Under Ultrahigh Vacuum Conditions Research Articles

Stability of the Fe3O4 (√2 x √2)R 45° Surface in 0.1 M NaOH Investigated by High-Energy Surface X-Ray Diffraction

Transition metal oxides, especially Fe, Co, and Ni oxides, are of great interest as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solution. However, detailed data on the atomic-scale oxide-electrolyte interface structure of these systems are rare. We present studies of a model oxide electrocatalyst, -reconstructed Fe3O4(001). This oxide surface has been extensively investigated under ultrahigh vacuum (UHV) conditions, leading to a detailed understanding of its atomic structure [2]. In previous in situ surface X-ray diffraction studies, we demonstrated that the reconstructed surface can survive transfer into electrochemical environment and determined the stability of the reconstruction as a function of potential [2]. Specifically, we showed that the ()R45° reconstructed Fe3O4(001) surface can be transferred into 0.1 M NaOH at 0.2 V vs. Ag/AgCl while maintaining surface structure and that the reconstruction is irreversibly lifted at sufficiently negative potentials.Here, we present more detailed in situ studies of this system, using the novel technique of High-Energy Surface X-Ray Diffraction (HESXRD) [3,4]. This technique enabled us to measure rapidly a huge reciprocal space volume, which allows to obtain very large sets of crystal truncation rods (CTRs) that can be analysed by surface crystallographic methods. These data provide detailed insights into the temporal stability of the surface at fixed potentials as well as potential-dependent changes in surface structure.In the experiments the Fe3O4(001) single crystal samples were prepared by sputtering and annealing in UHV, followed by sample transfer into the hanging-meniscus cell employed for the HESXRD studies. A new experimental procedure allowed to reduce the time of sample transfer and establishment of potential control to below one minute.The in situ HESXRD measurements were performed at beamline P21.2 of PETRA III at DESY. 7 symmetrically independent CTRs and 2 ()R45° fractional order rods could be measured. These data enabled detailed comparison of the oxide-electrolyte interface with the surface structure under UHV conditions [1]. The potential-dependent data indicate clear structural changes when applying more positive or more negative potentials than 0.2 V vs. Ag/AgCl, as illustrated in part A and B of Fig. 1. However, all surface structures were stable at constant potential over several hours, as indicated in part C of Fig. 1. The quality of the obtained results allows to determine quantitative analysis of the interface structure. First results of this analysis will be presented.We gratefully acknowledge financial support by the German ministry of Science and Education (BMBF) via projects 05K19FK3 and 05K22FK1.

Dünne Schichten metallorganischer Gerüstverbindungen als ideale Matrizes für die Azid‐Photolyse im Vakuum

AbstractStudien zu Reaktionen in Lösungen werden oft durch Lösungsmitteleffekte behindert. Zudem ist eine detaillierte Untersuchung der Kinetik auf den kleinen Temperaturbereich beschränkt, in dem das Lösungsmittel flüssig ist. Hier berichten wir über die spektroskopische in situ‐Beobachtung UV‐induzierter Reaktionen von Arylaziden in einer kristallinen Matrix im Vakuum. Die Matrizes werden durch Bindung der reaktiven Teile an ditopische Linker, die dann zu metallorganischen Gerüstverbindungen (MOFs) und oberflächenverankerten MOFs (SURMOFs) zusammengesetzt werden, gebildet. Diese porösen, kristallinen Gerüstverbindungen werden dann als Modellsysteme verwendet, um die chemischen Prozesse im Zusammenhang mit Aziden unter Ultrahochvakuum(UHV)‐Bedingungen – bei denen Lösungsmitteleffekte sicher ausgeschlossen werden können – und in einem großen Temperaturbereich zu untersuchen. Die Infrarot‐Reflexions‐Absorptions‐Spektroskopie (IRRAS) ermöglichte es uns, die Photoreaktion von Aziden in SURMOFs präzise zu überwachen. Die in situ‐IRRAS‐Daten zeigten in Verbindung mit XRD, MS und XPS, dass die Bestrahlung mit UV‐Licht zunächst zur Bildung eines Nitren‐Zwischenprodukts führt. Im zweiten Schritt findet eine intramolekulare Umlagerung statt, die ein Indoloindol‐Derivat ergibt. Diese Ergebnisse eröffnen einen neuartigen Weg für die präzise Untersuchung von chemischen Umwandlungen von Aziden. Referenzexperimente, die für lösungsmittelbeladene SURMOFs durchgeführt wurden, offenbaren eine enorme Vielfalt anderer Reaktionsschemata und unterstreichen daher den Bedarf an Modellsystemen, die unter UHV‐Bedingungen untersucht werden.

