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This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1

Molybdenum disulfide/pyrolytic carbon hybrid electrodes for scalable

hydrogen evolution

Hugo Nolan,a,b

Niall McEvoy,*a Maria O’Brien,

a,b Nina C. Berner,

a Chanyoung Yim,

a,b Toby Hallam,

a

Aidan R. McDonaldb and Georg S. Duesberg*

a,b

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX 5

DOI: 10.1039/b000000x

The electrochemical generation of hydrogen fuel via the proton reduction in the Hydrogen Evolution Reaction

(HER) in aqueous media is currently dependent on the use expensive noble metal catalysts for which alternatives

must be sought. Molybdenum disulfide (MoS2) has shown great promise as a suitable electrocatalyst in this regard.

While many lab-scale experiments on the HER activity of this material have demonstrated its viability and explored 10

some fundamental mechanistic features of HER at MoS2, these experimental techniques are often ill-suited to large

scale production of such electrodes. In this study we present work on the fabrication of MoS2/pyrolytic carbon (PyC)

electrodes via vapour phase sulfurization of Mo thin films. These hybrid electrodes combine the catalytic activity of

MoS2 with the conductivity and stability of PyC, whilst using industrially compatible processing techniques.

Structural defects in the sulfur lattice were found to be key catalytically active sites for HER and thinner MoS2 films 15

displayed a higher quantity of these defects and, hence, an improved HER activity. The observed Tafel slope of

95 mV/dec is comparable to previous literature works on MoS2 HER performance.

Introduction

Efficient and sustainable production of hydrogen is essential for a

hydrogen based economy to reduce our dependence on fossil fuel 20

energy sources.1 The electrochemical splitting of water via proton

reduction in the Hydrogen Evolution Reaction (HER) provides a

feasible route towards this without dependence on consumable

resources. However, to date, the best HER performance has been

achieved using prohibitively expensive platinum electrodes, 25

severely limiting the practical applications of this technology. If

the HER is to be exploited as a viable energy source, alternative

electrocatalysts to Pt must be sought.

Recent years have seen ever increasing focus placed on the

synthesis and characterisation of layered 2D materials, based 30

principally on their novel properties.2 Much work in the literature

at present is concerned with layered Transition Metal

Dichalcogenides (TMDs).2,3 This group of materials has been

proposed for a plethora of next generation uses with potential

applications in 2D electronics,4–6 gas sensing7–9

and, most 35

significantly for this study, catalysis of the HER,10–12

having

been demonstrated in recent years.

Thus far, molybdenum disulfide (MoS2) has been the most

heavily studied material in the context of catalysis, although

studies on the HER behaviour of other materials such as MoSe2, 40

WS2 and WSe2 have recently emerged.13–15

Depending on the

synthesis route undertaken, HER activity has been attributed to

different features of TMD layers. Where MoS2 flakes have been

exfoliated from bulk MoS2 via intercalation of Li-ions, it has

been demonstrated that the HER activity is related to the presence 45

of the so-called 1T polymorph.16,17

This is not found in bulk

MoS2, which consists solely of the 2H polymorph. Many other

studies have attributed HER activity of MoS2 to the presence of

unsaturated sulfur atoms along molybdenum edges of the MoS2

structure.18,19

Furthermore, theoretical studies have shown that 50

sulfur vacancies catalyse various reactions at the MoS2 surface.20

These defects, in the otherwise catalytically inactive basal plane

of the sulfur sub-lattice, act as catalytically active sites.

