Synthesis, Characterization, and Optical Properties of Well-Defined N

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Synthesis, Characterization, and Optical Properties of Well-Defined N-Doped, Hollow Silica/Titania Hybrid Microspheres Xuefeng Song and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China ReceiVed July 3, 2007. In Final Form: August 24, 2007 Well-defined nitrogen-doped, hollow SiO2/TiO2 hybrid spheres were successfully prepared through a two-step sol-gel synthesis combined calcination process using triethylamine as the nitrogen source. In this approach, polystyrene (PS)/silica microspheres were first synthesized. Subsequently, the amine-treated PS/SiO2/TiO2 hybrid spheres were obtained by sol-gel method. Finally, the elimination of the PS core, nitrogen-doping process, and crystallization of amorphous TiO2 were simultaneously conducted in the calcination process to acquire the final products. The asprepared hybrid spheres were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. The results of XRD, FTIR, and XPS spectra indicated that nitrogen was really doped into the anatase TiO2 shell and confirmed that most nitrogen dopants might be present in the chemical environments of N-Ti-O and Ti-N-O. It was found that the absorption shoulder of nitrogen-doped hollow SiO2/TiO2 hybrid spheres vastly shifted to the visible region up to around 530 nm. The photoluminescence (PL) bands showed spectral lines at about 421, 472, and 529 nm, which were attributed to the self-trapped excitons, F and F+ centers. Moreover, the intensity of the PL spectra band of hollow SiO2/TiO2 hybrid spheres increased with as the amount of titanium tetrabutoxide (TBOT) precursor increased. However, the doping of nitrogen into hollow SiO2/TiO2 hybrid spheres led to the drastic quenching of photoluminescence because of the increase in the separation efficiency of the photoinduced electron and hole pairs.

Introduction As an important kind of material, inorganic hollow spheres having tailored structural, optical, and surface properties may find a wide spectrum of applications such as drug-delivery vehicle systems, photonic crystals, fillers, and catalysts.1 Titanium dioxide (TiO2), which is nontoxic, highly chemically resistant, and available at low cost, is widely investigated and commonly applied as an inorganic shell material of hybrid spheres.2 To date, there have been two main methods for synthesizing TiO2 hollow spheres. The first is to form hollow spheres by a templating method including hard and soft templates. For instance, polymer colloid beads and silica spheres are commonly utilized in the hard templating method. TiO2 shells are usually coated onto the surface via a layer-by-layer technique3 or a sol-gel reaction4 whereas the soft templating method is generally to coat TiO2 nanoparticles onto the surface of vesicles or emulsion droplet by an emulsion/ interfacial polymerization strategy.5 The second is the template-free method for the controllable preparation of TiO2 * Corresponding author. E-mail: liangaoc@online. sh.cn. Fax: +8621-52413122. Tel: +86-21-52412718. (1) (a) Caruso, F. AdV. Mater. 2001, 13, 11. (b) Zhong, Z. Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. AdV. Mater. 2000, 12, 206. (c) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (d) Chen, M. H.; Gao, L. Inorg. Chem. 2006, 45, 5145. (e) Cheng, X. J.; Chen, M.; Wu, L. M.; Gu, G. X. Langmuir 2006, 22, 3858. (f) Chen, M.; Wu, L. M.; Zhou, S. Z.; You, B. AdV. Mater. 2006, 18, 801. (2) (a) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5206. (b) Yang, Z. Z.; Niu, Z. W.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (c) Yang, M.; Ma. J.; Niu, Z. W.; Dong, X., Xu, H. F.; Meng, Z. K.; Jin, Z. G.; Lu, Y. F.; Hu, Z. B.; Yang, Z. Z. Adv. Funct. Mater. 2005, 15, 1523. (d) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 2305. (3) (a) Caruso, F. Chem.sEur. J. 2000, 6, 413. (b) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (4) (a) Imhof, A. Langmuir 2001, 17, 3579. (b) Syoufian, A.; Satriya, O. H.; Nakashima, K. Catal. Commun. 2007, 8, 755. (c) Zhang, K.; Zhang, X. H.; Chen, H. T.; Chen, X.; Zheng, L. L.; Zhang, J. H.; Yang, B. Langmuir 2004, 20, 11312. (d) Guo, X. C.; Dong, P. Langmuir 1999, 15, 5535. (e) Jeong, U.; Wang, Y. L.; Ibisate, M.; Xia, Y. N. AdV. Funct. Mater. 2005, 15, 1907. (5) (a) Zhang, M.; Gao, G.; Li, C. Q.; Liu, F. Q. Langmuir 2004, 20, 1420. (b) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386.

