Polyoxometalate Cluster-Incorporated Metal-Organic Framework Hierarchical Nanotubes

Article abstract of DOI:10.1002/smll.201503695

A one-pot solvothermal strategy was designed to synthesize two kinds of bi-metallic hierarchical Fe/Co-BTC nanotubes modulated by Keggin-type POMs. Iron(iii) acetylacetonate and cobalt(iii) acetylacetonate were dissolved in a dimethylformamide (DMF)/decanol mixture. Then NaPW₁₂, 3.0 mL of water and 0.35 mL of 1,3,5-benzenetricarboxylic acid solution were added successively into the DMF/decanol solution. After stirring for several minutes, the starting solution was transferred into a 40-mL Teflon-lined stainless-steel autoclave. The sealed vessel was then heated at 100°C for 3 h before it was cooled down to room temperature. The electrochemical performance of the as-prepared products were measured using a two electrode cell. It was found that Tube1 exhibited higher catalytic properties than either MOF1 or P1 . In other words, the properties of the composite were better than those of the individual components, thus confirming the synergistic effect of the composite. Interestingly, the activity of Tube2 was almost the same as that of T1 , which may be attributed to its high specific surface area.

Full text of DOI:10.1002/smll.201503695

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Metal-Organic Composites
Polyoxometalate Cluster-Incorporated Metal-Organic
Framework Hierarchical Nanotubes
Xiaobin Xu, Shuangming Chen, Yifeng Chen, Hongyu Sun, Li Song, W e i H e ,
and Xun Wang*
Metal–organic frameworks (MOFs) are a class of porous agents[11] through host–guest supramolecular interactions.[12]
materials made from the co-assembly of metal ions with POMs may alter the equilibrium between metal cations and
organic linkers,[1] which have been widely used in gas organic ligands and affect the reactants, thus manipulating
storage,[2] separation,[3] and catalysis fields.[4] In contrast to the nucleation and growth processes of MOFs and therefore
bulk crystals, MOF nanocrystals can further enhance the the resulting structure and shape. Hence, controlled synthesis
inherent properties and fulfill specific needs,[5] such as for of MOF nanocrystals may be achieved by using POMs to
thin-film devices,[6] or drug-delivery systems.[7] Particularly, modulate the nucleation and growth processes.
the construction of special nanoMOFs, for example, hollow
Herein we utilized iron and cobalt ion as metallic nodes,
nanostructures, has gradually drawn more attention because 1,3,5-benzenetricarboxylic acid (H3BTC) as the organic
of their intriguing properties, such as, enlarged specific area, linker, and a N,N-dimethylformamide (DMF)/decanol mix-
low density, and cost reduction.[8] However, the synthesis of ture as the solvent. We specifically selected sodium phospho-
hollow MOF nanostructures, especially with high complexity, tungstate (Keggin-type POM, abbreviated as NaPW12) to
still remains a great challenge. Here for the first time we modulate the synthesis of MOF crystals. As a consequence,
report the synthesis of hierarchical MOF nanotubes by using uniform hierarchical Fe/Co-BTC nanotubes could be pre-
polyoxometalates (POMs) as structural modulators.
pared using a simple solvothermal process.
POMs are a class of inorganic clusters with a rich struc-
If the volume ratio of DMF/decanol was 1:1, Fe/Co-BTC
tural versatility and well-defined physical and chemical prop- nanosheet-based nanotubes (tube-1, or abbreviated T1)
erties.[9] POM-based MOF materials have attracted extensive were synthesized. As shown in the scanning electron micro-
interest because of their functional integrated and synergic scopy (SEM; Figure 1a–c), transmission electron microscopy
effects.[10] In the process of building MOF/POM composites, (TEM; Figure 1d), and high-angle annular dark-field scanning
POMs always act as anionic templates/structure-directing TEM (HAADF-STEM; Figure 1e) images, uniform tubes
with hollow cavities were clearly observed (inner diameter:
ca. 72 nm). More interestingly, there were numerous flex-
X. B. Xu, Prof. X. Wang
ible and interleaving nanosheets with a thickness below
10 nm on the surfaces of the inner nanotubes. Elemental
mapping (Figure 1f) using energy-dispersive X-ray spec-
troscopy (EDX) demonstrated the even distribution of Fe,
Co, O, P, and W along the tubes. X-ray photoelectron spec-
troscopy (XPS; Figure S1, Supporting Information) also
revealed that the nanotubes were composed of Fe, Co, C,
O, P, W. Moreover, the W element remained its highest
valence state (VI),[13] endowing it with a strongly oxidizing
nature. The composition of the nanotubes was determined
by inductively coupled plasma optical emission spectrometry
(ICP-OES) and C, H, N elemental analysis, which con-
firmed a chemical formula of Fe60Co4PW12C72H168O210Na7.
