III. RESULTS AND DISCUSSIONS

The XRD patterns of Fe₂O₃-R/GNS and Fe₂O₃-P/GNS composites are shown in Fig. 1. The broad (002) diffraction peak appears at 24.9° can be indexed into the disorderedly stacked GNS, which is in agreement with the diffraction peak of previously reported values of GNS. ^{Reference Cong, He, Lu and Yu23,Reference Zhou, Wang, Li, Zhang, Li, Wu, Wen and Cheng24} All the other diffraction peaks of two samples are in agreement with that of the standard hematite α-Fe₂O₃ (JCPDS No. 33-664, lattice constant a = b = 5.035 Å, and c = 3.749 Å), indicating that both products with relatively high crystal purity are synthesized. By comparing the full width half maximum (FWHM) of the diffraction peaks, one can easily find that the diffraction peaks of rhombohedra are broader than those of nanoparticles, which may be attributed to different particle sizes of the as-prepared Fe₂O₃ samples. According to the Scherrer's equation, the grain size of rhombohedra and particles are calculated to be 78.5 and 159.2 nm, respectively.

FIG. 1.

XRD patterns of (a) Fe₂O₃-R/GNS and (b) Fe₂O₃-P/GNS composites.

Figure 2 shows the FTIR spectra of graphene, bare Fe₂O₃, and Fe₂O₃-R/GNS composites. For graphene [Fig. 2(a)], the peak at 3473 cm⁻¹ can be attributed to O–H stretching vibrations of adsorbed water molecules and structural OH groups, and the peak at 1629 cm⁻¹ can be attributed to O–H bending vibrations. The presence of carboxyl and epoxy functional groups can also be detected at around 1737 and 1231 cm⁻¹, respectively. After Fe₂O₃ was integrated with GNS, a new strong Fe–O stretching vibration peak is observed at 559 cm⁻¹, and the absorption bands at 1583 cm⁻¹ reflecting the skeleton vibration of GNS enhance obviously. ^{Reference Mensing, Kerdcharoen, Sriprachuabwong, Sriprachuabwong, Wisitsoraat, Phokharatkul, Lomas and Tuantranont25} Meanwhile, the peaks in 1629 and 3473 cm⁻¹ related to oxidized group decrease significantly, indicating Fe³⁺ interactions with oxygen-containing groups and conversion to iron oxide in the subsequent reaction.

FIG. 2.

FTIR spectra of (a) bare Fe₂O₃, (b) Fe₂O₃-R/GNS, and (c) graphene.

Figure 3 shows the Raman spectra of graphene, Fe₂O₃-P/GNS and Fe₂O₃-R/GNS composites. The three samples show typical peaks at around 1340 and 1590 cm⁻¹, which correspond to the D band (κ-point phonons of A _1g symmetry) and G band (E _2g phonon of C sp ² atoms) of graphene. Besides the two peaks, the Raman band at 225 cm⁻¹ is assigned to A _1g mode and the bands at 290 and 407 cm⁻¹ are assigned to E _g vibration modes of hematite [Figs. 3(b) and 3(c)]. ^{Reference Lee, Deng, Fan, Mhaisalkar, Tan, Tok, Loh, Chin and Sow18} Such a result further confirms that the crystalline structure Fe₂O₃/graphene nanocomposites have been obtained.

FIG. 3.

Raman spectra of (a) graphene, (b) Fe₂O₃-P/GNS, and (c) Fe₂O₃-R/GNS composites.

Raman spectra with characteristic G and D bands are sensitive to defects, disorder, and carbon grain size, and have extensively been used to characterize carbon materials. D band is an indication of disorder in the Raman spectra of the GNS, originating from defects associated with vacancies, grain boundaries, and amorphous carbon species. And the I _D/I _G intensity ratio is a measure of disorder degree and average size of the sp ² domains. ^{Reference Zhou, Bao, Tang, Zhong and Loh26} In Fig. 3(a), it can be identified that the I _D/I _G of GNS calcined at 500 °C is 1.01, which is higher than that of pristine GO (approximately 0.75) reported in our previous study, ^{Reference Zhao, Liu, Jiang, Pan, Tao, Song, Fang and Xu22} indicating that a lot of carbon cavities are generated and the disorder degree increased during thermal decomposition of GO to form GNS in N₂. With the addition of Fe₂O₃ nanocrystallites, the disorder of the two composites increased to a certain extent possibly due to the insertion of Fe₂O₃ into GNS layers. This agrees well with the previous observation on GNS-based transitional metal oxide composites. ^{Reference Wang, Cui, Yang, Casalongue, Robinson, Liang, Cui and Dai15,Reference Xue, Ma, Cui and Xing21,Reference Zhang, Lei, Chen, Yu and Wang27} Especially, Fe₂O₃-R/GNS composite demonstrates more defects and disorder with an I _D/I _G ratio of 1.15, reflecting that a smaller size of nanorhombohedron is easier to be inserted between the GNS layers. Accordingly, this change would affect the physical and chemical properties of graphene-based composite materials. ^{Reference Pumera28}

