Magnetic recyclable CoFe2O4?PPy prepared by: In situ Fenton oxidization polymerization with advanced photo-Fenton performance

Article abstract of DOI:10.1039/c9ra09191b

Here we present a magnetic recyclable photo-Fenton catalyst CoFe₂O₄?PPy with uniform morphology and excellent dispersibility prepared via simple in situ Fenton oxidization polymerization. The CoFe₂O₄ core provides good magnetic recyclability for the catalysts as well as the ion source for catalyzed decomposition of H₂O₂ in PPy coating. The optimal catalytic effect can be obtained by adjusting the ratio of CoFe₂O₄ and PPy. Methylene blue, Methyl orange and Rhodamine B (RhB) employed as model pollutants certificated that the catalyst exhibits a wide range of photodegradability. The decoloration rates reach nearly 100% in the photodegradation of 10 mg L^-1 RhB after 2 h visible-light irradiation and only low toxicity small molecules are detected by LC-MS. Moreover, the catalytic activity remains after 5 cycles with decoloration rates up to 90%. The degradation measurement in the presence of scavengers of reactive species reveals that the positive holes (h⁺) and hydroxyl radical (·OH) are the main reactive oxygen species in the CoFe₂O₄?PPy system. The performance enhancement may be attributed to the combination of improved Fenton activity by coordinated Fe²⁺ and PPy redox pairs and photo-catalytic activity by broaden adsorption and photo-generated charge separation.

Full text of DOI:10.1039/c9ra09191b

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PAPER
Magnetic recyclable CoFe2O4@PPy prepared by in
situ Fenton oxidization polymerization with
advanced photo-Fenton performance†
Cite this: RSC Adv., 2020, 10, 1858
*
Yuanming Deng,
and Jiaoning Tang
Xiaoman Zhao,
Junxuan Luo, Zhong Wang
Here we present a magnetic recyclable photo-Fenton catalyst CoFe2O4@PPy with uniform morphology
and excellent dispersibility prepared via simple in situ Fenton oxidization polymerization. The CoFe2O4
core provides good magnetic recyclability for the catalysts as well as the ion source for catalyzed
decomposition of H2O2 in PPy coating. The optimal catalytic effect can be obtained by adjusting the
ratio of CoFe2O4 and PPy. Methylene blue, Methyl orange and Rhodamine B (RhB) employed as model
pollutants certificated that the catalyst exhibits a wide range of photodegradability. The decoloration
rates reach nearly 100% in the photodegradation of 10 mg Lꢀ1 RhB after 2 h visible-light irradiation and
only low toxicity small molecules are detected by LC-MS. Moreover, the catalytic activity remains after 5
cycles with decoloration rates up to 90%. The degradation measurement in the presence of scavengers
of reactive species reveals that the positive holes (h+) and hydroxyl radical ($OH) are the main reactive
oxygen species in the CoFe2O4@PPy system. The performance enhancement may be attributed to the
combination of improved Fenton activity by coordinated Fe2+ and PPy redox pairs and photo-catalytic
activity by broaden adsorption and photo-generated charge separation.
Received 6th November 2019
Accepted 28th December 2019
DOI: 10.1039/c9ra09191b
rsc.li/rsc-advances
energy is utilized to drive the reduction is a promising route to
conquer the sluggish kinetic. Aer the excitation of semi-
Introduction
conductors with photo-catalytic activity, photo-generated
electrons on conductive band play a role of reductant for
Fe(III) while the photo-generated holes play a role of additional
oxidant for organic pollutant.
Nowadays, organic pigments and dyes are increasingly
popular because of the booming chemical industry and the
pursuit of better life. The discharge of waste water containing
such organic pigments and dyes brings about enormous
threats to our environment and health. As an advanced
oxidation process, the Fenton reaction has advantages of
complete degradation and mineralization of organic pollut-
ants by in situ generated highly oxidative free hydroxyl radi-
cals. However, the traditional Fenton reaction encounters
problems such as low pH conditions, sluggish Fe3+ reduction
kinetics and secondary pollutants caused by mass production
of slurry containing Fe3+. Therefore, heterogeneous Fenton
systems1–3 in which Fe2+ of the catalyst was replaced by Fe(II)
active sites from solid iron metals,4 alloys,5–7 oxides7–9 and
other compounds are developed for better recyclability.
Nevertheless, the rapid re-generation of Fe(II) active site from
Fe(III) on the surface of heterogeneous Fenton catalysts
remains challenges. Photo-Fenton10–12 in which the solar
Transition mixed metal oxide, CoFe2O4, with spinel struc-
ture13 is regarded as one of effective heterogeneous photo-
Fenton catalysts along with magnetic recyclability14,15 as well
as unique electrical properties,16 physicochemical stability,17
abundant resources, and eco-friendliness.18 However, poor
electron transfer activity and dispersibility severely limit the
catalytic activity of the ferrite.19 Therefore, it is of particular
importance to transform the steric hindrance and electrical
conductivity of the nanomaterial by changing the surface of
the nanomaterial and appropriate doping, thereby obtaining
stable nanoparticles and facilitating electron conduction.20
Coating ferrite with inorganic layers e.g. silica,10,21 inhibits the
agglomeration of magnets, thus improves its dispersibility.
