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Langmuir 2008, 24, 7338-7345
Change of Adsorption Modes of Dyes on Fluorinated TiO2 and Its Effect on Photocatalytic Degradation of Dyes under Visible Irradiation Qi Wang, Chuncheng Chen,* Dan Zhao, Wanhong Ma, and Jincai Zhao* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China ReceiVed January 29, 2008. ReVised Manuscript ReceiVed April 30, 2008 Surface-fluorinated TiO2 (F-TiO2) particles were prepared via the HF etching method. The surface characteristics of fluorinated TiO2, the adsorption modes of dyes, and the reaction pathways for the photocatalytic degradation of dye pollutants under visible light irradiation were investigated. It was found that, in the treatment of TiO2 by HF etching, F- not only displaces surface HO- but also substitutes some surface lattice oxygen. Using zwitterionic Rhodamine B (RhB) dye as a model, the change of the adsorption mode of RhB on F-TiO2 relative to that on pure TiO2 was validated by adsorption isotherms, X-ray photoelectron spectroscopy (XPS), and IR techniques for the first time. RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, while its adsorption group is switched to the cationic moiety (-NEt2 group) on F-TiO2. Both the photocatalytic degradation kinetics and mechanisms were drastically changed after surface fluorination. Dyes with positively charged nitrogen-alkyl groups such as methylene blue (MB), malachite green (MG), Rhodamine 6G (Rh6G), and RhB all underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of dye chromophore ring structures predominated. The relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was also discussed in terms of the effect of fluorination on the surface adsorption of dyes and on the energy band structure of TiO2.
Introduction Heterogeneous photocatalysis based on TiO2 has been extensively studied in various fields, especially for the environmental application to the destruction of toxic pollutants.1,2 One obstacle toward its effective utilization is the relatively inefficient use of solar energy. Only less than 5% (UV light) of sunlight is absorbed by this photocatalyst. Photosensitization by dyes has been reported by our group and others to be one of the most effective ways to extend the photoresponse of TiO2 into the visible region.3–6 In this system, the dye instead of the TiO2 photocatalyst is excited by visible light irradiation. The excited dye molecule transfers electrons into the conduction band of TiO2, while the dye itself is converted to its cationic radical. The injected electrons can react with dioxygen adsorbed on the surface of TiO2 to generate a series of active oxygen species such as O2 · -, · OH, and H2O2. The subsequent radical chain reaction can lead to the degradation of the dye. It is well-known that heterogeneous photocatalytic reactions primarily take place on the surface and surface properties of TiO2 are critical for photocatalytic efficiencies. Accordingly, another important field in photocatalysis studies focuses on surface modification of TiO2 to improve its photocatalytic efficiency and to probe the reaction pathway.7–11 Among various surface * To whom correspondence should be addressed. Fax: 86-10-8261-6495. E-mail:
[email protected] (J.Z.);
[email protected] (C.C.). (1) Pichat, P. Photocatalytic degradation of pollutants in water and air: Basic concepts and applications. In Chemical Degradation Methods for Wastes and Pollutants: EnVironmental and Industrial Applications; Tarr, M. A., Ed.; Marcel Dekker: New York/Basel, 2003; pp 77-119. (2) Agrios, A. G.; Pichat, P. J. Appl. Electrochem. 2005, 35, 655–663. (3) Wu, T.; Lin, T.; Zhao, J.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 1999, 33, 1379–1387. (4) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845–5851. (5) Cho, Y.; Choi, W.; Lee, C. H.; Hyeon, T.; Lee, H. I. EnViron. Sci. Technol. 2001, 35, 966–970. (6) Xu, Y.; Langford, C. H. Langmuir 2001, 17, 897–902. (7) Zang, L.; Lange, C.; Abraham, I.; Storck, S.; Maier, W. F.; Kisch, H. J. Phys. Chem. B 1998, 102, 10765–10771. (8) Vohra, M. S.; Tanaka, K. EnViron. Sci. Technol. 2001, 35, 411–415.
modifications, surface fluorination has received particular attention.12–16 It is reported that the OH radical-mediated photocatalytic degradation reactions are greatly accelerated on FTiO2.12,15,16 For example, Pelizzetti and co-workers reported that surface fluorination of TiO2 could improve the photocatalytic oxidation rate of phenol under UV irradiation.14 Surface fluorination of TiO2 was also reported to enhance the formation of singlet oxygen and lead to the degradation of a recalcitrant organic substrate (cyanuric acid) under UV irradiation.17 However, photocatalytsis on surface-fluorinated TiO2 has only been investigated in the UV-irradiated system. As far as we know, the only work on a visible light responsive system is reported by Park and Choi.15 During their study on photocatalytic behaviors of fluorinated TiO2 under UV irradiation, they also found that visible light induced degradation of Acid Orange 7 on F-TiO2 was markedly reduced due to the hindered adsorption of substrates.15 Considering that TiO2-assisted photocatalytic degradation of dyes under visible irradiation does not involve the formation of photogenerated holes, the effect of surface fluorination on the photocatalytic degradation of dyes under visible light irradiation should be different from that under UV irradiation. In the visible light induced photocatalytic degradation of N-alkyl-containing dyes, two typical reaction pathways, namely N-dealkylation and direct cleavage of the conjugated chromophore (9) Schwitzgebel, J.; Ekerdt, J. G.; Sunada, F.; Lindquist, S. E.; Heller, A. J. Phys. Chem. B 1997, 101, 2621–2624. (10) Mutin, P. H.; Lafond, V.; Popa, A. F.; Granier, M.; Markey, L.; Dereux, A. Chem. Mater. 2004, 16, 5670–5675. (11) Liu, G.; Li, X.; Zhao, J.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2000, 34, 3982–3990. (12) Lee, J.; Choi, W.; Yoon, J. EnViron. Sci. Technol. 2005, 39, 6800–6807. (13) Lv, K.; Xu, Y. J. Phys. Chem. B 2006, 110, 6204–6212. (14) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632–2641. (15) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086–4093. (16) Vohra, M.; Kim, S.; Choi, W. J. Photochem. Photobiol., A 2003, 160, 55–60. (17) Janczyk, A.; Krakowska, E.; Stochel, G.; Macyk, W. J. Am. Chem. Soc. 2006, 128, 15574–15575.
