Shan Zhou

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 PdCl42- and PdBr42- by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl42- was reduced in the solution phase while PdBr42- was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr42- by ascorbic acid could be switched from surface to solution by raising the reaction temperature.

Site-selective growth of Ag nanocubes for sharpening their corners and edges, followed by elongation into nanobars through symmetry reduction

It remains a challenge to synthesize Ag nanocubes with sharp corners and edges while retaining a compact size below 20 nm. Here we demonstrate the use of site-selective growth to sharpen the corners and edges of truncated Ag nanocubes with sizes down to 18 nm, followed by their elongation into nanobars with aspect ratios up to 2. The key to the success of this synthesis is the site-selective deposition at corners and edges, as enabled by cetyltrimethylammonium chloride (CTAC). While CTA+ is an effective colloidal stabilizer, Cl− can react with Ag+ to generate AgCl precipitates, slowing down the reduction kinetics. In addition, Cl− can serve as a facet-selective capping agent towards the {100} side faces and thereby confine the growth mainly to corners and edges. Interestingly, once all the corners and edges have been sharpened, the growth is switched to an asymmetric mode to favor deposition on one of the six side faces only, leading to the formation of Ag nanobars with controllable aspect ratios. The symmetry reduction takes place as a result of the limited supply of Ag atoms, the strong capping of Cl− ions towards the {100} facets, and the possible involvement of localized oxidative etching caused by Cl−/O2. We also demonstrate that the Ag nanocubes with sharp corners and edges can serve as a better sacrificial template than their truncated counterparts in generating Au hollow nanostructures with ultrathin walls.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

Significance Controlling the shape of colloidal metal nanocrystals is central to the realization of their diverse applications in catalysis, photonics, electronics, and medicine. Here, we demonstrate that autocatalytic surface reduction can be employed to enable the formation of metal nanocrystals with well-controlled and predictable shapes through seed-mediated growth. Our quantitative analysis suggests that the kinetics of autocatalytic surface reduction is highly sensitive to the atomic structures on the surface of the seed, leading to different growth rates for different sites on the seed and eventually resulting in the evolution of nanocrystals into different shapes. The mechanistic insights into autocatalytic surface reduction obtained in this work can be extended to other systems involving nanocrystals with different compositions, facets, and structures. The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Facile Synthesis of Pd@Pt3−4L Core−Shell Octahedra with a Clean Surface and thus Enhanced Activity toward Oxygen Reduction

The presence of a capping agent or stabilizer in the synthesis of colloidal metal nanocrystals will compromise their performance if employed as electrocatalysts. Herein we demonstrate the synthesis of Pd@Pt3–4L core–shell octahedral nanocrystals with greatly enhanced activity toward the oxygen reduction reaction by eliminating the use of any capping agent or stabilizer. This was achieved by employing Pd octahedral seeds with well‐defined {1 1 1} facets and by dispersing them on a carbon black support prior to Pt deposition. Upon optimization of the reaction conditions, Pt ultrathin shells could be conformally deposited on the Pd octahedral seeds in a layer‐by‐layer fashion without involving self‐nucleation or island growth for the Pt atoms. The as‐obtained octahedral Pd@Pt3–4L/C catalyst exhibited a specific activity 50 % greater than that of a reference sample prepared in the presence of a polymer stabilizer such as poly(vinyl pyrrolidone). The polymer‐free catalyst also showed 5‐fold enhancement in specific activity if benchmarked against a commercial Pt/C catalyst.

Shape-controlled synthesis of CO-free Pd nanocrystals with the use of formic acid as a reducing agent

This paper reports the use of formic acid as a reducing agent for the shape-controlled synthesis of Pd nanocrystals with no chemisorption of CO on the surface, as confirmed by attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy.

Enabling Complete Ligand Exchange on the Surface of Gold Nanocrystals through the Deposition and Then Etching of Silver

We report an indirect method for the effective replacement of ligands on the surface of Au nanocrystals with different morphologies. The method involves the deposition of an ultrathin layer of Ag to remove a strong capping agent such as cetyltrimethylammonium chloride (CTAC), followed by selective etching of the Ag layer in the presence of citrate ions as a stabilizer. Using multiple characterization techniques, we confirm that the surface of the Au nanocrystals is covered by citrate ions after the indirect ligand exchange process, and there is essentially no aggregation during the entire process. We also demonstrate that this method is effective in suppressing the toxicity of Au nanospheres by completely replacing the initially used CTAC with citrate.