Spinel Phase Engineering Boosted Visible-Light Photocatalytic Activity in Co1–xSrxCr2O4

We synthesized Sr-doped spinel CoCr2O4 using the solution combustion method and characterized the structure, morphology, chemical state, and photocatalytic properties through different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrochemical impedance spectroscopy (EIS). 30–50 nm cuboid CoCr2O4 nanocrystals with Sr doping levels ranging from 0 to 0.6% were obtained; the increasing Sr doping deformed the coordination number of Co and Cr, transitioning to octahedral and tetrahedral units, inducing the phase transition from spinel to inverse spinel at 0.6% Sr content. This modification enhanced optical absorption, reduced the energy band gap, increased photoluminescence intensity, and maintained a high-spin state with oxygen vacancies. 0.6% Sr-doped CoCr2O4 demonstrated the highest photocatalytic efficiency at 93%. The XRD structure and photocatalytic activity remained at 87% over 7 cycles after 14 h. Employing degradation pathways and Mott–Schottky curves elucidated the enhancement mechanism.