Vladimír Havránek

Solid-water partitioning of elements in Czech freshwaters

Partitioning of 41 elements between solids and water was studied by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, KD, between suspended particulate matter (SPM) and filtrate (‘dissolved’ fraction) differed by 4 orders of magnitude. The highest KD values (log KD>2.0 l/g) were exhibited by Zr, Al, Ce, Pb, La, Ti, Fe, Sm, Th and Cr which are extremely insoluble in near-neutral water or generally poorly soluble (Zr,Ti). The KDs decrease with element and DOC loading due to the relative increase of the element in the low molecular fraction. Log KD mostly increased linearly with pH within a range from 3.5 to 9. KDU decreased at pH >6 due to carbonate complexation. The colloidal fraction (>1 kDa <0.4 μm) in a reservoir with a pH of 6.8 was mainly preferred by Fe, Pb, Be, Nb, Y, Al, Ni, U and Zr. When the colloidal fraction is not differentiated from true solution, then incorrect information about partitioning may be obtained and the highest KD may decrease.

Chemical Preparation of Graphene Materials Results in Extensive Unintentional Doping with Heteroatoms and Metals

Chemical synthesis of graphene relies on the usage of various chemical reagents. The initial synthesis step, in which graphite is oxidized to graphite oxide, is achieved by a combination of chemical oxidants and acids. A subsequent chemical reduction step eliminates/reduces most oxygen functionalities to yield graphene. We demonstrate here that these chemical treatments significantly contaminate graphene with heteroatoms/metals, depending on the procedures followed. Contaminations with heteroatoms (N, B, Cl, S) or metals (Mn, Al) were present at relatively high concentrations (up to 3 at%), with their chemical states dependent on the procedures. Such unintentional contaminations (unwanted doping) during chemical synthesis are rarely anticipated and reported, although the heteroatoms/metals may alter the electronic and catalytic properties of graphene. In fact, the levels of unintentionally introduced contaminants on graphene are often higher than typical levels found on intentionally doped graphene. Our findings are important for scientists applying chemical methods to prepare graphene.

Neutron depth profiling by large angle coincidence spectrometry

Abstract A measuring device for large angle coincidence neutron depth profiling was installed behind a 5.6 m long neutron guide. Two large area PIN diodes placed at a distance of 2 mm from the sample serve for the detection of particles emitted at different angles. Coincidence event registration and sorting was accomplished using a PC AT computer. A simple but effective technique for two-dimensional data evaluation is proposed. The feasibility of the method was tested on various standards. The detection limits and the depth resolution are determined and possible refinements of the technique are discussed.

Degradation of polyimide and polyethyleneterephtalate irradiated with 150 and 200 keV Ar+ ions, studied by RBS and ERD techniques

Abstract Polyimide (Upilex, PI) and polyethyleneterephtalate (Mylar, PET) were irradiated with 200 and 150 keV Ar + ions respectively to different fluences and the depth profiles of hydrogen and oxygen atoms in polymer surface layer were determined using standard Rutherford backscattering (RBS) and elastic recoil detection (ERD) techniques. Significant hydrogen and oxygen depletion is observed at the ion fluences above 1×10 14 cm −2 . Above 5×10 15 cm −2 a saturated state is achieved when another fluence increase does not result in significant changes of the H and O profile shape. At higher fluences the H and O depth profiles are rather flat with no local minima. This finding indicates only minor role of nuclear stopping in production of volatile degradation products.

Carboranedithiols: Building Blocks for Self-Assembled Monolayers on Copper Surfaces

Two different positional isomers of 1,2-dicarba-closo-dodecaboranedithiols, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (1) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.

