Novel hybrid Sr-doped TiO2/magnetic Ni0.6Zn0.4Fe2O4 for enhanced separation and photodegradation of organics under visible light

Article abstract of DOI:10.1039/c5ra00187k

Titanium dioxide (TiO₂) has been intensively used as a photocatalyst for the degradation of organic pollutants in water, but is typically obstacle by a low efficiency, costly separation, limited visible light response, and poor recyclability. Herein, we provided a reliable method to simultaneously tackle these four obstacles by developing an integrated and multifunctional hybrid photocatalyst/magnetic material, i.e., Sr-TiO₂/Ni_0.6Zn_0.4Fe₂O₄. This novel hybrid not only demonstrated a high efficiency (90-100%) and a good cycling performance (90% maintenance) for photodegradation of bisphenol A (BPA) under both UV and visible light irradiation, but it can also efficiently work at a wide pH range (4-10) and can be easily separated from water for reuse only by introducing an external magnetic field. The materials structure-to-activity correlation has also been investigated. It was found that doping Sr²⁺ and a coupling magnetic material with TiO₂ could extend the visible light response and create active defects in TiO₂, which were responsible for the nearly three times higher activity than that of commercial TiO₂(P25) under visible light. On the other hand, doping excessive Sr²⁺ lowered the surface area, enlarged the crystalline size and caused particle aggregation; thus, leading to a decrease in photocatalytic activity of the hybrid. These further modifications in the hybrid materials can provide a competitive alternative to control the organic pollutants in waste water.

Full text of DOI:10.1039/c5ra00187k

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PAPER
Novel hybrid Sr-doped TiO /magnetic
2
Ni Zn Fe O for enhanced separation and
0
.6
0.4 2 4
Cite this: RSC Adv., 2015, 5, 24056
photodegradation of organics under visible light
a
a
c
a
a
b
Fuming Liu, Yu Xie,* Changlin Yu, Xiaoming Liu, Yuhua Dai, Lianjun Liu*
a
and Yun Ling
2
Titanium dioxide (TiO ) has been intensively used as a photocatalyst for the degradation of organic
pollutants in water, but is typically obstacle by a low efficiency, costly separation, limited visible light
response, and poor recyclability. Herein, we provided a reliable method to simultaneously tackle these
four obstacles by developing an integrated and multifunctional hybrid photocatalyst/magnetic material,
2 2 4
i.e., Sr–TiO /Ni0.6Zn0.4Fe O . This novel hybrid not only demonstrated a high efficiency (90–100%) and a
good cycling performance (90% maintenance) for photodegradation of bisphenol A (BPA) under both UV
and visible light irradiation, but it can also efficiently work at a wide pH range (4–10) and can be easily
separated from water for reuse only by introducing an external magnetic field. The materials structure-
2+
to-activity correlation has also been investigated. It was found that doping Sr and a coupling magnetic
material with TiO could extend the visible light response and create active defects in TiO , which were
(P25) under visible
2
2
responsible for the nearly three times higher activity than that of commercial TiO
2
Received 11th January 2015
Accepted 18th February 2015
2+
light. On the other hand, doping excessive Sr lowered the surface area, enlarged the crystalline size
and caused particle aggregation; thus, leading to a decrease in photocatalytic activity of the hybrid.
These further modifications in the hybrid materials can provide a competitive alternative to control the
organic pollutants in waste water.
DOI: 10.1039/c5ra00187k
www.rsc.org/advances
organic pollutants, and it oen required a ltration or centri-
fugation process to separate the photocatalyst from water. This
1
. Introduction
Photodegradation of organic pollutants using photocatalysts process may increase the cost and brings a potential genotox-
2
,12
(
e.g., ZnO, TiO , WO , CdS) holds a great promise to purify water icity due to some residual organics adsorbed on the solids.
2 3
1
–3
and has been investigated intensively over the last decade.
Among the photocatalysts, TiO
has received great interest mance and easily recycled TiO
because of its chemical stability, high photocorrosion resis-
To improve the photocatalytic efficiency of TiO
has a have been made to modify TiO either by incorporating noble
very low quantum efficiency primarily because of fast recom- metals (e.g., Ag) or metal oxides (e.g., Fe ) to act as electron
Therefore, it is urgent and necessary to develop high perfor-
-based photocatalysts.
