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. 2026 Jan 5;11(2):3535–3543. doi: 10.1021/acsomega.5c11285

In Situ Surface-State Transformation on a Nickel Single Crystal for the Oxidation Process

Yaping Li †,‡,*, Xiaojun Li †, Hui-Qiong Wang ‡,§,*, Kefan Hu †, Sujun Guan †, Xinwei Zhao †, Kurash Ibrahim ∥,*, Jia-Ou Wang ∥, Rui Wu ∥, Hai-Jie Qian ∥, Jin-Cheng Zheng ‡,§, Junyong Kang ‡
PMCID: PMC12824708  PMID: 41585729

Abstract

Layer-by-layer oxidation of a Ni(111) single crystal was performed to investigate the interface electronic states at the buried NiO/Ni interface. The transformation of nickel from its metallic to oxidized states was studied using in situ X-ray photoelectron spectroscopy and synchrotron radiation X-ray absorption spectroscopy after oxidation at high temperatures in an oxygen atmosphere. Multiple electronic surface states were detected during the initial oxidation stage when the NiO thickness was below one monolayer. The oxidation process involved the coupled migration of Ni atoms from the interior to the surface and the concurrent penetration of oxygen atoms from the exterior to the interior. The presence of metallic nickel on the surface after oxidation indicates that the partial decomposition of NiO occurs at high temperatures under ultrahigh vacuum conditions.

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1. Introduction

The surface oxidation and stabilization mechanisms of transition metals have attracted significant attention due to their unique surface adsorption and catalytic properties. − As a member of the transition metal family, nickel possesses unique Ni 3d electronic states near the Fermi level. These states are characterized by incompletely filled 3d orbitals, which enable the atom to either gain or contribute electrons, thereby significantly enhancing its catalytic activity. Consequently, various nickel-related catalytic reactions have consistently remained a prominent research focus in the scientific community. ,,− The study of reactants and products in catalytic reactions at the atomic and molecular scale is highly valuable for understanding the catalytic mechanisms and enhancing the efficiency of nickel-based catalysts. ,, The heterogeneous catalytic reaction occurs on the catalyst surface; therefore, understanding the interactions between the catalyst surface and adsorbed molecules is crucial for comprehending the heterogeneous catalytic process. The surface thermal oxidation of nickel is scientifically important for elucidating metal–oxide interfacial physics and is widely applicable in key technological areas such as high-temperature oxidation, multiphase catalysis, exchange bias , and magnetoresistance applications. The realization of these functions relies on the oxidation process occurring on the surface of Ni metal or its alloysspecifically, the formation of an effective NiO phase. ,− Oxidizing metal single crystals in an oxygen atmosphere allows for precise control over the thickness and coverage of the oxide layer, thereby enhancing its controllability in industrial production. ,

Due to the pivotal role of the initial oxidation process in determining functional performance, numerous studies have focused on the early stages of surface oxidation. ,,, A three-stage model has been proposed to describe the kinetics of O2 interaction with Ni(111): (1) at low oxygen exposure, a 2 × 2 reconstructed structure forms with an oxygen coverage of 0.25 ML; (2) with higher exposure, a ( 3×3 )R30° reconstructed structure appears, corresponding to 0.33 ML coverage; (3) further oxidation leads to rapid surface oxidation of nickel, followed by slow thickening of the oxide layer as oxygen exposure increases. The saturated thickness of NiO formed on nickel metal in an oxygen atmosphere is typically 2–3 ML. , NiO can be grown layer-by-layer by controlling the oxidation time. , Greater thickness can be achieved by depositing nickel metal at elevated temperatures in an oxygen atmosphere. The formation of NiO is influenced by oxidation time, oxygen pressure, and the thickness of the nickel substrate when the temperature exceeds 250 °C, with a reported saturation thickness of approximately 6 atomic layers. Qu et al. and Wang et al. extensively investigated the nucleation and growth mechanisms of initial NiO oxidation at the atomic scale using high-resolution TEM. , The island nucleation behavior during early oxidation is affected by local mass transport at the interface and nearby surface structures such as step edges, indicating kinetically localized growth. Additionally, the formed NiO film decomposes into metallic nickel when annealed at temperatures reaching 400 °C.

