- Email: [email protected]
MWCNTs nanomaterials by a combined sol–gel and polyol process
Diamond & Related Materials 18 (2009) 312–315
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Synthesis of hybrid Pt/TiO2 (anatase)/MWCNTs nanomaterials by a combined sol–gel and polyol process K.N. Lin a, W.J. Liou a, T.Y. Yang b, H.M. Lin a,⁎, C.K. Lin c, S.H. Chien d, W.C. Chen e, S.H. Wu a a
Department of Materials Engineering, Tatung University, Taipei 104, Taiwan Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan d Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan e Department of Chemical and Material Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan b c
a r t i c l e
i n f o
Available online 5 August 2008 Keywords: MWCNTs Anatase Pt CO conversion
a b s t r a c t In the present study, hybrid Pt/TiO2/MWCNTs nanomaterials are prepared successfully by a combined sol–gel and polyol process. The as-prepared nanomaterials are characterized by X-ray diffraction, high resolution transmission electron microscopy, and thermogravimetry analysis. In addition, its catalytic performance by converting CO into CO2 is also evaluated. Experimental results show that the hybrid Pt/TiO2/MWCNTs nanomaterials exhibit a mixture of anatase TiO2 and Pt phases. Multi-wall carbon nanotubes serve as an excellent supporting material where anatase TiO2 nanoparticles are decorated with well-distributed Pt nanoparticles. Excellent catalytic performance can be revealed for the hybrid Pt/TiO2/MWCNTs nanomaterials. When compared with its Pt/TiO2 counterparts where ∼ 100% CO conversion occurred at 150 °C, almost 100% conversion of CO into CO2 can be observed at a temperature ranged from 30 °C to 100 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic–inorganic hybrid nanocomposite materials attracted considerable research interests in fundamental and application studies because of their potentially commercial applications [1]. Noble metals on ceramic nanoparticles are most notable in catalysis studies. Among them, the Pt/TiO2 nanocomposite has been studied on the hydrogenation and photocatalytic applications for several decades [2]. Pt nanoparticles on the matrix TiO2 are considered to play an important assistant catalyst role in the photocatalytic properties. Usually, the particle sizes of Pt and TiO2 are critical for the photocatalysis. Tanaka [3] showed that the more efficient photocatalytic action occurs in the larger TiO2 nanoparticles; because an electron and a positive hole can migrate a longer distance in the larger crystal than the smaller one, and accordingly separating more the reducing and oxidizing sites on the surface of the crystal. Lu [4] also showed that the anatase-phase titania will demonstrate the better photocatalytic activity for phenol oxidation than rutile phase because of the more surface-adsorbed water and hydroxyl groups. Generally speaking, anatase-phase TiO2 has attracted extensive attentions due to its excellent photocatalytic [5,6], photovoltaic [7], and gas sensing properties [8,9], etc. On the other side, the smaller size of platinum will provide the larger active areas and an electron trapper for catalysis, but the overload of platinum on the surface of titania will ⁎ Corresponding author. E-mail address: [email protected] (H.M. Lin). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.07.017
cover the photosensitive sites of titania [10]. In addition, carbon nanotubes (CNTs) base on the intriguingly physical and chemical properties are finding broad use as an ideal support for catalysts that improve obviously the stability of catalytic activity [11]. In the previous study, we have studied the hybrid TiO2/CNTs nanomaterial system prepared by sol–gel technique. The hybrid TiO2/CNTs nanomaterials exhibited anatase-phase TiO2 even at 1000 °C in Ar atmosphere, the phase transformation of anatase to rutile is inhibited by the pinning effect of the CNTs. The aim of the work is to prepare and characterize the size- and shape-controlled nanocomposite with homogeneous dispersion of Pt/TiO2 nanoparticles on multi-wall carbon nanotubes (MWCNTs) matrix. 2. Experimental procedures Multi-wall carbon nanotubes are commercially available (CTube 100, CNT Co., Ltd., Korea). MWCNTs are purified by concentrated HNO3 (Wako Pure Chemical Industries, Ltd. Japan) to remove the impurities and catalysts before coating TiO2 and Pt nanoparticles. For purification, 5 g of MWCNTs are boiled in 250 ml concentration HNO3 at 150 °C for 3 h, filtered and washed with distilled water to acid-free, and finally dried at 80 °C for 12 h to obtain the purified MWCNTs. Hybrid TiO2/MWCNTs nanomaterials are synthesized via a sol–gel process. Titanium tetra-isopropoxide and isopropyl alcohol (IPA), (CH3)2CHOH, are obtained from Wako Pure Chemical Industries, Ltd. Japan, with Wako 1st grade and JIS special grade, respectively. The solution of 0.1 g titanium tetra-isopropoxide dissolved in 50 ml IPA
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
Fig. 1. XRD patterns of purified multi-wall carbon nanotubes, hybrid TiO2/MWCNTs, Pt/ MWCNTs, and Pt/TiO2/MWCNTs nanomaterials.
