BiVO4/cobalt phthalocyanine (CoPc) nanofiber heterostructures: Synthesis, characterization and application in photodegradation of methylene blue

Article abstract of DOI:10.1039/c4ra08759c

BiVO₄/cobalt phthalocyanine (CoPc) hierarchical nanostructures were prepared. The structural and photo-chemical properties were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron micr

Full text of DOI:10.1039/c4ra08759c

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BiVO4/Cobalt Phthalocyanine (CoPc) Nanofiber Heterostructures:
Synthesis, Characterization and Application in Photodegradation of
Methylene Blue

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BiVO4/Cobalt Phthalocyanine (CoPc) Nanofiber Heterostructures:
Synthesis, Characterization and Application in Photodegradation of
Methylene Blue
Guoshuai Liu, Suwen Liu*, Qifang Lu, Haiyan Sun, Zhiliang Xiu
Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass
& Functional Ceramics, School of Material Science and Engineering, Qilu University
of Technology, Jinan, 250353, P. R. China
* Corresponding author: Tel: +86-531-89631231; Fax: +86-531-89631227
E-mail: liusw@qlu.edu.cn (Suwen Liu)

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BiVO4/cobalt phthalocyanine (CoPc) hierarchical nanostructureDsOI: 1w0.1e0r3e9/C4RA08759C
prepared. The structural and photo-chemical properties were characterized with
X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR),
scanning electron microscopy (SEM), UV-vis Diffuse reflectance spectroscopy
(DRS) and Brunauer-Emmett-Teller (BET) surface area test. The samples are
well-defined hierarchical nanostructures, and its absorption was extended to the
near-infrared region. The photocatalytic activities of the BiVO4/CoPc
photocatalysts were evaluated by the decomposition of methylene blue (MB)
under visible light irradiation and showed excellent visible light photocatalytic
performances than the BiVO4 nanofiber.
1. Introduction
TiO2 is the most investigated photocatalyst because of it owned properties
including nontoxicity, easy availability, low cost etc.1,2.Nevertheless, the energy band
gap of TiO2 (ca. 3.2 eV) limits its application because the electron-hole pairs can only
be exited by UV light with wavelength shorter than 380 nm3,4. Therefore, in view of
solar energy utilization, the search for semiconductor photocatalysts that can harvest
the wide spectrum of solar light, from ultraviolet to visible light wavelength, and
achieve efficient solar energy conversion remains one of the most meaningful
missions. Developing the new non titania types of photocatalysts considered to be
active under visible light is an attractive research area5. Bismuth vanadate species are
such promising kinds of materials and recently are being used as active visible-light
driven photocatalysts6-9. Its monoclinic phase has shown high photocatalytic activity
and it is also non-toxic and chemically stable in aqueous solution under irradiation.
However, monoclinic BiVO4 usually shows low photocatalytic activity due to the
poor charge-transport properties10. Phthalocyanine complexes are also known as a

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new type of visible light active materials11. These organometallic compounds are
appealing and have attracted many interests in various fields including homogeneous
and heterogeneous catalysis12. Metal phthalocyanines have been used as the
sensitizers for TiO2 and enhanced the photocatalytic activity13.
In this study, we prepared BiVO4 nanofiber by electrospinning and followed by
immobilized of cobalt phthalocyanine (CoPc) molecules onto the surface of the fiber.
Photochemical activity of the final products was studied from the degradation of MB
and compared with the unmodified catalysts. We found that the BiVO4/CoPc
samples enhanced the photocatalytic activity apparently compared pure BiVO4 fiber.
2. EXPERIMENTAL SECTION
2.1 Preparation of the BiVO4 nanofibers
All reagents were of the analytical purity and used without further treatment. In a
typical experimental procedure, 0.195 g Bi(NO3)3·5H2O (0.4 mmol) and 0.5 g citrate
acid were added into 1.4 mL deionized water with magnetic stirring to form a
suspension, and then 0.0468 g (0.4 mmol) NH4VO3 were dissolved in 2 mL deionized
water to form a solution. The above suspension and the solution were mixed, and
subsequently nitric acid was added to adjust the pH value to 1 form a solution. Finally,
it was mixed with 10 mL polyvinylpyrrolidone (PVP, Mw=1300 000) ethanol solution
(0.1 g/mL) to form the spinnable precursor sol. The precursor sol were drawn into the
needle tube, and the inner diameter of the needle is 0.5 mm. The electrospinning
voltage was set as 25 kV with a pumping speed of 0.0015 mm/s while the distance
between needle and collector is 30 cm. After electrospinning, the mat-like gel fibers

