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Solvothermal synthesis of nanocrystalline TiO2 in toluene with surfactant
ARTICLE IN PRESS
Journal of Crystal Growth 257 (2003) 309–315
Solvothermal synthesis of nanocrystalline TiO2 in toluene with surfactant Chung-Sik Kima, Byung Kee Moonb,*, Jong-Ho Parka, Byung-Chun Choib, Hyo-Jin Seob a
Basic Science Research Institute, Pukyong National University, Busan 608-737, South Korea b Department of Physics, Pukyong National University, Busan 608-737, South Korea Received 31 May 2003; accepted 17 June 2003 Communicated by M. Schieber
Abstract Synthesis of narrow-dispersed nanocrystalline TiO2 was investigated by surfactant-aided solvothermal synthetic method in toluene solutions. Titanium isopropoxide (TIP) was used as precursor, which was decomposed at high temperature in the surfactant-dissolved solution. After the solution was thermally treated at 250 C for 20 h in an autoclave, low-dispersed TiO2 nanocrystalline particles with average size of o6 nm were synthesized. When sufficient amount of TIP or surfactant was added in the solution, long dumbbell-shaped nanorods were formed, which may be due to the oriented growth of particles along [0 0 1] axis. Characterization of products was investigated by X-ray diffraction and transmission electron microscopy. r 2003 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.05.Ys; 81.10.Dn Keywords: A1. Crystallites; A2. Solvothermal crystal growth; B1. Nanomaterials; B1. Titanium compounds; B2. Semi-conducting materials
1. Introduction Nowadays as the concerns with global environmental issue increase, the application of TiO2 to the treatment of polluted air and wastewater has become more and more widespread because of its promising photocatalytic performance [1–6]. The photocatalytic activity of TiO2 is greatly influ*Corresponding author. Tel.:+82-51-620-6347; fax:+82-51611-6357. E-mail address: [email protected] (B.K. Moon).
enced by its crystal structure, particle size, surface area and porosity. With the decrease in particle size of powder to the nanometer scale, the catalytic activity is enhanced because the optical band gap is widened due to the quantum size effect, combined with the increased surface area [2,7–9]. The precise control of the morphology of semiconducting oxide materials at the nanometric scale is of basic importance for a fine-tuning of their physical properties such as electrical conductivity or magnetic, optical and mechanical characteristics. Hence, it is of great importance to improve
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0248(03)01468-4
ARTICLE IN PRESS C.-S. Kim et al. / Journal of Crystal Growth 257 (2003) 309–315
the preparation method of nanocrystalline TiO2 with narrow size distribution. Nanoparticle TiO2 has been prepared by the various methods such as the sol–gel method [10], the hydrothermal technique [11], and the reversed micelle method [12]. Although the sol–gel method is widely used to prepare nanometer TiO2, calcination process will inevitably cause the grain growth and reduction in specific surface area of particles and even induce phase transformation. Solvothermal synthesis, in which chemical reactions can occur in aqueous or organic media under the self-produced pressure at low temperature, (usually lower than 250 C) can solve those problems encountered during sol–gel process [13–16]. There have been many studies to stabilize and regulate the size of the nanoparticles by adding surfactant to solution to form reverse micelles [17–21]. However, in the case of the TiO2 nanoparticles, particle size uniformity and crystallinity are comparatively poor within authors’ knowledge [20,21]. In this study, a solvothermal synthetic method to TiO2 nanoparticle has been investigated in anhydrous toluene solutions with the aid of surfactant with expectation of obtaining the fine and low-dispersed nanoparticles. Oleic acid and titanium isopropoxide (TIP) were used as surfactant and precursor, respectively. Different compositions of precursor and surfactant in the solution were experimented to provide their effect on the formation of crystalline phase and diversity of the nanoparticles. The products were examined by Xray diffraction, transmission electron microscopy for characterizing their structure and particle size distribution.
