Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals

Article abstract of DOI:10.1021/ja210047e

This article presents a quantitative analysis of the role played by poly(vinylpyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm² for PVP of 55 000 and 10 000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm², respectively, when 100 nm Ag cubes were used as the seeds.

Full text of DOI:10.1021/ja210047e

Article
pubs.acs.org/JACS
Quantitative Analysis of the Role Played by Poly(vinylpyrrolidone) in
Seed-Mediated Growth of Ag Nanocrystals
†
,‡,§
†,§
†
‡
,†,⊥
Xiaohu Xia,
Jie Zeng, L. Kyle Oetjen, Qingge Li, and Younan Xia*
†
Department of Biomedical Engineering, Washington University, St. Louis, Missouri 63130, United States
‡
Engineering Research Center of Molecular Diagnostics, School of Life Sciences, Xiamen University, Xiamen, Fujian 361005, P. R.
China
*
S Supporting Information
ABSTRACT: This article presents a quantitative analysis of the
role played by poly(vinylpyrrolidone) (PVP) in seed-mediated
growth of Ag nanocrystals. Starting from Ag nanocubes encased
by {100} facets as the seeds, the resultant nanocrystals could
take different shapes depending on the concentration of PVP in
the solution. If the concentration was above a critical value, the
seeds simply grew into larger cubes still enclosed by {100}
facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100}
and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on
Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial
concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at
reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We
could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes
2
between these two samples. The coverage density was found to be 140 and 30 repeating units per nm for PVP of 55 000 and 10
0
2
00 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and
0 repeating units per nm , respectively, when 100 nm Ag cubes were used as the seeds.
2
1
. INTRODUCTION
which has also been refined by Yang and other groups in
conjunction with the concept of seed-mediated growth.
36
Noble-metal nanocrystals with controlled sizes and shapes have
received great interest in recent years due to the strong
correlations between the size and shape of a nanocrystal and its
electronic, optical, magnetic, and catalytic properties.
interest has also been augmented by the technological
applications of these nanocrystals in various areas including
Seed-mediated growth represents a simple and versatile route
to noble-metal nanocrystals with controlled sizes and shapes.
The concept of seed-mediated growth was initially conceived
by Murphy and co-workers for the synthesis of Au nanorods
1
−8
The
48
from Au seeds and by our group for the production of Ag
49
9
−14
15−18
19−22
23−27
nanowires from Pt seeds. In recent years, this approach has
been extended to other metals including Pd, Pt, Rh, and Cu, as
catalysis,
sensing,
imaging,
and biomedicine.
For Ag (or Au) nanocrystals, the applications are largely based
on their fascinating optical properties known as local surface
plasmon resonance (LSPR).
tional and experimental studies have shown that the shape of a
nanocrystal plays the most important role in determining the
number and position of LSPR modes. Thanks to the efforts
from many research groups, a wide variety of methods have
been developed for the synthesis of Ag nanocrystals with
different shapes, including sphere, cube, octahedron, bar,
3
4,40,50−54
well as their combinations.
As an advantage over
28−31
conventional methods based on homogeneous nucleation,
seed-mediated growth allows one to disentangle growth from
nucleation, making it easier to control both the size and shape
of a product by simply concentrating on the growth step. In the
absence of a capping agent, the free energies of various facets of
To this end, both computa-
a Ag polyhedron increase in the order of γ{111} < γ{100} < γ{110}
.
As such, when Ag cubes are employed as the seeds for growth
in the absence of a capping agent, the {100} facets will be
gradually replaced with the more stable {111} facets, leading to
the formation of truncated cubes, cuboctahedrons, and finally
32−40
bipyramid, plate, decahedron, rod, and wire.
them, polyhedrons with a single-crystal structure and enclosed
by a mix of {100} and {111} facets in different proportions
Among
3
,6,55
octahedrons.
In the presence of a capping agent, the seeds
(
e.g., cube, cuboctahedron, and octahedron) have gained
can be directed to grow into nanocrystals with different shapes
depending on the binding affinities of the capping agent for
different types of facets. These arguments are valid for seeds
particular interest due to their great potential as substrates for
plasmonic sensing
4
1,42
and surface-enhanced Raman scattering
4
0,43,44
45,46
(
SERS);
as building blocks for self-assembly;
and as
sacrificial templates for generating hollow structures made of
3
5,47
other noble metals.
These Ag polyhedrons could readily be
Received: October 25, 2011
Published: December 28, 2011
35
prepared using the polyol method developed by our group,
©
2011 American Chemical Society
1793
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Article
5
8,59
with both cubic and cuboctahedral shapes. For example, it has
been demonstrated that single-crystal seeds of Ag with a
cuboctahedral shape could be directed to grow into
octahedrons enclosed by {111} facets or cubes encased by
typically have a relatively broad distribution.
