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Effect of calcination temperature on the microstructure, crystallinity and photocatalytic activity of TiO2 hollow spheres
Journal of Alloys and Compounds 542 (2012) 32–36
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of calcination temperature on the microstructure, crystallinity and photocatalytic activity of TiO2 hollow spheres H.L. Shen a, H.H. Hu b, D.Y. Liang a, H.L. Meng a, P.G. Li a, W.H. Tang a, C. Cui a,⇑ a b
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China Zhejiang University City College, Hangzhou 310015, China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Accepted 16 July 2012 Available online 27 July 2012 Keywords: TiO2 hollow spheres Calcination Photocatalytic activity
a b s t r a c t Submicrometer-sized TiO2 hollow spheres (THS) were synthesized using a template method, which involves deposition of TiO2 on SiO2 spheres templates through sol–gel process, calcination to crystallize amorphous TiO2, and etching SiO2 templates by hydrothermal treatment in NaOH solution. The evolution of the microstructure, crystallinity and photocatalytic activity of the THS fabricated with TiO2@SiO2 calcined at various temperature were studied systematically. The calcination treatment allows crystallization of the amorphous TiO2 shell into anatase nanocrystals in temperature range of 400–800 °C, and the crystallinity of the anatase TiO2 nanocrystals is improved as the calcination temperature increases. At temperature above 1000 °C, partial anatase TiO2 transforms to rutile phases. The morphology and microstructure of THS also varied over the calcination temperature due to the growth and aggregation of TiO2 nanocrystals. Accordingly, the absorption spectra edges of the THS shift towards the long wavelength. The results of photodegradation of Rhodamine B (RhB) under ultraviolet (UV) light illumination indicated that the THS calcined at 800 °C exhibited the highest photocatalytic activity, benefiting from the high crystallinity of anatase TiO2 and hierarchical hollow spherical structure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured titania (TiO2) has been widely investigated for its potential applications in photocatalysts and photovoltaic cells. In order to improve the photoelectric performance, TiO2 with various morphologies, such as nanoparticles, nanorods, nanotubes, nanosheets and hollow spheres have been reported [1–6]. Among these structures, hierarchical TiO2 hollow spheres (THS) have received much attention because of their low bulk density, high specific surface area, reduced transport lengths for both mass and charge transport, good surface permeability and easy recycling [6–14]. In addition, it has been demonstrated that the porous structure endows THS with improved light harvest by reflecting UV-light within the hollow sphere interior, consequently enhances the photocatalytic property [15–17]. Up to now, many approaches have been developed for the synthesis of THS, including template-based or template-free approaches. The template-free approaches have been proved to be simple and high efficient, however, it is difficult to control the size and morphology of THS with these approaches [10–12]. So, the template-based methods are still promising for the synthesis of high quality THS with uniform morphology [13,14]. For TiO2 photocatalyst, surface area and crystallinity of TiO2 are two crucial factors that affect the photocatalytic activity. Large surface area results in more reactant adsorption-desorption sites for catalytic reaction, ⇑ Corresponding author. Tel.: +86 571 8684 3468. E-mail address: [email protected] (C. Cui). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.080
and provides more efficient transport channels for the reactant molecules. High crystallinity of TiO2 is essential to enhance the generation and migration of photogenerated electron/hole pairs in the bulk and surface of TiO2. Therefore, a well crystallized TiO2 nanostructure with large surface area is considered the preferred form of TiO2 photocatalyst. THS generally have an advantage of large surface area. However, the achievement of perfect crystallized THS usually conflict with the achievement of large surface area, because elevated calcination temperature applied to crystallize TiO2 may cause the further growth or agglomeration of TiO2 nanoparticles and finally reduce the specific surface area. In this paper, we studied the balance between achievements of large specific surface area and high crystallinity for THS photocatalyst by varying the calcination temperature. The variation of the microstructure, crystalliny as well as the photocatalytic activity of the THS fabricated with TiO2@SiO2 pre-calcined at different temperature has been studied comparatively. The results show that the THS fabricated with TiO2@SiO2 pre-calcined at 800 °C possess the highest photocatalytic activity, benefiting from the high anatase crystallinity and good hollow spherical structure.