The majority of studies on HER by TMDs deal with dispersions

of TMD flakes or nanoparticles which are then immobilised on an 55

electrode surface for HER measurements. Often these same

studies incorporate further processing conditions or stages to

enhance the HER activity of these films by establishing active

sites on the material. Li et al. used a solvothermal process to

synthesise MoS2 flakes and found that incorporating Reduced 60

Graphene Oxide (rGO) to create an MoS2/rGO hybrid improved

performance for HER.10

Xie et al. reported a synthetic chemical

route where the reaction precursors were controlled to create

defective MoS2 nanoflakes which exhibit favourable HER

activity.21

Wang et al. isolated MoS2 flakes exfoliated from 65

bulk22 and subsequently attributed increased HER performance of

smaller nanoparticles to an increase in available unsaturated

sulfur sites.23 Yu et al. found that the HER activity of MoS2

grown via Chemical Vapour Deposition (CVD) from an MoCl5

precursor improved with a reduction in the number of MoS2 70

layers.24 They correlated the activity to the hopping of electrons

between these layers. Typically characterisation of the HER

activity of such materials is carried out by depositing very small

amounts of the material on conductive substrates such as glassy

carbon. Merki et al. developed a technique whereby MoS2 flakes 75

can be electrochemically deposited on conductive substrates via

2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

repeated cyclic voltammetry in an electrolyte of (NH4)2[MoS4].25

Such direct deposition/synthesis techniques could conceivably

allow the large-scale fabrication of MoS2 electrodes for energy

applications.

Nonetheless, many techniques employed in literature studies on 5

HER at MoS2, while useful for lab-scale experiments, offer poor

scalability. Ultimately, if the implementation of TMDs as HER

catalysts is to be successful, large scale production of such

electrocatalysts for industrial applications must be viable. Here,

we propose an electrode fabrication process that involves the 10

deposition of thin films of metallic Mo directly on conductive

substrates followed by vapour phase sulfurisation. The poor

conductivity of MoS2 precludes the use of electrodes fabricated

entirely from the material. Mo films were evaporated on to

Pyrolytic Carbon (PyC) films grown via CVD. Our previous 15

experience with PyC has shown that this material is highly

suitable as an electrode material given its chemical, thermal and

mechanical durability with many reported electrochemical

applications.26–29

This investigation aims to show that such thin layers of MoS2, 20

grown via vapour phase sulfurisation directly on a substrate of

conductive PyC, present a suitable electrode for HER with few

limits to up-scaling production in an industrially compatible

process. In addition, details of the reaction mechanism are

discussed and correlated to spectroscopic and microscopic 25

characterisation of the MoS2/PyC films.

Results and Discussion

Electrodes were fabricated on SiO2/Si substrates with an initial

step of CVD growth of PyC. Subsequent deposition of Mo was

performed in a defined region of the sample to control the area of 30

the resultant electrode. Finally, vapour phase sulfurisation was

carried out to form MoS2 on the PyC coated substrate. All further

material characterisation and electrochemical measurements were

performed on such samples.

Material Characterisation 35

Raman Spectroscopy

MoS2 samples 4.5, 10 and 53 nm thick were investigated and

spectral features of both PyC and MoS2 are evident. The broad D

and G bands (~1353 and ~1583 cm-1, respectively) coupled with

the highly suppressed 2D band at 2700 cm-1 are indicative of a 40

disordered graphitic system such as PyC. The peaks observed at

~383 and ~408 cm-1 are characteristic of MoS2 and are denoted

the E12g and A1g peaks and arise due to in plane and out of plane

vibrations, respectively.30,31 Fig.1 (a) shows a comparison of the

Raman spectra of each sample, normalised to the intensity of the 45

A1g peak, with the D and G band region of PyC highlighted in

grey and the MoS2 peaks highlighted in cyan. It is immediately

apparent that the relative PyC characteristic signal is much

reduced as the MoS2 layer thickness increases. The greater

quantity of MoS2 present atop of the PyC surface causes a 50

decreased penetration of the Raman laser into the PyC substrate.

Fig 1: (a) Raman spectra for MoS2 films on PyC substrates. Also

included is the spectrum of the bare substrate. The cyan region marks

MoS2 characteristic peaks while the grey region marks PyC characteristic 55

peaks. (b) High spectral resolution scan of the MoS2 peaks with the

reduced peak separation of the thinner films indicated.

