Edge-On MoS2 Thin Films by Atomic Layer ... - ACS Publications

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Edge-On MoS2 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 (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 MoS2 cannot be inert basal planes. Therefore, MoS2 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 MoS2, MoS2 exhibits the nonideal mode of ALD growth on planar surfaces. As a model system, the ALD of MoCl5 and 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 MoS2 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 MoS2 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.

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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− → H2. 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 MoS2 has been demonstrated as an efficient, cheap, and earth-abundant electrocatalyst for HER.8,9 Interestingly, early works on the electrochemistry of MoS2 suggested that the bulk material is not active.10,11 The metallic edges of trigonal prismatic (2H) MoS2 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 MoS2. Since the use of MoS2 was first experimentally demonstrated for HERs,9 its active edge sites have become prerequisites for

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, MoS2 is known as a solid lubricant because of its low friction characteristics.1 Despite a tendency to easily degenerate, Bi2Te3 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, MoS2 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 H2 is produced © 2017 American Chemical Society

Received: July 30, 2017 Revised: August 19, 2017 Published: August 19, 2017 7604

DOI: 10.1021/acs.chemmater.7b03212 Chem. Mater. 2017, 29, 7604−7614

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Chemistry of Materials achieving efficient reactions,14,15 although recent work has objected to this assumption.16,17 Several strategies have been proposed to further improve the HER performance: (i) combining MoS2 with conductive materials to support fast electrical transport,18,19 (ii) engineering the phases of MoS2, for example, by converting the semiconducting 2H polymorph to its metallic 1T phase, which exhibited enhanced HER activities,20−25 and (iii) introducing defects or dopants into MoS2, which also dramatically enhanced its resulting catalytic activities.26,27 In recent decades, as a result, the HER performance of MoS2 has been greatly improved. For instance, an onset potential of approximately 100 mV and a Tafel slope of 40−60 mV/decade, which are close to those of Pt, have been obtained.28 Synthesizing MoS2 can be achieved by various methods, including hydrothermal, chemical vapor deposition, liquid exfoliation, wet chemical synthesis, and electrodeposition techniques.29−33 The HER performance of MoS2 is dependent on the preparation procedures, and correlating the performance with the variously fabricated structures is thus challenging in most cases. If a reliable methodology could not only produce crystalline MoS2 but also control the amounts of edge sites, we could formulate general design principles for applying MoS2 to HER. Tan and co-workers demonstrated the formation of crystalline MoS2 by atomic layer deposition (ALD) and subsequent high-temperature annealing (700 °C) under a sulfur environment.34 Similar ALD processes have been applied for lithium ion batteries and electrocatalysts for HER.35,36 Min and co-workers also reported ALD of MoS2, but the resulting structures were amorphous.34 ALD is advantageous in that it provides (i) control over film thicknesses at the level of atomic layers and (ii) the capability of conformal coating on many different substrates once the method has been well established.37,38 However, the direct growth of nanocrystalline MoS2 by ALD remains elusive.39,40 Moreover, whether ALD operates well for layered materials such as MoS2 has not yet been answered (see below). An open question is how ALD of layered materials works if a directional component predominates over the others or, in other words, how to control anisotropic growth during ALD. This would allow for a general understanding of ALD for layered materials and would help to solve issues on correlating the edge sites and the size. Scheme 1A illustrates highly textured in-plane growth via vdW attachments when growing layered materials by ALD. In contrast, Scheme 1B represents a vertical growth mode where the layer-by-layer growth along the basal plane with strong covalent bonding of MoS2 is common. Both are the result of the same ALD chemistry if vdW secondary nucleation does not take place (see the upper row of Scheme 1). However, each growth mode determines the final growth rate by which the mode operates, even though identical chemistry is employed (see the graphs of panels A and B). Here, we demonstrate that the nonideal growth mode operates when layered materials such as MoS2 are deposited by ALD on planar surfaces (see Scheme 1C). ALD of MoCl5 and H2S was studied as a model system, and nonideality was observed between the growth rate and the number of cycles. Remarkably, the method provided the ability to control the relative amounts of MoS2 edge sites and basal planes on the exposed surfaces. The amounts of MoS2 edge sites were correlated to the HER performance by carefully characterizing the geometric surface area along with the effective work function and impedance.

Scheme 1. (A, B) Two Distinctive Growth Modes of Layered Materials Such as MoS2 Grown by ALD and (C) So-Called “ALD Window” with Four Factors Affecting the Nonideality of ALD Processes (Condensation, Decomposition, Activation Energy, and Desorption)a

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This plot revisits the ALD process of layered materials by adding an additional factor, strong structural anisotropy (see the solid black line).

