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Nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 powders synthesized by a low temperature molten salt method
Materials Letters 59 (2005) 3059 – 3061 www.elsevier.com/locate/matlet
Nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 powders synthesized by a low temperature molten salt method Hua Tian a, Junfeng Ma a,b,*, Xiang Huang b, Lijin Xie a, Zhongqiang Zhao a, Jun Zhou a, Pingwei Wu b, Jinhui Dai b, Yingmo Hu b, Zhibin Zhu b, Hongfeng Wang b, Haiyan Chen b a
College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China Institute of Materials Sciences and Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China
b
Received 22 December 2004; accepted 24 May 2005 Available online 6 June 2005
Abstract Nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 powders were successfully synthesized by a low temperature molten salt method, where the precursor, a homogeneous precipitate containing titanium and tin cations was mixed with LiNO3 salt, nano-sized (Sn0.25,Ti0.75)O2 powders could be synthesized at 260 -C. The powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV – VIS, respectively. Experimental results showed that the product prepared was a pure phase (Sn0.25,Ti0.75)O2 of rutile structure without any other imparities, and composed of nano-sized and spherical particles of about 10 nm. In addition, the UV – VIS showed that the synthesized powders had a good UV absorption and visible light response. D 2005 Elsevier B.V. All rights reserved. Keywords: Coupled oxides; Nano-sized photocatalyst; Low temperature molten salt method; Titanium oxide; Tin oxide
1. Introduction SnO2 – TiO2 coupled oxide has received considerable attention as gas sensor [1– 4] and varistor ceramic [5], which is mainly due to the transport properties and electronic structure of the (Sn,Ti)O2 system [6 – 8]. In addition, SnO2 – TiO2 coupled photocatalyst is well known to be one of the most effective photocatalysts, and its photocatalytic behavior has been studied extensively in recent years [9– 12]. The relatively high photocatalytic activity of SnO2 –TiO2 coupled oxide should be attributed to a better charge separation due to fast electron transfer from TiO2 to SnO2. The proper placement of the individual semiconductor is essential for the charge separation to ensure high photo-
* Corresponding author. College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China. Tel.: +86 532 2031623; fax: +86 532 2031623. E-mail address: [email protected] (J. Ma). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.020
catalytic activity [9]. In general, the solid solution (Sn,Ti)O2 is of avail to high photocatalytic activity. Since in the solid solution, the photogenerated electron – hole pairs are effectively separated, these photocatalysts have a lower electron – hole recombination rate as compared with rutile TiO2. This lower recombination rate can be related to an increase in the bandgap of the solid solution. The elevation of the semiconductor’s bandgap causes an increase in its photocatalytic oxidation – reduction potential, which is the origin of the enhanced photocatalytic activity of (Sn,Ti)O2 [10 –17]. In addition, smaller crystalline size, better symmetrical and spherical crystallize mean better photocatalytic behavior [18]. SnO2 – TiO2 coupled oxides have been synthesized by various methods: the solid-state reaction [3], CVD [10], homogeneous precipitation [12]. But there still are limitations such as high reaction temperature and big particle size. The molten salt method is widely used for the preparation of unitary oxide and multicomponent oxides [13 –16], which can give a good reaction condition with high ion concentration and quick pervasion. To our
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H. Tian et al. / Materials Letters 59 (2005) 3059 – 3061
knowledge, there has been no known report on the preparation of SnO2 – TiO2 coupled oxides by a molten salt. If the processing conditions are properly controlled, it could be employed to synthesis desirable nano-sized photocatalysts. In this work, a nano-sized (Sn0.25,Ti0.75)O2 coupled photocatalyst was successfully synthesized by a low temperature molten salt method at 260 -C which is the lowest synthesis temperature compared with other analogous preparing methods [12]. Furthermore, the UV –VIS absorption spectra of the synthesized powders showed a good UV absorption and visible light response.
