Baran Eren

Activation of Cu(111) surface by decomposition into nanoclusters driven by CO adsorption

Nanoclusters just by adding CO The most closely packed surfaces of transition metals are usually stable under vacuum, but during catalytic reactions, energetic changes that result from adsorbing molecules could change the surface structure. Eren et al. present an extreme example for carbon monoxide (CO) adsorption on the (111) surface of copper at very low partial pressures. The surface decomposed into small nanoclusters (most containing 3 or 19 atoms). The surface was more reactive than the original and, for example, could dissociate adsorbed water at room temperature. Science, this issue p. 475 The Cu(111) surface decomposes into nanoclusters under CO at pressures in the Torr range at room temperature. 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

The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O2 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 O2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (∼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 (∼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(δ-), with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.

Graphene Membranes for Atmospheric Pressure Photoelectron Spectroscopy

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 Cu(2+) oxidation state is stable in O2 (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO2, N2, O2) in the pressure range 10-1500 mbar including species with low photoionization cross sections (He, H2). 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.

Pure hydrogen low-temperature plasma exposure of HOPG and graphene: Graphane formation?

Summary 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 °C.

The effect of low temperature deuterium plasma on molybdenum reflectivity

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 °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.

Towards plasma cleaning of ITER first mirrors

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 comparisonswithporousfilms. Thecomposition,morphologyandcrystalstructureofseveralfilmswerecharacterizedbymeans 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.

Friction force microscopy studies on SiO2 supported pristine and hydrogenated graphene

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.

Spectroscopic reflectometry of mirror surfaces during plasma exposure

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.

Morphological changes of tungsten surfaces by low-flux helium plasma treatment and helium incorporation via magnetron sputtering

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 × 10(19) 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.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO

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 clusters 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).