A Low-Band Gap, Nitrogen-Modified Titania Visible-Light Photocatalyst

Low-Band gap nitrogen-modified titania photocatalysts were prepared by calcining titanium hydroxide and urea at 400 °C. Different from previously kno...
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J. Phys. Chem. C 2007, 111, 11445-11449

11445

A Low-Band Gap, Nitrogen-Modified Titania Visible-Light Photocatalyst Horst Kisch,*,† Shanmugasundaram Sakthivel,† Marcin Janczarek,‡ and Dariusz Mitoraj† Institut fu¨r Anorganische Chemie der UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany, and Department of Chemical Technology, Gdansk UniVersity of Technology, ul. Narutowicza 11/12, 80-952 Gdansk, Poland ReceiVed: October 2, 2006; In Final Form: June 25, 2007

Low-Band gap nitrogen-modified titania photocatalysts were prepared by calcining titanium hydroxide and urea at 400 °C. Different from previously known nitrogen-doped titania (TiO2-N), which exhibits a strong band-to-band absorption in the ultraviolet and only a weak shoulder in the visible, the new materials (TiO2N1 and TiO2-N2) have an intense band-to-band absorption in the range of 400-500 nm, resulting in corresponding band gaps of 2.46 and 2.20 eV. As compared to unmodified titania, the quasi-Fermi level of electrons is anodically shifted by 0.07-0.16 eV. Whereas TiO2-N is inactive in visible-light mineralization (λ g 420 nm) of formic acid, TiO2-N1 is highly active.

Introduction Inspired by the principles of natural photosynthesis, the search for semiconducting materials catalyzing efficient visible-light photoredox reactions has become a central topic of basic and applied photochemistry. One promising approach seems to be the bulk or surface modification of well-known titania by maingroup elements like carbon1-3 and nitrogen.4-15 This generates localized electronic states (“surface states”) close to the valence band edge, which induce the appearance of a broad and weak visible absorption shoulder at the low-energy side (400600 nm) of the strong and steep band-to-band absorption of titania starting at about 400 nm. Accordingly, light absorption and photoactivity is much lower in the visible than in the UV. For a more efficient utilization of visible light in photocatalytic processes, it seems therefore worthwhile to search for a modified titania exhibiting a red-shifted and strong band-to-band absorption. Nitrogen-doped titania materials were prepared by various methods like precipitating titanium hydroxide from titanium salts or alcoholates with aqueous ammonia4,7 or ammonium salts.7,13,14 Further procedures are calcining TiO2 at 550-600 °C under an atmosphere of ammonia and argon,5 reactive DC magnetron sputtering,8 treating a titania sol with triethylamine at room temperature,16,17 oxidizing titanium nitride at 400-550 °C,6 and calcining solid mixtures of titania or titanium tetraisopropoxide and urea.15 For almost all of these materials, it was assumed that nitrogen became bound to titanium in the form of nitridic and NHx species, as evidenced by N 1s binding energies of 396.0-396.7 eV5,6,9,12 and 399.6 eV,12 respectively. Different from this, powders obtained by us employing a nitrogencontaining base for hydrolysis7 or by calcining the mixtures of titanium isopropoxide or titanium tetrachloride with thiourea at 400-600 °C contained the N 1s peak at 400.1 eV and an infrared absorption at 1387 cm-1.10 Only materials containing this IR absorption were active in visible-light photooxidation reactions. In the following, we report on preparation, photo* To whom correspondence should be addressed. Fax: (+49)91318527363. E-mail: [email protected]. † Institut fu ¨ r Anorganische Chemie der Universita¨t Erlangen-Nu¨rnberg. ‡ Gdansk University of Technology.

