Gas-surface interactions on two-dimensional crystals

Two-dimensional (2D) crystals have developed into a popular mainstream research topic which is interesting for basic research and many applications. Gas-surface interactions, as reviewed here, are important for catalysis including noble metal-free catalysts, materials science, and surface science as well as environmental and energy technologies. Basic science concerns fundamental differences of 2D crystals and bulk materials as well as e.g. how the substrate of epitaxial 2D crystals affects their surface properties.

Most of the attention so far obtained (gas-phase) water adsorption which always was an evergreen in surface science. However, studies about small inorganic/organic molecule adsorption (CO, CO₂, NO_x, O₂, H, rare gases, H₂S, SO₂, alkanes, benzene, alcohols, thiophene, etc.) and surface reactions on 2D crystals (CO oxidation, ethylene epoxidation, oxygen reduction reaction, SO₂ and H₂SO₃ oxidation) started to appear in the literature as well. This review describes all of these probe molecules, but focuses on experimental and theoretical surface science model studies usually conducted at ultra-high vacuum (UHV).

The review focusses on graphene and functionalized graphene (graphene oxide, N-graphene, etc.) since the bulk of the literature deals with that system. However, included in fair detail are also many other 2D crystals such as silicatene, zeolite films (doped silicatene), metal dichalcogenides (such as MoS₂, WS₂), boron nitride, MXenes, germanene, silicene, TiO₂, graphane, graphone, and portlandene.

As a prototypical example, in recent projects, the wetting properties of e.g. graphene for water were controversially discussed. Therefore, a long chapter is devoted to water on graphene. That dispute was originally based on contact angle measurements at ambient pressure. In the meanwhile detailed surface science works including theoretical modelling are available. Literature on other carboneous surfaces such as HOPG (see list of acronyms) will be considered as a reference. Related works are also visible for other inorganic 2D crystals such as silicatene, i.e., 2D-SiO₂, or 2D-MoS₂ as well as functionalized 2D crystals (i.e. graphene oxide, N-doped graphene, graphane, etc.). Hydrophobic systems also are interesting for applications.

Although included in this review, but not described in very detail are electro chemistry studies, projects in the liquid phase, photo-chemistry, high pressure catalysis, and pure engineering studies (membranes, separation, fuel cells). However, in comparison with 2D crystals and to perhaps motivate related UHV surface chemistry projects in the future many of these projects were included to some extent.

As a broader objective, this review summarizes the currently available knowledge needed to extend the use of 2D materials beyond the utilization of their remarkable electronic and mechanical properties.