The pursuit of efficient and durable photocatalysts for environmental remediation remains a critical challenge. This study introduces a novel carbon nitride (g-C3N5) photocatalyst, synthesized from 3-amino-1,2,4-triazole through solid-state combustion and ultrasonication, and integrated with bismuth oxide (Bi2O3) to form a Bi2O3/g-C3N5 heterojunction photocatalyst. Comprehensive characterization, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), was conducted to elucidate the structural, optical, and electronic properties of the individual components and their composite. The Bi2O3/g-C3N5 photocatalyst exhibited a reduced bandgap, enhancing visible light absorption and photocatalytic efficacy. It achieved 90% degradation of acridine orange within 60 min, demonstrating superior photocatalytic performance compared to the pure components. The enhanced activity is attributed to the effective separation of photogenerated charge carriers facilitated by the Z-scheme mechanism. Additionally, the Bi2O3/g-C3N5 composite showed exceptional stability, maintaining high photocatalytic activity through five reaction cycles and the Acridine orange degradation pathways were elucidated using intermediates detected by HRMS analysis. This work underscores the potential of Bi2O3/g-C3N5 composites in advancing photocatalytic technologies for environmental cleanup.