Experimental Elucidation of a Graphenothermal Reduction

Article pubs.acs.org/JPCC

Experimental Elucidation of a Graphenothermal Reduction Mechanism of Fe2O3: An Enhanced Anodic Behavior of an Exfoliated Reduced Graphene Oxide/Fe3O4 Composite in Li-Ion Batteries Shaikshavali Petnikota,†,‡,∥ Hussen Maseed,† V. V. S. S. Srikanth,*,† M. V. Reddy,*,‡,§ S. Adams,§ Madhavi Srinivasan,∥ and B. V. R. Chowdari‡,∥ J. Phys. Chem. C 2017.121:3778-3789. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/01/18. For personal use only.

†

School of Engineering Sciences and Technology, University of Hyderabad, Gachibowli, Hyderabad 500046, India Department of Physics and §Department of Materials Science and Engineering, National University of Singapore, Singapore 117546, Singapore ∥ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore ‡

S Supporting Information *

ABSTRACT: The graphenothermal reduction mechanism of Fe2O3 by graphene oxide (GO) is elucidated through careful experimental analysis. The degree of oxidation (DO) of GO plays a key role in controlling the reduction of Fe2O3 by GO. GO with low DO follows a conventional three-stage reaction path, i.e., ′2GO + Fe2O3 → EG/Fe3O4 (Stage I) → EG/FeO (Stage II) → EG/Fe (Stage III)′ (where EG is exfoliated reduced graphene oxide), at temperatures 650 and 750 °C to reduce Fe2O3, whereas the GO with higher DO transforms rapidly and ceases the reduction at Stage I, i.e., with the formation of EG/Fe3O4 at 650 °C. It is also found that slow thermal treatment of GO continues the reduction to Stage II and further to Stage III depending on time of heating and temperature. EG/Fe3O4 (synthesized at 550 °C, 5 h) by using GO with low DO showed superior cycling performance as an anode of Li-ion battery than its counterpart prepared (at 650 °C, 5 h) from GO with high DO owing to good contacts between EG and Fe3O4. EG/Fe3O4 (synthesized at 550 °C, 5 h) exhibited reversible capacity as high as 860 mAh/ g which is greater than the specific capacity of EG/Fe3O4 synthesized (at 650 °C, 5 h) by 150 mAh/g. Overall, EG/Fe3O4 (synthesized at 550 °C, 5 h) outperformed its counterpart (i.e., EG/Fe3O4 synthesized at 650 °C, 5 h) by exhibiting excellent cycling stability and rate capability at current rates ranging from 0.5 to 3.0 C. on FeO.12,13 We have recently shown that wrapping of graphene sheets on FeO will also give excellent results.8 In our recent work,8 an exfoliated reduced graphene oxide (EG)/FeO composite was synthesized using an easy solid state reduction process named as the Graphenothermal Reduction (GTR) process. Owing to the good synergy between EG and FeO, the EG/FeO composite performed far better than bare FeO.14 Here it should be mentioned that the EG/Fe3O4 composite was also prepared using the GTR process and was studied for its lithiation behavior.8 However, in the case of EG/ Fe3O4 composite, continuous capacity decay was noticed for which proper reasoning was not given in our previous work. In addition, a complete set of controlling parameters for the reduction of Fe2O3 in the GTR process were not identified due to the lack of understanding of the reduction mechanism. Here subsists the aim of this work, which is to experimentally elucidate the GTR mechanism of Fe2O3 by graphene oxide (GO). Further, it will also be shown how the understanding of the GTR mechanism led to the synthesis of large amounts of

1. INTRODUCTION Various anode materials for Li- and Na-ion batteries have been studied based on intercalation/deintercalation,1−4 conversion and alloying−dealloying reactions,5,6 and various other oxides which are summarized in Chemical Reviews.7 Among all iron oxides, those such as FeO and Fe3O4 are considered as suitable anode materials for lithium-ion batteries (LIBs) because of their characteristics like low weight, minimal volume changes during battery cycling, and low charging potentials.8 These characteristics also enable FeO and Fe3O4 to exhibit stable cycling performance with higher reversible capacities than Fe2O3 even though the theoretical capacity of Fe2O3 (1007 mAh/g) is slightly greater than that of FeO (744 mAh/g) and Fe3O4 (928 mAh/g).4−8 However, it should be noted that the performance of FeO and Fe3O4 is still limited owing to their poor intrinsic electronic conductivities and more importantly owing to their structural and phase instabilities. In order to overcome these limiting factors, FeO and Fe3O4 are supported by media such as highly conducting carbon additives or graphene. For example, there are reports on the preparation of graphene−Fe3O4 composites which performed extraordinarily as anode materials in LIBs.9−11 Similarly, the instability of FeO was resolved by making FeO/C composites in which C often acted as a capping © 2017 American Chemical Society

Received: December 10, 2016 Revised: January 17, 2017 Published: January 20, 2017 3778

DOI: 10.1021/acs.jpcc.6b12435 J. Phys. Chem. C 2017, 121, 3778−3789

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The Journal of Physical Chemistry C

Figure 1. (a) X-ray diffractograms of graphite, GO1 and GO2, (b) FTIR transmittance spectra of GO1 and GO2, and (c) Raman scattering spectra of graphite, GO1, and GO2.

was dispersed uniformly in 25 mL of H2SO4 (95%) at a temperature less than 5 °C. Next, 0.5 g of NaNO3 and 3 g of KMnO4 were mixed very slowly one after the other. After stirring for 30 min at room temperature 25 mL of distilled water was added to the reaction mixture. Consequently the temperature of the reaction mixture was raised to ∼98 °C. Stirring was continued at this temperature for 1 h. Subsequently, the reaction mixture was allowed to cool to room temperature. Finally, 1 mL of H2O2 (35%) was added to the reaction mixture resulting in sedimentation. The sediment was subjected to multiple washes with distilled water until supernatant’s pH became ∼7. The resultant material is GO1. In another independent experiment, to obtain GO with higher DO (GO2), the above-mentioned process was repeated except for using 98% H2SO4 and 50 mL of distilled water. GO1 and GO2 were then dried for 12 h in a hot air oven at 80 °C. Then, two reaction mixtures (RM), GO1 and Fe2O3 (RM1) and GO2 and Fe2O3 (RM2), were prepared by mechanical mixing for 30 min. In both RM1 and RM2 cases, 2 mol of GO was taken for 1 mol of Fe2O3. Molecular formula C2HO was used for GOs for molar calculations.8 Then both reaction powders, 6 g each, were taken into separate ceramic boats and heated separately in a tubular furnace (Carbolite, UK). Heating of both reaction powders was carried out in Ar environment at heating and cooling rates of 5 °C min−1. Thermal treatment of reaction powders was carried out in the temperature range 650−750 °C and time range 2−8 h. As a result of heating, black colored fine powders of EG/iron oxide or EG/iron composites of ∼3 g each were obtained. 2.2. Characterization of Materials. X-ray diffraction (XRD) patterns were recorded in the 2θ range 5−80° using Cu Kα as the X-ray source (λ = 1.54 Å) (Philips X’PERT MPD unit, PANalytical). XRD results were analyzed by Rietveld refinement via TOPAS (v2.1). Fourier transform infrared (FTIR) spectra were recorded in the wavenumber range 4000−

