N-Doped TiO2 Nanoparticle Based Visible Light Photocatalyst by

Publication Date (Web): August 26, 2008 ... University of Pune. , # .... and Electronic Structure of TiO2 Nanomaterials for Renewable Energy Applicati...
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J. Phys. Chem. C 2008, 112, 14595–14602

14595

N-Doped TiO2 Nanoparticle Based Visible Light Photocatalyst by Modified Peroxide Sol-Gel Method Tushar C. Jagadale,†,§ Shrikant P. Takale,# Ravindra S. Sonawane,# Hrushikesh M. Joshi,*,† Shankar I. Patil,§ Bharat B. Kale,*,# and Satishchandra B. Ogale*,† Physical and Materials Chemistry DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune 411008, India; Department of Physics, UniVersity of Pune, Pune 411008, India; and Center of Materials of Electronic Technology, Dr. Homi Bhabha Road, Pashan, Pune 411008, India ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: July 17, 2008

The peroxide gel route is employed to synthesize N-doped TiO2 nanoparticles (NP) at low temperature using titanium tetraisopropoxide, ethylmethylamine, and hydrogen peroxide as precursors. Structural studies show anatase phase in the undoped titania NPs as well as at 5 at. % N-doped titania NPs, although with a degree of matrix disorder in the latter case. The annealing of N-doped titania NPs at different temperatures shows that above 400 °C nitrogen escapes the O-Ti-O matrix and at 500 °C the sample becomes crystalline. Transmission electron microscopy reveals that the particle size is in the range of 20-30 nm for the undoped TiO2 but only 5-10 nm for N-doped TiO2. At higher nitrogen concentration (10 at. %) bubble-like agglomerates form. FTIR and photoluminescence quenching also confirm the incorporation of nitrogen in anatase TiO2. Optical properties reveal an extended tailing of the absorption edge toward the visible region upon nitrogen doping. X-ray photoelectron spectroscopy is used to examine the electronic state of doped nitrogen and the associated possible electronic modification of the TiO2 matrix. Under visible light irradiation the undoped TiO2 NPs do not show any significant photocatalytic activity, as expected; however, the 5 at. % N-doped TiO2 NPs show excellent activity. Introduction Controlled synthesis and characterization of functional metal oxide nanoparticles (NPs) is rapidly becoming a very important research area due to its direct impact on the technological sector.1-4 In recent years, TiO2 nanoparticles have attracted considerable attention because of the unique application-worthy physical and chemical properties of the compound, the possibility of realizing different phase forms, and the cost effectiveness of the material and its processing. The interesting properties of the compound include high efficiency of photocatalytic activity and related biosensing characteristics, chemical inertness, photostability, environmental compatibility, etc.5 Moreover, due to its photooxidation response, it has applications in key areas of water splitting and bacterial growth inhibition.6,7 The as-prepared pure TiO2 nanopowder absorbs radiation in the ultraviolet region of the electromagnetic spectrum. The band gap of TiO2 is about 3.2 eV, and for nanoparticles it gets even higher. Because the solar spectrum has very little (5-7%) contribution in the UV region, pure TiO2 nanoparticles cannot be utilized effectively for solar excitation or conversion. Clearly, the photoresponse application efficiency of the TiO2 nanoparticles can be enhanced dramatically by red shifting its absorption to the visible region (400-800 nm). One way to achieve this is doping of impurities (cationic as well as anionic) into the TiO2 matrix. However, cationic impurities such as transition metals have drawbacks of * Authors to whom correspondence should be addressed [e-mail (B.B.K.) [email protected]; (H.M.J.) [email protected]; (S.B.O.) sb.ogale@ ncl.res.in]. † National Chemical Laboratory. § University of Pune. # Center of Materials of Electronic Technology.

