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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