Synthesis of F-Doped Flower-like TiO2 Nanostructures with High

Mar 1, 2008 - The synthesized novel F-doped TiO2 flower-like nanomaterials possess good crystallinity and exhibit high photoelectrochemical activity f...
3 downloads 9 Views 994KB Size
Download PDF
Langmuir 2008, 24, 3503-3509

3503

Synthesis of F-Doped Flower-like TiO2 Nanostructures with High Photoelectrochemical Activity Guosheng Wu,† Jingpeng Wang,†,‡ Dan F. Thomas,‡ and Aicheng Chen*,† Department of Chemistry, Lakehead UniVersity, Thunder Bay, Ontario P7B 5E1, Canada, and Department of Chemistry, UniVersity of Guelph, Guelph, Ontario N1G 2W1, Canada ReceiVed October 6, 2007. In Final Form: December 18, 2007 We report on a novel and facile approach for the direct growth of F-doped flower-like TiO2 nanostructures on the surface of Ti in HF solutions under low-temperature hydrothermal conditions. The influence of the experimental parameters such as temperature, reaction duration, and the HF concentration on the morphology and photoelectrocatalytic activity of the formed F-doped flower-like TiO2 nanostructures was systematically studied. The presence of HF and the reaction time play an important role in the formation of the F-doped flower-like TiO2 nanostructures. The synthesized novel F-doped TiO2 flower-like nanomaterials possess good crystallinity and exhibit high photoelectrochemical activity for water-splitting and photodegradation of organic pollutants compared with P-25, which is currently considered to be one of the best commercial TiO2 photocatalysts. The approach described in this study provides a simple and novel method to synthesize F-doped TiO2 nanostructured materials that are ready for practical applications such as the photodegradation of wastewater.

Introduction Photocatalysis has gained considerable attention since the discovery of water-splitting on titania (TiO2) electrodes under UV light irradiation by Fujishima and Honda in 1972.1 Among various oxide semiconductors, TiO2 is currently one of the most promising photocatalysts because of its biological and chemical inertness, cost effectiveness, and the strong oxidizing power of its photogenerated holes. TiO2 has been widely used in photovoltaic cells,2 photocatalysis,3,4 sensors,5 and so forth. Various methods have been developed over the last two decades to fabricate different TiO2 nanostructures, for instance, the solgel process, hydrothermal methods, solvothermal methods, chemical vapor deposition, emulsion precipitation, and electrical oxidation.6,7 Different TiO2 nanostructures, including nanoparticles, nanowires, nanosheets, and nanotubes have been synthesized and tested. The main drawback of TiO2 is its wide band * To whom correspondence should be addressed. E-mail: aicheng.chen@ lakeheadu.ca. † Lakehead University. ‡ University of Guelph. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) (a) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737; (b) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (3) (a) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669; (b) Pal, B.; Torimoto, T.; Okazaki, K.; Ohtani, B. Chem. Commun. 2007, 483. (c) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. AdV. Funct. Mater. 2005, 15, 1291. (d) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (4) (a) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735758; (b) Chatterjee, D.; Dasgupta, S. J. Photochem. Photobiol. C 2005, 6, 186; (c) Zhang, X. T.; Jin, M.; Liu, Z. Y.; Nishimoto, S.; Saito, H.; Murakami, T.; Fujishima, A. Langmuir 2006, 22, 9477. (5) (a) Liu, S.; Chen, A. Langmuir 2005, 21, 8409; (b) Zhu, Y.; Shi, J.; Zhang, Z.; Zhang, C.; Zhang, X. Anal. Chem. 2002, 74, 120. (6) (a) Pan, D. C.; Zhao, N. N.; Wang, Q.; Jiang, S. C.; An, L. J. AdV. Mater. 2005, 17, 1991; (b) Peng, X. S.; Chen, A. AdV. Funct. Mater. 2006, 16, 1355; (c) Macak, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100; (d) Feng, X. J.; Macak, J. M.; Schmuki, P. Chem. Mater. 2007, 19, 1534; (e) Kim, Y. J.; Chai, S. Y.; Lee, W. I. Langmuir 2007, 23, 9567; (f) Chen, X.; Mao, S. Chem. ReV. 2007, 107, 2891. (7) (a) Tang, J.; Redl, F.; Zhu, Y.; Siegrist, T.; Brus, L. E.; Steigerwald, M. L. Nano. Lett. 2005, 5, 543; (b) Pal, M.; Serrano, J. G.; Santiago, P.; Pal, U. J. Phys. Chem. C 2007, 111, 2095; (c) Hosono, E.; Matsuda, H.; Honma, I.; Ichihara, M.; Zhou, H. Langmuir 2007, 7447; (d) Lin, J.; Lin, Y.; Liu, P.; Meziani, M. J.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2002, 124, 11514; (e) Ho, W.; Yu, J. C.; Yu, J. G. Langmuir 2005, 21, 3486.

