- Email: [email protected]
CVD-Titania on Fumed Silica Substrate
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
198, 141–156 (1998)
CS975270
CVD-Titania on Fumed Silica Substrate V. M. Gun’ko,* 1 V. I. Zarko,* V. V. Turov,* R. Leboda,† E. Chibowski,‡ L. Holysz,‡ E. M. Pakhlov,* E. F. Voronin,* V. V. Dudnik,* and Yu. I. Gornikov* *Institute of Surface Chemistry, 31 Prospect Nauki, 252022 Kiev, Ukraine; and †Department of Chemical Physics, and ‡Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received July 22, 1997; accepted October 24, 1997
A titania (anatase) was synthesized on a fumed silica substrate with the concentration of TiO2 (CTiO2 ) from 0.6 to 32.4 wt% using a chemical vapor deposition (CVD) technique. All studied characteristics of CVD-titania/silica (CVD-TS), such as abundance of active surface sites, amount of adsorbed water, particle size distribution, electrophoretic mobility, and free energy changes of interfacial water, depend nonlinearly on CTiO2 due to alterations in the structure and distribution of the titania phase in TS with increasing CTiO2 . The separated anatase phase is well defined in CVD-TS at CTiO2 É 5 wt%, which is lower than that for fumed TS (about 9 wt%). The anatase phase transition to rutile is inhibited by the silica matrix and the corresponding temperature increases. At relatively low temperatures (below 400 K) of CVDTS treatment, the amount of Brønsted acid sites is higher than that for fumed TS with analogous concentration of titania, but for T above 600 K, the opposite relationship is observed. The effective particle diameter for CVD-TS suspended in water decreases faster with increasing CTiO2 than for fumed TS. A particle size distribution in an aqueous suspension of TS differs from that for an original fumed silica suspension and the main contribution is given by the particles with sizes closely related to aggregates of primary silica particles. q 1998 Academic Press Key Words: CVD-titania/silica; particle structure; anatase on silica; electrophoretic mobility; particle size distribution; 1H NMR, interface modeling.
INTRODUCTION
Titania/silica (TS) materials with various compositions are widely used as fillers, pigments, catalysts, etc. (1–18). The structure and properties of TS strongly depend on the synthetic technique used (sol–gel, high-temperature hydrolysis, chemical vapor deposition (CVD), etc.). Titania/silica can possess (a) mixed structure in both the bulk and the surface layer as in fumed TS (8, 19) or (b) separated phases of TiO2 and SiO2 with a clear-cut phase boundary when one of these oxides is formed (using, e.g., CVD technique) on another which is a substrate. In the latter case two variants 1
To whom correspondence should be addressed. Fax: 380 44 264 0446.
are possible: the second phase forms (a) a continuous layer or (b) separated clusters and individual particles. Earlier we studied fumed TS (8, 10, 11, 19–22). The properties of CVD-TS and fumed TS differ due to alterations in the titania phase distribution and in the structure of the interfaces, where GSi{O{TiG or GSi{O(H){ TiG connections are formed. GSi{O(H){ TiG (Brønsted acid sites (B-sites) possessing catalytic activity (12, 14)) and GSi{O{TiG bonds were observed in fumed TS obtained by combustion of SiCl4 and TiCl4 in an oxygen–hydrogen flame (above 1300 K) (8, 19) as well as after TiCl4 interaction with a silica surface in vacuum (9). Titania (anatase) is used as a durable catalyst for decomposition of chemicals in wastewater under UV irradiation because of its sensitizer properties (14b). TS possessing this feature of titania can, however, possess stronger B-sites than original anatase. Also, TS and TS/Al2O3 are used as pigments in which the diameter (def ) of particles is approximately 0.3 mm (7, 23) or as additives for improvement of the polymer film properties (22). Silica and alumina applied for the coverage of the titania pigment surface can be deposited by the sol-gel method or CVD technique (24). The sol–gel technique allows to obtain relatively ‘‘smooth’’ coverage (25), but this route is expensive and give much wastewater. Some authors assumed that the CVD technique can give a continuous film with titania on oxide supports (26). However, features of formed films depend on the reactivity of the substrate and the new phase, their structures, etc., and in the case of higher reactivity of the second phase, such a continuous film can be absent due to preferable interaction of new portions of reagents with the formed clusters of the second phase. Therefore, it is of interest to study the structure of CVD-TS and its properties in comparison with those of fumed TS for various titania concentrations (CTiO2 ). EXPERIMENTAL
Materials. Titania synthesis was performed by the CVD method using fumed silica (Aerosil, 99.9%, ‘‘Chlorovinyl,’’ Kalush, Ukraine; specific surface area (S) about 270 m2 g 01
141
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
0021-9797/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
coida
142
GUN’KO ET AL.
TABLE 1 Some Properties of Synthesized CVD-Titania on Silica CTiO2
W a (g 101)
S (m2g01)
Def (mm)
pHb
PDc
nd
0 0.6 1.7 2.8 5 7.5 17 20 22 28 32.4
57.7 56.0 55.3 53.5 55.2 54.0 62.6 65.0 69.0 79.1 88.5
269 269 247 222 254 261 217 200 187 182 154
15.04 10.81 7.80 0.69 0.39 0.37 0.61 0.54 0.35 0.26 0.24
6.38 6.54 6.39 6.28 5.80 5.57 5.47 5.87 5.71 5.57 5.98
0.503 0.407 0.425 0.231 0.519 0.256 0.330 0.331 0.092 0.173 0.102
1 1 1 1 1 2 3 4 5 6 8
a
Apparent density. pH values for aqueous suspensions of 0.2 g of the solids per 1 l of deionized distilled water. c PD is polydispersity as a measure of the nonuniformity of the particle size distribution (PD õ 0.02 corresponds to a monodisperse distribution). d Number of synthetic cycles. b
and OH group concentration 0.6 mmol g 01 under synthetic conditions) as a substrate. A sample of fumed silica was placed in a glass reactor (volume 2 l) equipped with a mixer. The reactor was blown with dry air at 423 K for 3 h, then TiCl4 vapor was supplied and reacted at 423 K for 0.5 h; thereupon, the sample was blanketed by dry air at the reaction temperature for 0.5 h, then water vapor was used for hydrolysis of residual Ti-Cl bonds under the same conditions. The required amounts of TiCl4 and H2O were calculated according to the scheme: { GSiOH}n / TiCl4 r { GSiO}nTiCl40n / nHCl [1] { GSiO}nTiCl40n / (4 0 n)(H2O) r { GSiO}nTi(OH)40n / (4 0 n)HCl.
[2]
A series of samples with CTiO2 varied in the 0.6–32.4 wt% range (Table 1) was been synthesized using the TiCl4 chemisorption–hydrolysis cycles [1] and [2] from 1 to 8. The TiO2 concentration was determined by chemical analysis for Ti. The specific surface area (Table 1) was calculated using nitrogen adsorption at 78 K. X-ray study. The XRD study was performed using DRON-UM1 and DRON-4 (Russia) difractometers with CoKa radiation (30 kV, 20 mA, filtered by a Fe thin film). A monochromator with quartz monocrystal was placed after a sample. Quartz powder was used as a primary standard. Infrared spectroscopy. The IR spectra (400–4000 cm01 ) were recorded by a UR-20 (Germany) spectrophotometer in air and vacuum (10 04 Torr). In the first case, the samples were suspended in Vaseline and placed between KRS-5 glasses. In the second, the IR studies were performed
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
in special optical glass vessels using samples (8 1 28 mm) weighing 8–10 mg. Adsorption of water. Water adsorption on oxide samples (weighing 50–100 mg, pressed at approximately 10 4 Torr) was studied using an adsorption apparatus with a McBain– Bark scale. After evacuation to 10 03 Torr for 1–2 h, samples were heated at 613 K for 3–4 h to a constant weight, then cooled to 298.13 { 0.2 K, and adsorption of water vapor was studied at pressure (P) varied in the 0.06–0.95 P/Ps range. The measurement accuracy was 1 1 10 03 mg with relative mean error {5%. Optical spectroscopy. Dimethylaminoazobenzene (nDMAAB, pKa Å 3.3) was chosen as a color indicator for a study of acidic surface sites by optical spectroscopy. The diffuse reflection spectra of adsorbed DMAAB have been recorded using a SF-18 (Russia) spectrophotometer, then converted to the absorption spectra according to the Kubelka–Munk formula (27). The n-DMAAB adsorption from the gas phase on samples previously evacuated to 10 04 Torr and then heated in special optical glass vessels at defined temperatures for 1 h, was performed at 338 { 5 K for 2–4 h. The assignment of the DMAAB absorption bands has been done by the analogy to the spectra for this substance in neutral and acidic solutions (24). Five absorption bands of DMAAB adsorbed on mixed oxides are possible. They correspond to (1) 430–460 nm, physisorbed DMAAB (dDMAAB); (2) 480–490 nm, hydrogen-bonded complex (H-DMAAB); (3) 520–545 nm, complexes with H / transferred from the B-sites to DMAAD (H / -DMAAB); (4) 555–560 nm, complexes with the Lewis acid sites (LDMAAB); (5) 605–615 nm, dimers of the adsorbed DMAAB (DM-DMAAB) (19, 28, 29). Electrophoresis. Electrophoretic (30) and multimodal particle size distribution studies were performed on a Zeta Plus zeta potential apparatus (Brookhaven Instruments). Deionized distilled water (pH 6.95) and 0.2 g of the solids per liter of the water were used for suspension preparation in an ultrasonic bath for 2 h. The pH values were adjusted by addition of 0.1 M HCl or NaOH solutions. The pH values of the suspensions of oxides were measured by an OP-208/1 (Hungary) precision digital pH-meter. The average effective diameter (Def ) corresponding to the hydrodynamic diameter, i.e., the particle diameter plus an electrical double layer (EDL) thickness, and the polydispersity of the particles in aqueous suspensions were obtained by photon correlation spectroscopy using a Brookhaven MAS OPTION particle sizer (accuracy of {1–2% with monodisperse samples; repeatability of {1–2% with dust-free samples). 1 H NMR. The 1H NMR spectra were obtained by a highresolution WP-100 SY (Bruker) NMR spectrometer with a bandwidth of 50 kHz. Relative mean errors were {10% for signal intensity and {1 K for temperature. The amount of interfacial unfrozen water (CH2O ) in aqueous suspensions of
coida
CVD-TITANIA
143
oxides frozen at 200 õ T õ 273 K was estimated by comparison of its signal intensity (I) with that for water adsorbed on oxide powder at the gas phase boundary using a calibrated function I Å f (CH2O ) (31). The free energy changes ( DG) for the interfacial water layer were calculated using the tabulated dependence of the free energy of ice ( Gi ) on temperature (31, 32). We believe that water is frozen at the interfaces when G Å Gi and the value of DG Å DGi Å GiÉTÅ273K 0 Gi (T) corresponds to a decrease in the free energy due to water interaction with the solid surface (31, 32b). A capillary effect is practically absent upon water adsorption from air onto nonporous materials such as fumed oxides and the DG(CH2O ) function can be used for determination of a radial dependence of the free energy changes upon water adsorption from air on a layer thickness as a few molecular layers can be adsorbed. Assuming that an area occupied by a water molecule equals 0.09 nm2 , we obtained the DG(H) function, where H is the thickness of the unfrozen water layer in terms of the number of statistical monolayers of water (one layer thickness corresponds to 0.3 nm). RESULTS AND DISCUSSION
Structure of CVD-Titania/Silica The titania phase is not uniformly deposited on the silica surface in CVD-TS as individual clusters or patches with a crystalline structure are formed already for small CTiO2 ; i.e., a continuous titania layer is not formed. Inasmuch as according to the XRD data (Fig. 1a), the crystalline structure of anatase in CVD-TS appears for CTiO2 at about 3 wt% (for fumed TS, such process occurs for greater CTiO2 (19)) which is lower than the amount needed for monolayer coverage (about 17 wt% for silica with S Å 270 m2g 01 and d É 10 nm). Despite the clear-cut structure of anatase observed for CTiO2 ¢ 5 wt%, the rutile phase is not observed even for CTiO2 Å 32.4 wt% (Fig. 1b). This could be explained by the relatively low temperature of the CVD-TiO2 synthesis. However, the calcination of CVD-TS with CTiO2 Å 34.2 wt% at T É 1100 K (which is higher than the temperature for anatase phase transition to rutile (Tph )) for 2 h does not change the anatase structure (Fig. 1b). Consequently, this phase transition for CVD-TS is inhibited by the silica matrix. If we assume that initial tight contact between CVD-titania and silica substrate looks like the crystobalite–anatase interface (Fig. 2), then the formation of GTi{O{SiG bonds (5– 6 bonds per nm2 of contact area) leads to great distortions of the surface layers of the titania and silica lattices (Fig. 2, at the TS interface Si{O and Ti{O bonds can be elongated up to 0.177 nm and 0.210 nm, respectively; angles OTiO, OSiO, TiOTi, and SiOSi varying in the 3–207 range from the values for crystalline structures of anatase and crystobalite) increasing the probability of hydrolysis of these interfacial bridges due to the high exothermicity of such a process (19).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 1. X-ray data for CVD-TS at (a) CTiO2 Å 0.6 (1), 1.7 (2), 2.8 (3), 3.3 (4), 5 (5), and 7.5 wt% (6); (b) samples with CTiO2 Å 17 (1, 2) and 32.4 (3, 4) wt%; (1, 3) original samples and (2, 4) heated at 1100 K for 2 h. All peaks correspond to anatase.