2. Experimental 2.1. Synthesis of TiO2 hollow spheres Firstly, monodisperse SiO2 sphere templates with average diameter of 400 nm were synthesized using a modified Stöber method as demonstrated in Ref. [18]. Then, TiO2 were deposited on the SiO2 sphere templates by a sol–gel method to ob-
H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36 tain SiO2@TiO2 core-shell structure. In order to crystallize TiO2, SiO2@TiO2 was subjected to calcination treatment at 400, 600, 800, 1000, and 1200 °C for1 h, respectively. Finally, the SiO2 cores were removed from the pre-calcined SiO2@TiO2 core-shell particles by using hydrothermal treatment in NaOH solution. The THS fabricated with SiO2@TiO2 pre-calcined at 400, 600, 800, 1000, and 1200 °C are denoted as THS-400, THS-600, THS-800, THS-1000 and THS-1200. In a typical synthesis, 0.3 g SiO2 spheres were ultrasonic dispersed in 100 mL of ethanol to obtain solution A. 2 mL of Tetrabutyl titanate (TBT, 5.85 mmol) was dissolved in 100 mL of ethanol to obtain solution B. Solution B and 1.5 mL of aqueous ammonia were added to solution A, vigorously stirred at 60 °C for 3 h to obtain solid SiO2@TiO2 core-shell spheres. The SiO2@TiO2 core-shell spheres were centrifuged and carefully washed with ethanol and dried in air. After calcination treatment at different temperature for 1 h, the SiO2@TiO2 core-shell spheres were placed in a 50 mL Teflon-lined autoclave with stainless steel tank and then the autoclave was filled to 80% capacity with 1 mol/L NaOH aqueous solution. After completely mixing, the autoclave was sealed and heated at 80 °C for 4 h. The autoclave was then naturally cooled down to room temperature. Finally, the product was collected by centrifugation and washed for several times with distilled water.
2.2. Characterization The phase of the samples was determined by powder X-ray diffraction (XRD) analysis, which were carried out on a Bruker D8 Advance X-ray diffractometer using Cu-Ka (k = 1.5406 Å) radiation at 40 kV and 40 mA. The morphology and microstructure were examined by a Hitachi S-4800 field emission scanning electron microscope (SEM) and a JEM-2100 high-resolution transmission electron microscope (HRTEM) at an acceleration voltage of 200 kV. UV–vis diffuse reflectance spectra (DRS) were achieved using a Perkin Elmer Lambda 900 UV–vis spectrophotometer. UV–vis absorption spectra analysis of the RhB solution was performed on a Varian Cary 50 spectrophotometer.
2.3. Measurement of photocatalytic activity Photocatalytic activity of the THS samples was evaluated by degradation of RhB under the illumination of a 500 W Xe lamp equipped with a UV band pass filter (k 6 400 nm). 50 mg of THS was dispersed in a 50 mL RhB aqueous solution (2 105 mol/L). Prior to illumination, the suspensions were vigorously stirred in dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium between the THS and RhB (graphically represented as time 30 min in Fig. 6a). During illumination, a circulation of water through an external cooling coil was conducted to maintain the temperature of suspensions at about 25 °C. Oxygen under atmospheric pressure was bubbled through the reaction continuously. The suspensions were sampled at 20 min intervals and centrifuged. The concentration of RhB was estimated by measuring the absorbance at their maximum peaks (553 nm) with a UV–Vis spectrophotometer.
3. Result and discussion 3.1. Morphology, crystallinity and microstructure of THS Fig. 1 shows the SEM images of the SiO2 spheres templates and the as-prepared THS without calcination treatment. The monodispersed SiO2 spheres are about 400 nm in diameter. The as-prepared THS without calcination treatment remain good spherical morphology with a uniform diameter of around 500 nm. The bro-
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ken spheres in Fig. 1(b) demonstrate the formation of hollow spheres. In order to crystallize TiO2, the SiO2@TiO2 core-shell samples were subjected to calcination treatment at different temperature. After removal of the SiO2 cores in NaOH solution, the crystal structure of the THS was characterized by XRD measurement, as shown in Fig. 2. The TiO2 is amorphous phase in the sample without calcination. After calcination at 400 °C (THS-400), diffraction peaks related to anatase phase TiO2 (JCPDS No. 21–1272) are observed, indicating that the amorphous TiO2 transferred to anatase phase. As the calcination temperature increases from 400 to 800 °C, those peaks associated with anatase phase TiO2 become sharper, indicating that the higher calcination temperature promotes the forming of anatase TiO2 of high crystallinity. However, peaks related to rutile phase TiO2 (JCPDS NO. 21–1276) appear when the calcination temperature rises to 1000 °C, which denotes the transformation of anatase TiO2 to rutile TiO2 in THS-1000. At an even higher temperature of 1200 °C (THS-1200), more anatase TiO2 transforms to rutile TiO2. From the XRD spectra, the average crystallite sizes Kk can be calculated by the Debye–Scherrer formulae D ¼ b cos , where h D is the crystallite diameter in nm, K is the shape constant (0.89), k is the wavelength of X-ray (1.5406 Å), b is the peak width (in radians) at half-maximum height and h is the Bragg angle. The calculated TiO2 crystallite size is listed