small angle X-ray scattering

Article
Cite This: Chem. Mater. 2018, 30, 1127−1135
pubs.acs.org/cm
Tuning Precursor Reactivity toward Nanometer-Size Control in
Palladium Nanoparticles Studied by in Situ Small Angle X‐ray
Scattering
Liheng Wu, †,‡ Huada Lian, ‡ Joshua J. Willis, ‡,§ Emmett D. Goodman, ‡,§ Ian Salmon McKay, ‡ Jian Qin, ‡
Christopher J. Tassone,* ,† and Matteo Cargnello* ,‡,§
† Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
‡ Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States
§ SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States
*S Supporting Information
ABSTRACT: Synthesis of monodisperse nanoparticles (NPs) with precisely
controlled size is critical for understanding their size-dependent properties. Although
significant synthetic developments have been achieved, it is still challenging to
synthesize well-defined NPs in a predictive way due to a lack of in-depth mechanistic
understanding of reaction kinetics. Here we use synchrotron-based small-angle X-ray
scattering (SAXS) to monitor in situ the formation of palladium (Pd) NPs through
thermal decomposition of Pd−TOP (TOP: trioctylphosphine) complex via the
“heat-up” method. We systematically study the effects of different ligands, including
oleylamine, TOP, and oleic acid, on the formation kinetics of Pd NPs. Through
quantitative analysis of the real-time SAXS data, we are able to obtain a detailed
picture of the size, size distribution, and concentration of Pd NPs during the
syntheses, and these results show that different ligands strongly affect the precursor reactivity. We find that oleylamine does not
change the reactivity of the Pd−TOP complex but promote the formation of nuclei due to strong ligand−NP binding. On the
other hand, TOP and oleic acid substantially change the precursor reactivity over more than an order of magnitude, which
controls the nucleation kinetics and determines the final particle size. A theoretical model is used to demonstrate that the
nucleation and growth kinetics are dependent on both precursor reactivity and ligand−NP binding affinity, thus providing a
framework to explain the synthesis process and the effect of the reaction conditions. Quantitative understanding of the impacts of
different ligands enables the successful synthesis of a series of monodisperse Pd NPs in the broad size range from 3 to 11 nm with
nanometer-size control, which serve as a model system to study their size-dependent catalytic properties. The in situ SAXS
probing can be readily extended to other functional NPs to greatly advance their synthetic design.
1. INTRODUCTION
Rational design and synthesis of well-defined colloidal nanoparticles
(NPs) is of utmost importance for fundamentally
studying their intrinsic properties and for various technological
applications. 1−3 Over the past two decades, the synthetic
control of colloidal NPs has come to the level that by tuning
reaction conditions (e.g., reaction precursors, ligands, reaction
temperature, etc.), various sizes, shapes, compositions, and even
complex structures of NPs have been achieved. 4−8 However,
these syntheses are typically developed empirically, using trialand-error
approaches that cause substantial waste of time and
resources. Understanding the formation mechanism of these
NPs will provide guidelines for greatly accelerating their
synthesis with tailored properties. Unfortunately, it is still
challenging to study the fast nucleation and growth kinetics due
to the lack of proper experimental setups.
