Mesoporous TiO2 coated ZnFe2O4 nanocomposite loading on activated fly ash cenosphere for visible light photocatalysis

Article abstract of DOI:10.1039/c7ra11055c

Several activated fly ash cenosphere (AFAC) supporting TiO₂ coated ZnFe₂O₄ (TiO₂/ZnFe₂O₄/AFAC) photocatalysts were prepared by sol-gel and hydrothermal methods. These photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (UV-DRS) and nitrogen adsorption analyses for Brunauer-Emmett-Teller (BET) specific surface area measurements. We found that the main components of spherical AFAC were mullite (Al₆Si₂O₁₃) and SiO₂; the crystallite size of the TiO₂/ZnFe₂O₄ nanocomposite was less than 10 nm and its specific surface area was 162.18 m² g^-1. The TiO₂/ZnFe₂O₄ nanocomposite had a band-gap of 2.56 eV, which would photodegrade 95% of rhodamine B (RhB) under visible light within 75 min. When hybridized with 0.02 g AFAC, the TiO₂/ZnFe₂O₄/0.02 g AFAC photocatalyst with a band-gap of 2.50 eV could remove 97.1% of RhB and be reused three consecutive times with minor decrease in photocatalytic performance. However, the photocatalytic performance decreased to 91.0% on increasing the dosage of AFAC to 0.30 g. The mesoporous structure of all the photocatalysts and the strong adsorption ability of AFAC accounted for the notable performance.

Full text of DOI:10.1039/c7ra11055c

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Mesoporous TiO coated ZnFe O nanocomposite
2
2 4
loading on activated fly ash cenosphere for visible
light photocatalysis†
Cite this: RSC Adv., 2018, 8, 1398
abc
a
a
c
abc
abc
Hougang Fan, Dandan Chen, Xuefeng Ai, Shuo Han, Maobin Wei, Lili Yang,
abc
abc
Huilian Liu
and Jinghai Yang
*
Several activated fly ash cenosphere (AFAC) supporting TiO
2
coated ZnFe
2
O
4
(TiO
2
/ZnFe
2
O
4
/AFAC)
photocatalysts were prepared by sol–gel and hydrothermal methods. These photocatalysts were
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy
(
UV-DRS) and nitrogen adsorption analyses for Brunauer–Emmett–Teller (BET) specific surface area
measurements. We found that the main components of spherical AFAC were mullite (Al Si 13) and SiO
the crystallite size of the TiO /ZnFe nanocomposite was less than 10 nm and its specific surface area was
/ZnFe nanocomposite had a band-gap of 2.56 eV, which would photodegrade
5% of rhodamine B (RhB) under visible light within 75 min. When hybridized with 0.02 g AFAC, the TiO
/0.02 g AFAC photocatalyst with a band-gap of 2.50 eV could remove 97.1% of RhB and be reused
6
2
O
2
;
2
2 4
O
2
ꢀ1
1
62.18 m
g
. The TiO
2
2 4
O
9
2
/
Received 8th October 2017
Accepted 18th December 2017
2 4
ZnFe O
three consecutive times with minor decrease in photocatalytic performance. However, the photocatalytic
performance decreased to 91.0% on increasing the dosage of AFAC to 0.30 g. The mesoporous structure of
all the photocatalysts and the strong adsorption ability of AFAC accounted for the notable performance.
DOI: 10.1039/c7ra11055c
rsc.li/rsc-advances
which could prevent the transformation of TiO from anatase to
2
1
. Introduction
16,17
rutile.
With superior photocatalytic performance, high chemical band gap was about 1.8 eV. Hence, under visible light, the
2 4
stability, low cost and low-toxicity, TiO has the potential for utilization of a TiO /ZnFe O nanocomposite in aqueous
In addition, ZnFe O can absorb sunlight since its
2 4
18,19
2
2
applications in environmental purication and solar energy suspension has potential applications in wastewater treat-
1–3
18–21
conversion. However, the wide band gap of TiO (3.18 eV for ment.
2
Immobilizing TiO on adsorptive or hydrophobic
2
anatase), making it absorb no more than 5.0% of sunlight, materials would facilitate the oxygenation process throughout
22–25
together with the low quantum efficiency (high recombination the photocatalytic process.
Fly ash cenosphere (FAC) is the
rate of photogenerated electron–hole pair) hindered its appli- by-product generated in coal-red power plants and could be
4–6
6,26,27
cation. To extend the visible light response of TiO
2
, increasing used as a hydrophobic material or oating substrate,
which
efforts have been directed towards the improvement of the shows bifunctional properties with adsorption capability in
photocatalytic activity of TiO . Some of these methods are: addition to photocatalytic activity in loading semiconductor
2
2
8,29
doping ions, surface modication by noble metals and coupling nanoparticles.
