The palladium-hydrogen system plays a crucial role in catalysis, hydrogen production and storage, hydrogen embrittlement, and sensing technologies. Understanding the transition of palladium nanocrystals (NCs) from the hydrogen-poor (α) phase to the hydrogen-rich (β) phase is crucial for elucidating hydrogen absorption/desorption mechanisms as well as related phenomena such as hydrogen trapping. In this study, we carefully minimized undesired X-ray beam effects and used in situ Bragg coherent diffraction imaging under electrochemical control to map the strain and lattice parameter distribution within individual palladium NCs across electrochemical potentials relevant to hydrogen absorption and desorption. Lattice parameter changes in both α and β phases are tracked, and reversible strain inversion during the α-to-β phase transition is observed. Through strain and reciprocal space analysis and molecular simulations, a model for the α-to-β phase transition is proposed, which includes a hydrogen-saturated subsurface shell, hydrogen depletion from the α phase during β phase nucleation, and propagation of the β phase in a spherical-cap fashion.

Palladium hydrogen is a useful model in the study of both hydrogen absorption for energy storage, and lattice gas systems for fundamental thermodynamic models. Using in situ time-resolved X-ray nanodiffraction at the fourth generation Extremely Brilliant Source of the European Synchrotron (ESRF-EBS), the kinetics of hydrogen absorption in individual alpha phase Pd nanoparticles is examined. Hydrogen absorption kinetics in a gas reactor and an electrochemical cell are compared. Combining the individual nanoparticle X-ray measurements with chronoamperometry measurements, the kinetics of the ensemble of Pd nanoparticles on the glassy carbon substrate is compared with kinetics at the single nanoparticle level. Hydrogen absorption in alpha phase Pd in the electrochemical system is found to be slower than that of the gas system. Furthermore, the absorption in the electrochemical system slows down as the electrochemical potential is lowered. This slow down is found to be directly related to the increasing hydrogen absorption per step in electrode potential. Furthermore, differences between absorbed-quantity normalized absorption times is seen between the hydrogen and deuterium absorbates. Sieverts's law of absorption is also shown to hold for individual Pd nanoparticles in the alpha phase.

The contact resistance between the anode catalyst layer and the titanium (Ti)-based porous transport layer (PTL) of a proton exchange membrane water electrolyzer (PEMWE) can limit the efficiency of the system and its durability. Generally, the PTL side in contact with the anode is coated with a precious metal, such as platinum. This results in an increased overall cost of the system. Here, we report on the influence of various metal coatings (400 nm coatings of platinum and gold; 2, 10, 400 nm coatings of iridium) on the beginning of life performance and on the durability of a PEMWE device. The durability tests included varying the voltage between 1.5 and 2.2V and between 0 and 2.2V with a total test duration of 510 h by sample). The best beginning of life performances are obtained with a platinum coating but the best durability during start-stop events is obtained with a 10 nm iridium coating (representing less than 2% of the amount of iridium present in the system). The influence of the clamping stress is also evaluated. Whatever the nature of the coating, the electrical contact resistance decreases with an increase in the clamping stress and depends on the clamping history.

The mechanisms of oxidation of glucose, gluconic acid, and sorbitol have been studied on gold, platinum, and palladium using cyclic voltammetry (CV), differential electro-chemical mass spectrometry (DEMS), and in situ Fourier transform infrared (FTIR) spectroscopy. The nature of the reactant has a strong impact on the onset of the oxidation reaction. The anomeric function of glucose is oxidized at low potentials on the three surfaces, while gluconic acid and sorbitol poison the surface at low potentials. In addition, the nature of the metal surface leads to different reaction pathways. It is proposed that the oxidation of glucose initiates via the partial dissociative adsorption of glucose into glucose adsorbates and adsorbed H (Had) for the three metal surfaces. These adsorbates are partially combined into H2 on Au and oxidized into water on Pt and Pd. In addition, Au features the best activity, selectivity, and specificity for glucose oxidation into gluconate at low potentials. The study points out a reactant, catalyst, and potential dependent mechanism.

