J . Phys. Chem. 1985, 89, 4399-4404
in 2-1 electrolytes. The model fitting to the experimental data allows us to extract two independent parametrers, fi andfmd. The growth behavior of micelles-in 2-1 electrolytes is quite dTfierent from that in 1-1 electrolytes. The divalent counterions Mgz+ and Ca2+are more effective in the shielding of electrostatic interactions between micelles than the monovalent counterion Li+. The effective micellar surface charges are strongly affected by the added 2-1 electrolytes when the detergent concentration is low. The measured fexpl is in reasonable agreement with the E M N theory. The mechanism which is responsible for switching the intermicellar interaction from a repulsive to an attractive one is probably due to the gradual dominance of the van der Waals interaction as the
4399
double layer repulsion diminishes, and a further investigation on the nature of this attraction should be of great interest.I3 Acknowledgment. We are grateful to the Biology Department of BNL for use of their small angle spectrometer in this work. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. Registry No. LDS,2044-56-6; MgC12,7786-30-3; CaC12, 10043-52-4. (1 3) Israelachvili, J. N. "Intermolecularand Surface Forces"; Academic Press: New York, 1985.
NMR and Raman Study of the Hydrolysls Reaction in Sol-Gel Processes I. Artaki, M. Bradley, T. W. Zerda, and J. Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: April 5, 1985)
High-resolution NMR and Raman spectroscopy have been employed to investigatethe hydrolysis reaction in sol-gels containing chemical additives. These techniques have been shown to provide compatible results regarding the hydrolysis of tetramethylorthosilicate, Si(OCH3)& The differing rates of hydrolysis in formamide and methanol solutions have been tentatively explained as a function of viscosity and hydrogen-bonding characteristics. The initial stages of the condensation reaction have been followed by NMR and various low order polymeric species have been partially assigned.
Introduction The production of high-quality glasses and ceramics from silica gels rather than sand or clay has become an area of increasing interest.' The high temperatures required for firing in the latter technique (>1300 OC vs. 800 "C) and the high homogeneity and molding characteristics of the former make gelation a very attractive method. Consequently, a tremendous effort is currently being placed into understanding the elusive dynamics of gelation. Many techniques have been brought to bear on the problemsmall ~ , ~ size deangle X-ray,2 chemical test^,^^^ light ~ c a t t e r i n g ,pore termination7**-including recent NMR8,9and Raman8Jo results from this laboratory. This paper reports new experiments using the N M R and Raman techniques applied to the early and intermediate phases of gelation from silica sols. There are two ~ell-known'.~ stages of gelation when a silicon alkoxide is used to create a sol. These two stages, hydrolysis and condensation, are tightly interlocked in certain systems where they occur simultaneously and less so in others where one of the two is relatively fast." In all of the work here, the systems and experimental conditions were such that the hydrolysis and the condensation occurred on comparable time scales. (1) Brinker, C. J.; Clark, D. E.; Ulrich, D. R. Muter. Res. SOC.Symp. 1984, 32. (2) Schaefer, D. W.; Keefer, K. D. Phys. Reu. Lett. 1984, 53, 1383. (3) Iler, R. K. "The Chemistry of Silica"; Wiley: New York, 1979. (4) Iler, R. K. J . Colloid Interface Sci. 1980, 75, 138. (5) Marquee, J. A.; Deutch, J. M. J . Chem. Phys. 1981, 75, 5239. (6) Jamieson, A. M.; McDonnell, M. E. In 'Robing Polymer Structures", Kcenig, J. L. Ed.;American Chemical Society: Washington, DC, 1979; Adv. Chem. Ser., No. 174. (7) Orcel, G.; Hench, L. L. In Mater. Res. Soc. Symp. Proc. 1984,32,79. (8) Artaki, I.; Bradley, M.; Zerda, T. W.; Jonas, J.; Orcel, G.; Hench, L. L. In "Ultrastructure Processing of Ceramics, Glasses and Composites", Hench, L. L., Ulrich, D. R., Us.; Wiley: New York, 1985. (9) Artaki, I.; Sinha, S.; Jonas, J. Mater. Lett. 1984, 2, 448. (IO) Zerda, T. W.; Bradley, M.; Jonas, J. Mater. Lett. 1985, 3, 124. ( 1 1 ) Yamane, M.; Inoue, S.; Yasumori, A. J. Non-Cryst. Solids 1984, 63, 13.
