Synthesis and characterization of siloxane-titania materials

J O U R N A L OF

Journal of Non-Crystalline Solids 147&148 (1992) 62-66 North-Holland

NON-CRYffgNES01,IIIS

Synthesis and characterization of siloxane-titania materials S. D i r 6 a, F . B a b o n n e a u

b, G . C a r t u r a n

a a n d J. L i v a g e b

a Dipartimento di Ingegneria dei Materiali, Unicersitgt di Trento, 38050 Mesiano, Trento, Italy b Chimie de la Mati&e Condensde, Unicersitd de Paris 6, 75005 Paris, France

Diethoxydimethylsilane and titanium isopropoxide have been used for preparation, via the sol-gel process, of mixed siloxane-titania materials. Depending on the Ti/Si molar ratio, flexible to brittle gels can be obtained. The rheological properties of the sols can be adjusted in order to obtain films and transparent monoliths. Due to the high amount of hydrophobic methyl groups in these materials, thick films with an average thickness of 20 ixm can be obtained. These films have been used for laser dye matrices. A characterization of the various steps of the hydrolysis-condensation process as well as of the structure of the final solid gel determined by liquid and solid state NMR and TiK edge X-ray absorption spectroscopy are presented. A structural model for the gel is proposed mainly based on siloxane chains and TiO 2 based particles. Optical properties of Coumarin 4 and Rhodamin 6G incorporated in such films are discussed.

1. Introduction S o l - g e l c h e m i s t r y is m a i n l y b a s e d on i n o r g a n i c p o l y m e r i z a t i o n r e a c t i o n s l e a d i n g to a m a c r o m o l e c u l a r oxide n e t w o r k . In r e c e n t years, g r e a t i n t e r e s t has b e e n d e v o t e d to h y b r i d o r g a n i c - i n o r g a n i c m a t e r i a l s [1]. T y p i c a l p r e c u r s o r s for t h e s e systems a r e m o d i f i e d silicon alkoxides R'~Si( O R ) 4 _ x. If R ' is a n o n - h y d r o l y z a b l e group, it behaves as a network modifier. A great amount o f study has f o c u s e d on m a t e r i a l s p r e p a r e d with m o d i f i e d silicon p r e c u r s o r s a n d S i ( O R ) 4 as netw o r k f o r m e r [2,3]. T h e cross-linking o f t h e netw o r k can also b e o b t a i n e d with t r a n s i t i o n m e t a l alkoxides t r a d i t i o n a l l y k n o w n as n e t w o r k f o r m e r s . This p a p e r p r e s e n t s a study o n h y b r i d s i l o x a n e t i t a n i a systems o b t a i n e d f r o m co-hydrolysis of (CH3)2Si(OEt)2 and Ti(OPri)4 .

2. Experimental A m i x t u r e of d i e t h o x y d i m e t h y l s i l a n e ( D E D M S ) , a b s o l u t e e t h a n o l a n d w a t e r in a 1 : 1 : 1 m o l a r r a t i o was s t i r r e d for a few m i n u t e s b e f o r e a d d i n g t h e a p p r o p r i a t e a m o u n t s of T i ( O P r i ) 4 in o r d e r to o b t a i n T i : S i m o l a r ratios r a n g i n g f r o m

