doi: 10.1038/nmat3700 - nature · new jersey 08854, usa. 2university of ... 3rutgers university,...

Enhanced Catalytic Activity in Strained Chemically Exfoliated WS2 Nanosheets for Hydrogen Evolution

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Enhanced Catalytic Activity in Strained Chemically Exfoliated WS2 for Hydrogen

Evolution

Damien Voiry1, Hisato Yamaguchi1, Junwen Li2, Rafael Silva3, Diego C B Alves1,

Takeshi Fujita4,5, Mingwei Chen4,5, Tewodros Asefa3,6, Vivek B Shenoy2, Goki Eda7,8,

and Manish Chhowalla1

1Rutgers University, Materials Science and Engineering, 607 Taylor Road, Piscataway,

New Jersey 08854, USA. 2University of Pennsylvania, Materials Science and Engineering, 3231 Walnut St.,

Philadelphia, PA 19104, USA. 3Rutgers University, Department of Chemistry and Chemical Biology, 610 Taylor Road,

Piscataway, New Jersey 08854, USA 4WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577,

Japan. 5JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan 6Rutgers University, Department of Chemical and Biochemical Engineering, 98 Brett

Road, Piscataway, New Jersey 08854, USA. 7National University of Singapore, Physics Department and Graphene Research Centre,

2 Science Drive 3, Singapore 117542. 8National University of Singapore, Chemistry Department, 3 Science Drive 3, Singapore

117543.

**To whom correspondence should be addressed: [email protected]

Supplementary Information

Materials Synthesis

Bulk WS2 was intercalated with lithium by reacting WS2 powder (0.3 g, Alfa Aesar) with

n-butyllithium in hexane (1.6 M, 4 ml, Sigma-Aldrich) at 100° under argon. After 2 days,

Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution

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the suspension was filtered over a 450µm pore size membrane (Millipore) and washed 3

times with 50 ml of hexane (Sigma-Aldrich) giving a black powder of intercalated

LixWS2. Exfoliation in water was achieved immediately after intercalation. Dissolution

starts spontaneously but the solution was ultra-sonicated to facilitate the exfoliation of

the WS2 crystals. The solutions were then centrifuged several times to remove lithium

cations as well as the unexfoliated materials. Finally the solution was sonicated with a

sonicator horn at 25 W (40% amplitude) for 6 min.

Electrode Preparation

The electrodes for HER measurements for the 1T phase were prepared by simply drop

casting the as-exfoliated solution onto the glass carbon electrodes. For 2H HER

measurements, films of 1T WS2 were prepared by vacuum filtering 0.5 ml of the solution

over a 3.5 cm diameter membrane with 25 nm pores (equivalent to 6.5 µg/cm2). Films

were delaminated on water and transferred on silicon wafer covered with a 300-nm oxide

overlayer and annealed at 300°C under vacuum with a 70 sccm flow of a N2/H2 mixture.

Poly(methyl methacrylate) (PMMA) was then spin coated on the top of WS2 and the

silicon oxide layer was etched by a 1M NaOH solution. The delaminated films

(PMMA/WS2) were finally washed in water and transferred on the glassy carbon

electrode and the PMMA layer was removed in acetone. No chemical

modification/degradation (such as oxidation) was observed during this process as

confirmed by XPS. Platinum nanoparticles (1%) on carbon (Sigma-Aldrich) were

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dispersed in ethanol at 2 mg/ml. 10 µl was deposited on the working electrode

(equivalent to 0.28 mg/cm2) and then was covered by 7 µl of the 5% Nafion solution.

Materials Characterization

AFM data were obtained with a Digital Instruments Nanoscope IV in tapping mode with

standard cantilevers with spring constant of 40 N/m and tip curvature <10 nm. The TEM

images were taken in a JEOL 2000X at 200 kV. HAADF STEM imaging was performed

using JEOL JEM-2100F TEM/STEM with double spherical aberration (Cs) correctors

(CEOS GmbH, Heidelberg, Germany). The lens aberrations were optimized by

evaluating the Zemlin tableau of an amorphous carbon. The acceleration voltage was set

to 120 kV and the collecting angle was between 100 and 267 mrad.

