- Email: [email protected]
Luminescent and photocatalytic properties of hollow SnO2 nanospheres
Materials Science and Engineering B 178 (2013) 725–729
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Luminescent and photocatalytic properties of hollow SnO2 nanospheres Yanhua Zhu, Lingling Wang ∗ , Guifang Huang, Yifeng Chai, Xiang Zhai, Weiqing Huang School of Physics and Microelectronics and Key Lab for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 23 October 2012 Received in revised form 25 January 2013 Accepted 11 March 2013 Available online 26 March 2013 Keywords: Hollow nano-sphere Tin oxide Sacrificing template Specific surface area Photoluminescence Photocatalysis
a b s t r a c t Size tunable solid SnO2 (STO) and hollow SnO2 (HTO) nanospheres were prepared by a sacrificing template method. The peaks around 390 nm were observed in photoluminescence (PL) spectra. Based on the results, the PL intensity exhibits morphology-dependence and size-dependence, and HTO structure displays better optical properties than STO structure. The degradation of Methyl Orange (MO) in aqueous solution is selected as a probe reaction to evaluate the catalytic activity of nano-SnO2 . The result shows that HTO structure presents stronger photocatalytic (PC) activity. According to the result of specific surface area testing, the improved PL and PC properties of HTO structure can be mainly explained by the surface effect induced by large specific surface area. This work is meaningful for developing nanomaterials with enhanced optical and photochemical properties. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, great interest has been attracted on nanostructures owing to their unique properties and novel applications [1] due to the so-called quantum size effect. As a potential luminescent material, semiconductor nanoparticle shows fascinating optical properties with many technological applications in optics and optoelectronics [2]. In addition, semiconductor nanomaterial is also considered as potential candidate for chemical and environmental applications due to its high catalytic efficiency [3–5]. As an n-type semiconductor oxide with wide band gap (Eg 3.6 eV at 300 K), SnO2 is well-known for its potential applications in many fields. Plenty of papers have reported the optical properties of SnO2 nanostructures synthesized by these methods. The PL peaks at 355, 390, 460 and 580 nm have been observed [6,7]. In solar applications, SnO2 /Cu2 O solar cell has been reported in recent report [8]. Additionally, a number of reports are also linked to the chemical and environmental applications of nano-SnO2 [9]. The hetero-structural materials such as SnO2 /TiO2 [10], SnO2 /␣Fe2 O3 [11] and SnO2 /CuO [12] have been investigated extensively for their excellent performance in photocatalytic degradation of various organic substances.
Abbreviations: STO, solid tin oxide; HTO, hollow tin oxide; PL, photoluminescence; MO, methyl orange; PC, photo-catalysis; XRD, X-ray diffraction; FE-SEM, field emission scanning electron microscope; TEM, transmission electron microscope; UV, ultra-violet. ∗ Corresponding author. Tel.: +86 13508484833. E-mail address: [email protected] (L. Wang). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.03.006
Nonetheless, there is still room for improvement. As is known, surface defects are the most common defects on crystal surfaces. Such defects can directly or indirectly contribute to PL [13]. Therefore, enlarging the surface area of an optical material is an efficient way to enhance PL intensity. For normal solid nanostructures, the frequently-used method to increase specific surface area is to decrease the size of particles. However, due to some effects such as scattering and resonance, to fabricate ultra-small nanoparticles does not help enhance PL intensity. Thus, to find another method to get materials with large specific surface area is significant for improving the optical properties of nanomaterial [32–37]. Furthermore, for chemical or environmental application, such as organic degradation and water purification, most studies are mainly focused on nanofilms and nanosheets. However, because of the limitation of substrate, these substrate-based nanostructures are not design-friendly and efficient for some special use. On the other hand, normal non-substrated solid nanoparticles are imperfect as well. Owing to the dispersed granular morphology, it is hard to develop such a material with high adsorption of reactant molecules and high utilization of excitation light. Therefore, it is meaningful to develop a granulated nanomaterial with high PL intensity and strong PC activity. Based on these issues, the nanomaterials with hollow structure are considered as a potential solution and have received wide attention. The hollow Cu2 O nanocubes with unique optical properties have been fabricated [14]. The hollow spheres of Eu3+ -doped lanthanides compounds have been studied [15]. The enhanced PC properties of Co3 O4 /BiVO4 [16], TiO2 /PtCl4 [17], (BiO)2 CO3 [18–21] hollow microspheres have been reported. Herein, we report the luminescence and catalysis properties of HTO nanospheres, which
726
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
have large specific surface area and proper particle size. We believe that, compared with normal solid structure, the higher PL intensity and the stronger PC activity of HTO nanostructure will be much more potential and useful for optical and chemical applications.
2. Experimental 2.1. Synthesis The synthesis of STO nanospheres was followed by a low temperature combustion synthesis approach. SnCl4 ·5H2 O and concentrated nitric acid were mixed into a beaker at a molar ratio of 1:1 to transform into tin nitrate. Urea, boric acid and water were added in a proper ratio as well. The solution was handled by ultrasonic dispersion for 10 min, and then transferred into a preheated muffle furnace heating at 600 ◦ C for about 5 min. The product was cooled and grinded, and then the STO nanospheres were obtained. In these processes, we changed the ratio between urea and boric acid in order to prepare samples with different sizes. The HTO nanospheres were synthesized by a sacrificing template method. The general synthesis method mainly includes the adsorption of metal ions from solution to the surface layer of carbonaceous spheres and subsequent removal of the carbonaceous cores via calcinations [22]. The experiment procedures are as follow: Glucose solution (1 mol L−1 , 20 mL) was poured into a 25 mL autoclave, then the autoclave was transferred into an oven maintaining at a heating temperature between 180 and 250 ◦ C for 12 h. The product was centrifugal separated, washed three times with distilled water and anhydrous alcohol, and finally dried at 450 ◦ C for 6 h. The obtained colloidal carbonaceous spheres were used as templates. SnCl4 was mixed with ethanol at a molar ratio of 1:20, and the solution pH value was tuned by hydrochloric acid to different pH values, then tetraethyl orthosilicate (TEOS) was added into the solution. After a 30 min magnetic stirring, 200 mg colloidal carbonaceous spheres were put into the solution, and kept stirring for 24 h. In this process, Sn4+ ions combined with the unsaturated bonds on the surfaces of carbonaceous spheres and composed the shell of tin hydroxide outside carbonaceous core. Then the product was centrifugal separated and washed with distilled water and anhydrous alcohol, dried in an oven at 40 ◦ C for 12 h, and annealed at 450 ◦ C for 4 h. During the annealing, carbonaceous core were removed from the core-shell structure, and the reserved shell were oxidized and formed the hollow structure. It is worth mentioning that the sizes and structures of different metal oxide hollow spheres are predominantly determined by the templates [22]. Therefore, we can control the size of carbonaceous spheres by changing heating temperature in order to prepare size-tunable HTO nanospheres.
