visualization of the electrocatalytic activity of three...

Visualization of the electrocatalytic activity of three- dimensional MoSe2@reduced graphene oxide hybrid nanostructures for oxygen reduction reaction

Shuli Xin1,§, Zhengqing Liu2,§, Li Ma1, Yao Sun1, Chunhui Xiao1, Fei Li1,3 (), and Yaping Du2 ()

1 Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China 2 Frontier Institute of Science and Technology jointly with College of Science, State Key Laboratory for Mechanical Behavior of

Materials, Xi’an Jiaotong University, Xi’an 710049, China 3 Bioinspired Engineering and Biomechanics Center (BEBC), Xi’an Jiaotong University, Xi’an 710049, China § These authors contributed equally to this work.

Received: 15 June 2016

Revised: 21 July 2016

Accepted: 6 August 2016

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2016

KEYWORDS

MoSe2@rGO hybrid,

oxygen reduction reaction,

electrocatalyst,

scanning electrochemical

microscopy

ABSTRACT

Developments of nanostructured transition metal dichalcogenides (TMDs)

materials as novel electrocatalyst candidates for oxygen reduction reaction

(ORR) is a new strategy to promote the developments of non-precious metal ORR

catalysts. In this work, a three-dimensional (3D) hybrid of rosebud-like MoSe2

nanostructures supported on reduced graphene oxide (rGO) nanosheets was

successfully synthesized through a facile hydrothermal strategy. The prepared

MoSe2@rGO hybrid nanostructure showed enhanced electrocatalytic activity for

the ORR in alkaline medium compared to that of the pure MoSe2, rGO, and their

simple physical mixture, which could benefit from the excellent oxygen adsorption

ability of the abundantly exposed active edge sites of the ultrathin MoSe2 layers,

the conductivity and aggregation-limiting effect of the rGO platform, as well as

the unique 3D rosebud-like architecture of the hybrid material. The electrocatalytic

activity of the MoSe2@rGO hybrid towards ORR was comparable to that of com-

mercial Pt/C catalysts. And the promoted reaction was revealed to involve a nearly

four-electron-dominated ORR process by analysis of the obtained Koutecky–

Levich plots. The scanning electrochemical microscopy (SECM) technique, with

the advantages of investigating of the local catalytic activity of samples with high

spatial resolution and simultaneously evaluating activities of different catalysts in a

single experiment, was further applied to investigate the local ORR electrocatalytic

activity of MoSe2@rGO and compare it with those of other catalyst samples

through applying different sample potentials. The excellent stability and methanol

tolerance of the 3D nanostructured MoSe2@rGO hybrid against methanol further

prove the 3D nanostructured MoSe2@rGO hybrid as a promising ORR electrocatalyst

in alkaline solution for potential applications in fuel cells and metal–air batteries.

Nano Research

DOI 10.1007/s12274-016-1249-9

Address correspondence to Fei Li, [email protected]; Yaping Du, [email protected]

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

1 Introduction

The continuous increase in demand for energy supplies

and ongoing diminishment of fossil fuels on a global

scale urgently require for developing sustainable and

clean energy conversion and storage systems. Metal–

air batteries and fuel cells are two typical novel and

highly efficient energy conversion devices that function

by converting the chemical energy of fuels by oxidation

at an anode and reduction of oxygen at a cathode [1–5].

The oxygen reduction reaction (ORR) as the key

cathodic reaction in metal–air batteries and fuel cells

has been extensively studied both fundamentally and

for practical applications [4–11]. However, carbon-

supported platinum (Pt/C), which is traditionally used

as the catalyst for ORR, suffers from several drawbacks,

including its high cost, low stability, and problems

associated with fuel crossover and CO poisoning,

which hinder the further practical application of these

energy devices in our daily lives. Much research effort

has focused on the development of low-cost non-Pt

ORR catalysts with high electrocatalytic activity as an

alternative to commercial Pt/C [2, 4, 6, 7, 9, 10, 12, 13].

Transition metal dichalcogenides (TMDs), which are

important two-dimensional (2D) layered materials

represented by MX2 (M = Mo, W; X = S, Se), have

shown excellent performance in sensing [14–17],

catalysis [18–21], and energy conversion and storage

[16, 22], owing to their layered structures and ability

to accept electrons and protons. Nanostructured

MoS2 is the most studied TMD material and exhibits

excellent ORR catalytic activity owing to its abundant

exposed Mo edges for oxygen adsorption and

replacement, and the large surface area of layered

nanostructure [23–26]. More recently, the similarity of

the MoSe2 structure to that of MoS2 led to a pioneering

work of investigating the ORR electrocatalytic activity

of MoSe2, which revealed that ultrathin MoSe2 nano-

sheets also display highly efficient electrocatalytic

activity towards ORR [27].

However, the 2D MoSe2 nanosheets are inclined to

agglomerate and restack together due to their interlayer

van der Waals attraction and high surface energy,

resulting in the loss of their nanoscopic features

and a reduction in the density of exposed catalytic

active sites [28–32]. In addition, the intrinsically low

conductivity of TMDs and the poor electrical contact

between the active sites on the lying nanostructures

and the current collector also significantly suppress

their overall electrocatalytic rate and efficiency [33–35].

One efficient strategy to overcome these drawbacks is

to construct three-dimensional (3D) MoSe2 architectures

in order to prevent the aggregation of the MoSe2 layers

[31, 32]. Another strategy is to couple MoSe2 with

highly conductive materials, such as carbon fiber cloth

[36, 37], graphene [27, 38, 39], or reduced graphene

oxide (rGO) [40, 41], to provide more active reaction

sites and also facilitate electron transfer. Moreover,

the combination of these two approaches through

hybridization of 3D MoSe2 nanostructures with carbon

nanomaterials has also been used to prepare hybrid

nanostructures that exhibited superior electrocatalytic

performances to those of pure MoSe2 nanostructures

[31, 32]. However, these previous studies on 3D MoSe2

nanomaterials and their hybrids focused mainly

on their electrocatalytic performance for hydrogen

evolution reaction [31, 32]. Further discovering on

their electrocatalytic activities for ORR and their

potential application in fuel cells and metal–air batteries

is still rare and will become a research interest.

The electrocatalytic activity of TMDs hybrid nano-

structure is significantly affected by their surface

reactivity, which is strongly localized at the micrometer

and even nanometer scale. In addition, the increasing

interest in the development of TMD materials as

novel ORR electrocatalysts necessitates the rapid and

high throughput electrochemical screening methods.

However, the traditional electrochemical techniques

(e.g., cyclic voltammetry (CV) and rotating disk electrode

(RDE) techniques) investigate the entire electrode

surface and provide only average activity information

of catalysts. Furthermore, only one sample can be

studied through one CV or RDE measurement, which

can’t meet the need of rapidly finding the TMDs

candidate as good ORR catalyst from a large amount

of TMDs samples. Scanning electrochemical microscopy

(SECM), a kind of scanning probe microscopy, can

address these disadvantages [42–50]. By employing a

four-electrode system with a micrometer-sized electrode

as the SECM tip to record the redox current of the

catalytic reaction occurring on a substrate electrode,

the catalytic process on the catalyst sample can be

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

“visualized” in situ with high spatial and temporal

resolutions. And the electrocatalytic activities of

several catalysts can be evaluated in a single SECM

experiment [51–54]. Therefore, SECM has been

successfully employed to investigate the local elec-

trocatalytic activities of several ORR catalysts in

previous studies [55].

In this study, a 3D MoSe2@rGO hybrid nanostructure

is synthesized through anchoring hierarchical MoSe2

nanostructures to rGO nanosheets using a facile

hydrothermal strategy [32]. The rosebud-like nano-

structure and the composition of the prepared

MoSe2@rGO hybrid are thoroughly characterized by

transmission electron microscopy (TEM), scanning

electron microscopy (SEM), X-ray diffraction (XRD),

X-ray photoelectron spectroscopy (XPS), Raman spec-

troscopy, and thermogravimetric analysis (TGA). The

ORR electrocatalytic activity of the MoSe2@rGO

hybrid in 0.1 M KOH solution characterized by cyclic

voltammetry and rotating disk electrode measurements

demonstrate that the MoSe2@rGO hybrid has better

ORR electrocatalytic activity than the pure MoSe2,

pure rGO and the physically mixed MoSe2 and rGO,

which could be attributed to the abundant active

edge sites of the ultrathin MoSe2 layers and the high

conductivity of the rGO support. A nearly four-electron

oxygen reduction process occurring on the MoSe2@rGO

hybrid, as revealed through analysis of the Koutecky–

Levich (K–L) plots of the RDE data, confirms the

similarity of the ORR electrocatalytic mechanism of

the MoSe2@rGO hybrid with that of commercial

Pt/C catalysts in alkaline solution. And the excellent

methanol tolerance and stability of the MoSe2@rGO

hybrid further demonstrate its better ORR catalytic

performance than Pt/C. SECM analysis is performed

to provide quantitative information on the local ORR

electrocatalytic activities of MoSe2@rGO, pure MoSe2,

rGO, and physically mixed MoSe2 and rGO with

applying different polarized potentials. The results

of these experiments indicate that the prepared 3D

rosebud-like MoSe2@rGO hybrid nanostructures exhibit

efficient ORR electrocatalytic activity, good methanol

tolerance, and stability, making them a promising

ORR catalyst in the TMDs family for fuel cells and

metal–air batteries.

