Hydrogen-Bonded Networks in Surface-Bound Methanol

Fundamentally, methanol represents one of the simplest molecules for understanding extended hydrogen-bonded networks, and from a practical perspective it constitutes a very important chemical feedstock. The interaction of methanol with transition metal surfaces is of relevance to many catalytic processes. Gold-based catalysts show high selectivity for the partial oxidation of several species including, most recently, methanol. However, due to its weak interaction, it has not yet been possible to study intact methanol adsorption and ordering on any metal surface with molecular resolution. Using careful annealing treatments and low tunneling current, variable-temperature scanning tunneling microscopy, we show the basic bonding geometries of a range of hydrogen-bonded methanol structures as a function of surface coverage and temperature on Au(111). Like ice, methanol forms hexamer units; however, at all but very low coverages, chain structures dominate. Unlike the 2D honeycomb structure of ice, which is interconnected by hydrogen bonds on wettable metal surfaces, methanol chains interact weakly and actually repel one another until forced into close contact at high coverages. We report the three basic geometrical units of methanol that form the basis of all the observed structures. Annealing experiments reveal the reversibility of the assembly and demonstrate that the reported structures are thermodynamically formed. To demonstrate the generality of these results, we also present similar data for methanol on Cu(111), which has a very different lattice constant than Au.