FULL PAPER
Thin Films
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2D Heterostructure Membranes with Sunlight-Driven SelfCleaning Ability for Highly Efficient Oil–Water Separation
Yanan Liu, Yanlei Su, Jingyuan Guan, Jialin Cao, Runnan Zhang, Mingrui He, Kang Gao,
Linjie Zhou, and Zhongyi Jiang*
as energy saving, green, and continuous
operation.[6–9] Because the discharge of
oily wastewater discharging is rapidly
increasing, for example, a typical mining
operation can produce 140 KL oily wastewater per day,[10] the requirement of filtration membrane with high flux become
more urgent.
A variety of methods have been explored
to fabricate high-flux membranes, such
as phase separation, self-assembly, and
electrospinning.[7,11–17] Among them, selfassembly becomes a popular approach
because of its spontaneous organization
of building blocks into specific structures and patterns.[9,18–21] As an emergent
building block, graphene oxide (GO), a
functionalized graphene derivative, has
attracted tremendous attention in fabricating advanced membranes with tunable
mass transfer channels via intercalating
0-dimension and 1-dimension materials into adjacent GO to increase permeation flux.[15,21,22] For
instance, a nanostrand-channelled GO membranes with mass
transfer channel of 3–5 nm have been fabricated, and the permeance was tenfold of that of GO membrane.[15] However, high
flux often gives rise to serious membrane fouling, resulting in
high mass transfer resistance, due to significant adsorption
and deposition of foulants on membrane surface.[23,24] Hence,
a number of approaches have been explored to prevent the
foulants from adhering on membrane surface by constructing
hydrophilic or amphiphilic surface through surface coating, surface grafting, or surface segregation.[11,12,25,26] On account of the
complicated fouling mechanism of oil, hydrophilic membrane
surfaces often lose efficacy to resist fouling, and inspired by
self-cleaning surface of lotus leaf, the low surface energy materials are designed to prevent oil foulant from attaching membrane surface.[23,27] However, application of low surface energy
materials, such as silicon-containing and fluorine-containing
substances, often causes the drastic decrease of permeation
flux, a pronounced trade-off between permeation flux and antifouling properties. Therefore, how to effectively remove the oil
foulant adhered on membrane surface remains a critical issue.
Photocatalysis, a sustainable and green technology, is recognized as a new technology for water remediation, and coupling
membrane filtration with photocatalytic degradation of oil foulant becomes a research hotspot in oily water treatment.[28–36]
Currently, almost all the efforts are devoted to remove the oil
Introducing solar energy into membrane filtration to decrease energy and chemicals consumption represents a promising direction in membrane fields. In this
study, a kind of 0D/2D heterojunction is fabricated by depositing biomineralized
titanium dioxide (TiO2) nanoparticles with delaminated graphitic carbon nitride
(g-C3N4) nanosheets, and subsequently a kind of 2D heterostructure membrane is fabricated via intercalating g-C3N4@TiO2 heterojunctions into adjacent
graphene oxide (GO) nanosheets by a vacuum-assisted self-assembly process.
Due to the enlarged interlayer spacing of GO nanosheets, the initial permeation
flux of GO/g-C3N4@TiO2 membrane reaches to 4536 Lm−2 h−1 bar−1,
which is more than 40-fold of GO membranes (101 Lm−2 h−1 bar−1) when utilized for oil/water separation. To solve the sharp permeation flux decline, arising
from the adsorption of oil droplets, the a sunlight-driven self-cleaning process is
followed, maintaining a flux recovery ratio of more than 95% after ten cycles of
filtration experiment. The high permeation flux and excellent sunlight-driven flux
recovery of these heterostructure membranes manifest their attractive potential
application in water purification.
1. Introduction
The significant shortage of the clean water and clean energy
exerts the severe challenges for the modern societies and scientific research community.[1–3] Oil contamination, a main source
of water pollution, becomes a major environmental concern in
our dairy life and a wide range of industries, including leather,
textile, food, steel, petrochemical, mining, and metal finishing.
According to some statistics, the cost used to water disposal
is more than 38.3 billion US$ every year.[4,5] In recent years,
membrane technology has become a dominant technology to
deal with various oily wastewater, due to their advantages, such
Dr. Y. Liu, Prof. Y. Su, J. Guan, J. Cao, Dr. R. Zhang, Dr. M. He, K. Gao,
L. Zhou, Prof. Z. Jiang
Key Laboratory for Green Chemical Technology of Ministry of Education
School of Chemical Engineering and Technology
Tianjin University
Tianjin 300072, China
E-mail: zhyjiang@tju.edu.cn
Dr. Y. Liu, Prof. Y. Su, J. Guan, J. Cao, Dr. R. Zhang, Dr. M. He, K. Gao,
L. Zhou, Prof. Z. Jiang
Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin)
Tianjin 300072, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201706545.