gap (3.2 eV for anatase, 3.0 eV for rutile).8 Thus, only UV light can create electron-hole pairs and initiate photocatalytic processes.9 However, UV light accounts for only a small portion of solar energy compared to visible light. Much research is devoted to narrowing the band gap of TiO2 in order to maximize solar energy collection. One popular approach is to dope different elements into TiO2 nanostructures. Various metals have been doped into TiO2 nanomaterials,10-16 resulting, however, in thermal instability and increased carrier trapping.17 A number of nonmetal elements have also been recently doped into TiO2 nanostructures for band gap narrowing. Asahi et al. report N-doped TiO2 with a narrow band gap synthesized by sputtering TiO2 targets in a N2-Ar gas mixture followed by annealing in N2 gas at 550 °C.18 Recent studies have shown that carbon-doped TiO2 nanomaterials possess much higher photocurrent densities and are more efficient for water-splitting under visible-light illumination.19 Recent studies have also revealed that doping TiO2 with other elements, such as S,20 Cl, and Br,21 shifts the optical absorption edge to (8) Bendavid, A.; Martin, P. J.; Jamting, A.; Takikawa, H. Thin Solid Films 1999, 6, 355. (9) (a) Yin, S.; Zhang, Q. W.; Saito, F.; Sato, T. Chem. Lett. 2003, 32, 358; (b) Barborini, E.; Conti, A. M.; Kholmanov, I.; Piseri, P.; Podesta, A.; Milani, P.; Cepek, C.; Sakho, O.; Macovez, R.; Sancrotti, M. AdV. Mater. 2005, 17, 1842. (10) Frindell, K. L.; Bartl, M. H.; Robinson, M. R.; Bazan, G. C.; Popitsch, A.; Stucky, G. D. J. Solid State Chem. 2003, 172, 81. (11) (a) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669; (b) Thiel, J.; Pakstis, L.; Buzby, S.; Raffi, M.; Ni, C.; Pochan, D. J.; Ismat Shah, S. Small 2007, 3, 799. (12) Li, W.; Wang, Y.; Lin, H.; Shah, S. I.; Huang, C. P.; Doren, D. J.; Rykov, S. A.; Chen, J. G.; Barteau, M. A. Appl. Phys. Lett. 2003, 83, 4143. (13) Nagaveni, K.; Hegde, M. S.; Madras, G. J. J. Phys. Chem. B 2004, 108, 20204. (14) Salmi, M.; Tkachenko, N.; Lamminmaeki, R. J.; Karvinen, S.; Vehmanen, V.; Lemmetyinen, H. J. Photochem. Photobiol. A 2005, 175, 8. (15) Wu, C. G.; Chao, C. C.; Kuo, F. T. Catal. Today 2004, 97, 103. (16) Rodrigues, S.; Ranjit, K. T.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. AdV. Mater. 2005, 17, 2467. (17) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707. (18) Asahi, R.; Washizuka, T.; Yoshino, N.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (19) (a) Park, J. H.; Kim, S.; Bard, A. J. Nano. Lett. 2006, 6, 24; (b) Wu, G. S.; Nishikawa, T.; Ohtani, B.; Chen, A. Chem. Mater. 2007, 19, 4530. (20) (a) Umebayashi, T.; Yamaki, T.; Tanala, S.; Asai, K. Chem. Lett. 2003, 32, 330; (b) Ho, W. K.; Yu, J. C.; Lee, S. C. J. Solid State Chem. 2006, 179, 1171. (21) Luo, H.; Takata, T.; Lee, Y.; Zhao, J.; Domen, K.; Yan, Y. Chem. Mater. 2004, 16, 846.