, great efforts
2
2
2
3
–8
tance, non-toxicity, and low-cost. However, bare TiO
2
2
2 3
O
ꢀ
+
ꢀ
+
2,8,13–16
bination rates of electron and hole pair (e –h ) and limited traps and inhibit the recombination of e –h pairs,
or by
visible light responses (due to its wide band gap in the range of doping nonmetals (e.g., N) or co-doping with nonmetals and
3
+
2+
2+
2+
1
,8
3
.0–3.2 eV). The low efficiency of TiO is a major barrier for transition metal ions (e.g., Cr , Cu , Co , Mg ) to extend the
2
9,17–23
commercializing this photocatalytic technology. Complicated visible light response.
Alternatively, pure or modied
) and SrTiO /TiO hetero-
impede its practical application, but this challenge is seldom junctions have also been investigated as a candidate for the
separation of photocatalyst for regeneration and reuse may also perovskite strontium titanate (SrTiO
3
3
2
9
–11
23–27
addressed in the literature.
was suspended in a solution for the photodegradation of studies have been conducted on Sr doped TiO
todegradation of organic compounds. Very recently, Li et al.
Generally, a TiO
2
-based power photodegradation of organic pollutants.
However, a few
2
+
2
for the pho-
28
2
+
synthesized Sr -doped ZnO composites and attributed its
Department of Materials Chemistry, Nanchang Hangkong University, Nanchang, enhanced performance over ZnO for RhB degradation under
a
Jiangxi Province, 330063, PR China. E-mail: xieyu_121@163.com; Fax: +86 791
visible light to the narrowed band gap and the formation of
active defects to trap electrons. Along this line, it is expected
8
b
3953373; Tel: +86 791 83953373
Mechanical Engineering Department, University of Wisconsin–Milwaukee,
2+
that doping Sr into the TiO lattice will lead to an enhanced
2
Milwaukee, WI, 53211, USA. E-mail: liul@uwm.edu; Tel: +1 216 650 9842
c
photocatalytic degradation of bisphenol A (BPA).
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and
Technology, Ganzhou, Jiangxi Province, 341000, PR China
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To overcome the obstacle of TiO
2
powder separation from the sol was formed. The as-received sol was nally dried in an
ꢁ
solution, several efforts have been attempted to anchor TiO on oven at 100 C for 24 h.
2
4
,11,29–33
solid substrates (e.g., Ti mesh)
or incorporate TiO with
2
34–40
magnetic materials (e.g., ferrite).
Immobilizing TiO on solid 2.2 Synthesis of hybrid Sr–TiO /magnetic material
2
2
substrates is restricted to laboratory or small-scale applications
because of the complicated preparation process, weak attach-
The hybrid Sr–TiO /magnetic materials were prepared by a sol–
gel method. Typically, in a beaker A, 10 ml of tetrabutyl titanate
2
ment of TiO
sion and dispersion of TiO
2
on the foreign substrates, and decient immer-
in the slurry solution. On the other
(
C H O Ti), 1 g of Ni0.6Zn0.4Fe O nanoparticles, and 40 ml of
16 36 4 2 4
2
absolute ethyl alcohol were uniformly mixed. In a beaker B, a
certain amount of strontium nitrate (Sr(NO ) and starch (the
mass ratio of Sr(NO to starch is 0.02 : 0.5, 0.05 : 1, 0.1 : 1.5,
.2 : 2.5), 5 ml de-ionized water, and 15 ml acetic acid were
hand, magnetic separation is a promising route to recover the
used photocatalysts only by applying an external magnetic eld.
A number of studies have been attempted to fabricate core–shell
structured magnetic material/photocatalysts for the photo-
3 2
)
3 2
)
0
mixed. The mixture was heated for 5 min by microwave irradi-
ation to get a homogeneous solution. Next, the solution in the
beaker B was added dropwise to beaker A with mechanical
degradation of organics, including Fe/TiO
nickel ferrite/N–TiO , and strontium ferrite/N–TiO
Unfortunately, the nanosized core magnetic materials are easily
oxidized (e.g., Fe oxidized to Fe ) or rapidly transform in
the crystal phase (e.g., g-Fe ferromagnetic to a-Fe para-
2 2
/Ag, Fe/V/TiO ,
9,34,37,40
2
2
.
ꢁ
ꢁ
stirring for 2 h. The mixed solution was aged at 70 C for 24 h.