DOI: 10.1002/adfm.201706545
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foulant via the UV-driven self-cleaning ability of photocatalyst.[13,37] Sunlight-driven self-cleaning membrane for oil/water
separation, especially separation of oil-in-water emulsion, is
rarely reported.
Recently, graphitic carbon nitride (g-C3N4) with a layered
structure has received growing attention as a novel metal-free
photocatalyst, owing to visible light-driven bandgap (2.69 eV),
easy fabrication and high chemical and thermal stability.[38–40]
Theoretically, loading 0D semiconductor photocatalyst onto
2D nanosheets to form mixed-dimensional heterojunction
can suppress the recombination of photoexcited charges and
enhance the photocatalysis efficiency.[41,42] In this study, 2D
heterostructure membrane was fabricated by vacuum-assisted
self-assembly process via intercalating 0D/2D heterojunction of
titanium dioxide and graphitic carbon nitride (g-C3N4@TiO2)
into adjacent 2D GO nanosheets. The intercalation of the heterojunction into adjacent GO nanosheets could tune the interlayer distance of GO nanosheets as well as the surface topography of the 2D heterostructure GO/g-C3N4@TiO2 membrane,
resulting in high permeation flux and hierarchical nanostructure. And combining the hydrophilicity of TiO2 and hierarchical
nanostructure, the as-prepared membranes possessed superoleophobicity underwater. Because the surfactant-stabilized oilin-water emulsion had considerable absorption in the visible
light region, oil/water membrane separation, and photocatalytic membrane cleaning were conducted individually. Due to
the effective photodegradation of oil foulant adhered on membrane surface, the as-prepared membranes exhibited excellent
self-cleaning ability and flux recovery. Simultaneously, the flux
recovery ratio (FRR) could maintain more than 95% after ten
cycles of oil-in-water emulsion separation experiment.
2. Results and Discussion
2.1. Preparation and Characterization of 0D/2D Heterojunction
A series of g-C3N4@TiO2 0D/2D heterojunctions were fabricated by combining the biomimetic mineralization of TiO2
with the delamination of bulk g-C3N4, and the mass ratio
of g-C3N4 to TiO2 was controlled by changing the additive
amount of g-C3N4 during the synthesis process of g-C3N4@
TiO2 0D/2D heterojunction. The morphology of g-C3N4@TiO2
0D/2D heterojunction was investigated by transmission electron microscopy (TEM) and atomic force microscope (AFM)
(Figure 1). The as-prepared g-C3N4 demonstrated a nanosheetslike morphology with size of about 500 nm and thickness of
about 0.9 nm (Figure 1a). The TiO2 nanoparticles were distributed on the g-C3N4 nanosheets uniformly and showed an
average diameter of about 5 nm and the size and thickness of
g-C3N4@TiO2 0D/2D heterojunction were about 500 and 12.4 nm,
respectively (Figure 1a). Furthermore, there were no naked
g-C3N4 nanosheets and free TiO2 nanoparticles found. In the
high-resolution TEM image of g-C3N4@TiO2, a lattice distance
of 0.345 nm could be seen, corresponding to the (101) lattice
plane of anatase. Other g-C3N4@TiO2 0D/2D heterojunction
exhibited the similar nanostructure, as shown in Figure S1
(Supporting Information). Fourier transform infrared (FTIR)
spectrum and X-ray diffraction (XRD) were utilized to investigate the chemical structure and crystalline phase of as-prepared
g-C3N4@TiO2 0D/2D heterojunctions. There was the clear
TiOTi stretching vibration appearing around 500–700 cm−1
in TiO2 and g-C3N4@TiO2 0D/2D heterojunctions. Synchronously, the NH stretching, the typical stretching vibration of
Figure 1. a) TEM and AFM images, b) FTIR spectra, c) XRD patterns, and d) TGA curves of g-C3N4 nanosheets and g-C3N4@TiO2 0D/2D heterojunctions.