solvent is used as the precursors for sol–gel process. The TiO2 precursor sol is gradually peptized into the MWCNTs-suspended solution (0.03 g purified MWCNTs in 50 ml IPA) that is placed over a hot plate stirring with magnet. Hybrid TiO2/MWCNTs are obtained after drying for 24 h. Subsequently post heat treatments are performed at 1000 °C under Ar-controlled atmosphere for 1.5 hour. The polyol process is prepared by heating of ethylene glycol solution with the Pt precursor salt. TiO2/MWCNTs (0.03 g) are first dispersed into 15 ml ethylene glycol (Wako, 99.0%). Then, 1.8 ml aqueous solution of 0.05 M H2PtCl6·6H2O (Wako, 98.5%) and 1.2 ml of 0.4 M KOH (Shimakyu Pure Chemicals, 85.0%) are added continually and heated at 170 °C for 0.5 h. The final solution is filtered and calcined under argon atmosphere at 250 °C to obtain hybrid Pt/TiO2/MWCNTs nanomaterials. The hybrid Pt/TiO2/MWCNTs nanomaterials are characterized by X-ray diffraction (XRD), field-emission transmission electron microscopy (FE-TEM), and thermogravimetry analysis (TGA). The phase identification is performed using a MAC MXP 18 X-ray diffractometer with a monochromatic Cu Kα radiation. FE-TEM (JEOL, JEM-2100Fs) is operated at 200 kV to observe the bright field image of the hybrid nanomaterials. The thermogravimetry analysis is performed by SDT2960 Simultaneous DSC-TGA (TA Instruments) where the specimen is heated from room temperature to 1000 °C at a heating rate of 5 °C/min. The catalytic performance of the hybrid Pt/TiO2/MWCNTs nanomaterials and its counterparts is carried out in a continuous flow micro-reactor. The reaction gases, 10% O2/He and 4% CO/He, are fed chamber with 0.025 g catalyst (Pt) at a rate of 20 ml/min. The effluent gas is analyzed on-line using a Varian 3700 gas chromatograph with a carbosphere column.