3
O
4
2
O
3
The obtained gel was then dried and calcined at 550 C in air for
2
O
3
2 3
O
4
h. The nal hybrid materials were denoted as Sr
x
Ti/M
ꢁ
34,40
magnetic) when the calcination temperature is over 400 C.
(
M ¼ Ni0.6Zn0.4Fe
2 4
O
), where x is the nominal weight
Obviously, it is still a challenge to produce the TiO -coated
2+
2
percentage of Sr (i.e., 0.1 wt%, 0.25 wt%, 0.5 wt%, and 1 wt%).
For comparison, Ti/M and Sr0.25Ti were also prepared using a
same procedure as SrTi/M.
magnetic nanoparticles with concurrent good stability, high
visible light activity and good magnetic properties.
To simultaneously overcome the challenges in the catalytic
efficiency and separation of TiO
work, we designed a novel hybrid by integrating a photocatalyst
with magnetic material, i.e., Sr–TiO We
2
-based photocatalysts, in this
2
.3 Characterization
The crystal structure of Sr–TiO
characterized by X-ray diffraction (XRD) using a Bruker D8
Advance X-ray diffractometer with monochromated high-
2
2 4
/Ni0.6Zn0.4Fe O materials were
2
2 4
/Ni0.6Zn0.4Fe O .
2
+
hypothesize that (1) doping TiO with Sr could create some
2
active defects and promote visible light absorption, and thus
enhancing the charge separation and extending visible light
ꢀ
intensity CuKa radiation (l ¼ 0.15418 A) in the 2q range of
ꢁ
1
0–80 . The morphology and particle size of the materials were
response, (2) introducing Zn into NiFe
phase transition and assure good magnetic properties,
using Ni0.6Zn0.4Fe as magnetic core not only harvests visible
light but also facilitates photocatalyst separation, and (3) an
2 4
O could prevent the
41,42
identied by eld emission scanning electron microscopy
FESEM). Transmission electron microscopy (TEM) (JEOL TEM-
010) was used to approach the lattice fringes operating at an
and
(
3
2 4
O
2
+
accelerating voltage of 300 keV. The surface area and porosity of
the as-synthesized materials were examined by N adsorption/
desorption at 77 K using the Brunauer–Emmett–Teller (BET)
method (Micromeritics, ASAP 2020). The UV-vis diffuse reec-
tance spectra were recorded using a UV-vis-NIR spectrometer
interface may be formed between Sr -doped TiO
2
and magnetic
2
Ni0.6Zn0.4Fe O , and thus inducing a synergistic effect to
2
4
remove organics with an exceptional performance. To the best
of our knowledge, for the rst time this multifunctional mate-
rial for water treatment has been developed. All the materials
used (Ti, Sr, Ni, Zn, and Fe) to build the nanostructure are
inexpensive and earth-abundant. The sol–gel method employed
to prepare the hybrid is also simple and easy to scale-up.
Another feature in this work is to evaluate the cycling perfor-
mance of the hybrid photocatalyst/magnetic material under
both UV and visible light irradiation, and attempt to establish
the structure-to-activity relationships.
(
Cary 5000, Varian). The valence states of Ti and O were iden-
tied by X-ray photoelectron spectroscopy (XPS), using a PHI
000 versaprobe system using monochromatic Al KR radiation
1486.6 eV). All binding energies were referenced to the C 1s
5
(
peak at 284.6 eV. The magnetic properties of the photocatalyst
were evaluated at room temperature using a vibrating sample
magnetometer (VSM, 9600-1 LDJ, USA) at a maximum applied

eld of 10 kOe.
