Cr2O3 nanofiber: a high-performance electrocatalyst toward artificial N2 fixation to NH3 under ambient conditions

Article abstract of DOI:10.1039/c8cc07186a

NH₃ synthesis heavily depends on the energy-intensive Haber-Bosch process, which produces serious carbon emission. Electrocatalytic N₂ reduction emerges as an environmentally benign process for sustainable artificial N₂ fixation but requires efficient, stable and selective catalysts for the N₂ reduction reaction (NRR). Here, we report that Cr₂O₃ nanofiber behaves as a superb non-noble-metal NRR electrocatalyst for artificial N₂ fixation to NH₃, with excellent selectivity under ambient conditions. In 0.1 M HCl, this catalyst achieves a high Faradaic efficiency of 8.56% and a high NH₃ formation rate of 28.13 μg h^?1 mg_cat.^?1, placing it amongst the most active aqueous-based NRR electrocatalysts. Moreover, this catalyst also shows strong electrochemical durability during electrolysis and the recycling test. It opens a new avenue to explore the rational design of Cr-based nanostructures as advanced catalysts for N₂ fixation and other applications.

Full text of DOI:10.1039/c8cc07186a

INORGANIC CHEMISTRY
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RESEARCH ARTICLE
Design of atomically precise Au2Pd6 nanoclusters
for boosting electrocatalytic hydrogen evolution
on MoS2†
Cite this: Inorg. Chem. Front., 2018,
5, 2948
Yuanxin Du,‡a Ji Xiang,‡a Kun Ni,b Yapei Yun,a Guodong Sun,a Xiaoyou Yuan,a
a
Hongting Sheng,a Yanwu Zhu b and Manzhou Zhu
*
Atomically precise nanoclusters (NCs) have been widely used as catalysts in many reactions to investigate
the structure–activity relationship due to their ultrasmall sizes, well-defined structures and precise com-
positions, especially bimetallic NCs can further promote the catalytic activity by the synergistic effects of
heteroatoms. For electrocatalytic hydrogen evolution reaction (HER) catalysts, a common method to
improve the performance is coupling with a nano-metal, but the origin of the enhancement is still
unclear due to the diversity and complexity of the nanometal supported composites. Here, we take MoS2
(a star HER electrocatalyst) as an example, to report a strategy to boost the activity and give insight into
the activity enhancement of it by combining with bimetallic atomically precise NCs. The crystal structure
of this new NC is determined by X-ray crystallography, and its precise composition is identified as
Au2Pd6S4(PPh3)4(C6H4F2S)6 (Au2Pd6 for short). The Au2Pd6/MoS2 show significantly improved HER activity
and robust durability compared to the single component Pd3 or Au2/MoS2 and bare MoS2. This is attribu-
ted to the appropriate adsorption behavior of H atoms on Au2Pd6/MoS2 and the electronic interactions
between NCs and MoS2, according to the combination of experiment and theory. This study presents a
new strategy to improve the electrocatalytic activity of 2D materials such as MoS2 and sheds light on the
origin of the promotion effects at the atomic level.
Received 16th July 2018,
Accepted 27th September 2018
DOI: 10.1039/c8qi00697k
rsc.li/frontiers-inorganic
talysis systems.4,5 Based on the accurate controllability of the
geometrical structure, electronic structure and the surface
Introduction
Recently, atomically precise metal nanoclusters (NCs) have ligand of atomically precise NCs, one can carry out clear
attracted wide attention in the field of nano-catalysis due to exploration on various factors that influence the activity of the
their ultrasmall sizes, rich coordination unsaturated sites, catalytic reaction, such as the size effect,6–8 electronic
especially the definite crystal structures and compositions in effect,9–12 surface effect,13–19 etc. In particular, for bimetallic
comparison with the uncertain surface structures in tra- atomically precise NCs, they can further improve the catalytic
ditional metal nanoparticles (NPs).1–3 These advantages make activity and selectivity compared to monometallic NCs due to
them effective model catalysts which can be used to investigate the synergistic effects of heteroatoms.11,12,19–22 However, most
structure–property relationships at the atomic level, offering of the atomically precise alloy NCs were applied in organic cat-
opportunities to deeply understand the mechanism in nanoca- alysis, such as oxidation reaction,11,12,19,23,24 reduction reac-
tion,25,26 and coupling reaction,27–29 only the classical Pt1Au24
NCs recently reported by Kwak et al.30 have been used as elec-
trocatalysts for hydrogen production, which enrich the cataly-
aDepartment of Chemistry and Center for Atomic Engineering of Advanced Materials,
AnHui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid
sis fields of atomically precise NCs. Therefore, developing
Functionalized Materials, Anhui University, Hefei, Anhui 230601, China.
