Effective Cascade Modulation of Charge-Carrier Kinetics in the Well- Designed Multi-Component Nanofiber System for Highly-Efficient Photocatalytic Hydrogen Generation

The photocatalytic reduction of water to hydrogen (H2) over semiconductors potentially offers an economic way to alleviate the global energy crisis and environmental pollution. Optimal modulation of charge-carrier kinetics is of great importance for enhancing the photocatalytic activity of semiconductors for reducing water to green H2. The design and manufacture of semiconductor-based heterostructure systems have emerged as promising tactics for modulating charge -carrier kinetics based on sensitization either via the semiconductor heterojunction effect or localized surface plasmon resonance. However, the cascade modulation of charge-carrier kinetics is still difficult to achieve through rationally coupling the abovementioned sensitization processes in well-designed heterostructures for highly-efficient photocatalytic H2 generation. In this study, we developed a novel quaternary hetero-component nanofibers (HNFs) system by assembling plasmonic Ag nanoparticles (NPs) and two different semiconductors of Ag2S NPs and g-C3N4 nanosheets (NSs) into the electrospun TiO2 nanofibers (NFs) via in situ oxidation (for g-C3N4 exfoliation and Ag2S) and reduction (for Ag) reactions. By combining time-resolved photoluminescence spectroscopy, three-dimensional finite-difference-time-domain simulation, and control experiments, we found that the overlapping absorption peak of plasmonic Ag NPs and g-C3N4 NSs could induce plasmonic resonant energy transfer from the Ag NPs to the neighboring g-C3N4, thereby improving the generation of photoinduced charge carriers of g-C3N4 in the quaternary HNFs system. Simultaneously, plasmonic hot electrons could be generated on the Ag NPs and transferred to the near-by hetero-components of TiO2, g-C3N4, and Ag2S, to boost the generation and separation of photoinduced charge carriers in the system. Furthermore, the energy band structure at the g-C3N4/TiO2 hetero-interface belongs to the "type II" heterojunction, while the energy band structure at the TiO2/Ag2S hetero-interface can be classified as a "type I" heterojunction. This way, the successive "energy band step" could be constructed at the g-C3N4/TiO2/Ag2S hetero-interface, resulting in improved separation and migration of photoinduced charge carriers through the transfer of photoinduced electrons from g-C3N4 to Ag2S across TiO2. Thus, the plasmonic resonant energy transfer, hot electron transfer, and successive energy-band-step-induced charge separation processes were integrated into the as-synthesized quaternary Ag/Ag2S/g-C3N4/TiO2 HNFs system, thereby achieving the effective cascade modulation of the generation, separation, and migration of photoinduced charge carriers. As such, the photocatalytic H2-generation rate of the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs system was higher than that of the mechanically mixed TiO2 NFs, g-C3N4 NSs, Ag NPs, and Ag2S NPs, with the same amounts as the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs photocatalyst, by approximately 9-fold under simulated sunlight irradiation. This interesting cascade modulation of charge-carrier kinetics might open new avenues for the development of highly active semiconductor-based heterostructure system for solar-to-fuels conversion.