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Cause of Slow Phase Transformation of TiO2 Nanorods Y. Chen, K. S. Kang,* K. H. Yoo, N. Jyoti, and Jaehwan Kim Center for EAPap Actuator, Department of Mechanical Engineering, Inha UniVersity, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, South Korea ReceiVed: September 07, 2009; ReVised Manuscript ReceiVed: October 14, 2009
The phase transformation from anatase to rutile in nanocrystalline TiO2 nanorods comprised of length and diameter of approximately 35 and 4 nm, respectively, has been investigated. TiO2 nanorods were synthesized at 100 °C with titanium isopropoxide as a presusor, oleic acid (OLEA) as a surfactent, and trimethylamine as a catalyst. Characteristic absorption peaks of OLEA were observed from the OLEA-caped TiO2 nanorods after synthesis and disappeared after annealing process. No phase transformation (from anatase to rutile) were observed after annealing at 750 °C for 2 h. Only small portion of phase transformation occurred after annealing at 850 °C for 2 and 3 h. A relatively slow phase transformation from anatase to rutile might be due to the small diameter of TiO2 nanorods resulting favored surface free energy of the anatase phase. Introduction
Experimental Section
One-dimensional inorganic nanostructures such as wires, rods, and tubes exhibit a wide range of electrical and optical properties, which depend on their size and shape. Nanocrystalline TiO2 is one of the most extensively investigated oxide and exhibits high-refractive index (2.5 and 2.7 for anatase and rutile, respectively)1,2 and chemical and physical stability. Therefore, TiO2 has been considered as one of the most important metal oxide due to its potential application to photovoltaics, Schottky diodes, photocatalysts, photoelectrochemical cells, and optical devices.3-6 One-dimensional TiO2 crystals can be synthesized with several different methods including sol-gel template, surfactantdirected, and hydrothermal techniques. Although there have been numerous wet-chemistry synthetic techniques, preparation methods for nanosized and controlled crystalline TiO2 need to be further developed. Recently, the growth technique of spherical or rod shaped anatase TiO2 nanostructures were demonstrated at low temperature (80 °C).6 Since the rutile structure is thermally more stable than the anatase structure, annealing at high temperature leads to the phase transformation from anatase phase to rutile phase. Though a large number of studies have reported anatase to rutile transformation beyond 700 °C, the actual temperature range of this transformation crucially depends on the initial microstructure.7 As reported, the size was a key factor in the phase conversion. The critical diameter for the phase transformation from anatase to rutile was approximately 15 nm as reported, otherwise even annealing at 1100 °C, anatase phase will still remain in the crystal structure.8,9 In this study, we report the sythetic process of TiO2 nanorods at low temperature and the phase transformation of TiO2 from anatase phase to rutile phase depending on the annealing temperature. Relatively extremely slow phase transformation was obsered compared with other studies used nanoparticles and thin films.
Oleic acid (OLEA, 35 g) was dried at 120 °C for 1 h under vigorous stirring in a 50 mL three-neck flask and cooled down to 100 °C under nitrogen flow. Titanium isopropoxide (TIP, 10 mmol) was added to the dried OLEA and allowed to stir for 5 min. The solution turned from colorless to pale yellow. Trimethylamine (TMA) aqueous base solution (5 mL) was loaded into a syringe and then rapidly injected to the TIP and OLEA solution. The reaction temperature was kept at 100 °C, and the solution became slightly turbid. The solution was maintained in a closed system at 100 °C for 6 h to promote further hydrolysis and crystallization of TiO2 nanorods. TiO2 nanocrystals were readily precipitated upon addition of excess ethanol to the reaction mixture at room temperature. The resulting precipitate was isolated by centrifugation and washed twice with ethanol to remove residual surfactant. TiO2 nanorods were dried in hot oven at 100 °C for 24 h and annealed at 750 and 850 °C for various durations in the tube furnace (EM Tech) at ramping rate of 5 °C/min. Transmission electron microscope (TEM, Philips CM-200) and high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F) images were obtained with operating voltage at 150 and 200 kV, respectively. X-ray diffraction (XRD) patterns of TiO2 nanocrystals were collected with a Rigaku DMAX-2500 -powder X-ray Diffractomer. TiO2 nanorods were attached to a KBr plate for Fourier transform infrared (FTIR) spectra. FTIR spectra were recorded using a Varian Excalibur (3000 series) FTIR spectrometer.
* Author to whom correspondence should be addressed. E-mail:
[email protected].
10.1021/jp9086442 CCC: $40.75
Results and Discussion The size and morphology of TiO2 nanorods were characterized with TEM images. Figure 1a presents the partially aggregrated nanorods on TEM grid. Figure 1b exhibits the magnified image of Figure 1a and shows the diameter of approximately 4 nm and the length of about 35 nm. Detailed insight of the structure and crystallinity of TiO2 nanorods 2009 American Chemical Society
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Letters
Figure 1. (a) Large scale TEM micrographs of TiO2 nanorods after centrifuge, (b) TEM micrograph of TiO2 nanorods with 20 nm scale. Inset in Figure 1b shows the HRTEM image of the lattice frings
Figure 3. XRD spectra of TiO2 nanorods anneald at (a) 750 °C and (b) 850 °C for various times.
