Engineering the surface structure of noble-metal nanocrystals offers an effective route to the development of catalysts or electrocatalysts with greatly enhanced activity. Here, we report the synthesis of Pt-based icosahedral nanocages whose surface is enclosed by both {111} facets and twin boundaries while the wall thickness can be made as thin as six atomic layers. The nanocages are derived from Pd@Pt4.5L icosahedra by selectively etching away the Pd in the core. During etching, the multiply twinned structure can be fully retained whereas the Pt atoms in the wall reconstruct to eliminate the corrugated structure built in the original Pt shell. The Pt-based icosahedral nanocages show a specific activity of 3.50 mA cm(-2) toward the oxygen reduction reaction, much greater than those of the Pt-based octahedral nanocages (1.98 mA cm(-2)) and a state-of-the-art commercial Pt/C catalyst (0.35 mA cm(-2)). After 5000 cycles of accelerated durability test, the mass activity of the Pt-based icosahedral nanocages drops from 1.28 to 0.76 A mg(-1)Pt, which is still about four times greater than that of the original Pt/C catalyst (0.19 A mg(-1)Pt).

Nanocages have received considerable attention in recent years for catalytic applications owing to their high utilization efficiency of atoms and well-defined facets. Here we report, for the first time, the synthesis of Ru cubic nanocages with ultrathin walls, in which the atoms are crystallized in a face-centered cubic (fcc) rather than hexagonal close-packed (hcp) structure. The key to the success of this synthesis is to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temperature and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, we obtain Ru nanocages with an average wall thickness of 1.1 nm or about six atomic layers. Most importantly, the Ru nanocages adopt an fcc crystal structure rather than the hcp structure observed in bulk Ru. The synthesis has been successfully applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent density functional theory calculations indicate that these unique fcc-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp Ru(0001), on the basis of strengthened binding of atomic N and substantially reduced activation energies for N2 dissociation, which is the rate-determining step for ammonia synthesis on hcp Ru catalysts.

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.

We report the use of seed-mediated growth as a simple and versatile approach to the synthesis of penta-twinned Cu nanorods with uniform diameters and controllable aspect ratios. The success of this approach relies on our recently demonstrated synthesis of Pd decahedra as uniform samples, together with tightly controlled sizes in the range of 6-20 nm. When employed as a seed, the Pd decahedron can direct the heterogeneous nucleation and growth of Cu along the five-fold axis to produce a nanorod with a uniform diameter defined by the lateral dimension of the original seed.Due to a large mismatch in lattice constant between Cu and Pd (7.1%), the deposited Cu is forced to grow only along one side of the Pd decahedral seed, generating a nanorod with an asymmetric distribution of Cu, with the Pd seed being situated at one of the two ends. According to their extinction spectra, the as-obtained Cu nanorods could form a stable colloidal suspension in water and be stored in a capped vial under the ambient conditions for at least 6 months without noticeable degradation. This excellent stability allows us to systematically investigate the sizedependent surface plasmon resonance properties of the penta-twinned Cu nanorods. With their transverse modes being positioned at 560 nm, their longitudinal modes can be readily tuned from the visible to the near-infrared region by controlling their aspect ratios.

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.

Owing to the presence of {111} facets, twin boundaries, and strain fields on the surface, noble-metal nanocrystals with an icosahedral shape have been reported with stellar performance toward an array of catalytic reactions. Here, we report the successful synthesis of Ru icosahedral nanocages with a face-centered cubic (fcc) structure by conformally coating Pd icosahedral seeds with ultrathin Ru shells, followed by selective removal of the Pd cores via chemical etching. We discovered that the presence of bromide ions was critical to the layer-by-layer deposition of Ru atoms. According to in situ XRD, the fcc structure in the Ru nanocages could be retained up to 300 °C before it was transformed into the conventional hexagonal close-packed (hcp) structure. Additionally, the icosahedral shape of the Ru nanocages could be largely preserved up to 300 °C. The Ru icosahedral nanocages with twin boundaries on the surface exhibited greatly enhanced activities toward both the reduction of 4-nitrophenol and decomposition of hydrazine than their cubic and octahedral counterparts. When benchmarked against the parental Pd@Ru core–shell nanocrystals, all the Ru nanocages displayed superior catalytic activities. First-principles density functional theory calculations also suggest that the fcc-Ru icosahedral nanocages containing residual Pd atoms are more promising than the conventional hcp-Ru solid nanoparticles in catalyzing nitrogen reduction for ammonia synthesis. With the subsurface impurities of Pd, the twin boundary regions of the icosahedral nanocages are able to stabilize the N2 dissociation transition state, reducing the overall reaction barrier and promoting the competition with the N2 desorption process.

Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temperature were all instrumental to the formation of Pd@Ru core− shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five atomic layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent density functional theory calculations to study the adsorption and dissociation of N 2 as a means to predict the catalytic performance toward ammonia synthesis. Our results suggested that the small proportions of Pd atoms left behind in the walls during etching could play a key role in stabilizing the adsorption of N 2 as well as in reducing the activation energy barrier to N 2 dissociation.

As a solid precursor to O2 and hydrogen peroxide (H2O2), calcium peroxide (CaO2) has found widespread use in applications related to disinfection and contaminant degradation. The lack of uniform nanoparticles, however, greatly limits the potential use of this material in other applications related to medicine. Here, a new route to the facile synthesis of CaO2 nanocrystals and their spherical aggregates with uniform, controllable sizes is reported. The synthesis involves the reaction between CaCl2 and H2O2 to generate CaO2 primary nanocrystals of 2–15 nm in size in ethanol, followed by their aggregation into uniform, spherical particles with the aid of poly(vinyl pyrrolidone) (PVP). The average diameter of the spherical aggregates can be easily tuned in the range of 15–100 nm by varying the concentrations of CaCl2 and/or PVP. For the spherical aggregates with a smaller size, they release H2O2 and O2 more quickly when exposed to water, resulting in superior antimicrobial activity. This study not only demonstrates a new route to the synthesis of uniform CaO2 nanocrystals and their spherical aggregates but also offers a promising bacteriostatic agent with biodegradability.