Integrated Calendar

An extensive array of surface-sensitive characterization techniques that provide microscopic and spectroscopic information have revealed the structure of many crystal surfaces in their pristine clean state. Most of these studies are carried out in ultrahigh vacuum (UHV), which makes it possible to control sample composition and cleanliness to better than 0.1% of a monolayer. Starting from those by Irving Langmuir, such surface science studies constituted the core of our present understanding of solid surfaces. Under realistic ambient conditions, however, our knowledge is far less extensive. Of particular interest is the structure and chemical state of surfaces in dynamic equilibrium with gases and liquids at ambient conditions. Ambient pressure x-ray photoelectron spectroscopy (APXPS) and high pressure scanning tunneling microscopy (HPSTM) were developed for this purpose, i.e., understanding the atomic, electronic, and chemical structure of surfaces in the presence of reactant gases and liquids.
My talk will have three parts. The first part will be about ‘following surface structures’. I will show that the most compact and stable surfaces of Cu undergo massive reconstructions in the presence of CO at room temperature at pressures in the Torr range, and they decompose into two-dimensional nanoclusters, which is a double effect of low cohesive energy of Cu and the high gain in adsorption energy at the newly formed under-coordinated sites. Finally, it will be shown that the surfaces which are broken up into clusters are more active for water dissociation, a key step in the water gas shift reaction. Such a behavior opens a new paradigm, especially for other soft metals like gold, silver, zinc, etc., and it is clear that we need more of such studies. I will also briefly comment on the predictive power of my results and whether we can extrapolate it to other metals and reactants other than CO.
The second part will centered on ‘following surface reactions’ with APXPS. This technique is so powerful that it allows us to monitor the changes in the chemical state of the adsorbent, as well as coverage of adsorbates. As an example, I chose the CO oxidation reaction on Cu surfaces. It will be shown that if Cu remains metallic, the activation energy of the reactions scales with the oxygen binding energy, which is the manifestation of the Sabatier effect. In the presence of both CO and O2, however, the surface gets covered with 1-3 nm of Cu2O layer, which is more active than metallic Cu, but no CuO forms under the pressure and temperature range chosen in my study.

The final part will be about the possible future directions which I find important in the field for the next 5-10 years. Especially I will mention new approaches I plan to take for technique development which can offer measurements at higher pressures, as well as extend the outreach of the available techniques to other fields where solid-gas and solid-liquid interactions play an essential role.