A General and Programmable Synthesis of Graphene-Based Composite Aerogels by a Melamine-Sponge-Templated Hydrothermal Process

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2020A General and Programmable Synthesis of Graphene-Based Composite Aerogels by a Melamine-Sponge-Templated Hydrothermal Process Jin Ge†, Hong-Wu Zhu†, Yuan Yang, Yufang Xie, Gang Wang, Jin Huang, Lu-An Shi, Oliver G Schmidt and Shu-Hong Yu Jin Ge† Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China Institute for Integrative Nanosciences, IFW Dresden, 01609 Dresden, Germany †J. Ge and H.W. Zhu contributed equally to this work.Google Scholar More articles by this author , Hong-Wu Zhu† Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China †J. Ge and H.W. Zhu contributed equally to this work.Google Scholar More articles by this author , Yuan Yang Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China Google Scholar More articles by this author , Yufang Xie Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany Google Scholar More articles by this author , Gang Wang Center for Advancing Electronics Dresden (cfaed) and Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany Google Scholar More articles by this author , Jin Huang Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China Google Scholar More articles by this author , Lu-An Shi Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China Google Scholar More articles by this author , Oliver G Schmidt Institute for Integrative Nanosciences, IFW Dresden, 01609 Dresden, Germany Google Scholar More articles by this author and Shu-Hong Yu *Corresponding author: E-mail Address: [email protected] Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Institute of Biomimetic Materials and Chemistry, University of Science and Technology of China, Hefei 230026, China Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.201900073 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Three-dimensional (3D) graphene networks are performance boosters for functional nanostructures in energy-related fields. Although tremendous intriguing nanostructures-decorated 3D graphene networks have been realized, on-demand decoration of nanostructures in the specified position of interest within the whole 3D graphene skeleton is still out of reach, shedding limitations on constructing more sophisticated components with programmable structures which offer enormous potential for the enhancement of performance and exploration of new functions. Here, we report the melamine-sponge (MS)-templated hydrothermal method capable of realizing reduced graphene oxide (RGO)-nanostructure composite aerogels with programmable structures and compositions. The key of this method is using the MS template to preset the structures of choice through programmable solution-processed immobilization of graphene oxide (GO) and nanostructures. Remarkably, the hydrothermal treatment simultaneously removed the MS template and reduced the GO networks without changing the preset structures. We showcased nine typical RGO-nanostructures composite aerogels to demonstrate the versatility of the MS-templated hydrothermal method. Download figure Download PowerPoint Introduction Three-dimensional (3D)-graphene networks are intriguing hosts for functional nanostructures, particularly, in energy-related application fields.1–4 The large surface area of 3D graphene network profits the dispersion of the functional nanostructures, resulting in a substantial increase of active sites.5,6 Meanwhile, the conductive and interconnected skeletons, as well as the open-pore structure of the 3D graphene networks afforded the functional nanostructures with faster transportation of both electrons and ions.1,3 The collective effect of these advantages eventually enables the functional nanostructures with unprecedented performance, including their utilization in batteries,7–15 supercapacitors,16–22 water splitting,23–27 fuel cells,28–31 and others. Currently, several intriguing approaches of constructing nanostructures-decorated 3D graphene networks have been put forward, including the hydrothermal process promoted self-assembly,13,14,17,27,30–32 ions or nanostructures-induced self-assembly,33,34 in situ fabrication of nanostructures on existing 3D graphene networks,19,22,35–38 freeze-drying,8,18,39,40 and others. Though these methods have already produced incredibly high-performance 3D graphene nanostructure (3D-GN) aerogels in energy-related