title: Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals.
abstract: We report facile solution approaches for the phase-controlled synthesis of rock-salt cubic CoO (c-CoO) and wurtzite-type hexagonal CoO (h-CoO) nanocrystals. In the syntheses, the cobalt precursor cobalt (II) stearate is decomposed in 1-octadecene at 320 °C, and the crystalline phase of synthesized products depend critically on the amounts of H2O. While the presence of small amounts of H2O promotes the generation of c-CoO, h-CoO is obtained in the absence of H2O. The as-prepared c-CoO nanocrystals exhibit a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together whereas the h-CoO nanocrystals show a multi-rod structure with several rods growing on the same base facet along the c-axis. The formation mechanisms are discussed on the basis of FTIR spectrometry data and color changes of the reaction mixture. Finally the magnetic properties of as-prepared CoO nanocrystals are measured and the results show that c-CoO nanocrystals are intrinsically antiferromagnetic with a Néel temperature of about 300 K but the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Weak ferromagnetic contributions are also observed for both c-CoO and h-CoO nanocrystals with obvious magnetic hysteresis at 5 and 300 K. The uncompensated spins that can be induced by crystalline defects such as cation-vacancy may account for the observed weak ferromagnetism.
authors.affiliation: Department of Materials Science and Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, College of Materials, Xiamen University, Xiamen 361005, People's Republic of China.
authors.forename: Qiongqiong
authors.initials: Q
authors.lastname: Qi
authors.forename: Yuanzhi
authors.initials: Y
authors.lastname: Chen
authors.forename: Laisen
authors.initials: L
authors.lastname: Wang
authors.forename: Deqian
authors.initials: D
authors.lastname: Zeng
authors.forename: Dong-Liang
authors.initials: DL
authors.lastname: Peng