Fluorine- and Iron-Modified Hierarchical Anatase Microsphere

Nov 25, 2009 - High photocatalytic efficiency, easy recovery, and no biological toxicity are three key properties related to the practical application...
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Fluorine- and Iron-Modified Hierarchical Anatase Microsphere Photocatalyst for Water Cleaning: Facile Wet Chemical Synthesis and Wavelength-Sensitive Photocatalytic Reactivity Shaohong Liu,† Xudong Sun,*,† Ji-Guang Li,† Xiaodong Li,† Zhimeng Xiu,† He Yang,‡ and Xiangxin Xue‡ † Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China and ‡Institute of Metallurgical Resources and Environmental Engineering, School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China

Received September 15, 2009. Revised Manuscript Received November 4, 2009 High photocatalytic efficiency, easy recovery, and no biological toxicity are three key properties related to the practical application of anatase photocatalyst in water cleaning, but seem to be incompatible. Nanoparticlesconstructed hierarchical anatase microspheres with high crystallinity and good dispersion prepared in this study via one-step solution processing at 90 C under atmospheric pressure by using ammonium fluotitanate as the titanium source and urea as the precipitant can reconcile these three requirements. The hierarchical microspheres were found to grow via an aggregative mechanism, and contact recrystallization occurred at high additions of the FeCl3 electrolyte into the reaction system. Simultaneous incorporation of fluorine and iron into the TiO2 matrix was confirmed by combined analysis of X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis absorption spectroscopy. Surface structure and morphology changes of the microspheres induced by high-temperature annealing were clearly observed by field-emission scanning electron microscopy, especially for the phase-transformed particles. The original nanoparticles-constructed rough surfaces partially became smooth, resulting in a sharp drop in photocatalytic efficiency. Interestingly, iron loading has detrimental effects on the visible-light photocatalytic activity of both the as-prepared and the postannealed anatase microspheres but greatly enhances the photocatalytic activity of the as-prepared anatase microspheres under UV irradiation. No matter under UV or visible-light irradiation, the fluorineloaded anatase microspheres and especially the postannealed ones show excellent photocatalytic performance. The underlying mechanism of fluorine and iron loading on the photocatalytic efficacy of the anatase microspheres was discussed in detail. Beyond photocatalytic applications, this kind of material is of great importance to the assembling of photoactive photonic crystal that can control light motion.

1. Introduction Anatase TiO2 photocatalyst provides a route to remedy the problem of chemical waste. Constant vigorous research activities have been devoted to the investigation of cation- and anion-doped titania nanomaterials for the development of second- and thirdgeneration photocatalyst.1-6 Depositing or incorporating metal ion dopants into the titanium dioxide particles can influence the dynamics of electron/hole recombination and interfacial charge transfer.7 Fe3þ occupies a special position among the metallic dopants.8 When Fe3þ traps electrons, its stable electronic configuration (d5) is destroyed and stability decreases; thus, the trapped *Corresponding author: Tel þ86-24-83687787; Fax þ86-24-23906316; e-mail [email protected]. (1) Chen, X. B.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. Rev. 1995, 95, 735. (4) (a) Li, J.-G.; Ikeda, M.; Tang, C.; Moriyoshi, Y.; Hamanaka, H.; Ishigaki, T. J. Phys. Chem. C 2007, 111, 18018. (b) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049. (5) (a) Litter, M. I.; Navı´ o, J. A. J. Photochem. Photobiol. A 1996, 98, 171. (b) Choi, W.; Termin, A.; Hoffmann, M. R. Angew. Chem., Int. Ed. 1994, 33, 1091. (c) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Valentin, C. D.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666. (6) (a) Ozaki, H.; Iwamoto, S.; Inoue, M. J. Phys. Chem. C 2007, 111, 17061. (b) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (c) Wang, X. H.; Li, J.-G.; Kamiyama, H.; Katada, M.; Ohashi, N.; Moriyoshi, Y.; Ishigaki, T. J. Am. Chem. Soc. 2005, 127, 10982. (7) Beydoun, D.; Amal, R.; Low, G.; McEvoy, S. J. Nanopart. Res. 1999, 1, 439. (8) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33.

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electrons can easily be transferred to the oxygen adsorbed on the anatase surface. This may promote charge transfer and the efficient separation of the electrons and holes.8 Besides, for anatase TiO2, Karvinen et al.9 have theoretically demonstrated that the addition of Fe3þ caused significant band gap narrowing. Choi et al.10 have carried out a systematic study of the effects of 21 different metal ion dopants on nanocrystalline TiO2 and found that doping with Fe3þ at the 0.5 at. % level can significantly improve the photoreactivity of TiO2 for both oxidation and reduction. In addition, nonmetallic F doping can induce electronic change similar to O vacancy, thus reducing the effective band gap and improving visible-light photoresponse.11 Li et al.12 found that F doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity, and the increase of Ti3þ ions. Yu et al.13 found that F-doped TiO2 exhibited stronger absorption in the UV-vis range with a red shift in the band gap transition and showed high photocatalytic activity, even exceeding that of Degussa P25. Though anatase TiO2 doped with only iron or fluorine shows (9) Karvinen, S.; Hirva, P.; Pakkanen, T. A. J. Mol. Struct.: THEOCHEM 2003, 626, 271. (10) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (11) Yamaki, T.; Umebayashi, T.; Sumita, T.; Yamamoto, S.; Maekawa, M.; Kawasuso, A.; Itoh, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 206, 254. (12) Li, D.; Ohashi, N.; Hishita, S.; Kolodiazhnyi, T.; Haneda, H. J. Solid State Chem. 2005, 178, 3293. (13) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808.

