Visible-light photocatalytic performance, recovery and degradation mechanism of ternary magnetic Fe3O4/BiOBr/BiOI composite

Article abstract of DOI:10.1039/c9ra04412d

The ternary magnetic Fe₃O₄/BiOBr/BiOI (x : 3 : 1) photocatalysts were successfully synthesized by a facile solvothermal method. The samples were characterized by XRD, SEM, EDS, ICP-AES, XPS, UV-vis DRS, PL and VSM. Nitrogen-containing dye RhB was used as a degradation substrate to evaluate the photocatalytic degradation activities of the samples. The photocatalytic performance of Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) is superior to other Fe₃O₄/BiOBr/BiOI (x : 3 : 1). Compared with binary magnetic Fe₃O₄/BiOBr (0.5 : 1) prepared in our previous work, the Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) has obvious advantages in photocatalytic activity and adsorption capacity. And the specific surface area (48.30 m² g^-1) is much larger than that of the previous report (Fe₃O₄/BiOBr/BiOI (0.5 : 2 : 2)) synthesized by a co-precipitation method. Besides, after 25 s of magnetic field, Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) can be rapidly separated from water. After eight recycling cycles, the magnetic properties, photocatalytic activity, crystallization and morphology of the Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) catalyst remain good. The possible photocatalytic degradation mechanism of RhB under Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) photocatalyst was also proposed. The results indicate that the ternary magnetic Fe₃O₄/BiOBr/BiOI (0.4 : 3 : 1) composite with high photocatalytic degradation efficiency, good magnetic separation performance and excellent recyclability and stability has potential application prospect in wastewater.

Full text of DOI:10.1039/c9ra04412d

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Visible-light photocatalytic performance, recovery
and degradation mechanism of ternary magnetic
Fe O /BiOBr/BiOI composite†
Cite this: RSC Adv., 2019, 9, 23545
3
4
Jianhui Li, Fan Yang, Quan Zhou, Lijie Wu, Wenying Li, Ruipeng Ren*
and Yongkang Lv
*
3 4
The ternary magnetic Fe O /BiOBr/BiOI (x : 3 : 1) photocatalysts were successfully synthesized by a facile
solvothermal method. The samples were characterized by XRD, SEM, EDS, ICP-AES, XPS, UV-vis DRS, PL and
VSM. Nitrogen-containing dye RhB was used as a degradation substrate to evaluate the photocatalytic
degradation activities of the samples. The photocatalytic performance of Fe
superior to other Fe /BiOBr/BiOI (x : 3 : 1). Compared with binary magnetic Fe
in our previous work, the Fe /BiOBr/BiOI (0.4 : 3 : 1) has obvious advantages in photocatalytic activity and
adsorption capacity. And the specific surface area (48.30 m
report (Fe /BiOBr/BiOI (0.5 : 2 : 2)) synthesized by a co-precipitation method. Besides, after 25 s of
magnetic field, Fe /BiOBr/BiOI (0.4 : 3 : 1) can be rapidly separated from water. After eight recycling cycles,
the magnetic properties, photocatalytic activity, crystallization and morphology of the Fe /BiOBr/BiOI
0.4 : 3 : 1) catalyst remain good. The possible photocatalytic degradation mechanism of RhB under Fe
BiOBr/BiOI (0.4 : 3 : 1) photocatalyst was also proposed. The results indicate that the ternary magnetic Fe
3
O
4
/BiOBr/BiOI (0.4 : 3 : 1) is
3
O
4
3 4
O /BiOBr (0.5 : 1) prepared
3 4
O
2
ꢀ1
) is much larger than that of the previous
g
3 4
O
3
O
4
3 4
O
Received 12th June 2019
Accepted 22nd July 2019
(
3 4
O /
O /
3 4
DOI: 10.1039/c9ra04412d
BiOBr/BiOI (0.4 : 3 : 1) composite with high photocatalytic degradation efficiency, good magnetic separation
rsc.li/rsc-advances
performance and excellent recyclability and stability has potential application prospect in wastewater.
8–10
water body, etc.
Accordingly, it is necessary to exploit new
Introduction
highly efficient and easily separated visible-light-responsive
The organic pollutants seriously affect the water environment photocatalysts for degrading organics in water treatment.
and threaten human health. In particular, synthetic nitrogen-
In order to alleviate these problems, many attempts have
containing dyes (Rhodamine B (RhB), Methyl orange (MO), been developed, such as the synthesis of various structures,
11,12
Congo red, etc) are one of the largest organic pollutants. Many doping of metal elements, construction of heterojunctions.
treatment methods have been developed, such as biological The construction of heterojunction photocatalysts has been
methods, membrane separation technology, electrochemical regarded as an efficient way to solve the above-mentioned
1–3
methods, ozone oxidation and so on. Among them, photo- defects of single semiconductors. The previously reported het-
14
1
3
catalytic technology is considered to be a promising and effec- erojunction photocatalysts include BiVO
tive environmental pollutant treatment method with high ZnS/CdS, BiVO /Bi Ti O ,
4 2 2 7
4
/TiO
etc. Although this way improves
mineralization efficiency, no secondary pollution, energy the photocatalytic activities of the photocatalysts, the separa-
2
, BiOI/BiOBr,
15
16
4,5
savings and simple processes. In the past few years, many tion of the photocatalysts remains difficult.
active photocatalysts have been widely studied, and some of
Due to low-cost, high electrical conductivity (2 ꢁ 10 S m ),
4
ꢀ1
ꢀ1
6
them have been successfully industrialized. However, the excellent magnetic properties (58.0 emu g ), high theoretical
ꢀ1
photocatalytic activities of most semiconductor photocatalysts capacity (924 mA h g ), and eco-friendly properties, Fe
O
3 4
7
(TiO
2
, ZnO, CdS, ZnS, etc.) are low, and their practical appli- becomes one of the most promising used magnetic materials in
17
cations are greatly restricted by some defects, such as poor Fe
utilization rate of visible light, low separation rate of photo- Fe
3
O
O
4
, g-Fe
2 3 2 4
O and MFe O . Based on the above advantages,
can act as an electron transporter to inhibit recombina-
3
4
3 4
generated electron–hole pairs, and difficult separation from the tion of photogenerated electron–hole pairs. In addition, Fe O
particles have been modied with photocatalysts to improve
adsorption capacity and enlarge the specic surface area of the
Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi
Province, Taiyuan University of Technology, No. 79 Yingze West Street, Taiyuan
18–20
photocatalysts.
