Iodide-induced differential control of metal ion reduction rates: synthesis of terraced palladium-copper nanoparticles with dilute bimetallic surfaces

Article abstract of DOI:10.1039/C8TA06256K

Metal nanoparticles possessing a high density of atomic steps and edge sites provide an increased population of undercoordinated surface atoms, which can enhance the catalytic activity of these materials compared to low-index faceted or bulk materials. Simply increasing reactivity, however, can lead to a concurrent increase in undesirable, non-selective side products. The incorporation of a second metal at these reactive stepped features provides an ideal avenue for finely attenuating reactivity to increase selectivity. A major challenge in synthesizing bimetallic nanomaterials with tunable surface features that are desirable for fundamental catalytic studies is a need to bridge differences in precursor reduction potentials and metal lattice parameters in structures containing both a noble metal and a non-noble metal. We report the use of low micromolar concentrations of iodide ions as a means of differentially controlling the relative reduction rates of a noble metal (palladium) and a non-noble metal (copper). The iodide in this system increases the rate of reduction of palladium ions while concurrently slowing the rate of copper ion reduction, thus providing a degree of control that is not achievable using most other reported means of tuning metal ion reduction rate. This differential control of metal ion reduction afforded by iodide ions enables access to nanoparticle growth conditions in which control of palladium nanoparticle growth by copper underpotential deposition becomes possible, leading to the generation of unique terraced bimetallic particles. Because of their bimetallic surface composition, these terraced nanoparticles exhibit increased selectivity to acetaldehyde in gas phase ethanol oxidation.

Full text of DOI:10.1039/C8TA06256K

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ISSN 2050-7488
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Iodide-Induced Differential Control of Metal Ion Reduction Rates: Synthesis of Terraced
Palladium-Copper Nanoparticles with Dilute Bimetallic Surfaces
Melissa E. King and Michelle L. Personick*
Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459, United States
Email: mpersonick@wesleyan.edu
Abstract
Metal nanoparticles possessing a high density of atomic steps and edge sites provide an
increased population of undercoordinated surface atoms, which can enhance the catalytic activity
of these materials compared to lowꢀindex faceted or bulk materials. Simply increasing reactivity,
however, can lead to a concurrent increase in undesirable, nonꢀselective side products. The
incorporation of a second metal at these reactive stepped features provides an ideal avenue for
finely attenuating reactivity to increase selectivity. A major challenge in synthesizing the
bimetallic nanomaterials with tunable surface features that are desirable for fundamental
catalytic studies is a need to bridge differences in precursor reduction potentials and metal lattice
parameters in structures containing both a noble metal and a nonꢀnoble metal. We report the use
of low micromolar concentrations of iodide ions as a means of differentially controlling the
relative reduction rates of a noble metal (palladium) and a nonꢀnoble metal (copper). The iodide
in this system increases the rate of reduction of palladium ions while concurrently slowing the
rate of copper ion reduction, thus providing a degree of control that is not achievable using most
other reported means of tuning metal ion reduction rate. This differential control of metal ion
reduction afforded by iodide ions enables access to nanoparticle growth conditions in which
control of palladium nanoparticle growth by copper underpotential deposition becomes possible,
leading to the generation of unique terraced bimetallic particles. Because of their bimetallic
surface composition, these terraced nanoparticles exhibit increased selectivity to acetaldehyde in
gas phase ethanol oxidation.
1

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Introduction
The synthesis of wellꢀdefined metal nanoparticles with complex shapes that range beyond
Platonic solids has been the focus of ongoing research in nanoparticle synthesis due to the
potential applications of these exotic structures in plasmonics1ꢀ6 and catalysis.7ꢀ12 For catalytic
applications, bimetallic particles are especially favorable because they offer additional tunability
of surface composition, which can lead to greater activity and selectivity. The increased
functional flexibility of bimetallic particles relative to their monometallic counterparts may be
attributed to the synergistic effects that result from coupling the individual properties of each
metal.13ꢀ21 For example, platinum is an effective and active catalyst for the electrocatalytic
oxidation of small organic molecules, however, it has been shown that bimetallic structures show
greater sustained catalytic activity due to decreased surface poisoning in the presence of a
secondary metal such as lead or bismuth.22 Moreover, the deposition of a dilute surface coverage
of a secondary metal is advantageous for strategically enhancing or attenuating the activity of the
underlying metal in order to promote higher catalytic selectivity.17, 23 The prospect of enhanced
catalyst performance due to cooperative interactions at bimetallic surfaces has motivated the
synthesis of a number of bimetallic noble metal nanoparticle structures—including concave
cubes, stellated octahedra, concave hexoctahedra, and trisoctahedra—that are comprised of
various binary combinations of the metals gold, palladium, platinum, and silver.14, 24ꢀ27 Such
particles may have alloyed or coreꢀshell structures. Shaped nanoparticles with certain bimetallic
compositions, such as silverꢀgold and palladiumꢀplatinum, can be more readily synthesized due
to the small lattice mismatch between these pairs of noble metals.
