Experimental Elucidation of a Graphenothermal Reduction

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Experimental Elucidation of Graphenothermal Reduction Mechanism of FeO: An Enhanced Anodic Behavior of Exfoliated Reduced Graphene Oxide/FeO Composite in Li-Ion Batteries 2

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Shaikshavali Petnikota, Hussen Maseed, Vadali V. S. S. Srikanth, Mogalahalli Venkatashamy Reddy, Stefan Adams, Madhavi Srinivasan, and Bobba Venkateshwara Rao Chowdari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12435 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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The Journal of Physical Chemistry

Experimental Elucidation of Graphenothermal Reduction Mechanism of Fe2O3: An Enhanced Anodic Behavior of Exfoliated Reduced Graphene Oxide/Fe3O4 Composite in Li-Ion Batteries Shaikshavali Petnikota1,2,4, Hussen Maseed1, V. V. S. S. Srikanth1,*, M. V. Reddy2,3,†, S. Adams3, Madhavi Srinivasan4 and B. V. R. Chowdari2,4 1

School of Engineering Sciences and Technology, University of Hyderabad, Gachibowli,

Hyderabad 500046, India 2

Department of Physics, National University of Singapore, Singapore 117542, Singapore

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Department of Materials Science and Engineering, National University of Singapore, Singapore

117542, Singapore 4

School of Materials Science and Engineering, Nanyang Technological University, Singapore,

639798, Singapore ABSTRACT: 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 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/𝐹𝑒3 𝑂4 (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

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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 C to 3.0 C. 1. INTRODUCTION Various anode materials for Li and Na-ion batteries have been studied based on intercalation/deintercalation1-4, conversion and alloying-dealloying reactions5-6 and various other oxides which are are summarized in Chemical Reviews.7 Among all iron oxides such as FeO and Fe3O4 are considered as suitable anode materials for lithium-ion batteries (LIB) 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 high 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).8-4 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 preparation of graphene-Fe3O4 composites which performed extraordinarily as anode materials in LIB.9-11 Similarly, instability of FeO was resolved by making FeO/C composites in which C often acted as a capping 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 exfoliated reduced graphene oxide (EG)/FeO composite was synthesized using an easy solid state reduction process named as 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 EG/Fe3O4 composite was also prepared using 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, complete set of controlling parameters for the reduction of Fe2O3 in 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


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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 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 typically 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 amongst EG/Fe3O4, EG/FeO and EG/Fe composites. The importance of this work also lies in the bulk synthesis of 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,

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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 bio-medical applications37-40, gas barriers,41 CO2 capture,42 sensors,4345

desalination46 etc.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. GO was synthesized from graphite (flake size ≤ 47 μm) by 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 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 raised to ~98 °C. Stirring was continued at this temperature for 1 h. Subsequently, the reaction mixture was allowed to cool down 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



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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.1 Then both reaction powders, 6 g each were taken into separate ceramic boats and heated separately in a tube 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=1.54Å) (Philips X’PERT MPD unit, PANalytical). XRD results were analysed by Rietveld refinement via TOPAS (v2.1). Fourier transform infrared (FTIR) spectra were recorded in the wavenumber range 4000–400 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. CRM spectrometer equipped with a confocal microscope and 100X 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 AlKα radiation. The survey spectra were analyzed with Casa XPS software and charge referencing was carried out against adventitious carbon C (C1s binding energy = 284.6 eV). Thermogravimetric analysis (TGA) of GO1, GO2, RM1 and RM2 was performed in N2 environment in the temperature range 25–1000 °C and at a heating rate of 5 °C min-1 by using TA instrument 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. Energy Dispersive Spectroscope (EDS, AMETEK, APPOLO XLT2) in the TEM was used to record the composition of GO1 and GO2.


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2.3. Electrochemical Characterization. LIB in the form of coin cells (size 2016) were fabricated followed exactly the same procedure 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 C to 3.0 C (C = 750 mA/g) using NEWARE battery test system. 3. RESULTS AND DISCUSSION XRD patterns of graphite, GO1 and GO2 are shown in Fig. 1(a). In the case of Graphite, the sharp and intense Bragg’s peak at 26.5° corresponds to the diffraction from (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 shif is equivalent to an increased inter-planar 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 more number of functional groups than those of GO1. Based on the XRD and XPS (to be discussed) inferences4748

, DO of GO1 and GO2 are calculated as 48 and 63%, respectively which are in good agreement

with the reported values.48

Fig. 1. (a) X-ray diffractograms of graphite, GO1 and GO2, (b) FITR transmittance spectra of GO1 and GO2, and (c) Raman scattering spectra of Graphite, GO1 and GO2. 5 Environment ACS Paragon Plus