2. Experimental procedure TIP (Ti(OCH(CH3)2)4, 97%, Aldrich) was dissolved in anhydrous toluene(99.8%, Aldrich) with oleic acid (90%, Aldrich). All reagents were used without any further purification process and mixed in a glove box with argon atmosphere. Different amounts of TIP and oleic acid were dissolved in solvent (i) at the weight ratio of precursor:solvent
as 5:100, 10:100 and 20:100 keeping molar ratio of precursor to surfactant as 1:3 (referred to as 5:100 (1:3), 10:100 (1:3) and 20:100 (1:3), respectively) and (ii) at the molar ratio of precursor:surfactant as 1:1, 1:2, 1:3, 1:4 and 1:5 keeping weight ratio of precursor to solvent as 10:100 (refered to as 10:100 (1:1), 10:100 (1:2), 10:100 (1:4) and 10:100 (1:5), respectively). The mixture was vigorously stirred with a magnetic stirrer for 24 h and transferred into a stainless-steel autoclave with a teflon liner (130 ml capacity, 80% filling). And then it was heated to 250 C with a rate of 4 C/min and maintained for 20 h without stirring. After cooling gradually to room temperature, acetone was added to yield the precipitate, which was separated with centrifugal separator and then dried in vacuum. The collected products were characterized by Xray diffraction (X’Pert-MPD System, PHILIPS) and transmission electron microscopy (JEM-2010 JEOL, H-7500 HITACHI). Average particle size of nanocrystalline TiO2 was calculated from the width of the XRD peaks of (1 0 1) plane using the Scherrer equation [7,21]. Particle size was also measured by image analysis of TEM images. Fit for particle size distribution is achieved by the lognormal function. The lognormal function is very often used to describe the particle size distribution in various aspects of science [22–25].
(101) (004) (103) (112) (105) (211) (200)
Intensity (a.u.)
310
20:100(1:3)
10:100(1:3)
5:100(1:3)
20
30
40
50
60
70
2θ (deg) Fig. 1. X-ray diffraction patterns of nanoparticles produced at different concentrations of titanium precursor in solution keeping molar ratio of TIP:oleic acid as 1:3.
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(b)
(a)
(c) Fig. 2. TEM micrographs and electron diffraction patterns of the products prepared from the solution of (a) 5:100 (1:3), (b) 10:100 (1:3), and (c) 20:100 (1:3).
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70 60 50 40 30 20 10 0 0
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Fig. 3. (a) Distribution of particle size of 5:100 (1:3), which is measured from TEM images, (b) Distribution of diameter, and (c) length of TiO2 nanorods synthesized in solutions 20:100 (1:3) measured in the same way. Xc and w represent median value and width of the distribution fitted to lognormal function, respectively.
3. Results and discussion XRD patterns of the nanoparticles for the weight ratios of precursor to surfactant 1:3 are shown in Fig. 1. It can be obviously seen that the phase structure of the particles is crystalline and exclusively of anatase type (JCPDS 21-1272). Peak intensity at 38 of 20:100 (1:3) sample is higher than that of others. This is possibly due to the rod-shaped nature of particles, which will be discussed later. TEM micrographs and SAD patterns of the products 5:100 (1:3), 10:100 (1:3) and 20:100 (1:3) are shown in Figs. 2(a–c), respectively. Polyhedral shapes of particle morphology can be found (see the magnified
image in the inset) and electron diffraction rings are clearly defined. These facts indicate good crystallinity of the particles. Particles of 5:100 (1:3) show the fine granules, whose size is estimated by 4 nm by XRD and 6 nm by TEM measurement. In the 10:100 (1:3) sample, rodshape particles appear among the granular particles as can be seen in Fig. 2(b). The concentration of rods in the nanoparticle assembly increases as concentration of titanium precursor in the solution increases. Granular particles almost disappear in the 20:100 (1:3) sample (Fig. 2c). No isolated nanorod can be found and most of the nanorods have the long dumbbell shape as shown in the inset.
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Table 1 Sizes of nanoparticles synthesized in various compositions Sample identification 5:100 (1:3) 10:100 (1:3) 20:100 (1:3) 10:100 (1:1) 10:100 (1:2) 10:100 (1:4) 10:100 (1:5)
Weight ratio of precursor:solvent 5:100 10:100 20:100 10:100 10:100 10:100 10:100
Molar ratio of precursor:surfactant 1:3 1:3 1:3 1:1 1:2 1:4 1:5
Particle size (nm) XRD
TEM
4.070.2 3.870.1 3.470.1
5.970.4 3.970.3 2.870.1D (19.978.0)L
4.270.2 3.870.1 4.770.1
4.970.2 3.870.2 3.070.2D (25.0718.4)L
They are obtained from width of (1 0 1) peak in XRD pattern and image analysis in TEM images. (D: diameter, L: length).