The evolution of electronic structures during the oxidation process is also critically important for typical metal oxides, as it can significantly enhance their properties. The oxidation process of NiO has been systematically investigated using in situ photoelectron techniquessuch as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), and X-ray absorption spectroscopy (XAS)which are widely employed to study surface and interface electronic states and structures. ,,, In recent years, O K-edge X-ray absorption fine structure (XAFS) has been used to analyze oxygen in molecules and solids. A comprehensive analysis of 3d transition metal oxides, including crystal field effects and trends in oxidation state and covalency, is discussed in reviews on O K-edge XAS.

In this work, a nickel single crystal was oxidized in an oxygen atmosphere at high temperature. We aimed to conduct detailed electronic studies of the oxidation of nickel lattices, including the evolution of core-level and valence electron states during the process. The oxidation was controlled through multiple exposures to oxygen, and in situ XPS, UPS, and XAS measurements were performed to monitor changes in the surface electronic structure. The valence band, particularly Ni 3d states, was probed using UPS, while information on the conduction band and the hybridization of O 2p states was obtained from XAS. By comparing electronic structures at different oxidation times, we correlate the kinetic process of oxidation with spectral evolution. This study provides a detailed electronic structure analysis of the oxidation process in metal crystals, offering guidance for future research on the oxidation of transition metals.

2. Experimental Methods

The Ni(111) single crystal was cleaned via ultrasonication in acetone and ethanol, dried with nitrogen, and then transferred into an ultrahigh vacuum (UHV) chamber. The crystal was subsequently subjected to Ar+ sputtering at room temperature (p Ar = 1 × 10–6 Torr, beam energy = 1 kV) for 1 h, followed by transfer to an annealing chamber (1 × 10–8 Torr) for high temperature annealing at 873 K for 1 h. This sputtering and annealing cycle was repeated three times to ensure complete removal of surface contaminants such as carbon and oxygen compounds. No carbon or oxygen species were detected by XPS, and a clear low-energy electron diffraction (LEED) pattern was observed, indicating a well-ordered and clean Ni(111) surface. The LEED pattern and XPS survey spectrum of the freshly prepared surface are shown in Figure S1.

After surface cleaning, the sample was moved to an oxidation chamber and oxidized at 600 °C under an oxygen partial pressure of approximately 2 × 10–3 Pa (equivalent to 15 Langmuir, where 1 L = 1 × 10–6 Torr). It was then transferred to a high-vacuum analysis chamber (∼2 × 10–10 Torr) for in situ photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS) measurements. This oxidation–measurement cycle was repeated multiple times. The oxidation duration for each cycle was controlled by the oxygen exposure time, which was set to 10, 30, 60, 90, 3600, and 7200 s.

The entire experiment, including oxidation and measurements, was conducted at beamline 4B9B of the Beijing Synchrotron Radiation Facility (BSRF). PES measurements were performed using both synchrotron radiation and Mg Kα radiation sources. The core levels of Ni 2p, Ni 3p, O 1s, and the Ni LMM Auger transition were recorded. Additionally, synchrotron radiation with an incident photon energy of 36.2 eV was used for ultraviolet photoelectron spectroscopy (UPS). Oxygen K-edge XAS spectra were acquired in total electron yield (TEY) mode. In this mode, the sample current induced by synchrotron-excited electrons is measured. Due to the limited escape depth of electrons, TEY mode probes only the top few nanometers (∼4 nm) of the material, making it highly surface-sensitive. The top layer contributes approximately 10% to the spectral shape, rendering this technique particularly suitable for studying surface oxidation processes. The incident light was calibrated using Au references: the Au Fermi level for UPS, Au 4f for XPS, and Au spectra for XAS energy alignment.

3. Results and Discussion

After Ar+ ion bombardment and high-temperature annealing of the nickel crystal surface, in situ surface-sensitive low-energy electron diffraction (LEED) measurements were performed to characterize the crystal structure. A detailed analysis of the diffraction spot positions provides information about the surface crystal symmetry. The observed LEED pattern exhibits hexagonal symmetry, as shown in Figure S1 of the Supporting Information, confirming a clean and well-ordered Ni(111) surface.

