Bulk Ru Research Articles

Chemical mechanism of oxidative etching of ruthenium: Insights into continuous versus self-limiting conditions

Ruthenium (Ru) has emerged as a promising material for microelectronic applications that require precise modification and patterning of thin films at the nanoscale. However, the susceptibility of Ru to oxidative etching has raised challenges in achieving the desired profile of Ru thin films. In this study, we investigated the mechanisms and conditions required for oxidative etching of Ru using density functional theory (DFT) calculations. Our calculations examined the thermodynamics and kinetics of several pathways toward the formation of gaseous RuO4, including oxidations of bulk Ru and RuO2, from both surfaces of which surface reactions were considered. Our findings suggest that etching reactions under O3 exposure are more favored on a Ru surface than on RuO2. Experimental results obtained from etching of Ru using O3 further support our computational predictions, indicating that oxide formation leads to a reduction in the etching rate and that quasi-self-limiting etching conditions can be achieved towards oxidative ALE of Ru. Our study may contribute to the fundamental understanding of Ru surface phenomena and provides insights into the development of effective ALE techniques for Ru.

Studying the efficacy of hydrogen plasma treatment for enabling the etching of thermally annealed ruthenium in chemical solutions

Ruthenium (Ru) is a noble metal and is known to be resistant to many common chemicals and mixtures. We report in this study a controlled etching/recessing of Ru via wet processing and a combination of dry and wet process using metal-free chemical mixtures. The wet etching study was performed using both as-deposited and annealed atomic layer deposited ruthenium (Ru) films. The etching rate of Ru film in periodic acid mixtures was found to decrease substantially after being annealed at elevated temperatures (300 and 420 °C). X-ray photoelectron spectroscopy results indicated that this behavior could be explained by the formation of a thin surface ruthenium oxide layer induced by the thermal anneal. A short treatment in H2-based atmosphere at elevated temperature or in H2 plasma rendered the annealed Ru film etchable, explained by the reduction of RuOx film at the surface. The increase in surface roughness after wet etching was most likely caused by the gradual formation of oxidized Ru at the surface. Similarly, non-uniform etching of Ru was also observed for damascene Ru structure with the critical dimension ranging from ∼12 to 35 nm. An alternative approach using O2-based plasma achieved a good control of the etching (recessing) depth but also generated a thin layer of “residues” close to the interface between Ru and the liner (TiN). The results from this study have demonstrated that a short H2 plasma treatment prior to the wet process using the same chemical mixtures was able to etch the modified Ru surface layer together with the interfacial residues selectively to bulk Ru.

Role of dose optimization in Ru atomic layer deposition for low resistivity films

Ruthenium (Ru) is an alternative to copper (Cu) and cobalt (Co) interconnect layers in sub 20 nm features due to its low resistivity in scaled wires and low diffusion into porous low-K dielectrics (SiCOH). Two goals for a successful Ru atomic layer deposition (ALD) process are to enable films with resistivity values as close as possible to that of bulk Ru and to enable selective deposition to achieve bottom-up fill of vias. In this work, the effects of dose variation on resistivity and selectivity of the Ru ALD process using a dicarbonyl-bis(5-methyl-2, 4-hexanediketonato) Ru(II) precursor, Ru(IHD)2(CO)2 (“Carish”), and O2/He coreactant were investigated. Instead of varying the Carish precursor dose to optimize the growth rate per cycle, the precursor dose was optimized to reduce the film resistivity from 18.5 to 10.2 μΩ cm. By varying the O2/He coreactant dose, the substrate selectivity of the ALD process was successfully enhanced as evidenced by the increased nucleation delay on bis(N,N-dimethylamino)dimethylsilane passivated SiO2 over hydrofluoric acid-cleaned SiO2. These findings highlight the importance of dose optimization beyond the ALD saturation point in developing a selective and low resistivity Ru ALD process. Density functional theory calculations were performed to provide a mechanistic understanding of the underlying surface reactions of the Carish precursor and the roles of CH3 passivation and O2 coreactants.

