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Fabrication and photocatalytic properties of SnO2 double-shelled and triple-shelled hollow spheres
Solid State Sciences 56 (2016) 63e67
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Fabrication and photocatalytic properties of SnO2 double-shelled and triple-shelled hollow spheres Shanshan Niu a, b, Yong Wang a, *, Shan Lu a, Dongxia Wang a, Ping Wang c a
Department of Chemistry, Capital Normal University, Beijing 100048, China College of Resources Environment and Tourism, Capital Normal University, Beijing 100048, China c Department of Radiation Protection and Environmental Impact Assessment, Nuclear and Radiation Safety Center, Ministry of Environmental Protection, 100082, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2015 Received in revised form 28 March 2016 Accepted 22 April 2016 Available online 23 April 2016
SnO2 double-shelled and triple-shelled hollow spheres were tailored by adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous spheres. The structures of these SnO2 multi-shelled hollow spheres were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and their possible formation mechanism were also discussed. In virtue of triple-shelled hollow porous structure and higher specific surface area, SnO2 triple-shelled hollow spheres exhibited enhanced photocatalytic properties compared to SnO2 double-shelled hollow spheres. © 2016 Elsevier Masson SAS. All rights reserved.
Keywords: Nanostructured materials Chemical synthesis Multi-shell Hollow structure Photocatalytic properties
1. Introduction Hollow micro-/nanostructures with controllable complex structures have attracted extensive attention in recent years owing to their technical importance in wide applications, such as lithiumion batteries, photocatalysis and gas sensing [1e4]. In particular, multi-shelled metal oxide hollow structures, such as TiO2, WO3, Fe2O3 and CeO2, exhibit peculiar photocatalytic properties because of multi-reflection of the incident light, high surface area and low density of multi-shelled hollow materials [5e8]. Although template-free routes have been more frequently reported for the preparation of multi-shelled hollow particles [9,10], templating against colloidal particles is still the most effective and general method for the preparation of multi-shelled hollow particles with controllable size distribution and morphology [7,11,12]. For example, Hu et al. have demonstrated a new approach to fabricate multi-shelled a-Fe2O3 hollow spheres via a facile solvothermal method by using carbon spheres as templates [7]. Li et al. have reported triple-shelled Mn2O3 hollow nanocube synthesized through a programmed annealing treatment with cubic MnCO3 as
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.solidstatesciences.2016.04.005 1293-2558/© 2016 Elsevier Masson SAS. All rights reserved.
precursor [12]. However, it is still a big challenge to synthesize hollow spheres with controlled shell numbers by a simple templating process. Photocatalysis has attracted much interest because of its potential application in clean energy sources to degrade organic pollutants from water [13,14]. Various nanostructured semiconductor metal oxides, such as TiO2, ZnO, Fe2O3, WO3 and SnO2, have been developed as nanophotocatalysts for their ability to produce radicals and decompose most organic pollutants under UV irradiation or sunlight [5e8,14]. As a typical n-type semiconductor, SnO2 has received great attention because of its excellent stability, nontoxicity and low-cost [15e18]. Moreover, SnO2 has no adverse health effects and is poorly absorbed by the human body when injected or inhaled [19e21]. Thus, SnO2 is potentially an ideal photocatalyst [21]. Recently, SnO2 hollow structures have been demonstrated as high-efficiency photocatalysts owing to the large surface area and high light-harvesting efficiency [16,17]. To the best of our knowledge, however, there is no report on the photocatalytic properties of multi-shelled SnO2 hollow spheres. Herein, we give a systematic research on the controllable synthesis of double-shelled and triple-shelled SnO2 hollow spheres. By simply adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous microspheres,
