| 1. |
- Castner, Ashleigh T.
(author)
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From Peak to Peak: Exploring Electron Hopping and Mass Transport in Metal-Organic Frameworks
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Incorporating molecular electrocatalysts into solid support structures combines the best of two worlds: their molecular nature allows precise structural tunability for selective and efficient conversion of small molecule substrates, while their structural integrity is greatly improved by the heterogeneous support matrix to allow for long-term stability and potentially industrial-scale applications. Metal-organic frameworks (MOFs) represent a potential class of materials to act as viable support scaffolds for hosting molecular catalysts for sustainable energy conversion, exhibiting well-ordered porous structures that can support high loading densities of catalyst species. The utilization of such heterogeneous electrocatalyst materials then relies on the transport of both mass and charge throughout the MOF to sustain electrocatalytic reactions. The concomitant transport of charge to activate the embedded catalyst species and mass transport of substrate and product molecules, as well as charge-balancing ions, to these activated catalysts must all maintain a balance within the framework to optimize the catalytic efficiency of the MOF-based material. The aim of this thesis is to explore mass and charge transport behaviors in electroactive and electrocatalytic MOF materials to gain insight into the mechanisms for these interwoven transport-related processes.The first part of this thesis introduces two novel electrocatalytic MOF materials and discusses the charge transport behaviors exhibited in each. Potential limitations in mass and charge transport processes which could influence the catalytic efficiency of the material are identified in each system. The second part of this thesis is centered on discussions of two electroactive MOF materials without embedded catalysts to gain a mechanistic understanding of mass and charge transport processes. The first of these studies focuses on the influences of imposed mass transport properties on the observed charge transport in a MOF, revealing the potential for multiple mechanisms of charge transport to be exhibited in one framework. The second study presents the mediated charge transport through an electroactive framework to a dissolved acceptor species to drive a chemical process, and kinetic analysis of the model pseudo-catalytic reaction is discussed.The work in this thesis highlights the importance of understanding the influences of mass and charge transport in electroactive and electrocatalytic MOFs for understanding the underlying mechanisms of these processes. Such knowledge would allow for the optimization of transport phenomena and result in more efficient MOF-based electrocatalysts.
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| 2. |
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| 3. |
- Castner, Ashleigh T., et al.
(author)
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Microscopic Insights into Cation-Coupled Electron HoppingTransport in a Metal-Organic Framework br
- 2022
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:13, s. 5910-5920
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Journal article (peer-reviewed)abstract
- Electron transport through metal-organic frameworks by ahopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determinedapparent diffusion coefficients,Deapp, that characterize this form of chargetransport thus contain contributions from both processes. While this is wellestablished for MOFs, microscopic descriptions of this process are largelylacking. Herein, we systematically lay out different scenarios for cation-coupledelectron transfer processes that are at the heart of charge diffusion throughMOFs. Through systematic variations of solvents and electrolyte cations, it isshown that theDeappfor charge migration through a PIZOF-type MOF,Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide(NDI) linkers, spans over 2 orders of magnitude. More importantly, however,the microscopic mechanisms for cation-coupled electron propagation arecontingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers,either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, weshow that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes thatcan outcompete limitations from reduced cationflux.
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| 4. |
- Castner, Ashleigh T., et al.
