(A) study on surface engineering of functional nanomaterials for solar energy conversion

Abstract

Solar energy is regarded as the most potential energy source among renewable energy because it supplies 6,800 times more energy per second than the energy used by humankind. The engineering utilization of solar energy can be broadly divided into PV (Photovoltaic), which converts solar energy into electrical energy and PC (Photocatalytic), PEC (Photoelectrochemical), which convert solar energy into chemical energy. The purpose of this dissertation is to develop functional nanomaterials that convert solar energy into chemical energy and to improve catalytic properties through chemical and physical engineering of the functional nanomaterials surface. As the functional nanomaterials, metal (hydro)oxide nanostructures excluding precious noble metals that are not abundant on the earth were used as catalyst materials, and the phase of the specified metal (hydro)oxide was controlled according to the kind of solar fuel. The first research highlights on the carbon dioxide photo-reduction reaction through physical surface engineering of layered double hydroxide (LDH). The study of layered double hydroxides for carbon dioxide photo-reduction reactions has been limited to the synthesis of LDH containing only two types of metal cations because there is no optimized experimental method. Through this study, LDH containing more than four metal cations was synthesized for the first time in the world and applied to carbon dioxide photo-reduction by utilizing changed local electron structure. The quadruple NiGaMgAl LDH has a change in the local electronic structure of metal cations that affects the improvement of adsorption of carbon dioxide. The change of local electronic structure is attributed to electronegativity and it was finally found that the change of oxidation state of metal cations is a direct cause for increase of catalytic activity. In addition, a reaction mechanism for conversion of carbon dioxide into carbon monoxide was accurately demonstrated that carbon dioxide is reduced through an intermediate in the order of bidentate carbonate-cicarbonate in the LDH structure. The second research is on the water oxidation reaction through the chemical surface engineering of LDH. The biggest problem when using existing LDH as a water oxidation photocatalyst was that water was clogged in the anion layer of LDH, making it impossible to efficiently adsorb on the surface. Therefore, many studies have been conducted on the exfoliation method of the anionic layer, and a chemical etching method has been developed using formamide. However, chemical etching is not economical because not only expensive formamide is used, but also several and complicated steps are performed in the exfoliation process. This study developed a facile, inexpensive and efficient process by effectively etching the anion layer in LDH chemically through nitrogen plasma technology. As an additional effect of nitrogen plasma, oxygen vacancies serving as adsorption sites for water oxidation reactions were created, and nitrogen doped species which was efficient for charge transfer, was simultaneously generated in the metal cation layers. In the nitrogen plasma-treated NiAl LDH, it was confirmed that most of the anion layer was etched to change into a mono- or dual layered nanostructure having the thickness of 1-2 nm. In addition, oxygen vacancies and nitrogen doping generated in the plasma treatment process act as reaction sites for water molecules and have rapid electron transfer capability by changing the electronic structure of surrounding metal ions. The third research included the development of a dual-phase bismuth vanadate (BiVO$_4$)/graphitic carbon nitride (g-C$_3$N$_4$) composite photocatalysts and clarifying the role of tetragonal BiVO$_4$ as the charge mediator. Monoclinic BiVO$_4$ has been used as a classical photocatalyst for water oxidation reactions because Bi 6s orbitals and O 2p orbitals are mixed to form the hybrid orbitals resulted in having a low band gap of 2.4 eV. However, tetragonal BiVO$_4$ has been excluded from photocatalyst materials owing to a high band gap of 2.9 eV resulted from the absence of orbital hybridization due to its crystal structure. This study revealed that tetragonal BiVO$_4$, which is known to have little photocatalytic properties, acts as a chemical mediator between the monoclinic BiVO$_4$ and the g-C$_3$N$_4$. In addition, it was found that it effectively prevents recombination, which impairs the efficiency of the photocatalytic reaction. The fourth research is conducted on the synthesis of photocatalysts in which water oxidation and oxygen reduction reactions occur at different reaction sites at the same time by controlling the surface of tri-phase metal oxides inducing efficient hydrogen peroxide production. Hydrogen peroxide has been produced only in large quantities through the existing anthraquinone process, but there is a problem for contamination and reuse of organic matter in the production process, therefore a sustainable method was devised for producing hydrogen peroxide. In this study, a surface-controlled iron oxide-titanium oxide core-shell structure was engineered on cobalt-based nanosheets specialized for water oxidation reactions. Tri-phase metal oxide photocatalyst is designed to enable efficient light absorption in the order of ultraviolet-visible light through a core-shell structure. Especially, titanium oxide has an amorphous phase having chemical properties that are advantageous for hydrogen peroxide production through an oxygen reduction reaction. This dissertation presents a new methodology for inducing metal (hydro)oxide nanostructures to be applied to intended photocatalytic redox reactions through "Functional Material Development" and "Surface Engineering Technology".