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EXPERIMENTAL SECTION

Atomic Layer Deposition of MoS2 Films. MoS2 films were successfully grown on different substrates such as Au/Si, SiO2/Si wafer, and FTO-coated glass at 250−325 °C using a commercial ALD system (Lucida D-100, NCD, Daejeon, Korea). The ALD chamber was heated and stabilized for 30 min before the reactants were supplied. The MoS2 film was formed using a sequence of MoCl5 (99.6%, STREM Chemicals, Newburyport, MA) and H2S (3.99%, balance N2, JC Gas, Korea) as the reaction precursors under a carrier gas of N2 (5 N, JC Gas, Korea). The pulsing times of MoCl5 and H2S were controlled by high-temperature pneumatic valves (Kitz Co., Japan) with exactly 1 s of pulsing followed by 30 s of purging of N2 with a gas flow rate of 200 sccm. Approximately 200 nm of Au was deposited on a silicon wafer by e-beam evaporation. The uniform film of MoS2 was easily recognized by color changes on the substrates (uniform zone with an average width of 7.05 cm). FTO substrates were cut to 2 × 2 cm2 in size and exhibited the same zones as the Au wafer. General Characterization. The morphologies were observed by field-emission scanning electron microscopy (FESEM; JSM7500F, JEOL, Japan). Structures were investigated by energy-dispersive X-ray spectroscopy (Aztec, Oxford Instruments, United Kingdom) together with high-resolution transmission electron microscopy (HRTEM; JEM 2100F, JEOL, Japan). Cross-sectional samples were prepared by focus ion-beam etching (SMI305TB, SII, Japan). Surface chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS; ESCA Sigma Probe, Thermo VG Scientific). MoS2 formation was characterized by Raman spectroscopy (RM1000 microprobe, Renishaw, United Kingdom). Hall-effect measurements were performed with a commercially available measurement system (HMS-5300, Ecopia Corp., Korea). Electrochemical Characterization. All electrochemical measurements were performed with a three-electrode system with Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and MoS2 as the working electrode using either the SP-200 or VMP3 potentiostat/galvanostat, Bio-Logic. Cyclic voltammetry and linear sweep voltamperometry were performed with a scan rate of 5 mV/s in 0.5 M H2SO4 electrolyte. Electrochemical impedance spectra were measured over a frequency range of 106 to 0.1 Hz at a potential of 7605


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Figure 1. (A) Proposed nucleation mechanisms of MoS2 on Au via ALD. TEM micrographs showing the structures of MoS2 on (B) granular gold and (C) thin-film gold. Clearly, the MoS2 layers initially grew parallel to the Au surfaces. (D) SEM image of a fractured cross-section of thick MoS2 layers, indicative of the loss of texture orientation upon subsequent growth. −0.2 vs the reversible hydrogen electrode (RHE). Mott−Schottky impedance was performed at a frequency of 7.8 Hz from −0.6 to 0 V vs RHE in dark conditions. The reference electrode was calibrated to the RHE potential with an electrolyte of 0.5 M H2SO4, E(RHE) = E(Ag/AgCl) + 0.21 V. The typical electrode area was defined as 0.785 cm2, which is equal to the exposed area of the MoS2 electrode.

might occur as shown schematically in Figure 1A. Upon delivery of the MoCl5 precursor and sequentially with the sulfur source, H2S, the initial reaction would involve the formation of MoClnSx (MoCl5 → MoClnSx).41,42 During the course of the reaction with H2S, MoSx would form (MoClnSx → MoSx) at the surface. Because of the incomplete chemical reaction at the relatively low temperature, the MoClnSx will still exist, resulting in the strong coordination of the Cl 2p peak with Mo and S, as indicated below by XPS. Remarkably, the presence of chlorine inside the host MoS2 structure did not alter the distance of the vdW gaps, i.e., the position of the (002) peak, regardless of the MoS2 microstructure, including all 2H, 1T, and 3R phases (Figure S1, Supporting Information). However, the (100) diffraction peak clearly shifted to a lower diffraction angle of 31.42° instead of 32.68°. This indicates that the interlayer distance of the (100) plane changed to 2.84 Å and caused a 3.52% distortion of the crystal structure. It also indicates the formation of Cl inside the structure. From HRTEM lattice images of the as-deposited MoS2 on Au in Figure 1B,C, lattice fringes can be clearly observed, suggesting that the defined crystal structures of several MoS2 layers are formed on the surface of Au. An interplanar spacing of 6.15 Å was observed for the (002) plane. Notably, after several parallel layers away from the surface of the gold, the (002) plane apparently started to change its direction and was no longer parallel to the surface (Figure 1C,D). A higher resolution cross-sectional SEM image (inset in Figure 1D) clearly shows a relatively thick layer of initial MoS2 growth parallel to the surfaces (or randomly oriented). With the present methodology, we first confirmed the formation of layered MoS2 structures on various substrates, including SiO2/Si, Au/Si, and FTO/glass, by Raman spectroscopy (Figure 2A,B). The difference (Δ) of two characteristic Raman modes (in-plane, E12g, and out-of-plane, A1g) has been used to identify the number of layers in MoS2.43 The resulting MoS2 films have a Δ value of approximately 25 cm−1, which is assigned to the bulk of MoS2. Although this value did not change with the substrate type, the subtle spectral shifts might

