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XPS and FTIR Surface Characterization of TiO2 Particles Used in Polymer Encapsulation Bedri Erdem, Robert A. Hunsicker, Gary W. Simmons, E. David Sudol, Victoria L. Dimonie, and Mohamed S. El-Aasser* Emulsion Polymers Institute and Departments of Chemical Engineering and Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received October 30, 2000. In Final Form: February 13, 2001 The surfaces of hydrophilic (P25) and hydrophobic (T805) TiO2 particles were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to gain a better understanding of the adsorption mechanism of OLOA 370 (polybutene-succinimide pentamine) on TiO2 particles dispersed in styrene monomer prior to miniemulsion encapsulation polymerizations. XPS analysis revealed that both the P25 and T805 TiO2 particles had significant amounts of hydroxyl groups on their surfaces. The XPS results showed that the surface hydroxyl concentration on the hydrophilic (P25) particles was 3.3 OH/nm2, whereas the trimethoxy octyl silane (TMOS)-surface-modified hydrophobic (T805) particles unexpectedly contained 6.6 OH/nm2. This apparent increase in the hydroxyls was attributed to hydrolysis of -OCH3 on the TMOS. The majority of these groups, however, were apparently either not acidic or not accessible to the OLOA 370 in adsorption studies, where the concentration of reactive hydroxyls on the T805 particles was estimated to be 1.8 OH/nm2. FTIR analysis showed the existence of reactive hydroxyl groups on the surfaces of both the hydrophilic and hydrophobic TiO2 particles. Exposure of the particles to ammonia indicated a large reduction in the hydroxyl groups as detected by in situ FTIR measurements. New peaks characteristic of N-H stretching bands indicated strong interactions between the ammonia and hydroxyl groups on the surface of the TiO2 particles.
Introduction The polymer encapsulation of inorganic particles (i.e., TiO2) via miniemulsion polymerization first requires that these be dispersed in the monomer phase (i.e., styrene) in the form of small (approaching the primary particle size) and stable particles. The adsorption of a stabilizer on the inorganic particles can provide steric stabilization preventing interparticle approach and limited aggregation. The adsorption of the stabilizer onto the inorganic particles is strongly dependent upon the type and concentration of reactive sites (i.e., hydroxyls) on the particle surface. For instance, acid-base interactions between polar organic molecules (i.e., resins having functional reactive end groups) and oxide surfaces depend on the nature of the surface hydroxyls on the metal oxide surfaces.1 It has been reported in the literature that a layer of hydroxyl groups covers the outermost surface of an oxide or an oxide film.2,3 As shown in Figure 1A, the clean (unrelaxed) rutile TiO2 surface consists of the surface unit cell which has one 6-fold coordinated Ti atom, one 5-fold coordinated Ti atom, one 2-fold coordinating bridging atom above the truncated plane, and one 3-fold coordinated oxygen. The distance between successive (110) planes is 3.35 Å. The surface of the hydroxylated particles, shown in Figure 1B, is based on the model by Kurtz and Stockbauer.3 Approximately half of the 5-fold coordinated Ti atoms present at the clean surface become bonded to hydroxyl groups from water adsorption and dissociation. Similarly, about half of the 2-fold coordinated bridging O * To whom correspondence should be addressed. (1) Fluck, D. J. Dispersion Stability and Surface Charge Mechanism of Rutile Titanium Dioxide Dispersions in Low Dielectric Medium. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 1993. (2) Stumm, W. Chemistry of Solid-Water Interface; Wiley: New York, 1992; Chapter 2. Bolger, J. C. In Adhesion Aspects of Polymeric Coatings; Mittal, K. L., Ed.; Plenum Press: New York, 1983; p 3. (3) Kurtz R. L.; Stockbauer, T. E. Surf. Sci. 1989, 218, 190.