hollow spheres,6 which is facile and cost-effective. However, the methods mentioned above that are employed for hollow microspheres of TiO2 produce only 1-fold spheres that have one kind of inorganic particle used as the core or shell. There has been little research carried out on SiO2/TiO2 multilayer hollow hybrid microspheres. Recently, the optical properties of TiO2 and N-doped TiO2 have attracted increasing interest. Nitrogendoped titania has been exhibited to be an efficient way to extend the adsorption of light from the ultraviolet to visible region beacause the substitutional doping of N for O in the anatase TiO2 crystal would yield a band gap narrowing driven by mixing N 2p states with O 2p states.7 However, owing to the introduction of defects derived from the doping of N into the TiO2 lattice, the PL features of titania in relation to crystal surface oxygen vacancies and defects would be dramatically impacted. The photoluminescence of TiO2 materials has been reported previously for TiO2 nanowires, single crystals, and nanoparticles.8 However, few reports on the photoluminescence properties of SiO2/TiO2 or N-doped SiO2/TiO2 hollow hybrid microspheres are found. In the present work, we demonstrate a facile method for the fabrication of N-doped hollow SiO2/TiO2 hybrid microspheres via two-step sol-gel synthesis together with calcination processing using triethylamine as the nitrogen source. The novelties of our N-doped SiO2/TiO2 hollow hybrid microspheres, compared with the previous reports, mainly include the following aspects: (1) the fabrication of PS/SiO2/TiO2 hybrid microspheres could be employed easily via a two-step sol-gel method; (2) the (6) (a) Guo, C. W.; Cao, Y.; Xie, S. H.; Dai, W. L.; Fan, K. N. Chem. Commun. 2003, 700. (b) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (c) Li, Y. T.; Song, C. H.; Hu, Y. Z.; Wei, Y. J. Chem. Lett. 2006, 35, 1344. (7) Asahi. R.; Morikawa. T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (8) (a) Lei, Y.; Zhang, L. D.; Meng, G. W.; Li, G. H.; Zhang, X. Y.; Liang, C. H.; Chen, W.; Wang, S. X. Appl. Phys. Lett. 2001, 78, 1125. (b) Tang, H.; Berger, H.; Schmid, P. E.; Levy, F.; Burri, G. Solid State. Commun. 1993, 87, 847. (c) Shi, J. Y.; Chen, J.; Feng, Z. C.; Chen, T.; Lian, Y. X.; Wang, X. L.; Li, C. J. Phys. Chem. C 2007, 111, 693.

10.1021/la7019704 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Hollow Silica/Titania Hybrid Microspheres Scheme 1 Preparation of N-Doped, Hollow SiO2/TiO2 Hybrid Spheres