2.2. Characterization The morphology and crystal structure of as-synthesized nanoSnO2 were characterized by X-ray diffraction (XRD) using a Rigaku D/MAX-2400 utilizing Cu K␣ radiation, field emission scanning electron microscope (FE-SEM) employing a JEOL JSM-6700F electronic microscope, transmission electron microscope (TEM) images were collected with a JEOL JEM-2010 high-resolution transmission electron microscopy. The room temperature PL spectra of samples were recorded on a Hitachi F-4500 FL spectrophotometer with a Xe lamp as the excitation light source. The specific surface areas of HTO and STO were measured by BET (Brunauer, Emmett and Teller) method. 2.3. PC activity test The PC activity of HTO was evaluated by studying the photocatalytic degradation of aqueous MO solution. 50 mg nano-SnO2 (HTO or STO) was suspended in 50 mL of 10 mg L−1 MO aqueous solution. The solution was continuously stirred for about 30 min at room temperature to establish an adsorption-desorption equilibrium between the MO and photocatalyst. The PC reaction proceeded in an ultraviolet photochemical reactor (BL-GHX-V, Beijing Bilon Lab Equipment Co. Ltd). MO degradation rate was analyzed by UV–vis spectroscopy at its maximum absorption wavelength of 464 nm. 3. Results and discussion 3.1. Crystal structure The images of as-synthesized SnO2 are shown in Fig. 1, where Fig. 1(a) and (b) are the SEM images of unannealed nano-SnO2 , and Fig. 1(c) is the TEM image of HTO nanospheres, respectively. According to the observation, the size of particles is in the range of 100 to 200 nm, and we can find from Fig. 1(a) and (b) that the SnO2 nanoparticles are with similar size and spherical shape. However, the particles are still solid and undistributed. It is because of the products are not annealed and the carbonaceous cores which were used as templates were still inside of particles. Therefore, we removed carbonaceous cores by annealing and obtained the image of HTO nanospheres showed as Fig. 1(c). Generally, the nanoparticles present a porous spherical structure, and there is a brightness difference between center and border area, which indicates these structures are hollow. That is because the carbonaceous cores have transformed into carbon oxide (CO, CO2 ) and escaped through these micropores during annealing, and only porous SnO2 shells are reserved. Furthermore, the unbroken hollow spheres indicate carbon oxide gas escaped without any destruction to SnO2 shells.
Fig. 1. Images of HTO nanoparticles. (a and b) The SEM images of unannealed HTO structure, (a) 30,000×, (b) 100,000×, respectively. (c) The TEM image of HTO nanospheres.
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
727
Fig. 4. Specific surface area patterns of HTO and STO with different sizes. Fig. 2. XRD patterns for different SnO2 nanostructures. (a) XRD patterns of HTO nanospheres before annealing. (b) XRD patterns of HTO nanospheres after annealing. (c) XRD patterns of STO nanospheres.
The structures of the samples were characterized by XRD, and the patterns are displayed in Fig. 2, where the curves (a) and (b) are the XRD patterns of HTO nanospheres before and after annealing, and the curve (c) shows the pattern of STO nanospheres which is used for comparison. It is showed that there are SnO, SnOx , SnO2 and some other amorphous carbonic phases in curve (a), but there is only SnO2 phase in curve (b). It indicates that during the process of annealing, the SnO and SnOx phases were completely oxidized to SnO2 , and the residual carbon was transformed into carbon oxides and escaped to the air. According to curve (b) and (c), both HTO and STO diffraction lines are assigned to tetragonal rutile crystalline phases of tin oxide. The patterns are in good agreement with reference pattern of tin oxide (Powder Diffraction File No. 41-1445). 3.2. Photoluminescent property Fig. 3 shows the PL spectra of HTO and STO at room temperature using 302 nm laser. As is shown, all PL peaks are around 390 nm which is in the border between visible and ultraviolet range. Earlier reports indicate that SnO2 thin films exhibited a strong peak around 396 nm (3.1 eV) [23]. Since the energy of emission peak (390 nm, 3.2 eV) is lower than the band gap of SnO2 (3.6 eV), this observed
Fig. 3. PL spectra for STO and HTO. (a) STO with different sizes. (b) HTO with different sizes.