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RESULTS AND DISCUSSION ALD of MoS2. To prepare MoS2 via ALD, the most intuitive design for ALD chemistry would be to employ MoCl4 and H2S in the reaction MoCl4 + 2H2S → MoS2 + 4HCl. Note that crystalline MoS2 is chemically inert, so it is not attacked by the byproduct, HCl, upon reaction. However, choosing MoCl4 as the precursor is not practical for ALD systems since its melting point is approximately 552 °C (410 °C for MoCl3). An alternative could be MoCl5, with a melting point of approximately 194 °C. Nonetheless, the direct formation of crystalline MoS2 by ALD at reasonable ALD growth temperatures has not been reported to date in the literature. Here, MoS2 was grown on various substrates at 250 °C using a commercially available ALD system (Lucida D-100, NCD, Korea). The chamber is a so-called flow-through-type reactor, 6 in. in diameter. MoCl5 (99.6%, STREM Chemicals, Newburyport, MA) and H2S (3.99%, balance N2, JC Gas, Korea) were used as reactants. MoCl5 was kept in a stainless canister at 140 °C and delivered into the chamber. Ultra-high-purity N2 (5 N, JC Gas, Korea) was used as both the carrier and purging gas. The total flow rate was 200 standard cubic centimeters per minute (sccm). A full cycle of ALD consisted of injecting MoCl5 for 0.5−5 s and H2S for 1 s, followed by purging with N2 for 30 s. Increasing the pulse duration (tp) of the MoCl5 resulted in a larger deposition area, and a tp of ca. 5 s was chosen to cover wafers 4 in. in diameter and for our chamber design (see Figure 3A, for example). Initial surface-limited chemical reactions occur on the substrates in the sequence of exposing MoCl5 and H2S vapor at 250 °C and forming several MoS2 layers parallel to the surface of the substrates, for example, Au surfaces. The reaction 7606


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adventitious carbon maintains a similar position. The main peak at 284.5 eV is the expected C 1s major peak (C−C), and the left-side shoulder peaks at 285.6, 287.05, and 289.36 eV correspond to C−N, C−O and OCO. These results strongly indicate that our MoS2 contains the 1T phase. Simple differences in the symmetry of the molecular units (−S−Mo− S−) create two different 2H and 1T polymorphs, likely due to the presence of excess Cl during growth (Figure 1A). Notably, the XPS survey scans show a discernible Cl 2p peak in Figure 2C (∼8% on average). In the high-resolution scans, the parallel shift of Cl 2p1/2 and Cl 2p3/2 was also observed, similarly to the S 2p spectra (Figure 2F). The amounts of Cl were not significantly different from those in the growth zones (Figure S3, Supporting Information). Note that the resulting 1T phase was stable upon thermal annealing at 400 °C (Figure S4, Supporting Information). We observed 0.3 eV peak shifts to higher binding energies in the cases of Mo 3d, S 2p, and Cl 2p that might be a result of some of the internal chlorine diffusing and becoming Cl2 gas under such a reducing environment. The Mo 3d and S 2p peaks still indicate the formation of mixed phases of 1T and 2H. Porous, nanoflake-like morphologies are exhibited by the MoS2 films grown by ALD at 250 °C on 100 nm thick Au/Si substrates (with ∼10 nm thick Cr in between as an adhesive layer), as shown in Figure 3B,C. Figure 3A shows a photograph of the whole wafer of MoS2 films, wherein three distinctive regions from the inlet to the outlet are indicated as zones I, II, and III. Corresponding planar and cross-sectional SEM images show the morphological differences between zones I, II, and III in the same sample batch (in Figure 3B). Within the same zone, the morphology of the films is homogeneous. The size and density of the MoS2 nanoflakes increase from zone I to zone III. The MoS2 nanoflake thickness also increases from 22 to 60 nm from zone I to zone III, respectively. High fluctuations in the precursor pressure are expected near the inlet, so a smaller amount of MoS2 is formed. The precursor pressure is more stable near the outlet, which increased the density and size of the nanoflake MoS2, as shown in Figure 3B for a sample of 1000 ALD cycles of MoS2 on Au. Zone II is relatively larger than zones I and III and can be considered a uniform region during the ALD process. Different zones in the same batch of a sample can be easily identified, with the uniform zone II having a width of 7.05 ± 0.89 cm on average. Upon increasing the number of ALD cycles (500, 700, 1000, and 1500), the density and size of the nanoflakes also increased, as shown in Figure 3C. The thicknesses measured at the center of the wafer by varying the number of ALD cycles showed the linear relationship, indicative of the self-limiting behavior (Figure S5A, Supporting Information). Interestingly, few nanoflakes seem to be protruding from the flat films (500-cycle panel in Figure 3C). For the first few hundred ALD cycles, flat, layered MoS2 films are deposited, but subsequently, some nanoflakes, which are nearly parallel to the surface, start to nucleate and then protrude (see Figure 1). The homologous morphological structures were observed with MoS2 deposited on fluorinedoped tin oxide (FTO) substrates over various numbers of ALD cycles (Figure S5B), implying similar deposition processes onto both Au and FTO substrates. To evaluate the structures of as-grown MoS2, we carried out a grazing incidence X-ray diffraction (GI-XRD) analysis at different positions (Figure 3D) and with different numbers of ALD cycles (Figure 3E), as demonstrated by the GI-XRD results of the samples deposited with 50, 100, 200, 500, and

