Palladium nanofilm growth and surface modification for enhanced hydrogen desorption kinetics by electrochemical atomic layer deposition (E-ALD) and development of electroless atomic layer deposition (EL-ALD)
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This dissertation describes electrodeposition and electroless deposition of Pd. Pd thin films were grown by E-ALD on polycrystalline Au substrates. A scheme to modify the surface of the Pd with Pt was developed. The films were grown by surface limited redox replacement (SLRR). As the name suggests, surface limited reactions are used to grow conformal, thin films with precise coverage and controlled thickness. An atomic layer of Cu (sacrificial metal) was deposited at an underpotential and then replaced by Pd ions (noble metal) to form an atomic layer of Pd. This process was repeated to grow Pd thin films of the desired coverage. Following this, Cu was deposited at an underpotential on the Pd surface and then replaced by Pt ions to modify the Pd surface with Pt metal. The coverage was varied by tuning the UPD potential. Hydrogen sorption was studied by cyclic voltammetry (CV). Based on the CV, a scheme to charge and discharge the films with hydrogen using coulometry was developed. Enhancements in the rate of hydrogen desorption compared to bare Pd was observed even with low Pt coverage. In addition, the films maintained the ability to store hydrogen. Based on the results, an indirect mechanism for hydrogen sorption is proposed. Oxygen reduction in the films was also studied and an enhancement in the kinetics is reported. Electroless deposition involves the ability to deposit metal without applying any current. The technique is versatile and can be used on a variety of substrates. A two-step method for electroless atomic layer deposition (EL-ALD) of Pd was developed. Initially, Sn ions are adsorbed and then replaced by Pd ions in an SLRR reaction. CV was used to optimize the sequence and quantify the amount of Sn present. Deposition on Au, ITO, FTO and glass was achieved. EL-ALD Pd deposition was three times less than with E-ALD. AFM indicates a nucleation and growth model. Cu electrodeposited on Pd sensitized ITO was investigated and indicates a weak interaction between Pd and ITO under high currents. Pd deposition was found to be constant from cycle to cycle for EL-ALD just as in E-ALD.