  We report an in-depth study of the reduction of graphene oxide (GO) by
in-situ thermal transmission electron microscopy (TEM) analysis. In-situ
heating high-resolution TEM (HRTEM) imaging and electron energy-loss
spectroscopy (EELS) measurements have been combined to identify the
transformations of different oxygen functional groups, the desorption of
physisorbed and chemisorbed water and the graphitization as a function of the
temperature in the range from 70 up to 1200{\deg}C. A model for the general
removal of water and OFGs is proposed based on different chemical and physical
parameters that have been monitored. All this unique information provides a
detailed roadmap of the thermal behaviour of GO at an extended range of
temperature. This is not only of interest to understand the thermal reduction
process of GO but also of critical relevance to the response of GO in
applications when exposed to thermal effects.