Metal-Organic Framework Thin Films as Ideal Matrices for Azide Photolysis in Vacuum.

Studies on reactions in solutions are often hampered by solvent effects. In addition, detailed investigation on kinetics is limited to the small temperature regime where the solvent is liquid. Here, we report the in situ spectroscopic observation of UV-induced photochemical reactions of aryl azides within a crystalline matrix in vacuum. The matrices are formed by attaching the reactive moieties to ditopic linkers, which are then assembled to yield metal-organic frameworks (MOFs) and surface-mounted MOFs (SURMOFs). These porous, crystalline frameworks are then used as model systems to study azide-related chemical processes under ultrahigh vacuum (UHV) conditions, where solvent effects can be safely excluded and in a large temperature regime. Infrared reflection absorption spectroscopy (IRRAS) allowed us to monitor the photoreaction of azide in SURMOFs precisely. The in situ IRRAS data, in conjunction with XRD, MS, and XPS, reveal that illumination with UV light first leads to forming a nitrene intermediate. In the second step, an intramolecular rearrangement occurs, yielding an indoloindole derivative. These findings unveil a novel pathway for precisely studying azide-related chemical transformations. Reference experiments carried out for solvent-loaded SURMOFs reveal a huge diversity of other reaction schemes, thus highlighting the need for model systems studied under UHV conditions.

Enantiomer Adsorption in an Applied Magnetic Field: D‐ and L‐Aspartic Acid on Ni(100)

AbstractThere have been several reports of the enantiospecific adsorption of chiral compounds from solution onto ferromagnetic surfaces in applied magnetic fields. This work studies the kinetics of D‐ and L‐aspartic acid (Asp) adsorption onto a Ni(100) surface in a 0.45 T applied magnetic field under ultra‐high vacuum (UHV) conditions. Desorption studies performed after exposing the Ni(100) sample to L‐Asp revealed no influence of the applied magnetic field on the adsorption kinetics. Similarly, concurrent exposure of the Ni(100) surface to a mixture of isotopically labeled L*‐Asp and D‐Asp resulted in no detectable field induced differences in the relative enantiomer coverages adsorbed onto the surface. In summary, the presence of an applied magnetic field had no detectable influence on the adsorption kinetics of Asp enantiomers on a Ni(100) surface under UHV conditions. This is in contrast to the magnetic field induced adsorption enantiospecificity observed in solution phase. The possible origins of this discrepancy are considered.

Surroundings affect slip length dynamics in nanoscale friction through contact stiffness and damping

