Novel magnetically separable Fe3O4-WSe2/NG photocatalysts: Synthesis and photocatalytic performance under visible-light irradiation

Article abstract of DOI:10.1039/c8nj00406d

Visible light responsive Fe₃O₄-WSe₂/NG (nitrogen doped graphene oxide) heterojunction nanocomposites were synthesized by a hydrothermal synthesis route, in which Fe₃O₄ and WSe₂ particles were coated on the surface of NG. The samples were characterized by XRD, SEM, TEM, XPS, VSM, UV-vis DRS, TOC, and GC-MS techniques. The results showed that the photocatalytic activity of Fe₃O₄-WSe₂/NG photocatalysts under visible-light illumination was higher than that of WSe₂/NG owing to the narrow band gap energy and delayed electron-hole recombination, and Fe₃O₄ can be used in advanced oxidation processes (AOPs) to degrade organic pollutants at pH 3 and 3.0 mM H₂O₂. Moreover, the mass fraction of Fe₃O₄ had a great effect on the photocatalytic activity of Fe₃O₄-WSe₂/NG. Among the hybrid photocatalysts, 30 wt%Fe₃O₄-WSe₂/NG (30 wt% of Fe₃O₄) exhibited the highest photocatalytic activity with a rate constant of 0.05589 min^-1 for the degradation of MB in aqueous solution under visible light irradiation, and the removal efficiency reached 98.8% which was 1.6 times higher than that of WSe₂/NG. More importantly, the magnetic properties of 30 wt%Fe₃O₄-WSe₂/NG were analyzed and its reusability was also investigated after four successive runs by applying an external magnetic field.

Full text of DOI:10.1039/c8nj00406d

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Novel Magnetically Separable Fe O -WSe /NG Photocatalyst:
3
4
2
Synthesis and Photocatalytic Performance under Visible-light
Irradiation
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Baihong An, Yanan Liu , Chengcheng Xu , Han Wang , Jun Wan *
1 College of Environment and Safety Engineering, Key Laboratory of Ecoꢀchemical Engineering,
Ministry of Education, Qingdao University of Science and Technology, Qingdao 266042, China
2. Center of Chemical Examination of Qingdao Customs District of
PRC, Qingdao, 266000, China
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Abstract: Visible light responsive Fe O ꢀWSe /NG (Nitrogen doped graphene oxide)
3 4 2
heterojunction nanocomposites were synthesized by a hydrothermal synthesis route, in which
Fe and WSe particles were coated on the surface of NG. The samples were characterized by
O
3 4
2
XRD, SEM, TEM, XPS, VSM, UV–vis DRS, TOC, GCꢀMS techniques. The results showed that
the photocatalytic activity of Fe ꢀWSe /NG photocatalysts under visibleꢀlight illumination was
higher than that of WSe /NG owing to the narrow band gap energy and delayed electronꢀhole
3
O
4
2
2
3 4
recombination, and Fe O can be used for the advanced oxidation processes (AOPs) to degrade
2 2 3 4
organic pollutants at pH 3 and H O 3.0mM. Moreover, the mass fraction of Fe O had a great
3 4 2
effect on the photocatalytic activity of Fe O ꢀWSe /NG. Among the hybrid photocatalysts, 30
1
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1
3 4 2 3 4
wt%Fe O ꢀWSe /NG (30 wt% of Fe O ) exhibited the highest photocatalytic activity with a rate
ꢀ1
constant of 0.05589 min for the degradation of the MB in aqueous solution under visible light
irradiation, and the removal efficiency reached 98.8% which was 1.6 times higher than that of
2 3 4 2
WSe /NG. More importantly, the magnetic properties of 30 wt%Fe O ꢀWSe /NG were measured
and its reusability was also investigated after four successive runs applying an external magnetic
field.
Keywords: Fe O ; WSe ; graphene oxide; hydrothermal synthesis; photocatalytic degradation;
3
4
2
methylene blue
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1. Introduction
Common organic dye reagents have been widely used in printing and dyeing industries, and
the wastewater is often referred to as environmental pollutants. Their compositions are complex
and nonꢀbiodegradable, so it is necessary to find an effective way for treating dye wastewater. The
photocatalytic technology has gained greater attention ever since discovered of Water
decomposing under UV irradiation on the Nꢀtype semiconductor TiO electrodes [1].
2
Recently, many semiconductors have been used to degrade organic contaminants, e.g. CdS
3 4 2
[2], Ag PO [3], ZnO [4] and TiO [5,6], etc. Among these semiconductors, the twoꢀdimensional
2
transition metal dichalcogenide (TMDC) semiconductor WSe has attracted great attention [7,8].
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2
Due to its electronic properties [9] and smaller bandgap of 1.35 eV [10]. But the photocatalytic
activity of pure WSe is limited by its low efficiency of light absorption, low adsorption capacity
2
and high recombination rate of photoelectron hole pairs [11,12]. In order to promote the separation
of photoelectric charges of WSe
2
, heterojunction is usually used to change the photocatalysts
[13ꢀ16].
