Engineering of Microcage Carbon Nanotube Architectures with Decoupled Multimodal Porosity and Amplified Catalytic Performance

Article abstract of DOI:10.1002/adma.202008307

New approaches for the engineering of the 3D microstructure, pore modality, and chemical functionality of hierarchically porous nanocarbon assemblies are key to develop the next generation of functional aerogel and membrane materials. Here, interfacially driven assembly of carbon nanotubes (CNT) is exploited to fabricate structurally directed aerogels with highly controlled internal architectures, composed of pseudo-monolayer, CNT microcages. CNT Pickering emulsions enable engineering at fundamentally different length scales, whereby the microporosity, mesoporosity, and macroporosity are decoupled and individually controlled through CNT type, CNT number density, and process energy, respectively. In addition, metal nanocatalysts (Cu, Pd, and Ru) are embedded within the architectures through an elegant sublimation and shock-decomposition approach; introducing the first approach that enables through-volume functionalization of intricate, pre-designed aerogels without microstructural degradation. Catalytic structure–function relationships are explored in a pharma-important amidation reaction; providing insights on how the engineered frameworks enhance catalyst activity. A sophisticated array of advanced tomographic, spectroscopic, and microscopic techniques reveal an intricate 3D assembly of CNT building-blocks and their influence on the functional properties of the enhanced nanocatalysts. These advances set a basis to modulate structure and chemistry of functional aerogel materials independently in a controlled fashion for a variety of applications, including energy conversion and storage, smart electronics, and (electro)catalysis.

Full text of DOI:10.1002/adma.202008307

ReseaRch aRticle
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Engineering of Microcage Carbon Nanotube Architectures
with Decoupled Multimodal Porosity and Amplified
Catalytic Performance
Jamie Mannering, Rebecca Stones, Dong Xia, Daniel Sykes, Nicole Hondow,
Emmanuel Flahaut, Thomas W. Chamberlain, Rik Brydson, Gareth A. Cairns,
and Robert Menzel*
1. Introduction
New approaches for the engineering of the 3D microstructure, pore modality,
The discovery of atomically thin carbon
nanostructures, such as carbon nano-
tubes (CNTs) and graphene, with an array
materials. Here, interfacially driven assembly of carbon nanotubes (CNT)
of impressive and unique physiochemical
and chemical functionality of hierarchically porous nanocarbon assemblies
are key to develop the next generation of functional aerogel and membrane
properties[1–4] has led to intensive research
efforts into the development of a diverse
range of new nanocarbon-based applica-
tions and technologies, including energy
storage,[5] water treatment,[6] sensing,[7]
is exploited to fabricate structurally directed aerogels with highly controlled
internal architectures, composed of pseudo-monolayer, CNT microcages. CNT
Pickering emulsions enable engineering at fundamentally different length
scales, whereby the microporosity, mesoporosity, and macroporosity are
decoupled and individually controlled through CNT type, CNT number density,
and process energy, respectively. In addition, metal nanocatalysts (Cu, Pd,
and Ru) are embedded within the architectures through an elegant sublima-
tion and shock-decomposition approach; introducing the first approach that
enables through-volume functionalization of intricate, pre-designed aerogels
without microstructural degradation. Catalytic structure–function relationships
are explored in a pharma-important amidation reaction; providing insights
on how the engineered frameworks enhance catalyst activity. A sophisticated
array of advanced tomographic, spectroscopic, and microscopic techniques
reveal an intricate 3D assembly of CNT building-blocks and their influence on
the functional properties of the enhanced nanocatalysts. These advances set
a basis to modulate structure and chemistry of functional aerogel materials
independently in a controlled fashion for a variety of applications, including
energy conversion and storage, smart electronics, and (electro)catalysis.
structural
composites,[8]
actuators,[9]
neural cell growth frameworks,[10] and
many more.[11] Many of these applications
require macroscopic nanocarbon mate-
rial forms with well-defined structural and
chemical characteristics in order to enable
integration with existing technological
infrastructure. One way in which this
can be achieved is through the assembly
of individualized nanocarbons into gas-
filled, 3D porous architectures, known
as aerogels. Such nanocarbon aerogels
exhibit the highly desirable properties of
their carbon nanostructure building blocks
in a bulk phase while simultaneously
reaping the benefits of a hierarchically
J. Mannering, Dr. R. Stones, Dr. D. Xia, Dr. T. W. Chamberlain,
Dr. R. Menzel
D. Sykes
Henry Moseley X-Ray Imaging Facility
University of Manchester
School of Chemistry
University of Leeds
Leeds LS2 9JT, UK
E-mail: chmrme@leeds.ac.uk
Manchester M13 9PY, UK
Dr. N. Hondow, Prof. R. Brydson
School of Chemical and Process Engineering
University of Leeds
Leeds LS2 9JT, UK
Dr. E. Flahaut
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202008307.
CIRIMAT, Université de Toulouse, CNRS, INPT, UPS, UMR
CNRS-UPS-INP N 5085, Université Toulouse 3 Paul Sabatier
Bât. CIRIMAT
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cle under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
118, route de Narbonne, Toulouse 31062, France
Dr. G. A. Cairns
AWE plc
Aldermaston
Reading, Berkshire RG7 4PR, UK
DOI: 10.1002/adma.202008307
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porous system.[12,13] One of the main challenges in nanocarbon
aerogel research is to improve control over nanocarbon assembly
to produce well-defined 3D structures capable of optimization for
particular applications. Initial work on CNT aerogels resulted in
sheet-like macroscale structures such as vertically aligned nano-
tube forests and self-assembled 2D sheets that lack isotropic uni-
formity, scalability, and simplicity.[14–16] Early examples of true
3D nanocarbon aerogels (where all three aerogel dimensions are
at the same scale) required chemical vapor deposition onto poly-
meric or metallic substrates and foams.[17–20] These pioneering
approaches lay the foundation for numerous solution-based fab-
rication methods. There is now a thriving area of research into
free-standing low-density CNT aerogels fabricated via organoge-
lators capable of gelating the solvent and trapping the nanotubes
(e.g., ferrocene-grafted poly(p-phenyleneethynylene gelation of
chloroform)[21] and polymer-assisted assembly (e.g., MWCNT
embedded into poly(3,4-ethylenedioxythiophene)-poly(styren
esulfonate)).[22] These solution-phase fabrication methods are
the first demonstration of true 3D CNT aerogels and are physi-
cally characterized as stochastic isotropic architectures.[23–25]
Addressing the lack of detailed control over the internal aerogel
microstructure, emulsion-templating, that is, the directed
assembly of nanocarbons at the oil–water droplet interface in
Pickering emulsions, offers a highly attractive synthetic strategy.