On the other hand, the Tph shift is indirect evidence for the existence of bonding interaction between the silica and titania phases even after the interaction with water vapor. This may be due to residual GSi{O{TiG bonds formed during the CVD-TiO2 synthesis or to small silica particles embedded in the titania particles or caused by partial fusion of the titania and silica phases on sample heating to high temperatures. An analogous result on prevention of the phase transition of anatase to rutile was obtained for similar CVDTS materials (16). If a substrate structure corresponds to a deposited phase that phase transition temperature can be lower than for the individual oxide (33); however, crystallization of amorphous titania upon annealing can induce an increase in stress (34). Consequently, growth of crystalline anatase on the silica support and its annealing can increase stress at the interface that promotes its hydrolysis in air.
coida
144
GUN’KO ET AL.
FIG. 2. Model of the CVD-TS interface between the clusters of anatase and crystobalite used in the MM2 calculations; (a) initial crystalline structure and (b) the structure after full relaxation of the cluster geometry by MM2 method.
A band nSiO at 940–960 cm01 in the IR spectra of TS (Figs. 3 and 4) is linked to the GSi{O{TiG bridges (8, 20). Such SiO-stretching vibrations were observed for fumed TS (Fig. 3, curve 3), but they are practically absent for CVD-TS after its contact with water vapor (8, 9, 20), as Si{O{Ti bonds can be easily hydrolyzed, as well as other Si{O{ M (M x Si) bonds (35). Therefore, the number of such bonds in CVD-TS can be too low for observation
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
using IR spectroscopy. A small absorption in this region for parent silica (Fig. 3, curve 1) may be caused by water molecules bound to the surface as thermoevacuation leads to its disappearance for both silica (Fig. 4, curve 1 and Ref. (36)) and CVD-TS (Fig. 3, curve 4). We could assume that the calcination of CVD-TS at high temperatures causes an increase in the GSi{O{TiG number due to partial fusion of the particles. Therefore, we recorded the IR spectrum
coida
CVD-TITANIA
145
O(H){ Ti bonds in CVD-TS is too small to observe using IR spectroscopy, but optical spectroscopy of adsorbed color indicators is more sensitive to find the corresponding sites. Active Surface Sites of CVD-TS
FIG. 3. IR spectra of (1) fumed silica; (2) CVD-TS at CTiO2 Å 22 wt%; (3) fumed TS at CTiO2 Å 20 wt%; (4) CVD-TS sample after pretreatment in air at 1170 K for 7 h.
of CVD-TS heated at 1170 K over 7 h, but the band at 960 cm01 does not appear (Fig. 3, curve 4). However, the GSi{O{TiG bridges are observed upon reaction of TiCl4 with silica in vacuum (Fig. 4, curve 2, band at 940 cm01 ). After interaction of hexamethyldisiloxane with the surface, this band intensity grows due to the increased number of GSi{O{TiG bonds and the maximum shifts to 950 cm01 (Fig. 4, curve 3). The interaction of this sample with water vapor (at room temperature for 20 min and P/ Ps Å 0.3) magnifies the band intensity of SiO{H at 3750 cm01 , but the band at 950 cm01 disappears (Fig. 4). Additionally, a very broad (2800–3700 cm01 ) intensive band linked to adsorbed water is observed. Typically, only a small absorption appears in this region upon water adsorption on dehydrated fumed silica under such conditions (37); i.e., the surface formed via TiCl4 chemisorption on silica possesses a high adsorption ability for water. However, after the calcination of this sample, water adsorption is only slightly differs from adsorption on parent silica; i.e., changes in the TS structure occurs. Thus, the number of Si{O{Ti and Si{-
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
The comparison of the integral intensity of H / -DMAAB bands for fumed TS and CVD-TS shows that slightly heated samples differ more greatly (Fig. 5, curves 1 and 2) than after heating at higher temperatures (curves 3 and 4); i.e., the B-sites on CVD-TS are more sensitive to heating than that on fumed TS due to the alteration in the TiO2{SiO2 interfaces. The corresponding differences were observed for molecular and associative desorption of water from these oxides, i.e., upon removal of the B-sites (21). An increase in CTiO2 for CVD-TS reduces the DMAAB spectra intensity with elevating pretreatment temperature (Tp ) (Figs. 6 and 7). However, the relative contribution of d-DMAAB (Fig. 6, curves 1) and H-DMAAB (curves 2) increases with growing CTiO2 due to easy desorption of water from TS. At the same Tp , increasing CTiO2 magnifies the dehydroxylated (titania phase and interface) and dehydrated (silica phase) surface area (21), where DMAAB can be physisorbed (d-DMAAB) or forms the hydrogen bonds (HDMAAB). The relative contribution of H / -DMAAB complexes decreases with increasing CTiO2 (Fig. 6, curve 3, and Fig. 7a), but the L-DMAAB band intensity depends less strongly on Tp (Fig. 7b). Typically, intensity of the H / -DMAAB band decreases with increasing Tp (Figs. 6 and 7) except for CVD-TS with CTiO2 Å 17 wt%. The main fraction of molecular adsorbed water and a significant part of dissociatively adsorbed water can be desorbed from titania or TS at T õ 550 K (21). This is in good agreement with the strong decrease in H / DMAAB intensity at Tp ° 525 K (Fig. 7a). Adsorbed intact water can increase the stabilization of H / -DMAAB complexes (i.e., removal of intact water can decrease the possibility of the H / -DMAAB complex formation). A verification of this assumption is the small intensity of L-DMAAB (Fig. 7b), as associative desorption of water must be accompanied by removal of the B-sites (i.e., significant decrease in H / -DMAAB) and by formation of L-sites with increasing Tp . However, the decrease in H / -DMAAB band intensity is not accompanied by growth of L-DMAAB except for CVD-TS with CTiO2 Å 1.7 wt% (Fig. 7b, curve 1) when the titania phase is distributed as small clusters and crystalline anatase is not detected by XRD (Fig. 1a). Consequently, CVD-TS possesses low Lewis acidity, similarly to fumed titania, but fumed TS possesses higher acidity (19) than CVD-TS. A low DM-DMAAB band was observed only for CVD-TS with CTiO2 Å 17 wt% at Tp Å 703 K. The observed differences in acidity of CVD-TS and fumed TS can show up in the water adsorption.
coida
146
GUN’KO ET AL.
FIG. 4. IR spectra of (1) fumed silica thermoevacuated at 873 K; then after (2) reaction with TiCl4 at 473 K, (3) reaction with hexamethyldisiloxane at 573 K, and (4) adsorption of water at P/Ps Å 0.3 for 20 min.
Water Adsorption 2
The amount of adsorbed intact water per nm (is smaller on CVD-TS) than on fumed ST, but the amount of water dissociatively adsorbed on CVD-TS (for the same CTiO2 ) is greater due to the hydrolysis of the Si{O{Ti bridges between the titania phase and the silica substrate (21). These results are in agreement with the isotherms of water vapor adsorption on fumed TS and CVD-TS (Fig. 8). Also, the
FIG. 5. Dependence of integral intensity of the H / -DMAAB band on CTiO2 for CVD-TS (1 and 4) and fumed TS (2 and 3) at pretreatment temperatures (1) 353, (2) 373, and (3 and 4) 623 K.
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
amount of water adsorbed on CVD-TS depends slightly on CTiO2 due to formation of large titania particles (having relatively small S) and weak influence of the interfaces on the intact water adsorption. This conclusion is drawn because the isotherms for CVD-TS are similar to the isotherm for fumed silica used as a substrate for CVD-TiO2 , but water adsorption on fumed TS depends more strongly on CTiO2 and this dependence is nonlinear as well as for CVD-TS (Fig. 8). The higher integral acidity of fumed TS causes stronger water adsorption than by CVD-TS (due to a deeper decrease in the free energy of the interfacial water (8)). However, the local acidity of Si{O(H){ Ti or incompletely O-coordinated Ti (Lewis acid site) for fumed TS and CVD-TS can differ only slightly as the structures of the B- or L-sites are similar for these mixed oxides. Consequently, such an alteration in water adsorption is due to the difference in the interface structures and the concentrations of strong acidic sites. The small number of B-sites on CVD-TS is specified by hydrolysis not only of Si{O{Ti but also of Si{O(H){ Ti bonds and the content of dissociatively adsorbed water on CVD-TS is higher than that on fumed ST. These features of fumed TS and CVD-TS should influence the characteristics of their particles in aqueous suspensions. CVD-TS Particle Size Distribution in Aqueous Suspensions The specific surface area of CVD-TS decreases by half with increasing CTiO2 to 32.4 wt% (Table 1), which corre-
coida
CVD-TITANIA
147
FIG. 6. Optical spectra of DMAAB adsorbed on CVD-TS surface at CTiO2 Å 5 (a, c, e, g, and i) and 22 wt% (b, d, f, h, and j) for Tp Å 353 (a and b), 423 (c and d), 523 (e and f), 623 (g and h), and 703 K (i and j); bands: (1) d-DMAAB, (2) H-DMAAB, (3) H / -DMAAB, (4) L-DMAAB.
AID
JCIS 5270
/
6g39h$5270
01-30-98 10:52:22
coida
148
GUN’KO ET AL.
FIG. 7. Dependence of integral intensity of (a) H / -DMAAB and (b) L-DMAAB for CVD-TS on Tp at CTiO2 Å (1) 1.7, (2) 5, (3) 17, and (4) 22 wt%.
sponds to growth in the average size of the particles upon CVD-TiO2 synthesis. However, the Def value for CVD-TS is smaller than that for original fumed silica and decreases as CTiO2 increases; therewith, the Def diminution for CVDTS occurs faster than that for fumed TS (Fig. 9). Such alterations in the particle dispersity for CVD-TS (Table 1) can be explained by taking into account the complex structure of fumed oxides (19, 38) and the changes upon CVDTiO2 synthesis. For example, the primary spherical particles of fumed silica (diameter about 10 nm) form branched stable aggregates (0.1–0.2 mm, fractal dimension approximately 2.5, and apparent density about 30% of specific density) via the GSi{O{SiG connections due to sticking and reaction at relatively high temperatures. At lower temperatures, aggregates form loose agglomerates (above 5 mm, fractal dimension approximately 2.1, and apparent density about 1–3% of specific density) via the hydrogen-bonding
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 8. Adsorption isotherms of water vapor on (a) fumed TS at CTiO2 Å (1) 29, (2) 20, (3) 36, (4) 100—original titania, (5) 14, (6) 9, and (7) 0 wt%—fumed silica; (b) CVD-TS at CTiO2 Å (1) 33, (2) 17 wt%, (3) 0—fumed silica, (4) 20, (5) 2.8, and (6) 10 wt%.
FIG. 9. Dependence of Def on the concentration of titania for aqueous suspensions of CVD-TS (1) and fumed TS (2).
coida
CVD-TITANIA
and electrostatic interactions (38). More stable agglomerates can be formed upon long-term storage of fumed silica and such agglomerates (above 1 mm) can remain in aqueous suspension even after ultrasonic treatment (39). However, freshly prepared fumed silica possesses a low thickening ability in polar liquids (38), which corresponds to the decomposition of agglomerates formed in air. The Def value strongly decreases with increasing CTiO2 in TS (Fig. 9); however, for fumed alumina/silica, the opposite effect is observed (39, 40). The modification of fumed silica by organosilicon compounds also gives a Def value of about mean aggregate size (39). Consequently, a decrease in Def for CVD-TS is due to partial decomposition of the agglomerates upon CVD-TiO2 synthesis for small CTiO2 , and their entire disintegration is observed for CTiO2 Å 5 wt% (Fig. 10c, all particles with d õ 0.3 mm) when a well-defined individual TiO2 phase is formed (Fig. 1a). Break-up of agglomerates can be caused by GSiOH substitution of GSiOTiCl3 at the first stage of titania phase formation, which decreases the bonding interaction between aggregates. However, agglomerates are observed in suspensions of TS at different
149
CTiO2 ; for small CTiO2 , they are residual agglomerates of silica (Figs. 10a and 10b) and for CTiO2 ú 5 wt% (Figs. 10d and 10e), the formation of agglomerates can occur upon CVD-TiO2 synthesis or due to aggregate interaction in aqueous suspensions. However, for CTiO2 Å 32.4 wt%, only aggregates are observed (Fig. 10f). The TiO2 phase at CTiO2 Å 5 wt% can be present in CVDTS as separated particles with effective diameter (def ) about 0.08 mm (Fig. 10). The size of these particles increases with increasing CTiO2 . Considering that for CTiO2 Å 5 wt% def Å 0.08 mm, and assuming that an increase in CTiO2 leads to the growth only of already formed individual titania particles (in multicycle synthesis), then for CTiO2 Å 32.4 wt%, def of the titania particles should be about 0.15 mm. However, for CTiO2 Å 32.4 wt%, nearly a monodisperse distribution of the particles, with Def Å 0.237 mm, is observed and particles with def below 0.18 mn are not found (Fig. 10f). This might be a consequence of the fact that a fraction of small silica particles (e.g., primary particles or small aggregates) can be embedded into the larger titania particles or the population of individual titania particles decreases as CTiO2 grows due
FIG. 10. Particle size distribution (def ) in aqueous suspensions of CVD-TS at CTiO2 Å 0.6 (a), 2.8 (b), 5 (c), 7.5 (d), 17 (e), and 32.4 wt% (f ).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
coida
150
GUN’KO ET AL.
