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

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.