Development of Hierarchical Polymer@Pd Nanowire-Network: Synthesis and Application as Highly Active Recyclable Catalyst and Printable Conductive Ink

Article abstract of DOI:10.1002/open.201600009

A facile one-pot approach for preparing hierarchical nanowire-networks of hollow polymer@Pd nanospheres is reported. First, polymer@Pd hollow nanospheres were produced through metal-complexation-induced phase separation with functionalized graft copolymer

Full text of DOI:10.1002/open.201600009

DOI: 10.1002/open.201600009
Development of Hierarchical Polymer@Pd Nanowire-
Network: Synthesis and Application as Highly Active
Recyclable Catalyst and Printable Conductive Ink
[a]
Sajjad Husain Mir and Bungo Ochiai*
A facile one-pot approach for preparing hierarchical nanowire-
networks of hollow polymer@Pd nanospheres is reported. First,
polymer@Pd hollow nanospheres were produced through
metal-complexation-induced phase separation with functional-
ized graft copolymers and subsequent self-assembly of PdNPs.
The nanospheres hierarchically assembled into the nanowire-
network upon drying. The Pd nanowire-network served as an
active catalyst for Mizoroki–Heck and Suzuki–Miyaura coupling
reactions. As low as 500 mmol% Pd was sufficient for quantita-
tive reactions, and the origin of the high activity is ascribed to
the highly active sites originating from high-index facets, kinks,
and coalesced structures. The catalyst can be recycled via
simple filtration and washing, maintaining its high activity
owing to the micrometer-sized hierarchical structure of the
nanomaterial. The polymer@Pd nanosphere also served as
a printable conductive ink for a translucent grid pattern with
5
À1
excellent horizontal conductivity (7.5”10 Sm ).
Introduction
Nanoscale hybridization of functional organic and inorganic
components realizes a spectrum of applications, which rely on
the design of well-defined, multifunctional materials. Noble-
metallic nanoparticles (NPs) have been important constituents
for such hybrid materials because of their excellent applicabili-
ty in catalysis, gas sensors, energy conversion and fuel cells,
surface-enhanced Raman scattering, and printable conductive
leaching or degradation from repeated cycles, thus impairing
the reaction yields and limiting the reuse. Moreover, multistep
modifications such as chemical etching and thermolysis create
additional problems of processes and energy costs. Therefore,
exploration of facile and efficient methods to design and fabri-
cate such hierarchical structures for PdNPs is of critical impor-
tance. Another important potential applications of noble
metals is in fabrication of conductive materials by printed elec-
[1]
ink. The large surface-to-volume ratios and specific crystalline
structures of NPs result in a large share of atoms located on
the surface, which can significantly influence the material
[
7–8]
tronics.
These printed materials are employed in photovol-
[
8]
taics, transistors, displays, batteries, antennas, and sensors.
[
2,3]
properties.
For example, in catalysis, ultrafine NPs are typi-
However, metal NPs in conductive inks are typically stabilized
by surfactants and polymer additives. The stabilizers need to
be removed before or during the sintering process, either by
cally more reactive than bulk metal counterparts. However,
they are prone to aggregation due to high surface energy, and
recovering of NPs after completion of the reaction by simple
filtration is difficult. Over the past decade, research into the de-
velopment of hierarchical nanostructures has focused on en-
suring higher activity and reusability, especially for Pd nanopar-
[
9]
bulk heating or selective welding, in order to decrease the re-
0
sistance at the junction. Printed patterns of Pd were applied
0
as metallization catalysts for conductive patterns, since Pd is
an excellent catalyst for electroless plating of various
[4–5]
[10–12]
0
ticle (PdNP) catalysts.
metals.
Typically, fabrication of Pd patterns requires re-
2
+
Hollow spherical nanostructures form a unique class of hier-
archical architectures due to their high specific surface areas,
duction of Pd
photocatalyst printed on suitable sub-
[
13]
strates.
[
6]
enabling high metal payload and excellent catalytic activity.
