Reducing agent-structure-activity relationship of PdAg/C catalysts in formic acid decomposition for hydrogen generation

Article abstract of DOI:10.1166/jnn.2017.14009

The reducing agent effect on surface structure and then catalytic activity towards formic acid decomposition for hydrogen generation of PdAg/C nanoparticles was investigated. The PdAg/C catalysts were prepared by four different reducing agents including formic acid, hydrogen, sodium borohydride and ascorbic acid. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy were used to evaluate the physical structure of PdAg nanoparticles. It was found that reducing agents affected degree of alloying, particle size, atoms distribution and electronic state of the PdAg surface. The combination of structure with activity results indicated that the abundant and electron-rich Pd atoms benefited the catalytic activity. The PdAg/C catalyst prepared by using formic acid as a reducing agent showed high selectivity, high durability and the highest initial turnover frequency of 90 h^-1 for hydrogen generation from formic acid decomposition without any additive at room temperature.

Full text of DOI:10.1166/jnn.2017.14009

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 3798–3802, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Reducing Agent-Structure-Activity Relationship of
PdAg/C Catalysts in Formic Acid Decomposition for
Hydrogen Generation
1
1
2
1ꢀ∗
1ꢀ3ꢀ∗
Yunjie Huang , Junlei Xu , Tao Long , Qin Shuai , and Qingfeng Li
1
Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan), Wuhan 430074, P. R. China
2
Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University,
Huanggang 435500, P. R. China
Department of Energy Conversion and Storage, Kemitorvet 207, Technical University of Denmark, 2800 Lyngby, Denmark
3
The reducing agent effect on surface structure and then catalytic activity towards formic acid decom-
position for hydrogen generation of PdAg/C nanoparticles was investigated. The PdAg/C catalysts
were prepared by four different reducing agents including formic acid, hydrogen, sodium borohy-
dride and ascorbic acid. X-ray diffraction, transmission electron microscopy and X-ray photoelectron
spectroscopy were used to evaluate the physical structure of PdAg nanoparticles. It was found that
reducing agents affected degree of alloying, particle size, atoms distribution and electronic state of
the PdAg surface. The combination of structure with activity results indicated that the abundant and
IP: 79.110.19.91 On: Thu, 11 Oct 2018 05:23:32
electron-rich Pd atoms benefited the catalytic activity. The PdAg/C catalyst prepared by using formic
Copyright: American Scientific Publishers
acid as a reducing agent showed high selectivity, high durability and the highest initial turnover
−
1
Delivered by Ingenta
frequency of 90 h for hydrogen generation from formic acid decomposition without any additive
at room temperature.
Keywords: Formic Acid Decomposition, PdAg, Reducing Agent.
1
. INTRODUCTION
Pd shell containing 1–2 layers of atoms could be electron-
ically promoted by the Ag core, leading to significantly
enhanced catalytic properties towards FA decomposition.
Hydrogen generation from formic acid (FA) decomposi-
tion has attracted extensive research interests due to advan-
tages including bio-renewability, non-toxicity, high energy
density and high stability at room temperature of FA.
It is known that the active heterogeneous catalysts towards
1
6
Zhang and co-workers synthesized 2.2 nm monodisperse
Pd58Ag42 alloy nanoparticles by using oleylamine as a
reducing agent, which were supported onto carbon and
1
–3
4ꢀ5
−1
ꢀ
FA decomposition are noble metals such as Pd, Pt
showed a high initial TOF of 382 h at 50 C towards
FA without any additive. In another research, agglomer-
ated PdAg bimetal nanoparticles were simply prepared by
using sodium borohydride (SB) as a reducing agent. The
unsupported PdAg nanoparticles were reported to benefit
the separation of catalysts from the liquid system to control
and Au.6 Among these metals, Pd has been thought
to possess more interesting catalytic performance. Then,
much attention has been paid to develop bi/tri-metallic Pd-
based composite catalysts in order to improve the dehy-
–8
9
–19
drogenation performance of FA.
The enhancement was
1
7
attributed to factors such as changed Pd–Pd interatomic
distance, modified electron state, modified morphology
and distribution of metal atoms on the catalyst surface.
