pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Subsequently Light-Induced Route to Highly Efficient and Stable Sunlight-Driven Ag-AgBr Plasmonic Photocatalyst Long Kuai, Baoyou Geng,* Xiaoting Chen, Yanyan Zhao, and Yinchan Luo College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu, 241000, P. R. China Received October 6, 2010. Revised Manuscript Received November 15, 2010 In this paper, we successfully fabricate a stable and highly efficient direct sunlight plasmonic photocatalyst Ag-AgBr through a facile hydrothermal and subsequently sunlight-induced route. The diffuse reflectance spectra of Ag-AgBr indicate strong absorption in both UV and visible light region. The obtained photocatalyst shows excellent sunlightdriven photocatalytic performance. It can decompose organic dye within several minutes under direct sunlight irradiation and maintain a high level even though used five times. In addition, both the scanning electron microscopy images and X-ray photoelectron spectroscopy dates reveal the as-prepared photocatalyst to be very stable. Moreover, the mechanism suggests that the high photocatalytic activity and excellent stability result from the super sensitivity of AgBr to light, the surface plasmon resonance of Ag nanoparticles in the region of visible light, and the complexation between Agþ and nitrogen atom. Thus, the facile preparation and super performance of Ag-AgBr will make it available to utilize sunlight efficiently to remove organic pollutants, destroy bacteria, and so forth.
1. Introduction Currently, the “Green-Life” concept is inspiring enthusiasm to exploit efficient and stable photocatalysts in the visible light region. Compared with other physical, chemical, and biological methods, the photodecomposition approach is more acceptable in decomposing organic pollutants.1-4 TiO2 seems to be the most promising photocatalyst due to its stability, nontoxicity, and lowcost. However, its practical application is limited by its low utilization efficiency of solar because of the restricted absorption in ultraviolet (UV) region, which only account for 4% of the whole solar spectrum. Moreover, although many methods, including noble metal deposition, complex semiconductors, ion doping, and dye sensitization methods, have been explored to improve the properties of TiO2, there are still some other shortcomings.5 Therefore, it is necessary to develop some novel photocatalysts to remove the pollutants. Silver halide has been supposed to be a new visible light photocatalytie material for its good sensitivity to light. During its *Corresponding author. E-mail:
[email protected]. Fax: (þ86)553-3869303. (1) Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. J. Phys. Chem. C 2007, 111, 7574. (2) Huang, J. H.; Cui, Y. J.; Wang, X. C. Environ. Sci. Technol. 2010, 44, 3500. (3) Zeng, H. B.; Cai, W. P.; Li, Y.; Hu, J. L.; Liu, P. S. J. Phys. Chem. B 2005, 109, 18260. (4) Zeng, H. B.; Cai, W. P.; Liu, P. S.; Xu, X. X.; Zhou, H. J.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (5) Han, H.; Bai, R. B. Ind. Eng. Chem. Res. 2009, 48, 2891. (6) Hu, C.; Peng, T. W.; Hu, X. X.; Nie, Y. L.; Zhou, X. F.; Qu, J. H.; He, H. J. Am. Chem. Soc. 2010, 132, 857. (7) Huo, P. W.; Yan, Y. S.; Li, S. T.; Li, H. M.; Huang, W. H. Desalination 2010, 256, 196. (8) Elahifard, M. R.; Rahimnejad, S.; Haghighi, S.; Gholami, M. R. J. Am. Chem. Soc. 2007, 129, 9552. (9) Hu, C.; Lan, Y. Q.; Qu, J. H.; Hu, X. X.; Wang, A. M. J. Phys. Chem. B 2006, 110, 4066. (10) Zang, Y. J.; Farnood, R. Appl. Catal. B: Environ 2008, 79, 334. (11) Wang, P.; Huang, B. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Whangbo, M. H. Inorg. Chem. 2009, 48, 10697. (12) Li, G. T.; Wong, K. H.; Zhang, X. W.; Hu, C.; Yu, J. C.; Chan, R. C. Y.; Wong, P. K. Chemosphere 2009, 76, 1185.