emission cannot be ascribed to the direct recombination between a conduction electron in Sn4+ band and a hole in O2− valence band. In the light of the research of Lü’s group [24], the origin of this emission can be attributed to electron transition mediated by defects levels in the band gap, such as oxygen vacancies, owing to the temperature behavior of PL spectra of the nano-SnO2 powders, which is similar with nanoscaled SrTiO3 , TiO2 , ZnO, and BaTiO3 [25–28]. Based on the PL intensities of HTO and STO particles showed in Fig. 3, it is obvious that HTO structure exhibits stronger emission capacity. For similar particle sizes, the PL intensities of HTO particles are about 25% higher than those of STO. This result implies HTO has better optical properties than normal STO structures. It is well-known that, the intrinsic defects form defect levels in band gap in most semi-conductor materials, which trap electrons from valence band to contribute to luminescence. In semiconductor metal oxides, oxygen vacancy is an intrinsic donor and considered to be the most common defect, which usually acts as luminescent centers. In these material structures, luminescence is usually ascribed to the recombination of electrons in singly occupied oxygen vacancies with photo-excited holes in the valence band [29]. For SnO2 nanostructure, O2− is usually stable with several cationic surrounding in crystal lattices [30]. However, it is different on the surface of crystal that, the stability of O2− decreases rapidly as a result of the discontinuity of surface lattice, which is the so-called surface defect. Because of this defect, a large amount of oxygen vacancies are provided on the surface including the para•• •• magnetic VO• , which can transform into Vo when capture holes. Vo can recombine with photo-excited electrons, and plays a role of recombination center in luminescence processes. Therefore, we can describe the process of luminescence as below: when nano-SnO2 is illuminated by a laser with enough energy, electrons are photoexcited to conduction band from valence band and leave plenty of •• active holes. On the surface of nano-SnO2 , a number of Vo are gen• erated when holes are directly captured by Vo , they recombine with electrons and emit luminescence. Fig. 4 shows the BET surface area analysis results of assynthesized HTO and STO. It indicates that the specific surface area of HTO is clearly larger than that of STO, which means HTO has larger surface area that causes more surface defect. In accordance with the description in last paragraph, while illuminated by the ultraviolet laser, there are more oxygen vacancies generated in HTO •• structure due to its larger surface area. Hence more Vo are generated and recombine with electrons in the crystal, resulting in an enhancement of the luminescence intensity.
728
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
Fig. 5. MO degradation rate with UV irradiation time. (a) The time dependence of MO concentration under catalysis of as-synthesized nano-SnO2. (b) Reveals the PC abilities of HTO and STO with different sizes.
Moreover, Fig. 3 also indicates the size-dependent of PL intensity. As is shown, the PL intensity initially increases with the decrease of particle size and gets maximized with a size between 120 and 130 nm, and subsequently decreases as the size continues to decrease. This is potentially caused by the counterbalance between the improvement of PL intensity resulted in surface effect and the reduction of PL intensity induced by scattering effect and resonance effect [31]. With the decrease of particle size, the surface effect initially acts as the primary factor in the PL process, and then the scattering effect and the resonance effect become stronger and get dominant as the particles become ultra-small. In our results, the highest PL intensities are observed on the particles with a size between 120 nm and 130 nm, which indicates these positive and negative effects reach equilibrium with such particle size.
surface area, show a larger adsorption capacity and can be excited more efficiently with UV light. On the other hand, the special porous surface structure of the HTO particles which allows multiple reflections of UV light enhances photo-energy harvesting and benefits to improve the diffusion of reactants during PC reaction [18]. Both of these result in an enhanced PC activity. It is noteworthy that previous results indicate the contradiction between high PC activity and high PL intensity due to the inhibition of electron/hole pair recombination caused by the special structure [16,17]. However, unlike heterojunction materials, the undoped and unary crystal structure of SnO2 does not induce any special mechanism to inhibit the recombination of electron/hole pair. The enhancements of HTO structure on PL intensity and PC activity are resulted by the change of morphology.
3.3. Photocatalysis property
4. Conclusions
The time dependence of MO concentration under catalysis of as-synthesized nano-SnO2 is displayed in Fig. 5(a), where red and blue curves stand for HTO (100 nm) and STO (90 nm) respectively, green and purple curves give the degradation rate of reactions proceeded only with SnO2 catalyst or UV light, which indicates both catalyst and UV light are essential for MO degradation. Fig. 5(a) shows that after 4 h of reaction, in the case of HTO, the degradation rate of the MO concentration was about 75%, whereas in the case of STO, the degradation rate was less than 50%. It is evident that HTO performs a much more efficient PC activity than STO. Fig. 5(b) reveals the PC abilities of HTO and STO with different sizes. Based on Fig. 5(a), the MO degradation rates catalyzed by HTO are more than 50% higher than STO in the groups of similar sizes. Furthermore, the columns also indicate a dependence of the PC activity of the nano-SnO2 on size. But contrary to the PL intensity, the PC activity is related monotonously to particle size. In our results, the PC activity of the nano-SnO2 shows an enhancement with the decrease of particle size and reaches highest for the smallest particles. We can also explain these results by the difference of specific surface area. In a standard process of PC reaction, reactants are first adsorbed by catalyst particles. Then, on the surface of catalyst, the electrons are photo-excited by UV light and transfer energy to the reactant molecules, which make the reactants reacted and decomposed. In these processes, reactant adsorption and photo-excitation are considered as the most two important steps. Owing to this, adsorption capacity and utilization of excitation light are also considered to be the key factors for photocatalyst. The adsorption capacity of catalyst particles is mainly determined by their surface area. Generally, larger surface area leads to larger adsorption capacity. Therefore, HTO synthesized particles which have larger specific
