Defect-assisted synthesis of magneto-plasmonic silver-spinel ferrite heterostructures in a flower-like architecture

www.nature.com/scientificreports OPEN Defect‑assisted synthesis of magneto‑plasmonic silver‑spinel ferrite heterostructures in a flower‑like architecture Marco Sanna Angotzi1,2, Valentina Mameli1,2, Claudio Cara1,2, Vincenzo Grillo3, Stefano Enzo4, Anna Musinu1,2 & Carla Cannas1,2* Artificial nano-heterostructures (NHs) with controlled morphology, obtained by combining two or more components in several possible architectures, make them suitable for a wide range of applications. Here, we propose an oleate-based solvothermal approach to design silver-spinel ferrite flower-like NHs. Small oleate-coated silver nanoparticles were used as seeds for the growth of magnetic spinel ferrite (cobalt ferrite and spinel iron oxide) nanodomains on their surface. With the aim of producing homogeneous flower-like heterostructures, a careful study of the effect of the concentration of precursors, the reaction temperature, the presence of water, and the chemical nature of the spinel ferrite was carried out. The magnetic and optical properties of the NHs were also investigated. A heterogeneous growth of the spinel ferrite phase on the silver nanoparticles, through a possible defect-assisted mechanism, was suggested in the light of the high concentration of stacking faults (intrinsic and twins) in the silver seeds, revealed by Rietveld refinement of powder X-ray diffraction patterns and High-Resolution electron microscopy. Spinel ferrite-based nanoheterostructures (NHs) have attracted considerable interest in the last decades, thanks to the possibility of joining in a single material magnetic and other physical–chemical properties. Indeed, spinel ferrites represent a class of ferrimagnetic materials widely studied thanks to the chemical and mechanical stability, and the possibility to tune the hard or soft magnetic behaviour by changing the type of the divalent cation1–6. Furthermore, noble metals (e.g., Ag, Au) have numerous properties (optic, catalytic, antibacterial) that find application in several fields7. Between them, silver presents several advantages in terms of cost, availability, but also activity as a ntimicrobial8. Silver-ferrite NHs are used, for example, as substrates for surface-enhanced Raman spectroscopy (SERS), thanks to the high surface resonance effect of silver9–11. Electromagnetic enhancement, caused by the construction of “hot-spots” in aggregated NHs, can contribute to optimizing the SERS performances. In this context, magnetic-silver NHs represent an excellent material for SERS activity, since spinel ferrite nanoparticles can induce magnetic aggregation. Again, these kinds of NHs are employed as catalysts for purification of dye effluents11,12, exploiting the catalytic activities of silver, and the magnetic separation of spinel ferrite. Flower-like silver-magnetite heterostructures have been employed as an antibacterial against Escherichia coli12,13. Moreover, silver-spinel ferrite NHs have been utilized for combined photothermal and magnetic heating, exploiting the localized surface plasmon resonance effect of the noble metal part and the magnetic behavior of the spinel ferrite14–16. The most crucial and challenging aspect in designing heterostructures is the development of synthesis methodologies able to produce highly crystalline particles having low size dispersity, tuneable size, and shape, and defined