2 Experimental

2.1 Chemicals and materials

Ammonium molybdate ((NH4)2MoO4, 99.9%), selenium

powder (Se, 99.5%), and potassium hydroxide (KOH,

85%) were purchased from Alfa-Aesar. Oleic acid (OA,

90%) was purchased from Sigma-Aldrich. Dimethyl

formamide (DMF), ethanol, methanol, and cyclohexane

were purchased from Tianjin ZhiYuan Reagent Co.,

Ltd. Nafion (5 wt.%) and Pt/C (20 wt.%) were pur-

chased from Alfa-Aesar and E-TEK, respectively. The

glassy carbon electrode (GCE, Ø = 3 mm), Ag/AgCl

(KCl, saturated) electrode, and Pt wire electrode were

purchased from Xianren Co., Ltd. The glassy carbon

RDE with geometrical area of 0.247 cm2 was pur-

chased from Pine Instrument Co., Ltd. All chemicals

were used as received without further purification.

The aqueous solutions used in the electrochemical

measurements were prepared from Milli-Q reagent

water (Millipore Corp., resistivity > 18.2 MΩ·cm).

2.2 Synthesis of MoSe2 and MoSe2@rGO hybrid

nanostructures

2.2.1 Synthesis of layered MoSe2 nanostructures

A given amount of (NH4)2MoO4 (0.5 mmol) and Se

powder (1.0 mmol) were added to 18 mL of a 1:1 (v/v)

mixture of oleic acid and ethanol in a 20 mL Teflon-

lined autoclave. The autoclave was tightly sealed,

placed in an oven, heated at 160–200 °C for 72 h, and

then cooled to room temperature. The products were

separated from the solution by centrifugation, washed

three times with ethanol, and vacuum dried at 60 °C

for 12 h. Finally, the obtained samples were annealed

in Ar/H2 (95%:5%) at 500 °C for 1 h to remove the

surfactant (OA) and unreacted Se powder.

2.2.2 Synthesis of 3D MoSe2@rGO hybrid nanostructures

The MoSe2@rGO hybrid nanostructures were prepared

via a facile hydrothermal strategy according to our

previously reported method [32]. Firstly, 20 mg GO

made by a modified Hummers method [56] was

dispersed in 4 mL distilled water under constant

sonication at room temperature for ca. 60 min until a

clear and homogeneous solution was achieved. The


4 Nano Res.

synthesis of the MoSe2@rGO hybrid was then proceeded

in the same way as the preparation of the MoSe2

nanostructures, except with the addition of 4 mL of

the GO solution prepared above to a mixed solvent

containing 9 mL OA and 5 mL ethanol in a 20 mL

Teflon-lined autoclave.

2.3 Structural and composition characterizations

SEM images of the prepared samples were obtained

using a Quanta F250 scanning electron microscope

with an accelerating voltage of 10 kV. TEM, high-

resolution transmission electron microscopy (HRTEM)

and high-angle annular dark-field scanning TEM

(HAADF-STEM) analyses were performed on a Hitachi

HT-7700 transmission electron microscope (Japan)

with operating at an accelerating voltage of 100 kV, and

a Philips Tecnai F20 FEG-TEM (USA) with operating

at an accelerating voltage of 200 kV, respectively. XRD

patterns were obtained using a Rigaku D/MAX-RB

X-ray diffractometer with monochromatized CuKα

radiation (λ = 1.5418 Å) in the 2 range from 10 to

80. XPS spectra were conducted using a PHI Quantera

SXM instrument equipped with an Al X-ray excitation

source (1,486.6 eV). Binding energies of the samples

were referenced to the C 1s peaks at 284.6 eV. Raman

spectra were recorded using a LabRAM HR with a

633 nm laser excitation wavelength. TGA analysis

was performed using a Perkin-Elmer TGA7 thermo-

gravimetric analyzer over a temperature range of

100 to 900 °C at a heating rate of 10 °C·min−1 under air

flow.

2.4 Electrochemical measurements

2.4.1 Electrochemical impedance spectroscopy (EIS)

measurements

EIS measurements were performed on a CHI760D

electrochemical workstation (Chenhua Co., Ltd., China)

using a three-electrode cell with a sample-suspension-

modified GCE as the working electrode (WE), a

Ag/AgCl (KCl, saturated) electrode as the reference

electrode (RE) and a Pt wire as the counter electrode

(CE), respectively. Homogeneous catalyst suspensions

with a concentration of 4 mg·mL−1 were prepared by

dispersing 4 mg of catalyst samples in 1 mL of a 2:1

(v/v) mixture of DMF and deionized water, followed

by addition of 20 μL Nafion solution and sonication

for 10 min. A 5 μL aliquot of the catalyst suspension

was uniformly loaded onto the GCE surface using a

micropipette, and then dried in air to provide a

catalyst loading of ca. 0.285 mg·cm−2. The EIS Nyquist

plots of the samples were recorded in a 0.1 M KOH

aqueous solution at an overpotential of 250 mV in the

range of 100 kHz–0.1 Hz with amplitudes of 5 mV.

2.4.2 Cyclic voltammetry measurements

CV measurements were performed using the same

system as that described in the EIS measurements. A

9 μL aliquot of catalyst suspension was uniformly

loaded on the GCE surface using a micropipette and

then dried in air to provide a catalyst loading of ca.

0.51 mg·cm−2. For comparison, a commercial Pt/C

catalyst suspension was prepared using the same

quantity and method as those described above. All the

potentials reported in electrochemical measurements

were calibrated with respect to a reversible hydrogen

electrode (RHE) (details in the Electronic Supplemen-

tary Material (ESM)). All the CV measurements were

taken in an O2- or N2-saturated 0.1 M KOH aqueous

solution and scanned from 0.17 to 0.97 V at a scan

rate of 10 mV·s−1 at room temperature, except for the

comparison of the MoSe2@rGO and Pt/C samples,

which employed potentials from 0.17 to 1.07 V.

2.4.3 Linear sweep voltammetry (LSV) measurements

LSV measurements were carried out on a glassy carbon

rotating-disk electrode modified with 0.6 mg·cm−2 of

as-prepared sample suspensions or 50 μg·cm−2 Pt/C

suspension using a Pine biopotentiostat combined

with a rotation speed controller (Pine Instrument Co.

Ltd., USA). The LSV experiments were performed in

O2- or N2-saturated 0.1 M KOH aqueous solution and

scanned from 0.19 to 1.09 V at a scan rate of 10 mV·s−1

and various rotation speeds (400, 625, 900, 1,225, 1,600,

and 2,025 rpm, respectively).

The overall transfer electron numbers per oxygen

molecule in the ORR process were calculated based

on the slopes of Koutecky–Levich plots (J−1 vs. ω−1/2,

Eq. (1))


5 Nano Res.

1 2K L K

1 1 1 1 1

J J J J B (1)

where J is the measured current density, JK and JL are

the kinetic current density and the limiting diffusion

current density, respectively, and ω is the electrode

rotation speed. B was determined from the slope of the

K–L plot according to the Levich equation (Eq. (2)) [11]

2 2

2 31 6

O O0.2B nF D C (2)

where n is the overall number of electrons transferred

per oxygen molecule, F is the Faraday constant, DO2 is

the diffusion coefficient of O2 in 0.1 M KOH (1.73 ×

10−5 cm2·s−1), is the kinetic viscosity of the electrolyte

(0.01 cm2·s−1), and CO2 is the bulk concentration of O2

(1.21 × 10−6 mol·cm−3) [57].

