Shape Engineering of Biomass-Derived Nanoparticles from Hollow Spheres to Bowls through Solvent-Induced Buckling

Article abstract of DOI:10.1002/cssc.201801215

The realization of asymmetric hollow carbonaceous nanostructures remains a great challenge, especially when biomass is chosen as the carbon resource through hydrothermal carbonization (HTC). Herein, a simple and straightforward solvent-induced buckling strategy is demonstrated for the synthesis of asymmetric spherical and bowl-like carbonaceous nanomaterials. The formation of the bowl-like morphology was attributed to the buckling of the spherical shells induced by the dissolution of the oligomers. The bowl-like particles prepared through this solvent-driven approach demonstrated a well-controlled morphology and a uniform particle size of approximately 360 nm. The obtained nanospheres and nanobowls were loaded with CoS₂ nanoparticles to act as heterogeneous catalysts for the selective hydrogenation of aromatic nitro compounds. As expected, the CoS₂/nanobowls catalyst showed good tolerance to a wide scope of reducible groups and afforded both high activity and selectivity in almost all the tested substrates.

Full text of DOI:10.1002/cssc.201801215

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Accepted Article
Title: Shape Engineering of Biomass-Derived Nanoparticles from
Hollow Spheres to Bowls via Solvent-Induced Buckling
Authors: Chunhong Chen, Xuefeng Li, Deng Jiang, Zhe Wang, and
Yong Wang
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To be cited as: ChemSusChem 10.1002/cssc.201801215
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Shape Engineering of Biomass-Derived Nanoparticles from
Hollow Spheres to Bowls via Solvent-Induced Buckling
Chunhong Chen+, Xuefeng Li+, Jiang Deng, Zhe Wang, Yong Wang*
Dedication ((optional))
Abstract: To realize the asymmetry for the hollow carbonaceous
nanostructures remains to be a great challenge, especially when
biomass is chosen as the carbon resource via hydrothermal
carbonization (HTC). Herein, a simple and straightforward solvent
induced buckling strategy is demonstrated for the synthesis of
asymmetric spherical and bowllike carbonaceous nanomaterials. The
formation of the bowllike morphology was attributed to the buckling of
the spherical shells induced by the dissolution of the oligomers. The
bowllike particles made by this solvent-driven approach demonstrated
a well-controlled morphology and a uniform particle size of ~360 nm.
The obtained nanospheres and nanobowls can be loaded with CoS2
nanoparticles to act as novel heterogeneous catalysts for the selective
hydrogenation of aromatic nitro compounds. With the bowllike
structure in hand, as expected, the CoS2/nanobowls catalyst showed
good tolerance to a wide scope of reducible groups and afforded both
high activity and selectivity in almost all the tested substrates (14).
Among the different types of asymmetric nanocontainers, the
carbonaceous materials possess additional advantageous
properties such as high chemical and thermal stability, good
electrical conductivity, and accessible interior space etc.[22-26]
Although tremendous efforts have been made in fabrication of
hollow spheres, the strategy for the synthesis of asymmetric
carbonaceous nanocontainers is still in imperfect start stage. Only
a few methods have been reported for the rational synthesis of
these materials and the most mature strategy of which is the
templating method with such as resorcinol-formaldehyde (RE)
resin as the carbon source and silica as hard template in
general.[27-29] Although these methods are conceptually
straightforward in producing nanocontainers, they suffer from
fussy and tedious operating procedures, high cost, toxic reagents,
30]
and thus industrial unfeasible.[28,
Therefore, novel synthetic
protocols, which are faster, cheaper, and greener, for the
fabrication of asymmetric nanocontainers are especially welcome.
