The dual role of micelles as templates and reducing agents for the fabrication of catalytically active hollow silver nanospheres

Article abstract of DOI:10.1039/c4cc07872a

We report a simple and efficient protocol for fabrication of colloidal hollow silver nanospheres of size less than 30 nm using an ABC triblock copolymer poly(styrene-b-vinyl-2-pyridine-b-ethylene oxide) in the absence of any reducing agents. The colloidal silver hollow nanoparticles serve as an efficient heterogeneous catalyst for Baeyer-Villiger oxidation of ketones to the corresponding lactones in the presence of anhydrous tert-butylhydroperoxide under liquid-phase conditions.

Full text of DOI:10.1039/c4cc07872a

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Dual role of micelles as templates and reducing agent for the fabrication of
catalytically active hollow silver nanospheres
∗
∗
Manickam Sasidharan,a Chendrayan Senthil,a Vandana Kumarib and Asim Bhaumikb
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
challenging task to fabricate silver hollow nanospheres with a
few tens of nanometer with uniform particle size.
We report a simple and efficient protocol for fabrication of
colloidal hollow silver nanospheres of size less than 30 nm
using ABC triblock copolymer poly(styrene-b-vinyl-2-
pyridine-b-ethylene oxide) in the absence of any reducing
10 agents. The colloidal silver hollow nanoparticles serve as an
efficient heterogeneous catalysts for Baeyer-Villiger oxidation
of ketones to the corresponding lactones in the presence of
anhydrous tert-butylhydroxide under liquid-phase conditions.
Recently, a variety of inorganic hollow spherical nanospheres
60 were reported using asymmetric triblock copolymeric micells
with core-shell-corona architecture.8 The unique feature of these
micelles is that the core acts as a template for void formation,
shell domain serves as reaction site for inorganic precursors, and
corona stabilizes the composite micelles. Herein, we report an
15 Inorganic nanostructures with hollow interiors have been 65 efficient synthesis strategy for fabrication of uniform silver
extensively studied for tailoring the properties involving
applications like drug delivery, catalysis, surface enhanced
Raman scattering (SERS), and sensors.1 Noble metal
nanoparticles, in particular, silver has an interesting properties
20 that could be tuned or enhanced through the nanoscale control of
morphology. The synthesis of silver nanoparticles with well-
controlled morphology and a narrow size distribution is important
for uncovering their properties and achieving their practical
applications. Hollow nanostructures of metals are intriguing in
25 research because they exhibit surface plasmonic properties and
catalytic activities superior from their solid counterparts.2 For
example, silver is the most popular catalyst for oxidation of
ethylene to ethylene oxide and methanol to formaldehyde or
reduction of nitroarenes.3 Metal hollow nanoparticles have high
30 surface area to volume ratio vis-à-vis binding sites which are
mainly responsible for their catalytic activity in various organic
transformations. Given the range of applications in which silver is
important, better control of its morphology, size and properties
hollow nanospheres of size 29 ± 2 nm templated by micelles of
poly(styrene-b-vinyl-2-pyridine-b-ethylene oxide) (PS-PVP-
PEO) using AgNO3 as metal source in the absence of external
reducing agents under hydrothermal conditions. The
70 poly(ethylene oxide) block similar to ethylene glycol effectively
reduces AgNO3 at elevated temperature7a whereas the PVP
domain acts as capping agent for metallic silver and subsequent
shell formation. The synthesized silver hollow nanospheres acts
as an efficient heterogeneous catalyst for Baeyer–Villiger
75 oxidation, where various value-added lactones and esters can be
prepared in one pot from their respective carbonyl compound in
the presence of peroxides.9
The triblock copolymers with chain length of PS(20.1k) –
80 PVP(14.2k) – PEO(26k) was purchased from Polymer Source
Inc. and the number in the parentheses refer to molecular weights
of respective blocks. In a typical preparation of micelles, the
polymer was dissolved in a mixture of dimethylformamide
(DMF) and water and dialyzed against water to obtain micelles
85
would have profound impact in academia and industries.