2.4.4 SECM measurements

SECM measurements were performed on a CHI920C

SECM workstation (Chenhua Co., Ltd., China) using a

four-electrode system. A homemade 25 μm-diameter

Pt-disk microelectrode was used as the SECM probe

and also as working electrode 1 (WE1), and the GCE

loaded with the catalyst dispersions was used as

the substrate electrode and also as working electrode

2 (WE2). The RE and CE used in the SECM mea-

surements were the same as those used in the CV and

LSV experiments. The reported potential values applied

to both WE1 and WE2 in the SECM experiments were

also calculated with respect to the RHE. The catalyst

suspensions were prepared using the same method

as that described above. A 0.1 μL aliquot of catalyst

dispersion with a concentration of 4 mg·mL−1 was

dropped onto the surface of the GCE with a micro-

dispenser, and dried in air. All the SECM scanning

experiments were conducted using the redox com-

petition mode of SECM (RC-SECM) [51–54] (or the

equivalent shielding mode [55]) in an O2-saturated

0.1 M KOH aqueous solution with probe-to-sample

distances of ca. 50 μm. For area scan measurements,

the SECM probe applied with a potential of 0.17 V was

scanned in the XY plane over the substrate electrode

modified with catalyst spot array at the sample

potentials (ES) of 0.57, 0.62, 0.67, 0.72, and 0.77 V,

respectively, with an increment distance of 5 μm and

an increment time of 0.02 s. The x- and y-line SECM

scans were taken over the most active areas of the

catalyst spot arrays at a scan rate of 25 μm·s−1.

2.4.5 Stability and methanol tolerance measurements

Both the stability and methanol tolerance experiments

of MoSe2@rGO with respect to the commercial Pt/C

were carried out using chronoamperometry at 0.77 V

in an O2-saturated 0.1 M KOH solution employing

the same three-electrode system as that used in the CV

experiments. The stability experiments were performed

with prolonging chronoamperometry measurements

to 15,000 s and the methanol tolerance experiment was

tested after injection of 10% methanol at ca. 300 s.

3 Results and discussion

3.1 Structure and composition of MoSe2@rGO

hybrid nanostructures

The structures of the prepared MoSe2 and MoSe2@rGO

hybrid nanostructures can be observed by the obtained

SEM, TEM, and HAADF-STEM images (Figs. 1(a)–1(d)).

As shown in Fig. 1(a), the TEM image of pure MoSe2

shows the formation of aggregated MoSe2 consists of

layers. In the MoSe2@rGO hybrid, MoSe2 nanostructures

with an average size of ca. 150 ± 5 nm are shown

to be uniformly distributed on the rGO nanosheets

(Figs. 1(b)–1(d)), confirming that the rGO nanosheets

provide an ideal platform for growing MoSe2 layers.

Furthermore, the MoSe2@rGO hybrid exhibits a unique

rosebud-like 3D architecture. The possible mechanism

for the growth of MoSe2 layers on the rGO nanosheets

involves the precursors being anchored to the surface

of the rGO by carboxyl, hydroxyl, and epoxy functional

groups [58–60], and the defects of graphene nanosheets

acting as the nucleation sites [61]. Corrugations in the

MoSe2@rGO hybrid can be more clearly observed in

the inset of Fig. 1(b) and the HAADF-STEM image in

Fig. 1(d), which show that the MoSe2 in the MoSe2@rGO

hybrid is composed of thin layers. The energy disperse

spectroscopy data presented in Fig. S1(a) in the

Electronic Supplementary Material (ESM) further verify

the Mo:Se stoichiometric atomic ratio of 1:2.


6 Nano Res.

Figure 1 (a) TEM image of the as-prepared layered MoSe2 nano-structures. (b) SEM (inset: the enlarged SEM image of (b)), (c) TEM, and (d) STEM images of the as-prepared 3D MoSe2@rGO hybrid nanostructure.

The detailed crystal structure of the as-prepared

MoSe2@rGO hybrid was further characterized using

HRTEM (Figs. 2(a) and 2(b)). A single rosebud-like

MoSe2@rGO structure can be more clearly observed

Figure 2 (a) TEM image of MoSe2 grown on rGO to form the MoSe2@rGO hybrid nanostructure. (b) HRTEM image of MoSe2 taken from the rim of the MoSe2@rGO hybrid. Structural models of MoSe2 viewed from the (c) <010> and (d) <001> direction.

in Fig. 2(a). Figure 2(b) displays the HRTEM image

taken of the rim of the MoSe2 layers in the MoSe2@rGO

hybrid, in which MoSe2 exhibits defined crystal lattice

fringes with an interplanar spacing of 0.28 nm and an

interlayer spacing of 0.64 nm, corresponding to the

(100) and (002) planes of MoSe2, respectively. To provide

a deeper understanding of the MoSe2 nanostructure,

the atomic structure models with a normal interlayer

spacing of 0.64 nm were built (Figs. 2(c) and 2(d)). In

Fig. 2(c), the viewing from the <010> direction, the (100)

and (002) planes are marked, which are as shown

in the vertically stacked MoSe2 layers in the HRTEM

image in Fig. 2(b). The view from the <001> direction

exhibited in Fig. 2(d) indicates that the MoSe2 possesses

graphene-like structure. The (100) plane perfectly

matches the HRTEM image of the planar orientation

in Fig. 2(b).

The composition of the MoSe2@rGO hybrid nano-

structure was initially confirmed by XRD. In the XRD

patterns of pure MoSe2 and the MoSe2@rGO hybrid

nanostructure displayed in Fig. 3(a), all the diffraction

peaks of hexagonal MoSe2 (2H-type, space group:

P63/mmc, a = b = 0.329 nm, c = 1.293 nm, JCPDS Card

No. 29-0914) are observed in the MoSe2@rGO hybrid,

demonstrating that the rGO serves as the platform

for growing MoSe2. The chemical states of Mo, Se,

and C in the MoSe2@rGO hybrid were then analyzed

using XPS (Figs. 3(b) and 3(c), and Fig. S1(b) in the

ESM). In Fig. 3(b), the double peaks arising from the

core levels of Mo 3d3/2 and Mo 3d5/2 orbitals are

located at 232.2 and 228.9 eV, respectively, indicating

a characteristic of Mo4+ in the obtained MoSe2@rGO

hybrid [62, 63]. The Se 3d XPS spectra shown in Fig. 3(c)

show peaks at 55.0 and 54.2 eV, which are assigned to

the core levels of Se 3d3/2 and Se 3d5/2, respectively,

indicating the presence of Se2− [64, 65]. The C 1s

peaks at 284.6 eV in Fig. S1(b) (in the ESM) originate

from the graphene nanosheets with low oxygen content,

verifying the reduction of GO to rGO [66]. Raman

spectroscopy was further applied to characterize

the structure of the MoSe2@rGO hybrid. As displayed

in Fig. 3(d), the Raman peaks at 236 and 284 cm−1

correspond to the A1g and E12g of MoSe2 [32], respec-

tively, and the observed D, G, and 2D bands of graphene

in the hybrid [60] indicate that the MoSe2@rGO hybrid

is successfully synthesized.


7 Nano Res.

The loading percentage of the MoSe2@rGO hybrid

was determined by TGA. As demonstrated by our

previous report [32], MoSe2 is gradually oxidized

with increasing temperature. At the low temperature

of 200 °C, MoSe2 starts to pyrolyze and produces MoO3

and SeO2 according to the following reaction

2MoSe2 + 7O2 = 2MoO3 + 4SeO2 (3)

When the temperature reaches the sublimation tem-

perature of SeO2 (315 °C), SeO2 begins to gasify along

with the total weight residue oxide of MoO3. As

shown in Fig. 4(a), for the as-reduction to 80% of

the initial weight, and leads to the final prepared

MoSe2@rGO hybrid, the TGA curve exhibits a process

of two-stage weight loss, in which SeO2 volatilizes

at the first stage and rGO converts into CO2 at the

second stage. Through analysis of the TGA data, the

loading percentage of MoSe2 in the MoSe2@rGO hybrid

was calculated to be ca. 60.4%.

3.2 Electric resistance of MoSe2@rGO hybrid

nanostructures

The charge transfer resistance is an important

characteristic that affects the electrocatalytic perfor-

mance of ORR catalysts. Therefore, electrochemical

impedance spectroscopy measurements for the pure

MoSe2 and the MoSe2@rGO hybrid were conducted

in 0.1 M KOH solution in the range of 100 kHz–

0.1 Hz. The obtained EIS Nyquist plots of these two

samples are shown in Fig. 4(b). Both pure MoSe2 and

MoSe2@rGO exhibit arc-like profiles at high frequencies,

and long straight lines at low frequencies, which

correspond to the charge transfer resistance and mass

transfer resistance, respectively [67, 68]. The diameter

of the semi-circle at high frequencies is significantly

reduced for the plot of the MoSe2@rGO hybrid com-

pared with that of pure MoSe2. This indicates that the

charge-transfer resistance at the electrode/electrolyte

interface is significantly decreased upon combining

Figure 3 (a) XRD patterns, (b) and (c) XPS spectra, and (d) Raman spectrum of the as-obtained MoSe2@rGO hybrid nanostructures. (b) Mo 3d and (c) Se 3d signals of the MoSe2@rGO hybrid nanostructure.


