Highly Efficient Formation of Visible Light Tunable TiO2-xNx

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J. Phys. Chem. B 2004, 108, 1230-1240

Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale James L. Gole* and John D. Stout School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332

Clemens Burda,* Yongbing Lou, and Xiaobo Chen Department of Chemistry, Case Western UniVersity, CleVeland, Ohio 44106 ReceiVed: July 14, 2003; In Final Form: September 29, 2003

Using a simple nanoscale exclusive synthesis route, TiO2-xNx photocatalysts that can be tuned to absorb across the visible region are produced in seconds at room temperature. The photocatalysts are formed by employing the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. Depending on the degree of TiO2 nanoparticle agglomeration, catalytically active TiO2-xNx anatase structured particles are obtained whose absorption onset extends well into the visible region at λ ∼ 550 nm. The introduction of a small quantity of palladium in the form of the chloride or nitrate facilitates further nitrogen uptake, appears to lead to a partial phase transformation, displays a counterion effect when compared also to the acetate, and produces a material absorbing well into the near-infrared. The introduction of palladium via the chloride also facilitates the formation of small tetrahedral and octahedral palladium-based crystallites throughout the TiO2-xNx lattice. Surprisingly, no organics appear to be incorporated into the final TiO2-xNx products. The resulting photocatalysts readily photodegrade methylene blue and lead to the catalytic oxidation of ethylene as they are placed as gels on surfaces. In contrast to the current nitridation process, which is quite facile at the nanoscale, we observe a much slower nitration of Degussa P25 nanopowders and little or no direct nitridation of micrometer-sized anatase or rutile TiO2 powders at room temperature. We thus demonstrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron materials.

Introduction As an effective means of alleviating very low concentrations of pollutants including airborne pathogenic microorganisms, viruses, and volatile organic contaminants, semiconductor-based photocatalysis1 has attracted extensive interest, especially following the discovery of the photoinduced decomposition of water on TiO2.2,3 Because photocatalysts are capable of remediating concentrations that are difficult and/or expensive to eliminate with absorption, condensation, and thermal oxidation,2,4 they continue to be of considerable interest. Here, a continued focus on TiO2 has resulted because of its efficiency, low cost, chemical inertness, and photostability.5-17 However, the widespread technological use of a TiO2 photocatalyst has been hampered by its wide band gap (3.2 eV for anatase TiO2) and the requirement of ultraviolet radiation (λ < 380 nm) for photocatalytic activation. The sun can provide an abundant source of photons; however, UV light accounts for only a small fraction (∼5%) of the sun’s energy compared to the visible region (45%). An efficient process that shifts the optical response of active TiO2 from the UV to the visible spectral range and to longer wavelength can provide a framework to more easily incorporate the photocatalytic and solar efficiency of this material.18 Further, the cost and accessibility of UV photons make it desirable to develop photocatalysts that are active under visible light excitation using the solar spectrum or even interior room lighting. Here we detail19 a fast, cost efficient, means to * Authors to whom correspondence should be addressed. E-mail: J.L.G., [email protected]; C.B., [email protected].

address this problem as we outline a direct, room temperature process leading to the formation of nitrogen-doped, stable, and environmentally benign TiO2-xNx photocatalysts whose optical responses can be tuned across the visible to the near-infrared spectral region. The produced TiO2-xNx photocatalysts show an enhancement in the photodegredation efficiency of methylene blue under visible light irradiation that appears to exceed that previously attained with commercially available TiO2 photocatalysts or using more lengthy and complex doping approaches at elevated temperature. The utilization of a nanoscale synthesis is the key to the rapid formation of these photocatalysts at room temperature. Several attempts have been made to convert the TiO2 absorption onset from the ultraviolet to the visible region using transition metal doping20-23 or hydrogen reduction.24,25 Recently Asahi et al.18 have prepared TiO2-xNx films by (1) sputtering TiO2 targets for several hours in an N2 (40%)/Ar gas mixture and then annealing in N2 gas at 550 °C for 4 h, and (2) treating anatase TiO2 powders in an NH3 (67%)/Ar atmosphere at 600 °C for 3 h to produce photocatalysts whose absorption extends to wavelengths less than 500 nm. Khan et al.26 have recently used a flame pyrolysis at 850 °C to produce carbon-substituted n-TiO2 absorbing light at wavelengths up to 535 nm. Most recently, Irie et al.27 have presented an excellent discussion of previous nitrogen doping studies and considered the nitrogen concentration dependence on the photocatalytic activity of TiO2-xNx powders prepared by annealing anatase TiO2 under an ammonia flow at temperatures between 500 and 600 °C. In contrast, in the present study, at room temperature, we extend

10.1021/jp030843n CCC: $27.50 © 2004 American Chemical Society Published on Web 12/25/2003

