Controlled Synthesis of Recyclable, Porous FMO/C@TiO2 Core-Shell Nanofibers with High Adsorption and Photocatalysis Properties for the Efficient Treatment of Dye Wastewater

Article abstract of DOI:10.1002/cplu.201500534

One-dimensional magnetite/manganese iron oxide modified by carbon coating and with TiO₂ nanoparticles into core-shell composite nanofibers (FMO/C@TiO₂) with porous structure were fabricated using organometallic compounds as templates

Full text of DOI:10.1002/cplu.201500534

DOI: 10.1002/cplu.201500534
Full Papers
Controlled Synthesis of Recyclable, Porous FMO/C@TiO2
Core–Shell Nanofibers with High Adsorption and
Photocatalysis Properties for the Efficient Treatment of
Dye Wastewater**
[a, b]
[a]
[a]
[a]
[a]
Dezhi Chen,
Lin Guo*
Caixia Liu, Suhua Chen,* Weisong Shen, Xubiao Luo,* and
[
a, b]
One-dimensional magnetite/manganese iron oxide modified
The porous nanostructures and photoresponse range of the
composite nanofibers can be controlled by varying the propor-
tion of both template and titanium source. The resultant com-
posite nanofibers exhibited highly efficient removal of dye
from wastewater by combining adsorption and photocatalysis
processes. In addition, the composite nanofibers are superpar-
amagnetic, and can be recovered by magnet easily with
almost no decline in the removal efficiency. The facile synthesis
strategy used here might provide a universal and efficient
method to fabricate one-dimensional magnetic nanocompo-
sites with porous structures for various functional applications.
by carbon coating and with TiO nanoparticles into core–shell
2
composite nanofibers (FMO/C@TiO ) with porous structure
2
were fabricated using organometallic compounds as tem-
plates. The structure and physicochemical properties of the as-
obtained composite nanofibers were characterized by a series
of techniques, including X-ray diffraction, scanning electron mi-
croscopy, transmission electron microscopy, nitrogen adsorp-
tion–desorption isotherms, X-ray photoelectron spectroscopy
and UV/Vis diffuse reflectance. The results demonstrate that
the one-dimensional core–shell structure was formed by coat-
ing TiO nanoparticles onto a substrate of FMO/C nanofibers.
2
Introduction
[
1b,4]
Semiconductor materials have been widely investigated as het-
erogeneous photocatalysts that could be used for the removal
terials (such as magnetite and ferrite).
With the catalysts
immobilized on magnetic materials, convenient separation can
[
1]
of dyes in water. Among the various semiconductor photoca-
be achieved. However, compared with TiO , the magnetic
2
talysts, TiO has attracted great attention because of its advan-
oxide particles are much more sensitive and unstable. In order
to protect the magnetic particles from chemical degradation
or agglomeration in practical applications, it is necessary to
coat the magnetic material with an organic or inorganic
2
tages such as low toxicity, resistance to photocorrosion, chemi-
[
2]
cal stability, low-cost and high photocatalytic activity. Al-
though it has been shown that most dyes in water can be ef-
fectively mineralized under UV irradiation, the separation of
[
3,4f,g]
shell.
Furthermore, inserting a barrier layer between the
TiO particles from treated water is a major bottleneck that
magnetic particles and the photocatalysts is an effective way
to reduce the negative effects of electron–hole recombination
and photodissolution, which otherwise lower the photocatalyt-
ic activity. Additionally, new functions or properties might also
be introduced into the coatings, such as optical properties, cat-
2
limits the application of these photocatalytic processes for
[1b,3]
treating wastewaters.
To facilitate easy separation of catalysts, many efforts have
been devoted to combining photocatalysts with magnetic ma-
[
4f,g]
alysis or absorbing capacity.
