Hydrogen Evolution Catalyzed by A Molybdenum Sulfide Two

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Hydrogen Evolution Catalyzed by A Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes Tong Yang, Yang Bao, Wen Xiao, Jun Zhou, Jun Ding, Yuan Ping Feng, Kian Ping Loh, Ming Yang, and Shijie Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03977 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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ACS Applied Materials & Interfaces

Hydrogen Evolution Catalyzed by A Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes Tong Yang,†,‡,⊥ Yang Bao,¶,§,⊥ Wen Xiao,∥ Jun Zhou,† Jun Ding,∥ Yuan Ping Feng,†,§ Kian Ping Loh,¶,§ Ming Yang,∗,‡ and Shi Jie Wang∗,‡ †Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore. ‡Institute of Materials Research and Engineering, A∗ STAR, 2 Fusionopolis Way, Singapore 138634, Singapore. ¶Department of Chemistry, National University of Singapore, Singapore 117543, Singapore §Centre for Advanced 2D Materials and Graphene Research, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore. ∥Department of Materials Science & Engineering, National University of Singapore, Singapore 119260, Singapore ⊥These authors contributed equally to this work. E-mail: [email protected]; [email protected] Phone: +65 6319-4823; +65 6416-8953

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Abstract Molybdenum disulfide has been demonstrated as a promising catalyst for hydrogen evolution reaction (HER). However, its performance is limited by fractional active edge sites and the strong dependence on hydrogen coverage. In this study, we find an enhanced HER performance in a two-dimensional sub-stoichiometric molybdenum sulfide. Both first-principles calculations and experimental results demonstrate that the basal plane is catalytically active towards HER as evidenced by an optimum Gibbs free energy and a low reaction overpotential. More interestingly, the HER performance is insensitive to hydrogen coverage and can be improved under compressive in-plane biaxial strains. Our results suggest an improved HER performance of sub-stoichiometric molybdenum sulfide due to its chemical reactive basal plane, and also a way to tune the performance.

KEYWORDS: hydrogen evolution reaction; sub-stoichiometric molybdenum sulfide; electrocatalyst; two-dimensional materials; structure engineering

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Introduction Molecular hydrogen has been considered as one of the cleanest and renewable energy carriers. Hydrogen evolution reaction (HER) is a promising pathway to convert solar energies into chemical energies in the form of molecular hydrogen. 1 Currently, noble metals such as Pt are the most efficient catalysts for HER. 2,3 However, the scarcity and high-cost of noble metals prevent them from large-scale applications. Therefore, intensive efforts have been made to explore earth-abundant materials with the HER performances comparable to noble metals. Recently, molybdenum disulfide, 2H-MoS2 , has been demonstated to be promising non-noble metal catalysts. 4–7 However, the basal plane of pristine MoS2 is inert, which limits the observed HER activity at the edge sites of MoS2 flakes. 8,9 The HER performance is also significantly affected by the H coverage 8,10 and the poor electrical transport. 11,12 Hence, molybdenum sulfide films have been engineered to increase the active sites, which include defect-rich flakes, 13–17 mesoporous foams, 18 vertically aligned thin films, 19–21 and MoSx nanoparticles. 11,22–25 In addition, intrinsic defects or dopants have been introduced into 2H-MoS2 to activate its basal plane and enhance its conductivity. 26–28,28–32 Beyond the 2H phase, other phases of molybdenum sulfides have also attracted remarkable attentions, such as pristine 1T- 33–36 and 1T′ -MoS2 , 33,37,38 porous 1T-MoS2 39 and Mo3 S13 . 40 Both 1Tand 1T′ -MoS2 have HER active basal planes, but analogous to 2H-MoS2 , their activities strongly rely on hydrogen coverage. 33 The enhanced HER activity of porous 1T-MoS2 stems from its unique crystal structure with a higher density of edge states as well as sulfur vacancies, 39 whereas for Mo3 S13 , it is due to the under-coordinated S atoms. 24,40 More recently, two-dimenstional (2D) sub-stoichiometric molybdenum sulfide phase (s-MoSx , x < 2) has been synthesized on Cu (111) subtrate, which shows catalytic activity towards hydrogen adsorption. 41 However, the study on its HER application is limited. In this work, we show a superior HER performance of the two-dimensional sub-stoichiometric molybdenum sulfide (s-MoSx ) structure, which is insensitive to hydrogen coverage and can be further improved by applying compressive strain. Charge transfer rate in this s-MoSx sulfide is also expected to 3