Strong effect of crystal structure on the proximity effect between a superconductor and monolayer of cobalt

We present an unexpectedly strong influence of the proximity effect between the bulk Ru(0001) superconductor and atomically thin layers of Co on the crystal structure of the latter. The Co monolayer grows in two different modifications, such as hcp stacking and a reconstructed ε-like phase. While hcp islands show a weak proximity effect on Co and a little suppression of superconductivity in the substrate next to it, the more complex ε-like stacking becomes almost fully superconducting. We explain the weak proximity effect between Ru and hcp Co and the rather abrupt jump of the superconducting order parameter by a low transparency of the interface. In contrast, the strong proximity effect without a jump of the order parameter in the ε-like phase indicates a highly transparent interface. This work highlights that the proximity effect between a superconductor and a normal metal strongly depends on the crystal structure of the interface, which allows to engineer the proximity effect in hybrid structures.

Computational Insights into Ru, Pd and Pt fcc Nano-Catalysts from Density Functional Theory Calculations: The Influence of Long-Range Dispersion Corrections

Ruthenium, palladium and platinum fall within the group of noble metals that are widely used in catalysis, especially for the electrocatalytic production of hydrogen. The dominant phase of the bulk Ru metal is hexagonal close-packed (hcp), which has been studied extensively. However, significantly less attention has been paid to the face-centred cubic (fcc) phases, which have been observed in nanoparticles. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D2, DFT-D3 and DFT-D3-(BJ)] to investigate the lattice parameters, surface energies and work functions of the (001), (011) and (111) surfaces of Ru, Pd and Pt in the fcc phase. When investigating the surface properties of the three metals, we observed that the DFT-D2 method generally underestimated the lattice parameters by up to 2.2% for Pt and 2.8% for Ru. The surface energies followed the observed trend (111) < (001) < (011) for both Ru and Pd with all three methods, which is comparable to experimental data. For Pt the same trend was observed with DFT-D2 and DFT-D3(BJ), but it deviated to Pt (111) < Pt (011) < Pt (001) for the DFT-D3 method. DFT-D2 overestimated the surface energies for all three Miller Indexes by 82%, 73%, and 60%, when compared to experimental values. The best correlation for the surface energies was obtained with the DFT-D3 and DFT-D3(BJ) methods, both of which have deviate by less than 15% deviation for all surfaces with respect to experiment. The work function followed the trend of Φ (111) < Φ (001) < Φ (011) for all three metals and calculated by all three methods. Five different types of Ru, Pd and Pt nanoparticles were considered, including icosahedral, decahedral, cuboctahedral, cubic and spherical particles of different sizes. The bulk, surface and nanoparticle calculations showed that the DFT-D2 method for Pt overestimated the exchange-correlation, leading to higher energy values that can be contributed erroneously to a more stable structure. The calculations showed that as soon as the surface-to-bulk ratio > 1, the energy per atom resembles bulk energy values.

A ReaxFF Molecular Dynamics Study of Hydrogen Diffusion in Ruthenium-The Role of Grain Boundaries.

Ruthenium (Ru) thin films are used as protective caps for the multilayer mirrors in extreme ultraviolet lithography machines. When these mirrors are exposed to atomic hydrogen (H), it can permeate through Ru, leading to the formation of hydrogen-filled blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, but the nature of H diffusion through Ru and its contribution to the mechanisms of blistering remain unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behavior of H. For the Ru/H system, a ReaxFF force field which reproduces structures and energies obtained from quantum-mechanical calculations was parametrized. Molecular dynamics simulations have been performed with the newly developed force field to study the effect of tilt and twist grain boundaries on the overall diffusion behavior of H in Ru. Our simulations show that the tilt and twist grain boundaries provide energetically favorable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes and favor diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation.

