Inorganic-Organic Hybrid Materials

Organic-inorganic hybrid materials are materials prepared by combining organic and inorganic building blocks. The development of such materials, which have already found numerous applications, is one of the big achievements of sol-gel science. The notion is to create materials with new combinations of properties by combining inorganic and organic building blocks on a molecular level. In the so-called class I materials, the organic and inorganic entities interact only weakly, while they are linked through strong chemical bonds in class II hybrid materials. Our work is focused on class II sol-gel materials. Some of the materials we have developed in the past (partially in cooperation with various companies) are outlined in the following.

Aerogels

The preparation of organically modified aerogels led us to a detailed understanding on the structural development of gel networks from two different precursors, viz. Si(OR)₄ and R′Si(OR)₃ (R = functional or non-functional organic groups). Under basic conditions, Si(OR)₄ is hydrolyzed first and thus forms the basic structure of the gel. The R′Si(OR)₃ component reacts in a later stage and thus condenses on the existing network structure. This results in a kind of core-shell structure, where the network of aggregated primary particles (formed from Si(OR)₄) is covered with the organic groups R′ introduced through R′Si(OR)₃.

Nanocomposites

Our approach for the preparation of nanoparticles of composition B dispersed in a matrix phase A is to coordinate the precursor of the nanoparticle phase to the precursor of the matrix phase B by means of an organic spacer during sol-gel processing (Scheme 1). To this end, organically substituted alkoxysilanes of the type (RO)₃Si(CH₂)_nX are used for SiO₂ as the host phase, where X is a coordinating group. The groups X allow tethering of metal compounds to the silicate network during sol-gel processing due to the (intermediate) formation of compounds (RO)₃Si(CH₂)_nX-M, where M is a metal ion, a metal complex moiety, etc. The advantage of this method is that a two-component system (an alkoxysilane and a metal compound) is transformed in a single-source precursor. A high - ideally a molecular - dispersion is thus achieved during sol-gel processing. Nanocomposites, i.e. a nanometer-sized phase B dispersed in silica (phase A), are then obtained by controlled degradation of the organic tethers in a later step, and phase B particles are allowed to grow.

Scheme 1. Preparation of nanocomposites by sol-gel processing of organofunctional single-source precursors.

Metal / silica nanocomposites were developed by a three-step procedure. In the first step, metal ions are in situ coordinated to amino or diamino-substituted organotrialkoxysilanes. After sol-gel processing of the metal-containing precursor, the gel is heated in air to pyrolyze/oxidize the organic entities, and metal oxide nanoparticles are obtained. The metal oxide particles in the nanocomposite then can be reduced to metal nanoparticles.
This protocol provides metal/SiO₂ nanocomposite powders with the following properties:

The average metal particle diameter is in the lower nanometer range (typically 2-20 nm) and the particle size distribution is very narrow (even for high metal loading).
The metal proportion is variable and the kind of metal is easily varied (noble metals, Fe, Co, Ni, Cu, Cd, etc).
The metal particles are not agglomerated and are randomly distributed throughout the silica matrix.
The porosity of the nanocomposite powders is influenced by the complexing silane and can range from non-porous to highly porous.
The metal particles are formed at the inner surface of the matrix material (SiO₂).

The calcination step in air can be by-passed, if carbon-free composites are not required, or if part of the organic groups need to be retained for other reasons. For example, removal of the organic groups would be detrimental to obtaining composite films with sufficient properties. In such cases, the gels obtained by sol-gel processing can be directly treated with hydrogen at elevated temperatures.

Metal oxide/silica nanocomposites with more oxophilic metals, such as alumina, titania or zirconia, were prepared starting from a β-diketonate-substituted trialkoxysilane. The β-diketonate group of this functional silane allows coordinating metal alkoxide moieties (see Section on New Precursors for Sol-Gel Processing).