Figure S1: Schematics of WS2 monolayer structure as viewed down the c-axis for

corresponding STEM-HAADF in Figure 2b and 2c in the main MS. The left image

represents the distorted 1T superlattice structure while right image is the 2H structure.

The orange circles represent tungsten atoms while the light and dark green circles




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represent S atoms in the upper and lower layers, respectively. In the 2H structure, the two

layers of S atoms overlap in position and only those in the upper layer are shown.

The stress tensor map was calculated using the peak pair analysis (PPA) approach

outlined in Ref. 44 in the main text. The image analysis was conducted using

DigitalMicrograph software. A more detailed stress tensor map for the distorted 1T

superlattice obtained from the high resolution STEM analysis is shown in Figure S1.

Figure S2: Stress tensor map of distorted 1T superlattice phase calculated from peak pair

analysis of the high resolution TEM results. The lighter regions represent tensile stress

while the darker regions represent regions of compressive stress. The image is 7.45 nm x

7.45 nm. It should be noted that out-of-plane displacement of the atoms is assumed to be

negligibly small compared to the lateral displacement.

Raman spectra were measured using an InVia Raman microscope (Renishaw) at

excitation laser wavelength of 514 nm. X-Ray photoelectron spectroscopy (XPS)

experiments were performed with a Thermo Scientific K-Alpha spectrometer. All spectra

were taken using Al Kα micro focused monochromatized source (1486.6 eV) with a






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resolution of 0.6 eV and a 400 µm spot size. The WS2 thin films were measured on

silicon dioxide wafer and Si2p3 was taken as a reference at 103.4 eV.

Raman and XPS Analysis

Raman shift of as-deposited and annealed WS2 films are shown in Figure S3a. Spectrum

of bulk 2H WS2 is also provided for comparison. Two prominent peaks corresponding to

the in-plane E12g and out-of-plane A1g modes of 2H WS2 are observed for all three

samples. However, the as-exfoliated samples exhibit small peaks in the lower frequency

region, similar to what occurs in chemically exfoliated MoS2, that correspond to the

distorted 1T phase Raman active modes that are not allowed in the 2H phase. The

fraction of each phase was quantified using XPS. The W peaks at ~ 31 and 36eV

correspond to the 4f7/2 and 4f5/2 components of the 2H-WS2 phase. Deconvolution of

these peaks reveals that they are shifted to slightly lower energies compared to the 2H

phase, as indicated by the curve fits in Figure S3b. In the as-exfoliated films, the two

primary peaks correspond to the 1T phase. The contribution of this phase decreases upon

annealing at 300°C, where the spectrum matches closely the bulk 2H phase WS2. Similar

peak shifts are found in the sulfur peaks, indicating the presence of 1T phase. The

quantification of the structure using XPS reveals that the ~80% of the as-exfoliated WS2

sheets consist of the metallic 1T phase, in contrast with similar analysis of as-exfoliated

MoS2 which contains only 50% 1T component (Ref 31 in the MS). Upon annealing, the

concentration of the 1T phase decreases and the nanosheets consist primarily (~80%) of

the 2H phase. The large fraction of 1T phase is also confirmed by electrical






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measurements that show that the as-exfoliated WS2 nanosheets are electrically

conducting while the annealed material is semi-conducting.

Figure S3: a, Raman spectra of WS2 samples deposited on silicon dioxide. The J1, J2, and

J3 weak peaks in the shaded regions are only active in the as-exfoliated 1T WS2 phase.

The spectrum for WS2 nanosheets annealed at 300oC resembles that of 2H bulk phase that

is also shown. b, XPS spectra showing the W5p and W4f core level peak regions. W4f

peaks were deconvoluted with the 2H (light red) and 1T (light green) components. Based

on the deconvolutions, the 2H-1T content from the samples can be determined. As

deposited WS2 platelets are composed of ~ 80 % 1T phase whereas after annealing at

300oC 80% of the material is converted to 2H.

