Publications by authors named "Thomas W Hansen"

35 Publications

In situ manipulation of the active Au-TiO interface with atomic precision during CO oxidation.

Science 2021 01;371(6528):517-521

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027 China.

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO interface with the epitaxial rotation of gold nanoparticles on a TiO surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.
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http://dx.doi.org/10.1126/science.abe3558DOI Listing
January 2021

Visualizing HO molecules reacting at TiO active sites with transmission electron microscopy.

Science 2020 01;367(6476):428-430

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027 China.

Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium "active rows" to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water-gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.
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http://dx.doi.org/10.1126/science.aay2474DOI Listing
January 2020

Growth kinetics of single-walled carbon nanotubes with a (2, ) chirality selection.

Sci Adv 2019 12 13;5(12):eaav9668. Epub 2019 Dec 13.

Center for Multidimensional Carbon Materials, Institute for Basic Science, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea.

The growth kinetics play key roles in determining the chirality distribution of the grown single-walled carbon nanotubes (SWCNTs). However, the lack of comprehensive understandings on the SWCNT's growth mechanism at the atomic scale greatly hinders SWCNT chirality-selective synthesis. Here, we establish a general model, where the dislocation theory is a specific case, to describe the etching agent-dependent growth kinetics of SWCNTs on solid catalyst particles. In particular, the growth kinetics of SWCNTs in the absence of etching agent is validated by both in situ environmental transmission electron microscopy and ex situ chemical vapor deposition growth of SWCNTs. On the basis of the new theory of SWCNT's growth kinetics, we successfully explained the selective growth of (2, ) SWCNTs. This study provides another degree of freedom for SWCNT controlled synthesis and opens a new strategy to achieve chirality-selective synthesis of (2, ) SWCNTs using solid catalysts.
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http://dx.doi.org/10.1126/sciadv.aav9668DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6910834PMC
December 2019

Direct In Situ TEM Visualization and Insight into the Facet-Dependent Sintering Behaviors of Gold on TiO.

Angew Chem Int Ed Engl 2018 Dec 21;57(51):16827-16831. Epub 2018 Nov 21.

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China.

Preventing sintering of supported nanocatalysts is an important issue in nanocatalysis. A feasible way is to choose a suitable support. However, whether the metal-support interactions promote or prevent the sintering has not been fully identified. Now, completely different sintering behaviors of Au nanoparticles on distinct anatase TiO surfaces have been determined by in situ TEM. The full in situ sintering processes of Au nanoparticles were visualized on TiO (101) surface, which coupled the Ostwald ripening and particle migration coalescence. In contrast, no sintering of Au on TiO anatase (001) surface was observed under the same conditions. This facet-dependent sintering mechanism is fully explained by the density function theory calculations. This work not only offers direct evidence of the important role of supports in the sintering process, but also provides insightful information for the design of sintering-resistant nanocatalysts.
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http://dx.doi.org/10.1002/anie.201811933DOI Listing
December 2018

2D Transition Metal Carbides (MXenes) for Carbon Capture.

Adv Mater 2019 Jan 4;31(2):e1805472. Epub 2018 Nov 4.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever-increasing CO emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO . However, challenges remain because of limited CO /N selectivity and long-term stability. The effective adsorption of CO gas (≈12 mol kg ) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO /N selectivity, together with a chemical and thermal stability, identifies the archetype Ti C MXene as a new material for carbon capture (CC) applications.
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http://dx.doi.org/10.1002/adma.201805472DOI Listing
January 2019

Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles.

Adv Struct Chem Imaging 2017 25;3(1):14. Epub 2017 Oct 25.

Department of Physics, Technical University of Denmark, Fysikvej, Building 311, 2800 Kongens Lyngby, Denmark.

