Supporting Information. Enhanced Electrochemical Lithium-Ion Charge Storage of Iron Oxide. Nanosheets

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1 Supporting Information Enhanced Electrochemical Lithium-Ion Charge Storage of Iron Oxide Nanosheets Sibo Niu, 1 Ryan McFeron, 1 Fernando Godínez-Salomón, 1 Brian Chapman, 2 Craig A. Damin, 1 Joseph B. Tracy, 2 Veronica Augustyn, 2 and Christopher P. Rhodes 1,* 1 Department of Chemistry and Biochemistry, Texas State University, San Marcos, TX Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC Corresponding author: Tel.: , Fax: , address: cprhodes@txstate.edu S-1

2 Synthesis of γ-fe2o3 nanosheets Iron oxide nanosheets were synthesized though two steps as presented in Figure S1 (further details provided in the main text and experimental methods). For the synthesis of the γ-fe2o3 nanosheets, a precursor ferrous hydroxide, Fe(OH)2 nanosheet was first formed. The synthesis of Fe(OH)2 from a ferrous salt such as FeSO4 under basic conditions can be described by the following equation. 1-2 FeSO4(aq) + 2NaOH(aq) Fe(OH)2(aq) + Na2SO4(aq) (1) The aqueous synthesis involves hydrolysis and condensation reactions of Fe 2+ species, and the Fe 2+ coordination environment within aqueous solutions depends on the concentration, ph, anion, and temperature. 3-4 The precursor Fe(OH)2 material synthesized exhibited a nanosheet morphology from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figure S2), and the observed green color is consistent with previously reports of Fe(OH)2 which is also referred to as green rust. 1,5 For the second step in the process, the precursor Fe(OH)2 was treated with H2O2 to oxidize the Fe 2+ ions to Fe 3+ and form Fe2O3, as described by the equation below. 2Fe(OH)2 + H2O2(aq) Fe2O3 + 3H2O (2) Prior studies have reported that the Fe 2+ ions within Fe(OH)2 can oxidized to Fe 3+ by H2O2. 6 Step 1 Step 2 FeSO 4 H 2 SO 4 H 2 O NaOH EG H 2 O Fe(OH) 2 3wt% H 2 O 2 γ-fe 2 O 3 Figure S1. Representation of steps for synthesis of γ-fe2o3 nanosheets. S-2

3 (A) (C) 100 nm (B) 200 nm Figure S2. Analysis of Fe(OH)2 precursor: (A) transmission electron microscopy (TEM) image; (B) scanning electron microscopy (SEM) image; and (C) picture of solution showing green color of the Fe(OH)2 suspension. The Fe(OH)2 precursor was subsequently treated with H2O2 to form the γ-fe2o3 nanosheets. S-3

4 (A) (B) 1.2 nm 0.89 nm 1.05 nm Figure S3. Transmission electron microscopy (TEM) image (A) and expanded region (B) of asprepared FeOx nanosheets (EG/H2O-as-prep) showing representative thickness measurements (blue lines) of nanosheets. S-4

5 (B) (A) hole (pore) hole (pore) Figure S4. High resolution transmission electron microscopy (TEM) images (A,B) of iron oxide nanosheets prepared using ethylene glycol and H2O showing polycrystalline structure and holes (pores) within the nanosheet. S-5

6 Relative scattering intensity (arbitrary units) (A) EG/H 2 O-as prep γ-fe 2 O 3 Fe 3 O 4 T 2g A 1g Relative scattering intensity (arbitrary units) Relative scattering intensity (arbitrary units) (D) (C) Wavenumbers (cm -1 ) EG/H 2 O- as prep γ-fe 2 O 3 Wavenumbers (cm -1 ) Wavenumbers (cm -1 ) Relative scattering intensity (arbitrary units) (B) Fe 3 O 4 Wavenumbers (cm -1 ) Original data Fitted peak I Fitted peak II Figure S5. (A) Comparison of Raman spectra in the cm -1 spectra region of iron oxide samples prepared using ethylene glycol (EG) and H2O (EG/H2O-as prep) compared with commercially obtained samples of γ-fe2o3 and Fe3O4; curve-fitted bands of Raman spectra in the cm -1 spectra region of commercially obtained Fe3O4 (B), commercially obtained γ-fe2o3 (C), and synthesized iron oxide sample (EG/H2O-as prep) (D). S-6