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greater photoactivity than its undoped counterparts, it is still not clear whether fluorine- and iron-codoped anatase TiO2 has excellent photocatalytic performance. The synthesis route is one crucial factor affecting the photocatalytic efficiency and the practical application of the cation- and anion-doped titania particles.10,14 Previous work on TiO2 synthesis largely started with titanium(IV) compounds, mainly TiCl4 and alkoxides, which are highly sensitive to atmospheric moisture and therefore require special precautions.1,15 Besides, the reaction product is usually amorphous titania, which requires postannealing for crystallization.16 Postannealing, however, usually resulted in some unfavorable effects for cation- and anion-doped titania particles, such as surface dehydroxylation, hard aggregation, and segregation.14b,16 Therefore, development of new routes for the facile synthesis of anatase TiO2 with high crystallinity and good dispersion under atmospheric conditions is crucial. Recently, our group reported that ammonium fluotitanate can react with alkaline matters to directly generate anatase TiO2 at 90 C in solution under atmospheric pressure, which provides one novel route to the facile synthesis of cation- and anion-doped anatase photocatalyst.17 Nanosized anatase titania, due to its remarkable photocatalytic performance, has always been the focus of photocatalysis research.7 The recovery problem and potential biological toxicity of nanoparticles, however, hinder its practical application in water cleaning.18,19 Immobilization technology is preferred in liquidphase reactions to avoid a costly but necessary separation step after the photocatalytic reaction. However, the immobilization technique greatly decreases the photocatalytic efficiency of nanosized anatase TiO2 for the largely reduced specific surface area.20 Nanoparticles-constructed hierarchical microspheres, in bulk size but with the unique properties of nanoparticles, provide an alternative way to make the anatase photocatalyst easy to recover while simultaneously retain high photocatalytic activity. In addition, there is usually no space charge layers existing in nanosized anatase particles, so the separation of photogenerated charge carriers in anatase nanoparticles mainly occurs via diffusion race. The space charge layers existed in bulk-sized microspheres, however, can promote the separation of electron-hole pairs, thus prolonging lifetimes of the photogenerated charge carriers and consequently enhancing photocatalytic activity. Moreover, the antenna effect that allows energy/exciton to transfer in aggregated titania particles may probably exist in hierarchical microspheres, which can also improve the photocatalytic capability.21 (14) (a) Navı´ o, J. A.; Testa, J. J.; Djedjeian, P.; Padron, J. R.; Rodrı´ guez, D.; Litter, M. I. Appl. Catal., A 1999, 178, 191. (b) Bickley, R. I.; Gonzalez-Carre~no, T.; Gonzalez-Elipe, A. R.; Munuera, G.; Palmisano, L. J. Chem. Soc., Faraday Trans. 1994, 90, 2257. (15) (a) Kaneko, M.; Okura, I. Photocatalysis: Science and Technology; Springer: Berlin, 2002. (b) Li, J.-G.; Ishigaki, T.; Sun, X. J. Phys. Chem. C 2007, 111, 4969. (16) (a) Hirano, M.; Joji, T.; Inagaki, M. J. Am. Ceram. Soc. 2004, 87, 35. (b) Wang, J. A.; Limas-Ballesteros, R.; Lopez, T.; Moreno, A.; Gomez, R.; Novaro, O.; Bokhimi, X. J. Phys. Chem. B 2001, 105, 9692. (c) Perkas, N.; Palchik, O.; Brukental, I.; Nowik, I.; Gofer, Y.; Koltypin, Y.; Gedanken, A. J. Phys. Chem. B 2003, 107, 8772. (d) Zhang, Y. H.; Ebbinghaus, S. G.; Weidenkaff, A.; Kurz, T.; von Nidda, H. A. K.; Klar, P. J.; Gungerich, M.; Reller, A. Chem. Mater. 2003, 15, 4028. (17) Liu, S. H.; Sun, X. D.; Li, J.-G.; Li, X. D.; Xiu, Z. M.; Huo, D. Eur. J. Inorg. Chem. 2009, 2009, 1214. (18) (a) Dijkstra, M.; Panneman, H. J.; Winkelman, J.; Kelly, J. J.; Beenackers, A. Chem. Eng. Sci. 2002, 57, 4895. (b) Chang, H. T.; Wu, N. M.; Zhu, F. Water Res. 2000, 34, 407. (c) Rothen-Rutishauser, B. M.; Sch€urch, S.; Haenni, B.; Kapp, N.; Gehr, P. Environ. Sci. Technol. 2006, 40, 4353. (19) Brunner, T. J.; Wick, P.; Manser, P.; Spohn, P.; Grass, R. N.; Limbach, L. K.; Bruinink, A.; Stark, W. J. Environ. Sci. Technol. 2006, 40, 4374. (20) Rodrı´ guez, J. A.; Fernandez-Garcı´ a, M. Synthesis, Properties, and Applications of Oxide Nanomaterials; Wiley-Interscience: New York, 2007. (21) Wang, C.; B€ottcher, C.; Bahnemann, D. W.; Dohrmann, J. K. J. Mater. Chem. 2003, 13, 2322.