fields, they require the proper design of the reaction systems for each case and still encountering the challenges of precisely controlling the structures and compositions, especially, during codecoration of multitype nanostructures with determinable spatial distribution within the 3D graphene networks. There is no doubt that structure-definable 3D-GN aerogels are crucial for the clear-cut understanding of the structure–composition–property relations that are the indispensable guides for the design of 3D-GN aerogels with desired functions. Therefore, a programmable decoration method is highly desirable for the realization of 3D-GN aerogels with tight-controlled structures and compositions. Herein, we report a general melamine-sponge (MS)-templated hydrothermal method able to program the composition and structure of 3D-GN aerogels (Figure 1). The MS template performed the following vital functions: First, the 100 s micrometer open-pore system and robust mechanical property of the MS allow the immobilization of graphene oxide (GO) nanosheets and functional nanostructures on the skeleton of the sponge in the sequence of choice through a facile solution process. Second, the MS template removed quickly through a hydrothermal treatment, which could also reduce the GO coating, forming 3D-interconnected reduced graphene oxide (RGO) networks. More importantly, after the hydrothermal treatment, the stack sequence of the RGO layers and nanostructures followed the same sequence as the GO layers and nanostructures before the hydrothermal treatment, which allowed the generation of a defined wall structure (stack sequence) and wall composition (types of functional nanostructures) of the 3D-GN aerogels in a solution-processed dip-coating process. Third, the use of an MS template to aid the formation of 3D-RGO networks eradicated the aggregation problem of GO dispersion and limitation on the GO concentration for gelation to enable 3D-GN aerogels formation using ultralow GO with a mass/volume concentration of 0.05 mg/cm3. Fourth, the high mechanical and chemical stability of the MS-supported GO network made it compatible with a Layer-by-Layer (L-b-L) immobilization strategy, providing extraordinary flexibility in defining the structure and composition of the RGO-nanostructure composite aerogels. Finally, we showcased a variety of 3D-GN aerogels, in particular, well-defined compositions and structures, including nanostructures decorated on both sides of the RGO wall, nanostructures selectively decorated on the outer side of RGO wall, nanostructures selectively decorated on the inner side of the RGO wall, and defined RGO/nanostructures stack (Figure 1, bottom panel) that could hardly be realized by currently available methods. We envision that our MS-templated hydrothermal method would set up an upgraded platform for the deterministic fabrication of 3D-GN aerogels, benefiting both fundamental research and betterment of performance in energy-related fields. Figure 1 | Schematic illustration of the MS-templated hydrothermal method. Through Layer-by-Layer (L-b-L) dip-coating processes, GO and nanostructures are successively immobilized on the backbone of the commercially available MS. The GO nanosheets assemble along the skeleton of the MS and form 3D-interconnected tube networks and the nanostructures are confined to the specific layers (i.e., four examples in the middle of the panel). After the hydrothermal treatment, the MS are removed. Meanwhile, GO nanosheets on the sponge are reduced, forming nanostructures-decorated 3D-interconnected RGO tube networks. The decoration of the same nanostructure on the inner and outer of the RGO tube (bottom, first example) can be realized through one-step hydrothermal treatment of GO nanosheets coated MS sponge ([email protected]) in the precursor solution of the nanostructures. For other specific structures (bottom, three examples to the right), nanostructures are first immobilized on the [email protected] through L-b-L dip-coating processes. Then the following hydrothermal treatment removes the MS sponge and reduces the GO nanosheets. The layered structure of the resultant RGO-nanostructures tube walls totally follows their predecessor (GO-nanostructures tube walls). Ultimately, the structure and composition of RGO-nanostructures tube walls are programmed. GO, graphene oxide; RGO, reduced graphene oxide. Download figure Download PowerPoint Methods Fabrication of RGO