pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Fabrication and Optical Property of Hollow SnO2 Spheres and Their Application in Water Treatment Liang Shi*,† and Hailin Lin‡ †
Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China, and Department of Chemistry and Engineering, ZhongKai University of Agriculture Technology, Guangzhou, 510225, P. R. China
‡
Received September 20, 2010. Revised Manuscript Received October 20, 2010 Hollow SnO2 spheres with smooth surface have been fabricated by a low temperature template-free solution phase route via self-assembly of small nanocrystalline particles. These hollow spheres have a very thin shell thickness of about 10 nm and are built from SnO2 nanocrystals of an average size of 5.3 nm. The evacuation behavior of inside-out Ostwald ripening can be used to explain the formation of hollow spheres according to results of time-dependent reactions. The cathodoluminescence spectrum indicates a blue shift of the band gap emission peak of SnO2, originating from quantum confinement effect due to the nanoscle size of SnO2 particles. The as-prepared SnO2 hollow spheres were also found to exhibit excellent performance in wastewater treatment.
1. Introduction In recent years, inorganic hollow nanostructures have attracted increasing attention because they provide various excellent properties such as low density, large surface area, surface permeability, and so on, which enable them to be widely used in the fields of ionic intercalation, catalysts, photonic devices, chemical sensors, dyes, cosmetics, and capsules for controlled release of therapeutic agents.1-4 Up to date, hollow inorganic nanostructures, including oxides and metals, have been prepared with a variety of different approaches.5-8 Most of these reported approaches are based on templating methods with either hard or soft templates.9-11 In a typical procedure, the surface of the template is first coated with thin layers of desired materials by controlled surface precipitation of inorganic precursors or direct surface reaction utilizing special functional groups on the core, and the template core is then removed selectively by either calcination or solvent etching. Many methods require incorporation of surfactants, and the stability of the system limits their potential applications. The template removal process involves repetitious centrifugation, washing, and redispersion. The complexity associated with the templating methods is the main drawback, and it is not easy to overcome. Therefore, much effort has been dedicated to the design and fabrication of hollow nanostructures with simple and one-step methods without using any templates in order to avoid the *To whom correspondence should be addressed. Telephone: 86-5513607234. Fax: 86-551-3607402. E-mail:
[email protected]. (1) Hung, L. I.; Tsung, C. K.; Wenyu Huang, W. Y.; Yang, P. D. Adv. Mater. 2010, 22, 1. (2) Sun, Y.; Mayers, B.; Xia, Y. Adv. Mater. 2003, 15, 641. (3) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, D. A.; Weitz, A. R. Science 2002, 298, 1006. (4) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (5) Shi, L.; Lin, H. L.; Bao, K. Y.; Cao, J.; Qian, Y. T. Nanoscale Res. Lett. 2010, 5, 20. (6) Zhu, Y. C.; Bando, Y.; Yin, L. W.; Golberg, D. Chem.;Eur. J. 2004, 10, 3667. (7) Shi, L.; Gu, Y. L.; Chen, Y. L.; Yang, Z. H.; Ma, J. H.; Qian, Y. T. Chem. Lett. 2004, 33, 532. (8) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 2028. (9) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827. (10) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (11) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027.