Structural Evolution of Chemically Exfoliated WS2 Nanosheets

The as-exfoliated WS2 nanosheets predominantly exhibit distorted 1T structure. This has

been verified with extensive TEM, Raman and XPS analysis. Raman and XPS were used

to monitor the evolution of the WS2 phase from the distorted 1T to 2H. It can be seen

from Figure S4a that the contribution of the 1T phase decreases with temperature while






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the contribution from the 2H phase increases, as indicated by the fitting the W 4f7/2 and

4f5/2 peaks in XPS. The 1T phase contribution in Figure S4a is indicated by the light

green curve fits while the light red curve fits represent the 2H phase. The ratio of the two

phases as a function of the annealing temperature is summarized in Figure S4b. The

transition from 1T to 2H occurs at a transition temperature of around 200oC, similar to

what has been reported previously (See Ref 39 in the main text). It can be seen that the

as-exfoliated samples consist of ~80% 1T phase while the annealed samples consist of ~

80% 2H phase.

Figure S4: (a) XPS spectra of chemically exfoliated WS2 nanosheets as a function of

annealing temperature. The light green curve fits represent 1T contribution while the light

red curve fits represent the 2H phase contribution. (b) Summary of 1T phase

concentration and thin film sheet resistance (see below for details) as a function of

annealing temperature.

The Raman spectra were also monitored as a function of the annealing temperature, as

shown in Figure S5. The spectra reveal that the intensities of the Raman active modes






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from the 1T phase decrease with annealing temperature while the peaks corresponding to

the 2H phase become sharper. Spectrum of bulk 2H WS2 is also shown for comparison.

Figure S5: Raman spectra of chemically exfoliated WS2 nanosheets as a function of

annealing temperature.

Electrical Properties

Electrical and photoluminescence (PL) measurements were performed in order to monitor

the structural evolution during annealing. The current voltage (I-V) characteristics of thin

films of chemically exfoliated WS2 are shown in Figure S6a. The as-exfoliated material

exhibited linear I-V characteristics and low sheet resistance 430k�/sq. reflecting its more

conducting nature. Annealing leads to an increase in sheet resistance, especially above

200oC, consistent with the onset of transition from 1T to 2H phase. The resistance as a






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function of the annealing temperature (as well as the phase concentration) is shown in

Figure S4b. The transition to predominantly 2H phase was also corroborated by

photoluminescence measurements as shown in Figure S6b. Excitation by 514nm laser of

a monolayered film gave rise to PL at 2.01eV, which is in agreement with the direct band

gap of 2H WS2 monolayer. Very weak or no PL was observed in partially annealed WS2

nanosheets or fully annealed films with thicknesses substantially higher than a

monolayer, as indicated in Figure S6b. These results are similar to our findings on MoS2

(See Ref 31 in the main text).

Figure S6: (a) DC current versus voltage measurements as a function of annealing

temperature on WS2 thin films composed of chemically exfoliated nanosheets. (b) PL of

thick and thin WS2 films annealed at 300oC. The PL is only observed for the thin films,

which exhibit direct band gap. The inset shows the Raman peaks for a thick and thin WS2

films.






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1T and strain relationship:

We have correlated the strain values to the 1T phase concentration by measuring the

stress tensor maps in the TEM. We found that the strain is directly correlated with

annealing (i.e. decrease in 1T concentration). Our analysis suggests that 1T

concentrations of ~80%, ~50%, ~25%, and 0% would correspond to strains of ~ 3%,

~1%, ~ 0.3%, and 0%.

Figure S7: Stress tensor maps of WS2 nanosheets. Left most image represents as-

synthesized films containing maximum amount of 1T phase concentration. The images

represent decreasing 1T phase concentration with the right one representing 100% 2H

phase showing no strain.

Additional HER properties:

The key step in HER is the adsorption of the proton on the active site. To assess this, we

have varied the pH. We found that WS2 is active over a wide range of pH although the

activity decreases after pH=1 due to the strong diminution of the quantity of protons

available. Similar behavior has been reported in the case of amorphous molybdenum

sulfide with a strong decrease of the current density observed when increasing the pH

from 1 to 2 (Ref 16). Although the direct interpretation of the HER mechanism from the






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Tafel slope is limited to well-defined systems such as Pt, the dramatic increase of the

Tafel slope can be interpreted as decrease in hydrogen adsorption of WS2 as pH

increases.

