Electrochimica Acta 257 (2017) 49–55

Platinum-Group Metal Grown on Vertically Aligned MoS2 asElectrocatalysts for Hydrogen Evolution Reaction

Victor Shokhen, David Zitoun*Department of Chemistry, Bar Ilan Institute of Nanotechnology and Advanced Materials (BINA), Bar Ilan University, Ramat Gan, 5290002, Israel

A R T I C L E I N F O

Article history:Received 29 May 2017Received in revised form 28 September 2017Accepted 3 October 2017Available online 4 October 2017

Keywords:Hydrogen evolution reactionLayered transition metal dichalcogenidesNanoparticlesElectrodepositionMoS2

A B S T R A C T

In the past few years, research on Pt substitutes has focused on the electrocatalytic properties of layeredtransition metal dichalcogenides (LTMDs). The decoration of LTMDs with platinum-group metal (PGM)demonstrates a synergetic effect with PGM deposited on the LTMD basal plane. We report on thehydrogen evolution reaction (HER) electrocatalysis by PGM grown on vertically aligned (VA) MoS2. TheVA-MoS2 films were processed by chemical vapor deposition (CVD) and the PGM (Pt and Pd) wereelectrodeposited specifically on the edges of the MoS2 flakes from a simple metallic foil. Electronmicroscopy shows the formation of nm-scale metallic clusters binding through the dangling bonds of theunsaturated sulfides. The VA-MoS2 remains intact through the electrodeposition process, asdemonstrated by Raman spectroscopy and high-resolution transmission electron microscopy. In acidicmedia, both electrocatalysts VA-MoS2@Pt and VA-MoS2@Pd show a very low overpotential (91 and106 mV respectively at 10 mA cm�2) with high current density at 300 mV overpotential (318 and174 mA cm�2 respectively). Interestingly, the Tafel slope of VA-MoS2@Pt is even lower than that ofplatinum foil (24 vs 33 mV dec�1 respectively).

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1. Introduction

Research on electrocatalytic hydrogen evolution reaction (HER)has focused on the replacement of non-sustainable PGM electro-catalysts by earth-abundant ones [1–3]. Among the latter, muchattention has been drawn by the layered transition-metaldichalcogenides (LTMDs) [4–10]. Their HER efficiency results fromthe very low hydrogen binding energy of the LTMD edge atoms[11–13]. Recent reports have focused on the benefits of defect-induced HER activities [14,15]. The case of MoS2 has beenextensively studied and the theoretical and experimental studiesdemonstrate a higher activity of the edges compared to the basalplanes [16–18]. Chemical vapor deposition (CVD) has beenestablished as an efficient method for the preparation of largecrystals with high quality and controllable thickness [19,20], aviable alternative to atomic layer deposition (ALD) [21] andsolution processes [22]. In most cases, CVD growth yields 2Dmaterials with basal planes oriented parallel to the substrate [7].The preferential growth in vertically aligned orientation wasreported above a certain Mo thickness of several nm [23]. CVD haspreviously led to very thin films of VA-MoS2 (5–25 nm) by

* Corresponding author.

https://doi.org/10.1016/j.electacta.2017.10.0140013-4686/© 2017 Elsevier Ltd. All rights reserved.

sulfurization of thin Mo films, promoting the HER activity byvertical alignment [24,25,16]. We have recently applied thismethod to the synthesis of highly ordered thick film of VA-MoS2[26]. The HER activity of this oriented film shows a significantimprovement compared to randomly oriented LTMDs [27]. Thedoping with first-row transition metal enhances the HER activityand the thermal texturation of the flakes lowers the overpotentialto 170 mV at 10 mA cm�2 current density [28]. In summary,creating additional HER-active sites by designing defect-rich oredge-oriented structures has resulted in superior HER perfor-mance due to the high activity of unsaturated terrace and edgesites towards hydrogen.

Yet, in spite of these promising results, there are still challengesto close the gap with Pt in terms of current density andoverpotential since proton exchange membrane (PEM) electro-lysers are forecast to reach current densities of 2 A/cm2 in thecoming years [29]. Recently, theoretical studies supported theconcept of adsorption of MoS2 on metal layers or metal doping inMoS2 for promoting HER [30–32]. Several experimental attemptshave shown the benefits of modifying MoS2 with PGM nano-particles (NPs), mainly on its basal plane, by the epitaxial growthmethod, or by adsorption of colloidal NPs [33–39].

Here, we report on the HER activity of VA-MoS2 films processedby CVD, followed by Pt and Pd electrodeposition from a simple

Fig. 1. Scheme of the in chemical vapor deposition (CVD) process of VA-MoS2 (A); Electrodeposition of Pt or Pd on VA-MoS2 and photograph of the electrochemical cell (B).