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In this work, we report the preparation of fluorine and ironmodified hierarchical anatase microspheres with high photocatalytic efficiency. The effects of dopant, postannealing treatment, particle surface structure and morphology, and the excitation light source on the photocatalytic characteristics are investigated. All of these parameters were found to greatly influence the photocatalytic performance. Our results can furnish fresh perspectives on the investigation of anatase-based photocatalysts and provide new grounds for the following fundamental and applied research.

2. Experimental Section 2.1. Particle Synthesis. The titanium source for TiO2 synthesis was ammonium fluotitanate ((NH4)2TiF6, Reagent grade, Sinopharm Chemical Reagent Co.). Urea (CO(NH2)2, reagent grade, Sinopharm Chemical Reagent Co.), which releases ammonia via forced hydrolysis in water, was used to control the precipitation reaction. Anhydrous ferric chloride (FeCl3, reagent grade, Sinopharm Chemical Reagent Co.) was used as a source for Fe3þ loading. All the chemicals were used as received without further purification. For F- and Fe3þ coloaded titania, the desired amounts of ammonium fluotitanate, anhydrous ferric chloride, and urea were dissolved in water to make a solution of the intended concentration. The as-prepared solution was heated to 90 C under magnetic stirring and was held there for 30 min. After cooling naturally to room temperature, the resultant solids were recovered via centrifugation, washed repeatedly with distilled water via ultrasonic dispersion, and then dried at 60 C for 5 h. Specimens with different iron contents were prepared (0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, and 15.0 at. % in terms of nFe/nTi molar ratio) to investigate the effects of Fe3þ loading on the phase structure and physicochemical properties of the products. Samples are denoted by acronyms to indicate their composition and processing parameters. For example, TiO2(F) is the as-prepared F- loaded titania, and TiO2(F)-2%Fe-570 means F- and 2.0 at. % Fe3þ coloaded titania annealed in air at 570 C for 2 h. 2.2. Characterization Techniques. Phase identification was performed via X-ray diffractometry (XRD) on a Philips PW3040/ 60 diffractometer (Philips, Eindhoven, The Netherlands) operating at 40 kV/40 mA using Cu KR radiation. Particle morphology was observed via scanning electron microscopy (model SSX-550, Shimadzu Corp.) and field-emission scanning electron microscopy (FE-SEM, model JSM-7001F, Oxford Instrument HKL) operating at 15 kV. X-ray photoelectron spectroscopy (XPS) measurements were made on an ESCALAB250 (Thermo VG) machine with a monochromatic Al KR source, and all the binding energies were referenced to the C 1s peak at 284.6 eV. Thermogravimetry-differential scanning calorimetry analysis (TG-DSC) of the product was made using an STA 409 PC/PG analyzer (Netzsch, German) with a heating rate of 5 C/min under flowing nitrogen gas. Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer, Shelton, CT) of the powders was performed by the standard KBr method. UV-vis absorption spectra of the powders were obtained using a Lambda750 spectrophotometer (PerkinElemer, Shelton, CT) with a 60 mm integrating sphere. 2.3. Photocatalytic Evaluation. Photocatalytic activity was tested via bleaching 20 μM methyl orange (Reagent grade, Sinopharm Chemical Reagent Co.) solution under both UV and visible-light irradiation. The UV light was generated using a 500 W high-pressure mercury lamp with a 365 nm band-pass filter. Photocatalysis was performed by shining the UV light on the top surface of 10 mL dye solution (in a beaker of 15 mL capacity) with 5 mg of TiO2 particles ultrasonically dispersed in it. All the photocatalytic tests were conducted under magnetic stirring of the suspension. After being illuminated for a certain period of time, the suspension was immediately filtered through a 0.22 μm Millipore membrane to remove TiO2, and the relative concentration of methyl orange in the filtrate was determined through DOI: 10.1021/la903494r

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UV-vis spectroscopy by comparing its intensity of the 465 nm absorption with that of the original methyl orange solution. Visible-light-driven photocatalytic tests were similar to the above-mentioned UV test procedures, but with a different light source. In this case, a 500 W high-pressure xenon lamp, which emits a similar spectrum as sun, was used to simulate solar irradiation in the laboratory.