One interesting second metal is Ag, which shows a
similar lattice constant with Pd and a low price of only
about 3 percent of Pd. Tedsree et al. constructed well-
defined Pd@Ag core–shell bimetal nanoparticles by using
polyvinylpyrrolidone as a stabilizer and reported that the
the FA decomposition at any moment. Recently, Jiang
1
8
et al. reported a simple wet-chemical method for the syn-
thesis of PdAg hollow spheres anchored on graphene. The
hollow architecture with thin outer shells contributed to
high surface area of catalytic active sites and then showed
1
5
−1
a high initial TOF of 333 h at room temperature towards
the mixture solution of FA and sodium formate.
Obviously, the catalytic activity of PdAg bimetal cat-
alyst depends greatly on their preparation methods and
structures. It is significant to explore the structure-activity
∗
Authors to whom correspondence should be addressed.
3798
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 6
1533-4880/2017/17/3798/005
doi:10.1166/jnn.2017.14009

Huang et al.
Reducing Agent-Structure-Activity Relationship of PdAg/C Catalysts in Formic Acid Decomposition
relationship of PdAg for the development of high-
performance Pd-based catalysts towards FA decomposi-
tion. For bimetal nanoparticles, synthetic reducing agent
has been demonstrated to play a key role in the final
2.2. Physical Characterizations
X-ray diffraction (XRD, Bruker D8-Focus, Germany) mea-
surements were carried out with a scanning speed of
−1
2 degree min . X-ray photoelectron spectroscopy (XPS)
measurements were done by using a Kratos XSAM-
800 spectrometer with an Mg Kꢂ radiator. Transmis-
sion electron microscopy (TEM) was performed by using
2
0–24
bimetal structure.
The surface compositions, mor-
phologies and degrees of alloying of bimetal will be
influenced due to different reduction rates of two metal
precursors. In this study, the effect of reducing agents
on PdAg structure and then catalytic activity towards FA
decomposition was examined. The surface-clean PdAg/C
catalysts were prepared by a traditional impregnation-
reduction method with different reducing agents including
SB, FA, ascorbic acid (AA) and hydrogen. The combined
results of structure with catalytic activity indicated that
both the abundant Pd atoms and electron-rich state of Pd
atoms in the catalyst surface benefited the catalytic per-
formance towards FA decomposition. For example, the
highest activity was observed in the PdAg/C nanoparticles
prepared from FA and attributed to abundant Pd atoms in
the catalyst surface.
2
a FEI Titan G 60–300 Probe Cs Corrector HRSTEM
system. Fourier-transform infrared spectroscopy (FT-IR,
PerkinElmer Nicolet6700, USA) was used for CO deter-
mination. During the FT-IR analysis, a cylindrical vessel
with a volume of about 150 mL was used. Firstly, the ves-
sel was vacuumed by using a pump. Then, the gas for
detection was injected into the vessel by using an injector
until the vessel was full of the gas at an atmospheric pres-
sure. Finally, FT-IR spectra of the gas were recorded with
−1
a scan resolution of 2 cm . 32 cycles were made before
results came out. N was used for blank measurement.
2
2
.3. Measure the Activity of the Catalysts for
FA Decomposition
The FA decomposition reaction was performed in a test
tube containing 5 mL of 1 M FA at 25 C under ambient
atmosphere. The reforming gases from this reaction were
measured by using a gas burette. In order to check activa-
tion energy of the catalysis reaction, the reaction was also
2
. EXPERIMENTAL DETAILS
ꢀ
2
.1. Catalyst Preparation
All PdAg/C catalysts were prepared by a conventional
impregnation-reduction method. The PdAg/C catalyst from
the use of SB as a reducing agent was synthesized as fol-
ꢀ
checked at 45, 60 and 80 C, respectively.