The detailed reasons for choosing Fe O to
3 4
combine with photocatalysts are: (1) enhancement of specic
surface area can increase the active site of the catalyst; (2)
excellent magnetic character facilitates the recycling of the
0
†
1
30024, China. E-mail: renruipeng888@126.com; yongkanglv@163.com
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra04412d
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photocatalysts; (3) the existence of Fe–OH can improve the
adsorptive capacity of the photocatalysts. Many hetero- BiOI, BiOBr/BiOI (3 : 1) and Fe O /BiOBr/BiOI (x : 3 : 1)
Preparation of Fe O /BiOBr/BiOI composites. The BiOBr,
3
4
20
3
4
junction photocatalysts combined with Fe O particles such as composites (x represents the mole ratio of Fe O ) were also
3
4
20
3 4
2
1
Fe O /AgBr/ZnO,
BiVO /Bi Ti O /Fe O ,
g-C N /Fe O / synthesized by solvothermal method according to the published
3 4 3 4
3
4
4
2
2
7
3
4
22
23
25
MnWO
4
,
Ag
3
PO
4
/ZnO/Fe
3
O
4
,
etc. have been prepared to paper. In Fe O /BiOBr/BiOI (x : 3 : 1) composites, the mole
3 4
enhance the recycling ability. To the best of our knowledge, ratios of Fe O were chosen as 0.2, 0.3, 0.4 and 0.5. BiOBr/BIOI
3
4
limited research has been reported about ternary magnetic (3 : 1) and Fe O /BiOBr/BiOI (x : 3 : 1) composites with different
3
4
Fe
under visible light. Gao et al. have prepared Fe
0.5 : 2 : 2) composite by precipitation method to solve the (0.5 : 3 : 1), respectively. The specic dosages of the chemical
3 4
3 4
O /BiOBr/BiOI photocatalysts treating organic wastewater mole ratios of Fe O is abbreviated as Br/I (3 : 1), F/Br/I
24
3 4
O /BiOBr/BiOI (0.2 : 3 : 1), F/Br/I (0.3 : 3 : 1), F/Br/I (0.4 : 3 : 1) and F/Br/I
(
problem that the BiOBr/BiOI (2 : 2) heterojunction photo- reagents are shown in Table 1.
catalyst is difficult to be separated from aqueous phase.
3 4
However, Fe O /BiOBr/BiOI (0.5 : 2 : 2) composite synthesized
Characterization of the samples
by precipitation has the defects of non-uniform particle size and
small specic surface area. Because the products prepared by
solvothermal approach own high crystallinity, uniform particle
size distribution and high yield, solvothermal approach has
attracted more and more attention.
The phase purity and crystal structure of the prepared products
were characterized by X-ray diffraction (XRD) analysis. The
morphology and microstructures of the particles were investi-
gated by scanning electron microscope (SEM, JSM-7100F),
equipped with energy-dispersive X-ray spectroscopy (EDS) to
probe elemental analysis. Inductively couple plasma atomic
emission spectroscopy (ICP-AES, Varian 720) was used to detect
In this study, the ternary magnetic Fe O /BiOBr/BiOI
3
4
(
x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5) photocatalysts were
successfully synthesized by a facile solvothermal method. The
samples are characterized by X-ray diffraction (XRD), scanning
electron microscope (SEM), X-ray photoelectron spectroscopy
the contents of Fe and Bi elements in Fe
0.4 : 3 : 1). X-ray photoelectron spectroscopy (XPS) was used
to analyze the chemical valence and composition of the ternary
magnetic Fe /BiOBr/BiOI (0.4 : 3 : 1) composite. The surface
area and textural properties were determined by N adsorption–
desorption isotherms on a Quantachrome instrument (ASIQC
50200-6, USA). The absorption wavelengths of the samples
3 4
O /BiOBr/BiOI
(
(
XPS), Brunauer–Emmett–Teller (BET), UV-vis diffuse reection
spectroscopy (UV-vis DRS) and photoluminescence analysis
PL). Magnetic properties of magnetic samples were detected by
3 4
O
2
(
vibrating sample magnetometer (VSM). The photocatalytic
activities of the samples were evaluated by degrading RhB. The
0
were detected by UV-vis diffuse reection spectroscopy (UV-vis
DRS) on a Hitachi U-4100, Japan. The fate of photogenerated
carriers were investigated by photoluminescence analysis (PL)
on Elmer uorescence spectrophotometer. The magnetic
properties of magnetic materials were measured by a vibrating
sample magnetometer (VSM, LDJ9600).
3 4
ternary magnetic Fe O /BiOBr/BiOI (0.4 : 3 : 1) photocatalyst
was used in recycling experiments, and the crystallization,
morphology and magnetic properties before and aer recycling
3 4
were detected by XRD, SEM and VSM. Besides, Fe O /BiOBr/
BiOI (0.4 : 3 : 1) photocatalyst was compared the photo-
catalytic performance, adsorption capacity and magnetic prop-
erty with those of previous studies. In the end, the mechanism
of photocatalytic degradation of RhB under Fe O /BiOBr/BiOI Photocatalytic activity measurement
3
4
(
0.4 : 3 : 1) photocatalyst was analyzed in detail.
