Probing the transient fate of C-N bonding in hydrazine-treated carbon nanotubes by synchrotron photoelectron spectroscopy

The properties of hydrazine-treated carbon nanotubes (CNTs) were investigated by synchrotron photoelectron spectroscopy. The surfactant-free CNTs used in this study were synthesized by alcohol catalytic chemical vapor deposition. When the CNTs subject to the vapor-phase hydrazine treatment and the 80° C-baking treatment were probed by ultraviolet photoelectron spectroscopy (UPS), the results showed (i) damaged π-bonding and (ii) the shift of the CNTs' Fermi level toward the conduction band. A further 350°C-baking treatment on the hydrazine-treated CNTs could restore the damaged π-bonding and cause the CNTs' Fermi level to shift back toward the valence band. The results obtained from UPS indicated that the above interaction between hydrazine and CNTs was a thermally metastable chemical adsorption. When the CNTs subject to the vapor-phase hydrazine treatment and the 80° C-baking treatment were probed by X-ray photoelectron spectroscopy (XPS), the results showed a significant increase in the spectral intensity of the signal corresponding to C-N bonding in the XPS profile. A further 350° C-baking treatment on the hydrazine-treated CNTs could essentially eliminate the spectral intensity of the signal corresponding to C-N bonding in the XPS profile. Our experimental results show that the transient fate of the thermally metastable C-N bonding is associated with the nitrogenous radicals, such as nitrene and amidogen, thermally decomposed from hydrazine. The chemical association of nitrogenous radicals with CNTs generates metastable amino/aziridino derivatization on the surface of CNTs, which will disrupt the continuum of CNTs' graphitic domains. Upon further baking, the disruptive functionalization can be eliminated to restore the graphitic sp²-carbon bonding structure.