Effect of Surface Orientation on Methanol Adsorption and Thermally Induced Structural Transformations on Copper Surfaces(2021) The Journal of Physical Chemistry C. Abstract
Molecular adsorption of methanol on Cu(111), Cu(100), and Cu(110) surfaces in the 90–200 K temperature range is studied by a combination of infrared (IR) spectroscopy, thermal desorption analysis, and low-energy electron diffraction. Our results reveal the occurrence of a structural transformation in the H-bonded methanol assembly following heating from 90 to 120 K and demonstrate the effect of Cu surface orientation on the produced H-bonded structures, as well as thermal stability and ordering. At 90 K, the IR spectra indicate that similar H-bonded structures, presumably linear chains, are formed on all three surfaces. However, on Cu(111) the chains are assembled in ordered domains, whereas on Cu(100) and Cu(110) the chains are disordered. Heating to 120–130 K causes prominent changes in the IR spectrum of methanol on all surfaces, but there are significant differences between Cu(111) and the two other surfaces. We believe that such differences originate from different H-bonded methanol structures obtained on each surface after thermal annealing. On Cu(111) we suggest that cyclic structures (probably hexamers) are prevalent, whereas on Cu(100) and Cu(110) both cyclic and chain structures may coexist. The H-bonded structures produced at 120 K exhibit no long-range order on all three surfaces and show stronger adsorption (higher desorption temperate) on Cu(111). The higher stability and ordering (only at low temperatures) of adsorbed methanol on Cu(111) are attributed to the matching between the geometry of H-bonded methanol clusters and the surface structure.
(2021) Chemical Reviews. 121, 2, p. 962-1006 Abstract
This is a Review of recent studies on surface structures of crystalline materials in the presence of gases in the mTorr to atmospheric pressure range, which brings surface science into a brand new direction. Surface structure is not only a property of the material but also depends on the environment surrounding it. This Review emphasizes that high/ambient pressure goes hand-in-hand with ambient temperature, because weakly interacting species can be densely covering surfaces at room temperature only when in equilibrium with a sufficiently high gas pressure. At the same time, ambient temperatures help overcome activation barriers that impede diffusion and reactions. Even species with weak binding energy can have residence lifetimes on the surface that allow them to trigger reconstructions of the atomic structure. The consequences of this are far from trivial because under ambient conditions the structure of the surface dynamically adapts to its environment and as a result completely new structures are often formed. This new era of surface science emerged and spread rapidly after the retooling of characterization techniques that happened in the last two decades. This Review is focused on the new surface structures enabled particularly by one of the new tools: high-pressure scanning tunneling microscopy. We will cover several important surfaces that have been intensely scrutinized, including transition metals, oxides, and alloys.
Identifying the catalyst chemical state and adsorbed species during methanol conversion on copper using ambient pressure X-ray spectroscopies(2020) Physical Chemistry Chemical Physics. 22, 34, p. 18806-18814 Abstract
Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can generate a hydrogen-containing gas mixture. Understanding the chemical state of the catalyst over which these reactions occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions. Here we study polycrystalline Cu foils using ambient pressure X-ray spectroscopies to reveal the Cu oxidation state and identify the adsorbed species during partial oxidation (CH3OH + O2), steam reforming (CH3OH + H2O), and autothermal reforming (CH3OH + O2 + H2O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen-rich conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu2O. We rationalise this behaviour on the basis of the shift in equilibrium of the CH3OH* + O* ⇌ CH3O* + OH* reaction step caused by the addition of H2O.
(2020) Journal of the American Chemical Society. 142, 18, p. 8312–8322 Abstract[All authors]
The reaction of CO and O2 with submonolayer and multilayer CoOx films on Pt(111), to produce CO2, was investigated at room temperature in the mTorr pressure regime. Using operando ambient pressure X-ray photoelectron spectroscopy and high pressure scanning tunneling microscopy, as well as density functional theory calculations, we found that the presence of oxygen vacancies in partially oxidized CoOx films significantly enhances the CO oxidation activity to form CO2 upon exposure to mTorr pressures of CO at room temperature. In contrast, CoO films without O-vacancies are much less active for CO2 formation at RT, and CO only adsorbed in the form of carbonate species that are stable up to 260 °C. On submonolayer CoOx islands, the carbonates form preferentially at island edges, deactivating the edge sites for CO2 formation, even while the reaction proceeds inside the islands. These results provide a detailed understanding of CO oxidation pathways on systems where noble metals such as Pt interact with reducible oxides.