Emulsion-templating has been successfully exploited to produce
GO aerogels with defined, monomodal macroporous closed-cell
internal architectures.[26] An interesting feature of emulsion-
templated aerogel synthesis is its scalability, as demonstrated by
the fabrication of large scale, free-standing GO aerogel sheets
(1 m2), based on significant inhibition of crack formation and
propagation due to the disrupted alignment of GO liquid crys-
tals in the presence of a discontinuous oil-phase.[27] Another
advantage is the compatibility of emulsion-templated aerogel
synthesis with continuous processing microfluidic techniques to
generate superelastic reduced-GO aerogels.[28] The utilization of
advanced microfluidic techniques highlights the wide ranging
scope of emulsion-templating synthetic approaches, including
their great potential for highly engineered aerogel microstruc-
tures. Despite this potential, there are no examples which
explore interfacial nanocarbon chemistry for the engineering
and modulation of aerogel porosities at different length scales
or to achieve this through decoupled, independent processes. 1D
carbon allotropes (such as CNTs), which are yet to be utilized,
are uniquely advantageous in this regard with the opportunity to
yield macroscopic materials with independently controlled hier-
archical porosities due to their structural dimensionality.
very few approaches have been demonstrated so far. All of these
approaches are on stochastic aerogel architectures and utilize
wet-chemical methodologies which typically cause dramatic
microstructural changes within the aerogel architecture due to
capillary-force-driven porosity collapse.[34–36] There is currently
only one reported functionalization approach avoiding wet-
chemical aerogel treatment, based on aerogel functionalization
in supercritical CO2. However, the extreme pressure conditions
and highly turbulent flow environment of supercritical CO2
treatments are only suited for the most robust aerogels.[37] Here,
we propose an alternative, more widely applicable and gentle
sublimation–decomposition approach utilizing volatile metal-
complex precursors to enable full, through-volume, function-
alization while negating microstructural changes, even within
delicate, pre-designed aerogel architectures. Sublimation-based
functionalization has proven highly successful for the filling of
CNT powders with an assortment of important transition metal
NPs.[38,39] In context of fine-chemical catalytic reactions, pow-
ders of graphitic nanofibers and single-wall CNTs (SWCNTs),
functionalized through this approach, have been investigated
as confinement-based nanoreactors.[40] Developing a sublima-
tion-based functionalization approach has therefore not only
great potential to preserve the microstructure of pre-designed
functional porous materials, but also opens up exciting new
opportunities to explore nanoconfinement effects in macroscale
aerogel-based catalyst systems, potentially imparting them with
new and unique catalytic functions.
In this study, we utilize 1D carbon allotropes to produce
robust, low-density, double-wall CNT (DWCNT) and multi-
wall CNT (MWCNT) aerogels composed of tunable spherical
microcages, providing a novel approach toward structurally
controllable, uniform CNT aerogels. In addition, we adapt a ver-
satile precursor sublimation–deposition methodology to func-
tionalize the emulsion-templated aerogels with Cu–CuO, Pd,
and Ru metal NPs, introducing a novel, mild aerogel functional-
ization strategy that enables uniform NP decoration throughout
the whole aerogel volume while avoiding irreversible changes
in microstructure. The structural and chemical characteristics
of the decorated aerogels are characterized at multiple length
scales via a range of advanced microscopic, tomographic and
spectroscopic materials characterization techniques. To inves-
tigate aerogel-induced impact on NP functionality, Cu–CuO-
decorated DWCNT and MWCNT aerogels are assessed for their
catalytic activity in a heterogeneous oxidative amidation reac-
tion and compared with their equivalent unassembled powder
analogues.[41]
Beyond this, integration of inorganic nanoparticles (NPs) into
aerogels is a rapidly emerging area of interest which enables the
tuning of chemical aerogel functionality, important for a wide
range of applications, such as energy storage and conversion,
sorption, and heterogeneous chemical and electrochemical
catalysis.[29–31] NP-decorated aerogels provide significant func-
tional benefits over decorated nanocarbon powders, including
permanently fixed exfoliation and individualization of the
carbon nanostructures, thereby increasing the available active
surface of the embedded NPs. There is an extensive set of syn-
thetic techniques developed for the functionalization of unas-
sembled nanocarbon powders.[32,33] However, functionalization
of 3D nanocarbon architectures is a complex process and only
2. Results and Discussion
As a first nanotube model system, DWCNTs were selected
(Figure S1, Supporting Information) as they combine high
aspect ratio and high specific surface area (SSA) with a unique
two-wall morphology (allowing for acid-oxidation of the outer
tube while retaining the unique physical properties of the
inner tube). Specifically, acid-oxidized DWCNTs (oDWCNTs)
were integrated into an interfacially driven emulsion-tem-
plating aerogel fabrication process (Scheme 1). To this end, the
hydrophilic oDWCNT building blocks (Figure S1, Supporting
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Scheme 1. Emulsion templated fabrication (O/W) of uniform free-standing nanotube aerogels with microcage internal structure. Toluene droplets
template the formation of nanotube microcages. The emulsion-templated nanotubes are locked into position through unidirectional freezing followed
by lyophilization and thermal reduction. Crosslinked CNT aerogels exhibit decoupled multimodal porosity controlled by the selection of CNT inner
cavity (microporosity), CNT number density (mesoporosity), and droplet size (macroporosity).
Information) are dispersed and individualized within an addi-
tive-containing aqueous solution at a CNT concentration of
around 1.5 mg cm−3. At this concentration, a large degree of
nanotube de-bundeling can be achieved without compromising
through-volume nanotube percolation, required for successful
formation of robust 3D CNT aerogel networks. Emulsification
of the aqueous oDWCNT dispersion with an oil-phase (toluene,
25 vol%) under pH-controlled, acidic conditions leads to the
formation of nanotube-stabilized Pickering emulsions. In
essence, the spherical oil droplets in these emulsions are coated
by the nanotubes and thereby act as highly effective templates
for the formation of micrometer-sized nanotube microcages
within the final aerogels (Figure 1).[42,43] Careful acidification
of the aqueous phase is crucial to drive successful oDWCNT
assembly at the oil–water droplet interface via surface group
protonation and the resulting slow increase in nanotube hydro-
phobicity at low pH. The additives (poly(vinyl alcohol) and
sucrose) further increase emulsion stability through sites of
hydrogen bonding between the polymers and nanotube surface
groups; slowing the process of phase separation (Figure S2,
Supporting Information).[44] The successful formation of CNT
aerogels without shrinkage requires a delicate balance of organic
additives (OAs) to CNTs at a ratio of 1:2 OA:CNT (Figure S3,
Supporting Information). Further, the emulsion-templated
structure is locked in place through unidirectional freezing
in custom-made molds. Growing ice-crystals from the base
upward direct the microcages into vertical columns of align-
ment to improve macroscale uniformity (Figure S4, Supporting
Information). To prevent capillary-driven collapse, the solvent
is removed via freeze drying and the subsequent free-standing
aerogel monolith is reduced under thermal conditions to yield a
reduced DWCNT aerogel (Figure S5, Supporting Information)
with an ultralow envelope density of around 1.3 mg cm−3 (as a
comparison the density of air is around 1.2 mg cm−3).
