Synthesis of Metal- Organic Framework Hybrid Composites Based on Graphene Oxide and Carbon Nanotubes
Metal-organic frameworks (MOFs) are a class of versatile hybrid organic-inorganic porous materials and have attracted immense research interest in the last 20 years [1-4]. Owing to their exceptional properties such as large surface area, tunable structure, and high porosity, they are considered superior to other porous materials such as zeolites and silica [5,6]. MOFs have shown enormous potential in a myriad of applications such as gas separation and storage, sensing, catalysis, biomedicine, energy and environment, supercapacitors, and batteries [7-12]. Despite their considerable advancement in several applications, there are certain issues that need to be addressed to utilize MOFs to their true potential [12,13]. Firstly, MOFs in general are electrically non-conductive and unstable against high temperature and moisture, as well as strong electron beams . Upon exposure to moisture, the surface area of MOFs decreases drastically . Thus, the poor stability of the MOFs restricts their use in large-scale industrial applications, which require harsh conditions. Secondly, the MOF crystals are typically obtained in the form of pow'der which makes their subsequent processability a tedious job . Furthermore, the narrow micropores of MOFs restrict the rapid diffusion of gas inside the pores . Therefore, it is of utmost importance to address these issues to utilize the full potential of MOFs.
To overcome the above-mentioned challenges associated with MOFs, researchers developed three general strategies : (i) pre synthetic approach, which can involve doping metal ions or functionalization of the ligands, or selection of specific building blocks, (ii) post synthetic approach which might involve exchanging ligands and/ or metal ions or grafting active groups after synthesis of MOFs, and (iii) composite formation, which involves hybridization of the MOFs with some other material to develop a new hybrid functional material with superior properties. This latter strategy has recently attracted a lot of attention compared to the other tw'o strategies discussed.
MOF composites comprise the benefits of both the MOFs as well as the other functional material [12-33]. Moreover, fabrication of MOF composites is relatively easy. The composites can integrate the advantages of both the components and at the same time alleviate the disadvantages associated with each of the single components . Thus, MOF composite possess superior properties due to the synergistic effects of the individual components. Composites of MOFs with various other functional materials have been developed. MOF-metal nanoparticles/quantum dot composites [14-16]. MOF-silica composites [17-19], MOF-polymer composites [20-22]. MOF- polyoxometalates [23,24] and MOF-carbon composites [13,25-32] are widely studied. Among all these, MOF-carbon composites (composites of MOFs with carbon-based materials, such as carbon dots, carbon nanotubes, graphene, and graphene oxide) are special as MOFs are regarded as kind of state-of-the-art materials and carbon-based materials are known to be somewhat classical . These carbon-based materials are all unique in the sense that they possess several interesting properties such as high mechanical as well as elastic strength, excellent thermal and chemical stability, unique electronic and photophysical properties, low weight, biocompatibility as well as low cost. Such unique properties of the carbon-based materials have resulted in their use in sensing and catalysis, as well as energy and environmental applications [25-32]. Therefore, fabricating MOF-carbon composites will not only alleviate the disadvantages of MOFs but will also endow them w'ith several novel functionalities such as improved thermal conductivity, stability, etc. . In this chapter, however, we w'ill restrict ourselves to only MOF-graphene oxide (MOF-GO) and MOF-carbon nanotube (MOF-CNT) composites. CNTs are highly conductive ID materials and composite formation of MOFs w'ith CNTs can assist in the transportation of electrons from the outer circuit to the inner surface of the MOFs. GO, on the other hand, is a 2D material and it can function as a support to anchor and decorate the MOF nanocrystals on its surface enabling them to be employed in a variety of other applications. Both these carbon-based materials can suppress MOF aggregation and can control the properties, structure and morphology of MOFs.