Engineering photocatalysts towards high-performance solar hydrogen production

Abstract

The production of chemical fuels by solar energy conversion is regarded as one of the major strategies to address the aggravating energy and environmental problems. Particularly, photocatalytic water splitting has attracted tremendous attention since it represents a clean, cost-effective and environmental-benign technique for solar hydrogen (H2) production. The core challenge of this promising technology lies in the development of low-cost and environmentally-friendly photocatalyst/co-catalyst systems with high activity and stability. However, to date most of the photocatalysts and co-catalysts are based on transitional metals and noble metals (e.g. Pt), respectively, which are neither economic nor environmental-benign. Therefore, the development of metal-free photocatalysts and noble-metal-free co-catalysts are highly desirable. This thesis aims to design and fabricate different highly-active and stable metal-free photocatalyst and noble-metal-free co-catalyst to achieve high-efficient solar H2 production. The first part of this thesis focuses on developing high-performance and low-priced metal-free graphitic carbon nitride (g-C3N4) photocatalysts as an alternative to current metal-based photocatalysts. Porous P-doped g-C3N4 nanosheets (PCN-S) were firstly synthesized by combining P doping and thermal exfoliation strategies. The P-doped conjugated system and novel macroporous nanosheet morphology synergistically contribute to the outstanding photocatalytic H2-production activity of PCN-S under visible-light irradiation. Furthermore, the P doping was found to create empty midgap states in PCN-S, which greatly extend the light-responsive region; whilst the novel macroporous structure benefits the mass-transfer process and enhances light harvesting. This work not only demonstrates an easy, eco-friendly and scalable strategy to synthesize highly efficient porous g-C3N4 nanosheet photocatalysts, but also paves a new avenue for the rational design and synthesis of advanced photocatalysts by harnessing the strong synergistic effects through simultaneously tuning and optimizing the electronic, crystallographic, surface and textural structures. The second part of this thesis is to design and synthesize a series of earth-abundant co-catalysts for replacing rare and expensive Pt. Ni-based co-catalysts, e.g. NiS, metallic Ni and Ni(OH)2, were deposited on ZnxCd1-xS nano-particles (NPs) to greatly enhance their visible-light photocatalytic H2-production activity. Particularly, Ni(OH)2 loaded ZnxCd1-xS shows superior photocatalytic performance to Pt-loaded ZnxCd1-xS under the identical conditions. This outstanding performance originates from the notable synergetic effect between Ni(OH)2 and metallic Ni formed in situ during the photocatalytic reaction. We also for the first time designed and fabricated a novel MXene material, Ti3C2 NPs, as a highly-efficient co-catalysts. Ti3C2 NPs were rationally incorporated with CdS by a hydrothermal technique to achieve a super high visible-light photocatalytic H2-production performance. This remarkable performance results from the optimized Fermi level, efficient hydrogen evolution capacity and novel active sites on Ti3C2 NPs. Our work demonstrates the huge potential of earth-abundant MXene family materials to fabricate numerous high-performance and low-cost photocatalysts/photoelectrodes. The third part of this thesis aims to reveal the superior electron extracting capacity of Ti3C2 NPs on ZnxCd1-xS towards visible-light induced H2 production. Through combining the experimental techniques and theoretical computations, we have explored the critical role of Ti3C2 NPs loaded on the surface of ZnxCd1-xS, which greatly promotes the vectorial electron transfer from ZnxCd1-xS to Ti3C2 NPs. The as-synthesized Ti3C2 modified ZnxCd1-xS composite exhibits the highest photocatalytic H2-production activity of 7196 mol h-1 g-1 at the optimal loading content of 4 wt%. This work demonstrates the possibility of using Ti3C2 to replace expensive Pt in photocatalytic H2 production.