Eliška Mikmeková

Reductive Deprotection of Monolayer Protected Nanoclusters: An Efficient Route to Supported Ultrasmall Au Nanocatalysts for Selective Oxidation

Bulk gold has long been considered too inert to be a catalyst until the discovery that Au nanoparticles (AuNPs) supported on metal oxides such as TiO 2 , CeO 2 and Fe 2 O 3 could be very active for CO oxidation. [ 1 ] Supported Au and other NPs have now been successfully shown to catalyze various chemical reactions. [ 2 ] AuNP-catalyzed oxidation reactions, in particular, have attracted special attention because the reactions can lead to a range of important value-added oxygenated chemical products and pharmaceuticals, and also because oxidation (or epoxidation) of various alkenes, arenes and alcohols are proven to be effectively catalyzed by AuNPs. [ 3 ]

Yeast Cells-Derived Hollow Core/Shell Heteroatom-Doped Carbon Microparticles for Sustainable Electrocatalysis

The use of renewable resources to make various synthetic materials is increasing in order to meet some of our sustainability challenges. Yeast is one of the most common household ingredients, which is cheap and easy to reproduce. Herein we report that yeast cells can be thermally transformed into hollow, core-shell heteroatom-doped carbon microparticles that can effectively electrocatalyze the oxygen reduction and hydrazine oxidation reactions, reactions that are highly pertinent to fuel cells or renewable energy applications. We also show that yeast cell walls, which can easily be separated from the cells, can produce carbon materials with electrocatalytic activity for both reactions, albeit with lower activity compared with the ones obtained from intact yeast cells. The results reveal that the intracellular components of the yeast cells such as proteins, phospholipids, DNAs and RNAs are indirectly responsible for the latters higher electrocatalytic activity, by providing it with more heteroatom dopants. The synthetic method we report here can serve as a general route for the synthesis of (electro)catalysts using microorganisms as raw materials.

From ionic liquid-modified cellulose nanowhiskers to highly active metal-free nanostructured carbon catalysts for the hydrazine oxidation reaction

Ionic liquid (or [C4mim][CH3SO3])-modified cellulose nanowhiskers (CNWs) are synthesized and successfully used as precursors to make heteroatom (N and S)-doped nanostructured carbon catalysts. The catalysts can efficiently electrocatalyze the hydrazine oxidation reaction (HOR) with an onset potential close to the reactions thermodynamic value, or with a value better than those obtained for other related materials. The synthesis of these metal-free carbon electrocatalysts generally involves only a few, relatively less demanding synthetic steps. Based on relevant control experiments, the outstanding catalytic activity of the materials is attributed to the heteroatom dopants and defect sites in the materials, which form during carbonization due to the [C4mim][CH3SO3] placed around the CNWs. However, it is not necessarily the density of heteroatom dopant species introduced into the nanostructured carbon materials by the ILs that directly affect the electrocatalytic activity of these materials; it is rather the specific type of dopant-associated chemical moiety and vacancy site created in the materials, which are the main factors positively affecting the electrocatalytic activity of the materials toward the reaction. The surface areas of the materials play a relatively lesser role in affecting the electrocatalytic properties of the materials toward the HOR as well.

Counting graphene layers with very slow electrons

The study aimed at collection of data regarding the transmissivity of freestanding graphene for electrons across their full energy scale down to the lowest energies. Here, we show that the electron transmissivity of graphene drops with the decreasing energy of the electrons and remains below 10% for energies below 30 eV, and that the slow electron transmissivity value is suitable for reliable determination of the number of graphene layers. Moreover, electrons incident below 50 eV release adsorbed hydrocarbon molecules and effectively clean graphene in contrast to faster electrons that decompose these molecules and create carbonaceous contamination.

