Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High

Letter
pubs.acs.org/NanoLett
Molybdenum Sulﬁde/N-Doped CNT Forest Hybrid Catalysts for HighPerformance Hydrogen Evolution Reaction
Dong Jun Li,†,‡ Uday Narayan Maiti,†,‡ Joonwon Lim,†,‡ Dong Sung Choi,†,‡ Won Jun Lee,†,‡
Youngtak Oh,†,‡ Gil Yong Lee,†,‡ and Sang Ouk Kim*,†,‡
†
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea
‡
S Supporting Information
*
ABSTRACT: Cost eﬀective hydrogen evolution reaction
(HER) catalyst without using precious metallic elements is a
crucial demand for environment-benign energy production.
Molybdenum sulﬁde is one of the promising candidates for
such purpose, particularly in acidic condition, but its catalytic
performance is inherently limited by the sparse catalytic edge
sites and poor electrical conductivity. We report synthesis and
HER catalysis of hybrid catalysts composed of amorphous
molybdenum sulﬁde (MoSx) layer directly bound at vertical Ndoped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic
attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are speciﬁcally
deposited at NCNT surface under low-temperature wet chemical process. The synergistic eﬀect from the dense catalytic sites at
amorphous MoSx surface and ﬂuent charge transport along NCNT forest attains the excellent HER catalysis with onset
overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm2 current density, which is the highest HER activity
of molybdenum sulﬁde-based catalyst ever reported thus far.
KEYWORDS: Molybdenum sulﬁde, carbon nanotubes, hydrogen evolution, catalyst, doping
H
consisting of uncoordinated sulfur atoms with negative charges,
which facilitate proton adsorption and conversion into H2.
Nonetheless, due to the intrinsic low electrical conductivity,
amorphous MoSx catalysts suﬀer from retarded electron
transport while undergoing HER reaction, which may severely
deteriorate the overall electrocatalytic performance.22−29
In this work, we report highly HER active MoSx/NCNT
forest hybrid catalysts via straightforward low-temperature
precursor decomposition. Our hybrid catalyst has an ideal
morphology that ∼2 nm scale thick amorphous MoSx catalysts
are densely bound at electro-conductive NCNT forest,
vertically standing on glassy carbon substrate. Unlike widely
used oxygen functionalization, which severely damages the
electrical properties of graphitic carbons, electron-rich Ndoping provides additional electrons to graphitic carbons to
maintain high electroconductivity. Moreover, while pristine
graphitic carbon surface has a low surface energy and chemical
inertness, the N-doped graphitic plane30−34 with electronegative, readily protonated N shows greatly improved surface
energy and intrigues favorable reaction aﬃnity for MoSx
precursor molecules without any intermediate adhesive
layer.35,36 Besides, vertical NCNTs robustly bound at the
ydrogen is a promising alternative energy carrier that
holds great promise for clean and sustainable energy
technology.1 Electrochemical water splitting by clean energy
source or direct photoelectrolysis is attracting enormous
research attention for eﬃcient and eco-friendly generation of
hydrogen.2−4 Unfortunately, relatively large thermodynamic
overpotential for HER inherently deteriorates the overall
energy conversion eﬃciency. In this regard, various catalyst
materials, including Pt and noble analogues, have been
exploited to minimize the energy barrier. However, those
expensive rare earth materials are generally inappropriate for
conventional energy conversion devices.5
Recently, molybdenum sulﬁde featuring unique electronic
and optical properties6−10 is found to be an active nonmetallic
HER catalyst, which is potentially useful for acidic HER
condition with abundant protons.11−15 While crystalline MoS2
features HER catalytic sites along the edge of two-dimensional
sheet,16 amorphous MoSx shows catalytic activity at the
unsaturated sulfur atoms existing over the entire surface.14,17
For the further progress of molybdenum sulﬁde-based HER
catalysts, the foremost issue is how to expose and stabilize more
catalytic active sites at external surface.18−21 Previous research
eﬀorts have revealed that two-dimensional crystalline MoS2
platelets readily stack parallel at catalytic support surfaces.