aerogel and GO aerogel GO-Coated MS ([email protected]) was heated up at 200 °C for 2 h in Teflon reactors with deionized water (DIW) as the solution. Then, the resultant black monoliths were rinsed with DIW carefully to remove the hydrolysis products of the MS. Afterward, the RGO aerogels were obtained by freeze-drying the black monoliths. The fabrication of the GO aerogel was similar to RGO aerogel except for the use of HCl solution and decreasing the temperature to 140 °C for the hydrothermal treatment. Detailed synthesis procedures are described in ). Fabrication of [email protected]@N1 aerogels The [email protected] monoliths were put into Teflon reactors with N1 (N1 refers to one kind of nanostructures) precursor solutions and were heated at 180 °C for 6 h. Then, the resultant monoliths were rinsed with DIW carefully and freeze-dried. Examples include Fe2O3@[email protected]2O3 aerogels, Co3O4@[email protected]3O4 aerogels, and Mn3O4@[email protected]3O4 aerogels, shown in Figure 2. Detailed procedures for the synthesis are described in . Figure 2 | Decoration of nanostructures on the outer and inner sides of the RGO tube wall via one-step hydrothermal treatment. (a) Schematic illustration of the reactions during the hydrothermal process. SEM images of the (b, e) Fe2O3 nanoparticles, (c, f) Co3O4 nanoparticles, and (d, g) Mn3O4 nanoparticles decorated RGO aerogels. TEM images of the (h) Fe2O3 nanoparticles, (i) Co3O4 nanoparticles, and (j) Mn3O4 nanoparticles decorated RGO walls in (b, c, d), respectively. The scale bars of pictures inserted in (b, c, d) are 1 cm. RGO, reduced graphene oxide. Download figure Download PowerPoint Fabrication of [email protected] aerogels Nanostructures (N1) were immobilized on the surface of the [email protected] monoliths through either in situ growth or dip-coating prefabricated N1. Then the N1-decorated [email protected] monoliths were put into Teflon reactors with DIW as the solution and heated at 200 °C for 2 h. Afterward, the resultant monoliths were rinsed with DIW carefully and freeze-dried. Examples include [email protected] aerogels, [email protected] aerogels, [email protected] aerogels, and [email protected]2 aerogels, shown in Figures 3 and 4b,e. Detailed procedures for the synthesis are described in . Figure 3 | Decoration of nanostructures on the outer surface of the RGO tube network. (a) Schematic illustration of the decoration of nanostructures on the outer surface of the RGO tube network. SEM images of the Ag nanoparticles (b, e), Pt nanoparticles (c, f), and AuNPs (d, g) decorated RGO aerogels. TEM images of the Ag nanoparticles (h), Pt nanoparticles (i), and AuNPs (j) decorated RGO walls in (b, c, d), respectively. The scale bars of pictures inserted in (b, c, d) are 5 mm. RGO, reduced graphene oxide. Download figure Download PowerPoint Figure 4 | Fabrication of RGO-nanostructures composite aerogels with definable compositions and structures via L-b-L strategy. (a) Schematic illustration of three typical GO-nanostructures coatings on the MSs. Corresponding SEM images of (b, e) [email protected]2, (c, f) MnO2@RGO, (d, g) [email protected]2@[email protected] The scale bars of pictures inserted in (b, c, d) are 5 mm. RGO, reduced graphene oxide; GO, graphene oxide, MSs, melamine-sponge; SEM, scanning electron microscope. Download figure Download PowerPoint Fabrication of [email protected] aerogels Nanostructures (N1) were immobilized on the surface of the MSs. Then, the N1-decorated sponges were further coated with GO and heated at 200 °C for 2 h in Teflon reactors with DIW as the solution. Afterward, the resultant monoliths were rinsed with DIW carefully and freeze-dried. An example of MnO2@RGO aerogel is shown in Figure 4c,f. Detailed procedures of the synthesis are described in . Fabrication of [email protected]@[email protected] aerogel N1, GO, N2 (N2 refers to nanostructures different from N1), and GO were coated on the MSs one by one using L-b-L dip-coating strategy. Then, the resultant sponges were heated at 200 °C for 2 h in Teflon reactors with DIW as the solution. Afterward, the resultant monoliths were rinsed with DIW carefully and freeze-dried. An example of MnO2@[email protected]@RGO aerogel is shown in Figure 4d,g. Detailed procedures of the synthesis are described in . Results and Discussion Formation of interconnected RGO networks by hydrothermal treatment of GO-coated MS ([email protected]) The most basic of the MS-templated