10 : 90 to 50 : 50. T h e p H of the w a t e r was previously a d j u s t e d to 1 by a d d i t i o n o f h y d r o c h l o r i c acid. In this p a p e r , t h e s a m p l e s a r e l a b e l l e d D T i x , D for D E D M S a n d x for t h e m e t a l p e r c e n t a g e . T h e t r a n s p a r e n t sols w e r e well s t i r r e d a n d t h e n s t o r e d in o p e n vessels. In a few days, t r a n s p a r e n t b u l k p i e c e s w e r e o b t a i n e d . O n l y in t h e D T i 5 0 system was s e g r e g a t i o n of T i O 2 - b a s e d p a r t i c l e s o b s e r v e d . T h e m o n o l i t h i c x e r o g e l s w e r e flexible in t h e D T i l 0 system a n d b r i t t l e for the DTi30 composition. N M R s p e c t r a w e r e r e c o r d e d on a M S L 400 B r i i k e r s p e c t r o m e t e r . F o r 29Si liquid spectra, 60 p u l s e s w e r e a c c u m u l a t e d with 5 txs pulse width a n d 10 s r e l a x a t i o n delays. F o r solid N M R experiments, p u l s e w i d t h and r e l a x a t i o n delays w e r e respectively, 2.5 Ixs a n d 60 s for 29Si M A S - N M R , 5.7 p~s a n d 6 s for 13C C P - M A S - N M R ( c o n t a c t time: 1.5 ms) a n d 5.7 Ixs a n d 4 s for 1H M A S N M R . T h e solid s a m p l e s w e r e s p u n at 4 kHz. T h e X - r a y a b s o r p t i o n e x p e r i m e n t s w e r e cond u c t e d at L U R E (Orsay, F r a n c e ) using t h e E X A F S I I I line. A Si 311 m o n o c r y s t a l was u s e d as a monochromator. The XANES spectra were r e c o r d e d f r o m 4920 to 5070 e V with a 0.3 e V step while t h e E X A F S s p e c t r a w e r e r e c o r d e d from 4670 to 5670 e V with a 2 e V step. T h e e n e r g y

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S. Dir~ et al. / Siloxane-titania materials

calibration was carried out with a metallic titanium foil. The data have been analyzed according to the usual EXAFS formalism, using programs established by Michalowitz [4].

3. Characterization of the solution

A preliminary characterization was performed on the DTi30 composition. The hydrolysis-condensation process was followed by 29Si NMR spectroscopy. Figure 1 shows the evolution of the D E D M S solution after the Ti(OPri)4 addition. The simply hydrolyzed D E D M S solution shows the presence of mono-condensed species (D t) of the type RO-Si(CH3)2-O1. 5 (R = H, Et) at - 13

,

Jr,

C

u

Jt

i

-5

i

-15

-25

C H E M I C A L S H I F T (ppm) Fig. 1. 29Si N M R spectra of DTi30 solution after (a) 15 rain; (b) 45 rain; (c) 2 h; (d) 21 h.

63

and - 1 3 . 5 ppm. The presence of chains and cycles is pointed out by the peaks at - 2 1 . 7 ppm (D 2 chains) and - 8.9 and - 19.3 ppm (D 2 cycles) [5,6]. The presence of short chains in this system is consistent with the low H 2 0 / S i molar ratio and the observed intensity of the peak due to D 1 terminal units. A strong change of the spectrum is observed after Ti(OPri)4 addition (fig. l(a)). One peak with decreased intensity is observed for terminal units at - 1 3 . 5 ppm; peaks due to the cycles have almost disappeared and extra peaks are present at - 2 3 . 5 and - 2 4 . 2 ppm. The presence of these signals is more evident in fig. l(b) and 1(c). These peaks present a high field shift compared with the usual chemical shift of D 2 units ( - 2 2 ppm) in simply hydrolyzed solutions of DEDMS. They could be assigned to S i - O - T i bonds. This evidence disappears after longer reaction times. Figure l(d) shows a single peak at - 2 2 ppm for D 2 units, the signal of the monomeric species at - 4 . 8 ppm and one peak at - 1 3 . 4 ppm for D 1 units. The evolution of the hydrolysis-condensation process seems to proceed through the formation of intermediates containing bridging oxygens between Si and Ti species followed by a structural rearrangement. The final situation shows the presence of polydimethylsiloxane chains with no evidence of S i - O - T i bonds. Ti(OPri)4 is known to behave like a catalyst for the condensation process and to prevent cycles formation [7]. The local environment of Ti has been investigated by X-ray absorption spectroscopy at the Ti K-edge. The XANES spectrum and the Fourier transform of the EXAFS spectrum for the DTi30 solution after 24 h are reported in fig. 2. At the TiK-edge, the pre-edge features are strictly related to the symmetry of Ti sites. In the case of Ti(OPri)4, an intense singlet at 4966.9 eV is observed due to the fourfold coordination of Ti atoms [8], while TiO 2 anatase presents a triplet at 4966.1, 4969.1 and 4971.5 eV with relative intensities 0.09, 0.17 and 0.16 related to the Ti sixfold coordination. In the DTi30 solution (fig. 2(a)), a less intense peak (0.28) at 4967.4 eV is observed associated to a shoulder at 4971.6 eV (0.18). The peak could be assigned to a fivefold coordination [8], but the shift in energy and the presence of the

