Fabrication of 3D-GMOs

The growth process of our 3D-GMOs is illustrated in Figure 1 and NiCl 2 ·6H 2 O is used as the catalyst precursor of porous Ni skeleton (Fig. 1a). Firstly, the precursors are reduced to form 3D porous cross-linked Ni skeletons in Ar/H 2 atmosphere at 600°C (Fig. 1b). Next, the highly efficient growth is performed using methane as carbon source for only several seconds to minutes. Upon high-temperature annealing in the growth process, the tightly cross-linked catalyst skeletons with larger grain size are formed since the adjacent catalyst metals melted to come into being larger size at higher temperature (Fig. 1c). In contrast to the reported methods22,23, the advantages of our method to prepare 3D-GMOs lie in its fast growth process as well as its availability at atmospheric pressure. The effective growth is attributed to better catalytic reactivity and possibly high gas penetration through the porous 3D cross-linked Ni skeletons formed by the released gas (water vapor and hydrogen chloride) and melted metal at high temperature. Finally, the 3D-GMOs are obtained by removing Ni skeletons with FeCl 3 /HCl solution as etchant and followed by repeated water washes and freezing dry (Fig. 1d).

Figure 1 Schematic illustrations displaying the preparation process of 3D-GMOs. (a) A photograph of green NiCl 2 ·6H 2 O crystal precursors. (b) SEM image of porous cross-linked Ni catalysts after the reduction process. (c) SEM image of tightly cross-linked Ni catalysts covered by graphene layers. (d) SEM image of honeycomb-like graphene layers of the 3D-GMOs after etching Ni catalysts with FeCl 3 /HCl solution. Full size image

Characterization of the free-standing 3D-GMOs

We investigate the effect of growth temperatures on the prepared 3D-GMOs within the temperatures from 700 to 1000°C for a fixed growth time of 1.5 min. Typical Raman spectra and scanning electron microscopy (SEM) images show that the quality of 3D-GMOs is gradually improved with increasing growth temperature (Fig. 2a and Supplementary Fig. S1). This can be attributed to the improvement of crystalline quality of polycrystalline Ni with increasing growth temperature, which is conductive to reduce defects and form the thinner graphene layers. The thin graphene layers of 3D-GMOs grown for 1.5 min at 1000°C are investigated by SEM, Raman spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM). The free-standing 3D-GMOs show the honeycomb-like 3D interconnected morphology and curved graphene layers (Fig. 2c–e). The high conductivity (~12 S/cm) and the large specific surface area (~560 m2/g) are obtained by the two-probe method and the methylene blue adsorption, respectively. In addition, Figure 2b shows the typical Raman spectra of the 3D-GMOs. The number of graphene layers is estimated by 2D to G ratio (I 2D /I G ) together with 2D-band full-width at half maximum (FWHM, W 2D )46 and it can be seen that monolayer, bilayer and multilayer graphene coexist in the 3D-GMOs. The non-uniformity of layers may derive from that the individual Ni grains with varying sizes in polycrystalline porous Ni skeletons independently affecting the thickness of graphene in the CVD process15. Furthermore, the weak defect-related D peak reflects the high quality of the graphene layers. TEM is further employed to investigate the number of the graphene layers of the prepared 3D-GMOs directly. Low-magnification TEM in Figure 2f shows the thin curved morphology of graphene layers. Figure 2g–k indicates that the layer numbers of most of the 3D-GMOs are less than 10 layers determined by high-resolution TEM. In addition, we confirm the thickness of the graphene layers by AFM. The step heights in the AFM images are typically less than 3 nm calculated from the height difference between the surface of the graphene layers and the SiO 2 substrate, corresponding to the dominated range of graphene layer number of 1–7 L (Supplementary Fig. S2). Therefore, the layer thickness of the 3D-GMOs prepared by this strategy is conservatively estimated to be less than 4 nm.

Figure 2 Characterization the graphene layers of 3D-GMOs. (a) Typical Raman spectra of 3D-GMOs grown with different temperatures for 1.5 min. The Raman spectra show that the quality of 3D-GMOs is gradually improved with increasing the growth temperature up to 1000°C. (b) Typical Raman spectra of a 3D-GMO. Multi-layer, bilayer and monolayer graphene from bottom to top estimated by the intensity ratio of 2D peak to G peak, combining with 2D-band full-width at half maximum (FWHM, W 2D ). (c) A photograph of the free-standing 3D-GMO. (d, e) SEM images of honeycomb-like graphene layers after etching Ni template with FeCl 3 /HCl solution at different magnifications. (f) Low-resolution TEM image of the graphene layers in a 3D-GMO. (g–k) High resolution TEM images of different graphene layers in a 3D-GMO. (g) Monolayer. (h) Double layers and four layers. (i) Three layers. (j) Seven layers. (k) Ten layers. Full size image