The electronic structures of metallic nickel and nickel-related compounds have been extensively studied using photoelectron spectroscopy (PES). The relative peak intensity and spectral shape can effectively reflect the electronic structure of the material. Biesinger et al. quantified the content of different nickel species in samples using X-ray photoelectron spectroscopy (XPS). In this work, we performed XPS measurements on metallic nickel and after each oxidation step. Figure shows the survey spectra obtained throughout the oxidation process. The surface of the nickel crystal after Ar+ etching and vacuum annealing treatment was clean, with no detectable carbon or oxygen surface adsorbates. Only Ni and O signals were observed on the sample surface, and no other chemical elements were detected during oxidation.

1.

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(a) XPS survey spectra of a metallic Ni crystal and an oxidized Ni(111) single crystal after various oxidation durations. (b) XRD pattern of the Ni crystal after oxidation.

After 10 s of oxidation under an oxygen partial pressure of 2 × 10–3 Pa, a significant oxygen signal is clearly observed. As oxidation time increases, the intensity of the O signal continues to rise. To investigate the evolution of the electronic structure of the Ni single crystal during oxidation, we performed a detailed analysis of the surface for different oxidation durations. Figure b shows the XRD pattern of the oxidized sample. The diffraction peaks at 36.25° and 44.2° correspond to NiO(111) and Ni(111), respectively, indicating the formation of well-crystallized nickel oxide on the nickel single crystal via thermal oxidation. The NiO crystal is exclusively oriented along the [111] direction, which is consistent with previous studies reporting a preferred [111] orientation in oxidized Ni nanorods.

Quantitative absorption spectroscopy has become a widely used characterization technique with the advancement of synchrotron radiation technology. The distribution of electrons in transition-metal compounds is a critical factor influencing material properties, making the absorption structure at the transition-metal L-edge particularly significant.

Electron yield absorption spectroscopy is a surface-sensitive method, with a detection depth of approximately 2.5 nm at the Ni 2p absorption edge and about 1.9 nm at the O 1s absorption edge photon energy. Figure a shows the normalized absorption spectra of the Ni L-edge. The spectra have been aligned and offset to correct for incident photon energy variations and to facilitate comparison of the absorption fine structures.

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Experimental (a) Ni 2p XAS at different oxidation times; (b) comparison between experimental results and the corresponding fits obtained using the model described by eq ; (c) XAS-derived L3-edge multiplet fitting; and (d) comparison of the fitting parameters in Figure c.

From the Ni 2p absorption spectrum in Figure a, two main absorption edgeslabeled A and Dcan be resolved in metallic Ni. After oxidation, additional fine structures emerge as shoulders of the main edges, indicated as B, C, and E in Figure a. Two primary absorption peaks, A and D, located at 856.2 and 872.0 eV, respectively, are observed both before and after oxidation. These correspond to the Ni L3 and Ni L2 absorption edges. The absorption spectra for both metallic and oxidized Ni 2p are consistent with previously reported results, confirming the formation of crystalline nickel oxide on the surface. Resonance peaks B, C, and E appear after 10 s of oxidation in an oxygen atmosphere. The intensity of these satellite features near the main Ni L3 peak increases with prolonged oxidation time. The spectra obtained after 3600 and 7200 s exhibit similar line shapes, indicating that the electronic structure of the surface NiO has stabilized and closely resembles that of the bulk phase.

The L3 edge absorption peak intensity significantly increases with the extension of oxidation time, which results from an increase in the number of holes as the oxidation state increases. Prior studies have reported an exponential decay in the XAS signal intensity (measured by TEY) from the underlying substrate with increasing epitaxial layer thickness. , Based on this, the Ni metal XAS signal was taken as the signal from the underlying material, and the signal from the 7200 s sample was taken as that from the epitaxial layer. The signals from the 10 s, 30 s, and 90 s samples were fitted using the equation in eq . A comparison between the fitting results and the experimental data is presented in Figure b, showing an excellent agreement.

