Date of Award
Doctor of Philosophy (PhD)
A set of proof-of-concept in-situ synthesis of polyoxometalate-containing compounds is presented in this dissertation showing that room-temperature electrochemical crystal growth is a promising technique for the exploratory synthesis of new materials with functional architectures. Most polyoxometalate(POM)-based crystalline materials, if not all, have been synthesized via conventional hydrothermal/solvothermal methods. However, the high temperature and pressure required by these methods proves destructive towards thermally sensitive POM anions, resulting in greater uncertainty of the synthesis of crystalline materials based upon these plenary POM structures. We have explored a new approach for the “bench-top” synthesis of POM-containing materials at ambient conditions by employing electrochemical (e-chem) energy as a driving force for new compound formation. We have demonstrated that the e-chem approach allows for convenient synthesis of POM-containing compounds in aqueous solution without using any specialized reaction container. Compared to conventional hydrothermal/solvothermal synthesis, the new approach offers additional benefits especially towards the synthesis of POM materials that are otherwise subject to thermal decomposition. Furthermore, using e-chem methods for crystal growth facilitates a means for the selective synthesis of compounds with desired frameworks for electrical conductivity. We have had some success with e-chem crystal growth in which newly discovered compounds exhibit fascinating structures such as one-dimensional (1D), alternating POM anion and transition metal (M) cations, and two-dimensional (2D) frameworks featuring tethered POM clusters on metal-oxide chains. In addition to the use of X-ray diffraction methods to investigate
crystal structures, we have employed TGA/DSC (Thermogravimetric Analysis /Differential Scanning Calorimetry) methods to examine the thermal behaviors of new compounds, and XPS (X-ray Photoelectron Spectroscopy) to determine the oxidation states of the metal cations and etc. Inspired by previous work, an electrochemical synthetic system for the design of POM-based complex metal oxides has been adapted to explore organic-inorganic hybrid materials. This represents a key step in adaptation of the mild reaction system for use in the synthesis of polyoxometalate-organic-frameworks (POMOFs), a class of compounds previously dominated by synthesis via conventional hydrothermal or solvothermal methods, and more recently, ionothermal methods. Applications of such materials include use in lithium ion batteries owing to the multi-electron reduction of polyoxometalate clusters and tunability of pores allowing lithiation. Interestingly, the emerging electrochemical synthetic method enables synthesis of many micrometer scale single crystals and does not require a polymer matrix, differing from previous reports of electrodeposition used to grow thin film organic-inorganic hybrids. Most notably, conventional hydrothermal techniques prove destructive to POM anions with low thermal stability, precluding synthesis or creating reliance upon self-assembly of POM anions. This bench top, one-pot reaction system allows control of potential or current density, temperature, concentration, pH, timescale, and electrode material resulting in potential for enhanced tunability. Furthermore, the electrochemical pathway utilized allows greater selectivity for the synthesis of conductive materials.
Zhang, Qiuying, "Bench-Top Electrochemical Crystal Growth: In-Situ Synthesis of Extended Solids Containing Polyoxometalate Anions" (2020). All Dissertations. 2669.