(647c) Controlling Interfacial Curvature of Self-Assembled Mesoporous Thin Films by Controlling Cluster Size

Templated
ordered mesoporous metal oxides, since their discovery in the early 1990's¹,
 have attracted a great deal of attention due to their potential
applications in catalysis, adsorption, size-selective separation and
potentially in photovoltaic devices, sensors and thermoelectrics, where thin
films of these materials with open pore systems are desired. However,
synthesizing thin films with open and accessible pore systems has remained a
challenge.

Here, we report
a synthetic strategy wherein low-curvature phases with excellent accessibility
to the pore system (such as the double gyroid phase^{2, 3} with space
group Ia-3d) are synthesized by controlling the cluster size of the
silica clusters in the precursor solution⁴. With commercially
available non-ionic templates, a gradual evolution from high-curvature body
centered cubic to 2D hexagonal to gyroid is seen with increasing silica cluster
size. This trend is opposite to what is expected according to the charge
density matching arguments presented in literature⁵ for commonly
used ionic templates such as cetyl trimethyl ammonium bromide (CTAB) and is
investigated using Si²⁹ NMR and liquid phase small angle x-ray
scattering (SAXS). As the precursor solutions are aged, the cluster size
increases and ring and cage silica structures dominate in the solution over
linear silica oligomers. Large oligomers with radii of gyration up to 12Å are
seen in solution. Such large clusters do not swell the hydrophilic block as
effectively as smaller oligomers, thereby reducing the interfacial curvature.
The large oligomers are indispensable for making the gyroid phase, no gyroid
phase was seen right after mixing (when such clusters are absent) no matter
what the synthesis conditions were. Further, the gyroid films have very open
pore systems and allow electrodeposition of metals such as platinum, cobalt,
copper and semiconductors such as copper indium diselenide (CuInSe₂)
and cadmium telluride (CdTe) paving the way for making mesostructured thin
films of a variety of materials on a length scale wherein effects due to quantum
confinement can be seen.

References:

1.  
Beck, J. S. et al. Journal of the American Chemical Society 114, 10834-10843
(1992).

2.  
Hajduk, D. A. et al. Macromolecules 27, 4063-4075 (1994).

3.  
Schoen, A. H. NASA Technical Note #D5541 (1970).

4.  
Urade, V. N., Wei, T. C., Tate, M. P. & Hillhouse, H. W. Submitted
(2006).

5.  
Monnier, A. et al. Science 261, 1299-1303 (1993).