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Size and Stoichiometry Effects on the Reactivity of MoCy Nanoparticles Towards Ethylene
Molybdenum carbides are promising catalysts alternative to Pt-group metals for the hydrogenation of unsaturated hydrocarbons. Nanostructuring has been shown to be an efficient way to boost the catalytic activity of these materials with MoCy nanoparticles (NPs) exhibiting a good performance when encapsulated inside zeolites or dispersed on inert supports such as carbon or gold. In this work, we focus on a systematic DFT study of the interaction of MoCy NPs with ethylene (C 2 H 4 ), as a general and simple approach for examining binding and activation of C≡C bonds. Models for 14 NPs, with a Mo/C ratio in the 0.67 to 2.00 range, have been built following a cascade procedure. Several chemical descriptors, including the adsorption energy, structural NPs distortion, C≡C deformation, and C 2 H 4 attachment energy have been analyzed along with a meticulous geometric and electronic characterization of bare NPs and C2H4 binding. The present results show that 1:1 stoichiometric Mo 6 C 6 , Mo 12 C 12 , and Mo 24 C 24 , and the non-stochiometric Mo 4 C 6 , Mo 8 C 12 (MetCar), and Mo 14 C 13 (Nanocube) are excellent systems for the binding and activation of ethylene, exhibiting a much bigger reactivity than a bulk δ-MoC(001) surface with a similar Mo:C ratio. In addition, C 2 H 4 binding on the NPs with a Mo/C < 1.08 is advantageous since, apart from a rather large adsorption energy, implies low energy values for NPs deformation (from 0.00 to 0.31 eV), C≡C distortion (from 0.30 to 0.52 eV), and C 2 H 4 attachment (from -2.12 to -2.58 eV). These theoretical results point to the ideal MoCy size and composition for C 2 H 4 binding, providing a background for further experimental studies aimed at the preparation of MoCy NPs as hydrogenation catalysts.
Ring Size Effects on the Structures of Sandwich Compounds with a Stoichiometry of C 12 H 12 M (M = Ti–Ni)
Ring size effects on geometries and electronic structures were investigated for the (C n H n )M(C m H m ) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti–Ni) systems using density functional theory. The lowest-energy C 12 H 12 M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η 5 -C 5 H 5 )Ti(η 7 -C 7 H 7 ), doublet (η 5 -C 5 H 5 )V(η 7 -C 7 H 7 ), and singlet (η 6 -C 6 H 6 ) 2 Cr, respectively. The likewise experimentally known singlet (η 6 -C 6 H 6 ) 2 Ti, doublet (η 6 -C 6 H 6 ) 2 V, and singlet (η 5 -C 5 H 5 )Cr(η 7 -C 7 H 7 ) are the secondlowest- energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C 12 H 12 M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C 5 H 5 )M(C 7 H 7 ) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C 7 H 7 rings with one or two uncomplexed C=C bonds. The (C 4 H 4 )M(C 8 H 8 ) (M = Ti–Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C 5 H 5 )M(C 7 H 7 ) and (C 6 H 6 ) 2 M isomers.
Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe 2 Nanocrystals
CuInSe 2 nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more-toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states as well as unusual photophysics. In this work, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe 2 nanocrystals impart non-radiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs. electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (< 20 ps) followed by diphenylphosphine (< 500 ps) and finally tributylphosphine (> 2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are non-emissive.
Self-limiting stoichiometry in SnSe thin films
Raman spectroscopy showing the initial formation of SnSe 2 followed by the stabilization of SnSe with increased growth time.
Effect of local structure and stoichiometry on the dynamic behavior of bi-metal interfaces
Molecular dynamics (MD) simulations are performed to investigate the failure behavior of Al/Al, Al/Fe, and Al/AlFe bi-layers under high strain-rate shock loading. Additional simulations are performed to invesigate solely the tensile response of these systems and to isolate the effects of loading history on damage and failure. Specifically, the compression stage of the shock is excluded from the tensile simulations. This study shows that local structure variation, including the introduction of serrations, plays different roles in controlling damage nucleation depending on the specific loading condition. Under shock loading, the stress for void nucleation and the resulting void distribution is insensitive to an interface structure, whereas under pure tensile loading, the opposite is true, even though the strain rate is comparable. The above difference can be explained based on the corresponding change in the total deformation prior to damage nucleation under shock loading, which is missing from the pure tensile loading. Therefore, it is concluded that whether local interface structure matters under high strain-rate loading depends on the specific loading history and the stress state: the local structure variation matters only when it alters the deformation behavior, and subsequently, damage nucleation and evolution.
Stoichiometry-dependent surface electronic structure of SrTiO3 films grown by hybrid molecular beam epitaxy
We investigate the surface electronic structure of SrTiO3 (STO) films grown by a hybrid molecular beam epitaxy that are both stoichiometric and nonstoichiometric by means of x-ray photoelectron spectroscopy and electron energy loss spectroscopy. Increasing the fraction of the surface that is terminated with an SrO layer is correlated with a decrease in the chemical potential whereby the valence band maximum moves closer to the Fermi level, but without a significant change in the bandgap. Inasmuch as SrO-terminated STO (001) has previously been shown to act as an electron scavenger in which carriers from the bulk are trapped, we argue that the high fraction of SrO in the terminal layer is what lowers the chemical potential in Sr-rich STO. Our experimental results provide important insights into various physical phenomena that can occur on STO (001) surfaces and their effect on bulk electronic properties.