8 Nano Res.

MoSe2 with the conductive rGO platform, which leads

to improvement of the charge transfer kinetics of

the MoSe2 grown on the rGO support and further

contributes to the enhancement of the ORR electro-

catalytic activity of MoSe2.

3.3 ORR electrocatalytic activity of MoSe2@rGO


To evaluate the electrocatalytic activity of the prepared

MoSe2@rGO hybrid for ORR, and to compare its

electrocatalytic performance with those of pure MoSe2,

rGO, and their physical mixture (MoSe2+rGO), CV

measurements were performed in 0.1 M KOH solution

using a GCE modified with one of these four materials.

CV curves of a bare GCE and a Pt/C-modified GCE

were also recorded as control experiments. From the

obtained CVs of these samples shown in Figs. 5(a)

and 5(b), compared with the CV curves (dashed curves)

recorded in N2-saturated KOH solution, all the GCEs

exhibit obvious reduction currents in the potential

range of 0.17 to 1.07 V in the O2-saturated KOH solution

(solid curves), proving that the observed cathodic

currents are from the ORR at these GCEs. In Fig. 5(a),

the ORR peak potentials of these sample-modified

GCEs are in the order of MoSe2@rGO (0.77 V) > MoSe2

(0.72 V) > MoSe2+rGO (0.66 V) > rGO (0.62 V) > bare

GCE (0.58 V), from which we can obtain the following

information about these samples: Firstly, the rGO-

modified GCE has a more positive ORR peak potential

(i.e., better electrocatalytic activity for the ORR) than

the bare GCE, which could be due to that the large

surface area and the relatively good conductivity of

rGO contribute to increasing the oxygen adsorption

area of GCE and improving the electron transfer

kinetics from GCE to oxygen in solution [40, 41].

Secondly, the MoSe2-modified GCE has better ORR

electrocatalytic activity than those of the GCEs modified

with rGO and the physical mixture of MoSe2 and rGO.

This indicates that MoSe2 is the main contributor to

the electrocatalysis of the ORR, which could be due

Figure 4 (a) TGA analysis curve of the MoSe2@rGO hybrid. (b) EIS Nyquist plots of the pure MoSe2 and the MoSe2@rGO hybrid.

Figure 5 CV curves of different sample-modified GCEs in O2-saturated (solid curves) or N2-saturated (dashed curves) 0.1 M KOH aqueous solution at scan rates of 10 mV·s−1. (a) From top to bottom, the bare GCE, the rGO-modified GCE, the MoSe2+rGO-modified GCE, the MoSe2-modified GCE, and the MoSe2@rGO-modified GCE. (b) The GCEs modified with MoSe2@rGO and Pt/C.


9 Nano Res.

to the ultrathin MoSe2 layers with large surface area

providing numerous active sites for ORR, and the

abundant exposed Mo edges for adsorbing oxygen

from solution and thus achieving oxygen replacement

[27]. This result also indicates that the decreased

loading of MoSe2 in the mixture of MoSe2 and rGO

also results in a lower ORR electrocatalytic activity

compared to that of the same mass of pure MoSe2.

Thirdly, compared with the pure MoSe2, the ORR

peak potential of the MoSe2@rGO hybrid moves to

more positive potential by ca. 50 mV, demonstrating

the enhanced ORR electrocatalytic reactivity of MoSe2

grown on rGO. The substantially improved ORR

electrocatalytic activity of the MoSe2@rGO hybrid can

be ascribed to the rGO support efficiently ameliorating

the aggregation of layered MoSe2, and thus maintaining

its electrocatalytic activity during the ORR [38, 39]. As

proved by the above EIS results, the conductive rGO

support accelerates the electron transfer from the GCE

to oxygen through the MoSe2@rGO hybrid during the

ORR process. In addition, the 3D rosebud-like nano-

structure of the MoSe2@rGO hybrid also provides

excellent structural rigidity [38, 39], and the interlayer

MoSe2 spacing of 0.64 nm allows the MoSe2@rGO to

react with the electrolyte more efficiently and provides

abundant active edge sites accessible to oxygen

molecules [31, 32], both of which further enhance ORR

electrocatalytic performance [32]. Fourth, compared

to that of MoSe2+rGO, the ORR peak potential of

MoSe2@rGO moves to more positive potential by ca.

100 mV, further proving that the enhanced ORR

electrocatalytic activity of the MoSe2@rGO hybrid

benefits from the synergetic effect of MoSe2 and rGO

hybridization, rather than their simply physical mixing.

In Fig. 5(b), compared to the commercial Pt/C catalyst

with an ORR peak potential of 0.81 V, the ORR peak

potential of the MoSe2@rGO hybrid is only ca. 40 mV

smaller under the same mass loading, demonstrating

that the MoSe2@rGO hybrid presents as a potential

non-Pt ORR catalyst with favorable ORR electrocatalytic

activity.

LSV measurements using a RDE modified with

MoSe2@rGO in O2-saturated 0.1 M KOH solution were

performed to further understand the ORR process

occurring on the MoSe2@rGO hybrid (Fig. (6)). For

comparison, the LSV curves of RDEs modified with

Figure 6 (a) LSV curves of the RDE modified with different catalysts in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV·s−1

and a rotation speed of 1,600 rpm. From top to bottom: rGO, MoSe2+rGO, MoSe2, MoSe2@rGO, and Pt/C. (b) LSV curves of the RDE modified with the MoSe2@rGO hybrid at a scan rate of 10 mV·s−1 at different rotation speeds. (c) K–L plots of the MoSe2@rGO hybrid under different applied potentials and at different rotation speeds, and (d) Tafel plots for MoSe2@rGO and Pt/C.


10 Nano Res.

rGO, MoSe2, MoSe2+rGO, and commercial Pt/C were

also recorded under the same experimental conditions,

with a rotation rate of 1,600 rpm and a constant scan

rate of 10 mV·s−1. As shown in Fig. 6(a), the onset and

half-wave potentials of MoSe2@rGO are located at

0.90 and 0.73 V, respectively, which are more positive

than those of rGO (0.74 V, 0.58 V), MoSe2+rGO (0.83 V,

0.61 V), and MoSe2 (0.85 V, 0.63 V). The oxygen reduction

current densities of these catalysts at the potential of

0.5 V are in the order of MoSe2@rGO (3.18 mA·cm−2) >

MoSe2 (1.82 mA·cm−2) > MoSe2+rGO (0.75 mA·cm−2) >

rGO (0.24 mA·cm−2) under the same catalyst loading.

The more positive onset and half-wave potentials, and

the higher current density of the MoSe2@rGO hybrid,

further demonstrate its superior ORR electrocatalytic

performance to those of pure MoSe2, rGO, and their

physical mixture, which could also confirm the

synergetic effect between the ultrathin MoSe2 layers

with abundant ORR catalytically active edges and the

conductive rGO support to facilitate the electron

transfer from the less conductive MoSe2 to the RDE

[31, 32, 38, 39]. Compared to the commercial Pt/C,

a difference of 40 mV in both onset and half-wave

potentials is obtained for the MoSe2@rGO hybrid,

indicating the comparable ORR electrocatalytic activity

of MoSe2@rGO and Pt/C. In addition, the current pla-

teaus observed in the LSV curves also demonstrate that

the oxygen reduction processes is diffusion-controlled

on the catalysts modified GCE at the rotation rate.

The LSV results show that the ORR activities of these

catalysts increase in the order MoSe2@rGO > MoSe2 >

MoSe2+rGO > rGO, which is in good accordance with

the CV results and also provides a quantitative com-

parison of the ORR current densities of these samples.

To obtain the kinetic and mechanistic information

of the MoSe2@rGO-catalyzed ORR, LSV measurements

using a MoSe2@rGO-modified RDE were performed

at various rotation speeds with a constant scan rate.

As has been demonstrated [69], the shortened diffusion

distance at higher speeds enhances the diffusion of

oxygen to the RDE surface. Therefore, the limiting

current density increases with an increase in rotation

rates from 400 to 2,025 rpm, whereas the onset potentials

of the ORR catalyzed by MoSe2@rGO remain constant

(Fig. 6(b)). From the obtained LSVs of MoSe2@rGO

hybrid, the corresponding K–L plots at electrode

potentials from 0.45 to 0.65 V were calculated and

are presented in Fig. 6(c). All the linear K–L plots at

different potentials show inverse current density (j−1)

as a function of the inverse of the square root of the

rotation speed (ω−1/2) at these potentials, indicating

first-order reaction kinetics with respect to the con-

centration of dissolved oxygen, and revealing a similar

oxygen reduction-involved electron transfer number

per oxygen molecule (n) at different potentials [9, 70].

