(Invited) Nucleation and Growth on Pt(111) during Oxidation-Reduction-Cycles

Abstract

In order to realize the energy transition, and thus not to leave our children with a future problem, we need to switch to renewables and the introduction of H2-economy. Platinum is of crucial importance for this, as it is widely used as an electrocatalyst in several applications like fuel cells and electrolyzers, due to its high catalytic activity. However, platinum is scarce, expensive, and its economical feasibility is limited by its degradation under oxidizing conditions. Repeated oxidation and reduction of a platinum electrode, which can be compared to the switching ON and OFF of a device, leads to the roughening of the surface, caused by the nucleation and growth of nano-islands, which first grow only laterally (2D) and then exclusively in height (3D) [1,2]. Surprisingly, this particular nucleation and growth combines several remarkable aspects, discussed in this talk, that all deviate from standard text book- and literature-knowledge. It starts with the fact that the growth of these islands is promoted by the creation of adatom-vacancy pairs [3] such that not only growth processes but also etching processes have to be considered. As a precursor of these adatom-vacancy pairs, an adatom gas of so-called “place exchanged” (PE) atoms is created that is fully reversible and does not lead to any roughening, as long as one stays below a critical coverage and thus oxidizing potential [4-7]. With each added PE-atoms the surface stress is increased, until the critical value is reached, atoms are kicked out of the surface, and adatom-vacancy pairs are created [8]. A long ongoing debate about a possible ordering of these PE atoms into spokes (lines) [5-8] or even the famous “spoke wheels” [9] has just been settled with the conclusion that they do not exist [10]. Surprisingly, our most recent analysis of the island-island distance distribution function clearly shows a strong repulsive interaction during the nucleation stage, which can only be explained with the existence of the spokes [11]. Next to this mysterious nucleation, it is striking that the islands first grow only in 2D and upon touching at the base only grow further in height (3D) without any coalescence or Ostwald ripening that is expected to take place, as it would lower the free energy of the system. We unraveled the underlying atomic processes and pinpoint the reason to a growth instability for vacancies that stands in full analogy to the famous mound formation instability for adatoms based on the Ehrlich-Schwoebel barrier [3]. An Ehrlich-Schwoebel barrier for vacancies must exist, and we formulated a complete set of equivalent growth modes for vacancies (etching) next to the well-known homoepitaxial adatom growth modes [3]. Finally, the analytical description of the combined adatom and vacancy growth modes nicely fits our experimental observations, thereby verifying both their existence and our model. [1] Jacobse, L., Huang, Y., Koper, M.T.M., and Rost, M.J., Nat. Mater. 17, 277 (2018)[2] Jacobse, L., Rost, M.J., and Koper, M.T.M., ACS Cent. Sci. 5 (12), 1920 (2019) [3] Rost, M.J., Jacobse, L. and Koper, M.T.M., Nat. Commun. 10, 5233 (2019)[4] H. You, et al. , R. P., Proc. 6th Int. Symp. Electrode Processes (1996)[5] J. Drnec, et al., Electrochimica Acta, 224 220 (2017).[6] M. Ruge, et al., J. of The Electrochem. Soc., 164,9 H608 (2017).[7] M. Ruge, et al., J. of the Amer. Chemi. Soc. 139,12 4532 (2017).[8] Hanselman, S., McCrum, I., Rost, M.J. and Koper, M.T.M., Phys. Chem. Chem. Phys. 22, 10634 (2019).[9] M.A. van Spronsen, et al., Nature Commun. 8 429 (2017), https://doi.org/10.1038/s41467-017-00643-z [10] L. Jacobse, V. Vonk, I.T McCrum, C. Seitz, M.T.M. Koper, M.J. Rost, and A Stierle, Electrochimica Acta, submitted [11] M.J. Rost, L. Jacobse, M.T.M. Koper, Angew. Chem to be submitted