13 research outputs found
Toward a Quantitative Understanding of Symmetry Reduction Involved in the Seed-Mediated Growth of Pd Nanocrystals
We
report a quantitative analysis of the symmetry reduction phenomenon
involved in the seed-mediated growth of Pd nanocrystals under dropwise
addition of a precursor solution. In addition to the elimination of
self-nucleation, the dropwise approach allows for the formation of
a steady state for the number of precursor ions in the growth solution,
which only fluctuates in a narrow range defined by experimental parameters
such as the initial concentration of precursor solution and the injection
rate. We can deterministically control the growth mode (symmetric
vs asymmetric) of a seed by tuning these parameters to quantitatively
manipulate the reaction kinetics and thus the lower and upper limits
that define the steady state. We demonstrate that there exists a correlation
between the growth mode and the lower limit of precursor ions in the
steady state of a seed-mediated growth process. For the first few
drops of precursor solution, the resultant atoms will only be deposited
on a limited number of available sites on the seed if the lower limit
of the steady state is below a critical value. Afterward, the deposition
of atoms will be largely confined to these initially activated sites
to induce symmetry reduction if atom deposition is kept at a faster
rate than surface diffusion by controlling the lower limit of precursor
ions in the steady state. Otherwise, the migration of atoms to other
regions through surface diffusion can access other sites on the surface
of a seed and thus lead to the switch of growth mode from asymmetric
to symmetric. Our study suggests that symmetry reduction can only
be initiated and retained by keeping the atom deposition at a rate
slow enough to limit the number of initial nucleation sites on a seed
but fast enough to beat the surface diffusion process
Seed-Mediated Synthesis of Pd Nanocrystals: The Effect of Surface Capping on the Heterogeneous Nucleation and Growth
Seed-mediated
growth has emerged as an effective approach to the
synthesis of noble-metal nanocrystals with well-controlled sizes,
shapes, compositions, and structures. Although surface capping is
known to affect the growth pattern of a seed, its explicit role remains
to be fully understood. In this article, we applied the collision
model established for surface science to seed-mediated growth of nanocrystals
in an effort to account for the heterogeneous nucleation of atoms
on the surface of a seed and thus the growth pattern in the presence
or absence of a surface capping agent. Using Pd cubic seeds as a model
system, we demonstrated that the heterogeneous nucleation of Pd atoms
only occurred at the corner and edge sites when the {100} side faces
were selectively passivated by chemisorbed Br<sup>–</sup> ions.
In comparison, the Pd atoms were found to randomly nucleate on the
entire surface of similar Pd seeds if the Br<sup>–</sup> ions
were removed from the surface in advance. For a 1:1 mixture of Br<sup>–</sup>-capped and Br<sup>–</sup>-free Pd cubic seeds,
more atoms were added onto the Br<sup>–</sup>-free seeds due
to the involvement of a much larger bare surface with a higher sticking
coefficient. In addition, we found that the growth mode (island vs
layer-by-layer) of a seed was also highly sensitive to the surface
condition and could be altered by manipulating the rate of surface
diffusion. We further extended the collision model to account for
the growth behavior of other types of seeds whose surfaces were enclosed
by a mix of {111} and {100} facets in different proportions or characterized
by different internal structures, including Pd cuboctahedra and pentatwinned
nanowires. The mechanistic insights from this study clearly demonstrate
the role played by a surface capping agent in determining the sticking
coefficient of atoms and the morphology taken by nanocrystals in a
seed-mediated synthesis and should be extendable to other systems
involving different types or combinations of metals
Quantitative Analysis of the Coverage Density of Br<sup>–</sup> Ions on Pd{100} Facets and Its Role in Controlling the Shape of Pd Nanocrystals
We
report an approach based on a combination of inductively coupled
plasma mass spectrometry and X-ray photoelectron spectroscopy for
quantitative analysis of the role played by Br<sup>–</sup> ions
in the synthesis of Pd nanocrystals. The Br<sup>–</sup> ions
were found to adsorb onto Pd{100} facets selectively with a coverage
density of ca. 0.8 ion per surface Pd atom. The chemisorbed Br<sup>–</sup> ions could be removed via desorption at an elevated
temperature under reductive conditions. They could also be gradually
released from the surface when Pd cubic seeds grew into cuboctahedrons
and then octahedrons. On the basis of the coverage density information,
we were able to estimate the minimum concentration of Br<sup>–</sup> ions needed for the formation of Pd nanocubes with a specific size.
