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Facile Material Design Concept for Co-Free Lithium Excess Nickel-Manganese Oxide as High-Capacity Positive Electrode Material

High-capacity Li1+x(Ni0.3Mn0.7)1-xO2, (0 < x < 1/3) samples were synthesized by the coprecipitation–calcination method. Both electrochemical cycle and high-rate performances were drastically improved by selecting an N2 atmosphere as final calcination. Scanning transmission electron microscopy—energy dispersive X-ray spectroscopy analysis showed that the sample calcined in an N2 atmosphere had a more homogeneous transition metal distribution into primary particles than that calcined in air. The solid-state 7Li nuclear magnetic resonance data showed that electrochemically inactive domains were only diminished for the sample calcined in an N2 atmosphere after electrochemical activation. X-ray Rietveld analysis revealed that the suitable transition metal distribution and content of the samples were different from those of typical layered rock-salt materials. Only that calcined in an N2 atmosphere had no spinel formation during charging and no oxide ion insertion reaction during discharging. No positive Co substitution effect was observed under the optimized preparation conditions. At the 100th cycle, the discharge capacity was 216 mAh g−1, which corresponds to 87% of the initial capacity (251 mAh g−1) at optimizing synthetic condition.

A comparative study of the photocatalytic and optical properties of spinel-type titanates: A report for spinel sodium titanate

The recent discovery of Na3LiTi5O12 (NTO), which possesses spinel symmetry (, #227) with the 8a site occupied by Na, has enabled investigations into the effect of the 8a-site cation on the physical properties of spinel titanates. Hence, in this study, the optical and photocatalytic properties of NTO were investigated and compared with those of spinel Li4Ti5O12 (LTO) and rutile TiO2. The bandgaps were estimated theoretically using hybrid density functional theory and experimentally using the ultraviolet–visible spectroscopy, and the obtained results were similar for both methods and spinel titanates. The valence- and conduction-band components of the spinel-type titanates were similar to those of other titanium oxides, and both NTO and LTO exhibited photocatalytic activity for sacrificial H2 evolution from water. However, although they have similar band structures and optical properties, the NTO photocatalytic activity was clearly lower than that of LTO. This can be attributed to the surface roughness and ease of defect formation in the NTO system, which hindered charge separation. These results indicate that the optical properties of spinel-type titanates can be tuned by replacing the cations at the 8a sites.

Proton triple-quantum solid-state NMR spectroscopy at slow MAS ∼10 kHz

Solid-state NMR is a valuable tool for elucidating the structures and dynamics of materials at an atomic level. Proton multiple-quantum (MQ) /single-quantum (SQ) correlation NMR spectroscopy is widely used to probe spatial proximity among protons. In the triple-quantum (TQ)/SQ correlation experiment, the excitation of triple-quantum (TQ) coherences is traditionally achieved by a 90° pulse in conjugation with double-quantum (DQ) recoupling sequences. Nevertheless, such sequences often suffer from low TQ filtering efficiency and may lead to overlapping spinning sidebands in the indirect TQ dimension, especially at a slow MAS frequency. Herein, we design several supercycled symmetry-based RNnν γ-free TQ recoupling sequences and compare their performance via extensive numerical simulation and experiments. Experimental results further confirm that pulse sequence gives the highest TQ filtering efficiency of around 20% in the slow MAS regime (∼10 kHz). The 2D TQ/SQ spectrum at slow MAS is completely free of spinning sidebands in the TQ dimension due to its γ-free nature. We establish that such a γ-free pulse sequence is a superior candidate for TQ spectroscopy at slow MAS frequency.

A non-planar 2D covalent organic framework derived from a Z-shaped building unit

Herein, a novel non-planar 2D COF with a stair-stepped structure was constructed from a Z-shaped building block for the first time. Compared with its similar planar COF, the unique stair-stepped non-planar COF possesses larger surface area and stronger fluorescence, which was further applied for specific explosive detection through a fluorescence quenching mechanism. This work not only extends the traditional planar 2D COF structures to unique non-planar structures based on the bottom-up design principle, but also expands the potential applications of COF materials.

Signal-to-noise ratio in diffusion-ordered spectroscopy: how good is good enough?

Diffusion-ordered NMR spectroscopy (DOSY) constructs multidimensional spectra displaying signal strength as a function of Larmor frequency and of diffusion coefficient from experimental measurements using pulsed field gradient spin or stimulated echoes. Peak positions in the diffusion domain are determined by diffusion coefficients estimated by fitting experimental data to some variant of the Stejskal–Tanner equation, with the peak widths determined by the standard error estimated in the fitting process. The accuracy and reliability of the diffusion domain in DOSY spectra are therefore determined by the uncertainties in the experimental data and thus in part by the signal-to-noise ratio of the experimental spectra measured. Here the Cramér–Rao lower bound, Monte Carlo methods, and experimental data are used to investigate the relationship between signal-to-noise ratio, experimental parameters, and diffusion domain accuracy in 2D DOSY experiments. Experimental results confirm that sources of error other than noise put an upper limit on the improvement in diffusion domain accuracy obtainable by time averaging.