Figure 1. The clean rutile titanium dioxide (TiO2) (A) surface with the unit cell and the hydroxylated surface (B) showing a saturation coverage of one dissociated water molecule for every two surface unit cells as determined by Kurtz and Stockbauer (ref 3).
atoms at the surface become bonded to water-derived hydrogen. The dissociation of a water molecule at the surface thus results in two hydroxyl groups: one bonded to the previously 5-fold coordinated Ti, which is a basic site, and the other formed by bonding of a proton to the 2-fold coordinated bridging O which is an acidic site. The
10.1021/la0015213 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/03/2001
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unreacted 5-fold Ti sites behave as strong Lewis acid sites on the surface of the particles. Numerous techniques have been employed for the determination of surface hydroxyl concentration on many high surface area bulk oxides. The techniques used include loss upon ignition,4 isotope exchange,5 adsorption isotherms,6 and NMR spectroscopy.7 A number of publications have also been devoted to the IR characterization of titanium dioxide surfaces having different origins.8-14 Simmons and Beard pioneered the quantitative determination of the surface hydroxyl concentration on low specific surface area metal oxides such as TiO2 and Fe2O3 using X-ray photoelectron spectroscopy (XPS),15 which was then extended to other metal oxides such as Al2O3, Cr2O3, Ta2O5, and SiO2.16 XPS is also a powerful technique for studying the acid-base properties of metal oxides (Lewis and Brønsted17,18). The surface reactivities (acidic and basic) of several inorganic particles were also readily estimated by ammonia adsorption during in situ Fourier transform infrared (FTIR) studies.19 Investigations of highpurity materials have revealed both Lewis and Brønsted acid sites on the particles.10,20 In this paper, the surface characterization of two types of TiO2 particles will be reported using XPS and FTIR spectroscopies to determine the presence of hydroxyl groups. According to the manufacturer,21 the adsorption of trimethoxy octyl silane (TMOS) onto the surface of hydrophilic TiO2 particles (P25) produces surface-modified TiO2 particles (T805), which are hydrophobic in nature. Both TiO2 samples were in the form of a powder made up of irregularly shaped micron-size aggregates.22 Adsorption studies of OLOA 370 (polybutene-succinimide pentamine) stabilizer on the surface-modified TiO2 particles showed that these particles behaved as if there were still a significant number of reactive hydroxyl groups left on the surface.22 The motivation behind this study was to investigate the presence of hydroxyl groups on the surface of the hydrophobic (T805) TiO2 particles and, if found, to compare the number of the hydroxyl groups observed on the T805 to the number found on the P25 TiO2 particles. This information is relevant to understanding the adsorption behavior of the stabilizer (OLOA 370) onto the TiO2 particles during their dispersion in organic medium. The degree of dispersion was shown to have a strong impact on the encapsulation efficiencies resulting from subsequent miniemulsion encapsulation polymeriza(4) Morimoto, T.; Shiomi, K.; Tanaka, H. Bull. Chem. Soc. Jpn. 1964, 37, 396. (5) Li, Y.-X.; Klabunde, K. J. Chem. Mater. 1992, 4, 611. (6) McCafferty, E.; Zettlemoyer, A. C. Discuss. Faraday Soc. 1971, 52, 239. (7) Burmudez, V. M. J. Phys. Chem. 1970, 74, 4160. (8) Parfitt, G. D. Prog. Surf. Membr. Sci. 1976, 11, 181. (9) Knozinger, H. Adv. Catal. 1976, 25, 184. (10) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (11) Odenbrand, C. U. I.; Andersson, S. L. T.; Andersson, L. A. H.; Brandin, J. G. M.; Busca, G. J. Catal. 1990, 125, 541. (12) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75 (9), 1216. (13) Tsyganenko, A. A.; Filiminov, V. N. J. Mol. Struct. 1973, 19, 579. (14) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 149, 117. (15) Simmons, G. W.; Beard, B. C. J. Phys. Chem. 1987, 91, 1143. (16) McCafferty, E.; Wightman, J. P. Surf. Interface Anal. 1998, 26, 549. (17) Mullins, W. M.; Averbach, B. L. Surf. Sci. 1988, 206, 29. (18) Watts, J. F.; Gibson, E. M. Int. J. Adhes. Adhes. 1991, 11, 105. (19) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75 (9), 1221. (20) Hermann, M.; Boehm, H. P. Z. Anorg. Allg. Chem. 1969, 73, 368. (21) Materials Safety Data Sheet; Degussa Inc., 1998. (22) Erdem, B.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4419.
