Yolk@Shell Nanoreactors Carrying a Cluster of Metal Nanocrystals Stabilized Inside the Hollow Carbon Shell

Article abstract of DOI:10.1002/bkcs.12280

Synthesis and stabilization of ultrasmall metal nanocrystals at high temperature have always been challenging due to their self-aggregating behavior. Here, we introduce a strategy to synthesize a cluster of tiny metal nanocrystals (~3 nm) inside a hollow carbon shell as yolk@shell-type architecture following a metal-coordination-based nanocrystal stabilization at high temperature (500?^°C). The successful catalytic reduction of 2-amino-4-nitrophenol demonstrates the efficiency of such nanoreactor in chemical transformation. Further adaptation of this strategy helps in synthesizing bimetallic (Au/Pt, Au/Pd, and Au/Ru) nanocrystals, which unfolds the possibilities to design advanced new-generation catalysts.

Full text of DOI:10.1002/bkcs.12280

Communication
DOI: 10.1002/bkcs.12280
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A. Acharya et al.
KOREAN CHEMICAL SOCIETY
Yolk@Shell Nanoreactors Carrying a Cluster of Metal Nanocrystals
Stabilized Inside the Hollow Carbon Shell
*
Anubhab Acharya, Amit Kumar, and In Su Lee
Creative Research Initiative Center for Nanospace-confined Chemical Reactions (NCCR) and
Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673,
South Korea. *E-mail: insulee97@postech.ac.kr
Received March 10, 2021, Accepted April 7, 2021
Synthesis and stabilization of ultrasmall metal nanocrystals at high temperature have always been chal-
lenging due to their self-aggregating behavior. Here, we introduce a strategy to synthesize a cluster of tiny
metal nanocrystals (~3 nm) inside a hollow carbon shell as yolk@shell-type architecture following a
ꢀ
metal-coordination-based nanocrystal stabilization at high temperature (500 C). The successful catalytic
reduction of 2-amino-4-nitrophenol demonstrates the efficiency of such nanoreactor in chemical transfor-
mation. Further adaptation of this strategy helps in synthesizing bimetallic (Au/Pt, Au/Pd, and Au/Ru)
nanocrystals, which unfolds the possibilities to design advanced new-generation catalysts.
Keywords: Nanoreactor, Au nanocrystal, Hollow carbon, Nanocatalyst, Yolk@shell structure
Yolk@shell-type nanoarchitectures with noble metal core
inside hollow nanostructures have unveiled significantly
superior catalytic display with respect to normal core@shell-
type nanostructure.1–3 The fascinating interior cavity with
mobile core particles and void-space within the shell material
have notable contributions toward exposing greater number
of active sites from catalyst and better interactions with the
incoming reactants, which are key characteristics of
advanced nanoreactor design for catalysis, drug delivery,
electrocatalysis, sensing, and others.4–8 Considering the size
or morphology of the catalytic species, large-sized solid
cores of metal nanoparticles are mostly selected in such
yolk@shell-shaped catalytic nanoreactor.9 Extensive pro-
gress has been made so far for presenting improved activity
by utilizing ultrasmall metal nanocrystals (NCs), which have
been owing to their much increased catalytic surface area
compared to bulky core particles. Whereas, such systems are
rarely attempted as core part in yolk@shell catalysts, unless
proper nanoconfinement for stabilization was accomplished
under the given reaction condition.10,11 Generally, due to
their self-aggregating nature toward lowering the overall sur-
face energy, efforts to synthesize stable tiny NCs are mostly
guided by two major stabilizers: (1) capping agents and
(2) solid support.12–14 However, surfactant- or ligand-based
stabilization techniques for the ultrasmall NCs have encoun-
tered the deficiency in exposed catalytically active sites
through the blocking by the surface-ligands, required thermal
treatment for the removal of organic moieties, and thus, con-
sequently experienced poor catalytic efficiency.15 Therefore,
stabilization of tiny catalytic NCs with the assistance of solid
supports is regarded to be more effective method for
targeting efficient and durable catalysis. However, develop-
ing suitable support materials such as silica, carbon or metal
organic framework have several other challenges for the in-
house ultrasmall catalytic NCs such as, high thermal suscep-
tibility of the metal NCs toward sintering, and low NCs
loading in the final catalyst due to lack of proper coordinat-
ing sites and unsuitable pore structures for the diffusion of
substrate molecules by the support materials.16–18
Herein, we present a yolk–shell like nanoarchitecture
containing a stable cluster of tiny Au and Au alloy nano-
crystals inside hollow and porous carbon shell by
addressing the major challenge of synthesizing such
nanoreactor that involves the stabilization of surface-
ligand-free small NCs against any kind of potential
ꢀ
sintering even at high-temperature treatment (500 C) dur-
ing the carbon shell forming process. The spherical hollow
carbon shell featuring porous architecture, chemically
robust and confined platform to elevate the stability of the
clustered catalytic NCs, which was proved by the efficient
catalysis for the reduction of 2-amino-4-nitrophenol.
