Enhanced photocatalytic degradation of malachite green dye by NiCr2O4/TiO2 heterojunction under solar irradiation

Nickel chromite (NiCr₂O₄) was synthesized via the co-precipitation method and characterized for its physical and photoelectrochemical properties. Thermal gravimetry (TGA) and X-ray diffraction (XRD) analyses confirmed the formation of a single-phase cubic structure (space group: Fd-3 m) at temperatures above 850 °C. Transmission electron microscopy (TEM) revealed crystallite agglomeration, while diffuse reflectance spectroscopy indicated a direct optical transition with a bandgap energy of 1.76 eV. The material exhibited p-type behavior, with a flat band potential (E_fb) of 0.57 V vs. SCE, determined from capacitance-potential measurements. To enhance photocatalytic activity, a novel visible-light-responsive NiCr₂O₄/TiO₂ heterojunction was synthesized and evaluated for the degradation of malachite green (10 mg/L) at neutral pH. Individually, TiO₂ and NiCr₂O₄ achieved degradation rates of 42% and 58% after 180 min. However, the 50 wt %–50 wt % p-n NiCr₂O₄/TiO₂ heterojunction significantly improved performance, achieving a 90% degradation rate. This enhancement is attributed to improved charge separation and reduced electron–hole recombination, facilitating the generation of reactive species. Scavenger experiments with EDTA-2Na and ascorbic acid revealed that holes (h⁺) and superoxide radicals (O₂^•−) play a key role in the photocatalytic process.