(
1,4-Benzenedithiol
1,4-BDT, 98%) was purchased from Alfa Aesar. All aqueous solutions
were prepared using deionized (DI) water with a resistivity of 18.2
MΩ·cm. All syntheses of Ag nanocrystals were carried out in 100 mL
flasks with round bottom (ACE Glass).
Preparation of 40 nm Ag Cubes To Be Used as the Seeds.
The 40 nm cubic seeds of Ag were prepared using a recently reported
{
100} facets when sodium citrate or poly(vinylpyrrolidone)
(
PVP) was used as a capping agent while all other parameters
40
58
were kept the same. In these cases, the citrate ions and PVP
bind most strongly to {111} and {100} facets, respectively,
reducing their free energies and thus growth rates. In a sense,
the binding of a capping agent can alter the free energies of
various facets and even reverse their order. The facets expressed
on the final product should be a manifestation of the order of
free energies for different facets after the binding of capping
agent has been taken into account.
protocol with minor modifications. In a typical synthesis, 20 mL of
EG was added into a 100 mL flask and preheated under magnetic
stirring in an oil bath set to 150 °C. Other reagents dissolved in EG
were sequentially added into the flask using a pipet: 250 μL of NaHS
solution (3 mM) was first added, and after 2 min, 2 mL of HCl (3
mM) was added, followed by 5 mL of PVP55 (20 mg/mL). After
another 2 min, 1.5 mL of CF COOAg solution (282 mM) was added.
3
During the entire process, the flask was capped with a glass stopper
except during the addition of reagents. The 40 nm cubic seeds of Ag
were obtained by quenching the reaction with an ice-water bath when
the suspension had reached a brown color with a major LSPR peak
around 435 nm. After centrifugation and being washed with acetone
once and DI water twice, the seeds were redispersed in 2 mL of EG for
Although previous studies have demonstrated the capabilities
of various capping agents in generating Ag nanocrystals with
4
0,56,57
different shapes,
there is no quantitative analysis of the
explicit role played by a specific capping agent. Taking PVP as
an example, up to this point, it is still not clear whether the
initial concentration and molecular weight of this polymer will
have a major impact on the shape evolution of a Ag nanocrystal.
In this article, we conducted a set of experiments to achieve a
quantitative understanding of the explicit role played by PVP in
a seed-mediated synthesis. The experiments involved the use of
Ag nanocubes as seeds in ethylene glycol (EG) at an elevated
12
further use. The particle concentration was 5.0 × 10 particles/mL
based on the concentration of Ag measured using inductively coupled
plasma mass spectrometry (ICP-MS) and the particle size and shape
determined by electron microscopy.
Preparation of 100 nm Ag Cubes To Be Used as the Seeds.
The 100 nm cubic seeds of Ag were prepared using seed-mediated
growth. In a typical synthesis, 20 mL of EG was added into a 100 mL
flask and heated in an oil bath at 150 °C under magnetic stirring. After
0 min, 6 mL of PVP55 solution (20 mg/mL, in EG) was added using
a pipet. After another 10 min, 200 μL of the suspension of 40 nm Ag
cubes (5.0 × 10 particles/mL) in EG was introduced, followed by the
addition of 4 mL of AgNO solution (282 mM, in EG) using a pipet.
The 100 nm cubic seeds of Ag were obtained by quenching the
reaction with an ice-water bath when the major LSPR peak of the
product had reached ∼585 nm. The product was washed with acetone
once and DI water twice and then redispersed in 0.5 mL of EG for
further use. The particle concentration was 2.0 × 10 particles/mL.
Seed-Mediated Growth for Ag Polyhedrons. In a standard
synthesis, 15 mL of EG was added into a 100 mL flask and heated for
0 min under magnetic stirring in an oil bath set to 140 °C, and then
.5 mL of the suspension of 40 or 100 nm Ag cubes in EG was
introduced. After 2 min, 3 mL of PVP55 or PVP10 solution (in EG,
with a specific concentration noted in the text) and 2 mL of AgNO3
564 mM, in EG) were simultaneously added at rates of 1.0 and 0.5
6
0
1
temperature, with AgNO serving as a precursor to Ag and PVP
3
of two molecular weights and in different initial concentrations
serving as the capping agent. We found that well-defined
polyhedrons ranging from cubes (with enlarged sizes relative to
the seeds) to truncated cubes, cuboctahedrons, truncated
octahedrons, and octahedrons could all be obtained depending
on the initial concentration of PVP added to the reaction
solution. We also derived the coverage density (ϕ) of PVP on
Ag(100) surface by combining the results from two experi-
ments. In the first experiment, we followed the growth of Ag
12
3
12
3
0
cubic seeds at a fixed initial concentration (C ) of PVP and
1
found out the time (and thus the size of corresponding cubes)
at which {111} facets started to appear on the cubes. In the
second experiment, the same batch of Ag cubic seeds was
allowed to grow at a set of initial concentrations of PVP to
(
mL/min, respectively, using syringe pumps. The reaction time was
counted immediately after the addition of AgNO solution. Aliquots
figure out at which concentration (C ) {111} facets would start
2
3
to appear on the cubic seeds at the very beginning of growth.
