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Tuning the Electrochemical Properties of Metal Dopants Supported on Transition Metal Dichalcogenides
Metal-doped transition metal dichalcogenides (TMDs) have emerged as versatile optoelectronic, magnetic, and electrocatalytic materials due their tunable properties and two-dimensional structure. Functionalizing the surface of the TMD with catalytically relevant transition metal ions is a particularly intriguing strategy to generate single atom catalysts (SACs) with tunable local geometry and electronic properties. Although solution-phase methods have been developed to dope transition metal single atoms on TMDs surfaces, control over the local coordination environment of the doped metal atom remains a major challenge. In this dissertation, we develop a solution-phase synthetic method to controllably functionalize TMDs with transition metal ions. We are able to achieve control over the adsorbate morphology, ranging from single atoms to multimetallic clusters, and local coordination environment. We then utilize this range of doped TMDs to understand how metal atom coordination and clustering impact electrocatalytic activity in the oxygen evolution and oxygen reduction reactions.
We utilize colloidal WS2 nanosheets as the starting point for our synthetic method, which as synthesized, are completely inert toward metal functionalization on the basal plane. Therefore, we use n-butyllithium to generate nucleophilic sulfide sites on the basal planes that are capable of binding first-row transition metal ions. The dopants are subsequently introduced in solution and permitted to passively adsorb to the WS2 surface. Using NiCl2, it is possible to achieve Ni concentrations ranging from 9% to 47% with respect to W simply by varying the amount of NiCl2 introduced in solution. Through X-ray absorption spectroscopy (XAS) and high-resolution scanning transmission electron microscopy (HR-STEM) coupled to electron energy loss spectroscopy (EELS), we show that Ni single atoms predominant at low loadings of Ni (≤ 14 at.%) and mixtures of single atoms and multimetallic clusters exist at higher loadings. Electrochemical studies reveal that the single atom Ni-WS2 sample experiences the strongest electronic perturbation due to the WS2 nanosheet and thus, a much higher intrinsic activity for the alkaline oxygen evolution reaction.
Using the same solution-phase strategy, we dope isolated Co atoms onto the WS2 nanosheets and develop a post-sulfidation process in order to explicitly control the cobalt-sulfur coordination environment at the WS2 surface. Bulk cobalt sulfide materials are known catalysts for the electrochemical oxygen reduction reaction in neutral electrolyte, but the active site in these materials has been difficult to determine due to the non-uniform and dynamic nature of the cobalt and sulfur atoms at the surface of the bulk material. By studying our Co-doped WS2 materials with Co-S coordination ranging from monodentate to tetradentate, we show that the optimal active site for the oxygen reduction reaction features three-fold Co-S binding to the surface. These single atom Co-doped WS2 materials serve as important structural models for bulk cobalt sulfides and provide design principles for active site geometry and electronics in order to achieve more efficient electrocatalytic reactivity.