Soft magnetic Fe5C2-Fe3C@C as an electrocatalyst for the hydrogen evolution reaction

Article abstract of DOI:10.1039/c9dt00328b

Herein, cubic iron carbides encapsulated in an N-doped carbon shell (ICs@NC) were prepared by a simple two-step method. The two-step method included the preparation of iron oxalate dihydrate and the process of calcination with ethylenediamine. By changing the calcination temperature, we could control the type of iron carbide formed. Moreover, the prepared iron carbide@N-doped carbon core-shell particles exhibited regular cubic shapes and soft magnetic properties with high saturation magnetization. More importantly, we investigated the electrocatalytic activity of the iron carbide@N-doped carbon catalysts for the hydrogen evolution reaction (HER). The results show that the Fe₅C₂-Fe₃C@NC catalyst has efficient HER catalytic activity with an overpotential of 209 mV@10 mA cm^?2.

Full text of DOI:10.1039/c9dt00328b

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Soft magnetic Fe5C2–Fe3C@C as an electrocatalyst
for the hydrogen evolution reaction†
Cite this: DOI: 10.1039/c9dt00328b
Zhantong Ye,a Yaqin Qie,a Zhipeng Fan,a Yixuan Liu,a Zhan Shi b and
a
Hua Yang
*
Herein, cubic iron carbides encapsulated in an N-doped carbon shell (ICs@NC) were prepared by a
simple two-step method. The two-step method included the preparation of iron oxalate dihydrate and
the process of calcination with ethylenediamine. By changing the calcination temperature, we could
control the type of iron carbide formed. Moreover, the prepared iron carbide@N-doped carbon core–
shell particles exhibited regular cubic shapes and soft magnetic properties with high saturation magneti-
zation. More importantly, we investigated the electrocatalytic activity of the iron carbide@N-doped
carbon catalysts for the hydrogen evolution reaction (HER). The results show that the Fe5C2–Fe3C@NC
Received 24th January 2019,
Accepted 5th March 2019
DOI: 10.1039/c9dt00328b
rsc.li/dalton
catalyst has efficient HER catalytic activity with an overpotential of 209 mV@10 mA cm−2
.
ity, ICs are promising materials for biomedical applications
such as magnetic resonance imaging, drug delivery and mag-
1. Introduction
Magnetic materials, such as ferrites, SmCo5 and Nd–Fe–B, netic separation.9–12
have been widely applied in many fields, especially in energy-
In recent years, a number of synthetic methods, for
related applications, for example, in electric motors, electricity example, gas–solid method,13 sol–gel method,14–17 ball milling
generation and conversion technologies.1–6 Usually, magnetic method,18 and hot injection method,19,20 have been developed
materials can be classified into hard and soft magnetic for iron carbides. The conventional gas–solid reaction based
materials. Scientists are committed towards increasing the sat- on CO can result in the synthesis of many kinds of ICs, which
uration magnetization of soft magnetic materials as far as is a common method; however, nowadays, this method is
possible so as to reduce the amount of magnetic powders used being rarely used because of the toxicity of CO and the compli-
in practice by searching for new magnetic materials, changing cated tail gas treatment. The synthesis of ICs by the sol–gel
the synthesis methods and adjusting the experimental con- method using urea was first reported by Giordano et al., which
ditions; the advantage of high saturation magnetization is that is convenient, but only used to synthesize Fe3C.8 Based on the
it can reduce the amount of magnetic powders used in practice abovementioned two methods, a vaporized organic small
and make the development of miniaturized devices possible.3 molecule carbon source (ethylenediamine, triethylamine, etc.)
