Rutile RuO2 Research Articles

Metal Organic Framework-Based Alkaline Oxygen Evolution Reaction Electrocatalysts: Morphology, Metal Loading, and Durability

Electrochemical energy conversion technologies based on the oxygen evolution reaction (OER) are at the heart of many efforts to achieve a sustainable future, carbon-free fuel, and a circular economy. The sluggish kinetics of oxygen electrocatalysis, as well as the high overpotential required to attain practical current densities, limit the efficiency of several promising electrochemical technologies, including water and carbon dioxide electrolyzers, metal–oxygen batteries, and fuel cells. The most efficient OER catalysts are precious metals such as iridium- and ruthenium-based materials (i.e., IrO2 and RuO2). This fact represents a challenge against the cost-effective implementation of these electrolysis technologies.1-3 The rich chemistry of nanoporous materials provides an opportunity for the development of cost effective PGM-free OER catalysts, with equivalent or superior activity and durability to the PGM catalysts.4-7 Metal organic framework (MOF)-based OER catalysts with high activity and long-term stability in alkaline electrolytes will be discussed in this presentation. We will discuss the relationship between MOF morphology and metal-ion loading, and how their impact on OER activity.Acknowledgements: Argonne National Laboratory is managed for the U.S Department of Energy (DOE) by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357. This work was performed under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0), a DOE Office of Energy Efficiency, Hydrogen and Fuel Cell Technologies (HFTO) consortium. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.References Katsounaros, Ioannis, Serhiy Cherevko, Aleksandar R. Zeradjanin, and Karl JJ Mayrhofer. "Oxygen electrochemistry as a cornerstone for sustainable energy conversion." Angewandte Chemie International Edition53, no. 1 (2014): 102-121.Lee, Youngmin, Jin Suntivich, Kevin J. May, Erin E. Perry, and Yang Shao-Horn. "Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions." The journal of physical chemistry letters3, no. 3 (2012): 399-404.Cherevko, S. et al. Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Today 262, 170–180 (2016).Nahar, Lamia, Ahmed A. Farghaly, Richard J. Alan Esteves, and Indika U. Arachchige. "Shape controlled synthesis of Au/Ag/Pd nanoalloys and their oxidation-induced self-assembly into electrocatalytically active aerogel monoliths." Chemistry of Materials29, no. 18 (2017): 7704-7715.Farghaly, Ahmed A., Rezaul K. Khan, and Maryanne M. Collinson. "Biofouling-resistant platinum bimetallic alloys." ACS applied materials & interfaces10, no. 25 (2018): 21103-21112.Khan, Rezaul K., Ahmed A. Farghaly, Tiago A. Silva, Dexian Ye, and Maryanne M. Collinson. "Gold-Nanoparticle-Decorated Titanium Nitride Electrodes Prepared by Glancing-Angle Deposition for Sensing Applications." ACS Applied Nano Materials2, no. 3 (2019): 1562-1569.Farghaly, Ahmed A., Mai Lam, Christopher J. Freeman, Badharinadh Uppalapati, and Maryanne M. Collinson. "Potentiometric measurements in biofouling solutions: comparison of nanoporous gold to planar gold." Journal of The Electrochemical Society 163, no. 4 (2015): H3083.

Anti-Ferromagnetic RuO2: A Stable and Robust OER Catalyst over a Large Range of Surface Terminations