The HTO nanospheres were prepared by sacrificing template method. The size of fabricated HTO is between 100 and 200 nm. The PL measurements show that HTO particles emit stronger luminescence than normal STO ones, reaching the maximum PL intensity when the particle size is between 120 and 130 nm. The BET results indicate that both size-dependence and morphology-dependence of the PL intensity are mainly due to the specific surface area of the particles. In the study of PC property, HTO also exhibits higher catalytic activity, which can be attributed to its large specific surface area. This work provides a simple and efficient way to fabricate nanomaterials with higher PL intensity and stronger PC activity. We believe that our work is meaningful for the applications in optical, chemical and environmental fields. Authorship statement The submission of the manuscript has been approved by all coauthors. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11074069, 61176116), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120161130003), the Hunan Provincial Science and Technology Project of China (Grant Nos. 2011SK3217, 2012FJ4121), 2011 Graduate Science and Technology Innovation Program of Hunan Province (Grant No. CX2011B153), Henan Provincial Natural Science Foundation of China (Grant No. 122300410350), Aid Program for Science and Technology
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Hunan Provincial Natural Science Foundation of China (Grant No. 12JJ3009). References [1] Z.L. Wang, Applied Physics A 88 (2007) 7. [2] L.A. Patil, M.D. Shinde, A.R. Bari, V.V. Deo, Materials Science and Engineering B 176 (2011) 579. [3] W.H. Leng, H. Liu, S.A. Cheng, Journal of Photochemistry and Photobiology A: Chemistry 131 (2000) 125. [4] A.R. Kamali, D.J. Fray, Materials Science and Engineering B 177 (2012) 819. [5] Z.W. Seh, S. Liu, M. Low, S.Y. Zhang, Z. Liu, A. Mlayah, M.Y. Han, Advanced Materials 24 (2012) 2310. [6] L. Wang, J.W. Ren, X.H. Liu, G.Z. Lu, Y.Q. Wang, Materials Chemistry and Physics 127 (2011) 114. [7] G. Salviati, L. Lazzarini, Z.Z. Ming, V. Grillo, E. Carlino, Physical Status Solidi A 202 (2005) 2963. [8] Z.G. Zang, A. Nakamura, J. Temmyo, Materials Letters 92 (2013) 188. [9] G. Cheng, J.Y. Chen, H.Z. Ke, J. Shang, R. Chu, Materials Letters 65 (2011) 3327. [10] K. Vinodgopal, I. Bedja, P.V. Kamat, Chemistry of Materials 8 (1996) 2180. [11] M.T. Niu, F. Huang, L.F. Cui, P. Huang, Y.L. Yu, Y.S. Wang, ACS Nano 4 (2010) 681. [12] R. Jiang, H.Y. Zhu, Y.J. Guan, Y.Q. Fu, L. Xiao, Q.Q. Yuan, S.T. Jiang, Chemical Engineering & Technology 34 (2011) 179. [13] S.H. Luo, J.Y. Fan, W.L. Liu, M. Zhang, Z.T. Song, C.L. Lin, X.L. Wu, P.K. Chu, Nanotechnology 17 (2006) 1695. [14] H. Liu, Y. Zhou, S.A. Kulinich, J.J. Li, L.L. Han, S.Z. Qiao, X.W. Du, Journal of Materials Chemistry A 1 (2013) 302. [15] H.Y. Wang, R.J. Wang, X.M. Sun, R.X. Yan, Y.D. Li, Materials Research Bulletin 40 (2005) 911. [16] M.C. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, Journal of Physical Chemistry B 110 (2006) 20211.
729
[17] F. Dong, H.Q. Wang, G. Sen, Z.B. Wu, S.C. Lee, Journal of Hazardous Materials 187 (2011) 509. [18] F. Dong, H.T. Liu, W.K. Ho, M. Fu, Z.B. Wu, Chemical Engineering Journal 214 (2013) 198. [19] F. Dong, Y.J. Sun, M. Fu, W.K. Ho, S.C. Lee, Z.B. Wu, Langmuir 28 (2012) 766. [20] F. Dong, Y.J. Sun, W.K. Ho, Z.B. Wu, Dalton Transactions 41 (2012) 8270. [21] F. Dong, S.C. Lee, Z.B. Wu, Y. Huang, M. Fu, W.K. Ho, S.C. Zou, B. Wang, Journal of Hazardous Materials 195 (2011) 346. [22] B. Hu, K. Wang, L.H. Wu, S.H. Yu, M. Antonietti, M.M. Titirici, Advanced Materials 22 (2010) 1. [23] K. Nejati, Crystal Research and Technology 47 (2012) 567. [24] T.W. Kim, D.U. Lee, Y.S. Yoon, Journal of Applied Physics 88 (2000) 3759. [25] F. Gu, S.F. Wang, C.F. Song, M.K. Lü, Y.X. Qi, G.J. Zhou, D. Xu, D.R. Yuan, Chemical Physics Letters 372 (2003) 451. [26] W.F. Zhang, Z. Yin, M.S. Zhang, Z.L. Du, W.C. Chen, Journal of Physics: Condensed Matter 11 (1999) 5655. [27] W.F. Zhang, M.S. Zhang, Z. Yin, Q. Chen, Applied Physics B 70 (2000) 261. [28] W.F. Zhang, M.S. Zhang, Z. Yin, Physical Status Solidi (a) 179 (2000) 319. [29] Z.L. Liu, J.C. Deng, J.J. Deng, F.F. Li, Materials Science and Engineering B 150 (2008) 99. [30] M.L. Zhang, T.C. An, X.H. Hu, C. Wang, G.Y. Sheng, J.M. Fu, Applied Catalysis A: General 260 (2004) 215. [31] A.O. Govorov, H.H. Richardson, Nanotoday 2 (2007) 30. [32] O. Lupan, L. Chow, G. Chai, A. Schulte, S. Park, H. Heinrich, Materials Science and Engineering B 157 (2009) 101. [33] X.Y. Tao, I. Fsaifes, V. Koncar, C. Dufour, C. Lepers, L. Hay, B. Capoen, M. Bouazaoui, Applied Physics A 96 (2009) 741. [34] D.J. Sirbuly, A. Tao, P.D. Yang, Advanced Materials 19 (2007) 61. [35] C. Cheng, G. Xu, H. Zhang, Y. Li, Y. Luo, P. Zhang, Materials Science and Engineering B 147 (2008) 79. [36] M. Salehi, B. Janfeshan, S.K. Sadrnezhaad, Applied Physics A 97 (2009) 361. [37] E. Comini, S. Bianchi, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Applied Physics A 89 (2007) 73.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Luminescent and photocatalytic properties of hollow SnO2 nanospheres Yanhua Zhu, Lingling Wang ∗ , Guifang Huang, Yifeng Chai, Xiang Zhai, Weiqing Huang School of Physics and Microelectronics and Key Lab for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 23 October 2012 Received in revised form 25 January 2013 Accepted 11 March 2013 Available online 26 March 2013 Keywords: Hollow nano-sphere Tin oxide Sacrificing template Specific surface area Photoluminescence Photocatalysis
