Davide Bleiner

Laser-induced particulate as carrier of analytical information in LA-ICPMS direct solid microanalysis.

Laser ablation in combination with plasma spectrochemistry is an ideal technique for depth profiling analysis, based on signal profiles. However, signal profiles were found to be critically influenced by the characteristics of the ablated particles, especially their composition and size distribution, and consequently transport mechanism and plasma-assisted vaporization efficiency. Even for a refractory material like ceramic, relics of melting following laser irradiation were found, so that particles were non-stoichiometric as compared to the parent material. Estimates of transport efficiency showed that this is highly variable as a function of particle size. Large particles are likely to be lost in the sample chamber. Fine particles are prone to wall reaction, especially in Ar ambient. Variability in particle delivery to the ICP-MS was suspected to be the cause for an element-dependent analyte signal response. Fluctuation in particle vaporization degree as a consequence of plasma temperature instability was also responsible for element-dependent signal profile deviation. However, for a 10-fold higher mass load into the plasma, no direct fractionation effects were observed. Differential transport of chemically-differentiated analyte-carriers is suggested to be primary cause for element-dependent signal structure.

A novel gas inlet system for improved aerosol entrainment in laser ablation inductively coupled plasma mass spectrometry

In order to minimize the dead volume in large cells for laser ablation inductively coupled plasma mass spectrometry, and improve the aerosol entrainment characteristics, the gas inlet nozzle has been set in rotation. This allowed a wider volume to be swept than with the traditional static inlet nozzle approach. Therefore, sensitivity combined with site-to-site repeatability was improved by a factor of two, together with minimization of aerosol loss within the cell and signal dispersion.

Spatially resolved quantitative profiling of compositionally graded perovskite layers using laser ablation-inductively coupled plasma mass spectrometry

Fuel cell cathodes can be constructed as a stack of perovskite layers whose composition gradually changes over a few hundreds of µm. They are prepared by sintering a mixture of two ceramic powders (Mn-perovskite and Co-perovskite), where the proportions of the mixture contributes to the chemical gradation. Laser ablation-ICP-MS permitted the determination of the proportions of Mn-perovskite and Co-perovskite, in several depth profiles. The set-up and the laser operating conditions were specifically optimised so that correct elemental concentration profiles could be acquired, without beam induced artefacts. Lateral resolution below 100 µm and a depth resolution of 0.1–0.2 µm were obtained. Quantification was carried out from the proportions of the mixture of perovskites and the elemental composition of the individual perovskites (i.e., a “weighted summation”). The composition of the powder was previously determined via digestion and ICP-MS. Comparison with semi-quantitative data from SEM-EDX showed that the developed method provided reliable responses. Analysis of the signal structure of the depth profiles was performed by means of signal convolution and numerical differentiation. The occurrence of differential bands in conjugate pairs could be assessed and used for a realistic description of the sample structure. The fluctuation of analyte concentrations at low level (<1 µm) suggests that further improvements in the sampling thickness might conflict with robust and powerful quantification. Therefore, the determined pulse-related depth resolution of 100–200 nm seems to be a good compromise between spatially resolved analysis and quantification capability. The rapidity, flexibility and detection power of LA-ICP-MS are advantages that integrate and extend the analytical capabilities of other well-established beam-assisted techniques (i.e., XPS, AES, SIMS, SNMS, GD-OES/MS, SEM-EDX) and permit critical control of the quality of the fabricated products.

Closing the pressure gap in x-ray photoelectron spectroscopy by membrane hydrogenation

Comprehensive studies of gas-solid reactions require the in-situ interaction of the gas at a pressure beyond the operating pressure of ultrahigh vacuum (UHV) X-ray photoelectron spectroscopy (XPS). The recent progress of near ambient pressure XPS allows to dose gases to the sample up to a pressure of 20 mbar. The present work describes an alternative to this experimental challenge, with a focus on H2 as the interacting gas. Instead of exposing the sample under investigation to gaseous hydrogen, the sample is in contact with a hydrogen permeation membrane, through which hydrogen is transported from the outside to the sample as atomic hydrogen. Thereby, we can reach local hydrogen concentrations at the sample inside an UHV chamber, which is equipped with surface science tools, and this corresponds to a hydrogen pressure up to 1 bar without affecting the sensitivity or energy resolution of the spectrometer. This experimental approach is validated by two examples, that is, the reduction of a catalyst precursor for CO2 hydrogenation and the hydrogenation of a water reduction catalyst for photocatalytic H2 production, but it opens the possibility of the new in situ characterisation of energy materials and catalysts.