The most common and widely used soft magnetic materials was used to carbonate some precursors to obtain ICs.
are Fe3O4 and Fe; however, the low saturation magnetization of Importantly, the organic small molecule and the precursor are
Fe3O4 and the poor corrosion resistance of Fe limit their appli- placed in different ceramic boats; this significantly reduces the
cations. Therefore, researchers have found that iron carbides carbon content in the products and improves the magnetic
(ICs), in which carbon atoms enter the lattice of iron, form a properties of the ICs. Another advantage of this method is that
variety of structures depending on their carbon content. the type of ICs formed can be controlled by changing the reac-
Among these, χ-Fe5C2 and θ-Fe3C are popular magnetic iron tion temperature. Thus, we used this two-step method to
carbides due to their high theoretical saturation magnetiza- prepare several kinds of ICs (χ-Fe5C2 and θ-Fe3C).
tion, excellent mechanical performance and high chemical
More interestingly, in addition to the excellent magnetism,
stability.7,8 In addition, owing to their favorable biocompatibil- ICs have extraordinary electrochemical and chemical catalytic
properties. This may be attributed to their unique electronic
structure and special chemical bonding.21 It was reported that
aCollege of Chemistry, Jilin University, Changchun, 130012, PR China.
ICs could be applied in the Fischer–Tropsch synthesis,19,22
hydrogenation reaction,23 batteries and so on.24–26 Moreover,
E-mail: huayang86@sina.com
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
for electrochemical catalysis, the study on ICs has been limited
College of Chemistry, Jilin University, Changchun, 130012, PR China
to only the oxygen reduction reaction (ORR).27–29 Similarly, the
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
hydrogen evolution reaction (HER) is an important electro-
c9dt00328b
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catalytic process for the development and utilization of new 2.2 Preparation of the ICs@NC materials
energy sources.30 Pt and its alloys are still the main catalysts
The yellow powders (0.2 g) were transferred to a ceramic boat
in a tube-type furnace, and a ceramic boat with ethylenedia-
mine (4 mL) was placed in front of the abovementioned boat.
Then, the furnace was heated to the target temperature at the
ramping rate of 20 °C min−1 and maintained at the target
temperature for 1 hour under a N2 flow. After this, the furnace
was cooled down to room temperature. Finally, the black
product thus obtained was ground and collected. The entire
experimental process is shown in Scheme 1.
To remove the iron carbide phase, the Fe5C2–Fe3C@NC
catalyst was soaked in a 0.5 M H2SO4 solution for 10 h. The
product was obtained by centrifugation, repeatedly washed
several times with deionized water and ethanol, and then dried
at 80 °C. The product obtained was N-doped carbon (NC).
for the HER; however, their large-scale applications are limited
by their high cost and rare storage. Therefore, researchers are
actively developing some non-precious metal-based catalysts,
including transition metal sulfides, transition metal selenides,
transition metal carbides and so on, for the HER.31–33
However, these non-precious metal-based catalysts also face
many problems such as toxicity, poor stability, and a certain
disparity compared to Pt. Thus, the development of new HER
electrocatalysts will be the primary goal of researchers in the
next few years. Iron carbides, which are very important tran-
sition metal carbides, are used as HER catalysts; however, only
few studies have been reported on them.34,35 According to these
studies, iron carbides usually form composites with specific
carbon materials, such as graphene nanoribbons and carbon
nanofibers, to enhance their catalytic activity. Furthermore, iron
carbides and specific carbon materials have synergistic effects
on the catalytic activity for the HER. The results show that Fe3C-
based materials show certain catalytic activity for the HER;
however, the overpotential is not low enough in an alkaline
medium. Therefore, we designed cube-like structured particles
with iron carbides as the core and N-doped carbon as the shell
to improve the HER activity of iron carbides.
2.3 Characterization
Details of the physical and electrochemical characterization
methods of the resultant samples can be seen in the
Experimental section of the ESI.†
3. Results and discussion
In this study, we used a simple two-step method to prepare
cubic iron carbides encapsulated in a N-doped carbon shell by
changing the calcination temperature. In the beginning, iron
oxalate dihydrate, as a precursor, was synthesized through a
redox reaction between glucose and Fe(NO3)3. PVP was intro-
duced to obtain the precursor with cubic shapes. Then, under
an inert atmosphere (N2), with the increasing temperature,
ethylenediamine was used to carbonate the precursor to
obtain the ICs@NC materials; through a series of measure-
ment results, the iron carbide particles were shown to be sur-
rounded by a carbon shell and exhibited cubic shapes. Finally,
the cubic structured ICs@NC catalysts demonstrated soft mag-
netic properties and efficient catalytic activity for the HER by
electrochemical testing.
