N/S Co-doped Carbon Derived From Cotton as High Performance Anode Materials for Lithium Ion Batteries

Abstract

Highly porous carbon with large surface areas is prepared using cotton as carbon sources which derived from discard cotton balls. Subsequently, the sulfur-nitrogen co-doped carbon was obtained by heat treatment the carbon in presence of thiourea and evaluated as Lithium-ion batteries anode. Benefiting from the S, N co-doping, the obtained S, N co-doped carbon exhibits excellent electrochemical performance. As a result, the as-prepared S, N co-doped carbon can deliver a high reversible capacity of 1,101.1 mA h g⁻¹ after 150 cycles at 0.2 A g⁻¹, and a high capacity of 531.2 mA h g⁻¹ can be observed even after 5,000 cycles at 10.0 A g⁻¹. Moreover, excellently rate capability also can be observed, a high capacity of 689 mA h g⁻¹ can be obtained at 5.0 A g⁻¹. This superior lithium storage performance of S, N co-doped carbon make it as a promising low-cost and sustainable anode for high performance lithium ion batteries.

Full text

ORIGINAL RESEARCH
published: 26 April 2018
doi: 10.3389/fchem.2018.00078
Frontiers in Chemistry | www.frontiersin.org 1April 2018 | Volume 6 | Article 78
Edited by:
Qiaobao Zhang,
Xiamen University, China
Reviewed by:
Jinhu Yang,
Tongji University, China
Yunxiao Wang,
Institute for Superconducting and
Electronic Materials, Australia
Hui Xia,
Nanjing University of Science and
Technology, China
*Correspondence:
Chenghao Yang
esyangc@scut.edu.cn
Specialty section:
This article was submitted to
Physical Chemistry and Chemical
Physics,
a section of the journal
Frontiers in Chemistry
Received: 30 January 2018
Accepted: 08 March 2018
Published: 26 April 2018
Citation:
Xiong J, Pan Q, Zheng F, Xiong X,
Yang C, Hu D and Huang C (2018)
N/S Co-doped Carbon Derived From
Cotton as High Performance Anode
Materials for Lithium Ion Batteries.
Front. Chem. 6:78.
doi: 10.3389/fchem.2018.00078
N/S Co-doped Carbon Derived From
Cotton as High Performance Anode
Materials for Lithium Ion Batteries
Jiawen Xiong 1, Qichang Pan 1, Fenghua Zheng 1, Xunhui Xiong 1, Chenghao Yang1, 2
*,
Dongli Hu 3and Chunlai Huang 3
1Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of
Environment and Energy, South China University of Technology, Guangzhou, China, 2Guangdong Engineering and
Technology Research Center for Surface Chemistry of Energy Materials, New Energy Research Institute, School of
Environment and Energy, South China University of Technology, Guangzhou, China, 3Jiangsu Key Lab of Silicon Based
Electronic Materials, Jiangsu GCL Silicon Material Technology Development Co., Ltd, Xuzhou, China
Highly porous carbon with large surface areas is prepared using cotton as carbon
sources which derived from discard cotton balls. Subsequently, the sulfur-nitrogen
co-doped carbon was obtained by heat treatment the carbon in presence of thiourea
and evaluated as Lithium-ion batteries anode. Benefiting from the S, N co-doping, the
obtained S, N co-doped carbon exhibits excellent electrochemical performance. As a
result, the as-prepared S, N co-doped carbon can deliver a high reversible capacity of
1,101.1 mA h g−1after 150 cycles at 0.2 A g−1, and a high capacity of 531.2 mA h
g−1can be observed even after 5,000 cycles at 10.0 A g−1. Moreover, excellently rate
capability also can be observed, a high capacity of 689 mA h g−1can be obtained at
5.0 A g−1. This superior lithium storage performance of S, N co-doped carbon make it
as a promising low-cost and sustainable anode for high performance lithium ion batteries.