As an attempt to fabricate yolk–shell like nanoreactor
involving aggregated tiny Au NCs as core inside hollow car-
bon to acquire superior mass utilization of the catalytic core
than usual large-sized solid particles (Figure 1(a)). This
research is driven by the stabilization of ultrasmall Au NCs
under any reaction condition required for designing the
custom-made
catalysts.
Aminated-silica
(NH2SiO2)
nanospheres were employed as a template for depositing Au
NCs as providing them with requisite stabilization. First,
NH2SiO2 nanosphere of diameter [d] = 138 Æ 10 nm was
synthesized following a revised reverse-microemulsion tech-
nique using N-[3-(trimethoxysilyl)propyl]ethylenediamine
(TMSD) into a water-in-cyclohexane emulsion containing
tetraethyl orthosilicate (TEOS) and 3-(2-aminoethylamino)-
propyltriethoxysilane (TESD) mixture. Notably, the
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Figure 1. (a) Schematic illustration for the synthesis of Au NCs inside hollow Carbon shell. TEM images of the nanoparticles in stepwise
evolution containing (b) aminated silica (NH2SiO2), (c) Au deposited aminated silica (Au@NH2SiO2), (d) polydopamine coating
(Au@NH2SiO2@PDA), (e) high temperature-based reductive annealing (Au@SiO2@Carbon) and (f) chemical etching of silica from
core (Au@h-Carbon).
roughened NH2SiO2 spheres have inherent porosity
(~5 Æ 1 nm), as calculated from the corresponding transmis-
sion electron microscopy (TEM) image (Figure 1(b)). Next,
aqueous suspension of NH2SiO2 (10 mg/mL) with HAuCl4
(15 mM) forming yellow colored colloidal solution, before
quick injection of sodium borohydride (NaBH4) (100 mM)
as reducing agent was performed for Au NC formation. In
result, sudden change to deep brown color of the solution
was accompanied with homogeneous deposition of reduced
Au(0) species on the porous NH2SiO2. As shown in
Figure 1(c), Au NCs (d = 3 Æ 1 nm) are uniformly depos-
ited in both interior and exterior part of the NH2SiO2
nanospheres (Au@NH2SiO2). Interestingly, the nanopores
of NH2SiO2 are now occupied by the as-formed tiny Au
NCs which are possibly anchored through amine( NH2)
groups present in the silica architecture. Besides, absence
of free Au-growth outside and the undamaged spherical
morphology of NH2SiO2 have supported the usefulness of
our synthetic strategy for stabilizing the individual tiny
NCs over the aminated-silica surface.
Next, Au@NH2SiO2 were put through the polydopamine
(PDA) coating process via dopamine-polymerization tech-
nique that includes the immersion of aqueous dispersion of
Au@NH2SiO2 (10 mg/mL) in an air-exposed basic tris-
buffer solution (pH 9.0) containing dopamine solution
(5 mg/mL) under a mild vortex. The resultant black colored
solid of Au@NH2SiO2@PDA containing 20 Æ 1 nm-thick
smooth and continuous PDA-shell over each
Au@NH2SiO2 was noticed through TEM analysis
(Figure 1(d)). Apart from the size increase by ~20 nm, any
discernible change was not recognized for the core particles
after PDA coating, including the size of the Au NCs. The
shell thickness of PDA coating could be adjusted (5, 10, and
20 nm) by simply varying the amount of injected dopamine
solution (50, 100, and 200 μL, respectively) without hamper-
ing the pristine Au@NH2SiO2 structure (Supporting
Information Figure S1). Apparently, the ultraviolet-visible
(UV-Vis) absorbance peak for Au NCs in
Au@NH2SiO2@PDA was red-shifted from 526 to 545 nm
with increasing PDA-shell thickness due to the light absorp-
tion by PDA shells (Supporting Information Figure S2). In
control, an attempt to coat with tannic acid has resulted only
very thin TA-layer (~3 nm) despite several trials to increase
the shell thickness with varying the TA amount. Thus, high
compatibility toward easy and tunable coating on
Au@NH2SiO2 nanospheres by PDA is evident over other
polymeric coating.
By annealing the Au@NH2SiO2@PDA powders (with
ꢀ
20 nm-thick PDA shell), at 500 C under a flow of Ar + 4%
H2, the carbonization of PDA to nitrogen (N)-doped carbon
shell was induced for synthesizing Au@SiO2@Carbon.19
After the calcination, the well-isolated nanoparticles have
maintained their spherical shape, managed to get slightly
roughened surface and are shrunk by 5 nm in shell thickness
due to the decomposition of organic functional groups present
in PDA-shell at annealing temperature (Figure 1(e)). Notably,
the average size and distribution of the Au NCs inside the car-
bon shell remain unaffected in Au@SiO2@Carbon. Here,
the intended strong coordination of Au with the amine
( NH2) groups of dopamine molecule of PDA-shell are cru-
cial for stabilizing the ultrasmall Au NCs during the thermal
conversion of PDA into N-doped carbon shell. However, the
annealing with relatively thinner PDA-shell (~5 nm) resulted
the sintering of escaped Au NCs into larger crystals patched
at the outside due to lack of chemical interaction (Supporting
Information Figure S3). In addition, such a high temperature
annealing has induced the loss of organic moieties in
NH2SiO2; thus generating small pores in silica, which are
beneficial to provide the stability to tiny Au NCs in a con-
fined environment against any kind of diffusion of NCs or
merging into larger particles even at high annealing tempera-
ture.20 Control reaction involving the reductive annealing at a
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higher temperature (700 ꢀC) led to an increase in Au NCs size
(d = 6 Æ 2 nm) due to the promoted metal migration
followed by sintering at elevated temperature (Supporting
Information Figure S4). The annealing ꢀ with TA-coated
Au@NH2SiO2 (~3 nm thickness) at 500 C also gave the
sintered Au NCs (6 Æ 1 nm size) (Supporting Information
Figure S5).
