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Optical Quantum Confinement in Ultrasmall ZnO and the Effect of Size on Their Photocatalytic Activity

Cite this: J. Phys. Chem. C 2020, 124, 11, 6395–6404
Publication Date (Web):February 17, 2020
Copyright © 2020 American Chemical Society

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    Abstract Image

    Zinc oxide is a well-known metal oxide semiconductor with a wide direct band gap that offers a promising alternative to titanium oxide in photocatalytic applications. ZnO is studied here as quantum dots (QDs) in colloidal suspensions, where ultrasmall nanoparticles of ZnO show optical quantum confinement with a band gap opening for particles below 9 nm in diameter from the shift of the band edge energies. The optical properties of growing ZnO QDs are determined with Tauc analysis, and a system of QDs for the treatment and degradation of distributed threats is analyzed using an organic probe molecule, methylene blue, whose UV/vis spectrum is analyzed in some detail. The effect of optical properties of the QDs and the kinetics of dye degradation are quantified for low-dimensional ZnO materials in the range of 3–8 nm and show a substantial increase in photocatalytic activity compared to larger ZnO particles. This is attributed to a combined effect from the increased surface area as well as a quantum confinement effect that goes beyond the increased surface area. The results show a significantly higher photocatalytic activity for the QDs between 3 and 6 nm with a complete decolorization of the organic probe molecule, while QDs from 6 nm and upward in diameter show signs of competing reduction reactions. Our study shows that ultrasmall ZnO particles have a reactivity beyond that which is expected because of their increased surface area and also demonstrates size-dependent reaction pathways, which introduces the possibility for size-selective catalysis.

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    • Typical absorption spectrum of PMMA cuvette; absorbance spectra of methylene blue; conductivity of methylene blue aqueous solutions; self-degradation of MB(aq) and MB(H2O:EtOH); experimentally recorded absorbance spectra; and experimentally recorded absorbance spectra during photocatalysis(PDF)

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