in Table 1. From THS-400 to THS-1200, the size of anatase phase crystallite increases from 9.1 to 39.2 nm, and the size of rutile phase crystallite increases from 25.1 to 62.1 nm. Fig. 3 shows the SEM images of the THS samples fabricated with SiO2@TiO2 calcinated at different temperature. The diameters of the hollow spheres are about 500 nm. The rough surface structure reveals that the THS are hierarchical structure composing of nanoparticles. The hollow spheres keep in good spherical structure when the calcination temperature is below 800 °C. As the temperature rises above 800 °C, nanoparticles constituting the shell of the hollow spheres grow or agglomerate into larger particles, making the shell looser. In some case, the aggregation of the TiO2 nanoparticle inevitably lead to a collapse of the porous hollow sphere structure and hence remarkably decrease the specific surface area. As the THS-800 sample shows a good hollow spherical structure and high anatase crystallinity, more detailed microstructure of this THS is studied. Fig. 4(a) shows a panoramic SEM image of the THS800 sample, and we can see that almost all the hollow spheres remain good spherical morphology and dense shell structure. Fig. 4(b and c) show the TEM images of the sample. In Fig. 4(b), the color contrast between the central and fringe region of the spheres confirms the formation of hollow spherical structure with shells of about 50 nm. At a high resolution TEM image in Fig. 4(c), clear grain boundaries and crystal lattice fringes of crystallites with diameters of about 10 nm are observed. A set of crystal lattices
Fig. 1. SEM images of (a) SiO2 sphere templates and (b) as-prepared THS without pre-calcination.
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H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36
Fig. 2. XRD patterns of THS samples fabricated with TiO2@SiO2 pre-calcined at different temperature.
Table 1 Notations, crystallite sizes and photocatalytic reaction rate constant k for TiO2 hollow spheres. Sample notation
THS400
THS600
THS800
THS1000
THS1200
Crystallite size (nm) A:anatase R:rutile Reaction rate constant k (102 min1)
9.1(A)
9.9(A)
11.9(A)
0.67
4.42
12.0
22.3(A) 25.1(R) 3.78
39.2(A) 62.1(R) 0.97
with d-space equivalent to 0.352 nm corresponding to (1 0 1) planes of anatase TiO2 is found as the majority faces of individual crystallites throughout the area observed in Fig. 4(c). The TEM images reveal that the shells of THS-800 are composed of anatase nanocrystals with an average size of 10 nm, which is in good agreement with the results from the XRD analysis.
3.2. UV–vis DRS analysis of the THS UV–vis DRS spectra were measured to investigate the optical absorption property of the THS samples, as shown in Fig. 5. The
absorption onsets are determined by linear extrapolation from the inflection point of the curve to the baseline. The absorption onsets (inset of Fig. 5) for THS-400, THS-600 and THS-800 are around 390 nm, corresponding to bandgap energy of 3.2 eV. The THS-800 sample shows an enhanced absorbance in the spectral range of 320–390 nm, which can be attributed to the higher crystallinity of anatase phase TiO2 in THS-800 compared to THS-400 and THS600. Compared with that of THS-800, the absorption onsets of THS-1000 and THS-1200 slightly red shift. The absorption onsets for THS-1000 and THS-1200 are 410 nm (3.0 eV) and 417 nm (2.97 eV), respectively. In a recent report by Yin et al. [13], the Si in the SiO2 core could not enter the crystal lattice of TiO2 to act as a substitutional dopant during high temperature calcination. So, we can rule out the possibility of introduction of doping energy level, which usually cause the red shift of absorption onsets for doped-TiO2 [19]. Two possible reasons may lead to the red shift of the absorption onsets of the THS calcined at temperature above 1000 °C. The first reason is the obvious size augment of TiO2 nanocrystals that constitute THS. The average size of the TiO2 nanocrystals in THS-800 is about 11.9 nm, which is smaller than that in THS-1000 (22.3 nm for anatase phase and 25.1 nm for rutile phase) and THS-1200 (39.2 nm for anatase phase and 62.1 nm for rutile phase). Larger sizes of the TiO2 nanocrytals result in a slightly smaller bandgap of THS due to the weakened quantum-size effect. The second reason is the appearance of rutile phase TiO2 in the THS-1000 and THS-1200 samples, the rutile phase TiO2 has a smaller bandgap (3.0 eV) than that of anatase phase TiO2 (3.2 eV). 3.3. Photocatalytic activity of THS The photocatalytic activity of the THS samples for the degradation of RhB under UV light is revealed by the plots of C/C0 versus time t (Fig. 6a), where C is the concentration of RhB in aqueous solution at the time t, and C0 is the initial concentration. As we can see, the THS-800 sample shows the best photocatalytic activity which could achieve 100% degradation of RhB with UV light illumination for 60 min. The linear relationship between ln(C/C0) and t (Fig. 6b) indicates that the photocatalytic degradation reaction for all the samples follows pseudo-first-order kinetics. From Fig. 6b, the reaction rate constant k for the THS samples can be de-
Fig. 3. SEM images of THS samples fabricated with TiO2@SiO2 pre-calcined at different temperature.