thermal instability, higher probability to form charge carrier recombination centers, and expensive synthesis protocols.8 Hence, anionic impurities such as N, C, and S are being examined as better candidates for property engineering due to their closer position to oxygen in the periodic table.9 In 1986, Sato et al.10 reported for the first time visible light photocatalytic activity of NOx-doped TiO2. These authors concluded that NOx impurity, which was formed from NH4OH used in the preparation of titanium hydroxide, was responsible for the effect. In 2001, Asahi et al. used a sputtering method to synthesize N-doped TiO2 and specifically argued that N doping of TiO2 is the main cause of the observed visible photocatalysis effect.11 This interesting work was followed by several other studies, which mainly used anionic impurities such as C,12,13 N,11,14,15 F,16,17 S,18,19 and P.20,21 Nitrogen doping seems to be more attractive among all of these anionic elements because of its comparable atomic size with oxygen, small ionization energy, metastable center formation, and stability.5 To achieve appropriate nitrogen doping, there are several protocols already in use: physical techniques such as sputtering11,22,23 and ion implantation,24-27 chemical treatments of bare oxides, 28-30 and sintering of TiO2 under high temperature in a nitrogen-containing atmosphere,31 sol-gel process,14,32-35 or oxidation of titanium nitride,36 plasma processes,37,38 etc. In sputtering and ion implantation techniques, TiO2 films are directly treated with energetic nitrogen ions. In the sintering process, TiO2 (powder or films) is annealed at high temperature in the presence of nitrogen-containing precursors such as ammonia and urea. In the sol-gel process, synthesis of N-doped TiO2 is achieved by hydrolysis of titanium alkoxide precursors and aliphatic amines, NH4Cl, N2H4, NH4NO3, HNO3, or ammonia as a nitrogen source. Among these, the sol-gel process is the most

10.1021/jp803567f CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

14596 J. Phys. Chem. C, Vol. 112, No. 37, 2008 adopted method for the synthesis N-doped TiO2 nanoparticles because doping levels as well as the size of nanoparticles can be easily controlled depending on the reaction conditions such as the solvents used, pH, temperature, and hydrolysis rate. Several groups have already demonstrated successful incorporation of nitrogen in TiO2 using the sol-gel process.9,13,31-35 These groups have used either acids, bases, or other reagents such as sulfides, which need further washing to remove excess ions after nanoparticle formation. Some of them9 have also used higher processing temperatures up to 600 °C for incorporation of nitrogen into TiO2. Qui and Burda5 developed a hydrolysis approach in which amines have also been used for the formation of the N-doped TiO2 nanoparticles. However, hydrolysis is a complicated process involving many steps such as refluxing and pH control. In our work, nitrogen is incorporated in the peroxide gel itself. The novel aspect of the hydrogen peroxide based sol-gel method is the ability to synthesize good-quality N-doped TiO2 nanoparticles at low temperature, also facilitating the synthesis of highly transparent films of the material. Wu et al.39 have prepared well-crystallized anatase titania thin films at as low a temperature as 80 °C using H2O2. We have characterized the nanoparticles by UV-visible spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of nanoparticles is studied for methylene blue dye degradation. Experimental Section Synthesis of Nitrogen-Doped TiO2 Nanoparticles. Chemicals and materials used for the synthesis are titanium tetraisopropoxide [Ti (OC3H7)4, 99.5%, Aldrich], hydrogen peroxide (H2O2, 30% w/v, Qualigen India Ltd.), and ethylmethylamine (C3H9N, 99.0%, Merck make). Five grams of titanium tetraisopropoxide was dissolved in 10 mL of ethanol and then hydrolyzed with 50 mL of distilled water. The white precipitate of hydrous oxide was produced instantly, and the mixture was stirred for 10 min. The amorphous precipitate was separated by decantation and washed two or three times with distilled water for complete removal of alcohol. The supernatant liquid was then decanted. To this hydrous oxide precipitate was slowly added 10 mL of peroxide. The precipitate dissolved completely by reaction with peroxide and formed a transparent orange sol of titanium-hydrogen peroxide complex. This transparent orange sol slowly thickened with time and transformed into the gel, which on IR drying and subsequent calcination at 300 °C gave undoped TiO2 NPs. For the nitrogen doping case, the same sol was then stirred with the help of a magnetic stirrer followed by dropwise addition of ethylmethylamine solution to prepare nitrogen-containing TiO2 sol. After complete addition of the ethylmethylamine solution, the mixture was kept in a beaker. This sol slowly thickened with time and transformed to a viscous liquid at room temperature. The viscous gel was then further dried under an IR lamp for 6-8 h. This was followed by calcination at 300 °C. The choice of this temperature was based on the TGA analysis. Characterization Techniques. X-ray diffraction patterns of the powders were recorded using an X’pert pro PAN analytical diffractometer using Ni-filtered Cu KR radiation (λ ) 1.5418 Å). The crystallite size was estimated by applying the Scherrer formula to the fwhm of the (101) peak. Optical properties of the titania nanoparticles were studied by UV-visible-NIR spectrophotometer in diffuse reflectance