We then calculated the coverage density of PVP on Ag(100)
from the differences in concentration (C − C ) and total
were taken at different time points of a synthesis using glass pipets and
then quickly injected into cold acetone held in 1.5 mL centrifuge
tubes. Additionally, a small amount (several drops) of the solution was
taken with a glass pipet, diluted with DI water in a cuvette, and
subjected to measurement of UV−vis spectrum. These procedures
could be completed in 1 min, during which the Ag nanocrystals grew
very little in size. All the final products were collected by
centrifugation, followed by washing with acetone once and then DI
water three times to remove excess reagents. The final products were
dispersed in DI water for further characterizations.
1
2
surface area of Ag cubes between these two samples. We
compared the coverage densities for PVP with two different
molecular weights (55 000 and 10 000 g/mol) and Ag cubic
seeds of two different sizes (40 and 100 nm in edge length). To
our knowledge, this work represents the first attempt for a
quantitative analysis of the effect of a capping agent on the
shape evolution and growth habit of noble-metal nanocrystals.
The Ag polyhedrons with well-defined shapes and controllable
sizes also allowed for a systematic study of their LSPR
properties as a function of these two parameters.
Surface-Enhanced Raman Scattering (SERS) Measurements.
The as-prepared samples were diluted with water to obtain aqueous
suspensions with a particle concentration of 1.2 × 10 particles/mL.
11
For SERS measurements with 1,4-BDT as a probe molecule, the Ag
nanocrystals were functionalized with a 1 mM ethanol solution of 1,4-
BDT over a period of 2 h. Ethanol was used to wash the sample several
times before the particles were redispersed in water to obtain a
concentration of 1.2 × 10 particles/mL. Sample cells were
constructed by attaching the cap of a microcentrifuge tube to a glass
slide. The cap served as a vessel for the liquid sample, and a glass
coverslip (0.13−0.17 mm) was carefully placed on top to prevent
solvent evaporation and to serve as a reference point from which the
focal point was lowered to a depth of 0.2 mm into the sample.
2. EXPERIMENTAL SECTION
Chemicals and Materials. Ethylene glycol (EG, lot no. G32B27)
1
1
was obtained from J. T. Baker. Silver trifluoroacetate (CF COOAg,
3
≥
99.99%), silver nitrate (AgNO , 99+%), sodium hydrosulfide hydrate
3
(
NaHS·xH O), hydrochloric acid (HCl, 37% in water), 1-methyl-2-
2
pyrrolidinone (99%), and poly(vinylpyrrolidone) (PVP55 with MW
5 000 and PVP10 with MW ≈ 10 000) were all obtained from Sigma-
Aldrich. Note that the molecular weights of a commercial PVP
≈
5
1
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Figure 1. Schematic showing the growth of a Ag cubic seed with an edge length of a nm in the presence of PVP at a high concentration of C and a
1
critical concentration of C (C > C ), respectively. At C , enough free PVP in the solution ensures surface adsorption and thus good passivation of
2
1
2
1
the Ag(100) surface of the cubic seed (1), which can grow to a bigger cube (2), accompanied by reduction in PVP concentration due to its
adsorption onto the newly formed Ag surface. Eventually, when the concentration of free PVP drops to a critical value (C ), the PVP can no longer
2
passivate the Ag(100) surface, the free energies of {100} and {111} facets become the same, and {111} facets start to appear on the surface, leading
to the formation of a larger cube of b nm in size (3) with slight truncation at corners. Alternatively, when the initial concentration of PVP is reduced
to the critical value (C ), {111} facets start to appear on the surface of the cubic seed at the very beginning of growth, leading to the formation of a
2
slightly truncated cube (5) with essentially the same size as the initial seed.
We employed the 1,4-BDT SERS peak at 1562 cm−1 (the strongest
band in the spectra) to calculate the SERS EFs using the following
equation
light source at 514 nm was generated from an argon cw laser and used
with a holographic notch filter based on a grating of 1200 lines per
mm. The backscattered Raman signals were collected on a
thermoelectrically cooled (−60 °C) CCD detector. The SERS data
were collected with Plaser ≈ 2 mW and t = 30 s. All data analyses were
done with IGOR Pro software (Portland, OR).