In general, inorganic silica is
the most commonly used interlayer to fabricate magnetic, re-
[
a] D. Chen, C. Liu, Prof. S. Chen, W. Shen, Prof. X. Luo, Prof. L. Guo
Key Laboratory of Jiangxi Province for Persistent Pollutants
Control and Resources Recycle
School of Environmental and Chemical Engineering
Nanchang Hang Kong University, Nanchang 330063 (P. R. China)
E-mail: chensuhua@nchu.edu.cn
[
3]
cyclable, photocatalytic composites. Yin et al. reported that
the addition of a silica layer between an iron oxide core and
a TiO shell promotes the photocatalytic activity of the catalyst
2
by decreasing the adverse influence of the magnetic oxide
[
5]
luoxubiao@nchu.edu.cn
core. Yu et al. fabricated superparamagnetic g-Fe O /SiO /TiO
2 3 2 2
[
b] D. Chen, Prof. L. Guo
composite microspheres by using an effective three-step ap-
proach. The composite microspheres exhibited superior photo-
catalytic properties because of the enrichment of pollutant dye
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology
of Ministry of Education, School of Chemistry and Environment
Beihang University, Beijing 100191 (P. R. China)
E-mail: guolin@buaa.edu.cn
molecules by the SiO middle layer. Other magnetically separa-
2
[
6]
ble photocatalysts, such as Fe O /SiO /TiO –Ag, Fe O /C/
[
**] FMO=magnetite/manganese iron oxide
3
4
2
2
3
4
[
7]
[8]
[9]
Cu O, Fe O @SiO @Bi WO
and Ag–AgI/SiO /Fe O
4
have
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500534.
2
3
4
2
2
6
2
3
also been prepared.
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Compared to the widely used SiO material, carbon is a spe-
pore volume. However, little attention has been paid to the
preparation of 1D (such as rods, fibers, wires and chains) mag-
netically recyclable photocatalysts with core–shell structures,
mainly because of the laborious synthesis processes involved
and the lack of precursors with specific morphology.
2
cial and unique coating material and core–shell-type nano-
structures could thus be achieved with its use. In addition, the
nanosized pores distributed uniformly on the surface of the
carbon coating could not only increase the surface area, but
also facilitate penetration of reactive species. For example,
Herin, 1D, porous, magnetic magnetite/manganese iron
oxide (FMO)/C@TiO2 composite nanofibers with core–shell
structures were synthesized by a procedure combining solvo-
thermal methods with thermal treatments. The detailed syn-
thesis process is outlined in Scheme 1. First, 1D Fe/Mn-based
coordination compound nanofibers (FMNFs) were prepared by
[
10]
Wang, et al. reported that the as-prepared Fe O @C@Bi WO
6
3
4
2
nanocomposite exhibited high efficiency in the photocatalytic
decomposition of phenol with the assistance of H O , and
2
2
[11]
could be easily recovered by a magnet. Huang et al. report-
ed that yolk–shell-structured Fe O /C@F–TiO microspheres
3
4
2
with a fluorinated surface can be used as recyclable visible-
using a solvothermal method. Then, TiO was deposited uni-
2
[4a]
light-driven photocatalysts. Liu et al. reported that the pres-
ence of a carbon interlayer can prevent the occurrence of pho-
todissolution. The synthesized magnetically recoverable
Fe O @C@TiO nanocomposites showed a higher photoactivity
formly over the surface of these nanofibers to fabricate
FMNFs@TiO2 core–shell composites by another solvothermal
procedure. The FMO/C@TiO samples were produced by ther-
2
mal treatment of the FMNFs@TiO composites. During this final
3
4
2
2
than Fe O @TiO nanocomposites. Great success has been ach-
treatment, the FMNF cores were converted to carbon-modified
porous magnetic ferrite, which can be used to solve the recy-
3
4
2
ieved in the fabrication of magnetic recyclable photocatalysts,
however, most of those with a core–shell structure were of
a uniform sphere shape.
cling issues of traditional TiO -based materials. It was found
2
that the size of the TiO nanocrystals and the thickness of the
2
As a member of the core–shell-structured nanocomposites
family, 1D core–shell nanocomposites that are utilized as pho-
tocatalysts, have received a great deal of research interest, be-
shell layer could be tuned by varying the concentration of pre-
cursors. The synthesized FMO/C@TiO samples were termed S1,
2
S2, S3, S4 and S5 based on the mass ratio of TiO (55%, 65%,
2
[
4f,g,12]
cause of their unique structural and electronic properties.
D magnetically recyclable Fe O @C@CdS nanochains and
75%, 85% and 95%, respectively). In addition, the adsorption
and photocatalysis properties of the composites were also
studied. To the best of our knowledge, this work represents
the first example of preparing recyclable 1D core–shell compo-
site nanofibers and controlling the porous nanostructures and
photoresponse range by adjusting the proportion of the carrier
materials. It is expected that our work will represent a new op-
portunity for the preparation of recyclable 1D core–shell semi-
conductor composites.
