Synthesis of metal oxide semiconductor for enhanced visible light activity and photoelectrochemical water oxidation

Abstract

Since increasing the interest in clean energy sources, effective utilization of solar energy becomes one of the most significant issues to resolve the rising energy demand. Metal oxide semiconductors have attracted much attention because they are photoelectrocatalytically active under solar energy. Herein, we focused on the morphology control of metal oxide semiconductors and their surface modification with electrocatalysts for efficient photoelectrochemical reaction. Also, we discuss about electrocatalyst as cocatalyst and the formation of heterojunction structure with metal sulfide semiconductor. These materials are suitable for photoelectrochemical water and sulfite oxidation to produce O2 and H2 via direct solar energy conversion. In Chapter II, we described the tungsten oxide semiconductor ($WO_3$) and hydroxyl iron oxides (FeOOH), which play a role in photoelectrocatalyst and electrocatalyst, respectively. Single-crystalline $WO_3$ microplates with variable thickness grown directly on a fluorine-doped tin oxide (FTO) substrate followed by an annealing procedure. The resulting $WO_3$ microplate electrode, which had an average thickness of 470 nm and this value corresponding to the maximum hole diffusion length of $WO_3$, showed an improved photocurrent under 1 sun irradiation for water oxidation, with a significant improvement in the visible light region. This enhanced water oxidation activity originates from both single crystallinity with an optimum thickness of the $WO_3$ microplates and the oxygen-deficient characteristic. To improve the photochemical stability, FeOOH electrocatalysts were deposited on the $WO_3$ microplate sulface. The resulting $WO_3$/FeOOH composite electrode showed enhanced stability for water oxidation reaction. Furthermore, we investigated the evolution of the phase and morphology of FeOOH nanorods via X-ray diffraction and in situ transmission electron microscopy. The FeOOH nanorods were gradually converted from tetragonal structure ($\beta$-FeOOH) into a rhombohedral $Fe_2 O_3$ nanorod by a simple thermal treatment. The existence of an intermediate FeOOH structure with high lattice strains during the phase transition was identified and the electrochemical properties are investigated based on the crystal phases to elucidate their relative catalytic activities. The strained-FeOOH nanorods exhibited enhanced catalytic water oxidation activity and high electrochemical stability under neutral pH conditions. In Chapter III, metal oxide/ metal sulfide semiconductor heterojunction structure was designed for enhancing the photoelectrochemical activity in visible light region. The heterojunction structures were successfully synthesized by seed-mediated hydrothermal reaction of $WO_3$ nanoparticles onto fluorine-doped tin oxide (FTO) substrate, with successive reaction of a narrow band gap, 1.3-1.9 eV, visible light responding Bi2S3 nanowire by hydrothermal reaction. This synthetic method of heterojunction structure offers enhanced photoelectrochemical activity, including a significant improvement in the visible light region compared to previous reported synthetic method of heterojunction structure. Efficient formation of heterojunction, which the band position and band potential matching between $WO_3$ and $Bi_2 S_3$, is advantageous for charge separation and utilizing of the reduction-oxidation reaction.