3. Results and Discussion 3.1. Synthesis and Composition of Microspheres. During precipitation, urea decomposes at temperatures higher than about 80 C, as shown by 2ðNH2 Þ2 CO þ 5H2 O f 4NH4 þ þ OH - þ HCO3 - þ CO3 2 ð1Þ The generated NH4þ and OH- then react with ammonium fluotitanate to yield TiO2 according to the following equation: ðNH4 Þ2 TiF6 þ 4NH4 þ þ 4OH - f TiO2 þ 6NH4 F þ 2H2 O ð2Þ Conventionally, the product obtained at room temperature is a nongelatinous hydrous titanium(IV) oxide represented as γ-TiO2.22 By raising the reaction temperature to 90 C, anatase titania precipitates out directly. It is believed that the higher reaction temperature used in this work enhances mobility of the configurational ions and thus enables them to regularly deposit to form the anatase crystal structure.17 Figure 1 shows representative diffractograms for the asprepared TiO2 microspheres. It is seen that all the XRD patterns match the anatase type of TiO2 (JCPDS card no. 01-071-1166), with no trace of rutile or brookite impurity being observed. As iron loading concentration increased to 4.0 at. %, traces of iron oxide were detected, indicating that part of the Fe3þ ions precipitated out independently and thus TiO2/Fe2O3 mixtures were produced. Figure 2 and Figure S1 (see the Supporting Information) show the morphologies of the particles. It is seen that highly dispersed submicrometer anatase spheres with hierarchical structures are prepared. For the samples containing iron of below 5.0 at. %, the size of the spheres is in a range of 400-700 nm, which can be easily completely removed by filtration after photocatalytic reaction. Each submicrometer particle is constructed by a large number of rounded nanoparticles (Figure S1), implying an aggregative growth mechanism of microspheres. In addition, contact recrystallization occurred as the iron addition increased to 15.0 at. %, which was evidenced by the severe coalescence of particles shown in Figure 2f. 3.2. Effect of High-Temperature Annealing. Thermoanalytical measurement of the as-prepared specimen TiO2(F)-2%Fe was performed under flowing nitrogen gas with a heating rate of 5 C/min. The significant weight losses (18.56%) in the range 25-1000 C determined from the TG curve in Figure S2 (see the Supporting Information) are mainly due to the desorption of physically and chemically adsorbed water and the loss of crystal water. In addition, the broad exotherm recorded at 456 C on the DSC curve is attributed to the enhanced crystallization of anatase rather than anatase f rutile phase transition, which is evidenced by the XRD analysis (see the Supporting Information, Figure S3) (22) Moeller, T. Inorganic Synthesis; McGraw-Hill Book Co.: New York, 1957; Vol. V, p 79.

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Figure 1. XRD patterns of the as-prepared TiO2 microspheres. The bottom one is the F--loaded TiO2 while the above six ones with different iron contents are the F- and Fe3þ coloaded TiO2.

of sample TiO2(F)-2%Fe-570, where only anatase is detected. The anatase f rutile transition starts at 791 C, as sample TiO2(F)-2%Fe-810 (Figure S3) is mainly composed by anatase and rutile TiO2. The phase transition ends at 856 C because sample TiO2(F)-2%Fe-900 is only composed of rutile TiO2 (JCPDS card no. 01-089-0554) and pseudobrookite (JCPDS card no. 00-009-0182). Noticeably, the photoinactive pseudobrookite phase exists in both the 810 and 900 C annealed products. High-temperature annealing results not only in phase transition but also in structural and morphological changes. Parts a and b of Figure 3 show FE-SEM images of samples TiO2(F)-2%Fe570 and TiO2(F)-2%Fe-810, respectively. Compared with the structure and morphology of the as-prepared sample TiO2(F)-2% Fe (see the Supporting Information, Figure S1b), the annealed sample TiO2(F)-2%Fe-570 is still anatase TiO2, but the microsphere surface becomes less rough and more compact, possibly due to dehydroxylation, the loss of crystal water, and the sintering among the primary nanoparticles (building units of the sphere). Furthermore, once phase transition occurs, the surface morphology changes more drastically, as is shown in Figure 3b. Obviously, partial particle surfaces become extremely smooth with the disappearance of the rounded primary nanoparticles, and the newly formed rutile crystallites are in submicrometer range as can be determined from the smooth parts on the microsphere surface of sample TiO2(F)-2%Fe-810. It can also be seen from Figure S4 (see the Supporting Information) that annealing improved the crystallinity of each sample and the amorphous iron oxide phase existed in TiO2(F)-15%Fe disappeared after the annealing. Sample TiO2(F)-15%Fe-570, on the other hand, was composed of anatase TiO2 and crystalline iron oxide (JCPDS card no. 00-0021165). The as-prepared anatase TiO2 is hydrated, as evidenced by the strong absorption bands at ∼1640 cm-1 (the O-H bending mode) and ∼3400 cm-1 (the O-H stretching mode) on the FTIR curves (see the Supporting Information, Figure S5). The water content in the product sharply decreases by annealing at high temperatures, as evidenced by the gradual disappearance of the 1640 and 3400 cm-1 bands (Figure 4). The absorption bands at ∼1400 and ∼3166 cm-1 in Figure S5 may suggest the existence of some ammonium ions. As XRD analysis (Figure 1) of the F-loaded TiO2 only found anatase-type TiO2, it is reasonable to say that these ammonium ions come from some impurities adsorbed on TiO2 surfaces. These adsorbed impurities can be completely removed by annealing, as evidenced by the disappearance of Langmuir 2010, 26(6), 4546–4553