With growing global environmental awareness and increasing
shortage of fossil resource, energy and sustainability issues
nowadays have been seriously considered in designing synthetic
strategies.[31] In this respect, materials chemists will be required
to develop novel technologies and materials which are more
efficient for the required applications and can minimize both short-
and long-term impacts on the environment. Therefore, increased
attention has been paid to the development of new and alternative
carbons from renewable resources. Compared with organic
polymer-derived (e.g., RE) carbon materials, biomass-derived
carbonaceous materials synthesized from the hydrothermal
carbonization (HTC) approach are of special interest in this
consequence, as they are easy-processable, highly economical,
and environmentally benign.[32-33] The HTC procedure is
performed from renewable resources (e.g., glucose) at low
temperatures (160-250 oC) in aqueous medium under self-
generated pressure. However, the HTC materials prepared in this
straightforward water-based method are commonly irregular,
micrometer-sized, spherical particles.[34-35] Our recent work
demonstrated that the asymmetric, hollow, open, carbonaceous
nanoflasks can be successfully fabricated by using a double
templates HTC method from biomass, and the resulting
asymmetric materials, flasklike carbon, exhibited a much higher
energy storage ability for supercapacitors compared with the
symmetric hollow counterpart. This study demonstrated the
importance of preparing materials with desired morphology. To
continue addressing critical challenges existed in synthesizing
HTC carbon materials, new strategies to access materials with
special morphology that could not be easily prepared previously,
for example nanobowls, are necessary.
Introduction
Nanocontainers with controlled dimensions and intriguing
morphologies have been regarded as an important family
member of functional materials and attracted a great deal of
attention in nanoscience and technology.[1-9] With the deepening
of the nanoscience, controllable synthesis of asymmetric
nanomaterials with special nonspherical geometric morphologies
and compositions, such as nanobowls or nanoflasks, forms a new
tendency, since these novel morphologies may endow new
functionalities to the nanomaterials and create new opportunities
in nanocatalysis or energy related fields.[10-17] Indeed, many
interesting phenomena, such as improved catalytic performance,
highly effective diffusivity, controllable and efficient encapsulation,
and up-taking capacity for big guest molecules, have been
observed already due to the unique single-hole hollow structural
features compared to their closed-shell counterparts.[18-21]
[a]
[+]
C.Chen, X. Li, J. Deng, Z. Wang, Prof. Y. Wang
Advanced Materials and Catalysis Group, Institute of Catalysis,
Zhejiang University
Hangzhou 310028, P. R. China.
E-mail: chemwy@zju.edu.cn
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
In this contribution, the difficult task of fabricating asymmetric
bowllike nanoparticles from cheap biomass was accomplished
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based on a “solvent induced buckling” method. The HTC process
was firstly improved via a double-templating method to assist the
formation of uniform asymmetric hollow sphere structure from
centrosymmetric hemispherical morphology as indicated by both
TEM and SEM images (Figure 1d, e). These bowllike materials
demonstrated a thickness of the shell and a diameter of hollow
cavity of about 68 nm and 222 nm, respectively (Figure 1f).
o
inexpensive, harmless and naturally available sugars at 170 C
followed by
a post-solvent washing treatment to get the
anisotropic bowllike nanoparticle. The unexpected versatile
powers of this anisotropic bowllike materials were quickly
recognized and contribute significant values to the products. For
example, this HTC carbon was successfully coupled with metal
salts (CoS2) to produce novel nanohybrids, which served as a
cheap and active catalyst for the selective hydrogenation of
aromatic nitro compounds.[36-37]
Results and Discussion
Figure 2. Shape evolution along with the hydrothermal reaction time: TEM
images of samples fabricated with 6 h (a, e), 9 h (b, f), 12 h (c, g), and 24 h (d,
h), respectively (a-d: samples were treated with deionized water and alcohol; e-
h: the reaction solution freeze-dried directly).
To reveal the specific roles of the solvent treatment, the shape
evolution was then studied by varying the hydrothermal reaction
time, and the corresponding TEM images are listed in Figure 2
and SEM images listed in Figure S2. For comparison, as indicated
in Figure 2, samples a, b, c, and d were treated with deionized
water first and then with ethanol, and e, f, g, and h were samples
freeze-dried directly after HTC. Images a, b, e, and f in Figure 2
are samples after 6, 9, 12, and 24 h HTC treatment, respectively.
