Crystallization, cyanamide defect and ion induction of carbon nitride: Exciton polarization dissociation, charge transfer and surface electron density for enhanced hydrogen evolution

Abstract

Exciton polarization dissociation, charge transfer and surface electron density in photocatalysts are crucial for photocatalytic hydrogen evolution reactions (HERs). In this study, crystalline carbon nitride with cyanamide defect edges (crystalline CCN) was synthesized by one step polymerization of urea in the presence of KCl. The texture and electronic band structure of carbon nitrides could be facilely tailored by changing KCl dosage. The light absorption edge of crystalline CCN extended to 736 nm due to n→π* electron transition. The enhanced dielectric constants of crystalline carbon nitrides promoted exciton polarization dissociation. The small effective electron mass (me*) in crystalline CCN facilitated me* diffusion. The efficient separation of electrons and holes benefited the formation of internal electric field, showing an 8.56-fold promotion in electron transfer compared to pristine CN. Significantly, femtosecond time-resolved transient absorption demonstrated that the surface electron density on crystalline CCN was enhanced in the presence of salt ions (NaCl). As a result, crystalline CCN exhibited 14.9 times higher HER rate than pristine CN under visible light irradiation. The apparent quantum yield for H2 evolution on crystalline CCN reached to 42% at 420 nm and 9% at 500 nm. This study gets a comprehensive understanding of photocatalytic HERs using carbon nitride photocatalysts.