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Enhanced photocatalytic activity of C, F-codoped TiO2 loaded with AgCl
Journal of Alloys and Compounds 560 (2013) 20–26
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced photocatalytic activity of C, F-codoped TiO2 loaded with AgCl Hongjian Yan a, Saji Thomas Kochuveedu a, Li Na Quan a, Sang Soo Lee b, Dong Ha Kim a,⇑ a b
Department of Chemistry and Nano Science, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea Polymer Hybrid Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea
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Article history: Received 5 November 2012 Received in revised form 20 January 2013 Accepted 21 January 2013 Available online 13 February 2013 Keywords: Co-doped TiO2 C, F-doping Photocatalysis Visible light activity AgCl
a b s t r a c t A protocol is reported for the development of a unique type of a co-doped TiO2 with two kinds of nonmetal atoms, carbon (C) and fluorine (F), as a visible-light active photocatalyst system. C, F-codoped TiO2 nanoobjects with anatase phase were synthesized by calcination of a mixture of TiOF2 and sulfuric-acid-treated melamine. The C, F co-doping resulted in a red-shift of the absorption edge to 510 nm from the 390-nm value obtained for a control TiO2 photocatalyst. The C, F-codoped TiO2 shows photocatalytic activity under visible light irradiation. Furthermore, the loading of AgCl into the parent co-doped TiO2 enhanced its activity for degradation of methylene blue dye by about 3.5 times in the first hour irradiation. A brief mechanism responsible for the enhancement is proposed in terms of the transfer of holes from C, F-codoped TiO2 to the AgCl phase. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The photocatalytic purification and treatment of water and air using semiconductor nanostructures and sunlight is an effective and promising technology for low-cost environmental remediation. Titanium dioxide (TiO2) is a most promising semiconductor that has been investigated substantially for its excellent optical and electronic properties, long-term stability, low cost, and nontoxicity [1–3]. However, most as-prepared TiO2 is only responsive to ultraviolet (UV) light due to its relatively large band gap (3.2 eV for anatase) hindering the utilization of visible light, which occupies ca. 43% of the entire solar spectrum. There are several methods to improve the photocatalytic performance of TiO2: (1) Chemical doping of TiO2 with metallic (Cr, Fe, V) [4,5] or non-metallic (N, C, B, S or F) elements [6–11] to modify the electronic structures of semiconductors as well as their surface properties, thus extending their visible light absorbance. For example, C, W-codoped TiO2 exhibited higher visible-light photocatalytic activity than undoped TiO2, C-doped TiO2 or W-doped TiO2. The co-doping of C and W not only leads to the narrowing of the band gap of TiO2, but also increases the separation efficiency of the photo-generated electrons-hole pairs [11]; (2) Synthesizing TiO2 with certain exposed facets, which is more reactive due to the higher average surface energy (demonstrated with both theoretical and experimental evidence) [12–14]. TiO2 with certain exposed facets has a substantial effect on the surface separation
⇑ Corresponding author. Tel.: +82 2 3277 4517; fax: +82 2 3277 3419. E-mail address: [email protected] (D.H. Kim). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.155
and transfer of photogenerated electron–hole pairs, resulting in a significant enhancement of photocatalytic efficiency; (3) Coupling with other semiconductors to increase the separation efficiency of photogenerated electron–hole pairs during photocatalysis [15–20]. Recently, the composites comprising TiO2 and other semiconductors, such as WO3 [21], Co3O4 [22], SiC [15], and Fe2O3 [16], have been reported. Among these methods, doping TiO2 with a non-metal element is a promising approach to achieving visible light response in TiO2 photocatalysts. Furthermore, doping with two kinds of non-metal atoms has shown more beneficial effects. However, representative combinations of two non-metal atoms have mostly focused on S/N, C/N, B/N, and N/F pairs [23–27]. To the best of our knowledge, there have been few reports on the use of C and F as co-dopants. Silver chloride (AgCl), which has a direct band gap of 5.6 eV and an indirect band gap of 3.25 eV, is widely recognized as a photosensitive material, and is employed as a source material in photographic films [28,29]. Recently, a series of visible-light active composite photocatalysts containing silver halides have been developed for the degradation of organic pollutants [30–32]. In these photocatalysts, both metallic Ag nanoparticles and silver halides were generated, and it was revealed that the metallic component was responsible for the absorption of visible light and photocatalytic activity due to its surface plasmon resonance effect. Thus, it is a necessary and meaningful task to elucidate the role of Ag in the form of silver halides on the photocatalytic activity. We report on a facile synthetic protocol of TiO2 co-doped with C and F elements via calcination of a mixture of TiOF2 and sulfuricacid-treated melamine in Ar atmosphere. Further, we demonstrate that the C, F-codoped TiO2 shows enhanced absorption in the