64

S. Dird et al. / Siloxane-titania materials

i

,

)

5050

4950 Energy (eV)

w

0.0

I

I

I

4.o

i

r

)

8.o

Distance (~) Fig. 2. X-ray absorption spectra of DTi30 solution after 24 h: (a) XANES; (b) k3X(k) Fourier transform of the EXAFS spectrum.

shoulder suggest a superposition of a triplet due to a sixfold coordination. In fig. 2(b), the Fourier transform of the E X A F S spectrum is shown. Two T i - O distances are found at 1.86 and 2.08 A, and are assigned to terminal and bridging oxygens, respectively. For the distance at 3.17 A, a better simulation was obtained with a Ti as second neighbour rather than a Si. However, the presence of some T i - O - S i bonds cannot be rejected since the E X A F S technique is sensitive to the average coordination sphere thus favouring the main bonds. o

4. Characterization of the gel The xerogel has been studied by means of M A S - N M R and X-ray absorption spectroscopies.

The 29Si M A S - N M R spectrum of DTi30 gel, presented in fig. 3, exhibits a sharp peak at - 2 2 . 2 p p m and a small component around - 1 9 ppm due to the presence of cycles with four units [6]. The sharp peak is attributed to D 2 units and its chemical shift is near that observed in commercial polydimethylsiloxanes. The study of i H MASN M R and 13C C P M A S - N M R spectra gives some information about the hydrolysis process. Both spectra reveal intense peaks associated with methyl groups bonded to Si atoms. In the ~3C N M R spectrum, two small components at 17.9 and 23.3 p p m can be assigned to residual non-hydrolyzed alkoxide groups. However, the very low intensity of these signals is a proof of a rather complete hydrolysis process. Thus, the system seems to have mainly developed long chains of polydimethylsiloxane during the co-hydrolysis process. The local environment of Ti sites is shown by the X A N E S spectrum (fig. 4). A triplet at 4965.9, 4967.7 and 4971.3 eV with respective intensities 0.13, 0.29 and 0.18 is found. Once more, the pre-edge region, for the position and intensities of the peaks, can be described as a superposition of a triplet (sixfold coordination) and a singlet due to the fivefold coordination. The comparison with the X A N E S spectrum of the solution allows to assume an increase in the gel of Ti sites in sixfold coordination. From the EXAFSospectrum, two T i - O distances at 1.74 and 1.90 A and one T i - T i distance at 3.04 A can be extracted. The T i - O distance at 1.90 ,~ and the T i - T i distance at 3.04 A are in good agreement with those found

i

i

,

,

I

,

,

•

,

.F

,

,

L

0 -50 CHEMICAL SHIFT (ppm) Fig. 3.29Si MAS-NMR spectrum of DTi30 gel.

S. Dir~ et al. / Siloxane-titania materials

65

of bridges could be small and probably undetectable with the characterization techniques used.