Figure S8: Variation of current density versus the overpotential (RHE) as a function of

the pH. The highest current density is obtained for the lowest pH, consistent with the

solution having the highest proton concentration. The inset shows that the Tafel slope

also increases with pH indicating that the rate limiting proton adsorption step is strongly

influenced by the proton concentration. Overpotential = E (RHE) – (-0.059 x pH).

Active sites measurements:

The density of active sites was measured using underpotential deposition (UPD) of

copper using the method described by Green et al. in Ref 28 in the MS. Green et al. have






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demonstrated that the surface activity of platinum or ruthenium can be determined by

measuring the charges exchanged during copper stripping after deposition at the

underpotenial regions. By comparing the charges exchanged during hydrogen adsorption

and copper stripping in sulfuric acid in absence and presence of cupric sulfate (CuSO4) at

various potentials, they found that when shifting the potential toward underpotential

regions, the charge ratio QCu/QH decreases and reaches a plateau for QCu/QH equal to 2.

This ratio of 2 is expected when monolayer cupper is deposited at the same site as

hydrogen since copper stripping involves 2 electrons versus one for hydrogen adsorption.

At higher potentials the ratio decreases indicating that less copper is deposited compared

to the quantity of adsorbed hydrogen. We found that exfoliated WS2 has the ability to

reduce copper at higher potentials (underpotential deposition, UPD) than its

thermodynamic potential (overpotential deposition, OPD) and the method of Green et al.

was applied to quantify the number of active sites.

Figure S9: Cycling voltammetry of the exfoliated WS2 in presence of cupric sulfate

reveals the presence of overpotential stripping and deposition signal (IA and Ic

respectively) and broad peaks (IIA and IIC) corresponding to the underpotential regions






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between 300 mV and 650 mV vs. RHE. Under the same condition bulk WS2 shows a

small UPD signal close to the OPD potential between 250 and 300 mV vs. RHE whereas

glassy carbon does not exhibit any underpotential deposition signal, as shown.

Experimentally, the charges associated with the copper stripping were measured in three

steps. In a 0.1 M of H2SO4 and 2mM of CuSO4 solution, the electrode surface was

thoroughly electrochemically cleaned prior to performing any deposition by applying a

potential of 673 mV vs. RHE for 120 seconds. Then the potential was lowered and kept

constant for 100s to deposit copper after which it was progressively increased to 673 mV

vs. RHE to oxidize the copper. The current associated with the copper stripping was

measured. Hydrogen adsorption was measured the same way using a 0.1 M H2SO4

solution in absence of copper ions. The results agree perfectly with the observations

reported by Green et al. and the QCu/QH reaches at plateau of 2 after 525 mV vs. RHE

(See Figure S9). This indicates that at this potential the same quantity of copper as

hydrogen is deposited on the surface of the electrode at the active sites. From the quantity

of charges generated during the Cu stripping at a potential of deposition of 573 mV vs.

RHE, it can be calculated that 3.21 x 1013 mol of Cu (QCu/96500/2) have been deposited.

From this, an active site density ranging from 4.5 x 1014 sites/cm2 – 2.0 x 1015 sites/cm2

(nCu/Aelectrode) is obtained for hydrogen adsorption on 1T WS2 nanosheets. Similar

analyses on the annealed samples reveal lower active site density of 1.5 x 1014 sites/cm2.

The current exchange density was calculated using the method described previously in

Ref. 4. From the Tafel plot, we measured the current density to be 6.2 x 10-6






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A/cm2geometric. From the measured site density of 4.5 x 1014 cm-2, the exchange current

density per site is equal to 1.37 x 10-20 A/sites. This value was then multiplied by the

density of sites in case of platinum (1.5 x 1015 cm-2) for a direct comparison. Based on

this method, the exchange current density of strained chemically exfoliated WS2 was

found to be 2 x 10-5 A/cm2.

Figure S10: QCu/QH reaches at plateau of 2 after 525 mV vs. RHE indicating that at this

potential the same quantity of copper as hydrogen is deposited on the surface of the

electrode at the active sites.

Experimental details for the active sites measurements:

Electrodes for active site measurements were prepared in the exact same manner as for

HER experiments.