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complexity induced by templating methods.12-15 The direct fabrication, self-assembly, and self-organization of small particles into desired nanostructures is attractive because of both its special scientific interest and its practicality with simplicity, low cost, and energy savings over other techniques.16,17 This method has been employed to prepare hollow nanostructures including hollow spheres with high efficiency and advantages of simplicity and convenience.18-21 Tin oxide (SnO2) is a stable and n-type large bad gap semiconductor, having excellent optical and electrical properties such as peculiar optical transparency, low resistivity, and high theoretical specific capacity. It has been widely employed in gas sensors, heat mirrors, glass coatings, photocatalysts, transparent electrodes for solar cells, and storage applications. It is well-known that shape and size exert a significant influence on the physical, chemical, electronic, optical, and catalytic properties of nanoparticles. The organized assembly of SnO2 nanocrystals into hollow nanostructures may bring about novel properties and subsequently lead to functional improvements. Up to now, various hollow SnO2 sphere related nanostructures have been prepared by many methods such as sol-gel, microemulsion, hydrothermal, template, or surfactant-assisted approaches.22-28 Some of these reported approaches require either (12) Umar, A.; Hahn, Y. B. Appl. Surf. Sci. 2008, 254, 3339. (13) Zhang, Y.; Zhang, W.; Zheng, H. Scr. Mater. 2007, 57, 313. (14) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (15) Shen, L.; Bao, N.; Yanagisawa, K.; Gupta, A.; Domen, K.; Grimes, C. A. Cryst. Growth Des. 2007, 7, 2742. (16) Colfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (17) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisators, A. P. Nature 2000, 404, 59. (18) Liu, B.; Zeng, H. C. Chem. Mater. 2007, 19, 5824. (19) Wang, W. W.; Zhu, Y. J.; Yang, L. X. Adv. Funct. Mater. 2007, 17, 59. (20) Cui, G.; Hu, Y. S.; Zhi, L.; Wu, D.; Lieberwirth, I.; Maier, J.; Mullen, K. Small 2007, 3, 2066. (21) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (22) Gyger, F.; Hubner, M.; Feldmann, C.; Barsan, N.; Weimar, U. Chem. Mater. 2010, 22, 4821. (23) Liu, S. Q.; Li, Y. X.; Xie, M. J.; Guo, X. F.; Ji, W. J.; Ding, W. P.; Chen, Y. J. Nanosci. Nanotechnol. 2010, 10, 6725. (24) Yin, X. M.; Li, C. C.; Zhang, M.; Hao, Q. Y.; Liu, S.; Chen, L. B.; Wang, T. H. J. Phys. Chem. C 2010, 114, 8084. (25) Yang, R.; Gu, Y. G.; Li, Y. Q.; Zheng, J.; Li, X. G. Acta Mater. 2010, 58, 866.
Published on Web 11/10/2010
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high energy consumption or a relatively complicated procedure. In this work, we demonstrate a convenient single-step solution approach for fabrication of SnO2 hollow spheres via self-assembly of nanoscale particles based on an inside-out Ostwald ripening mechanism. The as-prepared spheres have a smooth surface in contrast to the more irregular coarse surface of the spheres reported in the literature. Furthermore, these hollow spheres have a thin shell thickness of about 10 nm and are built from very small SnO2 nanocrystals of an average size of 5.3 nm.
2. Experimental Section All reagents are analytical grade and used without further purification. In a typical procedure, 0.3 g of anhydrous SnCl2 (Aldrich, 99.9%) was added into 15 mL of distilled water with stirring in a 25 mL flask, and formation of a white precipitate was observed. Then, 0.20 mL of HCl (37%) was added into the above solution with magnetic stirring, the white precipitate disappeared, and the solution became transparent. Then, the above solution in the flask was heated at 90 °C for 12 h with continuous magnetic stirring before being cooled down to room temperature. During the reaction process, the solution became white opaque gradually, suggesting formation of SnO2. After synthesis, a white precipitate appeared in the solution and was collected by centrifugation, washed with absolute ethanol and distilled water several times, and finally dried in vacuum at 40 °C for 4 h. The overall crystallinity of the product was examined by X-ray diffraction (XRD, Rigakau RU-300 with Cu KR radiation). The general morphology of the products was characterized using scanning electron microscopy (SEM, LEO 1450VP). Detailed microstructure analysis was carried using transmission electron microscopy (TEM, PhilipsCM120). For TEM analysis, an appropriate amount of sample powder was ultrasonically dispersed in ethanol, and then a drop of the suspension was placed on the TEM copper grid. The TEM copper grid containing as-prepared nanostructures was then observed with transmission electron microscopy under 120 kV accelerating voltages. The chemical composition analysis was obtained by energy dispersive X-ray (EDX) spectrometry using an EDX spectrometer attached to the same micoscope. Room-temperature Raman spectra were measured using a microlaser Raman spectrometer (Renishaw) in a backscattering configuration, employing the 514.5 nm line of Ar laser as the excitation source. Cathodoluminescence (CL) spectra were taken in a scanning electron microscope (Cambridge S360) using a MonoCL system (Oxford Instrument). Fourier transformation infrared (FT-IR) spectra were measured with a Shimadzu IR-400 spectrometer with KBr pressed disks. Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. For conducting water treatment experiments, basic fuchsine was used as the organic contaminant in the wastewater and the as-prepared hollow SnO2 spheres were used as absorbent. A TU-1901 UV-vis spectrometer was used to monitor the basic fuchsine concentration remaining in the solutions. The control experiments are also performed on bulk SnO2 powders and commercial Degussa P25 TiO2 powders.
