Sodium citrate functionalized reusable Fe3O4@TiO2 photocatalyst for water purification

Article abstract of DOI:10.1016/j.cplett.2017.08.046

Easy-recycle photocatalysts are new materials for water treatment technologies. In order to improve the recyclable ability, we employed Fe₃O₄ particles, which were functionalized by sodium citrate, to serve as a substrate core to attract the deposition of a shell of TiO₂ particles. When compared to the calcining process for preparing the composite, the TiO₂ distributed homogeneously on the sodium citrate treated Fe₃O₄, forming a mesoporous shell layer. Due to the mesoporous structure, this Fe₃O₄@TiO₂ exhibited high photocatalytic degradation activity to Rhodamine B, and it was easily recycled using a magnetic field to recover the catalyst from solution.

Full text of DOI:10.1016/j.cplett.2017.08.046

Accepted Manuscript
Research paper
Sodium citrate functionalized reusable Fe3O4@TiO2 photocatalyst for water pu-
rification
Wenyu Li, Haoyi Wu
PII:
S0009-2614(17)30807-2
DOI:
http://dx.doi.org/10.1016/j.cplett.2017.08.046
CPLETT 35056
Reference:
Chemical Physics Letters
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Accepted Date:
6 May 2017
22 August 2017
Please cite this article as: W. Li, H. Wu, Sodium citrate functionalized reusable Fe3O4@TiO2 photocatalyst for water
purification, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.08.046
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Sodium Citrate Functionalized Reusable Fe3O4@TiO2 Photocatalyst for Water
Purification
Wenyu Lia* and Haoyi Wub*
aDepartment of Chemistry, Lishui University, Lishui 323000, China
bSchool of Physics and Optoelectronic Engineering, Guangdong University of
Technology, Guangzhou, 510006, China
Research highlights
1. A sodium citrate modified Fe3O4-TiO2 core-shell structure was fabricated.
2. The sodium citrate modified structure shows higher photocatalytic rate than
calcination sample.
3. The core-shell structure is recyclable by magnetic process.
Abstract
Easy-recycle photocatalysts are new materials for water treatment technologies. In
order to improve the recyclable ability, we employed Fe3O4 particles, which were
functionalized by sodium citrate, to serve as a substrate core to attract the deposition
of a shell of TiO2 particles. When compared to the calcining process for preparing the
composite, the TiO2 distributes homogeneously on the sodium citrate treated Fe3O4,
forming a mesoporous shell layer. Due to the mesoporous structure, this Fe3O4@TiO2
exhibited high photocatalytic degradation activity to Rhodamine B, and it was easily
recycled using a magnetic field to recover the catalyst from solution.
Keywords: TiO2; core-shell; photocatalysis
1

*Corresponding authors
Email: liwenyu_jnu@126.com (W. Li), manofchina@outlook.com (H. Wu)
Introduction
Photocatalysis is an energy conversion process where a semiconductor absorbs photon
energy and generates electron-hole pairs, followed by their migration to surface where
they react with the adsorbed molecules. This process is low-cost and consumes only
photon energy, which can be easily obtained from the sun or artificial lamps, so it is
an ideal method for removing organic pollutants from water. As one of the wide
bandgap semiconductors, TiO2 has become one of the best known and appreciated
photocatalytic materials, since its use was first reported in the photolysis of water in
1972 [1]. Over the course of the past decade, many researches have shown TiO2 to be
a superior photocatalytic material, because of its high degradation rates for many
organic pollutants [2-5]. In the application of water purification, TiO2 is usually
employed in the form of nanoparticles or thin films [6-8]. In general, it has been
found that the TiO2 nanoparticles offer a high degradation rate for organics, because
of the high surface area. However, this small particle size may result in a stabilized
dispersion of TiO2 which will encumber the collection of the particles from solution,
leading to secondary pollution.
In recent years, magnetic materials have been used in conjunction with TiO2
nanoparticles to easily separate the TiO2 from solution using a magnetic field, which
2