2
.4 Photocatalytic activity measurement
2
. Materials and methods
The photocatalytic degradation of BPA was carried out in a
hollow cylindrical photoreactor. The light source is a low-
a self- pressure mercury lamp (Beijing Ceaulight Co., Model CEL-
2.1 Synthesis of magnetic Ni0.6Zn0.4Fe
2
O
4
nanoparticles
Ni0.6Zn0.4Fe
O
2 4
nanoparticles were prepared by
propagating combustion method. In brief, 0.1 mol nickel LUV254, 10 W) that emits principally near 254 nm, or a long-
nitrate (Ni(NO $6H O), 0.067 mol zinc nitrate (Zn(NO $6H O) arc xenon lamp (Beijing Ceaulight Co., Model CEL-LAX500,
and 0.333 mol iron nitrate (Fe(NO $9H O) were dispersed into 500 W) to simulate visible light (>400 nm). The photoreactor
00 ml deionized water with vigorous stirring. Citric acid was cooled by circulating water through a quartz channel
)
3 2
2
)
3 2
2
)
3 3
2
1
(
0.5 mol) was then added into the solution, using ammonium inside, and the temperature was maintained at around 25 ꢂ
ꢁ
hydroxide to adjust the pH around 10.0. Next, the solution was 2 C. Prior to illumination, a 300 ml suspension with a certain
heated in a water bath at 70 C by microwave irradiation until amount of photocatalyst (0.5 g l ) was stirred for 30 min to
ꢁ
ꢀ1
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ensure the homogenous dispersion and full contact of BPA (10 diffraction peaks, which are indexed to a mixture of the
ppm) with catalysts. The pH of the BPA solution was controlled anatase phase of TiO2 (JCPDS no. 21-1272) and the spinel
4
3,44
using a NaOH solution (5 M, 1 M, 0.1 M) and an HNO solution phase of Ni0.6Zn0.4Fe O .
No additional peaks for perov-
3
2
4
(
2 M. 0.1 M) to prepare basic and acidic solutions, respectively. skite SrTiO or SrO crystallites appeared because of the low
3
Aer that, the lamp was turned on to irradiate the solution. A calcination temperature and low dopant concentration of
2
+
23,27
2+
sample of 0.5 ml of solution was withdrawn at every 30 min. Sr
Moreover, the pH of the BPA solution was recorded at different diffraction peaks. By contrast, Sr
reaction times using a pH meter (Mettler Toledo). The BPA and weakened diffraction peaks for both the TiO
concentration in each batch was measured by high- and magnetic Ni0.6Zn0.4Fe 4. This comparison indicated
.
Noticeably, Ti/M without Sr dopant exhibited sharp
Ti/M showed broadened
(101) plane
x
2
2
O
2
+
performance liquid chromatography (HPLC) equipped with a that doping Sr ions into the TiO lattice could stabilize the
2
C-18 column (LUNA 5u 100A, 4.6 mm ꢃ 250 mm, Phenomenex) crystal phase of TiO anatase and inhibit the aggregation and
2
and a diode array detector (SPD-M20A, Shimadzu). The isocratic growth of the particles, thus inducing a decrease in TiO2
methanol–water mixture (70 : 30, v/v) as an eluent was crystalline size. Among the samples, Sr0.1Ti/M showed the
ꢀ1
employed at a ow rate of 1 ml min . The residual BPA content smallest crystalline size (see Table 1). Increasing the
2
+
in the aqueous solution was determined with a standard curve concentration of Sr ions resulted in an increase in the TiO
2
2
(
R ¼ 0.9999) using a standard BPA solution for the calibration crystalline size from 15.7 nm to 18.8 nm, probably because
2+
(
i.e., 0, 0.2, 0.5, 1, 2, 5, 10 and 15 ppm). The degradation effi- the excessive Sr ions cannot be completely doped into the
ciency (DE) of BPA was calculated by the following equation: DE TiO lattice and caused particle agglomeration. Moreover, the
2
¼
C /C , where C and C are the initial and residual BPA uniformly dispersed Sr–TiO particles may fully attach on the
t
o
o
t
2
concentration (ppm) at different reaction times, respectively.
surface of Ni0.6Zn0.4Fe O and result in a decrease in its peak
2
4
intensity.
The morphology and particle size of the Sr
x
Ti/M samples
3
. Results and discussion
were characterized by SEM (Fig. 2) and TEM (Fig. 3). As shown in
Fig. 2, Sr0.1Ti/M and Sr0.2Ti/M have uniformly dispersed parti-
cles with an average size of 20 nm. By contrast, Sr1.0Ti/M has
randomly mixed small particles and big grains (over 100 nm).
The TEM image at a low magnication in Fig. 3a further
conrmed that Sr0.1Ti/M was composed of relatively uniform
spherical or rectangle particles with a diameter of ꢄ20 nm. The
TEM image at a high magnication in Fig. 3b clearly shows well-
3.1 Crystal structure, morphology and texture
The crystal structure of the hybrid was identied by XRD. As
shown in Fig. 1, all the samples displayed the same
faceted TiO
.352 nm that matches the (101) plane of anatase phase.
nanoparticles (ꢄ15 nm), located beyond TiO
2
nanocrystals with an interplanar spacing of
5,45,46
0
Ni0.6Zn0.4Fe
2
O
4
2
47
particles, was also observed with a lattice spacing of 0.187 nm.
