MSE Special Seminar - Dr. Jan Ingo Flege
In-situ investigation of cerium oxide growth and nanoscale reactivity on transition metal surfaces
Jan Ingo Flege
Host: Faisal Alamgir
Rare-earth oxides (REOs) are fascinating materials and exhibit a wide variability in terms of crystallographic and electronic structure as well as in their chemistry, making them integral components in existing and future applications in, e.g., catalysis, energy conversion and storage, sensing, and microelectronics. The physical origin of their inherent structural and electronic complexity is the unfilled shell of highly-localized 4f electrons in the valence band, giving rise to varying degrees of electron correlation and chemical reactivity. A primary example for a multivalent REO is cerium oxide, which offers facile switching between its two major oxidation states, i.e., Ce3+ (Ce2O3) and Ce4+ (CeO2) in reducing or oxidizing or conditions. Low-energy electron microscopy (LEEM) is a full field imaging technique and provides real-time information on the dynamics of surface processes with a lateral resolution of typically 10 nm. Here, we present an extensive study of the growth and geometric structure of ultrathin ceria films on late transition metal surfaces with low-energy electron microscopy and diffraction (LEEM/LEED) as well as soft x-ray photoemission electron microscopy, mainly focusing on cerium oxide on Ru(0001) [1,2]. Furthermore, we establish the approach of intensity-voltage (I(V))-LEEM for oxide phase identification  to access its local oxidation state on the nanometer scale, which is corroborated by resonant photoemission spectroscopy (RPES) and ab initio scattering theory . We demonstrate that, due to the distinct structure of the Ce-5d states, major differences for fully oxidized and reduced ceria, as confirmed by quantitative RPES, exist in the k||=0 projecte bandstructure, which determines the electron reflectivity, i.e., the I(V) energy dependence . Interestingly, this dependence also allows a distinction of the competing cubic (bixbyite) and hexagonal polymorphs for fully reduced ceria (Ce2O3). Its potential for local chemical characterization on the nanometer scale will be illustrated by LEEM experiments targeting methanol oxidation over nanoscale cerium oxide islands grown on Ru(0001), enabling imaging of local reduction upon methanol decomposition and subsequent thermal annealing . Further applications of the I(V)-LEEM technique shedding light on related inverse model catalyst systems  will be discussed.
Chem. C 117, 221 (2013).
2. J. I. Flege, B. Kaemena, S. D. Senanayake, J. Höcker, J. T. Sadowski, and J. Falta,
Ultramicroscopy 130, 87 (2013).
3. J. I. Flege, J. Hrbek, and P. Sutter, Phys. Rev. B 78, 165407 (2008).
4. J. I. Flege, A. Meyer, J. Falta, E. E. Krasovskii, Phys. Rev. B 84, 115441 (2011).
5. J. I. Flege, B. Kaemena, A. Meyer, J. Falta, S. D. Senanayake, J. T. Sadowski, R. D. Eithiraj,
and E. E. Krasovskii, Phys. Rev. B 88, 235428 (2008).
6. S. D. Senanayake, J. Sadowski, J. Evans, S. Kundu, S. Agnoli, F. Yang, D. Stacchiola, J. I.
Flege, J. Hrbek, J. A. Rodriguez, J. Phys. Chem. Lett. 3, 839 (2012).
Dr. Jan Ingo Flege received his PhD in Physics at the University of Hamburg,