ISSN 1998-0124 CN 11-5974/O4 2019, 12(8): 1781–1788 https://doi.org/10.1007/s12274-019-2431-7 Research Article Exploring the synthesis conditions to control the morphology of gold– iron oxide heterostructures Pablo Tancredi 1,§ , Luelc Souza da Costa 2,3,§ , Sebastian Calderon 4 , Oscar Moscoso-Londoño 5,6 , Leandro M. Socolovsky 7 , Paulo J. Ferreira 4,8 , Diego Muraca 5 , Daniela Zanchet 2 (  ), and Marcelo Knobel 5 (  ) 1 Laboratory of Amorphous Solids, INTECIN, Faculty of Engineering, University of Buenos Aires – CONICET, Buenos Aires, CP C1063ACV, Argentina 2 Department of Inorganic Chemistry, Institute of Chemistry, University of Campinas (UNICAMP), CP6154, 13083-970, Campinas, SP, Brazil 3 Brazilian Nanotechnology National Laboratory (LNNano), Rua Giuseppe Máximo Scolfaro 10000, Campinas, SP, Brazil 4 International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga s/n, 4715-330 Braga, Portugal 5 “Gleb Wataghin” Institute of Physics, University of Campinas (UNICAMP), CEP 13083-859, Campinas, SP, Brazil 6 Autonomous University of Manizales, Antigua Estación del Ferrocarril, Manizales, CP 170001, Colombia 7 Facultad Regional Santa Cruz, Universidad Tecnológica Nacional - CIT Santa Cruz (CONICET) – Av de los Inmigrantes 555, Río Gallegos, Santa Cruz, Argentina 8 Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, USA § Pablo Tancredi and Luelc Souza da Costa contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 26 November 2018 / Revised: 3 May 2019 / Accepted: 6 May 2019 ABSTRACT Gold–iron oxide nano-heterostructures with a clear and well-defined morphology were prepared via a seed-assisted method. The synthesis process and the events of heterogeneous nucleation during the decomposition of the iron precursor were carefully studied in order to understand the mechanism of the reaction and to tailor the architecture of the fabricated heterostructures. When employing Au seeds of 3 and 5 nm, nanoparticles with a dimer-like morphology were produced due to the occurrence of a single iron oxide nucleation event. Otherwise, multi-nucleation events could be favored by two mechanisms: (i) by the incorporation of a reducing agent and the slowing down of the heating protocol, leading to a core–shell system; (ii) by the increase of the Au seed size to 8 nm, leading to a flower-like system. Further increase of the Au seed size to 12 nm using similar synthesis conditions promotes the homogeneous nucleation and growth of the iron oxide phase, without formation of heterostructures. An in-depth study was performed on the gold–iron oxide heterostructures to confirm the epitaxial growth of the oxide domain over the Au seed and to analyze the elemental distribution of the components within the heterostructures. Finally, it was found that the modification of the plasmonic properties of the Au nanoparticles are strongly influenced by the architecture of the heterostructure, with a more pronounced damping effect for the systems produced after multi-nucleation events. KEYWORDS nanoparticles, gold–iron oxide heterostructures, heterogeneous nucleation, epitaxial growth, HR-microscopy 1 Introduction The ability to produce nanoscale heterostructures with a controlled and well-defined morphology has become one of the major challenges of the recent research in nanochemistry. This new kind of nanostructures exhibits an enormous potential for various technological applications as they can combine the properties of two or more inorganic materials in a single nanoscale entity [1–3]. Among the different strategies to produce nano-heterostructures, the seed-assisted method has become an effective alternative to create systems with a precise control of size and morphology [4–6]. This strategy relies on the heterogeneous nucleation and growth of a second solid phase onto the surface of pre-formed nanoparticles, called “seeds”. In an ideal seed-assisted procedure, the heterogeneous nucleation required to produce the heterostructure is favored over the homo- geneous nucleation because the presence of the seeds decreases the activation energy and the monomer's critical concentration needed to initiate the formation of the second phase [7, 8]. Under this framework, different approaches were proposed in order to identify the type of nucleation events that take place during the reaction. For example, concepts common to the nucleation on substrates from the vapor phase were employed to define a contact angle that determines the “wettability” of the components and the probability of heterogeneous nucleation [6, 8]. In a similar manner, the nucleation process can be analyzed in terms of surface energies and mismatch at the interface between the structures, an approach comparable to thin films growth theory [1, 9, 10]. In both cases, the relationship between the crystal structure and the lattice parameters of the two counterparts has a critical influence on the type of nucleation occurring throughout the reaction, as well as in the final morphology of the system. In recent years, Au and iron oxide (FeOx) heterostructures prepared by seed-assisted methods have gained considerable attention due to their plasmonic and magnetic properties with potential applications in fields such as catalysis, sensor design or biomedicine [11–21]. Typically, these heterostructures are formed after the thermal decomposition at high temperatures of an iron organo-metallic precursor over the surface of Au nanoparticles (AuNPs). Several Address correspondence to Daniela Zanchet, zanchet@unicamp.br; Marcelo Knobel, knobel@ifi.unicamp.br