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

Abstract

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 ∼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-<i>g</i>-poly­[(diethylene glycol methyl ether methacrylate)-<i>co</i>-poly­(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate)] [PT-<i>g</i>-P­(MeO₂MA-<i>co</i>-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 μm) increases in PG (0.95 ± 0.45 μm). 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>I</i>_D/<i>I</i>_G) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp² domain, and in PG, the π–π* 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 (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by <i>up-converted</i> 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 (<i>J</i>_sc) of 4.36 mA/cm², an open-circuit voltage (<i>V</i>_oc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both <i>V</i>_oc and <i>J</i>_sc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components