Nano and microstructured materials with chemical functions: Anisotropic particles for catalysis and photo-catalysis

For chemical reactions occurring on the surfaces, e.g., catalysis and photocatalysis, design and fabrication of structures in two different scales, i.e., angstrom/nanometer and micrometer ranges. The former regulates the adsorption and surface reaction of molecules, while the latter gives influence on the diffusion and mechanical strength of the material. In order to control the structures in both scale regions, use of hierarchical and/or anisotropic structure is beneficial. In this lecture, the author presents several examples of particles with hierarchical and/or anisotropic structures. First example is hollow core-shell (HCS) particles with a cadmium sulfide (CdS) nanoparticles core and nanoporous silica shell which drive efficient photocatalytic reaction without any coagulation of CdS nanoparticles. Similar HCS structured particles with a titania core and a silica shell were found to oxidize small organic compounds selectively under photoirradiation in the presence of oxygen. Third example is phase-boundary (PB) catalytic particles; zeolite or silica particles with hydrophilic and hydrophobic surface domains were prepared by newly developed methodology. The PB particles are spontaneously assembled at interface between aqueous and organic phases and thereby catalyze reaction of water-immiscible organic substrates with reagents dissolved in water, i.e., epoxidation of olefins with hydrogen peroxide without agitation or use of co-solvent. Flake-ball particles, assembly of nanometersized platelets, of bismuth tungstate (Bi2WO6), showed high-level of photocatalytic activities for both liquid and gas phase reactions under visible-light irradiation, presumably due to their high-crystallinity of platelets and relatively large surface area.