Visible Light Tunable TiO2-xNx Photocatalysts further into the nanometer regime to produce catalytically active TiO2-xNx nanoparticles that absorb well into the visible and near-infrared region. Particle size variation within the nanoscale regime would appear to facilitate the tuning of this catalyst absorption region. Experimental Section Synthesis and Characterization of Nitrogen-Doped TiO2. The TiO2 nanoparticle colloidal solutions were prepared, as previously described,28-30 through the controlled hydrolysis of titanium(IV) tetraisopropoxide in water. A 5 mL aliquot of Ti[OCH(CH3)2]4 (Aldrich 97%), dissolved in isopropyl alcohol (5:95 volume ratio), was added dropwise (1 mL/min) to 900 mL of distilled water at pH 2 (adjusted with HNO3). A clear nanocolloid solution is generated after continuous stirring of this mixture for 12 h. Using transmission electron microscopy (TEM), it has been demonstrated that nanoparticles ranging in size from 3 to 11 nm can be synthesized,30 which are stable for extended periods under refrigeration. In an alternate synthesis31 using acetic acid, 250 mL of doubly ionized water and 80 mL of acetic acid were combined in a 1 L flask as the mixture was cooled to 0 °C in an ice bath under vigorous stirring. A 10 mL aliquot of 2-propanol followed by 3.7 mL of Ti[OCH(CH3)2]4 is slowly added dropwise from a dropping funnel fixed to the flask, under a dry nitrogen atmosphere, and again vigorously stirred. Continued stirring of the initial mixture for 24 h produces a clear colloid solution with particle sizes ranging from 5 to 20 nm. This clear solution can also be maintained under refrigeration for several months. Extended exposure of the colloid solutions to air at room temperature or heating to 50 °C produces an agglomeration of the nanoparticles and results in the formation of a gel that can range from transparent to virtually opaque as the function of an increasing nanoparticle size distribution. We have treated both an initial nanoparticle colloidal solution and a partially agglomerated gel (solution) nitriding the TiO2 nanocolloid to TiO2-xNx. These experiments have been done in two modes, both of which involve the direct exposure of the nanocolloid to an alkylammonium salt. In one configuration, we place 5 mL of the nanocolloid solution or ∼0.5 g of the agglomerated gel into a 25 mL beaker. As the colloidal solution is mixed with a Teflon-coated magnetic stirrer, an excess of triethylamine (Alfa Aesar 99+%) is added, bringing the total solution volume to 15 mL. The agglomerated gel is treated directly with the amine, creating a less viscous solution that is thoroughly stirred. Reaction to form a nitrided TiO2-xNx nanocolloid is seen to take place in seconds coincidental with heat release. Metals,32 including Pd, can also be introduced to the system. In the present study we outline results obtained by introducing palladium in the form of 0.05 g of PdCl2 or Pd(C2H3O2)2 (Alfa Aesar metals grade 99.98%) or in the form of an 8.5 wt % Pd[NO3]2 solution (20 drops (1 drop ) 0.25 cm3)). After the introduction of these palladium compounds to the TiO2 nanocolloid, we observe their dispersal throughout the stirred solution. Subsequently, the introduction of an excess of triethylamine to this stirring solution again results in rapid reaction. Alternatively, and primarily for the nanocolloid solutions, the above mixtures can be prepared in small vials of total volume 2.5 mL. Here 1 mL of the TiO2 nanocolloid solution is treated33 with 1.5 mL of triethylamine and/or hydrazine (Alta Aesar 98.5%). The triethylamine when added to the colloid solution initially forms an observable interface. However, upon rapid agitation of the closed vial, reaction is induced with a heat release that can be readily discerned by hand as the vial is held.

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1231 Similarly, palladium can be introduced first to the colloid (in the form of the chloride (0.01 g), acetate, or nitrate) followed by the introduction of the amine and agitation of this mixture. The nanocolloid solution, after direct treatment with triethylamine (Et3N) forms a yellowish, partially opaque mixture. Upon evaporation of the Et3N, with continued stirring of the solution, and vacuum-drying (5 × 10-2 Torr) for several hours, the dried, treated, mixture forms deep yellow crystallites. The partially agglomerated nanoparticle gel solution, also similarly treated, including a final vacuum-drying, is found to form orange crystallites. The palladium-treated samples (when allowed to remain for several minutes) in air transform to a deep black solution. This transformation can be accelerated upon stirring of the reacted colloid-amine mixtures. The nitrided nanocolloids can be prepared to test their photocatalytic activity (1) as a solution, which can be made continuously more viscous with stirring (to evaporate excess amine), thus facilitating the distribution of this material over a surface to which it adheres and can be dried,34 or (2) as dried nanocrystallites, which can be brought into direct contact with those molecules whose conversion is to be achieved through photocatalysis. The nanocrystallite products of straightforward and rapid synthesis at the nanoscale have been characterized by electron microscopy (Philips CM20 TEM at 200 kV) and X-ray diffraction (XRD-Philips PW 3710), X-ray photoelectron (PHI 5600 XPS System), UV-visible reflectance (Cary 5000), laser Raman (Ocean Optics), and infrared spectroscopies. The reflectance spectra were measured using a diffuse reflectance accessory (from Varian) using, as background, the MgO spectrum. The samples, used as powders, were pressed in sample holders to be measured in reflectance mode. Evaluation of Photocatalytic Activity. The photocatalytic activity of the TiO2-xNx particles was evaluated at excitation wavelengths of 390 and 540 nm using a Clark MXR 2001 fs laser system. The laser beam (800 fs, 1 kHz, 120 fs laser pulse train) was sent either through a BBO crystal to generate second harmonic 390 nm (10 mW) light pulses or to an optical parametric amplifier to generate stable 540 nm (4 mW) pulses. The laser light intensity was adjusted with a neutral density filter wheel. The pulse train was guided into a quartz cuvette filled with a 2 mL aqueous solution of methylene blue (optical density ) 0.8) and 10 mg of the new catalyst, to excite a pump volume of about 5 nL (0.5 mm is the diameter of the excitation beam at the reaction cell). The decomposition of the solute was followed by measuring the decolorization of the methylene blue in solution with a Varian Cary Bio50 UV-vis spectrometer. The reported quantum yield for decomposition was based on the analysis of the photon flux, taking into account the volume of the excitation region and including the dilution factor for evaluating the optical density changes depicted. The process requires low intensity when compared with bright sunlight. Results The results of transmission electron microscopy studies (TEM, HRTEM) including selective area diffraction (SAD) on the yellow TiO2-xNx crystallites are depicted in Figure 1. Figure 1a corresponds to a low-resolution TEM image whose inset is magnified in Figure 1b. This figure corresponds to an HRTEM image that reveals the crystal structure of the nitrided sample confirmed also by SAD (inset) to be of the anatase form. We have also obtained evidence for nitrogen uptake using bright field TEM and EELS imaging. The study is further complemented by X-ray powder diffraction. The data in Figure 2a,b correspond to XRD powder patterns for the nitrided and partially

