Layered intercalation compounds: Mechanisms, new methodologies, and advanced applications
Introduction
Two-dimensional (2-D) layered materials such as graphene are currently a topic of intense interest because of their extraordinary physical and chemical properties [1], [2], [3], [4], [5]. 2-D layered materials share similar overall planar-like structures, with different sheet composition and crystal structure as the distinguishing features. The materials are in general characterized by strong intralayer bonding and weak interlayer interactions. The layers may be electrically neutral, or may be positively or negatively charged [6], [7], [8], [9], [10]. For neutral layered compounds, including graphite and some metal chalcogenides, metal oxides, and metal halides, the layers are held together via Van der Waals, π-stacking, or other noncovalent interactions, and the interlayer space is a network of empty lattice sites [11], [12]. For positively charged (such as layered double hydroxides (LDHs)) or negatively charged (such as metal phosphates, phosphonates, and silicates) layered materials, the layers are primarily held together by electrostatic forces and the interlayer space is filled by counter ions or by a combination of ions and solvent molecules such as water [13], [14]. Among the various 2-D materials, those with an intercalated structure have dominated the research due to their potential to offer unique properties through the combination of distinct features of guests (i.e., the intercalants) and hosts (i.e., the layers) within a single composite [15], [16]. Additionally, the interactions between guest and host may lead to new or enhanced phenomena, e.g., enhanced electronic, photonic and optoelectronic, thermal and thermoelectric, magnetic, and catalytic properties [1], [17], [18], [19], [20], [21]. Over the past few decades, there have been significant advancements in the understanding of intercalation chemistry [15], as well as the applications of intercalation materials, including, but not limited to, energy conversion and storage [22], [23], [24], [25], medical applications [26], [27], catalysis [1], [15], [28], sensors [29], display systems [15], nuclear waste treatment [30], and environmental remediation [13], [31], [32], [33].
The phenomenon of intercalation was first discovered ca. 600–700 CE in China [34]. At that time, Chinese researchers intercalated alkali metal ions into natural minerals, such as kaolin, to make porcelain [35]. The first intercalation phenomenon was reported in the literature in 1840 by Schafhäutl [36], in which it was attempted to dissolve graphite in sulfuric acid [36]. The modern era of intercalation research was initiated by a paper in 1926 by Fredenhagen et al., in which the uptake of potassium vapor into graphite was reported [37]. Since then, intercalation reactions have continued to fascinate chemists and material scientists.
To date, various approaches to achieve intercalation have been developed based on different physical and chemical reaction/interaction mechanisms [15], [16]. In addition, several key factors that influence the intercalation process have been verified, such as interlayer spacing and its regularity [38], [39]. The growing knowledge of intercalation mechanisms and influential factors has led to the attainment of intercalations at various interfaces, including solid-solid, solid-liquid, and solid-gas interfaces [15], [16], [40]. In some cases, the desired intercalated structure may be achieved in a single step by directly intercalating a layered host; however, some guest species are difficult or impossible to be directly intercalated because of size mismatch and/or lack of driving force. To solve such issues, usually multiple driving forces and/or multiple steps are necessary to achieve intercalation. Some non-conventional methods, including top down layer-by-layer self-assembly and one-step co-assembly methods, as well as bottom up synthetic methods have also been developed to address some special needs [15], [41].
In this review, we first give a brief perspective on the intercalation of 2-D layered materials concerning intercalation mechanisms, methodologies, and influential factors which may be paramount in better facilitating the development of new intercalation processes and novel and unique 2-D intercalation materials. We categorize the methodologies of the intercalation of 2-D materials into three groups: conventional intercalation methods, top down, and bottom up strategies (Fig. 1). Conventional methods generally utilize diffusion, ion exchange, intermolecular forces, acid-base reaction, electrochemical and redox-reactions, and some external factors to intercalate pre-existing 2-D materials. The section on top down and bottom up approaches will focus on the non-conventional intercalation strategies of layer-by-layer self-assembly, one-step co-assembly, and in situ one-pot synthesis of intercalation materials, among others. Finally, insights into the potential applications, challenges, and future opportunities of 2-D intercalated materials will be discussed.
Section snippets
Fundamental mechanisms and the corresponding intercalation approaches
In this part, we focus on the conventional intercalation methodologies of 2-D materials (as shown in Fig. 1a). The intercalation process consists of the insertion of molecules or ions (collectively referred to as the “guests”) into the interlayer of layered solids (collectively referred to as the “hosts”), resulting in the intercalation materials. The majority of intercalations rely on reactions/interactions between the guest and the host, including ion-exchange, acid-base reaction, hydrogen
Nonconventional techniques to achieve intercalation
As described in Fig. 1b and c, in addition to conventional intercalation approaches, special strategies of top down and bottom up have also been developed to prepare hybrids with an intercalated structure. Herein, we highlight a few representative top down strategies (including layer-by-layer self-assembly and one-step co-assembly) and bottom up methods (including in situ one-pot synthesis and calcination-rehydration approach).
Factors influencing the intercalation process
Interlayer distance is by far the most critical factor influencing the intercalation process and the ultimate results, as discussed in various examples above. Apart from interlayer distance, several other factors, such as arrangement of guests in the interlayer space, regularity of interlayer space, layer rigidity, among others, play an important role in intercalation.
Applications of intercalation compounds
The structural characteristics of 2-D materials result in unique physical, chemical, electronic, and optical properties, making them promising in a wide range of applications. Furthermore, the unique properties of individual 2-D materials can be fine-tuned and enhanced via intercalation [1]. With the rapid development in intercalation chemistry, the applications of various intercalation compounds are expected to become much wider in the near future. Herein, rather than giving a comprehensive
Summary and outlook
In summary, we highlighted the intercalation chemistry of 2-D materials. Various intercalation mechanisms, approaches, and influential factors, as well as potential applications of 2-D intercalation materials are discussed. The various intercalation mechanisms and the corresponding conventional intercalation methodologies based on reactions/interactions between the guest species and the host layered materials, including ion-exchange, acid-base reaction, hydrogen bonding, redox and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
L. Sun acknowledges the support from the National Science Foundation (CMMI-1562907). B. R. Martin acknowledges the support from the National Science Foundation (DMR-1205670). R. Zhu acknowledges the support from the National Natural Science Foundation of China (41572031). M. Laipan thanks the National Natural Science Foundation of China (41902039), the China Postdoctoral Science Foundation (2018M640832), and the China Scholarship Council (CSC) for financial support. The authors thank Anna Marie
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These authors contributed equally to this work.