Characterization of Monodisperse Wu ¨ stite Nanoparticles following Partial Oxidation Chih-Jung Chen, †,‡ Ray-Kuang Chiang, †, * Hsin-Yi Lai, ‡ and Chun-Rong Lin § Nanomaterials Laboratory, Department of Materials Science and Engineering, Far East UniVersity, Hsing-Shih, Tainan County 74448, Taiwan, Department of Mechanical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan, and Department of Mechanical Engineering, Southern Taiwan UniVersity of Technology, Tainan County 710, Taiwan, Republic of China ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: February 6, 2010 Monodisperse Fe 1-x O nanoparticles (NPs) with a mean size of 21.7 ( 2.1 nm were prepared by the thermal decomposition of iron(III) oleate complex at 380 °C using oleic acid as the solvent. Variation in their composition was monitored using XRD for a period of 120 days under ambient conditions, under which the dominant phase changed from wu ¨stite to a spinel-type iron oxide phase. HR-TEM images and absorption spectra of the 10-day sample further revealed an FeO/spinel-type phase core-shell structure. Exchange-bias coupling on the interfaces between the wu ¨stite and the spinel-type phase accompanied the variation in composition. The dependence of H E on temperature demonstrates that the H E onset temperature is approximately 200 K, which correlates with the T N of bulk FeO. 1. Introduction Monodisperse magnetic iron oxide nanoparticles (NPs) of Fe 1-x O, R-Fe 2 O 3 , γ-Fe 2 O 3 , and Fe 3 O 4 , with quantifiable size- and shape-dependent properties, have been intensively studied owing to their various potential applications, including in the separation of pollutants, as magnetic resonance imaging (MRI) agents, and as magnetic storage media, among others. 1 Recently, much attention has been paid to wu ¨stite Fe 1-x O(x ) 0.05-0.17) (hereinafter referred to as FeO) NPs, because of not only their complex defect structure and defect-related physical properties but also their interesting phase transformation to multiphase nanostructures 2 with modulated magnetic properties. 3 For example, FeO/Fe 3 O 4 core-shell nanostructures with ferromag- netic (FM)/ferrimagnetic (FIM) interfaces may entail interesting exchange anisotropy. Exchange bias was first discovered in Co/ CoO core-shell NPs by Meiklejohn and Bean in 1956. 4 Since then exchange bias has been widely studied in nanostructures such as thin films and core-shell NPs and practically applied in areas such as read-head and sensor technologies. 5 Recently the chance of “beating the superparamagnetic limit by exchange bias” has been exemplified in Co NPs embedded in a antifer- romagnetic CoO matrix by Skumryev and co-workers. 6 These have made exchange bias an appealing research topic. FeO is an antiferromagnet with a Ne ´el temperature (T N ) of approximately 200 K, below which the spins are antiparallel. However, bulk FeO is only stable above 560 °C and remains a metastable phase under ambient conditions when rapidly quenched from temperatures at which it is stable; this metastable phase can undergo a disproportionation reaction to Fe and Fe 3 O 4 or oxidation to other magnetic phases, such as magnetite or maghemite. 7 Traditionally, FeO NPs are prepared by the high- energy milling of a mixture of Fe 2 O 3 + Fe or Fe 3 O 4 + Fe powders at high temperature (>570 °C) under or not under high pressure. 8 However, the product has certain limitations: it is irregularly shaped and exhibits polydispersity. However, col- loidal FeO NPs are typically prepared by the thermal decom- position of soluble precursors in organic solvents at high temperature in the presence of capping agents. This approach can commonly yield NPs with controllable size, shape, and uniformity. Numerous precursors, such as Fe(CO) 5 , 9 Fe(OAc) 2 , Fe(acac) 2 , 10 Fe(acac) 3 , 11 and iron oleate complex 12 have been adopted in the synthesis of FeO NPs in various organic solvents, such as oleylamine, trioctylamine, 1-octadecene, and eicosane, among others. New approaches for preparing water-based FeO NPs have been developed, involving the low-temperature hydrolysis of the organometallic precursor {Fe[N(SiMe 3 ) 2 ] 2 } 13 or the pulse laser ablation of pure iron in liquid media. 14 The cited investigations have raised important issues such as the survival of the obtained metastable FeO NPs, and the evolution of the composition under ambient conditions. No quantitative work has yet addressed time-evolved FeO NP products under ambient conditions, even though time can have uncertain effects on the intrinsic properties of FeO NPs, including their magnetic, electric, and catalytic properties. 3,15 In this study, monodisperse FeO NPs with a size of ap- proximately 22 nm were initially synthesized and then adopted to study the time evolution of composition, structure, and magnetic properties under ambient conditions. An FeO/Fe 3 O 4 core-shell nanostructure was observed in a 10-day sample, and its interesting exchange-bias properties were studied based on the core-shell model. 2. Experimental Section Chemicals. Goethite (R-FeO(OH), 99.9%, Strem) and oleic acid (OA, 90%, Showa) were used as received without further purification. Synthesis of Iron(III) Oleate from Goethite Powder. The synthesis of iron oleate by the dissolution of goethite in oleic acids has been demonstrated in our earlier works. 16 Briefly, a mixture of goethite (FeO(OH), 3 mmol) and oleic acid (OA, 13.5 mmol) was loaded into a three-necked round-bottom flask, and then heated to 290 °C(∼15 °C/min) under flowing argon. The transformation was complete after 4 h, according to X-ray * To whom correspondence should be addressed. E-mail: rkc.chem@ msa.hinet.net. † Far East University. ‡ National Cheng Kung University. § Southern Taiwan University of Technology. J. Phys. Chem. C 2010, 114, 4258–4263 4258 10.1021/jp908153y  2010 American Chemical Society Published on Web 02/19/2010