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Photoelectron circular dichroism of aqueous phase alanine
Get PDFAmino acids and other small chiral molecules play key roles in biochemistry. However, in order to understand how these molecules behave in vivo, it is necessary to study them under aqueous phase conditions. Photoelectron circular dichroism PECD has emerged as an extremely sensitive probe of chiral molecules, but its suitability for application to aqueous solutions had not yet been proven. Here, we report on our PECD measurements of aqueous phase alanine, the simplest chiral amino acid. We demonstrate that the PECD response of alanine in water is different for each of alanine s carbon atoms, and is sensitive to molecular structure changes protonation states related to the solution pH. For C 1s photoionization of alanine s carboxylic acid group, we report PECD of comparable magnitude to that observed in valence band photoelectron spectroscopy of gas phase alanine. We identify key differences between PECD experiments from liquids and gases, discuss how PECD may provide information regarding solution specific phenomena for example the nature and chirality of the solvation shell surrounding chiral molecules in water and highlight liquid phase PECD as a powerful new tool for the study of aqueous phase chiral molecules of biological relevanc
Mono and sub monolayer films of a high T1 2 spin crossover molecule on HOPG temperature and light driven spin state transition
Get PDFWe investigated the properties of the [Fe H2B pz pypz 2] spin crossover complex deposited on highly oriented pyrolytic graphite HOPG by x ray absorption spectroscopy in the temperature range from 10 to 350 K for sample thicknesses from 0.9 1 to 1.3 2 ML. The highest temperature at which light can excite the system to a long lived metastable high spin HS state TLIESST and the temperature at which a thermal spin transition occurs T1 2 for the thin film samples are found as about 45 and 325 K, respectively, in agreement with the bulk values. The bulk phase has previously been studied by magnetic susceptibility measurements where it was found that two different polymorphic modifications exist. These findings suggest a potential correlation between TLIESST and the high T1 2 from the different film thicknesses of the tridentate [Fe H2B pz pypz 2] spin crossover molecule, with thinner samples exhibiting lower T1 2 values. Finally, the highest HS fraction achieved for [Fe H2B pz pypz 2] is 0.77 for samples of 0.9 1 and 1.1 1 ML on HOPG at 10 K and constant light irradiatio
A Radical Cationic Covalent Organic Framework to Accelerate Polysulfide Conversion for Long Durable Lithium Sulfur Batteries
Get PDFCovalent organic frameworks COFs have emerged as promising metal free sulfur hosts to facilitate the conversion kinetics and suppress the shuttling effect of lithium polysulfides LiPSs in lithium sulfur Li S batteries. However, constructing COFs with stable and high electrocatalytic functionality for LiPS conversion remains unexplored. Herein, we develop a radical cationic COF R TTF COF with superior electrical conductivity of 3.9 S m 1 at room temperature, which features both nucleophilic and electrophilic sites for effective LiPS chemisorption and conversion. With this novel radical based catalyst, the Li S battery achieves superior longevity of 1500 cycles with a capacity fading of 0.027 per cycle at a current density of 0.5 C. The capacity retention of the Li S battery based on R TTF COF at the current density of 2.0 C is nearly twice as high compared to a COF without radicals. The crucial role of radical cations in catalyzing LiPS conversion has been systematically elucidated through solid state nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, and theoretical simulations, which verify the reversible interactions between LiPSs and [TTF]2 moieties. This intriguing radical assisted mechanism opens a new avenue for designing efficient catalytic sulfur hosts using organic molecules, offering a significant step toward the practical application of Li S batterie
Transient Triplet Metallopnictinidenes M Pn M PdII, PtII; Pn P, As, Sb Characterization and Dimerization