Rutile RuO2 is a prime catalyst for the oxygen evolution reaction (OER) in water splitting. Whereas RuO2 is typically considered to be non-magnetic (NM), it has recently been established as being anti-ferromagnetic (AFM) at room temperature. The presence of magnetic moments on the Ru atoms signals an electronic configuration that is markedly different from what is commonly assumed, the effect of which on the OER is unknown. We use density functional theory (DFT) calculations within the DFT+U approach to model the OER process on NM and AFM RuO2(110) surfaces. In addition, we model the thermodynamic stability of possible O versus OH terminations of the RuO2(110) surface and their effect on the free energies of the OER steps. We find that the AFM RuO2(110) surface gives a consistently low overpotential in the range 0.4–0.5 V, irrespective of the O versus OH coverage, with the exception of a 100% OH-covered surface, which is, however, unlikely to be present under typical OER conditions. In contrast, the NM RuO2(110) surface gives a significantly higher overpotential of ∼0.7 V for mixed O/OH terminations. We conclude that the magnetic moment of RuO2 supplies an important contribution to obtaining a low overpotential and to its insensitivity to the exact O versus OH coverage of the (110) surface.

Data-efficient iterative training of Gaussian approximation potentials: Application to surface structure determination of rutile IrO2 and RuO2.

Machine-learning interatomic potentials, such as Gaussian Approximation Potentials (GAPs), constitute a powerful class of surrogate models to computationally involved first-principles calculations. At a similar predictive quality but significantly reduced cost, they could leverage otherwise barely tractable extensive sampling as in global surface structure determination (SSD). This efficiency is jeopardized though, if an a priori unknown structural and chemical search space as in SSD requires an excessive number of first-principles data for the GAP training. To this end, we present a general and data-efficient iterative training protocol that blends the creation of new training data with the actual surface exploration process. Demonstrating this protocol with the SSD of low-index facets of rutile IrO2 and RuO2, the involved simulated annealing on the basis of the refining GAP identifies a number of unknown terminations even in the restricted sub-space of (1 × 1) surface unit cells. Particularly in an O-poor environment, some of these, then metal-rich terminations, are thermodynamically most stable and are reminiscent of complexions as discussed for complex ceramic materials.

The regulation mechanism of V-shaped 103 nano-twin grain boundary on OER performance of rutile RuO2

During the oxygen evolution reaction (OER), surface oxygen adsorption plays a crucial role in electrocatalysis. The stability of surface adsorption is closely related to the surface morphology of the catalyst. As one of the V-shaped nanotwins, 103 nano-twin grain boundary (103-TGB) has a special roof structure, which results in the presence of double dangling-O atoms at the junction of 103-TGB with (110) surface. In contrast to the ideal RuO2, the d-band center of Ru atoms located on the surface of 103-TGB is shifted to the Fermi level by 1.66 eV. Moreover, the key intermediates, namely *OH, *O, and *OOH, have suitable adsorption energies, providing a lower overpotential of OER (ηOER), via adsorbate evolving mechanism (AEM), of just 0.45 eV. Therefore, the OER via AEM is strongly focused on site 02 with double dangling-O atoms, revealing a synergistic effect due to the presence of both the 103-TGB defect and (110) surface.

High Throughput Synthesis and Characterization of Multi-Metallic PGM-Free Aerogels for Catalyzing the Oxygen Evolution Reaction