At first, we characterized the structure of the prepared yellow
precursor and the as-prepared samples obtained at different
calcination temperatures, as shown in Fig. 1. As shown in
Fig. 1A, the X-ray powder diffraction (XRD) pattern of the
yellow precursor well-matched with the standard XRD pattern
of FeC2O4·2H2O (no. 22-0635); this proved that the precursor
was iron oxalate dihydrate.36 Then, the prepared iron oxalate
dihydrate cubes formed a series of compounds under the
action of ethylenediamine by thermal treatment under a N2
flow. The XRD patterns of the product obtained at different
calcination temperatures are shown in Fig. 1B. When the calci-
nation temperature was low (440 °C), FeC2O4·2H2O could first
react to form Fe3O4. When the calcination temperature was
increased, Fe3O4 and ethylenediamine reacted to form iron
carbides. When the calcination temperature was 520 °C, the
phase Fe5C2 was formed, as shown in curve b in Fig. 1B, which
2. Experimental
2.1 Preparation of iron oxalate dihydrate
In a typical process, glucose (25 mmol) and poly(vinyl pyrroli-
done) (PVP) (37.5 mmol) were dissolved in 25 mL deionized
water and slowly heated to 100 °C for about 20 min under con-
tinuous magnetic stirring. Subsequently, Fe(NO3)3·9H2O
(12.5 mmol), melted at 50 °C, was injected into the glucose–
PVP solution, and the mixture was kept at 100 °C for 1 hour.
In this process, the mixture produced large amount of gas.
The experimental process needed to be carried out in a fume
hood. After this, the mixture was naturally cooled down to
room temperature and separated by centrifugation. Finally, the
yellow precipitates obtained were washed several times with
deionized water and ethanol and dried in an oven at 60 °C for
12 hours. The obtained yellow powders were the precursor.
Scheme 1 Synthetic procedure for the IC@NC materials.
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Fig. 1 XRD pattern of (A) iron oxalate dihydrate and (B) as-prepared samples obtained at different calcination temperatures (a: 440 °C, b: 520 °C, c:
540 °C, d: 560 °C, and e: 620 °C).
well-matched with the standard XRD pattern of χ-Fe5C2 (no. surfactant that controls the morphology of the products. Iron
89-2544). When the calcination temperature was increased to oxalate dihydrate without PVP exhibits an irregular mor-
540 °C, a part of Fe5C2 was converted to Fe3C, as shown in the phology, as shown in Fig. S1.† Moreover, the SEM image of
curve c in Fig. 1B. Subsequently, Fe5C2 was completely trans- Fe5C2@NC is displayed in Fig. 3B, which shows cubic struc-
formed into Fe3C at 560 °C. Finally, at 620 °C, θ-Fe3C was tures. The edge size of the cubes obtained by heat treatment
stable. The crystal evolution process displayed a crystallization with ethylenediamine is ca. 1 µm, much shorter than the
and thermostability range from ferrite to iron carbides, as typical edge size of the original iron oxalate dihydrate
shown in Fig. 2; this suggested that the crystal transformation (2.5 µm). In Fig. 3C, the single particle of Fe5C2 shows a rec-
(Fe3O4 → Fe5C2 → Fe3C) adjusted the species and property of tangular shape, and there are some irregularities at the edges.