FIG. 11. Dependence of the effective diameter of CVD-TS particles in aqueous suspensions on pH at CTiO2 Å 28 wt%.
to their sticking together. The def values for individual titania particles in the 7.5–32.4 wt% range of CTiO2 mainly correspond to the size of silica aggregates. However, part of the titania phase can be formed as small clusters embedded into the structure of fractal silica aggregates; otherwise the CTiO2 effect on the CVD-TS properties should be linear. A small peak of Def at CTiO2 Å 17 wt% (Fig. 9, curve 1) can be due to increased synthetic cycle number (Table 1). The particle size distribution for CVD-TS at different pH corresponds to the agglomerate decomposition for pH about the isoelectric points (IEP) for silica or anatase (Fig. 11), where the surface charges ( s ) of the corresponding particular phases are close to zero and the EDL thickness is large (41); i.e., screening of the surface charges by the ions from a dense part of the EDL is weak and the particles can repel. For pH Å 2.5–5.6, the surface charges of the silica and titania phases are opposite, which leads to a strong attraction between them and, hence, Def increases due to particle agglomeration. The mechanism of CVD-TS particle agglomeration for pH ú 6 (Fig. 11) can be explained by attraction between the silica surface ( s õ 0) and some faces of anatase, on which a fraction of strong basic sites (GTi{OH) can have a positive charge due to the reaction GTi{OH r GTi / / OH 0
or GTi{OH / H / r GTiOH 2/ . Silica and anatase are characterized by different dependencies of the surface charges (potentials) on pH, which influences the agglomeration of the CVD-TS particles. The original aqueous suspensions of CVD-TS have natural pH about 6 (Table 1), which is close to IEPTiO2 , but under the same conditions, the silica surface has a negative charge. In this case, the EDL thickness equals approximately 100 nm
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
and such a thick ion-electrostatic barrier can inhibit the agglomeration of CVD-TS particles. With decreasing pH (via addition of HCl), the titania phase becomes positively charged (at pH õ IEPTiO2 ) and the aggregation of positively (TiO2 ) and negatively (SiO2 ) charged particles can occur. When pH is about 2 (about IEPSiO2 ), positively charged titania particles linked with the silica substrate form the potential barrier for the aggregation (as the charge of silica is close to 0) and Def decreases (Fig. 11). For pH ú 6, both phases are negatively charged; therefore, the agglomeration can occur for particles with the same sign charges but of different magnitudes if the attraction dispersion forces overcome the repulsive ones. For such a system, the energy barrier exists at a distance, but for shorter distances, attraction is observed between these particles. The energy of electrostatic interaction between two spherical particles (radii a1 and a2 ) having constant low ( C õ 25 mV) surface potentials (41) can be written as U bel Å
ea1a2 [( c1 / c2 ) 2 ln(1 / exp( 0 kH)) 4(a1 / a2 ) / ( c1 0 c2 ) 2 ln(1 0 exp( 0 kH))]
[3]
and barrier localization (H) is given by exp( kH) Å
1 2
S
c1 c2 / c2 c1
D
.
[4]
For pH ú 6, the surface potential of titania is lower than that for silica, and according to results obtained earlier (19), CTiO2 É 10 mV and CSiO2 É 20 mV; thus H É 0.22k 01 É 22 nm ( k is the Debye–Hu¨ckel parameter). If the CVDtitania particle size is about 100 nm that the barrier height from Eq. [3] equals 2.8 kT. Inasmuch as near the barrier (distance about H É 22 nm) the contributions of Hamaker attraction and structural repulsion are low (less than kT ), they are not considered for the U b estimation. Obviously the particles can overcome this barrier (about 3 kT ) due to Brownian motion to aggregate in the nearest potential minimum. Additionally, titania and silica particles can form large ‘‘dipoles,’’ as they possess opposite charges, which promotes particle aggregation at pH between IEPSiO2 and IEPTiO2 , where Def increases (Fig. 11). Inasmuch as the CVD-titania clusters partially cover the silica surface, the effect of the charge distribution on the titania phase in TS can be strong for all particles; e.g., a Def (pH) graph for CTiO2 Å 28 wt% has two minima corresponding to pH about IEPSiO2 and IEPTiO2 (Fig. 11). However, the particle size distributions for extreme points in this graph have some features (Fig. 12) due to the changes in the titania and silica surface charges for the corresponding pH values.
coida
151
CVD-TITANIA
FIG. 12. Particle size distribution in aqueous suspensions of CVD-TS at pH Å (a) 2.06, (b) 3.06, (c) 5.57, and (d) 10.11; CTiO2 Å 28 wt%.
Electrophoretic Mobility
1
Electrophoretic mobility (U) of TS particles depends nonlinearly on pH (Fig. 13) and CTiO2 (Fig. 14). The silica phase contribution to U is crucial, which obviously results from the dependence of U on CTiO2 at pH 2.5 (Fig. 14a); i.e., U depends weakly on CTiO2 for pH close to IEPSiO2 . U(CTiO2 ) increases with pH from 2.5 to 4, then for pH about IEPTiO2 , U(CTiO2 ) decreases (Fig. 14); i.e., the titania phase contribution is lower than that for silica. This effect is caused not only by a greater content of silica in CVD-TS but also by the structures of the silica and titania phases (e.g., the S value for silica is significantly larger than STiO2 ) as fumed silica is more fractal (primary silica particles about 10 nm are smaller than large CVD-TiO2 particles by a factor above 10). When s õ 0 for the silica and titania phases (pH ¢ 7), U weakly depends on CTiO2 (Fig. 14). Besides, the Def value increases as pH grows (Fig. 11), which leads to a decrease in mobility, as U Ç D 01 ef (30, 41). The IEP dependence on CTiO2 is also nonlinear (Fig. 15) and it is maximum for a small content of TiO2 , when the separated anatase phase is not observed and large titania particles are absent. The formation of well-defined anatase phase at CTiO2 Å 5 wt% reduces the influence of titania on the IEP of CVDTS. The IEP(CTiO2 ) graph in the 5–32.4 wt% range has another maximum at CTiO2 Å 17 wt% due to factors such as growing CTiO2 and Def . However, for this titania concentration, Def (CTiO2 ) has a small maximum (Fig. 9) as well as acidity of aqueous suspensions (Table 1).
The 1H NMR signals detected for frozen aqueous suspensions of oxides at T below 273 K are caused by interfacial water unfrozen due to disturbance by the oxide surface (8, 31, 40) and two regions for the DG(H) function can be found (Fig. 16 and Table 2). The first is a region of fast decrease in the thickness of the unfrozen water layer in a relatively narrow range of DG values at T slightly below 273 K and the second is a region where the H value decreases weakly in a wide range of DG values for greater changes of freezing temperature. A fraction of adsorbed water, which causes the appearance of the first region, may be determined as weakly bound water as the freezing temperature is slightly below 273 K. A long-range component of the radial function of change of the free energy (RFG) corresponds to this region. The second region for DG(H) corresponds to strongly bound water (as its freezing temperature is significantly below 273 K) and a short-range component of RFG. The thickness of the interfacial layer for each type of water (Hs and Hw for strongly and weakly bound water, respectively) and the maximum values of DG ( DG smax and w DG max in Table 2) can be estimated via extrapolation of the corresponding portions of the curves to the X and Y axes (8, 31). The value of total free energy change of the adsorbed water layers may be determined by a linear approach as
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
H NMR Study of Aqueous Suspensions of CVD-TS
DGS Å k( DGs Hs / DGw Hw )/2,
[5]
where k is the proportionality coefficient depending on pa-
coida
152
GUN’KO ET AL.
FIG. 13. Dependence of electrophoretic mobility of SiO2 (a, curve 1) and CVD-TS at CTiO2 Å 0.6 (a, curve 2), 1.7 (b, curve 1), 2.8 (b, curve 2), 5 (c, curve 1), 17 (c, curve 2), 28 (d, curve 1), and 32.4 wt% (d, curve 2) on pH.
rameter dimensions in Eq. [5]. The thickness of the water layer disturbed by the surface can reach to several tens of water molecular diameters (Table 2 and Fig. 16). This value is higher than a distance of effective intermolecular interactions but corresponds to the EDL thickness for TS particles. The H and DGS values are maximal for CTiO2 Å 1.7 wt% when the separated crystalline titania phase is not formed and TiO2 is distributed on the silica support in the form of small clusters. The values of DGS and H for fumed TS are higher than those for CVD-TS at CTiO2 ¢ 17 wt% (Table 2). This can be explained by the formation of large titania particles in CVD-TS at high CTiO2 giving a relatively small contribution to the specific surface area and having relatively slight contact with the silica substrate. For example, CVDTS at CTiO2 Å 17 wt% has a local small maximum of Def (Fig. 9) and gives the lowest DGS and H (Table 2) close to the corresponding values for Aerosil A-300. Also, the DGS value for this CVD-TS sample is close to the immersion enthalpy ( DHim ) for Cab-O-Sil with S Å 207 m2g 01 , and DGS for fumed silica used as the substrate (Table 2) is close to DHim for quartz (42). However, the difference for DHim (H) and DG(H) behavior is observed, as for DHim , the strong decrease is for the first water layer (42), but DG(H) reduces more slowly with increasing H (Fig. 16).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
For many CVD-TS samples, the DG smax values change slightly in comparison with that for silica (Table 2) as well as for other mixed oxides such as alumina/silica (40). The DGS value for fumed TS is greater than that for CVD-TS at high CTiO2 . This can be explained by a greater number of GSiO(H)TiG bonds as strong acidic sites on fumed TS than on CVD-TS, which manifests in optical spectroscopy studies (Fig. 5). Thus, all characteristics obtained in the 1H NMR study of aqueous suspensions of CVD-TS (Table 2) depend nonlinearly on CTiO2 . Theoretical Simulations The calculations of the cluster models of TS were performed using quantum chemical semiempirical NDDO method (19) and molecular mechanics MM2 (43). We modeled the CVD-TS interface (Figs. 2 and 17) using a cluster approach. The difference in the structures of fumed TS and CVDTS defines distinction in their surface characteristics. For fumed TS, the GSi{O(H){ TiG and GSi{O{TiG bridges have a strong influence on the surface properties, but in the case of CVD-TS, the number of such bonds is smaller because they are unstable due to distortions in the interface (Fig. 2, e.g., for bond lengths Dr/rmax É 0.09 and
coida
153
CVD-TITANIA
(Mi Å Si, Al, Ti, and Ge) using different semiempirical and ab initio methods. The obtained results testify that dissociative adsorption of water, G M1{O{ M2G / H2O r G M1 (OH){ O(H){ M2G,
occurs with lower activation energy if metal atoms Mi already bond hydroxyl groups. Besides, these reactions can be promoted by additional water molecules adsorbed on these sites. Different defects such as strained rings M » OO … Si (M Å Ti, Al, or Si) can interact with water molecules with a high exothermicity. The titania–silica interface in fumed TS is not so strained as in the original CVD-TS samples due to annealing of structural strains during the fumed TS synthesis. Removal of these strains at the interfaces of CVD-TS occurs via interaction with water upon its dissociative adsorption leading to disappearance of the Si{O{Ti bridges. All bonds formed between the titania and silica phases in CVD-TS (Fig. 2) can be strained as the lattice constants of these oxides differ markedly. Therefore, an increase in the TiO2{SiO2 contact area increases the bond strain at the interface and heat effect upon hydrolysis of such bonds can be higher than in the case of water adsorption on the surface of calcined individual oxides. The calculations by NDDO method show a high exothermicity of hydrolysis of even one strained Si{O{Ti bridge. For example, in the reaction of a water molecule with a small cluster (HO)3TiOSi(OH)3 , (HO)3Ti{O{Si(OH)3 / H2O r (HO)4Ti{O(H){ Si(OH)3 ,
[6]
the total energy change ( DEt ) equals 0103 kJ/mol (NDDO)
FIG. 14. Dependence of electrophoretic mobility of the CVD-TS particles on CTiO2 at pH Å 2.5 (a, curve 1), 3 (a, curve 2), 4 (a, curve 3), 5 (b, curve 1), 6 (b, curve 2), 7 (b, curve 3), 8 (c, curve 1), 9 (c, curve 2), and 10 (c, curve 3).
valence angles Df / fmax É 0.2 at the interface) and can be easily hydrolyzed. Previously (8, 10, 19, 21, 22, 39, 40, 44) we studied the interaction of water molecules with the M1{O{ M2 bridges
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 15. Dependence of the IEP value for aqueous suspensions of CVDTS on CTiO2 .
coida
154
GUN’KO ET AL.