Consequently, the eﬀective catalytic sites located along platelet
edges are hardly exposed externally. By contrast, amorphous
MoSx possesses abundant catalytic sites over entire surface
© 2014 American Chemical Society
Received: November 5, 2013
Revised: January 30, 2014
Published: February 6, 2014
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Figure 1. MoSx/NCNT forest hybrid catalyst. (a) Schematic illustration of three-dimensional hybrid catalyst synthesis. (b) FE-SEM image of bare
NCNT forest. (c) FE-SEM image of MoSx/NCNT forest hybrid. (d) Broad-ﬁeld FE-SEM image of (c). (e) TEM image of bare NCNT. (f) TEM
image of MoSx/NCNT. (g) TEM image of MoSx/NCNT after thermal annealing at 600 °C.
bottom glassy carbon substrate oﬀer ﬂuent charge transport
pathway directly connecting MoSx catalyst and external
electrochemical electrodes. Taking advantage of the synergistic
structure our hybrid catalyst exhibits outstanding HER catalytic
activity with a low overpotential of ∼75 mV.
Vertical NCNT forest were grown on SiO2/Si substrate by
plasma-enhanced chemical vapor deposition (PECVD) under
NH3 environment (see Supporting Information for synthetic
details) and transferred onto glassy carbon electrode substrates
(Supporting Information Figure S1). The mirror-polished
glassy carbon was chosen as the substrate material considering
its inertness to HER activity as well as strong chemical
resistance to corrosion. The electrical contact between NCNT
forest and glassy carbon substrate was conﬁrmed to be ohmic
by conduction AFM measurement (Supporting Information
Figure S2). For MoSx deposition, the template NCNT forest
were immersed in aqueous HCl solution (pH = 5) for 30 min
and ammonium tetrathiomolybdate ((NH4)2MoS4) precursor
solution was injected into the solution. The mixture solution
was kept at 90 °C for precursor decomposition. The hybrid
catalyst formation process is schematically illustrated in Figure
1a. Thiomolybdate anions are adsorbed on the NCNT wall by
electrostatic interaction and decomposed into amorphous
MoSx at the elevated temperature. Notably, the mechanical
contact between NCNT root and bottom glassy carbon
substrate is strong enough to withstand the wet chemical
reaction. Field-emission scanning electron microscopy (FESEM) images in Figure 1b,d contrast the morphological change
of NCNT forest before and after MoSx deposition. Smooth
NCNT surfaces are roughened with MoSx catalyst layer with
∼2 nm thickness. Vertical NCNT forest retains the large
surface area during MoSx deposition without any collapse or
aggregation of NCNT strands (Figure 1c). As such, this threedimensional catalyst/electrode hybrid structure demonstrates
an ideal morphology with maximum catalytic surface area and
ﬂuent charge transport pathway to external electrode. Moreover, its nanoscale morphology ensures ﬂuent transport of
reactants and products among individually separated vertical
catalyst strands.
