Abstract:

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.

Key words: Cascade modulation, Charge-carrier kinetics, Ag/Ag₂S/g-C₃N₄/TiO₂ hetero-component nanofibers, Broad spectral response, Photocatalytic H₂ generation