Coordination polymer submicrospheres: Fast microwave synthesis and their conversion under different atmospheres

Article abstract of DOI:10.1021/ic5005769

Rare earth (RE) based coordination polymer (CP) submicrospheres have been prepared from pyridine-2,5-dicarboxylic acid and RE(NO₃)₃ via a facile microwave heating method in 5 min, with N,N-dimethylformamide (DMF) as solvent. The submicrospheres have diameters of 100-400 nm. Furthermore, the surface of the microspheres is smooth and the microspheres are solid. Several CP submicrospheres (RE = La, Gd, Y) were selected and calcined under different atmospheres (including air, N₂, and NH₃). After calcination in air at 550 °C for 4 h, rare earth oxide (RE₂O ₃) submicrospheres were obtained. On calcination under an N ₂ atmosphere, LaN/La₂O₃/C composite spheres were obtained for La-based CPs. For Gd(Y)-based CPs, Gd₂O ₃(Y₂O₃)/C composite spheres were obtained. Porous carbon submicrospheres were obtained after the removal of RE ₂O₃ and REN from the composite spheres. Interestingly, under an NH₃ atmosphere, La₂O₂CN₂ submicrospheres were produced from the La-based CPs. In addition, the Gd-based and Y-based CPs submicrospheres gave Gd₂O₃/GdN/C and Y₂O₃/C submicrospheres, respectively. As examples of their potential applications, their upconversion properties and electrochemical properties of the as-prepared products were investigated. This facile microwave synthesis method may offer an attractive approach for the preparation of other RE-CP micro-/nanostructures, and many interesting materials may be derived.

Full text of DOI:10.1021/ic5005769

Article
pubs.acs.org/IC
Coordination Polymer Submicrospheres: Fast Microwave Synthesis
and Their Conversion under Different Atmospheres
Shengliang Zhong,* Hongyu Jing, Yuan Li, Shungao Yin, Chenghui Zeng, and Lei Wang*
College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People’s Republic of China
S
* Supporting Information
ABSTRACT: Rare earth (RE) based coordination polymer (CP)
submicrospheres have been prepared from pyridine-2,5-dicarbox-
ylic acid and RE(NO3)3 via a facile microwave heating method in 5
min, with N,N-dimethylformamide (DMF) as solvent. The
submicrospheres have diameters of 100−400 nm. Furthermore,
the surface of the microspheres is smooth and the microspheres are
solid. Several CP submicrospheres (RE = La, Gd, Y) were selected
and calcined under different atmospheres (including air, N2, and
NH3). After calcination in air at 550 °C for 4 h, rare earth oxide
(RE2O3) submicrospheres were obtained. On calcination under an
N2 atmosphere, LaN/La2O3/C composite spheres were obtained for La-based CPs. For Gd(Y)-based CPs, Gd2O3(Y2O3)/C
composite spheres were obtained. Porous carbon submicrospheres were obtained after the removal of RE2O3 and REN from the
composite spheres. Interestingly, under an NH3 atmosphere, La2O2CN2 submicrospheres were produced from the La-based CPs.
In addition, the Gd-based and Y-based CPs submicrospheres gave Gd2O3/GdN/C and Y2O3/C submicrospheres, respectively. As
examples of their potential applications, their upconversion properties and electrochemical properties of the as-prepared products
were investigated. This facile microwave synthesis method may offer an attractive approach for the preparation of other RE-CP
micro-/nanostructures, and many interesting materials may be derived.
submicrospheres have been prepared via a facile and fast
microwave heating method in 5 min by our group.10
INTRODUCTION
■
In the past decade, the preparation and application of CPs at
the micro-/nanoscale have aroused increasing interest.1 More
and more CPs with various morphologies and structures have
been prepared by various methods. For example, CP particles
with diameters of a few hundred nanometers were prepared by
simple addition of an initiation solvent to a precursor solution
of metal ions and metalloligands.2 Highly monodisperse MIII-
based soc-MOFs (M = In, Ga) with cubic and truncated cubic
morphologies were prepared by a conventional heating
method.3 Ultrathin 2D CP nanosheets were prepared by a
microemulsion method.4 CP hollow spheres with diameters of
260−500 nm have been prepared via a solvothermal method by
our group, and the formation mechanism was discussed.5 As is
well known, microwave can be employed to overcome vigorous
conditions and long reaction times, because it creates high
heating effects in a short time.6 At the same time, microwave
heating may have a beneficial effect on the properties and
performance of materials.7 Microwave heating methods have
also found application in the preparation of CP micro-/
nanomaterials, and it turned out that these methods could
abruptly shorten the preparation time. For instance, three
known MOFs, namely IRMOF1, IRMOF2, and IRMOF3, were
prepared through a rapid microwave-assisted methodology in
25 s.8 The morphology design of Cu3(btc)2n (btc = benzene-
1,3,5-tricarboxylate) was realized by microwave irradiation in 60
min. A morphological transition (octahedron−cuboctahedron−
cube) was observed.9 Uniform europium-based infinite CP
Recent work demonstrated that CPs are suitable precursors
for the production of nanoscale materials with optimized
morphologies and properties. By fine control of the conditions,
metals, metal oxides, metal sulfides, and other nanosized
materials with different morphologies can be obtained from CP
precursors.11 It turns out that the obtained nanomaterials from
nano-CP precursors show better and more useful properties.