be high due to its metallic character. The HER performance of s-MoSx is further evidenced by the measured low overpotential.

Method All calculations were performed with the spin-polarized density functional theory (DFT) using the generalized gradient approximation with Perdew-Burke-Ernzerhof (GGA-PBE) 42 format for the exchange-correlation interaction, as implemented in the Vienna ab initio simulation package (VASP). 43,44 The interaction between valence electrons and ionic cores was decribed by the project augmented wave (PAW) method. 45 The electronic wave functions were expanded in a plane wave basis with a cutoff energy of 500 eV. For the unit cell and the 2×2×1 supercell, the 16×16×1 and 8×8×1 Monkhorst-Pack k-point meshes were used to sample the first Brillouin zone, respectively, which ensures that the total energy converges within 1 meV. In order to minimize the spurious interlayer interaction, a 15 ˚ A vacuum layer was inserted in the direction perpendicular to the surface. The DFT-D3 method was applied to include the long-range van der Waals (vdW) interaction for the hydrogen adsorption on Mo6 S4 . 46,47 All geometry optimizations in this study were carried out until the HellmannFeynman force acting on each atom is less than 0.001 eV/˚ A. In addition, the thermal stability of the s-MoSx 2D structure was studied via canonical molecular dynamics (MD) simulations for a 3×3×1 supercell. The temperature is controlled at 300 K with the Nos´ e heat bath and the time step is set to 1 fs. For the dynamic stability of Mo6 S4 , the phonon dispersion was calculated using the finite displacement method 48 with a high electronic convergence criterion of 10−8 eV. The STM images were simulated using the Tersoff-Hamann method, 49 and were visualized with constant height mode where the distance between the tip and the Mo6 S4 2D surface is about 1.5 ˚ A. It has been found that the Gibbs free energy for adsorbed hydrogen,∆GH , is a good descriptor to evaluate the catalytic activity towards HER, 8,50 which is defined as ∆GH =

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∆EH +∆EZPE −T ∆SH . ∆EH is the hydrogen adsorption energy which can be obtained as follows: ∆EH = n1 [E(Mo6 S4 + nH) − E(Mo6 S4 ) − n2 E(H2 )], where E(Mo6 S4 + nH), E(Mo6 S4 ) and E(H2 ) are the energies of the Mo6 S4 with n hydrogen atoms adsorbed, the clean Mo6 S4 , and the gas phase H2 molecule, respectively. ∆EZPE and ∆SH stand for the change in the zero-point energy (ZPE) and the change in the entropy of hydrogen between the adsorbed state and the gas state. EZPE of hydrogen is calculated to be 0.133 eV for the gas phase and 0.173 eV for the adsorbed state, thereby ∆EZPE = 0.04 eV. We note that the ∆EZPE does not vary significantly with respect to the hydrogen coverage. Since the entropy of H in the adsorbed state is very small, T ∆SH ≈ T S 0 H = −0.202 eV at T = 298 K, 51 where S 0 H is the entropy in the gas phase. So ∆GH can be simplied as ∆GH = ∆EH + 0.242. The optimal HER activity can be achieved as ∆G goes to zero where both the hydrogen adsorption and the subsequent desorption can be facilitated. 8,50 The growth experiments were performed in an ultrahigh vacuum (UHV) chamber, which is connected a UHV STM chamber through a gate-valve. Both chambers were kept at a base pressure of 8×10−10 mbar). This experiment setup allows in-situ characterization of as-grown MoSx catalysts without contamination. The Cu (111) substrate (Mateck, GmbH) was cleaned by repeated argon ion sputtering at p(Ar) = 1 × 10−5 mbar, 1.5 keV, followed by annealing at 600◦ C. The Cu(111) substrate was subsequently sulfurized in H2 S (99.9% purity) at 400◦ C to form a Cu-S layer. Growth of s-MoSx islands was carried out by evaporating Mo atoms onto the Cu-S layer, followed by annealing in H2 S at elevated temperatures (600◦ C). STM experiments were carried out with a UHV STM unit (high-temperature STM 150 Aarhus, SPECS, GmbH), with typical sample biases ranging from -2 V to +2 V. VMP3 chemical workstation (Bio-logic Inc.) was used for electrochemical measurements. Typical three-electrode setup was adopted with a work electrode of the as-prepared catalyst, a counter electrode of graphite plate and a reference electrode of saturated calomel electrode (SCE). Linear sweep voltammogram (LSV) measurements were performed at room temperature at a scan rate of 10 mV/s in 1 M KOH (Sigma, anhydrous, ≥ 99.97% trace metals

