▎ 摘　　要

We investigated the conduction properties of pristine bulk graphene oxide (GO) with different oxidation degrees. GO with a low oxidation degree shows the electron/hole conduction behavior. Interestingly, however, GO with high oxidation degree exhibits mixed conduction behaviors (electron/hole conduction and proton conduction), depending on the water content. For GO with a high oxidation degree, electron/hole conduction dominates the electrical transport in the water-lean state, while the proton conduction dominates the electrical transport in the water-rich state. Among different oxygen-containing functional groups, the epoxide groups, which could influence the band gap, interlayer distance and water intercalation process, play a critical role in the conduction properties of GO. In electrical transport process, epoxide groups act as conduction barriers for electron/hole conduction while serving as active sites for proton conduction. Both graphite and GOs exhibit a clear anisotropic behavior in electrical transport.. These observations will provide insightful information for the applications of GO in fuel cells, humidity sensors and supercapacitors. (C) 2016 Elsevier Ltd. All rights reserved.