Analyses of Electrochemical Phenomena at Hg Ultramicroelectrodes

The focus of this thesis is the use of Hg ultramicroelectrodes (UMEs) as platforms to study electrochemical phenomena related to the electrodeposition of individual semiconductor micro/nano crystals by the electrochemical liquid-liquid-solid (ec-LLS) growth technique. ec-LLS is a non-energy intensive technique capable of producing crystalline semiconductor microwires at ambient conditions. ec-LLS growth to date has been carried out in the form of liquid metal electrode arrays, which results in microwire films with a range of morphologies. Voltammetric and chronoamperometric experiments were performed to investigate the electrodeposition of Ge by ec-LLS, including the associated adsorption and redox steps. The context of this work is to better understand ec-LLS in general by exploiting the advantages offered with small volumes of liquid metals. UMEs allow the opportunity to identify possible correlations between current transient and crystal growth. This thesis describes the following three primary findings. In Chapter 2, the data implicate that Ge crystal growth by ec-LLS is different at small Hg volumes as compared to bulk Hg pools. Recorded voltammetry with Hg UMEs showed unexpectedly slow introduction of Ge into the liquid metal. Unique morphologies never observed before in any electrodeposition context are detailed. In Chapter 3, the possibility that size affects adsorption of species to Hg interfaces was tested with both macrosize and microsize Hg electrodes. The data argue against any significant influence of electrocapillarity effects but do indicate that mass transport at small sizes can greatly alter adsorptive voltammetric features in some cases. In Chapter 4, a further assessment of the electrochemical reduction of GeO2 dissolved in aqueous electrolytes is presented. Voltammetric data for Hg UMEs in dilute aqueous solutions of dissolved GeO2 showed the rate of Ge introduction into the liquid metal, and the ratio between the anodic and cathodic charge argued for a complex and atom-inefficient process (with respect to introduction of Ge0 into liquid Hg). A final chapter is included in this thesis that describes several possible future directions for this work. Three topics are described. First, observations from preliminary attempts to perform Ge ec-LLS at Hg nanodroplets are described. Second, the preparation of UMEs with Ga is shown and discussed in terms of preparation challenges. Third, an alternative UME platform is demonstrated that consists of a single hole patterned in a photoresist film. Opportunities and challenges for this strategy are relayed.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137089/1/timzhang_1.pd