Through analyzing the slopes of the K–L plots according

to the K–L equation, the calculated average electron

transfer number per oxygen molecule for ORR at the

potential ranging from 0.45 to 0.65 V (vs. RHE) are

3.9~4.1, revealing a nearly four-electron dominated

ORR process for the MoSe2@rGO hybrid in alkaline

aqueous solution [6, 10, 11], similar to that of the

commercial Pt/C-catalyzed ORR. The good ORR elec-

trocatalytic activity of MoSe2@rGO is also confirmed

by the Tafel plot shown in Fig. 6(d). The kinetic

currents derived by mass-transport correction of the

RDE data give a Tafel slope of 62 mV per decade for

MoSe2@rGO at low overpotentials, which is close to

the 57 mV per decade of the commercial Pt/C catalyst.

The adjacent slopes observed for MoSe2@rGO and

Pt/C at low overpotentials in alkaline medium suggest

that the rate-determining step of both catalysts is

the first electron reduction of oxygen [71]. Moreover,

as illustrated in Table S1 (in the ESM), compared

with the previously reported nanosheet-structured

graphene–MoSe2 composite [27] and the MoS2

nanodot/N-graphene composite [24], the MoSe2@rGO

hybrid exhibits comparable electrocatalytic activity

for ORR, with closed onset and peak potentials for

oxygen reduction. Compared to other well-investigated

ORR catalysts, such as N-, B-, or Fe-doped graphene

[12, 13] and Co3O4 [6], the MoSe2@rGO hybrid has a

more positive peak potential for ORR and a larger

value of n close to 4.0. Even the ORR electrocatalytic

activity of MoSe2@rGO is still inferior to that of the

golden standard Pt/C catalyst [7]. However, considering

its advantages of low cost and the relatively efficient

ORR catalytic activity, the MoSe2@rGO composite

shows promise as a potential nonprecious metal ORR

catalyst.


11 Nano Res.

3.4 Quantitative comparison of local ORR electro-

catalytic activities of catalyst samples using SECM

The above CV and RDE results elucidated and com-

pared the average electrocatalytic activities of these

samples for ORR. To investigate the local catalytic

activity of MoSe2@rGO with high spatial resolution,

and to simultaneously compare its reactivity with the

other catalyst samples in a single experiment, SECM

in RC-SECM mode [51–54] (or the equivalent shielding

mode [55]) with a four-electrode system was employed.

Samples of MoSe2@rGO, MoSe2, MoSe2+rGO, and rGO

with the same mass loading were simultaneously

modified on the same GCE, and the substrate GCE

and the SECM probe were both applied with oxygen

reduction potentials in an O2-saturated 0.1 M KOH

solution. Catalyst samples with higher ORR electro-

catalytic activity consume more local oxygen, resulting

in a noticeable current decrease at the SECM probe

across the active catalyst surface. The ORR electro-

catalytic activities of different samples can then

be distinguished by comparing the recorded ORR

currents at the SECM probe (i.e., the color contrast in

the SECM image) across the corresponding sample

surfaces. And the difference in the ORR electrocatalytic

activities of samples can be more clearly observed by

applying different polarized potentials to the samples.

Figure 7(a) is the optical photograph of the GCE

surface loaded with spots of MoSe2@rGO, MoSe2,

MoSe2+rGO, and rGO. Figures 7(b)–7(f) are the corres-

ponding SECM images of these four catalyst samples

under different sample potentials (i.e., the polarized

potentials at the GCE) ranging from 0.57 to 0.77 V with

a gradient of 0.05 V, which were selected according to

the oxygen reduction peak potentials of these four

samples shown in Fig. 5(a). The currents presented in

the SECM images are the decreases of the ORR currents

recorded at the SECM probe (∆i) after subtraction of

the background currents at corresponding sample

potentials, which was defined as the SECM probe

currents over the bare GCE at the corresponding

applied potentials. As shown in Fig. 7(b), at a sample

potential (ES) of 0.57 V, all four sample spots exhibit

brighter images (yellow color) than that of the

background GCE (green color), which is due to the

large decrease in the ORR currents at the SECM probe

across the four active catalyst spots owing to the

consumption of oxygen around the SECM probe

by the electrocatalytic reaction. The images indicate

that all four samples show good ORR electrocatalytic

activities at this sample potential. The MoSe2@rGO

spot is more visible than the other three spots,

confirming that the MoSe2@rGO hybrid is more active

than MoSe2, MoSe2+rGO, and rGO towards ORR at

an ES of 0.57 V, which is consistent with the CV and

LSV results. Moreover, the local ORR electrocatalytic

activities of the MoSe2@rGO hybrid and the other

three samples can be more clearly distinguished by

comparing the values of ∆i at more positive sample

potentials. As shown in Figs. 7(c)–7(f), the color contrast

of the MoSe2@rGO hybrid with those of the MoSe2,

MoSe2+rGO, and rGO samples becomes more pro-

nounced when adjusting the sample potentials to

more positive values (i.e., from 0.62 to 0.77 V), further

proving that the MoSe2@rGO hybrid exhibits higher

ORR electrocatalytic activity than MoSe2, MoSe2+rGO,

and rGO at more positive sample potentials. For

example, at sample potentials of 0.72 and 0.77 V

(Figs. 7(e) and 7(f)), the MoSe2, MoSe2+rGO, and rGO

samples are hardly visualized compared to the

background GCE, which could be due to the ORR

peak potentials of the MoSe2, MoSe2+rGO, and rGO

samples all being lower than 0.72 V, thus not much

amount of O2 could be consumed by these three samples

at this potential. Whereas, the obtained maximum

values of ∆i for the MoSe2@rGO hybrid sample at ES =

0.72 and 0.77 V are 7.29 and 6.31 nA, respectively,

which is due to the ORR peak potential of MoSe2@rGO

hybrid being 0.77 V (Figs. 5(a) and 6(a)). This indicates

that the MoSe2@rGO hybrid still exhibits ORR elec-

trocatalytic activity at these two potentials.

To quantitatively compare the local ORR electro-

catalytic activities of the MoSe2@rGO hybrid with

those of MoSe2 and MoSe2+rGO, x-line scans along

the most active areas of MoSe2@rGO and MoSe2, and

y-line scans along the most active areas of MoSe2@rGO

and MoSe2+rGO were performed by recording the

oxygen reduction currents at the SECM probe under

different sample potentials. The differences in the local

electrocatalytic activities of MoSe2@rGO vs. MoSe2

and MoSe2+rGO at all five potentials can be more

clearly observed from the differences in ∆i in the


12 Nano Res.

RC-SECM x- and y-line scans and the extracted bar

graphs shown in Figs. 7(g)–7(j). In Figs. 7(g) and 7(i),

the values of ∆i for the MoSe2@rGO hybrid (from

10.70 to 3.73 nA) are higher than those of MoSe2

(from 10.07 to 2.35 nA) at all five sample potentials.

In Figs. 7(h) and 7(j), the values of ∆i for MoSe2@rGO

(from 14.00 to 3.73 nA) are also higher than those

of MoSe2+rGO (from 13.29 to 2.15 nA) at the same

Figure 7 (a) Optical photograph of the catalyst spots of MoSe2@rGO (upper-left), MoSe2 (upper-right), MoSe2+rGO (bottom-left) and rGO (bottom-right) loaded on a GCE. (b)–(f) Corresponding RC-SECM area scan images of the catalyst spots in O2-saturated 0.1 M KOH aqueous solution with different potentials applied to the GCE (ES): (b) 0.57 V, (c) 0.62 V, (d) 0.67 V, (e) 0.72 V, and (f) 0.77 V (vs.RHE). SECM probe diameter: 25 μm, probe-to-catalyst distances: ca. 50 μm, probe potential: 0.17 V (vs. RHE). (g) Background current-subtracted RC-SECM x-line scans (at y = 420 μm) along the most active sites of the MoSe2@rGO and MoSe2 spots at different catalyst potentials, and (i) the corresponding bar graph. (h) Background current-subtracted RC-SECM y-line scans (at x = 500 μm) along the most active areas of the MoSe2@rGO and MoSe2+rGO spots at different catalyst potentials, and (j) the corresponding bar diagram.


