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Received 18 Mar 2015 | Accepted 29 Jul 2015 | Published 7 Oct 2015

Stanley S. Chou1, Na Sai2, Ping Lu3, Eric N. Coker1, Sheng Liu4, Kateryna Artyushkova5, Ting S. Luk4, Bryan Kaehr1,5 & C. Jeffrey Brinker1,4,5

Establishing processingstructureproperty relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulde, a promising catalyst for articial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulde monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T0 (clustered Mo). These changes alter the energetics of molybdenum disulde interactions with hydrogen (DGH), and, with respect to catalysis, the 1T0 transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T0 molybdenum disulde and a lowering of DGH from 1.6 eV for 2H to 0.18 eV for 1T0, comparable to 2H molybdenum disulde

edges on Au(111), one of the most active hydrogen evolution catalysts known.

DOI: 10.1038/ncomms9311 OPEN

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

1 Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, USA. 2 Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA. 3 Department of Materials Characterization & Performance, Sandia National Laboratories, Albuquerque, New Mexico 87123, USA. 4 Center For Integrated Nanotechnologies (CINT), Sandia National Laboratories, Albuquerque, New Mexico 87123, USA. 5 Department of Chemical and Biological Engineering, The University of New Mexico, Albuquerque, New Mexico 87131, USA. Correspondence and requests for materials should be addressed to S.S.C. (email: mailto:[email protected]

Web End [email protected] ) or to C.J.B. (email: mailto:[email protected]

Web End [email protected] ).

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Improving the capacity and efciency of the Hydrogen Evolution Reaction (HER) is an enduring challenge of green energy production and articial photosynthesis1,2. Still, while

HER in organisms evolved with time and increasing complexity, articial HER aims to replicate the same with minimalism and simplicity. A cornerstone of the challenge is to mimic the function of natural hydrogenase enzymes, which catalyse HER in living systems. Indeed, it can be seen that without a catalyst like hydrogenase, HER does not proceed with the speed required for practical applications3,4. However, hydrogenases can be difcult to extract and purify, and can denature under non-natural operations5, consequently, inorganic alternatives are used for most applications. The most common example of this is platinum (Pt), which has served as the benchmark catalyst for HER due to its high catalytic efciency68. Nevertheless, because of the scarcity and cost of Pt, a more abundant alternative is needed for cost-effective implementation.

For this, MoS2, an earth abundant lamellar solid, has shown prominent HER catalysis nearing the efciency of platinum9,10. However, experiments using MoS2 grown on Au(111) indicated that this material is only catalytic on its edge sites9. Theoretical studies corroborated these results with the Gibbs free energy of hydrogen adsorption (DGH), a measure of HER efciency, to be feasible for catalysis only at MoS2 edges11,12; the basal plane of MoS2 does not appear to participate in catalysis, meaning the bulk of material is catalytically inert. Consequently, the maximization of MoS2 edges and mimicry of the edge structure has become a signicant topic1316.

Interestingly, recent studies have begun to show enhancement of MoS2 catalytic efciency following lithium intercalation and exfoliation1719. The Tafel-slope of these sheets, a benchmark of electrochemical efciency against applied overpotential, is nearly twice that of natural MoS2 after lithium treatment1719. As the lithium-exfoliation reactions increase the availability of basal plane surfaces but not edges, the catalytic improvements are postulated to be basal plane related. Nonetheless, because the basal plane structures of lithium-exfoliated monolayers, and indeed, many two-dimensional sheets, are difcult to characterize2022, the post lithiation and exfoliation structure of MoS2 remains nebulous and historically controversial, rendering the origin of this catalytic enhancement correlatively vague. To briey recount a history of the structural understanding of MoS2 exfoliated with the assistance of lithium, it can be seen that a structural change in Li-intercalated MoS2 has been reported as far back as 1973 (ref. 23), nevertheless, interpretations of the nal structure vary signicantly in the literature. For example, in 1973, Somoano et al.23, observed extensive layer displacements after intercalation, suggesting the resulting product is a mixture of disordered compounds. However, in 1983, Py et al.24 reported the alkali metal exfoliated structure to be crystalline, with a rst order phase transition from the natural trigonal prismatic (2H) to an octahedral (1T) phase. In 1989, a signicant distortion to a 2ao 2ao lattice was reported by Chrissas et al.25 In 1991, an