novel and high-efficiency bimetallic atomically precise NCs
E-mail: zmz@ahu.edu.cn
bKey Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences,
Department of Materials Science and Engineering, Hefei National Laboratory for
Physical Sciences at the Microscale, University of Science and Technology of China,
96 Jin Zhai Rd, Hefei, Anhui Province, 230026, China
and expanding their application in electrocatalysis still
remains meaningful and essential.
The electrocatalytic hydrogen evolution reaction (HER) is
important for clean and sustainable energy technology,
because it is an effective and accessible strategy to generate
hydrogen–a promising fuel due to its high energy density and
zero carbon release.31,32 For HER catalysts, a common and
†Electronic supplementary information (ESI) available: Fig. S1–S19 and Tables
S1–S4. CCDC 1813452. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c8qi00697k
‡These authors contributed equally.
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effective method to improve the activity is coupling with a gold (AuPPh3Cl) was added into the above solution under vig-
nano-metal.33–39 Taking the star HER catalyst-MoS2 as an orous stirring in an ice bath. After 15 min, 10 ml 3,4-difluoro-
example, researchers have modified MoS2 with various metal benzenethiol and 15 mg PPh4Br were added. After 30 min stir-
nanostructures, such as Pt NPs,34 Au nanorods,35 Pd nano- ring, 1 ml NaBH4 aqueous solution (45 mg ml−1) and 50 μl tri-
disks,36 etc., to obtain high HER performance. Some probable ethylamine were added quickly under vigorous stirring. The
reasons have been put forward to explain the improvement, reaction was aged for 12 h at 0 °C using an oil seal. The
including synergistic effects, promoted electron transport and mixture in the organic phase was rotary evaporated, and then
appropriate hydrogen adsorption. However, to date, the origin washed several times with methanol to remove the redundant
of the enhancement has been still puzzling due to the diversity ligand and byproducts. Black block crystals were crystallized
and complexity of the nanometal supported composites. from CH2Cl2/hexane after 7 days. The single crystal X-ray diffr-
Therefore, precise control of the morphology and component action apparatus was the Bruker Smart APEX II CCD diffract-
of the nanometal is absolutely essential in order to shed light ometer at 150 K, using Mo Kα radiation (λ = 0.71073 Å).
on the mechanism in the nanometal-induced improvement of
Synthesis of Au2 and Pd3
the HER activity.
Considering the model features of atomically precise NCs, Preparation of Au2: HAuCl4·3H2O (0.2 g, 0.5 mmol) was dis-
and the superior activity of bimetallic NCs, as well as the foun- solved in 1 ml H2O, and then added into 15 ml methanol.
dation of our previous studies (successful synthesis of the Then dimethyl sulfide (1.5 mmol, ∼3 equivalents per mole of
atomically precise Pd3 and Au2),40,41 herein, we design and syn- gold) was added into the above solution. After that, the result-
thesize a novel Au2Pd6 bimetallic atomically precise NC with a ing solution mixture was centrifuged to obtain the solids, and
definite structure via a Au2 unit connecting two Pd3 NCs (total then washed with methanol several times. Next, DPPM
composition of the NC is Au2Pd6S4(PPh3)4(C6H4F2S)6 deter- (0.3 mmol, ∼0.6 equivalents per mole of gold was added into
mined by X-ray crystallography). Then, we take MoS2 as an the above solution) was dissolved in 15 ml DCM, and the
example to report a strategy to boost the HER activity by coup- above solids were added. After about 30 min, the resulting
ling with bimetallic atomically precise NCs. Indeed, the solution was collected by using a rotary evaporation instru-
Au2Pd6/MoS2 exhibits remarkable enhancement for HER ment, and then washed several times with n-hexane.40 The syn-
activity and durability compared to the single component Pd3 thesis of atomically precise Pd3 followed previous reports.41
or Au2/MoS2 and bare MoS2. The precise structure and compo-
Synthesis of Au2Pd6 NCs/MoS2 nanocomposites
sition of the Au2Pd6 NC offers an exciting opportunity to gain
mechanistic insights into the catalytic enhancement by means Typically, a mass of 50 mg of MoS2 was dispersed in 25 ml iso-
of theory and experiment.
