Protonic surface conductivity and proton space-charge relaxation in hydrated fullerol

Abstract

The ac dielectric properties of both anhydrous fullerol (C-60(OH)(24)) and hydrated fullerol with 20% water mass content are investigated by means of temperature-dependent dielectric spectroscopy. Anhydrous polycrystalline fullerol exhibits charge transport mediated by hopping of electronic charge carriers. Hydrated fullerol has a dc conductivity higher by more than a factor of 10(3) than that of the anhydrous sample due to water-induced proton transport. Four distinct dielectric relaxation processes are observed in hydrated fullerol, two of which lie in the frequency range of the electrode polarization. The fastest relaxation is only observed below the melting point of pure water and is assigned to the migration of hydrogen-bond defects in the physisorbed H2O layers. The other three processes exhibit nonmonotonous temperature dependence upon dehydration by heating. The fastest of the three is present also in the anhydrous powder, and it is assigned to a space-charge relaxation due to accumulation of electronic charge carriers at samples heterogeneities such as grain boundaries. By studying the temperature dependence of the two slower relaxations across dehydration, we identify them as separate electrode polarization effects due to distinct charge carriers, namely electrons and protons. The electronic electrode polarization is also present in pure fullerol, while the proton space-charge relaxation is only present in the hydrated material. Our findings help elucidate the hitherto puzzling observation of more than one nonmonotonous relaxation process in hydrated and water-containing systems