In hydrolysis, the alkoxy groups on the silicon atom are replaced by hydroxy groups. This replacement does not occur on all four alkoxy units at once but proceeds rather in a series of steps. Thus, instead of forming just the totally hydrolyzed Si(OH).+,there are actually several possible reactions yielding a variety of silicon species." A general scheme for the chemical reactions which may occur must account for the simultaneous condensation reaction:
-
hydrolysis: Si(OR),
t
nH2O
Si(OR),-,,(OH),
t nROH
(1)
condensation:
I I I -Si-OH
-Si-OR
I
t HO-Si-
t HO-Si-
I I
I I
-
I
I
I I -Si-0-SiI
I I I
-Si-0-Si-
t ROH (2)
t H 2 0 (3)
Condensation may occur between partially hydrolyzed species which may in turn further hydrolyze. The mechanism behind hydrolysis (eq 1) consists of a nucleophilic substitution reaction wherein the hydroxy group attacks the silicon atom. Charge separation arguments require this attack to occur on the side opposite the leaving alkoxy group which may necessitate reorientation of the silicon.'* The equilibrium for these reactions will be affected by a wide range of experimental factors such as temperature, pH, and various constituent concentrations. In order to probe the time evolution of eq 1-3 and the effects of these factors on it, the concentrations of the various species must be attained. In both N M R and Raman, this may be achieved by observing the growth or decay of the peaks characteristic for each species. The utility of N M R derives from the differing chemical shifts of the silicon signal for each of the various (silicon containing) (12) Keefer, K. D. Mater. Res. SOC.Symp. Proc. 1984, 32, 15.
0022-3654/85/2089-4399$01.50/00 1985 American Chemical Society
4400
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
species in eq 1-3. Peaks due to the possible partially hydrolyzed monomeric units will thus be displaced from the dimer peaks and are further shifted according to the degree of hydrolysis. The case for the dimers is the same. Because of the chemical complexity produced by the continuing condensation, the lines extensively broaden due to the overlap of neighboring resonances. In contrast, Raman scattering provides data throughout the reaction and even well after gelatiomlo The differences in mass and force constants for the various partially hydrolyzed monomeric silicon species separates the Raman peaks sufficiently to allow their concentrations to be determined. Simultaneous changes in the methanol concentration are also observed. As noted above, the gelation technique is of interest because it allows preparation of homogeneous glasses at moderate temperatures. Unfortunately, during the drying process any solvent trapped within the gel may cause cracks to occur in the gel structure while escaping. These microscopic fractures may, if drying is done too rapidly, reduce the gel to a dry powder. Recently,’ use has been made of drying control chemical additives (DCCA’s) which significantly reduce fracturing. The effect of one such additive (formamide) on the condensation process is also under study.’) Formamide was found to influence the gelation process significantly, therefore indicating a change in the early stages of gelation. This study will investigate changes brought about in these earlier stages involving eq 1-3 by the DCCA formamide. Results from N M R and Raman will be compared and an analysis with and without the formamide performed.
Experimental Section Sample Preparation. There were two types of samples investigated. The first involved dropwise addition of an H20/ methanol mixture to the silicon initiating reagent, Si(OCH,).,, which had been previously mixed with the DCCA formamide. The second sol was the same except for the substitution of the DCCA by an equal amount of methanol. The solutions with and without formamide will be denoted as I and 11, respectively. The relative concentrations of each constituent was predetermined for a final silicon concentration of 1.6 M and 1 0 1 mole ratio of H20to silicon in both systems. For all of the N M R experiments and for part of the Raman experiments the temperature was set at 12 O C . The remaining Raman results were taken at 21 ‘C. Glass sample tubes were used for both types of experiments. N M R and Raman. Natural abundance 29SiN M R measurements were performed on a 250-MHz pulsed N M R spectrometer equipped with a quadrature detection system, spin-decoupler assembly, and variable-temperature capability. A commonly encountered problem in 29Si N M R studies is the presence of a broad resonance signal which arises from the glass sample tube or the probe insert and obscures a large portion of the high-resolution spectrum. This difficulty was eliminated by employing the spin-echo method which relies on the difference between the spinspin relaxation times of the Si nuclei in the solid glass phase and the Si nuclei in the liquid sol phase.14 The spin-echo method consists of the application of a 90°-~-1800 pulse sequence followed by the Fourier transformation of the free induction decay of the echo which forms at 27. The optimum 90’ pulse length and decay time T were determined to be 33 /JUSnd 50 ms, respectively. The spectra, recorded at spacings of 1 h, consisted of 1200 transients accumulated at 3-s intervals. Small amounts of the paramagnetic reagent Cr(acac)) were added to the solutions in order to suppress the excessively long spin-lattice relaxation times of 29Siand allow accummulations at 3-s intervals without saturation occurring. The Raman spectrometer, described previously,15consisted of a Spectra-Physica argon ion laser operating at 488 nm and 0.6-W output power. A 1.2-cm-’ slit and scanning interval of 4 cm-’ were used. The laser was polarized vertically with no analyzer. The digitized data taken at 30-min intervals were stored on floppy disks and transferred to a VAX-11 computer where processing (13) Artaki, I.; Zerda, T. W.; Jonas, J. J. Non-Crysr. Solids, in press. (14) Farrar, T. C.;Becker, E. D. “Pulse and Fourier Transform NMR”; Academic Press: New York, 1971. (15) Perry, S.; Zerda, T. W.; Jonas, J. J. Chem. Phys. 1981, 75, 4214.