Langmuir 2010, 26(24), 18723–18727
Scheme 1. The Production of Silver Nanoparticles on AgBr and the Proposed Photocatalytic Mechanism of Ag-AgBr Plasmonic Photocatalyst
application, silver halide is usually loaded on some other materials to perform its catalytic properties.6-12 However, this property of silver halide rarely plays a main role in these catalysts, resulting in not perfect efficiency. Considering the surface plasmon resonance (SPR) of noble metal nanoparticles, some highly efficient visiblelight plasmonic photocatalysts appear. For example, Farnood et al. prepared photocatalyst AgBr/Y-zeolite,13 which is highly efficient under sunlight irradiation, but it is so unstable that its practical application will be also limited. Some other visible-light photocatalysts, such as Ag@AgCl and Ag@AgBr have been developed recently.14-18 These catalysts display high photocatalytic activity and stability under visible-light due to the SPR of silver nanoparticles produced at the surface of silver halide. However, the fabrication method is multistep or time-consuming, and the produced silver nanoparticles are large and polydisperse, resulting in seriously weakening of the SPR of silver nanoparticles (13) Zang, Y. J.; Farnood, R.; Currie, J. Chem. Eng. Sci. 2009, 64, 2881. (14) Wang, P.; Huang, B. B.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Wei, J. Y.; Whangbo, M. H. Angew. Chem., Int. Ed. 2008, 47, 7931. (15) Wang, P.; Huang, B. B.; Zhang, X. Y.; Qin, X. Y.; Jin, H.; Dai, Y.; Wang, Z. Y.; Wei, J. Y.; Zhan, J.; Wang, S. Y.; Wang, J. P.; Whangbo, M. H. Chem.;Eur. J. 2009, 15, 1821. (16) An, C. H.; Peng, S.; Sun, Y. G. Adv. Mater. 2010, 22, 2570. (17) Wang, P.; Huang, B. B.; Lou, Z. Z.; Zhang, X. Y.; Qin, X. Y.; Dai, Y.; Zheng, Z. K.; Wang, X. N. Chem.;Eur. J. 2010, 16, 538. (18) Bi, Y. P.; Ye, J. H. Chem.;Eur. J. 2010, 16, 10327.
Published on Web 11/29/2010
DOI: 10.1021/la104022g
18723
Article
Kuai et al.
Figure 1. SEM images of the as-prepared Ag-AgBr before photocatalytic reaction: (a) low resolution; (b) high resolution (inset shows the surface SEM image of the products).
in the visible-light region because it is strongly dependent on the size, shape, and distribution of noble metal nanoparticles.19-22 To solve the problems discussed above, in this paper, we design a facile approach to prepare a stable and super highly efficient plasmonic photocatalyst Ag-AgBr based on SPR, which shows high efficiency under direct sunlight irradiation. Herein, first, AgBr microspheres are obtained by a simple hydrothermal process in the presence of hexadecyltrimethylammonium bromide (CTAB), silver nitric (AgNO3) and ammonia (NH3 3 H2O). Then the prepared AgBr microspheres are irradiated under sunlight. The Agþ ions on the surface of the microspheres are reduced, and next a certain amount of silver nanoparticles are produced naturally on the surface of AgBr microspheres. Because the localized SPR absorption of silver nanoparticles lies in the visible region, the Agþ ions inside of the AgBr microspheres will stop being reduced when the surfaces of AgBr microspheres are covered by a layer of silver nanoparticles. Thus, the high efficiency photocatalyst is obtained and displays powerful photodegradation ability to pollutants (as shown in Scheme 1). Additionally, we also enrich the photocatalytic mechanism from the point of complexation between Agþ and the nitrogen atom in methylic orange (MO). In our opinion, there are at least following four advantages in this work. (1) The Ag-AgBr photocatalyst is obtained by a onepot hydrothermal process and subsequent sunlight-induced formation of Ag nanoparticles, which makes the preparation more facile and less time-consuming. (2) The sunlight is used as the light source to decompose the organic dye, so the photocatalytic activity of Ag/AgBr is very close to that for the actual use. (3) The asprepared Ag-AgBr photocatalyst displays perfect photocatalytic activity under direct sunlight irradiation, because silver bromide with a 2.6 eV band gap is very sensitive to sunlight, and the silver nanoparticles distribute uniformly on the surface of AgBr, which can enhance the interfacial charge transfer and prevent the electron-hole from recombining efficiently. (4) Also, the obtained photocatalyst is very stable due to the SPR of the silver nanoparticles produced on the surface of AgBr.