However, most of these approaches are prone to catalyst
Herein, we present a unique functionalized graft copolymer,
poly(NVK-co-MAH)-g-(NH-(PPO -co-PEO ) [NVK = N-vinylcarba-
10
31
zole, MAH = maleic anhydride, PPO = polypropylene oxide,
PEO = polyethylene oxide], which facilitates controlled assem-
bly of ultrafine PdNPs. We also report its application to Mizoro-
ki–Heck and Suzuki–Miyaura reactions, where the quantitative
production of coupling compounds was achieved with trace
(500 mmol%) Pd. Furthermore, the catalyst could be easily recy-
cled for at least five cycles without the loss of high activity.
Inkjet printing of the polymer@Pd nanosphere ink produced
a translucent and highly conductive pattern.
[
a] S. H. Mir, Prof. B. Ochiai
Department of Chemistry and Chemical Engineering Faculty of Engineering
Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510 (Japan)
E-mail: ochiai@yz.yamagata-u.ac.jp
Supporting information for this article can be found under http://
dx.doi.org/10.1002/open.201600009.
ꢀ
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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TEM analysis was performed to investigate the assembly of
the PdNPs on the hollow spheres. It is quite evident from Fig-
ure 1c that the PdNPs have coalesced with each other, which
is a result of strong interactions. The scanning tunneling elec-
tron microscopy (STEM) image (Figure S3 in the Supporting In-
formation) also shows that the PdNP layer has grown over the
polymer shell, producing closed nanoshells, interconnected in
the nanowire-network fashion. It should be mentioned that
uncontrolled aggregation leading to large clusters did not
occur despite the strong interactions between PdNPs. The
nanowire-network is two-dimensional, as confirmed by the
atomic force microscopy (AFM) image (Figure S3).
Results and Discussion
In the approach taken here, a functionalized graft copolymer,
poly(NVK-co-MAH)-g-(NH-(PPO -co-PEO ) (Scheme S1 in the
1
0
31
Supporting Information), was used to produce extended well-
formed polymer@Pd nanowire networks. In a typical synthesis,
a 120 mm (CF COO) Pd/-tetrahydrofuran (THF) solution (2 mL)
3
2
was added to a 40 mm poly(NVK-co-MAH)-g-(NH-(PPO -co-
10
PEO )/-THF solution under an N atmosphere. The mixture was
31
2
2
+
stirred for 1 h at room temperature. For reduction of Pd
,
NaBH4 in methanol (4 mL, 4 mole equiv to Pd) was subse-
quently added under an N atmosphere, and the solution was
2
stirred for 1 h. The resulting complex solution was poured into
methanol, and the precipitate was filtered and dried under
vacuum. The Pd content incorporated onto the hollow spheres
was estimated as 21 wt% by thermogravimetric analysis (Fig-
ure S2 in the Supporting Information).
The Pd in the nanowire-network is almost monocrystalline,
as confirmed by the selected area electron diffraction (SAED)
pattern (inset in Figure 1d), which shows clear spots. The Pd
[
14f,g]
[14c]
[14c,e,g]
nanowire-network contains (553),
(310),
(111),
and
[
14c,g]
(221)
ure 1d.
high-index facets along the border as shown in Fig-
Various defective crystalline structures were also
[
4a,5g,14]
Figure 1 shows electron microscopic images of the material
generated according to this strategy. The samples were pre-
observed in the high-resolution (HR) TEM images. Figure 1e in-
dicates the presence of kinks formed between the coalescence
of Pd nanoparticles, which increase the coarseness of the
nanostructure. Figure 1 f shows twined (111) and (200) planes
originated from the coalescence of the PdNPs. Figure S5 in the
Supporting Information shows the intrafacial dislocation in the
lattice planes. These characteristic structures such as high-
index facets are known as very reactive sites enhancing the
pared by spin-casting
a THF solution of the product
À1
(
3 mgmL ) on aluminum foil and copper grids with carbon
coating for scanning electron microscopy (SEM) and transmis-
sion electron microscopy (TEM), respectively. The SEM image
(
Figure 1a) shows a networked structure consisting of spherical
particles with the average diameter of about 100 nm. The
magnified SEM image (Figure 1b) shows raspberry-like spheres
consisting of PdNPs with the average diameters of about 8 nm
connected to each other to form the networked structure. The
spheres are hollow, as shown in Figure 1b including a sphere
with defects.
0
catalytic performance of the Pd material as described latter.