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we have prepared SnO2 hollow spheres with controlled shell numbers. In addition, photocatalytic properties of the obtained double-shelled and triple-shelled SnO2 hollow spheres have also been investigated. To the best of our knowledge, this is the first report on the comparison of photocatalytic properties between double-shelled and triple-shelled SnO2 hollow spheres. 2. Material and methods 2.1. Preparation of carbonaceous microspheres In a typical synthesis, carbonaceous spheres are firstly obtained through a reported hydrothermal method [22]. Typically, a sucrose solution (40 mL, 1.5 M) was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was then heated in an air flow electric oven at 220 C for 1.5 h. After the autoclave cooled down naturally, the brown products were washed with three times with deionized water and ethanol before vacuum-drying at room temperature for 24 h. 2.2. Preparation of SnO2 triple-shelled and double-shelled hollow spheres Take the synthesis of SnO2 triple-shelled hollow spheres as an example, carbonaceous sphere powder was dispersed in 30 mL of 0.5 M tin (IV) chloride solution (water/ethanol ¼ 3:1, v/v, 30 mL) through sonication to form a uniform dispersion. After this, the resulting suspension was aged for 3 h to allow the sufficient penetration of tin (IV) ions into the carbonaceous spheres at room temperature (25 C). The precipitation was then filtered, washed with water and dried at 80 C for 12 h. The triple-shelled hollow spheres were generated through a thermal annealing treatment in air at the temperature of 500 C for 3 h with a heating rate of 1 C min1. SnO2 double-shelled hollow spheres were synthesized by changing the concentration of tin (IV) chloride solution (0.1 M) and using the same thermal processing procedure described above. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded with X-ray powder diffraction (XRD, Bruker, D8 ADVANCE). The morphology and structure of the samples were further investigated by field emission scanning electron microscopy (FESEM, Hitachi, S4800) with energy-dispersive X-ray (EDX) spectroscopy, and transmission electron microscopy (TEM, FEI Tecnai F20, 200 KV). The Brunauer-Emmett-Teller (BET) specific surface areas and pore
size distributions of the resultant products were measured with a Quantachrome NOVA 1000e analyzer. 2.4. Photocatalytic activity Methylene blue (MB) decomposition testings were carried out to study the photocatalytic ability of the as-synthesized products. On the basis of the Beer-Lambert law, the methylene blue aqueous solution with 105 M is linearly proportional to the intensity of the measured spectrum [5,13]. In the present study, 80 mL of a 10 mg L1 MB solution containing 80 mg of well-dispersed assynthesized products were illuminated employing a 250 W highpressure mercury lamp as the UV light source. Photocatalytic decomposition of the MB solutions was characterized by a UVevis spectrometer (UV-2550, Shimadzu, Japan). 3. Results and discussion Fig. 1 presents typical FESEM and TEM images of carbonaceous spheres. The size of carbonaceous microsphere templates is 1e1.5 mm. SnO2 double-shelled hollow spheres and triple-shelled hollow spheres can be controlled-synthesized by simply adjusting concentration of tin (IV) chloride solution. When the concentration of tin (IV) chloride solution at 25 C is 0.1 mol L1, SnO2 double-shelled hollow spheres of a 0.2 mm diameter porous shell with a porous outer shell of about 0.8 mm in diameter are yielded (Fig. 2aed). The diameters of the microspheres are ~0.8 mm, smaller than for the carbonaceous microspheres due to the inward shrinkage during the calcination process (Fig. 1). Interestingly, the double-shelled hollow spheres are in fact built from irregular nanoparticles, and the irregular nanoparticles spontaneously align with one another to form a spongelike porous double-shelled sphere with an interior cavity (Fig. 2bed and Fig. S1aeb). In addition, the size of the SnO2 irregular nanoparticles is 2e10 nm. Increasing the concentration of the tin (IV) chloride solution to 0.5 mol L1, SnO2 porous triple-shelled hollow spheres are obtained (Fig. 2eeh and Fig. S1ced). The phase purity and crystal structure of the as-obtained doubleshelled and triple-shelled hollow spheres have been examined by XRD. As shown in Fig. 3, all the diffraction peaks can be perfectly indexed as the tetragonal rutile phase of SnO2 (JCPDS card no. 41-1445, a ¼ 4.738 Å, c ¼ 3.187 Å) [15e17], and no other impurities are observed. Fig. 4 presents the nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the as-obtained double-shelled and triple-shelled hollow spheres. The Brunauer-Emmett-Teller (BET) specific surface area of the SnO2 triple-shelled spheres is 55.7 m2 g1,
Fig. 1. (a) FESEM and (b) TEM images of carbonaceous spheres.
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Fig. 2. FESEM images of (a) SnO2 double-shelled hollow spheres and (b) a broken SnO2 double-shelled hollow sphere; (ced) TEM images of SnO2 double-shelled hollow spheres; FESEM images of (e) SnO2 triple-shelled hollow spheres and (f) a broken SnO2 triple-shelled hollow sphere; (geh) TEM images of SnO2 triple-shelled hollow spheres.