propanol and sonicated for 30 min to obtain a well-dispersed
suspension. The synthesis of MoS2 followed previous reports.43
Then 2.5 mg Au2Pd6 NCs were dissolved in 5 ml dichloro-
methane and dropped into the MoS2 suspension. The mixture
was stirred for 1 h to ensure that the Au2Pd6 NCs were comple-
tely loaded onto the support and collected by centrifugation
and dried in a vacuum oven at 35 °C. Next, the mixture of pro-
ducts was placed into a tube furnace and annealed at 100 °C
under an argon flow to obtain solid connections. For compari-
son, Au2 NCs/MoS2, Pd3 NCs/MoS2, and Au2–Pd3 (Au2 and Pd3
mixture) NCs/MoS2 were prepared by the same procedure
(changing the dosage of different NCs to ensure that the
loading amounts of Pd and Au in all samples were the same).
Experimental
Materials
Palladium dichloride, tetrachloroauric(III) acid (HAuCl4·3H2O),
triphenylphosphine (PPh3), tetraphenylphosphonium bromide
(PPh4Br, 98%), dimethyl sulfide (C2H6S), bis-diphenyl-
phosphino
methane
(DPPM),
3,4-difluorobenzenethiol
(C6H4F2S, 98%), ammonium molybdate tetrahydrate
((NH4)6Mo7O24·4H2O), and thiourea (CH4N2S) were all pur-
chased from Aladdin. Other chemicals, unless specified, were
of reagent grade, and triply distilled water with a resistivity of
>18.0 MΩ cm was used in the preparation of aqueous
solutions.
Electrochemical measurements
The electrochemical measurements were performed in a three-
electrode system on a CHI760D electrochemical workstation
(Shanghai, Chenhua Co., China). Graphite rods and Ag/AgCl
(saturated KCl-filled) were used as the counter and reference
electrodes, respectively. A glassy carbon electrode (GCE) with a
diameter of 5 mm covered by a thin catalyst film was used as
the working electrode. Typically, 4 mg catalyst was suspended
in 1 ml isopropanol with 10 μl Nafion solution (5 wt%,
DuPont) and sonicated for 30 min to obtain a homogeneous
Synthesis of triphenyl phosphorus chloride gold (AuPPh3Cl)
800 μl HAuCl4·3H2O (0.2 mg ml−1) was dissolved in 10 ml
ethanol under vigorous stirring. After 10 min, 313 mg PPh3
was added into the solution. A white precipitate was obtained
by centrifugation.42
Synthesis of Au2Pd6
PdCl2 (20 mg) was dissolved in a mixed solution of 4 ml ink. Then, 5 μl of the ink was transferred onto the GCE and
ethanol and 4 ml dichloromethane (DCM) by an ultrasonic dis- dried in air at room temperature. HER tests were conducted in
solving method. Then, 10 mg triphenyl phosphorus chloride a N2-saturated 0.5 M H2SO4 electrolyte at room temperature.
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The polarization curves were obtained by sweeping the poten-
tial from −0.4 to 0.1 V with a sweep rate of 5 mV s−1. All the
data were recorded without iR compensation after applying a
number of potential sweeps until they became stable. Cyclic
voltammetry (CV) was conducted 5000 times between −0.4 and
0.1 V at 50 mV s−1 to investigate the cycling stability. The EIS
measurements were performed with the frequency range of
0.01 to 105 Hz, at a bias potential of −0.3 V. The estimation of
the effective active surface area of the samples was carried out
according to the literature by CV with various scan rates in the
0.3–0.4 V region.44,45 The number of active sites and turnover
frequency (TOF, s−1) of catalysts were calculated by CV
measurements in pH = 7 phosphate buffer at a scan rate of
50 mV s−1, according to the previous methods.45,46 All the final
potentials were calibrated with respect to a reversible hydrogen
electrode (RHE).
