Most Common Form of Tungsten Disulfide Nanomaterials: Nanosheets

Nanosheets of tungsten disulfide nanomaterials are the most common form, and the main synthetic strategies can be divided into two categories: top-down and bottom-up approaches. Top-down approaches allow the production of small amounts or single-layer samples at a lower cost, which is very beneficial for basic research. Among these top-down methods, mechanical peeling via Scotch tape is the simplest method, with only a few or single layers of WS₂ exfoliated via Scotch tape.

However, large-scale production is extremely challenging and yields are very limited. In addition, WS₂ can be exfoliated in the liquid phase, which requires the assistance of ultrasound or ion intercalation: ultrasonic treatment typically results in crystal flakes with large dimensions of several hundred nanometers. Therefore, a more feasible approach is to use ion intercalation into WS₂ atomic layers to separate the flakes

For the aforementioned yield and size limitations, several techniques of bottom-up strategies are reported for the controlled fabrication of nanosheets on a given substrate by staring from the constituent elements of WS₂. Most of the proposed processes utilize precursors in vapor form and transport selected species on the substrate via a carrier gas.

Chemical vapor deposition (CVD) is a widely used technique for preparing WS₂ nanosheets, which generally uses WO₃ as the W source and S powder or H₂S as the S source. For example, Hu et al. prepared WS₂ on SiO₂/Si substrates by a conventional low-pressure CVD method and proposed a corresponding growth mechanism.

In addition, atomic layer deposition (ALD) is a modified CVD technique in which two precursors are set up separately in the growth chamber, and its main advantage is the controllable thickness of WS₂ by varying the number of cycles of ALD. ALD is still not widely used to fabricate WS₂ nanosheets, mainly due to the lack of a suitable ALD chemistry.

Due to the instability of planar nano-S-W-S sheets of tungsten disulfide nanomaterials with a large number of overhanging bonds at the edges of the layers, closed structures such as IF-WS₂ and NT-WS₂ are formed by specific preparation methods. The preparation methods for producing IF-WS₂ nanoparticles and NT-WS₂ are broadly divided into two groups:   (1) processes such as arc discharge, electron beam irradiation, and laser ablation, and (2) chemical methods close to equilibrium, such as chemical gas-solid reactions.

Article Source: Sun, CB., Zhong, YW., Fu, WJ. et al. Tungsten disulfide nanomaterials for energy conversion and storage. Tungsten 2, 109–133 (2020).

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