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CN and CN heterocycles, and the s-triazine ring vibrations
appeared at 3000–3600, 1240–1640, and 808 cm−1 in g-C3N4
nanosheets and g-C3N4@TiO2 0D/2D heterojunctions, respectively. Similarly, the identified peaks of TiO2 and g-C3N4 could
be found in g-C3N4@TiO2 0D/2D heterojunction, and the TiO2
could be identified as anatase TiO2. Although ultrasonication
was utilized to prepare the TEM samples, there was no TiO2
nanoparticles peeled off from g-C3N4 nanosheets, which demonstrated that the composites of TiO2 nanoparticles and g-C3N4
nanosheets were in the form of heterojunction. The mass ratio
of TiO2 and g-C3N4 in g-C3N4@TiO2 0D/2D heterojunction was
measured by thermogravimetric analysis (TGA) with temperature range of 40–800 °C. As shown in Figure 1d, there was
almost no weight loss in TiO2 TGA from 40 to 800 °C, and pure
g-C3N4 nanosheets lost all weight from 500 to 720 °C. Hence, on
account of the combustion of g-C3N4 nanosheets, all g-C3N4@
TiO2 0D/2D heterojunction lost their weight rapidly from 500 to
610 °C. By calculating, the contents of g-C3N4 nanosheets
in g-C3N4@TiO2-x 0D/2D heterojunction (x = 1, 2, 3, 4) were
58.5, 43.3, 25.7, and 11.1 wt%, respectively.
2.2. Preparation and Morphology
of 2D Heterostructure Membranes
The 2D heterostructure membranes were fabricated by filtrating
the aqueous solution of GO nanosheets and g-C3N4@TiO2
0D/2D heterojunctions onto mixed cellulose membranes by
vacuum-assisted self-assembly process, as shown in Figure 2a.
The GO nanosheets used in this study were made by the typical
Hummers method and the thickness and size of GO nanosheets
were 0.8 nm and 1.5 µm, respectively (Figure S2, Supporting
Information). There were hydroxyl, epoxy, carboxyl, and carbonyl groups on GO nanosheets, which could anchor the intercalated nanomaterials at these functional sites.[43] Each layer
of g-C3N4 nanosheets comprised aromatic tri-s-triazine units
which was connected by tertiary amines, in practice, incomplete
condensation of precursors led to the existence of primary and
secondary amine groups on g-C3N4 nanosheets.[40,44] Hence,
the interaction between carboxyl groups on GO nanosheets
and amino groups on g-C3N4 nanosheets would bind the GO
nanosheets and g-C3N4@TiO2 0D/2D heterojunctions together;
Figure 2. a) Schematic illustration of the fabrication of GO/g-C3N4@TiO2 2D heterostructure membranes via vacuum-assisted self-assembly process,
b) SEM and AFM images of GO/g-C3N4@TiO2 2D heterostructure membranes.
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meanwhile, the interaction also could help crosslink neighboring GO together,[45,46] leading to well-aligned layer structure,
as shown in Figure S3 (Supporting Information). Meanwhile
the existence of TiO2 on g-C3N4@TiO2 0D/2D heterojunctions
could further influence the surface hydrophilicity and roughness of the as-prepared membranes.
The surface morphologies of 2D heterostructure membranes
with different TiO2 loading on g-C3N4@TiO2 0D/2D heterojunctions were shown in Figure 2b. The surface of GO membrane was
quite smooth and slight wrinkled corrugation existed. Due to the
intercalation of g-C3N4@TiO2 0D/2D heterojunctions, the surface
of the GO/g-C3N4@TiO2 membrane transferred to be rough. In
addition, as the amount of TiO2 on g-C3N4@TiO2 0D/2D heterojunctions increased, the membrane surface became rougher.
Because more loading of TiO2 on the 0D/2D heterojunctions
made g-C3N4 nanosheets more flat and inflexible, benefiting from
the lowered surface energy of the nanocomposites, as result of the
robust interactions between the two components,[41] the stacking of
flat and inflexible nanosheets was more cushy to produce embossment than the flexible nanosheets. Meanwhile, the loading of TiO2
on g-C3N4 nanosheets could prop up the compact interlayer of GO
nanosheets and g-C3N4 nanosheets partly; hence, the interlayer
spacing between GO nanosheets and g-C3N4 nanosheets became
large resulting in high surface roughness, as shown in scanning electron microscopy (SEM) and AFM images. Also, more
GO nanosheets were propped up as the loading of TiO2 on
g-C3N4 nanosheets increased, leading to rougher membrane
surface. Each membrane was measured more than twice to
confirm the surface roughness by AFM, and the surface roughness parameter Ra of as-prepared membranes were increased
from 5.5 ± 0.2 nm for GO membrane to 61.7 ± 2.7 nm for
GO/g-C3N4@TiO2-4 membrane, and the surface roughness parameter Rq of as-prepared membranes was increased
from 7.9 ± 0.3 nm for GO membrane to 79.3 ± 3.9 nm for GO/gC3N4@TiO2-4 membrane (Figure S4, Supporting Information).