0.45 0.50 0.55 0.6002468

10121416

QC

u/QH

Potential (V vs. RHE)

As-Deposited






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Active site measurements were performed using two 0.1 M H2SO4 solutions: one

containing only sulfuric acid, the second one containing the cupric sulfate at 2 mM.

Solutions were degassed with Ar prior the measurements and the measurement were done

under an Ar blanket. Cyclic voltamperometric were performed with a scan rate of 2 mV/s

and a saturated calomel electrode was used as the reference. Before each measurement,

electrodes were cleaned at 673 mV for 120 s and the potential of deposition was

maintained constant for 100 s. Stripping currents were measured by progressively

increasing the potential from the deposition potential up to 673 mV with a scan rate of 2

mV/s.

Discounting the presence of Lithium impurities:

We have ruled out any possibility of Li in our exfoliated samples by performing highly

sensitive Rutherford backscattering spectroscopy (RBS) analysis. The results shown in

Figure S11 clearly demonstrate that Li is not present in our samples.






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Figure S11: RBS results from an exfoliated WS2 nanosheet film showing no Li is present

(red curve). A curve from a Li intercalated sample is also shown to demonstrate that Li

can be detected with this technique. Curves are normalized to the W peak.

Stability of 1T Phase as a function of long term stressing:

To assess the electrochemical stability of the metastable 1T electrodes, we performed

over 10,000 cycles to monitor the overpotential. We also stressed the electrodes by

monitoring the current density for more than 100 hours at an applied cathodic potential of

0.3V. The variation in the current density, percent change in the overpotential, and the

change in 1T concentration as a function of the electrode operation time are shown in the






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main MS. Below we provide the XPS and Raman data showing that spectroscopically the

1T phase remains substantially stable during operation.

Figure S12: (a) W XPS peaks as a function of catalytic activity time. The fits represent

ratio of 1T phase which was found to remain ~ 80% +/- 5%. (b) Raman spectra showing

negligible change in 1T features at different catalyst operation times.

In addition to the long term testing, we also measured the concentration of 1T phase in

samples stored in our laboratory for over the past year to investigate whether the 1T

phase relaxes to the 2H phase.






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Figure S13: 1T phase concentration as a function of the number of days after exfoliation.

It can be seen that the 1T phase remains stable with time.

Impedance Measurements:

Impedance measurements were performed on electrodes consisting of 1T and 2H films,

bulk WS2 powder as well as glassy carbon reference.

0 100 200 3000

25

50

75

100

Films Solutions

1T c

onte

nt (a

t. %

)

Days






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Figure S14: Impedance spectra for glassy carbon electrode alone.

Comparison between Pt and carbon counter electrode:

Platinum contamination is a major risk in electrocatalysis since platinum would

artificially increases the activity of WS2. Figure S15 shows the polarization curves

obtained from as-deposited WS2 using platinum and carbon counter electrode. No

difference is noticeable and this rules out any Pt contamination during the measurements

(Figure S15).






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Figure S15: Polarization curves for as-deposited WS2 when using Pt or Carbon as counter

electrode.

Calculation of Turn Over Frequency:

The exchange current densities (i0) of the different samples were determined from the

Tafel plots. The linear fit of the Tafel plot for the as-deposited sample gives an exchange

current density of 6.2 x 10-5 A/cm2. Turn-over frequencies were calculated from the

density of active sites, which were explicitly measured using the method described above.

This results in a density of active sites of ~4.5 x 1014 cm-2. Turn-over frequencies were

then calculated using the following relation (See Ref 5 in the main text):

TOF (s-1) = (i, A/cm2) / ((4.5 x 1014 sites/cm2)*(1.602 x 10-19 C/e-) (2 e-/H2))

Replacing the current density (i, A/cm2) by the exchange current density determined from

the Tafel plot gives a turn-over frequency of 0.043s-1.






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Turn-over frequencies at the different potentials are calculated the same way using the

current density (i, A/cm2) instead of the exchange current, i0.

