共催化劑修飾於矽微米柱以提升光催化產氫之效率

Abstract

石化燃料快速之消耗，使可再生之替代能源更加被重視。近期以半導體材料進行光催化水分解之研究蓬勃發展，乃因光陰極產生之氫氣已被視為具發展性之替代能源。本實驗藉微影與乾蝕刻技術將矽基板蝕刻成矽微米柱結構(Si MWs)，以提升光陰極之吸光率與反應面積。然而其光生載子動能偏低與表面易生成氧化物之問題，使矽無法有效進行光催化水分解反應。 故第一部分實驗以簡易化學合成法，將共催化劑鈷二硫屬化物(CoX2)修飾於矽微米柱上於廣範pH值溶液進行光催化產氫反應，CoX2降低因矽照光所產生之電子電洞對之再結合，進而提升光催化水分解之效率，並因其核-殼狀結構隔絕矽表面與氧氣之接觸，抑制矽表面生成氧化層，進而提高光陰極材料之穩定性。實驗中修飾二硫化鈷之光陰極(Si@CoS2)於酸性溶液下表現優於修飾二硒化鈷之光陰極(Si@CoSe2)，乃因Si@CoS2具較大活性面積與較高之吸光率。然而，於中性與鹼性溶液之瞬態電流量測中，Si@CoS2具不穩定與過衝電流之現象產生。反觀地，Si@CoSe2穩定於廣範pH值溶液進行300分鐘反應，於鹼性溶液之計時伏安量測後，其光電流密度提升至-5.0 mA cm-2，乃因CoSe2轉為非晶相結構，使其裸露更多活性端與氫離子進行反應。 然而，因CoX2與p型矽之界面產生不適合之能帶彎曲，降低電子之傳輸效率，故於第二部分實驗選用能與p型矽產生p-n接面之共催化劑二硫化鉬(MoS2)，並對MoS2摻雜過渡金屬(MMoSx；M = Fe, Co, Ni)，使MoS2裸露出更多活性端。於X光繞射與拉曼圖譜發現，異金屬原子之摻雜破壞MoS2之晶格結構，呈現非晶相之結構。於酸性溶液下，修飾MoS2之光陰極(Si@MoS2)於外加偏壓為零伏下之光電流密度達到-8.41 mA cm-2，其中以摻雜鈷金屬原子之光陰極表現最佳，其光電流密度於外加偏壓為下達到-17.2 mA cm-2。藉X光吸收能譜發現，MMoSx中僅鈷與鎳金屬原子為以取代鉬原子之方式摻雜入MoS2內，其中以摻雜鈷原子之MoS2裸露出最多之活性端，於計時伏安量測亦可發現，Si@MoS2摻雜鈷原子後其法拉第常數由原本63%提升至81%。

Rapid depletion of fossil fuels have triggered an urgent demand for renewable alternative energy. Recently, the research of using semi-conductor for solar water splitting have flourished, because the products of hydrogen gas have been seen as a development for alternative energy. In this study, silicon substrate is etched to silicon microwires (Si MWs) by lithography and dry etching to enhance absorbance and reaction area. However, lower kinetic energy of photoinduced carries and oxide layer on Si MWs made it couldn’t effectively drive solar water splitting. Therefore, co-catalyst of coblat dichalcogenide (CoX2) decorated on Si MWs with simple chemical synthesis method for solar hydrogen evolution at wide range pH solution in the first part of study. The efficiency was enhanced, because the core-shell structure suppressed generation of silicon oxide and CoX2 reduced recombination of photoinduced electron-hole pairs. In acidic solution, the efficiency of photocathodes decorated by cobalt disulfide (Si@CoS2) is better than Si@CoSe2 due to better absorbance and active sites area. However, Si@CoS2 is unstable and have over-shoot photocurrent in neutral and basic solution in transient current measurement. On the other hand, Si@CoSe2 reacted 300 min of measurement stably in wide range pH solution. The photocurrent density enhance to -5.0 mA cm-2 in basic solution after chronoamperometry measurement, because CoSe2 transfers to amorphous phase exposing more active sites. However, unsuitable band alignment between CoX2 and p-type silicon block electron transmission. So the second part of study uses molybdenum disulfide (MoS2) which generates ohmic contact with p-type silicon. Besides, transition metal dope into MoS2 (MMoSx；M = Fe, Co, Ni) to make it expose more active sites. In X-ray diffraction and Raman spectrum, heterometals damage the lattice of MoS2, and turn into amorphous phase. In acidic solution, the photocurrent density of photocathodes decorated by MoS2 (Si@MoS2) achieves -8.41 mA cm-2 at 0 V. Si@CoMoSx is the best, because this photocurrent achieves -17.2 mA cm-2 at 0 V. In X-ray absorption spectroscopy, only cobalt and nickel atoms dope into MoS2 by substituting molybdenum atoms. Among these materials, the Si@CoMoS exposes the biggest area of active sites, so the faradic efficiency enhances to 81% in chronoamperometry measurement.