3. Results and Discussion Figure 1 shows typical SEM images of the SnO2 product. A large amount of smooth and uniform spheres can be observed in the sample. These spheres are discrete with a size distribution of 100-300 nm in diameter. Some broken spheres disclose their hollow structure. The inset of Figure 1 gives a magnified SEM (26) Miao, Z. J.; Wu, Y. Y.; Zhang, X. R.; Liu, Z. M.; Han, B. X.; Ding, K. L.; An, G. M. J. Mater. Chem. 2007, 17, 1791. (27) Xu, J. Q.; Wang, D.; Qin, L. P.; Yu, W. J.; Pan, Q. Y. Sens. Actuators, B 2009, 137, 490. (28) Zhao, Q. R.; Gao, Y.; Bai, X.; Wu, C. Z.; Xie, Y. Eur. J. Inorg. Chem. 2006, 8, 1643.
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Figure 1. SEM images of the as-prepared SnO2 hollow spheres.
Figure 2. Room-temperature XRD pattern (a) and Raman spectrum (b) of the as-prepared SnO2 product.
image of one broken sphere with a hole in the shell, indicating clearly its hollow nature. A typical XRD pattern of the as-prepared SnO2 product is shown in Figure 2a to characterize the crystal structure of the product. All diffraction peaks can be indexed to the rutile structured SnO2 with tetragonal lattice constant a=4.73 A˚ and c=3.18 A˚, which match well to the reported value for SnO 2 crystal (JCPDS card, no. 41-1445). No signals of impurities or byproducts were found in the product. The obvious broadening of XRD peaks suggests that the as-prepared SnO2 particles are of very small sizes. Based on the Scherrer equation, D = (0.89λ)/ β(cos θ), where λ is the wavelength for the KR1 (1.54056 A˚), β is the peak width at half-maximum in radians, and θ is the Bragg angle, the average particle size was calculated to be 5.3 nm. The particle size result is consistent with further structural analysis in later sections. Raman scattering spectroscopy was employed to investigate the crystal quality and vibration properties of the as-prepared SnO2 hollow spheres. Figure 2b shows a typical room-temperature Raman spectrum for the SnO2 product in the range of 400-900 cm-1. It is known that a unit cell of the typical rutile structure of SnO2 contains two tin ions and four oxygen ions and belongs to the space group P4n/mnm. According to the group theory, the active Raman modes B1g, Eg, A1g, and B2g can be observed in first-order spectrum. Among the four first-order active Raman modes of rutile SnO2, the A1g mode is much stronger, and the intensities of the other modes are relatively weaker. In Figure 2b, Raman peaks at 498, 625, 686, and 763 cm-1 can be observed. The 625 and 763 cm-1 peaks can be assigned to A1g and B2g modes, respectively. Here, A1g and B2g are related to the expansion and contraction vibration mode of Sn-O bonds. This gives evidence that the as-prepared SnO2 hollow spheres possess the main characteristic of tetragonal rutile structure. The A1g mode of the dominant peak at 625 cm-1 is sensitive to particle size.29 Here, the 625 cm-1 peak is observed to show line broadening; this should be originated from the finite size of the nanoparticles in the sample.30 The 498 and 686 cm-1 peaks in Figure 2b do not appear (29) Kravets, V. G. Opt. Spectrosc. 2007, 103, 766.
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Figure 4. Room temperature FT-IR transmission pattern of SnO2 hollow spheres.
