Journal of Sol-Gel Science and Technology, 2, 161-166 (1994) © 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.

M o l e c u l a r D e s i g n o f H y b r i d O r g a n i c - I n o r g a n i c Materials with Electronic Properties Code: BP12 C. SANCHEZ, B. ALONSO, F. CHAPUSOT AND F. RIBOT Laboratoire de Chimie de la Mati~re Condensde-U.R.A. C.N.R.S. 1466, Universitd Pierre et Marie Curie, 4, Place Jussieu, 75252-Paris Cedex 05, France

P. AUDEBERT Laboratoire d'Electrochimie Moldculaire-U.R.A. C.N.R.S. 438, Universit~ Pierre et Marie Curie, 2, Place Jussieu, 75252 - Paris Cedex 05, France

Abstract. The synthesis of two classes of hybrid organic-inorganic nanocomposites with electronic properties is reported. One is made of PDMS units cross linked with vanadium oxo-species where the vanadium coordination depends on the hydrolysis pH. Tetrahedral coordination is retained at neutral pH, while acidic conditions promote the segregation of five coordinated vanadium oxo-species. The reduction process depends also on the vanadium coordination. The second system is made of siloxane T units and polypyrrole oligomers, grafted and interpenetrated at a nano size level. Keywords: hybrids, siloxane, polypyrrole, vanadium (IV) oxo-species 1. Introduction The characteristics of the sol-gel process (metalloorganic precursors, organic solvents, low processing temperatures) allow to introduce "fragile" organic molecules into an inorganic network and therefore to synthesize hybrid organic-inorganic materials [1, 2]. The most commonly used precursors are organically modified silicon alkoxides: SiRx(OR')4_x, where R is any organic group attached to silicon by a Si-C bond. Indeed, the stability of the Si-C bond towards hydrolysis provides an easy way to introduce the organic moiety. Moreover, hydrolysis-condensation of alkoxysilane precursors SiRx(OR')4_x is known to lead to hybrid organic-inorganic materials that can be easily shaped. The introduction of electronic properties (conduction, redox properties) [3] can be achieved a priori in hybrid networks by using either the organic component (i.e, conductive polymers) or the inorganic component (i.e., transition metals oxides). Hybrid systems, obtained by intercalation of conductive polymers such as polypyrrole, polyaniline or polythiophene in a wide variety of host structures (V205 xerogels, MoS2,

clays, zeolites), have been reported [4]. The growth of polyaniline in silica-PDMS based gels has also been reported [5, 6]. The present work reports the molecular design, synthesis and preliminary characterization of hybrid organic-inorganic nanocomposites from modified metal alkoxide precursors. This paper proposes new synthetic approaches for two kinds of hybrid materials that exhibit electronic properties.

- i) Transition metals, such as vanadium, that can exhibit several valence states, are introduced at a molecular level by using alkoxides (VO(OAmt)3) as cross linking agents for siloxane based networks. Depending on the alkoxysilane precursor, DEDMS (Diethoxydimethylsilane) or DEMS (Diethoxymethylsilane), and the chemical conditions, hybrid materials can be produced in which vanadium exhibits a range of valence states. - i i ) In the second kind of hybrid materials, electronic properties are introduced by using precursors such as TMSPP (N-3 trimethoxy-silyl-propylpyrrole). This compound can be hydrolyzed and polymerized to yield polypyrrole polymers crosslinked with silicon-oxo-based species. This strategy

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preserves chemical bonds between the organic and the inorganic components.

2.

2.1.

Results and Discussion

Hybrids Based on Oxo-Vanadium/Siloxane Network

The hydrolysis of VO(OAmt)z/DEDMS (system 1) or VO(OAmt)z/DEMS (system 2) solutions yields sols and gels with significantly different redox behaviors. The second system leads to black sols and gels in a few seconds. This black color arises from the reduction of vanadium cations as shown by ESR and magnetic measurements [7]. The fast vanadium readuction is likely related to the strong reducing power of the hydrogen produced by the cleavage of the Si-H bonds which is catalyzed by transition metal alkoxides [8, 9]. Characterization of system 2 will be presented in a forthcoming paper [7]. In this article only system 1 will be described. It yields sols which can be easily deposited as coatings. The redox and mechanical behavior of these coatings strongly depend on the acidity of the hydrolysis solution.

