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.