Atomic-Scale Observation of NiO Nucleation during Nickel Oxidation

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The interaction between oxygen and metallic surfaces plays a critical role in various technological applications, including corrosion resistance, heterogeneous catalysis, and oxidation control. Despite of extensive research on the oxidation of metals, a significant knowledge gap persists regarding the atomistic mechanisms governing the initial stages of oxide nucleation. Traditional surface science tools, such as scanning tunneling microscopy and low-energy electron diffraction, have provided valuable insights into oxygen-adsorption-induced structure evolution of the topmost atomic layers of metal surfaces [1-3]. However, these techniques are limited in their inability to probe deeper atomic layers beneath the surface, where crucial processes such as oxygen dissolution and the early formation of oxide nuclei may occur. As a result, the atomic-sale dynamics of how oxygen penetrates the metal lattice and interacts with subsurface regions remain poorly understood. This gap in knowledge is critical, as the oxide nucleation period plays an important role in determining the long-term oxidation behavior and the properties of the oxide. To fully understand the oxidation process, advanced in-situ techniques are needed to probe the atomic dynamics beneath the topmost layers, where oxide nucleation and early-stage transformations are likely to occur. Environmental transmission electron microscopy (E-TEM) presents a unique platform to investigate metal oxidation processes by introducing O2 into the sample region while simultaneously probing the dynamic evolution of atomic structures across both the surface and subsurface region of the sample. Through E-TEM observations, we demonstrate the existence of a nucleation latency period, during which oxygen saturates multiple atomic layers of the Ni lattice—from the topmost surface to the subsurface— before triggering an abrupt transformation of these metallic layers into NiO. This transformation marks the onset of NiO nucleation, providing new insights into the early-stage oxidation process. Our E-TEM experiments were performed using an FEI Titan 80-300 microscope, equipped with an objective-lens spherical aberration corrector and differential pumping apertures, allowing for temperature-, time-, and pressure-resolved imaging of oxidation at the atomic scale. The in-situ oxidation experiments were carried out in two steps. First, a thin Ni foil, cut from a Ni single crystal using a focused ion beam (FIB) lift-out approach, was irradiated with a condensed electron beam within the TEM column, resulting in the tearing of the Ni film and exposing oxide-free surfaces. Next, the pristine Ni surfaces were exposed to 8.5×10-5 Torr O2 at room temperature, allowing us to capture the early stages of oxide formation in the cross-section view. Fig. 1(a) displays a clean Ni surface with a stepped surface morphology. At t₀ + 4.5 s after O2 exposure (Fig. 1(b)), slight displacements of atom columns were observed in the topmost layers (as marked by the yellow dashed lines), suggesting O adsorption at the Ni surface. However, the atomic layers extending from the topmost to the subsurface region retain the metallic Ni lattice, as shown by the simulated HRTEM image (inset in Fig. 1(b)), which is based on a DFT-relaxed structure model with 0.75 monolayer of adsorbed O on the Ni(111) surface. It is worth noting that introducing O2 into the TEM column caused sample drift, which was corrected by setting t₀ as the time after drift stabilization, ensuring clear imaging. In Fig. 1(c), within just 0.9 seconds, the top three atomic layers of the Ni lattice in both the right and left regions of the sample rapidly transformed into NiO, with the interplanar spacing of the oxidized layers measuring at 0.24 nm, matching the (111) plane of NiO. The resulting NiO(111) planes exhibit a slight tilt relative to the Ni(111) planes. To confirm these observations, similar experiments were performed on different sample regions. Fig. 1(d) shows the clean Ni surface before O2 exposure. At t₀ + 21.9 s, slight atomic displacements were observed in the outermost surface region (Fig. 1(e)). As seen in Fig. 1(f), within just 0.9 seconds, the top three to five atomic layers of Ni transformed into NiO. The Ni/NiO interface is highlighted by the red dashed line, with the resulting NiO lattice planes slightly tilted relative to the Ni lattice planes. The in-situ HRTEM imaging reveals a two-stage oxidation mechanism: (i) O atoms first adsorb onto the Ni surface, inducing localized displacements of surface Ni atoms (Figs. 1(b, e)), followed by subsurface diffusion into the FCC Ni lattice. (ii) Upon reaching a critical O concentration in the subsurface, the O-saturated Ni layers undergo an abrupt phase transformation into rock-salt NiO (Figs. 1(c, f)). To validate this pathway, DFT modeling in Fig. 2(a) demonstrates that incorporating a critical amount of O into octahedral interstitial sites of the top four Ni layers destabilizes the FCC lattice, spontaneously forming a NiO-like structure. This transformation is further confirmed by simulated HRTEM image (Fig. 2(c)) of the DFT-relaxed structure, showing a ∼ 8.9° tilt of NiO(111) planes relative to the (111) planes of the Ni substrate—a direct consequence of lattice mismatch accommodation at the coherent NiO/Ni interface. This tilt angle matches quantitatively with experimental HRTEM observations (Fig. 2(c)), confirming that oxygen dissolution drives coherent oxide nucleation [4]. In-situ HRTEM imaging of atomic-scale oxidation dynamics on Ni(111) surfaces, viewed along the [110] zone axis, under exposure to 8.5×10-5 Torr O2 at room temperature. (a-c) Time-resolved HRTEM images capturing the abrupt oxidation of a step-terrace region, where the topmost 3-4 atomic layers of metallic Ni are simultaneously transformed into NiO (rock-salt structure) after an incubation period of the O2 exposure. (d-f) Sequential HRTEM images from a distinct sample region, confirming the simultaneous transformation of multiple atomic layers of the Ni lattice, from the topmost layer to the subsurface region, following the incubation period of O2 exposure. Dashed yellow lines indicate slight displacements of atom columns of the topmost surface layer, caused by oxygen adsorption. Dashed red lines mark the NiO/Ni interface after the abrupt NiO formation. Atomic-scale mechanism of NiO nucleation via O dissolution in FCC Ni lattice revealed by DFT modeling and HRTEM simulations. (a) DFT-optimized structure illustrating spontaneous transformation of the Ni lattice into rock-salt NiO after O occupies octahedral sites in the top four atomic layers (critical O concentration: ∼ 22.5 at.%). (b) Simulated HRTEM image of the DFT-relaxed structure in (a), predicting a ∼ 8.9° tilt of NiO(111) planes (blue line) relative to Ni(111) (yellow line). (c) Experimental HRTEM image of the NiO/Ni interface region (yellow box in in Fig. 1(f)), showing the same tilt angle (∼ 8.9°). This agreement confirms that O dissolution into the Ni lattice drives coherent NiO nucleation. Scale bar: 0.5nm.

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