RESUMO
The magnetic properties and ozone (O3) gas-sensing activity of zinc ferrite (ZnFe2O4) nanoparticles (NPs) were discussed by the combination of the results acquired by experimental procedures and density functional theory simulations. The ZnFe2O4 NPs were synthesized via the microwave-assisted hydrothermal method by varying the reaction time in order to obtain ZnFe2O4 NPs with different exposed surfaces and evaluate the influence on its properties. Regardless of the reaction time employed in the synthesis, the zero-field-cooled and field-cooled magnetization measurements showed superparamagnetic ZnFe2O4 NPs with an average blocking temperature of 12 K. The (100), (110), (111), and (311) surfaces were computationally modeled, displaying the different undercoordinated surfaces. The good sensing activity of ZnFe2O4 NPs was discussed in relation to the presence of the (110) surface, which exhibited low (-0.69 eV) adsorption enthalpy, promoting reversibility and preventing the saturation of the sensor surface. Finally, the O3 gas-sensing mechanism could be explained based on the conduction changes of the ZnFe2O4 surface and the increase in the height of the electron-depletion layer upon exposure toward the target gas. The results obtained allowed us to propose a mechanism for understanding the relationship between the morphological changes and the magnetic and O3 gas-sensing properties of ZnFe2O4 NPs.
RESUMO
Novel magnetic metals and metal oxides that use both the spin and charge of an electron offer exciting technological applications. Their discovery could boost research on functional nanoscale materials. Here, for the first time, we report the magnetization of α-Ag2WO4 under electron beam and femtosecond laser irradiation. The formation and growth of silver oxides (AgO, Ag2O, and Ag3O4) and Ag nanofilaments can be observed on the surface of α-Ag2WO4 crystals. These features were also present in the composition of an extruded material and could open new avenues for surface magnetism studies. In order to understand these results, we used first-principles density functional theory calculations. This allowed us to investigate several potential scenarios for controlling magnetic properties. The effect of electron addition on the crystalline structures of α-Ag2WO4, Ag3O4, Ag2O, and AgO has been analyzed in detail. The creation of Ag and O vacancies on these compounds was also analyzed. Based on structural and electronic changes at the local coordination site of Ag, a mechanism was proposed. The mechanism illustrates the processes responsible for the formation and growth of metallic Ag and the magnetic response to electron beam irradiation.