Maneuvering charge polarization and transport in 2H- 2...

Maneuvering charge polarization and transport in 2H-MoS2 for enhanced electrocatalytic hydrogen evolutionreaction

Wei Ye, Chenhao Ren, Daobin Liu, Chengming Wang, Ning Zhang, Wensheng Yan, Li Song, and

Yujie Xiong ()

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for EnergyMaterials), Hefei Science Center (CAS), School of Chemistry and Materials Science, and National Synchrotron Radiation Laboratory,University of Science and Technology of China, Hefei 230026, China

Received: 5 March 2016

Revised: 13 May 2016

Accepted: 16 May 2016

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2016

KEYWORDS

hydrogen evolution,

molybdenum disulfide,

charge transport,

charge polarization,

nanowire

ABSTRACT

Semiconducting 2H-MoS2 with high chemical stability is a promising alternative

to the existing electrocatalysts for the hydrogen evolution reaction (HER);

however, the HER performance largely suffers from the limited number of active

S sites and low mobility for charge transport. In this work, we demonstrate that

the limitations of 2H-MoS2 for the HER can be overcome by forming hybrid

structures with metallic nanowires. Taking the integration with Pd as a proof-

of-concept, we show with solid experimental evidence that the one-dimensional

structure of metallic nanowires facilitates electron transport to active S sites,

while the interfacial charge polarization between MoS2 and Pd increases the

electron density of the S sites for improved activity. As a result, the hybrid

structure exhibits a current density of 122 mA·cm−2 at −300 mV versus RHE and

a Tafel slope of 44 mV·decade−1 with excellent durability, well exceeding the

performances of bare 2H-MoS2 and metallic 1T-MoS2. This work provides insights

into electrocatalyst design based on charge transport and polarization, which can

be extended to other hybrid structures.

1 Introduction

Renewable hydrogen energy is regarded as a next-

generation clean source that can be a potential solution

to the current energy crisis. Specifically, water splitting

to produce hydrogen is a promising protocol to convert

residual energy such as electricity, wind, or tide to

storable hydrogen energy. Electrocatalysts that can

reduce the overpotential and increase the conversion

efficiency play a key role in the important electro-

chemical reaction, hydrogen evolution reaction (HER,

2H+ + 2e− H2) [1]. Although Pt-based nanomaterials

Nano Research 2016, 9(9): 2662–2671

DOI 10.1007/s12274-016-1153-3

Address correspondence to [email protected]

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2663 Nano Res. 2016, 9(9): 2662–2671

have been proven to be the most effective catalysts

in the past decade, there is continuously increasing

interest in seeking alternative catalysts for application

in the HER.

Among the materials developed, molybdenum

disulfide (MoS2) with a layered structure has attracted

extensive attention owing to its promising application

in the HER [2–7]. So far, two different phases have been

identified for MoS2 electrocatalysts: semiconducting

2H (trigonal prismatic) and metallic 1T (octahedral)

phases. 2H-MoS2 exhibits impressive durability in

the HER; however, the number of active sites—the

unsaturated sulfur atoms located along the edges of

the MoS2 layers—is quite limited [5]. More importantly,

2H-MoS2 shows low mobility for charge transport

[2, 8]. Although 1T-MoS2 possesses more active sites on

its basal plane and higher carrier mobility for enhanced

HER performance [6, 7], this metastable phase may

easily be transformed into 2H- or amorphous phases

upon receiving energy in its applications [6, 9]. To

design stable MoS2 HER electrocatalysts, it is thus

imperative to improve the activity of each reaction site

and promote charge transport in 2H-MoS2.

In this work, we demonstrate that both the afore-

mentioned features can be achieved by simply forming

2H-MoS2-based hybrid structures. As a proof-of-concept

model, the hybrid structure is designed by integrating

2H-MoS2 with metallic nanowires, with full surface

coverage toward enhanced HER performance. The

one-dimensional structure of metallic nanowires

facilitates electron transport to active S sites owing to

their lower resistance. In the meantime, the interfacial

charge polarization between MoS2 and the metal

increases the electron density of the S sites for improved

HER activity. To better resolve the effects of charge

transport and polarization on the HER, we selected

Pd as the nanowire material and full surface coverage

as the hybrid structure configuration for two reasons.