(author)
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Mimicking the Electron Transport Chain and Active Site of [FeFe] Hydrogenases in One Metal-Organic Framework : Factors That Influence Charge Transport
- 2021
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:21, s. 7991-7999
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Journal article (peer-reviewed)abstract
- [FeFe] hydrogenase (H2ase) enzymes are effective proton reduction catalysts capable of forming molecular dihydrogen with a high turnover frequency at low overpotential. The active sites of these enzymes are buried within the protein structures, and substrates required for hydrogen evolution (both protons and electrons) are shuttled to the active sites through channels from the protein surface. Metal–organic frameworks (MOFs) provide a unique platform for mimicking such enzymes due to their inherent porosity which permits substrate diffusion and their structural tunability which allows for the incorporation of multiple functional linkers. Herein, we describe the preparation and characterization of a redox-active PCN-700-based MOF (PCN = porous coordination network) that features both a biomimetic model of the [FeFe] H2ase active site as well as a redox-active linker that acts as an electron mediator, thereby mimicking the function of [4Fe4S] clusters in the enzyme. Rigorous studies on the dual-functionalized MOF by cyclic voltammetry (CV) reveal similarities to the natural system but also important limitations in the MOF-enzyme analogy. Most importantly, and in contrast to the enzyme, restrictions apply to the total concentration of reduced linkers and charge-balancing counter cations that can be accommodated within the MOF. Successive charging of the MOF results in nonideal interactions between linkers and restricted mobility of charge-compensating redox-inactive counterions. Consequently, apparent diffusion coefficients are no longer constant, and expected redox features in the CVs of the materials are absent. Such nonlinear effects may play an important role in MOFs for (electro)catalytic applications.
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| 5. |
- Eliasson, Nora, et al.
(author)
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Ultrafast Dynamics in Cu-Deficient CuInS2 Quantum Dots : SubBandgap Transitions and Self-Assembled Molecular Catalysts
- 2021
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:27, s. 14751-14764
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Journal article (peer-reviewed)abstract
- The photophysical properties of Cu-deficient Cu01.2In1Sx quantum dots synthesized through a facile aqueous-based procedure have been investigated. Transient absorption experiments were carried out probing in the UV-vis, near-IR, and mid-IR regions, with the aim to (i) study the photophysical properties of the quantum dots and (ii) monitor kinetics of electron transfer to a molecular catalyst. When pumping subbandgap transitions, negative (bleach) signals were observed that were spectrally and kinetically distinct from those observed with bandgap pump wavelengths. Herein, these distinct contributions are suggested to result from the overlapping bleaching of state filling electrons and trapped holes. Such an interpretation highlights the importance of considering the hole-contributions to the bleach for the proper determination of carrier kinetics in similar systems. A model complex of the [Fe-2]-hydrogenase active site was introduced to explore the potential of the quantum dots as photosensitizers for molecular catalysts. The quantum dot photoluminescence was quenched upon catalyst addition, and direct evidence of the singly reduced catalyst was found by transient absorption in the UV-vis and mid-IR. The catalyst accepted reducing equivalents on a subpicosecond time scale upon photoexcitation of the quantum dots, despite no covalent linking chemistry being applied. This implies that charge transfer is not limited by diffusion rates, thus confirming the presence of spontaneous quantum dot and catalyst self-assembly.
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| 7. |
- McCarthy, Brian D., et al.
(author)
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Analysis of electrocatalytic metal-organic frameworks
- 2020
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In: Coordination chemistry reviews. - : ELSEVIER SCIENCE SA. - 0010-8545 .- 1873-3840. ; 406
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Research review (peer-reviewed)abstract
- The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.
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| 8. |
- Yang, Wenxing, et al.
(author)
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Photoinduced Fano Resonances between Quantum Confined Nanocrystals and Adsorbed Molecular Catalysts
- 2021
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:13, s. 5813-5818
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Journal article (peer-reviewed)abstract
- Interaction of surface adsorbate vibration and intraband electron absorption in nanocrystals has been reported to affect the photophysical properties of both nanocrystals and surface adsorbates and may affect the performance of hybrid photocatalysts composed of semiconductor nanocrystals and molecular catalysts. Here, by combining ultrafast transient visible and IR spectroscopic measurements, we report the observation of Fano resonances between the intraband transition of the photogenerated electrons in CdS and CdSe nanocrystals and CO stretching vibrational modes of adsorbed molecular catalysts, [Fe-2(cbdt)(CO)(6)] (FeFe; cbdt = 1-carboxyl-benzene-2,3-dithiolate), a molecular mimic for the active site of FeFe-hydrogenase. The occurrence of Fano resonances is independent of nanocrystal types (rods vs dots) or charge transfer character between the nanocrystal and FeFe, and is likely a general feature of nanocrystal and molecular catalyst hybrid systems. These results provide new insights into the fundamental interactions in these hybrid assemblies for artificial photosynthesis.
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