Platinum and palladium on carbon nanotubes:Experimental and theoretical studies

<p>Pristine and oxygen plasma functionalised carbon nanotubes (CNTs) were studied after the evaporation of Pt and Pd atoms. High resolution transmission electron microscopy shows the formation of metal nanoparticles at the CNT surface. Oxygen functional groups grafted by the plasma functionalization act as nucleation sites for metal nanoparticles. Analysis of the C1s core level spectra reveals that there is no covalent bonding between the Pt or Pd atoms and the CNT surface. Unlike other transition metals such as titanium and copper, neither Pd nor Pt show strong oxygen interaction or surface oxygen scavenging behaviour. (C) 2013 Elsevier B.V. All rights reserved.</p>

Muonium in Sulphur: mu SR's Oldest Puzzle

AbstractNew level–crossing resonance data shows the muonium defect centre in solid sulphur to have axial symmetry with hugedipolar anisotropyatlow cryogenic temperatures.Above100K,thetwo principalvaluesofthehyperfine tensor fall dramaticallyand pro rata, both apparently collapsingto zeroatthe melting point. Thefallis accompaniedby the onset of muon spin–lattice relaxation, visible on the microsecond timescale with low–field rates peaking around room temperature. In conjuction with old zero–field data, the low–T hyperfine parameters are determined accurately. New supercell density–functional calculations suggest their assignment to muonium at a bond–centre (BC) site in a closed–ringS8MuBC complex. The striking decrease of time–average parameters and the appearance of fluctuations causing relaxation are attributed to a dynamic equilibrium or chemical exchange with neutral configurations having muchlowerhyperfine coupling, accessedby smallcyclic displacements fromtheBC site.Time–average occupancy ofthissitefallswith temperatureandvanishesatthemeltingpoint

Muonium chemistry and spin dynamics in sulphur, modelling interstitial hydrogen

The nature of the elusive muonium centre in sulphur is re-examined in the light of new data on its level-crossing resonance and spin-lattice relaxation. The aim is to provide a model for the solid-state chemistry of interstitial hydrogen in this element, which is as yet unknown, as well as to solve one of the longest standing puzzles in μSR spectroscopy, namely the surprisingly strong depolarization of muons mimicking ion-implanted protons in this innocuous non-magnetic material. The paramagnetic muonium (and by inference hydrogen) centre is confirmed to have the character of a molecular radical, but with huge anisotropy at cryogenic temperatures and a striking shift of the resonance at ordinary temperatures, the hyperfine parameters appearing to collapse and vanish towards the melting point. New density-functional supercell calculations identify a number of possible structures for the defect centre, including a novel form of bond-centred muonium in a closed-ring S(8)Mu complex. Simulations of the spin dynamics and fits to the spectra suggest a dynamical equilibrium or chemical exchange between several configurations, with occupancy of the bond-centre site falling from unity at low cryogenic temperatures to zero near the melting point

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International audienceWhile changes in resonant Raman scattering measurements are commonly used to measure the effect of chemical functionalization on single-walled carbon nanotubes, the precise effects of functionalization on these spectra have yet to be clearly identified. In this density functional theory study, we explore the effects of functionalization on both the nanotube resonance energy and frequency shifts in radial breathing mode. Charge transfer effects cause a shift in the first Van Hove singularity spacings, and hence resonance excitation energy, and lead to a decrease in the radial breathing mode frequency, notably when the Fermi level decreases. By varying stochastically the effective mass of carbon atoms in the tube, we simulate the mass effect of functionalization on breathing mode frequency. Finally, full density functional calculations are performed for different nanotubes with varying functional group distribution and concentration using fluorination and hydrogenation, allowing us to determine overall effect on radial breathing mode and charge transfer. The results concur well with experiment, and we discuss the importance when using Raman spectroscopy to interpret experimental functionalization treatments

Boron- and nitrogen-doped multi-wall carbon nanotubes for gas detection

The response of pristine, nitrogen and boron doped carbon nanotube (CNT) sensors to NO2, CO, C2H4 and H2O at ppm concentrations was investigated at both room temperature and 150 °C. N-doped CNTs show the best sensitivity to nitrogen dioxide and carbon monoxide, while B-doped CNTs show the best sensitivity to ethylene. All tubes (including undoped) show strong humidity response. Sensing mechanisms are determined via comparison with density functional calculations of gas molecule absorption onto representative defect structures in N and B-doped graphene. N-CNTs show decreased sensitivity with temperature, and detection appears to occur via gas physisorption. B-CNTs appear to react chemically with many of the absorbed species as shown by their poor baseline recovery and increasing sensitivity with temperature. This limits their cyclability. Overall gas sensitivity is as good or better than post-growth functionalised nanotubes, and used in combination, CNTs, N-CNTs and B-CNTs appear highly promising candidates for cheap, low power, room temperature gas sensing applications