Modifying ferrite with metal nanoparticles acting as electron
acceptors such as Ag,22 Au,23 Pd,24 Cu25 etc. on the surface
enhances the separation of photo-generated electron–hole
pairs. Carbon nanotubes,26 graphene,27,28 C3N4 (ref. 29 and 30)
or other organic semiconductors decorated on ferrite, on the
one hand, can act as a collector and transmitter of excited
electrons because of its conjugated large P bond, on the other
hand, can improve the light absorption, thus enhance the
Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research
Center for Interfacial Engineering of Functional Materials, College of Materials
Science and Engineering, Shenzhen University, Shenzhen 518060, China. E-mail:
ymdeng@szu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09191b
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photo-catalytic activity. In recent years, thanks to the high
carrier mobility and chemical stability, conductive polymers
such as polythiophene (PTH),31 polyaniline (PANI),32,33 poly-
pyrrole (PPy),34,35 etc. are used to modify the ferrite, giving the
composites good electron transport capability in photo-
catalysis process.36 The synergistic effect is revealed by the
combination of the organic phase and the inorganic phase, as
improved photo-catalytic performance compared to each
single component is exhibited due to better electron transfer
ability through multiple redox pairs.
Scheme
1
Schematic illustration for the synthesis procedure of
CoFe2O4@PPy.
So far, PPy has been conrmed to effectively enhance the
photocatalytic performance of semiconductor photo-catalysts,37
such as BiOI@PPy,38 TiO2@PPy,39 Fe3O4@PPy,40 etc. Moreover,
Yu et al.41 found that PPy showed better enzyme-mimic activity
than that of horseradish peroxidase (HRP) in catalyzing the
decomposition of hydrogen peroxide. Yang et al.42 also tried to
combine Fe3O4 with PPy to prepare a peroxide nanozyme with
magnetic properties and excellent conductivity. It is deduced
that the enzyme-mimic activity similar with Fenton catalytic
ability may be related to its porphyrin-like structure. Accord-
ingly, combination of PPy and ferrites may provide novel cata-
lysts with enhanced performance in photo-Fenton degradation
of organic pollutants due to their improved dispersibility,
recyclability, light-absorption, and electron transfer. However,
so far as we know, there are few reports on the application of
such kind of catalysts.
Herein, we present a facile preparation of an effective
photo-Fenton catalyst with good recyclability and reusability.
The PPy coated CoFe2O4 microspheres (CoFe2O4@PPy) were
prepared via Fenton oxidation induced in situ polymerization
coating method. In brief, in situ generated Fe3+ and Co2+ by
dissolution of CoFe2O4 in acidic solution were used to catalyze
the decomposition of H2O2 for the oxidation polymerization of
PPy. The coating of PPy on the surface of CoFe2O4 produces
a ferromagnetic CoFe2O4@PPy with uniform morphology,
good dispersibility and recyclability. The catalyst shows
enhanced photo-Fenton activity in the removal of model water
pollutants RhB under neutral condition with 95% de-
coloration within 30 min irradiation in the presence of
45 mM H2O2. The catalytic activity remains 90% of de-
coloration rate aer 5 cycles of degradation, revealing good
reusability. The degradation of the two dyes, methylene blue
and methyl orange, demonstrates the lower selectivity of
catalysts.
prevent the agglomeration between polypyrrole coated ferrite
microspheres. Therefore, a well dispersed, spherical core–shell
structure with uniform particle size distribution was prepared
successfully.
Characterization of the catalyst
The amount of Py used for coating is varied as 2 mmol, 4 mmol,
6 mmol and 8 mmol. Accordingly, the obtained CoFe2O4@PPy
were named as S1, S2, S3 and S4. The morphologies of synthe-
sized catalysts S1, S2 and S3 characterized by SEM and TEM
were shown in Fig. 1a–c. It can be seen from Fig. 1 that the
prepared CoFe2O4@PPy exhibits spherical morphology with
diameter of 200 nm on average and excellent dispersibility.
