ABSTRACT
Defects and nanocrystalline grain structures play a critical role in graphene-enhanced Raman spectroscopy (GERS). In this study, we selected three types of few-layer, polycrystalline graphene films produced by chemical vapor deposition (CVD), and we tested them as GERS substrates. The graphene structure was controlled by decreasing the CVD temperature, thus obtaining (i) polycrystalline with negligible defect density, (ii) polycrystalline with high defect density, (iii) nanocrystalline. We applied rhodamine 6G as a probe molecule to investigate the Raman enhancement. Our results show that nanocrystalline graphene is the most sensitive GERS substrate, indicating that the GERS effect is primarily connected to the nanocrystalline structure, rather than to the presence of defects.
ABSTRACT
New carbon nanomaterials, i.e., carbon nanotubes and nanofibers, with special physico-chemical properties, are recently studied as support for methanol oxidation reaction electrocatalysts replacing the most widely used carbon black. Particularly, carbon fibrous structures with high surface area and available open edges are thought to be promising. Platelet type carbon nanofibers, which have the graphene layers oriented perpendicularly to the fiber axis, exhibit a high ratio of edge to basal atoms. Different types of carbon nanofibers (tubular and platelet) were grown by plasma enhanced chemical vapour deposition on carbon paper substrates. The process was controlled and optimised in term of growth pressure and temperature. Carbon nanofibers were characterised by high resolution scanning electron microscopy and X-ray photoelectron spectroscopy to assess the morphological properties. Then carbon nanofibers of both morphologies were used as substrates for Pt electrodeposition. High resolution scanning electron microscopy images showed that the Pt nanoparticles distribution was well controlled and the particles size went down to few nanometers. Pt/carbon nanofibers nanocomposites were tested as electrocatalysts for methanol oxidation reaction. Cyclic voltammetry in H2SO4 revealed a catalyst with a high surface area. Cyclic voltammetry in presence of methanol indicated a high electrochemical activity for methanol oxidation reaction and a good long time stability compared to a carbon black supported Pt catalyst.
ABSTRACT
We show how a varifocal pulsed gas lens, the colliding shock lens, can be used as an intracavity element to Q switch a ruby laser. By placement of the shock lens in tandem with a second lens, a giant pulse is obtained. The second lens may be a conventional glass lens or a continuous-wave gas lens.