Artaki et al.
0
/
I
,
/
,
1
,
12
IO
8
6
4
2
0
-
1
,
,
,
1
1
/
/
-2
-4
-6
-8
-10
-12
-14
-16
PPm
A
0
4d,,I
,
,
30 25 20
‘IL 4321 C,D
,
,
, ,
15
IO
5
,
,
I
,
,
,
,
I
,
,
,
,
-5 -10 -15 -20 -25 -30 -35 -40-45 -50-55 -60
0
Ppm
Figure 1. Natural abundance 29SiN M R spectrum of solution I acquired at (a, top) t / t e = 0.3, (b, bottom) t / t , = 0.6. P 0
/
12
-
8
IO
4
6
2
,
I
0
-2
-4
-6
,
1
1
1
/
,
,
-8
/
/
I
/
-10 -12
-14
-16
-18
PPm
A
4
1
1
25 2 0
B
1
1
1
l
1
15
IO
5
0
-5
1
,
1
,
l
l
I
-10 -15 -20 -25 -30 -35 - 4 0 -45 -50 -55 -60
PPm Figure 2. Natural abundance 29SiN M R spectrum of solution I1 acquired at (a, top) t/t, = 0.3, (b, bottom) t / t , = 0.6.
was done. Temperature control was accomplished by a Lauda Model RM6 water circulator and measured by using a thermocouple in contact with the glass sample cuvette. Between experiments, the apparatus was not changed (other than resetting wavenumbers) i.e. the laser was left on at constant power, the optics were not altered, and the cell was not disturbed. N o calibration within a series of spectra acquired in this manner was found necessary so the spectra presented here are raw, unsmoothed profiles. The viscosities of solutions I and I1 were measured at room temperature with a falling slug viscometer.I6 (16) Artaki, I.; Jonas, J. J . Chem. Phys. 1985,82, 3360.
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4401
Hydrolysis Reaction in Sol-Gel Processes TABLE I: Chemical Shifts of the Qo and Q1Structural Units Given in ppm with respect to TMOS &i
A1 A2 A3 A4
structure Si(OCH3), Si(OCH,),(OH) Si(OCHMW2 Si(OCH,)(OH), Si(OH)4
B1 B2 B3 B4
Si"(OCH,),-O-Si@Sin(OCH,)2(OH)-O-Si@Si"(OCH,) (OH) ,-O-Si@Sia(0H),-O-Si@-
-7.5 -6.1 -4.9 -4.0
A0
0
TABLE II: Chemical Shifts of the Q1Structural Units Given in ppm with respect to TMOS resonance line A&, resonance line ABsi B2
B4
a
2.0 3.4 4.5 5.3
b C
d
-3.934 -4.023 -4.084 -4.143
b C
d
-
C
d
e
B3 a
a b
-4.844 -4.930 -4.994 -5.067
f B1 a b C
El
d
c
e
E2
f g
-3
-4
-5
-6
-7
-e
-9
PPm
Figure 3. Expanded region of the Q' resonance peaks of solution I1 at t / r , = 0.4.
