Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based

Article
pubs.acs.org/cm
Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on
Mechanistic Understanding
Maria Wuithschick,† Benjamin Paul,‡ Ralf Bienert,§ Adnan Sarfraz,∥ Ulla Vainio,⊥ Michael Sztucki,▽
Ralph Kraehnert,‡ Peter Strasser,‡ Klaus Rademann,† Franziska Emmerling,§ and Jörg Polte*,†
†
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
Technische Chemie, Technische Universität Berlin, Straße des 17 Juni 124, 10623 Berlin, Germany
§
BAM Federal Institute of Materials Research and Testing, Richard-Willstätter-Straße 11, 12489 Berlin, Germany
∥
Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany
⊥
FS-DO at Deutsches Elektronen Synchrotron, Notkestraße 85, 22607 Hamburg, Germany
▽
European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France
‡
S Supporting Information
*
ABSTRACT: Metal nanoparticles have attracted much attention due to their unique
properties. Size control provides an effective key to an accurate adjustment of colloidal
properties. The common approach to size control is testing different sets of
parameters via trial and error. The actual particle growth mechanisms, and in
particular the influences of synthesis parameters on the growth process, remain a black
box. As a result, precise size control is rarely achieved for most metal nanoparticles.
This contribution presents an approach to size control that is based on mechanistic
knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the
reduction of AgClO4 with NaBH4. Conducting this approach allowed a well-directed
modification of this synthesis that enables, for the first time, the size-controlled
production of silver nanoparticles 4−8 nm in radius without addition of any
stabilization agent.
KEYWORDS: silver nanoparticles, growth mechanism, SAXS, size control, sodium borohydride
■
INTRODUCTION
Metal nanoparticles are used for a wide range of applications,1
for example, in spectroscopy,2,3 biomedicine,4−6 and catalysis,7−10 which is the result of their unique catalytic, optical,
electronic, and magnetic properties. These properties can be
adjusted by altering the nanoparticle size, composition, crystal
structure, and morphology.11,12 Consequently, size control
provides an effective key to an accurate adjustment of colloidal
properties.
The common synthetic procedure to obtain colloidal metal
nanoparticles is the chemical reduction of a precursor salt with
a reducing agent such as sodium citrate or sodium
borohydride.13 In general, the synthetic procedure itself is
relatively simple, whereas size control is often claimed but
rarely achieved.14 The exception might be gold nanoparticles,
but for silver, copper, and palladium, only a very few reliable
synthetic procedures rxost that deliver monodisperse colloids in
a size range of 1−20 nm.15−18 Moreover, most synthetic
procedures require additional stabilization agents that can
change relevant properties, such as biocompatibility or catalytic
activity, making them inappropriate for further use.19−21 The
most common approach to size control is testing different sets
of parameters via simple trial and error, which makes the
synthesis of nanoparticles “rather an art than a science”.22 The
© 2013 American Chemical Society
number of scientific contributions that investigate actual
nanoparticle growth mechanisms, and in particular the
influences of parameters on growth, is still very limited.14,23−26
As a result, nanoparticle growth processes often remain a black
box.16,17,27,28
A knowledge-based approach can be more effective to
achieve size control. This contribution presents such an
approach to size control, which comprises three steps as
depicted in Scheme 1: (A) investigation of the growth
mechanism in principle, including all relevant physicochemical
processes for one set of parameters; (B) investigation of
influences of synthesis parameters on the growth mechanism,
which leads to identification of size-determining parameters;
and (C) deliberate adjustment of the decisive reaction
parameters to obtain a desired final particle size distribution.
This approach can be applied to different nanoparticles (e.g.,
metallic, oxidic, or bimetallic particles), matrices (e.g., water,
organic solvents, glass),29 and preparation methods (e.g.,
chemical reduction, photochemical reduction). It requires
monitoring particle size distribution and concentration in situ
Received: June 6, 2013
Revised: October 8, 2013
Published: November 5, 2013
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evolution of the size distribution during the second coalescent
step (fourth step of the growth mechanism).
Detailed Investigation of Growth Processes during
Metastable State and Final Coalescence. Particle growth of
the standard synthesis (refers to simple 1:1 mixing of 0.5 mM
AgClO4 solution and 3 mM NaBH4 solution at ambient
conditions) was investigated with time-resolved SAXS at the
ID02 beamline of the European Synchrotron Radiation Facility
(ESRF) synchrotron storage ring. A free-liquid jet setup was
used for containerless measurement. The basic concept of this
setup is to extract a small flow of liquid sample continuously
from the reaction vessel. The solution is pumped to a nozzle,
resulting in a liquid jet at which the SAXS measurements are
conducted. Thus, the synthesis can be performed in its
undisturbed conventional environment (stirred batch reactor).
Furthermore, X-ray-induced effects are minimized and a high
time resolution at a low signal-to-noise ratio can be achieved
when a synchrotron light source is used. A detailed description
of the setup can be found elsewhere.34 To avoid agglomeration
of the silver nanoparticles inside the tubing, the distance
between reaction vessel and X-ray beam was minimized and
only Teflon tubing and connectors were used. From each
scattering curve, the particle mean radius, polydispersity,
relative volume fraction, and relative number of particles were
determined. The size distribution is assumed to be a Schulz−
Zimm distribution, which has been shown to be a suitable
approximation.39 Selected scattering curves and their corresponding theoretical fits are shown in Supporting Information
(section SI-1).
Figure 1a shows particle mean radius and volume fraction
versus time. The normalized volume fraction embodies the
whole volume of all particles. Polydispersity and number of
particles versus time are displayed in Figure 1b. The first
available scattering curve (t = 5 s) is already assigned to the
metastable state. The size distribution of the colloidal solution
remains constant during the entire metastable state (t = 5−520
s) with a particle mean radius of 1.5 nm and a polydispersity of
40%. The volume fraction is approximately 100% and remains
almost constant. These experimental findings reveal that
colloidal stability during the metastable state is sufficient to
prevent the nanoparticles from any further growth.