Figure 1e,f shows transmission electron microscopy (TEM)
images of pure NCNT and typical MoSx/NCNT hybrid
catalyst, respectively. Amorphous MoSx catalysts are uniformly
distributed along NCNT strand. Interestingly, tiny crystal
domains with typical size of less than 5 nm are observed in the
amorphous MoSx layer (Figure 1f inset). Fringe lattice spacing
of the crystal is 0.27 nm, which is consistent with lattice spacing
of (100) plane of crystalline MoS2 (Supporting Information
Figure S3a). The TEM observation clariﬁes that the NCNT
strands are coated with amorphous MoSx embedded with tiny
MoS2 crystalline domains. Signiﬁcantly, such an amorphous
MoSx with dense unsaturated S atoms expose dense catalytic
sites over entire surface. However, amorphous MoSx has an
intrinsic low electrical conductivity, which may bottleneck the
charge transport during electrocatalysis. TEM observation also
reveals that the nanoscale MoSx catalysts are intimately
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S2−, bridging S22−, terminal S2−, or S22− ligands.25,38 Taken
together, the XPS result veriﬁes amorphous MoSx phase
formation. Energy dispersive X-ray spectrometry (EDX) line
scanning proﬁle was performed to examine the elemental
distribution (Figure 2b). S elements are principally distributed
near two side walls of NCNT strand. Besides, Mo and N
element signals are also detected. The comparison of the peak
intensities between Mo and S shows that the S element is much
more than two times the Mo element. This amorphous MoSx
formation results from the decomposition of ammonium
tetrathiomolybdate precursor at the relatively low temperature
of 90 °C. The precursor decomposition is also accompanied
with the oxidation of S2− ligands, the reduction of Mo metal as
well as the thermal decomposition of intermediates.37 In
comparison to the as-synthesized MoSx, the thermally annealed
one shows diﬀerent XPS spectrum, as shown in Figure 2a. The
S 2p peak is shifted to a lower binding energy while an
additional peak grows at 163 eV. This is consistent with the
typical XPS spectrum of crystalline MoS2.26
The crucial role of N-dopants for MoSx deposition could be
illustrated by control experiment employing pristine CNTs
without heteroatom doping (experimental detail is described in
Supporting Information). Figure 3a and its inset ﬁgures
contrast the morphology change after molybdenum sulﬁde
deposition at pristine CNT and NCNT surfaces, respectively.
While small MoSx patches are sparsely deposited at pristine
CNT surface, nanoscale thick MoSx layer is densely coated at
NCNT surface. Figure 3b exhibits the diﬀerence of XPS proﬁle
between pristine CNT forest and NCNT forest. Owing to the
N-doping, NCNT possesses not only better wettability but also
more favorable interaction with precursor molecules.39−43 As
illustrated in Figure 3c, the protonation of lone-pair electrons at
pyridinic N-doping sites can attract thiomolybdate anions by
straightforward electrostatic attraction. The attracted thiomolybdate anions can be subsequently decomposed into
amorphous MoSx, while the nucleation barrier for molybdenum
sulﬁde formation is signiﬁcantly reduced at the slightly elevated
reaction temperature of 90 °C.
The electrochemical HER tests are performed using threeelectrode setup in the acidic condition of 0.5 M H2SO4 solution
(see Supporting Information for details). We note that all
polarization curves are not corrected for iR loss. Typical
polarization curve (I−V plot) demonstrates that MoSx/NCNT
forest hybrid electrode presents a low onset overpotential (η)
of ∼75 mV versus RHE for taking oﬀ HER activity (Figure 4b).
Further negative potential induces rapid rise of cathodic
current. The HER performances of commercial Pt/C catalyst,
amorphous MoSx, and bare NCNT forest are compared in the
same experimental setup (see Supporting Information for
synthesis approach). Commercial Pt/C catalyst shows the
highest HER activity with negligible overpotential. Amorphous
MoSx exhibits HER activity but the low electrical conductivity
limit the HER performance particularly in terms of current
density. As a typical reference metric for electrochemical
catalytic performance, the potential value for 10 mA/cm2
current density is frequently employed, while applying less
than 200 mV overpotential.17 Our hybrid catalysts only require
∼110 mV to achieve 10 mA/cm2, which is far better than any
other molybdenum sulﬁde-based HER catalysts ever reported
so far.14,17,22,27,39
To understand the detailed underlying mechanism of HER
activity, Tafel plots based on polarization curves are acquired, as
shown in Figure 4c. The linear regions of Tafel plots were ﬁt to
contacted with NCNT surface, which is highly desired for
direct charge transfer from NCNTs to catalytic sites.
