Raman study of gel-glass transformation in base-catalyzed silica

JOURNA

Journal of Non-Crystalline Solids 147&148 (1992) 251-255 North-Holland

L OF

NON-CRYSTALLISOLIDS NE

Raman study of gel-glass transformation in base-catalyzed silica K. D a h m o u c h e a, A. B o u k e n t e r b, C. B o v i e r a, j. D u m a s a, E. D u v a l b a n d J. S e r u g h e t t i a a D~parternent de physique des mat&iaux, UA 172 CNRS, and b Physicochimie des matdriaux luminescents, UA 442 CNRS, Universit~ Lyon 1, 43 Boulevard du 11 novembre 1918, 69622, Villeurbanne, France

T h e effects of heat treatments on silica xerogels prepared from base-catalyzed hydrolysis and condensation of T M O S were studied by low-frequency R a m a n scattering and electron microscopy. R a m a n scattering m e a s u r e d in the range 3-700 c m - 1 revealed a low-frequency band that arises from the discrete particulate character of base-catalyzed gels at very small size scales. A continuous evolution of the internal structure of particles is observed during thermal treatment and the R a m a n spectra are similar to the fused silica one at 1050°C. Electron microscopy observations of the nanostructure of the xerogels correlate with the particle R a m a n band maximum. T h e evolution of the sample as a whole (pores collapsing and densification) is described.

1. Introduction Recent low-frequency Raman scattering measurements of base-catalyzed silica aerogels showed the presence of a maximum at very low frequencies [1]. This maximum arises from scattering from surface modes of structural units or spherical particles. At frequencies higher than this maximum, the reduced intensity varies as w v, with v close to 1. This variation is interpreted as coming from fractons in structural units. For silica xerogels prepared in acid conditions and dried in atmosphere, similar behavior of w v between about 10 and 200 cm-1 was observed [2] but no low-frequency maximum was observed as in the base-catalyzed aerogels. Raman scattering from heat-treated basecatalyzed silica aerogels [3] showed three stages in the transformation to glass: (1) modification of the particle surface, (2) coalescence of particles, and (3) change of the internal structure and glass relaxation. From these results, it appears interesting to follow the transformation of a base-catalyzed xerogel with temperature, in order to observe similarities or differences with the previous densification process and with the structure of the previous amorphous materials. Experimental results of

low-frequency Raman scattering during the xerogel-glass transformation and scanning electron microscopy observations are reported here. The physical processes of densification are also discussed.

2. Experimental Gels were prepared by hydrolysis and condensation of a solution of TMOS, methanol and water, in the presence of N H 4 O H and dimethylformamide [4]. The sol was kept at 35°C for 7 h. After gelation, the temperature was raised from 35°C to 80°C in 45 h and held at this temperature for 5 h in order to age the wet gel. Drying of the gel was conducted by perforating the aluminium foil cover of the gel container. The temperature was increased to 160°C in 80 h and held there for 24 h. The xerogel obtained had a specific surface area about 617 m2/g, a pore volume of 1.45 cm3/g and mesopore radius of about 43 A, as determined by B E T measurements. The apparent deputy was about 0 5 4 g / c m 3. The Raman measurements were obtained with a Dilor monochromator using the 5145 A line of an argon ion laser as an excitation source and a cooled photomultiplier. The range investigated

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I( Dahmouche et al. / Gel-glass transformation

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varied from 3 to 700 cm-1. Fracture surfaces of Au20%Pt coated gels were observed with an Hitachi type S-800 scanning electron microscope.

3. Results

Figure 1 shows the Raman spectra up to 200 cm-1 for xerogels annealed in air between 160°C and 1050°C. A very low-frequency band at 17 cm -1 for the sample dried at 160°C is apparent (a). This type of peak has been observed with base-catalyzed aerogels [1] and can be attributed to surface-vibrational modes of particles which are agregated to form the gel. The frequency, w0, of the maximum is a function [5] of the sound velocity, v, in the particle and of the diameter,

~32

v

6

r

rr

Raman shiFi- (cm -1) Fig. 1. Low-frequency Raman scattering from base-catalyzed xerogels: (a) dried at 160°C; (b) annealed for 30 min at 700°C; (c) annealed for 30 min at 980°C; (d) annealed for 30 min at 1050°C. The spectrum of fused silica (e) is also represented.

2a: o~0= 0.8(v/2ac). Choosing a velocity v = 4000 m / s , a value slightly smaller than in silica, it is found that 2a = 60 A. The spectrum of this sample shows a line about 32 cm-1, probably due to the presence of D M F and organic compounds in the xerogel. The spectrum of the sample annealed at 700°C for 30 mn (b) shows no modification of the position of the low-frequency band but reveals a narrowing of it. The spectrum of the sample annealed at 980°C (c) shows again a narrowing and a small shift of the band to 14 cm-1. After the annealing at this temperature, the calculated diameter of the particle is 76 A. The spectrum of the sample annealed at 1050°C (d) is similar to the fused silica one (e). Now, it is interesting to consider the Raman scattering at higher frequencies, which comes from the vibrational modes inside the particles. Figure 2 presents the direct spectra up to 700 era-1 for the different previous heat treatments. We can see that the broad band (characteristic of the disorder at short distance) is located at 400 cm-1 for (a) and about 443 cm-1 for (b). There is an appearance of a line at 608 cm -1 (which corresponds to a threefold planar ring formed from three tetrahedra [6]) for the sample heattreated at 700°C. The line observed at 640 cm-1 for (a) is probably due to the presence of D M F and organic compounds in the dried gel at 160°C. Comparison of these two spectra also shows that the line at 478 cm -1 observed in (a) shifts to about 490 cm -1 in (b). These lines at 478 and about 490 cm-1 can be assigned to the fourfold planar ring, respectively at the surface and inside the particles [7]. Comparison between (b), (c) and (d) reveals that the line due to the fourfold planar ring (located about 490 cm -1) increases compared with the line at 608 cm-1 due to the threefold planar ring and also shows an increase of the intensity of the broad band compared to the line about 490 cm-1. At 1050°C, the spectrum (d) is similar to the fused silica one (e). Figure 3 represents the reduced intensities I(~o)w/(n(o~) + 1) versus the frequency, ~o, in a log-log plot. The spectrum of the sample dried at 160°C (a) is not characteristic of the structure of