(101)
(004) (103) (112)
(200)
Intensity (a.u.)
For the products of 10:100 (1:3) and 20:100 (1:3), brightness of ring pattern corresponding to (0 0 4) plane is higher than that of 5:100 (1:3). If [0 0 1] axis orients perpendicular to longitudinal direction of nanorods, intensity of electron diffraction on (0 0 4) plane will be low because of long pathlength along the nanorod for electron beam. Therefore, combined with results of other reports [26,27], where axis direction of TiO2 nanorods is [0 0 1], high intensity of the peak in (0 0 4) plane strongly implies that nanorods orient preferentially along [0 0 1] axis. The same reason could be used to explain high intensity at 38 for 20:100 (1:3) sample in XRD data (Fig. 1). Fig. 3(a) shows distribution of particle size obtained from TEM image of 5:100 (1:3) sample and Figs. 3(b) and (c) show distribution of diameter and length of TiO2 nanorods synthesized at 20:100 (1:3), respectively. Nanorods show narrow distribution in diameter and relatively broad distribution in length. The average particle size obtained by XRD and the median value of the lognormal function fitted for particle size distribution in TEM images are shown in Table 1. Average particle size is smaller and size distribution is narrower than particles synthesized without surfactant, which is synthesized in much shorter (3 h) reaction time [28]. X-ray diffraction patterns of nanoparticles produced at different molar ratios of 10:100 (1:1), 10:100 (1:2), 10:100 (1:3), 10:100 (1:4) and 10:100 (1:5) are shown in Fig. 4. Product of 1:1 has no diffraction peak and could be considered as
(105) (211)
10:100(1:5)
10:100(1:4)
10:100(1:3)
10:100(1:2) 10:100(1:1)
20
30
40
50
60
70
2θ (deg)
Fig. 4. X-ray diffraction patterns of nanoparticles produced at different molar concentration of titanium precursor in the solution keeping weight ratio of TIP:solvent as 10:100.
amorphous and others show crystalline anatase phase. Figs. 5(a–c) are TEM micrographs of the products prepared from the solution at 10:100 (1:2), 10:100 (1:4), 10:100 (1:5), respectively. Products from 10:100 (1:2) solution reveal that particles are scattered over cloudy background material, which indicates that reaction of reactants is not completed. Nanorods appear among nanoparticles in Fig. 5(b) and fully cover the whole area in Fig. 5(c). It can be concluded that anatase nanorods can be obtained from the solution with precursor:surfactant weight ratio of more than 1:3 for precursor:solvent weight ratio of 10:100 or the solution with precursor:solvent weight ratio of
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(a)
(b)
(c) Fig. 5. TEM micrographs of the products (a) 10:100 (1:2), (b) 10:100 (1:4), and (c) 10:100 (1:5).
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more than 20:100 for precursor:surfactant weight ratio of 1:3 in this experiment. Because of nanorods feature of 10:100 (1:5) sample, its XRD peak has little higher intensity at 38 than samples of other composition. But its extent was much less than the case of 20:100 (1:3) in Fig. 1 than the case of electron diffraction pattern in Fig. 2. The size of nanoparticles and diameter and length of nanorods obtained from image analysis are shown in Table 1. Size and aspect ratio of nanorods of 10:100 (1:5) is comparable to those of 20:100 (1:3). Growth of nanorods indicates that anatase phase has the characteristics of the oriented growth along preferential direction. The reason for failure to obtain crystalline nanoparticles at 10:10 (1:1) solution and the mechanism of synthesizing dumbbell shape of nanorods is still under study.
4. Conclusions In summary, anatase TiO2 nanoparticles are synthesized by a solvothermal route in surfactantadded solution. The different amount of precursor and surfactant were used in anhydrous toluene solutions. Size distribution of particles synthesized is narrower than particles synthesized without surfactant. High intensity of ring pattern corresponding to (0 0 4) plane in the SAD image strongly implies that nanorods orient preferentially along [0 0 1] axis. Anatase nanorods can be obtained from the solution with precursor:surfactant molar ratio of more than 1:3 for precursor:solvent weight ratio of 10:100 and the solution with precursor:solvent weight ratio of more than 20:100 for precursor:surfactant molar ratio of 1:3.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-070-C00042).