10.1021/la800313s CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
Adsorption Modes of Dyes on Fluorinated TiO2
structure, have been frequently observed by us and others.11,18–22 The relative preference of these two pathways was reported to be dependent on surface modification. For example, we found that the photodegradation of Sulforhodamine-B (SRB) on pure TiO2 predominantly led to cleavage of the dye chromophore structure, whereas the initial steps in the dye degradation occurred preferentially via the N-de-ethylation process after the addition of anionic surfactant (DBS) into the suspensions.11 Park and Choi18 observed the enhanced N-de-ethylation process during the visible-sensitized degradation of Rhodamine B (RhB) on Nafion-coated TiO2 relative to that on pure TiO2. However, the mechanism for such a change was not fully understood. In particular, no direct and conclusive evidence for the change of adsorption modes has been obtained so far, and the mechanistic relationship between the adsorption mode and degradation pathway has not been addressed. In the earlier reports, surface fluorinations were almost achieved by adding NaF into the dispersions under acidic conditions (pH 3-4), and the fluoride anion replaced surface hydroxyl groups.12–15 However, a TiO2 photocatalyst modified by this method will lose its function at higher pH. Interestingly, HF has been employed to dissolve anatase TiO2 and to etch the surface of rutile TiO2.23–25 It is expected that etching of the TiO2 surface by HF can change more strongly the surface properties than a simple exchange of surface OH-. In the present study, TiO2 was modified by immersing crystallized TiO2 particles into the HF solution assisted by ultrasonication. The adsorption modes of dyes on the surface of TiO2 and F-TiO2 were carefully examined and validated for the first time by direct evidence. The effect of surface fluorination of TiO2 on the photocatalytic degradation of various dyes under visible light irradiation was also investigated. It was observed that RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, leading to the direct cleavage of the RhB chromophore structure under visible irradiation, whereas, on F-TiO2, the adsorption of the dye occurs via the cationic moiety (-NEt2 group) and N-de-ethylation predominates before destruction of the chromophore structure. This study provides direct evidence for the inversion of adsorption modes and also helps to understand the mechanistic relationship between the adsorption mode and degradation pathway.
Experimental Section 1. Materials and Reagents. Titanium(IV) n-butoxide (99%) was purchased from Acros Organic Co. Isopropyl alcohol was of analytical grade, and hydrofluoric acid was of chemical reagent grade. The Rhodamine-B (RhB) dye, Rhodamine 101 (RhB101) dye, and Rhodamine 6G (Rh6G) dye were of laser grade quality. Alizarin red (AR), malachite green (MG), methylene blue (MB), fluorescein, Orange II (OrgII), and phenosafranine were of analytical grade. The molecular structure of RhB is shown as follows, and the other dye structures are given in Figure S1 in the Supporting Information. Deionized and doubly distilled water was used throughout the study. (18) Park, H.; Choi, W. J. Phys. Chem. B 2005, 109, 11667–11674. (19) Hu, X.; Mohamood, T.; Ma, W.; Chen, C.; Zhao, J. J. Phys. Chem. B 2006, 110, 26012–26018. (20) Chen, C.; Zhao, W.; Lei, P.; Zhao, J.; Serpone, N. Chem.sEur. J. 2004, 10, 1956–1965. (21) Yang, J.; Chen, C.; Ji, H.; Ma, W.; Zhao, J. J. Phys. Chem. B 2005, 109, 21900–21907. (22) Watanabe, T.; Takizawa, T.; Honda, K. J. Phys. Chem. 1977, 81, 1845– 1851. (23) Ohno, T.; Sarukawa, K.; Matsumura, M. J. Phys. Chem. B 2001, 105, 2417–2420. (24) Taguchi, T.; Saito, Y.; Sarukawa, K.; Ohnoz, T.; Matsumura, M. New J. Chem. 2003, 27, 1304–1306. (25) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. J. Am. Chem. Soc. 2005, 127, 12975–12983.
Langmuir, Vol. 24, No. 14, 2008 7339 The pH of the solutions was adjusted with 1 M HClO4 or 1 M NaOH.
