This work presents a systematic study of the ratio between the integrated intensities of the disorder-induced D and G Raman bands (ID∕IG) in nanographite samples with different crystallite sizes (La) and using different excitation laser energies. The crystallite size La of the nanographite samples was obtained both by x-ray diffraction using synchrotron radiation and directly from scanning tunneling microscopy images. A general equation for the determination of La using any laser energy in the visible range is obtained. Moreover, it is shown that ID∕IG is inversely proportional to the fourth power of the laser energy used in the experiment.
Graphene oxide, a two-dimensional aromatic scaffold decorated by oxygen-containing functional groups, possesses rich chemical properties and may present a green alternative to precious metal catalysts. Graphene oxide-based carbocatalysis has recently been demonstrated for aerobic oxidative reactions. However, its widespread application is hindered by the need for high catalyst loadings. Here we report a simple chemical treatment that can create and enlarge the defects in graphene oxide and impart on it enhanced catalytic activities for the oxidative coupling of amines to imines (up to 98% yield at 5 wt% catalyst loading, under solvent-free, open-air conditions). This study examines the origin of the enhanced catalytic activity, which can be linked to the synergistic effect of carboxylic acid groups and unpaired electrons at the edge defects. The discovery of a simple chemical processing step to synthesize highly active graphene oxide allows the premise of industrial-scale carbocatalysis to be explored.
This work reports the analysis of the G band profile in the Raman spectra of nanographites with different degrees of stacking order. Since the G band scattering coming from the 2D and 3D phases coexisting in the same sample can be nicely distinguished, the relative volumes of 3D and 2D graphite phases present in the samples can be estimated from their Raman spectra. The comparison between Raman scattering and X-Ray diffraction data shows that Raman spectroscopy can be used as an alternative tool for measuring the degree of stacking order of graphitic systems.
A theoretical model supported by experimental results explains the dependence of the Raman scattering signal on the evolution of structural parameters along the amorphization trajectory of polycrystalline graphene systems.Four parameters rule the scattering efficiencies, two structural and two related to the scattering dynamics. With the crystallite sizes previously defined from X-ray diffraction and microscopy experiments, the three other parameters (the average grain boundaries width, the phonon coherence length, and the electron coherence length) are extracted from the Raman data with the geometrical model proposed here. The broadly used intensity ratio between
A polarized Raman study of nanographite ribbons on a highly oriented pyrolytic graphite substrate is reported. The Raman peak of the nanographite ribbons exhibits an intensity dependence on the light polarization direction relative to the nanographite ribbon axis. This result is due to the quantum confinement of the electrons in the 1D band structure of the nanographite ribbons, combined with the anisotropy of the light absorption in 2D graphite, in agreement with theoretical predictions.
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