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were obtained. Then it was heated to 500 oC in a furnace with a heating rate 2 oC/min,
and kept for 1 h and finally cooled to room temperature naturally to obtain the BiVO4
nanofibers.
2.2 Synthesis of CoPc
6.67 g C6H4(CO)2O, 1.79 g CoCl3, 13.64 g CO(NH2)2 and 0.47 g
H24Mo7N6O24·4H2O were mixed in a 100 mL beaker, and the beaker was heated to
o
140 C in a furnace for 5 h. Taken out of the beaker, grinding the sample to granular
o
and then heated to 200 C in furnace for 2 h. After cooling, 20 mL ammonia added
into the beaker, cover with plastic wrap, heated to ammonia boiling for 15 minutes,
cooling and filtering. The products were washed with deionized water, ethanol and
o
toluene for several times and then dried at 150 C for 10 h, the CoPc was obtained.
The UV-vis spectrum of the CoPc as shown in Fig. 3b.
2.3 Fabrication of the CoPc/BiVO4 hierarchical nanostructures
0.5 g CoPc dissolved in 15 mL glycol was added into a 20 mL Teflon-lined
autoclave, then 0.15 g BiVO4 nanofibers were added into the system. The autoclave
o
was heated to 180 C for 18 h. The sample was washed with deionized water and
ethanol for several times, then dried at 70 oC for 10 h for further characterization.
2.4 Characterization
Fourier transform infrared spectroscopy (FT-IR) spectrum was tested on Infrared
Spectrometer (IR Prestige-21). X-ray diffraction (XRD) patterns of the samples were
measured on a D8 ADVANCE X-ray diffractometer (Bruker, Germany), using
monochromatized Cu Ka (λ = 0.15418 nm) radiation with a scan range from 10 to 90°.

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The accelerating voltage and the applied current were 40 kV and 40 mA. The
morphologies and microstructures of as-prepared samples were analyzed by a
FESEM-4800 field emission scanning electron microscope (SEM, Hitachi). The
Brunauer-Emmett-Teller (BET) surface area was determined by nitrogen sorption
using a Micromeritics ASAP 2020 analyzer. The UV-vis diffuse reflectance
spectroscopy (DRS) of the sample was recorded on a UV-2550 spectrophotometer
(Shimadzu) in the wavelength range of 200-800 nm.
2.5 Photocatalytic activity measurement
The photocatalytic activities of the samples were evaluated by the degradation of
MB under irradiation by using a 500 W Xe lamp with cutoff filter (λ≥ 400 nm), which
was set about 12 cm from the liquid surface of the MB suspension, and the irradiation
area was approximately 25 cm2. The initial concentration of MB solution was 20
mg/L. 0.06 g photocatalysts were put into 40 mL MB solution. Before the
photodegradation experiment was initiated, the suspension was magnetically stirred in
the dark for 30 min. Once the photodegradation experiment started, at given time
intervals, 4 mL solution was sampled and centrifuged to remove the photocatalysts.
The filtrates were analyzed by the variations of the maximum absorbance (664 nm) in
the UV-vis spectrum of MB with a UV-2550 spectrophotometer. The ratio (C/C0) of
MB concentrations was adopted to evaluate the photocatalytic performance. The
concentration is proportional to the absorbance of the MB, so we calculated the C/C0
by the ratio of the variations of the maximum absorbance of the MB, C0 represents the
concentration when the adsorption-desorption equilibrium. A radical trapping

