Initial atomic-scale oxidation pathways on a Ni\u201315Cr(100) alloy surface

William H. Blades,Petra Reinke,Matthew R. Barone

doi:10.1038/s41529-021-00164-7

William H. Blades, Petra Reinke + Show 1 more

Open Access

https://doi.org/10.1038/s41529-021-00164-7

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Abstract

To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni–15 wt%Cr(100) alloy surface using scanning tunneling microscopy/spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide–alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni–Cr(100) alloys within the pre-Cabrera–Mott regime.

Highlights

Early-stage oxidation of metals commences with the nucleation and growth of partial oxide layers or islands on the surface and results in significant structural and chemical changes at the metal–oxide interface[1,2,3,4,5,6]
It is evident from these images that the Ni–Cr(100) oxidation pathway is complex, and will be discussed in a step-by-step manner in this report starting with a closer look at the oxide clusters observed in the very first oxidation step
A general oxidation model is presented in Fig. 7, where the oxidation pathways of the Ni and Cr are partitioned and described over three stages

Summary

INTRODUCTION

Early-stage oxidation of metals commences with the nucleation and growth of partial oxide layers or islands on the surface and results in significant structural and chemical changes at the metal–oxide interface[1,2,3,4,5,6]. Experiments that target the initial progression of oxide growth, combined with surface-sensitive techniques, are necessary if the complex nature of early-stage oxidation is to be understood To this end, STM and STS have been employed to capture the atomicscale mechanisms of surface oxidation on a Ni–15 wt%Cr(100) sample at 500 °C and will be discussed in detail. Resolved DOS (density of states) maps reveal the origin of this behavior is attributed to the heterogeneity across the NiO superlattice, imprinted by the alloy–oxide interface These data are used to build a model for the nanoscale growth of the partial oxide layers Ni–Cr(100) surfaces. The usual assumption of a homogenous electric field between the alloy and oxide surface is

RESULTS

DISCUSSION

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