Carbon Monoxide Adsorption on Manganese Oxide/Cobalt: an Ambient Pressure X-ray Photoelectron Spectroscopy Study(2020) Journal of Physical Chemistry C. 124, 6, p. 3557-3563 Abstract
MnOx enhances the catalytic activity of Co during Fischer–Tropsch synthesis, increases selectivity toward C5+ products, and decreases methane formation. These desired traits are thought to result from a higher CO adsorption energy and, thus, potentially higher CO coverage. To investigate this, ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to probe the CO coverage of Co foil with increasing MnOx amounts at room temperature. The technique permits the quantification of chemical species on a surface from ultrahigh vacuum to the mbar pressure regime. CO was found to adsorb at both Co and MnOx sites. The electronic effect which results in the promotion of CO adsorption also promotes the adsorption of OH groups from background water vapor pressures. This process competes with CO adsorption, despite the water pressure being ∼8 orders of magnitude lower than the CO pressure at 1 mbar. Because water is a product of Fischer–Tropsch synthesis, this result has relevance to the understanding of MnOx as a promoter. This finding highlights the importance of considering unexpected contributions of background impurities in APXPS and other ambient pressure surface science techniques.
(2019) Energy Storage Materials. 20, p. 139-145 Abstract
Understanding interactions at the interfaces of carbon with ionic liquids (ILs) is crucially beneficial for the diagnostics and performance improvement of electrochemical devices containing carbon as active materials or conductive additives in electrodes and ILs as solvents or additives in electrolytes. The interfacial interactions of three typical imidazolium-based ILs, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (AMImTFSI) ILs having ethyl (C2), butyl (C4) and octyl (C8) chains in their cations, with highly oriented pyrolytic graphite (HOPG) were studied in-situ by electrochemical scanning tunneling microscopy (EC-STM). The etching of HOPG surface and the exfoliation of graphite/graphene flakes as well as cation intercalation were observed at the HOPG/C2MImTFSI interface. The etching also takes place in C4MImTFSI at −1.5 V vs Pt but only at step edges with a much slower rate, whereas C8MIm+ cations adsorbs strongly on the HOPG surface under similar conditions with no observable etching or intercalation. The EC-STM observations can be explained by the increase in van der Waals interaction between the cations and the graphite surface with increasing length of alkyl chains.
(2019) Journal of Physical Chemistry C. 123, 13, p. 8171-8176 Abstract
Scanning tunneling microscopy (STM) has proved to be a prime tool to characterize the atomic structure of crystal surfaces under UHV conditions. With the development of high-pressure scanning tunneling microscopy (HP-STM), the scope of this technique has been largely extended, as new structures were found to occur under gas phase chemical potentials achieved under ambient conditions. Particularly interesting is the substantial restructuring of initially flat and stable surfaces into new orientations by formation of nanoclusters. Here we discuss the possible generality of this phenomenon by analyzing cases where atomically flat surfaces of certain transition metals undergo such changes in the presence of CO at room temperature (RT) while some remain unchanged. From our analysis we argue that such changes can be predicted from thermodynamic data published in the literature, particularly from the difference in adsorption energy on low- and high-coordination sites, like terrace and step sites, which can be obtained from thermal desorption spectroscopy (TDS) measurements, and possibly also from theoretical calculations. For the vicinal surfaces with high Miller indices, changes in the repulsive elastic interactions between the ordered steps due to adsorbates may also play an important role.
(2019) Nature Catalysis. 2, 1, p. 78-85 Abstract
Bimetallic and multi-component catalysts typically exhibit composition-dependent activity and selectivity, and when optimized often outperform single-component catalysts. Here we used ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ and ex situ transmission electron microscopy (TEM) to elucidate the origin of composition dependence observed in the catalytic activities of monodisperse CoPd bimetallic nanocatalysts for CO oxidation. We found that the catalysis process induced a reconstruction of the catalysts, leaving CoOx on the nanoparticle surface. The synergy between Pd and CoOx coexisting on the surface promotes the catalytic activity of the bimetallic catalysts. This synergistic effect can be optimized by tuning the Co/Pd ratios in the nanoparticle synthesis, and it reaches a maximum at compositions near Co0.24Pd0.76, which achieves complete CO conversion at the lowest temperature. Our combined AP-XPS and TEM studies provide direct observation of the surface evolution of the bimetallic nanoparticles under catalytic conditions and show how this evolution correlates with catalytic properties.
(2018) Journal of the American Chemical Society. 140, 21, p. 6575-6581 Abstract
We studied the structure of the copper–cobalt (CuCo) surface alloy, formed by Co deposition on Cu(110), in dynamic equilibrium with CO. Using scanning tunneling microscopy (STM), we found that, in vacuum at room temperature and at low Co coverage, clusters of a few Co atoms substituting Cu atoms form at the surface. At CO pressures in the Torr range, we found that up to 2.5 CO molecules can bind on a single Co atom, in carbonyl-like configurations. Based on high-resolution STM images, together with density functional theory calculations, we determined the most stable CuCo cluster structures formed with bound CO. Such carbonyl-like formation manifests in shifts in the binding energy of the Co core-level peaks in X-ray photoelectron spectra, as well as shifts in the vibrational modes of adsorbed CO in infrared reflection absorption spectra. The multiple CO adsorption on a Co site weakens the Co–CO bond and thus reduces the C–O bond scission probability. Our results may explain the different product distribution, including higher selectivity toward alcohol formation, when bimetallic CuCo catalysts are used compared to pure Co.
