Band-Gap Engineered TiO₂–CuO/Hydroxyapatite Photocatalyst for Efficient Treatment and Dye Degradation in Wastewater Systems

Abstract

In this study, the photocatalytic degradation of methylene blue (MB) under sunlight irradiation was investigated using a TiO₂–CuO/hydroxyapatite (HAp) composite catalyst. To enhance the photocatalytic efficiency of conventional semiconductor photocatalysts, TiO₂–CuO/HAp composites were successfully synthesized via the sol–gel method. The physicochemical properties of the prepared composites were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDX), Thermogravimetric Analysis (TGA), Brunauer–Emmett–Teller (BET) surface area analysis, and UV–Vis Diffuse Reflectance Spectroscopy (UV–Vis DRS). Photocatalytic activity tests revealed that the degradation efficiency of the catalysts followed the order: HAp < CuO/HAp < TiO₂/HAp < TiO₂–CuO/HAp. The TiO₂–CuO/HAp composite, exhibiting the lowest bandgap energy (2.89 eV) as determined from the Tauc plot, achieved the highest photocatalytic performance, with a degradation efficiency of 99.83% for an initial MB concentration of 20 mg/L, a catalyst dose of 0.15 g, and an irradiation time of 150 minutes under natural sunlight. Furthermore, the TiO₂–CuO/HAp composite demonstrated the highest apparent first-order rate constant (kapp = 0.0430 min⁻¹) compared to other samples (HAp, CuO, TiO₂, CuO/HAp, and TiO₂/HAp). The reusability test over four successive cycles confirmed the structural stability and catalytic durability of TiO₂–CuO/HAp. Overall, the incorporation of CuO and HAp into the TiO₂ framework effectively reduced the bandgap energy and enhanced charge separation, thereby improving visible-light responsiveness. These findings underscore the potential of TiO₂–CuO/HAp as an efficient, reusable, and sunlight-driven photocatalyst for the degradation of organic pollutants in wastewater treatment applications.