Oxygen Overlayer Research Articles

Oxygen Adsorption Induced Superconductivity in Ultrathin FeTe Film on SrTiO3(001).

The phenomenon of oxygen incorporation-induced superconductivity in iron telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and quite different from the case of FeSe. Until now, the microscopic origin of the induced superconductivity and the role of oxygen are far from clear. Here, by combining in situ scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically adsorbed O2 molecules crystallized into c (2/3 × 2) structure as an oxygen overlayer at low temperature, which was vital for superconductivity. The O2 overlayer were not epitaxial on the FeTe lattice, which implied weak O2 –FeTe interaction but strong molecular interactions. The energy shift observed in the STS and XPS measurements indicated a hole doping effect from the O2 overlayer to the FeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi level. Our direct microscopic probe clarified the role of oxygen on FeTe and emphasized the importance of charge transfer effect to induce superconductivity in iron-chalcogenide thin films.

Thermal oxidation of Ru(0001) to RuO2(110) studied with ambient pressure x-ray photoelectron spectroscopy

The thermal oxidation of Ru(0001) has been extensively studied in the surface science community to determine the oxidation pathway towards ruthenium dioxide (RuO2(110)), improving the knowledge of Ru(0001) surface chemistry. Using time-lapsed ambient-pressure x-ray photoelectron spectroscopy (APXPS), we investigate the thermal oxidation of single-crystalline Ru(0001) films toward rutile RuO2(110) in situ. APXPS spectra were continuously collected while the Ru(0001) films were exposed to a fixed O2 partial pressure of 10−2 mbar and the sample temperature was increased stepwise from room temperature to 400 °C. We initially observe the removal of adventitious carbon and subsequent formation of a chemisorbed oxygen overlayer at 250 °C. Further annealing to 300 °C leads to an increase in thickness of the oxide layer and a shift in the Ru–O component of the Ru 3d spectra, indicating the presence of a metastable O–Ru–O trilayer structure. A rapid formation of the RuO2 rutile phase with an approximate thickness of at least 2.6 nm is formed about four minutes after stabilizing the temperature at 350 °C and subsequent annealing to 400 °C, signaled by a distinct binding energy shift in both the Ru 3d and O 1s spectra, as well as quantitative analysis of XPS intensities. This observed autocatalytic oxidation process agrees well with previous theoretical models and experimental studies, and the data provide the unambiguous spectral identification of one proposed metastable precursor required for full oxidation to rutile RuO2(110). Further ex situ characterization of the grown oxide with x-ray photoelectron diffraction confirms the presence of three rotated domains of rutile RuO2(110) and reveals their orientation relative to the substrate lattice.

Controlled growth of uniform two-dimensional ZnO overlayers on Au(111) and surface hydroxylation

Ultrathin ZnO nanostructures present interesting two-dimensional (2D) graphene-like structure in contrast to wurtzite structure in bulk ZnO. Growth on Au(111) has been regarded as a well-established route to the 2D ZnO layers while controlled growth of uniform ZnO nanostructures remains as a challenge. Here, reactive deposition of Zn in O3 and NO2 was employed, which is investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). We demonstrate that uniform ZnO monolayer nanoislands and films can be obtained on Au(111) using O3 and uniform ZnO bilayer nanoislands and films form on Au(111) using NO2, respectively. Formation of atomic oxygen overlayers on Au(111) via exposure to O3 is critical to the formation of uniform ZnO monolayer nanostructures atop. Near ambient pressure XPS studies revealed that nearly full hydroxylation occurs on monolayer ZnO structures upon exposure to near ambient pressure water vapor or atomic hydrogen species, while partial surface hydroxylation happens on bilayer ZnO under the same gaseous exposure conditions.

Infrared Spectroscopy of Ammonia on Iron: Adsorption, Synthesis, and the Influence of Oxygen

We report on reflection absorption infrared spectroscopy (RAIRS) investigations into the influence of oxygen on the surface chemistry of NH3 on Fe{111}. Preadsorption of oxygen is found to strengthen the interaction between ammonia and the surface, albeit at the lowest oxygen coverage examined the effect is only notable when the ammonia coverage becomes high. At higher oxygen coverage, the same effect is observed even with very low ammonia coverage. In cases where the oxygen overlayer is ordered, the effects are seen quite clearly, but for disordered overlayers, the surface heterogeneity makes assignment of absorption features more difficult; nevertheless, comparison with the ordered examples allows us to identify the same underlying behavior. When potassium is coadsorbed with oxygen, the effect on ammonia adsorption is three-fold, including potassium-dominated and oxygen-dominated features, alongside features that suggest either canceling or absent influences from the atomic adsorbates. Synthesis of ammo...