(2018) Encyclopedia of Interfacial Chemistry. Wandelt K.(eds.). 1st ed. Vol. 1. p. 645-657 Abstract
Low Miller-index copper surfaces break up into nanoclusters in the presence of reactant gases such as CO or CO2 in the Torr pressure range at room temperature. Such an atomistic phenomenon has a great significance in heterogeneous catalysis as it directly affects the electronic structure and thereby the chemical properties of the surface. The reason behind clustering of such compact surfaces is the high difference in adsorption energy at the low-coordinated Cu atoms (steps, kinks) and high-coordinated Cu atoms (terraces). Unlike CO and CO2, gas-phase methanol does not break up Cu into clusters because methoxy can already adsorb strongly on Cu terraces. These observations were made possible by the recent developments of high-pressure scanning tunneling microscopy and complementary spectroscopy techniques like ambient pressure X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy. This article provides scanning tunneling microscope images, corroborating spectra, and density functional theory calculations to summarize all the recent findings in this field.
Structure of the Clean and Oxygen-Covered Cu(100) Surface at Room Temperature in the Presence of Methanol Vapor in the 10–200 mTorr Pressure Range(2018) Journal of Physical Chemistry B. 122, 2, p. 548-554 Abstract
Using ambient pressure X-ray photoelectron spectroscopy (APXPS) and high pressure scanning tunneling microscopy (HPSTM), we show that in equilibrium with 0.01–0.2 Torr of methanol vapor, at room temperature, the Cu (100) surface is covered with methoxy species forming ac (2× 2) overlayer structure. In contrast, no methoxy is formed if the surface is saturated with an ordered oxygen layer, even when the methanol pressure is 0.2 Torr. At oxygen coverages below saturation, methanol dissociates and reacts with the atomic oxygen, producing methoxy and formate on the surface, and formaldehyde that desorbs to the gas phase. Unlike the case of pure carbon monoxide and carbon dioxide, methanol does not induce the restructuring of the Cu (100) surface. These results provide insight into catalytic anhydrous production of aldehydes.
(2017) Physical Chemistry Chemical Physics. 19, 24, p. 16251-16256 Abstract
Templating insulating surfaces at the nanoscale is an interesting prospect for applications that involve the adsorption of molecules or nanoparticles where electronic decoupling of the adsorbed species from the substrate is needed. In this study, we present a method to structure alkali halide surfaces at the nanoscale using a combination of low temperature plasma exposure and annealing, and characterize the surfaces by atomic force microscopy. We find that nanostructurating can be controlled by the duration of the exposure, the atomic mass of the plasma gas and the subsequent step-by-step annealing process. In contrast to previous studies with electron or high energy (few keV) ion irradiation, our approach of employing moderate particle energy (10-15 eV Ar+ or He+ ions) results in fine nanostructuring at length scales of nanometers and even single atom vacancies.
Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Cobalt Foil Model Catalyst under CO, H-2, and Their Mixtures(2017) ACS Catalysis. 7, 2, p. 1150-1157 Abstract
Ambient-pressure X-ray photoelectron spectroscopy (XPS) was used to investigate the reactions of CO, H-2, and their mixtures on Co foils. We found that CO adsorbs molecularly on the clean Co surface and desorbs intact in vacuum with increasing rate until similar to 90 degrees C where all CO desorbs in seconds. In equilibrium with 100 mTorr gas, CO dissociates above 120 degrees C, leaving carbide species on the surface but no oxides, because CO efficiently reduces the oxides at temperatures similar to 100 degrees C lower than H-2. Water as impurities or produced by reaction of CO and H-2 efficiently oxidizes Co even at room temperature. Under 97:3 CO/H-2 mixture and with increasing temperatures, the Co surface becomes more oxidized and covered by hydroxyl groups until similar to 150 degrees C where surface starts to get reduced, accompanied by carbide accumulation indicative of CO dissociation. A similar trend was observed for 9:1 and 1:1 mixtures, but surface reduction begins at higher temperatures.
A study of the O/Ag(111) system with scanning tunneling microscopy and x-ray photoelectron spectroscopy at ambient pressures(2016) Surface Science. 652, p. 51-57 Abstract
The interaction of O-2 with the Ag(111) surface was studied with scanning tunneling microscopy (STM) in the pressure range from 10(-9) Torr to 1 atm at room temperature and with X-ray photoelectron spectroscopy (XPS) up to 0.3 Torr O-2 in the temperature range from RT to 413 K. STM images show that the Ag(111) surface topography is little affected in regions with large flat terraces, except for the appearance of mobile features due to oxygen atoms at pressures above 0.01 Torr. In regions where the step density is high, the surface became rough under 0.01 Torr of O-2, due to the local oxidation of Ag. Various chemical states of oxygen due to chemisorbed, oxide and subsurface species were identified by XPS as a function of pressure and temperature. The findings from the STM images and XPS measurements indicate that formation of an oxide phase, the thermodynamically stable form at room temperature under ambient O-2 pressure, is kinetically hindered in the flat terrace areas but proceeds readily in regions with high-step density. (C) 2016 Elsevier B.V. All rights reserved.