elimination of the PS core, the nitrogen doping process, and the crystallization of TiO2 were simultaneously conducted in the calcination process to form N-doped hollow SiO2/TiO2 hybrid spheres directly; and (3) the photoluminescence phenomenon of SiO2/TiO2 and N-doped SiO2/TiO2 hollow hybrid microspheres was first investigated, the mechanism of the PL and the effect of the N concentration and TBOT content on the PL properties were also elaborated, which provided theoretical and practical guides to its potential applications in photocatalysis and optoelectronics. Experimental Section Materials. All chemicals were analytical grade and were used as received except for styrene, which was purified by distillation under reduced pressure. TBOT, styrene (St), and poly(vinyl pyrrolidone) (PVP) K30 (Mw ∼44 000-54 000) were purchased from Shanghai Chemical Regent Co., China. Cationic initiator 2,2-azobis-(isobutyramidine) dihydrochloride (AIBA) was obtained from Wako Pure Chem, Japan. Triethylamine was kindly provided by the Shanghai Linfeng Chemical Regent Co., China. Tetraethoxysilane (TEOS), 2-propanol, and an aqueous ammonia solution (∼28% by weight) were purchased from Jiangsu Yonghua Fine Chemistry Co., China. Deionized water was used for all polymerization and treatment processes. Typical Procedure. The fabrication of N-doped hollow silica/ titania hybrid spheres can be schematically described as shown in Scheme 1. It can be seen that the fabrication process mainly includes three steps, as follows: (1) Fabrication of Polystyrene Spheres. The cationic polystyrene cores were first fabricated by emulsifier-free emulsion polymerization using AIBA cationic initiator, as described by Goodwin.9 Typically, 20 mL of styrene was added to 200 mL of deionized water in a round-bottomed flask at room temperature. Then, 0.6 g of AIBA and 2.5 g of PVP were added at 70 °C under continuous vigorous stirring. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for 1 h and stirred for 24 h at 70 °C. The resultant PS spheres were then centrifuged and washed three times with ethanol and deionized water. (2) Fabrication of PS/SiO2 Spheres. A total of 1 mL of the PS spheres dispersion (3% by wt) was diluted with 15 mL of deionized water and 50 mL of 2-propanol. Then, when the pH value was modulated to ∼10 by dropping a certain amount of ammonia solution, 0.23 g of TEOS was added to the diluted PS dispersion under magnetic stirring. The reaction process was allowed to proceed for 2 h at room temperature under continuous stirring. The as-synthesized coreshell particles were recovered by centrifugation and washed three times with 2-propanol and then dried in vacuum at 60 °C. (3) Fabrication of the N-Doped, Hollow Silica/Titania Hybrid Spheres. Briefly, 0.03 g of as-prepared PS/SiO2 sphere powders, 0.15 g of deionized water, and 0.01 g of PVP surfactant were dissolved (9) Goodwin, J. W.; Ottewill, R. H.; Pelton, R. Colloid Polym. Sci. 1979, 257, 61.

Langmuir, Vol. 23, No. 23, 2007 11851 in 24 mL or 72 mL of ethanol and then sonicated for 15 min. With continuous stirring, a series of TBOT (0.06, 0.12, and 0.18 g) and triethylamine (0 or 0.3 mL) were added to the mixture. Triethylamine was employed as the nitrogen-doped source. The ultimate mixture dispersion was refluxed in a round-bottomed flask for 4 h at 80 °C. The precipitate was then centrifugated, redispersed in ethanol several times, and then dried at 60 °C for 8 h. For the experiment with 0 mL of triethylamine, hollow SiO2/TiO2 sphere powders were obtained after calcination of the dried precipitate at 550 °C for 3 h, which were denoted as HST-A, HST-B, and HST-C. However, for the experiment with 0.3 mL of triethylamine, N-doped hollow SiO2/ TiO2 hybrid microspheres were obtained after calcination of the dried precipitate at 550 °C for 3 or 9 h. The resultant N-doped products were marked as NHST-3 and NHST-9 (where the number represents the calcination time). The detailed experimental recipes are summarized in Table 1 Characterization. The crystalline structure of the samples was analyzed on a Japan Rigaku D/max 2550V X-ray diffractometer using Cu KR (λ ) 0.15406 nm) radiation at 40 kV and 60 mA. The crystal size of TiO2 was estimated by applying the Scherrer equation (φ ) kλ/βcosθ), where φ is the crystal size, λ is the wavelength of the X-ray irradiation, k is usually taken as 0.89 here, β is the peak width at half-maximum height of the (101) peak of anatase after subtracting the instrumental line broadening, and θ is the diffraction angle. The morphology and structure were probed by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) on a field-emission transmission electron microscope (JEM2100F; accelerating voltage, 200 kV). The average particle sizes of the various as-prepared microspheres were estimated using the Brookhaven ZetaPlus 1DA-033 average size analyzer. UV-vis absorption spectra analysis was performed on a Shimadzu UV3101PC spectrophotometer with dry-pressed disk samples at room temperature. PL (Photoluminescence) spectra analysis was performed in a Perkin-Elmer LS55 luminescence spectrometer with a Xe lamp as the excitation source at room temperature. The emission spectra were collected at an excitation wavelength of 350 nm. The charge state of N-doped hollow SiO2/TiO2 hybrid microspheres at the surface was investigated by X-ray photoelectron spectroscopy (XPS) using monochromated Al KR radiation (Kratos Axis Ultra DLD).

