MoS2 Decorated on Different Metal Oxide Nanotubular Structures with a High Density of Reactive Sites for HER Reactions
Authors: Zhou Xuemei , Jin Bowen , Yang Min , Schmuki Patrik 
Published: February 2020

MoS2 is intensively investigated for decades owing to some unique chemical and physical properties. 1 For many applications, the use of nanoscale geometries is highly advantageous owing to the high specific surface area and short carrier and ion diffusion pathways that can be established.2 This is to a large extent due to the weak van der Waals force between the stacked S-Mo-S units giving it a graphene-like layer structure. 3 This is not only the origin of outstanding lubrication properties but also the key to the insertion and extraction of small foreign ions into the free space of the S-Mo-S layers.4

In this study, we report the self-organized formation of anodic molybdenum oxide nanotube arrays.5 The amorphous tubes can be crystallized to MoO2 or MoO3 and be converted fully or partially into molybdenum sulfide. Vertically aligned MoOx/MoS2 nanotubes can be formed with defined MoS2 sheets in a layer by layer arrangement. These provide a high density of reactive stacking misalignments (defects). These core–shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS2 coating. Such structures are investigated for applications in electrocatalysis (hydrogen evolution).
Furthermore, nanoscaled MoS2 has a gap broadens up to 1.8 eV when reaching monolayer thickness with a conduction band more negative to the conduction band of TiO2. In our experiments, MoS2 can be decorated on the top of anodic anatase TiO2 nanotubes (TiNTs) site-selectively. 6 The layers can be used as co-catalysts for photocatalytic hydrogen evolution. A strongly enhanced H2 evolution activity can be observed using only a nominal 1 nm thick MoS2 decoration on top of a 6 μm thick TiNT layer. We ascribe this strong beneficial effect to two factors: (i) the thin molybdenum sulfide on the top acts as an electron transfer mediator, i.e. as an H2 evolution co-catalyst; and (ii) the underlying tube layer acts as a light-to-electron harvester.

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