lows. For the first step, 140 mg of Vulcan XC-72 carbon
IP: 79.110.19.91 On: Thu, 11 Oct 2018 05:23:32
powder was ultrasonically dispersed i Cn o4 p0 y0 ri gm hL t: oA fmwearti ec ra n Scientific Publishers
and subsequently mixed with 2×10 mol Pd(N OD eꢁliv ae nr de d by Ingenta
−
4
3
2
3. RESULTS AND DISCUSSION
The XRD patterns of the prepared catalysts were shown
−4
2
1
× 10 mol AgNO . Then, freshly prepared 20 mL of
3
00 mM SB aqueous solution was dropped into the above
in Figure 1. For all the catalysts, the broad peaks between
suspension. The mixture was stirred for another 8 h at
room temperature in order to deposit the bimetal parti-
cles onto the support absolutely. Finally, the desired cata-
lyst (referred to as PdAg/C–SB) from the suspension was
filtered and washed with water and then dried in vac-
ꢀ
ꢀ
2
0 to 30 were from graphite structure of carbon while
the other peaks were from face centered-cubic phase of
ꢀ
ꢀ
metals. For example, the 2ꢃ values of 38.1 and 44.3 in
Figure 2(a) were indexed to diffraction of (111) and (200)
ꢀ
uum at 80 C overnight. The PdAg/C catalysts from the
use of FA and AA as reducing agents were also syn-
thesized by the same wet-reduction method except that
FA/sodium formate (1:1) and AA were used, and referred
to as PdAg/C–FA and PdAg/C–AA, respectively. Another
PdAg/C catalyst with hydrogen used as a reducing agent
was also prepared by a gas-reduction method and referred
to as PdAg/C–H. 140 mg of Vulcan XC-72 carbon pow-
−
4
−4
der, 2 × 10 mol Pd(NO ꢁ and 2 × 10 mol AgNO3
3
2
were added into 10 mL of water. The mixture was stirred
at 60 C until the solvent was volatilized and then dried
at 60 C overnight. The obtained dry powder was ground
and treated by a hydrogen flow at 60 C for 2 h in
ꢀ
ꢀ
ꢀ
a tube furnace. Finally, the black powder was filtered
and washed with water and then dried in vacuum at
ꢀ
8
0 C overnight. For comparison, the monometallic cata-
lysts of Pd/C and Ag/C were also prepared separately by
using the same wet-reduction method and FA as reducing
agent.
Figure 1. XRD patterns of (a) Ag/C, (b) Pd/C, (c) PdAg/C–SB,
(d) PdAg/C–H, (e) PdAg/C–FA after three run of catalysis reaction,
(f) PdAg/C–FA and (g) PdAg/C–AA.
J. Nanosci. Nanotechnol. 17, 3798–3802, 2017
3799

Reducing Agent-Structure-Activity Relationship of PdAg/C Catalysts in Formic Acid Decomposition
Huang et al.
as-prepared catalysts. The calculated molar ratios of Pd
to Ag were shown in Table I. The obtained Pd/Ag molar
ratios from XPS results were found to be higher than those
from the precursors for the PdAg/C–AA, PdAg/C–FA,
and PdAg/C–H catalysts. This result meant the Pd atoms
trended to form in the surface of PdAg catalysts dur-
ing the preparation. For the same reason, one could con-
clude that Ag atoms trended to form in the surface of
the PdAg bimetal when SB was used as reducing agent,
which was also observed in the preparation of unsup-
ported PdAg nanoparticles by using the same reducing
1
5
agent. As shown in Figure 3, the binding energies of
Pd 3d in PdAg/C–FA shifted to higher values compared
to those in Pd/C, while the binding energy of Ag 3d in
PdAg/C–FA shifted to lower values compared to those in
Ag/C. This result demonstrated that some electrons were
transferred from the Pd to Ag atoms. This phenomenon
was also observed in the preparation of reported porous
1
9
PdAg nanoalloy catalyst. It was reported that the Pd/C
prepared by using AA as a reducing agent showed lower
2
5
binding energy of Pd than that by using SB. However,
the results shown in Table I indicated that AA brought
out higher and lower binding energy of Pd 3d and Ag
Figure 2. TEM images of (a) PdAg/C–SB, (b) PdAg/C–AA,
c) PdAg/C–FA and (d) PdAg/C–H.
(
3
d compared to SB, respectively. This phenomenon might
planes of Ag, respectively. For all the PdAg/C catalysts,
there existed diffraction peaks in the position between
the corresponding Ag and Pd peaks, indicating that the
formation of PdAg alloy in the catalyst. In addition, a
relate to the different reducing potential between Pd and
Ag precursors in the preparation of PdAg bimetal nanopar-
ticles. Then, the influences of reducing agents on the elec-
tronic properties of PdAg bimetal were more complicated
IP: 79.110.19.91 On: Thu, 11 Oct 2018 05:23:32
weak shoulder peak at the position of Pd diffraction could
Copyright: American S ct hi ea nn t itf hi ca tP ou fb ml is oh neo r ms etal and implied the opportunities to
also be observed in the XRD pattern of the PdAg/C–H
Delivered byt uI nn ge e bn i tma etal electronic properties by optimizing reducing
and indicated some unalloyed Pd crystals in the samples.
agents.
