Photoelectrochemistry of Ultrathin, Semitransparent, and Catalytic Gold Films Electrodeposited Epitaxially onto N-Silicon (111)

An ultrathin, epitaxial Au layer was electrochemically deposited on n-Si(111) to form a Schottky junction that was used as the photoanode in a regenerative photoelectrochemical cell. Au serves as a semitransparent contact that both stabilizes n-Si against photopassivation and catalyzes the oxidation of Fe ²⁺ to Fe ³⁺ . In this cell, Fe ²⁺ was oxidized at the n-Si(111)/Au(111) photoanode and Fe ³⁺ was reduced at the Au cathode, leading to the conversion of solar energy into electrical energy with no net chemical reaction. The photocurrent was limited to 11.9 mA·cm ^-2 because of the absorption of light by the Fe ²⁺ / ³⁺ redox couple. When a transparent solution of sulfite ion was oxidized at the photoanode, photocurrent densities as high as 28.5 mA·cm ^-2 were observed with AM 1.5 light of 100 mW·cm ^-2 intensity. One goal of the work was to determine the effect of the Au layer on the interfacial energetics as a function of the Au coverage. There was a decrease in the barrier height from 0.81 to 0.73 eV as the gold coverage was increased from island growth with 10% coverage to a dense Au film with a thickness of 11 nm. In all cases, the band-bending in n-Si was induced by the n-Si/Au Schottky junction instead of the energetic mismatch between the Fermi level of n-Si and the redox couple. The dense Au film gave the greatest stability. Although the photocurrent of the n-Si/Au photoanode with 10.2% island coverage dropped nearly to zero within 2 h, the photocurrent of the photoanode with a dense 11 nm thick Au film only decreased to 92% of its initial value after irradiation at open circuit with AM 1.5 light for 16 h. A 2.1 nm thick layer of SiO _x formed between the Au film and n-Si. With further irradiation, the fill factor decreased because of the increase of series resistance as the SiO _x layer thickness exceeded tunneling dimensions.