electrochemical characterization, and photocatalytic properties of nitrogen-doped titania exhibiting a strong red shift of the band-to-band absorption.18,19 Results and Discussion During work on the modification of titania by various nitrogen compounds,7,10 we replaced titania by titanium hydroxide, assuming this may lead to a better interaction with the modifier due to the higher surface OH group density as compared to that with titania. When the hydroxide powder was stirred in aqueous urea solution and subsequently calcined at 400 °C for 1 and 0.5 h, the samples TiO2-N1 and TiO2-N2, respectively, were obtained as yellow powders. Unmodified titania was prepared analogously, omitting urea. According to elemental analysis, TiO2-N1 and TiO2-N2 contain 1.01 and 11.70 wt % of nitrogen, respectively. Longer calcination times led to a white powder that was inactive in visible-light degradation of 4-chlorophenol (vide infra). This material contained 0.23 wt % of N in the form of nitrite and nitrate and exhibited a band gap of 3.14 eV, in agreement with the presence of unmodified titania. It is noted that replacing titanium hydroxide by self-prepared titania in the above procedure, under otherwise identical preparation conditions, affords a nitrogen-doped titania of high nitrogen content (13 wt %), which exhibits the same diffuse reflectance and IR spectrum as the recently reported material.10 According to XRD measurements, nitrogen-doped and -undoped titania, in addition to the major component anatase, contain also minor amounts of the brookite modification.20 From the (101) peak, a crystal size in the range of 7-10 nm is estimated for both samples through application of the Scherrer equation. A high specific surface area of 173 m2 g-1 was measured for TiO2-N1, whereas 67 and 167 m2 g-1 were obtained for TiO2-N2 and TiO2, respectively. The crystallites have diameters in the range of 5-10 nm and are packed to aggregates in the size range of 0.2-1.0 µm, as indicated by transmission electron micrography (Figure 1). Figure 2 displays the XPS N 1s spectra of TiO2-N1 and TiO2-N2. For both samples, before sputtering, the most intense peak was observed at 399.6-399.9 eV. Although this binding energy was previously proposed to arise from a hyponitrite

10.1021/jp066457y CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007

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Figure 1. TEM photographs of the TiO2-N1 powder; (a) crystallites, (b) aggregates.

Figure 2. N 1s XPS spectra of TiO2-N1 (A) and TiO2-N2 (B); a1 and a2 represent before and after sputtering, respectively.

species,21,22 by comparison with literature data, a final assignment cannot be made at the present (vide infra). IR spectra in KBr revealed clear differences between TiO2-N1, TiO2-N2, and self-prepared TiO2. Whereas the unmodified sample did not exhibit significant peaks between 700-2000 cm-1, various signals were observed for the two modified samples. In the case of TiO2-N1, a peak at 1329 cm-1 may be related to a nitrogen species in an oxidation state lower than three since it is transformed to nitrite and nitrate peaks upon prolonged calcination (vide supra). From this behavior and on the basis of comparison with the data reported for trans-hyponitrite adsorbed onto titania,21 its presence was postulated.10 However, it is unlikely that this species, even if complexed to titanium, will

survive a calcination temperature of 400 °C. Noteworthy, this peak was shifted to 1387 cm-1 in the previously reported nitrogen-doped titania.10 The TiO2-N1 spectrum additionally contains nitrite peaks at 1619, 1567, 1425, 1263, and 1172 cm-1, nitrate peaks at 1467 and 1502 cm-1, a fulminate (CNO-) peak at 2032 cm-1, and carbonate peaks at 1738 and 802 cm-1.3,21-25 The presence of the two latter species is also evidenced by the carbon content of 0.68 wt %, as determined by elemental analysis. The sample TiO2-N2 exhibited an almost identical IR spectrum. XPS measurements of TiO2-N1 revealed that upon argon ion sputtering for 2 min, the hyponitrite peak at 399.9 eV disappeared, and the concentration of nitrogen decreased from 2.44 to 0.00 atom %. This may suggest that TiO2-N1 is a coreshell particle consisting of a TiO2 core and a nitrogen-containing shell of a thickness of 3 nm or less. However, since the measurement does not involve a single crystallite but an aggregate of crystallites, this assumption remains speculative. In the case of TiO2-N2, a slightly shifted and a new peak appeared after 2 min of sputtering at 399.4 and 396.4 eV, respectively, and the nitrogen concentration changed from 18.65 to 5.83 atom %. After sputtering for 7 min, corresponding to the removal of an about 10 nm thick layer, TiO2-N2 did not exhibit any N 1s signal. The fact that nitridic nitrogen was observable only in TiO2-N2, the sample obtained at the shorter calcination time, suggests that this species is formed in the initial calcination phase and subsequently oxidized to an unknown nitrogen species and small amounts of nitrite and nitrate. In accord with this assumption is the fact that prolonged calcination affords an inactive material containing only nitrite and nitrate (vide supra). In Figure 3, the diffuse reflectance spectra of the two new materials are compared with undoped and conventionally doped titania, as prepared by a literature method (TiO2-N).5 Whereas the spectrum of TiO2-N (curve b) consists of a weak absorption shoulder in the visible followed by the strong titania band-toband absorption, the two novel powders (curves c and d) exhibit a strong and red-shifted band-to-band-type absorption arising from the nitrogen-modified surface layer. Assuming the materials to be indirect semiconductors, as is TiO2, a plot of the modified Kubelka-Munk function [F(R∞)E]1/2 versus the energy of absorbed light E26 affords band gap energies of 3.08, 2.46, and 2.20 eV for TiO2-N, TiO2-N1, and TiO2-N2, respectively (Figure 3 and Table 1).27 Thus, doping titania with 1.01 wt % of nitrogen affords a semiconductor of about the same band gap as cadmium sulfide (2.40 eV).

