Revealing the Spatially Resolved Charge Transfer and Ultrafast Charge Kinetics in 2D/2D S-Scheme Heterojunction Photocatalysts

Two-dimensional/two-dimensional (2D/2D) S-scheme heterojunctions with enlarged interfacial contact have garnered considerable interest as photocatalysts for hydrogen generation. However, the spatially resolved charge transfer and ultrafast charge kinetics in 2D/2D S-scheme heterojunctions remain unclear. To address this, a 2D/2D S-scheme heterojunction was prepared in this work through in situ growth of nitrogen-rich graphite carbon nitride (C 3 N 5 ) on sulfone-containing covalent organic framework (FS-COF) nanosheets (NSs), named as the C 3 N 5 /FS-COF heterojunction. Crucially, light-assisted Kelvin-probe force microscopy (KPFM) was employed to uncover the spatially resolved features of charge transfer at the nanoscale heterointerface, which demonstrates that the photogenerated electrons accumulate on the C 3 N 5 surface in the heterojunction, while the photogenerated holes are enriched on the FS-COF surface, resulting from the formation of a strong internal electric field (IEF) due to the work function difference between C 3 N 5 and FS-COF. In situ X-ray photoelectron spectroscopy (XPS) further confirms that, under visible-light irradiation, photogenerated electrons are retained on C 3 N 5 , whereas photogenerated holes are accumulated on FS-COF. Moreover, photodeposition experiments using Pt and RuO 2 as electron- and hole-trapping probes verify the preferential localization of photogenerated electrons on C 3 N 5 and photogenerated holes on FS-COF, respectively. Furthermore, femtosecond transient absorption spectroscopy (fs-TAS) was utilized to track the ultrafast charge kinetics. The fs-TAS results demonstrate the significantly prolonged lifetime of higher-energy excited-state electrons and the improved separation efficiency of photogenerated charge carriers. Consequently, the optimized photocatalyst, 75%-C 3 N 5 /FS-COF, exhibits a hydrogen production rate of 0.83 mmol h–1 at the stationary point with the photocatalyst concentration of 350 mg L–1, equivalent to the mass-normalized value of 23.7 mmol g–1 h–1, which is 98.8 and 1.9 times higher than those of C 3 N 5 and FS-COF, respectively. Overall, this work provides a multidimensional understanding of spatially resolved charge transfer and ultrafast charge kinetics, offering insights into the precise design of high-performance 2D/2D S-scheme photocatalysts.