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The type of functional groups attached to the basal planes in GO1 and GO2 are elucidated by FITR spectra shown in Fig. 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 whilst respective bands in GO2 are found at 1719.4, 1577.8, 1430.5, 1162.8 and 1059.4 cm-1 and in addition, 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 C=C band has been observed to decrease whereas absorbance of 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 subbands owing to 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 found at typically at ~1350 and ~1588 cm-1) in both GO1 and GO2 cases as shown in Fig. 1(c). No change in G-band’s position signifies that even though OD 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 same amount of functional groups despite of increased DO and further indicates that DO increased due to 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 increase in the concentration of H2SO4 rather than increasing oxidizers as pursued in other reports.47-49 But 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 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 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 GO1 case, which is an evidence for high DO in GO2.49, 55 Figures 2(a) and (b) show the high resolution C1s 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. C1s spectrum of GO1 could be deconvoluted into five peaks which correspond to C=C (sp2 C) centered at 284 eV, C–C/H (sp3


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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, C1s 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 C=C signal, appearance of –COOH signal, 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.

Fig. 2. XPS spectra of C1s in (a) GO1, (b) GO2, and remnants of (c) RM1 and (d) RM2 after thermal treatment at 650 °C for 5 h. The nature of remnants of GO1 and GO2 after thermal treatment of reaction mixtures RM1 and RM2 is studied by probing the corresponding C1s spectra (Figs. 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 C1s 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 (Fig. 2(a)). In the



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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 in proportion to those in the case of GO2 (Fig. 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 Fig. 3, GO2 is found to be more (semi) transparent than GO1which is an indication for higher oxidation. The selected area electron diffraction is a ring pattern (unlike a hexagonal pattern) in the case of GO1 sheet signifies that it as well oxidized. The ring pattern comprised of doublet and triplet atomic projections in the case of GO1. This is an indication for the presence of more 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 less number of graphene layers in comparison to GO1. Another inference it gives is that the basal plane is not much disordered59 i.e., less 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.

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


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in the case of GO2 (Fig. 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, Fig. 4(d) and RM2 = GO2+Fe2O3, Fig. 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, whilst 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 combine 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 Fig. 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 Fig. 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 plausibly 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 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/𝐹𝑒3 𝑂4 (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, reduction process may have been plausibly ceased at Stage I i.e., 2𝐺𝑂 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/𝐹𝑒3 𝑂4 (𝑆𝑡𝑎𝑔𝑒 𝐼) for the reason that a feeble weight loss is 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 (Figs. 5(a) and (b)) are considered for pin-pointing the reduction process



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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 (Fig. 5). At these temperatures, the type of reducing species (mainly CO and C) and their participation in reduction of Fe2O3 is well-known. However various types of plausible reduction reactions mentioned in earlier reports are shown in Supporting Information (SI) 1.

Fig. 4. TGA of GO1 and GO2 and the corresponding reaction mixtures RM1 and RM2 at 5 °C min-1 heating rate. As shown in Fig. 5(a), at 650 °C RM1 showed the co-existence of Fe3O4 and FeO for 2 to 4 h of heating whilst 5 h of heating appears to be the optimal heating time to retain pure FeO phase. Further prolonged heating say 6 h or more lead to the formation of Fe out of FeO. Similarly increase in the reaction temperature to say 750 °C lead 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% Fe3O4 phases along with trace amounts of unreacted Fe2O3 phase (indexed as α in Fig. 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 FeO phase to 99.6% whilst Fe3O4 phase decreased to 0.4%. After 8 h of prolonged heating, FeO phase decreased to 97.5% whilst Fe3O4 phase increased to 1.8% with evolution of 0.7% of 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 are in agreement with heating time and reduction process. On the other hand, at 650 °C, RM2 contained pure Fe3O4


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phase even for 2 h of heating. So-formed Fe3O4 phase has been found intact upon prolonged heating say up to 8 h as shown in Fig. 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 Fe3O4 phase could be retained even at higher temperatures beyond 650 °C as there is no significant weight loss as observed in 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.