A Low-Band Gap, TiO2-N Visible-Light Photocatalyst

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Figure 3. (A) Diffuse reflectance spectra of (a) TiO2, (b) TiO2-N, (c) TiO2-N1, and (d) TiO2-N2 in BaSO4. The Kubelka-Munk function F(R∞) is equivalent to absorbance. (B) Plot of the transformed Kubelka-Munk function versus the energy of light absorbed.

TABLE 1: Nitrogen Content, Photoelectrochemical Data, Band Gap Energies, and Initial Mineralization Rates (ri) of 4-Chlorophenol catalyst

N%

pH0a

Ebg (eV)b

* nEF (V, NHE)c

ri (10-4 mg L-1 s-1)

TiO2 TiO2-N1 TiO2-N2 TiO2-N

0.00 1.01 11.70 0.48

3.9 5.1 6.7 7.8

3.18 2.46 2.20 3.14

-0.63 -0.56 -0.47 -0.22

0.75 13.67 4.17 1.35

a Measured according to ref 29 and calculated for pH ) 7. Reproducibility was better than (0.01 eV. c Reproducibility was better than (0.01 V.

b

Figure 5. Variation of the photovoltage with the pH value for a suspension of TiO2-N in the presence of (DP)Br2.

Figure 4. Variation of the photovoltage with the pH value for a suspension of the photocatalyst powder in the presence of (MV)Cl2; (a) TiO2, (b) TiO2-N1, (c) TiO2-N2.

To find out whether a shift of the valence or conduction band edge is responsible for this decrease of the band gap energy, the position of the quasi-Fermi level of electrons (nEF*) was determined through measuring the photovoltage as a function of the suspension pH value (Figure 4). From the pH value of the inflection point (pH0), the quasi-Fermi level at pH 7 could be calculated.28,29 The value of -0.63 V found for undoped titania was shifted to -0.56 and -0.47 V in the case of TiO2N1 and TiO2-N2, respectively (Table 1). Thus, as observed for the band gap energy, also the shift of the quasi-Fermi level increased with increasing nitrogen content. The anodic shift of 70-160 mV is in agreement with our recent results on nitrogendoped titania having only the commonly observed visible absorption shoulder in the diffuse reflectance spectrum.7,10 Surprisingly, the known TiO2-N affords the inflection point at pH 7.80, resulting in a quasi-Fermi level of -0.22 V at pH 7 (Figure 5and Table 1).30 Assuming that the distance between the quasi-Fermi level of electrons and the conduction band edge is vanishing for these probably highly doped n-type materials, one can locate the

Figure 6. Electrochemical potentials (vs NHE) of band edges and surface states (shaded areas, as estimated from the onset of the curves of Figure 3B) at pH 7; (a) TiO2, (b) TiO2-N, (c) TiO2-N1, and (d) TiO2-N2.

