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Formation of Titania Nanofibers: A Direct Sol-Gel Route in Supercritical CO2 Ruohong Sui,† Amin S. Rizkalla,*,†,‡ and Paul A. Charpentier*,† Department of Chemical and Biochemical Engineering, Faculty of Engineering, and Division of Biomaterials Science, University of Western Ontario, London, Ontario, Canada N6A 5B9 Received March 6, 2005. In Final Form: May 25, 2005 In this letter, we present a new method to synthesize titania nanofibers with nanocrystallites via a sol-gel route in supercritical CO2. The nanofibers were formed by the esterification and condensation of titanium alkoxides using acetic acid as the polymerization agent in supercritical CO2 from 40 to 70 °C and 2500 to 8000 psia. The TiO2 nanofiber morphology was characterized by means of SEM and HRTEM, which indicated that the diameters ranged from 9 to 100 nm. N2 physisorption, and powder XRD showed that the nanofibers exhibited relatively high surface areas up to 400 m2/g and anatase and/or rutile nanocrystallites were formed after calcination.
The utilization of the green solvent supercritical carbon dioxide (scCO2) as a substitute for conventional organic solvents for the processing of materials has been widely reported.1-5 The reasons are due to scCO2 being nontoxic, nonflammable, inexpensive, naturally abundant, and environmentally benign. Its physical properties such as density and solubility can be ‘tuned’ within a wide range of processing conditions by varying both temperature and pressure (Pc ) 74 bar, Tc ) 31.1 °C).6 Low viscosity, high diffusivity, and negligible surface tension are considered highly effective for producing superior products of fine and uniform particles.7,8 In addition, removal of scCO2 can be easily achieved by venting; hence, no evaporation or drying processes are required.8,9 For aerogels, supercritical drying extinguishes the liquid surface tension that causes the shrinkage of the solid, hence maintaining the microstructure of the aerogel.10 Recently, scCO2 has attracted significant attention for synthesizing nanomaterials.11-13 Titania nanoparticles are of great interest for many applications, e.g., nanofibers for chemical sensors,14 porous * Authors to whom correspondence should be addressed. Email:
[email protected] (P.A.C.);
[email protected] (A.S.R.). Phone: (519) 661-3466 (P.A.C.); (519) 661-2111 x 82212 (A.S.R.). Fax: (519) 661-3498. † Department of Chemical and Biochemical Engineering, Faculty of Engineering. ‡ Division of Biomaterials Science. (1) Woods, H. M.; Silva, M. M. C. G.; Nouvel, C.; Shakesheff, K. M.; Howdle, S. M. J. Mater. Chem. 2004, 14, 1663. (2) O’Neil, A.; Watkins, J. J. Green Chem. 2004, 6, 363. (3) King, J. W.; Williams, L. L. Curr. Opin. Solid State Mater Sci. 2003, 7, 413. (4) Charpentier, P. A.; DeSimone, J. M.; Roberts, G. W. ACS Symposium Series 2002, 819, 113. (5) Sui, R.; Rizkalla, A. S.; Charpentier, P. A. J. Phys. Chem. B 2004, 108, 11886. (6) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth-Heinemann: Boston, 1994. (7) Debenedetti, P. G. AIChE 1990, 36, 1289. (8) Tom, J. W.; Lim, G. B.; Debenedetti, P. G.; Prud’homme, R. K. Applications of SCF in the Controlled Release of Drugs. In Supercritical Fluid Engineering Science; ACS Symposium Series 514; Kiran, E., Brennecke, J. F., Eds.;. 1993; p 238. (9) Tom, J. W.; Debenedetti, P. G. J. Aerosol Sci. 1991, 22, 555. (10) Pierre, A. C.; Pajonk, G. M. Chem. Rev. 2002, 102, 4243. (11) Johnston, K. P.; Shah, P. S. Science 2004, 303, 482. (12) Shah, P. S.; Novick, B. J.; Hwang, H. S.; Lim, K. T.; Carbonell, R. G.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2003, 3, 1671. (13) Pai, R. A.; Humayun, R.; Schulberg, M. T.; Sengupta, A.; Sun, J.-N.; Watkins, J. J. Science 2004, 303, 507. (14) Akbar, S.; Yoo, S. Chem. Sensors 2004, 20, 30.