We also carried out thermal annealing of the hybrid catalysts
at 600 °C under H2/Ar environment. As shown in TEM image
of Figure 1g, MoSx layer is transformed into few-layer stacked
MoS2 nanosheets. MoS2 formation could be also conﬁrmed by
Raman spectroscopy (Supporting Information Figure S4). Fast
Fourier transform (FFT) pattern (the upper inset of Figure 1g)
taken from selected area veriﬁes the MoS2 crystal structure. The
lattice spacing of 0.67 and 0.27 nm is measured, which is
consistent with the interlayer spacing of MoS2 sheets and
crystal lattice spacing within MoS 2 plane, respectively
(Supporting Information Figure S3b,c). Additionally, X-ray
diﬀraction (XRD) illustrates typical diﬀraction peaks for (002)
and (100) planes of MoS2. Because of the limited number of
layer stacking, the diﬀraction peak intensity for (200) is much
weaker than that of (100) plane (Supporting Information
Figure S5). TEM observation (Figure 1g) conﬁrms that double
layer stacking is dominant.
X-ray photoelectron spectroscopy (XPS) was utilized to
characterize the chemical nature and bonding state of MoSx at
NCNT surfaces (Figure 2a). The binding energy of Mo
Figure 2. Chemical composition analysis. (a) XPS spectra of Mo 3d
and S 2p in MoSx/NCNT hybrid, as-synthesized (top) and after
thermally annealed at 600 °C (bottom). (b) EDX line scanning proﬁle
for the as-synthesized sample.
features two principal peaks for Mo 3d5/2 = 229.6 eV and Mo
3d3/2 = 232.7 eV, which indicates the oxidation state of Mov.15
Besides, small shoulder signals for +6 oxidation state are
observed. Bulk MoS2 is known to primarily show S 2p spectrum
with a single doublets of S 2p2/3 = 162.0 and 163.3 eV.37 Our
MoSx, by contrast, exhibits a broad single peak at 163.4 eV,
indicating the existence of other binding signals, such as apical
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Figure 3. (a) TEM images of pristine CNTs and NCNTs after molybdenum sulﬁde deposition. (b) Nitrogen XPS spectra of pristine and N-doped
CNTs. (c) Proposed mechanism for MoSx layer formation at NCNT surface.
Tafel equation (η = b log j + a, where j is the current density
and b is the Tafel slope) to obtain slope, b.16,44 Tafel slopes of
34, 40, and 60 mV/decade are obtained for commercial Pt/C
catalyst, MoS x /NCNT hybrids, and amorphous MoS x,
respectively. The small Tafel slope of MoSx/NCNT forest
hybrid is advantageous for practical catalytic application, since it
leads to a rapid increase of HER rate with overpotential.38 In
general, three principal steps can be involved in a HER, as
usually noted by Volmer, the Heyrovsky, and the Tafel
steps.45−47 If the Volmer step associated with proton
absorption is rate-determining, a slope of ∼120 mV/decade
should be obtained, while Heyrovsky and Tafel steps should
give ∼40 and ∼30 mV/decade, respectively.47,48 Accordingly,
the HER mechanism of our hybrid catalyst follows Volmer−
Heyrovsky reaction where electrochemical desorption is the
rate-determining step. The low Tafel slope of our hybrid
catalyst is attributed to the strong electronic coupling between
nanoscale thin MoSx catalyst and NCNT surface (Figure 4a).
The ﬂuent charge transport could also be characterized by
electrochemical impedance spectroscopy (EIS), as shown in
Figure 4d. MoSx/NCNTs hybrid catalyst shows much lower
charge transfer resistance than pure amorphous MoSx.
Exchange current densities of various samples were also
obtained by the extrapolation of Tafel plots (Supporting
Information Figure S6 and Table S1). As listed in Supporting
Information Table S1, MoSx/NCNT displays the largest
exchange current density of 33.11 μA cm−2.
The intrinsic per-site activity of a catalyst is an important
metric to evaluate a catalyst material. We used electrochemical
capacitance surface area measurements to estimate the active
surface area of the catalyst ﬁlm,17 which was further used to
calculate the average activity of each site, namely, a per-site
turnover frequency (TOF) (detail calculation provided in the
Supporting Information). As summarized in Supporting
Information Table S2, the density of electrochemically
accessible site of MoSx/NCNT forest hybrid catalyst is
estimated to be ∼9.8 × 1016/cm2. The corresponding TOF
for each active site is calculated to be as high as 3.5 s−1 at η =
200 mV versus RHE and pH = 0.