7
–11
with photosensitive dye and other semiconductors.
these methods, coupling with other semiconductors could nanocomposite was synthesized by a sol–gel method, and then
efficiently improve the photocatalytic activity of TiO
under hybridized with different amounts of AFAC for the rst time.
has been Compared with the TiO /ZnFe nanocomposite, the TiO
loaded on AFAC (TiO /ZnFe /AFAC) photocatalyst
Among
2 2 4 2 2 4
In this study, the TiO coated ZnFe O (TiO /ZnFe O )
2
12–15
visible light irradiation.
In particular, ZnFe
2
O
4
2
2
O
4
/
2
considered as an interesting coupling semiconductor to TiO
2
,
ZnFe
O
2
4
2
O
2 4
exhibited better adsorption activity and photocatalytic activity
of removal of RhB under visible light.
a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of
Education, Jilin Normal University, Changchun 130103, PR China. E-mail:
jhyang1@jlnu.edu.cn
2
. Experimental
b
National Demonstration Center for Experimental Physics Education, Jilin Normal
2.1 Materials
University, Siping 136000, PR China
c
Fly ash was obtained from thermal power plants of Lingshou
city (Hebei province, China). All reagents including zinc acetate
dihydrate (Zn(CH COO) $2H O), sodium hydroxide (NaOH),
3 2 2
College of Physics, Jilin Normal University, Siping 136000, PR China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra11055c
1
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ferric chloride hexahydrate (FeCl
3
$6H
2
O), hydrazine hydrate spectrophotometer (UV-5800PC, Shanghai Metash Instruments
(
N H $H O), ethylene glycol ((CH OH) ), ethanol (C H OH), Co., Ltd).
2 4 2 2 2 2 5
cetyl trimethyl ammonium bromide (CTAB), tetrabutyl titanate
TBOT), isopropanol ((CH ) CHOH), nitric acid (HNO ) and
rhodamine B (RhB) used in the experiments were analytical
grade (purchased from Sinopharm Chemical Regent Co. Ltd)
and used without further purication. Deionized water was
used throughout the experiment.
(
3
2
3
2
.4 Photocatalytic activity test
The photocatalytic performance of the as-prepared photo-
catalysts was evaluated by the photocatalytic decomposition of
rhodamine B (RhB) aqueous solution. In the experiment, 50 mg
of the as-prepared photocatalyst was dispersed in 50.0 mL of
RhB solution (10 mg L ) in a 100 mL beaker. A 350 W Xe lamp
equipped with a 420 nm cut-off lter was used as a light source
along with a cooling device to eliminate the thermal effect. The
ꢀ
1
2.2 Synthesis
2
.2.1 Activating y ash. The acquired y ash (10 g) was
ꢀ2
ꢁ
average light energy density was estimated to be 68 mW cm .
activated by 200.0 mL HNO under stirring at 80 C for 2 h, and
3
The distance between the beaker containing the reaction
mixture and the light source was xed at 16 cm. First, the
reaction mixture was mechanically stirred for 30 min in the
dark to ensure the adsorption/desorption equilibrium of the
RhB on the surface of the photocatalysts. The reaction solutions
were sampled at 15 min illumination intervals, and the corre-
sponding UV-vis spectra (measured over the range of 300–800
nm) were recorded to monitor the progress of the degradation
of RhB using an UV-vis spectrophotometer. All the photo-
catalytic performances were tested under UV light, visible light
and sunlight. To investigate the stability of the TiO /ZnFe O /
then washed with deionized water until the pH of the solution
was 7. The particles suspended in the solution were collected
and dried in a vacuum oven at 80 C for 24 h.
ꢁ
2.2.2 Preparation of ZnFe
2
O
4
.
The mole ratio of
n(Zn) : n(Fe) ¼ 1 : 2 of the soluble salts was dissolved in
a mixture of ethylene glycol and deionized water solution. This
mixture was magnetically stirred for 30 min and named as
solution A. A certain amount of NaOH was dissolved in deion-
ized water and mixed with hydrazine hydrate, magnetically
stirred for 30 min, and the mixture was labeled as solution B.
Solutions A and B were mixed and vigorously stirred for 10 min,
with the injection of CTAB; magnetically stirring was continued
for 20 min. The reaction mixture was transferred into a 100 mL
2
2 4
0
.02 g AFAC photocatalyst, the composite was reused three
ꢀ
1
times for the photocatalytic degradation of RhB (10 mg L ).
ꢁ
Teon-lined stainless steel autoclave and kept at 150 C for 15 h.
