Toward the Synthesis of Sub-15 nm Ag Nanocubes with Sharp Corners and Edges: The Roles of Heterogeneous Nucleation and Surface Capping

We report a polyol method for the facile synthesis of Ag nanocubes having sharp corners and edges, together with edge lengths below 15 nm. The rapid nucleation of Ag atoms was facilitated through the addition of a trace amount of SH^– to generate Ag₂S clusters while the corners and edges of the nanocubes were sharpened through the introduction of Br^– as a regulator of the growth kinetics and a capping agent for the Ag(100) surface. Because of their much smaller size relative to the more commonly used capping agent based on poly­(vinylpyrrolidone), Br^– ions are more effective in passivating the {100} facets on very small Ag nanocubes. The mechanistic roles of these additives, along with the effects of their interactions with other species present in the reaction solution, were all systematically investigated. The concentration of SH^– was found to be a particularly effective parameter for tuning the edge length of the nanocubes. As a result of the understanding gained during the course of this study, Ag nanocubes with uniform edge lengths controllable in the range of 13–23 nm could be reliably produced. The nanocubes of 13.4 ± 0.4 nm in edge length constitute the smallest nanocrystals of this kind reported to date; they also possess sharper corners and edges relative to the limited examples of sub-20 nm Ag nanocubes reported in the literature. The availability of such small and sharp Ag nanocubes will open the door to an array of applications in plasmonics, catalysis, and biomedicine

Oxidative Etching of Pd Decahedral Nanocrystals with a Penta-twinned Structure and Its Impact on Their Growth Behavior

We report a systematic study of the oxidative etching of penta-twinned Pd decahedral nanocrystals by O₂/I^– under different conditions and its impact on their subsequent growth behavior. Analysis by transmission electron microscopy shows significant rounding of the decahedral structure. More specifically, the etching is found to begin at the equatorial vertices, due to their high surface free energy, and proceed along the adjacent, equatorial edges through the dissolution of low-coordination atoms. Comparison of the etching behaviors under different conditions reveals the critical role of a reductive environment for the initiation of oxidative etching, possibly due to the presence of a protective oxide layer on the surface of Pd decahedra. Overgrowth on the seeds with a rounded profile generates penta-twinned Pd nanorods with an asymmetric, tapered structure as a result of simultaneous axial and radial growth. In comparison, the original decahedral seeds only show axial growth, leading to the formation of penta-twinned nanorods with a uniform size along the axial direction. A good understanding of the etching and growth behaviors of Pd decahedral nanocrystals will be useful for the successful adoption of these nanomaterials in real-world applications, including their use as catalysts and as a platform for the development of more complex nanostructures

Assembly Behavior of Iron Oxide-Capped Janus Particles in a Magnetic Field

Three types of iron oxide Janus particles are obtained by varying the deposition rate of iron in a 3:1 Ar/O₂ atmosphere during physical vapor deposition. Each type of iron oxide Janus particle shows a distinct assembly behavior when an external magnetic field is applied, i.e., formation of staggered chains, double chains, or no assembly. A detailed deposition rate diagram is obtained to identify the relationship between deposition rate and assembly behavior. The extent of iron oxidation is identified as the key parameter in determining the assembly behavior. In addition, the effects of particle volume fraction, thickness of the iron oxide cap, and assembly time on the final assembly behavior are studied. Cap thickness is shown not to influence the assembly behavior, while particle volume fraction and assembly time affect the chain growth rate and the chain length, but not the overall assembly behavior. The samples are characterized by optical, scanning electron, and atomic force microscopies

Assembly Behavior of Iron Oxide-Capped Janus Particles in a Magnetic Field

Three types of iron oxide Janus particles are obtained by varying the deposition rate of iron in a 3:1 Ar/O₂ atmosphere during physical vapor deposition. Each type of iron oxide Janus particle shows a distinct assembly behavior when an external magnetic field is applied, i.e., formation of staggered chains, double chains, or no assembly. A detailed deposition rate diagram is obtained to identify the relationship between deposition rate and assembly behavior. The extent of iron oxidation is identified as the key parameter in determining the assembly behavior. In addition, the effects of particle volume fraction, thickness of the iron oxide cap, and assembly time on the final assembly behavior are studied. Cap thickness is shown not to influence the assembly behavior, while particle volume fraction and assembly time affect the chain growth rate and the chain length, but not the overall assembly behavior. The samples are characterized by optical, scanning electron, and atomic force microscopies

Assembly Behavior of Iron Oxide-Capped Janus Particles in a Magnetic Field

Three types of iron oxide Janus particles are obtained by varying the deposition rate of iron in a 3:1 Ar/O₂ atmosphere during physical vapor deposition. Each type of iron oxide Janus particle shows a distinct assembly behavior when an external magnetic field is applied, i.e., formation of staggered chains, double chains, or no assembly. A detailed deposition rate diagram is obtained to identify the relationship between deposition rate and assembly behavior. The extent of iron oxidation is identified as the key parameter in determining the assembly behavior. In addition, the effects of particle volume fraction, thickness of the iron oxide cap, and assembly time on the final assembly behavior are studied. Cap thickness is shown not to influence the assembly behavior, while particle volume fraction and assembly time affect the chain growth rate and the chain length, but not the overall assembly behavior. The samples are characterized by optical, scanning electron, and atomic force microscopies

A Droplet-Reactor System Capable of Automation for the Continuous and Scalable Production of Noble-Metal Nanocrystals

