We report the design and synthesis of multimetallic Au/Pt-bimetallic nanoparticles as a highly durable electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. This system was first studied on well-defined Pt and FePt thin films deposited on a Au(111) surface, which has guided the development of novel synthetic routes toward shape-controlled Au nanoparticles coated with a Pt-bimetallic alloy. It has been demonstrated that these multimetallic Au/FePt(3) nanoparticles possess both the high catalytic activity of Pt-bimetallic alloys and the superior durability of the tailored morphology and composition profile, with mass-activity enhancement of more than 1 order of magnitude over Pt catalysts. The reported synergy between well-defined surfaces and nanoparticle synthesis offers a persuasive approach toward advanced functional nanomaterials.

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [2] .The discovery of superconductivity in MgB 2 [3] at 39K sparked great interest in the fundamental physics and practical applications of this material. There has already been rapid progress in understanding the physical properties of this superconductor. Specific heat measurements [4,5] show that MgB 2 is an s-wave superconductor and the presence of the isotope effect [6,7] points towards phonon-mediated pairing. Tunneling and photoemission spectroscopy directly measures the superconducting energy gap and can provide further understanding of the origin of the superconductivity in this material. Earlier tunneling spectroscopy measurements show a large spread in the gap values [8][9][10] each consistent with the BCS form. More recent experiments, including STM tunneling spectroscopy [11], point-contact spectroscopy [12,13], specific heat measurements [4,5], and Raman spectroscopy [14] point towards the existence of two distinct gaps. This scenario has been predicted theoretically by Liu et al. [2]. First principle calculations show that the Fermi surface of MgB 2 consists of 2D cylindrical sheets arising from σ antibonding states of B p xy orbitals, and 3D tubular networks arising from π bonding and antibonding states of B p z orbitals. In this theoretical framework [2] two different energy gaps exist, the smaller one being an induced gap associated with the 3D bands and the larger one associated with the superconducting 2D bands. Furthermore both superconducting gaps should vanish at the bulk critical temperature T c . Due to this highly anisotropic band structure the superconducting gaps should be momentum-dependent reflecting the strength of the electron-phonon coupling of the carriers in the different bands. Up to now there has been no direct experimental evidence of the orientation dependence of the order parameter in this material. Moreover, the temperature dependence of the two gaps would give further insights into the nature of superconductivity in MgB 2 . Scanning tunneling spectroscopy is a unique technique that allows direct measure of the DOS near the Fermi energy with high...

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensity-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ≈ TCDW . Renormalized phonon energies are observed over a large wavevector range (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5). Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the chargedensity-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature.PACS numbers: 71.45. Lr, 63.20.kd, 63.20.dd, 63.20.dk The origin of charge-density-wave (CDW ) order, i.e., a periodic modulation of the electronic density, is a long-standing problem relevant to a number of important issues in condensed matter physics, such as the role of stripes in cuprates [1] and charge fluctuations in the colossal magnetoresistive manganites [2]. Chan and Heine derived the criterion for a stable CDW phase with a modulation wavevector q as [3]where η q is the electron-phonon coupling (EPC) matrix element associated with a mode at an unrenormalized energy of ω bare , χ q is the dielectric response of the conduction electrons, and U q and V q are their Coulomb and exchange interactions. Static CDW order typically is taken as a result of a divergent electronic susceptibility χ q due to nesting, i.e. parallel sheets of the Fermi surface (FS) separated by twice the Fermi wavevector 2k f . Electron-phonon coupling (EPC) is required to stabilize the structural distortion and, hence, an acoustic phonon mode at the CDW wavevector q CDW = 2k f softens to zero energy at the transition temperature T CDW [3,4]. However, when electronic probes reported only small and not well nested Fermi surfaces this scenario has been discarded for the prototypical CDW compound TiSe 2 [5][6][7].Alternative scenarios such as indirect or band-type Jahn-Teller effects [6][7][8] and, most prominently, exciton formation [5,9,10] are discussed in the theoretical as well as experimental literature. More recently, van Wezel et al. have invoked a model including exciton formation as well as EPC [11] and have shown that it can explain data from angle-resolved photoemission spectroscopy (ARPES) [12], formerly taken as evidence of an excitonic insulating phase in TiSe 2 [9]. Determining the origin of CDW formation in TiSe 2 is all the more important with respect to the nature of superconductivity, which emerges both as function of pressure [13] and Cu intercalation [14]. In particular, pressure induced superconductivity is expected to be closely linked to the nature of the parent CDW state.I...