Microstructure Effects on the Water Oxidation Activity of Co3O4/ Porous Silica Nanocomposites

We investigate the effect of microstructuring on the water oxidation (oxygen evolution) activity of two types of Co3O4/porous silica composites: Co3O4/porous SiO2 core/shell nanoparticles with varying shell thicknesses and surface areas, and Co3O4/mesoporous silica nanocomposites with various surface functionalities. Catalytic tests in the presence of Ru(bpy)3 2+ as a photosensitizer and S2O8 2- as a sacrificial electron acceptor show that porous silica shells of up to -20 nm in thickness lead to increased water oxidation activity. We attribute this effect to either (1) a combination of an effective increase in catalyst active area or consequent higher local concentration of Ru(bpy)3 2+; (2) a decrease in the permittivity of the medium surrounding the catalyst surface and a consequent increase in the rate of charge transfer; or both. Functionalized Co3O4/mesoporous silica nanocomposites show lower water oxidation activity compared with the parent nonfunctionalized catalyst, likely because of partial pore blocking of the silica support upon surface grafting. A more thorough understanding of the effects of microstructure and permittivity on water oxidation ability will enable the construction of next generation catalysts possessing optimal configuration and better efficiency for water splitting