Ruthenium Icosahedra and Ultrathin Platelets: The Role of Surface Chemistry on the Nanoparticle Structure

The control of the crystalline structure and shape (crystal habit) of nanoparticles (NPs) is the key to controlling their physical and chemical properties. Among the different metals, the crystallogenesis of ruthenium NPs has been less studied, and until recently, the Ru NP crystal structure and morphology have been considered as presenting less versatility than the face-centered cubic (fcc) metals of the platinum group. Here, we show that while the hydrogenation of [Ru(COD)(COT)] in solutions containing a long-chain amine (hexadecylamine, HDA) in large excess leads to isotropic NPs adopting the expected hexagonal close-packed (hcp) structure of bulk Ru, a long-chain carboxylic acid (lauric acid, LA) in large excess induces the formation of Ru nano-objects of two original structures: ultrathin platelets and icosahedra. The latter have never been produced so systematically by other methods. We show that carbon monoxide, produced in situ by the decarbonylation of lauric acid, plays a pivotal role in the stabilization of the Ru icosahedra. This result is supported by density functional theory (DFT) calculations, which show that above a critical surface coverage of CO, small Ru icosahedra are more stable than the Ru bipyramid polyhedra crystallizing in the hcp structure. Thus, in situ production of CO results in a competition between icosahedral and hcp seeds, which explains the mixture of icosahedra and ultrathin platelets. Another effect of the large excess of lauric acid is the stabilization of the ruthenium precursors in solution, limiting the nucleation extent and slowing down the NP growth. While the growth of the icosahedral seeds is limited because of the structural strains, the growth of the hcp seeds preferentially develops the (0001) facets, leading to ultrathin platelets and threefold stars.

(Battery Division Student Research Award Address Sponsored by Mercedes-Benz Research & Development) Towards Understanding and Controlling Bulk Anionic Redox in Li-ion Positive Electrode Materials Through Electronic Structure Tuning

Positive electrode materials such as layered lithium nickel, manganese and cobalt oxides (NMC)1 ,2 in Li-ion batteries typically store charge by relying on the redox activity of transition metal species, which is accompanied by the intercalation and deintercalation of Li ions into and out the host structure. Anionic redox in positive electrode materials can provide additional charge storage beyond the conventional metal redox. However, the physical origin of observed anion redox as well as the requirements for the reversible anionic redox activity remain under debate, hindering rational design of new electrode materials leveraging reversible anionic redox.In this talk, we first focus on understanding the cationic and anionic redox process in the positive electrode materials upon lithium deintercalation using X-ray absorption and emission spectroscopy (XAS and XES), X-ray photoelectron spectroscopy (XPS), coupled with density functional theory (DFT) calculations. We show electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-), in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. This lattice oxygen redox of Li2-xRuO3 was accompanied by bulk Ru reduction.3 This observed redox trend is in stark contrast of the observations in charged Ni-rich NMC upon charging. In Ni-rich NMC positive electrodes, nickel oxidation is primarily responsible for the charge capacity up to removing ~0.7 Li, beyond which is followed by Ni reduction near the surface (up to 100 nm) due to oxygen release, where there is no significant bulk metal reduction observed.4 The uniqueness of Ru-based system lies in the highly covalent nature of Ru-O bond, which stabilizes the O2-/(O2)n- intermediates, forbidding further oxygen release.We further discuss that a strong metal-oxygen covalency is needed to enhance a reversible anionic redox.5 Differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen release and quantify the reversibility of anionic redox of Li2Ru0.75 M 0.25O3 (M= Ti, Cr, Mn, Fe, Ru, Sn, Pt, Ir) upon first charge. We show through X-ray absorption spectroscopy that more ionic substituents and reduced metal-oxygen covalency introduce irreversible oxygen redox, accompanied with easier distortion of M-O octahedron and smaller barrier for forming oxygen dimer within the octahedron. We propose a universal electronic structure descriptor that could control bulk anionic redox, leading to a rational design strategy to enhance the cycling stability of Ni-rich NMC6 as well as Li-rich positive electrodes. Our study has laid a solid foundation for future high-throughput screening of novel and affordable metal oxides for battery and electrocatalysis applications. Reference (1) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li [NixCoyMnz] O2 (X= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for Lithium-Ion Batteries. Journal of power sources 2013, 233, 121–130.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries. Journal of The Electrochemical Society 2017, 164 (7), A1361–A1377.(3) Yu, Y.; Karayaylali, P.; Nowak, S. H.; Giordano, L.; Gauthier, M.; Hong, W.; Kou, R.; Li, Q.; Vinson, J.; Kroll, T. Revealing Electronic Signatures of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-Ion Battery Material Designs. Chemistry of Materials 2019, 31 (19), 7864–7876.(4) Yu, Y.; Karayaylali, P.; Giordano, L.; Corchado-García, J.; Hwang, J.; Sokaras, D.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries. ACS Applied Materials & Interfaces 2020.(5) Yu, Y.; Karayaylali, P.; Sokaras, D.; Giordano, L.; Kou, R.; Sun, C.-J.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Towards Controlling the Reversibility of Anionic Redox in Transition Metal Oxides for High-Energy Li-Ion Positive Electrodes. Energy & Environmental Science 2021, 14 (4), 2322–2334.(6) Yu, Y.; Zhang, Y.; Giordano, L.; Zhu, Y. G.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-horn, Y. Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification. Journal of The Electrochemical Society 2021.

