4.1 Functional Nanomaterials with an Equilibrium
Mesostructure
A widely followed approach for utilizing self-assembled BCP nanostructures for
fabrication of functional nanomaterials is to generate ordered porous BCP templates
via selective removal of one polymer block. Certain polymers are selectively removable via application of acid, base, heat, or ozone [36]. The resulting porous templates
can subsequently be backfilled with functional materials, e.g., via metal oxide
solutions or metal deposition. With the Steiner group at Cambridge University in
the lead, the Wiesner group helped to fabricate gyroidal titania electrodes for solidstate dye-sensitized solar cells [37] and gyroidal gold nanomaterials for metamaterial
applications [38]. Gyroidal minority networks formed by poly(L-lactic acid) were
selectively removed by an acid treatment for the former case [37], whereas
ozonolysis of polyisoprene was used in the latter [38]. Backfilling of titania into the
porous gyroidal template followed by incineration of the template resulted in
nanostructured titania electrodes, and electrodeposition of gold on the gyroidal
template led to gyroidal gold nanomaterials. This block removal strategy involves
two main difficulties. Complete selective etching of thick polymer monoliths can be
challenging since an etchant must reach a long-distant template center. Thorough
percolation of sacrificial polymer blocks is rare for large bulk samples due to the
randomized orientation of crystal axes. Thus, alignment of the crystal axes is usually
needed for better etching [39]. The other main difficulty is complete backfilling of a
large porous monolith with functional materials. Nanosized pores can easily become
clogged during a deposition process if functional materials are deposited, e.g., via
chemical vapor or electroless deposition. Furthermore, the use of nanostructured
porous BCPs as templates for fabrication of functional nanomaterials requires several
steps, including BCP SA, selective etching, and backfilling.
This multistep procedure can be simplified by directly employing BCPs as
structure-directing agents for functional inorganic materials in a single SA step.
In this approach, additive inorganic materials preferentially localize in one block of
the phase-separated nanostructures during BCP SA, usually in the more hydrophilic
block, thereby minimizing the enthalpic penalty of mixing with more repulsive
blocks. This strategy leads to nanostructured polymer/inorganic hybrid materials in
a “one-pot” fashion, without multiple and often tedious post-processing steps. The
disadvantage is that, depending on the inorganic additives, long post-SA annealing
steps may not be possible thus rendering achievement of long-range order of the
nanostructures challenging.
The Wiesner group has synthesized a variety of nanostructured oxide materials via
casting solutions of sol–gel-based oxide nanoparticles and a BCP. The sol–gel
process generated sol particles of a few nanometers in diameter, and BCPs directed
these particles into a single phase-separated block. This process has led to a variety of
nanostructures, including lamellar, cylindrical, ABCD woodpile, perforated lamellar,
double gyroid, alternating gyroid, and plumber’s nightmare structures. Dissolution
or disassembly of self-assembled oxide hybrids from majority polymer structures
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
271
Mesostructure
A widely followed approach for utilizing self-assembled BCP nanostructures for
fabrication of functional nanomaterials is to generate ordered porous BCP templates
via selective removal of one polymer block. Certain polymers are selectively removable via application of acid, base, heat, or ozone [36]. The resulting porous templates
can subsequently be backfilled with functional materials, e.g., via metal oxide
solutions or metal deposition. With the Steiner group at Cambridge University in
the lead, the Wiesner group helped to fabricate gyroidal titania electrodes for solidstate dye-sensitized solar cells [37] and gyroidal gold nanomaterials for metamaterial
applications [38]. Gyroidal minority networks formed by poly(L-lactic acid) were
selectively removed by an acid treatment for the former case [37], whereas
ozonolysis of polyisoprene was used in the latter [38]. Backfilling of titania into the
porous gyroidal template followed by incineration of the template resulted in
nanostructured titania electrodes, and electrodeposition of gold on the gyroidal
template led to gyroidal gold nanomaterials. This block removal strategy involves
two main difficulties. Complete selective etching of thick polymer monoliths can be
challenging since an etchant must reach a long-distant template center. Thorough
percolation of sacrificial polymer blocks is rare for large bulk samples due to the
randomized orientation of crystal axes. Thus, alignment of the crystal axes is usually
needed for better etching [39]. The other main difficulty is complete backfilling of a
large porous monolith with functional materials. Nanosized pores can easily become
clogged during a deposition process if functional materials are deposited, e.g., via
chemical vapor or electroless deposition. Furthermore, the use of nanostructured
porous BCPs as templates for fabrication of functional nanomaterials requires several
steps, including BCP SA, selective etching, and backfilling.
This multistep procedure can be simplified by directly employing BCPs as
structure-directing agents for functional inorganic materials in a single SA step.
In this approach, additive inorganic materials preferentially localize in one block of
the phase-separated nanostructures during BCP SA, usually in the more hydrophilic
block, thereby minimizing the enthalpic penalty of mixing with more repulsive
blocks. This strategy leads to nanostructured polymer/inorganic hybrid materials in
a “one-pot” fashion, without multiple and often tedious post-processing steps. The
disadvantage is that, depending on the inorganic additives, long post-SA annealing
steps may not be possible thus rendering achievement of long-range order of the
nanostructures challenging.
The Wiesner group has synthesized a variety of nanostructured oxide materials via
casting solutions of sol–gel-based oxide nanoparticles and a BCP. The sol–gel
process generated sol particles of a few nanometers in diameter, and BCPs directed
these particles into a single phase-separated block. This process has led to a variety of
nanostructures, including lamellar, cylindrical, ABCD woodpile, perforated lamellar,
double gyroid, alternating gyroid, and plumber’s nightmare structures. Dissolution
or disassembly of self-assembled oxide hybrids from majority polymer structures
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
271