NATURE OF THE 30 Å TEXTURE IN POLYMERIC TiO2 GEL

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NATURE OF THE 30 Å TEXTURE IN POLYMERIC

TiO2 GEL

A. Wright, S. Mukherjee, J. Epperson

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, supplément au n°12, Tome 46, décembre 1985 page C8-521

NATURE OF THE 30 A TEXTURE IN POLYMERIC TiO2 GEL

+ ++

A.F. Wright, S.P. Mukherjee and J.E. Epperson

Institut Laue-Langevin, 156 X, 38042 Grenoble Cedex, France

+Jet Propulsion Lab. Pasadena, California, U.S.A.

++M.S.T., Argonne National Laboratory, Illinois 60439, U.S.A.

Résumé - La polycondensation par hydrolyse d'une mole d'isopropoxide de titanium par 2 moles H2O produit un gel de TiO2 qui peut être transformé par séchage à 70°C en un solide amorphe transparent. La microstructure a été étudiée par diffusion des neutrons aux petits angles, mettant en évidence une inhomogénéité à l'échelle de 30 A. L'évolution de l'inhomogénéité a été suivie en fonction de la température et de l'humidité. Nous estimons qu'il existe des îlots de TiO2 pur dans une matrice de TiO2 contaminé par des résidus organiques. La phase pure de TiO2 peut être identifiée comme étant de l'anatase dès que la taille des particules atteint 50-80 A.

Abstract - Hydrolytic polycondensation of titanium isopropoxide using 2 moles H2O gives Ti02 gel which can be dried at 70°C to a transparent amorphous solid. Investigation of the microstructure by small angle neutron scattering reveals a texture on a length scale of 30 A. We have studied the development of this texture as a function of temperature and response to water vapour, using SANS and X-ray diffraction. We conclude that islands of pure TiO2 develop within a matrix of TiO2 contaminated with organic residues. The pure TiO? phase can be recognised as anatase once the particle size reaches 50-80 A.

I - INTRODUCTION

In recent years, the preparation of glasses/glass-ceratnics and fine grained ceramics by the sol-gel route /1,2/ has attracted increasing scientific and technological interest. Various single and multicomponent oxide systems which can hardly be pro-duced in the noncrystalline state in bulk by the conventional melt-quenching tech-nique are produced as non-crystalline transparent gels by the hydrolytic polyconden-sation of metalalkoxides /1,2/. When heated to moderate temperatures, the gel trans-forms to a non-crystalline inorganic solid and subsequently densifies to form a glass or crystallizes as a fine grained ceramic. Previous research work /3-6/ indi-cates that the microstructure and crystallinity of glass, glass-ceramics and cera-mics obtained by the sintering of gels are influenced by the physico-chemical para-meters of gel-preparation and gel processing which control the initial structure and microstructure of the gel. Hence, it is important to study the microstructure of the as-prepared gels in order to understand the mechanisms which control the later development. A considerable amount of work has been done on the synthesis of many types of gels and on their conversion to glasses and ceramics. But work doneon the texture of as-prepared gels and its subsequent evolution on thermal aging is very limited. Mukherjee /6/ made investigations on the microstructure and crystallization behaviour of non-crystalline titania gels using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Results of a transmission electron microscope study of powdered gels indicate that whilst it is possible to detect the presence of some extremely fine particles (<100 A).in the as-prepared titania gels, it is extremely difficult to get quantitative data about the size distribution of inhomogeneities by the TEM technique, and practically impossible to monitor the microstructural evolution during thermal aging.

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C8-522 JOURNAL DE PHYSIQUE

The work of Wright, Nunn and Brett /7/ on a study of the microstructure Zr02 gel clearly shows that small angle neutronoscattering is a powerful tool for monitoring microstructural changes in the 20-200 A range and for establishing the mechanism controlling the evolution during thermal aging.

The objectives of the work presented were (a) to synthesize non-crystalline transparent titania gels as a model oxide system; (b) to generate small angle neutron scattering data with titania gels as a function of thermal aging in the absence and presence of H20 which might act as a catalystfreactant in the microstructural/struc- tural evolution, and (c) to analyze the experimental scattering data on the basis of existing analytical models /7,8/ to establish the microstructural evolution and to understand the mechanism of the microstructural changes.

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GEL PREPARATION

Titanium isopropoxide in solution in isopropanol was hydrolysed by dropwise addition of a solution of two moles of H20 and 0.002 mole HCL in isopropanol. The sol was left to polymerise by elimination of bonded propanol groups for 15 hours. The gel product was dried at 70°C in air and formed transparent pieces, 2-5 mm in size. The X-ray diffraction pattern is shown in Fig. 1.

Fig. 1 - X-ray diffraction pattern of Ti02 gel.

(a) Air dried at 70°C, (b) Heated to 370°C - showing

amorphous phase plus anatase crystals,

(c) Heated to 850°C - showing amorphous phase plus rutile crystals.