50 V. Shokhen, D. Zitoun / Electrochimica Acta 257 (2017) 49–55

metallic foil, specifically at the edges of the MoS2 flakes. Themethodology reported here is highly reproducible and can beapplied to any heterostructure with a preferred orientation. Theresulting heterostructure demonstrates high current densities(above 200 mA cm�2) with a very low Tafel slope. In the case of VA-MoS2@Pt, the Tafel slope reaches 24 mV dec�1, even lower than forPt foil, measured under the same conditions.

2. Experimental

2.1. Growth of vertically aligned MoS2 (VA-MoS2)

Molybdenum foil (0.05 mm thick, 99.95%, STREM), platinumFoil (0.05 mm thick, 99.95%, Engelhard-CLAL) and palladium Foil(0.25 mm thick, 99.98%, Alfa Aesar) were cleaned as reportedbelow. Sulfur (Sigma-Aldrich), H2SO4 (96% analysis grade, CARLOERBA), ultrapure DI water (Millipore Milli-Q) were used during thesynthesis and electrochemical measurements.

The molybdenum foil was polished with 50-nm aluminasuspension and the polished foil was carefully washed with DIwater, cleaned (with acetone, isopropanol and 2 M HCl) andinserted into a one-inch quartz tube in the CVD furnace. Sulfurpowder was introduced into a boat in a different heat zone. TheCVD was purged with N2 at 100 sccm for 10 minutes. The Mo foilwas then heated to 500 �C and the sulfur powder was heated to145 �C. The MoS2 growth proceeded at 750 �C for 24 hours.

2.2. Electrodeposition of Platinum and Palladium

The electrodeposition was carried out in a three-electrode glassflask under N2 in 0.5 M H2SO4 with stirring at 200 rpm (no vortex).The MoS2 foil size is about 5 �14 mm2, the Pt (or Pd) foil is3 � 7 mm2 and the distance between the electrodes is 18 mm.

The electrodeposition was controlled by a Bio-Logic VMP3potentiostat (with impedance) with an Ag/AgCl reference elec-trode, VA-MoS2 foil as the working electrode and Pt or Pd foil ascounter electrode. The dissolution/deposition of Pt (or Pd)occurred while sweeping the potential from open circuit voltage(OCV) to +0.45 V and �0.6 V vs reference electrode at speed scanrate of 5 mV/s. The cycles were repeated until a stable HER activitywas reached (typically between 50 to 100 cycles).

2.3. Characterization

A focused-ion beam (FIB) (FEI, Helios 600) was used to cut thedeposited VA-MoS2 samples into lamellas for transmission

electron microscopy (TEM) observation. High-resolution TEM(HRTEM) images were obtained by a JEOL JEM-2100 (LaB6 or fieldemission) operated at 200 kV. Raman spectroscopy was carried outon Horiba LabRam HR evolution with a 532 nm laser. X-raydiffraction (XRD) images were collected on a Rigaku Smartlab XRDin Bragg-Brentano (u � 2u). The X-ray generator was operated at40 kV and 30 mA with Cu-Ka radiation (l = 1.54 Å). High-resolutionscanning electron microscopy (HR-SEM) images were collected onan FEI, Magellan 400L.

2.4. Electrochemical measurements

The HER activity was measured in 0.5 M H2SO4 under N2 with apotentiostat (Biologic VMP3) with an Ag/AgCl reference electrode,VA-MoS2 foil as the working electrode, and Pt foil as counterelectrode. For each voltammogram, the system was purged with N2

for 15 minutes. The HER measurements were corrected for theohmic loss (ZIR correction) and the scan rate set to 5 mV/sec. Theresults were reported as a function of the geometric area of eachworking electrode, typically around 1 cm2. The electrodes weretested for 1000 cycles.

3. Results

3.1. Vertically Aligned (VA) MoS2

The synthesis of the VA-MoS2 was carried out by placing apolished molybdenum foil in the CVD furnace (Fig. 1A). The samplewas purged by nitrogen gas in a thermal annealing step followedby the growth of a MoS2 film. During the process, sulfur vapor wasintroduced into the furnace from a boat filled with sulfur. After thereaction, the foil changed color from silver to dark purple. Thesurface displays vertically aligned (VA) flakes, homogeneouslydistributed on the foil (Fig. 2A–B). On the surface of the film, thelayered MoS2 opens in a flower-like morphology. X-ray diffraction(XRD) was carried out on VA-MoS2. The diffractogram shows twomain reflections: (100) of the 2H-MoS2 phase (ICCD #00-037-1492) and (200) of the Mo foil (Fig. 2C). The XRD pattern lacks allthe {002} reflections which demonstrates the vertical orientationgrowth observed in SEM. The {002} basal planes are verticallyaligned and the film grows along the (100) direction. Theorientation of the film is confirmed by Raman spectroscopy whichdisplays the typical in-plane Mo�S phonon mode (E12g) and theout-of-plane Mo-S phonon mode (A1g) (Fig. 2D). The A1g-to-E12gratio is close to 1:3 for the VA-MoS2 revealing the edge-terminatednature of the film, i.e. the vertical alignment [16].