octahedral structure was observed by Jimenez et al. and Qin et al., but a small distortion was noted, making the nal crystal a 2ao ao super lattice26,27. Atomic force microscopy images

obtained in 1993, by Schumacher et al., though, indicated no distortion or superlattice28. In 1998, Dungey et al. observed a 2ao 2ao lattice with trigonal Mo clustering29, but in 1999,

a 2ao ao superlattice was seen by Heising et al., with severe

distortion and formation of innite Mo-zig-zag chains30. A chain-of-diamonds motif was suggested by Petkov et al. in 2002 (ref. 31), but most recently, in 2013, Maitra et al. reported a distortion-free octahedral phase32. Chhowalla and coworkers observed similar variations, from a perfect octahedral phase with natural 2H MoS2 domains33, distrted phases19, coexistence of

various phases34, and also, a perfect octahedral phase35. Surprisingly, we note that the changed structures are almost always referred to as 1T, regardless of polymorphic structural differences. It is therefore not surprising to see recent catalytic improvement being attributed to varied structures, including octahedral17,18,32, and distorted Mo phases36,37. Part of the confusion is likely a simple consequence of incomplete transformation resulting in identication of partially transformed and transitional states due to variations between batches, however, there may be underlying issues of stability as well, making the resultant structure, and indeed, structural based predictions of its catalytic effectiveness difcult to handle38.

To address these long-standing issues and shed light on the catalytic origin of the transformed crystals, here we take a combined experimental/theoretical approach using controlled processing conditions to achieve stable phases and density functional theory calculations to investigate the stability of these polymorphs. With the solution processing advantages of these materials3941, we report the catalytic efciency of HER in homogenous reactions, with experimental H2 yield of these phases, and corroborate them with calculated

DGH. The analysis was then extended from MoS2 to WS2 to show similarly distorted crystal phases and basal plane catalytic activation.

ResultsExfoliation and phase transformation of MoS2. A typical lithium-exfoliation reaction consisted of immersing MoS2 powder in n-butyl lithium (1 M) for 72 h at room temperature31,33,42. The intercalated compound was then transferred to water and sonicated to yield exfoliated monolayers. After purication using centrifugation and dialysis, concentrations and purity of each batch were measured using Flame Atomic Absorption Spectrophotometry. Samples were then visualized using abberation-corrected scanning transmission electron microscopy.

As seen in Fig. 1, a typical sheet exfoliated using this method indeed yields a mixed phase (shown without false colouring in Supplementary Fig. 1). First, a trigonal prismatic (2H) phase that corresponds with an untransformed basal plane can be seen with symmetrical MoMo spacing of 2.980.05 . Second, an octahedral phase with displaced sulfur atoms and symmetrical Mo arrangements (2.950.06 ) is seen, albeit in small quantities. Last, large swatches of a visually distinct, tertiary phase is also present, with Mo atoms asymmetrically clustered to form one-dimensional lines of alternating light-and dark stripes. In this phase, MoMo distances of 3.550.16 and 2.920.16 were measured. On the basis of the above, we thus conclude that within one individual sheet, a microcosm of various phases are present, with each representing differing interpretations in previous work. Nevertheless, as the stability of each phase should be an intrinsic consequence of the energetics of distinct atomic arrangements, we sought to understand the relative stabilities of each phase. Because it was previously reported that materials revert back to the natural 2H phase42, we reasoned that the 2H is the overall energetic minimum, with the other polymorphs occupying differing metastable points in relation to the 2H.

To better dene this, we employed Density Functional Theory (DFT) to independently predict the structures of possible polymorphs (see Supplementary Information for computational details). As can be seen in Table 1, a total of four polymorphs were predicted based on structural optimization of unit cells. The rst two, consisting of 1 1 unit cells, were the 2H trigonal

prismatic phase and the 1T octahedral phase. In addition, a distorted phase with zig-zag chains consisting of a 2 1 supercell

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Figure 1 | Polymorphism and incomplete transformations. Polymorphisms within the basal plane exhibit coexistence of threedifferent phases, 2H (trigonal prismatic, false coloured as red), 1T (octahedral, false coloured as blue) and 1T0 (clustered Mo, false coloured as green).