EG/FeO and EG/Fe3O4 composites. More importantly, it should be noted that Fe2O3 is very complex and sensitive in nature, and therefore to control its reduction either by solid state or gaseous reducing species is difficult. Reduction of Fe2O3 by GO is totally different than the typical reduction process because GO offers its carbons, while gaseous species are produced during the thermal treatment; i.e., both solid and gaseous state reduction are possible at the same time. In such circumstances it is difficult to control the reduction, while the end product could be Fe3O4 or FeO or a mixture of both. It is therefore intriguing to understand the reduction mechanism of Fe2O3 by GO such that the desired iron oxides or their EG composites are obtained in scalable amounts. In this regard, in this work, the degree of oxidation (DO) of GO, heating time, and temperature have been found to be the controlling parameters for the reduction of Fe2O3 by GO. If the GTR process is done in a controlled manner, the end product can be only one among EG/Fe3O4, EG/FeO, and EG/Fe composites. The importance of this work also lies in the bulk synthesis of the stable EG/Fe3O4 composite because this composite can be of great help in other key applications such as removing various bacteria, oils from water,15,16 supercapacitors,17−19 electromechanical actuation,20 catalysis and electrocatalysis,11,21 magnetic applications,22−24 electromagnetic shielding,25−27 magnetic-controlled switches,28 magnetic phase separation,29,30 heavy metal and dye pollutant removal,31−36 biomedical applications,37−40 gas barriers,41 CO2 capture,42 sensors,43−45 desalination,46 etc.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. GO was synthesized from graphite (flake size ≤47 μm) by the modified Hummers method.8 Fe2O3 used in the present study was obtained by slowly (3 °C/min) heating FeC2O4·2H2O at 600 °C for 6 h in air. To prepare GO with low DO (GO1), 1 g of graphite flakes 3779


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Figure 2. XPS spectra of C 1s in (a) GO1, (b) GO2, and remnants of (c) RM1 and (d) RM2 after thermal treatment at 650 °C for 5 h.

400 cm−1 by using Bruker Equinox 55 FTIR instrument. Raman spectra were recorded in the range 500−3500 cm−1 using an Nd:YAG 532 nm laser in the back scattering geometry. The CRM spectrometer equipped with a confocal microscope and 100× objective (focal spot size diameter ∼1 μm) with a CCD detector (model Alpha 300 of WI Tec, Germany) was used to record Raman spectra. X-ray photoelectron spectroscopy (XPS) survey spectra in the range 0−1200 eV were recorded by using an AXIS ultra DLD spectrometer (Kratos Analytica) with monochromatic Al Kα radiation. The survey spectra were analyzed with Casa XPS software, and charge referencing was carried out against adventitious carbon C (C 1s binding energy = 284.6 eV). Thermogravimetric analysis (TGA) of GO1, GO2, RM1, and RM2 was performed in a N2 environment in the temperature range 25−1000 °C and at a heating rate of 5 °C min−1 by using TA Instruments 2960 (DTA-TGA). Field emission scanning electron microscope (FESEM, Ultra 55 of Carls Zeiss-Germany) images were recorded at an accelerating voltage of 5 kV. Transmission electron microscope (TEM, FEI Technai G2 S-Twin) images were recorded at an accelerating voltage of 200 kV. Energydispersive spectroscopy (EDS, AMETEK, APPOLO XLT2) in the TEM was used to record the composition of GO1 and GO2. 2.3. Electrochemical Characterization. LIBs in the form of coin cells (size 2016) were fabricated following exactly the same procedure as reported previously.1 Battery testing was carried out at room temperature after 8 h of relaxation. The cyclic voltammetry (CV) measurements were carried out in the voltage range 0.005−3.0 V at a scan rate of 0.1 mV/s using SOLARTRON 1470E equipment. The charge−discharge tests were carried out in the same voltage range as in the case of CV measurements but at current rates ranging from 0.1 to 3.0 C (C = 750 mA/g) using a NEWARE battery test system.

3. RESULTS AND DISCUSSION XRD patterns of graphite, GO1, and GO2 are shown in Figure 1(a). In the case of graphite, the sharp and intense Bragg’s peak at 26.5° corresponds to the diffraction from the (002) basal planes. In the case of GO1 and GO2, the Bragg’s peak corresponding to (002) basal planes appeared at 13.4 and 10.4°, respectively, as a consequence of various functional groups attached to the basal planes. The observed peak shift is equivalent to an increased interplanar distance of 0.66 nm in GO1 and 0.85 nm in GO2 when compared to 0.34 nm in graphite. The observed increase in interplanar distance signifies that the basal planes of GO2 are populated with a larger number of functional groups than those of GO1. Based on the XRD and XPS (to be discussed) inferences,47,48 DOs of GO1 and GO2 are calculated as 48 and 63%, respectively, which are in good agreement with the reported values.48 The type of functional groups attached to the basal planes in GO1 and GO2 is elucidated by FTIR spectra shown in Figure 1(b). GO1 contained strong absorption bands corresponding to the functional groups CO at 1723.7, CC at 1621.7, C− OH at 1400.8, C−O−C at 1220.9, and C−O at 1055.1 cm−1, while respective bands in GO2 are found at 1719.4, 1577.8, 1430.5, 1162.8, and 1059.4 cm−1; in addition, the OC−OH band was found at 1637.3 cm−1 in the case of GO2.47−54 In the case of GO2, the absorbance of the CC band has been observed to decrease, whereas the absorbance of the C−O band has been observed to increase in comparison to the same in the case of GO1. Further, in GO2 the regions of C−OH and C− O−C bands have been split into two or three sub-bands owing to an increase in their number as well as addition of new functional groups.49,51,54 These FTIR observations are an indication for increased oxidation in GO2 in good agreement with the XRD results. Raman spectroscopy results showed that DO has no influence on the positions of D and G bands (which are 3780


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Figure 3. TEM images of (a) GO1 and (b) GO2 with insets showing the corresponding electron diffraction patterns.

Figure 4. TGA of GO1 and GO2 and the corresponding reaction mixtures RM1 and RM2 at 5 °C min−1 heating rate.

found at typically at ∼1350 and ∼1588 cm−1) in both GO1 and GO2 cases as shown in Figure 1(c). No change in the G-band’s position signifies that even though DO was increased by 15% there are no considerable new in-plane sp3 carbon atoms in basal planes of graphene sheets.47,49 This inference implies that basal planes in both GO1 and GO2 are anchored with similar and the same amount of functional groups despite increased DO and further indicates that DO increased due to the attachment of more functional groups at edges and defect sites. This situation might plausibly be originated due to the effort to increase DO, i.e., due to an increase in the concentration of H2SO4 rather than increasing oxidizers as pursued in other reports.47−49 However, increased DO influenced the D-band’s intensity which decreased in good agreement with the reported work.49 The significant decrease in intensity of D-band in GO2 relative to GO1 also indicates that a high extent of oxidation took place in the case of GO2.49 Another important inference provided by Raman scattering analysis is the low discernibility of the 2D-band (in both GO1 and GO2 cases) which is an indication for complete stacking disorder along the crystallographic c-axis.49,55 Discernibility is poor in the case of GO2

when compared to the GO1 case, which is evidence for high DO in GO2.49,55 Figures 2(a) and (b) show the high-resolution C 1s spectra of GO1 and GO2, respectively. The spectra in both the cases appear similar, but peaks pertinent to GO2 are more intense than those to GO1. This is an indication for increased oxidation in GO2. The C 1s spectrum of GO1 could be deconvoluted into five peaks which correspond to CC (sp2 C) centered at 284 eV, C−C/H (sp3 C) centered at 284.8 eV, C−OH centered at 285.7 eV, C−O−C centered at 287 eV, and CO centered at 288.7 eV. On the other hand, the C 1s spectrum of GO2 could be deconvoluted too into five peaks which correspond to C−C/H centered at 284.8 eV, C−OH centered at 285.7 eV, C−O−C centered at 287 eV, CO centered at 288 eV, and C−OOH centered at 289 eV.47,49,53,56,57 The absence of a CC signal, appearance of a −COOH signal, and increased intensity count of C−OH and C−O−C peaks in the case of GO2 clearly indicate that GO2 is oxidized more than GO1. These observations are in tune with the inferences from FTIR spectroscopy studies. The nature of remnants of GO1 and GO2 after thermal treatment of reaction mixtures RM1 and RM2 is studied by 3781