The resulting hierarchical nanostructure would be in favor of the
formation of superoleophobicity underwater in the water/membrane interface.[47]
The GO/g-C3N4@TiO2 2D heterostructure membranes were
intact, free-standing, transparent, and flexible after the substrate
membrane was dissolved by dimethylacetamide, as shown in
Figure 3a and Figure S5 (Supporting Information). The average
membrane thickness of as-prepared membranes was measured
by SEM images using images analysis software with more than
seven positions for each membrane. The average thickness of
as-prepared membrane was in the range of 36–249 nm, and the
Figure 3. a) The photograph and b) membrane thickness of GO/g-C3N4@TiO2 2D heterostructure membranes, c) XRD pattern of GO membrane,
d) the interlayer distance of GO/g-C3N4@TiO2 2D heterostructure membranes, the scale bars of (a) and (b) represented 2 cm and 500 nm, respectively.
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intercalation of g-C3N4@TiO2 0D/2D heterojunctions increased
the membrane thickness. Also, as the loading of TiO2 on
g-C3N4@TiO2 0D/2D heterojunctions increased, the membrane
thickness increased. XRD was utilized to evaluate the interlayer
distance of GO membrane and diffraction peak was at 2θ =
10.9°. Notably, the interlayer distance of GO membrane was
0.81 nm. The intercalation of g-C3N4@TiO2 0D/2D heterojunctions into adjacent GO nanosheets would increase the interlayer
distance, which brought about some trouble to evaluate interlayer distance of GO by XRD pattern. Because the amount of GO
in the 2D heterostructure membranes was same, making use of
the membrane thickness and interlayer distance of GO in GO
membrane, the interlayer distance of GO in GO/g-C3N4@TiO2
2D heterostructure membranes was obtained. By calculating,
the interlayer distance of GO/g-C3N4@TiO2 2D heterostructure membranes was increased from 0.81 to 5.62 nm, as shown
in Figure 3d. Meanwhile, because the thickness of GO and
g-C3N4 used in this study was 0.8 and 0.9 nm, respectively, the
net interlayer distance of GO and g-C3N4 (calculated by the
interlayer distance of GO nanosheet and thickness of GO and
g-C3N4) was increased from 0.02 nm for GO/g-C3N4 membrane
to 1.96 nm for GO/g-C3N4@TiO2-4, which was beneficial for
the enhancement of membrane permeability.
2.3. Wettability Behavior of 2D Heterostructure Membranes
The membrane surface wettability of water in air and oil underwater was important property for membranes utilized for
wastewater treatment. As a result, apparent water contact angle
in air and soybean oil contact angle underwater were measured to estimate the wettability behavior of GO/g-C3N4@TiO2
2D heterostructure membranes. As shown in Figure 4a,
because the hydrophilicity of g-C3N4 nanosheets was lower than
GO nanosheets, the water contact angle of GO/g-C3N4 membrane
was a little higher than that of GO membrane. And because the
strong hydrophilicity of TiO2, as the loading of TiO2 on g-C3N4@
TiO2 0D/2D heterojunctions increased, the water contact angle
of GO/g-C3N4@TiO2 2D heterostructure membranes decreased.
At the same time, owing to high hydrophilicity and the surface roughness, more water would immerse into membrane
2.4. Permeation Performance of 2D Heterostructure Membranes
As a classical fluid dynamic theory, the Hagen–Poiseuille equation was utilized to analyze the liquid permeation flux through
an open pore. In the equation (J = επrp2Δp/8 µL), the permeation flux was described as a function of the surface porosity ε,
the pore radius rp, the pressure drop Δp, the liquid viscosity μ,
and the total distance L traveled by the liquid passing through
the membrane.[13,48] And the equation predicted that the permeation flux of membrane was in direct proportion to the square
of pore size and inverse proportion to membrane thickness.
