Edge-On MoS 2 Thin Films by Atomic Layer Deposition for Understanding the Interplay between the Active Area and Hydrogen Evolution Reaction Thi Anh Ho, † Changdeuck Bae,* ,†,‡ Seonhee Lee, † Myungjun Kim, † Josep M. Montero-Moreno, § Jong Hyeok Park, ∥ and Hyunjung Shin* ,† † Department of Energy Science and ‡ KNRF Shinjin Scientist Program, Sungkyunkwan University, Suwon 440-746, Republic of Korea § Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany ∥ Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea * S Supporting Information 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. ■ INTRODUCTION Layered metal chalcogenide materials with strong anisotropy show unique behavior in terms of charge transport and phonon propagation. Covalent bonds and van der Waals (vdW) interactions hold the molecular units orthogonal to each other, and the material properties are therefore extremely directional. This anisotropic nature offers great potential for application in many fields. For example, MoS 2 is known as a solid lubricant because of its low friction characteristics. 1 Despite a tendency to easily degenerate, Bi 2 Te 3 demonstrates moderate thermoelectric properties by balancing (high) electric and (low) thermal conductivities. 2 Its relatively wider vdW gaps could likely accommodate ionic species, indicative of promising battery materials. 3 Among electrocatalysts, including noble metals, MoS 2 is ranked as one of the most efficient catalysts because of its high number of so-called “edge sites”, which are known to be catalytically active. 4 The majority of commercially viable hydrogen is produced by methane and/or oil reforming and coal gasification technologies. Only ∼4% of current commercial H 2 is produced by water electrolysis. 5 For the hydrogen evolution reaction (HER), platinum is an archetypical but scarce catalyst for one of the half-reactions for water splitting, the reaction 2H + +e − → H 2 . Finding new HER catalysts that are inexpensive and stable over the long term is urgently required. Molybdenum sulfides are strong candidates to replace Pt. 6,7 MoS 2 has been demonstrated as an efficient, cheap, and earth-abundant electrocatalyst for HER. 8,9 Interestingly, early works on the electrochemistry of MoS 2 suggested that the bulk material is not active. 10,11 The metallic edges of trigonal prismatic (2H) MoS 2 crystals were known to be responsible for HERs, 12,13 while the basal planes were thought inactive. Calculation results by density functional theory show a low Gibbs free energy of absorbed atomic hydrogen on the active edges of MoS 2 . Since the use of MoS 2 was first experimentally demonstrated for HERs, 9 its active edge sites have become prerequisites for Received: July 30, 2017 Revised: August 19, 2017 Published: August 19, 2017 Article pubs.acs.org/cm © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.7b03212 Chem. Mater. XXXX, XXX, XXX−XXX