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Preparation of Ag-TiO2 hollow structures with enhanced photocatalytic activity
Materials Letters 65 (2011) 908–910
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of Ag-TiO2 hollow structures with enhanced photocatalytic activity Caixia Song a, Debao Wang b, Yaohua Xu b, Zhengshui Hu a,⁎ a b
College of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China Key Laboratory of Eco-chemical Engineering (Ministry of Education), Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
Article history: Received 6 August 2010 Accepted 7 December 2010 Available online 15 December 2010 Keywords: Microstructure Nanocomposites Photocatalytic Ag-TiO2
a b s t r a c t This paper describes a one-pot surfactant-assisted hydrothermal method for the synthesis of Ag-TiO2 composite hollow structures with high photocatalytic activity. TEM measurement reveals that the sample is composed of hollow spheres and their tube-like aggregates. The surfactant cetyltrimethylammonium bromide (CTAB) plays a key role in the construction of the Ag-TiO2 hollow structures. The Ag-TiO2 hollow structures showed effective photocatalytic activity for the decomposition of methyl orange. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TiO2 has been well known for its photocatalytic properties, which can find applications in various fields including water splitting, antibacterial, and degradation of various pollutants. However, the main drawbacks of low quantum yields and the lack of visible light utilization hinder its practical application. A variety of studies have been devoted to improving the photocatalytic efficiency and visible light utilization of TiO2 by doping of noble metals. Among them, Ag modified TiO2 composite particles have more significant practical values and attract more research interests [1,2]. Alternatively, nanosized TiO2 particles have also been explored to improve the photocatalytic performance [3]. However, the involvement of TiO2 and Ag-TiO2 nanoparticles make it difficult to recycle the catalysts. Syoufian et al. reported that photocatalytic activity of commercially available TiO2 was lower than that of the hollow particles [4]. Thus far, many methods have been explored to prepare TiO2 hollow spheres. These methods require hard template, self-sacrificial template, and oil/water droplets, as well as organometallic precursor of Ti(IV) [5–9]. Recently, F-doped TiO2 hollow microspheres and Fe3O4/TiO2 composite hollow spheres were synthesized showing high photocatalytic activity for degradation of organic dyes [10,11]. For economical and efficient use of Ag, preparation of Ag-doped TiO2 hollow structures may be an effective way to develop their photocatalytic activities. Although photocatalytic activities of Ag/TiO2 nanocomposite powders have been reported in literature [12–14], little work has concerned those of the Ag-doped TiO2 hollow structures. Previously, we prepared Ag-TiO2 hollow spheres using
⁎ Corresponding author. Tel.: +86 532 84022787. E-mail address: [email protected] (Z. Hu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.015
latex particles as template and find the as-prepared Ag-TiO2 hollow spheres showed broad and relatively strong absorption in visible region [15]. Herein, we develop a simple one-pot route for the synthesis of Ag-TiO2 hollow structures with high photocatalytic activities. To the best of our knowledge, few reports have concerned the photocatalytic activities of Ag-TiO2 hollow structures. 2. Experimental All chemicals were analytical grade and were used without further purification. In a typical procedure, 0.48 g Ti(SO4)2, 0.75 ml 30% H2O2, and 0.2 g CTAB were dissolved in 40 ml H2O in a 50 ml Teflon-lined stainless steel autoclave under agitation, then 1.6 ml 0.05 mol/L AgNO3 was added. The autoclave was sealed and put into a preheated oven. The reaction was conducted at 150 °C for 24 h, then, the autoclave was cooled to room temperature. The final products were collected, washed with de-ionized water and alcohol for several times. Experiments under other conditions were carried out following similar procedure. The X-ray powder diffraction (XRD) patterns of the products were recorded on a Bruker D8 advanced X-ray diffractmeter. The morphologies and structures were investigated by using a JSM-6700F field emission scanning electron microscope (SEM) and a JEM-2000EX transmission electron microscope (TEM). To investigate the photocatalytic activities of the sample, 5 mg of Ag-TiO2 photocatalyst (or TiO2) was suspended in 50 mL of methyl orange (MO) aqueous solution. The solution was continuously stirred for about 30 min in the dark to ensure the adsorption-desorption equilibrium among the photocatalyst, MO, and water. Then the mixture was exposed to UV irradiation from a home-lamp at room temperature. The samples were collected every 20 min to measure the MO degradation by UV–vis spectra using a UV/Vis/NIR Spectrometer.