The realization of polyhedral bimetallic nanostructures that incorporate less noble metals,
however, has been an ongoing challenge because these catalytically relevant metals generally
2

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have a substantial lattice mismatch with noble metals, which leads to illꢀdefined particles with
dendrite or island formation.28, 29 The incorporation of a second, less noble, metal during
nanoparticle synthesis also has a distinct possibility of causing undesired galvanic exchange in
which the noble metal completely displaces the secondary metal of interest in the final particle
morphology. This galvanic exchange process is sometimes used in an intentional, controlled
manner to circumvent issues that arise from the lattice mismatch between two noble metals.30ꢀ33
Copper or lead, for example, can be deposited electrochemically via underpotential deposition
onto preꢀsynthesized metal nanoparticles to serve as a sacrificial metal that will undergo galvanic
exchange with a more noble metal to produce coreꢀshell structures with controllable shell
thicknesses.26 Importantly, underpotential deposition may also be employed directly during nonꢀ
electrochemical colloidal synthesis to control particle morphology and generate bimetallic
surface compositions. Underpotential deposition, in which metal ions are reduced onto the
surface of a host metal at a potential more positive than their Nernst potential, yields a subꢀ
monolayer to monolayer coverage of a secondary metal at the particles’ surface.34 These
underpotentially deposited atoms have been shown to stabilize highꢀenergy surface features such
as step edges. As a result, underpotential deposition of silver or copper has proven to be a
powerful tool in the solutionꢀphase colloidal synthesis of highꢀindex faceted bimetallic gold
nanostructures, such as concave cubes, ditetragonal prisms, and tetrahexahedra.35ꢀ39 Conversely,
the underpotential deposition of copper onto the surface of palladium nanoparticles without
employing electrochemical methods has been challenging due to the low reduction potential of
copper.
Herein, we report a solutionꢀphase colloidal method for the synthesis of terraced palladium
particles possessing stepped surface features that are stabilized by underpotentially deposited
3

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copper. The shapeꢀcontrolling effects of the deposited copper atoms are finely balanced by
tuning the reduction rate of palladium ions via the addition of iodide anions to the growth
solution. Previous work from our group showed that low micromolar concentrations of iodide
ions facilitate palladium ion reduction during the synthesis of monometallic nanoparticles.40 The
deliberate incorporation of iodide in the current work similarly leads to an increased rate of
palladium ion reduction, which in turn shifts the copper to palladium ratio of the particles at the
key early stages of growth where shape is determined. Literature reports have shown that
bimetallic palladiumꢀcopper particles tend to form alloyed tripod morphologies with higher
copper content.41, 42 In contrast, the cooperative effects of iodide and copper in this system direct
particle formation to generate terraced bimetallic particles with a low copper content that
suggests subꢀmonolayer copper coverage at the palladium surface. The absence of either copper
or iodide results in lowꢀindex faceted morphologies or tripods, respectively. Together, these
results represent a successful framework for enabling the use of copper underpotential deposition
to control palladium nanoparticle shape.
Experimental
Chemicals
Hexadecyltrimethylammonium bromide (CTAB, BioXtra ≥ 99 %, lot no. SLBR1101V), Lꢀ
ascorbic acid (ACS reagent, ≥ 99 %), sodium tetrachloropalladate (II) (Na2PdCl4, ≥ 99.99 %
trace metal basis), and sodium borohydride (NaBH4, granular, 99.99 %) were purchased from
SigmaꢀAldrich. Bis(pꢀsulfonatophenyl)phenylphosphine dihydrate dipotassium salt (BSPP,
C6H5P(C6H4SO3K)2·2H2O, min. 97%) was purchased from STREM. Nitric acid concentrate
(HNO3, 100 mM), hydrochloric acid (HCl, TraceMetal™ Grade), nitric acid (HNO3,
TraceMetal™ Grade) and sodium iodide (NaI, Fisher Bioreagents, ≥ 99%) were purchased from
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Fisher Scientific. Copper (II) nitrate hydrate (Cu(NO3)2·H2O, Puratronic 99.999% metal basis)
was purchased from Alfa Aesar. All chemicals were used without further purification and all
solutions were prepared with deionized (DI) water (18.2 MΩ resistivity, Labconco Water Pro
PS). Deactivated borosilicate glass wool (Restek, Cat. #20789) was purchased from Fisher
Scientific and used without modification.
Synthesis of palladium seeds
In a typical synthesis, 0.250 mL of 10 mM Na2PdCl4 was added to 10.0 mL of 12.5 mM
CTAB in a 20 mL scintillation vial with vigorous stirring, yielding a light brown solution. Upon
thorough mixing, 0.600 mL of freshly prepared 10 mM NaBH4 was rapidly injected, inducing an
instantaneous darkening of the light brown solution. The seed solution was allowed to stir for an
additional minute to ensure uniformity in the dispersion of the reducing agent. The stirring was
then halted and the seed solution was allowed to sit for a period of two hours at room
temperature.
Synthesis of terraced palladium-copper particles
After allowing the seed reaction to age, the resultant seed solution was then serially diluted
by adding a 0.100 mL aliquot of the stock seed solution to 0.900 mL of 12.5 mM CTAB. From
this solution, two subsequent tenꢀfold dilutions were made to obtain an aliquot of seeds that was
1,000x diluted relative to the original seed solution. This diluted seed solution was then set aside
to be introduced to the growth solution in the final step. The growth solution was prepared by
first adding 10.0 mL of 50 mM CTAB to a 20 mL scintillation vial and allowing the surfactant
o
solution to equilibrate to 35 C in a water bath. Following this equilibration, all other reactants
were added to the 20 mL scintillation vial sequentially as follows with mixing after each
addition: 0.500 mL of 10 mM Na2PdCl4, 0.020 mL of 10 mM Cu(NO3)2, 0.050 mL of 1 mM NaI.
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To generate particles with more defined terraces, 0.050 mL of 100 mM HNO3 can also be added
to the growth solution. To this growth solution, 0.100 mL of 100 mM ascorbic acid was then
added and, finally, a 0.100 mL aliquot of the 1,000x diluted seed solution was injected to initiate
particle growth, with initial color developing at approximately 45 minutes. The final color of the
solution was grey with the particles settling at the bottom of the vial.
