The SnO 2 /GS Foam was fabricated by a chemical self-assembly strategy and a subsequent freeze-drying process (Fig. 1). At step 1, from the collochemistry, the metal oxide such as Fe 2 O 3 , TiO 2 and SnO 2 colloid was positively charged; the GO was negatively charged because GO has many oxygen containing functional groups on the surface35. The GO and SnO 2 nanoparticles were attracted by the electrostatic force so that the SnO 2 nanoparticles can distribute on the surface of GO and not departure by the ultrasonication and stir. At step 2, L-ascorbyl acid was used to reduce oxygen containing functional groups (e.g. carboxyl) on the surface GO, producing in situ reduced GO (rGO). rGO has smaller solubility in water than GO because the reduction of polar oxygen containing functional groups makes GO less hydrophilic. So, as GO sheet began to turn into the rGO, the delocalized π-bond’s conjugative effect would be increased and enlarged. The freshly formed rGO sheets would stack on other rGO sheets as a result of the π–π stacking interactions and self-assembled into a 3D structure. After the chemical reaction and at step 3, there are still many oxygen containing functional groups left on rGO sheets, thus, the SnO 2 nanoparticles with polar surfaces would interact with those functional groups via hydrogen bonding. By annealing at 550 °C, the hydrogen bonds may turn into oxygen bridges between SnO 2 and rGO, forming Sn-O-C bonds. Therefore, the SnO 2 nanoparticles are anchored strongly on the graphene surface through a C-O-Sn bridge, which facilitates the electron transfer and improve the electrode stability. Finally, we obtain a porous ASGF with relative density of ~19 mg cm−3.

Figure 1 Schematic Illustration of Preparation of ASGF. Full size image

The morphologies of the as-prepared ASGF were investigated by SEM and TEM. Typical SEM images in Fig. 2A,B show that ASGF possess a 3D structure with interconnected pores ranging from several nanometers to several micrometers. Moreover, the energy dispersive X-ray spectroscopy (EDX) measurement of the ASGF reveals that presence of Sn, O and C. (Fig. 2E). The TEM images (Fig. 2C,D) show that the SnO 2 nanoparticles with size in the range of 6–12 nm are distributed uniformly on the surface of a continuous 3D porous network made of ultrathin graphene sheets, in good agreement with the SEM observation above.

Figure 2 Morphology characterization of ASGF. SEM images (A,B) and EDX spectrum (E); TEM images with different magnifications (C,D) Full size image

The porous nature of SnO 2 -graphene architecture is further validated by nitrogen physisorption measurements and the results are shown in Fig. 3a. Clearly, for both the SGF and ASGF, the N 2 adsorption-desorption isotherms exhibit a typical II hysteresis loop at a relative pressure between 0.42 and 0.95, characteristic to pores with different pore sizes36. The surface area of the SGF and ASGF was determined to be 188 and 109 m2 g−1 by Brunauer–Emmett–Teller (BET) calculations, respectively. To gain insight into the chemical composition, thermo-gravimetric analysis (TGA) was performed on the 3D SnO 2 -graphene foam and typical result of SGF is shown in Fig. 3B. The sample is annealed under air at the 10 °C min−1 heating rate to remove the moisture and oxidize carbon to CO 2 . From the TGA data, the original content of SnO 2 is calculated to be 45.56 wt%. In combination with the analysis based on SEM and TEM images, we can safely conclude that the SnO 2 -garphene foam have a 3D graphene architectures that give rise to high surface areas and multilevel porous structures. It is this unique morphology that can greatly facilitate the access of electrolyte and the fast diffusion of lithium ion and electrons during lithium storage.

Figure 3 Nitrogen adsorption and desorption isotherms of ASGF (A) and SGF (B); TGA profile of SGF (C). Full size image

