André Vioux

Hybrid materials from organophosphorus coupling molecules

Organophosphorus acids and their derivatives (salts, esters) are quite complementary of organosilicon coupling molecules for the preparation of hybrid organic–inorganic materials, by sol–gel processing or surface modification. Organosilicon compounds are best suited for the anchoring of organic groups to silicon-containing inorganic matrices or supports, such as silica, silicates, silicon carbide, etc., whereas organophosphorus coupling molecules appear well adapted to matrices or supports based on metals or transition metals: oxides, hydroxides, as well as carbonates and phosphates. The different reactivity of organophosphorus coupling molecules leads to different structures and stabilities of the final hybrid materials and may provide decisive advantages in the sol–gel synthesis of homogeneous hybrids, the preparation of surface monolayers or the selective surface modification of nanopatterned supports.

Mixed oxides SiO2–ZrO2 and SiO2–TiO2 by a non-hydrolytic sol–gel route

SiO2–ZrO2 and SiO2–TiO2 mixed oxides with various metal contents have been prepared by a non-hydrolytic sol–gel route involving the condensation between chloride and isopropoxide functions at 110 °C. Well condensed, monolithic gels were obtained in one step, without the use of additives. The Si/M ratio of the oxide may be controlled easily by the composition of the starting mixture. The Si/Zr oxides remained amorphous after calcination for 5 h at 600 °C; IR and 29Si NMR spectroscopy showed a large amount of Si–O–Zr bonds, indicating a homogeneous distribution of the components on the atomic scale. The crystallization of tetragonal zirconia took place at higher temperature; the transformation of tetragonal to monoclinic zirconia was strongly retarded and did not take place after 2 h at 1300 °C. The crystallization of zircon (for a sample containing 50 mol% Zr) started at 1500°C and was completed after 20 h at 1500°C. IR spectroscopy indicated the presence of a limited number of Si–O–Ti bonds in all the Si/Ti oxides after calcination for 5 h at 500 °C. The sample within the stable glass region (5 mol% Ti) appeared perfectly homogeneous: it crystallized at 900 °C as single-phase cristobalite oxide, with Ti4+ ions substituting Si4+ ions at random. On the other hand, the precipitation of anatase was observed for the Si/Ti oxides with a high Ti content (20–50 mol% Ti), which are outside the stable glass region. The transformation of anatase to rutile was not observed even after 2 h at 1300 °C.

Ionic liquid as plasticizer for europium(III)-doped luminescent poly(methyl methacrylate) films

Flexible luminescent polymer films were obtained by doping europium(III) complexes in blends of poly(methyl methacrylate) (PMMA) and the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(6)mim][Tf(2)N]. Different europium(III) complexes have been incorporated in the polymer/ionic liquid matrix: [C(6)mim][Eu(nta)(4)], [C(6)mim][Eu(tta)(4)], [Eu(tta)(3)(phen)] and [choline](3)[Eu(dpa)(3)], where nta is 2-naphthoyltrifluoroacetonate, tta is 2-thenoyltrifluoroacetonate, phen is 1,10-phenanthroline, dpa is 2,6-pyridinedicarboxylate (dipicolinate) and choline is the 2-hydroxyethyltrimethyl ammonium cation. Bright red photoluminescence was observed for all the films upon irradiation with ultraviolet radiation. The luminescent films have been investigated by high-resolution steady-state luminescence spectroscopy and by time-resolved measurements. The polymer films doped with beta-diketonate complexes are characterized by a very intense (5)D(0)-->(7)F(2) transition (up to 15 times more intense than the (5)D(0)-->(7)F(1)) transition, whereas a marked feature of the PMMA films doped with [choline](3)[Eu(dpa)(3)] is the long lifetime of the (5)D(0) excited state (1.8 ms).

Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol–gel methods

Titania samples prepared by different non-hydrolytic sol–gel methods, mainly based on the etherolysis and alcoholysis of titanium tetrachloride, have been found to differ in both structure and texture. Thus, the reaction of diethyl ether with TiCl4 at 110 °C affords anatase, which begins to convert into rutile only around 1000 °C. The reaction of TiCl4 with ethanol leads to rutile as early as 110 °C, whereas the reaction of tert-butyl alcohol at 110 °C leads to the singular formation of brookite.

Non-hydrolytic sol–gel routes to silica

Abstract Silica was prepared by solvent-free non-hydrolytic condensation reactions from silicon tetrachloride with different oxygen donors: benzyl alcohol and tetrabenzyloxysilane (no catalysis), tetraethoxysilane, tetraisopropoxysilane, diethylether and diisopropylether (catalyzed by FeCl3, TiCl4, AlCl3 or ZrCl4). Highly condensed gels with a low hydroxyl content were obtained in high yields. A solution 29Si NMR study showed that in the presence of FeCl3 the condensation of SiCl4 with Si(OiPr)4 was accompanied by extensive redistribution of Si–Cl and Si–OiPr bonds, leading to increasingly complex mixtures. Non-hydrolytic silica xerogels were characterized by chemical analysis, IR and 29Si NMR spectroscopy and thermogravimetric analysis. Porosity and specific surface area were determined using nitrogen adsorption–desorption. Xerogels displayed a wide variety of textures depending on the nature of the oxygen donor and of the catalyst.

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Abstract Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)3 and aluminum halides AlX3, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by 27Al NMR spectroscopy.

Preparation of monolithic gels from silicon halides by a non-hydrolytic sol-gel process

Reactions of organic oxygen compounds, such as ter-butanol, benzylalcohol, dibenzylether and benzaldehyde, with silicon halides provide a novel non-hydrolytic sol-gel route to silica.

Materials chemistry communications. Preparation of monolithic metal oxide gels by a non-hydrolytic sol–gel process

Non-hydrolytic condensation between halide and alkoxide metal precursors leads to oxide gels; thus, Al2O3 and TiO2 were successfully synthesized as monolithic gels.

Organosilicon gels containing silicon-silicon bonds, precursors to novel silicon oxycarbide compositions

Organosilicon gel compounds were obtained from chlorodisilanes with the general formula (CH3)xCl6−xSi2 which may be available as industrial byproducts. Their thermal behavior was investigated by TG-MS analysis and by IR and NMR spectroscopy. Pyrolysis leads to high yields of silicon oxycarbides with novel oxygen-poor compositions. From NMR data, the entire range of tetrahedral units from SiC4 to SiO4 is observed as early as 800°C. Conversion into silicon carbide by carboreduction reaction occurs above 1400°C; it is associated with an extensive loss of CO and SiO needed to meet the SiC stoichiometry.

Silica xerogels containing a functional group at silicon

Modified silica xerogels can be obtained from several trialkoxysilanes containing a specific group attached to silicon ZSi(OR′)3 (where R′ means an alkyl group). We report here the use of Z = H. In basic catalyzed conditions, the hydrolysis induces a release of H2 gas coming from SiH bonds. However, in neutral or acidic conditions it is possible to prevent the cleavage of SiH bonds. Thus, the dried gel contains SiH bonds as shown both by spectroscopy and chemical analysis. This new family of gels reacts with oxygen gas from the atmosphere. During heating, gels show a 7% weight increase in the temperature range 250°C to 600°C.