Efficient Water Oxidation: Nanostructured Titanium Oxides vs. Iron-Group Metal Electrodeposited GaAs

Photoelectrochemical water splitting consists in the conversion of solar energy into chemical energy in the form of the hydrogen bond and simultaneously overcomes the intrinsic limitation of the intermittent property of the sunlight, allowing to provide energy in form of electricity reliably and continuously. Titanium oxide-based materials (such as TiO2) and single crystal semiconductors (such as Si or GaAs) are among the most attractive photoanode candidates for efficient water oxidation. TiO2 materials unfortunately suffer from the limited absorption of the solar spectrum, the poor charge transfer and the slow surface kinetics, while displaying a high stability and low cost. Si or GaAs in contrast have an ideal bandgap to maximize light absorption, but cannot be directly used as photoanodes, due to their limited stability and their susceptibility to photocorrosion, thus requiring a protective layer to enhance its functionality.
In the first part of the dissertation, we aim to investigate and improve the performance of titanium oxide nanomaterials by exploring (i) the generation of Ti3+/oxygen vacancies by ammonia treatment, (ii) TiO2-x sub-stoichiometries formation via extensive generation of oxygen vacancies and (iii) surface modification methods in order to widen the light absorption spectrum, improve charge transfer properties and increase interfacial water oxidation selectivity; finally efforts will be devoted to improve and optimize the various existing modification approaches simultaneously. The findings prove that there exists an optimum amount of oxygen deficiencies in TiO2-x that maximizes the photoelectrochemical performance in the form of a well-defined concentration of point defects in the anatase phase of TiO2; furthermore, surface modification with suitable water oxidation catalysts allows further improvement of the photoelectrochemical performance by accelerating the surface water oxidation kinetics, while minimizing the impact on the photo-transmittance; the optimization of the photoelectrochemical properties is a synergic effect of increasing the light absorption, reducing the surface recombination and improving the conductivity in the bulk. Various modification methods on TiO2 nanotube system were implemented to control the density of defects through oxygen removal; these include laser surface modification, introduction of Ti3+ by Ar/NH3 treatment, high-temperature thermal hydrogen reduction and electrodeposition of water oxidation catalysts. The stability of anatase TiO2 with nanotubular morphology is determined and various possible phase transformations of this materials are investigated. The density/types of the defects created were evaluated and quantified by using photoelectrochemical characterization, together with electrochemical analytical methods.
In the second part of the dissertation, we focus on investigating an electrodeposition method showing a self-limiting growth mechanism, in order to deposit Ni ultrathin film/nanoparticles on GaAs substrates. In addition, we extend the method to electrodeposit binary/ternary alloys following the same deposition behavior. We prove that the self-limiting deposition condition can be achieved for not only Ni layers, but also for other Iron-group mutual alloys such as Ni-Co, Ni-Fe and Ni-Fe-Co; the protective Ni layer allows an improved stability and surface water oxidation kinetics in an aqueous solution under photoelectrochemical measurements; the stability of this system can be further improved by adding a second or a third Iron-group element such as Co or Fe. Characterization methods including potential transient, XPS spectra and XPS depth profile, HR-TEM were applied to verify the self-limiting mechanism and to determine the thickness of the deposited protective layer. Photoelectrochemical and electrochemical characterization were used to evaluate the functionality of the metal-deposited GaAs as a photoanode candidate toward water oxidation and its performance.