Noble-metal nanocrystals with well-controlled shapes or morphologies are of great interest for a variety of applications. To utilize these nanomaterials in consumer products, one has to produce the colloidal nanocrystals in large quantities while maintaining good control over their physical parameters and properties. Droplet reactors have shown great potential for the continuous and scalable production of colloidal nanocrystals with controlled shapes. However, the efficiencies of most previously reported systems are still limited because of the complex post-treatment procedures. For example, the mixture of silicone oil and an aqueous suspension of solid products has to be separated by leveraging their miscibility and difference in density, while the solid products often need to be purified and concentrated by centrifugation. Herein, we report the design and construction of a droplet-reactor system that include new features such as a homemade unit for the automatic separation of silicone oil from the aqueous phase as well as a cross-flow filtration unit for the effective purification and concentration of the nanocrystals. Using various types of Pd nanocrystals as examples, we have demonstrated the feasibility of using this system to automatically produce and collect samples with uniform sizes and well-controlled shapes

Assembly Behavior of Iron Oxide-Capped Janus Particles in a Magnetic Field

Three types of iron oxide Janus particles are obtained by varying the deposition rate of iron in a 3:1 Ar/O₂ atmosphere during physical vapor deposition. Each type of iron oxide Janus particle shows a distinct assembly behavior when an external magnetic field is applied, i.e., formation of staggered chains, double chains, or no assembly. A detailed deposition rate diagram is obtained to identify the relationship between deposition rate and assembly behavior. The extent of iron oxidation is identified as the key parameter in determining the assembly behavior. In addition, the effects of particle volume fraction, thickness of the iron oxide cap, and assembly time on the final assembly behavior are studied. Cap thickness is shown not to influence the assembly behavior, while particle volume fraction and assembly time affect the chain growth rate and the chain length, but not the overall assembly behavior. The samples are characterized by optical, scanning electron, and atomic force microscopies

A Droplet-Reactor System Capable of Automation for the Continuous and Scalable Production of Noble-Metal Nanocrystals

Noble-metal nanocrystals with well-controlled shapes or morphologies are of great interest for a variety of applications. To utilize these nanomaterials in consumer products, one has to produce the colloidal nanocrystals in large quantities while maintaining good control over their physical parameters and properties. Droplet reactors have shown great potential for the continuous and scalable production of colloidal nanocrystals with controlled shapes. However, the efficiencies of most previously reported systems are still limited because of the complex post-treatment procedures. For example, the mixture of silicone oil and an aqueous suspension of solid products has to be separated by leveraging their miscibility and difference in density, while the solid products often need to be purified and concentrated by centrifugation. Herein, we report the design and construction of a droplet-reactor system that include new features such as a homemade unit for the automatic separation of silicone oil from the aqueous phase as well as a cross-flow filtration unit for the effective purification and concentration of the nanocrystals. Using various types of Pd nanocrystals as examples, we have demonstrated the feasibility of using this system to automatically produce and collect samples with uniform sizes and well-controlled shapes

Toward a Quantitative Understanding of the Sulfate-Mediated Synthesis of Pd Decahedral Nanocrystals with High Conversion and Morphology Yields

We report a systematic study of the sulfate-mediated polyol synthesis of Pd decahedra in an attempt to maximize both the conversion and morphology yields. Quantitative analyses of the as-obtained products reveal the presence of nanocrystal populations with distinct ranges of size. Samples collected in the early stage of a standard synthesis indicate that this size distribution is likely caused by the coalescence of the initially formed decahedral nuclei. Kinetic measurements clearly show the impact of the sulfate additive on the initial reduction rate and thus the yield of decahedral nuclei. Based on the mechanistic understanding developed over the course of this work, we were able to significantly improve the conversion and morphology yields of the Pd decahedra synthesis by introducing a second reducing agent after the completion of homogeneous nucleation. The improvement in yields for the Pd decahedra synthesis will increase the availability of these nanocrystals as catalytic materials and as a unique platform for the development of more complex nanostructures

One-Pot Synthesis of Penta-twinned Palladium Nanowires and Their Enhanced Electrocatalytic Properties

This article reports the design and successful implementation of a one-pot, polyol method for the synthesis of penta-twinned Pd nanowires with diameters below 8 nm and aspect ratios up to 100. The key to the success of this protocol is the controlled reduction of Na₂PdCl₄ by diethylene glycol and ascorbic acid through the introduction of NaI and HCl. The I^– and H⁺ ions can slow the reduction kinetics by forming PdI₄^2– and inhibiting the dissociation of ascorbic acid, respectively. When the initial reduction rate is tuned into the proper regime, Pd decahedral seeds with a penta-twinned structure appear during nucleation. In the presence of I^– ions as a selective capping agent toward the Pd(100) surface, the decahedral seeds can be directed to grow axially into penta-twinned nanorods and then nanowires. The Pd nanowires are found to evolve into multiply twinned particles if the reaction time is extended beyond 1.5 h, owing to the involvement of oxidative etching. When supported on carbon, the Pd nanowires show greatly enhanced specific electrocatalytic activities, more than five times the value for commercial Pd/C toward formic acid oxidation and three times the value for Pt/C toward oxygen reduction under an alkaline condition. In addition, the carbon-supported Pd nanowires exhibit greatly enhanced electrocatalytic durability toward both reactions. Furthermore, we demonstrate that the Pd nanowires can serve as sacrificial templates for the conformal deposition of Pt atoms to generate Pd@Pt core–sheath nanowires and then Pd–Pt nanotubes with a well-defined surface structure