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Sonochemical Preparation of Silane-Coated Titania Particles Kurikka V. P. M. Shafi,†,‡ Abraham Ulman,*,†,‡ Xingzhong Yan,‡,§ Nan-Loh Yang,‡,§ Michael Himmelhaus,| and Michael Grunze| Department of Chemical Engineering and Chemistry, Polytechnic University, Brooklyn, New York 11201, Department of Chemistry, CUNY at Staten Island, 2800 Victory Bulevard, Staten Island, New York 10314, The NSF MRSEC for Polymers at Engineered Interfaces, and Institute of Physical Chemistry, University of Heidelberg, INF 253, D-69120 Heidelberg, Germany Received August 30, 2000. In Final Form: December 21, 2000 We report that sonochemistry is a fast and efficient technique for coating of octadecyltrihydrosilane (CH3(CH2)17SiH3) on titania surfaces. Infrared spectroscopy as well as thermal analysis confirms that complete coating is achieved after 30 min. Solid-state 13C NMR spectroscopy establishes the bonding of trihydrosilane to the titania particles. Raman microscopy gives the expected rutile structure and further confirms the presence of an octadecyl monolayer. X-ray diffraction confirms that the rutile structure of the titania particles has not changed during sonication. Anatase titania undergoes the same reaction when sonicated in the presence of octadecyltrihydrosilane.
Introduction A recent communication by McCarthy and co-workers reported the reaction of trihydrosilanes (R-SiH3) with titanium and other metal oxide surfaces.1 This reaction represents a new type of self-assembly that can be used to functionalize nanoparticles by organic monolayers and thus may become important when the preparation of nanocomposites is limited by lack of compatibility between the inorganic nanoparticles and the organic matrix. The limitation of the reported route, however, is that maximum coverage is achieved only after a long reaction time, as was reported by the authors and was also found in our laboratory, thus making it less desirable for practical applications. We report here that a faster and more efficient coating of octadecyltrihydrosilanes (OTHS, CH3(CH2)17SiH3) on a titania surface (rutile) can be achieved by sonochemical means. Silica-coated titania particles can be further prepared by thermal decomposition of the silane-coated titania surfaces, removing the organic moieties and leaving a SiO2 layer on the TiO2 nanoparticle surface. The advantage of the new sonochemical method is that the reaction is completed within 30 min. Nanoparticles have been the subject of considerable interest because of their special properties, resulting from the nanoscale regime, such as a large surface-to-volume ratio, and increased surface reactivity as compared to that of the bulk material. This enables their use as catalysts, as well as in mechanical, electronic, and optical applications.2 Titanium dioxide is a most versatile material and has been extensively used over the past decade, because of its * To whom correspondence may be addressed. Phone: (718) 2603119. Fax: (718) 260-3125. E-fax: (810) 277-6217. E-mail: aulman@ poly.edu. † Polytechnic University. ‡ The NSF MRSEC for Polymers at Engineered Interfaces. § CUNY at Staten Island. | University of Heidelberg. (1) Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999, 121, 12184.
low cost, nontoxicity, photostability, and efficient photocatalytic properties.3 TiO2 is both biologically and chemically inert, and its photocatalytic properties are favorable for oxidation of hazardous chemicals,4 reduction of heavily metal ions,5 and photodestruction of bacteria and viruses in water. A high refractive index combined with a high degree of transparency in the visible region makes the TiO2 a unique choice for the pigment industry. The scattering efficiency for the visible light imparts whiteness, brightness, and opacity to the coatings. These properties have made TiO2 an important additive in cosmetic formulations as well. Sonochemistry has been used extensively to generate novel materials with unusual properties,6 because the method results in the formation of amorphous nanoparticles.7 The chemical effects of ultrasound are driven primarily from hot spots formed during acoustic cavitation, a process that dramatically concentrates the low-energy density of a sound field. Various experiments have demonstrated that the effective temperature reached during bubble collapse is 5000 K.8 When liquids that contain solids are irradiated with ultrasound, related phenomena can occur.9 There, cavitation occurs near an extended solid surface and cavity collapse is nonspherical and drives high-speed jets of liquid (2) (a) Agfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (b) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533. (c) Weller, H.; Eychmuller, A. Adv. Photochem. 1995, 165. (d) Serpone, N.; Khairutdinov, R. F. In Semiconductor Nanoclusters; Kamat, P. V., Meisel, D., Eds.; Studies in Surface Science and Catalysis, Vol. 103; Elsevier Science: New York, 1997; p 417. (e) Sailor, M. J.; Heinrich, J. L.; Lauerhaas, J. M. In Semiconductor Nanoclusters; Kamat, P. V., Meisel, D., Eds.; Studies in Surface Science and Catalysis, Vol. 103; Elsevier Science: New York, 1996; p 209. (3) Sawunyama, P.; Fujishima, A.; Hashimoto K. Langmuir 1999, 15, 3551. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (5) Chen, L. X.; Rajh, T.; Wang, Z.; Thurnauer, M. C. J. Phys. Chem. B 1997, 101, 10688. (6) Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: Weinheim, Germany, 1988. (7) (a) Shafi, K. V. P. M.; Gedanken, A.; Prozorov, R. Adv. Mater. 1998, 10, 590. (b) Shafi, K. V. P. M.; Felner, I.; Mastai, Y.; Gedanken, A. J. Phys. Chem B 1999, 103, 3358.
