Photochemical fabrication of hierarchical Ag nanoparticle arrays from domain-selective Ag+-loading on block copolymer templates

Article abstract of DOI:10.1039/b911173e

Well-ordered Ag nanoparticle arrays with high particle density were fabricated by photochemical reduction of domain-selective Ag⁺-loading on hydrophobic polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) self-assembled diblock copolymer templa

Full text of DOI:10.1039/b911173e

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Photochemical fabrication of hierarchical Ag nanoparticle arrays from
+
domain-selective Ag -loading on block copolymer templatesw
Yuanjun Liu, Longbin He, Changhui Xu and Min Han*
Received (in Cambridge, UK) 8th June 2009, Accepted 26th August 2009
First published as an Advance Article on the web 11th September 2009
DOI: 10.1039/b911173e
Well-ordered Ag nanoparticle arrays with high particle density
were fabricated by photochemical reduction of domain-selective
+
solid substrates. Recently Tomokazu Iyoda et al. fabricated
Ag nanoparticle arrays on amphiphilic diblock copolymer
films of PEO272-b-PMA(Az)102 and PEO114-b-PMA(Az)51
with vacuum ultraviolet irradiation, which drove both the
Ag -loading on hydrophobic polystyrene-block-poly(4-vinyl-
pyridine) (PS-b-P4VP) self-assembled diblock copolymer
+
templates.
photoreduction of Ag to Ag nanoparticles and the etching
1
5
of the template of the copolymer film in one step. It was
demonstrated that domain-selective photochemical reduction
in a phase-separated block copolymer template provides a
simple and flexible way for directly depositing regularly
arranged metal nanoparticle arrays on a solid substrate with
a tunable particle size and interparticle distance. In this paper,
a hierarchical self-assembled Ag nanoparticle array was
As one of the most important steps for the potential application
of nanomaterials in various fields such as electronics,
1
–3
photonics, sensors and catalysis,
the assembling of nano-
crystals into ordered two- and three-dimensional arrays is
highly required. Such strategies have become a hot topic in
recent years, upon the vast development on the synthesis of
nanocrystals in the past twenty years. Among various methods
for the assembling of nanocrystals, template-assisted deposition
is an important technique for synthesizing nanostructures
with controlled shape and size. Recently, block copolymer
templates have drawn much attention owing to their great
+
prepared by UV-assisted reduction of Ag on a PS-b-P4VP
diblock copolymer film. The nanocrystals were formed
through the diffusion–aggregation process of Ag atoms
+
photochemically reduced from Ag ions that were selectively
infiltrated into the nanocylinders of P4VP blocks, which act as
a good chelating agent for metal ions.
4
capability to self-assemble into well-ordered nanopatterns.
Such nanopatterns can also be used to manipulate the patterning
5
of inorganic materials. For example, Ag, Au and Pt nanoscale
PS-b-P4VP diblock copolymer films with self-assembled
nanopatterns were formed with a solvent annealing route.
Fig. 1(a) shows the atomic force microscope (AFM) height
image of an as-annealed PS-b-P4VP thin film on a silica glass
slice. Self-ordered, uniformly distributed, and high density pores
are evident on the surface. The pores can be assigned to the
P4VP domains, as is confirmed by the AFM phase image
(Fig. 1(b)), where the P4VP and PS domains present the brighter
patterns driven by polystyrene-block-poly(2-vinylpyridine)-
block-poly(ethylene oxide) triblock copolymer and PS-b-P4VP
6
,7
diblock copolymer templates
were prepared by galvanic
displacement on semiconductor surfaces. Similar aligned linear
metallic patterns were also obtained with oxygen plasmon
8
treatment. Surfactant covered Au nanoparticles can be assembled
16
in block copolymers through the interaction between the
,10
and the darker areas, respectively. The average pore diameter
is determined to be 45.7 nm, and the size distribution of the pore
is about 15 nm (inset of Fig. 1(b)). The density of the pores is
9
surfactant and one of the blocks of the copolymer.
Many
kinds of metals can be thermally evaporated onto the block
copolymer film to form highly ordered secondary patterns of
8
À2
+
about 3.11 Â 10 pores cm . After Ag was loaded in the film
and exposed to UV light for 1 h, particle-like structures could be
observed from the nanopores (Fig. 2(a)). Typically, the particle
has a size smaller than the diameter of the P4VP nanopore,
therefore the rims of the nanopores can be distinguished clearly.
