Formation of two-dimensional MoS₂ and one-dimensional MoO₂ nanowire hybrids

Abstract

Oxidation of two-dimensional (2D) transition metal dichalcogenides have received great interests because it significantly influences their electrical, optical, and catalytic properties. Monoclinic MoO₂ nanowires grow along the zigzag direction of 2D MoS₂ via thermal annealing at a high temperature with a low oxygen partial pressure. The hybrids of semiconducting 2D MoS₂ and metallic 1D MoO₂ nanowires have potential to be applied to various devices such as electrical devices, gas sensors, photodetectors, and catalysts.

Description

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have received great interests because of their outstanding electrical, optical, and catalytic properties. However, these properties are significantly influenced by oxidation. Moreover, heterostructure of transition metal oxides (TMOs) and TMDs are emerging as promising candidates for functional devices such as gas sensor, photodetector, and catalyst. Therefore, synthesizing multidimensional TMDs and TMOs hybrids are actively studied. (Molina-Mendoza et al., 2016).

Herein, we propose a new synthetic route for growth of 1D MoO₂ nanowires and 2D MoS₂ flakes hybrids via thermal oxidation of MoS₂. Oxidation into MoO₃ is common phenomenon in ambient conditions, but metallic MoO₂ nanowires grow at a high temperature with a low oxygen partial pressure. The hybrids are investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM imaging was performed with a FEI Titan Cube G2 60–300 instrument operated at 200 kV with an image aberration corrector, and SEM imaging was performed with Hitachi High-Technologies S-4800.

In Fig. 1a, SEM image clearly shows that 1D nanowires are grown along the edges of 2D MoS₂ flake, and the energy dispersive spectroscopy results in Fig. 1c confirm that they are molybdenum oxides. Moreover, Fig. 1d-e atomic-scale scanning TEM (STEM) images show that the molybdenum oxides are monoclinic MoO₂ with distorted rutile structure. The growth direction of nanowires is [100] which is same as the results in previous papers. (Vogl et al., 2019) In addition, MoO₂ nanowires are dominantly grown in six radial orientations at MoS₂ as indicated by the arrows in Fig. 1a. These orientations are consistent with the zigzag directions of MoS₂.

Fig. 1

(a) SEM, (b) STEM, and corresponding (c) EDS images of 1D MoO₂ nanowires and 2D MoS₂ hybrids. Atomic-scale STEM images (d) MoS₂ and (e) MoO₂ were taken from the marked position as a red box and blue box in (b), respectively

High contact resistance of TMDs is one of the key bottlenecks for applying TMDs to electrical devices. However, synthesizing the hybrids of semiconducting 2D MoS₂ and metallic 1D MoO₂ nanowires can be a solution. Metallic MoO₂ nanowires can be adopted as an electrode of MoS₂ directly, and hybridization between MoO₂ and MoS₂ might remove the Schottky barrier height and decrease the contact resistance. This simple synthetic route can be applied to many other 2D materials, and these 1D oxides-2D material hybrids can be used for not only electronic devices, but also catalytic, and optical devices.