Modifications of polypropylene induced by the implantation of iodine ions

The polypropylene (PP) samples implanted with I+ ions at the implantation energies of 50, 100 and 150 keV to the fluence of 1×1015 cm-2 were studied using standard RBS (Rutherford backscattering) technique. The surface of the implanted PP samples is heavily damaged and enriched in carbon. The measured projected ranges of iodine atoms are by 10 to 30% lower than the theoretical ones calculated using Monte-Carlo simulation code TRIM (transport of ions in matter) code for pristine PP. The differences may be due to structural changes of polymer during the ion implantation. The range stragglings exceed the theoretical TRIM estimations about twice. The profile broadening is probably due to the diffusion of the iodine atoms through the damaged PP surface layer. In repeated RBS measurements, the iodine diffusion was confirmed with diffusion coefficient of 2.7×10-18 cm2s-1. Noticeable oxidation of the implanted samples was observed, with majority of the oxygen atoms being trapped in the region of maximum radiation defects. The bulk oxygen concentration profile resembles that of iodine but it is shifted inward by 200-350A. The relative oxygen content in the bulk is an increasing function of the implantation energy and it exceeds considerably the stoichiometric value for common iodine oxides.

Pulse-shape discrimination in neutron depth profiling technique

Abstract Pulse-shape discrimination (PSD) is used for the first time for reduction of unwanted background in analyses of solid surfaces by neutron depth profiling method (NDP) based on the detection of charged particles from the (n, p) and (n, α) nuclear reactions induced by thermal neutrons on some light elements. The experimental PSD arrangement is described and its performance is demonstrated on the measurement of real sample. Background reduction by about two orders of magnitude in the energy region below 1 MeV leads to sensitivity improvement by about one order of magnitude and to extension of measurable depth region for some of light elements like N and Cl.

PIXE-INP computer code for PIXE analyses of thin and thick samples

Abstract The Pascal 6.0 program package PIXE-INP was developed at Institute of Nuclear Physics (Academy of Sciences of Czech Republic) for quantitative evaluation of PIXE spectra from thin and thick samples. The K-, L- and M-sensitivities are calculated using semi-empirical and theoretical ionization cross sections, tabulated relative X-ray emission probabilities, Coster-Kronig and fluorescence yields. The X-ray detector efficiency is determined by measuring a set of thin standards with known element content. The package is supplemented with the data bases on X-ray attenuation coefficients and proton stopping powers which are needed for the calculation of thick sample matrix corrections. The X-ray spectrum evaluation comprises an iterative background removal and non-linear least squares fitting of X-ray multiplets with Gaussian peak shape. The escape and summation peaks are taken into account. The performance of the program package is documented on measurements of reference materials.

RBS study of oxidation processes in polypropylene and polyethylene implanted with fluorine ions

Abstract The oxidation on polypropylene (PP) and polyethylene (PE) samples implanted with 50–200 keV F+ ions to the ion fluences from 1 × 1011–1 ×1015 cm−2 was studied using standard Rutherford backscattering (RBS) technique. The implanted samples were stored in the air and in darkness, and the oxidation process was followed by repeated RBS measurements in the time interval 1–150 days after the ion implantation. In PP, the oxidation, initially limited to the radiation damaged surface layer, proceeds steadily inward and a homogeneous oxidized layer is built up. The well defined sharp interface between the oxidized layer and the bulk PP moves inside with a mean velocity of 4.0 × 10−5 nms−1. The ion implanted PE samples are homogeneously oxidized from the sample surface up to the depths accessible to RBS technique. The surface ratio of oxygen and carbon atomic densities in ion implanted PP and PE samples increases from about 0.06 for the ion fluences below 1 × 1014 cm−2 to about 0.17 for higher ones.

Use of INAA, PIXE and XRF in Homogeneity Testing of New IAEA Reference Air Filters

About 250 sets of polycarbonate (Nuclepore, Costar) membrane filters loaded with two urban air particulate matter (APM) obtained in Vienna and Prague were prepared by filtration of a watter suspension of the above APM materials. The homogeneity of both bulk APM materials and the loaded filters was tested by determining a number of elements by instrumental neutron activation analysis (INAA), proton induced X-ray emission (PIXE) and micro-X-ray fluorescence (μ-XRF). Relative standard deviations due to inhomogeneity (sinh) were below 3% for many elements in the bulk APM materials. In the loaded filters, the sinh values increased significantly. Nevertheless, for up to 20 elements important in air pollution studies the sinh values were below 15%, allowing target values with a reasonably low uncertainties (up to 20%) to be derived for the future use of the filters in proficiency testing of laboratories involved in air pollution monitoring.