First, Pd is considered as a non-plasmonic metal,

and hence, we can differentiate our design from the

previously reported case of plasmonic hot electron

enhancement [10]. Second, we can exclude the

possibility of Pd directly participating in the HER [11],

as it is fully covered by MoS2.

2 Experimental

2.1 Synthesis of Pd nanowires (NWs)@2H-MoS2

hybrid structure

Pd NWs were used as templates to prepare the Pd

NWs@2H-MoS2 hybrid structure. Specifically, a 12 mL

DMF suspension of Pd NWs was mixed with 7.5 mg

(NH4)2MoS4. The mixture was stirred for 5 min and

then sonicated for another 30 min until all reactants

were fully mixed. The mixture was then transferred

and sealed in an autoclave with a Teflon liner, and

heated at 200 °C for 15 h. Then, it was naturally cooled

to room temperature. The black precipitate was

collected and washed with ethanol and water at least

three times to fully remove other impurities. The

product was dried in vacuum and characterized.

The growth of 2H-MoS2 on Cu nanowires, Ag

nanowires, and Pd nanocubes (NCs) was realized by

following the same procedure, except for the repla-

cement of metal nanostructure templates.

2.2 Electrochemical measurements

All the electrochemical measurements were performed

in a three-electrode system on an electrochemical

workstation (CHI 760E, Shanghai Chenhua, China) in

a 0.5 M H2SO4 electrolyte. The catalysts were dispersed

onto a glassy carbon rotating disk electrode (GC RDE,

PINE, PA, USA), used as the working electrode. A

graphite rod and a reversible hydrogen electrode

(RHE) served as the counter and reference electrodes,

respectively. The GC RDE had a diameter of 5 mm

and a geometric area of 0.19625 cm2. The electrode was

first polished with emery paper of decreasing grades

and then with Al2O3 powder with sizes down to

0.05 m. Prior to the deposition of the catalysts, the

electrode was immersed in ethanol and then thoroughly

rinsed with deionized (DI) water three times to remove

contaminants. To prepare the working electrode,

2 mg of the catalyst was dispersed in 2 mL of 3:1 v/v

water/isopropanol and sonicated for at least 30 min

to form a homogeneous ink. 27.7 μL of the catalyst

ink (containing 27.7 μg of the catalyst) was then

transferred to the GC RDE. Prior to electrocatalytic

measurements, the working electrode was cleaned

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2664 Nano Res. 2016, 9(9): 2662–2671

with RF plasma (Plasma Cleaner pdc-002, Harrick,

NY, USA) at a power level of 10.5 W for 2 min to

remove residue organics, and then covered with 10 μL

of Nafion dispersed in water (0.025%).

Linear sweep voltammetry from 0.1 to −0.3 V versus

RHE, with a scan rate of 2 mV·s−1, was performed under

Ar gas flow maintained over the electrolyte during

the HER experiment to eliminate dissolved oxygen.

The working electrode at the electrolyte level was

rotated at 1,600 rpm to remove hydrogen gas bubbles

formed at the catalyst surface. Cyclic voltammetry (CV)

was carried out between −0.3 and 0.1 V versus RHE

at 50 mV·s−1 for 6,000 cycles to investigate the cycling

stability.

3 Results and discussion

In our synthesis, we first prepared high-quality five-

fold twinned Pd NWs with an average diameter of

10 nm (see Fig. S1 in the ESM for morphology) through

a hydrothermal synthesis [12]. As illustrated in Fig. 1(a),

the Pd NWs were then coated by a universal method

for 2H-MoS2 involving (NH4)2MoS4 thermal decom-

position [13, 14]. Figures 1(b) and 1(c) show the

scanning and transmission electron microscopy (SEM

and TEM) images for the sample after MoS2 growth,

respectively, clearly revealing that the Pd NWs have

been covered by flake-like materials.

The energy-dispersive X-ray spectrum (EDS) of a

Figure 1 (a) Schematic illustration for the synthesis of Pd NWs@2H-MoS2 hybrid structure. (b) SEM, (c) TEM, and (d) HRTEM images of the synthesized Pd NWs@2H-MoS2 hybrid structure. (e) EDS elemental mapping profiles of S (cyan), Mo (dark yellow), andPd (magenta).