In past few years, the fast developments of in situ
experimental techniques have enabled the real time probing
of NP formation in solution. Direct visualization of NP growth
at the atomic resolution has been realized by in situ
transmission electron microscopy (TEM) using liquid environmental
cells. 9,10 Alternatively, synchrotron based X-ray
scattering techniques, due to high penetration of the X-ray
and fast data aquisition, 11,12 have advanced our understanding
of NP nucleation and growth during colloidal synthesis. 13−37
Taking Au NPs as an example, using in situ small-angle X-ray
scattering (SAXS), the nucleation kinetics of Au NPs
synthesized using different ligands were directly monitored. 16
The growth mechanism of Au NPs was also studied, 18,19 which
involves a rapid nucleation followed by NP growth driven by
both monomer attachment and particle coalescence. This in
situ technique has also been utilized to experimentally probe
NP formation under harsh reaction conditions, such as the
formation of CdSe quantum dots at 240 °C in a glass capillary,
in which the thermal activation of selenium precursor was
found to be the growth rate-determining step. 24 Despite this
Received: December 14, 2017
Revised: January 2, 2018
Published: January 3, 2018
© 2018 American Chemical Society 1127 DOI: 10.1021/acs.chemmater.7b05186
Chem. Mater. 2018, 30, 1127−1135

Chemistry of Materials
success, important questions still unanswered are whether the
measured nucleation and growth kinetics is valid in the lab-scale
batch synthesis and how the kinetics can be utilized for
designing better synthetic strategies toward well-defined NPs
for desired applications.
Here we report the in situ SAXS probing of the synthesis of
Pd NPs through thermal decomposition of Pd−TOP complex
in 1-octadecene solution using a “heat-up” method. 38−40 We
use a custom-made flask reactor to mimic typical laboratory
reaction conditions (e.g., high temperature, inert atmosphere)
for solution-phase colloidal synthesis. 41 We chose to study the
synthesis of Pd NPs because of their widespread catalytic
applications in many important chemical reactions, including
methane combustion reaction, 42 methane steam reforming
reaction, 43 and electrochemical oxidation of formic acid. 44,45
Understanding the size−activity relationship is critical for
designing better catalysts for those reactions. Although different
synthetic approaches have allowed for the preparation of
different-sized Pd NPs with narrow size distributions, 38,46−52
currently, there is still no general guideline for precisely
controlling their size due to the lack of insights into the
formation kinetics. By systematically studying the roles of
different ligands (i.e., trioctylphosphine (TOP), oleylamine,
and oleic acid) via in situ SAXS, we are able to quantitatively
explore the reaction kinetics. We find that the Pd−TOP
precursor reactivity is strongly affected by the type and amount
of ligands used, which controls the nucleation kinetics and
allows for the fine control of the final particle size. Theoretical
models suggest that the growth of Pd NPs is limited by surface
reaction between monomers and NPs or by thermal activation
of Pd-precursor depending on the precursor reactivity. The
mechanistic understanding of the effects of different ligands
enables the synthesis of Pd NPs in the size range of 3 to 11 nm
with nanometer-size control. We believe that the in situ SAXS
characterization coupled with the versatile reactor geometry
described here can be extended to accelerate the synthetic
developments of various functional NPs.
2. EXPERIMENTAL SECTION
2.1. Chemicals and Materials. Pd(acac) 2 (acac = acetylacetonate,
35% Pd) was purchased from Acros Organics. Oleic acid (90%),
oleylamine (70%), and 1-octadecene (90%) were purchased from
Sigma-Aldrich and degassed under vacuum at 100 °C for 1 h before
use. TOP (97%) was purchased from Sigma-Aldrich and used without
further purification. Hexanes and isopropanol were purchased from
Fisher Scientific.
2.2. Syntheses of Pd NPs. The syntheses were performed in a
custom-made flask reactor under inert Ar gas. Pd NPs were
synthesized via thermal decomposition of Pd−TOP complex modified
from previously reported procedures. 38,39 In a typical procedure, 1-
octadecene (10 mL) and Pd(acac) 2 (0.25 mmol) were mixed via
magnetic stirring under a gentle flow of Ar for 15 min. Then TOP was
injected, immediately forming a light yellow Pd−TOP complex, 53
followed by the addition of oleylamine or oleic acid depending on
experimental choices (Table 1). The mixture was kept at 60 °C for 30
min under the Ar flow to remove air and moisture and then heated to
280 °C using a heating rate of ∼15 °C min −1 (the heating rates for all
the reactions are very similar) and kept at the final temperature for up
to 30 min. After the reaction, the flask was cooled down to room
temperature by removing the heating tape. The obtained Pd NPs were
isolated by precipitation with 20 mL of 2-propanol followed by
centrifugation at 8000 rpm for 3 min. The precipitated Pd NPs were
further purified by another two cycles of precipitation (5 mL of
hexanes as the solvent and 20 mL of 2-propanol as the antisolvent).