a b s t r a c t Size tunable solid SnO2 (STO) and hollow SnO2 (HTO) nanospheres were prepared by a sacrificing template method. The peaks around 390 nm were observed in photoluminescence (PL) spectra. Based on the results, the PL intensity exhibits morphology-dependence and size-dependence, and HTO structure displays better optical properties than STO structure. The degradation of Methyl Orange (MO) in aqueous solution is selected as a probe reaction to evaluate the catalytic activity of nano-SnO2 . The result shows that HTO structure presents stronger photocatalytic (PC) activity. According to the result of specific surface area testing, the improved PL and PC properties of HTO structure can be mainly explained by the surface effect induced by large specific surface area. This work is meaningful for developing nanomaterials with enhanced optical and photochemical properties. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, great interest has been attracted on nanostructures owing to their unique properties and novel applications [1] due to the so-called quantum size effect. As a potential luminescent material, semiconductor nanoparticle shows fascinating optical properties with many technological applications in optics and optoelectronics [2]. In addition, semiconductor nanomaterial is also considered as potential candidate for chemical and environmental applications due to its high catalytic efficiency [3–5]. As an n-type semiconductor oxide with wide band gap (Eg 3.6 eV at 300 K), SnO2 is well-known for its potential applications in many fields. Plenty of papers have reported the optical properties of SnO2 nanostructures synthesized by these methods. The PL peaks at 355, 390, 460 and 580 nm have been observed [6,7]. In solar applications, SnO2 /Cu2 O solar cell has been reported in recent report [8]. Additionally, a number of reports are also linked to the chemical and environmental applications of nano-SnO2 [9]. The hetero-structural materials such as SnO2 /TiO2 [10], SnO2 /␣Fe2 O3 [11] and SnO2 /CuO [12] have been investigated extensively for their excellent performance in photocatalytic degradation of various organic substances.
Abbreviations: STO, solid tin oxide; HTO, hollow tin oxide; PL, photoluminescence; MO, methyl orange; PC, photo-catalysis; XRD, X-ray diffraction; FE-SEM, field emission scanning electron microscope; TEM, transmission electron microscope; UV, ultra-violet. ∗ Corresponding author. Tel.: +86 13508484833. E-mail address: [email protected] (L. Wang). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.03.006
Nonetheless, there is still room for improvement. As is known, surface defects are the most common defects on crystal surfaces. Such defects can directly or indirectly contribute to PL [13]. Therefore, enlarging the surface area of an optical material is an efficient way to enhance PL intensity. For normal solid nanostructures, the frequently-used method to increase specific surface area is to decrease the size of particles. However, due to some effects such as scattering and resonance, to fabricate ultra-small nanoparticles does not help enhance PL intensity. Thus, to find another method to get materials with large specific surface area is significant for improving the optical properties of nanomaterial [32–37]. Furthermore, for chemical or environmental application, such as organic degradation and water purification, most studies are mainly focused on nanofilms and nanosheets. However, because of the limitation of substrate, these substrate-based nanostructures are not design-friendly and efficient for some special use. On the other hand, normal non-substrated solid nanoparticles are imperfect as well. Owing to the dispersed granular morphology, it is hard to develop such a material with high adsorption of reactant molecules and high utilization of excitation light. Therefore, it is meaningful to develop a granulated nanomaterial with high PL intensity and strong PC activity. Based on these issues, the nanomaterials with hollow structure are considered as a potential solution and have received wide attention. The hollow Cu2 O nanocubes with unique optical properties have been fabricated [14]. The hollow spheres of Eu3+ -doped lanthanides compounds have been studied [15]. The enhanced PC properties of Co3 O4 /BiVO4 [16], TiO2 /PtCl4 [17], (BiO)2 CO3 [18–21] hollow microspheres have been reported. Herein, we report the luminescence and catalysis properties of HTO nanospheres, which
726
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
have large specific surface area and proper particle size. We believe that, compared with normal solid structure, the higher PL intensity and the stronger PC activity of HTO nanostructure will be much more potential and useful for optical and chemical applications.
2. Experimental 2.1. Synthesis The synthesis of STO nanospheres was followed by a low temperature combustion synthesis approach. SnCl4 ·5H2 O and concentrated nitric acid were mixed into a beaker at a molar ratio of 1:1 to transform into tin nitrate. Urea, boric acid and water were added in a proper ratio as well. The solution was handled by ultrasonic dispersion for 10 min, and then transferred into a preheated muffle furnace heating at 600 ◦ C for about 5 min. The product was cooled and grinded, and then the STO nanospheres were obtained. In these processes, we changed the ratio between urea and boric acid in order to prepare samples with different sizes. The HTO nanospheres were synthesized by a sacrificing template method. The general synthesis method mainly includes the adsorption of metal ions from solution to the surface layer of carbonaceous spheres and subsequent removal of the carbonaceous cores via calcinations [22]. The experiment procedures are as follow: Glucose solution (1 mol L−1 , 20 mL) was poured into a 25 mL autoclave, then the autoclave was transferred into an oven maintaining at a heating temperature between 180 and 250 ◦ C for 12 h. The product was centrifugal separated, washed three times with distilled water and anhydrous alcohol, and finally dried at 450 ◦ C for 6 h. The obtained colloidal carbonaceous spheres were used as templates. SnCl4 was mixed with ethanol at a molar ratio of 1:20, and the solution pH value was tuned by hydrochloric acid to different pH values, then tetraethyl orthosilicate (TEOS) was added into the solution. After a 30 min magnetic stirring, 200 mg colloidal carbonaceous spheres were put into the solution, and kept stirring for 24 h. In this process, Sn4+ ions combined with the unsaturated bonds on the surfaces of carbonaceous spheres and composed the shell of tin hydroxide outside carbonaceous core. Then the product was centrifugal separated and washed with distilled water and anhydrous alcohol, dried in an oven at 40 ◦ C for 12 h, and annealed at 450 ◦ C for 4 h. During the annealing, carbonaceous core were removed from the core-shell structure, and the reserved shell were oxidized and formed the hollow structure. It is worth mentioning that the sizes and structures of different metal oxide hollow spheres are predominantly determined by the templates [22]. Therefore, we can control the size of carbonaceous spheres by changing heating temperature in order to prepare size-tunable HTO nanospheres.