The stability decreases after approximately 520 s. The result
is a particle growth process with an increase in the mean radius
to 6.3 nm, accompanied by a successive decrease of the
polydispersity to 25% (see Figure 1a,b). The volume fraction of
the final colloidal solution is the same as during the metastable
state, which confirms that particle growth is a process of
coalescence. During the coalescent step, the volume fraction
decreases down to 65% (t = 530 s). This indicates that the
mathematical model used to fit the scattering curves cannot
describe the colloidal solution accurately at that point.
Obviously, the coalescing particles initially form irregular
objects that finally reorganize to spheres, whereas the model
assumes spherical morphology. On average, one final silver
nanoparticle is formed by the coalescence of approximately 50
smaller particles (see Figure 1b). Figure 1c illustrates the shift
of size distribution. The overlap of the size distribution before
and after coalescence is low, which indicates that all particles
participate in the growth process.
In conclusion, the high data quality of the synchrotron SAXS
investigations show that colloidal stability during the metastable
state is sufficient to prevent any particle aggregation and further
growth.. The chemical process of BH4− conversion, which is
Scheme 1. Approach to Size Control Based on Mechanistic
Knowledge
and time-resolved during the entire growth process. Such
experimental information can be obtained by applying several
lab-scale and synchrotron small-angle X-ray scattering (SAXS)
setups.24,30−37
The knowledge-based approach is exemplified for the
synthesis of silver nanoparticles which was adapted from Van
Hyning and Zukoski.38 It comprises the wet chemical reduction
of silver perchlorate (AgClO4) with sodium borohydride
(NaBH4) without any additional stabilizing agents.
■
RESULTS AND DISCUSSION
Step A: Growth Mechanism in Principle. In our recent
paper,39 we were able to deduce the nanoparticle growth
mechanism in principle of the herein-investigated silver
nanoparticle synthesis. However, important details are still
missing to gain a profound understanding of the growth
process. Thus, completing step A of the presented approach
constitutes the first part of this contribution.
Previously, it has been shown that the growth mechanism
comprises four steps: (1) rapid reduction of ionic silver to silver
atoms, which immediately form dimers, trimers etc.; (2)
coalescence of these preliminary formed clusters, resulting in
particles 2−3 nm in radius; (3) an intermediate phase of
stability, during which the particle mean radius remains
constant (referred to as metastable state); and (4) a second
coalescence that leads to the final colloids.39 The particle mean
radius ranges from 4 to 10 nm (at 20−30% polydispersity) and
is poorly reproducible. In addition, the duration of the
metastable state can vary between 5 and 20 min. The final
colloidal solutions showed no changes after several days. A
long-term stability test, in terms of months, was not performed.
It was assumed that the metastable state, and thus the growth
mechanism, is strongly influenced by the conversion of residual
BH4− to B(OH)4−.39 Experimental results indicate that particle
growth and chemical conversion of BH4− to B(OH)4− occur in
parallel (for details, see the supporting information of Polte et
al.).39 Further investigations are required to correlate particle
growth with associated physicochemical processes in the
colloidal solution. In particular, previous experimental results
were insufficient to exclude with certainty any growth of
particles during the metastable state. The limited data quality of
lab-scale SAXS experiments complicates the detection of very
small particles below 1 nm in diameter. In addition, lab-scale
SAXS experiments cannot provide detailed information on
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Although NaBH4 is a moderate reducing agent compared to
other metal hydrides, the reducing species H− is provided fast
enough to reduce metal ions such as Au3+ or Ag+ within
milliseconds.33 In comparison, hydrolysis is a much slower
process.41,42
According to eq i, 1 mol of Ag+ is reduced by the equivalent
amount of BH4−. To ensure complete reduction of the metal
precursor, the reducing agent is always used in excess (for the
standard synthesis, in 6-fold excess). Therefore, BH4− remains
in the colloidal solution after the ionic silver is reduced. The
remaining BH4− is converted during the nanoparticle growth
process. Previous experiments indicated that the metastable
state ends at the point of total BH4− consumption (see section
SI-6 of our previous work).39 It was shown that the addition of
small amounts of HAuCl4 during the metastable state leads to
the reduction of ionic gold by residual BH4−, which generates
gold nanoparticles besides the existent silver nanoparticles.
After the second coalescence, HAuCl4 is no longer reduced
when added to the colloidal silver solution. Another simple
experiment supports the correlation between amount of
residual BH4− and duration of the metastable state. Adding
fresh NaBH4 solution to the colloidal solution during the
metastable state extends the duration of this phase (for details,
see section SI-3 in Supporting Information).
The kinetics of BH4− consumption can be investigated by
time-resolved monitoring of H2 evolution from the reaction
vessel, since BH4− conversion is accompanied by the release of
hydrogen (see eqs i and ii). The released hydrogen can be
detected quantitatively by mass spectrometry. An according
setup is described in the Experimental Section and in section
SI-4 in Supporting Information. Two samples were investigated: (i) colloidal silver synthesis and (ii) a comparison
sample for which the NaBH4 solution is mixed with an equal
volume of water instead of silver precursor solution. Figure 2a
depicts the volume flow of released hydrogen versus time of the
colloidal solution (red line) and the corresponding NaBH4
solution (black line). Integration of the curves gives the total
volume of released hydrogen and is shown in Figure 2b.
For both samples, the maximum volume flow of hydrogen is
detected at the beginning. The maximum flow is 0.06 mL/min
for the colloidal solution and 0.017 mL/min for the NaBH4
solution. The flow decreases with increasing reaction time and
is zero after approximately 20 min for the colloidal solution and
after approximately 12 h (see section SI-4 in Supporting
Information) for the NaBH4 solution. The duration of the
metastable state can vary between 5 and 20 min for repeated
experiments. In this particular experiment, the second
coalescent step of the colloidal system was observed after
approximately 20 min; thus it coincides with the end of H2
detection. For the NaBH4 solution, the total volume of detected
hydrogen is 1.4 mL, which is in agreement with calculations for
a full BH4− conversion according to reaction ii and the ideal gas
law. The detected total volume of H2 for the colloidal solution
is only 0.7 mL, which is a result of the experimental setup: the
procedure of mixing the reactants and sealing the reaction
vessel takes 5 s. Since reaction i is a very fast process, H2
evolved from the reduction of Ag+ was not detected.
