Ru Islands Research Articles

Confining Flat Ru Islands into TiO2 Lattice with the Coexisting Ru-O-Ti and Ru-Ti Bonds for Ultra-Stable Hydrogen Evolution at Amperometric Current Density and Hydrogen Oxidation at High Potential.

Effective hydrogen evolution reaction (HER) under high current density and enhanced hydrogen oxidation reaction (HOR) over a wide potential range remain challenges for Ru-based electrocatalysts because its strong affinity to the adsorbed hydroxyl (OHad) inhibits the supply of the adsorbed hydrogen (Had). Herein, the coexisting RuOTi and Ru-Ti bonds are constructed by taking TiO2 crystal confined flat-Ru clusters (F-Ru@TiO2) to cope with above-mentioned obstacles. The different electronegativity (χTi = 1.54 < χRu = 2.20 < χO = 3.44) can endow Ti sites in RuOTi bonds with more positive charge and stabilize Ru atoms of RuTi bonds with the low-valence. The strength of Ru-OHad is then weakened by the oxophilicity of positively charged Ti atom in Ru-O-Ti bonds and the stronger Ti-OHad bond could release active Ru sites, especially for low-valence Ru in RuTi bonds, to serve as exclusive Had sites. As expected, F-Ru@TiO2 shows a low HER overpotential of 74 mV at 1000mA cm-2 and an ultrahigh mass activity of 3155A gRu -1 for HOR. More importantly, F-Ru@TiO2 can tolerate the HER current density of 1000mA cm-2 for 100h and the high anodic potential for HOR up to 0.5V versus RHE.

Active and stable ruthenium/osmium based electrocatalysts for hydrogen evolution by seawater splitting

Ruthenium/osmium based controlled size nanoparticles (NPs), supported on graphene, have been prepared by a simple synthetic strategy consisting in the thermolysis of a suitable precursor in organic solvent under a low oxygen enriched atmosphere. An excellent combination of a low Tafel slope of 28 mV/dec with a negligible overpotential was measured for the prepared nanocomposite (RuOs_G), due to metal oxides resulting in electron rich Ru and Os islands. The nanocatalyst shows an activity higher than that of common Pt. Additionally, the paper proves that it is possible to use the synthesized nanocatalyst to catalyse directly seawater splitting. RuOs_G resulted very stable and active in complex solutions during electrolysis under high current density.

Reverse-engineering of graphene on metal surfaces: a case study of embedded ruthenium

Using scanning tunneling microscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy, we show that Ru forms metallic nanoislands on graphite, covered by a graphene monolayer. These islands are air-stable, contain 2–4 layers of Ru, and have diameters on the order of 10 nm. To produce these nanoislands two conditions must be met during synthesis. The graphite surface must be ion-bombarded, and subsequently held at an elevated temperature (1000–1180 K) during Ru deposition. A coincidence lattice forms between the graphene overlayer and the Ru island top. Its characteristics—coincidence lattice constant, corrugation amplitude, and variation of carbon lattice appearance within the unit cell—closely resemble the well-established characteristics of single-layer graphene on the (0001) surface of bulk Ru. Quantitative analysis of the graphene lattice in relation to the coincidence lattice on the island tops show that the two-dimensional lattice constant of the underlying metal equals that of bulk Ru(0001), within experimental error. The embedded Ru islands are energetically favored over on-top (adsorbed) islands, based on density-functional-theory calculations for Ru films with 1–3 Ru layers. We propose a formation mechanism in which Ru atoms intercalate via defects that act as entry portals to the carbon galleries, followed by nucleation and growth in the galleries. In this model, high deposition temperature is necessary to prevent blockage of entry portals.