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Figure 1. TEM and HRTEM images of TiO2-xNx nanostructures: (a, left) low resolution TEM image; (b, right) HRTEM image showing the polycrystalline character and lattice planes for the region indicated in the low resolution image. The HRTEM image corresponds to an anatase crystal structure confirmed by the SAD pattern (inset).

Figure 2. XRD powder diffraction patterns for (a, top) untreated Degussa P25 TiO2 nanopowder showing a dominance of the anatase (/) over the rutile (o) form and (b, bottom) nitrided (TiO2-xNx) partially agglomerated nanoparticles corresponding to an anatase structure associated with the sharply rising reflectance spectrum at 550 nm in Figure 3.

TABLE 1: Summary of Lattice Parameters from Figure 2a sample

phase

a (Å)

4.5986 not-processed TiO2 rutile anatase 3.7862 orange TiO2 anatase 3.7942

a, std error (Å)

c (Å)

c, std error (Å)

0.0006 0.0004 0.0032

2.9634 9.5070 9.4676

0.0006 0.0011 0.0075

a

Lattice parameter calculations made using the CHEKCELL software program developed by Jean Laugier and Bernard Bochu (http// www.ccp14.ac.uk/tutorial/1mpg).

agglomerated TiO2-xNx nanoparticle gel (orange crystallites) and untreated Degussa P25 TiO2. The corresponding XRD data analysis is given in Table 1. The XRD and HRTEM results demonstrate that both the treated TiO2-xNx nanoparticle structures correspond dominantly to the anatase crystalline form of TiO2-xNx (as do the original nanoparticle crystallites). The Degussa P25 nanopowder sample, though dominantly of the anatase form, also contains some notable contribution from the rutile structure (ratio approximately 3:1). A clear peak broadening in Figure 2b, versus that for Degussa P25, results from the small size of the agglomerated nanoparticles and suggests that even the orange crystallites generated from the gel solution have a smaller average size than the Degussa nanopowder. As estimated from the Debye-Sherer

Figure 3. Reflection spectra for Degussa P25 TiO2 onsetting sharply at ∼380 nm, nitrided (TiO2-xNx) nanoparticles whose spectrum rises sharply at ∼450 nm, and nitrided (TiO2-xNx) partially agglomerated nanoparticles whose spectrum rises sharply at ∼550 nm. See text for discussion.

equation,35,36 the average grain size of the TiO2-xNx is close to 10 nm (vs 30 nm for Degussa). Figure 3 compares the optical reflectance spectrum for Degussa P25 TiO2 (reported at an average size of 30 nm) onsetting sharply at ∼380 nm, the reflectance spectrum for the nitrided (TiO2-xNx) crystallites generated from the nanoparticle solutions, rising sharply at ∼450 nm, and the corresponding spectrum for nitrided (TiO2-xNx), partially agglomerated nanoparticles, rising at ∼550 nm. The sites that give rise to this shift in reflectance may correspond to titanium ion (Ti4+)-oxygenhole centers, near the surface of the nanoparticles, which are highly perturbed by the nitrogen.37 We have also introduced

Visible Light Tunable TiO2-xNx Photocatalysts

Figure 4. Reflectance spectrum for palladium metal impregnated TiO2-xNx whose XPS spectra are depicted in Figures 8a and 9a.

Figure 5. TEM micrograph of nitrided Degussa P25 TiO2. The powder, which is light tan in color, incorporates nitrogen at a much slower rate than the TiO2 nanocolloid solutions and gels.

PdCl2 into the nitriding amine-TiO2 mixture. The corresponding reflectance spectrum, obtained for nitrided nanoparticles with palladium incorporation via PdCl2 (e0.05 g added to the nitriding solution, which produces an impregnation of the TiO2-xNx structure with reduced Pd nanocrystallites), appears to result in the conversion of some of the TiO2-xNx anatase structure to a material that displays an even broader response extending to considerably longer wavelength (Figure 4). The treatment of the nitriding amine-TiO2 mixture with Pd (acetate)2 or Pd(NO3)2 does not appear to lead to significant nanocrystallite formation31 although the transformation of these materials to a black solution also leads to an extended optical response. In contrast to the nanoparticle activity we have outlined above, no measurable reaction or heat release is observed as either distinct rutile or anatase TiO2 micropowders are treated directly with an excess of triethylamine. The treatment of DeGussa P25 “nanopowder” (mean distribution g30 nm) results in some conversion in a much slower transformation. A TEM micrograph of the treated particles is depicted in Figure 5. They form a pale brown crystalline material that gives rise to a complex reflectance spectrum.38 The TiO2 nanoparticle solutions also react strongly with hydrazine39 and to a lesser extent with ammonium hydroxide (NH3) solution. However, the reaction with N(Et)3 is found to be surprisingly facile, leading to rapid nitrogen incorporation (Figure 1 and following) into the TiO2 lattice to form TiO2-xNx when the direct nitridation process is carried out with 5-20 nm particles. Finally, it should be noted that as the nanocolloid is dried to a gel, it forms a considerably more transparent material than does Degussa P25 (whose visible light photocatalytic response the nanocolloid greatly exceeds).