Crosslinking of the 3D CNT network is based on
substantially increased van der Waals interactions between the
reduced DWCNTs due to markedly increased graphitic surface
crystallinity after thermal reduction (Figure S2, Supporting
Information).[45] X-ray photoelectron spectroscopy (XPS) reveals
oxygen functionality (hydroxyl and carboxyl) prior to reduc-
tion with a substantial loss in oxygen after thermal reduction
(≈1 at% residual oxygen) and marked increase in sp2 carbon
concentration, Figure S6 and Table S1, Supporting Informa-
tion).[46,47] Subsequent Raman analysis (Figure S7, Supporting
Information) confirms a significant increase in graphiticity of
the aerogel sample, as indicated by substantial increases in
IG/ID intensity ratio and sp2 cluster size upon thermal treat-
ment (Table S2, Supporting Information). While the vast
majority of the polymer additives are removed during thermal
reduction (>95%), some minor residues remain in graphitized
form (Figure S7C, Supporting Information) and act as addi-
tional covalent crosslinking agents. The resulting stability and
robustness of our ultralight nanotube aerogels is highlighted
by their ability to resist shrinkage during high-temperature
reduction. In contrast, graphene-based aerogels, produced at
the same low nanocarbon densities and under the exact same
fabrication conditions, significantly shrink and their microcel-
lular internal architecture completely collapses upon thermal
reduction (Figure S8, Supporting Information), indicating the
need for different conditions when fabricating microspherical
aerogel architectures from 2D rGO building blocks.[26,48] The
excellent resistance to microstructural changes highlights the
carbon nanotubes as ideal 3D network formers within aerogel
fabrication.[18] The excellent interconnectivity of 3D nanotube
networks also gives rise to reversible mechanical compressibility
(Figure S5, Supporting Information) and good through-volume
electrical conductivity in the DWCNT aerogels (0.28 S m−1);
properties not accessible in other porous materials, such as
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Figure 1. Tunable microcage internal structure of emulsion-templated DWCNT and MWCNT aerogels. A) Low-magnification SEM image of the internal
microcage structure of a DWCNT aerogel. B,C) Higher-magnification SEM images of an individual DWCNT microcage revealing an interconnected
monolayer of nanotubes. D,E) TEM bright-field images of the interwoven DWCNT in the microcage walls. F–J) Engineered macroporosity is achieved
through template control (F), resulting in: large (35 µm) (G,I) and small (3 µm) (H,J) microcages, depending on emulsification energy input.
zeolites or MOFs. Emulsion-templated aerogel fabrication
enables the creation of macroscopic nanotube materials with
high surface areas and well-controlled, hierarchical porosity
(Table 1; Figures S9,S10 and Table S3, Supporting Informa-
tion). During fabrication, the nanotubes remain well-exfoliated
and assemble around the oil droplets in a pseudo-monolayer
fashion, leading to the formation of DWCNT aerogels with a
high SSA (785 m2 g−1), approaching the theoretical limit of fully
individualized DWCNTs (800 m2 g−1 at the same diameter).[2]
In contrast, the SSA of an equivalent (oxidized, then thermally
reduced) DWCNT powder material is only 329 m2 g−1, high-
lighting the strength of emulsion-templated nanotube aerogels
to access the excellent properties of their nanoscale building
blocks in the bulk phase.
(Table 1; Figure S9, Supporting Information). The open internal
cavities of the nanotubes (i.e., inner nanotube channels)
in the microcage walls give rise to very well-defined micro-
porosity (Figure S1, Supporting Information) which can been
tuned through the selection of nanotube building block (e.g.,
inner DWCNT diameter ≈ 1.5 nm vs inner MWCNT diameter
4 nm, Figure S10, Supporting Information).[49] The interstitial
spaces between the nanotubes in the mesh-like microcage walls
(Figure 1B,C) generate a substantial degree of mesoporosity
(tunable through the selection of CNT type (10–43 nm) and
CNT concentration (43–105 µm), Figure S10, Supporting Infor-
mation). The hollow nanotube microcages create well-defined
macroporosity on the micrometer length scale (≈30 µm),
tunable during fabrication through the emulsification energy
input (Figure 1F; Figure S10, Supporting Information).
Further, the nanotube aerogels (both DWCNT and later
MWCNT) exhibit well-defined and tunable multimodal
porosity on the micro-, meso-, and macroporosity length scales
as indicated by electron microscopy (Scheme 1; Figure S10,
Supporting Information) and nitrogen sorption measurements
The extension of the emulsion-templating approach to other
1D nanocarbon allotropes was demonstrated with MWCNTs.
Utilizing the same synthetic conditions (Scheme 1), com-
mercially available, carboxylic acid functionalized MWCNTs
Table 1. Structural characteristics of emulsion templated DWCNT and MWCNT aerogels. Employing DWCNTs with much smaller diameters result in
high surface area aerogels with extensive graphiticity. Alternatively, employing MWCNTs lead to aerogels with significantly higher mesopore volume.
Macroscopic properties
Porosity
Graphiticity
sp2 [nm]
ρ [mg cm−3
]
SSA [m2 g−1
785
]
σ [S m−1
0.28
]
Microcage size [µm] VMeso [cm3 g−1
]
VMicro [cm3 g−1
0.066
]
IG/ID
8.36
0.79
DWCNT aerogel
MWCNT aerogel
1.3
1.5
30
35
1.38
1.83
37
3
416
0.72
0.012
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(Figure S1, Supporting Information) were assembled into
MWCNT aerogels with a spherical microcage internal archi-
tecture (Figure S11, Supporting Information). Despite having
almost identical envelope densities, the DWCNT aerogels and
MWCNT aerogels have distinctively different physical and
textural characteristics (see differences in SSA, micropore
volume (VMicro), mesopore volume (VMeso), and electrical
conductivity (σ) in Table 1), highlighting simple selection
of the CNT building blocks as a straightforward strategy to
engineer aerogel microstructure. Elements of these differences
are borne out in the microscopy images (Figure 1); a typical
internal microcage shows a smooth interconnected network of
finely woven DWCNT building blocks (Figure 1B,C). In con-
trast, the MWCNT aerogels exhibit internal microcages that are
less finely woven (Figure 1I,J), resulting in a larger mesopore
volume (Table 1). This variation in mesoporosity, at the same
carbon density, arises from the differences in diameter and
wall thickness of the two nanotube types, resulting in a lower
number density and, hence, larger spacing of nanotubes in
the MWCNT microcage walls. Selection of CNT type can also
be used to tune surface area for a given aerogel envelope den-
sity, with the MWCNT aerogel exhibiting a distinctly different,
smaller SSA (416 m2 g−1), compared to the DWCNT aerogel, at
the same density (Figure S9, Supporting Information). As for
the DWCNT, the SSA of the MWCNT aerogel approaches the
theoretical limit of fully individualized MWCNTs of around
500 m2 g−1. CNT type also impacts on electrical conductivity,
with the MWCNT aerogels (0.72 S m−1) performing significantly
better than DWCNT aerogels (0.28 S m−1), reflecting the pro-
nounced metallic character of their MWCNT building blocks.