Computational approach

The density functional calculations were carried out by using the Vienna ab initio

simulation package (VASP), with exchange-correlation functional described by Perdew-

Burke-Ernzerhof generalized gradient approximation (PBE-GGA) and interaction

between core electrons and valence electrons by the frozen-core projector-augmented

wave (PAW) method. An energy cutoff of 600 eV was used for plane wave basis

expansion. The fully relaxed lattice constants are a = 3.18 Å, b = 3.18 Å and a = 3.19 Å,

b = 6.54 Å for 2H-WS2 and distorted 1T-WS2, respectively. W-W bond length in 2H-

WS2 is 3.18 Å. In the distorted 1T-WS2 the W-W can have bond lengths of 2.77 Å and

3.19 Å. These values compare well with our experimental measurements. The hydrogen

adsorption energies have been calculated with a 4x2x1 supercell shown in Figure S16 for

1T-WS2. Usually denser k points are required to achieve convergence for metals.

However, the bands of 1T-WS2 are rather flat as shown in Figure S17 such that we can

sample the Brillouin zone by using Monkhorst-Pack 4x4x1 k-point grid. Denser sampling

only gives rise to a total energy change less than 2.5 meV as shown in Figure S18 for the

unit cell of 1T-WS2. We also carried out spin-polarized calculations for different H

coverage of 0% strain which only show total energy changes smaller than 1 meV with

respect to the non-spin-polarized results as indicated in Figure S19. A vaccum region of

greater than 18.00 Å was added along the direction normal to the sheet plane to avoid the

interaction between periodic supercells. We carried out relaxation for all supercells and






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all atoms in the supercells were allowed to move until the force on each atom is less than

0.02 eV/Å. Although the 2H-WS2 is more stable with total energy 0.537 eV lower than

that of the distorted 1T-WS2, there is a bigger energy barrier for the phase transformation.

The nudged elastic band [citation: H. Jonsson, G. Mills, and K.W. Jacobsen, in

Classical and Quantum Dynamics in Condensed Phase Simulations, edited by B. J.

Berne, G. Ciccotti, and D. F. Coker (World Scientific, Singapore, 1998)] was applied

to determine this energy barrier. Because of the lattice mismatch between 1T and 2H

structures, we employ the lattice constants of 1T phase for both 1T and 2H structures and

results are presented in Figure S20. The energy barrier is determined to be 0.92 eV. The

tensile strain experienced by the 2H structure makes the stabilization energy reduce to

0.49 eV. We also carried out calculations with lattice constants of 2H phase. The energy

barrier is 0.97 eV and the compressive strain experienced by the 1T structure increases

the stabilization energy to 0.59 eV.

The stability of hydrogen can be described by the differential hydrogen adsorption energy

ΔEH, which is defined as

ΔEH = EWS2 + nH - E WS2+(n-1)H – ��EH2,

Where EWS2+nH is the total energy for the WS2 system with n hydrogen atoms absorbed on

the surface, EWS2+(n-1)H is the total energy for (n-1) hydrogen atoms adsorbed on the

surface and EH2 is the energy for a hydrogen molecule in the gas phase. The ΔEH

obtained from the ground state calculations does not include the contributions from the






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vibrational motion and the entropy. The Gibbs free energy for hydrogen adsorption can

be calculated by including these corrections:

ΔGH = ΔEH + ΔEZPE - TΔSH

where ΔEH is the differential hydrogen adsorption energy, ΔEZPE is the difference in zero

point energy between the adsorbed hydrogen and hydrogen in the gas phase and ΔSH is

the entropy difference between the adsorbed state and the gas phase. The contribution

from the configurational entropy in the adsorbed state is small and is neglected. We can

take the entropy of hydrogen adsorption as ΔSH = 1/2SH2 where SH2 is the entropy of

molecule hydrogen in the gas phase at standard conditions[citation: NIST Chemistry

WebBook, NIST Standard Reference Database Number 69, edited by Linstrom, P. J.

& Mallard, W. G. (National Institute of Standards and Technology, Gaithersburg,

MD, 2009), http://webbook.nist.gov.] (1 bar of H2, pH = 0 and T = 300 K). We have

calculated the vibrational frequencies of H adsorbed on site 1 of the distorted 1T-WS2 by

using finite differences to determine the Hessian matrix. The calculated vibrational

frequencies are 2521 cm-1, 668 cm-1 and 562 cm-1. With these values the Gibbs free

energy is ΔGH = ΔEH + 0.28eV. Our calculations show that vibrational frequencies do not

exhibit sensitive dependence on strains studied in this paper, so we use the same value for

all corrections.






