarization or geometry of the titanium ions, causing a change in the Ti 3d orbitals. Beranek et al. attribute the anodic conduction band shift to a partial oxidation of surface Ti4+.99 In any case, further investigation into this matter is necessary to elucidate which mechanism contributes to the shifted conduction band edge.

doping level has been reported and backed up by experimentally obtained data. Additionally, based on the insights gained in this study, there is a high likelihood that some previous reports incorrectly attributed observed effects to tungsten doping rather than unintentional, undetected nitrogen doping.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ¶

DECHEMA Research Institute, Theodor-Heuss-Allee 25, 60468 Frankfurt am Main, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work reported in this paper has been funded by the European Union’s Seventh Framework Programme (FP7) Project Light2Cat (Grant No. 283062, Theme Environment).

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REFERENCES

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CONCLUSIONS Tungsten−nitrogen-codoped titania was systematically studied by preparing a total of 24 samples with different tungsten loadings ranging from 0.05 to 16.7 atom % and calcined at two different temperatures, 400 and 600 °C. These samples were subsequently analyzed for their physicochemical properties and position of their band edges. By also studying samples doped exclusively with nitrogen or tungsten, individual changes in the materials were assigned as a consequence of either nitrogen or tungsten doping. Out of all the observed effects, the only ones that could be attributed to the tungsten doping are the change in the lattice parameter c and the facilitation of nitrogen doping. The latter is more pronounced at 600 °C calcination temperature, where nitrogen doping is less energetically favored. At 400 °C on the other hand, even a very low amount of tungsten is sufficient to induce maximum nitrogen doping and there is no benefit in increasing it further. The nitrogen doping, however, even though mainly a consequence of the tungsten doping, is responsible for the majority of the observed effects. There is an additional absorption band in the visible region that is caused by transitions from a mid-band gap energy state located 0.7 to 0.9 eV above the valence band to the conduction band. Also, the conduction band edge is shifted positively and the band gap is decreased accordingly, linearly dependent on the nitrogendoping level. To the best of our knowledge, this is the first time a linear conduction band change in dependence of nitrogenI



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