One-Pot, Room-Temperature Conversion of CO2 into Porous Metal–Organic Frameworks

The conversion of CO2 into functional materials under ambient conditions is a major challenge to realize a carbon-neutral society. Metal–organic frameworks (MOFs) have been extensively studied as designable porous materials. Despite the fact that CO2 is an attractive renewable resource, the synthesis of MOFs from CO2 remains unexplored. Chemical inertness of CO2 has hampered its conversion into typical MOF linkers such as carboxylates without high energy reactants and/or harsh conditions. Here, we present a one-pot conversion of CO2 into highly porous crystalline MOFs at ambient temperature and pressure. Cubic [Zn4O(piperazine dicarbamate)3] is synthesized via in situ formation of bridging dicarbamate linkers from piperazines and CO2 and shows high surface areas (∼2366 m2 g–1) and CO2 contents (>30 wt %). Whereas the dicarbamate linkers are thermodynamically unstable by themselves and readily release CO2, the formation of an extended coordination network in the MOF lattices stabilizes the linker enough to demonstrate stable permanent porosity.

19F-detected dual-optimized inverted 1JCC 1,n-ADEQUATE

Modification of the recently reported 19F-detected 1,1-ADEQUATE experiment that incorporates dual-optimization to selectively invert a wide range of 1JCC correlations in a 1,n-ADEQUATE experiment is reported. Parameters for the dual-optimization segment of the pulse sequence were modified to accommodate the increased size of 1JCC homonuclear coupling constants of poly- and perfluorinated molecules relative to protonated molecules to allow broadband inversion of the 1JCC correlations. The observation and utility of isotope shifts are reported for the first time for 1,1- and 1,n-ADEQUATE correlations.

Metal-Templated Oligomeric Macrocyclization via Coupling for Metal-Doped π-Systems

A method for the synthesis of metal-doped aromatic macrocycles has been developed. The method, i.e., metal-templated oligomeric macrocyclization via coupling, adopts Ni as the template and assembles five pyridine units via a Ni-mediated coupling reaction to form aryl–aryl linkages. A pentameric oligopyridyl macrocycle was selectively obtained in good yield, and the reaction was also applicable to a gram-scale synthesis. The pentameric oligopyridyl macrocycle captured d8-Ni(II) at the center to form a paramagnetic pentagonal-bipyramidal complex. The method was applied to the synthesis of a large π-molecule to afford a nanometer-sized, bowl-shaped molecule having a unique combination of 120π and 8d electrons.

Optically active covalent organic frameworks and hyperbranched polymers with chirality induced by circularly polarized light

Axial chirality was induced by circularly polarized light to covalent organic frameworks as well as hyperbranched polymers composed of bezene-1,3,5-triyl core units and oligo(benzene-1,4-diyl) as linker units where variation in induction efficiency was rationally interpreted in terms of internal rotation dynamics studied through CPMAS 13C NMR experiments including CODEX measurements.

Structure Solution of Nano-Crystalline Small Molecules Using MicroED and Solid-State NMR Dipolar-Based Experiments

Three-dimensional electron diffraction crystallography (microED) can solve structures of sub-micrometer crystals, which are too small for single crystal X-ray crystallography. However, R factors for the microED-based structures are generally high because of dynamic scattering. That means R factor may not be reliable provided that kinetic analysis is used. Consequently, there remains ambiguity to locate hydrogens and to assign nuclei with close atomic numbers, like carbon, nitrogen, and oxygen. Herein, we employed microED and ssNMR dipolar-based experiments together with spin dynamics numerical simulations. The NMR dipolar-based experiments were 1H-14N phase-modulated rotational-echo saturation-pulse double-resonance (PM-S-RESPDOR) and 1H-1H selective recoupling of proton (SERP) experiments. The former examined the dephasing effect of a specific 1H resonance under multiple 1H-14N dipolar couplings. The latter examined the selective polarization transfer between a 1H-1H pair. The structure was solved by microED and then validated by evaluating the agreement between experimental and calculated dipolar-based NMR results. As the measurements were performed on 1H and 14N, the method can be employed for natural abundance samples. Furthermore, the whole validation procedure was conducted at 293 K unlike widely used chemical shift calculation at 0 K using the GIPAW method. This combined method was demonstrated on monoclinic l-histidine.

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Corona - Glow Discharge (DART Ion Source)

February 22, 2020