The mass spectrometric experiments reveal that H 2
evolution, and thus BH4− conversion, is highly accelerated in
the presence of silver nanoparticles (see Figure 2). This is not
surprising since catalytic activity of metal nanoparticles toward
the conversion of BH4− was observed in a variety of previous
studies.43−45 Tetraborohydride is converted during the entire
Figure 1. Results of synchrotron SAXS investigations of standard
colloidal synthesis: (a) particle mean radius and normalized volume
fraction (last data point set as 100%) vs reaction time; (b)
polydispersity and relative number of particles (last data point set as
1) vs reaction time; and (c) calculated particle size distributions for
selected reaction times.
assumed to cause the rapid loss of stability and initiates the
second coalescence, was investigated next
Chemical Conversion of BH4− during Particle Growth.
Two types of reaction can occur in an aqueous solution of
sodium borohydride: (i) BH4− can act as a source of
nucleophilic hydride H−, which can reduce a variety of metal
ions Mz+;40 and (ii) the H− ligands can be replaced by water
molecules (hydrolysis). The reactions can be described by the
following simplified equations (for details, see section SI-2 in
Supporting Information):
Mz + + z BH4 − + z 4H 2O
→ M 0 + z B(OH)4 − + z 3.5H 2 + z H+
BH4 − + 4H 2O → B(OH)4 − + 4H 2
(i)
(ii)
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residual BH4− and might be caused by oxidation of the
nanoparticle surface..
Step B: Influence of Reaction Parameters on Growth
Mechanism. In the previous section, the particle growth
mechanism was studied for one combination of synthesis
parameters ([AgClO4] = 0.5 mM, [NaBH4] = 3 mM, 1:1
mixing, stirring speed = 300 rpm, room temperature). In step B,
influences of different reaction parameters on the growth
mechanism, and thus on the final size distributions, are
investigated. A strategy to achieve a reproducible synthesis is
deduced, which is the basic requirement to develop sizecontrollable nanoparticle syntheses.
Influence of Reactant Concentrations on Growth Mechanism. Already in the 1990s, Glavee et al.51 pointed out that
the reduction of metal ions with BH4− is a complex interplay of
several chemical processes (reduction, hydrolysis, catalysis) that
is influenced by numerous parameters such as reactant
concentrations and pH value. Thus, it is not surprising that
the final particle size of the investigated synthesis is very
sensitive to concentrations of AgClO4 and NaBH4, temperature, ionic strength, and even parameters that are often not
considered to influence the synthesis, such as the type of
reaction vessel, stirring speed, and mixing procedure (for
details, see section SI-5 in Supporting Information). Therefore,
the same type of reaction vessel was used for all lab-scale
experiments, the stirring speed was kept constant (300 rpm),
and the same mixing procedure (1:1 mixing of 2 × 5 mL) was
used. The temperature was kept constant at 23 °C (±1 °C). In
addition, the reducing agent solution was always prepared
freshly and used within 1 min. Nevertheless, the exact
preparation of NaBH4 solution is difficult since NaBH4 is
hygroscopic and easily absorbs moisture.52 In section SI-6
(Supporting Information), it is demonstrated that the mass of
NaBH4 powder increases drastically (by up to 300%) if the
substance is not stored under water-free conditions. Thus, the
as-received NaBH4, powder was partitioned in small units and
stored under argon. However, even if all these precautions are
considered and samples are prepared simultaneously from
identical reactant solutions, the final size distribution is still
poorly reproducible. As a result, the final particle mean radius
obtained from the standard silver nanoparticle synthesis can
vary between 4 and 10 nm. A low level of reproducibility is also
apparent for other reactant concentrations, as illustrated by a
parameter variation study shown in Supporting Information
(section SI-7). The syntheses were carried out simultaneously
three times with identical reactants. However, standard
deviations of the mean radii are relatively high (between 0.3
and 1 nm). As a principal tendency, the average particle mean
radius increases with increasing AgClO4 and decreasing NaBH4
concentration.
Nevertheless, the insufficient reproducibility of the final
particle size demands further elucidation of which steps of the
growth mechanism are sensitive to synthesis parameters. It is
possible that small changes in the synthetic procedure (e.g.,
mixing conditions) already influence the outcome of the first
coalescent step considerably. Even small differences of the size
distribution after the first coalescent step might affect the
second coalescent step and therefore the final particle size.
Therefore, it is necessary to extend the parameter study by
mechanistic investigations. For five selected points of the
parameter study (highlighted in section SI-7 in Supporting
Information), the particle growth process was investigated
time-resolved with lab-scale SAXS.
Figure 2. Results of mass spectrometric investigations on hydrogen
release during silver nanoparticle synthesis and corresponding pure
NaBH4 solution: (a) volume flow of hydrogen vs reaction time and
(b) total volume of hydrogen vs reaction time.
metastable state. The end of H2 release indicates the complete
conversion of BH4− and coincides with the second coalescent
step. Coalescent processes result from a decrease of colloidal
stability. Therefore, the destabilization of the primary formed
nanoparticles is most likely associated with the progressing
conversion of BH4− to B(OH)4−. The chemical conversion
could influence the particle stability by the specific adsorption
of ionic species: stability might increase by adsorption of BH4−
or decrease by adsorption of B(OH)4− at the particle surface. A
paper by Andrieux et al.45 shows that BH4− adsorbs or even
dissociates on cobalt nanoparticle surfaces. However, the
amount of ions [BH4− and/or B(OH)4−] that could adsorb
at the silver particle surface changes gradually, whereas the
stability of the colloidal silver solution decreases relatively
abruptly.