After purification, the Pd NPs were redispersed in 5 mL of hexanes for
1128
Article
Table 1. Different Combinations of Ligands for the Synthesis
of Pd NPs in This Work a
reaction TOP oleylamine oleic acid
A 1.25 mmol - -
B 1.25 mmol 0.63 mmol -
C 1.25 mmol 2.50 mmol -
D 2.50 mmol 2.50 mmol -
E 1.25 mmol - 3.75 mmol
F 1.25 mmol - 9.40 mmol
G 1.25 mmol - 15.60 mmol
a The amount of Pd(acac) 2 used in each synthesis is 0.25 mmol.
further characterizations. Different combinations of ligands were
studied to systematically evaluate their effect on reaction kinetics and
are summarized in Table 1. For reactions E−G, due to large volume of
oleic acid used, the amount of 1-octadecene used was tuned such that
the total volume of 1-octadecene and oleic acid was 10 mL to maintain
similar precursor concentration for these reactions.
2.3. In Situ SAXS Characterization. In Situ SAXS measurements
were performed at Beamline 1-5 of Stanford Synchrotron Radiation
Lightsource (SSRL) using our custom-made setup (Figure S1). 41 The
X-ray path length within the reactor was 5 mm. The X-ray energy was
15.5 keV, and the beam spot size was 500 μm × 500 μm. The sampleto-detector
distance was calibrated to be 741.6 mm using a silver
behenate standard. For each data acquisition, an exposure time of 5 s
was applied. A Rayonix SX165 CCD area detector was used to collect
the two-dimensional (2D) scattering patterns. The 2D SAXS patterns
were reduced to 1D data, calibrated to absolute scale using a glassy
carbon standard (Figure S2a), 54 and were further analyzed using the
Irena package (available at usaxs.xray.aps.anl.gov/staff/ilavsky/irena.
html from the APS). 12 The size, size distribution, concentration, and
volume fraction of the Pd NPs were modeled in Irena package using a
spherical form factor (see details in the Supporting Information). The
yield (Y) of Pd NPs was obtained based on the equation Y = ϕ/ϕ max ,
where ϕ is the volume fraction of total Pd NPs at a specific reaction
time and ϕ max is the maximum volume fraction of Pd NPs in the
solution at 100% conversion of Pd-precursor to Pd nanoparticles.
2.4. Ex Situ Characterization. The purified Pd NPs were
characterized by TEM. TEM samples were prepared by drop-casting a
dilute NP dispersion in hexane onto carbon-coated 300 mesh Cu grids.
TEM images were collected on a FEI Tecnai transmission electron
microscope operated with an accelerating voltage of 200 kV. The
obtained superlattices of Pd NPs with the oleic acid ligand were
imaged using a FEI Magellan 400 XHR scanning electron microscopy
operating at 5 kV. Fourier transform infrared spectra of the NPs were
recorded on a Nicolet iS50 spectrometer.