Keywords: lithium-ion batteries, anode materials, sustainable, cotton, N/S co-doped carbon
INTRODUCTION
In the past decade, lithium-ion batteries (LIBs) have been widely used as power sources for
computing, communications, consumer batteries (3C battery), and electric vehicle. And LIBs with
a wide range of applications which due to its have high working voltage, high energy density,
and long service life (Goodenough and Kim, 2010; Goodenough and Park, 2013; Zheng et al.,
2015). Currently, graphite is mostly used as anode material for commercial LIBs due to its good
electronic conductivity, low cost and outstanding cycling stability. However, graphite can not meet
the increased energy and power density of high performance LIBs due to the low specific capacity
and poor rate performance (Huang et al., 2016; Wang et al., 2017; Pan et al., 2018). On the other
hand, the low lithiation/de-lithiation potentials (<0.3 V vs. Li+/Li) which resulting in seriously
security issue (Guo et al., 2011). Therefore, it is necessary to develop novel anode materials to
replace graphite anode for high-performance LIBs (Pan et al., 2017a).
Recently, amorphous carbon and hard carbon are attracted attentions as promising anode
to replace graphite anode for LIBs, which due to these carbonaceous materials exhibit higher
specific capacity and offer higher lithiation/de-lithiation potential (Wang et al., 2009; Casas and
Li, 2012; Tang et al., 2012). Moreover, these carbonaceous materials with partially graphitic
carbons which can accommodate Li+in the disordered interlayers as well as in the micropores,
and can exhibit excellent cycling stability and rate capability (Wu et al., 2003). On the other

Xiong et al. N/S Co-doped Carbon Anode Materials
hand, other carbonaceous materials such as carbon nanotube,
graphene and fullerenes were also developed as anode for LIBs,
and exhibit excellent electrochemical performance (Etacheri
et al., 2015; Wang et al., 2015). However, in order to synthesize
these carbonaceous materials which rely on hydrocarbon
precursors, resulting in expensive cost and commercially
nonviable. Therefore, it is necessary to explore a scalable and
inexpensive precursor as carbon sources for these carbonaceous
materials as anode materials for LIBs.
Nowadays, multitudinous biomass raw materials have been
extensively used as precursor for carbonaceous materials and
application in LIBs. So many biomass raw materials attracted
attention such as peanut (Ding et al., 2015), ramie (Jiang et al.,
2016), sisal (Yu et al., 2015), bamboo (Jiang et al., 2014), green
tea leaves (Han et al., 2014), peat moss (Ding et al., 2013), rice
husk (Wang et al., 2013), banana peel (Lotfabad et al., 2014),
and so on. However, cotton attracted more attention and have
been considered as the most promising compared with the other
biomass materials due to its abundant and low cost. On the
other hand, in China, cotton is planted around 550 Million tons
per year and giant cotton-products are abandoned which from
clothes, medical alcohol cotton, and so on. Moreover, lots of
abandoned cotton-products will bring lots of problems, such as
environmental pollution, safe question, and so on.
Herein, we addressed the above mentioned issues by prepared
high performance carbon-based anode materials using cotton
as precursor, as shown in Figure 1. Highly porous carbon with
large surface areas were prepared from cotton via s sample
method. And the N/S-coped carbon were further obtained using
thiourea as nitrogen and sulfur sources. When evaluated as anode
materials for LIBs, these carbon materials exhibit outstanding
rate capability and long-term cycling stability.
EXPERIMENTAL
Material Preparation
1.5 g cotton was dipped in homogeneous Mg(NO3)2solution
(8 mol L−1, 20 ml), then dried in the oven. After that, the obtained
cotton were annealed at 800◦C under N2atmosphere for 3 h with
a heating rate of 5◦C min−1. After cooling, the obtained cotton
carbon (denoted as CC) were washed with 1 M HCl and distilled
water several times, respectively, then dried in the oven.
To obtained the N, S co-doped carbon, the obtained CC were
immersed in 100 ml thiourea solution with ratio of 10:1. After
drying, the obtained powders were calcined at 800◦C under N2
atmosphere for 3 h. The N, S co-doped cotton carbon (denoted
as NS-CC) powders were obtained after cooling.
Material Characterizations
The XRD patterns of all samples were conducted on the Bruker
D8 Advance (Germany) (Cu, Kα,λ=1.5405 Å). Raman spectra
were obtained on a JOBIN-Yvon HR800 Raman spectrometer.
Morphology of the all samples were studied by SEM (FEI
Quanta 200 FEG) and TEM (Tecnai G2 F20 S-TWIN, Japan).
BET method and non-linear density functional theory (NLDFT)
(ASAP 2020 Micromeritics) were applied to test the specific
surface area and the pore size distribution.
