Langmuir 2008, 24, 8351-8357
8351
Highly Efficient Visible-Light-Induced Photocatalytic Activity of Nanostructured AgI/TiO2 Photocatalyst Yuanzhi Li,*,† Hua Zhang,† Zhimin Guo,† Jianjun Han,† Xiujian Zhao,† Qingnan Zhao,† and Sun-Jae Kim‡ Key Laboratory of Silicate Materials Science and Engineering (Wuhan UniVersity of Technology), Ministry of Education, 122 Luoshi Road, Wuhan 430070, People’s Republic of China, and Faculty of Nanotechnology and AdVanced Materials Engineering, Sejong UniVersity, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Korea ReceiVed April 3, 2008. ReVised Manuscript ReceiVed May 5, 2008 Nanostructured AgI/TiO2 photocatalyst was synthesized by a feasible approach with AgNO3, LiI, and Ti(OBu)4 and characterized by X-ray diffraction, transmission electron microscopy, angle-dependent X-ray photoelectron spectroscopy, diffusive reflectance UV-vis spectroscopy, Raman spectroscopy, photoluminescence, and the Brunauer-Emmett-Teller technique. The results of characterization reveal that the nanostructured AgI/TiO2 has a novel core/shell/shell nanostructure of AgI/Ag-I2/TiO2. Compared with TiO2 (P25) supported AgI, the formation of the nanostructure results in substantial shifting of the absorption edge of AgI to red, enhancement of the absorption intensity, and the appearance of a strong tail absorption above 490 nm, which is assigned to the absorption of I2 and Ag. Photocatalytic tests show that the nanostructured AgI/TiO2 photocatalyst exhibited very high visible-light-induced photocatalytic activity for the photodegradation of crystal violet and 4-chlorophenol, which is 4 and 6 times higher than that of P25 titania supported AgI, respectively. The highly efficient visible-light-induced photocatalytic activity of the nanostructured AgI/TiO2 is attributed to its strong absorption in the visible region and low recombination rate of the electron-hole pair due to the synergetic effect among the components of AgI, Ag, I2, and TiO2 in the nanostructured AgI/TiO2.
1. Introduction Nanostructured titania as a cheap, nontoxic, efficient photocatalyst for the detoxification of air and water pollutants has received much research attention during the past two decades. However, it is activated only under UV light irradiation because of its large band gap (3.2 eV for anatase and 3.0 eV for rutile). However, solar spectra only contain 5% UV. A total of 95% of the solar photons are useless for TiO2, which greatly limits its practical application in environmental decontamination. Therefore, it is crucial and of great challenge to explore efficient visiblelight-induced photocatalysts by the modification of titania, which is an international hot topic in the field of photocatalysis. To achieve such a goal, much scientific effort has been made during recent years to narrow its band gap or introduce stable optical sensitizers. These works include (1) doping TiO2 with various transition metals such as Pt, Au, Ag, Cr, V, etc.,1–5 (2) doping TiO2 with nonmetal atoms such as N, C, S, B, I, Br, F, etc.,6–19 and (3) anchoring an organic dye sensitizer molecule on the † ‡
Wuhan University of Technology. Sejong University.
(1) Kisch, H.; Zang, L.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D. Angew. Chem., Int. Ed. 1998, 37, 3034–3036. (2) Bosc, F.; Ayral, A.; Keller, N.; Keller, V. Appl. Catal., B 2007, 69, 133– 137. (3) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707–10711. (4) Wu, C. G.; Chao, C. C.; Kuo, F. T. Catal. Today 2004, 97, 103–112. (5) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505–516. (6) Asahi, R.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (7) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, J. L. Nano Lett. 2003, 3, 1049–1051. (8) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.; Pacchioni, G. J. Am. Chem. Soc. 2006, 128, 15666–15671. (9) Reyes-Garcia, E. A.; Sun, Y. P.; Reyes-Gil, K.; Raftery, D. J. Phys. Chem. C 2007, 111, 2738–2748. (10) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772–773. (11) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Catal. 2007, 246, 362–369.