Methanol oxidation on Ru/Pd(poly) in alkaline solution

Polycrystalline Pd electrode, Pd(poly), is modified by Ru nanoislands using spontaneous deposition method. Coverage of Pd(poly) electrode with the deposited Ru are approx. 20, 30 and 50% as estimated from phase atomic force microscopy images. The oxidation state of Pd substrate and the deposited Ru is determined by X-ray spectroscopy (XPS). Electrocatalytic activity of obtained Ru/Pd(poly) bimetallic electrodes is tested towards methanol oxidation in alkaline medium. Cyclic voltammetry and chronoamperometry experiments show the enhanced activity of Ru/Pd(poly) electrodes towards methanol electrooxidation with respect to bare Pd(poly). This is explained by the presence of Ru islands, which provided RuOH and Pd-RuOH sites, necessary for the oxidation of CO as the main intermediate during the oxidation of methanol at lower potentials. 30% Ru/Pd(poly) is the most active of all examined electrodes.

Cactus-Like Hollow Cu2-x S@Ru Nanoplates as Excellent and Robust Electrocatalysts for the Alkaline Hydrogen Evolution Reaction.

The development of Pt-free electrocatalysts for the hydrogen evolution reaction (HER) recently is a focus of great interest. While several strategies are developed to control the structural properties of non-Pt catalysts and boost their electrocatalytic activities for the HER, the generation of highly reactive defects or interfaces by combining a metal with other metals, or with metal oxides/sulfides, can lead to notably enhanced catalytic performance. Herein, the preparation of cactus-like hollow Cu2-x S@Ru nanoplates (NPs) that contain metal/metal sulfide heterojunctions and show excellent catalytic activity and durability for the HER in alkaline media is reported. The initial formation of Ru islands on presynthesized Cu1.94 S NPs, via cation exchange between three Cu+ ions and one Ru3+ , induces the growth of the Ru phase, which is concomitant with the dissolution of the Cu1.94 S nanotemplate, culminating in the formation of a hollow nanostructure with numerous thin Ru pillars. Hollow Cu2-x S@Ru NPs exhibit a small overpotential of 82 mV at a current density of -10 mA cm-2 and a low Tafel slope of 48 mV dec-1 under alkaline conditions; this catalyst is among state-of-the-art HER electrocatalysts in alkaline media. The excellent performance of hollow Cu2-x S@Ru NPs originates from the facile dissociation of water in the Volmer step.

Bimetallic Nickel/Ruthenium Catalysts Synthesized by Atomic Layer Deposition for Low-Temperature Direct Methanol Solid Oxide Fuel Cells.

Nickel and ruthenium bimetallic catalysts were heterogeneously synthesized via atomic layer deposition (ALD) for use as the anode of direct methanol solid oxide fuel cells (DMSOFCs) operating in a low-temperature range. The presence of highly dispersed ALD Ru islands over a porous Ni mesh was confirmed, and the Ni/ALD Ru anode microstructure was observed. Fuel cell tests were conducted using Ni-only and Ni/ALD Ru anodes with approximately 350 μm thick gadolinium-doped ceria electrolytes and platinum cathodes. The performance of fuel cells was assessed using pure methanol at operating temperatures of 300-400 °C. Micromorphological changes of the anode after cell operation were investigated, and the content of adsorbed carbon on the anode side of the operated samples was measured. The difference in the maximum power density between samples utilizing Ni/ALD Ru and Pt/ALD Ru, the latter being the best catalyst for direct methanol fuel cells, was observed to be less than 7% at 300 °C and 30% at 350 °C. The improved electrochemical activity of the Ni/ALD Ru anode compared to that of the Ni-only anode, along with the reduction of the number of catalytically active sites due to agglomeration of Ni and carbon formation on the Ni surface as compared to Pt, explains this decent performance.

Oxygen-Driven Porous Film Formation of Single-Crystalline Ru Deposited on Au(111).

We examined the interaction of oxygen with ultrathin Ru layers deposited on a Au(111) substrate using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The deposition of pure Ru below one monolayer (ML) at room temperature leads to the formation of clusters on the Au(111) surface, preferentially located at the elbow sites of the herringbone reconstruction. Subsequent exposure of molecular oxygen to such a Ru-covered Au(111) surface at 680 K results in the growth of two-layer-thick Ru islands that are embedded in the top Au(111) layer. This structural reorganization of Ru is driven by the minimization of surface energy and mediated by a mobile RuOx species. Deposition of an increasing amount of Ru at 620 K (0.5-10 ML, ML = monolayer) leads to a rough Ru film on Au(111). Subsequent oxygen treatment (10(-5) mbar) at 680 K creates either a porous Ru film (<4 ML) or a flat RuO2(110) film (>6 ML) depending on the thickness of the Ru film.