H. Yan et al. / Journal of Alloys and Compounds 560 (2013) 20–26
visible light region, leading to markedly enhanced photocatalytic activity, by loading a small amount of AgCl on its surface. 2. Experimental 2.1. Synthesis of C, F-codoped TiO2 TiOF2 was synthesized via a facile one-step hydrothermal reaction according to previous reports [33,34]. Typically, 35.0 ml of titanium butoxide (Ti(OBu)4), 78.0 ml of acetic acid (CH3COOH), and 11 mL of 47% hydrofluoric acid solution were mixed in a Teflon-lined autoclave with a volume of 150 ml under stirring. The autoclave was kept at 473 K for 12 h in an oven. After cooling down unassisted to room temperature, the as-obtained products were collected and washed with water and ethanol several times, and dried at 373 K for 10 h. The as-prepared TiOF2 was mixed with sulfuric-acid-treated melamine. Then, the mixture was heated at 400 °C for 2 h in Ar atmosphere, followed by heat treatment at 400 °C for 1 h in air. 2.2. Loading of AgCl The loading of AgCl on C, F-codoped TiO2 was performed by in situ precipitation method. Typically, 0.2 g of C, F-codoped TiO2 powder was impregnated in an aqueous solution containing a certain amount of silver acetate (Ag(CH3COO)). After stirring for 12 h, NH4Cl solution was added. The solution was then evaporated over a water bath at 80 °C. Finally, the obtained powder was calcined in air at 400 °C for 1 h. 2.3. Photocatalytic experiment The catalyst powder (10 mg) was dispersed in 30 mL of methylene blue (MB) solution (10 ppm). The samples were kept in the dark for 1 h before exposure to light, in order to ensure equilibrium between the adsorption and desorption of dye molecules on the surface of the catalyst. Then, the samples were irradiated under stirring by a 300-W Xe lamp (Newport Co., model 66984) equipped with a 420nm cutoff filter as a visible light source. A fixed amount of sample was withdrawn from the stock solution at regular intervals and centrifuged to remove the catalyst. To study the change in absorbance maxima of the dye, the specific characteristic absorbance was measured by UV–vis absorbance spectroscopy (Varian Cary 5000 UV–vis–NIR spectrophotometer). 2.4. Characterization The crystal phase of the samples was determined by X-ray diffractometry (D/ max RA, Rigaku) using nickel-filtered copper radiation (Cu Ka) at 40 kV and 30 mA, over a 2h range of 10°–80°. The morphology of the catalyst was investigated using a JEOL JSM2100-F TEM microscope operated at 100 kV. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PHI 5000 Versa Probe (Ulvac-PHI) system using an Al Ka (1486.6 eV) anode (25 W, 15 kV). The binding energies were calibrated using the carbon C 1s peak at 284.6 eV. UV–vis absorbance spectra were measured using a Sinco S-4100 spectrometer.
3. Results and discussion We first investigate the structural aspects of TiOF2 and C, F-codoped TiO2. Fig. 1a and b shows the crystal phases of the
Fig. 1. The XRD patterns of TiOF2 (a) and C, F-codoped TiO2 (b).
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as-prepared TiOF2 precursors and C, F-codoped TiO2, respectively. As shown in Fig. 1a, the diffraction peaks can be indexed as cubic phase with a space group of Pm-3 m, which is in good agreement with the reported compound, cubic TiOF2 (JCPDS No.77-0132). The characteristic peaks of TiOF2 disappear, and new anatase peaks appear (Fig. 1b) after heat treating the mixture of TiOF2 and sulfuric-acid-treated melamine. This clearly indicates that the TiOF2 precursors changed to anatase TiO2 phase. The chemical nature of the as-prepared samples was then investigated by X-ray photoelectron spectroscopy (XPS). The signals for C 1s, F 1s, Ti 3d, and O 1s, but not N 1s, were clearly observed in the survey scan spectra in the region of 0–1000 eV (Fig. 2a). The binding energy centered at around 684.5 eV, which is ascribed to F species, was observed for C, F-codoped TiO2 samples, indicating the incorporation of F in the TiO2 lattice (Fig. 2b). It is also observed in Fig. 2c that the C 1s peak exhibits two components with binding energy at 284.6 eV and 288 eV, respectively. The binding energy at 284.6 eV should be ascribed to ambient organic impurities adsorbed on the surface of the sample, and the one at 288 eV may be associated with the carbon species as an interstitial dopant [35,36]. No noticeable peaks due to nitrogen species were observed around 399 eV. These results lead us to conclude that the TiO2 was doped by C and F after heat treating the mixture of TiOF2 and sulfuric-acid-treated melamine. The loading of AgCl on C, F-codoped TiO2 was performed by an in situ precipitation method. Fig. 3 shows the XRD patterns of samples loaded with different amounts of AgCl, where typical XRD patterns of cubic AgCl (JCPDS No. 31-1238) can be observed. It is also shown in Fig. 3 that the loading of AgCl does not change the crystal phase of the doped TiO2. Furthermore, the intensity of AgCl increases with the increasing amount of AgCl loaded. Fig. 4 shows the XPS analysis result of a representative C, F-codoped TiO2 sample loaded with AgCl. The C 1s, F 1s, Ti 3d, O 1s, Ag 3d, and Cl 2p signals can be clearly observed in the survey scan spectrum (Fig. 4a). The peaks observed at 267.2 eV and 373.2 eV can be ascribed to Ag (I) species. No signal due to