Strain analysis from high-resolution transmission electron microscopy (HRTEM) images offers a convenient tool for measuring strain in materials at the atomic scale. In this paper we present a theoretical study of the precision and accuracy of surface strain measurements directly from aberration-corrected HRTEM images. We examine the influence of defocus, crystal tilt and noise, and find that absolute errors of at least 1-2% strain should be expected. The model structures include surface relaxations determined using molecular dynamics, and we show that this is important for correctly evaluating the errors introduced by image aberrations.
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http://dx.doi.org/10.1186/s40679-017-0047-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5656738PMC
October 2017

Interfacial charge distributions in carbon-supported palladium catalysts.

Nat Commun 2017 08 24;8(1):340. Epub 2017 Aug 24.

Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, 50011, USA.

Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers, hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry effects on catalytic performance. Our investigations reveal trends between the charge distribution at the palladium-carbon interface and the metal's selectivity for hydrogenation of multifunctional chemicals. These electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby providing a strategy to rationally design carbon-supported catalysts.Control over charge transfer in carbon-supported metal nanoparticles is essential for designing new catalysts. Here, the authors show that thermal treatments effectively tune the interfacial charge distribution in carbon-supported palladium catalysts with consequential changes in hydrogenation performance.
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http://dx.doi.org/10.1038/s41467-017-00421-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5569089PMC
August 2017

Growth Termination and Multiple Nucleation of Single-Wall Carbon Nanotubes Evidenced by in Situ Transmission Electron Microscopy.

ACS Nano 2017 05 17;11(5):4483-4493. Epub 2017 Apr 17.

Center for Electron Nanoscopy, Technical University of Denmark , Fysikvej 307, 2800, Kgs. Lyngby, Denmark.

In order to controllably grow single-wall carbon nanotubes (SWCNTs), a better understanding of the growth processes and how they are influenced by external parameters such as catalyst and gaseous environment is required. Here, we present direct evidence of growth termination of individual SWCNTs and successive growth of additional SWCNTs on Co catalyst particles supported on MgO by means of environmental transmission electron microscopy. Such in situ observations reveal the plethora of solid carbon formations at the local scale while it is happening and thereby elucidate the multitude of configurations resulting from identical external synthesis conditions, which should be considered in the quest for controlled SWCNT growth. Using CO and a mixture of CO and H as carbon sources, we show that the growth of SWCNTs terminates with a reduced tube-catalyst adhesion strength. Two main reasons for the cessation are proposed: insufficient active carbon species and a certain amount of stress exerted at the tube-catalyst interface. Interestingly, it was observed that catalyst particles stayed active in terms of nucleating additional solid carbon structures after growth termination of the first SWCNT. These observations elucidate the importance of an in-depth understanding of the role of catalysts and carbon sources in the continued growth of SWCNTs. Furthermore, it serves as a guide for further control of carbon nanostructure synthesis via catalyst engineering and synthesis optimization.
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http://dx.doi.org/10.1021/acsnano.6b05941DOI Listing
May 2017

Correction to: Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles.

Adv Struct Chem Imaging 2017 7;3(1):16. Epub 2017 Nov 7.

1Department of Physics, Technical University of Denmark, Fysikvej, Building 311, 2800 Kongens Lyngby, Denmark.

[This corrects the article DOI: 10.1186/s40679-017-0047-0.].
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http://dx.doi.org/10.1186/s40679-017-0049-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5676803PMC
November 2017

Tailoring ZSM-5 Zeolites for the Fast Pyrolysis of Biomass to Aromatic Hydrocarbons.

ChemSusChem 2016 06 11;9(12):1473-82. Epub 2016 May 11.

Department of Chemical and Biological Engineering, Iowa State University, 617 Bissell Road, 2138 Biorenewables Research Laboratory, Ames, IA, 50011, USA.

The production of aromatic hydrocarbons from cellulose by zeolite-catalyzed fast pyrolysis involves a complex reaction network sensitive to the zeolite structure, crystallinity, elemental composition, porosity, and acidity. The interplay of these parameters under the reaction conditions represents a major roadblock that has hampered significant improvement in catalyst design for over a decade. Here, we studied commercial and laboratory-synthesized ZSM-5 zeolites and combined data from 10 complementary characterization techniques in an attempt to identify parameters common to high-performance catalysts. Crystallinity and framework aluminum site accessibility were found to be critical to achieve high aromatic yields. These findings enabled us to synthesize a ZSM-5 catalyst with enhanced activity, which offers the highest aromatic hydrocarbon yield reported to date.
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http://dx.doi.org/10.1002/cssc.201600186DOI Listing
June 2016

Oxygen evolution on well-characterized mass-selected Ru and RuO nanoparticles.