Selective Catalytic Ammonia Oxidation to Nitrogen by Atomic Oxygen Species on Ag(111)

Ammonia-selective catalytic oxidation was studied on the planar Ag(111) single-crystal model catalyst surface under ultra-high-vacuum (UHV) conditions. A variety of oxygen species were prepared via ozone decomposition on pristine Ag(111). Surface coverages of oxygen species were quantified by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy techniques. Exposure of ozone on Ag(111) at 140 K led to a surface atomic oxygen (Oa) overlayer. Low-energy electron diffraction experiments revealed that annealing of this atomic oxygen-covered Ag(111) surface at 473 K in UHV resulted in the formation of ordered oxide surfaces (Oox) with p(5×1) or c(4×8) surface structures. Ammonia interactions with O/Ag(111) surfaces monitored by temperature-programmed reaction spectroscopy indicated that disordered surface atomic oxygen selectively catalyzed N–H bond cleavage, yielding mostly N2 along with minor amounts of NO and N2O. Higher coverage O/Ag(111) surfaces, whose structure was tentatively ass...

Mesoscopic organization of cobalt thin films on clean and oxygen-saturated Fe(001) surfaces

The different morphologies of Co films grown on either the clean Fe(001) surface and the oxygen-saturated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ substrate are investigated by means of scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory. The considered Co coverage range extends beyond the thickness at which layer-by-layer growth is destabilized by plastic deformations induced by the relaxation of the strain accumulated in the film. Our findings indicate that the oxygen overlayer of the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface floats on top of the growing Co film and strongly influences both the Co nucleation process and the film structural evolution. The layer-dependent islands nucleation of Co films grown on clean Fe(001) substrates, recently associated with a thickness-dependent adatom mobility [A. Picone et al., Phys. Rev. Lett. 113, 046102 (2014)], is found to be suppressed by the oxygen overlayer. The latter also significantly delays the layer-by-layer instability with respect to the oxygen-free growth. Furthermore, the body-centered-tetragonal/hexagonal-close-packed transition is not observed in the case of Co/Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ sample, replaced by the development of highly ordered surface undulations. These form a mesoscopic square pattern with the sides aligned to the $\mathrm{Fe}\ensuremath{\langle}110\ensuremath{\rangle}$ directions, while the surface atomic structure retains the square $p(1\ifmmode\times\else\texttimes\fi{}1)$ symmetry in registry with the substrate. Such undulations are likely generated by a highly ordered array of interfacial misfit dislocations running along the $\mathrm{Fe}\ensuremath{\langle}110\ensuremath{\rangle}$ directions.

Adsorption and thermal chemistry of formic acid on clean and oxygen-predosed Cu(110) single-crystal surfaces revisited

The thermal chemistry of formic acid on clean and oxygen-predosed Cu(110) single-crystal surfaces was studied under ultrahigh-vacuum (UHV) conditions by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Key results reported in the past were confirmed, including the partial switchover from H2 to H2O desorption upon oxygen addition on the surface and the development of a second decomposition regime at 420K, in addition to the one observed at 460K on the clean substrate. In addition, new observations were added, including the previously missed desorption of H2 at 420K and the existence of a normal kinetic isotope effect in both TPD peaks. Peak fitting of the XPS data afforded the identification of an asymmetric geometry for the formate intermediate, which was established to form by 200K, and the presence of coadsorbed molecular formic acid up to the temperatures of decomposition, probably in a second layer and held by hydrogen bonding. Quantitative analysis of the TPD data indicated a one-to-one correspondence between the increase in oxygen coverage beyond θO=0.5 ML and a decrease in formic acid uptake that mainly manifests itself in a decrease in the decomposition seen in the 460K TPD peak. All these observations were interpreted in terms of a simple decomposition mechanism involving hydrogen abstraction from adsorbed formate species, possibly aided by coadsorbed oxygen, and a change in reaction activation energy as a function of the structure of the oxygen overlayer, which reverts from a O-c(6×2) structure at high oxygen coverages to the O-(2×1) order seen at θO=0.5 ML.