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1 2 3 4

5 6 7 8

Figure S16 The supercell used in the calculations of hydrogen adsorption on thesurface of the distorted 1T-WS2. Gray and yellow spheres represent the W and Satoms, respectively. The numbers indicate the favorable sites of hydrogen binding.

Figure S17: Electronic band structure of distorted 1T WS2. The Fermi level is set to zeroand is indicated by blue solid line.






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Figure S18: Convergence test of k-point sampling. The 4x4x1 k grids used in ourcalculations for 4x2x1 supercell is equivalent to the 16x8x1 k grids for 1T unit cell.Denser k grids such as 24x12x1 and 32x16x1 only lead to total energy change less than2.5 meV.

Figure S19: Energy difference between spin-polarized and non-spin-polarizedcalculations for unstrained 4x2x1 supercell shown in Figure X. (1) corresponds to onehydrogen at site 1 and (1,2), (1,3), (1,5), and (1,6) correspond to two hydrogen atomswith one at site 1 and the other one at site 2, 3, 5, and 6, respectively.






26

We found that the hydrogen prefers to adsorb on the top of the S atoms which are

indicated in Figure S16. We studied two different hydrogen adsorption coverages (1/16

and 2/16) for the distorted 1T-WS2 and varied the tensile strain from 0% to 4%. The

calculated Gibbs free energies are shown in Table I and Figure S21. The strain can

strengthen the hydrogen binding on the surface. For small strain, the low coverage can

effectively evolve hydrogen. With higher strain, the high coverage will also be able to

turn over. We also analyzed the density of states of the distorted 1T-WS2 under different

strains and the results for 0% and 3% strains are depicted in Figure S22. Note that strain

Figure S20: Phase transformation energy barrier and stabilization energy between 1T and 2Hstructures. The lattice constants of 1T-WS2 were used for both 1T- and 2H-WS2 in the calculations. With lattice constants of 2H-WS2 for both 1T- and 2H-WS2 the energy barrier and stabilization energy are determined to be 0.97 eV and 0.59 eV, respectively.






27

significantly increases the density of states near the Fermi level. This may account for the

enhanced hydrogen binding and the catalytic performance.

We also studied the hydrogen adsorption on the surface of 2H-WS2 and a 4x4x1 supercell

is employed. The size of this supercell is close to that used in the study of distorted 1T-

WS2. Our calculations show that even under 4% tension, the Gibbs free energy for 1/16

coverage is still as high as 1.93 eV. This indicates that the tensile strain in the range

studied here can not make the surface of 2H-WS2 have good catalytic activity.

Table I: Calculated Gibbs free energy for H under strain ranging from 0.0% to 4.0%. The

1/16 coverage corresponds to one hydrogen at site 1 and the 2/16 coverage correspond to

two hydrogen atoms with one at site 1 and the other one at site 2, 3, 5, and 6, respectively.

Strain ΔGH (eV)

1

ΔGH (eV)

1, 2

ΔGH (eV)

1, 3

ΔGH (eV)

1, 5

ΔGH (eV)

1, 6

0.0% 0.28 0.49 0.48 0.41 0.36

1.0% 0.19 0.39 0.36 0.31 0.26

2.0% 0.10 0.32 0.25 0.20 0.16

3.0% -0.06 0.32 0.24 0.16 0.13

4.0% -0.15 0.32 0.24 0.11 0.09






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Figure S21: Calculated Gibbs free energy for H under strain ranging from 0.0% to 4.0%.(1) corresponds to 1/16 coverage with one hydrogen at site 1 and (1,2), (1,3), (1,5), and(1,6) show the Gibbs free energies of the 2/16 coverage corresponding to two hydrogenatoms with one at site 1 and the other one at site 2, 3, 5, and 6, respectively.






29

Figure S22: Density of states of the distorted 1T-WS2 without or with 3% strain. The

Fermi level is set to be zero and is indicated by the vertical line.





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