2. Experimental Section 2.1. Materials. Silver nitric (AgNO3), ammonia (NH3 3 H2O),
CTAB, and MO were obtained from Shanghai Reagents Company (Shanghai, China), and all the reagents were analytical grade
(19) El-Brolossy, T. A.; Abdallah, T.; Mohamed, M. B.; Abdallah, S.; Easawi, K.; Negm, S.; Talaat, H. Eur. Phys. J. Special Topics 2008, 153, 361. (20) Amendola, V.; Bakr, O. M.; Stellacci, F. Plasmonics 2010, 5, 85. (21) Roy, R. K.; Bandyopadhyaya, S.; Pal, A. K. Eur. Phys. J. B 2004, 39, 491. (22) Chen, M. W.; Chau, Y. F.; Tsai, D. P. Plasmonics 2008, 3, 157.
18724 DOI: 10.1021/la104022g
except AgNO3, which was is chemical grade. All the reagents were used as received without any further purification.
2.2. The Synthesis of Ag-AgBr Photocatalyst. The Preparation of AgBr. AgNO3 (0.105 g) is dissolved in 30 mL of deionized water, then 0.6 g of CTAB is added into the above solution. After 10 min of vigorous stirring, 9 mL of NH3 3 H2O (28%) is added, and a light yellow mixture is obtained. Then the mixture is transferred to a 75 mL Teflon-lined stainless-steel autoclave and kept for 8 h at 120 °C, and a light yellow precipitation is produced. After cooling to room temperature, the precipitation is collected and washed with deionized water and ethanol several times. The Preparation of Ag-AgBr. The obtained AgBr is dispersed into deionized water and irradiated under sunlight for about 2 h. When the color changes to gray, it indicates that silver appears and the Ag-AgBr photocatalyst is prepared, after which the photocatalyst is collected and dried at 60 °C for 12 h. 2.3. Characterization. As-prepared photocatalyst is characterized by scanning electron microscopy (SEM, Hitachi S-4800), X-ray diffraction (XRD, Philips X’Pert with Cu KR1 radiation (λ = 0.154056 nm), X-ray photoelectron spectroscopy (XPS, ESCALAB 250 with monochromatized Mg KR X-ray as the source), and room temperature UV-visible diffuse reflectance spectroscopy (UV-2450, SHIMADZU). 2.4. The Photodegradation of MO Dye. The photocatalytic activity of Ag-AgBr is evaluated through photodegrading MO dye. The experiments are carried out under direct sunlight irradiation (the sunlight intensity is about 50000 lx) with 20 mg of photocatalyst dispersed into 20 mL of 10 mg/L MO dye solution at room temperature, and the degradation results are monitored by UV-visible spectroscopy (U-3010 spectrophotometer).
3. Result and Discussion Figure 1 shows the typical SEM images of the as-prepared Ag-AgBr photocatalyst. As shown in Figure 1a, the Ag-AgBr photocatalyst has sphere-like morphology with diameters of 0.6-1.2 μm. Figure 1b is the SEM image of a single Ag-AgBr microsphere, and the inset is the high-magnification SEM image, in which it can be found that silver nanoparticles are distributed uniformly on the surface of the as-prepared spherical AgBr, and their size distribution is about 8-12 nm. Figure 2a shows the XRD patterns of the Ag-AgBr, pure AgBr, and Ag. Compared with the XRD pattern of the AgBr, the XRD pattern of Ag-AgBr obviously shows two additional peaks, which are indexed to the metal silver, indicating that metal silver appears after irradiating pure AgBr under sunlight. In addition, the appearance of metal silver also can be confirmed by the XPS. Figure 2b-d shows the XPS spectra of the as-prepared Ag-AgBr Langmuir 2010, 26(24), 18723–18727
Kuai et al.