With the increase of PPy content, the size of CoFe2O4@PPy
microspheres hardly increases but their surface becomes
increasingly rougher than that of pure CoFe2O4 (Fig. S1†),
revealing that the surface of CoFe2O4 is coated with a very thin
layer of PPy. However, there's no obvious PPy layer can be
resolved from TEM images as shown in Fig. 1 when the PPy
dosage no more than 6 mmol, implying the PPy layer on S1, S2
and S3 is too thin. Legible PPy coating layer is observed for S4
when the dosage of Py is up to 8 mmol. From the TEM as shown
in the inset of Fig. 2a, a boundary between PPy and CoFe2O4 is
appeared vividly. The magnied TEM image of S4 as shown in
Fig. 2b indicates that the surface of CoFe2O4 microspheres was
covered with an amorphous layer of about 10 nm thickness. To
get a deep insight on the structure of S4, high resolution TEM
and selected area electron diffraction (SAED) measurements
were performed. The SAED pattern as shown in Fig. 2c shows
a series of discontinuous diffraction rings with inter-planar
distance of 0.299, 0.254, 0.174, 0.166 and 0.150 nm which are
corresponding to the distance between crystal planes of {220},
Results and discussion
The schematic illustration of preparation procedure and prin-
ciple is shown in Scheme 1. By adding hydrochloric acid to the
reaction system, an acidic environment is created and partial of
CoFe2O4 is dissolved. The released Fe3+ was reduced to Fe2+ by
pyrrole and the polymerization is initiated. As soon as the
addition of hydrogen peroxide, it reacted with the resulted Fe2+
to generate a hydroxyl radical in Fenton mechanism, which act
as oxidant together with Fe3+ to oxidize the pyrrole to form
a polypyrrole shell on the surface of CoFe2O4. The presence of
Fig. 1 SEM images of S1 (a); S2 (b); S3 (c). Insets are TEM images
small amount of surfactant SDS in the solution is expected to corresponding to the SEM, respectively.
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surface of CoFe2O4, demonstrating that PPy is successfully
coated on CoFe2O4 microspheres.
The XRD patterns of original CoFe2O4, PPy and CoFe2O4@PPy
loaded with different content of PPy are depicted in Fig. 3a. The
diffraction peaks at 2q values of 30.084ꢁ, 35.437ꢁ, 43.058ꢁ,
53.445ꢁ, 56.973ꢁ, 62.585ꢁ, and 74.009ꢁ correspond to the stan-
dard peak positions of spinel CoFe2O4 structure (PDF no. 22-
1086). However, the pattern of PPy has only one broaden
diffraction peak with a 2q value of 26ꢁ due to the amorphous
structure of PPy polymer. The patterns of CoFe2O4@PPy have no
obvious PPy diffraction peak but strong diffraction signal of
CoFe2O4, which is attributed to the strong signal of CoFe2O4
tending to overwhelm the weak carbon peak. Thermogravimetric
analysis (TGA) in air was also carried out for further conrming
the existence of PPy and roughly estimating its amount. Due to
the weight loss caused by the volatilization of adsorbed water is
observed at around 100 ꢁC, the thermo-gravimetric analysis is
started from 100 ꢁC. The temperature was raised from 100 ꢁC to
800 ꢁC for the pyrolysis of PPy.43 As shown in Fig. 3b, PPy is
almost completely oxidized at 650 ꢁC and the weight loss is near
100%, whereas the curve for CoFe2O4 only shows a slight weight
loss (ꢂ5%) at 600 ꢁC. The weight loss for the composites
consecutively increases with the increase of the PPy content.
From the weight loss of composites, the content of PPy in S1, S2,
S3 and S4 is estimated to be 40, 23, 16 and 11 wt%. The Raman
spectra of CoFe2O4@PPy and bare PPy, CoFe2O4 are shown in
Fig. 3c. There are two distinct broad peaks ranging from
1200 cmꢀ1 to 1800 cmꢀ1 in CoFe2O4@PPy and pure PPy. The
peak at 1350 cmꢀ1 is assigned to the D band (C–N stretching
vibration), which mainly due to the presence of structural defects
Fig. 2 The SEM image of S4 (a); the TEM image of S4 (b); the SAED
pattern of S4 (c); the HRTEM image of S4 (d); the TEM-EDS image of S4
(e).
{311}, {422}, {511} and {440}, revealing a polycrystalline with
cubic lattice structure of CoFe2O4 microspheres. From HRTEM
image of Fig. 2d, the inter-planar distance is calculated to be
0.299 and 0.254 nm, corresponding to (220) and (311) plane of
CoFe2O4 respectively. Furthermore, the EDS mapping of a single
microsphere of S4 as shown in Fig. 4e clearly signies that
CoFe2O4@PPy contains ve elements of Co, Fe, O, C and N,
among which C and N elements evenly distributed on the
Fig. 3 Powder XRD patterns of PPy, CoFe2O4, and S4 (a); FT-IR spectra of PPy and S4 (b); Raman spectra (c) and TGA patterns (d) of PPy,
CoFe2O4, and S4.
