No abstract

Reduction of Pd ions by hydroquinone in the presence of gold nanoparticles and polyvinylpyrrolidone resulted in the formation of nanoflowers with a Au core and Pd petals. Addition of HCl to the synthesis halted the reduction by hydroquinone and enabled the acquisition of snapshots of the nanoflowers at different stages of growth. TEM images of the reaction after 10 s show that the nanoflower morphology resulted from the homogeneous nucleation of Pd clusters in solution and their subsequent attachment to gold seeds coated with a thin (0.8 ± 0.1 nm) shell of Pd. UV-visible spectra also indicate Pd clusters formed in the early stages of the reaction and disappeared as the nanoflowers grew. The speed at which this reaction can be halted is useful not only for producing a variety of bimetallic nanostructures with precisely controlled dimensions and morphologies but also for understanding the growth mechanism of these structures. The ability of the AuPd core-shell structure to catalyze the Suzuki coupling reaction of iodobenzene to phenylboronic acid was probed and compared against the activity of Pd nanocubes and thin-shelled AuPd core-shell nanoparticles. The results of this study suggest that Suzuki coupling was not affected by the surface structure or subsurface composition of the nanoparticles, but instead was primarily catalyzed by molecular Pd species that leached from the nanostructures.

Since first reported, layered ternary carbides and nitrides named "MAX phases" from M n+1 AX n (n = 1, 2 or 3) phases, where M is a transition metal, A is a group IIIA or IVA element, and X is carbon or nitrogen atoms, have attracted the attention of scientists and industry for their unique combination of metallic and ceramic properties. [1] For example, MAX phases show excellent resistance to oxidation, heat, and corrosion, high electrical conductivity, high strength and elastic modulus, and are easily produced, benefiting from their inherent lamellar structure with alternately arranged MX and A layers (Figure 1 a). [2][3][4] Currently, the study and application of MAX phases is still restricted to three-dimensional (3D) bulk samples. Nanosized materials, especially ultrathin twodimensional (2D) nanosheets, show enhanced properties with respect to their corresponding bulk counterpart. [5][6][7][8] For example, free-standing graphene shows remarkable in-plane thermal conductivity and mechanical strength, up to about 3000 W m À1 K À1 and 1060 GPa, respectively. [8,9] The thermal conductivity and mechanical strength of ultrathin boron nitride (BN) nanosheets are theoretically estimated as high as 2000 W m À1 K À1 and 800 GPa, [10] respectively, which are much higher than the value of the corresponding bulk materials. Benefiting from these properties, graphene and BN nanosheets are considered to be promising fillers in polymeric composites, to improve their thermal and mechanical properties. [11][12][13][14][15] Accordingly, we anticipate enhanced properties of ultrathin nanosheets of MAX phases with respect to the bulk materials. However, in contrast to inorganic graphene analogues (IGAs), with a weak van der Walls force between the layers, the MAX phases possess relatively strong bonds between the MX and A layers, and it has not been possible for these materials to be exfoliated into ultrathin nanosheets using simple exfoliation processes. [16,17] Recently, Naguib et al. have reported the preparation of "MXenes" by extraction of the A layers from the MAX phases in a solution of hydrofluoric acid (HF), [16,17] and the as-prepared MXenes show novel electronic and magnetic properties. [18,19] However, it is only when the A layer is aluminum that the MAX phases can be etched with HF and exfoliated into MXenes and the use of HF in the etching process is dangerous and toxic, both of these conditions will undoubtly restrict the application of this Figure 1. a) Crystal structure of 211, 312, and 413 MAX phases. b) Illustration of the substitutional solid solution based exfoliation process for the formation of free-standing nanosheets of MAX phases by activating the A layers.

Herein we report on the reversible electrochemical intercalation of Li ions into Ti2SC and Ti3SiC2, layered ternary transition-metal carbides, known as MAX phases. It is demonstrated that the particle size plays an important role in the electrochemical performance of MAX phases. Ti2SC particles with a size of hundreds of nanometers showed an initial reversible capacity of ∼80 mAh g–1 at 4 C, which increased to ∼180 mAh g–1 after 1000 cycles. This capacity was two times higher than that of Ti2SC with micrometer-sized particles. The increasing capacity with cycling was considered evidence for the reversible intercalation of Li ions into Ti2SC, as confirmed by transmission electron microscopy and Raman spectroscopy. In addition to Ti2SC, other MAX phases, such as Ti3SiC2, also exhibit promising Li-ion storage capability. This work suggests that the MAX phases, a large family of more than 70 stoichiometric phases and numerous solid solutions, are promising anode materials for Li-ion batteries and capacitors.