nanoparticulates as a semiconductor electrode in photoelectrochemical cells (PEC),15 and TiO2 as small as 10 nm enhances the photocatalytic reactivity due to the quantum size effect.16 The TiO2 nanoparticles are normally synthesized by a conventional sol-gel route, in which the hydrolysis and condensation of titanium alkoxides (TA) by water and acid are carefully controlled.17,18 Use of acetic acid allowed for submicrometer TiO2 fiber formation with diameters ranging from 100 nm to several micrometers.19 To prepare nanomaterials, another approach was performed by using scCO2 as a substitute for the organic solvent, e.g., preparing of TiO2 nanoparticles by dispersed water stabilized by hydrated reverse micelles20 or an anionic fluorinated surfactant,21 depositing TiO2 into the nanospace of activated carbon22 or coating TiO2 on alumina substrate.23 However, the sol-gel methods using water resulted in only spherical TiO2 nanoparticles. Hydrothermal reaction between TiO2 and alkaline solution was used to produce titanium oxide nanofibers; however, the resulting materials were hydrogen titanate rather than titania.24 A template method involving a sol-gel process in the nanoporous membrane was employed to synthesize titania nanofibers with diameters of 22 nm;25 the drawback of the method is the complex procedures of building the alumina template and chemical etching afterward and the resulting materials are polycrystalline with impurities.26 Electrospinning was also used for fabrication of titania nanofiber mats with diameters in the range from 20 to 200 nm.27 (15) Bisquert, J.; Cahen, D.; Hodes, G.; Ruhle, S.; Zaban, A. J. Phys. Chem. B 2003, 108, 8106. (16) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (17) Schneider, M.; Baiker, A. Catal. Today 1997, 35, 339. (18) Dawson, A.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 960. (19) Nagamine, S.; Masuda, I.; Ueda, T.; Su, C.; Sasaoka, E. J. Mater. Sci. Lett. 2003, 22, 1213. (20) Lim, K. T.; Hwang, H. S.; Ryoo, W.; Johnston, K. P. Langmuir 2004, 20, 2466. (21) Tadros, M. E. A., Carol L. J.; Russick, Edward M.; Youngman, Michael P. J. Supercrit. Fluids 1996, 9, 172. (22) Tatsuda, N.; Itahara, H.; Setoyama, N.; Fukushima, Y. J. Mater. Chem. 2004, 14, 3440. (23) Bocquet, J. F.; Chhor, K.; Pommier, C. Surf. Coat. Technol. 1994, 70, 73. (24) Yuan, Z.-Y.; Su, B.-L. Colloids Surf., A 2004, 241, 173. (25) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857. (26) Chu, S.-Z.; Wada, K.; Inoue, S.; Todoroki, S.-i. Chem. Mater. 2002, 14, 266. (27) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555.
10.1021/la0505972 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/11/2005
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Here we present a simple method of synthesizing mesoporous TiO2 nanofibers in scCO2, in which acetic acid was used as the polymerization agent. One of the attractive features associated with this method is that the titania nanofibers can be readily and reproducibly prepared in a large scale in an autoclave reactor, and pure titania with anatase and/or rutile crystalline phase can be obtained after calcination at different temperatures. Another feature is attributed to the benefit of the green chemistry synthetic conditions, as described earlier. Furthermore, the diameter of the titania nanofibers as small as 10 nm can be obtained, and the diameter can be tuned by varying the concentration of the precursor and acetic acid. To synthesize the TiO2 in scCO2, a 25 mL stainless steel view cell was connected with a syringe pump (ISCO 100 DM) for pumping CO2 from a dip tube. A check valve (HIP) next to the pump was used to prevent possible back flow from the view cell. The temperature and pressure in the view cell were measured and controlled by means of a T-type thermocouple (Omega), a heating tape, a pressure transducer (Omega), and a control valve (Badger), which were connected to a network interface (National Instruments). The network interface communicated with a computer by means of Labview software. Reagent grade 97% titanium(IV) butoxide (TBO), 97% titanium(IV) isopropoxide (TIP), and 99.7% acetic acid, from the Aldrich Chemical Co., were used without further purification. Instrument grade carbon dioxide (99.99%) was obtained from BOC Canada. In a typical experiment, the titanium alkoxide was quickly placed in the view cell, followed by addition of acetic acid and CO2 to the desired pressure and temperature. A magnetic stirrer was used for mixing the reaction mixture. It was found that the mixture of TBO/TIP and acetic acid was miscible with CO2 at the studied temperature range from 40 to 70 °C and pressure range from 2500 to 8000 psig. An amber transparent homogeneous phase was observed due to the original color of the TA. After the reaction mixture was stirred from several hours to days, the fluid in the view cell became semitransparent and then opaque, indicating the formation of solid phases. To ensure complete condensation of the precursor, a few drops of the reaction mixture were vented into water, where a white precipitate indicated that further reaction time was required. Typical conversions of 98%+ were obtained when the reaction was complete, based gravimetrically from the amount of starting TA and the weight of calcined TiO2 at 500 °C. SEM results illustrated that incomplete reaction time resulted in the formation of chunks instead of a fibrous structure. Normally, several days of aging were required for complete reaction. After aging, the formed gel was washed continuously using CO2 at a rate of approximately 0.5 mL/min, followed by controlled venting at 0.5 mL/min to prevent collapse of the solid network. The as-prepared TiO2 was then calcined in air using a heating rate for each calcination temperature of 10 °C/min, to set-point. The holding time was 2 h, and the cooling rate to room temperature was 0.5 °C/min. The resulting low-density aerogel-like materials (0.2 g/mL) were found to form an extremely fragile monolith that was easy to break apart into a fine powder. SEM results (recorded using a LEO 1530 operated at 3 kV) of the synthesized materials (see Figure 1) showed that both the as-prepared and the calcined TiO2 powders were composed of fiber clusters. The fiber surface of the asprepared material was smooth, while that of the fibers calcined above 380 °C became rough and were composed of connected beads. The diameters of the fibers were strongly affected by the HAc/TA ratio and the type of TA.