In this study, the intercalation of g-C3N4@TiO2 0D/2D heterojunctions could effectively increase the interlayer distance of
b
70
Oil contact angle(o)
Water contact angle (o)
a
pore; therefore, the water contact angle of GO/g-C3N4@TiO2-4
membrane decreased from 62° to 43° in 60 s, in contrast, the
water contact angle of GO membrane decreased from 70° to
62° in 60 s. Moreover, the hierarchical nanostructures and high
hydrophilicity contributed by the intercalation of g-C3N4@TiO2
0D/2D heterojunctions would entrap water within the interface
of water/membrane, which endowed as-prepared membranes
with a robust hydration layer. Synergistically adjusting the hierarchical nanostructures and hydrophilicity of membrane surface
would decrease the interaction between membrane surface and
oil droplet underwater, endowing the membrane with superoleophobicity underwater, synchronously, as shown in Figure 4b
and Figure S6 (Supporting Information). As the surface hydration capacity increased, the soybean oil contact angle underwater
was improved from 152° for GO membrane to 170 ° for GO/gC3N4@TiO2-4 membrane. When the oil droplet was approaching
the membrane surface with hierarchical nanostructure, the
water entrapped in the hierarchical nanostructure would form
a protective hydration structure, which could stop the oil droplet
from completely contacting with the membrane surface. As a
result, the three-phase contact interface of oil/water/membrane
became discrete, leading to significant decrease of effective contact area between membrane surface and oil droplet.[23] Hence,
the GO/g-C3N4@TiO2 membrane possessed superoleophobicity,
which could increase the antifouling properties and keep longterm operational stability of as-prepared membranes.
65
60
55
50
45
40
0
GO/g-C3N4
GO/g-C3N4@TiO2-1
GO/g-C3N4@TiO2-2
GO/g-C3N4@TiO2-3
GO/g-C3N4@TiO2-4
20
30
Time (s)
40
180
170
160
150
140
130
120
110
100
GO
10
190
50
90
GO
60
4
-4
-3
-2
-1
C 3N
O2
O2
O2
O2
/gTi
Ti
Ti
Ti
O
@
@
G
@
@
N4
N4
N4
N4
C3
C3
C3
C3
/g /g/ggO
/
GO
O
G
O
G
G
Figure 4. a) The water contact angle in air, b) soybean oil contact angle underwater of GO/g-C3N4@TiO2 2D heterostructure membranes.
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Table 1. Membrane thickness, interlayer distance of GO nanosheets, permeation flux, surface roughness, and oil contact angle underwater of asprepared membranes.
Thickness
[nm]
Interlayer distance
[nm]
Flux
[Lm−2 h−1 bar−1]
Roughness
GO
35.7 ± 7.0
0.81
101 ± 27
5.5 ± 0.1
7.9 ± 0.2
152.3 ± 2.6
GO/g-C3N4
78.8 ± 11.3
1.78
347 ± 46
8.7 ± 0.2
13.6 ± 0.4
150.6 ± 6.7
GO/g-C3N4@TiO2-1
108.8 ± 17.6
2.46
1085 ± 64
18.0 ± 0.4
23.4 ± 0.5
155.1 ± 3.0
Oil contact angle [°]
Ra [nm]
Rq [nm]
GO/g-C3N4@TiO2-2
167.5 ± 26.7
3.79
2507 ± 102
20.3 ± 0.2
27.2 ± 0.4
161.1 ± 2.2
GO/g-C3N4@TiO2-3
218.5 ± 25.6
4.94
4536 ± 156
55.4 ± 5.4
71.7 ± 7.5
165.1 ± 2.8
GO/g-C3N4@TiO2-4
248.6 ± 37.6
5.62
1397 ± 74
61.7 ± 2.7
79.3 ± 3.9
170.4 ± 2.0
GO nanosheets, via propping up GO nanosheets, and as the
loading of TiO2 increased, the net interlayer distance of GO
and g-C3N4 increased, as shown in Figure 3d; accordingly, the
pore size of as-prepared membrane was increased, which was
contributing to the enhancement of permeation flux. Nevertheless, more loading of TiO2 on g-C3N4 nanosheets would block
the water to pass through the channel between GO and g-C3N4
to increase mass transfer resistance, as shown in Figure S7
(Supporting Information). Simultaneously, the intercalation of
g-C3N4@TiO2 0D/2D heterojunctions would increase the membrane thickness, which was disadvantage for the enhancement
of permeation flux, as shown in Table 1. Therefore, because
of the synergetic effect of TiO2 loading on interlayer distance
and membrane thickness, the permeation flux existed a peak
value with the increase of TiO2 loading on g-C3N4@TiO2
0D/2D heterojunctions, as shown in Figure 5a. The stability of
a 5000
4500
80
3500
3000
60
2500
2000
40
1500
1000
20
500
0
GO
5500
c
5000
4500
4500
4000
4000
3500
3500
3000
3000
6000
5000
4000
2500
2500
3000
2000
2000
1500
1500
2000
1000
1000
1000
500
500
0
0
Flux' (Lm-2h-1)
Flux(Lm-2h-1bar-1)
b
N4
-2
-1
-3
-4
C3
2
O2
iO 2
iO 2
/gTiO
Ti
T
T
O
@
@
G
@
@
N4
N4
N4
N4
C3
C3
/g-C 3
-C 3
/ gg
g
O
/
/
O
G
G
GO
GO
0
1
3
7
pH
11
13
Flux(Lm-2 h-1 bar-1)
Flux (Lm-2h-1bar-1)
4000
Separation efficiency(%)
100
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pressure (bar)
Figure 5. a) Permeation flux and separation efficiency of GO/g-C3N4@TiO2 2D heterostructure membranes, b) permeation flux of GO/g-C3N4@TiO2-3
membrane at different pH, and c) under different applied transmembrane pressure.