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Figure 5. X-ray diffraction patterns: Effect of temperature and heating time on the structure of the resultant material in the case of (a) RM1 and (b) RM2.

functional groups and loss of carbon atoms, respectively, from basal planes.51−54,56,58 These two peaks are found to be centered at 197 and 549 °C, respectively, in the case of GO2 (Figure 4(b)). It is noteworthy that the area under peak1 of GO2 is greater than that of GO1 due to loss of more functional groups. This is in good agreement with XRD, FTIR, Raman, and XPS inferences. The area under peak 2 of GO1 is greater than that of GO2 which is an indication for sublimation of more backbone carbons from GO1 than from GO2. The weight loss features of reaction mixtures (RM1 = GO1 + Fe2O3, Figure 4(d), and RM2 = GO2 + Fe2O3, Figure 4(e)) appeared as replicas of respective individual GO characteristics with slight broadening of peaks 1 and 2. In the case of RM1, peaks 1 and 2 are centered at 189 and 499 °C, respectively, while in the case of RM2 they are centered at 194 and 492 °C, respectively. In addition, RM1 differs from RM2 by an extra small plateau type peak 3 centered around 680 °C. The combined plotting of TGA curves of GO1 and GO2 clearly showed that GO2 transformed rapidly at slightly lower temperatures (20−50 °C less) than GO1 as shown in Figure 4(c). Corresponding reaction mixtures too have shown similar weight losses but difference between maximum weight losses widened by 100− 200 °C as shown in Figure 4(f). In all four cases initial incomplete weight loss derivative peaks can be accredited to the evaporation of water molecules preferably from GOs.8,51−54,56,58 Now it is obvious that the weight loss (peak 1 related) in both RM1 and RM2 is contributed by respective GO alone by giving up functional groups, but RM1 lost its maximum weight in between 500 and 750 °C (peak 2 and 3) that corresponds to the reduction of both GO1 (exfoliation to EG) and Fe2O3 as well as subsequent formation of iron oxides like Fe3O4 and FeO. A considerable weight loss around 570 °C (peak 2) is a good enough evidence for the formation and stabilization of FeO. Further weight loss is plausible owing to the reduction of FeO to Fe (peak 3). Thus, overall weight loss can be represented by the conventional three-stage reduction path 2GO + Fe2O3 → EG/Fe3O4 (Stage I) → EG/FeO (Stage II) → EG/Fe (Stage III). On the other hand RM2 possesses its maximum weight loss below 500 °C (peak 2) which corresponds to formation of only Fe3O4 as temperature is not enough to form FeO and Fe (which is consistent with XRD findings, to be discussed). In this case, the reduction process may have been plausibly ceased at Stage I, i.e., 2GO + Fe2O3 → EG/Fe3O4 (Stage I) for the reason that a feeble weight loss is

probing the corresponding C 1s spectra (Figures 2(c) and (d), respectively), which revealed that in both cases the EG possessed residual functional groups in proportion to their starting materials GO1 and GO2. It is common to have residual functional groups such as hydroxyl (−OH), epoxide (C−O− C), and keto (CO) which are proven to be stable up to a temperature of 700 °C.50−52,54−59 In brief, the C 1s spectrum of EG/FeO (remnant of GO1) was deconvoluted into four peaks centered at 284.8, 285.3, 286.2, and 287.5 eV which correspond to graphitic carbons in sp2 and sp3 bonding modes, carbons bonded to hydroxyl and epoxide groups, respectively. These peaks are in proportion to those in the case of GO1 (Figure 2(a)). In the case of EG/Fe3O4, in addition to the peaks at 284.1, 285.1, 286.1, and 287.4 eV, there is an additional peak at 288.6 eV that corresponds to carbon bonded to oxygen (keto groups) that too is in proportion to those in the case of GO2 (Figure 2(b)). Thus, overall XPS study is in good agreement with XRD, FTIR, and Raman analysis. TEM observations of GO1 and GO2 are found to be in good agreement with XRD, FTIR, Raman, and XPS inferences. As shown in Figure 3, GO2 is found to be more (semi) transparent than GO1 which is an indication for higher oxidation. The selected area electron diffraction is a ring pattern (unlike a hexagonal pattern) in the case where the GO1 sheet signifies that it is well oxidized. The ring pattern is comprised of doublet and triplet atomic projections in the case of GO1. This is an indication for the presence of a larger number of graphene layers (basal planes) which are folded and crumpled. On the other hand, in the case of GO2, the ring pattern constituted blurred singlet atomic projections that signify that it has a smaller number of graphene layers in comparison to GO1. Another inference it gives is that the basal plane is not much disordered;59 i.e., a smaller number of functional groups are attached to the basal planes in strong agreement with Raman inferences. The elemental mapping showed that GO1 and GO2 possess O/C atomic ratios of 0.408 and 0.625, respectively, which is again an indication for increased oxidation in GO2. The temperature-dependent weight loss characteristics of GO1 and GO2 and RM1 and RM2 are studied in order to understand the reduction mechanism. To bring more clarity about thermal decomposition, first-order derivatives of weight losses are plotted as shown in Figures 4(a), (b), (d), and (e). GO1 showed two clear peaks centered at 179 °C (peak 1) and 592 °C (peak 2) that are representative of the breakup of 3782


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The Journal of Physical Chemistry C observed between 500 and 1000 °C. To know more about the type of iron oxides formed in the vicinity of peak 2 (as functional groups are released at peak 1 could be less effective in reducing Fe2O3 for the reason that the reaction temperature is less than 200 °C), XRD studies were carried out. X-ray diffractograms of heat-treated RM1 and RM2 at different times and temperatures (Figures 5(a) and (b)) are considered for pin-pointing the reduction process by identifying the types of iron compounds that are retained. In order to correlate with the inferences of TGA and good exfoliation of GO (above 600 °C8,50−52,54−60), the structure of end products of RM1 and RM2 at 650 and 750 °C are probed using XRD (Figure 5). At these temperatures, the type of reducing species (mainly CO and C) and their participation in reduction of Fe2O3 is wellknown. However, various types of plausible reduction reactions mentioned in earlier reports are shown in the Supporting Information (SI). As shown in Figure 5(a), at 650 °C RM1 showed the coexistence of Fe3O4 and FeO for 2 to 4 h of heating, while 5 h of heating appears to be the optimal heating time to retain the pure FeO phase. Further prolonged heating for say 6 h or more leads to the formation of Fe out of FeO. A similar increase in the reaction temperature to say 750 °C leads to the direct formation of Fe which is an indication of rapid reduction. In brief, RM1, after 2 h of heating at 650 °C contained 94.7% of FeO and 5.4% of Fe3O4 phases along with trace amounts of the unreacted Fe2O3 phase (indexed as α in Figure 5(a)) which is an indication that most of the reduction has completed at once. Further increase in heating time to 5 h resulted in the increase of the FeO phase to 99.6%, while the Fe3O4 phase decreased to 0.4%. After 8 h of prolonged heating, the FeO phase decreased to 97.5%, while the Fe3O4 phase increased to 1.8% with evolution of 0.7% of the Fe phase. 100% Fe phase formation was observed when heating temperature increased to 750 °C even for 5 h. Thus, heating at 650 °C up to 5 h is direct evidence for Stage I and Stage II reductions, whereas an increase in time to more than 5 h or an increase in temperature to 750 °C is a direct evidence for Stage III reduction. An increase or decrease in average crystallite sizes of individual phases is in agreement with heating time and reduction process. On the other hand, at 650 °C, RM2 contained the pure Fe3O4 phase even for 2 h of heating. The so-formed Fe3O4 phase has been found intact upon prolonged heating say up to 8 h as shown in Figure 5(b). An increase in average crystallite size with heating time is an indication for continuous agglomeration of individual Fe3O4 particles rather than phase change. Further, it can be expected that the Fe3O4 phase could be retained even at higher temperatures beyond 650 °C as there is no significant weight loss as observed in the TGA curve of RM2. Even doubling the amount of GO2 resulted again in the Fe3O4 formation which further confirms that an increase in GO2 weakly affects the reduction process. Thus, the XRD inferences in the case of RM2 are a direct evidence for Stage I reduction only. Detailed Rietveld refinement analysis of each XRD pattern is summarized in Tables S1 and S2 for RM1 and RM2, respectively. Thus, the overall XRD findings are consistent with TGA analysis. Investigation of morphology of EG/Fe3O4 and EG/FeO composites that are prepared at 650 °C by heating for 2 and 5 h, respectively, revealed the plausible causes for the already discussed XRD and TGA observations. Figures 6(a−c) show FESEM micrographs of the EG/Fe3O4 composite with poor contacts between EG and Fe3O4 nanoparticles. This situation