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Figure 2. Representative SEM images of the as-prepared TiO2 microspheres with different iron contents. (a) F--loaded TiO2 (TiO2(F)); (b) TiO2(F)-2%Fe; (c) TiO2(F)-3%Fe; (d) TiO2(F)-4%Fe; (e) TiO2(F)-5%Fe; (f) TiO2(F)-15%Fe.

Figure 3. Representative FE-SEM images of the annealed products of sample TiO2(F)-2%Fe: (a) TiO2(F)-2%Fe-570 and (b) TiO2(F)-2%Fe-810. Part of the particle surface, indicated by arrows, became smooth after annealing in air at 810 C for 2 h.

Figure 5. XPS survey Al KR photoelectron spectra of the asprepared specimen TiO2(F)-2%Fe.

Figure 4. FTIR spectra of the as-prepared sample TiO2(F)-2%Fe and its products annealed at three different temperatures of 570, 810, and 900 C.

absorption bands at ∼1400 and ∼3166 cm-1 from the FTIR spectra of sample TiO2(F)-2%Fe-570 (Figure 4). The absorption band at ∼634 cm-1 in Figure S5 indicates the existence of iron oxide in sample TiO2(F)-5%Fe, which coincides with the XRD analysis (Figure 1). The absorption band at ∼580 cm-1 corresponds to the anatase Ti-O stretching mode, and the strong ones at ∼654 and 430 cm-1 in Figure 4 correspond to the rutile Ti-O stretching mode. All these observations are consistent with the literature.23 From FTIR spectra in Figure 4, it can be seen that 570 C annealing did not cause phase transition, 810 C annealing led to rutile phase generation, and 900 C annealing led to a complete anatase to rutile phase transition. These results coincide well with that of the thermal analysis (Figure S2) and XRD analysis (Figure S3). 3.3. XPS Analysis. Figure 5 shows XPS spectrum of a typical sample TiO2(F)-2%Fe, which demonstrates the surface (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1997.

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chemical compositions and chemical states of the as-prepared powders. XPS survey peaks show that the as-prepared TiO2 powder contains only Ti, O, F, Fe, N, and C elements. The C element is ascribed to the adventitious hydrocarbon of the XPS instrument. High-resolution XPS spectra of the as-prepared sample TiO2(F)-2%Fe are shown in Figure 6. Figure 6a exhibits the Ti(2p) core level spectrum of sample TiO2(F)-2%Fe. The Ti2p1/2 and Ti2p3/2 spin-orbital splitting photoelectrons are located at binding energies of 464.2 and 458.5 eV, respectively. Ti3þ species were not observed. The peak positions of Ti2p1/2 and Ti2p3/2 are in excellent agreement with the reported values.24 The O(1s) core level spectrum (Figure 6b) shows two peaks at 529.7 and 532 eV, corresponding to O2- ions and surface hydroxyl groups or chemisorbed water molecules, respectively. The high intensity of the 532 eV peak implies the existence of a large amount of surface hydroxyls or water molecules, and this agrees well with the FTIR analysis (Figure S5). It is well established that the adsorbed states of water on titania surface have a marked influence on the photocatalytic activity in photoredox processes. The existence of such states, and of hydroxyl groups in particular, appears to be a prerequisite for the surfaces having appreciable activity.25,26 Figure 6c shows high-resolution XPS spectrum of the F(1s) region. The 683.8 eV peak is assigned to the adsorbed F- ions. (24) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (25) Bickley, R. I.; Munuera, G.; Stone, F. S. J. Catal. 1973, 31, 398. (26) Soria, J.; Conesa, J. C.; Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991, 95, 274.

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Figure 7. UV-vis absorption spectra of the as-prepared TiO2

samples. The bottom one is the F--loaded TiO2 while the above seven ones with different iron contents are the F- and Fe3þ coloaded TiO2.

Figure 6. High-resolution XPS spectra of the as-prepared sample TiO2(F)-2%Fe, with (a) the Ti(2p) core level spectra, (b) the O(1s) core level spectra, (c) the F(1s) core level spectra, (d) the Fe(2p) core level spectra, and (e) the N(1s) core level spectra.