Scanning Electron Microscopy with Samples in an Electric Field

The high negative bias of a sample in a scanning electron microscope constitutes the “cathode lens” with a strong electric field just above the sample surface. This mode offers a convenient tool for controlling the landing energy of electrons down to units or even fractions of electronvolts with only slight readjustments of the column. Moreover, the field accelerates and collimates the signal electrons to earthed detectors above and below the sample, thereby assuring high collection efficiency and high amplification of the image signal. One important feature is the ability to acquire the complete emission of the backscattered electrons, including those emitted at high angles with respect to the surface normal. The cathode lens aberrations are proportional to the landing energy of electrons so the spot size becomes nearly constant throughout the full energy scale. At low energies and with their complete angular distribution acquired, the backscattered electron images offer enhanced information about crystalline and electronic structures thanks to contrast mechanisms that are otherwise unavailable. Examples from various areas of materials science are presented.

Very low energy electron microscopy of graphene flakes

Commercially available graphene samples are examined by Raman spectroscopy and very low energy scanning transmission electron microscopy. Limited lateral resolution of Raman spectroscopy may produce a Raman spectrum corresponding to a single graphene layer even for flakes that can be identified by very low energy electron microscopy as an aggregate of smaller flakes of various thicknesses. In addition to diagnostics of graphene samples at larger dimensions, their electron transmittance can also be measured at very low energies.

Very low energy scanning electron microscopy in nanotechnology

The group of low energy electron microscopy at ISI AS CR in Brno has developed a methodology for very low energy scanning electron microscopy at high image resolution by means of an immersion electrostatic lens (the cathode lens) inserted between the illumination column of a conventional scanning electron microscope and the sample. In this way the microscope resolution can be preserved down to a landing energy of the electrons one or even fractions of an electronvolt. In the range of less than several tens of electronvolts the image signal generation processes include contrast mechanisms not met at higher energies, which respond to important features of the 3D inner potential of the target and visualise its local crystallinity as well as the electronic structure. The electron wavelength comparable with interatomic distances allows observation of various wave–optical phenomena in imaging. In addition, the cathode lens assembly secures acquisition of electrons backscattered from the sample at large angles with respect to the surface normal, which are abandoned in standard microscopes although they provide enhanced crystallinity information and surface sensitivity even at medium electron energies. The imaging method is described and illustrated with selected application examples.

Low energy TEM characterizations of ordered mesoporous silica-based nanocomposite materials for catalytic applications

1. Department of Materials Science and Engineering, Rutgers University, Piscataway NJ, USA 2. Institute of Scientific Instruments of the ASCR, v.v.i., Kralovopolska 147, Brno 612 64, Czech Republic 3. Department of Chemistry and Chemical Biology, Rutgers University, Piscataway NJ, USA 4. Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway NJ, USA 5. Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical Research and Development Institute (CMRDI), Helwan 11421, Cairo, Egypt 6. Department of Chemistry, University of Padova, 35131 Padova, Italy

Effect of Hydrogen on the Properties of Amorphous Carbon Nitride Films

The effect of hydrogen on the properties of amorphous carbon nitride films deposited onto Si substrates by magnetron sputtering device has been studied. The influence of hydrogen to roughness, porous character of films, composition and residual stress was investigated by atomic force microscopy, thermal desorption mass spectroscopy, X-ray photoelectron spectroscopy, scanning low energy electron microscopy and by goniometer equipped with Cu X-ray tube. The adding of hydrogen to nitrogen discharge a causes decrease in the high value of compressive stress (elimination of delamination of the films, increasing of nitrogen content in the bulk). On the other hand hydrogen increases roughness and porosity.

Practical Use of Scanning Low Energy Electron Microscope (SLEEM)

The high negative bias of a sample in a scanning electron microscope constitutes the cathode lens (CL), with a strong electric field just above the sample surface [1] offers a tool for controlling the landing energy of electrons down to units or even fractions of electronvolts. Moreover, the field accelerates and collimates the signal electrons to earthed detectors above and below the sample, thereby assuring high collection efficiency and high amplification of the image signal. One important feature is the ability to acquire the complete emission of the backscattered electrons, including those emitted at high angles with respect to the surface normal. The cathode lens aberrations are proportional to the landing energy of electrons, so the spot size becomes nearly constant throughout the full energy scale.