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 Fe2+ to Fe3+. In this cell, Fe2+ was oxidized at the n-Si(111)/Au(111) photoanode and Fe3+ 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 Fe2+/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.