Jagadale et al. mode over the spectral range of 190-1400 nm. The measurements were carried out on a Jasco V-570 spectrophotometer. HRTEM measurements were performed on a Tecnai F-30 instrument operated at an accelerating voltage of 300 kV. The samples for HRTEM analysis were prepared by drying the drops of the TiO2 nanoparticle solutions on carbon-coated copper grids. Image processing and interplanar distance evaluation were performed with the help of micrograph Gatan software. Powder samples were used for XPS measurements. The measurements were performed on a VG microtech ESCA 3000 instrument at a pressure of >1 × 10-9 Torr. The general scan, C1s, O1s, N1s, and Ti2p core level spectra were recorded with non-monochromatized Mg KR radiation (photon energy ) 1253.6 eV) at a pass energy of 50 eV and an electron takeoff angle (angle between electron emission direction and surface plane) of 55°. The overall resolution was 0.2 eV for XPS measurements. The core level spectra were background corrected using the Shirley algorithm, and chemically distinct species were resolved using nonlinear least-squares fitting procedure. Baseline correction and peak fitting for all of the samples were done using the software package XPS peak 41. The core level binding energies (BEs) were aligned with respect to the C1s binding energy of 285 eV. Equally weighed powder samples were used for PL measurements. The entrance and exit slit widths were kept the same. The measurements were performed on a Perkin-Elmer LS 55 spectrophotometer. These measurements were done on a Perkin-Elmer Spectrum One B spectrophotometer over the spectral range of 450-4000 cm-1. The samples for the measurement were prepared by mixing and grinding titaniabased sample powders with KBr. Photocatalytic Activity Measurements. Methylene blue dye (10-4 M) in DI water was used for the photocatalytic activity. Known amounts of TiO2 powder were taken and dispersed in given aqueous methylene blue solution. The mixtures were sonicated for 15 min before the photocatalytic activity measurements. Solutions containing the powder were placed in Petri dishes in equal amounts and placed in annular configuration below a mercury lamp (160 W) so that fluxes of the electromagnetic radiations were uniform on all samples. We used Petri dishes in these experiments with top surface optical exposure to avoid absorption of the electromagnetic radiation by glass. During the experiments, the Petri dishes were taken out after specific time intervals to monitor the degradation of methylene blue dye. A UV-vis spectrophotometer (Jasco V-570) was used to monitor changes in the spectral intensity distribution of the dye. Results and Discussion Figure 1A compares X-ray diffraction patterns for the NPs of undoped TiO2 (curve 1), and 5 at. % nitrogen-doped TiO2 (curve 2). The peak positions in Figure 1A identify with the anatase phase of TiO2. Clearly, the full width at half-maximum (fwhm) for pure TiO2 NPs is considerably smaller as compared to the peaks for N-doped TiO2 NPs. The crystallite size can be estimated using the Scherrer formula. For undoped TiO2, the crystallite size is ∼12 nm and for the N-doped case, it is ∼2 nm before heat treatment and ∼14 nm after heat treatment at 500 °C in air for 24 h. Whether the large line broadening in the case of N-doped TiO2 NPs represents much smaller sized NPs due to the potential influence of the N incorporation process on particle growth or internal structural incoherence due to dopant-induced distortions can become