Fig. 1 Crystal structure of the Au2Pd6 NC. (A) The two identical triangu-
lar Pd3 units of the Au2Pd6 NC, (B) the central Au2 unit linking the two
Pd3 units as the Au2Pd6 metal core, (C) the four S atoms protecting the
Au2Pd6 core, (D) the overall Au2Pd6S4(PPh3)4(C6H4F2S)6 structure. (Color
labels: yellow = Au, blue = Pd, red = S, deep yellow = P, green = F, gray
= C; all H atoms are not shown).
Calculation method
The plane wave basis set cutoff energy is 300 eV. A generalized
gradient approximation (GGA) with the Perdew–Burke–
Ernzerhof (PBE)47,48 function is used to describe the exchange
correlation interactions. Gaussian smearing with a sigma value
of 0.05 eV for geometry optimization and 0.2 eV for density of
states (DOS) calculation is employed. The energy convergence
tolerance for a self-consistent field (SCF) is 0.01 meV. The
force convergence tolerance for geometry optimization is con-
sidered as 0.03 eV per angstrom. All calculations were done
with spin unrestricted. The Gamma centered K point is
sampled as 1 × 1 × 1 for geometry optimization and 2 × 2 × 1
for single energy calculation.
Results and discussion
On the basis of our previous successful synthesis of atomically
precise Pd3 and Au2 NCs, and the consideration of the high
activity of bimetallic, we designed new Au2Pd6 NCs via a Au2
unit connecting with two Pd3. The details of the synthesis of
Au2Pd6 are provided in the Experimental section. The optical
absorption spectrum of Au2Pd6 showed only one peak at
385 nm (Fig. S1†). The structure analysis of Au2Pd6 is shown in
Fig. 1. The Au2Pd6 NC has an Au2Pd6 metal core, which was
built up by two identical triangular Pd3 units linked by an Au2
unit (Fig. 1A and B). The Au2Pd6 metal core is protected by
four S atoms (Fig. 1C). The four S atoms protect the Au2Pd6
core by two ways. One way is to link one S atom to two Pd
atoms and one Au atom, and another way is to link one S atom
to two Au atoms and one Pd atom (Fig. S2†). Finally, the
Au2Pd6S4 is protected by six –SC6H4F2 and four –PPh3
(Fig. 1D). In the Au2Pd6 core, the two triangular Pd3 units are
parallel in the structure (Fig. S3†), and the quadrangle of
Pd1Pd3Au1Au2Pd4Pd5 is twisty (Fig. S4 and S5†).
Fig. 2 (A) TEM image of Au2Pd6/MoS2, the inset showing a typical MoS2
layer distance of 0.63 nm; (B) HRTEM image of Au2Pd6/MoS2 (Au2Pd6
NCs marked by red circles); (C–H) TEM, high-angle annular bright field
(HAABF) STEM image and the corresponding elemental mapping analysis
of S, Mo, Pd, and Au; (I) the corresponding EDX spectrum of Au2Pd6/
Ultrathin MoS2 nanosheets were synthesized via a pre-
viously reported method43 as shown in Fig. 2 and S6A.† The
lamellar structure on the edge of the MoS2 nanosheets can be
observed from the high-resolution transmission electron MoS2, the inset is the content of these elements.
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microscopy (HRTEM) image in the inset of Fig. 2A. The
measured interlayer spacing is about 0.63 nm, which corres-
ponds to the (002) plane of MoS2,33 as also confirmed by the
X-ray diffraction (XRD) pattern (Fig. S6B†). Solid composite
connections NCs/MoS2 are obtained following our previously
reported methods, including mixed load, centrifugal collection
and low temperature annealing (see the details in
Experimental).4,19,49 As can be seen from the HRTEM image in
Fig. 2B and C and energy dispersive spectroscopy (EDS)
elemental mapping in Fig. 2D–I, the surface of MoS2 is suc-
cessfully decorated by the Au2Pd6 NCs and the NCs are uni-
formly dispersed, and the practical loading amount was con-
sistent with the raw ratio, and the atomic ratio of Pd/Au is
2.85, which is almost the same as that in Au2Pd6 NCs. The
MoS2 structure showed no evident change after supporting the
Au2Pd6 NCs (Fig. S7†).
