A visible-light-driven solid state photo-Fenton reagent based on magnetite/carboxylate-rich carbon spheres

Article abstract of DOI:10.1039/c2ce25834j

A visible-light-driven solid state photo-Fenton reagent based on magnetite/carboxylate-rich carbon spheres (MCRCSs) has been one-step fabricated via hydrothermal carbonization (HTC) process. Without using H₂O ₂, MCRCSs exhibit excell

Full text of DOI:10.1039/c2ce25834j

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COMMUNICATION
A visible-light-driven solid state photo-Fenton reagent based on
magnetite/carboxylate-rich carbon spheres{
Zhijun Luo,*ac Hongjun Tang,b Lingling Qu,b Tingting Hanb and Xiangyang Wu*a
Received 27th May 2012, Accepted 18th June 2012
DOI: 10.1039/c2ce25834j
hydrothermal carbonization (HTC) process is that rich functional
groups can be retained, which can greatly improve hydrophilicity
and chemical reactivity.6 However, the amount of carboxylic acid
groups contained in amorphous carbon obtained by HTC is rather
low, underlining the highly reductive conditions throughout the
hydrothermal, oxygen-free carbonization reaction (the constituting
aldoses transform into aldehydes onto the surface), which makes it
difficult to obtain the carboxylate-rich carbon materials.7 Titirici
et al. reported that carboxylate-rich carbon could be obtained via
the cycloaddition reaction between acrylic acid and glucose during
the HTC process.8
A visible-light-driven solid state photo-Fenton reagent based on
magnetite/carboxylate-rich carbon spheres (MCRCSs) has been
one-step fabricated via hydrothermal carbonization (HTC)
process. Without using H2O2, MCRCSs exhibit excellent
photodegradation activity at neutral pH under visible light
irradiation.
Recently, iron-based photocatalytic systems for wastewater treat-
ment have attracted much attention because of their low cost and
benignity to environment. Among varied iron-based photocataly-
tic systems, the photo-Fenton reaction is one of the most efficient
The motivation of the present research derived from the idea
that encapsulating Fe3O4 particles with an amorphous carbon
sphere rich in carboxylate groups can form ferricarboxylate
1
?
routes for wastewater treatment via hydroxyl radicals ( OH). The
classical homogeneous photo-Fenton system (Fe3+/H2O2/UV) has
some intrinsic drawbacks that limit its widespread application,
such as a narrow working pH range, high H2O2 dosage, and
difficulty of separation and recovery of the iron species.2 Iron
oxides as catalysts, the solidification of iron ions, and H2O2
together under light irradiation could set up a heterogeneous
photo-Fenton system, which can destroy organic pollutants
efficiently in a wider pH range with much less iron loss than the
homogeneous photo-Fenton system.3 Without using unstabilized
H2O2, another interesting photo-Fenton system is the combination
of carboxylic acids (such as oxalic, malic, citric, tartaric acids, etc.)
and iron (just dissolved or as oxides), which can form
ferricarboxylate complexes that absorb light irradiation with high
quantum yield to trigger radical chain mechanisms of oxidation.4
If the carboxylic acids can be solidified just like the solidification of
iron ions, it is feasible to fabricate the solid state photo-Fenton
reagent through the solidification of iron ions and carboxylic acids
at the same time, which is different from the homogeneous and
heterogeneous photo-Fenton reagent.
?
complexes, which can yield OH under visible light irradiation.
Herein, we report a facile one-step route to directly fabricate
MCRCSs. Without using H2O2, MCRCSs exhibit excellent
photodegradation activity for methylene blue (MB) at neutral
pH under visible light irradiation. In addition, it own unique
superiority in separation and recycling allows it to be collected
easily by applying an appropriate magnetic field. To the best of
our knowledge, this is the first report about the visible-light-driven
solid state photo-Fenton reagent based on MCRCSs.