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Figure 1. (A, B) X-ray diffraction (XRD) spectra of undoped (curve 1) and 5 at. % N-doped (curve 2) nanoparticles; (C) XRD spectra of undoped TiO2 (a), 5 at. % N-doped TiO2 NPs (b), 10 at. % N-doped TiO2 NPs (c), and 5 at. % N-doped TiO2 NPs sintered at different annealing temperatures for 24 h in air (d-f) [(g) corresponds to the 10 at. % N-doped TiO2 NPs at 500 °C for 24 h in air]; (D) X-ray diffraction (XRD) spectra of undoped (curve 1) and 5 at. % N-doped (curve 2) nanoparticles annealed at 500 °C.

clear only with TEM data as discussed later. Indeed, as will be discussed later, TEM shows the particle size for the undoped and N-doped nanoparticles to be in the ranges of ∼20-25 and ∼5-7 nm, respectively. It should be noted that XRD gives the crystallite size leading to coherent diffraction, whereas TEM shows the physical size. The presence of a disorder shell or incoherence within a particle can lead to such differences. As expected, annealing removes such disorder, and hence the XRD and TEM estimates approach each other. These observations suggest that the nitrogendoping protocol followed here affects the particle growth. Figure 1B depicts the enlarged version of the peak corresponding to (101) planes of anatase TiO2. A marginal shift in the peak position to higher angle (lower d value) can be noted to have resulted due to nitrogen incorporation into the TiO2 matrix. A lower d value implies compressive strain, which may emanate from the differences in the bonding characteristics between nitrogen and oxygen. This is important for significant modification to the electronic states due to the replacement of oxygen with nitrogen upon N doping, which is required for the desired substantial shift in the optical absorption toward the visible. This indeed is the case for our samples as will be shown and discussed later. To examine the thermal stability (metastability) of the nitrogen-incorporated state, the N-doped TiO2 NPs were annealed at different temperatures (300, 350, 400, or 500 °C) for 24 h in air. Figure 1C gives the corresponding XRD patterns. In Figure 1C, XRDs a, b, and c correspond to the undoped TiO2 and 5 and 10 at. % N-doped TiO2 NPs, respectively, annealed at 300 °C, whereas XRDs d, e, and f correspond to 5 at. % N-doped TiO2 NPs sintered at 350, 400, and 500 °C for 24 h. XRD g corresponds to 10 at. %

N-doped TiO2 NPs sintered at 500 °C. The XRD peaks are seen to get sharper with the increase in annealing temperature, as expected. Annealing in the air not only decomposes the organics from the surface of NPs and the deeper seated organics but also replaces the doped nitrogen. For annealing temperatures above 400 °C, the peaks are found to be almost similar to the undoped TiO2, indicating that the organic residues are removed along with the nitrogen from the O-Ti-O lattice. Clearly the peaks corresponding to the annealed N-doped TiO2 NPs are now much sharper as compared to those in Figure 1A (curve 2), whereas those corresponding to undoped TiO2 NPs have not changed appreciably. Thus, above 400 °C the crystallinity of the N-doped samples increases. From the XRD data on an expanded scale (Figure 1D), one can see that the peak positions for N-doped TiO2 NPs get back to almost the same position as that of pure TiO2 within experimental error. This confirms the suggested removal of incorporated nitrogen from the O-Ti-O lattice. We also explored the case of higher nitrogen incorporation by our process. The corresponding XRD result for 10 at. % N-doped TiO2 nanoparticle sample is shown in Figure 2. It shows broad features similar to the 5 at. % case; however, the rising background toward lower 2θ observed in the 5 at. % case is not seen in Figure 2. This implies enhanced longrange atomic correlation. The inset shows HRTEM micrograph for the 10 at. % case. It reveals bubble-like features separating regions of better crystallinity. It is possible that such an inhomogeneity is introduced because of the tendency of an extra nitrogen to segregate out, possibly due to enhanced strain. The presence of strain is suggested by the

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Figure 4. HRTEM images of (a) TiO2 and (b) 5 at. % N-doped TiO2 nanoparticles. The scales on both panels correspond to 20 nm. Figure 2. XRD result for 10 at. % N-doped TiO2 nanoparticle sample.

Figure 3. FTIR spectra of (a) TiO2, (b) 5 at. % N-doped TiO2, and (c) annealed 5 at. % N-doped TiO2 nanoparticles.

