Gadolinium-decorated functionalized multi-walled carbon nanotubes: synthesis, characterization, and photocatalytic degradation of organic dyes under UV irradiation

In this study, 3% gadolinium-decorated functionalized multi-walled carbon nanotubes (Gd/f-MWCNTs) were successfully synthesized through acid functionalization of pristine MWCNTs followed by a citric acid-assisted wet chemical deposition method and calcination at 300 °C. The nanomaterial was characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. The photocatalytic performance of the Gd/f-MWCNTs was evaluated for the degradation of Malachite Green (MG) and Methylene Blue (MB) dyes under UV irradiation. Under optimized conditions, 93.02% degradation of MG dye (0.20 g/L, pH 10, 40 min, k = 0.0666 min−1) and 87.32% degradation of MB dye (1.20 g/L, pH 2, 20 min) were achieved; an apparent pseudo-first-order rate constant of k = 0.1033 min−1 was estimated for MB. Scavenger studies revealed dye-specific reactive oxygen species hierarchies: for MG degradation, the order of contribution was photogenerated electrons (e−) > superoxide radicals (•O2−) > hydroxyl radicals (•OH) > holes (h+), whereas for MB degradation the order was e− > •OH > h+ > •O2−; in both cases, photogenerated electrons were the dominant reactive species. The catalyst maintained significant photocatalytic activity over four reuse cycles.