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References [1] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [2] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1. [3] Y. Ohko, I. Ando, C. Niwa, T. Tatsuma, T. Yamamura, T. Nakashima, Y. Kubota, A. Fujishima, Environ. Sci. Technol. 35 (2001) 2365. [4] J. Rodriguez, T. Jirsak, G. Liu, J. Hrbek, J. Dvorak, A. Maiti, J. Am. Chem. Soc. 123 (2001) 9597. [5] R.J. Farrauto, R.M. Heck, Catal. Today 55 (2000) 179. ! [6] J. Aguado, R. van Grieken, M.J. Lopez-Mun ˜ oz, J. Marug!an, Catal. Today 75 (2002) 95. [7] K.M. Reddy, C.V.G. Reddy, S.V. Manorama, J. Solid State Chem. 158 (2001) 180. [8] Taro Toyoda, Hiroshi Kawano, Qing Shen, Akihiko Kotera, Masahiro Ohmori, Jpn. J. Appl. Phys. Part 1 39 (2000) 3160. [9] G. Schmid, M. Baeumle, M. Geerkens, I. Heim, C. Osemann, T. Sawitowski, Chem. Soc. Rev. 28 (1999) 179. [10] B. Li, X. Wang, M. Yan, L. Li, Mater. Chem. Phys. 78 (2002) 184. [11] Y.V. Kolen’ko, A.A. Burukhin, B.R. Churagulov, N.N. Oleynikov, Mater. Lett. 57 (2003) 1124. [12] S.D. Romano, D.H. Kurlat, Chem. Phys. Lett. 323 (2000) 93. [13] M. Kang, J. Mol. Catal. A: Chem. 3856 (2002) 1. [14] C. Wang, Z. Deng, G. Zhang, S. Fan, Y. Li, Powder Technol. 125 (2002) 39. [15] S.J. Chen, L.H. Li, X.T. Chen, Z. Xue, J.M. Hong, X.Z. You, J. Crystal Growth 252 (2003) 184. [16] J. Liu, X. Chen, M. Shao, C. An, W. Yu, Y. Qian, J. Crystal Growth 252 (2003) 297. [17] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Am. Chem. Soc. 123 (2001) 12798. [18] M.L. Curri, A. Agostiano, L. Manna, M.D. Monica, M. Catalano, L. Chiavarone, V. Spagnolo, M. Lugara, J. Phys. Chem. B 104 (2000) 8391. [19] X. Fu, S. Qutubuddin, Colloids Surf. A 179 (2001) 65. [20] R. Zhang, L. Gao, Mater. Res. Bull. 37 (2002) 1659. [21] B.D. Cullity, Elements of X-ray Diffraction, 2nd Edition, Addison-Wesley, Reading, MA, 1978. [22] G.L. Li, G.H. Wang, Nanostruct. Mater. 11 (1999) 663. [23] H. Herrig, R. Hempelmann, Mater. Lett. 27 (1996) 287. [24] L.B. Kiss, J. Soederlund, G.A. Niklasson, C.G. Granqvist, Nanostruct. Mater. 12 (1999) 327. [25] Z. Wang, J. Li, X. Huang, L. Wang, J. Xu, K. Chen, Solid State Commun. 117 (2001) 383. [26] A. Chemseddine, T. Moritz, Eur. J. Inorg. Chem. (1999) 235. [27] Z.-Y. Yuan, J.-F. Colomer, B.-L. Su, Chem. Phys. Lett. 363 (2002) 362. [28] C.-S. Kim, B.K. Moon, J.-H. Park, S.T. Jung, S.-M. Son, J. Crystal Growth 254 (2003) 405.