The catalytic activity of all the prepared catalysts was
According to the diffraction peaks of PdAg alloy (111), the
average crystallite sizes were calculated to be 5.3, 6.6, and
tested by collecting the generated gas from the decom-
position of FA without any additive at room temperature.
The Ag/C seemed inactive because no gas or bubble was
observed while the Pd/C was distinctly active. The inac-
tive Ag catalysts towards FA decomposition have also been
7
.0 nm for PdAg/C–SB, PdAg/C–FA, and PdAg/C–AA in
sequence. Figure 2 shows the TEM images of PdAg/C
catalysts synthesized by different reducing agents. For all
the catalysts, PdAg nanoparticles (black dots) were well-
supported on the carbon (the gray base). The average par-
ticle sizes of the catalysts were given in Table I. The PdAg
nanoparticles from wet-chemical reduction method were
found to have comparable average particle size of about
1
1ꢀ16ꢀ26
reported by others.
Then, the active sites of PdAg
bimetal might be Pd rather than Ag towards the catalysis
reaction in this study. As shown in Figure 4, the catalytic
activity was distinct from the prepared different PdAg cat-
alysts. All the PdAg catalysts showed higher activity than
Pd/C catalyst except PdAg/C–AA. Then, it was feasible to
increase the catalyst performance of Pd by introducing Ag
into Pd with a suitable reducing agent.
5
nm while those from gas-chemical reduction method
have a larger average particle size with apparent particle
aggregation. These results agreed with the calculated aver-
age crystal size from the above XRD results.
XPS measurements were carried out to determine
both the compositions and electronic properties of the
The activity-structure relationship was discussed by
combining the above physical characterization with
Table I. The Pd/Ag molar ratio, binding energy of Pd and Ag of the prepared catalysts.
Catalyst
PdAg/C–FA
PdAg/C–SB
PdAg/C–H
PdAg/C–AA
Pd/C
Ag/C
Pd/Ag from XPS
Pd/Ag from precursors
2.34
1
0.81
1
2.48
1
2.97
1
–
–
–
–
Binding energy of Pd 3d5/2 (eV)
Binding energy of Ag 3d5/2 (eV)
Crystal size from XRD (nm)
Particle size from TEM (nm)
335.53
367.89
6.6
335.83
368.06
5.3
335.52
367.86
–
337.26
367.79
7.0
335.33
–
–
–
–
368.27
–
–
5.0
4.8
7.5
5.3
3800
J. Nanosci. Nanotechnol. 17, 3798–3802, 2017

Huang et al.
Reducing Agent-Structure-Activity Relationship of PdAg/C Catalysts in Formic Acid Decomposition
Figure 4. Volume change of reforming gas with time for 50 mg of the
ꢀ
synthesized catalyst in 5 ml of 5 M formic acid aqueous solution at 25 C.
alloyed state might benefit the catalytic activity toward FA
decomposition, which could be tuned by choosing suitable
reducing agents during the preparation.
Among all the prepared catalysts, PdAg/C–FA displayed
the highest catalytic performance and showed an initial
−1
TOF of 90 h , which was comparable with those of
reported active Pd based catalysts towards FA decomposi-
1
4ꢀ19ꢀ28
tion without any additive.
The activation energy was
also evaluated by controlling the temperature of decompo-
IP: 79.110.19.91 On: Thu, 11 Oct 2018 05:23:32
sition reaction on the PdAg/C–FA catalyst. It was deter-
Copyright: American Scientific Publishers
−1
mined to be as low as 29.6 kJ mol and indicated the
Delivered by Ingenta
active property of the PdAg nanoparticles. The recycle
test of the prepared PdAg/C–FA catalyst was also carried
out for the same decomposition reaction and the results
were shown in Figure 5. There was almost no deactiva-
tion after three runs of catalysis reactions. The XRD pat-
tern of the catalyst after three runs of catalysis reactions
was shown in Figure 1 and found to be very close to the
as-prepared fresh catalyst. These results denoted the good
Figure 3. XPS spectra of (a) Ag 3d and Pd 3d (b) of corresponding
Ag/C, AgPd/C and Pd/C catalysts.
activity results. PdAg/C–AA showed the highest Pd/Ag
molar ratio in the catalyst surface, which indicated more
active sites of Pd for the catalysis reaction. However, its
catalytic activity was the lowest among these four PdAg/C
catalysts and indicated low specific activity of Pd atoms
in PdAg/C–AA. This might be related to the poor-electron
state of Pd in PdAg/C–AA, which could be observed from
the highest binding energy of Pd 3d. In many research,
electron has been considered to benefit the catalytic activ-
1
4ꢀ27
ity of Pd towards FA decomposition.