Again, SEM and TEM images directly supported the XRD
2
+
observations that a small amount of Sr (below 0.25 wt%)
doped TiO particles were well-patched onto the magnetic
2
2
+
2 4
Ni0.6Zn0.4Fe O , while excessive Sr dopant easily induced
particles aggregation into large grains.
Table 1 also compared the measured BET surface area, pore
x
size, pore volume of the Sr Ti/M samples to explore the effect of
2
+
Sr -dopant content on the textural property. The presence of
mesopores in Sr Ti/M could result from the space between
x
particles, as evidenced by the SEM images in Fig. 2. At low
2
+
loadings of Sr , Sr0.1Ti/M and Sr0.25Ti/M displayed similar
surface areas, pore sizes, and pore volumes, which were
2
+
Fig.
2 2 4
1 XRD patterns for Sr -doped TiO /Ni0.6Zn0.4Fe O nano-
2+
composites with a series of concentrations of Sr
.
2+
Table 1 The BET surface area, pore volume, pore size, and crystallite size of TiO
2
for Sr -doped TiO
2
/M (M ¼ Ni0.6Zn0.4Fe
2 4
O )
BET surface area
Pore volume
Pore size
(nm)
Crystallite size of TiO
(nm)
2
2
ꢀ1
3
ꢀ1
Sample name
(m g
)
(cm g
)
Sr0.1Ti/M
Sr0.25Ti/M
Sr0.5Ti/M
Sr1.0Ti/M
64
66
44
46
0.205
0.198
0.063
0.071
12.7
12.0
5.6
15.7
15.9
17.5
18.8
6.1
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Fig. 2 SEM images for (a) Sr0.1Ti/M, (b) Sr0.25Ti/M, and (c) Sr1.0Ti/M
composites (M ¼ Ni0.6Zn0.4Fe
2 4
O ).
considerably higher than those of Sr0.5Ti/M and Sr1.0Ti/M at
2
+
high loadings of Sr . The data in Table 1 agrees well with XRD
2
+
and SEM observations that high contents of Sr results in the
aggregation of the particles and the growth in crystalline size,
which could block the micro/meso-pores within TiO
and decrease the BET surface area, pore size, and pore volume. Fig. 4 UV-vis spectra for Sr -doped TiO
2
particles
2
+
2
/Ni0.6Zn0.4Fe
2 4
O nano-
2
+
composites with different Sr concentrations.
3.2 Optical property and chemical state
UV-vis spectra, displayed in absorbance units, have been
associated with lattice oxygen (O ) bonded to metal ions, while
l
recorded to investigate the light response of the Sr Ti/M
x
the O 1s related to surface oxygenated species (O ) (e.g., OH
a
samples. As shown in Fig. 4, the absorption edge of bare
groups, adsorbed H O) remained at a same position at
2
TiO
gap of about 3.10 eV. By contrast, incorporating the magnetic
with TiO induced the appearance of a tail
extending to a longer wavelength of 600 nm. All the Sr Ti/M
2
(P25) is located around 400 nm, corresponding to a band
45
5
31.5 eV. Moreover, the relative amount of O
the peak area ratio of O /(O + O ) in Fig. 5b) in Sr0.01Ti/M
78.6%) was slightly higher than that in Ti/M (74.8%). Gener-
l
(by calculating
l
l
a
Ni0.6Zn0.4Fe
2
O
4
2
(
x
ally, the chemical shis in XPS spectra resulted from either the
formation of a new oxidation state or the changes in the local
chemical and physical environment. Because no shoulder peak
samples display similar broad bands extending to nearly 800
nm, which is consistent with the dark gray physical appearance
of the samples. Furthermore, the band edge of Sr0.1Ti/M red
shied to about 520 nm, corresponding to a small band gap of
3+
for the new state of Ti was observed in Sr0.1Ti/M, the slight
chemical shi is primarily related to a change in the local
2
.4 eV. The UV-vis results conrmed our original hypothesis
2
+
chemical environment of TiO
successfully doped into TiO
2
. In other words, Sr has been
2
+
that doping Sr into TiO
Ni0.6Zn0.4Fe
2
and coupling it with magnetic
and harvest
4
+
2
2
lattice. Because some Ti sites
have been substituted by Sr ions, some defects (e.g., oxygen
2
O
4
could narrow the band gap of TiO
2
+
visible light.