Among them, CPs have been demonstrated as promising
precursors to create functional nanoporous carbons. Several
porous carbon materials have been prepared by various
methods using coordination polymers as the precursors.12 For
instance, nanoporous carbon prepared by direct carbonization
of Al-based porous coordination polymers exhibits superior
sensing capabilities toward toxic aromatic substances.13 Porous
carbon with exceptionally high surface area, hydrogen sorption
capacity, and excellent electrochemical performance was
prepared from highly porous zeolite-type MOF (ZIF-8) and
furfuryl alcohol.14 Thus, far, several MOFs, such as MOF-5, Al-
PCP, and ZIF-8, have been demonstrated as promising
precursors, yielding highly nanoporous carbons showing
excellent properties in gas adsorption, electrochemical
capacitance, sensing, and catalysis. Research on the preparation
of carbons from CPs has opened up an exciting avenue toward
Received: March 12, 2014
Published: August 1, 2014
© 2014 American Chemical Society
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Magna-IR750 FTIR spectrometer as KBr pellets, scanning from 4000
to 400 cm−1 at room temperature. Thermogravimetric (TG) analysis
was carried out at a constant heating rate of 10 °C min−1 from room
temperature to 800 °C, using a TA-50 thermal analyzer. Photo-
luminescence (PL) excitation and emission spectra were measured
using a fluorospectrometer (FLUOROLOG-3-TAU). The upconver-
sion emission spectra were measured under 980 nm excitation using a
semiconductor laser (power set at 1000 mW) diode in a darkened
room. The photoluminescence measurements were measured in the
solid state. Before measurement, the samples were prepared by
pressing the samples in a stainless sample well.
Electrochemical Measurements. All electrochemical experi-
ments were carried out on a CHI660a electrochemical workstation
(Shanghai Chenhua Instruments Co.) at room temperature. The
working electrode was prepared by loading a slurry containing 90 wt %
of multihole carbon microspheres (about 2 mg), and 10 wt % of
poly(vinylidene fluoride) (PVDF) (in N-methylpyrrolidone). After the
electrode materials were loaded, the working electrode was pressed
and dried under vacuum at 80 °C for 12 h. In a three-electrode system,
the nickel foam loaded above as the working electrode was investigated
with a Pt counter electrode and Ag/AgCl reference electrode in 6 M
KOH solution as the electrolyte. CV curves were obtained in the
potential range of 0−0.6 V vs Ag/AgCl by varying the scan rate from 5
to 200 mV s−1. Charge/discharge measurements were carried out with
a constant current at 0.5−10.0 A g−1 with a potential window of 0−0.6
V.
functional carbon-based nanoporous materials. Owing to the
unique nature of nanoporous structures, nanoporous carbon is
used in many research fields, such as contamination removal,
gas storage, supercapacitors, and carriers for drug delivery
systems. For example, mesoporous carbon has good electrical
properties as an electrode material due to its high specific
surface area, plentiful mesoporous structure, and appropriate
pore size for rapid mass transfer as well as ion diffusion.15 The
nitrogen-doped porous carbon prepared directly by nitrogen-
rich precursors ensures an improved adsorption/absorption for
acidic gases such as CO2.16 In addition, aligned carbon
nanotube and polymer composites are a new class of
multifunctional chemical vapor sensors with low power
consumption, high sensitivity, good selectivity, and excellent
environmental stability, as detailed in a previous report.17
Because of their excellent properties and broad application,
carbon and carbon composites have been intensely investigated.
The preparation of carbon-based materials with excellent
properties via a facile process has been pursued by many
researchers.
Organic ligands in the CPs are abundant in carbon, which is
the optimal precursor of carbon composite materials. In
addition, the structures and components of the ligands in the
CPs are readily tuned. Therefore, CPs is also a good precursor
for carbon composite and carbon-related materials. On the
basis of the successful preparation by our group of uniform
europium-based CP submicrospheres via microwave heating,10
in this work, other RE-based CP submicrospheres have been
prepared by the same method. Subsequently, the as-prepared
CP submicrospheres were calcined under atmospheres of air,
N2, and NH3. A series of carbon and carbon-based materials
with interesting structures and compositions were obtained.
The as-prepared products were well characterized and analyzed,
and their properties were also measured.
RESULTS AND DISCUSSION
■
Characterization and Structure of the RE-Based CPs.
Figure 1 shows the SEM images of the products prepared after
5 min of microwave heating. It clearly shows that the product is
composed of uniform microspheres with diameters ranging
from 100 to 400 nm, which is similar to those for the europium-
EXPERIMENTAL SECTIONS
■
Preparation. The RE-based CP submicrospheres were prepared by
the same method that we reported recently.10 The detailed process is
as follows: 32 mL of a DMF solution containing 0.1 mmol of
RE(NO3)3·nH2O and 0.3 mmol of pyridine-2,5-dicarboxylic acid was
poured into a 100 mL round-bottom flask. The resulting mixture was
stirred for 20 min at room temperature and then was heated in a
microwave oven for 5 min. The microwave power and frequency were
set at 150 W and 2450 MHz. The microwave oven used in our
experiments is a modified household microwave oven equipped with a
refluxing apparatus. After irradiation, the flask was naturally cooled to
room temperature. The precipitates were centrifuged, washed several
times with ethanol, and then dried under vacuum at 60 °C overnight.
