Cyclodextrin-Directed Synthesis of Gold-Modified TiO2 Materials and Evaluation of Their Photocatalytic Activity in the Removal of a Pesticide from Water: Effect of Porosity and Particle Size

Visible-light-driven plasmonic photocatalysts are promising candidates for environmental cleanup applications owing to their high solar energy utilization efficiency. In this Letter, we describe a facile and versatile cyclodextrin (CD)-driven colloidal self-assembly approach toward visible-light-responsive Au/TiO2 photocatalysts for the degradation of the herbicide phenoxyacetic acid (PAA) in water. The effect of several cyclodextrins, natives and modified, is investigated. Our results reveal that these cyclic oligosaccharides exert antagonistic actions on the size of metal particles and the porosity of composite materials. Indeed, because of their surface-active properties and weak intermolecular interactions, the randomly methylated ss-CD and the 2-hydropropoxyl ss-CD produce large Au particles dispersed over a highly porous TiO2 material, whereas the native alpha-CD, ss-CD and ss-CD, owing to their ability to self-assemble into cage-type structures through intermolecular hydrogen-bond interactions, allow us to fabricate small Au particles dispersed over a dense and compact network. Among the various cyclodextrins investigated, the randomly methylated ss-CD leads to Au/TiO2 composites with an appropriate combination of interconnected pore structure, large surface area, high crystallinity and optimum Au particle size for the best photocatalytic activity under visible light irradiation. Our findings highlight the pivotal role played by the chemical nature of the substituents in the CD ring on the structural and textural characteristics of the composites as well as on their photocatalytic degradation efficiency.