에너지 분야의 응용을 위한 산화물 박막의 격자 변형의 역할에 관한 연구

Abstract

Energy and environmental issues such as energy supplies, energy costs, environmental diseases and greenhouse gas emissions are spurring the demand for environmentally harmless renewable energy sources, including solar energy, hydropower energy, wind energy, tidal energy, biomass energy, geothermal energy. Recently, the strain in oxide thin film heterostructures has known that it greatly affect various physical properties, such as ferroelectricity, magnetic ordering, ion conductivity, electron mobility. The reason for this diverse change in the physical properties is mostly due to their strongly correlated properties in metal oxide thin films. In this regards, this thesis focuses on the energy related research based on metal oxide thin films using the flexibility of the property alteration. Among the renewable energy fields, the role of epitaxial strain in ferroelectric photovoltaics and exsolution for fuel cells will be discussed. The first topic is about ferroelectric photoltaic effect. Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. Among various factors influencing the FPV efficiency, narrow Eg and large polarization are known as the two most important factors. This is because the absorption of sun light by ferroelectric materials is limited by their Eg values and large polarization is crucial to efficiently separate the photo-generated exciton (electron-hole) pairs. Here, we present an easy and simple method of simultaneously achieving the above noted two important goals of FPVs, i.e., reduced Eg and enhanced polarization, by applying a film strain to epitaxially grown hexagonal YbFeO3 (h-YbFO) thin-film heterostructures. We further show the switchable FPV effect of these h-YbFO thin-film devices. The crystal structure of h-YbFO is featured by an alternative stacking of two distinct layers: one layer of corner-linked FeO5 bipyramids and the other layer of trivalent Yb3+ cations (Figure 1a). We demonstrate enhanced FPV efficiency by suitably exploiting the substrate-induced film strain. More explicitly, a compressive-strained h-YbFO/Pt/MgO heterojunction device shows ~3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device or multiferroic BiFeO3 which is known as the prototypic FPV. The origin of this enhanced PCE is investigated by examining the substrate-dependent band gap (Eg) and polarization (Pr), in conjunction with first-principles calculations. The second theme is about the exsolution of nanoparticles for renewable energy and catalysis. Catalytic metal particles dispersed on oxide surfaces play a key role in catalysis, energy conversion, and energy storage industries, including batteries, fuel cells, electrolysis cells. These catalytic metal particles have been prepared mainly by deposition techniques. However, the deposited particles mostly show limitation of particle size and distribution control, and degradation by agglomeration or carbon coking. The exsolution of the B-site ions from perovskite lattices (ABO3) under reduction conditions is emerging as an alternative, which shows possibility of in situ growth of nanoparticles. Compared to deposition procedures, this process shows higher cost- or time-efficiency, good thermal stability, and resistance to coking problems. However, the exsolution from stoichiometric ABO3 perovskites has primarily shown (i) a limited number of active cations, (ii) preferential occurrence within bulk (inside) rather than surface, and (iii) a slow speed of particle generation. To overcome these problems, the A-site deficient perovskites were employed to promote the B-site caion exsolution on surfaces, leading to both A-site and oxygen deficiencies that make easier ion diffusion and electron generation during the reduction by hydrogen. It was further revealed that voltage-driven reduction is two orders of magnitude faster than conventional reduction and yields a small particle size (~15 nm) and the highest population density of ~400 particles∙μm−2, resulting in outstanding electrochemical activity. To date, the studies on such exsolution have been performed mainly in bulk polycrystalline ceramics. Unlike bulk systems, thin-film heterostructures can induce strain in a film lattice because of the lattice mismatch between substrate and film, which affects various physical properties such as ferroelectricity, electron mobility, ionic conductivity and electrocatalysis. In particular, thin-film oxide fuel cells are currently attracting renewed attention owing to advantages of low temperature operation and portable device applications. Here, we demonstrate an unprecedentedly high degree of exsolution of nanoparticles in strained epitaxial thin films (particle density of 1177 particles∙μm−2, size of 5 nm) at a temperature as low as 550 oC. Compressive-stained films show a larger number of exsolved particles and higher thermal stability than tensile-strained films. The strain-driven in situ exsolution further offers rapid release of particles, good coking resistance, numerous particles even in μm-thickness, and wide tunability of particle growths. This finding opens a new way to design and development of catalytic functional materials and portable energy applications. The present studies will (i) offer a new standard for selecting substrates towards optimal design of ferroelectric optoelectronics and photovoltaic devices and (ii) opens a new way to design and development of catalytic functional materials and portable energy applications.