Greatly enhanced H2S sensitivity using defect-rich titanium oxide films

Defect-rich titanium oxide films revealed overlayers of chemi-sorbed oxygen, producing a significant enhancement in sensitivity towards H2S gas with greater stability, better selectivity and quicker recovery.

Influence of oxygen and hydrogen adsorption on the magnetic structure of an ultrathin iron film on an Ir(001) surface

We present a detailed ab initio study of the electronic structure and magnetic order of an Fe monolayer on the Ir(001) surface covered by adsorbed oxygen and hydrogen. The results are compared to the clean Fe/Ir(001) system, where recent intensive studies indicated a strong tendency towards an antiferromagnetic order and complex magnetic structures. The adsorption of an oxygen overlayer significantly increases interlayer distance between the Fe layer and the Ir substrate, while the effect of hydrogen is much weaker. We show that the adsorption of oxygen (and also of hydrogen) leads to a $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic order of the Fe moments, which is also supported by an investigation based on a disordered local moment state. Simulated scanning tunneling images using the simple Tersoff-Hamann model hint that the proposed $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic order could be detected even by nonmagnetic tips.

First-Principles Study of the Formamide Adsorption to the Oxygen-Covered (0001) Surface of Ruthenium

Density functional theory has been used to study the adsorption of formamide over the (0001) surface of ruthenium metal with a coadsorbed oxygen (1 × 2) overlayer. Molecular adsorption is found to be energetically preferred and in the case of a perfectly ordered oxygen overlayer proceeds mainly through hydrogen bonding between the amino hydrogen and surface oxygen as well as some weak coordinate bonding between the carbonyl oxygen and surface ruthenium. The most energetically favored dissociative mode is found to involve the transfer of a proton to metal HCP sites and coordinate bonding between both the formamide N and O atoms and surface Ru. The resulting adsorption energies are found to be too small to explain experimental desorption temperatures but may be improved somewhat by the inclusion of an empirical dispersion correction. The effect of vacancies and disorder in the oxygen overlayer on formamide adsorption has also been investigated. It is found that disorder in the oxygen overlayer yields higher...

A density functional theory study of ordered oxygen overlayers on Ir(100)

Using density-functional theory (DFT) calculations, we study the adsorption of oxygen on Ir(100) as a function of coverage up to 1.0 monolayer and compare the results with available findings for other systems. The p(1 × 2)-O overlayer is shown to be most favorable on Ir(100), in good agreement with experimental findings. The stability of oxygen overlayers and induced work function changes are analyzed. Coverage-dependent modifications of the surface d-band electronic structures are discussed in detail and we find a near-linear correlation between the surface d-band center and the d-band width for the O/Ir(100). The dependence of the adsorption energy on the d-band center exhibits some exceptions in comparison with that of O/Pt(111).

The presence of a (1 × 1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 × 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.

Oxygen-induced effects on the morphology of the Fe(001) surface in out-of-equilibrium conditions

The oxygen-induced effects on the morphology of out-of-equilibrium iron surfaces have been studied by means of scanning tunneling microscopy and spectroscopy applied to either the Fe(001) or the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface. With respect to the pristine Fe(100) surface, step coalescence is observed upon oxygen exposure when the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface is formed, with a final morphology characterized by wide terraces separated by steps bunches. We have also investigated the surfactant properties of the chemisorbed oxygen by comparing Fe/Fe(001) and Fe/Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O homoepitaxy and analyzing the influence of the oxygen overlayer on the growth kinetics. The intralayer mass transport is found to be unaffected by the presence of oxygen, suggesting oxygen-enhanced interlayer mass transport as a mechanism able to explain the surfactant effect. We attribute both phenomena to the presence of a strained FeO overlayer.

