Enhanced magnetic ordering, and microwave-shielding and photocatalytic performance in hydrogenated ZnO nanoparticles

Optical, photocatalytic, magnetic and microwave-shielding properties correlated with electronic structures of ZnO nanoparticles (NPs) hydrogenated at the annealing temperature (T_an) ranging from 400 to 900 °C have been investigated. All fabricated samples are hexagonally monophasic with more lattice defects generated by hydrogenation that have changed the bandgap energy, and the features of luminescence and Raman-scattering spectra. Hydrogenation-induced defects also stimulated an anomalous Raman mode (AM) peaked at ∼ 477 cm⁻¹. Interestingly, all samples exhibit ferromagnetism. Magnetic ordering gradually increases when T_an increases from 400 to 700 °C, but decreases for T_an > 700 °C. Upon the results obtained from analyzing X-ray absorption, magnetic-resonant and luminescent spectra, we believe that hydrogenation-induced ferromagnetism and AM are mainly due to Zn-related defects. These together with other hydrogen-related defects could form clusters to cause the magnetic coercivity. For T_an > 700 °C, the defects migrate to grain surfaces/boundaries while unstable defects move out of the ZnO lattice that reduce magnetic ordering. We have also found ZnO NPs hydrogenated at 700 °C with suitable thicknesses absorbing above 99.5 % incident microwaves at frequencies 8.6 and 10.4 GHz, corresponding to reflection-loss magnitudes of 25 ∼ 38 dB. Concurrently, this sample shows the best photocatalytic performance of Rhodamine-B (RhB) degradation, about 67 % RhB decomposed under visible light irradiation for 3 h. Though its photodegradation kinetics obey the pseudo-first-order model, their pseudo-rate constant is dependent on irradiation time, which is related to the location of accessible-active sites on both surface and internal layers of NPs.