Among the other
three PdAg/C catalysts, the observation of the lowest activ-
ity in PdAg/C–SB was reasonable because it showed the
highest binding energy and lowest Pd/Ag molar ratio in the
catalyst surface. In addition, the PdAg/C–FA showed close
binding energy and Pd/Ag molar ratio to the PdAg/C–H.
However, the catalytic activity of PdAg/C–FA was higher
than that of PdAg/C–H. The poor alloy state, relatively
large particle size and bad dispersion of nanoparticles
in PdAg/C–H might be the reason. Therefore, the PdAg
bimetal surface with abundant Pd atoms, electron-rich and
Figure 5. Durability test for the dehydrogenation of FA over PdAg/C–
FA catalyst at 25 C.
ꢀ
J. Nanosci. Nanotechnol. 17, 3798–3802, 2017
3801

Reducing Agent-Structure-Activity Relationship of PdAg/C Catalysts in Formic Acid Decomposition
Huang et al.
Hubei Key Laboratory for Processing and Application of
Catalytic Materials (CH201410).
References and Notes
1
2
3
4
5
. D. A. Bulushev, S. Beloshapkin, and J. R. H. Ross, Catal. Today
154, 7 (2010).
. X. C. Zhou, Y. J. Huang, W. Xing, C. P. Liu, J. H. Liao, and T. H.
Lu, Chem. Commun. 30, 3540 (2008).
. K. Jiang, K. Xu, S. Z. Zou, and W. B. Cai, J. Am. Chem. Soc.
1
36, 4861 (2014).
. G. Q. Lu, A. Crown, and A. Wieckowski, J. Phys. Chem. B
03, 9700 (1999).
1
. L. Jia, D. A. Bulushev, O. Y. Podyacheva, A. I. Boronin, L. S. Kibis,
E. Y. Gerasimov, S. Beloshapkin, I. A. Seryak, Z. R. Ismagilov, and
J. R. H. Ross, J. Catal. 307, 94 (2013).
6. M. Ojeda and E. Iglesia, Angewandte Chemie-International Edition
48, 4800 (2009).
7
. Q.-Y. Bi, X.-L. Du, Y.-M. Liu, Y. Cao, H.-Y. He, and K.-N. Fan,
J. Am. Chem. Soc. 134, 8926 (2012).
Figure 6. FT-IR spectra for CO detection: (a) 100 ppm CO, (b) 75 ppm
CO, (c) 50 ppm CO, (d) 30 ppm CO and (e) the reforming gas from the
ꢀ
8. S. Singh, S. Li, R. Carrasquillo-Flores, A. C. Alba-Rubio, J. A.
Dumesic, and M. Mavrikakis, Aiche J. 60, 1303 (2014).
FA decomposition on PdAg/C–FA catalyst at 25 C.
9
. Y. Huang, X. Zhou, M. Yin, C. Liu, and W. Xing, Chem. Mater.
22, 5122 (2010).
stability of the PdAg/C catalyst during the FA decom-
position. The CO content of the reforming gas from FA
decomposition on the PdAg/C–FA catalyst was examined
by FT-IR. As shown in Figure 6, there was no obser-
vation of characteristic adsorption peaks of CO between
10. K. Tedsree, T. Li, S. Jones, C. W. A. Chan, K. M. K. Yu, P. A. J.
Bagot, E. A. Marquis, G. D. W. Smith, and S. C. E. Tsang, Nat.
Nanotechnol. 6, 302 (2011).
11. S. Zhang, O. Metin, D. Su, and S. H. Sun, Angewandte Chemie-
International Edition 52, 3681 (2013).
−1
2
040 and 2200 cm . This result indicated high selectivity
12. J. Cho, S. Lee, J. Han, S. P. Yoon, S. W. Nam, S. H. Choi, S.-A.
Hong, K.-Y. Lee, and H. C. Ham, J. Nanosci. Nanotechnol. 15, 8233
of decomposition pathway and high quality of reforming
gases, which was always a favor in fuel cell applications.