Fig. 5. Effect of temperature and heating time on the structure of the resultant material in the case of (a) RM1 and (b) RM2. 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 EG/Fe3O4 composite with poor contacts between EG and Fe3O4 nanoparticles. This situation 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 pushed 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 lose contacts between EG and Fe3O4 clarifies that 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 °C to 500 °C) to form various



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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 either by gaseous or solid state reducing agents ceases and EG/Fe3O4 is retained even for prolonged heating times and high temperatures. On the other hand Fig. 6(d) shows existence of EG flakes and FeO particles together in a manner that they are tightly adhered with each other in EG/FeO composite. A close observation of FeO nanoparticles shows that they are aggregated as shown in Fig. 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 in between them as shown in Figs. 6(a-c). Additionally, morphology of FeO and Fe3O4 particles is totally different in comparison to parent Fe2O3 material as shown in Fig. S1. The starting Fe2O3 powder was found to contain large slablike particulates (2–20 µm long). After reduction their size reduced to a great extent i.e., to few sub-microns 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 shape and crystallinity changes which was also noticed in the other metal oxide case studies.60-62

Fig. 6. FESEM micrographs of (a-c) EG/Fe3O4-650 °C-2 h and (d) EG/FeO-650 °C-5 h. 12 Environment ACS Paragon Plus

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Close examination (TEM and HRTEM images) of individual iron oxides particles revealed that FeO particles are either wrapped or covered by FLG (Figs. 7(a-b)) whilst Fe3O4 particles are in their purest form (Figs. 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 cooling. Nonetheless, it is known that 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 reduction process in RM1 continued through all stages

i.e.,

2𝐺𝑂1 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/𝐹𝑒3 𝑂4 (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 whilst in RM2 reduction stopped at stage I i.e. 2𝐺𝑂2 + 𝐹𝑒3 𝑂4 → 𝐸𝐺/𝐹𝑒2 𝑂3 (𝑆𝑡𝑎𝑔𝑒 𝐼) for all the considered heating times and even for all temperatures above 500 °C as illustrated by XRD and TGA analysis.

Fig. 7. HRTEM images of (a–b) EG/FeO-650 °C-5 h and (c–d) EG/Fe3O4-650 °C-2 h. Further, post synthesis XPS studies of EG/FeO and EG/Fe3O4 those prepared at 650 °C and 5h heating corroborated with the XRD studies. Fe 2p spectrum pertaining to EG/FeO looks similar to that of EG/Fe3O4 (pure Fe3O4) owing to the presence of trace amounts of Fe3O4 phase in it.



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But they mainly differ in their peak positions as shown in Figs. 8(a) and (c). The binding energies of characteristic doublet of Fe 2p in EG/FeO are found 1 eV lesser than EG/Fe3O4 in agreement with the available reports.63-67 The positions of 2p3/2 and 2p1/2 peaks are centered at 710 and 723.8 eV, respectively 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) in-between 2p3/2 and 2p1/2 doublet is an indication for complete conversion of starting Fe2O3 to either FeO or Fe3O4.64-67,

69

Further, O1s spectra (Figs. 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 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 functional groups of EG and adsorbed moisture, respectively.66, 70

Fig. 8. High resolution XPS spectra of Fe 2p (top left) and O1s (top right) in EG/FeO and Fe 2p (bottom left) and O1s (bottom right) in EG/Fe3O4. In view of the above understanding the reduction mechanism as well as success and failure story 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 5h, respectively. In this case (EG/Fe3O4–550 °C–5h) size and shape of Fe3O4 nanoparticles as shown in Fig. 9, are found similar to those observed in EG/Fe3O4–650 °C–5h (Fig.6). But here Fe3O4 particles are spread across surfaces (Fig. 9 (a) and 14 Environment ACS Paragon Plus

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(c)) and formed a network around edges of EG (Fig. 9 (b) and (d)). The presence of thin EG flakes in-between Fe3O4 particles signifies that they adhered well to EG unlike as shown in Fig. 6. Thus, the contacts between EG and Fe3O4 phases in EG/Fe3O4–550 °C–5h seem to have far improved than in EG/Fe3O4–650 °C–5h. 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–5h (Fig. S2) and EG/Fe3O4–650 °C–5h (Fig. 5(b)) that suggest that EG/Fe3O4–550 °C–5h contains pure Fe3O4 phase. It possess an average crystallite size of 57.6 nm with lattice parameter a = 8.4086 Å when refined w.r.t space group Fd-3m (PDF#75-449). Individual weight contents of EG/Fe3O4–550 °C–5h are estimated from TGA (Fig. 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 LIB.