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basis). Potentials were calibrated with respective to reversible hydrogen electrode (RHE). The onset overpotential is estimated from the intersection point of the tangent of the beginning linear regime of overpential and the Tafel slope. The current density was normalized to the area of s-MoSx . Compensation value of 85 % was applied for IR correction.

Results and Discussions Mo6 S4 2D Structure Since a Mo-S stretching mode that is characteristic to Mo6 S8 clusters has been detected in the grown s-MoSx 2D film, 41 this is indicative that the s-MoSx 2D structure is likely derived from Mo6 S8 Chevrel-phase clusters. The Mo6 S8 cluster is electron deficient, and thus it has to be stabilized either by forming compounds with metal cations or inter-cluster bonding. 52 It is worth noting that Mo6 S8 Chevrel phase based 1D, 2D and 3D compounds have been experimentally synthesized. 41,53,54 Moreover, for the grown s-MoSx 2D structure, the STM measurement shows square surface lattices. Thus, we consider three potential structures by inter-clustering the Mo6 S8 Chevrel units into 2D square structures, which are denoted as Mo6 S4 , Mo8 S8 , and Mo6 S6 , respectively, as shown in Figure S1. The relative stability of the proposed 2D structures can be implied by comparing the cohesive energy:

Ecoh (Mox Sy ) = [E(Mox Sy ) − xE(Mo) − yE(S)]/(x + y),

(1)

where E(M ox Sy ), E(M o) and E(S) are the energies of Mox Sy , free Mo and S atoms. The cohesive energy of Mo6 S4 is around 0.1 eV/atom and 0.9 eV/atom lower than those of Mo8 S8 and Mo6 S6 , respectively, indicating that Mo6 S4 is the most stable. Therefore, our study is focused on the Mo6 S4 2D structure. The Mo6 S4 2D structure consists of square lattices with three atomic layers and a symmetry group of P4mmm (Figure 1a). There are 6 Mo and 4 S atoms in a unit cell. The

