Synthesis of phase-pure superparamagnetic nanoparticles of ZnFe2O4 via thermal decomposition of zinc-iron layered double hydroxysulphate

Abstract Superparamagnetic nanoparticles of zinc ferrite ZnFe2O4 were obtained by air heat treatment of iron and zinc sulfated layered double hydroxide (SLDH) Zn2Fe4(OH)12SO4·8H2O at 600°С for 1 h. The initial sulfated hydroxide was obtained on a silicon substrate as a result of 30 cycles of successive ionic layer deposition from 0.01 M solutions of the Mohr salt (NH4)2Fe(SO4)2 and zinc ammonia complex [Zn(NH3)4](NO3)2. Iron and zinc SLDH nanocrystals have the nanosheets morphology with an average thickness of 3–7 nm and a diameter of about 17 nm which are oriented perpendicular to the silicon substrate plane. Physico-chemical processes which occur during thermal treatment (25–600°С) of iron and zinc sulfated layered double hydroxide and lead to the formation of zinc ferrite at 550°С are investigated by simultaneous thermal analysis. It was found that ZnFe2O4 nanocrystals with an average size of about 30 nm and isometric morphology, which partially preserve the morphological motifs of the original SLDH nanosheets, were formed as a result of the thermal treatment of sulfated hydroxide. The obtained ZnFe2O4 nanocrystals are chemically and phase-pure, have lattice parameters a = b = c = 8.4394 A and behave like superparamagnetic nanoparticles: they are characterized by the magnetization of 1.85 (298 K) and 12.79 (77 K) emu/g with 18 kOe applied field, and the effective magnetic moment is μeff = 24.3 μB per formula unit. This phase-pure superparamagnetic ZnFe2O4 nanoparticles can be potentially used as the basis for magnetic fluids, gas sensors, drugs for hyperthermia and magnetic resonance imaging diagnostics.