Search

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.

In this work, the intramolecular structure of a pristine graphitic carbon nitride (GCN) was modified to investigate how it would affect the photocatalytic activity towards hydrogen evolution reaction (HER) under simulated solar irradiation. The intramolecular bonding of GCN was broken by subjecting it to a simple thermal treatment under a controlled environment. Both experimental and computational studies revealed that removal of inter-heptazine CN(H)C groups from the pristine structure induced an amorphous phase and additional energy bands to GCN. It was also observed that a mid-gap state was formed between the conduction band (CB) of the amorphous carbon nitride (ACN) and the H+/H2 reduction potential. The presence of the mid-gap state not only served as an additional reduction site for HER, but also acted as a buffer to reduce the recombination rate of photogenerated charge carriers. The optimum ACN sample displayed an enhanced photocatalytic performance achieving a HER rate of 789 µmol/gcat, which was about 2-fold higher than that of pristine GCN. This could be ascribed to the synergistic effect of improved light absorption, increased surface area, suppressed charge recombination and increased charge carrier densities for the ACN samples to drive the overall reduction–oxidation process.

In recent years, graphitic carbon nitride (gC3N4) has hit the limelight as a bright, metal-free and visible-light responsive photocatalyst. Pristine gC3N4, however, suffers from inadequate light absorption profile, limited surface area and significant recombination of photo-induced electron-hole pairs, thus requiring reformative strategies. Herein, this work has employed elemental doping of oxygen to gC3N4 (O-gC3N4) and uncovered the evolution of properties that arises in conjunction with increasing doping level. These intrusive changes in properties by increasing oxygen doping levels are then evaluated based on the trends observed from photocatalytic hydrogen (H₂) evolution. It is revealed that oxygen doping had a competing dual effect to the photocatalytic activity. On one hand, oxygen doping introduced more porosity and added sub-gap impurity states in its electronic band structure, which resulted in enhanced light harvesting capabilities. On the other hand, impurity levels from high density oxygen groups behave detrimentally as potent electron-holes recombination centers. This became the governing factor for higher doping levels O-gC3N4, which thereby resulted in a weakened photoactivity compared to pristine gC3N4. In overall, it is hoped that the findings of this study can provide a new understanding of the rational design and oxygen-doping strategy for gC3N4 for its photocatalytic enhancement.