Fig. 3. XRD patterns of (a) double-shelled hollow spheres and (b) triple-shelled hollow spheres.
which is higher than that of the double-shelled spheres (44.5 m2 g1). The Barret-Joyner-Halenda (BJH) pore sizes of the samples are distributed from several nanometers to more than 30 nm. Quantitative calculation shows that the as-prepared SnO2 triple-shelled spheres and double-shelled spheres possess pore volumes of 0.211 and 0.13 cm3 g1, respectively. These different SnO2 hollow spheres with porous structures permit the organic pollutants to easily penetrate through the pores and make close contact with the inner-outer surfaces of numerous primary particles, possibly resulting in a considerable improvement in the photocatalytic properties [5,14,16,17]. The element composition is further confirmed with energy dispersive X-ray (EDX) spectroscopy analysis under FESEM and TEM. EDX point spectra, taken from the center point of a sphere, show strong Sn and O signals (Fig. 5). In addition, no C signals are
Fig. 4. The nitrogen adsorption/desorption isotherms and pore size distribution curves (insets) of (a) double-shelled hollow spheres; (b) triple-shelled hollow spheres.
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found in the EDX point spectra under FESEM (Fig. 5), which demonstrates removal of carbonaceous templates. The above investigations indicate that the carbonaceous templates are almost removed and hollow spheres with controlled shell numbers are produced, which conforms to the above XRD analysis. On the basis of the above experimental results, we can deduce that the concentration of tin (IV) chloride solution concentration plays an important role in the present controlled-synthesis of double-shelled hollow spheres and triple-shelled hollow spheres. The shell number for the hollow spheres can be manipulated by adjusting the concentration of tin (IV) chloride solution, which can generate double-shelled and triple-shell SnO2 hollow spheres. It is well-known that such carbonaceous spheres contain abundant hydroxyl (OH) and carbonyl (C]O) functional groups [7,22]. Therefore, tin (IV) ions would diffuse into the carbonaceous spheres through capillary action, and coordinated by the OH and C]O groups, form Sn@carbonaceous composite spheres. During calcination in air, the tin (IV) ions in the carbonaceous microsphere are oxidized to SnO2 nanoparticles by the oxygen of air, while the carbon in the carbonaceous microsphere is combusted [6e8]. When the concentration of tin (IV) chloride solution is 0.1 M, the penetration depth of tin (IV) ions within the carbonaceous microspheres is large enough, which result in the separation of tin (IV) ions within the carbonaceous spheres. When the heating temperature is increased in air, some tin (IV) ions adsorbed on the exterior of carbonaceous microsphere gradually turn to metal oxide nanoparticles and are compacted to form SnO2 porous outer shells, whereas the others adsorbed on the interior of carbonaceous microsphere supply tin (IV) resource for the creation of porous
inner shells until the carbonaceous microsphere is completely combusted and a double-shelled hollow structure is formed (Fig. 6a). Higher tin (IV) solution concentration (0.5 M) possibly leads to larger penetration depth of tin (IV) ions within the carbonaceous microspheres. As a result, more porous shells are created via a similar template process and finally triple-shelled porous hollow spheres are formed (Fig. 6b). The photocatalytic activity of the as-obtained double-shelled and triple-shelled SnO2 hollow spheres was evaluated by monitoring the degradation of organic dye methylene blue (MB) in aqueous solution under UV irradiation. Fig. 7 displays the photodegradation behaviors of MB in the presence of SnO2 samples as well as without a photocatalyst for comparison, where C is the concentration of dye after different light irradiation times and C0 is
Fig. 6. Schematic illustration of the formation process of (a) double-shelled hollow spheres; (b) triple-shelled hollow spheres.
Fig. 5. FESEM images (insets) and EDX point spectra of (a) double-shelled hollow sphere; (b) triple-shelled hollow sphere. Al peaks are attributed to the aluminum foil used as the holder. The Au peaks are from the conducting layer of Au used for FESEM characterization. The scale bar of insets in (a) and (b) is 200 nm.