2. Experimental 2.1. Preparation for the precursor of (Sn0.25,Ti0.75)O2 with a homogeneous co-precipitation method SnCl4I5H2O and Ti(OC4H9)4 were used as the starting materials, which were of analytic reagent grade. HCl (37 wt.%) solution as reaction medium and ammonia solution (25 wt.%) as a co-precipitant, SnCl4I5H2O and Ti(OC4H9)4 in a molar ratio of 1 : 3 were dissolved in the HCl solution (pH = 1), respectively. Then, the ammonia solution was slowly added to the above solution containing titanium and tin cations to adjust its pH value to 8, and a white precipitate was formed. The precipitate was filtered and washed with ammonia solution that could avoid the formation of sol. Then, the precipitate was dried at about 70 -C in air overnight to form the precursor for (Sn0.25,Ti0.75)O2. 2.2. Preparation for nano-sized (Sn0.25,Ti0.75)O2 powders with the low temperature molten salt method (Sn0.25,Ti0.75)O2 precursor was mixed by ball milling in absolute ethanol with LiNO3 salt whose melting point is 254 -C [18]. The weight ratio of LiNO3 salt to (Sn0.25,Ti0.75)O2 precursor was selected as 4.
Fig. 2. TEM micrograph of (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
After the mixture was dried at about 70 -C in air, it was placed in a crucible, and heat treated at 260 -C for 2 h. Finally, the resulting product was washed with a deionized water for several times to remove the alkali metal salt, and washed with absolute ethanol, and dried at 70 -C for 3 h. 2.3. Characterization of samples XRD pattens of the samples were recorded on a D8 advance diffractometer (Bruker, Germany) using Cu Ka radiation. JEM-1200 EX transmission electron microscope
180 160 Intensity (a.u.)
Intensity (a.u.)
140 120 100 80 60 40 20 0 20
30
40
50 2θ (deg.)
60
70
Fig. 1. XRD pattern of the (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
120 110 100 90 80 70 60 50 40 30 20 10 20
30
40 50 2θ (deg.)
60
70
Fig. 3. XRD pattern of the (Sn0.25,Ti0.75)O2 precursor synthesized by the homogeneous co-precipitation method.
H. Tian et al. / Materials Letters 59 (2005) 3059 – 3061
The UV – VIS absorption spectrum of the (Sn0.25,Ti0.75)O2 is given in Fig. 4. It shows that the sample, compared with the relative reports [9,10], not only has stronger UV absorption, but also enhanced absorption to the visible light.
2.2 2.0 Absorbance(a.u)
3061
1.8 1.6 1.4
4. Conclusions
1.2 1.0 0.8 0.6
400
600 Wavelength(nm)
800
Fig. 4. UV – VIS absorption spectrum of the (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
(Tokyo, Japan) was employed to examine the morphology of the nanoparticles. And the UV – VIS absorption spectra of the samples were recorded on a U-3010 spectrophotometer (Tokyo, Japan).
3. Result and discussion Fig. 1 shows X-ray diffraction (XRD) pattern of the (Sn0.25,Ti0.75)O2 powder synthesized by our method at 260 -C for 2 h. All the peaks could be indexed as (Sn0.25,Ti0.75)O2 of the rutile structure, which was consistent with the literature [5]. No TiO2 or SnO2 exists whether anatase or rutile, which indicated that a solid solution with the rutile structure was formed [3]. Fig. 2 is a typical transmission electron microscopy (TEM) micrograph of the (Sn0.25,Ti0.75)O2 powder. It is showed that the (Sn0.25,Ti0.75)O2 powder synthesized is composed of nano-sized particles with the average size of about 10 nm. The particle size is smaller than the average particle size of (Sn0.25,Ti0.75)O2 in the literature [12], which was the smallest size that had been reported. Fig. 2 also displays that each particle is nearly spherical in shape and dense in density and no visible aggregation can be found. The reason for the formation of nano-sized (Sn0.25,Ti0.75)O2 powder seemed to be a little complicated. We presumed that it was related to (Sn0.25,Ti0.75)O2 precursor prepared by the homogeneous co-precipitation method and the reaction conditions in the molten salt process. The precipitate formed in the homogeneous coprecipitation system consisted of hydrates of TiO2 and SnO2, which is likely to be nano-sized. It could be confirmed by XRD pattern of the (Sn0.25,Ti0.75)O2 precursor in Fig. 3. The excessive LiNO3 salt could limit the mass transport rate and consequently prevent the further growing up of (Sn0.25,Ti0.75)O2 phase, even though the free energy of formation of (Sn0.25,Ti0.75)O2 phase was decreased, which lead to greatly lower its synthetic temperature.
A nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 was successfully synthesized by a molten salt method at a temperature as low as 260 -C, which might be known as the lowest synthetic temperature compared with analogous preparing methods. The (Sn0.25,Ti0.75)O2 powder was composed of nearly spherical particles with the average size of about 10 nm, and had good visible light responses.
References [1] W.-Y. Chung, D.-D. Lee, B.-K. Sohn, Thin Solid Films 221 (1992) 304. [2] M. Radecka, K. Zakrzewska, M. Rekas, Sens. Actuators, B, Chem. 47 (1998) 194. [3] Marta Radecka, Katarzyna Zakrzewska, Mieczysaaw Rekas, Sens. Actuators, B, Chem. 47 (1998) 194. [4] M. Radecka, J. Przewoznik, K. Zakrzewska, Thin Solid Films 391 (2001) 247. [5] P.R. Bueno, M.R. Cassia-Santos, L.G.P. Simoes, J.W. Gomes, E. Longo, J.A. Varela, J. Am. Ceram. Soc. 85 (2002) 282. [6] M. Radecka, P. Pasierb, K. Zakrzewska, M. Rekas, Solid State Ionics 119 (1999) 43. [7] P.R. Bueno, E.R. Leite, L.O.S. Bulhoes, E. Longo, C.O. Paiva-Santos, J. Eur. Ceram. Soc. 23 (2003) 887. [8] Fabricio R. Sensato, Rogerio Custodio, Elson Longo, Armando Beltran, Juan Andres, Catal. Today 85 (2003) 145. [9] Jing Shang, Wenqing Yao, Yongfa Zhu, Nianzu Wu, Appl. Catal., A Gen. 257 (2004) 25. [10] Sarah Pilkenton, Daniel Raftery, Magn. Reson. 24 (2003) 236. [11] N. Kanai, T. Nuida, K. Ueta, K. Hashimoto, T. Watanabe, H. Ohsaki, Vacuum 74 (2004) 723. [12] Liyi Shia, Chunzhong Li, Hongcheng Gu, Dingye Fang, Mater. Chem. Phys. 62 (2000) 62. [13] Yanmei Kan, Xihai Jin, Peiling Wang, Yongxiang Li, Yi-Bing Cheng, Dongsheng Yan, Mater. Res. Bull. 38 (2003) 567. [14] V. Harle, M. Vrinat, J.P. Scharff, B. Durand, J.P. Deloume, Appl. Catal., A Gen. 196 (2000) 261. [15] Congkang Xua, Xiaolin Zhao, Sheng Liu, Guanghou Wang, Solid State Commun. 125 (2003) 301. [16] A.V. Gorokhovsky, J.I. Escalante-Garc, T. Sanchez-Monjaras, C.A. Gutierrez-Chavarria, J. Eur. Ceram. Soc. 24 (2004) 3541. [17] Jun Lin, Jimmy C. Yu, 1 D. Lo, S.K. Lam, J. Catal. 183 (1999) 368. [18] Ruren Xu, Synthesis and Preparation in Inorganic Chemistry, Higher Education Press, 2001, p. 199.
Nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 powders synthesized by a low temperature molten salt method Hua Tian a, Junfeng Ma a,b,*, Xiang Huang b, Lijin Xie a, Zhongqiang Zhao a, Jun Zhou a, Pingwei Wu b, Jinhui Dai b, Yingmo Hu b, Zhibin Zhu b, Hongfeng Wang b, Haiyan Chen b a
College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China Institute of Materials Sciences and Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China
b
Received 22 December 2004; accepted 24 May 2005 Available online 6 June 2005
Abstract Nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 powders were successfully synthesized by a low temperature molten salt method, where the precursor, a homogeneous precipitate containing titanium and tin cations was mixed with LiNO3 salt, nano-sized (Sn0.25,Ti0.75)O2 powders could be synthesized at 260 -C. The powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV – VIS, respectively. Experimental results showed that the product prepared was a pure phase (Sn0.25,Ti0.75)O2 of rutile structure without any other imparities, and composed of nano-sized and spherical particles of about 10 nm. In addition, the UV – VIS showed that the synthesized powders had a good UV absorption and visible light response. D 2005 Elsevier B.V. All rights reserved. Keywords: Coupled oxides; Nano-sized photocatalyst; Low temperature molten salt method; Titanium oxide; Tin oxide
1. Introduction SnO2 – TiO2 coupled oxide has received considerable attention as gas sensor [1– 4] and varistor ceramic [5], which is mainly due to the transport properties and electronic structure of the (Sn,Ti)O2 system [6 – 8]. In addition, SnO2 – TiO2 coupled photocatalyst is well known to be one of the most effective photocatalysts, and its photocatalytic behavior has been studied extensively in recent years [9– 12]. The relatively high photocatalytic activity of SnO2 –TiO2 coupled oxide should be attributed to a better charge separation due to fast electron transfer from TiO2 to SnO2. The proper placement of the individual semiconductor is essential for the charge separation to ensure high photo-
* Corresponding author. College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yu Shan Road, Qingdao 266003, People’s Republic of China. Tel.: +86 532 2031623; fax: +86 532 2031623. E-mail address: [email protected] (J. Ma). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.020
catalytic activity [9]. In general, the solid solution (Sn,Ti)O2 is of avail to high photocatalytic activity. Since in the solid solution, the photogenerated electron – hole pairs are effectively separated, these photocatalysts have a lower electron – hole recombination rate as compared with rutile TiO2. This lower recombination rate can be related to an increase in the bandgap of the solid solution. The elevation of the semiconductor’s bandgap causes an increase in its photocatalytic oxidation – reduction potential, which is the origin of the enhanced photocatalytic activity of (Sn,Ti)O2 [10 –17]. In addition, smaller crystalline size, better symmetrical and spherical crystallize mean better photocatalytic behavior [18]. SnO2 – TiO2 coupled oxides have been synthesized by various methods: the solid-state reaction [3], CVD [10], homogeneous precipitation [12]. But there still are limitations such as high reaction temperature and big particle size. The molten salt method is widely used for the preparation of unitary oxide and multicomponent oxides [13 –16], which can give a good reaction condition with high ion concentration and quick pervasion. To our
3060
H. Tian et al. / Materials Letters 59 (2005) 3059 – 3061
knowledge, there has been no known report on the preparation of SnO2 – TiO2 coupled oxides by a molten salt. If the processing conditions are properly controlled, it could be employed to synthesis desirable nano-sized photocatalysts. In this work, a nano-sized (Sn0.25,Ti0.75)O2 coupled photocatalyst was successfully synthesized by a low temperature molten salt method at 260 -C which is the lowest synthesis temperature compared with other analogous preparing methods [12]. Furthermore, the UV –VIS absorption spectra of the synthesized powders showed a good UV absorption and visible light response.
