Reticular Metalation in Metal-Organic Frameworks for Nanocasting and Interface Construction
Metal-organic frameworks (MOFs) are a class of porous, crystalline materials constructed by extending the linkages between inorganic and organic molecular building blocks through strong bonds. Because of their high porosities, accessible interconnected pore structures, and designable backbones, MOFs have been used in applications ranging from as catalyst supports, membrane substrates, sensing layers, and casting templates for metals. There are many methods that leverage the tunability of MOFs to introduce extraneous metals, or templating, including (i) metal addition on MOFs’ backbones; (ii) metal-ion exchange in MOFs; (iii) cation metathesis in MOFs; and (iv) growth or encapsulation of metal nanoparticles in MOFs’ pores.
Despite the significant efforts devoted to the reticular metalation in MOFs, there are still numerous challenges that need to be addressed. To name a few, in terms of anchoring individual metal atoms onto MOF’s building units, little is known about the local structures inside metallated MOF’s pores, especially for factors such as metal binding modes, oxidation states, and existing coordination forms. With regards to casting/encapsulating metal nanostructures in MOFs, the accurate control and fundamental understanding of the process have yet to be established. As for constructing interfaces between the metallated-MOF and substrate, well-designed synthetic routes need further investigations. The work presented in this thesis aims to answer these questions associated with the reticular metalation in MOFs, from the viewpoints of material synthesis, characterizations, as well as application aspects.
Chapter 1 introduces the concept, the-state-of-the-arts, and unresolved problems of the reticular metalation in MOFs. Various illustrations are utilized for detailed discussions.
Chapter 2 reports examples of anchoring metal atoms into a Zr (IV)-based MOF, MOF808, to afford MOF-808-M (M = Pd, Pt, Au, Rh, Ru, Ir). The structural features of MOF-808-M are thoroughly characterized by X-ray diffraction and CO-IR spectroscopy, and it is confirmed that the strong coordinative interaction between metals and MOFs’ backbones is allowing the visualization of the atomic structures of introduced metal species.
Chapter 3 leverages the discoveries in Chapter 2 and develops a general strategy for casting nanoporous metals into MOF templates. This Chapter begins with X-ray diffraction and transmission electron microscopy analyses to identify the casting process, followed by using various MOFs with tailorable dimensions and topologies to grow nanoporous platinum as examples. The generality of MOF-templated nanocasting is additionally explored afterwards.
Chapter 4 switches focus from the previous chapters and starts to look at the pristine interface between the metallated-MOFs and the supportive substrate. In this Chapter, a novel twostep synthetic route is demonstrated to design a well-defined Al-TCPP-Co MOF/carbon cloth composite during the reticular metalation in MOFs, and a clear interfacial chemistry is illustrated combining the analyses of X-ray photoelectron spectroscopy, X-ray scattering, and scanning electron microscopy.
Chapter 5 utilizes two examples from Chapter 3 (Nanocasting) and Chapter 4 (Interface construction) to demonstrate the power of the reticular metalation in MOFs for applications such as electrochemical catalysis and sensing. Specifically, MOF-templated nanoporous platinum (NPPt) is selected as the anodic catalyst for the methanol electrooxidation reaction, and Al-TCPPCo/CC composite is applied as the sensing electrode for the selective detection of hydrogen peroxide. By investigating the electrochemistry incurred in applications, comprehensive discussions are provided with regards to the properties enhanced during the reticular metalation process.
Chapter 6 finally provides some challenges and opportunities for the reticular metalation in MOFs, including the stability issue and bio-inspired metalation. To further develop the reticular metalation in MOFs, it is believed that more fundamental and characterization-oriented studies will be crucial.