SEM and TEM observations revealed the morphologies of α-Fe₂O₃ nanoparticles and rhombohedra composites, as shown in Fig. 4. In the system without adding any inorganic salt additive, Fe₂O₃ particles showed irregular shape and congregated together forming an irregular aggregate despite the presence of GNS support [Fig. 4(a)]. The magnified TEM image reveals that the Fe₂O₃ particles are around 150–200 nm in size [Fig. 4(b)]. However, when the system was added with CH₃COONa and reacted for the same 6 h, well dispersed and uniform Fe₂O₃ nanorhombohedra with average diameters of 80 nm were obtained, which is consistent with the results of XRD calculations. Obviously, the size, morphology, and distribution of Fe₂O₃ rhombohedra on graphene are much better than those of Fe₂O₃-P/GNS composite. According to the TEM image [Fig. 4(d)], the graphene layer can be clearly distinguished from the background. And a few particles seem to be enveloped by a thin film as some particles locate on the back of monolayer graphene. Although not fully verified, it has been proposed that the epoxy and C–OH functional groups lie above and below each carbon layer. ^{Reference Bourlinos, Gournis, Petridis, Szabo, Szeri and Dekany29} The Fe₂O₃ nanorhombohedra appear on both sides of the support: some locate above the graphene nanosheet, while others lie on the back of the sheet.

FIG. 4.

SEM and TEM images of (a, b) Fe₂O₃-P/GNS composite and (c, d) Fe₂O₃-R/GNS composite, respectively, (e) higher magnification SEM image of the edge of Fe₂O₃-R/GNS and its EDS spectrum (f).

Figure 4(e) shows the higher magnification SEM image of the edge of graphene in Fe₂O₃-R/GNS composite. The graphene nanosheets are of inherent wrinkled or crumpled thin voile-like structures with folds, originated from the exfoliation of GO precursors during the pyrolysis process. These transparent flakes composed of high quality multilayer graphene sheets are closely attached with the oxides, suggesting good electrical conductivity of the composite. The corresponding energy-dispersive x-ray spectrometry (EDS) is shown in Fig. 4(f). A few elements such as C, O, and Fe were present in the composite. The Si element was observed because the Si substrate was used to disperse SEM sample, thus removing the carbon effect from the common substrate (carbon paste). The weight percentage of Fe₂O₃ is calculated to be about 45.9%, which is slightly different from the theoretical value (50%), indicating that most Fe³⁺ ions are eventually generated into Fe₂O₃ on the graphene surface.

Previous studies have demonstrated that inorganic species could play as the capping agents or structural agents to control the morphology and size of nanomaterials besides organic surfactants and polymers. Owing to their relatively small size, it is expected that inorganic ions might have a more pronounced influence on the nucleation and growth process of nanocrystallites than organic surfactants and polymers. Take the synthesis system of rhombohedra for example, inorganic salts might change the pH and ionic strength of the solution. And the inorganic anions can coordinate with existing reactant cations via different coordination modes and strengths. Moreover, the selective adsorption of inorganic ions on the surface of the formed crystal nuclei and primary nanoparticles can alter its surface termination and atomic arrangement. ^{Reference Wu, Guo, Wan and Hu7} All these changes may affect the anisotropic growth process and result in a different size and shape of the final nanostructures.