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Figure 1. SEM: (a) and (b) titania nanofibers of TiO2-1 calcined at 380 °C under different magnifications; (c) TiO2-2 calcined at 380 °C; (d) and (e) TiO2-3 calcined at 380 °C under different magnifications; (f) TiO2-4 calcined at 380 °C. All the samples were examined without grinding or gold coating.
Generally, usage of TIP gave a smaller-diameter fiber compared with TBO. Typically, at 40 °C and 6000 psi, with initial concentrations of 1.1 mol/L TBO and 6.1 mol/L acetic acid, titania (TiO2-1) fibers with a diameter of about 100 nm and lengths from 2 to 4 µm were obtained (Figure 1a,b). According to the SEM results, by varying the magnification, the aerogel was composed of individual nanofibers with very similar diameters and aspect ratios. Figure 1c shows the fiber nature of the particulate (TiO2-2) synthesized using a lower system pressure of 2500 psig. In general, we found no obvious change of fiber morphology and dimension when adjusting the reaction temperature from 40 to 70 °C and the pressure from 2500 to 8000 psi using the above initial concentrations. However, the fiber diameter decreased when TIP was used as the alkoxide precursor. At 60 °C and 6000 psi, with initial concentrations of 1.5 mol/L TIP and 6.1 mol/L acetic acid, titania (TiO2-3) fibers with approximately 10 nm diameters and lengths from 1 to 2 µm were obtained (Figure 1d,e). At 60 °C and 6000 psi, with initial concentrations of 1.1 mol/L TIP and 6.1 mol/L acetic acid, titania (TiO2-4) fibers with a diameter of about 40 nm and lengths from 2 to 4 µm were obtained (Figure 1f). HRTEM (JEOL 2010F operated at 100 or 200 kV) (Figure 2a) provides further microstructural and crystalline details of the nanofibers. Figure 2b shows that the fibers of TiO2-4 are composed of 10 nm nanocrystallites or amorphous spheres, while Figure 2c shows the anatase diffraction pattern of the 10 nm diameter fibers. Figure 3a shows the wide-angle powder XRD pattern (Bruker D8 Discover Diffractometer with GADDS, the type of radiation: Cu KR1 + KR2 ) 1.54184 Å) for sample TiO2-1. This sample exhibited a rutile phase at 380 °C, while an increased calcination temperature resulted in the formation of anatase until 600 °C, whereas further heat treatment at higher temperatures resulted in the
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Figure 2. HRTEM (a) TiO2-4 calcined at 380 °C; (b) and (c) TiO2-3 calcined at 500 °C under different magnifications.
Figure 3. XRD: (a) powder XRD pattern of TiO2-1 calcined at A, 380 °C; B, 500 °C; C, 600 °C; D, 800 °C; and E, 1000 °C; (b) powder XRD pattern of TiO2-3 calcined at F, 380 °C; G, 500 °C; and H, 600 °C. Note: a, anatase; r, rutile.
formation of rutile again. However, TiO2-3 exhibited anatase nanocrystallites from 380 to 500 °C and multiple phases of 56% rutile and 44% anatase at 600 °C (Figure 3b). Anatase was the main crystalline phase in TiO2-3 at relatively low calcination temperatures, even though rutile is more thermodynamically stable under ambient pressure compared with anatase and brookite. The XRD results showed that the crystalline phase is particle-size dependent. According to Zhang et al.,28 both experimental and theoretical analysis show that, when the particle size of TiO2 drops to 14 nm, the total free energy of rutile is higher than that of anatase. This makes the relative phase stability of anatase and rutile reverse, and anatase becomes the more stable phase. Our XRD results agree with Zhang’s conclusion, given the fact that the fiber diameter of TiO2-3 is much smaller than that of TiO2-1. By using Scherer’s equation,29 the crystallite sizes of the TiO2-1 and TiO2-3 samples calcined at 380 °C were determined to be 21 ( 4 and 10 nm ( 2 nm, respectively, which are quite close to the TEM observations. The samples were also examined in the small-angle XRD region between 0 and 20 2θ. No discernible peaks were observed, indicating the mesophases were not in a regular pattern, such as those produced by templating methods which yield cubic mesostructures of TiO2.30 (28) Zhang, H.; Banfield, J. F. J. Mater. Chem. 1998, 8, 2073. (29) Weibel, A.; Bouchet, R.; Boulc’h, F.; Knauth, P. Chem. Mater. 2005, 7, 2378. (30) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152.