Ag (0) could be found from the high-resolution Ag 3d XPS spectrum. Therefore, the XRD and XPS results confirm that cubic AgCl phase was deposited on the surface of C, F-codoped TiO2 rather than metallic Ag. The morphology of TiOF2, C, F-codoped TiO2, and AgCl-loaded C, F-codoped TiO2 was characterized by TEM. In Fig. 5a, it is observed that well-defined sub-micron cube structures with a uniform edge length of around 300–400 nm are developed for the TiOF2. After calcination with sulfuric-acid-treated melamine, part of the cubic TiOF2 morphology was changed to particles, as shown in Fig. 5b. Furthermore, some small particles of size around 60–100 nm could be observed. The interplanar spacing of C, F-codoped TiO2 is 3.52 Å (HRTEM, insert in Fig. 5b), which corresponds to the d-spacing of the (1 0 1) plane from XRD measurements. As shown in Fig. 5c, the loading of AgCl has little effect on the morphology of C, F-codoped TiO2. Small particles with 4–10-nm diameter could be observed on the surface of C, F-codoped TiO2. Fig. 5d shows the HRTEM of a selected particle loading on the surface of C, F-codoped TiO2. The interplanar spacing of the small particle is about 2.0 Å, which corresponds to the (2 2 0) d-spacing of AgCl. Therefore, the TEM image clearly shows that the AgCl nanoparticles are loaded on the surface of C, F-codoped TiO2. The absorption properties of TiOF2, C, F-codoped TiO2, and AgClloaded C, F-codoped TiO2 were then investigated by UV–vis diffuse reflectance spectroscopy. The absorption edge of the TiOF2 sample occurs at ca. 390 nm, corresponding to a band gap of 3.2 eV (Fig. 6a). After calcination of the mixture of TiOF2 and sulfuricacid-treated melamine, the absorption edge of the resulting C, Fcodoped TiO2 sample was red-shifted to a lower energy region at about 500 nm, corresponding to a band gap of about 2.48 eV (Fig. 6b). The absorption edge has little change after loading with
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Fig. 2. The XPS spectra of C, F-codoped TiO2. (a) Survey scan; (b) high-resolution F 1s; (c) high-resolution C 1s and (d) high-resolution N 1s.
Fig. 3. The XRD patterns of C, F-codoped TiO2 loaded with different amounts of AgCl.
AgCl, although the intensity of the absorption was increased, as shown in Fig. 6c. Photocatalytic activity was demonstrated in terms of the degradation of methylene blue (MB) under visible light irradiation (Fig. 7). The ratios of the intensities of the characteristic absorbance peak of MB at a wavelength of about 664 nm were plotted after irradiation with visible light for a specific period of time (C) and prior to irradiation (C0), as shown in Fig. 7e. All the samples show visible-light-responsive photocatalytic activity for the degradation of MB. The photocatalytic activity of C, F-codoped TiO2 was further enhanced by loading with AgCl. With increasing the amount of AgCl, the photocatalytic activity is increased, and the performance achieves a maximum when the amount of AgCl is
about 7 wt.%. Further increasing the amount of AgCl leads to a decrease in photocatalytic activity. The decrease in the photocatalytic activity at larger amounts of AgCl loading may be ascribed to the shielding effect of the visible light absorption by AgCl with a larger band gap. The enhanced photocatalytic activity upon the loading of AgCl was also examined in terms of the degradation of 4-nitrophenol (see Fig. S1 in the Supporting information). The enhancement of the photocatalytic performance of C, Fcodoped TiO2 by loading AgCl is attributed mainly to the effective separation of photogenerated electron–hole pairs. This mechanism was supported by comparing the photoluminescence (PL) spectra of neat C, F-codoped TiO2 with those of C, F-codoped TiO2 loaded with 7 wt.% AgCl (Fig. S2). The PL of C, F-codoped TiO2 loaded with 7 wt.% AgCl was slightly quenched compared with the AgCl-free sample, indicating that the separation of photogenerated electron–hole pairs is more effective in the presence of the AgCl. A concise scheme for the visible-light-driven electron–hole separation of the AgCl-loaded C, F-codoped TiO2 materials is proposed in Fig. 8. In general, the conduction band (CB) and valence band (VB) of non-metal-doped TiO2 is composed by Ti 3d, and the hybrid of O 2p and the p orbital of the non-metal element, respectively. Therefore, the CB and VB of C, F-codoped TiO2 are estimated to be 0.29 eV and +2.19 eV, respectively (with respect to the standard hydrogen electrode potential, SHE). For reference, it was reported that the CB and VB of AgCl are about 1.15 eV and +2.1 eV (versus SHE), respectively [21]. The electron–hole pairs are first generated upon excitation by visible light absorbed in C, F-codoped TiO2, and the holes are subsequently transferred to the AgCl surface, thereby resulting in the oxidation of Cl ions to Cl0 atoms. The Cl0 atoms in turn act as reactive species for the oxidation of dye molecules. At the same time, the photoinduced electrons are suspended in C, Fcodoped TiO2 and reduce the O2 to O2 radicals. However, since
H. Yan et al. / Journal of Alloys and Compounds 560 (2013) 20–26
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Fig. 4. The XPS spectra of C, F-codoped TiO2 loaded with 7 wt.% AgCl. (a) Survey scan; (b) high-resolution Ag 3d and (c) high-resolution Cl 2p.