Friction force microscopy (FFM) explores the interaction in a sliding contact on the nanoscale, providing information on the frictional dynamics and lateral contact stiffness with lattice resolution. Recent FFM measurements on a NaCl crystal immersed in liquid (ethanol) surroundings displayed an increase of the effective contact stiffness, Keff, with the applied load, differently from similar measurements performed under ultra-high vacuum (UHV) conditions, where Keff showed negligible load dependency. Additionally, under UHV conditions multiple slip length friction with increasing load was reported, while in ethanol surroundings only single (lattice unit length) slips were observed. Our current understanding of this behavior relates the transition from single jumps to multiple jumps dynamics to the normal load (manifested through the amplitude of the interaction potential at the contact, U0) and to the damping of the system. Here we have incorporated the effect of the load dependency on both U0 and Keff within Prandtl—Tomlinson based simulations, accompanied by variations in the damping coefficient of the system. Introducing the experimentally observed load dependency to Keff resulted indeed in single slip jumps at critical damping, while multiple slip jumps were obtained at constant Keff. The average slip length increased with the normal load, particularly when the system became underdamped. Our work provides a glimpse on the relation between the characteristic observables in atomic-scale sliding friction (maximal slip forces, stiffness, and slip dynamics) with respect to their governing parameters (corrugation energy, effective stiffness, and damping). While common understanding in nanotribology relates the effect of surrounding media mainly to the interaction potential at the contact, here we show that the media can also greatly affect the elastic interaction, and consequently play an important role on the transition from single to multiple stick-slip.

Hydrogen plasma enhanced oxide removal on GaSb planar and nanowire surfaces

Due to its high hole-mobility, GaSb is a highly promising candidate for high-speed p-channels in electronic devices. However, GaSb exhibits a comparably thick native oxide causing detrimental interface defects, which has been proven difficult to remove. Here we present full oxide removal from GaSb surfaces using effective hydrogen plasma cleaning, studied in-situ by synchrotron-based X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV). GaSb nanowires turn out to be cleaned faster and more efficiently than planar substrates. Since the UHV conditions are not scalable for industrial sample processing, H-plasma cleaning is furthermore used as pre-treatment prior to atomic layer deposition (ALD) of a protective high-k layer to demonstrate the use of the cleaning step in a more realistic fabrication situation. We observe a cleaning effect of the H-plasma even in the ALD environment, but we also find residual Ga- and Sb-oxides at the GaSb-high-k interface, which we attribute to re-oxidation of the cleaned surface. Our results indicate that an improved control of the ALD reactor vacuum environment could realize oxide- and defect-free interfaces in GaSb-based electronics.

Electrical Double Layer on the Pt(111) Electrode Modeled under Ultrahigh Vacuum Conditions

An adsorbed hydroxide (OHad) layer is formed on platinum electrodes at positive potentials, which affects many important electrochemical reactions, such as the oxygen reduction reaction. The stability of the OHad layer is strongly governed by the hydrophilicity of the cation interacting with OHad in the electrical double layer (EDL). Therefore, elucidating the detailed hydration structure and role of cations in the EDL is a key research goal. In this study, the structures of H2O, OHad, and Li were investigated using infrared spectroscopy and density functional theory calculations. By optimizing the coverage of OHad (θOH) and Li (θLi), the bending mode of the PtOH (δPtOH) band on the Pt(111) surface under ultrahigh vacuum (UHV) conditions matches with that on the Pt(111) electrode at 0.8–0.9 V versus reversible hydrogen electrode (RHE) in LiOH solution, indicating that a quasi-EDL composed of OHad species interacting with hydrated Li+ on Pt(111) was successfully modeled under UHV conditions. According to the interfacial structure modeled under UHV conditions, θOH = 0.3, and Li interacts directly with OHad at a stoichiometric ratio (θOH/θLi) of 2. Modeling of the EDL, including the outer Helmholtz plane, is beneficial for identifying the microscopic details of the EDL under electrochemical conditions.

Novel two-dimensional materials and their heterostructures constructed in ultra-high vacuum

Compared with the three-dimensional bulk materials, two-dimensional (2D) materials exhibit superior electronic, optical, thermal, and mechanical properties due to the reduced dimensionality. The quantum confinement effect of 2D materials gives rise to exotic physical properties, and receives extensive attention of the scientists. Lots of routes to fabricate the 2D materials have been proposed by the material scientists, including the traditional mechanical exfoliation, chemical vapor deposition, molecular beam epitaxy under ultra-high vacuum (UHV), and so on. Among them, fabricating materials under ultra-high vacuum has the advantages of constructing large-scale and high-quality samples, and is therefore widely adopted in the 2D material growth. In this paper, we review three different strategies of growing 2D materials under UHV conditions, including molecular beam epitaxy, graphene intercalation and manual manipulation by nano probes. We compare the advantages and drawbacks among those methods in creating 2D materials, and try to provide some guidance to the community, especially those who are new to the field.