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experiment was performed in the same condition of degradation for MB. TheDtOraI:p10p.1i0n3g9/C4RA08759C
molecule concentration was 1 mM L-1, and the stabilization time was 30 minutes. The
dye mineralization degree was monitored by analyzing the total organic carbon
content (TOC) in the solutions at different irradiation times. The samples were
analyzed in a Shimadzu VSCN8 TOC analyzer.
3. RESULTS AND DISCUSSION
X-ray diffraction patterns of the prepared samples are shown in Fig. 1. The
characteristic diffraction peaks for the monoclinic BiVO4 phase are confirmed by
comparing the XRD data with those reported in literature14. It is obvious that
monoclinic phase is the dominant phase. The main XRD peaks for the monoclinic
phase appear at around 18.7, 19, 28.9 and 30.6° (attributed to [101], [011], [112] and
[004] crystal planes, respectively). As for the BiVO4/CoPc samples, the diffraction
peaks of the CoPc were not observed, which might be due to the low mass of the
CoPc.
Fig. 1 The X-ray diffraction patterns of BiVO4 and BiVO4/CoPc samples.
The FT-IR spectra of the prepared samples are shown in Fig. 2. The pure BiVO4 has
a broad band between 650 and 850 cm-1, which is attributed to Bi-O and V-O
vibrations15. From the curve of the BiVO4/CoPc samples, the sample appeared several

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absorption peaks at around 1090, 1165, 1280, 1420, 1460, and 1605 cm-1, which
might be assigned to phthalocyanine skeletal and metal-ligand vibrations,
respectively16,17. And, the other absorption peaks at 1502 and 1605 cm-1 may be
assigned to the C=N stretching and symmetric C=C stretching on the aromatic ring
structure of the CoPc molecule. And, these peaks were also observed in the FT-IR
spectrum of pure CoPc.
Fig. 2 The FT-IR spectra of BiVO4, CoPc and BiVO4/CoPc samples.
The DRS of the prepared BiVO4, BiVO4/CoPc and CoPc samples are shown in Fig.
3. The BiVO4 sample exhibit the absorption in the visible light range and we can
ascertain the band gap of BiVO4 fiber in this paper by the equation9: Eg=1240/λ, λ is
the value of the target line to the X-axis, the band gap of BiVO4 fiber and
BiVO4/CoPc samples prepared in this paper were about 2.4 eV and 2.39 eV,
respectively. The CoPc sample is present mainly as monomers, characterized by the
sharp absorption bands in the Soret (350 nm) and in the Q-band region (around 680
nm)13. Furthermore, for the BiVO4/CoPc samples, the adsorb edge shifted slightly
toward higher wavelengths compared with BiVO4 and the composite exhibits
remarkable absorbance in near-IR region. the distinguished bands around 650 and 600
nm are attributed to the monomeric and dimeric forms of cobalt phthalocyanine

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species, respectively18,19
.
Fig. 3 UV-vis Diffuse reflectance spectra of BiVO4, BiVO4/CoPc (a) and CoPc (b) samples.
The SEM micrographs of the samples are shown in Fig. 4. Gel fiber (a) showed
well defined fiber morphologies, the lengths of these randomly oriented gel fibers can
reach several micrometers, and the diameters of those nanofibers are around 250 nm.
The BiVO4 fiber (b) almost the same morphologies with gel fibers but the diameter
reduced to around 200 nm caused by the removal of the PVP in the calcined
procedure, and the surface is smooth. As for the BiVO4/CoPc samples (c), the CoPc
nanoparticles with a diameter of ca. 4 nm were grew twining on the surface of BiVO4
nanofibers and formed the BiVO4/CoPc nanofiber heterostructures. The EDS analysis
(d) of the BiVO4/CoPc samples, revealing that the samples are composed of Bi, V, O,
Co, C and O elements. The results of FT-IR, DRS, SEM and EDS analysis indicate
that the BiVO4/CoPc nanofiber heterostructures samples were successfully
synthesized.
Fig. 4 SEM images of the gel fibers (a), the BiVO4 nanofibers (b), the BiVO4/CoPc sample (C)