Structural investigations were also carried out, initially by
employing X-ray powder diffraction (XRD). However, the
XRD pattern (Figure S2, Supporting Information) did not
give any valuable information. Fortunately, FT-IR analysis
(Figure 2a, Figure S3, Supporting Information) was a useful
tool in this case. The carboxylate stretching frequency red
shifted to 1615 cm−1 from 1722 cm−1 for the non-coordinated
H3BTC (Figure S3, Supporting Information), confirming the
coordination of the carboxylate groups to the metal ions[14]
and thus indicating that T1 formed a coordination polymer.
Key Lab of Organic Optoelectronics
and Molecular Engineering
Department of Chemistry
Tsinghua University
Beijing 100084, P. R. China
E-mail: wangxun@mail.tsinghua.edu.cn
S. M. Chen, Prof. L. Song
National Synchrotron Radiation Laboratory
Hefei Science Center CAS
University of Science and Technology of China
Hefei Anhui 230029, P. R. China
Y. F. Chen, Prof. W. He
School of Medicine and Tsinghua-Peking
Joint Centers for Life Science
Tsinghua University
Beijing 100084, P. R. China
H. Y. Sun
Beijing National Center for Electron Microscopy
School of Materials Science and Engineering
The State Key Laboratory of New Ceramics and Fine Processing
Key Laboratory of Advanced Materials (MOE)
Tsinghua University
Beijing 100084, P. R. China
DOI: 10.1002/smll.201503695
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Figure 1. a–c) SEM (scale bars 500 nm, 500 nm, and 100 nm, respectively), d) TEM (scale bar 100 nm), e) HAADF-STEM of T1 (scale bar 50 nm).
f) EDX mapping images of (e). g) nitrogen adsorption (squares) and desorption (circles) isotherms of T1 measured at 77 K. The inset shows the
corresponding pore-size distribution.
Moreover, the characteristic peaks of the Keggin-structure spectroscopy (XAS) was recorded at the XAFS beamline of
were clearly illustrated (Figure 2a), indicating that the POM the Shanghai synchrotron radiation facility (see Experimental
ions were retained in the final product and their structural Section). From the W L3-edge X-ray absorption near-edge
integrity was maintained. This result was perfectly consistent structure (XANES) spectrum (Figure 2b), it was found that
with the elemental analysis by ICP-OES as the measured the energy positions, the shape, and intensity of the absorp-
atomic ratio of P/W (i.e., 1:12) was equal to the calculated tion edges in T1 were almost the same as those of the refer-
value in the Keggin structure. Nonetheless, the connectivities ence compound (NaPW12 ). Moreover, the refined parameters
in the structure were still not totally clear.
of the extended X-ray absorption fine structure (EXAFS)
Therefore, in order to further gain insight into the spectra of the first coordination shell around the W atoms in
local structure and atomic environment, X-ray absorption T1 and NaPW12 (Table 1) also showed little difference. All of
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in the second coordination sphere of the
Fe atom, indicating that the Fe atoms were
not coordinated to the POM ions. Based
on the above results, many structural
assumptions could be unambiguously con-
firmed, such as that T1 is a kind of coordi-
nation polymer formed by Fe ions and the
organic trimesic acid (H3BTC), whereby
the POM ions were encapsulated in the
framework and there were no interac-
tions between the iron ions and the POM
anions.