Chem Sci 2015 Jan 26;6(1):190-196. Epub 2014 Sep 26.

Center for Individual Nanoparticle Functionality (CINF) , Department of Physics , Kgs. Lyngby DK-2800 , Denmark . Email:

Oxygen evolution was investigated on model, mass-selected RuO nanoparticles in acid, prepared by magnetron sputtering. Our investigations include electrochemical measurements, electron microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy. We show that the stability and activity of nanoparticulate RuO is highly sensitive to its surface pretreatment. At 0.25 V overpotential, the catalysts show a mass activity of up to 0.6 A mg and a turnover frequency of 0.65 s, one order of magnitude higher than the current state-of-the-art.
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http://dx.doi.org/10.1039/c4sc02685cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5424673PMC
January 2015

Structure identification in high-resolution transmission electron microscopic images: an example on graphene.

Microsc Microanal 2014 Dec 12;20(6):1772-81. Epub 2014 Nov 12.

1Department of Applied Mathematics and Computer Science,Technical University of Denmark,Building 324/130,Richard Petersens Plads,2800 Kgs Lyngby,Denmark.

A connection between microscopic structure and macroscopic properties is expected for almost all material systems. High-resolution transmission electron microscopy is a technique offering insight into the atomic structure, but the analysis of large image series can be time consuming. The present work describes a method to automatically estimate the atomic structure in two-dimensional materials. As an example graphene is chosen, in which the positions of the carbon atoms are reconstructed. Lattice parameters are extracted in the frequency domain and an initial atom positioning is estimated. Next, a plausible neighborhood structure is estimated. Finally, atom positions are adjusted by simulation of a Markov random field model, integrating image evidence and the strong geometric prior. A pristine sample with high regularity and a sample with an induced hole are analyzed. False discovery rate-controlled large-scale simultaneous hypothesis testing is used as a statistical framework for interpretation of results. The first sample yields, as expected, a homogeneous distribution of carbon-carbon (C-C) bond lengths. The second sample exhibits regions of shorter C-C bond lengths with a preferred orientation, suggesting either strain in the structure or a buckling of the graphene sheet. The precision of the method is demonstrated on simulated model structures and by its application to multiple exposures of the two graphene samples.
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http://dx.doi.org/10.1017/S1431927614013464DOI Listing
December 2014

Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles.

Nat Commun 2014 Oct 10;5:5154. Epub 2014 Oct 10.

Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, Jülich D-52425, Germany.

Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.
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http://dx.doi.org/10.1038/ncomms6154DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4214405PMC
October 2014

In situ study of noncatalytic metal oxide nanowire growth.

Nano Lett 2014 Oct 24;14(10):5810-3. Epub 2014 Sep 24.

Department of Applied Physics, Aalto University School of Science , Puumiehenkuja 2, 00076 Espoo, Finland.

The majority of the nanowire synthesis methods utilize catalyst particles to guide the nanowire geometry. In contrast, catalyst-free methods are attractive for facile fabrication of pure nanowires without the need for catalyst preparation. Nonetheless, how nanowire growth is guided without a catalyst is still widely disputed and unclear. Here, we show that the nanowire growth during metal oxidation is limited by a nucleation of a new layer. On the basis of in situ transmission electron microscope investigations we found that the growth occurs layer by layer at the lowest specific surface energy planes. Atomic layers nucleate at the edge of twin boundary ridges and form a long-range ordering along the twin boundary. We anticipate our study to be a starting point to employ defects for nanowire growth control and consequently shaping the geometry of nanowires in a similar manner as in the catalyst-assisted growth method.
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http://dx.doi.org/10.1021/nl502687sDOI Listing
October 2014

High sintering resistance of size-selected platinum cluster catalysts by suppressed Ostwald ripening.

