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TiO2 photocatalyst by ball milling
Materials Chemistry and Physics 98 (2006) 116–120
The preparation of coupled SnO2/TiO2 photocatalyst by ball milling Chen Shifu a,c,∗ , Chen Lei b , Gao Shen b , Cao Gengyu c a
Department of Chemistry, Huaibei Coal Normal College, Anhui, Huaibei 235000, People’s Republic of China Department of Physics, University of Science and Technology of China, Anhui, Hefei 230026, People’s Republic of China c Department of Chemical Physics, University of Science and Technology of China, Anhui, Hefei 230026, People’s Republic of China b
Received 30 May 2004; received in revised form 21 August 2005; accepted 31 August 2005
Abstract The coupled photocatalyst SnO2 /TiO2 is prepared by ball milling through doping SnO2 into TiO2 and using H2 O solution as disperser. The coupled photocatalyst SnO2 /TiO2 is characterized by the UV–vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The results show that the optimum percentage of SnO2 doped is 5 wt.% and that the photocatalytic activity of the coupled SnO2 /TiO2 photocatalyst is much higher than that of TiO2 and SnO2 –TiO2 without ball milling. Compared with TiO2 , the photoexcited wavelength range of the coupled SnO2 /TiO2 red shifts of about 20 nm and the light absorption intensity is also improved. The crystal phase of TiO2 is not changed and new crystal phases are not found by ball milling. SnO2 and TiO2 coupled highly, forming the SnO2 /TiO2 photocatalyst. The increased photocatalytic activity of the coupled photocatalyst may be attributed to the enhance charge separation efficiency and extend the wavelength range of photoexcitation. © 2005 Elsevier B.V. All rights reserved. Keywords: Coupled photocatalyst; SnO2 /TiO2 ; Semiconductors; Characterization
1. Introduction In recent years, photocatalytic degradation of various kinds of organic and inorganic pollutants using semiconductor powders as photocatalysts has been extensively studied [1,2]. Owing to its relatively high photocatalytic activity, biological and chemical stability, low cost, nonpoisonous and long stable life, TiO2 has been used widely as a photocatalyst [3–5]. However, the photocatalytic activity of TiO2 (the band gap is 3.2 eV and it can be excited by photons with wavelengths below 387 nm) is limited to irradiation wavelengths in the UV region, so that the effective utilization of solar energy is limited to about 3–5% of the total solar spectrum. Also, some problems still exist in its application, like the fast recombination of photogenerated electron–hole pairs. Therefore, improving photocatalytic activity by modification has become a hot topic among researchers in recent years [6,7]. The study of composite photocatalysts is one of the approaches [8,9]. Now, the improved sol–gel method and co-precipitation method [10–12] are used to prepare composite semiconductor–semiconductor photocatalysts
∗
Corresponding author. Tel.: +86 561 3803279; fax: +86 561 3803141. E-mail address: [email protected] (C. Shifu).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.073
such as TiO2 -CdS, TiO2 -SnO2 , TiO2 -WO3 , TiO2 -ZnO, etc. And the SnO2 /TiO2 photocatalyst prepared by chemical methods has been studied extensively [13–15]. The results show that nearly all the composite semiconductors have presented higher photocatalytic activity than single ones. In this study, we use H2 O as disperser and dope a small amount of SnO2 intoTiO2 and prepare coupled SnO2 /TiO2 photocatalysts by ball milling. The photocatalyst is characterized by the UV–vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The mechanisms of increasing photocatalytic activity are also discussed. 2. Experimental 2.1. Materials and characterization The TiO2 (100% anatase, with the crystallite size of about 30 nm), used in the experiments, is prepared by thermal decomposition and calcinations of colloidal solution made from hydrolysis of titanium tetra-iso-propoxide [Ti(isoOC3 H7 )4 ] in our laboratory, SnO2 (with the crystallite size of about 20 nm) is also prepared by using SnCl4 ·5H2 O as the starting material. Other chemicals used in the experiments are of analytically pure grade. Deionized and doubly distilled water is used throughout this study. Monocrotophos is supplied by Qingdao Pesticides Factory (purity 99.9%). For reference, the structure of monocrotophos
C. Shifu et al. / Materials Chemistry and Physics 98 (2006) 116–120 is shown below:
The photodegradation efficiency of monocrotophos is calculated by the following equation: η=
UV–vis diffuse reflectance spectroscopy measurement is carried out using a Hitachi UV-365 spectrophotometer equipped with an integrating sphere attachment. The analyzed range is 240–600 nm, and BaSO4 is used as a reflectance standard. In order to determine the crystal phase and the crystallite size of the photocatalysts, X-ray diffraction measurement is carried out at room temperature using a Philips MPD 18801 X-ray powder diffraction spectroscope with Cu K␣ radiation and the scanning speed of 3◦ min−1 . The accelerating voltage and emission current are 35 kV and 20 mA, respectively. The crystallite size is calculated by X-ray line broadening analysis using the Scherrer equation. The crystallite size and shape of the photocatalysts are also observed by using (H-800 Instrument, Japan) transmission electron microscopy. X-ray photoelectron spectroscopic examination is carried out on the ESCALABMKII multifunctional spectrometer (VG Scientific, England) using Al K␣ radiation. The irradiation powers of the monochromatic light are measured by using a UV radiometer. The monochromatic lights are made by monochromatic optical filter.