Scale bar, 5 nm.

Table 1 | Structural polymorphs of MoS2 and summary of parameters as predicted by Density Functional Theory.

2H 1T 1T 1T

Phase Relative stability (eV f.u. 1) a () b () a () MoMo () Eg (eV)2H 0 3.18 3.18 120 3.18 1.71T 0.82 3.17 3.17 120 3.17 Metallic1T0 0.55 6.55 3.18 119 2.77, 3.18 0.01 (PBE) 0.26 (HSE06)

1T00 0.63 6.44 119.5 2.77, 3.26 0.1 (PBE) 0.01 (HSE06)

(Top) Mo atoms are represented as teal spheres, S atoms as gold spheres. (Bottom) From left to right, relative stability in eV, lattice parameters a, b and a, Mo-Mo distance, and band gap E (eV) based

on PerdewBurkeErnzerhof (PBE) and Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation functionals.

and a phase with MoMo atoms clustered into trimerized pockets in a 2 2 supercell were predicted. We dub the two later phases,

1T0 and 1T00, respectively.

Next, the stabilities of each DFT polymorph were calculated. It can be seen that the symmetrical 1T representation has the least stable ground-state ( 0.82 eV versus 2H). Indeed, in a 2 2 or

4 4 supercell, we nd the 1T phase eventually relaxes into 1T00,

which is second in stability, at 0.63 eV, conforming with the

dynamical phonon instability previously predicted in the 1T phase38. Nevertheless, the 1T0 phase was signicantly more stable than 1T and 1T00 ( 0.55 eV versus 2H), with a barrier against 2H

reversion of 0.73 eV f.u. 1 (formula unit, Supplementary Fig. 2, Supplementary Table 1). By the DFT calculations, the transformed portions within a sample, if allowed to reach metaequilibria, should therefore preferentially form 1T0 instead of the other polymorphs.

To investigate these results experimentally, we prepared samples to induce equilibria by extending the lithium intercalation period. As seen in Fig. 2a, a preferential transformation to 1T0 is possible on the entirety of the basal plane. This therefore

suggests that the various polymorphs observed previously are likely consequences of incomplete reactions, such as partial intercalation, with parts of the basal plane yet to undergo sufcient conversion to the 1T0. Indeed, we have observed that when intercalation times are extended from 72 to 240 h, a complete transformation to the 1T0 can be achieved (Supplementary Fig. 3).

Given the DFT prediction of 1T0 as a metastable phase, we examined the ability to covert 1T0 back to 2H under electron beam-illumination. As shown in Fig. 2c, sequential frames taken at 60 s intervals (acquired at 40 s/frame at an electron dose rate of 2,800 electron 2 s 1) show gradual relaxation of Mo atoms from 1T0 lines to symmetrical 2H spacing. A detailed investigation of the structures again showed the 1T0 phase with Mo spacings of3.55 and 2.92 , and the relaxed structure displayed trigonal prismatically coordinated sulfur atoms with MoMo spacing of2.95 , in agreement with the 2H structure (Fig. 2d). These results corroborate the 1T0 as the energetically preferred metastable phase that will relax back to 2H when energetic input, for example, heat, exceeds that of the metastable barrier energy. It further suggests that

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a b

c d

Figure 2 | Basal plane transformations. (a) MoS2 basal plane showing preferential transformation to 1T0 with increased lithiation, in accordance with the calculated relative stability of the metastable phases. (b) On electron beam illumination, the basal plane exhibits a restoration to a symmetrical Mo arrangement indicative of 2H reversion. (c) Sequential frames taken 60 s apart, showing phase evolution under the electron beam. Electron dose rate was 2,800 e 2 s 1, with each frame requiring 40 s to capture. (d) An electron dosed region with sulfur atoms resolved, demonstrating 1T0 to 2H phase reversion after illumination.

