Photocathodic Hydrogen Evolution Reactions at p‐Type CaFe2 O 4 Electrodes with Fermi Level Pinning

The photocathodic properties of p‐type electrodes were studied in detail. The Fermi level pinning of this electrode was demonstrated from measurements of its photoelectric and photoelectrochemical properties. The large and stable cathodic photocurrent for the hydrogen evolution reaction was observed in acidic solutions. It was found that the band bending in the space charge layer increased with a decrease in p H of the solution. The large photocurrent in acidic solutions is based on the large band bending. The change of the band bending with p H is due to the reaction of the surface state with H + or OH − in the solution. The surface state causing the Fermi level pinning is the redox level of Fe 3+ /Fe 2+ or Fe 4+ /Fe 3+ in the oxide surface lattice. The change of the redox level with p H is discussed in detail. A maximum in the photocurrent was observed at an electrode coated with Pt‐Pd alloy. This is based on the electrocatalytic effect of this metal. The increase in the photocurrent of electrodes coated with metals thicker than about 50Å were also observed under the same intensity of compensated light.