2. Photocatalyst Preparation. TiO2 was synthesized by a sol-gel method. A total of 0.02 mol titanium(iv) n-butoxide (99%) was dissolved in 150 mL of isopropyl alcohol, and the mixture was magnetically stirred. Distilled water (15 mL) was added dropwise to the mixture at 0 °C under vigorous stirring. After complete hydrolysis, the white suspension was kept at 80 °C for 1 h, separated by centrifugation, and washed with 20 mL of distilled water for 10 times to remove any impurity. The powders were dried at 100 °C in the oven for 4 h and sintered at 430 °C for 2 h. As-prepared TiO2 powders (500 mg) were suspended in 5 mL of 5% hydrofluoric acid under ultrasonication for 0.5 h, separated by centrifugation, and then dried at 80 °C. The dried powders were washed by distilled water until no F- was detected in the filtrate and dried again at 100 °C. The pure TiO2 nanoparticles were prepared with the same procedure except that no hydrofluoric acid was used before washing with water. 3. Characterization. X-ray diffraction measurements were performed on a Regaku D/Max-2500 diffractometer with Cu KR radiation (1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a ESCA laboratory 220i-XL spectrometer with an Al KR (1486.6 eV) X-ray source and a charge neutralizer, and all the spectra were calibrated to the C 1s peak at 284.8 eV. For XPS experiments detecting the changes of the N 1s binding energy in a dye, RhB was first adsorbed on TiO2 and F-TiO2 followed by centrifugation and separation, and then the solid catalysts were washed completely to remove the physically adsorbed dye molecules. The washed samples were dried under vacuum at room temperature. The above process was carried out in the dark to ensure that no photoreaction would occur during the sample treatment. The infrared spectra of RhB dye adsorbed on the catalysts was obtained on a TENSOR 27 (Bruker) Fourier transform infrared (FT-IR) spectrometer in the mode of diffuse reflectance using TiO2 and F-TiO2, as the background references. The samples used in this experiment were prepared by the same process as that in the XPS measurements. The electrophoretic mobilities of TiO2 and F-TiO2 particles in aqueous suspensions (0.1 g/L) were measured to determine their ζ-potentials as a function of pH using a Zetasizer 2000 instrument (Malvern Co., U.K.). 4. Photoreactor and Light Source. A 500 W halogen lamp as the visible light source was put in a cylindrical glass vessel with a recycling water glass jacket, meanwhile a cutoff filter was placed outside the water jacket to completely remove any radiation at wavelengths below 450 nm, thereby ensuring illumination by visible light only. 5. Photocatalytic Degradation of Dyes. The aqueous dispersions were typically prepared by addition of 50 mg of TiO2 or F-TiO2 to a 50 mL aqueous solution containing 2 × 10-5 M dyes. Prior to irradiation, the suspensions were magnetically stirred in the dark for ∼30 min to establish an adsorption/desorption equilibrium. The dispersions were kept under constant air-equilibrated conditions before and during irradiation. At given time intervals, 5 mL aliquot dispersions were sampled, centrifuged, and filtered through a Millipore filter (pore size 0.2 µm). A Lambda Bio 20 spectrophotometer (Perkin-Elmer Co.) and high-performance liquid chromatography (HPLC) (Dionex P580 pump and UVD340S diode array detector) were used to record the temporal variations of the dyes. The N-de-ethylated intermediates of RhB were identified by HPLC according to our previous report.20
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Wang et al. Table 1. Relative Intensity of Fitted Peaks of the High-Resolution XPS Spectra of F- Modified TiO2 for the F 1s Region and the O 1s Regiona F 1s (%) samples
O 1s (%)
684.6 eV 689.6 eV 529.8 eV 531.9 eV
about 100 trace F-TiO2 (unwashed) F-TiO2 (water-washed) 78.1 21.9 F-TiO2 (NaOH-washed) 12.5 87.5
87.8 88.8 83.5
12.2 11.2 16.5
a Percentages were obtained by the ratio of each peak area to the corresponding total area of fluorine or oxygen peaks.
Figure 1. XPS spectra of F 1s: (a) catalyst prepared without washing with water; (b) washed catalyst with distilled water until no F- was detected in the filtrate; and (c) catalyst washed further with 1 M NaOH.
Results and Discussion 1. Structures and Properties of F-TiO2 The X-ray diffraction patterns (Figure S2 in the Supporting Information) indicate that the as-prepared TiO2 consists of a single anatase crystalline phase. No diffraction peaks due to other phases (for example, rutile or brookite TiO2) were observed. After treating with 5% HF, no new diffraction peaks appeared and positions of the characteristic peaks of anatase TiO2 also remained unchanged, indicating that the etching of TiO2 by HF under the present conditions cannot change the crystalline phase and that F- is not incorporated into the bulk of TiO2 particles. Although TiO2 can be transformed to TiOF2 by treating with a high concentration of aqueous HF (50%),26–28 HF at low concentration (5%) under the present experimental conditions seems to etch only the surface of anatase TiO2 and, maybe, to dissolve some amorphous TiO2. The asprepared TiO2 exhibited a Brunauer-Emmett-Teller (BET) surface area of 65 m2/g, and the treatment by HF hardly changed the surface area (62 m2/g). The XPS technique was further employed to make clear the effect of HF etching on the surface of TiO2. Figure 1 shows the F 1s XPS spectra of F-TiO2 treated through different procedures. For F-TiO2 without washing, only one F 1s peak (peak 1) at 684.6 eV was observed (spectrum a), which is attributed to Fadsorbed on the surface of TiO2 (physically adsorbed F- or Fthat replaces surface hydroxyl groups).15 When F-TiO2 was washed thoroughly with water to remove the physically adsorbed F-, besides the peak at 684.6 eV, a new peak at higher band energy (689.6 eV, peak 2) appeared (spectrum b). When the water-washed F-TiO2 was further washed with 1 M NaOH solution, the relative peak intensity for peak 1 became weaker (spectrum c), and its contribution to the total peak area decreased from 78.1% for water-washed F-TiO2 to 12.5% for NaOH-washed F-TiO2 (Table 1). It was reported that the surface-exchanged Fon TiO2 can be easily replaced by OH- at alkaline pHs.14–16 Accordingly, the decrease in the relative intensity for peak 1 by washing with 1 M NaOH is attributed to the replacement of (26) Li, D.; Haneda, H.; Labhsetwar, N.; Hishita, S.; Ohashi, N. Chem. Phys. Lett. 2005, 401, 579–584. (27) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2588– 2595. (28) Vorres, K. S.; Dutton, F. B. J. Am. Chem. Soc. 1955, 77, 2019–2019.