surface of the photocatalyst.20,21 Because of the instability of the metal-doped titania, relatively low absorption coefficient of the nonmetal-doped titania in the visible light region, and toxicity and self-photodegradation of the organic sensitizer, challenges do exist in finding a new material system that can effectively utilize visible light. Recently, sensitizing titania by colorful inorganic materials received scientific attention because of their stability and relatively strong absorption in the visible light region. Usseglio et al. encapsulated (I2)n inside nanovoid-structured titania and reported that titania sensitized by I2 exhibited 10 times higher photocatalytic activity for the photodegradation of methylene blue than the P25 commercial photocatalyst under irradiation of sunlight.22 Silver halides are known as photosensitive materials and are widely used in photographic films. The photographic process in silver halides comprises basically the following. Absorption of a photon (12) Tachikawa, T.; Tojo, S.; Kawai, K.; Endo, M.; Fujitsuka, M.; Ohno, T.; Nishijima, K.; Miyamoto, Z.; Majima, T. J. Phys. Chem. B 2004, 108, 19299– 19306. (13) Li, Y.; Hwang, D.; Lee, N. H.; Kim, S. J. Chem. Phys. Lett. 2005, 404, 25–29. (14) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454–456. (15) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. J. Am. Chem. Soc. 2004, 126, 4782–4782. (16) Hong, X. T.; Wang, Z. P.; Cai, W. M.; Lu, F.; Zhang, J.; Yang, Y. Z.; Ma, N.; Liu, Y. J. Chem. Mater. 2005, 17, 548–1552. (17) Liu, G.; Chen, Z. G. ; Dong, C. L.; Zhao, Y. N.; Li, F.; Lu, G. Q.; Cheng, H. M. J. Phys. Chem. B 2006, 110, 20823–20828. (18) Luo, H. M.; Takata, T.; Lee, Y. G.; Zhao, J. F.; Domen, K.; Yan, Y. Chem. Mater. 2004, 16, 846–849. (19) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N. Chem. Mater. 2005, 17, 2596– 2602. (20) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J. Phys. Chem. B 2004, 108, 14093–14101. (21) Chen, F.; Deng, Z. G.; Li, X. P.; Zhang, J. L.; Zhao, J. C. Chem. Phys. Lett. 2005, 415, 85–88. (22) Usseglio, S.; Damin, A.; Scarano, D.; Bordiga, S.; Zecchina, A.; Lamberti, C. J. Am. Chem. Soc. 2007, 129, 2822–2828.
10.1021/la801046u CCC: $40.75 2008 American Chemical Society Published on Web 07/08/2008
8352 Langmuir, Vol. 24, No. 15, 2008
liberates an electron and a positive hole. The electrons will combine with mobile interstitial silver ions, leading to separation of a silver atom. Upon repeated absorption of photons, a cluster of silver atoms can be ultimately formed.23 If the photographic process is inhibited, the photogenerated electron and hole could be used in the photocatalytic process. It has been proved that the presence of a support (e.g., silica, zeolite) could stabilize silver halide by inhibiting the photographic process and make silver halide photocatalytically active.23 AgBr supported on Al-MCM41 has been reported to exhibit photocatalytic activity and stability under visible light irradiation.24 Hu et al. prepared P25 titania supported AgBr and AgI by deposition-precipitation and found that the photocatalysts AgBr/TiO2 and AgI/TiO2 are very stable under light irradiation and showed high efficiency for the degradation of azo dyes and killing of bacteria.25,26 On the other hand, it has been reported that efficient photocatalytic activity could be achieved by carefully designing a nanostructured photocatalyst of a titania composite, such as mesoporous Au/ TiO2,27 I2/TiO2,22 etc. Here we developed an approach to synthesize nanostructured AgI/TiO2 photocatalyst with a novel core/shell/shell nanostructure of AgI/Ag-I2/TiO2. It was found that the nanostructured AgI/TiO2 photocatalyst exhibited very high visible-light-induced phtotocatalytic activity for photodegradation of crystal violet and 4-chlorophenol.
2. Experimental Section 2.1. Materials. AgNO3 was a product of Guangzhou Lixin Chemical Co. LiI · 3H2O was purchased from Zigong Jindian Chemical Co. Ethanol was purchased from Shanghai Zhenxing Chemical Co. Titanium butoxide (Ti(OBu)4) was purchased from Shanghai Aisi Chemical Reagent Co. Silica was from Shanghai Aijian Chemical Reagent Co. Titania P25 (TiO2, ca. 80% anatase, 20% rutile) was purchased from the Degussa Co. Crystal violet, 4-chlorophenol, and iodine were purchased from Shanghai Chemical Co. All of these chemicals were used without further purification. 2.2. Preparation of Photocatalysts. LiI-ethanol solution was obtained by dissolving 0.1880 g of LiI · 3H2O in 4 mL of ethanol. AgNO3-ethanol solution was obtained by adding 0.1690 g of AgNO3 to 15 mL of ethanol followed by sonication until it dissolved. The LiI solution was dropped into the AgNO3 solution under magnetic stirring. Then 1 mL of Ti(OBu)4 was added to the mixture and the resulting mixture heated at 70-80 °C under stirring until all of the solvent was almost evaporated. After the slurry of the mixture was cooled to ambient temperature, 8 mL of ethanol was added to it. A 1 mL sample of distilled water was dropped into the mixture under magnetic stirring. The formed precipitate was filtered, washed thoroughly with distilled water and then with acetone, dried at 90 °C for 5 h, and finally calcined at 450 °C for 2 h. The obtained AgI/TiO2 was a yellow powder. Pure titania (TiO2) was prepared by a procedure similar to that of the preparation of the above AgI/ TiO2 but without addition of LiI and AgNO3 solution. Titania supported metallic silver (Ag/TiO2) was prepared as follows. Ammonia ethanol solution was obtained by adding 1 mL of 28% NH3 · H2O to 9 mL of ethanol. A 0.3400 g sample of AgNO3 was dissolved in 34 mL of ethanol by sonication. A 1.4 mL sample of the ammonia-ethanol solution was dropped into the AgNO3 solution under magnetic stirring. Then 2 mL of Ti(OBu)4 was added to the mixture and the resulting mixture heated at 70-80 °C under (23) Kakuta, N.; Goto, N.; Ohkita, H.; Mizushima, T. J. Phys. Chem. B 1999, 103, 5917–5919. (24) Rodrigues, S.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. J. Catal. 2005, 233, 405–410. (25) (a) Hu, C.; Lan, Y. Q.; Qu, J. H.; Hu, X. X.; Wang, A. M. J. Phys. Chem. B 2006, 110, 4066–4072. (b) Hu, C.; Hu, X. X.; Wang, L. S.; Qu, J. H.; Wang, A. M. EnViron. Sci. Technol. 2006, 40, 7903–7907. (26) Elahifard, M. R.; Rahimnejad, S.; Haghighi, S.; Gholami, M. R. J. Am. Chem. Soc. 2007, 129, 9552–9553. (27) Li, H. X.; Bian, Z. F.; Zhu, J.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 4538–4539.