Results and Discussion Morphology Observation of the N-Doped, Hollow SiO2/ TiO2 Hybrid Spheres. According to our previous work,10 the silica midlayer could facilitate the PS spheres to be completely coated with TiO2 particles, which was due to the change in surface conditions on PS spheres because the TiO2 particles have a higher affinity with SiO2 than with PS particles. Figure 1 demonstrates the morphology of the obtained multifarious spheres. Obviously, it could be seen that a well-defined core-shell structure with a PS bead as the core and SiO2 or SiO2/TiO2 as the shell was indeed formed. The average diameter of the PS/SiO2 and amineadded PS/SiO2/TiO2 hybrid spheres was around 282 and 330 nm, respectively, which were almost in agreement with Figure 1b,c, also indicating that PS/SiO2 spheres as the core were covered completely by amine-added TiO2. Chen et al.11 had pointed out how triethylamine is incorporated into the titania layer in the doping mechanism. The doping chemistry involved a two-step reaction process: hydrolysis and condensation. A hydroxyl group first undergoes nucleophilic substitution on the metal center, resulting in the exchange of the alkyl group. Then, the condensation reaction proceeded, which formed the Ti-O-NTi bonds. In addition, the dispersivity of the as-obtained hybrid spheres was improved because 0.01 g of surfactant PVP having a steric blocking effect could effectively hold back the mutual (10) Song, X. F.; Gao, L. J. Phys. Chem. C 2007, 111, 8180. (11) Chen, X. B.; Lou, Y. B.; Samia, A. C. S.; Burda, C.; Gole, J. L. AdV. Funct. Mater. 2005, 15, 41.

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Song and Gao Table 1. Detailed Experimental Recipes Information

sample HST-A HST-B HST-C NHST-3 NHST-9

triethylamine (mL) 0 0 0 0.3 0.3

PS/SiO2 powder (g)

deionized water (g)

TBOT (g)

PVP (g)

ethanol (mL)

calcination time (h)

0.03 0.03 0.03 0.03 0.03

0.15 0.15 0.15 0.15 0.15

0.06 0.12 0.18 0.18 0.18

0.01 0.01 0.01 0.01 0.01

24 24 24 72 72

3 3 3 3 9

contact of hybrid spheres as indicated by comparing parts a-c of Figure 1. The TEM images of the final products of N-doped, hollow SiO2/TiO2 double-shell hybrid microspheres were shown in Figure 2a,b. The obvious difference in color contrast between the central region and the verge presented a hollow spherical structure with an ∼30 nm wall thickness, indicating the removal of PS cores from core-shell particles. However, it could be seen that the hollow spheres tended to agglomerate as a result of the heat treatment. The SAED pattern showed several Debye-Scherrer rings (Figure 2a, inset), corresponding to the reflections of the TiO2 polycrystals. In the high-magnification TEM image (Figure 2b), we could apparently observe two layers in the shell part in which there were SiO2 and N-doped TiO2 shells. The SEM image (Figure 2c) showed not only that the obtained hybrid microspheres retained their spherical morphology even after the calcination at

Figure 1. TEM images of the various colloids: (a) pure PS, (b) PS/SiO2 hybrid spheres, and (c) amine-added PS/SiO2/TiO2 bilayer hybrid spheres.

550 °C for 9 h but also that their surface was quite even and dense. The broken spheres (Figure 2d) further verified the welldefined N-doped, hollow SiO2/TiO2 bilayer hybrid spheres that were effectively formed after the removal of the PS cores by calcination at 550 °C. XRD Patterns. Figure 3 displays the XRD patterns for nonheated samples HST-C, NHST-3, and NHST-9. X-ray diffraction showed that the outermost shell of a sample before being annealed was the precursor of N-doped TiO2 added to triethylamine, which was amorphous. However, for the annealing sample all of the diffraction peaks could be assigned to wellcrystallized anatase TiO2 (JCPDS 21-1272). There were no nitride peaks, which was due to the small amount of N incorporated into the TiO2 layer. It was clear from the XRD patterns that the

Figure 2. (a) TEM image of N-doped hollow SiO2/TiO2 hybrid spheres. (Inset) Selected-area electron diffraction pattern of TiO2 polycrystals. (b) Magnification of single N-doped hollow SiO2/TiO2 hybrid spheres. (c) SEM images of N-doped hollow SiO2/TiO2 hybrid spheres for the NHST-9 sample. (d) Image of typical rupture hollow hybrid spheres.