Fig. 2. Scanning electron microscopy (SEM) images of VA-MoS2 (A-B). X-ray diffraction (XRD) pattern of VA-MoS2 (C); Raman spectrum of VA-MoS2 (D).

V. Shokhen, D. Zitoun / Electrochimica Acta 257 (2017) 49–55 51

3.2. Selective electrodeposition of Pt and Pd

The electrodeposition of Pt, or Pd, is performed by cathodicdissolution of a Pt, or Pd, in the three-electrode glass cell in 0.5 MH2SO4 (photograph in Fig. 1). The electrodeposition can be easilyfollowed on the linear sweep voltammogram, which shows theenhancement of the HER activity upon cycling (Fig. 3A–B). Thedeposition is stopped when a stabilized voltammogram isobserved, typically after 90 cycles for platinum and 50 cycles forpalladium. Between each cycle, we measured the cyclic voltam-metry for the samples and followed the deposition. The Pt cyclicvoltammogram is typical of adsorbed hydrogen with the

Fig. 3. Electrodeposition cycles on VA-MoS2 with platinum (A) and palladium (B); cycl

adsorption and desorption below 0.2 V vs RHE (Fig. 3C); theoxygen adsorption/desorption could not be measured since the Mofoil does not sustain a potential above 0.6 V vs RHE. The HUPD

charge was calculated from the adsorption peak at 0.1 V vs RHE; byusing the standard value of 212 mC cm�2 for Pt, the calculatedsurface is 0.23 cm2 which is close to the geometric surface area(0.3 cm2).

The Pd cyclic voltammogram shows a large peak only for slowscan rate (5 mV s�1) (Fig. 3D). Indeed, Pd can absorb significantamount of hydrogen. The ratio H/Pd is about 0.6 when the a ! bphase transition is completed. Oxidation of adsorbed and absorbedhydrogen takes place at almost the same potential; therefore, the

ic voltammetry of VA-MoS2@Pt at 100 mV/s (C) and VA-MoS2@Pd at 5 mV s�1 (D).

Fig. 4. High-resolution scanning electron microscopy (HR-SEM) in-plane and 4500 tilted view of VA-MoS2@Pt (A-C) and VA-MoS2@Pd (B-D).

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charges associated with the adsorbed (QHads) and absorbed (QHabs)hydrogen cannot be distinguished. Only the total charge of sorbedhydrogen (QHsorb = QHabs + QHads) can be measured. [40], In our case,QHsorb = 45 � 5 mC for a foil of 0.27 cm2. A simple calculation givesthe number of electrons and therefore Pd atoms (while assumingH/Pd � 0.6): 1.1 �0.1 mg cm�2. By analogy, the Pt content can becalculated �2.0 � 0.2 mg cm�2. A measurement by ICP-MAS doesnot give a reproducible value due to the sulfur impurities from thedissolution of MoS2.

The platinum deposition occurs almost exclusively on the edgesof the MoS2 flakes (Fig. 4A–C). Individual platinum nanoparticlesdecorate the edges with a size below 5 nm, too small to preciselycalculate the size distribution from HRSEM imaging. The palladiumelectrodeposition gives similar results with individual Pd NPsdeposited on the edges of the VA-MoS2 (Fig. 4B–D). The NP meanparticle size is slightly larger in the case of Pd electrodepositioncompared with Pt. An HRTEM study gives a closer look at theheterostructure.

A slice of VA-MoS2@Pt was cut by FIB and the cross-section wasimaged and analyzed by bright field HRTEM (Fig. 5A–B). The cross-section shows the MoS2 substrates with Pt NPs deposited on theedges of the flakes. Due to the constraints on the experiments, anamorphous carbon film and a platinum film were deposited by e-

Fig. 5. Transmission electron microscopy (TEM) of the cross-

beam evaporation on top of the sample (Fig. 5A). The HRTEMshows the vertical orientation of the (002) basal planes of the 2H-MoS2 phase with lattice fringes of 6.3 Å. The crystalline layers aresurrounded by single crystalline FCC Pt NPs without a clearpreferred crystallographic orientation. The Pt nanocrystals growfrom the edges in a hemispherical shape with a 4.4 � 0.5 nmthickness and a 5.3 � 0.5 nm diameter. The growth does not showany epitaxial relationship between the substrate and the Ptnanocrystals.

