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

Abstract 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 (C3N5) on sulfone-containing covalent organic framework (FS-COF) nanosheets (NSs), named as the C3N5/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 C3N5 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 C3N5 and FS-COF. In situ X-ray photoelectron spectroscopy (XPS) further confirms that, under visible-light irradiation, photogenerated electrons are retained on C3N5, whereas photogenerated holes are accumulated on FS-COF. Moreover, photodeposition experiments using Pt and RuO2 as electron- and hole-trapping probes verify the preferential localization of photogenerated electrons on C3N5 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%-C3N5/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 C3N5 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.