With the irradiation time increased to 2 h, the porous surface of
block copolymer film cannot be seen, instead, a high density
array of nanoparticles appears. The diameter of the nano-
particles is around 50 nm (Fig. 2(b)). However, viewing from
the section plane (Fig. 2(d)), we see that the height of the
particles is much smaller than their lateral dimensions, i.e., the
particles resemble disks rather than spheres. The lateral dimensions
of the nanoparticles continuously increase with irradiation time,
and gradually exceed the original diameters of the P4VP pores, as
can be seen from the AFM image for the film with 3 h irradiation
(Fig. 2(c)). On the other hand, UV irradiation on bare PS-b-P4VP
thin film induces no changes either on morphology or on the
nanopore size. Clearly, the nanoparticles directly grew from
1
1,12
nanoparticles.
The templating relies on chemical differences
1
between the blocks in the block copolymer. Shi et al. also
3
demonstrated that silver nanoclusters obliquely deposited onto
polystyrene-block-polybutadiene-block-polystyrene
triblock
copolymer templates can be hierarchically assembled to form
ordered linear arrays of nanoparticles. However, the complex
process and instrumentations limit the applications. Assembling
of metal nanocrystals into two-dimensional patterns by an
easy route is still a challenge.
Although it is known that noble metal nanoparticles can be
4
1
formed in solution by UV irradiation, there are few reports
on photochemical fabrication of metal nanoparticle arrays on
National Laboratory of Solid State Microstructures and Department
of Materials Science and Engineering, Nanjing University,
Nanjing 210093, China. E-mail: sjhanmin@nju.edu.cn;
Fax: 86-25-83686248; Tel: 86-25-83686248
w Electronic supplementary information (ESI) available: Details of
experiments and HRTEM images, EDS spectrum and SAED pattern.
See DOI: 10.1039/b911173e
+
the Ag -loaded P4VP domains, where silver was enriched due
+
to the strong Ag chelating ability of P4VP.
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566 | Chem. Commun., 2009, 6566–6568
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Fig. 1 AFM images (1 mm  1 mm) of an as-annealed PS-b-P4VP thin
film: (a) height image, (b) phase image and associated pore diameter
distribution (inset).
+
Fig. 3 TEM images of Ag -loaded PS-b-P4VP templates with
UV-irradiation for (a) 1 h and (b) 2 h.
nanoparticles in Fig. 3 are confined within the P4VP domains,
and appear as disk-shaped nanoparticles in the AFM
characterization. High-resolution TEM (HRTEM), selected-
area electron diffraction (SAED), and energy dispersive
spectrum (EDS) all confirm the metallic silver nature of the
nanoparticles in the clusters (see ESI,w Fig. S1).
+
The Ag -loaded PS-b-P4VP film was further verified by
transmission electron microscopy (TEM). As shown in Fig. 3,
hierarchical self-assembled nanostructures appear on the film.
On the length scale of about 50 nm, the film can be viewed as a
uniform array of two-dimensional circular clusters. However,
on a finer inspection, each cluster is seen to be composed of an
aggregate of a few tens of nanoparticles with much smaller
diameters of about 5 nm. Such nanoparticles formed closely-
spaced arrays within the cluster. Although the phase-separated
PS and P4VP domains are impossible to be distinguished from
the TEM image due to their low contrasts, it is a logical
argument that the nanoparticle clusters in the TEM images of
Fig. 3 correspond to the nanoparticles observed in the AFM
images of Fig. 2, since they have a similar size and planar
morphology. For example, an analysis of the cluster diameters
from the TEM image for the film with 2 h irradiation, as
shown in Fig. 3(b), gives an average value of 50 nm, which is
very close to the value obtained from AFM. When the UV
irradiation time increases, an expansion of the cluster size
simultaneous with the growth of the constituent nanoparticles
can be observed from TEM, which is also consistent with the
AFM observation. We therefore conclude that the clusters of
The evolution of the metallic Ag nanoparticle array during
the photochemical process was investigated with in situ
UV-Vis absorption spectrum measurement. The extinction
+
spectra of the Ag -loaded PS-b-P4VP film on silica glass
measured at different UV irradiation stages are shown in
Fig. 4. Before UV irradiation, the spectrum of the silver
PS-b-P4VP complex is indistinguishable from that of the bare
+
PS-b-P4VP block copolymer film. UV exposure of the Ag -
loaded PS-b-P4VP film results in a new extinction peak
between 350 and 600 nm, which can be assigned to the
extinction of the surface plasmon resonance (SPR) of metallic
1
7,18
Ag nanoparticles.