1
3
4
Fe O @C@ZnO microrods with core–shell structures were syn-
3
4
[4f,g]
thesized by magnetic-field-induced assembly,
and showed
high activity as recyclable photocatalysts for the degradation
of organic dyes. Compared with nanoparticles (NPs) or bulk
materials, 1D structural core–shell nanocomposites have the
[4f,g,12a,b]
following advantages:
1) the 1D geometry gives rise to
fast and long-distance electron transport, 2) the light absorp-
tion and scattering can be enhanced because of the high
length-to-diameter ratio of the 1D structure, and 3) 1D struc-
tures are expected to have larger specific surface area and
2
Scheme 1. The synthesis of 1D FMO/C@TiO .
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that these anatase crystals are highly crystalline. With the in-
Results and Discussion
crease of TiO content, the main diffraction peaks of ferrites
2
were replaced gradually by the anatase diffraction patterns.
The XRD results indicate the successful immobilization of TiO2
on the surface of FMO/C nanofibers by our synthetic routes.
Structure and morphology
The crystallographic structures of the as-prepared 1D bare
FMO/C and FMO/C@TiO2 core–shell nanocomposites were
characterized by XRD measurements. The XRD patterns of the
as-prepared samples are shown in Figure 1. The XRD pattern
Furthermore, the average sizes of the as-prepared TiO nano-
2
crystals on the surface of FMO/C deduced from Sherrer’s for-
mula for the strongest peak (101) were 16, 17, 20, 25 and
2
8 nm for S1–S5, respectively.
The evolution of the morphology from FMNFs to FMO/C or
FMO/C@TiO2 was further revealed by SEM. Figure 2a shows
the SEM image of FMNFs obtained after hydrothermal reaction
for 6 h at 1808C. It is apparent that the as-prepared FMNFs ex-
hibited the morphology of 1D nanofibers. The average diame-
ter of FMNFs was about 150 nm and the lengths were up to
several tens of micrometers. The surface of the as-synthesized
nanofibers was smooth and no isolated NPs were observed.
Figure 2b shows the SEM image of the FMO/C prepared by
the calcination of FMNFs at 5008C. We can observe that the
FMO/C maintained the morphology of 1D nanofibers with di-
ameters of approximately 150 nm and lengths ranging from
several micrometers to several tens of micrometers, indicating
good structure stability. However, the surface of FMO/C is
rough because a highly porous structure was formed after cal-
cination, which was further confirmed by the nitrogen adsorp-
tion measurements. Figure 2c–h shows typical SEM images of
1D FMO/C@TiO2 resulting from the coating of TiO2 onto
FMNFs. As seen in Figure 2d–h, the as-prepared FMO/C@TiO2
exhibits the 1D morphology of fibers even under strong ultra-
Figure 1. XRD patterns of a) bare FMO/C, b) S1, c) S2, d) S3, e) S4 and f) S5.
of bare FMO/C (Figure 1a) can be indexed to magnetite (Fe O ,
3
4
JCPDS
No. 75-1609)
and
manganese
iron
oxide
[
(Mn0.983Fe ) O , JCPDS No. 24-0507]. Compared with that of
0.017 2
3
the bare FMO/C, the diffraction peaks with 2q values of 25.38,
7.18, 37.98, 38.78, 48.28, 53.98, 55.18, 62.28, 62.88 and 68.98 in
Figure 1b–f can be perfectly indexed to the (101), (004), (200),
105), (201), (204), (204), (116), (220), and (215) crystal planes of
3
sonic treatment. The length of FMO/C@TiO was approximately
2
5–8 mm and the diameter approximately 200–500 nm. The
(
spherical TiO NPs with uniform size of approximately 30 nm
2
anatase TiO (JCPDS No. 21-1272), respectively. Specifically, the
were densely anchored onto the surface of FMO/C. The larger
size observed in the SEM image than that calculated from XRD
2
diffraction peaks assigned to TiO2 are sharp, which suggest
Figure 2. Typical SEM images of as-prepared samples of a) FMNFs, b) bare FMO/C, and FMO/C@TiO
2
samples c) and d) S1, e) S2, f) S3, g) S4, and h) S5.