Collisional mixing between inner and outer solar system planetesimals inferred from the Nedagolla iron meteorite

AbstractThe ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the noncarbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf‐W chronology of Nedagolla indicates that this mixed NC–CC isotopic composition was established relatively late, more than 7 Myr after solar system formation. The mixed NC–CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re‐equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit‐and‐run collision between two differentiated bodies, which also provides a possible pathway for Nedagolla's extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiter's growth.

Electrodeposition of Ru onto Ru and Au Seed Layers from Solutions of Ruthenium Nitrosyl Sulfate and Ruthenium Chloride

The electrodeposition of Ru was investigated from solutions of ruthenium(III) nitrosyl sulfate and ruthenium(III) chloride onto seed layers of epitaxial and polycrystalline Ru and epitaxial Au. Using both galvanostatic and potentiostatic deposition modes, metallic Ru was found to electrodeposit as a porous layer comprised of (0001) oriented Ru crystallites, the presence of which was discovered and confirmed by X-ray and scanning transmission and transmission electron microscope (S/TEM) analyses. This finding was independent of the Ru salt and seed layer used. Using X-ray reflectivity (XRR), the average film density of the porous electrodeposited Ru layer was measured as less than the density of bulk Ru (14.414 g cm−3). Increasing the magnitude of the applied current density from −100 μA cm−2 to −10 mA cm−2 in solutions of Ru nitrosyl sulfate increased the from 7.4 g cm−3 to 9.7 g cm−2 while the current efficiency decreased from 9.4% to 4.3%.

Electronic Structure Based Intuitive Design Principle of Single‐Atom Catalysts for Efficient Electrolytic Nitrogen Reduction

AbstractAs an alternative to cost‐ and energy intensive Haber‐Bosch process, the implementation of electrolytic ammonia synthesis from dinitrogen molecule has been a long‐sought goal. State‐of‐the‐art electrocatalysts for nitrogen reduction reaction (NRR) face not only activity but also selectivity problem with the competitive hydrogen evolution reaction (HER). Recently, single‐atom catalysts (SACs) have emerged as promising for various reactions as they combine the best of homogenous and heterogenous catalysts. The reason for their high activity compared to their bulk and nanoparticle counterparts are yet to be completely understood. Inspired by the structure of nitrogenase FeMo cofactor, here we studied 13 transition metals anchored on MoS2 monolayer at Mo‐top positions, as possible electrolytic NRR catalysts using first‐principles methods. Employing the implicit solvation model, we calculated free energy barriers for proton abstraction by N2 molecule in end‐on configuration and adsorption free energy of hydrogen on all SACs. Based on these two parameters, Fe, Co, and Ru were found to be the most active and highly selective electrolytic NRR catalysts. Compared with other mechanisms, the limiting potentials (and hence activity) for enzymatic mechanism were found to be higher on these three SACs, with Ru SAC having a very low overpotential of 0.38 V vs SHE. Bader charge transferred from transition metal to N2 molecule and group number of transition metals correlate strongly with the NRR activity and hence emerge as two key descriptors for catalytic activity. These intuitive principles for rational designing of promising alternatives to the currently used bulk Ru(0001) catalyst could accelerate the search for highly efficient and selective SACs for electrolytic NRR.

