24378 research outputs found
Sort by
A MOF modified NaCrO2 cathode for high rate and wide temperature applications in sodium ion batteries
No full textSodium ion batteries SIBs are considered promising candidates for energy storage devices due to their abundant resources and low cost. In this study, Cr based metal organic frameworks MOFs are chosen to modify the NaCrO2 cathode material during the synthesis process for achieving prominent electrochemical performances. The modified NaCrO2 demonstrates significantly superior rate performance compared to pristine NaCrO2; for instance, while pristine NaCrO2 struggles to charge and discharge at 50C, the coated material retains a capacity of 72.9 mA h g amp; 8722;1 even at this high rate. Even at an elevated temperature of 55 C, the coated NaCrO2 exhibits excellent cycling stability, retaining 85 of its initial capacity after 200 cycles at 0.5C, demonstrating its robust performance under challenging conditions. Comprehensive characterization, including Neutron Powder Diffraction NPD , X ray Absorption Spectroscopy XAS , and in situ X ray Diffraction XRD , reveals that the Cr2O3 C coating on the surface of the NaCrO2 cathode significantly enhances surface ionic transport while minimizing side reactions with the electrolyte by effectively isolating direct contact between the electrode and the electrolyte. The MOF modified strategy improves cycling stability by suppressing interfacial side reactions and optimizing the phase transition process, while enhancing rate capability by facilitating ion transport. This work provides new insights for the design of high rate, wide temperature, and low cost large scale energy storage system
Single Atom Sites in Ga Ni Supported Catalytically Active Liquid Metal Solutions SCALMS for Selective Ethylene Oligomerization
Get PDFSupported catalytically active liquid metal solutions SCALMS are materials composed of a liquid metal alloy deposited on a porous support. Due to the dynamic properties of the liquid metal alloy, these systems are suggested to form single atom sites, resulting in unique catalytic properties. Ga amp; 8722;Ni SCALMS were successfully applied to ethylene oligomerization, yielding catalysts that were stable up to 120 amp; 8197;h time on stream. A workflow based on synchrotron based X ray photoelectron spectroscopy XPS , transmission electron microscopy TEM as well as density function theory DFT and ab initio molecular dynamics AIMD simulations was applied to investigate the nature of the active species in these materials. The combination of XPS with DFT calculations indeed indicates the presence of isolated single Ni atoms on the liquid metal surface, while TEM measurements show high dynamics in the liquid metal with intermetallic phase dissolution and transformation. Furthermore, DFT AIMD methods allowed for rationalizing the role of hydrogen pretreatment in enriching the Ni atom at the surface of the liquid metal allo
Description of excitonic absorption using the Sommerfeld enhancement factor and band fluctuations
No full textOne of the challenges of excitonic materials is the accurate determination of the exciton bindingenergy and bandgap from optical measurements. The difficulty arises from the overlap of thediscrete and continuous excitonic absorption at the band edge. Many researches have modeledthe shape of the absorption edge of such materials on the seminal formulation proposed byElliott in 1957 Phys. Rev.1081384 9 and its several modifications such as non parabolicbands, magnetic potentials and electron hole polaron interactions. However, exciton bindingenergies obtained from optical absorption often vary strongly depending on the chosen Elliottformula . Here, we propose an alternative and rather simple approach, which has previouslybeen successful in the determination of the optical bandgap of amorphous, direct and indirectsemiconductors, based on the band fluctuations BFs model. In this model, the fluctuations dueto disorder, temperature or lattice vibrations give rise to the well known exponential shape ofband tail states. The formulation results in an analytic equation for the fundamental absorptionwith 6 parameters only. To test it, the binding energy and optical bandgap of GaAs and thefamily of tri halide perovskites MAPbX3 , X Br,I,Cl, over a wide range of temperatures, areobtained by fitting the modified Elliott model. The results for the bandgap, linewidth andexciton binding energy are in good agreement with reports based on non optical measurements.Moreover, due to the polar nature of perovskites, the retrieved binding energies can becompared with those computed with a model proposed by Kane 1978Phys. Rev.B186849 . Inthe latter model, the exciton is surrounded by a cloud of virtual phonons interacting via theFr amp; ouml;lich interaction. As a consequence, the upper bound for the binding energy of theexciton polaron system can be estimated. These results are in good agreement with the opticalparameters obtained with the proposed Elliott equation including BF
Dynamics of Palladium and Palladium Gold Thin Film Catalysts during Acetylene Semihydrogenation Revealed by Synchrotron Grazing Incidence X ray Diffraction