exchanged F- by OH-. It was reported that the fluorine that substitutes surface lattice oxygen (lattice F) exhibited a higher binding energy than that of the adsorbed F- or replaced F- in TiOF2.29,30 In addition, no change in the position of the F 1s XPS spectra under both the acidic (pH ) 3.6) and alkaline conditions (pH ) 10.6) was observed in the work by Park and Choi,15 suggesting that pH has a negligible effect on the binding energy of F 1s. Accordingly, peak 2 can be assigned to lattice F, which cannot be replaced by OH- even under alkaline conditions. It was surprising somewhat that a little signal at the position of peak 2 was observed for unwashed samples (spectrum a). To be more confirmative, TiO2 was etched using different concentrations of HF (5-30%). Little signals at the position of peak 2 were observed for the unwashed samples treated with 5-20% HF (Figure S8 in the Supporting Information). Considering that the XPS spectra give simply the information on the relative intensity of one peak and that the washing processes with water or NaOH make it impossible to incorporate F- into the TiO2 lattice, the imperceptibleness of peak 2 for the unwashed F-TiO2 samples is probably due to the existence of a substantial amount of adsorbed F-. After washing with water or NaOH, the adsorbed F- was removed to some extent. As a result, the lattice F became observable by XPS. Moreover, for O 1s, the XPS spectrum was asymmetric (Figure S3 in the Supporting Information), which was derived from the presence of two different environments for the O element. The main contribution is attributed to lattice oxygen (Ti-O-Ti) with a XPS peak at 529.8 eV, and the other is oxygen in surface hydroxyl groups or water with a peak at around 531.9 eV.31,32 It can be seen that the surface hydroxyl content after NaOH washing was higher than that before washing, and the fraction of oxygen in HO- increased from 11.2 to 16.5%, which further confirms that the surface fluoride (tTi-F) is replaced by -OH after NaOH washing. In the previous reference, surface-fluorinated TiO2 was achieved by simply adding NaF into the dispersion of TiO2. In these cases, F- replaces the surface OH- of TiO2, but it is difficult to incorporate into the lattice of TiO2.15 However, by treating with a high concentration of HF (50%), TiO2 can be transformed to TiOF2.26–28 Under our experimental conditions, TiO2 was treated by diluted HF (5%) assisted by ultrasonication. No diffraction peaks in the XRD pattern for TiOF2 were observed, but the XPS results demonstrated the existence of lattice F-. Considering that the XRD and XPS techniques reflect the characteristics of bulk and surfaces of materials, respectively, it is reasonable to propose that, under the moderate conditions in our study, the effect of HF etching on the structure of TiO2 lies between the NaF and concentrated HF treatments. In other words, F- not only displaces surface OH- but also substitutes some (29) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. J. Fluorine Chem. 2005, 126, 69–77. (30) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808–3816. (31) Yu, J. C.; Yu, J.; Zhao, J. Appl. Catal., B 2002, 36, 31–43. (32) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918–5920.
Adsorption Modes of Dyes on Fluorinated TiO2 Scheme 1. Effect of HF Etching and Subsequent NaOH Treatment on the Surface Structure of TiO2
lattice oxygen. However, F- cannot enter deeply into TiO2 to affect the crystalline phase of bulk TiO2. The substitution of lattice oxygen only occurs on the surface (Scheme 1). 2. Visible Light Induced Photocatalytic Degradation of Dyes. The RhB dye contains two N-ethyl groups at each side of the xanthene ring, which is stable in aqueous solution under visible light irradiation. In the presence of pure TiO2 or F-TiO2 (unless otherwise noted, F-TiO2 denotes the HF-etched TiO2 which was washed completely with water), RhB underwent pronounced photocatalytic degradation upon visible light irradiation. The UV-vis spectral changes during the photodegradation of RhB on TiO2 and F-TiO2 are illustrated in Figure 2A and B, respectively. The most noticeable difference in the spectral changes between the pure TiO2 and F-TiO2 systems was the extent of the blue shift in the maximum absorbance of RhB. In the pure TiO2 dispersion, as reported in our earlier studies,20 the characteristic absorption band of the dye around 554 nm decreased gradually during visible irradiation and very little shift of the maximum absorbance of RhB was observed (Figure 2A), indicating the rather facile cleavage of the whole conjugated chromophore structure. In contrast, the maximum absorbance of RhB exhibited a marked blue shift (from 554 to 499 nm) in the F-TiO2 system. This blue shift in the maximum absorbance has been proven to be derived from the formation of N-de-ethylated intermediates in the photocatalytic degradation of RhB.19,20 Therefore, it is clear that direct cleavage of the RhB chromophore structure predominates in the RhB/TiO2 system, whereas N-deethylation occurs preferentially before destruction of the chromophore structure in the RhB/F-TiO2 system. Due to the spectral overlap between the original dye and its degradation intermediates, which makes it difficult to quantify the decay rate of the original dye, HPLC was further used to separate and analyze the mixed solution during photocatalytic degradation. Besides the original dye, five kinds of N-de-ethylated intermediates were