the product. According to Fig. 1B and 2, we can infer the reac- This can be attributed to the collapse and epitaxy of the
tion mechanism. When the temperature increases, ethylene- sample during the calcination process. The high-resolution
diamine evaporates into gas and fills the quartz tube; TEM (HRTEM) image of the cube edges shows the surface of
FeC2O4·2H2O is decomposed into Fe3O4, and then, the ethyle- the Fe5C2 particles is covered with a thin carbon shell. This is
nediamine gas carbonizes Fe3O4 to form χ-Fe5C2; then, with a consistent with the XPS results. The lattice spacing of the
further increase in temperature, crystal transformation occurs, neighboring fringes is 0.205 nm, corresponding to the (510)
and finally, θ-Fe3C is formed. Obviously, the reaction takes lattice spacing in χ-Fe5C2, as shown in Fig. 3D, which is con-
place from FeC2O4 to Fe3O4 to χ-Fe5C2 to θ-Fe3C.
sistent with the XRD results. The morphology of the
The morphology of iron oxalate dihydrate and Fe5C2@NC Fe3C-620@NC materials is also similar to that of Fe5C2@NC,
materials was characterized by scanning electron microscopy as shown in Fig. S2.†
(SEM) and transmission electron microscopy (TEM), as shown
The X-ray photoelectron spectroscopy (XPS) full spectrum of
in Fig. 3. Moreover, according to Fig. 3A, the particles of iron the as-synthesized χ-Fe5C2@NC materials is displayed in
oxalate dihydrate exhibit uniform cubic shapes with an Fig. 4A, indicating that χ-Fe5C2@NC mainly consists of C, N,
average size of 2.5 µm. In the synthesis process, PVP acts as a O, and Fe and no other elements. Oxygen mainly originates
from the adsorption of oxygen in the air. The content of the C,
N, O, and Fe elements on the surface of the materials is shown
in Table S1.† Since XPS is quite a surface technique, the main
component of the surface of χ-Fe5C2@NC has been found to
be N-doped carbon. Moreover, low Fe content indicates that
iron is concentrated in the core of the particles. Moreover,
Fig. 4B shows the high-resolution Fe 2p spectrum of the
χ-Fe5C2@NC materials. The peak at 707 eV is attributed to the
statistical Fe–C and metallic Fe bonding.20 In addition, the
peaks at 709.9, 713, 724.4 and 726.4 eV are attributed to Fe2+
and Fe3+.37,38 The same results are displayed in the XPS
spectra of the Fe3C-620@NC materials in Fig. S3.† The high-
resolution C 1s spectrum is shown in Fig. 4C. It is important
that the carbon element is divided into four peaks. A peak at
283.5 eV has been detected that represents the typical metallic
Fe–C bonding.20 This also proves the existence of Fe–C from
Fig. 2 Summary of the crystal evolution at different temperatures.
another aspect. Moreover, the peak at 286.4 eV is attributed to
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Fig. 3 SEM images of (A) iron oxalate dihydrate and (B) Fe5C2@NC materials and (C) TEM and (D) HRTEM image of the Fe5C2@NC materials.
Fig. 4 (A) XPS full spectrum of the Fe5C2@NC materials and corresponding high-resolution spectra of the (B) Fe 2p, (C) C 1s, and (D) N 1s.
the sp2-hybridized carbons (CvC). Furthermore, the peaks 399.3 and 398.3 eV, attributed to graphitic-N, pyrrolic-N, and
located at 285.8 and 288.3 eV could be ascribed to the organic pyridinic-N, respectively, indicating that the nitrogen atoms
N–C and O–C species, respectively.39 Finally, in Fig. 4D, the are doped into carbon in the materials.40 Moreover, graphitic-
high-resolution N 1s spectrum displays three peaks at 400.9, N and pyridinic-N are good for the HER activity.
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uration magnetization, which can significantly reduce the
amount of magnetic powders used in practical applications,
such as in soft magnetic composites, rotating electrical
machines and high-frequency operations; this favors the devel-
opment of miniaturized devices.
Furthermore, we measured the catalytic activity of the as-
prepared ICs@NC catalysts for the HER in a typical three-elec-
trode cell using 1 M KOH electrolyte and maintained the same
catalyst loading density (0.57 mg cm−2) on the polished glassy
carbon electrode. To better illustrate the activity of the catalyst,
the Fe5C2–Fe3C@NC catalyst was soaked in 0.5 M H2SO4 to
remove iron carbides, and N-doped carbon (NC) was obtained.
