Positron Annihilation Lifetime Spectroscopy Research Articles

Capture of Volatile Iodine Gas and Identification of Adsorption Sites in the Pore Network of Unsubstituted Imidazole Linker-Incorporated Zeolitic Imidazolate Framework-8.

The incorporation of unsubstituted imidazole (Im) in zeolitic imidazolate framework-8 (ZIF-8) crystallized in sodalite topology is proposed to improve gas capture and gas separation performance drastically. However, the incorporation of unsubstituted Im in ZIF-8 has remained challenging due to the thermodynamic instability of zinc and Im bonding in sodalite topology. We have incorporated up to 24.4 (mol %) Im linker in highly crystalline ZIF-8 with similar morphology and sodalite topology using a delayed linker addition strategy at room temperature. Im incorporation brings significant tuning to the pore architecture of ZIF-8, as confirmed by positron annihilation lifetime spectroscopy. The modifications in the pore architecture are primarily due to linker defects produced in the frameworks during crystallization and the elimination of steric hindrance due to the absence of a methyl group on Im. The Im-incorporated ZIF-8 shows significant enhancement in iodine capture as well as higher crystal structure stability under iodine vapor exposure as compared to pristine ZIF-8. Through ortho-positronium interaction with the adsorbed molecular iodine in the pore network of the frameworks, it is confirmed that iodine is preferentially adsorbed at cavity and intercrystalline voids, whereas aperture sites remain unoccupied by iodine molecules.

Hydrated doped-BaZrO3 proton conductors studied by positron annihilation lifetime spectroscopy

The study of defect chemistry for doped BaZrO3 proton conductors is of particular interest because of defect interactions that can affect the proton conductivity of the material. Protons incorporated due to the material's hydration can be trapped by negatively charged immobile dopants, reducing proton mobility. The reduction of the proton conduction impedes the use of BaZrO3 materials in energy conversion applications at intermediate temperatures (300 °C – 600 °C). The probing of proton trapping in doped BaZrO3 is hindered by the limited availability of techniques sensitive to defect chemistries. In this work, we used positron annihilation lifetime spectroscopy (PALS) to study the defect chemistry of Y-doped and Sc-doped BaZrO3. Using a two-state positron trapping model we showed that PALS can be applied to study the defect chemistry of hydrated dense proton conductors. Positron trapping rates and lifetimes were correlated with doping levels of the materials. Probability significance t-tests were carried out for PALS parameters to verify whether there are differences/similarities for various populations: non-doped/doped, level and type of doping, high temperature, and surface effects. The results revealed that the initial doping generates a significant number of traps available for positrons. Doping with yttrium increased the positron trapping rate, while this effect was not observed with scandium. Low-temperature hydration affects specimens significantly inhibiting positron trapping at undoped BaZrO3 material and highly doped specimens. Positronium formation in rough surface layers, and highly doped specimens was detected but does not exceed 1%.

Unravelling the Water Adsorption Mechanism in Hierarchical MOFs: Insights from In Situ Positron Annihilation Lifetime Studies.

Atmospheric water harvesting with metal-organic frameworks (MOFs) is a new technology providing a clean, long-term water supply in arid areas. In-situ positron annihilation lifetime spectroscopy (PALS) is proposed as a valid methodology for the mechanistic understanding of water sorption in MOFs and the selection of prospective candidates for desired applications. DUT-67-Zr and DUT-67-Hf frameworks are used as model systems for method validation because of their hierarchical pore structure, high adsorption capacity, and chemical stability. Both frameworks are characterized using complementary techniques, such as nitrogen (77 K) and water vapor (298 K) physisorption, SEM, and PXRD. DUT-67-Zr and DUT-67-Hf are investigated by PALS upon exposure to humidity for the first time, demonstrating the stepwise pore filling mechanism by water molecules for both MOFs. In addition to exploring the potential of PALS as a tool for probing MOFs during in situ water loading, this work offers perspectives on the design and use of MOFs for water harvesting.

