Nanoscale Visualization of The Field-Assisted Fe Oxidation by Operando Atom Probe

Physics governing surface chemical reactions and interfaces involved in heterogeneous catalysis and corrosion fundamentally depends on the synergistic interactions between reactive gases and specific surface structures. A deep comprehension of those mechanisms at the nanoscale is a mandatory requirement to design high performance chemical systems and sustain the development of green chemical industry. Surface science techniques are continuously evolving to help bridge knowledge gaps between fundamental research and real-world applications, opening opportunities to observe ongoing chemical reactions. In the past decade, an increasing number of analytical techniques successfully achieved their evolution towards an in situ and operando version of themselves, and recently such approaches are being developed for atom probe microscopy (APM) techniques^1,2. In this work, we will present the recent advances in the conversion of Atom Probe Tomography (APT) to study surface dynamics of O₂/Fe using two different APM techniques and modifications: Field Ion Microscopy (FIM), and Operando Atom Probe (OAP).<br/>APM techniques are capable of imaging the surface of sharp needles’ apices with nanometric lateral resolution³. Those apices have similar shape and size than single nanoparticles, exposing a large variety of surface structures. Specifically, FIM is used to image such surface with atomic resolution and identify the crystal orientation. The resulting FIM image of a Fe specimen corresponds to a stereographical projection of the apex and allows the identification of the crystal orientations with atomic resolution (0.2 nm lateral). However, FIM does not provide any information regarding the chemical nature of the observable. OAP is based on the combination of FIM and time-of-flight mass spectroscopy (ToFMS), and on field evaporation. OAP applies a combination of static and pulsed electric fields to extract surface species from the surface and identify their chemical nature by ToFMS. OAP allows to observe in real-time the progressive oxidation of a Fe nanoparticle while a strong electric field (&gt;20 V/nm) is applied on the specimen.<br/>In this work, we use a (011) oriented Fe sample with a central Fe(011) facet surrounded by two pairs of Fe{244} and Fe{024} facets, and a set of four Fe{112} facets. OAP analysis is performed on Fe specimen while being exposed to reactive gas (~1.1×10^-7mbar of pure O₂ (99.993%)) at 300K. As soon as the oxygen is introduced to the chamber, we can measure the surface formation of Fe oxides by monitoring the local concentration of O atoms collected over the surface. Over an hour of exposure in the presence of a strong electric field (~23 V/nm) we observe local oxidation over open facets structures, such as Fe{244} and Fe{112}, while central Fe(011) and the Fe{024} show significantly higher resistance toward oxidation. OAP results allow us to reconstruct the full movie of the surface oxidation in real-time and show how intense electric fields (&gt;10V/nm) play a central role in surface chemistry.<br/><br/>References<br/>1.Lambeets S.V. et al., J. Phys Chem. Lett. 11 (2020) 3144-3451<br/>2.Lambeets, S.V., Kautz, E.J., Wirth, M.G. et al. Top. Catal. 63, 1606–1622 (2020)<br/>3.Gault. B. et al. Atom Probe Microscopy (Springer Ed.) New York Heidelberg Dordrecht London, 2012