2.2. The preparation of coupled SnO2 /TiO2 photocatalyst The preparation of the SnO2 /TiO2 photocatalyst is carried out in a QM1F ball miller (made in Nanjing University). TiO2 powder and agate ball are mixed in the agate ball milling tank with a ratio of 1:10, and then a certain amount of SnO2 and H2 O are added. After being milled for a certain time at a speed of 300 rpm, the wet powder is dried at a temperature of 110 ◦ C in air. The final samples are used for the determination of photocatalytic activity and characterization.
2.3. Photoreactor and procedure Experiments are carried out in a photochemical quartz reactor. The cylindrical annular-type of reactor consists of three parts [16]. The first part is an empty chamber in which a 375 W medium pressure mercury lamp (Institute of Electric Light Source, Beijing) with a maximum emission at about 365 nm is hung. The second part is an inside thimble with running water passing through it to cool the reaction solution. Owing to the continuous cooling, the temperature of the reaction solution is maintained at approximately 30 ◦ C. The third part is an outside thimble (volume, 400 cm3 ). At the start of the experiment, the reaction solution containing monocrotophos and photocatalyst is put in the thimble. Air is introduced into the reaction solution through the gas entry at the base of the reactor. In order to determine the photocatalytic activity of the samples under the illumination at different wavelengths (UV and visible light), the light source is fixed at the top of the outside thimble wrapped by black paper. The irradiation area of monochromatic light is of about 12.8 cm2 . The distance between the light source and the surface of the reaction solution is 11 cm. In the experiments, an initial pH of the reaction solution is 5.0; flow rate of air is 0.01 m3 h−1 ; the amount of the photocatalyst used is 2.0 g L−1 ; the initial concentration of monocrotophos is 1.0 × 10−4 mol L−1 . In order to disperse the photocatalyst powder, the suspensions are ultrasonically vibrated for 20 min prior to irradiation. After the illumination, samples (volume of each sample is 5 cm3 ) are taken from the reaction suspension and centrifuged at 7000 rpm for 10 min, and filtered through a 0.2 m millipore filter to remove the particles. The filtrate is then analyzed.
2.4. The activity evaluation of the photocatalysts
117
Pt × 100% Po
where η is the photodegradation efficiency, Pt the amount of phosphate in solution after illumination time t and Po is the total amount of organophosphate in solution before illumination.
3. Results and discussion 3.1. Evaluation of the photocatalytic activity The fixed illumination time of each experiment is 10 min, and the ball milling time of each sample is 12 h. The photocatalytic activities of SnO2 /TiO2 photocatalysts prepared with or without ball milling with different contents of doped-SnO2 are shown in Table 1. From Table 1, it can be seen that for the pure TiO2 photocatalyst, the photocatalytic activity increases slowly by ball milling. The reason is that with the increase in the ball milling time, the specific surface area of the TiO2 increases (TiO2 , without and with being ball milled for 12 h, the specific surface area are 46.7 m2 g−1 and 49.8 m2 g−1 , respectively). And without the condition of ball milling, the photocatalytic activities of the photocatalysts have no remarkable enhancement and the activities of the samples are even lower than that of pure TiO2 photocatalyst. However, with ball milling, the photocatalytic activities of the SnO2 /TiO2 photocatalysts increase with increasing amount of doped-SnO2 up to 5 wt.%. From the results, it is known that without ball milling, TiO2 and SnO2 only play their own photocatalytic role, and that coupled photocatalysts are not formed, but after ball milling, SnO2 and TiO2 can form coupled SnO2 /TiO2 photocatalysts. In Refs. [12,14], the coated coupled SnO2 /TiO2 photocatalyst prepared by the sol–gel method also show that the activity of the photocatalyst increases with increasing the amount of doped-SnO2 , but it will decrease remarkably with doped-SnO2 higher than a certain amount. Under this experimental condition, the optimum percentage of SnO2 doped is 5 wt.%. If the amount of doped-SnO2 is greater than 5 wt.%, the photocatalytic activities of the coupled SnO2 /TiO2 photocatalysts decrease obviously. It is proposed that, when the amount of SnO2 is lower than its optimum amount of coupling, with the increase of the amount of SnO2 , both SnO2 and TiO2 can form the coupled SnO2 /TiO2 photocatalysts by ball milling. Therefore, the photocatalytic activity increases. But when the amount of SnO2 is higher than its optimum amount of coupling, the photocatalytic activity decreases because the photocatalytic activity of SnO2 is lower than that of TiO2 . Table 1 The activity of SnO2 /TiO2 photocatalyst SnO2 content (wt.%)
The photocatalyst activity is determined by the photocatalytic degradation efficiency of monocrotophos. One of the final degradation products of monocrotophos is PO4 3− , its formation rate can express the rate of complete degradation of monocrotophos. The determination of PO4 3− is performed colorimetrically by the molybdenum blue method.
