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 $1330{\mathrm{cm}}^{-1},$ a well-defined peak at $1620{\mathrm{cm}}^{-1},$ and the disappearance of the second-order $2660{\mathrm{cm}}^{-1}$ band. Furthermore, it is shown that the $E_{2g2}$ graphite Raman band at $1580{\mathrm{cm}}^{-1}$ is shifted to $1590{\mathrm{cm}}^{-1}$ due to B doping. The appearance of a new weak Raman band in the B-doped mMPCF’s near $1320{\mathrm{cm}}^{-1}$ is closely related to the B-C stretching mode in the graphite lattice. These results are associated with the breakdown of the $k=0\mathrm{}$ 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 $1330{\mathrm{cm}}^{-1}$ to the Raman line near $1590{\mathrm{cm}}^{-1}$ 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 Å, which could correspond to the distance between boron atoms substituted for C atoms in a graphene layer of the fibers.