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and the EDS analysis (d) of the BiVO4/CoPc sample.
The possible formation mechanismof the BiVO4/CoPc hierarchical nanostructure:
at the early stage, the CoPc in the form of molecules uniformly distribute in the
ethylene glycol solution, grew into the primary seeds on the surface of the BiVO4
nanofibers through the hydrogen bonds, which formed by the nitro groups of CoPc
and the surface hydroxyl groups of BiVO4 nanofibers, Finally, after further growth,
the BiVO4/CoPc hierarchical nanostructured was prepared.
The Brunauer-Emmett-Teller (BET) specific surface areas of the BiVO4 and
BiVO4/CoPc samples were investigated by using nitrogen adsorption and desorption
isotherms. As shown in Figure 5, the BET specific surface area of BiVO4/CoPc
sample was calculated to be 32.51m2/g, which was much larger than the BiVO4
sample (16.63m2/g). The BiVO4/CoPc samples exhibit larger specific surface than the
BiVO4 fiber samples for the immobilization of cobalt phthalocyanine on the BiVO4
surface.
Fig. 5 Nitrogen absorption-desorption isotherms of BiVO4 (a) and BiVO4/CoPc (b) samples.
To prove the photocatalytic activity of the BiVO4/CoPc nanofiber heterostructures,
the decomposition of MB in water under visible light irradiation as a function of
irradiation time were investigated ( Figure 6 a). For comparison, decomposition
abilities of BiVO4 nanofibers and MB solution with out photocatalysts were measured
under the same experimental conditions. As figure 6 a shows, when assembling CoPc

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particles on the surface of BiVO4 nanofibers, the visible photocatalytic perfDoOrIm: 10a.n10c3e9/C4RA08759C
of the BiVO4/CoPc nanofiber heterostructures is dramatically improved. The
corresponding decomposition rate increases to 85% under 120 min irradiation, which
is much higher than that of BiVO4 nanofibers (65%). The MB solution exhibited little
photolysis, indicating the photolysis of MB could be ignored in the photocatalysis.
The BiVO4/CoPc nanofiber heterostructures possess enhanced visible photocatalytic
activities compared with BiVO4 fiber samples.
Fig. 6 The photodegradation MB of the BiVO4/CoPc under visible light irradiation (a), UV-vis
spectra of MB solution degraded by the BiVO4/CoPc sample(b).
To further make the reaction mechanism clear, the effects of various additives on
the photodegradation processes of the BiVO4/CoPc nanofiber heterostructures for MB
are examined. It is well known that EDTA is a very effective trap for holes and
tert-butanol (TBA) is widely used for ·OH scavenger14. From Fig. 7, we can see that
EDTA and TBA all lead to a marked suppression of the degradation rate of MB. The
results confirm that ·OH and holes play important roles in the mineralization of MB.

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Fig. 7 Various additives on the photodegradation processes of the BiVO4/CoPc nanofiber
heterostructures for MB.
The conduction band of the investigated phthalocyanine complexes is estimated to
be -0.9 V and the band gap is about 2 eV20. As calculated in the DRS analysis, the Eg
of BiVO4 is about 2.4 eV, we can calculate the conduction and valence band positions
theoretically through the following equation: EVB = X - Ee + 0.5Eg, where Ee is the
energy of free electrons on the hydrogen scale (≈4.5 eV), and X is the Mulliken′s
electronegativities. For BiVO4, the value of X is 6.035 and Eg is the band gap14. The
Ecb of BiVO4 prepared in this paper is estimated to be 0.38 eV.
On the basis of the above results and the earlier reports on the dye-sensitized
photocatalytic oxidation of pollutants21,22, a proposed mechanism of visible
light-induced photodegradation of MB with the BiVO4/CoPc hierarchical
nanostructures was elucidated schematically in Fig. 8. Upon irradiation with visible
light (λ > 400 nm), the phthalocyanine particles and the BiVO4 nanofibers are excited
and generated the excited charge (e-) and the hole (h+), the excited charge of CoPc
then injected from the excited state of the CoPc particle into the conduction band of
the BiVO4. The hole (h+) of BiVO4 then injected from the valence band of BiVO4
nanofibers into the valence band of CoPc. Dissolved oxygen molecules reacted with

