Unraveling the roles of single transition metal atom anchored on equivalent stoichiometry graphitic carbon nitride (gC₆N₆) for carbon dioxide reduction: a density functional theory study

CO₂ photoreduction into hydrocarbon fuels is a promising strategy in closing the carbon cycle to realize a sustainable energy economy. Among the many photocatalysts that have been developed thus far, porous graphitic carbon nitride (gC₆N₆) has emerged as a potential photocatalyst candidate in view of its unique optoelectronic properties, metal-free nature and two-dimensional versatile structure that can be easily modified. In this work, the enhancement of equivalent stoichiometry carbon nitride (gC₆N₆) through single transition metal atom modification was systematically studied from first principles density functional theory calculations. The formation energy calculations revealed that incorporating single Co, Cu, Ni or Pd atom into gC₆N₆ is energetically favorable, with the exception of Pt. The computed density of states plot indicates that a greater degree of hybridization of the transition metal atom d-orbitals with the p-orbitals of O atom from CO₂ will lead to stronger adsorption interaction. The optical absorption spectra show that Cu, Pd, and Pt promotes greater light absorption by extending the optical absorption to the NIR region. The presence of additional dopant states near the Fermi surface was found to have affected the optical absorption. The band structures of the Co,Cu,Pd,Pt@gC₆N₆ show bandgap narrowing due to the shifting of conduction band edge closer to the Fermi level. Contrastingly, Ni@gC₆N₆ exhibits bandgap narrowing through the shifting of the valence band edge to the Fermi level. The band edge positions suggest that anchoring gC₆N₆ with single Co, Cu, Ni, Pd and Pt atom dopants possesses the capability to reduce CO₂ into C1 products. Among all the transition metals studied, Pd@gC₆N₆ and Cu@gC₆N₆ are identified as the most promising single-atom photocatalysts for CO₂ reduction due to their energetically favorable formation energy, stable CO₂ adsorption configuration, narrow bandgap, low charge carrier recombination, extended light absorption range and suitable band edge positions.