Fabrication and characterization of $Cu_2O/TiO_2$ photocathodes for efficient photoelectrochemical water splitting

Abstract

While the utilization of immense amount of solar energy is indispensible, there is always the issue on the incompatibility between our continuous demands for power and irregular solar irradiation. The photoelectrochemical (PEC) production of chemical fuels by solar energy conversion is an attractive solution to the energy problem. One of the chemical fuels, hydrogen, is being greatly investigated in the pathways for converting solar to electrical and chemical energy owing to its high mass energy density, and is an non-polluting by-product. $Cu_2O$ has received much attention as the most promising candidate for hydrogen generation due to a direct bandgap of 2 eV which guarantees a large portion of visible light absorption, the favorable position of conduction band for reducing water, non-toxicity, and the natural abundance of copper for a large scale fabrication. It has been theoretically predicted 18 % for a light-to-hydrogen conversion efficiency. Indeed, $Cu_2O$ as photocathodes recently has re-ceived much interests, and prior research has mainly been focused on protective layer degra-dation, interlayer, and catalyst degradation. Although these researches used $TiO_2$ surface layer in common to protect photoabsorber $Cu_2O$ in photoelectrochemical environments, durability studies using $TiO_2$ layer without any buffer layers, another important property of $TiO_2$ as an anti-reflection coating, and interfacial band-edge engineering of $TiO_2$ layer have not been ad-dressed for simplifying the photoelectrodes structures, suppresing light reflection from $Cu_2O$ surface, and enhancing charge transfer, respectiviely. This thesis provides the fabrication and chacterization of $Cu_2O/TiO_2$ photocathodes for enhancing PEC properties by addressing such issues. The research has been well performed with three steps as planned. First, we fabricated highly active $Cu_2O$ films as photocathodes. This is realized by electrodeposition of $Cu_2O$ on various substrates, structural analysis of the $Cu_2O$ films, and the optimization of PEC characteristics. In addition, we showed that ALD $TiO_2$ alone could stabilized the buried $Cu_2O$ films when its thickness is thicker than 20 nm. Second, we introduced another important role of $TiO_2$ as an anti-reflection coating for enhancing $Cu_2O$ -based PEC water splitting systems. The proof-of-concenpt study succesfully showed that $Cu_2O$ coated with a $TiO_2$ layer of 45 nm, which was proven to give an enhnaced light absorption, exhibits significantly improved photocurrent density. These results suggest that $Cu_2O$ films coated with anti-reflective $TiO_2$ layer are efficient for photoelectrochemical water splitting. Furthermore, our findings expect that various phtoocathodes with unstability and low light utilization efficiency can solve their limitations through adopting anti-reflective $TiO_2$ layer. Third, we introduced a facile process to manage interfacial band-edge energetics for water reduction through controlling the deposition temperature of $TiO_2$ by ALD. The low-temperatured $TiO_2$ provides efficient charge transfer across the surface due to energetically favorable conduction band edges of $TiO_2$ layer. Therefore, the $Cu_2O/TiO_2-80^\circ C$ showed to $-2.2 mA/cm^2$ at 0 V vs. RHE, which is the highest value reported for $Cu_2O$ -based materials without the use of any interlayer. In addition, the energetically favorable band alignment near the surface is a highly dominant factor for stabilizing buried $Cu_2O$ films as well as producing hydrogen efficiently. Since the energetically favorable band alignment near the surface has revealed to be a primary important factor for overall PEC responses, this facile approach will no doubt be beneficial for other unstable photoelectrodes such as Si, InP, and GaAs designed to have $TiO_2$2 surface layer. The significant approaches based on 1) enhanced PEC properties of electrodeposited P-type $Cu_2O$ photocathodes and durability studies with $TiO_2$ layer, 2) light absorption enhancements of $Cu_2O$ photocathodes using another important property of $TiO_2$ as an anti-reflection coating, and 3) interfacial band-engineering of $TiO_2$ layer on $Cu_2O$ films for PEC responses. We expect that these methodologies can be highly beneficial for other photoelectrodes due to their pro-cessibility, scalability, and versatility.