Materials characterization
Scanning electron microscopy (SEM) images were taken on a Hitachi SUꢀ70 and a Hitachi
SUꢀ5000 field emission microscope. Scanning transmission electron microscopy (STEM) images
and energy dispersive Xꢀray spectroscopy (STEMꢀEDS) maps were obtained using a
FEI Tecnai Osiris 200kV TEM. A beryllium sample holder and gold TEM grids were used
during collection of STEMꢀEDS maps to avoid interference from background copper signals.
Samples for SEM and STEMꢀEDS imaging were prepared by centrifuging an aliquot of
nanoparticles at 6,000 rpm for 4 minutes, removing the supernatant and then washing the
particles with DI water. This washing process was repeated once more to remove excess
surfactant. After the second wash the particles were resuspended in a small amount of water and
then dropꢀcast onto a slice of silicon wafer (for SEM) or a gold TEM grid (for STEMꢀEDS).
Inductively coupled plasma mass spectrometry (ICPꢀMS) analysis was carried out using a
Perkin Elmer ICPꢀMS Elan DRCꢀe. Separate but identical reaction solutions were used for each
time point and were immediately quenched by injecting 0.100 mL of 55 mM BSPP followed by
vigorous mixing to ensure uniform dispersion. A 1.0 mL aliquot was taken from each reaction
vial and the samples were then centrifuged at 14,800 rpm for 10 minutes. The supernatant was
removed with care to ensure the particles at the bottom of the tube were undisturbed. The
particles were then washed with DI water and the previous procedure was repeated. After
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washing, the particles were resuspended in 1.0 mL of DI water which was then transferred to a
15 mL conical tube and digested with 2.0 mL of aqua regia (1:3 nitric acid to hydrochloric acid)
made from concentrated trace metal acids (Caution: strong acid). The particles were allowed to
digest for several hours and were then carefully diluted to 10.0 mL with DI water.
Xꢀray photoelectron spectroscopy (XPS) spectra were collected using a monochromatic
1486.7 eV Al Kα Xꢀray source on a PHI VersaProbe II Xꢀray photoelectron spectrometer with
0.47 eV system resolution. Spectra were charge corrected to the C1s peak of adventitious carbon
(284.8 eV). Samples were prepared on a slice of silicon wafer in a manner analogous to that used
for SEM imaging, except that an entire 1 mL aliquot of particles was concentrated and dropꢀcast
to generate a dense coverage of particles for XPS analysis.
Catalytic evaluation
Samples were prepared for catalytic evaluation by washing 80 mL of nanoparticle growth
solution twice with water, centrifuging for 20 minutes at 7,830 rpm in 50 mL conical tubes each
time. A concentrated solution of these nanoparticles was then mixed with a small piece of glass
wool (~0.15 g). The nanoparticles were prepared for catalytic studies after six hours of growth,
when particle formation was completed, to prevent galvanic exchange of surface copper.
Catalysts were dried for at least four hours at 100 °C in an oven to remove water prior to
catalytic testing. Dried catalysts were then placed into individual fritted quartz reactor tubes (0.4
inch internal diameter) and loaded into a flow reactor tube furnace. Reaction conditions for
ethanol oxidation were 5% ethanol and 20% oxygen (O2) with a total flow rate of 50 mL minꢀ1 in
helium (He) at 150 °C. Reaction products were monitored with an inꢀline gas chromatograph
mass spectrometer (GCꢀMS, Agilent 7890B/5977A) and a thermal conductivity detector (TCD).
Percent conversion and product selectivity were determined from the peak areas of species in the
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total ion chromatogram, with adjustment for MS ionization crossꢀsections.43 ICPꢀMS of the
catalysts postꢀreaction was used to determine catalyst mass for rate calculations.
Results and Discussion
Terraced palladiumꢀcopper nanoparticles were synthesized via a twoꢀstep seedꢀmediated
method. Palladium seed particles were produced through the rapid reduction of sodium
tetrachloropalladate (Na2PdCl4, 0.25 mL, 10 mM) in hexadecyltrimethylammonium bromide
(CTAB, 10 mL, 50 mM) by injecting sodium borohydride (0.6 mL, 10 mM) with vigorous
stirring and were subsequently aged for two hours at room temperature. A separate “growth
solution” was prepared in the second step and consisted of Na2PdCl4 (0.5 mL, 10 mM), copper
nitrate (Cu(NO3)2, 0.02 mL, 10 mM), sodium iodide (NaI, 0.05 mL, 1 mM), and ascorbic acid
(0.1 mL, 100 mM) consecutively added to CTAB (10 mL, 50 mM) which had been preheated to
o
35 C in a 20 mL scintillation vial. The mixture was swirled after each addition to ensure
uniform mixing. Finally, particle growth was initiated through the injection of 0.1 mL of aged
seed particles that had been serially diluted 1,000x in CTAB (12.5 mM) prior to injection. The
o
growth solutions were allowed to react in a water bath at 35 C and began to scatter light at
approximately 45 minutes, causing the growth solution to appear greyꢀbrown in color and
indicating the onset of significant particle growth. The growth solutions were allowed to react
o
overnight at 35 C. Noticeable settling of particles to the bottom of the vials occurred over the
course of the reaction indicating a final particle size greater than 50 nm. The final color of the
reaction solution was predominantly grey. The prepared particles were characterized via
scanning electron microscopy (SEM) to determine particle morphology and by inductively
coupled plasma mass spectrometry (ICPꢀMS), Xꢀray photoelectron spectroscopy (XPS) and
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scanning transmission electron microscopy energy dispersive Xꢀray spectroscopy (STEMꢀEDS)
to determine elemental composition.