Proton-exchange membrane water electrolyzers (PEMWEs) electrodes use scarce and costly platinum group metals (PGMs), but only such devices are able to meet the requirements associated with renewable energies (large amplitude and frequent and rapid variations in the current applied to the cell). Catalysts represent only 8 % to the overall stack cost, 6 % being associated with the high iridium (Ir) loading used to electrocatalyze the anodic oxygen evolution reaction (OER). High surface area supported Ir oxide (IrOx) catalysts thus represent a promising strategy to reduce the cost of this technology and limit the geological pressure on Ir. However, the Gibbs-Thompson effect, which controls the electrochemical stability of nanomaterials casts a doubt on the viability of this approach. To shed light into the benefits and limitations of supported and unsupported IrOx catalysts, we benchmarked commercial materials (unsupported IrO2, Ir/C), unsupported porous IrOx microparticles, and IrOx nanoparticles (NPs) supported on carbon black or on doped tin oxide aerogels (AG)/nanofibers (NFs). Transmission electron microscopy (TEM) and identical-location transmission electron microscopy (IL-TEM) provided changes in morphology during accelerated stress testing. Complementarily, a flow cell connected to an inductively-coupled mass spectrometer (FC-ICP-MS) was used to assess their stability number (S-number, see Figure 1). The results show that supported IrOx nanocatalysts are extremely active toward the OER because they feature mixed Ir oxidation states and a high density of active sites (small crystallite/particle size). The lack of robustness of their supports, however, prevents the nanocatalysts from sustaining this high OER activity [1, 2]. Similar to what was observed on extended surfaces, we report that mild thermal annealing (450°C) leads to lower Ir atom dissolution rate. Overall, the best compromise between OER activity and stability was obtained for unsupported porous IrOx microparticles after mild thermal annealing under air at 450°C [2]. On the cathode side, IL-TEM measurements revealed mild changes in morphology for Pt/C nanoparticles [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033). References S. Abbou, R. Chattot, V. Martin, F. Claudel, L. Solà-Hernández, C. Beauger, L. Dubau, F. Maillard, ACS Catal. 10 (2020) 7283-7284. C. Daiane Ferreira da Silva, F. Claudel, V. Martin, R. Chattot, S. Abbou, K. Kumar, I. Jiménez-Morales, S. Cavaliere, D. Jones, J. Rozière, L. Solà-Hernandez, C. Beauger, M. Faustini, J. Peron, B. Gilles, C. Beauger, L. Piccolo, F. H. Barros de Lima, L. Dubau, F. Maillard, ACS Catal. 11 (2021) 4107-4116. A. Viola, L. Dubau, F. Maillard, in preparation. Figure 1. S-number values calculated for supported and unsupported IrOx electrocatalysts during a galvanostatic accelerated stress test (j = 10 mA cm-2 geo, T = 80 °C, U cut-off = 2 V vs. RHE) performed in Ar-saturated 0.05 M H2SO4. Reprinted with permission from ref. [2]. Copyright 2021 American Chemical Society. Figure 1

A broad variety of defects has been observed in two-dimensional materials. Many of these defects can be created by top-down methods such as electron irradiation or chemical etching, while a few of them are created along bottom-up processes, in particular during the growth of the material, in which case avoiding their formation can be challenging. This occurs e.g. with dislocations, Stone-Wales defects, or atomic vacancies in graphene. Here we address a defect that has been observed repeatedly since 2007 in epitaxial graphene on metal surfaces like Ru(0001) and Re(0001), but whose nature has remained elusive thus far. This defect has the appearance of a vacant hill in the periodically nanorippled topography of graphene, which comes together with a moir{\'e} pattern. Based on atomistic simulations and scanning tunneling microscopy/spectroscopy measurements, we argue that such defects are topological in nature and that their core is a stacking fault patch, either in graphene, surrounded by loops of non-hexagonal carbon rings, or in the underlying metal. We discuss the possible origin of these defects in relation with recent reports of metastable polycyclic carbon molecules forming upon graphene growth. Like other defects, the vacant hills may be considered as deleterious in the perspective of producing high quality graphene. However, provided they can be organized in graphene, they might allow novel optical, spin, or electronic properties to be engineered.

