C. Strebel

Technical University of Denmark, København, Capital Region, Denmark

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Publications (9)49.73 Total impact

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    ABSTRACT: Mass-selected nanoparticles can be conveniently produced using magnetron sputtering and aggregation techniques. However, numerous pitfalls can compromise the quality of the samples, e.g. double or triple mass production, dendritic structure formation or unpredicted particle composition. We stress the importance of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) for verifying the morphology, size distribution and chemical composition of the nanoparticles. Furthermore, we correlate the morphology and the composition of the PtxY nanoparticles with their catalytic properties for the oxygen reduction reaction. Finally, we propose a completely general diagnostic method, which allows us to minimize the occurrence of undesired masses.
    Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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    ABSTRACT: Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy Pt x Y is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model Pt x Y nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mg Pt −1 at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of Pt x Y over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core. P olymer electrolyte membrane fuel cells (PEMFCs) hold the potential to provide a zero-emission power source for future automotive applications. However, their widespread commer-cialization is hindered by the high loadings of Pt required to catalyse the oxygen reduction reaction (ORR) at the cathode 1–4 . An order of magnitude increase in ORR mass activity (that is, current density per unit mass Pt) over state-of-the art commercial pure Pt catalysts would bring the precious-metal loading in fuel cells to a similar level to that used for emission control in internal combustion engines 3,5 . Some alloys of PtX (X = Co, Ni, Cu) show higher ORR activity in comparison to pure Pt (refs 1–4), but typically their long-term performance is compromised by their poor stability against dealloying 6,7 . In recent years, progress has been made towards the stabilization of Pt-based catalysts 8–12
    Nature Chemistry 01/2014; · 21.76 Impact Factor
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    ABSTRACT: Using temperature-programmed desorption experiments, we have studied the coordination dependent adsorption of CO on a platinum (Pt) single crystal, and mass-selected Pt nanoparticles in the size range of 3 to 11 nm, for CO dosing pressures in 10–7 mbar and mbar ranges. From low pressure CO adsorption experiments on the Pt(111) crystal, we establish a clear link between the degree of presputtering of the surface prior to CO adsorption, and the amount of CO bound at high temperature. It was found that for rougher surfaces, i.e., with more undercoordinated surface atoms, a feature appears in the CO desorption spectra at high temperature. The result is consistent with literature results from stepped single crystals that have found high temperature CO desorption features due to the presence of undercoordinated step and kink sites on the crystal facets. For the nanoparticles, a study of the dependence of the CO desorption profile with particle size found more prominent high temperature CO desorption features as the nanoparticle size was decreased, consistent with the expectation for a higher proportion of undercoordinated sites at smaller particle sizes. Thus, for both systems there is a clear relation between surface atom coordination, and the desorption temperature of CO. Investigation of these structural features was then made for CO dosing pressures in the mbar range. Intriguingly, from the mbar pressure experiments it was observed that elevated CO pressures enhanced the annealing of the Pt(111) surface, but on the otherhand, caused an apparent roughening of the nanoparticles.
    The Journal of Physical Chemistry C. 07/2012; 116(29):15353–15360.
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    ABSTRACT: The active sites for CO dissociation were probed on mass-selected Ru nanoparticles on a HOPG support by temperature programmed desorption spectroscopy using isotopically labelled CO. Combined with transmission electron microscopy we gain insight on how the size and morphology of the nanoparticles affect the CO dissociation activity. The Ru nanoparticles were synthesized in a UHV chamber by gas-aggregation magnetron sputtering in the size range from 3 to 15 nm and the morphology was investigated in situ by scanning tunneling microscopy and ex situ by high resolution transmission electron microscopy. Surprisingly, it was found that larger particles were more active per surface area for CO dissociation. It is suggested that this is due to larger particles exposing a more rough surface than the smaller particles, giving rise to a higher relative amount of under-coordinated adsorption sites on the larger particles. The induced surface roughness is proposed to be a consequence of the growth processes in the gas-aggregation chamber.
    Physical Chemistry Chemical Physics 05/2012; 14(22):8005-12. · 4.20 Impact Factor
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    ABSTRACT: A matter of size: The particle size effect on the activity of the oxygen reduction reaction of size-selected platinum clusters was studied. The ORR activity decreased with decreasing Pt nanoparticle size, corresponding to a decrease in the fraction of terraces on the surfaces of the Pt nanoparticles (j(k) =kinetic current density, see picture).
    Angewandte Chemie International Edition 03/2012; 51(19):4641-3. · 11.34 Impact Factor
