The materials used in this study were Iron (III) chloride hexahydrate (AR grade) and

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1 Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry 2017 Supporting Information Synthesis of Metastable Hard-Magnetic ε-fe 2 O 3 Nanoparticles from Silica-Coated Akaganeite Nanorods Marin Tadic a, Irena Milosevic b, Slavko Kralj c, Miodrag Mitric a, Darko Makovec c, Marie-Louise Saboungi d, e, and Laurence Motte f a. Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Science, University of Belgrade, POB 522, Belgrade, Serbia. marint@vinca.rs b. Powder Technology Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. irena.markovic@epfl.ch c. Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. d. IMPMC, Sorbonne Univ UPMC Univ Paris 06, UMR CNRS 7590, Museum National d Histoire Naturelle, IRD UMR 206, 4 Place Jussieu, F Paris, France; e. Soochow Univ, InstFunct Nano & Soft Mat FUNSOM, Suzhou , Jiangsu, Peoples R China. f. Inserm, U1148, Laboratory for Vascular Translational Science, UFR SMBH, Université Paris 13, Sorbonne Paris Cité, F Bobigny, France. The materials used in this study were Iron (III) chloride hexahydrate (AR grade) and dopamine (MB grade). Tetraethoxysilane (TEOS, 99.9%) and polyvinylpyrrolidone (PVP; MW ~ ) were purchased from Alfa Aesar. Acetone (AppliChem GmbH), ethanol absolute (Carlo Erba, reagent - USP), NH 4 OH (aq) (Fluka, p.a., 25%), NaOH 1M and HCl 1M (p.a., Riedl-de-Haën) were used as received. These materials were used as such without any further purification. First, bare akaganeite nanorods were synthesized by forced hydrolysis of aqueous FeCl 3 /HCl solution in the presence of dopamine as shape controlling agent as previously described [43,46,47]. Briefly, 10 ml FeCl 3.6H 2 O (0.5 mol.l -1 ) solution was mixed with 10 ml of 0.04 mol.l -1 HCl solution. 1 ml of dopamine (1.6 mg.ml -1 ) was added in the mixture and stirred magnetically at 500 rpm for few minutes. After that 180 ml of water at C was poured into 1

2 the flask. The solution was mixed at this temperature for 2 hours under reflux. The suspension was then cooled down to room temperature and precipitated by addition of NaOH 1mol.L -1. The precipitate was separated by centrifugation and washed with deionized water several times. An orange colloidal suspension of β-feooh nanorods is obtained. Fig. S1 show TEM micrographs of bare akaganeite nanorods. Second, silica coated nanorods were prepared by modified Stöber method based on hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) as described previously [43]. The as-synthesized akaganeite nanorods were coated with a ~ 5-nm-thick silica shell. In brief, 10 ml of the suspension containing as-synthesized β-feooh nanorods (159 mm, Fe ions) were transferred into 30 ml of ethanol solution containing 0.6 ml of aqueous ammonia (25 %) and 90 mg of polyvinyl pyrrolidone (PVP). Then, the mixture of 0.35 ml of tetraethoxysilane (TEOS) and 2 ml of ethanol were added drop-by-drop into the above suspension over a period of 10 minutes, while vigorously stirring. The silica coated akaganeite nanorods (SiO were obtained after 8 hours of stirring, followed by washing with acetone and distilled water using centrifuge (10 minutes, g). Third, controlled thermal treatment of the silica coated akaganeite nanorods was done (Figure S2). The silica coated akaganeite nanorods were dried by freeze-drying. This assynthesized dry powder sample was consecutively annealed at 200, 300, 400, 500, 800 and 1000 ºC, keeping the temperature constant at each step for 3 h (Figure S2). The as-synthesized nanoparticles were examined with a transmission electron microscopy (JEOL, JEM 2100) operating at 200 kv. For the TEM investigations the drop of nanoparticle suspension was deposited by drying a suspension on a copper-grid-supported, perforated, 2

3 transparent carbon foil. The X-ray powder diffractometer (Phillips 1050) employing Cu Kα (λ= Å, 2θ= ) radiation was used to characterize the crystal structure of the samples. Magnetic measurements were performed on a commercial vibrating sample magnetometer (VSM, Quantum Design, Versalab) in the wide range of temperatures ( K) and applied DC fields (up to 3 T). Fig. S1 TEM micrographs of bare akaganeite nanorods. 3

4 Temperature [ o C] Time [hours] Fig. S2 Schematic diagram of the heat treatment process. Table S1. Refinement parameters of β-feooh nanoparticles at room temperature. Atom positions, B-temperature factor and occupations Atom x y z Occupancy B[Å -2 ] Fe Fe O O O O Cl Rietveld refinement results a[å] b[å] c [Å] [ ] Crystal size [nm] 20.2 Microstrain [%] Preference orientation along [010] direction Rwp Rexp

5 Table S2. Refinement parameters of ε-fe 2 O 3 nanoparticles at room temperature. Atom positions, B-temperature factor and occupations Atom x y z Occupancy B[Å -2 ] Fe Fe Fe Fe O O O O O O Rietveld refinement results a[å] b[å] c [Å] Crystal size [nm] 23.8 Microstrain [%] Preference orientation along [010] direction Rwp Rexp /M [g/emu] FeOOH Linear fit T N =253 K T [K] Fig. S3 Temperature dependence of the inverse magnetization of β-feooh. The linear fit to the high-temperature part gives the Neel temperature of 253 K. 5

6 0.10 -Fe 2 O 3 dm/dt T [K] Fig. S4 Differential magnetization curve of the ε-fe 2 O 3 sample. Fig. S5 Low-field magnetization dependence of the demagnetization curve of ε-fe 2 O 3 and corresponding differential curve. 6