61
EF = (ISERS × Nbulk)/(Ibulk × NSERS
)
(1)
where ISERS and Ibulk are the intensities of the same band for the SERS
and ordinary spectra from a bulk sample, Nbulk is the number of
molecules probed for a bulk sample, and NSERS is the number of
molecules probed in SERS. Nbulk was determined based on the Raman
spectrum of an aqueous solution containing 0.1 M 1,4-BDT and 12 M
NaOH and the focal volume (1.48 pL) of our Raman system. When
determining NSERS, we assumed that the 1,4-BDT molecules were
3. RESULTS AND DISCUSSION
Quantitative Analysis of the Surface Coverage
Density of PVP. Figure 1 schematically illustrates how to
experimentally determine the coverage density (ϕ) of PVP on
Ag(100) surface. As expected, PVP will adsorb onto the surface
of Ag nanocrystals upon contacting each other, and the surface
coverage density of PVP on a Ag nanocrystal is dependent on
the concentration of free PVP in the solution. During the
growth of Ag nanocrystals, the concentration of free PVP drops
with time due to the increase in total surface area for the
nanocrystals and thereby increase of surface-bound PVP. If the
2
62
adsorbed as a monolayer with a molecular footprint of 0.54 nm , and
4
4
2
surface areas of 3.4 × 10 and 1.9 × 10 nm were used for a nanocube
and a octahedron both of 75 nm in edge length based on SEM
imaging.
Instrumentation. SEM images were captured using a Nova
NanoSEM 230 field-emission microscope (FEI, Hillsboro, OR)
operated at 15 kV. Transmission electron microscopy (TEM) images
were taken using a Tecnai G2 Spirit Twin microscope (FEI, Hillsboro,
OR) operated at 120 kV. The concentration of Ag was determined
using an ICP-MS (Perkin-Elmer Elan DRC II ICP-MS) and then
converted into the concentration of Ag nanocrystals once the particle
size and shape had been determined by TEM and SEM imaging. The
samples for ICP-MS were prepared by dissolving 10 μL of the
initial concentration (C ) of PVP is high enough, there will be
1
sufficient free PVP in the solution to maintain the coverage
density of PVP on the surfaces of all Ag cubic seeds as they
grow in size. In this case, the cubic seed (a nm in edge length)
will grow in size as a function of time without experiencing any
change to its shape (Figure 1, 1 to 2). However, the growth will
eventually lead the concentration of free PVP in the solution to
drop to a critical level, at which surface passivation by PVP can
no longer be maintained. At this point, the surface free energies
of {100} facets and {111} facets will become the same and
{111} facets will start to appear on the surface, leading to the
suspension of Ag nanocrystals with 30 μL of concentrated HNO . The
3
resultant solution was further diluted to the 100 ppb level for ICP-MS
analysis. Extinction spectra of all the Ag nanocrystals were recorded
using a UV−vis spectrometer (Varian, Cary 50). The SERS spectra
were recorded using a Renishaw in Via confocal Raman spectrometer
coupled to a Leica microscope with a 50× objective (N.A. = 0.90). The
1
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formation of a cube (b nm in edge length) with slight
truncation at corners (Figure 1, 3). Alternatively, one could
gradually reduce the initial concentration of PVP used for a
approximately 6, 4, and 4 nm, respectively, for the 40 nm seed,
55 nm cube, and 75 nm cube, see Figure S2). These results
imply that the PVP55 could effectively passivate the Ag(100)
surface via adsorption, leading to the formation of Ag
nanocubes enclosed by {100} facets. In this case, the cubic
seed was merely enlarged in size with almost no development
of {111} facets. However, as the reaction was prolonged to t =
synthesis until a specific value (C ) is reached, at which {111}
2
facets will start to appear on the seed at the very beginning of
growth (Figure 1, 4 to 5).
Since the concentrations of free PVP and coverage densities
of PVP on the Ag(100) surface should be the same for samples
2
0 min (Figure 2C), we obtained Ag nanocubes with slight
3
and 5, the coverage density can now be calculated from the
truncation and thus formation of eight {111} facets at the
corners.
differences in initial concentration of PVP and the total surface
area (ΔSAg)
When the reaction time was further extended, truncated
cubes, truncated octahedrons, and octahedrons were observed
at t = 25, 45, and 75 min, respectively (Figure S3). During this
period of time (20−75 min), the total area of {111} facets was
constantly enlarged at the expense of {100} facets, which
eventually disappeared. When {111} facets started to appear at
t = 20 min, it implies that the surface of nanocrystals could no
longer be passivated by PVP and therefore {111} facets would
start to grow at the expense of {100} facets. At t = 75 min, a
perfect Ag octahedron with an edge length of 220 nm was
developed (Figure S3C), and no additional growth was
observed thereafter for the octahedrons due to the self-limiting
ϕ = (C − C )·V·N /ΔS
1
2
A
Ag
(2)
where V is the volume of the reaction solution that was 0.02 L
for all of our experiments and N is Avogadro’s number (6.02 ×
A
23
12
1
0 ). ΔS equals the total number (1.0 × 10 ) of cubic seeds
Ag
2 2 2
multiplied by 6(b − a ) nm , the increase in surface area
between the two different cubes.