Fig. 5. TEM images of a series of samples used in this study. (a) TiOF2; (b) C, F-codoped TiO2; (c) 7 wt.% AgCl-loaded C, F-codoped TiO2 and (d) HRTEM of C, F-codoped TiO2.
the conduction band (CB) of AgCl ( 1.15 eV versus SHE) lies above the CB of C, F-codoped TiO2 ( 0.3 eV versus SHE), the electrons can also reduce the Ag (I) to metallic Ag.
It is well known that Ag@AgCl has been shown to be an effective visible-light active photocatalyst for the surface plasmon resonance effect of Ag nanoparticles. However, there is an intrinsic
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H. Yan et al. / Journal of Alloys and Compounds 560 (2013) 20–26
Fig. 6. The UV–vis spectra of TiOF2 (a), C, F-codoped TiO2 (b) and AgCl-loaded C, F-codoped TiO2 (c).
difference between our materials and Ag@AgCl. In Ag@AgCl, the Ag nanoparticles are responsible for the absorption of visible light, because of its surface plasmon resonance effect, and the photogenerated electron–hole pairs. However, in the case of our photocatalysts, AgCl-loaded C, F-codoped TiO2, the C, F-codoped TiO2 is
Fig. 8. Schematic diagram of the photocatalytic mechanism of C, F-codoped TiO2 loaded with AgCl.
responsible for the absorption of visible light and the generation of electron–hole pairs, because of the narrow band gap. The AgCl in our system also acts as a hole acceptor. The holes generated were transferred to the AgCl surface, and the VB edge potential of AgCl is slightly negative compared to that of C, F-codoped TiO2.
Fig. 7. The photocatalytic degradation of MB over C, F-codoped TiO2 loaded with different amount of AgCl under visible light (k > 420 nm) irradiation. (a) 0 wt.% AgCl, (b) 3 wt.% AgCl, (c) 5 wt.% AgCl, (d) 7 wt.% AgCl and (e) Comparison of the visible light photocatalytic activity using C/C0 versus time plot.
H. Yan et al. / Journal of Alloys and Compounds 560 (2013) 20–26
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Fig. 9. The stability test result of C, F-codoped TiO2 for the degradation of MB. Fig. 10. The XRD patterns of C, F-codoped TiO2 before and after irradiation with visible light for different amount of time.
Next, the stability of the AgCl-loaded catalysts was investigated. As shown in Fig. 9, the photocatalytic activity decreases in the first three cycling runs. As the cycling runs increase further, the photocatalytic activity progresses steadily. To better understand the cause of the decrease in the activity, XRD and XPS analyses were carried out for the samples after irradiation. In Fig. 10, a typical XRD pattern characteristic of metallic Ag could be observed in the sample after 1 h of irradiation. It is also observed that the XRD intensity of atomic Ag increases with increasing irradiation time. Fig. 11 shows the XPS analysis result of a sample loaded with AgCl after irradiation. The C 1s, F 1s, Ti 3d, O 1s, Ag 3d, and Cl 2p signals can be clearly observed in the survey scan spectrum (Fig. 11a). However, except for the binding energy ascribed to Ag(I) species, the binding energy for metallic Ag(0) at 366 eV and 372 eV can be found from the high-resolution Ag 3d XPS spectrum (Fig. 11b). There is no noticeable difference in the high-resolution
Cl 2p XPS spectrum (Fig. 11c). The XPS results also confirm the existence of metallic Ag in the sample after irradiation, indicating that part of the AgCl was reduced to metallic Ag during extended irradiation. The atomic concentrations of Ag and Cl in the catalysts before and after irradiation were also analyzed by XPS. They are determined to be 0.58 (before) and 0.55 (after) for Ag, and 0.55 (before) and 0.21 (after) for Cl, respectively. This indicates that there is a leaching of Cl during the photocatalytic degradation of the dye. To evaluate the effect of the existence of metallic Ag on the photocatalytic activity compared with AgCl, Ag particles were loaded on the surface of C, F-codoped TiO2 by reducing AgCH3COO with N2H4. The XRD pattern confirms that metallic Ag is deposited on C, F-codoped TiO2 by this reducing method (Fig. 12). The Ag-loaded C, F-codoped TiO2 exhibits significantly lower photocatalytic
Fig. 11. The XPS spectra of C, F-codoped TiO2 after irradiation with visible light. (a) Survey scan; (b) high-resolution Ag 3d and (c) high-resolution Cl 2p.
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Acknowledgment This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2011-0029409; 2012-0009649). H. Yan was supported by the RP-Grant 2012 of Ewha Womans University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013. 01.155. References Fig. 12. The XRD pattern of Ag-loaded C, F-codoped TiO2.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 13. Comparison of the photocatalytic activity of C, F-codoped TiO2, AgCl-loaded C, F-codoped TiO2 and Ag-loaded C, F-codoped TiO2.