More recently, nitrogen doped graphene oxide (NG), a metalꢀfree, stable, large surface area
and inexpensive polymer semiconductor, is attracting more and more attention [17ꢀ19]. What’s
more, the modest bandgap energy of NG enables direct use of visible light. Nitrogenꢀdoped
graphene oxide is synthesized by hydrothermal method from graphene oxide and urea, the
graphite type N in the nitrogen doped graphene oxide (NG) can increase the conductivity of GO
and improve the adsorption capacity of the dye molecules [20]. In addition, when NG modified
other materials to form heterojunctions, in the photocatalytic process, the transferred photoelectric
energy can participate fast in the reduction reaction and promote the separation of photoelectric
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charges, thus significantly improve the photocatalytic performance of NG modified photocatalysts.
Recently, the improved photocatalytic properties of NG modified conventional semiconductor
3 4 2 2 3 3 4 4 3 4
have been studied, such as gꢀC N /Bi O CO [21], gꢀC N /BiIO [22], ZnO/gꢀC N [23] and
3 4 2 6
gꢀC N /Bi MoO [24]. Although photocatalysts compound can improve photocatalytic properties,
these improvements do not solve the issues of photocatalysts for separation and recovery. In
previous works, WSe /NG photocatalyst was generally suspended in water for photocatalytic
2
reaction and was difficult to be separated, because the light and fine particles were difficult to
stabilize.
Magnetic separation is a very convenient and effective method for removing and recovering a
1
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2
2
3 4
magnetic catalyst by applying an external magnetic field. Fe O has high superꢀmagnetism for
effective recycling by magnet that aroused our attention [25ꢀ29]. He et al [30] used Ti (SO ) Ti as
4
2
2 3 4
the initial raw material, prepared the coreꢀshell structure TiO @Fe O catalysts by precipitation
method, they had a high photocatalytic activity and easy recovery, but the The synthesis process
2 2 3 4
was cumbersome. Liu et al [31] composited TiO /SiO @Fe O magnetic microspheres and used
them as photocatalytic degraded dyestuff. In this study, We have synthesized Fe
a simple hydrothermal method and the difficult problem of recovery of photocatalyst would be
solved. What’s more, Fe can also be used for AOPs, the characteristic is to produce high
3 4 2
O ꢀWSe /NG by
3 4
O
activity hydroxyl radical (HO·), can unselectively degrade, have been successfully used in the
field of Many kinds of organic wastewater treatment [32ꢀ34]
3 4 2
In this artical, preparation of a high quality Fe O ꢀWSe /NG nanocomposites was used by
hydrothermal synthesis. Take advantage of the excellent performance and complementarity of the
three ingredient. Compared with WSe
2
/NG, in the presence of pH 3 and H
2
O
2
3.0mM,
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Fe O ꢀWSe /NG nanocomposites exhibited great enhancement in photocatalytic degradation of
3 4 2
3 4 2
methylene blue (MB) under visible light irradiation. Fe O ꢀWSe /NG as photocatalyst can
improve the photocatalytic efficiency effectively and the degradation of MB (32 mg/L) was
98.8%.
2. Experimental
2.1 Reagents and instruments
4 2 2
KMnO was purchased from Sanhe Chemical Reagent Co. H O was brought from Tianjin
4 2 2 4 2 3 3 2 2
DingShengXin Chemical Co. LTD. FeSO ·7H O, Na WO ·2H O, NaNO , NH , CO(NH ) and
2 4 2 3 2
N H ·H O were purchased from Tianjin BASF Chemical Co. LTD. FeCl ·6H O was purchased
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from Tianjin Damao Chemical Reagent Factory. Se powder, Pꢀ25 and MB were obtained from
Aladdin Reagent Co. NaOH, HNO (65 wt%)，H SO (98 wt%) and HCl (36~38wt%) were
3
2
4
supplied by Laiyang Fine Chemical Factory. Graphite powder was purchased from Sinopharm
Chemical Reagent Co. LTD. αꢀDꢀgrucose was purchased from Tianjin Bodi chemical Co. LTD.
All chemicals are used without further processed. Doubly distilled water was used throughout all
of this work.
The general morphology and microstructure of the prepared catalysts were investigated using
a scanning electron microscopy (SEM, JEOL 6700ꢀF) and a transmission electron microscope
(TEM, JEM 2100). The crystallographic structures of samples were examined by Xꢀray diffraction
(XRD) measurements using a D/Maxꢀ2400 powder diffractometer with Cu Ka X ray sources
(Japanese Physical Co.). The XPS analysis was performed on a Thermo Scientific Escalab 250xl.
T=300K magneticꢀhysteresis (MꢀH) loops of the samples were obtained using vibrating sample
magnetometer. The photocatalytic reactions were carried out in a JOYNꢀGHXꢀA photocatalytic
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reactor (Qiaoyue Electronics Co.). UVꢀvis diffuse reflectance spectra of the samples were obtained
on a UVꢀvis spectrophotometer (TUꢀ2600). Elemental analyses were obtained by a EuroEA
Elemental Analyser. Energyꢀdispersive xꢀray spectroscopy (EDS) mapping was carried out to
verify the element distribution. Qualitative quantitative analyses of dye components after
degradation were provided by a GCꢀMS (Thermo Scientific Co. LTD.). The total organic carbon
(TOC) of dye was obtained on Total Organic Carbon Analyzer (SHIMADZU, SSMꢀ5000A).
