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42751-AC10
Multifunctional Porous Nanocomposites
3DOM/m carbon monoliths as electrodes and supports: Monolithic pieces of carbon with interconnected macropores (a few hundred nm diameters) are of interest as electrodes for power sources and sensors, as well as supports for sorption and catalysis. Addition of mesopores (a few nm diameters) within the walls significantly enhances surface areas (e.g., for supercapacitor applications) and provides spaces for selective uptake of guests or for incorporation of additional phases to produce a nanocomposite structure. We previously prepared three-dimensionally ordered macroporous/mesoporous (3DOM/m) carbon by first forming a 3DOM/m silica structure as a mold, filling its pores with polymer precursor, carbonizing the polymer and etching away the silica mold. During this reporting period, we made significant improvements in our ability to structure 3DOM/m carbon and carbon-silica composites with hierarchical porosity using two more direct and simpler methods. One approach employs a tri-constituent precursor (a source of polymer, a source of silica and a surfactant to template mesopores) which is infiltrated into a colloidal crystal and processed to form porous carbon. This method eliminates formation of a separate silica mold but still requires removal of silica at the end. An even more efficient method excludes silica completely so that a final silica extraction is avoided. All of these hierarchically structured carbon samples were characterized in detail by diffraction, microscopy, nitrogen sorption, chemical analysis and electrochemical characterization. Through further modification with carbon, tin oxide, or silicon/silica phases within the mesopores, we were able to tune properties such as surface areas and pore structure over wide ranges. Addition of tin oxide to the mesopores in 3DOM/m carbon increased the specific capacity for lithiation of this sample. We also used 3DOM carbon electrodes as the intermediate layer between an ionophore-doped solvent polymeric membrane and a metal contact as a novel approach to solid-contact ion-selective electrodes (SC-ISEs). Due to the well-interconnected pore and wall structure of 3DOM carbon, filling of the 3DOM pores with an electrolyte solution resulted in a nanostructured material that exhibited high ionic and electric conductivity. The long-term drift of freshly prepared SC-ISEs with 3DOM carbon contacts was only 11.7 μV/h, making this the most stable SC-ISE reported so far. The electrodes show good resistance to the interference from oxygen. Moreover, in contrast to previously reported SC-ISEs with conducting polymers as the intermediate layer, 3DOM carbon is an electron conductor rather than a semiconductor, eliminating any light interference.
Bifunctional catalyst designed for direct conversion of synthesis gas to liquid fuels: 3DOM tungstated zirconia was synthesized by colloidal crystal templating using zirconium and tungsten precursors, together with promoters based on iron(III) nitrate or hydrogen hexachloroplatinate. The product was coated with multiple polyelectrolyte layers before zeolite NaY nanocrystals were grown hydrothermally on the zirconia walls. At this stage, pore surfaces were coated with zeolite particles ca. 70 nm in average diameter. Ruthenium was introduced by ion exchange and reduction reactions. Products were characterized by XRD, TEM, SEM and XPS analyses. Catalytic properties of the materials will be evaluated during the no-cost extension period.
Shaped porous and non-porous nanoparticles: As we optimized syntheses of materials with hierarchical porosity, we discovered a novel way of preparing monodisperse, discrete nanoparticles (NPs) of a variety of compositions (silica, carbon, polymers, metal oxides) with specific shapes (cubes, tetrapods, spheres). These particles can be mesoporous, thereby enhancing surface areas of the particles and opening up possibilities for host-guest applications. The method is based on a combination of self-assembly and templating methods in tandem with controlled disassembly of macroporous solids into their structural building blocks. Polymeric colloidal crystal templates with a face-centered cubic structure are infiltrated with a solution containing a non-ionic surfactant, an inorganic or polymer precursor and an acid. This mesophasic precursor mixture adopts the shape of the interstitial space between polymer spheres after gelation. Within certain, reproducible experimental parameters, disassembly of the structure occurs during calcination and a bimodal dispersion of NPs corresponding to replicas of the octahedral and tetrahedral voids in the colloidal crystal is obtained. We have identified critical parameters that lead to disassembly, including the inorganic precursor, aging time and calcination time. The sizes of the mesoporous particles correlate linearly with the diameters of the polymer spheres. Partial fractionation of the larger particles is possible through density gradient centrifugation. The shaped porous nanoparticles can be replicated by established nanocasting processes. This facile hard-templating strategy provides vigorous control over shape and size of the mesoporous silica NPs, and minimizes the agglomeration effect common in traditional hydrothermal syntheses. The porous nano-objects are particularly interesting, for example, as capsules for enzymes, a means of drug delivery. They also provide a starting point for more complex nanostructures (e.g., 3D arrays) with potential applications in power storage, detection and pharmaceutics.
