Current Nanoscience, 2010, 6, 00-00
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Preparation and Characterization of Nanocrystalline Titanium Dioxide with a Surfactant-mediated Method Lay Gaik Teoh1, Ying-Chieh Lee2, Yee Shin Chang3*, Te-Hua Fang4 and Hong Ming Chen1 1
Department of Mechanical Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 912, Taiwan.
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Department of Materials Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 912, Taiwan.
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Department of Electronic Engineering, 4Institute of Mechanical and Electro-Mechanical Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Abstract: Using a surfactant-mediated method, nanocrystalline titanium dioxide with a high BET surface area was generated within the template of the nonionic surfactant micelle assembly from the anhydrous metal chloride, TiCl4 . The results indicate that the block copolymer can control the particle size and inhibit the agglomeration of anatase titanium dioxide in the calcining process, and simultaneously enhance phase transformation from amorphous titanium dioxide to anatase phase. The pore diameter and the BET surface area of the material, evaluated from the N2 adsorption-desorption isotherm, indicate average pore diameter of about 6.6 nm, and BET surface area about 115.04 m2/g for calcination at 400. The resulting particles were highly crystalline and largely monodisperse oxide particles in the nanometer range of 18-22 nm.
Key Words: Nanocrystalline, titanium dioxide, surfactant-mediated method. INTRODUCTION Nanocrystalline materials are gaining increasing importance in many fields by virtue of their unique and interesting properties [13]. Of all the transition metaloxides, nanocrystalline titanium dioxide is one of the most studied semiconductors for use in photocatalytic reactions and as a gas sensor [4-6]. Its low cost, ease of handling, resistance to photoinduced corrosion, and harmlessness to humans and the environment are the main properties that make this material so interesting [7]. In addition to its conventional applications in white pigments, new applications are being developed, such as dye-sensitized nanocrystalline titanium dioxide as an energy converter in solar cells [8-10], doped or undoped nanocrystalline titanium dioxide used to photochemically degrade toxic chemicals [11], and the nanocrystalline anatase phase for use in electrodes in lithium batteries [12]. The properties of most interest for these applications generally depend on the materials morphology, crystal phase, crystallinity and surface area. The particle size of nanocrystalline titanium dioxide plays an important role in the physical and chemical behavior of the material. This is because the surface area, the chemical stability and the chemical reactivity of the material are all correlated with the particle size [11]. Commercial crystalline titanium dioxide, however, has a very low surface area and a minute particle size, making its recovery quite difficult. For many of the potential applications of nanocrystalline titanium dioxide, preparing a high surface area and well-controlled particle size are essential. In this study, a block copolymer plays an important role in the preparation of nanocrystalline titanium dioxide nanoparticles, and it is shown that nanoparticulate titanium dioxide with a high surface area and wellcontrolled particle size can be formed as the hydrolysis product of titanate precursor materials in the presence of a block copolymer. EXPERIMENTAL PROCEDURE Poly (alkylene oxide) block copolymer F108 was used as an organic template material with 1.5 g dissolved in 10 ml of anhydrous ethanol. 0.01 mole of the inorganic precursor, TiCl4 (Acros 99 %),
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Author correspondence to the authors at the Institute of Mechanical and ElectroMechanical Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan; Tel: +886-5-6315684, Fax: +886-5-6315643; E-mail:
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was then added to the F108 ethanol solution and stirred vigorously for 1 h. The resulting sol solution was gelled in an open petri dish at 60 ℃ in air. The as-made sample was then calcined at 400℃ for 5 h to remove the block copolymer. Powder XRD data was measured with a Rigaku D/max-Ⅳ diffractometer with Cu K radiation (=1.5418 Å). The sample was scanned from 20o to 60o (2) in steps of 4 o/min. The crystallite domain size (D) was examined from the XRD peak based on the Scherrer equation [13], D=0.9/A cos , where is the x-ray wavelength, is the Bragg diffraction angle, and A is the true half-peak width of the XRD lines. FTIR spectra, in the range of 4000-400 cm1, were recorded on a Perkin–Elmer Spectrum GX infrared spectrophotometer. XPS was carried out at room temperature in a Perkin–Elmer PHI 5300 XPS/630 SAM system. The binding energy was calibrated with reference to a C 1s peak (285.0 eV). The TEM micrographs were made with a JEOL 3010 transmission electron microscope operated at 200 keV. The sample for TEM was prepared by dispersing the final powder in ethanol, and this dispersion was then dropped on carbon-copper grids. A SEM image was obtained using a Hitachi 3000N, and the sample was prepared by dispersing the final powder in conductive glue, and this dispersion was then sprayed with carbon. The N2 adsorption-desorption isotherm was recorded on a Micromeritics ASAP 2010 automated sorption analyser. The sample was outgassed for 7 h at 150 before the analysis. The Barrett-JoynerHalenda (BJH) and Brunauer-Emmett-Teller (BET) methods were applied to determine the pore size and BET surface area, respectively. The structure and bonding type of the titanium dioxide was evaluated by Raman spectrometry (LabRAM HR, Jobin Yvon), and the Raman spectra were obtained from the excitation of a 532 nm argon laser focused with a spot diameter of 1 Am. RESULTS AND DISCUSSION The FTIR spectra in the range 4000-400 cm-1 of the block copolymer and with the material calcined at 400 for 5 h are shown in Fig. (1). Some bands around 2950 and 1480 cm-1 that originated from the vibrations of –CH2– and –CH3 [14] of block copolymer are clearly seen in Fig. (1a). After calcination at 400, no bands due to organic species were present, showing that the block copolymer template was completely removed by calcination. The results are in good agreement with those reported in our previous study [15] on mesoporous tungsten oxide. The bands due to the oxide structure appeared in the region between 400 and 850 cm1 [16]. The two peaks at 3400 and 1650 cm-1 correspond to the surface adsorbed © 2010 Bentham Science Publishers Ltd.