16,29
vacancy, V
O
) were formed for charge compensation.
This is
XPS has been conducted to approach the chemical states of
also supported by the XPS quantitative analysis that Ti/M has a
higher ratio of O/(Ti + Zn + Ni + Fe) (2.61) than does Sr0.1Ti/M
TiO with and without doping Sr. As shown in Fig. 5a, Ti/M
2
displayed Ti 2p3/2 and the Ti 2P1/2 binding energies at
(
2.02). Hence, the change in the chemical environment of
4
58.6 and 464.2 eV, respectively, corresponding to a typical
4
+
TiO was possibly caused by the partial replacement of Ti by
Sr dopants and the formation of V in TiO .
O 2
4
+
16,40
2
characteristic of the Ti oxidation state.
Interestingly, the Ti
2
+
2
p binding energies of Sr0.1Ti/M slightly shied to a lower level
2
+
(by 0.4 eV). Similarly, doping Sr into TiO
2
also led to a
chemical shi in the O 1s binding energy at 529.9 eV that is
3
.3 Photocatalytic performance measurement
Photocatalytic degradation of BPA over the hybrid Sr
rst conducted under UV (254 nm) irradiation. Fig. 6 compares
x
Ti/M was

the photodegradation efficiency of various Sr Ti/M. Right before
x
the photodegradation, a background experiment was carried
out without UV irradiation in the presence of Sr0.1Ti/M. The
result showed that in the dark process, the concentration of BPA
almost remained same as the initial one within 4 h, indicating
that no photocatalytic reaction took place without photo-
2
+
illumination. At low contents of Sr , Sr0.1Ti/M and Sr0.25Ti/M
exhibited similar photodegradation activity. Furthermore,
2
+
increasing the concentration of Sr resulted in a decrease in the
activity over Sr0.5Ti/M and Sr1.0Ti/M. By correlating the structure
and property of the materials (see the results in Table 1, Fig. 1
Fig.
3
(a) TEM and (b) HRTEM images for Sr0.1Ti/M (M
).
¼
2
+
Ni0.6Zn0.4Fe
2
O
4
and 2), the inferior activity at a high content of Sr is probably
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2+
Fig. 6 Photodegradation of BPA by various Sr -doped TiO
2
/M
(
M ¼ Ni0.6Zn0.4Fe
2 4
O
) catalysts under UV-254 nm irradiation as a
ꢀ1
function of time (catalyst dosage ¼ 0.5 mg ml , initial BPA concen-
tration ¼ 10 ppm, near neutral pH).
was found that either at basic (pH ¼ 10) or acidic conditions
(
pH ¼ 4), Sr Ti/M displayed a higher efficiency that at near
0
.25
neutral conditions (pH ¼ 6–8) within the beginning 2 h, while
the efficiency eventually remained almost the same at the later
stage and reached as high as nearly 99% at 3.5 h in the entire pH
range (4–10). This result strongly suggested that our novel
x
hybrid Sr Ti/M is able to efficiently work at a wide pH range.
Moreover, the pH evolution during the photocatalytic reaction
was also monitored, as shown in Fig. 7b. At a basic condition,
the pH gradually decreased and reached a steady state to near
neutral, while at an acidic condition, the initial pH gradually
increased. If starting with a near neutral solution, the pH only
slowly decreased. The evolution of solution pH revealed that
2 2 4
Fig. 5 XPS spectra for undoped TiO /Ni0.6Zn0.4Fe O and Sr0.1Ti/
Ni0.6Zn0.4Fe O composites: (a) Ti 2p, and (b) O 1s.
2 4
because Sr0.5Ti/M and Sr1.0Ti/M have larger crystalline sizes,
smaller specic surface areas and pore volumes, and less
uniform particle dispersions. These drawbacks on Sr0.5Ti/M and
Sr1.0Ti/M could hinder the adsorption and activation of BPA on
+
both proton (H ) and OHc radicals play a critical role in the
photodegradation of BPA. A possible mechanism has been
proposed to explain the pH impacts, as shown in Fig. 8. The
2
+
the surface. Excessive Sr may also induce the formation of a
ꢀ
ꢀ
+
starting solution with a high pH contains more OH anions,
new e –h recombination center that shortened the lifetime of
+
28
which could act as hole (h ) scavengers and react with photo-
photogenerated charge carriers. Another important result in
Fig. 6 is that Sr0.1Ti/M and Sr0.25Ti/M displayed a considerably
higher efficiency compared to Ti/M, Sr0.25Ti, commercial
+
16,27
generated h to form active OHc radicals (Fig. 8a).