H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36
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Fig. 4. SEM (a), TEM (b) and HRTEM (c) images of the THS-800 sample.
duced from the slope of the lines and they are listed in Table 1. THS-800 shows a superior reaction rate constant of 12.0 102 min1, which is much higher than those for THS-600 (4.42 102 min1) and THS-400 (0.67 102 min1). The high reaction rate constant of THS-800 can be primarily attributed to the formation of highly crystallized anatase TiO2 nanocrystals, which can enhance the generation and migration of photogenerated electron/hole pairs and therefore increase the redox property. It has been reported that the mixture of anatase and rutile phase TiO2 exhibited a better photocatalytic activity than the sole anatase or rutile phase TiO2 [1,20]. In this study, the mixture of anatase and rutile phase appeared in THS-1000 and THS-1200, however the photocatalytic reaction rate constants of these two samples are smaller (3.78 102 and 0.97 102 min1 for THS1000 and THS-1200) than that of THS-800. This may be attributed to the smaller specific surface area of THS-1000 and THS-1200, as a result of the growth and aggregation of the nanocrystals constituting the hollow spheres at the high calcination temperature (Fig. 3).
Fig. 6. (a) Evolution of RhB concentration versus UV illumination time in the presence different THS samples, and (b) linear transform ln(C/C0) = kt of the kinetic curves of the degradation of RhB versus time.
Fig. 5. UV–vis diffuse reflectance absorption (UV–DRS) spectra for different THS samples.
Therefore, we can conclude that the THS-800 sample achieves a balance of large specific surface area and high crystallinity for THS photocatalyst.
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H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36
4. Conclusions THS with diameter about 500 nm have been synthesized by a SiO2 spheres template method followed by calcination and etching of the SiO2 by hydrothermal treatment in NaOH solution. To achieve the optimum properties in terms of morphological feature, crystallinity and photocatalytic activity, THS fabricated with TiO2@SiO2 pre-calcined at deferent temperature were investigated by means of SEM, TEM, XRD, UV–vis DRS as well as the photodegradation of RhB under UV light illumination. It is found that the THS exhibit good hollow spherical morphology as the calcination temperature is below 800 °C, while the crystallinity of anatase TiO2 phase improves as the temperature creases from 400 to 800 °C. At temperature above 1000 °C, partial anatase TiO2 transforms to rutile phases, and the morphology of THS also varied due to the growth and aggregation of TiO2 nanocrystals at high temperature. The experiment of the photodegradation of RhB shows that the THS fabricated with TiO2@SiO2 pre-calcined at 800 °C exhibits the highest photocatalytic activity. This is attributed to the superior hierarchical hollow spherical structure and high anatase crystallinity, causing the high adsorption of reactant, high light harvest, and enhanced generation and migration of photogenerated electron/hole pairs. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos.60806045, and 11074220), Zhejiang Provincial Natural
Science Foundation of China (Nos. Y4100310, Y1110519 and R4090058) and the Teachers’ Scientific Research Fund of the Zhejiang University City College (No. J-12009). References [1] P. Roy, S. Berger, P. Schmuki, Angew. Chem., Int. Ed. 50 (2011) 2904–2939. [2] A.M. Luís, M.C. Neves, M.H. Mendonc, O.C. Monteiro, Mater. Chem. Phys. 125 (2011) 20–25. [3] R.M. Mohamed, E.S. Aazam, J. Alloys Compd. 509 (2011) 10132–10138. [4] J.Z.Y. Tan, Y. Fernández, D. Liu, M. Maroto-Valer, J.C. Bian, X.W. Zhang, Chem. Phys. Lett. 531 (2012) 149–154. [5] J.G. Yu, G.P. Dai, Q.J. Xiang, M. Jaroniec, J. Mater. Chem. 21 (2011) 1049–1057. [6] X.Y. Lai, J.E. Halpert, D. Wang, Energy Environ. Sci. 5 (2012) 5604–5618. [7] Y.W. Zhang, J. Zhang, P.Q. Wang, G.T. Yang, Q. Sun, J. Zheng, et al., Mater. Chem. Phys. 123 (2010) 595–600. [8] A. Syoufian, O.H. Satriya, K. Nakashima, Catal. Commun. 8 (2007) 755–759. [9] C.Y. Song, W.J. Yu, B. Zhao, H.L. Zhang, C.J. Tang, K.Q. Sun, X.C. Wu, L. Dong, Y. Chen, Catal. Commun. 10 (2009) 650–654. [10] J.G. Yu, J.J. Fan, B. Cheng, J. Power Sources 196 (2011) 7891–7898. [11] B. Song, S.W. Liu, J.K. Jian, M. Lei, X.J. Wang, H. Li, J.G. Yu, X.L. Chen, J. Power Sources 180 (2008) 869–874. [12] J.G. Yu, J. Zhang, Dalton Trans. 39 (2010) 5860–5867. [13] J.B. Joo, Q. Zhang, I. Lee, M. Dahl, F. Zaera, Y.D. Yin, Adv. Funct. Mater. 22 (2012) 166–174. [14] D.P. Wang, H.C. Zeng, Chem. Mater. 21 (2009) 4811–4823. [15] Y. Kondo, H. Yoshikawa, K. Awaga, M. Murayama, T. Mori, K. Sunada, S. Bandow, S. Iijima, Langmuir 24 (2008) 547–550. [16] S.C. Lee, C.W. Lee, S.C. Lee, J.S. Lee, Mater. Lett. 62 (2008) 564–566. [17] Q. Zhang, W. Li, S.X. Liu, Powder Technol. 212 (2011) 145–150. [18] H.L. Meng, C. Cui, H.L. Shen, D.Y. Liang, Y.Z. Xue, P.G. Li, W.H. Tang, J. Alloys Compd. 527 (2012) 30–35. [19] K.S. Yang, Y. Dai, B.B. Huang, Chem. Phys. Lett. 456 (2008) 71–75. [20] T. Ohno, K. Tokieda, S. Higashida, M. Matsumura, Appl. Catal., A. 244 (2003) 383–391.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of calcination temperature on the microstructure, crystallinity and photocatalytic activity of TiO2 hollow spheres H.L. Shen a, H.H. Hu b, D.Y. Liang a, H.L. Meng a, P.G. Li a, W.H. Tang a, C. Cui a,⇑ a b
Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China Zhejiang University City College, Hangzhou 310015, China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Accepted 16 July 2012 Available online 27 July 2012 Keywords: TiO2 hollow spheres Calcination Photocatalytic activity
a b s t r a c t Submicrometer-sized TiO2 hollow spheres (THS) were synthesized using a template method, which involves deposition of TiO2 on SiO2 spheres templates through sol–gel process, calcination to crystallize amorphous TiO2, and etching SiO2 templates by hydrothermal treatment in NaOH solution. The evolution of the microstructure, crystallinity and photocatalytic activity of the THS fabricated with TiO2@SiO2 calcined at various temperature were studied systematically. The calcination treatment allows crystallization of the amorphous TiO2 shell into anatase nanocrystals in temperature range of 400–800 °C, and the crystallinity of the anatase TiO2 nanocrystals is improved as the calcination temperature increases. At temperature above 1000 °C, partial anatase TiO2 transforms to rutile phases. The morphology and microstructure of THS also varied over the calcination temperature due to the growth and aggregation of TiO2 nanocrystals. Accordingly, the absorption spectra edges of the THS shift towards the long wavelength. The results of photodegradation of Rhodamine B (RhB) under ultraviolet (UV) light illumination indicated that the THS calcined at 800 °C exhibited the highest photocatalytic activity, benefiting from the high crystallinity of anatase TiO2 and hierarchical hollow spherical structure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured titania (TiO2) has been widely investigated for its potential applications in photocatalysts and photovoltaic cells. In order to improve the photoelectric performance, TiO2 with various morphologies, such as nanoparticles, nanorods, nanotubes, nanosheets and hollow spheres have been reported [1–6]. Among these structures, hierarchical TiO2 hollow spheres (THS) have received much attention because of their low bulk density, high specific surface area, reduced transport lengths for both mass and charge transport, good surface permeability and easy recycling [6–14]. In addition, it has been demonstrated that the porous structure endows THS with improved light harvest by reflecting UV-light within the hollow sphere interior, consequently enhances the photocatalytic property [15–17]. Up to now, many approaches have been developed for the synthesis of THS, including template-based or template-free approaches. The template-free approaches have been proved to be simple and high efficient, however, it is difficult to control the size and morphology of THS with these approaches [10–12]. So, the template-based methods are still promising for the synthesis of high quality THS with uniform morphology [13,14]. For TiO2 photocatalyst, surface area and crystallinity of TiO2 are two crucial factors that affect the photocatalytic activity. Large surface area results in more reactant adsorption-desorption sites for catalytic reaction, ⇑ Corresponding author. Tel.: +86 571 8684 3468. E-mail address: [email protected] (C. Cui). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.080
and provides more efficient transport channels for the reactant molecules. High crystallinity of TiO2 is essential to enhance the generation and migration of photogenerated electron/hole pairs in the bulk and surface of TiO2. Therefore, a well crystallized TiO2 nanostructure with large surface area is considered the preferred form of TiO2 photocatalyst. THS generally have an advantage of large surface area. However, the achievement of perfect crystallized THS usually conflict with the achievement of large surface area, because elevated calcination temperature applied to crystallize TiO2 may cause the further growth or agglomeration of TiO2 nanoparticles and finally reduce the specific surface area. In this paper, we studied the balance between achievements of large specific surface area and high crystallinity for THS photocatalyst by varying the calcination temperature. The variation of the microstructure, crystalliny as well as the photocatalytic activity of the THS fabricated with TiO2@SiO2 pre-calcined at different temperature has been studied comparatively. The results show that the THS fabricated with TiO2@SiO2 pre-calcined at 800 °C possess the highest photocatalytic activity, benefiting from the high anatase crystallinity and good hollow spherical structure.