2.1.1. Synthesis Diethoxydimethylsilane (DEDMS) and VO(OAmt)3 (1:0.1 molar ratio) were mixed for a few minutes by magnetic stirring. Water was then added with the hydrolysis ratio, [H20]/[Si] = 1. The water pH was 6.5 (sol or gel B) or adjusted with hydrochloric acid to pH = 1 (sol or gel A). Immediately after water addition, both solutions A and B exhibit an orange color which is characteristic of vanadium (V) oxo-alkoxo species in which vanadium is in a distorted square pyramidal environment [ 10]. After ten minutes of fast stirring, sol A exhibits a yellow colour, while sol B turns colorless. Sols A and B are then poured in petri vessels. Under such conditions, gelation takes 5 days. When hydrolysis is carried out with acidic pH (sol A), coatings turn green in 2 days, then crack and are finally transformed into green powders. Conversely, when hydrolysis is performed at pH = 6.5 (sol B) crack-free, transparent coatings are obtained. The hydrolysis-condensation process of these two preparations were followed by 29Si and 51V, liquid and solid state MAS NMR.

2.1.2. Solution Study The 29Si NMR spectra of sol A exhibits three main resonances at - 21.4, - 18.9 and - 1 2 . 7 ppm. They are respectively assigned to D2 units (Si-O-Si(CH3)2-O-Si), D2 units in D42 cyclic species, and D1 terminal units, ZO-Si(CH3)2-O-Si (Z = H, R). A weak resonance at -11.5 ppm is also observed. Such resonance is only observed upon hydrolysis of DEDMS in the presence of another metal alkoxide [11]. It can be assigned to D1 units bonded to vanadium oxo-species V-O-Sji (CH3)2-O-Si. The 29Si NMR spectra of sol B exhibits the same general features. However, the component located at -11.5 ppm presents a much higher intensity than for sol A. These results indicate the formation of Si-O-V bonds in the early stage of the process and suggest that these bonds are more stable in sol B prepared at pH 6.5 than in sol A prepared at pH 1. Such a conclusion is clearly confirmed by SlV NMR experiments (Figure 1). Figures la, lb, lc show the 51V NMR spectra of sol A, sol B, and a reference sol C made by mixing hydroxy terminated PDMS chains (Mw = 1750) (1 mole) with VO(OAmt)3 (0.5 mole), respectively. A large number of 51V NMR resonances, regrouped in well-defined zones, appear from - 6 4 0 to - 7 4 0 ppm (chemical shifts referenced to VOCI3). The position of the zones depends on the vanadium coordination number and on the nature of the surrounding oxygenated ligands. Their assignments have been made from literature [12-15] and are reported in Figure 1. Figure lc exhibits three resonances characteristic of vanadium in a pseudo tetrahedral coordination environment: "O = VO3". These resonances are due to different degrees of reaction of the teramyloxy groups with the terminal hydroxyl groups of the PDMS chains. A high shift is observed as the degree of substitution increases (O = V(OSi)(OAmt)2: = - 7 0 0 ppm, O=V(OSi)2(OAmt): 5 = - 7 1 7 ppm, O = V ( O S i ) 3 : 6 = - 7 3 6 ppm). The pseudo tetrahedral precursor, O = V(OAmt)3, has a resonance at - 6 8 4 ppm [12]. Figures la and lb, corresponding to sols A and B, are much more complex because the presence of water and DEDMS generates a large set of species. Hydrolysis-condensation reactions yield V-OH, Si-OH and numerous metal oxooligomers. The DEDMS precursor also undergoes alcoholysis reactions (substitution of ethoxy groups by ter-amyloxy groups), which introduces numerous additional resonances. Consequently, compared with Figure lc, Figures la and lb exhibit three sets of ex-