Analysis of the MoS2 characteristic peaks divulges information

regarding the nature of the MoS2 films. Fig.1 (b) shows that the

spectral region of interest for MoS2 is rather similar for all 60

samples. However, closer inspection shows a shift in peak

position which is dependent on MoS2 thickness. The coupling

between electronic transitions and phonons is modified as the

MoS2 thickness is decreased, causing a narrowing of peak

separation between the E12g and A1g modes. This behaviour is 65

reflected here with the dotted lines highlighting a reduction in

peak separation of ~2 cm-1 on going from 53 nm to 4.5 nm thick

MoS2. From this it is apparent that the 4.5 nm film can be

considered distinct from bulk MoS2.

X-Ray Photoelectron Spectroscopy 70

XPS analysis of the MoS2/PyC films revealed a wealth of

chemical information. Survey spectra of each MoS2 film and the

bare PyC substrate are shown in Fig.2 (a) with characteristic

photoemission peaks corresponding to C, S and Mo highlighted

in grey, yellow and cyan, respectively. It is apparent, as with the 75

Raman spectroscopy, that the spectral contributions from the PyC

substrate diminish as the MoS2 layer thickness increases.

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

Fig 2: (a) XPS survey scans of the MoS2/PyC electrodes investigated in

this study. Carbon characteristic core level peaks marked by grey, Mo

peaks by cyan and the S peaks by yellow. (b) S2p core level peak for

4.5 nm MoS2 film with spectral contributions fitted. 5

To gain a greater insight into the chemical nature of the film,

higher resolution scans of the S2p and Mo3d spectral regions

were recorded and spectral contributions were fitted to the

measured data. Fig.2 (b) shows the S2p region for the 4.5 nm film

including peak contributions, which are consistent with the 10

binding energy values found by Eda et al.32 The two key features

are the doublet peak which corresponds to sulfur in bulk MoS2

(marked in blue) and a smaller doublet peak at a slightly higher

binding energy which is associated with unsaturated sulfur in

edge crystallographic states (marked in green). It was found that 15

this “edge-sulfur” contribution was appreciably smaller for

thicker MoS2 films. The concentration of edge sulfur as

determined by XPS was found to increase from approximately

7.6% in the thickest film to approximately 13.6% in the thinnest

film. A detailed discussion of the S2p fitting and a comparison of 20

all three MoS2 film thicknesses is included in the ESI in Fig.S1.

From analysis of the Mo3d spectral region it was found that low

levels of oxides in the form of MoO2 and MoO3 are present in the

MoS2. However, these quantities remain invariant with MoS2 film

thickness and, as such, do not influence any trends in HER 25

activity of the MoS2/PyC electrodes (see ESI for detailed

discussion, Fig.S2).

Electron Microscopy

Fig.3 (a) displays a cross-sectional TEM image of the 53 nm

MoS2/PyC film. The indicated Au/Pd and Pt are part of a 30

protective capping layer involved in the fabrication of the TEM

lamella. A uniform thickness is observed throughout the film as

shown. The polycrystalline nature of the MoS2 layer is evident in

the higher magnification image in Fig.3 (b) where randomly

oriented crystallites can be discerned by virtue of the crystalline 35

planes. Further analysis of MoS2/PyC cross sections is included

in the ESI where an MoS2 interlayer spacing of approximately

6.6 Å was found, consistent with previously reported values.33

Further TEM analysis was carried out on MoS2 grown on SiO2

using an identical technique as used for that grown on PyC. This 40

substrate allowed the MoS2 film to be transferred to TEM grids

via a polymer supported transfer method.

A top-down image of such a MoS2 film is shown in Fig.3 (c).

Hexagonal symmetry is clearly visible in the lattice. Inset in this

panel is a Fast Fourier Transform (FFT) corresponding to this 45

image. The clear hexagonal pattern further confirms the structure

of the film with the presence of multiple MoS2 layers evidenced

by the outer ring in the FFT spot pattern.

Fig 3: (a) Cross sectional TEM image showing uniformity of the 53 nm 50

MoS2/PyC hybrid electrode. (b) HR-TEM of the PyC/MoS2 interface.