The obtained dark green products were washed three times with
ethanol and deionized water before being dried at 60 C for 12 h
3
3
. Results and discussion
ꢁ
.1 Structure and morphology of photocatalysts
Fig. 1 shows the X-ray diffraction patterns of AFAC, ZnFe
nano- TiO /ZnFe nanocomposite and the TiO /ZnFe /0.02 g
composite was prepared using the sol–gel method. Initially, AFAC sample. For the AFAC sample, most diffraction peaks
4.0 mL TBOT was diluted in 8.5 mL isopropoxide solution belonged to mullite (Al Si 13, JCPDF no. 15-0776); the amor-
under stirring. The mixture was added dropwise into 340 mL phous phase located at 22 primarily composed of SiO (JCPDF
water with vigorous stirring and the pH was adjusted to 3 with no. 88-1535). For the as-prepared ZnFe sample, all the
(JCPDF no. 82-1049). For
and then crushed in an agate mortar to obtain ZnFe
powders.
2 4
O
O ,
2 4
2.2.3 Photocatalysts preparation. The TiO
2
/ZnFe
2
O
4
2
O
2
4
2
O
2 4
3
6
2
O
ꢁ
2
2 4
O
HNO . The mixture was placed in a 500 mL ask and heated in diffraction peaks belonged to ZnFe
2
O
4
3
ꢁ
a water bath at 75 C for 12 h. Finally, pure TiO
2
sol was ob-
powder was dispersed in
sol in an ultrasonic bath for 30 min. Then, it was
30
tained. Further, 0.35 g ZnFe
2 4
O
300.0 mL TiO
2
dried into a powder in a rotatory evaporator under vacuum at
ꢁ
7
5 C. The obtained powder was named as TiO
2
coated ZnFe
/ZnFe load-
ings on AFAC (TiO /ZnFe O /AFAC) photocatalysts were ob-
2 4
O
nanocomposite (TiO
2
/ZnFe
2
O
4
). The nal TiO
2
2 4
O
2
2 4
tained by triturating TiO /ZnFe O powder and AFAC with mass
2
2 4
ratio of 1 : 0.02 and 1 : 0.30, respectively.
2.3 Characterization
The crystalline structure of the samples was determined using
X-ray diffraction (XRD MAC Science, MXP18, Japan). The
morphology of the as-synthesized samples was characterized by

eld emission scanning electron microscopy (FESEM, JEOL
JEM-2010HR) and transmission electron microscopy (TEM,
JEOL JEM-2010HR). N2 adsorption isotherms were measured
using an AUTOSORB-IQ (Quantachrome Instruments, USA). UV-
2 4 2 2 4 2
Fig. 1 The XRD spectra of AFAC, ZnFe O , TiO /ZnFe O and TiO /
2 4
vis absorption spectra were acquired with an UV-vis ZnFe O /0.02 g AFAC samples.
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Table 1 The crystallite size and band gap of different samples
Sample
ZnFe
2
O
4
TiO
2
/ZnFe
2
O
4
TiO
2
2
/ZnFe O
4
/0.02 g AFAC
2 2 4
TiO /ZnFe O /0.30 g AFAC
Crystallite size (nm)
Band gap (eV)
25.0 ꢂ 2.3
8.7 ꢂ 0.6
8.4 ꢂ 0.5
9.1 ꢂ 0.9
1.33
2.56
2.50
2.44
the TiO
ZnFe
286), while ve diffraction peaks of ZnFe
observed at lower intensity, which demonstrated that the have a large specic surface area.
amount of ZnFe O accounted for a small proportion. For the Fig. 2A–C show the SEM and TEM images of AFAC. In Fig. 2A
TiO /ZnFe O /0.02 g AFAC sample, the diffraction peaks of and B we can observe that the particle shape of AFAC was
2
/ZnFe
2
O
4
nanocomposite, the TiO
2
on the surface of Table 1. The crystallite sizes of TiO
/ZnFe /0.30 g AFAC samples were all
2 4
O could also be less than 10 nm, which implied that the above samples would
2 2 4 2 2 4
/ZnFe O , TiO /ZnFe O /
2
O
4
exhibited a dominant anatase phase (JCPDF no. 84- 0.02 g AFAC and TiO
2
2 4
O
1
2
4
2
2 4
Al Si O , ZnFe O and TiO could all be observed and marked spherical and their diameters ranged from about 1 mm to nearly
6
2
13
2
4
2
6,26–28
by different symbols. However, the peak intensities of mullite 10 mm, which was similar to previous studies.
were relatively small, which suggested that the amount of AFAC the perfect AFAC sphere is presented in the TEM image with
in the TiO /ZnFe /0.02 g AFAC sample was small. The crys- some tiny particles on its surface, which implied that the AFAC
tallite sizes of the above samples were determined from the had adsorption ability. Fig. 2D shows the SEM image of
broadening of the corresponding X-ray diffraction peaks by ZnFe , whose crystallite size is about 20–30 nm, coinciding
In Fig. 2C,
2
2 4
O
2 4
O
using Scherer's formula D ¼ (Kl)/(b cos q), where l is the with the results summarized in Table 1. Fig. 2E and F represent
wavelength of the X-ray radiation (l ¼ 0.15406 nm), K is the the SEM and TEM images of the TiO /ZnFe O nanocomposite.