Figure 3. (a) Low and (b) high-magnification TEM images, (d, e) HRTEM images, and (c) EDX spectrum of SnO2 hollow spheres. Inset of (b) shows the SAED pattern for the product.
in bulk SnO2, and we believe they are induced by the size effect of the very small SnO2 nanocrystals. It has been reported that relaxation of the k=0 selection rule will be progressive if the size decreases or the rate of disorder increases.31 Infrared (IR) modes will become weakly active as a result of the structural changes resulting from disorder and size effects. Therefore, the weak Raman peaks at 498 and 686 cm-1 should be induced by the IR-active A2u modes due to the size effect of the very small SnO2 nanocrystals. Similar behavior has been reported for SnO2 nanostructures.32 TEM studies accompanied by selected area electron diffraction (SAED) and EDX spectrometry were performed to characterize the morphology, structures, and chemical composition of the SnO2 products, as depicted in Figure 3. A TEM image in Figure 3a indicates that the as-prepared spheres are of smooth surface. The obvious contrast between the dark edge and the pale center of the spheres confirms their hollow nature. Figure 3b shows a magnified TEM image of a hollow sphere, indicating the shell thickness of the hollow spheres is as thin as 10 nm. The diffraction rings of the SAED pattern for the product, as displayed in the inset of Figure 3b, reveal the polycrystalline nature of these hollow spheres. The brightest diffraction ring can be indexed to (110) planes. The EDX spectrum (Figure 3c) from the SnO2 product shows intense peaks of Sn and O, displaying the composition as Sn and O only. The Cu and carbon signals come from the supporting TEM grid. EDX quantitative analysis gives an average Sn/O composition of 1:2 within the accuracy of the technique, in accordance with the stoichiometry of SnO2. A highresolution TEM (HRTEM) image (Figure 3d) shows that the asprepared hollow spheres are actually built from very fine nanocrystalline grains with size as small as about 5 nm, as already evidenced by the above XRD and Raman results. The lattice fringe spacing of each nanocrystalline particle is measured to be 0.335 nm, matching the d value for (110) planes of rutile SnO2. Moreover, we can observe some amorphous substance between the SnO2 particles by careful examination, as shown by the arrows in Figure 3d. It is shown clearly in the HRTEM image that the (30) Dieguez, A.; Romano-Rodriguez, A.; Vila, A.; Morante, J. R. J. Appl. Phys. 2001, 90, 1550. (31) Abello, L.; Bochu, B.; Gaskov, A.; Koudryavtseva, S.; Lucazeau, G.; Roumyantseva, M. J. Solid State Chem. 1998, 135, 78. (32) Peng, X. S.; Zhang, L. D.; Meng, G. W.; Tian, Y. T.; Lin, Y.; Geng, B. Y.; Sun, S. H. J. Appl. Phys. 2003, 93, 1760.
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SnO2 nanoparticles are all covered with this kind of amorphous material. It is known that nanocrystals have high surface energy and they tend to contact and grow together or fuse into larger particles to decrease the surface energy. If this amorphous material happens not to be present between the interfacial grains, as shown between the A and B particles in Figure 3e (a arrow indicating the interface), the oriented attachment takes place and leads to the elimination of the common grain boundary. They tend to coalesce into a larger single particle. Here, we believe the SnO2 crystal growth is inhibited due to isolation by this amorphous material and the nanoscale size of the SnO2 nanocrystals can be preserved as a result. FT-IR studies can give more information about this amorphous material covering the SnO2 nanocrystalline particles. A FT-IR transmission pattern of SnO2 hollow spheres is shown in Figure 4. It indicates that the Sn-O-Sn vibration appeared in the range of 400-700 cm-1 as the result of condensation reaction.33 The peak located at 665 cm-1 is induced by the Sn-O-Sn antisymmetric vibrations. The peak at 563 cm-1 is a character of the very small particle size, disclosing the nanosize effect, and could be attributed to surface modes.31 Absorption peaks at 3395 and 1634 cm-1 are originated from the vibration of hydroxyl. The strong peak of 3395 cm-1 means that there are many O-H bonds in the sample, suggesting the surfaces of the particles are covered with hydroxyl species. In order to reveal the formation mechanism of hollow SnO2 spheres, time-dependent reactions were carried out to investigate the growth process in detail and distinct evolution stages were obtained. Figure 5 shows the typical TEM images of the products prepared at certain reaction time intervals. As shown in Figure 5a and a magnified TEM image in Figure 5b, very fine nanoparticles with sizes of about several nanometers appeared first at the initial stage of 3 h. As the reaction was prolonged to 5 h, nanoparticles aggregated into solid spheres (Figure 5c). With the reaction progressing further, some little voids can be found by careful observation inside part of these solid spheres, as shown in Figure 5d. This was induced by the beginning of evacuation process. After 9 h reaction, hollow space appears evidently inside each sphere and most part of the interior part of these spheres became hollow. Finally, over the whole 12 h reaction duration, hollow spheres with smooth thin shells were formed (Figure 5f). Based on the above results of time-dependent reactions, it is proposed that the hollow spheres are formed through three consecutive steps: (1) first production of very fine nanocrystals; (2) formation of solid spheres via aggregation of the previously nanoparticles; (3) evacuation of solid spheres and growth into perfect hollow spheres based on an inside-out Ostwald ripening mechanism.34 During the initial stage of the reaction process, (33) Cukov, L. M.; Tsuzuki, T.; McCormick, P. G. Scr. Mater. 2001, 44, 1787. (34) Zeng, H. C. Curr. Nanosci. 2007, 3, 177.