313
Pt/TiO2/MWCNTs is smaller than that in Pt/MWCNTs. Since the same polyol process is used to prepare these two hybrid materials, it is suggested that the reduction of Pt on MWCNTs will be preferred than that on TiO2/MWCNTs. Fig. 2 shows a field-emission transition electron microscope image of hybrid Pt/MWCNTs. It can be observed that Pt nanoparticles are well dispersed on multi-wall carbon nanotubes. Only limited agglomeration of Pt nanoparticles can be revealed even after sintering at 250 °C. Fig. 3 shows FE-TEM images of Pt/TiO2/MWCNTs at various magnifications. In a previous study, we have demonstrated that a distinct hybrid TiO2/MWCNTs nanomaterials, where TiO2 nanoparticles are adhered individually to the uncovered MWCNTs, can be synthesized by sol–gel process followed by post annealing at 1000 °C under an Ar-protected environment. Fig. 3(a), Pt/TiO2/MWCNTs, shows a similar image as that of TiO2/MWCNTs. One of the adhered nanoparticles, indicated by the arrow in Fig. 3(a), is examined using a higher magnification. As shown in Fig. 3(b), it can be noted that Pt nanoparticles with a size of 2–3 nm are well dispersed on TiO2 nanoparticles (∼ 40 nm). Fig. 3(c) and (d) shows the well dispersion and arrangement of Pt on TiO2 surface. In order to measure the relative amount of platinum and titania deposited on the purified multi-wall carbon nanotubes, thermogravimetry analysis is performed under air. Fig. 4 shows the TGA curves of the purified multi-wall carbon nanotubes (curve (1)), hybrid TiO2/ MWCNT (curve (2)), Pt/MWCNTs (curve (3)), and Pt/TiO2/MWCNTs (curve (4)) from room temperature to 1000 °C at a heating rate of 5 °C/ min. It can be noticed from curve (1) that MWCNTs almost burn out completely at a temperature higher than ∼ 560 °C. The residual weight percent was ∼ 1.3%. Hybrid TiO2/MWCNTs, curve (2), shows a similar trend as that of pure MWCNTs, but the residual weight percent of 67.43% due to the existence of sol–gel TiO2 nanoparticles is apparently higher than that of pure ones. In addition, comparing curves (1) and (2), the oxidation of MWCNTs are delayed slightly to higher temperature due to TiO2 particles. With polyol process to deposit the Pt nanoparticles, the TGA curves of hybrid Pt/MWCNTs and Pt/ TiO2/MWCNTs nanomaterials, curves (3) and (4), exhibit weight lose at a temperature of ∼350 °C, lower than that of the previous two samples. This can be attributed to the catalytic activity of Pt where the oxidation temperature of MWCNTs is lowered. After calculating the residual weight percentages of the MWCNTs, TiO2/MWCNTs, and Pt/ TiO2/MWCNTs, and assuming limited reactions among Pt, TiO2, and MWCNTs, relative amounts of TiO2 and Pt components within the hybrid Pt/TiO2/MWCNTs are 66.13 and 8.32%, respectively.
3. Results and discussion In the present study, hybrid Pt/TiO2/MWCNTs nanomaterials are prepared by a combined sol–gel and polyol process. MWCNTs are first purified by HNO3 for 3 h, followed by sol–gel TiO2, and then Pt nanoparticles are deposited by polyol process. Fig. 1 shows a series of X-ray diffraction patterns at various stages of preparation. It can be noted that anatase-phase TiO2 exhibited in TiO2/MWCNTs (curve (2) in Fig. 1) and Pt/TiO2/MWCNTs (curve (4)), even heat treating at 1000 °C after sol–gel process. Relative amounts of Pt nanoparticles with and without TiO2, however, are quite different. Strong Pt crystalline peaks can be noticed in Pt/MWCNTs (curve (3) in Fig. 1). With the sol–gel TiO2, Pt peak intensities in hybrid Pt/TiO2/MWCNTs nanomaterials are decreased significantly. This indicates that the relative amount of Pt in
Fig. 2. Field-emission transmission electron microscopy image of hybrid Pt/MWCNTs nanomaterials.
314
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
Fig. 3. Field-emission transmission electron microscopy images of hybrid Pt/TiO2/MWCNTs nanomaterials at various magnifications.
So far we have demonstrated that MWCNTs serve as good supports for the TiO2 and Pt nanoparticles. Various hybrid nanomaterials based on multi-wall carbon nanotubes can be synthesized. In TiO2/MWCNTs, TiO2 may increase the initial oxidation temperature of MWCNTs;
while, in contrast, Pt nanoparticles in either Pt/MWCNTs or Pt/TiO2/ MWCNTs will catalyze the oxidation of MWCNTs and lower the oxidation temperature. The catalytic properties of hybrid Pt/TiO2/ MWCNTs are further evaluated by converting CO into CO2. Fig. 5 shows the CO conversion performance of various materials investigated in
Fig. 4. TGA curves of purified multi-wall carbon nanotubes, hybrid TiO2/MWCNTs, Pt/ MWCNTs, and Pt/TiO2/MWCNTs nanomaterials.
