Band Gap Engineering of ZnO using Core/Shell Morphology
with Environmentally Benign Ag<sub>2</sub>S Sensitizer for Efficient
Light Harvesting and Enhanced Visible-Light Photocatalysis
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Abstract
Band gap engineering offers tunable
optical and electronic properties of semiconductors in the development
of efficient photovoltaic cells and photocatalysts. Our study demonstrates
the band gap engineering of ZnO nanorods to develop a highly efficient
visible-light photocatalyst. We engineered the band gap of ZnO nanorods
by introducing the core/shell geometry with Ag<sub>2</sub>S sensitizer
as the shell. Introduction of the core/shell geometry evinces great
promise for expanding the light-harvesting range and substantial suppression
of charge carrier recombination, which are of supreme importance in
the realm of photocatalysis. To unveil the superiority of Ag<sub>2</sub>S as a sensitizer in engineering the band gap of ZnO in comparison
to the Cd-based sensitizers, we also designed ZnO/CdS core/shell nanostructures
having the same shell thickness. The photocatalytic performance of
the resultant core/shell nanostructures toward methylene blue (MB)
dye degradation has been studied. The results imply that the ZnO/Ag<sub>2</sub>S core/shell nanostructures reveal 40- and 2-fold enhancement
in degradation constant in comparison to the pure ZnO and ZnO/CdS
core/shell nanostructures, respectively. This high efficiency is elucidated
in terms of (i) efficient light harvesting owing to the incorporation
of Ag<sub>2</sub>S and (ii) smaller conduction band offset between
ZnO and Ag<sub>2</sub>S, promoting more efficient charge separation
at the core/shell interface. A credible photodegradation mechanism
for the MB dye deploying ZnO/Ag<sub>2</sub>S core/shell nanostructures
is proposed from the analysis of involved active species such as hydroxyl
radicals (OH<sup>•</sup>), electrons (e<sup>–</sup><sub>CB</sub>), holes (h<sup><b>+</b></sup><sub>VB</sub>), and superoxide
radical anions (O<sub>2</sub><sup>•–</sup>) in the photodegradation
process utilizing various active species scavengers and EPR spectroscopy.
The findings show that the MB oxidation is directed mainly by the
assistance of hydroxyl radicals (OH<sup>•</sup>). The results
presented here provide new insights for developing band gap engineered
semiconductor nanostructures for energy-harvesting applications and
demonstrate Ag<sub>2</sub>S to be a potential sensitizer to supersede
Cd-based sensitizers for eco-friendly applications