Two water-stable
zirconium-based metal–organic frameworks
(MOFs) (NU-1000 and UiO-67) have been synthesized in various size
scales (100–2000 nm) for the adsorptive removal of glyphosate
from the aqueous media. Both NU-1000 and UiO-67 possess a three-dimensional
structure; NU-1000 consists of triangular micropores and wide mesoporous
channels (31 Å), whereas UiO-67 has cage-like pores [octahedral
(16 Å) and tetrahedral (14 Å) cages]. NU-1000 comprises
Zr
6
(μ
3
-O)
4
(μ
3
-OH)
4
(H
2
O)
4
(OH)
4
, and
UiO-67 contains Zr
6
O
4
(OH)
4
as secondary
building units. These units act as Lewis acid nodes and can interact
with the Lewis base phosphate group of the glyphosate. The time taken
for reaching equilibrium is found to be reduced considerably as the
size of the MOF decreases. The smaller the particle size, the lesser
is the diffusion barrier for the analyte, which enhances the interaction
between Lewis acidic metal nodes and the Lewis basic center of the
glyphosate molecule. NU-1000 was found to be better compared to UiO-67,
both in terms of efficiency and reusability. This might be due to
the larger pore diameters of the NU-1000. Theoretical calculations
revealed that the interaction energy of glyphosate with the nodes
of NU-1000 is higher (−37.63 KJ mol
–1
) compared
to UiO-67 (−17.37 KJ mol
–1
), which might
be the possible reason for the higher efficiency of NU-1000.
Chemical bonding
in 2D layered materials and van der Waals solids
is central to understanding and harnessing their unique electronic,
magnetic, optical, thermal, and superconducting properties. Here,
we report the discovery of spontaneous, bidirectional, bilayer twisting
(twist angle ∼4.5°) in the metallic kagomé MgCo
6
Ge
6
at
T
= 100(2) K via X-ray
diffraction measurements, enabled by the preparation of single crystals
by the Laser Bridgman method. Despite the appearance of static twisting
on cooling from
T
∼300 to 100 K, no evidence
for a phase transition was found in physical property measurements.
Combined with the presence of an Einstein phonon mode contribution
in the specific heat, this implies that the twisting exists at all
temperatures but is thermally fluctuating at room temperature. Crystal
Orbital Hamilton Population analysis demonstrates that the cooperative
twisting between layers stabilizes the Co-kagomé network when
coupled to strongly bonded and rigid (Ge
2
) dimers that
connect adjacent layers. Further modeling of the displacive disorder
in the crystal structure shows the presence of a second, Mg-deficient,
stacking sequence. This alternative stacking sequence also exhibits
interlayer twisting, but with a different pattern, consistent with
the change in electron count due to the removal of Mg. Magnetization,
resistivity, and low-temperature specific heat measurements are all
consistent with a Pauli paramagnetic, strongly correlated metal. Our
results provide crucial insight into how chemical concepts lead to
interesting electronic structures and behaviors in layered materials.
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