Temperature-induced transformation of electrochemically formed hydrous RuO2 layers over Ru(0001) model electrodes.

Hydrous RuO2 reveals excellent performance both as a supercapacitor and as a heterogeneous oxidation catalyst. Molecular understanding of these processes needs, however, a model system with preferably low structural and morphological complexity. This goal is partly accomplished here by using single crystalline Ru(0001) as a template on which hydrous RuO2 is electrochemically formed. The hydrous RuO2 layers on Ru(0001) and their temperature induced transformation under ultra high vacuum (UHV) conditions are comprehensively characterized by scanning electron microscopy and X-ray photoemission spectroscopy. The hydrous RuO2 layer grows with an intricate morphology governed by the presence of step bunching regions of the Ru(0001) surface. Upon annealing to 200 °C in UHV the hydrous RuO2 layer transforms mostly into flat metallic Ru islands and occasionally into (100) and (111) oriented RuO2 particles aligned along the high symmetry direction of Ru(0001).

Epitaxial Growth of Co and Ru on Pt(111)

The epitaxial growth of Co/Ru on a well-defined Pt(111) electrode surface has been studied as a function of deposition potentials by the reflection high-energy electron diffraction (RHEED). For ∼1 to 2 ML coverage, Co growth on Pt(111) proceeds initially via nucleation and formation of patchy islands with various levels, and the grain boundaries likely enhance the CoO and Co3O4 cluster formation after emersion. At ∼3 to 4 ML coverage, 2D islands covered by CoO surface layer, in incoherent epitaxy on Pt(111), are observed, and the twinned Co clusters are for the first time detected at ∼5 ML thick islands, showing a Stranski–Krastanov growth mode. Greater than 5 to 6 ML coverage leads to 3D cluster formation growth in the (111) orientation aligned along the Co [110] to the Pt [110] direction. For Ru on Pt(111), at ∼0.2 ML coverage Ru islands are mainly monolayer high with a pseudomorphic growth; beyond this coverage (∼0.6 ML), concurrent growth of higher layers leads to predominantly bilayer island formation, showing an incommensurate growth. Greater than 1 ML coverage develops multilayer islands growing in the (0001) orientation and aligned the [11–20] direction to the [110] Pt direction. The acquired reciprocal lattice net of Co(110)*fcc differs considerably from that of an Ru(110)hcp*, which allows a reliable phase determination of Co/Ru deposits on Pt(111).

Electrochemical stability and restructuring and its impact on the electro-oxidation of CO: Pt modified Ru(0001) electrodes

Structural modifications during electrochemical measurements on well defined Pt modified Ru(0001) electrode surfaces, which were prepared and characterized under ultrahigh vacuum (UHV) conditions, and the influence of the restructuring on the CO oxidation reaction have been investigated in a set-up combining surface preparation and scanning tunneling microscopy characterization under UHV conditions (UHV-STM) and electrochemical flow cell measurements. Bare Ru(0001) and Pt monolayer island modified Ru(0001) electrodes with different Pt coverages were investigated, together with a Pt0.3Ru0.7/Ru(0001) monolayer surface alloy for comparison. Comparing bulk CO oxidation measurements performed upon cycling in base electrolyte (0.5M H2SO4) to 0.90VRHE with similar measurements performed after potential cycling to 1.05VRHE, we find pronounced differences in the current–voltage characteristics, with a distinct new peak at low potentials in the positive-going scan in the latter case, which is centered at 0.67VRHE. STM imaging performed before and after the electrocatalytic measurements revealed a distinct restructuring of the Pt monolayer island modified Ru(0001) surfaces upon potential cycling to 1.05VRHE, while cycling to 0.90VRHE maintains the original structure and morphology of the bimetallic surface. In contrast, for the bare Ru(0001) electrode, restructuring of steps is observed already upon potential cycling to 0.9VRHE. Implications of these findings on the electrochemical stability of the electrodes as well as on the mechanistic understanding of the CO oxidation reaction on bimetallic PtRu electrode surfaces and on the activity of different mono- and bimetallic nanostructures are discussed.

