Electrical Characteristic and HER Mechanism of Tungsten Disulfide Nanomaterials

Due to the promising applications of tungsten disulfide nanomaterials in the field of energy conversion and storage, efforts have been made to study and improve its electrical characteristic and HER mechanism of WS₂, such as carrier concentration (p), mobility (μ), and resistivity (ρ). According to theoretical predictions, WS₂ has the highest electron mobility in semiconductor TMDCs due to the reduced effective mass.

The electrical transport parameters of bulk 2H-WS₂ measured at room temperature were found to be: ρ = 3.37 Ω^-cm; p = 7 × 1016 cm^-3; μ = 30 cm2 -V^-1-s^-1. The electron mobility can be improved when the thickness of WS₂ is reduced to a single layer. For example, the electron mobility of multilayer and monolayer WS₂ is 44 cm² -V^-1.s^-1 at room temperature.

Similarly, the researchers studied the electronic properties of monolayer and bilayer WS₂, obtaining field-effect mobility of 50 ± 7 cm² -V^-1.s^-1 at room temperature. It should be noted that water molecules and other atoms absorbed on the surface of WS₂ can act as traps for charge carriers, which can strongly affect the electrical properties of WS₂. Therefore, in situ annealing can be used to remove adsorbed materials in order to study the intrinsic electrical characteristic of WS₂.

In the case of WS₂ nanotubes (NTs), the carrier mobility of NTs is six orders of magnitude smaller than that of bulk WS₂ due to the non-ideal field-effect behavior. the carrier concentration of NTs varies from 3.0 × 1017 to 1.6 × 1018 cm^-3, which is higher than that of bulk WS₂ (1 × 1017 cm^-3). The possible reason for the high carrier concentration of WS₂ NTs compared to the bulk may be due to the strain and defects in the curved layer.

HER mechanism, the cathodic half-reaction of water decomposition, is one of the most intensively studied electrochemical reactions. To drive the reaction, additional energy is required to overcome the activation barrier, such as electrical energy. In the presence of an applied current, the electrocatalytic HER process of WS₂ proceeds mainly through hydrogen adsorption (Volmer reaction) and desorption (Heyrovsky or Tafel reaction).

The ideal catalyst has a free energy of adsorbed atomic hydrogen close to thermal neutrality (GH≈0), which may affect the first step of the HER mechanism process. Based on DFT calculations, the researchers calculated the hydrogen binding energy of WS₂, where the S and W sides are equal (GH ≈ 0.22 eV), which indicates that WS₂ should be a reasonably good hydrogen evolution catalyst.

The edge of WS₂ is the active site and the crystal structure has a considerable influence on the activity of HER, especially the 1T structure. For example, Voiry et al. found that the metal 1T site is an important factor in enhancing the catalytic activity of tungsten disulfide nanomaterials. This can be attributed to the fact that 1T-WS₂ becomes highly distorted with a large percentage of zigzag strain in the lattice. the atomic hydrogen adsorption free energy of 1T-WS₂ monolayers can be modulated by the strain.

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

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