Edge-On MoS2Thin Films by Atomic Layer Deposition for Understanding the Interplay between the Active Area and Hydrogen Evolution Reaction

The edge sites of molybdenum disulfide (MoS2) 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 MoS2. 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 MoS2cannot be inert basal planes. Therefore, MoS2may 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 MoS2, MoS2exhibits the nonideal mode of ALD growth on planar surfaces. As a model system, the ALD of MoCl5and H2S 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 MoS2to 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 MoS2films, we monitored the resulting catalytic performance, finding current densities of up to 20 mA cm–2at −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.