Aqueous solution synthesis of reduced graphene oxide-germanium nanoparticles and their electrical property testing
© Yin et al.; licensee Springer. 2013
Received: 1 July 2013
Accepted: 24 September 2013
Published: 17 October 2013
Aqueous solution synthesis of reduced graphene oxide-germanium nanoparticles (RGO-GeNPs) was developed using graphene oxide (GO) as stabilizer, which could be conducive to obtain better excellent electrical properties. The information about morphology and chemical composition of the nanomaterials were obtained by TEM, FTIR, EDS, and XRD measurements. Stable aqueous dispersibility of RGO-GeNPs was further improved by poly(sodium 4-styrenesulfonate) (PSS) to obtain amphiphilic polymer-coated RGO-GeNPs (PSS-RGO-GeNPs). A possible mechanism to interpret the formation of RGO-GeNPs was proposed. The as-synthesized RGO-GeNPs showed excellent battery performance when used as an anode material for Li ion batteries. The resulting nanocomposites exhibited high specific capacity and good cycling stability after 80 cycles. This study showed a facile strategy to synthetize graphene and Ge nanocomposites which can be a hopeful anode material with excellent electrical properties for lithium ion batteries.
KeywordsAqueous solution synthesis Reduced graphene oxide-germanium nanoparticles Dispersibility
With the advent of nanoscience and nanotechnology, semiconductor nanomaterials have received much attention due to their unique physical properties and potential applications in electronics, catalysts, sensors, and optical devices . The group IV semiconductors such as silicon (Si) and germanium (Ge) were unique materials with a wide range of technological applications. Ge or Ge-based nanomaterials have shown valuable physical properties for various applications in solar cells, optoelectronics, bio-imaging, energy conversion, and storage .
In recent years, a variety of strategies have been developed to synthesize functional GeNPs physically and chemically [3–7]. Nevertheless, synthesis and application of Ge nanomaterials have suffered from serious limitations such as some stiff experimental conditions, high temperatures, toxic precursors, and complex synthesis process . Furthermore, the application of Ge nanomaterials was often hampered by the aggregation and lowered physical properties, as these facts directly determine the applications of Ge nanomaterials. Though Ge nanomaterials have excited an attractive prospect, the majority of synthetic strategies did not provide facile aqueous solution routes. Moreover, organic and inorganic substances such as PVP , (CH3)3SiCl , amino acid [11, 12], and graphene  have been employed to stabilize Ge nanomaterials and to develop nanomaterials with variant morphologies; these strategies could partly improved the physical performance and stability of the Ge nanomaterials.
Graphene is a single-atom-thick two-dimensional graphitic carbon material, which possesses extraordinary large surface area and chemical stability . Recently, graphene has been used as an excellent substance to acquire variously functional nanomaterials, including graphene-silver nanoparticles , graphene-gold nanoparticles , graphene-TiO2 nanomaterials , and graphene-palladium nanoparticles . Recently, some works have reported about synthetizing and studying the electrochemical performance of graphene mixed with Ge nanomaterials [19–23]. For instance, Cheng and Du  reported the synthesis of graphene-Ge nanocomposites from expensive GeCl4 and graphene oxide as precursor. Although the nanocomposites exhibited a high specific capacity as anode materials for lithium ion batteries (LIBs), this strategy did not acquire a material with long cycle life. Ren et al.  reported the synthesis of graphene-Ge nanocomposite by chemical vapor deposition (CVD), which exhibited a good capacity retention behavior and long cycle life as anode materials. However, the strategy did not provide a facile route for synthesis. Moreover, the loss of stability and electrochemical properties often inevitably occurred due to irreversible agglomeration and poor dispersions of graphene-Ge nanocomposites in aqueous solution. Therefore, it was important to find a new synthesized method to prepare water-dispersable Ge nanocomposites with excellent electrical properties.