Li et al. magnetic stirring until all of the solvent was almost evaporated. After the slurry of the mixture was cooled to ambient temperature, 16 mL of ethanol was added to it. A white precipitate was formed after 2 mL of distilled water was dropped into the mixture under magnetic stirring. The next procedure was similar to that of the preparation of the above AgI/TiO2. Finally, a black powder was obtained. Titania (P25) supported AgI (AgI/P25) was prepared as follows according to the deposition-precipitation method developed by Hu et al.26 A 1.0 g sample of P25 TiO2 was added to 100 mL of distilled water, and the suspension was sonicated for 30 min. A 0.2050 g sample of KI was added to the suspension, and the mixture was stirred magnetically for 30 min. Then 0.2100 g of AgNO3 in 2.3 mL of NH4OH solution (25 wt % NH3) was quickly added to the mixture. The product was filtered, then washed with water, and finally dried at 70 °C for 12 h. Silica supported AgI (AgI/SiO2) was prepared by the same procedure as the preparation of AgI/P25 except for using 1.0 g of silica instead of TiO2 (P25). Nanovoid-structured titania with encapsulated I2 (I2/TiO2) was synthesized according to the method developed by Usseglio et al.22 A 0.1 g sample of I2 crystal was dissolved in 3 mL of Ti(OBu)4 in a small beaker. The liquid was kept under continuous stirring at ambient temperature. After 36 h, the obtained powder was dried at 100 °C in an oven and then calcined in air at 500 °C for 6 h. 2.3. Characterization. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-IIIA X-ray diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) images were obtained by using a JEM-100CX electron microscope. Angledependent X-ray photoelectron spectroscopy (XPS) spectra were acquired by an ESCALab MK2 X-ray photoelectron spectrometer. Mg KR radiation was selected as the X-ray source. The spectra of the samples were collected and corrected by referencing the binding energy to carbon (C1s, 284.6 eV). Raman spectra were recorded on a Renishaw inVia Raman microscope. Diffusive reflectance UV-vis (DRUV-vis) absorption spectra were recorded on a UV-240 UV-vis spectrophotometer. Photoluminescence (PL) spectra were recorded at room temperature on a Shimadzu RF-5301 PC spectrometer using 420 nm excitation light. The Brunauer-Emmett-Teller (BET) surface area was measured on Autosorb-1 using N2 adsorption at -196 °C for the sample predegassed at 200 °C in a vacuum for 2 h. 2.4. Photocatalytic Activity. The photocatalytic activity of the as-synthesized samples was evaluated by photodegradation of crystal violet (CV) or 4-chlorophenol under irradiation of visible light (λ > 420 nm). The light source was a 125 W high-pressure Hg lamp (Shanghai Yayuan Lighting Appliance Co. Ltd.). Light passed through a cutoff filter and then was focused onto a 100 mL beaker. The reaction was maintained at ambient temperature. In a typical experiment, aqueous suspensions of CV or 4-chlorophenol (50 mL, 2 × 10-4 mol/L) and 0.2000 g of the photocatalyst powder were placed in the beaker. Prior to irradiation, the suspension was magnetically stirred in the dark to ensure the establishment of an adsorption/desorption equilibrium. The suspension was kept under constant air-equilibrated conditions. At given irradiation time intervals, 1 mL of the suspension was collected and centrifuged to remove the particles. A 0.9 mL sample of the filtrate was diluted with 3 mL of distilled water. The dye concentration was determined by measuring the UV-vis absorbance of the dye aqueous solution.