2.2. Characterization The morphology and crystal structure of as-synthesized nanoSnO2 were characterized by X-ray diffraction (XRD) using a Rigaku D/MAX-2400 utilizing Cu K␣ radiation, field emission scanning electron microscope (FE-SEM) employing a JEOL JSM-6700F electronic microscope, transmission electron microscope (TEM) images were collected with a JEOL JEM-2010 high-resolution transmission electron microscopy. The room temperature PL spectra of samples were recorded on a Hitachi F-4500 FL spectrophotometer with a Xe lamp as the excitation light source. The specific surface areas of HTO and STO were measured by BET (Brunauer, Emmett and Teller) method. 2.3. PC activity test The PC activity of HTO was evaluated by studying the photocatalytic degradation of aqueous MO solution. 50 mg nano-SnO2 (HTO or STO) was suspended in 50 mL of 10 mg L−1 MO aqueous solution. The solution was continuously stirred for about 30 min at room temperature to establish an adsorption-desorption equilibrium between the MO and photocatalyst. The PC reaction proceeded in an ultraviolet photochemical reactor (BL-GHX-V, Beijing Bilon Lab Equipment Co. Ltd). MO degradation rate was analyzed by UV–vis spectroscopy at its maximum absorption wavelength of 464 nm. 3. Results and discussion 3.1. Crystal structure The images of as-synthesized SnO2 are shown in Fig. 1, where Fig. 1(a) and (b) are the SEM images of unannealed nano-SnO2 , and Fig. 1(c) is the TEM image of HTO nanospheres, respectively. According to the observation, the size of particles is in the range of 100 to 200 nm, and we can find from Fig. 1(a) and (b) that the SnO2 nanoparticles are with similar size and spherical shape. However, the particles are still solid and undistributed. It is because of the products are not annealed and the carbonaceous cores which were used as templates were still inside of particles. Therefore, we removed carbonaceous cores by annealing and obtained the image of HTO nanospheres showed as Fig. 1(c). Generally, the nanoparticles present a porous spherical structure, and there is a brightness difference between center and border area, which indicates these structures are hollow. That is because the carbonaceous cores have transformed into carbon oxide (CO, CO2 ) and escaped through these micropores during annealing, and only porous SnO2 shells are reserved. Furthermore, the unbroken hollow spheres indicate carbon oxide gas escaped without any destruction to SnO2 shells.
Fig. 1. Images of HTO nanoparticles. (a and b) The SEM images of unannealed HTO structure, (a) 30,000×, (b) 100,000×, respectively. (c) The TEM image of HTO nanospheres.
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
727
Fig. 4. Specific surface area patterns of HTO and STO with different sizes. Fig. 2. XRD patterns for different SnO2 nanostructures. (a) XRD patterns of HTO nanospheres before annealing. (b) XRD patterns of HTO nanospheres after annealing. (c) XRD patterns of STO nanospheres.
The structures of the samples were characterized by XRD, and the patterns are displayed in Fig. 2, where the curves (a) and (b) are the XRD patterns of HTO nanospheres before and after annealing, and the curve (c) shows the pattern of STO nanospheres which is used for comparison. It is showed that there are SnO, SnOx , SnO2 and some other amorphous carbonic phases in curve (a), but there is only SnO2 phase in curve (b). It indicates that during the process of annealing, the SnO and SnOx phases were completely oxidized to SnO2 , and the residual carbon was transformed into carbon oxides and escaped to the air. According to curve (b) and (c), both HTO and STO diffraction lines are assigned to tetragonal rutile crystalline phases of tin oxide. The patterns are in good agreement with reference pattern of tin oxide (Powder Diffraction File No. 41-1445). 3.2. Photoluminescent property Fig. 3 shows the PL spectra of HTO and STO at room temperature using 302 nm laser. As is shown, all PL peaks are around 390 nm which is in the border between visible and ultraviolet range. Earlier reports indicate that SnO2 thin films exhibited a strong peak around 396 nm (3.1 eV) [23]. Since the energy of emission peak (390 nm, 3.2 eV) is lower than the band gap of SnO2 (3.6 eV), this observed
Fig. 3. PL spectra for STO and HTO. (a) STO with different sizes. (b) HTO with different sizes.