After that, the as-obtained products were thermally treated under
different conditions at 550 °C for 4 h.
Characterization. Elemental analysis of C, H, and RE in the solid
samples was carried out on a VarioEL instrument (Elementar
Analysensysteme GmbH) and an inductive coupled plasma (ICP)
atomic emission spectrometer (POEMS, TJA), respectively. X-ray
photoelectron spectroscopy (XPS) was performed using a PerkinElm-
er RBD upgraded PHI-5000C ESCA system with monochromatic Mg
Kα excitation, and a charge neutralizer was used to investigate the
surface electronic states of the samples. Powder X-ray diffraction
(XRD) patterns were obtained with a Rigaku/Max-3A X-ray
diffractometer with Cu Kα radiation (λ 1.54178 Å); the operation
voltage and current were maintained at 40 kV and 40 mA, respectively.
Field emission scanning electron microscopy (FE-SEM) images were
obtained with a JEOL JSM-6330F instrument operated at a beam
energy of 15.0 kV. Transmission electron microscopy (TEM) images
were obtained with a JEOL-2010 TEM instrument at an accelerating
voltage of 200 kV. Infrared (IR) spectra were obtained with on a
Figure 1. SEM images of RE-CPs obtained after microwave irradiation
at 150 W for 5 min.
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based CP submicrospheres reported by us.10 As is well known,
La, Gd, and Y can be denoted as light rare earth, middle rare
earth, and heavy rare earth elements, respectively. In this work,
the products built from La, Gd, and Y were selected as the
representative samples and XRD, IR, TG-DTA, and XPS were
used to investigate the composition and structure of the
products.
1014), and cubic Y2O3 (JCPDS No. 43-0661), respectively.
Figure 3 shows the SEM and TEM images of the rare earth
Figure S1 (Supporting Information) shows the representative
XRD pattern of the as-prepared La-based, Gd-based, and Y-
based products, clearly indicating that they are all amorphous
and noncrystalline.2 The coordination of the carboxylate groups
to La3+, Gd3+, and Y3+ ions was confirmed by the IR spectra
(Figure S2, Supporting Information), as evidenced by a shift in
the CO stretching frequency to 1654 cm−1 (La-CPs), 1618
cm−1 (Gd-CPs), and 1593 cm−1 (Y-CPs) from 1736 cm−1 for
the uncoordinated building block pyridine-2,5-dicarboxylate.10
The TG curves taken in air exhibit two main weight losses
(Figure S3, Supporting Information). The first weight loss of
about 12% before 150 °C corresponds to the loss of water
molecules in the product. The second weight loss from 150 to
800 °C is due to the burnout of organic molecules. The survey
XPS spectrum reveals that the microspheres are composed of
RE, C, N, and O (Figure S4, Supporting Information). From
the elemental analysis for La-CPs, Gd-CPs, and Y-CPs, the
following data were found: La 29.08%, Gd 32.83%, Y 21.16%;
C 27.90%, 23.73%, and 28.68%, respectively; H 2.56%, 2.94%,
and 4.90%, respectively; N 4.94%, 4.11%, and 4.67%,
respectively. On the basis of the above considerations, the
products can be presumed to have the compositions
La2(C7H3NO4)3·nH2O, Gd2(C7H3NO4)3·nH2O, and
Y2(C7H3NO4)3·nH2O. On the basis of the above analysis, it
is safe to draw a conclusion that RE-based CP microspheres
have been prepared.
Figure 3. SEM images (a−c) and TEM images (d−f) of the products
obtained by calcining the RE-CP microspheres at 550 °C for 4 h: (a,
d) La-CP; (b, e) Gd-CP; (c, f) Y-CP.
oxide obtained by calcining the RE-CP microspheres at 550 °C
for 4 h. It can be seen from Figure 3 that the oxides still
maintain spherical structures, indicating that the calcination will
not change the morphology of the product. However, their
particle size decreased to some extent. Interestingly, by fine
observation, the Y2O3 spheres are obviously porous.
Calcination under an N2 Atmosphere. After Calcination
under an N2 Atmosphere. Figure 4 shows the SEM and TEM
Characterization and Properties of the Product
Obtained after Calcination under Different Conditions.
Calcination in the Air. As discussed before, CPs are suitable
precursors for the preparation of nanoamaterials with different
compositions by finely controlling the processing conditions. In
this work, La-CPs, Gd-CPs, and Y-CPs were selected and
calcined under different conditions. Figure 2 is the XRD pattern
of the products obtained by calcining the RE-CPs microspheres
at 550 °C for 4 h in air. The X-ray diffraction data clearly show
that the products are RE2O3. All diffraction peaks of the XRD
patterns for each sample can be readily indexed as hexagonal
La2O3 (JCPDS No. 05-0602), cubic Gd2O3 (JCPDS No. 43-
Figure 4. SEM images (a−c) and TEM images (d−f) of the products
obtained by calcination of the RE-CP microspheres at 550 °C for 4 h
under an N2 atmosphere: (a, d) La-CP; (b, e) Gd-CP; (c, f) Y-CP.
images of the products after calcination of the RE-CPs under an
N2 atmosphere at 550 °C for 4 h. It clearly demonstrates that
the appearance of the products is still spherelike. Elemental
analysis results indicate that N (0.87%) and C (6.78%) of the
La-CPs are preserved. However, for Gd-based and Y-based
CPs, N is 0.14% and 0.23% and C is 4.46% and 5.31%,
Figure 2. XRD patterns of the products prepared after calcination of
the RE-CPs in the air: (a) La-CP; (b) Gd-CP; (c) Y-CP.