η1 (%) η2 (%)
0
1
3
5
10
27.6 28.1
27.6 35.0
26.3 39.7
25.6 43.6
24.3 40.1
Experimental conditions: η1 without ball milling and η2 with ball milling.
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It is known from the heterogeneous photocatalytic mechanisms that the quantum efficiency of the photocatalytic reaction depends on two key processes: one is the competition between the recombination and capture of photogenerated electrons and holes; the other is the competition between the recombination of captured electrons and holes and interface charge transfer. The lengthening recombination time of electrons and holes and increasing interface electron transfer rate are both favorable for the enhancement of quantum efficiency. Therefore, in order to increase the quantum efficiency of photocatalytic reaction, the recombination of the photogenerated electron–hole pairs must be decreased. It is known that the band gaps of SnO2 and TiO2 are 3.8 eV and 3.2 eV, respectively. TiO2 can be excited by photons with wavelengths below 387 nm, which produces photogenerated electron hole pairs and show photocatalytic activity. For SnO2 , theoretically, it can be excited by photons with the wavelengths below 326 nm, but it shows only low photocatalytic activity under UV light in the present experimental conditions. But when SnO2 and TiO2 form a coupled photocatalyst, as the electric potential of SnO2 conduction band is about ECB = 0.45 eV and TiO2 about −0.5 eV (the potential is relative to hydrogen electrode, pH 1), the CB of SnO2 is lower than that of TiO2 , the former can act as a sink for the photogenerated electrons. When TiO2 and SnO2 are excited simultaneously under UV illumination, the photogenerated electrons of the TiO2 conduction band will be transferred to the conduction band of SnO2 . Since the holes move in the opposite direction from the electrons and at the same time photogenerated holes might be trapped within the TiO2 particle, which make charge separation more efficient, resulting in the SnO2 /TiO2 coupled photocatalyst exhibiting even higher photocatalytic activity than that of TiO2 . When TiO2 is only excited by UV, the electron–hole pairs are produced on the TiO2 surface, photogenerated electrons will be transferred toward SnO2 conduction band, while holes will remain in the valence-band of TiO2 and then be captured to take part in the reaction, which make charge separation more efficient, and thus the SnO2 /TiO2 coupled photocatalyst shows higher photocatalytic activity. At the same time, the coupled SnO2 /TiO2 photocatalyst, compared with TiO2 , the absorption wavelength range red shifts of about 20 nm, and it is the other important reason for the enhancement of the photocatalytic activity. In order to determine the photocatalytic activity of the samples under illumination at different wavelengths (UV and visible light), monochromatic lights of 365 nm and 405 nm are chosen to represent the different wavelengths. The irradiation intensities of 365 nm and 405 nm are 0.16 mW cm−2 and 0.083 mW cm−2 , respectively. The results show that after illumination for 4 h at 365 nm, the photodegradation efficiencies of the TiO2 and the SnO2 /TiO2 (the amount of doped-SnO2 is 5 wt.%) are 17.6% and 36.7%, respectively. And after illumination for 6 h at 405 nm, the photodegradation efficiencies are 0.97% and 9.6%, respectively. The photocatalytic activities of the SnO2 /TiO2 are shown to be superior to that of the TiO2 in both UV and visible ranges of irradiation.
Fig. 1. XRD patterns of SnO2 /TiO2 and TiO2 : (a) TiO2 and (b) SnO2 (5 wt.%)/TiO2 .