Crystalline planes are visible in the MoS2 layer. (c) High resolution top-

4 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

down image of MoS2 film on TEM grid with evident hexagonal

symmetry. Inset: Corresponding FFT image.

HER Catalysis

Linear Sweep Voltammetry (LSV) was performed at a scan rate

of 5 mV s-1 for several MoS2/PyC samples. The voltammetry data 5

is presented in Fig.4.

An understanding of the mechanism by which HER can proceed

is required to fully characterise the HER activity of different

electrode materials. A Tafel plot displays voltammetric data on a

log –linear plot. The linear portion of the curve on a Tafel plot is 10

described by the Tafel equation.

Where η is the overpotential, b is the Tafel slope, j is the current

density and a captures several thermodynamic constants. The

kinetics of different catalytic reactions can be related to the slope.

As a result, analysis of the Tafel slope then provides an insight 15

into the HER mechanism at the electrode surface. The following

is covered in detail by Compton and Banks34

and also Conway

and Tilak35

. In acidic media, an initial step involves the reduction

of a proton in solution followed by adsorption of intermediate

Hads to the electrode surface as follows: 20

(Volmer reaction)

A Tafel slope of approximately 120 mV/dec is associated with

this reaction step. This is followed by desorption of H2 by either

of the following reactions:

(Heyrovsky reaction) 25

(Tafel reaction)

According to Conway and Tilak,35

the latter reaction has a Tafel

slope of approximately 30 mV/dec. The former has a slope of

approximately 120 mV/dec when surface coverage of Hads is

relatively low. In cases of higher surface coverage, this reduces to 30

approximately 40 mV/dec. This is caused by the different steps of

the mechanism dominating at the different Hads surface

coverages.

It is immediately apparent from Fig.4 (a) that the MoS2 films

have a dramatically increased HER activity compared to the bare 35

PyC substrate. Given the inertness of the PyC, any contributions

from it towards the performance of the MoS2 films can be

considered negligible. The onset potential for the different

materials was determined by measuring the overpotential at

which a current density of 2 mA cm-2 was achieved. It was found 40

that the bare PyC substrate only achieved this current density at

an overpotential of approximately 870 mV. Pt is known to have

excellent HER catalytic performance and, as expected, a near-

zero overpotential was observed. Interestingly, the onset potential

of the MoS2/PyC films was found to decrease with decreasing 45

film thickness. For 53 nm MoS2 an onset potential of 540 mV

was recorded, the 10 nm film displayed a potential of 520 mV

while an onset of 503 mV was observed for the 4.5 nm sample.

This is a small but clear reduction of the onset potential which

trends with films thickness. 50

Fig 4: (a) Linear sweep voltammograms for MoS2/PyC hybrid electrodes

compared to the bare PyC substrate and a platinum electrode. (b)

Corresponding Tafel plots with fitted slopes.

Further information about the HER activity of the various 55

samples was garnered by analysing the linear region of the Tafel

plots shown in Fig.4 (b). As previously mentioned, some details

about the reaction mechanism at the electrode surface can be

extracted from this data.

It is known that the HER at a Pt electrode proceeds via the so-60

called Volmer-Tafel mechanism with a Tafel slope of 30 mV/dec.