(2016) Surface Science. 651, p. 210-214 Abstract
The bulk terminated Cu(100) surface becomes unstable in the presence of CO at room temperature when the pressure reaches the mbar range. Scanning tunneling microscopy images show that above 0.25 mbar the surface forms nanoclusters with CO attached to peripheral Cu atoms. At 20 mbar and above 3-atom wide one-dimensional nanoclusters parallel to <001 > directions cover the surface, with CO on every Cu atom, increasing in density up to 115 mbar. Density functional theory explains the findings as a result of the detachment of Cu atoms from step edges caused by the stronger binding of CO relative to that on flat terraces. (C) 2016 Elsevier B.V. All rights reserved.
Dissociative Carbon Dioxide Adsorption and Morphological Changes on Cu(100) and Cu(111) at Ambient Pressures(2016) Journal of the American Chemical Society. 138, 26, p. 8207-8211 Abstract
Ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling microscopy (HPSTM) were used to study the structure and chemistry of model Cu(100) and Cu(111) catalyst surfaces in the adsorption and dissociation of CO2. It was found that the (100) face is more active in dissociating CO2 than the (111) face. Atomic oxygen formed after the dissociation of CO2 poisons the surface by blocking further adsorption of CO2. This "self-poisoning" mechanism explains the need to mix CO into the industrial feed for methanol production from CO2, as it scavenges the chemisorbed O. The HPSTM images show that the (100) surface breaks up into nanoclusters in the presence of CO2 at 20 Torr and above, producing active kink and step sites. If the surface is precovered with atomic oxygen, no such nanoclustering occurs.
(2016) Langmuir. 32, 22, p. 5526-5531 Abstract
The supramolecular self-assembly of copper(II) octaethyl-porphyrin (CuOEP) and octaethylporphyrin (H2OEP) on graphitic surfaces immersed in organic solvents (dichlorobenzene, dodecane) is studied using scanning tunneling microscopy (STM) and Raman spectroscopy. STM reveals that the self-assembled structure of CuOEP in 1,2-dichlorobenzene is significantly altered by dissolved oxygen within the solvent. Raman spectroscopy reveals that the presence of the oxygen alters the molecule substrate interaction, which is attributed to the adsorption of oxygen on the Cu center of the CuOEP, which is facilitated by electron transfer from the graphitic surface. Such oxygen-induced changes are not observed for H2OEP, indicating that the metal center of CuOEP plays a critical role. When the solvent is dodecane, we find that solvation effects dominate. CuOEP adsorbed on graphitic surfaces provides a model system relevant to the study of the transport and activation of oxygen by enzymes and other complexes.
(2016) Journal of Physical Chemistry Letters. 7, 9, p. 1622-1627 Abstract
Atmospheric pressure X-ray photoelectron spectroscopy (XPS) is demonstrated using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude. The graphene serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu2+ oxidation state is stable in O-2 (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO2, N-2, O-2) in the pressure range 10-1500 mbar including species with low photoionization cross sections (He, H-2). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect atmospheric pressure XPS based on this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis.
Structural Changes of Cu(110) and Cu(110)-(2 x 1)-O Surfaces under Carbon Monoxide in the Torr Pressure Range Studied with Scanning Tunneling Microscopy and Infrared Reflection Absorption Spectroscopy(2016) Journal of Physical Chemistry C. 120, 15, p. 8227-8231 Abstract
The atomic structure of the clean Cu(110) and the oxygen covered Cu(110) surfaces in the presence of carbon monoxide (CO) gas in the Torr pressure range at 298 K is studied using scanning tunneling microscopy (STM) and infrared reflection adsorption spectroscopy (IRRAS). We found that the initially clean surface reconstructs to form short rows of Cu atoms along the [1-10] direction separated by missing rows. The adsorbed CO molecules show two different C-O stretch vibration modes originating from molecules bound to Cu atoms with different coordination numbers, in the middle and at the end of the atomic rows. On the oxygen covered p(2 X 1) surface, adsorbed CO is observed only after removal of surface O atoms by reaction with CO. In the presence of 1:5 and 1:1 mixtures of O-2 and CO at 298 K, the p(2 X 1)-O reconstructed surface transforms into Cu2O, instead of reducing to metallic Cu.
Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review(2016) Topics in Catalysis. 59, 5-7, p. 405-419 Abstract
Understanding the reaction mechanism of various heterogeneous catalytic reactions is of fundamental importance in catalysis science. In the past, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have proved to be powerful surface-sensitive techniques to characterize surface reactions on model catalysts under UHV conditions. The recent development of high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has largely extended the application of these two excellent surface-sensitive imaging and electron spectroscopy techniques to a variety of catalytic systems under realistic conditions. In this mini review, we will review a series of catalytic systems studied by HP-STM and AP-XPS, including reactant coadsorption systems, coadsorption + reaction systems, and poisoned reaction systems. We will also illustrate one of the main difficulties in the practical execution of experiments where the initial surface cleanliness is easily compromised by the adsorption of adventitious contaminants. All of these examples will demonstrate that the combined use of HP-STM and AP-XPS can provide a deeper understanding of the structure and dynamics of reactant coadsorption on model catalysts, although great care has to been taken to maintain the cleanness of the in situ instrumentation.
Work function of few layer graphene covered nickel thin films measured with Kelvin probe force microscopy(2016) Applied Physics Letters. 108, 4, 041602. Abstract
Few layer graphene and graphite are simultaneously grown on a similar to 100 nm thick polycrystalline nickel film. The work function of few layer graphene/Ni is found to be 4.15 eV with a variation of 50 meV by local measurements with Kelvin probe force microscopy. This value is lower than the work function of free standing graphene due to peculiar electronic structure resulting from metal 3d-carbon 2p(pi) hybridization. (C) 2016 AIP Publishing LLC.
(2016) Science. 351, 6272, p. 475-478 Abstract
The (111) surface of copper (Cu), its most compact and lowest energy surface, became unstable when exposed to carbon monoxide (CO) gas. Scanning tunneling microscopy revealed that at room temperature in the pressure range 0.1 to 100 Torr, the surface decomposed into clusters decorated by CO molecules attached to edge atoms. Between 0.2 and a few Torr CO, the clusters became mobile in the scale of minutes. Density functional theory showed that the energy gain from CO binding to low-coordinated Cu atoms and the weakening of binding of Cu to neighboring atoms help drive this process. Particularly for softer metals, the optimal balance of these two effects occurs near reaction conditions. Cluster formation activated the surface for water dissociation, an important step in the water-gas shift reaction.
Catalyst Chemical State during CO Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy and Near Edge X-ray Adsorption Fine Structure Spectroscopy(2015) Journal of the American Chemical Society. 137, 34, p. 11186-11190 Abstract
The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O-2 pressure range of 10-300 mTorr at 298-413 K was studied in situ using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O-2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (similar to 1 nm) Cu2O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu2O film increases in thickness and exceeds the escape depth (similar to 3-4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO2 delta-, with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.
Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X-ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study(2015) Journal of Physical Chemistry C. 119, 26, p. 14669-14674 Abstract
The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01-0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface.
(2015) Nuclear Fusion. 55, 6, 063020. Abstract
To avoid reflectivity losses in ITER's optical diagnostic systems, on-site cleaning of metallic first mirrors via plasma sputtering is foreseen to remove deposit build-ups migrating from the main wall. In this work, the influence of aluminium and tungsten deposits on the reflectivity of molybdenum mirrors as well as the possibility to clean them with plasma exposure is investigated. Porous ITER-like deposits are grown to mimic the edge conditions expected in ITER, and a severe degradation in the specular reflectivity is observed as these deposits build up on the mirror surface. In addition, dense oxide films are produced for comparisons with porous films. The composition, morphology and crystal structure of several films were characterized by means of scanning electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction and secondary ion mass spectrometry. The cleaning of the deposits and the restoration of the mirrors' optical properties are possible either with a Kaufman source or radio frequency directly applied to the mirror (or radio frequency plasma generated directly around the mirror surface). Accelerating ions of an external plasma source through a direct current applied onto the mirror does not remove deposits composed of oxides. A possible implementation of plasma cleaning in ITER is addressed.
Spectroscopic ellipsometry on Si/SiO2/graphene tri-layer system exposed to downstream hydrogen plasma: Effects of hydrogenation and chemical sputtering(2015) Applied Physics Letters. 106, 1, 011904. Abstract
In this work, the optical response of graphene to hydrogen plasma treatment is investigated with spectroscopic ellipsometry measurements. Although the electronic transport properties and Raman spectrum of graphene change after plasma hydrogenation, ellipsometric parameters of the Si/SiO2/graphene tri-layer system do not change. This is attributed to plasma hydrogenated graphene still being electrically conductive, since the light absorption of conducting 2D materials does not depend on the electronic band structure. A change in the light transmission can only be observed when higher energy hydrogen ions (30 eV) are employed, which chemically sputter the graphene layer. An optical contrast is still apparent after sputtering due to the remaining traces of graphene and hydrocarbons on the surface. In brief, plasma treatment does not change the light transmission of graphene; and when it does, this is actually due to plasma damage rather than plasma hydrogenation. (C) 2015 AIP Publishing LLC.
Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO(2014) Journal of Physical Chemistry Letters. 5, 15, p. 2626-2631 Abstract
We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the dusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).[All authors]
(2014) Chemical Physics Letters. 609, p. 82-87 Abstract
In this work, carbon nanotubes (CNTs) are grown from Ni and Fe nanoparticles supported on a rough AlN surface. Although, identical experimental parameters are used during dewetting (island formation) via thermal treatment, Ni particles appear metallic and larger, whereas Fe particles are smaller and slightly oxidized. This difference in the nanoparticle chemical state and morphology reflects to CNTs during catalytic chemical vapor deposition in terms of their CNT growth mode and size: tip-growth mode for Ni catalyst with CNT diameters of up to 40 nm, whereas base-growth mode for Fe with CNT diameters typically less than 10 nm are observed. (C) 2014 Elsevier B.V. All rights reserved.
Morphological Changes of Tungsten Surfaces by Low-Flux Helium Plasma Treatment and Helium Incorporation via Magnetron Sputtering(2014) ACS applied materials & interfaces. 6, 14, p. 11609-11616 Abstract
The effect of helium on the tungsten microstructure was investigated first by exposure to a radio frequency driven helium plasma with fluxes of the order of 1 X 1019 m(-2) s(-1) and second by helium incorporation via magnetron sputtering. Roughening of the surface and the creation of pinholes were observed when exposing poly- and nanocrystalline tungsten samples to low-flux plasma. A coating process using an excess of helium besides argon in the process gas mixture leads to a porous thin film and a granular surface structure whereas gas mixture ratios of up to 50% He/Ar (in terms of their partial pressures) lead to a dense structure. The presence of helium in the deposited film was confirmed with glow-discharge optical emission spectroscopy and thermal desorption measurements. Latter revealed that the highest fraction of the embedded helium atoms desorb at approximately 1500 K. Identical plasma treatments at various temperatures showed strongest modifications of the surface at 1500 K, which is attributed to the massive activation of helium singly bond to a single vacancy inside the film. Thus, an efficient way of preparing nanostructured tungsten surfaces and porous tungsten films at low fluxes was found.
(2014) ACS Nano. 8, 6, p. 5932-5938 Abstract
Graphene was synthesized from pentacenequinone molecules on a Cu(111) surface using a three-step thermal treatment process: (1) self-assembly of a single layer molecular film at 190 degrees C, (2) formation of covalent bonding between adjacent molecules at intermediate temperatures, (3) thermal dehydrogenation and in-plane carbon diffusion at 600 degrees C. Transformation of the surface conformation was monitored with bimodal atomic force microscopy at the atomic scale and was corroborated with core-level X-ray photoelectron spectroscopy. A strong C=0 ... H-C hydrogen bonding involving the quinone moiety plays a key role in graphene growth, whereas conventional pentacene simply desorbs from the substrate during the same process. The most significant achievement of this proposed technique is obtaining graphene a couple of hundred degrees lower than standard techniques. Intrinsic defects due to carbon deficiency and the defects intentionally introduced by the microscope tip were also investigated with atomic-scale imaging.
(2014) Journal of Nuclear Materials. 446, 1-3, p. 106-112 Abstract
The behavior of rhodium film mirrors with different crystal structure and morphology toward a deuterium plasma is presented. The specular reflectivity of rhodium films was monitored before, during and after exposure. To understand the reflectivity behavior of the rhodium films during exposure, samples were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Crystal structure and morphology of rhodium films strongly affect the change of the specular reflectivity during deuterium plasma exposure. In particular, films with few nm crystallite size and granular-like morphology prevent the reflectivity degradation, probably as a consequence of the inhibition of rhodium deuteride sub-superficial layer formation. (C) 2013 Elsevier B.V. All rights reserved.
(2014) Journal Of Physics D-Applied Physics. 47, 2, 025302. Abstract
In this work, 100 nm gold films with -30 +/- 6 MPa residual compressive stress grown by the Volmer-Weber mechanism are exposed to low-flux, low-temperature hydrogen plasma. The films, which were free of any type of blisters prior to plasma treatment, exhibited plenty of buckling zones localized inside circular boundaries after the treatment. This is attributed to compressive stress exerted by the over-pressurized hydrogen gas at the trap zones in the film and at the coating interface. The geometrical parameters of the circular buckling zones indicate a compressive stress of -1.2 +/- 0.3 GPa. The findings reveal a serious concern for technological applications involving hydrogen plasma treatment of samples containing thin gold films, but from an optimistic perspective, suggest an efficient cleavage technique for such films. Several methods including reducing the ion impact energy, increasing the sample temperature and changing the substrate material are investigated to suppress hydrogen-induced buckling. Among these, reducing the impact energy of the ions appeared to be the only effective method.