3. RESULTS AND DISCUSSION
Pd NPs were synthesized through thermal decomposition of
Pd−TOP complex in 1-octadecene (see Experimental Section
for details). The synthesis procedure using only TOP as
surfactant (reaction A in Table 1) is taken as standard. The
Pd−TOP complex solution was heated up from 60 to 280 °C
and kept at 280 °C for up to 30 min. During the reaction, SAXS
patterns were acquired at an exposure time of 5 s for each
pattern. The obtained 2D SAXS data were integrated into 1D
data, and the background signal from the reactor and solvent
were subtracted. As reaction temperature increases up to 230
°C, the scattering from nuclei with diameter larger than 1 nm
appears. This temperature, defined as the nucleation temperature,
is taken as t = 0 s. Representative background-subtracted
SAXS patterns are shown in Figures 1a and S2b. As the reaction
proceeds, the scattering intensity at low scattering vector q
increases quickly, indicating increased particle size. Obvious
oscillation peaks appear as well, suggesting the narrowing of the
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Figure 1. (a) Representative SAXS data (colored plots) and the
corresponding fits (black plots) of Pd NPs at different reaction times
during the synthesis using only TOP as surfactant (reaction A,
Pd:TOP molar ratio = 1:5). The scattering intensities are offset for
clarity. Time t = 0 s is set when nucleation starts. (b) Quantitative
analysis of the concentration of Pd NPs as a function of reaction time.
(c) Size (dots) and size distribution (bars) of Pd NPs as measured by
in situ SAXS. (d) Reaction temperature profile during the synthesis.
Article
NP size distribution. To obtain more information about size,
size distribution, and concentration of the Pd NPs, the SAXS
data were fitted using a spherical NP model in the Irena
package (see SI for details). Figure 1b−d shows the quantitative
analysis of the mean diameter, polydispersity, and concentration
of Pd NPs as a function of reaction time. Based on the
analysis of particle concentration, burst nucleation occurs in the
first 50 s, during which the NP concentration increases with a
constant nucleation rate of 1.22 × 10 14 mL −1 s −1 in the case of
the standard reaction A (Table 1). Meanwhile, the polydispersity
of the formed nuclei dropped significantly to ∼14%. The
narrow size distribution of the nuclei is critical for their further
simultaneous growth into uniform NPs. During this nucleation
stage ∼27% of the Pd atoms in the precursor are incorporated
into Pd NPs (Figure 2c). Between 50 to 115 s, the Pd NPs
continue to slowly grow in size from 2.8 to 3.7 nm. Although a
small concentration of new nuclei is formed during this period,
the much slower nucleation rate (2.4 × 10 13 mL −1 s −1 )
guarantees that the fraction of newly formed nuclei is relatively
small and the overall polydispersity was not increased but
instead dropped to ∼10%. Between 115 and 160 s Pd NPs
slowly grow to 3.9 nm at nearly constant particle concentration
due to much decreased monomer concentration. After 160 s,
there is a slight decrease in particle concentration (Figure 1b),
suggesting the existence of NP−NP coalescence or Ostwald
ripening, which is common in the colloidal synthesis of
NPs. 18,24,55,56 After 5 min, Pd NPs with the final size of 4.0 ±
0.4 nm are obtained, and their size and concentration do not
change during further aging process. The final Pd NPs were
also characterized by TEM, as shown in Figure 2d. The size
based on TEM characterization was measured to be 3.7 ± 0.3
nm, which is in good agreement with the SAXS measurement
(Figure 2e,f and Table 2).
To study the effect of different ligands on the synthesis of Pd
NPs, we started by comparing the standard reaction condition
(reaction A) with those where oleylamine, a widely used
stabilizing ligand for metallic NPs, 57 is present. Real-time SAXS
patterns using different amounts of oleylamine are shown in
Figure 3. We observed a similar nucleation temperature at
Figure 2. (a−c). Quantitative analysis of (a) size, (b) concentration, and (c) yield of Pd NPs with different amounts of oleylamine as measured via in
situ SAXS. (d) TEM images of the final Pd NPs synthesized with different ligands. (e) SAXS patterns (colored plots) and the corresponding fits
(black plots) of the as-synthesized Pd NPs. (f) Size and size distribution measured from TEM (histogram) and SAXS (dotted plots, normalized to
the maximum of the histogram). Results from reactions A, B, and C are shown in red, blue, and purple, respectively.