This is reflected in our experimental data where the Pt electrode

exhibited an almost ideal Tafel slope of 31 mV/dec. Of the MoS2

films investigated in this study, the thinnest (4.5 nm) displayed

the lowest Tafel slope of 95 mV/dec. The thicker the MoS2 film, 65

the greater the Tafel slope observed. Li et al. proposed that the

HER at MoS2 electrodes proceeds via a Volmer-Heyrovsky

mechanism.10

A modification of the structural morphology of the

MoS2 flakes used in their MoS2/rGO electrodes caused a

reduction in Tafel slope which suggested an increase in site 70

density for Hads, as is consistent with this mechanism. In addition,

it was proposed that more effective electronic coupling between

the rGO and MoS2 flakes contributed to the enhanced HER

performance. Our investigations indicate a reduction in Tafel

slope as the film thickness decreases. It is likely that decreasing 75

the MoS2 film thickness exposes a greater density of active edge

sites on the surface of the film which, in turn, provides a greater

number of accessible sites for Hads. This is consistent with

behaviour observed by Wang et al., where smaller MoS2 flakes

displayed enhanced HER activity compared to larger ones which 80

was attributed to an increase in unsaturated sulfur sites at the

MoS2 flakes.23 The Tafel slope of 95 mV/dec for our thinnest film

is well below that of 110 mV/dec reported for untreated

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 5

exfoliated MoS2 flakes16 and comparable to that observed by Xie

et al. for calcined synthetic MoS2 nanosheets36

and by Li et al. for

synthetic MoS2 particles.10

The MoS2/rGO hybrid material of Ma

et al. exhibits an identical Tafel slope.37 It should be noted that a

secondary phenomenon may contribute to the improved HER 5

performance of the thinner MoS2 films in this study whereby the

thinner films facilitate electron migration to the MoS2 surface.

While the results presented here do not match the best

performance of MoS2 materials for HER available in the

literature, a greater understanding of the mechanism by which the 10

reaction proceeds has been achieved. Sufficiently thin MoS2

layers exhibit behaviour distinct from bulk and a greater number

of defects in the sulfur lattice is seen. Importantly, our electrodes

display both good reproducibility and stability as shown in Fig.S6

of the supplementary information. It is envisaged that further 15

modification of the MoS2/PyC hybrid stack, by chemical or

physical means, could result in additional enhancement of HER

catalysis. In addition, the facile fabrication of MoS2 electrodes on

conductive substrates renders this process highly applicable for

large scale production. 20

Conclusions

In conclusion, we have developed a robust method of fabricating

electrodes of MoS2 on conductive PyC substrates and

demonstrated encouraging results when this is employed as a

HER catalyst. This combination of materials leads to a 25

mechanically durable electrode assembly which can easily be

manufactured on a large scale, unlike many techniques which are

used in the literature to investigate HER at TMDs. Spectroscopic

techniques confirm the stoichiometry of the MoS2 film and also

indicate that our thinnest films behave in a manner distinct from 30

bulk MoS2. The improved performance of the thinner MoS2 films

over thicker ones is correlated to an apparent increase in sulfur

defects in the thinner sample. We propose that this increase in

crystalline defects allows for an increased surface coverage of

adsorbed H (Hads) on the electrode surface. Controlling the 35

thickness of the starting Mo layer is sufficient to enhance the

HER performance of the resultant electrode. The performance of

the MoS2 catalyst is comparable to other published results for

untreated MoS2 materials. Further engineering of the MoS2/PyC

electrode presented here to increase surface roughness and defect 40

levels could further enhance the HER performance. Additionally,

our methods could readily be extended to synthesise electrodes

from other members of the TMD family (WS2, MoSe2 etc.),

potentially improving their catalytic performance.

Experimental 45

Electrode Fabrication

All chemicals used throughout the synthesis and characterisation

were purchased from Sigma-Aldrich and used as-received. PyC

films were grown by CVD of acetylene at 950° C for 30 minutes

to a thickness of 300-400 nm on 300 nm thermal SiO2 on Si 50

substrates in a hot wall quartz tube furnace as previously

reported.26

Following CVD of PyC, the samples were cooled

under Ar atmosphere to room temperature. Subsequently,

electron beam evaporation was carried out in a Temescal FC-

2000 to deposit Mo to thicknesses of approximately 2 nm, 4 nm 55

and 20 nm. Areas of the PyC substrates were physically masked

so that only a 1 cm2 area of the PyC sample was exposed to the

Mo flux. This ensured a reproducible electrode area from sample

to sample. Sulfurisation was then performed as previously

reported.9,33

Briefly, the Mo/PyC samples were placed in a two 60

zone hot wall quartz tube furnace and heated to 750° C under Ar

flow (150 sccm). Sulfur powder was thermally sublimed

upstream of the samples at a temperature of 120° C and

sulfurisation of the Mo took place in the higher temperature

region. Samples were then cooled to room temperature under Ar 65

flow.