allows the reuse of the photoactive catalyst [9-11]. Due to its paramagnetic feature,
Fe3O4 has been recognized as one of the promising supporting materials for with the
recycle of photocatalytic particles, including WO3, CeO2, BiOI and TiO2, which
promotes their magnetic collection. Constructing a Fe3O4-TiO2 core-shell
photocatalytic structure has been widely reported. For example, Alvarez et al. coated
TiO2 onto Fe3O4 particles to fabricate Fe3O4@TiO2 particles. These particles exhibited
considerable photocatalytic performance and they were easily collected using a
magnet, making them ready for reuse [12]. Chalasani et al. reported a
cyclodextrin-functionalized Fe3O4@TiO2 structure that could photocatalytically
degrade organics, and was then magnetically separated from the dispersion [13].
These reported fabrication methods involved chemically wrapping a Ti-containing
precursor on the Fe3O4 particles followed by crystallization of the TiO2 at high
temperature. Reported herein, we detail a facile method to fabricate Fe3O4@TiO2 by
hydrogen bonding with the assistance of sodium citrate. The particles obtained by this
method exhibited better photocatalytic performance than those synthesized by
calcination because the forming of mesoporous structure, and the subject magnetic
photocatalysts were easily recycled thanks to their magnetic core.
Experimental
Fe3O4 synthesis To synthesize the Fe3O4 nanoparticles, FeSO4 (AR, Sigma) was
dissolved in H2O2 to oxidize Fe2+ to Fe3+, which produced a Fe2(SO4)3 solution
(0.0833mol/L). Next, the FeSO4 and Fe2(SO4)3 solutions were combined in volume
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ration of 1:2. Then an ammonia solution (1.5 mol/L, Beijing Innochem) was added to
the mixture with stirring, and the pH value of the solution was adjusted to 11 using
dilute NaOH. After 30 min, a precipitate was collected using a magnet and it was
washed several times with DI water. The pH value of the final dispersion was adjusted
to 7. The final powder was collected by filtration and dried in air 40 oC.
Fe3O4 functionalization The resulting Fe3O4 particles (2 g) were dispersed in 100
ml of an aqueous solution with sodium citrate (0.1 mol/L) that was degassed with
bubbling N2 gas for 15 min. Then the dispersion was sealed and sonicated for 30 min.
o
Following this, the dispersion was transferred to a 60 C water bath and stirred for 12
hours. The powder was magnetically collected after being washed with acetone and
water.
TiO2 synthesis To prepare the TiO2, TiO(OH)2 (AR. Hubei Kangbaotai) was first
dissolved in concentrated H2SO4 (AR. Shenyang Xinxing) and then the solution was
diluted with DI water. The diluted solution was stirred for 30 min and then a select
amount of activated carbon powder was added and the mixture was allowed to
incubate. The activated carbon was then filtered from the mixture to produce a
TiOSO4 solution. An ammonia solution was added to the TiOSO4 solution to
o
precipitate the Ti4+ as Ti(OH)4, followed by a calcination process at 550 C for 2.5
hours. This last step produced the final TiO2 powder product.
Functionalized Fe3O4@TiO2 fabrication The functionalized Fe3O4 particles were
dispersed in 100 mL of DI water by sonication for 30 min. Then TiO2 powder was
added to the Fe3O4 solution and kept stirring for 30 min. The final product was
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collected by separation after aging and precipitation. This product was denoted as
SC-FOT. The amount of Fe in the product was determined by reducing the Fe3+ to
Fe2+ and then titrating the solution using sodium diphenylamine sulfonate and 0.0083
mol/L K2Cr2O7 aqueous solution. The results indicated that the estimated amount of
Fe in the product was 17 %.
Control Fe3O4@TiO2 fabrication A control Fe3O4@TiO2 sample was fabricated
by calcination. Typically, 30 mL Fe2(SO4)3 solution and 30 ml of a FeSO4 solution
were combined, followed by addition of a small amount of sodium dodecyl sulfate.
The resulting reaction solution was placed in a water bath and maintained 65 oC. Then
dilute NaOH (1 mol/L) was added drop wise until the pH of the solution reached 12.
After stirring for 30 min, the powder was magnetically collected after being washed
o
with DI water, followed by drying at 100 C for 3 hours in air. Then the resulting
Fe3O4 particles were combined with the TiO2 particles in a solution that was
o
continuously stirred at 70 C for 3 hours. The final mixture was washed and dried. In
the last step, the powder was sintered at 550 oC in a N2 atmosphere and the amount of
Fe in the mixture was estimated to be 18 %. This control was denoted as FOT.
Characterizations The X-ray powder diffraction patterns of the product were
obtained using a TDM-3500 (Tongda) XRD analyzer, with Cu Kα irradiation (λ =
1.5406 Å). A CX-9900 (PE) Fourier Transform Infrared spectrophotometer was
employed to investigate the surface functional groups of the samples. Transmission
electron microscopic (TEM) images and the Energy Dispersive X-Ray Spectroscopy
(EDX) of the test samples were recorded using a TF20 JEOL 2100 microscope (FEI).
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The surface area was evaluated by Brunauer-Emmett-Teller (BET) method from
isotherm measured by a ASPS2020M physisorption analyzer (Micromeritics). The
photocatalytic activity of the synthesized catalysts was evaluated by the degradation
of Rhodamine B (RhB) (0.01 mmol/L, 50 ml) using 50 mg of the photocatalysts.
Results and Discussion
The crystal phases of the obtained materials were identified by XRD analysis. As
shown in Figure 1, pure TiO2 and Fe3O4 powders exhibited anatase and spinel
structures that were indicative of the TiO2 and Fe3O4, according to the JCPDS
standard card nos. 21–1272 and 65-3107. The SC-FOT sample exhibited both TiO2
and Fe3O4 phases, suggesting that Fe3O4/TiO2 composites were successfully prepared.
The FT-IR spectra of the test materials are shown in Figure 2. As can be seen, the
Fe3O4@TiO2 samples exhibited a broad absorption band at 400-800 cm-1, which was
attributed to the stretching vibration of the Fe-O bond, which overlapped with the
absorption bands for the Ti-O-Ti bonds [10,14]. A small absorption peak at 2800 cm-1
was observed, indicating the appearance of the –C-H bond, and the peaks at 1630 and
1400 were attributed to the -COO- groups. The appearance of -C-H and –COO-
agrees well to the sodium citrate functionalization on the surface of Fe3O4.
The morphology of the samples was investigated using TEM analysis. As shown in
Figures 3 (a) - (d), the FOT and SC-FOT samples consisted of two kinds of particles
including Fe3O4 and TiO2, which was confirmed by the EDX results. The size of the
Fe3O4 particles was distributed in a range from 30 - 50 nm, where the TiO2 particles
6