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and marginal unsaturated carbon, while the characteristic peak
The valence state and element ratio in CoFe2O4@PPy and
at 1580 cmꢀ1 is attributed to the G band (C]C stretching CoFe2O4 were analyzed by XPS as shown in Fig. 5. From the full
vibration), representing the in-plane stretching vibration of XPS spectra shown in Fig. 5a, the peaks of elements C, O, Fe, Co
carbon atom sp2 hybrid.34,44 It can be seen from Fig. 3c that were observed at nearly 280, 530, 720 and 780 eV successively.
unloaded CoFe2O4 has no obvious G band or D band, whereas Apart from those observed for CoFe2O4, the signal for CoFe2-
there is a signicant double peaks aer PPy loading, indicating O4@PPy emerged at approximately 400 eV is ascribed to
the existence of PPy in the composites. In addition to the existing nitrogen element in coated PPy layer. The magnication spectra
D band and G band, two peaks at 1045 and 977 cmꢀ1 are of signals assigned to specic element C, N, O, Co and Fe were
observed because of the existence of PPy with the polaron (NH+) shown in Fig. 5b–f. C 1s XPS spectra in Fig. 5b was tted by four
and bipolaron structures (N+). It is worth noting that aer the component peaks at 284.4, 285, 286, and 288.5 eV, corre-
addition of CoFe2O4 to the PPy matrix, the characteristic sponding to C–C, C–H, C–N, C]O groups49 respectively. The
absorption peak of PPy at 1579 cmꢀ1 was blue-shied to main peak at 399.8 eV in the deconvoluted N 1s XPS spectrum
1592 cmꢀ1. This transfer may be owing to the interaction between (Fig. 5c) is assigned to neutral amine nitrogen(–NH–),50,51
CoFe2O4 and PPy backbones. The pure polypyrrole has a sharp G whereas the small peak at 401.6 eV could be originated from
band and a at D band, while for CoFe2O4@PPy, the relative iron–nitrogen related bond (Fe–N type) which is regarded as
intensity of the D band and the G band decreases, manifesting catalytic active Fe–N sites. The O 1s spectra as shown in Fig. 5d
that the degree of disorder of PPy increases due to the intro- can be divided into several peaks at around 530.36, 531.73 and
duction of CoFe2O4 nanoparticle.45 FT-IR measurement was also 533.1 eV, which can be assigned to the surface adsorbed
performed. As shown in Fig. 3d, the absorption peak at hydroxyl oxygen, water and crystal lattice oxygen O2ꢀ (ref. 16)
1550 cmꢀ1 is attributed to the anti-symmetric stretching of respectively. For CoFe2O4@PPy, the peak locating at 530.36 eV is
pyrrole ring due to the stretching vibration of the C]C bond; the weakened due to the cover of PPy layer. Notably, an additional
absorption peaks observed at 1400 and 1043 cmꢀ1 correspond to peak at 531.1 eV appeared, probably due to the formation of Fe–
the N–H and C–H deformation vibrations; and the peak at O–C bond between PPy and Fe2+, which is benecial for the
1217 cmꢀ1 is derived from C–N+ stretching,46,47 moreover rapid transfer of electrons during the photo-Fenton reaction.52
940 cmꢀ1 and 790 cmꢀ1 are the C–C bond vibration and the out- The PPy coating also reduced the relative strength of Fe and Co.
of-plane vibration of C–H bond,41 respectively.
From Fig. 5e, the three characteristic peaks with binding ener-
The specic surface area and pore size distribution of the gies of 780.05 ꢃ 0.2 eV, 781.89 ꢃ 0.2 eV and 785.9 eV are
samples can be determined by isothermal nitrogen adsorption– ascribed to Co 2p3/2. The binding energies of 780.05 ꢃ 0.2 eV
desorption curves. The results as shown in Fig. 4a indicate that the and 781.89 ꢃ 0.2 eV are Co2+ ions at octahedral and tetrahedral
curves of CoFe2O4, PPy, CoFe2O4@PPy own hysteresis loop positions,53 respectively, conrming the chemical state as Co(II).
consistent with the IV type adsorption–desorption behavior. The The signal at 785.9 eV is characterized to be the satellite peak of
BET surface areas of pure CoFe2O4, PPy, CoFe2O4@PPy calculated Co 2p3/2 main line. In the Fe 2p spectrum of CoFe2O4 (Fig. 5f),
by isotherms are 90.48, 8.65, 69.86 cm2 gꢀ1, respectively. The signals appear at 711.10 and 724.7 eV with the separation of
coating of PPy reduces the specic surface area of CoFe2O4@PPy, ꢂ13 eV are ascribed as Fe 2p3/2 and Fe 2p1/2, respectively,
which may be attributed to the lling or covering of the micro- indicating the presence of Fe3+. Furthermore, a satellite peak
pores of CoFe2O4. CoFe2O4 and CoFe2O4@PPy have a slight appears at the binding energy of 720.14 eV, which is a typical
capillary condensation step in the relative pressure range of 0.44– feature of Fe3+. However, the two signicant peak positions of
0.87, indicating that the material has a large mesopores.48 In CoFe2O4@PPy shied to a low binding energy by 1.35 eV with
addition, the hysteresis loop in the curve shows high permeability the separation of ꢂ6 eV,54 demonstrating the existence of Fe2+
.
between the mesopores. Fig. 4b shows that the pore size is mainly The appearance of Fe2+ may be due to the reduction of Fe3+ by Py
distributed at 1–20 nm, which suggests that CoFe2O4 and CoFe2- during the in situ growth of Py and then Fe2+ strongly coordi-
O4@PPy are mainly mesoporous structures, giving them physical nated with the nitrogen atom in PPy,35 controlled growth of PPy
adsorption capacity.