2665 Nano Res. 2016, 9(9): 2662–2671

hybrid structure (Fig. S2 in the ESM) indicates the

co-existence of Pd, Mo, and S elements with a molar

ratio of Mo:S = 1:2. Furthermore, X-ray photoelectron

spectroscopy (XPS, Fig. S3 in the ESM) verifies the

valence states of Mo4+ and S2– [2]. Thus, it can be

concluded that the newly formed material is MoS2.

Although X-ray diffraction (XRD, Fig. S4 in the ESM)

cannot clearly resolve the crystal structure of the

MoS2 in hybrid structures [14, 15], the locations of Pd

and MoS2 are further resolved by high-resolution

TEM (HRTEM, Fig. 1(d)). The lattice fringes with

spacings of 0.23 and 0.62 nm in the core and shell

regions can be attributed to (111) of face-centered

cubic (fcc) Pd (JCPDS 46-1043) and (002) of 2H-MoS2

(JCPDS 37-1492), respectively. This suggests that the

cores of the Pd NWs are covered by the 2H-MoS2

flakes-composed shells, which agrees with the results

of EDS elemental mapping (Fig. 1(e)). We thus name

this hybrid structure “Pd NWs@2H-MoS2”. According

to inductively coupled plasma-mass spectrometry

(ICP-MS) measurements, this Pd NWs@2H-MoS2

sample contains about 30% Pd by weight. Specifically,

ICP-MS measurements reveal that the molar ratio of

Pd:Mo:S is 12.6:28.8:58.6. Based on the molar ratio of

S to Mo at 2 in MoS2, we can estimate that less than

8% of Pd (namely, approximately 1–2 atomic layers

according to the diameter of Pd NWs) has been

vulcanized. Such an ultrathin layer of PdSx has not

been resolved by HRTEM.

The template of Pd NWs with high chemical stability

is indispensable for the formation of the hybrid

structure. When Pd NWs are replaced by Ag and Cu

NWs in the synthesis, only flower-like Ag@MoS2 and

Cu@MoS2 hybrid structures (Fig. S5 in the ESM), in

which Ag and Cu nanoparticles are surrounded by

MoS2 flakes, respectively, can be obtained. It implies

that the Ag and Cu NWs have been etched into the

nanoparticles during sulfidation [16, 17].

The one-dimensional core–shell structure provides

an excellent platform for investigating the role of the

Pd NWs in the HER. To assess the contribution from

the Pd NWs, we prepared a bare 2H-MoS2 sample by

following a similar protocol, except for the absence

of Pd NWs. As shown in Fig. S6 (in the ESM), the

resulting 2H-MoS2 possesses the same flake-like

structure as Pd NWs@2H-MoS2, which is a typical

feature for the thermal decomposition of (NH4)2MoS4

[3, 14]. The HER performance is evaluated using

GC RDEs at a rotating rate of 1,600 rpm in a 0.5 M

H2SO4(aq) electrolyte. Figure 2(a) shows the polarization

curves of Pd NWs@2H-MoS2 benchmarked against

bare 2H-MoS2, Pd NWs, and commercial Pt/C catalyst

(20 wt.% Pt). The Pd NWs@2H-MoS2 sample exhibits

an overpotential of 60 mV, which is obviously smaller

than those for the bare 2H-MoS2 (150 mV) and Pd

NWs (100 mV), demonstrating the higher catalytic

activity of our hybrid structure. The enhanced HER

performance is better reflected by the comparison

of current densities. At −300 mV versus RHE, the

Pd NWs@2H-MoS2 achieves a current density of

122.2 mA·cm−2, which is 1.8 and 2.3 times larger than

those of the bare 2H-MoS2 and Pd NWs, respectively.

As an indicator for the rate-determining reactions

[3, 18], the Tafel slope has been further analyzed. As

indicated in Fig. 2(b), the Pd NWs@2H-MoS2 sample

shows a Tafel slope of 44 mV·decade–1, which is smaller

than that of the bare 2H-MoS2 (59 mV·decade–1). This

result suggests that the HER is maneuvered by the

addition of Pd NWs. In principle, three key steps

are involved in the HER in acidic medium [3, 18]: a

primary discharge step (i.e., Volmer step, Eq. (1)),

followed by two electronic desorption steps (i.e., the

Heyrovsky step, Eq. (2); and the Tafel step, Eq. (3)).