Langmuir, Vol. 17, No. 9, 2001 2665 Table 1. Properties of the TiO2 Particles Used in the FTIR and XPS Characterizations and Polymer Encapsulation Studies commercial name Degussa P25
nature of the surface
20% rutile; 80%anatase (hydrophilic)a Degussa T805 trimethoxy octyl silane (hydrophobic)
particle surface size area density nmd m2/gb g/cm3 pHc 50
∼4.1
6.5
∼29
50
4.1
7.1
29
a Aerosil process of the hydrolysis of gaseous titanium tetrachloride: TiCl4 + 2H2 + 2O2 f TiO2 + 4HCl. b Determined by nitrogen adsorption method. c In aqueous dispersion. d Average primary particle size by TEM.
tions.23 Unmodified (P25, Degussa) and TMOS-surfacemodified (T805, Degussa) TiO2 particles were analyzed by XPS. Experimental Procedure XPS. The XPS experiments were performed in an ultrahigh vacuum (UHV) using the Scienta ESCA-300 high-resolution X-ray photoelectron spectrometer (HR-XPS).24 A rotating anode serves to generate an Al KR X-ray beam of 7.6 kW (photon energy of 1486.8 eV). The X-ray beam is monochromatized using seven bent quartz crystals and focused onto the sample resting on an automated manipulator/goniometer. The detector system consists of a 300 mm mean radius hemispherical energy analyzer and a multichannel plate detector and provides an overall energy resolution of 0.27 eV as determined at room temperature by the Fermi level edge of Ag. The properties of the two TiO2 samples are shown in Table 1. The TiO2 powders were dehydrated under vacuum prior to XPS analysis. The dehydrated specimens were then lightly pressed onto individual pieces of tin foil using a hydraulic ram press. The prepared specimens were then attached on flat electron spectroscopy for chemical analysis (ESCA) stubs. The analysis stub was inserted into the fast entry chamber of the ESCA apparatus, where the pressure was reduced to about 1 × 10-6 Torr. The stub was then transferred into the analysis chamber of the ESCA apparatus, where the background pressures were on the order of 1.5 × 10-9 Torr. The surfaces of the pressed TiO2 samples were scraped in situ to expose fresh powder surfaces. Both survey and narrow region scans of Ti 2p, C 1s, Si 2p, and C 1s were conducted at 150 eV pass energy, with an incremental step size of 1 eV for survey scans and 0.05 eV for the narrow scans, and a 0.8 mm slit width. All spectra were taken at polar angle θ ) 0, that is, normal to the surface plane of the tin foil. The TiO2 samples were sufficiently conducting that it was not necessary to supply electrons for charge compensation. FTIR. The surfaces of the TiO2 particles were also investigated by FTIR to determine the presence of the hydroxyl groups and their reactivities. The analysis was carried out under O2/He gas at 120 °C and 1 × 10-4 Torr to remove any water adsorbed on the surface of the particles. Infrared spectra were obtained by pressing the titanium dioxide powder into self-supporting wafers (5-6 mg) and mounting the samples in an in situ FTIR cell (Harrick HTC-100). Spectra were recorded with a BioRad FTS40A FTIR spectrophotometer (DTGS detector) using 250 signalaveraged scans at a resolution of 2 cm-1. All spectra were normalized to 5 mg and smoothed (Savitsky-Golay algorithm, degree 2 with 20 points) to enhance the apparent signal-to-noise ratio. The pretreatment inside the IR cell consisted of first heating the sample to 120 °C under vacuum (10-4-10-5 Torr) followed by cooling to 25 °C for spectrum acquisition. After the original specimens were analyzed, the samples were allowed to interact with ammonia/helium (NH3/He ) 30:70 v/v) to determine the reactivity of the surface hydroxyls. The surface of the particles was saturated via a 33.5 Torr gas mixture for 10 min and then (23) Erdem, B.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4441. (24) Scienta ESCA 300 User’s Manual; Scienta: Uppsala, Sweden.
