Nanostructured, nitrogen-doped carbon materials for hydrogen storage

Abstract

An astonishing feature of carbon materials is the variety of morphological forms created by growth processes. We report here on a growth process originating from CVD diamond synthesis. The addition of nitrogen gas to the microwave plasma ignited in a CH 4/H 2 mixture has a profound influence on the crystal structure and morphology of diamond and graphite. Graphite appears in several forms: a single-crystal net-like planar form, nanoparticulates, and new nanotubes with perpendicularly stacked graphite rings. Nitrogen incorporation into these forms is approximately 1 at.%. Nanotubes are also formed in the process of etching graphite by nitrogen plasma. Nanotubes and particulates are built from structural units that are graphite nanocrystals with dimensions as small as 2.5×2.5 nm 2. The self-assembling tendency of these units results in different forms: rings, nanotubes or flat particles with a diameter of 70, 60 and 25 nm, respectively. Electron diffraction of material which contain 10 at.% N indicates a lattice expansion of graphitic C–N nanocrystals compared to pure graphite. Creation of active sites for hydrogen-molecule adsorption by incorporation of nitrogen atoms into the graphitic network is the subject of speculation. Hydrogen intake for materials grown from the C–H–N gas system is 0.7–0.8 wt.%. Measurements have been performed using a thermogravimetric method at room temperature and under hydrogen pressure of 7 MPa. TGA-150 Cahn microbalance measurements have been corrected for the buoyancy effect.