Publications by authors named "Johannes Bernardi"

42 Publications

Steering the Methane Dry Reforming Reactivity of Ni/LaO Catalysts by Controlled In Situ Decomposition of Doped LaNiO Precursor Structures.

ACS Catal 2021 Jan 11;11(1):43-59. Epub 2020 Dec 11.

Department of Physical Chemistry, University of Innsbruck, Innrain 52c, A-6020 Innsbruck, Austria.

The influence of A- and/or B-site doping of Ruddlesden-Popper perovskite materials on the crystal structure, stability, and dry reforming of methane (DRM) reactivity of specific ABO phases (A = La, Ba; B = Cu, Ni) has been evaluated by a combination of catalytic experiments, in situ X-ray diffraction, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and aberration-corrected electron microscopy. At room temperature, B-site doping of LaNiO with Cu stabilizes the orthorhombic structure () of the perovskite, while A-site doping with Ba yields a tetragonal space group (4/). We observed the orthorhombic-to-tetragonal transformation above 170 °C for LaNiCuO and LaNiCuO, slightly higher than for undoped LaNiO. Loss of oxygen in interstitial sites of the tetragonal structure causes further structure transformations for all samples before decomposition in the temperature range of 400 °C-600 °C. Controlled in situ decomposition of the parent or A/B-site doped perovskite structures in a DRM mixture (CH:CO = 1:1) in all cases yields an active phase consisting of exsolved nanocrystalline metallic Ni particles in contact with hexagonal LaO and a mixture of (oxy)carbonate phases (hexagonal and monoclinic LaOCO, BaCO). Differences in the catalytic activity evolve because of (i) the in situ formation of Ni-Cu alloy phases (in a composition of >7:1 = Ni:Cu) for LaNiCuO, LaNiCuO, and LaBaNiCuO, (ii) the resulting Ni particle size and amount of exsolved Ni, and (iii) the inherently different reactivity of the present (oxy)carbonate species. Based on the onset temperature of catalytic DRM activity, the latter decreases in the order of LaNiCuO ∼ LaNiCuO ≥ LaBaNiCuO > LaNiO > LaBaNiO. Simple A-site doped LaBaNiO is essentially DRM inactive. The Ni particle size can be efficiently influenced by introducing Ba into the A site of the respective Ruddlesden-Popper structures, allowing us to control the Ni particle size between 10 nm and 30 nm both for simple B-site and A-site doped structures. Hence, it is possible to steer both the extent of the metal-oxide-(oxy)carbonate interface and its chemical composition and reactivity. Counteracting the limitation of the larger Ni particle size, the activity can, however, be improved by additional Cu-doping on the B-site, enhancing the carbon reactivity. Exemplified for the LaNiO based systems, we show how the delicate antagonistic balance of doping with Cu (rendering the LaNiO structure less stable and suppressing coking by efficiently removing surface carbon) and Ba (rendering the LaNiO structure more stable and forming unreactive surface or interfacial carbonates) can be used to tailor prospective DRM-active catalysts.
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http://dx.doi.org/10.1021/acscatal.0c04290DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7783868PMC
January 2021

Method for the production of pure and C-doped nanoboron powders tailored for superconductive applications.

Nanotechnology 2020 Dec;31(49):494001

CNR-SPIN, C.so M.F. Perrone 24, 16152 Genova, Italy. Physic Department of Genoa University, Via Dodecaneso 33, 16146 Genova, Italy.

The present paper describes the improvement of the performances of boron powder obtained applying the freeze-drying process (FDP) for the nanostructuration and doping of BO, which is here used as boron precursor. After the nanostructuration process, BO is reduced to elemental nanoboron (nB) through magnesiothermic reaction with Mg. For this work, the usefulness of the process was tested focusing on the carbon-doping (C-doping), using C, inulin and haemoglobin as C sources. The choice of these molecules, their concentration, size and shape, aims at producing improvements in the final compound of boron: in this case the superconductive magnesium diboride, which has been prepared and characterized both as powder and wire. The characteristics of BO, B and MgB powder, as well as MgB wire were tested and compared with that obtained using the best commercial precursors: H. C. Starck micrometric boron and Pavezyum nanometric boron. Both the FDP and the magnesiothermic reaction were carried out with simplicity and a great variety of doping sources, i.e. elements or compounds, which can be organic or inorganic and soluble or insoluble. The FDP allows to produce nB suitable for numerous applications. This process is also very competitive in terms of scalability and production costs if compared to the via gas technique adopted by nanoboron producers currently available on the world market.
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http://dx.doi.org/10.1088/1361-6528/abb269DOI Listing
December 2020

Understanding electrochemical switchability of perovskite-type exsolution catalysts.

Nat Commun 2020 Sep 23;11(1):4801. Epub 2020 Sep 23.

TU Wien, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164-EC, 1060, Vienna, Austria.

Exsolution of metal nanoparticles from perovskite-type oxides is a very promising approach to obtain catalysts with superior properties. One particularly interesting property of exsolution catalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized LaSrFeO thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high and low activity state is accompanied by a phase change of exsolved particles between metallic α--Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on LaSrFeO electrodes affect the H oxidation and HO splitting mechanism and why the particle size plays a minor role.
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http://dx.doi.org/10.1038/s41467-020-18563-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7511332PMC
September 2020

Influence of experimental constraints on micromechanical assessment of micromachined hard-tissue samples.