C. Song et al. / Materials Letters 65 (2011) 908–910
909
Fig. 1. SEM (a) and TEM (b) images, ED (c) and XRD (d) patterns of the sample with 5.4% Ag.
3. Results and discussion Fig. 1a displays the SEM image of the Ag-TiO2 sample. It shows that the sample exhibits the aggregates of microspheres with diameters of about 500 nm. In the inset of Fig. 1a, a cracked microsphere is given to reveal the hollow structures of the aggregated microspheres. To further confirm the hollow nature of the product, a typical TEM image is shown in Fig. 1b. It can be seen that there exist some tube-like structures together with the aggregated microspheres. The contrast between the dark edge and pale center is the evidence for their hollow nature. The wall thickness of these hollow spheres is in the range of 20–40 nm. Because of the significant difference in the image contrast of silver from TiO2, Ag nanoparticles could be easily identified as dark dots superimposed on TiO2 background [16,17]. The ring-like diffraction pattern in Fig. 1c can be indexed to tetragonal phase anatase TiO2 and cubic phase silver, suggesting the formation of crystalline Ag-TiO2 structures. Fig. 1d shows XRD pattern of the as-obtained sample. The diffraction peaks are labeled and can be indexed to tetragonal phase anatase TiO2 (JCPDS Card 83-2243) together with the diffraction of (111) and (200) planes of fcc silver (JCPDS Card 4-783), indicating the formation of Ag-TiO2 composites. The line broadening of the diffraction peaks results from the small grain size and structural defects of the sample, which benefits the catalytic activity of the catalyst. It is known that surfactant CTAB can form spherical micelles in aqueous solution above the critical micelle concentration [18]. It is expected that the CTAB micelles serve as a soft template for Ag-TiO2 hollow structures, however the size and shape of the hollow structures do not match those of CTAB micelles. It has been reported that both the shapes and the size of the CTAB micelles will change with the increase of the surfactant concentration or addition of inorganic salts to the solution. Rod-like and worm-like micelles can also be formed [19]
together with the aggregation of the micelles. The schematic diagram of the possible formation mechanism for Ag-TiO2 hollow structures is illustrated in Fig. 2. In the aqueous solution, CTAB molecules selfassemble into spherical micelles. After adding the reactants, these micelles will aggregate and their shape and size will change. It is known that Ti(IV) hydrolyses to form TiO2·xH2O sols, which exist as negative + 2x− colloids in solution due to its charged{[TiO2]mnTiO2− 3 ,2(n − x)H } weak acidity. The negative charged colloids, then, adsorb onto the surface of the positive charged CTA+ micelles by the electrostatic interactions. Thus, TiO2·xH2O hollow structures are produced on the surface of the CTA+ micelles after reaching equilibrium conditions. Meanwhile, Ag+ ions are reduced to metal silver. After hydrothermal crystallization, Ag-TiO2 hollow structures are obtained.
Fig. 2. Schematic diagram of the formation mechanism of the hollow structures.