35
Hollow metal nanospheres are generally fabricated by coating
the surfaces of colloidal particles (e.g., silica beads, polymer
latexes etc.) with thin layer of desired metal precursors followed
by selective removal of the colloidal templates through
40 calcinations or wet chemical etching.4 Caruso et al. adapted
different strategy which involves layer-by-layer adsorption of
polyelectrolytes and charged metal colloids to form shell-like
structures around colloidal template particles to hollow spherical
particles.5 Mann and co-workers described a self-supporting
45 porous framework of silver using dextran as a sacrificial
template.6 Recently, there are few reports about the preparation of
metal nanoparticles using soft templates and liposome.7 However,
hollow spherical particles prepared by the above methods are
associated with problems such as rough surfaces,
50 polycrystallinity, non-uniformity in shell thickness, poorly
defined composition, and difficulty in removing the colloidal
90
95
Scheme 1. Fabrication of silver hollow nanosphere using PS–
templates without breaking the shells. More importantly, the 100 PVP–PEO.
with concentration of 1 gL-1 (Experimental details, ESI†). Scheme
1 illustrates the typical formation of polymeric micelles and silver
hollow nanospheres by redox reactions. Dynamic light scattering
hollow metal spheres obtained by these methods ranges from few
hundred nanometers to a few micrometers and difficult to obtain
55 hollow nanospheres of size less than 50 nm. Therefore, it is still a
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and electrophoretic light scattering experiments of micelles
revealed the values of hydrodynamic diameter Dh (63 ± 2 nm),
and the ζ-potential (+ 41 mV), respectively. However, TEM
observation of the micelles stained with phosphotungstic acid
suggested a nearly monodispersed spherical micelles with
average diameter ca. 38 ± 2 nm (Fig. 1A). The observed size
discrepancy in the micelle particles’s between DLS and TEM is
due to the fact that the latter accounts for only the core-shell part
and excludes the corona part of the micelles.8 The white contrast
volume and shell thickness. The void volume expanded to 21 ± 1
nm using polymer with increased PS block length (45k) whereas
the shell thickness increased to 11 ± 0.5 nm by using high silver
precursors, PVP/Ag = 15 (Fig. S4 & Experimental sectionn;
70 ESI†). Though the polymer micelles, Ag/polymer (Fig.1 A & B)
are less prone to aggregation, the final Ag-hollow particles
exhibit pronounced agglomeration (Fig. S5, ESI†) due to very
high free-energy of nanoparticles. The high resolution TEM
image (Fig. 1D) reveals that the silver hollow nanoparticles are
5
10 in the TEM image indicates hydrophobic PS-core whereas both 75 highly crystalline and the fringe spacing of 2.4, 2.0, and 1.4 Å
PVP and PEO are hydrophilic domains. On addition of AgNO3
(PVP/Ag =7) to the micelle solution at pH 4 at room temperature,
the transparent clear micelles turn to light brown color (Fig. S1,
ESI†) during the commencement of nucleation and growth of
correspond to the (111), (200), and (220) planes, respectively of
face-centered cubic structure.14 The well-resolved interference
fringe patterns confirm the single crystalline structure of silver
hollow nanospheres. The selected area electron diffraction pattern
15 nanoparticles and the PVP units serve as capping agent, whereas 80 (SAED, Fig. 1D, inset picture) from the same nanoparticles also
hydrophilic poly(ethylene-oxide) of the template micelles
facilitates reduction of Ag+ to Ag. On treatment of the resulting
colloidal suspension at elevated temperature (120 °C) under
hydrothermal conditions further facilitates the reduction as well
20 as assembly of nanostructures. The polyethylene glycol unimers
reduce the silver ions and this reduction behavior of polyethylene
glycol have already been well established for synthesis of silver
nanostructures with different morphologies.7a Therefore,
polyethylene glycol domain of the template micelles play a dual
25 role as stabilization of micelle architecture as well as reducing
agents similar to polyvinyl alcohol (PVA) which simultaneously
reduce metal ions and also acts as capping agents.10
indicates its high crystallinity. The powder XRD pattern (Fig. S6,
ESI†) also exhibits (111), (200) and (220) peaks and all the
diffraction peaks can be indexed to cubic face centered silver
(JCPDS 04-0783) and no obvious impurity phase, such as Ag2O
85 is detected.15 The BET surface area estimated from nitrogen
adsorption isotherms was found to be 67 m2 g-1. High BET
surface area of the synthesized hollow nanoparticles has
motivated us to explore their catalytic activity in Baeyer-Villiger
(BV) oxidation reaction under liquid-phase reaction conditions.