In terms of aerogel macroporosity, control over the nanotube
microcage size in the final aerogels is readily achieved through
variation of the shear mixing rate (i.e., energy input, Figure 1F)
during the initial emulsification process (Scheme 1, Figure
S10, Supporting Information) in order to tune the size of the
emulsion droplet templates. Variation of the shear mixing rate
in a MWCNT emulsion system was used to reduce internal
microcage size by one order of magnitude, from 35 µm at an
energy input of 10 J dm−3 (Figure 1G,I) down to 3 µm at an
energy input of 200 J dm−3 (Figure 1H,J). It is worth noting
that increasing shear rate during emulsification also results
in a much narrower microcage size distribution (Figure S12,
Supporting Information). In addition to controlling the micro-,
meso-, and macroporosity through processing parameters
(Figure S10, Supporting Information), a fourth parameter
(demonstrated here for the first time with emulsion templated
CNT aerogels) of controlling the discontinuous toluene volume
fraction (demonstrated between 25–50 vol%, Figure S10,
Supporting Information) enables engineering of the funda-
mental architectural design through an inverse transformation
from an open-cell microstructural architecture to a closed-cell
architecture with wide-ranging implications on the function-
ality of percolation networks. Controlling these microstructural
parameters, which subsequently enable full control over aerogel
porosity and percolation, will inevitably lead to changes in mass
transport and provides new opportunities to control and opti-
mize aerogel performance in a given application.
aerogel materials. Introduction of metal NPs requires a non-
aggressive synthetic methodology, capable of functionalizing the
aerogel throughout its entire internal volume without collapsing
the carefully designed and intricate microstructure. To achieve
this, a stepwise process of sublimation–deposition of readily
available transition metal complexes (carrying acetylacetonate
or carbonyl ligands) as volatile NP precursors, followed by
thermal-shock decomposition into metal NPs, was employed
(Figure 2A). Adapting a well-established gas-phase decorative
approach, 3D macroscale aerogel structures are subjected to the
same sublimation–deposition conditions (Figure 2A) that have
proven highly successful for the external and internal func-
tionalization of carbon nanotube and nanofiber powders.[38,39]
In the context of structurally engineered nanocarbon aerogels,
sublimation–deposition of precursor complex compounds have
two crucial advantages: i) the NP precursor can readily diffuse
through the macroscopic, porous aerogel in the gas-phase with
little mass-transfer resistance (improving through-volume
decoration uniformity), and ii) precursor deposition can be
carried out without exerting any capillary-forces (so avoiding
microstructural aerogel changes). Following precursor
sublimation–deposition, a rapid, thermal-shock-decomposition
process is then applied to efficiently decompose the volatile
precursors into metallic NPs (Figure 2A).
As a first model system, this gas-phase aerogel functionali-
zation approach is demonstrated for the sublimable precursor
copper acetylacetonate (Cu(acac)2) to decorate emulsion-
templated DWCNT aerogels with Cu–CuO core–shell NPs
(a low-cost and elementally abundant metal with a wide range
of applications, especially in chemical and electrochemical
catalysis).[50–52] Stoichiometry of aerogel and precursor are
chosen to yield a final Cu–CuO loading of around 8 wt% in the
aerogels, a typical degree (between 5–10 wt%) of NP decoration
in many inorganic/nanocarbon hybrid powder systems used
in catalysis. In the first stage of the aerogel functionalization
process, the organometallic precursor is sublimed at mod-
erate temperatures under high-vacuum conditions and left
to diffuse through the aerogel. Precursor deposition onto the
aerogel framework is then induced through rapid cooling in
an ice-water bath. Sublimation was carried out here over three
days to ensure full precursor utilization; however sublimation
times can be readily reduced to hours without any impact on
functionalization uniformity (Figure S13, Supporting
Information). In fact, this first sublimation stage could be
shortened even further (<1 h) through the use of pre-processed
precursor powders with smaller grain sizes (to increase speed
of sublimation). In the second stage, Cu–CuO core–shell NP
formation is achieved by thermal-shock heating (800 °C for
1 min) under a low-pressure nitrogen atmosphere (Figure S14,
Supporting
Information),
inducing
rapid
precursor
decomposition.[53] Decomposition was also explored at more
conventional heating conditions, including lower temperatures
and longer treatment durations; however these decomposi-
tion conditions led to relatively large NPs (>70 nm, Figure S15,
Supporting Information) or only partial precursor decompo-
sition (Figure S16, Supporting Information). The optimized
shock-decomposition approach, which is applicable to many
other metal precursors, yields DWCNT aerogels uniformly
embedded with Cu–CuO core–shell NPs (Cu@DWCNT
Beyond this, NP decoration is a crucial strategy to widen and
tune the chemical functionality of the structurally engineered
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Figure 2. Decoration of CNT aerogels with “naked” metallic Cu–CuO core–shell NPs (Cu@CNT aerogel) through a structure-preserving, gas-phase
functionalization approach. A) Schematic for the sublimation–deposition of a typical, volatile metal-complex precursor (Cu(acac)2) onto the framework
of a nanotube aerogel, followed by thermal-shock precursor decomposition into Cu–CuO core–shell NPs. B) Synchrotron source X-ray absorption
spectroscopy and C) X-ray photoelectron spectroscopy of Cu@DWCNT aerogel reveal physiochemical information on the composition of the metallic
Cu–CuO NPs. D) SEM backscattered electron mode (BSE). E,F) Bright-field TEM and G) STEM-EDX images of Cu@DWCNT aerogel show the structural
preservation of aerogel microstructure, Cu–CuO NP distribution. and elemental composition, respectively.
aerogel). The thermal-shock-decomposition approach mitigated
NP aggregation at longer heating durations (a strategy that has
proven highly successful in the synthesis of high-entropy-alloy
NPs).[54] The sublimation/shock-decomposition approach
thereby enables the formation of “naked” Cu–CuO core–shell
NPs without any stabilizing capping agents or surfactants;
allowing the exposed NP surface to interact directly with the
local environment.[55]
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) imaging of the final Cu@DWCNT
aerogel (sampled from the internal core of the aerogel mono-
lith, Figure 2D–G) confirm the formation of well-dispersed,
unaggregated Cu–CuO NPs and highlight that the gaseous pre-
cursor is able to diffuse fully into the aerogel interior. Impor-
tantly, SEM imaging collected at lower magnification reveals
a fully preserved emulsion-templated aerogel microstructure
after NP decoration (Figure 2D). Bright-field (BF) TEM and
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) reveal that the formed NPs adopt
an isotropic, spherical morphology with an average diameter of
20 7 nm (Figure 2E,F; Figure S17, Supporting Information).