Adjustable pipettes and automatic dual water distiller were purchased from Brocade Equipment
Co. LTD. All measurements were recorded at room temperature.
2.2 Synthesis of GO
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GO was prepared by using graphite powder as a precursor through the Hummers method
[35,36] with a minor modification. NaNO (0.75 g) and concentrated H SO (33.75 ml) were
3
2
4
o
added into a 250 ml flask and the mixture was cooled to 0 C using an ice bath. 1.0 g graphite
powder and 4.5 g KMnO
4
were added slowly into the mixture and the reaction temperature was
o
o
maintained below 20 C, the reaction was then warmed (30 C) with stirring for 1 h, the mixture
became a dark brown mushy material and was kept stirring slowly for 12 h. Then the above
mixture was poured into 46 mL deionized water. 30 min later, it was poured into 140 mL
2 2 4
deionized water containing 2.5 mL 30% H O in order to react with excess KMnO and produced
2
MnO , and the color of the mixture immediately turned bright yellow. The mixture was then
o
dialysed until the pH added to 7. At last, the resulting product was dried at 40 C in a vacuum oven
for 12 h.
2.3 Synthesis of the WSe2
2
WSe photocatalyst was prepared by oneꢀstep hydrothermal method. In the classical
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procedure, 0.66 g Na WO ·2H O and 0.32 g Se powder were dissolved in 45 mL deionized water,
2 4 2
2 4 2
then 25 ml HCl and 10 ml N H ·H O were poured into the solution successively, stirred for 2h.
Above suspension was transferred into an 80 mL Teflonꢀlined autoclave and was maintained at
o
200 C for 48 h. After being cooled down naturally, then treated with 25% NaOH solution to
remove residual Se powder, the products were washed with ethanol and distilled water for six
o
times and dried at 105 C in a vacuum oven for 12 h.
3 4
2.4 Synthesis of Fe O nanoparticles
3 4
Fe O nanoparticles were synthesized by the classical liquid phase coprecipitation
4 2 3 2
method[37]. A certain amount of FeSO ·7H O, FeCl ·6H O, aꢀDꢀgrucose dissolved in the second
1
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1
2
2
2
3 2 4 2
distilled water respectively, FeCl ·6H O(5ml 0.3mol/L), FeSO ·7H O (11.25ml 0.1mol/L) and
aꢀDꢀgrucose (5ml 0.05mol/L) mixture were added into 50ml dry three bottles and the mixture was
o
o
stirred at 80 C for 20 minutes, then the temperature was adjusted to 60 C, stirring rapidy while
dropping 1mol/L NH ·H O solution slowly, when the mixed solution PH was 10, then the solution
3
2
became black immediately, continue stirring for 30min, the nanoꢀferroferric oxide solution was
obtained, the whole reaction process were carried out in nitrogen protection. The product was
separated with a permanent magnet, the precipitate was washed with distilled water and ethanol
o
for several times and dried at 50 C in a vacuum oven for 8h.
3 4 2 3 4 2 2
2.5 Synthesis of Fe O -WSe /GO, Fe O -WSe /NG nanocomposites and WSe /NG
nanocomposites
A typical synthesis of Fe
3 4 2
O ꢀWSe /NG nanocomposites was as follows. The prepared GO
powder (0.25 g) was dissolved in 500 mL distilled water and was ultrasonicated for 3 h, then it
2 2 3 4
was centrifuged and the GO supernatant was taken. 7.5 g CO(NH ) , 0.25g Fe O and 0.25 g
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WSe were added into the GO supernatant and stirred for 1 h. Then the mixture was transferred
2
o
into a Teflonꢀlined autoclave and was maintained at 200 C for 12 h. After cooled it to room
temperature, the solid powders were filtered and washed with distilled water and ethanol for six
3 4 2
times and dried at 35 ℃ in a vacuum oven for 12 h. A series of Fe O ꢀWSe /NG nanocomposites
with different mass fraction of Fe
3 4
O (10, 20, 30 and 50 wt%) from the actual feed were prepared
under the same situation, which were marked as 10 wt%Fe O ꢀWSe /NG, 20 wt%Fe O ꢀWSe /NG,
3
4
2
3
4
2
3 4 2 3 4 2
30 wt%Fe O ꢀWSe /NG and 50 wt%Fe O ꢀWSe /NG, respectively, and the mass ratio of GO to
2 3 4 2
WSe is 1:1. The schematic diagram of Fe O ꢀWSe /NG nanocomposites preparation process is
3 4 2
depicted in Fig. 1. In order to make a comparison, Fe O ꢀWSe /GO nanocomposites was also
1
1
obtained by the same hydrothermal method without nitrogen source and WSe
2
/NG
nanocomposites was also obtained by the same hydrothermal method without Fe O .