Generally, the fabrication of Fe3O4/carbon composites require
two steps, the synthesis of an Fe3O4 core and the subsequent
coating of the carbon shell, which makes the synthesis route
complicated and render the catalysts too expensive for widespread
industrial use.9 In the present study, we develop a facile one-step
synthesis of MCRCSs, which only needs two reagents: the single
iron source FeCl3 and sodium gluconate. Different functional
groups of sodium gluconate play synergetic roles in the formation
of MCRCSs as illustrated in Scheme 1: (1) the hydrolysis of
sodium gluconate, which is the strong base–weak acid salt,
provides controlled quantities of OH2 ions for the formation of
Fe3O4; (2) Fe3+ can be reduced into Fe2+ in the presence of large
numbers of reductive hydroxyl groups of sodium gluconate under
the hydrothermal treatment; (3) sodium gluconate as the derivative
of glucose could form the carboxylate-rich carbon via the
aromatization and carbonization reaction during the HTC process.
In addition, the strong coordinating ability of carboxylate group to
metal cations means the gluconate can strongly bond with the Fe
atom of surface of Fe3O4 particles, which is propitious to the in situ
carbon coating on Fe3O4 particles.
Compared with the intensive research of fullerenes, carbon
nanotubes (CNTs), and graphene, amorphous carbon, another
allotrope of carbon, has been neglected in photocatalytic
applications.5 The merit of amorphous carbon obtained by a
aSchool of The Environment, Jiangsu University, Zhenjiang, 212013,
P. R. China. E-mail: lzj@ujs.edu.cn
bSchool of Chemistry and Chemical Engineering, Jiangsu University,
Zhenjiang, 212013, P. R. China
cState Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing, 210093, P. R. China
{ Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ce25834j
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Scheme 1 Functional groups of sodium gluconate play synergetic roles
in the formation of MCRCSs.
The XRD pattern (Fig. 1a) of the as-prepared samples could be
indexed as Fd3m cubic Fe3O4 (JCPDS card No. 11-0614). No
other characteristic peaks such as the impurities of hematite or
hydroxides were detected, suggesting high purity of the as-
synthesized Fe3O4 in MCRCSs. No obvious diffraction peak for
the carbon is observed, suggesting the carboxylate-rich carbon is
amorphous carbon. The Raman spectra (Fig. 1b) for MCRCSs
shows the characteristic wide D and G bands around 1360 and
1580 cm21, respectively, typical for amorphous carbons. The large
ID/IG value (0.8) indicates the low degree of graphitization of the
carboxylate-rich carbon. The carbon content is determined by
thermogravimetric analysis (TGA, Fig. S1{). It can be noticed that
the weight loss below 190 uC could probably be attributed to the
evaporation of the adsorbed moisture or gaseous molecules, and
the major weight loss takes between 190 uC and 450 uC, giving rise
to a large weight loss of about 59.28 wt%. While considering that
Fe3O4 will be converted into Fe2O3 when heated in air, the actual
carbon content in the MCRCSs can be estimated to be about
64.16%. Fig. 1c presents a panoramic scanning electron micro-
scopy (SEM) image of the MCRCSs. It can be seen that the
MCRCSs have a relatively uniform diameter of about 5 mm. The
high-resolution SEM image of an individual sphere (Fig. 1d)
shows that these spheres are constructed of numerous particles.
Fig. 2 FT-IR and XPS spectra of MCRCSs: (a) FT-IR, (b) Fe 2p, (c) C
1s, (d) O 1s.
FT-IR spectroscopy was further used to investigate the
functional groups remaining on the surface of MCRCSs. For
the purpose of comparing MCRCSs with Fe3O4 nanoparticles,
Fe3O4 nanoparticles were synthesized and the XRD pattern and
TEM image are shown in Fig. S2 and Fig. S3,{ respectively.