Derivation of Surface Coverage for PVP55 and 40 nm
Cubic Seeds. We started our measurements with Ag cubes of
4
0 nm in edge length as the seeds and PVP55 of various molar
concentrations as the capping agent. Here the concentration of
PVP was calculated in terms of the repeating unit of PVP with a
molecular weight of 111 g/mol. As shown by the SEM and
TEM images in Figure S1(A) of the Supporting Information,
the cubic seeds were 40 ± 1 nm in edge length, together with
uniform distributions for both size and shape. The initial
63
feature of {111} facets. At the same time, irregular particles
started to appear in the product (Figure S3D) due to
homogeneous nucleation and growth. When we increased the
initial molar concentration of PVP from 1.0 to 2.0 mM, 130 nm
sharp cubes with no truncation at corners were obtained at t =
20 min (Figure S4), confirming our assumption that the
presence of sufficient PVP55 in the solution was a key
parameter in maintaining good passivation for {100} facets and
thus their dominance over {111} facets during the growth
process.
concentration (C ) of PVP55 in the solution was first set to 1.0
1
mM. At t = 5 and 10 min, the 40 nm cubic seeds had evolved
into Ag cubes with average edge lengths of 55 and 75 nm,
respectively, as shown in Figure 2, A and B. The corners of
The result in Figure 2C indicates that the 40 nm cubic seed
(
a = 40 nm) would grow into a cube (b = 120 nm) with slight
corner truncation at C = 1.0 mM and t = 20 min. In order to
1
obtain the concentration of PVP at which {111} facets will start
to appear at corners from the very beginning and thus calculate
ϕ, we conducted a set of syntheses with decreasing initial
concentrations for PVP. Aliquots were taken from the reaction
solutions at a very early stage (t = 5 min) and analyzed by SEM
to monitor the growth process. Interestingly, when the
concentration of PVP was reduced to a critical value (C ) of
2
0
.1 mM, {111} facets would start to appear from the very
beginning (Figure 2D). Further growth of these truncated
cubes eventually led to the formation of cuboctahedrons
(
Figure 2E) and perfect octahedrons (Figure 2F). These results
imply that C was likely around 0.1 mM for PVP55 and 40 nm
2
seeds. From these data and eq 2, the value of ϕ for PVP55 and
4
0 nm seeds was calculated to be approximately 140 repeating
2
units per nm . Since the area occupied by a PVP monomer (i.e.,
N-vinypyrrolidone) is around 0.21 nm (see Figure S5 for
2
detailed calculation), the number of repeating units in each
segment of PVP folded on the Ag surface is approximately 29.
In an ideal situation where growth is limited to the [100]
directions only, the edge lengths of the resultant octahedron
Figure 2. SEM images of Ag polyhedrons grown from 40 nm cubic
seeds in the presence of (A−C) 1.0 mM and (D−F) 0.1 mM PVP55,
respectively, as the capping agent. The samples were collected from
the reaction solutions at different time points: (A, D) 5 min, (B, E) 10
min, and (C, F) 20 min. The inset shows the corresponding 3D model
for each type of polyhedron.
64
and the cubic seed should be related by a factor of 2.12. From
the TEM images (Figure S6), we found that the average edge
length (75 nm) of the octahedrons shown in Figure 2F was
8.5% smaller than the edge length of 82 nm expected for
octahedrons grown from 40 nm cubes under an ideal situation.
This small discrepancy can be attributed to the slight truncation
at corners for the initial cubic seeds (see Figure S7).
these cubes were even slightly sharper than those of the original
cubic seeds (with the radii of curvature at corner sites being
1
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By further reducing the initial molar concentration of PVP to
octahedrons had similar SERS EFs for 1,4-BDT, the stronger
intensity of PVP SERS peak from the Ag nanocubes could be
attributed to a higher coverage density of PVP on the surface of
Ag nanocubes than Ag octahedrons.
The Effect of PVP Molecular Weight on the Surface
Coverage Density. Besides the concentration, the molecular
weight of PVP was also found to have an impact on the shape
evolution of a Ag nanocrystal. To single out the role of
molecular weight, we repeated the experiments performed for
PVP55 under similar conditions (e.g., by using the same type
and concentration of seeds, precursor, temperature, and PVP
concentration) except for the use of PVP with a molecular
the range of 0.02−0.1 mM, we found that the results were more
or less similar to what was observed for C = 0.1 mM (data not
2
shown). However, when the concentration was below 0.02 mM
or no PVP55 was added into the reaction solution, Ag cubes
with different degrees of truncation at corners and cuboctahe-
drons with round corners (Figure S8) were observed at t = 5
and 10 min, respectively. In these cases, there was not sufficient
PVP to passivate the newly formed Ag(100) surface on Ag
nanocrystals. The new morphologies were likely products of
preferential growth of Ag on those sites where the surfaces were
65
poorly covered by PVP.
