Green Synthesis of Ferromagnetic Metal-Doped TiO2 Nanoparticles Using Spent-Coffee-Grounds for Efficient Degradation of Methylene Blue Dye

Semiconductor nanoparticles (NPs) serve as promising method to degrade organic dyes via their photocatalytic properties. However, their resource-intensive syntheses, inefficiencies under visible-light, and recovering the NPs remain challenges to viable solutions. Our study combines the benefits of green synthesis and oxygen-vacancy induced ferromagnetism to synthesize Fe-, Ni-, and Co-Doped TiO2 NPs as effective photocatalysts for degrading Methylene Blue (MB) dye under visible-light. Green synthesis was done using Spent-Coffee-Ground extract, with varying dopant concentrations (0% mol, 2% mol, and 4% mol) and a novel Air-Nitrogen 2-step calcination was employed to produce oxygen vacancies in the NPs. The identities of the metal-doped TiO2 NPs were confirmed using X-Ray Diffraction. Scanning Electron Microscopy analysis showed our NPs were spherical with 91 nm average diameter. Desorption of MB at the start of light irradiation, an uncommonly reported phenomenon, was observed and analysed in detail. Experimental data was fitted to a pseudo-first order model to reflect both desorption and degradation processes. We found that Ni 2% mol TiO2 NPs possess the best degradation rate constant of 0.022 min−1 with a net removal of 93% of MB dye. Our work highlights the potential for magnetically recoverable metal-doped TiO2 NPs for a scalable and green solution to critical challenges in wastewater treatment.