It was shown for many silver nanoparticle systems that a
silver oxide layer is formed upon storage in ozone but also in
aqueous solution at ambient conditions.46−50Therefore, the
decrease of colloidal stability could result from a sudden
formation of a silver oxide layer at the nanoparticle surface.
Residual BH4− might continuously reverse any surface
oxidation of the particles during the metastable state. Its
complete consumption at the end of the metastable state could
initiate a collective surface modification of all particles and
consequently a “simultaneous” decrease of colloidal stability of
all particles. As a result, the particles undergo further growth
due to coalescence until a stable size is reached.
Summary of Step A. It was shown that colloidal stability
remains sufficient to inhibit any particle growth during the
metastable state. The decrease of particle stability that initiates
the second coalescent step correlates with full conversion of
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Figure 3. Results of time-resolved SAXS investigations: mean radius (polydispersity = 50% before and 25% after final coalescence) vs reaction time
for varied (a) silver perchlorate concentration and (b) reducing agent concentration. The displayed concentrations refer to solutions used for the
syntheses and mixed 1:1. Different symbols refer to different synthesis repetitions. Light and dark gray bars highlight particle sizes after the first and
second coalescent steps, respectively. The blue dotted line highlights the duration of the metastable state. Results of the standard synthesis are
displayed twice (middle panels).
impossible: (i) the hygroscopy of solid NaBH4 and its fast
change of chemical composition upon dissolving in water
impede an exact adjustment of the reducing agent concentration; (ii) identical mixing and stirring conditions are hard to
achieve; (iii) small temperature variations can hardly be
eliminated; and (iv) the exact amount of dissolved oxygen
can hardly be controlled.55
Instead of attempting to control BH4− consumption, a much
more practical approach to achieve a reproducible nanoparticle
synthesis could be elimination of the complex second
coalescent step. For this purpose, the synthesis needs to be
modified so that the two separated coalescent steps merge to
one single step, thus eliminating the metastable state. The
results displayed in Figure 3 clearly show that the duration of
the metastable state (highlighted by blue dotted lines)
correlates with the amount of residual BH4−. The duration
decreases with decreasing amount of residual BH4−. From the
results of the mechanistic investigations, it can be expected that
the metastable state will vanish if the NaBH4 concentration is
reduced to the concentration of AgClO4. This can be achieved
(i) by reducing the quantity of dissolved NaBH4 or (ii) by aging
the as-used reducing agent solution, which has a NaBH4 excess
(hydrolysis leads to a decrease of BH4− concentration).42
Merging the two coalescent steps by use of a reduced
quantity of NaBH4 was examined for an AgClO4 concentration
of 0.5 mM. The concentration of NaBH4 was successively
reduced from a ratio R = [BH4−]/[Ag+] of 2 to 1. The final
colloidal solutions were investigated with lab-scale SAXS and
UV−vis spectroscopy. The results are shown in Supporting
Information (section SI-9). The duration of the metastable
state decreases to zero exactly for R = 1. However, the colloidal
stability at this ratio is low, which leads to precipitation within 2
min.
The alternative approach is based on aging of the NaBH4
solution. The BH4− concentration decreases exponentially due
to hydrolysis,42 and complete conversion in the absence of
nanoparticles proceeds within hours. In the following, the
The results of the mathematical modeling are displayed in
Figure 3 (note that the standard synthesis is displayed twice).
Selected scattering curves and their corresponding theoretical
fits are shown in Supporting Information (section SI-8).
The SAXS investigations reveal that differences between final
particle sizes of repeated syntheses and between syntheses with
concentration variations are caused by the second coalescent
step. Particle size after the first coalescent step (highlighted by
light gray bars) is about the same for all investigated systems
(mean radius approximately 2 nm at 50% polydispersity). After
the second coalescent step, the mean radii vary considerably
(highlighted by dark gray bars). For example, mean radii after
the first coalescent step of the standard synthesis (diagram in
the middle panels) are nearly identical for all three experimental
runs, but the final mean radii deviate between approximately 7
and 10 nm. As a consequence, reproducibility and particle size
control can be achieved only by controlling the second step of
coalescence.
The second coalescent step is a very complex process. It
comprises aggregation of spherical nanoparticles with a broad
size distribution (polydispersity approximately 50%) that must
reorganize to give again spherical-shaped colloids. The
aggregation is caused by a loss of colloidal stability, which
correlates with the full conversion of BH4−. The driving force
for reorganization of the aggregated nanoparticles is a gain of
energy (surface energy vs bulk energy).53 The process is
strongly affected by particle surface chemistry,54 such as the
presumed surface oxidation. Thus, both processesaggregation and reorganizationand consequently the second
coalescent step are dependent on the kinetics of the BH4− →
B(OH)4− conversion and the associated oxidation of the
particle surface. The conversion rate of BH4− depends on many
parameters, such as reactant concentrations, reaction temperature, stirring speed, and catalytic properties of the nanoparticles formed after the first coalescent step.41,42 Therefore,
these parameters have to be precisely controlled to adjust the
particle size distribution. However, absolutely accurate control
of these parameters under normal lab conditions is almost
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the final size distribution is reproducible and the mean radius
increases from 6 to 9.3 nm. Within a period of more than 90
min (ta approximately 650−740 min), the mean radius even
remains constant at 9.3 nm. This period is referred to as size
plateau. For ta > 800 min (phase III), the residual amount of
BH4− is insufficient to reduce Ag+ completely, which is
illustrated by a decreasing volume fraction (reflects the total
volume of all silver nanoparticles). In phase III the particle
mean radius decreases significantly with increasing ta.
The duration of the metastable state (reaction time until a
significant color change is observed; see Polte et al.39) versus ta
is displayed in Figure 4b. During phase I, the duration of the
metastable state decreases with ta. The color change, which
indicates the second coalescence, disappears with the beginning
of phase II. Exemplarily, section SI-11 (Supporting Information) shows two time-resolved UV−vis investigations for
colloidal syntheses of phase I (ta = 90 min) and phase II (ta
= 500 min). For ta = 90 min, a significant change of the
maximum absorbance and wavelength is observed at 240 s,
while the UV−vis spectrum for ta = 500 min remains constant.