The extinction band peaks at 428 nm
with 5 min irradiation time, and monotonously shifts to longer
wavelength when increasing the UV exposure time. A red-shift
of about 27 nm is observed for the film under 3 h irradiation.
Along with the red-shift of the SPR wavelength, UV irradiation
also induces an increase of the SPR peak intensity. The red-
shift of the SPR wavelength can be attributed to the growth of
the silver nanoparticles, which varies both the size and inter-
spacing of the nanoparticles. For silver nanoparticles as small
as 5 nm in diameter, the SPR wavelength will be influenced by
1
9
the change of the nanoparticle size. For closely-spaced silver
nanoparticles, the change of the interspace will also influence
the near field coupling between the nanoparticles and result in
2
0
a change on the SPR wavelength. According to Mie
1
9
theory, the extinction cross sections of metal nanoparticles
near the SPR frequency is directly proportional to their mass.
Therefore the increase of the SPR peak intensity can also be
attributed to the growth of the Ag nanoparticles.
The formation of Ag nanoparticles is a result of aggregation
+
of Ag atoms photochemically reduced from Ag loaded on
PS-b-P4VP diblock copolymer templates. It is well known that
+
P4VP can chelate Ag owing to its electron-rich pyridine ring,
which is caused by the lone pair electrons on N atoms and
3
conjugation effect in the ring. When AgNO solution is dipped
+
onto the PS-b-P4VP template film, Ag will be absorbed
+
preferentially on the P4VP domain and an Ag -enriched
+
Fig. 2 AFM height images (1 mm  1 mm) of Ag -loaded PS-b-P4VP
region is formed. UV light exposure will excite the pyridine
ring and produce electron–hole pairs. The electrons have
+
templates upon UV-irradiation for (a) 1, (b) 2 and (c) 3 h. For (b) and
(c), the particle diameter distribution is given as an inset. (d) Cross-
section analyses of the AFM height images (cross sections were taken
along the lines shown in (a), (b), (c)).
strong reducing ability and reduce Ag to Ag rapidly.
Therefore the P4VP domains become Ag enriched and metallic
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of about 50 nm, while each cluster is composed of a closely-
spaced aggregate of a few tens of nanocrystals with diameters
of about 5 nm. The nanocrystals were formed through the
diffusion–aggregation process of Ag atoms obtained by photo-
+
chemical reduction from Ag selectively infiltrated into the
nanocylinders of P4VP domains of the diblock copolymer
+
templates. The loading of Ag into the P4VP domains was
+
achieved by a simple interaction of Ag with the pyridine
ring assisted with the solvent ethanol. The surface plasmon
resonance of the metallic Ag nanoparticle array shows mono-
tonous red-shifting with increasing of the UV exposure time,
and the red-shift up to 27 nm was attributed to the growth of
the silver nanoparticles, which varies both the size and inter-
spacing of the nanoparticles. This simple metal nanoparticle
array fabrication may be extended to other metals, oxides and
sulfides for potential applications ranging from photonics to
block copolymer photolithography.
+
Fig. 4 Optical extinction spectra of Ag -loaded PS-b-P4VP template
with different UV-irradiation times.
Ag nanoparticles can be formed through diffusion–aggregation.
Metallic Ag also has stronger interaction with P4VP than that
with PS, so that the Ag nanoparticles will also be confined
within the P4VP domains and possess low mobility on the
surface. Surface diffusion plays an important role in determining
the morphology of the nanoparticle assemblies. Under
appropriate UV light flux, a number of growth centers can
be seeded simultaneously inside a P4VP domain and gradually
develop into the nanoparticle clusters. As the irradiation
proceeds, more Ag is reduced and fused into the Ag nano-
particles, therefore the size of the nanoparticles will increase
with irradiation time. On the other hand, the pyridine ring is
efficiently excited by UV light to produce electron–hole pairs,
and so the density of reduced Ag will be higher at the center of
the P4VP domain since the reduction rate is higher there. The
growth rate of the nanoparticles is directly related to the
density of the monomers, therefore the nanoparticle clusters
will be visible in TEM and AFM first from the center of the
P4VP domain and gradually expand along with the increase of
We thank the financial support from NSFC (Grant Nos.