Nano Lett 2014 Oct 11;14(10):5803-9. Epub 2014 Sep 11.

Department of Applied Physics, Chalmers University of Technology , 41296 Göteborg, Sweden.

Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.
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http://dx.doi.org/10.1021/nl502686uDOI Listing
October 2014

Mass-selected nanoparticles of PtxY as model catalysts for oxygen electroreduction.

Nat Chem 2014 Aug 13;6(8):732-8. Epub 2014 Jul 13.

Center for Individual Nanoparticle Functionality, Department of Physics, Technical University of Denmark, Kgs Lyngby DK-2800, Denmark.

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 PtxY 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 PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.
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http://dx.doi.org/10.1038/nchem.2001DOI Listing
August 2014

Controlled environment specimen transfer.

Microsc Microanal 2014 Aug 14;20(4):1038-45. Epub 2014 May 14.

1Center for Electron Nanoscopy,Technical University of Denmark,Kgs. Lyngby DK-2800,Denmark.

Specimen transfer under controlled environment conditions, such as temperature, pressure, and gas composition, is necessary to conduct successive complementary in situ characterization of materials sensitive to ambient conditions. The in situ transfer concept is introduced by linking an environmental transmission electron microscope to an in situ X-ray diffractometer through a dedicated transmission electron microscope specimen transfer holder, capable of sealing the specimen in a gaseous environment at elevated temperatures. Two catalyst material systems have been investigated; Cu/ZnO/Al2O3 catalyst for methanol synthesis and a Co/Al2O3 catalyst for Fischer-Tropsch synthesis. Both systems are sensitive to ambient atmosphere as they will oxidize after relatively short air exposure. The Cu/ZnO/Al2O3 catalyst, was reduced in the in situ X-ray diffractometer set-up, and subsequently, successfully transferred in a reactive environment to the environmental transmission electron microscope where further analysis on the local scale were conducted. The Co/Al2O3 catalyst was reduced in the environmental microscope and successfully kept reduced outside the microscope in a reactive environment. The in situ transfer holder facilitates complimentary in situ experiments of the same specimen without changing the specimen state during transfer.
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http://dx.doi.org/10.1017/S1431927614000853DOI Listing
August 2014

Trends in the electrochemical synthesis of H2O2: enhancing activity and selectivity by electrocatalytic site engineering.

Nano Lett 2014 Mar 12;14(3):1603-8. Epub 2014 Feb 12.

Center for Individual Nanoparticle Functionality, Department of Physics, ‡Center for Electron Nanoscopy, and §Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark , DK-2800 Kongens Lyngby, Denmark.

The direct electrochemical synthesis of hydrogen peroxide is a promising alternative to currently used batch synthesis methods. Its industrial viability is dependent on the effective catalysis of the reduction of oxygen at the cathode. Herein, we study the factors controlling activity and selectivity for H2O2 production on metal surfaces. Using this approach, we discover two new catalysts for the reaction, Ag-Hg and Pd-Hg, with unique electrocatalytic properties both of which exhibit performance that far exceeds the current state-of-the art.
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http://dx.doi.org/10.1021/nl500037xDOI Listing
March 2014

Measurements of local chemistry and structure in Ni(O)-YSZ composites during reduction using energy-filtered environmental TEM.

Chem Commun (Camb) 2014 Feb 8;50(15):1808-10. Epub 2014 Jan 8.

Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.

Energy-filtered transmission electron microscopy images are acquired during the reduction of a NiO-YSZ composite in H2 up to 600 °C. Temperature-resolved quantitative information about both chemistry and structure is extracted with nm spatial resolution from the data, paving the way for the development of detailed reduction models.
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http://dx.doi.org/10.1039/c3cc46682eDOI Listing
February 2014

Enabling direct H2O2 production through rational electrocatalyst design.

Nat Mater 2013 Dec 17;12(12):1137-43. Epub 2013 Nov 17.

1] Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2].