The photocatalytic activities of the samples were evaluated by
degrading RhB under visible light irradiation. A 500 W Xe lamp
with a cutoff lter (l > 400 nm) was used as light source. The
distance between the liquid surface and the light source is
Experimental
Materials and synthesis of the photocatalysts
1
5 cm. 0.035 g of the prepared catalysts were added to 100 mL
Materials. Ferric(III) chloride hexahydrate (FeCl
Rhodamine B (RhB) were obtained from Aladdin. Sodium
acetate (NaHCO ), potassium bromide (KBr), potassium iodide
KI), ethanol, ethylene glycol (EG) were gotten from Kermel
Chemical Reagent, Tianjin. Bismuth nitrate (Bi(NO ) $5H O)
3
$6H
2
O) and
ꢀ1
RhB solution (20 mg L ). The mixture was mechanically stirred
in the dark for 60 minutes to reach absorption–desorption
equilibrium between catalyst particles and RhB molecules. The
light was then directed at the dispersion at room temperature.
About 5 mL dispersion was token and centrifugally separated
out catalyst powder. A 52 UV-vis spectrophotometer (Spectrum
instruments, Shanghai) was used to analyze the concentrations
of RhB. In the recycling experiment, the Fe O /BiOBr/BiOI
3
(
3
3
2
was purchased from Macklin. All the chemical reagents were
used as received without any further purication.
Synthesis of Fe
prepared by a facile solvothermal method. 1 g (0.005 mol)
FeCl $6H O were dissolved in 80 ml of EG. 3.6 g NaHCO was
added in the stirring process. Then, the mixed solution was
heated to 50 C, and stirred evenly, then transferred to 100 ml
Teon-lined autoclave at 180 C for 12 h. The black solid deposit
was washed for three times with ethanol and water, respectively,
3 4 3 4
O nanoparticles. The Fe O particles were
3
4
(0.4 : 3 : 1) was separated from the dispersion with a magnet
3
2
3
and washed with ethanol and ultrapure water several times
sequentially.
ꢂ
ꢂ
Detection of nal products
and separated by magnetic eld. The black particles were dried RhB molecule contains nitrogen atoms, so the nal products
ꢂ
+
ꢀ
ꢀ
in vacuum at 70 C for 12 h.
(NH , NO or NO ) of nitrogen atoms can be used to measure
4 2
3
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Table 1 The specific dosages of the chemical reagents
3 3 2
Bi(NO ) $5H O
Samples
Fe
3
O
4
(mmol)
(mmol)
KBr (mmol)
KI (mmol)
BiOBr
—
—
—
0.8
1.2
1.6
2.0
16
16
16
16
16
16
16
16
—
12
12
12
12
12
—
16
4
4
4
BiOIa
Br/I (3 : 1)
b
F/Br/I (0.2 : 3 : 1)
F/Br/I (0.3 : 3 : 1)
F/Br/I (0.4 : 3 : 1)
F/Br/I (0.5 : 3 : 1)
4
4
a
b
3 4
BiOBr/BiOI. Fe O /BiOBr/BiOI.
+
the degradation of RhB. Ammonia nitrogen (NH –N), nitrate corresponded to JCPDS card no. 19-0629. As for the BiOBr/BiOI
4
ꢀ
ꢀ
nitrogen (NO3 –N) or nitrite nitrogen (NO2 –N) can be detected (3 : 1) and Fe O /BiOBr/BiOI (x : 3 : 1) composites, all peaks are
3
4
by SP-752 visible spectrophotometer (Spectrum instruments, well indexed to tetragonal phase BiOBr, BiOI and the crystal
+
Shanghai). Ammonia nitrogen (NH
4 3 4
–N) was detected by Ness- Fe O , and no characteristic diffraction peaks of other impuri-
ler's Reagent Spectrometry at 420 nm. Salicylic acid- ties are observed. The results indicate that the prepared binary
concentrated sulfuric acid Spectrophotometry was utilized to BiOBr/BiOI (3 : 1) and ternary magnetic Fe
3
O
4
/BiOBr/BiOI
ꢀ
determine the concentration of nitrate nitrogen (NO
3
–N) at (x : 3 : 1) (x ¼ 0.2, 0.3,0.4 and 0.5) composites are only
4
10 nm. N-(1-Naphthyl)-ethylenediamine Spectrophotometry composed of BiOBr, BiOI and Fe
3
O
4
phases. In addition, the
ꢀ
was used to analyze nitrite nitrogen (NO2 –N) at 540 nm.
intensity of diffraction peaks of BiOBr and BiOI weaken grad-
ually with the increase of Fe O mole ratios in ternary magnetic
3
4
Fe O /BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3,0.4 and 0.5) composites,
3
4
Results and discussion
Characterization results
implying that Fe
on the surface of BiOBr/BiOI (3 : 1) composite.
Morphology and composition characterization. As shown in
Fig. 2a–c, the prepared pure Fe , BiOBr and BiOI exhibit
3 4
O particles have been successfully deposited
26
Crystal structure. Fig. 1 shows the XRD patterns of BiOBr,
3 4
O
BiOI, Fe
3 4 3 4 3 4
O and Fe O /BiOBr/BiOI (x : 3 : 1) with different Fe O
uniform nanospheres of about 600–700 nm, regular nanosheets
of about 100–250 nm and irregular nanosheet structures,
respectively. Compounded BiOBr and BiOI with the mole ratio
of 3 : 1, the binary BiOBr/BiOI (3 : 1) composite shows
a uniform ower-like hierarchical microsphere structure with
(
x) mole ratios. The strong and sharp diffraction peaks impli-
cate that all prepared products are highly crystalline. The all
characteristic peaks of the prepared pure BiOBr and BiOI are
well indexed to the tetragonal phase BiOBr (JCPDS no. 73-2061)
and BiOI (JCPDS no. 10-0445), respectively. Meanwhile, all
diffraction peaks of the prepared Fe O can be clearly
average diameter of about 1600 nm. As for Fe
3 4
O /BiOBr/BiOI
3
4
(x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5) composites, the ower-like
Fig. 2 SEM image of (a) Fe
/BiOBr/BiOI (e) Fe /BiOBr/BiOI (0.2 : 3 : 1), (f) F/Br/I (0.3 : 3 : 1), (g) F/Br/I
(x) molar ratios (0.4 : 3 : 1) and (h) F/Br/I (0.5 : 3 : 1); (i) EDS spectrum of F/Br/I
(0.4 : 3 : 1) (Br/I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/BiOI).