13 Nano Res.

potentials. And with increasing ES from 0.57 to 0.77 V,

the ∆i values of these three samples all display

decreasing trends, which is due to the three samples

becoming less active for electrocatalyzing ORR at

more anodic sample potentials. The differences in

∆i between MoSe2@rGO and MoSe2, and MoSe2@rGO

and MoSe2+rGO, become more apparent, further

confirming that the MoSe2@rGO hybrid exhibits

superior ORR electrocatalytic performance than those

of MoSe2+rGO and pure MoSe2. The SECM area and

line scan results above provide a comparison of the

local ORR electrocatalytic activities of these catalyst

samples from both qualitative and quantitative points

of view. Furthermore, the SECM results confirm that

the MoSe2@rGO hybrid exhibits improved ORR elec-

trocatalytic activity compared with those of the other

three, supporting the CV and LSV results, and com-

plementing the CV and LSV measurements by providing

local ORR electrocatalytic information for the catalyst

samples. In addition, compared to the CV and LSV

techniques, SECM also has the advantages of only

needing a small amount of sample and the capability

of distinguishing the catalytic activities of samples

with close ORR peak potentials in a single SECM

measurement by applying different sample potentials.

3.5 Stability and methanol tolerance of MoSe2@rGO


For practical application in fuel cells, the stability and

tolerance to methanol of an ORR catalyst are the

other two crucial parameters. The stability of the

MoSe2@rGO hybrid compared with that of the

commercial Pt/C in O2-saturated 0.1 M KOH was

investigated with chronoamperometry. As shown in

Fig. 8(a), at a constant ORR potential of 0.77 V, both

the current densities of MoSe2@rGO and Pt/C exhibit

decaying trends with time. However, the current–time

(i–t) response of MoSe2@rGO exhibits a relatively

slow attenuation with high current retention (91.3%)

over 15,000 s of continuous ORR processing, whereas

the Pt/C catalyst exhibits relatively low stability with

a low current retention (86.0%) under the same

experimental conditions. These results indicate the

better electrochemical stability of MoSe2@rGO than

that of Pt/C, which could also be due to the com-

bination of MoSe2 with the rGO substrate ameliorating

the possible aggregation of layered MoSe2 nanostruc-

tures and therefore maintaining the ORR catalytically

active sites of the MoSe2 [27]. The methanol tolerance

of the MoSe2@rGO hybrid and Pt/C were comparatively

tested using chronoamperometry in O2-saturated

0.1 M KOH at a constant potential of 0.77 V (Fig. 8(b)).

Initially, before injecting methanol, both MoSe2@rGO

and Pt/C generate negative currents at 0.77 V due to

only the ORR occurring. After injection of 10 vol.%

methanol into the solution at ca. 300 s, a sharp current

density change to positive can be observed for the

commercial Pt/C catalyst, which could be due to

methanol adsorption and oxidation on the Pt/C surface

[9, 10]. However, the MoSe2@rGO hybrid exhibits only

a slight current change, and the current response

recovers rapidly, indicating the resistance of MoSe2@rGO

to methanol [27]. The above results of the stability

and methanol tolerance of MoSe2@rGO proves that it

is a promising ORR electrocatalyst in alkaline solution

for practical fuel cell applications.

Figure 8 Chronoamperometry curves of the MoSe2@rGO hybrid and commercial Pt/C on GCEs in O2-saturated 0.1 M KOH at 0.77 V (vs. RHE) (a) for 15,000 s and (b) before and after adding 10 vol.% methanol at ca. 300 s.


14 Nano Res.

4 Conclusions

A 3D rosebud-like MoSe2@rGO hybrid nanostructure

was successfully synthesized through a simple and

facile hydrothermal approach. Its structure, composition,

and electric resistance were thoroughly characterized

using TEM, SEM, XRD, XPS, Raman spectroscopy,

TGA, and EIS. The superior ORR catalytic activity

of the MoSe2@rGO hybrid than those of the pure

MoSe2, rGO, and physically mixed MoSe2 and rGO

was confirmed through CV and LSV measurements,

which could be attributed to the synergetic effect of

the abundantly active edge sites of the layered MoSe2

nanostructures, the highly conductive rGO nanosheet

support, and the unique 3D nanostructure of the hybrid.

The nearly four-electron dominated ORR process, as

confirmed through K–L plots, and the close slopes

of the Tafel plots drawn from the RDE data further

confirmed the comparable ORR catalytic performances

of the MoSe2@rGO hybrid and the commercial Pt/C

catalyst in alkaline medium. Furthermore, the results

of methanol tolerance and stability testing illustrated

the superior robustness of the MoSe2@rGO hybrid to

that of the commercial Pt/C. The SECM technique

was also applied to provide both qualitative and

quantitative comparison of the local ORR catalytic

activities of the MoSe2@rGO hybrid, pure MoSe2, rGO,

and physically mixed MoSe2 and rGO under different

polarized potentials. This work indicates that MoSe2

is a promising new TMD ORR catalyst for energy

conversion, and its ORR electrocatalytic performance

can be improved through the use of highly conductive

carbon nanomaterials as supports.

Acknowledgements

This work was financially supported by the National

Natural Science Foundation of China (Nos. 21105079

and 21405119), the Fundamental Research Funds for

the Central Universities of China (Nos. 0109-1191320016

and cxtd2015003), the Scientific Research Foundation

for the Returned Overseas Chinese Scholars by the

State Education Ministry of China, and the Interna-

tional Science and Technology Cooperation and

Exchange Program of Shaanxi Province of China (No.

2016KW-064). Yaping Du gratefully acknowledges

the financial support from the start-up funding from

Xi'an Jiaotong University, the Fundamental Research

Funds for the Central Universities of China (No.

2015qngz12), and the the National Natural Science

Foundation of China (Nos. 21522106 and 21371140).

Electronic Supplementary Material: Supplementary

material (calibration of reversible hydrogen electrode;

EDX of MoSe2, and XPS of MoSe2@rGO hybrid nano-

structures; Table S1) is available in the online version

of this article at http://dx.doi.org/10.1007/016-1249-9.

References

[1] Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.;

Goodenough, J. B.; Shao-Horn, Y. Design principles for

oxygen-reduction activity on perovskite oxide catalysts

for fuel cells and metal–air batteries. Nat. Chem. 2011, 3,

546–550.

[2] Wang, S. Y.; Iyyamperumal, E.; Roy, A.; Xue, Y. H.; Yu, D. S.;

Dai, L. M. Vertically aligned BCN nanotubes as efficient

metal-free electrocatalysts for the oxygen reduction reaction:

A synergetic effect by Co-doping with boron and nitrogen.

Angew. Chem., Int. Ed. 2011, 50, 11756–11760.

[3] Debe, M. K. Electrocatalyst approaches and challenges for

automotive fuel cells. Nature 2012, 486, 43–51.

[4] Dai, L. M.; Xue, Y. H.; Qu, L. T.; Choi, H. J.; Baek, J. B.

Metal-free catalysts for oxygen reduction reaction. Chem.

Rev. 2015, 115, 4823–4892.

[5] Wang, Y. J.; Zhao, N. N.; Fang, B. Z.; Li, H.; Bi, X. T.;

Wang, H. J. Carbon-supported Pt-based alloy electrocatalysts

for the oxygen reduction reaction in polymer electrolyte

membrane fuel cells: Particle size, shape, and composition

manipulation and their impact to activity. Chem. Rev. 2015,

115, 3433–3467.

[6] Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.;

Regier, T.; Dai, H. J. Co3O4 nanocrystals on graphene as a

synergistic catalyst for oxygen reduction reaction. Nat.

Mater. 2011, 10, 780–786.

[7] Lin, L.; Zhu, Q.; Xu, A. W. Noble-metal-free Fe−N/C

catalyst for highly efficient oxygen reduction reaction under

both alkaline and acidic conditions. J. Am. Chem. Soc. 2014,

136, 11027−11033.

[8] Xia, W.; Mahmood, A.; Liang, Z. B.; Zou, R. Q.; Guo, S. J.

Earth-abundant nanomaterials for oxygen reduction. Angew.

Chem., Int. Ed. 2016, 55, 2650–2676.

[9] Niu, W. H.; Li, L. G.; Liu, X. J.; Wang, N.; Liu, J.; Zhou,

W. J.; Tang, Z. H.; Chen, S. W. Mesoporous N-doped

carbons prepared with thermally removable nanoparticle


15 Nano Res.

templates: An efficient electrocatalyst for oxygen reduction

reaction. J. Am. Chem. Soc. 2015, 137, 5555–5562.

[10] Jin, H. L.; Huang, H. H.; He, Y. H.; Feng, X.; Wang, S.;

Dai, L. M.; Wang, J. C. Graphene quantum dots supported

by graphene nanoribbons with ultrahigh electrocatalytic

performance for oxygen reduction. J. Am. Chem. Soc. 2015,

137, 7588–7591.

[11] Wang, M.; Wang, J. Z.; Hou, Y. Y.; Shi, D. Q.; Wexler, D.;

Poynton, S. D.; Slade, R. C. T.; Zhang W. M.; Liu, H. K.;

Chen, J. N-doped crumpled graphene derived from vapor

phase deposition of PPy on graphene aerogel as an efficient

oxygen reduction reaction electrocatalyst. ACS Appl. Mater.

