High-density three-dimension graphene macroscopic objects for high-capacity removal of heavy metal ions

Abstract

The chemical vapor deposition (CVD) fabrication of high-density three-dimension graphene macroscopic objects (3D-GMOs) with a relatively low porosity has not yet been realized, although they are desirable for applications in which high mechanical and electrical properties are required. Here, we explore a method to rapidly prepare the high-density 3D-GMOs using nickel chloride hexahydrate (NiCl₂·6H₂O) as a catalyst precursor by CVD process at atmospheric pressure. Further, the free-standing 3D-GMOs are employed as electrolytic electrodes to remove various heavy metal ions. The robust 3D structure, high conductivity (~12 S/cm) and large specific surface area (~560 m²/g) enable ultra-high electrical adsorption capacities (Cd²⁺ ~ 434 mg/g, Pb²⁺~ 882 mg/g, Ni²⁺ ~ 1,683 mg/g, Cu²⁺ ~ 3,820 mg/g) from aqueous solutions and fast desorption. The current work has significance in the studies of both the fabrication of high-density 3D-GMOs and the removal of heavy metal ions.

Figure 1. Schematic illustrations displaying the preparation process of 3D-GMOs.

(a) A photograph of green NiCl₂·6H₂O 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 FeCl₃/HCl solution.

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 FeCl₃/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.

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.

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.

Figure 5. Electrolytic deposition on 3D-GMOs removing heavy metal ions.

(a) Schematic illustrations of the electrolytic deposition on 3D-GMOs removing heavy metal ions. (b) Typical adsorption capacity of 3D-GMOs versus time for the various heavy metals ions, Cd²⁺ (red line), Pb²⁺ (green line), Ni²⁺ (blue line) and Cu²⁺ (black line). SEM images of the deposited products of Cd²⁺ (c), Pb²⁺ (d), Cu²⁺ (e) and Ni²⁺ (f) on 3D-GMOs for the same deposition time of 20 min, showing 3D porous structures derived from the 3D-GMOs templates.