1
0674063, 90606002, 10674056, 10775070), the Hi-tech
Research and Development Program of China under contract
number 2006AA03Z316, the National Basic Research Program
of China (973 Program, contract NO. 2009CB930501), as well
as the Provincial Hi-tech Research Program of Jiangsu in
China (BG2007041).
Notes and references
1
C. P. Collier, R. J. Satkally, J. J. Shiang, S. E. Henrichs and
J. R. Heath, Science, 1997, 277, 1978.
2 G. Schmid and B. Corain, Eur. J. Inorg. Chem., 2003, 3081.
3
4
5
M. Geissler and Y. N. Xia, Adv. Mater., 2004, 16, 1249.
G. Krausch and R. Magerle, Adv. Mater., 2002, 14, 1579.
S. B. Darling, Prog. Polym. Sci., 2007, 32, 1152.
+
UV exposure dose. However, due to the consumption of Ag ,
6 M. Aizawa and J. M. Buriak, J. Am. Chem. Soc., 2006, 128, 5877.
7
8
M. Aizawa and J. M. Buriak, J. Am. Chem. Soc., 2005, 127, 8932.
J. N. Chai, D. Wang, X. N. Fan and J. M. Buriak, Nat. Nano-
technol., 2007, 2, 500.
Q. F. Li, J. B. He, E. Glogowski, X. F. Li, T. Emrick and
T. P. Russell, Adv. Mater., 2008, 20, 1462.
the reduction rate decreases with the UV irradiation time and
results in a slowing of the growth rate of the Ag nanoparticle
cluster. In Fig. 4 it is obvious that the SPR peak intensity
increases much more slowly at longer exposure time.
9
1
0 S. Watanabe, R. Fujiwara, M. Hada, Y. Okazaki and T. Iyoda,
Angew. Chem., Int. Ed., 2007, 46, 1120.
1 W. A. Lopes and H. M. Jaeger, Nature, 2001, 414, 735.
The presence of ethanol is very important for the absorption
+
and enrichment of Ag preferentially on the PS-b-P4VP
1
diblock copolymer film. In our experiment, when AgNO3
12 S. Park, J. Y. Wang, B. Kim and T. P. Russell, Nano Lett., 2008, 8,
1667.
aqueous solution was used to immerse the film, no Ag
1
1
1
1
3 Z. T. Shi, M. Han, F. Q. Song, J. F. Zhou, J. G. Wan and
G. H. Wang, J. Phys. Chem. B, 2006, 110, 18154.
4 Y. Zhou, S. H. Yu, C. Y. Wang, X. G. Li, Y. R. Zhu and
Z. Y. Chen, Adv. Mater., 1999, 11, 850.
5 J. Z. Li, K. Kamata, S. Watanabe and T. Iyoda, Adv. Mater., 2007,
19, 1267.
6 S. Park, J. Y. Wang, B. Kim, W. Chen and T. P. Russell, Macro-
molecules, 2007, 40, 9059.
17 S. Porel, S. Singh, S. S. Harsha, D. N. Rao and
nanoparticles were obtained with UV irradiation. The nano-
scale P4VP domain could not be wetted by the aqueous
solution since it lies below the vertical level of the hydrophobic
PS phase. On the contrary, ethanol is a good solvent for P4VP
2
1
but not so for PS. Therefore, when ethanol is used as solvent,
the AgNO solution preferentially wets the P4VP segment,
3
+
which makes Ag exclusively adsorbed on P4VP domains.
In summary, a well-ordered Ag nanoparticle array with a
high particle density was successfully fabricated over a large
T. P. Radhakrishnan, Chem. Mater., 2005, 17, 9.
1
1
8 Y. Lu, G. L. Liu and L. P. Lee, Nano Lett., 2005, 5, 5.
9 U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters,
Springer Verlag, Berlin, 1995.
+
area by photoreduction of domain-selective Ag -doping in a
porous hydrophobic film of phase-segregated PS-b-P4VP
diblock copolymer. Hierarchical self-assembling nanostructures
of metallic silver were revealed, which appear as a uniform
array of two-dimensional circular clusters on the length scale
20 L. A. Sweatlock, S. A. Maier, H. A. Atwater, J. J. Penninkhof and
A. Polman, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71,
2
35408.
2
1 S. Park, B. Kim, J. Y. Wang and T. P. Russell, Adv. Mater., 2008,
20, 681.
6
568 | Chem. Commun., 2009, 6566–6568
This journal is ꢀc The Royal Society of Chemistry 2009