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Optical and photocatalytic properties of TiO2 with hollow nanostructure
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Materials Letters 62 (2008) 564 – 566 www.elsevier.com/locate/matlet
Optical and photocatalytic properties of TiO2 with hollow nanostructure Seung-Chul Lee, Chang-Woo Lee, Sunyong Caroline Lee, Jai-Sung Lee ⁎ Department of Materials Engineering, Hanyang University, Ansan 426-791, South Korea Received 26 August 2007; accepted 10 September 2007 Available online 14 September 2007
Abstract The effect of hollow nanostructure of TiO2 on the degradation of organic compounds and optical transmittance in the visible-light range was investigated. TiO2 hollow nanoparticles (HNP) consisting of a mixed phase of anatase and rutile in the ratio of 6:4, had a mean size of 70 nm and shell thickness of 5–7 nm. Spectrophotometry study showed that the transmittance of hollow particles was less than 30% in the wavelength range of 320–800 nm. Also, degradation experiment revealed hollow particles exhibited relatively poorer degradation compared to that of commercial P25 powder. These results indicate that movement of electron and light propagation can be affected by the hollow nanostructure. © 2007 Elsevier B.V. All rights reserved. Keywords: Catalysts; Nanomaterials; Hollow structure; Optical materials and properties; Surfaces
1. Introduction Ceramic hollow particles have attracted considerable interests due to their typical properties such as large surface area, low density, and high strength [1–3]. Especially, many researchers have focused on enhancing optical, chemical, and electrical properties of hollow particles by reducing particle size below 100 nm to facilitate a dramatic increase in surface area to volume ratio [4–6]. Recently, the authors reported the formation of hollow nanoparticles (HNP) of Fe2O3, Al2O3, and TiO2 during chemical vapor condensation (CVC) process [7–10]. In those reports, it was suggested that the formation of the hollow structure depends on how quickly metal ions are liberated from acetylacetonate groups and move to the surface of the precursor in order to react with oxygen [7–10]. For TiO2 HNP, it was found that the band gap energy was irrelevant to a particle shape and optical transmittance was low compared to that of commercial P25 powder (Degussa) in the visible-light range [9]. The low transmittance provides an important practical meaning in the application of TiO2 HNP as a photocatalyst or an electrode material. Therefore, this study concentrates on understanding the light propagation through close-packed HNP by measuring their transmittance using UV– visible spectrophotometer. Also, the effect of hollow structure ⁎ Corresponding author. Tel.: +82 31 400 5225; fax: +82 31 406 5170. E-mail address: [email protected] (J.-S. Lee). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.008
on light absorption for the photocatalytic reaction was studied by conducting a decomposition experiment of methylene blue as an organic compound. 2. Experimental In order to synthesize TiO2 HNP, titanium oxide acetylacetonate (TiO(C5H7O2)2, Sigma-Aldrich Co.) was used as a precursor [9]. Precursor vaporized, at 300 °C, was delivered into the CVC reactor using He carrier gas at a flow rate of 1 l/min while the pressure in the reactor was held at 400 mbar. Simultaneously, the reaction gas, O2, was delivered into the reactor at a flow rate of 2 l/min. The reaction temperature was maintained at 800 °C. As-received HNP were sampled at the collector site, held at room temperature. The crystal phase and the morphology of the titanium oxide nanoparticles were characterized by X-ray diffractometry (XRD, Rigaku D/Max 2500) and transmission electron microscopy (TEM, JEM-2010). Also, optical transmittance of powder samples was measured by UV–visible spectrophotometer (JASCO V-550) equipped with an integration sphere detection system. The degradation experiment of organic compound was carried out using methylene blue solution (2.0 × 10− 5 mol/20 ml). For the light absorption, a visible-light lamp (150 W, UV cut-off filter) within a wavelength range of 400–700 nm for 30, 60, and 120 min. irradiated the hollow particles, respectively. For the photocatalytic characterizations of
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Fig. 1. a) TEM micrograph and b) XRD pattern of TiO2 hollow nanoparticles (HNP) synthesized by chemical vapor condensation (CVC) at 800 °C and 400 mbar.