Interfaces 2015, 7, 7066–7072.

[12] Parvez, K.; Yang, S. B.; Hernandez, Y.; Winter, A.; Turchanin,

A.; Feng, X. L.; Müllen, K. Nitrogen-doped graphene

and its iron-based composite as efficient electrocatalysts for

oxygen reduction reaction. ACS Nano 2012, 6, 9541–9550.

[13] Wang, Z. J.; Cao, X. H.; Ping, J. F.; Wang, Y. X.; Lin, T. T.;

Huang, X.; Ma, Q. L.; Wang, F. K.; He, C. B.; Zhang, H.

Electrochemical doping of three-dimensional graphene

networks used as efficient electrocatalysts for oxygen reduction

reaction. Nanoscale 2015, 7, 9394–9398.

[14] Huang, X.; Zeng, Z. Y.; Zhang, H. Metal dichalcogenide

nanosheets: Preparation, properties and applications. Chem.

Soc. Rev. 2013, 42, 1934–1946.

[15] Sun, Y. F.; Gao, S.; Lei, F. C.; Xiao, C.; Xie, Y. Ultrathin

two-dimensional inorganic materials: New opportunities for

solid state nanochemistry. Acc. Chem. Res. 2015, 48, 3–12.

[16] Rao, C. N. R.; Gopalakrishnan, K.; Maitra, U. Comparative

study of potential applications of graphene, MoS2, and other

two-dimensional materials in energy devices, sensors, and

related areas. ACS Appl. Mater. Interfaces 2015, 7, 7809–

7832.

[17] Tan, C. L.; Liu, Z. D.; Huang, W.; Zhang, H. Non-volatile

resistive memory devices based on solution-processed ultrathin

two-dimensional nanomaterials. Chem. Soc. Rev. 2015, 44,

2615–2628.

[18] Gao, M. R.; Jiang, J.; Yu, S. H. Solution-based synthesis and

design of late transition metal chalcogenide materials for

oxygen reduction reaction (ORR). Small 2012, 8, 13–27.

[19] Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.;

Zhang, H. The chemistry of two-dimensional layered

transition metal dichalcogenide nanosheets. Nat. Chem. 2013,

5, 263–275.

[20] Min, Y. L.; He, G. Q.; Xu, Q. J.; Chen, Y. C. Dual-functional

MoS2 sheet-modified CdS branch-like heterostructures with

enhanced photostability and photocatalytic activity. J. Mater.

Chem. A 2014, 2, 2578–2584.

[21] Zhang, Y. J.; Gong, Q. F.; Li, L.; Yang, H. C.; Li, Y. G.;

Wang, Q. B. MoSe2 porous microspheres comprising

monolayer flakes with high electrocatalytic activity. Nano

Res. 2015, 8, 1108–1115.

[22] Gao, M. R.; Xu, Y. F.; Jiang, J.; Yu, S. H. Nanostructured

metal chalcogenides: Synthesis, modification, and applications

in energy conversion and storage devices. Chem. Soc. Rev.

2013, 42, 2986–3017.

[23] Wang, T. Y.; Zhuo, J. Q.; Chen, Y.; Du, K. Z.; Apakons-

tantinou, P.; Zhu, Z. W.; Shao, Y. H.; Li, M. X. Synergistic

catalytic effect of MoS2 nanoparticles supported on gold

nanoparticle films for a highly efficient oxygen reduction

reaction. ChemCatChem 2014, 6, 1877–1881.

[24] Du, C. C.; Huang, H.; Feng, X.; Wu, S. Y.; Song, W. B.

Confining MoS2 nanodots in 3D porous nitrogen-doped

graphene with amendable ORR performance. J. Mater. Chem.

A 2015, 3, 7616–7622.

[25] Huang, H.; Feng, X.; Du, C. C.; Song, W. B. High-quality

phosphorus-doped MoS2 ultrathin nanosheets with amenable

ORR catalytic activity. Chem. Commun. 2015, 51, 7903–7906.

[26] Xiao, B. B.; Zhang, P.; Han, L. P.; Wen, Z. Functional MoS2

by the Co/Ni doping as the catalyst for oxygen reduction

reaction. Appl. Surf. Sci. 2015, 354, 221–228.

[27] Guo, J. H.; Shi, Y. T.; Bai, X. G.; Wang, X. C.; Ma, T. L.

Atomically thin MoSe2/graphene and WSe2/graphene nano-

sheets for the highly efficient oxygen reduction reaction. J.

Mater. Chem. A 2015, 3, 24397–24404.

[28] Liao, L.; Zhu, J.; Bian, X. J.; Zhu, L. N.; Scanlon, M. D.;

Girault, H. H.; Liu, B. H. MoS2 formed on mesoporous

graphene as a highly active catalyst for hydrogen evolution.

Adv. Funct. Mater. 2013, 23, 5326–5333.

[29] Eng, A. Y. S.; Ambrosi, A.; Sofer, Z.; Šimek, P.; Pumera,

M. Electrochemistry of transition metal dichalcogenides:

Strong dependence on the metal-to-chalcogen composition

and exfoliation method. ACS Nano 2014, 8, 12185–12198.

[30] Gong, Q. F.; Cheng, L.; Liu, C. H.; Zhang, M.; Feng, Q. L.;

Ye, H. L.; Zeng, M.; Xie, L. M.; Liu, Z.; Li, Y. G. Ultrathin

MoS2(1–x)Se2x alloy nanoflakes for electrocatalytic hydrogen

evolution reaction. ACS Catal. 2015, 5, 2213–2219.

[31] Xu, S. J.; Lei, Z. Y.; Wu, P. Y. Facile preparation of 3D

MoS2/MoSe2 nanosheet–graphene networks as efficient

electrocatalysts for the hydrogen evolution reaction. J. Mater.

Chem. A 2015, 3, 16337–16347.

[32] Liu, Z. Q.; Li, N.; Zhao, H. Y.; Du, Y. P. Colloidally

synthesized MoSe2/graphene hybrid nanostructures as

efficient electrocatalysts for hydrogen evolution. J. Mater.

Chem. A 2015, 3, 19706–19710.

[33] Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.;

Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K.

Biomimetic hydrogen evolution: MoS2 nanoparticles as

catalyst for hydrogen evolution. J. Am. Chem. Soc. 2005,

127, 5308–5309.


16 Nano Res.

[34] Yan, Y.; Ge, X. M.; Liu, Z. L.; Wang, J. Y.; Lee, J. M.;

Wang, X. Facile synthesis of low crystalline MoS2

nanosheet-coated CNTs for enhanced hydrogen evolution

reaction. Nanoscale 2013, 5, 7768–7771.

[35] Li, H.; Tsai, C.; Koh, A. L.; Cai, L. L.; Contryman, A. W.;

Fragapane, A. H.; Zhao, J. H.; Han, H. S.; Manoharan, H. C.;

Abild-Pedersen, F. et al. Activating and optimizing MoS2

basal planes for hydrogen evolution through the formation

of strained sulphur vacancies. Nat. Mater. 2016, 15, 48–53.

[36] Qu, B.; Yu, X. B.; Chen, Y. J.; Zhu, C. L.; Li, C. Y.; Yin, Z.

X.; Zhang, X. T. Ultrathin MoSe2 nanosheets decorated

on carbon fiber cloth as binder-free and high-performance

electrocatalyst for hydrogen evolution. ACS Appl. Mater.

Interfaces 2015, 7, 14170–14175.

[37] Zhang, Y.; Liu, Z. Q.; Zhao, H. Y.; Du, Y. P. MoSe2

nanosheets grown on carbon cloth with superior electrochemical

performance as flexible electrode for sodium ion batteries.

RSC Adv. 2016, 6, 1440–1444.

[38] Tang, H.; Dou, K. P.; Kaun, C. C.; Kuang, Q.; Yang, S. H.

MoSe2 nanosheets and their graphene hybrids: Synthesis,

characterization and hydrogen evolution reaction studies. J.

Mater. Chem. A 2014, 2, 360–364.

[39] Mao, S.; Wen, Z. H.; Ci, S. Q.; Guo, X. R.; Ostrikov, K. K.;

Chen, J. H. Perpendicularly oriented MoSe2/graphene nano-

sheets as advanced electrocatalysts for hydrogen evolution.

Small 2015, 11, 414–419.

[40] Jia, L. P.; Sun, X.; Jiang, Y. M.; Yu, S. J.; Wang, C. M. A

novel MoSe2-reduced graphene oxide/polyimide composite

film for applications in electrocatalysis and photoelectrocatalysis

hydrogen evolution. Adv. Funct. Mater. 2015, 25, 1814–1820.

[41] Zhang, Z. A.; Fu, Y.; Yang, X.; Qu, Y. H.; Zhang, Z. Y.