Owing to the sluggish reaction kinetics of the glucose HTC, only
baby particles can be seen from these images, but even at this
initial stage, differences can be observed when compared images
b and image f in Figure 2. Obviously, Figure 2b demonstrated
obviously buckling (crumpled) structure whereas Figure 2f
showed hollow sphere morphology with one opening (Figure S2b).
When the HTC time was prolonged to 12 hours, the above
mentioned asymmetric hollow spheres and bowllike structure
were observed, respectively. However, further prolonging to 24
hours, the samples after ethanol treatment turned to be relative
uniform hollow spherical morphology, despite the freeze-dried
counterparts still showed an asymmetric hollow spherical
structure. These shape evolution against HTC time clearly
revealed that the ethanol treatment of the HTC samples is a key
factor that can control the final morphologies of the products. We
then deduced this bowllike morphology obtained by buckling of
the spherical shells. The buckling was induced by the dissolving
of the oligomers (formation during the glucose HTC process)
enclosed in the shells in the ethanol.[40-41] This may cause a stress
comparable to an anisotropic pressure on a spherical airproof
shell.[42] After post-buckling, the colloidal shells show bowllike
morphology. Besides, this buckling resulted shapes is also
affected by the HTC time. At the initial stage, for example 9 h, the
hollow sphere texture is fragile, therefore the dissolving of the
oligomers in the ethanol will lead to the collapse of the whole
structure. While 12 h seems to be the suitable time to form the
bowllike structure as both polymerization degree and shell
thickness will increase with the increase of the HTC time.
Figure 1. Morphologies characterization: SEM and TEM images for the non-
centrosymmetric nanospheres (a, b) and nanobowls (d, e); Statistical
distributions: the shell diameters and cavity diameters for nanospheres (c) and
nanobowls (f).
The HTC reactions were performed at 170 oC with glucose, a
cheap and renewable sugar, chosen as carbon source and Poly
(ethylene glycol)-block-poly (propylene glycol)-block-poly
(ethylene glycol) (EO20-PO70-EO20, P123) and sodium dodecyl
sulfate (SDS) as double templates. Post solvents treatment after
HTC is crucial for the formation of different morphologies as
demonstrated in Figure 1. After 12 h HTC treatment, uniform
hollow nanospheres with mean size around 340 nm are prepared
by washing with water for several times (Figure 1a, b, c). The
formation of these hollow nanospheres is mainly based on the
cooperative formation of P123/SDS/glucose composite micelles
under the acidic hydrothermal process.[38-39] It should be noted
that the ratio of P123 will affect the final morphology of the
products greatly, overloading of P123 would lead to the formation
of flasklike carbon (Figure S1) as reported previous.[10] However,
different from the hollow spheres reported previous, these hollow
spheres are non-centrosymmetric with a thinner shell about 57
nm, a thicker shell about 100 nm, and a cavity diameter around
181 nm, as indicated in Figure 1b and c. The formation
mechanism of these non-centrosymmetric structure will be
discussed later. Interestingly, when further washed with ethanol
after HTC and water washing, significant changes occurred,
thereby giving products with uniform bowllike non-
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Observed from the side of the bowllike structure, the interspace
after buckling can be further observed (Figure S3 c). However,
when further increasing the polymerization time, although some
oligomers inside the shells were dissolved in the ethanol, the
shells were still strong enough to resist the buckling press,
therefore the intact hollow sphere like structure can be kept
(Figure 2d). With this deduce of the oligomers inside the shells in
hand, the asymmetric hollow spheres structure (freeze-dried
directly after HTC, Figure 2g, h) can be reasonably attributed to
the flowing of the oligomers inside the hollow shells. We then
conducted the HTC reaction using a rotated homogeneous
reactor instead of the still ones, the obtained samples still
demonstrated asymmetric spherical morphology (Figure S4b).
