Supporting Information. Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation

Transcription

1 Supporting Information Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation Artur Feld,*,,,, Agnes Weimer,, Andreas Kornowski, Naomi Winckelmans, Jan-Philip Merkl,, Hauke Kloust, Robert Zierold, Christian Schmidtke, Theo Schotten, Maria Riedner#, Sara Bals and Horst Weller*,,,, Institute of Physical Chemistry, Hamburg University, Grindelallee 117, D Hamburg, Germany. The Hamburg Center for Ultrafast Imaging, Hamburg University, Luruper Chaussee 149, D Hamburg, Germany. Fraunhofer-CAN, Grindelallee 117, D Hamburg, Germany. Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O BOX Jeddah 21589, Saudi Arabia. # Department of Chemistry, Hamburg University, Martin-Luther-King-Platz 6, D Hamburg, Germany. Electron Microscopy for Materials Science (EMAT), Department Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium. Center for Hybrid Nanostructures, Hamburg University, Luruper Chaussee 149, Hamburg, Germany. 1

2 1. Synthesis of iron sources 1.1 Synthesis of FeCO3 - iron(ii) source The reaction is described by the following chemical equation: FeSO4 7 H2O + Na2CO3 FeCO3 + Na2SO4 + 7 H2O (1) The resulting grey precipitate is shown in Figure S1. Figure S1. Preparation and washing step of the FeCO3: A) Synthesis of FeCO3. A->B) The precipitate was transferred into a Schlenk-frit and was washed four times with water. C) It remained a purified precipitate. 1.2 Synthesis of Fe2(CO3)3 iron(iii) source The reaction is described by the following chemical equation: Fe2(SO4)3 x H2O + 3 Na2CO3 Fe2(CO3)3 + 3 Na2SO4+x H2O (2) 2

3 The resulting precipitate is shown in Figure S2. Figure S2. Preparation and washing step of the Fe2(CO3)3: A) The synthesis of Fe2(CO3)3 was carried out in a Schlenk flask A->B) The Fe2(CO3)3 was transferred into a Schlenk-frit and washed four times with water. It remained a purified reddish-brown precipitate. 2. Synthesis of iron oleates 2.1 Synthesis of Fe(II) oleate The reaction is described by the following chemical equation: FeCO3 + 2RCOOH [Fe 2+ (RCOO - )2] + CO2 + H2O (3) 3

4 Figure S3. Image of the iron(ii) oleate/water-emulsion after the reaction of FeCO3 with oleic acid (left figure) and the same solution after the drying process (right figure). 2.2 Synthesis of Fe(III) oleate The reactions are described by the following chemical equations: Fe2(CO3)3 + 6RCOOH 2[Fe 3+ (RCOO - )3] + 3CO2 + 3H2O (4) FeO(OH) + RCOOH [Fe 3+ O(RCOO - )] + H2O (5) Fe(OH)3 + 3RCOOH [Fe 3+ (RCOO - )3] + 3H2O (6) 4

5 Figure S4. Image of the Iron(III) oleate/water-emulsion after the reaction of Fe2(CO3)3 with oleic acid (left figure) and the same solution after the drying process (right figure). 3. Nanocrystal synthesis Figure S5. Standard experimental setup for the synthesis of iron oxide nanocrystals. 5

6 4. MALDI-TOF MS measurements Intens. [a.u.] x m/z Figure S6. Representative MALDI-TOF MS spectra of iron(ii) oleate. Intens. [a.u.] ,18 281,69 37_1 VV - 9-NA - RP m/z Figure S7. MS/MS of m/z = peak, correlating to a composition of [(Fe II )2(OA - )3] +. The complex fragmented into one oleate ion [ Da] and two Fe(OA) species [ Da]. 6

7 4.1 Influence of the degree of purity of the used oleic acid on the MALDI-TOF spectra a) m/z = [(Fe II ) 2 (OA - ) 3 ] + b) (OA: tech. grade) (OA: pure) simulated measured Normalized Normalized Figure S8. a) MALDI-TOF MS spectra of m/z = peak with technical grade oleic acid. b) MALDI-TOF MS spectra of m/z = peak with pure oleic acid. 50 m/z: 955 theoretical prediction [(Fe II ) 2 (OA - ) 3 ] + m/z: 955 measurement sum [(Fe II ) 2 (OA - ) 3 ] + and [(Fe II ) 2 (OA - ) 2 (LA - )] + 40 rel % m/z Figure S9. Refined calculations of the m/z = peak by including a fraction of 10% of linoleic acid (LA), providing an even better fit for the experimental data of the reactions with technical grade oleic acid. 7

