Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties

Article abstract of DOI:10.1021/jacs.1c02734

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H2 selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.

Full text of DOI:10.1021/jacs.1c02734

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Article
Kinetically Controlled Synthesis of Rhodium Nanocrystals with
Different Shapes and a Comparison Study of Their Thermal and
Catalytic Properties
Ming Zhao,$ Zitao Chen,$ Yifeng Shi, Zachary D. Hood, Zhiheng Lyu, Minghao Xie, Miaofang Chi,
and Younan Xia*
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ABSTRACT: We report the synthesis of Rh nanocrystals with
different shapes by controlling the kinetics involved in the growth
of preformed Rh cubic seeds. Specifically, Rh nanocrystals with
cubic, cuboctahedral, and octahedral shapes can all be obtained
from the same cubic seeds under suitable reduction kinetics for the
precursor. The success of such a synthesis also relies on the use of
a halide-free precursor to avoid oxidative etching, as well as the
involvement of a sufficiently high temperature to remove Br− ions
from the seeds while ensuring adequate surface diffusion. The
availability of Rh nanocrystals with cubic and octahedral shapes
allows for an evaluation of the facet dependences of their thermal
and catalytic properties. The data from in situ electron microscopy
studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively.
When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H2
selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about
three times more active than the octahedral sample.
shapes would make it easy to examine the facet- or shape-
dependence of a catalytic reaction.18−20
INTRODUCTION
■
Colloidal metal nanocrystals with controlled shapes have found
use in a wide variety of fundamental studies and
applications.1−5 In the context of catalysis, such nanocrystals
provide well-defined surface structures for the establishment of
structure−property relationship to guide the rational develop-
ment of improved or new catalysts toward various reactions.6−9
Specifically, it is a viable idea to optimize the catalytic activity
and/or selectivity of nanocrystals toward a structure-sensitive
reaction through shape-controlled synthesis.10−13 This concept
has been extensively explored using different systems,14−17 but
the simplest study should be based on a direct comparison of
nanocrystals made of the same metal while taking cubic,
cuboctahedral, and octahedral shapes to present the surface as
a mix of {100} and {111} facets in different proportions. When
made of Pt and evaluated as catalysts toward the oxygen
reduction reaction in a perchlorate electrolyte, the octahedral
nanocrystals were found to be five times more active than their
cubic counterparts.16 When tested as catalysts toward the
hydrogenation of benzene, only cyclohexene was produced on
nanocubes encased by {100} facets whereas both cyclohexane
and cyclohexene were formed on cuboctahedral nanocrystals
covered by a mix of {111} and {100} facets.17 These and many
other examples demonstrate that the availability of metal
nanocrystals featuring cubic, cuboctahedral, and octahedral
Thanks to the efforts from many research groups, now we
can easily access nanocrystals with cubic, cuboctahedral, and
octahedral shapes for a number of metals, including Ag, Au,
Pd, and Pt.4,19,20 In the case of Rh, despite the availability of
well-established protocols for the synthesis of cubes, it is still
challenging to produce cuboctahedral and octahedral nano-
crystals in high quality and uniformity (Figure S1).21−24 As a
result, it has been difficult to investigate the structure−
property relationship of Rh nanocrystals in terms of facet or
shape dependence. The predicament can be understood from
several aspects. First, the formation of Rh octahedral
nanocrystals requires a fast reduction rate at the beginning
for the generation of single-crystal seeds and then switching to
a slow reduction rate for the expression of {111} facets favored
by thermodynamics. For a one-pot synthesis involving
Received: March 12, 2021
Published: April 14, 2021
https://doi.org/10.1021/jacs.1c02734
J. Am. Chem. Soc. 2021, 143, 6293−6302
© 2021 American Chemical Society
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Article
another EG solution containing 46.2 mg of Na3RhCl6 was added into
the flask at 60 mL h−1 for the first 1.1 and 4 mL h−1 for the remaining
4.9 mL, respectively. After 3 h, the solid products were collected by
centrifugation, washed once with acetone and three times with a
mixture of ethanol and acetone.
homogeneous nucleation, it is nontrivial to independently
adjust the reduction kinetics for the nucleation and growth
steps.25,26 Second, the halide ions, either used as an additive or
derived from the precursor, are detrimental to the formation of
Rh nanocrystals uniform in size and shape.27−30 The halide
ions can combine with O2 from air to cause oxidative etching
during the nucleation and growth steps, resulting in the
production of polydispersed samples.27,28 The strong binding
of halide ions to a specific type of facet, for example, Br− ions
for the Rh{100} facets, tends to make the capped facets more
favorable for expression in the final product.29,30 Third, the
bond energy of Rh−Rh (285 kJ mol−1) is much greater than
those of Pd (100 kJ mol−1) and Ag (160 kJ mol−1),31 creating a
higher energy barrier to the diffusion of Rh adatoms and thus
impeding the formation of a smooth surface.32−34
In addressing these issues, we switched to seed-mediated
growth and further demonstrated the synthesis of Rh
nanocrystals with different shapes, including octahedra,
cuboctahedra, and cubes. The success of such a synthesis
critically relied on the manipulation of reduction kinetics
through the use of a programmable syringe pump or a polyol
with proper reduction power. To avoid possible oxidative
etching and surface capping commonly associated with halide
ions, it was necessary to use a halide-free Rh(III) precursor
such as Rh(acac)3. Typically, the synthesis was conducted at
220 °C to help remove the Br− ions adsorbed on the
preformed cubic seeds while ensuring adequate surface
diffusion for the formation of a smooth surface. By a simple
increase in the volume of the reaction solution, the protocol
based on triethylene glycol (TEG) and syringe pump allowed
us to produce Rh octahedral nanocrystals of 8.9 0.8 nm in
edge length at a scale of roughly 5 mg per batch. By replacing
TEG with tetraethylene glycol (TTEG) and adding an
adequate amount of poly(vinylpyrrolidone) (PVP) into the
reaction mixture to tune the reaction kinetics, we also
demonstrated the synthesis of Rh octahedra in the one-shot
setting. A mechanistic study was further conducted to elucidate
the impacts of TTEG and PVP on the reduction kinetics of the
Rh(III) precursor. Such a one-pot synthesis can be potentially
conducted in a continuous-flow reactor for scalable produc-
tion. The as-obtained Rh nanocubes and octahedra allowed us
to systematically evaluate the shape-dependent thermal and
catalytic properties of Rh nanocrystals.
Synthesis of Rh Octahedral Nanocrystals. In the standard
protocol, 0.2 mg of the as-prepared 4.5 nm Rh cubes and 25 mg of
PVP were mixed in 2 mL of TEG. The mixture was then transferred
into a 20 mL vial and heated at 220 °C for 10 min under magnetic
stirring (380 rpm). Meanwhile, 2.4 mL of a TEG solution containing
Rh(acac)3 (1 mg mL−1) was added into the growth solution at a rate
of 5 mL h−1. After all the precursor had been added, the reaction was
continued for 1.5 h before quenching in an ice−water bath. The solid
products were collected by centrifugation and washed once with
acetone and three times with a mixture of ethanol and acetone. Figure
S2 shows a schematic illustration of the setup used for the synthesis.