J Mech Behav Biomed Mater 2020 06 24;106:103741. Epub 2020 Mar 24.

Institute of Lightweight Design and Structural Biomechanics, TU Wien, 1060, Vienna, Austria. Electronic address:

Continuing technological advancement of mechanical characterization at the microscale has enabled the isolation of micron-sized specimens and their direct mechanical characterization. Such techniques, initially developed for engineering materials and MEMS, can also be applied on hard biological materials. Bone is a material with a complex hierarchical structure ranging from the macro- all the way down to the nanoscale. To fully understand bone tissue mechanics, knowledge of the mechanics of all structural elements i.e. at every length scale is necessary. Particularly, the mechanical properties of microstructural elements, such as bone lamellae are still largely unknown. In the last decade, testing protocols have been devised to close this gap including bending and compression of micrometer-sized bone specimens. However, the precision and accuracy of results obtained have not been discussed. In this study, we aim to do exactly this: we validate microbeam bending by testing silicon microbeams with known mechanical constants, and evaluate the precision and sources of errors in both microbeam bending and micropillar compression by means of finite element (FE) modeling. Bending of Si-microbeams reproduced the expected value for the bending modulus within 17% accuracy, although the effect of geometrical uncertainties was estimated to result in relative errors of up to 50%. The deformation of constraining bulk material had a smaller influence, with relative errors of 11%, for microbeam bending and 25% for micropillar compression. For the latter this error could be sufficiently eliminated by the Sneddon correction. The tapering of micropillars had a negligible effect on overall apparent stiffness, but induced inhomogeneous stress state within micropillars may lead to superposed structural deformation mechanisms and be responsible for failure patterns observed in past studies.
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http://dx.doi.org/10.1016/j.jmbbm.2020.103741DOI Listing
June 2020

Epitaxial GeSn Nanowires for Nanoscale Mid-Infrared Emitters.

ACS Nano 2019 Jul 12;13(7):8047-8054. Epub 2019 Jul 12.

Institute of Materials Chemistry , TU Wien , Getreidemarkt 9/BC/02 , A-1060 Vienna , Austria.

Highly oriented GeSn nanowires have been synthesized by a low-temperature chemical vapor deposition growth technique. The nanostructures form by a self-seeded vapor-liquid-solid mechanism. In this process, liquid metallic Sn seeds enable the anisotropic crystal growth and act as a sole source of Sn for the formation of the metastable GeSn semiconductor material. The strain relaxation for a lattice mismatch of ε = 2.94% between the Ge (111) substrate and the constant GeSn composition of nanowires is confined to a transition zone of <100 nm. In contrast, GeSn structures with diameters in the micrometer range show a 5-fold longer compositional gradient very similar to epitaxial thin-film growth. Effects of the Sn growth promoters' dimensions on the morphological and compositional evolution of GeSn are described. The temperature- and laser power-dependent photoluminescence analyses verify the formation of a direct band gap material with emission in the mid-infrared region and values expected for unstrained GeSn (.., band gap of 0.3 eV at room temperature). These materials  hold promise in applications such as thermal imaging and photodetection as well as building blocks for group IV-based mid- to near-IR photonics.
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http://dx.doi.org/10.1021/acsnano.9b02843DOI Listing
July 2019

From Anhydrous Zinc Oxide Nanoparticle Powders to Aqueous Colloids: Impact of Water Condensation and Organic Salt Adsorption on Free Exciton Emission.

Langmuir 2019 Jul 21;35(26):8741-8747. Epub 2019 Jun 21.

Department of Chemistry and Physics of Materials , University of Salzburg , Jakob-Haringer-Strasse 2a , 5020 Salzburg , Austria.

Variations in the composition and structure of ZnO nanoparticle interfaces have a key influence on the materials' optoelectronic properties and are responsible for high number of discrepant results reported for ZnO-based nanomaterials. Here, we conduct a systematic study of the room-temperature photoluminescence of anhydrous ZnO nanocrystals, as synthesized in the gas phase and processed in water-free atmosphere, and of their colloidal derivatives in aqueous dispersions with varying amounts of organic salt admixtures. A free exciton band at hν = 3.3 eV is essentially absent in the anhydrous ZnO nanocrystal powders measured in vacuum or in oxygen atmosphere. Surface hydration of the nanoparticles during colloid formation leads to the emergence of the free exciton band at hν = 3.3 eV and induces a small but significant release in lattice strain as detected by X-ray diffraction. Most importantly, admixture of acetate or citrate ions to the aqueous colloidal dispersions not only allows for the control of the ζ-potential but also affects the intensity of the free exciton emission in a correlated manner. The buildup of negative charge at the solid-liquid interface, as produced by citrate adsorption, increases the free exciton emission. This effect is attributed to the suppression of electron trapping in the near-surface region, which counteracts nonradiative exciton recombination. Using well-defined ZnO nanoparticles as model systems for interface chemistry studies, our findings highlight water-induced key effects that depend on the composition of the aqueous solution shell around the semiconducting metal oxide nanoparticles.
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http://dx.doi.org/10.1021/acs.langmuir.9b00656DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7116045PMC
July 2019

Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes.

