Surface Functionalization of 2D Transition Metal Oxides and Dichalcogenides via Covalent and Non-covalent Bonding for Sustainable Energy and Biomedical Applications

The interest in two-dimensional (2D) nanomaterials has been increased rapidly after the discovery of graphene. The 2D graphene analogs (2DGAs) with ultrathin-layered feature exhibit a unique property which their bulk counterparts cannot possess. Among various 2D nanomaterials, the transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) have attracted considerable attention from scientific communities. The availability of various transition metals and their binding states in 2D TMDs and TMOs enable them to show a wide spectrum of properties such as metals to wide band gap semiconductors. Although properties of 2D TMDs and TMOs are excellent, further enhancement of their properties is still required for cutting-edge applications. The surface functionalization of 2DGAs is a potential route for enhancing their properties. The surfaces of 2D TMDs and TMOs could be functionalized with various types of organic or inorganic nanomaterials via covalent or non-covalent interactions. The functionalization may alter the Fermi level of 2D nanomaterials, also having this potential to add extra functionalities to the basis of the fabricated devices using them. In this review, we first introduce and specify the characteristics of 2D TMDs and TMOs as two important 2DGAs. Then, we discuss their limitations and how surface functionalization may enhance their performance. The covalent and non-covalent modes of surface functionalization with organic and inorganic nanomaterials on both 2D TMDs and TMOs are then considered. The current state and challenge of the functionalized 2D TMDs and TMOs are also discussed by their application point of view in energy storage and conversion, sensing, biomedical, catalytic combustion, and (opto)electronic devices.