Enhancement of commercial ZnO adsorption and photocatalytic degradation capacity of methylene blue by oxygen vacancy modification: Kinetic study

Abstract

The effect of the introduction of oxygen vacancies on commercial ZnO by hydrogen reduction on the adsorption and photocatalytic degradation of methylene blue is investigated. Commercial ZnO was reduced at 500 °C by a 10% H2/Ar gas with a temperature programmed reduction method. Both pristine and reduced photocatalysts were characterized by TPR, FE-SEM, EDS, XPS, XRD, BET, and PL analyses. The bandgap of the photocatalysts was estimated by Tauc plot method. The characterization results indicated that the morphology and crystallinity of the nanoparticles remained unchanged after the reduction treatment, while the ratio of the oxygen vacancy to the lattice oxygen was increased. Furthermore, the maximum valence band of the photocatalyst experienced an upward shift from 2.43 to 2.67 eV which resulted in a decrease of the bandgap from 3.22 eV in commercial ZnO to 3.07 eV in the reduced photocatalyst. To investigate the effect of hydrogen reduction on the adsorption in the dark and photocatalytic degradation of methylene blue, experiments were performed using both commercial and reduced photocatalysts at a pH range between 3 and 11. Our recently developed kinetic model, taking into account non-equilibrium adsorption and photocatalytic degradation via radicals (in solution) and electron-holes (at the photocatalyst surface), was used to describe the experimental data. The results showed that the concentration of active sites of the photocatalyst increased by ∼ 20% after the hydrogen reduction, resulting in higher adsorption and photocatalytic degradation. The highest adsorption takes place at pH 3, while the degradation rate and quantum yield reach their maximal value at pH 11. Finally, the definition of quantum yield is revisited in terms of actual degradation, rather than liquid concentration differences.