Figure 2. Raman spectra of ALD-grown MoS2 on (A) Si and (B) Au and FTO substrates. The characteristic Raman modes (E12g and A1g) are labeled. (C−F) High-resolution XPS spectra of MoS2 on Au: (C) survey scan and (D) Mo 3d, (E) S 2p, and (F) Cl 2p core level peaks.

be a result of the presence of Cl, as discussed later. The lack of Raman peaks exhibited by MoS2 films after 50 cycles of ALD (solid green line, Figure 2A) implies either the presence of an incubation period of the present processes or the sparse nucleation probability on the Si substrate with native oxides. The incorporation of chlorine likely presents as an impurity inside the layers as a result of the low growth temperature. Moreover, such an inclusion could also affect the structural evolution during crystal growth. Accordingly, we investigated the XPS spectra of our MoS2 layers for the Mo 3d, S 2p, and Cl 2p regions, as shown in Figure 2C−F. The spectrum of the Mo 3d core level in Figure 2D shows the major doublet at binding energies (Eb) of 228.7 and 231.9 eV (solid blue lines) as well as the contribution of two shoulder peaks at 229.6 and 232.8 eV (solid green line). Note that the Eb of 229.6 eV could be assigned to 2H MoS2, indicative of the oxidation state of Mo4+.44 Our Mo 3d spectrum displays two major doublets shifted to the lower Eb of about 0.9 eV, which is attributed to the appearance of the 1T phase.45 Note that the higher oxidation states, such as 5+ and 6+, have large binding energies with respect to the position of the 4+ oxidation state, while those from 1T MoS2 exhibited red shifts. Such a trend was also detected in the S 2p region of the spectra (Figure 2E). The parallel spectral shift of these peaks to 162.62 and 161.5 eV, respectively, suggests the creation of the 1T phase (solid blue lines). Therefore, the resulting MoS2 layers grown by ALD exhibit a mixture of 2H and 1T polymorphs. For further confirmation, we compared the XPS data of our MoS2 with those of commercial 2H MoS2 (Figure S2, Supporting Information). Remarkably, the results of the high-resolution XPS of Mo 3d and S 2p indicated that our samples possess a lower binding energy compared with that of commercial 2H MoS2 by approximately 0.9 eV, while the C 1s peak of 7607


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Figure 3. (A) Photograph of MoS2 thin films grown on a 4 in. Au/Si wafer. Corresponding SEM micrographs and XRD patterns (B and D, respectively) at different positions on the wafer and (C and E, respectively) with varying numbers of ALD cycles. The density and size of the flakes increase with increasing number of cycles.