Figure 6. FESEM micrographs of (a−c) EG/Fe3O4-650 °C-2 h and (d) EG/FeO-650 °C-5 h.

might have arisen due to rapid transformation of GO, i.e., its rapid exfoliation to form EG, and the fast gust of various gaseous species those degassing from EG due to decomposition of various functional groups could be collectively pushing freshly formed Fe3O4 particles away from contacting EG. Otherwise the topographic nature of EG does not readily allow freshly formed Fe3O4 particles to accommodate on it. Thus, formed loose contacts between EG and Fe3O4 clarify why the reduction process stopped at stage I even for 2 to 8 h of heating time and negligible weight loss of RM2 beyond 500 °C as observed in TGA analysis. It appears that highly oxidized GO loses its functional groups rapidly around 500 °C (within 20 min from 400 to 500 °C) to form various gaseous species including H*/CO which might have simultaneously reduced Fe2O3 to Fe3O4 along with carbon atoms that are in contact with EG prior to exfoliation. As observed in TGA, after 500 °C no more weight loss of reaction mixture means neither gaseous species nor carbon atoms are available for reduction as the temperature and time of heating increased in agreement with XRD results. Thus, further reduction by either the gaseous or solid state reducing agents ceases, and EG/Fe3O4 is retained even for prolonged heating times and high temperatures. On the other hand Figure 6(d) shows the existence of EG flakes and FeO particles together in a manner that they are tightly adhered with each other in the EG/FeO composite. A close observation of FeO nanoparticles shows that they are aggregated as shown in Figure 6(d). This situation might be possible as a consequence of relatively slow transformation of GO1 (in comparison to GO2), which exfoliates slowly and releases less gaseous species compared to GO2 and provides good contacts between reduced iron compounds and EG throughout the heating time and temperatures as observed in XRD and TGA studies. In contrast to FeO, Fe3O4 particles possessed very smooth surface finish, and there is no sign of the presence of EG between them as shown in Figures 6(a−c). Additionally, the morphology of FeO and Fe3O4 particles is totally different in comparison to parent Fe2O3 material as shown in Figure S1. The starting Fe2O3 powder was found to contain large slab-like particulates (2−20 μm long). After reduction their size reduced to a great extent, i.e., to a few submicrons in EG/FeO and few nanometers in EG/Fe3O4 composites, respectively, along with shape change. Thus, GO, irrespective of its DO, not only reduces Fe2O3 but also induces 3783


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cooling. Nonetheless, it is known that the FeO phase readily converts either to Fe3O4 or Fe2O3. Thus, EG or FLG acts like a protecting layer to FeO similar to carbon coating or organic surfactants. Combined morphology observations of FESEM and TEM suggest that the reduction process in RM1 continued through all stages, i.e., 2GO1 + Fe2O3 → EG/Fe3O4 (Stage I) → EG/FeO (Stage II) → EG/Fe (Stage III), depending on time of heating and temperature owing to good contacts between iron oxides and EG, while in RM2 reduction stopped at stage I, i.e., 2GO2 + Fe2O3 → EG/Fe3O4 (Stage I), for all the considered heating times and even for all temperatures above 500 °C as illustrated by XRD and TGA analysis. Further, postsynthesis XPS studies of EG/FeO and EG/ Fe3O4 prepared at 650 °C and 5 h heating corroborated with the XRD studies. The Fe 2p spectrum pertaining to EG/FeO looks similar to that of EG/Fe3O4 (pure Fe3O4) owing to the presence of trace amounts of the Fe3O4 phase in it, but they mainly differ in their peak positions as shown in Figures 8(a) and (c). The binding energies of a characteristic doublet of Fe 2p in EG/FeO are found to be 1 eV lesser than EG/Fe3O4 in agreement with the available reports.63−67 The positions of the 2p3/2 and 2p1/2 peaks are centered at 710 and 723.8 eV, respectively, and in the case of EG/FeO those are matching well with reported values of Fe2+ in FeO.63−68 The respective peaks in EG/Fe3O4 are found centered at 711 and 724.7 eV, consistent with Fe2.5+ in Fe3O4.63−67,69 Moreover the absence of satellite peak(s) between 2p3/2 and 2p1/2 doublets is an indication for complete conversion of starting Fe2O3 to either FeO or Fe3O4.64−67,69 Further, O 1s spectra (Figures 8(b) and (d)) show peaks at 530.1 and 530.4 eV pertaining to EG/FeO and EG/Fe3O4, respectively, that are linked to the oxygen directly bonded to Fe in lattices of FeO and Fe3O4, respectively.65−67,69,70 The other two peaks at 531.8 and 533.3 eV correspond to oxygen present in the residual

shape and crystallinity changes which was also noticed in the other metal oxide case studies.60−62 Close examination (TEM and HRTEM images) of individual iron oxide particles revealed that FeO particles are either wrapped or covered by FLG (Figures 7(a,b)), while Fe3O4

Figure 7. HRTEM images of (a,b) EG/FeO-650 °C-5 h and (c,d) EG/Fe3O4-650 °C-2 h.

particles are in their purest form (Figures 7(c,d)). Here, in situ stabilization of FeO particles will be achieved when they are either wrapped or covered by FLG during either heating or

Figure 8. High-resolution XPS spectra of Fe 2p (top left) and O 1s (top right) in EG/FeO and Fe 2p (bottom left) and O 1s (bottom right) in EG/ Fe3O4. 3784