hydrothermal method toward the fabrication of the 3D-GN aerogel is the formation of RGO aerogel with an interconnected RGO network after a hydrothermal treatment of the [email protected] The interconnected RGO network fixed the dispersed nanostructures and served as the skeleton of the RGO-nanostructures composite aerogels. The main composition of MS sponge is the melamine formaldehyde, which consists of 3D–cross-linked polymer networks (Figure 5a). The ether bridge (indicated as Linkage A) and methylene bridge (Linkage B) are hydrolyzable in water and acidic solution. The speed of hydrolysis increased with a rise in reaction temperature.41–43 The hydrolysis products, including small melamine formaldehyde (MF) polymer networks, melamine, and formaldehyde, were water-soluble. We found that the hydrothermal treatment could boost the hydrolysis speed of MF, which dissolved in water within 2 h. Since the hydrothermal treatment could also reduce GO, the removal of the template could be, potentially, coupled with the reduction of GO. We performed the hydrothermal treatment of [email protected], aiming to obtain RGO aerogel (Figure 5b). The [email protected] monoliths were fabricated through the centrifugation-assisted dip-coating process.19,44,45 The open-pore structure of the MS sponge was protected after a dip-coating of GO (Figure 5d). The wrinkles on the MS surface indicated a thin layer of GO coating on the skeleton surface (Figure 5g). After heating the [email protected] in a hydrothermal reactor with pure water as the solution at 200 °C for 2 h, we observed a highly shrank black monolith (see the photo inserted in Figure 5c). After the sponge template was removed, 3D-interconnected wrinkled tube networks were formed (Figure 5c,f). Raman spectra indicated a reduction in the GO coating (see ). The change of surface energy and interaction force between RGO nanosheets during the hydrothermal treatment caused shrinkage of the GO networks. With the reduction of GO nanosheets, the GO coating became hydrophobic, and the surface energy of the GO/water interface increased, which promoted a self-folding of the RGO coating by virtue of increased π–π interaction. Notably, the hydrolysis products of the MS reacted with the GO nanosheets. Among them, formaldehyde promoted the reduction of the GO nanosheets.46 Melamine reacted with the GO nanosheets, ultimately resulting in nitrogen-doped RGO.47 Figure 5 | Coupled hydrolysis of the MS and reduction of the GO during the hydrothermal process. (a) Hydrolysis mechanism of the melamine formaldehyde polymer networks. (b) Schematic illustration of the hydrothermal treatment of GO-coated MS ([email protected]) in neutral and acid solutions, respectively. SEM images of the (c, f) RGO aerogel, (d, g) [email protected], and (e, h) GO aerogel in low and higher magnification, respectively. The scale bars of pictures inserted in (c, d, e) are 1 cm. MS, melamine-sponge; GO, graphene oxide; RGO, reduced graphene oxide. Download figure Download PowerPoint As the hydrolysis of MS sponge could be improved by decreasing the pH of the water solution, we could remove the MS template with low temperature and decrease the degree of reduction of GO, in which case, the shrinkage of the GO network could be avoided to enable the realization of GO aerogel with ultralow density. We performed the hydrothermal treatment of [email protected] in HCl solution with a lower temperature (140 °C). The resultant brown monolith almost has the same size as the pristine [email protected] (see the photo inserted in Figure 5e). The MS template was removed totally without noticeable shrinkage on the GO tube networks (Figure 5e,h). The Raman spectrum indicated a partial reduction of the GO coating (see ). The resultant GO tube networks could be reduced further by thermal reduction (e.g., 800 °C for 2 h in argon atmosphere), resulting in an ultralight RGO aerogel (∼1.54 mg/cm3) with 3D-interconnected tube networks (see ). The conductivity of this ultralight RGO aerogel is roughly 15 S/m, much higher than the conductivity of the RGO aerogel (0.6 S/m) shown in Figure 5c. In particular, all of these RGO aerogels did not exhibit resilience after compression due to their large pore size and thin skeleton of only a few RGO layers. The resilient property of the RGO aerogels could be improved potentially by embedding carbon nanotube into the RGO walls.48 Since it is feasible to fabricate RGO aerogel through the