The electrocatalytic HER activity of the Au2Pd6/MoS2 com-
posites was evaluated by electrochemical measurements in N2-
saturated 0.5 M H2SO4 solution33,35,36 with a typical three-elec-
trode system (see the details in Experimental). In order to
investigate the superiority of the bimetallic NCs, we also tested
Pd3 (with a precise triangle structure but a single component),
Au2 and their mixture supported on MoS2 for comparison (the
contents of Pd or Au in all samples are the same in order to
avoid the mass loading difference). The synthesis of Pd3 and
Au2 followed the previously reported methods40,41 and the
corresponding characterization is shown in Fig. S8 and S9.†
Au2Pd6/MoS2 shows the best HER activity with a small onset
overpotential of 127 mV, which decreases by 14, 21, 53, and
91 mV relative to Au2–Pd3/MoS2 (141 mV), Pd3/MoS2 (148 mV),
Au2/MoS2 (180 mV) and MoS2 (218 mV), respectively (Fig. 3A).
We also compared the current density at −0.4 V vs. RHE for all
the considered catalysts in order to quantitatively investigate
Fig. 3 (A) Polarization curves of bare MoS2 and NC modified MoS2; (B)
comparison of the overpotential at 10 mA cm−2 current density (red
column on the right) and the current density at −0.4 V vs. RHE (black
column on the left); (C) Tafel plots and (D) Nyquist plots of various
samples, the inset is the corresponding equivalent circuit diagram; (E)
scan rate dependence of the current density at 0.35 V for various
samples; (F) durability measurement of Au2Pd6/MoS2; inset: potential
values recorded initially and after every 1000 cycles at 5 mA cm−2
10 mA cm−2 and 20 mA cm−2 current density, respectively.
,
the HER activities. Au2Pd6/MoS2 displays prominent activity plays the largest exchange current density of 9.88 μA cm−2
,
with a current density of 91.0 mA cm−2, which is 1.47, 1.70, which is 4.8 times larger than that of MoS2 nanosheets, indi-
3.02 and 4.44 times higher than those of Au2–Pd3/MoS2 cating the remarkable HER activity (Table S1†).
(61.7 mA cm−2), Pd3/MoS2 (53.7 mA cm−2), Au2/MoS2 (30.1 mA Electrochemical impedance spectroscopy (EIS) can character-
cm−2) and MoS2 (20.5 mA cm−2) (Fig. 3B). The overpotential at ize the electrode kinetics and interface reactions.36,37 The inset
10 mA cm−2 current density is a metric related to solar fuel of Fig. 3D is the corresponding equivalent circuit diagram. The
synthesis that usually acts as a key parameter to evaluate the semicircle at high frequency corresponds to the charge trans-
HER performance.5,43 As shown in Fig. 3B, the Au2Pd6/MoS2 fer process, and the semicircle diameter equals the charge
exhibits the lowest overpotential that is much smaller than transfer resistance (Rct). Generally, a smaller value of Rct
those of other composites with single component Au or Pd means a faster charge transfer kinetics. As shown in Fig. 3D, a
NCs and their mixture, indicating the highest HER activity significant decrease in Rct from 403 Ω (MoS2 nanosheets) to
after Au2Pd6 were deposited on MoS2.
163 Ω (Au2Pd6/MoS2), indicating a faster electron catalytic reac-
Tafel slopes of various catalysts were constructed to under- tion rate for the HER on the Au2Pd6/MoS2 electrode,36,37 is
stand the inherent HER kinetics. The much smaller Tafel attributed to the improvement of the inter-domain conduc-
slope of Au2Pd6/MoS2 (67 mV dec−1, indicating a Volmer– tivity upon modification by Au2Pd6 NCs, consistent with the
Heyrovsky mechanism) than those of other catalysts (Au2–Pd3/ DOS results illustrated below.