3 4
O , (b) BiOBr, (c) BiOI, (d) BiOBr/BiOI (3 : 1),
Fig. 1 XRD patterns of BiOBr, BiOI, Fe
3
O
4
and Fe
x : 3 : 1) (x ¼ 0.2, 0.3,0.4 and 0.5) with different Fe
Br/I : BiOBr/BiOI; F/Br/I : Fe /BiOBr/BiOI).
3
O
4
3 4
O
(
(
3 4
O
O
3 4
3 4
O
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2
8
hierarchical microsphere structure (Fig. 2e–h) has not changed catalyst. It can be seen from Fig. 3e, the I 3d shows two
along with the adding of different mole ratios of Fe O . As seen strong peaks at binding energies of 619.6 and 631.2 eV,
3
4
from the enlarged image of Fig. 2g and h, with the increase of which are matched with I 3d5/2 and I 3d3/2, respectively. The
mole ratios of Fe O , it is obvious that Fe O particles adhere to result conrms the ꢀ1 valence of I in the ternary magnetic
3
4
3 4
the surface of the binary BiOBr/BiOI (3 : 1) composite, which is composite. In addition, the Fe 2p binding energies of 711.3
consistent with the result of XRD analysis.
The chemical composition of the ternary magnetic Fe
BiOBr/BiOI (0.4 : 3 : 1) composite was carried out by SEM-EDS Through the XPS analysis, Fe
and 725.0 eV are attributed to Fe 2p3/2 and Fe 2p1/2, con-
3
+
2+
3
O
4
/
rming the existence of Fe
and Fe
, BiOBr and BiOI coexist in
4
O /BiOBr/BiOI (0.4 : 3 : 1)
in the sample.
3 4
O
elemental mapping when the SEM in scanning mode was the ternary magnetic Fe
3
operated. The EDS spectrum (Fig. 2i and S1†) shows strong composite, which corresponds to the results of XRD and
signals of Bi, O, Br, I and Fe elements without detecting any SEM analysis.
other elements, which demonstrates that the ternary magnetic
BET surface area analysis. The results of surface area and
Fe /BiOBr/BiOI (0.4 : 3 : 1) composite is composed of Bi, O, textural properties are shown in Fig. 4 and Table 2. As shown in
3 4
O
Br, I and Fe elements. What is more, the quantitative result Fig. 4, the isotherms of all as-prepared samples correspond to
presents the atomic of Bi : O : Br : I : Fe in the ternary magnetic typical IV-type curves with a distinct H3 hysteresis loop (IUPAC
29
Fe
which is in line with theoretical chemometrics ratio of result of average pore diameters (Table 2) conrms this statement.
0 : 28 : 15 : 5 : 6 for Bi : O : Br : I : Fe in Fe O /BiOBr/BiOI The pure BiOBr and BiOI display relatively small specic surface
3 4
O /BiOBr/BiOI (0.4 : 3 : 1) composite is 19 : 33 : 15 : 48 : 57, classication), indicating the presence of 2–50 nm mesopores. The
2
3
4
2
ꢀ1
(0.4 : 3 : 1) in view of instrumental error, implying that the area of 14.41 and 14.03 m g , and relatively large average pore
mole ratio of Fe O , BiOBr and BiOI in the ternary magnetic diameters of 52.76 and 51.61 nm. In addition, the specic surface
3
4
2
ꢀ1
Fe
3
O
4
/BiOBr/BiOI (0.4 : 3 : 1) composite is 0.4 : 3 : 1.
ICP-AES analysis (Table S1†) shows that the mass content and average pore diameter are relatively small. Aer compounded
the mass of elements in catalyst per kilogram) of Fe and Bi are BiOBr with BiOI, the specic surface area of the formed BiOBr/BiOI
3 4
area of Fe O is largest with 96.91 m g , while the pore volume
(
6
ꢀ
1
2
ꢀ1
71 821.2 and 49 761.7 mg kg in Fe
3
O
4
/BiOBr/BiOI (0.4 : 3 : 1) (3 : 1) increases signicantly to 44.21 m g , and the average pore
and
to be 2.77 : 10, which is basically consistent with stoichiometric BiOBr/BiOI (3 : 1) are compounded to form ternary magnetic Fe O /
3 4
composite, respectively. The molar ratio of the two is calculated diameter decreases obviously to 21.90 nm. And when Fe O
3
4
ratio of 3 : 10.
BiOBr/BiOI (x : 3 : 1) composite, and the specic surface area of
XPS analysis. The typical XPS survey spectrum (Fig. 3a) Fe O /BiOBr/BiOI (x : 3 : 1) increases with the increase of the mole
3
4
shows Bi, O, Br, I, Fe and C elements are detected, proving ratios of Fe
once again that the ternary magnetic Fe /BiOBr/BiOI accordingly.
0.4 : 3 : 1) composite is composed of Bi, O, Br, I and Fe UV-vis DRS analysis. The absorption wavelengths of the
ve elements. While C1s at 284.6 eV as the reference for the photocatalysts can be used to measure the absorptivity to light.
3 4
O , and the pore volume and pore diameter decrease
3
O
4
(

binding energies of other elements is the adventitious The UV-vis diffuse reectance spectra of BiOBr, BiOI and Fe
3 4
O /
carbon. As seen in Fig. 3b, two peaks with binding energies BiOBr/BiOI (x : 3 : 1) with different Fe O (x) mole ratios are
3
4
at 68.8 and 69.2 eV are Br 3d and Br 3d3/2 peaks, respec- shown in Fig. 5. The absorption wavelengths of all samples are
5
/2
2
7
tively. From the spectra shown in Fig. 3c, the peaks for Bi 4f in Fig. 5. The absorption wavelengths of all samples are in the
are observed, and contain Bi 4f7/2 (Bi(III)) and Bi 4f5/2 (Bi(V))
2
4
peaks. As for the O 1s, two different oxygen species are
presented, the strong peak located at 530.2 eV corresponds
to lattice oxygen in Fe O /BiOBr/BiOI (0.4 : 3 : 1), and the
3 4
binding energy situated at 531.5 eV is attributed to the
hydroxyl groups attached to the surface of the magnetic
Fig. 4
N
2
adsorption–desorption tests of BiOBr, BiOI, Fe
/BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5) with different
(x) molar ratios (Br/I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/
3 4
O and
Fe
/BiOBr/BiOI (0.4 : 3 : 1) sample: (a) survey Fe
of the photocatalyst; (b) Br 3d; (c) Bi 4f; (d) O 1s; (e) Fe 2p; (f) I 3d.