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Figure 4. N2 adsorption/desorption isotherm of the TiO2-1 calcined at 500 °C. Inset is the BJH pore size distribution.
The N2 physisorption (Micromeritics ASAP 2010 at 77 K) showed that all the TiO2 nanofibers exhibited a type IV isotherm, indicating the presence of mesopores.31 The surface area of the TiO2-1 samples for as-prepared, calcined at 380 °C, and 500 °C were 308, 105, and 83 m2/g, respectively, and those for TiO2-3 were 415, 143, and 77 m2/g, respectively. The surface area of TiO2-4 calcined at 380 °C was as high as 270 m2/g. The N2 physisorption isotherm and the BJH pore size distribution of the TiO2-1 nanofibers calcined at 500 °C are shown in Figure 4. The adsorption in the low-relative-pressure region is contributed by the fiber surface. The large hysteresis loop from 0.42 to 0.77 P/P0 is due to the mesopores inside the fibers, and the small loop from 0.9 to 1 P/P0 is assigned to the interparticle porosity due to the agglomeration of the nanofibers.32 It is clear that the majority of the nanofiber surface area is due to the mesopores. The mesopores exhibited a narrow pore-size distribution (Figure 4 inset) with a mean diameter of 4.5 nm. The narrow pore-size distribution is likely due to the interstitial space between the crystallites, which was measured to be 21 nm by XRD, as described earlier. The mechanism of fiber formation is quite complicated, as it consists of both condensation and self-assembly steps. According to the studies by Livage et al. and Sanchez et al., under ambient conditions,33-35 a bridging complex is formed between the titanium atom of the alkoxide monomer and acetic acid, which leads to one-dimensional step-growth of the polymer molecule. To lower the surface tension while the polymer grows, a concentrated spherical region (called coacervates) is formed. The linear polymers in the coacervates then become ordered due to chain-chain interactions, resulting in the formation of tactoids. The tactoids can further gel to form ordered structures called crystalloids or fibers (observed in SEM/HRTEM), in which the polymers chains are irreversibly bonded with each other. As well, hydrogen (31) Sing, K. S. W.; Evertt, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (32) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area, and porosity, 2nd ed.; Academic Press: London; Toronto, 1982. (33) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (34) Livage, J.; Sanchez, C.; Henry, M.; Doeuff, S. Solid State Ionics 1989, 32/33, 633. (35) Sanchez, C.; Babonneau, F.; Doeuff, S.; Leaustic, A. Ultrastructure Processing of Advanced Ceramics; Wiley: New York, 1988.
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bonding between CO2 and acetic acid36 remarkably slows down the condensation process and facilitates the formation of nanostructures. During washing of the gel with scCO2, the ester and alcohol condensate products are removed, along with any unreacted acetic acid. Due to the low surface tension of CO2, careful venting maintains the pore structure of the solid network, allowing the mesoporous fibers to be obtained. Further studies on the details of the mechanism are being conducted in our laboratories in order to control morphology and surface functionality. Our research shows a new approach to the synthesis of titania nanofibers by the direct polymerization of titanium alkoxides in scCO2, which provides for mesoporous nanofibers with high surface areas. The use of a reactor makes this technique attractive for scale-up, while the resulting materials have many exciting (36) Mu, T.; Han, B.; Zhang, J.; Li, Z.; Liu, Z.; Du, J.; Liu, D. J. Supercrit. Fluids 2004, 30, 17.
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applications in the fields of alternative energy and advanced materials. Acknowledgment. We would like to thank Dr. Todd Simpson for SEM analysis at the UWO Nanotechnology Centre, Fred Pearson from the Brockhouse Institute, McMaster University, for HRTEM, Ms. Tatiana Karamaneva for the powder XRD, and M. Mozahar Hossain and Nicole Persaud for their help on BET measurements. This work was financially supported by the Canadian Natural Science and Engineering Research Council (NSERC), the Materials and Manufacturing Ontario Emerging Materials program (MMO-EMK), the Canadian Foundation for Innovation (CFI), and the UWO Academic Development Fund (ADF). Supporting Information Available: Schematic of experimental setup. This material is available free of charge via the Internet at http://pubs.acs.org. LA0505972
