Speaker
Description
Surface Action Spectroscopy (SAS) is a method derived from the messenger action spectroscopy typically applied to aggregates in the gas phase to elucidate their structure. To this end, weakly bound atoms, or molecules, the ‘messengers’, are attached to the aggregates as indicators of a vibrational excitation. They may desorb when a vibrational mode is excited with infrared light, which usually stems from a free-electron laser (FEL). The approach permits to record vibrational spectra, which are a fingerprint of the surface structure - comparison with simulated vibrational spectra may reveal it. The same approach can be applied to thin film or single crystal surface structures. We have performed studies of the surface structure of Co$_3$O$_4$(100) films and the stabilization of superoxide by surface defects, supported by theoretical calculations from the Pentcheva group in Duisburg [1]. In this context, SAS spectra for different degrees of surface reduction, and of superoxide stabilized by oxygen vacancies were recorded and in part modelled theoretically.
A further activity was the structural characterization of Ni and Au single atoms on Fe$_3$O$_4$(100) [2]. The central questions in this study were whether SAS would be able to determine the location of the adatoms on an atomic scale, and whether a simplified structural determination procedure, where just the modification of vibrations with large amplitudes by adatoms is considered, would be able to safely determine the adatom bonding sites. Spectra of the adatom-covered surface could be successfully modelled by theoretical computations (Sergey Levchenko/Zhongkang Han), which permitted to reveal the adatom adsorption site. However, the simplified structure determination approach turned out to work only with a limited level of reliability. A relevant result of this study was the insight that the theoretically simulated vibrational spectra depend very critically on many parameters, including the starting point for the DFT structural optimization, which demands great care and a critical review when such spectra are computed to avoid failure of the structure determination.
A technical step forward was the implementation of He as a messenger gas, which requires sample temperatures below 2K. This was achieved by an improved shielding of the transferable sample holder against thermal infrared radiation and precooling of the wires to the sample holder as well as a thorough improvement of the thermal heat transfer between the sample and the cryostat. Still, a careful setup of the sample holder is required, but standard commercial transferable sample holder plates suffice.
Presently, the structure of a Ni-N-C model catalyst based on an epitaxial graphite film is investigated, aiming to gain insight into structure-reactivity relationships for the carbon dioxide electrocatalytic reduction. Continuation of the successful single atom catalyst structure studies will be another direction for future studies.
References
[1] Liu, Y, Y. Peng, M. Naschitzki, S. Gewinner, W. Schöllkopf, H. Kuhlenbeck, R. Pentcheva, B. Roldan Cuenya: Surface oxygen Vacancies on Reduced Co$_3$O$_4$(100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation. Angewandte Chem. Intl. Ed., 60 (30), 16514-16520 (2021).
[2] Liu, Y., Z. Han, S. Gewinner, W. Schöllkopf, S.V. Levchenko, H. Kuhlenbeck, B. Roldan Cuenya: Adatom bonding sites in a Ni-Fe$_3$O$_4$(001) single atom model catalyst and O$_2$ reactivity unveiled by surface action spectroscopy with infrared free-electron laser light. Angew. Chem. Intl. Ed. 61 (28), e202202561 (2022).
| Abstract Number (department-wise) | ISC 14 |
|---|---|
| Department | ISC (Roldán) |