emission cannot be ascribed to the direct recombination between a conduction electron in Sn4+ band and a hole in O2− valence band. In the light of the research of Lü’s group [24], the origin of this emission can be attributed to electron transition mediated by defects levels in the band gap, such as oxygen vacancies, owing to the temperature behavior of PL spectra of the nano-SnO2 powders, which is similar with nanoscaled SrTiO3 , TiO2 , ZnO, and BaTiO3 [25–28]. Based on the PL intensities of HTO and STO particles showed in Fig. 3, it is obvious that HTO structure exhibits stronger emission capacity. For similar particle sizes, the PL intensities of HTO particles are about 25% higher than those of STO. This result implies HTO has better optical properties than normal STO structures. It is well-known that, the intrinsic defects form defect levels in band gap in most semi-conductor materials, which trap electrons from valence band to contribute to luminescence. In semiconductor metal oxides, oxygen vacancy is an intrinsic donor and considered to be the most common defect, which usually acts as luminescent centers. In these material structures, luminescence is usually ascribed to the recombination of electrons in singly occupied oxygen vacancies with photo-excited holes in the valence band [29]. For SnO2 nanostructure, O2− is usually stable with several cationic surrounding in crystal lattices [30]. However, it is different on the surface of crystal that, the stability of O2− decreases rapidly as a result of the discontinuity of surface lattice, which is the so-called surface defect. Because of this defect, a large amount of oxygen vacancies are provided on the surface including the para•• •• magnetic VO• , which can transform into Vo when capture holes. Vo can recombine with photo-excited electrons, and plays a role of recombination center in luminescence processes. Therefore, we can describe the process of luminescence as below: when nano-SnO2 is illuminated by a laser with enough energy, electrons are photoexcited to conduction band from valence band and leave plenty of •• active holes. On the surface of nano-SnO2 , a number of Vo are gen• erated when holes are directly captured by Vo , they recombine with electrons and emit luminescence. Fig. 4 shows the BET surface area analysis results of assynthesized HTO and STO. It indicates that the specific surface area of HTO is clearly larger than that of STO, which means HTO has larger surface area that causes more surface defect. In accordance with the description in last paragraph, while illuminated by the ultraviolet laser, there are more oxygen vacancies generated in HTO •• structure due to its larger surface area. Hence more Vo are generated and recombine with electrons in the crystal, resulting in an enhancement of the luminescence intensity.
728
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
Fig. 5. MO degradation rate with UV irradiation time. (a) The time dependence of MO concentration under catalysis of as-synthesized nano-SnO2. (b) Reveals the PC abilities of HTO and STO with different sizes.
Moreover, Fig. 3 also indicates the size-dependent of PL intensity. As is shown, the PL intensity initially increases with the decrease of particle size and gets maximized with a size between 120 and 130 nm, and subsequently decreases as the size continues to decrease. This is potentially caused by the counterbalance between the improvement of PL intensity resulted in surface effect and the reduction of PL intensity induced by scattering effect and resonance effect [31]. With the decrease of particle size, the surface effect initially acts as the primary factor in the PL process, and then the scattering effect and the resonance effect become stronger and get dominant as the particles become ultra-small. In our results, the highest PL intensities are observed on the particles with a size between 120 nm and 130 nm, which indicates these positive and negative effects reach equilibrium with such particle size.
surface area, show a larger adsorption capacity and can be excited more efficiently with UV light. On the other hand, the special porous surface structure of the HTO particles which allows multiple reflections of UV light enhances photo-energy harvesting and benefits to improve the diffusion of reactants during PC reaction [18]. Both of these result in an enhanced PC activity. It is noteworthy that previous results indicate the contradiction between high PC activity and high PL intensity due to the inhibition of electron/hole pair recombination caused by the special structure [16,17]. However, unlike heterojunction materials, the undoped and unary crystal structure of SnO2 does not induce any special mechanism to inhibit the recombination of electron/hole pair. The enhancements of HTO structure on PL intensity and PC activity are resulted by the change of morphology.
3.3. Photocatalysis property
4. Conclusions
The time dependence of MO concentration under catalysis of as-synthesized nano-SnO2 is displayed in Fig. 5(a), where red and blue curves stand for HTO (100 nm) and STO (90 nm) respectively, green and purple curves give the degradation rate of reactions proceeded only with SnO2 catalyst or UV light, which indicates both catalyst and UV light are essential for MO degradation. Fig. 5(a) shows that after 4 h of reaction, in the case of HTO, the degradation rate of the MO concentration was about 75%, whereas in the case of STO, the degradation rate was less than 50%. It is evident that HTO performs a much more efficient PC activity than STO. Fig. 5(b) reveals the PC abilities of HTO and STO with different sizes. Based on Fig. 5(a), the MO degradation rates catalyzed by HTO are more than 50% higher than STO in the groups of similar sizes. Furthermore, the columns also indicate a dependence of the PC activity of the nano-SnO2 on size. But contrary to the PL intensity, the PC activity is related monotonously to particle size. In our results, the PC activity of the nano-SnO2 