Fig. 2a shows the FT-IR spectra curves of Fe3O4 nanoparticles
and MCRCSs. The band at 1700 cm21 is associated with carbonyl
in carboxylate group.8 Two additional vibration bands located at
1590 and 1408 cm21 can be assigned to the asymmetric and
symmetric stretching vibration of COO2 groups, confirming that
large amounts of carboxylate groups from carbon were coordi-
nated strongly to the iron cations.10 The band at 580 cm21 is
attributed to the Fe–O stretch, which indicates the existence of
Fe3O4.11 The bands at 3400 cm21 imply the existence of large
numbers of hydroxyl groups. The bands at 2923 and 2854 cm21
should be attributed to the stretching vibration of –CH2–.10,12
Surface analysis of the MCRCSs was carried out using X-ray
photoelectron spectroscopy (XPS). In the partial XPS spectrum of
Fe 2p (Fig. 2b), the peaks for Fe 2p3/2 and Fe 2p1/2 were observed
at 711.2 and 724.5 eV, which is also indicative of the formation of
a Fe3O4 phase in MCRCSs. The partial XPS spectrum of C1s
(Fig. 2c) can be deconvoluted into three component peaks with
binding energies at about 284.4, 285.1 and 287 eV, which are
attributed to the C–C, C–O, and CLO bonds, respectively.13 For
the O 1s spectrum represented in Fig. 2d, three component peaks
with binding energies at about 531, 532.1 and 533 eV were
identified. They are attributed to the Fe–O, CLO, and C–O bonds,
respectively.12,13 According to the FT-IR and XPS spectra results,
it was demonstrated that a large number of carboxylate groups
and hydroxyl groups remained in the MCRCSs and carboxylate
groups from carbon were coordinated strongly to the iron cations.
As is well known, Fe3O4 nanoparticles are usually hydrophobic,
which limits their application in the photocatalytic degradation of
organic pollutants. However, these hydrophilic groups, carbox-
ylate groups and hydroxyl groups, can endow the MCRCSs with
excellent aqueous dispersibility (videos S1 and S2, ESI{).
To shed light on the formation mechanism of MCRCSs, the
time-dependent experiments were carried out. Fig. S4{ shows
SEM images of the samples obtained during the HTC at 3 h, 6 h,
Fig. 1 (a) XRD pattern, (b) Raman spectrum, and (c and d) SEM
images of MCRCSs.
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12 h, and 48 h. Fig. S4a{ shows the morphology of the products
obtained at 3 h, the product was mainly microspheres composed of
particles and the diameter of microspheres is about 3.5 mm. These
particles have a diameter of hundreds of nanometers. These
particles were analyzed by energy dispersive X-ray spectroscopy
(EDS). The result shown in Fig. S5{ reveals that only the elements
of Fe, C, and O are contained in the particles, which suggests that
these particles were the Fe3O4/carbon composites. Microspheres
composed of particles were not the exclusive morphology in the
products. From the Fig. S4b,{ we can clearly observe that
agglomerated particles were encapsulated with carbon spheres.
When the reaction time was prolonged to 6 h, more Fe3O4/carbon
particles integrated into microspheres and most of microspheres
grow, in which the diameter of microspheres increased to about
4 mm (Fig. S4c and d{). After 12 h of reaction, these microspheres
composed of Fe3O4/carbon particles were gradually coated by
carbon (Fig. S4e and f{). The diameter of Fe3O4/carbon micro-
spheres did not increase but the rough surface of Fe3O4/carbon
microspheres become smooth (Fig. S4f{). After hydrothermal
treatment for 48 h, large numbers of MCRCSs were obtained and
Fe3O4 particles were enwrapped by carbon more compactly as
uniform microspheres (Fig. S4g and h{). EDS analysis on the
Fe3O4/carbon microspheres obtained at 48 h suggests that only the
elements of Fe, C, and O are contained in the microspheres (Fig.
S5{). However, the intensity of C element on the surface of
MCRCSs obtained at 48 h is higher than that of Fe3O4/carbon
composites obtained at 3 h, which implies that the carbon content
of product obtained at 48 h is larger than that of product obtained
at 3 h.
Fig. 3 (a) UV-vis spectra of MB vs. photoreaction time; (b) photo-
degradation of MB over blank, pure Fe3O4 nanoparticles, and MCRCSs;
?
(c) OH-trapping PL spectra of Fe3O4/carboxylate-rich carbon micro-
spheres/TA solution; (d) field-dependent magnetization curve of Fe3O4/
carboxylate-rich carbon microspheres at room temperature, inset shows
the strong attraction of the particles suspended in water toward a magnet.
presence of MCRCSs, MB degradation reaches about 96% after
4 h of visible light irradiation without using H2O2.