Results and Discussion The very narrow line widths typically observed in aqueous silicate solutions allow the resolution of resonances from the same isotopic species present in different chemical environments. Due to the low natural abundance of the %i isotope and because proton broad-band decoupling has been employed, electron coupled spin-spin interactions which give rise to hyperfine splittings of the resonance lines can be neglected. Therefore, each observed resonance signal corresponds to an individual, chemically discrete nucleus. Figures 1 and 2 show typical N M R spectra obtained for solutions I and 11, respectively, at two stages in the polymerization process: (a) t / t , = 0.3 and (b) t / t , = 0.6 where t, denotes the gelation time. All chemical shifts are reported relative to TMOS (Si(OCH,),), with positive shifts to higher frequency. For clarity, Figures l a and 2a have been expanded about the region in which the silicon resonances are present. For the assignment of the individual peaks the letter convention of previous publication^^^^^^ has been adopted. The symbol Q' is used to represent specific silicate structural units, where i refers to the number of siloxane bonds attached to the silicon atom. The resonances which belong to the group labeled A correspond to the monomeric units, Qo. Depending upon the extent of hydrolysis, a total of five monomeric units of the type [Si(OCH3),(OH),,] may exist where x can vary from 0 to 4. The exact assignment of the individual resonances of group A was based on the relative position of fully hydrolyzed tetrasilicic acid, Si(OH)41Ewith respect to unreacted TMOS, [Si(OCH,),]. All chemical shifts are summarized in Table I. The resonances of the group labeled B are assigned to end group silicons, denoted by Q'. Again, depending upon the character of the substituents directly attached to the silicon nucleus giving rise to the signal, four resonances may exist, as listed in Table I. However, each of these resonances exhibits fine structure, as shown in Figure 3. The fine structure is due to the additional chemical shift caused by the nature of the substituents attached to the silicon adjacent to the silicon nucleus giving rise to the signal. It is (17) Harris, R.K.; Knight, C.T. a,;Hull, w.E.In *soluble silicatw", Falcone, J. S., Ed.; American Chemical Society: Washington,DC,1982; ACS Symp. Ser., No. 194. (18) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Till, A. Z . Anorg. Allg. Chem. 1911, 450, 5 .
-6.012 -6.062 -6.099 -6.160 -6.198 -6.247 -7.535 -7.583 -7.620 -7.657 -7.746 -7.816 -7.977
assumed that the end groups originate to a large extent from linear dimeric and trimeric species." For a given configuration of silicon a,[Sia(OH),(0CH,),~,-O-Si-] a total of four different degrees of hydroxylation of Si8 are possible in the case of a dimer [Sia(OH),(OCH3)3-x-O-Si~(OH)y(OCH3)3-y] and only three in the case of a trimer [Siu(OH),(OCH3)3-,-O-Si(OH),(OCH3)2-,,-0-Si-]. Despite S/N limitations, comparison of several spectra obtained at different stages of the polymerization reaction allowed the elimination of spurious noise spikes and identification of actual resonance peaks, as marked in Figure 3. Unfortunately, the information content of Figure 3 is not sufficient for the assignment of the individual resonances in terms of the silicon species described above. A precise assignment would require semiempirical calculations for the determination of the electron distribution around the silicon nuclei and also accurate predictions regarding bond angles, bond orders, and polarities. This task is beyond the scope of the present publication. A detailed listing of the chemical shifts of all observed resonances belonging to the Q' structural unit is given in Table 11. Figure 3 illustrates the wealth of information which 29SiN M R can provide. An extremely important point connected with this is that, once the assignment is completed and each resonance line of Figure 3 attributed to a specific chemical species, its intensity and thus the concentration can be monitored as a function of time up to the gelation point. However, as also noted by Assink et al.,I9 in order to extract the kinetic rate constants extensive mathematical computations would be needed for solving the very large number of coupled, nonlinear differential equations. The bands labeled C and D in Figures 1 and 2 correspond to Band C is attributed to the central the Q2structural Si atom of linear trimers and band D to cyclotetramers. Bands E and F are assigned to Q3and Q4 structural units, respectively. Typical Q3and Q4units are prismatic hexamers and cubic octamers, respectively, although other configurations and conformations can be placed in this category as well. The breadth of the C, D, E, and F bands is due to the overlap of a large number of single resonance lines produced as a result of the highly varied chemical environment surrounding the silicon nuclei of condensed oligomeric species. For instance, depending upon the degree of hydroxylation, a total of 48 chemically different linear trimers and 8 1 chemically different cyclotetramers are possible. Therefore the use of 29Si N M R spectroscopy as a structure identification tool is limited to the study of the initial stage of the sol-gel process, before the formation of the highly condensed oligomeric species. The progress of the polycondensation reaction can be qualitatively inferred by comparing Figure 1, a and b, or Figure 2, a and b. Two distinct transformations occur: (i) the intensity of the partially hydrolyzed monomeric units increases at the expense of the TMOS intensity, reflecting the hydrolysis reaction; (ii) (19) Assink,R.A.; Kay, B. D. Mater. Res. SOC.Symp. Proc. 1984, 32, 301.