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Figure 3. XRD patterns for (a) a nonheated sample, (b) HST-C, (c) NHST-3, and (d) NHST-9. Figure 5. FTIR transmission spectra of (a) an HST-C sample, (b) an NHST-9 sample, and (c) an NHST-3 sample.

Figure 4. UV-vis spectra of (a) PS spheres, (b) a PS/SiO2 coreshell sphere, (c) HST-C, (d) HST-B, (e) HST-A, (f) NHST-3, and (g) NHST-9.

nitrogen-doped SiO2/TiO2 hollow hybrid microspheres samples (NHST-3) exhibited peak broadening and that the anatase phase was retained in comparison with that of hollow SiO2/TiO2 hybrid spheres (HST-C). The average crystallite sizes of HST-C, NHST3, and NHST-9 were calculated to be 10.1, 5.7, and 8.5 nm, respectively, which indicated that the particle size decreased after nitrogen doping. With prolonged heating, the particle size increased, which was due to the fact that the longer heating time resulted in a larger crystallite size. The decrease in the average size of TiO2 nanoparticles after the addition of triethylamine and calcination might be attributed to the fact that the alcoholysis process of TBOT was accelerated to form a larger amount of TiO2 nuclei as a result of the hydrolysis of triethylamine during the refluxing process. Thus, the TiO2 crystallite size would be decreased when the total amount of TiO2 in solution was the same. Very recently, Chi et al.12 observed a similar phenomenon and gave the analogous explanation for the urea-added products. UV-Vis Absorption Spectra. UV-vis absorption spectroscopy had been carried out to investigate the optical property of the various obtained hybrid spheres. Simultaneously, the spectra of polystyrene template spheres and PS/SiO2 spheres were also investigated for reference. Figure 4 illustrates the UV-vis absorption spectra of the original PS spheres, PS/SiO2 coreshell spheres, and hollow SiO2/TiO2 spheres (HST-A, HST-B, HST-C) as well as that of N-doped hollow SiO2/TiO2 hybrid spheres (NHST-3, NHST-9). For PS template spheres and PS/ SiO2 core-shell spheres, there was an obvious absorbance edge around 275 nm (curves a and b). However, the absorption shoulder shifted to the visible region when the silica-coated PS template spheres were coated with titania and the onset wavelength in the (12) Chi, B.; Zhao, L.; Jin, T. J. Phys. Chem. C 2007, 111, 6189.

spectra of c-e was around 410 nm. Combining the observations of the TEM images, this red shift in the absorption spectra indicated that TiO2 nanoparticles perfectly coated the surface of the SiO2 shell. It was found that the onset wavelength underwent no obvious changes as the amount of TBOT increased. Obviously, the undoped spheres exhibited little light absorption in the visiblelight range because nanoscale titanium dioxide is transparent to visible light. Curves f and g denoted, respectively, the absorption spectra of NHST-3 and NHST-9 products, evidently showing the absorption edge markedly extended up to about 530 and 500 nm and exhibiting strong visible-light absorption. The absorption spectra of two samples changed acutely in the visible-light region in comparison with that of the hollow titania-coated SiO2 spheres, which is predominantly due to nitrogen doping and is of great significance with respect to its practical application. The substitutional doping of N was the most effective at improving the visible-light photocatalytic activity of anatase TiO2 crystal by decreasing the band gap with N-TiO2 as a result of either mixing O 2p states with its 2p states7 or creating isolated N 2p states above the valence band maximum of TiO2.13,14 The narrow band gap demonstrated that the necessary energy required to create an electron-hole pair was lower than the broadened band gap. Hence, the absorption band exhibited a red shift in the absorption spectra of N-doped, hollow SiO2/TiO2 hybrid spheres. Furthermore, as shown in Figure 4, the NHST-3 product possessed the strongest absorption ability of visible light between the N-doped hollow SiO2/TiO2 hybrid spheres. The analysis of nitrogen content for NHST-3 and NHST-9 showed that the N content decreased from 1.31 to 0.46 atom % with the increase in treatment time. Consequently, the visible-light absorption ability decreased with increasing calcination time. In our method, the nitrogen content could be controlled by facilely adjusting the treatment time in the ambient atmosphere. Fourier Transform Infrared (FTIR) Spectroscopy. Figure 5 shows the FTIR spectra of N-doped, hollow SiO2/TiO2 hybrid spheres (NHST-3 and NHST-9) obtained at different treatment times together with that of undoped, hollow SiO2/TiO2 spheres (HST-C). The characteristic peak at about 1628 cm-1 was attributed to the O-H bending vibration of adsorbed water molecules.15 The broad adsorption band centered at 3410 cm-1 corresponded to the O-H stretching region.16 Chen et al.11 stated that the signals in the 400-1200 cm-1 range were characteristic of the formation of an O-Ti-O lattice. The characteristic Si-O (13) Lee, J. Y.; Park, J.; Cho, J. H. Appl. Phys. Lett. 2005, 87, 011904011906. (14) Long, M.; Cai, W.; Wang, Z.; Liu, G. Chem. Phys. Lett. 2006, 420, 71. (15) Deng, C.; James, P. F.; Wright, P. V. J. Mater. Chem. 1998, 8, 153. (16) Zheng, M.; Gu, M.; Jin, Y.; Jin, G. Mater. Sci. Eng., B 2000, 77, 55.