Get PDFCatalytic thin films are an exciting concept in the field of heterogeneous catalysis that produce 2D structures with precisely engineered active sites to generate fundamental knowledge and optimize catalytic performance. Acetylene semihydrogenation on Pd catalysts is an industrially crucial chemical reaction known for its complex subsurface chemistry competitive occupation of the metal s interstitials between atomic hydrogen and carbon leads to substantial changes in reaction selectivity, the key performance indicator. In the present work, we monitor the dynamics of supported thin films of Pd and PdAu during the reaction using a combination of synchrotron in situ GIXRD, pre and post reaction XPS and TEM. PdHx formation occurs at room temperature, with a significant increase in the lattice parameter 2.9 4.1 for Pd, whereas PdAu only marginally expands 0.4 upon interaction with molecular H2. Changes in the partial pressure of H2 result in differences in the formation of PdH amp; 945; and or PdH amp; 946;. At a partial pressure of 0.1 bar, both the amp; 945; and amp; 946; phases coexist in the 20 nm Pd film. The extreme sensitivity of the in situ GIXRD approach allows to monitor the transition from Pd metallic amp; 8594; PdH amp; 945; amp; 8594; PdH amp; 946;, its reversibility, and the stability of the films upon cycles of H2. Exposure to C2H2 alone or mixtures of C2H2 H2 at elevated temperatures results in the irreversible expansion of the Pd unit cell 2.5 3.5 , demonstrating the formation of a PdCx interstitial solution with homogeneous composition along the thickness of Pd , which hinders almost completely later hydride formation. PdAu forms an interstitial solution with C, although to a much lesser extent than that of the Pd films. The current work generates fundamental knowledge using thin film catalysts, which represent a step between model and industrial systems. Our experiments close the pressure gap compared to previous experiments performed with ideal surfaces at very low pressures and allow to monitor the competitive dynamics of PdHx and PdCx formation in the catalytic subsurfac
Trigonal distortion and orbital polarization of Co2 in a honeycomb cobaltate film grown on a SrTiO3 substrate
No full textHigh quality single crystal films of CoTi amp; 8290;O3 were fabricated on a SrTi amp; 8290;O3 substrate using pulsed laser deposition methods, allowing for a detailed investigation of the electronic structure and orbital occupancy via x ray linear dichroism in both in plane and out of plane configurations. The combination of measurements and semiempirical calculations revealed that the trigonal distortion of Co amp; 8290;O6 octahedra deviating from the cubic crystal field gives rise to an additional trigonal crystal field of amp; 916;trig 45meV, with a low lying amp; 119890; amp; 120587; amp; 119892; orbital of the energetic splitting of the amp; 119905;2 amp; 8290; amp; 119892; band. Additionally, the x ray magnetic circular dichroism XMCD at the Co amp; 119871;2,3 edge captured significant magnetic anisotropy and unquenched orbital angular momentum inherent in the film. An in plane exchange field of amp; 8722;0.65 meV is required in the multiplet simulation to well reproduce the XMCD spectra obtained with field applied in the in and out of plane directions. These findings provide insights into the potential of CoTi amp; 8290;O3 to align with the Kitaev model when it is presented as a thin film, which is crucial for the further realization of quantum spin liquids via strain engineerin
Size and Surface Effects in the Ultrafast Dynamics of Strongly Cooperative Spin Crossover Nanoparticles
Get PDFCooperative photoinduced switching of molecular materials at the nanoscale is still in its infancy. Strongly cooperative spin crossover nanomaterials are arguably the best prototypes of photomagnetic and volume changing materials that can be manipulated by short pulses of light. Open questions remain regarding their non equilibrium dynamics upon light excitation and the role of cooperative elastic interactions in nanoscale systems that are characterized by large surface volume ratios. Femtosecond resolved broadband spectroscopy is performed on nanorods of the strongly cooperative Fe triazole, which undergoes a reversible low spin to high spin HS phase transition amp; 8776;360 K. Supported by density functional theory and mechano elastic Monte Carlo simulations, a marked difference is observed in the photoswitching dynamics at the surface of the nanoparticles compared with the core. Surprisingly, under low excitation lt;2 conditions, there occurs a transient increase in the HS population at the surface on the picosecond time scale, while the HS population in the core decays concomitantly. These results shed light onto the importance of surface properties and dynamical size limits of nanoscale photoresponsive nanomaterials that can be used in a broad range of application
Importance of Electrostatic Effects for Interpretation of X ray Photoemission Spectra of Self Assembled Monolayers