physically attached together in a ligand-free environment
supported by the N-doped carbon shell (Supporting Infor-
mation Figure S7). As shown in Figure 2(c) and (d),
Au@h-Carbon, on treatment with HCl (0.01 M) at room
temperature, has experienced the tiny Au NCs to be trans-
formed into large dimension (9 Æ 2 nm) (Supporting Infor-
mation Figure S8), and the corresponding time-course
study reveals that such acid catalyzed reshaping of Au NCs
are very fast and resultant particles’ size remains stable
even after long reaction time up to 24 h (Supporting Infor-
mation Figure S9). However, TEM images after increasing
HCl concentration demonstrated that the relative size of the
transformed NCs could be increased proportionately, for-
ming Au-lumps (~60 nm) at higher (1 M) HCl concentra-
tion (Supporting Information Figure S10). Such
transformations may be reasoned by the Au NCs dissolu-
tion followed by reshaping caused by infused chloride ions
into the carbon shell.21 In order to evaluate the catalytic
performance of such clustered Au NCs of Au@h-Carbon,
we performed the reduction of 2-amino-4-nitrophenol (NP;
1.2 mM) by Au@h-Carbon catalyst.22 The real-time moni-
toring of the reduction process in every 10 min interval for
the next 1 h revealed the gradual discoloration of the deep
yellow solution with successive lowering of UV-vis absor-
bance spectra at 443 nm and evolution of new peak at
323 nm indicating the formation of 2,4-diaminophenol
(AP) (Figure 3). The efficient catalytic performance demon-
strates the high enough catalytic activity of Au NCs even
in the clustered state and the permeable nature of carbon
shell allowing the reactants/products to pass through while
protecting the catalytic Au-core from dissolution or
leaching during catalysis.
Next, Au@SiO2@Carbon was treated in hydrogen fluo-
ride (HF) solution, which completely removed the solid sil-
ica core and left
a spherical hollow carbon shell
(d ~70 Æ 10 nm and thickness ~20 nm) containing tiny Au
NCs inside (Au@h-Carbon). As shown in Figure 1(f), the
surfactant-free Au NCs are fully settled down forming an
aggregate inside the concave hollow surface of carbon,
while the size (3 Æ 1 nm) and shape of the individual NCs
remain unaltered. The product, Au@h-Carbon was further
characterized by energy dispersive X-ray spectroscopy
(EDS)-based elemental mapping to verify the stable aggre-
gates of Au NCs (Figure 2(a) and (b)). As per the high–
resolution TEM (HRTEM) image, each Au NCs are single
crystalline, showing 2.3 Å lattice fringe for (111) plane of
the stable metallic Au (0). The UV-Vis absorption spectra
of Au@h-Carbon suggests the broad absorption of light
by the resultant carbon shell to hamper the usual UV-Vis
adsorption of the inner Au NCs, while Au peak was still
visible in Au@NH2SiO2@PDA (Supporting Information
Figure S6). Furthermore, the stability of these uniformly
aggregated NCs inside carbon shell were tested against var-
ious chemical environments. Reaction mixtures of Au@h-
Carbon with ethanedithol and disodium sulfide have
resulted in scattered Au NCs distribution with increase in
inter-particle distance implying that Au NCs are just
Inspired by this result, we next extended our protocol
toward introducing other various catalytic noble metals
(M = Pt, Pd, Ru) within the h-carbon shell as bimetallic
hybrids [M]-Au@h-Carbon containing different atomic%
ratio (EDS-estimated) of M and Au (Supporting Informa-
tion Figure S11 and S12). Interestingly, when [Ru]-Au@h-
Carbon was reacted with HCl solution (0.01 M) at RT, Au
NCs were reshaped into larger size while Ru NCs were
remain unaffected and managed to link to the Au particles
forming satellite-like nanostructure. Corresponding TEM
image, STEM-EDS elemental mapping and line profiling
Figure 3. (a) Catalytic reduction of 2-amino-4-nitrophenol (NP) to
2,4-diaminophenol (AP) by Au@h-Carbon through decolouration
of yellow solution. (b) Time dependant UV-Vis spectra of the
reduction reaction.
Figure 2. TEM and STEM-EDS-elemental mapping images of
Au@h-Carbon (a and b) before and (c and d) after HCl
treatment.
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Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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Acknowledgment. This work was supported by the Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Sci-
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