Surface organometallic chemistry for ALD growth of ultra-thin films of WS2 and their photo(electro)catalytic performances

SSCI-VIDE+ING+MZH:EQDInternational audienceElongated nanostructures with a high-aspect-ratio are known to strike a balance between large surface area and minimized charge recombination in energy conversion applications.1 Their surface functionalization with a thin catalytic layer can significantly enhance their performance. Atomic Layer Deposition (ALD) is an established method for achieving uniform coating of high-aspect-ratio surfaces with a conformal thin to ultra-thin film. ALD is based on the succession of two (or more) different self-limiting surface reactions. Understanding the surface chemistry during ALD growth, especially in the first cycles, is important for proper selection of suitable precursors, avoidance of undesired by-products, optimization of deposition conditions as well as film quality when ultra-thin films are targeted.1We here introduce a methodology of studying the surface chemistry of an ALD growth of WS2 via modeling the deposition reactions by molecular compounds in solution and on the surface of high-surface-area 3D-type substrates. The molecular model part of this method is inspired by Surface Organometallic Chemistry (SOMC)2, which brings a large range of spectroscopic and analytic tools to gain insight into the mechanism of ALD reactions, as recently shown by our group on ulktra thin film MoS2 growth.3 Bis(tert-butylimido)bis(dimethylamido)tungsten (VI) (BTBMW) and 1,2-ethanedithiol (EDT) served as tungsten and sulfur precursors, respectively. BTBMW was chosen as a tungsten precursor as there was a precedent in the literature (in collaboration with us) showing successful ALD growth of WS2 while coupling with H2S.4 EDT is an interesting sulfur alternative to H2S and provides a robust analytic handle for the molecular level monitoring of the reaction at each half-cycle. Replication of the surface chemistry in solution using a silica model, triphenylsilanol (Ph3SiOH), as well as on high-surface-area 3D silica powder as a model of silicon wafer5,6 adds complementary molecular precision in the ALD modeling. All results are compared and contrasted with the complement XPS and Raman studies that are conducted on wafers, silica powders and triphenylsiloxy derivatives, en route to molecular level comprehension of the very first stages of WS2 growth from W (VI) precursor.The developed ALD growth method was applied onto (semi)conducting 2D substrates like a Ti disk coated with photoactive TiO2 nanotubes. Then, the ALD-modified and pristine Ti disks were measured in photocurrent production tests.References: 1.Bachmann, J. Atomic layer deposition, a unique method for the preparation of energy conversion devices. Beilstein Journal of Nanotechnology vol. 5 245–248 (2014).2.Copéret, C. et al. Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities. Chem. Rev. 116, 323–421 (2016).3.Cadot, S. et al. A novel 2-step ALD route to ultra-thin MoS2 films on SiO2 through a surface organometallic intermediate. Nanoscale 9, 538–546 (2017).4.Wu, Y. et al. A Self-Limited Atomic Layer Deposition of WS 2 Based on the Chemisorption and Reduction of Bis( t -butylimino)bis(dimethylamino) Complexes. Chem. Mater. 31, 1881–1890 (2019).5.Sneh, O. & George, S. M. Thermal Stability of Hydroxyl Groups on a Well-Defined Silica Surface. J. Phys. Chem. 99, 4639–4647 (1995).6.Nyns, L. et al. HfO2 Atomic Layer Deposition Using HfCl4/H2O: The First Reaction Cycle. ECS Trans. 16, 257–267 (2019)