Proficient capture of carbon dioxide (CO2) is considered to be a backbone for environment protection through countering the climate change caused by mounting carbon content. Here we present a comprehensive mechanism to design novel functional nanostructures capable of capturing a large amount of CO2 efficiently. By means of van der Waals corrected density functional theory calculations, we have studied the structural, electronic and CO2 storage properties of carbon nitride (g-C6N8) nano sheets functionalized with a range of transition metal (TM) dopants ranging from Sc to Zn. The considered TMs bind strongly to the nano sheets with binding energies exceeding their respective cohesive energies, thus abolishing the possibility of metal cluster formation. Uniformly dispersed TMs change the electronic properties of semiconducting g-C6N8 through the transfer of valence charges from the former to the latter. This leaves all the TM dopants with significant positive charges, which are beneficial for CO2 adsorption. We have found that each TM's dopants anchor a maximum of four CO2 molecules with suitable adsorption energies (-0.15 to -1.0 eV) for ambient condition applications. Thus g-C6N8 nano sheets functionalized with selected TMs could serve as an ideal sorbent for CO2 capture.