The XRD pattern of N-doped carbon is shown in Fig. S3,†
proving that Fe5C2–Fe3C is completely corroded by H2SO4.
Moreover, the HER activity of NC was measured. As shown in
Fig. 6A, the Fe5C2–Fe3C@NC catalyst shows efficient HER cata-
Fig. 5 Hysteresis loops of the as-prepared samples at room tempera-
ture (a: Fe3O4@NC, b: Fe5C2@NC, c: Fe3C-560@NC, and d:
Fe3C-620@NC).
lytic activity with an overpotential (η) of 209 mV@10 mA cm−2
.
As shown in Fig. 5, the magnetism measurements were
studied by the M–H loops with an applied magnetic field of 1.7
T at room temperature. The detailed magnetic data of the as-
prepared samples are listed in Table S2.† It is obvious that
Fe3O4@NC, obtained at the calcination temperature of 440 °C,
displays superparamagnetic properties, whereas Fe5C2@NC,
Fe3C-560@NC and Fe3C-620@NC exhibit soft magnetic pro-
perties. Moreover, the saturation magnetization (MS) value
(119.29 emu g−1) of Fe5C2@NC is slightly lower than the
theoretical value of the bulk materials, which is found to be
The overpotential of the Fe5C2–Fe3C@NC catalyst is lower than
that of the Fe5C2@NC catalyst (287 mV@10 mA cm−2),
Fe3C-560@NC catalyst (252 mV@10 mA cm−2
) and NC
(353 mV@10 mA cm−2); this indicates that the Fe5C2–
Fe3C@NC catalyst has unique catalytic activity. This may be
mainly attributed to the unique structure of the catalyst when
it is in the intermediate state of phase transition between two
kinds of carbides. Moreover, this noticeable intermediate state
may have more defects caused by phase transformation than
single-phase iron carbide, and there is also a synergistic effect
of these two kinds of carbides on the HER catalytic activity.
The high overpotential of N-doped carbon is due to the low
nitrogen content in the materials. Moreover, the η value of
Fe3C-620@NC is higher by 36 mV than that of the
Fe3C-560@NC catalyst; this may be caused by the reduction of
the active sites due to the increase in the calcination tempera-
ture. The corresponding Tafel plots of the ICs@NC catalysts
are shown in Fig. 6B. The result of the Tafel slope of the
ICs@NC catalysts is slightly different, and the value of the
Tafel slope of the Fe5C2–Fe3C@NC catalyst is between the
values of Fe5C2@NC and Fe3C@NC, which is about 155 mV
dec−1, indicating that iron carbides are good for the kinetics of
the HER.
ca. 140 emu g−1 5
. Similarly, the MS values of Fe3C-560@NC
and Fe3C-620@NC are very close to the theoretical value of
pure θ-Fe3C (ca. 150 emu g−1).5 This may be attributed to the
large particle size and low free carbon content distributed on
the surface of the particles. Compared with that of the ICs pre-
pared by other methods listed in Table 1, the saturation mag-
netization of the ICs prepared herein has been significantly
improved. There are two reasons for this enhancement: one is
that the sizes of the ICs particles reported in this study are
relatively larger, i.e., the magnetic domain of the ICs prepared
in this study is larger; the other reason is that the content of
free carbon in the materials prepared herein is lower than that
in the materials prepared by other methods. Moreover, we
have compared the recently reported state-of-the-art Fe-based
materials with those reported in this study in Table 1. It is
obvious that the synthesized ICs@NC materials have high sat-
To test the effective surface areas of the ICs@NC catalysts,
double-layer capacitances (Cdl) at the solid–liquid interface
were measured by CV and are shown in Fig. S5.† The double-
Table 1 Comparison of the materials reported in this study with the recently reported state-of-the-art Fe-based materials
Materials
Synthesis
MS (emu g−1
)
Ref.