Gaining fundamental insights into the formation and the stability of iridium (Ir) surface oxides is pivotal to efficient and sustainable oxygen evolution reaction (OER) electrocatalysis, a key reaction in the field of power-to-fuels and power-to-chemicals. However, proton-exchange membrane water electrolysers (PEMWE) systems currently suffer from their high investment and operational costs, which are partly due to the use of micrometer-sized iridium oxide (IrOx) particles to electrocatalyze the OER. A passage to IrOx nanoparticles would be highly desirable but these nanomaterials face time-dependent changes in structure and chemical composition in OER conditions. To shed fundamental light into the mechanisms at stake, we used electrochemical techniques in combination to X-ray photoelectron spectroscopy (XPS), and inductively coupled mass plasma spectrometry (ICP-MS). Experiments on Ir(111), Ir(210) and nanostructured Ir(210) surfaces showed that the oxy-hydroxides layers forming in the pre-OER region feature mixed Ir oxidation states (presence of Ir(0), Ir(+III) and Ir(+IV) species), and that the fraction of each oxidation state depends on the crystallographic orientation. In the OER region, Ir(+III) species progressively dissolve leading to an enrichment of the surface and near-surface regions of the single crystals into Ir(+IV) species, and resulting in a decrease of their intrinsic activity towards the OER. The results indicate a convergence towards a more stable but less active surface state, which does not depend neither on the initial arrangement of surface atoms (crystallographic orientation, proportion of high- and low-coordinated atoms) nor on their oxidation state (initial state vs. electrochemically-activated Ir(hkl) surfaces) [1]. These findings were confirmed on more technologically relevant materials such as IrOx nanoparticles supported onto antimony-doped (ATO), niobium-doped (NTO) or tantalum-doped (TaTO) tin oxide aerogels [2]. Moreover, the combination of identical-location transmission electron microscopy (IL-TEM) and in situ ICP-MS helped in identifying which potential ranges are critical to the stability of IrOx nanocatalysts and their supports, and provided practical guidelines for the development of more active and more stable PEMWE anodes [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033). References 1. M. Scohy, S. Abbou, V. Martin, B. Gilles, E. Sibert, L. Dubau, F. Maillard, ACS Catal. 9 (2019) 9859-9869. 2. F. Claudel, L. Dubau, G. Berthomé, L. Solà-Hernandez, C. Beauger, L. Piccolo, F. Maillard, ACS Catal. 9 (2019) 4688-4698. 3. S. Abbou, R. Chattot,F. Claudel, L. Dubau, L. Solà-Hernandez, C. Beauger, L. Dubau, F. Maillard, in preparation. Figure 1. Schematics of the main findings obtained on Ir(hkl) single crystal surfaces: the initial crystallographic orientation and the fraction of high- and low-coordinated atoms influence the nature and the oxidation state of Ir surface atoms, thus determining their initial OER activity. The Janus nature of Ir(III) species is demonstrated: these species are the most active towards the OER but they get easily dissolved, leading eventually to a less active yet more stable surface state. Figure 1

Gaining fundamental insights into the formation and the stability of surface oxides on iridium (Ir) surfaces is pivotal to oxygen evolution reaction (OER) electrocatalysis. Herein, we examined the potential-dependent structural and chemical changes occurring on planar Ir(111), Ir(210), and nanofaceted Ir(210) single-crystal surfaces using electrochemistry, scanning probe microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. We show that, after polarization in OER conditions, Ir surface atoms feature mixed oxidation states—(0), (+III), and (+IV)—and then enrich into Ir(+IV) due to the dissolution of Ir(+III) species. The rate of surface and near-surface layer enrichment in Ir(+IV) species depends on the modulation mode of the potential (linear potential sweeps vs. potential steps) and is faster on opened surfaces. By combining fits derived from the XPS spectra and OER activity measurements, we found that the OER specific activity varies with the Ir oxidation state and is closely related to the fraction of Ir(+III) species.

Chemical vapor deposition (CVD) on metals is so far the best suited method to produce high-quality, large-area graphene. We discovered an unprecedentedly large family of small size-selective carbon clusters that form together with graphene during CVD. Using scanning tunneling microscopy (STM) and density functional theory (DFT), we unambiguously determine their atomic structure. For that purpose, we use grids based on a graphene moiré and a dilute atomic lattice that unambiguously reveal the binding geometry of the clusters. We find that the observed clusters bind in metastable configurations on the substrate, while the thermodynamically stable configurations are not observed. We argue that the clusters are formed under kinetic control and establish that the evolution of the smallest clusters is blocked. They are hence products of surface reactions in competition with graphene growth, rather than intermediary species to the formation of extended graphene, as often assumed in the literature. We expect such obstacles to the synthesis of perfect graphene to be ubiquitous on a variety of metallic surfaces.

We report on the electrochemical behavior of Pt/W(111) nanfaceted surface in sulfuric acid. The Pt/W(111) nanofaceted surface was characterized by X-ray photelectron spectroscopy (XPS) and Scanning Tunneling Micrsocopy (STM) before and after the electrochemical measurements. The X-ray photoelectron spectroscopy (XPS) have shown a partial oxidation of W(111) during electrochemical measurements. Comparison of STM images before and after the electrochemical measurements enabled to locate the region where the oxidation process took place.

The heteroepitaxial interface formed by copper deposited onto the tantalum (001) surface is studied by surface x-ray diffraction and ab initio calculations. The analysis of the crystal truncation rods reveals the presence of a wetting layer of copper made of two atomic planes pseudomorphic to the tantalum substrate, with the upper most atomic planes significantly deformed. These findings are in total agreement with the results of density-functional-theory calculations. The presence of the wetting layer confirms a Stranski-Krastanov growth mode and is thought to explain the extremely fast atomic diffusion of copper during the dewetting process in the solid state at high temperature.