FIG. 16. Dependencies of G0-G on the thickness of the unfrozen interfacial water layer for (a) silica (1), CVD-TS at CTiO2 Å 0.6 (2), 1.7 (3); and (b) CVD-TS at CTiO2 Å 5 (1), 17 (2), and 32.4 (3) wt%.
and in the reaction with a strained ring with no decomposition,
between anatase and crystobalite clusters in Fig. 2b. Consequently, the exothermicity of dissociative water adsorption with no Si{O{Ti bond breaking (Eqs. [6] and [7]) is less than that in the reactions with strain relieving as in Eq. [8]. Also, this effect increases upon cleavage of a few neighboring strained bonds at the TS interfaces as in Fig. 2. The NDDO calculation of hydrolysis of three neighboring Si{O{Ti bonds (Fig. 17a r Fig. 17b) give DEt about 0500 kJ/mol per strained bond (the NDDO method can overestimate DEt for water chemisorption as well as other semiempirical methods (45)). Dissociative adsorption of water molecule on pure crystalline anatase gives DEt Å 0255 kJ/mol by NDDO (19). Thus, DEt for the hydrolysis (Fig. 17a r Fig. 17b) per bond is higher than DEt for Eq. [8]; i.e., relaxation of neighboring bonds, which do not take part in the hydrolysis, gives an essential contribution (about 20%) to the total reaction exothermicity. The MM2 calculations give a relatively small effect ( DEt Å 027 kJ/mol per hydrolyzed Si{O{Ti bond) in the process (Fig. 17a r Fig. 17b). This can be explained by a weaker influence of the angle deformations in the MM2 force field on the total energy in comparison with the NDDO method. Besides, MM calculations do not explicitly treat the electrons in a molecular system, as electronic effects are included in force fields only via its parameterization; therefore, a portion of the energy of chemical bond transformations can be lost in the MM calculations. It seems likely that calculated deformations (until 10% for bonds and 20% for valence and dihedral angles) can give the main contribution to the exothermicity caused by strain relieving via hydrolysis of the CVD-TS interface. Because of this, for freshly prepared CVD-TS, the amount of adsorbed water and its free energy changes can be more greater than analogous values for individual oxides. The MM2 calculations of the TS clusters (Fig. 17) covered by a solvation shell with 122 water molecules give a deeper stabilization of the initial cluster
TABLE 2 Characteristics of the Interfacial Water Layers for Aqueous Suspensions of CVD-TS
(HO)2Ti » OO … Si(OH)2 / H2O r (HO)3Ti »
O(H) O
… Si(OH)2 ,
[7]
DEt Å 0106 kJ/mol, but upon strained bond hydrolysis,
(HO)3Ti » OO ( H ) … Si(OH)2 / H2O r (HO)3Ti{O(H){ Si(OH)4 ,
[8]
DEt Å 0397 kJ/mol. The bond elongation in the strained ring Ti » OO … Si is 0.017 nm for Si{O and 0.025 nm for Ti{O, which is close to the elongation for the boundary
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
Sample
CTiO2
Silicaa Silicab CVD-TS CVD-TS CVD-TS CVD-TS CVD-TS Fumed ST
0
a b
0.6 1.7 5.0 17.0 32.4 20.0
DGSmax (kJ/mol)
DGWmax (kJ/mol)
HS
HW
DGS (mJ/m2)
3.8 3.5 3.5 3.9 2.9 3.5 4.2 2.7
1.0 1.3 1.5 1.0 1.2 2.0 0.9 0.8
3.2 8.0 9.5 21.0 11.0 4.2 7.5 10.5
8.4 17.0 11.5 40.0 13.0 5.0 17.0 25.0
185 451 455 1097 428 222 421 435
Silica Aerosil A-300 (34). Silica used as a substrate for CVD-TiO2 .
coida
CVD-TITANIA
155
FIG. 17. Cluster model of CVD-TS with crystobalite/anatase: (a) initial structure and (b) after hydrolysis of three Si{O{Ti bonds and geometry relaxation calculated by NDDO method.
(Fig. 17a) in the water shell than post-hydrolysis separated clusters of TiO2 and SiO2 (Fig. 17b): DDEt Å 050 kJ/mol. CONCLUSION
CVD-titania does not form a continuous layer on the silica substrate independent of the concentration. Its significant fracture appears as nanoscaled anatase particles (70–250 nm at high CTiO2 ), which have weak contact with the silica phase via hydrogen bonding and electrostatic interaction and a small number of Si{O{Ti bonds. However, the silica substrate inhibits the anatase phase transition to rutile. The titania concentration in calcinated CVD-TS has a minor influence on the amount of adsorbed water due to a small contribution of the titania phase in the specific surface area of CVD-TS. However, a fraction of titania can be distributed over the silica matrix as the small clusters (especially at CTiO2 õ 3 wt%), which affect not only the structure and the free energy of adsorbed (interfacial) water (the effect is at a maximum for CTiO2 Å 1.7 wt%) but also the concentration of B- and L-sites and the particle (agglomerates) size distribution in aqueous suspensions of CVD-TS. The graph of Def on pH for CVD-TS has two minima corresponding to IEP of silica and titania. The CTiO2 increase leads to decrease in Def to the size of silica agglomerates about 0.2 mm. ACKNOWLEDGMENTS Some of the authors are grateful to the Sherwin–Williams Co. (U.S.A.) for partial financial support of this work. V.M.G. thanks the Swiss National Science Foundation (Grant 7UKPJ 48657) for financial support. The authors thank the referees for useful remarks.
REFERENCES 1. Fernandez, A., Gonzalez-Elipe, A. R., Real, C., Caballero, A., and Munuera, G., Langmuir 9, 121 (1993).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
2. (a) Dutoit, D. C. M., Schneider, M., and Baiker, A., J. Catal. 153, 165 (1995); (b) Hutter, R., Mallat, T., Dutoit, D., and Baiker, A., Top. Catal. 3, 421 (1996); (c) Dartt, C. B., and Davis, M. E., Appl. Catal. A 143, 53 (1996). 3. Hutter, R., Mallat, T., and Baiker, A., J. Catal. 153, 177 (1995). 4. Imamura, S., Nakai, T., Kanai, H., and Ito, T., J. Chem. Soc. Faraday Trans. 91, 1261 (1995). 5. Jehng, J.-M., Wachs, I. E., Weckhuysen, B. M., and Schoonheydt, R. A., J. Chem. Soc. Faraday Trans. 91, 953 (1995). 6. Arndt, J., Phys. Chem. Glasses 24, 104 (1983). 7. Matijevic, E., Langmuir 10, 8 (1994). 8. Gun’ko, V. M., Zarko, V. I., Turov, V. V., Voronin, E. F., Tischenko, V. A., and Chuiko, A. A., Langmuir 11, 2115 (1995). 9. Voronin, E. F., Pakhlov, E. M., and Chuiko, A. A., Colloids Surf. A 101, 123 (1995). 10. Zarko, V. I., and Gun’ko, V. M., Functional Mater. 2, 110 (1995). 11. Zarko, V. I., Kozub, G. M., Pokrovskiy, V. A., and Chuiko, A. A., Teor. Eksper. Khim. 27, 735 (1991). 12. Contesu, C., Popa, V. T., Miller, J. B., Ko, E. I., and Schwarz, J. A., J. Catal. 157, 244 (1995). 13. Miller, J. B., Johnston, S. T., and Ko, E. I., J. Catal. 150, 311 (1994). 14. (a) Liu, Z., Tabora, J., and Pavis, R. J., J. Catal. 149, 117 (1994); (b) Hoffmann, M. R., Martin, S. T., Choi, W., and Bahnemann, D. W., Chem. Rev. 95, 69 (1995). 15. Khonw, C. B., and Dartt, C. B., J. Catal. 149, 195 (1994). 16. Sushko, R. V., Gette, A. V., Mironyuk, I. F., and Chuiko, A. A., Zh. Priklad. Khim. 56, 1230 (1983). 17. Schraml-Marth, M., Walther, K. L., Wokaun, A., Handy, B. E., and Baiker, A., J. Non-Cryst. Solids 143, 93 (1992). 18. Greegor, R. B., Lytle, F. W., Sandstrom, D. R., Wong, J., and Schultz, R., J. Non-Cryst. Solids 55, 27 (1983). 19. Gun’ko, V. M., Zarko, V. I., Chibowski, E., Dudnik, V. V., Leboda, R., and Zaets, V. A., J. Colloid Interface Sci. 188, 39 (1997). 20. (a) Pakhlov, E. M., Voronin, E. F., and Sushko, R. V., Zh. Prikl. Spectrosk. 47, 311 (1987); (b) Pakhlov, E. M., Zarko, V. I., and Voronin, E. F., Ukr. Khim. Zh. 59, 373 (1993). 21. Gun’ko, V. M., Zarko, V. I., Chuikov, B. A., Dudnik, V. V., Ptushinskii, Yu. G., Voronin, E. F., Pakhlov, E. M., and Chuiko, A. A., Int. J. Mass Spectrom. Ion Process., in press. 22. Gun’ko, V. M., Voronin, E. F., Zarko, V. I., Pakhlov, E. M., and Chuiko, A. A., J. Adhesion Sci. Technol. 11, 627 (1997). 23. (a) Stieg, F. B., J. Coat. Technol. 61, 67 (1989); (b) Kerker, M., Cooke, D. D., and Ross, W. D., J. Paint Technol. 47, 333 (1975).
coida
156
GUN’KO ET AL.
24. Powell, Q. H., Kodas, T. T., and Anderson, B. M., Chem. Vapor Depos. 2, 179 (1996). 25. (a) Hanprasopwattana, A., Reiker, T., Sault, A. G., and Datye, A. K., Catal. Lett. 45, 165 (1997); (b) Hanprasopwattana, A., Srinivasan, S., Sault, A. G., and Datye, A. K., Langmuir 12, 3173 (1996). 26. (a) Aleskovskiy, V. B., and Yuffa, A. Ya., J. Vses. Khim. Obsh. Mendeleev. 34, 317 (1989); (b) Aleskovskiy, V. B., ‘‘Stereochemistry and Synthesis of Solid Compounds.’’ Nauka, Leningrad, 1976. 27. Kubelka, P., and Munk, F., Z. Tech. Phys. 12, 593 (1931). 28. (a) Tanabe, K., ‘‘Solid Acids and Bases.’’ Kodansha, Tokyo, 1970; (b) Brazdil, J. F., and Yeager, E. B., J. Phys. Chem. 85, 1005 (1981); (c) Sivalov, E. G., and Tarasevich, Yu. I., Adsorption Adsorbents N10, 49 (1982). 29. (a) Zarko, V. I., Kozub, G. M., Sivalov, E. G., and Chuiko, A. A., Ukr. Khim. Zh. 54, 1144 (1988); (b) Tarasevich, Yu. I., and Sivalov, E. G., Dokl. Akad. Nauk Ukr. Ser. B N3, 252 (1978). 30. Hunter, R. J., ‘‘Zeta Potential in Colloid Sciences.’’ Academic Press, London/San Francisco, 1981. 31. (a) Gun’ko, V. M., Turov, V. V., Zarko, V. I., Dudnik, V. V., Tischenko, V. A., Kazakova, O. A., Voronin, E. F., Siltchenko, S. S., Barvinchenko, V. N., and Chuiko, A. A., J. Colloid Interface Sci. 192, 166 (1997); (b) Turov, V. V., Leboda, R., Bogillo, V. I., and Skubishewska-Zieba, J., Langmuir 13, 1237 (1997); (c) Turov, V. V., Zarko, V. I., and Chuiko, A. A., Zh. Fiz. Khim. 69, 677 (1995). 32. (a) V. P. Glushko (Ed.), ‘‘Handbook of Thermodynamic Properties of Individual Substances.’’ Nauka, Moskow, 1978; (b) Mank, v. V., and Lebovka, N. I., ‘‘NMR Spectroscopy of Water in Heterogeneous Systems.’’ Naukova Dumka, Kiev, 1988. 33. Byun, C., Jang, J. W., Kim, I. T., Hong, K. S., and Lee, B. W., Mater. Research Bull. 32, 431 (1997).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
34. Otterman, C. R., and Bange, K., Thin Solid Film 286, 32 (1996). 35. Voronkov, M. G., Maletina, E. A., and Poman, V. K., ‘‘Heterosiloxanes.’’ Nauka, Novosibirsk, 1984. 36. Tertykh, V. A., Ogenko, V. M., Voronin, E. F., and Chuiko, A. A., Zh. Prikl. Spectrosk. 23, 464 (1975). 37. (a) Little, L. H., ‘‘Infrared Spectra of Adsorbed Species.’’ Academic Press, London, 1966; (b) Kiselev, A. V., and Lygin, V. I., ‘‘Infrared Spectra of Surface Compounds and Adsorbed Matters.’’ Nauka, Moscow, 1972. 38. (a) Barthel, H., Colloids. Surf. A 101, 217 (1995); (b) Barthel, H., Rosch, L., and Weis, J., in ‘‘Organosilicon Chemistry. II. From Molecules to Materials’’ (N. Auner and J. Weis, Eds.), pp. 761–778. VCH, Weinheim, 1996. 39. (a) Zarko, V. I., Gun’ko, V. M., Chibowski, E., Dudnik, V. V., and Leboda, R., Colloids Surf. A, 127, 11 (1997); (b) Gun’ko, V. M., Zarko, V. I., Leboda, R., Voronin, E. F., Chibowski, E., and Pakhlov, E. M., Colloids-Surf. A, in press. 40. Gun’ko, V. M., Zarko, V. I., Turov, V. V., Voronin, E. F., Tischenko, V. A., Dudnik, V. V., Pakhlov, E. M., and Chuiko, A. A., Langmuir 13, 1529 (1997). 41. Derjaguin, B. V., Churaev, N. V., and Muller, V. M., ‘‘Surface Forces.’’ Plenum, New York, 1990. 42. (a) Douillard, J. M., J. Colloid Interface Sci. 188, 511 (1997), and 182, 308 (1996); (b) Douillard, J. M., Zoungrana, T., and Partyka, S., Petrol. Sci. Eng. 14, 51 (1995); Malandrini, H., Sarraf, R., Faucompre, B., Partyka, S., and Douillard, J. M., Langmuir 13, 1337 (1997). 43. Burket, U., and Allinger, N. L., ‘‘Molecular Mechanics.’’ Am. Chem. Soc., Washington, DC, 1982. 44. Gun’ko, V. M., Voronin, E. F., Zarko, V. I., and Pakhlov, E. M., Langmuir 13, 250 (1997). 45. Bredow, Th., and Jug, K., Surf. Sci. 327, 398 (1995).
coida
198, 141–156 (1998)
CS975270
CVD-Titania on Fumed Silica Substrate V. M. Gun’ko,* 1 V. I. Zarko,* V. V. Turov,* R. Leboda,† E. Chibowski,‡ L. Holysz,‡ E. M. Pakhlov,* E. F. Voronin,* V. V. Dudnik,* and Yu. I. Gornikov* *Institute of Surface Chemistry, 31 Prospect Nauki, 252022 Kiev, Ukraine; and †Department of Chemical Physics, and ‡Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received July 22, 1997; accepted October 24, 1997
A titania (anatase) was synthesized on a fumed silica substrate with the concentration of TiO2 (CTiO2 ) from 0.6 to 32.4 wt% using a chemical vapor deposition (CVD) technique. All studied characteristics of CVD-titania/silica (CVD-TS), such as abundance of active surface sites, amount of adsorbed water, particle size distribution, electrophoretic mobility, and free energy changes of interfacial water, depend nonlinearly on CTiO2 due to alterations in the structure and distribution of the titania phase in TS with increasing CTiO2 . The separated anatase phase is well defined in CVD-TS at CTiO2 É 5 wt%, which is lower than that for fumed TS (about 9 wt%). The anatase phase transition to rutile is inhibited by the silica matrix and the corresponding temperature increases. At relatively low temperatures (below 400 K) of CVDTS treatment, the amount of Brønsted acid sites is higher than that for fumed TS with analogous concentration of titania, but for T above 600 K, the opposite relationship is observed. The effective particle diameter for CVD-TS suspended in water decreases faster with increasing CTiO2 than for fumed TS. A particle size distribution in an aqueous suspension of TS differs from that for an original fumed silica suspension and the main contribution is given by the particles with sizes closely related to aggregates of primary silica particles. q 1998 Academic Press Key Words: CVD-titania/silica; particle structure; anatase on silica; electrophoretic mobility; particle size distribution; 1H NMR, interface modeling.