ChemNanoMat 2019 May 3;5(5):634-641. Epub 2019 Apr 3.

Department of Chemistry and Physics of Materials University of Salzburg Jakob-Haringer-Strasse 2a 5020 Salzburg Austria.

Metal oxide nanocomposites are non-equilibrium solids and promising precursors for functional materials. Annealing of such materials can provide control over impurity segregation and, depending on the level of consolidation, represents a versatile approach to engineer free surfaces, particle-particle interfaces and grain boundaries. Starting with indium-magnesium-oxide nanoparticle powders obtained via injection of an indium organic precursor into the magnesium combustion flame and subsequent particle quenching in argon, we investigated the stability of the trivalent In ions in the host lattice of MgO nanoparticles by determining grain growth, morphology evolution and impurity segregation. The latter process is initiated by vacuum annealing at 873 K and can be tracked at 1173 K on a time scale of minutes. In the first instance the surface segregated indium wets the nanoparticle interfaces. After prolonged annealing indium evaporates and leaves the powder via the gas phase. Resulting MgO nanocubes are devoid of residual indium, regain their high morphological definition and show spectroscopic fingerprints (UV Diffuse Reflectance and Photoluminescence emission) that are characteristic of electronically unperturbed MgO cube corner and edge features. The results of this combined XRD, TEM, and spectroscopy study reveal the parameter window within which control over indium segregation is used to introduce a semiconducting metal oxide component into the intergranular region between insulating MgO nanograins.
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http://dx.doi.org/10.1002/cnma.201900077DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6563704PMC
May 2019

Reactive metal-support interaction in the Cu-InO system: intermetallic compound formation and its consequences for CO-selective methanol steam reforming.

Sci Technol Adv Mater 2019 25;20(1):356-366. Epub 2019 Apr 25.

Department of Physical Chemistry, University of Innsbruck, Innsbruck, Austria.

The reactive metal-support interaction in the Cu-InO system and its implications on the CO selectivity in methanol steam reforming (MSR) have been assessed using nanosized Cu particles on a powdered cubic InO support. Reduction in hydrogen at 300 °C resulted in the formation of metallic Cu particles on InO. This system already represents a highly CO-selective MSR catalyst with ~93% selectivity, but only 56% methanol conversion and a maximum H formation rate of 1.3 µmol g s. After reduction at 400 °C, the system enters an InO-supported intermetallic compound state with CuIn as the majority phase. CuIn exhibits markedly different self-activating properties at equally pronounced CO selectivities between 92% and 94%. A methanol conversion improvement from roughly 64% to 84% accompanied by an increase in the maximum hydrogen formation rate from 1.8 to 3.8 µmol g s has been observed from the first to the fourth consecutive runs. The presented results directly show the prospective properties of a new class of Cu-based intermetallic materials, beneficially combining the MSR properties of the catalyst's constituents Cu and InO. In essence, the results also open up the pathway to in-depth development of potentially CO-selective bulk intermetallic Cu-In compounds with well-defined stoichiometry in MSR.
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http://dx.doi.org/10.1080/14686996.2019.1590127DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6493314PMC
April 2019

Surface-Structure Libraries: Multifrequential Oscillations in Catalytic Hydrogen Oxidation on Rhodium.

J Phys Chem C Nanomater Interfaces 2019 Feb 23;123(7):4217-4227. Epub 2019 Jan 23.

Institute of Materials Chemistry, Technische Universität Wien, Getreidemarkt 9, 1060 Vienna, Austria.

Multifrequential oscillating spatiotemporal patterns in the catalytic hydrogen oxidation on rhodium have been observed in situ in the 10 mbar pressure range using photoemission electron microscopy. The effect is manifested by periodic chemical waves, which travel over the polycrystalline Rh surface and change their oscillation frequency while crossing boundaries between different Rh() domains. Each crystallographically specific μm-sized Rh() domain exhibits an individual wave pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction, altogether creating a structure library. This unique reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as a feedback mechanism of kinetic oscillations. Formation of a network of subsurface oxygen as a result of colliding reaction fronts was observed in situ. Microkinetic model analysis was used to rationalize the observed effects and to reveal the relation between the barriers for surface oxidation and oscillation frequency. Structural limits of the oscillations, the existence range of oscillations, as well as the effect of varying hydrogen pressure are demonstrated.
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http://dx.doi.org/10.1021/acs.jpcc.8b11421DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6494118PMC
February 2019

Hybrid synthesis of zirconium oxycarbide nanopowders with defined and controlled composition.

RSC Adv 2019 Jan 23;9(6):3151-3156. Epub 2019 Jan 23.

Institute of Physical Chemistry, University of Innsbruck, Innrain 52c, A-6020 Innsbruck, Austria. Email: Tel: +43 512 507 58003.