I=I0Niexp(−d/λ)+I0NiO[1−exp(−d/λ)] 1

For 3d transition metals, the electronic ground state consists of a full filled 2p6 core–shell and a partially filled 3d N shell. An electron is excited from the core–shell to the 3d shell according to the transition: 2p63d N → 2p53d N+1. The enhancement of the resonance structure (peak B, C, and E) primarily arises from the formation of Ni–O covalent bonds, which induce a crystal field effect around Ni − and facilitate spin-state transitions. Metallic Ni in a low-spin state with a 2p63d8 configuration possesses only one unoccupied orbital, resulting in a wide single peak at the L3 edge. In contrast, high-spin Ni species (e.g., NiO) exhibit two partially filled 3d orbitals, leading to more complex multiplet structures. The main L3 peak originates from a nonresonant 2p → 3d transition, while the shoulder features result from resonant 2p → 3d excitations. Consequently, the high-energy shoulder of the L3 edge is often used as a spectral marker for high-spin Ni­(II). ,, The multiple fine structures observed after oxidation can be attributed to ligand-field multiplet effects and spin–orbital coupling interactions at the surface.

Multiplet fitting was taken for the Ni L3 edge XAS, as shown in Figure c, the spectrum from Nickel metal contains two subabsorption peaks, while the oxidation sample contains three subabsorption peaks, and the fitting parameters were compared in Figure d. Subabsorption peak3 appears as oxidation begins, and its intensity increases along with the main L3 peak. However, at the 10 s sample, both the energy difference between peak3 and peak2 and the intensity ratio of peak3 to peak1 are smaller than those in the later oxidation stages. This phenomenon can be attributed to the incomplete formation of the NiO layer on the surface during the initial oxidation stage, which leads to symmetry breaking in the surface oxide.

The formation process of Ni–O covalent bonds can also be analyzed using the O 1s absorption spectrum. As shown in Figure , four main absorption peaksA, B, C, and Dare observed in the oxidized samples. The peak at 531.5 eV corresponds to transitions into empty O 2p states hybridized with Ni 3d orbitals. The features at higher photon energies (>534 eV) are attributed to O 2p character mixed with Ni 4sp-derived states. , These four absorption peaks match those reported for NiO single crystals, indicating the formation of well-defined Ni–O covalent bonds even in the early stages of oxidation.

3.

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O 1s absorption spectrum.

In the initial oxidation period (10–30 s), a weak pre-edge feature is observed at 529.7 eV, which disappears completely after 90 s. This pre-edge signal may originate from splitting of the e_g energy levels (dz 2 and dx 2+y 2) due to symmetry breakingsuch as in submonolayer NiO epitaxial films or nanoparticles, where the local coordination deviates from ideal octahedral symmetry. Additionally, the absorption peaks in the later oxidation stage (after 3600 s) are shifted approximately 0.2 eV to higher energies compared to those in the early stage (before 90 s). This shift is consistent with previously reported strain effects: the hybridization-related O 2p–Ni 3d peak can shift relative to bulk NiO under biaxial strain-induced asymmetric lattice interactions, a phenomenon also observed in photoelectron spectroscopy studies.

In situ photoelectron spectroscopy was employed to further analyze the elemental core-level energies during the oxidation process. Consistent with the XAS results, a shift in the O 1s emission peak position is observed, as shown in Figure a. The main O 1s emission peak from NiO is located at 530.0 eV. In the early oxidation stage (10–90 s), this peak is shifted approximately 0.1 eV toward lower binding energy compared to the later stage (3600 and 7200 s).

4.

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XPS spectra with a synchrotron light source of 720 eV (a) for O 1sand (c) Ni 3p. (b) and (d) Magnified images from the area outlined by the squares in panels (a) and (c).

An additional emission peak is resolved at 532.2 eV in Figure b, which is commonly attributed to surface adsorbates such as OH–, typically appearing at higher binding energies. This peak is barely detectable at 10 s, becomes more pronounced between 30 and 90 s, and decreases in intensity thereafter. After extended oxidation, most of the surface adsorbed oxygen has incorporated into the lattice through reaction with nickel, forming stable lattice oxygen. As a result, the adsorbate-related peak diminishes significantly and becomes nearly undetectable in the later oxidation stages.

Figure c,d shows the Ni 3p emission peak and its corresponding enlarged view, respectively. As seen in the figures, the satellite peak at 85.5 eV is enhanced in the oxidized samples, while the intensity of the peak at 78.2 eV is significantly reduced after 30 s of oxidation.