INTRODUCTION
Titania/silica (TS) materials with various compositions are widely used as fillers, pigments, catalysts, etc. (1–18). The structure and properties of TS strongly depend on the synthetic technique used (sol–gel, high-temperature hydrolysis, chemical vapor deposition (CVD), etc.). Titania/silica can possess (a) mixed structure in both the bulk and the surface layer as in fumed TS (8, 19) or (b) separated phases of TiO2 and SiO2 with a clear-cut phase boundary when one of these oxides is formed (using, e.g., CVD technique) on another which is a substrate. In the latter case two variants 1
To whom correspondence should be addressed. Fax: 380 44 264 0446.
are possible: the second phase forms (a) a continuous layer or (b) separated clusters and individual particles. Earlier we studied fumed TS (8, 10, 11, 19–22). The properties of CVD-TS and fumed TS differ due to alterations in the titania phase distribution and in the structure of the interfaces, where GSi{O{TiG or GSi{O(H){ TiG connections are formed. GSi{O(H){ TiG (Brønsted acid sites (B-sites) possessing catalytic activity (12, 14)) and GSi{O{TiG bonds were observed in fumed TS obtained by combustion of SiCl4 and TiCl4 in an oxygen–hydrogen flame (above 1300 K) (8, 19) as well as after TiCl4 interaction with a silica surface in vacuum (9). Titania (anatase) is used as a durable catalyst for decomposition of chemicals in wastewater under UV irradiation because of its sensitizer properties (14b). TS possessing this feature of titania can, however, possess stronger B-sites than original anatase. Also, TS and TS/Al2O3 are used as pigments in which the diameter (def ) of particles is approximately 0.3 mm (7, 23) or as additives for improvement of the polymer film properties (22). Silica and alumina applied for the coverage of the titania pigment surface can be deposited by the sol-gel method or CVD technique (24). The sol–gel technique allows to obtain relatively ‘‘smooth’’ coverage (25), but this route is expensive and give much wastewater. Some authors assumed that the CVD technique can give a continuous film with titania on oxide supports (26). However, features of formed films depend on the reactivity of the substrate and the new phase, their structures, etc., and in the case of higher reactivity of the second phase, such a continuous film can be absent due to preferable interaction of new portions of reagents with the formed clusters of the second phase. Therefore, it is of interest to study the structure of CVD-TS and its properties in comparison with those of fumed TS for various titania concentrations (CTiO2 ). EXPERIMENTAL
Materials. Titania synthesis was performed by the CVD method using fumed silica (Aerosil, 99.9%, ‘‘Chlorovinyl,’’ Kalush, Ukraine; specific surface area (S) about 270 m2 g 01
141
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
0021-9797/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
coida
142
GUN’KO ET AL.
TABLE 1 Some Properties of Synthesized CVD-Titania on Silica CTiO2
W a (g 101)
S (m2g01)
Def (mm)
pHb
PDc
nd
0 0.6 1.7 2.8 5 7.5 17 20 22 28 32.4
57.7 56.0 55.3 53.5 55.2 54.0 62.6 65.0 69.0 79.1 88.5
269 269 247 222 254 261 217 200 187 182 154
15.04 10.81 7.80 0.69 0.39 0.37 0.61 0.54 0.35 0.26 0.24
6.38 6.54 6.39 6.28 5.80 5.57 5.47 5.87 5.71 5.57 5.98
0.503 0.407 0.425 0.231 0.519 0.256 0.330 0.331 0.092 0.173 0.102
1 1 1 1 1 2 3 4 5 6 8
a
Apparent density. pH values for aqueous suspensions of 0.2 g of the solids per 1 l of deionized distilled water. c PD is polydispersity as a measure of the nonuniformity of the particle size distribution (PD õ 0.02 corresponds to a monodisperse distribution). d Number of synthetic cycles. b
and OH group concentration 0.6 mmol g 01 under synthetic conditions) as a substrate. A sample of fumed silica was placed in a glass reactor (volume 2 l) equipped with a mixer. The reactor was blown with dry air at 423 K for 3 h, then TiCl4 vapor was supplied and reacted at 423 K for 0.5 h; thereupon, the sample was blanketed by dry air at the reaction temperature for 0.5 h, then water vapor was used for hydrolysis of residual Ti-Cl bonds under the same conditions. The required amounts of TiCl4 and H2O were calculated according to the scheme: { GSiOH}n / TiCl4 r { GSiO}nTiCl40n / nHCl [1] { GSiO}nTiCl40n / (4 0 n)(H2O) r { GSiO}nTi(OH)40n / (4 0 n)HCl.
[2]
A series of samples with CTiO2 varied in the 0.6–32.4 wt% range (Table 1) was been synthesized using the TiCl4 chemisorption–hydrolysis cycles [1] and [2] from 1 to 8. The TiO2 concentration was determined by chemical analysis for Ti. The specific surface area (Table 1) was calculated using nitrogen adsorption at 78 K. X-ray study. The XRD study was performed using DRON-UM1 and DRON-4 (Russia) difractometers with CoKa radiation (30 kV, 20 mA, filtered by a Fe thin film). A monochromator with quartz monocrystal was placed after a sample. Quartz powder was used as a primary standard. Infrared spectroscopy. The IR spectra (400–4000 cm01 ) were recorded by a UR-20 (Germany) spectrophotometer in air and vacuum (10 04 Torr). In the first case, the samples were suspended in Vaseline and placed between KRS-5 glasses. In the second, the IR studies were performed
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
in special optical glass vessels using samples (8 1 28 mm) weighing 8–10 mg. Adsorption of water. Water adsorption on oxide samples (weighing 50–100 mg, pressed at approximately 10 4 Torr) was studied using an adsorption apparatus with a McBain– Bark scale. After evacuation to 10 03 Torr for 1–2 h, samples were heated at 613 K for 3–4 h to a constant weight, then cooled to 298.13 { 0.2 K, and adsorption of water vapor was studied at pressure (P) varied in the 0.06–0.95 P/Ps range. The measurement accuracy was 1 1 10 03 mg with relative mean error {5%. Optical spectroscopy. Dimethylaminoazobenzene (nDMAAB, pKa Å 3.3) was chosen as a color indicator for a study of acidic surface sites by optical spectroscopy. The diffuse reflection spectra of adsorbed DMAAB have been recorded using a SF-18 (Russia) spectrophotometer, then converted to the absorption spectra according to the Kubelka–Munk formula (27). The n-DMAAB adsorption from the gas phase on samples previously evacuated to 10 04 Torr and then heated in special optical glass vessels at defined temperatures for 1 h, was performed at 338 { 5 K for 2–4 h. The assignment of the DMAAB absorption bands has been done by the analogy to the spectra for this substance in neutral and acidic solutions (24). Five absorption bands of DMAAB adsorbed on mixed oxides are possible. They correspond to (1) 430–460 nm, physisorbed DMAAB (dDMAAB); (2) 480–490 nm, hydrogen-bonded complex (H-DMAAB); (3) 520–545 nm, complexes with H / transferred from the B-sites to DMAAD (H / -DMAAB); (4) 555–560 nm, complexes with the Lewis acid sites (LDMAAB); (5) 605–615 nm, dimers of the adsorbed DMAAB (DM-DMAAB) (19, 28, 29). Electrophoresis. Electrophoretic (30) and multimodal particle size distribution studies were performed on a Zeta Plus zeta potential apparatus (Brookhaven Instruments). Deionized distilled water (pH 6.95) and 0.2 g of the solids per liter of the water were used for suspension preparation in an ultrasonic bath for 2 h. The pH values were adjusted by addition of 0.1 M HCl or NaOH solutions. The pH values of the suspensions of oxides were measured by an OP-208/1 (Hungary) precision digital pH-meter. The average effective diameter (Def ) corresponding to the hydrodynamic diameter, i.e., the particle diameter plus an electrical double layer (EDL) thickness, and the polydispersity of the particles in aqueous suspensions were obtained by photon correlation spectroscopy using a Brookhaven MAS OPTION particle sizer (accuracy of {1–2% with monodisperse samples; repeatability of {1–2% with dust-free samples). 1 H NMR. The 1H NMR spectra were obtained by a highresolution WP-100 SY (Bruker) NMR spectrometer with a bandwidth of 50 kHz. Relative mean errors were {10% for signal intensity and {1 K for temperature. The amount of interfacial unfrozen water (CH2O ) in aqueous suspensions of
coida
CVD-TITANIA
143
oxides frozen at 200 õ T õ 273 K was estimated by comparison of its signal intensity (I) with that for water adsorbed on oxide powder at the gas phase boundary using a calibrated function I Å f (CH2O ) (31). The free energy changes ( DG) for the interfacial water layer were calculated using the tabulated dependence of the free energy of ice ( Gi ) on temperature (31, 32). We believe that water is frozen at the interfaces when G Å Gi and the value of DG Å DGi Å GiÉTÅ273K 0 Gi (T) corresponds to a decrease in the free energy due to water interaction with the solid surface (31, 32b). A capillary effect is practically absent upon water adsorption from air onto nonporous materials such as fumed oxides and the DG(CH2O ) function can be used for determination of a radial dependence of the free energy changes upon water adsorption from air on a layer thickness as a few molecular layers can be adsorbed. Assuming that an area occupied by a water molecule equals 0.09 nm2 , we obtained the DG(H) function, where H is the thickness of the unfrozen water layer in terms of the number of statistical monolayers of water (one layer thickness corresponds to 0.3 nm). RESULTS AND DISCUSSION
Structure of CVD-Titania/Silica The titania phase is not uniformly deposited on the silica surface in CVD-TS as individual clusters or patches with a crystalline structure are formed already for small CTiO2 ; i.e., a continuous titania layer is not formed. Inasmuch as according to the XRD data (Fig. 1a), the crystalline structure of anatase in CVD-TS appears for CTiO2 at about 3 wt% (for fumed TS, such process occurs for greater CTiO2 (19)) which is lower than the amount needed for monolayer coverage (about 17 wt% for silica with S Å 270 m2g 01 and d É 10 nm). Despite the clear-cut structure of anatase observed for CTiO2 ¢ 5 wt%, the rutile phase is not observed even for CTiO2 Å 32.4 wt% (Fig. 1b). This could be explained by the relatively low temperature of the CVD-TiO2 synthesis. However, the calcination of CVD-TS with CTiO2 Å 34.2 wt% at T É 1100 K (which is higher than the temperature for anatase phase transition to rutile (Tph )) for 2 h does not change the anatase structure (Fig. 1b). Consequently, this phase transition for CVD-TS is inhibited by the silica matrix. If we assume that initial tight contact between CVD-titania and silica substrate looks like the crystobalite–anatase interface (Fig. 2), then the formation of GTi{O{SiG bonds (5– 6 bonds per nm2 of contact area) leads to great distortions of the surface layers of the titania and silica lattices (Fig. 2, at the TS interface Si{O and Ti{O bonds can be elongated up to 0.177 nm and 0.210 nm, respectively; angles OTiO, OSiO, TiOTi, and SiOSi varying in the 3–207 range from the values for crystalline structures of anatase and crystobalite) increasing the probability of hydrolysis of these interfacial bridges due to the high exothermicity of such a process (19).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 1. X-ray data for CVD-TS at (a) CTiO2 Å 0.6 (1), 1.7 (2), 2.8 (3), 3.3 (4), 5 (5), and 7.5 wt% (6); (b) samples with CTiO2 Å 17 (1, 2) and 32.4 (3, 4) wt%; (1, 3) original samples and (2, 4) heated at 1100 K for 2 h. All peaks correspond to anatase.
