Edge-On MoS₂ Thin Films by Atomic Layer Deposition for Understanding the Interplay between the Active Area and Hydrogen Evolution Reaction

Abstract

The edge sites of molybdenum disulfide (MoS 2 ) have been shown to be efficient electrocatalysts for the hydrogen evolution reaction (HER). To utilize these structures, two main strategies have been proposed. The first strategy is to use amorphous structures, which should be beneficial in maximizing the area of the edge-site moieties of MoS 2 . However, these structures experience structural instability during HER. The other strategy is nanostructuring, in which, to enhance the resulting HER performance, the exposed surfaces of MoS 2 cannot be inert basal planes. Therefore, MoS 2 may need critical nanocrystallinity to produce the desired facets. Here, we first describe that when atomic layer deposition (ALD) is applied to layered materials such as MoS 2 , MoS 2 exhibits the nonideal mode of ALD growth on planar surfaces. As a model system, the ALD of MoCl 5 and H 2 S was studied. This nonideality does not allow for the conventional linear relationship between the growth thickness and the number of cycles. Instead, it provides the ability to control the relative ratios of the edge sites and basal planes of MoS 2 to the exposed surfaces. The number of edge sites produced was carefully characterized in terms of the geometric surface area and effective work function and was correlated to the HER performance, including Tafel slopes and exchange current densities. We also discussed how, as a result of the low growth temperature, the incorporation of chlorine impurities affected the electron doping and formation of mixed 2H and 1T phases. Remarkably, the resulting 1T phase was stable even upon thermal annealing at 400 °C. With the simple, planar MoS 2 films, we monitored the resulting catalytic performance, finding current densities of up to 20 mA cm −2 at −0.3 V versus the reversible hydrogen electrode (RHE), a Tafel slope of 50−60 mV/decade, and an onset potential of 143 mV versus RHE.

Two-Dimensional Material Molybdenum Disulfides as Electrocatalysts for Hydrogen Evolution

“…More interestingly, contrary to the traditional view that the number of edge sites is important, some studies also point out that the hopping efficiency of electrons in the vertical direction or the resistance plays a key role in the development of high-efficiency two-dimensional material catalysts [75][76][77]. Cao et al have investigated layer-dependent MoS 2 electrocatalytic activity by controlling the number of layers during the synthesis [76].…”

“…As the potential barriers exit in the interlayer gap, the electron transfer in the perpendicular direction is through hopping (Figure 9b), which makes the thicker sample shows poor electrocatalytic performance. Recently, Shin's group also investigated the electrocatalytic performance of MoS 2 with a different number of edge sites [77]. Their experimental results indicated that the thicker zone with the maximum number of edge sites did not exhibit the best HER performance, while the thin zone consisting of basal planes resulted in the best performance.…”

Creating Competitive Active Sites on CNTs Walls by N‐Doping and Sublayer Co4N Encapsulating for Efficient Hydrogen Evolution Reaction

“…It is imperative to develop a low‐cost hydrogen evolution reaction (HER) catalyst . Transition metals alloys and transition metals compounds, such as metal alloys, metal phosphide, metal nitride, metal carbide, and metal sulfide/fluoride with defect have shown their outstanding HER catalysis activity. Carbon materials such as heteroatom‐doped graphene, C 3 N 4 @N doped graphene and metal‐free covalent organic polymer (COPs) also showed excellent catalytic ability for hydrogen evolution reaction with easily prepared process.…”

Polarized Raman spectroscopy to elucidate the texture of synthesized MoS2

“…atomic-layer deposition (ALD) and chemical vapour deposition (CVD). 12,15,17,[27][28][29][30][31] Synthetic MoS 2 films can be amorphous or crystalline with a texture of the crystalline material ranging from in-plane oriented grains (i.e. horizontally aligned layers) to complex 3D structures with out-of-plane oriented (OoPO) grains (i.e.…”

“…15,17,27 It is well known that the differences in grain orientation of these highly anisotropic materials significantly impact film properties. 2,27,[31][32][33] Horizontally aligned grains are typically well suited for electronic applications, while OoPO grains are very efficient electrocatalysts. It is therefore of paramount importance to be able to characterize the texture of synthesized TMDs to optimize performance.…”