Fig. 9. (a-d) FESEM micrographs Fe3O4 nanoparticles well adhered to EG surfaces in EG/Fe3O4–550 °C–5h. Electrochemical characterization results of EG/Fe3O4–550 °C–5h as an anode material in LIB in a typical half-cell configuration are shown in Fig. 10. The observed CV (Fig. 10(a)) and chargedischarge (Fig. 10(b)) characteristics are consistent with each other. For example large reduction



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current and redox couple noticed in CV are in accordance with large discharge capacity and charge-discharging potentials of galvanostatic cycling, respectively. The large reduction current in 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 first anodic scan and first charging onwards. 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 non-zero current and capacity at all voltages other than redox reactions of iron oxide originates 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/Fe3O4–550 °C–5h is found capable of storing lithium up to maximum of 1364 mAh/g but out of this, 860 mAh/g of capacity can be reversible during first charge that is equivalent to 63% of Coulombic efficiency (QE). Soon after first few cycles onwards QE values are found about 98% till the end of cycling as shown in Fig. 10(c) for 0.5 C cycling. The rapid capacity decay that occurred up to first five cycles of 0.5 C cycling is a consequence of electrode formation effect in which 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 first cycle are also responsible for the observed capacity decay. Once the electrode formation completes, stable capacity was achieved as observed in Fig. 10(c). The combination of loses increases with increasing current rate and for this reason 0.1 C (Fig. 10 (b)) cycling differs slightly from 0.5 C (Fig. 10(c)). Here EG/Fe3O4–550 °C–5h showed good cycling stability with 150 mAh/g of more capacity than EG/Fe3O4–650 °C–5h. EG/Fe3O4–550 °C–5h exhibited good rate capability at current rates ranging from 0.5 to 3.0 C as shown in Fig. 10(d). When current was switched back to 0.5 C from 3.0 C, the full capacity was not regained but increasing capacity trend at both 0.5 and 0.1 C signifies it takes longer aging time to come back to the starting capacity values. The observed capacity increase is a consequence either from insitu electrochemical exfoliation of EG72 or enhancement of iron oxides reversibility or contribution from both the factors simultaneously. EG/Fe3O4–550 °C–5h showed better capacity values than its class of materials such as EG/FeO, graphene-Fe3O4 composites.8-9 EG/Fe3O4–550 °C–5h could outperform other advanced graphene-Fe3O4 composites11,


73-78

if clustering is

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avoided and uniform distribution of Fe3O4 nanoparticles on EG surfaces is achieved. Such improvements can be pursued by mixing initial reagents in solution medium.

Fig. 10. Electrochemical characterization results of EG/Fe3O4–550 °C–5h as an anode material in LIB in half cell configuration; (a) CV curves at 0.1 mV/s, (b) charge-discharge curves at 0.1 C, (c) cyclic performance at 0.5 C in comparison of EG/Fe3O4–650 °C–5h and (d) rate capability. 4. CONCLUSIONS In summary, 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 ꞌ2𝐺𝑂1 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/𝐹𝑒3 𝑂4 (𝑆𝑡𝑎𝑔𝑒 𝐼) → 𝐸𝐺/𝐹𝑒𝑂(𝑆𝑡𝑎𝑔𝑒 𝐼𝐼) → 𝐸𝐺/𝐹𝑒(𝑆𝑡𝑎𝑔𝑒 𝐼𝐼𝐼)ꞌ. In contrast, GO with high DO anneals rapidly and restricts to only first stage reduction ꞌ2𝐺𝑂2 + 𝐹𝑒2 𝑂3 → 𝐸𝐺/ 𝐹𝑒3 𝑂4 (𝑆𝑡𝑎𝑔𝑒 𝐼)ꞌ i.e., it forms only Fe3O4 for all times of heating. Even doubling the amount of GO2 was found to be ineffective on the reduction process. Thus it is possible to control the reduction of Fe2O3 with GO by varying GOꞌs 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. EG/Fe3O4 composite prepared in this work can perform even better if uniform distribution of Fe3O4 nanoparticles on EG surfaces is achieved. 17 Environment ACS Paragon Plus


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ASSOCIATED CONTENT Supporting Information. 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 XRD pattern of EG/Fe3O4–550°C–5h, TGA analysis of EG/Fe3O4–550°C–5h. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel: +91 40 2313 4453; E-mail address: [email protected] (V. V. S. S. Srikanth). †

Corresponding author. Tel: +65 6516 2605; E-mail address: [email protected];

[email protected]; [email protected] (M. V. Reddy). 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. ACKNOWLEDGMENT PSS and HM are grateful to Government of India for providing financial support through Maulana Azad National Fellowship (PSS: 201112-MANF-MUS-AND-213 and HM: 201213MANF-2012-13-MUS-AND-16231) to pursue research studies at the University of Hyderabad. PSS is thankful to NUS for financial support through 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). 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 Li-Cycling Properties. Cryst. Eng. Comm. 2012, 14 (3), 978-986. 18 Environment ACS Paragon Plus

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TOC Graphic


Experimental Elucidation of a Graphenothermal Reduction

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