Herein, we demonstrate a simple and mild method to fabricate the RGO-GeNPs in aqueous solution. Stable aqueous dispersions of nanocomposites were synthesized by the reduction of exfoliated graphite oxide and GeO2 precursor. Poly(sodium 4-styrenesulfonate) (PSS) was employed to obtain aqueous dispersibility of PSS-RGO-GeNPs, which was hopeful to further improve its electrochemical properties. The study provided a strategy to synthetize RGO-GeNPs which could be served as promising anode materials for LIBs.
All reagents in this work were of analytical grade and were used as received without further purification. GeO2, PSS (analytically pure), and graphite powders (spectral pure) were purchased from Sinopharm Chemical Reagent Beijing Co. NaBH4, the reducing agent, was obtained from Aladdin Chemical Co., Ltd. (China). All the aqueous solutions were prepared with double-distilled water.
Preparation of RGO-GeNPs and PSS-RGO-GeNPs
Graphene oxide (GO) was prepared by oxidizing natural graphite powder based on a modified Hummers and Offeman method  as originally presented by Kovtyukhova et al. . The RGO-GeNPs were synthesized by the following method:10 mL of as-prepared GO supernatant (20 mg/mL) was distributed in 40 mL of ultrapure water to obtain a homogeneous, stable dispersion with the aid of ultrasonication in a water bath (KQ218, 60 W), named 'A solution’. A 0.08 g GeO2 was dissolved completely in 10 mL 0.64 M NaOH solution to form Na2GeO3 liquid precursors, then the pH of the solution was adjusted by 0.5 M HCl solution to be 7 to 8, named 'B solution’. Next, both suspensions were mixed together under constant stirring for 1.0 h. The mixture solution was, in the first, instance put into a water bath at 60°C.Then, under a nitrogen atmosphere and continuous magnetic stirring, fresh NaBH4 solution (10 mL, 0.1 M) was added dropwise into the mixture solution. This solution was stirred for 4.0 h more. Afterwards, the solution was dialyzed against deionized water for 3 days. Then, the RGO-GeNPs were freeze-dried and collected in a powder form. When the reduction was carried out in the presence of poly(sodium 4-styrenesulfonate), a stable black PSS-RGO-GeNPs solution was obtained.
Characterization technique and electrical properties testing
The absorption spectra were recorded on a Cary 5000 UV-visible spectrophotometer (Varian Technology Co., Ltd., Palo Alto, CA, USA). Powder X-ray diffraction (XRD) data were collected using a Bruker D8 Advance X-ray diffractometer (Ettlingen, Germany) equipped with CuKα radiation. The FTIR samples were recorded on Equinox 55 IR spectrometer (Bruker) in the range from 4,000 to 400 cm-1 using the KBr-disk method. The TEM micrographs were obtained on Hitachi (H-7650, Tokyo, Japan) for TEM operated at an accelerating voltage at 80 kV. Energy-dispersive X-ray spectroscopy (EDS) was carried out during the transmission electron microscopy (TEM) measurement.
Electrochemical measurements were performed using CR2032 coin-type cells assembled in an argon-filled glove box. For the preparation of RGO-GeNPs, Super carbon black and polyimide (PI) binder (dissolved in N-methylpyrrolidone) were mixed in a mass ratio of 85:8:7. The resultant slurry was then uniformly coated on a Cu foil current collector and dried overnight under vacuum. The electrochemical cells were assembled with RGO-GeNP electrode or PSS-RGO-GeNP electrode as cathode, metallic lithium foil as anode, and Celgard 2325 porous film (Charlotte, North Carolina) as separator. The electrolyte used in this work was a solution of 1.2 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) (3:7 by volume). In addition, 10 wt% fluoroethylene carbonate (FEC) was added into the above electrolyte as additive. Galvanostatic electrochemical experiments were carried out in a Maccor Series 4000 battery system (Tulsa, OK, USA). The electrochemical tests were performed between 0.01 and 1.5 V vs. lithium at ambient temperature.
