Heterojunction effect for photocatalytic enhancement and electrochemical study of g-C₃N₄/TiO₂ nanoparticles for RR4 dye

Titanium dioxide (TiO₂) is widely recognized as a promising photocatalyst due to its strong oxidizing power, chemical stability, and low cost; however, its wide band gap (3.2 eV) and rapid electron-hole recombination restrict its photocatalytic activity under visible light. To address these limitations, graphitic carbon nitride (g- C₃N₄), a visible-light-responsive and metal-free semiconductor, was coupled with TiO2 to form a heterojunction composite with enhanced charge separation efficiency. This study aims to synthesize g- C₃N₄/TiO₂ composite catalysts via an in-situ sol-gel hydrothermal method using titanium butoxide and urea as precursors for improved heterojunction formation, to elucidate their physicochemical and optoelectronic properties, and to evaluate their photocatalytic efficiency for the degradation of Reactive Red 4 (RR4) dye and recyclability. The g- C₃N₄/TiO₂ composite (denoted as TCN) was successfully synthesized, with TCN₅ (5 wt% g- C₃N₄) exhibiting the highest photocatalytic performance. Under 55 W visible-light irradiation, TCN₅ achieved a degradation efficiency of 99.73% and an apparent rate constant (k) of 0.0920 min⁻¹ for 30 mg L⁻¹ RR4 dye. Structural and compositional analyses (FESEM-EDX, FTIR, XRD, and XPS) confirmed the successful incorporation of g- C₃N₄ into the TiO₂ matrix. Optical analyses using UV-Vis DRS and PL revealed a narrowed band gap energy of 3.07 eV and reduced PL intensity, indicating suppressed electron–hole recombination. Electrochemical analyses further demonstrated enhanced charge transport, with a conductivity of 2.8 × 10⁻³ Ω and a photocurrent density of 3.5 × 10⁻⁶ A/cm². Mott–Schottky plots identified conduction band positions of –0.45 eV for TiO₂ and –0.61 eV for g- C₃N₄, confirming the formation of a type-II heterojunction. The recyclability test revealed excellent durability of the in-situ synthesized TCN5, maintaining 73.94% degradation efficiency after eight cycles, outperforming the batchwise-prepared sample. Overall, the in-situ sol–gel hydrothermal synthesis effectively enhanced interfacial charge transfer, photocatalytic activity, and material stability, demonstrating the potential of g- C₃N₄/TiO₂ heterojunctions for environmental remediation applications.