3.2. Characterization of SnO2 /TiO2 photocatalysts The results show that when the amount of SnO2 doped is less, the diffraction peaks of SnO2 cannot be found in XRD patterns. This illustrates that SnO2 is highly dispersed in the bulk phase of the catalyst. When the amount of SnO2 doped is high, the diffraction peaks of SnO2 can be found in XRD patterns. Since no new crystal phases are found, it can be concluded that a new solid is not formed in the ball milling process of TiO2 and SnO2 . The XRD patterns of the SnO2 /TiO2 and the TiO2 are shown in Fig. 1, the amount of doped-SnO2 is 5 wt.%, “+” in the figure stands for diffraction peaks of SnO2 . It is known from the calculation of the Scherrer equation that the diameter of the photocatalyst is not obviously changed. The crystallite size is of about 30 nm. The result is the same as that of TEM. Fig. 2 shows TEM images of before and after ball milling photocatalysts. The crystallites are spherical particles with a crystallite size of about 20–30 nm. Fig. 3 shows UV–vis diffuse reflection spectra of SnO2 /TiO2 and TiO2 , respectively. From Fig. 3, it can be seen that absorption wavelength range of the coupled photocatalyst red shifts of about 20 nm, compared with pure TiO2 photocatalyst, and its absorption intensity is also increased. Because of the absorption wavelength range red shifts of about 20 nm and absorption intensity increases, the formation rate of electron–hole pairs on the photocatalyst surface also increases greatly, resulting in the coupled photocatalyst SnO2 /TiO2 exhibiting higher photocatalytic activity. The reason for the absorption wavelength range red shifts of the SnO2 /TiO2 could probably be attributed to the formation of defect energy level in the particles during highenergy ball milling process. In the process of ball milling, the crystal lattices of the TiO2 and SnO2 undergo severe plastic deformation, producing stresses and strains. This creates a crystal lattice distortion, while at the same time forming many defects inside particles. These defects have high lattice distortion energy and surface energy. This makes the activation energy for diffusion of elements decrease markedly, and allows for atomic or ionic marked diffusion among elements at room temperature. When the activity of the powder system is high enough, during the ball milling
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Fig. 2. TEM patterns of photocatalysts: (a) without ball milling and (b) with ball milling.
process, the collision between balls and grains of the powder will produce a rise in the interface temperature, which will induce the coupling reaction mentioned here. So, when the interfaces of the TiO2 and SnO2 contact with each other, the coupled photocatalyst SnO2 /TiO2 can be formed. Fig. 4 shows XPS spectra of the SnO2 /TiO2 and high resolution scanning XPS spectra of O 1s. From XPS spectra of the SnO2 /TiO2, it can be seen that the sample surface is composed of Ti, O, C and Sn, which means that SnO2 and TiO2 are highly and evenly mixed, and that the SnO2 /TiO2 photocatalyst has coupled. The photoelectron peak for Ti 2p appears clearly at the binding energy of 458 eV and C 1s at 284 eV. Here, C is generated from C compounds adsorbed by samples from air and oily dirt from apparatuses. The photoelectron peaks of O 1s and Sn 3d are at the binding energy of 529 eV and 486 eV, respectively. From high resolution scanning XPS spectra of O 1s in Fig. 4, it can be seen that oxygen on the sample surface exists at least in three forms: at the binding energy of 529.30 eV corresponds to oxygen in TiO2 lattice; 531.40 eV corresponds to oxygen in TiO2 surface adsorption of (–OH); 532.90 eV corresponds to oxygen in TiO2 surface adsorption of H2 O. It is just because so much adsorption oxygen exists on the TiO2 sur-
Fig. 4. XPS spectra of SnO2 /TiO2 and high resolution scanning XPS spectra of O 1s.
face, that they become captives of photogenerated electron–hole pairs directly or indirectly. So that recombination of the photogenerated electron–hole pairs is suppressed and, therefore, the quantum efficiency of photocatalytic reaction is improved. This may be other reason for the enhancement of the photocatalytic activity. 4. Conclusions Fig. 3. UV–vis diffuse reflection spectrum: (a) TiO2 ball milling in H2 O and (b) SnO2 (5 wt.%)/TiO2 .
The coupled SnO2 /TiO2 photocatalyst with higher photocatalytic activity is prepared by ball milling. Compared with TiO2 ,
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the photoexcited wavelength range of the coupled SnO2 /TiO2 red shifts of about 20 nm and the light absorption intensity is also improved. The crystal phase of TiO2 is not changed and the new crystal phases are not found by ball milling. The optimum percentage of SnO2 doped is 5 wt.%. The increased photocatalytic activity of the coupled photocatalyst may be attributed to the enhancement of the charge separation efficiency and the extension of the wavelength range of photoexcitation. Acknowledgements This work is supported by the Natural Science Foundation of Anhui Province (Contract No. 01045304) and the Natural Science Foundation of Education Committee of Anhui Province (Contract No. 2005Kj018ZD). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69.