2
2 4
Scherer constant (K ¼ 0.9), q is the X-ray diffraction peak and It could be observed that the dark ZnFe O particles with
2
4
b is the full-width at half maximum (FWHM) of the peak (in a crystallite size of about 25 nm are coated by the outer grey
radians), which was corrected for the instrumental broadening homogeneous layer of TiO particles with a crystallite size of
¼ 0.00122 rad) prior to the calculation of its crystallite size less than 10 nm, which coincided with the results summarized
broadening. The crystallite sizes were calculated and listed in in Table 1. Fig. 2G and H show the SEM image and EDS results
2
(b
0
Fig. 2 SEM images (A, B) and TEM image (C) of AFAC, SEM image (D) of ZnFe
2 4 2 2 4
O , SEM images (E) and TEM image (F) of TiO /ZnFe O nano-
composite, SEM image (G) and EDS (H) of ZnFe /TiO /0.30 g AFAC sample.
O
2 4
2
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of the ZnFe
2
O
4
/TiO
2
/0.30 g AFAC sample. It can be observed that isotherms and the corresponding pore size distribution histo-
the AFAC particle is semi-spherical and the ZnFe O /TiO
2
gram of the above samples. According to the International
2
4
particles are loaded on the AFAC surface. The EDS elemental Union of Pure and Applied Chemistry (IUPAC) classication, all
analysis shows that most of the elements are oxygen (70%), the samples exhibited reversible type IV isotherms with H3-type
3
4,35
silicon (14%), aluminum (11%) and titanium (2.3%), which hysteresis loop,
which indicated the formation of meso-
36
corresponds to the compounds SiO
respectively. AFAC primarily consisted of SiO
other oxides, such as Fe . Only 0.05% of zinc and 0.29% of intra-aggregated pores formed between intra-agglomerated
iron were found, which corresponded to the compounds Fe primary particles in the above composites. The pore diameter
and ZnFe O , which might be because the added amount of distribution histogram was calculated using the adsorption
2
, Al
6
Si
2
O
13 and TiO
2
,
porous materials. The hysteresis loop in the relative pressure
2
, Al Si 13, and range between 0.4 and 0.9 was probably related to the ner
6
2
O
2 3
O
2 3
O
2
4
ZnFe O was small (3 wt%, wt% is an abbreviation of weight branch of the isotherm, which indicated the presence of mes-
2
4
percentage) and the ZnFe O particles were coated by TiO2 opores (from 30 to 50 nm) and macropores (from 50 to 120 nm).
2
4
particles.
The mesoporous structure could be related to the pores formed
between stacked TiO particles coating on the ZnFe parti-
2
2 4
O
cles. The data summarized in Table 2 indicated that the BET
3
.2 FT-IR spectrum
FT-IR analysis was performed on TiO
.02 g AFAC and TiO /ZnFe O /0.30 g AFAC composites to
specic surface area of the above photocatalysts were all more
2
/ZnFe
2
O
4
, TiO
2
/ZnFe
2
O
4
/
2
ꢀ1
than 150.00 m
g
and larger than that reported in some
0
2
ꢀ1
2
2 4
previous studies (less than 100.00 m g ), which inferred that
all the composites would have a strong adsorption ability.
further prove the coating of TiO on ZnFe O and the existence
of AFAC. Fig. 3 shows their FT-IR spectra recorded over the
range of 4000–500 cm . In all the spectra, the band centered at
2
2 4
ꢀ1
ꢀ
1
551 cm can be assigned to the Zn–O stretching of ZnFe
2
O
4
3.4 UV-vis photoresponse and band gap determination
ꢀ1
(ref. 31) and the band centered at 1384 cm can be attributed
The UV-vis diffuse reectance spectra (UV-DRS) of the samples
are depicted in Fig. 5. It is clearly shown that the as-prepared
TiO /ZnFe O /AFAC samples primarily absorbed UV light and
2 2 4
part of the visible light. There are three intrinsic adsorption
edges located at 380 nm, 460 nm and 645 nm, which correspond
32
2
to the Ti–O–Ti vibration observed in TiO . The band centered
at 1630 cm can be assigned to the bending vibration of the
O–H bond of chemisorbed water, while the band centered at
ꢀ
1
33
ꢀ
1
1053 cm in the spectra of the TiO /ZnFe O /0.02 g AFAC and
2 2 4
2 2 4
TiO /ZnFe O /0.30 g AFAC composites could be assigned to the
asymmetric stretching of Si–O–Si groups of AFAC, which could
not be observed in the spectrum of the TiO
nanocomposite.