Molecular Design of Hybrid Organic-Inorganic Materials

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tra resonances, located around - 6 8 0 ppm, - 6 6 5 ppm and - 6 3 8 pro, which correspond to vanadium alkoxide, vanadium hydroxo-alkoxide and vanadium oxoalkoxide oligomers (VO(OAmt)z(OEt)~(OH)vOk, z + y + z + 2k = 3). Figures la and lb also exhibit a set of resonances close to those observed in the reference sol C ({5 = - 7 0 0 ppm, - 7 1 7 ppm, - 7 3 6 ppm) which correspond to PDMS substituted vanadium oxospecies. The splitting of each resonance group is due to chemical modifications of the first and second vanadium coordination spheres (the bonded oxygenated groups can be OH, OEt, OAm t and in situ generated PDMS chains of various length and conformations). The most obvious feature is the fact that in sol B the pseudo-tetrahedral vanadium species, which are di- and tri-substituted with siloxane groups like O = V(OSi)2 (O-) and O = V(OSi)3, are dominant and their proportions are much higher than in sol A.

2.1.3. Characterization of the Gels 29Si and 81V NMR MAS spectra of the green gels A could not be recorded with standard NMR sequences [11, 12, 14]

probably because of the presence of numerous magnetic species provided by the reduction of vanadium atoms. However, the reduced species can be readily identified by ESR spectroscopy, which shows a strong resonance, 270 Gauss wide, located at 9 = 1.96 together with a superimposed eight line spectrum due to the hyperfine coupling with 51V. The weak anisotropic hyperfine signal has the following magnetic parameters: g// = 1.963, A// = 203 Gauss, g± = 1.983, Aa = 80 Gauss which are characteristic of vanadium(IV) in a square pyramidal environment close to those reported for vanadyl species [16]. Most of the V(IV) exchanges strong spin-spin interactions which lead to the observed strong resonance characteristic of vanadium (IV) oxo-species clustering [16, 17]. 29Si and 51V NMR MAS spectra of gel B are reported in Figures 2a and 2b. The 29Si NMR spectra, recorded at a spinning speed of 4 kHz, exhibit four main resonances at -21.7, - 2 0 , - 1 9 and -11.5 ppm. They can be respectively assigned to D2 units (Si-OSi(CHa)2-O-Si), D2 units bond to vanadium species (V-O-Si-O-Si(CHa)2-O-Si), D2 units in D42 cyclic

164

Sanchez, Alonso, Chapusot, Ribot and Audebert

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species, and D1 units bonded to vanadium species (VO-Si(CH3)2-O-Si). The MAS 51V NMR spectra of gel B (Fig. 2b) (recorded at 15 kHz) exhibits two main resonances at - 7 2 9 ppm (54%) and - 7 0 9 ppm (35%). These resonances correspond to O--V(OSi)3 and O--V(OSi)2(X) species, respectively (X is essentially residual ter-

amyloxy groups as evidenced by solid state 1H and 13C NMR experiments [7]), where vanadium atoms have a pseudo-tetrahedral environment. 2.1.4. Discussion The NMR experiments indicate that vanadium and DEDMS co-condensation occurs in the early stage of the process at higher pH (6.5 for sol