We also investigated on the HER performance of MoS2/
NCNT forest hybrid catalysts obtained after thermal annealing
at 600 °C (see Supporting Information for more details, Figure
S8). The HER performance has been noticeably degraded from
the original amorphous MoSx based catalyst. Crystalline MoS2
sheets stack parallel with NCNT surface such that catalytically
inactive basal planes are predominantly exposed at surface
instead of active edge sites.
Catalytic stability is another signiﬁcant criterion for HER
catalysts. The catalytic stability of our MoSx/NCNT forest
catalyst is characterized by continuous cyclic voltammetry
performed between −0.2 and 0.2 V vs RHE at 50 m/s scan rate
(Figure 4e). Only a minor deterioration of cathodic current is
observed after 1000 cycling. By contrast, MoSx/CNT catalyst
without N-doping shows serious fade of HER activity in the
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Figure 4. Electrochemical catalytic activities. (a) HER scheme for MoSx/NCNT forest hybrid catalyst. (b) Polarization curves and (c) Tafe plots of
diﬀerent catalysts. (d) Electrochemical impedance spectra of MoSx/NCNT and bare MoSx at −0.2 V versus RHE from 5 MHz to 10 mHz. Cycle
stabilities of (e) MoSx/NCNT and (f) MoSx/CNT forest catalysts.
widely useful for nonmetallic catalysts suﬀering from limited
catalytic surface area and ineﬀective electron transport.
same condition (Figure 4f). In general, the durability of
supported catalyst is known to strongly depend on the
catalyst−support interaction.49,50 Previous simulation and
experimental results have established that strong interaction
of d-orbital of transition metal with the p-orbital of N-dopants
may give rise to robust bonding.49,51−53 It is noteworthy that
due to the low wettability and no speciﬁc functional groups at
pristine CNT forest, MoSx was principally deposited at the top
surface of CNT forest. Accordingly, the initial HER performance of MoSx/CNT is much inferior to that of MoSx/NCNT
hybrid catalyst.
We have demonstrated facile wet chemical synthesis and
remarkable HER activity of MoSx/NCNT forest hybrid
catalysts. N-dopants at graphitic surfaces strongly interact
with anionic precursors to generate dense nanoscale
amorphous MoSx layer via simple low-temperature solution
process. The best HER catalytic activity with a low overpotential is obtained among molybdenum sulﬁde based
catalysts reported thus far. The synergistic interplay between
the nanometer thick amorphous MoSx layer with dense
catalytic active sites and the carefully engineered NCNT forest
electrodes attained such a noticeable catalytic performance.
Moreover, NCNT oﬀers excellent durability to the composite
catalyst; a critical criteria for any supported catalyst. Our
approach oﬀers a general route to idealized N-doped graphitic
carbon based nanohybrid catalyst structures, which can be
■
ASSOCIATED CONTENT
S Supporting Information
*
Experimental, methods, additional SEM and photo images, and
additional measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] Oﬃce phone: +82-42-3503339. Fax: +82-42-350-3310.
Notes
The authors declare no competing ﬁnancial interest.
■
ACKNOWLEDGMENTS
This work was ﬁnancially supported by Institute for Basic
Science (IBS) (CA1401-02).
■
REFERENCES
(1) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414 (6861), 332−
337.
(2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103
(43), 15729−15735.
1232
dx.doi.org/10.1021/nl404108a | Nano Lett. 2014, 14, 1228−1233
Nano Letters
Letter
(34) Lee, D. H.; Lee, W. J.; Lee, W. J.; Kim, S. O.; Kim, Y. H. Phys.
Rev. Lett. 2011, 106 (17), 175502−175505.
(35) Maiti, U. N.; Lee, W. J.; Lee, J. M.; Oh, Y.; Kim, J. Y.; Kim, J. E.;
Shim, J.; Han, T. H.; Kim, S. O. Adv. Mater. 2014, 26 (1), 40−67.