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Fig. (1). FTIR spectra of: (a) block copolymer and (b) nanocrystalline titanium dioxide calcined at 400℃ for 5 h. Fig. (3). Raman spectrum of nanocrystalline titanium dioxide calcined at 400℃ for 5 h. anatase phase in the calcined sample and crystallite size of about 20 nm. Fig. (3) shows the Raman spectrum of the titanium dioxide calcined at a temperature of 400. It can be seen that the crystallization was complete, and the formation of an anatase phase is evident from the characteristic anatase Raman modes at 150, 403, 515, and 638 cm-1. A careful analysis of the Raman spectrum of titanium dioxide prepared at 400 shows no evidence of brookite or rutile phases.
Fig. (2). Powder XRD pattern of nanocrystalline titanium dioxide. water and the hydroxyl groups, respectively [17]. Moreover, Cl ions seem to be removed by the calcination process. As shown in the FTIR spectra, there are no peaks related to the hydrogen chloride of the calcined sample in Fig. (1b), and the stretching vibration of HCl is at 2719.5 cm1. Calcination at 400 for 5 h was conducted to remove the block copolymer template from the titanium dioxide particles. At this temperature, the majority of the block copolymer is removed (as evidenced by the FTIR result). It is also seen that this takes place without much structural distortion caused by nanocrystallite growth within the titanium dioxide framework. The XRD pattern in Fig. (2) shows several well resolved peaks, indexed as the (101), (004), (200), (105) and (211) planes, which are typical of anatase phase titanium dioxide (JCPDS card No. 21-1272). This clearly shows the presence of nanocrystalline anatase in the calcined sample. The crystallite size calculated from the Scherrer equation using the (101) diffraction peak of anatase is about 20 nm. TEM observation further substantiates the XRD results, showing the presence of the
The particle morphology of the titanium dioxide powder calcined at 400 for 5 h was observed by SEM (Fig. 4a). After calcination, the particles do not seem to be sintered, and the surface of the particles is rough, suggesting the formation of a porous structure created by the aggregation of the surfactant micelles. Such a structure is likely to facilitate the catalysis process, because of the capillary pores and large surface area. Further information on the microstructure of the titanium dioxide is provided by TEM investigations, as shown in Fig. (4b) and c, which confirm the nanoscale size of the titanium dioxide. The corresponding selected area electron diffraction (SAED) pattern proves the high crystallinity of the titanium dioxide framework, with only one crystalline phase present, namely anatase (Fig. 4b inset). The size of the primary nanoparticles can be determined from dark-field imaging by TEM in Fig. (4c). It can be observed that the titanium dioxide calcined at 400 is totally composed of highly crystalline and monodisperse anatase nanoparticles, with a particle size of about 20 nm in the TEM image. This is in good agreement with the XRD results. The nanocrystalline titanium dioxide is shown to be homogeneous, and the mesopores are observed as white areas in the images. Table 1 shows a summary of the particle size of a nanocrystalline anatase titanium dioxide calculated by the Scherrer formula, obtained based on TEM data and estimated from the BET value. The BET surface area of one nanocrystalline anatase sample is found to be 115.04 m2/g. The particle size obtained from the TEM method agrees very well with the result given by X-ray line broadening. The result from the BET method is larger than that from the TEM and X-ray methods. This might be because there is sometimes some particle aggregation, and so it is not unexpected that the measured BET size is greater than the corresponding XRD size and TEM observation. Fig. (5) shows the particle size distribution of
Preparation and Characterization of Nanocrystalline
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Fig. (5). Mean particle size and particle distribution of nanocrystalline titanium dioxide obtained from TEM.