Fig. 4 Nitrogen absorption–desorption isotherms (a) and the pore size distribution curve (b) of pure CoFe2O4, PPy and S4 composite.
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Fig. 5 XPS spectra of the pure CoFe2O4 and S4. Survey of the samples (a); C 1s narrow scan (b); N 1s narrow scan (c); O 1s narrow scan (d); Co 2p
narrow scan (e) and Fe 2p narrow scan (f).
into the surface of CoFe2O4. Accordingly, Fe2+ is embedded in Fig. 6a–c. It can be seen from Fig. 6a that the adsorption of RhB
the interior of PPy which is consistent with the result of EDS.
is signicantly increased with the incorporation of PPy, which
may be attributed to the strong interaction between RhB and
PPy. Compared with CoFe2O4 and PPy, PPy modied CoFe2O4
showed obvious dark Fenton activity when H2O2 was added in,
as the concentration of RhB decreased gradually because of the
Fenton-like degradation. The improved catalytic performance of
CoFe2O4@PPy and PPy may be attributed to the presence of Fe2+
The catalytic activity and reusability of catalysts in Fenton
degradation of RhB
The abilities of different samples to degrade organic substance
under dark Fenton, photocatalysis, photo-Fenton were detected
using RhB as a model pollutant, and the results were shown as
Fig. 6 Rhodamine degradation ability under dark Fenton for all samples (a); the photocatalytic degradation of RhB over all samples under visible
light irradiation (b), the photo-Fenton degradation of RhB solution with the presence of H2O2 under visible light irradiation (c) (CRhB ¼ 5 mg Lꢀ1
CH O ¼ 45 mM, pH ¼ 7).
,
2
2
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coordinated in PPy, which may be dissolved and released into content. Since S4 is the composite with the best synergy effect,
the solution to catalyze the decomposition of H2O2, similar with we chose S4 as the main research object in subsequent experi-
that reported in the enzyme-mimic behavior. It is worth ments. In order to prove the non-selectivity of catalytic degra-
mentioning that PPy decolorized RhB by 50% in the rst 10 dation, we also selected methyl orange and methylene blue as
minutes of dark Fenton. This may be due to the large amount of substances to be degraded, which showed outstanding photo-
Fe2+ anchored into PPy during synthesis process. With the degradation effect (Fig. S2–S4†).
addition of H2O2 during the dark Fenton process, it is
The reusability of CoFe2O4@PPy was investigated by cycling
consumed in a short time. Subsequently, the ability of PPy to test. To reduce the inuence of RhB adsorption in the degra-
degrade RhB tends to be gentle, while other composites still dation, the RhB concentration is increased to 10 mg Lꢀ1 and the
maintain a stable activity. Among them, the dark Fenton ability time of each cycle is set as 2 h. As shown in Fig. 7, the degra-
of S1 and S2 is poor, which may be due to the relatively small dation efficiencies of RhB in these ve cycles are 99.7%, 99.5%,
amount of PPy coating. Although S3 has a higher degree of 92.1%, 91.4% and 91.2%, respectively. Although the degrada-
decoloration at 60 minutes compared with S4, we believe this is tion efficiency was slightly reduced, the overall decolorization
mainly due to the strong adsorption effect during dark process. effect was still signicant, demonstrating that the stability of
Comparing the dark Fenton activity, S4 has a better catalytic the system is still brilliant. The slight decrease of decolorization
ability, and the photocatalysis is consistent with this result. rate may be owing to the adsorption of small molecules on the
Fig. 6b shows that the photocatalytic ability of S4 is better than surface of CoFe2O4@PPy aer decomposition of RhB molecule,
that of S3. This strongly indicates that S4 has improved elec- occupying the catalytic position on the surface active site. We
tron–hole transport ability and more oxidizing substances are compare this work with others, as shown in Table 1, which
generated. The photo-Fenton degradation behavior shows demonstrates that CoFe2O4@PPy has better photocatalytic
signicantly better performance than that of dark Fenton and ability.
photocatalysis shown as Fig. 6c. Similarly, the activity of
CoFe2O4@PPy increases with the increase of polypyrrole
Degradation pathway and species of RhB
The photo-Fenton degradation mechanism of RhB may be that
the RhB molecule is eventually oxidized.55 As the reaction pro-
gressed, the RhB solution decolored from rose red to a colorless
chromophore. However, the decolorization of RhB does not
mean complete mineralization, and the intermediate products
produced during the degradation process may also be
contaminated and toxic, and cannot achieve the purpose of our
initial preparation of the catalyst. Therefore, during the exper-
iment, we chose to scan a wavelength range of 200–650 nm.