On the other hand, the Tph shift is indirect evidence for the existence of bonding interaction between the silica and titania phases even after the interaction with water vapor. This may be due to residual GSi{O{TiG bonds formed during the CVD-TiO2 synthesis or to small silica particles embedded in the titania particles or caused by partial fusion of the titania and silica phases on sample heating to high temperatures. An analogous result on prevention of the phase transition of anatase to rutile was obtained for similar CVDTS materials (16). If a substrate structure corresponds to a deposited phase that phase transition temperature can be lower than for the individual oxide (33); however, crystallization of amorphous titania upon annealing can induce an increase in stress (34). Consequently, growth of crystalline anatase on the silica support and its annealing can increase stress at the interface that promotes its hydrolysis in air.
coida
144
GUN’KO ET AL.
FIG. 2. Model of the CVD-TS interface between the clusters of anatase and crystobalite used in the MM2 calculations; (a) initial crystalline structure and (b) the structure after full relaxation of the cluster geometry by MM2 method.
A band nSiO at 940–960 cm01 in the IR spectra of TS (Figs. 3 and 4) is linked to the GSi{O{TiG bridges (8, 20). Such SiO-stretching vibrations were observed for fumed TS (Fig. 3, curve 3), but they are practically absent for CVD-TS after its contact with water vapor (8, 9, 20), as Si{O{Ti bonds can be easily hydrolyzed, as well as other Si{O{ M (M x Si) bonds (35). Therefore, the number of such bonds in CVD-TS can be too low for observation
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
using IR spectroscopy. A small absorption in this region for parent silica (Fig. 3, curve 1) may be caused by water molecules bound to the surface as thermoevacuation leads to its disappearance for both silica (Fig. 4, curve 1 and Ref. (36)) and CVD-TS (Fig. 3, curve 4). We could assume that the calcination of CVD-TS at high temperatures causes an increase in the GSi{O{TiG number due to partial fusion of the particles. Therefore, we recorded the IR spectrum
coida
CVD-TITANIA
145
O(H){ Ti bonds in CVD-TS is too small to observe using IR spectroscopy, but optical spectroscopy of adsorbed color indicators is more sensitive to find the corresponding sites. Active Surface Sites of CVD-TS
FIG. 3. IR spectra of (1) fumed silica; (2) CVD-TS at CTiO2 Å 22 wt%; (3) fumed TS at CTiO2 Å 20 wt%; (4) CVD-TS sample after pretreatment in air at 1170 K for 7 h.
of CVD-TS heated at 1170 K over 7 h, but the band at 960 cm01 does not appear (Fig. 3, curve 4). However, the GSi{O{TiG bridges are observed upon reaction of TiCl4 with silica in vacuum (Fig. 4, curve 2, band at 940 cm01 ). After interaction of hexamethyldisiloxane with the surface, this band intensity grows due to the increased number of GSi{O{TiG bonds and the maximum shifts to 950 cm01 (Fig. 4, curve 3). The interaction of this sample with water vapor (at room temperature for 20 min and P/ Ps Å 0.3) magnifies the band intensity of SiO{H at 3750 cm01 , but the band at 950 cm01 disappears (Fig. 4). Additionally, a very broad (2800–3700 cm01 ) intensive band linked to adsorbed water is observed. Typically, only a small absorption appears in this region upon water adsorption on dehydrated fumed silica under such conditions (37); i.e., the surface formed via TiCl4 chemisorption on silica possesses a high adsorption ability for water. However, after the calcination of this sample, water adsorption is only slightly differs from adsorption on parent silica; i.e., changes in the TS structure occurs. Thus, the number of Si{O{Ti and Si{-
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
The comparison of the integral intensity of H / -DMAAB bands for fumed TS and CVD-TS shows that slightly heated samples differ more greatly (Fig. 5, curves 1 and 2) than after heating at higher temperatures (curves 3 and 4); i.e., the B-sites on CVD-TS are more sensitive to heating than that on fumed TS due to the alteration in the TiO2{SiO2 interfaces. The corresponding differences were observed for molecular and associative desorption of water from these oxides, i.e., upon removal of the B-sites (21). An increase in CTiO2 for CVD-TS reduces the DMAAB spectra intensity with elevating pretreatment temperature (Tp ) (Figs. 6 and 7). However, the relative contribution of d-DMAAB (Fig. 6, curves 1) and H-DMAAB (curves 2) increases with growing CTiO2 due to easy desorption of water from TS. At the same Tp , increasing CTiO2 magnifies the dehydroxylated (titania phase and interface) and dehydrated (silica phase) surface area (21), where DMAAB can be physisorbed (d-DMAAB) or forms the hydrogen bonds (HDMAAB). The relative contribution of H / -DMAAB complexes decreases with increasing CTiO2 (Fig. 6, curve 3, and Fig. 7a), but the L-DMAAB band intensity depends less strongly on Tp (Fig. 7b). Typically, intensity of the H / -DMAAB band decreases with increasing Tp (Figs. 6 and 7) except for CVD-TS with CTiO2 Å 17 wt%. The main fraction of molecular adsorbed water and a significant part of dissociatively adsorbed water can be desorbed from titania or TS at T õ 550 K (21). This is in good agreement with the strong decrease in H / DMAAB intensity at Tp ° 525 K (Fig. 7a). Adsorbed intact water can increase the stabilization of H / -DMAAB complexes (i.e., removal of intact water can decrease the possibility of the H / -DMAAB complex formation). A verification of this assumption is the small intensity of L-DMAAB (Fig. 7b), as associative desorption of water must be accompanied by removal of the B-sites (i.e., significant decrease in H / -DMAAB) and by formation of L-sites with increasing Tp . However, the decrease in H / -DMAAB band intensity is not accompanied by growth of L-DMAAB except for CVD-TS with CTiO2 Å 1.7 wt% (Fig. 7b, curve 1) when the titania phase is distributed as small clusters and crystalline anatase is not detected by XRD (Fig. 1a). Consequently, CVD-TS possesses low Lewis acidity, similarly to fumed titania, but fumed TS possesses higher acidity (19) than CVD-TS. A low DM-DMAAB band was observed only for CVD-TS with CTiO2 Å 17 wt% at Tp Å 703 K. The observed differences in acidity of CVD-TS and fumed TS can show up in the water adsorption.
coida
146
GUN’KO ET AL.
FIG. 4. IR spectra of (1) fumed silica thermoevacuated at 873 K; then after (2) reaction with TiCl4 at 473 K, (3) reaction with hexamethyldisiloxane at 573 K, and (4) adsorption of water at P/Ps Å 0.3 for 20 min.
Water Adsorption 2
The amount of adsorbed intact water per nm (is smaller on CVD-TS) than on fumed ST, but the amount of water dissociatively adsorbed on CVD-TS (for the same CTiO2 ) is greater due to the hydrolysis of the Si{O{Ti bridges between the titania phase and the silica substrate (21). These results are in agreement with the isotherms of water vapor adsorption on fumed TS and CVD-TS (Fig. 8). Also, the
FIG. 5. Dependence of integral intensity of the H / -DMAAB band on CTiO2 for CVD-TS (1 and 4) and fumed TS (2 and 3) at pretreatment temperatures (1) 353, (2) 373, and (3 and 4) 623 K.
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
amount of water adsorbed on CVD-TS depends slightly on CTiO2 due to formation of large titania particles (having relatively small S) and weak influence of the interfaces on the intact water adsorption. This conclusion is drawn because the isotherms for CVD-TS are similar to the isotherm for fumed silica used as a substrate for CVD-TiO2 , but water adsorption on fumed TS depends more strongly on CTiO2 and this dependence is nonlinear as well as for CVD-TS (Fig. 8). The higher integral acidity of fumed TS causes stronger water adsorption than by CVD-TS (due to a deeper decrease in the free energy of the interfacial water (8)). However, the local acidity of Si{O(H){ Ti or incompletely O-coordinated Ti (Lewis acid site) for fumed TS and CVD-TS can differ only slightly as the structures of the B- or L-sites are similar for these mixed oxides. Consequently, such an alteration in water adsorption is due to the difference in the interface structures and the concentrations of strong acidic sites. The small number of B-sites on CVD-TS is specified by hydrolysis not only of Si{O{Ti but also of Si{O(H){ Ti bonds and the content of dissociatively adsorbed water on CVD-TS is higher than that on fumed ST. These features of fumed TS and CVD-TS should influence the characteristics of their particles in aqueous suspensions. CVD-TS Particle Size Distribution in Aqueous Suspensions The specific surface area of CVD-TS decreases by half with increasing CTiO2 to 32.4 wt% (Table 1), which corre-
coida
CVD-TITANIA
147
FIG. 6. Optical spectra of DMAAB adsorbed on CVD-TS surface at CTiO2 Å 5 (a, c, e, g, and i) and 22 wt% (b, d, f, h, and j) for Tp Å 353 (a and b), 423 (c and d), 523 (e and f), 623 (g and h), and 703 K (i and j); bands: (1) d-DMAAB, (2) H-DMAAB, (3) H / -DMAAB, (4) L-DMAAB.
AID
JCIS 5270
/
6g39h$5270
01-30-98 10:52:22
coida
148
GUN’KO ET AL.
FIG. 7. Dependence of integral intensity of (a) H / -DMAAB and (b) L-DMAAB for CVD-TS on Tp at CTiO2 Å (1) 1.7, (2) 5, (3) 17, and (4) 22 wt%.
sponds to growth in the average size of the particles upon CVD-TiO2 synthesis. However, the Def value for CVD-TS is smaller than that for original fumed silica and decreases as CTiO2 increases; therewith, the Def diminution for CVDTS occurs faster than that for fumed TS (Fig. 9). Such alterations in the particle dispersity for CVD-TS (Table 1) can be explained by taking into account the complex structure of fumed oxides (19, 38) and the changes upon CVDTiO2 synthesis. For example, the primary spherical particles of fumed silica (diameter about 10 nm) form branched stable aggregates (0.1–0.2 mm, fractal dimension approximately 2.5, and apparent density about 30% of specific density) via the GSi{O{SiG connections due to sticking and reaction at relatively high temperatures. At lower temperatures, aggregates form loose agglomerates (above 5 mm, fractal dimension approximately 2.1, and apparent density about 1–3% of specific density) via the hydrogen-bonding
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 8. Adsorption isotherms of water vapor on (a) fumed TS at CTiO2 Å (1) 29, (2) 20, (3) 36, (4) 100—original titania, (5) 14, (6) 9, and (7) 0 wt%—fumed silica; (b) CVD-TS at CTiO2 Å (1) 33, (2) 17 wt%, (3) 0—fumed silica, (4) 20, (5) 2.8, and (6) 10 wt%.
FIG. 9. Dependence of Def on the concentration of titania for aqueous suspensions of CVD-TS (1) and fumed TS (2).
coida
CVD-TITANIA
and electrostatic interactions (38). More stable agglomerates can be formed upon long-term storage of fumed silica and such agglomerates (above 1 mm) can remain in aqueous suspension even after ultrasonic treatment (39). However, freshly prepared fumed silica possesses a low thickening ability in polar liquids (38), which corresponds to the decomposition of agglomerates formed in air. The Def value strongly decreases with increasing CTiO2 in TS (Fig. 9); however, for fumed alumina/silica, the opposite effect is observed (39, 40). The modification of fumed silica by organosilicon compounds also gives a Def value of about mean aggregate size (39). Consequently, a decrease in Def for CVD-TS is due to partial decomposition of the agglomerates upon CVD-TiO2 synthesis for small CTiO2 , and their entire disintegration is observed for CTiO2 Å 5 wt% (Fig. 10c, all particles with d õ 0.3 mm) when a well-defined individual TiO2 phase is formed (Fig. 1a). Break-up of agglomerates can be caused by GSiOH substitution of GSiOTiCl3 at the first stage of titania phase formation, which decreases the bonding interaction between aggregates. However, agglomerates are observed in suspensions of TS at different
149
CTiO2 ; for small CTiO2 , they are residual agglomerates of silica (Figs. 10a and 10b) and for CTiO2 ú 5 wt% (Figs. 10d and 10e), the formation of agglomerates can occur upon CVD-TiO2 synthesis or due to aggregate interaction in aqueous suspensions. However, for CTiO2 Å 32.4 wt%, only aggregates are observed (Fig. 10f). The TiO2 phase at CTiO2 Å 5 wt% can be present in CVDTS as separated particles with effective diameter (def ) about 0.08 mm (Fig. 10). The size of these particles increases with increasing CTiO2 . Considering that for CTiO2 Å 5 wt% def Å 0.08 mm, and assuming that an increase in CTiO2 leads to the growth only of already formed individual titania particles (in multicycle synthesis), then for CTiO2 Å 32.4 wt%, def of the titania particles should be about 0.15 mm. However, for CTiO2 Å 32.4 wt%, nearly a monodisperse distribution of the particles, with Def Å 0.237 mm, is observed and particles with def below 0.18 mn are not found (Fig. 10f). This might be a consequence of the fact that a fraction of small silica particles (e.g., primary particles or small aggregates) can be embedded into the larger titania particles or the population of individual titania particles decreases as CTiO2 grows due
FIG. 10. Particle size distribution (def ) in aqueous suspensions of CVD-TS at CTiO2 Å 0.6 (a), 2.8 (b), 5 (c), 7.5 (d), 17 (e), and 32.4 wt% (f ).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
coida
150
GUN’KO ET AL.