10.1021/la001252g CCC: $20.00 © 2001 American Chemical Society Published on Web 02/01/2001
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into the surface. These jets and associated shock waves can cause substantial surface damage and expose fresh, highly heated surfaces. While sonochemistry has been rarely used in the presence of small organic molecules because of the possible decomposition, we have found in numerous cases that different surfactants are stable in the sonochemical treatment and that this technique can be used to promote a chemical reaction at the surface of nanoparticles, rather than to “clean” them.10 The shock wave that hits the surface of the solid particles causes local heating at the surface, which provides the activation energy for surface reactions. Inorganic surface treatments can provide improvements in properties of TiO2, such as, dispensability in water and in organic solvents, hiding power efficiency, and the chalk and discoloration resistance. Chalking is the photochemical degradation of the organic binder (usually a high molecular weight polymer) at the pigment surface in the presence of water and oxygen causing a white chalky appearance. These inorganic surface modifiers are often precipitated coatings of silica and alumina, and a good control of the type, amount, and the method of deposition of these coatings will influence the performance of the titania as a good pigment. In this scenario, our sonochemical technique becomes more significant, as a better control on the amount and the uniformity of the surface coating can be achieved than one obtained in the conventional precipitation method, normally used in the paint industry. The pigment particles are encased in an impervious shell of silica, and this tight coating prevents the photochemical degradation. The surface coating also acts as a physical spacer, maintaining good separation between adjacent TiO2 particles and thus minimizing the losses in diffractive light-scattering efficiency as pigment concentration is increased. Experimental Section Materials. Titania samples (rutile and anatase) were obtained as a gift from Estee Lauder. Specifications provided with the samples indicate average particle size of ∼300 nm. Heptane (anhydrous), pentane (anhydrous), and OTHS were obtained from Aldrich and used as received. Preparation of silane coated titania particles was carried out by ultrasonic irradiation of the slurry of titania in heptane solution of OTHS at 273 K with high-intensity ultrasonic probe (Sonics Vibra Cell, model VC 601, 1.25 cm Ti horn, 20 kHz, 100 W/cm2). After 10 min of irradiation a colloidal suspension was obtained which was then centrifuged (9000 rpm, 5 min, 10 °C) and washed with dry pentane. Centrifugation and washing were repeated at least five times to remove the excess of OTHS, and the product was then dried under vacuum. The irradiation process was repeated for 30 min, and for 1, 2 and 3 h, respectively. The as prepared products were examined by thermal analysis infrared spectroscopy, Raman microscopy, and NMR spectroscopy. Solid-state 13C NMR spectra were acquired on a Varian (Palo Alto, CA) Unityplus wide-bore spectrometer operating at a 13C frequency of 75 MHz. A Doty 7 mm XC5 probe was used to perform cross polarization magic angle spinning (CP/MAS) experiments at room temperature using an XPOLAR1 sequence. (8) (a) Mason, T. J.; Lorimer, J. P. Sonochemistry: theory, applications and uses of ultrasound in chemistry; Ellis Horwood Ltd.: Chichester, 1988. (b) Sonochemistry: the uses of ultrasound in sonochemistry; Mason, T. J., Ed.; Royal Society of Chemistry: London, 1990. (c) Current trends in sonochemistry; Price, G. J., Ed.; Royal Society of Chemistry: London, 1992. (d) Ultrasound: its chemical, physical and biological effects; Suslick, K. S., Ed.; VCH Publishers Inc.: New York, 1988. (e) Crum, L. A. Bubbles hotter than the sun. New Sci. 1995, 146, 36. (f) Hiller, R.; Putterman, S. J.; Barber, B. P. Phys. Rev. Lett. 1992, 69, 1182. (9) Suslick K. S. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; J. Wiley & Sons: New York, 1998; Vol. 26, pp 517-541. (10) Shafi, K. V. P. M.; Ulman, A.; Yan. X.; Yang, N.-L.; Estournes, C.; White, H.; Rafailovich, M. Submitted for publication in J. Phys. Chem.
