Effective Cascade Modulation of Charge-Carriers Kinetics in the Well-Designed Multi-Component Nanofiber System for Highly-Efficient Photocatalytic H₂ Generation

Abstract

Photocatalytic reduction of water into H2 over semiconductors offers a potential economic way to alleviate the global energy crisis and environmental pollution. Optimal modulation of the charge-carriers kinetics behavior is of great importance to enhance the photocatalytic activity of semiconductors for reducing water into the green fuel of H2. Nowadays, design and manufacture of semiconductor-based heterostructure systems have emerged as a classic tactic to modulate the charge-carriers kinetics behavior based on the sensitization process of either the semiconductor heterojunction effect or localized surface plasmon resonance. However, it is still an arduous task to the cascade modulation of chargecarriers kinetics by rationally coupling the above sensitization processes in the welldesigned heterostructure towards highly-efficient photocatalytic H2 generation. In this contribution, we develop the novel quaternary hetero-component nanofibers (HNFs) system through assembling the plasmonic Ag nanoparticles (NPs) and two different semiconductors of Ag2S NPs and g-C3N4 nanosheets (NSs) into the electrospun TiO2 nanofibers (NFs) via the in situ oxidation (for g-C3N4 exfoliation and Ag2S) and reduction (for Ag) reactions. By combining the time-resolved photoluminescence spectroscopy, 3D finite-difference-time-domain simulation and control experiments, we demonstrate that the overlapped absorption spectra between plasmonic Ag NPs and g-C3N4 NSs could induce the plasmonic resonant energy transfer from Ag NPs to their neighboring g-C3N4, thereby improving the generation of the photoinduced charge-carriers of g-C3N4 in the above quaternary HNFs system. Meanwhile, the plasmonic hotelectrons could be also generated over Ag NPs, which further transfer to the contacted hetero-components of TiO2, g-C3N4, and Ag2S for boosting the generation and separation of the 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 should be classified as the "type I" heterojunction. In this way, the successive "energy band step" could be constructed at the g-C3N4/TiO2/Ag2S hetero-interface, which thereby results in the promotion of separation and migration of the photoinduced charge-carriers through the photo-induced electrons transfer from g-C3N4 to Ag2S across TiO2. Thus, the plasmonic resonant energy transfer, hot electron transfer processes, and the successive "energy band step"-induced charge separation process are integrated into the as-synthesized quaternary Ag/Ag2S/g-C3N4/TiO2 HNFs, thereby achieving the effective cascade modulation of the generation, separation and migration of the photoinduced charge-carriers. As such, the photocatalytic H2-generation rate of the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs is higher than that of the random mechanically mixed TiO2 NFs, g-C3N4 NSs, Ag NPs and Ag2S NPs with the same amounts to the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs by ~9 times upon...

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