8 5. Gas chromatography measurements N2 O2 Figure S10. Gas chromatogram of iron oleate synthesized with iron(iii) source at a reaction temperature of 60 C. The first peak (RT 0.76) was identified as oxygen by a control experiment with air. The integrated peak area corresponds to a value of 4.85%. The second peak (RT 0.92) was identified as nitrogen with an integrated peak area of 95.1%. This measurement demonstrates the release of oxygen due to a redox process in early stages of the reaction. 8

9 O 2 20 Integrated Peak Area Temperature / C Figure S11. GC measurement of the control experiment using pure oleic acid under air atmosphere in a closed flask. An increased consumption of oxygen occurs only after heating above 140 C. 6. Partial post-synthetic surface oxidation To determine which iron oxide phases are present after routinely purification, a Fourier analysis of a high resolution STEM image (Fig. S11) was performed. The FFT in Figure S12 c was acquired in the middle of the particle and can be indexed according to the [100] zone axis of wustite (FeO). A second FFT, obtained near the edge, can be indexed according to the [100] zone of either magnetite (Fe3O4) or maghemite (ɣ-fe2o3). Since both phases have the same crystal space group (Fd-3m) and very close lattice parameters (8.33Å and 8.36Å respectively) 1,2, high resolution imaging or electron diffraction is not able to distinguish between both phases. 9

10 Figure S12. a) High resolution HAADF-STEM image of an octapod shaped nanocrystal. b) Zoom in of image a. c-d) FFT of regions indicated by red squares in image b. The FFT indicate that the nanostructures contain multiple types of iron oxide, nl. FeO (c) and Fe3O4 and/or γ-fe2o3 (d). 10

11 7. SEM measurements a) b) 20 nm 100 nm c) d) 200 nm 100 nm Figure S13. SEM images of octapod shaped SPIONs. Images of the tilted sample at different magnification (a-c) and an overview of the non-tilted sample (d). 8. Size and shape control The size and shape control of the NC was achieved by diluting the iron oleate with 1-octadecene (ODE) and varying the heating time, while the molar ratio of iron: oleic acid has been kept constant at 1:7 (Fe/OA). Octapod star shaped NC were obtained shortly after nucleation (kinetically favored product) and underwent a metamorphism towards cubic shaped NC. Figure S14 illustrates the influence of both: reaction time and factor of dilution on the size and shape of the resulting NC. 11

12 ODE dilution [vol%] 1:7 Fe/OA 56% 21 nm 22 nm 21 nm 23 nm 30% 37 nm 32 nm 31 nm 28 nm 8% 45 nm 38 nm 37 nm 40 nm no ODE 82 nm 75 nm reaction time [min] Figure S14. Schematic representation of the influence of reaction time (after nucleation) and factor of dilution on the size and shape of the resulting NC. 45 nm 60 nm 12

13 5 nm 21 nm 100 nm 100 nm 37 nm 10 nm 10 nm 20 nm 45 nm 100 nm 100 nm 82 nm Figure S15. TEM images of octapod-star shaped NC corresponding to a time window of 2-6 min after nucleation (Figure S14). The cubic shaped NC were obtained within a time window of min as a product of the shape transformation process. The corresponding sizes are shown in the Figure S16. 13

14 100 nm 100 nm 23 nm 10 nm 28 nm 10 nm 100 nm 100 nm 40 nm 10 nm 60 nm 10 nm Figure S16. TEM images of cubic shaped NC corresponding to a time window of min after nucleation (Figure S14). Alternatively, the size of the cubic shaped NC can be changed by varying the molar ratio of Fe:OA, while keeping dilution with 1-octadecene over 66 vol%. Hereby, it is possible to synthesize cubic NC in a size range of 8-22 nm (Fig. S17). 14

15 molar ratio Fe : OA 1:2.5 1:4 1:5 1:7 50 nm 7.5 nm ± 10.2 % 100 nm 8.5 nm ± 9.0 % 50 nm 50 nm 50 nm 13.3 nm ± 4.8 % 21.5 nm ± 6.0 % Figure S17. TEM images of cubic shaped NC and schematic illustration of the influence of the molar ratio of Fe:OA on the size of cubic shaped NC. 0.5 min 2 min 4 min 8 min 13 min 19 min 56 min 20 nm 10 nm 5 nm Figure S18. TEM images of NC demonstrating the metamorphism from octapod star shape towards cubic shape. Reaction conditions: molar ratio of Fe:OA at 1:7 and 1-octadecene dilution of 8 vol%. The size and shape control of the NC was achieved by diluting the iron oleate with 1-octadecene (ODE) and varying the heating time, while the molar ratio of iron: oleic acid has been kept constant 15