Fig. 1. Scanning electron microscopy (SEM) images and corresponding models representing (A)
terraced cubes, (B) tripods, and (C) rounded cubes. The particles in (A) were synthesized with
the addition of 0.05 mL of 100 mM nitric acid to improve definition of the surface features.
Scale bars: 200 nm.
The resultant palladiumꢀcopper particles possess an underlying cubic structure with outward
protrusions at each of the faces that form stepped terraces with a thickness on the order of a few
nm (Fig. 1A, Fig. S1AꢀB and S2†). The emergence of these terraced surface features in the final
particle morphology is critically dependent on the concentrations of both copper and iodide ions
in the growth solution. The ideal concentrations of copper and iodide are 0.02 mM and 0.005
mM respectively. In the absence of iodide anions, the incorporation of copper leads to a mixture
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of multiply twinned particles including tripods, dendritic rods, and quasispherical particles with
no distinct morphology (Fig. 1B, Fig. S1C†). Similarly, in the absence of copper, the
incorporation of iodide anions yields a cubic morphology with truncation at the edges and
corners bound by a mixture of lowꢀindex facets (Fig. 1C, Fig. S1D†). Deliberate incorporation of
iodide in this system drives the overall particle morphology towards a more prominent
population of single crystalline lowꢀindex faceted structures rather than the polydisperse twinned
dendritic structures that predominate when only copper is present to direct the shape. Overall, the
incorporation of both copper and iodide in this growth system enables dual tuning of
nanoparticle shape through both kinetic control, by using iodide to increase the rate of reduction
of palladium ions, and selective surface passivation, in which edges and steps are stabilized
through the underpotential deposition of copper. Together these two effects lead to the
emergence of the terraced palladiumꢀcopper nanoparticles.
When the palladium ion reduction rates are probed via ICPꢀMS in the absence of copper, it
becomes readily apparent that the rate of palladium ion reduction is drastically increased in the
presence of iodide (Fig. 2A, Fig. S3†). One potential mechanism for this iodideꢀassisted
reduction is an oxidationꢀreduction reaction between surfaceꢀbound iodide ions and palladium
ions, in which iodide ions are oxidized to form triiodide and donate two electrons to reduce a
palladium ion (Eq. 1).40 Triiodide can then be reduced by ascorbic acid and reꢀadsorb to the
palladium surface to
–
3I– + Pd2+ → I3 + Pd0
(Eq. 1)
continue the cycle. Importantly, this rate increase leads to a distinct difference in the overall
particle morphology. When iodide is omitted, the reaction yields multiply twinned structures that
are bound by {100} facets, including right bipyramids and pentatwinned rods (Fig. 2B). In
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contrast, the incorporation of iodide directs growth toward largely single crystalline structures
that have a rounded cubic morphology (Fig. 2C). The structures that result from these control
experiments provide an understanding of a key aspect of the underlying shape evolution of the
more complex particles that form in the presence copper. The basic shape of the multiply
twinned structures generated when only copper and palladium are present (i.e. in the absence of
iodide) is analogous to that of the particles formed in the palladiumꢀonly control. Likewise, the
ideal terraced structures that form when both copper and iodide are added can be related to the
rounded cubic morphology from the palladium and iodide control. While the surface features of
the copperꢀcontaining particles are different from the copperꢀfree controls, this comparison of
fundamental shape in the presence and absence of iodide indicates that the core morphology is
dictated by the rate of palladium ion reduction.
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Fig. 2. (A) Graph of palladium incorporation during particle growth as determined by
inductively coupled plasma mass spectrometry (ICPꢀMS) for palladium particles formed in the
presence (green diamonds) and absence (black triangles) of iodide, (B) corresponding SEM
image of palladium particles grown in the absence of iodide, and (C) corresponding SEM image
of palladium particles grown in the presence of iodide. Scale bars: 200 nm.
Fig. 3. Graph of palladium incorporation during particle growth as determined by ICPꢀMS for
palladium particles formed in the presence (blue triangles) and absence (black triangles) of
copper.
Investigation of the impact of copper on palladium ion reduction shows that the addition of
copper alone does not affect the rate of reduction of palladium in this system, and suggests that
copper has a surface passivating effect rather than a kinetic influence. Indeed, elemental analysis
of the growth solution shows that the rates of reduction of palladium in the presence and absence
of copper are almost identical (Fig. 3). Iodide, however, does slow the rate of copper ion
reduction slightly, indicating that iodide has the ability to differentially control palladium and
copper ion reduction. By comparing the rate of reduction of copper in the systems with and
without iodide, it is readily apparent that the initial rate of copper ion reduction and therefore the
initial incorporation of copper is greater in the system in which no iodide is present (Fig. 4A).
Copper has a lower standard reduction potential than palladium: 0.60 V for [PdCl4]2ꢀ compared
to 0.34 V for Cu2+, both vs. standard hydrogen electrode (SHE).44 Therefore, since the standard
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potential for the reduction of triiodide to iodide is 0.536 eV vs. SHE,44 iodide is not a strong
enough reducing agent to reduce copper ions. The differential control of metal ion reduction rate
provided by iodide anions enables an increase in palladium ion reduction rate while
simultaneously slowing the rate of copper ion reduction, which decreases the amount of copper
in the terraced particles during the initial stages of growth. This is further evidence that the early
formation of a larger palladium core is essential to the evolution of the terraced palladiumꢀcopper
particles.