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Table 2. Average Pd NP Sizes Measured from TEM and SAXS
reaction A B C D E F G
SAXS (nm) 4.0 ± 0.4 3.8 ± 0.4 3.4 ± 0.4 5.5 ± 0.6 7.5 ± 0.5 9.2 ± 0.6 11.0 ± 0.9
TEM (nm) 3.7 ± 0.3 3.5 ± 0.3 3.3 ± 0.3 5.2 ± 0.7 7.3 ± 0.3 8.7 ± 0.6 10.5 ± 0.9
Figure 3. Representative SAXS patterns (colored plots) and the
corresponding fits (black plots) of Pd NPs at different reaction times
in the presence of (a) TOP 1.25 mmol + oleylamine 0.65 mmol
(reaction B); (b) TOP 1.25 mmol + oleylamine 2.50 mmol (reaction
C). The scattering intensities are offset for clarity. Time t = 0 s is set
when nucleation starts and the nucleation temperature is at ∼230 °C
for both reactions.
Although oleylamine changes the nucleation rate, the change is
small which leads to limited tunability of final particle size. We
also tested the effect of final reaction temperature on the
formation of Pd NPs and found that it has minimal or no effect
on particle size and size distribution, as long as it is above the
critical nucleation temperature that enables burst nucleation.
For example, there is no obvious difference in the final size of
Pd NPs ex situ synthesized using the same ligand mixture from
reaction C but at different final reaction temperatures of 240
and 290 °C (Figure S4).
In contrast to oleylamine, TOP substantially changes the
reactivity of Pd precursor. It has been demonstrated that Pd(II)
forms a stable Pd II −(TOP) 4 complex and a large excess of TOP
retards the thermal decomposition of the Pd−TOP complex. 53
Taking advantage of the in situ SAXS, we quantitatively studied
the effect of TOP by doubling its amount (molar ratio of
Pd:TOP increases from 1:5 to 1:10, reaction D). We observed
the formation of Pd NPs at 255 °C rather than 230 °C due to
the shift in the reaction equilibrium of the decomposition of
Pd−TOP complex into Pd NPs, which requires higher energy
to activate it. Quantitative analysis of the SAXS data (Figure
S5) shows that the formation rate of Pd NPs is reduced by a
factor of 5 (Figure 4c). Due to a depressed precursor reactivity
in the presence of large excess of TOP which cannot support a
∼230 °C, indicating that oleylamine does not affect the
formation of monomer species or the chemistry of the Pd−
TOP complex. The Pd NP size and concentration are derived
from the fits of the SAXS results, and their quantitative
comparisons are shown in Figure 2a−c. Similar to the case
without oleylamine, a rapid nucleation occurred in the first ∼50
s, followed by a slow growth of NP from ∼2.8 nm in size. The
similar yield of Pd NPs over reaction times suggests that
oleylamine does not affect precursor reactivity (Figure 2c).
However, slightly higher nucleation rate (1.37 × 10 14 mL −1 s −1 )
was observed when 0.63 mmol of oleylamine was used
(reaction B), generating more nuclei. Since the precursor
amount is constant, the more nuclei formed, the smaller the
final Pd NPs, as confirmed by the smaller size measured from
SAXS (3.8 ± 0.4 nm) and TEM (3.5 ± 0.3 nm) (Figure 2d−f
and Table 2). The difference in the nucleation rate in the
presence of oleylamine is likely due to stronger binding
between oleylamine and the Pd 0 nuclei, which facilitates the
formation of nuclei by stabilizing them with reduced nuclei−
solvent interfacial tension. This observation is also supported
by the fact that the TOP ligand on the Pd surface can be easily
exchanged with oleylamine (Figure S3) and further corroborated
by previous studies involving NMR. 53 Further increasing
the amount of oleylamine from 0.63 to 2.50 mmol
correspondingly increases the nucleation rate to 1.86 × 10 14
mL −1 s −1 (Figure 2), leading to even smaller Pd NPs with an
average diameter of 3.4 ± 0.4 nm from SAXS (Figure 2d−f).