1T 1T 1T

Simulated

Experimental

HAADF image Line Profile

10.5 0

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Experimental

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0.2

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Figure 3 | Different possible phases for MoS2. (a) Simulated and experimental images of different possible phases for MoS2. FFT mask lters have been employed for clarity. (b) line scan comparisons between 2H and 1T using the simulated and experimental images, showing modulation of sulfur contrast.

the partial or incomplete phase transformations observed previously may be due to energetic conditions reverting the material to 2H during the characterization process. Supplementary Fig. 4 shows the nal product of 1T0 heated under inert atmosphere is a 2H lattice.

These observations support previous electrical characterizations of the material33,42 with 1T0 having greatly reduced resistivity compared to 2H (calculated bandgap of 0.01 and 1.7 eV, respectively, shown in Table 1).

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a b

800

1 )

600

H 2flux (nmol min

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Time (ns) Time (ns) Time (ns)

15%TEOA

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EY only

Data

0 2 4 6

Figure 4 | Hydrogen evolution. With dye sensitization (eosin Y, EY), MoS2 can be used to conduct homogenous hydrogen evolution reactions in solution. (a) Real time H2 ux at incremental MoS2 concentrations. Diminished reactions are indicative of proton exhaustion, and can be restarted with acid addition. (b) Corresponding cumulative H2 yield (For a and b note key bottom left). (c) Incremental concentration of MoS2 and correlative overlay of cumulative H2 yield (black) with uorescence quenching of dye (red) in solution. (d) uorescence decay of eosin Y, (e) reaction saturated with eosin Y and MoS2 showing similar decay, indicative of static-quenching and ground-state complex, and (f) collisional quenching between TEOA and eosin Y resulting in second order decay and attenuated lifetime. Error bars denote s.d.

To corroborate the above, High-angle annular Dark-eld (HAADF) images were simulated using coordinates from DFT. As seen in Fig. 3, good agreement was seen between experiment and simulations. We note in samples that are predominantly 1T0 (Fig. 2a), lines modulated in two-dimensions can be seen at intersections of 1T0. This may be interpreted as the 1T00, however, its occurrence is very localized and defective (o5%), and consequently, not of practical signicance.

Dye-sensitized HER. Following these structural insights, HER catalysis of these MoS2 polymorphs was examined. For this, we measured H2 formation using MoS2 in a homogenous photo-catalysis reaction. In a typical reaction, a solution of MoS2 was sensitized with Eosin Y (EY), and Triethanolamine (TEOA) was added as a sacricial electron donor43,44. The reaction mixture was then purged of atmospheric gasses, and irradiated with a xenon lamp tuned to 1 sun. The ow entering and exiting the reaction ask was then monitored on a gas chromatograph to quantify H2 formation inside the ask.

The reaction proceeded via photoexcitation of EY and subsequent intersystem crossing to yield a triplet excited state (EY3*). Acceptance of an electron via reductive quenching from the sacricial electron donation (TEOA) then yields a radical EY 32,43,4547. The highly reductive EY can then reduce a proton to form H0, return the electron to an oxidized TEOA, or transfer the electron to another catalyst such as the MoS2. As seen

in Fig. 4a, in the absence of MoS2, direct reduction of protons by radicalized EY occurs by collisional reductive quenching with TEOA (Supplementary Fig. 5), and generates a peak H2 ux of 150 nmol min 1. Over the course of 100 min, a yield of 7.5 mmols was observed, giving a typical activity of 4.5 mmol h 1 (Fig. 4b).

Sequential addition of 1T0 (for example, fully transformed MoS2) gradually raised the peak ux, until saturation was reached with 40 p.p.m. of MoS2, yielding a peak ux of 620 nmol min 1. With the latter reaction, 40 p.p.m. of MoS2 raised overall yield to 30 mmol in the rst 100 min, giving a typical activity of 18 mmol h 1, which is four times higher than the

autocatalytic EY dye under the same conditions43. In both cases, addition of small amounts of HCl regenerates a stopped reaction, suggesting reaction termination is due to proton depletion (Supplementary Fig. 6).

