Nano Today
ReviewA review of graphene-based 3D van der Waals hybrids and their energy applications
Graphical abstract
Introduction
The recent advances in energy research can be boosted by the exploration of emerging building blocks for highly efficient energy materials. Since graphene was firstly isolated in 2004 [1], the family of two-dimensional (2D) materials has been strongly considered worldwide and their applications in energy conversion and storage are also highly concerned. Great efforts have been made on the investigation of graphene-analogous 2D materials, including graphene materials, graphitic carbon nitride (g-C3N4), transition-metal dichalcogenide (TMD), layered double hydroxide (LDH), etc., in the fields of energy electrocatalysis [2,3], supercapacitors [[4], [5], [6]], metal-ion batteries [[7], [8], [9], [10]], and so on [[11], [12], [13], [14], [15], [16]]. In addition to the direct employment of an individual 2D material due to its unique properties, the merging of different 2D components to form van der Waals (vdW) heterostructures towards favorable structural and electronic varieties is emerging as a versatile route for materials innovation [17]. The assembly of different 2D material layers is analogous to the building of LEGO blocks, but via the interplanar van der Waals interactions. In fact, the van der Waals interactions are not limited to interplanar interactions in layered materials, but also between any passivated, dangling-bond-free surface [18]. Consequently, various vdW heterostructures can be fabricated through the rational hybridization of different 2D materials, which can effectively combine multi-functionalities, compensate individual weakness, and even endow new properties, leading to significantly enhanced performances in energy-related applications [[19], [20], [21], [22], [23]].
Theoretically, graphene is featured with outstanding intrinsic properties as an atomic-thick 2D material, such as high electrical and thermal conductivity, and high mechanical strength. [[24], [25], [26], [27]] However, the graphene products obtained and utilized for energy-related applications are always in the form of powder, which is difficult to fully maintain and demonstrate the intrinsic features [28]. Recently, the concept of three-dimensional (3D) graphene has been proposed and verified [[29], [30], [31]]. By constructing 3D graphene framework with sp2 configuration, the excellent intrinsic properties of 2D graphene can be inherited into 3D nanostructures [32]. Based on the 3D graphene material instead of its 2D parent, 3D vdW hybrids of other 2D materials supported by 3D graphene can be therefore constructed. The concept of 3D vdW hybrids is an extension of the vdW hybrids of 2D materials, in which 2D materials are likewise hybridized by interplanar vdW interactions, while a 3D structure is formed through curving, stacking, or pore-creating. The integration of 2D and 3D materials is a challenging topic with great significance [33], and 3D vdW hybrids are promising to build a bridge between two- and three-dimensional materials. Benefited by the good electrical conductivity and abundant pore structures of 3D graphene for accelerated electron and mass transfer, the graphene-based 3D vdW hybrids will explore more possibilities of 2D materials, leading to unexpected performances in energy-related applications. Generally, the 3D graphene framework used for constructing graphene-based 3D vdW hybrids is constructed by continuous graphene layers or interconnected graphene nanosheets, exhibiting a 3D hierarchical porous structure. The nanosheets of other 2D materials are in-situ grown or post-deposited on the graphene framework, forming vdW heterostructures. The application performance of the obtained 3D vdW hybrids are mainly determined by the various properties of the additional 2D materials supported by graphene, meanwhile the 3D graphene framework can enhance the electrical conductivity, enlarge the surface area, and make the nanosheets of the 2D material more uniformly dispersed and fully accessible, thus boosting the superb performance in energy-related applications.
Though the concept of 3D vdW hybrids emerges very recently, there were some reported nanocomposites with the configuration of 2D materials combined with 3D graphene before the 3D vdW concept was proposed. For a better understanding of the present advances and new insights for the further development in this field, herein we summarize the graphene-based 3D vdW hybrids, mainly classified into the hybrid between graphene and TMDs, other transition metal compounds, metal-free 2D materials, and multiple 2D materials, together with their energy applications, as illustrated in Fig. 1. The synthetic strategies, unique properties, and electrochemical performances of each family of 3D vdW hybrids are reviewed in details, with some perspectives included. We hope to provide an overview of the recent progress of the emerging graphene-based 3D vdW hybrids, and also expect to inspire more works on the smart design of 2D materials for various energy applications.