Electrochemical Measurements
The samples were executed in CR2025 coin cells. The mingled
ratio of sample CC, sample NS-CC and carbon black and poly
(vinylidenedifluoride) were 7:2:1 (the loaded active electrode
materials are about 0.5 mg cm−2). The mixture was coating on
copper foil to prepare electrodes. Metal lithium boil as counter
electrode and 1 M LiPF6dissolved in ethylene carbonate (EC)
and dimethyl carbonate (DMC) (1:1, v/v) as electrolyte. CV and
EIS were conducted on a CHI660A electrochemical workstation.
Galvanostatic charge/discharge and cycling performance were
executed at 25◦C based on the active electrode material
corresponding specific capacity.
RESULTS AND DISCUSSIONS
The morphology of the all samples was characterized by
SEM firstly. Figures 2A,C exhibit the SEM images of cotton
carbon, which shows micron size bulk materials and composed
of nanosheets. Figures 2B–D shows the SEM images of the
cotton carbon after N and S co-doped, which exhibits similar
morphology to cotton carbon. Moreover, cotton carbon and S,
N co-doped cotton carbon with a large number of pores in the
nanosheets according to the HRSEM (Figures 2C,D). On the
other hand, EDS element mapping of S, N co-doped cotton
carbon were investigated, which indicated that C, N, O, and
S elements exist S, N co-doped cotton carbon. Furthermore, S
and N elements evenly distributed (Figures 2G–J) in the carbon
matrix. Therefore, the results indicated that the N and S elements
were successfully doped into cotton carbon after heat treatment
the carbon in presence of thiourea. The microstructure of CC and
NS-CC was further studied by TEM, and the results are shown in
Figures 2E,F. It clearly illustrates that the samples are amorphous
carbon with nano/meso porous structure (Chen et al., 2014).
Furthermore, typical selected area electron diffraction further
proved that the carbon were amorphous, which corresponding
to TEM results (Zhu and Akiyama, 2016).
The XRD patterns of the CC and NSCC are showed
in Figure 3A. There are no obvious peaks for magnesium
compounds in CC sample, which indicated that the Mg(NO3)2or
MgO are removed completely by washed with dilute hydrochloric
acid. And two broad peaks at around 23 and 43◦can be observed
both at CC and NS-CC samples, which can be attributed to
(002) and (100) graphitic planes, respectively (Hou et al., 2015;
Chen et al., 2017). The Raman spectra of the CC and NS-
CC are shown in Figure 3B, two peaks at around 1,361 and
1,596 cm−1were obtained, which corresponding to the D band
and G band for carbon materials, respectively (Li et al., 2015; Gao
et al., 2017). Furthermore, the D band arises from edges, defects,
and disordered carbon, whereas the G band is ascribed to sp2-
hybridized carbon (Pan et al., 2017b). Therefore, a high ID/IG
band intensity ratio indicates the generation of large amounts
of defects. The ID/IGratio for NS-CC is higher than that of
CC, which indicated that more vacancies and defects generated
by doping N and S atoms into the carbon material. More
importantly, more vacancies and defects are beneficial for the
transmission of Li-ion and offer more active site for Li storage,
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Xiong et al. N/S Co-doped Carbon Anode Materials
FIGURE 1 | Schematic of the fabrication of porous carbon sheet anode form wasted cotton for Lithium-ion battery anode powering blue light emitting diode (LED).
which resulting in improved electrochemical performance (Qie
et al., 2015; Lu et al., 2017).
The XPS measurement was conducted to confirmed that
the presence of N and S elements in NS-CC. As shown in
Figures S1A,B, the survey spectrum for NS-CC and CC exhibit
two predominant peak at around 284.8 and 532 eV can be
observed, which can be assigned to C and O. It certified
for the NS-CC that N and S atoms were successfully doped
corresponding with EDS element mapping. In Figures S1C,D, the
C 1s XPS spectrum for both CC and NS-CC can be deconvoluted
into five peaks, which corresponding to C =C, C-C/C =N, C-
O, C =O and π-π∗(Liu et al., 2017), respectively. The high
resolution N 1s spectrum as shown in Figure 3E, the N 1s speak
can be fitted by three component peaks at around 401.9, 400.5,
and 398.3 eV, which can be ascribed to graphitic N, pyrrolic N and
pyridinic N, respectively (Ou et al., 2015). As for high resolution
of S 2p spectrum (Figure 3F), there were five peaks attributed
to -C-S-C- bond and -C-SOX-C- bond. Therefore, these results
indicated that the N and S has been successfully incorporated
into the carbon structure of NS-CC. And the content of N and
S in NS-CC were confirmed for 3.0 and 1.4%, respectively. The
heteroatoms N and S can effectively enlarge the interlayer space
because of their lager radius than C atom, resulting in forming the
defects and providing more active sites for Li-ions on the carbon
materials (Xu et al., 2015, 2016; Xiong et al., 2016). Moreover,
pyridinic N and quaternary N are favorable for Li+and electrons
and the doped S in the carbon materials can participate in the
redox reactions contribute to the reversible capacity (Ma et al.,
2018).