interfaces17. In the literature, silver-spinel ferrite NHs have been synthesized in forms of many architectures, from core-shell18,19 to dimers18–24 and flower-like9–11,25–28, prepared via one-pot9,10,28 or twopot syntheses (Table 1S)9–11,18–28. For example, Fe3O4@Ag flower-like NHs have been synthesized by a seedmediated growth approach in organic solvents starting from magnetite NPs with silver nitrate or silver oleate as Ag precursors25. Dimer Ag-ferrite NHs have also been synthesized by various authors20–24, through thermal 1 Department of Chemical and Geological Sciences, University of Cagliari, S.S. 554 Bivio per Sestu, 09042 Monserrato, Italy. 2Consorzio Interuniversitario Nazionale Per La Scienza e Tecnologia Dei Materiali (INSTM), Via Giuseppe Giusti 9, 50121 Florence, Italy. 3Istituto Nanoscienze Consiglio Nazionale delle Ricerche (CNR-NANO), Via G. Campi 213/a, 41125 Modena, Italy. 4Department of Chemistry and Pharmacy, University of Sassari, Via Vienna 2, 07100 Sassari, Italy. *email: Scientific Reports | (2020) 10:17015 | https://doi.org/10.1038/s41598-020-73502-5 1 Vol.:(0123456789) www.nature.com/scientificreports/ Sample Ag@Co1 Ag@Co2 Ag@Co3 Ag@Co4 Ag@Co5 Ag@Co6 Ag@Co7 Ag@Co8 Ag@Co9 Ag@Co10 Ag@Fe1 Ol (mmol) 0.5 P (mL) 5 0.25 0.125 0.125 0.125 0.125 0.125 0.125 0.125 0.125 0.125 5 5 5 5 10 10 10 10 10 10 T (mL) 5 5 5 5 5 10 10 10 10 10 10 W (mL) 0 0 0 0 0 0 0 0 0 0.1 0 T (°C) 140 140 140 180 200 140 180 200 220 200 200 Phase a (Å) % w/w <ε> < DXRD > (nm) < DTEM_V > (nm) CoFe2O4 8.413 76 1∙10-6 7(1) 7(1) Ag FCC 4.089 24 2∙10-3 19(2) 20(5) CoFe2O4 8.450 48 8∙10-3 6(1) 5(1) Ag FCC 4.100 52 6∙10-7 15(1) 15(4) CoFe2O4 8.436 25 5∙10-3 8(1) 6(1) Ag FCC 4.090 75 1∙10-5 20(1) 18(5) CoFe2O4 8.405 28 4∙10-3 9(1) 7(1) Ag FCC 4.089 72 2∙10-5 26(1) 25(2) CoFe2O4 8.390 24 4∙10-3 12(1) 10(2) Ag FCC 4.089 76 1∙10-4 28(2) 31(6) CoFe2O4 8.470 25 2∙10-3 6(1) 5(1) Ag FCC 4.096 75 6∙10-8 22(2) 18(3) CoFe2O4 8.437 30 5∙10-3 9(1) 6(1) Ag FCC 4.092 70 7∙10-6 24(2) 23(5) CoFe2O4 8.418 34 6∙10-3 11(1) 7(1) Ag FCC 4.091 66 1∙10-5 29(2) 20(3) CoFe2O4 8.391 42 3∙10-3 10(1) 9(1) Ag FCC 4.089 58 1∙10-4 80(10) 30(7) CoFe2O4 8.396 41 7∙10-5 6(1) 10(2) Ag FCC 4.089 59 3∙10-6 > 100 100(20) Fe3O4 8.397 32 2∙10-3 10(1) 7(1) Ag FCC 4.092 68 1∙10-6 20(2) 21(2) Table 1.  Synthesis conditions and features of silver-based spinel ferrite heterostructures. The reaction time was 10 h, the silver seeds were 0.2 mmol. Ol: metal oleate; P: 1-pentanol; T: toluene; W: water; T: temperature. < ε > : microstrain; a: cell parameter; %w/w: weight percentage calculated by Rietveld refinement; < DXRD > : crystallite size; < DTEM_V > : volume-weighted particle size calculated from TEM images. decomposition of iron acetylacetonates or oleates in the presence of silver NPs (seed-mediated growth). Other less conventional approaches concern the surface reduction of silver nitrate in the presence of a reducing agent (e.g., glucose, oleylamine, etc.) on spinel ferrite NPs to produce dimer and flower-like NHs11,26,27, or surface oxidation of Ag@Fe core–shell NPs in the air for the production of core–shell (or flower-like) NHs having a silver core and magnetite/maghemite shell18,19. Most of the above-cited syntheses are afforded through the well-known surfactants-assisted high-temperature decomposition of organic complexes in high boiling organic solvents. Nevertheless, the necessity of more eco-friendly strategies that use less amount of toxic organic sol (...truncated)