The powder X-ray diffraction (XRD) pattern of this network
(Figure 2) also shows peaks assignable only to fcc Pd (JCPDS
a
Figure 2. X-ray diffraction (XRD) pattern of the Pd nanowire-network (Cu-K ).
no. 05-0681), indicating the presence of pure metallic Pd crys-
tals. The sharp (111) peak also provides evidence of pure crys-
tallinity. The crystallite size was estimated as 10 nm from the
full width at half maximum (FWHM) of the (111) diffraction by
applying Scherrer’s equation (D=kl/bcosq, where D is the
average particle size in Š, b is FWHM of the diffraction peak in
radian, q is the peak position (2q=388), l is the wavelength of
the radiation (1.54056 Š for Cu-Ka radiation), and k is the
shape factor of 0.89 for spherical crystals). The identical sizes
that were calculated from the SEM image and the XRD diffrac-
tion also support the monocrystalline nature.
A plausible mechanism for the formation of the Pd nano-
wire-network is illustrated in Figure 3. The functionalized graft
Figure 1. SEM (a,b), TEM (c), HRTEM (d–f), and selected area electron diffrac-
tion (SAED) pattern (inset in d) images of the Pd nanowire-network.
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Figure 3. Plausible mechanism of hollow sphere formation with dynamic light scattering (DLS) profiles in THF.
copolymer poly(NVK-co-MAH)-g-(NH-(PPO -co-PEO ), carries
1
0
31
Table 1. Mizoroki–Heck reaction with polymer@Pd nanowire-network.
2
+
carboxylic acid groups capable of coordinating with Pd ions.
The resulting ionic structure around the stem chain has poor
affinity with THF, whereas the polyether chains have high affin-
ity with THF. This difference in affinity promotes complexation-
induced phase separation to form nanosized vesicles. Chemical
2
+
reduction with NaBH produces PdNPs from Pd embedded
4
on the sphere as seeds, forming polymer@Pd nanospheres.
The dynamic light scattering (DLS) profiles in Figure 3 support
1
2
Entry
1 or 4 [R ]
Alkene [R ]
3
Yield [%]
this mechanism. The hydrodynamic diameter (D ) of poly(NVK-
[a]
n
h
1
1a (H)
1a (H)
1a (H)
1a (H)
1a (H)
1b (CH
4a (H)
4b (CH
2 (CO Bu )
3a
3a
3a
3a
3’a
3’b
3’a
3’b
3’c
3’d
98
98
97
2
[
a,d]
n
co-MAH)-g-(NH-(PPO -co-PEO ) in THF is 13 nm (Figure 3a),
2 (2nd use)
3 (4th use)
2 (CO
2
Bu )
10
31
[
a,d]
n
2
+
2 (CO Bu )
and the complexation with Pd results in an increase of D to
2
h
[
a,d]
n
[c]
4
5
6
7
8
9
1
(5th use)
2 (CO
2
Bu )
97
6
0 nm (Figure 3b), due to the electrostatic repulsion forces be-
[
a]
2’ (Ph)
2’ (Ph)
2’ (Ph)
2’ (Ph)
2’ (Ph)
2’ (Ph)
99
85
85
90
95
92
2
+
tween Pd ions. The NP formation via reduction concentrates
[a]
[b]
3
O)
O)
2
+
the Pd in solution onto the sphere surface, thus leading to
[
[
b]
b]
[
their expansion (D =130 nm) (Figure 3c). Upon drying, the
3
h
4c (CH
4d (CHO)
3
CO)
stiff shell consisting of PdNPs maintains its shape, whereas the
inner polyether chain shrinks towards the shell. As a result,
hollow particles are produced. The self-assembly of PdNPs into
the nanowire-network can be ascribed to the strong interparti-
cle interactions originating from the high surface energy and
b]
0
[
a] 1 (1.0 mol equiv), 2 (2.0 mol equiv), Et
monium acetate (TBAA, 0.2 mol equiv), Pd (500 mmol%), 1-BuOH, 1008C,
4 h. [b] 4 (1.0 mol equiv), 2’ (2.0 mol equiv), NaOAc (0.6 mol equiv), tetra-
butylammonium bromide (TBAB, 0.2 mol equiv), Pd (500 mmol%), 1008C,
4 h. [c] Pd concentration detected by ICP-MS in the supernatant of the
3
N (3.0 mol equiv), tetrabutyllam-
2
[
15]
2
elasticity of PdNPs.
reaction mixture was below 1 ppb. [d] The catalyst was recycled by filtra-
tion and washing with EtOAc.