The large amount of surficial F- ions may occur not only by electrostatic adsorption but also by nucleophilic substitution reaction of F- ions and surficial hydroxyl groups.27 The 691.1 eV peak may be attributed to the incorporated F- ions in titania lattice. When F- ions replace the O2- ions in the titania lattice, a charge imbalance would be created, and the extra positive charge around the incorporated F- ions would increase its core level binding energy. Some literature suggested that fluorinated TiO2 shows enhanced photocatalytic performance due to the reduced electron-hole recombination rate.4a,13,27-30 The peaks of Fe2p3/2 and Fe2p1/2 shown in Figure 6d are located at 710.3 and 723.4 eV, respectively, corresponding well to the binding energy of Fe3þ.24 The low intensity of the Fe2p peaks suggests that the incorporated Fe3þ ions do not accumulate at the microsphere surface layers but uniformly distribute in the titania matrix. To investigate whether nitrogen is incorporated in the titania lattice or not, the N(1s) core level spectrum of sample TiO2(F)-2%Fe was recorded (Figure 6e). A N(1s) peak of weak intensity is observed at about 400 eV, which can be assigned to the surface adsorbed ammonium ions, and this result agrees well with that of the FTIR analysis (Figure S5). 3.4. Optical Properties. Figure 7 shows UV-vis absorption spectra of the as-prepared TiO2 specimens. The absorbance in the visible region largely increased by iron loading. It is well established that the formation of iron-rich layers or distinct Fe(III) species on titania surfaces would enhance the visible absorption but meanwhile result in a poor photocatalytic ability. Only good degrees of Fe3þ dispersion in a single-phase system were found to be photoactive.14b,21 The combined XPS and XRD analysis of specimen TiO2(F)-2%Fe does not find the accumulation of ironrich layers or distinct iron species, which means that the improved visible absorption is due to the homogeneous distribution of Fe3þ ions in the anatase matrix. In addition, XRD analysis shows that (27) Minero, C.; Mariella, G.; Maurino, V.; Vione, D.; Pelizzetti, E. Langmuir 2000, 16, 8964. (28) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632. (29) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086. (30) Li, D.; Haneda, H.; Labhsetwar, N. K.; Hishita, S.; Ohashi, N. Chem. Phys. Lett. 2005, 401, 579.

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the specimens containing iron of below 3.0 at. % are solid solutions, and thus constantly enhanced visible absorptions are observed as the iron contents increased from 1.0 to 3.0 at. % (Figure 7). Iron oxide phase is observed in the specimens containing iron of above 3.0 at. %, and therefore their ultraintense visible absorptions are the joint actions of iron ions dispersed in the titania matrix and the distinct iron oxide phase. The red shift of the absorption edge in the F- and Fe3þ coloaded TiO2 specimens is due to the excitation of the 3d Fe3þ electrons to the TiO2 conduction band (charge-transfer transition).31-33 The broad band that appears at ca. 470 nm can be ascribed to the d-d transition 2T2g f 2A2g, 2T1g or to the charge-transfer transitions among the dopant ions via the conduction band (Fe3þ þ Fe3þ f Fe4þ þ Fe2þ). The increased absorption in the visible regime can be due to the transitions implicating surface states or native defects in the lattice.10,33-35 Figure 8 shows UV-vis absorption spectra of the specimens annealed in air at 570 C for 2 h. In contrast to the as-prepared specimens containing iron of below 5.0 at. %, markedly enhanced absorbance in the visible region was observed for each annealed specimen. This is because annealing not only greatly enhances the crystallinity of the sample but also promotes the diffusion of iron ions into the titania matrix. In addition, the remarkable visible absorption of the annealed specimens containing iron of above 10.0 at. % is mainly due to the existence of a large amount of crystalline iron oxide. XPS analysis reveals that a large amount of fluorine ions exist on the as-prepared titania surface. After postannealing, these surficial fluorine ions would diffuse into the titania lattice, hence enhancing the absorption in the visible region, as evidenced by the greatly enhanced visible absorption of sample TiO2(F)-570 shown in Figure 8. 3.5. Photocatalytic Reactivity. Photocatalytic performances of the TiO2 powders synthesized in this work were tested via bleaching 20 μM methyl orange aqueous solutions under UV and visible light irradiation, respectively. Figure 9 shows degradation kinetics of the methyl orange solutions over the asprepared TiO2 microspheres under 365 nm UV irradiation. A blank test indicates that in the absence of TiO2 photocatalyst (31) Johnson, O. W.; Ohlsen, W. D.; Kingsbury, P. I. Phys. Rev. 1968, 175, 1102. (32) Moser, J.; Gr€atzel, M.; Gallay, R. Helv. Chim. Acta 1987, 70, 1596. (33) Mizushima, K.; Tanaka, M.; Asai, A.; Iida, S.; Goodenough, J. B. J. Phys. Chem. Solids 1979, 40, 1129. (34) Navı´ o, J.; Colon, G.; Litter, M. I.; Bianco, G. N. J. Mol. Catal. A: Chem. 1996, 106, 267. (35) Serpone, N.; Lawless, D.; Disdier, J.; Herrmann, J.-M. Langmuir 1994, 10, 643.