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Figure 6. Infrared spectra for (a) triethylamine showing a clear C-H stretch region and (b) nitrided nanocolloid particles corresponding to the yellow crystallites whose reflection spectrum rises sharply at 450 nm.

The infrared spectra40 depicted in Figure 6 demonstrate no evidence for hydrocarbon incorporation in the final doped TiO2 product. The IR spectrum (a) corresponds to that for the trialkylamine, demonstrating, among other features, the alkyl C-H stretch region. In contrast, the IR spectrum (b), corresponding to the yellow TiO2-xNx nanocrystallites (yielding the ∼450 nm reflectance spectrum) pressed into a KBr pellet, shows virtually no infrared spectrum, especially in the C-H stretch region. This indicates virtually no residual organic incorporation after the air and vacuum-drying processes outlined in the Experimental Section have been performed on the nitrided TiO2 nanoparticles. This observation is consistent both with photoelectron (XPS) and X-ray diffraction (XRD) studies. XPS studies, with Ar+ sputtering, which probe the surface region of the nanoparticles, indicate the presence of nitrogen not only at the surface but also incorporated into the sublayers of the TiO2-xNx nanoparticle agglomerates. These studies suggest a range between 3.6 and 5.1 atomic % increasing to between 7.3 and 17.6% for the Pd(Cl)2- and Pd(NO3)2-treated samples, respectively, where the Pd appears to facilitate nitrogen uptake albeit at different levels depending upon the palladium compound introduced to the nanocolloid. Appropriate XPS spectra for TiO2 and TiO2-xNx are compared in Figures 7 and 8. Figure 7a corresponds to that for the untreated TiO2 nanocolloid and shows no discernible N 1s peaks. Figure 7b corresponds to that resulting from a nitrided TiO2-xNx nanocolloid solution, indicating the region of the nitrogen 1s peaks. A more detailed scan of the N 1s region shows a strong feature peaking at 400.7 eV and a very weak 396 eV feature. Figure 8 demonstrates the enhancement of intensity and modification of the N 1s peak region when palladium is incorporated into the aminating mixture using PdCl2, Pd(acetate)2, and Pd(NO3)2. Figures 9 correspond to higher resolution scans of the N 1s region showing a virtual absence of the 396 eV peak associated with the formation of β-N and TiN films by Saha et al.41 However, a broad peak combination extending from ∼397 to 404 and peaking at ∼400.7 eV is observed for the PdCl2-treated samples (as well as for doped TiO2-xNx, which is untreated). This peak is slightly skewed to lower binding energy. In contrast, a broad feature extending from ∼398 to 403 and peaking at 400.7 eV is found to be strongly skewed to higher binding energy for the sample treated with Pd (acetate)2. The Pd(NO3)2-

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Figure 7. XPS spectra for (a) untreated TiO2 nanoparticles formed as a nanocolloid and (b) nitrided TiO2 nanoparticles (TiO2-xNx) in the same size range, which result in the 450 nm reflectance spectrum in Figure 3. A more detailed scan of the N 1s region shows that the N 1s peak is at 400.7 eV. See Table 2 and text for discussion.

TABLE 2: Elemental Percentages Determined from XPS TiO2 TiO2-xNx C%b 24.8 O% 52.3 Ti% 22.4 Pd% 0 Cl% 0 N% 0.6 N 1s peak position (eV)

33.2 43.2 18.5 0 0 5.1 400.7

Pd(NO3)2a 44.1 33.4 13.8 1.2 0 17.6 401.6, 406.5

Pd(acetate)2a PdCl2a 32.9 47.7 18.0 0.3 0 3.0 ∼400.7

46.1 28.7 8.4 3.1 6.2 7.3 400.7

a Indicates palladium compound added to aminating TiO nano2 colloid. b Samples on a carbon background.

treated samples display a strong nearly symmetric peak extending from ∼397 to 403 and peaking at 401.6 eV. However, a distinct peak also extends from 405 to 408.8 and peaks at 406.5 eV for this Pd(NO3)2-treated aminated sample (Figure 9c). The 396 eV peak that is thought to signal the presence of substitutional nitrogen associated with the formation of a TiN bond in films and at surfaces may not be apparent, but Raman spectra42 of the nitrided samples consistent with the XPS data suggest nitrogen incorporation in the form of several monolayers of the oxynitride. The data in Figure 10 demonstrate the fit to the Raman lines of nitrided TiO2, associated with the 450 nm reflectance spectrum, showing evidence for a TiOxNx-1 feature that has been attributed to the first-order scattering of nonstoichiometric titanium nitride.43 Further, we note broad features extending from the oxygen 1s and Ti 2F peaks (Figures 7b and 8a-c) that may also be consistent with the oxynitride.43 In addition, considerable satellite structure follows the N 1s peak for the PdCl2- and Pd(NO3)2-treated samples. The combination of data summarized in Figures 7-10 suggest the ready incorporation and partial substitution of nitrogen for oxygen in the initially treated TiO2 nanocolloids. The parameters associated with the data in Figures 7-10 are summarized in Table 2, where we also note the signals for palladium and its counterions in the respective spectra for the PdX2-treated (X ) Cl, acetate, NO3) samples.