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pattern is due to the spray treatment with gold. The energy-
dispersive X-ray spectroscopy (EDX) analysis was performed to
To investigate the specific surface area and pore characteris-
tics of the as-synthesized samples, nitrogen adsorption–de-
sorption measurements were performed (Figure 4). The nitro-
gen adsorption–desorption isotherm (Figure 4a) of the pure
TiO and FMO/C@TiO samples are type-IV isotherms, which in-
confirm the elemental composition of FMO/C@TiO (Figure S2),
2
which indicates the presence of Ti, Fe, Mn, O and C. The elec-
tron mapping images were used to further verify the elemental
2
2
distribution of FMO/C@TiO (Figure S2), and show that all the
dicate monolayer adsorption at low pressure and multilayer
adsorption at high pressure. The hysteresis loops between ad-
sorption and desorption indicate a porous structure, which is
confirmed by the pore distribution curves (Figure 4b). On the
basis of the Brunauer–Emmett–Teller (BET) equation, the specif-
2
elements in FMO/C@TiO are homogeneously distributed. The
2
diameter of FMO/C@TiO was observed to increase with an in-
2
crease of mass ratio of TiO2 (Figure 2d–h). For comparison,
pure TiO NPs were synthesized under the same conditions,
2
and typical SEM images (Figure S3) indicate that the morpholo-
ic surface area of S1, S2, S3, S4, S5 and bare TiO NPs are 56.72,
2
2
À1
gy of these NPs is consistent with the TiO NPs anchored on
51.90, 40.10, 37.01, 37.19 and 42.24 m g , respectively. The
corresponding pore size distributions of the products, calculat-
ed from the Barrett–Joyner–Halenda (BJH) model show wide
peaks centered at 10.13, 22.26, 22.09, 22.00, 22.45 and
21.71 nm, respectively. The details of nitrogen adsorption–de-
sorption measurements for bare TiO and FMO/C@TiO samples
2
the FMO/C.
To visualize the microstructure, TEM and high resolution
(
HR)-TEM images of the sample (S2) were acquired and typical
examples are shown in Figure 3. From the TEM image shown
in Figure 3a, it is apparent that the FMO/C@TiO nanofiber was
2
2
2
composed of many NPs, and the porous structure can be ob-
are listed in Table S1 in the Supporting Information.
served throughout the whole FMO/C@TiO nanofiber. The HR-
2
TEM image presented in Figure 3b exhibits a clear interface
Element analysis
between the TiO NPs and FMO/C, and the average size of TiO2
2
NPs anchored on the FMO/C is approximately 20 nm, which is
consistent with the corresponding XRD pattern (see Figure 1c).
Figure 3b (inset) shows clear lattice fringes with the spacing
value of 3.52 Š, which are assigned to the (101) crystalline
Figure 5 shows typical XPS spectra of a FMO/C/TiO sample
2
(S2). The sharp peaks in Figure 5a at 285.2, 458.2, 530.2, 642.2,
and 711.2 eV correspond to the characteristic peaks of C1s,
Ti 2p, O1s, Mn2p and Fe2p, respectively, indicating the exis-
tence of C, Ti, O, Mn and Fe in the sample. Figure 5b shows
the high-resolution XPS spectrum for the Ti 2p binding energy
regions. The peaks located at 458.6 eV (Ti2p ) and 464.4 eV
planes of anatase TiO (PDF No. 21-1272). In addition, scanning
2
transmission electron microscopy (STEM) imaging (Figure 3c)
and EDX mapping (Figure 3d–h) further reveal the core–shell
3/2
[
13]
structure of the FMO/C@TiO . The EDX elemental line-scanning
(Ti2p ) were assigned to the OÀTi bond in TiO . The high-
2
1/2
2
results (Figure 3i) also indicates that the FMO/C core is sur-
resolution Fe2p spectrum (Figure 5c) exhibits two peaks at
711.2 and 724.6 eV, corresponding to Fe2p3/2 and Fe2p1/2, re-
rounded by a TiO shell.
2
Figure 3. a) TEM, b) HRTEM and c) STEM images, the corresponding EDX elemental mapping (d–h) and i) the 3D line-scanning image of a sample of S2.
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2 2 2
Figure 4. N adsorption–desorption isotherms and the pore size distribution curves of the pure TiO and as-prepared FMO/C@TiO samples.
2
Figure 5. XPS survey spectra of the S2 FMO/C@TiO sample: a) wide scan, b) Ti 2p, c) Fe2p, d) Mn2p, e) O1s, and f) C1s.
spectively, which are in good agreement with the reported
energy peak at 529.9 eV is assigned to the oxide in TiÀO link-
[
14]
[17]
values for Fe O . The satellite peak situated at approximately
ages of TiO . The peak appearing at 531.2 eV can be attribut-
3
4
2
3
+
[18]
7
19.0 eV is characteristic of Fe in g-Fe O , suggesting that
ed to the O=C or OH groups. The peak appearing around
2
15]
3
[
the Fe O4 NPs were partially oxidized.
