Recuced graphene oxide sheet carry net negative charge because of the numerous oxygen-containing groups still in their structure despite they were reduced by some reductant. Strictly speaking, graphene oxide is hard to restore completely. Some references titled "Solutions of Negatively Charged Graphene Sheets and Ribbons" and "Processable aqueous dispersions of graphene nanosheets"

Recuced graphene oxide sheet carry net negative charge because of the numerous oxygen-containing groups still in their structure despite they were reduced by some reductant. Strictly speaking, graphene oxide is hard to restore completely. Some references titled "Solutions of Negatively Charged Graphene Sheets and Ribbons" and "Processable aqueous dispersions of graphene nanosheets"

You can use zeta potential analyzer to measure the surface potential of your sample (ex. disperse graphene oxide in solvent first) , and most of time it is the surface charge (+or-) of your sample

It should be negative due to the residual of oxygen-containing functionalities. Furthermore, in dispersions it should be pH-dependent.

I agree with Mr. Szacilowski, the charge should be negative due to the residual oxygen content. If you have a theoretically 100% reduction, then the charge should be neutral.

we  used zeta potential analyzer to measure the surface potential of rGO and it was negative as the other told.

thank you for all.. I studied zeta potential for rGO. Its negative only.

Would have thought that neat graphene has also negative charge due to pi electrons.

Have a work in different solvents measured zeta potential?

I am also agreed with Prof Konrad Szacilowski. Also, If you have specific sample it would be better to measure it by using Zeta potential.

Additionally you can read the attached article

2014 carbon-The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis