Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.
The intercalation of tetrabutylammonium (TBA) cations into graphite by cation exchange from a sodium-ethylenediamine graphite intercalation compound yields a single-phase first-stage product, C(44)TBA, with a gallery expansion of 0.47 nm. The gallery dimension requires an anisotropic "flattened" cation conformation.
Graphite intercalation compounds (GICs) of a series of symmetric or asymmetric tetraalkylammonium (TAA) intercalates are obtained from stage-1 [Na(en)1.0]C15 via cation exchange. The prepared dull-black TAA-GICs contain either flattened monolayer or bilayer galleries, with significant cointercalation of the dimethylsulfoxide (DMSO) solvent in the bilayer galleries. The TAA-GIC products obtained are characterized by X-ray diffraction and related structural modeling, compositional analyses, and Raman spectroscopy. [(C4H9)4N]C43 is obtained as a pure stage-1 GIC with the flattened monolayer structure. The larger symmetric TAA cations, (C6H13)4N, (C7H15)4N, (C8H17)4N, and the asymmetric TAA cations, (C12H25)(CH3)3N, (C18H37)(CH3)3N, (C18H37)2(CH3)2N, all form pure stage-1 GICs with flattened bilayer conformations. Thermogravimetric analyses combined with mass spectrometry and elemental analyses indicate the presence of ∼1-2 DMSO cointercalates per bilayer cation. The intercalate layers in these TAA-GICs have expansions along the stacking direction of ∼0.40 nm. Raman data confirm the low graphene sheet charge densities in the obtained TAA-GICs.
New graphite intercalation compounds
(GICs) containing N,N-n-alkyl substituted
pyrrolidinium cation intercalates (Py
n.m
, n, m = alkyl
chain lengths) are obtained via cationic exchange from stage-1 donor-type
GIC [Na(ethylenediamine)1.0]C15. Powder
X-ray diffraction and thermogravimetric analyses are used to determine
the GIC structures and compositions. [Py4.8]C47·0.71DMSO and [Py8.8]C48 with intercalate
monolayers are obtained as stage-1 GICs with gallery expansions of
0.48 nm, whereas [Py1.18]C47 and [Py12.12]C80·0.25DMSO form stage-1 GICs with intercalate
bilayers and gallery expansions of 0.81 nm. The gallery dimensions
require that alkyl chain substituents orient parallel to the encasing
graphene sheets. Smaller intercalate cations such as Py1.4, Py4.4, and Py1.8 either form high-stage GICs
or do not form stable intercalation compounds. These results, along
with those reported for graphite intercalation of other quaternary
ammonium cations, indicate trends in graphite chemistry where larger
intercalates form more stable and lower-stage GICs, and the graphene
sheet charge densities can be correlated to the intercalate footprint
areas.
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