Advances in poly(heptazine imide)/poly(triazine imide) photocatalyst

Polymeric carbon nitride (PCN) has garnered increasing attention as a metal-free photocatalyst with a suitable band gap. In efforts to enhance its photocatalytic performance, researchers have examined various PCN materials, including poly(heptazine imide) (PHI) and poly(triazine imide) (PTI), two isomers within the PCN family that exhibit distinct and superior photocatalytic activity compared to other forms. The challenge, however, lies in the common practice among researchers to categorize PHI and PTI along with other PCN types under the overarching term “g-C3N4,” which significantly impedes optimization efforts. The objective of this review is to provide comprehensive insights into the structural features, photoelectrochemical properties, and effective characterization methods employed for distinguishing between PHI and PTI materials. The review also summarizes various optimization strategies, such as crystallinity adjustments, defect engineering, morphology control, constructing heterojunction, and atomic-level metal loading dispersion, to elevate the photocatalytic activity of PHI and PTI, in addition to summarizing the history of carbon nitride development. Furthermore, this review highlights the primary applications of PHI and PTI, encompassing nitrogen fixation, biomass conversion, organic synthesis, CO2 reduction, pollutant degradation, H2O2 production, and photocatalytic water splitting. Lastly, the prospects and challenges associated with further advancing PHI and PTI are thoroughly examined.