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In this article, graphene quantum dots (GQDs) with tunable surface chemistry (increasing oxidation degree) were prepared by an efficient two-step method. The GQDs have tunable fluorescence induced by the degree of surface oxidation, fine solubility, high stability and applicable up-conversion photoluminescence (PL). The PL mechanism was investigated based on the surface structure and PL behaviors. More importantly, the GQDs have acid-base response property and can be applied as pH sensors.

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Graphene of 1–5 layers was synthesized on a polycrystalline Co film by radio-frequency plasma-enhanced chemical vapour deposition at a relatively low temperature of 800 °C for only 40 s in a mixture of gases of CH4/H2/Ar (1/5/20, with a total gas flow rate of 78 sccm). The obtained graphene is of high quality as revealed by Raman spectroscopy and x-ray photoelectron spectroscopy. It exhibits a high optical transmittance of more than 70% in the wavelength range 500–1200 nm, and a sheet resistivity of 2661 Ω/sq. A possible formation mechanism is proposed. The significant influence of octahedral and tetrahedral lattice interstitial sites in the Co crystallites on the formation of graphene is discussed.