A combined synthesis strategy involving a carbothermal reduction and gelation approach with glycine as gelating agent was used to obtain Zr-based (oxy)carbide materials with defined and controlled composition. A comparatively low temperature approach (1500 °C) allows exploration of the ZrC-ZrO phase diagram and reproducibly leads to zirconium (oxy)carbide phases with different C/Zr ratios, as confirmed by combined X-ray diffraction (XRD) and transmission electron microscopy (TEM) data. The latter also indicates a chemically very homogeneous distribution of oxygen and carbon throughout the sample bulk, a prerequisite for further characterization of its intrinsic physico-chemical properties. Due to the general variability of the synthesis procedure - variation of metal precursor, amount of gelating agent and carbon precursor source - it is expected that the method can be easily adapted and transferred to other metal - oxycarbide materials.
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http://dx.doi.org/10.1039/c8ra09584aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6394884PMC
January 2019

Crystallographic and electronic evolution of lanthanum strontium ferrite (LaSrFeO) thin film and bulk model systems during iron exsolution.

Phys Chem Chem Phys 2019 Feb;21(7):3781-3794

Department of Physical Chemistry, Universität Innsbruck, A-6020 Innsbruck, Austria.

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.
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http://dx.doi.org/10.1039/c8cp07743fDOI Listing
February 2019

Thin water films and particle morphology evolution in nanocrystalline MgO.

J Am Ceram Soc 2018 Nov 30;101(11):4994-5003. Epub 2018 May 30.

Department of Chemistry and Physics of Materials Paris-Lodron University Salzburg Salzburg Austria.

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing-induced morphology changes on powders of MgO nanocubes in different gaseous HO environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(HO) = 10 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two-dimensional solvent for Mg ion hydration. In addition, we obtained direct evidence for hydroxylation-induced stabilization of (110) faces and step edges of the grain surfaces.
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http://dx.doi.org/10.1111/jace.15775DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6175089PMC
November 2018

Visualizing catalyst heterogeneity by a  multifrequential oscillating reaction.

Nat Commun 2018 02 9;9(1):600. Epub 2018 Feb 9.

Institute of Materials Chemistry, Technische Universität Wien, 1060, Vienna, Austria.

It is well documented that different surface structures of catalytically active metals may exhibit different catalytic properties. This is typically examined by comparing the catalytic activities and/or selectivities of various well-defined smooth and stepped/kinked single crystal surfaces. Here we report the direct observation of the heterogeneity of active polycrystalline surfaces under reaction conditions, which is manifested by multifrequential oscillations during hydrogen oxidation over rhodium, imaged in situ by photoemission electron microscopy. Each specific surface structure, i.e. the crystallographically different µm-sized domains of rhodium, exhibits an individual spiral pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction. This reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as feedback mechanism of the observed oscillations. The current experimental findings, backed by microkinetic modeling, may open an alternative approach towards addressing the structure-sensitivity of heterogeneous surfaces.
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http://dx.doi.org/10.1038/s41467-018-03007-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5807506PMC
February 2018

Setting Directions: Anisotropy in Hierarchically Organized Porous Silica.

Chem Mater 2017 Sep 31;29(18):7969-7975. Epub 2017 Aug 31.

Chemistry and Physics of Materials, Paris Lodron University Salzburg, 5020 Salzburg, Austria.

Structural hierarchy, porosity, and isotropy/anisotropy are highly relevant factors for mechanical properties and thereby the functionality of porous materials. However, even though anisotropic and hierarchically organized, porous materials are well known in nature, such as bone or wood, producing the synthetic counterparts in the laboratory is difficult. We report for the first time a straightforward combination of sol-gel processing and shear-induced alignment to create hierarchical silica monoliths exhibiting anisotropy on the levels of both, meso- and macropores. The resulting material consists of an anisotropic macroporous network of struts comprising 2D hexagonally organized cylindrical mesopores. While the anisotropy of the mesopores is an inherent feature of the pores formed by liquid crystal templating, the anisotropy of the macropores is induced by shearing of the network. Scanning electron microscopy and small-angle X-ray scattering show that the majority of network forming struts is oriented towards the shearing direction; a quantitative analysis of scattering data confirms that roughly 40% of the strut volume exhibits a preferred orientation. The anisotropy of the material's macroporosity is also reflected in its mechanical properties; i.e., the Young's modulus differs by nearly a factor of 2 between the directions of shear application and perpendicular to it. Unexpectedly, the adsorption-induced strain of the material exhibits little to no anisotropy.
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http://dx.doi.org/10.1021/acs.chemmater.7b03032DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5627989PMC
September 2017

Dislocations Accelerate Oxygen Ion Diffusion in LaSrMnO Epitaxial Thin Films.

ACS Nano 2017 11 16;11(11):11475-11487. Epub 2017 Oct 16.

Institute of Chemical Technologies and Analytics, Vienna University of Technology , Getreidemarkt 9, Vienna A-1060, Austria.

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO and SrTiO substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.
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http://dx.doi.org/10.1021/acsnano.7b06228DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5707630PMC
November 2017

Surface Spectroscopy on UHV-Grown and Technological Ni-ZrO Reforming Catalysts: From UHV to Operando Conditions.

Top Catal 2016 12;59(17):1614-1627. Epub 2016 Aug 12.