Promotion of Hydrogen Desorption from Palladium Surfaces by Fluoropolymer Coating

The catalytic activity of Pd surfaces towards hydrogen desorption was significantly improved by a nanometer‐thin polytetrafluoroethylene (PTFE) layer, as shown by an enhancement in the permeability of a Pd membrane coated on the permeate side. The origin of this effect was found to be due to a lowering of the barrier for hydrogen desorption, as evidenced by a change in the rate‐limiting mechanism of hydrogen permeation through the membrane from desorption (un‐coated) to diffusion controlled. In situ X‐ray photoelectron spectroscopy (XPS) revealed the electronic structure of the sputtered PTFE. Apart from C–Fn subunits (n=1, 2, 3), we found that nonsaturated carbon atoms became hydrogenated during hydrogen permeation, which was indicative of an interaction between Pd and PTFE. This interaction was weak; no Pd−F bonds were formed. We thus attributed the effect to an increase in the hydrophobicity of the surface by the porous PTFE layer and to a promoter effect of hydrogen desorption as a result of electrostatic interactions between chemisorbed hydrogen and physisorbed PTFE.

Carboxylate Functional Groups Mediate Interaction with Silver Nanoparticles in Biofilm Matrix

Biofilms causing medical conditions or interfering with technical applications can prove undesirably resistant to silver nanoparticle (AgNP)-based antimicrobial treatment, whereas beneficial biofilms may be adversely affected by the released silver nanoparticles. Isolated biofilm matrices can induce reduction of silver ions and stabilization of the formed nanosilver, thus altering the exposure conditions. We thus study the reduction of silver nitrate solution in model experiments under chemically defined conditions as well as in stream biofilms. Formed silver nanoparticles are characterized by state-of-the art methods. We find that isolated biopolymer fractions of biofilm organic matrix are capable of reducing ionic Ag, whereas other isolated fractions are not, meaning that biopolymer fractions contain both reducing agent and nucleation seed sites. In all of the investigated systems, we find that silver nanoparticle–biopolymer interface is dominated by carboxylate functional groups. This suggests that the mechanism of nanoparticle formation is of general nature. Moreover, we find that glucose concentration within the biofilm organic matrix correlates strongly with the nanoparticle formation rate. We propose a simple mechanistic explanation based on earlier literature and the experimental findings. The observed generality of the extracellular polymeric substance/AgNP system could be used to improve the understanding of impact of Ag+ on aqueous ecosystems, and consequently, to develop biofilm-specific medicines and bio-inspired water decontaminants.

X-ray absorption spectroscopy probing hydrogen in metals

X-ray absorption spectroscopy (XAS) is a widely used technique for determining the electronic structure of matter. In contrast to X-ray photoelectron spectroscopy (XPS), XAS makes use of photons only, and therefore suffers less from absorption of the probe beam, i.e., electrons or photons, respectively. This is true for hard X-rays probing, e.g., the Kedges of d-metals in metal hydrides (albeit with limited chemical information). Soft X-rays, which are suited to analyze the electronic structure of hydrogen in solids, have a limited absorption length in gases. Photons with energies of less than 50 eV (“hydrogen K-edge” <;20 eV) are absorbed in less than 1 mm at ambient pressure, which is needed for technical hydrides. Recently, we developed a membrane-based approach to study materials exposed to high hydrogen “pressures” while keeping analysis chamber under high vacuum - thus effectively achieving high pressure XPS analysis. In this paper, we demonstrate that the membrane approach originally designed for XPS can be equally well used for XAS. We show first results on the electronic structure of hydrogen in Pd-Ag alloy as measured by in situ XAS using a laboratory extreme ultraviolet (EUV) source.

Table-top two-color soft x-ray laser from Ni-like Mo plasma

Two-color laser pulses in a laboratory setup are interesting for enabling a number of advanced spectroscopy techniques. The generation by means of Ni-like laser-produced plasmas is promising for scaling down the wavelength towards the soft X-ray. The occurrence of a two-color signal has been documented in the 1990s and a detailed atomic physics study interpreted the pulse as due to concomitant collisional pumping (color-1) and self-photoexcitation (color-2). If this framework is on the atomic physics basis valid, its kinetic and hydrodynamic quantitative aspects need to be better understood. In particular, experimental observations leave room for a number of open points. Here we propose am amplified Raman scattering (ARS) scheme, to explain the growth of color-2 and its sensitivity to the plasma irradiation scheme.