Catalytic properties of nanoparticles can be significantly enhanced by controlling nanoscale alloying and its structure. In this work, by using a facet-controlled Pt@Ni core-shell octahedron nanoparticle, we show that the nanoscale phase segregation can have directionality and be geometrically controlled to produce a Ni octahedron that is penetrated by Pt atoms along three orthogonal Cartesian axes and is coated by Pt atoms along its edges. This peculiar anisotropic diffusion of Pt core atoms along the ⟨100⟩ vertex, and then toward the ⟨110⟩ edges, is explained via the minimum strain energy for Ni-Ni pair interactions. The selective removal of the Ni-rich phase by etching then results in structurally fortified Pt-rich skeletal PtNi alloy framework nanostructures. Electrochemical evaluation of this hollow nanoframe suggests that the oxygen reduction reaction (ORR) activity is greatly improved compared to conventional Pt catalysts.

The ultrasensitive detection of cancer in its earliest stage would greatly help the ensuing treatment process, and therefore various imaging modalities and image-enhancing methods are being developed.[1] In particular, metal oxide nanoparticles prove to be promising contrast agents in magnetic resonance imaging (MRI) for the ultrasensitive detection of cancer, and the principles for enhancing MRI contrast have been deciphered recently.[2] For example, it is advantageous to employ superparamagnetic metal oxide nanoparticles with high magnetization values (emu g À1 ) for improved T 2 image contrast.[3] In addition, clusters of superparamagnetic nanoparticles exhibit greater T 2 contrast abilities than individual nanoparticles.[4] Therefore, the clustering of magnetic nanoparticles with high magnetization values is advantageous because of both improved T 2 contrast and the frugal usage of targeting moieties. For enhanced T 1 contrast, nanoparticles should have numerous high-spin metal ions exposed on the surface for facilitated interactions with the surrounding water molecules. [5,6] This calls for the use of smaller nanoparticles with a high surface-to-volume ratio, but simply using a high number of small nanoparticles is not compatible with the frugal usage of targeting moieties. In the case of large nanoparticles, the non-exposed metal ions in the core cannot contribute to the MRI T 1 contrast; the T 1 -weighted image obtained with metal ions is much poorer than that from conventional ion-based contrast agents.We reasoned that the highest surface area for a nanoparticle of a given diameter would be provided by an urchinlike morphology as shown in Figure 1. As a model system to prove our concept, manganese oxides were investigated that had been previously used as an MRI T 1 contrast agent. This system is particularly interesting because of the easy conversion of MnO to Mn 3 O 4 and the different stabilities of these two manganese oxide phases under physiological conditions. It is envisaged that the MnO nanoparticle trapped in the thin shell of an urchin-shaped stable Mn 3 O 4 phase can be unloaded in the form of Mn II ions to the low-pH sites (< pH 7) in the tumor. While the low pH of tumor cells has been exploited for the fabrication of numerous activatable drug-delivery systems, [7] a nanoparticle-based pH-activatible MRI agent is unprecedented to our knowledge. The combination of the T 1 contrast effect from the empty Mn 3 O 4 urchin shell with a high surface area and the released Mn II ions should make the MnO@Mn 3 O 4 nanourchin a powerful MRI T 1 contrast agent. Herein we report the synthesis of the MnO@Mn 3 O 4 nanourchin through facet-selective etching as well as its successful application as a pH-responsive activatable T 1 contrast agent,

Noble metal binary alloy nanoframes have emerged as a new class of fuel cell electrocatalysts because of their intrinsic high catalytic surface area and accompanied high catalytic activity. However, their inferior structural and compositional stability during catalysis pose as formidable huddles to their practical applications. Herein, it is reported that introduction of an additional component to the binary catalytic system may serve as a simple and effective means of enhancing the structural and compositional stability of nanoframe‐based electrocatalysts. It is demonstrated that in situ doping of Co to the PtCu alloy nanoframe yields a ternary PtCuCo rhombic dodecahedral nanoframe (Co‐PtCu RNF) with a reinforced vertex structure. Co‐PtCu RNF exhibits superior electrocatalytic activity and durability for the oxygen reduction reaction to those of PtCu rhombic dodecahedral nanoframe (PtCu RNF) and Pt/C catalysts, due to its ternary composition and vertex‐strengthened frame structure. Furthermore, Co‐PtCu RNF shows enhanced activity for the methanol oxidation reaction as compared to PtCu RNF and Pt/C.

Pt-based nanoframe catalysts have been explored extensively due to their superior activity toward the oxygen reduction reaction (ORR). Herein, we report the synthesis of Pt-Ni multiframes, which exhibit the unique structure of tightly fused multiple nanoframes and reinforced by an embedded dendrite. Rapid reduction and deposition of Ni atoms on Pt-Ni nanodendrites induce the alloying/dealloying of Pt and Ni in the overall nanostructures. After chemical etching of Ni, the newly formed dendrite-embedded Pt-Ni multiframes show an electrochemically active surface area (ECSA) of 73.4 m g and a mass ORR activity of 1.51 A mg at 0.93 V, which is 30-fold higher than that of the state-of-the-art Pt/C catalyst. We suggest that high ECSA and ORR performances of dendrite-embedded Pt-Ni multiframes/C can be attributed to the porous nanostructure and numerous active sites exposed on surface grain boundaries and high-indexed facets.