XRD line shift. In the following, therefore, we have focused on the 5 at. % case. Figure 3 shows the FTIR spectra for the undoped and 5 at. % N-doped TiO2. The signals in the range of 400-1250 cm-1 are characteristic of the formation of the O-Ti-O lattice. It may be noted that in curve b, which corresponds to N-doped TiO2, the structure in this region is characteristically different as compared to the cases of undoped and annealed samples. For the latter, the band is rather flat below ∼800 cm-1. This is a clear signature of incorporated nitrogen in the sample represented by curve b. The inset shows the spectrum taken after an increase in the number of scans followed by the so-called data tune-up mode (Perkin-Elmer Spectrum One B spectrophotometer) highlighting the signal in the characteristic region of O-Ti-O lattice for undoped and 5 at. % N-doped samples. It clearly reveals that the host matrix has been modified significantly after N doping. The similarity of features between the undoped and annealed N-doped TiO2 samples implies that nitrogen effuses out by such treatment. No peak corresponding to hydrocarbon is observed in the IR spectrum. We did not find any evidence of formation of any amine complexes with the titanium. The strong signal at 1390 cm-1, which is present in the 5 at. % N-doped TiO2 sample (b), corresponds to the symmetric stretch weak IR band of nitro compounds, indicating amine treatment and the presence of nitrogen. The common peak at 1630 cm-1 corresponds to OH.40 The HRTEM images recorded for pure TiO2 and for 5 at. % N-doped TiO2 NPs are shown in Figure 4. From Figure 4a it is clear that the morphology of the undoped TiO2 is somewhat faceted and that the size distribution of the particles is between 20 and 30 nm.

Figure 5. HRTEM micrographs of (a) pure TiO2, (b) 5 at. % N-doped TiO2, and (c, d) FFT processed images corresponding to (a) and (b) along with line profile diagrams.

Figure 4b shows the morphology of the N-doped TiO2, revealing particle size ranging from 5 to 10 nm. This implies that the nitrogen incorporation process employed here decreases the mean particle size. This result is consistent with the XRD data presented earlier. Indeed, the TEM result confirms that the broadening of the XRD line width does represent nanoparticle size reduction. Figure 5 shows high-resolution TEM micrographs of (a) TiO2 and (b) nitrogen-doped TiO2 nanoparticles. Panels c and d of Figure 5 are the corresponding FFT processed images and line profiles. From Figure 5a,c the interplaner distance is 0.365 nm in undoped TiO2 NPs, which matches exactly with the primary (101) family of planes of the anatase TiO2 matrix, as already indicated by the XRD data. In the case of 5 at. % N-doped TiO2 NPs, the planes appear to be considerably incoherently distorted as compared to undoped TiO2 NPs. This may be because of somewhat coordinated nitrogen incorporation in the O-Ti-O lattice via dopant-dopant interactions. Figure 6 summarizes the XPS results on the synthesized nanoparticles. The Ti 2p3/2 core level for TiO2 and 5 at. % N-doped TiO2 is seen to appear at 458.72 and 458.22 eV, respectively. A shift to lower binding energy for the primary Ti 2p peak indicates a difference in the electronic interaction of Ti with anions in pure TiO2 and N-doped TiO2. This implies

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Figure 6. XPS spectra of (A) TiO2 and (B) 5 at. % N-doped TiO2 nanoparticles.

Figure 8. PL spectra of (a) TiO2 and (b) 5 at. % N-doped TiO2 nanoparticles, respectively. (Inset) Photos for 0.5 mg/mL solutions of (i) pure (white) and (ii) 5 at. % N-doped TiO2 (colored) samples.