K. Dahmouche et al. / Gel-glass transformation

253

5.5 5.0

~+4.5

o

.~4.0

2.5-

o

6 v "5 C" a_. C-

1_o910 (C0(cm-1)) Fig. 3. Log-log plot of reduced intensities versus the frequency, w, of Roman scattering from base-catalyzed xerogels: (a)-(e) same samples as in fig. 1.

0

200

400

600

Roman shiF~ (cm -1) Fig. 2. Roman scattering from base-catalyzed xerogels: (a)-(e) same samples as in fig. 1.

the materials because of the presence of DMF and organic compounds in this non-heat-treated xerogel, but the spectra of the samples annealed at 700°C (b) and 980°C (c) for 30 min shows that the plot is linear from about 35 to 160 c m - l . In this interval, the reduced intensity varies as w ~.

Fig. 4. (a) Scanning electron microscopy image of the xerogel dried at 160°C. (b) Scanning electron microscopy image of the xerogel annealed for 30 min at 980°C.

K. Dahmouche et al. / Gel-glass transformation

254

Table 1 BET measurements and determination of apparent density Temperature (°C)

Radius of mesopores

Specific area (cm2/g)

(~,,) 160 700 980 1050

43 43 37 -

Apparent density ( g / c m 3)

617 570 310 0

0.54 0.56 1 2.19

The spectrum of the sample annealed at 1050°C (d) presents no linear zone and is similar to the fused silica one (e). SEM study shows a microstructure consisting of spherical dusters with a mean diameter of about 300 A for the xerogel dried at 160°C (fig. 4(a)). The image also shows that these clusters are constituted of particles with a diameter of about 60 A which correlates with Raman measurements. The image of the sample annealed at 980°C (fig. 4(b)) shows a decrease of the porosity of the materials, but the size of the clusters is not very different from fig. 4(a), showing that the conversion of the xerogel to glass is realized in a very small domain of temperature (between 980 and 1050°C). B E T measurements and determination of apparent density provide the values given in table 1, reflecting the evolution from the xerogel to the glass.

tance. The apparition of the line located at 608 cm-1 (fig. 2(b)) reflects the modification of the surface of the particles which was rough but becomes smooth [9]. The shift of the line located at 478 cm-1 at 160°C (fig. 2(a)) to 490 cm-1 at 700°C (fig. 2(b)) again reflects modification of the surface of the silica particles from rough to smooth. The increase of the line at 490 c m - 1 compared with the line at 608 c m - 1 with increasing temperature shows progressive evolution of the structure of xerogel to glass, also reflected by the increasing of the intensity of the broad band compared with the line about 490 cm-1. The spectra in fig. 3 show that plots of the samples annealed at 700°C (b) and 980°C (c) for 30 min are linear from about 35 to 160 cm -~ and the reduced intensity varies as w ~. Such Raman scattering was interpreted as coming from fractons [10]. The Raman exponent, v, was expressed [2] as a function of the fractal and fracton dimensionalities, respectively D and d, and of the superlocalization exponent, d&. This exponent is such that the fracton wave function decays [11] as e x p ( - a r e 4 0 versus the Euclidian distance, r. The expression of v was obtained assuming that the scattering from the different points in the fractal localization volume is incoherent: v = d((2d&/D)

The small shift of the position of the lowfrequency band observed in fig. 1 and the narrowing of it shows the growth of the largest particles at the expense of smaller ones [8] between 160°C (a) and 980°C (c). At 1050°C (d), the similarity of the spectra to that of fused silica confirms the transformation of xerogel to glass. In fig. 2, the modification of the position of the broad band between 160°C (a) and 700°C (b) reveals that there is a modification of the internal structure in the particles in this interval of temperature which change the disorder at short dis-

1) - 1.

If the Raman scattering is supposed to be coherent, one would find [12] v = d(2dcb/D)

4. Discussion

+

-

1.

The hypothesis of incoherent scattering is chosen because we have to do with disordered fractal materials. From theoretical results [13] it was assumed 1 < d& < 1.3. The experimentally measured fractal dimensions for gels or porous silica are higher than 1.75 and smaller than 3 [14-18]. Taking d & = l and the medium value D = 2 . 3 7 5 it is found that d =0.98

for 700°C

(v = 0.8),

d = 1.36

for 980°C

(v = 1.5).

The increase of the spectral dimension, d, with temperature could be due to the modification of the connectivity inside the particles (d = 3 for a

K. Dahmouche et al. / Gel-glass transformation

uniform, fully connected structure of dense SiO 2 [191).

5. Conclusion

Raman scattering, SEM, and BET measurements allow the process of densification of basecatalyzed xerogels to be described. Phenomena which can be distinguished include smoothing of the particles, small growth of their size, modification of their internal structure, and decrease of porosity.

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