Molecular Design of Hybrid Organic-Inorganic Materials

B), to yield stable Si-O-V bridges. Even if numerous species are in equilibria in the sol (as evidenced by 51V NMR), upon drying equilibrium is mainly shifted towards O = V(OSi)3 and O = V(OSi)2(X) species. This results in the covalent bonding of discrete tetrahedraUy coordinated vanadium (V) oxo-species to polysiloxane moieties. The sequestering of vanadium (V) in a tetrahedral coordination probably inhibits its reduction by alcohol and light. V(IV), which is larger than V(V), is known to be stabilized in oxygen coordinated polyhedra that have higher coordination numbers such as octahedra or square pyramids [ 18]. On the other hand, even though co-condensation occurs in the early stage of acid sol preparation (Sol A), the degree of co-condensation is less and the chemical conditions do not favor vanadium in tetrahedral coordination. Moreover, they favor the cleavage of Si-O-V bonds by promoting better leaving groups via the protonation of oxygen atoms. Consequently, vanadium is much less sequestered by siloxane species and can segregate into vanadium oxo-polymeric species. This segregation allows the vanadium (V) to increase its coordination number (the vanadium environment is probably close to a distorted square pyramid) and therefore favors the reduction process by stabilizing V(IV) oxo-compounds.

2.2.

Hybrids Based on Polypyrrole/Siloxane Networks

A second class of hybrid materials with electronic properties has been synthesized by using TMSPP (N-3 trimethoxy-silyl-propyl-pyrrole) as a precursor. TMSPP was hydrolyzed and polymerized (FeC13 as oxidant) to yield polypyrrole polymers cross linked with silicon oxo-based species. Two kinds of gels have been prepared.

2.2.1. Synthesis Gel Py A: A solution of TMSPP, pyrrole and THF in a 1:1:1 molar ratio was hydrolyzed (H20/Si = 3) in the presence of a nucleophilic catalyst ([NH4F] = 10 -1 mol.1-1) to yield a transparent sol [19]. A nucleophilic catalyst was preferred because polymerization of pyrrole in presence of proton is more complex and less efficient. This sol was stirred for twenty minutes, and anhydrous FeCla was slowly added until the molar ratio between pyrrole groups and

165

iron was stoichiometric. A black gel (gel Py A) was obtained. The black colour is characteristic of the polymerization process of polypyrrole derivatives. Gel Py B: A solution of TMSPP, pyrrole, TMOS and THF in a 1:1:1:1 molar ratio was hydrolyzed (H20/Si = 3) in the presence of a nucleophilic catalyst ([NH4F]= 10 -1 mol.l-1). The transparent sol obtained was stirred for ten minutes and anhydrous FeCI3 was slowly added until the molar ratio between pyrrole groups and iron was stoichiometric. A black gel (gel Py B) is obtained. Both gels Py A and Py B were washed several times with THF to remove the iron salt.

2.2.2. Characterization of the Composites The 29Si MAS NMR spectra of gels Py A and Py B exhibit one resonance at - 6 5 ppm and two resonances located at - 6 4 and - 1 1 2 ppm, respectively. These resonances can be assigned to T 3 and Ta + Q4 units, respectively. Their presence indicates a high degree of condensation for the silicon oxo-species inside both composite gels. The ZgSi MAS NMR spectra of both composite gels are characterized by the presence of strong and numerous spinning side bands which are related to the electron distribution created by delocalized electrons and/or paramagnetic centers [20]. The composite gels have been electrochemically characterized with the help of a carbon paste electrode, as it has been previously shown in the case of classical polypyrroles [21]. The cyclic voltammograms show that there is a redox system at 0 V (vs SCE), which is in the characteristic potential domain of polypyrroles. However, the redox system appears less stable than in the case of classical polypyrrole and the peak intensities decrease noticeably upon successive sweeps (scan rate 10 mV/s). A possible explanation could be that the composites contain mainly short oligomers, instead of long polymer chains, which are more easily degraded in the reduced state. Cold pressed pellets have been made from these composites. Their permittivity is around 2 and they exhibit an electronic conductivity of 6 x 10 -3 S.m -1 at a frequency of 1 GHz. These results show that the degree of percolation of the polypyrrole through the silicon oxo-network is lower than reported for PVC-polypyrrole blends [22]. The conductivity of these composites must be increased by two orders of magnitude in order to obtain interesting microwave absorbing materials.

166

3.