The porosity is another important
aspect of MOF crystals. We therefore
analyzed the surface area and pore-size
distribution of T1 by Brunauer–Emmett–
Teller (BET) measurements. As shown in
Figure 1g, T1 exhibited a typical type-IV
isotherm, characteristic of mesoporous
materials. The pore-size distribution cal-
culated from the Barrett–Joyner–Halenda
(BJH) method showed that it contained
mesopores with diameters of approxi-
mately 3.83 nm. The total BET and Lang-
muir surface area of T1 were 131.4 and
94.5 m2 g−1, respectively.
Figure 2. a) Magnified FT-IR spectrum of T1. b) Normalized W L3-edge XANES spectra of T1,
T2, and the reference compound (NaPW12 ). c) Normalized Fe K-edge XANES spectra of T1, T2.
d) The k3-weighted Fourier transform Fe K-edge EXAFS spectra of T1. The experimental data
(dots) are presented along with best fits of the FT magnitude (solid line).
To further explore facile approaches
these results confirmed the integrity of the Keggin anions. On to fabricate hollow nanoMOFs with complex configuration,
the other hand, in the Fe K-edge XANES spectrum (Figure 2 the synthetic conditions were carefully altered and optimized.
c), the energy positions of the absorption edges revealed that Significantly, by changing the volume ratio of DMF/decanol
the oxidation state of iron was 3+,[15] which was in accord- to 7:1, another kind of well-defined nanotubes, namely,
ance with the XPS results. Furthermore, the shape and inten- uneven-surfaced hierarchical ones (marked as sample tube-2,
sity of the pre-edge feature indicated that the Fe ions were abbreviated T2), were achieved.
in an octahedral coordination.[15] The EXAFS analysis of the
The morphology, composition, and structure of T2 were
Fe atom (Figure 2d, Table 1) revealed that the Fe ions were similar to those of T1.As shown in the SEM (Figure 3a,c) and
coordinated by six oxygen atoms and also verified the pres- TEM (Figure 3b,d) images, the product consisted of uniform
ence of C atoms at distances of approximately 3.0 Å. This hierarchical tubes (total diameter: ca. 108 nm, wall thickness:
result suggested that the BTC linkers were connected to the ca. 48 nm) with hollow cavities and an uneven surface. Both
Fe ions. In addition, the best fit of the Fe-K edge indicated EDX mapping (Figure 3e) and XPS analysis (Figure S4, Sup-
two iron atoms in the second coordination shell, which meant porting Information) revealed that the samples were com-
that the Fe atoms were bonded to each other to form larger posed of Fe, Co, P, W, C, and O, however, the amount of POM
structure. However, there was no evidence of any W atoms in T2 was much lower than that in T1. Elemental analyses
Table 1. R e fi ned EXAFS structural parameters of Fe and W atoms; type of neighboring atom, bond distance R, coordination number N, and Debye–
Waller factor, σ2. Error bounds (accuracies) were estimated as R 1%; N 5%; σ2 1%.
Sample
Central atom
W
Neighbor type
R [Å]
1.70
1.89
2.25
1.71
1.86
2.24
1.98
3.05
3.30
3.41
N
1.5
3.0
1.5
1.1
2.8
1.9
6
σ
2 [10−3 Å2]
2.2
Reference compound
O
O
O
O
O
O
O
C
1.3
1.1
T1
T1
W
1.3
5.6
7.1
Fe
10.4
2.8
5
O
Fe
5
3.4
2
6.3
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Figure 3. a,c) SEM; b,d) TEM, e) HAADF-STEM (scale bar 50 nm) and EDX elemental mapping of T2. f) The k3-weighted Fourier transform Fe K-edge
EXAFS spectra of T2. The experimental data (dots) are presented along with a best fit of the FT magnitude (solid line). g) SEM, and h) TEM images
of FeBTC nanotubes.
confirmed a formula of Fe60Co6C100H169O154.5P0.31W3.84Na8.6
.
of the W atoms, it was found that the POM was intact.