Oxygen vacancy induced luminescence in Y2Zr2O7 and its removal on Eu3+ doping leading to enhanced quantum efficiency

To circumvent the low color rendering index and high color temperature of existing white light emitting diodes (LEDs), a lot of attention has been given on designing efficient red emitting luminophores for phosphor converted LEDs (pcLEDs). Utilizing the favorable properties of Y2Zr2O7 (YZO) pyrochlore and Eu3+ as dopant ion we could produce bluish white light emitting YZO and red light emitting YZO:Eu3+ (YZOE). Based on monoexponential luminescent decay, we could infer europium ion occupies Y3+ site which is in line with density functional theory (DFT) calculated defect formation energy and positron annihilation lifetime spectroscopy (PALS). YZO on UV irradiation generates bluish white light endowed by both neutral and singly ionized oxygen vacancies but with very meager photoluminescence quantum yield (PLQY∼1.2%). On doping Eu3+ ion in YZO; oxygen vacancy is completely removed with efficient host to dopant energy transfer leading to bright red emission with PLQY∼ 8.2%. The feasibility of YZO to Eu energy transfer is well established using DFT calculations. Reduction in density of oxygen vacancies on doping is well corroborated using electron paramagnetic resonance (EPR) and thermoluminescence (TL) spectroscopy and very well supported by DFT calculations. In this work by harnessing the potential of pyrochlore lattice defects and doping engineering we could enable almost 8-fold increase in PLQY in YZOE.

Artificial neural network as an effective tool to calculate parameters of positron annihilation lifetime spectra

The paper presents the application of the multi-layer perceptron regressor model for predicting the parameters of positron annihilation lifetime spectra using the example of alkanes in the solid phase. Good agreement of calculation results was found when the approach is compared with the commonly used methods, e.g., LT. The presented method can be used as an alternative quick and accurate tool for the decomposition of positron annihilation lifetime spectroscopy (PALS) spectra in general. The advantages and disadvantages of this new method are discussed. We show the preliminary results where the trained network can give better outcomes than the results yielded by programs based on an analysis of a single PALS spectrum.

Impacts of sintering temperature on the structure and physical properties of MnWO4 ceramics

The effects of sintering temperature on the structure, morphology, vacancy defects, magnetic and dielectric characteristics of MnWO4 ceramics synthesized by solid-state reaction were investigated. The results revealed that the increase of sintering temperature caused lattice deformation, but had no effect on the monoclinic lattice structure of all MnWO4 samples. SEM analysis showed that all of the synthesized samples were relatively dense, and higher sintering temperature was conducive to grain growth. Positron annihilation lifetime spectroscopy revealed that the vacancy size and concentration were regulated by sintering temperature. The properties measurements showed that appropriate increase of sintering temperature increased the dielectric constant, magnetization, and expanded existing temperature range of AF2 phase for MnWO4 system. The relationship between structure and performances was discussed in this paper.

In Situ Heating Positron Annihilation Lifetime Spectroscopy Experiments on an Al–Mg Alloy

The binding between vacancies and Mg atoms in an aluminum solid solution is not fully understood but essential for understanding its role in age hardening of many Al alloys. After annealing and quenching, Mg prevents the loss of excess vacancies during natural ageing and forms complexes containing one, possibly two, vacancies, and various Mg atoms. By heating the alloy after natural ageing, these complexes are dissolved, i.e., natural ageing is reverted. This reversion process is studied by in situ positron annihilation lifetime spectroscopy utilizing the very high count rate at the accelerator driven facility ELBE. Positron spectra are continuously acquired during heating at rates between 3 and 50 K min−1. After correcting for the contributions of the oxidized surface and decomposing spectra into components, the process can be followed in detail and is found to take place in distinct stages: first, the number of vacancy–Mg complexes is reduced and then the liberated vacancies agglomerate into clusters that eventually dissolve at even higher temperatures.

Evaluating the catalytic performances of supported transition metal catalysts by positron annihilation lifetime spectroscopy

Supported transition metal catalysts have played a critical role in the modern synthetic chemical industry. Herein, MCM-41 was chemically modified with amino groups and then used as the support of six transition metal cations. The chemical structures, microstructures and dispersions of these supported transition metal cations were carefully characterized by XRD, TEM and BET. The chelations of modified MCM-41 support with these transition metal cations were confirmed by FT-IR and XPS. The reduction reaction of 4-nitrophenol into 4-aminophenol was employed to evaluate the catalytic activities and recyclabilities of these supported transition metal catalysts. Meanwhile, their reactivities with o-Ps species were assessed by positron annihilation lifetime spectroscopy (PALS). It was found that the reduced o-Ps annihilation intensities (ΔI3) were well associated with their catalytic activities while the o-Ps annihilation lifetime values (τ3) were related to their catalytic recyclabilities. In all, the catalytic activities and recyclabilities of supported transition metal cations can be conveniently evaluated by their o-Ps annihilation properties. This method holds great potential in the design and preparation of highly active and stable supported transition metal catalysts.