To reveal the growth mechanism of the rhombohedra α-Fe₂O₃/GNS in more detail, the products were collected at certain reaction time intervals for TEM observations (Fig. 5) and the formation of Fe₂O₃ rhombohedra could be illustrated by three processes. (i) Nucleation process. As is known, perfect rhombohedra gradually grew up from crystal nucleus at the center. Firstly, Fe³⁺ ions are selectively adsorbed with the carboxyl groups of the graphene surface through mutual electrostatic attraction. Then, the Fe³⁺ ions, which were adsorbed on both sides of graphene, reacted with OH⁻ (produced by the hydrolysis of CH₃COO⁻) and oxygen at room temperature to form α-FeOOH amorphous particles, as shown in Fig. 5(a). Then α-FeOOH particles dehydrated to form spherical iron oxide nucleus under hydrothermal condition. This kind of spherical iron oxide nucleus is in the unsteady state due to its poor crystallinity and large surface energy. ^{Reference Voorhees30,Reference Yang and Zeng31} (ii) Growth process. One ferric iron (Fe³⁺) could chelate three CH₃COO⁻ to form octahedral molecular structures, and selectively adsorb in spherical iron oxide nucleus in different directions. The different adsorption ability of CH₃COO⁻ on different planes of iron oxide resulting in anisotropic growth and oriented attachment, which then eventually form this particular rhombohedra structure. After hydrothermal reaction for 30 min [Fig. 5(b)], there are transparent areas appear around the spherical iron oxide nucleus significantly getting larger. The phenomenon of small particles dissolution, while larger particles grow up agrees with Ostwald ripening. ^{Reference Siegfried and Choi32,Reference Filankembo and Pileni33} And finally, small spherical iron oxide would grow into rhombohedra after heating for 2 h [Fig. 5(c)]. It is notable that some small primary nanocrystals still existed in this sample, indicating that oriented attachment is still underway. (iii) Growth arrest. The adsorption of CH₃COO⁻ on the surface of the rhombohedra can make the particles negatively charged. Due to the same negative charge, interactional repulsion exists among the particles. As a result, rhombohedra were uniformly monodispersed on the graphene surface in this 6 h sample, as shown in Fig. 5(d). The grain size remained basically the same when the standing time prolong to 24 h, indicating that CH₃COO⁻ can play the role of capping agents which have a pronounced influence on the uniformity of the as-grown rhombohedra.

FIG. 5.

TEM images of Fe₂O₃-R/GNS sample at different hydrothermal reaction time intervals (a) 0 min, (b) 30 min, (c) 2 h, and (d) 6 h.

To investigate the particle size and shape effects on the electrochemical properties of the α-Fe₂O₃ nanostructures, we mixed these materials with acetylene black and PTFE as working electrodes. Figures 6(a) and 6(b) show the first and second charge–discharge curves of Fe₂O₃-P/GNS and Fe₂O₃-R/graphene composites at a current density of 100 mA g⁻¹ in the voltage range of 5 mV to 3 V. It was found that the shape of the two curves is very similar, both samples have two voltage plateaus at 1.1–1.52 V and 0.78–0.95 V in the first discharge curves. The voltage plateau at around 1.52 V can be described as a small amount of lithium insertion into the crystal structure of Fe₂O₃ to form Li _x Fe₂O₃ intermediate. The discharge plateau at approximately 0.95 V reflects Li _x Fe₂O₃ inserted a certain amount of lithium and turned into Li₂Fe₂O₃, meanwhile the crystal structure of Fe₂O₃ transformed into cubic from hexagonal stacking. The long plateau at around 0.78 V is largely due to the reduction reaction of Fe³⁺ and the irreversible formation of solid electrolyte interphase (SEI) layers because of the decomposition of the solvent in the electrolyte. ^{Reference Tian, Zhuang, Li, Wu, Shi and Sun34} These two plateau voltages are clearly reflected as peaks in the CV curves shown in Figs. 6(c) and 6(d). The magnitude of the current density of the redox peaks is significantly large in the case of the Fe₂O₃-R/GNS composite electrode, demonstrating excellent electrical conductivity between Fe₂O₃ nanorhombohedra and GNS matrix. The initial discharge capacities based on the total mass are 1270.2 and 1356.9 mAh g⁻¹ for Fe₂O₃-P/GNS and Fe₂O₃-R/GNS composites, respectively. The corresponding reversible charge capacities are 839.6 and 905.3 mAh g⁻¹. Obviously, Fe₂O₃-R/GNS obtains higher specific capacity than that of Fe₂O₃-P/GNS composite. The value is comparable to those obtained in previous Fe₂O₃/graphene hybrid materials. ^{Reference Wu, Guo, Wan and Hu7,Reference Xue, Ma, Cui and Xing21,Reference Liu and Tay35,Reference Wang, Liu, Luo, Zhao, Ren, Bai and Wang36} This should be readily attributed to the smaller size and larger surface area for the Fe₂O₃-R/GNS sample. Meanwhile, small particles are much easier to be inserted in the graphene layers [as seen in Fig. 4(e)], by this way, can prevent the stack of graphene and improve the specific surface area and porosity, thus providing sufficient contact area for the reaction of lithium intercalation to Fe₂O₃-R/GNS composite. ^{Reference Wang, Tian, Liu, Zhi, Bando and Golberg37}

FIG. 6.

The galvanostatic charge–discharge profiles and CV curves of (a, c) Fe₂O₃-P/GNS and (b, d) Fe₂O₃-R/GNS composites, respectively.