the titanium oxide HNP, commercial P25 nanoparticles with 15–25 nm in size, in a solid particle shape, were also tested for comparison [11]. 3. Results and discussion Fig. 1a shows the TEM micrograph of the titanium oxide nanoparticles synthesized by CVC process. As shown in the figure, the hollow particles have a mean size of 70 nm and a shell thickness of 5–7 nm. The XRD analysis in Fig. 1b shows that the HNP consist of a mixed phase of anatase and rutile. The mean crystallite sizes calculated using the Scherrer formula [12], was 14 and 25 nm for anatase and rutile, respectively. An approximate ratio of anatase to rutile phase was obtained by estimating the intensity ratio of the (101) peak intensity of the anatase to the (110) peak intensity of the rutile. The volume percent of the rutile of TiO2 HNP was 44%, which is larger than that of P25 based on the previous report [11]. In order to understand the effect of particle shape on an optical property, the optical transmittance for the two kinds of TiO2 nanoparticles, which are HNP and P25, was measured by UV–visible spectrophotometer (Fig. 2). The transmittance of HNP sample showed similar tendency to that of commercial P25 from the UV-B (280– 320 nm) to the UV-C (b 280 nm) regions. However, HNP sample exhibited lower transmittance which was less than 30% in the UV-A (320–400 nm) regions and in the visible-light (400–800 nm) regions, compared to P25 sample having a transmittance over 90% in the
Fig. 2. Optical transmittance spectra of TiO2 HNP and commercial P25 nanoparticles.
visible-light range. From the results, it can be suspected that the effect of low transmittance might be due to two following reasons; first, TiO2 HNP has a high absorbance or second, this phenomenon relates reflectance depending on hollow structure. Fig. 3 shows photocatalytic efficiency of HNP and P25 samples after the irradiation in the wavelength of 400–800 nm for 30 to 120 min. As shown in Fig. 3, degradation of methylene blue solution using HNP sample was preceded slower than that of P25 sample. This result shows that the degradation efficiency of HNP sample for organic compound is poorer than that of solid particles and the low transmittance of hollow structure is not exactly equal to the light absorption. Then, some contribution from the reflection or scattering can be considered.
Fig. 3. The absorbance of methylene blue solution including a) TiO2 HNP and b) commercial P25 nanoparticles after the light illumination using visible-light for 30 to 120 min.
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4. Conclusion
Fig. 4. Schematic suggestion of the movement of electron-hole pairs toward to two-side surfaces and specular (regular) or diffuse reflection of light happening in the hollow nanostructures.
In this study, we suggested that both of the light trap in the visible-light range and the photocatalytic reaction in hollow nanostructure have been demonstrated for the first time. TiO2 hollow nanoparticles consisting of a mixed phase of anatase and rutile were synthesized by CVC process. According to low transmittance in both UV-A and visible-light range, we observed a relation between hollow structure and behavior of incident-light. It was found that low transmittance has no influence on increasing light absorbance in decomposition experiment of methylene blue. However, it is expected that penetrated incident-light is trapped inside hollow nanostructure and continuous reflection within shells can lead to the extinction or the transmission with lost energy level. Further studies on analyzing and understanding the optics in hollow nanostructure to the phenomena will be preceded. Acknowledgement
Moreover, we need to consider partial photocatalytic reaction of HNP sample, since the irradiation wavelength range (400–800 nm) includes a narrow region for excitation of TiO2. This is the reason we need to divide our focuses into two regions. One is the wavelength range below 420 nm which corresponds to the band gap energy of TiO2, 3.0–3.2 eV [13–15] for excitation and decomposition of organic compounds. The other one exists behind 420 nm which belongs to visible-light range. First, it is well known that electron-hole pairs generated in TiO2 nanoparticles by the light corresponding to energy band gap, move to surfaces interacting with outer gas or liquid phases at the early stage of photocatalytic reaction. However, it should be noted that the hollow nanostructure of TiO2 in this study consists of both the inside and outside surfaces of a particle. This means that the electron-hole pairs moving to the outer interface can only react with target compounds and the other pairs on inside surface can be neutralized by recombination (Fig. 4). This can be an explanation for the poor degradation efficiency of HNP sample. Secondly, if we assume that visible-light does not contribute to photocatalytic reaction at all, then it is expected that penetrated incident-light is trapped inside hollow nanostructure and continuous reflection within shells leads to the extinction or the transmission with lost energy level (Fig. 4). Some phenomenon on the light behavior in hollow structure can be explained by well known concepts, such as optical mirror and diffuse reflection [16]. However, the present studies suggest that both of the light trap phenomena in the visible-light range and the photocatalytic reaction in hollow nanostructure have been demonstrated for the first time.