Scaling Up the Synthesis of Rh Octahedral Nanocrystals by
Five Times. In a typical synthesis, 1 mg of the 4.5 nm Rh cubes, 125
mg of PVP, and 10 mL of TEG were mixed in a three-necked flask.
The flask was then placed in an oil bath and heated to 220 °C under
magnetic stirring (800 rpm). After 10 min, 12 mL of a TEG solution
containing Rh(acac)3 (1 mg mL−1) was dropwise added into the flask
at a rate of 25 mL h−1. The reaction was allowed to continue for 1.5 h,
after all the precursor had been added, and then quenched in an ice−
water bath. The solid products were collected by centrifugation and
washed once with acetone and three times with a mixture of ethanol
and acetone.
Synthesis of Rh Octahedral Nanocrystals by Adding the
Rh(III) Precursor in One Shot. Typically, 0.2 mg of the 4.5 nm Rh
cubes, 1000 mg of PVP, and 2 mL of TTEG were mixed in a 20 mL
vial and heated at 220 °C for 10 min under magnetic stirring (380
rpm). Meanwhile, 2.4 mL of a TTEG solution containing Rh(acac)3
(1 mg mL−1) was added into the growth solution in one shot. After 3
h, the solid products were collected by centrifugation and washed
once with acetone and three times with a mixture of ethanol and
acetone.
Characterizations. TEM images were taken on a Hitachi 7700
microscope. High-angle annular dark-field (HAADF) and bright-field
(BF) scanning transmission electron microscopy (STEM) images
were acquired on a Cs-corrected FEI Titan 80/300 kV microscope at
Oak Ridge National Laboratory (ORNL). X-ray diffraction (XRD)
patterns were recorded using a PANalytical X’Pert PRO Alpha-1
diffractometer using a 1.8 kW Ceramic Copper tube source. An
inductively coupled plasma mass spectrometer (ICP-MS, NexION
300Q, PerkinElmer) was used to quantify the metal contents. The X-
ray photoelectron spectroscopy (XPS) data were recorded on a
Thermo K-Alpha spectrometer with an Al Kα source.
Quantitative Analysis of Reduction Kinetics. In a typical
study, 0.1 mL was sampled from the reaction solution every 10 min
during a synthesis. The aliquot was then mixed with 0.9 mL of
acetone to precipitate out all the particles, followed by centrifugation
to leave behind the unreacted Rh(III) ions in the supernatant. The
supernatant was then collected and diluted for ICP-MS analysis.
Evaluation of Thermal Stability. The thermal stability of the Rh
nanocrystals was evaluated using in situ HRTEM coupled with a
Protochips Aduro heating holder. Typically, an aqueous suspension of
the as-prepared Rh nanocrystals was drop-cast onto the Aduro
thermal device prior to drying under ambient conditions. The sample
was then heated to various temperatures up to 800 °C at an interval of
100 °C. The heating rate was 1000 °C ms−1, and the sample was held
at each specified temperature for 1 h.
EXPERIMENTAL SECTION
■
Chemicals and Materials. Rhodium(III) acetylacetonate (Rh-
(acac)3, 97%) was obtained from Acros Organics. Ethylene glycol
(EG, 99%) was purchased from J. T. Baker. Sodium
hexachlororhodate(III) (Na3RhCl6, 97%), rhodium(III) chloride
hydrate (RhCl3·xH2O, 99.98%), PVP (MW ≈ 55,000), perchloric
acid (HClO4, 70%, PPT grade, Veritas), L-ascorbic acid (AA, 99%),
potassium bromide (KBr, 99%), potassium hydroxide (KOH), TEG
(99%), TTEG (99%), and hydrazine monohydrate (N2H4·H2O, 98%)
were all purchased from Sigma-Aldrich. Ethanol (C2H5OH,
anhydrous) was obtained from KOPTEC. Syringes and syringe
pump were purchased from KD Scientific. Polyvinyl chloride tubing
and capillaries were purchased from Thermo Scientific. The
temperatures of all syntheses were monitored using a thermal sensor
purchased from ACE Glass. Aqueous solutions were prepared using
deionized (DI) water featuring a resistivity of 18.2 MΩ cm at room
temperature.
Hydrazine Decomposition Experiments. In a typical measure-
ment, 0.2 mg of the Rh catalyst was added into a two-necked flask,
with one opening connected to a gas buret. Afterward, 10 μL of N2H4·
H2O was injected into the flask under magnetic stirring at room
temperature. The volume of the produced gases was monitored using
the gas buret after passing through 1.0 M HCl solution to ensure the
complete removal of ammonia.
Synthesis of 4.5 nm Rh Cubic Seeds. Typically, 13 mL of an
EG solution containing AA (52.8 mg), KBr (108 mg), and PVP (133
mg) was transferred into a three-neck flask and heated at 140 °C
under magnetic stirring (380 rpm) for 1 h. Meanwhile, 6 mL of
Electrochemical Measurements. The electrochemical measure-
ments were conducted in a three-electrode cell using an electro-
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chemical workstation (CHI 600E potentiostat). A Pt mesh and
reversible hydrogen electrode (RHE) were used as the counter and
reference electrodes, respectively. The as-synthesized Rh nanocrystals
were loaded on a carbon support (Ketjenblack EC-300J, AkzoNobel)
with a weight percentage of ca. 20 wt % and then used as the working
electrode. Cyclic voltammograms (CVs) were recorded in a N2-
saturated 0.1 M HClO4 solution, in the potential range of 0.08−1.1 V
and at a scanning rate of 50 mV s−1. The linear sweep voltammograms
(LSVs) were measured in a mixture of 1.0 M ethanol and 1.0 M KOH
aqueous solutions, in the potential range of 0.08−1.2 V and at a
scanning rate of 50 mV s−1. The potentials involved in this work were
derived from E(RHE) and presented as E(SCE), where SCE denotes
the saturated calomel electrode, according to the formula E(RHE) =
E(SCE) + 0.2412 + 0.05916xpH.
and the ratio between their areas kept increasing as the volume
of the Rh(acac)3 solution was increased. Eventually, Rh
octahedral nanocrystals encased by {111} facets were obtained,
and the average edge length was increased to 9.0 0.8 nm
(Figure 1d). Figure 1e shows a HAADF-STEM image of two
such octahedral nanocrystals. Figure 1f shows an atomic-
resolution ABF-STEM image of an individual octahedral
nanocrystal, where the lattice fringe spacing of 1.9 and 2.2 Å
could be indexed to the {200} and {111} planes, respectively,
of face-centered cubic (fcc) Rh. Figure S4 shows an XRD
pattern of the as-synthesized Rh octahedral nanocrystals,
further confirming the fcc phase.
Mechanistic Understanding of the Synthesis. During
the synthesis of Rh octahedral nanocrystals via seed-mediated
growth, the injection rate of the Rh(acac)3 precursor and thus
the deposition rate of Rh atoms had to be carefully controlled
in order to avoid homogeneous nucleation while ensuring
heterogeneous nucleation and continuous growth on the seeds.
To this end, we varied the injection rate of the precursor to
examine its impact on the formation of Rh octahedral
nanocrystals, as illustrated in Figure 2a−d. When the precursor
was added in one shot instead of dropwise addition, the
products contained a mix of icosahedra, nanoplates, and
irregular nanoparticles, in addition to the desired octahedra.
