Supplementary Information. Gamma-iron phase stabilized at room temperature by thermally processed graphene oxide

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1 Supplementary Information Gamma-iron phase stabilized at room temperature by thermally processed graphene oxide Artur Khannanov, [a] Airat Kiiamov, [a],[b] Alina Valimukhametova, [a] Dmitrii A. Tayurskii, [a],[b] Felix Börrnert, [c] Ute Kaiser, [c] Siegfried Eigler, [d] * Farit G. Vagizov, [b] * and Ayrat M. Dimiev [a] * [a] Laboratory for Advanced Carbon Nanomaterials, Kazan Federal University. AMDimiev@kpfu.ru; [b] Institute of Physics, Kazan Federal University, Kremlyovskaya str. 18, Kazan , Russian Federation. vagizovf@gmail.com; [c] Materialwissenschaftliche Elektronenmikroskopie, Universität Ulm, Albert-Einstein-Allee 11,89081 Ulm, Germany. [d] Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustraße 3, Berlin, Germany. siegfried.eigler@fu-berlin.de 1. Experimental Part 1.1 Synthesis of graphene oxide Graphite flakes (10 g, 832 mmol) were dispersed in 96% sulfuric acid (680 ml) at room temperature using a mechanical stirrer. After 10 min of stirring, 1 wtequiv of KMnO4 (10 g, 63.2 mmol) was added. The mixture became green due to the formation of the oxidizing agent MnO3 +. Additional portions of KMnO4 (10 g, 63.2 mmol each) were added when the green color of MnO3 + was diminished, indicating that the oxidizing agent was consumed. A total of 4 wtequiv of KMnO4 were sequentially added in total. The end of the oxidation was determined by the disappearance of the green color after each KMnO4 addition. After complete consumption of KMnO4, the reaction was quenched with 1400 ml of ice-water mixture, and then H2O2 solution (16 ml, 30%) was added to convert manganese by-products to soluble colorless Mn (II) ions. The reaction mixture was centrifuged 15 min at 8900 rpm to separate GO from acid. For purification, the GO precipitate was redispersed in distilled water, stirred 30 min, and centrifuged 20 min at 8900 rpm to separate purified GO from washing waters. This constituted one purification cycle. Four more purification cycles were performed consecutively: one time with distilled water and three times with HCl (4%). The GO precipitate after the last washing was dried in air g of air-dryed GO was obtained. 1.2 Preparation of Fe 3+ -GO composites (impregnation step) In a typical procedure, a Fe(NO3)3 solution (20 ml, 2%) was added to GO solution (20.0 ml, 2.0%) with stirring. The dispersion was further stirred on a magnetic stirrer for 24 hours. The resulted dispersion was centrifuged ~40 min at 5000 rpm until complete separation of particles, and the supernatant was decanted. For purification, the jelly Fe/GO precipitate was redispersed S1

2 in 400 ml DI water, stirred for 30 min and centrifuged as above. This constitutes one purification cycle. Six purification cycles were conducted. The purified jelly samples were dried at ambient condition and used as the air-dried solid samples. Nitrate removed. The powder X-ray diffraction (XRD) pattern of Fe 3+ -GO is completely featureless except for the signal at ~10º (2θ), attributed to the interlayer distance of GO (Figure S2). Importantly, neither FTIR (see Figure S1), nor XRD detects any traces of iron (III) nitrate; the anion is thus efficiently removed by the purification of as-prepared Fe 3+ -GO. 1.3 The growth of Fe-NPs (annealing) The air-dried Fe 3+ -GO sample, was introduced into the tube furnace, purged with nitrogen to remove all the oxygen, and was heated to a selected temperature. The heating ramp was 5 C/min. The sample was maintained at the selected temperature for 20 min. Then, the oven was turned off, and cooled down naturally to ~100 C in the flow of nitrogen. After cooling, the sample was removed from the oven and used for characterization. After the characterization, the same sample was annealed again at the higher temperature. The temperature range was from 200 C through 950 C. The step size was 50 C and lower. In an alternative experiments, described in sections 4 and 5 of this ESI, the sample was heated straight to 800 C, and maintained at this temperature for a certain time. 1.4 Characterization of obtained samples The scanning electron microscopy (SEM) images were acquired with a field-emission highresolution scanning electron microscope Merlin from Carl Zeiss at accelerating voltage of incident electrons of 5 kv and a current probe of 300 pa. The transmission electron microscopy (TEM) imaging and local electron energy-loss spectroscopy (EELS) was performed with the SALVE instrument at an electron accelerating voltage of 80 kv. The powder X-ray diffraction (XRD) was acquired with Bruker D8 Advance with Cu Kα irradiation (λ= Å) in the Bragg-Brentano geometry; the rate was 0.18 º/min; the range of 2θ angle was from 7º through 100º; the step was 0.015º. The thermogravimetric analysis (TGA) data was collected with the STA 449 F5 Jupiter analyzer from Netzsch in both Ar and synthetic air atmosphere. The FTIR spectra were acquired with the Spectrum 400 FT-IR spectrometer (PerkinElmer Inc.) with a Diamond KRS-5 attenuated total internal reflectance attachment (resolution 0.5 cm 1, 32 scans, wavelength range cm 1 ). The Mössbauer effect measurements were carried out mainly at room temperature, using a conventional constant-acceleration spectrometer, produced by WissEl (Germany). Commercial Mössbauer source of 57 Co in rhodium matrix (Ritverc isotope products, Saint Petersburg, Russia) with an activity of about 40 mci was used as the γ-radiation source. Low-temperature measurements were carried out with a continuous flow cryostat (model CFICEV from ICE Oxford, UK), equipped with Cryo-Con temperature controller (Model 32B); the sample temperature was kept with an accuracy of ± 0.1K. The absorber was prepared by uniformly packing the sample under study into a holder closed by thin aluminum foil. The experimental spectra were least-squares fitted with the assumption that line shapes are Lorentzian to yield the hyperfine parameters, namely isomer shift (IS), quadrupole splitting (QS), and hyperfine field (HHF). A metallic-iron foil at RT was used for velocity calibration of the Mössbauer spectrometer. Isomer shifts were referred to α-fe at RT. S2