To better understand how PVP binds to the {100} and
111} facets, we obtained SERS spectra of PVP55 adsorbed on
the surfaces of Ag nanocubes enclosed by {100} facets (shown
in Figure 2B) and Ag octahedrons enclosed by {111} facets
weight of 10 000 (PVP10). At C = 1.0 mM, 65, 90, and 130
nm sharp cubes were obtained at t = 5, 10, and 20 min,
respectively (Figure 4, A−C), indicating that {100} facets were
1
{
(shown in Figure 2F). Figure 3A shows the SERS spectra
Figure 4. SEM images of Ag polyhedrons grown from 40 nm cubic
seeds in the presence of (A−C) 1.0 mM and (D−F) 0.1 mM PVP10
as the capping agent. The samples were collected from the reaction
solutions at different reaction times: (A, D) 5 min, (B, E) 10 min, and
(C, F) 20 min. The inset shows the corresponding 3D model for each
type of polyhedron.
Figure 3. Solution-phase SERS spectra recorded from (A) as-prepared
Ag nanocubes (Figure 2B) and octahedrons (Figure 2F) prepared in
the presence of PVP and (B) the same samples of nanocubes and
octahedrons after their surfaces had been functionalized with 1,4-BDT.
Band assignments for PVP and 1,4-BDT are labeled above the peaks.
All samples were suspended in water, and the suspensions had roughly
the same particle concentration. cps = counts per second.
lower in energy than {111} facets at least until t = 20 min. Note
that {111} facets already started to appear at this time point
when PVP55 was used at the same concentration. When C was
reduced to 0.1 mM, sharp cubes, slightly truncated cubes, and
1
truncated octahedrons (Figure 4, D−F), were obtained at t = 5,
1
0, and 20 min, respectively. In these cases, there were still
{
100} facets on the surface at t = 20 min, while they had
recorded from aqueous suspensions with roughly the same
particle concentration (1.2 × 10 particles/mL). Clearly, the
1
1
completely disappeared in the case of PVP55. These results
indicate that PVP10 was more effective in reducing the surface
free energy of Ag(100) than PVP55 when they were used at the
same concentration in terms of repeating unit. The difference
might be caused by the larger size and thus steric effect of
PVP55 that tended to make it harder for the polymer chains to
adsorb and pack on the surface of a Ag nanocrystal.
Interestingly, we found that the cubic seeds grew into round
particles at t = 10 min when 0.1 mM 1-methyl-2-pyrrolidinone,
a monomer analogue of PVP, was used to replace the PVP10,
with other reaction conditions being kept the same (see Figure
6
6,67
intensity of characteristic CO peak
for PVP molecules on
the Ag nanocubes was much higher than those on the Ag
octahedrons, implying a higher surface coverage density of PVP
on the nanocubes. Figure 3B shows SERS spectra recorded
from the same samples of Ag nanocubes and octahedrons after
their surfaces had been functionalized with monolayers of 1,4-
Based on the phenyl ring stretching mode at 1562
cm , the enhancement factors (EFs) were estimated to be 2.3
10 and 2.7 × 10 for the nanocubes and octahedrons,
respectively, based on eq 1. Since the Ag nanocubes and
4
3,68
BDT.
−
1
5
5
×
1
797
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Journal of the American Chemical Society
Article
S9). This result is similar to the case when no PVP was added
into the reaction solution. The lack of a polyvalency effect for
the monomer analogue of PVP might be responsible for the
reduction in binding affinity for the {100} facets and thus the
nm slightly truncated cubes at t = 15, 45, and 80 min,
respectively, were obtained. The cubic seed with a = 100 nm
started to show truncation at t = 80 min when its edge length
became b = 240 nm (Figure 5C). We then gradually reduced
the initial molar concentration of PVP55 and found that slightly
truncated cubes of 120 nm in edge length were formed at t = 15
min when the PVP concentration was reduced to 0.3 mM. As
the reaction proceeded, truncated octahedrons and octahedrons
(Figure 5, E and F) were formed at t = 45 and 80 min,
respectively. The edge length (210 nm, as shown in Figure 5F)
of the octahedron was longer than that of the 100 nm cubic
seed by a factor of 2.10, indicating that the octahedron was
grown from the cubic seed under a more or less ideal situation.
69,70
disappearance of {100} facets during growth.
To quantitatively understand how more effective PVP10 was
in reducing the surface free energy of Ag(100), we also derived
the value of ϕ for PVP10 and 40 nm cubic seeds. The images in
Figure 4 suggest that the cubic seed with a = 40 nm started to
show truncation at corners once its size had grown to b = 85
nm by t = 10 min at C = 0.1 mM (Figure 4E). To obtain C2
1
and thus calculate the ϕ of PVP10, we also performed
syntheses at gradually decreasing concentrations of PVP with
other parameters being kept the same. Results similar to what
were shown in Figure 2, D−F, were observed when the initial
concentration of PVP10 was reduced to 0.015 mM (data not
These results indicate that C was 0.3 mM for this system.
2
According to eq 2, the value of ϕ for PVP55 and 100 nm seeds
2
became approximately 100 repeating units per nm , which was
very close to what was derived for 40 nm cubic seeds.