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respectively. These results suggest that parts of N and C are still
in the product after calcination of RE-CPs under an N2
atmosphere.
XRD results (Figure 5) show that the Gd-CP transforms into
Gd2O3 (cubic, JCPDS No. 43-1014) after calcination under an
Figure 6. SEM (a, c) and TEM images (b, d) of the products obtained
by calcination of La-CP spheres under an N2 atmosphere after acid
treatment: (a, b) samples treated for 1 day; (c, d) samples treated for 3
days.
product with 6 mol L−1 NaOH to remove the LaN, and then
Nessler’s reagent was added to the centrifuged supernatant to
test NH4+ (Figure 7c). The transparent supernatant generated a
tan precipitate immediately, which further proved the existence
of LaN. After alkali treatment, the product was a mixture of
La2O3 and La(OH)3 (Figure 7d).
Raman spectroscopy was used for the characterization of the
electronic structure of carbon products, which were treated
with acid for 3 days. A change in Raman band intensity and
shifts provide information on the nature of C−C bonds and
defects. The Raman spectra in Figure 7b show the characteristic
D and G bands at 1344 and 1597 cm−1 found in the composite.
The D band is a common feature for sp3 defects in carbon, and
the G band provides information on in-plane vibrations of sp2-
bonded carbons. The intensity ratio of the D band to the G
band usually reflects the order of defects in graphite. The
calculated ID/IG ratio of the sample was 1.22, and it has been
previously reported that an increase in the ID/IG ratio during
solvothermal processing or chemical reduction is also due to
the fragmentation of sp2 domains in comparison with pure
graphite.18,19 Therefore, the product obtained after calcining
La-CPs under N2 was a mixture of La2O3, LaN, and carbon.
Under an NH3 Atmosphere. After Calcination under NH3.
As is well known, calcining active metal compounds under an
NH3 atmosphere always leads to the formation of nitrides. To
the best of our knowledge, calcining MOFs under an NH3
atmosphere has been rarely studied. In this case, RE-CPs have
been calcined under an NH3 atmosphere at 550 °C for 4 h.
Figure 8 gives the SEM and TEM images of the products that
were RE-CPs after calcination under an NH3 atmosphere at
550 °C for 4 h. It can be seen that the appearance of the
products was retained. We found that the percentages of N
were significantly improved: for La the sample was 11.58% N
(0.87% under an N2 atmosphere), the Gd sample was 2.35% N
(0.14% under an N2 atmosphere), and the Y sample was 0.39%
N (0.23% under an N2 atmosphere). At the same time, the
percentage of C was improved as well: for La the sample was
12.86% C (6.78% under an N2 atmosphere), the Gd sample
was 14.00% C (4.46% under an N2 atmosphere), and the Y
sample was 9.92% C (5.31% under an N2 atmosphere). On the
basis of the above analysis, we conjectured that Gd-nitride
might form after calcination under the the NH3 atmosphere.
Figure 9 shows the XRD patterns of the products after
calcination of La-CP, Gd-CP, and Y-CP under an NH3
atmosphere. Unexpectedly, La2O2CN2 (tetragonal, JCPDS
No. 49-1162) was obtained. The synthesis and crystal structure
Figure 5. XRD patterns of the products obtained after calcination of
the RE-CP microspheres at 550 °C for 4 h under an N2 atmosphere:
(a) La-CP; (b) Gd-CP; (c) Y-CP.
N2 atmosphere. Furthermore, the peaks are sharp, indicating
that the product is quite crystalline. For Y-CP, Y2O3 (cubic,
JCPDS No. 43-0661) was produced. However, the peaks are
broadened to some extent, indicating that the product is not
very crystalline. To optimize the calcination protocol, we
calcined the Gd-CP at 450, 650, and 750 °C under an N2
atmosphere for 4 h, respectively. From the XRD patterns (as
shown in Figure S5, Supporting Information), the product
obtained after calcination at 450 °C is not well crystallized and
the product obtained at 650 °C is similar to the product
prepared at 550 °C. When the calcination temperature is
increased to 750 °C, a mixture of cubic and monoclinic Gd2O3
is produced. We can draw the conclusion that the calcination
temperature has a great effect on the phase and crystallinitiy of
the final product. For La-CP, the major phase is La2O3.
However, some uncertain impurity peaks are found. As the
content of N was high in the product, we think the nitride
compound of La may have formed, which will be discussed
later.
After Acid Treatment. The La2O3 was then etched away
from the La sample with 6 mol L−1 HNO3. Figure 6a is the
TEM image of the sample treated for 1 day, and Figure 6b is
that for 3 days. The morphology of the samples was similar to
that of La-CP. However, the products were porous due to the
removal of the oxide.
