Modulating charge carrier dynamics in potassium-poly(heptazine imide) <i>via</i> aqueous KCl solutions probed by femtosecond transient absorption spectroscopy

The significantly enhanced photocatalytic hydrogen evolution (PHE) performance of graphitic carbon nitride (GCN)-based materials in alkali metal salt solutions has garnered increasing attention. However, the influence of these salt solutions on the intrinsic carrier dynamics within such materials remains poorly understood. In this study, we synthesized potassium-poly(heptazine imide) (K-PHI) and systematically investigated its femtosecond transient absorption (fs-TA) spectra in both pure water and 1.0 M KCl solution. Notably, the presence of 1.0 M KCl induces a distinct stimulated emission in the fs-TA spectra of K-PHI, accompanied by the emergence of a delayed photoluminescence process with a characteristic formation time of ∼3.5 ns. This phenomenon is ascribed to the detrapping of photogenerated electrons from shallow trap states. Furthermore, the lifetime of shallow-trapped electrons in K-PHI undergoes a significant change, from 39 ps (<i>β</i> ∼ 0.49) in pure water to 15 ps (<i>β</i> ∼ 0.27) in 1.0 M KCl solution. These findings suggest that KCl solution modulates the trap-state energy distribution of K-PHI, thereby optimizing its intrinsic carrier dynamics and contributing to its superior PHE performance. This study provides fundamental insights into the role of salt solutions in regulating the photophysical properties and charge carrier behavior of GCN-based photocatalysts, offering a mechanistic basis for the rational design of high-performance photocatalytic systems.