2 2 2 4 2 4
to anatase TiO , TiO /ZnFe O /AFAC sample and the ZnFe O
33
material, respectively. On further addition of AFAC, the light
absorption of TiO /ZnFe O /AFAC sample slightly enhanced.
2 2 4
/ZnFe O
2
2 4
Based on the optical absorption edge obtained from UV-DRS,
the energy band gaps for direct band-gap semiconductor of
3.3 Pore structure and surface area determination
1/2
6,27,37
different samples were calculated by plotting (Ahn) to hn,
N adsorption measurement was carried out to characterize the where A is the absorption coefficient, hn is the photon energy
2
porous structures and specic surface area of the TiO
TiO /ZnFe /0.02 g AFAC and TiO /ZnFe
/0.30 g AFAC values of the samples are listed in Table 1. The estimated E
photocatalysts. Fig. 4A–C display the N
adsorption–desorption values of the as-prepared ZnFe TiO /ZnFe TiO
ZnFe O /0.02 g AFAC and TiO /ZnFe O /0.30 g AFAC samples
2
/ZnFe
2
O
4
,
g
and E is the energy band gap; the measured energy band gap
2
2
O
4
2
2
O
4
g
O
4
,
2
2
O
4
,
2
/
2
2
2
4
2
2 4
were 1.33 eV, 2.56 eV, 2.50 eV and 2.44 eV, respectively, which
indicated that all the above samples would possess photo-
catalytic ability under visible light irradiation. The energy band-
gap of anatase TiO
narrow band-gap semiconductor ZnFe
2
is 3.18 eV; when hybridized with 0.35 g
, it decreases greatly
nanocomposite, which is similar to
When hybridized with 0.30 g AFAC, the
2 4
O
to 2.56 eV in TiO
previous reports.
2 2 4
/ZnFe O
1
7–21
band-gap decreases to 2.44 eV, which had a less signicant
6
effect on the band-gap than that reported by Huo et al. Hence,
ZnFe
of AFAC in our as-prepared TiO
The visible red shi of the adsorption of TiO
samples resulted from two factors. One factor was the mixing
effect of band gaps of ZnFe and TiO semiconductors dis-
cussed above. Since the added amount of ZnFe was small
3 wt%), the mixing effect alone cannot account for the entire
large red shi. The other factor considered was the interface
effect. Due to the interfacial coupling effect between ZnFe
and TiO grains, ZnFe can induce lattice defects on the
2
O
4
has more inuence on the band gap of TiO
/ZnFe /AFAC photocatalysts.
/ZnFe /AFAC
2
than that
2
2 4
O
2
2 4
O
2
O
4
2
2 4
O
(
2 4
O
Fig. 3 The FT-IR spectra of TiO
2
2 4 2 2 4
/ZnFe O , TiO /ZnFe O /0.02 g
AFAC and TiO /ZnFe /0.30 g AFAC samples.
2
2
O
4
2 4
O
2
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2 2 4 2 2 4 2 2 4
Fig. 4 BET isotherm and pore diameter distribution of different samples: (A) TiO /ZnFe O ; (B) TiO /ZnFe O /0.30 g AFAC; (C) TiO /ZnFe O /
0
.02 g AFAC (inset illustrates the graph of the corresponding pore diameter distributions).
Table 2 BET specific surface area of different samples
quickly in the rst 15 min and achieved maximum adsorption
aer 30 min. Their adsorption kinetics of RhB shown in Fig. 6B
could be tted well with a pseudo-second-order adsorption
Pore volume BET specic
Half pore
width (nm) error
Fitting
3
ꢀ1
2
ꢀ1
Sample
(cm g
)
surface (m g
)
27
model. The adsorption pseudo second-order kinetic constants
2
TiO
TiO
2
/ZnFe
/ZnFe
2
O
O
4
4
0.213
0.209
162.183
151.122
25.935
24.210
2
2.495% (k ) and their standard error, and the regression coefficient R
2
2
/
/
2.618% are listed in Table 3. Therefore, the photocatalysts were rst
0
.02 g AFAC
TiO /ZnFe
.30 g AFAC
placed in dark for 30 min to reach the adsorption equilibrium
and then were irradiated by the Xe lamp for 75 min for photo-
2
2
O
4
0.219
167.447
26.500
0.474%
0
catalytic degradation. It was noteworthy that the adsorption
activity of TiO /ZnFe O /0.30 g AFAC photocatalyst was not even
2
2 4
2 2 4
as good as that of the TiO /ZnFe O nanocomposite, which
would result in disadvantageous inuence on the nal removal
rate of RhB.
surface TiO
6
octahedra, which may serve as the centers of
38
bound excitons. These two factors together resulted in the red
shi of the band gap of TiO /ZnFe O /AFAC photocatalysts.