2. Experimental 2.1. Synthesis of TiO2 hollow spheres Firstly, monodisperse SiO2 sphere templates with average diameter of 400 nm were synthesized using a modified Stöber method as demonstrated in Ref. [18]. Then, TiO2 were deposited on the SiO2 sphere templates by a sol–gel method to ob-
H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36 tain SiO2@TiO2 core-shell structure. In order to crystallize TiO2, SiO2@TiO2 was subjected to calcination treatment at 400, 600, 800, 1000, and 1200 °C for1 h, respectively. Finally, the SiO2 cores were removed from the pre-calcined SiO2@TiO2 core-shell particles by using hydrothermal treatment in NaOH solution. The THS fabricated with SiO2@TiO2 pre-calcined at 400, 600, 800, 1000, and 1200 °C are denoted as THS-400, THS-600, THS-800, THS-1000 and THS-1200. In a typical synthesis, 0.3 g SiO2 spheres were ultrasonic dispersed in 100 mL of ethanol to obtain solution A. 2 mL of Tetrabutyl titanate (TBT, 5.85 mmol) was dissolved in 100 mL of ethanol to obtain solution B. Solution B and 1.5 mL of aqueous ammonia were added to solution A, vigorously stirred at 60 °C for 3 h to obtain solid SiO2@TiO2 core-shell spheres. The SiO2@TiO2 core-shell spheres were centrifuged and carefully washed with ethanol and dried in air. After calcination treatment at different temperature for 1 h, the SiO2@TiO2 core-shell spheres were placed in a 50 mL Teflon-lined autoclave with stainless steel tank and then the autoclave was filled to 80% capacity with 1 mol/L NaOH aqueous solution. After completely mixing, the autoclave was sealed and heated at 80 °C for 4 h. The autoclave was then naturally cooled down to room temperature. Finally, the product was collected by centrifugation and washed for several times with distilled water.
2.2. Characterization The phase of the samples was determined by powder X-ray diffraction (XRD) analysis, which were carried out on a Bruker D8 Advance X-ray diffractometer using Cu-Ka (k = 1.5406 Å) radiation at 40 kV and 40 mA. The morphology and microstructure were examined by a Hitachi S-4800 field emission scanning electron microscope (SEM) and a JEM-2100 high-resolution transmission electron microscope (HRTEM) at an acceleration voltage of 200 kV. UV–vis diffuse reflectance spectra (DRS) were achieved using a Perkin Elmer Lambda 900 UV–vis spectrophotometer. UV–vis absorption spectra analysis of the RhB solution was performed on a Varian Cary 50 spectrophotometer.
2.3. Measurement of photocatalytic activity Photocatalytic activity of the THS samples was evaluated by degradation of RhB under the illumination of a 500 W Xe lamp equipped with a UV band pass filter (k 6 400 nm). 50 mg of THS was dispersed in a 50 mL RhB aqueous solution (2 105 mol/L). Prior to illumination, the suspensions were vigorously stirred in dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium between the THS and RhB (graphically represented as time 30 min in Fig. 6a). During illumination, a circulation of water through an external cooling coil was conducted to maintain the temperature of suspensions at about 25 °C. Oxygen under atmospheric pressure was bubbled through the reaction continuously. The suspensions were sampled at 20 min intervals and centrifuged. The concentration of RhB was estimated by measuring the absorbance at their maximum peaks (553 nm) with a UV–Vis spectrophotometer.