We then employed the Pd nanowire-network to the Mizoro-
ki–Heck (Table 1) and the Suzuki–Miyaura coupling (Table 2) re-
actions, since these reactions have excellent applications in
synthetic organic chemistry and the fabrication of electronic
[
15,16]
materials.
First, the Mizoroki–Heck reaction of iodobenzene
methoxy iodobenzene (1b) with styrene (2’) also proceeded
smoothly to give the corresponding stilbenes, 3’a and 3’b, in
85 and 99% yields (entries 5 and 6), respectively. Moreover,
aryl bromides may also be used as substrates for this coupling.
When the reaction of bromobenzene (4a) and 2’ was per-
formed in the presence of sodium acetate and tetrabutylam-
monium bromide (TBAB), the reaction gave 3’a in an 85%
(
1a) and n-butyl acrylate (2) was carried out with the nanowire
network (500 mmol% of Pd to 1a) in 1-butanol in the presence
of triethylamine (Et N) for neutralization and tetrabutyllammo-
3
[
17]
nium acetate (TBAA) as an efficient base at 1008C. The cou-
pling proceeded smoothly to give n-butyl cinnamate (3a) in
a 98% yield (entry 1, Table 1). The reactions of 1a and p-
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The high activity of the polymer@Pd nanowire-network to-
wards aryl chlorides identical to the aryl bromides and aryl io-
dides can be presumably ascribed to the steric restrictions on
approach of the substrates towards the Pd nanowire-net-
Table 2. Suzuki–Miyaura cross-coupling reactions with polymer@Pd nano-
wire-network.
[a]
[
19]
work. The affinity of the polymer@Pd nanowire-network to-
wards the reaction solvents and the substrates, for example, 1-
BuOH, EtOH/H O, and bromobenzene, is very low as confirmed
2
by the negligible swelling ratios (<5%). This low affinity, made
the approach as the rate-determining step in spite of the ex-
cellent chemical activity originating from the high active sites.
This low affinity of the polymer@Pd nanowire-network is also
a possible reason for the improvement of the yield by the ad-
dition of TBAB.
1
Entry
1
1, 4, 6 [R ]
Time [h]
5
Yield [%]
4a (H)
4a
6
6
6
6
6
6
4
4
6
8
6
6
6
5a
5a
5a
5a
5a
5b
5c
5d
5e
5 f
5a
5b
5c
>99
98
2
3
4
5
6
7
8
9
1
(2nd use)
(4th use)
(5th use)
4a
97
95
98
[c]
[b]
4a
1a (H)
4b (CH
4c (COCH
4e (NO
4 f (CH
Since the nanowire-network efficiently promoted the Mizor-
oki–Heck and Suzuki–Miyaura coupling reactions, recycling of
the polymer@Pd nanowire-network was investigated for both
reactions under identical conditions. After the reactions in en-
tries 1 of Tables 1 and 2 were completed, the nanowirenetwork
was recovered by simple filtration and washing with ethyl ace-
tate owing to the microscaled size, and reused for the identical
reactions. The catalysts of both reactions were reused five
times without the loss of catalytic activity, as indicated in en-
[
c]
[b]
3
O)
99
99
99
99
96
90
95
3
)
)
2
)
3
)
0
1g (OH)
6a (H)
6b (CH
1
1
1
1
2
3
3
O)
6c (COCH
[
c]
[b]
3
95
[
(
5
a] Reaction conditions: phenylboronic acid (0.55 mmol), aryl bromide
0.50 mmol), CO
mmol%), Pd (500 mmol%), EtOH/H
detected by ICP-MS in the supernatant of the reaction mixture was below
ppb [c] The catalyst was recycled by filtration and washing with EtOAc.