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Table 2. Location of Binding Energies, fwhm, and Mix (of Lorentzian and Gaussian curves Used in Fitting) for the Core Level Signals of the Hydrophilic (P25) and Hydrophobic (T805) TiO2 Particle Photoemissions Ti eV sample P25 fwhm mix T805 fwhm mix
C eV
Si eV
2p1/2
2p3/2
O 1s eV bulk O2-
459.36 465.28 530.64 1.2 1.12 0.94 285.10 102.55 459.33 465.28 530.61 1.2 1.11 1.0
OH
Si-O-Tia
531.86 1.14 0.40 531.80 532.49 1.14 1.51 0.46 0.2
a The peak also contains a Si-O-Si bond at 532.7 eV and a Si-O-CH3 bond at 532.8 eV.
Figure 3. High-resolution XPS spectra of the Ti 2p peaks of the P25 and T805 TiO2 samples.
Figure 2. XPS survey Al KR photoelectron spectra of the P25 and T805 TiO2 samples. evacuated to remove the ammonia. The sample surfaces were again analyzed to observe any changes in their spectra.
Experimental Results XPS Results and Discussion. Quantitative XPS analysis was performed on the P25 and T805 titanium dioxide particles. Typical survey and high-resolution spectra are presented in Figures 2-6. The observed binding energies and full-widths-at-half-maximum (fwhm) are summarized in Table 2. The survey spectra of the P25 and T805 particles contain the Ti 2p and O 1s peaks of the titanium dioxide (Figure 2). The survey spectrum of the T805 particles also contains C 1s and Si 2p peaks in addition to the Ti 2p and O 1s peaks, confirming the presence of the TMOS surface modifier. The Ti 2p1/2 and Ti 2p3/2 spin-orbital splitting photoelectrons for both samples are located at binding energies of 465.28 and 459.36 eV, respectively, as seen in Figure 3. The peak separation of 5.92 eV between the Ti 2p1/2 and Ti 2p3/2 signals is in excellent agreement with the reported literature values.16 The fwhm of the Ti 2p3/2 signal was 1.2 eV for both samples (Figure 3). The O 1s signal for hydrophilic (P25) and hydrophobic (T805) TiO2 particles is shown in Figure 4, indicating a peak at 530.64 eV and a shoulder located toward the side of higher binding energies. The same fwhm and Lorent-
Figure 4. High-resolution XPS spectra of the O 1s peaks of the P25 and T805 TiO2 samples.
zian/Gaussian mix values were used in the curve resolution of the individual O 1s peaks in the two spectra. The curveresolved O 1s signal of the P25 sample resulted in a second peak located at a binding energy of 531.86 eV. This secondary peak was assigned to OH species on the surface. The locations of the binding energies for these peaks agree well with the reported values for bulk oxide (O2-) and hydroxyl (OH) species as listed in Table 3. The values of the fwhm of the lower and higher binding energy peaks were 1.12 and 1.14 eV, respectively. The difference between the binding energies of the assigned hydroxyl (OH) and oxide (O2-) species was 1.24 eV, which is close to reported differences of 1.5-1.9 eV (see Table 3). The binding energy
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Table 3. Comparison of the Assignment of the Peaks for OH and Bulk O2- with Literature Values reference
bulk O2- eV
OH
difference eV
Fisicaro (ref 29) Sanjines (ref 30) McCafferty (ref 16) Simmons (ref 15) Riakar (ref 31) Erdem et al. (current work)
529.7 530.0 530.6 529.7 529.5 530.62
531.6 531.5 532.4 531.5 531.0 531.86
1.9 1.5 1.8 1.8 1.5 1.24
Table 4. Comparison of the Assignment of the Peaks for Ti and Bulk O2- with Literature Values reference
bulk O2eV
Ti 2p1/2 eV
difference eV
Riakar (ref 31) Sanjines (ref 30) Fisicaro (ref 29) Gao (ref 32) Handbook (ref 33) Erdem et al. (current work)
530.9 530.0 529.6 530.7 531.0 530.61
458.2 458.7 458.5 458.5 458.8 459.33
72.70 71.28 71.15 72.20 71.20 71.28