H3O+ + e− Hads + H2O (1)

Hads + H3O+ + e− H2 + H2O (2)

Hads + Hads H2 (3)

The Tafel slope is an inherent feature of the catalyst,

and is determined by the rate-limiting steps of

the HER. Ideally, the Tafel slopes are 120, 40, and

30 mV·decade–1 when the Volmer, Heyrovsky, and

Tafel steps determine the rate, respectively. However,

because of the complicated situation in reality, hydrogen

evolution is the combination of Volmer–Tafel or

Volmer–Heyrovsky reactions. In our case, the Tafel

slope down to 44 mV·decade–1 for Pd NWs@2H-MoS2

suggests that the Heyrovsky step dominates in the

Volmer–Heyrovsky mechanism [3, 14, 18].

The electrochemical stability of the samples is another

important parameter for HER. In order to investigate

the stability of the catalysts, durability tests for up


2666 Nano Res. 2016, 9(9): 2662–2671

to 6,000 cycles have been performed by running

continuous cyclic voltammetry curves between −300

and 100 mV versus RHE in 0.5 M H2SO4 at a scanning

rate of 50 mV·s–1 (Fig. 2(c)). While the other catalysts

show relatively weak stability, negligible decay in

the HER activity is observed for Pd NWs@2H-MoS2

and the bare 2H-MoS2 between the curves measured

at the initial cycle and after 6,000 CV cycles. This

demonstrates the excellent durability of our hybrid

structure during long-term cycling, which is the

typical feature for chemically stable 2H-MoS2 [2–4].

The excellent performance durability envisions the

advantage of the Pd NWs@2H-MoS2 hybrid structure

over the chemically exfoliated 1T-phase metallic MoS2

nanosheets, which have been considered the best

MoS2 catalyst for the HER (see performance in Fig. S7

in the ESM) [6]. Despite their lower initial performance

(i.e., current density of 80.4 mA·cm−2 for 1T-MoS2 versus

122.2 mA·cm−2 for Pd NWs@2H-MoS2), the 1T-MoS2

nanosheets show a dramatically decayed current density

of 28.7 mA·cm−2 after 6,000 CV cycles.

Upon identifying the improved HER performance,

we are now in a position to address the critical

question: What factors induce the increase in current

density and the reduction in Tafel slope when

integrating 2H-MoS2 with Pd NWs? We first examine

the role of the Pd NWs in reducing charge transfer

resistance, given that the limited mobility for charge

transport is a bottleneck for 2H-MoS2. In order to

investigate the intrinsic charge transport ability, we

performed electrochemical impedance spectroscopy

(EIS) measurements on the electrodes modified by

the catalysts. As displayed in the Nyquist plots

(Fig. 2(d)), the charge-transfer resistance of 2H-MoS2

was substantially reduced in the hybrid structure,

which could be attributed to the high conductivity of

Pd NWs. The electrons can be transported through

the Pd NWs and thus reach the active sites across the

interface of Pd-MoS2.

To demonstrate the role of Pd NWs in facilitating

charge transport, we replaced the one-dimensional

cores with Pd NCs (Fig. 3(a)). As expected, the Pd

NCs@2H-MoS2 sample showed higher charge-transfer

resistance than the one-dimensional structure (Fig. 3(b)),

Figure 2 (a) Polarization curves obtained from GC RDEs deposited with Pd NWs@2H-MoS2 hybrid structure with reference to bare 2H-MoS2, Pd NWs, and Pt/C at the catalyst loading of 0.142 mg·cm−2 in a 0.5 M H2SO4(aq) electrolyte. (b) The corresponding Tafel plots. The dashed lines indicate the linear regions. (c) HER durability tests and (d) EIS Nyquist plots for the samples in (a).


2667 Nano Res. 2016, 9(9): 2662–2671

owing to the low efficiency of electron transport

between the nanoparticles. As a result, the sample

exhibited reduced HER performance in terms of current

density and Tafel slope (Figs. 3(c) and 3(d)).