Although the XPS spectra suggest that the incorporation of nitrogen dominates the nitridation process, we cannot unequivocally rule out the incorporation of some carbon26 resulting from the use of an organic agent in the nitriding process or through the presence of the acetate counterion. The XRD data taken for Degussa P25 TiO2 when compared to that for the nitrided partially agglomerated TiO2 gel solution (Figure 2, Table 1) also suggest an expansion of the “a” lattice parameter for the orange nanocrystallites, due presumably to nitrogen incorporation. We note that XRD can be a sensitive tool for determining whether nitrogen, as a dopant, is actually incorporated on interstitial lattice sites of the TiO2 particles or merely absorbed at the surface. The existence of interstitially dissolved nitrogen and the perturbation of the TiO2 lattice structure38,44 might be associated, at least in part, with γ-N2 features that contribute to the XPS spectra (see following discussion). Further, the nitrogen incorporation indicated by the XPS and Raman spectra are consistent with both the XRD and additional EELS analyses. The analysis of both the XRD (Figure 2) and HRTEM (Figure 1b) data clearly demonstrates the presence of a dominant anatase phase. We find no evidence for any degree of conversion from the anatase to the rutile structure on nitrogen doping18 for either the initial TiO2 nanocolloid or agglomerated gel solutions. Figure 11 corresponds to transmission electron micrographs of the products of simultaneously palladium-treated and nitrogendoped nanocolloid solutions where the palladium is introduced in the form of PdCl2 (Figure 11a,b), (Pd(C2H3O2)2)3 (Figure 11c), and dilute Pd(NO3)2 (Figure 11d), respectively. Parts a and b of Figure 11 demonstrate both the impregnation of the doped TiO2-xNx structure with smaller “reduced” palladiumbased nanocrystallites and also suggest the possibility of a significant alteration in structure consistent with a broad and complex XRD pattern38 (the dark regions indicated in Figure 11 may correspond to the octahedrite (TiO2) analogue of the oxynitride). Closer examination of the impregnated regions reveals a combination of what appear to be dominating tetrahedral (appearing triangular) and octahedral palladium-based

Visible Light Tunable TiO2-xNx Photocatalysts

Figure 8. XPS spectra for palladium-treated and nitrided TiO2 where the palladium is obtained from (a) PdCl2, (b) Pd(acetate)2, and (c) Pd(NO3)2, respectively.

crystallites interspersed nearly uniformly throughout the TiO2-xNx nanoparticle framework. To a decreasing extent, these palladium crystallites can also be identified in the TEM micrographs of

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1235 the structures generated by introducing the acetates and nitrates of palladium. In the latter case, however, this may result from the decreased palladium concentration inherent to the nitrate solution. Finally, we note the distinctly different effect of the acetate and nitrate in the aminating process. Although the acetate appears to produce an agglomeration and annealing of the TiO2-xNx nanostructures providing a low contrast TEM image and larger structural framework, the nitrate-treated sample clearly displays a structure consisting of a significant grouping of nanostructures, suggesting a considerably enhanced effective surface area. Photocatalytic Activity of Nitrogen-Substituted TiO2-xNx. Photocatalytic activity of the doped TiO2-xNx nanoparticles was evaluated by adding 10 mg of the powder into a 3 mL quartz cuvette filled with methylene blue solution in DI water. The mixture was stirred until all powder appeared to be completely dissolved. The filled cuvette was then exposed to irradiations of various kinds from filtered xenon light to laser pulse irradiation. We preferred to conduct the experiments under illumination with femtosecond laser pulse trains because this mode of excitation allowed us to work under readily reproducible conditions, which allowed us to tune more easily. Following the exposure to light, the decolorization of methylene blue, at its absorption maximum (650 nm), was recorded as a function of time. The generation of electron-hole pairs following light excitation affords the possibility of both reductive reaction by electrons and oxidative reaction by holes. Mills and Wang45a have demonstrated that methylene blue can react by competing reaction mechanisms under either aerobic or anaerobic conditions to decolorize the methylene blue. By the aerobic mechanism, an electron from the conduction band reduces molecular oxygen to its radical anion, and this species then reacts further, resulting in the oxidation of methylene blue. Ollis45b amplifies this mechanism by saying that OH radicals are formed directly by electron-hole pairs at the titania surface either (1) as an OH anion is oxidized to form the OH radical by the action of a hole or (2) as an electron reduces oxygen to form the oxygen radical anion that then reacts with water to form the OH radical. We favor the aerobic mechanism. The redox reaction takes place when the target molecules come in contact with the surface of the photoexcited nanoparticles. Possible energy loss mechanisms that can impair this process are recombination of electrons and holes at lattice defect sites, or on the nanoparticle surface. Efficiencies and the observed photophysics can well be different for excitation using continuous wave and ultrafast light sources. We have found that the cleanest photochemistry is conducted using femtosecond laser pulses, which allow simple photon counting and wavelength tuning for reaction yield calculations. Using 120 fs pulses, the excitation of the doped TiO2-xNx sample was varied between 390 and 540 nm, employing either an optical parametric amplifier to obtain tunable wavelengths in the visible spectrum or a second harmonic generation crystal (BBO) to produce 390 nm photons. Excitation powers were adjusted using a neutral density filter wheel. Figure 12 demonstrates the photodegradation observed at 390 and 540 nm for methylene blue at pH 7 in the presence of various Ti-ON-based catalysts. The data for the nitrided samples as well as the palladiumtreated samples referred to above show a notably enhanced photocatalytic activity for the TiO2-xNx particles at 390 nm. In contrast, the undoped TiO2 nanoparticles (Degussa P25) showed minimal activity under visible light radiation compared to a reference experiment without nanoparticles. The significant