Table 5. Comparison of the Assignment (eV) of the Peaks, fwhm, and Mix (Lorentzian and Gaussian) Values between Hydrophilic (P25) and Hydrophobic (T805) TiO2 Particles Ti eV sample P25 fwhm mix T805 fwhm mix
C eV
Si eV Ti 2p1/2 Ti 2p3/2 bulk
O 1s eV O2-
459.36 465.28 530.64 1.12 0.94 285.10 102.55 459.33 465.28 530.61 1.11 1.0
OH
Si-O-Ti
531.86 1.14 0.40 531.80 532.49 1.14 1.5093 0.46 0.2
difference of 71.28 eV between the observed peak positions of Ti 2p1/2 and O 1s (oxide) is also in excellent agreement with reported literature values of 72.9-71.2 eV (see Table 4). The ratio of titanium to oxygen in P25 was determined by integrating the areas under the Ti 2p and O 1s peaks and correcting the areas by the respective Scofield photoionization cross sections of the core level photoelectrons.25 The titanium-to-oxygen ratio was 0.53:1, which is close to that expected from the stoichiometry of TiO2. Curve resolution of the O 1s signal of the T805 sample (Figure 4, bottom) indicated the presence of two additional peaks located at 531.86 and 532.49 eV as reported in Table 5. The observed peaks at 530.64 and 531.86 eV were again assigned to bulk oxide (O2-) and hydroxyl (OH) species. The O 1s peak at 532.49 eV is in agreement with the reported O 1s binding energy of the Si-O-Ti species, which reflects the bonding of TMOS to the surface of the TiO2 particles. The fwhm values of the O 1s binding energy peaks were 1.11, 1.14, and 1.51 eV. The Si-O-Ti peak has a slightly larger fwhm, which is attributed to the presence of Si-O-C from unreacted (or unhydrolyzed) OCH3 and from Si-O-Si cross-linking between two neighboring TMOS molecules. The T805 TiO2 particles therefore appear to contain significant amounts of hydroxyls on their surface even though their surface was pretreated with TMOS. The C 1s peak at 285.10 eV signifies the presence of the octyl (C8H17) portion of the TMOS structure. The Si 2p peak at 102.55 eV also reflects the presence of TMOS on the surface of these particles. The titanium-to-oxygen stoichiometric ratio was 0.58:1, slightly higher than expected from the stoichiometry of TiO2. The C 1s signal for the T805 sample is located at a binding energy of 285.10 eV (Figure 5). The Si 2p signal is located at a binding energy of 102.55 eV. The carbon(25) Scofield, J. H. J. Electron Spectrosc. 1978, 8, 129.
Figure 5. High-resolution XPS spectrum of the C 1s peak of the T805 TiO2 sample.
to-silicon stoichiometric ratio was 7.9:1.0, which is close to the expected ratio of carbon-to-silica for TMOS (C8H17Si). It was expected, based on the amounts of stabilizer adsorbed on the two types of TiO2 particles, that the hydrophobic T805 particles would have fewer hydroxyls (“adsorption” sites) compared to the hydrophilic P25 particles because of coverage of the hydroxyl groups by the TMOS surface modifier. The estimation of the number of hydroxyl groups per unit area was made on the basis of the method proposed by Dreiling26 and then developed by Simmons and Beard.15 Using the measured relative intensities of the O 1s photoelectron signals from the hydroxyl and oxide, the hydroxyl concentration of the hydrophilic TiO2 (P25) particles was found to be 3.3 hydroxyl/nm2. This value is reasonable compared with those reported in the literature.6 The O and Ti photoelectrons emitted from the T805 particles experienced isotropic intensity attenuation because of inelastic scattering by the TMOS overlayer, which resulted in the reduced intensities shown in Figure 6 and listed in Table 6. Because O 1s photoelectrons from oxide and hydroxyl have nearly the same kinetic energy, the IOH/Ioxide ratio is not significantly affected by the overlayer. Accordingly, a quantitative comparison of the number of hydroxyl groups on the surface of the P25 and T805 