position of the valence band edge by adding the band gap energy to the quasi-Fermi level value. Potentials of 2.55, 1.90, and 1.73 V for TiO2, TiO2-N1, and TiO2-N2, respectively, were obtained for pH ) 7 (Figure 6). These strong cathodic shifts of 0.65 and 0.82 V differ significantly from previously obtained nitrogen-doped samples, which all exhibited only the weak visible absorption shoulder and a very small shift of the valence band edge.7,10 This new phenomenon of a strong band gap shift induced by doping titania with a main-group element apparently is not coupled to a high nitrogen content but rather to the detailed structure of the nitrogen dopant as evidenced, for example, by the similar nitrogen contents of 1.01 and 0.80% found for TiO2N1 and TiO2-NU, a sample prepared with thiourea as the nitrogen source.10 Whereas both in TiO2-N1 and TiO2-N2 the

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Kisch et al. accord with its smaller surface area and the poorer oxidizing and reducing properties of the light-generated charges (see Figure 6).31 The superior visible-light activity of the new photocatalysts is further demonstrated by their ability to induce the mineralization of hydroquinone, trichloroethylene, and formic acid (Figure 7B). Air pollutants like acetaldehyde, benzene, and carbon monoxide are also degraded (Figure 7C). Unmodified titania exhibited only a very low activity under identical experimental conditions. Note that conventionally prepared TiO2-N did not catalyze formic acid degradation (curve d), as also recently reported.19 Experimental Section

Figure 7. (A) Photomineralization of 4-chlorophenol (c ) 2.5 × 10-4 mol L-1; TOC0 and TOCt ) total organic carbon content at times 0 and t); (a) TiO2, (b) TiO2-N, (c) TiO2-N1, and (d) TiO2-N2. (B) Photomineralization of other pollutants by TiO2-N1; (a) hydroquinone (2.5 × 10-4 mol L-1), (b) formic acid (1 × 10-3 mol L-1), (c) trichloroethylene (2.5 × 10-4 mol L-1), and (d) photomineralization of formic acid (1 × 10-3 mol L-1) by TiO2-N. (C) Gas-phase photodegradation of acetaldehyde (a) (5 vol %), benzene (b) (5 vol %), and carbon monoxide (c) (5 vol %) in air in the presence of TiO2N1; λ g 420 nm.

doping nitrogen species exhibited an IR absorption peak at 1329 cm-1, it occurred at 1387 cm-1 in TiO2-NU, a material which contains only the common weak shoulder in its visible absorption spectrum. To explore the visible-light photocatalytic activity of the novel low-band gap semiconductors, degradation of the pollutants 4-chlorophenol, formic acid, trichloroethylene, hydroquinone, acetaldehyde, benzene, and carbon monoxide were investigated. Figure 7A illustrates the photomineralization of the ubiquitous water pollutant 4-chlorophenol with visible light (λ g 420 nm). Whereas unmodified and conventionally doped titania (TiO2N) after 6 h induced only 9 and 18% degradation (curves a and b), values of 75 and 38% were obtained in the case of TiO2N1 and TiO2-N2 (curves c and d), respectively. The lower activity of TiO2-N2 as compared to that of TiO2-N1 is in