formed in both systems, and the amount of the N-de-ethylated intermediates in the F-TiO2 system was much greater than that in the pure TiO2 system during the reaction process (Figure S4 in the Supporting Information), which is consistent with the results in Figure 2. The decay of the original RhB obtained by HPLC is shown in Figure 3. The decrease in the concentration of RhB in the bulk solutions before irradiation reflects the extent of adsorption of the dye molecules onto the catalyst surface. It was found that the amount of adsorbed dye in the F-TiO2 system (28%) was much greater than that in the pure TiO2 case (10%) (also see Table 2 below). The decay rate under visible light irradiation in the former was also much greater than that in the latter. The RhB dye in the F-TiO2 dispersion disappeared completely within 30 min of irradiation, with an initial decay rate constant of k = 0.18 min-1 (pseudo-first-order kinetics). However, only about 52% of RhB was degraded after 120 min of irradiation in the pure TiO2 case (k = 0.026 min-1). To investigate how the surface fluoride modification influences the kinetics and mechanisms in the photodegradation of dyes, the photodegradation of other kinds of dyes with different charges and functional groups (their structures are shown in Figure S1 in the Supporting Information) were also examined, and the results
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are summarized in Table 2. It was found that all of the cationic dyes such as methylene blue (MB), malachite green (MG), phenosafranine, Rhodamine 6G (Rh6G), and Rhodamine B (RhB) (entries 1-5) showed similar adsorption and degradation characteristics: greater adsorption amount and faster degradation rate on surface-fluorinated TiO2 than on pure TiO2. In contrast, for the anionic dyes such as alizarin red (AR), fluorescein, and Orange II (OrgII) (entries 7-9), both the adsorption amount and photocatalytic degradation rate were retarded greatly on F-TiO2 compared to those on pure TiO2. These results indicate that different dyes tend to exhibit different adsorption and degradation properties on fluorinated TiO2 and pure TiO2. Moreover, fluorinated TiO2 seems to have a selective interaction with the positively charged nitrogen-alkyl groups. 3. Adsorption Modes of RhB on F-TiO2 and Pure TiO2: Diethylamine Group Adsorption versus Carboxyl Group Adsorption. Langmuir Adsorption Isotherms. By monitoring the concentration of RhB in the bulk solution after the adsorption/ desorption equilibrium was reached from various initial concentrations of dye, the adsorption isotherms of RhB in aqueous TiO2 and F-TiO2 dispersions were examined, and the results are shown in Figure 4. The adsorption isotherms in both systems showed the typical Langmuir adsorption/desorption behaviors, and they can be expressed by the following equation:
Γ ) ΓsβCeq ⁄ (1 + βCeq)
(1)
where Γ is the adsorption amount (mol/g), Γs is the saturation amount of adsorption, β (M-1) represents the equilibrium constant for the adsorption process, and Ceq is the equilibrium concentration of RhB in the bulk solution. Γs could be estimated from the slope of the linear plot of Ceq/Γ versus Ceq, and β can be determined from the intercept of the line (at Ceq ) 0). It is found that the saturation coverage of RhB on F-TiO2 was fitted to be Γs ) 19.2 × 10-7 mol/g (18.6 × 10-3 molecules/nm2) and the adsorption constant β was 7.1 × 105 M-1. For the pure TiO2 particles, a saturation coverage of 4.6 × 10-7 mol/g (4.3 × 10-3 molecules/ nm2) and an adsorption constant of 2.9 × 105 M-1 were obtained. These results indicate that surface fluorination of TiO2 increases both the saturation amount and the adsorption strength for RhB by a factor of about 3.2 and 1.5, respectively. Since there was little difference in the BET surface areas between F-TiO2 and pure TiO2, this could exclude the possibility that the enhancement in the saturation adsorption amount of RhB on the F-TiO2 surface is derived from change of surface areas by HF etching. This argument can be further supported by the fact that F-TiO2 exhibited less adsorption abilities for anionic dyes relative to pure TiO2 (Table 2). It is more reasonable to attribute the difference in adsorption abilities for RhB to the change of surface characteristics after surface fluorination, that is, the change in the adsorption mode and adsorption site. In addition, the good linear relationship (with a standard deviation R2 value of 0.9996 and 0.9987 for F-TiO2 and pure TiO2 systems, respectively) of the Langmuir isotherms also suggests that RhB tends to adsorb on the surface of both fluorinated and pure TiO2 in monolayer and single-site modes. XPS Study. Figure 5 shows the N 1s XPS spectra of RhB in different systems. The XPS peak of N 1s in pure solid RhB dye with Cl- as the counteranion was located at 399.4 eV. After the adsorption of RhB on pure TiO2, the N 1s peak of the dye exhibited a little shift to a higher binding energy (399.7 eV). Interestingly, once RhB was adsorbed on the surface of F-TiO2, a new peak appeared at 401.5 eV (spectrum b). In a control experiment, RhB was recrystallized in 10% HF solution to examine the effect of F- on the XPS spectrum. A strong N 1s peak appeared at a
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Figure 2. Temporal UV-vis absorption spectral changes during the photocatalytic degradation of RhB (2 × 10-5 M, 50 mL, pH ) 3) in aqueous TiO2 (A) and F-TiO2 (B) suspensions (1 g/L).