[2] S.U.M. Khan, M. Al-shahry, W.B. Lngler Jr., Science 297 (2002) 2243. [3] T. Zhang, T. Oyama, S. Horikoshi, J. Zhao, N. Serpone, H. Hidaka, Appl. Catal. B: Environ. 42 (2003) 13. [4] N. San, A. Hatipoglu, G. Kocturk, Z. Cinar, J. Photochem. Photobiol. A: Chem. 146 (2002) 189. [5] S.F. Chen, G.Y. Cao, Chemosphere 60 (2005) 1308. [6] A.K. Axelsson, L.J. Dunne, J. Photochem. Photobiol. A: Chem. 144 (2001) 205. [7] J. Yu, J.C. Yu, M.K.P. Leung, W. Ho, J. Catal. 217 (2003) 69. [8] S.F. Chen, X. Liang, M.Y. Zhao, Photographic Sci. Photochem. 17 (1999) 85 (in Chinese). [9] S.F. Chen, G.Y. Cao, Solar Energy 73 (2002) 15. [10] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [11] C. Wang, J.C. Zhao, X. Wang, B.X. Mai, G.Y. Sheng, P.A. Peng, J.M. Fu, Appl. Catal. B: Environ. 39 (2002) 269. [12] L.Y. Shi, C.Z. Li, J. Chem. Phys. 13 (2000) 336 (in Chinese). [13] N. Kanai, T. Nuika, K. Ueta, K. Hashimoto, T. Watanabe, H. Ohsaki, Vacuum 74 (2004) 723. [14] J. Shang, W. Yao, Y. Zhu, N. Wu, Appl. Catal. A: Gen. 257 (2004) 25. [15] S. Pilkenton, D. Raftery, Solid State Nucl. Magn. Reson. 24 (2003) 236. [16] S.F. Chen, X.L. Cheng, J. Chem. Technol. Biotechnol. 73 (1998) 264.
The preparation of coupled SnO2/TiO2 photocatalyst by ball milling Chen Shifu a,c,∗ , Chen Lei b , Gao Shen b , Cao Gengyu c a
Department of Chemistry, Huaibei Coal Normal College, Anhui, Huaibei 235000, People’s Republic of China Department of Physics, University of Science and Technology of China, Anhui, Hefei 230026, People’s Republic of China c Department of Chemical Physics, University of Science and Technology of China, Anhui, Hefei 230026, People’s Republic of China b
Received 30 May 2004; received in revised form 21 August 2005; accepted 31 August 2005
Abstract The coupled photocatalyst SnO2 /TiO2 is prepared by ball milling through doping SnO2 into TiO2 and using H2 O solution as disperser. The coupled photocatalyst SnO2 /TiO2 is characterized by the UV–vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The results show that the optimum percentage of SnO2 doped is 5 wt.% and that the photocatalytic activity of the coupled SnO2 /TiO2 photocatalyst is much higher than that of TiO2 and SnO2 –TiO2 without ball milling. Compared with TiO2 , the photoexcited wavelength range of the coupled SnO2 /TiO2 red shifts of about 20 nm and the light absorption intensity is also improved. The crystal phase of TiO2 is not changed and new crystal phases are not found by ball milling. SnO2 and TiO2 coupled highly, forming the SnO2 /TiO2 photocatalyst. The increased photocatalytic activity of the coupled photocatalyst may be attributed to the enhance charge separation efficiency and extend the wavelength range of photoexcitation. © 2005 Elsevier B.V. All rights reserved. Keywords: Coupled photocatalyst; SnO2 /TiO2 ; Semiconductors; Characterization
1. Introduction In recent years, photocatalytic degradation of various kinds of organic and inorganic pollutants using semiconductor powders as photocatalysts has been extensively studied [1,2]. Owing to its relatively high photocatalytic activity, biological and chemical stability, low cost, nonpoisonous and long stable life, TiO2 has been used widely as a photocatalyst [3–5]. However, the photocatalytic activity of TiO2 (the band gap is 3.2 eV and it can be excited by photons with wavelengths below 387 nm) is limited to irradiation wavelengths in the UV region, so that the effective utilization of solar energy is limited to about 3–5% of the total solar spectrum. Also, some problems still exist in its application, like the fast recombination of photogenerated electron–hole pairs. Therefore, improving photocatalytic activity by modification has become a hot topic among researchers in recent years [6,7]. The study of composite photocatalysts is one of the approaches [8,9]. Now, the improved sol–gel method and co-precipitation method [10–12] are used to prepare composite semiconductor–semiconductor photocatalysts
∗
Corresponding author. Tel.: +86 561 3803279; fax: +86 561 3803141. E-mail address: [email protected] (C. Shifu).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.073
such as TiO2 -CdS, TiO2 -SnO2 , TiO2 -WO3 , TiO2 -ZnO, etc. And the SnO2 /TiO2 photocatalyst prepared by chemical methods has been studied extensively [13–15]. The results show that nearly all the composite semiconductors have presented higher photocatalytic activity than single ones. In this study, we use H2 O as disperser and dope a small amount of SnO2 intoTiO2 and prepare coupled SnO2 /TiO2 photocatalysts by ball milling. The photocatalyst is characterized by the UV–vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The mechanisms of increasing photocatalytic activity are also discussed. 2. Experimental 2.1. Materials and characterization The TiO2 (100% anatase, with the crystallite size of about 30 nm), used in the experiments, is prepared by thermal decomposition and calcinations of colloidal solution made from hydrolysis of titanium tetra-iso-propoxide [Ti(isoOC3 H7 )4 ] in our laboratory, SnO2 (with the crystallite size of about 20 nm) is also prepared by using SnCl4 ·5H2 O as the starting material. Other chemicals used in the experiments are of analytically pure grade. Deionized and doubly distilled water is used throughout this study. Monocrotophos is supplied by Qingdao Pesticides Factory (purity 99.9%). For reference, the structure of monocrotophos
C. Shifu et al. / Materials Chemistry and Physics 98 (2006) 116–120 is shown below:
The photodegradation efficiency of monocrotophos is calculated by the following equation: η=
UV–vis diffuse reflectance spectroscopy measurement is carried out using a Hitachi UV-365 spectrophotometer equipped with an integrating sphere attachment. The analyzed range is 240–600 nm, and BaSO4 is used as a reflectance standard. In order to determine the crystal phase and the crystallite size of the photocatalysts, X-ray diffraction measurement is carried out at room temperature using a Philips MPD 18801 X-ray powder diffraction spectroscope with Cu K␣ radiation and the scanning speed of 3◦ min−1 . The accelerating voltage and emission current are 35 kV and 20 mA, respectively. The crystallite size is calculated by X-ray line broadening analysis using the Scherrer equation. The crystallite size and shape of the photocatalysts are also observed by using (H-800 Instrument, Japan) transmission electron microscopy. X-ray photoelectron spectroscopic examination is carried out on the ESCALABMKII multifunctional spectrometer (VG Scientific, England) using Al K␣ radiation. The irradiation powers of the monochromatic light are measured by using a UV radiometer. The monochromatic lights are made by monochromatic optical filter.
