Rare-Earth-Based MIS Type Core-Shell Nanospheres with Visible-Light-Driven Photocatalytic Activity through an Electron Hopping-Trapping Mechanism.

Abstract

A bilayered rare-earth-based metal–insulator–semiconductor, Dy2O3@SiO2@ZnO core–shell nanospheres, was synthesized by a stepwise synthesis for enhanced visible photocatalytic activity. The prepared material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, ultraviolet–visible diffuse reflectance spectroscopy, field-emission scanning electron microscopy, energy-dispersive spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller, and electron paramagnetic resonance techniques. Dy2O3@SiO2@ZnO core–shell nanospheres were found be in a spherically arranged cauliflower-like morphology (40–60 nm). The high-resolution transmission electron microscopy analysis proved the core–shell morphology of the prepared material with a single Dy2O3 core and two shells comprising SiO2 and ZnO. The material possessed a surface roughness of 4. 98 nm (2 × 2 μm area) and a band gap energy of 2.82 eV. The in situ generation of OH radicals was confirmed by electron paramagnetic resonance. Electron hopping through the SiO2 layer from ZnO to Dy2O3 played a major role in trapping electrons in the f-shells of lanthanides, thus, preventing the recombination of electron–hole pair. X-ray photoelectron spectroscopy studies proved the band alignment of the material. Brunauer–Emmett–Teller analysis further showed the core–shell surface area was 14 m2/g. The visible photocatalytic activity was tested against 2,4-D (2,4-dichlorophenoxyacetic acid), an endocrine disruptor. The kinetic studies showed that the photocatalytic degradation process followed a pseudo-first-order pathway. The photocatalyst was found to be reusable even up to the third cycle.