Mesoporous Co3O4 nanostructured material synthesized by one-step soft-templating: A magnetic study
Mesoporous Co3O4 nanostructured material synthesized by one-step soft-templating: A magnetic study
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DOI：
10.1063/1.4868680
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发表时间：
2014-03
影响因子：
3.2
通讯作者：
A. Poyraz;W. Hines;Chung-Hao Kuo;Nan Li;D. Perry;S. Suib
中科院分区：
文献类型：
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作者：
A. Poyraz;W. Hines;Chung-Hao Kuo;Nan Li;D. Perry;S. Suib
对最近发展起来的一类高有序介孔纳米结构材料——介孔Co3O4（命名为UCT-8，康涅狄格大学介孔材料）进行了磁化和零场59Co自旋回波核磁共振（NMR）研究。采用反胶束填充法一步软模板法合成了该材料。通过粉末x射线衍射和电子显微镜对UCT-8进行了表征，结果表明，UCT-8由直径约为12 nm的Co3O4纳米颗粒组成。UCT-8的N2吸附等温线为ⅳ型，滞回线为H1型，其BET表面积为134 m2/g， BJH解吸孔径为7.7 nm。讨论了热处理对组织的影响。反铁磁性Co3O4纳米颗粒的尼尔温度为TN = 27 K，略低于本体。在75 K≤T≤300 K的温度范围内，Co2+离子的有效磁矩为μeff = 4.36 μB，表明有一定的轨道贡献，居里-魏斯温度Θ =−93.5 K，符合反铁磁有序。亚晶格间和亚晶格内Co2+离子的交换常数分别为J1/kB =(−)4.75 K和J2/kB =(−)0.87 K，均对应于反铁磁耦合。在TN以下观察到无补偿表面自旋的存在，并伴有迟滞回路的移位，即交换偏置效应。UCT-8的59Co核磁共振波谱在约55 MHz （T = 4.2 K）处出现不对称展宽，这是由于四面体A位的Co2+离子引起的。由于在a位处存在立方对称，因此展宽表明由于未补偿的表面自旋而导致的磁场分布。光谱与纳米尺寸和单畴的反铁磁有序粒子相一致。
A combined magnetization and zero-field 59Co 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 Co3O4 (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 N2 sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m2/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 Co3O4 nanoparticles have a Neel temperature TN = 27 K, 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 μeff = 4.36 μB for the Co2+ 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 Co2+ ions are J1/kB = (−)4.75 K and J2/kB = (−)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below TN with shifts in the hysteresis loops, i.e., an exchange-bias effect. The 59Co NMR spectrum for UCT-8, which is attributed to Co2+ 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.