Figure 7. (a) Optical diffuse reflection spectra of (1) TiO2 and (2) 5 at. % N-doped TiO2 nanoparticles along with the corresponding Tauc’s plots shown in (b). The blue curve in panel a corresponds to the annealed N-doped sample.

considerable modification of the TiO2 matrix due to nitrogen doping possibly caused by dopant-induced strain, which can also be the cause of incoherent strain distributions seen in XRD and HRTEM analyses. Lowering of the binding energy of Ti 2p core level in the case of N-doped TiO2 may also be because of the increase in the covalent nature of the bond between Ti and N. These results are consistent with earlier

reports.14,15 Additionally a small contribution can be noted near 456 eV. This can be attributed to the presence of the Ti3+ state in the system.41 The corresponding electronic states can also have significant implications for optical absorption. However, a mixed phase TiO2-TiN can be ruled out because X-ray diffraction does not show any signature of an impurity phase such as TiN. In both cases, the oxygen 1s core level peak appears at the same place, indicating the nature of oxygen to be similar. However, a small broadening of the O 1s core level is clearly visible in the case of 5 at. % N-doped TiO2 sample. This might be due to the presence of oxygen and nitrogen in the same lattice units of TiO2. The spectra are deconvoluted into two peaks, one at ∼530 eV and the other at ∼532 eV. The primary peak at 530 eV can be attributed to O-Ti-O type of oxygen coordination, whereas the small contribution at 532 eV may arise from the chemisorbed species from the ambient.42 Figure 6A2 shows the small contribution (enhanced for clarity) of the N 1s level. The presence of the nitrogen core 1s contribution in undoped TiO2 (although small in intensity) can be attributed to the presence of chemisorbed nitrogen.

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Figure 9. Photocatalytic degradation of methylene blue dye in the presence of undoped TiO2 and 5 at. % N-doped TiO2 nanoparticles. [UV-vis spectral data (a) of undoped TiO2, (b) 5 at. % N-TiO2, and (c) annealed N-TiO2 at 400 °C for 24 h; (d) profile of percentage decomposition vs time for undoped TiO2 and N-doped TiO2); (e, f) photographs of degraded MB using undoped TiO2 and N-doped TiO2, respectively].

Several authors11,43,44 have shown this contribution and attributed it to molecularly chemisorbed γ-nitrogen. On the other hand, the nitrogen 1s core level from 5 at. % N-doped TiO2 NPs (Figure 6B2) shows significant peak asymmetry and can be deconvoluted into two peaks, which appear at 398.8 and 400.8 eV. As reported earlier, the peak at 398.8 eV may be because of anionic N incorporated in TiO2 in O-Ti-N linkages. From the reports published earlier, the higher binding energy peak at 400.8 eV can be attributed to oxidized nitrogen in the form of Ti-O-N or Ti-N-O linkages.14 The absence of a peak at or near 396 eV for the N 1s core level45 once again reaffirms that the TiN phase is not formed in the nanomaterials. Instead, Ti3+ is present, which acts as a color center and absorbs visible light. Quantitative estimates of the N/O ratios were also made by XPS measurements (see the Table SI-1 in the Supporting Information). It was found that the nitrogen concentration in nominally 5 at. % N-doped TiO2 NPs is ∼4.2% (increased to ∼6.4% from ∼2.2% in undoped one representing chemisorbed N), which represents a fairly good match between the expected and experimentally observed values. In the Supporting Information (Figure SI-2) we give the C 1s data for the undoped, 5 at. % N-doped, and 5 at. % N-doped annealed cases. Because the main carbon peak is used as an internal energy standard, we focused on the satellites and their evolution. We noted that the satellites are broadly similar in the undoped and N-doped cases, suggesting that although the carbon moieties may have some role, this role does not appear to be significant. Upon annealing, however, one of the satellites evolves differently. Before we proceed to present the results of photocatalysis, it is instructive to discuss the optical properties of undoped and N-doped titania nanoparticles because this is a reasonably transparent way to reveal dopant-induced electronic state effects.