Electrochemical energy conversion and storage technologies are at the heart of the societal pursuit towards circular economy, greenhouse gas emission-free fuel and sustainable energy future. The oxygen evolution reaction (OER) plays a central role in many of these electrochemical technologies, such as electrolyzers and metal-air batteries. The sluggish kinetics of the 4-electron oxidation of water to O2 and the resulting large overpotential limit the efficiency of these devices. The low abundance, prohibitive cost, and moderate stability of state-of-the-art OER catalysts, iridium- and ruthenium-based materials (i.e., IrO2 and RuO2) are obstacles against the cost-effective implementation of electrolysis technologies.1-3 To address these concerns and take the advantage of the intriguing properties and rich chemistry of nanoporous materials,4-7 we have developed a high throughput synthesis route for multi-metallic PGM-free aerogels for catalyzing the OER. The synthesis route is capable of producing 100 samples per batch using the High-Throughput Research Facility at Argonne National Laboratory. The effect of electrocatalyst composition (metal ion ratio) as well as the nature of the metal centers on the OER electrocatalytic activity have been evaluated.AcknowledgementsThis work was supported by the U.S. Department of Energy, Advanced Research Projects Agency Energy (ARPA-E) under the DIFFERENTIATE program. This work was authored in part by Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.References Katsounaros, Ioannis, Serhiy Cherevko, Aleksandar R. Zeradjanin, and Karl JJ Mayrhofer. "Oxygen electrochemistry as a cornerstone for sustainable energy conversion." Angewandte Chemie International Edition53, no. 1 (2014): 102-121.Lee, Youngmin, Jin Suntivich, Kevin J. May, Erin E. Perry, and Yang Shao-Horn. "Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions." The journal of physical chemistry letters3, no. 3 (2012): 399-404.Cherevko, S. et al. Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Today 262, 170–180 (2016).Nahar, Lamia, Ahmed A. Farghaly, Richard J. Alan Esteves, and Indika U. Arachchige. "Shape controlled synthesis of Au/Ag/Pd nanoalloys and their oxidation-induced self-assembly into electrocatalytically active aerogel monoliths." Chemistry of Materials29, no. 18 (2017): 7704-7715.Farghaly, Ahmed A., Rezaul K. Khan, and Maryanne M. Collinson. "Biofouling-resistant platinum bimetallic alloys." ACS applied materials & interfaces10, no. 25 (2018): 21103-21112.Khan, Rezaul K., Ahmed A. Farghaly, Tiago A. Silva, Dexian Ye, and Maryanne M. Collinson. "Gold-Nanoparticle-Decorated Titanium Nitride Electrodes Prepared by Glancing-Angle Deposition for Sensing Applications." ACS Applied Nano Materials2, no. 3 (2019): 1562-1569.Farghaly, Ahmed A., Mai Lam, Christopher J. Freeman, Badharinadh Uppalapati, and Maryanne M. Collinson. "Potentiometric measurements in biofouling solutions: comparison of nanoporous gold to planar gold." Journal of The Electrochemical Society163, no. 4 (2015): H3083.

Machine Learning Driven Discovery and Optimization of Multi-Component Oxygen Evolution Reaction Catalysts

