Transition metal oxides on reduced graphene oxide (TMO@rGO) nanocomposites were successfully prepared via a very simple one-step solvothermal process, involving the simultaneous (thermal) reduction of graphene oxide to graphene and the deposition of TMO nanoparticles over its surface. Texture and morphology, microstructure, and chemical and surface compositions of the nanocomposites were investigated via scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. The results prove that Fe2O3@rGO, CoFe2O4@rGO, and CoO@rGO are obtained by using Fe and/or Co acetates as oxide precursors, with the TMO nanoparticles uniformly anchored onto the surface of graphene sheets. The electrochemical performance of the most promising nanocomposite was evaluated as anode material for sodium ion batteries. The preliminary results of galvanostatic cycling prove that Fe2O3@rGO nanocomposite exhibits better rate capability and stability than both bare Fe2O3 and Fe2O3+rGO physical mixture.
Recently, graphene-based nanocomposites have captured considerable attention due to their unique properties and the large variety of possible applications, ranging from sensing and energy storage to heterogeneous, electro-, and photocatalysis [
Graphite powders represent nowadays the standard anode material for commercial secondary Li+ ion batteries. Recently, many metal oxides have been evaluated as alternative anode materials both in Li+ and Na+ ion batteries (LIBs and SIBs) [
In this work, we report the synthesis of nanostructured composites, consisting of cobalt and/or iron oxide anchored on GO, by the one-step solvothermal approach. The main advantage of this method is that since the reduction of GO to rGO occurs simultaneously to the deposition of the TMO NPs, it does not require the use of any chemical-reducing agent. Textural, structural, and morphological properties of the nanocomposites are studied by several complementary analysis techniques in view of their possible application as electrode materials in rechargeable batteries. Preliminary results relative to the electrochemical performance of the most promising of them are presented revealing that the carbon oxide nanocomposite shows good electrochemical performance in terms of specific capacity and capacity retention.
Sodium nitrate (NaNO3, 99.5% purity), sulfuric acid (H2SO4, 95-97% purity), and propylene carbonate (anhydrous,
Graphene oxide was prepared by exfoliation of graphite powder following a slightly modified Hummers method [
The nanocomposites were obtained by the solvothermal treatment sketched in Figure
The sample texture and morphology were investigated by scanning electron microscopy (SEM). Analysis was performed using a Phenom ProX scanning electron microscope equipped with an energy-dispersive X-ray (EDX) spectrometer.
The formation of GO and its subsequent reduction to rGO upon thermal treatment were ascertained by means of X-ray powder diffraction (XRD) and micro-Raman spectroscopy (MRS). The XRD patterns were recorded at RT by using the Ni
The crystalline phase of the oxide was identified via XRD and MRS analyses. Surface composition of the samples and chemical environment of the component species were investigated by X-ray photoelectron spectroscopy (XPS). Spectra were acquired using a K-Alpha system of Thermo Scientific, equipped with a monochromatic Al-K
Galvanostatic cycling with potential limitation (GCPL) was carried out with the Bio-Logic VSP-300 multichannel potentiostat/galvanostat on two-electrode coin cells CR2032 assembled in an argon-filled glove box (MBraun), with sodium acting both as counter and reference electrodes. The working electrode for galvanostatic cycling was prepared mixing Fe2O3@rGO composite, the carbon matrix (Super P, MM Carbon), and the polymer binder (polyacrylic acid Mw ~450,000, Sigma-Aldrich) in the weight ratio 8 : 1 : 1, respectively. A second electrode was made by using a physical mixture of Fe2O3 powder and rGO (Fe2O3+rGO) as an active material. For this purpose, Fe2O3 powder and rGO, synthesized separately, were mixed in the same ratio as Fe2O3@rGO nanocomposite. Finally, a third electrode was prepared by adding a higher carbon black content to bare Fe2O3 in order to obtain a comparable total carbon content with respect to the other electrodes. The conductive carbon matrix used in the preparation of the electrode was pretreated at 800°C under argon atmosphere, to remove the adsorbed water and impurities, reducing the typical irreversibility that occurs during the first charge and discharge in such systems [
The formation of GO from graphite was ascertained by means of XRD (compare patterns a and b in Figure
After the solvothermal treatment, the characteristic peak of GO disappears, confirming the occurrence of the thermal reduction to rGO. A very broad and intense peak around
The results of MRS and XPS analyses confirm the formation of rGO upon solvothermal reaction. The Raman spectra of GO and rGO (Figure
The D/G integrated intensity ratio (
The high-resolution photoelectron profiles of the C 1s core levels in GO and rGO (Figure
Figure
SEM images of (a) CoO@rGO, (b) CoFe2O4@rGO, and (c–f) Fe2O3@rGO nanocomposites.