To obtain ϕ for PVP10 and 100 nm cubic seeds, we also
performed a set of experiments under the same conditions as
shown). This observation indicates that C was 0.015 mM for
2
PVP10 and 40 nm cubic seeds. Using the same method
developed for PVP55, the value of ϕ was calculated to be
2
above, except for the use of PVP10 as a capping agent. At C =
approximately 30 repeating units per nm for PVP10 and 40
1
2
.5 mM, 125, 200, and 275 nm sharp cubes (Figure 6, A−C)
nm cubic seeds. Note that the value of ϕ for PVP10 was only
about one-fourth of that for PVP55, indicating that PVP with a
lower molecular weight was more effective in passivating the
Ag(100) surface likely due to more efficient adsorption and
packing.
were obtained at t = 15, 45, and 80 min, respectively. When C
1
The Effect of Seed Size on Surface Coverage Density.
To examine the effect of seed size on ϕ, we conducted a set of
syntheses by replacing the 40 nm Ag cubes with 100 nm Ag
cubes as the seeds while all other parameters were kept the
same except for the PVP concentration. Figure S1B shows SEM
and TEM images of the 100 nm cubic seeds, which had sharp
corners and were 100 ± 2 nm in edge length. We first used
PVP55 as a capping agent in the syntheses. As shown in Figure
5
, A−C, for C = 2.5 mM, 130, 190 nm sharp cubes and 240
1
Figure 6. SEM images of Ag polyhedrons grown from 100 nm cubic
seeds in the presence of (A−C) 2.5 mM and (D−F) 0.3 mM PVP10
as the capping agent. The samples were collected from the reaction
solutions at different reaction times: (A, D) 15 min, (B, E) 45 min,
and (C, F) 80 min, respectively.
was reduced to 0.3 mM, 120 nm sharp cubes, 190 nm truncated
cubes, and 260 nm cuboctahedrons (Figure 6, D−F) were
formed at t = 15, 45, and 80 min, respectively. By gradually
reducing the initial concentration of PVP10, the cubes started
to show truncation at t = 0 min when it was reduced to C =
2
0
.03 mM (data not shown). Accordingly, the ϕ value for
PVP10 and 100 nm cubic seeds was calculated to be
approximately 20 repeating units per nm . The values of ϕ
for PVP55 and PVP10 adsorbed on the 100 nm cubic seeds
were of the same orders of magnitude as the values derived for
the 40 nm cubic seeds. Taken together, it is clear that ϕ has a
Figure 5. SEM images of Ag polyhedrons grown from 100 nm cubic
seeds in the presence of (A−C) 2.5 mM and (D−F) 0.3 mM PVP55
as the capping agent. The samples were collected from the reaction
solutions at different reaction times: (A, D) 15 min, (B, E) 45 min,
and (C, F) 80 min.
2
1
798
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Journal of the American Chemical Society
Article
much stronger dependence on the molecular weight of PVP
than the size of the cubic seeds.
mM, which was much faster than what we observed for 0.1 mM
PVP10.
Reaction Kinetics of the Seed-Mediated Growth. We
also studied the reaction kinetics to better understand the roles
played by both the concentration and molecular weight of PVP
in shape evolution during the growth of Ag cubic seeds. We
first plotted the sizes (L) of Ag nanocrystals as a function of
reaction time (t). Here L was defined as the distance between
two opposite {100} facets as illustrated in Figure S10. When
PVP was used as a capping agent and 40 nm cubes as the seeds
A similar trend was also observed when 100 nm cubes were
used as the seeds and PVP in different molecular weights and
concentrations as the capping agent (Figures S11). The growth
rate (1.9−2.5 nm/min) for the syntheses with 100 nm seeds
was roughly two times lower than the rate (3.1−4.1 nm/min)
obtained for the 40 nm seeds. This can be explained by the fact
that a larger seed has a larger surface area and thus requires
more Ag atoms to achieve a growth rate the same as the case
with a smaller seed.
(
Figures 2 and 4), there was a more or less linear relationship
between the size of Ag nanocrystals and reaction time for both
PVP55 and PVP10 at two different concentrations (Figure 7A).
Optical Properties of the Ag Nanocrystals. The Ag
nanocrystals with controlled sizes and well-defined shapes
allowed us to systematically investigate the effects of size and
shape on LSPR. Figure 8A shows normalized UV−vis spectra
Figure 7. Plots of (A) the size (L) and (B) the ratio between areas of
{
111} and {100} facets (denoted as A{111} and A{100}, respectively) for
Figure 8. Normalized UV−vis spectra of samples shown in Figure 2
the Ag nanocrystals shown in Figures 2 and 4 as a function of reaction
time.
(as marked on the spectra). Dashed curves correspond to UV−vis
spectra of the 40 nm Ag cubes serving as the seeds. The peak positions
are labeled on each spectrum, and their assignments can be found in
the text.
Their slopes fell into the range of 3.1−4.1 nm/min, which
corresponded to the growth rate along the [100] directions.