Electrochemical energy conversion and storage technologies are in the heart of the societal pursuit towards circular economy, greenhouse gas emission-free fuel and sustainable energy future. Oxygen evolution reaction (OER) plays a central role in many of these electrochemical technologies such as electrolyzers and metal-air batteries. However, the sluggish kinetics of 4-electron water oxidation to O2, the resulting large overpotential and severe energy loss limit the development of energy efficient devices. The low-abundance, prohibitive cost, and moderate stability of state-of-the-art OER catalysts, iridium- and ruthenium-based materials i.e. IrO2 and RuO2, also limit the cost effective implementation of electrolyzers.1-3 This presentation will describe our research to address these concerns, taking advantage of the intriguing properties and rich chemistry of nanoporous materials, the demonstrated capability of machine learning (ML)-guided materials discovery, and the high OER electrocatalytic activity of perovskites especially in alkaline media.4-10 We will describe our work in simulating over 8,000 perovskites across a variety of cell sizes, space groups, and compositions using density functional theory (DFT). By mining this simulation data, we found that experimentally-known perovskites have low energy above the thermodynamic convex hull. Then, using efficient search algorithms, deep learning-based models, and DFT calculations, the space of perovskite oxides is explored to produce novel compositions with tailored electronic descriptors. Promising compositions designed for high activity and stability are then selected for high throughput automated synthesis using the High-Throughput Research Facility at Argonne National Laboratory.AcknowledgementsThis work was supported by the U.S. Department of Energy, Advanced Research Projects Agency-Energy (ARPA-E) under the DIFFERENTIATE program. This work was authored in part by Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.References Katsounaros, Ioannis, Serhiy Cherevko, Aleksandar R. Zeradjanin, and Karl JJ Mayrhofer. "Oxygen electrochemistry as a cornerstone for sustainable energy conversion." Angewandte Chemie International Edition53, no. 1 (2014): 102-121.Lee, Youngmin, Jin Suntivich, Kevin J. May, Erin E. Perry, and Yang Shao-Horn. "Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions." The journal of physical chemistry letters3, no. 3 (2012): 399-404.Cherevko, S. et al. Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Today 262, 170–180 (2016).Nahar, Lamia, Ahmed A. Farghaly, Richard J. Alan Esteves, and Indika U. Arachchige. "Shape controlled synthesis of Au/Ag/Pd nanoalloys and their oxidation-induced self-assembly into electrocatalytically active aerogel monoliths." Chemistry of Materials29, no. 18 (2017): 7704-7715.Farghaly, Ahmed A., Rezaul K. Khan, and Maryanne M. Collinson. "Biofouling-resistant platinum bimetallic alloys." ACS applied materials & interfaces10, no. 25 (2018): 21103-21112.Khan, Rezaul K., Ahmed A. Farghaly, Tiago A. Silva, Dexian Ye, and Maryanne M. Collinson. "Gold-Nanoparticle-Decorated Titanium Nitride Electrodes Prepared by Glancing-Angle Deposition for Sensing Applications." ACS Applied Nano Materials2, no. 3 (2019): 1562-1569.Farghaly, Ahmed A., Mai Lam, Christopher J. Freeman, Badharinadh Uppalapati, and Maryanne M. Collinson. "Potentiometric measurements in biofouling solutions: comparison of nanoporous gold to planar gold." Journal of The Electrochemical Society163, no. 4 (2015): H3083.Suntivich, Jin, Kevin J. May, Hubert A. Gasteiger, John B. Goodenough, and Yang Shao-Horn. "A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles." Science334, no. 6061 (2011): 1383-1385.Hwang, Jonathan, Zhenxing Feng, Nenian Charles, Xiao Renshaw Wang, Dongkyu Lee, Kelsey A. Stoerzinger, Sokseiha Muy et al. "Tuning perovskite oxides by strain: electronic structure, properties, and functions in (electro) catalysis and ferroelectricity." Materials Today31 (2019): 100-118.Gómez-Bombarelli, Rafael, Jennifer N. Wei, David Duvenaud, José Miguel Hernández-Lobato, Benjamín Sánchez-Lengeling, Dennis Sheberla, Jorge Aguilera-Iparraguirre, Timothy D. Hirzel, Ryan P. Adams, and Alán Aspuru-Guzik. "Automatic chemical design using a data-driven continuous representation of molecules." ACS central science4, no. 2 (2018): 268-276.

Atomic Layer Deposition of Ruthenium Dioxide Thin Films Using RuO4 and Alcohols As Reactants