The XRD patterns (Figure 7a) of the products obtained after
acid treatment indicated that only amorphous carbon was
found after 3 days. In addition, a composite of LaN (cubic,
JCPDS No. 15-0892) and amorphous carbon was obtained
after treatment for 1 day. LaN has attracted a great deal of
research because of its high hardness, brittleness, high melting
point, and superconducting transition temperature. This is also
interesting from an optical, electronic, magnetic, vibrational,
and mechanical properties point of view. We treated the
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Figure 7. (a) XRD patterns of the product obtained after acid treatment of the sample prepared by calcination of the La-CPs under an N2
atmosphere: (1) 1 day; (2) 3 days. (b) Raman spectra of the product obtained after acid treatment for 3 days excited with a UV laser at 514 nm. (c)
Photographs of the supernatant with alkali treatment before and after addition of Nessler’s reagent. (d) XRD pattern of the product obtained after
alkali treatment.
our experimental results, we found a method that was efficient
and had low energy consumption to obtain Ln2O2CN2. For Gd-
CP and Y-CP, the major phases are Gd2O3 and Y2O3,
respectively. However, some unknown peaks were also found,
which will be discussed later.
We treated the Gd sample with 6 mol L−1 HNO3 for 3 days
to remove Gd2O3. The XRD patterns in Figure 10 showed that
the product treated for 1 day was GdN (cubic, JCPDS No. 15-
0888) and the product treated for 3 days was carbon, similar to
the product obtained after calcining La-CP under an N2
atmosphere. The results showed that light rare earth elements
formed nitrides under an N2 atmosphere more readily than
heavy rare earth elements, and the middle rare earth elements
can also form nitrides under an NH3 atmosphere. However, it is
difficult for heavy rare earth elements to form nitrides. We also
treated the product of Gd-CP calcination under an NH3
atmosphere with 6 mol L−1 NaOH and Nessler’s reagent; the
result was consistent with La-CP calcination under an N2
atmosphere.
The FTIR spectra of the products that were calcined under
different atmospheres are given in Figure S6 (Supporting
Information). The peak at about 3500 cm−1 is attributed to the
stretching modes of OH groups associated with the water
molecules, and the band at 1500−1600 cm−1 belongs to the
hydroxyl deformation mode of the water molecules. The strong
peaks at 1394, 1384, and 1364 cm−1 (Figure S6a) are due to the
vibration mode of CO32− from CO2 absorbed on the surface of
the materials. The vibration mode of CN in La2O3 obtained
under an N2 atmosphere resulted in a peak at 1472 cm−1, but it
did not appear in the curves of the Gd and Y samples, testifying
that no nitride was formed. The existence of Gd-nitride in the
product obtained after calcination under NH3 can be proved by
the band at 1457 cm−1 in Figure S6c. There are two
Figure 8. SEM images (a−c) and TEM images (d−f) of the products
obtained by calcination of the RE-CP microspheres at 550 °C for 4 h
under an NH3 atmosphere: (a, d) La-CP; (b, e) Gd-CP; (c, f) Y-CP.
refined by the Rietveld method of La2O2CN2 have already been
reported.20 Usually, the synthesis required more than 900 °C
and 10 h under flowing ammonia. However, Ln2O2CN2 exhibits
both powerful nitridation and carburization abilities, and several
metal nitride and carbide nanoparticles can be easily obtained at
temperatures lower than those reported for the conventional
methods.21 In comparison to metal oxides, metal nitride and
carbide nanoparticles have superior properties in some respects,
such as excellent electronic characteristics, extreme corrosion
and wear resistance, and good catalytic properties. According to
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Figure 9. XRD patterns of the products obtained by calcination of the RE-CP microspheres at 550 °C for 4 h under an NH3 atmosphere: (a) La-CP;
(b) Gd-CP; (c) Y-CP.
Figure 10. XRD patterns of the Gd (a1) and Y (a2) samples obtained by calcination under an NH3 atmosphere after acid treatment: (a) 1 day for
both samples; (b) 3 days for the Gd sample.
characteristic peaks of La2O2CN2 at 2000 cm−1, which is the
stretching mode of CN, and at 1462 cm−1, which is the
vibration mode of CN (Figure S6c1). For the Y sample, the
main peaks still appear from 1500 to 1600 cm−1 after
calcination under N2 and NH3; the above results indicated
that the nitride was not generated. The peak at 1383 cm−1 is
the symmetric vibration of R-NO2 (Figure S6d), and the peak
at 1602 cm−1 is due to the vibration mode of CC (sp2),
which is the characteristic peak of graphite.
Chemical surface analysis of the product obtained after
calcining La-CP samples under N2 and after acid treatment was
performed using XPS, as shown in Figure S7 (Supporting
Information; the inset gives the spectrum of La 3d). From the
C 1s XPS spectrum of samples after acid treatment the peak at
the binding energy of 286.7 eV is assigned to C−O and C−O−
C, the signal at 288.5 eV is attributed to the O−CO oxygen-
containing carbonaceous band, and the deconvoluted peak
centered at a binding energy of 284.6 eV is assigned to the C−
C, CC, and C−H bonds (sp2). The XPS N 1s spectra can be
deconvoluted into two peaks centered at 400 and 398.4 eV; the
peak around 400 eV is assigned to the pyrrolic-N species,
whereas that at 398.4 eV is attributed to the presence of
pyridinic-N species.22
As an example of the applications of the RE-based
coordination polymers, 15 mol % Yb3+ and 5 mol % Er3+
codoped Gd-CPs were prepared, and then the Gd-CPs with
Yb3+ and Er3+ were calcined under atmospheres of air, N2, and
NH3. We tested the upconversion abilities of all products upon
excitation with a 980 nm laser. As can be seen in the spectrum
(Figure 11), the products showed a red single peak emission
band centered at λ 675 nm, corresponding to an energy-transfer
4
4
process from the excited state F9/2 to the ground state I15/2
.