(36) Czerw, R.; Terrones, M.; Charlier, J. C.; Blase, X.; Foley, B.;
Kamalakaran, R.; Grobert, N.; Terrones, H.; Tekleab, D.; Ajayan, P.
M.; Blau, W.; Rühle, M.; Carroll, D. L. Nano Lett. 2001, 1 (9), 457−
460.
(37) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.;
Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M.
Nano Lett. 2003, 3 (3), 275−277.
(38) Shi, Y. M.; Wang, Y.; Wong, J. I.; Tan, A. Y. S.; Hsu, C. L.; Li, L.
J.; Lu, Y. C.; Yang, H. Y. Sci. Rep. 2013, 3 (2169), 1−8.
(39) Merki, D.; Hu, X. L. Energy Environ. Sci. 2011, 4 (10), 3878−
3888.
(40) Lee, W. J.; Lee, J. M.; Kochuveedu, S. T.; Han, T. H.; Jeong, H.
Y.; Park, M.; Yun, J. M.; Kwon, J.; No, K.; Kim, D. H.; Kim, S. O. ACS
Nano 2012, 6 (1), 935−943.
(41) Lee, W. J.; Lee, D. H.; Han, T. H.; Lee, S. H.; Moon, H. S.; Lee,
J. A.; Kim, S. O. Chem. Commun. 2011, 47 (1), 535−537.
(42) Lee, J. M.; Kwon, B. H.; Park, H. I.; Kim, H.; Kim, M. G.; Park,
J. S.; Kim, E. S.; Yoo, S.; Jeon, D. Y.; Kim, S. O. Adv. Mater. 2013, 25
(14), 2011−2017.
(43) Lee, D. H.; Lee, W. J.; Kim, S. O. Nano Lett. 2009, 9 (4), 1427−
1432.
(44) Haq, A. U.; Lim, J.; Yun, J. M.; Lee, W. J.; Han, T. H.; Kim, S.
O. Small 2013, 9 (22), 3829−3833.
(45) Trasatti, S. J. Electroanal. Chem. Interfacial Electrochem. 1972, 39
(1), 163−184.
(46) Pentland, N.; Bockris, J. O. M.; Sheldon, E. J. Electrochem. Soc.
1957, 104 (3), 182−194.
(47) Conway, B. E.; Tilak, B. V. Electrochim. Acta 2002, 47 (22−23),
3571−3594.
(48) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. J. Electrochem. Soc.
2010, 157 (11), B1529−B1536.
(49) Groves, M. N.; Malardier-Jugroot, C.; Jugroot, M. J. Phys. Chem.
C 2012, 116 (19), 10548−10556.
(50) Chen, Y. G.; Wang, J. J.; Liu, H.; Li, R. Y.; Sun, X. L.; Ye, S. Y.;
Knights, S. Electrochem. Commun. 2009, 11 (10), 2071−2076.
(51) Shang, Y.; Zhao, J. X.; Wu, H.; Cai, Q. H.; Wang, X. G.; Wang,
X. Z. Theor. Chem. Acc. 2010, 127 (5−6), 727−733.
(52) An, W.; Turner, C. H. J. Phys. Chem. C 2009, 113 (17), 7069−
7078.
(53) Yang, S. H.; Shin, W. H.; Lee, J. W.; Kim, H. S.; Kang, J. K.;
Kim, Y. K. Appl. Phys. Lett. 2007, 90 (1), 013101−013103.
(3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi,
Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110 (11), 6446−6473.
(4) Turner, J. A. Science 2004, 305 (5686), 972−974.
(5) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.;
Norskov, J. K. Nat. Mater. 2006, 5 (11), 909−913.
(6) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.;
Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.;
Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.;
Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones,
M.; Windl, W.; Goldberger, J. E. ACS Nano 2013, 7 (4), 2898−2926.
(7) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Phys. Rev. Lett.
2010, 105 (13), 136805−136808.