FIG. 11. Dependence of the effective diameter of CVD-TS particles in aqueous suspensions on pH at CTiO2 Å 28 wt%.
to their sticking together. The def values for individual titania particles in the 7.5–32.4 wt% range of CTiO2 mainly correspond to the size of silica aggregates. However, part of the titania phase can be formed as small clusters embedded into the structure of fractal silica aggregates; otherwise the CTiO2 effect on the CVD-TS properties should be linear. A small peak of Def at CTiO2 Å 17 wt% (Fig. 9, curve 1) can be due to increased synthetic cycle number (Table 1). The particle size distribution for CVD-TS at different pH corresponds to the agglomerate decomposition for pH about the isoelectric points (IEP) for silica or anatase (Fig. 11), where the surface charges ( s ) of the corresponding particular phases are close to zero and the EDL thickness is large (41); i.e., screening of the surface charges by the ions from a dense part of the EDL is weak and the particles can repel. For pH Å 2.5–5.6, the surface charges of the silica and titania phases are opposite, which leads to a strong attraction between them and, hence, Def increases due to particle agglomeration. The mechanism of CVD-TS particle agglomeration for pH ú 6 (Fig. 11) can be explained by attraction between the silica surface ( s õ 0) and some faces of anatase, on which a fraction of strong basic sites (GTi{OH) can have a positive charge due to the reaction GTi{OH r GTi / / OH 0
or GTi{OH / H / r GTiOH 2/ . Silica and anatase are characterized by different dependencies of the surface charges (potentials) on pH, which influences the agglomeration of the CVD-TS particles. The original aqueous suspensions of CVD-TS have natural pH about 6 (Table 1), which is close to IEPTiO2 , but under the same conditions, the silica surface has a negative charge. In this case, the EDL thickness equals approximately 100 nm
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
and such a thick ion-electrostatic barrier can inhibit the agglomeration of CVD-TS particles. With decreasing pH (via addition of HCl), the titania phase becomes positively charged (at pH õ IEPTiO2 ) and the aggregation of positively (TiO2 ) and negatively (SiO2 ) charged particles can occur. When pH is about 2 (about IEPSiO2 ), positively charged titania particles linked with the silica substrate form the potential barrier for the aggregation (as the charge of silica is close to 0) and Def decreases (Fig. 11). For pH ú 6, both phases are negatively charged; therefore, the agglomeration can occur for particles with the same sign charges but of different magnitudes if the attraction dispersion forces overcome the repulsive ones. For such a system, the energy barrier exists at a distance, but for shorter distances, attraction is observed between these particles. The energy of electrostatic interaction between two spherical particles (radii a1 and a2 ) having constant low ( C õ 25 mV) surface potentials (41) can be written as U bel Å
ea1a2 [( c1 / c2 ) 2 ln(1 / exp( 0 kH)) 4(a1 / a2 ) / ( c1 0 c2 ) 2 ln(1 0 exp( 0 kH))]
[3]
and barrier localization (H) is given by exp( kH) Å
1 2
S
c1 c2 / c2 c1
D
.
[4]
For pH ú 6, the surface potential of titania is lower than that for silica, and according to results obtained earlier (19), CTiO2 É 10 mV and CSiO2 É 20 mV; thus H É 0.22k 01 É 22 nm ( k is the Debye–Hu¨ckel parameter). If the CVDtitania particle size is about 100 nm that the barrier height from Eq. [3] equals 2.8 kT. Inasmuch as near the barrier (distance about H É 22 nm) the contributions of Hamaker attraction and structural repulsion are low (less than kT ), they are not considered for the U b estimation. Obviously the particles can overcome this barrier (about 3 kT ) due to Brownian motion to aggregate in the nearest potential minimum. Additionally, titania and silica particles can form large ‘‘dipoles,’’ as they possess opposite charges, which promotes particle aggregation at pH between IEPSiO2 and IEPTiO2 , where Def increases (Fig. 11). Inasmuch as the CVD-titania clusters partially cover the silica surface, the effect of the charge distribution on the titania phase in TS can be strong for all particles; e.g., a Def (pH) graph for CTiO2 Å 28 wt% has two minima corresponding to pH about IEPSiO2 and IEPTiO2 (Fig. 11). However, the particle size distributions for extreme points in this graph have some features (Fig. 12) due to the changes in the titania and silica surface charges for the corresponding pH values.
coida
151
CVD-TITANIA
FIG. 12. Particle size distribution in aqueous suspensions of CVD-TS at pH Å (a) 2.06, (b) 3.06, (c) 5.57, and (d) 10.11; CTiO2 Å 28 wt%.
Electrophoretic Mobility
1
Electrophoretic mobility (U) of TS particles depends nonlinearly on pH (Fig. 13) and CTiO2 (Fig. 14). The silica phase contribution to U is crucial, which obviously results from the dependence of U on CTiO2 at pH 2.5 (Fig. 14a); i.e., U depends weakly on CTiO2 for pH close to IEPSiO2 . U(CTiO2 ) increases with pH from 2.5 to 4, then for pH about IEPTiO2 , U(CTiO2 ) decreases (Fig. 14); i.e., the titania phase contribution is lower than that for silica. This effect is caused not only by a greater content of silica in CVD-TS but also by the structures of the silica and titania phases (e.g., the S value for silica is significantly larger than STiO2 ) as fumed silica is more fractal (primary silica particles about 10 nm are smaller than large CVD-TiO2 particles by a factor above 10). When s õ 0 for the silica and titania phases (pH ¢ 7), U weakly depends on CTiO2 (Fig. 14). Besides, the Def value increases as pH grows (Fig. 11), which leads to a decrease in mobility, as U Ç D 01 ef (30, 41). The IEP dependence on CTiO2 is also nonlinear (Fig. 15) and it is maximum for a small content of TiO2 , when the separated anatase phase is not observed and large titania particles are absent. The formation of well-defined anatase phase at CTiO2 Å 5 wt% reduces the influence of titania on the IEP of CVDTS. The IEP(CTiO2 ) graph in the 5–32.4 wt% range has another maximum at CTiO2 Å 17 wt% due to factors such as growing CTiO2 and Def . However, for this titania concentration, Def (CTiO2 ) has a small maximum (Fig. 9) as well as acidity of aqueous suspensions (Table 1).
The 1H NMR signals detected for frozen aqueous suspensions of oxides at T below 273 K are caused by interfacial water unfrozen due to disturbance by the oxide surface (8, 31, 40) and two regions for the DG(H) function can be found (Fig. 16 and Table 2). The first is a region of fast decrease in the thickness of the unfrozen water layer in a relatively narrow range of DG values at T slightly below 273 K and the second is a region where the H value decreases weakly in a wide range of DG values for greater changes of freezing temperature. A fraction of adsorbed water, which causes the appearance of the first region, may be determined as weakly bound water as the freezing temperature is slightly below 273 K. A long-range component of the radial function of change of the free energy (RFG) corresponds to this region. The second region for DG(H) corresponds to strongly bound water (as its freezing temperature is significantly below 273 K) and a short-range component of RFG. The thickness of the interfacial layer for each type of water (Hs and Hw for strongly and weakly bound water, respectively) and the maximum values of DG ( DG smax and w DG max in Table 2) can be estimated via extrapolation of the corresponding portions of the curves to the X and Y axes (8, 31). The value of total free energy change of the adsorbed water layers may be determined by a linear approach as
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
H NMR Study of Aqueous Suspensions of CVD-TS
DGS Å k( DGs Hs / DGw Hw )/2,
[5]
where k is the proportionality coefficient depending on pa-
coida
152
GUN’KO ET AL.
FIG. 13. Dependence of electrophoretic mobility of SiO2 (a, curve 1) and CVD-TS at CTiO2 Å 0.6 (a, curve 2), 1.7 (b, curve 1), 2.8 (b, curve 2), 5 (c, curve 1), 17 (c, curve 2), 28 (d, curve 1), and 32.4 wt% (d, curve 2) on pH.
rameter dimensions in Eq. [5]. The thickness of the water layer disturbed by the surface can reach to several tens of water molecular diameters (Table 2 and Fig. 16). This value is higher than a distance of effective intermolecular interactions but corresponds to the EDL thickness for TS particles. The H and DGS values are maximal for CTiO2 Å 1.7 wt% when the separated crystalline titania phase is not formed and TiO2 is distributed on the silica support in the form of small clusters. The values of DGS and H for fumed TS are higher than those for CVD-TS at CTiO2 ¢ 17 wt% (Table 2). This can be explained by the formation of large titania particles in CVD-TS at high CTiO2 giving a relatively small contribution to the specific surface area and having relatively slight contact with the silica substrate. For example, CVDTS at CTiO2 Å 17 wt% has a local small maximum of Def (Fig. 9) and gives the lowest DGS and H (Table 2) close to the corresponding values for Aerosil A-300. Also, the DGS value for this CVD-TS sample is close to the immersion enthalpy ( DHim ) for Cab-O-Sil with S Å 207 m2g 01 , and DGS for fumed silica used as the substrate (Table 2) is close to DHim for quartz (42). However, the difference for DHim (H) and DG(H) behavior is observed, as for DHim , the strong decrease is for the first water layer (42), but DG(H) reduces more slowly with increasing H (Fig. 16).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
For many CVD-TS samples, the DG smax values change slightly in comparison with that for silica (Table 2) as well as for other mixed oxides such as alumina/silica (40). The DGS value for fumed TS is greater than that for CVD-TS at high CTiO2 . This can be explained by a greater number of GSiO(H)TiG bonds as strong acidic sites on fumed TS than on CVD-TS, which manifests in optical spectroscopy studies (Fig. 5). Thus, all characteristics obtained in the 1H NMR study of aqueous suspensions of CVD-TS (Table 2) depend nonlinearly on CTiO2 . Theoretical Simulations The calculations of the cluster models of TS were performed using quantum chemical semiempirical NDDO method (19) and molecular mechanics MM2 (43). We modeled the CVD-TS interface (Figs. 2 and 17) using a cluster approach. The difference in the structures of fumed TS and CVDTS defines distinction in their surface characteristics. For fumed TS, the GSi{O(H){ TiG and GSi{O{TiG bridges have a strong influence on the surface properties, but in the case of CVD-TS, the number of such bonds is smaller because they are unstable due to distortions in the interface (Fig. 2, e.g., for bond lengths Dr/rmax É 0.09 and
coida
153
CVD-TITANIA
(Mi Å Si, Al, Ti, and Ge) using different semiempirical and ab initio methods. The obtained results testify that dissociative adsorption of water, G M1{O{ M2G / H2O r G M1 (OH){ O(H){ M2G,
occurs with lower activation energy if metal atoms Mi already bond hydroxyl groups. Besides, these reactions can be promoted by additional water molecules adsorbed on these sites. Different defects such as strained rings M » OO … Si (M Å Ti, Al, or Si) can interact with water molecules with a high exothermicity. The titania–silica interface in fumed TS is not so strained as in the original CVD-TS samples due to annealing of structural strains during the fumed TS synthesis. Removal of these strains at the interfaces of CVD-TS occurs via interaction with water upon its dissociative adsorption leading to disappearance of the Si{O{Ti bridges. All bonds formed between the titania and silica phases in CVD-TS (Fig. 2) can be strained as the lattice constants of these oxides differ markedly. Therefore, an increase in the TiO2{SiO2 contact area increases the bond strain at the interface and heat effect upon hydrolysis of such bonds can be higher than in the case of water adsorption on the surface of calcined individual oxides. The calculations by NDDO method show a high exothermicity of hydrolysis of even one strained Si{O{Ti bridge. For example, in the reaction of a water molecule with a small cluster (HO)3TiOSi(OH)3 , (HO)3Ti{O{Si(OH)3 / H2O r (HO)4Ti{O(H){ Si(OH)3 ,
[6]
the total energy change ( DEt ) equals 0103 kJ/mol (NDDO)
FIG. 14. Dependence of electrophoretic mobility of the CVD-TS particles on CTiO2 at pH Å 2.5 (a, curve 1), 3 (a, curve 2), 4 (a, curve 3), 5 (b, curve 1), 6 (b, curve 2), 7 (b, curve 3), 8 (c, curve 1), 9 (c, curve 2), and 10 (c, curve 3).
valence angles Df / fmax É 0.2 at the interface) and can be easily hydrolyzed. Previously (8, 10, 19, 21, 22, 39, 40, 44) we studied the interaction of water molecules with the M1{O{ M2 bridges
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
FIG. 15. Dependence of the IEP value for aqueous suspensions of CVDTS on CTiO2 .
coida
154
GUN’KO ET AL.
