Oxidation of gold by ultraviolet light and ozone at 25 °C

Gold surfaces have been found to be hydrophilic only after exhaustive preparation and with the ultimate care in sample preparation and treatment. The use of a combination of ultraviolet ~UV! light and ozone has been described as a viable method of producing a clean, hydrophilic, gold surface. We have found that gold surfaces, which have been either stored in the laboratory after vacuum deposition or purchased as high purity standards, are oxidized by a combination of UV light and ozone generated from a mercury lamp. The samples were characterized with x-ray photoelectron spectroscopy ~XPS! and ion scattering spectroscopy ~ISS! prior to and after exposure to UV/ozone in a stainless steel box in laboratory air. After the cleaning process gold surfaces were found by XPS to contain less carbon and to be enriched in oxygen. The O 1s on the cleaned surface, which was not present on the untreated surface, consisted of two peaks that are attributed to gold oxide and hydroxyl. The oxide layer was found to be 1764 Å thick by variable angle XPS depth profiling with an initial stoichiometry of Au2O3. The oxide was found to be stable to extended exposure to UHV and water and ethanol rinses. ISS compositional depth profiles confirmed the oxide layer thickness and that the hydrated surface layer is removed in the initial sputtering of the oxidized gold. Implications of these results related to the mechanism of self-assembly of thiols on gold are discussed.