Fe5C2@NC
Fe3C-620@NC
θ-Fe3C
Mono-Fe5C2
ortho-Fe3C
θ-Fe3C
ε-Fe3N
Iron nitride
H-Fe3O4
Simple two-step method
Simple two-step method
Sol–gel method
Hot injection method
Hot injection method
Hydrothermal method
Sol–gel method
Electrospinning technique
Hydrothermal method
Hydrothermal method
119.29
134.10
62.8
This work
This work
16
20
20
41
42
43
44
45
95.0
101.2
123.03
102.13
122
78.9
34.73
CoFe2O4 NC
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Fig. 6 (A) Polarization curves of the as-prepared sample electrodes in 1 M KOH. (B) Corresponding Tafel plots. (C) The capacitive currents as a func-
tion of scan rate for the as-prepared sample electrodes. (D) Polarization curves of Fe5C2–Fe3C@NC obtained initially and after 1000 cycles.
layer charging current is plotted versus the scan rate in Fig. 6C, active sites of catalysis and (b) graphitic-N and pyridinic-N are
and the slopes are defined as Cdl. Notably, the Cdl of the beneficial to catalysis. However, poor conductivity of the
Fe5C2–Fe3C@NC catalyst is 1.13 mF cm−2, which is the largest materials leads to poor Cdl and EASA. Thus, improvement of
of all the reported data. According to the calculation formula the electrical conductivity of materials has become the key to
of the electrochemically active surface area (EASA) in the ESI,† improving the electrocatalytic activity of the IC materials.
the EASA of the Fe5C2–Fe3C@NC catalyst is 51.36. For the Cdl
and EASA, the main reason this value is not high is due to the
poor conductivity of the materials. Furthermore, the durability
of the catalysts is another important parameter. After the
4. Conclusions
1000-cycle test, the polarization curve obtained for the Fe5C2–
Fe3C@NC catalyst is basically the same as the initial curve, as
shown in Fig. 6D; this illustrates that the catalyst has good
In summary, cubic iron carbides encapsulated in an N-doped
carbon shell were successfully synthesized by a simple two-
step method. Importantly, χ-Fe5C2 and θ-Fe3C were acquired
stability. We have compared the recent HER catalysts with the
by changing the calcination temperature. Careful analysis of
materials reported in this study in Table 2. The results show
the SEM and TEM images shows that the prepared iron
that the ICs@NC catalyst has efficient HER catalytic activity,
carbide materials have regular cubic shapes and a thin carbon
shell. Furthermore, both the χ-Fe5C2@NC and the θ-Fe3C@NC
materials exhibited soft magnetic properties with high satur-
which may be attributed to the following facts: (a) ICs,
especially the mixed phase of two kinds of carbides, are the
ation magnetization. Moreover, we measured the electro-
catalytic activity of the prepared iron carbide@N-doped carbon
Table 2 Comparison of the electrochemical catalytic performances of
the recently reported HER catalysts with that of the catalyst reported
herein
catalysts for the HER, and the Fe5C2–Fe3C@NC catalyst showed
an efficient HER catalytic activity (209 mV@10 mA cm−2).
Finally, this simple and universal method should inspire and
guide the synthesis and exploration of various types of metal
carbides.
η (mV vs. RHE)@
Catalyst
10 mA cm−2
Electrolyte
Ref.
Fe5C2–Fe3C@NC
Fe-CACNF
Ni–Co–P HNBs
Co/NR CNTs
o-CoSe2/CC
PNC/Co
209
330
270
370
270
300
1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
This work
35
46
47
48
49
Conflicts of interest
There are no conflicts to declare.
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26 Z. Xu, Y. Du, D. Liu, Y. Wang, W. Ma, Y. Wang, P. Xu and
Acknowledgements
X. Han, ACS Appl. Mater. Interfaces, 2019, 11, 4268–4277.
This work was supported by the National Natural Science 27 W. Yang, X. Liu, X. Yue, J. Jia and S. Guo, J. Am. Chem. Soc.,
Foundation of China (NFSC) (no. 51872111) and Natural
Science Foundation of Jilin Province (no. 20190201253JC).
2015, 137, 1436.