MoS2: 86 mV dec−1, Pd3/MoS2: 88 mV dec−1, Au2/MoS2: 94 mV
We calculated the effective active surface area of these cata-
dec−1 and MoS2: 97 mV dec−1) demonstrates its better kinetic lysts by measuring electrochemical double-layer capacitances
process, thus leading to a faster increase of the HER rate with (Cdl) (Fig. 3E and S11†) to further evaluate the catalytic
increasing overpotential, which is important in practical appli- activity.45,51 Au2Pd6/MoS2 shows
a much higher Cdl of
cations (Fig. 3C).50–52 Exchange current density values of 32.08 mF cm−2 than other composites, suggesting that much
various samples were obtained by applying an extrapolation more effective active sites were exposed for the enhanced HER
method to the Tafel plots (Fig. S10†). The Au2Pd6/MoS2 dis- activity. The turnover frequency (TOF), which is the best figure
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of metric to estimate the intrinsic activity, is further investi- son of Pd 3d spectra (before and after Au2Pd6 NCs deposited
gated. The total number of active sites (determined by the inte- on MoS2) reveals that the Pd0 peak has a significant positive
grated charges based on the cyclic voltammetry in pH = 7 shift (Fig. 4C), indicating the tight bonding between the NCs
(phosphate buffer) minus the blank value for bare GCE37,45) is and MoS2 and the electron transfer from the Au2Pd6 NCs to
1.673 × 10−3, 2.425 × 10−3, 3.373 × 10−3, 3.472 × 10−3 and 4.02 the MoS2.14,34–36 The same phenomenon also appeared after
× 10−3 mol g−1 for MoS2, Au2/MoS2, Pd3/MoS2, Au2–Pd3/MoS2, Pd3 NCs were anchored on MoS2 (Fig. S15†). The binding
and Au2Pd6/MoS2, respectively (Fig. S12†). These values are energy of Au 4f in Au2Pd6/MoS2 also shows a positive shift
gradually increased and the trend is consistent with the result (0.41 eV) compared to that of bare Au2Pd6 NCs (Fig. 4D). These
of Cdl (Fig. 3E), which further proves that the Au2Pd6 NCs effec- results are well consistent with other nanometal/MoS2 cata-
tively increase the activity of the MoS2 sites. Furthermore, the lysts.14,34,35 The effect of deposited NCs on the electronic pro-
calculated TOFs for each active site of Au2Pd6/MoS2 reached perties of MoS2 is also reflected in the Raman spectroscopic
1.15 s−1 at 400 mV, which is 1.43 and 1.92 times higher than analysis.5,35 In MoS2-based materials, A1g phonons couple
those of Pd3/MoS2 (0.80 s−1) and MoS2 (0.60 s−1), respectively, (corresponding to the S atoms oscillating in the antiphase out-
suggesting the best intrinsic HER activity of Au2Pd6/MoS2 of-plane) much more tightly with electrons than E1 phonons
2g
(Fig. S13 and Table S1†). These results show that the Au2Pd6 (corresponding to the S and Mo atoms oscillating in the anti-
NCs greatly promote the HER performance of MoS2 compared phase parallel to the crystal plane).53–55 As shown in Fig. S16,†
with those previously reported (Table S2†). In addition, the an obvious blue shift of about 1.5 cm−1 of the A1g mode is
Au2Pd6/MoS2 catalysts also exhibit a very stable performance in observed after the Au2Pd6 NC assembly, while the position of
the accelerated degradation measurements by 5000 CV sweeps the E1 mode almost has no obvious change, indicating a
2g
(Fig. 3F). As shown in the inset of Fig. 3F, from the potential synergistic electronic interaction between NCs and
values recorded at different current densities with different CV MoS2.35,53,56
sweeps, Au2Pd6/MoS2 showed only a slight degradation in
activity. After long-time tests, the Au2Pd6/MoS2 showed no sig- to obtain further insight into the HER activity origin of the
nificant changes as shown in Fig. S14.† Au2Pd6/MoS2. We used the simplified Au2Pd6S4(PH3)4(SH)6
Density functional theory (DFT) simulation was employed
In order to probe the intrinsic reasons of the improvement model to represent the Au2Pd6 NC (simplifying PPh3 to PH3
