Electron spin resonance of carbon nanotubes under hydrogen adsorption

Carbon nanotubes provided by different manufacturers and synthesized by a variety of methods were subjected to the same oxidative purification procedure. Electron spin resonance (ESR) was used to investigate changes in the electronic structure before and after purification and exposure to hydrogen gas at a pressure of 136 kPa. The ESR signal in single-wall carbon nanotubes was due to paramagnetic impurities and diminished in intensity upon hydrogen adsorption. The conduction electrons of multiwall carbon nanotubes gave rise to signals with Dysonian line shapes. Here, the signal intensity increased upon hydrogen adsorption and the asymmetry parameter as well as the g factor were affected, suggesting a decrease in band gap. In samples with large metal content a ferromagnetic resonance was observable which disappeared upon purification. Some samples yielded no observable ESR signal due to an increased relaxation time of the electrons upon interaction with residual metal catalyst particles and possibly a large proportion of semiconducting nanotubes.