(8) Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.;
Chen, X.; Zhang, H. ACS Nano 2012, 6 (1), 74−80.
(9) Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Nat. Nanotechnol. 2012,
7 (8), 494−498.
(10) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.;
Strano, M. S. Nat. Nanotechnol. 2012, 7 (11), 699−712.
(11) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.;
Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. J. Am. Chem.
Soc. 2005, 127 (15), 5308−5309.
(12) Hou, Y. D.; Abrams, B. L.; Vesborg, P. C. K.; Bjorketun, M. E.;
Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.;
Hansen, O.; Rossmeisl, J.; Dahl, S.; Norskov, J. K.; Chorkendorff, I.
Nat. Mater. 2011, 10 (6), 434−438.
(13) Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.;
Chang, C. J. Science 2012, 335 (6069), 698−702.
(14) Merki, D.; Fierro, S.; Vrubel, H.; Hu, X. Chem. Sci. 2011, 2 (7),
1262−1267.
(15) Tang, M. L.; Grauer, D. C.; Lassalle-Kaiser, B.; Yachandra, V. K.;
Amirav, L.; Long, J. R.; Yano, J.; Alivisatos, A. P. Angew. Chem., Int. Ed.
2011, 50 (43), 10203−10207.
(16) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.;
Horch, S.; Chorkendorff, I. Science 2007, 317 (5834), 100−102.
(17) Benck, J. D.; Chen, Z. B.; Kuritzky, L. Y.; Forman, A. J.;
Jaramillo, T. F. ACS Catal. 2012, 2 (9), 1916−1923.
(18) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Nat.
Mater. 2012, 11 (11), 963−969.
(19) Wang, H. T.; Kong, D. S.; Johanes, P.; Cha, J. J.; Zheng, G. Y.;
Yan, K.; Liu, N. A.; Cui, Y. Nano Lett. 2013, 13 (7), 3426−3433.
(20) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.;
Cui, Y. Nano Lett. 2013, 13 (3), 1341−1347.
(21) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.;
Lou, X. W.; Xie, Y. Adv. Mater. 2013, 25 (40), 5807−5813.
(22) Vrubel, H.; Moehl, T.; Gratzel, M.; Hu, X. Chem. Commun.
2013, 49 (79), 8985−8987.
(23) Chen, Z. B.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara,
M. K.; Jaramillo, T. F. Nano Lett. 2011, 11 (10), 4168−4175.
(24) Kim, J.; Byun, S.; Smith, A. J.; Yu, J.; Huang, J. J. Phys. Chem.
Lett. 2013, 4 (8), 1227−1232.
(25) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.;
Jin, S. J. Am. Chem. Soc. 2013, 135 (28), 10274−10277.
(26) Chang, Y.-H.; Lin, C.-T.; Chen, T.-Y.; Hsu, C.-L.; Lee, Y.-H.;
Zhang, W.; Wei, K.-H.; Li, L.-J. Adv. Mater. 2013, 25 (5), 756−760.
(27) Vrubel, H.; Merki, D.; Hu, X. Energy Environ. Sci. 2012, 5 (3),
6136−6144.
(28) Laursen, A. B.; Kegnaes, S.; Dahl, S.; Chorkendorff, I. Energy
Environ. Sci. 2012, 5 (2), 5577−5591.
(29) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am.
Chem. Soc. 2011, 133 (19), 7296−7299.
(30) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang,
H. Nature Chem. 2013, 5 (4), 263−275.
(31) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.;
Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Energy Environ. Sci. 2012, 5 (7),
7936−7942.
(32) Jin, Z.; Yao, J.; Kittrell, C.; Tour, J. M. ACS Nano 2011, 5 (5),
4112−4117.
(33) Pint, C. L.; Sun, Z.; Moghazy, S.; Xu, Y.-Q.; Tour, J. M.; Hauge,
R. H. ACS Nano 2011, 5 (9), 6925−6934.
1233
dx.doi.org/10.1021/nl404108a | Nano Lett. 2014, 14, 1228−1233

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