Fig. 8a shows that the peak at 553 nm is decreasing, and the
peak at 259 nm, 310 nm, and 320 nm gradually disappears,
indicating that the aromatic chain, N-position ethyl group and
conjugated structure in the RhB molecule are gradually
destroyed during the reaction resulting in small organic mole-
cules.55 Due to the destruction of the C]N and C]O
Fig. 7 Cycling runs for the photodegradation of RhB in the presence
of S4 under visible light irradiation. (CRhB ¼ 10 mg Lꢀ1, CH O ¼ 45 mM,
2
2
pH ¼ 7).
Table 1 The comparison of this work with others
Catalyst
Dye
Reaction conditions
Degradation
99.5% (30 min) 5th (90%)
6th (70.2%) 62
Reusability Ref.
CoFe2O4@PPy
RhB
[Dye] ¼ 5 mg L; [H2O2] ¼ 45 mM;
pH ¼ 7; catalyst ¼ 0.01 g
This work
Fe3O4/P(MAA-MBAA)-PPy/Au/void/TiO2 RhB
Ag@CoFe2O4/Fe2O3
[Dye] ¼ 5 mg L; pH ¼ 3; catalyst ¼ 4 mg 100% (90 min)
RhB
[Dye] ¼ 5 mg L; [H2O2] ¼ 225 mM;
99.15% (1 h)
63
pH ¼ 7; catalyst ¼ 2 cm ꢄ 2 cm square
Reduced graphene oxide-CoFe2O4
Malachite green [Dye] ¼ 5 ꢄ 10ꢀ6 M; [H2O2] ¼ 112.5 mM; 100% (30 min)
64
pH ¼ 7;
catalyst ¼ 0.05 g
rGO/PB/PPy
RhB
[Dye] ¼ 10 mg L; [H2O2] ¼ 22.5 mM;
pH ¼ 6.18; catalyst ¼ 0.01 g
[Dye] ¼ 10 mg L; [H2O2] ¼ 180 mM;
pH ¼ 7; catalyst ¼ 0.03 g
95.2% (30 min) 4th (91.5%) 65
100% (150 min) 5th (94.6%) 66
rGS/FexOy/NCL
PPy NF/Zn–Fe LDH
RhB
Safranin
[Dye] ¼ 5 mg L; catalyst ¼ 0.02 g;
pH ¼ 7
88% (120 min)
6th (58%)
67
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Fig. 8 The photocatalytic degradation of RhB over S4 under visible light irradiation (CRhB ¼ 10 mg Lꢀ1, CH O ¼ 45 mM, pH ¼ 7) (a). LC/MS spectra
2
2
of end products of the photo-Fenton degradation of RhB (b), the degradation path of possible intermediates (c).
conjugated structures, the characteristic absorption peak at into small molecules. Further mineralization into H2O and CO2
553 nm gradually decreases.56 The disappearance of the eventually.
absorption peak at 259 nm indicates that the structure of the
benzene ring has been destroyed. However, the peak at 225 nm
The mechanism in performance enhancement
has been rising. It is speculated that the conjugated structure of
To deep understand the mechanism in performance enhance-
the RhB molecule is gradually destroyed during the reaction,
ment, PL tests and quench experiments were conducted. The
and other non-polluting small molecules may be formed by de-
room temperature PL spectrum was used to study the separa-
tion and recombination rate of photoexcited charges of charge
carriers. As shown in Fig. 9a, the PL spectrum of CoFe2O4 shows
only one emission peak at 427 nm. Whereas, that of CoFe2-
O4@PPy shows two emission peaks at 427 nm and 620 nm,
which are attributed to the band PL and the exciton PL
respectively.57 Compared to the PL of CoFe2O4, the lower
ethylation at the N-position. In order to gain a deeper under-
standing of the colorless products aer degradation, we con-
ducted a LC/MS test. Aer two hours of visible light irradiation,
the nal colorless product of Rhodamine B degradation was
used for LC-MS testing to analyze the components. From
Fig. 8b, the nal products were mainly small molecule
compounds, which means RhB has been completely decom-
intensity of band PL at 427 nm in PL of CoFe2O4@PPy indicates
posed into low-toxic or even non-toxic substances. Based on the
that the load PPy contributes to the separation of the photo-
results of LC-MS, the structures of major intermediates can be
generated charges, resulting less charge recombination. The
incorporation of PPy also introduces in a broad emission peak
range from 500 to 750 nm, which can be attributed to the
oxygen-related defects (such as oxygen gaps).58 This observation
inferred as Fig. 8c. The degradation process mainly involves: (i)
at the beginning, the peaks differ exactly by 28 mass units
successively, indicating sequential N-de-ethylation of RhB. (ii)
The intermediate is further broken down into several
reveals that the coating of PPy results defect-rich surface and
compounds by the cleavage of the azo (C–N) bond. Fragmented
oxygen vacancy of CoFe2O4, which are also evidenced by the XPS
compounds have no chromophore, resulting in colorless. A
results. Quenching experiments were carried out to investigate
ring-opening reaction occurs, decomposing the intermediate
the active species of photocatalytic properties. In the photo-
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CoFe2O4 reveals that the factor response for photo-Fenton