Fig. 4. Graphs of (A) copper incorporation and (B) composition during particle growth as
determined by ICPꢀMS for palladium particles formed in the presence (red squares) and absence
(blue triangles) of iodide.
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Correlation of SEM images to the elemental analysis data at different time points confirms
that the overall particle morphology with and without iodide is dictated by the relative rates of
metal ion reduction early in the reaction (Fig. S4ꢀS7†). In both cases, the final copper content of
the palladiumꢀcopper particles is approximately 1ꢀ2%, but there is a noticeable difference in the
copper content during the earlier time points (Fig. 4B). Indeed, the amount of copper that is
Fig. 5. SEM images of the formation of palladiumꢀcopper particles in the absence (AꢀE) and
presence (FꢀJ) of iodide at discrete time points (30 minutes, 45 minutes, 1 hour, 2 hours, and 3
hours.) Samples were obtained by quenching individual growth solutions with bis(pꢀ
sulfonatophenyl) phenylphosphine (BSPP). Scale bars: 200 nm.
incorporated in the absence of iodide during these early time points indicates a slightly faster
initial deposition of copper that coincides with a significantly slower reduction of palladium and
leads to an overall increased early copper content. When comparing the products of the
palladium and copper growth solution at thirty minutes (Fig. 5AꢀE) to the growth solution
containing iodide anions (Fig. 5FꢀJ), the former yields multiply twinned particles with prominent
growth at the edges and vertices which is evidence of likely copper deposition at these sites.
Moreover, as the copper in these particles is galvanically replaced by palladium during the
course of the reaction there is a distinct increase in definition at these sites as well as growth of
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ridges, resulting in the tripod and dendritic rod morphologies previously discussed. In contrast,
increasing the rate of palladium ion reduction through the deliberate incorporation of low
micromolar concentrations of iodide anions leads to an initial large core structure that appears to
be single crystalline in nature. During later stages of particle growth, the copper is
underpotentially deposited at the step edges of this structure, stabilizing edges and vertices.
Copper is then galvanically replaced or otherwise exchanged by its nobler palladium counterpart.
Subsequent cycles of palladium deposition, underpotential deposition of copper, and exchange of
copper with palladium persist through the rest of the growth period (Fig. 6). Importantly, the
displacement of copper by palladium is slow relative to the direct deposition of palladium and
the step sites are passivated relative to the terrace sites, thereby directing the formation of
multilayered surface features.
Fig. 6. Simplified schematic representation of the stabilization of palladium step sites by
underpotentially deposited copper (Cu UPD), leading to the formation of terraces. The
displacement of copper by palladium is slower than the direct reduction of palladium ions onto
the particle surface, resulting in slowed growth outward from step sites. As a result, these
copperꢀpassivated steps are maintained in the final particle morphology.
This mechanistic description of particle growth is supported by XPS analysis of the surfaces
of the nanoparticles formed under each of the different growth conditions (Fig. 7, Fig. S8†).
Highꢀresolution spectra of the copper 2p region for the palladiumꢀcopper particles formed in the
absence of iodide indicates that no copper is present at the surface of the particles (Fig. 7A). This
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is likely a consequence of the rapid reduction of copper early in particle growth followed by
subsequent galvanic exchange of copper by palladium. This lack of copper at the particle surface
is also supported by elemental mapping of the particles via EDS (Fig. S9†), which suggests that
the copper in these particles is more localized in the core as a copperꢀpalladium alloy surrounded
by a palladium shell. However, it is important to note that the bulk copper content for these
particles approaches the limit of detection for EDS (approximately 1%) and submonolayer
coverages of copper are not mappable using this approach. Therefore, the XPS results are more
conclusive in determining surface composition. In contrast, for the terraced particles, in which
the onset of copper deposition is delayed, copper is still present at the particle surface at the
conclusion of particle growth (Fig. 7A). The surface coverage of copper in these particles is low,
which is expected for submonolayer formation through underpotential deposition. EDS mapping
shows copper extending to the surface of the particles (Fig. S10ꢀS11†), though, as before, the
submonolayer coverage is too dilute to map conclusively using this technique. Interestingly,
according to XPS characterization, the coverage of iodide at the surface of the terraced particles
is low in comparison to the monometallic palladium particles formed in the presence of iodide
(Fig. 7B). This suggests that the coverage of iodide at the particle surface is high at early stages
of growth, thereby contributing to enhanced palladium ion reduction and the formation of a
single crystalline core, and that it is largely displaced by copper at later stages of growth.
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Fig. 7. Highꢀresolution XPS spectra of (A) the copper 2p region for palladiumꢀcopper particles
formed in the absence and presence of iodide and (B) the iodide 3d region for palladium and
palladiumꢀcopper particles formed in the presence of iodide. Particles were prepared for analysis
after 6 hours of reaction, when particle growth is complete.