3
4
1
1
2
3
Fig. 1. Schematic diagram for the preparation of Fe
2.6 Adsorption and photocatalytic performance measurements
The adsorptive and photocatalytic performances for Pꢀ25, WSe
nanocomposites and Fe O ꢀWSe /NG nanocomposites were assessed by calculating the adsorption
3 4 2
O ꢀWSe /NG nanocomposites.
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2
,
2
WSe /NG
3
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2
and photocatalytic degradation rate of MB solution in the dark and then under visible light
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irradiation, respectively. A photocatalyst sample (50 mg) was distributed in 50 mL MB aqueous
solution (32 mg/L), add a certain amount of H and nitric acid, and stirred 30 min in dark to
2
O
2
achieve the adsorptionꢀdesorption equilibrium. Then, the suspension was under irradiation by a
500W Xe lamp with a cutꢀoff filter (> 400 nm) as the visible light source. 5 mL sample solution
was taken at 10 min interval and remove the catalyst completely used a centrifuge, then the
supernatant was measured at 665 nm by UV–vis spectrophotometry. The removal efficiency of
MB was measured by D = (C − C ) / C ×100% , where D is the removal efficiency of MB,
0
t
0
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C
0
is the starting concentration of MB; and C
Generally, the photocatalytic degradation of a dye followed the pseudoꢀfirst order kinetics and
rate constant is determined by ln(C / C) = kt ; k is calculated from a graph of ln(C / C)
t
is the MB concentration at time t.
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1
0
0
and time, where C and C denote the MB concentrations at illumination time 0 and t, respectively.
0
Following the above procedure, different operating conditions such as initial concentration of
2 2
the H O and initial pH were investigated.
2.7 Total organic carbon experiments
The degree of mineralization for MB was calculated as: mineralization
degree= (TOC − TOC ) / TOC ×100% , where TOC
0
was the content of TOC (MB) in the
0
t
0
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2
2
2
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0
1
2
t
initial solution and TOC was the content of TOC (MB) after light irradiation at time t.
2.8 Recycling tests
In practical application, the photocatalysts’ activity, stability and reusability are very
important [38]. Hence, the used catalysts were separated with a permanent magnet, washed with
o
ethanol and distilled water for several times, dried in vacuum at 50 C for 8 h, and then four
3 4 2
consecutive recycling tests for MB degradation were finished by Fe O ꢀWSe /NG.
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3. Results and discussion
3 4 2
3.1 Characterization of Fe O -WSe /NG photocatalysts
2 2
The XRD analysis was used to study the crystalline structure of pure WSe , WSe /NG and
3 4 2
Fe O ꢀWSe /NG nanocomposites. As shown in Fig. 2a, several major diffraction peaks are
observed in the pure WSe
41.64°, 47.28°, 55.92°, 57.74°, corresponding to the (002), (100), (103), (006), (105), (110) ,
(112)agreed well with the typical hexagonalstructure of WSe (JCPDS No. 38ꢀ1388). From Fig. 2c,
2 2
, the feature peak of WSe are at around 2θ=13.54°, 31.44°, 37.78°,
2
it can be seen that the feature peak of Fe
62.58°, corresponding to the(220), (311), (400), (511) and (420) planes were consistent with the
standard Fe XRD spectrum (JCPDS No. 19ꢀ629). There are no other miscellaneous peaks on
the XRD spectrum, indicating that the prepared Fe O particles have higher purity. In addition, the
3 4
O are at around 2θ=30.76°, 35.7°, 43.06°, 57.08° and
1
1
1
1
1
1
3 4
O
3
4
feature peak of NG are at around 2θ=23.5°, 28°, 43.8° and 52° are attributed to the (002) , (100),
(004) in Fig. 2c. Fig. 2d compares with Fig. 2a, Fig. 2b and Fig. 2c, The XRD analysis of
3 4 2 2 3 4
Fe O ꢀWSe /NG composite in Fig.2d contains all the feature peaks of WSe , Fe O and NG,
3 4 2
implying that the Fe O ꢀWSe /NG composite synthesized successfully.
WSe (JCPDS No. 38-1388)
2
Fe O (JCPDS No.19-629)
3
4
002
103
100
110
0
06 105
1
12
a
b
2
20 311 400
511 440
002 002
100
004
c
d
10
20
30
40
50
60
70
2
Theta (degree)
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Fig. 2. XRD patterns of WSe (a), Fe O (b), WSe /NG(c) and Fe O ꢀWSe /NG
2 3 4 2 3 4 2
nanocomposites(d) .
and Fe ꢀWSe /NG nanocomposites are provided in Fig. 3(a)
The TEM images of Fe
O
3 4
O
3 4
2
3 4
and 3(b). From Fig. 3(a), the Fe O nanoparticles are mostly spherical, due to the magnetic force
between the particles and electrostatic force. Fig. 4(b) shows the particle size distribution of the
Fe O nanoparticles in Fig. 3(a). It exhibits that the particle size ranges from 9 to 16 nm, so the
3
4
Fe
3
O
4
nanoparticles owned a small size distribution. As shown in Fig. 3(b), WSe
2
and Fe
3
O
4
is
successfully.