The byproducts were indicative of homogeneous nucleation,
primarily due to the high concentration of Rh atoms caused by
the one-shot injection of the precursor (Figure 2a). When we
added the precursor at a rate of 30 mL h−1, the proportions of
twinned and irregular nanoparticles decreased whereas Rh
octahedra prevailed in the products (Figure 2b). As the
pumping rate was reduced to 1 mL h−1, cuboctahedral
nanocrystals enclosed by a mix of {111} and {100} facets were
obtained as the main product (Figure 2c). If we further
reduced the pumping rate to 0.5 mL h−1, the resultant Rh
nanocrystals took the same cubic shape as that of the seeds,
showing well-defined {100} facets and an enlarged size of 9.1
1.1 nm in terms of edge length (Figure 2d).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Rh Octahedral
Nanocrystals. The synthesis started with the preparation of
Rh nanocubes with an average edge length of 4.5 0.4 nm
(Figure S3).25 The as-obtained nanocubes were then mixed
with PVP in TEG held at 220 °C, followed by the dropwise
addition of Rh(acac)3 precursor solution to initiate heteroge-
neous nucleation and then growth. Figure 1a−d shows TEM
images of the Rh nanocrystals obtained after the addition of
0.5, 1.0, 2.0, and 2.4 mL of the Rh(acac)3 solution, indicating
gradual evolution from cubes to truncated cubes, cuboctahe-
dra, truncated octahedra, and octahedra, respectively. All these
nanocrystals were enclosed by a mix of {111} and {100} facets,
It should be pointed out that during the synthesis of 4.5 nm
Rh cubic seeds, Br− ions were used as a capping agent for the
preferential expression of {100} facets on the surface. After the
washing process, the surface of the cubic seeds should still be
covered by some Br− ions because of the strong binding
strength involved. Figure S5 shows a Br 3d XPS spectrum of
the Rh cubic seeds, in which the peak located at around 69 eV
confirms the presence of residual Br− ions. Due to the
involvement of a temperature as high as 220 °C, the adsorbed
Br− ions were expected to desorb from the surface during the
synthesis. As verified by the XPS result (Figure S5), the peak
associated with Br− ions essentially disappeared after the seeds
had been heated at 220 °C for 2 h. Considering the extremely
small amount of residual Br− ions (at a level of nano mole for a
full coverage on all the Rh seeds involved) and the desorption
of most Br− ions from the surface at 220 °C, the Br− ions
should not be able to exert a remarkable capping or etching
effect on the growth process. This argument was validated by
the formation of well-defined {111} facets on the Rh
octahedral nanocrystals (Figure 1d−f). Figure S6 compares
Br 3d XPS spectra of the Rh cubic, cuboctahedral, and
octahedral nanocrystals (Figures 1d, and 2c,d) prepared from
the 4.5 nm Rh cubes, confirming the absence of Br− ions in
these samples.
Figure 1. TEM images of the Rh nanocrystals prepared using the
standard protocol except that different volumes of the Rh(acac)3
solution were added into the reaction solution: (a) 0.5, (b) 1.0, (c)
2.0, and (d) 2.4 mL. The scale bar in panel a also applies to panels b−
d. The insets in panels a−c indicate the shape taken by nanocrystals in
each sample while those in panel d show models of an octahedron at
different orientations. The green and yellow colors represent {100}
and {111} facets, respectively. (e) HAADF-STEM image of two Rh
octahedra. (f) Atomic-resolution STEM image recorded from one Rh
octahedron.
Figure 2e shows a schematic illustration of the mechanism
underlying the formation of Rh polyhedral nanocrystals with
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Figure 2. TEM images of the Rh nanocrystals prepared using the standard protocol except that the Rh(acac)3 solution was added (a) in one shot
and (b−d) at different pumping rates: (b) 30, (c) 1, and (d) 0.5 mL h−1. The scale bar in panel a applies to all panels b−d. (e) Schematic
illustration showing the formation of various types of Rh polyhedral nanocrystals at different pumping rates for the precursor while keeping all other
parameters the same as those in the standard synthesis.
diverse shapes at different pumping rates for the precursor in
the case of heterogeneous nucleation only. Given the absence
of capping agent and thus the accessibility of the entire surface
for atom deposition, the newly generated atoms would be
preferentially deposited on the side faces of the seeds relative
to the corners owing to the higher surface energy of {100}
facets than that of {111} facets.34,35 When the pumping rate of
the precursor was relatively fast (i.e., 5 mL h−1), the atoms
would pile up on the {100} facets in the form of pyramids for
the generation of {111} facets with a lower surface energy than
that of {100} facets, giving rise to an octahedral shape. If the
precursor was added at a moderate rate of 1 mL h−1, an
increased number of the adatoms were able to diffuse to the
corners and edges, resulting in the formation of cuboctahedral
nanocrystals. This argument was further confirmed when a
slow pumping rate of 0.5 mL h−1 was involved. In this case, a
large portion of the deposited Rh atoms would have sufficient
time to diffuse across the entire surface and thereby replicate
the cubic shape of the underlying seeds. Taken together, by
varying the pumping rate of the Rh(acac)3 precursor, we were
able to control the growth pattern of Rh atoms in an kinetic
manner and attain nanocrystals with a variety of shapes
including octahedra, cuboctahedra, and cubes.
The reaction temperature is another parameter crucial to
nanoparticles synthesis which affects both the reduction
kinetics (i.e., deposition rate) of the metal precursor and
surface diffusion kinetics of adatoms. In the content of
generating well-defined Rh octahedral nanocrystals, an
appropriate temperature should be leveraged not only to
generate a proper number of atoms for continuous
heterogeneous growth but also to ensure the preferential
surface diffusion over atom deposition for the formation of a
smooth surface. Figure S7 shows TEM images of the Rh
nanocrystals prepared using the standard protocol except for
the variation in reaction temperature. When the temperature
was set to 160 °C, the resultant Rh nanocrystals took a cubic
shape, with a size similar to that of the original cubic seeds
(Figure S7a). This result could be attributed to the slow
reduction kinetics of the precursor at such a low temperature,
resulting in limited growth. If the temperature was elevated to
180 °C, the products showed obvious growth but took a rough
surface owing to the lack of adequate surface diffusion (Figure
S7b). When the temperature was further increased to 200 °C,
truncated octahedral nanocrystals were obtained as the
prevalent products and their surface became smoother relative
to those obtained at 180 °C (Figure S7c). If the synthesis was
conducted at 240 °C, although surface diffusion was greatly
enhanced, the generation of Rh atoms was also accelerated
substantially, giving rise to the formation of diverse Rh
nanocrystals through homogeneous nucleation (Figure S7d). It
is worth noting that for Ag and Pd octahedra synthesized via
seed-mediated growth, a smooth surface could be achieved at
room temperature and 60 °C, respectively, whereas the
expression of well-defined {111} surface for Rh octahedra
required a reaction temperature as high as 220 °C.36,37 The
sharp contrast emphasizes the importance to optimize the
reaction temperature in overcoming the high diffusion energy
barrier of Rh atoms for the formation of a smooth surface.31
According to our prior work, oxidative etching enabled by
the halide ions from the Rh precursor could greatly affect the
shape of the final products.38,39 Here, we also utilized different
types of Rh precursors to examine their effect on the synthesis
of Rh octahedra. Figure S8a,b shows TEM images of the Rh
nanocrystals prepared using the standard protocol except for
the involvement of Na3RhCl6 and RhCl3 as precursors,
respectively. Compared with the standard sample shown in
Figure 1d, there existed a large number of irregular
nanoparticles and octahedral nanocrystals with a bumpy
surface in both samples (as indicated by the arrows in Figure
S8). This observation could be rationalized by the oxidative
etching enabled by the Cl− ions from the precursors.38,39
During the synthesis, the Cl− ions from Na3RhCl6 or RhCl3
served as a coordination ligand and the O2 from air served as
an oxidant, forming an etchant for the Rh nanocrystals. In the
etching process, the surface of the Rh nanocrystal acted as a
cathode for the reduction of oxygen, while the Cl− ions served
as a charge carrier and coordination ligand for the anodic
oxidation and dissolution of metal atoms.38,39 The involvement
of oxidative etching could significantly affect the growth
pattern of Rh atoms on the seeds, resulting in deviations from
the well-defined octahedral shape. Additionally, because the
molar ratio of Cl− to Rh3+ in Na3RhCl6 was two times greater
than that in RhCl3 (6:1 vs 3:1), oxidative etching was more
significant for the resultant Rh nanocrystals prepared using
Na3RhCl6 as the precursor, further confirming the role of Cl−
ions in oxidative etching (Figure S8a). This result is also
consistent with the studies reported in literature, in which the
involvement of RhCl3 as the precursor would produce
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nanocrystals that were notably deviated from well-defined
octahedra (Figure S1).21,22 Altogether, in order to prevent the
occurrence of oxidative etching during the synthesis, a halide-
free Rh precursor such as Rh(acac)3 should be employed to
attain Rh octahedral nanocrystals in high quality and with a
smooth surface.