Figure 4. TEM results of MoS2 layers at different positions on the same substrate to investigate the orientation of the basal planes of our ALD-grown MoS2: (A) photograph of MoS2 thin films as given in Figure 3A; (B) low- and (H) high magnification of MoS2 at zone I; (C) low and (I) high magnification at zone II; (D) low and (J) high magnification at zone III; (E−G) electron diffraction patterns corresponding to panels B−D, respectively.

changed at different positions (Figure 3D) after sufficient ALD cycles (i.e., 1000 cycles). Thinner MoS2 areas (positions 1 and 2, which are closer to the chamber inlet in the inset of Figure 3D) exhibited stronger (002) diffraction and vice versa (position 4 close to the chamber outlet). This would imply that, in addition to the resulting thicknesses, the underlying microstructures of MoS2 develop differently at different

1000 ALD cycles on the Au/Si wafer. After 50 cycles, there are almost no MoS2 peaks, but broad, well-defined (002) plane peaks started to appear after 100 cycles. The intensity of the MoS2 (002) peaks gradually increases with the number of ALD cycles. A strong (002) diffraction peak in all the XRD spectra illustrates the ordered stacking of MoS2 single layers in the structure.46 Remarkably, the intensities of the (002) peaks 7608


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Figure 5. HRTEM micrographs of MoS2 layers (1000 ALD cycles) grown on 500 nm thick SiO2/Si in zone III. The vertically aligned basal planes were grown onto random patches of MoS2 that had initially been nucleated.

the surface leads to discontinuous films on the imperfectly flat surface of SiO2/Si substrates. In the discontinuous MoS2 films, new layers can be nucleated and grown in different directions, thus finally forming the vertical nanoflake films at zone III. While the randomly oriented nanoflake MoS2 makes the film (meso)porous and basal plane terminated (zone I, Figure 4B,H), vertically grown single-nanoflake MoS2 consists of a large amount of exposed edge sites (zone III, Figure 4D,J). Accordingly, our ALD-grown nanoflake MoS2 films with this unique morphology would guarantee high catalytic activity for HERs. Moreover, the incorporation of 1T MoS2 should lead to synergetic contributions for HERs, yielding a profound understanding of the correlation between the charge transfer, the edge sites, and the crystallinity, as will be discussed. Given that large amounts of chloride ions (approximately 5− 10 atom %) were detected and modified the resulting structures into the mixture of 2H and 1T phases, a detailed investigation into the functions of chloride ions is required. Indeed, a Mott− Schottky analysis showed the presence of significant amounts of negatively charged species, such as Cl ions, that had a density of ∼1023 cm−3, as shown in Figure 6A. This order of magnitude is in good agreement with the chemical analysis results. The doping density was measured by the Hall measurement technique with van der Pauw geometry. Our mixed-phase MoS2 layers showed typical semiconducting behavior by demonstrating increased conductivity with increasing temperatures. However, our MoS2 layers have not exhibited photoactivity under simulated solar light (data not shown). Indeed, the results exhibited an electron density of ∼1019 cm−3 (Figure 6B,C) and indicate that the Cl dopants are not fully thermally activated. Under the assumption that the defect levels are in the band gap of MoS2, the Arrhenius equation was applied and resulted in the presence of shallow levels (∼50 meV) beneath the bulk conduction band minima (Figure 6D). Therefore, the presence of Cl in MoS2 in the resulting formation of the 1T phase contributed to the high conductivity of our MoS2 film, which could not be found in only the 2H phase itself. Hydrogen Evolution Reaction. Not only do our nanoflake MoS2 films show a high density of exposed edge sites and bring advantages in terms of electron/proton transfer, but they also easily drive off the produced gas bubbles.48 All the different MoS2 nanoflake morphologies were used as catalytic electrodes for HER in an electrolyte solution of 0.5 M H2SO4. Generally,

positions. If thinner MoS2 has a textured orientation along the surface of the Au, the (002) diffraction can be pronounced. Therefore, this picture might provide a clue to the growth mechanisms of layered materials in accordance with the flow coming from the inlet. Note that the ALD of typical oxides, including Al2O3 and TiO2, did not result in such thickness gradients when grown in the same chamber. The average crystallite size of MoS2 in the 500- and 1000-cycle samples, which is determined by the broadening of the (002) diffraction peak using Scherrer analysis, 47 was 6.1 ± 0.16 nm, corresponding to approximately 10 layers. These results have a grain size similar to the size calculated from SEM images of the 500- and 1000-cycle samples (Figure 3B,C). As discussed in Scheme 1, the ideality of layered materials fabricated via ALD closely pertains to the nucleation and growth along their basal planes. Without the use of certain host substrates, such as single crystals for epitaxial growth, however, the orientation of nucleation and growth is expected to be random, rendering polycrystalline layers. As probed in the XRD results, the formation of textured MoS2 around the inlet area suggests rather complex growing mechanisms. One indicator is much thinner layers of 10−20 nm thickness in zone I as compared to ∼100 nm in zone II with the same number of ALD cycles (1000 cycles, Figure 4). This implies that, in addition to the adsorption of precursors, desorption processes are also dominant near the inlet (that is, zone I) during growth. In contrast with the ALD of oxides where hydroxyl moieties are repeatedly recovered, there exists no clear chemistry for strong binding of MoS2 onto Au. Thus, physisorption is a major process in introducing precursor molecules onto the surfaces, and the in-plane growth is more favorable (Scheme 1A), while vertical growth is suppressed via the desorption of precursors around zone I. Near the outlet (zone III), the gas flow becomes stable, which can promote the vertical growth. As a result, different growth zones (I, II, and III) were observed with a thickness gradient, as shown in Scheme 1 and Figure 4. Note that the MoS2 in zone III is edge terminated, while in zone I it is nearly all basal plane terminated on the surface, as shown in the cross-sectional TEM image and described above. That is, the exposure of the MoS2 edge sites can be tailored by controlling the growth mode. Moreover, even for zone III, where the vertical growth takes place, nuclei lie at the root via the in-plane growth (see Figure 5A). Since the ALD temperature is low, the inefficient diffusion of precursors to 7609