(
a
)
b
( )
c
( )
d
( )
9
0
1
1
1
1
Fig. 3. TEM images of WSe
2 3 4 3 4 2
(a), NG (b), Fe O (c) and Fe O ꢀWSe /NG nanocomposites
1
2
3
(contains a picture of a magnification)(d).
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30
(
a
)
(
b
)
25
20
20
15
15
10
10
5
0
5
0
800 1000 1200 1400 1600 1800 2000
9
10
11
12
13
14
15
16
Particlesize/nm
Particlesize/nm
1
2
2 3 4
Fig. 4 The particle size distribution of the WSe (a), the Fe O nanoparticles (b).
3
4
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8
9
In order to further understand the composition of these particles’ surface, XPS of the samples
was measured. The XPS spectra of Fe ꢀWSe /GO and Fe ꢀWSe /NG composites are shown
O
3 4
2
3
O
4
2
in Figure 5. The spectra of N 1s with the binding energy at 399.1, 400.1 and 402.0 eV, respectively.
2
The peak at binding energy of 399.1 eV is attributed to the sp ꢀhybridized nitrogen (C=NꢀC) and
3
the two peaks of 400.1 and 402.0 eV can be assigned to tertiary nitrogen (Nꢀ(C) ) and amino
functional group with a hydrogen atom (CꢀNꢀH). The XPS spectra of Fe
the success of nitrogen doping.
3 4 2
O ꢀWSe /NG confirmed
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Fe O -WSe /GO
3
4
2
Fe O -WSe /NG
400000
300000
200000
100000
0
3
4
2
C 1s
O 1s
Fe 2p
N 1s
Se 3d
W4f
0
200
400
600
800
1000
1200
1400
B.E/eV
1
2
Fig. 5. XPS spectra of Fe O ꢀWSe /NG nanocomposites and Fe O ꢀWSe /GO nanocomposites.
3
4
2
3
4
2
3
4
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9
0
1
2
3
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5
6
Fig. 6 is the photo showing magnetic capture of the Fe
nanocomposites and their MꢀH curve at T=300K. A photo of Fe
addition to magnetic field is as shown in Fig. 6a, it can be seen from the figure that the prepared
Fe nanoꢀparticles are greatly influenced by the applied magnetic field, which is beneficial for
recycling. Fig. 6b is a photo of Fe O ꢀWSe /NG with aqueous solution in addition to magnetic
3
O
4
nanoparticle, the Fe
3 4 2
O ꢀWSe /NG
3
O
4
with aqueous solution in
3 4
O
3
4
2
field. It shows that the further preparation of Fe
can be effectively recycled under the effect of magnetic field. From Fig. 6c, it shows that no
hysteresis loop is formed in the magnetization of Fe and Fe ꢀWSe /NG particles , and the
3 4 2
O ꢀWSe /NG have good magnetic properties and
1
1
1
1
1
1
1
3
O
4
O
3 4
2
magnetization curve and the demagnetization curve coincide perfectly. When external magnetic
field is at H = 0 Oe, the residual magnetization and coercivity of Fe O and Fe O ꢀWSe /NG
3
4
3
4
2
particles are zero, displays the standard superparamagnetic characteristics. The results show that
the prepared Fe and Fe ꢀWSe /NG particles have obvious super paramagnetic properties at
3
O
4
3
O
4
2
3 4 3 4 2
T=300K. What’s more, the saturation magnetization of Fe O and Fe O ꢀWSe /NG particles are
60.5emu/g and 42.5 emu/g respectively.
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(b)
(a)
1
80
60
T=300K
(c)
40
20
0
-
20
40
60
Fe O -WSe /NG
3
4
2
-
Fe O4
3
-
-80
-10000
-5000
0
5000
10000
H (Oe)
2
3
Fig. 6. The photo showing magnetic capture of the Fe
nanocomposites (b) and their MꢀH curve at T=300K (c).
3.2 Properties of Fe -WSe /NG nanocomposites
3.2.1 Visible light responsive property
2
The visible light responsive properties of Pꢀ25, WSe WSe /NG and 30
3 4 3 4 2
O nanoparticle (a), Fe O ꢀWSe /NG
4
5
6
7
8
9
0
3
O
4
2
2
,
3 4 2
wt%Fe O ꢀWSe /NG were expressed by UVꢀvis diffuse reflectance spectra in Fig. 7. From Fig. 7a,
Pꢀ25 showed an absorption edge at 420 nm and WSe showed an absorption edge at 760 nm,
2
1
showing these two catalysts are ultraviolet light active and visibleꢀlight active semiconductors,
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4

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1
2
3
4
respectively. WSe /NG presented a similar UVꢀvis spectrum as that of WSe and showed an
2
2
2
absorption edge at 950 nm. It not only compensates for the absorption of WSe in the ultraviolet
3 4
area, but also responds well in the visible region. With the addition of Fe O , the absorption edge
reaches to1300nm. For crystalline semiconductor, the bandgap can be derived from the Kubelka
1
n
5
6
Munk equation: (ahv) = A(hv− Eg) , where a, v, Eg, and A stand for the absorption
2
coefficient, light frequency, band gap energy, and a constant, respectively [39]. The plots of (ahv)
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
and photon energy (hv) are exhibited in Fig. 7b; from which we can estimate the bandgap energy
2 2 3 4 2
of Pꢀ25, WSe , WSe /NG and 30 wt%Fe O ꢀWSe /NG are 3.05, 1.35, 0.85 eV and 0.35ev,
respectively. They are consistent with the reported values [6,10].