K
2
3
(2 equiv), tetrabutylammonium bromide (TBAB, tries 2–4 in both Tables 1 and 2. The negligible leaching of Pd
O (v/v=1/1), 808C [b] Pd concentration
2
species was confirmed in the reaction mixtures after the
th reuse cycles of the Pd network catalysts by inductively cou-
5
1
pled plasma-mass spectrometry (ICP-MS) analysis (entries 4 in
Tables 1 and 2). The concentrations of Pd species in the super-
natants of both reactions were ꢀ1 ppb, which is as low as the
ICP-MS detection limit and lower than previously reported cat-
alyst systems that plausibly proceeded with ppm-order of
yield. Both electron-rich and electron-deficient aryl bromides
4b–d) may also be reacted with 2b to give the corresponding
(
[
20]
cinnamates, 3’b–3’d, in 90–95% yields (entries 7–10). We as-
cribed the excellent catalytic activity enabling catalysis with
trace amounts of Pd to the characteristic structures such as
high-index facets observed in the HRTEM images.
leached Pd species. The ease of redeployment of the catalyst
and the quantitative conversions over several efficient cycles
testifies to the versatility of the Pd nanowire-network as an
active immobilized catalyst.
The Suzuki–Miyaura coupling reaction was also examined
with the polymer@Pd nanowire-network (Table 2). We used 4a
and phenylboronic acid as the initial substrates. This cross-cou-
pling reaction was also carried out using 500 mmol% of the Pd
nanowire-network as the catalyst in a mixed solvent of water
and ethanol (v/v=1/1) under reflux conditions. The yield was
only 53% but was enhanced considerably to >99% by the ad-
dition of 5 mmol% TBAB (run 1), served as a phase transfer cat-
A hot filtration test was conducted to investigate the leach-
ing behavior for the reaction of the 4a and phenylboronic acid
conducted under identical conditions with entry 1 in Table 2
[
21]
(Figure S8 in the Supporting Information). The reaction mix-
ture was hot-filtered at 808C after 1 h to remove the insoluble
Pd network. As a result, the conversion of 4a stayed constant
at 55% after filtration. The ICP-MS analysis and the hot filtra-
tion test manifested negligible leaching. Moreover, the TEM
images of the reused catalysts (Figure S9 and S10 in the Sup-
porting Information) indicate that the Pd nanowire-network
maintained the tertiary nanostructure during the reactions.
These results indicate that this catalysis proceeded on the sur-
face of the PdNPs or with the undetectable amount of leached
Pd species, which was quantitatively captured via release–
[
18,20d]
alyst.
Table 2 shows that the nanowire-networks are
highly active for various electron-rich and electron-deficient
aryl bromides in a similar manner as the Mizoroki–Heck reac-
tion (entries 5–9). Moreover, less reactive aryl chlorides may
also be used as the substrates for this coupling. Cross-coupling
reaction of chlorobenzene (6a) and phenylboronic acid was
performed under identical conditions, and the reaction gave
[
22]
catch mechanism. Heterogeneous catalysts often suffer from
leaching of the active metal species during reactions, and
eventually lose their catalytic activity. Leaching is also responsi-
ble for product contamination with metals and loss of the
noble metal. The negligible leaching of Pd species from this
catalyst is an important advantage for maintaining high cata-
lytic activity, as well as avoiding outflow of and contamination
by Pd.
5
a in 90% yield. Both electron-rich and electron-deficient
chlorides (6b–c) may also be reacted to give the correspond-
ing biphenyls, 5b–5c, in 95% yields. Figure S6 and S7 in the
Supporting Information show plots of conversion versus reac-
tion time for the Mizoroki–Heck and Suzuki–Miyaura reactions,
in which the conversions increase with time reaching quantita-
tive yields in 24 and 6 h, respectively.
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The polymer@Pd nanosphere was also applied as a conduc-
Keywords: hollow nanoparticles · immobilized catalysts · Pd
nanowire-network · printable conductive ink · self-assembly
À1
tive ink. The polymer@Pd nanospheres ink in THF (20 mgmL )
was prepared by dispersing the polymer@Pd nanosphere iso-
lated via precipitation into methanol. The ink was stable up to
one week without aggregation. The resulting ink was inkjet-
printed on a cleaned glass substrate as a grid pattern with
a 50 mm spacing and a 15 mm line width in a 1”1 cm square
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PEO ) as the template for complexation-induced phase separa-
31
tion and spontaneous coassembly. A well-controlled hierarchi-
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spheres comprising PdNPs. This Pd nanowire-network has the
advantages of a facile preparative protocol, excellent catalytic
activity and recyclability in Mizoroki–Heck and Suzuki–Miyaura
coupling reactions, and application for printable and conduc-
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dure can be extended to the fabrication of various nano
hybrid materials with designed functionalities for catalysis,
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Received: January 25, 2016
Published online on March 1, 2016
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