Effects of Selected Plant Extracts on the Quality and Functional Properties of Gelatin and Furcellaran-Based Double-Layer Films

AbstractIn this study, for the first time, double-layer films based on furcellaran and gelatin were obtained, in which the 1st layer of furcellaran was enriched with various plant extracts. The aim of this work was to improve the composition of the developed innovative packaging material by using two layers of biopolymer complexes and analysing the composites to determine the effect of different model plant extracts on the physicochemical properties of the double-layered materials. Furthermore, the hypothesis that positron annihilation lifetime spectroscopy (PALS) could be correlated with gas barrier properties was verified. The addition of plant extracts significantly influenced the physicochemical parameters of the obtained double-layered materials. The lowest solubility was noted in films with the addition of garlic extract (58.40%) and black pepper extract (59.26%) which also had the lowest water content (8.88%). The film with garlic extract also had the lowest oxygen transfer rate (OTR) value after 6 months of storage which is consistent with PALS. The presented data suggest the possibility of correlating PALS results with the gas barrier properties. Tests carried out using this method will facilitate the selection of new potential ingredients when designing packaging materials based on biopolymers, thus, making it possible to obtain new-generation matrices.

Mapping positron annihilation lifetime spectroscopy data of a polymer to classical molecular dynamics simulations without shifting the glass transition temperature.

Positron annihilation lifetime spectroscopy (PALS) enables the nondestructive measurement of nanoscale cavities in materials. In this study, a strategy was proposed for mapping PALS measurement data of isotactic polypropylene to classical molecular dynamics (CMD) simulations. The discrepancy between simulated and experimental glass transition temperatures was resolved by shortening the polymer chains, rather than adjusting for the temperature, using the Williams-Landel-Ferry (WLF) equation. The effective probe radii of ortho-positronium (o-Ps), determined by comparing PALS data with CMD simulations, were ∼0.8nm, which was consistent with the o-Ps size given by the solution of the Schrödinger equation. The free-volume fraction corresponding to the effective probe radius was 12.3% at the glass transition temperature, close to the value estimated using Simha-Boyer theory. The cavity number density was proportional to the effective probe radius and decreased with temperature. The o-Ps effective probe radius was proportional to both the critical probe radius and the -1/3 power of the monomer number density, and increased with increasing temperature. These findings suggest that combining PALS measurements with CMD simulations may provide insight into cavities in polymeric materials without relying on the WLF equation.

Leptothrix biofilms and the formation of oxygen and hydrogen molecules in structure

In this work, the crystal structure of Leptothrix biosamples was studied using positron annihilation lifetime spectroscopy, MIKA modeling package and X-ray diffraction method. The structural analysis results showed that free cubic and orthorhombic phase Fe and trigonal Fe2O3 and cubic Fe3O4 phases were detected in Leptothrix biosamples. It has been determined that the concentration of cubic Fe3O4 phases in biosamples is higher than other phases. The theoretical bases of the regularity of distribution of hydrogen and oxygen atoms in the form of free and vacancies in the trigonal Fe2O3 and cubic Fe3O4 phases were calculated. On the other hand, the lifetime of vacancies in Fe, Fe2O3 and Fe3O4 structures has been determined.

Evolution of Local Structure and Pore Architecture during Zeolitic Imidazolate Framework-L to Zeolitic Imidazolate Framework-8 Phase Transformation Investigated Using Raman, Extended X-ray Absorption, and Positron Annihilation Lifetime Spectroscopy

Evolution of Local Structure and Pore Architecture during Zeolitic Imidazolate Framework-L to Zeolitic Imidazolate Framework-8 Phase Transformation Investigated Using Raman, Extended X-ray Absorption, and Positron Annihilation Lifetime Spectroscopy

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Exposure: Observations with Positron Annihilation Lifetime Spectroscopy.

