Preparation of water soluble graphene using polyethylene glycol: Comparison of covalent approach and noncovalent approach

Formulation of stable aqueous graphene solution is attempted either through esterification of GO with PEG and subsequent reduction in covalent approach or in situ chemical reduction of aqueous GO solution in the presence of NH2–PEG–NH2 in noncovalent approach. While covalent approach produces insoluble graphite-like precipitation, noncovalent approach produces practically homogeneous reduced graphene solution for several months without any precipitate. Noncovalent interaction between NH2–PEG–NH2 and graphene is crucial for formulating fully reduced graphene solution with high stability. Various wet process such as spin-coating, ink-jet printing, and spraying can be directly utilized to formulate graphene-based electronic devices such as transparent electrode or anti-static coating without any additional reduction process which is necessary when insulating GO solution is used instead. Non-conducting NH2–PEG–NH2 can be simply removed from conducting graphene film by simple filtration and washing process to formulate pure graphene film because interaction between graphene and NH2–PEG–NH2 is not covalent. The initial surface resistance of graphene thin film with NH2–PEG–NH2 is in the range of 106 Ω/□ but it decreases to 104 Ω/□ after removing NH2–PEG–NH2.