太陽能光催化水分解已成為一新展望，作為一取代石化燃料之目標，乃因其具有零碳排放與空氣零汙染等優點。眾多半導體材料中以矽具備較窄之能隙與適當之傳導帶位置，其傳導帶約負於氫之還原電位，被視為於太陽能產氫燃料中最具潛力候選人之一。
本研究所使用矽微米柱乃藉由微影製程技術與乾式蝕刻製程完成，柱子長度與直徑分別約為10 μm和0.85 μm。然而目前研究領域上，水分解過程中矽光電極其穩定性仍然為一備受考驗之挑戰。在本研究中，我們選擇以二硫化鈷(CoS2)作為實驗中矽光陰極之共觸媒，其藉由對前驅物Co(OH)2以熱硫化之簡易方法即可製得。所製備之二硫化鈷修飾於矽之光陰極於電子顯微鏡下觀看，形貌呈現出一微米尺度之核-殼結構，二硫化鈷完全包覆於光觸媒矽之表層。研究中，矽光陰極於太陽模擬光照射(100 mW/cm2)、標準氫電極電位(reversible hydrogen electrode)RHE為0 V下並無法顯示出光反應，然而當沉積4小時之二硫化鈷修飾矽微米柱表面後，光電流可提升至0.666 mA/cm2。所有樣品之最佳參數為沉積6小時之CoS2-Si光陰極，其餘外加偏壓為0 V時之光電流大小可達3.22 mA/cm2，而起始電位正偏移至0.248 V，若進一步將二硫化鈷修飾量增加，則可能加長光生載子所需傳輸路徑，反而增加載子再結合機率。另一方面CoS2-Si-6h光陰極其穩定性於0 V(vs. RHE)可維持9小時而不具明顯衰減。
光催化水分解之表現實驗中由交流阻抗法進行分析，量測到用不同時間沉積二硫化鈷所得之阻抗值與光電流特性曲線具有相同趨勢。同時我們以同步輻射光源進行x光吸收能譜，期望更深入瞭解光電子於矽與二硫化鈷間之傳導訊息，研究中我們亦以x光光電子能譜以研究二硫化鈷修飾於矽、矽與白金修飾於矽等三種光電極差異。證明出矽與鉑-矽光電極之光催化效率衰減來自於實驗過程中矽於水溶液中產生氧化所致，而以二硫化鈷修飾矽後，則因二硫化鈷扮演一完全覆蓋之保護層，使得反應前後之光陰極之訊號不存在差異性。
研究結果顯示CoS2-Si於本研究中作為一光催化水分解上之良好光陰極，同時作為促進載子傳輸效率之共觸媒，與能夠避免矽產生氧化之鈍化層。利用CoS2-Si-6h作為光陰極進行光催化水分解之產氫與產氧效率分別為0.833與0.414 μmol/min。

關鍵字：水分解、光觸媒、光陰極

Solar water splitting has been thought a new prospect to replace fossil fuel due to its no carbon emission and air pollution. Silicon (Si), among the numerous semiconductor materials, is considered as the most potential photocathodes for hydrogen evolution of solar fuel production because of its narrow band gap and negative conduction band edge.
Here Si microwires (MWs) were fabricated by the photolithography technique and dry etching process. The diameter and length of Si MWs are about 1.7 μm and 10.0 μm, respectively. However, the stability of Si is an ongoing challenge to be solved. In presented study, cobalt disulfide (CoS₂) synthesized by thermal sulfidation of Co(OH)₂, was played as a co-catalyst, and the morphology of CoS2-Si photocathodes formed a core-shell microstructure by SEM examined. The bare Si MWs showed no photoresponse under solar stimulation (100 mW/cm²) at 0 V (v.s RHE), but the photocurrent of Si MWs with 4 hrs CoS₂ deposition increased to 0.666 mA/cm². The optimized photocurrent of CoS₂ decorated on Si MWs was 3.22 mA/cm² after 6 hrs deposited, and the onset potential, which is defined as the potential at photocurrent density reached -1 mA/cm2, is 0.248 V also. Further increasing the loading amount of CoS₂ co-catalyst prolong the path which carriers need to pass by, may increase the recombination probability. The stability measurement of Si MWs with 6 hrs CoS₂ modification can maintain for 9 hrs at 0 V (vs. RHE) with no obviously degradation.
The performance of the solar water splitting was examined by EIS analysis, which showed the same trend about impedance in different loading time photocathodes. The 6 hrs CoS2 deposition time had the best charge transfer efficiency. And XAS was also used to investigate the charge behavior between Si and CoS2.
XPS was conducted to study the stability among the CoS2-Si-6h, bare Si and Pt-Si photocathode. The decay in bare Si and Pt-Si electrode were proof by the Si oxidation signal increase, and there was no difference whether CoS2-Si-6h electrode performing experiment or not.
These results revealed our CoS2-Si served as a good photocathode in solar water splitting, it not only can play a role of co-catalyst to promote charge transfer efficiency, but also can be a passivation layer to protect Si MWs from oxidation. Finally, the hydrogen and oxygen evolution rate were 0.833 and 0.414 μmol/min respectively.

Key words: water splitting、photocatalyst、photocathode

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