The effect of hydrochloric acid on photocatalytic hydrogen production of crystalline graphitic carbon nitride

Crystalline graphitic carbon nitride (CCN) synthesized by molten salt method has been widely used in many fields. However, the CCN prepared by the molten salt method always contains K ions. The influence of these K ions on the photocatalytic performance of CCN is still not clear. In this paper, three kinds of CCN with different structures were prepared by the molten salt method, including heptazine-based crystalline graphitic carbon nitride (HCN), triazine-based graphitic carbon nitride (TCN), and heptazine/triazine-based composite crystalline graphitic carbon nitride (HTCN). The present study elucidates the inhibitory mechanism of unstable potassium ions in carbonized carbon nitride (CCN) on photocatalytic hydrogen evolution through systematic acid treatment experiments. Notably, hydrochloric acid treatment effectively removed excess K⁺ ions from HTCN and HCN materials, significantly enhancing their photocatalytic performance. Specifically, the hydrogen evolution rate of HTCN-H (1170.4 μmol h⁻¹ g⁻¹) exhibited a 32.1% increase compared to pristine HTCN (885.9 μmol h⁻¹ g⁻¹). Mechanistic investigations revealed two critical functions of residual K⁺ ions: firstly, elevating the valence band position of CCN materials, thereby reducing their redox potential and electron transfer capability; secondly, occupying active catalytic sites that are essential for proton reduction reactions. However, there is no K ion in the structure of TCN, so hydrochloric acid treatment has no significant effect on its photocatalytic activity (from 49.1 to 54.9 μmol h⁻¹ g⁻¹). These findings not only clarify the dual negative effects of alkali metal residues in CCN-based photocatalysts but also provide a strategic approach for optimizing material performance through targeted ion removal.