Simultaneous Electrochemical Exfoliation and Functionalization of 2H-MoS₂ for Supercapacitor Electrodes

MoS₂ is a promising semiconducting material that has been widely studied for applications in catalysis and energy storage. The covalent chemical functionalization of MoS₂ can be used to tune the optoelectronic and chemical properties of MoS₂ for different applications. However, 2H-MoS₂ is typically chemically inert and difficult to functionalize directly and thus requires pretreatments such as a phase transition to 1T-MoS₂ or argon plasma bombardment to introduce reactive defects. Apart from being inefficient and inconvenient, these methods can cause degradation of the desirable properties and introduce unwanted defects. Here, we demonstrate that 2H-MoS₂ can be simultaneously electrochemically exfoliated and chemically functionalized in a facile and scalable procedure to fabricate functionalized thin (∼4 nm) MoS₂ layers. The aryl diazonium salts used for functionalization have not only been successfully covalently grafted onto the 2H-MoS₂, as verified by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, but also aid the exfoliation process by increasing the interlayer spacing and preventing restacking. Electrochemical energy storage is one application area to which this material is particularly suited, and characterization of supercapacitor electrodes using this exfoliated and functionalized material revealed that the specific capacitance was increased by ∼25% when functionalized. The methodology demonstrated for the simultaneous production and functionalization of two-dimensional (2D) materials is significant, as it allows for control over the flake morphology with increased repeatability. This electrochemical functionalization technique could also be extended to other types of transition-metal dichalcogenides (TMDs), which are also typically chemically inert with different functional species to adjust to specific applications.