If the concentration of Br<sup>–</sup> ions was below this
minimum value, not all of the {100} facets could be stabilized by
the capping agent, leading to the formation of nanocubes with truncated
corners. The quantitative analysis developed in this study is potentially
extendable to other systems involving chemisorbed capping agents
Use of Reduction Rate as a Quantitative Knob for Controlling the Twin Structure and Shape of Palladium Nanocrystals
Kinetic control is a powerful
means for maneuvering the twin structure and shape of metal nanocrystals
and thus optimizing their performance in a variety of applications.
However, there is only a vague understanding of the explicit roles
played by reaction kinetics due to the lack of quantitative information
about the kinetic parameters. With Pd as an example, here we demonstrate
that kinetic parameters, including rate constant and activation energy,
can be derived from spectroscopic measurements and then used to calculate
the initial reduction rate and further have this parameter quantitatively
correlated with the twin structure of a seed and nanocrystal. On a
quantitative basis, we were able to determine the ranges of initial
reduction rates required for the formation of nanocrystals with a
specific twin structure, including single-crystal, multiply twinned,
and stacking fault-lined. This work represents a major step forward
toward the deterministic syntheses of colloidal noble-metal nanocrystals
with specific twin structures and shapes
Facile Synthesis of Gold Wavy Nanowires and Investigation of Their Growth Mechanism
We describe a synthesis of Au wavy nanowires in an aqueous
solution
in the presence of cetyltrimethylammonium bromide (CTAB). The resultant
Au nanowires automatically separated from the solution and floated
at the air/water interface. We investigated the formation mechanism
by characterizing the samples obtained at different stages of the
synthesis. Both particle attachment and cold welding were found to
be involved in the formation of such nanowires. Based on X-ray photoelectron
spectroscopy and thermogravimetric analysis, the CTAB molecules adsorbed
on the surface of a Au nanostructure went through a change in structure
from a bilayer to a monolayer, converting the Au surface from hydrophilic
to hydrophobic. As a result, the Au wavy nanowires were driven to
the air/water interface during the synthesis. This growth mechanism
is potentially extendable to many other systems involving small surfactant
molecules
Facile Synthesis of Iridium Nanocrystals with Well-Controlled Facets Using Seed-Mediated Growth
Iridium
nanoparticles have only been reported with roughly spherical
shapes and sizes of 1–5 nm, making it impossible to investigate
their facet-dependent catalytic properties. Here we report for the
first time a simple method based on seed-mediated growth for the facile
synthesis of Ir nanocrystals with well-controlled facets. The essence
of this approach is to coat an ultrathin conformal shell of Ir on
a Pd seed with a well-defined shape at a relatively high temperature
to ensure fast surface diffusion. In this way, the facets on the initial
Pd seed are faithfully replicated in the resultant Pd@Ir core–shell
nanocrystal. With 6 nm Pd cubes and octahedra encased by {100} and
{111} facets, respectively, as the seeds, we have successfully generated
Pd@Ir cubes and octahedra covered by Ir{100} and Ir{111} facets. The
Pd@Ir cubes showed higher H<sub>2</sub> selectivity (31.8% vs 8.9%)
toward the decomposition of hydrazine compared with Pd@Ir octahedra
with roughly the same size
Confining the Nucleation and Overgrowth of Rh to the {111} Facets of Pd Nanocrystal Seeds: The Roles of Capping Agent and Surface Diffusion
This article describes a systematic
study of the spatially confined
growth of Rh atoms on Pd nanocrystal seeds, with a focus on the blocking
effect of a surface capping agent and the surface diffusion of adatoms.
We initially used Pd cuboctahedrons as the seeds to illustrate the
concept and to demonstrate the capabilities of our approach. Because
the Pd{100} facets were selectively capped by a layer of chemisorbed
Br<sup>–</sup> or I<sup>–</sup> ions, we were able to
confine the nucleation and deposition of Rh atoms solely on the {111}
facets of a Pd seed. When the synthesis was conducted at a relatively
low temperature, the deposition of Rh atoms followed an island growth
mode because of the high Rh–Rh interatomic binding energy.
