Heterojunction Engineering of the Cr/gC ₃ N ₄ Semiconductor for Electrochemical Dye Sensing and Photocatalytic Degradation of Cibacron Deep Red under Sunlight

Cibacron Deep Red (CDR) dye stands as a water-soluble reactive azo dye extensively used in the textile industry and poses a significant threat to human health and the environment. In the present work, we reported detection and photodegradation of CDR dye by means of the synthesis of pure sheet-like gC3N4 and Cr/gC3N4 by the thermal decomposition method. The structural, optical, electrical, operational charge separation, conductivity, Mott–Schottky analysis, and other characteristic properties of the material were examined by PXRD, IR, UV–visible, FE-SEM, HRTEM, EDS, PL, EIS, M–S, and CV performances. Chromium effectively enhanced the electrochemical dye sensing and photocatalytic degradation of Cibacron Deep Red (CDR) dye in comparison to gC3N4. Cr/gC3N4-modified CPE demonstrated a significant current response indicating its capability toward detection of 10 ppm of CDR dye at pH 5. The study of photoexcited charge transfer mechanism has long been one of the photocatalysis focal points. Mott–Schottky analysis revealed that the lowest conduction band edge of gC3N4 is at −0.388 eV (vs RHE), while the highest valence band is found to be at 2.381 eV (vs RHE); similarly, for Cr/gC3N4, the calculated CB is at −0.488 eV (vs RHE). As a result, the change in Vfb (shift of the Fermi level) causes the electric field to form over the heterojunctions. Cr/gC3N4 shows an excellent photocatalytic degradation efficiency of 96% for CDR dye, as evaluated by UV–visible spectroscopic analysis in 150 min under natural sunlight irradiation. The effect of experimental parameters designed for dye degradation includes an initial dye concentration of 10 mg/L and a catalyst dose of 5 mg at natural pH. Catalyst recycling was also investigated to optimize conditions. The electrochemical and photocatalytic mechanisms were also explained. Furthermore, to support degradation, liquid chromatography–mass spectrometry (LC–MS) was used to find the photocatalytic degradation intermediate products or by-products.