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simultaneous conduct of oil/water separation and photocatalysis,
the as-prepared membranes were evaluated by water–emulsion–
water–water four stage filtration experiment, and initial permeation flux of deionized water (Jwater), the permeation flux of
surfactant-stabilized soybean oil-in-water emulsion (Joil), the
recovered permeation flux of deionized water (Jrecovery-rinsing) after
20 min rinsing and the recovered permeation flux of deionized
water (Jrecovery-light irradiation) after 60 min light irradiation were
acquired, as shown in Figure 6a. And the GO/g-C3N4@TiO2
membrane exhibited excellent antifouling properties compared
with the GO membrane. The oil/water separation experiment
proceeded with a transmembrane pressure of 0.5 bar and a
near-surface stirring speed of 200 rpm. The oil content in permeation was determined by UV–vis spectrophotometer, and the
oil rejection ratio of all as-prepared membrane was higher than
99.9% in Figure 5a (Figure S8, Supporting Information). The
flux recovery ratios of the GO/g-C3N4@TiO2 membranes were
gradually increased from 50% for the GO membrane to 100%
for the GO/g-C3N4@TiO2-4 membrane with the increase of TiO2
loading on g-C3N4@TiO2 0D/2D heterojunctions, as shown
in Figure S9 (Supporting Information). Because with increase
of TiO2 loading on g-C3N4@TiO2 0D/2D heterojunctions,
GO/g-C3N4@TiO2 2D heterostructure membrane was evaluated
by the water permeation flux with water pH of 1.0–13.0 and the
dependence of permeation flux to the applied pressure in the
range of 0.1–1.0 bar. As shown in Figure 5b, the membrane
possessed stable water permeation flux at pH range of 1.0–13.0.
It could be known that the permeation flux exhibited a gradual
increase with the increase of applied transmembrane pressure
(where Flux’ respected the permeation flux independent of the
applied transmembrane pressure). However, the increasing
ratio of permeation flux was decreased, which implied that the
interlayer distance of GO nanosheets formed by the intercalation of g-C3N4@TiO2 0D/2D heterojunctions was compressed
to some extent according to the increase of transmembrane
pressure.
2.5. Self-Cleaning Ability of 2D Heterostructure Membranes
Self-cleaning ability of membrane was vital for membrane
process during separating oil-in-water emulsion. In this
study, because the surfactant-stabilized oil-in-water emulsions
had absorption in the visible light region, which limited the
Flux (Lm-2h-1bar-1)
a
5000
4500
Jwater
4000
Joil
Jrecovery-rinsing
3500
Jrecovery-light irradiation
3000
2500
2000
1500
1000
500
0
GO
-1
-2 -1
N
C3
/g O
G
@
4
4500
-1
O2
Ti
C
/gGO
c
5000
4000
Flux (Lm h bar )
N4
g-C 3
Jwater
N
3
-2
O2
Ti
@
4
C
/gGO
N
3
-3
O2
Ti
@
4
C
/gGO
N
3
@
4
-4
O2
Ti
105
100
95
3500
90
3000
85
FRR(%)
b
/
GO
2500
2000
80
75
70
1500
65
1000
60
Joil
500
55
0
-1
0
1
2
3
4
5
6
Cycle Number
7
8
9
10
50
Soybean oil
Diesel
Gasoline
Hexane Petroleum ether
Figure 6. a) Permeation flux of the 2D heterostructure membranes in water–emulsion–water–water four stages filtration experiments, b) permeation
flux of GO/g-C3N4@TiO2-3 membrane for cyclic separation experiment including permeation flux of deionized water and permeation flux of surfactantstabilized soybean oil-in-water emulsion, and (3) the flux recovery ratio of GO/g-C3N4@TiO2-3 membrane for different surfactant-stabilized oil-in-water
emulsions.