In a similar process, a slice of VA-MoS2@Pd was cut by FIB andthe cross-section was imaged and analyzed by bright-field HRTEM(Fig. 6A–B). Pd nanocrystals grow on the edges of the (002) basalplanes of the 2H-MoS2 phase. Pd displays a well-defined crystallineFCC pattern, and the particle size is slightly larger than Pt with a5.5 � 0.5 nm thickness and a 7.2 � 0.7 nm diameter. Here, also, thePd growth does not seem to follow an epitaxial relationship.

The integrity of the VA-MoS2 was confirmed by Ramanspectroscopy of the VA-MoS2@Pt and VA-MoS2@Pd samples(Fig. 6). The Raman spectra show the vibration peaks of MoS2,and the orientation of the film is confirmed by Raman spectrosco-py, which displays the typical Mo�S in-plane phonon mode (E12g)at wavenumber 385 cm�1 and out-of-plane phonon mode (A1g) atwavenumber

section (A) and high resolution TEM of VA-MoS2@Pt (B).

Fig. 6. High resolution transmission electron microscopy (HRTEM) of the cross-section (A) and HRTEM of VA-MoS2@Pd (B).

V. Shokhen, D. Zitoun / Electrochimica Acta 257 (2017) 49–55 53

409 cm�1. The VA-MoS2 displays a 1:3 ratio revealing the edge-terminated nature of the film. Those peaks were unchanged afterthe electrodeposition (Fig. 7).

3.3. Hydrogen evolution reaction (HER)

The HER activity of the electrocatalysts has been checked at300 K in 0.5 M H2SO4 for samples of 1 cm2. The results are reportedafter ohmic loss correction (iR free) up to 200 mA cm�2. All theresults are compared with the HER activity of Pt foil and plotted vsthe normal hydrogen electrode potential. Pristine VA-MoS2displays a rather low impedance of 24.83 V at 100 kHz comparedto previous reports, due to the vertical alignment and excellentadhesion of the CVD grown MoS2 on the Mo foil. Afterelectrodeposition, the impedance measured at 100 kHz for VA-MoS2@Pt and VA-MoS2@Pd (respectively 1.108 and 1.21 V) isdrastically reduced compared to that of pristine VA-MoS2. Pt andPd electrodeposition results in an excellent electronic conductivity.Pristine VA-MoS2 does not reach 10 mA cm�2 even at 0.4 Voverpotential, while the VA-MoS2@Pd provides 10 mA cm�2 at0.127 V overpotential and the VA-MoS2@Pt at 0.096 V overpotentialclose to the Pt foil (0.076 V, Fig. 8A).

For a given overpotential of 300 mV, the VA-MoS2@Pt and VA-MoS2@Pd give current densities of 318 and 174 mA cm�2 respec-tively (Fig. 8B). As discussed below, these values are much higherthan any reported values for a MoS2-based HER electrocatalyst.

Fig. 7. Raman spectra of VA-MoS2@Pt (red curve) and VA-MoS2@Pd (blue curve).

Fig. 8C shows the Tafel plots to reveal their HER kinetics. Wecalculated the Tafel slopes of VA-MoS2@Pt and VA-MoS2@Pd(respectively 24 and 48 mV dec�1) and compared them with Pt foil;VA-MoS2@Pt shows the lowest value, lower than any reportedvalue for MoS2-based HER catalysts. This value indicates aHeyrovsky or Tafel rate-determining-step mechanism for theHER, rather than the common Volmer reaction. We also extrapo-late the overpotential values from the Tafel plots; Pt foil: h = 42 mV,VA-MoS2@Pt: h = 65 mV, VA-MoS2@Pd: h = 58 mV. All calculatedoverpotential values are very low. Interestingly, the lowest value isfound for VA-MoS2@Pd and is comparable with the best valuereported so far [35]. The HER activity was found to be stable for1000 cycles with a slightly higher activity for high current densityabove 25 mA cm�2 for VA-MoS2@Pd and 67 mA cm�2 for VA-MoS2@Pt.