10.1021/la703098g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

3504 Langmuir, Vol. 24, No. 7, 2008

longer wavelengths. It has also been reported that F-doped TiO2 nanostructures can improve the visible response of TiO2 photocatalysts.22 However, the traditional methods used in the fabrication of F-doped TiO2 nanostructures require a fluorine atmosphere and high temperatures. In addition, F-doped TiO2 nanostructures prepared by the sol-gel method are deleteriously amorphous in nature. In this paper, we report for the first time a one-step, templateless and seedless method to directly grow F-doped flower-like TiO2 nanostructures in HF solutions using a low-temperature hydrothermal method. The influence of the experimental parameters such as temperature, reaction duration, and HF concentration on the morphology and photocatalytic activity of the formed F-doped TiO2 flower-like nanostructure was systematically studied. The synthesized novel F-doped TiO2 flower-like nanomaterials possess good crystalline TiO2 and exhibit high photocatalytic activity for water-splitting and the photodegradation of organic pollutants. Experimental Section Titanium plates of 1.0 × 12.5 × 8 mm were first degreased using acetone and then washed with distilled water, etched in 18% HCl at 85 °C for 15 min to remove the oxide layer on the surface, completely washed with distilled water, and finally dried in a vacuum oven at 40 °C. In a typical synthesis, the Ti plates and a 15 mL solution of HF with different concentrations were transferred to a 23 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at preselected temperatures and for preselected times. After the hydrothermal treatment, the samples were completely washed with distilled water several times and dried in an oven at 100 °C. For comparison, 1 mM NaF, 1 mM HNO3, and a mixture of 1 mM NaF and 1 mM HNO3 solutions were used instead of HF solutions for the hydrothermal process and all other conditions were kept the same. The morphology of the F-doped flower-like TiO2 nanostructures was analyzed by scanning electron microscopy (SEM) (JEOL JSM 5900LV) with energy dispersive X-ray spectrometry (EDS) (Oxford Links ISIS). The X-ray diffraction (XRD) measurements were performed with a Philips PW 1050-3710 diffractometer with Cu KR radiation. Photoelectrochemical experiments were carried out in a 0.5 M Na2SO4 electrolyte using a three-electrode configuration with a Pt coil counter electrode and a saturated Ag/AgCl reference electrode (PGZ 301, Radiometer analytical). The UV source was Cure Spot 50 (ADAC systems) equipped with an Hg lamp. The wavelength range was from 280 to 450 nm, and the light intensity was 2 mW/ cm2. For visible light irradiance, the source light was passed through an optical filter, which cut off wavelengths below 420 nm. The intensity of the resulting visible light was ∼0.015 mW/cm2. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS System with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to the C 1s peak at 284.4 eV of the surface adventitious carbon. The photoelectrocatalytic activity of the synthesized F-doped flower-like TiO2 nanostructures was further investigated using 4-nitrophenol as a model pollutant. We started with an initial concentration of 7.5 × 10-5 M 4-nitrophenol; 0.5 M NaOH was employed as the supporting electrolyte. The change of the concentration of 4-nitrophenol in the course of the photodegradation was monitored by in-situ UV-vis spectroscopy (EPP StellarNet Inc). For comparison, a commercially available TiO2 powder (Degussa P-25) was used to prepare thin particulate films on titanium plate and the prepared particulate films were also tested in this study. To fabricate the P-25 thin film, P-25 ethanolic suspension (25 g/L) was deposited on the pretreated Ti plates by spin-coating first and then (22) (a) Li, D.; Haneda, H.; Labhsetwar, N. K.; Hishita, S.; Ohashi, N. Chem. Phys. Lett. 2005, 401, 579; (b) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808.