Fig. (4). (a) is the SEM micrograph of nanocrystalline titanium dioxide calcined at 400℃ for 5 h, (b) and (c) are the bright field and dark field of the TEM micrograph of nanocrystalline titanium dioxide, respectively. titanium dioxide nanoparticles. It can be seen that titanium dioxide has a well-controlled size distribution of particles, with a modal size of about 20 nm, and 90% of the particles having a size of 18-22 nm, as derived by manual measurement from the TEM image.
Fig. (6). N2 adsorption(x)-desorption(o) isotherm and BJH pore size distribution curve for nanocrystalline titanium dioxide calcined at 400℃ for 5 h.
The N2 adsorption-desorption isotherm and BJH pore size distribution curve of the sample calcined at 400 is depicted in Fig. (6), in which the sample still exhibits a type IV isotherm with a large hysteresis loop between the H1- and H2-types, characteristic of mesoporous materials [18]. The narrow pore diameter distribu-
tion, calculated using the BJH model (obtained from the desorption branch of the isotherm), is centered at approximately 6.6 nm (Fig. 6 inset). The pore volume of the sample calcined at 400 is 0.27 cm3/g. The existence of the uniform mesopores in the calcined sample implies that at least some of the mesoporous structure of the
Table 1. The Particle Size of Nanocrystalline Titanium Dioxide as Measured Using Three Different Techniques BET
X-ray
TEM
BET Surface Area (m2/g)
Average Particle Size on Average Particle Size on
Crystalline Size from Scherrer’s Equation (nm)
Average Particle Size from TEM (nm)
115.04
22.7
20.0
20.0
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Lai et al. [19], when the mesopore collapsed and sintering occurred, Ostwald ripening probably caused the grain continue to grow to the larger ones by decreasing the interfacial energy. In addition, some small particles aggregate into secondary particles because of their extremely small dimensions and high surface energy. The surface area of the titanium dioxide nanoparticles prepared by the surfactant-mediated method is high is due to the large number of pores formed by the surfactant and smaller particle size. Therefore, addition of the surfactant plays an important role in the preparation of titanium dioxide nanoparticles. More detailed studies about the factors of controlling particle size, as well as the effects of nucleation growth process, are currently in progress. CONCLUSIONS In this paper we have described the synthesis of a nanostructured material titanium dioxide with well-controlled particle distribution and an average 20 nm particle size. Structural characterization indicates that after being calcined at 400°C these nanocrystalline titanium dioxide particles still have a mesoporous structure with anatase crystallites. The average pore size and BET surface area of the nanocrystalline titanium dioxide is 6.6 nm and 115.04 m2/g, respectively. Block copolymer is used to produce a smaller particle size and larger BET surface area for the highly crystalline dispersed particles in the synthesis process. The present investigations prove that the surfactant-mediated method is useful for the preparation of titanium dioxide nanoparticles. The findings of this work can be applied to the synthesis of nanostructured materials prepared by the surfactant-mediated method. ACKNOWLEDGMENTS This work was financially supported by the National Science Council of Taiwan, the Republic of China, grant No. NSC 98-2221E-020-020, which is gratefully acknowledged. REFERENCES [1]
Fig. (7). XPS electron spectra of nanocrystalline titanium dioxide calcined at 400℃ for 5 h.
[2]
as-synthesized sample was maintained when the block copolymer was thermally removed.
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XPS measurements were performed to characterize the surface compositions up to a depth of about 5 nm. The surface and nearsurface chemical compositions of the samples analysed by XPS are surveyed in the binding energy range of 0-1000 eV, as shown in Fig. (7). Quantitative XPS analysis was performed on the titanium dioxide particles, and a typical wide scan XPS survey spectrum is shown in Fig. (7a). Distinct photoelectron peaks are clearly observed in this figure for various titanium, oxygen and carbon electron orbitals on the characteristic stepwise background. Fig. (7b) shows the O 1s, Ti 2p and C 1s lines of the sample after calcination at 400. The Ti 2p3/2 line is composed of a single peak at a binding energy of 459.2 eV, with a full width at half maximum of 1.5 eV. The separation between the Ti 2p3/2 and Ti 2p1/2 peaks is 5.7 eV, while the O 1s binding energy is 530.5 eV. These values, proving the existence of titanium dioxide as well as the fact that titanium is present with only one valence (Ti4+), are characteristic of a stoichiometric surface. The titanium-to-oxygen ratio is 0.53:1, which is close to that expected from the stoichiometry of titanium dioxide. A carbon C 1s peak at a binding energy of 285 eV is observed on the sample, and this is related to surface pollution, which corresponds to the fact that the sample was exposed to air before the XPS experiments. The results reveal that the addition of surfactant greatly influences the particle size and aggregation. In the model proposed by
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Accepted: September 01, 2009