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Figure 6. XPS spectra of the O 1s, N 1s, and Ti 2p core levels in nitrogen-doped, hollow SiO2/TiO2 hybrid spheres.

band at around 1080 cm-1 was observed, and Molinari et al.17 assigned it to the asymmetric stretching mode of the Si-O band. The peak at about 1401 cm-1 was clearly observed for nitrogendoped, hollow SiO2/TiO2 hybrid spheres. This peak should be assigned to the bending vibration mode of the N-H band, which might be formed by doping nitrogen with the absorbed H2O.18 The intensity of the N-H absorption peak decreased with increasing treatment time, which was likely to be a result of the removal of nitrogen with the elongation of the treatment time. The characteristic absorption bands at 3000 cm-1 for the C-H stretch and 1490 cm-1 for the aromatic C-C stretch were not present in the spectra of hollow SiO2/TiO2 hybrid spheres and nitrogen-doped hollow SiO2/TiO2 hybrid spheres. The infrared spectra indicated that the PS spheres were completely removed and demonstrated that the nitrogen element was indeed incorporated into the TiO2 shells. XPS Spectra. To further realize the effect of doping nitrogen atoms on the optical properties of N-doped, hollow SiO2/TiO2 hybrid spheres, Ti, O, and N bonding information was determined using XPS. Figure 6 shows the XPS spectra of the O 1s, Ti 2p, and N 1s core levels in nitrogen-doped, hollow SiO2/TiO2 hybrid spheres. The nitrogen 1s core level showed two peaks at about 396.3 and 400.2 eV, and to date, the assignment of the XPS peak of N 1s had been under debate. Previous reports in the literature on the XPS analysis of N-doped TiO2 powders showed the N 1s core level at a binding energy of between 396 and 397 eV, which were attributed to either the nitrogen anion (N-)19 or atomic β-N.20 Saha et al. assigned the peak as atomic β-N (396 eV) for substitutional N, and the additional N 1s peak was assigned to molecularly chemisorbed γ-N2 (between 400 and 402 eV) for interstitial N. However, Chen et al.21 argued that the peaks of substitutional nitrogen were situated at higher binding energies of about 402 eV. We attributed the peak (396.3 eV) to the formation of O-Ti-N, which was consistent with previous conclusions for nitrogen-doped TiO2 particles.22,23 The peak at (17) Molinari, M.; Rinnert, H.; Vergnat, M. Appl. Phys. Lett. 2003, 82, 3877. (18) Wang, J. W.; Zhu, W.; Zhang, Y. Q.; Liu, S. X. J. Phys. Chem. C 2007, 111, 1010. (19) Diwald, O.; Thompson, T. L.; Goralski, E. G.; Walck, S. D.; Yates, J. T. J. Phys. Chem. B 2004, 108, 52. (20) Saha, N. C.; Tompkins, H. G. J. Appl. Phys. 1992, 72, 3072. (21) Chen, X. B.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (22) Sathish, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349. (23) Irie, H.; Watanabe, Y.; Hashmoto, K. J. Phys. Chem. B 2004, 107, 5483.