Ruthenium dioxide (RuO2) is the most stable oxide of Ru and has attracted tremendous attention in different applications such as supercapacitors, catalysis, and electrochemical devices. The high conductivity, good chemical stability, a work function value that is even higher than metallic Ru are few properties that make this material so demanding1. Although RuO2 is such an interesting material, atomic layer deposition (ALD) literature reports on this material are scarce. The existing ALD processes based on metalorganic Ru precursors necessitate careful tuning of the O2 partial pressure in order to deposit RuO2 films, as Ru metal is produced at lower O2 partial pressures. Furthermore, reported processes often require high temperature (>180°C) depositions and suffer from high nucleation delays, up to several hundred cycles in some cases.2 In this scenario, we present a novel ALD strategy solely for the synthesis of RuO2 films that takes advantage of the reaction between various alcohols and ruthenium tetroxide (RuO4) without any significant nucleation delay. The process, which uses methanol and RuO4 as reactants, exhibits a growth per cycle (GPC) of 1 angstrom (Å) per cycle (Figure 1a and 1b) at a substrate temperature as low as 60 °C. A constant GPC of 1Å is obtained in the temperature window of 60 °C-120 °C, while the RuO4 precursor is known to decompose above 125 °C.3 The depositions result in amorphous RuO2 films, as confirmed with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The films can be transformed into crystalline rutile RuO2 by annealing in helium or air at around 400 °C (Figure 1c). The films are conductive, as is evident from the resistivity value of 230 µΩ.cm for a 20 nm film, and the conductivity improved slightly after the anneal.Interestingly, the reaction of other alcohol molecules such as ethanol, 1-propanol, and 2-propanol with RuO4 results in a higher GPC compared to the methanol-based process and it is concluded that the GPC can be increased by increasing the number of carbon atoms in the alcohol chain (Figure 1d). However, all these processes employing different alcohols display typical self-saturating ALD properties. The reaction mechanism of the developed ALD approach was studied in depth using in situ mass spectrometry and in vacuo XPS studies. A reaction mechanism is proposed based on these learnings, wherein during the first half-cycle, alcohol molecules are oxidized into aldehyde and H2O on a RuO2 surface. This in turn causes the partial reduction of the surface RuO2 layer into RuOx or Ru. During the next half-cycle RuO4 oxidizes the surface back to RuO2 and additional RuO2 is deposited on the surface. References Ryden, W.; Lawson, A.; Sartain, C. C., Electrical Transport Properties of Ir O 2 and Ru O 2. Physical Review B 1970, 1 (4), 1494.Austin, D. Z.; Jenkins, M. A.; Allman, D.; Hose, S.; Price, D.; Dezelah, C. L.; Conley Jr, J. F., Atomic layer deposition of ruthenium and ruthenium oxide using a zero-oxidation state precursor. Chemistry of Materials 2017, 29 (3), 1107-1115.Minjauw, M. M.; Dendooven, J.; Capon, B.; Schaekers, M.; Detavernier, C., Atomic layer deposition of ruthenium at 100° C using the RuO 4-precursor and H 2. Journal of Materials Chemistry C 2015, 3 (1), 132-137. This work is funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No.765378. Figure 1

First-principles investigation of the metal-insulator transition in rutile RuO2

The electronic and magnetic properties of rutile ruthenium dioxide (RuO2) are extensively investigated by employing density functional theory. The material RuO2 is observed as a nonmagnetic metal. The sharing of a single Ru 4d electron by the dyz/xz orbitals is responsible for the metallicity of RuO2. The ground-state properties of this material change significantly upon the application of on-site Coulomb interaction U. RuO2 preserves its nonmagnetic metallic character up to U=3 eV. Nevertheless, RuO2 encounters a first step transition from nonmagnetic metal to a ferromagnetic metal (FM) at U=4 eV. The present system remains in its FM phase till U=5 eV. It exhibits a second step transition from the FM to a half-metallic ferromagnetic phase at U=6 eV. Eventually, RuO2 encounters metal-insulator transition (MIT) at U=7 eV with a band gap of Eg ∼0.3 eV upholding ferromagnetism. It is revealed that the spin polarization arising from the strong dynamical electron correlation due to the application of U is responsible for the metal to half-metal and MIT in RuO2.

Explicit solvent effects on (1 1 0) ruthenium oxide surface wettability: Structural, electronic and mechanical properties of rutile RuO2 by means of spin-polarized DFT-MD

The structural, electronic and mechanical properties of crystalline ruthenium dioxide RuO2 in its rutile structure have been calculated via forefront spin-polarized Density Functional Theory (DFT) and DFT-Molecular Dynamics (DFT-MD) simulations. Notwithstanding RuO2 is known as a highly active catalyst for a wide number of (photo) electrochemical reactions in aqueous/humid environments, the study of the interaction of RuO2 surfaces with water has been confined largely to (static) surface science and water adsorption calculations. Herein, an atomistic understanding of bulk rutile RuO2 and explicit solvent effects on (110)-RuO2 facet are provided. We especially focus on the comprehension of the mechanistic interplay between surface wettability, interfacial water dynamics and surface chemical activity.Analysis and characterization of the interfacial water and H-bond environment reveal how explicit liquid water and its dynamics play a role in the surface reconstruction and in the hydrophobic nature of the (110)-RuO2 facet with a prevailing H-bond acceptor character. Moreover, we provide a dependence of physical and chemical properties, such as surface electric field and work function, from different degrees of surface wettability of the (110)-RuO2 facet. Results on bulk properties of crystalline ruthenium dioxide RuO2 are in good agreement with the nowadays available experimental evidences and previous DFT studies.