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Fig. 7. (a) UVevisible spectra of the solutions at 10 mg L1 of MB in the presence of SnO2 triple-shelled hollow spheres; (b) Photocatalytic activities of SnO2 double-shelled hollow spheres and triple-shelled hollow spheres.
the initial concentration of dye (C0 ¼ 10 mg L1). Obviously, only a weak activity can be obtained for the decolorization of MB aqueous solution without SnO2 photocatalysts, which confirms the photostability of the dye. The addition of SnO2 photocatalysts leads to obvious degradation of MB, and the photocatalytic activity depends on the morphology of the SnO2 samples (Fig. 7b). After UV irradiation for 180 min, the decomposition rate of MB for triple-shelled SnO2 hollow spheres is 71.8% (C ¼ 2.82 mg L1); and the value is 64.2% for double-shelled SnO2 hollow spheres (C ¼ 3.58 mg L1), respectively. Therefore, it is clear that the photocatalytic activities of triple-shelled SnO2 hollow spheres are higher than that of double-shelled SnO2 hollow spheres. Based on the above results, the higher photocatalytic activity of triple-shelled SnO2 hollow spheres may be ascribed to tripleshelled hollow porous structure and higher specific surface area. It is believed that a high surface area of triple-shelled SnO2 hollow spheres are responsible for providing more surface sites, in which free radicals could be generated by the reaction between the photogenerated charge carriers and adsorbed molecules [5e8,16,17]. As shown in Fig. 4, the specific surface areas of SnO2 triple-shelled spheres and double-shelled spheres are 55.7 and 44.5 m2 g1, respectively. The above results confirm that a decrease in specific surface area results in a decrease in the photocatalytic activity. Recent researches have also demonstrated that hollow structured materials with multiple-shells, such as WO3, Fe2O3 and CeO2, are promising candidates in the photocatalytic field [6e8]. Advantages include (1) the multi-shelled hollow porous structure provides more active sites at multiple shells, allowing efficient transport of the reactant molecules and thus leading to higher photodegradation efficiency; (2) multiple reflection of the illumination light may occur at the multiple shells of the porous spheres, resulting in more efficient absorption of the light [6e8,10]. In our case, triple-shelled SnO2 hollow spheres show higher photocatalytic activity as compared with double-shelled SnO2 hollow spheres, which conforms to the above conclusion. 4. Conclusions In summary, we developed a facile method for the controlledsynthesis of uniform SnO2 double-shelled and triple-shelled hollow spheres. SnO2 double-shelled and triple-shelled hollow spheres could be tailored by simply adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous microspheres. Photocatalytic properties of the asobtained SnO2 samples were also investigated. Triple-shelled SnO2 hollow spheres exhibited enhanced photocatalytic properties as compared with double-shelled SnO2 hollow spheres. Investigations confirmed that the higher photocatalytic activity of
triple-shelled SnO2 hollow spheres might be ascribed to tripleshelled hollow porous structure and higher specific surface area. Acknowledgments This work was supported by Beijing Municipal Natural Science Foundation (2152010) and the General Program of Science and Technology Development Project of Beijing Municipal Education Commission (KM201210028019). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2016.04.005. References [1] W.S. Kim, Y. Hwa, J.H. Jeun, H.J. Sohn, S.H. Hong, J. Power Sources 225 (2013) 108e112. [2] Q. Shao, L.Y. Wang, X.J. Wang, M.C. Yang, S.S. Ge, X.K. Yang, J.X. Wang, Solid State Sci. 20 (2013) 29e35. [3] X.C. Ma, H.Y. Song, C.S. Guan, Sens. Actuators. B 177 (2013) 196e204. [4] X.M. Li, L.F. Jiang, C. Zhou, J.P. Liu, H.B. Zeng, NPG Asia Mater. 7 (2015) e165ee173. [5] X.J. Xu, X.S. Fang, T.Y. Zhai, H.B. Zeng, B.D. Liu, X.Y. Hu, Y. Bando, D. Golberg, Small 7 (2011) 445e449. [6] G.C. Xi, Y. Yan, Q. Ma, J.F. Li, H.F. Yang, X.J. Lu, C. Wang, Chem. Eur. J. 18 (2012) 13949e13953. [7] Y. Liu, C.Y. Yu, W. Dai, X.H. Gao, H.S. Qian, Y. Hua, X. Hu, J. Alloys Compd. 