Rh(acac)3 solution at an appropriate rate using a program-
mable syringe pump. We also attempted to develop a one-shot
method for the synthesis of Rh octahedra by optimizing the
reagents to achieve the appropriate kinetic condition.
Compared with the synthesis involving dropwise addition of
the precursor solution, one-shot synthesis has additional
benefits for easy operation and mechanistic study as well as
potentials in a continuous-flow reactor for scaleup production.
The one-shot synthesis was realized by switching from TEG to
TTEG, as the solvent and reductant, and introducing a large
amount of PVP into the synthesis, both of which have been
demonstrated to decelerate the reduction kinetics of metal
precursors through weakening the reducing capability and
enhancing the steric hindrance, respectively.42,43 Figure 3a
The use of preformed seeds was also essential to the
synthesis of Rh octahedral nanocrystals.40,41 The presence of
monodispersed cubic seeds is advantageous not only in
avoiding the potential heterogeneity arising from the
homogeneous nucleation process but also in ensuring an
identical growth behavior for each seed, giving rise to the
formation of products in high quality and uniformity. In sharp
contrast, if the standard synthesis was conducted in the
absence of seeds, the products contained a mix of nanoplates,
decahedra, icosahedra, and irregular nanoparticles (Figure S9),
confirming the vital role of Rh seeds in directing the evolution
of the octahedral shape. Additionally, the size of the seeds also
played a critical role in controlling the formation of Rh
octahedral nanocrystals because it determined the distance that
the deposited Rh atoms would need to diffuse in order to
generate a smooth surface. In general, small seeds are better
suited for the adatoms to cover the entire surface by diffusion
while large ones impose a long distance for the adatoms to
diffuse across. Figure S10a shows the product prepared using
the standard protocol except for the involvement of 9 nm Rh
cubic seeds (Figure 2d). The resultant nanocrystals took
irregular shapes and a rough surface, indicative of inadequate
surface diffusion. When we decreased the pumping rate of the
precursor to 0.5 mL h−1, Rh cubes with a concave surface were
obtained (Figure S10b). At such a slow pumping rate, the
diffusion of deposited atoms would be enhanced to form a
smoother surface relative to the sample shown in Figure S10a,
but still insufficient to drive the adatoms deposited on the
edges or boundaries to diffuse across the large side faces,
leading to the generation of a concave structure.
Scale-Up the Synthesis of Rh Octahedral Nanocryst-
als. Despite the success in the shape-controlled synthesis of
noble-metal nanocrystals, it remains a grand challenge to
bridge the gap between academic studies and industrial
applications owing to the inability to produce the nanocrystals
at a large scale while maintaining the tight control over their
shapes. To mitigate this issue, we attempted to scale our
synthesis up to five times by proportionally increasing the
volumes of reaction solutions. Noticeably, the pumping rate of
the precursor was also increased by five times to 25 mL h−1 in
order to generate adequate Rh atoms for the continuous
growth on the seeds. Simultaneously, the reaction solution was
magnetically stirred at an increased speed of 800 rpm to ensure
the uniform dispersal of the injected precursor while avoiding
local supersaturation of free Rh atoms for the initiation of
homogeneous nucleation. Figure S11a shows a TEM image of
the Rh nanocrystals synthesized using the scale-up protocol
and they featured a well-defined octahedral shape and a
uniform size distribution. According to the histogram of size
distribution shown in Figure S11b, the average size of the
resultant Rh octahedral nanocrystals was 8.9 0.8 nm in edge
length, consistent with that of the product from a standard
synthesis.
Figure 3. TEM images of the Rh nanocrystals prepared by adding the
Rh(acac)3 solution into the TTEG solution in one shot, together with
the use of PVP at different amounts: (a) 25, (b) 100, (c) 300, and (d)
1000 mg. The scale bar in panel a also applies to panels b−d.
shows a TEM image of the Rh nanocrystals prepared using the
standard protocol except for the involvement of TTEG and the
addition of the Rh(acac)3 solution in one shot. The products
contained icosahedra, nanoplates, and irregular nanocrystals, in
addition to a small portion of octahedra. When the amount of
PVP was increased to 100 mg and further to 300 mg, the
proportion of twinned nanocrystals decreased whereas
octahedral nanocrystals prevailed in the products (panels b
and c of Figure 3, respectively). When 1000 mg of PVP was
added into the synthesis, the products were dominated by Rh
octahedral nanocrystals with well-defined {111} facets and an
average size of 8.9 0.9 nm (Figure 3d), almost identical to
the standard sample shown in Figure 1d.
To achieve a quantitative understanding of the mechanism
involved in the one-shot synthesis of Rh octahedra, we
conducted a kinetic study on the reduction of Rh(III) ions
under different reaction conditions. Figure 4a shows the
normalized concentrations of Rh(acac)3 remaining in the
solution during different syntheses, and they all decreased as a
function of reaction time. Despite the same trend, the
instantaneous concentrations of Rh(acac)3 differed remarkably
from each other. When the one-shot synthesis was conducted
in TEG, 90% of the precursor was depleted within 0.5 h. In
comparison, only 64% of the precursor was consumed in 0.5 h
when switching from TEG to TTEG, verifying the role of
TTEG in decelerating the reduction kinetics of Rh(III) ions.
As the amount of PVP was increased to 100, 300, and 1000 mg
in the TTEG system, the conversion of the precursor in 0.5 h
dropped to 54, 42, and 26%, respectively, suggesting that
One-Shot Synthesis of Rh Octahedral Nanocrystals
and the Kinetic Study. For the synthesis of Rh octahedral
nanocrystals based upon seed-mediated growth, it was critical
to manipulate the growth kinetics by dropwise adding the
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for heterogeneous nucleation and growth on the seeds,
enabling one-shot synthesis of Rh octahedral nanocrystals.