Institute of Materials Chemistry, Technische Universität Wien, Getreidemarkt 9/BC/01, 1060 Vienna, Austria.

Ni nanoparticles supported on ZrO are a prototypical system for reforming catalysis converting methane to synthesis gas. Herein, we examine this catalyst on a fundamental level using a 2-fold approach employing industrial-grade catalysts as well as surface science based model catalysts. In both cases we examine the atomic (HRTEM/XRD/LEED) and electronic (XPS) structure, as well as the adsorption properties (FTIR/PM-IRAS), with emphasis on in situ/operando studies under atmospheric pressure conditions. For technological Ni-ZrO the rather large Ni nanoparticles (about 20 nm diameter) were evenly distributed over the monoclinic zirconia support. In situ FTIR spectroscopy and ex situ XRD revealed that even upon H exposure at 673 K no full reduction of the nickel surface was achieved. CO adsorbed reversibly on metallic and oxidic Ni sites but no CO dissociation was observed at room temperature, most likely because the Ni particle edges/steps comprised Ni oxide. CO desorption temperatures were in line with single crystal data, due to the large size of the nanoparticles. During methane dry reforming at 873 K carbon species were deposited on the Ni surface within the first 3 h but the CH and CO conversion hardly changed even during 24 h. Post reaction TEM and TPO suggest the formation of graphitic and whisker-type carbon that do not significantly block the Ni surface but rather physically block the tube reactor. Reverse water gas shift decreased the H/CO ratio. Operando studies of methane steam reforming, simultaneously recording FTIR and MS data, detected activated CH (CH and CH), activated water (OH), as well as different bidentate (bi)carbonate species, with the latter being involved in the water gas shift side reaction. Surface science Ni-ZrO model catalysts were prepared by first growing an ultrathin "trilayer" (O-Zr-O) ZrO support on an PdZr alloy substrate, and subsequently depositing Ni, with the process being monitored by XPS and LEED. Apart from the trilayer oxide, there is a small fraction of ZrO clusters with more bulk-like properties. When CO was adsorbed on the (fully metallic) Ni particles at pressures up to 100 mbar, both PM-IRAS and XPS indicated CO dissociation around room temperature and blocking of the Ni surface by carbon (note that on the partially oxidized technological Ni particles, CO dissociation was absent). The Ni nanoparticles were stable up to 550 K but annealing to higher temperatures induced Ni migration through the ultrathin ZrO support into the PdZr alloy. Both approaches have their benefits and limitations but enable us to address specific questions on a molecular level.
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http://dx.doi.org/10.1007/s11244-016-0678-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5153820PMC
August 2016

Hydroxylation Induced Alignment of Metal Oxide Nanocubes.

Angew Chem Int Ed Engl 2017 01 22;56(5):1407-1410. Epub 2016 Dec 22.

Department of Chemistry and Physics of Materials, Paris Lodron University of Salzburg, Hellbrunnerstrasse 34/III, 5020, Salzburg, Austria.

Water vapor is ubiquitous under ambient conditions and may alter the shape of nanoparticles. How to utilize water adsorption for nanomaterial functionality and structure formation, however, is a yet unexplored field. Herein, we report the use of water vapor to induce the self-organization of MgO nanocubes into regularly staggered one-dimensional structures. This transformation evolves via an initial alignment of the MgO cubes, the formation of intermediate elongated Mg(OH) structures, and their reconversion into MgO cubes arranged in staggered structures. Ab initio DFT modelling identifies surface-energy changes associated with the cube surface hydration and hydroxylation to promote the uncommon staggered stacked assembly of the cubes. This first observation of metal oxide nanoparticle self-organization occurring outside a bulk solution may pave novel routes for inducing texture in ceramics and represents a great test-bed for new surface-science concepts.
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http://dx.doi.org/10.1002/anie.201608538DOI Listing
January 2017

Changing interfaces: Photoluminescent ZnO nanoparticle powders in different aqueous environments.

Surf Sci 2016 Oct;652:253-260

Department of Chemistry and Physics of Materials, Paris Lodron University of Salzburg, Hellbrunnerstrasse 34/III, A - 5020, Salzburg, Austria.

We transformed vapor phase grown ZnO nanoparticle powders into aqueous ZnO nanoparticle dispersions and studied the impact of associated microstructure and interface property changes on their spectroscopic properties. With photoluminescence (PL) spectroscopy, we probed oxygen interstitials in the near surface region and tracked their specific PL emission response at hv = 2.1 eV during the controlled conversion of the solid-vacuum into the solid-liquid interface. While oxygen adsorption via the gas phase does affect the intensity of the PL emission bands, the O contact with ZnO nanoparticles across the solid-liquid interface does not. Moreover, we found that the near band edge emission feature at hv = 3.2 eV gains relative intensity with regard to the PL emission features in the visible light region. Searching for potential PL indicators that are specific to early stages of particle dissolution, we addressed for aqueous ZnO nanoparticle dispersions the effect of formic acid adsorption. In the absence of related spectroscopic features, we were able to consistently track ZnO nanoparticle dissolution and the concomitant formation of sol- vated Zinc formate species by means of PL and FT-IR spectroscopy, dynamic light scattering, and zeta potential measurements. For a more consistent and robust assessment of nanoparticle properties in different continuous phases, we discuss characterization challenges and potential pitfalls that arise upon replacing the solid-gas with the solid-liquid interface.
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http://dx.doi.org/10.1016/j.susc.2016.02.019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7116034PMC
October 2016

Mechanism of Rare Earth Incorporation and Crystal Growth of Rare Earth Containing Type-I Clathrates.