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Figure 6. (A) Mott−Schottky analysis of capacitance−voltage measurements of ALD-grown MoS2/Au(100 nm)/Si at 4.8 Hz under dark conditions, (B) carrier density vs temperature, (C) conductivity vs temperature, and (D) Arrhenius plot of the resistivity from Hall-effect measurements on ALD-grown MoS2/SiO2(500 nm)/ Si.

Figure 7. Growth-zone-dependent HER performance of our ALDgrown MoS2 on different electrode substrates at a fixed number of ALD cycles (i.e., 1000) for comparison. (A, C) Linear sweep voltammetry and (B, D) Tafel curves of MoS2 on (A, B) Au/Si and (C, D) FTO/glass.

the electrochemical capacitance surface area measurement was performed to estimate the active area of the films. The capacitive current was measured as a function of the scan rate, which was then used to calculate a roughness factor of RF = 1.71 in the case of 1000 ALD cycles of MoS2 on a Au wafer (Figure S6, Supporting Information). Figure 6A shows the electrochemical impedance Mott−Schottky analysis, which shows the n-type behavior of MoS2, with a flat-band potential of approximately −0.25 V vs RHE. This flat-band potential has been predicted to be close to the onset potential for the electrocatalytic activity of MoS2.49 To demonstrate the catalytic activities of the MoS2 electrode, parts A and B of Figure 7 present the polarization curves and the corresponding Tafel plots of a bare Au electrode, ALD-grown nanoflake MoS2 films on Au, and a sputtered Pt film on Si. As a result of the optimum nanocrystallinity, our MoS2 electrodes show one of the best performances among planar MoS2 electrodes. The onset potential of ∼143 mV is close to the approximately 0 V onset potential of Pt, thus demonstrating promising catalytic performance comparable with that of expensive Pt electrocatalysts. It also exhibits a higher current density of 10 mA/cm2 at a bias of 260 mV. Moreover, a smaller Tafel slope is more advantageous for practical catalytic applications because it is caused by a rapid increase in HER rate vs overpotential.50,51 By fitting the Tafel plots to the equation η = δ log j + log jo (where η is the overpotential, j is the current density, jo is the exchange current density, and δ is the Tafel slope), slopes of approximately 130, 60, and 30 mV/decade were obtained for Au wafers, nanoflake MoS2 films on Au, and Pt, respectively. These Tafel slopes suggest that the kinetics of HER on MoS2 films occur via both Volmer−Heyrovsky and Volmer−Tafel reaction mechanisms, as indicated by slopes of 60 mV/decade and higher. Together with the direct contact of MoS2 on conductive electrodes, such kinetics were caused by a low ohmic contact resistance and rapid charge transfer, resulting in higher HER efficiency.