In addition, the vanishing of the metastable state was observed
by time-resolved SAXS by applying the free-liquid jet setup at
the ID02 beamline of the synchrotron light source ESRF.
Exemplarily, Figure 5 depicts the size distribution versus time
influence of the NaBH4 aging process on the nanoparticle
growth mechanism is investigated.
NaBH4 Aging: Key to Reproducibility. The chemical
composition of the reducing agent solution changes with
time, due to the conversion of BH4− to B(OH)4−. The
conversion starts immediately after NaBH4 is dissolved in
water. To investigate how the hydrolysis progress of the
reducing agent solution (referred as aging) influences the
nanoparticle growth process, a standard NaBH4 solution (3
mM) was prepared and stored at ambient conditions (T = 23
°C). For various times between 0 and 1020 min after dissolving
the NaBH4 powder (referred as aging time ta), the solution was
used to prepare simultaneously three colloidal silver solutions
by reduction of a standard 0.5 mM AgClO4 solution. Size
distributions of the final nanoparticless were determined with
lab-scale SAXS. Selected scattering curves and corresponding
fits are shown in Supporting Information (section SI-10).
Results of the mathematical modeling are displayed in Figure 4.
Figure 4. Results of lab-scale SAXS investigations on the influence of
the NaBH4 aging process on nanoparticle synthesis. (a) Final particle
mean radius (polydispersity = 30% and for the last three aging times
25%) and normalized volume fraction (first data point set as 100%) vs
aging time. (b) Duration of the metastable state vs aging time. The
diagram can be divided into three parts: a phase with poor
reproducibility of the final size distribution (I), a phase with good
reproducibility (II), and a phase of incomplete precursor reduction
(III).
Figure 5. Results of time-resolved SAXS investigations on the growth
mechanism of silver colloids with aged NaBH4 solution as reducing
agent. Particle mean radius and polydispersity vs time for aging times
of (a) 240 min and (b) 660 min are shown.
Figure 4a depicts the evolution of final particle mean radius
and relative volume fraction versus aging time ta. The diagram
can be divided into three parts. For 0 min < ta < 400 min
(phase I), the final particle mean radius is poorly reproducible
but decreases with increasing ta from approximately 8 to 6 nm.
The volume fraction remains almost constant, which indicates a
full precursor reduction. The volume fraction remains also
constant for 400 min < ta < 800 min (phase II). In this phase,
for two colloidal syntheses during phase I (ta = 240 min) and
phase II (ta = 660 min). Selected scattering curves and
corresponding fits are displayed in section SI-12 of Supporting
Information.
For ta = 240 min, the particle mean radius at the first
available measuring point (5 s) is approximately 3 nm
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(polydispersity of 50%) and increases to approximately 5 nm
(polydispersity of 30%) at a reaction time of around 150 s. In
contrast, for synthesis with the 660-min-old NaBH4 , the
second coalescent step vanishes. The particle mean radius
remains constant at 8.5 nm (polydispersity of 35%) from the
first measurement point (5 s). These experiments show that the
particle growth mechanism changes due to aging of the
reducing agent solution. For ta > 400 min, only one fast
coalescent step is observed. Actually, this is a surprising result
since the silver precursor is still reduced completely between
400 and 800 min. This means the molar ratio between the
NaBH4 solution used and Ag+ is at least equimolar, and most
likely above 1, during phase II. In contrast, the results presented
in Figure 2 suggest that the second coalescence coincides with a
complete depletion of BH4−. However, in this experiment no
B(OH)4− is present when the reactants are mixed. Therefore,
the vanishing of the second coalescent step during phase II
might be connected to the B(OH)4− anion or the ratio BH4−/
B(OH)4−. However, the improvement of reproducibility of the
final size distribution coincides with the merging of the two
separated coalescent steps as expected. In addition, the
obtained final size distribution remains almost constant within
a size plateau. This period of time is ideal for conducting a
parameter variation study that aims to identify size-determining
parameters.
Parameter Variation within Size Plateau. It was shown that
reproducibility of the synthesis can be improved significantly by
aging the NaBH4 solution. This observation can be used to
conduct a reliable parametric study. An aged NaBH4 solution
(ta within the size plateau) was used to reduce a standard 0.5
mM AgClO4 solution. This parameter study includes variations
of AgClO4 concentration, ionic strength [Na+, ClO4−,
B(OH)4−], temperature, and pH. Size distributions of the
final colloids were investigated with lab-scale SAXS. Selected
scattering curves and their corresponding fits can be found in
Supporting Information (section SI-13).
Figure 6a displays the results of AgClO4 concentration
variation. For decreasing silver precursor concentration, the
final mean radius decreases (for [AgClO4] = 0.25 mM, to r =
7.8 nm). In Figure 6b,c, the results of Na+ and ClO4−
concentration variation are displayed. The final mean radius
increases only slightly with increasing ionic strength. Figure 6d
displays the results of temperature variation. The final particle
mean radius decreases for decreasing temperature. For 0.5 and
10 °C, a particle mean radius of 8.3 and 8.6 nm, respectively, is
obtained. Figure 6e depicts the final mean radius obtained upon
addition of perchloric acid (HClO4). The mean radius remains
almost constant even for the addition of 250 μL of acid (pH =
3). Figure 6f illustrates the influence of additional B(OH)4− on
the final particle size. Unfortunately, it is not possible to
determine the total concentration of B(OH)4− present during
the Ag+ reduction, since the exact chemical composition of the
reducing agent (exact ratio [BH4−]/[B(OH)4−]) is unknown.
