| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 105 === 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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