551 (2013) 440e443. [8] J. Qi, K. Zhao, G.D. Li, Y. Gao, H.J. Zhao, R.B. Yu, Z.Y. Tang, Nanoscale 6 (2014) 4072e4077. [9] Z.G. Wu, Y.J. Zhong, J.T. Li, X.D. Guo, L. Huang, B.H. Zhong, S.G. Sun, J. Mater. Chem. A 2 (2014) 12361e12367. [10] Z.C. Ma, L.M. Wang, D.Q. Chu, H.M. Sun, A.X. Wang, RSC Adv. 5 (2015) 8223e8227. [11] J.Y. Wang, N.L. Yang, H.J. Tang, Z.H. Dong, Q. Jin, M. Yang, D. Kisailus, H.J. Zhao, Z.Y. Tang, D. Wang, Angew. Chem. Int. Ed. 52 (2013) 6417e6420. [12] H.B. Lin, H.B. Rong, W.Z. Huang, Y.H. Liao, L.D. Xing, M.Q. Xu, X.P. Li, W.S. Li, J. Mater. Chem. A 2 (2014) 14189e14194. [13] N. Brahiti, T. Hadjersi, H. Menari, S. Amirouche, O. El Kechai, Mater. Res. Bull. 62 (2015) 30e36. [14] T. Leshuk, S. Linley, G. Baxter, F. Gu, ACS Appl. Mater. Interfaces 4 (2012) 6062e6070. [15] K. Vignesh, R. Hariharan, M. Rajarajan, A. Suganthi, Solid State Sci. 21 (2013) 91e99. [16] X. Wang, H.Q. Fan, P.R. Ren, Catal. Commun. 31 (2013) 37e41. [17] Y.H. Zhu, L.L. Wang, G.F. Huang, Y.F. Chai, X. Zhai, W.Q. Huang, Mater. Sci. Eng. B 178 (2013) 725e729. [18] P. Zhang, L.J. Wang, X. Zhang, J.H. Hu, G.S. Shao, Nano-Micro Lett. 7 (2015) 86e95. [19] A. Bhattacharjee, M. Ahmaruzzaman, T. Sinha, Spectrochim. Acta A 136 (2015) 751e760. [20] G. Elango, S.M. Kumaran, S.S. Kumar, S. Muthuraja, S.M. Roopan, Spectrochim. Acta A 145 (2015) 176e180. [21] S.P. Kim, M.Y. Choi, H.C. Choi, Mater. Res. Bull. 74 (2016) 85e89. [22] Q. Wang, H. Li, L.Q. Chen, X.J. Huang, Carbon 39 (2001) 2211e2214.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Fabrication and photocatalytic properties of SnO2 double-shelled and triple-shelled hollow spheres Shanshan Niu a, b, Yong Wang a, *, Shan Lu a, Dongxia Wang a, Ping Wang c a
Department of Chemistry, Capital Normal University, Beijing 100048, China College of Resources Environment and Tourism, Capital Normal University, Beijing 100048, China c Department of Radiation Protection and Environmental Impact Assessment, Nuclear and Radiation Safety Center, Ministry of Environmental Protection, 100082, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2015 Received in revised form 28 March 2016 Accepted 22 April 2016 Available online 23 April 2016
SnO2 double-shelled and triple-shelled hollow spheres were tailored by adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous spheres. The structures of these SnO2 multi-shelled hollow spheres were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and their possible formation mechanism were also discussed. In virtue of triple-shelled hollow porous structure and higher specific surface area, SnO2 triple-shelled hollow spheres exhibited enhanced photocatalytic properties compared to SnO2 double-shelled hollow spheres. © 2016 Elsevier Masson SAS. All rights reserved.
Keywords: Nanostructured materials Chemical synthesis Multi-shell Hollow structure Photocatalytic properties
1. Introduction Hollow micro-/nanostructures with controllable complex structures have attracted extensive attention in recent years owing to their technical importance in wide applications, such as lithiumion batteries, photocatalysis and gas sensing [1e4]. In particular, multi-shelled metal oxide hollow structures, such as TiO2, WO3, Fe2O3 and CeO2, exhibit peculiar photocatalytic properties because of multi-reflection of the incident light, high surface area and low density of multi-shelled hollow materials [5e8]. Although template-free routes have been more frequently reported for the preparation of multi-shelled hollow particles [9,10], templating against colloidal particles is still the most effective and general method for the preparation of multi-shelled hollow particles with controllable size distribution and morphology [7,11,12]. For example, Hu et al. have demonstrated a new approach to fabricate multi-shelled a-Fe2O3 hollow spheres via a facile solvothermal method by using carbon spheres as templates [7]. Li et al. have reported triple-shelled Mn2O3 hollow nanocube synthesized through a programmed annealing treatment with cubic MnCO3 as
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.solidstatesciences.2016.04.005 1293-2558/© 2016 Elsevier Masson SAS. All rights reserved.
