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

Abstract

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.