Boosted photoredox capability of visible light-active P-doped C3N4 with efficient harvesting of electron–hole pairs†

Abstract

Photocatalytic production of solar fuels and high-value chemicals by photogenerated carriers has been at the forefront as one of the promising sustainable approaches. However, most of the studies focus only on one of the half reactions, either photoreduction or photooxidation, leading to underutilization of the potentiality of photocatalysis due to inefficient harvesting of electron–hole pairs. Herein, the efficient utilization of photogenerated electron–hole pairs was demonstrated by employing phosphorous-doped graphitic carbon nitride (P-doped g-C₃N₄) as a visible light-active photocatalyst that is capable of simultaneously producing hydrogen and benzaldehyde from benzyl alcohol. P-doping into g-C₃N₄ was achieved using an eco-friendly P source. P doping induced changes in the light-harvesting capacity of g-C₃N₄, and its consequence on the dual-functional photocatalytic activity of P-doped g-C₃N₄ was systematically investigated using various characterization techniques. P-doped g-C₃N₄ exhibited an ≈3-fold increase in photocatalytic activity in the production of H₂ and benzaldehyde as compared to that of pristine g-C₃N₄. Density functional theory (DFT) studies reveal that the P-dopant preferentially replaces the corner C-site as compared to the N site of the tri-s-triazine ring of g-C₃N₄, which results in the creation of mid-gap states that enable the enhanced visible light absorption of P-doped g-C₃N₄. Mechanistic investigation studies suggest that photogenerated holes drive the selective oxidation of benzyl alcohol to benzaldehyde while photogenerated electrons drive H₂ evolution, leading to concomitant production of H₂ and benzaldehyde by P-doped g-C₃N₄. The selective conversion of benzyl alcohol proceeds through a carbon-centred radical mechanism, according to experimental and DFT studies. This work elucidates the importance of P-doping in g-C₃N₄ for the simultaneous production of solar fuels (such as H₂) and high-value chemicals (such as benzaldehyde).