Given the boosted HER performance, it reasons that in the presence of fully transformed MoS2 a preferential charge transfer occurred between the radical EY and the MoS2. Indeed, with the reductive potential of EY at 0.8 V versus normal hydrogen

electrode (NHE), and MoS2s conduction band situated at 0.2 V versus NHE, such a transfer is feasible48,49. To support this, the charge transfer was examined using uorescence quenching correlations between EY and the Li-exfoliated MoS2 (Fig. 4c). It can be seen that the uorescence quenching interactions between EY and MoS2 are predictive of the eventual H2 yield, with both curves rising to saturation in the presence of 40 p.p.m. of MoS2.

The uorescence quenching following a saturation reaction tting, vis--vis a continual diminishing of uorescence with increasing quencher concentration (for example, collisional quenching) indicates the formation of a ground-state complex between EY and lithium-exfoliated MoS2, facilitating effectual charge transfer. To further elucidate these interactions, we monitored the excited state interactions via uorescence decay. As seen in Fig. 4d, the uorescence lifetime of a singlet excited EY, in the absence of secondary interactions, is B1 ns. However, interactions with reductive quenchers, such as TEOA, via collisional quenching results in uorescence lifetime shortening that ts a two exponential decay (Fig. 4e)50. Interactions between EY and MoS2 did not follow this behavior. Instead, the lifetime of

EY was maintained at B 1 ns (Fig. 4f), while the uorescence intensity was attenuated by 75% at saturation. This indicates adsorption of EY onto MoS2 to form a non-uorescent ground-state complex that facilitates charge transfer within the complex with an excited state lifetime following the behavior of the 25% minority free dyes50.

As the above were performed with the fully transformed 1T0, effect of partial 1T0 transformation was then investigated. Here, two additional MoS2 batches with diminishing 1T0

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a b

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Energy/Mo atom

76% 1T'/40 p.p.m.

46% 1T'/40 p.p.m.

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G H(eV)

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0.6

Mo-edge/Au(111) Mo-edge/

graphene

[afii9835] =0.50 [afii9835] =0.25

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1T'MoS2

G H(eV)

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Figure 5 | Energetics of Hydrogen Evolution. (a) Cumulative H2 yield with MoS2 undergoing differing degrees of 1T0 transformation. (b) DFT calculated Gibbs free energy of hydrogen adsorption (DGH) as a function of H coverage. (c) DFTcalculated stability of the MoS2 structures per Mo atoms as a function of H coverage. As the H coverage exceeds 0.4, the stability of the 1T0 phase (red dashes) surpasses that of the 2H (black dots). (d) Free-energy diagram of hydrogen evolution reaction at 1/2 and 1/4 H coverage, with 1T0 MoS2 basal plane, 2H MoS2 edge on graphene and 2H MoS2 on Au(111) as reference. (e)

Structural representation of 1T0 at 1/2 H coverage.

transformation were used in identical reactions. As shown in Fig. 5a, the MoS2 batches with diminished 1T0 transformation yielded signicantly less H2. Indeed, it can be seen that H2 yield scaled proportionately with 1T0 signatures, which dominates the basal plane of the exfoliated sheets (Supplementary Fig. 7, Supplementary Table 2). Given that 1T0 has shown empirical evidence of improved electrical conductivity, it is therefore possible that the improved catalysis seen with the fully transformed MoS2 is a consequence of better electron conduction from the EY charge transfer site to the catalytic sites of MoS2.

However, it is also possible that the 1T0 transformation fundamentally alters the catalytic mechanism of the MoS2 sheets, possibly giving additional active sites for catalysis.