3 4
These results also demonstrated that the addition of NG and Fe O could narrow the
1
1
1
1
1
1
1
1
1
1
2
2
2
bandgap of WSe , which is attributed to the formation of FeꢀWꢀSeꢀCꢀN chemical bonding in 30
2
3 4 2
wt% Fe O ꢀWSe /NG. Moreover, it can be observed that a broad absorption peak in the visible
region for 30 wt% Fe
3
O
4
ꢀWSe
2
/NG nanocomposites [6].
1.5
70
P-25
WSe2
P-25
WSe2
60
a
b
1.2
WSe/NG
WSe/NG
2
2
50
30wt%FeO-WSe/NG
30wt%FeO-WSe/NG
3
4
2
3
4
2
0.9
40
0.6
30
20
0.3
10
0.0
0
200 300 400 500 600 700 800 900 1000 1100 1200 1300
0
1
2
3
4
5
6
hv(ev)
Wavelenghth(nm)
Fig. 7. UVꢀVis absorption spectra (a) and estimated band gaps (b) of Pꢀ25, WSe , WSe /NG and
2
2
3 4 2
30 wt%Fe O ꢀWSe /NG nanocomposites.
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1
2
3
4
5
6
3.2.2 Photocatalytic performance and analysis
According to previous work, the optimum synthesis ratio of WSe
photocatalytic performance of commercial Pꢀ25 was measured by the degradation of MB as a
model reaction in dark and under ultraviolet light, the asꢀprepared WSe , WSe /NG and
2
and NG was 1:1. The
2
2
Fe
3 4 2
O ꢀWSe /NG nanocomposites were also measured by degradation MB in dark and under
visible light irradiation. The results are shown in Fig. 8.
1.0
1.0
5
0wt%Fe O-WSe/NG
5
0wt%FeO-WSe/NG
3 4 2
3
4
2
3
0wt%Fe O-WSe/NG
3
0wt%FeO-WSe/NG
b
3 4 2
a
3
4
2
0.8
20wt%Fe O-WSe/NG
20wt% FeO-WSe/NG
3 4 2
3
4
2
1
0wt %FeO-WSe /NG
1
0wt%FeO-WSe/NG
3
4
2
3
4
2
0.8
0.6
0.4
0.6
0.2
0.4
0.0
0
5
10
15
20
25
30
35
-30
0
30
60
Time(min)
Time(min)
1
.0
4
WSe2
P-25
c
d
WSe /NG
2
0.8
3
2
1
0
30wt %Fe O-WSe /GO
3 4 2
30wt%Fe O-WSe /NG
3
4
2
-1
0.6
k=0.05589min
-
1
k=0.02481min
0.4
NoCatalyst
WSe2
-
1
k=0.0123min
-1
P-25
k=0.0056min
0.2
WSe/NG
2
-1
3
0wt%FeO-WSe/NG
3
4
2
k=0.00179min
0.0
0
10
20
30
40
50 60
-30
0
30
60
Time(min)
Time(min)
7
8
Fig. 8. The absorbance variation curves of MB over different Fe O photocatalysts in the dark (a)
3
4
9
and P.hotodegradation curves of MB under visible light irradiation (b). Photodegradation curves of
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6

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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
MB over different photocatalysts under visible light irradiation and Pꢀ25 under UVꢀlight
irradiation (c). Photodegradation kinetics of different catalysts on MB (d).
3 4 2
As shown in Fig. 8a, the majority of Fe O ꢀWSe /NG nanocomposites showed high adsorption
capacity, and the mass fraction of NG increases and the adsorption capacity of Fe
3
O
4
ꢀWSe
2
/NG
nanocomposites also increases. The strong adsorption capacity of Fe
3
O
4
ꢀWSe
2
/NG
nanocomposites may be ascribed to the strong interactions between azo dye molecules and NG
[40]. Obviously, the concentrations of MB in all samples showed that there are almost no changed
after being stirring for 40 min in the dark, implying that adsorptionꢀdesorption equilibrium formed.
3 4
As shown in Fig. 8b, photodegradation curves of MB over different Fe O photocatalysts under
1
1
1
1
1
1
1
1
1
1
2
2
2
visible light irradiation. Most of them have high photodegradation efficiency of MB, it can be
attributed to the larger surface area and unique electronic structure of NG. The addition of Fe O
3
4
into WSe
2
/NG can promote the photocatalytic performance. With the mass fraction of Fe
3
O
4
3 4 2
increasing, the photocatalytic activity of Fe O ꢀWSe /NG nanocomposites increases and then
3 2
decreases. The 30 wt%Fe O4ꢀWSe /NG nanocomposites showed the highest photocatalytic
performance and the degradation rate of MB was 98.8% within 60 min under visible light
irradiation. At last, 30 wt% of Fe O was chosen as the optimum mass fraction.
