CVD Molybdenum Disulfide/Graphene Heterostructures: Synthesis and Electrochemical Applications
thesisposted on 01.07.2016 by Amirhossein Behranginia
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There is a rising demand to substitute fossil fuels with environmental-friendly energy resources. Providing hydrogen resource through a water splitting process can be one way to satisfy this critical need. The high cost of the conventional water splitting catalysts such as platinum makes this process inapplicable for industry utilization. Hence, there is a need to find low cost catalysts to make this process applicable for future applications. In this respect, earth abundant and inexpensive MoS2 nanostructures with sulfur (s) terminated edge atoms have previously been tested for HER activity. However, results indicate less efficient HERs on these structures which is mainly attributed to: (i) a low electrochemical activity of sulfur edge atoms, and (ii) a high charge transfer resistance between semiconductor MoS2 and electrode due to the Schottky barrier formation at their interface. In this work, a large-area crystalline 3D structured MoS2/Graphene (Gr) heterostructure is directly grown on the glass carbon electrodes using a large-scale atmospheric pressure chemical vapor deposition (APCVD) technique to considerably increase the active edge atoms of the MoS2 and improve the charge transfer toward active edge sites. The grown 3D structured MoS2 with Mo terminated edges has shown remarkable electrochemical activity due to the high density of d orbital electrons of the edge atoms (Mo). Our recent work published in chemistry of materials, which is widely reviewed in the second and third chapters of this dissertation, has fully investigated the growth mechanisms of this structure and the applicability of its heterostructure with graphene for HER activity. The control experiments and characterizations have revealed that the 3D structured MoS2 follows the Stranski-Krastanov growth mode in which the transition from 2D to 3D happens after a critical thickness. The turn over frequency (TOF) measurements have shown a very high value for the 3D structured MoS2 demonstrating high chemical activity of this structure. Also, the electrochemical impedance spectroscopy (EIS) measurements have shown that the charge transfer resistance is much lower in 3D structured MoS2 grown on graphene film than those grown on glassy carbon and transferred to glassy carbon. These results confirm that the growth of this structure on top of the graphene film considerably improved the contact resistance between MoS2 and electrode.