Structural analysis of the B-doped mesophase pitch-based graphite fibers by Raman spectroscopy

Milled B-doped mesophase pitch-based carbon fibers (mMPCF's) prepared from a melt-blown petroleum mesophase pitch precursor material have been developed for enhanced Li uptake capacity in Li ion batteries. Raman spectroscopy has been used to investigate the structure of graphitized and B-doped mMPCF's using 632.8-nm HeNe laser excitation. The B-doped mMPCF's show a strong Raman peak near 1330 cm(-1), a well-defined peak at 1620 cm(-1), and the disappearance of the second-order 2660 cm(-1) band. Furthermore, it is shown that the E-2g2 graphite Raman band at 1580 cm(-1) is shifted to 1590 cm(-1) due to B doping. The appearance of a new weak Raman band in the B-doped mMPCF's near 1320 cm(-1) is closely related to the B-C stretching mode in the graphite lattice. These results are associated with the breakdown of the k=0 selection rules by a local distortion of the graphite lattice due to substitutional boron doping. On the basis of the integrated intensity ratio R of the disorder-induced line near 1330 cm(-1) to the Raman line near 1590 cm(-1) after 2.66 at. % boron doping, it is suggested that the substitutional boron in the mMPCF's is homogeneously distributed within the graphene layer in the fiber form. The crystallite domain size L-a parallel and perpendicular to the fiber axis on the surface of the fiber is estimated to be about 60 Angstrom, which could correspond to:the distance between boron atoms substituted for C atoms in a graphene layer of the fibers. [SO163-1829(98)08538-5].