Literally dozens(if not hundreds) of techniques available.. simplest of them all is sodium citrate/NaBH4 or urea etc.

You can use electrochemical method.

Thanks. Since, Graphene is chemically inert. How can we functionlize it using a simple chemical? 

Thanks for you cooperation.

Please go through the paper RSC Advances 2014,4, 25904-25911 (DOI:10.1039/c4ra01860e. It gives an account of SnO2 synthesized along with reduced grapheme oxide composite as a shield against EMI.

Typical procedure is as follows:

Synthesis of SnO2 decorated RGO sheets. The chemical reduction of graphene oxide (GO) was carried out in the presence of SnCl2 to fabricate SnO2 decorated RGO. Prior to this, GO has been synthesised. The detailed method of GO synthesis was discussed in our earlier report.In brief, commercially available graphite powder (5 g) and NaNO3 (5 g) was mixed into concentrated H2SO4 (230 mL). KMnO4 (40 g) was gradually added with stirring and cooling, so that the temperature of the mixture did not reach beyond 20 C. The mixture was then stirred at 35 C for 2 h, and deionized water (200 mL) added. The reaction was stirred for 1 h followed by the addition of a large amount of deionized water (300 mL) and 30% H2O2 solution (30 mL), causing violent effervescence and an increase in temperature to 100 C, aer which the color of the suspension changes to bright yellow. The suspension was washed with a solution of HCl in order to remove the metal ions. The paste collected was dried at 60 C. 2 g of the GO powder was dispersed in 500 mL of distilled water followed by stirring and ultrasonication for 1 h to prepare the solution of GO.

A typical preparation process for SnO2 decorated RGO is as follows: 14 g of SnCl2$2H2O was added to 500 mL HCl solution (100 mL L1 of 37 wt% HCl). Then, this solution was mixed with the GO solution and sonicated for 1 h. The resulting mixture was stirred at 90 C for 6 h. SnO2@RGO was collected after several washings with water through centrifugation, then dried in a vacuum oven at 100 C. Calcination (400 C for 5 h under a N2 atmosphere) was carried out to improve the crystallity of SnO2.

The possible reaction mechanism can be written as follows:

SnCl2 + 2H2O + GO + H2O + 2HCl / SnO2@RGO

Thanks Sir Sundeep but I do not have graphite powder. I have Graphene platelets and I have to use it for nanocomposites. Can you please tell me a simple way to convert graphene to graphene oxide ?

All researchers try to make graphene from GO. Nobody likes to convert grphene to GO. However, if you are interested we can send you some GO prepared by us for your R&D.