FIG. 16. Dependencies of G0-G on the thickness of the unfrozen interfacial water layer for (a) silica (1), CVD-TS at CTiO2 Å 0.6 (2), 1.7 (3); and (b) CVD-TS at CTiO2 Å 5 (1), 17 (2), and 32.4 (3) wt%.
and in the reaction with a strained ring with no decomposition,
between anatase and crystobalite clusters in Fig. 2b. Consequently, the exothermicity of dissociative water adsorption with no Si{O{Ti bond breaking (Eqs. [6] and [7]) is less than that in the reactions with strain relieving as in Eq. [8]. Also, this effect increases upon cleavage of a few neighboring strained bonds at the TS interfaces as in Fig. 2. The NDDO calculation of hydrolysis of three neighboring Si{O{Ti bonds (Fig. 17a r Fig. 17b) give DEt about 0500 kJ/mol per strained bond (the NDDO method can overestimate DEt for water chemisorption as well as other semiempirical methods (45)). Dissociative adsorption of water molecule on pure crystalline anatase gives DEt Å 0255 kJ/mol by NDDO (19). Thus, DEt for the hydrolysis (Fig. 17a r Fig. 17b) per bond is higher than DEt for Eq. [8]; i.e., relaxation of neighboring bonds, which do not take part in the hydrolysis, gives an essential contribution (about 20%) to the total reaction exothermicity. The MM2 calculations give a relatively small effect ( DEt Å 027 kJ/mol per hydrolyzed Si{O{Ti bond) in the process (Fig. 17a r Fig. 17b). This can be explained by a weaker influence of the angle deformations in the MM2 force field on the total energy in comparison with the NDDO method. Besides, MM calculations do not explicitly treat the electrons in a molecular system, as electronic effects are included in force fields only via its parameterization; therefore, a portion of the energy of chemical bond transformations can be lost in the MM calculations. It seems likely that calculated deformations (until 10% for bonds and 20% for valence and dihedral angles) can give the main contribution to the exothermicity caused by strain relieving via hydrolysis of the CVD-TS interface. Because of this, for freshly prepared CVD-TS, the amount of adsorbed water and its free energy changes can be more greater than analogous values for individual oxides. The MM2 calculations of the TS clusters (Fig. 17) covered by a solvation shell with 122 water molecules give a deeper stabilization of the initial cluster
TABLE 2 Characteristics of the Interfacial Water Layers for Aqueous Suspensions of CVD-TS
(HO)2Ti » OO … Si(OH)2 / H2O r (HO)3Ti »
O(H) O
… Si(OH)2 ,
[7]
DEt Å 0106 kJ/mol, but upon strained bond hydrolysis,
(HO)3Ti » OO ( H ) … Si(OH)2 / H2O r (HO)3Ti{O(H){ Si(OH)4 ,
[8]
DEt Å 0397 kJ/mol. The bond elongation in the strained ring Ti » OO … Si is 0.017 nm for Si{O and 0.025 nm for Ti{O, which is close to the elongation for the boundary
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
Sample
CTiO2
Silicaa Silicab CVD-TS CVD-TS CVD-TS CVD-TS CVD-TS Fumed ST
0
a b
0.6 1.7 5.0 17.0 32.4 20.0
DGSmax (kJ/mol)
DGWmax (kJ/mol)
HS
HW
DGS (mJ/m2)
3.8 3.5 3.5 3.9 2.9 3.5 4.2 2.7
1.0 1.3 1.5 1.0 1.2 2.0 0.9 0.8
3.2 8.0 9.5 21.0 11.0 4.2 7.5 10.5
8.4 17.0 11.5 40.0 13.0 5.0 17.0 25.0
185 451 455 1097 428 222 421 435
Silica Aerosil A-300 (34). Silica used as a substrate for CVD-TiO2 .
coida
CVD-TITANIA
155
FIG. 17. Cluster model of CVD-TS with crystobalite/anatase: (a) initial structure and (b) after hydrolysis of three Si{O{Ti bonds and geometry relaxation calculated by NDDO method.
(Fig. 17a) in the water shell than post-hydrolysis separated clusters of TiO2 and SiO2 (Fig. 17b): DDEt Å 050 kJ/mol. CONCLUSION
CVD-titania does not form a continuous layer on the silica substrate independent of the concentration. Its significant fracture appears as nanoscaled anatase particles (70–250 nm at high CTiO2 ), which have weak contact with the silica phase via hydrogen bonding and electrostatic interaction and a small number of Si{O{Ti bonds. However, the silica substrate inhibits the anatase phase transition to rutile. The titania concentration in calcinated CVD-TS has a minor influence on the amount of adsorbed water due to a small contribution of the titania phase in the specific surface area of CVD-TS. However, a fraction of titania can be distributed over the silica matrix as the small clusters (especially at CTiO2 õ 3 wt%), which affect not only the structure and the free energy of adsorbed (interfacial) water (the effect is at a maximum for CTiO2 Å 1.7 wt%) but also the concentration of B- and L-sites and the particle (agglomerates) size distribution in aqueous suspensions of CVD-TS. The graph of Def on pH for CVD-TS has two minima corresponding to IEP of silica and titania. The CTiO2 increase leads to decrease in Def to the size of silica agglomerates about 0.2 mm. ACKNOWLEDGMENTS Some of the authors are grateful to the Sherwin–Williams Co. (U.S.A.) for partial financial support of this work. V.M.G. thanks the Swiss National Science Foundation (Grant 7UKPJ 48657) for financial support. The authors thank the referees for useful remarks.
REFERENCES 1. Fernandez, A., Gonzalez-Elipe, A. R., Real, C., Caballero, A., and Munuera, G., Langmuir 9, 121 (1993).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
2. (a) Dutoit, D. C. M., Schneider, M., and Baiker, A., J. Catal. 153, 165 (1995); (b) Hutter, R., Mallat, T., Dutoit, D., and Baiker, A., Top. Catal. 3, 421 (1996); (c) Dartt, C. B., and Davis, M. E., Appl. Catal. A 143, 53 (1996). 3. Hutter, R., Mallat, T., and Baiker, A., J. Catal. 153, 177 (1995). 4. Imamura, S., Nakai, T., Kanai, H., and Ito, T., J. Chem. Soc. Faraday Trans. 91, 1261 (1995). 5. Jehng, J.-M., Wachs, I. E., Weckhuysen, B. M., and Schoonheydt, R. A., J. Chem. Soc. Faraday Trans. 91, 953 (1995). 6. Arndt, J., Phys. Chem. Glasses 24, 104 (1983). 7. Matijevic, E., Langmuir 10, 8 (1994). 8. Gun’ko, V. M., Zarko, V. I., Turov, V. V., Voronin, E. F., Tischenko, V. A., and Chuiko, A. A., Langmuir 11, 2115 (1995). 9. Voronin, E. F., Pakhlov, E. M., and Chuiko, A. A., Colloids Surf. A 101, 123 (1995). 10. Zarko, V. I., and Gun’ko, V. M., Functional Mater. 2, 110 (1995). 11. Zarko, V. I., Kozub, G. M., Pokrovskiy, V. A., and Chuiko, A. A., Teor. Eksper. Khim. 27, 735 (1991). 12. Contesu, C., Popa, V. T., Miller, J. B., Ko, E. I., and Schwarz, J. A., J. Catal. 157, 244 (1995). 13. Miller, J. B., Johnston, S. T., and Ko, E. I., J. Catal. 150, 311 (1994). 14. (a) Liu, Z., Tabora, J., and Pavis, R. J., J. Catal. 149, 117 (1994); (b) Hoffmann, M. R., Martin, S. T., Choi, W., and Bahnemann, D. W., Chem. Rev. 95, 69 (1995). 15. Khonw, C. B., and Dartt, C. B., J. Catal. 149, 195 (1994). 16. Sushko, R. V., Gette, A. V., Mironyuk, I. F., and Chuiko, A. A., Zh. Priklad. Khim. 56, 1230 (1983). 17. Schraml-Marth, M., Walther, K. L., Wokaun, A., Handy, B. E., and Baiker, A., J. Non-Cryst. Solids 143, 93 (1992). 18. Greegor, R. B., Lytle, F. W., Sandstrom, D. R., Wong, J., and Schultz, R., J. Non-Cryst. Solids 55, 27 (1983). 19. Gun’ko, V. M., Zarko, V. I., Chibowski, E., Dudnik, V. V., Leboda, R., and Zaets, V. A., J. Colloid Interface Sci. 188, 39 (1997). 20. (a) Pakhlov, E. M., Voronin, E. F., and Sushko, R. V., Zh. Prikl. Spectrosk. 47, 311 (1987); (b) Pakhlov, E. M., Zarko, V. I., and Voronin, E. F., Ukr. Khim. Zh. 59, 373 (1993). 21. Gun’ko, V. M., Zarko, V. I., Chuikov, B. A., Dudnik, V. V., Ptushinskii, Yu. G., Voronin, E. F., Pakhlov, E. M., and Chuiko, A. A., Int. J. Mass Spectrom. Ion Process., in press. 22. Gun’ko, V. M., Voronin, E. F., Zarko, V. I., Pakhlov, E. M., and Chuiko, A. A., J. Adhesion Sci. Technol. 11, 627 (1997). 23. (a) Stieg, F. B., J. Coat. Technol. 61, 67 (1989); (b) Kerker, M., Cooke, D. D., and Ross, W. D., J. Paint Technol. 47, 333 (1975).
coida
156
GUN’KO ET AL.
24. Powell, Q. H., Kodas, T. T., and Anderson, B. M., Chem. Vapor Depos. 2, 179 (1996). 25. (a) Hanprasopwattana, A., Reiker, T., Sault, A. G., and Datye, A. K., Catal. Lett. 45, 165 (1997); (b) Hanprasopwattana, A., Srinivasan, S., Sault, A. G., and Datye, A. K., Langmuir 12, 3173 (1996). 26. (a) Aleskovskiy, V. B., and Yuffa, A. Ya., J. Vses. Khim. Obsh. Mendeleev. 34, 317 (1989); (b) Aleskovskiy, V. B., ‘‘Stereochemistry and Synthesis of Solid Compounds.’’ Nauka, Leningrad, 1976. 27. Kubelka, P., and Munk, F., Z. Tech. Phys. 12, 593 (1931). 28. (a) Tanabe, K., ‘‘Solid Acids and Bases.’’ Kodansha, Tokyo, 1970; (b) Brazdil, J. F., and Yeager, E. B., J. Phys. Chem. 85, 1005 (1981); (c) Sivalov, E. G., and Tarasevich, Yu. I., Adsorption Adsorbents N10, 49 (1982). 29. (a) Zarko, V. I., Kozub, G. M., Sivalov, E. G., and Chuiko, A. A., Ukr. Khim. Zh. 54, 1144 (1988); (b) Tarasevich, Yu. I., and Sivalov, E. G., Dokl. Akad. Nauk Ukr. Ser. B N3, 252 (1978). 30. Hunter, R. J., ‘‘Zeta Potential in Colloid Sciences.’’ Academic Press, London/San Francisco, 1981. 31. (a) Gun’ko, V. M., Turov, V. V., Zarko, V. I., Dudnik, V. V., Tischenko, V. A., Kazakova, O. A., Voronin, E. F., Siltchenko, S. S., Barvinchenko, V. N., and Chuiko, A. A., J. Colloid Interface Sci. 192, 166 (1997); (b) Turov, V. V., Leboda, R., Bogillo, V. I., and Skubishewska-Zieba, J., Langmuir 13, 1237 (1997); (c) Turov, V. V., Zarko, V. I., and Chuiko, A. A., Zh. Fiz. Khim. 69, 677 (1995). 32. (a) V. P. Glushko (Ed.), ‘‘Handbook of Thermodynamic Properties of Individual Substances.’’ Nauka, Moskow, 1978; (b) Mank, v. V., and Lebovka, N. I., ‘‘NMR Spectroscopy of Water in Heterogeneous Systems.’’ Naukova Dumka, Kiev, 1988. 33. Byun, C., Jang, J. W., Kim, I. T., Hong, K. S., and Lee, B. W., Mater. Research Bull. 32, 431 (1997).
AID
JCIS 5270
/
6g39h$$301
01-30-98 10:52:22
34. Otterman, C. R., and Bange, K., Thin Solid Film 286, 32 (1996). 35. Voronkov, M. G., Maletina, E. A., and Poman, V. K., ‘‘Heterosiloxanes.’’ Nauka, Novosibirsk, 1984. 36. Tertykh, V. A., Ogenko, V. M., Voronin, E. F., and Chuiko, A. A., Zh. Prikl. Spectrosk. 23, 464 (1975). 37. (a) Little, L. H., ‘‘Infrared Spectra of Adsorbed Species.’’ Academic Press, London, 1966; (b) Kiselev, A. V., and Lygin, V. I., ‘‘Infrared Spectra of Surface Compounds and Adsorbed Matters.’’ Nauka, Moscow, 1972. 38. (a) Barthel, H., Colloids. Surf. A 101, 217 (1995); (b) Barthel, H., Rosch, L., and Weis, J., in ‘‘Organosilicon Chemistry. II. From Molecules to Materials’’ (N. Auner and J. Weis, Eds.), pp. 761–778. VCH, Weinheim, 1996. 39. (a) Zarko, V. I., Gun’ko, V. M., Chibowski, E., Dudnik, V. V., and Leboda, R., Colloids Surf. A, 127, 11 (1997); (b) Gun’ko, V. M., Zarko, V. I., Leboda, R., Voronin, E. F., Chibowski, E., and Pakhlov, E. M., Colloids-Surf. A, in press. 40. Gun’ko, V. M., Zarko, V. I., Turov, V. V., Voronin, E. F., Tischenko, V. A., Dudnik, V. V., Pakhlov, E. M., and Chuiko, A. A., Langmuir 13, 1529 (1997). 41. Derjaguin, B. V., Churaev, N. V., and Muller, V. M., ‘‘Surface Forces.’’ Plenum, New York, 1990. 42. (a) Douillard, J. M., J. Colloid Interface Sci. 188, 511 (1997), and 182, 308 (1996); (b) Douillard, J. M., Zoungrana, T., and Partyka, S., Petrol. Sci. Eng. 14, 51 (1995); Malandrini, H., Sarraf, R., Faucompre, B., Partyka, S., and Douillard, J. M., Langmuir 13, 1337 (1997). 43. Burket, U., and Allinger, N. L., ‘‘Molecular Mechanics.’’ Am. Chem. Soc., Washington, DC, 1982. 44. Gun’ko, V. M., Voronin, E. F., Zarko, V. I., and Pakhlov, E. M., Langmuir 13, 250 (1997). 45. Bredow, Th., and Jug, K., Surf. Sci. 327, 398 (1995).
coida