?
The hydroxyl radical ( OH) has been considered as a key species
in the photodegradation of many hazardous chemical compounds
for its high reaction ability to attack any organic molecule.
Terephthalic acid photoluminescence probing technique (TAPL), a
On the basis of the information we have gathered, the
synthetic strategy of MCRCSs is shown in Scheme S1.{ A two
stage growth mechanism of MCRCSs has been proposed. The
formation of the MCRCSs is divided into two growth stages,
i.e., the formation of the Fe3O4/carbon microsphere and the
further carbon coating process. In the former stage, there is a
hydrolysis equilibrium of sodium gluconate in water, which
provides OH2 ions for the formation of Fe3O4 particles. In
addition, in situ carbon coating on Fe3O4 particle occurs, due
to carboxylate group of sodium gluconate strongly coordinat-
ing to the Fe atom and HTC process of sodium gluconate.
Then these Fe3O4/carbon particles agglomerate and coalesce
together to form Fe3O4/carbon microspheres. The latter stage
is the further carbon coating process, Fe3O4/carbon micro-
sphere composed of Fe3O4/carbon particles were further
enwrapped by carbon to form MCRCSs.
highly sensitive and simple method, has been widely used in
3b,14
?
detection of OH.
?
The terephthalic acid (TA) probed OH to
form 2-hydroxylterepthalic acid (HTA) during the photocatalytic
process, which was measured with a fluorescence spectrophot-
ometer. The HTA exhibited a strong fluorescence peak (lex
=
315 nm, lem = 426 nm). Thus, we measured the fluorescence
?
intensity of HTA to detect OH indirectly. The photoluminescence
spectra of MCRCSs–TA solution under visible light irradiation is
shown in Fig. 3c. The photoluminescence peak of HTA (lem = 426
nm) increased steadily with the irradiation time, suggesting that the
?
?
OH increased steadily, which elucidated that OH on MCRCSs
was produced under visible light irradiation without using H2O2.
After the photo-Fenton reaction ended, the MCRCSs could be
rapidly separated under an applied magnetic field. As shown, the
MCRCSs are strongly attracted to a permanent magnet (inset in
Fig. 3d). The magnetic properties of MCRCSs were studied by
using a superconducting quantum interference device (SQUID)
magnetometer at room temperature. Fig. 3d displays hysteresis
loops of MCRCSs, which indicates it possess magnetic saturation
The visible light photocatalytic activity of MCRCSs was
evaluated with the photodegradation of MB in aqueous solution
at room temperature. The UV-vis spectra of MB aqueous solution
with MCRCSs under visible light irradiation (l . 420 nm) at
room temperature for different durations are shown in Fig. 3a.
The main absorption peak located at 665 nm, which corresponds
to the MB molecule, decreases rapidly with extension of the
exposure time, and was almost completely disappeared after 4 h of
visible light irradiation. Further experiments were carried out to
compare the photocatalytic activities of pure Fe3O4 nanoparticles
with MCRCSs. Fig. 3b shows the curves of the degradation
efficiency with irradiation time. As can be seen, without any
catalyst or with pure Fe3O4 nanoparticles, nearly no degradation
of MB was detected under visible light irradiation. However, in the
(Ms) values of about 25 emu g21
.
In summary, we demonstrate a feasible, economical and green
synthetic route for the synthesis of highly water-dispersible
MCRCSs. The solidification of iron ions and carboxylic acids at
the same time was realized via the fabrication of MCRCSs.
Carboxylates in the amorphous carbon can form complexes with
?
Fe3O4, which can produce OH under visible light irradiation.
Therefore, without using H2O2, MCRCSs possess significantly
enhanced photocatalytic activity under visible light irradiation and
can be conveniently separated by using an applied magnetic field.
5712 | CrystEngComm, 2012, 14, 5710–5713
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Acknowledgements
This work was supported by the Universities Natural Science
Foundation of Jiangsu Province (11KJB480001), Postdoctoral
Foundation of Jiangsu Province (1102125C), Jiangsu Natural
Science Fund of China (SBK201222389), and Highly Qualified
Professional Initial Funding of Jiangsu University (10JDG120).
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