Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as similar to 5.6 +/- 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P-(MeO(2)MA-co-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core-shell morphology and the average particle size of P (0.16 +/- 0.08 mu m) increases in PG (0.95 +/- 0.45 mu m). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between -OH and -COOH groups of GQDs and -NMe2 groups of P. A decrease of the intensity ratio of Raman D and G bands (I-D/I-G) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) domain, and in PG, the pi-pi* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (similar to 430 nm) with increasing aging time for energy transfer from P to GQDs followed by up-converted emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium-titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (J(sc)) of 4.36 mA/cm(2), an open-circuit voltage (V-oc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, eta) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both V-oc and J(sc) increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.