The high density of 3D-GMOs

For the comparison with the graphene layers grown on Ni foams, we perform the growth of graphene layers on our 3D cross-linked Ni skeletons and commercial Ni foams in the same condition for 1.5 min at 1000°C, respectively (Fig. 3a,c). By contrast, 3D-GMOs show higher density of ~22 mg/cm3, which is one order of magnitude larger than that of Ni foam-grown graphene layers (~1 mg/cm3). The pore size of commercial Ni foam-grown graphene is 1–2 orders of magnitude larger than that of our porous cross-linked Ni-grown graphene (Fig. 3b,d). To explore the effect of growth time on the morphology and density of 3D-GMOs, we investigate the growth time ranging from 30 s to 10 min (Fig. 4a). We find that when the growth time is more than 3 min, there exist many graphite microspheres and the more intact graphite microspheres are formed with the growth time increasing, suggesting the growth of graphene layers is highly efficient in such a short time. The morphology difference may be attributed to that when the growth time is shorter, the typical graphene layers wrapped on Ni particle skeletons are thinner and not strong enough to hold the “body” after the Ni skeletons being removed, resulting in the collapse of graphene layers. With prolonging the growth time, the graphene layers are thicker and stronger enough to avoid collapsing without the support of Ni skeletons, producing many graphite microspheres. Moreover, the density of 3D-GMOs increases linearly with the growth time. The higher density of 3D-GMOs of up to 100 mg/cm3 can be obtained when grown for 10 min (Fig. 4b,c).

Figure 3 Comparison between commercial Ni foam-grown graphene and our porous cross-linked Ni-grown graphene. Ni foams are not removed (a) and removed (b). The porous Ni catalysts are not etched (c) and etched (d). The insets are the magnified images of (c) and (d), respectively. SEM images show that the pore size of commercial Ni foam-grown graphene is 1–2 orders of magnitude larger than that of our porous cross-linked Ni-grown graphene. Full size image

Figure 4 The effect of growth time on the morphology and density of 3D-GMOs. (a) SEM images of the morphologies of 3D-GMOs at growth time of 30 s, 1.5 min, 3 min, 5 min, 8 min, 10 min, respectively. When the growth time is more than 3 min, there exist many graphite microspheres and more intact graphite microspheres are formed with the growth time increasing. All the scale bars are 5 μm. (b) The mass of 3D-GMOs versus the growth time at 1000°C per 1.0 g Ni catalyst skeletons. (c) The density of 3D-GMOs increases nearly linearly with the growth time from 30 s to 10 min. Full size image

High-capacity removal of heavy metal ions

The 3D-GMOs as an electrode of the electrolytic deposition are investigated to remove heavy metal ions from aqueous solutions. The electric capture process of heavy metal ions is schematically illustrated in Figure 5a. We employ platinum (Pt) foils as anodes and free-standing 3D-GMOs as cathodes. Aqueous solutions containing single heavy metal ions (e.g. Cd2+, Pb2+, Cu2+, Ni2+) are used as electrolytes. Electrolytic deposition process is performed under a constant current of 0.05 A for 5 min, 10 min, 15 min and 20 min, respectively and the final concentrations of the single heavy metal ions mentioned above in the aqueous solutions are measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). As shown in Figure 5b, the adsorption capacities of 3D-GMOs increase with deposition time (Supplementary Fig. S3a–d). After 20-min deposition, the adsorption capacities are 434 mg/g, 882 mg/g, 1,683 mg/g and 3,820 mg/g corresponding to Cd2+, Pb2+, Ni2+ and Cu2+, respectively, which are much higher than that of the reported typical results using active carbon-based materials as adsorbents24,25,26,27. In addition, these values are higher than those of the reported graphene based adsorbents: 106.3 mg/g and 145.48 mg/g for Cd(II)33,36, 479 mg/g and 842 mg/g for Pb (II)31,40, 46.55 mg/g for Ni (II)43, 130 mg/g and 46.6 mg/g for Cu (II)34,35 (Supplementary Table S1) Interestingly, the deposited products on 3D-GMOs for 20 min show 3D porous structures derived from the 3D-GMOs templates in Figure 5c–f characterized by SEM (Supplementary Fig. S4) and the deposited products of Cd2+, Pb2+, Ni2+ and Cu2+are Cd(OH) 2 , PbO, Ni and Cu/Cu 2 O, respectively, determined by X-ray diffraction (XRD) patterns and energy dispersive X-ray (EDX) spectra (Supplementary Figs. S5–S8). We speculate that the high adsorption capacities for heavy metal ions originate from the following reasons. Firstly, the honeycomb-like 3D-GMOs provide the large-area templates for the deposited products of the metal ions, which continuously offer the 3D porous templates for the subsequent electrolytic deposition (Supplementary Fig. S3c–g). Secondly, the high conductivity of 3D-GMOs could enable the high electrolytic deposition rate. Lastly, the high density and effectively cross-linked structure of graphene layers sustain the free-standing monoliths and protect them from collapsing during the deposition process. Further, to investigate recovery performance of the 3D-GMOs, a desorption process is performed. In terms of the desorption of the deposited product of Cd2+, the free-standing 3D-GMO (deposited for 20 min) and the Pt foil are the anode and the cathode, respectively. A high concentration of 1.0 g/L (pH = 2.0) Cd (NO 3 ) 2 aqueous solution is electrolyte. After a constant current of 0.1 A applied for 1 min between the two electrodes, the deposited products on the 3D-GMO almost disappear (a desorption efficiency >96%) (Supplementary Fig. S9).