Reactivity with Water and Bulk Ruthenium Redox of Lithium Ruthenate in Basic Solutions

AbstractThe reactivity of water with Li‐rich layered Li2RuO3 and partial exchange of Li2O with H2O within the structure is studied under aqueous (electro)chemical conditions. Upon slow delithiation in water over long time periods, micron‐sized Li2RuO3 particles structurally transform from an O3 structure to an O1 structure with a corresponding loss of 1.25 Li ions per formula unit. The O1 stacking of the honeycomb Ru layers is imaged using high‐resolution high‐angle annular dark‐field scanning transmission electron microscopy, and the resulting structure is solved by X‐ray powder diffraction and electron diffraction. In situ X‐ray absorption spectroscopy suggests that reversible oxidation/reduction of bulk Ru sites is realized on potential cycling between 0.4 and 1.25 VRHE in basic solutions. In addition to surface redox pseudocapacitance, the partially delithiated phase of Li2RuO3 shows high capacity, which can be attributed to bulk Ru redox in the structure. This work demonstrates that the interaction of aqueous electrolytes with Li‐rich layered oxides can result in the formation of new phases with (electro)chemical properties that are distinct from the parent material. This understanding is important for the design of aqueous batteries, electrochemical capacitors, and chemically stable cathode materials for Li‐ion batteries.

Bipolar Energetics and Bifunctional Catalytic Activity of a Nanocrystalline Ru Thin-Film Enable High-Performance Photoelectrochemical Water Reduction and Oxidation.

Photoelectrochemical (PEC) cells, which represent a promising technology for the production of hydrogen fuel through water splitting reactions, must meet two criteria to achieve high-performance operation: (i) a high thermodynamic open-circuit potential and (ii) a low kinetic overpotential. Herein, we achieved these criteria in both an oxygen-evolving n-Si photoanode and hydrogen-evolving p-Si photocathode by simple electrodeposition of a nanocrystalline thin film of Ru. The bifunctional electrocatalytic activity of the nanocrystalline Ru led to low overpotentials in both the acidic oxygen evolution reaction (0.27 V) and alkaline hydrogen evolution reaction (0.04 V). In addition, the nanocrystalline Ru/Si junctions influenced the interface energetics via the induction of an extrinsic electrochemical potential on the surface of the Ru nanocrystals through a redox reaction rather than the chemical potential of the electrons (work function) of bulk Ru. The nanocrystalline Ru film exhibited bipolar applicability, enabling both Ru/n-Si and Ru/p-Si junctions with high Voc values of 0.63 and 0.5 V, respectively. As a result, the n-Si photoanode in the acidic electrolyte and the p-Si photocathode in the alkaline electrolyte generated a photocurrent of 10 mA/cm2 at record values of 0.87 and 0.42 V versus the reversible hydrogen electrode, respectively. These results provide insight into the development of high-performance PEC cells based on a nanocrystalline electrocatalyst.

Capturing the Equilibration Pathway of Nanomaterials Metastable in Both Crystal Structure and Morphology

In this issue of Matter, Li et al. used in situ electron microscopy to investigate the equilibration pathway of Au nanoribbons that are metastable in terms of both crystal structure and morphology when they are subjected to thermal stress.