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Figure 9. High-resolution XPS spectra for the N 1s region showing (a) a broad feature centered at 400.7 eV after treatment with PdCl2, (b) a broad feature centered at approximately 400.7 eV after treatment with Pd(acetate)2, and (c) features peaking at 400.7 and 406.5 eV after treatment with Pd(NO3)2. See also Figure 8 and text for discussion.

Visible Light Tunable TiO2-xNx Photocatalysts

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1237 counterions Cl-, acetate-, and NO3-. These studies were carried out to consider, at least initially, whether the counterion constituents, if still present (as evidenced, in part, by XPS results; Table 2), might influence the photocatalytic properties. The results shown in Figure 12c below suggest that there is a counterion effect. The sample prepared from Pd(NO3-)2 is the most active photocatalyst for 390 nm excitation. Discussion

Figure 10. Raman spectrum and fit for TiO2-xNx nitrided nanoparticles (corresponding to the 450 nm reflectance spectrum). The fit to anatase features and the TiOxN1-x first-order scattering of nonstoichiometric titanium nitride is indicated.

decrease in optical activity for methylene blue in the presence of the current TiO2-xNx photocatalysts signals a conversion process considerably exceeding that of previously employed photocatalysts.18 Analysis of the monochromatic photon flux taking into account the volume of the excitation region shows that approximately one photon/particle was used for excitation. At 540 nm, all of the TiO2-xNx and palladium-treated samples still display a notable activity relative to a blank and TiO2 sample but differences in activity are muted in this less sensitive absorption region.46 In contrast, at wavelengths below 350 nm, the activity of both the TiO2 and nitrided samples would appear to be comparable. The nitrided TiO2-xNx samples are catalytically more active at considerably longer wavelength than TiO2. The TiO2-xNx samples were treated with a variety of palladium compounds with the palladium ion initially bound to the

Extending the Absorption Region and Tuning the Onset of Absorption. The results we have obtained in this study demonstrate that by forming and adjusting an initial TiO2 nanoparticle size distribution and mode of nanoparticle treatment, it is possible to extend and tune the absorption of a doped TiO2-xNx nanoparticle sample well into the visible region. Further, the current study demonstrates that an important modification of a TiO2 photocatalyst can be made considerably simpler and more efficient by extension to the nanometer regime. The current process can produce submicron agglomerates of visible light absorbing TiO2-xNx nanoparticles via a room temperature procedure that otherwise is highly inefficient if not inoperative at the micron scale. Further, these materials have maintained stability for a minimum of eight months and, in contrast to dye-treated TiO2, they are environmentally benign. It also appears possible to extend this approach for substitutional nitration by continuing to operate at the nanoscale, albeit with some minor modification. We have now begun to successfully nitride structures of several additional oxides including ZrO2, HfO2, SiO2, and SnO2. Future experiments can be envisioned to test not only the use of these nanoscale based doping procedures to produce photocatalysts but also potential solar cell materials with, for example, the introduction of silver nitrate.47 In both of these applications, it will be appropriate to optimize (chemometrics) particle size distribution-structure-absorption wavelength relations to take full advantage of these relatively simple procedures

Figure 11. TEM micrographs of (a) palladium metal crystallite impregnated TiO2-xNx nanostructures and (b) dark brown-black crystal phases accompanying the Pd nanocrystal impregnated nanostructures. This structure results from the introduction of PdCl2 into the nitriding nanocolloid solution; (c) and (d) also correspond to palladium-treated TiO2-xNx nanostructures where the palladium source is respectively the acetate and a dilute nitrate solution. See text for discussion.

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Figure 12. Comparison of the photocatalytic decomposition (decolorization) of methylene blue after (a) 390 nm laser excitation and (b) 540 nm excitation, monitored at 650 nm, and catalyzed by undopedTiO2 (open diamonds) and nitrogen-doped TiO2 nanocrystals, and (c) laser excitation at 390 nm as a function of the introduction of palladium via palladium acetate, chloride, and nitrate. The inset in (a) shows the photodegradation of methylene blue in water at neutral pH. Note the significant decrease in optical density after excitation of a 5 nL volume of a 2 mL methylene blue (ODinitial ) 0.8) solution. The photodegradation observed with palladium seeding displays a counterion effect with increasing catalytic activity from the acetate to the chloride to the nitrate. Please see also Figure 11 and text for discussion.

to produce active materials in the visible region. We will be especially interested in the behavior of these systems as a function of metal (Pd, Pt, transition metals) loading where we have already observed32 the effects of nitrogen doping on (1) the nature of dispersal of seeded metals in the titanium oxynitride vs titanium oxide lattice48 and (2) the resulting