4402
Artaki et al.
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
>.
k v) z
\
W
I-
z
I
600
700
800 900 1000 FREQUENCY
I
1100
I
1200
I
1
500
Figure 4. Time evolution of the Raman spectra of sol-gels containing formamide (solution I). Spectra were taken at hourly intervals and are offset for clarity. The experimental conditions were left unchanged between separate runs.
600
800
900 FREQUENCY
700
io00
1100
1200
Figure 5. Time evolution of the Raman spectra of sol-gels in solution 11. Spectra were taken at hourly intervals.
TABLE 111: Assignment of Raman Bands in the Sot-Gel Spectra between 600 and 1050 cm-' assignment u , cm-'
610 645 675 695 720 795 830 1029
formamide Si(OCH3)4 Si(OCH3)30H Si(OCHddOH), Si(OCH,)(OH), silica dimers silica network methanol
Si-0-C Si-0-C Si-0-C Si-0-C Si-0-Si Si-0-Si
'
c-0
resonan= due to the Q2,Q3,and Q4units become more prominent reflecting the condensation reaction. These transformations are significantly more pronounced in solution I1 than in I, indicating faster kinetics, a subject to which we will return. It should also be noted that the characteristic features of the spectra, such as the nature of the N M R resonances, along with their corresponding chemical shifts, are identical for solutions I and 11. The possibility that formamide chemically bonds to the silicon network through the scissioning of stressed bridging oxygen bonds' was considered. Since the practical chemical shift range of most functional groups (including -Si-N) is about 250 ppm, spectra were also recorded over a 300-ppm spectral width. N o additional resonance signals were detected. Therefore the N M R evidence cannot support the possibility of formamide being chemically bonded to silicon polymers. As pointed out in eq 1, the TMOS initially present in each of the sols will be hydrolyzed into various species involving a mixture of OR and OH groups. Since each of these partially or fully hydroxylated molecules have differing reduced masses and varying bond strengths, as pointed out in the Introduction, the Raman peaks will display a shift relative to the unsubstituted TMOS. Because the reduced mass is decreased when an OR is replaced by an O H group, this shift will be to higher frequency. This behavior is clearly illustrated in Figures 4 and 5 and in the region from 640 to 740 cm-' for both sols type I and 11, respectively. Table 111 lists the assignments for all the peaks observed in these figures. For a Raman experiment of the geometry used here, the intensity of a given peak is proportional to the concentration of scatterer^.'^ A ratio method may thus be used to calculate the concentration C ( t ) at anytime t after mixing given the initial intensity I ( 0 ) and concentration C(0): (4) where all intensities are background corrected. Thus, the rate of hydrolysis of TMOS may be followed by the decrease in its 640-cm-' peak. Conversely, the simultaneous formation of
t
40 6~
n "
0 . 2REDUCED 0.4TIME (t/ig) 0.6 I
3,4
0.8
I
I
I
6,8
10,2
13.6
TIME (hours)
Figure 6. Comparison between concentrations of Si(OCH,)., and Si(0H)(OCH3)3in solution I, obtained by two techniques: ( 0 )Raman, (D) NMR.