were about 10 nm in size. In the case of the FOT sample, the TiO2 loading on the
surface of Fe3O4, produced an aggregated composite with two kinds of materials
(Figure 3 (b)). For the SC-FOT sample, it was found that the TiO2 particles were
homogeneously wrapped around the Fe3O4 forming a core-shell structure. The
thickness of TiO2 shell was about 20 nm. In other words, the sodium citric treatment
assisted in the aggregation of TiO2 on the Fe3O4.
The isotherms in Figure 4 show the surface area of the materials. The surface area of
the TiO2 was reduced after coating on Fe3O4. A mesoporous structure was observed
from CS-FOT, with pore volume of 0.13 cm3/g, which is higher than the one in FOT
(0.05 cm3/g). This mesoporous structure renders the sample a higher surface area
(54.7 m2/g) than that of FOT (15.1 m2/g). This structure was formed by the advantage
of physical attraction of sodium citric surface of Fe3O4 which aggregated TiO2
particles as a shell. As shown in Figure 5 (a), when compared to the calcination
procedure, the core with sodium citrate appeared to be more effective in the
fabrication of the TiO2@Fe3O4 core-shell structure.
Since the mesoporous Fe3O4@TiO2 core-shell structure of SC-FOT was formed
without calcination, the resulting product had different photocatalytic properties.
Therefore, the photocatalytic performance of the novel photocatalyst was investigated
by monitoring the photo-decomposition of Rhodamine B (RhB) in solution. As shown
in Figure 5, the SC-FOT catalyst exhibited a faster degradation of to RhB, as
compared to the FOT catalyst. We used a pseudo first-order kinetic equation to
evaluate the photodegradation behavior [15] in this reaction:
7

