AN APPROACH TO IDEAL SEMICONDUCTOR ELECTRODES FOR EFFICIENT PHOTOELECTROCHEMICAL REDUCTION OF CARBON-DIOXIDE BY MODIFICATION WITH SMALL METAL PARTICLES

Photoelectrochemical reduction of carbon dioxide (CO2) on p-type silic on (p-Si) electrodes modified with small metal (Cu, Ag, or Au) particl es has been studied. The electrodes in CO2-saturated aqueous electroly te under illumination produce methane, ethylene, carbon monoxide, etc. , similar to the metal (Cu, Ag, or Au) electrodes, but at ca. 0.5 V mo re positive potentials than the corresponding metal electrodes, contra ry to continuous-metal-coated p-Si electrodes. The results clearly sho w that the metal-particle-coated p-Si electrodes not only have high ca talytic activity far electrode reactions but also generate high photov oltages and thus work as an ideal type semiconductor electrode. It is discussed that the CO2 photoreduction proceeds with an upward shift of the surface band energies of p-Si in order to get energy level matchi ng between the semiconductor and solution reactants, though hydrogen p hotoevolution occurs without such an upward shift. It is also discusse d that the control of surface structure on a nanometer-sized level, as well as on an atomic stale, is important for getting higher efficienc ies.