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Figure 5. TEM images of the products obtained from the timedependent reactions for the synthesis of hollow SnO2 spheres: (a, b) 3 h, (c) 5 h, (d) 7 h, (e) 9 h, and (f) 12 h.
Figure 6. Room temperature CL spectrum of SnO2 hollow spheres.
oxidation of Sn2þ by oxygen produced SnO2 nanoparticles with diameter of ∼5 nm.35 Then, these nanoparticles aggregated into solid spheres for the minimization of overall system energy. It should be noted that these solid spheres are all composed of very small nanocrystalline particles, just as shown in Figure 5b. The particles located in the inner region should be packed more loosely than those in the outer layer. Therefore, the smaller crystallites located at central cores have higher surface energy, and they tend to dissolve and relocate themselves to the shell parts during the Ostwald ripening which processes with recrystallization and hollowing. The central void space is getting bigger with continuing evacuation, and as a result the central part of solid spheres is evacuated and the dense packing of the shell is developed. Finally, hollow spheres are formed. The driving force for evacuation behavior by inside-out Ostwald ripening comes from decreasing the total surface energy. Similar behavior has been observed and demonstrated in the preparation of other hollow structures, such as hollow Cu2O cubes and hollow TiO2 spheres.36,37 Figure 6 shows the room-temperature CL spectrum of the asprepared hollow SnO2 spheres. A low-energy stronger emission centered at around 536 nm and a high-energy emission located at (35) Han, W. Q.; Zettl., A. J. Am. Chem. Soc. 2003, 125, 2062. (36) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (37) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492.
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331 nm can be observed in the pattern. The low-energy yellow emission is believed to result from crystal defects or defect levels associated with oxygen vacancies, or tin interstitials formed during the growth.38-41 The obviously much higher intensity of 536 nm peak suggests a lot of crystal defects and higher defect levels in it, possibly due to the hollow structure and very small size nanocrystals. It is believed that the oxygen vacancies are located on the surfaces of particles and interact with interfacial tin vacancies. Therefore, many trapped states within the band gap are formed and give rise to a higher CL intensity peak of the lowenergy emission for hollow SnO2 spheres. The high-energy emission should be originated from the band-to-band emission of the as-prepared SnO2 product. The energy gap of the bulk SnO2 is 3.62 eV at room temperature, and the corresponding emission peak is at 340 nm.42,43 The peak’s position of the present band-toband emission is 331 nm, smaller than 340 nm, indicating a significant blue shift from the band gap energy of bulk SnO2, owing to the quantum confinement effects of nanoscale particles.44,45 We also investigated the potential application of the asprepared hollow spheres to be used as an adsorbent in wastewater treatment. Basic fuchsine was chosen as a typical organic contaminant in the wastewater because it is often used as a dye in the textile industry. Fuchsine molecules will adsorb on the metal oxide surface in the solution due to the coordination effect between metal ions and amine groups of basic fuchsine molecules. Based on this adsorption approach, the higher the surface area of SnO2 hollow spheres, the greater the amount of basic fuchsine molecules that can be removed. The specific surface area of SnO2 hollow spheres was evaluated to be 88.6 m2 g-1 based on the BET result. This high surface area would be very beneficial for adsorbing and removing organic contaminant. UV-vis absorption spectroscopy was used to record the adsorption behavior of the solution after adding 20 mg of as-prepared SnO2 hollow spheres under stirring in the dark. Figure 7a shows the time-dependent absorption