This research was supported by the Ministry of Education and Human Resources through Brain Korea 21 (BK 21) program. Also, the authors are grateful to Dr. Meng-Dawn Cheng for making valuable comments. References [1] M.S. Morey, J.D. Bryan, S. Schwarz, G.D. Stucky, Chem. Mater. 12 (2000) 3435–3444. [2] K. Okada, et al., Microporous Mesoporous Mater. 21 (1998) 289–296. [3] J.K. Cochran, Curr. Opin. Solid State Mater. Sci. 3 (1998) 474–479. [4] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453–457. [5] J.A. Zasadzinski, E. Kisak, C. Evans, Curr. Opin. Colloid Interface Sci. 6 (2001) 85–90. [6] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [7] J.S. Lee, et al., J. Nanopart. Res. 6 (2004) 627–631. [8] C.W. Lee, S.G. Kim, J.S. Lee, Key Eng. Mater. 317–8 (2006) 219–222. [9] C.W. Lee, J.S. Lee, J. Ceram. Soc. Japan 114 (2006) 923–928. [10] C.W. Lee, J.S. Lee, Intern. J. Mater. Res. 98 (2007) 21–25. [11] J.F. Porter, Y.G. Li, C.K. Chan, J. Mater. Sci. 34 (1999) 1523–1531. [12] A. Taylor, X-ray Metallography, Wiley, New York, 1961. [13] J. Pascual, J. Camassel, H. Mathieu, Phys. Rev., B 18 (1978) 5606–5614. [14] H. Tang, H. Berger, P.E. Schmid, F. Levy, G. Burri, Solid State Commun. 87 (1993) 847–850. [15] S.D. Mo, W.Y. Ching, Phys. Rev., B 51 (1995) 13023–13032. [16] I. Ricárdez-Vargas, M. Iturbe-Castillo, R. Ramos-García, K. VolkeSepúlveda, V. Ruíz-Cortés, Opt. Express 13 (2005) 968–976.
Materials Letters 62 (2008) 564 – 566 www.elsevier.com/locate/matlet
Optical and photocatalytic properties of TiO2 with hollow nanostructure Seung-Chul Lee, Chang-Woo Lee, Sunyong Caroline Lee, Jai-Sung Lee ⁎ Department of Materials Engineering, Hanyang University, Ansan 426-791, South Korea Received 26 August 2007; accepted 10 September 2007 Available online 14 September 2007
Abstract The effect of hollow nanostructure of TiO2 on the degradation of organic compounds and optical transmittance in the visible-light range was investigated. TiO2 hollow nanoparticles (HNP) consisting of a mixed phase of anatase and rutile in the ratio of 6:4, had a mean size of 70 nm and shell thickness of 5–7 nm. Spectrophotometry study showed that the transmittance of hollow particles was less than 30% in the wavelength range of 320–800 nm. Also, degradation experiment revealed hollow particles exhibited relatively poorer degradation compared to that of commercial P25 powder. These results indicate that movement of electron and light propagation can be affected by the hollow nanostructure. © 2007 Elsevier B.V. All rights reserved. Keywords: Catalysts; Nanomaterials; Hollow structure; Optical materials and properties; Surfaces
1. Introduction Ceramic hollow particles have attracted considerable interests due to their typical properties such as large surface area, low density, and high strength [1–3]. Especially, many researchers have focused on enhancing optical, chemical, and electrical properties of hollow particles by reducing particle size below 100 nm to facilitate a dramatic increase in surface area to volume ratio [4–6]. Recently, the authors reported the formation of hollow nanoparticles (HNP) of Fe2O3, Al2O3, and TiO2 during chemical vapor condensation (CVC) process [7–10]. In those reports, it was suggested that the formation of the hollow structure depends on how quickly metal ions are liberated from acetylacetonate groups and move to the surface of the precursor in order to react with oxygen [7–10]. For TiO2 HNP, it was found that the band gap energy was irrelevant to a particle shape and optical transmittance was low compared to that of commercial P25 powder (Degussa) in the visible-light range [9]. The low transmittance provides an important practical meaning in the application of TiO2 HNP as a photocatalyst or an electrode material. Therefore, this study concentrates on understanding the light propagation through close-packed HNP by measuring their transmittance using UV– visible spectrophotometer. Also, the effect of hollow structure ⁎ Corresponding author. Tel.: +82 31 400 5225; fax: +82 31 406 5170. E-mail address: [email protected] (J.-S. Lee). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.008