Similarly, the structural information was obtained from Moreover, C atoms were observed in the second coordina-
FT-IR and XAS. Both the characteristic peaks of the Keggin tion shell of the Fe atoms without any evidence of W atoms
structure and a red shift of the carboxylate stretching fre- in the same coordination shell (Figure 3f, Table S1). Thus it
quency were identified by FT-IR (Figure S6, Supporting was confirmed that the structure of T2 was similar to T1: it
Information).[14] From the XANES spectrum (Figure 2b) was also a kind of coordination polymer and POM ions were
and EXAFS analysis (Table S1, Supporting Information) located in the pores of the framework. From the nitrogen
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adsorption–desorption isotherms and pore-size distribution
measurements (Figure S7, Supporting Information), a total
BET surface area of T2 of 373.2 m2 g−1 and mesopores with
diameters of approximately 3.82 nm were found.
acac− + H2O = Hacac +OH−
(2)
The acac− was subject to hydrolysis in water, forming
acetylacetone (Hacac) and the hydroxyl ion (OH−). The
We then tried to investigate the formation mechanisms released OH− greatly increased the rate of deprotonation
of T1 and T2. Because of its simpler structure, compared to of the organic ligand (H3BTC). After this, complexation
that of T1, we first studied the T2 system. Interestingly, in the between the metal ions and the organic ligands occurred,
absence of the cobalt precursor, while keeping the other syn- leading to the formation of the coordination polymers.
thetic conditions the same, the resultant morphology of the
To gain further insight into the evolution process, we car-
T2 compound showed little difference. Well-defined nano- ried out time-dependent experiments and monitored the
tubes could still be prepared (Figure 3g,h). However, in the solvothermal process using various characterization tech-
absence of POMs the reaction yielded irregular solid aggre- niques, for example, TEM and EDX analysis to verify its
gates (marked as MOF1, abbreviated M1, Figure S8, Sup- morphology and compositional change respectively. For the
porting Information). Therefore, it was speculated that the T2 system (Figure 4c), at a reaction time of 10 min, the prod-
determining factor in the formation process of the nanotubes ucts were uniform solid nanorods with a diameter of around
was the presence of POMs instead of the cobalt precursor.
52 nm (Figure 4c1). The EDX mapping of a single nanorod
Considering that POMs tend to act as an anionic tem- (Figure S10, Supporting Information) showed that the P, W,
plate/structure-directing agent through host–guest supra- Fe, and Co elements were distributed evenly. Prolonging
molecular interactions in the process of building MOF/ the reaction time to 20 min did not alter the morphology,
POM composites,[16] it seems possible that the POMs altered however, the diameter of the nanorods increased to around
the equilibrium between the metal cations and the organic 102 nm (Figure 4c2). Interestingly, after careful observation an
ligands and thus regulated the synthesis of the MOFs. Hence, image contrast could be found across the individual nanorods.
we tried to study the mechanism from the perspective of Moreover, the EDX line-scanning profile of the Fe element
the supramolecular interactions between the metal and the (Figure 4c3) showed a valley-like structure: the intensity at
POM ions. In order to simplify the problem, we investigated the edge was much higher than at the center. Therefore, the
the homometallic (Fe) system based on the fact that the final nanorod proved to have a core–shell structure. That is, the ini-
results of homo- (Fe) and hetero-(Fe/Co) metallic systems tial nanorod (the core, dark area) was surrounded by a uni-
showed little difference (Figure 3). Direct evidence of the form coating (light area). As time increased, the core dissolved
interaction was obtained from 31P NMR spectra of solutions gradually, leading to the formation of cavities (Figure 4c4).
prepared from Fe(NO3)3·9H2O and NaPW12. Upon adding When the core dissolved completely, only the outer layer was
FeIII cations, the 31P resonance of the POM anions showed a left, and, as such, nanotubes were obtained (Figure 4c5).