As shown in Fig-
532.6 eV can be ascribed to the OÀC group. The weak intensity
peak at 534.2 eV can be attributed the OÀC group or adsorbed
H O molecule on the FMO/C@TiO sample. The high-resolution
3
ure 5d, Mn2p3/2 and Mn2p1/2 peaks at 642.2 and 653.7 eV, re-
II
[16]
spectively, correspond to Mn in manganese iron oxide. The
high-resolution O1s spectrum (Figure 5e) can be fitted into
four peaks at 529.9, 531.2, 532.6 and 534.2 eV. The lower-
2
2
scan (Figure 5 f) of the C1s region indicates that carbon atoms
are present in five different chemical environments, corre-
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sponding to graphite CÀC bonds at 285 eV, CÀO at 286.1 eV,
[19]
C=O at 287.5 eV and COOH at 289.3 eV.
Optical properties
The optical measurements of the samples using the UV/Vis dif-
fuse reflectance spectroscopy (DRS; Figure 6a), provided evi-
dence that the property of light absorption of FMO/C@TiO2
can be finely tuned by the addition of different amounts by
weight of FMO/C. In particular, the integration of TiO2 and
FMO/C enables the extension of its light absorption to the visi-
ble light region, and the absorption intensity of the compo-
sites in the visible light region is gradually enhanced with the
increase of the mass ratio of FMO/C. A plot obtained by the
transformation based on the Kubelka–Munk function versus
the energy of light is shown in Figure 6b. Compared with the
2 2
Figure 7. PL spectra of FMO/C@TiO and pure TiO NPs.
pure TiO NPs, the band gap of hybrid nanofibers is significant-
2
ly decreased, and the absorption edge generates an obvious
redshift. This redshift is attributed to the charge-transfer transi-
tion between the electrons of the FMO/C NPs and the conduc-
Figure 7, we found that the intensity of PL signals obtained for
all the FMO/C@TiO samples is weaker than that of pure TiO2
2
NPs; the suppression effect is in the order S1>S2>S3>S4>
tion band (or valence band) of TiO . FMO/C NPs can increase
2
S5, which matches that of the FMO/C content of these materi-
als.
energy spacing of the conduction band in TiO , leading to the
2
quantization of energy levels and causing absorption in the
visible region. The estimated band gap values are 2.17, 2.38,
2
.60, 2.87, 2.58, 3.11, and 3.21 eV corresponding to bare FMO/
Removal of dyes
C, S1, S2, S3, S4, S5 and pure TiO NPs, respectively.
2
Photoluminescence (PL) is used primarily to determine the
effectiveness of trapping, migration and transfer of charge car-
riers, as well as to understand the fate of the photo-induced
To demonstrate the potential application of FMO/C@TiO com-
2
posites, the degradation of Congo red was used to evaluate
the photocatalytic activity of the as-prepared FMO/C@TiO2
composites by combining adsorption in the dark and then
photocatalysis under simulated solar light. Figure 8a shows the
[20]
electrons and holes pairs in semiconductors. PL is the result
of the recombination of excited electrons and holes, and the
[
21]
lower the PL intensity is, the lower the recombination rate.
relative concentration changes (C/C ) of Congo red at a given
0
Figure 7 shows the PL spectra of FMO/C@TiO composites and
time interval. Firstly, all the samples showed fast adsorption
rates and strong adsorption capacities, the S1 composite could
adsorb 76.8% of dyes in water. However, the adsorption capa-
2
pure TiO NPs with the excitation wavelength at 320 nm. It is
2
apparent that all the samples exhibit broad PL signals in the
wavelength range 350–550 nm and the spectra are similar in
shape. Two obvious PL peaks around 410 and 470 nm were
observed. These spectra are in good agreement with the re-
bility of FMO/C@TiO composites declines with the decrease of
2
FMO/C content in the composite. With the help of adsorption
processes, 82.9%, 78.6%, 57.0%, 53.5% 53.4%, and 68.2% of
Congo red was decomposed under simulated solar light after
ported PL spectrum of TiO . From the PL spectra shown in
2
Figure 6. a) UV/Vis diffuse reflectance spectra and b) the corresponding plot of the transformed Kubelka–Munk function versus the energy of light for FMO/C
and FMO/C@TiO
2
.
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Figure 8. a) Curve and b) bar plot of the efficiency of removal of Congo red from pure TiO
order plots for Congo red adsorbed on pure TiO NPs and FMO/C@TiO composites. d) The pseudo-first-order plots of photocatalytic kinetics curves of Congo
red on pure TiO NPs and FMO/C@TiO composites.
2 2
NPs and the FMO/C@TiO composites. c) The pseudo-second-
2
2
2
2
the value estimated from the pseudo-second-order kinetic
model.