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those observed in EG/Fe3O4-650 °C-5 h (Figure 6). However, here Fe3O4 particles are spread across surfaces (Figure 9 (a) and (c)) and formed a network around edges of EG (Figure 9(b) and (d)). The presence of thin EG flakes between Fe3O4 particles signifies that they adhered well to EG unlike as shown in Figure 6. Thus, the contacts between EG and Fe3O4 phases in EG/Fe3O4-550 °C-5 h seem to have far improved than in EG/Fe3O4-650 °C-5 h. This shows that DO of GO plays a major role in establishing contacts between EG and reduced iron oxides. There are no substantial differences found between XRD patterns of EG/Fe3O4-550 °C-5 h (Figure S2) and EG/ Fe3O4-650 °C-5 h (Figure 5(b)) that suggest that EG/Fe3O4550 °C-5 h contains a pure Fe3O4 phase. It possesses an average crystallite size of 57.6 nm with lattice parameter a = 8.4086 Å when refined w.r.t. space group Fd3̅m (PDF#75-449). Individual weight contents of EG/Fe3O4-550 °C-5 h are estimated from TGA (Figure S3) by subtracting theoretical weight gain (3.3%) due to oxidation of Fe3O4 (to Fe2O3) from final residual mass left. The estimated weight percentages of EG and Fe3O4 are 30.8 and 69.2, respectively, and are equivalent to a theoretical capacity of approximately 753 mAh/g. In view of the above observations and inferences, it is now apparent that EG/Fe3O4-550 °C-5h should exhibit far better behavior (than other composites) as an anode material in LIBs. Electrochemical characterization results of EG/Fe3O4-550 °C-5 h as an anode material in LIBs in a typical half-cell configuration are shown in Figure 10. The observed CV (Figure 10(a)) and charge−discharge (Figure 10(b)) characteristics are consistent with each other. For example large reduction current and redox couple noticed in CV are in accordance with large

functional groups of EG and adsorbed moisture, respectively.66,70 In view of the above understanding of the reduction mechanism as well as success and failure stories of EG/FeO and EG/Fe3O4 as anode materials in LIB, respectively, EG/ Fe3O4 was synthesized from RM1 by heating it at 550 °C for 5 h. In this case (EG/Fe3O4-550 °C-5 h) size and shape of Fe3O4 nanoparticles as shown in Figure 9 are found to be similar to

Figure 9. (a−d) FESEM micrographs of Fe3O4 nanoparticles well adhered to EG surfaces in EG/Fe3O4-550 °C-5 h.

Figure 10. Electrochemical characterization results of EG/Fe3O4-550 °C-5 h as an anode material in half-cell configuration. (a) CV curves at 0.1 mV/s, (b) charge−discharge curves at 0.1 C, (c) capacity versus cycle number at 0.5 C in comparison of EG/Fe3O4-650 °C-5 h, and (d) rate capability studies. 3785


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reduction process. Thus, it is possible to control the reduction of Fe2O3 with GO by varying GO’s DO and heat treatment time and temperature. DO of GO is a key factor to decide the nature of EG-reduced iron oxide composites and their electrochemistry. EG/Fe3O4 composite prepared by using GO1 exhibited better cycling performance with reversible capacity as high as 860 mAh/g owing to good contacts between EG and Fe3O4 phases. The EG/Fe3O4 composite prepared in this work can perform even better if uniform distribution of Fe3O4 nanoparticles on EG surfaces is achieved.

discharge capacity and charge−discharging potentials of galvanostatic cycling, respectively. The large reduction current in the first cathodic scan of CV and large first discharge capacity are a combined consequence of electrolyte decomposition as well as electrochemical reaction between Li and Fe3O4 to form elemental Fe and Li2O compounds.8,10,11 These characteristic reactions are partially reversible, and that is why they do not appear from the first anodic scan and first charging onward. The broad current peaks centered on 0.9 and 0.18 V correspond to multiple redox reactions between Li/Fe3O4 and Li2O/Fe, respectively.8,10,11 In both testing modes the entire nonzero current and capacity at all voltages other than redox reactions of iron oxide originate from (de)lithiation by EG.71,72 These overall electrochemical features are commensurate with other graphene−Fe3O4 composites reported elsewhere.8,10,11 During first discharging at 0.1 C rate EG/Fe3O4550 °C-5 h is found capable of storing lithium up to a maximum of 1364 mAh/g, but out of this, 860 mAh/g of capacity can be reversible during the first charge that is equivalent to 63% of Coulombic efficiency (QE). Soon after the first few cycles onward QE values are found at about 98% until the end of cycling as shown in Figure 10(c) for 0.5 C cycling. The rapid capacity decay that occurred up to the first five cycles of 0.5 C cycling is a consequence of electrode formation effect in which the anode matrix undergoes structural changes to achieve improved electrical contacts and proper percolation of the electrolyte. In addition, SEI formation and partly irreversible electrochemical rupturing of Fe3O4 crystals during the first cycle are also responsible for the observed capacity decay. Once the electrode formation completes, stable capacity was achieved as observed in Figure 10(c). The combination of losses increases with increasing current rate, and for this reason 0.1 C (Figure 10(b)) cycling differs slightly from 0.5 C (Figure 10(c)). Here EG/Fe3O4-550 °C-5 h showed good cycling stability with 150 mAh/g of more capacity than EG/Fe3O4-650 °C-5 h. EG/Fe3O4-550 °C-5 h exhibited good rate capability at current rates ranging from 0.5 to 3.0 C as shown in Figure 10(d). When current was switched back to 0.5 C from 3.0 C, the full capacity was not regained, but the increasing capacity trend at both 0.5 and 0.1 C signifies it takes a longer aging time to come back to the starting capacity values. The observed capacity increase is a consequence either from in situ electrochemical exfoliation of EG72 or from enhancement of iron oxide reversibility or contribution from both the factors simultaneously. EG/Fe3O4-550 °C-5 h showed better capacity values than its class of materials such as EG/FeO, graphene− Fe3O4 composites.8,9 EG/Fe3O4-550 °C-5 h could outperform other advanced graphene−Fe3O4 composites11,73−78 if clustering is avoided and uniform distribution of Fe3O4 nanoparticles on EG surfaces is achieved. Such improvements can be pursued by mixing initial reagents in solution medium.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12435. Plausible reduction reactions between GO and Fe2O3, Rietveld analysis of XRD patterns of RM1and RM2, SEM images of the starting Fe2O3 bulk powder, Rietveld refinement analysis of the XRD pattern of EG/Fe3O4550 °C-5 h, and TGA analysis of EG/Fe3O4-550 °C-5 h (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +91 40 2313 4453 E-mail: [email protected] (V. V. S. S. Srikanth). *Tel: +65 6516 2605. E-mail: [email protected]; [email protected]; [email protected] (M. V. Reddy). ORCID

V. V. S. S. Srikanth: 0000-0002-3021-0987 M. V. Reddy: 0000-0002-6979-5345 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS PSS and HM are grateful to the Government of India for providing financial support through Maulana Azad National Fellowship (PSS: 201112-MANF-MUS-AND-213 and HM: 201213-MANF-2012-13-MUS-AND-16231) to pursue research studies at the University of Hyderabad. PSS is thankful to NUS for financial support through the India research initiative (NUS-IRI) fund (WBS No: R069000006646). MVR thanks National Research Foundation, Prime Minister’s Office, Singapore, for the research support under its Competitive Research Programme (CRP Award No. NRF-CRP 10-2012-6).

