Ultrafast Electron Transfer from Crystalline g-C3N4 to Pt Revealed by Femtosecond Transient Absorption Spectroscopy

Increasing the crystallinity of g-C3N4 is an effective strategy to simultaneously accelerate charge carrier mobility and reduce structural defects to ultimately boost photocatalytic performance. However, current studies mainly focus on basic characterizations such as spectral absorption and morphology control, whereas experimental evidence on charge carrier dynamics is yet to be provided. Herein, crystalline g-C3N4 was prepared by post-treatment of bulk g-C3N4 in KCl–LiCl eutectic mixtures at 550 °C to unravel the charge carrier dynamics in controlling photocatalytic performance. We found that the as-prepared crystalline g-C3N4 achieved a photocatalytic hydrogen evolution rate of 37.0 μmol·h–1, a ca. 28-time enhancement of pristine g-C3N4 without KCl–LiCl treatment. Femtosecond transient absorption spectroscopy demonstrated that 59.5% of the photogenerated electrons were transferred to Pt within 1.6 ps for crystalline g-C3N4, whereas only 21.3% were transferred at a longer 11.8 ps for pristine g-C3N4. Hence, more electrons are transferred and ultrafast electron mobility is achieved for crystalline g-C3N4, which is responsible for its enhanced performance. Intriguingly, two shallow-trapped species were identified in crystalline g-C3N4, while a deep-trapped species and a species associated with charge carrier recombination were observed in pristine g-C3N4. The shallow-trapped species could then migrate to participate in the following proton reduction, providing further evidence of its superior activity. Therefore, this study highlights the effectiveness of crystalline g-C3N4 in promoting charge carrier migration and suppressing charge carrier recombination to boost photocatalytic hydrogen evolution.