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interface electron transfer ecb- ðTi3þ Þ þ Ox f Ox-

ð8Þ

hvb þ ðæOH 3 Þ þ Red f Redþ

ð9Þ

where Ox is an electron acceptor (oxidant) and Red is an electron donor (reductant). Irradiation of the iron-loaded specimens can lead to the following additional step. charge trapping Fe3þ þ e - f Fe2þ

Figure 8. UV-vis absorption spectra of the annealed TiO2 specimens. The bottom one is the F--loaded TiO2 while the above seven ones with different iron contents are the F- and Fe3þ coloaded TiO2. Each specimen was calcined in air at 570 C for 2 h.

Figure 9. Degradation kinetics of 20 μM methyl orange solutions over the as-prepared TiO2 photocatalysts under 365 nm UV irradiation.

decoloration of the dye solution is negligible within the tested period. In the presence of TiO2 powders, irradiation of the suspension with UV light results in decoloration in all the cases, suggesting destruction of the absorption band of methyl orange. Under sufficient irradiation, electron-hole pairs are produced in the conduction and valence bands of anatase TiO2, and the following successive events take place.5a,14b,21 charge-pair generation TiO2 þ hυ f ecb- þ hvb þ

ð3Þ

Ti4þ þ ecb- f Ti3þ

ð4Þ

charge trapping

æOH - þ hvb þ f Ti4þ þ æOH 3

Thus, for solid-solution specimens containing iron of below 3.0 at. %, due to the enhanced separation of charge carriers resulted from iron loading, the photocatalytic degradation of methyl orange by hþ is gradually enhanced as the iron contents increased. The best performance is observed for the 3.0 at. % Fe3þ-loaded specimen (TiO2(F)-3%Fe), which causes an almost complete decoloration of the methyl orange solution within 15 min. The as-prepared specimen containing iron of above 3.0 at. %, in which an iron oxide phase is present, is less photoactive. It is known that the incorporation of fluorine into the titania lattice not only enhances crystallinity but also narrows bandgap, and the Ti3þ centers induced by charge compensation can promote e-/hþ separation.4a,13,36 It might be for this reason that the specimen TiO2(F) exhibits a good photocatalytic ability. The iron in the titania lattice has a stronger ability to separate charge carriers, and consequently the F- and Fe3þ coloaded specimens exhibit a higher photocatalytic activity than the F--loaded specimen of TiO2(F). Postannealing is usually required in wet chemical synthesis of anatase TiO2, which either transforms the amorphous precipitate into photoactive products or further improves crystallinity. Hightemperature annealing, however, may meanwhile change the particle surface constituents and surface structure that are vital to photocatalytic reactions. Figure 10 shows the degradation kinetics of methyl orange solutions over the annealed specimens under UV irradiation. When comparing the photocatalytic activities of the as-prepared and annealed specimens, interesting things are found. After annealing in air at 570 C for 2 h, the F--loaded specimen shows greatly enhanced photocatalytic performance, and the sample TiO2(F)-570 causes nearly complete decoloration of the methyl orange solution within 15 min. This is due to the improved crystallinity and the incorporation of a large amount of fluorine ions into the titania lattice by annealing. In contrast, the F- and Fe3þ coloaded samples show drastically decreased photocatalytic efficiency by annealing, especially for the 3.0 at. % Fe3þloaded specimen (TiO2(F)-3%Fe). Some literature states that enrichment of the surface by iron species commences at temperatures as low as 500 C, though not detectable by XRD.5a,14b,21 Once annealing causes the formation of surface iron-rich layers, the following steps would predominate:

ð5Þ 4þ

Fe

recombination

ð10Þ

Fe3þ þ e - f Fe2þ

ð11Þ

Fe3þ þ hþ f Fe4þ þ Fe2þ f 2Fe3þ þ heat

ð12Þ ð13Þ

ecb- þ hvb þ f heat

ð6Þ

That is, the surface-segregated iron ions arising from annealing serve as recombination centers for the photoexcited electrons and

Ti3þ þ æOH 3 f Ti4þ þ æOH -

ð7Þ

(36) Czoska, A. M.; Livraghi, S.; Chiesa, M.; Giamello, E.; Agnoli, S.; Granozzi, G.; Finazzi, E.; Valentin, C. D.; Pacchioni, G. J. Phys. Chem. C 2008, 112, 8951.