ln(C0/C) = kt
where C and C0 are the time-dependence concentration and original concentration
of RhB, k is the pseudo first order rate constant and t represents time. The fitted
experimental results produced the k values for the FOT and SC-FOT catalyst, which
were calculated to be 0.044 and 0.056. Therefore, the SC-FOT catalyst appeared to
exhibit a higher photodegradation efficiency for RhB than the FOT. This improvement
is attributed to the enhanced surface area with mesoporous structure of the TiO2 shell,
which adsorbs organic molecules more efficient than the individual particles. Thus
sodium citrate functionalization provides a new method form mesoporous core-shell
structure to improve the adsorbing ability of the materials, so that photocatalytic
degradation rate of organic pollutants is enhanced.
In addition, it was noted that the Fe3O4@TiO2 material was also ferromagnetic, which
made it easy to recover from solution using a magnet. As revealed in Figure 5 (c) and
(d), about 90 % of the particles were collected and then reused to assess their
performance in a new cycle of photodegradation of RhB. In the second test, the rate of
RhB degradation exhibited a slight decrease, as a result of the smaller quantity of
recovered TiO2 particles in the test. After 5 cycles, more than 90 % of the SC-FOT
material was preserved, and the degradation rate was about 0.041, which
demonstrated a stable catalytic material with good recyclability.
Conclusions
In summary, sodium citrate was employed to functionalize Fe3O4 particles, which
8

then were easily wrapped by TiO2. When compared to the calcination preparation
process, the sodium citrate served as a surfactant that functionalized the Fe3O4
particles, assisting in the formation of a mesoporous Fe3O4@TiO2 core-shell structure.
Therefore, due to the improved adsorbing ability of mesoporous structure, the
material showed an efficient photocatalytic decomposition to RhB. Moreover, the
material was easily recycled using a magnet, and could be reused exhibiting that it is
highly suited for water purification applications.
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Figure captions
Figure 1. XRD patterns of Fe2O3, TiO2 and SC-FOT.
Figure 2. FT-IR of and the TiO2 coating sodium citrate functionalized Fe3O4.
Figure 3. TEM of (a) (b) FOT and (c) (d) SC-FOT, (e) EDX patterns of the SC-FOT.
Figure 4. Isotherms for pure TiO2 (), FOT () and SC-FOT (△).
Figure 5. (a) Methods for preparation of FOT and SC-FOT, (b) degradation curves of RhB with
FOT and SC-FOT, (c) RhB degradation after 5 cycles, (d) recycling ratio of the SC-FOT catalyst
through 5 degradation tests (Wn: Weight of the sample collected in the nth cycling, Wn-1: Weight
of the sample collected in the n-1th cycling).
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Figures
Figure 1. XRD patterns of Fe2O3, TiO2 and SC-FOT
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Figure 2. FT-IR of and the TiO2 coating sodium citrate functionalized Fe3O4.
12

Figure 3. TEM of the (a) (b) FOT and (c) (d) SC-FOT, (e) EDX patterns of the SC-FOT
13

Figure 4. Isotherms for pure TiO2 (), FOT () and SC-FOT (△).
14

Figure 5. (a) Methods for preparation of FOT and SC-FOT, (b) degradation curves of RhB with
FOT and SC-FOT, (c) RhB degradation through 5 cycles, (d) recycling ratio of the SC-FOT
catalyst through 5 RhB degradation tests (Wn: Weight of the sample collected in the nth cycling,
Wn-1: Weight of the sample collected in the n-1th cycling).
15