Infrared reflection absorption spectroscopy and temperature-programmed desorption studies of CO adsorption on Ni/CeO2(111) thin films: The role of the ceria support

Ceria-supported Ni has shown unique catalytic activity due to unique properties of small Ni particles and strong metal-support interaction. Identification of adsorption sites and understanding the chemical interaction over Ni-ceria at the fundamental level provide crucial insights into the reaction pathways of complex catalytic processes. In this study, to probe the surface sites, the adsorption of CO was carried out with model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films using infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) under ultrahigh vacuum (UHV) conditions. Fully oxidized CeO2(111) (Ce4+) and partially reduced CeO1.75(111) (Ce4+/Ce3+) thin films were grown on Ru(0001) to examine the role of the ceria support. Ni with low coverages (e.g., 0.2 ML) grows forming small two-dimensional particles on ceria at 300 K, which develop into three-dimensional clusters after heating to 700 K. In the absence of Ni, CO adsorption at 1 mTorr at 100 K shows distinct IR bands at 2158 cm−1 on CeO2 and 2165 cm−1 on CeO1.75. Bridging and atop IR bands associated with CO adsorption over metallic Ni were observed on the Ni-CeO1.75 surface at 300 K under UHV conditions. CO adsorption over Ni0 was also observed over as-deposited Ni on CeO2. However, a new IR band at 2146 cm−1 due to CO adsorption over Ni2+ species was detected at 100 K over the annealed Ni particles on CeO2. CO IRRAS data suggest the oxidation of Ni to Ni2+ on CeO2 and the formation of predominant Ni2+ species with heating, which is further confirmed with CO TPD data and previous x-ray photoelectron spectroscopy results.

Retraction to: Self-assembly of nickel icosahedrons and truncated octahedral nanocrystals on a SrTiO3 (111) support.

Nickel nanocrystals have received much attention for their ferromagnetic properties. The crystal properties are strongly dependent on their facets and therefore detailed study of their morphology, facets and orientation is critical for magnetic applications. In this work, equilibrium crystal shapes of self-assembled nickel nanocrystals on the (111) termination of strontium titanate (SrTiO3) at room temperature and under ultra-high vacuum (UHV) conditions have been investigated using scanning tunneling microscope (STM). SrTiO3 (111) substrate was sputtered (0.5 keV, 2.5 µA, 10 min) and annealed (900 °C, 1 h) under UHV conditions. Three different periodicities were observed: (2.21 ± 0.01) nm corresponding to (4 × 4) reconstruction, (3.31 ± 0.02) nm corresponding to (6 × 6) reconstruction, and (2.85 ± 0.05) nm, rotated at 30° with respect to (4 × 4) reconstruction, corresponding to (3√3 × 3√3)R30° reconstruction. Nickel (~1 ML) was deposited using an e-beam evaporator on the substrate preheated to 320 °C and the sample was post-annealed multiple times. Nickel took platonic shapes of supported icosahedron comprising of (111) facets and truncated octahedron comprising of (001) and (111) facets. Based on surface energy ratios of truncated octahedrons at equilibrium the work of adhesion was calculated to be (3.889 ± 0.167) J/m2.