16 at 1:7. Octapod star shaped NC were obtained shortly after nucleation (kinetically favored product) and underwent a metamorphism towards cubic shaped NC. 9. Magnetic measurements Figure S19. Magnetization characterization of star-shaped nanoparticles similar to Fig 3a. The exchange bias is obvious in magnetization isotherm measurements after field-cooled at 20 koe. Temperature-dependent measurement in warm-up after saturation at low temperature reveals two features which can be attributed to the Verwey transition in magnetite and the Néel transition in wustite. Literature values are highlighted by vertical lines. Magnetization measurements of star- and cube-shaped a nanoparticles are shown in Figure S19 and S20, respectively. In the hysteresis curves, temperature-dependent exchange bias is obvious. 16

17 The hysteresis at 300 K remains for both types of nanoparticles open, thus indicating a collective, ferromagnetic state. Note, at temperatures above 200 K the exchange bias vanishes (zoom-in S19 upper right). This fact is in good agreement with the temperature-dependent measurement of the magnetic moment (remanence in S19 and ZFC in S20). Two transitions can be identified to the Verwey and the Néel transition in magnetite and wustite, respectively. 3,4 The small offset to higher temperatures compared to the literature values (vertical line in S19) is caused by the measurement procedure. The base temperature of the cryostat is in warm-up measurement higher than the actual temperature at the sample; thus, shifting the apparent transition to higher values. A reduced sweep rate would help to minimize this artefact Figure S20. Magnetization characterization of cube-shaped nanoparticles similar to Fig 3b. The hysteresis remains open up to 300 K with an obvious remanence and coercive field. A warm-up measurement of the magnetic moment with an applied magnetic field of 50 Oe after zero-field cool (ZFC) reveals again the two transitions which can be attributed to the magnetite and wustite, respectively. 17

18 Figure S21. Magnetization characterization of smaller nanoparticles (smaller ODE dilution). Upper and lower row of star- and cubic-shaped particles, respectively. On the left side, ZFC measurements at different applied magnetic fields (color-coded) are shown. On the right side, insets in magnetization isothermal measurements. When smaller nanoparticles are used, a maximum in the ZFC measurement is obvious for starand cube-like nanoparticles (Figure S21, left column). The maximum shifts with increasing magnetic field to lower temperatures as expected for superparamagnetic nanoparticles. One may assume two transitions at around 125 K and 200 K which indicating the appearance of the magnetite and the wustite phase. The steep rise at about 200 K might be related to the vanishing coupling of the antiferromagnetic wustite and the ferromagnetic magnetite at temperatures larger than the Néel temperature. Whereas the blocking temperature of the star-like nanoparticles at small magnetic fields is below 300 K (upper panel), the blocking temperature of the cubes is about at 400 K (lower panel). This observation is supported by the magnetization isotherm measurements: 18

19 The star-like nanoparticles display vanishing remanence and coercive field, whereas cubic nanoparticles still show ferromagnetic behavior. Again exchange bias can be observed pointing to the core-shell like structure. Note, there is a jump in the hysteresis curve which cannot be explained by classical reversal mechanism in single-domain or multi-domain nanoparticles. One may speculate about a vortexlike state at zero-field in the ferromagnetic shell which has been predicted by simulations of spherical magnetite particles with similar dimensions. 5 References (1) Pecharroman, C.; Gonzalezcarreno, T.; Iglesias, J. E. The Infrared Dielectric-Properties of Maghemite, Gamma-Fe2O3, From Reflectance Measurement on Pressed Powders. Phys. Chem. Miner. 1995, 22, (2) Solano, E.; Frontera, C.; Puig, T.; Obradors, X.; Ricart, S.; Ros, J. Neutron and X-Ray Diffraction Study of Ferrite Nanocrystals Obtained by Microwave-Assisted Growth. A Structural Comparison with the Thermal Synthetic Route. J. Appl. Crystallogr. 2014, 47, (3) Schrettle, F.; Kant, C.; Lunkenheimer, P.; Mayr, F.; Deisenhofer, J.; Loidl, A. Wüstite: Electric, Thermodynamic and Optical Properties of FeO. Eur. Phys. J. B 2012, 85, 164. (4) Walz, F. The Verwey Transition - a Topical Review. J. Phys. Condens. Matter 2002, 14, R285 R340. (5) Betto, D.; Coey, J. M. D. Vortex State in Ferromagnetic Nanoparticles. J. Appl. Phys. 2014, 115, 17D