Figure 2. FT-IR spectrum of OLEA, OLEA capped-TiO2 nanorods after synthesis, and OLEA capped-TiO2 nanorods after annealing.
illustrates inset Figure 1b and shows a well-crystallized structure with lattice frings of about 3.5 Å. FTIR spectra of OLEA, TiO2 nanorod after synthesis, and TiO2 nanorods after annealing in the region of 4000-400 cm-1 were shown in Figure 2. The characteristic absorption peaks of OLEA at 2927 and 2854 cm-1 assign antisymmetric and symmetric C-H stretching vibrations, respectively. A weak and small absorption peak at 3008 cm-1 attributes the olefinic C-H stretching vibrations. Strong absorption peak at 1712 cm-1 can be assigned CdO stretching vibrations. The absorption peaks at 2924, 2852, and 3008 of OLEA-capped TiO2 nanocrystals correspond to the OLEA C-H stretching vibrations and olefinic C-H stretching, respectively. The absorption peaks at 1523 and 1433 cm-1 correspond to the COO anion antisymmetric and symmetric stretching vibrations, respectively, complexed with surface Ti centers. The lack of free CdO stretching band (1650-1720 cm-1) indicates the chelating bidendate complex between COO- and TiO2 surface. Broad absorption band centered at 3433 cm-1 can be assigned absorbed H2O or titanol stretching. The characteristic absorption peaks of Ti-O-Ti
network can be seen in below 950 cm-1. After annealing, there is no characteristic absorption peaks of OLEA, indicating complete removal of capping agent. Under the standard pressure, rutile phase is relatively more stable than the anatase phase. According to the calculated result, however, the relative phase stability reverses when the particle size is less than 8 nm when the surface stress is not considered and 14 nm when the surface stress equals the surface free energy. In between 350-500 °C, transformation from anatase to rutile is minor even after 100-450 h calcination with TiO2 nanoparticles having diameter of less than 14 nm. Approximately 9.6% transformation from anatase to rutile10 was observed with TiO2 nanoparticles having diameter of 17.6 nm after 1.2 h annealing at 525 °C. Complete phase transformation from anatase to rutile was observed for TiO2 films11 annealed at 700 °C for 4 h. The phase transformation ratios of 65.8, 55.1, and 43.5% from anatase to rutile were observed with TiO2 nanoparticles having diameters of 12, 17, and 23 nm, respectively, after annealing at 800 °C for 1 h.12 Figure 3a,b shows XRD patterns of TiO2 nanorods (length = 35 nm, diameter = 4 nm) annealed for various times at 750 and 850 °C, respectively. No rutile phase deffraction peak of TiO2 nanorods was observed even after 2 h annealing at 750 °C. Approximately 7 and 11% of rutile peak of TiO2 nanorods were observed after annealing at 850 °C for 2 and 3 h, respectively. This phase transformation rate is extremely slow compared with many other references. This slow phase transformation might be due to the small diameter of the nanorods resulting lower surface free energy for the anatase phase than that of rutile phase.
Letters Conclusions In summary, TiO2 nanorods comprised of length and diameter of approximately 35 and 4 nm, respectively, were synthesized at l00 °C. FTIR spectra show characteristic OLEA-capped TiO2 nanorods. As-synthesized TiO2 nanorods show characteristic anatase phase. Compared with other research results, thermal phase transformation of TiO2 nanorods was very slow. Only 7 and 11% of phase transformation from anatase to rutile was observed for TiO2 nanorods annealed at 850 °C for 2 and 3 h, respectively. These slow phase transformation might be due to the small diameter of TiO2 nanorods resulting favored surface free energy of the anatase phase. Acknowledgment. This work was supported by Inha University Research Grant.
J. Phys. Chem. C, Vol. 113, No. 46, 2009 19755 References and Notes (1) Zhang, Y.; Wang, J.; Li, J. J. Electroceram. 2006, 16, 499. (2) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121. (3) Yuji, T.; Akatsuka, H.; Mungkung, N.; Park, B. W.; Sung, Y. M. Vacuum 2008, 83, 124. (4) Jun, Y.; Casula, M. F.; Sim, J.; Kim, S. Y.; Cheon, J.; Alicisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (5) Kamat, P. V. J. Phys. Chem B 2002, 106, 7729. (6) Cozzoli, P. D.; Kornowski, Weller, A. H. J. Am. Chem. Soc. 2003, 125, 14539. (7) Chen, C. A.; Chen, Y. M.; Korotcov, A.; Huang, Y. S.; Tsai, D. S.; Tiong, K. K. Nanotechnology 2008, 19, 075611. (8) Yamamoto, S.; Yamaki, T.; Naramoto, H.; Tanaka, S. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 206, 268. (9) Phani, A. R.; Santucci, S. J. Phys.: Condens. Matter 2006, 18, 6965. (10) Zhang, H.; Banfield, J. F. J. Mater. Chem. 1998, 8, 2073. (11) Hung, W. C.; Fu, S. H.; Tseng, J. J.; Chu, H.; Ko, T. H. Chemosphere 2007, 66, 2142. (12) Li, W.; Ni, C.; Lin, H.; Huang, C. P.; Shah, S. I. J. Appl. Phys. 2004, 96, 6663.
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