Titanium Hydroxide. A solution of 0.25 mol L-1 of TiOSO4 (Alfa Aesar) was prepared by stirring the required quantity in 200 mL of doubly distilled water at room temperature until a clear solution was obtained. Thereafter, it was heated to 80 °C, and a solution of 1.25 mol L-1 of NaOH was slowly added until a pH value of 5.5 was reached. The formed gels were kept at 80 °C for 1 h under stirring followed by aging for 24 h at room temperature. Finally, titanium hydroxide was filtered off and dried under air at 70 °C. To obtain titania, 1 g of this powder was calcined for 1 h at 400 °C. N-Doped Titania. To a suspension of 3 g of titanium hydroxide in 50 mL of H2O was added 6 g of urea. After sonicating for 15 min and subsequent stirring for 24 h, water was removed in vacuo at 60 °C, and the residue was dried again in an oven at 100 °C for 1 h. Then, 1 g of the resulting powder was calcined in a 15 cm Schlenk tube under air at 400 °C for 1.0 and 0.5 h, affording TiO2-N1 and TiO2-N2, respectively; the calcination temperature was attained at a heating rate of about 5 °C min-1. The resulting powder was washed three times with water to remove urea decomposition products and finally dried under air at 70 °C. The particle morphology was observed with an analytical transmission electron microscope (Philips CM 30 T/STEM) at an acceleration voltage of 300 kV. The chemical composition of the samples was characterized by elemental analysis (Carlo Erba, CHNSO, E.A.1108) and X-ray photoelectron spectroscopy (PHI 5600 XPS). The samples for XPS measurements were pressed pellets of the powder attached to aluminum foil by silver lacquer. Argon ion sputtering was performed using a Penning source (Specs PS IQP 10/63; p ) 10-8 Torr; voltage ) 3.5 kV), and the sputtering rate was estimated by calibration using a SiO2 standard of known thickness. All XPS spectra were referenced to the C 1s peak at 284.8 eV from the adventitious hydrocarbon contamination. Fitting of the XPS data was accomplished using XPSPEAK41 software. A Shirley-type background subtraction was used. The surface area was determined by the BET (Brunauer-Emmett-Teller) method using a Gemini 2370 V.01 instrument. Diffuse reflectance spectra were recorded relative to barium sulfate from samples which were prepared by grinding mixtures of 0.25 g (Figure 2a, c, d) or 0.10 g (Figure 2b) of the titania powder with 2 g of BaSO4 in a mortar. A Shimadzu UV-2401 UV/vis spectrophotometer was used. Quasi-Fermi energies (nEF*) were measured according to the literature 28a using methylviologen dichloride (MV2+, Ered ) -0.44 V) or ethane-1,2-diyl-bridged diazapyrenium dibromide (DP2+, Ered ) -0.27 V) 29b as pHindependent redox systems. The obtained pH0 values were converted to the Fermi potential at pH 7 by the equation * 28a Reproducibility of the nEF (pH 7) ) Ered + 0.059(pH0-7). pH0 values was better than 0.2 pH units. In a typical experiment,

A Low-Band Gap, TiO2-N Visible-Light Photocatalyst 30 mg of catalyst and 6 mg of methylviologen dichloride were suspended in a 100 mL two-necked flask in 50 mL of 0.1 M KNO3. A platinum flag and Ag/AgCl served as working and reference electrodes and a pH meter for recording the proton concentration. HNO3 (0.1 M) and NaOH (0.1 M) were used to adjust the pH value. The suspension was magnetically stirred and purged with nitrogen gas throughout the experiment. Initially, the pH of the suspension was adjusted to pH 1 before measurement. The light source was the same as that used in the photodegradation. Stable photovoltages were recorded about 30 min after changing the pH value. Photodegradation experiments were carried out in a jacketed cylindrical 15 mL quartz cuvette attached to an optical train. Irradiation was performed with an Osram XBO 150 W xenon arc lamp installed in a light-condensing lamp housing (PTI, A1010S). A water filter and a 420 nm cutoff filter were placed in front of the cuvette. Running water was circulated through the jacket to ensure constant temperature of the reaction mixture, which was stirred magnetically. TOC measurements were made on a Shimadzu total carbon analyzer TOC-500/5050 with a NDIR optical system detector. Initial reaction rates were calculated from the disappearance rate of 4-CP; comparison of these rates is reliable since the photocatalyst concentration was high enough (1.0-2.0 g/l) to ensure complete light absorption in all individual runs. The concentration of 4-CP was monitored through its absorbance at 224 nm. A Varian CARY 50 Conc UV/vis spectrophotometer was used for recording absorption spectra. Gas-phase degradations were conducted in a 100 mL cylindrical vessel containing a TLC plate (Kieselgel 60/ Kieselgur F 254, Merck; 9 × 2.5 cm), onto which a suspension of 50 mg of TiO2-N1 in 3 mL of water had been cast onto and dried overnight at room temperature. The disappearance of substrates and appearance of carbon dioxide were followed by IR spectroscopy Acknowledgment. This work was supported by Deutsche Forschungsgemeinschaft. We thank Dr. Ing. Gerhard Frank for TEM measurements. References and Notes (1) Lettmann, C.; Hildenbrand, K.; Kisch, H.; Macyk, W.; Maier, W. F. Appl. Catal. B 2001, 32, 215. (2) Khan, S.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 5590. (3) (a) Sakthivel, S.; Kisch, H. Angew. Chem. 2003, 115, 5057. Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (4) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, A.; Taga, Y. Science 2001, 293, 269, and references therein. (6) Morikawa, T.; Asahi, R.; Ohwaki, T.; Aoki, A.; Taga, Y. Jpn. J. Appl. Phys. 2001, 40, 561.