Evaluation of the Shape-Dependent Thermal Stabil-
ity. Rh nanocrystals are well-documented as effective
heterogeneous catalysts for a myriad of catalytic processes
including syngas production and exhaust treatment,44−46 which
are typically operated under elevated temperatures. To study
the shape-dependent response to thermal stress, we subjected
the Rh cubic and octahedral nanocrystals with a similar size of
9 nm to in situ heating using HRTEM. As shown in Figure 5,
Figure 5. HRTEM images recorded from the same Rh (a) cube and
(b) octahedron, respectively, when heated in situ to different
temperatures in the range of 400−800 °C. The heating rate was
1000 °C ms−1 and the nanocrystal was held at each marked
temperature for 1 h before ramping to the next temperature. The scale
bar is 5 nm and applies to all panels.
the original shapes of both the nanocrystals could be retained
when heated up to 400 °C. However, as the temperature was
further elevated, the two samples started to differ in terms of
response. At 500 °C, the corners of the Rh cubic nanocrystal
became slightly truncated while its side faces were still
dominated by {100} facets (Figure 5a). The truncation
became more significant as the temperature reached 700 °C.
When the temperature reached 800 °C, the Rh cube evolved
into a cuboctahedron, with its surface covered by a mix of
{100} and {111} facets at roughly the same proportion (Figure
5a). With regard to the octahedral nanocrystal, its shape was
largely maintained even after heating at 500 °C for 1 h, except
for the minor rounding at the corners (Figure 5b). When the
sample was heated to 600 °C, the nanocrystal showed major
deviations from a well-defined octahedron and the {111} facets
were greatly reduced in proportion. When the temperature was
further elevated to 700 and then 800 °C, the particle eventually
evolved into a spheroidal nanocrystal (Figure 5b). Taken
together, the cubic and octahedral nanocrystals enclosed by
{100} and {111} facets, respectively, could largely retain their
original shapes up to 700 and 500 °C when subjected to
heating under vacuum.
The superior thermal stability of cubic nanocrystals relative
to the octahedral counterparts could be attributed to their
different surface reconstruction behaviors.47−50 According to
density functional theory (DFT) calculations, atoms on {111}
facets typically preferred diffusion via hopping across the
surface (i.e., the hopping mechanism) whereas those on the
{100} facets tended to replace surface atoms by ejecting them
to neighboring surface sites (i.e., the exchange mecha-
nism).47−50 Since the energy barrier involved in the hopping
process is lower than that in the exchange mechanism, the
movement of adatoms on the {111} facets should be more
facile than that on the {100} facets. In general, octahedral
Figure 4. Quantitative analysis of the reduction kinetics of Rh(III)
ions involved in the one-shot synthesis of Rh octahedra, where the
type of polyol and the amount of PVP were varied. (a) Plots showing
the normalized percentage of Rh(acac)3 remaining in the reaction
solution as a function of reaction time. (b) Plots showing the linear
relationship between -ln([Rh(acac)3]) and reaction time and the
fitting based on the pseudo-first-order kinetics.
adding a larger amount of PVP could further retard the
reduction kinetics of Rh(III) ions.
For the one-shot synthesis of Rh nanocrystals, the reduction
of Rh(III) ions is expected to follow the second-order rate law
due to the involvement of collision and electron transfer
between the Rh(III) ions and reductant.42 Given that the
reductant (i.e., TEG or TTEG) was in great excess in the
present study, its concentration could be assumed as
unchanged during the synthesis. Therefore, the reaction rate
could be fitted using the pseudo-first-order kinetics.41 Figure
4b shows the linear dependence of -ln([Rh(acac)3]) on the
reaction time under different conditions, from which the rate
constants were derived. By switching from TEG to TTEG
while keeping other reaction conditions unchanged, the rate
constant exhibited a 2.3-times drop from 8.8 × 10−2 to 3.8 ×
10−2 min−1. When the amount of PVP was increased to 100,
300, and 1000 mg, the rate constant further decreased to 3.1 ×
10−2, 2.4 × 10−2, and 1.5 × 10−2 min−1, respectively.
Collectively, by replacing TEG with TTEG and adding 1000
mg of PVP into the reaction solution, we were able to decrease
the reduction rate constant of Rh(III) ions by 5.9 times relative
to the standard synthesis. As such, the concentration of the
newly formed Rh atoms could be kept at a level below the
supersaturation for homogeneous nucleation but high enough
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Figure 6. Comparison of the catalytic performance of Rh cubes, cuboctahedra, and octahedra toward hydrazine decomposition and ethanol
oxidation. (a) Time-course plots for hydrazine decomposition in the presence of different types of Rh catalysts. (b) Selectivity for hydrogen
generation from hydrazine decomposition catalyzed by different types of Rh nanocrystals. (c) CV curves measured for different types of Rh
catalysts in a 0.1 M HClO4 solution at a scanning rate of 50 mV s−1. (d) LSV curves of ethanol oxidation recorded in a mixture of 1.0 M ethanol
and 1.0 M KOH solutions at a scanning rate of 50 mV s−1.
nanocrystals would be more vulnerable to thermal deformation
than their cubic counterparts.
of ethanol. Figure 6c shows the CV curves recorded in an
aqueous HClO4 solution over different Rh catalysts, from
which the electrochemical surface areas (ECSAs) could be
Evaluation of the Facet-Dependent Catalytic Per-
formance. The catalytic performance of Rh nanocrystals has
been proven to show strong shape dependence, whereas a
study on Rh octahedral nanocrystals has not been accom-
plished due to the lack of Rh octahedra in high quality and
uniformity. Here, we evaluated the facet-dependent perform-
ance of Rh nanocrystals in hydrazine decomposition by
employing Rh nanocrystals with a similar size of 9 nm but
covered by various shapes as the catalysts.50 Figure 6a shows
the plots for hydrazine decomposition in the presence of Rh
cubes, cuboctahedra, and octahedra, which are enclosed by
{100} facets, a mix of {100} and {111} facets, and {111}
facets, respectively (Figures 1d, and 2c,d). The molar ratios of
the generated H2 and N2 over the initially added hydrazine,
denoted n(N2 + H2)/n(N2H4), were derived as 0.6, 1.0, and
1.5 for the cubes, cuboctahedra, and octahedra, corresponding
to H2 selectivity of 11.3, 26.2, and 44.4%, respectively (Figure
6b). In particular, the Rh octahedra enclosed by {111} facets
exhibited 3.9- and 1.7-fold enhancements in terms of H2
selectivity relative to their cubic and cuboctahedral counter-
parts, respectively. The enhanced selectivity of Rh octahedra
could be rationalized by the lower activation energy for the
dehydrogenation of hydrazine molecules on Rh{111} facets
relative to that on Rh{100} facets.51−53 Additionally, the
decreased transient Rh−H bond strength and adsorption
affinity of intermediates on Rh{111} facets were also found to
be advantageous in promoting H2 generation from hydrazine
decomposition.51−53
−1
derived as 13.1, 18.0, and 17.2 m2 gRh for Rh nanocrystals
with cubic, cuboctahedral, and octahedral shapes, respectively.