activity is far beyond the specic surface area. The quench
experiments also indicate that in CoFe2O4@PPy catalyzed
photo-Fenton, the contributor on degradation is more than
hydroxyl radicals. Therefore, the improvement of degradation
performance may be attributed to several factors. Firstly, there
is an appropriate amount of Co2+, Fe2+ and Fe3+ embedded in
the interior of PPy during the polymerization, which may
accelerate the decomposition of H2O2 through Fenton way,
which may mainly contribute to the formation of hydroxyl
radicals. Secondly, oxygen vacancy and lattice defect may be
introduced into the CoFe2O4 during the PPy coating. The exis-
tence of oxygen vacancy enhances the adsorption of substrates,
Fig. 9 PL spectra of CoFe2O4 and S4 (a); trapping experiment of active
species during the photocatalytic degradation of RhB over S4 under
visible light irradiation (b) (CRhB ¼ 10 mg Lꢀ1, CH O ¼ 45 mM, pH ¼ 7).
2
2
Fenton process, superoxide radicals from oxygen reduction, provides more active sites, and promotes the photo-generated
photo-generated holes, and hydroxyl radicals from H2O2 charge separation.61 Thirdly, as an organic semiconductor,
decomposition and water oxidation are responsible for the PPy not only promotes the light harvesting, but also accelerates
degradation of organic pollutants. Therefore, EDTA as the electron transfer in the re-generation of active site for Fenton
electron donor, ethanol (EA) containing a-H as the free radical reaction through redox pairs. In summary, the improvement on
scavenger and benzoquinone (BQ) as the superoxide radical degradation activity of photo-Fenton can be attributed to the
quencher were added into the photo-Fenton system individu- combination of improved Fenton oxidation and additional
ally. As shown in Fig. 9b, compared with the control experiment photo-catalyzed degradation provided by coated PPy layer on
without any quencher, the RhB decolorization rate decreased by CoFe2O4 microspheres.
57.74%, 69.25% and 76.16% aer adding EA, EDTA and BQ
respectively, revealing a complicate contribution of decoloriza-
tion. According to the results of the quenching test, we specu-
lated the mechanism of performance improvement. CoFe2O4
Experimental
Materials and reagents
and PPy will generate photo-induced electrons and holes under
light irradiation59 (reaction (1)). Under light irradiation, the
photo-generated electrons of CoFe2O4 move to the PPy shell and
react with oxygen attached to the surface to generate $OO-
ꢀ(reaction (2)), which contributed to the degradation of organic
matter. Simultaneously, Fe2+ located between the core and shell
can form a Fenton reaction system with H2O2 to form Fe3+(re-
action (3)). Hydrogen peroxide can be captured by Fe3+, gener-
ating Fe2+ and performing Fe2+/Fe3+ cycles (reaction (4)–(6)),
which makes recycling and stability possible. Moreover, $OH
generated in this process is the most important active species
according to the trapping experiment. The accumulated h+ can
oxide the organic substrates directly60 (reaction (7)).
CoSO4$7H2O, FeCl3$6H2O, ethylene glycol (EG), pyrrole (Py),
rhodamine B (RhB), hydrogen peroxide (H2O2, 30%), hydro-
chloric acid (HCl) were obtained from Aladdin (China). Sodium
dodecyl sulfate (SDS), sodium acetate anhydrous (NaAc, AR),
polyethylene glycol (PEG), ethylenediaminetetraacetic acid
(EDTA), methanol (MA) and benzoquinone (BQ) were purchased
from Macklin (China). All chemicals were used without further
purication.
Preparation of CoFe2O4@PPy
CoFe2O4 was prepared according to the previous work,43,68 and
the details are shown in the ESI.† The CoFe2O4@PPy was
synthesized via in situ Fenton oxidation polymerization of
pyrrole. Typically, 50 mg CoFe2O4, 100 mg SDS and various
amounts of pyrroles were mixed with 40 mL water under
magnetic stirring to form a homogeneous brown dispersion. To
the dispersion cooled under ice bath, 280 mL HCl and 2 mL 30%
H2O2 were successively dropped in, and vigorous stirring was
continued for other 3 hours. The product was collected by
centrifugation and repeated washed, then vacuum dried at
65 ꢁC for 12 h. For comparison, PPy nanoparticles were also
prepared according to the previous report. In this synthesis
procedure, the molar ratios of CoFe2O4/PPy were adjusted at
1 : 2, 1 : 4, 1 : 6 and 1 : 8, and the corresponding CoFe2O4@PPy
samples were named Sx (x ¼ 1–4).