The nitrogen adsorption/desorption isotherms and the pore
size distribution of CC and NS-CC are shown in Figures 3C,D.
As shown in Figure 3C both of the two samples showed type IV
isotherms (Islam et al., 2017). The BET specific surface area of
CC and NS-CC are 1235.35 and 1326.20 m2g−1, respectively.
The BET specific surface areas of NS-CC increased compare than
the CC after N and S atoms doping. In addition, the highly
porous structure of the two samples were further evaluated by
Barrett-Joyner-Halenda (BJH) calculations (Figure 3D). It can be
seen that the two samples exhibit a broad pore size distribution,
and the pore size of the two samples were centered at around 2
and 5 nm, respectively. Therefore, NS-CC exhibits larger specific
surface area which can provide more active sites for lithium ion
storage. Moreover, the highly porous structure of the two samples
can greatly shorten the diffusion distance of both electrons and
ions, which resulting in improved rate performance (Hao et al.,
2014).
The electrochemical performance of CC and NS-CC was first
measured by cyclic voltammetry (CV) with voltage range of 3.0–
0.01 V. Figure 4A exhibits the CV curves of the NS-CC sample.
During the first discharge cycle, three cathodic peaks at around
1.4 and 0.65 V can be obtained and disappeared in the subsequent
cycles, which corresponding to formation of a solid-electrolyte
interphase (SEI) film (Wang et al., 2011; Jiang et al., 2013) as
well as some irreversible and side reactions associated with the
decomposition of electrolyte (Yoshio et al., 2000). Moreover,
the CV profiles almost overlapped after the initial scanning
cycle, which indicates that the structural stability of the NS-CC
electrode during the subsequently cycling. On the other hand, the
CV curves of CC electrode (Figure S2A) are similar to the NS-CC
electrode.
The charge and discharge profiles of CC and NS-CC electrode
at 0.1 A g−1with cutoff voltage window of 0.01–3.0 V were
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Xiong et al. N/S Co-doped Carbon Anode Materials
FIGURE 2 | SEM images of (A,C) CC and NS-CC (B,D); TEM images of (E) CC and (F) NS-CC (the inset of part is the SAED pattern); (G–J) EDS mapping of NS-CC.
studied. As shown in Figure 4B and Figure S2B, two plateaus
at around 1.4 and 0.65 V can be observed during the first
discharge process, which corresponding to the forming of SEI
layer and decomposition of the electrolyte, in good agreement
with the above CV results. The first discharge capacity of CC
and NS-CC are as high as 3,275.1 and 4,179.1 mA h g−1, while
the initial re versible capacity are only 1,512.3 and 1,823.3 mA h
g−1, corresponding to low initial Coulombic efficiency of 46.2
and 43.6%, respectively. The large irreversible capacity loss for
the CC and NS-CC electrode can be ascribed to formation of a
SEI layer on the relatively large specific surface area (Jiang et al.,
2013). Furthermore, a mass of reduction of oxygen functionalities
on the carbon materials surface (Bhattacharjya et al., 2014) and
reduction of electrolyte components on the active electrode of the
CC and NS-CC electrode (Hu et al., 2007) also contributed to the
irreversible capacity loss.
The cycling performance of the CC and NS-CC electrode were
evaluated at 0.2 A g−1(Figure 4D). NS-CC electrode exhibits
excellent cycling stability and a high reversible capacity of
1101.1 mA h g−1can be observed after 150 cycles, but for CC
electrode, a lower reversible capacity of 637.1 mA h g−1can be
obtained. Rate performance is very important for LIBs, especially
application in electric vehicles. Therefore, the rate performance
of the samples were evaluated at various current densities from
0.1 to 5.0 A g−1. As seen in Figure 4C, NS-CC electrode can
deliver reversible capacities of 1,443, 1,035, 954, 884, 802, and
689 mA h g−1at 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g−1, respectively.