28 X. Wang, P. Zhang, W. Wang, X. Lei and H. Yang, Chem. –
Eur. J., 2016, 22, 4863–4869.
29 V. Vij, J. N. Tiwari, W. G. Lee, T. Yoon and K. S. Kim, Sci.
Rep., 2016, 6, 20132.
30 C. G. Moralesguio, L. A. Stern and X. Hu, Chem. Soc. Rev.,
2014, 43, 6555–6569.
References
1 F. Liu, Y. Hou and S. Gao, Chem. Soc. Rev., 2014, 43, 8098–8113.
2 L. Wu, A. Mendoza-Garcia, Q. Li and S. Sun, Chem. Rev., 31 X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148–5180.
2016, 116, 10473–10512.
32 Y. Liu, Y. Li, H. Kang, T. Jin and L. Jiao, Mater. Horiz., 2016,
3, 402–421.
33 S. Li, P. Ren, C. Yang, X. Liu, Z. Yin, W. Li, H. Yang, J. Li,
X. Wang, Y. Wang, R. Cao, L. Lin, S. Yao, X. Wen and
D. Ma, Sci. Bull., 2018, 63, 1358–1363.
3 O. Gutfleisch, M. A. Willard, E. Bruck, C. H. Chen,
S. G. Sankar and J. P. Liu, Adv. Mater., 2011, 23, 821–842.
4 K. Zhu, Y. Ju, J. Xu, Z. Yang, S. Gao and Y. Hou, Acc. Chem.
Res., 2018, 51, 404–413.
5 Z. Ye, P. Zhang, X. Lei, X. Wang, N. Zhao and H. Yang, 34 X. Fan, Z. Peng, R. Ye, H. Zhou and X. Guo, ACS Nano,
Chem. – Eur. J., 2018, 24, 8922–8940. 2015, 9, 7407–7418.
6 W. Yang, S. Rehman, X. Chu, Y. Hou and S. Gao, 35 D. Ji, S. Peng, J. Lu, L. Li, S. Yang, G. Yang, X. Qin,
ChemNanoMat, 2015, 1, 376–398.
7 H. I. Faraoun, Y. D. Zhang, C. Esling and H. Aourag, J. Appl.
Phys., 2006, 99, 576.
8 C. Giordano, A. Kraupner, S. C. Wimbush and
M. Antonietti, Small, 2010, 6, 1859–1862.
M. Srinivasan and S. Ramakrishna, J. Mater. Chem. A, 2017,
5, 7507–7515.
36 S. Y. Hong, D. H. Chun, J.-I. Yang, H. Jung, H.-T. Lee,
S. Hong, S. Jang, J. T. Lim, C. S. Kim and J. C. Park,
Nanoscale, 2015, 7, 16616–16620.
9 J. Yu, F. Chen, W. Gao, Y. Ju, X. Chu, S. Che, F. Sheng and 37 P. Zhao, W. Xu, X. Hua, W. Luo, S. Chen and G. Cheng,
Y. Hou, Nanoscale Horiz., 2017, 2, 81–88. J. Phys. Chem. C, 2016, 120, 11006–11013.
10 W. Tang, Z. Zhen, C. Yang, L. Wang, T. Cowger, H. Chen, 38 R. Marega, F. De Leo, F. Pineux, J. Sgrignani, A. Magistrato,
T. Todd, K. Hekmatyar, Q. Zhao and Y. Hou, Small, 2014,
10, 1245–1249.
A. D. Naik, Y. Garcia, L. Flamant, C. Michiels and
D. Bonifazi, Adv. Funct. Mater., 2013, 23, 3173–3184.
11 J. Yu, C. Yang, J. Li, Y. Ding, L. Zhang, M. Z. Yousaf, J. Lin, 39 P. Wu, Y. Qian, P. Du, H. Zhang and C. Cai, J. Mater. Chem.,
R. Pang, L. Wei and L. Xu, Adv. Mater., 2014, 26, 4114–4120. 2012, 22, 6402–6412.
12 Y. Jing, Y. Ju, L. Zhao, C. Xin, W. Yang, Y. Tian, F. Sheng, 40 P. Chen, L.-K. Wang, G. Wang, M.-R. Gao, J. Ge, W.-J. Yuan,
L. Jian, L. Fei and Y. Dong, ACS Nano, 2015, 10, 159.
13 S. I. Hirano and S. Tajima, J. Mater. Sci., 1990, 25, 4457–4461.
Y.-H. Shen, A.-J. Xie and S.-H. Yu, Energy Environ. Sci., 2014,
7, 4095–4103.
