二硫化鉬被認為是下一代奈米技術最有前途的材料之一。由於其獨特的二維結構、優異的物理化學特性，因此近年來許多研究都致力於開發新的二硫化鉬合成及其製備方法，以探討二硫化鉬的物理與化學性質以及其可能的應用性。其中，二硫化鉬材料的半導體特性、導電性及化學穩定性，使得二硫化鉬被視為理想的電催化析氫材料。然而，二硫化鉬仍有很大的改善空間。因此，透過不同製程方法以及後續處理的二硫化鉬材料特性被廣泛的研究。

從模擬計算結果中證實二硫化鉬的催化特性與處理後的硫缺陷數量有正相關，然而實際上要觀察經處理後的硫缺陷對二硫化鉬其表面形貌與表面催化特性影響的證據探討有相當程度地困難，因此截至目前為止鮮有研究對此現象做有系統地探討。本研究得力於結合數種不同種類的顯微觀察技術，尤其是具有原子級影像的掃描穿隧式顯微鏡。使學生製備的二硫化鉬與經氫電漿處理後的樣品，能觀察表面形貌與微區電化學特性，並有系統地討論經氫電漿處理後的硫缺陷對二硫化鉬的形貌改變與電化學催化機制的影響。

本實驗透過改變氫氣電漿處理時間製備具有不同硫缺陷（S-Vacancy）的二硫化鉬薄片。透過成分分析可知，二硫化鉬系列樣品隨著時間增加S/Mo的比例有系統性的下降，在拉曼光譜中也有發現到相對應硫缺陷造成的特徵峰位移趨勢。為了討論不同數量的硫缺陷對催化活性之影響，利用氫氣產生的反應做為催化反應活性的標的。結果顯示，當二硫化鉬具有最佳的硫缺陷數目時，其塔佛斜率會從148.3 mV/dec減少為89 mV/dec，顯示其產氫的機制由Volmer機制控制並獲得改善，同時其反應過電位（η1）亦從783 mV減少為524 mV。本研究結果顯示，透過改變硫缺陷的活性位置數目能有效地增加催化劑之活性，同時本研究亦為首次利用顯微觀察技術擷取到原子級解析度缺陷影像，將成為未來研究的一項利器。

MoS2 is one of the earth-abundant and nontoxic 2D materials, which has recently garnered a lot of research interest. Few layers of MoS2 were synthesized by a simple Chemical Vapor Deposition method. Here, few layers MoS2 were grown by Mo foil and S powder using low pressure argon environment. The size of a single layer can be controlled from 10 to 100 µm by changing the growth conditions. The optical microscopy (OM) and atomic force microscopy (AFM) show the morphology of single layer MoS2. The thickness of single layer MoS2 is around 0.6 to 0.7 nm.

The MoS2 has been considered as a promising catalyst for hydrogen evolution reaction for some time already. Unfortunately, the catalytic activity of MoS2 is still lower than Pt, and necessary to improve its activity by further modification. Here, we present a simple hydrogen plasma treatment to create defect sites on the surface of a few layers of MoS2, which shows help improved its hydrogen evolution reaction (HER) activity. The scanning tunneling microscopy (STM) image of pristine few layer MoS2 on highly oriented pyrolytic graphite (HOPG), 10 min and 40 min hydrogen plasma treated MoS2. The STM of pristine MoS2 shows clear Moire pattern due to a lattice mismatch between MoS2 and HOPG. The lattice constant measured here is around 0.315 nm which can be attributed to the S atom of MoS2. STM images of MoS2 after 10 min plasma treatment show irregular orientation of S atom, which might be related to the S-vacancy created by the hydrogen plasma. STM images of MoS2 after 40 min plasma treatment show that the surface is severely damaged by the plasma treatment. With higher treatment time, the XPS analysis shows S/Mo ratio of MoS2 systematically changed. The Raman spectra also shows the structural evolution of MoS2 after plasma treatment. The E2g and A1g mode shows red shifts and blue shifts, respectively. These shifts are indicative of the presence of defects in MoS2 and might be correlated with the increase in HER activity.

Finally, the results show that with increasing S-vacancies of MoS2, the Tafel slope of HER will change from 148.3 to 89 mV/dec, which reveal the mechanics of hydrogen evolution reaction is Volmer mechanism and η1 also decrease from 783 to 524 mV. In this study, we demonstrate the observation of S-vacancies on MoS2. It is worth to note that we illustrate how S-vacancies affects the catalytic properties, for the first time, in a systematic experiment instead of simulations.

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