Figure 4. Quantitative analysis of (a) size, (b) concentration, and (c)
yield of Pd NPs as measured via in situ SAXS during the syntheses
with different amounts of TOP. (d) Representative TEM image of the
final Pd NPs. (e) SAXS pattern (colored plot) and the corresponding
fit (black plot) of the as-synthesized Pd NPs. (f) Size and size
distribution measured from TEM (histogram) and SAXS (dotted
plots, normalized to the maximum of the histogram).
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Figure 5. (a, b) Quantitative analysis of (a) size and (b) yield of Pd NPs measured via in situ SAXS at varying molar ratios of oleic acid:TOP
(reactions E−G). (c) Represenative TEM images of the dispersed Pd NPs synthesized from reaction E (upper), reaction F (middle), and reaction G
(lower). Scale bars are 20 nm. (d) SAXS patterns (colored plots) and corresponding fits (black line) of the as-synthesized Pd NPs. (e) Size and size
distribution measured from TEM (histogram) and SAXS (dotted plots, normalized to the maximum of the histogram). Results from reactions E, F,
and G are shown in red, blue, and purple, respectively.
Figure 6. Classical modeling of the formation kinetics of Pd NPs using different ligands. (a) Only TOP (reaction A). (b) A mixture of TOP and
oleylamine (reaction C). (c) A large excess of TOP (reaction D). (d) A mixture of oleic acid and TOP (reaction F). The red lines are from the
classical model, and the size and size distribution are experimental results from SAXS.
large amount of monomer, the nucleation rate (1.01 × 10 13
mL −1 s −1 ) is more than an order of magnitude smaller
compared to that for reaction C (1.86 × 10 14 mL −1 s −1 ), and
the nucleation process lasts much longer (∼290 s) compared to
the rapid nucleation (∼50 s) for reaction C (Figure 4a,b). As a
result, fewer nuclei are formed (1.9 × 10 15 mL −1 vs 8.5 × 10 15
mL −1 ) and the final Pd NPs are larger in diameter (5.5 ± 0.6
nm for reaction D vs 3.4 ± 0.4 nm for reaction C) from SAXS,
Figure 4e,f), which are also confirmed by ex situ TEM (Figure
4d). It is worth mentioning that, due to much longer nucleation
process in reaction D, the particle size distribution is wider
(Figure 4d), thus highlighting the importance of controlling
precursor reactivity for burst nucleation in order to obtain
narrow size distribution.
Recently we reported that, in stark contrast to conventional
colloidal synthesis, the Pd NPs rapidly crystallize into threedimensional
superlattices in the presence of oleic acid rather
than dispersed NPs, and weak binding between oleic acid and
Pd NPs plays a critical role in the crystallization. 41 We found
that, in the presence of oleic acid (1:1 molar ratio of oleic
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Table 3. Fitting Parameters Used for Modelling the Nucleation and Growth Kinetics
reaction A B C D E F G
γ (J m −2 ) 1.155 1.150 1.142 1.245 1.160 1.175 1.192
ξ 100000 115000 135000 80000 56700 50000 40000
k r /T (nm s −1 K −1 ) 0.407 0.355 0.305 0.566 0.714 0.799 0.985
acid:TOP), Pd NPs start to form at 200 °C, which is ∼30 °C
lower than the standard reaction A with only TOP. Oleic acid
acts like a “catalyst” for the thermal decomposition of the Pd−
TOP complex, which is probably due to either the active
involvement of oleic acid in the thermal decomposition of Pd−
TOP complex or the formation of a Pd(oleate) x (TOP) y
complex that decomposes at lower temperatures compared to
pure Pd−TOP complex. 58−60 The reduced reaction temperature
resulted in a smaller nucleation rate, and consequently,
larger particles of 6.8 ± 0.4 nm, which is ∼2.8 nm larger than
the Pd NPs synthesized without oleic acid (reaction A). 41 By
increasing the amount of oleic acid, we observed similar in situ
assembly of Pd NPs into face-centered cubic superlattices,
followed by postgrowth of Pd NPs inside the superlattices as
described before 41 (Figure S6−S9). The mean diameter and
the yield of Pd NPs as a function of reaction time derived from
SAXS are shown in Figure 5a,b. Interestingly, increasing the
molar ratio of oleic acid:TOP results in even larger Pd NPs
with narrow size distributions. Pd NPs of 7.5 ± 0.5 nm, 9.2 ±
0.6 nm, and 11.0 ± 0.9 nm average diameters from SAXS were
synthesized using 3:1, 7.5:1, and 12.5:1 molar ratios of oleic
acid:TOP, respectively (Figure 5c−e). We ascribe the large
tunability of size to different nucleation rates caused by
different binding affinity of oleic acid/TOP on Pd NPs.
