Electrochemical synthesis and nanoscale characterization of polymorphous molybdenum oxide
This dissertation describes the electrochemical synthesis and characterization of molybdenum oxide thin films. In addition to conventional ensemble-based spectroscopic and electrochemical investigations, high-resolution spectroelectrochemical imaging and scanning probe microscopy techniques are implemented to study localized ion/charge transfer reactivity. Chapter 1 provides a general overview of the material to be discussed in the dissertation. Chapter 2 describes investigations regarding the electrochemical synthesis of hydrated, amorphous, sub-stoichiometric thin films of molybdenum oxide. Chronocoulometry, X-ray photoelectron spectroscopy, spectroelectrochemistry and electrochemical quartz crystal microgravimetry are used to establish corresponding deposition mechanisms for films grown at different potentials from both iso- and peroxo-polymolybdate solutions. Chapter 3 details the effects of post-deposition heat treatment on the physical, electrochemical and spectroscopic properties of electrodeposited molybdenum oxide thin films. X-ray diffraction, Xray photoelectron spectroscopy, and atomic force microscopy indicate that heat treatment at 250 ºC induces a phase transition from an amorphous structure to one that is structurally disordered and comprises discrete α-MoO3, β-MoO3 and intermixed α-/β-MoO3 domains. These thermally induced structural changes strongly influence the ion storage/coloration properties due to changes in film morphology and chemical composition. Chapter 4 describes the localized measurement of electronic conductivity and chemical composition in polymorphous molybdenum oxide using conductive probe atomic force microscopy and Raman Microprobe spectroscopy, respectively. From their conductivity and spectroscopic signatures chemically distinct phases of molybdenum oxide are identified and found to contribute unequally to the overall coloration/-insertion response. Chapter 5 introduces a novel optical imaging methodology for quantitative measurement of spatially resolved ion/charge transfer dynamics in polymorphous molybdenum oxide. Optical imaging coupled with chronoamperometry and cyclic voltammetry are used to study the associated phase-specific ion/charge transfer reactivity. Lithium diffusion coefficients, conductivities and insertion ratios are estimated for each identified phase (e.g., α- MoO3, β-MoO3, or mixed α-/β-MoO3) in polymorphous molybdenum oxide. A detailed derivation of the equation relating the time-dependent optical density change to the lithium diffusion coefficient is provided in the Appendix.