Interestingly, the DWCNT aerogel induces the formation of 50%
smaller NPs with much narrower size distribution, compared
to an equivalent DWCNT powder support (Figure S18,
Supporting Information). This substantial reduction in NP
size (also observed in Cu@MWCNT) and improvement in
NP homogeneity likely stems from the much-improved acces-
sibility of the nanotube’s surfaces in the open, porous aerogel
network, potentially providing a larger number of more widely
spaced NP nucleation sites compared to the heavily aggregated
nanotube powders. Increased nucleation and interparticle
distances potentially reduce the NP sizes and limit the impact
of NP sintering, allowing for aerogel functionalization with
smaller NPs relative to the powders. Moreover, high-resolution
TEM imaging and STEM-EDX mapping (Figure 2G; Figure S17,
Supporting Information) confirm that the observed NPs consist
of highly crystalline Cu–CuO and are well-embedded within
the carbon nanotube network. The crystallinity of the Cu–CuO
NPs is evidenced by the presence of clear lattice spacings in the
image and spots in the Fourier-transformed power spectrum
(Figure 2F).
In order to probe the chemical state of the formed Cu–CuO
core–shell NPs, a Cu@DWCNT aerogel was studied via X-ray
absorption near-edge spectroscopy (XANES), utilizing a high-
energy synchrotron X-ray source to collect sufficient Cu signal
from the challenging ultralow-density aerogel sample. XANES
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Figure 3. Emulsion-templated nanotube aerogels decorated with Pd and Ru NPs. A) SEM (BSE mode) and B) SEM-EDX elemental mapping of
Pd@DWCNT aerogel. C) Low-magnification bright-field TEM and D) HR-TEM reveal an average Pd NP size of 11 2 nm in an FCC crystal phase.
Similarly, E,F) SEM-BSE and SEM-EDX elemental mapping of the Ru@DWCNT aerogel. G,H) Average Ru NP size of 3 2 nm in an HCP crystal phase,
determined through TEM.
confirms, on a bulk scale, the presence of metallic Cu0 NPs
in the decorated aerogels, as indicated by the characteristic
Cu K-edge position (8979 eV) and post-edge features of the
Cu@DWCNT aerogel spectrum (Figure 2B).[56] In addition,
XANES also confirms the absence of CuII precursor species
(as evidenced by the absence of the CuII characteristic pre-edge
peak at 8978 eV stemming from the Cu(acac)2),[57] indicating
that the decorated aerogels are free of any unconverted
precursor and highlighting the efficacy of the thermal-shock-
decomposition process in generating Cu–CuO NPs. The post
K-edge features (the extended fine structure) appear slightly
shifted (Figure 2B, position b and c) for the aerogel-supported
Cu–CuO when compared to an unsupported Cu0 reference
sample. This shift suggests a slight difference in local chem-
ical environment, likely arising from factors such as degree of
surface oxidation or interaction with the DWCNT framework.
However, detailed fine structure modelling (beyond the scope
of this study) is required to confirm such effects. To corroborate
the XANES results, XPS characterization of the Cu@DWCNT
aerogel was employed as a complimentary technique, probing
the surface, rather than bulk, of the NPs. The deconvoluted
high-resolution XPS Cu 2p spectrum (Figure 2C) indicates
that most Cu in the NP surface layer is present as metallic
Cu0 (main peak positions at 932.6 and 952.4 eV). However, in
contrast to XANES, XPS suggests the additional presence of
smaller quantities of CuII species, which is most likely CuO
(weak satellite at ≈943 eV and shoulders in the two main XPS
peaks).[58] These results suggest a biphasic Cu–CuO which is
likely to be in a core–shell composition with a large metallic
Cu core and a thin CuO shell, formed upon air exposure. In
addition, aerogel decoration via precursor sublimation/shock-
decomposition can be easily translated to other nanotube
aerogels (e.g., MWCNT aerogels, Figure S19, Supporting Infor-
mation) and other metal NPs (e.g., Pd and Ru NPs, Figure 3;
Figure S20, Supporting Information). For example, emulsion-
templated nanotube aerogels can also be readily decorated with
metal NPs of Pd and Ru (Figure 3), when simple sublimable
transition metal complexes, such as Pd(F6acac)2 or Ru3(CO)12,
are used under the same sublimation–deposition conditions,
outlined in Figure 2A. Decoration is again highly uniform
across the aerogel framework, as evidenced by SEM–EDX
mapping (Figure 3B,F). The formed Pd (11 nm) and Ru (3 nm)
NPs are highly crystalline and present in their pure metallic
form without any oxide shell, as indicated by high-resolution
TEM (Figure 3D,H) and XPS (Figure S21, Supporting
Information).
These results highlight the broad applicability of the
sublimation/shock-decomposition decoration approach utilizing
a toolbox of simple metallic complexes and its general com-
patibility with 3D porous structures.
To image the true 3D aerogel microstructure, X-ray computed
micro- and nanotomography was conducted on a Cu–CuO
decorated MWCNT aerogel (Figure 4; Figure S22, Supporting
Information). X-ray tomography provides an extremely
valuable, non-destructive tool to probe 3D aerogel morphology
and structural uniformity at the micrometer scale without
the potentially distorting effects of destructive 3D imaging
methodologies, such as FIB-SEM. The deposited Cu–CuO NPs
introduce considerably higher atomic number contrast, thereby
acting in effect as contrast agents. The resulting increase
in X-ray attenuation enables faster and more detailed X-ray
tomographic imaging of the emulsion-templated microstructure
(which is very challenging otherwise due to the ultralow-
density of the purely carbon-based undecorated aerogels). X-ray
microtomography (Figure S22, Supporting Information) shows
that the internal CNT microcages are positioned uniformly
throughout the aerogel with no observed structural gradients or
larger macroscopic voids. Pushing the resolution boundaries in
X-ray imaging, the nanotomography reconstruction (Figure 4) is
able to resolve the finer details of a typical nanotube microcage
(Figure 4, left panel). The high-resolution 3D reconstruction
(Figure 4, central image) reveals excellent 3D uniformity of the
internal microcages, which is further confirmed by the highly
spherical color maps of a typical MWCNT microcage in three
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Figure 4. Advanced tomographic characterization on Cu@MWCNT aerogel. Left) Comparison of X-ray 3D and electron 2D (SEM) images of a typical
nanotube microcage. Center) High-resolution 3D reconstruction of X-ray nanotomography orthoslices, revealing highly uniform microcages without
the presence of large Cu–CuO NPs. Right) Multidimensional analysis of a typical microcage through three different orthogonal planes, reaffirming the
highly spherical shape adopted by the nanotubes in emulsion templating.
orthogonal planes (Figure 4, right panel). While the tomography
methodologies employed here do not allow for spatial resolu-
tion of individual NPs, they do confirm that the sublimation/
shock-decomposition method results in excellent 3D uniformity
of Cu–CuO decoration, with the Cu–CuO well-dispersed around
the individual MWCNT microcages.