Journal of Crystal Growth 257 (2003) 309–315
Solvothermal synthesis of nanocrystalline TiO2 in toluene with surfactant Chung-Sik Kima, Byung Kee Moonb,*, Jong-Ho Parka, Byung-Chun Choib, Hyo-Jin Seob a
Basic Science Research Institute, Pukyong National University, Busan 608-737, South Korea b Department of Physics, Pukyong National University, Busan 608-737, South Korea Received 31 May 2003; accepted 17 June 2003 Communicated by M. Schieber
Abstract Synthesis of narrow-dispersed nanocrystalline TiO2 was investigated by surfactant-aided solvothermal synthetic method in toluene solutions. Titanium isopropoxide (TIP) was used as precursor, which was decomposed at high temperature in the surfactant-dissolved solution. After the solution was thermally treated at 250 C for 20 h in an autoclave, low-dispersed TiO2 nanocrystalline particles with average size of o6 nm were synthesized. When sufficient amount of TIP or surfactant was added in the solution, long dumbbell-shaped nanorods were formed, which may be due to the oriented growth of particles along [0 0 1] axis. Characterization of products was investigated by X-ray diffraction and transmission electron microscopy. r 2003 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.05.Ys; 81.10.Dn Keywords: A1. Crystallites; A2. Solvothermal crystal growth; B1. Nanomaterials; B1. Titanium compounds; B2. Semi-conducting materials
1. Introduction Nowadays as the concerns with global environmental issue increase, the application of TiO2 to the treatment of polluted air and wastewater has become more and more widespread because of its promising photocatalytic performance [1–6]. The photocatalytic activity of TiO2 is greatly influ*Corresponding author. Tel.:+82-51-620-6347; fax:+82-51611-6357. E-mail address: [email protected] (B.K. Moon).
enced by its crystal structure, particle size, surface area and porosity. With the decrease in particle size of powder to the nanometer scale, the catalytic activity is enhanced because the optical band gap is widened due to the quantum size effect, combined with the increased surface area [2,7–9]. The precise control of the morphology of semiconducting oxide materials at the nanometric scale is of basic importance for a fine-tuning of their physical properties such as electrical conductivity or magnetic, optical and mechanical characteristics. Hence, it is of great importance to improve
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0248(03)01468-4
ARTICLE IN PRESS C.-S. Kim et al. / Journal of Crystal Growth 257 (2003) 309–315
the preparation method of nanocrystalline TiO2 with narrow size distribution. Nanoparticle TiO2 has been prepared by the various methods such as the sol–gel method [10], the hydrothermal technique [11], and the reversed micelle method [12]. Although the sol–gel method is widely used to prepare nanometer TiO2, calcination process will inevitably cause the grain growth and reduction in specific surface area of particles and even induce phase transformation. Solvothermal synthesis, in which chemical reactions can occur in aqueous or organic media under the self-produced pressure at low temperature, (usually lower than 250 C) can solve those problems encountered during sol–gel process [13–16]. There have been many studies to stabilize and regulate the size of the nanoparticles by adding surfactant to solution to form reverse micelles [17–21]. However, in the case of the TiO2 nanoparticles, particle size uniformity and crystallinity are comparatively poor within authors’ knowledge [20,21]. In this study, a solvothermal synthetic method to TiO2 nanoparticle has been investigated in anhydrous toluene solutions with the aid of surfactant with expectation of obtaining the fine and low-dispersed nanoparticles. Oleic acid and titanium isopropoxide (TIP) were used as surfactant and precursor, respectively. Different compositions of precursor and surfactant in the solution were experimented to provide their effect on the formation of crystalline phase and diversity of the nanoparticles. The products were examined by Xray diffraction, transmission electron microscopy for characterizing their structure and particle size distribution.