6
8
0 min by S1–S5 and bare TiO , respectively, and 90.4%,
2
3.8%, 70.4%, 63.0%, 73.2%, 91.4% of Congo red could be re-
moved by S1–S5 and bare TiO after 120 min, respectively. The
For the photocatalytic process, the Langmuir–Hinshelwood
mechanism, simplified for low concentrations as pseudo-first-
order process, was tested [Eq. (2)]:
2
amount of Congo red removed by adsorption and photode-
gradation FMO/C@TiO composites is shown in Figure 8b. Fur-
2
thermore, the total organic carbon (TOC) content of the Congo
red solution after adsorption and photodegradation by bare
TiO and FMO/C@TiO samples (Table S2) is consistent with the
C0
C
ln ¼ kt
ð2Þ
2
2
results shown in Figure 8a.
To further investigate the mechanism of the adsorption pro-
cess, kinetic studies were developed. The most widely used
model for describing the adsorption process is of pseudo-
second order that assumes comparable concentrations of the
surface active sites and of the pollutant molecules/species
where k is the apparent rate constant, C is the concentration
0
of dye before illumination and C the concentration of dye at
t
time t. The plots of Àln(C/C ) versus t for various TiO loadings
0
2
are shown in Figure 8d. All curves show good linear behavior,
revealing that the kinetics of the degradation reaction are con-
trolled by a pseudo-first-order reaction, which is consistent
[
22]
[
Eq. (1)]:
[
6]
with previous reported results. Furthermore, k was found to
be dependent on the TiO loading. This is reasonable because
2
t
t
1
the Fe O NPs will screen and weaken the UV light that irradi-
¼
þ
ð1Þ
3
4
qt qe k2q2e
[3,23]
ates the TiO2 NPs in FMO/C@TiO2.
As expected, it was
found that the photocatalytic activities of these FMO/C@TiO2
nanocomposites towards Conge red degradation decrease
with increasing FMO/C loading. In order to analyze the activity
of FMO/C@TiO and bare TiO NPs in the photocatalytic pro-
À1
where q [mgg ] is the amount of adsorbate at time t [min],
t
À1
qe [mgg ] is the equilibrium adsorption capacity and k
2
À1
À1
[
gmg min ] is the pseudo-second-order rate constant. The
2
2
plots of the above model are shown in Figure 8c, and the
slope and the intercept of each linear plot were used to calcu-
late k and q . The adsorption kinetic constants and regression
cess, the electrochemical impedance spectroscopy (EIS) Ny-
quist plots of bare TiO2 NPs and the FMO/C@TiO2 samples
were obtained (Figure 9a). The radius of the EIS spectra deter-
mines the reaction rate occurring at the surface of the elec-
trode, and it was observed that the semicircle size in the plot
reduced with the increase of FMO/C loading, which indicated
an effective separation of photogenerated electron–hole pairs
and fast interfacial charge transfer between the electron donor
2
e
coefficients are summarized in Table S3. The correlation coeffi-
2
cient (R ) of the pseudo-second-order adsorption kinetics is
high, suggesting the kinetics of adsorption of Congo red on
FMO/C@TiO composites and pure TiO NPs follow a pseudo-
2
2
second-order model. The experimental q value is also close to
e
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and electron acceptor. However, the photocatalytic activity of
the samples was inconsistent with the corresponding EIS re-
sults. Therefore, the formation of hydroxyl radicals (·OH) on the
surface of UV-illuminated samples was measured by the PL
spectroscopy using terephthalic acid (TA) as a probe molecule.
As shown in Figure 9b, ·OH radicals were formed in the solu-
tions containing different photocatalysts under the same con-
ditions. With increasing FMO/C loading, a gradual decrease in
the PL intensity at approximately 430 nm was observed, and
the formation of ·OH in S1 was greatly suppressed. This might
have been because Fe O particles blocked some of the TiO
2
havior at room temperature. The superparamagnetic behavior
implies that the composites can be easily recovered using
a magnet, which enhances their feasibility for practical use. To
evaluate the recycling stability of adsorption/photocatalysis on
S2, we performed repeatedly the removal experiment six
times. The magnetization curve of S2 after six adsorption–pho-
tocatalysis cycles is shown in Figure 10a. The magnetic satura-
tion value of S2 after the sixth reuse, measured at room tem-
À1
perature, is 9.89 emug , indicating the high magnetic stability
of the as-prepared composite. Despite the magnetic strength
having somewhat reduced, S2 could still be collected after the
water treatment (Figure 10a, inset). The results shown in Fig-
ure 10b indicated that the composites are stable under repeat-
ed application without obvious decrease in dye removal rate
for the treatment of wastewater. By comparison, the cycling
3
4
[23]
active sites and absorbed a portion of the UV light. Further-
more, the small amount of carbon is insufficient to inhibit the
negative effect of Fe O on the photocatalytic activity of TiO .
