High Surface Area MoS2 and WS2 As Active HER Catalysts

Abstract

The ever-increasing demand for clean energy necessitates a widespread adoption of clean energy carriers, namely green hydrogen. However, its production via electrolysis remains expensive. To achieve the needed drop in production cost, efficient catalyst materials are sought after to overcome the energy barrier of water splitting. Transition metal dichalcogenides (TMDCs) such as tungsten and molybdenum disulfide (WS2, MoS2) have been identified as promising catalysts because of their remarkably low Gibbs free energy for hydrogen adsorption.[1] The main challenge for these materials is the precise fabrication of continuous (mono)layers to achieve the desired functional properties.[2] Bottom-up material fabrication approaches like the vapor phase methods CVD and ALD offer several advantages. The use of chemical reactions of gas phase species allows precise thickness control, high quality films and allow large areas to be coated.This study focused on the fabrication of WS2 and MoS2 layers via metal organic chemical vapor deposition (MOCVD) and subsequent investigation of the properties of the resulting films in relation to the applied process conditions. Depositions were performed on FTO substrates and their thickness dependent capability to catalyze the electrochemical hydrogen evolution reaction (HER) were investigated in detail.For WS2 the selective growth of the semiconducting hexagonal phase with the targeted stoichiometry at a deposition temperature of 600°C was achieved (Fig 1 a). Similarly, in the case of MoS2, the hexagonal phase could be selectively deposited from 400 °C onwards and with pronounced crystallinity at 600°C (Fig. 1 b) which also showed the best stochiometric match and highest purity. The thin film characterization was further supported by Raman spectroscopy which serves as probe to verify the formed phase and layer number for 2D-materials. UV-VIS spectroscopy was employed to assess the semiconducting behavior allowing for band gap determination via the Tauc plot. Morphology and surface topography, especially interesting in the realm of HER catalysis, were assessed by SEM and HR-TEM showing the typical flakelike features known for these materials in their hexagonal phase (Fig 1 c). Electrochemical LSV experiments of WS2 in H2SO4 showed thickness dependence of the overpotentials which correlates with the flake density and the availability of vertical edge sites most active for HER (Fig 1 d). Similar studies for MoS2 are ongoing. These results are especially noteworthy as previous studies on WS2 deposited onto FTO have reported notably higher overpotentials.[3,4] This study demonstrates the potential of large area and high surface area TMDCs as effective HER electrocatalysts.[1] S. H. Noh, J. Hwang, J. Kang, M. H. Seo, D. Choi, B. Han, J. Mater. Chem. A 2018, 6, 20005–20014.[2] K. S. Novoselov, D. Jiang, A. K. Geim, et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453.[3] J.-L. Wree, J.-P. Glauber, A. Devi, et al. J. Mater. Chem. C 2021, 9, 10254–10265.[4] J.-L. Wree, E. Ciftyurek, A. Devi, et al. Dalton Trans. 2020, 49, 13462–13474. Figure 1