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surface layers are composed of a square Mo and a square S sub-lattices, which sandwich a square Mo sub-lattice in the middle. The interlayer Mo-S bond slightly shifts the surface S sub-lattices towards the middle, as can be seen in Figure 1b. The optimized lattice constant is 5.32 ˚ A. The surface and interlayer Mo-S bonds are 2.66 ˚ A and 2.57 ˚ A, respectively, which ˚) 55 and 1T-MoS2 (2.43 ˚ are slightly larger than those of 2H-MoS2 (2.41 A A). 34 Figure 1c is √ √ the simulated STM image of the ( 2 × 2) 2D Mo6 S4 supercell under a bias voltage of -1 V, which shows two sets of square lattices with 7.52 ˚ A spacing and different intensity. These are in agreement with experimental observations (see Figure 1d). These results suggest that the predicted Mo6 S4 2D structure is a promising candidate to resolve the recent synthesized sub-stoichiometric molybdenum sulfide 2D phase. 41 The stability of the Mo6 S4 2D structure is also verified by the calculated phonon dispersion, as shown in Figure 2a. The absence of imaginary frequency in the dispersion spectrum suggests that this 2D structure is dynamically stable. We further carry out the ab initio molecular dynamics (MD) simulation at the temperature of 300 K to check the thermal stability of Mo6 S4 (Figure 2b). During the simulation time of 6 ps, the average total energy converges quickly (See Figure S2a). The bond length between S and the middle-layered Mo is fluctuating around the equilibrium within an amplitude of 0.3 ˚ A. We note that Mo6 S4 remains the square lattice pattern after 6 ps (See Figure S2b). Thus, all these results suggest that the proposed Mo6 S4 2D structure is stable.

Electronic Properties The bonding character of the Mo6 S4 2D structure is investigated by calculating its electron localization function (ELF). Figure 2c shows the contour plot of ELF in the (001) plane cutting through the atoms in the top layer of Mo6 S4 . The ionic bonding character dominates between Mo and S atoms. On the other hand, near the center of the triangle formed by the surface Mo and its two nearest S atoms, the electron localization function has a small but noticeable value of around 0.25, which might be due to the hybridization among t2g states 7



of the surface Mo atom, eg states of the Mo atom in the middle layer, and the delocalized s states of the S atom. The ionic bond is also the main character between the surface S atom and the nearest middle-layer Mo atom (See Figure S3). The Bader charge analysis 56 suggests that Mo6 S4 has an ionic formula of Mo6 0.6+ S4 0.9− . Compared with 2H-Mo1.2+ S2 0.6− and 1T-Mo1.26+ S2 0.63− , each S atom in Mo6 S4 gains more electrons, while every Mo atom loses less owing to the high Mo/S ratio. The Mo6 S4 2D structure is metallic as suggested by the projected density of states (PDOS) in Figure 2d. The electrical conductivity is mostly contributed by the d orbitals of Mo atoms. While, the p orbitals of surface S atoms are mainly distributed far below the Fermi energy, suggesting that the surface S sites may not be chemically reactive. The metallic property is favorable for the electron injection from an cathode to the catalyst surface, where intermediate protons induced by water dissociation in alkaline solutions are reduced and adsorbed on the catalyst-covered cathode. 1 The combination of under-coordinated Mo atoms and the metallic character of the 2D Mo6 S4 can be expected to lead to a good HER application.

HER Here, we adopt a (2×2) Mo6 S4 2D supercell to study its HER performance. On the basal plane of Mo6 S4 , there are three potential hydrogen adsorption sites, i.e, Mo top site, S top site and hollow site. The hydrogen adsorption energies on these sites are calculated and summarized in Table 1. The Mo top site is the most stable adsorption site with ∆EH = −0.342 eV. The adsorption of hydrogen is much weak for the hollow site (-0.066 eV) and is unfavorable for the S top site (1.989 eV). Therefore, we will only focus on the Mo top site hereafter. Taking the correction from the zero-point energy and the entropy into consideration, we obtain the Gibbs free energy ∆GH = −0.1 eV for the Mo top site, indicating that the surface Mo site is catalytically active for HER. Although the basal plane of pristine 2H-MoS2 mono-