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Figure 1. Transmission electron micrographs of rutile titania nanoparticles before and after sonication in heptane with OTHS. The application of TOSS (total sideband suppression) was found to reduce significantly background noise from the Doty Si3N4 thick-wall rotor used at a spinning rate of 6 kHz (Varian speed controller). Contact times of 0.5-2.5 ms were examined to arrive at the optimum spectral results. A contact time of 1.0 ms and a recycled time of 2 s were used for the spectrum reported below with 30k transients, showing reasonable resolution, acceptable signal-to-noise ratio, and both methylene trans and gauche bands. Infrared Spectroscopy. Samples of equal weight were prepared in KBr, and the spectra were recorded on a Nicolet 760 spectrophotometer equipped with a He-Ne laser and an MCT detector, in 2 cm-1 resolution. Raman Spectroscopy. For acquisition of Raman spectra, a Dilor LabRAM I Raman microscope was applied. Lumps of nanoparticles of several tens of millimeters in diameter were placed onto a roughened, diffusive scattering metal plate, and then a proper, i.e., smooth and horizontal, surface on one of the lumps was selected. We note that the metal plate itself did not exhibit any resonant features when probed without nanoparticles on top. Further, the lumps were too large in size to benefit from surface enhancement due to the rough metal surface. For excitation of the particles the 514.5 nm line of an Ar-ion laser (OmNichrome, model 532-AP-AO1) was coupled into the microscope. The incident power was kept constant at 21 mW. The beam was focused onto the sample’s surface via a 10-fold magnifying lens, yielding a spot size of 20 µm on the sample’s surface. Higher magnification up to ×100sthus corresponding to a spot size of 2 µmsdid not lead to any significant changes in the spectra. To obtain good signal-to-noise (S/N) ratio, each 50 scans of 20 s in acquisition time were accumulated for one spectral window. One complete spectrumsfrom 100 to 3500 cm-1shas been composed of three different spectral windows. The slit width was set to 120 µm yielding a spectral resolution of ∼6 cm-1 at 3000 cm-1. Thermal Analysis. The samples were heated to 900 °C at a rate of 10 deg/min, under a nitrogen atmosphere on a TA 2950 Hi Res thermogravimetric analyzer. X-ray Diffraction (XRD). X-ray diffraction patterns of bare and silane-coated TiO2 powders were collected using a Philips PW1729 X-ray diffractometer operating at 40 kV and 30 mA. The interval of data collection was 0.02 deg/s and the range of 2θ was between 20 and 80°. Figure 2 shows an illustration of the resulting diffraction pattern.
Results and Discussion Figure 1 presents transmission electron microscopy (TEM) images of the rutile titania before, and after sonication in heptane for 3 h in the presence of OTHS. No change in size and size distribution could be observed. Figure 2 shows X-ray diffraction traces of the same two samples, indicating that the sonication process does not alter the titania structure. The infrared spectra of titania (rutile structure) and OTHS are presented in Figure 3, while Figure 4A shows the infrared spectra of the same titania after a sonochemical reaction with OTHS for various irradiation time periods and Figure 4B presents only the C-H stretching
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Figure 2. X-ray diffraction traces of rutile titania nanoparticles before and after sonication in heptane with OTHS.
Figure 4. Infrared spectra of TiO2 particles after a sonochemical reaction with OTHS for various irradiation times. A shows the entire IR scan (400-4000 cm-1), while B shows the C-H stretching region (2800-3000 cm-1). Figure 3. Infrared spectra of titania (rutile structure) and OTHS.
vibration range of the 3 h sonicated sample. The presence of peaks for hydrocarbon stretching (2950-2850 cm-1) and bending (1475-1375 cm-1) in the irradiated samples confirms that the silane has reacted with TiO2. The νa(CH3) and νs(CH3) bands appear at 2963 and 2931 cm-1, respectively. The νs(CH2) band appears at 2855 cm-1, while bands at 2926 and 2922 cm-1 are assigned to the νa(CH2). This suggests that the octadecyl chains in the coating might be in two different environments, one which may be a disordered solidlike (νa(CH2) at 2922 cm-1) and the other disordered liquidlike (νa(CH2) at 2926 cm-1). These values for the asymmetric C-H stretch of the methylene groups are the result of the large SiO3 headgroup formed after the reaction of the Si-H groups with the surface Ti-OH groups, as well as the roughness of the nanoparticles surfaces. The very small peaks at 2150 and 925 cm-1 (Si-H stretch and H-Si-H bend, respectively) suggest that some Si-H groups could not find the proper orientation for efficient binding to the TiO2 surface. The broad band between 1200 and 1000 cm-1, which is present in all the samples including the uncoated titania, can be inferred to the presence of Ti-O-Si, Si-O-Si, and Ti-O-Ti species. The infrared spectra of the 30 min and the 3 h irradiated samples are identical, implying that the reaction is complete within 30 min of irradiation time. Raman microscopy measurements were carried out on both OTHS coated and uncoated titania particles. Figure 5 shows the photograph of an aggregate of particles that was used in the experiment. No laser damage could be
Figure 5. Photograph of an aggregate of TiO2 particles used for Raman spectroscopy studies. The beam was focused at the center.