3 2. Additional Characteristics of Fe 3+ -GO composite formed during the impregnation step Figure S1. FTIR spectra for Fe 3+ -GO as compared to parent GO. Figure S2. XRD data for Fe 3+ -GO and Fe(NO3)3 9H2O precursor. S3

4 3. Mössbauer spectroscopy data Transmission, % Fe 3+ Fe 2+ Fe 3+ Fe oxides Velocity (mm sec -1 ) Figure S3. Room temperature Mössbauer spectrum of Fe-NP/tpGO obtained by annealing at 300 C and model subspectra Transmition / % g-fe Fe 2,5+ Fe 3+ a-fe Fe oxides Velocity / mm sec -1 Figure S4. Room temperature Mössbauer spectrum of Fe-NP/tpGO obtained by annealing at 700 C and model subspectra. S4

5 104 Fe x C y 22,5% 100 Transmission, % a-fe, 45.5% g-fe 26.7% Velocity, mm/sec Figure S5. Room temperature Mössbauer spectrum of Fe-NP/tpGO obtained by annealing at 800 C and model subspectra. Fe x C y 15.6% 100 Transmission, % 96 a-fe, 35% g-fe 49.4% Velocity, mm/sec Figure S6. Room temperature Mössbauer spectrum of the Fe-NP/tpGO obtained by annealing at 900 C, and model subspectra. S5

6 Figure S7. Powder XRD patterns for the Fe-NP/tpGO samples annealed at 700 C, 800 C and 850 C. S6

7 Figure S8. The photograph of Fe 3+ -GO composite obtained after the impregnation step. Figure S9. The photograph of Fe-GO composite annealed at 350 С. S7

8 Figure S10. The original TEM image with high resolution, shown in Fig. 4E. oxygen iron Figure S11. Electron energy-loss spectroscopy (EELS) performed locally on the nanoparticle shown in Fig. S10. S8

9 3. Determining the carbon content in γ-fe-np from the XRD data There is a well-established relationship between the carbon content and the lattice parameters for the austenitic steels, dating back in the mid of the 20th century.[1-3] We calculated the lattice parameters for our γ-fe from the experimental XRD data (Fig. 3, Fig. S7, Fig. S12) as an average for the reflexes (111), (200), (220), and (311). The lattice parameters were , , and for the samples annealed at 850 ºC, 900 ºC, and 950 ºC, respectively. This values were compared with the calibration curves build based on the literature data. The asdetermined carbon content was 0.82%, 0.88%, and 0.93%, respectively. The lattice parameter for the sample prepared by annealing at 800 ºC for 8 h (see Figure S12B in the section 4 below) was The carbon content was 0.60%. Thus, the carbon content in the gamma-phase of our samples vary from 0.60% through 0.93%, which is on the lower end for the austenitic steels. 4. Alternative experiment on growing Fe-NPs. In the alternative experiment, the annealing procedure was changed. The same Fe 3+ -GO precursor as in the experiment described in the main text, was annealed at 800 ºC for 8 h. Figure S12 demonstrates the characteristics of as-obtained sample. The XRD data shows that gammaphase forms in majority. The Mössbauer data shows 64.1 % γ-fe, 16.7% α-fe, and 19.2% FexCy, very close to the sample obtained after annealing at 950 ºC, described in the main text. However, the size and the shape of the Fe-NPs in this sample is very different from that presented in the main text (Fig. 3). First of all, the size of the particles is significantly smaller: from 30 through 55 nm. Secondly, the shape of the NPs is very different: about half of the NPs have cubic shape, and another half are spherical multi-facet crystals. The cubic shape is naturally intrinsic for the cubic lattice. The spherical particle might form by fusing several smaller cube-shaped NPs. Comparing this sample with that obtained at 950 ºC, one can conclude that at temperatures above 800 ºC, particles migrate and fuse to form larger spherically shaped particles. S9

