Heteroelectrode Structure for Solar Water Splitting: Integrated Cobalt Ditelluride across TiO2-passivated Silicon Microwire Array

• Main Authors: Lin, Yu-Chen, 林育辰
• Other Authors: Hu, Shu-Fen
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/mua9w6

碩士 === 國立臺灣師範大學 === 物理學系 === 105 === Global energy approximately consumed 15 TW per years, and the energy demand is gradual increasing over time. So the worldwide country develops clean and renewable energy is more and more important. And efficiently solar water splitting is one of the goals which is people chasing. Moreover, that fossil fuels even can replace with hydrogen energy in the coming decades, and reach to no pollutants (e.g., mainly CO2) be produced. The semiconductor which can act as a photocatalyst producing photogenerated carriers to reduce hydrogen ions or oxidize water. And it's required that conduction band of photocatalyst should negative than hydrogen reduction potential. Silicon as a semiconductor possesses beneficial properties. Silicon, with a band gap of 1.1 eV and a theoretical maximum photocurrent density of 44 mA cm-2, is a promising potential material for photoelectrochemical (PEC) applications. In this work, silicon microwire arrays (Si-MWs) were verified to have scalable potential as a platform in PEC water splitting devices because of its optical absorption (high aspect-ratio structure) and increasing reaction surface area and also improving electrical transport (shorter diffusion length for minor carriers). The approaches of fabrication microwire are photolithography and dry-etching, and the result of the experiment shows the length and diameter were 10 μm and 0.85 μm, respectively. However, silicon suffers from two major problems, slow photogenerated carrier kinetics and oxide layer formed to efficiently block photocharges. Herein, decorating cobalt ditelluride (transition metal dichalcogenide) act as a cocatalyst, trap photogenerated electrons from Si-MWs and provide active sites to combine hydrogen ions reduced on the surface to molecular hydrogen. And the cobalt ditelluride which synthesized by simple cation-exchange reaction coated on the microwire surface. In order to prevent Si-MWs contacting with electrolyte, coating on 10 nm titanium dioxide with atomic layer deposited (ALD). The role of titanium dioxide is passivation layer between the silicon and liquid electrolyte. By passivating the surface, the surface recombination rate and electrode corrosion are suppressed and the interfacial charge-transfer rate for hydrogen reduction is improved. Since some group has presented it before, who used HMDS (hexamethyldisilazane) for improving surface tension and changing the surface to hydrophobic character. It addressed the nonhomogeneous distribution of cocatalyst and loose binding on the interface. The result of HMDS pre-treatment checked with XPS, the Si-O bond ratio decreased also. Using optimal condition of cocatalyst volume (30μL drop-casting) to measure linear sweep voltammetry, the result performing 0.17 V(vs. RHE) positive shift of onset potential and producing higher photocurrent density -24 mA cm-2 at 0 V vs. RHE. For long-term hydrogen evolution reaction, the result shows more than 5 hours stability and faradaic efficiency (ηF) roughly reached to 80% with CoTe2@TiO2@Si-MWs photo-electrode.