Catalytic Behavior of Molybdenum Sulfide for the Hydrogen Evolution Reaction as a Function of Crystallinity and Particle Size Using Carbon Multiwall Nanotubes as Substrates

Abstract

Abstract Molybdenum sulfide is of interest as a noble metal-free catalyst for the hydrogen evolution reaction (HER). In crystallized form, it shows a typical stacking of planar S–Mo–S layers whereas the catalytically active centers are situated on the edges of these entities characterized by non-saturated bonds of the molybdenum atoms. In this study, 2H-MoS 2 is investigated as HER catalyst as a function of particle size using powder electrodes of different grain sizes and morphology. HER was also determined as a function of growth defects (bending of layers) and as a function of active sites employing MoS 2 nanoparticles (NP). To study the influence of the substrate on the perfection of the transition metal disulfide, MoS 2 nanosheets were deposited on multi-walled carbon nanotubes (MWCNTs) of different diameters. Highest activity was found for MoS 2 nanosheets deposited on MWCNTs with a diameter smaller than 8 nm. At diameters larger than 10 nm, a wrapping of the nanotubes by partially bended stacks of S–Mo–S layers occurs, while at diameters smaller than 10 nm, individual MoS 2 nanosheets of 3–5 S–Mo–S stacks of 3–4 nm in height and 10–20 nm in lateral extension surround the carbon nanotubes in form of hexagonal cylinders. The ratio of catalytically active non-van-der-Waals and hexagonal basal planes was determined electrochemically by electro-oxidation and correlated with HER activity.