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Abstract: X-ray absorption spectroscopy of gases is seldom used for structural analysis, except in as much as the extended X-ray absorption fine-structure (EXAFS) signal is used to monitor some specific molecular process. Gas spectroscopy, however, has been the main tool in the systematic study of higher-order photoabsorption processes, multielectron photoexcitations (MEEs), which were introduced with an experiment at the K edge of argon. Along with noble gases, where MEEs are studied in the pure form, free of the structural signal, measurements have been extended to monatomic metal vapours, although at the cost of considerable experimental difficulties. Several types of absorption cells have been devised, among them the ceramic double cell and the heat-pipe cell for experiments in the low-energy region. In a high-temperature oven with considerable technical refinement, edge profiles of nonvolatile metals were measured up to 2500 K. Edge profiles and MEEs, which are strongly dependent on electron correlation, provide sensitive tests of the theory of atomic and molecular structure.
Keywords: X-ray absorption spectroscopy, absorption cells, gas and vapour samples, multielectron photoexcitation, effects of electron correlation
Published in RUNG: 05.09.2024; Views: 633; Downloads: 4
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Abstract: Technologies for mass production require cheap and abundant materials such as ferrous chloride (FeCl2). The literature survey shows the lack of experimental studies to validate theoretical conclusions related to the population of ionic Fe-species in the aqueous FeCl2 solution. Here, we present an in situ X-ray absorption study of the structure of the ionic species in the FeCl2 aqueous solution at different concentrations (1–4 molL−1) and temperatures (25–80 ◦C). We found that at low temperature and low FeCl2 concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl2(H2O)4]0. The observed substitutional mechanism is facilitated by the presence of the intramolecular hydrogen bonds as well as entropic reasons. The transition from the single charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models.
Keywords: structure, population, ionic species, aqueous ferrous chloride, in situ X-ray absorption spectroscopy
Published in RUNG: 24.01.2022; Views: 2692; Downloads: 43  (1 vote)
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Abstract: The mechanism of Cu2+ loading into commercially available natural HEU-type and synthetic LTA-type zeolites for their possible use in environmental processes, such as water and air treatment applications, was studied. Elemental analysis, SEM/EDXS, XRD, XAS and XPS analyses revealed 4-fold coordination of Cu2+ cations with oxygen atoms in the pores, a predominant location of copper atoms on the surface of crystallites and retained crystallinity of zeolites throughout the processes. The post-treatment of Cu2+-loaded samples with HCl and/or NaCl solutions confirmed the predominantly reversible sorption of copper on zeolites from aqueous solutions by ion-exchange mechanism and, therefore, excellent regeneration possibilities for both types of zeolites. Furthermore, with the calcination of exhaust metal-loaded zeolites, catalysts for total toluene oxidation reaction, as a model VOC pollutant, were obtained.
Keywords: Cu2+ ion exchange, Total toluene oxidation, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, Zeolite
Published in RUNG: 03.06.2021; Views: 2725; Downloads: 154
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Abstract: An absorption cell is constructed for x-ray absorption spectroscopy of reactive, unstable or hazardous gases at room temperature. In conjunction with in-situ micro-synthesis technique relying on handling the gas in syringes it enabled a first measurement of x-ray absorption spectra in the region of K and L edges for the series of hydrides of 5p elements (SnH4, SbH3, TeH2, HI). The signal-to-noise ratio above 103 was achieved, whereby fine detail is discerned in the spectra, in particular the small sharp features above each absorption edge, testifying of coexcitations of outer electrons in the core photoeffect.
