單層二硫化鉬經氫電漿修飾後在電化學產氫之效應及具有原子介面之二硒化鎢/二硫化鉬平面異質結構之成長及應用

Abstract

在這篇論文中，我們使用化學氣相沈積法 (CVD) 製備二維材料 (TMDs)，並分別探討單層二硫化鉬 (MoS2) 在電化學產氫之應用及二硫化硒/二硫化鉬 (MoS2 / WSe2) 具原子介面解析之水平異質材料 (Heterostructures) 在電子和光學方面的潛力。過渡金屬硫化物二維材料被視為低成本及高效率之催化材料，其二維型態之晶體邊緣 (crystal edges) 被視為催化活性點 (active cites) 並具有良好的催化能力；但其在晶體平面 (basal planes) 不具有催化能力，因此無法有效增加其活性點之表面積以提高催化效能。本研究利用簡易且高效之氫電漿方法 (Hydrogen plasma)，在單層MoS2晶體平面上，將上層之硫元素S部份移除產生硫空缺 (S-vacancies)，藉以XPS得知其催化性能最佳之化學組成比例；在移除上層硫元素之同時，不僅可提高活性點之密度，並由原子力顯微鏡 (AFM) 顯示出可維持晶體形貌和結構之完整性是本研究最顯著之優勢。活性點控制在產氫效能中扮演非常重要的角色，透過電化學量測，其極化曲線 (polarization curves) 之過電位由727mV顯著改善至183mV，Tafel值也由164顯著優化至77mV/dec 。透過EDLC量測，我們得知其電容量有數倍之增加 (Capacitance)，並由穩定性測試 (Stability test) 得知其催化穩定性良好。 另一方面，我們透過化學氣相沈積法製備MoS2 / WSe2 heterostructure，首度克服在直接成長製程 (Direct growth) 其熱力學傾向於形成合金 (Alloy) 之問題，並製備出具有原子解析介面 (Atomically sharp interface) 之異質材料；藉由電流-電壓曲線得知，其具有明顯的光伏效應及p-n junction之性質。關鍵優勢在於具有單層之p-n介面 (p-n junction) 可用來製作整流及發光二極體、光電裝置及雙極介面電晶體。

Two-dimensional layered transition metal dichalcogenides (TMDs) materials such as Molybdenum disufide (MoS2) have been recognized as one of the low-cost and highly efficient electrocatalysts for hydrogen evolution reaction (HER). On the other hand, TMDs have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. The crystal edges, rather than the basal planes, of MoS2 have been identified as the active sites for HER performance, but they only account for a small percentage of the surface area, of MoS2 monolayer. Here, we report a simple and efficient approach that involves using a remote hydrogen-plasma process to creating S-vacancies on the basal plane of a monolayer crystalline MoS2; this process not only can generate high density of S-vacancies but also can maintain the morphology and structure of MoS2 monolayer, as confirmed with Atomic force microscopy (AFM) characterizations. The density of S-vacancies (defects) on the basal plane of MoS2 monolayers resulting from the remote hydrogen-plasma process can be tuned and play a critical role in HER, as evidenced by the results of electrical measurements. A lowered overpotential, from 727mV to 183mV, and a decreased Tafel slope, from 164mV/dec to 77mV/dec, as compared to those of a pristine MoS2 monolayer are observed. We found several times enhancement in the capacitance of the hydrogen- plasma-treated MoS2 monolayer from the electrical double layer capacitance (EDLC) measurement, Moreover, the stability test shows these materials have high durability in acid environment. The H2-plasma-treated MoS2 also provides an excellent platform for systematic and fundamental study of defect-property relationships in TMDs, which provides insights for future applications including electrical, optical and magnetic devices. Second, we report a two-step epitaxial growth of lateral heterojunction WSe2-MoS2 monolayer with an atomically sharp interface, instead of preferred TMD alloy, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface which can be evidenced by high resolution TEM. From the electrical transport curves, we found the lateral heterostructure WSe2-MoS2 monolayer display apparent p-n junction and thus photovoltaic effect. Our spatially connected TMD lateral heterojunctions are potential candidates for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors.