Molecular Modifications of Crystalline Poly(triazine imide) for Advancing its Structure-Property Relationships in Light-Driven Catalysis

Carbon-nitride materials represent light-absorbing structures composed of earth abundant elements capable of being leveraged for semiconductor photocatalysis at their surfaces. This study systematically investigates the addition of molecular modifiers in the synthesis of crystalline carbon nitride, primarily in the form of poly(triazine imide) lithium chloride (PTI-LiCl), to assess their effects on the materials’ structure, optical bandgap, and photocatalytic activity for hydrogen (H2) and oxygen (O2) evolution under ultraviolet and visible light irradiation. Melamine and five pyrimidine-centered analogs were employed as building blocks to modify various heteroatoms within its polymeric framework. The new, modified materials were characterized with attention to the differences introduced by the monomeric modifiers and their influence on the resulting structures and compositions. Findings indicate that these modifications significantly broaden the visible-light absorption range, albeit with the gradual loss of the bulk crystalline structure. As the loading of modifiers increased beyond 50%, a predominantly amorphous form of carbon nitride emerges. XPS and SEM analyses corroborated the changes, which were attributed to modifications of the elemental composition, specifically a reduced amount of Li cations and charge-balancing Cl anions due to fewer binding sites in the intralayer cavities. In photocatalytic measurements under an ultraviolet 390 nm LED, the unmodified PTI-LiCl framework demonstrated the highest H2 evolution rate (HER; 3.44 mmol·g-1·h-1) at an apparent quantum yield of 5.4%, along with total water splitting at rates of 163 µmol of H2·g-1·h-1 and 75.6 µmol·O2 g-1·h-1. While PTI-LiCl showed trace activity under a visible-light 440 nm LED, all modified materials exhibited enhanced reactivity with as low as 5% molecular modifier. The photocatalytic rates peaked at a 15% modification level when using 2,4,6-triaminopyrimidine, with rates of 33 µmol·g-1·h-1 for HER, along with 19.7 µmol of H2·g-1·h-1 and 8.7 µmol of O2·g-1·h-1 for total water splitting. Density functional theory calculations were used to probe electronic band structure changes resulting from the modifications. Thus, these findings elucidate the structural, optical and electronic changes arising from the five selected molecular modifiers and their impact on the semiconductors’ photocatalytic properties.