3
O
O
4
Fig. 3 XPS spectra of Fe
3
O
4
3
4
3 4
O
BiOI).
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Table 2 Characteristics of the prepared samples by N
desorption tests
2
adsorption–
A (nm)
2
ꢀ1
3
ꢀ1
Samples
S (m g
)
P (cm g
)
BiOBr
14.41
14.03
44.21
44.82
46.67
48.30
52.59
96.91
0.2152
0.1810
0.2421
0.2251
0.2184
0.2193
0.1825
0.0888
52.76
51.61
21.90
17.29
19.58
16.18
16.16
18.32
BiOIa
Br/I (3 : 1)
b
F/Br/I (0.2 : 3 : 1)
F/Br/I (0.3 : 3 : 1)
F/Br/I (0.4 : 3 : 1)
F/Br/I (0.5 : 3 : 1)
Fe
3 4
O
a
b
3 4
BiOBr/BiOI. Fe O /BiOBr/BiOI; S: specic surface area; P: pore
volume; A: average pore diameter.
Fig. 6 Photoluminescence (PL) spectra of BiOBr, BiOI, Fe
3
O
4
and
/BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5) with different
(x) molar ratios. (Br/I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/
Fe
visible region. The band edge of BiOBr/BiOI (3 : 1) lies between Fe
3
O
O
4
3
4
3 4
O
BiOI).
the pure BiOBr (425 nm) and BiOI (670 nm). In comparison with
BiOBr/BiOI (3 : 1) composite, the absorption wavelengths of the
ternary magnetic Fe
shi, indicating that the introduction of Fe
absorptivity of the photocatalysts to visible light.
PL analysis. The PL analysis results are shown in Fig. 6. The present good ferromagnetic behavior. It can be clearly realized
intensity of the diffraction peak can be applied to estimate the that the magnetism of Fe /BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3,
recombination probability of photogenerated electron–hole 0.4 and 0.5) increases with the increase of Fe mole ratio
pairs. The stronger the peak is, the greater the recombination (Fig. 7a (inset)). The saturation magnetization (MS) of pure
probability is. Based on this principle, it can be seen from Fig. 6, Fe and Fe /BiOBr/BiOI (x : 3 : 1) composite (x ¼ 0.2, 0.3,
the emission peak of the pure BiOBr is the strongest, indicating 0.4 and 0.5) are 58.28, 2.64, 3.89, 5.10 and 6.27 emu g
3
O
4
/BiOBr/BiOI (x : 3 : 1) exhibit slightly red
can enhance the samples (Fe
Fe (x) mole ratios) are shown in Fig. 7. All magnetic samples
Magnetic analysis. The magnetic properties of the magnetic
, and Fe /BiOBr/BiOI (x : 3 : 1) with different
3
O
4
3
O
4
O
3 4
O
3 4
33
3 4
O
3 4
O
3
O
4
3 4
O
ꢀ1
,
that the photogenerated electron–hole pairs, recombination respectively. Seen from Fig. 7b, the color of the pure BiOBr and
rate is the largest, and that of BiOBr/BiOI (3 : 1) is second, BiOI are greyish-white and red-brown, and BiOBr and BiOI
30,31
between BiOBr and BiOI.
pure Fe O is the weakest, predicting the smallest recombina- catalyst. The color of Fe
While the emission peak of the compound with a mole ratio of 3 : 1 to form bright yellow
is black, when Fe combines with
tion rate of photogenerated electron–hole pairs. The result BiOBr/BiOI (3 : 1), the color of the ternary magnetic Fe
illustrates that Fe O owns strong electronic transmission BiOBr/BiOI (x : 3 : 1) composites gradually shi from light
3
4
3
O
4
3 4
O
3 4
O /
3
4
32
capacity. As can be seen from Fig. 6, the peak strength of BiOI brown to reddish brown. The Fig. 7b also shows the separation
is second only to that of Fe . Loaded Fe
on BiOBr/BiOI ability of magnetic samples under an external magnetic eld.
3 : 1) composite, the photogenerated electron–hole pairs, Powder samples dispersed in water present turbidity. Aer 25 s
3
O
4
3 4
O
(
3 4
recombination rate of Fe O
/BiOBr/BiOI (x : 3 : 1) decreases of applied magnetic eld, the turbid suspensions become clear
signicantly, which indirectly indicates that the utilization rate to varying degrees. The suspensions of the ternary magnetic
of photogenerated carriers is improved.
Fe
stay a little cloudy, and those of Fe /BiOBr/BiOI (0.4 : 3 : 1),
3 4
Fe /BiOBr/BiOI (0.5 : 3 : 1) and Fe O become very clear. This
3 4 3 4
O /BiOBr/BiOI (0.2 : 3 : 1) and Fe O /BiOBr/BiOI (0.3 : 3 : 1)
3
O
4
3
O
4
phenomenon illustrates that when the mole ratio of Fe
3 4
O is
greater than 0.3, the synthesized ternary magnetic Fe /BiOBr/
3 4
O
BiOI (x : 3 : 1) composites have good separation ability in the
aqueous solution under an external magnetic eld.
Fig. 5 UV-vis diffuse reflectance spectra of BiOBr, BiOI and Fe
O
3 4
/
Fig. 7 (a) VSM curves of the magnetic samples; (b) pictures of the
BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5) with different Fe
3
O
4
(x) samples and magnetic separation performance of the magnetic
samples (Br/I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/BiOI).
molar ratios. (Br/I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/BiOI).