Cryst Growth Des 2016 Jan 2;16(1):25-33. Epub 2015 Dec 2.

Institute of Solid State Physics, Vienna University of Technology , Wiedner Hauptstrasse 8-10, Vienna 1040, Austria.

Type-I clathrates possess extremely low thermal conductivities, a property that makes them promising materials for thermoelectric applications. The incorporation of cerium into one such clathrate has recently been shown to lead to a drastic enhancement of the thermopower, another property determining the thermoelectric efficiency. Here we explore the mechanism of the incorporation of rare earth elements into type-I clathrates. Our investigation of the crystal growth and the composition of the phase Ba RE TM Si (RE = rare earth element; TM = Au, Pd, Pt) reveals that the RE content is mainly governed by two factors, the free cage space and the electron balance.
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http://dx.doi.org/10.1021/acs.cgd.5b00461DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4718404PMC
January 2016

Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems.

Chemistry 2016 Jan 18;22(5):1744-9. Epub 2015 Dec 18.

Department of Chemistry & Physics of Materials, Paris-Lodron University Salzburg, Salzburg, Austria.

We show that both single-crystalline and nanostructured MgO surfaces convert free-base tetraphenyl porphyrin (2HTPP) into magnesium tetraphenyl porphyrin (MgTPP) at room temperature. The reaction can be viewed as an ion exchange between the two aminic protons of the 2HTPP molecule with a Mg(2+) ion from the surface. The driving force for the reaction is the strong stability of the formed hydroxyl groups along the steps and at defects on the MgO surface. We have used an integrated characterization approach that includes UV/Vis diffuse reflectance measurements on nanostructured powders, X-ray photoelectron spectroscopic investigation of atomically clean MgO(100) single-crystalline thin films, and density functional theory (DFT) calculations on model systems. The DFT calculations demonstrate that MgTPP formation is strongly exothermic at the corners, edges and steps, but slightly endothermic on terrace sites. This agrees well with the UV/Vis diffuse reflectance, which upon adsorption of 2HTPP shows a decrease in the absorption band associated with corner and edge sites on MgO nanocube powders.
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http://dx.doi.org/10.1002/chem.201503661DOI Listing
January 2016

Pt-B System Revisited: Pt2B, a New Structure Type of Binary Borides. Ternary WAl12-Type Derivative Borides.

Inorg Chem 2015 Nov 2;54(22):10958-65. Epub 2015 Nov 2.

Institute of Solid State Physics, TU Wien , A-1040 Vienna, Austria.

On the basis of a detailed study applying X-ray single-crystal and powder diffraction, differential scanning calorimetry, and scanning electron microscopy analysis, it was possible to resolve existing uncertainties in the Pt-rich section (≥65 atom % Pt) of the binary Pt-B phase diagram above 600 °C. The formation of a unique structure has been observed for Pt2B [X-ray single-crystal data: space group C2/m, a = 1.62717(11) nm, b = 0.32788(2) nm, c = 0.44200(3) nm, β = 104.401(4)°, RF2 = 0.030]. Within the homogeneity range of "Pt3B", X-ray powder diffraction phase analysis prompted two structural modifications as a function of temperature. The crystal structure of "hT-Pt3B" complies with the hitherto reported structure of anti-MoS2 [space group P63/mmc, a = 0.279377(2) nm, c = 1.04895(1) nm, RF = 0.075, RI = 0.090]. The structure of the new "[Formula: see text]T-Pt3B" is still unknown. The formation of previously reported Pt∼4B has not been confirmed from binary samples. Exploration of the Pt-rich section of the Pt-Cu-B system at 600 °C revealed a new ternary compound, Pt12CuB6-y [X-ray single-crystal data: space group Im3̅, a = 0.75790(2) nm, y = 3, RF2 = 0.0129], which exhibits the filled WAl12-type structure accommodating boron in the interstitial trigonal-prismatic site 12e. The isotypic platinum-aluminum-boride was synthesized and studied. The solubility of copper in binary platinum borides has been found to attain ∼7 atom % Cu for Pt2B but to be insignificant for "[Formula: see text]T-Pt3B". The architecture of the new Pt2B structure combines puckered layers of boron-filled and empty [Pt6] octahedra (anti-CaCl2-type fragment) alternating along the x axis with a double layer of boron-semifilled [Pt6] trigonal prisms interbedded with a layer of empty tetrahedra and tetragonal pyramids (B-deficient α-T[Formula: see text]I fragment). Assuming boron vacancies ordering (space group R3), the Pt12CuB6-y structure exhibits serpentine-like columns of edge-connected boron-filled [Pt6] trigonal prisms running infinitely along the z axis and embedding the icosahedrally coordinated Cu atom. Pt2B, (Pt1-yCuy)2B (y = 0.045), and Pt12CuB6-y (y = 3) behave metallically, as revealed by temperature-dependent electrical resistivity measurements.
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http://dx.doi.org/10.1021/acs.inorgchem.5b01998DOI Listing
November 2015

Porphyrin Metalation at the MgO Nanocube/Toluene Interface.

ACS Appl Mater Interfaces 2015 Oct 12;7(41):22962-9. Epub 2015 Oct 12.

Department of Materials Science and Physics, Paris Lodron University of Salzburg , Hellbrunnerstrasse 34/III, A-5020 Salzburg, Austria.

Molecular insights into porphyrin adsorption on nanostructured metal oxide surfaces and associated ion exchange reactions are key to the development of functional hybrids for energy conversion, sensing, and light emission devices. Here we investigated the adsorption of tetraphenyl-porphyrin (2HTPP) from toluene solution on two types of MgO powder. We compare MgO nanocubes with an average size d < 10 nm and MgO cubes with 10 nm ≤ d ≤ 1000 nm. Using molecular spectroscopy techniques such as UV/vis transmission and diffuse reflectance (DR), photoluminescence (PL), and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy in combination with structural characterization techniques (powder X-ray diffraction and transmission electron microscopy, TEM), we identified a new room temperature metalation reaction that converts 2HTPP into magnesium tetraphenyl-porphyrin (MgTPP). Mg(2+) uptake from the MgO nanocube surfaces and the concomitant protonation of the oxide surface level off at a concentration that corresponds to roughly one monolayer equivalent adsorbed on the MgO nanocubes. Larger MgO cubes, in contrast, show suppressed exchange, and only traces of MgTPP can be detected by photoluminescence.
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http://dx.doi.org/10.1021/acsami.5b08123DOI Listing
October 2015

Microwave-assisted solution-liquid-solid growth of Ge1-xSnx nanowires with high tin content.

Chem Commun (Camb) 2015 Aug;51(61):12282-5

Vienna University of Technology, Institute of Materials Chemistry, Getreidemarkt 9/BC/02, 1060 Vienna, Austria.

A microwave assisted growth procedure for the first bottom-up synthesis of germanium tin alloy (Ge1-xSnx) nanowires with constant diameter along their axis was developed. Ge1-xSnx nanowires with mean diameters of 190 ± 30 nm and a homogeneous distribution of 12.4 ± 0.7% Sn in Ge have been synthesized.
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http://dx.doi.org/10.1039/c5cc03639aDOI Listing
August 2015

Extracellular bone matrix exhibits hardening elastoplasticity and more than double cortical strength: Evidence from homogeneous compression of non-tapered single micron-sized pillars welded to a rigid substrate.

J Mech Behav Biomed Mater 2015 Dec 7;52:51-62. Epub 2015 Mar 7.

Institute for Mechanics of Materials and Structures, Vienna University of Technology (TU Wien), Karlsplatz 13/202, 1040 Vienna, Austria. Electronic address:

We here report an improved experimental technique for the determination of Young׳s modulus and uniaxial strength of extracellular bone matrix at the single micrometer scale, giving direct access to the (homogeneous) deformation (or strain) states of the tested samples and to the corresponding mechanically recoverable energy, called potential or elastic energy. Therefore, a new protocol for Focused Ion Beam milling of prismatic non-tapered micropillars, and attaching them to a rigid substrate, was developed. Uniaxial strength turns out as at least twice that measured macroscopically, and respective ultimate stresses are preceded by hardening elastoplastic states, already at very low load levels. The unloading portion of quasi-static load-displacement curves revealed Young׳s modulus of 29GPa in bovine extracellular bone matrix. This value is impressively confirmed by the corresponding prediction of a multiscale mechanics model for bone, which has been comprehensively validated at various other observation scales, across tissues from the entire vertebrate animal kingdom.
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http://dx.doi.org/10.1016/j.jmbbm.2015.03.001DOI Listing
December 2015

Surface modification processes during methane decomposition on Cu-promoted Ni-ZrO catalysts.

Catal Sci Technol 2015 Feb 27;5(2):967-978. Epub 2014 Oct 27.

Institute of Materials Chemistry , Vienna University of Technology , Getreidemarkt 9 , 1020 Wien , Austria . Email: ; ; Tel: +43 1 165110.

The surface chemistry of methane on Ni-ZrO and bimetallic CuNi-ZrO catalysts and the stability of the CuNi alloy under reaction conditions of methane decomposition were investigated by combining reactivity measurements and synchrotron-based near-ambient pressure XPS. Cu was selected as an exemplary promoter for modifying the reactivity of Ni and enhancing the resistance against coke formation. We observed an activation process occurring in methane between 650 and 735 K with the exact temperature depending on the composition which resulted in an irreversible modification of the catalytic performance of the bimetallic catalysts towards a Ni-like behaviour. The sudden increase in catalytic activity could be explained by an increase in the concentration of reduced Ni atoms at the catalyst surface in the active state, likely as a consequence of the interaction with methane. Cu addition to Ni improved the desired resistance against carbon deposition by lowering the amount of coke formed. As a key conclusion, the CuNi alloy shows limited stability under relevant reaction conditions. This system is stable only in a limited range of temperature up to ~700 K in methane. Beyond this temperature, segregation of Ni species causes a fast increase in methane decomposition rate. In view of the applicability of this system, a detailed understanding of the stability and surface composition of the bimetallic phases present and the influence of the Cu promoter on the surface chemistry under relevant reaction conditions are essential.
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http://dx.doi.org/10.1039/c4cy00988fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4348800PMC
February 2015

Synthesis and thermal behavior of tin-based alloy (Sn-Ag-Cu) nanoparticles.