To investigate, we calculated, via DFT, the free energy of hydrogen adsorption (DGH), for the basal plane of each polymorph. From previous reports, it is known that |DGH|E0

describes an optimal adsorption condition for catalysis10. Signicant deviation in the exothermic direction (DGHo0) can

indicate irreversible adsorption, consequently, an efcient catalyst such as the 2H MoS2 edge site, tends towards being slightly endothermic (DGH40). From our calculations, DGH for the 2H basal plane is 4 1.6 eV regardless of the H coverage and the

absorption site. As a result, the adsorption of H on the 2H basal plane is strongly unfavorable. This is consistent with the 2H surface being catalytically inert9. However, when the basal plane is converted to 1T0, the hydrogen binding energy becomes negative (adsorption occurs). As seen in Fig. 5b, the free energy of adsorption here for 1T0 MoS2 is reasonably close to the optimum value of \0 eV (ref. 12), emerging at DGH 0.13 eV at 1/16

coverage and growing to 0.25 eV as H coverage increased to 1/2. This indicates that the 1T0 transformed surface is amenable for catalysis, and indeed, the HER improvements with 1T0 fraction is due to catalytic activation of the MoS2 basal plane. Most interestingly, we also observe adsorption of hydrogen to stabilize the 1T0. Indeed, Fig. 5c shows that when H coverage exceeds 0.4, 1T0 becomes more stable than 2H. As signicant benchmark work with 2H MoS2 was performed with Au (111)

supports9, we compare 1T0 with 2H Mo-edge sites, when supported on gold. It can be seen here that |DGH| on the 1T0 basal plane is comparable to the 2H Mo-edge supported on Au (111) and graphene at 0.25 coverage and slightly outperforms Mo-edges on Au (111) at 0.5 coverage (Fig. 5d,e)51. Above all, the shear increase in surface area availability after exfoliation to monolayers (1001,000-fold greater than bulk solids)52 renders the basal plane improvements even more signicant.

As WS2 can undergo analogous phase transformations, its catalytic efciency was similarly analyzed. We show in Supplementary Fig. 8, the H2 ux with WS2 was a third lower than MoS2. For peak H2 ux, WS2s 400 nmol min 1 showed approximately threefold improvement over controls, but weaker than the 750 nmol min 1 observed for MoS2. Similarly, H2 yield with WS2 was 18 mmol at t 100 min, which is 33% lower than

MoS2. As uorescence lifetime measurements revealed an analogous molecular charge transfer pathway, the reduced HER efciency was thus indicative of WS2 being a less efcient catalyst. From Fig. 5b, it can be seen that the DGH of 1T0 transformed WS2 was 0.15 to 0.2 eV higher than MoS2. Nevertheless, this value represents a 10-fold reduction over

DGH of 2H WS2, indicating catalytic activation of the basal plane comparable to MoS2.

DiscussionWe investigated the structure of MoS2 exfoliated with lithiation intercalation and directly correlated the varying physical structures to the catalytic origin for HER. Particularly, we demonstrated via abberation-corrected scanning transmission electron microscopy that modulated lithiation can lead to complete 1T0 transformation in the basal plane, and corroborated the experimental ndings with DFT calculated phase stabilities. It was shown that the Mo atoms of MoS2 became asymmetrically spaced with increased lithiation, thus forming the 1T0 phase. In DFT, the 1T0 was shown to be energetically favorable compared to other possible polymorphs. Nevertheless, external

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perturbations within the environment, including electron imaging parameters, can induce a reversion to 2H.

With regard to catalysis, we have shown that the 1T0 transformation rendered the normally inert basal plane amenable towards hydrogen adsorption and H2 evolution. Indeed, DGH on the basal plane of 1T0 MoS2 showed catalytic activation and was lowered from 1.6 eV in the 2H to 0.18 eV. Moreover, when

H coverage became 40.4, DFT showed 1T0 phase stability surpasses that of 2H. To put this in perspective, the DGH of

0.18 eV is comparable to 2H MoS2 edges on Au(111), one of the most active HER catalysts yet characterized. This makes the 1T0 MoS2 a state-of-the-art catalyst. In addition, as exfoliation to monolayers increases the surface area by as much as 1,000-fold52,53, the basal plane activation provides non-trivial increases in catalytic efciency compared with the edge only catalysis of the 2H (refs 9,15). We demonstrate this by H2 evolution studies in basic solution (pH 11), via self-assembled photocatalytic charge transfer complexes for HER. Here it can be seen that MoS2 catalysts can boost H2 yield of the HER catalytic dye, EY, by fourfold. Moreover, MoS2 itself does not fatigue over the course of the reaction. A clearer understanding of these materials, and the underlying relationship between structure, properties and performance, provides a pathway towards quantitative engineering of MX2 to enable the emerging green energy

economy.

