Co oxide (Co3O4) nanotubes are shown to act as efficient water oxidation catalyst when driven with a visible light sensitizer (pH 7). The nanotubes form the core of a Co3O4-SiO2 core-shell nanotube design for separating the carbon dioxide photoreduction from the oxygen evolution reaction. Amorphous dense phase silica of a few nanometer depth is shown to conduct protons while blocking molecular oxygen. Organic molecular wires embedded in the silica shell provide controlled charge transport between the light absorber on one side and the Co3O4 catalyst on the other side. Hence, the silica shell is suitable as a membrane of an assembly for closing the photosynthetic cycle on the nanometer scale under product separation.