3
4
2
As such, it is not hard to understand why the photocatalytic
activity of FMO/C@TiO is slightly lower than that of bare TiO2
degradation of bare TiO was also stable (Figure S4), but these
2
2
NPs.
NPs are difficult to collect from the treated water. Figure S5
shows XRD patterns of the recycled S2, compared to the XRD
curve of the initial cycle. The diffraction peaks after the sixth
cycle were unchanged, which demonstrated that S2 main-
tained its crystallographic structure. In addition, the main-
tained 1D nanostructure was observed by SEM (Figure S6). The
corresponding EDX spectra and mapping images (Figure S6),
further demonstrate the feasibility of the as-prepared compo-
sites for practical use.
Magnetic properties and recycling
To further explore the potential application of FMO/C@TiO2
composites, the magnetic properties and recycling stability of
the as-prepared composites were investigated. Figure 10
shows the magnetization curves of as-synthesized S2 compo-
À1
sites. The magnetic saturation value was 10.25 emug before
adsorption/photodegradation. In addition, the low value of re-
manent magnetization and coercivity suggests that the pre-
pared FMO/C@TiO composites exhibit superparamagnetic be-
2
Figure 9. a) EIS Nyquist plots of bare TiO
reaction of terephthalic acid (TA) with ·OH radicals generated from bare TiO
2
NPs and the FMO/C@TiO
2
composites. b) Fluorescence spectra of 2-hydroxy-terephthalic acid (TAOH) formed by the
and FMO/C@TiO samples.
2
2
Figure 10. a) The magnetization hysteresis of S2 FMO/C@TiO
b) Cyclic degradation rate for Congo red using S2 FMO/C@TiO
2
composites, before (black line) and after (red line) six adsorption–photodegradation cycles.
composites.
2
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3
020 surface area and pore size analyzer (Micromeritics, Atlanta,
Conclusion
USA), and the specific surface area and pore volume distribution
were calculated using the BET and BJH methods, respectively. X-ray
photoelectron spectroscopy (XPS) was measured on an Axis Ultra
spectrometer (Kratos, Manchester, UK). The binding energies ob-
tained from the XPS analysis were calibrated for specimen charg-
ing by referencing the C1s peak to 285 eV. The optical properties
of the samples were analyzed over the range 200–800 nm by UV/
Vis DRS using a UV/Vis spectrophotometer (Hitachi U-3900H), for
In summary, 1D porous magnetic FMO/C@TiO core–shell com-
2
posite nanofibers were synthesized using a simple two-step
solvothermal method. By changing the amount of nanofiber
precursor added during the second step of the preparation,
the porous nanostructures and photoresponse range of the
composite nanofibers could be controlled. The as-prepared 1D
composite nanofibers exhibited high adsorption and photoca-
talysis performance for the efficient removal of a dye from
which BaSO was used as the internal standard. The PL spectra
4
were measured with a Hitachi F-7000 FL spectrophotometer at
room temperature. EIS was measured using an electrochemical an-
alyzer (CHI 660D, Chenhua Instrument Company, Shanghai, China)
with a conventional three-electrode cell. The resultant electrode
served as the working electrode, with a graphite rod as the coun-
ter electrode and a saturated calomel electrode as the reference
water. Unlike the pure TiO NPs, the 1D composite nanofibers
2
are superparamagnetic, which makes recovery convenient for
their cyclical use for water treatment. It is hoped that our work
could not only offer useful information on the synthesis of vari-
ous porous 1D core–shell semiconductor nanocomposites, but
also help bring forward new ideas on the practical treatment
of organic wastewater by using their magnetic properties.
electrode.
A Na SO4 (0.50m) solution containing K [Fe(CN) ]
2 3 6
(2.5 mm) and K [Fe(CN) ] (2.5 mm) was used as electrolyte, and the
4
6
obtained curves were recorded with an ac perturbation signal of
mV over the frequency range 0.1 Hz to 100 kHz.
