Does a hydration structure above gold imply a clean surface?

Solid-liquid interfaces are pivotal in a broad range of fields, including electrochemistry, catalysis and geochemistry. In electrochemistry, the molecular structure of the electrode-water interface is decisive for understanding and improving electrochemical reactions. In this context, the Au(111) surface constitutes a prototypical model system as gold is believed to be a relatively inert and well-characterized sample material. So far, however, little is known about the hydration structure at the Au(111)-water interface at the molecular level. Here, we present atomic force microscopy data on a gold sample carefully cleaned according to standard electrochemistry preparation recipes. Our three-dimensional atomic force microscopy data reveal a vertically ordered hydration structure with layer-to-layer distances of 0.36 nm as expected for water. Strikingly, despite our extensive cleaning efforts, our two-dimensional data (images) reveal stripe-like structures at the Au(111)-water interface. The dimensions of these structures differ from the well-known herringbone reconstruction of the gold surface, which is why we interpret them as surface contamination. Our work, hence, demonstrates that the presence of a hydration structure is no evidence for a clean gold surface.