We report on the structure, the chemisorption and the electrocatalytic properties of multilayer (2.2, 3.3 and 5.5 physical monolayers) Pt films deposited on W(111) elaborated by molecular beam epitaxy. The Pt/W(111) surfaces were characterized by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). Pronounced changes of the surface reactivity were noticed as the Pt coverage is decreased. In particular, the affinity for under-potentially deposited hydrogen (Hupd) and hydroxyl (OHads) species and the ability to electrooxidize a monolayer of COads were depreciated in agreement with strain and ligand effects.

In this study, we show that the platinum electrode preparation procedure influences its behavior towards the borohydride electrooxidation reaction (BOR) mechanism. Cycling a smooth polycrystalline Pt electrode in alkaline electrolyte within the water stability domain prior to the BOR characterization radically changes the shape of the BOR voltammogram obtained in hydrodynamic conditions using the rotating disk electrode (RDE) setup, compared to the “classical” one measured on a smooth polycrystalline Pt electrode just polished before the BOR RDE study. This particular BOR voltammogram is reversibly brought back to the “classical” one after voltammetric cycling in borohydride alkaline media. These changes in the BOR voltammogram highlight the sensitivity of the BOR mechanism towards the Pt surface morphology. A first comparison of the Pt electrode surface before and after the voltammetric cycling in alkaline media using tapping mode atomic force microscopy (AFM) shows no morphological differences between the two surfaces within the AFM observation range, suggesting a very fine atomic structure disordering of the Pt surface. Such strong dependence of the BOR mechanism on Pt regarding the electrode atomic structuring opens the way to future studies focusing on the BOR on well-defined Pt single crystals.

We report a new way to strongly couple graphene to a superconductor. The graphene monolayer has been grown directly on top of a superconducting Re(0001) thin film and characterized by scanning tunneling microscopy and spectroscopy. We observed a moiré pattern due to the mismatch between Re and graphene lattice parameters that we have simulated with ab initio calculations. The density of states around the Fermi energy appears to be position dependent on this moiré pattern. Tunneling spectroscopy performed at 50 mK shows that the superconducting behavior of graphene on Re is well described by the Bardeen-Cooper-Schrieffer theory and stands for a very good interface between the graphene and its metallic substrate.

A method to fabricate large-area superconducting hybrid tunnel junctions with a suspended central normal metal part is presented. The samples are fabricated by combining photo-lithography and chemical etch of a superconductor—insulator—normal metal multilayer. The process involves few fabrication steps, is reliable and produces extremely high-quality tunnel junctions. Under an appropriate voltage bias, a significant electronic cooling is demonstrated. We analyze semi-quantitatively the thermal behavior of a typical device.

Hard X-ray lens-less microscopy holds the promise of a resolution power meeting the need of nanoscience, owing to the possibility of circumventing the limits of state-of-the-art X-ray lenses [1].Beyond the resolution issue, the complex-valued wavefield is imaged, hence ensuring truly quantitative information on the sample scattering contrast.Furthermore, combining this approach to the Bragg geometry allows providing images of defects and strains in nanocrystals, in a non-destructive manner [2].Lens-less microscopy makes use of far-field coherent intensity patterns produced by third generation synchrotron sources.Instead of lenses, numerical approaches are employed to retrieve the exit-field at the sample position [1].An overview of the capabilities and the actual limits will be given in this presentation, in the specific case of crystalline imaging.In particular Bragg coherent diffraction imaging, Fourier transform holography [3] and ptychography [4] will be discussed and compared.The accurate and detailed knowledge of the crystalline structures at the nanoscale is highly desirable for its potential to bring new insight and understanding in a large variety of nanoscience material problems: this challenge is expected to be met by Bragg lens-less X-ray microscopy.

In this study, we investigated the role of oxygen in the faceting of the W(111) surface at temperatures close to T = 2000°C. For that purpose, we characterized the W(111) surface before and after the annealing step by low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES). It is found that W(111) undergoes a massive reconstruction to form three sided pyramids of nanometer dimensions with mainly {211} planes as facet sides. Interestingly, the facetted W(111) surface is deprived from oxygen. We then show how the facetted W(111) surface can be used as a template to deposit platinum by molecular beam epitaxy.

We have measured the transport properties of ferromagnet-superconductor nanostructures, where two superconducting aluminum (Al) electrodes are connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find that, below the superconducting critical temperature of Al, the resistance depends on the relative alignment of the ferromagnets' magnetization. This spin-valve effect is analyzed in terms of spin accumulation in the superconducting electrode submitted to inverse proximity effect.