Browsing by Author "Gibbons, Bradley"

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    Photoelectrochemical water oxidation by a MOF/semiconductor composite
    Gibbons, Bradley; Cairnie, Daniel R.; Thomas, Benjamin; Yang, Xiaozhou; Ilic, Stefan; Morris, Amanda J. (Royal Society of Chemistry, 2023-05)
    Artificial photosynthesis is one of the most promising forms of renewable fuel production, due to the abundance of water, carbon dioxide, and sunlight. However, the water oxidation reaction remains a significant bottleneck due to the high thermodynamic and kinetic requirements of the four-electron process. While significant work has been done on the development of catalysts for water splitting, many of the catalysts reported to date operate at high overpotentials or with the use of sacrificial oxidants to drive the reaction. Here, we present a catalyst embedded metal-organic framework (MOF)/ semiconductor composite that performs photoelectrochemical oxidation of water at a formal underpotential. Ru-UiO-67 (where Ru stands for the water oxidation catalyst [Ru(tpy)(dcbpy)OH2](2+) (tpy = 2,2':6',2''-terpyridine, dcbpy = 5,5-dicarboxy-2,2'-bipyridine)) has been previously shown to be active for water oxidation under both chemical and electrochemical conditions, but here we demonstrate, for the first time, incorporation of a light harvesting n-type semiconductor as a base photoelectrode. RuUiO-67/WO3 is active for photoelectrochemical water oxidation at a thermodynamic underpotential ( h approximate to 200 mV; E-onset = 600 mV vs. NHE), and incorporation of a molecular catalyst onto the oxide layer increases efficiency of charge transport and separation over bare WO3. The charge-separation process was evaluated with ultrafast transient absorption spectroscopy (ufTA) and photocurrent density measurements. These studies suggest that a key contributor to the photocatalytic process involves a hole transfer from excited WO* (3) to Ru-UiO-67. To our knowledge, this is the first report of a MOFbased catalyst active for water oxidation at a thermodynamic underpotential, a key step towards lightdriven water oxidation.
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    Vibrational spectroscopy investigation of defects in Zr- and Hf-UiO-66
    Yost, Brandon T.; Gibbons, Bradley; Wilson, Addison; Morris, Amanda J.; McNeil, L. E. (Royal Society of Chemistry, 2022-08-10)
    Defect engineering in metal-organic framework compounds has allowed for improvements in catalysis-based functionalities, gas sensing, and gas storage. Metal-organic framework UiO-66 compounds with Zr- and Hf-based metal secondary building units were studied with Raman and infrared vibrational spectroscopy. Missing linker and missing cluster defects were engineered into the crystal structure via a modulated synthesis technique. Missing cluster defects in Hf-UiO-66 are first characterized by powder X-ray diffraction (PXRD) whereby two low-angle peaks were fit to extract the relative quantity of reo topology in four defective samples. A monotonic red-shift of the Raman-active Hf-O coordination bond vibration is interpreted as a signature of missing cluster defects, resulting from less-rigid charge-balancing monocarboxylate formate ions replacing the dicarboxylate linker molecule. This signature is hypothesized to be independent of the topology in which the defects appear. Missing linker defects in Zr-UiO-66 are characterized by infrared absorption spectroscopy by the quenching of C-C and C-H vibrational modes confined to the linker molecule. Together, Raman and infrared vibrational spectroscopies coupled with standard characterization techniques are employed to directly probe the nature of defects as well as offer new characterization tools for missing cluster defects in UiO-66.