precursor [12]. However, it is still a big challenge to synthesize hollow spheres with controlled shell numbers by a simple templating process. Photocatalysis has attracted much interest because of its potential application in clean energy sources to degrade organic pollutants from water [13,14]. Various nanostructured semiconductor metal oxides, such as TiO2, ZnO, Fe2O3, WO3 and SnO2, have been developed as nanophotocatalysts for their ability to produce radicals and decompose most organic pollutants under UV irradiation or sunlight [5e8,14]. As a typical n-type semiconductor, SnO2 has received great attention because of its excellent stability, nontoxicity and low-cost [15e18]. Moreover, SnO2 has no adverse health effects and is poorly absorbed by the human body when injected or inhaled [19e21]. Thus, SnO2 is potentially an ideal photocatalyst [21]. Recently, SnO2 hollow structures have been demonstrated as high-efficiency photocatalysts owing to the large surface area and high light-harvesting efficiency [16,17]. To the best of our knowledge, however, there is no report on the photocatalytic properties of multi-shelled SnO2 hollow spheres. Herein, we give a systematic research on the controllable synthesis of double-shelled and triple-shelled SnO2 hollow spheres. By simply adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous microspheres,
64
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we have prepared SnO2 hollow spheres with controlled shell numbers. In addition, photocatalytic properties of the obtained double-shelled and triple-shelled SnO2 hollow spheres have also been investigated. To the best of our knowledge, this is the first report on the comparison of photocatalytic properties between double-shelled and triple-shelled SnO2 hollow spheres. 2. Material and methods 2.1. Preparation of carbonaceous microspheres In a typical synthesis, carbonaceous spheres are firstly obtained through a reported hydrothermal method [22]. Typically, a sucrose solution (40 mL, 1.5 M) was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was then heated in an air flow electric oven at 220 C for 1.5 h. After the autoclave cooled down naturally, the brown products were washed with three times with deionized water and ethanol before vacuum-drying at room temperature for 24 h. 2.2. Preparation of SnO2 triple-shelled and double-shelled hollow spheres Take the synthesis of SnO2 triple-shelled hollow spheres as an example, carbonaceous sphere powder was dispersed in 30 mL of 0.5 M tin (IV) chloride solution (water/ethanol ¼ 3:1, v/v, 30 mL) through sonication to form a uniform dispersion. After this, the resulting suspension was aged for 3 h to allow the sufficient penetration of tin (IV) ions into the carbonaceous spheres at room temperature (25 C). The precipitation was then filtered, washed with water and dried at 80 C for 12 h. The triple-shelled hollow spheres were generated through a thermal annealing treatment in air at the temperature of 500 C for 3 h with a heating rate of 1 C min1. SnO2 double-shelled hollow spheres were synthesized by changing the concentration of tin (IV) chloride solution (0.1 M) and using the same thermal processing procedure described above. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded with X-ray powder diffraction (XRD, Bruker, D8 ADVANCE). The morphology and structure of the samples were further investigated by field emission scanning electron microscopy (FESEM, Hitachi, S4800) with energy-dispersive X-ray (EDX) spectroscopy, and transmission electron microscopy (TEM, FEI Tecnai F20, 200 KV). The Brunauer-Emmett-Teller (BET) specific surface areas and pore
size distributions of the resultant products were measured with a Quantachrome NOVA 1000e analyzer. 2.4. Photocatalytic activity Methylene blue (MB) decomposition testings were carried out to study the photocatalytic ability of the as-synthesized products. On the basis of the Beer-Lambert law, the methylene blue aqueous solution with 105 M is linearly proportional to the intensity of the measured spectrum [5,13]. In the present study, 80 mL of a 10 mg L1 MB solution containing 80 mg of well-dispersed assynthesized products were illuminated employing a 250 W highpressure mercury lamp as the UV light source. Photocatalytic decomposition of the MB solutions was characterized by a UVevis spectrometer (UV-2550, Shimadzu, Japan). 3. Results and discussion Fig. 1 presents typical FESEM and TEM images of carbonaceous spheres. The size of carbonaceous microsphere templates is 1e1.5 mm. SnO2 double-shelled hollow spheres and triple-shelled hollow spheres can be controlled-synthesized by simply adjusting concentration of tin (IV) chloride solution. When the concentration of tin (IV) chloride solution at 25 C is 0.1 mol L1, SnO2 double-shelled hollow spheres of a 0.2 mm diameter porous shell with a porous outer shell of about 0.8 mm in diameter are yielded (Fig. 2aed). The diameters of the microspheres are ~0.8 mm, smaller than for the carbonaceous microspheres due to the inward shrinkage during the calcination process (Fig. 1). Interestingly, the double-shelled hollow spheres are in fact built from irregular nanoparticles, and the irregular nanoparticles spontaneously align with one another to form a spongelike porous double-shelled sphere with an interior cavity (Fig. 2bed and Fig. S1aeb). In addition, the size of the SnO2 irregular nanoparticles is 2e10 nm. Increasing the concentration of the tin (IV) chloride solution to 0.5 mol L1, SnO2 porous triple-shelled hollow spheres are obtained (Fig. 2eeh and Fig. S1ced). The phase purity and crystal structure of the as-obtained doubleshelled and triple-shelled hollow spheres have been examined by XRD. As shown in Fig. 3, all the diffraction peaks can be perfectly indexed as the tetragonal rutile phase of SnO2 (JCPDS card no. 41-1445, a ¼ 4.738 Å, c ¼ 3.187 Å) [15e17], and no other impurities are observed. Fig. 4 presents the nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the as-obtained double-shelled and triple-shelled hollow spheres. The Brunauer-Emmett-Teller (BET) specific surface area of the SnO2 triple-shelled spheres is 55.7 m2 g1,
Fig. 1. (a) FESEM and (b) TEM images of carbonaceous spheres.
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Fig. 2. FESEM images of (a) SnO2 double-shelled hollow spheres and (b) a broken SnO2 double-shelled hollow sphere; (ced) TEM images of SnO2 double-shelled hollow spheres; FESEM images of (e) SnO2 triple-shelled hollow spheres and (f) a broken SnO2 triple-shelled hollow sphere; (geh) TEM images of SnO2 triple-shelled hollow spheres.
Fig. 3. XRD patterns of (a) double-shelled hollow spheres and (b) triple-shelled hollow spheres.
which is higher than that of the double-shelled spheres (44.5 m2 g1). The Barret-Joyner-Halenda (BJH) pore sizes of the samples are distributed from several nanometers to more than 30 nm. Quantitative calculation shows that the as-prepared SnO2 triple-shelled spheres and double-shelled spheres possess pore volumes of 0.211 and 0.13 cm3 g1, respectively. These different SnO2 hollow spheres with porous structures permit the organic pollutants to easily penetrate through the pores and make close contact with the inner-outer surfaces of numerous primary particles, possibly resulting in a considerable improvement in the photocatalytic properties [5,14,16,17]. The element composition is further confirmed with energy dispersive X-ray (EDX) spectroscopy analysis under FESEM and TEM. EDX point spectra, taken from the center point of a sphere, show strong Sn and O signals (Fig. 5). In addition, no C signals are
Fig. 4. The nitrogen adsorption/desorption isotherms and pore size distribution curves (insets) of (a) double-shelled hollow spheres; (b) triple-shelled hollow spheres.
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found in the EDX point spectra under FESEM (Fig. 5), which demonstrates removal of carbonaceous templates. The above investigations indicate that the carbonaceous templates are almost removed and hollow spheres with controlled shell numbers are produced, which conforms to the above XRD analysis. On the basis of the above experimental results, we can deduce that the concentration of tin (IV) chloride solution concentration plays an important role in the present controlled-synthesis of double-shelled hollow spheres and triple-shelled hollow spheres. The shell number for the hollow spheres can be manipulated by adjusting the concentration of tin (IV) chloride solution, which can generate double-shelled and triple-shell SnO2 hollow spheres. It is well-known that such carbonaceous spheres contain abundant hydroxyl (OH) and carbonyl (C]O) functional groups [7,22]. Therefore, tin (IV) ions would diffuse into the carbonaceous spheres through capillary action, and coordinated by the OH and C]O groups, form Sn@carbonaceous composite spheres. During calcination in air, the tin (IV) ions in the carbonaceous microsphere are oxidized to SnO2 nanoparticles by the oxygen of air, while the carbon in the carbonaceous microsphere is combusted [6e8]. When the concentration of tin (IV) chloride solution is 0.1 M, the penetration depth of tin (IV) ions within the carbonaceous microspheres is large enough, which result in the separation of tin (IV) ions within the carbonaceous spheres. When the heating temperature is increased in air, some tin (IV) ions adsorbed on the exterior of carbonaceous microsphere gradually turn to metal oxide nanoparticles and are compacted to form SnO2 porous outer shells, whereas the others adsorbed on the interior of carbonaceous microsphere supply tin (IV) resource for the creation of porous
inner shells until the carbonaceous microsphere is completely combusted and a double-shelled hollow structure is formed (Fig. 6a). Higher tin (IV) solution concentration (0.5 M) possibly leads to larger penetration depth of tin (IV) ions within the carbonaceous microspheres. As a result, more porous shells are created via a similar template process and finally triple-shelled porous hollow spheres are formed (Fig. 6b). The photocatalytic activity of the as-obtained double-shelled and triple-shelled SnO2 hollow spheres was evaluated by monitoring the degradation of organic dye methylene blue (MB) in aqueous solution under UV irradiation. Fig. 7 displays the photodegradation behaviors of MB in the presence of SnO2 samples as well as without a photocatalyst for comparison, where C is the concentration of dye after different light irradiation times and C0 is
Fig. 6. Schematic illustration of the formation process of (a) double-shelled hollow spheres; (b) triple-shelled hollow spheres.