Figure 6d shows the ECSA-normalized LSV curves measured
in a mixture of ethanol and KOH solutions. In the positive-
going curves, two characteristic oxidation peaks could be
observed for different Rh catalysts, which were positioned at
−0.3 and −0.1 V vs. SCE, respectively. The peak at −0.3 V
could be attributed to the oxidation of ethanol to acetaldehyde
and CO2, while the peak at −0.1 V corresponds to the
involvement of acetic acid formation.54,55 Additionally, the
ECSA-normalized peak current density (i.e., specific activity)
of Rh cubes was derived as 2.6 mA cm−2, which was 1.6 and
2.9 times as high as those of their cuboctahedral (1.6 mA
cm−2) and octahedral (0.9 mA cm−2) counterparts, respec-
tively. This result validated that the {100} facets on Rh
nanocubes were more active than the {111} facets in catalyzing
ethanol oxidation. The superior performance of Rh{100} facets
is attributable to their higher surface free energy relative to
Rh{111} facets, which was instrumental to promoting the C−
C bond cleavage during the oxidation of ethanol molecules.54
DFT calculations also suggested that compared with the close-
packed {111} facets, the low-coordination atoms on the {100}
facets exhibited a stronger binding strength and thus a lower
energy barrier to both the dehydrogenation and C−C scission
of ethanol molecules, leading to the boosted performance
toward ethanol oxidation.54
We also assessed the catalytic activity of different Rh
polyhedral nanocrystals toward the electrochemical oxidation
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United States; Center for Nanophase Materials Sciences, Oak
Ridge National Laboratory, Oak Ridge, Tennessee 37831,
United States
Zhiheng Lyu − School of Chemistry and Biochemistry, Georgia
Institute of Technology, Atlanta, Georgia 30332, United
States; orcid.org/0000-0002-1343-4057
Minghao Xie − School of Chemistry and Biochemistry, Georgia
Institute of Technology, Atlanta, Georgia 30332, United
States; orcid.org/0000-0003-3781-5118
Miaofang Chi − Center for Nanophase Materials Sciences,
Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831, United States; orcid.org/0000-0003-0764-1567
CONCLUSIONS
■
We have developed a robust protocol based on seed-mediated
growth for the synthesis of Rh nanocrystals with three basic
shapes of cubes, cuboctahedra, and octahedra. It was critical to
exclude the etching and capping effects associated with halide
ions by using Rh(acac)3 as the precursor while ensuring
adequate surface diffusion at a high enough temperature. In the
case of TEG, it was also pivotal to add the precursor solution
into a suspension of the cubic seeds with the use of a syringe
pump to manipulate the reduction kinetics. With substituting
TEG with TTEG while adding adequate PVP into the reaction
solution to modulate the reduction kinetics, it was feasible to
obtain Rh octahedra by directly adding the precursor solution
in one shot. Our in situ electron microscopy studies indicate
that the original shape of Rh nanocubes could be largely
maintained up to 700 °C, 200 °C higher than their octahedral
counterparts. When tested as catalysts for hydrazine decom-
position, the {111} facets on Rh octahedral nanocrystals were
found to be almost four times more selective toward H2
production when benchmarked against the {100} facets on
Rh nanocubes. As for ethanol oxidation, Rh nanocubes
outperformed their octahedral counterparts by almost three
times in terms of activity. Taken together, this work provides
an effective strategy based on kinetic manipulation for the
synthesis of Rh nanocrystals with diverse but controllable
shapes, together with insights into their shape-dependent
thermal and catalytic properties.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02734
Author Contributions
$These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a grant from NSF (CHE-
1804970) and start-up funds from the Georgia Institute of
Technology. Microscopy and XRD analyses were conducted at
Institute of Electronics and Nanotechnology (IEN, Georgia
Institute of Technology) supported by the NSF (ECCS-
1542174). Part of the electron microscopy analyses was carried
out in the Center for Nanophase Materials Sciences in ORNL,
which is a DOE Office of Science User Facility (M.C. and
Z.D.H.). Z.D.H. gratefully acknowledges support from the
NSF Graduate Research Fellowship (DGE-1650044) and the
Georgia Tech-ORNL Fellowship.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02734.
■
sı
TEM images and XPS data of Rh cubic seeds and Rh
nanocrystals prepared under different experimental
conditions are provided. XRD pattern of the Rh
octahedra prepared using the standard protocol is also
included (PDF)
REFERENCES
■
(1) Guntern, Y. T.; Okatenko, V.; Pankhurst, J.; Varandili, S. B.;
Iyengar, P.; Koolen, C.; Stoian, D.; Vavra, J.; Buonsanti, R. Colloidal
Nanocrystals as Electrocatalysts with Tunable Activity and Selectivity.
ACS Catal. 2021, 11, 1248−1295.
(2) You, H.; Yang, S.; Ding, B.; Yang, H. Synthesis of colloidal metal
and metal alloy nanoparticles for electrochemical energy applications.
Chem. Soc. Rev. 2013, 42, 2880−2904.
(3) Xie, C.; Niu, Z.; Kim, D.; Li, M.; Yang, P. Surface and Interface
Control in Nanoparticle Catalysis. Chem. Rev. 2020, 120, 1184−1249.
(4) Shi, Y.; Lyu, Z.; Zhao, M.; Chen, R.; Nguyen, Q. N.; Xia, Y.
Noble-Metal Nanocrystals with Controlled Shapes for Catalytic and
Electrocatalytic Applications. Chem. Rev. 2021, 121, 649−735.
(5) Chen, Q.; Jia, Y.; Xie, S.; Xie, Z. Well-faceted noble-metal
nanocrystals with nonconvex polyhedral shapes. Chem. Soc. Rev. 2016,
45, 3207−3220.
(6) Strasser, P.; Gliech, M.; Kuehl, S.; Moeller, T. Electrochemical
process on solid shaped nanoparticles with defined facets. Chem. Soc.
Rev. 2018, 47, 715−735.
(7) Zhu, W.; Kattel, S.; Jiao, F.; Chen, J. G. Shape-Controlled CO2
Electrochemical Reduction on Nanosized Pd Hydride Cubes and
Octahedra. Adv. Energy Mater. 2019, 9, 1802840.
(8) Lyu, Z.; Zhu, S.; Xu, L.; Chen, Z.; Zhang, Y.; Xie, M.; Li, T.;
Zhou, S.; Liu, J.; Chi, M.; Shao, M.; Mavrikakis, M.; Xia, Y. Kinetically
Controlled Synthesis of Pd−Cu Janus Nanocrystals with Enriched
Surface Structures and Enhanced Catalytic Activities toward CO2
Reduction. J. Am. Chem. Soc. 2021, 143, 149−162.
(9) Collins, G.; Schmidt, M.; O’Dwyer, C.; Holmes, J. D.;
McGlacken, G. P. The Origin of Shape Selectivity in Palladium-
Catalyzed Suzuki-Miyaura Cross Coupling Reactions. Angew. Chem.,
Int. Ed. 2014, 53, 4142−4145.