Fig. 5. CO conversion performance of hybrid Pt/TiO 2 and Pt/TiO 2 /MWCNTs nanomaterials.
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
the present study. No CO conversion can be noticed for MWCNTs (also TiO2/MWCNTs, though not shown). It is worthy to note that ∼100% conversion of CO can be noticed at temperatures ranged from 30 °C to 100 °C. In order to verify the contribution of MWCNTs, hybrid Pt/TiO2 samples are prepared and its CO conversion curve is shown in Fig. 5 for comparison. It can be noticed that CO conversion does not occur when it is tested at 50 °C. Limited CO conversion of 7.7% and 12.8% can be noticed when the reaction temperature is set at 100 °C and 125 °C, respectively. At a temperature of 150 °C or higher, ∼ 100% CO conversion can be revealed. This indicates that MWCNTs serve as a good support material where sol–gel TiO2 can be uniformly adhered and further Pt nanoparticles can be dispersed uniformly by polyol process. 4. Conclusions Using purified multi-wall carbon nanotubes as supporting materials, various hybrid TiO2/MWCNTs, Pt/MWCNTs, and Pt/TiO2/MWCNTs nanomaterials are prepared successfully by a combined sol–gel and polyol process. TiO2 nanoparticles with a size of ∼ 40 nm are adhered individually onto purified MWCNTs by sol–gel process followed by post annealing at 1000 °C under an Ar-protected environment. These TiO2 nanoparticles are deposited with well-distributed Pt nanoparticles (2–3 nm) by polyol process. Relative weight percentages of TiO2 and Pt components are 66.13% and 8.32%, respectively. The hybrid Pt/
315
TiO2/MWCNTs nanomaterials exhibit excellent catalytic performance where not only ∼100% CO conversion can be observed, but the reacting temperature can be decreased to 30 °C. Acknowledgement The authors deeply acknowledge the grant of the National Science Council of Taiwan to support this research. References [1] Hari Singh Nalwa, Handbook of Organic–Inorganic Hybrid Materials and Nanocomposites, Vol. 1 Hybrid Materials, American Scientific Publishers, 2003. [2] Ulrike Diebold, Surface Science Reports 48 (2003) 53. [3] Keiichi Tanaka, F.V. Mario, Capule, Teruaki Hisanaga, Chemical Physics Letters 187 (1991) 73. [4] Z. Ding, G.Q. Lu, P.F. Greenfield, Journal of Physical Chemistry B 104 (2000) 4815. [5] C. He, Y. Yu, X. Hu, A. Larbot, Applications of Surface Science. 200 (2002) 239. [6] C.C. Chang, C.K. Lin, C.C. Chan, C.S. Hsu, C.Y Chen, Thin Solid Films 494 (2006) 274. [7] B.O Regan, M. Grätzel, Nature 353 (1991) 737. [8] P.K. Dutta, A. Ginwalla, B.R. Patton, B. Chwieroth, Z. Liang, P. Gouma, M. Mills, S. Akbar, Journal of Physical Chemistry B 103 (1999) 4412. [9] P.I. Gouma, P.K. Dutta, M.J. Mills, NanoStructured Materials 11 (8) (1999) 1231. [10] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Journal of Photochemistry and Photobiology A: Chemistry 89 (1995) 177. [11] Magdalena Bonarowska, Kuan-Nan Lin, Marta Legawiec-Jarzyna, Leszek Stobinski, Wojciech Juszczyk, Zbigniew Kaszkur, Zbigniew Karpiñski, Hong-Ming Lin, Solid State Phenomena 128 (2007) 261.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Synthesis of hybrid Pt/TiO2 (anatase)/MWCNTs nanomaterials by a combined sol–gel and polyol process K.N. Lin a, W.J. Liou a, T.Y. Yang b, H.M. Lin a,⁎, C.K. Lin c, S.H. Chien d, W.C. Chen e, S.H. Wu a a
Department of Materials Engineering, Tatung University, Taipei 104, Taiwan Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan d Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan e Department of Chemical and Material Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan b c
a r t i c l e
i n f o
Available online 5 August 2008 Keywords: MWCNTs Anatase Pt CO conversion