methanol may be observed through the rising intensity of the C-O mode at 1030 cm-I. Again, as in the N M R discussion above, no drastic change in the spectral features is produced upon addition of formamide except (unlike N M R ) for the appearance of the vibrational peaks of the formamide itself. Notably, there is no detectable shift or change in profile of these formamide peaks between the neat liquid and in the sol, reaffirming the conclusion above that formamide is not chemically bound to the silicon polymers. While N M R and Raman techniques in general provide complimentary information, their mutual capability to calculate the time dependence of the TMOS concentration and, to a lesser extent for Raman, the partially hydroxylated Si(OR),(OH), concentrations allows a comparison between them. In Figure 6, the time evolution of the concentrations for TMOS and the Si(OCH,)3(OH) species calculated by both techniques are shown for type I sol. The excellent agreement between the separate results (which is also seen for type I1 sols) conclusively shows the two techniques may be used in tandem to study the hydrolysis reactions. The initial Raman peak height is proportional to the amount of methanol present at preparation. If we assume that the entire 1.6 M TMOS, each molecule of which has four OCH3 groups attached, hydrolyzes, a further 6.4 M methanol will be produced, increasing the C-O peak height by a factor predicted from eq 4. The observed height after gelation matches within error the final
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4403
Hydrolysis Reaction in Sol-Gel Processes TABLE IV: Concentration of Alkoxy Groups Attached to Silicon Moieties“
time. h 1 2 3 4 5 6 7
CMn
C’(t)
time, h
6.21 6.07 5.95 5.55 4.89 4.44 3.92
6.20 6.1 5.9 5.6 5.1 4.6 4.2
8 9 10 11 12 14
C M ~ C’(t) 3.55 2.91 2.41 1.87 1.385 0.84
3.7
3.2 2.1 2.3 1.8 1.2
‘See text for explanation of symbols. intensity calculated by using eq 4 and assuming this total conversion occurs. This leaves the problem of following the O R s during the intermediate stages of reaction which may be accomplished by comparing the concentration of O R still attached to silicon monomers and dimers CM,D with the amount of OR not yet converted to methanol C’(t) at any time t . The concentration values for CM,D and C’(t) reported in Table IV are all molarities of O R groups calculated from the various spectra of formamide sols at 12 O C . CM,p is calculated from the N M R spectra by realizing, for example, that one TMOS molecule represents four O R groups by the relation CM,D = 4[Si(OCH3),] 3[Si(OCH3),0H] ... 3[(CH30),Si-O-Si-] ... ( 5 )
+
+ +
+
where the brackets [...I indicate concentration. All of the OR containing species listed in Table I must be taken into account. the concentration of the methanol at time t , C ( t ) ,consists of the initial amount C(0) plus that produced via chemical reaction (eq 1 and 2). The amount of O R not yet hydrolyzed, C’(t),will thus consist of the maximum producible (6.4 M, as discussed above) less that already converted C’(t) = 6.4 M - [C(t) - C(O)] (6) where C ( t ) is calculated via eq 4. As seen by comparing C(OR)M,Dand C(t) in Table IV, almost all of the [OR] not converted to methanol a t any time t is tied up in the silicon monomers or dimers. There are thus very few O R side chains observed attached to the Si network, especially since the small difference between CM,D and C’(t) is well within the experimental error involved. The quantitative effect on hydrolysis caused by the addition of formamide is evaluated in Figure 7. Here, the rate of disappearance of the starting reagent TMOS and the time evolution of the partially hydrolyzed monomer Si(OH)(OCH3), have been plotted vs. reduced time t / t , . A significantly slower rate of TMOS disappearance in solution I indicates an inhibition of hydrolysis by the formamide. The formation rate of the partially hydrolyzed monomer also reflects the hydrolysis speed; however, this molecule may disappear either by hydrolyzing further (eq 1) or by forming dimers (eq 2 or 3) or higher polymers. Comparing the formation and disappearance rates of the Si(OH)(OCH3)3molecule in solution I and I1 further indicates a hindering of hydrolysis in the formamide sol. Immediately following the preparation of the sol, TMOS can only participate in the hydrolysis reaction via eq 1 with n = 1 and rate constant of kH. Since an excess of water is present, the disappearance of TMOS follows first-order kinetics and therefore the initial slope of the In [Si(OCH,),] vs. time curve can be used to predict the magnitude of kH. At later times, TMOS may participate in condensation through eq 2 so the curve will deviate from linearity with a positive curvature (faster disappearance of TMOS) as seen in Figure 7. Calculations of kH based on the real rather than reduced time give values of 1.0 X lo4 and 1.6 X lo4 s-’ for solutions I and 11, respectively (at 12 OC, t, = 17 h for solution I and 37 h for 11). These slopes indicate a strong decelerating effect of formamide on hydrolysis relative to solution 11. One possible source of this slowing of hydrolysis by formamide may be found in the higher viscosity of solution I. The viscosity measured immediately after mixing was found to be 25% greater
20
0 REDUCED TIME Wtg)
Figure 7. Time evolution of Si(OCHJ4 and Si(OH)(OCH,), in solutions I (m) and I1 (0).
