High-pressure torsion to induce oxygen vacancies in nanocrystals of magnesium oxide: Enhanced light absorbance, photocatalysis and significance in geology

Magnesium oxide (MgO), which is an insulator ceramic with high significance in geology and earth sciences, is not generally considered as a photocatalyst due to its wide bandgap as similar to 7.8 eV. Experimental studies show that nanocrystalline MgO can have optical bandgaps down to 5.25-5.95 eV and density functional theory calculations suggest that the introduction of oxygen vacancies can reduce the optical bandgap below 5 eV. In this study, oxygen monovacancies and divacancies (F, F+ and F-2(+) centers) are experimentally introduced in nanocrystalline MgO by severe plastic deformation via the high-pressure torsion (HPT) method. The overall optical bandgap is successfully reduced to 3.9 eV after HPT processing. The HPT-processed samples exhibit photocatalytic dye degradation, while the degradation rate improves with increasing the imposed strain. These results not only confirm that the simultaneous introduction of oxygen vacancies and nanocrystals is quite effective to tune the optical properties of nanocrystalline MgO as a nature-friendly photocatalyst, but also introduce a pressure-strain-based mechanism for the formation of vacancy defects in MgO-based minerals.