Due to the limited photon energy available from synchrotron radiation, Mg Kα X-rays were selected as the excitation source to analyze the Ni 2p electrons and Ni LMM Auger electrons. The corresponding spectra are shown in Figure a,b, respectively. As can be observed, the Ni 2p3/2 emission peak before oxidation is located at 852.7 eV, which corresponds to metallic nickel. After extended oxidation, this peak shifts to 854.7 eV, consistent with the characteristic binding energy of NiO. These results clearly indicate the formation of lattice oxygen on the surface of the nickel single crystal.

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Ni 2p (a) and LMM (b) Auger XPS spectra with the Mg Kα excitation source.

Correspondingly, even at the later stages of oxidation, the metallic Ni 2p3/2 signal remains detectable. The ratio of metallic to oxidized nickel components shows little change between 3600 and 7200 s, indicating that the system has reached a stable state. This persistent metallic signal can be attributed to minor surface reduction occurring in the vacuum environment after oxidation, leading to partial desorption of oxygen and the reappearance of a small fraction of metallic nickel on the surface.

The Ni 2p spectrum exhibits four emission peaks, consisting of the Ni 2p3/2 and Ni 2p1/2 main peaks along with their corresponding satellite features, consistent with previously reported results. These peaks reflect the complexity of the strongly correlated electronic structure in the valence band, arising from multiplet structures and final-state effects involving 3d electrons. Each spin–orbit multiplet can be decomposed into several components, as illustrated in Figure a. The emission of Ni 2p photoelectrons may be accompanied by the rearrangement of electrons in the valence band.

The ground state is reported to be the combination of 3d8 + 3d9 L + 3d10 L 2, which broaden the emission peak. The main peak of 854 eV originates from the local screening effect 2p53d9 L, where L denotes a hole in the O 2p energy level induced by the screening effect. The satellite peak at 855 eV originates from nonlocal screening effect with a hole from adjacent NiO6 clusters, and this nonlocal screening effect could be enhanced in NiO nanoparticles and surface states of this film. −

Compared to soft X-ray PES, the hard X-ray PES is more sensitive to the bulk. Taguchi et al. investigated the Ni 2p XPS by the energies of 7.93 and 1.25 keV with an effect penetration depth of 6 nm, and their results are similar to those of Soriano et al. Considering the huge difference in the incident light energies, the difference in the satellite peak at 856 eV due to nonlocal screening effect is weak, but instead the intensity change of the main peak is much stronger, which is also consistent with the results of Soriano et al. The main peak of nanoparticle NiO has a lower relative ratio than that of bulk materials, demonstrating that hard X-ray photoelectron spectroscopy is bulk-sensitive. The intensity ratios of the satellite peaks to the main peak could be used to calculate the elemental stoichiometric ratios in NiO. The Ni LMM Auger Spectrum broadens a lot for NiO, which is the result of local and nonlocal screening effects in NiO.

The valence band electronic structure was characterized using in situ synchrotron radiation UPS with a photon energy of 36.2 eV, and its evolution during oxidation is presented in Figure . According to the QUASES-IMFP-TPP2M code developed by Tougaard, the inelastic mean free path (IMFP) of 36.2 eV photoelectrons is approximately 0.265 nm in Ni and 0.256 nm in NiO, respectively. In metallic Ni single crystals, the valence band is dominated by Ni 3d electrons, located about 0.6 eV below the Fermi level, as shown in Figure a. The inset in Figure a provides a detailed view of the electronic structures near the Fermi level, , where a metallic state remains detectable even after 7200 s of oxidation. This persistent metallic signature may originate from oxygen desorption from the surface, leading to the reappearance of metallic nickel, or from the underlying unoxidized Ni substrate.

6.

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Synchrotron-based UPS spectra acquired at an excitation energy of 36.2 eV: (a) valence band structures throughout the oxidation process; the inset shows an enlarged view of the region near the Fermi level; (b) comparison between experimental results and the corresponding fits obtained using the model described by eq ; (c) spectra of each growth stage using successive differences method; and (d) difference spectra between the overlayers at different stages and the bulk NiO (7200s) phase.