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11449 (7) Sakthivel, S.; Kisch, H. ChemPhysChem 2003, 4, 487. (8) Lindgren, T.; Mwabora, J. M.; Avendano, E.; Jansson, J.; Hoel, A.; Granqvist, C.-G.; Lindquist, S.-E. J. Phys. Chem. B 2003, 107, 5709. (9) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (10) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384. (11) Miyauchi, M.; Ikezawa, A.; Tobimatsu, H.; Irie, H.; Hashimoto, K. Phys. Chem. Chem. Phys. 2004, 6, 865. (12) Diwald, O.; Thomson, T. L.; Zubkov, T.; Goralski, Ed. G.; Walck, S. D.; Yates, J. T. J. Phys. Chem. B 2004, 108, 6004. (13) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Mater. Sci. Eng., B 2005, 117, 67. (14) Livraghi, S.; Votta, A.; Paganini, M. C.; Giamello, E. Chem. Commun. 2005, 498. (15) (a) Nosaka, Y.; Matsushita, M.; Nishino, J.; Nosaka, A. Y. Sci. Technol. AdV. Mater. 2005, 6, 143. (b) Bacsa, R.; Kiwi, J.; Ohno, T.; Albers, P.; Nadtochenko, V. J. Phys. Chem. B 2005, 109, 5994. (16) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J. D.; Gole, J. L. Nano Lett. 2003, 3, 1049. (17) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen, X. J. Phys. Chem. B 2004, 108, 1230. (18) The only exception was reported in ref 3. However, upon repetition of this work, we and others19 obtained a material showing also only the weak visible absorption. Furthermore, upon irradiation (λ g 320 nm) of this material in an aqueous suspension for 3 h, its nitrogen content decreased from 3.13 to 0.19 wt %. For comparison, the same value changed only from 1.01 to 0.85 wt % when the novel TiO2-N1 was irradiated even for 10 h under identical experimental conditions. (19) Mrowetz, M.; Balcerski, W.; Colussi, A. K.; Hoffmann, M. R. J. Phys. Chem. B. 2004, 108, 17269. (20) The identification of the different crystalline phases was accomplished using the JCPDS database. (21) Navio, J. A.; Cerrillos, C. C.; Real, C. Surf. Interface Anal. 1996, 24, 355. (22) Gablenz, S.; Abicht, H.-P.; Pippel, E.; Lichtenberger, O.; Woltersdorf, J. J. Eur. Ceram. Soc. 2000, 20, 1053. (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley: New York, 1969. (24) Hadjiivanov, K.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2000, 2, 2803. (25) It is noted that these carbon species do not induce visible-light activity; see ref 3. (26) Karvaly, B.; Hevesy, I. Z. Naturforsch., A: Astrophys, Phys. Phys. Chem. 1971, 26, 245. (27) This corresponds to absorption onsets of 403, 504, and 563 nm, respectively. (28) (a) Roy, A. M.; De, G. C.; Sasmal, N.; Bhattacharyya, S. S. Int. J. Hydrogen Energy 1995, 20, 627. (b) Kisch, H.; Burgeth, G.; Macyk, W. AdV. Inorg. Chem. 2004, 56, 241. (29) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 27. (30) When nitrogen doping was not performed to completeness, the resulting powder exhibited pH0 values of 6.7 and 4.3, indicating that it consists of a mixture of unmodified (pH0 ) 4.3) and modified (pH0 ) 6.7) titania. This is a further example of the usefulness of this method for testing the homogeneity of semiconductor powders; see, for example: Rusina, O; Macyk, W.; Kisch, H. J. Phys. Chem. B 2005, 109, 10858. (31) It is noted that different from a previous report,32 the self-prepared, unmodified titania does not catalyze visible-light degradation of 4-chlorophenol. (32) Kim, S.; Choi, W. J. Phys. Chem. B 2005, 109, 5143.