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

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

Electrooxidation of 2-propanol on the mixture of nanoparticles of Pt and RuO2 supported on Ti

Abstract A thermal process was employed to prepare a catalyst consisting of a mixture of metallic-Pt and rutile RuO2 nanocrystals. This catalyst was used for the electrooxidation of 2-propanol in an alkaline solution. The effect of the catalyst composition on its microstructure, surface properties and catalytic activity was examined. With increasing the RuO2 content, the catalytic activity increases, reaches its maximum and then decreases. The catalytic effect is a result of the bifunctional mechanism of the mixture of Pt and RuO2 nanocrystals. The RuOHad particles are formed on Ru atoms of the RuO2 nanocrystals at potentials more negative than on Pt atoms. These oxy-species facilitate the dehydrogenation, breaking of C–C bonds and oxidation of both 2-propanol and its intermediates, adsorbed on assemblies of adjacent Pt atoms.

Oxygen evolution reaction: Bifunctional mechanism breaking the linear scaling relationship.

The bifunctional mechanism for the oxygen evolution reaction (OER) involving two distinct reaction sites is studied through the computational hydrogen electrode method for a set of catalyst materials including rutile TiO2(110), anatase TiO2(101), SnO2(110), RuO2(110), IrO2(110), Ni2P(0001), and BiVO4(001). The calculations are performed both at the semilocal level and at the hybrid functional level. Moreover, anodic conditions are modeled and their effect on the OER free energy steps is evaluated. The free energies of the reaction steps indicate that for specific combinations of catalysts, the limitations due to the linear scaling relationship can be overcome, leading to smaller overpotentials for the overall OER. At the same time, a detailed analysis of the results reveals a strong dependence on the adopted functional. For both functionals, it is shown that the energy level of the highest occupied electronic state can serve as a descriptor to guide the search for the optimal catalyst acting as a hydrogen acceptor. These results support the bifunctional mechanism as a means to break the linear scaling relationship and to further reduce the overpotential of the OER.

Subnano Ruthenium Species Anchored on Tin Dioxide Surface for Efficient Alkaline Hydrogen Evolution Reaction

Summary Single-atom catalysts with unique electronic structures are drawing increasing attention as compared with nano-catalysts. However, subnano-catalysts, falling between the two categories, may match the demands of catalytic reactions better due to their tunable electronic structure, but they have rarely been studied. Here, we report a subnano-ruthenium species anchored on nano-SnO2 (Ru@SnO2). Although rutile SnO2 and RuO2 are isostructural and tend to form a spinodal structure in bulk materials, the Ru@SnO2 nano-structure is successfully prepared by our newly developed micro-etching technique. The optimized sample displays high activity for alkaline hydrogen evolution reaction with low overpotential and flat Tafel slope, superior to commercial Pt/C (20 wt%), while density functional theory investigations on the hydrogen-binding energy and Gibbs free energy are consistent with these results. The inherent design and synthesis strategies reported herein may open a new avenue for further catalyst development.

Hydrogen peroxide synthesis from water and oxygen using a three-component nanohybrid photocatalyst consisting of Au particle-loaded rutile TiO2 and RuO2 with a heteroepitaxial junction.

