Quantum confinement effect in NiO and Co–NiO modified with CNTs for photocatalytic removal of bromocresol purple

The fabrication of doped and composite-based photocatalysts is gaining attention due to their higher surface area via the formation of crystal defects, vacancies, and excellent physicochemical properties to mineralize the wastewater pollutants. Therefore, we synthesized NiO, cobalt-doped NiO (Co–NiO), and its composite with allotropic form of carbon, i.e., CNTs (Co–NiO/CNTs) to degrade bromocresol purple (BCP) dye. The photocatalysts were fabricated by the hydrothermal and ultrasonication approach to degrade bromocresol purple (BCP). Through the analysis, we find that all the results, including the shift towards higher theta values, the reduction in crystallite size, the blue shift in optical spectra, and the larger bandgap, are consistent with the quantum confinement approach. The recombination rate of photo-induced species is suppressed by the addition of cobalt into NiO, which led to a higher degradation rate (k = 0.0581 min⁻¹, 88.3 %) than pristine NiO (k = 0.037 min⁻¹, 73.6 %). The quenching experiment, the dosage effect, pH effect, and stability tests were carried out by using the composite material as a reference. The results revealed that the effective concentration of photocatalyst, the existence of CNTs with Co–NiO, offered a higher number of active sites and served as electron mediators, as well as the stability of the material demonstrated that how doping and composite formation can enhance the degradation rate of dyes from water bodies.