Electrochemical activation of small molecules by monolayer molybdenum disulphide

Molybdenum disulphide (MoS2) is an earth-abundant layered semiconductor that can be exfoliated into monolayer nanosheets. Due to its potential applications in catalysing electrochemical reactions, recent researches have been focused on modifying MoS2 monolayer into a highly active and cost-effective heterogeneous electrocatalyst. In this thesis, high-quality monolayer MoS2 with different polytypes was functionalised by doping different single heteroatoms onto the surface. Subsequently, the catalytic activity of as-modified MoS2 samples for electrochemical hydrogen production (HER) and carbon dioxide reduction (CO2RR) was examined. In-depth characterisations with the support of computational simulations were conducted to investigate their structural and electronic properties.
MoS2 nanosheets with the semiconducting polytype (2H), prepared via BuLi exfoliation of bulk MoS2, have been doped with single transition metal atoms to produce 2D monolayer catalysts used for the electrochemical hydrogen evolution reaction. High-angle annular dark field imaging scanning transmission electron microscopy (HAADF-STEM) show the direct proof of single transition metal atoms reside on the surface basal sites. The incorporated atoms subtly modify the electrocatalytic activities of the monolayer MoS2, which is dependent on their electronic and stereospecific properties. It is found that these dopants play important roles in tuning the hydrogen adsorption enthalpies on exposed surface S atoms and Mo atoms in HER. Electrochemical testing, characterization and computational modelling demonstrate that Co dopants can significantly enhance the HER activity by the dominant Co S interaction whereas Ni dopants substantially lowers the HER rate due to the Ni-Mo interaction on the same basal site. The two transition metal dopants show opposite doping behavior even though they are neighbors in the periodic table.
The study of the uncommon metallic MoS2 and particularly the doping effect with transition metal (TM) is rather limited due to the instability of this polytype (1T) at high temperature. A simple ambient temperature modification method using sonication was developed to dope the monolayer metallic MoS2 with various TM precursors. It is found that 1T-SMoS2 is more superior than corresponding semiconducting monolayer MoS2 (2H-SMoS2) and the inclusion of 3 wt% Pt or Pd can also further enhance the HER activity. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and X-ray adsorption spectroscopy (XAS) show the active single TM atom doping on this surface is to account for the high activity. Kinetic and density functional theory (DFT) analyses also illustrate that the metallic nature of 1T-SMoS2 greatly facilitates the first proton reduction step from water, rendering it non-rate limiting as contrast to that of 2H-SMoSM2. The inclusion of TM single doper such as Pd, despite at low loading, can offer the dramatic acceleration on the rate limiting recombination of H to H2. As a result, a bifunctional catalysis for HER over this tailored composite structure is demonstrated. The electrocatalytic ability of heteroatoms doped 1T-SMoS2 was further tested for electrochemical CO2RR. Electrochemical system coupled with in-situ Fourier-transform infrared spectrometer (FTIR) shows that *CO intermediate formed on the Re and In doped 1T-SMoS2 can be further reduced into hydrocarbons upon increasing the applied potential, suggesting that the reaction is potential dependent. The NMR results suggested that Re and In can help to facilitate the production of higher-order alcohols from the CO2RR. DFT calculations demonstrated that the incorporation of these two single atom dopants does not encourage the competitive hydrogen evolution reaction while facilitate CO2RR in our aqueous electrochemical system.

Advisor: Foord, JS
Tsang, ESC