spectra of the fuchine solution, and the characteristic absorption of basic fuchsine at approximately 545 nm was used to monitor the process of adsorption. We can see that the intensity of the 545 nm peak decreased very quickly once SnO2 hollow spheres were added. After only 30 min, this peak became too weak to be observed, suggesting the high efficiency for removing fuchsine molecules. Further experiments were carried out to compare the adsorption activity of the as-prepared SnO2 hollow spheres to bulk SnO2 powders and commercial Degussa P25 TiO2 powders. Figure 7b shows the adsorption efficiency of basic fuchsine versus reaction time for these three samples. Here, C0 and C are concentrations of basic fuchsine before and after treatment, respectively. It discloses clearly that the SnO2 hollow spheres show a higher efficiency than that of other two materials. This should be induced by the much larger surface area resulted from the hollow spheres structure and its being made up of fine nanocrstalline particles with very small size. Therefore, the as-prepared SnO2 hollow spheres may find applications in wastewater treatment for the environmental protection. (38) Wang, B.; Yang, Y. H.; Wang, C. X.; Yang, G. W. Chem. Phys. Lett. 2005, 407, 347. (39) Huang, L. S.; Pu, L.; Shi, Y.; Zhang, R.; Gu, B. X.; Du, Y. W. Appl. Phys. Lett. 2005, 87, 163124. (40) Zhang, X. T.; Liu, Z.; Ip, K. M.; Leung, Y. P.; Li, Q.; Hark, S. K. J. Appl. Phys. 2004, 95, 5752. (41) Zhang, X. T.; Liu, Z.; Li, Q.; Hark, S. K. J. Phys. Chem. B 2005, 109, 17913. (42) Tatsuyama, C.; Ichimura, S. Jpn. J. Appl. Phys. 1976, 15, 843. (43) Aoki, A.; Sasakura, H. Jpn. J. Appl. Phys. 1970, 9, 582. (44) Banyai, L.; Hu, Y. Z.; Lindberg, M.; Koch, S. W. Phys. Rev. B 1988, 38, 8142. (45) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226.
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Figure 7. (a) Absorption spectra of basic fuchsine aqueous solution (0.1 mM, 20 mL) in the presence of SnO2 hollow spheres at different intervals. (b) Adsorption rate of basic fuchsine on as-prepared SnO2 hollow spheres, bulk SnO2 powders, and commercial Degussa P25 TiO2 powders.
In summary, direct fabrication of hollow SnO2 spheres has been carried out via self-assembly of small nanocrystalline particles with a solution phase route. The morphology, structure, composition, and luminescence properties were systematically characterized by SEM, XRD, Raman, FT-IR, TEM, SAED, EDX, HTREM, and CL. The size of these hollow smooth spheres is in a range of 100-300 nm. The as-prepared spheres have a smooth surface in contrast to the more irregular coarse surface of the spheres reported in the literature. Furthermore, these hollow spheres have a thin shell thickness of about 10 nm and are built from very small SnO2 nanocrystals of an average size of 5.3 nm. The formation process for the hollow spheres has been illustrated in detail, and inside-out Ostwald ripening can be used to explain the production of hollow structures from evacuation of solid spheres. Surfaces of the nanoparticles are found to be covered
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with amorphous hydroxyl species, which is believed to prevent SnO2 nanocrystals from growth by isolation of them. Quantum confinement effect is found to occur induced by the small size of these SnO2 nanocrystals based on the CL analysis results. The present single-step approach has the advantage of simplicity and low cost for the preparation of SnO2 hollow spheres and may be extended to fabrication of hollow structure for other oxides system. The as-prepared SnO2 hollow spheres were found to exhibit excellent adsorption performance of basic fuchsine in aqueous solution, and they are expected to be employed in wastewater treatment for environmental cleaning. Acknowledgment. The financial support of this work by the National Natural Science Foundation of China (Grant No. 20771096) is gratefully acknowledged.
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