Hierarchical MoSe2 nanosheets/reduced graphene oxide

composites as anodes for lithium-ion and sodium-ion batteries

with enhanced electrochemical performance. ChemNanoMat

2015, 1, 409–414.

[42] Bard, A. J.; Fan, F. R. F.; Kwak, J.; Lev, O. Scanning

electrochemical microscopy. Introduction and principles.

Anal. Chem. 1989, 61, 132–138.

[43] Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C.

Scanning electrochemical microscopy for direct imaging of

reaction rates. Angew. Chem., Int. Ed. 2007, 46, 1584–1617.

[44] Li, F.; Bertoncello, P.; Ciani, I.; Mantovani, G.; Unwin, P.

R. Incorporation of functionalized palladium nanoparticles

within ultrathin nafion films: A nanostructured composite

for electrolytic and redox-mediated hydrogen evolution.

Adv. Funct. Mater. 2008, 18, 1685–1693.

[45] Bertoncello, P. Advances on scanning electrochemical

microscopy (SECM) for energy. Energy Environ. Sci. 2010,

3, 1620–1633.

[46] Lai, S. C. S.; Macpherson, J. V.; Unwin, P. R. In situ scanning

electrochemical probe microscopy for energy applications.

MRS Bull. 2012, 37, 668–674.

[47] Wain, A. J. Scanning electrochemical microscopy for com-

binatorial screening applications: A mini-review. Electrochem.

Commun. 2014, 46, 9–12.

[48] Byers, J. C.; Güell, A. G.; Unwin, P. R. Nanoscale

electrocatalysis: Visualizing oxygen reduction at pristine,

kinked, and oxidized sites on individual carbon nanotubes.

J. Am. Chem. Soc. 2014, 136, 11252–11255.

[49] Zhang, B. Y.; Yuan, H. L.; Zhang, X. F.; Huang, D. K.; Li,

S. H.; Wang, M. K.; Shen, Y. Investigation of regeneration

kinetics in quantum-dots-sensitized solar cells with scanning

electrochemical microscopy. ACS Appl. Mater. Interfaces

2014, 6, 20913–20918.

[50] Zhang, B. Y.; Zhang, X. F.; Xiao, X.; Shen, Y. Photo-

electrochemical water splitting system—A study of interfacial

charge transfer with scanning electrochemical microscopy.

ACS Appl. Mater. Interfaces 2016, 8, 1606–1614.

[51] Eckhard, K.; Chen, X. X.; Turcu, F.; Schuhmann, W. Redox

competition mode of scanning electrochemical microscopy

(RC-SECM) for visualisation of local catalytic activity.

Phys. Chem. Chem. Phys. 2006, 8, 5359–5365.

[52] Chen, X. X.; Eckhard, K.; Zhou, M.; Bron, M.; Schuhmann,

W. Electrocatalytic activity of spots of electrodeposited

noble-metal catalysts on carbon nanotubes modified glassy

carbon. Anal. Chem. 2009, 81, 7597–7603.

[53] Kundu, S.; Nagaiah, T. C.; Xia, W.; Wang, Y. M.; van

Dommele, S.; Bitter, J. H.; Santa, M.; Grundmeier, G.;

Bron, M.; Schuhmann, W. et al. Electrocatalytic activity

and stability of nitrogen-containing carbon nanotubes in the

oxygen reduction reaction. J. Phys. Chem. C 2009, 113,

14302–14310.

[54] Ma, L.; Zhou, H.; Xin, S. L.; Xiao, C. H.; Li, F.; Ding, S. J.

Characterization of local electrocatalytical activity of

nanosheet-structured ZnCo2O4/carbon nanotubes composite

for oxygen reduction reaction with scanning electrochemical

microscopy. Electrochim. Acta 2015, 178, 767–777.

[55] Fonseca, S. M.; Barker, A. L.; Ahmed, S.; Kemp, T. J.;

Unwin, P. R. Direct observation of oxygen depletion and

product formation during photocatalysis at a TiO2 surface using

scanning electrochemical microscopy. Chem. Commun. 2003,

1002–1003.

[56] Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.;

Sun, Z. Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M.

Improved synthesis of graphene oxide. ACS Nano 2010, 4,

4806–4814.

[57] Lai, L. F.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.;

Tang, C. H.; Gong, H.; Shen, Z. X.; Lin, J. Y.; Ruoff, R. S.

Exploration of the active center structure of nitrogen-doped

graphene-based catalysts for oxygen reduction reaction.


17 Nano Res.

Energy Environ. Sci. 2012, 5, 7936–7942.

[58] Wang, H. L.; Cui, L. F.; Yang, Y.; Casalongue, H. S.;

Robinson, J. T.; Liang, Y. Y.; Cui, Y.; Dai, H. J. Mn3O4-

graphene hybrid as a high-capacity anode material for

lithium ion batteries. J. Am. Chem. Soc. 2010, 132, 13978–

13980.

[59] Wang, H. L.; Robinson, J. T.; Diankov, G.; Dai, H. J.

Nanocrystal growth on graphene with various degrees of

oxidation. J. Am. Chem. Soc. 2010, 132, 3270–3271.

[60] Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. G.; Hong, G.

S.; Dai, H. J. MoS2 nanoparticles grown on graphene: An

advanced catalyst for the hydrogen evolution reaction. J. Am.

Chem. Soc. 2011, 133, 7296–7299.

[61] Shi, Y. M.; Zhou, W.; Lu, A. Y.; Fang, W. J.; Lee, Y. H.;

Hsu, A. L.; Kim, S. M.; Kim, K. K.; Yang, H. Y.; Li, L. J.

et al. Van der Waals epitaxy of MoS2 layers using graphene

as growth templates. Nano Lett. 2012, 12, 2784–2791.

[62] Kibsgaard, J.; Chen, Z. B.; Reinecke, B. N.; Jaramillo, T. F.

Engineering the surface structure of MoS2 to preferentially

expose active edge sites for electrocatalysis. Nat. Mater.

2012, 11, 963–969.

[63] Vrubel, H.; Merki, D.; Hu, X. L. Hydrogen evolution catalyzed

by MoS3 and MoS2 particles. Energy Environ. Sci. 2012, 5,

6136–6144.

[64] Abdallah, W. A.; Nelson, A. E. Characterization of

MoSe2(0001) and ion-sputtered MoSe2 by XPS. J. Mater.

Sci. 2005, 40, 2679–2681.

[65] Boscher, N. D.; Carmalt, C. J.; Parkin, I. P. Atmospheric

pressure chemical vapor deposition of WSe2 thin films on

glass-highly hydrophobic sticky surfaces. J. Mater. Chem.

2006, 16, 122–127.

[66] Wan, D. Y.; Yang, C. Y.; Lin, T. Q.; Tang, Y. F.; Zhou, M.

Zhong, Y. J.; Huang, F. Q.; Lin, J. H. Low-temperature

aluminum reduction of graphene oxide, electrical properties,

surface wettability, and energy storage applications. ACS

Nano 2012, 6, 9068–9078.

[67] Lu, L.; Hao, Q. L.; Lei, W.; Xia, X. F.; Liu, P.; Sun, D. P.;

Wang, X.; Yang, X. J. Well-combined magnetically separable

hybrid cobalt ferrite/nitrogen-doped graphene as efficient

catalyst with superior performance for oxygen reduction

reaction. Small 2015, 11, 5833–5843.

[68] Li, R.; Wei, Z. D.; Gou, X. L. Nitrogen and phosphorus

dual-doped graphene/carbon nanosheets as bifunctional

electrocatalysts for oxygen reduction and evolution. ACS

Catal. 2015, 5, 4133−4142.

[69] Zhang, Y. J.; Chu, M.; Yang, L.; Deng, W. F.; Tan, Y. M.;

Ma, M.; Xie, Q. J. Synthesis and oxygen reduction properties

of three-dimensional sulfur-doped graphene networks. Chem.

Commun. 2014, 50, 6382–6385.

[70] Yadav, R. M.; Wu, J. J.; Kochandra, R.; Ma, L. L.; Tiwary,

C. S.; Ge, L. H.; Ye, G. L.; Vajtai, R.; Lou, J.; Ajayan, P. M.

Carbon nitrogen nanotubes as efficient bifunctional electro-

catalysts for oxygen reduction and evolution reactions. ACS

Appl. Mater. Interfaces 2015, 7, 11991–12000.

[71] Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.;

Bjerrum, N. J.; Li, Q. F. Hollow spheres of iron carbide

nanoparticles encased in graphitic layers as oxygen reduction

catalysts. Angew. Chem., Int. Ed. 2014, 53, 3675–3679.

visualization of the electrocatalytic activity of three...

Documents