If graphene is ever going to live up to the promises of future nanoelectronic devices, an easy and cheap route for mass production is an essential requirement. A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented. Micrometre-wide flakes consisting of four to six atomic layers of stacked graphene sheets have been synthesized by controlled recombination of carbon radicals in a microwave plasma. A simple and highly reproducible technique is essential, since the resulting flakes can be synthesized without the need for a catalyst on the surface of any substrate that withstands elevated temperatures up to 700• C. A thorough structural analysis of the flakes is performed with electron microscopy, x-ray diffraction, Raman spectroscopy and scanning tunnelling microscopy. The resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process, as revealed by the combined analysis of electron microscopy and x-ray photoelectron spectroscopy.

Although cellulose, as an abundant and renewable resource, offers a promising alternative for the production of biofuels and platform chemicals, [1] there have thus far only been a few studies that have reported its aqueous-phase conversion into polyols by solid chemocatalysts. [2][3][4][5] The principal reason is that these polymeric biomolecules with a semicrystalline structure cannot penetrate the pores of conventional heterogeneous catalysts. Advances in the conversion of cellulose therefore require the design of efficient multifunctional catalysts with sterically accessible sites.[6] Herein, we demonstrate that sugar alcohols can be selectively produced from cellulose in a one-pot catalytic process over reshaped Ni particles at the tip of carbon nanofibers.One elegant strategy to valorize cellulose into polyols is inspired by a previous report on starch conversion. [7] One-pot catalytic approaches of this kind rely on proton-catalyzed hydrolysis of the glycoside bonds, followed by fast metal-catalyzed hydrogenation of the released glucose units into sorbitol. A high sorbitol yield is only guaranteed if hydrolysis is the rate-limiting step (preventing undesirable glucose degradation). The first reports on the application of such bifunctional catalysis to cellulose fractions appeared in the pioneering work of Fukuoka and Dhepe.[2] They demonstrated the selective conversion of cellulose into sugar alcohols by using supported precious-metal catalysts at elevated temperatures. Pt and Ru gave the highest hexitol yields. For example, Pt/g-Al 2 O 3 catalyzed the conversion of cellulose to yield 25 % and 6 % of sorbitol and mannitol, respectively. Alternatively, on a Ru/C catalyst Luo et al. reported yields of 30 % and 10 % of sorbitol and mannitol, respectively.[3] Most notably, in such studies less-expensive Ni catalysts consistently exhibit inferior performances towards sugar alcohol production (Supporting Information, Table S1), [2,[4][5] in agreement with their known unselective hydrogenolysis behavior. [8] The main difficulty in using conventional heterogeneous catalysts for bulky cellulose substrates is the limited accessibility of the active catalytic sites. Their performance seems to be governed by the restricted space inside the pore systems, preventing polymeric biomolecules from penetrating to the metal sites.[6] To overcome this incompatibility between substrate and catalyst, the present study uses carbon nanofibers instead of porous solids to support Ni at the tip of the carbon filaments. In a typical synthesis, the Ni-containing carbon nanofibers (Ni/CNF) were formed by catalytic vapor deposition (CVD) of methane over a catalyst consisting of Ni nanoclusters supported on g-alumina (Puralox, 155 m 2 g À1).[9] The textural properties of the catalysts were investigated by scanning electron microscopy (SEM) and N 2 physisorption.As can be seen in Figure 1 a, the obtained sample of carbon nanofibers grown over supported nickel (Ni/CNF) showed an entangled "spaghetti-like" morphology with a fiber diameter ...

In this work, a detailed structural and spectroscopic study of nanocrystalline diamond (NCD) thin films grown by a continuous bias assisted CVD growth technique is reported. This technique allows the tuning of grain size and phase purity in the deposited material. The crystalline properties of the films are characterized by SEM, TEM, EELS, and Raman spectroscopy. A clear improvement of the crystalline structure of the nanograined diamond film is observed for low negative bias voltages, while high bias voltages lead to thin films consisting of diamond grains of only ∼10 nm nanometer in size, showing remarkable similarities with so‐called ultrananocrystalline diamond. These layers arecharacterized by an increasing amount of sp2‐bonded carbon content of the matrix in which the diamond grains are embedded. Classical molecular dynamics simulations support the observed experimental data, giving insight in the underlying mechanism for the observed increase in deposition rate with bias voltage. Furthermore, a high atomic concentration of hydrogen has been determined in these films. Finally, Raman scattering analyses confirm that the Raman line observed at ∼1150 cm−1 cannot be attributed to trans‐poly‐acetylene, which continues to be reported in literature, reassigning it to a deformation mode of CHx bonds in NCD.

No abstract

No abstract