2. Experimental procedure TIP (Ti(OCH(CH3)2)4, 97%, Aldrich) was dissolved in anhydrous toluene(99.8%, Aldrich) with oleic acid (90%, Aldrich). All reagents were used without any further purification process and mixed in a glove box with argon atmosphere. Different amounts of TIP and oleic acid were dissolved in solvent (i) at the weight ratio of precursor:solvent
as 5:100, 10:100 and 20:100 keeping molar ratio of precursor to surfactant as 1:3 (referred to as 5:100 (1:3), 10:100 (1:3) and 20:100 (1:3), respectively) and (ii) at the molar ratio of precursor:surfactant as 1:1, 1:2, 1:3, 1:4 and 1:5 keeping weight ratio of precursor to solvent as 10:100 (refered to as 10:100 (1:1), 10:100 (1:2), 10:100 (1:4) and 10:100 (1:5), respectively). The mixture was vigorously stirred with a magnetic stirrer for 24 h and transferred into a stainless-steel autoclave with a teflon liner (130 ml capacity, 80% filling). And then it was heated to 250 C with a rate of 4 C/min and maintained for 20 h without stirring. After cooling gradually to room temperature, acetone was added to yield the precipitate, which was separated with centrifugal separator and then dried in vacuum. The collected products were characterized by Xray diffraction (X’Pert-MPD System, PHILIPS) and transmission electron microscopy (JEM-2010 JEOL, H-7500 HITACHI). Average particle size of nanocrystalline TiO2 was calculated from the width of the XRD peaks of (1 0 1) plane using the Scherrer equation [7,21]. Particle size was also measured by image analysis of TEM images. Fit for particle size distribution is achieved by the lognormal function. The lognormal function is very often used to describe the particle size distribution in various aspects of science [22–25].
(101) (004) (103) (112) (105) (211) (200)
Intensity (a.u.)
310
20:100(1:3)
10:100(1:3)
5:100(1:3)
20
30
40
50
60
70
2θ (deg) Fig. 1. X-ray diffraction patterns of nanoparticles produced at different concentrations of titanium precursor in solution keeping molar ratio of TIP:oleic acid as 1:3.
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(b)
(a)
(c) Fig. 2. TEM micrographs and electron diffraction patterns of the products prepared from the solution of (a) 5:100 (1:3), (b) 10:100 (1:3), and (c) 20:100 (1:3).
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70 60 50 40 30 20 10 0 0
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Fig. 3. (a) Distribution of particle size of 5:100 (1:3), which is measured from TEM images, (b) Distribution of diameter, and (c) length of TiO2 nanorods synthesized in solutions 20:100 (1:3) measured in the same way. Xc and w represent median value and width of the distribution fitted to lognormal function, respectively.
3. Results and discussion XRD patterns of the nanoparticles for the weight ratios of precursor to surfactant 1:3 are shown in Fig. 1. It can be obviously seen that the phase structure of the particles is crystalline and exclusively of anatase type (JCPDS 21-1272). Peak intensity at 38 of 20:100 (1:3) sample is higher than that of others. This is possibly due to the rod-shaped nature of particles, which will be discussed later. TEM micrographs and SAD patterns of the products 5:100 (1:3), 10:100 (1:3) and 20:100 (1:3) are shown in Figs. 2(a–c), respectively. Polyhedral shapes of particle morphology can be found (see the magnified
image in the inset) and electron diffraction rings are clearly defined. These facts indicate good crystallinity of the particles. Particles of 5:100 (1:3) show the fine granules, whose size is estimated by 4 nm by XRD and 6 nm by TEM measurement. In the 10:100 (1:3) sample, rodshape particles appear among the granular particles as can be seen in Fig. 2(b). The concentration of rods in the nanoparticle assembly increases as concentration of titanium precursor in the solution increases. Granular particles almost disappear in the 20:100 (1:3) sample (Fig. 2c). No isolated nanorod can be found and most of the nanorods have the long dumbbell shape as shown in the inset.
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Table 1 Sizes of nanoparticles synthesized in various compositions Sample identification 5:100 (1:3) 10:100 (1:3) 20:100 (1:3) 10:100 (1:1) 10:100 (1:2) 10:100 (1:4) 10:100 (1:5)
Weight ratio of precursor:solvent 5:100 10:100 20:100 10:100 10:100 10:100 10:100
Molar ratio of precursor:surfactant 1:3 1:3 1:3 1:1 1:2 1:4 1:5
Particle size (nm) XRD
TEM
4.070.2 3.870.1 3.470.1
5.970.4 3.970.3 2.870.1D (19.978.0)L
4.270.2 3.870.1 4.770.1
4.970.2 3.870.2 3.070.2D (25.0718.4)L
They are obtained from width of (1 0 1) peak in XRD pattern and image analysis in TEM images. (D: diameter, L: length).
(101)
(004) (103) (112)
(200)
Intensity (a.u.)