Future generations require more efficient and localized processes for energy conversion and chemical synthesis. The continuous on-site production of hydrogen peroxide would provide an attractive alternative to the present state-of-the-art, which is based on the complex anthraquinone process. The electrochemical reduction of oxygen to hydrogen peroxide is a particularly promising means of achieving this aim. However, it would require active, selective and stable materials to catalyse the reaction. Although progress has been made in this respect, further improvements through the development of new electrocatalysts are needed. Using density functional theory calculations, we identify Pt-Hg as a promising candidate. Electrochemical measurements on Pt-Hg nanoparticles show more than an order of magnitude improvement in mass activity, that is, A g(-1) precious metal, for H2O2 production, over the best performing catalysts in the literature.
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http://dx.doi.org/10.1038/nmat3795DOI Listing
December 2013

The role of electron-stimulated desorption in focused electron beam induced deposition.

Beilstein J Nanotechnol 2013 14;4:474-80. Epub 2013 Aug 14.

Materials Science, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.

We present the results of our study about the deposition rate of focused electron beam induced processing (FEBIP) as a function of the substrate temperature with the substrate being an electron-transparent amorphous carbon membrane. When W(CO)6 is used as a precursor it is observed that the growth rate is lower at higher substrate temperatures. From Arrhenius plots we calculated the activation energy for desorption, E des, of W(CO)6. We found an average value for E des of 20.3 kJ or 0.21 eV, which is 2.5-3.0 times lower than literature values. This difference between estimates for E des from FEBIP experiments compared to literature values is consistent with earlier findings by other authors. The discrepancy is attributed to electron-stimulated desorption, which is known to occur during electron irradiation. The data suggest that, of the W(CO)6 molecules that are affected by the electron irradiation, the majority desorbs from the surface rather than dissociates to contribute to the deposit. It is important to take this into account during FEBIP experiments, for instance when determining fundamental process parameters such as the activation energy for desorption.
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http://dx.doi.org/10.3762/bjnano.4.56DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3778412PMC
September 2013

Sintering of catalytic nanoparticles: particle migration or Ostwald ripening?

Acc Chem Res 2013 Aug 1;46(8):1720-30. Epub 2013 May 1.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.

Metal nanoparticles contain the active sites in heterogeneous catalysts, which are important for many industrial applications including the production of clean fuels, chemicals and pharmaceuticals, and the cleanup of exhaust from automobiles and stationary power plants. Sintering, or thermal deactivation, is an important mechanism for the loss of catalyst activity. This is especially true for high temperature catalytic processes, such as steam reforming, automotive exhaust treatment, or catalytic combustion. With dwindling supplies of precious metals and increasing demand, fundamental understanding of catalyst sintering is very important for achieving clean energy and a clean environment, and for efficient chemical conversion processes with atom selectivity. Scientists have proposed two mechanisms for sintering of nanoparticles: particle migration and coalescence (PMC) and Ostwald ripening (OR). PMC involves the mobility of particles in a Brownian-like motion on the support surface, with subsequent coalescence leading to nanoparticle growth. In contrast, OR involves the migration of adatoms or mobile molecular species, driven by differences in free energy and local adatom concentrations on the support surface. In this Account, we divide the process of sintering into three phases. Phase I involves rapid loss in catalyst activity (or surface area), phase II is where sintering slows down, and phase III is where the catalyst may reach a stable performance. Much of the previous work is based on inferences from catalysts that were observed before and after long term treatments. While the general phenomena can be captured correctly, the mechanisms cannot be determined. Advancements in the techniques of in situ TEM allow us to observe catalysts at elevated temperatures under working conditions. We review recent evidence obtained via in situ methods to determine the relative importance of PMC and OR in each of these phases of catalyst sintering. The evidence suggests that, in phase I, OR is responsible for the rapid loss of activity that occurs when particles are very small. Surprisingly, very little PMC is observed in this phase. Instead, the rapid loss of activity is caused by the disappearance of the smallest particles. These findings are in good agreement with representative atomistic simulations of sintering. In phase II, sintering slows down since the smallest particles have disappeared. We now see a combination of PMC and OR, but do not fully understand the relative contribution of each of these processes to the overall rates of sintering. In phase III, the particles have grown large and other parasitic phenomena, such as support restructuring, can become important, especially at high temperatures. Examining the evolution of particle size and surface area with time, we do not see a stable or equilibrium state, especially for catalysts operating at elevated temperatures. In conclusion, the recent literature, especially on in situ studies, shows that OR is the dominant process causing the growth of nanoparticle size. Consequently, this leads to the loss of surface area and activity. While particle migration could be controlled through suitable structuring of catalyst supports, it is more difficult to control the mobility of atomically dispersed species. These insights into the mechanisms of sintering could help to develop sinter-resistant catalysts, with the ultimate goal of designing catalysts that are self-healing.
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http://dx.doi.org/10.1021/ar3002427DOI Listing
August 2013