TiO2 particles was possible. In contrast to expectations, the O 1s region indicated a greater amount of hydroxyls on the surface of the modified TiO2 (T805) particles (IOH/Ioxide ) 0.154 or 6.6 OH/nm2) compared to the P25 TiO2 particles (IOH/Ioxide ) 0.077 or 3.3 OH/nm2). This doubling in the amount of hydroxyl groups is attributed to the hydrolysis of the -OCH3 groups of the TMOS (i.e., two hydroxyl groups or silanols generated for each TMOS interacting with a surface hydroxyl) as reported in the literature.27 The silanols of the hydrolyzed TMOS are either not sufficiently acidic or are not accessible for the interaction with the amine end groups of the OLOA 370 as determined from adsorption studies.28 In the latter (26) Dreiling, M. J. Surf. Sci. 1978, 71, 231. (27) Angst, D. L. The Adsorption of Water by Thin Films of SiO2 on Silicon and at the SiO2 Organosilane Interface. Ph.D. Dissertation, Lehigh University: Bethlehem, PA, 1992. (28) Erdem, B. Encapsulation of Inorganic Particles via Miniemulsion Polymerization. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 2000. (29) Fisicaro, E.; Visca, M.; Garbassi, F.; Ceresa, E. M. J. Phys. Chem. 1970, 74, 4160. (30) Sanjines, R.; Tang, H.; Berger, F.; Gozzo, F.; Margaritonto, G.; Levy, F. J. Appl. Phys. 1994, 75 (6), 2945. (31) Riakar, G. N.; Gregory, J. C.; Ong, J. L.; Lucas, L. C.; Lemons, J. E.; Kawahara, D.; Nakamura, M. J. Vac. Sci. Technol. 1995, 13 (5), 2633. (32) Gao, X.; Simon, R. B.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 5653.
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Figure 7. FTIR spectra showing the presence of hydroxyl groups on the hydrophilic TiO2 particles (P25) before (solid line) and after (dashed line) exposure to ammonia.
Figure 6. Comparison of high-resolution XPS spectra of O 1s (top) and Ti 2p (bottom) for the hydrophilic (P25) and hydrophobic (T805) TiO2 particles. Table 6. Quantification of the Surface Species on the Hydrophilic (P25) and Hydrophobic (T805) TiO2 Particles by XPS O 1s sample O2- % OH % Ti-O-Si % Ti 2p % Si 2p % C 1s % P25 T805
60.16 38.98
4.65 6.02
3.989
35.19 20.49
3.44
27.08
studies, the area occupied by the adsorbed OLOA 370 on the hydrophilic TiO2 particles at saturation was found to be 0.24-0.26 nm2/molecule, which corresponds to about 4 molecules of OLOA 370 per nm2. Comparison of the latter with the number of hydroxyls per nm2 as determined by the XPS measurements (3.3 OH/nm2) on the hydrophilic particles suggests that each hydroxyl group is anchoring about one OLOA 370 molecule. Using the relative numbers of the OLOA 370 molecules irreversibly adsorbed on the surface of the hydrophilic TiO2 particles (1.35 × 1020 molecules/g TiO2) and on the surface of the hydrophobic TiO2 particles (5.59 × 1019 molecules/g TiO2), the number of acidic hydroxyl groups on the surface of the hydrophobic TiO2 was estimated to be 1.8 OH/nm2.28 Although the hydrolyzed TMOS increases the number of hydroxyl groups, silanols, at the TMOS-TiO2 interface, as determined by XPS, the surface modification nevertheless reduces the number of accessible acidic groups by nearly a factor of 2. FTIR Results and Discussion. The IR spectrum of the pure P25 TiO2 specimen shown in Figure 7 shows ν(OH) stretching bands in the region between 3740 and 3300 cm -1. The sharp bands observed between 3630 and 3680 cm-1 and the broader bands at 3500 and 3420 cm-1 have been reported in the literature9 and are attributed to hydroxyl groups on different sites and to varying interactions between hydroxyl groups on TiO2. Ammonia readily interacts with the most acidic hydroxyl groups (33) Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; Physical Electronics: Eden Prairie, MN, 1992.