For the products of 10:100 (1:3) and 20:100 (1:3), brightness of ring pattern corresponding to (0 0 4) plane is higher than that of 5:100 (1:3). If [0 0 1] axis orients perpendicular to longitudinal direction of nanorods, intensity of electron diffraction on (0 0 4) plane will be low because of long pathlength along the nanorod for electron beam. Therefore, combined with results of other reports [26,27], where axis direction of TiO2 nanorods is [0 0 1], high intensity of the peak in (0 0 4) plane strongly implies that nanorods orient preferentially along [0 0 1] axis. The same reason could be used to explain high intensity at 38 for 20:100 (1:3) sample in XRD data (Fig. 1). Fig. 3(a) shows distribution of particle size obtained from TEM image of 5:100 (1:3) sample and Figs. 3(b) and (c) show distribution of diameter and length of TiO2 nanorods synthesized at 20:100 (1:3), respectively. Nanorods show narrow distribution in diameter and relatively broad distribution in length. The average particle size obtained by XRD and the median value of the lognormal function fitted for particle size distribution in TEM images are shown in Table 1. Average particle size is smaller and size distribution is narrower than particles synthesized without surfactant, which is synthesized in much shorter (3 h) reaction time [28]. X-ray diffraction patterns of nanoparticles produced at different molar ratios of 10:100 (1:1), 10:100 (1:2), 10:100 (1:3), 10:100 (1:4) and 10:100 (1:5) are shown in Fig. 4. Product of 1:1 has no diffraction peak and could be considered as
(105) (211)
10:100(1:5)
10:100(1:4)
10:100(1:3)
10:100(1:2) 10:100(1:1)
20
30
40
50
60
70
2θ (deg)
Fig. 4. X-ray diffraction patterns of nanoparticles produced at different molar concentration of titanium precursor in the solution keeping weight ratio of TIP:solvent as 10:100.
amorphous and others show crystalline anatase phase. Figs. 5(a–c) are TEM micrographs of the products prepared from the solution at 10:100 (1:2), 10:100 (1:4), 10:100 (1:5), respectively. Products from 10:100 (1:2) solution reveal that particles are scattered over cloudy background material, which indicates that reaction of reactants is not completed. Nanorods appear among nanoparticles in Fig. 5(b) and fully cover the whole area in Fig. 5(c). It can be concluded that anatase nanorods can be obtained from the solution with precursor:surfactant weight ratio of more than 1:3 for precursor:solvent weight ratio of 10:100 or the solution with precursor:solvent weight ratio of
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(a)
(b)
(c) Fig. 5. TEM micrographs of the products (a) 10:100 (1:2), (b) 10:100 (1:4), and (c) 10:100 (1:5).
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more than 20:100 for precursor:surfactant weight ratio of 1:3 in this experiment. Because of nanorods feature of 10:100 (1:5) sample, its XRD peak has little higher intensity at 38 than samples of other composition. But its extent was much less than the case of 20:100 (1:3) in Fig. 1 than the case of electron diffraction pattern in Fig. 2. The size of nanoparticles and diameter and length of nanorods obtained from image analysis are shown in Table 1. Size and aspect ratio of nanorods of 10:100 (1:5) is comparable to those of 20:100 (1:3). Growth of nanorods indicates that anatase phase has the characteristics of the oriented growth along preferential direction. The reason for failure to obtain crystalline nanoparticles at 10:10 (1:1) solution and the mechanism of synthesizing dumbbell shape of nanorods is still under study.
4. Conclusions In summary, anatase TiO2 nanoparticles are synthesized by a solvothermal route in surfactantadded solution. The different amount of precursor and surfactant were used in anhydrous toluene solutions. Size distribution of particles synthesized is narrower than particles synthesized without surfactant. High intensity of ring pattern corresponding to (0 0 4) plane in the SAD image strongly implies that nanorods orient preferentially along [0 0 1] axis. Anatase nanorods can be obtained from the solution with precursor:surfactant molar ratio of more than 1:3 for precursor:solvent weight ratio of 10:100 and the solution with precursor:solvent weight ratio of more than 20:100 for precursor:surfactant molar ratio of 1:3.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-070-C00042).
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