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layer is inert, it has been shown that the edges of 2H-MoS2 flakes are chemically reactive towards HER due to the under-coordinated S atoms at the edges. 8,33,35 Similarly, Tran et al. recently showed that the unsaturated Mo sites could enhance HER activity of amorphous molybdenum sulfide (a-MoSx ). 57 Li et al. also revealed that introducing S vacancies to 2HMoS2 can make the neighboring Mo atoms unsaturated and exposed, resulting in improved HER performance. 27 These works indicate that unsaturated Mo sites of Mo6 S4 are possibly HER active. In order to benchmark our calculations, we utilized the same method to calculate the Gibbs free energies of hydrogen adsorption for the S site on Mo edge of 2H-MoS2 and the lower S top site in the basal plane of 1T′ -MoS2 . The same models as Hinnemann et al. (∆GH = 0.08 eV) 8 and Fan et al. (∆GH = 0.06 eV) 33 are taken for 2H-MoS2 and 1T′ -MoS2 , respectively. Our calculated Gibbs free energy is 0.11 eV for 2H-MoS2 and 0.07 eV for 1T′ -MoS2 , which are similar to literature values as shown above and therefore imply our calculations are reliable. In order to elucidate the adsorption mechanism on the Mo top site, we further analyze the charge transfer, which is defined as ∆Q = Q(Mo6 S4 + H) − Q(Mo6 S4 ) − Q(H), where Q(Mo6 S4 + H), Q(Mo6 S4 ) and Q(H) are the charge densities of the Mo6 S4 with a hydrogen adsorbed, the clean Mo6 S4 and a single hydrogen atom, respectively. As Figure 3b shows, charges are transferred from the Mo atom into the adsorbed hydrogen. We also can see that the transferred electrons are mostly from the Mo dz2 orbitals, as suggested by the visualized charge-density depletion around the Mo atom. The Bader charge analysis reveals that the adsorbed hydrogen obtains around 0.4 electrons, which is mainly contributed by the Mo atom right below the H atom. On the other hand, the PDOSs before and after the adsorption are compared (see Figure 3a for Mo dz2 and Figure S4 for other Mo orbitals). Prior to the adsorption, the delta-like H s orbital is 4.12 eV below the Fermi level, whereas the dz2 orbital of the Mo atom right below H has a high intensity at the Fermi level. Upon the hydrogen adsorption, the H s orbital hybridizes with the Mo dz2 orbital, forming the bonding and anti-bonding Mo dz2 -H s states. The bonding Mo dz2 -H s state is more than 2 eV below

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the Fermi level and is occupied. Meanwhile,the noticeable high intensity of Mo dz2 orbital around the Fermi level vanishes owing to the hydrogen adsorption. This dramatic change of Mo dz2 orbital is ascribed to the hybridization with H and the charge transfer from Mo to H atoms as well. Both effects eventually lead the hydrogen-adsorbed Mo6 S4 to a lower energy state compared to the clean Mo6 S4 and the gas phase H2 molecule.

Hydrogen Coverage and Strain Effects Next, we explore the hydrogen coverage effect on the HER catalytic performance of Mo6 S4 . We define the coverage of hydrogen as the fraction of a monolayer with respect to the number of available surface Mo atoms in the basal plane

θH (ML) =

nH #of Mosurface

(2)

For the 2×2×1 Mo6 S4 supercell, θH(ML) can be 25%, 50%, 75% and 100%, and the calculated Gibbs free energies are summarized in Table 2. Interestingly, the variation of the Gibbs free energy is very small with the increase of H coverage. Such slight variation indicates that ∆GH is insensitive to the hydrogen coverage. In contrast, the HER catalytic activity of either 2H-or 1T/1T′ -MoS2 strongly relies on the hydrogen coverage. 10,33,58 The insensitive behavior can be understood because it is the Mo dz2 orbital that plays the key role in the hydrogen adsorption. The Mo dz2 is oriented perpendicular to the surface and thus the effect of additions or removals of hydrogen at the neighboring Mo sites is very small. At the hydrogen coverage of 50%, the two hydrogen atoms could be on top of either the neighboring (θH(ML) = 50%Hn ) or diagonal (θH(ML) = 50%Hd ) Mo atoms. The calculated Gibbs free energy of the 50%Hn configuration is 0.025 eV lower than that of the 50%Hd configuration. As a result, the population of configuration 50%Hn is expected to be higher than that of configuration 50%Hd by a factor of constant e, according to the Boltzmann distribution.