Methods

Lithium intercalation and exfoliation. For MoS2, lithium intercalation was accomplished by immersing 1 g of MoS2 powder in 10 ml of 1 M n-butyl lithium. The mixture was stirred vigorously in an inert atmosphere glovebox for 3 to 10 days. After, the compound was washed over three layers of Whatman lter paper (#51, ashless) with 200 ml of hexane and then collected in a bottle. Three hundred millilitres of H2O was then added, and the mixture was sonicated for 3 m to extricate the intercalated MoS2 powder from the lter paper. The lter paper was then removed from the bottle, and the solution was sonicated for an additional1.5 h. Unexfoliated portions were removed with centrifugation at 100 g for 3 min. The supernatant was then collected, and puried by centrifugation and washing with H2O (3 10,000 g for 1.5 h, followed by resuspension in water after each

centrifugation cycle). The solution was then transferred in to a dialysis bag (Fisher Scientic, MW 5,000), and dialysed against running water for 3 days. The solution was then again centrifuged at 500 g for 15 m to remove aggregates, and the resultant samples were used as is. For WS2, Li intercalation was performed at 100 C in a

Parr bomb. Exfoliation and purication of WS2 was the same as MoS2. Atomic force microscopy images of exfoliates sheets are shown in Supplementary Fig. 9.

STEM microscopy and analysis. A FEI Titan G2 80200 S/TEM with a Cs probe corrector operated at 200 kV was used in this study. High-angle annular dark-eld (HAADF) S/TEM images were acquired with an electron probe size of 0.8 , convergence angle of 18.1 mrad, and current of B100 pA with an annular detector with a collection range of 60160 mrad. The high resolution HAADF images were typically taken at 1,800 k magnication, yielding a pixel size of about 0.23 or a frame size of 48 48 nm for 2,048 2,048 pixels per frame. Such conditions gave

rise to an equivalent electron dose rate of B2,800 electrons 2 s 1. Acquisition of the HAADF image (frame of 2,048 2,048 pixels) took B40 s at a dwell time of

B10 ms/pixel. In sub-gures of Figs 1, 2 and 3, the ltered images were obtained by Fast Fourier transformation (FFT) of the HAADF images into reciprocal space, forming FFT patterns. Reciprocal spots from the patterns (typically {100} type reections) were selected, masked with a 60 pixels lter, and then transformed into real space with inverse FFT. Gatan Digital Micrograph was used for the image processing. Additional details on TEM imaging and simulation are shown in Supplementary Methods

Computational details. All density functional calculations were carried out using the Vienna ab initio simulation package54 with plane wave basis set and projector augmented-wave pseudopotentials55. All energies were calculated with Perdew BurkeErnzerhof exchange-correlation potentials56. For selected structures, we applied the hybrid HSE06 functional57 to calculate the electronic band gap. The MoX2 (X Mo,W) monolayers were modeled using surface supercells separated in

the periodic direction by a 20 -thick vacuum slab. We applied a plane wave energy cutoff of 600 eV and G-centered 24 24 1, 12 24 1, 12 12 1, and

6 6 1 k-points grids for Brillouin zone sampling of the 1 1 unit cell (2H and

1T), and the 2 1 (1T0), 2 2 (1T0), and 4 4 (H/XS2) supercells, respectively.

The criteria of convergence for energy and force were set as 10 4 eV and

3 10 3 eV/. The hydrogen adsorption energies were calculated using the 4 4

surface supercell containing 16 Mo atoms and 32 S atoms. A dipole correction was applied to cancel the electrostatic interaction between the periodic slabs. The DFT binding energies were calculated as DEH 1n E surf nH

E surf

n2 E H2

, where E(surf nH), E(surf), and E(H2) are the total energies for the MoS2 surface

with n hydrogen atoms adsorbed, the clean MoS2 surface, and the molecular hydrogen in the gas phase, respectively. The most stable H binding site in the basal plane of MoS2 is on the top of the S atoms. We dene H coverage as the ratio of the number of H and Mo atoms in the basal plane. For 1T0 MoS2, we obtained binding energies of 0.16 eV, 0.13 eV, and 0.11 eV, and 0.035 eV at the 1/16H,