AUTHOR INFORMATION
Corresponding Author
■
Younan Xia − School of Chemistry and Biochemistry and
School of Chemical and Biomolecular Engineering, Georgia
Institute of Technology, Atlanta, Georgia 30332, United
States; The Wallace H. Coulter Department of Biomedical
Engineering, Georgia Institute of Technology and Emory
University, Atlanta, Georgia 30332, United States;
orcid.org/0000-0003-2431-7048; Email: younan.xia@
bme.gatech.edu
Authors
Ming Zhao − School of Chemistry and Biochemistry, Georgia
Institute of Technology, Atlanta, Georgia 30332, United
States; orcid.org/0000-0003-0127-1470
Zitao Chen − The Wallace H. Coulter Department of
Biomedical Engineering, Georgia Institute of Technology and
Emory University, Atlanta, Georgia 30332, United States;
orcid.org/0000-0002-6111-7750
Yifeng Shi − School of Chemical and Biomolecular
Engineering, Georgia Institute of Technology, Atlanta,
Georgia 30332, United States
Zachary D. Hood − School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, Georgia 30332,
6300
https://doi.org/10.1021/jacs.1c02734
J. Am. Chem. Soc. 2021, 143, 6293−6302

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(10) Li, Z.; Ji, S.; Liu, Y.; Cao, X.; Tian, S.; Chen, Y.; Niu, Z.; Li, Y.
Well-Defined Materials for Heterogeneous Catalysis: From Nano-
particles to Isolated Single-Atom Sites. Chem. Rev. 2020, 120, 623−
682.
(11) Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for
the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52,
8526−8544.
(12) Watt, J.; Cheong, S.; Tilley, R. D. How to control the shape of
metal nanostructures in organic solution phase synthesis for
plasmonics and catalysis? Nano Today 2013, 8, 198−215.
(13) Wu, Y.; Wang, D.; Li, Y. Nanocrystals from solutions: catalysts.
Chem. Soc. Rev. 2014, 43, 2112−2124.
(29) Biacchi, A. J.; Schaak, R. E. Ligand-Induced Fate of Embryonic
Species in the Shape-Controlled Synthesis of Rhodium Nanoparticles.
ACS Nano 2015, 9, 1707−1720.
(30) Ghosh, S.; Manna, L. The Many “Facets” of Halide Ions in the
Chemistry of Colloidal Inorganic Nanocrystals. Chem. Rev. 2018, 118,
7804−7864.
(31) Lide, D. CRC Handbook of Chemistry and Physics, 84th ed.;
CRC Press: Boca Raton, FL, 2003; Vol. 9, pp 52−64.
(32) Xie, S.; Lu, N.; Xie, Z.; Wang, J.; Kim, M. J.; Xia, Y. Synthesis of
Pd−Rh Core−Frame Concave Nanocubes and Their Conversion to
Rh Cubic Nanoframes by Selective Etching of the Pd Cores. Angew.
Chem., Int. Ed. 2012, 51, 10266−10270.
(33) Xie, S.; Peng, H.-C.; Lu, N.; Wang, J.; Kim, M. J.; Xie, Z.; Xia,
Y. Confining the Nucleation and Overgrowth of Rh to the {111}
Facets of Pd nanocrystal Seeds: The Roles of Capping Agent and
Surface Diffusion. J. Am. Chem. Soc. 2013, 135, 16658−16667.
(34) Sneed, B. T.; Brodsky, C. N.; Kuo, C.-H.; Lamontagne, L. K.;
Jiang, Y.; Wang, Y.; Tao, F.; Huang, W.; Tsung, C.-K. Nanoscale-
Phase-Separated Pd−Rh Boxes Synthesized via Metal Migration: An
Archetype for Studying Lattice Strain and Composition Effects in
Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 14691−14700.
(35) Xia, X.; Xie, S.; Liu, M.; Peng, H.; Lu, N.; Wang, J.; Kim, M. J.;
Xia, Y. On the role of surface diffusion in determining the shape or
morphology of noble-metal nanocrystals. Proc. Natl. Acad. Sci. U. S. A.
2013, 110, 6669−6673.
(14) Wu, Y.; Cai, S.; Wang, D.; He, W.; Li, Y. Synthesis of Water-
Soluble Octahedral, Truncated Octahedral, and Cubic Pt−Ni
Nanocrystals and Their Structure-Activity Study in Model Hydro-
genation Reactions. J. Am. Chem. Soc. 2012, 134, 8975−8981.
(15) Yu, N.; Tian, N.; Zhou, Z.; Sheng, T.; Lin, W.; Ye, J.; Liu, S.;
Ma, H.; Sun, S. Pd Nanocrystals with Continuously Tunable High-
Index Facets as a Model Nanocatalyst. ACS Catal. 2019, 9, 3144−
3152.
(16) Lee, C.-T.; Yang, X.; Vara, M.; Gilroy, K. D.; Xia, Y. Water-
Based Synthesis of Sub-10 nm Pt Octahedra and Their Performance
towards the Oxygen Reduction Reaction. ChemNanoMat 2017, 3,
879−884.
(17) Bratile, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai,
G. A. Platinum Nanoparticle Shape Effects on Benzene Hydro-
genation Selectivity. Nano Lett. 2007, 7, 3097−3101.
(18) Xie, S.; Liu, X.; Xia, Y. Shape-controlled syntheses of rhodium
nanocrystals for the enhancement of their catalytic properties. Nano
Res. 2015, 8, 82−96.
(36) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Palladium nanocrystals
enclosed by {100} and {111} facets in controlled proportions and
their catalytic activities for formic acid oxidation. Energy Environ. Sci.
2012, 5, 6352−6357.
(37) Wang, Y.; Wan, D.; Xie, S.; Xia, X.; Huang, C.; Xia, Y. Synthesis
of Silver Octahedra with Controlled Sizes and Optical Properties via
Seed-Mediated Growth. ACS Nano 2013, 7, 4586−4594.
(38) Zhang, H.; Xia, X.; Li, W.; Zeng, J.; Dai, Y.; Yang, D.; Xia, Y.
Facile Synthesis of Five-fold Twinned, Starfish-like Rhodium
Nanocrystals by Eliminating Oxidative Etching with a Chloride-Free
Precursor. Angew. Chem., Int. Ed. 2010, 49, 5296−5300.
(39) Long, R.; Zhou, S.; Wiley, B. J.; Xiong, Y. Oxidative etching for
controlled synthesis of metal nanocrystals: atomic addition and
subtraction. Chem. Soc. Rev. 2014, 43, 6288−6310.
(19) Kang, Y.; Yang, P.; Markovic, N. M.; Stamenkovic, V. R.
Shaping electrocatalysis through tailored nanomaterials. Nano Today
2016, 11, 587−600.
(20) Wu, B.; Zheng, N. Surface and interface control of noble metal
nanocrystals for catalytic and electrocatalytic applications. Nano
Today 2013, 8, 168−197.
(21) Biacchi, A. J.; Schaak, R. E. The Solvent Matters: Kinetic versus
Thermodynamic Shape Control in the Polyol Synthesis of Rhodium
Nanoparticles. ACS Nano 2011, 5, 8089−8099.
(40) Xia, Y.; Gilroy, K. D.; Peng, H.-C.; Xia, X. Seed-Mediated
Growth of Colloidal Metal Nanocrystals. Angew. Chem., Int. Ed. 2017,
56, 60−95.
(22) Long, N. V.; Chien, N. D.; Hirata, H.; Matsubara, T.; Ohtaki,
M.; Nogami, M. Highly monodisperse cubic and octahedral rhodium
nanocrystals: Their evolutions from sharp polyhedrons into branched
nanostructures and surface-enhanced Raman scattering. J. Cryst.