detected using the microscope. The spectra in Figure 6A show that sonochemistry does not change the rutile structure of the sample.11 The monolayer of octadecyl chains is clearly observed. Figure 6B presents the spectra before and after coating, and Figure 6C shows the difference spectrum. The shoulder at 2960 cm-1 is the νa (ip) C-H stretching of the CH3 groups. The peak at 2877 cm-1 is the νs(C-H) stretching mode of the CH3 groups. (11) Gonzalez, R. J. Ph.D. Dissertation, Virginia Polytechnic Institute, 1998.
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Figure 7. Thermograms of the various OTHS-coated titania samples.
Figure 8. Solid-state 13C NMR spectrum of OTHS-coated TiO2 particles after 3 h of sonication. 30 000 scans were collected.
Figure 6. Raman spectra of the titania samples sonicated with and without OTHS in heptane for 3 h.
The band at 2922 cm-1 is the νa(C-H) stretching modes of the CH2 groups. The band at 2845 cm-1 is probably the stretching frequency of gauche CH2.12 The thermograms of the titania samples, reacted for various time with OTHS, are shown in Figure 7. The mass loss, as well as the high desorption temperature, confirms strong binding between the OTHS molecules and the titania particles. The thermograms of the 30 min to 3 h irradiated samples show the same weight loss, corroborating that the reaction is complete within 30 min of ultrasound exposure. The total weight loss of the silanecoated titania samples is 0.85-0.9%, which indicates, after accounting for the weight loss for uncoated TiO2 particles (12) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1980; Volume 2, p 4.
(∼0.25%), a full surface coverage of titania particles of average 0.3 µm in size.13 Preliminary investigation (thermal studies) on the silane-coated anatase titania particles shows greater weight loss than in that seen with the rutile phase, and this indicates that the anatase phase is more efficient in coating. Further studies are in progress, and the results would be published in the forthcoming paper. The solid-state CP/MAS 13C NMR spectrum of the modified TiO2 sample sonicated for 3 h (Figure 8) shows typical characteristics of broad resonances for organic chains attached in high density to inorganic nanoparticles.14 The chemical shifts of the carbon atoms for the 18-carbon chain indicate bonding of the silane in high density to the nanoparticle. Under the experimental conditions used, the results cannot be considered quantitative. However, from the resonances in the region 2934 ppm for 1 ms contact time, one can suggest semiquantitatively ca. 50% of the methylene carbons are closely packed (all-trans) while others are in gauche conformational environments. This is consistent with the Raman spectrum in Figure 6C. For the coated particles, a chemical shift of C-1 (5.0 ppm) is slightly upfield to the C-1 of the free ligand, CH3(CH2)17SiH3 in solution (6.13 ppm). The (13) This calculation assumes 18 Å2 as the area per octadecyl chain and 3.9 g cm-3 as the TiO2 density. (14) (a) Badia, A.; Gao, W.; Singh, S.; Demers, L.; L. Cuccia, L.; Reven, L. Langmuir 1996, 12 (5), 1262-1269. (b) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1997, 13, 115.
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higher shielding of C-1, due to SiO3 on the solid surface compared with SiH3 in solution, can be attributed to (p f d) π bonding of the silicon atom with three polar ligands as in the shielding of the carbon atom (-16 ppm) in CH3SiCl3.15 It would require inordinately long spectrometer time to conduct a series of contact time variation experiment for quantitative extrapolation. It was observed, however, that at 2.5 ms contact time the trans band appears to be significantly weaken compared with the gauche band. Conclusion In conclusion, we reported that sonochemistry is a fast and efficient technique for coating of octadecyltrihydro(15) Hunter, B. K.; Reeves, L. W. Can. J. Chem. 1968, 46, 1399.
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silanes (CH3(CH2)17SiH3) on a titania surface. Infrared spectroscopy, as well as thermal analysis confirm that complete coating is achieved after 30 min. Solid-state NMR spectroscopy establishes the bonding of trihydrosilane to the TiO2 particles. Both 13C NMR spectroscopy and Raman microscopy suggest that the alkyl chains form both close packed assemblies with all-trans chains and loosely packed assemblies with significant gauche conformations. Acknowledgment. This work was funded by the NSF Garcia MRSEC for Polymers at Engineered interfaces. We thank Polytechnic University for the support of K.V.P.M.S. A.U. thanks the Alexander von Humboldt Foundation for supporting his stay in Heidelberg. LA001252G