Thin heteroepitaxial (HEPI) layers of RuO2 were selectively formed on the TiO2 surface of Au nanoparticle-loaded rutile TiO2 particles (RuO2#TiO2-Au) with an orientation of RuO2(110)//TiO2(110) by a hydrothermal method, and the three-component nanohybrid exhibits a high photocatalytic activity far exceeding that of Au/TiO2 for hydrogen peroxide generation from water and oxygen due to the HEPI junction-induced unique morphology of RuO2.

Temperature controlled Ru and RuO2 growth via O* radical-enhanced atomic layer deposition with Ru(EtCp)2.

This work demonstrates by in vacuo X-ray photoelectron spectroscopy and grazing-incidence X-ray diffraction that Ru(EtCp)2 and O* radical-enhanced atomic layer deposition, where EtCp means the ethylcyclopentadienyl group, provides the growth of either RuO2 or Ru thin films depending on the deposition temperature (Tdep), while different mechanisms are responsible for the growth of RuO2 and Ru. The thin films deposited at temperatures ranging from 200 to 260 °C consisted of polycrystalline rutile RuO2 phase revealing, according to atomic force microscopy and the four-point probe method, a low roughness (∼1.7 nm at 15 nm film thickness) and a resistivity of ≈83 µΩ cm. This low-temperature RuO2 growth was based on Ru(EtCp)2 adsorption, subsequent ligand removal, and Ru oxidation by active oxygen. The clear saturative behavior with regard to the precursor and reactant doses and each purge time, as well as the good step coverage of the film growth onto 3D structures, inherent to genuine surface-controlled atomic layer deposition, were confirmed for the lowest Tdep of 200 °C. However, at Tdep = 260 °C, a competition between film growth and etching was found, resulted in not-saturative growth. At higher deposition temperatures (300-340 °C), the growth of metallic Ru thin films with a resistivity down to ≈12 µΩ cm was demonstrated, where the film growth was proved to follow a combustion mechanism known for molecular oxygen-based Ru growth processes. However, this process lacked the truly saturative growth with regard to the precursor and reactant doses due to the etching predominance.

Engineering the Atomic Layer of RuO2 on PdO Nanosheets Boosts Oxygen Evolution Catalysis.

We report an atomic-scale controllable synthesis of the face-centered cubic Ru overlayers on Pd nanosheets (Pd@Ru NSs) by a solution-based epitaxial growth method. The thickness of Ru overlayers can be accurately tuned at an atomic level, which has been confirmed by atomic force microscopy and high-angle annular dark-field scanning transmission electron microscopy. After annealing in air, the Pd@Ru NSs were transformed to PdO@RuO2 NSs with rutile RuO2 epitaxially grown on the PdO. The oxygen evolution reaction (OER) activity and stability strongly depend on the atomic layers of RuO2 and ∼4 atomic layers of RuO2 (PdO@RuO2-4layers) exhibit superior stability and optimal activity for OER with only 257 mV of the overpotential to reach 10 mA cm-2. Density functional theory calculations well reproduce the thickness dependence of OER activity and reveal that O* binds more weakly on the PdO@RuO2-4layers that boosts the rate-determining step for formation of HOO*, assuring the best OER performance.

Intrinsic Thermal Shock Behavior of Common Rutile Oxides

Rutile TiO2, VO2, CrO2, MnO2, NbO2, RuO2, RhO2, TaO2, OsO2, IrO2, SnO2, PbO2, SiO2, and GeO2 (space group P42/mnm) were explored for thermal shock resistance applications using density functional theory in conjunction with acoustic phonon models. Four relevant thermomechanical properties were calculated, namely thermal conductivity, Poisson’s ratio, the linear coefficient of thermal expansion, and elastic modulus. The thermal conductivity exhibited a parabolic relationship with the linear coefficient of thermal expansion and the extremes were delineated by SiO2 (the smallest linear coefficient of thermal expansion and the largest thermal conductivity) and PbO2 (vice versa). It is suggested that stronger bonding in SiO2 than PbO2 is responsible for such behavior. This also gave rise to the largest elastic modulus of SiO2 in this group of rutile oxides. Finally, the intrinsic thermal shock resistance was the largest for SiO2, exceeding some of the competitive phases such as Al2O3 and nanolaminated Ti3SiC2.