(Figure S25, Supporting Information, Cu@DWCNT aerogel
catalyzes oxidative amidation at double the reaction yield and
three-times the turn-over-number (TON, see also Table S4 and
Figure S26, Supporting Information), indicating markedly
improved copper utilization.
The final product yield of the Cu@CNT aerogel catalysts
is also enhanced (increase in yield by around 15%) when
compared against equivalent Cu@CNT powder analogues (i.e.,
Cu–CuO NP catalysts supported at the same weight loading
on nanotube powders rather than aerogels, Figure 5A). This
increase in product yield likely originates from the improved
structural features of the aerogel-based catalyst systems,
including larger SSA, higher meso/macroporosity, and smaller
NP size compared to the more aggregated powder analogues
(Figure S18, Supporting Information). It is worth noting that
initial rates for reagent conversion and product formation
show less pronounced differences (independent on stirring
rate, Figure S24, Supporting Information). Further, reaction
yields are limited to about 70%, even for the best performing
Cu@DWCNT aerogel catalysts. Post-catalysis XPS (Figure S27,
Supporting Information) suggests that copper oxidation at pro-
longed reaction times might contribute to limiting the yields.
Future reaction optimization (temperature, oxidizing agent)
could therefore be utilized to increase final yields even further.
In terms of reaction selectivity, both Cu@CNT aerogel
catalysts exhibit a higher selectivity toward the desired amide
product compared to their powder analogues—an effect
more pronounced for the Cu@DWCNT catalysts (Figure 5B).
Full mechanistic understanding of this selectivity improve-
ment in the aerogel-based catalysts is complex. However, it is
Controlling aerogel 3D microstructure and NP decoration
are crucial tools to tailor aerogel materials to specific applica-
tions. To probe how structural nanotube assembly impacts
on chemical functionality, the emulsion-templated, Cu–CuO
decorated CNT aerogels were studied as catalysts in an oxidative
amidation reaction (Scheme S1, Supporting Information).
In this context, oxidative amidation (specifically, the reaction
of a tertiary amine and an anhydride in the presence of an
oxidizing agent)[59] provides a particularly robust and repeat-
able catalytic reaction model, due to the relatively small number
of side-products and the straightforward one step nature of
the reaction (reaction scheme Figure 5A) with efficient atom
economy. Oxidative amidation reactions are also of great
practical importance as amide bond formations are key features
in many pharmaceutical compounds (e.g., Lisinopril, Valsartan)
and biomolecular structures (e.g., proteins) with 16% of all
industrial pharmaceutical reactions involving an amide bond
formation stage.[60–62]
In this model reaction system, Cu–CuO decorated CNT
aerogels show enhanced product formation yields at equilib-
rium in comparison to both unsupported NP catalysts as well
as Cu@CNT powder analogues (Figure 5A product formation
plots, Figure S23, Supporting Information). Moreover, com-
pared to an unsupported, commercial Cu nanopowder catalyst
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Figure 5. Catalytic performance and solvent-induced shrinkage-expansion cycling of emulsion-templated CNT aerogels decorated with Cu–CuO core–
shell NPs. A) Comparison of product formation evolution in an oxidative amidation model reaction, induced by Cu–CuO catalysts supported on dif-
ferent CNT powders and aerogels (Cu loading ≈ 8 wt%), revealing enhanced performance of the Cu–CuO catalyst on the CNT aerogel supports. B)
Selectivity based on proportion of the desired product formed in comparison to unreacted starting material and by-products for different Cu@CNT
catalysts, highlighting the excellent catalytic activity of the Cu@DWCNT aerogel. C) Shrinkage–expansion cycling of the Cu@DWCNT aerogel upon
drying and addition of acetonitrile (repeated 20 times)—scale bar: 1 cm.
plausible to relate selectivity enhancement back to the smaller
NP size (potentially leading to changes in type and composi-
tion of active sites), higher porosity (potentially impacting mass
transport and reaction kinetics) and increased surface accessi-
bility (potentially boosting pre-concentration and confinement
effects) in the aerogel-based catalysts.[63]
the shrunken aerogels fully and rapidly (within seconds) expand
back to their original size and shape. This solvent-induced,
dynamic shape-recovery process is repeatable over 20 cycles
without loss of aerogel shape or structural integrity. Shrinkage
occurs through the capillary action of evaporation, inducing
collapse of the voids within the aerogel. Inversely, a wicking
effect is observed on solvent addition, the inner voids balloon
which favorably allows shape recovery. This solvent-induced
shape recovery of the aerogels is potentially facilitated by the
residual oxygen within the carbon lattice (Figure S6, Supporting
Information), improving wettability of the aerogel by the
solvent. The flexible network of the high-aspect-ratio 1D CNT
building blocks crucially enable this dynamic behavior and
ensures structural aerogel integrity over repeated shrinkage
and expansion cycles. This unusual feature is particularly
pronounced in the DWCNT aerogel and to a lesser extent in the
MWCNT aerogels. This dynamic structural behavior demon-
strates the true multifunctional nature of the nanotube aerogel
supports, providing functionality completely inaccessible in
unassembled nanocarbon powder materials. The shrinkage/
expansion behavior is likely to prove particularly useful for inte-
gration of the aerogel catalysts into flow chemistry processes,
for example, the aerogel could be easily introduced into a
tubular (or more complex) flow reactor architecture and then
be expanded upon solvent exposure to fill a predefined volume
and shape without any gaps. More generally, we envision future
integration of our aerogel catalyst systems into chemical flow
processes where they provide a wide range of advantages over
catalyst powder systems. The aerogels investigated here show
Interestingly, catalytic aerogel performance can be readily
tuned through selection of nanotube building block. The
Cu–CuO NP catalysts on the DWCNT aerogel perform
considerably better than those on the MWCNT aerogel, as
evidenced by a product yield increase of 20% and a doubling
of TON (similar increases are reflected in the powder
analogues, Table S4, Supporting Information). These differ-
ences in catalytic performance likely reflect the markedly dif-
ferent meso- and microporosity of the DWCNT and MWCNT
aerogel supports (Table 1), impacting on reagent mass transfer
and molecular confinement effects. Another potential
contributing factor could stem from nanotube-specific metal–
support interactions, originating from the intrinsic electronic
and structural differences of the two nanotube types in terms
of surface curvature,[64] helicity, structural defects,[65] and
conductivity.