The terraced particles can be tuned to have sharper edges and vertices through the addition of
nitric acid to the growth solution to slow the reduction of both metal ions by decreasing the
reducing power of ascorbic acid. Further decreasing the pH through the incorporation of
additional nitric acid has a drastic impact on overall particle morphology even though all other
conditions are held constant. In the absence of nitric acid, the ideal copper and iodide
concentrations yield a large proportion of the terraced palladium and copper nanoparticles that
are approximately 175 nm in size (Fig. 8A). The addition of 25 or 50 µL of 100 mM nitric acid
under otherwise identical conditions—Figs. 8B and 8C, respectively—gives rise to enhanced
definition of the terraces. The emergence of sharper features is not unexpected, as the onset of
copper underpotential deposition is shifted to later time points, leading to a larger overall particle
size which stabilizes sharp features at the particles’ surface by increasing the radius of curvature
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of the terrace edges (Fig. S12†). Further increasing the volume of acid added to 75 µL results in
particles that are considerably more rounded and exhibit greater proportions of {111} surface
features, which suggests a change in the mechanism of growth (Fig. 8D). Indeed, the addition of
100 – 175 µL of nitric acid (Fig. 8EꢀH) leads to a complete shift toward the evolution of concave
octahedra rather than terraced cubes. These concave octahedral particles resemble monometallic
palladium particles reported in the presence of iodide,45 suggesting that ascorbic acid is no longer
strong enough to reduce copper via underpotential deposition.
Fig. 8. SEM images illustrating a distinct change in particle morphology from terraced
palladiumꢀcopper particles to concave octahedra with increasing amounts of 100 mM nitric acid
in the growth solution: (A) no acid; (B) 25 µL of HNO3; (C) 50 µL of HNO3; (D) 75 µL of
HNO3; (E) 100 µL of HNO3; (F) 125 µL of HNO3; (G) 150 µL of HNO3; and (H) 175 µL of
HNO3. Scale bars: 200 nm.
To probe this trend more thoroughly, the elemental composition of these bimetallic structures
at different growth stages was analyzed in the presence of 0, 50, and 175 µL of 100 mM nitric
acid to determine the underlying cause of the shape change (Fig. 9). As hypothesized, the
resultant concave octahedral shaped particles that form in the presence of 175 µL of nitric acid
have no copper content throughout the entire reaction (Fig. 9B). The incorporation of nitric acid
weakens the reducing power of ascorbic acid, preventing the reduction of copper completely, and
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results in particles that possess a concave octahedral morphology that forms when copper is
omitted from the growth solution under analogous conditions (Fig. S13†). Similarly, when 50 µL
of acid is present in the growth solution there is a slight decrease in the rate of copper reduction
(Fig. 9B), however, the observed reduction of palladium occurs at close to the same rate as the
experiments where no acid is present (Fig. 9A). This slight slowing in the rate of copper ion
reduction leads to faster core growth yielding a larger overall particle morphology that is better
able to stabilize sharper edge features.
Fig. 9. Graphs of (A) palladium and (B) copper incorporation during particle growth as
determined by ICPꢀMS for particles formed with increasing volumes of added 100 mM nitric
acid.
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The catalytic performance of these terraced copperꢀpalladium nanoparticles for the gas phase
oxidation of ethanol was evaluated and compared to the monometallic palladium nanoparticles
synthesized in the presence and absence of iodide to probe the effects of changes in surface
structure and composition on the catalytic behavior of these materials. Each of the nanoparticle
materials was dispersed and dried on deactivated glass wool as an inert support, and catalytic
experiments were carried out under flowing, ambient pressure conditions. Three oxidation
products were observed: acetaldehyde, ethyl acetate, and acetic acid (Fig. S14ꢀ15†). While the
{100}ꢀfaceted palladium nanoparticles synthesized in the absence of iodide exhibited the highest
overall activity (0.33 mmol sꢀ1 gꢀ1 relative to 0.20 mmol sꢀ1 gꢀ1 for the palladium nanoparticles
synthesized with iodide and 0.21 mmol sꢀ1 gꢀ1 for the terraced palladiumꢀcopper nanoparticles),
the palladiumꢀcopper nanoparticles were the most selective to a single product. At comparable
conversions of close to 16% (15.6ꢀ16.2%), the terraced palladiumꢀcopper particles were 93.4%
selective to acetaldehyde, with the remaining product being the oxidative coupling product, ethyl
acetate (Fig. 10A). In contrast, the oxidation of ethanol over palladium nanoparticles with and
without iodide at the surface yielded acetic acid in addition to acetaldehyde and ethyl acetate.
The monometallic palladium particles with no iodide showed 76.8% selectivity to acetaldehyde
production, with 18.5% selectivity to ethyl acetate and a small amount of acetic acid formation
(4.7% selectivity). Interestingly, when compared to the palladium nanoparticles without iodide,
the palladium nanoparticles with surface iodide had a greater selectivity to acetic acid (8.2%),
along with a higher selectivity to acetaldehyde (81.4%) and a lower selectivity to ethyl acetate
(10.4%). At the maximum observed conversions, the selectivity to acetaldehyde for the terraced
copperꢀpalladium particles decreased slightly, to 86.9% (Fig. 10B). The selectivity to
acetaldehyde for the palladium particles with iodide decreased more significantly, to 71.7%,
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while that of the palladium particles with no iodide decreased dramatically, to 53.5%, coupled
with a significant increase in ethyl acetate and acetic acid production.
Fig. 10. Graphs of product selectivity at (A) approximately 16% conversion and (B) maximum
conversion in the catalytic oxidation of ethanol by palladium nanoparticles synthesized with and
without iodide as well as palladiumꢀcopper terraced particles synthesized with the addition of 50
ꢁL of 100 mM nitric acid. Reaction conditions: 5% ethanol and 20% O2 in He (50 mL minꢀ1),
150 °C, 1.9ꢀ2.5 mg of catalyst.