The metallic state of Ni is also evident in the Ni 2p spectrum obtained using an incident energy of 1253.7 eV. The penetration depth of X-rays for Ni 2p is nearly eight times greater than that of UPS with 36.2 eV incident energy. Therefore, any metallic signal originating from the underlying bulk Ni substrate should not be detectable in the UPS measurements. The observation of a metallic Ni state in the UPS spectra thus indicates the presence of metallic nickel on the surface, likely due to decomposition of the outermost NiO layer.

Based on the signal attenuation model, the thickness of the surface oxide layer can be approximately estimated using the eq . The parameter d represents the thickness of the NiO layer, and λ denotes the inelastic mean free path of electrons (approximately 0.7 nm). The thickness values used in the fitting were set to 0.26, 0.60, and 1.00 nm for the 10, 30, and 90 s spectra, respectively. Furthermore, the electronic signal from thin NiO films differs from that of the bulk phase owing to reduced surface lattice periodicity.

A comparison between the fitted and experimental spectra reveals that while the intensity of the Ni 3d band is well reproduced, the modeled O 2p intensity shows deviations from the experimental data, particularly being underestimated in the 4–6 eV region. This discrepancy suggests the presence of additional adsorbed oxygen species on the sample surface.

Ioverlayer=Itotal−Iunderlayer·exp(−doverlayerλ) 2
Idiff=Ioverlayer−INiO 3

The valence band spectra after oxidation were obtained by subtracting the underlying signal using the differential method in eq , and the results are plotted in Figure c. The difference of data in Figure c and NiO bulk are obtained by eq and plotted in Figure d. Figure c,d reveals that in the energy range of 3–7 eV below the Fermi level, the spectrum from the first 10 s of oxidation exhibit significant differences compared to bulk NiO. Specifically, the 0–10 s spectrum shows a single broad feature, whereas the spectra at other oxidation times display two distinct features. These two features are the result of a nonlocal screening effect in NiO, while this effect is greatly reduced for the first 10s spectrum because of the broken octahedral symmetry at the initial oxidation stage. The Ni 2p XPS spectra were also fitted using eq . However, for the value of Mg Kα, λ was set to 1.2 nm, the fitting thickness in Ni 2p is consistent with the thickness used here. The fitting results show good agreement with the experimental data, and the comparative plot is displayed in Figure S2.

According to the fitting results, NiO grows in a layer-by-layer mode rather than following the Stranski–Krastanov growth mode, which is consistent with the layered oxidation behavior reported in ref . However, a key difference is that the literature describes the oxidation process of Ni nanoparticles as proceeding from the interior outward, whereas in our case, oxidation occurs from the exterior inward.

4. Conclusions

A detailed analysis of the evolution of core-level electrons, valence band electrons, and conduction band electrons during the oxidation of nickel metal was conducted for the first time in this work using in situ synchrotron radiation PES (photoemission spectroscopy) and XAS (X-ray absorption spectroscopy). The oxidation process follows a layer-by-layer mechanism proceeding from the exterior inward, ultimately forming a well-crystallized thin NiO layer. The valence band electronic structure of the fully oxidized nickel closely resembles that of bulk NiO, confirming that high-temperature thermal oxidation is an effective method for preparing highly crystalline NiO.

A metallic nickel phase was observed on the outermost surface, resulting from the partial decomposition of surface NiO. This decomposition leads to the formation of a sandwich-like structure on the Ni single crystal surface, which may contribute to the high carrier mobility observed in NiO-based thin-film transistors (TFTs). Our findings provide insight into the transformation mechanism of nickel oxidation and offer guidance for understanding charge transport processes in catalytic systems.

Supplementary Material

ao5c11285_si_001.pdf (248.5KB, pdf)

Acknowledgments

This work was supported by a scientific research fund project for high-level talents of Henan University of Technology 2019BS065, 2019BS066, 0004-31401803, 0004-31401758, College-Enterprise Cooperation Project 0004-51100256. The authors thank Chen Liu, Jiali Zhao, Jinmei Li, and Tao Lei for the assistance with the synchrotron-based experiments.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c11285.

  • LEED diffraction patterns and XPS survey plo of the clean Ni(111) plane; comparison of experimental Ni 2p XPS results and the corresponding fits for samples with oxidation times within 90 s (PDF)

The authors declare no competing financial interest.

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