Keywords: X-ray absorption spectroscopy Micro-synthesis absorption cell Gaseous hydrides Multielectron photoexcitations
Published in RUNG: 10.02.2020; Views: 4504; Downloads: 0
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Abstract: In this work, we experimentally studied the influence of photoexcitation energy influence on the charge transport in organic semiconductors. Organic semiconductors were small molecules like corannulene, perylene and pentacene derivatives, polymers such as polythiophene and benzothiophene derivatives, and graphene, along with combinations of these materials in heterojunctions or composites. The first part of this study is focused on the photoexcitation energy influence on the transient photoconductivity of non-crystalline curved π-conjugated corannulene layers. The enhanced photoconductivity, in the energy range where optical absorption is absent, is deduced from theoretical predictions of corannulene gas-phase excited state spectra. Theoretical analysis reveals a consistent contribution involving transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse orbitals typical of curved π-conjugated molecules. More, the photoconductivity of the curved corannulene was compared to the π-conjugated planar N,N′-1H,1H- perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN2), where the photoexcitation energy dependence of photocurrent closely follows the optical absorption spectrum. We next characterized charge transport in poly(3-hexylthiophene) (P3HT) layers deposited from solution. Our results indicate that time-of-flight (TOF) mobility depends on the photoexcitation energy. It is 0.4× 10 −3 cm 2 /Vs at 2.3 eV (530 nm) and doubles at 4.8 eV (260 nm). TOF mobility was compared to field-effect (FET) mobility of P3HT field-effect transistors (OFETs). The FET mobility was similar to the 2.3 eV excitation TOF mobility. In order to improve charge mobility, graphene nanoparticles were blended within a P3HT solution before the deposition. We found that the mobility significantly improves upon the addition of graphene nanoparticles of a weight ratio as low as 0.2 %. FET mobility increases with graphene concentration up to a value of 2.3× 10 −2 cm 2 /Vs at 3.2 %. The results demonstrate that phase segregation starts to influence charge transport at graphene concentration of 0.8 % and above. Hence, the graphene cannot form a bridged conduction channel between electrodes, which would cancel the semiconducting effect of the polymer composite. An alternative approach to enhance mobility is to optimize the molecular ordering of organic semiconductors. For that purpose, we studied an innovative nanomesh device. Free-standing nanomesh devices were used to form nanojunctions of N,N′- iiDioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires and crystalline bis(triisopropylsilylethinyl)pentacene (TIPS-PEN). We characterized the photocurrent response time of this novel nanomesh scaffold device. The photoresponse time depends on the photon energy. It is between 4.5 − 5.6 ns at 500 nm excitation wavelength and between 6.7 − 7.7 ns at 700 nm excitation wavelength. In addition, we found that thermal annealing reduces charge carrier trapping in crystalline nanowires. This confirms that the structural defects are crucial to obtaining high photon-to-charge conversion efficiency and subsequent transport from pn junction in heterostructured materials. Structural defects also influence the power conversion efficiency of organic heterostructured photovoltaics (OPVs). Anticipating that polymers with different backbone lengths produce different level of structural defects, we examined charge transport dependence on the molecular weight of poly[4,8-bis(5-(2- ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2- ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th) from 50 kDa to 300 kDa. We found p-type hopping transport in PTB7-Th, characterized by 0.1 – 3× 10 −2 cm 2 /Vs mobility, which increases with temperature and electric field. The polymer molecular weight exhibits a non-trivial influence on charge transport. FET mobility in the saturation regime increases with molecular weight. A similar trend is observed in TOF mobility and FET mobility in the linear regime, except for the 100kDa polymer, which manifests in the highest mobility due to reduced charge trapping. The lowest trapping at the dielectric interface of OFET is observed at 200 kDa. In addition, the 200 kDa polymer exhibits the lowest activation energy of the charge transport. Although the 100 kDa polymer indicates the highest mobility, OPVs using the 200 kDa polymer exhibit the best performance in terms of power conversion efficiency.
Keywords: organic semiconductors, optical absorption spectroscopy, time-of-flight photoconductivity, transient photocurrent spectroscopy, organic thin film transistors, atomic force microscopy, superatomic molecular orbitals, pn heterojunction, organic nanowires, graphene, composites, charge mobility, charge trapping, temperature dependence, photodetector, photovoltaic, solar cell, organic electronics
Published in RUNG: 08.10.2018; Views: 6769; Downloads: 170
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