of catalytic activity, we performed XPS and Raman analyses to and C6H4F2S to HS), similarly to previous work.57,58 As shown
investigate the interactions between NCs and MoS2 in Fig. 5A, when Au2Pd6 NCs combined with MoS2, the calcu-
nanosheets. The binding energies of Mo 3d in Au2Pd6/MoS2 lated ΔG*H for a H atom adsorbed at site 4 is −0.01 eV, while
show a larger negative shift (0.41 eV) than in Pd3/MoS2 (0.19 for Au2Pd6 NC and MoS2, the ΔG*H is −0.04 and 1.83 eV,
eV) compared to that of bare MoS2, indicating greater electron respectively (Fig. S17 and S18 and Tables S3 and S4†). This
interactions between Au2Pd6 and MoS2 (Fig. 4A).14 Such indicates that the HER performance has been improved after
obvious negative shifts (0.34 eV) are also observed in the depositing Au2Pd6 NC on MoS2. More importantly, in Au2Pd6/
binding energies of S 2p in Au2Pd6/MoS2 (Fig. 4B). A compari- MoS2, not only Au atoms (site 4 and 6) but also the S atom
(site 5) have the appropriate ΔG*H (−0.01 eV at site 4 in Fig. 5A,
0.02 eV at site 5, and −0.03 eV at site 6 in Fig. S19†), while in
Au2Pd6 NCs, only one Au–Pd bridge site has the ΔG*H nearby
zero (−0.04 eV at site 4 in Fig. S20†), and in defect-free MoS2,
the ΔG*H on the S atom is 1.83 eV. It is evident that the number
of active sites for the HER in Au2Pd6/MoS2 significantly
Fig. 5 (A) H adsorption configuration in the Au2Pd6/MoS2 system (from
different orientations) with the best ΔG*H value at site 4. (B) Density of
states of the MoS2 and Au2Pd6/MoS2 system. The inset is the partial
Fig. 4 (A) Mo 3d and (B) S 2p XPS spectra of MoS2, Pd3/MoS2 and
Au2Pd6/MoS2; (C) Pd 3d and (D) Au 4f XPS spectra of MoS2 and Au2Pd6/
MoS2.
charge density of the defect state near the Fermi level. Yellow balls: S,
purple balls: Mo, blue balls: Pd, orange balls: Au, pink balls: P, green
balls: H, red balls: the adsorption H.
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increased compared to that of Au2Pd6 NCs and MoS2, which
was beneficial for the rapid HER kinetics (Tables S3 and S4†).
For comparison, we also calculated the ΔG*H at the different
adsorption sites in Pd3 NCs, Pd3/MoS2, Au2 NC, and Au2/MoS2
systems, but all of them showed lower HER activities than that
of Au2Pd6/MoS2 (Fig. S21, S22 and Tables S5–S8†). Besides,
DOS analysis has been employed to obtain electronic structure
information of the Au2Pd6/MoS2. As shown in Fig. 5B, the
Au2Pd6/MoS2 clearly has a defect state near the Fermi level,
while MoS2 does not have the corresponding state. This defect
state narrows the band gap of the material and leads to a
better electronic conductivity (Fig. 5B and S23†). This con-
clusion is well consistent with our experiment. Furthermore,
from the perspective of charge deformation density analysis,
after depositing Au2Pd6 NCs on MoS2, the charge density of
Au2Pd6 NCs is decreased while that of MoS2 is increased,
which indicated the charge transfer from NCs to MoS2, which
is also consistent with the experiments (Fig. S24†).
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Conclusions
In summary, we report here a novel Au–Pd bimetallic atomic-
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Conflicts of interest
19 Y. Wang, X.-K. Wan, L. Ren, H. Su, G. Li, S. Malola, S. Lin,
Z. Tang, H. Häkkinen, B. K. Teo, Q.-M. Wang and
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20 X. Yuan, X. Y. Dou, K. Y. Zheng and J. P. Xie, Part. Part.
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There are no conflicts to declare.
Acknowledgements
21 J. Fang, B. Zhang, Q. F. Yao, Y. Yang, J. P. Xie and N. Yan,
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the Doctoral Scientific Research Foundation of Anhui 24 M. Takahashi, H. Koizumi, W.-J. Chun, M. Kori, T. Imaoka
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