We also facilitated the surface diffusion of deposited Rh atoms by
increasing the reaction temperature and decreasing the injection rate
for the Rh precursor. Under these conditions, the deposition of Rh
on the Pd{111} facets was switched to a layered growth mode. We further
successfully extended this approach to a variety of other types of
Pd polyhedral seeds that contained Pd{111} and Pd{100} facets in different
proportions on the surface. As expected, a series of Pd–Rh
bimetallic nanocrystals with distinctive elemental distributions were
obtained. We could remove the Pd cores through selective chemical
etching to generate Rh hollow nanoframes with different types and
degrees of porosity. This study clearly demonstrates the importance
of facet capping, surface diffusion, and reaction kinetics in controlling
the morphologies of bimetallic nanocrystals during a seed-mediated
process. It also provides a new direction for the rational design
and synthesis of nanocrystals with spatially controlled distributions
of elements for a variety of applications
Seed-Mediated Growth of Gold Nanocrystals: Changes to the Crystallinity or Morphology as Induced by the Treatment of Seeds with a Sulfur Species
We report our observation of changes
to the crystallinity or morphology
during seed-mediated growth of Au nanocrystals. When single-crystal
Au seeds with a spherical or rod-like shape were treated with a chemical
species such as S<sub>2</sub>O<sub>3</sub><sup>2–</sup> ions,
twin defects were developed during the growth process to generate
multiply twinned nanostructures. X-ray photoelectron spectroscopy
analysis indicated that the S<sub>2</sub>O<sub>3</sub><sup>2–</sup> ions were chemisorbed on the surfaces of the seeds during the treatment.
The chemisorbed S<sub>2</sub>O<sub>3</sub><sup>2–</sup> ions
somehow influenced the crystallization of Au atoms added onto the
surface during a growth process, leading to the formation of twin
defects. In contrast to the spherical and rod-like Au seeds, the single-crystal
structure was retained to generate a concave morphology when single-crystal
Au seeds with a cubic or octahedral shape were used for a similar
treatment and then seed-mediated growth. The different outcomes are
likely related to the difference in spatial distribution of S<sub>2</sub>O<sub>3</sub><sup>2–</sup> ions chemisorbed on the
surface of a seed. This approach based on surface modification is
potentially extendable to other noble metals for engineering the crystallinity
and morphology of nanocrystals formed via seed-mediated growth
Polyol Syntheses of Palladium Decahedra and Icosahedra as Pure Samples by Maneuvering the Reaction Kinetics with Additives
This article reports a robust method based upon polyol reduction for the deterministic synthesis of Pd decahedra or icosahedra with tunable sizes and a purity approaching 100%. The success of such a selective synthesis relies on an ability to fine-tune the reaction kinetics through the addition of Na<sub>2</sub>SO<sub>4</sub> and HCl for decahedra and icosahedra, respectively. In the absence of any additive, the product of a similar synthesis in diethylene glycol contained 10% decahedra and 90% icosahedra. By optimizing the amount of Na<sub>2</sub>SO<sub>4</sub> (or HCl) added into the reaction solution, the percent of decahedra (or icosahedra) in the product could be increased up to 100%. The roles of Na<sub>2</sub>SO<sub>4</sub> and HCl were also investigated in great detail, and two plausible mechanisms were proposed and validated through a set of experiments. In general, a faster reduction rate is needed for the synthesis of Pd decahedra when compared with what is needed for Pd icosahedra. This work not only offers a simple approach to the deterministic syntheses of Pd decahedra and icosahedra but also provides an in-depth understanding of the mechanisms involved in shape-controlled syntheses of noble-metal nanocrystals from the perspective of reaction kinetics. On the basis of the mechanistic understanding, we have also achieved successful synthesis of Pd decahedra as pure samples by adding a proper amount of NaOH into the system to speed up the reduction kinetics
Toward a Quantitative Understanding of the Reduction Pathways of a Salt Precursor in the Synthesis of Metal Nanocrystals
Despite the pivotal
role played by the reduction of a salt precursor in the synthesis
of metal nanocrystals, it is still unclear how the precursor is reduced.
The precursor can be reduced to an atom in the solution phase, followed
by its deposition onto the surface of a growing nanocrystal. Alternatively,
the precursor can adsorb onto the surface of a growing nanocrystal,
followed by reduction through an autocatalytic process. With Pd as
an example, here we demonstrate that the pathway has a correlation
with the reduction kinetics involved. Our quantitative analyses of
the reduction kinetics of PdCl<sub>4</sub><sup>2–</sup> and
PdBr<sub>4</sub><sup>2–</sup> by ascorbic acid at room temperature
in the absence and presence of Pd nanocubes, respectively, suggest
that PdCl<sub>4</sub><sup>2–</sup> was reduced in the solution
phase while PdBr<sub>4</sub><sup>2–</sup> was reduced on the
surface of a growing nanocrystal. Our results also demonstrate that
the reduction pathway of PdBr<sub>4</sub><sup>2–</sup> by ascorbic
acid could be switched from surface to solution by raising the reaction
temperature