Computational and experimental characterization of structural properties in nanoporous materials using optical absorption spectroscopy

  1. Computational and experimental characterization of structural properties in nanoporous materials using optical absorption spectroscopy

    19SPEC01 / Spectroscopy
    Promotor(en): V. Van Speybroeck, H. Vrielinck / Begeleider(s): A.E.J. Hoffman, S. Khelifi

    In recent years, crystalline nanoporous materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been widely investigated for their potential use in various applications such as gas storage, gas separation, catalysis of chemical reactions and drug delivery systems. The main features of these materials responsible for such applications are their large internal pores and long-range ordered structure, allowing them to accommodate for various guest molecules inside these pores. MOFs are built from metal-oxide centers connected through organic linkers. This hybrid nature gives rise to fascinating structural properties. For example, it allows to include defects in the material easily, which can improve its performance as a catalyst for various chemical processes [1]. Moreover, the weak bond between organic and inorganic units can introduce flexibility in the material [2]. In contrast to MOFs, COFs consist completely of organic building blocks. They may be fabricated from planar 2D sheets or from non-planar building units, yielding 2D or 3D COFs respectively [3]. Due to the absence of metals, COFs are much lighter than MOFs, while they still possess very high surface areas and are chemically and thermally very stable. Although the absence of metals also limits their use as catalysts, the large internal pores allow them to adsorb large activated molecules and fix them on the framework, a process called anchoring, which in turn opens up new pathways for including new functionalities to the materials. Despite the suitable properties of MOFs and COFs, their application in present-day applications remains limited due to stability issues. Therefore, an in-depth structural investigation of these materials is required.

    The chemical versatility with which these materials can be synthesized leads to an enormous number of possible MOFs and COFs. As a result, it is impossible to experimentally scan all possible candidates for a certain application. Therefore, to exploit the full potential of these extraordinary materials, scientists typically apply both experimental and computational techniques, combining the strengths of both. For example, the resulting diffraction patterns from X-ray diffraction (XRD) experiments contains information about the geometry and structure of the material and can be compared with the equilibrium geometry predicted by quantum mechanical calculations, while lattice vibrations and the electronic band structure are directly related to various spectroscopic techniques such as infrared (IR) absorption spectroscopy and ultraviolet-visible (UV/VIS) spectroscopy. Furthermore, by means of far-IR spectroscopy, we can focus on the low-frequency vibrations, which correspond to delocalized degrees of freedom related to various forms of framework flexibility such as breathing in MOFs and shearing in 2D COFs [4].

    Goal

    In this thesis subject, both experimental and computational techniques will be applied to investigate the structural properties of various MOFs and COFs via vibrational spectroscopy. For MOFs, the focus will be on frameworks exhibiting defects or flexibility. A thorough investigation of the specific vibrational fingerprint regions will lead to a better understanding of these phenomena. Concerning COFs, the aim will be to structurally characterize several frameworks by estimating the size of the pores/channels and identifying the vibrational degrees of freedom. Specifically for 2D COFs, the shearing of the sheets will be investigated, both without and with anchored molecules, allowing to elaborate on the impact of anchoring on the mechanical flexibility of the material.

    From the experimental point of view, samples will be characterized with X-ray diffraction (XRD), infrared-visible optical absorption spectroscopy and Raman scattering. The XRD pattern provides an easy quality check for the crystallinity of nanoporous materials and also allows to determine their crystal structure. To obtain a deeper insight in the structural properties of the examined materials, optical absorption spectra will be recorded extending from the far-IR over the mid-IR to the near-IR region. Each of these regions offer specific information on the material under study. The far-IR region, containing vibrations which extend over the complete framework, will be of interest for the characterization of flexibility in MOFs and the detection of shear vibrational modes in COFs. Next, the mid-IR region contains fingerprint vibrations of molecular building blocks, which can be used to detect and characterize defects in MOFs. Furthermore, the near-IR region can be measured to reveal electronic transitions appearing in metal complexes anchored at COFs.

    By means of molecular modeling, one can investigate the structure and vibrational modes on a molecular level. Using quantum mechanical calculations based on Density Functional Theory (DFT), one can compute the geometric and electronic structure of these materials. As a result, we can compute several observables such as the XRD pattern as well as the IR spectrum and compare it with the experimental results. In order to describe certain phenomena, such as the shearing of sheets in 2D COFs on a large time and length scale, quantum mechanical calculations may turn out to be computationally too expensive. As a result, one usually applies so-called force fields, which are a mathematical expression for the interaction between the atoms in the molecule (the electrons are not described explicitly) based on simple spring models. The unknown parameters occurring in the energy expression, such as the force constants and rest lengths of the springs, can be estimated either from experimental or quantum mechanical input. Using these force fields, one can perform molecular dynamic simulations that can trace the behavior of the materials through long periods of time, allowing to characterize the dynamical behavior.

    This thesis comprises both experiments and computational modeling. Depending on the interest of the student, the focus can be shifted towards one or the other. The materials will be synthesized at COMOC (Center for Ordered Materials, Organometallics and Catalysis, UGent, prof. P. Van Der Voort), hence this research will be performed in close collaboration with this research group.

    Aspects
    Master of Science in Engineering Physics: This thesis subject is closely related to the following clusters of elective courses: Nano and Modeling. Engineering aspects: the student will apply several experimental and computational techniques to characterize new materials and investigate their potential for various applications. Physics aspects: the student will use several techniques and interpret their results, which requires a fundamental understanding of quantum mechanics, solid state physics and atomic and molecular physics