In addition to their tunable catalytic activity (Figure S23,
Supporting Information), our metal-decorated aerogel catalysts
also display an interesting and novel dynamic structural
behavior on the macroscale (Figure 5C), induced by solvent
exposure and removal. Thermal evaporation of the reaction
solvent causes the aerogel to shrink significantly and densify.
Remarkably, upon addition of fresh solvent/reaction medium
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highly controlled porosity (tunable on different length scales)
which provides opportunities to control flow properties such as
pressure drop, mixing, and residence time, in continuous flow
processes. The pronounced (and stable) electrical conductivity
of the interconnected CNT aerogel network will also allow for
the exploration of electrical current/potential utilization in flow
reactions, for example, in context of electrochemical organic
flow synthesis, in the future.
for exciting future exploitation of the engineered materials as
well as electrocatalysis for renewable energy vectors and energy
storage technologies.
In addition to aerogel-enhanced catalytic performance, this
work provides a blueprint that allows for wider exploitation
and integration of microcage CNT aerogels in diverse areas of
chemistry and materials science. For example, the capsule-like
internal architectures lend themselves to areas such as drug
confinement where microcage wall density and functionality
could be tuned to determine drug loading and release rate.
The flexibility of the aerogel fabrication process should also
prove highly advantageous for future sacrificial templating
strategies to produce highly ordered, carbon-free metal aero-
gels with engineered microstructures inherited from the sac-
rificial microcage CNT aerogels. Integration into chemical
flow technologies (chemical and electrochemical) is another
important future application area envisioned, where the capa-
bility to control aerogel structure at different length scales will
be highly beneficial, for example by providing unprecedented
opportunities to independently tailor through-flow properties
on the mesoscale (enabling control over residence time, rea-
gent mixing, pressure drops, etc.) and molecular confinement
on the nanoscale (enabling control of reaction pathways and
product selectivity). The discoveries unveiled in this work there-
fore highlight the potential of these next generation 3D archi-
tectures in many well-established and emerging applications
and technologies.
3. Conclusion
High-surface-area aerogel materials with unique micro-
structural features, based on nanotube microcages, have
been successfully produced through an emulsion-templating
methodology. CNT emulsion-templating allows for tuning of
aerogel microstructure across a wide range of length scales
through nanotube type selection, nanotube number density,
emulsion droplet size and toluene volume fraction. This
unique fabrication design provides a multitude of enhanced
features such as a SSA approaching the theoretical limit of the
selected nanotubes and large, controllable multimodal porosi-
ties. Beyond this, the free-standing aerogels are decorated with
metallic NPs through an organometallic precursor sublimation/
shock-decomposition approach. This gas-phase functionali-
zation approach, demonstrated here in aerogels for the first
time, is capable of full diffusion and deposition on the entire
aerogel framework without altering its structural design. The
rapid, energy efficient and facile shock-based decomposition
produces uncapped, highly crystalline, metallic NPs (Cu–CuO,
Pd, and Ru). This sublimation-based aerogel functionalization
approach can be easily extended to include a toolbox of other
simple metallic precursors to achieve aerogel decoration with
functional mono- or multi-metallic NPs with excellent 3D
uniformity. The microstructure-preserving sublimation/shock-
decomposition methodology can also be readily translated to
a wide variety of other structurally engineered macroscopic
material forms, such as foams, membranes or woven, fiber-
based fabrics, providing great scope for the development of
advanced functional porous materials.
A sophisticated array of advanced characterization techniques
spanning all pore size domains (X-ray nanotomography, SEM,
HR-TEM) enable detailed imaging of the 3D aerogel micro-
structure while spectroscopic measurements on the bulk and
nanoscale (XANES, XPS, STEM-EDX) enable in-depth under-
standing of NP distribution and chemical state. In addition,
we explore how the nanotube framework and type of nanotube
building block impact on the activity of supported Cu–CuO
NPs when they are employed in a pharmaceutical catalytic
oxidative amidation reaction. The aerogel supports enhance
catalytic performance, with Cu–CuO supported on DWCNTs
performing better (higher yields, higher TON, improved selec-
tivity) than the MWCNTs. These findings highlight how simple
changes in the nanocarbon building blocks resonate into effects
on catalytic performance. The microstructural and chemical
tunablity of emulsion-templated NP@CNT aerogels lay the
foundation for future catalyst tailoring for classic catalyzed fine-
chemical synthesis while the aerogels’ electrical responsiveness
(enabled by the electrically conducting CNT framework) allow
4. Experimental Section
Materials: Catalytic chemical vapor deposition (CCVD) grown,
carboxylic-acid-functionalized MWCNTs, Cu nanopowder (60–80 nm),
N,N-dimethylaniline, dodecane, toluene, acetonitrile, Cu(acac)2,
Pd(F6acac)2, and Ru3(CO)12 were purchased from Sigma-Aldrich.
Poly(vinyl alcohol) and sucrose were purchased from Fischer. Acetic
anhydride and tert-butyl hydroperoxide were purchased from Merck.
DWCNTs were grown (via CCVD) and acid-oxidized (3 m HNO3, 130 °C,
24 h) using previously reported synthetic methodologies.[49,66]
Preparation of CNT Aerogels: Poly(vinyl alcohol) (0.38 mg cm−3)
and sucrose (0.38 mg cm−3) were fully dissolved in water through
mild heating. The concentrations of these additives were optimized to
maximize mechanical strength while minimizing impact on the nanotube
surface-area (see Figure S3, Supporting Information, and supplementary
statement for additive optimization). The acid-oxidized MWCNTs or
DWCNTs (1.5 mg cm−3) were added followed by probe-tip sonication
(8 × 10 min, 30% power) with intervals of stirring to produce well-
dispersed aqueous CNTs. HCl (0.1 m, 0.005 vol%) was then added
dropwise while under continuous stirring to protonate the surface
functional groups, which is required for successful emulsion-templating.
Toluene was added (25 vol%) as the discontinuous phase. The oil/water/
CNT mixture was then either vortex mixed (benchtop vortex mixer,
Fischerbrand, 10 min at 30 000 RPM) to generate larger microcages or
shear mixed (benchtop shear mixer, Silverson L5 fitted with a square hole
high shear screen, 10 min, 6000 RPM) to generate small microcages.