Figure 7a shows diffused reflectance spectroscopy (DRS) data for the undoped and nitrogen-doped TiO2 NPs. These data reveal that nitrogen doping in TiO2 nanoparticles shifts the absorption edge toward higher wavelength, that is, toward the visible region with a substantially long band tailing, possibly reflecting site disorder effects compounded with dopant-defect states. Figure 7b shows the corresponding Tauc’s plot. The mean band gap values (more of a mobility edge value for N-doped case) are found to be 3.45 and 2.75 eV for undoped and doped samples, respectively. It can be seen that after annealing at 500 °C for 24 h, the long band tail completely disappears, suggesting liberation of the incorporated nitrogen. The band tailing seen in Figure 7a is rather interesting and has been the subject of lively debate in recent literature. Generally speaking, it could be ascribed to the compound effect of dopant-induced midgap electronic states and lattice disorder effects. Band gap narrowing has also been suggested to be a possibility11,46 but appears to be generally ruled out by mounting evidence.4748 The lattice disorder effects can be expected on the basis of the differences in the bonding characteristics between nitrogen and oxygen along with random distribution of nitrogen or random distribution of nitrogen segment lengths in the case of spatially coordinated nitrogen as suggested by the TEM data. However, the issues of midgap states and band gap narrowing have been quite controversial. Serpone48 has discussed these in some details. Briefly, the theoretical work by Valentin et al. 49,50 suggests that the visible response of N-doped TiO2 system arises from occupied localized N2p states located above the valence band by about 0.7 eV. N doping of TiO2 is also suggested to create localized states below the conduction band edge.51 Additionally, the role of color centers associated with oxygen vacancies created by charge imbalance due to N doping has also been discussed. Indeed, as Serpone48 argues, the large

N-Doped TiO2 Nanoparticles body of evidence suggests that these oxygen vacancies have a major role to play in the red shift of the band edge. The charge states of such vacancies (such as neutral, singly charged, and doubly charged) can lead to electronic states of different energies within the band. Clearly, the presence of lattice disorder can cause broadening of such levels and lead to the extended tailing as observed. It is to be expected that the specifics of tailing are a function of the sample preparation method, which therefore highlights the significance of different methods of synthesis. Figure 8 shows the PL spectra of undoped and nitrogendoped TiO2. The sample quantity used for the measurement was the same (80 mg). The sample was excited at a wavelength of 260 nm, and this selection of the wavelength was decided from the excitation plot. The spectra show four distinct peaks at about 390, 490, 520, and 595 nm. The first emission at 390 nm is attributed to the direct transition from the conduction band to the valence band, whereas the latter two at 490 and 520 nm are emission signals originating from the charge transfer transition from an oxygen vacancy trapped electron.52 The last prominent signal at 595 nm originates from the recombination of self-trapped exciton.48 Interestingly, the PL spectrum for N-doped TiO2 shows a significant decrease in the peak intensity. Because the PL emission is the result of radiative recombination of excited electrons and holes, the lower PL intensity of the N-doped sample clearly implies enhanced contribution of nonradiative processes in the N-doped TiO2 case. This effective quenching of the photoluminescence can be attributed to the two pathways: (1) The excited electron is trapped by the oxygen vacancy, whereas the hole is trapped by the doped nitrogen, which reduces the recombination rate. (2) The excited electron can transfer from the valence band to the new defect levels introduced by nitrogen doping that exists near the conduction band minimum and can also decrease the PL intensity.53 The inset shows the photographs of the samples (i) TiO2 and (ii) 5 at. % N-doped TiO2 dispersed in water with a concentration of 0.5 mg/mL, giving perception of the change in the color after N doping. Figure 9 shows the photodegradation of methylene blue in the presence of TiO2 nanoparticles and N-doped TiO2 nanoparticles monitored in water at neutral pH. Photocatalytic measurements were done using a mercury lamp (160 W), which has much less emission in the UV range, thereby not inviting special efforts to filter the same. Thus, pure TiO2 NPs showed very little photocatalytic activity (Figure 9a), as expected. On the other hand, the data in Figure 9b, which corresponds to the case of 5 at. % N-doped TiO2 NPs, clearly show a significant decrease in the spectral intensity, indicating substantial degradation of the dye under the visible light dominated illumination.10 Figure 9d compares the percentage dye degradation for the cases of photocatalysis with undoped and N-doped TiO2 NPs measured at ∼660 nm. Photographs of progressive dye degradation shown in Figure 9e,f at comparable times for the same quantity of material bring out this comparison visually and also help to rule out the possibility of the adsorption of the methylene blue dye on the nanocatalyst surface or the dye-sensitized photocatalysis or photobleaching process under visible light irradiation. Conclusion It is shown that good-quality N-doped titania nanoparticles exhibiting visible light photocatalysis can be synthesized at