Article
Figure 2. (a) XRD patterns of the Ag (black line), pure AgBr (red line), and as-prepared Ag-AgBr (blue line). (b) The overview XPS spectrum of the as-prepared Ag-AgBr. (c) Br3d and (d) Ag3d XPS spectra (black line: before photocatalysis; red line: after photocatalysis).
photocatalyst. Figure 2b displays the whole XPS spectrum of the as-prepared Ag-AgBr, which mainly exhibits the peaks of Ag, Br, and C. The sources of the peaks are AgNO3 and CTAB, respectively. Figure 2d further suggests that metal silver is produced on the surface of AgBr, because the XPS peaks at 367.5 and 373.6 eV are all assigned to Ag03d, which is consistent with the result of XRD analysis. Figure 3 displays the typical UV-visible diffuse reflectance spectra of pure AgBr and the as-prepared Ag-AgBr photocatalyst. As is vividly depicted in Figure 3, the as-prepared photocatalyst Ag-AgBr has a much stronger absorption in the visiblelight area than that of pure AgBr, resulting from the SPR of the silver nanoparticles.14 Therefore, the as-prepared photocatalyst must have high photocatalytic activity in the whole sunlight region. The photocatalytic activity and stability of the as-prepared Ag-AgBr photocatalyst are evaluated by photodegradation MO dye under direct sunlight irradiation. As shown in the Figure 4a inset, the peak intensity decreases rapidly at wavelengths of 464 and 270 nm, which are assigned to the function groups of azo and phenyl, respectively. What’s more, the MO dye can be decomposed more than 83% within 2 min with the presence of the asprepared Ag-AgBr photocatalyst and almost decomposed completely after 5 min of irradiation under direct sunlight. However, there is almost no degradation of MO with N-doped TiO2 or without any photocatalyst. It is worth mentioning that the photocatalytic reaction is taken under direct sunlight irradiation, which is much more perfect than other light sources, such as Xe lamp and high-pressure mercury lamp. In addition, another group of new absorption peaks is found at the wavelength of 246 nm, and their intensity increases with the degradation of MO Langmuir 2010, 26(24), 18723–18727
Figure 3. UV-visible diffuse reflectance spectra of pure AgBr (red line) and the as-prepared Ag-AgBr (black line).
dye, which may belong to the hydrazine derivative generated from the MO reduction process.13,23 The photocatalytic stability is also evaluated by the cycling degradation experiments. As shown in Figure 4b, the photocatalytic activity of the as-prepared Ag-AgBr photocatalyst still maintains a high level, even though it was used 5 times during our research. Herein, the MO dye is degraded more than 95% at each recycling within 5 min, although the photocatalytic rate decreases slightly at the outset of the photodegradation process. (23) Anandan, S.; Yoon, M. J. Photochem. Photobio. C: Photochem. Rev. 2003, 4, 5.
DOI: 10.1021/la104022g
18725
Article
Kuai et al.
Figure 4. (a) The photodecomposition curve of MO. C0 is the original concentration of MO (10 mg/L), and C is concentration of the remaining MO at time t. The inset on the right shows the UV-visible spectrum of MO dye after the corresponding degradation time. (b) Five cycling experimental results of the as-prepared Ag-AgBr photocatalyst. The low-magnification (c) and high-magnification (d) SEM images of the Ag-AgBr photocatalyst after photocatalytic reaction (inset shows the surface SEM image of the photocatalyst).