Get PDFNitrenes R N have been subject to a large body of experimental and theoretical studies. The fundamental reactivity of this important class of transient intermediates has been attributed to their electronic structures, particularly the accessibility of triplet vs singlet states. In contrast, electronic structure trends along the heavier pnictinidene analogues R Pn; Pn P Bi are much less systematically explored. We here report the synthesis of a series of metallodipnictenes, M Pn amp; 9552;Pn M M PdII, PtII; Pn P, As, Sb, Bi and the characterization of the transient metallopnictinidene intermediates, M Pn for Pn P, As, Sb. Structural, spectroscopic, and computational analysis revealed spin triplet ground states for the metallopnictinidenes with characteristic electronic structure trends along the series. In comparison to the nitrene, the heavier pnictinidenes exhibit lower lying ground state SOMOs and singlet excited states, thus suggesting increased electrophilic reactivity. Furthermore, the splitting of the triplet magnetic microstates is beyond the phosphinidenes M P dominated by heavy pnictogen atom induced spin orbit couplin
Defects in Al Doped ZnO in Reactions with CO2 and H2 An In Situ Operando cw ESR Study
Get PDFIn this study, Al doped ZnO AZO is investigated by combining in situ and operando continuous wave cw electron spin resonance ESR measurements with other methods, including microwave cavity perturbation technique MCPT , magnetization, TGA, 27Al NMR, transmission electron microscopy, and XRD measurements. As Al doped ZnO AZO is the support of Cu ZnO Al CZA , an industrially used catalyst for methanol synthesis, studying the evolution of defects and changes in electronic structure under conditions used for reductive activation of the CZA catalyst or typical conditions for the r WGS reaction is of interest to gain further insight into the complex interplay between the different components in the CZA catalyst. Next to paramagnetic defect sites, a ferromagnetic phase was observed forming upon activation in reducing atmospheres, which is modified, but not destroyed in re oxidizing conditions. The results prove the oxidizing effect of CO2 on the paramagnetic defects as well as the ferromagnetic phase of an activated AZO. Comparison with prior results on CZA provides insight into the effect of Cu on AZO and enhances the understanding of the synergistic effects in the catalys
Optical Simulations of Nanotextured All Perovskite Tandem Solar Cells
Get PDFThis numerical study investigates, how textures at various locations of all perovskite tandem solar cells affect their optical performance. For this, hexagonal sinusoidal textures with 750 nm period and aspect ratios height to period of 27 moderate and 54 pronounced are considered. The optical simulations are performed with the finite element method and an algorithm to correct for the thick glass superstrate. The complex refractive index data of the wide bandgap WBG and narrow bandgap NBG perovskites with spectroscopic ellipsometry is determined. Texturing between the glass superstrate and the WBG perovskite top cell has an antireflective effect across the whole wavelength region. In contrast, texturing between the WBG perovskite top cell and the NBG perovskite bottom cell has no additional effect for a moderate texture but leads to light trapping in the NBG perovskite for a pronounced texture. Moderate texturing between the NBG perovskite absorber and the metal back contact leads to light trapping in the NBG perovskite but also excites surface plasmons in the copper back contact. Dielectric interlayers between the NBG perovskite and the metal back contact can reduce the plasmonic absorption losses. Texturing potentially allows to increase the current matched short circuit current density beyond 17 mA cm
Improved RF Performance of Niobium Cavities via In situ Vacuum Heat Treatment Technique
Get PDFVacuum thermal treatments baking are known to improve the superconducting properties of the RF surface layer of niobium cavities, and are employed as a last processing step to increase their efficiency determined by intrinsic quality factor Q0. A specific method to perform the baking has been used. It consists in annealing of an evacuated cavity with the local heaters installed on its outer surface in a cryostat which ensures an exterior vacuum and protects the outer cavity surface from oxidation. Such a set up has a number of advantages as it does not require to cool the cavity flanges during baking, and allows to perform the cold RF characterization of the cavity in situ, immediately after the thermal treatment without disassembly of heating elements. Moreover, the air exposure