CSEM Scientific and Technical Report 2008

3D Hierarchical Porous Coatings for Multifunctional Surfaces with Enhanced Properties
E. Scolan, V. Monnier, K. Zeng, R. Pugin
Sol-gel processes are very versatile and well-suited for the deposition of layers with controlled homogeneity, thickness, porosity and associated
surface structures. Their high surface area can be enhanced by introducing porosity on different length scales: these hierarchically porous surfaces
are advantageous for a large variety of applications, such as catalysis, absorption, electrodes and drug delivery.
Over the last 15 years there has been an intensive effort to
develop porous materials with a wide range of pore sizes up
to 1 mm. These porous materials create new opportunities in
catalysis, adsorption and separation technology. In parallel,
the progress in creating inorganic-organic composites is
significant because of their potential applications in catalysis,
photonics, and electronics as well as their use as structural
and biomedical materials. The hierarchical organisation of
composites is one of the most challenging issues, leading to
the emergence of multifunctional materials and specific
physical properties often observed in biological systems.
Sol-gel processes [1] can be broadly defined as the preparation
of metal oxide materials at the nanometer scale (including
fibres, powders/particles, shaped/moulded bulks, (thin) films).
This gentle wet chemistry route to ceramics has many
advantages: prior to gelation, metal oxide nanoparticle
dispersions are ideal for preparing porous thin films, with pore
sizes ranging from a few Ångstrom up to 1 mm; Moreover, the
sol step enables the mixing with soluble organic and/or
biological species to prepare hybrid organic-inorganic
materials.
Microporous materials (pore size < 2 nm) are usually prepared
by a templating process using organic additives, leading to
zeolitic structures. Larger mesopores (2 < pore size < 50 nm)
can be obtained by using surfactant arrays, emulsion droplets
as templates, leaching techniques or nanoparticles
aggregation [ 2 ] . Finally macroporous systems (pore size
> 50 nm) require a phase separation mechanism or the use of
colloidal crystals, vesicles, foams or biological species as
templates. The self-assembly ability of all these templates
often adds an advantageous structural order to the materials.
After the formation of the inorganic metal oxide backbone, the
organic template can be removed by dissolution or calcination.
Nanostructured coatings have been developed and
extensively studied at CSEM using two approaches: one
based on a hot water leaching [ 3] and the other on polymer
crosslinked nanoparticulate films [2] . The combination of these
nanostructuration processes with a totally compatible
templating approach using sub-micron polymeric beads has
enabled the formation of meso- and macroporous metal oxide
coatings. The hot water leaching process (cf. Figure 1a) has
led to thick macroporous aluminum oxide films exhibiting both
external and internal surface nanostructures.
The introduction of polymeric beads into crosslinkable metal
oxide nanoparticles dispersions generates macropores into
the mesoporous coatings (cf. Figure 1b and 1c). The
mesoporosity is induced by the aggregation of discrete solid
metal oxide nanoparticles used as nano-building blocks.
50
(a)
(b) (c)
Figure 1: SEM pictures of (a) Aluminium, (b) Silicon and (c) Titanium
oxide hierarchically structured coatings
These bimodal porous coatings have enhanced surface area
compared to either purely macro- or mesoporous films (by the
fine tuning of pore sizes, connectivities or pore ordering). This
means that enhanced performances can be obtained through
larger contact surface, easier and faster fluid transport and by
combining the functionalities which occur at different scales
(e.g. optical properties of macroporous porosity of inverse
opals and drug delivery of mesoporosity). Many advanced
applications, linked with highly efficient smart materials
(e.g. photovoltaic and fuel cells, sensors and affinity
membranes) or green applications (energy saving for hybrid
vehicles, filtration or separation membranes; (photo) catalysis;
water purification; self-cleaning surfaces) will benefit from this
easy and versatile approach to prepare multimodal porous
coatings.
CSEM would like to thank the European Commission
(www.napolyde.org) for their financial support.
[ 1 ] J. Brinker, G. Scherer, “Sol-Gel Science, The Physics and
Chemistry of Sol-Gel Processing”, Academic Press, San Diego
(1990)
[2] E. Scolan, V. Monnier, R. Steiger, R. Pugin, “3D Hierarchical
Porous Coatings for Multifunctional Surfaces with Enhanced
Properties”, in this report, page 50
[3] K. Tadanaga, N. Katata, T. Minami, “Super-Water-Repellent
Al2O3 Coating Films with High Transparency”, J. Am. Ceram.
Soc., 80 (1997), 1040