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C. Song et al. / Materials Letters 65 (2011) 908–910
is shown in the inset of Fig. 3. It can be seen that the Ag-TiO2 hollow structures exhibit much enhanced photocatalytic activities in comparison with the TiO2 microstructures. Such significant improvement of photocatalytic activity of Ag-TiO2 hollow structures can be attributed to the more effective electron-hole separation of the AgTiO2 composite structures [1,2]. 4. Conclusion We have developed a one-pot procedure for facile synthesis Ag-TiO2 composite hollow structures with the assistance of surfactant CTAB. The Ag-TiO2 hollow structures show high photocatlytic activity to MO, which makes it a promising candidate material for environmental applications. Acknowledgement
Fig. 3. Variation of the UV–vis spectra for MO solution in the presence of Ag-TiO2 hollow structures at different intervals. The inset is the comparison of photocatalytic activities of different samples.
This work was financially supported by the Natural Science Found of Shandong Province (ZR2009BL018) and the Foundation of Education Department of Shandong Province (J10LB11). References
A series of experiments have been carried out to investigate the formation mechanism. For example, to confirm the templating role of CTAB, the experiments are carried out in presence of different dose of CTAB. SEM observation indicates that less hollow structures are obtained with addition of less CTAB. Experimental results also show H2O2 plays an important role in the formation of the hollow structures. It is expected that the formation of [TiO(H2O2)]2+ complex decreases the hydrolysis ratio of Ti(IV), which makes it possible to adsorb the TiO2·xH2O colloids on the surface of CTAB micelles. If no H2O2 is added, SEM observation shows that irregular TiO2 aggregates are obtained. The photocatalytic activities of the Ag-TiO2 samples were evaluated by the degradation of methyl orange (MO) in an aqueous solution. Fig. 3 shows the variation in the UV–Vis spectra of MO at different irradiation intervals. It can be clearly seen that the maximum absorbance at 462 nm disappears almost completely after irradiation for 60 min. Since no new peak was observed during the whole process, a degradation reaction of MO has occurred. The degradation rate of MO on commercial TiO2 catalyst, as-prepared TiO2 and Ag-TiO2
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Xin B, Jing L, Ren Z, Wang B, Fu H. J Phys Chem B 2005;109:2805–9. Hu C, Lan Y, Qu J, Hu X, Wang A. J Phys Chem B 2006;110:4066–72. Zhang DY, Yang D, Zhang HJ, Lu CH, Qi LM. Chem Mater 2006;18:3477–85. Syoufian A, Nakashima K. J Colloid Interface Sci 2007;313:213–8. Ma YR, Qi LM. J Colloid Interface Sci 2009;335:1–10. Yu JG, Liu W, Yu HG. Cryst Growth Des 2008;8:930–4. Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. Du X, He JH, Zhao YQ. J Phys Chem C 2009;113:14151–8. Wang YD, Zhou AN, Yang ZY. Mater Lett 2008;62:1930–2Inorg Chem 2008;45: 3493–5. Xuan SH, Jiang WQ, Gong XL, Hu Y, Chen ZY. J Phys Chem C 2009;113:553–8. Zhou JK, Lv L, Yu JQ, Li HL, Guo PZ, Sun H, Zhao XS. J Phys Chem C 2008;112: 5316–21. Koh JK, Seo JA, Koh JH, Kim JH. Mater Lett 2009;63:1360–2. Liu WJ, Zhao XM, Cao LY, Zou B, Zhang G, Lu LH, Cui HN. Mater Lett 2009;63: 2456–8. Lai YK, Chen YC, Zhuang HF, Lin CJ. Mater Lett 2008;62:3688–90. Song CX, Wang DB, Gu GH, Lin YS, Yang JY, Chen L, Fu X, Hu ZS. J Colloid Interface Sci 2004;272:340–4. Cozzoli PD, Comparelli R, Fanizza E, Curri ML, Agostiano A, Laub D. J Am Chem Soc 2004;126:3868–79. Guin D, Manorama SV, Latha JNL, Singh S. J Phys Chem C 2007;111:13393–7. Li F, Li GZ, Wang HQ, Xue QJ. Colloids Surf A 1997;127:89–96. Zhang W, Li G, Mu J, Shen Q, Zheng L, Liang H, Wu C. Chin Sci Bull 2000;45:1854–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of Ag-TiO2 hollow structures with enhanced photocatalytic activity Caixia Song a, Debao Wang b, Yaohua Xu b, Zhengshui Hu a,⁎ a b
College of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China Key Laboratory of Eco-chemical Engineering (Ministry of Education), Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
Article history: Received 6 August 2010 Accepted 7 December 2010 Available online 15 December 2010 Keywords: Microstructure Nanocomposites Photocatalytic Ag-TiO2