However, when these samples were further ultrasonically treated
using water as solvent, obviously less asymmetric hollow shells
were yielded (Figure S4c and Figure S5). These controlled
experiments clearly indicated that the flowing oligomers inside the
shells caused the asymmetric hollow spheres structure.
degree of the colloid shells. Furthermore, due to the coating of
these oligomers, the decomposition rate of surfactants in
nanospheres obviously slower than that in nanobowls (Figure 3e,
Figure S6b). Solid state NMR results further convinced the
residual of oligomers in the nano-sphere as showed in Figure 3f.
The solvents as used were then extended to CH3OH, CH3CN, and
C2H4(OH)2, as showed in Figure S9, all these solvents lead to the
formation of bowllike structure like ethanol. However, if the
alcohols with longer side alky chain were used (such as C4H9OH,
and C5H11OH), with the decreased oligomers solubility, both
nanobowls and nanospheres were observed (Figure S10).
Figure 4. Schematic illustration of the different shape evolution paths via post-
treatment of solvents.
On the basis of the above study, we proposed that the formation
of the nanobowls and nanospheres in Figure 4. In the first stage,
P123 and SDS acted as double templates and induced the
formation of hollow spheres structures during the acidic HTC
reaction. With different HTC time, the polymerization degree of
the glucose will be different (Figure S11), and therefore leading to
different morphology with or without ethanol treatment. For
example, 9 h HTC of glucose and then washed with ethanol leads
to the formation of crumpled structure for the dissolution of the
large amount of oligomers at this early stage. When prolong the
HTC time to 12 h, asymmetric hollow spheres are prepared with
the help of the flowing oligomers inside the hollow spheres.
Dissolving of the oligomers in ethanol induced the buckling of the
shells and therefore the formation of asymmetric bowllike
morphology. Simultaneously, the HTC time is also a key factor to
affect the morphology of the final products, if the HTC time was
prolonged to above 24 h (with enough high polymerization
degree), even with the treatment of ethanol, only hollow spheres
can be observed for the high carbonization degree of the shells.
To illustrate the usefulness of these novel materials with
different morphologies, CoS2 nanopartilces on nanospheres
(CoS2/NS) and nanobowls (CoS2/NB) were then synthesized and
selective hydrogenation of aromatic nitro compounds was tested.
Powder X-ray diffraction (XRD) pattern of CoS2/NS and CoS2/NB
validated the formation of high crysalline CoS2 pyrite (JCPDS 41-
1471). The morphologies of CoS2/NB and CoS2/NS composite
materials have also been investigated by TEM (Figure 5). From
the TEM images well-dispersed CoS2 particles with mean size of
10.4 nm (CoS2/NB) and 14.8 nm (CoS2/NS) were found (Figure
5). It should be noted that the original morphology nanobowl and
nanosphere remained well after coupling with CoS2. The textural
Figure 3. Morphological evolution: samples obtained by (a) freeze-dried
reaction solution directly, (b) filtered with ultrapure water, and (c) further filtered
with ethanol. Note: the inset pictures are the water filtrate (b) and ethanol filtrate
(c). Characterization: the comparison of the FTIR spectra (d), the TGA (e), and
13C direct polarization spectra (f) between the nanospheres and nanobowls.
Morphological evolution with the types of solvents were then
examined in detail. After 12 h HTC treatment, the samples were
divided into three parts, one was freeze-dried directly, another
was filtered and washed with water, and the others were filtered
and washed with both water and ethanol. The TEM images of
these samples are listed in Figure 3. As expected, the samples
without washing or washing with only water gave asymmetric
spherical morphology, and the sample washed with additional
ethanol demonstrated bowllike structure. It should be noted that
the filtrates showed totally different color (Figure 3, inside image
b and c), the water solution show pale yellow color and the ethanol
solution gave brownish red color. These color differences may
cause by the different dissolving ability of oligomers into the two
different solvents. As an organic solvent, ethanol is a good solvent
for oligomers, therefore more oligomers will be removed away by
ethanol from the HTC products, and the residual samples should
be the polymers with high polymerization degree. IR spectrum of
the two structures were then conducted for comparison. The IR of
the nanobowls showed increased aromatic C-H and C=C peaks
but decreased aliphatic C-H peaks com-pared with the
nanospheres counterparts, indicating different polymerization
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