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4. CONCLUSIONS In summary, the reducing ability of GO w.r.t. to Fe2O3 can be tailored with its DO. GO with low DO anneals slowly and simultaneously reduces Fe2O3 to Fe3O4, FeO, and Fe depending on heating time and temperature. Thus, it leads to a three-stage reduction represented by ′2GO1 + Fe2O3 → EG/ Fe3O4 (Stage I) → EG/FeO (Stage II) → EG/Fe (Stage III)′. In contrast, GO with high DO anneals rapidly and restricts to only the first stage reduction ′2GO2 + Fe2O3 → EG/Fe3O4 (Stage I)′; i.e., it forms only Fe3O4 for all times of heating. Even doubling the amount of GO2 was found to be ineffective in the

REFERENCES

(1) Cherian, C. T.; Reddy, M. V.; Magdaleno, T.; Sow, C. H.; Ramanujachary, K. V.; Subba Rao, G. V.; Chowdari, B. V. R. (N,F)Co-Doped TiO(2): Synthesis, Anatase-Rutile Conversion and LiCycling Properties. CrystEngComm 2012, 14 (3), 978−986. (2) Reddy, M. V.; Teoh, X. W. V.; Nguyen, T. B.; Lim, Y. Y. M.; Chowdari, B. V. R. Effect of 0.5 M NaNO3: 0.5 M KNO3 and 0.88 M LiNO3:0.12 M LiCl Molten Salts, and Heat Treatment on Electrochemical Properties of TiO2. J. Electrochem. Soc. 2012, 159 (6), A762−A769. (3) Zhang, Y.; Foster, C. W.; Banks, C. E.; Shao, L.; Hou, H.; Zou, G.; Chen, J.; Huang, Z.; Ji, X. Graphene-Rich Wrapped Petal-Like 3786


Article


(22) Su, J.; Cao, M.; Ren, L.; Hu, C. Fe3O4−Graphene Nanocomposites with Improved Lithium Storage and Magnetism Properties. J. Phys. Chem. C 2011, 115, 14469−14477. (23) Liao, Z.-M.; Wu, H.-C.; Wang, J.-J.; Cross, G. L. W.; Kumar, S.; Shvets, I. V.; Duesberg, G. S. Magnetoresistance of Fe3O4-grapheneFe3O4 Junctions. Appl. Phys. Lett. 2011, 98, 052511. (24) He, H.; Gao, C. Supraparamagnetic, Conductive, and Processable Multifunctional Graphene Nanosheets Coated with High-Density Fe3O4 Nanoparticles. ACS Appl. Mater. Interfaces 2010, 2, 3201−3210. (25) Jian, X.; Wu, B.; Wei, Y.; Dou, S. X.; Wang, X.; He, W.; Mahmood, N. Facile Synthesis of Fe3O4/GCs Composites and Their Enhanced Microwave Absorption Properties. ACS Appl. Mater. Interfaces 2016, 8, 6101−6109. (26) Shen, B.; Zhai, W.; Tao, M.; Ling, J.; Zheng, W. Lightweight, Multifunctional Polyetherimide/Graphene@Fe3O4 Composite Foams for Shielding of Electromagnetic Pollution. ACS Appl. Mater. Interfaces 2013, 5, 11383−11391. (27) Qu, B.; Zhu, C.; Li, C.; Zhang, X.; Chen, Y. Coupling Hollow Fe3O4−Fe Nanoparticles with Graphene Sheets for High-Performance Electromagnetic Wave Absorbing Material. ACS Appl. Mater. Interfaces 2016, 8, 3730−3735. (28) Liang, J.; Xu, Y.; Sui, D.; Zhang, L.; Huang, Y.; Ma, Y.; Li, F.; Chen, Y. Flexible, Magnetic, and Electrically Conductive Graphene/ Fe3O4 Paper and Its Application for Magnetic-Controlled Switches. J. Phys. Chem. C 2010, 114, 17465−17471. (29) Liu, Q.; Shi, J.; Wang, T.; Guo, F.; Liu, L.; Jiang, G. Hemimicelles/Admicelles Supported on Magnetic Graphene Sheets for Enhanced Magnetic Solid-Phase Extraction. J. Chromatogr., A 2012, 1257, 1−8. (30) Li, X.; Wang, X.; Song, S.; Liu, D.; Zhang, H. Selectively Deposited Noble Metal Nanoparticles on Fe3O4/Graphene Composites: Stable, Recyclable, and Magnetically Separable Catalysts. Chem. Eur. J. 2012, 18, 7601−7607. (31) Xie, G.; Xi, P.; Liu, H.; Chen, F.; Huang, L.; Shi, Y.; Hou, F.; Zeng, Z.; Shao, C.; Wang, J. A Facile Chemical Method to Produce Superparamagnetic Graphene Oxide-Fe3O4 Hybrid Composite and its Application in the Removal of Dyes From Aqueous Solution. J. Mater. Chem. 2012, 22, 1033−1039. (32) Dubey, R.; Bajpai, J.; Bajpai, A. K. Green Synthesis of Graphene sand Composite (GSC) as Novel Adsorbent for Efficient Removal of Cr (VI) Ions From Aqueous Solution. J. Water Process Eng. 2015, 5, 83−94. (33) Geng, Z.; Lin, Y.; Yu, X.; Shen, Q.; Ma, L.; Li, Z.; Pan, N.; Wang, X. Highly Efficient Dye Adsorption and Removal: A Functional Hybrid of Reduced Graphene Oxide-Fe3O4 Nanoparticles as an Easily Regenerative Adsorbent. J. Mater. Chem. 2012, 22, 3527−3535. (34) Li, J.; Zhang, S.; Chen, C.; Zhao, G.; Yang, X.; Li, J.; Wang, X. Removal of Cu(II) and Fulvic Acid by Graphene Oxide Nanosheets Decorated with Fe3O4 Nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 4991−5000. (35) Mishra, A. K.; Ramaprabhu, S. Ultrahigh Arsenic Sorption Using Iron Oxide-Graphene Nanocomposite Supercapacitor Assembly. J. Appl. Phys. 2012, 112, 104315. (36) Venkateswarlu, S.; Lee, D.; Yoon, M. Bioinspired 2D-Carbon Flakes and Fe3O4 Nanoparticles Composite for Arsenite Removal. ACS Appl. Mater. Interfaces 2016, 8, 23876−23885. (37) Ma, X.; Tao, H.; Yang, K.; Feng, L.; Cheng, L.; Shi, X.; Li, Y.; Guo, L.; Liu, Z. A Functionalized Graphene Oxide-Iron Oxide Nanocomposite for Magnetically Targeted Drug Delivery, Photothermal Therapy, and Magnetic Resonance Imaging. Nano Res. 2012, 5, 199−212. (38) Deng, L.; Li, Q.; Al-Rehili, S. a.; Omar, H.; Almalik, A.; Alshamsan, A.; Zhang, J.; Khashab, N. M. Hybrid Iron Oxide− Graphene Oxide−Polysaccharides Microcapsule: A Micro-Matryoshka for On-Demand Drug Release and Antitumor Therapy In Vivo. ACS Appl. Mater. Interfaces 2016, 8, 6859−6868. (39) Chen, W.; Yi, P.; Zhang, Y.; Zhang, L.; Deng, Z.; Zhang, Z. Composites of Aminodextran-Coated Fe3O4 Nanoparticles and