In contrast, the reversible capacities are 1,020, 655, 541, 478, 427,
and 370 mA h g−1at 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g−1for
CC electrode. Obviously, the NS-CC electrode exhibits excellent
rate capability. Therefore, the long-term cycling at high current
density was also tested for NS-CC electrode, the results as shown
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Xiong et al. N/S Co-doped Carbon Anode Materials
FIGURE 3 | (A) XRD patterns (B) Raman spectrum (C) Nitrogen adsorption/desorption isotherm (D) Pore size distribution of CC and NS-CC. High-resolution scans of
(E) N spectrum (F) S2p spectrum electrons of NS-CC.
in Figure 4E. NS-CC electrode can deliver a high reversible
capacity of 531.2 mA h g−1can be obtained even after 5,000
cycles at 10.0 A g−1. However, CC electrode only delver a lower
reversible of 283 mA h g−1after 5,000 cycles at the same current
density. Therefore, NS-CC electrode exhibits excellent rate
performance and long-term cycling stability, which shows better
electrochemical performance than previous reported carbon-
based materials, as shown in Table S1. As a result, The NS-
CC electrode delivered amazing electrochemical performance
especially with ultrahigh specific capacity and rate capability
which can be given rise to the following reasons: (1) The carbon
materials interlayer spacing are expanded by N and S successfully
co-doping which benefit Li-ions diffusion. (2) The marked
large surface of carbon materials offer plentiful micropores and
mesopores structure shorten the diffusion distance, sufficient
contact between electrolyte and electrode and active sites for
lithium ion storage. (3) The ample pyridinic N, pyrrolic N and -S-
C-S- covalent bonds built adequate active sites to improve surface
capacity contribution (Xia et al., 2017).
The electrochemical impedance spectroscopy for CC and
NS-CC electrode were further investigated to understand the
significantly improved electrochemical performance. As shown
in Figure S3, the Nyquist plots of CC and NS-CC electrode have
shown the typical characteristics of one semicircle and a sloping
straight line (Liu et al., 2016). The diameter of the semicircle
is reduced in the plots of the NS-CC electrode compared with
that of the CC electrode, indicating the decreased charge-transfer
resistance at the electrode/electrolyte interface after doping of N,
S atoms into the carbon. On the other hand, the charge transfer
resistance presents a decreasing trend along with the cycles for
both CC and NS-CC electrode, which due to formation of stable
SEI film and the process of activation after cycling (Pan et al.,
2016).
In order to further understand the high-rate performance,
the capacitive behavior of the NS-CC and CC electrode were
investigated and their kinetics were also analyzed with CV
measurements. Figure 5A and Figure S4A show the CV curves
of NS-CC electrode at various scan rates ranging from 0.1 to
10 mV s−1. All of the curves display a similar shape, two cathodic
peaks and one anodic peak are evidently on each curve. The peak
current is not proportional to the square root of the sweep rate
(v), indicating that the charge/discharge process is comprised of
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Xiong et al. N/S Co-doped Carbon Anode Materials
FIGURE 4 | (A) CV curves and (B) first 10 cycles of charge-discharge profiles of NS-CC; (C) Rate performance and (D) cyclic performance at 0.2 A g−1;(E)
long-term cyclic performance at 10 A g−1of CC and NS-CC.
faradic and non-faradic processes. According to the equation of
the relationship of iand v:
i=avb(1)
or
log(i)=b×log(v)+log(a)′(1)′
Here, a and b are constants. The process is an ionic diffusion
controlled behavior when b-value is equal to 0.5, while is Li+
capacitive behavior when b-value is equal to 1.0. Figure 5B
presents log(i)-log(v) plots for NS-CC electrode on the CV
curves at peak 1, 2 and 3 potentials and the b-value are
0.84, 0.75 and 0.77, respectively. And for CC the b-value are
0.75, 0.71, and 0.97, respectively (Figure S4B). Therefore, It
can be seen that all these values of b indicate fast kinetics
resulting from the pseudocapacitive effect. Moreover, To quantify
the pseudocapacitive contribution, we can divide the current
response i at a fixed potential V into pseudocapacitive (k1v) and
diffusion-controlled contributions (k2v0.5) by following equation
(Muller et al., 2015).
