Surface Organometallic Chemistry for Atomic Layer Deposition Growth of Ultra-Thin Films of WS2 and Their Photo(electro)catalytic Performances

A new Atomic Layer deposition (ALD) method for the growth of ultra-thin films of WS2 was investigated by testing two tungsten precursors (hexakis(dimethylamido) ditungsten (III) and bis(tert-butylimido)bis(dimethylamido) tungsten (VI)) and one sulfur precursor (1,2-ethanedithiol). The growth chemistry of WS2 ALD was first studied by model reactions in molecular and surface organometallic chemistry (SOMC). Several SOMC tools (infrared (IR), Raman, solid- and liquid state nuclear magnetic resonance (NMR), elemental analysis, in situ quantitative determinations of by-products at the gas-solid interphase) applicable to high-surface-area powder silica provided an understanding of the surface chemistry during the initial ALD cycles, which allowed to propose possible structures of the surface sites. A successful ALD-like sequence of reactions with bis(tert-butylimido)bis(dimethylamido) tungsten (VI) and ethandithiol on porous high-surface-area silica nanobeads afforded to perform the growth of WS2 on 2D substrates (silica-covered silicon wafers, SiO2@Siwafer, and silicon oxides-covered copper grids, SiOx@Cu grid, titania nanotubes on titanium disk, TNTs@Ti disk, and carbon nanotubes CNT). X-Ray photoelectron spectroscopy XPS study conducted on the ALD-modified silicon wafers coupled with analogous XPS insight gained on the molecular models brought new insights into the oxidation state evolution and environment of tungsten during the ALD process. Gradual reduction of tungsten’s oxidation number from the initial (VI) to targeted (IV) was first observed upon the thiol pulse, and then completed during the annealing step. Monitoring the surface in situ by high-resolution transmission electron microscopy (HRTEM) supported the tungsten reduction by the formation of characteristic layered patterns and well-ordered crystalline nanodomains. Deposition onto (semi)conducting 2D substrates like a Ti disk coated with photoactive TiO2 nanotubes (TNTs@Ti disk) and conducting carbon nanotubes allowed to initiate the photocurrent and CO2 electro reduction measurement, respectively.