Symmetry Breaking during Seeded Growth of Nanocrystals

Currently, most of the reported noble-metal nanocrystals are limited to a high level of symmetry, as constrained by the inherent, face-centered cubic (fcc) lattice of these metals. In this paper, we report, for the first time, a facile and versatile approach (backed up by a clear mechanistic understanding) for breaking the symmetry of an fcc lattice and thus obtaining nanocrystals with highly unsymmetrical shapes. The key strategy is to induce and direct the growth of nanocrystal seeds into unsymmetrical modes by manipulating the reduction kinetics. With silver as an example, we demonstrated that the diversity of possible shapes taken by noble-metal nanocrystals could be greatly expanded by incorporating a series of new shapes drastically deviated from the fcc lattice. This work provides a new method to investigate shape-controlled synthesis of metal nanocrystal

Novel Nanostructures of Rutile Fabricated by Templating against Yarns of Polystyrene Nanofibrils and Their Catalytic Applications

This Article describes a facile approach to the synthesis of rutile nanostructures in the form of porous fibers or bundles of nanotubes by maneuvering the surface wettability of yarns made of polystyrene nanofibrils. Specifically, hierarchically porous fibers were obtained by hydrolyzing titanium tetraisopropoxide to form TiO₂ nanoparticles in the void spaces among hydrophobic nanofibrils in each yarn. After calcination in air at 800 °C, the resultant fibers were comprised of many interconnected rutile nanoparticles whose diameters were in the range 20–80 nm. After converting the nanofibrils and yarns into hydrophilic surfaces through plasma treatment, however, the TiO₂ formed conformal coatings on the surfaces of nanofibrils in each yarn during hydrolysis instead of just filling the void spaces among the nanofibrils. As a result, bundles of rutile nanotubes were obtained after the sample had been calcined in air at 800 °C. The thermodynamically stable rutile nanostructures were then explored as supports for Pt nanoparticles whose catalytic activity was evaluated using the reduction of <i>p</i>-nitrophenol by NaBH₄. The Pt supported on porous rutile fibers exhibited a better performance than the Pt on rutile nanotubes in terms of both induction time (<i>t</i>_ind) and apparent rate constant (<i>k</i>_app)

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

A Sinter-Resistant Catalytic System Fabricated by Maneuvering the Selectivity of SiO₂ Deposition onto the TiO₂ Surface versus the Pt Nanoparticle Surface

A triphasic catalytic system (Pt/TiO₂–SiO₂) with an “islands in the sea” configuration was fabricated by controlling the selectivity of SiO₂ deposition onto the surface of TiO₂ versus the surface of Pt nanoparticles. The Pt surface was exposed, while the nanoparticles were supported on TiO₂ and isolated from each other by SiO₂ to achieve both significantly improved sinter resistance up to 700 °C and outstanding activity after high-temperature calcination. This work not only demonstrates the feasibility of using a new triphasic system with uncovered catalyst to maximize the thermal stability and catalytic activity but also offers a general approach to the synthesis of high-performance catalytic systems with tunable compositions

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

Development of Highly-Active Catalysts toward Oxygen Reduction by Controlling the Shape and Composition of Pt–Ni Nanocrystals

Electrocatalysts comprised of Pt–Ni alloy nanocrystals have garnered substantial attention due to their outstanding performance in catalyzing the oxygen reduction reaction (ORR). Herein, we present the synthesis of Pt–Ni nanocrystals with a variety of controlled shapes and compositions in an effort to investigate the impact of the Ni content on the formation of {111} facets and thereby the ORR activity. By completely excluding O2 from the reaction system, we could prevent the generation of Ni(OH)2 on the surface of the nanocrystals and thereby achieve a linear relationship between the atomic ratio of Pt to Ni in the nanocrystals and the feeding ratio of the precursors. The atomic ratio of Pt to Ni in the Pt–Ni nanocrystals was tunable within the range of 1.2–7.2, while their average sizes were kept around 9 nm in terms of edge length. In addition, a quantitative correlation between the area ratio of {111} to {100} facets and the feeding ratio of Pt(II) to Ni(II) was obtained by adjusting the mole fraction of the Ni(II) precursor in the reaction mixture. For the catalysts comprising octahedral nanocrystals, their specific ORR activities exhibited a positive correlation with the Pt/Ni atomic ratio. After the accelerated durability test, both specific and mass activity displayed a volcano-type trend with a peak value at a Pt/Ni atomic ratio of 1.6

Seed-Mediated Synthesis of Silver Nanocrystals with Controlled Sizes and Shapes in Droplet Microreactors Separated by Air