activity than that of AgCl-loaded C, F-codoped TiO2, as shown in Fig. 13, indicating that the incorporation of Ag element as a form of AgCl plays a significant role in enhancing the C, F-codoped TiO2. 4. Conclusion In conclusion, C, F-codoped TiO2 photocatalysts with anatase phase were synthesized by calcination of the mixture of TiOF2 and sulfuric-acid-treated melamine. The C, F-codoping resulted in a red-shift of the absorption edge from 390 nm for TiO2 to 510 nm for C, F-codoped TiO2. The C, F-codoped TiO2 shows effective photocatalytic activity for the degradation of MB dye under visible light irradiation. The loading of AgCl further enhanced the visible light photocatalytic activity of C, F-codoped TiO2 by the effective separation of photogenerated electron–hole pairs. This work demonstrated an unprecedented method for enhancing the photocatalytic activity of doped TiO2 nanostructures by loading AgCl.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced photocatalytic activity of C, F-codoped TiO2 loaded with AgCl Hongjian Yan a, Saji Thomas Kochuveedu a, Li Na Quan a, Sang Soo Lee b, Dong Ha Kim a,⇑ a b
Department of Chemistry and Nano Science, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Republic of Korea Polymer Hybrid Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 20 January 2013 Accepted 21 January 2013 Available online 13 February 2013 Keywords: Co-doped TiO2 C, F-doping Photocatalysis Visible light activity AgCl
a b s t r a c t A protocol is reported for the development of a unique type of a co-doped TiO2 with two kinds of nonmetal atoms, carbon (C) and fluorine (F), as a visible-light active photocatalyst system. C, F-codoped TiO2 nanoobjects with anatase phase were synthesized by calcination of a mixture of TiOF2 and sulfuric-acid-treated melamine. The C, F co-doping resulted in a red-shift of the absorption edge to 510 nm from the 390-nm value obtained for a control TiO2 photocatalyst. The C, F-codoped TiO2 shows photocatalytic activity under visible light irradiation. Furthermore, the loading of AgCl into the parent co-doped TiO2 enhanced its activity for degradation of methylene blue dye by about 3.5 times in the first hour irradiation. A brief mechanism responsible for the enhancement is proposed in terms of the transfer of holes from C, F-codoped TiO2 to the AgCl phase. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The photocatalytic purification and treatment of water and air using semiconductor nanostructures and sunlight is an effective and promising technology for low-cost environmental remediation. Titanium dioxide (TiO2) is a most promising semiconductor that has been investigated substantially for its excellent optical and electronic properties, long-term stability, low cost, and nontoxicity [1–3]. However, most as-prepared TiO2 is only responsive to ultraviolet (UV) light due to its relatively large band gap (3.2 eV for anatase) hindering the utilization of visible light, which occupies ca. 43% of the entire solar spectrum. There are several methods to improve the photocatalytic performance of TiO2: (1) Chemical doping of TiO2 with metallic (Cr, Fe, V) [4,5] or non-metallic (N, C, B, S or F) elements [6–11] to modify the electronic structures of semiconductors as well as their surface properties, thus extending their visible light absorbance. For example, C, W-codoped TiO2 exhibited higher visible-light photocatalytic activity than undoped TiO2, C-doped TiO2 or W-doped TiO2. The co-doping of C and W not only leads to the narrowing of the band gap of TiO2, but also increases the separation efficiency of the photo-generated electrons-hole pairs [11]; (2) Synthesizing TiO2 with certain exposed facets, which is more reactive due to the higher average surface energy (demonstrated with both theoretical and experimental evidence) [12–14]. TiO2 with certain exposed facets has a substantial effect on the surface separation
⇑ Corresponding author. Tel.: +82 2 3277 4517; fax: +82 2 3277 3419. E-mail address: [email protected] (D.H. Kim). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.155
and transfer of photogenerated electron–hole pairs, resulting in a significant enhancement of photocatalytic efficiency; (3) Coupling with other semiconductors to increase the separation efficiency of photogenerated electron–hole pairs during photocatalysis [15–20]. Recently, the composites comprising TiO2 and other semiconductors, such as WO3 [21], Co3O4 [22], SiC [15], and Fe2O3 [16], have been reported. Among these methods, doping TiO2 with a non-metal element is a promising approach to achieving visible light response in TiO2 photocatalysts. Furthermore, doping with two kinds of non-metal atoms has shown more beneficial effects. However, representative combinations of two non-metal atoms have mostly focused on S/N, C/N, B/N, and N/F pairs [23–27]. To the best of our knowledge, there have been few reports on the use of C and F as co-dopants. Silver chloride (AgCl), which has a direct band gap of 5.6 eV and an indirect band gap of 3.25 eV, is widely recognized as a photosensitive material, and is employed as a source material in photographic films [28,29]. Recently, a series of visible-light active composite photocatalysts containing silver halides have been developed for the degradation of organic pollutants [30–32]. In these photocatalysts, both metallic Ag nanoparticles and silver halides were generated, and it was revealed that the metallic component was responsible for the absorption of visible light and photocatalytic activity due to its surface plasmon resonance effect. Thus, it is a necessary and meaningful task to elucidate the role of Ag in the form of silver halides on the photocatalytic activity. We report on a facile synthetic protocol of TiO2 co-doped with C and F elements via calcination of a mixture of TiOF2 and sulfuricacid-treated melamine in Ar atmosphere. Further, we demonstrate that the C, F-codoped TiO2 shows enhanced absorption in the