In-situ analysis on the initial growth of ultra-thin ruthenium films with atomic layer deposition

The initial growth behavior of ruthenium during the thermal-activated atomic layer deposition (ALD) using [(ethylcyclopentadienyl)(pyrrolyl)ruthenium(II)] (ECPR) and molecular oxygen was investigated in a cluster tool combining an ALD reactor with a surface analysis unit under high vacuum conditions. A direct qualification and quantification of the chemical surface composition by X-ray photoelectron spectroscopy (XPS) and a determination of the surface topography by atomic force microscopy (AFM) were conducted in the course of the ALD cycles without vacuum break. XPS revealed a substrate-inhibited Ru growth on a hydrogen-terminated silicon surface, which was preceded by an incubation period of 20 ALD cycles. The Si surface oxidized during the first 50 cycles. AFM measurements showed a roughness maximum around the 40th ALD cycle, which suggested an island growth mode and thus corresponded with the substrate inhibition. As verified from the AFM data with an analytical model by Nilsen et al. the Ru islands coalesced between the 40th and 50th ALD cycle. The ALD growth initiation of Ru was also investigated on aluminum oxide and tantalum nitride. XPS revealed a similarly inhibited growth behavior on all the investigated substrates. However, the ECPR adsorption during the very first Ru precursor pulse differed as the amount of chemisorbed Ru on the NHy-terminated TaNx(O, C) surface was much higher compared to the OH-terminated Al2O3 and the H-terminated Si surface. Summarizing, we demonstrated that in-vacuo XPS and AFM as well as the combination of both are ideally suited for studying the ALD growth initiation of Ru. Furthermore, we provided important chemical information about the initial Ru precursor adsorption on several foreign substrate materials, which will direct further investigations towards a non-inhibited Ru growth.

μSR studies of superconductivity in eutectically grown mixed ruthenates

The low-temperature magnetic behavior of the double-layered ruthenate Sr${}_{3}$Ru${}_{2}$O${}_{7}$, as grown from a eutectic Sr${}_{2}$RuO${}_{4}$-Sr${}_{3}$Ru${}_{2}$O${}_{7}$ system, was investigated via zero- and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the ${T}_{c}$ of the single phase Sr${}_{2}$RuO${}_{4}$, provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr${}_{2}$RuO${}_{4}$ and Sr${}_{3}$Ru${}_{2}$O${}_{7}$ domains, or even inside the Sr${}_{3}$Ru${}_{2}$O${}_{7}$ phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with change-over at about $T=2.5$ K and $T=1.5$ K. The comparison with zero-field $\ensuremath{\mu}$SR data in the Ru-Sr${}_{2}$RuO${}_{4}$ eutectic system rules out the possibility of spurious effects due to embedded Ru islands.

Electrochemical redox pattern and allied interactive behaviour of FAD on a ruthenium-modified glassy carbon electrode surface

The redox and interactive behaviour of flavin adenine dinucleotide (FAD) with a ruthenium (Ru)-modified glassy carbon electrode (GCE) was investigated. The electron-transfer kinetics on the Ru-modified GCE gives an apparent electron-transfer coefficient, α app of 0.56, and an apparent heterogeneous electron transfer rate constant, k app of 2.32 s−1, respectively. The cyclic voltammetry (CV) complemented by alternating cyclic voltammetry (ACV) shows reduction of FAD to be a quasi-reversible reaction involving FAD adsorption. The adsorption of FAD on the Ru-modified GCE fits a Langmuir adsorption isotherm. The large apparent negative Gibbs energy of adsorption ΔG ads (−38.2 kJ mol−1) of FAD onto the Ru-modified GCE confirmed a strong chemical adsorption of FAD on the surface. The deposited Ru islands block surface sites for FAD adsorption and the electron-transfer communication between FAD and the electrode surface does not significantly improve with a deposited Ru monolayer.