Silver nanocrystals with uniform sizes were synthesized in droplet microreactors through seed-mediated growth. The key to the success of this synthesis is the use of air as a carrier phase to generate the droplets. The air not only separates the reaction solution into droplets but also provides O₂ for the generation of reducing agent (glycolaldehyde). It also serves as a buffer space for the diffusion of NO, which is formed <i>in situ</i> due to the oxidative etching of Ag nanocrystals with twin defects. For the first time, we were able to generate Ag nanocrystals with controlled sizes and shapes in continuous production by using droplet microreactors. For Ag nanocubes, their edge lengths could be readily controlled in the range of 30–100 nm by varying the reaction time, the amount of seeds, and the concentration of AgNO₃ in the droplets. Furthermore, we demonstrated the synthesis of Ag octahedra in the droplet microreactors. We believe that the air-driven droplet generation device can be extended to other noble metals for the production of nanocrystals with controlled sizes and shapes

Synthesis of Ag Nanocubes 18–32 nm in Edge Length: The Effects of Polyol on Reduction Kinetics, Size Control, and Reproducibility

This article describes a robust method for the facile synthesis of small Ag nanocubes with edge lengths controlled in the range of 18–32 nm. The success of this new method relies on the substitution of ethylene glycol (EG)the solvent most commonly used in a polyol synthesiswith diethylene glycol (DEG). Owing to the increase in hydrocarbon chain length, DEG possesses a higher viscosity and a lower reducing power relative to EG. As a result, we were able to achieve a nucleation burst in the early stage to generate a large number of seeds and a relatively slow growth rate thereafter; both factors were critical to the formation of Ag nanocubes with small sizes and in high purity (>95%). The edge length of the Ag nanocubes could be easily tailored in the range of 18–32 nm by quenching the reaction at different time points. For the first time, we were able to produce uniform sub-20 nm Ag nanocubes in a hydrophilic medium and on a scale of ∼20 mg per batch. It is also worth pointing out that the present protocol was remarkably robust, showing good reproducibility between different batches and even for DEGs obtained from different vendors. Our results suggest that the high sensitivity of synthesis outcomes to the trace amounts of impurities in a polyol, a major issue for reproducibility and scale up synthesis, did not exist in the present system

Synthesis of Colloidal Metal Nanocrystals in Droplet Reactors: The Pros and Cons of Interfacial Adsorption

Droplet reactors have received considerable attention in recent years as an alternative route to the synthesis and potentially high-volume production of colloidal metal nanocrystals. Interfacial adsorption will immediately become an important issue to address when one seeks to translate a nanocrystal synthesis from batch reactors to droplet reactors due to the involvement of higher surface-to-volume ratios for the droplets and the fact that nanocrystals tend to be concentrated at the water–oil interface. Here we report a systematic study to compare the pros and cons of interfacial adsorption of metal nanocrystals during their synthesis in droplet reactors. On the one hand, interfacial adsorption can be used to generate nanocrystals with asymmetric shapes or structures, including one-sixth-truncated Ag octahedra and Au–Ag nanocups. On the other hand, interfacial adsorption has to be mitigated to obtain nanocrystals with uniform sizes and controlled shapes. We confirmed that Triton X-100, a nonionic surfactant, could effectively alleviate interfacial adsorption while imposing no impact on the capping agent typically needed for a shape-controlled synthesis. With the introduction of a proper surfactant, droplet reactors offer an attractive platform for the continuous production of colloidal metal nanocrystals

Seed-Mediated Synthesis of Silver Nanocrystals with Controlled Sizes and Shapes in Droplet Microreactors Separated by Air

Silver nanocrystals with uniform sizes were synthesized in droplet microreactors through seed-mediated growth. The key to the success of this synthesis is the use of air as a carrier phase to generate the droplets. The air not only separates the reaction solution into droplets but also provides O₂ for the generation of reducing agent (glycolaldehyde). It also serves as a buffer space for the diffusion of NO, which is formed <i>in situ</i> due to the oxidative etching of Ag nanocrystals with twin defects. For the first time, we were able to generate Ag nanocrystals with controlled sizes and shapes in continuous production by using droplet microreactors. For Ag nanocubes, their edge lengths could be readily controlled in the range of 30–100 nm by varying the reaction time, the amount of seeds, and the concentration of AgNO₃ in the droplets. Furthermore, we demonstrated the synthesis of Ag octahedra in the droplet microreactors. We believe that the air-driven droplet generation device can be extended to other noble metals for the production of nanocrystals with controlled sizes and shapes