Moreover, the investigation on the surface chemical composition and morphology after catalytic reaction further illustrates that the as-prepared Ag-AgBr photocatalyst is very stable. Figure 4c,d shows the typical SEM images of the as-prepared photocatalyst after photocatalytic reactions, in which it can be found easily that the spherical structure with the silver nanoparticles distributed on the surface is not destroyed after photocatalytic reactions. In addition, according to the XPS dates, the atomic rate of silver and bromide is 1.28 and 1.35 before and after the photocatalytic reactions, respectively, which reveals that the contents of Ag0 or the surface chemical composition rarely change after the photocatalytic reactions (Figure 2c,d). The above results reveal that the as-prepared Ag-AgBr photocatalyst exhibits high stability. A possible mechanism is proposed to explain the reasons of the high photocatalytic activity and stability of the as-prepared Ag-AgBr photocatalyst. It has been confirmed that the silver nanoparticles make great contribution to the high visible light photocatalytic activity as a result of their SPR produced by the collective oscillations of surface electrons.9,14,15,24,25 The SPR of silver locates at the visible light region, which leads to the strong absorption to the sunlight. Additionally, the excellent conductivity of silver nanoparticles can enhance the electron translation so as to enhance the interfacial charge transfer and stop the recombination of electron-hole pairs efficiently. As for the as-prepared Ag-AgBr, the sunlight can be absorbed efficiently both by silver nanoparticles and AgBr, then a large amount of electron-hole (e--hþ) pairs are generated (as shown in Scheme 1). On the one hand, the hþ generated on AgBr can oxidize Br- to Br0, and Br0 is the reactive radical specie and oxidizes MO dye molecule, which has been reported by Huang at el.;14 on the other hand, the hþ generated on silver nanoparticles can also oxidize the MO dye molecule directly.9,26 In addition, the timely absorption of MO (24) Jin, R.; Cao, Y. C.; Mirkin, C. A.; Kelly, K. L.; Schatzand, G. C.; Zheng, J. G. Science 2001, 294, 1901. (25) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (26) Zhou, X. F.; Hu, C.; Hu, X. X.; Peng, T. W.; Qu, J. H. J. Phys. Chem. C 2010, 114, 2746.
18726 DOI: 10.1021/la104022g
dye molecule to the photocatalyst plays another important role in the fast degradation rate, which is due to the complexation of Agþ with nitrogen in the MO molecule. Thus, when one combines the properties of silver nanoparticles, AgBr, and the complexation of Agþ, the photocatalytic rate is promoted exceedingly. Meanwhile, the noticeable behavior of electron decides the stability of as-prepared photocatalyst. Because of the dipolar property of SPR of silver nanoparticles, the absorbed photon is separated into an electron and a hole. Due to the excellent conductivity of silver nanoparticles, the electron can be transferred quickly and induced away from AgBr as far as possible rather than being trapped by the Agþ of AgBr, so that Agþ inside the AgBr microspheres can escape from the reduction of the electrons to the maximum extent. Then these electrons go to reduce the MO molecules, or are trapped by O2 and H2O at the surface of photocatalyst or in the solution to form O2-, O2-• and other reactive oxygen species. These reactive oxygen species also help the degradation of MO dye. These actions of electrons not only prevent the recombination of the hole and electron efficiently, but also almost avoid being captured by Agþ. Thus, the activity is kept at a high level and, more importantly, the stability is ensured at the same time.14,26
4. Conclusion In summary, the stable and highly active Ag-AgBr photocatalyst is prepared by a facile hydrothermal and subsequently sunlight-induced formation method. The as-prepared Ag-AgBr photocatalyst can be used under direct sunlight to remove the pollutants, and the MO dye can be decomposed more than 83% within 2 min of sunlight irradiation. Moreover, the photocatalyst still keeps a high level of activity even though it is used five times, and the MO dye can be removed more than 95% every time. The XPS dates indicate that the surface chemical composition rarely changes after the photocatalytic reactions, because the atomic rate of silver and bromide nearly remains consistent before and after the photocatalytic reactions. The superiority of the asprepared photocatalyst should be attributed to the super sensitivity Langmuir 2010, 26(24), 18723–18727
Kuai et al.
of AgBr to light, the SPR of Ag nanoparticles in the region of visible light, and the complexation between Agþ and the nitrogen atom. Therefore, the as-prepared Ag-AgBr photocatalyst has potential application on pollutant degradation under direct sunlight as well as the destruction of bacteria and photovoltaic fuel cells.
Langmuir 2010, 26(24), 18723–18727
Article
Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671003, 20971003), the Key Project of the Chinese Ministry of Education (209060), the Science and Technological Fund of Anhui Province for Outstanding Youth (10040606Y32) and the Program for Innovative Research Team at Anhui Normal University.
DOI: 10.1021/la104022g
18727