Suppression of Proximity Effect and the Enhancement ofp-Wave Superconductivity in theSr2RuO4-Ru System

We report unexpected phenomena observed on the Sr2RuO4-Ru eutectic phase featuring Ru islands embedded in a bulk crystal of the chiral p-wave superconductor Sr2RuO4. It was found that the Sr2RuO4/Ru interface is atomically sharp, terminated uniformly by a Sr/O layer. Surprisingly, the proximity-induced p-wave superconducting energy gap predicted by theory was not detected inside Ru islands. Our results suggest that the previously observed enhancement of superconductivity in this eutectic phase occurs away from rather than near the Sr2RuO4/Ru interface, where dislocations and phonon hardening were found.

Contrast in Metal−Ligand Effects on PtnM Electrocatalysts with M Equal Ru vs Mo and Sn As Exhibited by in Situ XANES and EXAFS Measurements in Methanol

In situ X-ray absorption spectroscopy (XAS) measurements, at the Pt L3 edge (XANES and EXAFS), were carried out on carbon-supported PtnMo and PtSn electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO, OH, and H relative adsorbate coverages on Pt are determined as a function of the applied potential via the ΔXANES technique and compared with comparable data previously reported for PtnRu. The more reactive Sn and Mo atoms on the Pt surface form the oxide over the potentials of interest, while Ru has variable oxide content depending on Ru island size and potential. The strength of the electronic ligand effect appears to increase in the order Ru < MoOn < SnOn < RuOn, where the Pt−CO bond strength is found to decrease and the Pt−OH bond strength increase with ligand effect. In the Sn and Mo bimetallics, the ligand effect is found to be sufficiently strong to allow CO replacement by H2 at low potentials. These widely different ligand effects may provide a straightforward explanat...

From bilayer to monolayer growth: Temperature effects in the growth of Ru on Pt(1 1 1)

At temperatures around 373 K, Ru growth on Pt(1 1 1) proceeds via nucleation and growth of bilayer islands [H.E. Hoster et al., Phys. Chem. Chem. Phys. 3 (2001) 337]. The influence of the deposition temperature on the Ru growth behavior on Pt(1 1 1) was studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) in the temperature range between 303 and 773 K. The data reveal a distinct change in the growth characteristics, most important the change from the growth of bilayer Ru islands to monolayer islands, at temperatures between 523 and 573 K. Based on AES data and on atomic resolution STM images, these changes are associated with the onset and increasing contribution of surface alloy formation via Pt–Ru exchange and, at T > 673 K, alloy formation in near surface regions. Consequences of these data for the mechanism of bilayer growth and the underlying physical origin are discussed.

Oxidation of formaldehyde and ethanol on Au(1 1 1) and Au(1 1 1) modified by spontaneously deposited Ru in sulfuric acid solution

The oxidation of formaldehyde and ethanol on both pure Au(1 1 1) and Au(1 1 1) modified by approximately 0.3 monolayer (ML) of spontaneously deposited Ru was studied by cyclic voltammetry (CV) in 0.5 M H 2SO 4 solution containing either 0.25 M formaldehyde or 0.35 M ethanol. In situ scanning tunneling microscopy (STM) and CV were employed to characterize the Au(1 1 1) and Ru/Au(1 1 1) surfaces. The oxidation of HCHO on Ru/Au(1 1 1) commences at 0.1 V more negative potential than on pure Au(1 1 1). From 0.25 to 0.55 V vs. (Ag/AgCl), the reaction occurs with increasing current, showing a peak at a potential of 0.43 V. It is assumed that the increasing anodic activity of the Ru/Au(1 1 1) surface is associated with the oxidation of some reaction intermediates, facilitated by the presence of Ru in its metallic state. On the other hand, the oxidation of ethanol on Ru/Au(1 1 1) commences at 0.1 V more positive potential than on pure Au(1 1 1), and proceeds in the potential region from 0.2 to 0.5 V with significantly smaller currents, showing a peak at 0.43 V. This inhibiting effect is explained by the deactivation of the most active Au(1 1 1) step sites by high coverage with Ru islands. The appearance of a small peak at 0.43 V is most likely associated to the oxidation of some intermediates during ethanol oxidation at the Ru/Au step sites formed on the Au(1 1 1) terraces by the presence of a small coverage with Ru islands.

