Epitaxial Growth and Characterization of Metal Oxide Thin Films and Their Heterostructures-Photoanode for Efficient Photoelectrochemical Water SPlitting

Abstract

Photoelectrochemical (PEC) water splitting has attracted great interest as the most ideal method for environment-friendly hydrogen production, due to its potential to harvest sustainable and clean energy by the splitting of hydrogen and oxygen in water using solar energy. However, until now, the low solar-to-hydrogen conversion efficiency of the photoelectrodes limited commercialization due to poor charge transport properties. Various attempts have been made to enhance the charge transport efficiency, including the formation of heterojunctions, exerting control over surface morphology, doping of electron donors, and the introduction of the oxygen evolution catalysts. However, new approaches are required for further enhancement. To that end, the exploration of the fundamental properties of epitaxial metal oxide thin film photoelectrodes can maximize their potential and seek further breakthrough development. For example, epitaxial thin films enable an in-depth understanding of the PEC properties of the photoelectrodes themselves by minimizing any other suppressing factors. In this thesis, it is strongly suggested that the new development and tailoring of functionalities in epitaxial complex oxide thin films could provide new routes for significantly improving photo-efficiency for solar water splitting. Charge separation and collection by an electric field in the photoactive material is one of the most important factors to improve conversion efficiency. Hence, ferroelectric oxides, in which electrons are the majority carriers, are considered to be promising photoanode materials because their high built-in-potential, due to their spontaneous polarizations, can largely enhance the separation and drift of photo-generated carriers. In the first part of this dissertation, the PEC properties of thin film BiFeO3 photoanodes with different crystallographic orientations and consequent ferroelectric domain structures are investigated. As the crystallographic orientation changes from (001) via (110) to (111), ferroelastic domains in epitaxial BiFeO3 thin films become mono-variant and the spontaneous polarization levels increase up to 110 μC cm-2. Consequently, the photocurrent density at 0 V vs. Ag/AgCl increases by approximately 5.3 times and the onset potential decreases by 0.180 V in the downward polarization states. It is further demonstrated that ferroelectric switching in the BiFeO3 (111) thin film photoanode leads to an approximate change of 8000% in the photocurrent density and a shift of 0.330 V in the onset potential. This study strongly suggests that domain-engineered ferroelectric material can be used as an effective charge separation and collection layer for efficient solar water splitting photoanodes. In the second part of this dissertation, we report the fundamental approach for BiVO4 thin film photoanodes by fabricating epitaxial oxide thin films with different crystallographic orientations for PEC water splitting. The crystalline anisotropy generally reveals distinct physical phenomena along different crystallographic orientations. In terms of the anisotropic properties of BiVO4, the electrical conductivity of BiVO4 is greater along ab-plane than along the c-axis. Consequently, as the crystallographic orientation of the BiVO4 thin film changes from (001) to (010), the charge transport properties in the epitaxial BiVO4 thin film are significantly enhanced. Thus, at 1.23 VRHE, the photocurrent density of the epitaxial BiVO4 (010) thin film is much higher than that of the epitaxial BiVO4 (001) thin film because of a significant enhancement in charge transport properties, even in undoped BiVO4. These results strongly suggest that the growth of epitaxial BiVO4 thin films, with a specific crystallographic orientation, have great potential to considerably improve the charge transport efficiency of photoanodes for solar water splitting. In the last part of this dissertation, we report on an in-situ high-quality epitaxial BiVO4/Bi4V2O11 type-II heterojunction thin film photoanode fabricated by using pulsed laser deposition (PLD). This material was deposited on the basis of only one BiVO4 ceramic target, using the transition between BiVO4 and Bi4V2O11 crystalline phases. Herein, for the first time, we report on the structural and chemical transition between monoclinic BiVO4 (010) and orthorhombic Bi4V2O11 (001) crystalline phases by simply controlling the oxygen partial pressure. Subsequently, the growth of an epitaxial BiVO4/Bi4V2O11 heterojunction thin film is achieved by controlling only the oxygen partial pressure based on band alignment. At 1.23 VRHE, the photocurrent density of the heterojunction BiVO4/Bi4V2O11 structure is also significantly higher than that of the epitaxial BiVO4 thin film, owing to the effective charge transfer in the Bi4V2O11 thin film. This study strongly suggests that the non-equilibrium deposition of epitaxial BiVO4 thin films can propose a new paradigm in the structural design of photoanodes.