The next question would be as follows: What else

can the presence of Pd NWs induce for facilitating

the HER in addition to charge transport? The tunable

Tafel slopes observed in our work indicate that the

HER occurring at the S sites has been altered by the

addition of Pd components [18]. During the process,

the bond formation and cleavage of the site-H should

have a strong correlation with the electron density of

the active sites [19, 20]. We thus focus on the interface

of Pd with MoS2 and hypothesize that the difference

in work functions (5.12 eV for Pd versus 6.53 eV for

MoS2) will induce the accumulation of negative charges

on MoS2 through charge polarization at the Pd–MoS2

interface [21, 22]. Although PdSx with a work function

of 3.59 eV is formed at the interface [23, 24], electrons

should be allowed tunneling through the PdSx as it is

sufficiently thin (i.e., 1–2 atomic layers).

Such a charge accumulation has been experimentally

verified by three different characterization techniques.

In the first characterization, we employ XPS to

characterize the binding energies of Pd and S in Pd

NWs@2H-MoS2. As shown in Fig. 4(a), the two groups

of Pd 3d3/2 and 3d5/2 peaks for the Pd NWs@2H-MoS2

can be attributed to Pd(0) and Pd(+2), respectively [25].

In comparison with the spectrum of the bare Pd NWs,

the additional set of peaks for Pd(+2) at 336.8 eV (3d5/2)

and 342.1 eV (3d3/2) should result from the sulfidation

at the Pd-MoS2 interface, as superfluous S2– ions

formed by the decomposition of (NH4)2MoS4 at high

temperature can react with the surface atoms of the

Pd NWs [26]. As the interfacial PdSx is not mainly

responsible for the HER enhancement (see Fig. S8 in

the ESM), we pay special attention to the position shift

of the Pd(0) peaks with respect to those for the bare

Pd NWs. The Pd(0) peaks are shifted from 334.3 eV

(3d5/2) and 339.6 eV (3d3/2) in Pd NWs to 335.8 eV (3d5/2)

and 341.1 eV (3d3/2) in Pd NWs@2H-MoS2, respectively.

In principle, the shift of XPS peaks toward higher

Figure 3 (a) TEM image of the synthesized Pd NCs@2H-MoS2 hybrid structure. (b) EIS Nyquist plots for Pd NCs@2H-MoS2 and Pd NWs@2H-MoS2 hybrid structures. (c) Polarization curves obtained from GC RDEs deposited with the samples in (b) at the catalystloading of 0.142 mg·cm−2 in a 0.5 M H2SO4(aq) electrolyte. (d) The corresponding Tafel plots. The dashed lines indicate the linearregions.


2668 Nano Res. 2016, 9(9): 2662–2671

binding energies should be ascribed to the reduction

in electron density. Thus, this result suggests that

electrons may have been transferred from Pd to MoS2

due to charge polarization [10, 22, 27].

Since MoS2 accepts electrons from Pd through the

interface, it is anticipated that XPS peak shift should

also occur in MoS2. Figure 4(b) gives the S 2p spectrum

for Pd NWs@2H-MoS2, which can be divided into

four peaks. In addition to bridging S22– and apical S2–,

the peaks at 161.3 and 161.9 eV should be attributed

to the terminal S22– and S2– which are generally the

active sites for HER [14, 28]. Apparently these binding

energies are 0.2 eV lower than those for bare 2H-MoS2,

indicating an increase in electron density.

Once the electrons are accumulated on the terminal

S22– and S2–, their atom vibration and electron transitions

will be in turn affected, which thus can be probed by

Raman spectroscopy and near-edge X-ray absorption

fine structure (NEXAFS) spectroscopy, respectively.

Figure 4(c) shows the Raman spectra, in which the A1g

mode of MoS2 represents the out-of-plane vibration of

the S atoms [29]. As compared with the bare 2H-MoS2,

there is a blue-shift of 3.6 cm–1 for the A1g mode of Pd

NWs@2H-MoS2, indicating the stiffened vibration

along with the increase in electron density [30]. The S

L2,3-edge NEXAFS spectra in Fig. 4(d) also reveal that

the transitions of S 2p electrons to an S s-like state

(peak A) and to an empty S 3d state (peak B) have

lower intensities when the MoS2 is integrated with Pd.

