Combinatorial Engineering Toward Improved Photoelectrochemcial-water-oxidation Activity of Hematite Photoanode

• Main Authors: Yi-Hsuan Wu, 吳以璿
• Other Authors: Jeng-Kuei Chang
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/yd9gvg

碩士 === 國立中央大學 === 材料科學與工程研究所 === 105 === Hydrogen has been expected to replace fossil fuel as major energy fuels in future human’s community. With its high gravimetric energy density, grid production and transportation are desired. Within several possible pathways, solar water splitting, especially photoelectrochemical water splitting, can entirely utilize water to evolve oxygen and hydrogen at anode and cathode respectively beyond sunlight illumination with small external bias. This technique has been intensively studied and expected as promising method to link up fossil-fuel community in future. In this thesis, the study is separated into three parts. First part interprets conceptual knowledge of photoelectrochemical cell (PEC), including general knowledge of semiconductor photoelectrochemistry, semiconductor-liquid junction, and physics of charge carriers. Though a complete water splitting combines oxidation and reduction, we here focus on four-electron process of water oxidation at photoanode. Comparing with several transition metal oxides such as TiO2, BiVO4, and WO3, hematite (-Fe2O3) is selected as target material owning to its proper band-gap width for visible-light absorption, dominating theoretical quantum efficiency and stability within reaction, environmental friendly character, and low-cost for production. Second part of study lies on aliovalent-ion doping. Foreign-ion doping to hematite has been approved to enhance conductivity and lifetime of charge carriers. We wet-chemically impregnated In3+, Mn2+, Ca2+ and Ti4+ as p-/n-type dopants during hydrothermal synthesis and found promising improvement with titanium ion doping. Meanwhile, the self Sn-doping from back contact (F:SnO2) is an intrinsic way to eliminate drawbacks of synthesized hematite. With Sn4+ assisted, the photocurrent increases from nearly 0.0 mA/cm2 to 0.42 mA/cm2 at 1.23 VRHE under AM 1.5G illumination. Additional improvement was by help of Ti4+ which enhances photocurrent to 0.87 mA/cm2 at 1.23 VRHE under light harvesting. By EIS, XPS, and XAS, the results are ascribed to near-surface nano TiO2 and/or composition heterogeneous (Fe2-xTixO3) in material which may prolong electron survival lifetime as well as diffusion length. The third part of study was to decorate surface with phosphate (Pi) nanoparticles or thin film by dip coating. The surface overlayer produced by mild-annealing was found to facilitate hole transportation from tail of space-charge layer to liquid site. Phosphorus can bridge and replace surface –OH and take up intrinsic surface states. This nonmetal treatment presents no effect of bulk doping and set its own debate on semiconductor-liquid junction. In EIS, the eliminated semi-ellipsoid exhibits surface effect of Pi decoration; In J-V diagram the overlayer also emphasize its role in reaction by evidence of ignorable increase of photocurrent in Ti-doped (surface-activated) electrodes. XRD patterns indexes Kx+2(PxO3x+1) complex as composition of overlayer itself. This study addresses a different conclusion toward debate on role of Ti4+ in doped hematite and capability of phosphate ion in bulk iron-oxide. Ti4+ in Ti-doped Fe2O3 didn’t act as electron donor (by Mott-Schotky) but tune near-surface composition and formed TiO2 nanostructure within Fe-O crystal. Phosphorus made by phosphate salt serves sound ability for hole transportation from bulk to solid-liquid interface. However, phosphate surface doping is not suggested. The conclusion contributes to renewed suggestions on several aspects of Fe2O3 photoanode.