2
2 4
3
.6 Photocatalytic degradation of RhB
The photocatalytic activity of the pristine TiO
The adsorption ability would affect the photocatalysis perfor- TiO /ZnFe /0.02 g AFAC and TiO /ZnFe /0.30 g AFAC were
3.5 Adsorption of RhB
2 2 2 4
, TiO /ZnFe O ,
2
2
O
4
2
2 4
O
mance greatly. Hence, rhodamine B (RhB) was chosen as the evaluated by degrading RhB under visible light, simulated
simulated pollutant to evaluate the adsorption activity of the as- sunlight (without the 420 nm cutoff lter) and UV light irradi-
prepared photocatalysts. Fig. 6A shows the adsorption process ation. The results are shown in Fig. S1.† Under UV light irra-
of RhB removal on different TiO
in the dark for 120 min; all the samples adsorbed the RhB TiO
2
/ZnFe
2
O
4
/AFAC photocatalysts diation, RhB was almost thoroughly degraded in 15 min by pure
; however, it was evident that TiO was impotent for the
2
2
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Table
3
Adsorption pseudo second-order kinetic constant and
related parameters of different samples
ꢀ1
2
Samples
k
2
(g mg
)
Standard error
R
TiO
TiO
TiO
2
2
2
/ZnFe
/ZnFe
/ZnFe
2
O
2
O
2
O
4
4
4
0.14284
0.15692
0.20229
0.00611
0.00152
0.00333
0.98732
0.99935
0.99810
/0.02 g AFAC
/0.30 g AFAC
degradation of RhB under visible light irradiation. Hence, we
focused on the photocatalytic performance of the as-prepared
composites under visible light, which would be enlightening
for future application. Fig. 7 shows the processes of RhB
removal on different TiO
2 2 4
/ZnFe O /AFAC photocatalysts under
visible light irradiation. The TiO /ZnFe O nanocomposite
2
2 4
Fig. 5 UV-vis DRS of ZnFe
AFAC and TiO /ZnFe O /0.30 g AFAC samples.
2 2 4
2
O
4
, TiO
2
/ZnFe
2
O
4
, TiO
2
/ZnFe
2
O
4
/0.02 g removed 95.1% of the RhB. When hybridized with 0.02 g AFAC,
the TiO /ZnFe O /0.02 g AFAC photocatalyst removed 97.1% of
2
2 4
the RhB with the highest photocatalytic rate except for the
abnormal upturn point located at 30 min, which is demon-
strated in Fig. 8. When the amount of AFAC was increased to
0.30 g, the ratio of the removed RhB decreased to 91%, which
resulted from its relatively weak adsorption activity of RhB as
mentioned before. With the appropriate band gap, large specic
surface area and good adsorption ability, the TiO /ZnFe O /
2
2 4
0.02 g AFAC photocatalyst had the best photocatalytic perfor-
mance on the degradation of RhB among all our samples.
RhB could hardly be photodegraded under visible light
39
irradiation; therefore the removal of RhB was primarily
attributed to the photocatalytic degradation by the photo-
catalyst. Fig. 8 shows the spectra of photocatalytic degradation
of RhB by the three photocatalysts. When the simulated RhB
wastewater was irradiated under visible light, RhB was clearly
photodegraded in 75 min. The color of the wastewater changed
gradually from magenta to colorless (Fig. 8A). The maximum
absorption peak shied gradually from 552 nm to 495 nm,
which implied that there were different intermediate
2 2 4
products. Under visible light irradiation, the TiO /ZnFe O
Fig. 6 (A) Adsorption kinetics of RhB onto TiO
composite, TiO /ZnFe /0.02 g AFAC and TiO
2
/ZnFe nano-
2
O
4
2
2
O
4
2
/ZnFe O /0.30 g
2 4
AFAC photocatalysts; (B) pseudo second-order kinetic plots for RhB
removal.
Fig. 7 The photodegradation rate of RhB with different photocatalysts
under visible light.