The mystery of gold's chemical activity: local bonding, morphology and reactivity of atomic oxygen

Recently, gold has been intensely studied as a catalyst for key synthetic reactions. Gold is an attractive catalyst because, surprisingly, it is highly active and very selective for partial oxidation processes suggesting promise for energy-efficient "green" chemistry. The underlying origin of the high activity of Au is a controversial subject since metallic gold is commonly thought to be inert. Herein, we establish that one origin of the high activity for gold catalysis is the extremely reactive nature of atomic oxygen bound in 3-fold coordination sites on metallic gold. This is the predominant form of O at low concentrations on the surface, which is a strong indication that it is most relevant to catalytic conditions. Atomic oxygen bound to metallic Au in 3-fold sites has high activity for CO oxidation, oxidation of olefins, and oxidative transformations of alcohols and amines. Among the factors identified as important in Au-O interaction are the morphology of the surface, the local binding site of oxygen, and the degree of order of the oxygen overlayer. In this Perspective, we present an overview of both theory and experiments that identify the reactive forms of O and their associated charge density distributions and bond strengths. We also analyze and model the release of Au atoms induced by O binding to the surface. This rough surface also has the potential for O(2) dissociation, which is a critical step if Au is to be activated catalytically. We further show the strong parallels between product distributions and reactivity for O-covered Au at low pressure (ultrahigh vacuum) and for nanoporous Au catalysts operating at atmospheric pressure as evidence that atomic O is the active species under working catalytic conditions when metallic Au is present. We briefly discuss the possible contributions of oxidants that may contain intact O-O bonds and of the Au-metal oxide support interface in Au catalysis. Finally, the challenges and future directions for fully understanding the activity of gold are considered.

Water-induced surface reconstruction of oxygen(2×1)covered Ru(0001)

Low-temperature scanning tunneling microscopy and density-functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with $(2\ifmmode\times\else\texttimes\fi{}1)$ periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the $\ensuremath{\sim}200\text{ }\text{meV}$ water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a $(4\ifmmode\times\else\texttimes\fi{}2)$ superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original $(2\ifmmode\times\else\texttimes\fi{}1)$. This stable structure is not recovered until after heating to temperatures close to 260 K.

Oxidation of NO with O2 on Pt(111) and Pt(321) Large Single Crystals

The kinetics of the oxidation of NO by O(2) was studied on 1 cm diameter single crystals, Pt(111) and Pt(321), at atmospheric pressure. The surface of the (321) crystal is composed of 20% kink, 20% step, and 60% terrace atoms and simulates small 1-3 nm size Pt particles on supported catalysts, while the (111) surface simulates the most stable plane found on large, >5 nm, particles. The turnover rates (TORs), that is, rate normalized by the exposed platinum, on the two single crystals differ by less than a factor of 2 over the range of conditions studied and are also similar to the TOR on a supported catalyst with an average particle size of 9 nm. Both surfaces show a dynamic kinetic behavior as evidenced by a change in the apparent activation energy and reaction orders as a function of reaction conditions. The oxygen coverage after initial rate experiments on Pt(111) was 0.6 monolayer (ML) on average which is similar to that measured previously by in situ X-ray photoelectron spectroscopy (XPS) under similar conditions. This oxygen overlayer, which is likely controlled by the relative presence of NO and NO(2), inhibits O(2) dissociation but lowers the binding energy of reactants enough to allow the catalysis. Long-term stability studies on Pt(111) correlate catalyst deactivation with irreversibly bound oxygen on the surface at coverages over 1 ML, as measured after reaction. Ex situ Auger electron spectroscopy (AES) and XPS results suggest that the surface defect sites on Pt(321) begin to oxidize relative to atoms on the (111) plane at lower NO(2) to NO ratios.

Tunable Metallicity of theLa5/8Ca3/8MnO3(001)Surface by an Oxygen Overlayer

We studied the surface structure of La_{5/8}Ca_{3/8}MnO_{3}(001) thin films using in situ scanning tunneling microscopy (STM). Atomically resolved STM images reveal that a (sqrt[2]xsqrt[2])R45;{ degrees } reconstructed surface and a (1x1) surface can be converted back and forth through adsorption and desorption of oxygen at the surface. The electrical properties of the surfaces are investigated by scanning tunneling spectroscopy. I-V curves clearly show that the presence of an oxygen overlayer renders the surface insulating while the (1x1) surface without the oxygen overlayer is metallic.