Electrochemical Measurements

Electrochemical measurements were carried out to evaluate the

performance of MoS2/PyC films as electrodes for HER. Sulfuric

acid (0.5 M) was used as the electrolyte. A three electrode 70

configuration was employed with MoS2 as the working electrode,

a graphite counter electrode and a Reference Hydrogen Electrode

(RHE) as the reference electrode. A Gamry Reference 600

potentiostat was used for all electrochemical measurements.

Spectroscopy 75

Raman spectroscopy was performed using a Witec Alpha 300 R

confocal Raman microscope with a laser wavelength of 532 nm at

a power of ~ 0.5 mW. For wide spectral scans a grating with 600

lines/mm was used. Higher spectral resolution measurements

required the use of a 1800 lines/mm grating. 80

XPS was carried out on a VG Scientific ESCAlab MKII system,

using Al Kα X-rays (1486.7 eV) as incident light. An analyser

pass energy of 50 eV was used for survey scans while a pass

energy of 20 eV was used to obtain high resolution spectra of

characteristic core levels. 85

Thickness measurements of as-deposited Mo and sulfurised MoS2

films were performed by spectroscopic ellipsometry as described

in a previous work.38 Bare SiO2 substrates were included in the

same deposition runs as the PyC samples of each thickness and

these samples were used to calibrate the actual film thicknesses 90

for each sample set. Mo films of 2, 4 and 20 nm were found to

increase in thickness by a factor of approximately 2.5 after

sulfurisation. The resultant MoS2 films were found to be

approximately 4.5, 10 and 53 nm thick, respectively. Detailed

discussion of these measurements is included in the ESI. 95

Microscopy

Cross sections of the MoS2 on PyC were fabricated by Focussed

Ion Beam (FIB) milling of lamellar cross sections followed by

TEM imaging. A PMMA polymer support layer was spun on 100

before the SiO2 substrate was etched using 2M NaOH. The

resultant films were transferred to a TEM grid and the PMMA

layer dissolved in acetone. A Carl Zeiss Auriga FIB was used for

lamella fabrication and TEM images were recorded using an FEI

Titan TEM at an accelerating voltage of 300 kV. Scanning 105

Electron Microscopy (SEM) was performed with a Carl Zeiss

Ultra FE SEM at low accelerating voltages with a secondary

electron detector.

Atomic Force Microscopy (AFM) images were obtained using a

6 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

Veeco Dimension 3100 atomic force microscope in tapping mode

with 300 kHz resonant Si Tips.

Acknowledgements

This work is supported by the SFI under Contract No.

12/RC/2278 and PI_10/IN.1/I3030. NM acknowledges the EU 5

under FP7-2010-PPP Green Cars (Electrograph No. 266391) and

MO is supported by the Irish Research Council under the

Enterprise Partnership Scheme, Project #201517, Award #12508.

The authors thank Mr. Davide Betto for his assistance with

electron beam evaporation. Mr. Dermot Daly and Mr. Clive 10

Downing, of the Advanced Microscopy Laboratory, are thanked

for their assistance with FIB and TEM techniques, respectively.

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Notes 95

a Centre for Research on Adaptive Nanostructures and Nanodevices

(CRANN) & Advanced Materials Bio-Engineering Research Centre

(AMBER),Trinity College Dublin, Dublin 2, Ireland.

E-mail: duesberg@tcd.ie, nmcevoy@tcd.ie b School of Chemistry, Trinity College Dublin, Dublin 2, Ireland. 100

† Electronic Supplementary Information (ESI) available: [Detailed XPS

analysis, further TEM, ellipsometry measurements, AFM]. See

DOI: 10.1039/b000000x/