Mesoporous Co_3O_4 nanostructured materiatsynthesized by one-step soft-templating: A magnetic study

摘要

A combined magnetization and zero-field ~(59)Co spin-echo nuclear magnetic resonance (NMR) study has been carried out on one member of a recently developed class of highly ordered mesoporous nanostructured materials, mesoporous Co_3O_4 (designated UCT-8, University of Connecticut, mesoporous materials). The material was synthesized using one-step soft-templating by an inverse micelles packing approach. Characterization of UCT-8 by powder x-ray diffraction and electron microscopy reveals that the mesostructure consists of random close-packed CO3O4 nanoparticles ≈ 12 nm in diameter. The N_2 sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m~2/g BET surface area and a 7.7 nm BJH desorption pore diameter. The effect of heat treatment on the structure is discussed. The antiferromagnetic Co_3O_4 nanoparticles have a Neel temperature T_N = 27K, somewhat lower than the bulk. A fit to the Curie-Weiss law over the temperature range 75 K ≤ T ≤ 300 K yields an effective magnetic moment of μ_(eff) = 4.36μ_B for the Co~(2+) ions, indicative of some orbital contribution, and a Curie-Weiss temperature Θ = -93.5 K, consistent with antiferromagnetic ordering. The inter-sublattice and intra-sublattice exchange constants for the Co~(2+) ions are J_1/k_B = (-)4.75 K and J_2/k_B= (-)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below T_N with shifts in the hysteresis loops, i.e., an exchange-bias effect. The ~(59)Co NMR spectrum for UCT-8, which is attributed to Co~(2+) ions at the tetrahedral A sites, is asymmetrically broadened with a peak at ≈55 MHz (T = 4.2 K). Since there is cubic symmetry at the A-sites, the broadening is indicative of a magnetic field distribution due to the uncompensated surface spins. The spectrum is consistent with antiferromagnetically ordered particles that are nanometer in size and single domain.