significant shift from −10.159 ppm to 40.736 ppm (Figure 4a),
Based on the above results, it could be concluded that the
indicating a change in the local environment of the central formation of nanotubes was caused by a dissolution of the
P atom in the POM anion. Such impact was the result of core. In the initial stage, due to complexation of the metal
the interaction with the FeIII cations. The peak broadening ions and POM anions, H3BTC was quickly deprotonated by
might be attributed to the intricate or random orientation of the released OH−. Driven by the reaction kinetics, the depro-
the FeIII cations around the surface of the POM anions.[17] tonated BTC rapidly coordinated to the metal ions, leading
Substantial evidence was further obtained from UV-vis spec- to the formation of coordination polymers with encapsulated
troscopy measurements in DMF/decanol (v/v = 1:1) solution. POM anions. Then a shell started to form on the surface of
[UV-vis measurements in DMF/decanol (v/v = 7:1) solution the core. Interestingly, the POM content in the shell was
are shown in Figure S9, Supporting Information].As shown in lower than that in the core, which was verified by a decrease
Figure 4b (line A), iron(iii) acetylacetonate [Fe(acac)3] pos- in the tungsten/iron molar ratio as the shell grew (Figure S11,
sessed strong electronic absorptions in the 300–600 nm spec- Supporting Information, and Figure 4c6). After that, the core
tral region. The individual bands near 350 and 440 nm could started to dissolve, maybe because the core is thermodynami-
be assigned to metal-to-ligand charge transfer (MLCT) and cally unstable in this synthetic system. In accordance with the
ligand-to-metal charge transfer (LMCT) transitions, respec- above analysis, the relative amount of W to Fe decreased fur-
tively.[18] After addition of the POM solution, the absorption ther during this process. Through formation and coalescence
intensities of the peaks around 350 and 440 nm significantly of vacancies, hollow voids were formed and the content of W
weakened, accompanied by a slight blue shift in the peaks reached its minimal value.
(Figure 4b, line B–D). This would indicate the occurrence of a
The evolution processes for the T1 structure were similar
ligand-exchange reaction (acac− in Fe(acac)3 was replaced by to that of T2: the pre-formed nanorods were coated with a
POM anions), which could further be verified by an increase uniform layer, which then dissolved gradually, leading to
in the pH after addition of the POM aqueous solution. The hollow nanorods. At the same time, nanosheets grew on the
possible reactions were as follows:
surface of these hollow nanorods. Finally, perfect nanosheet-
based hierarchical nanotubes were obtained. (See Supporting
Information for a detailed description.)
Considering that MOFs show promoting effect on catalytic
reactions involving gases,[19] we chose the removal of sulfur-
based harmful industrial chemicals under mild conditions with
Fe acac 3 + xPOMn−
=
(
)
(xn−3+y)−
−
(1)
⎡
⎣
⎤
Fe acac POM
)y (
+ 3− y acac
(
(
)
(
)
)
x
⎦
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Figure 4. a) 31P NMR spectra of a solution of A) NaPW12; B) Fe(NO3)3·9H2O and NaPW12. b) UV–vis spectra of A) DMF/decanol solution of Fe(acac)3;
B–D) solution (A) plus added 0.1 mL NaPW12 aqueous solution collected at different times. c) TEM (scale bars 200 nm) images of T2 collected at
different reaction times: c1) 10 min, c2) 20 min, c4) 1 h, c5) 2 h. c3) EDX line-scanning profiles for Fe and W along a single nanorod collected at
20 min. (Other elements are omitted for clarity.) c6) Change in percentage of W during 3 h of reaction time based on EDX results.
O2, which is a significant reaction in the chemical and petro- the oxidation of thiols to less toxic and less odorous disulfides
leum industries, as a laboratory test reaction to assess the was examined (Figure 5a). Reports have shown that the Fe,[20]
catalytic capabilities of these hierachical nanotubes. In detail, Co[21] centers, and POM unit[10b] can all act as active sites for
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Figure 5. a) Aerobic oxidation of thiophenol to diphenyldisulfide. Reaction conditions: thiophenol (1 mmol), catalyst (12.5 mg, preactivated at
140 °C for 3 h), acetonitrile (5 mL), 60 °C. b) Conversion (%) as a function of time in the oxidation of p-toluenethiol over T1 (A), T2 (B), M1 (C),
and P1 (D). c) Recyclability of T1 for the oxidation of p-toluenethiol using O2. d) Products and yields of isolated products from the oxidation of
p-toluenethiol to disulfides catalyzed by T1 for 12 h. e) Charge–discharge voltage profiles of T3 for the 1st, 2nd, 3rd, 5th, and 10th cycles in
the voltage range 0.01–3 V at a current rate of 200 mA g−1. f) Capacity and Coulombic efficiency versus cycle number of T3 at a current rate of
200 mA g−1
.