To gain more insights on the formation of Pd NPs with
different ligands, we modeled the nucleation and growth
kinetics using classical models 61 in which three steps are
involved: (i) decomposition of Pd precursor into monomers;
(ii) reaction of monomers to form nuclei; and (iii) growth of
nuclei with addition of monomers (see details in SI). Fitting the
experimental results to these models provides useful insights on
the nucleation kinetics and the growth-limiting mechanism of
Pd NPs in the presence of different ligands. The nucleation rate
is dependent on the degree of supersaturation, reaction
temperature, and interfacial tension between NP and the
solvent. The growth rate can be expressed by the equation
⎡ 2γV
⎤
DV C ⎣S
− exp
r
(
m
d
m 0
rRT ) ⎦
=
, where D is the diffusion coefficient of
dt
r+
D/
kr
Pd monomers, V m is the molar volume of Pd NPs, C 0 is
monomer solubility, S is the dimensionless parameter
describing the supersaturation of Pd monomer in the solution,
r is the radius of Pd NPs, γ is interfacial tension between Pd NP
and the solvent, k r is the surface reaction rate of the
incorporation of monomers into the NPs, R is gas constant
(8.314 J K −1 mol −1 ), and T is the reaction temperature. 62−64
This equation is converted into dimensionless form
* *
dr S−
exp(1 / r )
=
dτ
*
r + ξ
with three dimensionless parameters: the
*
RT
reduced radius r = γ
r, the reduced kinetic length
2 V m
D
RT
RT
( )
ξ = , and the reduced time τ = ( kr
2γVm
γ
VC ) Dt
2 V m 0 .By
m
adjusting the parameters, we obtained a good fit to
experimental results in both nucleation and growth regimes
of reaction A with γ = 1.155 J m −2 , k r /T = 4.07 × 10 −1 nm s −1
K −1 , and ξ = 1.0 × 10 5 (Figure 6a). The large ξ ≫ 1 indicates
2
that k r·r ≪ D, suggesting that the growth of Pd NPs is limited
by surface reaction between monomers and Pd NPs rather than
by diffusion of monomers to the particle surface. 62 As discussed
before, oleylamine does not change the Pd−TOP precursor
reactivity, and thus good fits of the growth kinetics were
obtained showing similar nucleation and growth mode (Figures
6b and S10 and Table 3). The slight decrease of the γ value in
the presence of oleylamine is in line with our hypothesis that
stronger binding ligand covers NPs more densely reducing the
NP−solvent interfacial tension. However, in the presence of
large excess of TOP, a classical growth kinetic model does not
provide a good fit to the experimental results due to substantial
decrease of precursor reactivity (Figure 6c), suggesting that the
particle growth is limited neither by the monomer diffusion nor
by surface reaction but instead by the thermal activation of Pd
precursor, which is confirmed by our experimental observations.