hydrothermal treatment of [email protected], the next crucial step was to decorate the nanostructures on the RGO tube networks. Using MS sponge as the template became advantageous, and the 100 s micrometer open pores allowed the fast diffusion of ions and facilitated the reaction processes or absorptions onto the surface of both [email protected] and RGO aerogels. Meanwhile, the robust mechanical properties of the MS sponge allowed solution-processed manipulation without breaking the connectivity of GO networks. Additionally, the MS sponge could be removed easily through hydrothermal treatment. Other types of polymer sponges such as polyurethane and polyvinyl formal sponges, either did not have the perfect 3D network structure like the MS or could not be removed. Nickel foams had interconnected open-pore structures and could be removed easily, but their large pore size led to a 3D graphene framework with a low surface area. The cost of nickel foams was also much higher than MS sponge. Further, it is worth mentioning that the centrifugation-assisted dip-coating used to prepare [email protected] was a time- and material-saving process, and could be scaled-up easily (see ), making the [email protected] a versatile platform for the construction of 3D-GN aerogels by hydrothermal treatment. Decorating the inner and outer surfaces of nanostructures of the RGO tube wall The hydrothermal treatment, used for the preparation of RGO aerogel, is also a well-known method for the fabrication of nanostructures such as metal oxides. Moreover, the oxygen-containing functional groups of the GO are ideal nucleation sites for anchoring the nanostructures. Thus, we envisaged that it might be possible to realize the immobilization of the metal oxides nanostructures on the backbone of the RGO networks via one-step hydrothermal treatment. Our supposition of the reaction mechanism for the decoration of nanostructures on both sides of the RGO tube wallis illustrated in Figure 2a, as follows: The metal ions in the precursor solution absorb on the GO surface through electrostatic interaction or hydrogen-bond interaction. With the hydrothermal process going on, the metal oxide nanostructures (MexOy) formed on the outside of the GO tube. Meanwhile, the MS sponge is gradually hydrolyzed, leaving a gap between the MS fiber and the GO tube wall. The gap allows the metal ions to diffuse into the inner side of the GO tube. At this stage, the GO is partially reduced and still has the oxygen-containing functional groups to anchor the metal oxide nanostructures. Finally, the MS sponge template is removed, GO tube is reduced into RGO tube, and MexOy nanostructures grow on both sides of the RGO wall. To demonstrate our supposition, we select FexOy, CoxOy, and MnxOy nanostructures as examples. The [email protected] monoliths were put into the waterborne precursor solutions [FeSO4, Co(CH3COO)2, and Mn(CH3COO)2] and the same hydrothermal treatment, used for the fabrication of RGO aerogel, was performed. Figure 2b–d shows the resultant monoliths. All of them have the 3D-interconnected RGO networks with an open-porous structure. Figure 2e–j shows well-dispersed nanostructures immobilized on the RGO surfaces. In particular, Figure 2e shows that the nanostructures are immobilized on both sides of the RGO wall. The X-ray powder diffraction (XRD) analysis of the resultant monoliths indicated that the nanostructures immobilized on the RGO tube networks were Fe2O3, Co3O4, and Mn3O4, respectively (see ). These results demonstrated that it is feasible to immobilize nanostructures on all the exposed surfaces of the RGO aerogels through the one-step MS-templated hydrothermal reaction. The MS sponge template not only endowed the GO with large and robust open-pore structure enabling the fast diffusion of ions to the surface of GO for chemical reactions but also eliminated the difficulty in gelling RGO-nanostructures composites that exists in the commonly used template-free one-pot hydrothermal reactions.6,49 The content of the GO in the MS sponge template was only 0.05 mg/mL. With such a low concentration of GO dispersion, it is impossible to fabricate RGO-nanostructures aerogel by using the template-free hydrothermal method (see ). In most circumstances, this latter method demands a higher concentration of GO dispersion (>0.5 mg/mL) for the gelation of RGO nanostructures and requires a long time to realize the homogeneous dispersion of metal salts in the