Optimization of Enhanced Photocatalytic Degradation of Turquoise Blue G (TBG) Using a Cu-N:ZnO/TiO2 Nano-Heterojunction Photocatalyst via Response Surface Methodology (RSM)

A novel Cu-N co-doped ZnO/TiO2 (7% Cu-N:ZnO/TiO2) modified Type-II nano-heterojunction photocatalyst was synthesized using a cost effective room temperature sol-gel (RTSG) method to enhance dye degradation performance. Nitrogen doping levels (1-5 wt%) were optimized relative to the 7% Cu doped ZnO/TiO2 system. Structural, morphological, optical, and surface chemical properties were systematically characterized using XRD, FE-SEM, UV-Vis, XPS and FTIR. Among all, the 3 wt% N-doped catalyst, 7% Cu-N(3%):ZnO/TiO2, exhibited the significant red shift (388.71 → 417.5 nm) and notable band-gap reduction from 3.19 eV (pure TiO2) to 2.97 eV, enabling superior visible-light absorption. For the first time, this optimized photocatalyst was applied for the photocatalytic degradation of Turquoise Blue G (TBG) dye, achieving an excellent degradation efficiency of 98.07% within 150 minutes-1.57 times higher than pure TiO2 and reducing COD to a level 5.5 times lower than untreated wastewater, confirming effective mineralization. Compared with our earlier Cu doped ZnO/TiO2 system, nitrogen incorporation further enhanced charge separation and reaction kinetics. Additionally, RSM using Central Composite Design (CCD) was employed to model, analyze, and optimize key process parameters, showing strong predictive accuracy. This study demonstrates that Cu-N co-doping provides a highly efficient and scalable strategy for sustainable photocatalytic treatment of dye wastewater. Highlights 1) A novel 7% Cu-N:ZnO/TiO₂ modified type-II heterojunction photocatalyst was successfully synthesized using a cost-effective room-temperature sol-gel (RTSG) method. 2) Nitrogen co-doping (1–5 wt%) induced a strong red shift, reducing the band gap to 2.97 eV, significantly enhancing visible-light absorption. 3) For the first time, the 7% Cu-N(3%):ZnO/TiO₂ photocatalyst achieved an outstanding 98.07% degradation efficiency toward Turquoise Blue G (TBG) dye. 4) RSM-CCD optimization identified nitrogen doping (2.887 wt%) and degradation time (2.494 h) as the optimal conditions for maximum efficiency. 5) The developed quadratic model (R² = 0.9923) exhibited excellent predictive accuracy, confirming the effectiveness of Cu–N co-doping with RSM optimization for dye wastewater treatment.