3. Result and discussion 3.1. Morphology, crystallinity and microstructure of THS Fig. 1 shows the SEM images of the SiO2 spheres templates and the as-prepared THS without calcination treatment. The monodispersed SiO2 spheres are about 400 nm in diameter. The as-prepared THS without calcination treatment remain good spherical morphology with a uniform diameter of around 500 nm. The bro-
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ken spheres in Fig. 1(b) demonstrate the formation of hollow spheres. In order to crystallize TiO2, the SiO2@TiO2 core-shell samples were subjected to calcination treatment at different temperature. After removal of the SiO2 cores in NaOH solution, the crystal structure of the THS was characterized by XRD measurement, as shown in Fig. 2. The TiO2 is amorphous phase in the sample without calcination. After calcination at 400 °C (THS-400), diffraction peaks related to anatase phase TiO2 (JCPDS No. 21–1272) are observed, indicating that the amorphous TiO2 transferred to anatase phase. As the calcination temperature increases from 400 to 800 °C, those peaks associated with anatase phase TiO2 become sharper, indicating that the higher calcination temperature promotes the forming of anatase TiO2 of high crystallinity. However, peaks related to rutile phase TiO2 (JCPDS NO. 21–1276) appear when the calcination temperature rises to 1000 °C, which denotes the transformation of anatase TiO2 to rutile TiO2 in THS-1000. At an even higher temperature of 1200 °C (THS-1200), more anatase TiO2 transforms to rutile TiO2. From the XRD spectra, the average crystallite sizes Kk can be calculated by the Debye–Scherrer formulae D ¼ b cos , where h D is the crystallite diameter in nm, K is the shape constant (0.89), k is the wavelength of X-ray (1.5406 Å), b is the peak width (in radians) at half-maximum height and h is the Bragg angle. The calculated TiO2 crystallite size is listed in Table 1. From THS-400 to THS-1200, the size of anatase phase crystallite increases from 9.1 to 39.2 nm, and the size of rutile phase crystallite increases from 25.1 to 62.1 nm. Fig. 3 shows the SEM images of the THS samples fabricated with SiO2@TiO2 calcinated at different temperature. The diameters of the hollow spheres are about 500 nm. The rough surface structure reveals that the THS are hierarchical structure composing of nanoparticles. The hollow spheres keep in good spherical structure when the calcination temperature is below 800 °C. As the temperature rises above 800 °C, nanoparticles constituting the shell of the hollow spheres grow or agglomerate into larger particles, making the shell looser. In some case, the aggregation of the TiO2 nanoparticle inevitably lead to a collapse of the porous hollow sphere structure and hence remarkably decrease the specific surface area. As the THS-800 sample shows a good hollow spherical structure and high anatase crystallinity, more detailed microstructure of this THS is studied. Fig. 4(a) shows a panoramic SEM image of the THS800 sample, and we can see that almost all the hollow spheres remain good spherical morphology and dense shell structure. Fig. 4(b and c) show the TEM images of the sample. In Fig. 4(b), the color contrast between the central and fringe region of the spheres confirms the formation of hollow spherical structure with shells of about 50 nm. At a high resolution TEM image in Fig. 4(c), clear grain boundaries and crystal lattice fringes of crystallites with diameters of about 10 nm are observed. A set of crystal lattices
Fig. 1. SEM images of (a) SiO2 sphere templates and (b) as-prepared THS without pre-calcination.
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H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36
Fig. 2. XRD patterns of THS samples fabricated with TiO2@SiO2 pre-calcined at different temperature.
Table 1 Notations, crystallite sizes and photocatalytic reaction rate constant k for TiO2 hollow spheres. Sample notation
THS400
THS600
THS800
THS1000
THS1200
Crystallite size (nm) A:anatase R:rutile Reaction rate constant k (102 min1)
9.1(A)
9.9(A)
11.9(A)
0.67
4.42
12.0
22.3(A) 25.1(R) 3.78
39.2(A) 62.1(R) 0.97
with d-space equivalent to 0.352 nm corresponding to (1 0 1) planes of anatase TiO2 is found as the majority faces of individual crystallites throughout the area observed in Fig. 4(c). The TEM images reveal that the shells of THS-800 are composed of anatase nanocrystals with an average size of 10 nm, which is in good agreement with the results from the XRD analysis.