Figure 3. RhB concentration changes during photocatalytic degradation of RhB (2 × 10-5 M, 50 mL, pH ) 3) in aqueous TiO2 (a) and F-TiO2 (b) suspensions (1 g/L). C0 is the initial RhB concentration before adsorption, and C is the temporal concentration of RhB after equilibrium adsorption. Table 2. Adsorption amounts and photodegradation rates of various dyes on pure TiO2 and F-TiO2 adsorption amount (%) dye conversion (%) entry 1 2 3 4 5 6 7 8 9
a
dye
d
Rhodamine B Rhodamine 6Ge methylene bluef malachite geenb,f phenosafraninef Rhodamine 101b,e fluoresceind alizarin redc,d Orange IIf
TiO2
F-TiO2
TiO2
F-TiO2
10.1 10.8 4.8 10.4 4.2 15.9 23.5 30.2 62.1
28.4 18.3 15.0 20.6 10.3 28.3 15.0 13.6 18.5
21.2 19.5 13.3 15.4 17.5 24.0 60.0 25.0 31.1
95.6 74.8 43.1 49.2 25.7 14.7 29.3 16.1 10.0
a Unless otherwise mentioned, the reaction conditions are as follows: initial concentration, 2.0 × 10-5 M; catalyst, 1 g/L; pH ) 3. b Initial concentration: 1.0 × 10-5 M. c Initial concentration: 4.0 × 10-4 M. d The dye conversion was obtained after visible irradiation for 30 min. e The dye conversion was obtained after visible irradiation for 60 min. f The dye conversion was obtained after visible irradiation for 120 min.
higher binding energy (402.0 eV, spectrum c); that is, the shifts were about 2.6, 2.3, and 0.5 eV relative to RhB with the Clcounterion, adsorbed on pure TiO2, and adsorbed on F-TiO2, respectively. It is indicative that the N 1s binding energy is very sensitive to the chemical environment of the -N(Et)2 group and the XPS technique provides a good tool for the study of interaction modes of RhB with the photocatalyst. For the pure TiO2 system, though there was a little binding energy difference in the N 1s peak between the solid RhB with
Figure 4. Adsorption isotherms of RhB in aqueous TiO2 and F-TiO2 suspensions. (inset) Plots of CeqΓ-1 versus Ceq for TiO2 and F-TiO2. The symbols are experimental data, and the solid lines are fitted according to Langmuir adsorption isotherm equation.
Cl- (399.4 eV) and the pure TiO2 adsorbed dye (399.7 eV), the specific interaction of the -N(Et)2 group with the TiO2 surface site can be excluded based on the following facts: (1) as illustrated in Figure 5 (a), only one symmetric N 1s peak was observed and no new N 1s peak or shoulder appeared and (2) RhB has been reported to anchor strongly on the surface of pure TiO2 via the carboxylic (-COOH) group.33 However, in the earlier report,34 it has been proven that the counterion can have a great influence on the binding energy of the tetraalkylammonium salts. The large shift of the N 1s peak between F-/RhB and Cl-/RhB is attributed convincingly to the interaction between the Fcounterion and the -N(Et)2 group of RhB, since the F- ion can shift the N 1s peak to a higher binding energy than that of Cl-.34 A positive shift of 1.8 eV in the N 1s binding energy of RhB on the F-TiO2 surface with respect to that on pure TiO2 demonstrates unambiguously that the chemical environment of nitrogen is markedly different between the two systems. As mentioned above, RhB underwent a rapid N-de-ethylation process on the surface of F-TiO2. When the fully de-ethylated product (Rhodamine 101), instead of RhB, was adsorbed on pure TiO2, only one XPS peak centered at 399.7 eV was observed. Thus, the possibility that the new XPS peak of N 1s in the F-TiO2 system is derived from the N-de-ethylated intermediates formed during sample preparation can be excluded. Considering that the difference in the N 1s binding energy between F-/RhB and (33) Zhao, D.; Chen, C.; Wang, Y.; Ma, W.; Zhao, J.; Rajh, T.; Zang, L. EnViron. Sci. Technol. 2008, 42, 308–314. (34) Jack, J. J.; Hercules, D. M. Anal. Chem. 1971, 43, 729–736.
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Figure 5. N 1s XPS spectra of (a) RhB adsorbed on pure TiO2; (b) RhB adsorbed on F-TiO2; and (c) RhB recrystallized in 10% HF solution. Scheme 2. Proposed Adsorption Modes of RhB in Aqueous F-TiO2 (A) and Pure TiO2 (B) Dispersions
F-TiO2/RhB systems is 0.5 eV and that the F-TiO2 sample for XPS measurement has been washed thoroughly by water, the N 1s peak at 401.5 eV cannot be attributed to the interaction between the free F- ion and RhB. Therefore, in the F-TiO2 system, the dye should adsorb on the surface F site through the -N(Et)2 group (as illustrated in Scheme 2). Fourier Transform Infrared (FT-IR) Spectroscopy. The different adsorption modes are expected to result in changes in the specific vibrational modes, which can give more information on the interaction between the dye and surface of the catalyst. In this study, the FT-IR spectra were also examined to further confirm the adsorption modes of RhB on F-TiO2 and pure TiO2, and the results are displayed in Figure 6. From the literature,11,33,35 the following vibrational modes for the principal bands in the IR spectra of pure RhB (curve a) are assigned: the stretching vibration band of the carboxylic group is located at 1697 cm-1, the aromatic ring and heterocycle vibrations are in the region of 1450-1600 cm-1, the 1334 cm-1 band is due to C-aryl bond vibrations, and the peak at 1649 cm-1 is attributed to vibrations of the carbon-nitrogen bond. In the earlier study,33 we have shown that the carboxylic group can bind to the surface Ti site of pure TiO2 through monodentate (35) McHedlov-Petrossyan, N. O.; Shapovalov, S. A.; Egorova, S. I.; Kleshchevnikova, V. N.; Arias Cordova, E. Dyes Pigm. 1995, 28, 7–18.
Figure 6. FT-IR spectra of (a) dye RhB powder, (b) RhB adsorbed on pure TiO2, and (c) RhB adsorbed on F-TiO2.