90
Figure 1B shows the TEM image of the polymer/silver
30 composite micelles and the average size of the particles were
found be 51 ± 3 nm. The thermal analyses (TG/DTA) of
polymer/silver composite particles revealed about 13 wt.%
weight loss for as-synthesized samples (Fig. S2, ESI†). It is worth
to note that a part of organic polymers was washed away during
35 initial centrifugation and washing with ethanol. The
polymer/silver composite particles were further extracted with
dimethylformamide or toluene at 100 °C for 12 h under stirring to
remove the remaining polymeric template and obtain template-
free silver hollow nanospheres. The formation of Ag nanocrystals
40 was characterized by UV-vis spectra which illustrate the Plasmon
resonance peak intrinsic to silver nanoparticles at different period
of time (Fig. S3, ESI†).11 After commencement of reaction at
room temperature (at 2 hour), the solution gave a weak peak
centered at 418 nm suggesting presence of very little Ag
45 nanoparticles (Fig. S3, A). Further treatment of reaction solutions
at elevated temperature (110 °C) for 8 h resulted in very high
intensity peak with similar band width indicating substantial
increase in the concentration of Ag nanoparticles (Fig. S3, B) and
further extended period of reaction did not increase absorption
50 peak intensity (Fig. S3, C). Thus Ag particles formation and their
distribution appears to depend on varying extent of chemical
reaction as well as a digestive processes at high temperature.12
Fig.1. TEM images of nanoparticles: (A) PS-PVP-PEO micelle; (B)
Ag/polymer composite particles; (C) Ag hollow nanospheres and (D)
HRTEM and SAED of sample C.
95
The Bayer-Villiger oxidation is a very important organic
reactions for production of industrially important lactones and
esters from their corresponding carbonyl compounds in the
presence oxidizing agents.16 Scheme 1 depicts the Ag hollow
nanospheres catalyzed Baeyer-Villiger oxidation under liquid
Fig. 1C shows TEM image of Ag hollow nanospheres and
55 average particle size and void space diameter were found to be 29
± 2 nm and 15 ± 1 nm, respectively. The wall thickness estimated
from TEM observation was approximately 7 ± 0.5 nm. The PS
block core size estimated from TEM image (white contrast) was
found to 25 nm ± 1 nm (Fig. 1A) and size of the composite
60 particle increases to 51 ± 3 nm on treatment with AgNO3;
however, after solvent extraction of the composite particles in
DMF, the particle size shrinks and the void space diameter was
found to be 15 ± 1 nm, resulting in shrinkage of about 40 % of
initial size similar to metal oxide hollow nanospheres.13 The
65 versatility of this method was also demonstrated by tuning void
100 phase conditions and data for various cyclic and acylic substrates
are given in Table 1. Among the solvents toluene, THF, and
acetonitrile (entries 1–3) employed, acetonitrile exhibited
consistently good activity consistent with earlier reports17 and
therefore used as solvent for subsequent investigation. The
105 anhydrous tert-butylhydroperoxide (TBHP) exhibited higher
reactivity than benzaldehyde/O2 system under similar conditions
(entries 3 and 4) for conversion of cyclohexanone to ε-
caprolactone. It is likely that the exposed silver surface atoms
may be poisoned and/or converted to Ag2O in presence of oxygen
2
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aSRM Research Institute, SRM University, Kattankulathur, Chennai,
45 603203, India, E-mail:sasidharan.m@res.srmuniv.ac.in.