Adv. Funct. Mater. 2018, 1706545
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the photocatalytic activity of g-C3N4@TiO2 0D/2D heterojunctions was improved, as shown in Figure S10 (Supporting
Information). In particular, the initial permeation flux of the
GO/g-C3N4@TiO2-3 membrane was 4536 Lm−2 h−1 bar−1, and
decreased to 316 Lm−2 h−1 bar−1 for soybean oil-in-water emulsion separation. And after simulated-sunlight irradiation, the
permeation flux could recover to 4523 Lm−2 h−1 bar−1, which
was 99.7% of the initial permeation flux. Hence, the GO/gC3N4@TiO2-3 membrane was chosen for the further test for
the comprehensive performance including permeation flux
and antifouling properties. Cyclic filtration experiment proceeded to evaluate the long-term antifouling properties of the
GO/g-C3N4@TiO2-3 membrane, as shown in Figure 6b, after
ten cycles of separation experiment, the flux recovery ratio was
more than 95%. Moreover, as a nature light source, sunlight
was utilized to irradiate the fouled membrane, and after 2 h
sunlight irradiation from 12 am to 14 pm, the flux recovery ratio
was more than 92%. Meanwhile, some other kinds of emulsions including surfactant-stabilized diesel-in-water emulsion,
surfactant-stabilized gasoline-in-water emulsion, surfactantstabilized hexane-in-water emulsion and surfactant-stabilized
petroleum ether-in-water emulsion were utilized to evaluate
the self-cleaning ability of as-prepared membrane. After light
irradiation for 1 h, the flux recovery ratio for these emulsions
was all more than 90%, as shown in Figure 6c. All these filtration experiments demonstrated that the GO/g-C3N4@TiO2 2D
heterostructure membranes were equipped with excellent antifouling properties and could be utilized to separate surfactantstabilized oil-in-water emulsion, efficiently.
The photocatalysis of g-C3N4@TiO2 0D/2D heterojunctions
for degrading organic compounds endowed the GO/g-C3N4@
TiO2 2D heterostructure membranes with excellent antifouling
properties and self-cleaning ability, which was crucial for practical application. The photocatalysis of g-C3N4@TiO2 0D/2D
heterojunctions for degrading soybean oil was confirmed by
treating 1000 ppm surfactant-stabilized soybean oil-in-water
emulsion with 1 h simulated-light irradiation, and the results
showed that g-C3N4@TiO2 0D/2D heterojunctions possessed
photocatalytic degradation of soybean oil, especially g-C3N4@
TiO2-3 and g-C3N4@TiO2-4 (Figure S10, Supporting Information). To test the photocatalytic degradation of the GO/g-C3N4@
TiO2 2D heterostructure membranes, a series of oil contact
angles underwater and FTIR spectra were measured, as shown in
Figure 7. The original GO/g-C3N4@TiO2-3 membrane exhibited
Figure 7. a) Oil contact angle underwater after fouling by soybean oil and subsequently irradiation by light of GO/g-C3N4@TiO2-3 membrane and b) GO
membrane, and c) FTIR spectra of the original GO/g-C3N4@TiO2-3 membrane, adhered by soybean oil and subsequently irradiated by simulated sunlight.
Adv. Funct. Mater. 2018, 1706545
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superoleophobicity underwater with the oil contact angle of
165°. After fouling by soybean oil, the superoleophobicity
underwater disappeared and the oil contact angle underwater
was 109°. When the fouling membrane was irradiated by simulated sunlight for 1 h, the membrane recovered to be superoleophobicity underwater, and the oil contact angle underwater
become 161°. For comparison, the same experiment was proceeded with GO membrane, and it could not recover to be
superoleophobicity underwater after simulated-sunlight irradiation. These results were accordant with the results of FTIR
spectra, as shown in Figure 7c. The original GO/g-C3N4@
TiO2-3 membrane had no obvious peaks in the range of 2800–
3000 and 1470 cm−1, after fouled by soybean oil, the obvious
peaks appeared at 2800–3000 and 1470 cm−1 for the stretching
vibration of CH2 and CO, respectively. Also, these characteristic peaks disappeared completely after simulated-sunlight
irradiation, testifying the soybean oil was degraded by g-C3N4@
TiO2 0D/2D heterojunction.
