Role of vanadium ions, oxygen vacancies, and interstitial zinc in room temperature ferromagnetism on ZnO-V(2)O(5) nanoparticles

In this work, we present the role of vanadium ions (V(+5) and V(+3)), oxygen vacancies (V(O)), and interstitial zinc (Zn(i)) to the contribution of specific magnetization for a mixture of ZnO-V(2)O(5) nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10(−3) to 3.5?×?10(−3) emu/gr. Pure ZnO powders (1.34?×?10(−3) emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zn(i), while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of V(O) concentration. For the ZnO-V(2)O(5) system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V(2)O(3) and secondary phases containing V(+3) ions; at the same time, an increase of V(O) is observed with an abrupt fall of magnetization to σ?~?0.7?×?10(−3) emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including V(O) and Zn(i) depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zn(i) at the same time that V(2)O(5) NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of V(O) and Zn(i) was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.