Ordered Macroporous Materials by Colloidal Assembly: A Possible Route to Photonic Bandgap Materials
G. Subramanian, Vinothan N. Manoharan, James D. Thorne, and David J. Pine Advanced Materials 11, 1261-1264 (1999).
Soft Matter Keywords
This paper describes a fabrication technique for large scale macroporous oxide structures using polystyrene microparticles and colloidal dispersions of the oxides, using a Sol-gel like process. Macroporous oxide structures have unique optical properties and can be used to make photonic bandgap crystals. Previous fabrication strategies for such structures involved the use of alcoxide precursors, however as the alcoxide causes dramatic shrinkage of the polystyrene, these have been limited to structures of less than 0.5 mm in size. Thus, this paper offers a significant contribution to the fabrication of useful photonic bandgap crystals.
Monodisperse polystyrene spheres are used as a template to define the structure of the pores. The authors mix in ultrafine (<100 nm) colloidal particles of either silica or titania in water to the polystyrene. The water in the mixture is then slowly evaporated, and the polystyrene particles organize themselves into an ordered lattice due to gradual increases in their concentrations. As the polystyrene particles and oxide dispersion are both negatively charged, this leads to a stable suspension. The oxide particles pack tightly into the voids between the larger polystyrene particles, somewhat like a gel. The polystyrene particles are then removed by calcination, and a macroporous structure of the oxide is left to remain.
Scanning electron microscope images of the macroporous structures are shown in Figure 1. It can be seen that the pores are highly ordered in a 3D lattice over a large distance.
Conclusions and Soft Matter Discussion
The use of self-assembled templates is a versatile and inexpensive way of producing ordered macroporous ceramics of many different materials. As well as applications in photonics, such structures may have application as catalytic surfaces and supports, adsorbents, chromatographic materials, filters, light-weight structural materials, and thermal, acoustic and electrical insulators .
Packing of spheres is an interesting area of research in soft matter . The particles in this paper pack in either a face-centered cubic, or hexagonal lattice. It is known that colloidal crystals pack in an fcc structures through sedimentation, however the authors note other crystal structures may be realized by changing the distribution of colloid particle sizes.
Understanding how colloidal particles pack together is extremely useful for development of technologies using colloids, as it may one day allow the engineering of colloidal structures in specific shapes. Colloidal materials have unique properties, such as shear thickening, which is ideal for damping and shock adsorption. The US army is already investigating the use of colloidal materials in new types of body armor. When Kevlar is made from colloidal silica, and colloidal PMMA it has a strength of 2.5 and 1.5 times respectively the strength of regular Kevlar which is made from polymer chains.
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