2.2. The preparation of coupled SnO2 /TiO2 photocatalyst The preparation of the SnO2 /TiO2 photocatalyst is carried out in a QM1F ball miller (made in Nanjing University). TiO2 powder and agate ball are mixed in the agate ball milling tank with a ratio of 1:10, and then a certain amount of SnO2 and H2 O are added. After being milled for a certain time at a speed of 300 rpm, the wet powder is dried at a temperature of 110 ◦ C in air. The final samples are used for the determination of photocatalytic activity and characterization.
2.3. Photoreactor and procedure Experiments are carried out in a photochemical quartz reactor. The cylindrical annular-type of reactor consists of three parts [16]. The first part is an empty chamber in which a 375 W medium pressure mercury lamp (Institute of Electric Light Source, Beijing) with a maximum emission at about 365 nm is hung. The second part is an inside thimble with running water passing through it to cool the reaction solution. Owing to the continuous cooling, the temperature of the reaction solution is maintained at approximately 30 ◦ C. The third part is an outside thimble (volume, 400 cm3 ). At the start of the experiment, the reaction solution containing monocrotophos and photocatalyst is put in the thimble. Air is introduced into the reaction solution through the gas entry at the base of the reactor. In order to determine the photocatalytic activity of the samples under the illumination at different wavelengths (UV and visible light), the light source is fixed at the top of the outside thimble wrapped by black paper. The irradiation area of monochromatic light is of about 12.8 cm2 . The distance between the light source and the surface of the reaction solution is 11 cm. In the experiments, an initial pH of the reaction solution is 5.0; flow rate of air is 0.01 m3 h−1 ; the amount of the photocatalyst used is 2.0 g L−1 ; the initial concentration of monocrotophos is 1.0 × 10−4 mol L−1 . In order to disperse the photocatalyst powder, the suspensions are ultrasonically vibrated for 20 min prior to irradiation. After the illumination, samples (volume of each sample is 5 cm3 ) are taken from the reaction suspension and centrifuged at 7000 rpm for 10 min, and filtered through a 0.2 m millipore filter to remove the particles. The filtrate is then analyzed.
2.4. The activity evaluation of the photocatalysts
117
Pt × 100% Po
where η is the photodegradation efficiency, Pt the amount of phosphate in solution after illumination time t and Po is the total amount of organophosphate in solution before illumination.