a b s t r a c t In the present study, hybrid Pt/TiO2/MWCNTs nanomaterials are prepared successfully by a combined sol–gel and polyol process. The as-prepared nanomaterials are characterized by X-ray diffraction, high resolution transmission electron microscopy, and thermogravimetry analysis. In addition, its catalytic performance by converting CO into CO2 is also evaluated. Experimental results show that the hybrid Pt/TiO2/MWCNTs nanomaterials exhibit a mixture of anatase TiO2 and Pt phases. Multi-wall carbon nanotubes serve as an excellent supporting material where anatase TiO2 nanoparticles are decorated with well-distributed Pt nanoparticles. Excellent catalytic performance can be revealed for the hybrid Pt/TiO2/MWCNTs nanomaterials. When compared with its Pt/TiO2 counterparts where ∼ 100% CO conversion occurred at 150 °C, almost 100% conversion of CO into CO2 can be observed at a temperature ranged from 30 °C to 100 °C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic–inorganic hybrid nanocomposite materials attracted considerable research interests in fundamental and application studies because of their potentially commercial applications [1]. Noble metals on ceramic nanoparticles are most notable in catalysis studies. Among them, the Pt/TiO2 nanocomposite has been studied on the hydrogenation and photocatalytic applications for several decades [2]. Pt nanoparticles on the matrix TiO2 are considered to play an important assistant catalyst role in the photocatalytic properties. Usually, the particle sizes of Pt and TiO2 are critical for the photocatalysis. Tanaka [3] showed that the more efficient photocatalytic action occurs in the larger TiO2 nanoparticles; because an electron and a positive hole can migrate a longer distance in the larger crystal than the smaller one, and accordingly separating more the reducing and oxidizing sites on the surface of the crystal. Lu [4] also showed that the anatase-phase titania will demonstrate the better photocatalytic activity for phenol oxidation than rutile phase because of the more surface-adsorbed water and hydroxyl groups. Generally speaking, anatase-phase TiO2 has attracted extensive attentions due to its excellent photocatalytic [5,6], photovoltaic [7], and gas sensing properties [8,9], etc. On the other side, the smaller size of platinum will provide the larger active areas and an electron trapper for catalysis, but the overload of platinum on the surface of titania will ⁎ Corresponding author. E-mail address: [email protected] (H.M. Lin). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.07.017
cover the photosensitive sites of titania [10]. In addition, carbon nanotubes (CNTs) base on the intriguingly physical and chemical properties are finding broad use as an ideal support for catalysts that improve obviously the stability of catalytic activity [11]. In the previous study, we have studied the hybrid TiO2/CNTs nanomaterial system prepared by sol–gel technique. The hybrid TiO2/CNTs nanomaterials exhibited anatase-phase TiO2 even at 1000 °C in Ar atmosphere, the phase transformation of anatase to rutile is inhibited by the pinning effect of the CNTs. The aim of the work is to prepare and characterize the size- and shape-controlled nanocomposite with homogeneous dispersion of Pt/TiO2 nanoparticles on multi-wall carbon nanotubes (MWCNTs) matrix. 2. Experimental procedures Multi-wall carbon nanotubes are commercially available (CTube 100, CNT Co., Ltd., Korea). MWCNTs are purified by concentrated HNO3 (Wako Pure Chemical Industries, Ltd. Japan) to remove the impurities and catalysts before coating TiO2 and Pt nanoparticles. For purification, 5 g of MWCNTs are boiled in 250 ml concentration HNO3 at 150 °C for 3 h, filtered and washed with distilled water to acid-free, and finally dried at 80 °C for 12 h to obtain the purified MWCNTs. Hybrid TiO2/MWCNTs nanomaterials are synthesized via a sol–gel process. Titanium tetra-isopropoxide and isopropyl alcohol (IPA), (CH3)2CHOH, are obtained from Wako Pure Chemical Industries, Ltd. Japan, with Wako 1st grade and JIS special grade, respectively. The solution of 0.1 g titanium tetra-isopropoxide dissolved in 50 ml IPA
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
Fig. 1. XRD patterns of purified multi-wall carbon nanotubes, hybrid TiO2/MWCNTs, Pt/ MWCNTs, and Pt/TiO2/MWCNTs nanomaterials.