The reduced transition intensity indicates that some

electrons have occupied the upper energy levels,

because of the charge polarization between MoS2 and

Figure 4 XPS spectra of Pd 3d and S 2p in Pd NWs@2H-MoS2 hybrid structure with reference to bare 2H-MoS2 and Pd NWs: (a) Pd 3dhigh-resolution spectra, and (b) S 2p high-resolution spectra. (c) Raman spectra and (d) S L2,3-edge NEXAFS spectra of Pd NWs@2H-MoS2

hybrid structure and bare 2H-MoS2. (e) Schematic illustration of the working mechanisms involved in the Pd NWs@2H-MoS2 HER electrocatalyst.


2669 Nano Res. 2016, 9(9): 2662–2671

Pd [31, 32]. Note that the L2,3-edge peaks here do not

originate from the interfacial PdSx (see Fig. S9 in the

ESM for details). The above results clearly demonstrate

that the ultrathin PdSx at the interface does not hinder

the electron transfer from Pd to MoS2.

As the terminal S22– and S2– are the catalytic sites for

hydrogen evolution, the polarized electrons on the S

sites are anticipated to promote the Heyrovsky step

[22, 33]. As illustrated in Fig. 4(e), the electrons can be

efficiently transported along the Pd NWs, and then

be supplied to the terminal S22– and S2– sites on MoS2

driven by interfacial charge polarization. The facilitated

electron transport toward the reaction sites will

enhance the overall HER performance. According to

this mechanism, one can recognize that the weight

content of Pd NWs in the hybrid structure must be

kept at a certain level; otherwise, the long distance

from the edge S sites to the Pd cores will reduce the

efficiency of charge transport and polarization (see

Fig. S10 in the ESM).

It is worth pointing out that Pd was not considered

a suitable candidate for the HER, as the formation of

PdHx reduces the stripping rate of hydrogen from the

catalyst [34]. In our case, owing to the complete

cladding by MoS2, PdHx is not formed in Pd NWs@2H-

MoS2 during the HER (Fig. S11 in the ESM). For this

reason, the presence of Pd NWs would not compromise

the performance durability of Pd NWs@2H-MoS2.

This finding also verifies that the Pd surface is fully

covered by MoS2 in the hybrid structure, which has

been further proven by CO stripping tests (Fig. S12 in

the ESM). The CO stripping peak at about 0.9 V vs.

RHE, which is a typical feature for bare Pd surfaces [35],

is absent for the Pd NWs@2H-MoS2 hybrid structure,

thus demonstrating complete coverage by MoS2.

4 Conclusions

In summary, we have designed a Pd NWs@2H-MoS2

hybrid structure for HER application. The one-

dimensional structure of the metallic NWs facilitates

electron transport to the active S sites, while the

interfacial charge polarization between MoS2 and Pd

increases the electron density of the S sites. These

two effects together dramatically enhanced the HER

performance. The hybrid structure exhibits a small

overpotential of 60 mV, a large current density of

122 mA·cm−2 at −300 mV versus RHE, and a low Tafel

slope of 44 mV·decade–1, with excellent durability.

Although the material cost of the metallic cores is yet

to be reduced, we envision that this work would

provide fresh insights into the rational design of hybrid

electrocatalysts for the HER, from the viewpoint of

charge behavior.

Acknowledgements

This work was financially supported by the National

Natural Science Foundation of China (Nos. 21471141,

U1532135, 11375198, 11574280, and U1532112), the

Recruitment Program of Global Experts, the CAS

Hundred Talent Program, the Hefei Science Center

CAS (Nos. 2015HSC-UP009 and 2015HSC-UP020), and

the Fundamental Research Funds for the Central

Universities (Nos. WK2060190025 and WK2310000035).

NEXAFS experiments were performed at the MCD

Endstation at the BL12B-a beamline in the National

Synchrotron Radiation Laboratory (NSRL) in Hefei,

China.

Electronic Supplementary Material: Supplementary

material (TEM images, EDS spectrum, XPS spectra,

XRD patterns, NEXAFS spectra, and CV curves) is

available in the online version of this article at

http://dx.doi.org/10.1007/s12274-016-1153-3.

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