7 (A) FeO x -H 2 O-as prep Synthesized in H 2 O (B) FeO x -EG/H 2 O-as prep Synthesized in H 2 O:ethylene glycol 100 nm 100 nm Figure S6. Transmission electron microscopy (TEM) images of as-prepared iron oxide samples prepared using either (A) H2O or (B) ethylene glycol (EG) and H2O. S-7

8 EG/H 2 O-as prep Relative intensity (counts) H 2 O-as prep (100) (002) (101) δ-feooh: JCPDS # (012) (110) (220) (311) (222) (400) Fe 3 O 4 : JCPDS # (422) (511) (440) (210) (211) (220) (311) (222) (400) γ-fe 2 O 3 : JCPDS # (422) (511) (440) (101) (103) (111) α-feooh: JCPDS # (114) (115) θ Figure S7. Comparison of powder x-ray diffraction (XRD) data of as-prepared iron oxide samples prepared using either H2O (H2O-as prep) or ethylene glycol and H2O (EG/H2O-as prep); shown also are patterns for reference phases, as labeled. S-8

9 Specific discharge capacity (mah/g) Cycle number Figure S8. Capacity retention upon cycling for iron oxide samples prepared using H2O (FeOx nanorods) Mass percent (%) EG/H 2 O-as prep EG/H 2 O-120v EG/H 2 O-200 Region I Region II Temperature ( o C) Figure S9. Thermogravimetric (TG) data of iron oxide samples prepared with ethylene glycol and H2O: as-prepared, treated at 120 C under vacuum, and treated at 200 C. S-9

10 Mechanisms of Mass Loss The general formula presented within the text for the iron oxide nanomaterials is Fe2O3(C2H6O2)y(OH)z(OH2)w. Three species (H2O, OH, and ethylene glycol) may plausibly be removed from the Fe2O3 structure from room temperature to 600 C. The following equations (3-6) describe loss of H2O, OH, and ethylene glycol, respectively, from the Fe2O3 structure. Within Region I corresponding to the temperature window between 25 C and 400 C (Figure S9), mass loss may be described by equation 3 and equation 4. Equation 5 represents removal of OH groups followed by formation of a bridging oxygen (not represented in equation 5 for simplicity) and loss of H2O. Full coordination of the Fe is not represented in equation 5. Mass loss occurring within Region II corresponding to the temperature window between 400 C and 600 C may be described by equation 6. Additional details are provided in the main text. Fe2O3(C2H6O2)y(OH)z(OH2)w Fe2O3(C2H6O2)y(OH)z(OH2)w-a + ah2o (3) Fe2O3(C2H6O2)y(OH)z(OH2)w Fe2O3(C2H6O2)y(OH)z-b(OH2)w + bh2o (4) Fe-OH +Fe-OH Fe-O-Fe + H2O (5) Fe2O3(C2H6O2)y(OH)z(OH2)w Fe2O3(C2H6O2)y-c(OH)z(OH2)w-a + cc2h6o2 (6) S-10

11 Table S1. Comparison of the molar composition of FeOx nanomaterials based on analysis of thermogravimetic data; *mass loss region used to calculate the amount of H2O within the structure (Equations 3 and 4); ** mass loss region used to calculate the amount of ethylene glycol (C2H6O2) within the structure (Equation 6); additional details are provided in the text. Sample Percent average mass loss from C Percent average mass loss from C ( Region I )* Percent average mass loss from C ( Region II )** Calculated molar composition (from TGA) EG/H 2O-as prep 18.7 ± ± ± 1.0 Fe 2O 3(C 2H 6O 2) 0.16(OH 2) 1.50 EG/H 2O-120v 15.9 ± ± ± 2.5 Fe 2O 3 (C 2H 6O 2) 0.17(OH 2) 1.08 EG/H 2O ± ± ± 2.7 Fe 2O 3 (C 2H 6O 2) 0.15(OH 2) 0.71 ATR-FT-IR spectroscopy of FeOx nanomaterials ATR-FT-IR spectroscopy was used to evaluate the local structure of H2O and ethylene glycol with the FeOx nanomaterials and determine changes upon heating. The infrared spectra of 200 C and 450 C-treated samples and liquid ethylene glycol are presented in Figure S10. Fe-O stretching modes are observed in the ~ cm -1 region in the infrared spectra of the 200 C and 450 C-treated samples. 7 The infrared spectrum of the 200 C-treated FeOx sample shows the presence of prominent absorptions observed at 1150 and 1205 cm -1 (Figure S10B). No bands from either H2O or γ-fe2o3 are observed within this spectral range. The infrared spectrum of the neat, liquid-phase ethylene glycol (Figure S10B) possesses two strong absorptions at 1035 cm -1 and 1084 cm -1 that have been previously assigned to the asymmetric C-O stretching mode, νas(co), and symmetric C-O stretching mode, νs(co), respectively. 8 Based on the comparison between the infrared spectra of pure ethylene glycol and the 200 o C-treated FeOx sample, absorptions located at 1150 and 1204 cm -1 in the treated sample are attributed to the vibrational modes of ethylene S-11

