Unique templating of and applications enabled through mesoporous colloids

This work describes the templating process and applications enabled by utilizing mesoporous colloids, colloidal crystals, and films. Silica colloidal crystals, having colloids of less than 300 nm, were fabricated and used as a template for silicon deposition. Brillouin Light Scattering (BLS) experiments conducted on a silicon templated silica colloidal crystal show unusual behavior in the BLS spectrum where the Stokes and anti-Stokes phonon intensities were asymmetrical. The dispersion relationship between the phonon frequencies and wave vectors had no phononic band bending, possibility due to the unusual phenomenon. Mesoporous silica was used as a template for platinum and silicon deposition. After a successful colloidal crystal fabrication from mesoporous silica colloids, platinum was deposited and partially infiltrated the mesopores. Removing the silica template revealed a mesoporous platinum shell, replicating the mesopores of the colloids and the colloidal crystal structures. The silicon-silica composite was used to study the thermal conductivity of the confined silicon. Polycrystalline silicon, deposited as a reference, was shown to have a thermal conductivity of 38 W m-1K-1; the silicon-silica composite had an effective thermal conductivity between 2.2 and 2.4 W m-1K-1, an order of magnitude less than the polycrystalline film. Material thermal conductivity of the silicon could not be determined with simple modeling; accurate results need a more detailed investigation. The first successful carbon colloidal crystal is fabricated through tailoring the surface charge of the individual colloids through a partial oxidation. This colloidal crystal was used as a unique template for a variety of materials. The templated materials penetrate deep into the mesopores, creating a nanostructured inverse opal after carbon removal. Crucially, the carbon removal is completely orthogonal to the deposited material removal, allowing thorough template replication and preserving the fine-scale mesostructure of the colloid. This approach can be generalized for templating materials, which are inherently difficult to template with conventional colloidal crystals. Two major applications were investigated utilizing the mesoporous colloids. The first involved the templating of titania for improvement in dye-sensitized solar cells. A carbon colloidal crystal could successfully be fabricated on the conductive glass substrate after an initial titania deposition. Titania was deposited and infiltrated the mesoporous carbon colloidal crystal template. After titania crystallization, the carbon was removed orthogonally through a simple oxidation, followed by solar cell assembly. The performance of the cell indicated low dye loading in the titania nanostructure; however, experiments to increase the dye loading were unsuccessful. The second major application investigated also utilized the carbon colloidal crystal. Lithium battery anodes were fabricated by having the active materials, silicon and Fe3O4¬, inside the mesopores. The mesoporous carbon is unique in that it provides a porous, conductive network, allowing both electrolyte diffusion and electron extraction, and confines the active materials. This confinement minimizes capacity loss through electrical isolation of the active materials and suppresses any pulverization or migration. The silicon-carbon anode showed stability through 85 cycles after the addition of vinylene carbonate to the electrolyte, effectively stabilizing the solid-electrolyte interface. The Fe3O4-carbon anode showed excellent coulombic efficiency with a dramatic decrease in the charge hysteresis, from 0.90 V in conventional systems to 0.60 V in the templated, confined anode, attributed to the confinement of the Fe3O4 active material. Preliminary magnetic measurements were also conducted on the deposited iron-carbon composite, showing unique behavior