thiol oxidation. Figure 5b shows the oxygen-based oxidation of of both T1 and T2 were retained after the catalytic reaction, as
p-toluenethiol in acetonitrile solution catalyzed by T1, T2, M1, revealed by TEM (Figure S14, Supporting Information). After
or NaPW12 (marked as POM1, abbreviated P1) under identical catalysis, the T1 component was separated by centrifugation,
conditions. It was found that T1 exhibited higher catalytic prop- washed with acetonitrile, and then reused. The corresponding
erties than either M1 or P1. In other words, the properties of the product yields are shown in Figure 5c. It could be seen that the
composite were better than those of the individual components, catalyst showed an excellent reusability: the yield (87.5%) in
thus confirming the synergistic effect of the composite. Interest- the fifth cycle was only slightly lower than that in the first cycle
ingly, the activity of T2 was almost the same as that of T1, which (92.5%). Furthermore, substrates with different kinds [including
may be attributed to its high specific surface area (related to the electron-donating (1a-e) and -withdrawing ((1)f) groups] and
adsorption and storage captivity of oxygen). The morphologies position [including ortho- ((1)a), meta- ((1)b), and para- (1c-f)
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0.35 mL of 1,3,5-benzenetricarboxylic acid (H3BTC) solution were
added successively into the DMF/decanol solution. After stirring
for several minutes, the starting solution was transferred into a
40-mL Teflon-lined stainless-steel autoclave. The sealed vessel
was then heated at 100 °C for 3 h before it was cooled down to
room temperature. The products were separated via centrifugation
at 8000 rpm for 6 minutes and further purified with DMF and eth-
anol for several times. The same procedures were also applied to
the preparation of T2, unless otherwise noted.
Preparation of T2: Fe(acac)3 (80.4 mg), Co(acac)3 (10.1 mg), DMF/
decanol mixture (32 mL, v/v = 7:1), NaPW12 (110 mg), 2.6 mL of water
and 0.35 mL of H3BTC were used in the same procedure as for T1.
Preparation of T3: The as-formed T1 was thermally treated at
500 °C at a heating rate of 1 °C min−1 in N2, and the final product
of T3 was obtained.
Aerobic Oxidation of Thiophenol: Thiophenol (1 mmol), T1,
or T2 (12.5 mg, preactivated at 140 °C for 3 h) dispersed in ace-
tonitrile (5 mL) were added to an oven-dried Schlenk tube, and
the system was evacuated and refilled with O2 (balloon) for three
times. The reaction was assumed to start when the reactor was
introduced into an oil bath at 60 °C and vigorously stirred (ca.
1600 rpm). Aliquots were taken at specific times to determine the
component concentrations by gas chromatography−mass spec-
trometry (GCMS-QP 2010 SE, Shimadzu, Japan).
Electrochemical Measurements: The electrochemical perfor-
mance of the as-prepared products were measured using a two-
electrode cell. To prepare the working electrode, a mixture of the
active materials, carbon black, and polyvinylidene fluoride (PVDF) in
a weight ratio of 70:20:10 was ground in a mortar with N-methy1-
2-pyrrolidone (NMP) as the solvent to make a slurry. The mass
loading of the electrodes was about 2 mg cm−2. To assemble the
Li-ion batteries a Li foil was used as the counter electrode and a solu-
tion of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC)
(1:1 in volume) was used as the electrolyte. Cyclic voltammetry (CV)
was performed in the voltage range of 30.01 V (vs. Li/Li+) at a sweep
rate of 0.2 mV s−1. The charge/discharge curves and cycling capacity
were evaluated at the cut-off voltages of 0.01 and 3.0 V for a current
density of 200 mA g−1 for 50 cycles. For the high rate testing, the
discharge current was gradually increased from 200 mA g−1 to 1000,
2000, and 5000 mA g−1, then decreased again to 200 mA g−1 in a
step-by-step manner (10 cycles for each). All the charge/discharge
measurements were performed symmetrically at room temperature.