results in diminished reactivity. Entries 5 and 6 exhibit difference
in migratory aptitude of substituted cyclohexanones; 2-
methylcyclohexanone yields 7-methyloxepane-2-one as the sole
product whereas 3-methylcyclohexanone leads mixture of
lactones (ca. 6-methyloxepan-2-one and 4-methyloxepan-2-one)
b Department of Material Science, Indian Association for the Cultivation
of Science, Jadavpur, Kolkata 700 032, India, E-mail: msab@iacs.res.in
† Electronic Supplementary Information (ESI) available: [Preparation of
micelles and hollow nanospheres. TEM, SEM, XRD, TG-DTA, and UV-
5
similar to literature reports.18 Cycloheptanone forms the 50 visible data]. See DOI: 10.1039/b000000x/
Acknowledgements. We thank SERB-DST for their partial research
supports (Grand nos. SB/S1/PC-043/2013 and SR/S1/IC-61/2012).
corresponding lactone at high turn-over number (entry 7). The
acyclic methyl ethyl ketones and isobutyl methyl ketone also lead
to formation of corresponding ester in very high yields (entries 8
1
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10 and 9). Entry 10 shows the activity of Ag nanoparticles of size 8
± 1nm that exhibits lower activity than Ag hollow particles
possibly due to presence of PVP stabilizer (Fig. S7 ESI†) .
Furthermore, we have also scrutinized the micron-sized Ag
hollow nanoparticles (entry 11, Fig. S8, ESI†) synthesized by
15 layer-layer technique realized lower activity due to less catalytic
binding sites of larger particles. We have recycled the catalyst for
cyclohexanone oxidation for 5 repetitive cycles after activating
the catalyst with nitrogen containing 5 % H2 at 120 °C for 3 h and
Ag particles also retain the crystallinity (Fig. S9 ESI†). In the fifth
20 cycle the conversion and TON were nearly maintained compared
to the pristine catalysts under similar conditions (entry 12).
55
60
65
70
75
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Table 1. Baeyer-Villiger oxidation of ketonesa
O
O
Ag-catalyst, 85 oC
O
4
5
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6
7
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Conversion, TON
mole %
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1b
2c
3d
4e
5
6
7
8
9
Cyclohexanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
3-methylcyclohexanone
4-methylcyclohexanone
Cycloheptanone
Ethyl methyl ketone
Methyl isobutyl ketone
Cyclohexanone
63.8
78.3
98.8
83.0
93.2
95.6
91.7
96.5
92.8
74.7
74.5
98.1
10.4
12.7
16.1
13.5
15.0
15.4
14.9
15.6
15.0
12.1
13.0
15.8
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10f
11g
12h
Cyclohexanone
Cyclohexanone
25 aReaction conditions: ketone = 3 mmol (0.28 g); tert-butylhydroperoxide
= 3.3 mmol (70 % anhydrous solution), acetonitrile = 5 mL, 10 wt %
catalysts with respect to substrate, 0.1 mmol naphthalene was used as
internal standard, Reaction time = 4 h and temperature 85°C and the
reaction was carried out in N2 atmosphere. btoluene solvent; cTHF solvent
30 dacetonitrile solvent; cbenzaldehyde/O2 as oxidant; fDense silver
nanoparticles ( 8 ± 1 nm) as catalysts; ghollow silver microsphere as
catalysts; and hreused catalyst after 5 cycles.
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In conclusion, we have demonstrated novel strategy for
efficient and simple fabrication of silver hollow nanospheres
35 using PS-b-PVP-b-PEO polymeric micelles with core-shell-
corona architectures. The present method enables us to prepare
hollow spherical particles of size less than 30 nm with high
crystallinity, size tenability, and monodispersity. The silver
hollow particles efficiently catalyze various cyclic- and acyclic-
40 ketones to the corresponding lactones and esters at high yields
under liquid-phase conditions, suggesting future potential of this
unique Ag nanoarchitecture in efficient catalysis.
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Notes and references
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