2. Experimental 2.1. Preparation for the precursor of (Sn0.25,Ti0.75)O2 with a homogeneous co-precipitation method SnCl4I5H2O and Ti(OC4H9)4 were used as the starting materials, which were of analytic reagent grade. HCl (37 wt.%) solution as reaction medium and ammonia solution (25 wt.%) as a co-precipitant, SnCl4I5H2O and Ti(OC4H9)4 in a molar ratio of 1 : 3 were dissolved in the HCl solution (pH = 1), respectively. Then, the ammonia solution was slowly added to the above solution containing titanium and tin cations to adjust its pH value to 8, and a white precipitate was formed. The precipitate was filtered and washed with ammonia solution that could avoid the formation of sol. Then, the precipitate was dried at about 70 -C in air overnight to form the precursor for (Sn0.25,Ti0.75)O2. 2.2. Preparation for nano-sized (Sn0.25,Ti0.75)O2 powders with the low temperature molten salt method (Sn0.25,Ti0.75)O2 precursor was mixed by ball milling in absolute ethanol with LiNO3 salt whose melting point is 254 -C [18]. The weight ratio of LiNO3 salt to (Sn0.25,Ti0.75)O2 precursor was selected as 4.
Fig. 2. TEM micrograph of (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
After the mixture was dried at about 70 -C in air, it was placed in a crucible, and heat treated at 260 -C for 2 h. Finally, the resulting product was washed with a deionized water for several times to remove the alkali metal salt, and washed with absolute ethanol, and dried at 70 -C for 3 h. 2.3. Characterization of samples XRD pattens of the samples were recorded on a D8 advance diffractometer (Bruker, Germany) using Cu Ka radiation. JEM-1200 EX transmission electron microscope
180 160 Intensity (a.u.)
Intensity (a.u.)
140 120 100 80 60 40 20 0 20
30
40
50 2θ (deg.)
60
70
Fig. 1. XRD pattern of the (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
120 110 100 90 80 70 60 50 40 30 20 10 20
30
40 50 2θ (deg.)
60
70
Fig. 3. XRD pattern of the (Sn0.25,Ti0.75)O2 precursor synthesized by the homogeneous co-precipitation method.
H. Tian et al. / Materials Letters 59 (2005) 3059 – 3061
The UV – VIS absorption spectrum of the (Sn0.25,Ti0.75)O2 is given in Fig. 4. It shows that the sample, compared with the relative reports [9,10], not only has stronger UV absorption, but also enhanced absorption to the visible light.
2.2 2.0 Absorbance(a.u)
3061
1.8 1.6 1.4
4. Conclusions
1.2 1.0 0.8 0.6
400
600 Wavelength(nm)
800
Fig. 4. UV – VIS absorption spectrum of the (Sn0.25,Ti0.75)O2 powder synthesized by the molten salt method at 260 -C for 2 h.
(Tokyo, Japan) was employed to examine the morphology of the nanoparticles. And the UV – VIS absorption spectra of the samples were recorded on a U-3010 spectrophotometer (Tokyo, Japan).
3. Result and discussion Fig. 1 shows X-ray diffraction (XRD) pattern of the (Sn0.25,Ti0.75)O2 powder synthesized by our method at 260 -C for 2 h. All the peaks could be indexed as (Sn0.25,Ti0.75)O2 of the rutile structure, which was consistent with the literature [5]. No TiO2 or SnO2 exists whether anatase or rutile, which indicated that a solid solution with the rutile structure was formed [3]. Fig. 2 is a typical transmission electron microscopy (TEM) micrograph of the (Sn0.25,Ti0.75)O2 powder. It is showed that the (Sn0.25,Ti0.75)O2 powder synthesized is composed of nano-sized particles with the average size of about 10 nm. The particle size is smaller than the average particle size of (Sn0.25,Ti0.75)O2 in the literature [12], which was the smallest size that had been reported. Fig. 2 also displays that each particle is nearly spherical in shape and dense in density and no visible aggregation can be found. The reason for the formation of nano-sized (Sn0.25,Ti0.75)O2 powder seemed to be a little complicated. We presumed that it was related to (Sn0.25,Ti0.75)O2 precursor prepared by the homogeneous co-precipitation method and the reaction conditions in the molten salt process. The precipitate formed in the homogeneous coprecipitation system consisted of hydrates of TiO2 and SnO2, which is likely to be nano-sized. It could be confirmed by XRD pattern of the (Sn0.25,Ti0.75)O2 precursor in Fig. 3. The excessive LiNO3 salt could limit the mass transport rate and consequently prevent the further growing up of (Sn0.25,Ti0.75)O2 phase, even though the free energy of formation of (Sn0.25,Ti0.75)O2 phase was decreased, which lead to greatly lower its synthetic temperature.