(2014) Applied Physics Letters. 104, 4, 041910. Abstract
A graphene sample supported on SiO2 with pristine and plasma-hydrogenated parts is investigated by friction force microscopy. An initial contrast in friction is apparent between the two regions. A tip induced cleaning of the surface in the course of continuous scanning results in a very clean surface accompanied with a reduction of the friction force by a factor of up to 4. The contamination is adhering stronger to hydrogenated regions, but once cleaned, the frictional behavior is the same on pristine and hydrogenated graphene. Raman imaging demonstrates that the hydrogenation remains intact under the mechanical treatment. (C) 2014 AIP Publishing LLC.
Roughening and reflection performance of molybdenum coatings exposed to a high-flux deuterium plasma(2013) Nuclear Fusion. 53, 11, 113013. Abstract
Optical diagnostic systems of ITER are foreseen to include metallic, plasma-facing, electromagnetic radiation reflecting components called first mirrors (FMs). Molybdenum coatings are important candidates for these components. Depending on the local plasma parameters of the reactor, the mirrors may be under net erosion or deposition conditions. In this work, we exposed molybdenum coatings to a high-flux deuterium plasma in order to test their roughening limits under erosion conditions. The high energy of deuterium ions (500 eV on average) results in more vigorous roughening of the surface compared with lower energy ions (200 eV). Longer exposure (3x10(20) ions cm(-2)) of the 200 eV ions results in only a slightly increased roughness compared with shorter exposure (6.8x10(19) ions cm(-2)). Both phenomena match to the theory regarding roughening dynamics of physical sputtering. A comparison of results in this work with previous studies gives support to the hypothesis that roughening is flux and temperature dependent. Partial delamination of the coatings is observed upon exposure at room temperature, but not at an elevated temperature (200 degrees C). In summary, Mo coatings will remain functional in the ITER environment under the expected conditions. However, changes in the expected conditions such as 500 eV mean energy of impinging charge exchange neutrals or <100 degrees C surface temperature of the mirrors can lead to gradual or sudden failure of the coatings.[All authors]
Can aluminium or magnesium be a surrogate for beryllium: A critical investigation of their chemistry(2013) Fusion Engineering and Design. 88, 9-10, p. 1718-1721 Abstract
The use of beryllium is still an existing question according to the studies concerning the plasma-wall interactions which are expected to occur in ITER. Prediction of erosion and co-deposition processes for ITER is necessary for the design and the material choice of the first wall. In the current configuration, it is expected that co-deposited layers containing Be, tungsten and possibly carbon will be formed. However, the toxicity of Be limits its use in many experimental facilities around the world. Using aluminium or magnesium as Be replacements in laboratory experiments would solve this problem of toxicity and handling of Be mixed materials. A critical question which automatically arises is the relevance to use Al or Mg regarding the physical and chemical properties of both elements in comparison to the codeposited layers expected in ITER. This work provides a review of the chemical and physical properties of Al and Mg, in the respect of comparing these properties to those of Be. Thanks to the similarity of its electronegativity to Be, Al can successfully resemble Be in terms of formation of compounds, especially the oxides and possibly the hydrides. However, due to the difference in the nature of the bonding, Mg cannot be.a replacement for a possible hydride deposit formation. (C) 2013 Elsevier B.V. All rights reserved.
In situ evaluation of the reflectivity of molybdenum and rhodium coatings in an ITER-like mixed environment(2013) Journal of Nuclear Materials. 438, p. S852-S855 Abstract
Molybdenum and rhodium are foreseen to be utilized in ITER for the light reflecting, plasma facing components called first mirrors (FMs). In this work, the plasma and impurity conditions which FMs are expected to be subjected to were simulated experimentally, while monitoring their reflectivity. Experiments include deuterium plasma exposure with tungsten-carbon and tungsten-aluminum impurities, where aluminum was employed as a proxy for beryllium. The surface composition and morphology of the mirrors were characterized with XPS and SEM. When carbon was present in the plasma, the molybdenum surface became carbidized, while this effect was not observed for rhodium. Aluminum impurities were deposited as oxides, whereas tungsten was either oxidized or carbidized depending on the presence of carbon in the plasma. SEM results show the deposits to be amorphous. The mirrors in erosion conditions showed no critical decrease in the reflectivity, whereas the degradation was severe in net deposition conditions involving carbon. Cleaning techniques have to be developed for mirrors in deposition conditions, which should be part of ITER's routine operation. (C) 2013 Elsevier B. V. All rights reserved.
