Effect of Environmental Contaminants on the Interfacial Properties of Two-Dimensional Materials

Abstract

ConspectusThe surface of 2D materials can spontaneously adsorb and react with molecules in the environment during their processing and storage. This effect, while having a significant impact on many properties of 2D materials, is not always recognized and accounted for in the research involving them. This Account summarizes our recent work in understanding how the ambient environment impacts the properties of 2D materials and its mitigation strategies. We highlight graphene and hydrocarbons in our discussion and complement it with selected studies involving other 2D materials as well as water and oxygen.When graphene and graphite are exposed to air and water, their surfaces adsorb the residue hydrocarbons, typically at part-per-trillion to part-per-billion levels, in the environment. The adsorption of hydrocarbons reduces the surface energy of graphene and graphite and creates a barrier between them and the electrolyte. As a result, the wettability and electrochemical properties of graphene and graphite can be significantly altered by mere exposure to the ambient environment. These changes can be very significant yet highly variable depending on the local environment: several hours of air exposure can increase the water contact angle of graphene by up to 40° and reduce the double-layer capacitance of graphite by up to 50%! The high hydrophobicity and poor electrochemical performance of pristine graphitic carbons, once believed to be intrinsic properties of these materials, are largely due to unintentional surface contamination. The same type of hydrocarbon adsorption was reported for many other 2D materials, such as MoS2, hexagonal BN, and mica. In the case of mica, which is highly ionic in nature, the adsorption of hydrocarbons disrupts its interaction with ionic liquid and alters the self-assembly structure of ionic liquid at the mica surface. Similarly, water also impacts the surface properties of graphene in several ways. Water vapor can compete with hydrocarbons for adsorption onto the surface of graphene, thus reducing the rate of hydrocarbon contamination. Water can intercalate between graphene and some of its supporting substrate, altering their interactions. Finally, water enhances the doping of 2D materials by O2 by promoting an electrochemical doping mechanism involving the O2/H2O redox couple.Reducing and reversing the surface contamination of 2D materials can greatly enhance material and device performances. While completely stopping the contamination is still challenging, a high-humidity environment is shown to reduce the rate of contamination, as mentioned above. For samples already contaminated by airborne hydrocarbons, their surface properties can be partially restored by treatment in high-vacuum, high-temperature, or mildly oxidative environments.