However, the absolute amount of ionic silver reduced is 2.5
μmol. The amount of additional B(OH)4− is of the same
magnitude. Addition of NaB(OH)4 results in a significant
decrease of the particle mean radius. The mean radius decreases
to approximately 7.3 nm if the silver precursor solution is
adjusted to a NaB(OH)4 concentration of 15 mM. These
experimental results show that temperature and ratio between
the concentrations of Ag+, BH4−, and B(OH)4− have a major
influence on the final size distribution.
Figure 6. Results of SAXS investigations on the influences of reaction
parameters on final size distribution. For all syntheses, aged NaBH4
solution (within plateau) was used. Final particle mean radius is shown
vs (a) silver perchlorate concentration, (b) concentration of Na+, (c)
concentration of ClO4−, (d) temperature, (e) added volume of HClO4,
and (f) added amount of NaB(OH)4. Polydispersity stayed constant at
30%. NaClO4 and HClO4 were added to the silver precursor solution
to give the displayed concentrations. The silver precursor solutions
were then mixed 1:1 with the aged reducing agent solution. Results of
standard precursor solutions are highlighted (●).
Step C: Size Control. From mechanistic investigations, it
was deduced that the second coalescent step and therefore the
final particle size can hardly be controlled. It was shown that
the reproducibility can be improved significantly by merging
the two coalescent steps. This can be achieved by aging the
NaBH4 solution (see Figures 4 and 5). In fact, aging NaBH4
represents a decreasing ratio of BH 4 − to B(OH) 4 − .
Furthermore, this decreasing ratio leads to an increasing final
particle size (see phase II in Figure 4). As a result, NaBH4 aging
enables a reproducible and size-controlled synthesis of silver
colloids.
However, aging the NaBH4 solution is very laborious since
the reducing agent solution has to be prepared at least 5 h in
advance (begin of phase II; see Figure 4). Furthermore, it is
very difficult for a size control to obtain exactly the demanded
aging and thus the demanded BH4−/B(OH)4− ratio. Chemical
conversion of BH4− to B(OH)4− can be faster or slower, for
example, due to small temperature variation during storage.
Imitation of NaBH4 Aging. The alternative is an imitation of
the NaBH4 aging process. A variation of the BH4−/B(OH)4−
ratio can also be achieved by simply mixing B(OH)4− with fresh
BH4− solution. B(OH)4− solution can be obtained from longer
storage of BH4− solution due to the hydrolysis of BH4−. The
following synthetic procedure imitates the NaBH4 aging
described in step B (see Figure 4): a 3 mM NaBH4 solution
is prepared and stored for at least 1 day. The obtained
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B(OH)4− solution is mixed with freshly prepared 3 mM NaBH4
solution in different ratios and immediately used to reduce a 0.5
mM AgClO4 solution (1:1 mixing). Figure 7 shows the results
merging of the two coalescent steps. This means that the
growth mechanism changes from a four-step to a two-step
mechanism similar to the corresponding gold nanoparticle
synthesis.33 The BH4−/B(OH)4− ratio can be precisely adjusted
by well-defined aging of the reducing agent solution or a simple
imitation of this aging process. Although the exact role of the
BH4−/B(OH)4− ratio remains unclear, modification of the
synthesis leads to a reproducible growth process.
As a result, the gained mechanistic knowledge enabled a welldirected modification of the synthesis that allows reproducible
and size-controlled production of silver nanoparticles in the
range of 4−8 nm in radius (polydispersity = 30%). This
represents substantial progress for the synthesis of metal
colloids, since syntheses of silver nanoparticles with size control
are rare, especially in aqueous solution and without addition of
stabilizing agents.13,56−60 Our work proves that mechanistic
studies are not only of academic interest but can be the key to
improve the current state of nanoparticle syntheses.
Figure 7. Results of silver nanoparticle syntheses using a mixture of
aged 3 mM and fresh 3 mM NaBH4 solution as reducing agent. Final
mean radius (polydispersity = 30%) vs percentage of fresh NaBH4 is
plotted. The x-axis is inverted since a decreasing percentage of fresh
NaBH4 imitates an increasing aging time.
■
EXPERIMENTAL SECTION
Colloidal Syntheses. In this paper, the standard procedure for
synthesis of colloidal silver nanoparticles in water refers to 1:1 mixing
of 0.5 mM AgClO4·H2O and 3 mM NaBH4 solution. The reactant
solutions were obtained by dissolving 225.33 mg of AgClO4·H2O
(Sigma−Aldrich, 99.999%) in 2 L and 113.5 mg of NaBH4 powder
(Alfa Aesar, 98%) in 1 L of ultrapure water (18.2 MΩ·cm, Millipore).
The silver precursor solution was stored in the dark. The reducing
agent solution was prepared freshly and used within 1 min. The asreceived NaBH4 was divided into small portions under inert gas. A
new portion was used for each experiment. The colloidal synthesis was
carried out at ambient conditions (23 °C ± 1 °C). The stirring speed
was kept constant at 300 rpm.
For lab-scale syntheses, 5 mL portions of each reactant solution
(total volume = 10 mL) were mixed by use of two Eppendorf pipettes.
For each synthesis, an unused small glass container (20 mL volume)
was used as reaction vessel.
For investigations at the synchrotron beamlines, the synthesis was
scaled up to give a total volume of 400 mL of colloidal solution. The
reactant solutions were filled into two glass flasks with a selfmanufactured outlet at the bottom and mixed 1:1 within 3 s by a handoperated pump. A beaker was used as reaction vessel. After use, all
glassware was cleaned with concentrated nitric acid and rinsed with
generous amounts of ultrapure water.
For reactant concentration studies, 1 mM AgClO4 stock solution
was prepared by dissolving 225.33 mg of AgClO4·H2O in 1 L of water
and diluted to give 0.25, 0.4. 0.5, 0.6, and 0.75 mM precursor solutions.
NaBH4 solutions were prepared freshly by dissolving appropriate
amounts of NaBH4 in 1 L of water: 56.75 mg (1.5 mM), 85.13 mg
(2.25 mM), 113.5 mg (3 mM), 141.88 mg (3.75 mM), and 170.25 mg
(4.5 mM).