XAS analysis: i) Data collection: W L3-edge and Fe K-edge EXAFS
measurements were carried out at beamline 14W1 of the Shanghai
Synchrotron Radiation Facility (SSRF). The X-ray was monochroma-
tized by a double-crystal Si (111) monochromator. The storage ring
of SSRF was operated at 3.5 GeV with a current of 300 mA. ii) Data
analysis: the acquired EXAFS data were processed according to
standard procedures using the WinXAS3.1 program.[25] Theoret-
ical amplitudes and phase-shift functions were calculated using
FEFF8.2 code[26] by integrating the structural parameters of the
Fe(C2O4)(H2O)2 and NaH2(PW12O40)(H2O)12WO3 foil.
position] of their substituent groups could all be oxidized to
give the corresponding oxidation products (Figure 5d).
MOFs and their derivatives may serve as a platform for
clean and renewable energy, such as in hydrogen produc-
tion and storage, and in lithium-ion batteries (LIBs).[19] The
hierarchical tubular architecture has a high specific surface
area, which is beneficial for mass transport, increases the
active sites, and provides shorter Li+-ion diffusion channels,
thus making this architecture a promising candidate for LIBs
anode.[22] Hence, we carried out electrochemical measure-
ments to evaluate the performance of the thermally annealed
product of T1 (marked as T3). As the TEM image (Figure S15,
Supporting Information) shows, after thermal annealing in N2,
the tubular structure was mostly preserved. Combined with
EDX, Raman, and XRD results, T3 was shown to be a mixture
of carbon, iron/cobalt oxide, and tungstates (Figure S16–S18).
Figure 5e displays the voltage capacity profiles for the first,
second, third, fifth, and tenth cycles at a current density of a
200 mA g−1 over a potential range between 0.01 V and 3 V (vs.
Li+/Li). The first discharge and charge capacities were 859.1
and 568.5 mA h g−1, delivering an initial Coulombic efficiency
of 66.2%. From the third cycle onward, the T3 exhibited a
high Coulombic efficiency of around 95% (Figure 5f), indi-
cating the excellent reversible Li+ insertion/extraction of the
composite electrode. The cycling stability tested at a current of
200 mA g−1 is shown in Figure 5f. The irreversible capacity loss
during the initial discharge/charge process might be related to
lithium loss due to the formation of a solid electrolyte inter-
face (SEI) layer[23] and incomplete conversion reaction.[24]
From the third cycle onwards, the reversible capacity started
to increase gradually, which is common for metal-oxide
anode materials.[24] This phenomenon probably is related to
the gradual electrochemical activation process of the anode
electrodes, such as the wetting of the electrolyte into the
electrode,[23b] as well as interfacial Li storage.[23a,c] Afterwards,
the capacity was well retained. Even after 50 cycles, the elec-
trode could still deliver a capacity of 639.3 mA h g−1.
In summary, we have rationally designed a one-pot solvo-
thermal strategy to synthesize two kinds of bi-metallic hier-
archical Fe/Co-BTC nanotubes modulated by Keggin-type
POMs. Considering that both POMs and MOFs are two large
classes of species with various compositions and structures,
we expect our endeavor may further the study of constructing
novel MOF-based nanostructures. On the other hand, these
hierarchical tubes possess diverse intriguing properties, such
as low density, enlarged specific area and capacious pores,
which benefit the process of mass transport and the increase
of active sites; thus making these architectures promising can-
didates for various applications. As a proof-of-concept, these
nanotubes show an excellent catalytic performance for the
detoxification of sulfur compounds with O2 and a remarkable
cycling stability as anode materials for lithium-ion batteries.
Experimental Section
Preparation of T1: Iron(iii) acetylacetonate (Fe(acac)3,
80.4 mg) and cobalt(iii) acetylacetonate (Co(acac)3, 10.1 mg)
were dissolved in a dimethylformamide (DMF)/decanol mixture
(32 mL, v/v = 1:1). Then NaPW12 (110 mg), 3.0 mL of water and
Supporting Information
Supporting Information is available from the Wiley Online Library
or from the author.
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Acknowledgements
This work was supported by NSFC (21431003, 21521091,
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Received: December 7, 2015
Revised: February 19, 2016
Published online: April 21, 2016
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2990
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