Bio-inspired Hierarchical CdS/Au/TiO₂Photocatalysts For Z-scheme Type Water Splitting: Synthesis And Photocatalytic Properties

The development of human society based on fossil fuels results in asignificant negative impact on environment, and fossil fuels are coming todepletion in21stcentury, forcing people to seek for clean and renewableenergy. As clean and almost infinitive, solar energy provides a possible wayto relieve these issues. Hydrogen, as another clean energy, has found itsapplication in industrial production and fuel cells. Photocatalytic watersplitting provides a promising way of unitizing solar energy and converting itto hydrogen, and it has always been a focus topic around world thatdeveloping efficient photo catalysts for water splitting.Inspired by the way in which natural species utilize solar energy throughthe combination of components and structure, we combined CdS/Au/TiO₂Z-scheme type photosystem with biological hierarchical structure toaccelerate water splitting efficiency. The construction of biomorphicCdS/Au/TiO₂Z-scheme type photosystem with hierarchical structures is a more comprehensive way of mimicking natural Z-scheme type photosynthesis.In this article, we used natural leaves and black wings of butterflies astemplates to synthesis CdS/Au/TiO₂Z-scheme type photosystem withbiological hierarchical structures, and compared photocatalytic H2evolutionrate of samples with different components. The main contents and results asfollows:（1） Inspired by the way in which natural leaves utilizes solar energy throughthe combination of photosynthesis natural leaves’ hierarchical structure, theleaves of Cherry blossom were used as templates to synthesizeCdS/Au/N-TiO₂artificial Z-scheme type photosysytem. The uniquearchitectures-hierarchical macro/mesoporous morphology of natural leavesare retained in the synthesized systems to enhance overall light harvesting andto offer more absorption and reaction sites for the catalytic reactions. Thephotocatalytic modules—CdS（shell）/Au（core）/N-TiO₂heterostructures areserved as a prototype here to demonstrate this concept, in which N-TiO₂andCdS serve as PS II and PS I, respectively, while Au acts as the electrontransfer mediator, contributing to the enhancement of electron hole separationand interfacial charge transfer. The contents and thicknesses of CdS shells arecontrolled to obtain an optimized activity. The H2evolution rates of optimizedCdS/Au/N-TiO₂heterostructures are about2.6times of N-TiO₂under UV/visible light, and about270times of Au/N-TiO₂under visible lightirradiation. The systems have high visible light harvesting, high hydrogenevolution rate and long electron-hole lifetimes compared withnon-incorporated systems. The design of this system is based on bothbiological morphology and mechanism paradigms, which would provide aproof of concept for the bio-inspired design of artificial photosynthetic systemfor enhanced performance.（2） Inspired by the antireflection architecture of butterfly wings and naturalphotosynthesis, the wings of butterfly Papilio nephelus Boisduva were usedas templates to synthesize CdS/Au/TiO₂Z-scheme photosystem with thebutterfly wing architecture. This combination of artificial Z-schemephotosystem and butterfly wing’s hierarchical architecture was expected toenhance the light harvesting and water splitting efficiency. Thefinite-difference time-domain （FDTD） stimulation was applied to demonstratethe optical function of the architecture inherited from butterfly wingtheoretically, and UV-vis spectra and photocatalytic H2evolution rate werefurther recorded to experimentally demonstrate the coupled effect of butterflywing architecture and CdS/Au/TiO₂Z-scheme components. The FDTDstimulation shows that the architecture of the wing scale TiO₂effectively reduced the UV light reflection by about40%. Meanwhile, the wing scalearchitecture model exhibited lower UV reflection and transmission in waterthan those in air, which can be attributed to the stronger diffuse reflection inwater. UV-vis spectra and photocatalytic H2evolution experiments confirmedthat the combination of the wing scale architecture and CdS/Au/TiO₂Z-scheme components contributed to the enhancement of the light harvestingability and improved the water-splitting efficiency by200%compared to thenone-architecture TiO₂.Compared to those which focused on either components or structuraloptimization, the combination of the two is a more comprehensive way ofmimicking natural photosynthesis. By exploring the basic principle behindthe phenomena that Nature utilizes solar energy, we can extract morearchitecture prototypes and find out the chemical working mechanisms for theconstruction of more efficient solar cells and photocatalysts. Moreover, asNature has such a great variety of life forms, exploration into them willprovide inexhaustible inspiration for our strategies to address a serious ofpressing issues facing human today like energy crisis and environmentalpollution.