As many −COOH and −NH2 groups are on the surface of the
CP spheres, they have good biocompatibility and potential
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and surface defects on the surface of materials. These increased
the instability of ions on the surfaces of the nanoparticles,
reduced the symmetry of the cells, and resulted in disorder of
the local area environment. These results degraded the crystal
field on the surface of the material. Moreover, the surface
defects acted as a path of nonradiative transition of the RE ions,
which made the lifetime of RE ions shorter.25 For the above
reasons, the samples obtained in N2 and NH3 have low
upconversion luminescence efficiency. We also tested the
upconversion abilities of the products which were obtained
from the Gd-CP with Yb3+ and Er3+ calcined at 750 °C under
an N2 atmosphere. As can be seen in the spectrum (Figure S8,
Supporting Information), the peak emission band was seriously
splitt, which can be ascribed to the fact that the calcined
product has two different kinds of crystal phases. On the basis
of all the results above, a higher calcination temperature did not
perfect the products or their properties.
Up to now, carbon electrodes have played important roles in
supercapacitors on account of their good electric conductivity,
long cycle stability, and stable physicochemical properties. The
carbon materials store energy through charge accumulation at
the electrode/electrolyte interfaces, which allows very fast
energy uptake and delivery and therefore a better power
performance. The porous carbon submicrospheres have good
conductivity and keep the porous structure of the original
polymers, providing a short movement path for both electrolyte
ions and electrons during the electrochemical process.
Therefore, the obtained carbon materials may have great
potential in energy storage. The porous carbon microspheres
(obtained after acid treatment of the product produced after
calcination of the La-CP under an N2 atmosphere) were further
applied as electrode materials for supercapacitors (Figure 12).
The electrochemical properties of multihole carbon micro-
Figure 11. Upconversion luminescence spectrum of Gd-CP with Yb3+
and Er3+ and the products of Gd-CP with Yb3+ and Er3+ calcined under
different atmospheres upon excitation with a 980 nm laser: (1) Gd-CP
with Yb3+ and Er3+; (2) calcination in air; (3) calcination under an N2
atmosphere; (4) calcination under an NH3 atmosphere.
bioapplications.23 From the spectrum we can see that the signal
of the sample calcined in air was the strongest, the precursor
was secondary, and the signals of the samples calcined under N2
and NH3 atmospheres were very weak. Although the signal of
Gd-CPs with Yb3+ and Er3+ was stronger than those for the
samples calcined in air and under N2 (Figure 11(3)), it was
much weaker in comparison with that of the sample calcined in
the air (Figure 11(4)). The obviously stronger upconversion
efficiency of the product calcined in air in comparison to that of
Gd-CP was due to crystalline materials having stronger crystal
field strength in comparison to materials having low
crystallinity and amorphous materials. Previous research
showed that the the stronger the crystal field strength, the
higher the upconversion efficiency.24 After calcination under N2
and NH3, the obtained products contained carbon and GdN,
and these components led to an amount of lattice distortion
Figure 12. (a) Representative cyclic voltammetric (CV) curves of a multihole carbon microsphere electrode. (b) Calculated specific capacitances
from the CV curves at different scan rates. (c) Charge/discharge curves of the multihole carbon microsphere electrode at different constant current
densities. (d) Cycle life of the multihole carbon microsphere electrode at a charge/discharge current density of 1 A g−1 for 1000 cycles.
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Inorganic Chemistry
Article
spheres were investigated by using cyclic voltammogram (CV)
and galvanostatic charge/discharge measurements within the
potential window of 0−0.6 V in 6 M KOH aqueous solution
with a three-electrode system. The CV curves measured at
potential sweep rates of 5−200 mV s−1 are shown in Figure
12a. The specific capacitances of multihole carbon micro-
spheres obtained from the CV curves are calculated by the
equation C = (∫ I dv)/(υmV), where C is the specific
capacitance (F g−1), I is the current (A), V is the potential
window (V), υ is the scan rate (mV s−1), and m is the mass of
the sample used for the electrochemical test (g).26 The
calculated specific capacitances at different scan rates are given
in Figure 12b. The multihole carbon microspheres show
specific capacitances of 220.8 and 117.7 F g−1 at scan rates of 5
and 100 mV s−1, respectively. The charge/discharge behavior of
the porous carbon microsphere electrode was also tested at
current densities from 0.5 to 10 A g−1. The observation of
nearly symmetric potential−time curves at all current densities
implies low polarization of the unique electrode.27 The specific
capacitances of multihole carbon microspheres can be
calculated from the discharge curves by the equation C =
(I Δt)/(m ΔV),28 where I is the constant discharge current (A),
Δt is the discharging time (s), and ΔV is the discharge voltage.