Section snippets
3D vdW hybrids of transition metal dichalcogenides and graphene
Transition metal dichalcogenides are an important family of 2D materials, and are widely investigated in the fields of optoelectronics [34], semiconductors [35], electrochemical energy storage [16,36,37], etc. [38]. The dichalcogenides of Mo and W (for instance, MoS2, MoSe2, WS2, etc.) are acknowledged for their high reactivity for hydrogen evolution reaction (HER) [[39], [40], [41]]. Therefore, these 2D TMDs are regarded as promising alternatives for Pt-based electrocatalysts in
3D vdW hybrids of other transition metal compounds and graphene
Metal compounds beyond dichalcogenides also contribute as an important part of the 2D material family [17]. Metal oxides [53,54], hydroxides [13], nitrides and carbides (MXenes) [55,56], and other types of metal compounds with 2D crystal structure [[57], [58], [59]] have exhibited their significant roles on energy-related applications. Herein, research progresses on the 3D hybrids of these metal compound 2D materials and graphene are reviewed.
The widely investigated 2D metal oxide materials in
3D vdW hybrids of metal-free 2D materials and graphene
2D metal-free graphene analogues, including graphitic carbon nitride (g-C3N4) [[83], [84], [85]], hexagonal boron nitride (hBN) [86], etc. [87,88], have proven their significance in the fields of semiconductor [89,90], electrochemical energy storage [91,92], and so on. By the hybridization with graphene materials, the conductivity of these 2D materials can be improved, and the morphology can also be regulated. The g-C3N4 material can be regarded as a 2D carbon nanomaterial with high nitrogen
3D vdW hybrids of multiple 2D materials and graphene
Besides using a single 2D material to fabricate 3D vdW hybrids with graphene, the hybrids of graphene and more than one kinds of 2D materials can further extend the types of their applications. The reported combinations of the 2D materials include graphene/g-C3N4/MoS2 [[102], [103], [104], [105]], graphene/g-C3N4/Ni(OH)2 [106], etc. However, most of the reported works on the hybridization of multiple 2D materials and graphene did not aim at fabricating vdW hybrids. The direct evidence for the
Summary and outlook
Fabricating graphene-based 3D vdW hybrids is an emerging and effective strategy for the design of 2D-material-based energy materials. The combination of 2D nanomaterials and 3D graphene framework can take the advantages of the features of 3D graphene, including high electrical conductivity, porous structure, and high specific surface area. Through the rational integration of 2D materials and 3D graphene, the 2D materials can be transformed into 3D nanostructure, thus bringing more possibilities
Acknowledgements
This work was supported by National Key Research and Development Program (2016YFA0202500 and 2016YFA0200101) and National Natural Scientific Foundation of China (21676160 and 21825501), and Tsinghua University Initiative Scientific Research Program. We thank Bo-Quan Li and Bin Wang for helpful discussion.
Hao-Fan Wang received his B.S. and Ph.D. degree in 2013 and 2018, respectively, both from the Department of Chemical Engineering, Tsinghua University, under the supervision of Prof. Qiang Zhang. His research focuses on advanced energy materials for the applications on bifunctional oxygen electrocatalysis, metal–air batteries, etc.
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Hao-Fan Wang received his B.S. and Ph.D. degree in 2013 and 2018, respectively, both from the Department of Chemical Engineering, Tsinghua University, under the supervision of Prof. Qiang Zhang. His research focuses on advanced energy materials for the applications on bifunctional oxygen electrocatalysis, metal–air batteries, etc.
Cheng Tang received his Bachelor and Ph.D. degrees from the Department of Chemical Engineering, Tsinghua University in year of 2013 and 2018, respectively. His research interests focus on nanomaterials and energy electrocatalysis, including 3D graphene, hierarchical nanomaterials, oxygen reduction/evolution, hydrogen evolution, nitrogen reduction, etc.
Qiang Zhang received his bachelor and Ph.D. degree from Tsinghua University in 2004 and 2009, respectively. After a stay in Case Western Reserve University, USA, and Fritz Haber Institute of the Max Planck Society, Germany, he was appointed a faculty in Tsinghua University at 2011. His interests focus on energy materials, includes Li-S batteries, Li metal anode, 3D graphene, and electrocatalysts. He has been awarded The National Science Fund for Distinguished Young Scholars, Young Top-Notch Talent from China, and Newton Advanced Fellowship from Royal Society, UK. Currently, he is the associate editor of Journal of Energy Chemistry and sitting on the advisory board of Matter, Advanced Energy Materials, Advanced Materials Interfaces, Philosophical Transactions A, Science China Materials, and so on.