Together, these results suggest that copper at the surface of the terraced palladiumꢀcopper
nanoparticles is passivating a portion of the active surface sites and modifying the activity of
palladium to decrease the production of ethyl acetate and acetic acid. While the presence of
iodide at the palladium surface does decrease formation of ethyl acetate to some extent, it does
not prevent acetic acid formation. Thus, the addition of iodide alone is not sufficient to account
for the increased selectivity of the terraced particles toward acetaldehyde. This indicates that
surface copper is necessary to inhibit acetic acid formation. Similar suppression of acetic acid
formation by copper has also been observed for small, spherical palladiumꢀcopper alloys.46 The
ability of the terraced palladiumꢀcopper nanoparticles to maintain a significant conversion rate
while also exhibiting enhanced selectivity points to the broader potential opportunities for dilute
bimetallic nanoparticles with highꢀenergy surfaces in tailoring catalyst performance.
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Conclusion
Overall, this work highlights the utility of iodide ions as a tool for differentially tuning the
reduction rates of two dissimilar metal ion precursors, here palladium and copper. This is a
degree of control that is not achievable with most standard methods of modifying metal ion
reduction rates, such as adjusting the solution pH, raising or lowering the reaction temperature,
or changing the surfactant counterion. In the current system, by adding a low micromolar
concentration of iodide ions to the nanoparticle growth solution, it is possible to increase the rate
of palladium ion reduction while simultaneously decreasing the rate of copper ion reduction,
thereby reaching the optimum relative reduction rates for enabling the copper underpotential
deposition driven formation of terraced palladiumꢀcopper nanostructures. The resulting
submonolayer coverage of copper at the surface of these terraced nanoparticles enhances their
selectivity in the catalytic gas phase oxidation of ethanol by blocking pathways to acetic acid
production. This approach also has the potential to be more broadly applicable to the synthesis of
other dilute bimetallic nanomaterials involving a more easily reduced noble metal and a more
catalytically active but less easily reduced nonꢀnoble transition metal. Work to expand the scope
of this iodideꢀassisted method is ongoing in our research group. Nanostructures that are bound by
wellꢀdefined surface facets and have mixed noble metal/nonꢀnoble metal compositions are of
significant interest in fundamental research focused on enhancing selectivity and sustainability in
catalytic transformations. This work provides a promising new avenue for addressing the
significant synthetic challenges inherent in generating such complex functional materials.
Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgements
Acknowledgement is made to the donors of The American Chemical Society Petroleum
Research Fund for partial support of this research. This work was also supported by startꢀup
funding from Wesleyan University. SEM imaging at Wesleyan University was supported by the
National Science Foundation Major Research Instrumentation program under Grant No.
1725491. STEMꢀEDS and additional SEM characterization carried out at Yale University were
supported by YINQE and NSF MRSEC DMR 1119826. ICPꢀMS measurements were performed
at the Yale Analytical and Stable Isotope Center (YASIC), a Yale Institute for Biospheric
Studies (YIBS) research center. XPS analysis was conducted at the Yale University West
Campus Materials Characterization Core and the authors thank Dr. Min Li for assistance in
acquiring the XPS spectra.
† Electronic supplementary information (ESI) available.
Notes and references
1.
N. Zhou, V. LopezꢀPuente, Q. Wang, L. Polavarapu, I. PastorizaꢀSantos and Q.ꢀH. Xu,
RSC Adv., 2015, 5, 29076ꢀ29097.
2.
3.
4.
5.
6.
7.
8.
M. A. Boles, M. Engel and D. V. Talapin, Chem. Rev., 2016, 116, 11220ꢀ11289.
S. Chavez, U. Aslam and S. Linic, ACS Energy Lett., 2018, 3, 1590ꢀ1596.
S. Linic, P. Christopher and D. B. Ingram, Nat. Mater., 2011, 10, 911ꢀ921.
D. Schaadt, B. Feng and E. Yu, Appl. Phys. Lett., 2005, 86, 063106.
H. Zhang and A. O. Govorov, J. Phys. Chem. C, 2014, 118, 7606ꢀ7614.
K. An and G. A. Somorjai, ChemCatChem, 2012, 4, 1512ꢀ1524.
S. Linic, P. Christopher, H. Xin and A. Marimuthu, Acc. Chem. Res., 2013, 46, 1890ꢀ
1899.
9.
L. Liu and A. Corma, Chem. Rev., 2018, 118, 4981ꢀ5079.
10.
11.
B. Roldan Cuenya and F. Behafarid, Surf. Sci. Rep., 2015, 70, 135ꢀ187.
Z. Wang, H. Wang, Z. Zhang, G. Yang, T. He, Y. Yin and M. Jin, ACS Nano, 2017, 11,
163ꢀ170.
12.
13.
J. Zhang, Q. Kuang, Y. Jiang and Z. Xie, Nano Today, 2016, 11, 661ꢀ677.
D. Kim, C. Xie, N. Becknell, Y. Yu, M. Karamad, K. Chan, E. J. Crumlin, J. K. Nørskov
and P. Yang, J. Am. Chem. Soc., 2017, 139, 8329ꢀ8336.
14.
15.
L. Zhang, Z. Xie and J. Gong, Chem. Soc. Rev., 2016, 45, 3916ꢀ3934.
L. Zhang, Q. Chen, X. Wang and Z. Jiang, Nanoscale, 2016, 8, 2819ꢀ2825.
23

Journal of Materials Chemistry A
Page 24 of 25
View Article Online
DOI: 10.1039/C8TA06256K
16.
17.
18.
S. Rej, C.ꢀF. Hsia, T.ꢀY. Chen, F.ꢀC. Lin, J.ꢀS. Huang and M. H. Huang, Angew. Chem.
Int. Ed., 2016, 55, 7222ꢀ7226.