Rutile TiO2 tuned by Carbon Dots for High-Performance Sodium Storage. Adv. Mater. 2016, 28, 9391−9399. (4) Zhang, Y.; Yang, Y.; Hou, H.; Yang, X.; Chen, J.; Jing, M.; Jia, X.; Ji, X. Enhanced sodium storage behavior of carbon coated anatase TiO2 hollow spheres. J. Mater. Chem. A 2015, 3, 18944−18952. (5) (a) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions. Adv. Mater. 2010, 22 (35), E170−E192. (b) Reddy, M. V.; Yu, C.; Fan, J. H.; Loh, K. P.; Chowdari, B. V. R. Li-Cycling Properties of Molten Salt Method Prepared Nano/Submicrometer and Micrometer-Sized CuO for Lithium Batteries. ACS Appl. Mater. Interfaces 2013, 5 (10), 4361−4366. (6) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for LiIon Batteries. Chem. Rev. 2014, 114 (23), 11444−11502. (7) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113 (7), 5364−5457. (8) Petnikota, S.; Marka, S. K.; Banerjee, A.; Reddy, M. V.; Srikanth, V. V. S. S.; Chowdari, B. V. R. Graphenothermal Reduction Synthesis of ‘Exfoliated Graphene Oxide/Iron (II) Oxide’ Composite for Anode Application in Lithium Ion Batteries. J. Power Sources 2015, 293, 253− 263. (9) Wu, Z.-S.; Zhou, G.; Yin, L.-C.; Ren, W.; Li, F.; Cheng, H.-M. Graphene/Metal Oxide Composite Electrode Materials For Energy Storage. Nano Energy 2012, 1, 107−131. (10) Hameed, A. S.; Reddy, M. V.; Chowdari, B. V. R.; Vittal, J. J. Preparation of rGO-Wrapped Magnetite Nanocomposites and Their Energy Storage Properties. RSC Adv. 2014, 4, 64142−64150. (11) Zhao, B.; Zheng, Y.; Ye, F.; Deng, X.; Xu, X.; Liu, M.; Shao, Z. Multifunctional Iron Oxide Nanoflake/Graphene Composites Derived from Mechanochemical Synthesis for Enhanced Lithium Storage and Electrocatalysis. ACS Appl. Mater. Interfaces 2015, 7, 14446−14455. (12) Gao, M.; Zhou, P.; Wang, P.; Wang, J.; Liang, C.; Zhang, J.; Liu, Y. FeO/C Anode Materials of High Capacity and Cycle Stability for Lithium-Ion Batteries Synthesized by Carbothermal Reduction. J. Alloys Compd. 2013, 565, 97−103. (13) Hou, Y.; Xu, Z.; Sun, S. Controlled Synthesis and Chemical Conversions of FeO Nanoparticles. Angew. Chem., Int. Ed. 2007, 46, 6329−6332. (14) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized Transition-Metal Oxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496−499. (15) Cong, H.-P.; Ren, X.-C.; Wang, P.; Yu, S.-H. Macroscopic Multifunctional Graphene-Based Hydrogels and Aerogels by a Metal Ion Induced Self-Assembly Process. ACS Nano 2012, 6, 2693−2703. (16) Zhan, S.; Zhu, D.; Ma, S.; Yu, W.; Jia, Y.; Li, Y.; Yu, H.; Shen, Z. Highly Efficient Removal of Pathogenic Bacteria with Magnetic Graphene Composite. ACS Appl. Mater. Interfaces 2015, 7, 4290− 4298. (17) Qu, Q.; Yang, S.; Feng, X. 2D Sandwich-like Sheets of Iron Oxide Grown on Graphene as High Energy Anode Material for Supercapacitors. Adv. Mater. 2011, 23, 5574−5580. (18) Shi, W.; Zhu, J.; Sim, D. H.; Tay, Y. Y.; Lu, Z.; Zhang, X.; Sharma, Y.; Srinivasan, M.; Zhang, H.; Hng, H. H.; Yan, Q. Achieving High Specific Charge Capacitances in Fe3O4/Reduced Graphene Oxide Nanocomposites. J. Mater. Chem. 2011, 21, 3422−3427. (19) Zhao, C.; Shao, X.; Zhang, Y.; Qian, X. Fe2O3/Reduced Graphene Oxide/Fe3O4 Composite in Situ Grown on Fe Foil for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 30133−30142. (20) Liang, J.; Huang, Y.; Oh, J.; Kozlov, M.; Sui, D.; Fang, S.; Baughman, R. H.; Ma, Y.; Chen, Y. Electromechanical Actuators Based on Graphene and Graphene/Fe3O4 Hybrid Paper. Adv. Funct. Mater. 2011, 21, 3778−3784. (21) Ding, Y.; Cheng, H.; Zhou, C.; Fan, Y.; Zhu, J.; Shao, H.; Qu, L. Functional Microspheres of Graphene Quantum Dots. Nanotechnology 2012, 23, 255605. 3787


Article

The Journal of Physical Chemistry C Graphene Oxide for Cellular Magnetic Resonance Imaging. ACS Appl. Mater. Interfaces 2011, 3, 4085−4091. (40) Swain, A. K.; Pradhan, L.; Bahadur, D. Polymer Stabilized Fe3O4-Graphene as an Amphiphilic Drug Carrier for ThermoChemotherapy of Cancer. ACS Appl. Mater. Interfaces 2015, 7, 8013−8022. (41) Jiao, W.; Shioya, M.; Wang, R.; Yang, F.; Hao, L.; Niu, Y.; Liu, W.; Zheng, L.; Yuan, F.; Wan, L.; He, X. Improving the Gas Barrier Properties of Fe3O4/Graphite Nanoplatelet Reinforced Nanocomposites by a Low Magnetic Field Induced Alignment. Compos. Sci. Technol. 2014, 99, 124−130. (42) Mishra, A.; Ramaprabhu, S. Enhanced CO2 Capture in Fe3O4Graphene Nanocomposite by Physicochemical Adsorption. J. Appl. Phys. 2014, 116, 064306. (43) Song, Y.; He, Z.; Hou, H.; Wang, X.; Wang, L. Architecture of Fe3O4−Graphene Oxide Nanocomposite and its Application as a Platform for Amino Acid Biosensing. Electrochim. Acta 2012, 71, 58− 65. (44) Ye, Y.; Kong, T.; Yu, X.; Wu, Y.; Zhang, K.; Wang, X. Enhanced Nonenzymatic Hydrogen Peroxide Sensing With Reduced Graphene Oxide/Ferroferric Oxide Nanocomposites. Talanta 2012, 89, 417− 421. (45) Wang, L.; Zhang, Y.; Cheng, C.; Liu, X.; Jiang, H.; Wang, X. Highly Sensitive Electrochemical Biosensor for Evaluation of Oxidative Stress Based on the Nanointerface of Graphene Nanocomposites Blended with Gold, Fe3O4, and Platinum Nanoparticles. ACS Appl. Mater. Interfaces 2015, 7, 18441−18449. (46) Narayanan, T. N.; Liu, Z.; Lakshmy, P. R.; Gao, W.; Nagaoka, Y.; Sakthi Kumar, D.; Lou, J.; Vajtai, R.; Ajayan, P. M. Synthesis of reduced graphene oxide−Fe3O4 Multifunctional Freestanding Membranes and Their Temperature Dependent Electronic Transport Properties. Carbon 2012, 50 (3), 1338−1345. (47) Kadam, M. M.; Lokare, O. R.; Kireeti, K. V. M. K.; Gaikar, V. G.; Jha, N. Impact of the Degree of Functionalization of Graphene Oxide on the Electrochemical Charge Storage Property and Metal Ion Adsorption. RSC Adv. 2014, 4, 62737−62745. (48) Yan, H.; Tao, X.; Yang, Z.; Li, K.; Yang, H.; Li, A.; Cheng, R. Effects of the Oxidation Degree of Graphene Oxide on the Adsorption of Methylene Blue. J. Hazard. Mater. 2014, 268, 191−198. (49) Krishnamoorthy, K.; Veerapandian, M.; Yun, K.; Kim, S. J. The Chemical and Structural Analysis of Graphene Oxide With Different Degrees of Oxidation. Carbon 2013, 53, 38−49. (50) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural Evolution During the Reduction of Chemically Derived Graphene Oxide. Nat. Chem. 2010, 2 (7), 581− 587. (51) Acik, M.; Mattevi, C.; Gong, C.; Lee, G.; Cho, K.; Chhowalla, M.; Chabal, Y. J. The Role of Intercalated Water in Multilayered Graphene Oxide. ACS Nano 2010, 4, 5861−5868. (52) Jeong, H.-K.; Lee, Y. P.; Jin, M. H.; Kim, E. S.; Bae, J. J.; Lee, Y. H. Thermal Stability of Graphite Oxide. Chem. Phys. Lett. 2009, 470, 255−258. (53) Su, C.; Acik, M.; Takai, K.; Lu, J.; Hao, S.-j.; Zheng, Y.; Wu, P.; Bao, Q.; Enoki, T.; Chabal, Y. J.; Ping Loh, K. Probing the Catalytic Activity of Porous Graphene Oxide and the Origin of This Behaviour. Nat. Commun. 2012, 3, 1298. (54) Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. The Role of Oxygen during Thermal Reduction of Graphene Oxide Studied by Infrared Absorption Spectroscopy. J. Phys. Chem. C 2011, 115, 19761−19781. (55) González, Z.; Botas, C.; Á lvarez, P.; Roldán, S.; Blanco, C.; Santamaría, R.; Granda, M.; Menéndez, R. Thermally Reduced Graphite Oxide as Positive Electrode in Vanadium Redox Flow Batteries. Carbon 2012, 50, 828−834. (56) Ganguly, A.; Sharma, S.; Papakonstantinou, P.; Hamilton, J. Probing the Thermal Deoxygenation of Graphene Oxide Using HighResolution In Situ X-ray-Based Spectroscopies. J. Phys. Chem. C 2011, 115, 17009−17019.