3. Results and Discussion 3.1. Structure and Morphology. 3.1.1. XRD. Figure 1 shows the XRD patterns of as-synthesized AgI/P25 and AgI/TiO2. It can be seen that AgI supported on P25 titania (AgI/P25) was a mixture of β-AgI and γ-AgI and TiO2 is a mixture of rutile and anatase. For AgI/TiO2, TiO2 exists in the form of pure anatase. Although AgI in AgI/TiO2 has an XRD pattern similar to that of AgI/P25, its intensity ratio at 2θ ) 22.38° and 23.70° (52/ 100), which is almost the same as that of β-AgI (60/100, ICSD 9-374), is much larger than that of AgI/P25 (24/100). This
Photocatalytic ActiVity of AgI/TiO2 Photocatalyst
Langmuir, Vol. 24, No. 15, 2008 8353 Table 1. Angle-Independent XPS Analysis of the AgI/TiO2 Photocatalyst atomic composition (%)
Figure 1. XRD patterns of as-synthesized AgI/TiO2 samples.
Figure 2. TEM images of the as-synthesized AgI/TiO2 (A) and HRTEM image of the interface between AgI and TiO2 nanoparticles (B).
observation suggests that β-AgI is the predominant phase in AgI/TiO2. Hu et al.26 reported that irradiation led to sharpening of the XRD peaks of β-AgI(002) and γ-AgI(111), indicating that AgI crystals were grown to be larger under irradiation. In contrast, irradiation of AgI/TiO2 with a 125 W high-pressure lamp for 4 h does not lead to any detectable sharpening of the peak of β(002) or γ(111) and the appearance of metallic Ag. This observation indicates that the as-synthesized AgI/TiO2 is photostable without reduction of Ag+ and oxidation of I-. The result shows that AgI particles in AgI/TiO2 are stabilized by the surrounding TiO2. 3.1.2. TEM. Figure 2 shows TEM images of the as-synthesized AgI/TiO2. As shown in Figure 2A, AgI nanoparticles (black image) with sizes of 5-30 nm contact titania nanoparticles (bright image). To further obtain more structural information on the interface of AgI and TiO2 nanoparticles, the HRTEM image of the interface was taken (Figure 2B). The interplanar spacing of TiO2 is 0.344 nm, which corresponds to the (101) planes of anatase. The interplanar spacing (0.352 nm) on the AgI nanoparticle is less than that of the preferential (002) planes of β-AgI (0.375 nm). As indicated in Figure 1, XRD peaks of β-AgI(101) and anatase (101) are overlapped because of their very close spacing. Although the spacing (0.352 nm) is very close to that of the (101) planes of β-AgI (0.351 nm), we could not exclude the following possibilities. One is that the lattice match of AgI and TiO2 on the interface leads to the shrinkage of the interplanar spacing of the (002) planes of the exposed β-AgI nanoparticles. Another is that AgI nanoparticles are encapsulated within a titania layer, the interplanar spacing of the (101) planes of anatase of which is enlarged due to the lattice match. HRTEM could not distinguish the possibilities, so further characterization is needed. 3.1.3. Angle-ResolVed XPS. XPS is a very useful technique of surface analysis. Angle-resolved XPS can provide more structural information on the surface as reducing the photoelectron takeoff angle (measured from the sample surface) reduces the
atomic ratio
takeoff angle (deg)
Ti
O
Ag
I
Ag/I
O/Ti
25 85
24.9 23.6
50.7 48
16.8 19.3
7.7 9.0
2.19 2.14
2.03 2.03
depth from which the XPS information is obtained.28–30 The result of angle-dependent XPS is tabulated in Table 1. At a takeoff angle of 25°, the atomic abundances of Ti, O, Ag, and I in AgI/ TiO2 are 24.9%, 50.7%, 16.8%, and 7.7%, respectively. The atomic ratio of O to Ti is 2.03, which corresponds to the stoichiometric ratio of TiO2. With an increase of the takeoff angle to 85°, the atomic abundance of Ti and O decreases from 24.9% and 50.7% to 23.6% and 48.0%, respectively, but the atomic ratio of O to Ti remains unchanged. On the other hand, the atomic abundance of Ag and I increases from 16.8% and 7.7% to 19.3% and 9.0%, respectively. These results indicate that AgI nanoparticles are encapsulated within a titania shell, because the atomic abundance of Ti and O would remain unchanged or increase if the AgI nanoparticles were exposed. Therefore, the interplanar spacing (0.352 nm) on AgI nanoparticles observed by HRTEM is ascribed to the enlarged spacing of the (101) planes of anatase from 0.344 nm. The enlargement of interplanar spacing is most probably due to the lattice match of β-AgI(002) and TiO2(101) on the interface of the core/shell of AgI/ TiO2. The thickness of the titania shell is estimated according to the formula as follows:28,29