GO dispersion. In contrast, our use of the MS sponge template resulted in the immediate realization of the homogeneous absorption of ion on the GO surface when immersing the [email protected] monolith in the metal salts solutions. Though we only showcased the decoration of Fe2O3, Co3O4, and Mn3O4 nanostructures on the RGO aerogel, the one-step MS-templated hydrothermal method could be extended to the decoration of the RGO aerogel with other kinds of nanostructures [e.g., Ni(OH)2,17 SnO2,50–52 RuO2,53 noble metals,54 and so on], which could be fabricated by one-pot waterborne hydrothermal reaction. Selectively decorating nanostructures on the outer surface of RGO tube wall The direct exposure of functional nanostructures to the interconnected open pores to enable fast mass transfer, is highly preferable in heterogeneous catalytic reactions. Currently, template-free hydrothermal methods could successfully fix the functional nanostructures throughout the RGO frameworks homogeneously, but parts of the embedded nanostructures remained unutilized.31,32,39 Thus, the low material utilization efficiency is likely to dramatically increase the cost, in particular, when using noble metals as the catalysts. Accordingly, we put forward a two-step MS-templated hydrothermal method to selectively decorate the functional nanostructures on the surfaces of graphene frameworks expose directly to the interconnected open pores. As is shown in Figure 3a, the first step was to immobilize the nanostructures on the surfaces of [email protected] either by in situ reaction or by absorption of prefabricated nanostructures. Since there is no gap available between the GO coating and the skeleton of the MS sponge, all the nanostructures immobilized, selectively, on the outer surface of the GO coating. The second step was to apply the hydrothermal treatment on the nanostructures-decorated [email protected], which removed the MS sponge template and reduced the GO coating. During this step, the nanostructures were always fixed initially by the GO coating and afterward, the RGO wall. Therefore, all the nanostructures only stood on the outer surface of the RGO tube wall after the whole process. Taking the noble metals such as Ag, Pt, Au nanostructures as examples, we first fixed the Ag nanoparticles (AgNPs), Pt nanoparticles (PtNPs), and Au nanoparticles (AuNPs) on the surface of [email protected] separately either through in situ reaction or the physical absorption of prefabricated nanoparticles. Then we put the nanoparticles-decorated [email protected] monoliths in the hydrothermal reactors and heated them at 200 °C for 2 h. Figure 3b–d shows the resultant monoliths, which all displayed interconnected RGO networks of the open-pore system. The scanning electron microscopy (SEM) images (Figure 3e–g), transmission electron microscopy (TEM) images (Figure 3h–j), and XRD spectra (see ) demonstrated that AgNPs, PtNPs, and AuNPs were immobilized on the surface of the RGO aerogels, respectively. This two-step MS-templated hydrothermal method could be applied potentially to other nanostructures beyond AgNPs, PtNPs, and AuNPs, shown in Figure 3. Nonetheless, we should note the limitation of this method regarding nanostructures in metastable states (see ) and some high aspect ratio nanostructures (see ) of which the morphologies and crystal structures of the nanostructures might experience significant change during the hydrothermal treatment. Fortunately, our current method is promising for such nanostructures fabrication through the hydrothermal reaction, as revealed by the MnO2 nanowire shown in Figure 4. Inspired by these nanocrystals fabrication, we foresaw that the use of a capping agent might aid in improving their stability during the hydrothermal process. Programmable decoration of the RGO tube wall with diverse nanostructures It has been reported that the synergistic effects of different types of nanostructures decorated on the same RGO nanosheets could enhance performance, compared with using a single type of nanostructures.55–57 This finding motivated researchers to develop diverse nanostructures-codecorated RGO aerogel with the aim of exploring new properties or improving their performance. The commonly used template-free hydrothermal method could realize the codecoration of different types of nanostructures on the same RGO frameworks. However, the nanostructures were distributed randomly throughout the RGO frameworks. In this