on light absorption for the photocatalytic reaction was studied by conducting a decomposition experiment of methylene blue as an organic compound. 2. Experimental In order to synthesize TiO2 HNP, titanium oxide acetylacetonate (TiO(C5H7O2)2, Sigma-Aldrich Co.) was used as a precursor [9]. Precursor vaporized, at 300 °C, was delivered into the CVC reactor using He carrier gas at a flow rate of 1 l/min while the pressure in the reactor was held at 400 mbar. Simultaneously, the reaction gas, O2, was delivered into the reactor at a flow rate of 2 l/min. The reaction temperature was maintained at 800 °C. As-received HNP were sampled at the collector site, held at room temperature. The crystal phase and the morphology of the titanium oxide nanoparticles were characterized by X-ray diffractometry (XRD, Rigaku D/Max 2500) and transmission electron microscopy (TEM, JEM-2010). Also, optical transmittance of powder samples was measured by UV–visible spectrophotometer (JASCO V-550) equipped with an integration sphere detection system. The degradation experiment of organic compound was carried out using methylene blue solution (2.0 × 10− 5 mol/20 ml). For the light absorption, a visible-light lamp (150 W, UV cut-off filter) within a wavelength range of 400–700 nm for 30, 60, and 120 min. irradiated the hollow particles, respectively. For the photocatalytic characterizations of
S.-C. Lee et al. / Materials Letters 62 (2008) 564–566
565
Fig. 1. a) TEM micrograph and b) XRD pattern of TiO2 hollow nanoparticles (HNP) synthesized by chemical vapor condensation (CVC) at 800 °C and 400 mbar.
the titanium oxide HNP, commercial P25 nanoparticles with 15–25 nm in size, in a solid particle shape, were also tested for comparison [11]. 3. Results and discussion Fig. 1a shows the TEM micrograph of the titanium oxide nanoparticles synthesized by CVC process. As shown in the figure, the hollow particles have a mean size of 70 nm and a shell thickness of 5–7 nm. The XRD analysis in Fig. 1b shows that the HNP consist of a mixed phase of anatase and rutile. The mean crystallite sizes calculated using the Scherrer formula [12], was 14 and 25 nm for anatase and rutile, respectively. An approximate ratio of anatase to rutile phase was obtained by estimating the intensity ratio of the (101) peak intensity of the anatase to the (110) peak intensity of the rutile. The volume percent of the rutile of TiO2 HNP was 44%, which is larger than that of P25 based on the previous report [11]. In order to understand the effect of particle shape on an optical property, the optical transmittance for the two kinds of TiO2 nanoparticles, which are HNP and P25, was measured by UV–visible spectrophotometer (Fig. 2). The transmittance of HNP sample showed similar tendency to that of commercial P25 from the UV-B (280– 320 nm) to the UV-C (b 280 nm) regions. However, HNP sample exhibited lower transmittance which was less than 30% in the UV-A (320–400 nm) regions and in the visible-light (400–800 nm) regions, compared to P25 sample having a transmittance over 90% in the
Fig. 2. Optical transmittance spectra of TiO2 HNP and commercial P25 nanoparticles.
visible-light range. From the results, it can be suspected that the effect of low transmittance might be due to two following reasons; first, TiO2 HNP has a high absorbance or second, this phenomenon relates reflectance depending on hollow structure. Fig. 3 shows photocatalytic efficiency of HNP and P25 samples after the irradiation in the wavelength of 400–800 nm for 30 to 120 min. As shown in Fig. 3, degradation of methylene blue solution using HNP sample was preceded slower than that of P25 sample. This result shows that the degradation efficiency of HNP sample for organic compound is poorer than that of solid particles and the low transmittance of hollow structure is not exactly equal to the light absorption. Then, some contribution from the reflection or scattering can be considered.
Fig. 3. The absorbance of methylene blue solution including a) TiO2 HNP and b) commercial P25 nanoparticles after the light illumination using visible-light for 30 to 120 min.