3
4
The Fig. 8c showed that MB aqueous solution without catalyst showed little change under
visible light, the removal efficiency of MB was only 5% after 60 min, which exhibited that in the
absence of a photocatalyst, the degradation of MB is extremely slow under visibleꢀlight
illumination. We can see that the degradation performance of different photocatalysts decreases
follow this sequence of 30 wt% Fe O ꢀWSe /NG > WSe /NG > P25 > WSe . Among them, the 30
3
4
2
2
2
3 4 2
wt% Fe O ꢀWSe /NG nanocomposites showed the highest photocatalytic performance. This is due
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7

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1
2
3
4
5
to the introduction of Fe O can be used for the AOPs in the presence of low pH and hydrogen
3 4
peroxide and can produce high activity hydroxyl radical(HO·) which is conducive to degradation
of organic pollutants.
In addition, the kinetics of MB degradation reactions are showed in Fig. 8d. In general, the
organic dyes conform to the obvious firstꢀorder kinetics which is in good consistency with a
kKC
6
general langmuirꢀhinshelwood mechanism, γ = ꢀdC / dt =
,where,
γ
is the
1
+ kC
7
8
9
0
1
2
3
4
5
6
degradation rate of the reactant (mg/L·min), C is the concentration of reactant (mg/L), t is the
illumination time, K is the adsorption coefficient of the reactant (L/mg) and k is the reaction rate
constant (mg/L·min). If C is very small then the above equation could be reduced to
ln(C /C)=kKt≈k t. In general, a plot of ln(C / C) and time represents a straight line and the
1
1
1
1
1
1
1
0
app
0
slope is equal to the apparent first order rate constant kapp. From Fig. 8d, we can see that the
largest obvious firstꢀorder reaction rate constant kapp is from 30 wt%Fe
3
O
4
2
ꢀWSe /NG (0.05589
ꢀ1
min ), which is a few folds higher than others, including Pꢀ25. What’s more, the firstꢀorder
3 4 2
reaction rate constant kapp of 30 wt%Fe O ꢀWSe /NG is much higher than that 30
wt%Fe
3 2
O4ꢀWSe /GO, it suggests that the N dopping content has very important effect on the
activity of photocatalyst.
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1.0
1.0
2
.0mM
3.0mM
4.0mM
5.0mM
PH3
PH5
PH7
PH9
PH11
a
b
0.8
0.8
0.6
0.4
0.2
0.6
0.4
0.2
0.0
0.0
-30
0
30
60
-30
0
30
60
Time(min)
Time(min)
1
2
Fig. 9. The Photodegradation curves of MB over 30 wt%Fe O ꢀWSe /NG photocatalysts
3
4
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
2 2
under different pH conditions (a) and under different H O conditions (b).
The dye industrial wastewater generally has a wide range of pH values. Furthermore, the
generation of hydroxyl radicals is a function of pH. Thus pH plays an important role in the
performance of photocatalytic process. Hence, photodegradation of the synthetic wastewater using
30 wt%Fe O ꢀWSe /NG photocatalyst was investigated under different initial pH values (Fig. 9a).
3
4
2
The highest photodegradation rate was obtained at pH 3. H
photocatalytic process, which affects the amount of ·OH, From Fig. 9b, we can see that with the
concentration of H increasing, the photocatalytic activity of 30 wt%Fe ꢀWSe /NG
2 2
O also plays an important role in
1
1
1
1
1
1
1
O
2 2
3
O
4
2
composites increases and then decreases. The highest photodegradation rate was obtained at H
3.0mM.
2 2
O
3 4 2
3.3 The reaction mechanism of Fe O -WSe /NG catalyzed MB degradation
A good photocatalyst not only has good photoactivity, but also the high conversion
(mineralization degree) in its practical application which is very important. TOC of MB solution
3 4 2
were analysed to characterize the mineralization degree of 30 wt%Fe O ꢀWSe /NG
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
nanocomposites. The results of TOC analysis showed that 30 wt%Fe O ꢀWSe /NG had the highest
3 4 2
mineralization degree of 87.7% under visible light irradiation. GC/MS analysis was used to
detect the intermediate compounds produced during the MB photodegradation for 30 wt%
3 4 2
Fe O ꢀWSe /NG under visible light irradiation could help us to understand the photodegradation
process of MB. The result showed that MB had been almost completely mineralized to CO
2
and
H O, and other products were isopropyl acetate and Dꢀglucopyranuronic acid. This reveals that 30
2
3 4 2
wt%Fe O ꢀWSe /NG has excellent photocatalytic performance.