To achieve the best HER performance, remarkably, we observed a trade-off between the quantity of edge sites and the charge transfer resistance (Figure 7A). As analyzed earlier, MoS2 films in zone III have the maximum number of edge sites (Figure 4D,J). However, they did not produce the best HER results. Although the exposed surfaces of MoS2 films in zone I consist of basal planes (Figure 4B,H), they resulted in the best performance. Therefore, future studies should take into account, for example, the resistance due to the thickness and/or surface roughness. Similar behaviors were observed from the series of nanoflake MoS2 samples that were deposited on FTO (Figure 7C,D). The MoS2 films grown on Au in zone I by different numbers of ALD cycles show different catalytic performances, as shown in Figure 8A,B. The current density gradually increases with the number of ALD cycles, except for the thickest samples investigated (∼1500 ALD cycles). Typically, the MoS2 films from 1000 ALD cycles displayed the best performance with higher current densities and lower Tafel slopes for both substrates (Au and FTO). Remarkably, there exists a critical thickness of MoS2 layers to optimize HER performance (black arrows of panels A and C). This could simply be attributed to the increasing material resistance of the thicker layers, thereby limiting electron transfer for HERs.52 Further correlating the resulting HER performance to the morphological developments during ALD is also necessary to fully understand our observations. Since we assumed that the catalytic activities of MoS2 films are influenced by the number of exposed edge sites, the dependence of the nanoflake density on the number of ALD cycles can lead to the catalytic activities of MoS2, as indicated by the current densities. The active area is plotted together with the flake density, which is calculated on the scale of SEM images and normalized to the whole electrode area with different samples (Figure S7, Supporting Information). The active area exhibits a linear fit and demonstrates the dependence of the increase in the number of exposed edge sites 7610


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activities of the sample grown at 250 °C exhibited the highest catalytic efficacy by demonstrating the highest current density and lowest Tafel slope among all samples investigated (Figure S11, Supporting Information). Stability is also critical in evaluating MoS2 catalysts for HERs; although amorphous structures are beneficial in maximizing the area of the edge-site moieties of MoS2, they exhibited structural and compositional instability during HER. Figure 9 shows the

Figure 8. Thickness-dependent HER performance of our MoS2 films taken from zone I on (A, B) Au/Si and (C, D) FTO/glass. (A, C) Linear sweep voltammetry and (B, D) Tafel curves of ALD-grown MoS2. Solid black arrows indicate the presence of a critical thickness for optimizing the HER performance.

on the number of ALD cycles. However, this was not the case, as shown in Figure 8. Therefore, we suspect that, in addition to the resistance, a certain degree of surface corrugation is needed for efficient HERs to influence the release of the H2 bubbles. Indeed, as the MoS2 continued to grow, the surfaces became smooth (compare panels B−D with the others in Figure 4), although the quantity of exposed edges increased. Note that the overall conductivity of the system limits the resultant electrochemical performance via the current densities, regardless of the electrode (Figure S8, Supporting Information). Unsurprisingly, the charge transfer and internal resistance of a 1000-cycle MoS2 on a Au wafer showed lower values than those of 1000-cycle MoS2 on FTO. Therefore, the spectroscopy revealed the role of the substrate in the series resistance of the system in addition to the material and solution resistances. The influence of the growth temperature and postannealing was also investigated. For example, samples of ALD cycles annealed at 500 °C for 1 h showed a similar trend in the catalytic activity compared with those of the as-deposited samples, but with much smaller current densities (Figure S9, Supporting Information). We believe that the thermal annealing results in phase transformation of 1T structures, restoring partially back to 2H MoS2 as known in chemically exfoliated MoS2.53 The HER activities of MoS2 grown at various temperatures (250, 275, 300, or 325 °C) were also studied. In the XRD patterns, stronger (100) diffractions were observed, while the (002) diffraction peak diminished with increasing growth temperatures (Figure S10, Supporting Information). Specifically, the diffraction peaks of samples grown at 275, 300, and 325 °C are, respectively, at 32.0°, 32.1°, and 32.17° instead of 32.68° in the JCPDF card of 2H MoS2. These results suggest that the growth temperature strongly affects the incorporation of Cl inside the structure. By observing a rapid growth of the XRD peaks with increasing growth temperatures, we could suspect the formation of alloy structures with Cl rather than doping. If the chemical vapor deposition (CVD) components dominate the growth, the mechanism could be different from that of ALD. The catalytic

Figure 9. (A) Polarization curves, (B) XPS spectra of 1000 cycles of MoS2 taken before and after the 1000-CV-cycle stability test, and (C) 50 h stability test of MoS2 (1000 ALD cycles) on Au/Si at a constant potential of −0.5 V vs RHE in 0.5 M H2SO4 solutions (pH 0.4).