3.2. UV–vis DRS analysis of the THS UV–vis DRS spectra were measured to investigate the optical absorption property of the THS samples, as shown in Fig. 5. The
absorption onsets are determined by linear extrapolation from the inflection point of the curve to the baseline. The absorption onsets (inset of Fig. 5) for THS-400, THS-600 and THS-800 are around 390 nm, corresponding to bandgap energy of 3.2 eV. The THS-800 sample shows an enhanced absorbance in the spectral range of 320–390 nm, which can be attributed to the higher crystallinity of anatase phase TiO2 in THS-800 compared to THS-400 and THS600. Compared with that of THS-800, the absorption onsets of THS-1000 and THS-1200 slightly red shift. The absorption onsets for THS-1000 and THS-1200 are 410 nm (3.0 eV) and 417 nm (2.97 eV), respectively. In a recent report by Yin et al. [13], the Si in the SiO2 core could not enter the crystal lattice of TiO2 to act as a substitutional dopant during high temperature calcination. So, we can rule out the possibility of introduction of doping energy level, which usually cause the red shift of absorption onsets for doped-TiO2 [19]. Two possible reasons may lead to the red shift of the absorption onsets of the THS calcined at temperature above 1000 °C. The first reason is the obvious size augment of TiO2 nanocrystals that constitute THS. The average size of the TiO2 nanocrystals in THS-800 is about 11.9 nm, which is smaller than that in THS-1000 (22.3 nm for anatase phase and 25.1 nm for rutile phase) and THS-1200 (39.2 nm for anatase phase and 62.1 nm for rutile phase). Larger sizes of the TiO2 nanocrytals result in a slightly smaller bandgap of THS due to the weakened quantum-size effect. The second reason is the appearance of rutile phase TiO2 in the THS-1000 and THS-1200 samples, the rutile phase TiO2 has a smaller bandgap (3.0 eV) than that of anatase phase TiO2 (3.2 eV). 3.3. Photocatalytic activity of THS The photocatalytic activity of the THS samples for the degradation of RhB under UV light is revealed by the plots of C/C0 versus time t (Fig. 6a), where C is the concentration of RhB in aqueous solution at the time t, and C0 is the initial concentration. As we can see, the THS-800 sample shows the best photocatalytic activity which could achieve 100% degradation of RhB with UV light illumination for 60 min. The linear relationship between ln(C/C0) and t (Fig. 6b) indicates that the photocatalytic degradation reaction for all the samples follows pseudo-first-order kinetics. From Fig. 6b, the reaction rate constant k for the THS samples can be de-
Fig. 3. SEM images of THS samples fabricated with TiO2@SiO2 pre-calcined at different temperature.
H.L. Shen et al. / Journal of Alloys and Compounds 542 (2012) 32–36
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Fig. 4. SEM (a), TEM (b) and HRTEM (c) images of the THS-800 sample.
duced from the slope of the lines and they are listed in Table 1. THS-800 shows a superior reaction rate constant of 12.0 102 min1, which is much higher than those for THS-600 (4.42 102 min1) and THS-400 (0.67 102 min1). The high reaction rate constant of THS-800 can be primarily attributed to the formation of highly crystallized anatase TiO2 nanocrystals, which can enhance the generation and migration of photogenerated electron/hole pairs and therefore increase the redox property. It has been reported that the mixture of anatase and rutile phase TiO2 exhibited a better photocatalytic activity than the sole anatase or rutile phase TiO2 [1,20]. In this study, the mixture of anatase and rutile phase appeared in THS-1000 and THS-1200, however the photocatalytic reaction rate constants of these two samples are smaller (3.78 102 and 0.97 102 min1 for THS1000 and THS-1200) than that of THS-800. This may be attributed to the smaller specific surface area of THS-1000 and THS-1200, as a result of the growth and aggregation of the nanocrystals constituting the hollow spheres at the high calcination temperature (Fig. 3).
Fig. 6. (a) Evolution of RhB concentration versus UV illumination time in the presence different THS samples, and (b) linear transform ln(C/C0) = kt of the kinetic curves of the degradation of RhB versus time.
Fig. 5. UV–vis diffuse reflectance absorption (UV–DRS) spectra for different THS samples.
Therefore, we can conclude that the THS-800 sample achieves a balance of large specific surface area and high crystallinity for THS photocatalyst.
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4. Conclusions THS with diameter about 500 nm have been synthesized by a SiO2 spheres template method followed by calcination and etching of the SiO2 by hydrothermal treatment in NaOH solution. To achieve the optimum properties in terms of morphological feature, crystallinity and photocatalytic activity, THS fabricated with TiO2@SiO2 pre-calcined at deferent temperature were investigated by means of SEM, TEM, XRD, UV–vis DRS as well as the photodegradation of RhB under UV light illumination. It is found that the THS exhibit good hollow spherical morphology as the calcination temperature is below 800 °C, while the crystallinity of anatase TiO2 phase improves as the temperature creases from 400 to 800 °C. At temperature above 1000 °C, partial anatase TiO2 transforms to rutile phases, and the morphology of THS also varied due to the growth and aggregation of TiO2 nanocrystals at high temperature. The experiment of the photodegradation of RhB shows that the THS fabricated with TiO2@SiO2 pre-calcined at 800 °C exhibits the highest photocatalytic activity. This is attributed to the superior hierarchical hollow spherical structure and high anatase crystallinity, causing the high adsorption of reactant, high light harvest, and enhanced generation and migration of photogenerated electron/hole pairs. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos.60806045, and 11074220), Zhejiang Provincial Natural
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