Gole et al. oxidation state of the metal ions present in the lattice.49 However, one must be cautiously optimistic on the basis of the extensive literature on metallized TiO2. Transition metal doping has been used to tune the properties of TiO2 with high expectations;50-52 however, metal doping can result in increased carrier trapping.53,54 It remains to be determined whether these deleterious effects can be minimized at the nanoscale. From an alternate perspective, it will be important to assess the interaction of the doped titanium oxynitrides with organic dyes. An open question relates to the assessment of how the observed shifts in absorption characteristic of the TiO2-xNx system are related to a change in band structure or to the introduction of new donor/acceptor levels. Asahi et al.,18 on the basis of band structure calculations, insist that the visible light sensitivity of nitrogen-doped TiO2 is due to the narrowing of the band gap by mixing the N 2p and O 2p states. Irie et al.27 suggest that an isolated narrow band (formed above the valence band) is responsible for the visible light response of their oxynitride powders. As we now readily introduce dopants to the TiO2 nanoparticles, do we change their molecular electronic structure as we form the doped material through substitutional chemistry? Here, it will be important to characterize the nature of the “band gap” change to evaluate whether there is structure associated with the excitation in the conduction band region. In concert with these studies, we also hope to carry out surface based conductivity measurements. Photocatalytic Activity. Asahi et al.18 have associated the catalytic activity they observe with the signal from the N 1s XPS peak at 396 eV attributed to the bonding of atomic nitrogen to titanium.55 Their samples are dominated by N 1s region features peaking at 400 and 402 eV, which do not change significantly, and the 396 eV feature, which increases with increased catalytic activity. Not only do we find the photocatalytic activity that we have outlined for methylene blue photodegredation but also preliminary experiments56 indicate that slurries of the prepared TiO2-xNx and Pd-treated TiO2-xNx placed on the surface of a stop-flow tube reactor allow the photocatalytic oxidation of ethylene to CO2 and H2 using a simple incandescent lamp excitation in the visible region. In view of these results, we were surprised to find little evidence for the 396 eV XPS peak, previously attributed to atomic nitrogen binding, associated with any of the TiO2-xNx or Pdtreated oxynitride samples produced in this study. Our XPS observations are, however, consistent with the evidence obtained for a nonstoichiometric surface-based Ti-O-N bonding region.42,43 Saha and Tomkins,41 in their XPS study of the oxidation of titanium nitride films observed three dramatically changing spectral features at 396, 400, and 402 eV. With an increase in oxidation time and temperature, the low binding energy peak at 396 eV became the dominant N 1s peak and the two higher energy peaks, though also clearly formed,57 could be annealed away in a vacuum at temperatures in excess of 500 °C. Saha and Tomkins41 compared their work to that of Shinn and Tsang,55 who observed four distinctly varying peaks associated with the N 1s XPS spectrum of chemisorbed nitrogen on Cr/W films. These authors assigned the 396 eV feature to β-N (TiN), 400, and 405 eV peaks to terminally bonded, well screened, and poorly screened, respectively, γ-N2 states, and a 397.5 eV peak to RN2. They found that, at a much lower temperature, 300 K, all “molecular” chemisorbed nitrogen, was desorbed, leaving only the 396 eV feature. Saha and Tomkins41 have suggested that the features observed in their spectrum associate well with those of Shinn and Tsang;55 however, this requires

Visible Light Tunable TiO2-xNx Photocatalysts that they correlate the 402 eV feature with that observed by Shinn and Tang at 405 eV, implying a considerable difference in the screening of nitrogen. Thus changes in the nitrogen environment can produce significant changes in the nitrogen 1s XPS spectral region. In contrast to Saha and Tompkins,41 Shinn and Tsang,55 and Asahi et al.,18 the present study does not involve the variation of nitrogen and oxygen content in oxide/nitride thin films but, in fact, at the surface of nanocolloids. The TiO2 nanocolloids must represent a distinctly different environment for the nitration process, at least in the fact that more confined regions of potentially higher surface tension (surface free energy) are formed. As we nitride several monolayers of the TiO2 nanocolloid, it is not difficult to envision that the XPS spectra of the present systems will differ significantly from those of the previous work.18,41,55 We have found that the present nitration process, though operative at room temperature and rapid, will not provide a means to nitride micron-sized TiO2 particles. These results imply that the correlation of those features (Figures 8 and 9), which we observe in the range from ∼397 to 408.8 eV, with the assignments given on the basis of film studies by previous authors18,41,55 may be tenuous. Further, as Gyorgy et al.43 have noted in their recent study of the surface nitration of titanium, TiN is stable over a large range of stoichiometry with several different phases possible. It is not clear which, if any, of these equilibrium phases is associated with the present nitration process. Rather, it is most likely that we have observed the formation of a nonstoichiometric titanium oxynitride layer. In fact, in a more recent study involving the XPS depth profiling characterization of the surface layer obtained by pulsed laser irradiation of titanium in nitrogen, Gyorgy et al.58 have studied the Ti 2p, O 1s, and N 1s regions. They note that the presence of Ti-O-N bonds complicates the interpretation of the Ti 2p region as the binding energy range of TiOxNy compounds superimposes with that of an energy loss feature41,59,60 situated at approximately a 1.7 ( 0.2 eV higher binding energy from the elastic TiN peaks. In complement, their oxygen 1s spectrum also displays a broad feature extending to higher binding energy and previously attributed61,62 to the presence of Ti-O-N bonds. These results would suggest that a few electron volts shift in the present XPS spectra from those of previous workers for a confined nanocolloid would not be surprising. For these reasons, we feel it highly unlikely that the N 1s XPS features we have observed for the TiO2-xNx and Pd-treated TiO2-xNx samples (formed by nitrating TiO2 nanocolloids) are correlated exclusively with chemisorbed N2 and not, at least in part, with the formation of oxynitride and Ti-N bond formation. The relative photocatalytic efficiency observed in the TiO2-xNx-Pd based experiments with change in palladium counterion from acetate to chloride to nitrate (Figure 12c) is intriguing and bears consideration. The impetus for these experiments lies in the relatively smaller photocatalytic efficiency for the TiO2-xNx-Pd system vs TiO2-xNx when the palladium is introduced as the chloride. The TEM micrographs of Figure 11 suggest, at least in part, that both the chloride and nitrate treatments lead to (preserve) a distribution of TiO2-xNx nanoparticle structures. These appear to be well developed in the case of the nitrate and, though distinct, are further enhanced through the presence of palladiumbased nanocrystallites following treatment with palladium chloride. In contrast, the acetate appears to promote the formation of either a much more homogeneous agglomerated structure, which must certainly drastically reduce the surfaceto-volume ratio, or a fine dispersal of the palladium, which could