For investigations on the minimal excess of fresh NaBH4 (R =
[NaBH4]/[AgClO4]) required to receive a stable colloidal solution, 0.5
mM AgClO4 solution was reduced. NaBH4 solutions were prepared
freshly by dissolving appropriate amounts of NaBH4 in 1 L of water:
18.9 mg (0.5 mM, R = 1), 20.8 mg (0.55 mM, R = 1.1), 22.7 mg (0.6
mM, R = 1.2), 28.4 mg (0.75 mM, R = 1.5), and 37.8 mg (1 mM, R =
2).
To study the influence of the NaBH4 aging process, 1 L of 3 mM
NaBH4 solution was prepared freshly and stored at ambient conditions
(open to atmosphere). After certain aging times, three colloidal
solutions were prepared (standard lab-scale synthetic procedure with a
total volume of 10 mL).
The parameter study within the size plateau (see Figure 4) was
carried out by use of 3 mM NaBH4 solution that was stored 10 h at
ambient conditions as reducing agent. The silver precursor solutions
were adjusted for different variations:
of lab-scale SAXS investigations of final colloidal silver solutions
that were synthesized with BH4−/B(OH)4− mixtures containing
15−35% fresh NaBH4. For details of this synthetic procedure,
see the Experimental Section. Selected scattering curves and
their corresponding fits can be found in Supporting
Information (section SI-14).
For a mixture of 35% BH4− and 65% B(OH)4−, two
separated coalescent steps are observed during the colloidal
synthesis, whereas a mixture that contains only 32.5% BH4−
leads to a nanoparticle growth mechanism that comprises only
one single coalescent step. Thus, this mixture corresponds to
the beginning of phase II of the aging experiment (see Figure
4). In accordance with the aging experiment, a decreasing
percentage of BH4− leads to an increasing final mean radius.
The mean radius increases from 4 to 8 nm, whereas the mean
radius of the corresponding aging experiment is slightly bigger.
Nevertheless, this procedure enables a very simple and
reproducible access to colloidal silver nanoparticles with
accurate size control between 4 and 8 nm in radius
(polydispersity = 30%). To the best of our knowledge, this is
the first size-controlled synthesis of colloidal silver nanoparticles that does not require an additional stabilization agent.
■
CONCLUSIONS
This paper presents an approach to size control based on
mechanistic knowledge that was exemplified for a common
silver nanoparticle synthesis (reduction of AgClO4 with an
excess of NaBH4 in aqueous solution). It comprises an
understanding of the nanoparticle growth mechanism and the
influences of synthesis parameters on the growth. The growth
mechanism consists of four steps and includes two separated
steps of coalescence. It is shown that the final growth step (the
second coalescent step) correlates with the conversion of
residual BH4− to B(OH)4−. The depletion of BH4− could cause
a surface oxidation of the preliminary nanoparticles formed in
the first coalescent step. This would lead to a decrease of
colloidal stability, which initiates the further growth due to
coalescence. The second coalescence is a complex process that
can hardly be controlled. As a consequence, the final particle
size is not reproducible. From the mechanistic studies, it was
deduced that a decreasing ratio of BH4− to B(OH)4− leads to a
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(i) AgClO4 stock solution (1 mM) was diluted to give 0.5, 0.45, 0.4
and 0.25 mM silver precursor solutions.
(ii) AgClO4 stock solution (1 mM) was diluted 1:1 with 0.6, 1.2,
and 3 mM sodium perchlorate solutions. NaClO4·H2O (<98%) was
purchased from Sigma−Aldrich.
(iii) Perchloric acid (Sigma−Aldrich, 20%, p.a.) was diluted to
adjust a pH value of 3.0. To samples of 0.5 mM AgClO4 solution (25
mL each) were added 50, 100, and 250 μL of HClO4.
(iv) AgClO4 stock solution (1 mM) was diluted 1:1 with 0.6, 1.2, 3,
and 30 mM solutions of sodium tetrahydroxyborate. The NaB(OH)4
solutions were obtained by diluting 30 mM stock solution. To prepare
the stock solution, 1.13 g of NaBH4 were dissolved in 1 L of ultrapure
water and stored at ambient conditions for 1 week. During this time,
the borohydride species converts to tetrahydroxyborate. The
completeness of this conversion can be proved by adding AgClO4.
The solution should remain colorless (no formation of silver
nanoparticles), indicating the absence of any BH4−.
(v) For temperature variation, 0.5 mM AgClO4 solution and a
sample of the aged reducing agent solution were cooled to 0.5 and 10
°C, respectively. The syntheses were carried out in temperaturecontrolled water baths.
For the aging imitation experiment (see Figure 5), 113.5 mg of
NaBH4 granulate (Sigma−Aldrich, 98%) was dissolved in 1 L of MilliQ water (3 mM solution). The solution was stored at ambient
conditions for 1 day. During this time, the borohydride species
converts to tetrahydroxyborate. The completeness of this conversion
can be proved by adding AgClO4 to a sample of the B(OH)4− solution.
The solution should remain colorless (no reduction and thus no
formation of silver nanoparticles), indicating the absence of any BH4−.
Portions (85, 82.5, 80, 77.5, 75, 72.5, 70, 67.5, and 65 mL) of the
obtained B(OH)4− solution were filled up to 100 mL with freshly
prepared 3 mM NaBH4 solution. The obtained solutions were used to
reduce 0.5 mM AgClO4 solution (standard lab-scale synthetic
procedure with a total volume of 10 mL).
Note: The concentrations displayed in the diagrams always refer to
the solutions that are reacted 1:1 with the corresponding reactant
solution.