12 glycol that interact with the FeOx surface. The ethylene glycol-feox interactions may alter the local potential energy environment compared with the liquid phase leading to shifts in the positions of the ethylene glycol absorptions. Prior work by Jansen et al. showed that vibrational modes of ethylene glycol, including the νs(c-o) mode, are shifted to higher frequencies upon interaction with the surfaces of a Rh(100) single crystal. 9 For the 450 C-treated FeOx sample, no absorptions corresponding to ethylene glycol are observed at appreciable intensities over the ~ cm - 1 range when compared to the normalized relative intensities of the Fe-O stretching modes that are observed in the cm -1 region. 7 S-12

13 (A) EG/H 2 O-450 Relative percent transmittance (arbitrary intensity units) EG/H 2 O-200 Ethylene glycol (neat liquid) Wavenumbers (cm -1 ) (B) EG/H 2 O-450 Relative percent transmittance (arbitrary intensity units) EG/H 2 O-200 Ethylene glycol (neat liquid) ν s (C-O) ν as (C-O) ν(fe-o) region Wavenumbers (cm -1 ) Figure S10. ATR-FT-IR spectra of iron oxide nanomaterials and neat liquid ethylene glycol (EG) within the (A) cm -1 spectral range and (B) the expanded cm -1 spectral range; details for the notations used for the materials are provided in the main text. Features observed in the ~ cm -1 region are attributed to uncompensated adsorptions of the diamond ATR crystal. S-13

14 200 nm Figure S11. Scanning electron microscopy (SEM) image of iron oxide nanosheets prepared using ethylene glycol (EG) and H2O treated at 120 C under vacuum. S-14

15 EG/H 2 O-120v Relative intensity (counts) (100) (002) (101) EG/H 2 O-as prep δ-feooh: JCPDS # (012) (110) (220) (311) (222) (400) Fe 3 O 4 : JCPDS # (422) (511) (440) (210) (211) (220) (311) (222) (400) γ-fe 2 O 3 : JCPDS # (422) (511) (440) θ Figure S12. Comparison of powder X-ray diffraction (XRD) data of iron oxide samples prepared using ethylene glycol (EG) and H2O, as-prepared (EG/H2O-as prep), and treated at 120 C under vacuum (EG/H2O-120v); shown also are patterns for reference phases, as labeled. S-15

16 Relative scattering intensity (arbitrary units) EG/H 2 O- 120v EG/H 2 O- as prep Wavenumbers (cm -1 ) Figure S13. Comparison of Raman spectra in the cm -1 spectra region of iron oxide samples prepared using ethylene glycol (EG) and H2O, as prepared (EG/H2O-as prep), and treated at 120 C under vacuum (EG/H2O-120v). S-16

17 Voltage (V vs Li) (A) EG/H 2 O-as prep First cycle Second cycle Fifth cycle Charge Discharge Voltage (V vs Li) (B) EG/H 2 O-200 Charge Discharge Firsty cycle Second cycle Fifth cycle Specific capacity(mah/g) Specific capacity(mah/g) Voltage (V vs Li) (C) EG/H 2 O-350 Charge First cycle Second cycle fifth Fifth cycle Discharge Specific capacity(mah/g) Voltage (V vs Li) (D) EG/H 2 O-450 Charge First cycle Second cycle Fifth cycle Discharge Specific capacity(mah/g) Figure S14. Comparison of the voltage profile and specific capacities of iron oxide nanomaterials at 1 st, 2 nd and 5 th charge and discharge cycles; mass-normalized current of 3 ma g -1 : (A) EG/H2Oas-prep, (B) EG/H2O-200, (C) EG/H2O-350, and (D) EG/H2O-450; details for the notations used for the materials is provided in the main text. S-17

18 200 nm Figure S15. Scanning electron microscopy (SEM) image of iron oxide electrode (EG/H2O- 120v) after 10 charge/discharge cycles showing nanosheet morphology is retained after cycling. Sample was in the discharged state (1.5 V vs Li). Additional details are provided in the text and experimental section. S-18