a b s t r a c t This paper describes a one-pot surfactant-assisted hydrothermal method for the synthesis of Ag-TiO2 composite hollow structures with high photocatalytic activity. TEM measurement reveals that the sample is composed of hollow spheres and their tube-like aggregates. The surfactant cetyltrimethylammonium bromide (CTAB) plays a key role in the construction of the Ag-TiO2 hollow structures. The Ag-TiO2 hollow structures showed effective photocatalytic activity for the decomposition of methyl orange. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TiO2 has been well known for its photocatalytic properties, which can find applications in various fields including water splitting, antibacterial, and degradation of various pollutants. However, the main drawbacks of low quantum yields and the lack of visible light utilization hinder its practical application. A variety of studies have been devoted to improving the photocatalytic efficiency and visible light utilization of TiO2 by doping of noble metals. Among them, Ag modified TiO2 composite particles have more significant practical values and attract more research interests [1,2]. Alternatively, nanosized TiO2 particles have also been explored to improve the photocatalytic performance [3]. However, the involvement of TiO2 and Ag-TiO2 nanoparticles make it difficult to recycle the catalysts. Syoufian et al. reported that photocatalytic activity of commercially available TiO2 was lower than that of the hollow particles [4]. Thus far, many methods have been explored to prepare TiO2 hollow spheres. These methods require hard template, self-sacrificial template, and oil/water droplets, as well as organometallic precursor of Ti(IV) [5–9]. Recently, F-doped TiO2 hollow microspheres and Fe3O4/TiO2 composite hollow spheres were synthesized showing high photocatalytic activity for degradation of organic dyes [10,11]. For economical and efficient use of Ag, preparation of Ag-doped TiO2 hollow structures may be an effective way to develop their photocatalytic activities. Although photocatalytic activities of Ag/TiO2 nanocomposite powders have been reported in literature [12–14], little work has concerned those of the Ag-doped TiO2 hollow structures. Previously, we prepared Ag-TiO2 hollow spheres using
⁎ Corresponding author. Tel.: +86 532 84022787. E-mail address: [email protected] (Z. Hu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.015
latex particles as template and find the as-prepared Ag-TiO2 hollow spheres showed broad and relatively strong absorption in visible region [15]. Herein, we develop a simple one-pot route for the synthesis of Ag-TiO2 hollow structures with high photocatalytic activities. To the best of our knowledge, few reports have concerned the photocatalytic activities of Ag-TiO2 hollow structures. 2. Experimental All chemicals were analytical grade and were used without further purification. In a typical procedure, 0.48 g Ti(SO4)2, 0.75 ml 30% H2O2, and 0.2 g CTAB were dissolved in 40 ml H2O in a 50 ml Teflon-lined stainless steel autoclave under agitation, then 1.6 ml 0.05 mol/L AgNO3 was added. The autoclave was sealed and put into a preheated oven. The reaction was conducted at 150 °C for 24 h, then, the autoclave was cooled to room temperature. The final products were collected, washed with de-ionized water and alcohol for several times. Experiments under other conditions were carried out following similar procedure. The X-ray powder diffraction (XRD) patterns of the products were recorded on a Bruker D8 advanced X-ray diffractmeter. The morphologies and structures were investigated by using a JSM-6700F field emission scanning electron microscope (SEM) and a JEM-2000EX transmission electron microscope (TEM). To investigate the photocatalytic activities of the sample, 5 mg of Ag-TiO2 photocatalyst (or TiO2) was suspended in 50 mL of methyl orange (MO) aqueous solution. The solution was continuously stirred for about 30 min in the dark to ensure the adsorption-desorption equilibrium among the photocatalyst, MO, and water. Then the mixture was exposed to UV irradiation from a home-lamp at room temperature. The samples were collected every 20 min to measure the MO degradation by UV–vis spectra using a UV/Vis/NIR Spectrometer.