Wu et al. dried in air at 105 °C. This procedure was repeated several times until the thickness of the P-25 film reached ca. 1 µm. The sample size is 1.0 cm2, the irradiation area is around 0.5 cm2, and the volume of the 4-NPh solution is 12 mL.

Results and Discussion Figure 1 shows the SEM images of F-doped TiO2 flower-like nanostructures prepared at 100 °C for 5 h under different HF concentrations: (A) 1 mM, (B) 5 mM, and (C) 20 mM. The insets to Figure 1A-C are their corresponding high-magnification SEM images. As seen in the SEM images, the Ti substrate was well-covered by the uniform flower-like TiO2 nanostructures. When increasing the HF concentration from 1 to 20 mM, the size of the flower-like TiO2 nanostructures slightly decreases. To determine the composition of the synthesized flower-like nanostructures, EDS spectra for the three samples were recorded and are presented in Figure 1D. Ti and O are observed with an estimated atomic ratio of 1:2, showing the formation of TiO2. An F peak is also observed, and the intensity of the F signal increases with the increase of the HF concentration. However, the EDS results cannot distinguish the origin of the F signal, which may arise from the F doped into the TiO2 nanostructures and/or from the F on the surface, although the samples were completely washed with distilled water several times before the EDS analysis. In order to decipher the origin of the F signal, a high-resolution XPS spectrum of the F 1s region of the sample prepared in 1 mM HF at 100 °C for 5 h was measured and is presented in Figure 2. The F 1s region is composed of two peaks. The large peak located at 685.2 eV can be attributed to the Fions physically adsorbed on the surface of the flower-like TiO2 nanostructures, while the other small peak located at 688.2 eV can be attributed to the substitutional F-atoms in the TiO2.22 This second peak originates from the F-atoms doped into the TiO2 crystal lattice during the hydrothermal process. Quantitative analysis shows that the doped F vs Ti is 0.37 atomic %. Increasing the concentration of the HF solution slightly increases the amount of doped F. The amount of doped F is 0.50 and 0.63 atomic % for sample B and C, respectively. We further investigated the effect of temperature on the formation of the F-doped TiO2 flower-like nanostructures in 1 mM HF for 5 h. Figure 3 presents the SEM images of the four samples synthesized at (A) 100 °C, (B) 120 °C, (c) 150 °C, and (D) 180 °C. Obviously, flower-like TiO2 nanostructures were formed under all four temperatures. The size of the flower-like TiO2 nanostuctures slightly decreases with the increase of temperature from 100 to 180 °C. The corresponding X-ray diffraction (XRD) patterns of the four samples are presented in Figure 4. The peaks marked by a star are derived from the Ti substrate. It can be seen that anatase TiO2 (JCPDS file No. 211272) is the dominant phase in all these samples. Upon increasing the temperature from 100 to 180 °C, the diffraction peaks attributed to TiO2 became stronger and stronger and the peaks marked by a star became weaker and weaker, indicating that more and more TiO2 is formed on the substrate. It is interesting to note that a very weak rutile phase diffraction (110) was also detected for the samples synthesized at 150 and 180 °C, indicating that the high pressure in the autoclave induces the phase transformation at a much lower temperature. Normally, the phase transformation temperature in air from anatase to rutile TiO2 is between 600 and 750 °C.6b,23 In addition, our further XPS analysis shows that the doped F vs Ti of all these four samples prepared (23) Madras, G.; McCoy, B. J.; Navrotsky, A. J. Am. Ceram. Soc. 2007, 90, 250.

