Nanoscopic methods for ultrathin metal chalcogenide film electrocatalysts

The urgent need for sustainable energy technologies and the global demand for cost-effective, earth-abundant electrocatalysts for the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) motivate this thesis. The work bridges the gap between idealized model systems and realistic electrochemical conditions through advanced operando methodologies, focusing on atomic-scale phase and morphological transformations as well as on the catalytic role of point defects in two-dimensional (2D) materials. Model systems were designed using ultrathin films and nanostructures of Transition Metal Dichalcogenides (TMDCs), namely MoS2 and RuSx, as well as magnetite (Fe3O4), epitaxially grown on inert Au(111) substrates. These materials were chosen due to their relevance in applied catalysis. MoS2, a promising PGM-free electrocatalyst for HER, exhibits activity at edges and defects, while its basal plane is inert. However, the catalytic role of point defects has largely been addressed only theoretically. Here Scanning Tunneling Microscopy (STM), in both UHV and operando electrochemical configurations (EC-STM), enabled real-time visualization of catalytic and corrosive processes at the electrified solid/liquid interface. Epitaxial monolayer MoS2 was prepared with systematically engineered point defects, from single sulfur vacancies to multiatomic vacancies. Operando EC-STM during HER revealed the activity of sulfur and molybdenum vacancies (VS, VMo), directly visualizing catalytic dynamics and showing how sulfo-reductive conditions reshape active edges, ultimately leading to catalyst aging. The importance of layered 2D materials in catalysis and electronics inspired the development of a novel RuSx phase with hexagonal symmetry and 1T crystal structure, distinct from bulk RuS2. Systematic UHV and electrochemical analyses revealed its structural and electronic properties. STM identified point defects analogous to MoS2, while ARPES showed a valence band structure resembling monolayer 1T-FeS2. EC-STM demonstrated its stability in acidic and alkaline environments. Upon UHV annealing, the 1T-RuSx film transformed into a honeycomb structure with near 1:1 Ru:S stoichiometry, which proved unstable in acidic electrolytes. On the anodic side of the water splitting reaction, the EC-STM methods were employed to observe the transformations of Fe3O4 nanoislands during OER in alkaline conditions. Iron oxides are fundamental materials for alkaline OER, both as standalone catalysts and as enhancers. However, previous works struggled to determine which specific phase of iron oxide or oxohydroxide is performing the reaction at the interface with the solution. High-resolution images show that the active phase symmetry is critically dependent on the support morphology and the applied potential. On flat, wide terraces magnetite transforms topotactically into the isostructural γ-Fe2O3 (maghemite), maintaining hexagonal symmetry. Conversely, on narrow and corrugated terraces (as a result of substrate oxidation at high anodic potentials) the Fe3O4 fragments and restructures into nanometric-sized domains exhibiting previously unobserved square or rectangular symmetries. The rectangular symmetry is tentatively associated with the α-FeOOH (010) (Goethite) termination. Finally, this research utilizes atomic-resolution operando microscopy to establish quantitative structure-activity relationships for key electrocatalytic materials (MoS2, RuSx, and Fe3O4), providing fundamental insights into the role of specific defects and interfacial transformations necessary for the rational design of durable and efficient 2D and nanostructured electrocatalysts.