The surface chemistry of the ASGF was characterized using XPS and the results are depicted in Fig. 4. As shown in Fig. 4A, the general XPS spectrum proves the presence of carbon, oxygen and tin and no other elements are detected. The peaks of Sn 3d, 4d, 3p and 4s from SnO 2 are observed. The peak of C 1s is attributed mainly to graphene. The Sn 3d spectra of both SGF and ASGF, as shown in Fig. 4B, consist of two peaks at around 487.6 eV and 496.l eV, corresponding to Sn 3d 5/2 and Sn 3d 3/2 spin-orbit peaks of SnO 2 , respectively, confirming the formation of SnO 2 nanoparticles on the surface of graphene sheets, the minute difference between SGF and ASGF indicates the different chemical environment of SnO 2 nanoparticles37. Figure 4C shows that the O 1s core level peak of SGF can be resolved into two components centered at 531.2 eV and 532.8 eV, which can be assigned to Sn-O and/or C=O bonds and C-OH and/or C-O-C groups (hydroxyl and/or epoxy), respectively. On contrast, Fig. 4D shows that the O 1s core level peak of ASGF consists of three components centered at 531.3, 532.2 and 533.3 eV, which can be assigned to Sn-O and/or C=O bonds, Sn-C-O bonds and C-OH and/or C-O-C groups (hydroxyl and/or epoxy)38. Clearly, the annealing treatment induces the formation of new bonds — the Sn-C-O bonds, confirming the strong interaction between the SnO 2 nanoparticles and the graphene surface, which is the key to have the synergistic effect to improve the electrochemical properties38,39. Besides, the annealing treatment also decrease the C-OH and/or C-O-C groups (hydroxyl and/or epoxy) in the rGO that also helps enhance the electrical conductivity of ASGF40. The electrochemical properties of ASGF and SGF were systematically evaluated by galvanostatic discharge (lithium insertion)-charge (lithium extraction) measurements. Figure 5A,B compare the cycling performance for ASGF, SGF and pure SnO 2 nanoparticles at specific currents of 200 mA g−1 and 1000 mAh g−1 between 0.01 and 3 V vs. Li+/Li. The initial discharge and charge capacities of the ASGF at 200 mA g−1 are 1653 mAh g−1 and 984.2 mAh g−1, respectively, with a Coulombic efficiency (CE) around 60%. The CE of the second cycle increases to be 94.7% and maintains thereafter about 97% after 3 cycles (Fig. S1). After 50 cycles at 200 mA g−1, the ASGF electrode still exhibits a reversible capacity of 845 mAh g−1, which is 89.7% of the value of the second cycle.

Figure 4 XPS spectra of SGF and ASGF. (A) general XPS spectrum of ASGF; (B) Sn 3d XPS spectrum; (C) O1s XPS spectrum of SGF; (D) O1 s XPS spectrum of ASGF Full size image

Figure 5 Cycling performances of ASGF, SGF and pure SnO 2 nanoparticles at specific currents of 200 mA g−1 (A) and 1000 mA g−1 (B); rate capability of ASGF and SGF at current densities from 100 mA g−1 to 3000 mA g−1 (C); CV curves of the first three cycles of ASGF at a scanning rate of 0.5 mV s−1 (D). Full size image

By contrast in Fig. 5A, the cycling profiles for the SGF and the pure SnO 2 nanoparticles show continuous and progressive capacity decay along with cycling processes. In specific, the discharge and charge capacities of the SGF after the first cycle are 1592.7 mAh g−1 and 919.8 mAh g−1, respectively; after the second cycle, are 918.1 and 858.4 mAh g−1, respectively; after 50 cycles, 638.3 mAh g−1 and 634.6 mAh g−1, respectively. The capacity retention of the SGF after the 50th cycle is 73.9% with respect to that after the second cycle. For the pure SnO 2 nanoparticles, the capacity retention is much worse compared to SGF and the specific capacity drops to as low as 201.8 mAh g−1 after 25 cycles and remains thereafter.

Figure 5B shows that the capacity-retention advantage of the ASGF over the SGF and the pure SnO 2 nanoparticles is more obvious at higher current densities. After 150 cycles at the specific current of 1000 mAh g−1, the capacity of ASGF electrode is 533.7 mAh g−1 which is much better than SGF (294.8 mAh g−1) and pure SnO 2 nanoparticles (44.1 mAh g−1).

Figure 5C shows the rate capability of the ASGF and SGF electrodes. As the current densities increase stepwise from 100 to 200, 500, 1500 and 3000 mA g−1, the ASGF electrode delivers stable capacities varying from 922.0 to 770.5, 672.3, 582.8 and 480.3 mAh g−1, respectively; the SGF electrode from 784.5 to 693.7, 587.0, 396.9 and 251.2 mAh g−1, respectively. The specific capacities of the ASGF electrode is 17.4%, 11.1%, 14.5%, 46.8%, 91.2% higher than those of the SGF electrode at 100, 200, 500, 1500 and 3000 mA g−1. When the specific current returns to 100 mA g−1, the capacity recovers to 890.9 mA g−1 for the ASGF electrode, close to that after the 10th cycle at 100 mA g−1. The result shows that the ASGF has higher reversible capacity at the high current rates and better rate capability compared with the SGF.

Figure S2 shows the charge-discharge profiles of the ASGF(A) and SGF(B) electrode at a specific current of 200 mA g−1. The voltage profiles present sloping lines during both charge and discharge processes, in accordance with the broad peaks observed during CV scans. Moreover, both charge and discharge profiles exhibit little change from the second to the 50th cycles, demonstrating that the ASGF electrodes are very stable during cycling41,42.