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Fig. 8 (A) The color of the simulated wastewater after different times under visible light by TiO
/ZnFe O
2 2 4
/0.02 g AFAC photocatalyst; the spectra
of photocatalytic degradation of RhB by (B) TiO
2 2
/ZnFe O
4 2 2 4 2 2 4
; (C) TiO /ZnFe O /0.02 g AFAC and (D) TiO /ZnFe O /0.30 g AFAC photocatalysts.
0
nanocomposite rst photodegraded RhB (550 nm) to N,N,N - effects of the adsorption ability and photocatalytic ability of the
triethyl rhodamine (537 nm) aer 15 min, then to rhodamine ZnFe
2 4 2
O /TiO /AFAC photocatalyst improve the removal of RhB
(495 nm) aer 30 min and the peak of rhodamine sharply in the solution.
decreased aer 75 min, which indicated that RhB was photo-
degraded. Moreover, the TiO /ZnFe O /0.02 g AFAC photo-
catalyst photodegraded RhB (550 nm) rst to N,N -diethyl
rhodamine (521 nm) aer 15 min and then to rhodamine
2
2 4
3
.7 Recyclability
To evaluate the recyclability of the as-prepared catalysts, the
ZnFe /TiO /0.02 g AFAC photocatalyst was selected to test the
0
2
O
4
2
(
495 nm) aer 30 min. The TiO
catalyst photodegraded RhB (550 nm) rst to N-ethyl rhodamine
505 nm) aer 15 min and then to rhodamine (495 nm) aer
0 min. The above results were similar to those reported by
2 2 4
/ZnFe O /0.30 g AFAC photo-
recycle experiments. As shown in Fig. 9, aer two and three
runs, about 83.4% and 73.7% of RhB was photodegraded,
respectively. The photocatalytic performance of the as-prepared
catalyst decreased slightly during the photodegradation
process. The decrease in the RhB removal efficiency might be
attributed to the accumulation of RhB on the photocatalyst
surface, which blocked the mesopores gradually, and then the
adsorption of RhB from the solution was restrained. Therefore,
to improve the photocatalytic performance of the reused cata-
lyst, appropriate prolonging of the oxidation time is benecial.
(
3
40
Watanabe et al., in which the absorption of the intermediate
products changed from RhB (555 nm), to N,N,N -triethyl
rhodamine (539 nm), N,N -diethyl rhodamine (522 nm), N-ethyl
0
0
rhodamine (510 nm) and rhodamine (498 nm). Fig. 8C shows an
0
abnormal upturn from N,N -diethyl rhodamine (521 nm) to
rhodamine (495 nm) aer 30 min, corresponding to the
abnormal upturn of the TiO
catalyst (Fig. 7), which implied that the formation of rhodamine
495 nm) was critical to the photocatalysis process.
The process of RhB removal on the ZnFe O /TiO /AFAC
2 2 4
/ZnFe O /0.02 g AFAC photo-
(
3.8 Photocatalytic mechanism
2
4
2
Under visible light, the impressive photocatalytic performance
of the ZnFe O /TiO /0.02 g AFAC photocatalyst primarily
2 4 2
resulted from the following three factors: (1) the crystallite size
photocatalyst surface could be summarized as follows: (1) when
the ZnFe /TiO nanocomposite was loaded on AFAC, the
2
O
4
2
surface adsorption of RhB on the photocatalyst could be
enhanced, which causes a concentration effect in photo-
degrading RhB; (2) the photodegradation of RhB would empty
the adsorption sites on the surface of the photocatalyst, which
would continue adsorbing molecular RhB. The synergistic
of the as-prepared anatase TiO was less than 10 nm, which
2
indicated that the specic surface area of anatase TiO would be
2
large; (2) when TiO particles were coated on ZnFe O particles,
2
2 4
2 2 4
the band-gap of the TiO /ZnFe O nanocomposite decreased
from 3.18 to 2.56 eV, so the nanocomposite could photodegrade
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Fig. 9 The photocatalytic degradation of RhB for three cycles by TiO /
2 4
ZnFe O /0.02 g AFAC photocatalyst.
Fig. 10 Schematic diagram of photocatalytic degradation of RhB on
2 2 4
TiO /ZnFe O /AFAC photocatalyst under visible light irradiation.