results of the stability test of 1000 ALD cycles of MoS2 and chronogalvanometry at a constant applied potential of −0.5 V vs RHE in an acidic electrolyte solution (0.5 M H2SO4). The measurements were carried out under continuous evolution of H2 conditions. There were many bubbles on the electrode surface, which required minimal stirring of the solution at 200 rpm to dislodge the bubbles, thus allowing the reaction to proceed. We observed an excellent durability over 1000 cycles of cyclic voltammetry (CV) measurement of the 1000-ALDcycled sample. The current density and onset potential remain the same in such a highly acidic electrolyte. Figure 9C shows the current density as a function of time. After 24 h of measurement, the current density showed less than 10% degradation. After the electrolyte was refreshed, the current density stabilized with a slight improvement. The XPS investigation before and after the stability tests confirmed the stable operation during measurements. After 1000 cycles of CV measurement, the elemental composition of the sample was carefully analyzed (Figure 9B). No large changes were observed after the stability measurements. The increase in the O 1s peak and the decrease in the Cl 2p peak were evident. This could be explained by surface oxidation during the HER in the acidic solution. Correlation between Structure and HER Performance. We further investigated the correlation between the resulting structures and HER performance by both surface work function (Φ) measurements and an impedance analysis. Kelvin probe force microscopy (KPFM) was employed to study Φ on the edge-site portions of MoS2 layers. The geometrical areas of edge sites were quantitatively estimated at different zones with electron micrographs. The corresponding Φ values were 7611


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importance for HERs because it can more easily expel the gas bubbles produced on the surfaces of MoS2.

measured by means of KPFM spectroscopy at least 50 locations, following our previously published methodology.54,55 Parts A and B of Figure 10 summarize the results. While the

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CONCLUSIONS We demonstrated ALD of MoS2 as a representative example of a layered material. The nonideal mode of ALD growth on planar surfaces is described for the first time in the present work. This nonideality has not been observed before in the conventional linearity between the growth thickness and the number of cycles, and it provided control over the relative ratios of edge sites and basal planes of MoS2 to the exposed surfaces. As a result of the low growth temperature, the incorporation of chlorine impurities was observed. The number of edge sites was carefully characterized in terms of geometric surface area and effective work function and correlated to the HER performance, including Tafel slopes and exchange current densities. Indeed, the present study revisits the ALD processes of layered materials and provides insight into the growth mechanisms of growing layered materials in general. Moreover, we were able to determine the key factors in the HER performance of MoS2 by demonstrating the growth kinetics in a controlled manner on the same substrate using ALD.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03212. Detailed XRD, XPS, and SEM analyses of the MoS2 films and electrochemical impedance and capacitance measurements (PDF)

Figure 10. (A, B) Correlation between the geometrical area of MoS2 edge sites and the local work function by KPFM measurements at different growth zones (i.e., 1000 ALD cycles). (C, D) Impedance analysis of MoS2 layers grown on Au/Si substrates as functions of the different zones and the number of ALD cycles.

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edge-site portions of MoS2 layers increased with the distance from the chamber inlet as described earlier, the corresponding Φ values decreased. These results can be commonly understood within the framework of the growth mechanisms. In zone I, accordingly, basal planes were exposed, resulting in smaller edge-site areas and stable, high-Φ surfaces, whereas, farther from the inlet, the opposite was observed. Note that the Φ values of the best performing area (zone I) are close to those of noble metals such as Au and Pt, in accordance with our KPFM results. Electrochemical impedance spectroscopy (EIS) analysis provided further insights into the electrode kinetics. Typical Nyquist plots of MoS2 films on Au wafers with different zones and varied numbers of ALD cycles are presented in Figure 10C,D. The contribution of the internal resistances remained the same with samples from different positions. The influence of the contact resistance to the internal resistance becomes more apparent according to the film thickness. Therefore, the effect of contact resistance was observed to follow the decreasing charge transfer resistance with a critical thickness of MoS2 films. The EIS results from the different zones indicate that zone I has the smallest charge transfer resistance, most likely due to the thinner film thicknesses when the same number of ALD cycles were applied (Figure 10C). When the ALD cycles are varied, there exists a critical optimal thickness for HER (here, approximately 1000 ALD cycles). This could well explain the electrochemical performance that we attained. Consequently, we conclude that (i) if the MoS2 layers are too thick, their charge transfer resistances are disadvantageous for HERs even if the surfaces are edge terminated, (ii) if MoS2 is too thin, partially covered substrates may have an effect (Figure 10D), and (iii) the role of surface corrugation is of great

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Changdeuck Bae: 0000-0001-5013-2288 Jong Hyeok Park: 0000-0002-6629-3147 Author Contributions

T.A.H. and C.B. contributed equally to this work. C.B. and H.S. conceived the project. C.B. developed the present chemistry. C.B. and T.A.H. prepared the samples. C.B., T.A.H., M.J., S.L., J.M.M.-M., J.H.P., and H.S. analyzed the structures. C.B., J.M.M.-M., and T.A.H. carried out the electrochemical experiments. C.B., T.A.H., and H.S. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the grant by the Samsung Science and Technology Foundation (SRFCMA1502-09). REFERENCES

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