Insights on the impact of doping levels in oxygen-doped gC3N4 and its effects on photocatalytic activity

In recent years, graphitic carbon nitride (gC(3)N(4)) has hit the limelight as a bright, metal-free and visible-light responsive photocatalyst. Pristine gC(3)N(4), 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 gC(3)N(4) (O-gC(3)N(4)) 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 (H-2) 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-gC(3)N(4), which thereby resulted in a weakened photoactivity compared to pristine gC(3)N(4). 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 gC(3)N(4) for its photocatalytic enhancement.