14 A. Kraupner, M. Antonietti, R. Palkovits, K. Schlicht and 41 X. Lei, W. Wang, Z. Ye, N. Zhao and H. Yang, Dyes Pigm.,
C. Giordano, J. Mater. Chem., 2010, 20, 6019–6022. 2017, 139, 448–452.
15 Z. Schnepp, S. C. Wimbush, M. Antonietti and 42 P. Zhang, X. Wang, W. Wang, X. Lei and H. Yang, J. Mater.
C. Giordano, Chem. Mater., 2010, 22, 5340–5344. Chem. A, 2015, 3, 6464–6469.
16 X. Wang, P. Zhang, J. Gao, X. Chen and H. Yang, Dyes 43 A. Garcia-Marquez, S. Glatzel, A. Kraupner, K. Kiefer,
Pigm., 2015, 112, 305–310.
17 A. E. Danks, S. R. Hall and Z. Schnepp, Mater. Horiz., 2016,
3, 91–112.
18 B. Ghosh and S. K. Pradhan, J. Alloys Compd., 2009, 477,
127–132.
19 C. Yang, H. Zhao, Y. Hou and D. Ma, J. Am. Chem. Soc.,
2012, 134, 15814–15821.
K. Siemensmeyer and C. Giordano, Chem. – Eur. J., 2018,
24, 4895–4901.
44 Y. Wang, L. Chen, H. Liu, Z. Xiong, L. Zhao, S. Liu,
C. Huang and Y. Zhao, Chem. Eng. J., 2019, 356, 746–755.
45 S. Yang, X. Qiu, P. Jin, M. Dzakpasu, X. C. Wang, Q. Zhang,
L. Zhang, L. Yang, D. Ding, W. Wang and K. Wu, Chem.
Eng. J., 2018, 353, 329–339.
20 Z. Yang, T. Zhao, X. Huang, X. Chu, T. Tang, Y. Ju, 46 E. Hu, Y. Feng, J. Nai, D. Zhao, Y. Hu and X. W. Lou, Energy
Q. Wang, Y. Hou and S. Gao, Chem. Sci., 2017, 8, 473.
21 D. J. Ham and J. S. Lee, Energies, 2009, 2, 873–899.
22 T. Qiu, L. Wang, S. Lv, B. Sun, Y. Zhang, Z. Liu, W. Yang
and J. Li, Fuel, 2017, 203, 811–816.
Environ. Sci., 2018, 11, 872–880.
47 X. Zou, X. Huang, A. Goswami, R. Silva, B. R. Sathe,
E. Mikmekovã and T. Asefa, Angew. Chem., Int. Ed., 2014,
53, 4372–4376.
23 J. Shi, Y. Wang, W. Du and Z. Hou, Carbon, 2016, 99, 330–337. 48 P. Chen, K. Xu, S. Tao, T. Zhou, Y. Tong, H. Ding, L. Zhang,
24 X. Cai, L. Lai, J. Lin and Z. Shen, Mater. Horiz., 2017, 4,
945–976.
W. Chu, C. Wu and Y. Xie, Adv. Mater., 2016, 28, 7527–
7532.
25 Y. Wang, M. Li, L. Xu, T. Tang, Z. Ali, X. Huang, Y. Hou and 49 X. Li, Z. Niu, J. Jiang and L. Ai, J. Mater. Chem. A, 2016, 4,
S. Zhang, Chem. Eng. J., 2019, 358, 962–968.
3204–3209.
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