shows an enhancement with the decrease of particle size and reaches highest for the smallest particles. We can also explain these results by the difference of specific surface area. In a standard process of PC reaction, reactants are first adsorbed by catalyst particles. Then, on the surface of catalyst, the electrons are photo-excited by UV light and transfer energy to the reactant molecules, which make the reactants reacted and decomposed. In these processes, reactant adsorption and photo-excitation are considered as the most two important steps. Owing to this, adsorption capacity and utilization of excitation light are also considered to be the key factors for photocatalyst. The adsorption capacity of catalyst particles is mainly determined by their surface area. Generally, larger surface area leads to larger adsorption capacity. Therefore, HTO synthesized particles which have larger specific
The HTO nanospheres were prepared by sacrificing template method. The size of fabricated HTO is between 100 and 200 nm. The PL measurements show that HTO particles emit stronger luminescence than normal STO ones, reaching the maximum PL intensity when the particle size is between 120 and 130 nm. The BET results indicate that both size-dependence and morphology-dependence of the PL intensity are mainly due to the specific surface area of the particles. In the study of PC property, HTO also exhibits higher catalytic activity, which can be attributed to its large specific surface area. This work provides a simple and efficient way to fabricate nanomaterials with higher PL intensity and stronger PC activity. We believe that our work is meaningful for the applications in optical, chemical and environmental fields. Authorship statement The submission of the manuscript has been approved by all coauthors. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11074069, 61176116), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120161130003), the Hunan Provincial Science and Technology Project of China (Grant Nos. 2011SK3217, 2012FJ4121), 2011 Graduate Science and Technology Innovation Program of Hunan Province (Grant No. CX2011B153), Henan Provincial Natural Science Foundation of China (Grant No. 122300410350), Aid Program for Science and Technology
Y. Zhu et al. / Materials Science and Engineering B 178 (2013) 725–729
Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Hunan Provincial Natural Science Foundation of China (Grant No. 12JJ3009). References [1] Z.L. Wang, Applied Physics A 88 (2007) 7. [2] L.A. Patil, M.D. Shinde, A.R. Bari, V.V. Deo, Materials Science and Engineering B 176 (2011) 579. [3] W.H. Leng, H. Liu, S.A. Cheng, Journal of Photochemistry and Photobiology A: Chemistry 131 (2000) 125. [4] A.R. Kamali, D.J. Fray, Materials Science and Engineering B 177 (2012) 819. [5] Z.W. Seh, S. Liu, M. Low, S.Y. Zhang, Z. Liu, A. Mlayah, M.Y. Han, Advanced Materials 24 (2012) 2310. [6] L. Wang, J.W. Ren, X.H. Liu, G.Z. Lu, Y.Q. Wang, Materials Chemistry and Physics 127 (2011) 114. [7] G. Salviati, L. Lazzarini, Z.Z. Ming, V. Grillo, E. Carlino, Physical Status Solidi A 202 (2005) 2963. [8] Z.G. Zang, A. Nakamura, J. Temmyo, Materials Letters 92 (2013) 188. [9] G. Cheng, J.Y. Chen, H.Z. Ke, J. Shang, R. Chu, Materials Letters 65 (2011) 3327. [10] K. Vinodgopal, I. Bedja, P.V. Kamat, Chemistry of Materials 8 (1996) 2180. [11] M.T. Niu, F. Huang, L.F. Cui, P. Huang, Y.L. Yu, Y.S. Wang, ACS Nano 4 (2010) 681. [12] R. Jiang, H.Y. Zhu, Y.J. Guan, Y.Q. Fu, L. Xiao, Q.Q. Yuan, S.T. Jiang, Chemical Engineering & Technology 34 (2011) 179. [13] S.H. Luo, J.Y. Fan, W.L. Liu, M. Zhang, Z.T. Song, C.L. Lin, X.L. Wu, P.K. Chu, Nanotechnology 17 (2006) 1695. [14] H. Liu, Y. Zhou, S.A. Kulinich, J.J. Li, L.L. Han, S.Z. Qiao, X.W. Du, Journal of Materials Chemistry A 1 (2013) 302. [15] H.Y. Wang, R.J. Wang, X.M. Sun, R.X. Yan, Y.D. Li, Materials Research Bulletin 40 (2005) 911. [16] M.C. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, Journal of Physical Chemistry B 110 (2006) 20211.
729
[17] F. Dong, H.Q. Wang, G. Sen, Z.B. Wu, S.C. Lee, Journal of Hazardous Materials 187 (2011) 509. [18] F. Dong, H.T. Liu, W.K. Ho, M. Fu, Z.B. Wu, Chemical Engineering Journal 214 (2013) 198. [19] F. Dong, Y.J. Sun, M. Fu, W.K. Ho, S.C. Lee, Z.B. Wu, Langmuir 28 (2012) 766. [20] F. Dong, Y.J. Sun, W.K. Ho, Z.B. Wu, Dalton Transactions 41 (2012) 8270. [21] F. Dong, S.C. Lee, Z.B. Wu, Y. Huang, M. Fu, W.K. Ho, S.C. Zou, B. Wang, Journal of Hazardous Materials 195 (2011) 346. [22] B. Hu, K. Wang, L.H. Wu, S.H. Yu, M. Antonietti, M.M. Titirici, Advanced Materials 22 (2010) 1. [23] K. Nejati, Crystal Research and Technology 47 (2012) 567. [24] T.W. Kim, D.U. Lee, Y.S. Yoon, Journal of Applied Physics 88 (2000) 3759. [25] F. Gu, S.F. Wang, C.F. Song, M.K. Lü, Y.X. Qi, G.J. Zhou, D. Xu, D.R. Yuan, Chemical Physics Letters 372 (2003) 451. [26] W.F. Zhang, Z. Yin, M.S. Zhang, Z.L. Du, W.C. Chen, Journal of Physics: Condensed Matter 11 (1999) 5655. [27] W.F. Zhang, M.S. Zhang, Z. Yin, Q. Chen, Applied Physics B 70 (2000) 261. [28] W.F. Zhang, M.S. Zhang, Z. Yin, Physical Status Solidi (a) 179 (2000) 319. [29] Z.L. Liu, J.C. Deng, J.J. Deng, F.F. Li, Materials Science and Engineering B 150 (2008) 99. [30] M.L. Zhang, T.C. An, X.H. Hu, C. Wang, G.Y. Sheng, J.M. Fu, Applied Catalysis A: General 260 (2004) 215. [31] A.O. Govorov, H.H. Richardson, Nanotoday 2 (2007) 30. [32] O. Lupan, L. Chow, G. Chai, A. Schulte, S. Park, H. Heinrich, Materials Science and Engineering B 157 (2009) 101. [33] X.Y. Tao, I. Fsaifes, V. Koncar, C. Dufour, C. Lepers, L. Hay, B. Capoen, M. Bouazaoui, Applied Physics A 96 (2009) 741. [34] D.J. Sirbuly, A. Tao, P.D. Yang, Advanced Materials 19 (2007) 61. [35] C. Cheng, G. Xu, H. Zhang, Y. Li, Y. Luo, P. Zhang, Materials Science and Engineering B 147 (2008) 79. [36] M. Salehi, B. Janfeshan, S.K. Sadrnezhaad, Applied Physics A 97 (2009) 361. [37] E. Comini, S. Bianchi, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Applied Physics A 89 (2007) 73.