Direct Exfoliation of Conductive MoS2 Using...

Abstract

Two-dimensional (2D) materials have attracted much attention over the last decade due to their high performance in nanoelectronic devices. The discovery of graphene opened up many opportunities to investigate and explore other 2D materials. There has been a drive to expand the toolbox of 2D materials to also include insulators and semiconductors with a variety of bandgaps. As a result, a wide range of materials have been discovered or predicted, with molybdenum disulfide (MoS 2 ) being particularly popular. Using the semiconducting phase of MoS 2 (2H-MoS 2 ) requires a relatively high voltage to get sufficient conductivity due to the presence of a band gap. However, for applications in batteries, supercapacitors, electrocatalysts and solar cells, a substantially increased conductivity is required in order to achieve reasonable currents. 1 The most common source of conductive MoS 2 is metallic MoS 2 (1T-MoS 2 ) that has been prepared via the lithium intercalation process, which requires inert atmosphere processing and safety procedures. 2 Hence, there is a desire to develop a safer and more efficient process to yield conductive MoS 2 . Defects play a very important role in modulating the electrical properties of MoS 2 . Sonication of MoS 2 in an appropriate solvent results in many disordered structural defects. The most common defects on MoS 2 are sulfur defects. These defects increase the energy level of the gap state and eventually deteriorate the device performance. Thiol based molecules are commonly used to reduce the number of sulfur defects on MoS 2 . Other molecules such as oxygen or organic super acids like bis(trifluoromethane) sulfonamide (TFSI) have also been reported to passivate the surface defect. 3 Past research has mainly focused on the theoretical study of defective MoS 2 and how to utilize those defects for improving photoluminescent efficiency. However, those defects can also be utilized to improve the conductivity of MoS 2 as a safer alternative for applications in batteries, supercapacitors, solar cells, electrocatalyst and sensors. Conductive MoS 2 (c-MoS 2 ) can be used as active material for low-cost solid-state chemiresistive pH sensors. 4 In chemiresistive sensors, conductivity changes are observed based on direct interactions between the active material and the analyte. 5 Even though chemiresistive pH sensors based on exfoliated graphene, carbon nanotubes, or graphitic materials are available, their sensing response is limited to less than 20%. 6 On the other hand, MoS 2 has attracted great attention as a promising electrocatalyst for the hydrogen evaluation reaction (HER) because of abundant active sites at edge sites and on the basal plane for facilitating hydrogen production. Water splitting is the simplest and most convenient method to generate hydrogen. In industrial applications, an electrocatalyst is commonly used to accelerate the HER and reduce the overpotential. Even though 2H-MoS 2 has good catalytic activity at the edge sites, its low electrical conductivity limits the achievable current density, resulting in a high Tafel value and making it unsuitable for practical application in HER. In this work, we show a simple and effective way to prepare few layer c-MoS 2 under ambient conditions using 0.06 vol% aqueous hydrogen peroxide. We have demonstrated that the bulk conductivity of the conductive MoS 2 that we prepared is up to seven orders of magnitude higher than that of the semiconducting phase of MoS 2 . The samples were also characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy which showed that hydrogen molybdenum bronze (H x MoO 3 ) and substoichiometric MoO 3−y help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. C-MoS 2 was further functionalized with thiols to determine the number of residual reactive sites. An important goal of our work is to control the conductivity of the MoS 2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors in liquid environments. We fabricated chemiresistive pH sensors with centimeter channel lengths while maintaining low measurement voltages. We further measured the catalytic activity of c-MoS 2 films in 0.5 M H 2 SO 4 electrolyte solution with three electrode systems using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm 2 current density. The lower Tafel value demonstrated that c-MoS 2 has potential to use as catalyst for HER. Our study furthers the understanding of conductive forms of MoS 2 , and also opens up a new pathway for next generation electronic and energy conversion devices. References: Saha. D; Selvaganapathy. P R; Kruse. P, J. Electrochem. Soc., 167 , 126517 (2020). Eda. G et al ., Nano Lett ., 11 , 5111−5116 (2011). Lu. H et al. APL Mater ., 6 , 066104 (2018). Saha. D; Kruse. P, ACS Appl. Nano Mater ., 3 , 10864-10877 (2020). Kruse. P, J. Phys., D , 51 , 203002 (2018). Gou, P et al ., Sci. Rep., 4 , 4468, (2015). Figure 1