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^– ions. In comparison, the Pd atoms were found to randomly nucleate on the entire surface of similar Pd seeds if the Br^– ions were removed from the surface in advance. For a 1:1 mixture of Br^–-capped and Br^–-free Pd cubic seeds, more atoms were added onto the Br^–-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^– 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^– ions in the synthesis of Pd nanocrystals. The Br^– 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^– 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^– ions needed for the formation of Pd nanocubes with a specific size. If the concentration of Br^– 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 H₂ 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^– or I^– 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 S₂O₃^2– ions, twin defects were developed during the growth process to generate multiply twinned nanostructures. X-ray photoelectron spectroscopy analysis indicated that the S₂O₃^2– ions were chemisorbed on the surfaces of the seeds during the treatment. The chemisorbed S₂O₃^2– 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 S₂O₃^2– 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 Na₂SO₄ 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 Na₂SO₄ (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 Na₂SO₄ 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 PdCl₄^2– and PdBr₄^2– by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl₄^2– was reduced in the solution phase while PdBr₄^2– was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr₄^2– by ascorbic acid could be switched from surface to solution by raising the reaction temperature