F-Doped Flower-like TiO2 Nanostructures

Langmuir, Vol. 24, No. 7, 2008 3505

Figure 1. SEM images of the samples prepared at 100 °C for 5 h using different concentrations of HF: (A) 1 mM, (B) 5 mM, and (C) 20 mM. (D) EDS of the flower-like TiO2 nanostructures. The insets show the corresponding high-magnification images.

for the different time periods are presented in Figure 5. Before the hydrothermal process, the surface of the Ti substrate is relatively smooth. After 60 min of hydrothermal treatment (Figure 5A), the Ti substrate is chemically etched and the surface is rough. A few nanoparticles are also found on the surface. Increasing to 90 min (Figure 5B), a large number of TiO2 nanoparticles appear on the Ti substrate. The formed TiO2 nanoparticles increase in size when the hydrothermal time is increased to 120 min (Figure 5C). Flower-like TiO2 nanostructures are formed after 180 min of the hydrothermal treatment, as shown in Figure 5D. Further increasing the hydrothermal time to 240 min (Figure 5E) and 600 min (Figure 5F), the size of the flowerlike TiO2 nanostructures steadily decreases, which is consistent with the observation in Figure 1 when the HF concentration was increased. All the above results indicate that the formation of F-doped flower-like TiO2 nanostructures involves several steps. First, the Ti substrate reacts with HF under the hydrothermal condition, forming H2TiF6: Figure 2. F 1s high-resolution XPS spectrum of the F-doped flowerlike TiO2 nanostructures prepared using the hydrothermal method at 150 °C in 1 mM HF for 5 h.

under the same HF concentration but at different temperature is very similar: 0.38 (A), 0.36 (B), 0.33 (C), and 0.35 atomic % (D). In order to elucidate the formation mechanism of the flowerlike TiO2 nanostructures, we investigated the influence of reaction time while the concentration of HF was kept at 1 mM and the temperature at 100 °C. The SEM images of samples synthesized

Ti + 6HF f H2TiF6 + 2H2v

(1)

As reaction 1 continues, more H2TiF6 is produced. Subsequently, H2TiF6 combines with H2O, forming Ti(OH)4:

H2TiF6 + 4H2O f Ti(OH)4 + 6HF

(2)

The formed Ti(OH)4 turns to TiO2 initially, nucleates, and grows into TiO2 nanoparticles under the hydrothermal conditions:

Ti(OH)4 f TiO2 + 2H2O

(3)

3506 Langmuir, Vol. 24, No. 7, 2008

Wu et al.

Figure 3. SEM images of the samples prepared in 1 mM HF for 5 h at different temperatures: (A) 100 °C, (B) 120 °C, (C) 150 °C, and (D) 180 °C.

Figure 4. XRD patterns of samples illustrated in Figure 3 prepared by using 1 mM HF for 5 h at different temperatures. The peaks marked by a star are derived from the Ti substrate.

It is known that HF is a corrosive chemical. The formed TiO2 can also be etched by HF:

TiO2 + 6HF f H2TiF6 + 2H2O

(4)

Meanwhile, the produced H2TiF6 species diffuses to the surface of the TiO2 and new TiO2 is formed and aggregated on the former

