Direct Exfoliation of Conductive MoS2 Using Peroxide for Solid State Sensor and Catalytic Applications

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 (MoS2) being particularly popular. Using the semiconducting phase of MoS2 (2H-MoS2) 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 MoS2 is metallic MoS2 (1T-MoS2) 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 MoS2. Defects play a very important role in modulating the electrical properties of MoS2. Sonication of MoS2 in an appropriate solvent results in many disordered structural defects. The most common defects on MoS2 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 MoS2. 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 MoS2 and how to utilize those defects for improving photoluminescent efficiency. However, those defects can also be utilized to improve the conductivity of MoS2 as a safer alternative for applications in batteries, supercapacitors, solar cells, electrocatalyst and sensors.Conductive MoS2 (c-MoS2) 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, MoS2 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-MoS2 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-MoS2 under ambient conditions using 0.06 vol% aqueous hydrogen peroxide. We have demonstrated that the bulk conductivity of the conductive MoS2 that we prepared is up to seven orders of magnitude higher than that of the semiconducting phase of MoS2. The samples were also characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy which showed that hydrogen molybdenum bronze (HxMoO3) and substoichiometric MoO3−y help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. C-MoS2 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 MoS2 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-MoS2 films in 0.5 M H2SO4 electrolyte solution with three electrode systems using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm2 current density. The lower Tafel value demonstrated that c-MoS2 has potential to use as catalyst for HER. Our study furthers the understanding of conductive forms of MoS2, and also opens up a new pathway for next generation electronic and energy conversion devices.