Figure 8. FTIR spectra showing the presence of hydroxyl groups on the hydrophobic TiO2 particles (T805) before (solid line) and their absence after (dashed line) exposure to ammonia.
and remains on the surface of the particles even after evacuation of the reaction chamber as evidenced by the significant reduction of the IR intensity of the ν(OH) band upon exposure of the pure P25 TiO2 to ammonia and the concomitant appearance of ν(NH) stretching bands at 3346 and 3400 cm-1. The intensities of the bands at 3500 and 3420 cm-1, which have been attributed to a monodentate hydroxyl group and a bridging hydroxyl group, respectively, were reduced completely by the ammonia. The residual intensity of the band in the 3630-3680 cm-1 region of the spectrum is attributed to weak Lewis and Brønsted sites from which the ammonia desorbed after evacuation of the chamber.10 The IR spectrum of the surface-modified hydrophobic T805 TiO2 specimen shows ν(CH) stretching modes from the -CH2- and CH3- of the TMOS in the region of 3000 and 2800 cm-1 (Figure 8). Although the specimen is hydrophobic, the broad band appearing from 3420 to 3680 cm-1 indicates the presence of hydroxyl groups. In contrast to unmodified P25 TiO2, however, the absence of the sharp bands in the region of 3630-3680 cm-1 suggests that the hydroxyl groups represented by these bands are completely modified by the TMOS via surface modification. The band for the bridging hydroxyl group, 3420 cm-1, is still evident, but the band for the monodentate hydroxyl group, 3500 cm-1, is not resolved. The integrated intensity of the ν(OH) band, between 3740 and 3300 cm-1, is greater for modified than for unmodified TiO2, in agreement with the XPS results. The silanol ν(OH) band of TMOS is centered near 3500 cm-1 and explains the relatively high IR intensity in this region compared to the spectrum of the unmodified TiO2. Exposure of modified TiO2 to ammonia and subsequent evacuation caused a reduction in intensity over the entire
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ν(OH) stretching band, 3740 and 3300 cm-1, and the appearance of the ν(NH) stretching band, 3345 and 3400 cm-1. The changes in intensity of the ν(OH) band are not distinct enough to estimate the relative extent of reaction of ammonia with the silanols of adsorbed TMOS and the accessible hydroxyl groups on the modified TiO2 surface. The comparable intensities of the ν(NH) bands upon ammonia adsorption on both the modified and unmodified TiO2 suggests that some of the silanols, indeed, react with ammonia. An estimation of the free hydroxyl groups on modified TiO2 by ammonia is ambiguous owing to the presence of TMOS silanols, which also react with ammonia. Because the larger OLOA 370 molecule would have limited access to these silanols, OLOA 370 adsorption is a more reliable method for determining the number of free hydroxyl groups. Conclusions Surface characterization of the hydrophilic (P25) and hydrophobic (T805) TiO2 particles by XPS and FTIR showed that both particles have hydroxyl groups on their surfaces. By XPS, the number of hydroxyls per unit area was determined to be 3.3 OH/nm2 on the hydrophilic P25 TiO2 particles and 6.6 OH/nm2 on the hydrophobic T805 TiO2 particles. The latter was unexpected based on prior
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adsorption studies and was attributed to the hydrolysis of the -OCH3 groups on the TMOS-modified hydrophobic particles. The majority of these hydroxyls represent silanols which are either not acidic or are not accessible for interaction with the adsorbing OLOA 370 stabilizer (containing amine groups) used in their dispersion in organic media. On the basis of the adsorption studies, the number of reactive hydroxyls on the surface of TMOSmodified T805 TiO2 particles was estimated to be 1.8 OH/ nm2. FTIR characterization confirmed the presence of hydroxyl groups on the surfaces of the P25 and T805 TiO2 particles. Upon exposure to ammonia in the FTIR interaction chamber, the area under the peaks representing the hydroxyls was greatly reduced. The concomitant appearance of new N-H bands at lower frequencies (3345 and 3400 cm-1) indicated that the hydroxyls had indeed reacted with the ammonia. Acknowledgment. The authors acknowledge the helpful discussions and technical assistance of Dr. Alfred Miller in the XPS studies and Dr. Lloyd Burcham in the FTIR studies. LA0015213