400.2 eV was attributed to the binding energy of chemisorbed N2 molecules. The substitution of N for O in TiO2 crystals was further verified by the results of XPS spectra of O 1s and Ti 2p peaks. As for the Ti 2p, the typical binding energy of the Ti 2p3/2 peak in the TiO2 shell is 457.6 eV, which is significantly lower than that in P25 powder (459.7 eV)21 and pure TiO2 powders (459.3),22 also indicating the nitrogen incorporation into the nitrogen-doped, hollow SiO2/TiO2 hybrid spheres. The lower binding energy of Ti 2p in the hollow hybrid spheres showed that the electronic interaction of Ti with anions was significantly different from that of pure TiO2 particles and P25 powders. The decrease in binding energy of the Ti 2p3/2 peak after calcination could be explained in terms of the difference in the electronic interaction of Ti with different anions. As we all know, the lower electronegativity of nitrogen compared with that of oxygen resulted in decreases in percent ionicity and electron density around the N anion, causing an increase in electron density on Ti.22 It was evident that the O 1s peak appeared at around 528.7 eV, which was derived from the O in Ti-O. In comparison with the O 1s peak of the P25 sample (530.8 eV) and pure nanocolloid TiO2 (530.4), the changes in binding energy were 2.1 and 1.7 eV, respectively, suggesting a greater perturbation of the TiO2 lattice in the TiO2 shell upon nitrogen incorporation.11 Additionally, broadening on the higher bonding energy band for N-TiO2 at 531.4 could be found. The appearance of this additional peak was a consistent feature of the nitridation process. This feature was previously assigned to the presence of another kind of oxygen in N-TiO2. Saha et al.20 first observed this feature and Gyorgy et al.24 comprehensively described the characteristic line of this peak in their depth profiling characterization on TiN surfaces. They attributed this feature to the formation of oxidized Ti-N, which led to the Ti-N-O structure. This feature might be related to the presence of oxygen and nitrogen from the same lattice units in TiO2.22 On the basis of the above results and analysis, XPS indicated that the chemical environments for doping nitrogen were N-Ti-O and Ti-N-O. PL Emission Spectra. PL spectroscopy is a powerful tool for understanding the electro-optic and photoelectric properties of materials and for investigating the fate of electron-hole pairs in semiconductor particles because PL emission resulted from (24) Gyorgy, E.; Perez del pino, A.; Serra, P.; Morenza, J. L. Surf. Coat. Technol. 2003, 173, 265.

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Figure 7. Photoluminescence spectra for (a) hollow SiO2/TiO2 hybrid microspheres. (Inset) PL spectra of HST-C and their Gaussian fit band with three peaks (1, 2, and 3). (b) Influence of N content on the PL spectra of nitrogen-doped hollow SiO2/TiO2 hybrid spheres (excitation wavelength 350 nm, room temperature.)

the recombination of free carriers.25-27 The PL spectra of hollow SiO2/TiO2 hybrid spheres obtained for different amounts of TBOT precursor are shown in Figure 7a. Obviously, the intensity of the PL spectra band increased with an increasing amount of TBOT precursor, which was due to the increases in density and content of TiO2 nanocrystals in the shell.28 With increasing content of the TBOT loaded, the titania layer covering the surface of the SiO2 shell gradually became denser, which should be a result of the fact that the latter fine titania nanoparticles fully filled the former larger pores,10 suggesting that the density of the TiO2 shell was increased. The inset of Figure 7a shows the PL spectrum of HST-C, a broad band including three observed peaks. The best Gaussian fit of the PL band gave three peaks located at about 421, 472, and 529 nm. These peaks were consistent with previous conclusions by Lei et al.,8a who reported that the peak positions of the anatase TiO2 nanowires were located at 425, 465, and 525 nm and were attributed to self-trapped F and F+ centers. Generally,

the PL spectra of anatase TiO2 materials were attributed to three kinds of physical origins: self-trapped excitons, oxygen vacancies, and the surface state.8,29,30 Peak 1 at 421 nm should be attributed to self-trapped excitons located on the TiO6 octahedral. This is an intrinsic property of the TiO2 nanocrystal structure. PL bands at longer wavelengths in anatase TiO2 had been attributed to the oxygen vacancies. The role of shallow traps or deep traps relative to oxygen vacancies has been discussed.29,31 The emission of shallow traps acting as the radiative centers took place between 452 and 576 nm, with energy levels ranging from 0.27 to 0.87 eV below the conduction band.32 In our case, peak 2 (472 nm) and peak 3 (529 nm) could be attributed to such traps. Therefore, peak 2 at 472 nm was assigned to the oxygen vacancy with two trapped electrons (i.e., a F center). Peak 3 at 529 nm should originate from the oxygen vacancy with one trapped electron (i.e., a F+-type color center). As shown in Figure 7b, it is clear that the doping of nitrogen into hollow SiO2/TiO2 hybrid spheres led to the drastic quenching

