  Graphene oxide (GO) is reduced by Joule heating using in-situ transmission
electron microscopy (TEM). The approach allows the simultaneous study of GO
conductivity by electrical measurements and of its composition and structural
properties throughout the reduction process by TEM, electron diffraction and
electron energy-loss spectroscopy. The small changes of GO properties observed
at low applied electric currents are attributed to the promotion of diffusion
processes. The actual reduction process starts from an applied power density of
about 2 1014 Wm-3 and occurs in a highly uniform and localized manner. The
conductivity increases more than 4 orders of magnitude reaching a value of 3
103 Sm-1 with a final O content of less than 1%. We discuss differences between
the reduction by thermal annealing and Joule heating.