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14601 low temperature by the peroxide gel route by incorporating a nitrogen precursor in the sol itself. Nitrogen incorporation in the O-Ti-O matrix and its evolution upon thermal annealing treatment are brought out by various techniques. Specifically, the optical absorption of the nitrogen-incorporated TiO2 NPs shifts to the visible region in the form of an extended band tailing. It is found that above 400 °C, nitrogen escapes the O-Ti-O matrix. Acknowledgment. S.B.O. acknowledges support from the Department of Science and Technology (DST) nanoscience program and for the award of a Ramanujan fellowship, government of India. B.B.K. is grateful to the DST, New Delhi, for providing financial support and also thanks the Director, Executive Director, C-MET and all members of nanocrystalline Materials/Glass group, C-MET, Pune. Supporting Information Available: Area under the curve of all spectra and XPS of Cls core levels of the varioius nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ito, A.; Shinkai, M.; Honda, H.; Kobayashi, T. J. Biosci. Bioeng. 2005, 100, 1. (2) Rout, C. S.; Raju, A. R.; Govindraj, A.; Rao, C. N. R. Solid State Commun. 2006, 138, 136. (3) Wang, X.; Song, J.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (4) Martinez, C. J.; Hockey, B.; Montgomery, C. B.; Semancik, S. Langmuir 2005, 21, 7937. (5) Qui, X.; Burda, C. Chem. Phys. 2007, 339, 1. (6) Torres, G. R.; Lindgren, T.; Lu, J.; Granqvist, C. G.; Lindquist, S. E. J. Phys. Chem. B 2004, 108, 5995. (7) Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. EnViron. Sci. Technol. 1998, 32, 726. (8) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. B 1994, 98, 13669. (9) Reddy, K. M.; Baruwati, B.; Jayalakshmi, M.; Rao, M. M.; Manorama, S. V. J. Solid State Chem. 2005, 178, 3352. (10) Sato et al. Chem. Phys. Lett. 1986, 123, 126. (11) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (12) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (13) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772. (14) Sathish, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349. (15) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049. (16) Ho, W.; Yu, J. C.; Lee, S. Chem Commun. 2006, 10, 1115. (17) Yamaki, T.; Sumita, T.; Yamamoto, S. J. Mater. Sci. Lett. 2002, 21, 33. (18) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454. (19) Umebayashi, T.; Yamaki, T.; Yamamoto, S.; Miyashita, A.; Tanaka, S.; Sumita, T.; Asai, K. J. Appl. Phys. 2003, 93, 5156. (20) Yu, J. C.; Zhang, L.; Zheng, Z.; Zhao, J. Chem. Mater. 2003, 15, 2280. (21) Shi, Q.; Yang, D.; Jiang, Z.; Li, J. Mol. Catal. B 2006, 43, 44. (22) Nakano, Y.; Morikawa, T.; Ohwaki, T.; Yaga, Y. Appl. Phys. Lett. 2005, 86, 132104. (23) Mwabora, J. M.; Lindgren, T.; Avendano, E.; Jaramillo, T. F.; Lu, J.; Lindqusit, S. E.; Granqvist, C. G. J. Phys. Chem. B 2004, 108, 20193. (24) Premkumar, J. Chem. Mater. 2006, 16, 3980. (25) Diwald, O.; Thompson, T. L.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 52. (26) Ghicov, A.; Macak, J. M.; Tsuchiya, H.; Kunze, J.; Haeublein, V.; Frey, L.; Schmuki, P. Nano Lett. 2006, 6, 1080. (27) Batzill, M.; Morales, E. H.; Diebold, U. Chem. Phys. 2007, 339, 36. (28) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (29) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D., Jr. J. Phys. Chem. B 2004, 108, 6004. (30) Nosaka, Y.; Matsushita, M.; Nasino, J.; Nosaka, A. Y. Sci. Technol. AdV. Mat. 2005, 6, 143.

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