Morphological Instability of Metallic Surfaces: Adsorbate Induced Nanoscale Faceting

AbstractHeterogeneous catalysis is a field of major interest in the surface science and development of new supported model catalysts with a narrow size distribution. The possibility to create narrow size distributions is to use the faceted surface structures as self assembled templates on which metal nanoclusters can grow. In the present study, new aspects of adsorbate induced faceting and nanoscale phenomena on adsorbate-covered metallic surfaces are studied on atomically-rough morphologically unstable metallic fcc (Rh) and hcp (Re) surfaces. Formation of oxygen induced faceting of atomically-rough Rh (210) surface has been studied and characterized by means of LEED, STM and AES. The LEED studies confirm the formation of three sided faceted (nanoscale pyramidal structure) Rh(210) surface when the oxygen covered Rh surface annealed to temperature higher than 550K. The facet orientations of the nanopyramid are characterized as two {731} face and a reconstructed (110) face. The excess oxygen overlayer can be removed from pyramidal faceted Rh(210) surface via catalytic reaction at low temperature using CO oxidation or H2 reaction while preserving the pyramidal structure. The average size of the nanopyramids is observed to be dependent on annealing temperature and vary from 12nm to 21nm. The atomically resolved STM images confirm the LEED observations and also revealed that (110) face of nanopyramids exhibits various reconstructions (1×n, n = 2-4) depending on oxygen coverage. Further, oxygen induced nanoscale faceted Re (12-31) surface has been formed which consist of nano-trenches (two sided ridges) with facets in (11-21) and (01-10) direction. These two- sided faceted Re (12-31) surface is used as a template to grow gold nanostructures. Under controlled growth conditions, the gold induced nanostructures are formed on oxygen induced nano-trenched Re (12-31) surface. It is observed that gold grows in the form of islands on the faceted rhenium substrate i.e. at lower coverages (≥0.8ML), it forms 2D islands whereby for higher coverages 3D islands are formed on top of the nano-ridges. The surfaces morphology of the gold covered faceted Re surface changes drastically on annealing, for the temperature >870 K, gold atoms wet the rhenium template while annealing to higher temperature (˜970K) led to the development of a three sided nanopyramids. On further annealing to higher temperatures (>1100K) cause the complete destruction of the faceted structure. LEED and STM studies revealed that the gold induced three sided nanopyramids consists of two faces of original two sided ridges with (11-20), (01-10) orientation and an additional facet in (12-32) direction. The high resolution STM images revealed that the (12-32) facet of the nanopyramid is fully decorated with single atomic zigzag gold nanochain. These faceted surfaces can be potential template for heterogeneous catalysis and development of new supported model catalysts with a narrow size distribution.

Molecular dynamics of hydrogen dissociation on an oxygen covered Pt(111) surface

The dissociation of hydrogen on a Pt(111) surface covered with a p(2 x 2) oxygen phase was investigated using quasiclassical, six-dimensional molecular dynamics. The potential energy surface (PES) used in these simulations was obtained by an iterative novelty sampling algorithm. Compared to molecular beam experiments performed under similar conditions, the simulations give an accurate prediction of the reaction probability via a direct dissociation pathway. When compared to previously reported reaction probability curves for the clean Pt(111) surface, we find that the presence of an oxygen overlayer inhibits the direct pathway to hydrogen dissociation. This inhibition is a function of incident energy and cannot be described by a simple site blocking model. An indirect pathway to dissociation, which was observed in experiments, is not properly captured by the PES. Spatially resolved "reaction maps" indicate that the preferred site for hydrogen dissociation on an oxygen covered Pt surface is the top site of the Pt atom farthest from the adsorbed oxygen atom.

Investigation of the Shockley surface state on clean and air-exposed Au (1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) carried out in vacuum and air were used to study the electronic structure of the Au (1 1 1) surface in the range of 0.0–0.7 eV below the Fermi level. The STS experiment carried out in UHV showed the existence of the Shockley surface state (SS) located 0.48 eV below the Fermi level. STS carried out in air showed strong local maximum located 0.35 eV below the Fermi level. This maximum was ascribed to the SS shifted toward lower energy due to carbon and oxygen overlayer. To confirm that the SS could exist on the sample exposed to air we did ultraviolet photoemission spectroscopy (UPS) experiment on air-treated and clean Au (1 1 1). Our results suggest that the SS position initially measured at 0.38 eV below the Fermi level was shifted to 0.27 eV after air treatment. Additionally, the level of contamination was measured using X-ray photoelectron spectroscopy (XPS).