The emulsion was then unidirectionally frozen in custom-made molds
(15 min), followed by freeze-drying for 48 h (0.075 mBar, −52 °C,
Labconco FreeZone 1). The resulting aerogels were thermally reduced
in a tube furnace (100 °C, 5 °C min−1, 30 min followed by 1000 °C,
5 °C min−1, 2 h) under an H2 (5%)/N2 atmosphere to facilitate network
crosslinking and to restore the graphitic properties of the CNTs.
Preparation of NPs@CNT Aerogels: DWCNT or MWCNT aerogel and
precursor (Cu(acac)2, Pd(F6acac)6, or Ru3(CO)12) (quantity based on desired
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weight loading onto aerogel support) were sealed in a Pyrex ampoule
(×10−6 mbar) on a custom-made high-vacuum filling rig. The ampoules
were submerged in an oil bath for 3 days at 130 °C followed by rapid cooling
through ampoule submersion into ice-water. The precursor@CNT aerogels
were then resealed in a new ampoule under N2 (50 mbar) and heated in a
muffle furnace (800 °C, 1 min) to form metal decorated CNT aerogels. The
same procedure was followed for the CNT powder supports.
Catalytic Amide Bond Forming Reaction: Acetonitrile (5 mL), dodecane
internal standard (0.09 mmol), N,N-dimethylaniline (0.24 mmol), and
acetic anhydride (0.29 mmol) were added to an RB flask. A sample
(0.2 mL) was taken from the flask and added to CDCl3 (0.03% TMS)
(0.8 mL) for 1H-NMR analysis (0 h sample). Tertbutyl hydroperoxide
(0.19 mmol) was then added followed by the addition of Cu@DWCNT
(8 mg), Cu@MWCNT (8 mg), or Cu nanopowder (0.64 mg) catalysts
(8 wt% Cu loading). The flask was then placed into an oil bath to
initiate the reaction (70 °C, stirring rate 100 RPM under air and standard
atmospheric pressure conditions). All further samples were taken with a
volume of 0.2 mL and mixed with 0.8 mL of CDCl3 (0.03% TMS).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors are grateful to Stuart Micklethwaite and the Meldrum
Research Group for the guidance and support. The authors wish to
acknowledge financial support by the Atomic Weapons Establishment
and by the Engineering and Physical Research Council (EPSRC, PhD
CASE award 1799924). The authors also like to acknowledge support by
the Diamond Light Source (beamline I20 proposal SP21167), Henry Royce
Institute, and Leeds Electron Microscopy and Spectroscopy Centre. The
Royce Ph.D. Equipment Access Scheme enabled access to the Henry
Moseley X-Ray Imaging Facility at the University of Manchester (EPSRC
grant EP/R00661X/1). The authors further acknowledge support from the
Henry Royce Institute (EPSRC grants: EP/P022464/1, EP/R00661X/1),
which funded the VXSF Facilities within the Bragg Centre for Materials
Research at Leeds. This publication is UK Ministry of Defence © Crown
copyright 2021.
Characterization: High-resolution transmission electron microscopy
(HR-TEM) was conducted on a FEI Titan3 Themis G2 operating at 300 kV
fitted with 4 EDX silicon drift detectors, multiple STEM detectors and
a Gatan One-View CCD camera. EDX spectroscopy and mapping were
undertaken using Velox software. A FEI Tecnai TF20 field-emission gun
transmission electron microscope (FEG-TEM) operating at 200 kV and
equipped with an Oxford Instruments 80 mm2 X-Max EDX detector
was also used to collect several global view images in order to obtain
NP size distributions. Microstructural features were analyzed using
a Hitachi SU8230 cold field-emission scanning electron microscope
(FE-SEM) equipped with an Oxford Instruments 80 mm2 X-Max energy-
dispersive X-ray (EDX) detector and a photodiode-backscattered electron
(PD-BSE) detector (2–15 kV). Microscopy images were processed using
Digital Micrograph (version 3.30.16.0). X-ray computed nanotomography
was performed using a Zeiss Xradia 810 Ultra with a chromium K alpha
source. It was operated at 5.4 keV and Zernike phase contrast was
achieved using a phase ring. The sample was centered and a total of 510
equi-angularly spaced X-ray projections were taken over a 180° rotation. An
exposure time of 90 s/projection was used giving a total acquisition time
of 12.75 h (exposure × projections). Volumetric data was reconstructed
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
with voxel size of 129 nm, using a filtered back projection algorithm. Keywords
X-ray computed microtomography was performed using a Zeiss Xradia
carbon nanotube aerogels, catalysis, emulsion templating, nanoparticle
functionalization
520 Versa, operated at 60 kV and 5 W. A 10× objective lens was used.
A total of 1201 X-ray projections were collected, over 360° rotation. An
exposure time of 15 s/projection was used giving a total acquisition time
of 5 h. Volumetric data was reconstructed with a voxel size of 410 nm,
using a filtered back projection algorithm. All tomography data processing
was conducted using Dragonfly (version 4.1.0.467) software. XANES
measurements were taken using the I20 high-resolution X-ray emission
spectrometer equipped with three Ge(555) analyzer crystals and using
the Si(111) monochromator crystal cut.[67] Solid samples were mounted
between two pieces of Kapton tape and were kept in place by a plastic
O-ring. The spectrometer was calibrated using a Cu foil, measuring
the Cu K-edge. X-ray photoelectron spectroscopy was undertaken on a
Kratos Axis Ultra (Kratos Analytical, UK) spectrometer equipped with a
monochromatic Al Kα source (1486.6 eV) and processed using CasaXPS
software (version 2.3.22). Surface area and porosity measurements were
made on a Micromeritics Tristar 3000 using the Brunauer–Emmett–
Teller and the Barrett–Joyner–Halenda methods. Nanocarbon aerogels
were degassed on a Micromeritics FlowPrep 060 at 120 °C under a
stream of N2 for 24 h. Nitrogen sorption isotherm measurements (at
a temperature of 77 K) were performed at a relative pressure (P/P0)
between 0.05 and 1 with 29 points of measurement in the adsorption
stage and 17 points in the desorption stage. Thermogravimetric analysis
(TGA) was performed on a Shimadzu TGA-50 in 50 mL min−1 of air flow
up to 900 °C (10 °C min−1 with a hold at 100 °C for 20 min). Raman
spectroscopy was conducted on a Renishaw InVia using a 532 nm laser
between 400 and 4000 cm−1. H-NMR was conducted on a Bruker AV3HD
400 MHz spectrometer with a Broadband Observe probe. NMR data was
processed using Mnova software (version 14.1.2).
Received: December 9, 2020
Revised: March 23, 2021
Published online:
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