Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) are a fascinating class of nanomaterials that have the potential for application in catalysis, electronics, photonics, energy storage, and sensing. TMDs are rather inert, and thus pose problems for chemical derivatization. However, to further modify the properties of TMDs and fully harness their capabilities, routes towards their chemical functionalization must be identified. In this research news article we critically review recent efforts towards the chemical (bond-forming) functionalization of 2D TMDs.We highlight recent successes and also areas where further detailed analyses and experimentation are required. As detailed herein, this burgeoning field is very much in its infancy but has already provided several important breakthroughs. Graphical Abstract3

Identifying cheap, yet effective, oxygen evolution catalysts is critical to the advancement of water splitting. Using liquid exfoliated Co(OH)2 nanosheets as a model system, we developed a simple procedure to maximise the activity of any OER nano-catalyst. We first confirmed the nanosheet edges as the active areas by analysing the catalytic activity as a function of nanosheet size. This allowed us to select the smallest nanosheets (length~50 nm) as the best performing catalysts. While the number of active sites per unit electrode area can be increased via the electrode thickness, we found this to be impossible beyond ~10 m due to mechanical instabilities. However, adding carbon nanotubes increased both toughness and conductivity significantly.These enhancements meant that composite electrodes consisting of small Co(OH)2 nanosheets and 10wt% nanotubes could be made into free-standing films with thickness of up to 120 m with no apparent electrical limitations. The presence of diffusion limitations resulted in an optimum electrode thickness of 70 m, yielding a current density of 50 mA cm -2 at an overpotential of 235 mV, close to the state of the art in the field. Applying this procedure to a high performance catalyst such as NiFeOx should significantly surpass the state-of-the-art.Keywords: nano-catalyst, layered material, exfoliation, oxygen evolution reaction, sizedependence 2 ToC figToC text: Liquid exfoliation of Co(OH)2 yields suspensions of nanosheets which are easily processed and so optimised for OER catalysis. This processability has allowed the variation of nanosheet size and the production of catalytic electrodes with controlled thickness as well as the addition of carbon nanotubes to enhance electrode conductivity and strength. This has resulted in an optimised electrode design with near record performance.

Thiol-chemistry directed techniques have been extensively employed to modify the properties of two-dimensional MoS2 nanosheets, aiming to heal or functionalize sulfur vacancies. However, the exact nature of the organic thiol/exfoliated MoS2 interaction remains under dispute. Herein, the reactions between 2H-MoS2 and organic thiols are explored in detail. We quantitatively monitor the consumption of 1-octanethiol in the presence of exfoliated 2H-MoS2 nanosheets using 1H nuclear magnetic resonance spectroscopy. The generation of dioctyl-disulfide product was detected as the only product of the 2H-MoS2/1-octanethiol reaction. Furthermore, it was found that this reaction was catalytic, and was not caused by atmospheric dioxygen. In addition, we found that the following affected the kinetics of the reaction: the thickness of the exfoliated 2H-MoS2 nanosheets, the use of deuterated substrate (kinetic isotope effect = 2), and the electron donating nature of the thiol substrate. The disulfide product was postulated to be formed via the intermolecular exchange between free thiols and either adsorbed thiolate (or thiyl radicals) or desorbed thiyl radicals. Our studies demonstrate that the “functionalization” of 2H-MoS2 nanosheets with organic thiols is likely simply 2H-MoS2 catalyzed oxidation of the thiol, with no evidence for any bond-forming (covalent or dative) interaction between the thiols and MoS2.