Hence,
ꢀ
the consumption of OH led to the decrease in pH over the
reaction time. The OHc radicals have a strong oxidation ability
to degrade BPA to CO and H O, which could explain why the
2 2
TiO
1
4
2
(P25), and bare M. Sr0.1Ti/M is able to eliminate nearly
00% BPA at 4 h, while TiO (P25) and bare M can only reach
0% and 25%, respectively. In addition, Sr0.25Ti and Ti/M was
(P25) and M. In this regard, it is
2
activity is higher at pH ¼ 10. On the other hand, the starting
+
solution with a low pH contains more H ions, which serves as
even more active than bare TiO
believed that there is a synergy between Sr dopant and
magnetic M that remarkably prevents the charge recombination
2
ꢀ
+
2
+
electron scavengers and inhibits the recombination of e –h
+
pairs (Fig. 8b). Moreover, the Hc radicals, generated from H +
ꢀ
e
2
/ Hc, could react with O to form hydrogen peroxide that
in TiO , as a consequence of leading to a superior activity of
2
48
decomposes into oxidative OHc under UV irradiation. Thus,
the suppressed recombination and the formation of OHc leads
to an increase in pH over time and a higher efficiency than at
neutral pH values.
Sr0.1Ti/M.
We also performed photocatalytic degradation of BPA at
different pH conditions using a Sr0.25Ti/M composite under UV
irradiation because the waste water may be acidic or basic. The
initial pH of the reactant solution was controlled by adding a
The photocatalytic performance of the Sr Ti/M samples was
x
also evaluated under visible light (400–1000 nm) irradiation, as
shown in Fig. 9. The order of activity of the hybrids was Sr0.1Ti/
desired amount of NaOH or HNO
3
. Fig. 7a compares the pho-
tocatalytic performance of Sr0.25Ti/M in a pH range of 4–10. It
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2+
Fig. 9 Photodegradation of BPA by various Sr -doped TiO
2
/M
(
(
M
¼
Ni0.6Zn0.4Fe
2 4
O ) catalysts under visible light irradiation
ꢀ1
400–1000 nm) as a function of time (catalyst dosage ¼ 0.5 mg ml
,
initial BPA concentration ¼ 10 ppm, near neutral pH).
at 4 h, nearly 4 times higher than bare TiO
2
(P25) and M (only
15–20%). In addition to the larger surface area, smaller crys-
talline size, and more uniform particle dispersion, the superior
performance of Sr0.1Ti/M under visible light is probably also
due to another two reasons. First, incorporating an appropriate
2
+
amount of Sr dopant and magnetic M with TiO narrowed its
2
band gap (see UV-vis spectra in Fig. 4); thus, enhancing the
harvest and utilization of visible light. Second, as evidenced by
2
+
2
XPS results, doping Sr into TiO also created some active
defect sites (oxygen vacancies), which may induce the formation
of the new energy state located below the conduction band
Fig. 7 Photodegradation of BPA by Sr0.25Ti/M (M ¼ Ni0.6Zn0.4Fe
2 4
O )
minimum of TiO . The active defect sites could facilitate charge
2
under UV-254 nm irradiation (a) at different pH environments and (b)
16,28,49
separation and trap electrons even under visible light.
The
ꢀ1
the evolution of pH as a function of time (catalyst dosage ¼ 0.5 mg ml
initial BPA concentration ¼ 10 ppm).
,
electrons accumulated at defect sites could easily attach on O
molecules to produce powerful superoxide radicals (cO ) that
2
2
ꢀ
promote the activation and oxidation of BPA molecules.
