Just reduce your GO films in Hydrazine vapour followed by 400C temperature.

Hummers`s method provide you non-conductive form of graphene - graphene oxide. If you wish to obtain back conductive material you have to reduce it to graphene. Some pathways are described in CurrOrgChem 2011 ( https://www.researchgate.net/publication/233598324_Preparation_of_Functionalized_Graphene_Sheets?ev=prf_pub ).

However you will never received conductivity as in starting graphene.

Article Preparation of Functionalized Graphene Sheets

Thanks Christian Punckt and Zdenko Spitalsky... we have already tried to reduce the oxide content from graphene oxide using a annealing process in a tube furnace at 500C, However still could not achieve the desired conductivity. The heating temperature is limited by the substrate properties to a maximum of 500C.

any comments please?

the conductivity will depend on the purity of your product for which you need to do chemical analysis. Depending on the nature of constituents or substituents, you can decide further purification steps and concentration of graphene.

You can do thermal or chemical reduction. Thermal reduction of 500 deg C may be too low. Try to use chemical reduction or combine it with your thermal reduction. Or maybe change the atmosphere of your furnace to a more reducing environment. 

Yasir F joya, yes I agree, we need to do detailed chemical analysis. 

Christene, as I mentioned earlier, this is one of the limitation of the material that we are using, can not heat more than that.

Debmalya Roy, Yes i believe we should do the liquid exfoliation.

Thank you every one for your suggestions, Based on your expert opinion, do you think that performing chemical reduction after thermal reduction will be effective to increase conductivity of graphene?

I remember having improved mobility of graphene FET by doing laser annealing (ie. 2mW/um2 swept along the sample) while pumping. 

Another method is the current annealing : http://digital.csic.es/bitstream/10261/10475/1/APL-2007.pdf

Hi Muhammad, there are studies on chemical reduction of GO and maybe you can combine it with your annealing process. I would prefer doing chemical reduction first before thermal reduction if you prefer to do them separately. Just ensure that the substrate is inert to your reducing agents.

On the other hand, I would suggest that you try first to do your annealing in a more reducing environment.

Thanks Antonie and Christine, I will try the suggested methods based on availability of facilities.

Hi, if reduction cannot reach the conductivity you want, maybe you can try to add some conductive molecule, such as 1-Pyrenesulfonic acid, pedot, or the other matters.

Thanks shu'xuan, I will try to do that if needed.

Just reduce your GO films in Hydrazine vapour followed by 400C temperature.

We previously did a combination of chemical reduction (at low temp to reduce reduction reaction rate, and facilitates max O2 reduction) and thermal reduction (vac annealing at 1900 C for few hrs under E-6 torr pressure). This resulted RGO membrane to a reassembled graphite structure containing negligible oxygen content (O 1s/C 1s = 0.005), and high electrical conductivity (up to 1.6 × 105 S/m) without disintegration of its self-standing feature. If you are looking for similar things then find more details here.

Article Solution-Processed Graphite Membrane from Reassembled Graphene Oxide