WITHDRAWN: Austenite Stabilized Iron Nanocrystals on SrTiO3 (111)

This article has been withdrawn at the request of the author. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal

Solution-Based Alkyne–Azide Coupling on Functionalized Si(001) Prepared under UHV Conditions

Synthesis of organic bi-layers on silicon was realized by a combination of surface functionalization under ultra-high vacuum (UHV) conditions and solution-based click chemistry. The silicon (001) surface was prepared with a high degree of perfection in UHV and functionalized via chemoselective adsorption of ethinyl cyclopropyl cyclooctyne from the gas phase. A second organic layer was then coupled in acetonitrile via the copper-catalyzed alkyne azide click reaction. The samples were directly transferred from UHV via the vapour phase of the solvent into the solution of reactants and back to UHV without contact to ambient conditions. Each reaction step was monitored by means of X-ray photoelectron spectroscopy in UHV; the N 1s spectra clearly indicated the click reaction of the azide group in the two test molecules employed, i.e., methyl-subsituted benzylazide and azide substituted pyrene. In both cases, up to 50 - 60 % of the ethinyl cyclopropyl cyclooctyne molecules on the surface were reacted.

Impact of defects on the decomposition of chemical warfare agent simulants in Zr‐based metal organic frameworks

AbstractMetal organic frameworks (MOFs) containing zirconium secondary building units (SBUs) in UiO‐67 and related MOFs, are highly active for neutralizing both the chemical warfare agents and simulants, such as dimethyl methylphosphonate (DMMP). However, two recent publications gave conflicting reports of DMMP reaction with UiO‐67 under ultra high vacuum (UHV) conditions, with one reporting chemisorption and reaction (Wang et al., J Phys Chem C, 2017, 121, 11261–11272) and the other reporting only physisorption and reversible desorption (Ruffley et al., J Phys Chem C, 2019, 123, 19748–19758) from very similar temperature programmed desorption experiments. We show that the discrepancy between these experiments may be explained by different levels of missing linker defects in the UiO‐67 samples. We present density functional theory calculations showing that SBU sites having two‐adjacent missing linkers exhibit reaction barriers that are about 30 kJ/mol lower than SBU sites having a single missing linker. We also show that topology of the undercoordinated sites plays an important role in the reaction barrier under UHV conditions.

Self-assembly of nickel icosahedrons and truncated octahedral nanocrystals on a SrTiO3 (111) support.

Nickel nanocrystals have received much attention for their ferromagnetic properties. The crystal properties are strongly dependent on their facets and therefore detailed study of their morphology, facets and orientation is critical for magnetic applications. In this work, equilibrium crystal shapes of self-assembled nickel nanocrystals on the (111) termination of strontium titanate (SrTiO3) at room temperature and under ultra-high vacuum (UHV) conditions have been investigated using scanning tunneling microscope. SrTiO3 (111) substrate was sputtered (0.5 keV, 2.5 µA, 10 min) and annealed (900°C, 1 h) under UHV conditions. Three different periodicities were observed: 2.21 ± 0.01 nm corresponding to (4 × 4) reconstruction, 3.31 ± 0.02 nm corresponding to (6 × 6) reconstruction and 2.85 ± 0.05 nm, rotated at 30° with respect to (4 × 4) reconstruction, corresponding to (3√3 × 3√3)R30° reconstruction. Nickel (∼1 ml) was deposited using an e-beam evaporator on the substrate preheated to 320°C and the sample was post-annealed multiple times. Nickel took platonic shapes of supported icosahedron comprising of (111) facets and truncated octahedron comprising of (001) and (111) facets. Based on surface energy ratios of truncated octahedrons at equilibrium, the work of adhesion was calculated to be 3.889 ± 0.167 J/m2.

Superlubricity between Graphite Layers in Ultrahigh Vacuum

Graphite has been conventionally believed to exhibit an inferior lubricating performance with significantly larger friction coefficient and wear rate in a vacuum environment than in ambient air. Dangling bonds at the edge planes of graphite, accounting for the high friction in inert atmosphere are saturated by chemisorbed vapor molecules in air, which contributes to low surface adhesion and low friction. However, there is still a lack of direct experimental evidence whether basal planes of graphite excluding the negative effects of edges or dangling bonds shows intrinsic lubricity when sliding under ultrahigh vacuum (UHV) conditions. By the interlayer friction measurement enabled by graphite flake-wrapped atomic force microscope tips in UHV, we show a record-low friction coefficient of 4 × 10-5 (slope of friction vs normal force curve) when sliding between graphite layers, which is much lower than that in ambient air. This discrepancy manifests the intrinsic sliding frictional behavior between the graphite basal planes when the tribo-materials and experimental conditions are well-designed and strictly controlled. In addition, the temperature dependence of the kinetic friction between the graphite layers has been investigated under UHV conditions over the temperature range of 125-448 K, which is consistent with the thermally activated process.

2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film.

We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF4). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF4 at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis.

Morphology and Magneto‐Transport in Exfoliated Graphene on Ultrathin Crystalline β‐Si3N4(0001)/Si(111)

AbstractThis work reports the first experimental study of graphene transferred on β‐Si3N4(0001)/Si(111). A comprehensive quantitative understanding of the physics of ultrathin Si3N4 as a gate dielectric for graphene‐based devices is provided. The Si3N4 film is grown on Si(111) under ultra‐high vacuum (UHV) conditions and investigated by scanning tunneling microscopy (STM). Subsequently, a graphene flake is deposited on top of it by a polymer‐based transfer technique, and a Hall bar device is fabricated from the graphene flake. STM is employed again to study the graphene flake under UHV conditions after device fabrication and shows that the surface quality is preserved. Electrical transport measurements, carried out at low temperature in magnetic field, reveal back gate modulation of carrier density in the graphene channel and show the occurrence of weak localization. Under these experimental conditions, no leakage current between back gate and graphene channel is detected.

Near ambient pressure adsorption of nickel carbonyl contaminated CO on Cu(111) surface

Formation of volatile nickel carbonyls with CO in catalytic reaction is one of the mechanisms of catalyst deactivation. CO is one of the most popular probe molecules to study the surface properties in model catalysis. Under ultra-high vacuum (UHV) conditions, the problem of nickel carbonyl impurity almost does not exist in the case that a high purity of CO is used directly. While in the near ambient pressure (NAP) range, nickel carbonyl is easily found on the surface by passing through the Ni containing tubes. Here, the NAP techniques such as NAP-X-ray photoelectron spectroscopy and NAP-scanning tunneling microscopy are used to study the adsorption of nickel carbonyl contaminated CO gas on Cu(111) surface in UHV and NAP conditions. By controlling the pressure of contaminated CO, the Ni-Cu bimetallic catalyst can form on Cu(111) surface. Furthermore, we investigate the process of CO adsorption and dissociation on the formed Ni-Cu bi-metal surface, and several high-pressure phases of CO structures are reported. This work contributes to understanding the interaction of nickel carbonyl with Cu(111) at room temperature, and reminds the consideration of CO molecules contaminated by nickel carbonyl especially in the NAP range study.

Growth of Ultrathin Single-Crystalline IrO2(110) Films on a TiO2(110) Single Crystal.

The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10-4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10-6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10-4 mbar of O2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10-4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)-TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.

Reversible Oxygen‐Driven Nickel Oxide Structural Transition on the Nickel(1 1 1) Surface at Near‐Ambient Pressure

AbstractNiO cluster formation with strictly controlled O2 exposure on a Ni(1 1 1) surface has been investigated extensively for decades under ultra‐high vacuum (UHV) conditions. The classical model of three‐stage Ni oxidation refers to the relationship between NiO cluster evolution and the kinetics of O2 exposure; however, this information has a critical inherent limitation because of the “pressure gap” between UHV and real reaction conditions. Here, we report reversible NiO phase transitions on the Ni(1 1 1) surface at near‐ambient pressure by using scanning tunneling microscopy at room temperature. The restricted kinetic growth of NiO cluster evolution expands unexpectedly to oxide multi‐layer formation at 100 mTorr of O2. Furthermore, metastable NiO islands can be manipulated by varying the partial CO pressure of the gas mixture. The interplay between the CO and O2 molecules on the Ni(1 1 1) is correlated definitely to either surface oxide formation or competitive CO adsorption on the defect‐laden multi‐layered NiO interface.