Surface alloy formation, short-range order, and deuterium adsorption properties of monolayer PdRu/Ru(0 0 0 1) surface alloys

The formation and structure of monolayer PdRu/Ru(0 0 0 1) surface alloys and their adsorption properties with respect to deuterium adsorption were investigated by atomic resolution scanning tunneling microscopy and by temperature programmed desorption. Surface alloys, prepared by deposition of up to one monolayer of Pd and flash annealing to 1150 K show (i) negligible loss of Pd by desorption or diffusion into the Ru bulk during this procedure and (ii) dominant phase separation into 2D Pd and Ru islands, in contrast to the random distribution in PtRu/Ru(0 0 0 1) surface alloys [H.E. Hoster, A. Bergbreiter, P.M. Erne, T. Hager, H. Rauscher, R.J. Behm, Phys. Chem. Chem. Phys. 10 (2008) 3812]. 2D short-range order parameters and the abundance of specific adsorption ensembles were evaluated for different Pd contents, effective pair interaction (EPI) energies were derived from Monte Carlo simulations. Deuterium adsorption on Pd monolayer films shows a pronounced weakening of adsorption bond, which is attributed to compressive strain and metal–metal interactions between Pd and underlying Ru atoms (‘vertical ligand effect’). Mixed adsorption ensembles containing both Pd and Ru atoms give rise to D 2 desorption in the intermediate temperature regime. The impact of the specific lateral distribution of the two metal species on the chemical surface properties is illustrated by comparison with deuterium adsorption on dispersed PtRu/Ru(0 0 0 1) surface alloys [T. Diemant, H. Rauscher, R.J. Behm, J. Phys. Chem. C 112 (2008) 8381].

Observation of PtRu Particle Aging in Methanol with X-ray Absorption Spectroscopy

The dissolution/agglomeration of Ru and Pt from supported PtRu/C in an electrochemical cell, with potential cycling between 0.02 and 0.8 V, are reported. Two different commercial PtRu black catalysts (Johnson-Matthey and Tanaka) were utilized in 1M TFMSA and 0.3 M methanol and their properties compared. The Tanaka catalyst had relatively large RuOn islands on the surface; and underwent little Ru dissolution/agglomeration after 40 cycles, while the Johnson-Matthey catalysts with very small Ru islands underwent considerable Ru dissolution/agglomeration and thus, degradation of the catalyst. The Tanaka catalysts showed some Pt dissolution/agglomeration, but this appeared to increase the rate of CO oxidation (improve performance) after 40 cycles.

Kinetics of CO Poisoning in Simulated Reformate and Effect of Ru Island Morphology on PtRu Fuel Cell Catalysts As Determined by Operando X-ray Absorption Near Edge Spectroscopy

In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS), were carried out on commercially produced Pt and PtRu bimetallic electrocatalysts as well as on a mechanically mixed PtRu bimetallic electrocatalyst in an operating fuel cell in H2 doped with 150 ppm CO. By use of the novel ΔXANES technique, the coverages of CO and ontop and n-fold H (overpotential deposited and underpotential deposited hydrogen) are obtained and compared for the three catalysts, and the results are correlated with PtRu cluster morphology. The mechanical mixing process used to create the bimetallic PtRu catalyst is found to maximize CO tolerance, although the PtRu commercial electrocatalyst exhibits an increased electronic effect, most probably due to the presence of Ru(O)x islands at the catalyst surface. The mobility of the CO on both Ru and Pt is found to be sharply dependent on the CO coverage, decreasing dramati...