Fig. 5. FESEM images (insets) and EDX point spectra of (a) double-shelled hollow sphere; (b) triple-shelled hollow sphere. Al peaks are attributed to the aluminum foil used as the holder. The Au peaks are from the conducting layer of Au used for FESEM characterization. The scale bar of insets in (a) and (b) is 200 nm.
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Fig. 7. (a) UVevisible spectra of the solutions at 10 mg L1 of MB in the presence of SnO2 triple-shelled hollow spheres; (b) Photocatalytic activities of SnO2 double-shelled hollow spheres and triple-shelled hollow spheres.
the initial concentration of dye (C0 ¼ 10 mg L1). Obviously, only a weak activity can be obtained for the decolorization of MB aqueous solution without SnO2 photocatalysts, which confirms the photostability of the dye. The addition of SnO2 photocatalysts leads to obvious degradation of MB, and the photocatalytic activity depends on the morphology of the SnO2 samples (Fig. 7b). After UV irradiation for 180 min, the decomposition rate of MB for triple-shelled SnO2 hollow spheres is 71.8% (C ¼ 2.82 mg L1); and the value is 64.2% for double-shelled SnO2 hollow spheres (C ¼ 3.58 mg L1), respectively. Therefore, it is clear that the photocatalytic activities of triple-shelled SnO2 hollow spheres are higher than that of double-shelled SnO2 hollow spheres. Based on the above results, the higher photocatalytic activity of triple-shelled SnO2 hollow spheres may be ascribed to tripleshelled hollow porous structure and higher specific surface area. It is believed that a high surface area of triple-shelled SnO2 hollow spheres are responsible for providing more surface sites, in which free radicals could be generated by the reaction between the photogenerated charge carriers and adsorbed molecules [5e8,16,17]. As shown in Fig. 4, the specific surface areas of SnO2 triple-shelled spheres and double-shelled spheres are 55.7 and 44.5 m2 g1, respectively. The above results confirm that a decrease in specific surface area results in a decrease in the photocatalytic activity. Recent researches have also demonstrated that hollow structured materials with multiple-shells, such as WO3, Fe2O3 and CeO2, are promising candidates in the photocatalytic field [6e8]. Advantages include (1) the multi-shelled hollow porous structure provides more active sites at multiple shells, allowing efficient transport of the reactant molecules and thus leading to higher photodegradation efficiency; (2) multiple reflection of the illumination light may occur at the multiple shells of the porous spheres, resulting in more efficient absorption of the light [6e8,10]. In our case, triple-shelled SnO2 hollow spheres show higher photocatalytic activity as compared with double-shelled SnO2 hollow spheres, which conforms to the above conclusion. 4. Conclusions In summary, we developed a facile method for the controlledsynthesis of uniform SnO2 double-shelled and triple-shelled hollow spheres. SnO2 double-shelled and triple-shelled hollow spheres could be tailored by simply adjusting concentration of tin (IV) chloride solution during the process of the tin (IV) ions infused carbonaceous microspheres. Photocatalytic properties of the asobtained SnO2 samples were also investigated. Triple-shelled SnO2 hollow spheres exhibited enhanced photocatalytic properties as compared with double-shelled SnO2 hollow spheres. Investigations confirmed that the higher photocatalytic activity of
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