that causes partial degradation of Q0 by surface reoxidation is avoided. The heat treatment of a single cell 1.3 GHz niobium cavity at 230 C for 24 h demonstrated the doubling of Q0 at Eacc 10 MV m from 1.20 1010 to 2.4 1010 and retained the maximal accelerating field of 35 MV m without quenching. The selection of treatment parameters is based on our previous XPS studies. This treatment ensures incomplete dissolution of the native oxide by oxygen diffusion, thereby preventing interaction of niobium surface with external contaminants. We propose to bake the cavities directly in a cryomodule, which would allow to use the treatment to improve their performance. The potential impact of material parameters on the components of surface resistance has been briefly examine
High average power, few cycle, 2.1 m OPCPA laser driver for soft X ray high harmonic generation
Get PDFRepetition rate upscaling of intense, femtosecond duration pulses in the short wave infrared is necessary to further develop and apply tabletop, ultrafast soft X ray sources. Here, we present a 20 fs pulse duration, 2.1 m central wavelength, optical parametric chirped pulse amplification laser, which outputs 52 W of amplified signal power at a repetition rate of 52.6 kHz. Despite the potential for deleterious thermal processes, the laser output exhibits excellent spatial and temporal profiles in a 45 W beam at a soft X ray generation target after amp; 8764;5 m of propagation. In argon gas, this enables high harmonic generation up to amp; 8764;190 eV photon energies, demonstrating the system s potential for ultrashort soft X ray pulse productio
Regulated Crystallization Through Intermolecular Interactions Bridging for Efficient Tin Based Perovskite Solar Cells
No full textTin halide perovskite THP has emerged as a promising lead free material for high performance solar cells, attracting significant attention for their potential use for energy conversion. However, the rapid crystallization of THP due to its high Lewis acidity and easy oxidation of Sn2 leads to poor morphology and rampant defects in the resulting perovskite films. These strongly hamper the advances in efficiency and stability in THP solar cells. Herein, a comprehensive crystallization regulation strategy is demonstrated by introducing methyl carbazate C2H6N2O2, MeC to regulate the crystallization kinetics of perovskite through inter molecular interactions. The coordination bonds O Sn and hydrogen bonds N amp; 9472;H O between MeC and perovskite bridge the perovskite lattice together, helping suppress the oxidation of Sn2 , meanwhile, restraining the fast crystallization of perovskite in the precursor solution, by enhancing nucleation sites. More importantly, the connection by MeC can reduce the deep level trap state defect density, significantly restraining non radiative recombination and improving the carrier lifetime. Consequently, this facile strategy offers valuable insights into THP crystallization kinetics and allows an enhanced high power conversion efficiency from 10.43 to 14.02 to be achieved with good stabilit
NH3 induced activation of hydrophilic Fe N C nanocages for enhanced oxygen reduction reaction
Get PDFNon noble metal electrocatalysts for the oxygen reduction reaction ORR are urgently needed in metal air batteries, seawater batteries and fuel cells. Fe N C materials are among the most active catalysts for the ORR. Fe N C synthesis usually requires post heat treatment after pyrolysis which is time consuming and inevitably triggers inactive aggregate Fe species due to difficulties in controllable atom level modulation. Here, highly active Fe N C catalysts were prepared by a simple process involving an ammonia etching treatment by using ZIF 8 as a hard template and a mixture of FeSO4 and 2 methylimidazole as the Fe, N and C precursors. The direct ammonia treatment modulates N and Fe active species and removes the unstable carbon framework to form pyrolyzed Fe N C nanocages with a well dispersed pore structure. The obtained Fe N C exhibits a potential of 0.89 V vs. RHE at a kinetic current density of amp; 8722;1 mA cm amp; 8722;2 E amp; 8722;1 for the ORR, similar to commercial Pt C, but outperforming it in terms of stability and methanol tolerance. In situ electrochemical Raman and density functional theory provide insights into the origin of the activity of Fe N C materials and the underlying ORR electrocatalytic mechanisms at the molecular leve