( 1 ) Hydrolysis

(2) Polycondensation

111 - SMALL ANGLE NEUTRON SCATTERING FROM A PULSED SOURCE

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sample is now spread out in time (the shorter wavelengths arrive earlier) and con- tinues to spread further on its way to the detector. The small angle diffraction instruments has a path length of eleven metres in total with three meters from the sample to the detector. The detector is 18 x 18 cm2 with a spatial resolution of 3 mm. The counting of scattered neutrons begins when 0.5 neutrons have time to reach the detector. The scattered neutron data is stored in 64 equally spaced time slices corresponding to 0.1 19 wavelength steps up to 8 A for each of the 64 x 6 4 detector elements. The range in scattering vector is 0.0085

<

q

<

0.2

A-I.

Data from each pulse is summed to give the complete spectrum. In the present case measurements were made over a period of 2 hours for each data set.

The gel samples were loaded into a quartz sample cell of 1 cm diameter and path length 2 mm, which was housed in a simple tube furnace aligned with the neutron beam.

IV - RESULTS (a) Dry gel

The gel dried at 70°C gave weak scattering with a well defined interference ma- ximum at q = 0.07

I-'.

Although the counting statistics were quite poor, the analysis clearly indigated the presence of particles of 30 A in diameter, well separated from each other. Upon hea:ing for 2 hours at 250°C in vacuum, the scattering was stronger and gave an ex- cellent fit to the model over the whole range. For a description of the data analysis and simulation see ref. (8). In Fig. 2a we show the experimental data and the calculated cyrve corresponding to spheres of 34 A in diameter dispersed by a weak screened re- pulsion potential. Fig. 2b shows the static pair correlation function g(r) for the dispersed phase. This indicates zero probability of spheres in contact (at R/diam = 1) and an exclusion zone extending about one radius beyond contact. X-ray diffraction curves were virtually identical to those of the gel dried at 70°C. Upon more severe heating the SANS

curves become more intense and sharp- er, but werequalitatively similar

(fig. 3). The peak is slowly displaced toward lower q. The curve analysis shows a steadily growing particle size, and a particle separation which scales with the particle size.

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JOURNAL

DE

PHYSIQUE

Beyond 370°C X-ray diffraction peaks corresponding to the anatase form of Ti02 could be identified and at even higher temperature the anatase transformed to rutile (Fig. 1). It is important to note however that the broad peak centred near d = 7 characteristic of the amorphous phase in the 70°C gel was still substan- tially present after even the most severe heat-treatment. The crystalline phase remained in the minority.

(b) Sensitivity to water

The introduction of water to the gel gave no significant change on scattering at

room temperature. Gentle heating however Fig. 3

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Small angle neutron scattering gave to a sharp increase in scatter- curves obtained during thermal ageing of ing compared to the dry thermal ageing. Ti02 gel.

At all temperatures the inhomogeneities showed considerably increased size and more rapid development, but was qualita- tively unchanged.

The amor~hous matrix must differ in

1

The coniinuity in the evolution of the SANS data from 70°C to 400°C suggests that the 30 A texture of the dry gel and the well defined anatase precipitate are closely related. We can investigate just how closely by looking quantitatively at the kinetics of the process. In Fig. 4 we plot the variation of I(Q=O) derived from the simulation /7,8/ with the particle volume and obtain a linear relationship which passes through the origin. These re-

sults correspond to an Ostwald ripe-

some way from pure Ti02. We believe I I

that it is formed from impure TiO? 200 400 600 S D O I ~ ~ I ~ I A ~ X I O ~ I

ning process where the total volume fraction of disperse phase remains s o -

constant. Data for the dry gel at 70°C also fits this equation. The

inhomogeneities at 70°C therefore

:

have the same volume fraction and _{2 0 -} density as anatase but do not give identifiable diffraction peaks. This may in part be due to their size, but could also be due to poor

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crystalline order.

with some organic residues still bonded to the titania. This sug-

gests that the condensation pro- Fig. 4 - Correlation between I(Q = 0) and the cess is incomplete and that the particle volume corresponding to an Oswald impurities stabilise the amorphous ripening process.

matrix.

-

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Ti Ti bonds through the formation of nun-bridging hydroxyl bonds could favour the migration. The activation energy drops from 83 kjoules mole-I to 30 kjoules mole-I with the addition of water.

REFERENCES

/I/ Wills, R.R., Markle, R.A. and Mukherjee, S.P., Bull. Amer. Cermaic Soc. Vol. 62 8 904-915 (1983).

/ 2 / "Glass

...

Current Issues" Ed. A.F. Wright and J. Dupuy, pp. 203-280. Martinus Nijhoff, 1985 The Netherlands.

/3/ Mukherjee, S.P. and Sharma, K., to be published in proceedings of the conference on "Effects of Modes of Formation on the Structure of Glass. Nashville Tenn. 1984 /4/ Mukherjee, S.P., Earzycki, J . and Traverse, J. Mat. Sci. 11, 341-355, 1976. /5/ Mukherjee, S.P. and Zarzycki, J., J. Amer. Ceramic Soc. 62, p. 1-4, 1979. /6/ Mukherjee, S.P., "Emergent process Methods for high Technology carmics, Ed. R.F. Davis, H. Palmour, and R.L. Porter 95-109, Plenum (1984).

/ 7 / Wright, A.F., Nunn, S. and Brett, N.N., "Science and Technology of Zirconia I1 Ed. N. Claussen, American Ceramic Society (1985).