The nanocrystals obtained at a higher concentration of PVP
were always slightly larger than those obtained at a lower
concentration of PVP with the same molecular weight at a
specific reaction time. Meanwhile, the nanocrystals obtained
with PVP10 were slightly larger in size than those prepared
with PVP55 at the same concentration and reaction time.
These data also imply that more Ag atoms were added onto the
seeds when PVP at a higher concentration or with a lower
molecule weight was used in the synthesis. These results can be
explained by the coordination effect of PVP with Ag ions (thus
effectively lowering the reduction potential of Ag ). The same
phenomenon was also observed when citrate was used as a
capping agent for Ag nanoplates. In addition, Ag −PVP10
complex may form more easily than Ag −PVP55 due to a
weaker steric effect for the shorter polymer chains. We also
plotted the ratio between the areas of {111} and {100} facets
expressed on the surface as a function of reaction time. As
shown in Figure 7B, the ratio increased rapidly as the reaction
proceeded when PVP55 was used at a concentration of 0.1
recorded from aqueous suspensions of the Ag nanocrystals
shown in Figure 2, A−C. The major LSPR peaks displayed a
constant red-shift from 435 to 470, 515, and 615 nm as the size
of nanocrystals increased. Three LSPR peaks, which are dipole
resonance modes, were present for the 50 nm cubes (Figure
2
A), while four peaks, including a small shoulder at ∼450 nm
(
next to the primary LSPR peak, a quadrupole resonance
mode), can be resolved for the 70 nm cubes (Figure 2B). These
observations are consistent with the characteristic dipole and
+
36,72
+
quadrupole resonance peaks for Ag cubes at these sizes.
The 120 nm truncated Ag cubes (Figure 2C) showed relatively
broad LSPR peaks, which might be attributed to their large
sizes and thus the involvement of multipode excitations in
71
+
+
36,72
addition to dipole excitation.
Figure 8B shows UV−vis spectra of the Ag nanocrystals
shown in Figure 2, D−F. We found that the major LSPR peak
was red-shifted very little from 435 to 445 nm at the beginning
of the reaction, and the truncated cubes and cuboctahedrons
1
799
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Journal of the American Chemical Society
Article
showed major LSPR peaks at the sample position of 445 nm. In
comparison with the cubic seeds, there are only two peaks for
the truncated cubes, with the peak observed at ∼380 nm for the
cubic seeds disappearing in the spectra of the truncated cubes.
For the spectrum of cuboctahedrons, both peaks located at
AUTHOR INFORMATION
■
Corresponding Author
younan.xia@bme.gatech.edu
Present Address
⊥
∼
345 and 380 nm disappeared, mainly because a cuboctahe-
The Wallace H. Coulter Department of Biomedical Engineer-
ing, Georgia Institute of Technology and Emory University
Medical School, and School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, Georgia 30332,
United States.
dron has higher symmetry than a cube. It is interesting to note
that there were four LSPR peaks for the Ag octahedron with an
average edge length of 75 nm (Figure 2F), including three weak
peaks located at 350, 380, and 415 nm, and one primary peak at
40
5
00 nm corresponding to one of the dipole modes.
Author Contributions
These authors contributed equally to this work.
The LSPR peaks of Ag nanocrystals with larger sizes (100−
00 nm, as shown in Figure 5) were relatively broad and thus
§
3
less distinguishable (Figure S12). In practice, one can obtain Ag
nanocrystals with a specific morphology and size by
continuously monitoring changes to the UV−vis spectra
taken at different reaction stages.
ACKNOWLEDGMENTS
■
This work was supported in part by the NSF (DMR-1104614)
and NIH (R01 CA13852701). As a jointly supervised Ph.D.
student from Xiamen University, X.X. was also partially
supported by a Fellowship from the China Scholarship Council.
4. CONCLUSION
We have systematically investigated the role of PVP as a
capping agent in controlling the shape evolution of Ag
nanocrystals during a seed-mediated synthesis. It can be
concluded that the coverage density of PVP adsorbed on the
surface of a Ag nanocrystal plays a pivotal role in determining
the surface free energies of Ag(100) and Ag(111) facets and
thus the formation of nanocrystals with different shapes. We
have also obtained the critical concentration of PVP in the
solution and the coverage density of PVP on the surface, at
which the order of free energies between Ag(100) and Ag(111)
are reversed. Our results indicate that PVP with a relatively
lower molecular weight was more effective in reducing the free
energy of Ag(100) via surface adsorption than PVP with a
higher molecular weight. By simply controlling the initial
concentration of PVP added into a reaction solution, a series of
Ag polyhedrons from enlarged cubes to truncated cubes,
cuboctahedrons, truncated octahedrons, and octahedrons could
all be easily obtained from cubic seeds. This work not only
advances our understanding of the role played by PVP in
controlling the shape evolution of Ag nanocrystals but also
allows us to investigate the effects of both size and shape on the
LSPR properties. The quantitative analysis reported in this
article could also be extended to other systems involving
different types of seeds and/or capping agents.
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