Atomic oxygen (AO) is one of the dominant components of the residual atmosphere in low Earth orbit. AO collides with spacecraft with a translational energy of 5 eV, forming nanoscale protrusions on polymeric materials. To clarify the influences of a polymer's chemical structure on the formation of AO-induced microstructures, this study investigated the size of free-volume holes and the layer thickness that interacted with AO for polyethylene (PE), polypropylene (PP), and polystyrene (PS) by positron annihilation lifetime spectroscopy. The injection energies of positrons varied from 1.3 to 10 keV to adjust the injection depth (range) into the polymers (40 nm-1.6 μm). For the pristine films, the lifetime of ortho-positronium (o-Ps, τ3) was longer in the order of PS, PP, and PE regardless of the injection energy of positrons, showing the different sizes of free-volume holes with radii of 0.29, 0.31, and 0.32 nm, respectively. The fraction of the decay component corresponding to o-Ps in all decay components (relative intensity of o-Ps, I3) was used to investigate the chemical change induced by AO exposure. The I3 values for the three polymers were decreased by AO exposure of (2-5) × 1018 atoms/cm2 or more at a depth of 40-48 nm, obtained by 1.3 keV positrons. This indicates that AO formed polar groups (i.e., an oxidized layer) on the polymer surfaces. The maximum depths of such chemical change for PE and PP were deeper than that for PS. The different sizes of free-volume holes would affect the diffusion or ballistic penetration of AO, resulting in the difference in the oxidized layers' thicknesses and surface morphologies.

Nanoformulation of lipase from Porcine pancreas by electrospinning as a novel alternative for enzyme-based per os therapies

Pancreatic enzyme replacement therapy (PERT) treats pancreatic insufficiency, causing a lack of digestive enzymes due to pancreatic damage or dysfunction. However, traditional PERT faces challenges such as poor stability and limited efficiency of the enzymes. Lipase from Porcine pancreas (PpL) is commonly applied for PERT, however only classic formulas such as tablets or capsules produced from physical mixture of the enzyme and excipients had been developed. Electrospinning produces nanofibers that are promising as drug carriers, due to their high surface area-to-volume ratio and tunable physicochemical properties. In this study we investigated the direct nanoformulation of PpL applying polymer nanofibers. The effect of water-soluble and water-resist polymers (PVA: polyvinyl alcohol, PVP: polyvinylpyrrolidone and PLA: polylactic acid) on the physicochemical properties of precursor for electrospinning and on the nanofibers produced from them was systematically investigated. The immobilization of PpL using electrospun nanofibers by entrapment and adsorption methods was developed and compared the enzymatic activities of the nanofibrous formulas. It was found that immobilization using nanofibers in all cases caused a large increase in the enzymatic activity of PpL. Our research demonstrates that nanoformulation of PpL could be efficient by electrospinning technique and the polymer has dominant effect on the enzymatic and structural properties of formulas since PpL entrapped in PLA showed the highest enzymatic activity. Nanofibrous enzyme loaded formulations were comprehensively studied by rheology, scanning electron (SEM) and Raman microscopy, water content determination, positron annihilation lifetime spectroscopy (PALS) and enzyme activity assay of ester hydrolysis.

Quantitative Contribution of Oxygen Vacancy Defects to Arsenate Immobilization on Hematite.

Hematite is a common iron oxide in natural environments, which has been observed to influence the transport and fate of arsenate by its association with hematite. Although oxygen vacancies were demonstrated to exist in hematite, their contributions to the arsenate immobilization have not been quantified. In this study, hematite samples with tunable oxygen vacancy defect (OVD) concentrations were synthesized by treating defect-free hematite using different NaBH4 solutions. The vacancy defects were characterized by positron annihilation lifetime spectroscopy, Doppler broadening of annihilation radiation, extended X-ray absorption fine structure (EXAFS), thermogravimetric mass spectrometry (TG-MS), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The results revealed that oxygen vacancy was the primary defect type existing on the hematite surface. TG-MS combined with EPR analysis allowed quantification of OVD concentrations in hematite. Batch experiments revealed that OVDs had a positive effect on arsenate adsorption, which could be quantitatively described by a linear relationship between the OVD concentration (Cdef, mmol m-2) and the enhanced arsenate adsorption amount caused by defects (ΔQm, μmol m-2) (ΔQm = 20.94 Cdef, R2 = 0.9813). NH3-diffuse reflectance infrared Fourier transform (NH3-DRIFT) analysis and density functional theory (DFT) calculations demonstrated that OVDs in hematite were beneficial to the improvement in adsorption strength of surface-active sites, thus considerably promoting the immobilization of arsenate.

