結合二碲化鈷與二氧化鈦保護層之矽微米柱異質結構應用於光催化水分解

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2017

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全球每年平均能源消耗約15兆瓦,且對於能源之需求與日俱增,故各國積極開發乾淨之替代能源變得越來越重要,而有效利用太陽能進行光催化水分解為一新之展望,其可取代化石燃料,以達到無碳排放與零污染產物等特點。應用於光催化水分解之光觸媒須滿足特定條件,首先須為半導體材料,且其導電帶位置須負於氫氣之還原電位,此研究以矽為光觸媒,因其具窄能隙,故可利用大部分之可見光,為太陽能產氫能源建立新之里程碑。 此研究使用矽微米柱陣列結構作為光捕捉之利用與增加反應表面積,並減少電子擴散路徑,其藉由黃光微影製程技術與乾式蝕刻製作而成,完成之柱長與直徑分別約為10 μm和0.85 μm。然而於目前之研究領域中,矽基光電極仍有許多問題存在,大致上為光生載子動能不足與氧化物生成造成不穩定之結果。此研究利用修飾過渡金屬二硫屬化合物(transition metal dichalcogenide;TMD)為軸,以二碲化鈷(CoTe2)作為共觸媒,並使用原子層沉積(atomic layer deposition;ALD)生成二氧化鈦(TiO2)作為保護層以解決上述之問題。藉由簡易之陽離子交換反應法,以鈷離子置換前驅物亞碲酸鈉(Na2TeO3)之鈉離子,並經氫氣還原反應後得二碲化鈷。而經原子層沉積後,可於電子顯微鏡下觀察到二氧化鈦包覆於矽微米柱表面,以避免電解液與矽基材直接接觸。 於異質介面上探討能帶彎曲對稱情形,使介面能障消失,更以六甲基二矽氮烷(hexamethyldisilazane;HMDS)進行預處理,減少表面張力並對表面進行改質,可有效改善共觸媒不均勻分散於微米柱結構之情況,並可降低介面阻抗,其結果於電子顯微鏡下觀察到粒子聚集情況降低,且以X光光電子能譜分析可得矽-氧鍵之比例減少,進而改善光生載子傳輸效率,以降低載子再結合發生機率。 光電流特性則於模擬太陽光照射(100 mW/cm2)下,以標準氫電極電勢(reversible hydrogen electrode;RHE)為0 V下量測其光反應,其結果顯示於定量下30 μL共觸媒前驅物所合成之二碲化鈷具最佳光電流特性,於0 V vs. RHE下光電流可達24 mA/cm2,同時起始電位正偏移至0.17 V。而沉積二氧化鈦保護層後,進行長時間產氫量之量測,經計算後可得約80%之法拉第效率(Faradaic efficiency;ηF),且其穩定性於酸性電解液環境中可維持5小時無明顯衰減。
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.

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水分解, , 產氫, 二硫屬化物, 二碲化鈷, water splitting, silicon, hydrogen evolution, dichalcogenide, cobalt ditelluride

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