Improved photocatalytic decolorization of reactive black 5 dye through synthesis of graphene quantum dots-nitrogen-doped TiO2

Abstract

Graphene quantum dots (GQDs), a new solid-state electron transfer material was anchored to nitrogen-doped TiO₂ via sol gel method. The introduction of GQDs effectively extended light absorption of TiO₂ from UV to visible region. GQD-N-TiO₂ demonstrated lower PL intensity at excitation wavelengths of 320 to 450 nm confirming enhanced exciton lifespan. GQD-N-TiO₂-300 revealed higher surface area (191.91m² g^-1), pore diameter (1.94 nm), TEM particle size distribution (4.88 ± 1.26 nm) with lattice spacing of 0.45 nm and bandgap (2.91 eV). In addition, GQDs incorporation shifted XPS spectrum of Ti 2p to lower binding energy level (458.36 eV), while substitution of oxygen sites in TiO₂ lattice by carbon were confirmed through deconvolution of C 1 s spectrum. Photocatalytic reaction followed the pseudo first order reaction and continuous reductions in apparent rate constant (K_app) with incremental increase in RB5 concentration. Langmuir-Hinshelwood model showed surface reaction rate constants K_C = 1.95 mg L^-1 min^-1 and K_LH = 0.76 L mg^-1. The active species trapping, and mechanism studies indicated the photocatalytic decolorization of RB5 through GQD-N-TiO₂ was governed by type II heterojunction. Overall, the photodecolorization reactions were triggered by the formation of holes and reactive oxygen species. The presence of •OH, ¹O₂, and O₂• during the photocatalytic process were confirmed through EPR analysis. The excellent photocatalytic decolorization of the synthesized nanocomposite against RB5 can be ascribed to the presence of GQDs in the TiO₂ lattice that acted as excellent electron transporter and photosensitizer. This study provides a basis for using nonmetal, abundant, and benign materials like graphene quantum dots to enhance the TiO₂ photocatalytic efficiency, opening new possibilities for environmental applications.

Keywords: Graphene quantum dots; Heterogeneous reaction; Photosensitizer; Reaction kinetics; Type II heterojunction.