Chiral-selective growth of single-walled carbon nanotubes on lattice-mismatched epitaxial cobalt nanoparticles.

Sci Rep 2013 ;3:1460

Department of Biotechnology and Chemical Technology, School of Chemical Technology, Aalto University, Aalto, Finland.

Controlling chirality in growth of single-walled carbon nanotubes (SWNTs) is important for exploiting their practical applications. For long it has been conceptually conceived that the structural control of SWNTs is potentially achievable by fabricating nanoparticle catalysts with proper structures on crystalline substrates via epitaxial growth techniques. Here, we have accomplished epitaxial formation of monometallic Co nanoparticles with well-defined crystal structure, and its use as a catalyst in the selective growth of SWNTs. Dynamics of Co nanoparticles formation and SWNT growth inside an atomic-resolution environmental transmission electron microscope at a low CO pressure was recorded. We achieved highly preferential growth of semiconducting SWNTs (~90%) with an exceptionally large population of (6, 5) tubes (53%) in an ambient CO atmosphere. Particularly, we also demonstrated high enrichment in (7, 6) and (9, 4) at a low growth temperature. These findings open new perspectives both for structural control of SWNTs and for elucidating the growth mechanisms.
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http://dx.doi.org/10.1038/srep01460DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3597999PMC
September 2013

Molecule-by-molecule writing using a focused electron beam.

ACS Nano 2012 Nov 31;6(11):10076-81. Epub 2012 Oct 31.

Applied Physics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.

The resolution of lithography techniques needs to be extended beyond their current limits to continue the trend of miniaturization and enable new applications. But what is the ultimate spatial resolution? It is known that single atoms can be imaged with a highly focused electron beam. Can single atoms also be written with an electron beam? We verify this with focused electron-beam-induced deposition (FEBID), a direct-write technique that has the current record for the smallest feature written by (electron) optical lithography. We show that the deposition of an organometallic precursor on graphene can be followed molecule-by-molecule with FEBID. The results show that mechanisms that are inherent to the process inhibit a further increase in control over the process. Hence, our results present the resolution limit of (electron) optical lithography techniques. The writing of isolated, subnanometer features with nanometer precision can be used, for instance, for the local modification of graphene and for catalysis.
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http://dx.doi.org/10.1021/nn303793wDOI Listing
November 2012

Environmental transmission electron microscopy in an aberration-corrected environment.

Microsc Microanal 2012 Aug 12;18(4):684-90. Epub 2012 Jun 12.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.

The increasing use of environmental transmission electron microscopy (ETEM) in materials science provides exciting new possibilities for investigating chemical reactions and understanding both the interaction of fast electrons with gas molecules and the effect of the presence of gas on high-resolution imaging. A gaseous atmosphere in the pole-piece gap of the objective lens of the microscope alters both the incoming electron wave prior to interaction with the sample and the outgoing wave below the sample. Whereas conventional TEM samples are usually thin (below 100 nm), the gas in the environmental cell fills the entire gap between the pole pieces and is thus not spatially localized. By using an FEI Titan environmental transmission electron microscope equipped with a monochromator and an aberration corrector on the objective lens, we have investigated the effects on imaging and spectroscopy caused by the presence of the gas.
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http://dx.doi.org/10.1017/S1431927612000293DOI Listing
August 2012

Questions about dexmedetomidine as a preanesthetic in cats.