(2015).
1
3. S. Duan, Y. F. Ji, P. P. Fang, Y. X. Chen, X. Xu, Y. Luo, and Z. Q.
IP: 79.110.19.91 On: Thu, 11 Oct 2018 05:23:32
Tian, PCCP 15, 4625 (2013).
Copyright: American Scientific Publishers
1
4. Z. L. Wang, J. M. Yan, Y. Ping, H. L. Wang, W. T. Zheng,
4
. CONCLUSIONS
Delivered by Ingenta
and Q. Jiang, Angewandte Chemie-International Edition 52, 4406
2013).
15. J. Liu, L. Lan, R. Li, X. Liu, and C. Wu, Int. J. Hydrogen Energy
1, 951 (2016).
The structure-activity relationship of PdAg/C catalysts was
studied by tuning the reducing agents during the prepa-
ration. It was found that the reducing agents influenced
the degree of alloying, particle size, atoms distribution and
electronic state of the PdAg surface. For example, all the
reducing agents except SB promoted the formation of Pd
in the PdAg surface. The particle sizes of PdAg nanoparti-
cles from the gas-reduction method were larger than those
from the wet-reduction method. The weak reducing agent
of AA produced electron-rich Ag atoms and electron-
poor Pd atoms. The combination of PdAg nanoparticle
structure with activity results indicated that the abundant
and electron-rich Pd atoms in the PdAg bimetal surface
benefited the catalytic activity, which could be possibly
designed by selecting a suitable reducing agent. The best
PdAg/C catalyst was prepared from FA and showed high
activity, selectivity and durability for hydrogen genera-
tion from FA decomposition without any additive at room
temperature.
(
4
1
6. Y. Jiang, X. Fan, X. Xiao, T. Qin, L. Zhang, F. Jiang, M. Li, S. Li,
H. Ge, and L. Chen, Journal of Materials Chemistry A 4, 657 (2016).
7. J. Cho, S. Lee, J. Han, S. P. Yoon, S. W. Nam, S. H. Choi, K. Y.
Lee, and H. C. Ham, Journal of Physical Chemistry C 118, 22553
(2014).
1
18. L. Yang, X. Hua, J. Su, W. Luo, S. Chen, and G. Cheng, Applied
Catalysis B-Environmental 168, 423 (2015).
19. K. Mandal, D. Bhattacharjee, and S. Dasgupta, Int. J. Hydrogen
Energy 40, 4786 (2015).
20. N. R. Mathe, S. S. Nkosi, D. E. Motaung, M. R. Scriba, and N. J.
Coville, Electrocatalysis 6, 274 (2015).
2
2
2
2
2
2
2
1. L. Zhang, K. C. Lee, and J. J. Zhang, Electrochim. Acta 52, 7964
(2007).
2. Y. C. Wei, C. W. Liu, Y. W. Chang, C. M. Lai, P. Y. Lim, L. D.
Tsai, and K. W. Wang, Int. J. Hydrogen Energy 35, 1864 (2010).
3. B. J. Hwang, S. M. S. Kumar, C. H. Chen, R. W. Chang, D. G. Liu,
and J. F. Lee, Journal of Physical Chemistry C 112, 2370 (2008).
4. H. B. Bae, S. H. Oh, J. C. Woo, and S. H. Choi, J. Nanosci. Nan-
otechnol. 10, 6901 (2010).
5. Y. Holade, C. Canaff, S. Poulin, T. W. Napporn, K. Servat, and K. B.
Kokoh, Rsc Advances 6, 12627 (2016).
6. Y. Y. Cai, X. H. Li, Y. N. Zhang, X. Wei, K. X. Wang, and J. S.
Chen, Angewandte Chemie-International Edition 52, 11822 (2013).
7. J. Cheng, X. Gu, X. Sheng, P. Liu, and H. Su, Journal of Materials
Chemistry A 4, 1887 (2016).
Acknowledgments: National Natural Science Founda-
tion of China (21303172), the Fundamental Research
Funds for the Central Universities, China University of
Geosciences (Wuhan) (CUG140610 and CUG140414) and
28. O. Metin, X. L. Sun, and S. H. Sun, Nanoscale 5, 910 (2013).
Received: 31 May 2016. Accepted: 6 September 2016.
3802
J. Nanosci. Nanotechnol. 17, 3798–3802, 2017