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DOI: 10.1021/la903494r

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Figure 10. Degradation kinetics of 20 μM methyl orange solutions over the annealed TiO2 photocatalysts under 365 nm UV irradiation.

holes, thus resulting in the drastic reduction of photocatalytic efficiency of the annealed specimens containing iron. Anatase/rutile composite photocatalyst, like Degussa P25, usually shows high photocatalytic performance. Though specimen TiO2(F)-2%Fe-810 has the anatase/rutile composite structure (Figure 3b), its photocatalytic efficiency (Figure 10) is extremely low. This abnormal phenomenon may be ascribed to the relatively smooth particle surfaces, the existence of photoinactive pseudobrookite (as revealed by XRD in Figure S3), and the recombination centers arising from the surface-segregated iron ions. Similar reasons resulted in the lower photocatalytic efficiency of the higher temperature annealed specimen TiO2(F)2%Fe-900. In order to determine whether the tendency found under UV irradiation agrees with that under visible light irradiation, the photocatalytic kinetics under visible light irradiation have also been investigated. Figure 11 shows the degradation kinetics of 20 μM methyl orange solutions over the as-prepared TiO2 samples, which are obviously largely different from that shown in Figure 9. Under 365 nm UV irradiation, the best performance is observed for the F- and 3.0 at. % Fe3þ coloaded sample (TiO2(F)-3%Fe) in Figure 9. Under visible light irradiation, however, the best one is the F--loaded specimen (TiO2(F), Figure 11). Fluorine loading narrows the bandgap of anatase TiO2 and thus enhances its photoresponse under visible light irradiation. This effect is more significant especially after postannealing, as evidenced by the results that specimen TiO2(F)-570 shows stronger visible light absorption (Figure 8) and greatly enhanced photocatalytic activity that causes nearly complete decoloration of the methyl orange solution within 2 h (Figure 12). For the F- and Fe3þ coloaded samples, the photocatalytic performances under visible-light irradiation decrease appreciably along with increased iron loading (Figure 11), and this is contrary to the tendency observed under UV irradiation (Figure 9). It is known that there are two effects induced by iron loading in titania. One is the increased absorption of UV and visible light, which would generate more charge carriers and thus tend to improve the photocatalytic performance. The other one is the formation of recombination centers, which would decrease the photocatalytic performance. The photocatalytic activity of Fand Fe3þ coloaded samples is therefore the joint actions of these two effects. Under UV irradiation, due to the strong photon energy of UV, the concentration of survived charge carriers in the F- and Fe3þ coloaded solid solution samples may be largely higher than that in the Fe3þ unloaded samples, and thus the 4552 DOI: 10.1021/la903494r

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Figure 11. Photodecomposition kinetics of 20 μM methyl orange solutions over the as-prepared TiO2 photocatalysts under visiblelight irradiation.

Figure 12. Photodecomposition kinetics of 20 μM methyl orange solutions over the annealed TiO2 photocatalysts under visible-light irradiation.

photocatalytic activity of the F- and Fe3þ coloaded samples improves as the iron content increases up to about 3.0 at. %. Under visible light irradiation, carrier concentration of the Fand Fe3þ coloaded samples would also increase. Because of the weak irradiation energy of the visible light, however, the net increase in carrier concentration would be rather limited. In such a case, carrier recombination would predominate; thus, the activity of the doubly loaded samples decreases as the iron content increases. Another noteworthy result is that postannealing decreases the visible-light photocatalytic activity of each F- and Fe3þ coloaded sample (Figure 12), and this further confirms the detrimental effects of postannealing on the photocatalytic activity of the anatase microspheres containing iron.

4. Conclusions By using ammonium fluotitanate as the titanium source, hierarchical anatase microspheres modified with iron and fluorine loading were successfully prepared via a urea-based homogeneous precipitation method. The nanoparticles-constructed rough surfaces are beneficial to improve photocatalytic performance. Anatase/rutile composite structure shows extremely low photocatalytic performance due to the relatively smooth surfaces of the microspheres as well as the existence of photoinactive pseudobrookite phase and iron-rich surficial layers. Interestingly, postannealing lowered the photocatalytic performance of the anatase microspheres with iron loading but greatly increased that of the fluorine-loaded anatase microspheres. Fe3þ loading constantly Langmuir 2010, 26(6), 4546–4553

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led to a decrease in the visible light photocatalytic activity of the as-prepared anatase microspheres, but up to 3.0 at. % of Fe3þ led to an increase in the activity of the as-prepared anatase microspheres under UV irradiation. In contrast, fluorine-loaded anatase microspheres, especially the postannealed ones, show excellent photocatalytic efficiencies under both UV and visiblelight irradiation. The anatase microspheres prepared in this work are not only a beautiful powder photocatalyst but also may be further assembled into photoactive opal-structure photonic crystal that can control the motion of light. Acknowledgment. This work was supported by the National Natural Science Fund for Distinguished Young Scholars (50425413), the Program for New Century Excellent Talents in

Langmuir 2010, 26(6), 4546–4553

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University (NCET-25-0290), the National Natural Science Foundation (Major Program) of China (50990303), and the Program for Changjiang Scholars and Innovative Research Teams in University (PCSIRT, IRT0713). Supporting Information Available: FE-SEM images of the as-prepared hierarchical anatase microspheres (Figure S1), TGDSC curves of the as-prepared sample TiO2(F)-2%Fe (Figure S2), XRD patterns of the as-prepared sample TiO2(F)-2%Fe and its annealed products (Figure S3), XRD patterns of the annealed TiO2 samples with different iron contents (Figure S4), and FTIR spectra of the as-prepared TiO2 samples with different iron contents (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la903494r

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