No full textThis paper reviews the relevant work regarding electrostatic effects in X ray photoemission from self assembled monolayers SAMs which are application relevant ultrathin molecular films, coupled over a suitable anchoring group to the substrate. Whereas, in most cases, the standard concept of chemical shift is fully sufficient to describe X ray photoelectron spectra of these systems, consideration of electrostatic effects is frequently necessary for their proper interpretation. Due to the insulator character of the SAM matrix, decoupled electronically from the substrate, the introduction of a dipolar sheet at the SAM substrate interface or within this matrix creates a potential discontinuity shifting the energy levels above the sheet with respect to those below it. This shift is reflected then in the matrix related spectra, resulting in shifts and splitting of the characteristic photoemission peaks. Several representative examples in this context are provided and discussed in detail. These examples and other relevant literature data underline the importance of electrostatic effects in photoemission and suggest that they should be considered on the equal footing as the chemical shift one
Dual orthogonal metal complexes and their utilization for the versatile fabrication of smart interpenetrating polymer networks
Get PDFThis article presents a versatile method for the preparation of smart interpenetrating polymer networks IPNs . Based on two different orthogonal binding motives, IPNs are generated by simply mixing ligand containing polymers and corresponding metal salts leading to a polymeric material featuring shape memory abilities. Due to the utilization of heteroleptic complexes as crosslinks in one of the two networks, simple tuning of the crosslinking density, as well as the generation of IPNs made out of three different main polymers is possible. The structure property relationships of the resulting IPNs are investigated in a detailed fashion applying FT Raman spectroscopy, thermogravimetric analysis TGA , dynamic mechanical thermal analyses DMTA as well as thermo mechanical analyses TMA . In the scope of thermo mechanical analysis TMA excellent shape memory properties, with strain fixity rates near 100 and strain recovery rates up to 93 , could be observe
Facile Synthesis of CuxS Electrocatalysts for CO2 Conversion into Formate and Study of Relations Between Cu and S with the Selectivity
Get PDFThe conversion of CO2 into formate HCOO amp; 8722; , a techno economically feasible product, can be achieved using earth abundant CuxS electrocatalysts, but questions remain regarding how catalyst structure, composition, and reaction environment influence product selectivity. A novel synthesis method based on electrodeposition of Cu foam and its subsequent sulfidation via immersion in sulfur saturated toluene solution resulted in CuxS foams. Catalytic activity studies found that HCOO amp; 8722; selectivity is dependent on electrochemical activation at higher overpotentials. To understand the effects of activation, determine the active forms of the catalysts, and identify the role of sulfur, the electrodes are carefully characterized as well as gaseous and sulfur dissolved in electrolyte. This included study of the effects of intentional addition of solution sulfur species, identification of the sulfur loss, determination of the electrode composition and relating sulfur speciation to observed product selectivity. It is found that residual sulfur stabilizes Cu during electrolysis at potentials favoring HCOO amp; 8722; production, in contrast to pristine Cu that undergoes complete reduction and shows poor HCOO amp; 8722; selectivity. Sulfur in both the catalyst and dissolved in electrolyte are of dynamic nature, and surface residues of SO42 amp; 8722; species are identified in all activated catalysts which correspond with enhanced HCOO amp; 8722; productio
Investigations of free electrons in doped silicon crystals derived from Fourier transformed infrared measurements and ab initio calculations
Get PDFThe reflection spectra of n and p type silicon crystals doped with phosphorus and boron were measured for the free carrier concentrations of 1.1 1015 cm amp; 8722;3 amp; 8722; 1.2 1020 cm amp; 8722;3 in the far and mid infrared range between 20 3000 cm amp; 8722;1 using synchrotron radiation and Fourier transformed infrared technique. Transmittance spectra could be measured for lower sample carrier concentrations from the range studied. The measured reflection spectra were fitted by using the Drude relation and the parameters of free electron conductivity electron effective mass m and momentum scattering time amp; 61556; were obtained for the n and p type doped silicon. Additionally, the calculations of the band electronic structure and the electric conductivity amp; 61555; of the crystals were performed in the framework of the density functional theory for different carrier concentrations and temperatures. The study main findings are 1 the substantial decrease of the momentum scattering time amp; 61556; and 2 the clear increase of the electron effective mass m with an increase of the carrier concentrations Nc for both n and p type doped silicon crystal