4. Discussion

A CVD process has been developed to grow 2H-MoS2 films withvertically aligned (VA) stacking directly from a Mo foil. The VA-MoS2 shows HER activity with an onset potential of 450 mV for10 mA cm�2 current density. On the other hand, a simpleelectrodeposition process from a platinum (or palladium) foilyields a unique heterostructure consisting of Pt (or Pd) hemi-spherical nanoparticles growing on the edges of the foils. The VA-MoS2@Pt and VA-MoS2@Pd display an intact vertical orientation ofthe layered phase 2H-MoS2 and the NPs grow selectively from thedangling bonds of MoS2. The heterostructure shows state-of-the-art HER activity which can be compared with the previous reportsof MoS2 heterostructures with Pd and Pt (Table 1). VA-MoS2@Pdshows the highest reported current density at 300 mV (174 mAcm�2), more than twice the previous value reported for Pdnanorods deposited on the basal planes of defect rich MoS2.

In the case of VA-MoS2@Pt, the current density at 300 mV isnearly double (318 mA cm�2), higher than any reported values toour knowledge. The mass activity of the catalysts can be calculatedfollowing the analysis of the cyclic voltammetry. At 300 mV, themass activity reaches 159 mA mgPt�1 and 174 mA mgPd�1.

The excellent HER activity can be explained by the unprece-dently low Tafel slope of 24 mV dec�1, which shows significantenhancement compared to our standard Pt foil (33 mV dec�1) andcan be compared with the Pt decorated exfoliated MoS2 [38]. Thehigh activity of the Pt and Pd NPs electrodeposited on the edges ofVA-MoS2 may be due to the optimal size (�5 nm) and location (onthe tip) of the catalyst. Another important factor is the low chargetransfer resistance from the heterostructure. Contrary to previous

Fig. 8. Linear sweep voltammetry in 0.5 M H2SO4 (iR free, 5 mV s�1) of VA-MoS2 (pink), Pt foil (black), VA-MoS2@Pt (red) and VA-MoS2@Pd (blue) (A-B); Tafel plots of thecorresponding catalysts (C); linear sweep voltammetry after 1000 cycles (iR free, 5 mV s�1) (D).

Table 1HER parameters and comparison with the literature.

Materials hb [mV] Tafel Slope [mV dec�1] Ja [mA cm�2] Reference

VA-MoS2 �450 184 1.5 This workVA-MoS2@Pt 91 24 318 This workVA-MoS2@Pd 106 48 174 This workPt foil 75 33 >500 This workPd ND/DR-MoS2 103 41 83 [35]MoS2@Pt Not reported Not reported 260 [36]

a Cathodic current density (j) was recorded at h = 300 mV.b Overpotential (h) was recorded at j = 10 mA cm�2.

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reports on colloidal routes towards the Pd nanorods [35], theelectrodeposition bottom-up approach anchors the Pd (or Pt)particles on the edge of MoS2, thereby limiting the contactresistance between the HER active NPs and the substrate. Thestability of the heterostructure can be also derived from the stableelectrochemical behavior upon cycling with no degradation of theHER activity after 1000 cycles.

5. Conclusions

In this article, we report on the HER electrocatalysis by Pd and Ptgrown on vertically aligned (VA) MoS2. The VA-MoS2 films wereprocessed by CVD and platinum or palladium was electrodepositedspecifically on the edges of the MoS2 flakes from a simple metallicfoil. HRTEM shows the formation of Pd or Pt nanocrystals at about5 nm scale binding through the dangling bonds of the unsaturatedsulfides. The VA-MoS2 remains intact through the electrodeposi-tion process, as demonstrated by Raman spectroscopy and HRTEM.In acidic medium, both electrocatalysts VA-MoS2@Pt and VA-MoS2@Pd show a very low overpotential at 10 mA cm�2 (91 and106 mV respectively) with high current density at 300 mV over-potential (318 and 174 mA cm�2 respectively, mass activity of159 mA mgPt�1 and 174 mA mgPd�1). Interestingly, the Tafel slope ofVA-MoS2@Pt is lower even with respect to the platinum foil (24 vs33 mV dec�1).

Acknowledgments

The authors are deeply grateful to Dr Doron Naveh for fruitfuldiscussions and access to his facilities, Chen Stern for herassistance with the MoS2 growth and Noam Gotlib for the Ramanspectroscopy. The authors thank Dr Vladimir Ezersky and Dr IlanaPerelshtein for the HRTEM, Prof. Arlene Gordon and Yuval Elias forproofreading. The authors thank Dr Yuval Elias and Yulia Kostikovfor proofreading the article. D.Z. acknowledges partial financialsupport from the Israel Council for Higher Education through theIsrael National Research center for Electrochemical Propulsion(INREP) and the Israel Ministry of Science and Technology.

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