Structure And Composition Of Bismuth And Silver Bismuth Films Electrodeposited From Deep Eutectic Solvents
Supervisor(s)Hillman, Robert A.; Ryder, Karl S.
Date of award2019-06-26
Author affiliationDepartment of Chemistry
Awarding institutionUniversity of Leicester
NotesElectrodeposited coatings of silver and bismuth (as individual metals, alloyed or multi-layered) are used extensively in a range of engineering applications. Fabrication of these coatings is commonly achieved via electrochemical deposition using aqueous electrolytes containing strong alkali or cyanide that present environmental issues. We investigate the possibility of bismuth metal deposition and silver-bismuth co-deposition from an environmentally benign, deep eutectic solvent (DES), ethaline 200 (choline chloride: ethylene glycol in 1:2 stoichiometric ratio). We report the first application of a range of electroanalytical techniques (potential step voltammetry, linear sweep voltammetry, cyclic voltammetry, electrogravimetry using a quartz crystal microbalance) to quantitatively characterise the properties of electrochemical reactions for these metal systems in a DES. These techniques were based on both static and hydrodynamic electrodes (i.e. rotating disk electrode (RDE)). A fundamental study into nucleation mechanisms and growth kinetics of silver, bismuth and silver/ bismuth deposition was carried out. We found nucleation for silver and bismuth to be instantaneous at high overpotentials and progressive at low overpotentials; both were followed by diffusion controlled 3D growth. The average stripping efficiencies for silver and bismuth had a range between 74 -89% while stripping efficiency for silver/ bismuth co-deposit was ca. 36%. RDE methods were used to calculate diffusion coefficients, heterogeneous rate constants, transfer coefficients and standard rate constants for these metal systems. The metal films deposited were analysed using simultaneous thickness and electrochemical potential (STEP) measurements for multi-layer metals, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDAX) and atomic force microscopy (AFM). From these methods we were able to identify intermetallics of silver/ bismuth, crystalline phases formed, composition (Ag: Bi ratio), surface morphology (including surface roughness) and film thickness respectively. We report observations of deposition and dissolution of these metals using the novel, non-invasive techniques: combined electrochemical quartz crystal microbalance (EQCM)/ probe beam deflection (PBD). Electrochemistry (E) provides control of the interfacial reactions and gives an overall measure of rate, the acoustic wave (QCM) response yields the deposited mass at the interface and the optical signal (PBD) provides information on solution chemistry of the reactants and products.The experimental protocol was shown to be successful in analysis of metal systems in DES. The combined data were used to deduce mechanistic details e.g. changes in speciation of metals involved during each potential region, underpotential deposition regions of bismuth, mole fractions of Ag:Bi during co-deposition.