O
3 4
3 4
O
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Table 3 The adsorption capacity of RhB under the prepared samples
Photocatalytic performance
Photocatalytic degradability. Fig. 8a presents the photo-
degradation of RhB with illumination time over different pho-
tocatalysts under visible light. And the kinetic curves transform
and rst-order kinetic constants (K) of RhB degradation are
shown in Fig. 8b and c. When the adsorption–desorption
equilibrium reaches, the adsorption capacity of RhB by the
ꢀ1
ꢀ1
Samples
Fe
q
e
(mg g
)
Samples
e
q (mg g )
b
3
O
4
0.87
1.86
F/Br/I (0.2 : 3 : 1)
35.45
38.72
39.31
37.14
BiOBr
F/Br/I (0.3 : 3 : 1)
F/Br/I (0.4 : 3 : 1)
F/Br/I (0.5 : 3 : 1)
BiOIa
12.79
35.42
Br/I (3 : 1)
a
b
ꢀ
1
3 4
BiOBr/BiOI. Fe O /BiOBr/BiOI.
samples can be calculated using q (mg g ).
e
q
e
¼ (C
0
ꢀ C) V/m
(1)
3 4
of RhB under the pure Fe O can be also almost negligible. In
where C
0
and C are the initial and equilibrium concentrations the case of pure BiOBr and BiOI, RhB is obviously degraded.
of RhB in the RhB solution (ppm), V is the solution volume (L), And the photodegradation rates of BiOBr and BiOI are 20.8%
and m is the mass of the catalysts (g). The adsorption capacity of and 29.4%, respectively. While the corresponding K values are
ꢀ1
RhB by the as-prepared samples within 60 min is presented in only 0.00252 and 0.00134 min . The low degradation rate of
the Table 3. The adsorption rates of BiOBr and BiOI to RhB are BiOI seems to be contrary to PL results, which may be due to the
1
.5 and 18.1%, and the corresponding adsorption capacities of low photogenerated electron–holes produced by BiOI itself.
ꢀ
1
them are 1.86 and 12.79 mg g . Compared to the pure BiOBr This has a lot to do with the small specic surface area and the
and BiOI, the adsorption capacity of BiOBr/BiOI (3 : 1) increases agglomerated morphology. It is worth mentioning that the
ꢀ1
greatly to 35.42 mg g . It is obvious that the adsorption photocatalytic activity of the BiOBr/BiOI (3 : 1) composite is
capacities of Fe
/BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and dramatically enhanced when the BiOBr and BiOI combine with
.5) is stronger than that of BiOBr/BiOI (3 : 1), and increases the mole ratio of 3 : 1, and the degradation rate and K of RhB
3 4
O
0
ꢀ1
with the increase of Fe O when the mole ratio of Fe O is reach to 99.8% and 0.0712 min , severally, which is attributed
3
4
3 4
smaller than 0.5. The reason for this phenomenon may be that to its uniform ower-like hierarchical microsphere structure,
the load of Fe increases the specic surface area of the larger specic surface area and smaller electron–holes recom-
photocatalyst and forms Fe–OH on the surface of Fe /BiOBr/ bination rate. Aer combined Fe O with BiOBr/BiOI (3 : 1), the
3 4
O
3 4
O
3
4
20
BiOI (x : 3 : 1) composite, but the adsorption capacity of Fe
itself is very small, too much addition will cover the adsorption 0.3, 0.4 and 0.5) decrease slightly, and the degradation rates of
3
O
4
photocatalytic activities of Fe O /BiOBr/BiOI (x : 3 : 1) (x ¼ 0.2,
3
4
site of the photocatalyst. The maximum adsorption capacity of RhB are 97.1, 99.3, 99.2 and 98.9%, and K values are 0.0376,
ꢀ1
ꢀ1
Fe O /BiOBr/BiOI (0.4 : 3 : 1) is 39.31 mg g
.
0.0518, 0.0507 and 0.0478 min , severally. This result seems to
3
4
Irradiation in visible light for 80 minutes, in the absence of be contrary to the wide absorption band edge and the low
any catalyst, RhB is difficult to decompose and exhibits excel- photogenerated carrier recombination rate of Fe O /BiOBr/BiOI
3
4
lent chemical stability, which indicates that the direct photol- (x : 3 : 1) (x ¼ 0.2, 0.3, 0.4 and 0.5). The reason for this result is
ysis is neglectable. In addition, the photodegradation efficiency that the pure Fe O has no photocatalytic activity on RhB, and
3
4
3 4
the presence of Fe O covers part of the active sites. To sum up,
the inhibitory effect of Fe O on photocatalytic degradation is
3
4
stronger than the promoting effect. Data analysis shows that the
load of Fe O improves the adsorption capacity of the photo-
3
4
catalyst, but slightly inhibits the photocatalytic activity.
The color change of RhB solution with irradiation time (min)
over Fe O /BiOBr/BiOI (0.4 : 3 : 1) under visible light is shown
3 4
in Fig. 8d. It is evident that the color of RhB solution becomes
lighter with the increase of illumination time, until becomes
colorless. The colorless RhB solution was detected to conrm
+
ꢀ
ꢀ
the generation of NH –N, NO –N or NO3 –N by SP-752 visible
4
2
spectrophotometer. The test result exhibits the existence of
+
ꢀ
ꢀ
NH
4
and NO
3
, and no NO
2
was detected. It is noteworthy
+
ꢀ
3
–N are
that the detected concentrations of NH –N and NO
.74 mg L and 0.46 mg L , respectively. The total concen-
tration of N atoms in NH
4
ꢀ1
ꢀ1
0
+
ꢀ
ꢀ1
4
and NO
3
is 1.20 mg L . The
concentration value is basically the same as the concentration
ꢀ
1
ꢀ1
of N atoms (1.26 mg L ) in RhB solution (20 mg L ), inferring
that nitrogen atoms of RhB in the solution is almost completely
degraded.
Fig. 8 (a) Photodegradation of RhB over different samples under
visible light; (b) the kinetic curves transform of RhB degradation over
different samples; (c) first-order kinetic constants (K) of RhB degra-
dation over different samples; (d) the colour change of RhB solution
3 4
Reusability of the Fe O /BiOBr/BiOI (0.4 : 3 : 1) composite.