surface of TiO2 nanoparticles.6c The continuous repeating of the dissolution and deposition processes results in the formation of the F-doped flower-like TiO2 nanostructures. In this hydrothermal process, HF not only etches the Ti substrate, providing a Ti source for the formation of TiO2 nanostructures, but also serves as the source of F-dopant. To further confirm the above proposed mechanism, we did three control experiments under the same hydrothermal condition but using (a) 1 mM HNO3 solution, (b) 1 mM NaF solution, and (c) a mixture of 1 mM HNO3 + 1 mM NaF solutions instead of the HF solution. No TiO2 nanostructures were formed in both experiments a and b; similar flower-like TiO2 nanostructures were produced in experiment c, demonstrating that HF plays a crucial role in the formation of the flowerlike TiO2 nanostructures. We further studied the photoelectrocatalytic activity of the synthesized flower-like TiO2 nanostructures. For comparison, P-25 was used as the benchmark to evaluate the photoelectrocatalytic activity of the synthesized flower-like TiO2 nanostructures. P-25 is a mixture of anatase (∼79%) and rutile (∼21%) TiO2 and it is currently considered to be one of the best commercial TiO2 photocatalysts. Figure 6 shows a comparison of the photocurrent density vs applied potential curves for the F-doped flower-like TiO2 nanostructures prepared at different temperatures in 1 mM HF for 5 h and a P-25 film with the same thickness as the flower-like TiO2 nanostructures prepared at 150 °C under Hg lamp irradiation (∼2 mW/cm2). The electrolyte used in this photoelectrochemical study was 0.5 M Na2SO4. The reaction investigated here is thus the water-splitting reaction:

2H2O f O2v + 2H2v

(5)

F-Doped Flower-like TiO2 Nanostructures

Langmuir, Vol. 24, No. 7, 2008 3507

Figure 5. SEM images of the samples prepared by using 1 mM HF at 100 °C for different times: (A) 60 min, (B) 90 min, (C) 120 min, (D) 180 min, (E) 240 min, and (F) 600 min.

Figure 6. Variation of photocurrent density vs applied electrode potential for P25 and the F-doped TiO2 flower-like nanostructures prepared at different temperatures in 0.5 M Na2SO4 under Hg lamp irradiation (∼2 mW/cm2).

The photocurrent intensity increases with the increase of the applied electrode potential, which reduces the recombination of the photogenerated electrons and holes. This is consistent with the previous literature results.19 Figure 6 shows that all the flowerlike TiO2 nanostructures exhibit higher photocurrent than the P-25 film and that the sample synthesized at 150 °C possesses the highest photocurrent. The higher photocurrent intensity shows

that the synthesized F-doped flower-like TiO2 nanostructures have higher photoelectrocatalytic activity than the P-25 coated sample, mainly due to the larger surface area of the flower-like nanostructure. As seen in the X-ray diffraction patterns presented in Figure 4, increasing the temperature from 100 to 180 °C increases the intensity of the TiO2 peaks, indicating the increase of the thickness of the formed F-doped flower-like TiO2 nanostructures. TiO2 is a semiconductor with low conductivity. If the formed TiO2 film is too thick, the resulting photocurrent lowers. In addition, increasing the temperature increases the rutile phase. Only anatase phase is observed for the samples prepared under 100 and 120 °C; however, rutile phase appears in the samples synthesized under 150 and 180 °C. It is well-known that anatase TiO2 has higher photocatalytic activity than that in the rutile phase. We further investigated their response to visible light. Figure 7 presents the photocurrents of the F-doped flower-like TiO2 nanostructures fabricated at different temperatures and the P-25 film under visible light irradiation with a wavelength of λ > 420 nm and intensity equal to 0.015 mW/cm2. The trend of the changes of the photocurrent of the different samples under visible light irradiation is similar to the observation from Figure 6; the sample fabricated at 150 °C also possesses the highest photocurrent. As expected, the photocurrent of P-25 is very low and does not change with the increase of the applied potential. Interestingly, the photocurrent of the F-doped flower-like TiO2 nanostructures increases with the increase of the electrode potential and is much higher than that of the P-25 film, demonstrating that the F-doped

3508 Langmuir, Vol. 24, No. 7, 2008

Wu et al.

flower-like TiO2 nanostructures can more effectively harvest visible light.

Figure 7. Variation of photocurrent density vs applied electrode potential for P25 and F-doped TiO2 flower-like nanostructures prepared by using 1 mM HF for 5 h at different temperatures in 0.5 M Na2SO4 under visible light (∼0.015 mW/cm2) irradiation (λ > 420 nm).

