Unravelling the charge dynamics of a molecular photosensitizer of ZnPc on Polymeric g-C₃N₄ for superior photocatalytic hydrogen evolution from seawater splitting

This work aims to provide deep insights into the fundamental workings of zinc phthalocyanine (ZnPc) as a photosensitizer for amorphous carbon nitride (ACN) in photocatalytic hydrogen (H₂) evolution from artificial seawater conditions. In this regard, ACN was doped selectively with ZnPc and platinum (Pt) to yield different photocatalyst composites, which include (i) ZnPc/ACN, (ii) Pt/ACN, and (iii) ZnPc/ACN/Pt with different degrees of ZnPc loading. The structural and optoelectronic properties and H-adsorption behaviors of the composites were studied experimentally and computationally via density functional theory. The results show that doping ZnPc onto ACN can extend the maximal light absorption by inheriting the Q-band absorbance from ZnPc with improved conductivity owing to the establishment of π-π conjugated interaction between ACN and ZnPc. In addition, the charge recombination of the ZnPc/ACN/Pt composite is suppressed due to facilitated charge transfer and renders a closer to thermoneutral free energy of H adsorption (ΔG_H ∼ 0) for H₂ evolution reactions (HER) with enhanced charge localization at the surface-active sites. As a result, optimal 2-ZnPc/ACN/Pt achieved the highest HER activity of 2.26 mmol g_cat^-1 in 6 h under simulated solar irradiation. Moreover, the charge transfer pathway and reaction mechanisms are postulated in relation to the electronic configuration of the composites. In this aspect, ZnPc bestows a photosensitizing effect and injects additional electrons into ACN upon light irradiation. On top of that, the diminishment in the H adsorption-desorption barrier corroborates the significance of ZnPc in engineering the adsorption kinetics to achieve highly efficient HER.