Journal of the American Chemical Society
is highly desired, which potentially provides a productive
avenue for NPs based porous solids.
Here, we show that self-assembled NP superstructures with
an open porous feature can be formed by confined assembly of
NPs within nanotubes (NTs), akin to the molecular host−
guest assembly. Tuning the size ratios of NTs-to-NPs allows
for the structural engineering of these open porous NP
superstructures, with symmetries such as C , zigzag, C , C , and
1
2
4
C5. Additionally, assembly of NPs either at the internal surface
or external surface of NTs can be deliberately controlled
through the introduction of competitive molecular additives,
leading to the competitive assembly.
Last but not least, these 1D porous NPs superstructures
show enhanced catalytic activity for silane alcoholysis and
semihydrogenation reactions.
RESULTS AND DISCUSSION
■
Assembly of NPs into Porous NPs Superstructures.
Co(OH) NTs tethered with a layer of octylamine were
2
synthesized according to a previous published method with
3
1
modifications. Transmission electron microscopy (TEM)
reveals that the particles are tubular structures with external
diameter and wall thickness of ∼18.6 nm and ∼1.0 nm,
respectively (Figure 1a−c). The length of these NTs is in the
range of hundreds of nanometers (Figure 1d). These NTs are
able to disperse in a nonpolar organic solvent, i.e., hexane.
The octylamine grafting density, determined from thermog-
ravimetric analysis (TGA), is measured to be ∼2.4 octylamine
2
Figure 1. Design concept for NCP 1D assembly of NPs within
per nm , assuming that the grafting density of octylamine
Co(OH) NTs. (a) The schematic diagram of Co(OH) NTs with
2
2
distinct curvatures at their internal and external surfaces. (b, c) TEM
(
Supporting Information, SI Note S1). Importantly, the
images of Co(OH) NTs. (d) Size histogram of diameter and length
2
geometric curvature of the external and internal surface of
the NTs differs, which consequently modifies the respective
molecular orientations. For the external surface with positive
curvature, the molecular packing diverges, and the distance
in the tail region (Figure 1e and SI Note S1). In sharp contrast,
for the internal surface with negative curvature, the molecular
packing converges and the distance between two neighboring
molecules decreases in the tail region (Figure 1e). It is this
connection between local curvature and molecular orientations
of NTs that provides the basis for the current construct. A
spherical NP would prefer to attach to the internal surface of
the NTs rather than the external surface with optimized van
Note S2). Therefore, these NPs would spontaneously assemble
within the NTs, giving rise to NP superstructures within the
NTs, resembling the molecular host−guest assembly (left,
Figure 1f). However, when the intermolecular space at the
internal surface of NTs is blocked by molecular additives, the
affinity between NPs and NTs is largely diminished, which
consequently drives the assembly of NPs at the external surface
of NTs (middle, Figure 1f) or self-sorted assembly (right,
Figure 1f), termed as competitive self-assembly and phase
separation, respectively.
of Co(OH) NTs. (e) Ligand packing density at the external and
2
internal surface of Co(OH) NTs. (f) Schematic representations of
2
possible modes of assembly of NPs and NTs. NPs can interact with
either internal or external surface of NTs, leading to respective host−
guest or competitive self-assembly; aggregation of NTs would lead to
phase separation.
Co(OH)2 NTs was carried out through a bad solvent
permeation method. Au5 NPs and NTs with a mass ratio of
1
:1 produce the best assembly structure (Figure S7). Typically,
Au5 NPs and NTs with a mass ratio of 1:1 were dispersed in
°
−
9
−1
of hexane with a rate of ∼6.0 × 10 mol h (Figure S8g).
After 8 h, nearly all the Au5 NPs were encapsulated into NTs,
resulting in Au5/NTs composites, as can be observed in TEM
images (Figures 2a,b and S8f). Several different orientations
reveal that these Au5 NPs are self-assembled into 1D
superstructures with C symmetry, and the unit cell consists
2
of four NPs, where two NPs occupy the circular cross section
of the NTs and the other two NPs are packed orthogonally
(
Figure 2c). Importantly, this C symmetric packing of Au5
The host−guest assembly was first engineered with Au NPs
2
3
2
NPs within NTs gives rise to the porous structures, as
evidenced by the nitrogen sorption isotherm with a mode of
type IV (Figure 2d). The pore size distribution based on the
Non Localized Density Functional Theory (NLDFT) method
clearly reveals the sizes of mesopores centered at 4.0 nm
(Figure 2e), which is much smaller than that of bare NTs
without the encapsulation of Au5 NPs.
d
NT
diameter ratio (γ) between NTs and NPs, γ =
≈ 2.0 (the
dNP
effective diameter of a NP (d ) is determined by the sum of
NP
the metal core diameter and twice the thickness of the organic
ligand (Figure S6c). The assembly of Au5 NPs within
1
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J. Am. Chem. Soc. 2021, 143, 11662−11669