I1/I2 ) [1 - exp(-d/(λ sin θ1))]/[1 - exp(-d/(λ sin θ2))] where I1 and I2 are the XPS intensities at takeoff angles of θ1 and θ2, respectively, d is the thickness of the layer, and λ is the inelastic mean free path of the photoelectron, which can be calculated from the Bethe equation.30,31 The calculated λ of the Ti2p photoelectron in titania is 1.97 nm. The average thickness of the titania shell in AgI/TiO2 is estimated to be 3.7 nm by numerical calculation. As shown in Table 1, the atomic ratio of Ag to I in AgI/TiO2 is as high as 2.19 at a takeoff angle of 25°. The atomic ratio of Ag to I slightly decreases to 2.14 with evolution of the takeoff angle from 25° to 85°. However, the molar chemical composition of Ag and I is 1:1 in AgI/TiO2. The much higher atomic ratio of Ag to I suggests that a AgI nanoparticle as the core is probably covered by a shell of metallic Ag. Figure 3 shows XPS spectra of AgI/TiO2 and AgI/P25. In AgI/P25, Ag species existed in the form of pure AgI without detectable Ag0 species by XPS, XRD, etc. The binding energy of Ag3d5/2 (367.4 eV) is assigned to Ag+ of AgI26 (Figure 3A). However, the binding energy of Ag3d5/2 (368.8 eV) in AgI/TiO2 is much larger than that in AgI/P25. Luo et al.32 studied the growth and interaction of Ag with the TiO2(110) surface by XPS, low-energy ion spectroscopy (LEIS), and scanning tunneling microscopy (STM). They found that the binding energy of Ag3d5/2, which was dependent on the sizes (e.g., diameter and height) of the Ag nanocluster, increased from (28) Yan, S. P.; Wang, G. H.; Li, F.; Gu, G. Y. J. Hefei UniV. Technol. 2003, 26(3), 452–455. (29) Hazell, L. B.; Brown, I. S.; Freisinger, F. Surf. Interface Anal. 1986, 8, 25–31. (30) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1994, 21, 165–176. (31) Jablonski, A.; Lesiak, B.; Zemek, J.; Jiricek, P. Surf. Sci. 2005, 595, 1–5. (32) Luo, K.; St. Clair, T. P.; Lai, X.; Goodman, D. W. J. Phys. Chem. B 2000, 104, 3050–3057.
8354 Langmuir, Vol. 24, No. 15, 2008
Li et al.
Figure 4. Raman spectra of the photocatalysts and solid iodine.
Figure 3. XPS spectra of Ag3d (A) and I3d (B) for AgI/TiO2 and AgI/ P25.
368.1 eV for bulk metallic Ag to at most 369.3 eV with a decrease of the size of the Ag nanocluster to 2-3.5 nm in diameter with a height of 0.6-0.7 nm. They attributed the higher Ag binding energy shift to both initial and final state effects. A binding energy of Ag3d5/2 as high as 368.8 eV (takeoff angle 85°) in the present AgI/TiO2 indicates that Ag0 nanoclusters constitute the very thin metallic Ag shell on AgI cores. We could not estimate the thickness of the metallic Ag shell by angle-dependent XPS because the presence of the TiO2 shell outside the Ag shell makes the estimation complicated. However, according to the dependence of the binding energy of Ag3d5/2 on the size of the Ag nanocluster on TiO2,32 the thickness of the shell of Ag nanoclusters in AgI/TiO2 is estimated to be 0.6-1.0 nm. The reason why the Ag nanoclusters were not observed by XRD is probably due to their amorphous or less crystalline state, and a small amount of Ag existed in AgI/TiO2. As our results proved that titania supported AgI is photostable, it is concluded that the formation of a Ag shell occurred accompanying the formation of AgI by photolysis during the preparation of the photocatalyst AgI/TiO2. Figure 3B shows XPS spectra of I3d. For AgI/P25, the XPS peak of I3d5/2 appears at 618.4 eV, which is assigned to I- in AgI. In contrast, AgI/TiO2 exhibits a much higher I3d5/2 binding energy of 620.1 eV assigned to I2.33,34 The formed I2 is probably adsorbed on the metallic Ag shell during the photolysis of AgI. 3.1.4. Raman Spectra. Figure 4 shows the Raman spectra of the as-synthesized samples and solid iodine. AgI/P25 exhibits strong bands at 143, 395, 513, and 637 cm-1, which are attributed to the Eg, B1g, A1g, and Eg modes of anatase, respectively, and (33) Salaneck, W. R.; Thomas, H. R.; Bigelow, R. W.; Duke, C. B.; Plummer, E. W.; Heeger, A. J.; MacDiarmid, A. G. J. Chem. Phys. 1980, 72, 3674–3678. (34) Kaufmann, R.; Klewe-Nebenius, H.; Pfennig, G.; Ache, H. J. Fresenius’ Z. Anal. Chem. 1989, 333, 398–400.