C. Song et al. / Materials Letters 65 (2011) 908–910
909
Fig. 1. SEM (a) and TEM (b) images, ED (c) and XRD (d) patterns of the sample with 5.4% Ag.
3. Results and discussion Fig. 1a displays the SEM image of the Ag-TiO2 sample. It shows that the sample exhibits the aggregates of microspheres with diameters of about 500 nm. In the inset of Fig. 1a, a cracked microsphere is given to reveal the hollow structures of the aggregated microspheres. To further confirm the hollow nature of the product, a typical TEM image is shown in Fig. 1b. It can be seen that there exist some tube-like structures together with the aggregated microspheres. The contrast between the dark edge and pale center is the evidence for their hollow nature. The wall thickness of these hollow spheres is in the range of 20–40 nm. Because of the significant difference in the image contrast of silver from TiO2, Ag nanoparticles could be easily identified as dark dots superimposed on TiO2 background [16,17]. The ring-like diffraction pattern in Fig. 1c can be indexed to tetragonal phase anatase TiO2 and cubic phase silver, suggesting the formation of crystalline Ag-TiO2 structures. Fig. 1d shows XRD pattern of the as-obtained sample. The diffraction peaks are labeled and can be indexed to tetragonal phase anatase TiO2 (JCPDS Card 83-2243) together with the diffraction of (111) and (200) planes of fcc silver (JCPDS Card 4-783), indicating the formation of Ag-TiO2 composites. The line broadening of the diffraction peaks results from the small grain size and structural defects of the sample, which benefits the catalytic activity of the catalyst. It is known that surfactant CTAB can form spherical micelles in aqueous solution above the critical micelle concentration [18]. It is expected that the CTAB micelles serve as a soft template for Ag-TiO2 hollow structures, however the size and shape of the hollow structures do not match those of CTAB micelles. It has been reported that both the shapes and the size of the CTAB micelles will change with the increase of the surfactant concentration or addition of inorganic salts to the solution. Rod-like and worm-like micelles can also be formed [19]
together with the aggregation of the micelles. The schematic diagram of the possible formation mechanism for Ag-TiO2 hollow structures is illustrated in Fig. 2. In the aqueous solution, CTAB molecules selfassemble into spherical micelles. After adding the reactants, these micelles will aggregate and their shape and size will change. It is known that Ti(IV) hydrolyses to form TiO2·xH2O sols, which exist as negative + 2x− colloids in solution due to its charged{[TiO2]mnTiO2− 3 ,2(n − x)H } weak acidity. The negative charged colloids, then, adsorb onto the surface of the positive charged CTA+ micelles by the electrostatic interactions. Thus, TiO2·xH2O hollow structures are produced on the surface of the CTA+ micelles after reaching equilibrium conditions. Meanwhile, Ag+ ions are reduced to metal silver. After hydrothermal crystallization, Ag-TiO2 hollow structures are obtained.
Fig. 2. Schematic diagram of the formation mechanism of the hollow structures.
