Three Colors Emission from S,N Co-doped Graphene Quantum Dots for Visible Light H2 Production and Bioimaging

A facile solvothermal route to synthesize S,N co-doped graphene quantum dots (S,N-GQDs) with unique optical properties is demonstrated. Three absorption bands are observed at 338, 467, and 557 nm, which is different from any previous reports. The photoluminescent spectra display emissions in three primary colors that are independent of the excitation wavelength, within the excitation wavelength ranges of 340-420 nm, 460-540 nm, and 560-620 nm. The PL excitation spectra indicate that each emission is related to a single excitation band. It is proposed that three independent luminescent centers coexist in S, N-GQDs because the doping with S and N may change the chemical environment of the GQDs. However, energy-transfer processes usually do not occur among the independent luminescent centers under different wavelength light excitation. Heteroatom-doping of GQDs provides an attractive means of effectively tuning their optical properties for the purpose of exploiting new applications in visible-light photocatalytic and bioimaging. S,N-GQDs/TiO2 composites exhibit better hydrogen production activities under visible light (lambda > 420 nm) than commercial TiO2 (P25), owing to the presence of characteristic absorption bands in the visible region. Furthermore, the S,N-GQDs have a pronounced biocompatibility and bioimaging ability under long-wavelengths excitation for live A549 cells.