The specific capacitances of porous carbon microspheres from
the discharge curve are calculated to be 307.7 and 516.7 F g−1
at current densities of 1 and 0.5 A g−1, respectively. The specific
capacitance of porous carbon obtained at a current density of 1
A g−1 is comparable to those of N-doped hollow graphitic
carbon spheres (260 F g−1, 2 M H2SO4 electrolyte)29 and N-
doped carbon fibers (202.0 F g−1, 6 M KOH electrolyte)30 and
is higher than that of many other carbon-based materials such
as Co3O4/r-GO composites (163.8 F g−1, 6 M KOH
electrolyte).31 The electrochemical performance of porous
carbon can be explained by its porous structure and the
existence of nitro groups. The porous structure expedites the
transport of ions and electrons between the electrodes due to it
possessing high surface area and high porosity.32 In addition,
carbon materials containing some nitrogen groups can provide
pseudocapacitance and enhance the surface wettability by the
electrolyte, as well as ensure a complete utilization of the
exposed surface for charge storage.33 In addition, pyridinic
nitrogen, pyrrolic nitrogen, and quinone oxygen were
confirmed to have the most pronounced influence on the
capacitance due to their pseudocapacitive contributions.34
AUTHOR INFORMATION
■
Corresponding Authors
*S.Z.: tel, +86 791 88120386; fax, +86 791 88120386; e-mail,
slzhong@jxnu.edu.cn.
*L.W.: e-mail, wangleifly2006@126.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.Z. acknowledges the support of projects from the National
Natural Science Foundation of China (Nos. 21201089,
21261010, 61201104), Jiangxi Provincial Department of
Science and Technology (No. 2010BJB01100), and Jiangxi
Provincial Education Department (Nos. GJJ11382,
KJLD13021).
REFERENCES
■
(1) (a) Della Rocca, J.; Liu, D. M.; Lin, W. B. Acc. Chem. Res. 2011,
44, 957−968. (b) Carne, A.; Carbonell, C.; Imaz, I.; Maspoch, D.
Chem. Soc. Rev. 2011, 40, 291−305. (c) Spokoyny, A. M.; Kim, D.;
Sumrein, A.; Mirkin, C. A. Chem. Soc. Rev. 2009, 38, 1218−1227.
(2) Oh, M.; Mirkin, C. A. Nature 2005, 438, 651−654.
(3) Pang, M.; Cairns, A. J.; Liu, Y.; Belmabkhout, Y.; Zeng, H. C.;
Eddaoudi, M. J. Am. Chem. Soc. 2012, 134, 13176−13179.
(4) Junggeburth, S. C.; Diehl, L.; Werner, S.; Duppel, V.; Sigle, W.;
Lotsch, B. V. J. Am. Chem. Soc. 2013, 135, 6157−6164.
(5) Zhong, S. L.; Xu, R.; Zhang, L. F.; Qu, W. G.; Gao, G. Q.; Wu, X.
L.; Xu, A. W. J. Mater. Chem. 2011, 21, 16574−16580.
(6) (a) Leonelli, C.; Mason, T. Chem. Eng. Process. 2010, 49, 885−
900. (b) Dallinger, D.; Kappe, C. O. Chem. Rev. 2007, 107, 2563−
2591. (c) Polshettiwar, V.; Varma, R. S. Chem. Soc. Rev. 2008, 37,
1456−1557. (d) Baghbanzadeh, M.; Carbone, L.; Cozzoli, P. D.;
Kappe, C. O. Angew. Chem., Int. Ed. 2011, 50, 2−50. (e) Muthuswamy,
E.; Iskandar, A. S.; Amador, M. M.; Kauzlarich, S. M. Chem. Mater.
2013, 25, 1416−1422.
(7) Jeon, Y. M.; Armatas, G. S.; Heo, J.; Kanatzidis, M. G.; Mirkin, C.
A. Adv. Mater. 2008, 20, 2105−2110.
(8) Ni, Z.; Masel, R. I. J. Am. Chem. Soc. 2006, 128, 12394−12395.
(9) Umemura, A.; Diriing, S.; Furukawa, S.; Uehara, H.; Tsuruoka,
T.; Kitagawa, S. J. Am. Chem. Soc. 2011, 133, 15506−15513.
(10) Zhao, D.; Wang, L.; Li, Y.; Zhang, L.; Lv, Y.; Zhong, S. Inorg.
Chem. Commun. 2012, 20, 97−100.
(11) Masoomi, M. Y.; Morsali, A. Coord. Chem. Rev. 2012, 256,
2921−2943.
(12) Chaikittisilp, W.; Ariga, K.; Yamauchi, Y. J. Mater. Chem. A
2013, 1, 14−19.
CONCLUSIONS
(13) Hu, M.; Reboul, J.; Furukawa, S.; Torad, N. L.; Ji, Q.; Srinivasu,
P.; Ariga, K.; Kitagawa, S.; Yamauchi, Y. J. Am. Chem. Soc. 2012, 134,
2864−2867.
■
In conclusion, RE-based CP microspheres have been prepared
by a facile microwave method, and they were calcined under
different atmospheres. RE2O3, REN/RE2O3/C, REN/C,
RE2O2CN, and carbon submicrospheres can be obtained after
calcination and posttreatment. Results show that light rare earth
element based CPs easily form REN on calcination under N2
and NH3. This method may be extended to the preparation of
various composite materials with excellent properties via a facile
process. There is still much room for development.