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1239 promote kelating by the acetate. We speculate that this effect may lead to the significant reduction in nitrogen uptake and photocatalytic activity for the acetate-treated samples relative to TiO2-xNx and the corresponding chloride and nitrate-treated samples. Although both the PdCl2- and Pd(NO3)2-treated aminated samples display a significant catalytic activity, it is clear that the activity of the chloride system lags that of the nitrided TiO2-xNx and subsequently nitrated (Pd) systems. The chloride treatment of the TiO2-xNx nanocolloid introduces strongly electron attracting chlorine atoms or molecules within the nanostructure framework (Figure 11a,b). We suggest that these halogen sites may inhibit the availability of electrons for the photocatalytic reduction process. These Pd-based systems will be the subject of further study in our laboratory. Acknowledgment. We acknowledge helpful discussions with Professors M. G. White, M. K. Barefield, and T. Lian and funding by ACS-PRF, the Case Western Reserve Center for Chemical Dynamics, and the National Science Foundation. This paper is dedicated to the memory of John Margrave, a man of the highest integrity and a research advisor par excellence. References and Notes (1) Sabria, M.; Garcia-Nunez, M.; Pedro-Botet, M. L.; Sopena, N.; Gimeno, J. M.; Reynaga, E.; Morere, J.; Rey-Joly, C. Presence and Chromosomal Subtyping of Legionella Species in Potable Water Systems in 20 Hospitals of Catalonia, Spain. Infect. Control Hosp. Epidemiol. 2001, 22 (11), 673-676. (2) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37. (3) Anrow, P. M.; Bakir, M.; Thompson, K.; Bova, J. L. Endemic Contamination of Clinical Specimens by Mycobacterium Gordonae. Clin. Infect Dis. 2000, 31, 472-476. Panwalker, A. P.; Fuhse, E. Nosocomial Mycobacterium Gordonae Pseudoinfection from Contaminated Ice Machines. Infect. Control 1986, 7, 67-70. (4) Schiavello, M., Ed. Photoelectrochemistry, Photocatalysis, and Photoreactors: Fundamentals and DeVelopments; NATO ASI Series; D. Reidel Publishing Co.: Dordrecht/Boston/Lancaster, 1985. (5) Bolton, J. R. Sol. Energy 1996, 57, 37. (6) Khan, S. U. M.; Akikusa, J. J. Phys. Chem. B 1999, 103, 7184. (7) Akikusa, J.; Khan, S. U. M. J. Electrochem. Soc. 1998, 145, 89. (8) Khan, S. U. M.; Akikusa, J. Int. J. Hydrogen Energy 2002, 27, 863. (9) Khaselev, O.; Turner, J. A. Science 1998, 280, 425. (10) Licht, S.; et al. J. Phys. Chem. B 2000, 104, 8920. (11) Hoffman, M. R.; et al. Chem. ReV. 1995, 95, 69. (12) Ollis, D. S., Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (13) Wilcoxon, J. P. Photocatalysis Using Semiconductor Nanoclusters; Advanced Catalytic Materials; MRS: Boston, MA, 1998. (14) Serpone, N., Pelizzetti, E., Eds. Photocatalysis: Fundamentals and Applications; Wiley: New York, 1989. (15) Kozhukharov, V.; Vitanov, P.; Stefchev, P.; Kabasanova, E.; Kabasanov, K.; Machkova, M.; Blaskov, V.; Simeonov, D.; Tzaneva, G. TiO2-photocatalyzed Oxidative Water Pollutants Degradation: a Review of the State-of-art. J. EnViron. Protect. Ecol. 2001, 2 (1), 107. (16) Schiavello, M., Dordrecht, H., Eds. Photoelectrochemistry, Photocatalysis, and Photoreactors: Fundamentals and DeVelopments; Kluwer Academic: Boston, 1985. (17) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. ReV. 1995, 95, 735. (18) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269. (19) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049. (20) Ghosh, A. K.; Maruska, G. P. J. Electrochem. Soc. 1977, 24, 1516. (21) Choi, W.; Termin, A.; Hoffman, M. R. J. Phys. Chem. 1994, 98, 13669. (22) Anpo, M. Catal. SurV. Jpn. 1997, 1, 169. (23) Akikusa, J. Thesis, Duquesne University, 1997. (24) Breckenridge, R. G.; Hosler, W. R. Phys. ReV. 1953, 91, 793. (25) Cronemeyer, D. C. Phys. ReV. 1959, 113, 1222. (26) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243.

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