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visible light region, leading to markedly enhanced photocatalytic activity, by loading a small amount of AgCl on its surface. 2. Experimental 2.1. Synthesis of C, F-codoped TiO2 TiOF2 was synthesized via a facile one-step hydrothermal reaction according to previous reports [33,34]. Typically, 35.0 ml of titanium butoxide (Ti(OBu)4), 78.0 ml of acetic acid (CH3COOH), and 11 mL of 47% hydrofluoric acid solution were mixed in a Teflon-lined autoclave with a volume of 150 ml under stirring. The autoclave was kept at 473 K for 12 h in an oven. After cooling down unassisted to room temperature, the as-obtained products were collected and washed with water and ethanol several times, and dried at 373 K for 10 h. The as-prepared TiOF2 was mixed with sulfuric-acid-treated melamine. Then, the mixture was heated at 400 °C for 2 h in Ar atmosphere, followed by heat treatment at 400 °C for 1 h in air. 2.2. Loading of AgCl The loading of AgCl on C, F-codoped TiO2 was performed by in situ precipitation method. Typically, 0.2 g of C, F-codoped TiO2 powder was impregnated in an aqueous solution containing a certain amount of silver acetate (Ag(CH3COO)). After stirring for 12 h, NH4Cl solution was added. The solution was then evaporated over a water bath at 80 °C. Finally, the obtained powder was calcined in air at 400 °C for 1 h. 2.3. Photocatalytic experiment The catalyst powder (10 mg) was dispersed in 30 mL of methylene blue (MB) solution (10 ppm). The samples were kept in the dark for 1 h before exposure to light, in order to ensure equilibrium between the adsorption and desorption of dye molecules on the surface of the catalyst. Then, the samples were irradiated under stirring by a 300-W Xe lamp (Newport Co., model 66984) equipped with a 420nm cutoff filter as a visible light source. A fixed amount of sample was withdrawn from the stock solution at regular intervals and centrifuged to remove the catalyst. To study the change in absorbance maxima of the dye, the specific characteristic absorbance was measured by UV–vis absorbance spectroscopy (Varian Cary 5000 UV–vis–NIR spectrophotometer). 2.4. Characterization The crystal phase of the samples was determined by X-ray diffractometry (D/ max RA, Rigaku) using nickel-filtered copper radiation (Cu Ka) at 40 kV and 30 mA, over a 2h range of 10°–80°. The morphology of the catalyst was investigated using a JEOL JSM2100-F TEM microscope operated at 100 kV. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PHI 5000 Versa Probe (Ulvac-PHI) system using an Al Ka (1486.6 eV) anode (25 W, 15 kV). The binding energies were calibrated using the carbon C 1s peak at 284.6 eV. UV–vis absorbance spectra were measured using a Sinco S-4100 spectrometer.
3. Results and discussion We first investigate the structural aspects of TiOF2 and C, F-codoped TiO2. Fig. 1a and b shows the crystal phases of the
Fig. 1. The XRD patterns of TiOF2 (a) and C, F-codoped TiO2 (b).
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as-prepared TiOF2 precursors and C, F-codoped TiO2, respectively. As shown in Fig. 1a, the diffraction peaks can be indexed as cubic phase with a space group of Pm-3 m, which is in good agreement with the reported compound, cubic TiOF2 (JCPDS No.77-0132). The characteristic peaks of TiOF2 disappear, and new anatase peaks appear (Fig. 1b) after heat treating the mixture of TiOF2 and sulfuric-acid-treated melamine. This clearly indicates that the TiOF2 precursors changed to anatase TiO2 phase. The chemical nature of the as-prepared samples was then investigated by X-ray photoelectron spectroscopy (XPS). The signals for C 1s, F 1s, Ti 3d, and O 1s, but not N 1s, were clearly observed in the survey scan spectra in the region of 0–1000 eV (Fig. 2a). The binding energy centered at around 684.5 eV, which is ascribed to F species, was observed for C, F-codoped TiO2 samples, indicating the incorporation of F in the TiO2 lattice (Fig. 2b). It is also observed in Fig. 2c that the C 1s peak exhibits two components with binding energy at 284.6 eV and 288 eV, respectively. The binding energy at 284.6 eV should be ascribed to ambient organic impurities adsorbed on the surface of the sample, and the one at 288 eV may be associated with the carbon species as an interstitial dopant [35,36]. No noticeable peaks due to nitrogen species were observed around 399 eV. These results lead us to conclude that the TiO2 was doped by C and F after heat treating the mixture of TiOF2 and sulfuric-acid-treated melamine. The loading of AgCl on C, F-codoped TiO2 was performed by an in situ precipitation method. Fig. 3 shows the XRD patterns of samples loaded with different amounts of AgCl, where typical XRD patterns of cubic AgCl (JCPDS No. 31-1238) can be observed. It is also shown in Fig. 3 that the loading of AgCl does not change the crystal phase of the doped TiO2. Furthermore, the intensity of AgCl increases with the increasing amount of AgCl loaded. Fig. 4 shows the XPS analysis result of a representative C, F-codoped TiO2 sample loaded with AgCl. The C 1s, F 1s, Ti 3d, O 1s, Ag 3d, and Cl 2p signals can be clearly observed in the survey scan spectrum (Fig. 4a). The peaks observed at 267.2 eV and 373.2 eV can be ascribed to Ag (I) species. No