Sanchez,Alonso, Chapusot, Ribot and Audebert

Conclusion

Two types of hybrid organic-inorganic materials with electronic properties have been investigated. In the first one, made by co-condensing VO(OAmt)3 with DEDMS, the hydrolysis pH shows a great influence on the nanostructure of the gels and xerogels. Acidic conditions promote the formation of vanadium oxospecies, in which vanadium exhibits a coordination number of 5 (distorted square pyramid). In such oxospecies, V(V) is easily reduced in V(IV). Neutral hydrolysis conditions favor the sequestering of vanadium in 4 fold coordination and therefore make more difficult the reduction of V(V). The second type of materials is based on pyrrole grafted to siloxane T units precursors (TMSPP). Hydrolysis-condensation and polymerization yield polypyrrole attached on a siloxane backbone. Characterizations show that the siloxane network is highly condensed, but only small oligomers ofpolypyrrole are formed. The resulting electronic conductivity is quite low and has to be improved for applications.

5. Kramer, S.J., Colby M.W., Mackenzie, J.D., Mattes, B.R., and Kaner, R.B., in ChemicalProcessing of Advanced Ceramics, edited by Hench, L.L., and West, J.K., (J. Wiley & Sons, New York, 1992), p. 737. 6. Dunn, B., and Zink, J.I., J. Mater. Chem 1(6), 903 (1991). 7. Alonso, B., Ribot, E, and Sauchez, C., J. Mater. Science (submited). 8. Campostrini, R. and Dir6, S., Proceedings of Eurogel 92 (in print). 9. Panthe, M., Phalippou, J,, Belot, V., Corriu, R., Leclercq, D., and Vioux, A., J. Non-Cryst. Solids 125, 187 (1990). 10. Crans, D.C., Chen, H., and Felty, R., J. Am. Chem. Soc. 114, 4553 (1992). 11. Dir6, S., Babonnean, E, Caruran, G., and Livage, J., J. NonCryst. Solids 147.148, 62 (1992). 12. Feher, EJ,, and Walzer, J.E, Inorg. Chem. 30, 1689 (1991). 13. Feher, EJ. and BlanskY, R.L., Organometallics 12, 958 (1993). 14. Sanchez, C., Nabavi, M., and Tanllele, F., in Better Ceramics through ChemistryI11,edited by C.J. Brinker, D.E. Clark and D.R. Ulrich (MRS, Pittsburg, 1988), p. 96. 15. Rehder, D., Bull. Magn. Res, 4, 33 (1982), 16. (a) Babonneau, E, Barboux, P., Josien, F.A., and Livage, J., Journal de Chimie Physique 7-8, 82 (1985). (b) Balhausen, C.J., Gray, H,B., and Inorg. Chem, 1, I l l (1962).

Acknowledgments

17. Vandenborre, M.T., Sanchez, C., and Politi, A., Nouveau Journal de Chimie 9(7), 45 (1985).

J. Maquet and D, Massiot for MAS NMR experiments.

18. Nabavi, M., Sanchez, C., and Livage, J., Phil. Mag B 63(4), 941 (1991).

References

19. Corriu, R.J.P., Leclercq, D., Vioux, A., Pauthe, M., and Phalippou, J., in Ultrastructnre Processing of Advanced Ceramics, edited by Mackenzie, J.D. and Ulrich, D.R. (J. Wiley & Sons, New York, 1988), p. 113.

1. Schmidt, H., Scholze, H., and Kaiser, A., J. Non-Cryst. Solids 63, 1 (1984). 2. Sanchez, C. and In, M., J. Non-Cryst. Solids 147-148,1 (1992). 3. Bretscheidel, B., Zieder, J. and Schubert, U., Chem. Mater. 3, 559 (1991). 4. Ruiz-Hitchky, E., Adv, Mater. 5, 334 (1993).

20. Hatfield, G.R. and Cardurer, K.R., J. Mater. Science 24, 4209 (1989). 21. Audebert, P. and Bidan, G., Synth. Met. 4, 71 (1986). 22. Olmedo, L., Hourquebie, P., and Jousse, F., Adv. Mater. 5(5), 373 (1993).