The photoelectrocatalytic activity of the synthesized F-doped flower-like TiO2 nanostructures was further tested using 4-nitrophenol (4-NPh) as a model pollutant. Nitrophenols are among the most common toxic persistent pollutants in industrial and agricultural wastewater. They are considered to be hazardous waste and priority toxic pollutants by the U.S. Environmental Protection Agency. Figure 8A,B presents the in situ kinetics studied by recording the UV-vis spectra at 30-min intervals during the photoelectrochemical degradation of 4-NPh on the F-doped flower-like TiO2 nanostructures prepared at 150 °C (A) and the P-25 TiO2 film (B) under ∼2 mW/cm2 UV and visible irradiation. An anodic bias of 600 mV vs Ag/AgCl was applied. The main absorption band centered at ∼ 400 nm significantly decreases with the UV irradiation time. The changes in the concentration of 4-NPh under UV and visible irradiation are shown in Figure 8C. The linear relationship of ln c/co vs time (Figure 8D) shows that the photoelectrocatalytic degradation of 4-NPh follows pseudo-first-order kinetics:

Figure 8. (A) UV-vis spectra of F-doped flower-like TiO2 samples taken during the photoelectrochemical degradation of 0.075 mM 4-nitrophenol basic solution. (B) UV-vis spectra of P-25 samples taken during the photoelectrochemical degradation of 0.075 mM 4-nitrophenol alkaline solution. (C) The variation of 4-nitrophenol concentration by photoelectrocatalytic reaction with flower-like TiO2 and P-25. (D) Pseudo-first-order kinetic rate plots for the photoelectrochemical degradation of 4-nitrophenol.

F-Doped Flower-like TiO2 Nanostructures

c ln ) kt c0

Langmuir, Vol. 24, No. 7, 2008 3509

Conclusions

(6)

where c/c0 is the normalized 4-NPh concentration, t is the reaction time, and k is the apparent reaction rate in terms of min-1. The apparent photoelectrochemical degradation rate constant for the F-doped flower-like TiO2 nanostructures is 5.49 × 10-3 min-1, which is about 30% higher than that for the P-25 film, 3.95 × 10-3 min-1, further confirming that the F-doped flower-like TiO2 nanostructures exhibit high photoelectrocatalytic efficiency for promising environmental applications. It is known that the use of aqueous suspensions limits practical applications, because of the difficulties in separating TiO2 fine particles and recycling the photocatalyst. The micrometric size of particles makes it very difficult for them to be separated from water, particularly from wastewaters. Many techniques have been proposed for the immobilization of TiO2 on solid substrates to overcome this problem. Generally speaking, all the proposed methods involve a tedious procedure: prepare TiO2 nanostructures from solution; redisperse them into certain agents; and precipitate, paste, or spin-coat them on a substrate. Please note that the F-doped flowerlike TiO2 nanomaterials fabricated in this study were directly grown on the Ti substrate, thus not requiring separation and being ready for practical applications.

In summary, novel F-doped flower-like TiO2 nanostructures have been successfully synthesized in the presence of HF by a mild hydrothermal process for the first time. Our study has demonstrated that HF plays a crucial role in the growth of the flower-like TiO2 nanostructures. These synthesized flower-like TiO2 nanostructures possess good crystallinity and exhibit high photoelectrochemical activity for water-splitting and the photodegradation of organic pollutants, making them promising candidates for environmental applications in wastewater treatment and the photoinduced splitting of water into hydrogen, a green energy source. The approach described in this study provides a simple and novel method to synthesize F-doped TiO2 nanostructured materials, ready for practical applications such as the photodegradation of wastewater. Acknowledgment. This research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A.C. acknowledges NSERC and the Canada Foundation for Innovation (CFI) for the Canada Research Chair Award in Material and Environmental Chemistry. LA703098G