19 Specific capacity (mah/g) γ-fe 2 O 3 nanosheets 30 ma/g 60 ma/g 30 ma/g 150 ma/g 300 ma/g 600 ma/g Filter paper Separator Cycle number Figure S16. Comparison of the capacities of γ-fe2o3 nanosheets at different mass-normalized rates as noted within the figure, using either commercial separator (MTI Corp.) or glass microfiber filter paper; 5 charge/discharge cycles were run at each rate. Table S2. Comparison of the capacities of γ-fe2o3 nanosheets at different rates using either commercial separator (MTI Corp.) or filter paper; average discharge capacity (mah g -1 ) from the 2 nd cycle at different mass normalized currents of 3, 30, 60, 150, 300 and 600 ma g -1. Material ID Average discharge capacity (mah/g) at mass normalized current 3 ma/g 30 ma/g 60 ma/g 150 ma/g 300 ma/g 600 ma/g EG/H 2O-as prep MTI separator EG/H 2O-as prep filter paper separator N/A S-19

20 Specific discharge capacity (mah g-1) 160 EG/H2O-as-prep EG/H2O EG/H2O EG/H2O BET surface area (m g-1) Figure S17. Plot of specific discharge capacity (mah g-1) at mass-normalized current of 3 ma g-1 vs. BET surface areas of the different iron oxide nanomaterials: as-prepared nanosheets EG/H2Oas-prep and thermally-treated samples, EG/H2O-200, EG/H2O-350, and EG/H2O mv s-1 5 mv s Current (A) 1 mv s mv s mv s Voltage (V vs Li) Figure S18. Comparison of CVs of commercial γ-fe2o3 at different scan rates of 0.1, 0.5, 1.0, 5.0 and 10 mv s-1; electrolyte: 1 M LiPF6 in EC:DEC; 1:1 v/v; counter/reference: metallic Li. S-20

21 mv s -1 5 mv s -1 Current (A) mv s mv s -1 1 mv s Voltage (V vs Li) Figure S19. Comparison of CVs of iron oxide NPs (EG/H2O-200) at different scan rates of 0.1, 0.5, 1.0, 5.0 and 10 mv s -1 ; electrolyte: 1 M LiPF6 in EC:DEC; 1:1 v/v; counter/reference: metallic Li. S-21

22 References (1) Refait, P.; Genin, J. M. R. The Oxidation of Ferrous Hydroxide in Chloride-Containting Aqueous Media and Pourbaix Diagrams of Green Rust Ore. Corrosion Sci. 1993, 34, (2) Gilbert, F.; Refait, P.; Leveque, F.; Remazeilles, C.; Conforto, E. Synthesis of Goethite from Fe(OH)2 Precipitates: Influence of Fe(II) Concentration and Stirring Speed. J. Phys. Chem. Solids 2008, 69, (3) Livage, J.; Henry, M.; Sanchez, C. Sol-gel Chemistry of Transition-metal Oxides. Prog. Solid State Chem. 1988, 18, (4) Pedrosa, J.; Costa, B. F. O.; Portugal, A.; Duraes, L. Controlled Phase Formation of Nanocrystalline Iron Oxides/Hydroxides in Solution - An Insight on the Phase Transformation Mechanisms. Mater. Chem. Phys. 2015, 163, (5) Ma, M.; Zhang, Y.; Guo, Z. R.; Gu, N. Facile Synthesis of Ultrathin Magnetic Iron Oxide Nanoplates by Schikorr reaction. Nanoscale Res. Lett. 2013, 8, 1-7. (6) Remazeilles, C.; Refait, P. Formation, fast oxidation and thermodynamic data of Fe(II) hydroxychlorides. Corrosion Sci. 2008, 50, (7) Duraes, L.; Moutinho, A.; Seabra, I. J.; Costa, B. F. O.; de Sousa, H. C.; Portugal, A. Characterization of iron(iii) oxide/hydroxide nanostructured materials produced by sol-gel technology based on the Fe(NO3)3 9H2O-C2H5OH-CH3CHCH2O system. Mater. Chem. Phys. 2011, 130, (8) Sawodny, W.; Niedenzu, K.; Dawson, J. W. Vibrational Spectrum of Ethylene Glycol. Spectrochim. Acta Mol. Biomol. Spectrosc. 1967, 23A, (9) Jansen, M. M. M.; Nieuwenhuys, B. E.; Niemantsverdriet, H. Chemistry of Ethylene Glycol on a Rh(100) Single-Crystal Surface. ChemSusChem 2009, 2, S-22