MS-1-O-2359 Temperature-induced core-shell reconfiguration of FexO/CoFe2O4 nanocrystals in ordered 2D nanocrystal arrays*†
A large variety of single- and multi-component nanocrystals (NCs) can now be synthesized and integrated into nanocrystal superlattices.1,2 These superstructures and their components have a limited thermal and temporal stability, though, which often hampers their application as functional devices. On the other hand, temperature-induced reconstructions can also reveal opportunities to manipulate properties and access new types of nanostructures.3-5 In-situ atomically resolved monitoring of nanomaterials provides insight into the temperature induced evolution of the individual NC constituents within these superstructures at the atomic level.6 Here, we investigate the effect of temperature annealing on 2D square and hexagonal arrays of FexO/CoFe2O4 core/shell NCs (Figure 1) as a model for many complex oxides by in-situ heating in a transmission electron microscope (TEM). The FexO core has a rock salt structure with some cation deficiencies (x = 0.83-0.95)7 and the lattice constant varies between 0.4255 nm and 0.4294 nm, depending on the oxidation state.7 The CoFe2O4 shell has a spinel crystal structure (lattice constant 0.846 nm).8 Both structures have a face centered cubic (FCC) oxygen sublattice with a lattice mismatch of only 3 %.7 Both cubic and spherical NCs undergo a core-shell reconfiguration at a temperature of approximately 300 ⁰C, whereby the FexO core material segregates at the exterior of the CoFe2O4 shell, forming ‘snowman’-like particles (asymmetric dumbbells) with a small FexO domain attached to a larger CoFe2O4 domain (Figure 2). During reconfiguration, the core volume is filled by the CoFe2O4 shell material. Upon continued annealing, the segregated FexO domains form bridges between the CoFe2O4 domains, followed by coalescence of all domains resulting in loss of ordering in the 2D arrays. Annealed FexO domains contain Co traces as well (Figure 3).
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[6] van Huis, M.A. et al. Advanced Materials 2009, 21, 4992–4995.
[7] Pichon, B.P. et al. Chemistry of Materials 2011, 23, 2886–2900.
[8] Song, Q. & Zhang, Z.J. Journal of the American Chemical Society 2004, 126, 6164–6168.
* Yalcin, A.O. et al. Nanotechnology 2014, 25, 055601.
† This work has appeared on the journal cover (Nanotechnology Volume 25, Issue 5) as the featured article (http://ej.iop.org/pdf/nano/vol25/na2505-webcover.pdf).
This work is part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organization for Scientific Research (NWO).
Fig. 1: TEM images of different types of 2D arrays. a) cubic FexO/CoFe2O4 core/shell NCs forming a square 2D array. Inset figure was taken with an objective aperture inserted (diffraction contrast). Core-shell contrast is observed better in this way. b) Spherical FexO/CoFe2O4 core/shell NCs array forming a hexagonal 2D array. |
Fig. 2: TEM images of FexO/CoFe2O4 ‘initially’ core/shell NC arrays; a) cubic NCs at 335 ⁰C, and b) spherical NCs at 360 ⁰C. The insets in the images were taken when the objective aperture was inserted (diffraction contrast). The (200) spacing of FexO is 0.21 nm, and (220) and (311) spacings of CoFe2O4 are 0.3 nm and 0.255 nm respectively. |
Fig. 3: Energy filtered TEM Co-mapping of initially spherical NCs. Figure 3a is the zero-loss image and Figure 3b is the corresponding Co map. The dotted lines were used to clarify different domains. Arrows show the FexO domain with Co presence. |
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