Growth 2011, 320, 78−89.
(41) Niu, W.; Zhang, L.; Xu, G. Seed-mediated growth of noble
metal nanocrystals: crystal growth and shape control. Nanoscale 2013,
5, 3172−3181.
(42) Wang, Y.; He, J.; Liu, C.; Chong, W. H.; Chen, H.
Thermodynamics versus Kinetics in Nanosynthesis. Angew. Chem.,
Int. Ed. 2015, 54, 2022−2051.
(23) Luo, L.; Li, H.; Peng, Y.; Feng, C.; Zeng, J. Rh-Based
Nanocatalysts for Heterogeneous Reactions. ChemNanoMat 2018, 4,
451−466.
(24) Zhang, Y.; Grass, M. E.; Kuhn, J. N.; Tao, F.; Habas, S. E.;
Huang, W.; Yang, P.; Somorjai, G. A. Highly Selective Synthesis of
Catalytically Active Monodisperse Rhodium Nanocubes. J. Am. Chem.
Soc. 2008, 130, 5868−5869.
(25) Zhang, H.; Li, W.; Jin, M.; Zeng, J.; Yu, T.; Yang, D.; Xia, Y.
Controlling the Morphology of Rhodium Nanocrystals by Manipulat-
ing the Growth Kinetics with a Syringe Pump. Nano Lett. 2011, 11,
898−903.
(26) Duan, H.; Yan, N.; Yu, R.; Chang, C.; Zhou, G.; Hu, H.; Rong,
H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y.
Ultrathin rhodium nanosheets. Nat. Commun. 2014, 5, 3093.
(27) Zhang, N.; Shao, Q.; Pi, Y.; Guo, J.; Huang, X. Solvent-
Mediated Shape Tuning of Well-Defined Rhodium Nanocrystals for
Efficient Electrochemical Water Splitting. Chem. Mater. 2017, 29,
5009−5015.
(43) Koczkur, K. M.; Mourdikoudis, S.; Polavarapu, L.; Skrabalak, S.
E. Polyvinylpyrrolidone (PVP) in nanoparticle synthesis. Dalton
Trans. 2015, 44, 17883−17905.
(44) Kyriakou, V.; Neagu, D.; Zafeiropoulos, G.; Sharma, R. K.;
Tang, C.; Kousi, K.; Metcalfe, I. S.; van de Sanden, M. C. M.;
Tasmpas, M. N. Symmetrical Exsolution of Rh Nanoparticles in Solid
Oxide Cells for Efficient Syngas Production from Greenhouse Gases.
ACS Catal. 2020, 10, 1278−1288.
(45) Lambert, C. K. Current state of the art and future needs for
automotive exhaust catalysis. Nat. Catal. 2019, 2, 554−557.
(46) Asakura, H.; Hosokawa, S.; Ina, T.; Kato, K.; Nitta, K.; Uera,
K.; Uruga, T.; Miura, H.; Shishido, T.; Ohyama, J.; Satsuma, A.; Sato,
K.; Yamamoto, A.; Hinokuma, S.; Yoshida, H.; Machida, M.;
Yamazoe, S.; Tsukuda, T.; Teramura, K.; Tanaka, T. Dynamic
Behavior of Rh Species in Rh/Al2O3 Model Catalyst during Three-
Way Catalytic Reaction: An Operando X-ray Absorption Spectrosco-
py Study. J. Am. Chem. Soc. 2018, 140, 176−184.
(47) Zhao, M.; Chen, Z.; Lyu, Z.; Hood, Z. D.; Xie, M.; Vara, M.;
Chi, M.; Xia, Y. Ru Octahedral Nanocrystals with a Face-Centered
Cubic Structure, {111} Facets, Thermal Stability up to 400 °C, and
Enhanced Catalytic Activity. J. Am. Chem. Soc. 2019, 141, 7028−7036.
(28) Kuo, C.; Kao, C.; Chen, W.; Lu, Y.; Cullen, D. A.; Sneed, B. T.;
Chuang, Y.; Yu, C.; Kuo, C. Aqueous Synthesis of Concave Rh
Nanotetrahedra with Defect-Rich Surfaces: Insights into Growth-,
Defect-, and Plasmon-Enhanced Catalytic Energy Conversion. Chem.
Mater. 2018, 30, 4448−4458.
6301
https://doi.org/10.1021/jacs.1c02734
J. Am. Chem. Soc. 2021, 143, 6293−6302

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(48) Vara, M.; Roling, L. T.; Wang, X.; Elnabawy, A. O.; Hood, Z.
D.; Chi, M.; Mavrikakis, M.; Xia, Y. Understanding the Thermal
Stability of Palladium−Platinum Core−Shell Nanocrystals by In Situ
Transmission Electron Microscopy and Density Functional Theory.
ACS Nano 2017, 11, 4571−4581.
(49) Chi, M.; Wang, C.; Lei, Y.; Wang, G.; Li, D.; More, K. L.;
Lupini, A.; Allard, L. F.; Markovic, N. M.; Stamenkovic, V. R. Surface
faceting and elemental diffusion behavior at atomic scale for alloy
nanoparticles during in situ annealing. Nat. Commun. 2015, 6, 8925.
(50) Singh, S. K.; Zhang, X.; Xu, Q. Room-Temperature Hydrogen
Generation from Hydrous Hydrazine for Chemical Hydrogen Storage.
J. Am. Chem. Soc. 2009, 131, 9894−9895.
(51) Bai, J.; Xu, G.; Xing, S.; Zeng, J.; Jiang, J.; Chen, Y.
Hydrothermal Synthesis and Catalytic Application of Ultrathin
Rhodium Nanosheet Nanoassemblies. ACS Appl. Mater. Interfaces
2016, 8, 33635−33641.
(52) Deng, Z.; Lu, X.; Wen, Z.; Wei, S.; Liu, Y.; Fu, D.; Zhao, L.;
Guo, W. Mechanistic insight into the hydrazine decomposition on
Rh(111): effect of reaction intermediate on catalytic activity. Phys.
Chem. Chem. Phys. 2013, 15, 16172−16182.
(53) Kang, Y.; Xue, Q.; Peng, R.; Jin, P.; Zeng, J.; Jiang, J.; Chen, Y.
Bimetallic AuRh nanodendrites consisting of Au icosahedron cores
and atomically ultrathin Rh nanoplate shells: synthesis and light-
enhanced catalytic activity. NPG Asia Mater. 2017, 9, No. e407.
(54) Zhang, J.; Ye, J.; Fan, Q.; Jiang, Y.; Zhu, Y.; Li, H.; Cao, Z.;
Kuang, Q.; Cheng, J.; Zheng, J.; Xie, Z. Cyclic Penta-Twinned
Rhodium Nanobranches as Superior Catalysts for Ethanol Electro-
oxidation. J. Am. Chem. Soc. 2018, 140, 11232−11240.
(55) Li, H.; Fan, Q.; Ye, J.; Cao, Z.; Ma, Z.; Jiang, Y.; Zhang, J.;
Cheng, J.; Xie, Z.; Zheng, L. Excavated Rh nanobranches boost
ethanol electro-oxidation. Mater. Today Energy 2019, 11, 120−127.
6302
https://doi.org/10.1021/jacs.1c02734
J. Am. Chem. Soc. 2021, 143, 6293−6302