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conduction band electrons (e-) in the BiVO4 to yield superoxide radical anions (O2·-),
and then protonated to generate the hydroxyl radicals (OH·) which has strong
oxidization ability to decompose the MB. The hole (h+) in the valence band of CoPc
reacted with MB directly and decompose the MB to products. Because the interior
charge recombination is avoided in the BiVO4/CoPc hierarchical nanostructures, and
the BiVO4/CoPc samples owned larger specific surface than the BiVO4 fiber samples
can absorb more active species and reactants on their surface, so the visible
photocatalytic activity of BiVO4/CoPc hierarchical nanostructures enhance apparently
than the BiVO4 fiber samples.
Fig.8 Postulate Mechanism of the Visible-Light-Induced Photodegradation of MB with the
BiVO4/CoPc Heterostructures sample.
In practical applications, the efficient mineralization of organic dyes and pollutants
is very important for preventing secondary pollution14. In order to study the
mineralization of the dyes, TOC (Total Organic Carbon ) for the MB solution with
different irradiation times in the photocatalytic procedure by the BiVO4/CoPc sample
was also investigated. As shown in Fig. 9a, the TOC content decreased to ca. 47%
after the irradiation of 120 min. Compared to the degradation in photocatalytic
experiment, the value was higher, which is because the bleaching of the dye solution

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goes beyond the inactivation of the chromophore groups in the molecular structure.
The complete inactivation of the chromophore groups of MB leads to bleaching of the
solution, but not necessarily to mineralization of the dye. The stability of the
BiVO4/CoPc was also evaluated by recycling the catalyst experiment and the
photocatalytic activity was examined in each cycle. As shown in Fig. 9b, the
photocatalytic activity of the BiVO4/CoPc does not exhibit obvious loss. After four
recycles, the degradation ration for the BiVO4/CoPc sample is still 79.2%. The
decrease of its activity may be due to the slight weight loss in the cycle experiment.
The BiVO4/CoPc sample exhibits high stability and excellent reusability in catalytic
process, which facilitates in practical application.
Fig.9 Variation of TOC removal versus time for the MB dye solution (a) and .
cycling degradation of MB mediated by BiVO4/CoPc (b).
4. Conclusions
The BiVO4/CoPc heterostructured nanofibers were prepared. The heterostructured
BiVO4/CoPc nanofibers extend the light adsorption to near IR region, owned large
specific surface area and possess enhanced visible light photocatalytic activities
compared with BiVO4. This study demonstrate that BiVO4/CoPc nanocomposite is a
very promising candidate for development of high photocatalysis performance for

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environmental and energy applications.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(Grant No. 51172133), Key Project of Chinese Ministry of Education (Grant No.
211098), Project of Independent Innovation of University Institute of Jinan (Grant No.
201311034) and the Project of Shandong Province Higher Educational Science and
Technology Program (Grant No. J13LA01). The authors also thank the Analytical
Center of Qilu University of Technology for technological support.
Notes and references
Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass
& Functional Ceramics, School of Material Science and Engineering, Qilu University
of Technology, Jinan, 250353, P. R. China.
Electronic Supplementary Information (ESI) available: The UV-vis spectrum of the
CoPc. Various additives on the photodegradation processes of the BiVO4/CoPc
nanofiber heterostructures for MB. Variation of TOC removal versus time for the MB
dye solution. Cycling degradation of MB mediated by BiVO4/CoPc.
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