The Coordination Chemistry and Stoichiometry of Extracted Diglycolamide Complexes of Lanthanides in Extraction Chromatography Materials
Industrial rare earth element (REE) separations predominantly utilize solvent extraction processes tailored toward conventional resources such as bastnäsite, monazite, and ion adsorption clays. Advances in diglycolamide (DGA) chemistry have shown effective extraction characteristics for REE separations. However, limitations associated with traditional DGA solvent extraction techniques, such as third-phase formation and gelling, have hindered commercial viability. By supporting DGA extractants on porous resins such as polystyrene divinyl benzene (PS-DVB), the desirable combination of solvent extraction selectivity and ease of operation of sorbent columns can be achieved. To design a low-cost model for such solid-supported DGAs, extraction characteristics as influenced by the underlying coordination chemistry must be explored to achieve efficient functional systems. Within this study, we report novel DGA resin materials, each incorporating one of the DGAs N,N,N’,N’-tetra-(1-octyl)-3-oxapentane-1,5-diamide (TODGA), N,N'-dimethyl-N,N'-dioctyl-3-oxapentane-1,5-diamide (DMDODGA), and 2,2'-oxybis(1-(3-(((2-ethylhexyl)thio)methyl)-4-methylpyrrolidin-1-yl)ethan-1-one) (DEHPDGA). The affinity of DGAs across the lanthanide (Ln) series was evaluated for both hydrochloric acid and nitric acid media with varying Ln feed concentrations to study distribution ratios and loading characteristics. Focusing on dysprosium, extended X-Ray Absorption Fine Structure (EXAFS) and density functional theory (DFT) calculations were also utilized to explore coordination chemistry and their effects on ligand performance. The general trend for both acid media resulted in DMDODGA having the highest extraction strength of all three DGAs at varying acid concentrations. Coordination-chemistry analysis supported by loading data, DFT calculations, and EXAFS results under forced loading conditions posited less than the expected 3:1 ligand-to-metal coordination.
Tuning of charge density wave transitions in LaAu x Sb 2 by pressure and Au stoichiometry
Two charge density wave transition can be detected in La Au Sb 2 at ~ 110 and ~ 90 K by careful electrical transport measurements. Whereas control of the Au site occupancy in La Au x Sb 2 (for 0.9 ≲ x ≲ 1.0 ) can suppress each of these transitions by ~ 80 K , the application of hydrostatic pressure can completely suppress the lower transition by ~ 7.5 kbar and the upper transition by ~ 17 kbar . Here, clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero.
Adsorption-controlled growth of MnTe(Bi2Te3)n by molecular beam epitaxy exhibiting stoichiometry-controlled magnetism
We report the growth of the intrinsic magnetic topological system MnTe ( Bi 2 Te 3 ) n by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the n = 1 phase Mn Bi 2 Te 4 with 2.0 < Bi : Mn < 2.6 at 225°C. In this work, the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn≥4.5) it is found that the n = 2 Mn Bi 4 Te 7 phase is stabilized. Transport measurements indicate that the Mn Bi 2 Te 4 and Mn Bi 4 Te 7 undergo magnetic transitions around 25 and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn<2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped Mn Bi 2 Te 4 . Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.
Oxygen non-stoichiometry and point defect equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3-δ
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Tuning Surface Stoichiometry of SOFC Electrodes at the Molecular and Nano-scale for Enhanced Performance and Durability
This project achieved the following objectives. Different cation segregation behaviors of different common SOFC cathodes, including La 0.6 Sr 0.4 Co0.2Fe 0.8 O 3-δ (LSCF), Sr 0.5 Sm 0.5 O 3-δ (SSC), and PrBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O5 +δ (PBSCF), were determined. We showed how oxygen partial pressure and gas impurities impact the stability of SOFC cathodes. We developed atomic layer deposition (ALD) coating techniques for electrodes and showed that by introducing different elements and different ALD oxidizers, electrode surface chemistry can be altered, resulting in enhanced oxygen reduction kinetics. In addition, we developed solution infiltration technique to enhance performance and durability of electrodes. Different infiltrates as well as different thermal treatment processes were screened to identify the optimal surface modification process that yields both low impedance and high durability. The modified cathode shows excellent stability and has a low area specific resistance (ASR) of only 0.2 Ωcm 2 at 600 °C after over 2000 hours of operation. Further, we developed ceramic anodes, SrFe(Co,Mo)O 3 (SFCM) and SrFe(Ni,Mo)O 3 (SFNM), and enhanced anode oxidation kinetics by solution infiltration or in situ catalyst exsolution from the ceramic anode surface. The modified anodes showed improved performance in full SOFCs, and the optimized anode modification shows high stability for over 400 hours at 550 °C. Moreover, the modified anode also demonstrates high durability in methane. This work provides fundamental understanding of electrode surface chemistry and demonstrates a simple, facile, cost-effective approach to enhance catalytic activity and durability of SOFC electrodes.
The Role of Stoichiometry in Mn1-xZnxFe2O4 Ferrite Microwave Absorbers.
Abstract not provided.
Oxygen non-stoichiometry and point defect equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3-δ at 1200 ? 1450 oC.
Abstract not provided.
Oxygen non-stoichiometry and point defect equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3-δ at 1200 – 1450 oC
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The Role of Stoichiometry in Mn1-xZnxFe2O4 Ferrite Microwave Absorbers
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The Role of Stoichiometry in Mn1-xZnxFe2O4 Ferrite Microwave Absorbers
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