RSM-driven optimization and charge carrier dynamics for a novel NixCo1.5-xFe1.5O4 photoelectrocatalyst as a rapid photoanode

Novel nickel-substituted cobalt ferrite (NixCo1.5-xFe1.5O4, x = 0, 0.2, 0.4) particles were synthesized hydrothermally and deposited on indium tin oxide (ITO) to design visible-light photoanodes for Rhodamine B (RhB) degradation. Structural, optical and electrochemical analyses (XRD, TEM, BET, DRS, PL/TRPL, EIS, and photocurrent) revealed that incorporating Ni contracts the spinel lattice, raises the specific surface area to 129 m² g⁻¹, widens the band gap to 2.53 eV, and shifts the flat-band potential towards more negative values, and significantly improves charge carrier separation. Under 449 nm LED irradiation, the Ni0.4Co1.1Fe1.5O4 electrode achieved 99.66 ± 0.49% RhB discoloration within 8 min under the optimized conditions with a pseudo-first-order rate constant of k = 0.255 min⁻¹, which is 2.6 times higher than the unsubstituted Co1.5Fe1.5O4 ferrite. Response Surface Methodology (RSM) was employed to optimize the operational parameters of the photoelectrocatalytic (PEC) process: Na2SO4 concentration, applied current density, initial dye load, and irradiation time. The optimal conditions deduced from the model (0.26 M Na2SO4, 4 mA·cm–2, 16 ppm of RhB, time = 8 min) were tested experimentally and yielded a degradation efficiency consistent with the theoretical prediction. Scavenger tests identified holes (h⁺), hydroxyl radicals (•OH), and superoxide radicals (O2•⁻) as dominant oxidants in the catalytic process for the x = 0.4 composition. TOC analysis showed 95% TOC removal, indicating advanced mineralization rather than simple dye discoloration. The optimized photoanode also showed good operational stability over seven consecutive cycles, with negligible metal leaching and no obvious morphological deterioration after reuse. These results demonstrate that the partial substitution of cobalt with nickel combined process optimization via RSM improves the charge-transfer behavior and visible-light photoelectrochemical activity of cobalt ferrite photoanodes for dye degradation.