A heterojunction semiconductor of MoO3:Cu in conjunction with g-C3N4 was fabricated using a green synthesis approach facilitated by leaves extract of Murraya paniculata. Numerous methods of characterisation, including XPS, FTIR spectroscopy, XRD, BET, HR-TEM, FE-SEM, EDAX, and UV–visible spectrophotometry were employed to analyse the materials. The results from XRD and HRTEM indicated the highly crystalline nanoparticles with a mean particle size of 5.42 nm for MoO3 nanomaterials with doping of 5.62 nm Cu particles. The two-dimensional g-C3N4 sheeted structure acted as a translucent layer, facilitating self-assembly on the spherical MoO3:Cu nanoparticles, thereby promoting efficient charge flow and enhancing the photodegradation capability of the heterojunction semiconductor. XPS analysis confirmed the presence of Mo, Cu, N, O, and C in the fabricated heterojunction. UV–Visible and PL spectra demonstrated the efficient suppression of exciton annihilation in the heterojunction semiconductor. Remarkably, the heterojunction exhibited a high photocatalytic degradation performance of 97 % for the photodegradation of Crystal violet (CV) dye. Additionally, the effects of pH and dose of catalyst on photodegradation, as well as the recyclability of the catalyst, were evaluated. Kinetic analysis revealed a pseudo first-order reaction with 0.617 min−1 rate constant, while scavenger experiments confirmed the involvement of h+ and .O2− as active species in the degradation process.