Authors:
Thomas W Hansen

J Am Vet Med Assoc 2012 Apr;240(8):931

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April 2012

Stability of Porous Platinum Nanoparticles: Combined In Situ TEM and Theoretical Study.

J Phys Chem Lett 2012 May 12;3(9):1106-10. Epub 2012 Apr 12.

∥Department of Geoscience, and Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States.

Porous platinum nanoparticles provide a route for the development of catalysts that use less platinum without sacrificing catalytic performance. Here, we examine porous platinum nanoparticles using a combination of in situ transmission electron microscopy and calculations based on a first-principles-parametrized thermodynamic model. Our experimental observations show that the initially irregular morphologies of the as-sythesized porous nanoparticles undergo changes at high temperatures to morphologies having faceted external surfaces with voids present in the interior of the particles. The increasing size of stable voids with increasing temperature, as predicted by the theoretical calculations, shows excellent agreement with the experimental findings. The results indicate that hollow-structured nanoparticles with an appropriate void-to-total-volume ratio can be stable at high temperatures.
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http://dx.doi.org/10.1021/jz3001823DOI Listing
May 2012

Exploring the environmental transmission electron microscope.

Micron 2012 Nov 8;43(11):1169-75. Epub 2012 Mar 8.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.

The increasing interest and development in the field of in situ techniques have now reached a level where the idea of performing measurements under near realistic conditions has become feasible for transmission electron microscopy (TEM) while maintaining high spatial resolution. In this paper, some of the opportunities that the environmental TEM (ETEM) offers when combined with other in situ techniques will be explored, directly in the microscope, by combining electron-based and photon-based techniques and phenomena. In addition, application of adjacent setups using sophisticated transfer methods for transferring the specimen between specialized in situ equipment without compromising the concept of in situ measurements will be exploited. The opportunities and techniques are illustrated by studies of materials systems of Au/MgO and Cu(2)O in different gaseous environments.
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http://dx.doi.org/10.1016/j.micron.2012.02.008DOI Listing
November 2012

Relating rates of catalyst sintering to the disappearance of individual nanoparticles during Ostwald ripening.

J Am Chem Soc 2011 Dec 1;133(51):20672-5. Epub 2011 Dec 1.

University of New Mexico, Albuquerque, New Mexico 87131, USA.

Sintering of nanoparticles (NPs) of Ni supported on MgAl(2)O(4) was monitored in situ using transmission electron microscopy (TEM) during exposure to an equimolar mixture of H(2) and H(2)O at a pressure of 3.6 mbar at 750 °C, conditions relevant to methane steam reforming. The TEM images revealed an increase in the mean particle size due to disappearance of smaller, immobile NPs and the resultant growth of the larger NPs. A new approach for predicting the long-term sintering of NPs is presented wherein microscopic observations of the ripening of individual NPs (over a span of a few seconds) are used to extract energetic parameters that allow a description of the collective behavior of the entire population of NPs (over several tens of minutes).
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http://dx.doi.org/10.1021/ja208324nDOI Listing
December 2011

Discrete dynamics of nanoparticle channelling in suspended graphene.

Nano Lett 2011 Jul 9;11(7):2689-92. Epub 2011 Jun 9.

DTU Nanotech, Kgs. Lyngby, Denmark.

We have observed a previously undescribed stepwise oxidation of mono- and few layer suspended graphene by silver nanoparticles in situ at subnanometer scale in an environmental transmission electron microscope. Over the range of 600-850 K, we observe crystallographically oriented channelling with rates in the range 0.01-1 nm/s and calculate an activation energy of 0.557 ± 0.016 eV. We present a discrete statistical model for this process and discuss the implications for accurate nanoscale patterning of nanoscale systems.
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http://dx.doi.org/10.1021/nl200928kDOI Listing
July 2011
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