/

5. Optical properties of films doped with Rhodamin 6G (R6G) and Coumarin 4 (C4)

,. ~ 4950

r

,

r 5050

,

>

E n e r g y (eV) Fig. 4. X A N E S s p e c t r u m of DTi30 gel.

in TiO 2 anatase. The short Ti-O distance at 1.74 could correspond to terminal oxygen atoms and be related to the formation of small TiO zbased particles. According to MAS-NMR and X-ray absorption results, the system seems to be formed with polysiloxane chains and small TiO2-based particles. At present, we are not able to give a better description of the DTi system than the structural model proposed in fig. 5. The system could be described as a nanocomposite with chains of polydimethylsiloxane and TiOz-based particles where the chains could act as bridges between the inorganic particles. This model presumes the presence of Ti-O-Si bonds that have not been demonstrated in this study. However, the amount

Si~ 0

Si

Si

/ 0

/

O

The rheological properties of the sols prepared in this work allow easy deposition of films on glass sheets. Siloxane-oxide hybrid coatings have been prepared with various metal alkoxides (M(OR),, M = Ti, Zr, A1, Si) [9]. Due to the presence of siloxane chains and hydrophobic methyl groups, films with an average thickness of 20 p~m with no cracks have been obtained. Organic dyes (R6G, C4) can be incorporated in the films reaching high dye concentrations (C _> 10 .2 M). Absorption and emission experiments have been performed with R6G and C4. R6G gives strong fluorescence in all matrices even for very high dye concentrations. This is probably related to the minimization of dimers responsible for the quenching of fluorescence in water and alcoholic mediums. This effect, in the gel, is assigned to the formation of polymeric cages that prevent aggregation between dye molecules [10]. This is enhanced in siloxane-oxide hybrid materials for the large amount of hydrophobic methyl groups favouring interactions with the dye. Coumarin 4 shows absorption and emission features depending on the nature of the matrices. Matrices with Zr and Si give the strongest fluorescence. A specific interaction between the dye and Ti has been pointed out by 47'49Ti NMR experiments. The reactivity of Ti species towards C4 is responsible of the enhanced absorption and the emission quenching of the dye.

O~

\s,

j Si

\o

s,

I o...

O

si

\Si

O

\

O Fig. 5. P r o p o s e d structural model for D T / x gels.

\

6. Conclusions

Mixed siloxane-titania materials have been prepared starting from diethoxydimethylsilane and titanium isopropoxide. The evolution of the hydrolysis-condensation process, followed by 29Si NMR, seems to proceed through the formation of intermediates containing Si-O-Ti bonds. Struc-

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S. Dir6 et al. / Siloxane-titania materials

tural rearrangement leads to polydimethylsiloxane chains with no evidence of S i - O - T i bonds. Ti(OPri)4 seems to behave like a catalyst for the condensation process. Depending on the T i / S i molar ratio, flexible to brittle gels can be obtained, According to 29Si MAS-NMR and TiK-edge X-ray absorption spectroscopy results, the xerogels appear to be composed of chains of polydimethylsiloxane and small TiO2-based particles. Rheological properties of the sols can be adjusted to prepare transparent films and monoliths. Crack-free films with an average thickness of 20 ~m have been obtained. Organic dyes can be incorporated in these films in high concentrations. Rhodamine 6G shows strong emission for very high concentration while Coumarin 4 shows absorption and emission features depending on the nature of the matrix.

References [1] H. Schmidt, Mater. Res. Soc. Syrup. Proc. 180 (1990) 961, and references therein. [2] H. Scholze, J. Non-Cryst. Solids 73 (1985) 669. [3] H. Schmidt and H. Wolter, J. Non-Cryst. Solids 121 (1990) 428. [4] A. Michalowicz, in: Proc. Structures Fines d'Absorption en Chimie (Absorption Fine Structures in Chemistry) CNRS Summer School, ed. M. Verdaguer and H. Dexpert (CNRS, Paris, 1988). [5] G. Engelhardt and H. Jancke, J. Organomet. Chem. 210 (1981) 295. [6] R.K. Harris and M.L. Robins, Polymer 19 (1978) 1123. [7] W. Noll, Chemistry and Technology of Silicones (Academic Press, New York, 1968). [8l F. Babonneau, S. Doeuff, A. L~austic, C. Sanchez, C. Cartier and M. Verdaguer, Inorg. Chem. 27 (1988) 3166. [9] S. Dir~, F. Babonneau, C. Sanchez and J. Livage, J. Mater. Chem. 2 (1992) 239. [10] D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem. 88 (1984) 5956.