Figure
XRD patterns of (a) Fe2O3@rGO, (b) CoO@rGO, and (c) CoFe2O4@ rGO nanocomposites.
As for the crystalline phase of the TMOs, the formation of hematite is observed when Fe(CH3COO)2 alone is used as a precursor (Figure
When Co(CH3COO)2·4H2O alone is utilized, no evident diffraction peaks are detected in the XRD pattern of the nanocomposite, probably due to the amorphous nature of the oxide formed (Figure
Figure
Micro-Raman spectra of (a) bare Fe2O3, (b) Fe2O3@rGO, (c) CoO@rGO, and (d) CoFe2O4@rGO nanocomposites.
The spectra of TMO@rGO nanocomposites (Figures
Cobalt ferrite with a cubic structure belongs to the space group Fd3m [
Figures
High-resolution photoelectron spectra of the Fe 2p core levels in (a) Fe2O3@rGO and (b) CoFe2O4@rGO nanocomposites.
High-resolution photoelectron spectra of the Co 2p core levels in (a) CoO@rGO and (b) CoFe2O4@rGO nanocomposites.
The asymmetry featuring the spectral profile of the Co 2p core level photoelectron spectrum in CoFe2O4@rGO nanocomposite (Figures
The indications emerging from XPS analysis allow understanding the lack of oxide-related contributions in the Raman spectrum of CoO@rGO nanocomposite (Figure
Additional information is inferred from the high-resolution photoelectron spectra of the O 1s core level (Figure
The relative weight of the transition metal oxide (rGO) in the investigated nanocomposites, as estimated on the basis of the XPS compositional analysis (Table
The results of rate capability tests on bare Fe2O3, Fe2O3@rGO nanocomposite, and Fe2O3+rGO physical mixture are presented in Figure
(a) Anodic specific capacity and columbic efficiency as a function of the number of cycles, as obtained from the GCPL for electrodes based on bare Fe2O3, Fe2O3@rGO nanocomposite, and Fe2O3+rGO physical mixture. (b) Voltage versus specific capacity for selected cycles of the Fe2O3@rGO nanocomposite.
The comparison between specific capacities of Fe2O3@rGO nanocomposite and Fe2O3+rGO physical mixture demonstrates the importance of an intimate connection between the carbonaceous support and the oxide anchored on its surface. As a matter of fact, in the first cycles, the specific capacities of Fe2O3@rGO and Fe2O3+rGO are comparable, but at higher rates, the difference becomes more evident. At C/2 and 2C rates, the nanocomposite outperforms the physical mixture. This is particularly evident at the highest rate. At 2C, in Fe2O3+rGO, the specific capacity decreases down to 20 mAh/g, whereas Fe2O3@rGO can still deliver 65 mAh/g. Moreover, when the current is set back to C/20, a better stability of the nanocomposite is registered, which proves its greater ability to buffer the volume changes occurring during the sodiation/desodiation process (i.e., a superior structural stability) with respect to the physical mixture. Although a long-term cycling analysis is still needed, these preliminary results clearly prove that the
The low columbic efficiency (CE) that all active materials exhibit in the first cycle is generally attributed to the decomposition of the electrolyte and the formation of the solid electrolyte interface (SEI) [
In the next cycles, CE rapidly increases reaching values higher than 99%. Finally, after the first cycle, during the sodiation, a two-step process takes place, which could be assigned to the insertion of sodium in the oxide and the conversion of iron [
As known from the literature [
In conclusion, transition metal oxides on reduced graphene oxide (TMO@rGO) nanocomposites, consisting of TMO nanoparticles uniformly anchored onto the surface of graphene nanosheets, were successfully synthesized via the one-step solvothermal process and characterized by means of a combination of complementary techniques. Fe2O3@rGO, CoFe2O4@rGO, and CoO@rGO were obtained by using Fe and/or Co acetates as oxide precursors.
The results of preliminary tests aimed at evaluating the electrochemical performance of Fe2O3@rGO as anode material for sodium ion rechargeable batteries demonstrate that the intimate contact between the carbonaceous support and the oxide anchored on its surface plays a crucial role. Fe2O3@rGO nanocomposite exhibits better rate capability and stability with respect to both bare Fe2O3 and Fe2O3+rGO physical mixture, as it is able to buffer the volume changes occurring during the sodiation/desodiation process.
The data (SEM, XRD, micro-Raman, and XPS (Fe 2p and Co 2p) relative to composites) used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this paper.
Supplementary Materials include a schematic representation of the solvothermal treatment leading to the formation of the TMO@rGO nanocomposites; the elemental composition of the nanocomposites estimated from quantitative analysis of the XPS spectra; results of XRD, MRS, XPS, and SEM analyses on GO and rGO; EDX spectrum of CoFe2O4@rGO nanocomposite; and photoelectron spectra of the O 1s and C 1s core levels in the nanocomposites.