a small band at 450 cm-1 assigned to the Eg mode of rutile.35 In the case of AgI/TiO2, only Raman modes of anatase were observed, indicating its pure anatase crystalline structure, which is in agreement with the result obtained by XRD. Compared to AgI/P25, the Raman peak of the Eg mode of anatase broadens and has a blue shift of 13 cm-1 for AgI/TiO2. This shift and broadening are attributed to the size effect on the Raman spectra36 because of the presence of the 3.7 nm thick titania shell in AgI/ TiO2. The bulk iodine exhibits Raman peaks at 179, 188, and 212 cm-1, which are assigned to the Ag and B3g modes and I-I stretching mode of I2, respectively.37 Such Raman modes of iodine were not observed for AgI/P25, but two shoulders at 188 and 212 cm-1 were observed for AgI/TiO2. These observations indicate that I2 is present in AgI/TiO2, which is in agreement with the results of XPS. On the basis of the above results and discussion, we concluded that the AgI/TiO2 photocatalyst has a core/shell/shell nanostructure (schematically illustrated in the inset in Figure 2) with AgI nanoparticles as the core (5-30 nm), a composite of Ag and I2 as the medium shell (0.6-1 nm), and titania as the outside shell (3.7 nm). The formation of the nanostructure is easily understood by our preparation procedure. First, AgI nanoparticles were prepared, and then Ti(OBu)4 was added to the suspension of AgI nanoparticles in ethanol. The Ti(OBu)4 molecules were adsorbed on the surface of the AgI nanoparticles and slowly hydrolyzed to become a titania layer on the AgI nanoparticles by moisture. The formed AgI nanoparticles are unstable in the absence of TiO2 and photolyzed to become Ag nanoclusters before addition of Ti(OBu)4. Thus, there is a shell of Ag nanoclusters between the AgI core and outside TiO2 shell. 3.2. UV-Vis Absorption. Figure 5 gives the diffusive reflectance UV-vis spectra of the photocatalysts. As can be seen from Figure 5A, AgI/SiO2 shows an absorption edge at 442 nm similar to that of AgI/P25. Because both TiO2 (P25) and SiO2 have no adsorption at around this wavelength, the absorption edge is attributed to the contribution of AgI. Compared to AgI/ P25, the formation of the AgI/Ag-I2/TiO2 core/shell/shell nanostructure leads to a substantial red shifting of the absorption edge as well as enhancement of the absorption intensity. The plot of the transformed Kubelka-Munch function versus the energy of light affords a band gap of 2.80 eV (442 nm) and 2.52 eV (490 nm) for AgI/P25 and AgI/TiO2, respectively (Figure 5B). As the AgI nanoparticle in the nanostructured AgI/TiO2 directly contacts a shell of Ag and I2 composite, the lower band (35) Zhang, Y. H.; Ebbinghaus, S. G.; Weidenkaff, A.; Kurz, T.; von Nidda, H. A. K.; Klar, P. J.; Gungerich, M.; Reller, A. Chem. Mater. 2003, 15, 4028– 4023. (36) Kelly, S.; Pollak, F. H.; Tomkiewicz, M. J. Phys. Chem. B 1997, 101, 2730–2734. (37) Anderson, A.; Sun, T. S. Chem. Phys. Lett. 1970, 6, 611.
Photocatalytic ActiVity of AgI/TiO2 Photocatalyst
Langmuir, Vol. 24, No. 15, 2008 8355
Figure 6. Room temperature photoluminescence spectra with excitation at 420 nm for the photocatalysts.
Figure 5. Diffusive reflectance UV-vis spectra of the photocatalysts: (A) absorbance versus the wavelength of the light absorbed; (B) Kubelka-Munk function versus the energy of the absorbed light.
gap and stronger absorption of the nanostructured AgI/TiO2 are most probably due to the strong interaction between the core AgI and the medium shell of the Ag and I2 composite. Furthermore, the nanostructured AgI/TiO2 exhibits a strong absorption tail above 490 nm, which covers the whole visible light region. As shown in Figure 5A, Ag/TiO2 shows a broadening surface plasma absorption corresponding to metallic Ag particles38 in the visible light region with its maximum absorption centered at 580 nm. I2/TiO2 exhibits a platform absorption above 420 nm. The absorption tail looks like the overlapping of the absorption of Ag and I2. Consequently, the strong absorption above 490 nm for AgI/TiO2 is reasonably attributed to the contribution from the medium shell of the Ag and I2 composite. 3.3. Photoluminescence. In this work, we focused on searching for visible-light-induced photocatalysts of the titania composite. It is well-known that their photocatalytic activity is greatly affected by the recombination rate of excited electrons and holes. PL spectra are a useful tool to investigate such a recombination. A low PL intensity indicates a decrease in the recombination rate.39,40 As TiO2 shows PL emission when excited by UV light, to avoid interference from titania, PL spectra for the as-synthesized AgI/SiO2, AgI/P25, and AgI/TiO2 were recorded with excitation at 420 nm because titania cannot be excited at this excitation wavelength. As shown in Figure 6, AgI/P25 and AgI/SiO2 show almost the same PL spectra except for a decrease of the PL intensity at 442 nm for the former. The emission at 442 nm (2.80 eV) is attributed to the fact that (38) Stathatos, E.; Lianos, P.; Falaras, P.; Siokou, A. Langmuir 2000, 16, 2398–2400. (39) Li, F. B.; Li, X. Z. Chemosphere 2002, 48, 1103–1111. (40) Yu, J. G.; Yu, H. G.; Cheng, B.; Zhao, X. J.; Yu, J. C.; Ho, W. K. J. Phys. Chem. B 2003, 107, 13871–13879.