Titania surface chemistry and its influence on supported metal catalysts

This work characterized the surface chemistry of a number of different titania samples including four commercial anatase samples, an anatase sample that we synthesized, the pyrogenic titania samples P25 and P90, and a commercial rutile sample. X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), were used to identified surface species that might interfere with the acid/base properties of the surface hydroxyls. All commercial anatase samples were contaminated by sulfur, which diminished their effectiveness as metal oxide supports for heterogeneous catalysis and has implications for their utility as photocatalysts. Hydrogen-bonded surface hydroxyls remained after calcination up to 400 °C for all anatase samples, in contrast to rutile and the pyrogenic titania materials P25 and P90, in which they were eliminated. Ru(0) catalysts on titania without hydrogen-bonded surface hydroxyls showed enhanced CO hydrogenolysis selectivity in the presence of water while Ru(0) catalysts on titania with hydrogen-bonded surface hydroxyls showed diminished selectivity in water, suggesting that surface hydrophilicity is important for this reaction. Heteroepitaxy between rutile RuO2 and rutile TiO2 is not essential for the creation of small evenly-spaced supported Ru(0) nanoparticles, which are important in many catalytic reactions.

Rutile RuO2 dispersion on rutile and anatase TiO2 supports: The effects of support crystalline phase structure on the dispersion behaviors of the supported metal oxides

Abstract In this study, to investigate the influence of support crystalline phase structure on the dispersion behaviors of the supported metal oxides, tetragonal rutile RuO2 supported on both tetragonal anatase a-TiO2 and tetragonal rutile r-TiO2 supports with different loadings have been prepared, characterized and probed by CO oxidation. It has been discovered that the monolayer dispersion capacity of RuO2 on r-TiO2 support is 0.84 mmol per 100 m2 surface, which is much higher than 0.15 mmol per 100 m2 surface on a-TiO2 support. Due to the close matching of the crystalline phase and lattice parameters between RuO2 and r-TiO2, Ru O Ti interfacial bonds can be effectively formed, thus facilitating the RuO2 dispersion and resulting in a high monolayer dispersion capacity. On the contrary, the big difference of the crystalline phase and lattice parameters between RuO2 and a-TiO2 support impedes the formation of Ru O Ti interfacial bonds and RuO2 dispersion, resulting in a low monolayer dispersion capacity. As a result, RuO2 has a much stronger interaction with r-TiO2 than with a-TiO2 support, which can generate more abundant mobile oxygen sites. Therefore, in comparison with RuO2/a-TiO2, the intrinsic activity of RuO2/r-TiO2 for CO oxidation is remarkably enhanced. On the basis of the results, it is rational to propose that a like-disperses-like phenomenon is present in the supported metal oxide systems.

Surface morphology and electrochemical properties of RuO2-doped Ti/IrO2-ZrO2 anodes for oxygen evolution reaction

RuO2-doped IrO2-ZrO2 ternary oxide coatings with different Ru/Ir/Zr mole ratios were synthesized using a sol-gel route. The surface morphology was determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS) analysis. The electrochemical behavior was investigated using linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and accelerated life test (ALT). The results showed that the compositions of the coatings are rutile IrO2 and RuO2. ZrO2 is present as an amorphous phase in the coatings. The coating with Ru content of 21 mol% shows a compact structure with needle-like crystals dispersed. With the increase in Ru content, the porous and cracked structure of the ternary oxide coating of increased active surface area is obtained. Increasing Ru content improves the electrocatalytic activity, but worsens the stability of the coating in OER. The coating with Ru content of 21 mol% shows good electrocatalytic activity and the longest service lifetime in OER.