than in solution 11. Hydrolysis proceeds as noted earlier by a nucleophilic attack on a silicon atom by a negatively charged hydroxyl group. This may require a reorientation of the silicon nucleus in order to permit the maximum charge separation with the attacking and leaving groups situated on opposite sides.’* The mobility of the silicon molecules and thus their ability to reorient will be adversely affected by the increase in viscosity caused by formamide. Further, the approach of the nucleophiles themselves will be slowed. This decrease in the hydrolysis rate will result in a larger proportion of bulky dimers because of the existence of more partially hydrolyzed monomers. Further hydrolysis of these dimers will then be slowed due to steric hindrances which result from these bulkier alkoxy groups. Spectroscopic evidence for this is seen in Figures l b and 2b where a larger percentage of fully hydrolyzed dimers is observed in solution 11. The different rates of the hydrolysis reaction may be explained in terms of the hydrogen-bonding properties of formamide and methanol. Formamide is known to form stronger hydrogen bonds with SiOH and SiOR groups than methanol. This will limit the attack of the OH nucleophile on the O R groups attached to Si atoms and thus slow hydrolysis (eq 1). To explore this hydrogen-bonding effect on the hydrolysis reaction, we are currently undertaking further studies with different additives of various hydrogen-bonding properties. Conclusions N M R has frequently been used to study the hydrolysis reaction in sol-gels. For the first time Raman and N M R were shown in this paper to provide information on partially hydrolyzed TMOS. The various partially hydrolyzed TMOS molecules were found to give rise to separate peaks at different frequencies. The intensities of these peaks evolve in time later disappearing as a result of condensation. In both cases, the peak heights may be related to the concentrations of the species being probed. The results of the two techniques were found to yield identically the same results. The hydrolysis rate was found from the initial slopes of the TMOS decay curve. Use of the DCCA formamide was found to slow the hydrolysis process. Only a tentative explanation of this phenomena has been proposed. The high viscosity found in the formamide sols will slow the reorientation necessary for the nucleophilic attack to occur. The stronger hydrogen bond formed between formamide and the silicon species will also slow hydrolysis due to steric hindrances. A combined analysis of the N M R and Raman results led to the conclusion that no O R groups remain in the higher order
4404
J . Phys. Chem. 1985, 89, 4404-4408
polymers. Thus, most of the O R groups attached to silicon exist only up to the trimeric species. High-resolution 29SiN M R allowed the distinguishing of various species of silicon originating from differing chemical environments. The influence of the substituents on neighboring silicons attached by an oxygen bridge to the probed nucleus was spectroscopically detected and a partial assignment provided.
Acknowledgment. The authors thank Drs. L. Hench and G. Orcel for stimulating this study. This work was partially supported by the Air Force Office of Scientific Research under Grant USAFOSR 8 1-0010. Registry No. Si(OCH3),,,681-84-5; formamide, 75-12-7; methanol, 67-56-1.
Hydrated Surfaces of Particulate Titanium Dioxide Prepared by Pyrolysis of Alkoxide K. Morishige,* F. Kanno, S. Ogawara, and S. Sasaki Department of Chemistry, Faculty of Science, Okayama University of Science, 1 - 1 Ridaicho, Okayama 700, Japan (Received: May I , 1985)
The hydration state of the surfaces of particulate Ti02 prepared by pyrolysis of alkoxide was studied by thermal desorption, infrared spectroscopy, and adsorption of CHI. Only one desorption peak with the peak maximum around 100 OC appeared on anatase while two desorption peaks with the peak maxima at 1 15 and 245 O C appeared on rutile. The corresponding surface species of the peak maximum at 245 OC on rutile give rise to two strong infrared bands at 3650 and 3410 cm-I, which are assigned to the two distinguishable surface hydroxyls on a (1 10) plane. The physisorption isotherms of CHI showed that ca. 40% of the surface of rutile is occupied by the crystal faces with no chemisorbed water. After evacuation at room temperature, anatase of high surface area exhibited small infrared bands at 3660, 3640, and 1615 cm-' together with a very broad band centered at 3200 cm-' while anatase of medium surface area showed infrared bands at 3715, 3670, 3640, 3450, 1620, and 1540 cm-I as well as a very broad band centered at 3200 cm-'. The bands in the region 3750-3600 cm-' are ascribed to isolated hydroxyl groups attached to different crystal faces of anatase, and the very broad band is ascribed to the molecular water. Anatase of high surface area exhibited the Bronsted acidity toward NH3 molecule while anatase of medium surface area and rutile did not show it.
