Polarons in Fe2O3: Affecting H2O Diffusion and Desorption

Matthias Meier (TU Vienna)

Dec 10. 2019, 17:00 — 19:00

Matthias Meier a,b , Michele Reticcioli b , Florian Kraushofer a , Zdenek Jakub a , Jan Hulva a , Manuel Ulreich a , Roland Bliem a , Martin Setvin a , Michael Schmid a , Ulrike Diebold a , Cesare Franchini b , and Gareth S. Parkinson a a Institute of Applied Physics, TU Wien, b Center for Computational Materials Science, Faculty of Physics, University of Vienna Author Email: meier.matthias.001@gmail.com Hematite (Fe2O3) is a promising candidate for photo-electrochemical water splitting. For this purpose, the interaction of its surfaces with water is of interest. Previously, the so-called “R-cut” surface [1] has been investigated, utilizing a combination of UHV surface techniques and DFT calculations. It was followed by an H2O adsorption study [2], where partially dissociated water dimers were found to be the most stable configuration, fully covering the surface along Fe-rows, which then desorb as seen by temperature-programmed desorption (TPD), with peaks at 240 and 340 K. The water and hydroxyl molecules can easily dissociate and recombine without any rate-limiting barrier to be overcome or requiring larger diffusion processes. This allows every third water molecule to directly desorb from the surface, after-which the remaining molecules can simply reorganize into new partially dissociated water dimers, now separated by an empty adsorption site in-between neighboring dimers. These desorbing molecules result in the first desorption peak observed at 240 K. The desorption of the remaining dimers gives then rise to the second desorption peak at 340 K. Surprisingly, DFT calculations indicate very small changes in desorption energies between both the above mentioned coverages, therefore in disagreement with the 100 K shift in temperature measured experimentally. We propose that this change in temperature is related to polaronic effects, known in hematite [3,4], reducing the adsorption energies of every third water molecules in the initial fully covered surface, leading them to desorb first. The second peak at 340 K consists of the desorption of both molecules in each remaining dimer. After the first molecular water molecule desorbs, OH+H recombines and can diffuse along the Fe-rows before its desorption. If another such molecule from a neighboring dimer - also in the process of desorbing - is reachable, it can form a new partially dissociated water dimer. This process involving water diffusion would then be repeated, until all molecules are either desorbed or its diffusion hindered (e.g. by step edges, defects ...). This diffusion process along the Fe-rows is nevertheless impossible if the presence of polarons is taken into account and their repulsive effects on water. This forces the water diffusion into an alternative path, perpendicularly to the Fe-row, involving an O exchange between the molecule and the surface. Rather than the molecule diffusing as en entity on top of the surface, to overcome the large Fe-Fe distance between neighboring Fe-rows and reduce its cost, the H are detached, diffuse and form a new water molecule with an O surf atom. This exchange can be observed directly experimentally where an O 18 isotopic labeled surface leads to a change in mass of every second water molecule desorbing in the 340 K peak. Keywords: Polaron, Hematite, Fe2O3 , H2O, TPD, DFT, Omol - Osurf exchange References [1] F. Kraushofer, Z. Jakub, M. Bichler, J. Hulva, P. Drmota, M. Weinold, M. Schmid, M. Setvin, U. Diebold, P. Blaha, and G. S. Parkinson, J. Phys. Chem. C 122, 1657 (2018). [2] Z. Jakub, F. Kraushofer, M. Bichler, J. Balajka, J. Hulva, J. Pavelec, I. Sokolović, M. Müllner, M. Setvin, M. Schmid, U. Diebold, P. Blaha, and G. S. Parkinson, ACS Energy Lett. 4, 390 (2019). [3] J. E. Katz, X. Zhang, K. Attenkofer, K. W. Chapman, C. Frandsen, P. Zarzycki, K. M. Rosso, R. W. Falcone, G. A. Waychunas, and B. Gilbert, Science 337, 1200 (2012). [4] K. M. Rosso, D. M. A. Smith, and M. Dupuis, J. Chem. Phys. 118, 6455 (2003).

Further Information
ESI Boltzmann Lecture Hall
Associated Event:
Polarons in the 21st Century (Workshop)
Jozef Devreese (U Antwerpen)
Cesare Franchini (U of Vienna)
Georg Kresse (U of Vienna)
Jacques Tempere (U Antwerpen)