10 Figure S12. Fe-NP/tpGO grown in an alternative experiment. (A) Mössbauer spectra; (B) Powder XRD data; (C) TEM images; (D) SEM image. The Fe 3+ -GO precursor was annealed at 800 ºC for 8 h. Electron beam diffraction acquired from the large area of this sample shows presence of both gamma iron and alpha iron. Note, electron beam cannot go through the larger particles. This is why the diffraction pattern was acquired from the edges, where the particles are relatively thin (Figure S13C). The diffraction pattern suggests presence of mostly gamma iron. The smaller particles were transparent for the beam (Figure S13D). The diffraction pattern reveals presence of alpha iron without the gamma phase. Thus, this experiment revealed that the two iron phases exist as the separate particles. Note, for the larger particles we could investigate the particles edges only. Strictly, one cannot fully exclude presence of alpha phase in the centre of the large particles. S10

11 Figure S13. Electron beam diffraction from Fe-NP/tpGO. (A) TEM image of the area. (B) Diffraction pattern acquired from the entire area shown on (A). (C) and (D) Diffraction patterns acquired from the selected NPs shown on pane (A). 5. Growing Fe-NPs on alternative carbon supports In the alternative experiments we tried to elucidate the role of GO in the formation of the gamma-iron phase. Among other carbon sources, graphite and carbon nanotubes did not work since less than 1 wt% iron adsorbed on their surface. This is why the two carbon sources with the developed pore structure have been used. First is the Cseal-F, sold by Mi SWACO company as an additive for drilling fluids; previously we successfully used this carbon source to fabricate oxidatively modified carbon material for water remediation [4]. The second carbon source is a bituminous coal mined by a Russian company "Belkommerc". The two carbon sources were used as is and being oxidatively modified. The oxidative modification procedure, as well as characterization of the materials is provided in our previous paper [4]. The impregnation procedure was similar to that used with GO, except that only one washing was performed to keep more iron on the surface. The impregnated samples were annealed at 800 ºC for 2 h, Figure S14 shows the resulted data. The use of Cseal-F and oxidized Cseal-F did not result in formation of S11

12 any detectable gamma-iron phase (Figure S14a,b,c). The use of the bituminous coal resulted in formation of up to 26% gamma-iron according to the Mössbauer spectroscopy data (Figure S14d,e,f). At the same time the powder XRD revealed very weak signals (Figure S14d), suggesting semi-amorphous nature of the iron-based products. Note, the low iron content in the resulted composite products, related to the low surface area and low affinity of the two alternative carbon sources when compared to GO. Figure S14. Growing Fe-NPs on alternative carbon support: (A-C) on Cseal-F and oxidatively modified Cseal-F). (D-F) on bituminous coal and oxidatively modified bituminous coal. 6. Raman spectroscopy data for Fe-NP/tpGO obtained by annealing at different temperatures. S12

13 Figure S15. Raman spectroscopy data for Fe-NP/tpGO obtained by annealing at different temperatures. The Raman spectra of the tpgo support does not change significantly with annealing temperature. This is in line with the well documented GO behavior upon annealing. The density of the defects in the original GO is already above the threshold level, detectable by the Raman spectroscopy. References 1. W.J. Wrazej, Lattice spacing of retained austenite in iron carbon alloy. Nature, 1949, 163, J. Mazur, Lattice parameters of martensite and austenite. Nature, 1960, 166, Cheng et al. Lattice parameters of iron-carbon and iron-nitrogen martensites and austenites, Scr. Mater., 1990, 24, Khannanov A.; Neklyudov, V.; Gareev, B.; Kiiamov, A.; Dimiev, A.M. Oxidatively modified carbon as efficient material for removing radionuclides from water. Carbon, 2017, 115, S13