Pd‐Ru Alloy Nanocages with a Face‐Centered Cubic Structure and Their Enhanced Activity toward the Oxidation of Ethylene Glycol and Glycerol

AbstractThis article reports a facile method for the synthesis of Pd‐Ru nanocages by activating the galvanic replacement reaction between Pd nanocrystals and a Ru(III) precursor with I‐ ions. The as‐synthesized nanocages feature a hollow interior, ultrathin wall of ≈2.5 nm in thickness, and a cubic shape. Our quantitative study suggests that the reduction rate of the Ru(III) precursor can be substantially accelerated upon the introduction of I‐ ions and then retarded as the ratio of I‐/Ru3+ is increased. The Pd‐Ru nanocages take an alloy structure, with the Ru atoms in the nanocages crystallized in a face‐centered cubic structure instead of the hexagonal close‐packed phase taken by bulk Ru. Using Pd nanocubes with different edge lengths, the dimensions of the nanocages in the range of 6−18 nm can readily be tuned. When tested as catalysts toward the electro‐oxidation of ethylene glycol and glycerol, respectively, the Pd‐Ru cubic nanocages prepared from 18 nm Pd cubes exhibit 5.1‐ and 6.2‐fold enhancements in terms of mass activity relative to the commercial Pd/C. After 1000 cycles of accelerated durability test, the mass activities of the nanocages are still 3.3 and 3.7 times as high as that of the pristine commercial Pd/C catalyst, respectively.

Tuning the Catalytic Preference of Ruthenium Catalysts for Nitrogen Reduction by Atomic Dispersion

AbstractDeveloping cost‐effective, high‐performance nitrogen reduction reaction (NRR) electrocatalysts is required for the production of green and low‐cost ammonia under ambient conditions. Here, a strategy is proposed to adjust the reaction preference of noble metals by tuning the size and local chemical environment of the active sites. This proof‐of‐concept model is realized by single ruthenium atoms distributed in a matrix of graphitic carbon nitride (Ru SAs/g‐C3N4). This model is compared, in terms of the NRR activity, to bulk Ru. The as‐synthesized Ru SAs/g‐C3N4 exhibits excellent catalytic activity and selectivity with an NH3 yield rate of 23.0 µg mgcat−1 h−1 and a Faradaic efficiency as high as 8.3% at a low overpotential (0.05 V vs the reversible hydrogen electrode), which is far better than that of the bulk Ru counterpart. Moreover, the Ru SAs/g‐C3N4 displays a high stability during five recycling tests and a 12 h potentiostatic test. Density functional theory calculations reveal that compared to bulk Ru surfaces, Ru SAs/g‐C3N4 has more facile reaction thermodynamics, and the enhanced NRR performance of Ru SAs/g‐C3N4 originates from a tuning of the d‐electron energies from that of the bulk to a single‐atom, causing an up‐shift of the d‐band center toward the Fermi level.

Observation of the Formation Processes of Hexagonal Close-packed and Face-centered Cubic Ru Nanoparticles

Face-centered cubic (fcc) Ru nanoparticles (NPs) have been recently synthesized by reducing Ru(III) acetylacetonate, despite bulk Ru having a hexagonal close-packed (hcp) structure. We observed the...

Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance.

Owing to their highly open structure and a large number of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decomposition. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br- ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of Pd@Ru core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystallized in a face-centered cubic (fcc) phase instead of the hexagonal close-packed (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chemical etching, we obtain Ru cuboctahedral nanoframes. By varying the amount of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few atomic layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300 °C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H2 selectivity toward hydrazine decomposition. This work offers the opportunity to engineer both the morphology and crystal phase of Ru nanocrystals for catalytic applications.

Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400 °C, and Enhanced Catalytic Activity.

Ruthenium nanocrystals with both a face-centered cubic ( fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepared as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the atomic packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hexagonal close-packed ( hcp) lattice associated with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400 °C, which is more than 100 °C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.

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.