1/8H, 1/4H, and 1/2H coverage. For the 4 4 supercell, this corresponds to n 1,

2, 4, and 8, respectively. The adsorption free energy was calculated by adding a thermal correction to the binding energy DGH DEH DEZPE TDSH, where

DEZPE and TDSH are, respectively, the differences in the zero point energy and entropy contribution between the H adsorbed state and H2 in the gas phase. We took DSH S(H2), where S(H2) is the entropy of H2 in the gas phase at

standard conditions (T 298.15 K, Pressure 1 atm). We used the assumption

that the vibrational entropy in the adsorbed state is small58. For H/1T0 MoS2, EZPE 0.228 eV (vibrational frequencies 2,530.1 cm1, 637.6 cm1, 519.8 cm 1)

and EZPE 0.271 eV for H2. With these values, the Gibbs free energy was calculated

as DGH DEH 0.29 eV. Additional details are available in Supplementary

Methods.

Photocatalysis. Stock mixtures of 30% v/v TEOA and 10 mg ml 1 Eosin Y were freshly prepared for each reaction. MoS2 (and WS2) were prepared at twice the desired nal concentration (for example, 80 p.p.m.). To prepare the reaction mixture, 35 ml of the 30% v/v TEOA was mixed with 35 ml of the MoS2 or WS2, reaching the nal concentration of 15% v/v TEOA and the predetermined MoS2/WS2 concentration (for example, 40 p.p.m.). The mixture was then transferred to a 250 ml two-neck ask, to which 1 ml of the 10 mg ml 1 Eosin Y was added. The ask was then covered in aluminum foil, stirred and purged with continuous Ar ow (20 c.c.m.) for 20 min. After Ar purge, the aluminum foil covering was removed, and the mixture was illuminated at 1 sun. With the Ar ow kept at 20 c.c.m., gas samples were continuously monitored at 90 s intervals using an Incon 3,000 micro GC gas analyzer until reaction termination.

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Acknowledgements

This work was supported by the U.S. Department of Energy, Ofce of Science, Ofce of Basic Energy Sciences, Division of Materials Sciences and Engineering. XPS and gas chromatography experiments were supported by the U.S. Department of Energy, Ofce of Science, Ofce of Basic Energy Sciences, Catalysis Science Program grant DE-FG02-02ER15368. Fluorescence decay measurements were performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Ofce of Basic Energy Sciences user facility. Computing resources were provided by the National Energy Research Scientic Computing Center and the Texas Advanced Computing Center (TACC). N.S. was partially supported with funding Sandia National Laboratories. We thank Ana B. Trujillo and James A. Ohlhausen for AFM use. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energys National Nuclear Security Administration under contract DE-AC04-94AL85000.

Author contributions

The study concept and design was provided by S.C., N.S. and C.J.B. Materials synthesis was performed by S.C. Catalysis experiments were performed by S.C. and E.C. Spectroscopy experiments were performed by S.L., T.L., K.A., B.K. and S.C. DFT calculations were performed by N.S. STEM imaging was performed by P.L. with S.C. STEM simulations were performed by P.L. All authors discussed the results and contributed to the analysis of the data. Critical revision of the article for intellectual content was conducted by all authors.

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Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T' (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔG_H ), and, with respect to catalysis, the 1T' transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T' molybdenum disulfide and a lowering of ΔG_H from +1.6 eV for 2H to +0.18 eV for 1T', comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

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Title

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

Author

Chou, Stanley S; Sai, Na; Lu, Ping; Coker, Eric N; Liu, Sheng; Artyushkova, Kateryna; Luk, Ting S; Kaehr, Bryan; Brinker, C Jeffrey

Pages

8311

Publication year

2015

Publication date

Oct 2015

Publisher

Nature Publishing Group

e-ISSN

20411723

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1038/ncomms9311

ProQuest document ID

1719425186

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

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