<p>Dyes represent a major class of water pollutants worldwide, posing significant threats to aquatic ecosystems and the environment due to their discharge into industrial wastewater without treatment. Thus, before releasing organic colors and their derivatives into rivers, they must be degraded using heterogeneous photocatalysts. Similarly, burning fossil fuels releases massive amounts of CO₂ into the atmosphere, exacerbating global warming and environmental devastation. Herein, we report the design and fabrication of Ag/CdSe/g-C₃N₄ (i.e., Ag/CdSe/CN) as a highly efficient photocatalyst for the degradation of Toluidine Blue dye and CO₂ conversion to value-added products. X-ray diffraction analysis confirmed the crystalline nature of the as-synthesized photocatalyst. SEM and TEM analysis revealed that Ag and CdSe nanoparticles are uniformly dispersed on the surface of a 2D sheet-like structure for CN. The photocatalytic performance of the photocatalysts was explored at different parameters such as dye and photocatalyst doses, pH, and time. The as-fabricated Ag/CdSe/CN photocatalyst revealed exceptional performance for Toluidine Blue dye degradation under visible light irradiation, with a notable 92.3% degradation at pH 13, which is remarkably higher than those of the CdSe/CN, CdSe, and CN photocatalysts. Additionally, the Ag/CdSe/CN photocatalyst revealed outstanding performance for visible light catalytic CO₂ conversion to CH₄ (i.e., 9&#xa0;μmol&#xa0;g⁻¹&#xa0;h⁻¹) and CO (28&#xa0;μmol&#xa0;g⁻¹&#xa0;h⁻¹), in comparison to the reference samples. This work will provide a new route for the design and fabrication of highly efficient photocatalysts for environmental applications.</p> Graphical abstract <p>Graphical abstract showing the design of Ag/CdSe/g-C₃N₄ nanocomposite for CO₂ conversion and dye degradation</p> <p></p>