Abstract: Crystalline carbon nitride (CCN) prepared by a molten-salt method is attracting increased attention because of its promising properties and excellent photocatalytic activity. In this work, we further improve the crystallinity of CCN through synthesis by the molten-salt method under the action of aqueous hydrochloric acid (HCl) solution. Our results showed that the crystallinity of the as-prepared samples increased with increasing HCl concentration and reached the maximum value at 0.1 mol L^-1. This can be attributed to the removal of some potassium ions (K⁺) from the terminal amino groups of CCN by the aqueous HCl solution, which results in a release of the polymerization sites. As a result, the crystallinity of the as-prepared samples further increased. Moreover, the obtained 0.1 highly crystalline carbon nitride (0.1HCCN; treated with 0.1 mol L^-1 aqueous HCl solution) exhibited an excellent photocatalytic hydrogen evolution of 683.54 µmol h^-1 g^-1 and a quantum efficiency of 6.6% at 420 nm with triethanolamine as the sacrificial agent. This photocatalytic hydrogen evolution was 2 and 10 times higher than those of CCN and bulk carbon nitride, respectively. The enhanced photocatalytic activity was attributed to the improved crystallinity and intercalation of K⁺ into the xHCCN interlayer. The improved crystallinity can decrease the number of surface defects and hydrogen bonds in the as-prepared sample, thereby increasing the mobility of the photoinduced carriers and reducing the recombination sites of the electron-hole pairs. The K⁺ intercalated into the xHCCN interlayer also promoted the transfer of the photoinduced electrons because these ions can increase the electronic delocalization and extend the π-conjugated systems. This study may provide new insights into the further development of the molten-salt method.

Key words: Crystalline carbon nitride, Molten-salt method, Photocatalytic hydrogen evolution, Potassium ions, Aqueous hydrochloric acid solution