Introduction The study of the hydration state of T i 0 2 surface is important, since the chemisorbed water (hydroxyl groups and undissociated molecular water) on T i 0 2 plays an important role in its photocatalysis.I4 The hydration state of T i 0 2 surface has been investigated by various methods including specific chemical reactions,5 adsorption measurement,6 lieat of immersion,'** thermal desorption spectros~opy,~-'~ and nuclear magnetic resonance spectroscopy.12 Among them, the most detailed information has been obtained by infrared absorption ~ p e c t r o s c o p y . ~ ~ ~Nev'~-~'
(1) Boonstra, A. H.; Mutsaers, C. A. H. A. J. Phys. Chem. 1975,79,2025. (2) Schrauzer, G. N.; Guth, T. D. J. Am. Chem. SOC.1977, 99, 7189. (3) Van Damme, H.; Hall, W. K. J. Am. Chem. SOC.1979, 101, 4373. (4) Anpo, M.; Aikawa, N.; Kodama, S.;Kubokawa, Y. J. Phys. Chem. 1984,88, 2569. ( 5 ) Boehm, H. P. Discuss. Faraday SOC.1971, 52, 264. ( 6 ) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243. (7) Morimoto, T.; Nagao, M.; Omori, T. Bull. Chem. SOC. Jpn. 1969,42, 943. (8) Omori, T.; Imai, J.; Nagao, M.; Morimoto, T. Bull. Chem. SOC.Jpn. 1969, 42, 2198. (9) Munuera, G.; Stone, F. S . Discuss. Faraday SOC.1971, 52, 205. (10) Munuera, G.; Moreno, F.; Gonzalez, F. Proc. 7th Int. Symp. Reactiu. Solids, Bristol, July 1972 1972, 681. (11) Egashira, M.; Kawasumi, S.;Kagawa, S.; Seiyama, T. Bull. Chem. SOC.Jpn. 1978, 51, 3144. (12) Dartmieux-Morin, C.; Enriquez, M.-A.; Sanz, J.; Fraissard, J. J. Colloid Interface Sci. 1983,95, 502. (13) Yates, D. J. C. J. Phys. Chem. 1961, 65, 746. (14) Parkyns, N. D. In "Chemisorption and Catalysis"; Hepple, P., Ed.; Elsevier: Amsterdam, 1971; p 150. (15) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216. (16) Jackson, P.; Parfitt, G. D. Trans Faraday SOC.1971, 67, 2469.
ertheless, there seems to be remaining several problems which should be solved. These include the conflict between the infrared results on anatase, the distinction between hydroxyl groups and strongly adsorbed molecular water, and the difference between anatase and rutile. The surface impurities such as chlorideI6 or sulfate ion,22923as well as porosity,24 may modify the intrinsic properties of the chemisorbed water on Ti02 Pyrolysis of metal alkoxides enables the preparation of the oxide particles of ultrahigh purity, simple texture, and of wide variation in size.25,26The aim of our studies is to investigate the hydration state of the surface of particulate TiO, prepared by the pyrolysis of the alkoxide by measuring the thermal desorption and infrared spectra of the chemisorbed water, as well as the physisorption isotherm of CH4 which gives the information concerning surface homogeneity.
Experimental Section Materials. The preparation apparatus is shown in Figure 1. Titanium tetraisopropoxide (TTIP) in the vaporizer maintained at 100 O C was bubbled by passing a carrier gas (N2or a mixture (17) Jones,P.; Hockey, J. A. Trans Faraday SOC.1971,67, 2669. (18) Jones, P.; Hockey, J. A. J . Chem. Soc., Faraday Trans. 1 1972,68, 907. (19) Griffiths, D. M.; Rochester, C. H. J. Chem. SOC.,Faraday Trans. J 1977, 73, 1510. (20) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 736. (21) Tanaka, K.;White, J. M. J. Phys. Chem. 1982,86, 4708. (22) Morishige, K. Ph.D. Thesis, the University of Hokkaido, 1976. (23) Morterra, C.; Ghiotti, G.; Garrone, E.; Fisicaro, E. J. Chem. SOC., Faraday Trans. 1 1980, 76, 2102. (24) El-Akkad, T. M. J. Colloid Interface Sci. 1980, 76, 6 7 . (25) Mazdiyasni, K. S.;Lynch, C. T.; Smith, J. S. J. Am. Ceram. SOC. 1965, 48, 372.(26) Komiyama, H.; Kanai, T.; Inoue, H. Chem. Lett. 1984, 1283
0022-3654/85/2089-4404$01.50/00 1985 American Chemical Society