signal due to Ag (0) could be found from the high-resolution Ag 3d XPS spectrum. Therefore, the XRD and XPS results confirm that cubic AgCl phase was deposited on the surface of C, F-codoped TiO2 rather than metallic Ag. The morphology of TiOF2, C, F-codoped TiO2, and AgCl-loaded C, F-codoped TiO2 was characterized by TEM. In Fig. 5a, it is observed that well-defined sub-micron cube structures with a uniform edge length of around 300–400 nm are developed for the TiOF2. After calcination with sulfuric-acid-treated melamine, part of the cubic TiOF2 morphology was changed to particles, as shown in Fig. 5b. Furthermore, some small particles of size around 60–100 nm could be observed. The interplanar spacing of C, F-codoped TiO2 is 3.52 Å (HRTEM, insert in Fig. 5b), which corresponds to the d-spacing of the (1 0 1) plane from XRD measurements. As shown in Fig. 5c, the loading of AgCl has little effect on the morphology of C, F-codoped TiO2. Small particles with 4–10-nm diameter could be observed on the surface of C, F-codoped TiO2. Fig. 5d shows the HRTEM of a selected particle loading on the surface of C, F-codoped TiO2. The interplanar spacing of the small particle is about 2.0 Å, which corresponds to the (2 2 0) d-spacing of AgCl. Therefore, the TEM image clearly shows that the AgCl nanoparticles are loaded on the surface of C, F-codoped TiO2. The absorption properties of TiOF2, C, F-codoped TiO2, and AgClloaded C, F-codoped TiO2 were then investigated by UV–vis diffuse reflectance spectroscopy. The absorption edge of the TiOF2 sample occurs at ca. 390 nm, corresponding to a band gap of 3.2 eV (Fig. 6a). After calcination of the mixture of TiOF2 and sulfuricacid-treated melamine, the absorption edge of the resulting C, Fcodoped TiO2 sample was red-shifted to a lower energy region at about 500 nm, corresponding to a band gap of about 2.48 eV (Fig. 6b). The absorption edge has little change after loading with
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Fig. 2. The XPS spectra of C, F-codoped TiO2. (a) Survey scan; (b) high-resolution F 1s; (c) high-resolution C 1s and (d) high-resolution N 1s.
Fig. 3. The XRD patterns of C, F-codoped TiO2 loaded with different amounts of AgCl.
AgCl, although the intensity of the absorption was increased, as shown in Fig. 6c. Photocatalytic activity was demonstrated in terms of the degradation of methylene blue (MB) under visible light irradiation (Fig. 7). The ratios of the intensities of the characteristic absorbance peak of MB at a wavelength of about 664 nm were plotted after irradiation with visible light for a specific period of time (C) and prior to irradiation (C0), as shown in Fig. 7e. All the samples show visible-light-responsive photocatalytic activity for the degradation of MB. The photocatalytic activity of C, F-codoped TiO2 was further enhanced by loading with AgCl. With increasing the amount of AgCl, the photocatalytic activity is increased, and the performance achieves a maximum when the amount of AgCl is
about 7 wt.%. Further increasing the amount of AgCl leads to a decrease in photocatalytic activity. The decrease in the photocatalytic activity at larger amounts of AgCl loading may be ascribed to the shielding effect of the visible light absorption by AgCl with a larger band gap. The enhanced photocatalytic activity upon the loading of AgCl was also examined in terms of the degradation of 4-nitrophenol (see Fig. S1 in the Supporting information). The enhancement of the photocatalytic performance of C, Fcodoped TiO2 by loading AgCl is attributed mainly to the effective separation of photogenerated electron–hole pairs. This mechanism was supported by comparing the photoluminescence (PL) spectra of neat C, F-codoped TiO2 with those of C, F-codoped TiO2 loaded with 7 wt.% AgCl (Fig. S2). The PL of C, F-codoped TiO2 loaded with 7 wt.% AgCl was slightly quenched compared with the AgCl-free sample, indicating that the separation of photogenerated electron–hole pairs is more effective in the presence of the AgCl. A concise scheme for the visible-light-driven electron–hole separation of the AgCl-loaded C, F-codoped TiO2 materials is proposed in Fig. 8. In general, the conduction band (CB) and valence band (VB) of non-metal-doped TiO2 is composed by Ti 3d, and the hybrid of O 2p and the p orbital of the non-metal element, respectively. Therefore, the CB and VB of C, F-codoped TiO2 are estimated to be 0.29 eV and +2.19 eV, respectively (with respect to the standard hydrogen electrode potential, SHE). For reference, it was reported that the CB and VB of AgCl are about 1.15 eV and +2.1 eV (versus SHE), respectively [21]. The electron–hole pairs are first generated upon excitation by visible light absorbed in C, F-codoped TiO2, and the holes are subsequently transferred to the AgCl surface, thereby resulting in the oxidation of Cl ions to Cl0 atoms. The Cl0 atoms in turn act as reactive species for the oxidation of dye molecules. At the same time, the photoinduced electrons are suspended in C, Fcodoped TiO2 and reduce the O2 to O2 radicals. However, since
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Fig. 4. The XPS spectra of C, F-codoped TiO2 loaded with 7 wt.% AgCl. (a) Survey scan; (b) high-resolution Ag 3d and (c) high-resolution Cl 2p.
Fig. 5. TEM images of a series of samples used in this study. (a) TiOF2; (b) C, F-codoped TiO2; (c) 7 wt.% AgCl-loaded C, F-codoped TiO2 and (d) HRTEM of C, F-codoped TiO2.
the conduction band (CB) of AgCl ( 1.15 eV versus SHE) lies above the CB of C, F-codoped TiO2 ( 0.3 eV versus SHE), the electrons can also reduce the Ag (I) to metallic Ag.
