Growth and Characterization of Oxide Heterostructure Thin Films for Photoelectrochemical Water Splitting

Abstract

An increase of energy demand and concerns about environmental problems of fossil fuels accelerate development of renewable energy resource replacing fossil fuels. Hydrogen is an alternative energy source of fossil fuels for realizing low carbon society. However, the most of hydrogen production pathways over 90 % currently depend on the fossil fuels paradoxically. Therefore, it is important to develop an eco-friendly hydrogen production way toward sustainable hydrogen society. Solar driven water-splitting is one way to produce eco-friendly hydrogen production using water and solar energy. In particular, photoelectrochemical water-splitting is practical model system for solar hydrogen production facilities using semiconductor absorber layer in the solar driven water-splitting approaches. However, while photoelectrochemical tandem cell system theoretically achieve a maximum solar-to hydrogen (STH) conversion efficiency of approximately 30 %, it is difficult to accomplish the theoretical STH conversion efficiency due to absence of ideal semiconductor material, which have a narrow band gap without photocurrent loss and suitable band position with respect to water redox potential in the semiconductor photoelectrode, realistically. Thus, the systematic exploration and fundamental investigation of the semiconductor photoelectrode are essential to enhance photocurrent and photostability for approaching the theoretical STH conversion efficiency. While the p-type semiconductor CuBi2O4 was proposed as a photocathode material with high visible-light responsiveness in 2007, there have been no studies of CuBi2O4 thin film photocathodes grown by physical vapor deposition. In this thesis, the polycrystalline, single-crystal CuBi2O4/NiO heterostructure thin film growth via pulsed laser deposition (PLD) and their photoelectrochemical properties are investigated and discussed. In the first part of this thesis, the single layer CuBi2O4 and CuBi2O4/NiO heterojunction photocathode using a physical vapor deposition technique of PLD are investigated. Both CuBi2O4 and CuBi2O4/NiO thin films exhibited highly dense, uniform and homogeneous surface morphology, which is confirmed by x-ray reflectivity and surface scanning electron microscopy and atomic force microscopy roughness measurements. In particular, CuBi2O4 in coupling with the intermediate layer of NiO to form type-II band alignment enhance the charge transport efficiency of CuBi2O4 photocathode. As a result, at 0.4 VRHE, the photocurrent density of CuBi2O4/NiO thin film (1.5 mA/cm2) is much higher than that of bare CuBi2O4 thin film (1.0 mA/cm2) due to the significant improvement of charge transport properties at a uniform interface. Above all, the chemical stability of both the bare CuBi2O4 and CuBi2O4/NiO heterojunction thin films was confirmed to have reasonable photoactivity properties during the chemical stability test for 8 hours, thanks to its advantages of PLD technique. These results strongly suggest that the use of highly crystalline and dense long-term sustainable CuBi2O4 /NiO type-II heterojunction thin films fabricated by PLD could be one of the most efficient ways to simultaneously achieve both improved photocatalytic property and chemical stability in the practical application of CuBi2O4 photocathode for solar water splitting. In the second part of this thesis, a-oriented CuBi2O4 single-crystal thin film using a NiO template layer grown on a SrTiO3 (001) substrate via PLD is presented. The domain-matching epitaxy relationship and microstructure of a-oriented CuBi2O4/NiO heterostructure thin film were demonstrated by synchrotron radiation reciprocal space mapping and high-resolution transmission electron microscopy. Consequently, the a-oriented CuBi2O4 single-crystal thin film photocathode exhibited photocurrent density of -0.4 mA/cm2 at even 0 VRHE, with no severe dark current in a 0.1-M potassium phosphate buffer solution, which is approximately 10 times better than that of recently reported pure CuBi2O4 polycrystalline thin film photocathode. We believe that the dense, uniform and homogeneous CuBi2O4/NiO polycrystalline thin film and CuBi2O4 single-crystal thin film photocathode grown by PLD will afford us opportunities to solve the challenges of CuBi2O4 photocathodes in photoelectrochemical water-splitting devices. Our results also provide the basic information for deep understanding the photoelectrochemical properties of CuBi2O4 single-crystal thin film photocathodes toward efficient water-splitting.