The rate capabilities and high rate cycling performance of the two composites are also investigated and shown in Fig. 7. Fe₂O₃-R/GNS composite shows rather good rate capability with an average discharge capacity as high as 1025.2 mAh g⁻¹ at a current density of 0.1 C, 780.3 mAh g⁻¹ at 0.2 C, 426.7 mAh g⁻¹ at 1 C and finally resume 871.2 mAh g⁻¹ when the current density is reduced back to 0.1 C. While Fe₂O₃ particle/graphene exhibits lower charge capacities of 891.8, 648.2, 302.4, and 724.2 mAh g⁻¹ at the corresponding rates, respectively. These results show that the Fe₂O₃-R/GNS composites are indeed promising architectures, which are able to improve the electrochemical performance of the electrodes. To understand the reason behind the superb performance of the Fe₂O₃-R/GNS electrodes compared to the Fe₂O₃-P/GNS electrodes, comparative electrochemical impedance spectroscopy (EIS) data were collected. Nyquist plots (Fig. 8) demonstrate that a diameter of the semicircle of Fe₂O₃-R/GNS electrodes in the high-medium frequency region is much smaller than that of Fe₂O₃-P/GNS electrodes, thus suggesting that Fe₂O₃-R/GNS composites possess the lower contact and charge-transfer impedance. The result indicates that the Fe₂O₃-R/GNS composites possess a high electrical conductivity, resulting in the better rate capability and higher reversible capacity in comparison with the Fe₂O₃-P/GNS composites.

FIG. 7.

Comparison of rate capability of Fe₂O₃-P/GNS and Fe₂O₃-R/GNS composites.

FIG. 8.

Nyquist plots of electrodes made of Fe₂O₃-P/GNS and Fe₂O₃-R/GNS composites.

Figure 9 shows the cycling performance of bare Fe₂O₃, Fe₂O₃ particles/graphene, and Fe₂O₃ rhombohedra/graphene composite in the voltage range of 5 mV to 3 V at 0.1 C rate. It shows that the capacity of the bare Fe₂O₃ rapidly decreases from 869.8 to 254.3 mAh g⁻¹ after 50 cycles, resulting in 1.42% capacity loss per cycle. For Fe₂O₃-P/GNS, the discharge specific capacity fades down from 839.6 to 478.4 mAh g⁻¹, about 57.1% retention of the initial capacity is maintained. In a sharp contrast, Fe₂O₃-R/GNS could still deliver a specific capacity of 775.4 mAh g⁻¹, exhibits excellent cyclic performance, and retains 85.7% of its initial capacity after 50 cycles. It is worth noting that the present Fe₂O₃-R/GNS revealed excellent electrochemical stability compared with many recently reported Fe₂O₃/graphene hybrid structures. ^{Reference Lee, Deng, Fan, Mhaisalkar, Tan, Tok, Loh, Chin and Sow18-Reference Xue, Ma, Cui and Xing21,Reference Zhang, Lei, Chen, Yu and Wang27,Reference Tian, Zhuang, Li, Wu, Shi and Sun34-Reference Zou, Kan and Wang38} These Fe₂O₃ products in these mentioned composites included nanoparticles, nanorice, nanotubes, nanoplatelets, and hollow spheres; and they were also anchored on the surface of graphene. Therefore, the superior electrochemical stability improvement for our sample is believed to be resulted from the synergetic effect of rhombohedra morphology, small diffusion lengths, and flexible confinement graphene. First, a superflexible graphene coating covering α-Fe₂O₃ nanocrystals not only provides an elastic buffer space to accommodate the volume change upon lithiation/delithiation, but also efficiently prevents the crumbling of the electrode material. Even though the volume expansion occurred, the electrode will not be cracked since the graphene sheets can deform resilient to buffer such volume variation. Second, apart from preventing the detachment and agglomeration of pulverized F₂O₃, graphene can also provide effective conductive network and excellent Li⁺ transport, which decrease the inner resistance of lithium-ion batteries and are favorable for stabilizing the electronic and ionic conductivity. ^{Reference Zou, Kan and Wang38} Third, it may be related to the different morphologies of the α-Fe₂O₃ nanostructures, which holds different crystalline surfaces exposed to electrolyte. Finally, compared with the particles, rhombohedron has the advantages of being fairly uniform in nuclear size (80 nm), solid combination, and good dispersion on the graphene surface, which would shorten the path length for Li-ion transport, increase materials/electrolyte contact area, and facilitate the Li-ion diffusing to the sites on the surface of α-Fe₂O₃ nanostructures, therefore leading to high specific capacity and cyclability.

FIG. 9.

Cycling performance of (a) bare Fe₂O₃, (b) Fe₂O₃-P/GNS and (c) Fe₂O₃-R/GNS composites at 0.1 C rate in the voltage range of 5 mV to 3 V.