5
Experimental Section
Adsorption and photocatalytic properties
Synthesis of 1D FMNFs
The adsorption and photocatalytic properties of the as-prepared
samples were evaluated by removal of Congo red. The prepared
In a typical process, stoichiometric amounts of FeSO ·7H O,
4
2
MnSO ·H O and N(CH COOH) were dissolved with vigorous stirring
4
2
2
3
composite (50 mg) was added to a 200 mL quartz photoreactor
in deionized water (40 mL) and isopropanol at a volume ratio of
:1. The molar ratio of FeSO ·7H O/MnSO ·H O was 3:1. Subse-
À1
that contained an aqueous solution of Congo red (35 mgL
,
7
4
2
4
2
1
00 mL). The mixed solution was stirred at 258C using a mechanical
quently, the mixture was transferred to a Teflon-lined stainless steel
autoclave. The autoclave was heated gradually to 1808C in an elec-
tric oven and maintained at this temperature for 6 h. After the au-
toclave had been cooled to room temperature, white precipitates
were collected by centrifugation and washed with water and etha-
nol several times before being dried at 608C for 12 h.
stirrer at a speed of 300 rpm in the dark. Analytical samples (1 mL)
were taken from the mixture at given time intervals and immedi-
ately filtered to remove the composite. Prior to illumination, the
suspensions were mechanically stirred in the dark for 1 h to ensure
the establishment of an adsorption–desorption equilibrium of
Congo red on the composite surface. Photocatalysis was per-
formed using a solar simulator 300 W Xe lamp (Perfectlight, Beijing,
China), which provides UV/Vis light ranging from 360 to 700 nm.
Once the photodegradation experiment started, an analytical
sample (1 mL) was collected at a given time interval and the pho-
tocatalysts were separated immediately using a magnet or by cen-
trifugation. The Congo red concentration was measured by using
a Hitachi U-3900H spectrophotometer. Furthermore, the TOC con-
tent of the irradiated samples was analyzed using a Shimadzu TOC
analyzer (TOC-L CSH). To further study the recyclability of the 1D
FMO/C@TiO2 core–shell composite nanofibers, we also examined
the adsorption/photocatalysis properties of FMO/C@TiO2 (S2) for
six rounds. The recycled samples were reused without any post-
treatment except they were washed with ethanol and distilled
water three times after each removal of Congo red.
Synthesis of 1D FMO/C@TiO composites
2
Typically, different amounts of FMNFs were dispersed ultrasonically
in absolute ethanol (20 mL) for 30 min, followed by the addition of
tetrabutyl titanate (5 mL) and HF (40 wt.%, 0.4 mL). After continu-
ous stirring, the mixture was transferred into a Teflon-lined stain-
less steel autoclave and heated at 1808C for 24 h. After being al-
lowed to cool to room temperature, the products were collected
and washed with ethanol and deionized water. The final products
were dried and calcined at 5008C under an N2 atmosphere for
3
0 min to produce the 1D FMO/C@TiO composites. The synthe-
2
sized samples were designated S1, S2, S3, S4 and S5 based on the
mass ratio of TiO (55%, 65%, 75%, 85% and 95%, respectively).
2
For comparison, pure TiO2 NPs were prepared in a similar way
without the addition of FMNFs. The bare FMO/C nanofibers were
synthesized by direct calcination of FMNFs.
Measurement of hydroxyl radicals
FMO/C@TiO (5 mg) with different TiO content and pure TiO NPs
2
2
2
Characterization
were dispersed in an aqueous solution (10 mL) containing NaOH
(10 mm) and TA (5 mm). The suspension was stirred in the dark for
60 min, and then irradiated under a solar simulator 300 W Xe lamp
The crystal phase of the as-prepared materials was analyzed by
powder X-ray diffraction (XRD) on a Bruker D8 X-ray powder dif-
fractometer using CuKa radiation (l=1.5418 nm) at 40 kV and
(Perfectlight, Beijing, China). After 30 min, the solid FMO/C@TiO or
2
4
0 mA, with 2q ranging from 108 to 808. SEM and TEM were used
pure TiO NPs were collected with a magnet or by centrifugation,
2
to determine the morphology of the samples on an FEI Nova
NANOSEM 450 scanning electron microscope and a JEOL 2100
transmission electron microscope, respectively. The samples S1–S5
were sprayed with gold before SEM observation. Nitrogen adsorp-
tion–desorption isotherms were measured by using a TriStar II
and the rest of solution was used for fluorescence spectroscopy
measurements. A fluorescence spectrophotometer (Hitachi F-7000)
was then used to measure the fluorescence signal of the generat-
ed 2-hydroxyterephthalic acid (TAOH). The excitation light wave-
length used in recording the fluorescence spectra was 320 nm.
ChemPlusChem 2016, 81, 282 – 291
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Manuscript received: November 24, 2015
Revised: January 10, 2016
Accepted Article published: January 11, 2016
Final Article published: February 2, 2016
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