RhB under visible light irradiation; (3) when hybridized with
.02 g AFAC, the TiO /ZnFe O /0.02 g AFAC exhibited a band-
gap of 2.50 eV, the number of the macropores in the photo-
catalyst decreased and the majority of the pores were meso-
ꢀ
+
ZnFe
2
O
4
/TiO
2
/AFAC + hn / e + h
(1)
(2)
(3)
(4)
0
2
2 4
ꢀ
ꢃ
ꢀ
2
e
+ O / O
2
pores, which resulted in
a
competitive phenomenon:
+
h + H
ꢃ
ꢃ
2
O / OH + H
a dominant positive effect of the enhancement of the adsorp-
tion ability and a negligible negative effect of a decrease in the
ꢃ
+
ꢀ
RhB + hn / RhB* / RhB + e
2
ꢀ1
2
ꢀ1
specic surface area from 162.18 m g to 151.12 m g . The
+
ꢃ
ꢀ
ꢃ
ꢃ
+
0
combination of the above three positive factors resulted in h , O , OH + RhB, RhB / N,N,N -triethyl rhodamine /
2
0
a synergistic effect, which led to the slight enhancement of the
photocatalytic performance from 95% to 97.1%. When hybrid-
ized with 0.30 g AFAC, although the TiO /ZnFe O /0.30 g AFAC
N,N -diethyl rhodamine / N-ethyl rhodamine /
rhodamine / CO , H O, etc.
2
2
(5)
2
2 4
2
ꢀ1
photocatalyst had a bigger specic surface area of 167.44 m g
and a smaller band-gap of 2.44 eV, which were both positive
effects for photocatalytic performance, the removal efficiency of
RhB decreased to 91%. The reason might be as follows: the
more AFAC was added, the more chances that AFAC may absorb
and reect the visible light energy, so less energy would be
absorbed by the TiO /ZnFe O photocatalyst and eventually the
4
. Conclusions
A series of TiO
the basis of the sol–gel and hydrothermal method. By coating
ZnFe nanoparticles with TiO nanoparticles, the band gap of
TiO /ZnFe nanocomposite was narrowed to 2.56 eV, which
would possess photocatalytic ability under visible light. When the
AFAC was hybridized with the TiO /ZnFe O nanocomposite,
their adsorption capacity and absorption ability were enhanced.
Under visible light irradiation for 75 min, the removal rate of RhB
on the TiO
AFAC photocatalyst were 95.0% and 97.1%. However, on
hybridization with more AFAC, the TiO /ZnFe /0.30 g AFAC
photocatalyst had a lower removal rate of 91%. The impressive
photocatalytic performance originated from the synergistic effect
of small crystallite size, narrow band-gap and mesoporous
structure of the photocatalysts. Recycling experiments indicated
that the photocatalyst retained higher activity aer three cycles.
2 2 4
/ZnFe O /AFAC photocatalysts were prepared on
O
2 4
2
2
2 4
2
2 4
O
photocatalytic performance decreased.
A probable photocatalytic mechanism is schematically
illustrated in Fig. 10. Under visible light irradiation, the elec-
2
2 4
ꢀ
trons (e ) would be promoted to the conduction band (CB),
+
while some corresponding positive holes (h ) would emerge in
2 2 4 2 2 4
/ZnFe O nanocomposite and TiO /ZnFe O /0.02 g
1
6,41,42
the valence band (VB) as described in eqn (1).
The pho-
togenerated electrons could migrate to the surface and adsorb
2
2 4
O
ꢃ
ꢀ
molecular oxygen to generate the superoxide radical ( O
)
2
(
eqn (2)) and the photogenerated holes could react with the
ꢃ
adsorbed H
(
2
O molecules to generate hydroxyl radical ( OH)
4
3
eqn (3)). The RhB adsorbed on the photocatalyst surface
4
4
could be promoted to its excited state (RhB*); then, RhB*
would promote an electron to the photocatalyst and RhB
ꢃ
+
Therefore, the TiO
potential application in environmental remediation.
2 2 4
/ZnFe O /AFAC photocatalysts would have
would be formed (eqn (4)). The generated superoxide radicals,
hydroxyl radicals and the photogenerated holes of the valence
ꢃ
+
band could degrade RhB (and RhB ) into carbon dioxide and
water (eqn (5)). The above process can be summarized as Conflicts of interest
follows:
There are no conicts to declare.
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2
2 J. Zhang, X. Wang, X. Wang, J. Song, J. Huang,
B. Louangsouphom and J. Zhao, RSC Adv., 2015, 5, 71922–
71931.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (Grant No. 61775081, 61475063, 21576111) and 23 X. Wang, W. Wang, X. Wang, J. Zhang, Z. Gu, L. Zhou and
the Graduate Innovation Research Fund at the Jilin Normal
University (No. 201628).
J. Zhao, Appl. Surf. Sci., 2015, 349, 264–271.
24 S. Hosseini, S. Borghei, M. Vossoughi and N. Taghavinia,
Appl. Catal., B, 2007, 74, 53–62.
2
5 L. C. R. Machado, C. B. Torchia and R. M. Lago, Catal.
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