X. Li, X. Wang, M. Liu, H. Liu, Q. Chen, Y. Yin and M. Jin, Nano Res., 2018, 11, 780ꢀ
790.
A. ZaleskaꢀMedynska, M. Marchelek, M. Diak and E. Grabowska, Adv. Colloid Interface
Sci., 2016, 229, 80ꢀ107.
19.
20.
W. Yu, M. D. Porosoff and J. G. Chen, Chem. Rev., 2012, 112, 5780ꢀ5817.
M. Morad, E. Nowicka, M. Douthwaite, S. Iqbal, P. Miedziak, J. K. Edwards, G. L. Brett,
Q. He, D. Morgan, H. Alshammari, D. Bethell, D. W. Knight, M. Sankar and G. J.
Hutchings, Catal. Sci. Technol., 2017, 7, 1928ꢀ1936.
21.
22.
M. Laskar and S. E. Skrabalak, J. Mater. Chem. A, 2016, 4, 6911ꢀ6918.
C. Roychowdhury, F. Matsumoto, V. B. Zeldovich, S. C. Warren, P. F. Mutolo, M.
Ballesteros, U. Wiesner, H. D. Abruña and F. J. DiSalvo, Chem. Mater., 2006, 18, 3365ꢀ
3372.
23.
24.
25.
26.
D. D. Robertson, M. E. King and M. L. Personick, Top. Catal., 2018, 61, 348ꢀ356.
J. Gu, Y.ꢀW. Zhang and F. Tao, Chem. Soc. Rev., 2012, 41, 8050ꢀ8065.
Q. Chen, Y. Jia, S. Xie and Z. Xie, Chem. Soc. Rev., 2016, 45, 3207ꢀ3220.
K. D. Gilroy, A. Ruditskiy, H.ꢀC. Peng, D. Qin and Y. Xia, Chem. Rev., 2016, 116,
10414ꢀ10472.
27.
28.
R. G. Weiner, M. R. Kunz and S. E. Skrabalak, Acc. Chem. Res., 2015, 48, 2688ꢀ2695.
L. Han, P. Cui, H. He, H. Liu, Z. Peng and J. Yang, J. Power Sources, 2015, 286, 488ꢀ
494.
29.
30.
M. Gong, G. Fu, Y. Chen, Y. Tang and T. Lu, ACS Appl. Mater. Interfaces, 2014, 6,
7301ꢀ7308.
J. Chen, B. Wiley, J. McLellan, Y. Xiong, Z.ꢀY. Li and Y. Xia, Nano. Lett., 2005, 5,
2058ꢀ2062.
31.
32.
H. Jing and H. Wang, Chem. Mater., 2015, 27, 2172ꢀ2180.
K. D. Gilroy, P. Farzinpour, A. Sundar, R. A. Hughes and S. Neretina, Chem. Mater.,
2014, 26, 3340ꢀ3347.
33.
L. Zhang, J. Zhang, Q. Kuang, S. Xie, Z. Jiang, Z. Xie and L. Zheng, J. Am. Chem. Soc.,
2011, 133, 17114ꢀ17117.
34.
35.
O. A. Oviedo, P. Vélez, V. A. Macagno and E. P. M. Leiva, Surf. Sci., 2015, 631, 23ꢀ34.
J. Zhang, M. R. Langille, M. L. Personick, K. Zhang, S. Li and C. A. Mirkin, J. Am.
Chem. Soc., 2010, 132, 14012ꢀ14014.
36.
37.
L. Zhang, Q. Chen, Z. Jiang, Z. Xie and L. Zheng, CrystEngComm, 2015, 17, 5556ꢀ5561.
M. R. Langille, M. L. Personick, J. Zhang and C. A. Mirkin, J. Am. Chem. Soc., 2012,
134, 14542ꢀ14554.
38.
39.
M. Liu and P. GuyotꢀSionnest, J. Phys. Chem. B, 2005, 109, 22192ꢀ22200.
T. Ming, W. Feng, Q. Tang, F. Wang, L. Sun, J. Wang and C. Yan, J. Am. Chem. Soc.,
2009, 131, 16350ꢀ16351.
40.
41.
M. E. King and M. L. Personick, Part. Part. Syst. Char., 2017, 34, 1600422.
L. Zhang, S.ꢀI. Choi, J. Tao, H.ꢀC. Peng, S. Xie, Y. Zhu, Z. Xie and Y. Xia, Adv. Funct.
Mater., 2014, 24, 7520ꢀ7529.
42.
43.
Y.ꢀT. Chu, K. Chanda, P.ꢀH. Lin and M. H. Huang, Langmuir, 2012, 28, 11258ꢀ11264.
B. Xu, R. J. Madix and C. M. Friend, J. Am. Chem. Soc., 2010, 132, 16571ꢀ16580.
24

Page 25 of 25
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA06256K
44.
A. J. Bard, R. Parsons and J. Jordan, Standard Potentials in Aqueous Solution, Marcel
Dekker, New York, 1985.
45.
46.
W. Niu, L. Zhang and G. Xu, ACS Nano, 2010, 4, 1987ꢀ1996.
J. Yin, S. Shan, M. S. Ng, L. Yang, D. Mott, W. Fang, N. Kang, J. Luo and C.ꢀJ. Zhong,
Langmuir, 2013, 29, 9249ꢀ9258.
Table of Contents
Iodideꢀinduced kinetic control enables selective surface passivation of palladium through
underpotential deposition of copper to generate terraced palladiumꢀcopper particles.
25