Monodisperse Ni_xFe_3-xO₄ nanospheres: Metal-ion-steered size/composition control mechanism, static magnetic and enhanced microwave absorbing properties

Highlights: • A metal-ion-steered solvothermal method for synthesizing Ni_xFe_3-xO₄ nanospheres. • Proposing an in situ-reduction, coordination-precipitation transformation mechanism. • Investigating size- and composition-dependent static magnetic properties. • Investigating size- and composition-dependent microwave absorbing properties. - Abstract: An easy metal-ion-steered solvothermal method was developed for the one-step synthesis of monodisperse, uniform Ni_xFe_3-xO₄ polycrystalline nanospheres with tunable sphere diameter (40–400 nm) and composition (0 ≤ x ≤ 0.245) via changing just Ni²⁺/Fe³⁺ molar ratio (γ). With g increased from 0:1 to 2:1, sphere diameter gradually decreased and crystal size exhibited an inversed U-shaped change tendency, followed by increased Ni/Fe atom ratio from 0% to 0.0888%. An in situ-reduction, coordination-precipitation transformation mechanism was proposed to interpret the metal-ion-steered growth. Size- and composition-dependent static magnetic and microwave absorbing properties were systematically investigated. Saturation magnetization declines with g in a Boltzmann model due to the changes of crystal size, sphere diameter, and Ni content. The coercivity reaches a maximum at γ = 0.75:1 because of the critical size of Fe₃O₄ single domain (25 nm). Studies on microwave absorption reveal that 150–400 nm Fe₃O₄ nanospheres mainly obey the quarter-wavelength cancellation model with the single-band absorption; 40–135 nm Ni_xFe_3-xO₄ nanospheres (0 ≤ x ≤ 0.245) obey the one and three quarter-wavelength cancellation model with the multi-band absorption. 150 nm Fe₃O₄ nanospheres exhibit the optimal EM wave-absorbing property with an absorbing band of 8.94 GHz and the maximum R_L of −50.11 dB.