3 4
with irradiation time (min) over Fe O
/BiOBr/BiOI (0.4 : 3 : 1) (Br/ The reusability of the catalyst is also an important index to
I : BiOBr/BiOI (3 : 1); F/Br/I : Fe /BiOBr/BiOI).
3 4
O
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measure the performance of the catalyst. As shown in Fig. 9a, where c, h, l
0
are the velocity of light, Planck constant and the
aer eight recycles of photocatalytic degradation of RhB over maximum absorption wavelength, respectively. The E of BiOBr
g
the ternary magnetic Fe O /BiOBr/BiOI (0.4 : 3 : 1) composite, and BiOI are calculated to be 2.92 and 1.85 eV, which is
3
4
27,37
the degradation rate decreases slightly, from 99.2% to 82.3%. consistent with the literature report.
The magnetic change of the Fe /BiOBr/BiOI (0.4 : 3 : 1) cata- edge position and the conduction band (CB) edge position of
lyst before and aer recycling is presented in Fig. 9a (inset), the BiOBr and BiOI can be calculated by Mulliken electronegativity
The valence band (VB)
3 4
O
ꢀ1
saturation magnetization decreases from 5.10 to 4.54 emu g
,
theory via formulas (3) and (4):
the decrease is only 11% before and aer recycling. The changes
of crystalline phase and morphology of the catalysts before and
aer reaction were observed by XRD and SEM. Seen from
Fig. 9b, the Fe O /BiOBr/BiOI (0.4 : 3 : 1) catalyst maintains
ECB ¼ X ꢀ E ꢀ 0.5E
(3)
(4)
c
g
E
VB ¼ E
g
+ ECB
3
4
good crystallinity before and aer reaction, and its morphology
do not change signicantly. The above analysis results illustrate
that the ternary magnetic Fe O /BiOBr/BiOI (0.4 : 3 : 1)
3 4
c
here E is the energy of free electrons on the hydrogen scale,
about 4.5 eV. And X is the absolute electronegativity of the
samples. The X values of BiOBr and BiOI are 6.21 and 6.45 eV.
According to the formulas (3) and (4), the ECB and EVB of BiOI
are 0.78 and 2.63 eV, respectively, and those of BiOBr are 0.49
and 3.42 eV. The band edge potentials of the VB and CB of
BiOBr and BiOI are presented in Fig. 10a.
composite has good reusability and stability.
Comparation with previous work. The quality of the catalyst
can be reected when compared with other catalysts. In our
previous work, the binary magnetic Fe O /BiOBr (0.5 : 1) pho-
3
4
25
34–36
tocatalyst with better performance than other literatures
Under light illumination, since the energy of photons is
greater than the difference between LUMO and HOMO of dyes,
24
has been developed. Compared with literature (Table 4),
although the saturation magnetization of Fe /BiOBr/BiOI
0.4 : 3 : 1) is smaller than that of Fe /BiOBr (0.5 : 1), the
specic surface area, adsorption capacity, degradation activity
3 4
and K value are larger than those of binary magnetic Fe O /
3 4
O
ꢀ
the transition of electrons (e ) in dyes molecules occurs to form
(
3 4
O
+
dyes which can continue to decompose into small molecules.
+
However, the electrons and dyes are very easy to recombine to
form dye molecules, which is the reason that the RhB is almost
not degraded in the absence of photocatalyst. In the presence of
photocatalysts, the situation will be greatly changed.
In RhB aqueous solution with Fe O /BiOBr/BiOI (0.4 : 3 : 1)
3 4
photocatalyst under visible light illumination (l > 400 nm,
24
BiOBr (0.5 : 1) photocatalyst. Previous study has also synthe-
sized ternary magnetic Fe O /BiOBr/BiOI (0.5 : 2 : 2) composite
3
4
by precipitation method. As presented in Table 4, the saturation
magnetization of Fe O /BiOBr/BiOI (0.4 : 3 : 1) is slightly
3
4
smaller than that of Fe
3
O
4
/BiOBr/BiOI (0.5 : 2 : 2), but Fe
3
O
4
/
energy less than 3.10 eV), the energy of photons is larger than
BiOBr/BiOI (0.4 : 3 : 1) can also be well separated from water
ꢀ
the E of BiOI (1.85 eV) and BiOBr (2.92 eV), the e on the VB of
g
under magnetic eld within 25 s. Besides, the specic surface
BiOI (2.63 eV) and BiOBr (3.42 eV) transmits to the CB of BiOI
2
ꢀ1
area of Fe
than that of Fe
specic surface area of this paper is 3 times that of literature,
3 4
O /BiOBr/BiOI (0.5 : 2 : 2) (28.21 m g ) is much less
(
0.78 eV) and BiOBr (0.49 eV), so this will produce photoelec-
2
ꢀ1
3 4
O /BiOBr/BiOI (0.4 : 3 : 1) (48.30 m g ). The
ꢀ
+
tron–hole pairs (e –h ) (reaction (5)). As shown in Fig. 10a, the
CB edge potentials of BiOI (0.78 eV) and BiOBr (0.49 eV) seem to
be detrimental to the transmission of electrons. In fact, it can be
seen from the Fig. 10b, the electrons on the VB of BiOI (2.63 eV)
and BiOBr (3.42 eV) can be stimulated to the more negative
potentials (BiOI of ꢀ0.47 eV, and BiOBr of 0.32 eV). At this
point, the CB of BiOBr (0.32 eV) is more positive potential than
that of BiOI (ꢀ0.47 eV), which makes it possible for electrons to
transfer from the CB of BiOI to the CB of BiOBr. Absorbed
photons (energy less than 3.10 eV) with greater energy than the
difference between LUMO (ꢀ1.42 eV) and HOMO (0.95 eV) of
24
which may indicate higher adsorption capacity and degradation
activity of Fe O /BiOBr/BiOI (0.4 : 3 : 1).