3 4 2
Three typed of factors enable Fe O ꢀWSe /NG nanocomposites’ generally high photocatalytic
activity for MB degradation. First, the efficient adsorption. Second, the efficient transport and
separation of charge carriers. Third, Fe can be used for AOPs. The possible mechanism of
separation and transportation of electronꢀhole pairs at the Fe O ꢀWSe /NG photocatalyst interface
1
1
1
1
1
1
1
1
1
1
2
2
2
3 4
O
3
4
2
3 4
under visible light and Fe O for AOPs are proposed in Fig. 10. Under visible light irradiation,
both WSe
conduction band (CB) potentials of NG are 1.52 eV and ꢀ 0.9 eV [41]. Due to the CB edge
potential of WSe (ꢀ1.04eV) is more negative than that of NG (ꢀ 0.9 eV) [42ꢀ44], the charge
transfer between WSe and NG heterojunction causes that the CB electrons of WSe are injected
2
and NG can absorb visible light to produce charge carriers. The valence band (VB) and
2
2
2
into the NG via the wellꢀdeveloped interface, meanwhile the holes on the NG surface can migrate
to WSe in a similar way. NG acts as an electron trap that promotes the separation of
2
photogenerated electrons, and also facilitate the interfacial electron transfer process. The
separation of electronꢀhole pairs is also driven by the internal tectonic field, it reduces the
recombination of the electronꢀhole and results in the buildup of electrons and holes on the surfaces
ꢀ
of NG and WSe
2
, respectively [45,46]. Then the photogenerated electrons (e ) react with oxygen to
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2
ꢀ
1
2
3
4
5
6
generate ·O on the surface of NG, ·OH is formed on the surface of WSe nanosheets by the
2
+
reaction of holes (h ) with the adsorbed water, hydroxide, etc. These radicals initiate redox
reactions with MB molecules, then are degraded [6,47]. The graphite type N can increase the
adsorption capacity of the reactants and improve the nanocomposite’s conductivity [20], thus, NG
modified WSe
2
greatly improves the charge separation efficiency, while reduces the recombination
probability of photogenerated electrons and holes. In addition, Fe O for AOPs is as follows: the
3
4
2
+
7
8
catalyzed decomposition of H O by Fe to produce ·OH at low pH, Eq. (1), using
2 2
3
+
lightꢀassisted Fe regeneration 2+,
Fe
(2). [48ꢀ51], these ·OH initiate redox reactions with MB
Eq.
9
0
molecules, then are degraded (3). Due to the synergy of these components, the photocatalytic
performance of Fe ꢀWSe /NG nanocomposites are improved significantly.
1
3
O
4
2
1
1
1
2
Fig. 10. Proposed photocatalytic mechanism of Fe O -WSe /NG.
3
4
2
1
1
3
4
3.4 Recycling tests
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1

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100
1.0
3
0wt%FeO-WSe/NG
3
4
2
3
rd
4th
1
st
2nd
WSe/NG
2
a
b
80
0.8
60
0.6
40
0.4
2
0
0.2
0.0
0
1
2
3
4
0
60
0
60
0
60
0
60
Recoverytimes
Time(min)
1
2
Fig.11. Recovery (a) and recycling tests (b) for 30 wt%Fe3O4-WSe2/NG under visible light
irradiation.
3
4
5
6
7
8
9
0
1
2
3
4
5
According to Fig. 11a, the average recovery rate of 30 wt%Fe O ꢀWSe /NG is 90% after 4
3
4
2
2
recycling. The average recovery rate of WSe /NG is only 70.3%, significantly lower than that of
3 4 2
30 wt%Fe O ꢀWSe /NG. The results of four successive recycling tests for degradation of MB by
3 4 2
30 wt%Fe O ꢀWSe /NG are shown in Fig. 11b. Each run was carried out under the identical
conditions. As displayed in Fig. 11b, after four times repeated use, the removal efficiency of MB
still preserved above 85%, which presented excellent magnetic property and good reusability of
the photocatalyst.
1
1
1
1
1
1
4. Conclusions
3 4 2
Novel magnetically separable Fe O ꢀWSe /NG nanocomposite photocatalysts were
hydrothermally synthesized. Compared with WSe /NG and Pꢀ25, Fe O ꢀWSe /NG
2
3
4
2
nanocomposites exhibited excellent photocatalytic activity for degradation of MB under visible
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2

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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
light irradiation, which is attributed to the superior adsorption performance for MB, the excellent
electrical conductivity, the efficient transport and separation of charge carriers of NG, and Fe O
4
3
2 2
can be used for the AOPs to degradation of organic pollutants in the presence of pH 3 and H O
3 4 2
3.0mM. The 30 wt%Fe O ꢀWSe /NG nanocomposites degraded MB (32 mg/L) by 98.8% in 1
hour. Moreover, it showed higher magnetic property and good reusability, and the removal
efficiency of MB remained above 85% after four times repeated runs applying an external
magnetic field.
Acknowledgements
This work was supported by the Shandong Province Natural Science Foundation
(ZR2017MB049) and the Shandong Province University of Science and Technology Plan Projects
(J17KA106 and J17KA108).
1
1
1
1
1
1
1
1
1
1
2
2
2
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