Results and discussion
We have prepared the RGO-GeNPs by a one-step approach. Under the present experimental conditions, GO was suitable for the preparation of RGO-GeNP hybrid because of its large surface area and chemical stability.
Formation mechanism of RGO-GeNPs
Electrical properties testing
The theoretical researches showed that Ge exhibits a huge theoretical capacity (1,600 mAhg-1) and faster diffusivity of Li compared with Si . Ge can be expected to exhibit excellent electrical properties as anode material for LBIs. Graphene also was a good candidate for Li ion batteries because of its high electrical conductivity, specific wrinkled structures, and flexibility, which made graphene suppress local stress and large volume expansions/shrinkages during a lithiation/delithiation process and alleviate the aggregation or pulverization problems . Therefore, by combining with Ge nanomaterials, the RGO-GeNPs could have enhanced electrical properties, which would be promising materials for various kinds of market-demanded LIBs.
In our study, the RGO-GeNPs and RGO-Ge were also tested for comparison. As shown in Figure 5b, the PSS-RGO-GeNPs exhibited a higher specific capacity and better cycling stability than RGO-GeNPs and pristine RGO-Ge. The PSS-RGO-GeNPs still retained a reversible capacity of 760 mAhg-1 after 80 duty cycles under a current density of 50 mAg-1. PSS was employed to obtain aqueous dispersibility of PSS-RGO-GeNPs, which could further improve the electrochemical properties of RGO-GeNPs because of the smaller size and better dispersibility of the GeNPs. The theoretical capacity of PSS-RGO-GeNPs was about two times higher than that of the RGO-Ge. It clearly illustrated that the use of nanosized germanium can effectively overcome the shortcoming of poor cyclability and rapidly declining capacity during the Li uptake and release process.
High rate capabilities and good cycling stability were also observed in the PSS-RGO-GeNPs. As shown in Figure 5c, the PSS-RGO-GeNPs showed a much higher capacity than the RGO-GeNPs and pristine RGO-Ge at different investigated current densities of 0.1 c, 0.2 c, 0.5 c, 1 c, 2 c, and 5 c. Even under the very high current density of 5c, the PSS-RGO-GeNPs still exhibited a favorable specific capacity of 574 mAhg-1 after 10 duty cycles. Importantly, the capacity could be recovered to the initial reversible values when the rate was returned to 0.1c, implying their good duty cycling stability and indicating their potential application as promising candidates for the development of high-performance LIBs.
The electrochemical impedance spectra of the PSS-RGO-GeNPs, RGO-GeNPs, and pristine RGO-Ge were demonstrated in Figure 5d. Apparently, the PSS-RGO-GeNP electrode showed a much lower charge transfer resistance Rct than the RGO-Ge electrode on the basis of the modified Randles equivalent circuit given in the inset of Figure 5d. This result indicated that the PSS-RGO-GeNP electrode possesses a high electrical conductivity, resulting in the better rate capability and higher reversible capacity in comparison with pristine RGO-Ge.
In conclusion, we have developed a simple, convenient, and aqueous solution synthesis method to fabricate the RGO-GeNPs under mild conditions. PSS was employed to obtain aqueous dispersibility of PSS-RGO-GeNPs, which was hopeful to further improve its electrical properties. The PSS-RGO-GeNPs exhibited excellent battery performance in comparison with the RGO-GeNPs and pristine RGO-Ge. The resulting nanocomposites exhibit high specific capacity and good cycling stability after 80 cycles, which could be attributed to the electronically conductive and elastic RGO networks, as well as the carbon shells and the small size of the GeNPs. The study provided a strategy to synthetize RGO-GeNPs which could serve as promising anode materials for LIBs.
Energy-dispersive X-ray spectroscopy
Fourier transform infrared
Lithium ion batteries
Reduced graphene oxide-germanium nanoparticles
Solid electrolyte interface
Transmission electron microscopy
This work was supported by the grants from the National Natural Science Foundation of China (no. 21071064 and no.21375048).
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