(14) Jiang, H. L.; Liu, B.; Lan, Y. Q.; Kuratani, K.; Akita, T.;
Shioyama, H.; Zong, F.; Xu, Q. J. Am. Chem. Soc. 2011, 133, 11854−
11857.
(15) Yuan, D. S.; Chen, J. X.; Tan, S. X.; Xia, N. N.; Liu, Y. L.
Electrochem. Commun. 2009, 11, 1191−1194.
(16) (a) Jurgens, B.; Irran, E.; Senker, J.; Kroll, P.; Muller, H.;
̈
̈
Schnick, W. J. Am. Chem. Soc. 2003, 125, 10288−10300. (b) Hulicova,
D.; Yamashita, J.; Soneda, Y.; Hatori, H.; Kodama, M. Chem. Mater.
2005, 17, 1241−1247.
(17) Wei, C.; Dai, L. M.; Roy, A.; Tolle, T. B. J. Am. Chem. Soc. 2006,
128, 1412−1413.
ASSOCIATED CONTENT
* Supporting Information
■
S
(18) Shen, J. F.; Yan, B.; Shi, M.; Ma, H. W.; Li, N.; Ye, M. X. J.
Mater. Chem. 2011, 21, 3415−3421.
Figures giving detailed characterization data for the products
obtained in the form of XRD spectra, FTIR spectra, XPS
spectra, TG-DTG curves, and an upconversion luminescence
spectrum. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) Zhou, K. F.; Zhu, Y. H.; Yang, X. L.; Jiang, X.; Li, C. Z. New J.
Chem. 2011, 35, 353−359.
(20) Hashimoto, Y.; Takahashi, M.; Kikkawa, S.; Kanamaru, F. J. Solid
State Chem. 1995, 114, 592−594.
8285
dx.doi.org/10.1021/ic5005769 | Inorg. Chem. 2014, 53, 8278−8286

Inorganic Chemistry
Article
(21) Lei, M.; Zhao, H. Z.; Yang, H.; Li, P. G.; Tang, H. L.; Song, B.;
Tang, W. H. Diamond Relat. Mater. 2007, 16, 1974−1981.
́
(22) Casanovas, J.; Ricart, J. M.; Rubio, J.; Illas, F.; Jimenez-Mateos, J.
M. J. Am. Chem. Soc. 1996, 118, 8071−8076.
(23) (a) Zhong, S.; Ji, Y.; Xie, Q.; Wang, L.; Li, Y.; Jeong, J. H. Mater.
Lett. 2013, 102−103, 19−21. (b) Zhou, J.; Liu, Z.; Li, F. Chem. Soc.
Rev. 2012, 40, 1323−1349.
(24) Yi, G.; Lu, H. C.; Zhao, S. Y.; Ge, Y.; Yang, W. J.; Chen, D. P.;
Guo, L. H. Nano Lett. 2004, 4, 2191−2196.
(25) Peng, H. S.; Song, H. W.; Chen, B. J.; Lu, S. Z.; Huang, S. H.
Chem. Phys. Lett. 2003, 370, 485−489.
(26) Wu, X. L.; Wen, T.; Guo, H. L.; Yang, S. B.; Wang, X. K.; Xu, A.
W. ACS Nano 2013, 7, 3589−3597.
(27) Yuan, C. Z.; Li, J. Y.; Hou, L. R.; Zhang, X. G.; Shen, L. F.; Lou,
X. W. Adv. Funct. Mater. 2012, 22, 4592−4597.
(28) Lei, Z. B.; Shi, F. F.; Lu, L. ACS Appl. Mater. Interfaces 2012, 4,
1058−1064.
(29) Ma, F. W.; Zhao, H.; Sun, L. P.; Li, Q.; Huo, L. H.; Xia, T.; Gao,
S.; Pang, G. S.; Shi, Z.; Feng, S. H. J. Mater. Chem. 2012, 22, 13464−
13468.
(30) Chen, L. F.; Zhang, X. D.; Liang, H. W.; Kong, M.; Guan, Q. F.;
Chen, P.; Wu, Z. Y.; Yu, S. H. ACS Nano 2012, 6, 7092−7102.
(31) Zhou, W.; Liu, J.; Chen, T.; Tan, K. S.; Jia, X.; Luo, Z.; Cong,
C.; Yang, H.; Li, C. M.; Yu, T. Phys. Chem. Chem. Phys. 2011, 13, 14.
(32) Chen, Z.; Wen, J.; Yan, C. Z.; Rice, L.; Sohn, H.; Shen, M. Q.;
Cai, M.; Dunn, B.; Lu, Y. F. Adv. Energy Mater. 2011, 1, 551−556.
(33) Li, W. R.; Chen, D. H.; Li, Z.; Shi, Y. F.; Wan, Y.; Huang, J. J.;
Yang, J. J.; Zhao, D. Y.; Jiang, Z. Y. Electrochem. Commun. 2007, 9,
569−573.
(34) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J.
Adv. Funct. Mater. 2009, 19, 438−447.
8286
dx.doi.org/10.1021/ic5005769 | Inorg. Chem. 2014, 53, 8278−8286