Laser damage thresholds of ITER mirror materials and first results on in situ laser cleaning of stainless steel mirrors(2013) Fusion Engineering and Design. 88, 5, p. 388-399 Abstract
A laser ablation system has been constructed and used to determine the damage threshold of stainless steel, rhodium and single-, poly- and nanocrystalline molybdenum in vacuum, at a number of wavelengths between 220 nm and 1064 nm using 5 ns pulses. All materials show an increase of the damage threshold with decreasing wavelength below 400 nm. Tests in a nitrogen atmosphere showed a decrease of the damage threshold by a factor of 2-3. Cleaning tests have been performed in vacuum on stainless steel samples after applying mixed Al/W/C/D coatings using magnetron sputtering. In situ XPS analysis during the cleaning process as well ex situ reflectivity measurements demonstrate near complete removal of the coating and a substantial recovery of the reflectivity. The first results also show that the reflectivity obtained through cleaning at 532 nm may be further increased by additional exposure to UV light, in this case 230 nm, an effect which is attributed to the removal of tungsten dust from the surface. (c) 2013 Elsevier B.V. All rights reserved.
Deuterium plasma exposure on rhodium: Reflectivity monitoring and evidence of subsurface deuteride formation(2013) Applied Surface Science. 273, p. 94-100 Abstract
The effects of low flux, low temperature deuterium plasma (LTP) exposure on nanocrystalline rhodium (Rh) films are investigated. The exposures do not cause any surface damage on the nanoscale and the specific electrical resistivity of the films remains invariant during exposures. However, the spectral reflectivity of Rh decreases during exposure and recovers very slowly during subsequent storage in high vacuum. This drop in the reflectivity can be associated with a formation of a subsurface rhodium deuteride (RhDx, x
(2013) Applied Physics Letters. 102, 7, 071602. Abstract
In this work, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of single layer graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793197][All authors]
(2012) Beilstein Journal of Nanotechnology. 3, p. 852-859 Abstract
Single- and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to a pure hydrogen low-temperature plasma (LTP). Characterizations include various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence-band structure, which suggests double-sided hydrogenation. With the scanning tunneling microscopy technique, various atomic-scale charge-density patterns were observed, which may be associated with different C-H conformers. Hydrogen-LTP-exposed graphene on SiO2 has a Raman spectrum in which the D peak to G peak ratio is over 4, associated with hydrogenation on both sides. A very low defect density was observed in the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen-LTP-exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing at over 1000 degrees C.
(2012) Review of Scientific Instruments. 83, 1, 013509. Abstract
An in situ spectroscopic reflectometry system has been built to investigate the evolution of the specular reflectivity spectrum of ITER first mirror samples during plasma exposure. Results are presented for three different types of molybdenum mirror samples that were exposed to deuterium plasma, including single crystalline, nanocrystalline, and polycrystalline molybdenum. The results show good agreement with ex situ measurements of the reflectivity spectrum before and after exposure and extend the results obtained in previous experiments. (C) 2012 American Institute of Physics. [doi:10.1063/1.3678640]
(2011) Nuclear Fusion. 51, 10, 103025. Abstract
Metallic first mirrors (FMs) are foreseen to play a crucial role for all optical diagnostics in ITER. It is highly important for the FMs to maintain a good reflectivity both in erosion and deposition zones in the harsh ITER environment. Molybdenum mirrors, which are important candidates for the FMs, exhibit a reflectivity spectrum different from that of bulk molybdenum after exposure to low temperature (4-5 eV) deuterium plasma. This difference is mainly due to the presence of deuterium and deuterium-induced defects in the metal. The results presented show that these reflectivity changes are similar for single and nanocrystalline molybdenum mirrors. Moreover, exposure of magnetron sputtered nanocrystalline molybdenum films to deuterium plasma revealed that after a certain deviation of the spectrum has been reached, the reflectivity remains constant upon further exposure. Exposures were carried out in a range of fluences corresponding to up to 18 ITER discharges in equatorial ports and 38 discharges in the upper ports in the first wall positions. Constant conditions of -200 V bias and 150 degrees C temperature were maintained on the samples. Further exposures performed in a tokamak result in reflectivity changes that are comparable to those obtained with deuterium plasma exposure. No mechanical damage, such as blistering and increase in roughness, is observed on the coated molybdenum films upon any of the mentioned exposures. The complex permittivity of the exposed molybdenum is determined from ellipsometry measurements and corroborated with core and valence level photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy and surface resistivity measurements.
(2011) Fusion Engineering and Design. 86, 9-11, p. 2593-2596 Abstract
Metallic mirrors are foreseen to play a crucial role for all optical diagnostics in ITER. Therefore, the development of reliable techniques for the production of mirrors which are able to maintain their optical properties in the harsh ITER environment is highly important. By applying magnetron sputtering and evaporation techniques, rhodium and molybdenum films have been prepared for tokamak tests. The films were characterised in terms of chemical composition, surface roughness, crystallite structure, reflectivity and adhesion. No impurities were detected on the surface after deposition. The effects of deposition parameters and substrate temperature on the resulting crystallite structure, surface roughness and hence on the reflectivity, were investigated. The films are found to exhibit nanometric crystallites with a dense columnar structure. Open boundaries between the crystallite columns, which are sometimes present after evaporation, are found to reduce the reflectivity as compared to rhodium or molybdenum references. (C) 2010 Elsevier B.V. All rights reserved.[All authors]