3. Results and discussion 3.1. Evaluation of the photocatalytic activity The fixed illumination time of each experiment is 10 min, and the ball milling time of each sample is 12 h. The photocatalytic activities of SnO2 /TiO2 photocatalysts prepared with or without ball milling with different contents of doped-SnO2 are shown in Table 1. From Table 1, it can be seen that for the pure TiO2 photocatalyst, the photocatalytic activity increases slowly by ball milling. The reason is that with the increase in the ball milling time, the specific surface area of the TiO2 increases (TiO2 , without and with being ball milled for 12 h, the specific surface area are 46.7 m2 g−1 and 49.8 m2 g−1 , respectively). And without the condition of ball milling, the photocatalytic activities of the photocatalysts have no remarkable enhancement and the activities of the samples are even lower than that of pure TiO2 photocatalyst. However, with ball milling, the photocatalytic activities of the SnO2 /TiO2 photocatalysts increase with increasing amount of doped-SnO2 up to 5 wt.%. From the results, it is known that without ball milling, TiO2 and SnO2 only play their own photocatalytic role, and that coupled photocatalysts are not formed, but after ball milling, SnO2 and TiO2 can form coupled SnO2 /TiO2 photocatalysts. In Refs. [12,14], the coated coupled SnO2 /TiO2 photocatalyst prepared by the sol–gel method also show that the activity of the photocatalyst increases with increasing the amount of doped-SnO2 , but it will decrease remarkably with doped-SnO2 higher than a certain amount. Under this experimental condition, the optimum percentage of SnO2 doped is 5 wt.%. If the amount of doped-SnO2 is greater than 5 wt.%, the photocatalytic activities of the coupled SnO2 /TiO2 photocatalysts decrease obviously. It is proposed that, when the amount of SnO2 is lower than its optimum amount of coupling, with the increase of the amount of SnO2 , both SnO2 and TiO2 can form the coupled SnO2 /TiO2 photocatalysts by ball milling. Therefore, the photocatalytic activity increases. But when the amount of SnO2 is higher than its optimum amount of coupling, the photocatalytic activity decreases because the photocatalytic activity of SnO2 is lower than that of TiO2 . Table 1 The activity of SnO2 /TiO2 photocatalyst SnO2 content (wt.%)
The photocatalyst activity is determined by the photocatalytic degradation efficiency of monocrotophos. One of the final degradation products of monocrotophos is PO4 3− , its formation rate can express the rate of complete degradation of monocrotophos. The determination of PO4 3− is performed colorimetrically by the molybdenum blue method.
η1 (%) η2 (%)
0
1
3
5
10
27.6 28.1
27.6 35.0
26.3 39.7
25.6 43.6
24.3 40.1
Experimental conditions: η1 without ball milling and η2 with ball milling.
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It is known from the heterogeneous photocatalytic mechanisms that the quantum efficiency of the photocatalytic reaction depends on two key processes: one is the competition between the recombination and capture of photogenerated electrons and holes; the other is the competition between the recombination of captured electrons and holes and interface charge transfer. The lengthening recombination time of electrons and holes and increasing interface electron transfer rate are both favorable for the enhancement of quantum efficiency. Therefore, in order to increase the quantum efficiency of photocatalytic reaction, the recombination of the photogenerated electron–hole pairs must be decreased. It is known that the band gaps of SnO2 and TiO2 are 3.8 eV and 3.2 eV, respectively. TiO2 can be excited by photons with wavelengths below 387 nm, which produces photogenerated electron hole pairs and show photocatalytic activity. For SnO2 , theoretically, it can be excited by photons with the wavelengths below 326 nm, but it shows only low photocatalytic activity under UV light in the present experimental conditions. But when SnO2 and TiO2 form a coupled photocatalyst, as the electric potential of SnO2 conduction band is about ECB = 0.45 eV and TiO2 about −0.5 eV (the potential is relative to hydrogen electrode, pH 1), the CB of SnO2 is lower than that of TiO2 , the former can act as a sink for the photogenerated electrons. When TiO2 and SnO2 are excited simultaneously under UV illumination, the photogenerated electrons of the TiO2 conduction band will be transferred to the conduction band of SnO2 . Since the holes move in the opposite direction from the electrons and at the same time photogenerated holes might be trapped within the TiO2 particle, which make charge separation more efficient, resulting in the SnO2 /TiO2 coupled photocatalyst exhibiting even higher photocatalytic activity than that of TiO2 . When TiO2 is only excited by UV, the electron–hole pairs are produced on the TiO2 surface, photogenerated electrons will be transferred toward SnO2 conduction band, while holes will remain in the valence-band of TiO2 and then be captured to take part in the reaction, which make charge separation more efficient, and thus the SnO2 /TiO2 coupled photocatalyst shows higher photocatalytic activity. At the same time, the coupled SnO2 /TiO2 photocatalyst, compared with TiO2 , the absorption wavelength range red shifts of about 20 nm, and it is the other important reason for the enhancement of the photocatalytic activity. In order to determine the photocatalytic activity of the samples under illumination at different wavelengths (UV and visible light), monochromatic lights of 365 nm and 405 nm are chosen to represent the different wavelengths. The irradiation intensities of 365 nm and 405 nm are 0.16 mW cm−2 and 0.083 mW cm−2 , respectively. The results show that after illumination for 4 h at 365 nm, the photodegradation efficiencies of the TiO2 and the SnO2 /TiO2 (the amount of doped-SnO2 is 5 wt.%) are 17.6% and 36.7%, respectively. And after illumination for 6 h at 405 nm, the photodegradation efficiencies are 0.97% and 9.6%, respectively. The photocatalytic activities of the SnO2 /TiO2 are shown to be superior to that of the TiO2 in both UV and visible ranges of irradiation.
Fig. 1. XRD patterns of SnO2 /TiO2 and TiO2 : (a) TiO2 and (b) SnO2 (5 wt.%)/TiO2 .
