The efficient synthesis of 2D polymers (2DPs) with tailorable structures and properties is highly desired but remains a considerable challenge. Here, the first solution synthesis of millimeter-size crystalline covalent triazine frameworks (CTFs) with a clear lamellar structure, which can be exfoliated into micrometer-size few-layer 2DP sheets via both micromechanical cleavage and liquid sonication, is reported. The obtained CTFs or 2DPs show a unique staggered AB stacking with a dominant pore size of ≈0.6 nm, which is different from the common eclipsed AA stacking in various covalent organic frameworks. The preference for AB stacking is due to the specific interaction of triflic acid with CTFs as revealed computationally. When explored as new polymeric anodes for sodium-ion batteries, both crystalline bulk CTF and exfoliated 2DP exhibit very high capacities (225 and 262 mA h g at 0.1 A g , respectively), impressive rate capabilities (67 and 119 mA h g at 5.0 A g , respectively), and excellent cycling stability (95% capacity retention after 1200 cycles) due to their robust conjugated porous structure, outperforming most organic/polymeric sodium-ion battery anodes ever reported.
Silicene, a 2D silicon allotrope with unique low-buckled structure, has attracted increasing attention in recent years due to its many superior properties. So far, epitaxial growth is one of the very limited ways to obtain high-quality silicene, which severely impedes the research and application of silicene. Therefore, large-scale synthesis of silicene is a great challenge, yet urgently desired. Herein, the first scalable preparation of free-standing high-quality silicene nanosheets via liquid oxidation and exfoliation of CaSi is reported. This new synthesis strategy successfully induces mild oxidation of the (Si ) layers in CaSi into neutral Si layers without damage of pristine silicene structure and promotes the exfoliation of stacked silicene layers. The obtained silicene sheets are dispersible and ultrathin ones with monolayer or few-layer thickness and exhibit excellent crystallinity. As a unique 2D layered silicon allotrope, the silicene nanosheets are further explored as new anodes for lithium-ion batteries and exhibit a nearly theoretical capacity of 721 mAh g at 0.1 A g and an extraordinary cycling stability with no capacity decay after 1800 cycles in contrast to previous most silicon anodes showing rapid capacity decay, thus holding great promise for energy storage and beyond.
The activation of Cu+ ions for NO binding by the zeolite “ligand” is analyzed. Density functional theory (DFT) calculations show that the interaction of NO with isolated Cu+ ions and with Cu+ ions in zeolites is of different character. In gas-phase Cu+−NO complexes interactions of the a‘ singly occupied orbital (SOMO) on NO with the unoccupied 4s and the occupied 3d orbitals on Cu+ depopulate the antibonding SOMO and this strengthens the NO bond. In the Cu/zeolite system electrostatic attraction increases the Pauli repulsion and pushes up the (occupied) 3d orbitals, which can interact with the a‘ ‘ LUMO of NO. This interaction populates the antibonding LUMO of NO and, hence, weakens the NO bond. The binding of NO onto Cu+ sites in high-silica zeolites MFI and FER is investigated by the combined quantum mechanics/interatomic potential function approach that describes the particular framework topology. Two types of Cu+ sites are studied, on the channel wall (coordination to 3−4 framework O atoms) and on the channel intersection (coordination to 2 O atoms). Upon the interaction with NO the Cu+ ion stays coordinated to only two framework oxygen atoms belonging to the AlO4 tetrahedron and the structures of all ON−Cu/zeolite adsorption complexes become very similar. EPR hyperfine coupling constants (HFCC) were calculated and compared with experiment. The isotropic component of HFCC strongly depends on CuNO angle, but due to small variations of this angle, it will be difficult to resolve EPR signals (or other spectroscopic signals) for different sites. The interaction of NO with the Cu+ sites on the channel intersection is significantly stronger (29.5−27.1 kcal/mol) than the interaction with the Cu+ sites on the channel wall (22.6−15.0 kcal/mol) because framework deformation into a state prepared for bonding NO costs (deformation) energy.
Interaction of CO with K-FER zeolite was investigated by a combination of variable-temperature IR spectroscopy and computational study. Calculations were performed using omega(CO)/r(CO) correlation method in combination with a periodic density functional theory model. On the basis of agreement between experimental and calculated results, the following carbonyl complexes were identified: (i) mono- and dicarbonyl C-down complexes on single K(+) sites characterized by IR absorption bands at 2163 and 2161 cm(-1), respectively; (ii) complexes formed by CO bridging two K(+) ions separated by about 7-8 A (dual sites) characterized by a band at 2148 cm(-1); and (iii) isocarbonyl (O-down) complexes characterized by a band at 2116 cm(-1). The bridged carbonyl complexes on dual K(+) sites are about 5 kJ/mol more stable than monodentate (monocarbonyl) CO complexes. The C-O stretching frequency of monocarbonyl species in K-FER depends on K(+) location in the zeolite, and not on K(+) coordination to the framework. A combination of theoretical calculations using a periodic density functional model and experimental results showed formation of two types of monocarbonyls. The most abundant type appears at 2163 cm(-1), and the less abundant one at 2172 cm(-1). These experimentally determined wavenumber values coincide, within +/-2 cm(-1), with those derived from theoretical calculations.
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