(ester-like) linkage. In the present study, the stretching vibration band of the carboxylic group (1697 cm-1) in RhB after adsorption on the pure TiO2 surface was observed to be split into two peaks: one is the antisymmetric vibration (νas, 1679 cm-1), and the other is the symmetric vibration (νs, 1406 cm-1). The frequency difference between the antisymmetric and symmetric stretching vibrations (∆ ) νas - νs) 273 cm-1) also fell into the range of monodentate linkage.33,36,37 However, for the F-TiO2 system (curve c), the stretching vibration band at 1697 cm-1 of the carboxylic group tended to remain untouched, and little splitting of the vibration was observed, which indicates that RhB molecules adsorb on F-TiO2 through other groups rather than the carboxylic group. Interestingly, after adsorption of RhB on the surface of F-TiO2 (curve c), besides the original C-N vibration band at 1649 cm-1, a new vibration band appeared at 1616 cm-1. This peak can be assigned to the C-N vibration in the diethylamine group which has interaction with the surface of F-TiO2. However, in the RhB/TiO2 system, the vibrations of the carbon-nitrogen bond remained nearly unchanged. These facts suggest again that RhB adsorbs on the F-TiO2 surface through the diethylamine group but binds to the surface of pure TiO2 via the carboxylic groups (Scheme 2). In fact, the surface hydroxyl on the Ti site is indispensable for the formation of an ester-like bond between the carboxylic group and surface Ti site of TiO2. On the surface of F-TiO2, as indicated above, the surface hydroxyl is displaced by F-, which prohibits the esterification between the surface hydroxyl group and the carboxylic group of RhB. Consequently, the fluorination of TiO2 prevents the carboxylic groups of RhB from interaction with the catalyst surface. On the other hand, the negative charges on the surface of F-TiO2 derived from the surface F- ion can effectively attract the positive-charged diethylamine group, which results in the adsorption of RhB on F-TiO2 through the diethylamine group. This argument is further supported by the fact that other dyes (36) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227. (37) Weng, Y. X.; Li, L.; Liu, Y.; Wang, L.; Yang, G. Z. J. Phys. Chem. B 2003, 107, 4356–4363.
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with positively charged nitrogen-alkyl groups also exhibited a stronger adsorption on the surface of F-TiO2 than on pure TiO2 (Table 2). Furthermore, as shown in Figure S5 in the Supporting Information, the ζ-potential measurements showed that the point of zero charge (pzc) of pure TiO2 was estimated to be about pH 6.2. After surface fluorination, the pzc shifted to pH 4.6, indicating that the surface positive charges under acidic conditions are greatly decreased by surface fluorination.15 This result can also rationalize the inversion of the adsorption mode of RhB on F-TiO2 compared to that on pure TiO2. There are three possible specific interaction modes between the diethylamine group of RhB and the surface of F-TiO2: (1) RhB stands on the surface of the catalyst through one diethylamine group; (2) RhB stands on the surface via two -N(Et)2 groups; or (3) RhB lies on the surface of the catalyst via two -N(Et)2 groups. Although the present experimental results do not give direct information to distinguish the three modes, we prefer the first one from the following considerations: (1) The adsorption isotherms of RhB exhibited typical Langmuir adsorption/ desorption behaviors, which suggests that RhB tends to cover the surface of F-TiO2 in a single-site way. (2) Both the N 1s peaks in the XPS spectra (Figure 5b) and C-N vibration bands in the IR spectra (Figure 6c) of the F-TiO2 adsorbed RhB dye revealed two kinds of N species. It is reasonable to ascribe one to direct-adsorbed -N(Et)2 and the other primarily to the untouched -N(Et)2 in the identical RhB molecule. (3) It is known that the positive charge on RhB is resonant between the two diethylamine groups of the RhB chromophore. Once one of the diethylamine groups adsorbs on the negative F site, the resonance should be destroyed, which causes the positive charge to center at the adsorbed diethylamine and favors the mode of singlediethylamine adsorption. Based on such proposed singlediethylamine and stand-up adsorption modes, we can estimate, from the fitted peak area of the N 1s XPS spectra, the relative amount of diethylamine-adsorbed dye and the free or carboxylicadsorbed dye on the surface of F-TiO2. It is estimated that 82% of RhB dye molecules adsorb on the surface of F-TiO2 through the -N(Et)2 group. 4. Effect of TiO2 Surface Fluorination on Its Photocatalytic Activity. As implicated and discussed above, surface fluorination can markedly change the surface charge and specific site of TiO2, and consequently vary the adsorption site, mode and strength of dyes. Since TiO2-assisted photocatalytic degradation of dyes under visible irradiation requires the direct interaction between the dyes and the surface of TiO2 to achieve efficient charge injection, the change in the adsorption properties by fluorination should have great influence on the photocatalytic pathway and degradation rate. The increase in both the adsorption amount and the adsorption strength for cationic dyes on F-TiO2 (Table 2) can promote the electron transfer from the excited dyes to TiO2, which is one important reason for observed enhancement in the degradation of these N-alkyl-containing cationic dyes (Figure 3, and Table 2, entries 1-5). The retarded degradation of anionic dyes on F-TiO2 (Table 2, entries 7-9) can also be attributed to the suppression in their adsorption. To examine the effect of fluorination on the energy band structure of TiO2, The flat-band potentials (Efb) for pure TiO2 and F-TiO2 particles were also measured by the slurry method (Figure S6 in the Supporting Information).38,39 The Efb value measured for pure TiO2 was approximately -0.50 V vs NHE at pH 7, which is in good agreement with -0.47 V reported for (38) Sakthivel, H.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908–4911. (39) Kim, S.; Hwang, S. J.; Choi, W. J. Phys. Chem. B 2005, 109, 24260– 24267.