It is well known that Ag@AgCl has been shown to be an effective visible-light active photocatalyst for the surface plasmon resonance effect of Ag nanoparticles. However, there is an intrinsic
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Fig. 6. The UV–vis spectra of TiOF2 (a), C, F-codoped TiO2 (b) and AgCl-loaded C, F-codoped TiO2 (c).
difference between our materials and Ag@AgCl. In Ag@AgCl, the Ag nanoparticles are responsible for the absorption of visible light, because of its surface plasmon resonance effect, and the photogenerated electron–hole pairs. However, in the case of our photocatalysts, AgCl-loaded C, F-codoped TiO2, the C, F-codoped TiO2 is
Fig. 8. Schematic diagram of the photocatalytic mechanism of C, F-codoped TiO2 loaded with AgCl.
responsible for the absorption of visible light and the generation of electron–hole pairs, because of the narrow band gap. The AgCl in our system also acts as a hole acceptor. The holes generated were transferred to the AgCl surface, and the VB edge potential of AgCl is slightly negative compared to that of C, F-codoped TiO2.
Fig. 7. The photocatalytic degradation of MB over C, F-codoped TiO2 loaded with different amount of AgCl under visible light (k > 420 nm) irradiation. (a) 0 wt.% AgCl, (b) 3 wt.% AgCl, (c) 5 wt.% AgCl, (d) 7 wt.% AgCl and (e) Comparison of the visible light photocatalytic activity using C/C0 versus time plot.
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Fig. 9. The stability test result of C, F-codoped TiO2 for the degradation of MB. Fig. 10. The XRD patterns of C, F-codoped TiO2 before and after irradiation with visible light for different amount of time.
Next, the stability of the AgCl-loaded catalysts was investigated. As shown in Fig. 9, the photocatalytic activity decreases in the first three cycling runs. As the cycling runs increase further, the photocatalytic activity progresses steadily. To better understand the cause of the decrease in the activity, XRD and XPS analyses were carried out for the samples after irradiation. In Fig. 10, a typical XRD pattern characteristic of metallic Ag could be observed in the sample after 1 h of irradiation. It is also observed that the XRD intensity of atomic Ag increases with increasing irradiation time. Fig. 11 shows the XPS analysis result of a sample loaded with AgCl after irradiation. The C 1s, F 1s, Ti 3d, O 1s, Ag 3d, and Cl 2p signals can be clearly observed in the survey scan spectrum (Fig. 11a). However, except for the binding energy ascribed to Ag(I) species, the binding energy for metallic Ag(0) at 366 eV and 372 eV can be found from the high-resolution Ag 3d XPS spectrum (Fig. 11b). There is no noticeable difference in the high-resolution
Cl 2p XPS spectrum (Fig. 11c). The XPS results also confirm the existence of metallic Ag in the sample after irradiation, indicating that part of the AgCl was reduced to metallic Ag during extended irradiation. The atomic concentrations of Ag and Cl in the catalysts before and after irradiation were also analyzed by XPS. They are determined to be 0.58 (before) and 0.55 (after) for Ag, and 0.55 (before) and 0.21 (after) for Cl, respectively. This indicates that there is a leaching of Cl during the photocatalytic degradation of the dye. To evaluate the effect of the existence of metallic Ag on the photocatalytic activity compared with AgCl, Ag particles were loaded on the surface of C, F-codoped TiO2 by reducing AgCH3COO with N2H4. The XRD pattern confirms that metallic Ag is deposited on C, F-codoped TiO2 by this reducing method (Fig. 12). The Ag-loaded C, F-codoped TiO2 exhibits significantly lower photocatalytic
Fig. 11. The XPS spectra of C, F-codoped TiO2 after irradiation with visible light. (a) Survey scan; (b) high-resolution Ag 3d and (c) high-resolution Cl 2p.
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Acknowledgment This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2011-0029409; 2012-0009649). H. Yan was supported by the RP-Grant 2012 of Ewha Womans University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013. 01.155. References Fig. 12. The XRD pattern of Ag-loaded C, F-codoped TiO2.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 13. Comparison of the photocatalytic activity of C, F-codoped TiO2, AgCl-loaded C, F-codoped TiO2 and Ag-loaded C, F-codoped TiO2.
activity than that of AgCl-loaded C, F-codoped TiO2, as shown in Fig. 13, indicating that the incorporation of Ag element as a form of AgCl plays a significant role in enhancing the C, F-codoped TiO2. 4. Conclusion In conclusion, C, F-codoped TiO2 photocatalysts with anatase phase were synthesized by calcination of the mixture of TiOF2 and sulfuric-acid-treated melamine. The C, F-codoping resulted in a red-shift of the absorption edge from 390 nm for TiO2 to 510 nm for C, F-codoped TiO2. The C, F-codoped TiO2 shows effective photocatalytic activity for the degradation of MB dye under visible light irradiation. The loading of AgCl further enhanced the visible light photocatalytic activity of C, F-codoped TiO2 by the effective separation of photogenerated electron–hole pairs. This work demonstrated an unprecedented method for enhancing the photocatalytic activity of doped TiO2 nanostructures by loading AgCl.
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