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Due to the nature of atomic thicknesses of 2D materials, their structures are flexible to external influences such as strain, which can be induced by substrates. 59–61 Recently, Li et al. 27 experimentally showed the basal plane of 2H-MoS2 can be engineered reactive towards HER by applying the strain and introducing S vacancies. So we investigate the strain effect on the HER catalytic activity of Mo6 S4 . Both tensile and compressive biaxial strains are applied to Mo6 S4 in the range from -4% to +4%, and the related Gibbs free energies are calculated and shown in Figure 4b. It is found that the Gibbs free energy becomes more negative with the increase of the tensile strain, and it is approaching 0 under the compressive strain. This trend suggests that compressive strains can enhance the catalytic activity towards HER. As can be seen in Figure 4b, by applying 4% compressive strain, the Gibbs free energy is about -0.06 eV. It suggests that the HER activity of Mo6 S4 is comparative to that of 1T′ -MoS2 (0.06 eV) and even better than that of 2H-MoS2 (0.08 eV), but it is still inferior to the HER activity of Pt (-0.03 eV), 50 as shown in Figure 4a. Besides, it is worth noting that the HER activity of the basal plane of pristine 2H-MoS2 cannot be enhanced simply by applying strains. 27 The trend of the Gibbs free energy against the biaxial strain can be qualitatively understood through the Bader charge analysis. As shown in Figure 4b, Mo loses less (more) charges with the increase of the compressive (tensile) strain, giving rise to a weaker (stronger) bond between Mo and H atoms. It is clear that the ionic bonding component of H-Mo bond plays a significant role in the HER performance of Mo6 S4 .

Synthesis and HER Activity The S-enriched Cu (111) substrate enables the formation of s-MoSx 2D layer. The as-grown s-MoSx and its monolayer thickness were confirmed by the STM investigation (see Figure 1d and Figure S5). The 2D s-MoSx layer is constituted by two sets of square lattices, with lattice spacing about 0.7 ˚ A and a surface coverage of 18% (see Figure S6). The STM measured lattice pattern and lattice spacing of the s-MoSx 2D layer resemble those of the above Mo6 S4 2D structure. Its electrocatalytic HER activities were subsequently evaluated 11



in 1 M KOH, where the current density is normalized to the surface area of s-MoSx . The onset overpotential was determined to be 33 mV, indicating that s-MoSx is indeed active towards HER (see Figure S7). The current density of 10 mA cm−2 can be reached under the overpotential of 116 mV (Figure 5a). The low Tafel slope (∼58 mV/decade, see Figure 5b) further corroborates the above measurements, which implies that the HER reaction may take the Volmer-Heyrovsky step. These results indicate that the kinetics of the water dissociation step is effectively facilitated in the alkaline environment. Such HER performance of the 2D s-MoSx is much superior to that of MoS2 (>200 mV for the overpotential;96 mV/decade for the Tafel slope) 62,63 in the alkaline environment. It is even slightly better than the performance of the MoS2 catalyst in the acidic condition (∼170 mV for the overpetial). 27,64,65 We note that transition metal doped MoS2 films have been proposed to improve the HER performance. 63 In terms of the measured overpotential and Tafel slope, the HER performance of the 2D s-MoSx is also better than those of Fedoped (163 mV for the overpotential; 181 mV/decade for the Tafel slope) and Co-doped MoS2 (203 mV for the overpotential; 201 mV/decade for the Tafel slope), and comparable with those of Ni-doped MoS2 (98 mV for the overpotential; 60 mV/decade for the Tafel slope) in alkaline conditions. 63 While compared with Pt (

Hydrogen Evolution Catalyzed by A Molybdenum Sulfide Two

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