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

Abstract

We investigate the effect of microstructuring on the water oxidation (oxygen evolution) activity of two types of Co 3 O 4 /porous silica composites: Co 3 O 4 /porous SiO 2 core/shell nanoparticles with varying shell thicknesses and surface areas, and Co 3 O 4 /mesoporous silica nanocomposites with various surface functionalities. Catalytic tests in the presence of Ru(bpy )3 2+ as a photosensitizer and S 2 O 8 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 Co 3 O 4 /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. KeywordsCo3O4/SiO2 core/shells, microstructure effects, nanocatalysts, nanocomposites, water oxidation Disciplines Chemistry CommentsReprinted ( 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 Co 3 O 4 / 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. KEYWORDS: Co 3 O 4 /SiO 2 core/shells, nanocomposites, nanocatalysts, water oxidation, microstructure effects ■ INTRODUCTIONElectrochemical and photochemical water splitting are ways to produce molecular hydrogen gas, H 2 , a potentially valuable and clean-burning fuel. Water oxidation is the most difficult halfreaction in water splitting, involving the transfer of four electrons and the formation of oxygen−oxygen bonds. 1−4 After many studies devoted to developing more efficient and economic water oxidation catalysts, 5 cobalt-based materials have been identified as some of the most promising due to their relative abundance, high activity, and...

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