3.4 Material separation and cycling performance
The above photocatalytic activity results indicated that among
all the tested samples, Sr0.1Ti/M has an outstanding perfor-
mance under both UV and visible light irradiation. To further
understand its superiority, we measured the cycling perfor-
mance of Sr0.1Ti/M during three runs of photodegradation
alternations. In between each cycle, the spent Sr0.1Ti/M was
separated only by adding an external magnetic eld around the
solution and washed with water without any high temperature
Fig. 8 The possible mechanism for BPA photodegradation under (a) treatment or centrifugation. Above all, we measured the
basic and (b) acidic conditions.
magnetic property of Sr0.1Ti/M at room temperature using
vibrating magnetometer. Fig. 10 shows the magnetization
(
M)–magnetic eld (H) loop of Sr0.1Ti/M. It was found
M > Sr0.25Ti/M z Sr0.5Ti/M z Sr1.0Ti/M > Sr0.25Ti > Ti/M > P25 >
M, which is very similar to the trend under UV irradiation (see
Fig. 6). Even under visible light irradiation, the photo-
degradation efficiency of Sr0.1Ti/M could reach as high as 90%
that Sr0.1Ti/M exhibited a typical so ferrite behavior with a
saturated magnetization (M ) of 19.04 emu g , the number
of which was lower than the literature reported for pure
Ni0.6Zn0.4Fe
ꢀ
1
s
42,43
2
O
4
.
2 4
This is because our Ni0.6Zn0.4Fe O was
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separation and harvest visible light concurrently. This good
cycling performance also conrms our original hypothesis that
integrating metal ion doped photocatalysts (e.g., Sr–TiO ) and
2
magnetic materials (e.g., Ni0.6Zn0.4Fe O ) is a reliable and
2
4
convenient method to simultaneously enhance material sepa-
ration and visible light activity for water purication.
4. Conclusions
In this work, we designed a novel hybrid photocatalyst/
2
+
2 2 4
magnetic material, i.e., Sr doped TiO /Ni0.6Zn0.4Fe O , in
order to enhance the efficiency of photo-degrading organic
pollutants under visible light and to easily separate and reuse
the material. The hybrid was synthesized by a simple sol–gel
2
+
method. We found that doping a low concentration of Sr
below 0.25 wt%) induced a smaller crystalline size, larger
surface area and pore volume, and more uniform particle
(
2
+
dispersion than did loading a high concentration of Sr .
Fig. 10 Room temperature hysteresis behaviour of Sr0.1Ti/M
2
+
(
M ¼ Ni0.6Zn0.4Fe
2 4
O
). Inset photograph shows a separation process of Moreover, doping Sr ions and coupling a magnetic material
Sr0.1Ti/M from aqueous solution using a magnet.
with TiO
2
narrowed the band gap and induced the generation of
defect sites in TiO . As a result, the integrated hybrid with a low
2
2
+
loading of Sr not only demonstrated a high efficiency (over
encapsulated with nonmagnetic Sr0.1TiO
2
nanoparticles. 90%) and a good recycling performance (90% maintenance)
Although Sr0.1Ti/M has a relatively low M , it still can be easily under both UV and visible light irradiation, but also can effi-
s
magnetically separated for reuse. As evidenced by the inset ciently work at a wide pH range (4–10) and be easily separated
photograph in Fig. 10, the suspended solution quickly became only by adding an external magnetic eld. Furthermore, the
clear once placing a magnet near the bottle wall for 30 s, and the hybrid Sr–TiO /Ni0.6Zn0.4Fe O showed an over two-times
2
2 4
powder was accumulated and attached on the wall.
Fig. 11 compares the photodegradation efficiency of Sr0.1Ti/ TiO (P25) and bare Ni0.6Zn0.4Fe O , as well as 50% higher
2 2 4
higher activity than those of TiO /Ni0.6Zn0.4Fe O , commercial
2
2 4
M in each cycle under both UV and visible light irradiation activity than that of Sr-doped TiO , indicating that there is a
2
rd
aer 4 h. Aer three cycles, the efficiency in the 3 run only synergy between the doped photocatalyst and magnetic mate-
slightly dropped at both conditions, but still maintained as high rial. The ndings in this work suggest a new direction to engi-
as 89% and 78% under UV and visible light irradiation, neer a smart photocatalyst/magnetic material heterojunction
respectively. Obviously, Sr0.1Ti/M demonstrated a good stability and control the interface between them, and it sheds light on
and recyclability, where M could provide a magnetic eld for the material's application in other aqueous-solid phase photo-
catalytic reactions.
Acknowledgements
This work was nancially supported by National Natural
Science Foundation of China (no. 51468043), Natural Science
Foundation of Jiangxi Province (no. 20132BAB203018), Jiangxi
Province Youth Scientists Cultivating Object Program (no.
20112BCB23017), Key Laboratory of Photochemical Conversion
and Optoelectronic Materials, TIPC, CSA (no. PCOM201401).
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