ZnO quantum dots decorated 3DOM TiO2 nanocomposites: Symbiose of quantum size effects and photonic structure for highly enhanced photocatalytic degradation of organic pollutants

Three dimensionally ordered macroporous inverse opal TiO2 nanocomposites decorated by ZnO quantum dots (ZnO [email protected] TiO2) with an intimate contact were successfully synthesized using the sol-gel technique and characterized in terms of structure, porosity, chemical composition and optical properties. The photocatalytic activity of ZnO [email protected] TiO2 nanocomposites with different ZnO QDs amounts was evaluated in the aqueous phase of dye pollutant molecules and compared with the state-of-the-art 3DOM TiO2 and P25 photocatalysts. The symbiotic effect of ZnO QDs and 3DOM photonic structure on the light absorption and further on the photocatalytic activity of the nanocomposites was observed. The sample with the highest ZnO QDs amount exhibits extraordinarily high photocatalytic activity, which is attributed to firstly, the formation of an intimate junction between the two semiconductors, hence favoring the separation of photo-introduced electron–hole pairs in ZnO-TiO2 photocatalyst, and, secondly, to the quantum size effect (QSE). The QSE results in an increase in the width of the forbidden electronic band, which increases the energy of electrons (holes) in the conduction (valence) and particularly leads to the displacement of the conduction band potentials of ZnO to more negative energy values compared to TiO2. Thanks to the heterojuction formed between ZnO QDs and 3DOM TiO2, the energy difference between conduction bands of both semiconductors acts as a driving force for rapid electron/hole transfer between the coupled materials. Due to the extremely short diffusion time, the lifetime of photogenerated charge carriers is extended and the effectiveness of reduction and oxidation process is increased with faster reaction kinetics. Increasing the amount of ZnO QDs can boost the photocatalytic activity. On the other hand, 3DOM photonic structure of TiO2 with its open meso-macroporosity can facilite the diffusion of dye molecules and light propagation. This first successful example of symbiose of a series of physical effects can open a new window for solar energy conversion by the synergitic association of QSEs, photonic effect and other effects such as plasmonic effects, in one solid material to develop highly efficient solar light havesting system to enhance solar energy conversion effeciency for photocatalysis and photovoltaics.