(57) Pei, S.; Cheng, H.-M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210−3228. (58) Botas, C.; Á lvarez, P.; Blanco, C.; Santamaría, R.; Granda, M.; Gutiérrez, M. D.; Rodríguez-Reinoso, F.; Menéndez, R. Critical Temperatures in the Synthesis of Graphene-Like Materials by Thermal Exfoliation−Reduction Of Graphite Oxide. Carbon 2013, 52, 476− 485. (59) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New Insights Into the Structure and Reduction Of Graphite Oxide. Nat. Chem. 2009, 1, 403−408. (60) Petnikota, S.; Srikanth, V. V. S. S.; Nithyadharseni, P.; Reddy, M. V.; Adams, S.; Chowdari, B. V. R. Sustainable Graphenothermal Reduction Chemistry to Obtain MnO Nanonetwork Supported Exfoliated Graphene Oxide Composite and its Electrochemical Characteristics. ACS Sustainable Chem. Eng. 2015, 3, 3205−3213. (61) Petnikota, S.; Teo, K. W.; Chen, L.; Sim, A.; Marka, S. K.; Reddy, M. V.; Srikanth, V. V. S. S.; Adams, S.; Chowdari, B. V. R. Exfoliated Graphene Oxide/MoO2 Composites as Anode Materials in Lithium-Ion Batteries: An Insight into Intercalation of Li and Conversion Mechanism of MoO2. ACS Appl. Mater. Interfaces 2016, 8, 10884−10896. (62) Petnikota, S.; Marka, S. K.; Srikanth, V. V. S. S.; Reddy, M. V.; Chowdari, B. V. R. Elucidation of Few Layered Graphene-Complex Metal Oxide (A2Mo3O8, A = Co, Mn and Zn) Composites As Robust Anode Materials in Li Ion Batteries. Electrochim. Acta 2015, 178, 699− 708. (63) Swiatkowska-Warkocka, Z.; Kawaguchi, K.; Wang, H.; Katou, Y.; Koshizaki, N. Controlling Exchange Bias in Fe3O4/FeO Composite Particles Prepared by Pulsed Laser Irradiation. Nanoscale Res. Lett. 2011, 6, 226. (64) Moussy, J.-B. From Epitaxial Growth of Ferrite Thin Films to Spin-Polarized Tunnelling. J. Phys. D: Appl. Phys. 2013, 46, 143001. (65) Joseph, Y.; Ketteler, G.; Kuhrs, C.; Ranke, W.; Weiss, W.; Schlogl, R. On the Preparation and Composition of Potassium Promoted Iron Oxide Model Catalyst Films. Phys. Chem. Chem. Phys. 2001, 3 (18), 4141−4153. (66) Yamashita, T.; Hayes, P. Analysis of XPS Spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441−2449. (67) Abreu, G. J. P.; Paniago, R.; Pfannes, H. D. Growth of UltraThin FeO(100) Films on Ag(100): A Combined XPS, LEED and CEMS Study. J. Magn. Magn. Mater. 2014, 349, 235−239. (68) Merte, L. R.; Shipilin, M.; Ataran, S.; Blomberg, S.; Zhang, C.; Mikkelsen, A.; Gustafson, J.; Lundgren, E. Growth of Ultrathin Iron Oxide Films on Ag(100). J. Phys. Chem. C 2015, 119 (5), 2572−2582. (69) Fujii, T.; De Groot, F.; Sawatzky, G.; Voogt, F.; Hibma, T.; Okada, K. In situ XPS Analysis of Various Iron Oxide Films Grown by NO2-Assisted Molecular-Beam Epitaxy. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 59, 3195−3202. (70) Eltouny, N.; Ariya, P. A. Competing Reactions of Selected Atmospheric Gases on Fe3O4 Nanoparticles Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 23056−23066. (71) Petnikota, S.; Rotte, N.; Srikanth, V. S. S.; Kota, B. R.; Reddy, M. V.; Loh, K.; Chowdari, B. V. R. Electrochemical Studies of FewLayered Graphene as an Anode Material for Li Ion Batteries. J. Solid State Electrochem. 2014, 18, 941−949. (72) Petnikota, S.; Rotte, N. K.; Reddy, M. V.; Srikanth, V. V. S. S.; Chowdari, B. V. R. MgO-Decorated Few-Layered Graphene as an Anode for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 2301− 2309. (73) Chen, D.; Ji, G.; Ma, Y.; Lee, J. Y.; Lu, J. Graphene-Encapsulated Hollow Fe3O4 Nanoparticle Aggregates As a High-Performance Anode Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2011, 3, 3078−3083. (74) Zhao, J.; Yang, B.; Zheng, Z.; Yang, J.; Yang, Z.; Zhang, P.; Ren, W.; Yan, X. Facile Preparation of One-Dimensional Wrapping Structure: Graphene Nanoscroll-Wrapped of Fe3O4 Nanoparticles and Its Application for Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2014, 6, 9890−9896. 3788


Article

The Journal of Physical Chemistry C (75) Yang, Y.; Li, J.; Chen, D.; Zhao, J. A Facile Electrophoretic Deposition Route to the Fe3O4/CNTs/rGO Composite Electrode as a Binder-Free Anode for Lithium Ion Battery. ACS Appl. Mater. Interfaces 2016, 8, 26730−26739. (76) Qi, L.; Xin, Y.; Zuo, Z.; Yang, C.; Wu, K.; Wu, B.; Zhou, H. Grape-Like Fe3O4 Agglomerates Grown on Graphene Nanosheets for Ultrafast and Stable Lithium Storage. ACS Appl. Mater. Interfaces 2016, 8, 17245−17252. (77) Zhao, L.; Gao, M.; Yue, W.; Jiang, Y.; Wang, Y.; Ren, Y.; Hu, F. Sandwich-Structured Graphene-Fe3O4@Carbon Nanocomposites for High-Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 9709−9715. (78) Hu, X.; Ma, M.; Zeng, M.; Sun, Y.; Chen, L.; Xue, Y.; Zhang, T.; Ai, X.; Mendes, R. G.; Rümmeli, M. H.; Fu, L. Supercritical Carbon Dioxide Anchored Fe3O4 Nanoparticles on Graphene Foam and Lithium Battery Performance. ACS Appl. Mater. Interfaces 2014, 6, 22527−22533.

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Experimental Elucidation of a Graphenothermal Reduction

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