Investigation of Nd3+:Y2Si2O7 Phosphors Using Photoluminescence and Positron Annihilation Lifetime Spectroscopy

Neodymium-doped Y2Si2O7 nanopowders were synthesized by sol-gel and annealed under varying annealing temperature conditions. The phase transformation of Y2Si2O7 and the crystalline size change were observed with annealing temperature. Photoluminescence and the decay patterns of the 4F3/2→ 4I9/2, 4I11/2, and 4I13/2 transitions were examined to reveal the spectroscopic characteristics of the powders. The spectral outputs of Nd3+:Y2Si2O7 phosphors remain strongly connected to the phase properties of Y2Si2O7. The positron lifetimes (τ 2, τ 3, and τ 4) and intensities (I 2, I 3, and I 4) obtained by positron annihilation lifetime spectroscopy were discussed for neodymium-doped Y2Si2O7 phosphors to relate the structural and boundary changes in terms of the phases and molar contents of the Nd3+ ions. The τ 2 and I 2 components were related to the structural open space in the grain, the τ 3 and I 3 components were correlated with vacancy-like defects in the grain boundaries, and the τ 4 and I 4 components were associated with vacancy defects in the grain, together with the effect of the increasing content of the Nd3+ ions.

Free volume can control biodegradation rate of polylactic acid

We evaluated whether the enzymatic degradation rate of biodegradable plastics is related to the amorphous structure of the polymer. The “free-volume,” meaning the cavity between the polymer chains in the amorphous part, is known to influence various physical properties. In this study, enzymatic degradation tests were performed on polylactic acid (PLA) films with free-volume hole sizes estimated by positron annihilation lifetime spectroscopy (PALS). Films with different free-volume hole sizes near the surface region degraded in different modes, and a positive correlation, with a correlation coefficient of 0.713, was observed between the degradation rate in the initial 30 min and the free volume size near the surface. These results suggest the possibility of controlling the degradation rate of the amorphous region by controlling the free volume size, which is important for future applications of biodegradable plastics in a wide range of fields.

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10-7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

Super high counting rate of positron annihilation lifetime spectrometer achieved via multiple detectors

Positron annihilation lifetime (PAL) spectroscopy has long been considered as a powerful approach to characterize atomic-scale vacancy-type defects and supermicropores, which affect the macroscopic properties of materials. In standard PAL measurements, several million annihilation events are required to achieve accuracy. For a conventional PAL spectrometer which consists of two scintillation detectors and analytical electronics, the two detectors are usually perpendicularly positioned to diminish spectra distortions (caused by backscattering and pileup). The perpendicular placement of detectors leads to a counting rate of 100 – 300 counts/second (cps) while using a 30 μCi 22Na source (3.3 – 10 cps/μCi). Consequently, collecting one PAL spectrum with total counts of one million takes several hours. The long time limits its potential to observe changes in the microstructure. To increase the counting rate of PAL measurements, we propose a novel multi-detector PAL spectrometer, which includes three or four detectors, a digital oscilloscope, and an algorithm for any detector to record both start and stop signals. The spectrometer achieves a high counting rate of around 2144 cps (71.5 cps/μCi) with four detectors, which is about ten times that of a conventional one with two detectors perpendicularly positioned. This work enables us to perform in-situ PAL measurements to disclose the minute-scale evolution of microstructure more precisely during fast physical and chemical processes in the future.

Challenges in porosity characterization of thin films: Cross-evaluation of different techniques

The review article provides an overview of the most important and popular techniques for evaluating the porosity of thin films developed for various applications. These methods include ellipsometric porosimetry (EP), positron annihilation (lifetime) spectroscopy (PAS/PALS), and grazing incidence small-angle x-ray scattering (GISAXS). Special attention is given to the challenges associated with interpreting the measured data and the inherent limitations of each method. It is demonstrated that EP, GISAXS, and PALS are all informative for studying the pore structure in thin films, with each method offering unique insights. GISAXS, in particular, allows for the evaluation of three-dimensional mesostructures, including pore arrangement, pore spacing, and structural order. On the other hand, PALS has a unique advantage in its capability to analyze extremely small isolated pores (free volume). The advantage of EP lies in its simplicity and the possibility to analyze multiple properties from the same set of measurements. The cross-evaluation of different methods offers important insights into the complex pore structure of materials, highlighting the significance of appropriate modeling and interpretation of data.