solvent is used as the precursors for sol–gel process. The TiO2 precursor sol is gradually peptized into the MWCNTs-suspended solution (0.03 g purified MWCNTs in 50 ml IPA) that is placed over a hot plate stirring with magnet. Hybrid TiO2/MWCNTs are obtained after drying for 24 h. Subsequently post heat treatments are performed at 1000 °C under Ar-controlled atmosphere for 1.5 hour. The polyol process is prepared by heating of ethylene glycol solution with the Pt precursor salt. TiO2/MWCNTs (0.03 g) are first dispersed into 15 ml ethylene glycol (Wako, 99.0%). Then, 1.8 ml aqueous solution of 0.05 M H2PtCl6·6H2O (Wako, 98.5%) and 1.2 ml of 0.4 M KOH (Shimakyu Pure Chemicals, 85.0%) are added continually and heated at 170 °C for 0.5 h. The final solution is filtered and calcined under argon atmosphere at 250 °C to obtain hybrid Pt/TiO2/MWCNTs nanomaterials. The hybrid Pt/TiO2/MWCNTs nanomaterials are characterized by X-ray diffraction (XRD), field-emission transmission electron microscopy (FE-TEM), and thermogravimetry analysis (TGA). The phase identification is performed using a MAC MXP 18 X-ray diffractometer with a monochromatic Cu Kα radiation. FE-TEM (JEOL, JEM-2100Fs) is operated at 200 kV to observe the bright field image of the hybrid nanomaterials. The thermogravimetry analysis is performed by SDT2960 Simultaneous DSC-TGA (TA Instruments) where the specimen is heated from room temperature to 1000 °C at a heating rate of 5 °C/min. The catalytic performance of the hybrid Pt/TiO2/MWCNTs nanomaterials and its counterparts is carried out in a continuous flow micro-reactor. The reaction gases, 10% O2/He and 4% CO/He, are fed chamber with 0.025 g catalyst (Pt) at a rate of 20 ml/min. The effluent gas is analyzed on-line using a Varian 3700 gas chromatograph with a carbosphere column.
