We studied the chemical processes that take place during hydrothermal gelation of graphene oxide (GO), quantifying the reaction products generated during hydrothermal reduction. The gelation proceeds with disproportionation of GO yielding a large amount of CO2 (about a quarter of the original mass of GO), organic acidic fragments, and CO. The CO2 that is formed is trapped in the hydrogel creating macroscopic voids which can lead to cracking of the hydrogel during compression. We were able to quantify the amount of CO2 produced in situ by adding ammonia during the synthesis, and converting CO2 into ionic carbonate species that we could easily quantify by titration. We used titration to evaluate the formation of organic acidic fragments too and evaluated the amount of H2O and CO produced by thermogravimetric analysis and mass balance. The conversion of CO2 into ionic species allowed us to produce void-free hydrogels which remain structurally stable after extensive compression. However, such hydrogels on average showed lower mechanical strength and electrical conductivity than the hydrogels with voids. This is a result of the difference in chemistry and morphology between hydrogels reduced under acidic pH and basic pH. Our work provides for the first time a clear quantitative estimate of CO2 evolution and organic fragment formation during hydrothermal reduction of GO, an overall picture of the reaction products, and a deepened understanding of the conditions that can be used to prepare stronger and more conductive graphene hydrogels and aerogels.

Graphene-nanoparticle (NP) composites have shown potential in applications ranging from batteries to, more recently, tissue engineering. Graphene and NPs should be integrated into uniform free-standing structures for best results. However, to date, this has been achieved only in few examples; in most cases, graphene/NP powders lacking three-dimensional (3D) structure were produced. Here we report a facile and universal method that can be used to synthesize such structures based on colloidal chemistry. We start from aqueous suspensions of both graphene oxide nanosheets and citrate-stabilized hydroxyapatite (HA) NPs. Hydrothermal treatment of the mixtures of both suspensions reduces graphene oxide to graphene, and entraps colloidal HA NPs into the 3D graphene network thanks to a self-assembled graphite-like shell formed around it. Dialysis through this shell causes uniform NP deposition onto the graphene walls. The resulting graphene-HA gels are highly porous, strong, electrically conductive and biocompatible, making them promising scaffolds for bone tissue engineering. This method can be applied to produce a variety of free-standing 3D graphene-based nanocomposites with unprecedented homogeneity.

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