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C. Song et al. / Materials Letters 65 (2011) 908–910
is shown in the inset of Fig. 3. It can be seen that the Ag-TiO2 hollow structures exhibit much enhanced photocatalytic activities in comparison with the TiO2 microstructures. Such significant improvement of photocatalytic activity of Ag-TiO2 hollow structures can be attributed to the more effective electron-hole separation of the AgTiO2 composite structures [1,2]. 4. Conclusion We have developed a one-pot procedure for facile synthesis Ag-TiO2 composite hollow structures with the assistance of surfactant CTAB. The Ag-TiO2 hollow structures show high photocatlytic activity to MO, which makes it a promising candidate material for environmental applications. Acknowledgement
Fig. 3. Variation of the UV–vis spectra for MO solution in the presence of Ag-TiO2 hollow structures at different intervals. The inset is the comparison of photocatalytic activities of different samples.
This work was financially supported by the Natural Science Found of Shandong Province (ZR2009BL018) and the Foundation of Education Department of Shandong Province (J10LB11). References
A series of experiments have been carried out to investigate the formation mechanism. For example, to confirm the templating role of CTAB, the experiments are carried out in presence of different dose of CTAB. SEM observation indicates that less hollow structures are obtained with addition of less CTAB. Experimental results also show H2O2 plays an important role in the formation of the hollow structures. It is expected that the formation of [TiO(H2O2)]2+ complex decreases the hydrolysis ratio of Ti(IV), which makes it possible to adsorb the TiO2·xH2O colloids on the surface of CTAB micelles. If no H2O2 is added, SEM observation shows that irregular TiO2 aggregates are obtained. The photocatalytic activities of the Ag-TiO2 samples were evaluated by the degradation of methyl orange (MO) in an aqueous solution. Fig. 3 shows the variation in the UV–Vis spectra of MO at different irradiation intervals. It can be clearly seen that the maximum absorbance at 462 nm disappears almost completely after irradiation for 60 min. Since no new peak was observed during the whole process, a degradation reaction of MO has occurred. The degradation rate of MO on commercial TiO2 catalyst, as-prepared TiO2 and Ag-TiO2
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Xin B, Jing L, Ren Z, Wang B, Fu H. J Phys Chem B 2005;109:2805–9. Hu C, Lan Y, Qu J, Hu X, Wang A. J Phys Chem B 2006;110:4066–72. Zhang DY, Yang D, Zhang HJ, Lu CH, Qi LM. Chem Mater 2006;18:3477–85. Syoufian A, Nakashima K. J Colloid Interface Sci 2007;313:213–8. Ma YR, Qi LM. J Colloid Interface Sci 2009;335:1–10. Yu JG, Liu W, Yu HG. Cryst Growth Des 2008;8:930–4. Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. Du X, He JH, Zhao YQ. J Phys Chem C 2009;113:14151–8. Wang YD, Zhou AN, Yang ZY. Mater Lett 2008;62:1930–2Inorg Chem 2008;45: 3493–5. Xuan SH, Jiang WQ, Gong XL, Hu Y, Chen ZY. J Phys Chem C 2009;113:553–8. Zhou JK, Lv L, Yu JQ, Li HL, Guo PZ, Sun H, Zhao XS. J Phys Chem C 2008;112: 5316–21. Koh JK, Seo JA, Koh JH, Kim JH. Mater Lett 2009;63:1360–2. Liu WJ, Zhao XM, Cao LY, Zou B, Zhang G, Lu LH, Cui HN. Mater Lett 2009;63: 2456–8. Lai YK, Chen YC, Zhuang HF, Lin CJ. Mater Lett 2008;62:3688–90. Song CX, Wang DB, Gu GH, Lin YS, Yang JY, Chen L, Fu X, Hu ZS. J Colloid Interface Sci 2004;272:340–4. Cozzoli PD, Comparelli R, Fanizza E, Curri ML, Agostiano A, Laub D. J Am Chem Soc 2004;126:3868–79. Guin D, Manorama SV, Latha JNL, Singh S. J Phys Chem C 2007;111:13393–7. Li F, Li GZ, Wang HQ, Xue QJ. Colloids Surf A 1997;127:89–96. Zhang W, Li G, Mu J, Shen Q, Zheng L, Liang H, Wu C. Chin Sci Bull 2000;45:1854–7.