Wang et al.
anatase powder.40 At pH 3 (typical photocatalytic degradation conditions used in this study), the Efb value was estimated as -0.26 and -0.20 V vs NHE for pure TiO2 and F-TiO2, respectively. The positive shift (about 60 mV) of the flat-band potential after surface fluorination can increase the driving force for electron injection from the excited state of the dye to the conduction band. Moreover, Park and Choi15 have proposed that the surface tTi-F group acts as an electron-trapping site due to the strong electronegativity of fluorine. Accordingly, the driving force for the electron recombination from the conduction band to the formed dye cationic radicals would also decrease. Both changes tend to enhance the electron separation. This is probably another reason underlying the promotion of the degradation of dyes. In addition, Macyk et al.17 have proposed that surface fluorination of TiO2 can enhance the formation of singlet oxygen on the UV-irradiated F-TiO2 surface. However, under our experimental conditions (visible light), little singlet oxygen was observed by spin-trapped electron spin resonance (ESR) measurements, and singlet oxygen scavengers (NaN3) had little effect on both the degradation rate and the wavelength shift rate (as illustrated in Figures S9 and S10 in the Supporting Information), implying that singlet oxygen plays a negligible role in the present system. Our experimental results also demonstrate that, besides enhancing the degradation rate, surface fluorination changed markedly the pathway for the photocatalytic degradation of N-alkyl-containing cationic dyes; that is, direct cleavage of the chromophore structure predominated during photocatalytic degradation in the pure TiO2 system, whereas N-dealkylation of the dyes occurred preferentially in the F-TiO2 system (Figure 2). To make clear the cause for the change in the degradation pathway, the photocatalytic degradation of the cationic dye of Rh6G was also examined in the polyelectrolyte modified TiO2 dispersion (Figure S7 in the Supporting Information). It was found that, after addition of anionic polyelectrolytes (such as poly(acrylic acid) and poly(sodium 4-styrene-sulfonate), both the N-deethylation and degradation rate were greatly enhanced, whereas cationic polyelectrolytes (poly(diallyldimethylammonium chloride) and poly(allylamine hydrochloride) suppressed the overall photocatalytic degradation of the dye. Moreover, a similar N-deethylation process was also observed on CF3COOH pretreated TiO2 (data not shown). In the previous studies of visible light induced degradation of dyes,11,19–22 the N-dealkylation process was also observed to become dominant on the TiO2 surface modified by an anionic surfactant11 and Nafion (a perfluorinated polymer with anionic sulfonate groups).18 The common characteristics of the present and previously reported systems are that all the modifications can make the surface charge of TiO2 more negative. They would hence share the analogical mechanism for the enhancement of N-dealkylation. The present experimental results from XPS can shed some new light on the mechanism. It has been reported that the binding energy of an electron is often found to be inversely proportional to the electron density of the detected atom.34 The change of the N 1s binding energy upon adsorption of RhB on F-TiO2 (Figure 5) suggests that the electron density and the energy level structure of the ground state of the dye are greatly influenced by the inverse adsorption modes. We can expect that the electron distribution and the stability of the excited state and cationic radical of the dye could also be changed by the different adsorption modes. Therefore, it is reasonable to propose that the changes in the electron structure could exert a great influence on both the photoinduced electron (40) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 27–31.
Adsorption Modes of Dyes on Fluorinated TiO2
injection from the excited state of the dye to the conduction band and the electron recombination from the conduction band to the dye cationic radical. Consequently, the efficiency of electron separation, the pathway, and the rate of photocatalytic degradation of the dyes could be changed. It should also be pointed out that, as mentioned above, surface modification by anionic species may have a complex influence on the TiO2 surface properties, such as the flat-band potential, the surface site, and the surface acidity. These factors can in turn influence the reduction of dioxygen by the conduction electron and the formation of superoxide, which remain unclear and need to be investigated further.
Conclusion Fluorination of anatase TiO2 by the HF etching method hardly changed the crystalline phase and BET surface area but significantly influenced the surface properties of TiO2. F- not only displaced the surface OH- of TiO2 but also substituted some surface lattice oxygen. As a result, both the adsorption modes and degradation of the dyes were greatly altered. The adsorption mode of zwitterionic dyes was reversed from anionic group adsorption on pure TiO2 to nitrogen-alkyl group adsorption on F-TiO2. Dyes with positively charged nitrogen-alkyl groups
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underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of the dye chromophore ring structures predominated. It was proposed that the increase in the degradation kinetics after fluorination was derived from the enhanced adsorption and the lowered flat-band potential. The promotion of N-dealkylation is likely to be due to the changes in the electron density, the energy level structure of the dye excited state, and the dye cationic radical after inversion of the adsorption modes. This proposition may be also applicable to other anionic modified TiO2 systems. Acknowledgment. Generous financial support by the Ministry of Science and Technology of China (2007CB613306 and 2007AA061402), by the National Science Foundation of China (20537010 and 50436040), and by the Chinese Academy of Sciences is gratefully acknowledged. Supporting Information Available: Structures of all the other dyes used, X-ray diffraction patterns, O 1s XPS spectra, flat-band potentials, ζ-potentials, distribution of N-de-ethylated products, effects of polyelectrolyte on photocatalysis, effects of HF concentration on the XPS spectra, and ESR measurements detecting singlet oxygen and addition of singlet oxygen scavenger on the degradation kinetics. This material is available free of charge via the Internet at http://pubs.acs.org. LA800313S