Mixed-metal oxide nanocomposites (e.g., Si/Ti, Si/Zr or Si/Al) can be prepared from these single source precursors by sol-gel processing without the phase separation problems associated with Si(OR)₄/M(OR)n mixtures. Time-resolved small-angle scattering (SAXS) investigations through all stages of the preparation process showed that the M(OR)_n groups are hydrolyzed first. This results initially in small M/O clusters which are covered by the (MeO)₃SiCH₂CH₂CH₂CH[C(Me)O]₂ groups. Hydrolytic polycondensation of the (MeO)₃Si groups, i.e. formation of the surrounding silica matrix, occurs with some delay. When the gels are heated in air to pyrolyze/thermolyze the organic groups, titania, zirconia or alumina nanoparticles are formed in the silica matrix.

Coatings

We have developed several coatings based on class II inorganic-organic hybrid materials. Recent examples:

A protective sol-gel layer on aluminum pigments was developed which prevents the aluminum flakes to react with water and thus allows the use of the coated pigments in water-based highly reflective lacquer formulations.
Magnesium with a density of only 65% of aluminum could be a breakthrough technology in the aerospace industry if used for cost efficient, low weight components and airframe structures. However, to use this low weight material the mechanical and technological properties have to be improved. A key issue is the protection of the magnesium structures against corrosion. In an EU project (Aeromag), a new anti-corrosive coating for magnesium alloys based on sol-gel technology was developed.
Metal/silica nanocomposite films on glass substrates were developed by the "single-source precursor method" outlined in the previous paragraph.
Polymer films with crosslinking metal oxide clusters were obtained by a modification of the methods outlined in the section on Cluster-Reinforced Polymers.
Films with polymodal porosity (macro-, meso- and micropores) as well as complex materials compositions (different metals, different organic groups) were prepared by using multiple porogens, mixtures of modified and unmodified metal alkoxides as well as post-synthesis modification of the gels by organic groups. Such coatings will be used as contamination traps under high-vacuum conditions.

Selected review articles on inorganic-organic hybrid materials

U. Schubert, New J. Chem. 1994, 18, 1049-1058; "Catalysts made of organic-inorganic hybrid materials".
U. Schubert, N. Hüsing, A. Lorenz, Chem. Mater. 1995, 7, 2010-2027; "Hybrid Inorganic-Organic Materials by Sol-Gel Processing of Organofunctional Metal Alkoxides".
D. Avnir, L. C. Klein, D. Levy, U. Schubert, A. B. Wojcik, "Organo-silica sol-gel materials" in "The Chemistry of Organic Silicon Compounds, Vol.2, Part 3", Eds. Z. Rappoport, Y. Apeloig, J.Wiley & Sons (1998), p.2317-2362.
U. Schubert, F. Schwertfeger, C. Görsmann, ACS Symp.Ser. 1996, 622, 366-381; "Sol-Gel Materials with Controlled Nano-Heterogeneity".
U. Schubert, J. Chem. Soc. Dalton 1996, 3343-3348; "New Materials by Sol-Gel Processing: Design at the Molecular Level".
N. Hüsing, U. Schubert, Angew.Chem. 1998, 110, 22-47; Angew.Chem.Int.Ed.Engl. 1998, 37, 22-45; "Aerogels - airy materials: chemistry, structure and properties".
U. Schubert, G.Kickelbick, N.Hüsing, Mol.Cryst. Liquid Cryst. 2000, 354, 107-122; "Nanoscale Structures of Sol-Gel Materials".
U. Schubert, J.Sol-Gel Sci.Technol. 2003, 26, 47-55; "Silica-based and transition metal-based inorganic-organic hybrid materials - a comparison".
U. Schubert, Polymer Intern. 2009, in press; "Preparation of Metal Oxide or Metal Nanoparticles in Silica via Metal Coordination to Organofunctional Trialkoxysilanes"

This work was supported by several companies, the Forschungsförderungsgesellschaft (FFG) and the European Union (projects Nanoreflex, Aeromag and IP Napolyde).