Structural and magnetic properties of highly Fe-doped ZnO nanoparticles synthesized by one-step solution plasma process

Abstract

Among many preparation methods for the synthesis of Fe-doped ZnO nanoparticles which have been widely investigated due to versatile properties and potential applications in optoelectronics, photovoltaics, and spintronic devices, solution-based methods such as sol-gel, hydrothermal, and precipitation processes usually consume many reagents and involve numerous steps. In this study, we report a one-step synthesis of highly Fe-doped ZnO nanoparticles (NPs) by a solution plasma process (SPP) using FeCl2 and FeCl3 as precursors without any addition of chemicals. Breakdown of water bubbles caused by plasma discharge led to the formation of hydroxyl radicals, which reacted with the dissolved Zn2+ ions producing Zn(OH)2 that was indeed converted to ZnO through thermal dehydration. Meanwhile, Fe ions were incorporated into the host ZnO lattice during the synthesis process. Fe-doping levels in Zn1-xFexO NPs are easily controllable by changing the Fe precursor concentration and plasma discharging time. We achieved a high doping content (x = 0.14–0.46) within 30 min, which is difficult to achieve using traditional solution-based synthesis approaches. All the Fe-doped NPs exhibit ferromagnetic behavior, but the magnitude is strongly dependent on the Fe content and doping ratio of Fe2+ to Fe3+ ions, which are explained by the dominance and competition between the ferromagnetic and antiferromagnetic exchange interactions through the presence of mixed Fe2+ and Fe3+ ions. The observed optimal ratio of Fe2+ to Fe3+ for maximizing the magnetization is approximately 3:7. It is noteworthy that the magnetization of Fe-doped ZnO NPs can be simply controlled by manipulating the Fe content and composition of Fe2+ and Fe3+ ions achieved through the SPP. This result provides a platform for studying fundamental magnetic properties of TM-doped semiconductors for potential spintronic applications. Thus, the SPP has great potential as an alternative strategy for the synthesis of highly Fe-doped ZnO NPs, which can be expanded to the synthesis of other doped metal-oxide nanostructures for a broad range of research applications.