the photoexcited electrons at the conduction band edge recombine directly with holes at the valence band edge in AgI/P25.41 The PL peaks above 442 nm arise from shallow and deep exciton traps involving electron-phonon interactions or crystalline defects and impurities.41 The lower PL intensity of AgI/P25 than that of AgI/SiO2 is due to the transfer of the excited electron from the conduction band of AgI to that of titania.42 Compared to that in AgI/P25 and AgI/SiO2, the PL intensity of AgI in AgI/TiO2 is greatly decreased, indicating that the recombination rate of the electron-hole pair is greatly decreased due to the formation of the nanostructured AgI/TiO2.43 As no PL emission was detected for I2 under the same conditions, we could not probe the recombination rate of excited electrons and holes in I2/TiO2. It is known that pure AgI nanoparticles could be photodecomposed into a metallic Ag cluster through the photographic process. Once a composite of AgI and TiO2 is formed, the transfer of photoexcited electrons from the conduction band of AgI to that of TiO2 is beneficial for the stabilization of AgI. After the electron reaches the conduction band of TiO2, it potential (band edge position of TiO2 at pH 0, -0.12 V)42 is insufficient for reducing AgI to Ag (E°AgI/Ag ) -0.1522 V). That is one of the reasons why the nanostructured AgI/TiO2 and AgI/P25 are photostable. 3.4. Photocatalytic Activity. Dye effluents from textile industries are becoming a serious environmental problem because of their unacceptable color, high chemical oxygen demand content, and resistance to chemical, photochemical, and biological degradation. Effective utilization of solar light to degrade organic wastes and spent dyes in the presence of titania is expected to provide an attractive approach of environmental remediation.44 We chose photodegradation of crystal violet with irradiation of visible light to evaluate the photocatalytic activity of the photocatalysts. Figure 7A shows the time course of the decrease in the dye concentration with the irradiation of the visible light (λ > 420 nm). Crystal violet adsorbed on pure titania is slowly photodegraded due to the self-photosensitization effect of titania sensitized by the dye.45 Compared with the pure titania, supporting AgI on P25 TiO2 results in substantial enhancement of photocatalytic activity. In contrast, the nanostructured AgI/TiO2 exhibits much larger photocatalytic activity than AgI/P25. Crystal violet (41) (a) Kumar, P. S.; Sunandana, C. S. Nano Lett. 2002, 2, 431–434. (b) Kumar, P. S.; Sunandana, C. S. Nano Lett. 2002, 2, 975–978. (42) Fitzmaurice, D.; Frei, H.; Rabani, J. J. Phys. Chem. 1995, 99, 9176–9181. (43) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K. J. Photochem. Photobiol. A 2001, 141, 209–217. (44) Yanagisawa, K.; Ovenstone, J. J. Phys. Chem. B 1999, 103, 7781–7787. (45) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1999, 103, 4862–4867.
8356 Langmuir, Vol. 24, No. 15, 2008
Li et al.
Figure 8. Time course of the decrease in the 4-chlorophenol concentration with irradiation of visible light (λ > 420 nm).
Figure 7. Time course of the decrease in the dye concentration (A) and ln(C0/C) (B) with irradiation of visible light (λ > 420 nm).
is completely decolored on AgI/TiO2 under the irradiation of visible light for 120 min. It is well-known that photocatalytic oxidation of organic pollutants follows first-order kinetics.46 Figure 7B confirms that the photodegradation of crystal violet on the photocatalysts follows first-order kinetics. The rate constant for the photodegradation of the dye on the nanostructured AgI/ TiO2 (0.031 min-1) is 15 and 4 times higher than that on pure titania and on AgI/P25, respectively. Hu et al.26 reported that the AgI/P25 photocatalyst can be efficiently excited by visible light to create electron-hole pairs to form reactive oxygen species of the •OH radical. Our result of PL and the work of Rabani et al.42 show that the electron in AgI excited by visible light could transfer from the conduction band of AgI to that of TiO2. Once the electron reaches the TiO2 conduction band, it subsequently induces the generation of active oxygen species, which result in the degradation of crystal violet. As discussed above, the recombination rate of the photogenerated electron-hole pairs in the nanostructured AgI/TiO2 is much lower than that in AgI/P25, which results in the higher photocatalytic activity of the former than that of the latter. Moreover, compared to AgI supported on P25, the formation of AgI/Ag-I2/TiO2 core/shell/shell nanostructure results in considerable enhancement of the absorption intensity in the visible light region. The enhanced absorption indicates that the nanostructured AgI/TiO2 exhibits a higher efficiency for the generation of electron-hole pairs than AgI/ P25, which further contributes to the enhancement of photocatalytic activity for the nanostructured AgI/TiO2. CV has strong absorption in the visible light region, which leads to the photodegradation of the dye through the selfphotosensitization mechanism in the presence of TiO2 or AgI. (46) Hong, S. S.; Lee, M. S.; Park, S. S.; Lee, G. D. Catal. Today 2003, 87, 99–105.