A nano-sized coupled photocatalyst (Sn0.25,Ti0.75)O2 was successfully synthesized by a molten salt method at a temperature as low as 260 -C, which might be known as the lowest synthetic temperature compared with analogous preparing methods. The (Sn0.25,Ti0.75)O2 powder was composed of nearly spherical particles with the average size of about 10 nm, and had good visible light responses.
References [1] W.-Y. Chung, D.-D. Lee, B.-K. Sohn, Thin Solid Films 221 (1992) 304. [2] M. Radecka, K. Zakrzewska, M. Rekas, Sens. Actuators, B, Chem. 47 (1998) 194. [3] Marta Radecka, Katarzyna Zakrzewska, Mieczysaaw Rekas, Sens. Actuators, B, Chem. 47 (1998) 194. [4] M. Radecka, J. Przewoznik, K. Zakrzewska, Thin Solid Films 391 (2001) 247. [5] P.R. Bueno, M.R. Cassia-Santos, L.G.P. Simoes, J.W. Gomes, E. Longo, J.A. Varela, J. Am. Ceram. Soc. 85 (2002) 282. [6] M. Radecka, P. Pasierb, K. Zakrzewska, M. Rekas, Solid State Ionics 119 (1999) 43. [7] P.R. Bueno, E.R. Leite, L.O.S. Bulhoes, E. Longo, C.O. Paiva-Santos, J. Eur. Ceram. Soc. 23 (2003) 887. [8] Fabricio R. Sensato, Rogerio Custodio, Elson Longo, Armando Beltran, Juan Andres, Catal. Today 85 (2003) 145. [9] Jing Shang, Wenqing Yao, Yongfa Zhu, Nianzu Wu, Appl. Catal., A Gen. 257 (2004) 25. [10] Sarah Pilkenton, Daniel Raftery, Magn. Reson. 24 (2003) 236. [11] N. Kanai, T. Nuida, K. Ueta, K. Hashimoto, T. Watanabe, H. Ohsaki, Vacuum 74 (2004) 723. [12] Liyi Shia, Chunzhong Li, Hongcheng Gu, Dingye Fang, Mater. Chem. Phys. 62 (2000) 62. [13] Yanmei Kan, Xihai Jin, Peiling Wang, Yongxiang Li, Yi-Bing Cheng, Dongsheng Yan, Mater. Res. Bull. 38 (2003) 567. [14] V. Harle, M. Vrinat, J.P. Scharff, B. Durand, J.P. Deloume, Appl. Catal., A Gen. 196 (2000) 261. [15] Congkang Xua, Xiaolin Zhao, Sheng Liu, Guanghou Wang, Solid State Commun. 125 (2003) 301. [16] A.V. Gorokhovsky, J.I. Escalante-Garc, T. Sanchez-Monjaras, C.A. Gutierrez-Chavarria, J. Eur. Ceram. Soc. 24 (2004) 3541. [17] Jun Lin, Jimmy C. Yu, 1 D. Lo, S.K. Lam, J. Catal. 183 (1999) 368. [18] Ruren Xu, Synthesis and Preparation in Inorganic Chemistry, Higher Education Press, 2001, p. 199.