(25) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2596. (26) Zhang, J.; Hu, Y.; Matsuoka, M.; Yamashita, H.; Minagawa, M.; Hidaka, H.; Anpo, M. J. Phys. Chem. B 2001, 105, 8395. (27) Tang, H.; Prasad, K.; Sanjines, R.; Schmidd, P. E.; Levy, F. J. Appl. Phys. 1994, 75, 2042. (28) Sun, Z. C.; Kim, D. H.; Wolkenhauer, M.; Bumbu, G. G.; Knoll, W.; Gutmann, J. S. ChemPhysChem 2006, 7, 370.

(29) Serpone, N.; Lawless, D.; Khairutdinov, R. J. Phys. Chem. 1995, 99, 16646. (30) Zhang, W. F.; Zhang, M. S.; Yin, Z.; Chen, Q. Appl. Phys. B 2000, 70, 261. (31) Rothschild, A.; Levakov, A.; Shapira, Y.; Ashkenasy, N.; Komem, Y. Surf. Sci. 2003, 532-535, 456. (32) Maestre, D.; Cremades, A.; Piqueras, J. Nanotechnology 2006, 17, 1584.

11856 Langmuir, Vol. 23, No. 23, 2007

of photoluminescence. The intensity of HST-C was higher than that of NHST-3 or NHST-9, indicating that N doping resulted in a decrease in the recombination of electron-hole pairs. Nitrogen doping caused the large reduction in the formation cost of an oxygen vacancy. Valentin et al.33 corroborated that the oxygen vacancy formation energy dropped from 4.2 to 1.0 eV upon nitrogen doping whereas the cost of oxygen substitution with nitrogen dropped from 9.7 to 6.4 eV. The nitrogen doping process generates a number of oxygen vacancies, so the electrons were easily trapped in the oxygen vacancies. With the increase in the content of nitrogen incorporated into the TiO2 shell, the probability of holes being trapped by nitrogen increased. Cong et al.34 pointed out that the decrease in PL intensity should be a result of the fact that the electron and the hole were trapped by the oxygen vacancy and the doped nitrogen, respectively. The doped nitrogen could increase the separation efficiency of the photoinduced electron and hole. Thereby, the doping of nitrogen into hollow SiO2/TiO2 hybrid spheres led to drastic quenching of the photoluminescence, and the higher the content of nitrogen doping, the lower the intensity of PL. (33) Valentin, C. D.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. J. Phys. Chem. B 2005, 109, 11414. (34) Cong, Y.; Zhang, J. L.; Chen, F.; Anpo, M. J. Phys. Chem. C 2007, 111, 6976.

Song and Gao

Conclusions Nitrogen-doped, hollow SiO2/TiO2 hybrid spheres were successfully prepared by a two-step sol-gel synthesis and calcination process using triethylamine as the nitrogen source. The results of XRD, FTIR, and XPS spectra demonstrated that the nitrogen element was indeed incorporated into the anatase TiO2 shell and the chemical environments of nitrogen doping were N-Ti-O and Ti-N-O. A stronger visible absorption of N-doped, hollow SiO2/TiO2 hybrid spheres was observed with increasing amounts of nitrogen doping. The PL bands showed spectral lines at about 421, 472, and 529 nm, which were attributed to the self-trapped excitons (i.e., F and F+ centers). In particular, the doping of nitrogen into hollow SiO2/TiO2 hybrid spheres led to the drastic quenching of photoluminescence, indicating that nitrogen doping increased the separation efficiency of the photoinduced electron and hole pairs. This work further demonstrated that photoluminescence is a powerful technique for investigating the mechanism of photocatalytic reaction and provided a practical guide. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (grant no. 2005CB623605), the National Natural Science Foundation of China (no. 50572116), and the Shanghai Nanotechnology Promotion Center (nos. 0552nm045 and 0652nm022). LA7019704