To exclude the effect of self-photosensitization and confirm whether AgI/TiO2 is photoactive for photodegradation of other organic pollutants without absorption in the visible light region, we tested the photocatalytic activity of the photocatalysts for the photodegradation of 4-chlorophenol under visible light irradiation (λ > 420 nm). As shown in Figure 8, pure titania (P25) is inactive for the photodegradation of 4-chlorophenol because it could not be activated by visible light due to its large band gap. AgI/P25 shows considerable photocatalytic activity for the photodegradation of 4-chlorophenol. In contrast, the nanostructured AgI/ TiO2 also exhibits much larger photocatalytic activity than AgI/ P25. The rate constant for the photodegradation of 4-chlorophenol on the nanostructured AgI/TiO2 (0.011 min-1) is 6 times higher than that on AgI/P25. As the nanostructured AgI/TiO2 is composed of AgI, Ag, I2, and TiO2, to elucidate the role of each component, we prepared the photocatalysts of Ag/TiO2, AgI/SiO2, and I2/TiO2 and tested their photocatalytic properties. Compared with pure titania, all of them show higher photocatalytic activity in order as follows: Ag/TiO2 > AgI/SiO2 > I2/TiO2. The enhancement of photocatalytic activity with irradiation of visible light for I2/TiO2 is attributed to the photosensitization of titania by I2. I2 absorbing a visible photon is promoted into an excited I2*, from which an electron can be transferred into the conduction band of TiO2; thus, the photodegradation process can occur according to the standard mechanism.22 Its slight enhancement as compared to TiO2 in our case is probably due to its lower specific area (4.2 m2/g). It has been reported that AgBr supported on silica or Al-MCM-41 exhibited photocatalytic activity and stability under visible light irradiation.23,24 Our experiment proved that AgI supported on silica also exhibits visible-light-induced photocatalytic activity. The rate constant on the nanostructured AgI/ TiO2 is 7, 8.8, and 13 times higher than that on Ag/TiO2, AgI/ SiO2, and I2/TiO2, respectively. The BET surface area of AgI/ TiO2, Ag/TiO2, and I2/TiO2 is 11.8, 7.2, and 4.2 m2/g, respectively. The difference in their surface areas could not give a reasonable explanation of the large difference in their photocatalytic activities. These results suggest that there is a synergetic effect among the components of AgI, Ag, I2, and TiO2 in the nanostructured AgI/ TiO2 instead of a simple mixture of them.
4. Conclusion In summary, nanostructured AgI/TiO2 with a novel core/shell/ shell nanostructure of AgI/Ag-I2/TiO2 was synthesized by a feasible approach. The nanostructured AgI/TiO2 photocatalyst exhibits strong absorption in the whole visible light region. Compared with TiO2 (P25) supported AgI, the formation of the nanostructure results in substantial shifting of the absorption
Photocatalytic ActiVity of AgI/TiO2 Photocatalyst
edge of AgI to red, great enhancement of absorption intensity, and the appearance of a strong absorption tail above 490 nm, which is assigned to the absorption of I2 and Ag. The nanostructured AgI/TiO2 photocatalyst exhibited much higher photocatalytic activity for the photodegradation of crystal violet and 4-chlorophenol under visible light irradiation than pure titania, AgI/SiO2, Ag/TiO2, and P25 supported AgI. The highly efficient visible-light-induced photocatalytic activity of the nanostructured AgI/TiO2 is attributed to its novel core/shell/shell nanostructure, which results in its strong absorption in the visible region and low recombination rate of electron-hole pairs due to the synergetic effect among the components of AgI, Ag, I2, and
Langmuir, Vol. 24, No. 15, 2008 8357
TiO2. Our work provides a novel pathway to synthesize visiblelight-induced photocatalyst by designing a nanostructured titania composite. Acknowledgment. This work was supported by the National Science Foundation (Grant 20743001), the Talented Young Scientist Foundation (Grant 2007ABB034) of Hubei Province, China, the Program for New Teacher from the Ministry of Education (Grant 20070497003), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT0547). LA801046U