313
Pt/TiO2/MWCNTs is smaller than that in Pt/MWCNTs. Since the same polyol process is used to prepare these two hybrid materials, it is suggested that the reduction of Pt on MWCNTs will be preferred than that on TiO2/MWCNTs. Fig. 2 shows a field-emission transition electron microscope image of hybrid Pt/MWCNTs. It can be observed that Pt nanoparticles are well dispersed on multi-wall carbon nanotubes. Only limited agglomeration of Pt nanoparticles can be revealed even after sintering at 250 °C. Fig. 3 shows FE-TEM images of Pt/TiO2/MWCNTs at various magnifications. In a previous study, we have demonstrated that a distinct hybrid TiO2/MWCNTs nanomaterials, where TiO2 nanoparticles are adhered individually to the uncovered MWCNTs, can be synthesized by sol–gel process followed by post annealing at 1000 °C under an Ar-protected environment. Fig. 3(a), Pt/TiO2/MWCNTs, shows a similar image as that of TiO2/MWCNTs. One of the adhered nanoparticles, indicated by the arrow in Fig. 3(a), is examined using a higher magnification. As shown in Fig. 3(b), it can be noted that Pt nanoparticles with a size of 2–3 nm are well dispersed on TiO2 nanoparticles (∼ 40 nm). Fig. 3(c) and (d) shows the well dispersion and arrangement of Pt on TiO2 surface. In order to measure the relative amount of platinum and titania deposited on the purified multi-wall carbon nanotubes, thermogravimetry analysis is performed under air. Fig. 4 shows the TGA curves of the purified multi-wall carbon nanotubes (curve (1)), hybrid TiO2/ MWCNT (curve (2)), Pt/MWCNTs (curve (3)), and Pt/TiO2/MWCNTs (curve (4)) from room temperature to 1000 °C at a heating rate of 5 °C/ min. It can be noticed from curve (1) that MWCNTs almost burn out completely at a temperature higher than ∼ 560 °C. The residual weight percent was ∼ 1.3%. Hybrid TiO2/MWCNTs, curve (2), shows a similar trend as that of pure MWCNTs, but the residual weight percent of 67.43% due to the existence of sol–gel TiO2 nanoparticles is apparently higher than that of pure ones. In addition, comparing curves (1) and (2), the oxidation of MWCNTs are delayed slightly to higher temperature due to TiO2 particles. With polyol process to deposit the Pt nanoparticles, the TGA curves of hybrid Pt/MWCNTs and Pt/ TiO2/MWCNTs nanomaterials, curves (3) and (4), exhibit weight lose at a temperature of ∼350 °C, lower than that of the previous two samples. This can be attributed to the catalytic activity of Pt where the oxidation temperature of MWCNTs is lowered. After calculating the residual weight percentages of the MWCNTs, TiO2/MWCNTs, and Pt/ TiO2/MWCNTs, and assuming limited reactions among Pt, TiO2, and MWCNTs, relative amounts of TiO2 and Pt components within the hybrid Pt/TiO2/MWCNTs are 66.13 and 8.32%, respectively.
3. Results and discussion In the present study, hybrid Pt/TiO2/MWCNTs nanomaterials are prepared by a combined sol–gel and polyol process. MWCNTs are first purified by HNO3 for 3 h, followed by sol–gel TiO2, and then Pt nanoparticles are deposited by polyol process. Fig. 1 shows a series of X-ray diffraction patterns at various stages of preparation. It can be noted that anatase-phase TiO2 exhibited in TiO2/MWCNTs (curve (2) in Fig. 1) and Pt/TiO2/MWCNTs (curve (4)), even heat treating at 1000 °C after sol–gel process. Relative amounts of Pt nanoparticles with and without TiO2, however, are quite different. Strong Pt crystalline peaks can be noticed in Pt/MWCNTs (curve (3) in Fig. 1). With the sol–gel TiO2, Pt peak intensities in hybrid Pt/TiO2/MWCNTs nanomaterials are decreased significantly. This indicates that the relative amount of Pt in
Fig. 2. Field-emission transmission electron microscopy image of hybrid Pt/MWCNTs nanomaterials.
314
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
Fig. 3. Field-emission transmission electron microscopy images of hybrid Pt/TiO2/MWCNTs nanomaterials at various magnifications.
So far we have demonstrated that MWCNTs serve as good supports for the TiO2 and Pt nanoparticles. Various hybrid nanomaterials based on multi-wall carbon nanotubes can be synthesized. In TiO2/MWCNTs, TiO2 may increase the initial oxidation temperature of MWCNTs;
while, in contrast, Pt nanoparticles in either Pt/MWCNTs or Pt/TiO2/ MWCNTs will catalyze the oxidation of MWCNTs and lower the oxidation temperature. The catalytic properties of hybrid Pt/TiO2/ MWCNTs are further evaluated by converting CO into CO2. Fig. 5 shows the CO conversion performance of various materials investigated in
Fig. 4. TGA curves of purified multi-wall carbon nanotubes, hybrid TiO2/MWCNTs, Pt/ MWCNTs, and Pt/TiO2/MWCNTs nanomaterials.