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    ABSTRACT: Using model catalysts, we demonstrate that CO desorption from Ru surfaces can be switched from that typical of single crystal surfaces to one more characteristic of supported nanoparticles. First, the CO desorption behaviour from Ru nanoparticles supported on highly oriented pyrolytic graphite was studied. Both mass-selected and thermally evaporated nanoparticles were deposited. TPD spectra from the mass-selected nanoparticles exhibit a desorption peak located around 410 K with a broad shoulder extending from around 480 K to 600 K, while spectra obtained from thermally evaporated nanoparticles exhibit a single broad feature from ∼350 K to ∼450 K. A room temperature deposited 50 Å thick Ru film displays a characteristic nanoparticle-like spectrum with a broad desorption feature at ∼420 K and a shoulder extending from ∼450 K to ∼600 K. Subsequent annealing of this film at 900 K produced a polycrystalline morphology of flat Ru(001) terraces separated by monatomic steps. The CO desorption spectrum from this surface resembles that obtained on single crystal Ru(001) with two large desorption features located at 390 K and 450 K due to molecular desorption from terrace sites, and a much smaller peak at ∼530 K due to desorption of dissociatively adsorbed CO at step sites. In a second experiment, ion sputtering was used to create surface defects on a Ru(0 1 54) single crystal surface. A gradual shift away from the desorption spectrum typical of a Ru(001) surface towards one resembling desorption from supported Ru nanoparticles was observed with increasing sputter time.
    Physical Chemistry Chemical Physics 06/2011; 13(21):10333-41. · 4.20 Impact Factor
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    ABSTRACT: The scanning tunneling microscopy and temperature programmed oxidation methods were used to study the catalytic oxidation of graphite by mass-selected Ru nanoparticles. Channeling by the nanoparticles was observed on the unsputtered HOPG basal surface at temperatures above 750 °C in 10−6 mbar O2. Ar+ ion bombardment was used to create layers of disordered carbon of various depths on the HOPG surface. The channel propagation rate in the disordered carbon layer was found to increase for larger nanoparticles. The depth of the interface between the disordered carbon layer and the graphite determined whether the nanoparticles etched paths parallel or perpendicular to the surface. The gasification onset temperature depended on the degree of graphitisation of the surface, with more heavily sputtered surfaces undergoing gasification at much lower temperatures than the unsputtered surface.Graphical abstractRu nanoparticles act as catalysts for the oxidation of substrate carbon. The etch channels follow random paths when the nanoparticles etch disordered carbon but are highly directional when the nanoparticles are in contact with the graphite basal plane. The propagation rate increases with particle size.Research highlights► Size-selected Ru nanoparticles catalytically oxidise sputtered HOPG surfaces. ► Ru nanoparticles channel randomly through the disordered carbon layers. ► The nanoparticles become trapped at the graphite/disordered carbon interface. ► The onset temperature for the etching depends on how much the surface is pre-sputtered. ► The etch rate scales with the nanoparticle size.
    Carbon. 01/2011; 49(2):376-385.
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    ABSTRACT: Scanning tunneling microscopy was used to compare the morphologies of Ru nanoparticles deposited onto highly-oriented graphite surfaces using two different physical vapour deposition methods; (1) pre-formed mass-selected Ru nanoparticles with diameters between 2 nm and 15 nm were soft-landed onto HOPG surfaces using a gas-aggregation source and (2) nanoparticles were formed by e-beam evaporation of Ru films onto HOPG. The particles generated by the gas-aggregation source are round in shape with evidence of facets resolved on the larger particles. Annealing these nanoparticles when they are supported on unsputtered HOPG resulted in the sintering of smaller nanoparticles, while larger particles remained immobile. Nanoparticles deposited onto sputtered HOPG surfaces were found to be stable against sintering when annealed. The size and shape of nanoparticles deposited by e-beam evaporation depend to a large extent on the state of the graphite support and the temperature. Ru deposition onto unsputtered HOPG is characterised by bimodal growth with large flat particles formed on the substrate terraces and smaller diameter particles aligned along the substrate steps. Evaporation onto sputtered HOPG results in the formation of 2 nm round particles with a narrow size distribution. Finally, thermal deposition onto both sputtered and unsputtered HOPG at 660 °C results in larger particles showing a flat Ru(0 0 0 1) top facet.
    Surface Science 01/2009; 603(24):3420-3430. · 1.87 Impact Factor
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    ABSTRACT: We have investigated the morphology of mass selected ruthenium nanoparticles produced with a magnetron-sputter gas-aggregation source. The nanoparticles are mass selected using a quadrupole mass filter, resulting in narrow size distributions and average diameters between 2 and 15nm. The particles are imaged in situ by scanning electron microscopy and scanning tunneling microscopy (STM) as well as ex-situ using transmission electron microscopy (TEM). For each distribution of mass selected nanoparticles, the height determined by STM and the width determined by TEM are seen to be similar throughout the mass range investigated. The particles are found to have a well-defined morphology for diameters below approximately 6nm. Larger nanoparticles are less well-defined having rough surfaces, unlike the equilibrium morphology determined from the Wulff construction. The morphology of the particles is, in general, believed to be determined by the conditions inside the gas-aggregation source and the morphology is retained as the particles are soft-landed on the substrate. KeywordsHOPG-Magnetron-sputter gas-aggregation source-Mass selected nanoparticles-Nanoparticle morphology-Scanning tunneling microscopy-Synthesis and characterization-Transmission electron microscopy-Ruthenium
    Journal of Nanoparticle Research 12(4):1249-1262. · 2.18 Impact Factor