i(V)=k1v+K2v1/2
By calculating both k1 and k2 constants, the overall contribution
of pseudocapacitor at various scan rates can be obtained. The
detail pseudocapacitive contribution of NS-CC and CC electrode
at 10 mV s−1as shown in Figure 5C and Figure S4C, in which
78.9% as capacitive (red region). Therefore, all contribution ratios
of the capacitive capacity at scan rates of 0.1, 0.2, 0.5, 1, 2,
and 5 mV s−1were also obtained. Figure 5D and Figure S4D
shows contributions of the pseudocapacitive behaviors at various
scan rates. The proportion of capacitive contribution for NS-
CC electrode are 29.8, 31.7, 36.9, 43.5, 52.1, 64.7, and 78.9% at
0.1, 0.2, 0.5, 1, 2, 5, and 10.0 mV s−1, respectively. Furthermore,
the contribution for CC electrode are 27.1, 28.2, 32.3, 41.3, 50.1,
62.6, and 75.6% at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and10.0 mv s−1. As a
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Xiong et al. N/S Co-doped Carbon Anode Materials
FIGURE 5 | (A) CV curves measured between 0.01 and 3.0 V at various scan rate from 0.1 to 10 mV s−1.(B) The b-value determined by using the relationship
between peak current and scan rate. (C) CV curve with the pseudocapacitive fraction shown by red and diffusion shown by black at a scan rate of 10 mV s−1.(D) Bar
chart showing the percentage of pseudocapacitive contribution at vs. scan of NS-CC.
result, the capacitive contribution for CC is smaller than the NS-
CC electrode, which can attribute to the NS-CC electrode with
larger specific area after N and S atoms co-doping, resulting in
enhanced pseudocapacitive contribution (Augustyn et al., 2014;
Chao et al., 2016).
CONCLUSION
In summary, highly porous carbon were prepared by using cotton
as precursor with a sample method. Subsequently, the sulfur-
nitrogen co-doped carbon were obtained via heat treatment the
carbon in presence of thiourea, which can induce the defects and
the expanded interlayer of the carbon. Therefore, the expanded
interlayer and defects can reduce the diffusion distance of Li
ions as well as offer more active sites for lithium storage. As a
result, S, N co-doped carbon exhibits excellent electrochemical
performance when evaluated as anode materials for Lithium-ion
materials. The as-prepared S, N co-doped carbon can deliver a
high reversible capacity of 546.4 mA h g−1even after 5,000 cycles
at 10 A g−1. Moreover, excellently rate capability also can be
observed, a high capacity of 600 mA h g−1can be obtained at
5.0 A g−1. This superior lithium storage performance of S, N co-
doped carbon make it as a promising low-cost and sustainable
anode material for lithium ion batteries.
AUTHOR CONTRIBUTIONS
JX conducted the experiments CY is the supervisor of this
research work. JX and QP helped writing. JX, FZ, XX, DH, and
CH performed the characterization and data analysis. All authors
involved the analysis of experimental data and manuscript
preparation.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from the
Science and Technology Planning Project of Guangdong
Province, China (No. 2017B090916002), Guangdong
Natural Science Funds for Distinguished Young Scholar
(2016A030306010), Guangdong Innovative and Entrepreneurial
Research Team Program (2014ZT05N200) and Fundamental
Research Funds for Central Universities, China (2017
ZX010).
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fchem.
2018.00078/full#supplementary-material
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Xiong et al. N/S Co-doped Carbon Anode Materials
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Conflict of Interest Statement: DH and CH were employed by company Jiangsu
Key Lab of Silicon Based Electronic Materials.
The other authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
Copyright © 2018 Xiong, Pan, Zheng, Xiong, Yang, Hu and Huang. This is an
open-access article distributed under the terms of the Creative Commons Attribution
License (CC BY). The use, distribution or reproduction in other forums is permitted,
provided the original author(s) and the copyright owner are credited and that the
original publication in this journal is cited, in accordance with accepted academic
practice. No use, distribution or reproduction is permitted which does not comply
with these terms.
Frontiers in Chemistry | www.frontiersin.org 9April 2018 | Volume 6 | Article 78