ꢀ
+
PPy + CoFe2O4 + hn / eCB + hVB
(1)
(2)
(3)
(4)
(5)
(6)
(7)
ꢀ
ꢀ
O2 + eCB /$O2
Fe2+ + H2O2 / Fe3+ + $OH + OHꢀ
Fe3+ + H2O2 / Fe2+ + $OOH + H+
Fe2+ + $OOH / Fe3+ + $OOHꢀ
Fe3+ + eCB / Fe2+
ꢀ
ꢀ
$OH + $O2 + h+ + RhB / degraded products
Characterization
Generally, the larger specic surface area can provide more
active sites for photodegradation and promote photocatalytic
reaction. However, the relatively poor catalytic performance of
The morphology and composition of samples were observed by
scanning electron microscope (SEM, SU-70) equipped with EDS
and Transmission electron microscope (TEM) (JEM-21200F).
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The magnetic properties of CoFe2O4 and CoFe2O4@PPy reactive species and PL spectra of catalysts. Photodegradation
composites were measured in a vibrating sample magnetometer experiments of methyl orange and methylene blue conrm the
(Quantum Design Corporation, USA) with a maximum applied wide applicability of the catalyst. It is deduced to be attributed
eld of ꢃ2 T. X-ray diffraction (XRD) curves were recorded with to the synergistic effects of the following factors: rstly, the
an X-ray diffractometer (Smart Lab) using Cu Ka radiation (l ¼ rapid Fenton oxidation brings about massive defects in PPy
˚
1.5418 A). The chemical structure of samples was characterized layer, thereby strongly improves its adsorption to RhB;
by FT-IR spectrometer.
secondly, the in situ dissolved Fe3+ is also reduced to Fe2+ and
(Nicolet 6700) and FT Raman Microscope (RENIDHAW embedded into the PPy layer through N–Fe coordination thus
invia). The chemical valence state of samples was analyzed by improves the activity in Fenton reaction; thirdly, the presence of
XPS (PHI5000 Versa Probe II). TGA was performed to under- PPy coating layer prevents the agglomeration of magnetic
stand the content of PPyꢀo1n a thermal analyzer (TGA 55) at particles; the last but the most important, the PPy layer on
a heating rate of 10ꢁC min from ambient to 800 ꢁC in air. The CoFe2O4 enhances the absorption of light, inhibits the combi-
nitrogen adsorption desorption isotherms were measured at 77 nation of photo-generated charge carriers and promotes the
K using a Micromeritics ASAP 2460 system, and the surface area electron transfer for H2O2 decomposition.
and pore size distribution were calculated using the Brunauer–
Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
method, respectively. The UV-Vis absorption spectrum (UV-Vis)
Conflicts of interest
was characterized with a UV-Vis spectrometer (PerkinElmer
Lambda 950). Liquid chromatography tandem mass spectrom-
There are no conicts to declare.
etry (LCMS) with QTRAP6500+ was employed to determine the
nal product of rhodamine decolorization. Photoluminescence Acknowledgements
(PL) emission spectra was performed on a Renishaw in via
The authors would like to thank the support of Instrumental
spectrometer with 285 nm UV light as the excitation source.
Analysis Center of Shenzhen University (Xili Campus), Natural
and
Science
Foundation
of
Guandong
Province
Degradation of RhB by heterogeneous catalyst
(2015A030310238), Knowledge Innovation Program of Shenz-
hen City (Grant: JCYJ20160422102919963) and Natural and
Science Foundation of China (51778639).
The photo Fenton activity was assessed by the degradation of
rhodamine B (Rh B) under visible light irradiation with a 300 W
xenon lamp with a 420 nm cut-off lter. Typically, 10 mg sample
was dispersed in 50 mL 10 mg Lꢀ1 of RhB in a baker with cycling
water cooling and magnetic stirring. Aer 30 min for adsorption
in dark, 200 mL of 30% H2O2 was added in and light on. Solution
was taken out at preset intervals and centrifuged prior to UV-
visible spectrometry analysis. The dark Fenton activity was
assessed similarly except lighting off. The catalyst aer 1 cycle
of 2 h degradation test was washed with ethanol and dried
thoroughly, then used for the other cycle of degradation to
evaluate its reusability. Photo degradation in the presence of
ethylene diamine tetra-acetic acid (EDTA), methanol (MA) and
benzoquinone (BQ) as a quencher for holes, hydroxyl radicals
and superoxide radicals respectively, was also carried out to
study the active species during degradation.
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