Photocatalytic degradation of Congo red dye using zinc oxide nanoparticles prepared using Carica papaya leaf extract

Abstract

In this research, the photocatalytic degradation of Congo red (CR) dye was investigated using hexagonal wurtzite structured zinc (Zn) oxide nanoparticles (ZnO NPs: crystallite size of ∼21 nm and band gap energy of 2.88 eV) prepared through the reduction of a Zn salt using Carica (C.) papaya leaf extract. The Fourier transform infrared (FTIR) spectra revealed the presence of several organic moieties (supplied by the C. papaya leaf extract) on the ZnO surface. High-resolution transmission electron microscope (HRTEM) images showed non-agglomerated and multi-structured (e.g., spherical, semi-spherical, hexagonal, and rod-like) ZnO NPs. The BET analysis indicated the mesoporous structure of ZnO NPs with a high specific surface area of 34 m2/g. The ZnO NPs showed 99.9% degradation of CR dye (120 mg L−1) within 80 min under ultraviolet (UV) irradiation. The ZnO NPs exhibited excellent photocatalytic performance (e.g., quantum yield: 3.18 × 10−4Φ, kinetic reaction rate: 1.43 × 10+3 μmol g−1 h−1, space time yield: 3.18 × 10−5 molecules photon−1 mg−1, and figure-of-merit: 5.53 × 10−9 mol L J−1 g−1 h−1) with a minimum energy consumption of 3.26 × 1022 J mol−1 in comparison to other photocatalytic systems. The photocatalytic reaction pathways of CR dyes are addressed, along with a discussion on its degradation products based on gas chromatography-mass spectrometry (GC-MS) analysis. The overall results of this research indicate that C. papaya-mediated ZnO NPs can be used as an effective option for the degradation of industrial dyes.