3.2. Characterization of SnO2 /TiO2 photocatalysts The results show that when the amount of SnO2 doped is less, the diffraction peaks of SnO2 cannot be found in XRD patterns. This illustrates that SnO2 is highly dispersed in the bulk phase of the catalyst. When the amount of SnO2 doped is high, the diffraction peaks of SnO2 can be found in XRD patterns. Since no new crystal phases are found, it can be concluded that a new solid is not formed in the ball milling process of TiO2 and SnO2 . The XRD patterns of the SnO2 /TiO2 and the TiO2 are shown in Fig. 1, the amount of doped-SnO2 is 5 wt.%, “+” in the figure stands for diffraction peaks of SnO2 . It is known from the calculation of the Scherrer equation that the diameter of the photocatalyst is not obviously changed. The crystallite size is of about 30 nm. The result is the same as that of TEM. Fig. 2 shows TEM images of before and after ball milling photocatalysts. The crystallites are spherical particles with a crystallite size of about 20–30 nm. Fig. 3 shows UV–vis diffuse reflection spectra of SnO2 /TiO2 and TiO2 , respectively. From Fig. 3, it can be seen that absorption wavelength range of the coupled photocatalyst red shifts of about 20 nm, compared with pure TiO2 photocatalyst, and its absorption intensity is also increased. Because of the absorption wavelength range red shifts of about 20 nm and absorption intensity increases, the formation rate of electron–hole pairs on the photocatalyst surface also increases greatly, resulting in the coupled photocatalyst SnO2 /TiO2 exhibiting higher photocatalytic activity. The reason for the absorption wavelength range red shifts of the SnO2 /TiO2 could probably be attributed to the formation of defect energy level in the particles during highenergy ball milling process. In the process of ball milling, the crystal lattices of the TiO2 and SnO2 undergo severe plastic deformation, producing stresses and strains. This creates a crystal lattice distortion, while at the same time forming many defects inside particles. These defects have high lattice distortion energy and surface energy. This makes the activation energy for diffusion of elements decrease markedly, and allows for atomic or ionic marked diffusion among elements at room temperature. When the activity of the powder system is high enough, during the ball milling
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Fig. 2. TEM patterns of photocatalysts: (a) without ball milling and (b) with ball milling.
process, the collision between balls and grains of the powder will produce a rise in the interface temperature, which will induce the coupling reaction mentioned here. So, when the interfaces of the TiO2 and SnO2 contact with each other, the coupled photocatalyst SnO2 /TiO2 can be formed. Fig. 4 shows XPS spectra of the SnO2 /TiO2 and high resolution scanning XPS spectra of O 1s. From XPS spectra of the SnO2 /TiO2, it can be seen that the sample surface is composed of Ti, O, C and Sn, which means that SnO2 and TiO2 are highly and evenly mixed, and that the SnO2 /TiO2 photocatalyst has coupled. The photoelectron peak for Ti 2p appears clearly at the binding energy of 458 eV and C 1s at 284 eV. Here, C is generated from C compounds adsorbed by samples from air and oily dirt from apparatuses. The photoelectron peaks of O 1s and Sn 3d are at the binding energy of 529 eV and 486 eV, respectively. From high resolution scanning XPS spectra of O 1s in Fig. 4, it can be seen that oxygen on the sample surface exists at least in three forms: at the binding energy of 529.30 eV corresponds to oxygen in TiO2 lattice; 531.40 eV corresponds to oxygen in TiO2 surface adsorption of (–OH); 532.90 eV corresponds to oxygen in TiO2 surface adsorption of H2 O. It is just because so much adsorption oxygen exists on the TiO2 sur-
Fig. 4. XPS spectra of SnO2 /TiO2 and high resolution scanning XPS spectra of O 1s.
face, that they become captives of photogenerated electron–hole pairs directly or indirectly. So that recombination of the photogenerated electron–hole pairs is suppressed and, therefore, the quantum efficiency of photocatalytic reaction is improved. This may be other reason for the enhancement of the photocatalytic activity. 4. Conclusions Fig. 3. UV–vis diffuse reflection spectrum: (a) TiO2 ball milling in H2 O and (b) SnO2 (5 wt.%)/TiO2 .
The coupled SnO2 /TiO2 photocatalyst with higher photocatalytic activity is prepared by ball milling. Compared with TiO2 ,
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the photoexcited wavelength range of the coupled SnO2 /TiO2 red shifts of about 20 nm and the light absorption intensity is also improved. The crystal phase of TiO2 is not changed and the new crystal phases are not found by ball milling. The optimum percentage of SnO2 doped is 5 wt.%. The increased photocatalytic activity of the coupled photocatalyst may be attributed to the enhancement of the charge separation efficiency and the extension of the wavelength range of photoexcitation. Acknowledgements This work is supported by the Natural Science Foundation of Anhui Province (Contract No. 01045304) and the Natural Science Foundation of Education Committee of Anhui Province (Contract No. 2005Kj018ZD). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69.
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