3
4
Mechanism of photocatalytic activity
The band gap energy (E ) of the semiconductor can be evaluated
g
by the following calculation formula:
E
g
¼ hc/l
0
¼ 1240/l
0
(2)
+
RhB, the electrons of RhB are excited to produce RhB (reaction
(6)), and actively migrate to the CB position of BiOI due to the
more negative potential of LUMO (ꢀ1.42 eV) than the CB
+
potential of BiOI (ꢀ0.47 eV). And RhB can decompose into
ꢀ
+
small molecules or CO , H O, NO and NH (reaction (7)). At
2 2 3 4
+
the same time, the h on the VB of BiOBr automatically transfers
to the VB of BiOI because of the more positive VB potential of
BiOBr (3.42 eV) than that of BiOI (2.63 eV). Through the above
principle analysis, the ow of electron–hole pairs establishes an
external circuit to form a virtuous cycle, which promotes the
continuation of photocatalytic process.
Fig. 9 (a) Recycling utilization experiments and magnetism of Fe
BiOBr/BiOI (0.4 : 3 : 1) composite before and after recycling (inset); (b)
3 4
O /
Next comes the production of active free radicals. The holes
on the VB of BiOI will react with H O molecules to emerge $OH
2
XRD patterns and SEM images (inset) of Fe
composite before and after recycling.
3 4
O /BiOBr/BiOI (0.4 : 3 : 1)
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25
24
a
3 4
Table 4 Compare this paper with Fe O /BiOBr (0.5 : 1) in our previous work and literature
M
(emu g
ꢀ
1
2
ꢀ1
ꢀ1
ꢀ1
Magnetic photocatalyst
)
S (m g
)
q
e
(mg g
)
D (%)
K (min
)
2
2
5
4
17.3
7.2
5.1
34.11
28.21
48.30
14.53
—
39.31
78.4
—
99.2
0.035
—
0.0507
This paper
a
M: magnetization; S: specic surface area; D: degradation activity.
+
ꢀ
RhB and RhB + $OH, $O and H O
2 2
(
reaction (8)), which is due to the more positive potential VB of
2
ꢀ
ꢀ
38
ꢀ
+
BiOI than the redox potential of OH /$OH (1.99 eV). While e
can be delivered by the Fe O particles with strong electron
/ CO + H O + NO + NH4
(13)
2
2
3
3
4
transfer ability (reaction (9)). Because the CB potential of BiOI
ꢀ
(
ꢀ0.47 eV) is more negative than the potential of O
2 2
/$O
39
(ꢀ0.046 eV), the electrons transferred by Fe
3 4
O particles
(reaction (10)), and
ꢀ
Conclusions
combine with O
2
molecules to form $O
2
subsequently produce H
2
O
2
ꢀ
(O
2
/H
2
O
2
, 0.682 eV) (reaction (11)),
The ternary magnetic Fe
were synthesized by a facile solvothermal method. The cha`r-
acterization results reveal that all the Fe /BiOBr/BiOI
x : 3 : 1) samples possess good visible light response and
3 4
O /BiOBr/BiOI (x : 3 : 1) composites
ꢀ
which can reaction with e to emerge $OH and OH (reaction
4
0
ꢀ
(
12)). $OH, $O2 and H O are oxidants, which can oxidize RhB
2 2
3 4
O
+
molecules and RhB into small molecules, until RhB molecules
and RhB are oxidized into CO , H O, NH and NO3 (reaction
(
+
+
ꢀ
ꢀ1
2
2
4
large specic surface area. The RhB (20 mg L ) solution can be
(13)). The possible photocatalytic reactions of the ternary
degraded to colorless within 80 min over Fe O /BiOBr/BiOI
3
4
magnetic Fe
follows:
3 4
O /BiOBr/BiOI (0.4 : 3 : 1) photocatalyst are as
3 4
(0.4 : 3 : 1) photocatalyst. Fe O /BiOBr/BiOI (0.4 : 3 : 1) can be
rapidly separated from water within 25 s under external
magnetic eld. Aer 8 times recycles, the degradation rate
decreases slightly, from 99.2% to 82.3%, the saturation
magnetization decreases only 11%, and the crystallinity and
morphology have not changed. Compared with binary magnetic
+
ꢀ
BiOBr/BiOI + hn / BiOBr/BiOI (h + e )
(5)
(6)
(7)
+
ꢀ
RhB + hn / RhB + e
+
3ꢀ
+
4
Fe O /BiOBr (0.5 : 1) prepared by the same method in our
RhB / intermediates + CO
2
+ H
2
O + NO +NH
3 4
previous work and the previous Fe O /BiOBr/BiOI (0.5 : 2 : 2)
3
4
+
+
H O + BiOBr/BiOI (h ) / BiOBr/BiOI + H + $OH
2
(8) synthesized by co-precipitation method, the photocatalytic
activity and adsorption capacity of Fe /BiOBr/BiOI (0.4 : 3 : 1)
(e ) (9)
3 4
O
ꢀ
ꢀ
BiOBr/BiOI (e ) + Fe
3
O
4
/ BiOBr/BiOI + Fe
3
O
4
are superior to them. All the results show the high photo-
catalytic degradation efficiency, good magnetic separation
performance and excellent recyclability and stability of ternary
ꢀ
ꢀ
Fe O (e ) + O / Fe O + $O
(10)
(11)
(12)
3
4
2
3
4
2
magnetic Fe O /BiOBr/BiOI (0.4 : 3 : 1) photocatalyst. Photo-
2ꢀ
+
3 4
2
$O + 2H / O
2
+ H
2
O
2
catalytic mechanism analysis presents that RhB is degraded by
ꢀ
ꢀ
ꢀ
oxidation of $OH, $O2 and H O .
2
2
H
2
O
2
+ Fe
3
O
4
(e ) / $OH + OH + Fe
3
O
4
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work has been supported by the National Natural Science
Foundation of China (21776196, 51778397 and 21707098), the
National
Key
Research
and
Development
Project
(2016YFB0600502) and Innovative Projects for Post-graduate
Education of Shanxi Province (No. 2018BY046).
References
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ꢀ2
optical power with 250 mW cm ).
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23552 | RSC Adv., 2019, 9, 23545–23553
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Paper
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