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S.-C. Lee et al. / Materials Letters 62 (2008) 564–566
4. Conclusion
Fig. 4. Schematic suggestion of the movement of electron-hole pairs toward to two-side surfaces and specular (regular) or diffuse reflection of light happening in the hollow nanostructures.
In this study, we suggested that both of the light trap in the visible-light range and the photocatalytic reaction in hollow nanostructure have been demonstrated for the first time. TiO2 hollow nanoparticles consisting of a mixed phase of anatase and rutile were synthesized by CVC process. According to low transmittance in both UV-A and visible-light range, we observed a relation between hollow structure and behavior of incident-light. It was found that low transmittance has no influence on increasing light absorbance in decomposition experiment of methylene blue. However, it is expected that penetrated incident-light is trapped inside hollow nanostructure and continuous reflection within shells can lead to the extinction or the transmission with lost energy level. Further studies on analyzing and understanding the optics in hollow nanostructure to the phenomena will be preceded. Acknowledgement
Moreover, we need to consider partial photocatalytic reaction of HNP sample, since the irradiation wavelength range (400–800 nm) includes a narrow region for excitation of TiO2. This is the reason we need to divide our focuses into two regions. One is the wavelength range below 420 nm which corresponds to the band gap energy of TiO2, 3.0–3.2 eV [13–15] for excitation and decomposition of organic compounds. The other one exists behind 420 nm which belongs to visible-light range. First, it is well known that electron-hole pairs generated in TiO2 nanoparticles by the light corresponding to energy band gap, move to surfaces interacting with outer gas or liquid phases at the early stage of photocatalytic reaction. However, it should be noted that the hollow nanostructure of TiO2 in this study consists of both the inside and outside surfaces of a particle. This means that the electron-hole pairs moving to the outer interface can only react with target compounds and the other pairs on inside surface can be neutralized by recombination (Fig. 4). This can be an explanation for the poor degradation efficiency of HNP sample. Secondly, if we assume that visible-light does not contribute to photocatalytic reaction at all, then it is expected that penetrated incident-light is trapped inside hollow nanostructure and continuous reflection within shells leads to the extinction or the transmission with lost energy level (Fig. 4). Some phenomenon on the light behavior in hollow structure can be explained by well known concepts, such as optical mirror and diffuse reflection [16]. However, the present studies suggest that both of the light trap phenomena in the visible-light range and the photocatalytic reaction in hollow nanostructure have been demonstrated for the first time.
This research was supported by the Ministry of Education and Human Resources through Brain Korea 21 (BK 21) program. Also, the authors are grateful to Dr. Meng-Dawn Cheng for making valuable comments. References [1] M.S. Morey, J.D. Bryan, S. Schwarz, G.D. Stucky, Chem. Mater. 12 (2000) 3435–3444. [2] K. Okada, et al., Microporous Mesoporous Mater. 21 (1998) 289–296. [3] J.K. Cochran, Curr. Opin. Solid State Mater. Sci. 3 (1998) 474–479. [4] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453–457. [5] J.A. Zasadzinski, E. Kisak, C. Evans, Curr. Opin. Colloid Interface Sci. 6 (2001) 85–90. [6] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [7] J.S. Lee, et al., J. Nanopart. Res. 6 (2004) 627–631. [8] C.W. Lee, S.G. Kim, J.S. Lee, Key Eng. Mater. 317–8 (2006) 219–222. [9] C.W. Lee, J.S. Lee, J. Ceram. Soc. Japan 114 (2006) 923–928. [10] C.W. Lee, J.S. Lee, Intern. J. Mater. Res. 98 (2007) 21–25. [11] J.F. Porter, Y.G. Li, C.K. Chan, J. Mater. Sci. 34 (1999) 1523–1531. [12] A. Taylor, X-ray Metallography, Wiley, New York, 1961. [13] J. Pascual, J. Camassel, H. Mathieu, Phys. Rev., B 18 (1978) 5606–5614. [14] H. Tang, H. Berger, P.E. Schmid, F. Levy, G. Burri, Solid State Commun. 87 (1993) 847–850. [15] S.D. Mo, W.Y. Ching, Phys. Rev., B 51 (1995) 13023–13032. [16] I. Ricárdez-Vargas, M. Iturbe-Castillo, R. Ramos-García, K. VolkeSepúlveda, V. Ruíz-Cortés, Opt. Express 13 (2005) 968–976.