In Situ Small-Angle X-ray Scattering Investigations. Synchrotron SAXS Investigations. Synchrotron SAXS investigations were
performed at the ID02 beamline (ESRF) with a free-liquid jet setup.34
The distance between reaction vessel and jet was minimized to achieve
a low dead time (approximately 5 s) and to avoid agglomeration of
particles inside the tubing. The technique offers the possibility to
follow nanoparticle growth in situ with a time resolution that is limited
just by the photon flux and the acquisition time of the detector. In
addition, X-ray-induced effects are minimized and contamination
problems (contamination of capillary walls) are eliminated.
Lab-Scale SAXS Investigations of Final Colloidal Solutions.
Scattering curves of the final colloidal solutions were recorded by
extracting the samples from the batch solution and inserting them in a
flow cell of a SAXS instrument (SAXSess, Anton Paar GmbH).
Time-Resolved Lab-Scale SAXS Investigations. Mechanistic studies
at lab scale were performed by use of a SAXS instrument (SAXSess,
Anton Paar GmbH). The colloidal solution was pumped via Teflon
tubing into a flow cell that was cooled to 5 °C to suppress particle
agglomeration inside the quartz cell.
Small-Angle X-ray Scattering Evaluation. Scattering curves of
the colloidal solution were analyzed with the assumptions of spherical
shape, homogeneous electron density, and a Schulz−Zimm size
distribution. The Schulz−Zimm distribution is given by
f (r ) = (z + 1)z + 1x 2
exp[− (z + 1)x]
R avg Γ(z + 1)
I(q) = NIpart(q) = NVpart 2P(q)
⎧ 3[sin(qR ) − qR cos(qR )] ⎫2
⎬
= NVpart 2⎨Δρ
qR
⎩
⎭
(2)
In the case of polydisperse spherical particles, one has to sum the
scattering intensities over all particle sizes weighted by their frequency
or integrate by use of a size distribution function. It is common to use
the Schulz−Zimm distribution for polydisperse particles. Hence, the
scattering intensity is given by
I(q) = N
∫0
∞
f (r )Vpart 2P(q) dr
(3)
An analytical solution of the integral can be found in Kotlarchyk et
al.61 In order to analyze the nucleation and growth mechanism of
nanoparticles, the number of particles is important. This information
can be obtained from the general relation of I(q = 0) for a single
particle, which is independent of its shape and size, that is, I =
(Δρ)2V2. Thus the scattered intensity I(q = 0) of polydisperse particles
can be written as
I(q = 0) = N ⟨V 2⟩(Δρ)2
(4)
where N is the number of particles and ⟨V ⟩ is the mean value of V2.
Due to the overlapping of scattering intensity with the primary beam,
I(q = 0) cannot be measured directly, but it is accessible via
extrapolation of I(q) for q → 0.
Hydrogen Monitoring. Mass spectrometry was used to monitor
the hydrogen release from the reaction vessel. A scheme of the setup
can be found in section SI-4a in Supporting Information. The reactants
were mixed in the reaction vessel. Immediately, the vessel was sealed
with a septum and flushed by compressed air. The flow rate was
adjusted to V̇ in = 10.46 mL/min by use of a mass flow controller, F201D-FAC-33-P (Bronkhorst Mättig GmbH). A mass spectrometer,
OmniStarGSD301C (Pfeiffer Vacuum, Asslar, Germany), was attached
to the reaction vessel. The spectrometer was programmed for multiion monitoring in order to record the relative ion intensities of
hydrogen (m/z = 2), nitrogen (m/z = 28), and oxygen (m/z = 32)
with a time resolution of 1.7 s. The volume flow of the released
hydrogen V̇ H2 can be calculated from the percentage of hydrogen PH2:
2
VḢ 2 =
PH2Viṅ
1 − PH2
(5)
UV−Visible Spectroscopy. UV−vis spectra of the colloidal
solutions were recorded on an AvaSpec-2048TEC-2 equipped with a
deuterium halogen light source (Avantes, Broomfield, CO), connected
to a 10 mm optical path length cuvette holder via fiber optic cables.
The investigations were carried out in a standard 1 mL UV cuvette.
Colloidal solutions (200 μL) were extracted and mixed with 300 μL of
poly(vinylpyrrolidone) solution (MW = 40 000, 6 mg dissolved in 50
mL of water) to avoid agglomeration/aggregation of the nanoparticles
inside the cuvette. The time delay between extraction and actual
measurement was below 5 s.
■
ASSOCIATED CONTENT
S Supporting Information
*
Additional text and equations; 14 figures showing selected
scattering curves with linear rather than logarithmic y-scale,
selected scattering curves for different reaction times, UV−vis
spectra showing extended duration of metastable state by
addition of BH4−, schematic setup for time-resolved hydrogen
monitoring via MS spectrometry and total volume of released
H2 vs time, selected scattering curves for different mixing
conditions, mass vs time for relative humidities of 40% and
55%, SAXS data from variation of AgClO4 and NaBH4
concentrations, selected scattering curves from time-resolved
investigations, UV−vis spectra of selected colloidal solutions,
influence of NaBH4 aging time on final size distribution, time-
(1)
where Ravg is the mean radius, x = (r/Ravg), z is related to the
polydispersity p (p = σ/Ravg) by z = (1/p2 − 1), and σ2 is the variance
of the distribution. The scattering intensity of nonaggregated particles
can be assumed to be proportional to the form factor of a single
particle P(q). Thus, the scattering intensity of N monodisperse spheres
with homogeneous electron density with volume Vpart is given by
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resolved UV−vis investigations using aged reducing agent
solution, influence of NaBH4 aging time on particle growth
mechanism, parameter variation study within the plateau upon
variation of the reducing agent solution (mixing fresh and aged
NaBH4), and SAXS data from variation of the reducing agent
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■
AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
J.P. acknowledges generous funding by the Deutsche
Forschungsgemeinschaft within Project PO 1744/1-1. M.W.
also acknowledges financial support by the Fonds der
Chemischen Industrie. We acknowledge the European
Synchrotron Radiation Facility for provision of synchrotron
radiation facilities, and we thank Dr. T. Narayanan for
assistance in using beamline ID02.
■
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