In the case with oleic acid, the classical model fits well
with the experimental results (Figures 6d and S10), suggesting
the same surface-reaction limited growth mechanism. The
larger γ value in the case of oleic acid suggests less dense ligand
coverage of Pd NPs, which was confirmed by weaker binding
between oleic acid and Pd NPs compared to oleylamine or
TOP. 41 The same trend of larger γ with oleic acid compared to
oleylamine was also observed in the Au NPs. 16,64 We should
emphasize that increased value of γ with oleic acid affects the
nucleation and growth kinetics much more considerably, thus
offering wider tunability of the particle size. These results
together with the quantitative results from in situ SAXS
highlight the importance of tailoring the precursor reactivity
and ligand−NP binding affinity to rationally tune the final NP
size. 65,66
4. CONCLUSIONS
Using in situ synchrotron-based SAXS, we have systematically
studied the effect of different ligands (i.e., oleylamine, TOP,
and oleic acid) on the synthesis of Pd NPs. Through
quantitative analysis we have shown that nucleation kinetics
is strongly dependent on the Pd−TOP precursor reactivity and
ligand−NP binding affinity, which determines the final particle
size and quality. Due to the formation of thermally stable Pd−
TOP complex, an excess amount of TOP significantly retards
the precursor decomposition and slows down the nucleation
rate by more than an order of magnitude, and larger and more
polydisperse NPs are synthesized. Oleylamine does not affect
the reactivity of Pd−TOP precursor but slightly facilitates the
formation of nuclei due to stronger binding between oleylamine
and Pd NPs, leading to smaller NPs in the presence of
oleylamine. In contrast, oleic acid strongly influences the
reactivity of the Pd−TOP complex and the nucleation kinetics,
and larger Pd NPs are synthesized in the presence of more oleic
acid. The quantitative understanding of the nucleation kinetics
with different ligands studied by in situ SAXS enables the
synthesis of a library of monodisperse Pd NPs (polydispersity <
10%) with a wide size range from 3 to 11 nm. These welldefined
Pd NPs serve as a model system for studying their sizedependent
catalysis for methane combustion reaction which is
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presented in a separate publication. 67 The in situ SAXS
measurement coupled with the versatile flask reactor can be
readily extended to a broad variety of functional NPs to
accelerate their synthetic developments for both fundamental
research
■
and technological applications.
ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemmater.7b05186.
Details on the SAXS data analysis, kinetic models on the
formation of nanoparticles, and additional supplementary
Figures S1−S10 (PDF)
■ AUTHOR INFORMATION
Corresponding Authors
*(C.J.T.) E-mail: tassone@slac.stanford.edu.
*(M.C.) E-mail: mcargnello@stanford.edu.
ORCID
Jian Qin: 0000-0001-6271-068X
Matteo Cargnello: 0000-0002-7344-9031
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy
(DOE), Laboratory Directed Research and Development
program, at SLAC National Accelerator Laboratory under
Contract No. DE-AC02-76SF00515. In situ SAXS experiments
were performed at the Beamline 1-5 at the Stanford
Synchrotron Radiation Lightsource (SSRL) of SLAC, and use
of the SSRL is supported by the DOE Office of Science, Office
of Basic Energy Sciences, under Contract No. DE-AC02-
76SF00515. J.J.W. acknowledges support from the U.S.
Department of Energy, Office of Sciences, Office of Basic
Energy Sciences, to the SUNCAT Center for Interface Science
and Catalysis. M.C. acknowledges support from the School of
Engineering at Stanford University and from a Terman Faculty
Fellowship. J.Q. acknowledges support from the 3M Non-
Tenured Faculty Award and the Hellman Scholar Award.
E.D.G. acknowledges support from the National Science
Foundation Graduate Research Fellowship under Grant
DGE-1656518. I.S.M. was supported by the Department of
Defense through the National Defense Science & Engineering
Graduate Fellowship Program and by the Fannie and John
Hertz Foundation through a Hertz Foundation Fellowship.
L.W. and C.J.T. thank T.J. Dunn from SSRL for his assistance
during the experiments. The electron microscopy characterization
was performed at the Stanford Nano Shared Facilities
(SNSF) at Stanford University.
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