Nanoscale 2015 Mar;7(13):5843-51

Department of Inorganic Chemistry (Materials Chemistry), University of Vienna, A-1090 Vienna, Austria.

The prominent melting point depression of nanoparticles has been the subject of a considerable amount of research. For their promising applications in electronics, tin-based nano-alloys such as near-eutectic Sn-Ag-Cu (SAC) alloys have been synthesized via various techniques. However, due to issues such as particle aggregation and oxidation or introduced impurities, the application of these nano-size particles has been confined or aborted. For instance, thermal investigations by DTA/DSC in a large number of studies revealed exothermic peaks in the range of 240-500 °C, i.e. above the melting point of SAC nanoparticles, with different and quite controversial explanations for this unclear phenomenon. This represents a considerable drawback for the application of nanoparticles. Correspondingly, in the current study, the thermal stability of SAC nanoparticles has been investigated via electron microscopy, XRD, FTIR, and DSC/TG analysis. It was found that the nanoparticles consist mainly of a metallic β-Sn core and an amorphous tin hydroxide shell structure. The SnO crystalline phase formation from this amorphous shell has been associated with the exothermic peaks on the first heating cycle of the nanoparticles, followed by a disproportionation reaction into metallic Sn and SnO₂.The results also revealed that the surfactant and reducing agent cannot only affect the size and size distribution of the nanoparticles, they might also alter the ratio between the amorphous shell and the crystalline core in the structure of particles.
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http://dx.doi.org/10.1039/c5nr00462dDOI Listing
March 2015

Electrochemical properties of LaSrCoO thin films investigated by complementary impedance spectroscopy and isotope exchange depth profiling.

Solid State Ion 2014 Mar;256:38-44

Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria.

The oxygen exchange and diffusion properties of LaSrCoO thin films on yttria stabilized zirconia were analyzed by impedance spectroscopy and O tracer experiments. The investigations were performed on the same thin film samples and at the same temperature (400 °C) in order to get complementary information by the two methods. Electrochemical impedance spectroscopy can reveal resistive and capacitive contributions of such systems, but an exact interpretation of the spectra of complex oxide electrodes is often difficult from impedance data alone. It is shown that additional isotope exchange depth profiling can significantly help interpreting impedance spectra by giving reliable information on the individual contribution and exact location of resistances (surface, electrode bulk, interface). The measurements also allowed quantitative comparison of electrode polarization resistances obtained by different methods.
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http://dx.doi.org/10.1016/j.ssi.2013.12.016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4986314PMC
March 2014

Cation diffusion in La(0.6)Sr(0.4)CoO(3-δ) below 800 °C and its relevance for Sr segregation.

Phys Chem Chem Phys 2014 Feb;16(6):2715-26

Institute of Chemical Technologies and Analytics Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria.

Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.
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http://dx.doi.org/10.1039/c3cp51906fDOI Listing
February 2014

Ionic bis-nanoparticle networks.

Monatsh Chem 2012;143(4):519-525. Epub 2012 Jan 21.

Transmission Electron Microscopy Centre, USTEM, Vienna University of Technology, Vienna, Austria.

Abstract: A newly arising challenge in the field of nanoparticle research concerns the control and the understanding of the interparticle interactions and interparticle properties. This should allow the development of materials based on nanoparticle assemblies which represents a great opportunity to exploit nanoparticle collective properties. Although some nanoparticle networks have been reported, few works are addressing the highly exciting problem of forming bis-nanoparticle assemblies in which two different types of nanoparticles are present. In this article we report an original synthesis pathway for the formation of an ionic bis-nanoparticle network, silica/silver, based on the formation of an imidazolium bridging unit. The reaction used for the formation of the bridging imidazolium can be considered as click-like chemistry. The synthesis of the metal/metal oxide hybrid composite material starts from the formation of a metal oxide nanoparticle modified with an imidazole ligand. This composite formation is therefore very general and could be extended to other metal/metal oxide composites.

Graphical Abstract: .
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http://dx.doi.org/10.1007/s00706-011-0709-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495052PMC
January 2012

Solid-solid interface formation in TiO2 nanoparticle networks.

Langmuir 2011 Mar 25;27(5):1946-53. Epub 2011 Jan 25.

Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany.

Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO(2) nanoparticle networks, we subjected entirely different TiO(2-x) precursor structures to vacuum annealing. The transformation of an amorphous TiO(2-x) gel--obtained by sol-gel processing of an ethylene glycol-modified titanium precursor--into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO(2-x) networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.
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http://dx.doi.org/10.1021/la104213dDOI Listing
March 2011