Deep eutectic solvent routes to photoactive metal oxides
Date
2021-08
Authors
Hong, Sangki
Major Professor
Advisor
Zaikina, Yulia
Miller, Gordon
Smith, Emily
Rossini, Aaron
Slowing, Igor
Committee Member
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Abstract
To address synthetic challenges of synthesis of technologically important metal oxides, deep eutectic solvents (DESs) are utilized as reaction media. DESs, prepared by mixing a hydrogen bond donor (quaternary ammonium salt) and acceptor (urea), displays strong solvation capability of stable binary metal oxides such as ZnO and V2O5. Moreover, the solvents’ components are inexpensive, nontoxic, and biodegradable, making DESs as sustainable alternative to other toxic and corrosive solvents in oxide syntheses. A combustion reaction of thoroughly mixed metal precursors in DESs leads to a successful synthesis of M2V2O7 and MV2O6 (M = Zn and Cu). According to in situ and ex situ experiments, ternary oxide intermediates (e.g., Cu2-xV2O7, x ~ 0.27) and/or final products are quickly formed by this route. Interestingly, the unique reaction environment provided by the evolved ammonia allows for stabilization of oxygen vacancies and reduced oxidation states of metal cations. The mid-gap states as well as narrowing band gaps are prevalent in the oxygen-deficient vanadates due to the modification of their electronic structure. Local structural disorder created by the defects also alters the lattice vibrational modes in corresponding Raman spectra. The oxygen-vacancy-rich vanadates are prone to electron trapping and/or charge recombination at the reduced metal center. A review of chemically reduced metal oxides provides insight into the complex structural modification upon removal and replacement of oxygen anions.
We demonstrated versatility of DESs by employing them in a precipitation route synthesis. With simple addition of a precipitation agent to a solution of metal precursors in DESs, zinc-rich or copper-rich vanadates are prepared. This is the first example of ternary compounds synthesized by this route. Key experimental parameters were identified to control size and morphology of precipitates. Based on thermal analysis, we found ternary metal oxyhydroxides to be reactive precursors in the oxide synthesis. Surprisingly, the metastable Zn4V2O9 is synthesizable within a minute of heat treatment of the precipitate. In the CuO-V2O5 system, we were able to access compositionally closely spaced oxide phases via kinetic control of Cu/V metal ratio in the precipitate. By utilizing an in situ experiment, the formation mechanism of the synthetically challenging Cu11V6O26 was determined. Overall, this work expands our understanding of DES-involved synthesis and structure-property relationship of ternary vanadates in energy-related applications. It will offer a base of future opportunity in rational design of material synthesis and potentially oxide material discovery.
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dissertation