Fig. 5. CO conversion performance of hybrid Pt/TiO 2 and Pt/TiO 2 /MWCNTs nanomaterials.
K.N. Lin et al. / Diamond & Related Materials 18 (2009) 312–315
the present study. No CO conversion can be noticed for MWCNTs (also TiO2/MWCNTs, though not shown). It is worthy to note that ∼100% conversion of CO can be noticed at temperatures ranged from 30 °C to 100 °C. In order to verify the contribution of MWCNTs, hybrid Pt/TiO2 samples are prepared and its CO conversion curve is shown in Fig. 5 for comparison. It can be noticed that CO conversion does not occur when it is tested at 50 °C. Limited CO conversion of 7.7% and 12.8% can be noticed when the reaction temperature is set at 100 °C and 125 °C, respectively. At a temperature of 150 °C or higher, ∼ 100% CO conversion can be revealed. This indicates that MWCNTs serve as a good support material where sol–gel TiO2 can be uniformly adhered and further Pt nanoparticles can be dispersed uniformly by polyol process. 4. Conclusions Using purified multi-wall carbon nanotubes as supporting materials, various hybrid TiO2/MWCNTs, Pt/MWCNTs, and Pt/TiO2/MWCNTs nanomaterials are prepared successfully by a combined sol–gel and polyol process. TiO2 nanoparticles with a size of ∼ 40 nm are adhered individually onto purified MWCNTs by sol–gel process followed by post annealing at 1000 °C under an Ar-protected environment. These TiO2 nanoparticles are deposited with well-distributed Pt nanoparticles (2–3 nm) by polyol process. Relative weight percentages of TiO2 and Pt components are 66.13% and 8.32%, respectively. The hybrid Pt/
315
TiO2/MWCNTs nanomaterials exhibit excellent catalytic performance where not only ∼100% CO conversion can be observed, but the reacting temperature can be decreased to 30 °C. Acknowledgement The authors deeply acknowledge the grant of the National Science Council of Taiwan to support this research. References [1] Hari Singh Nalwa, Handbook of Organic–Inorganic Hybrid Materials and Nanocomposites, Vol. 1 Hybrid Materials, American Scientific Publishers, 2003. [2] Ulrike Diebold, Surface Science Reports 48 (2003) 53. [3] Keiichi Tanaka, F.V. Mario, Capule, Teruaki Hisanaga, Chemical Physics Letters 187 (1991) 73. [4] Z. Ding, G.Q. Lu, P.F. Greenfield, Journal of Physical Chemistry B 104 (2000) 4815. [5] C. He, Y. Yu, X. Hu, A. Larbot, Applications of Surface Science. 200 (2002) 239. [6] C.C. Chang, C.K. Lin, C.C. Chan, C.S. Hsu, C.Y Chen, Thin Solid Films 494 (2006) 274. [7] B.O Regan, M. Grätzel, Nature 353 (1991) 737. [8] P.K. Dutta, A. Ginwalla, B.R. Patton, B. Chwieroth, Z. Liang, P. Gouma, M. Mills, S. Akbar, Journal of Physical Chemistry B 103 (1999) 4412. [9] P.I. Gouma, P.K. Dutta, M.J. Mills, NanoStructured Materials 11 (8) (1999) 1231. [10] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Journal of Photochemistry and Photobiology A: Chemistry 89 (1995) 177. [11] Magdalena Bonarowska, Kuan-Nan Lin, Marta Legawiec-Jarzyna, Leszek Stobinski, Wojciech Juszczyk, Zbigniew Kaszkur, Zbigniew Karpiñski, Hong-Ming Lin, Solid State Phenomena 128 (2007) 261.