J=
3. Conclusions
We have successfully fabricated GO/g-C3N4@TiO2 2D heterostructure membranes by vacuum-assisted self-assemble
process. The resulting heterostructure membranes displayed
high permeation flux and excellent self-cleaning ability for surfactant-stabilized oil-in-water emulsion separation. The intercalation of g-C3N4@TiO2 0D/2D heterojunctions into adjacent
GO nanosheets could increase the interlayer distance of
GO nanosheets from 0.81 to 5.62 nm, enhancing the permeation flux from 101 Lm−2 h−1 bar−1 for GO membrane to
4536 Lm−2 h−1 bar−1 for GO/g-C3N4@TiO2-3 membrane. The
synergistic effect of hydrophilicity of TiO2 and hierarchical
nanostructure formed by the assembly of GO nanosheets and
g-C3N4@TiO2 0D/2D heterojunctions endowed the 2D heterostructure membranes with superoleophobicity underwater.
Meanwhile, the photocatalysis of g-C3N4@TiO2 0D/2D heterojunctions could degrade the oil foulants adhered on membrane
surface endowing GO/g-C3N4@TiO2 2D heterostructure membranes with excellent self-cleaning ability, and FRR maintained
more than 95% after ten cycles of oil-in-water emulsion separation experiment. The current study offers a useful attempt of
combining the photocatalysis with membrane filtration to treat
oily wastewater efficiently.
4. Experimental Section
Materials: GO nanosheets were fabricated via the typical Hummers
method from graphite powders. The graphitic carbon nitride (g-C3N4)
nanosheets and nanocomposites of g-C3N4 and titanium dioxide
(g-C3N4@TiO2) were fabricated according to the literature.[39] The details
of fabrication process of GO, g-C3N4 and g-C3N4@TiO2 could be found
in the Supporting Information. All the chemicals utilized for fabrication
of GO, g-C3N4, and g-C3N4@TiO2 were purchased from Sigma-Aldrich
(Beijing, China). Deionized water (>18 MΩ cm−1) was used throughout
the experiments which was purified by Milli-Q system.
Fabrication of GO and GO/g-C3N4@TiO2 Membranes: The GO and
GO/g-C3N4@TiO2 membranes were fabricated by vacuum-assisted
self-assembly process. The GO nanosheets were dispersed into
deionized water with the concentration of 0.1 mg L−1, and the g-C3N4
Adv. Funct. Mater. 2018, 1706545
and g-C3N4@TiO2 nanosheets were dispersed into deionized water with
the concentration of 0.1 mg L−1, respectively (the concentration was
calculated by the amount of g-C3N4 in g-C3N4@TiO2 nanosheets). The GO
dispersion (20 mL) or the mixture of GO dispersion (20 mL) and g-C3N4@
TiO2 dispersion (20 mL) were treated with ultrasonication for 5 min
and vacuum-filtrated onto mixed cellulose ester (MCE) membranes
with pore size of 200 nm. The as-prepared membranes were dried at
60 °C for 12 h. Free-standing GO and GO/g-C3N4@TiO2 membranes
could be obtained by immersing in dimethylacetamide to dissolve MCE
membranes and transferred onto nonwoven fabrics (average pore size of
30 µm, Figure S11, Supporting Information) for performance evaluation.
Membrane Performance: The permeation performances of as-prepared
membranes were evaluated by a dead-end filtration cell (Millipore Model
8003) with effective filtration area of 0.9 cm2, which was equipped with
nitrogen gas cylinder and solution reservoir. In order to ensure high
reproducibility of membrane performance, each membrane was test at
least three times. First, each membrane was precompacted by a specific
operating pressure with deionized water until the flux reached a stable
value. The membrane performance was achieved by a transmembrane
pressure of 0.5 bar. The permeate fluxes J (L m−2 h−1 bar−1) of each
membrane were calculated by 5 min filtration of deionized water
according to the following equation for more than three times
V
A ∆ t∆ p
(1)
where V (L) was the permeated water volume, A (m2) was the efficient
membrane area, Δt (h) and Δp (bar) were the recorded time and
the pressure drop, respectively. To evaluate antifouling properties of
as-prepared membranes in oil-in-water emulsion separation, 1.0 g soybean
oil was added in 1000 mL deionized water which was stabilized by 0.1 g
sodium dodecyl sulfate to form surfactant-stabilized oil-in-water emulsion
under mechanical stirring for more than 12 h. The filtration of surfactantstabilized oil-in-water emulsion underwent 10 min, and the oil content in
the filtrate and feed solution was determined by a UV-spectrophotometer
(UV-9200). The antifouling properties of as-prepared membranes were
evaluated by the four stages filtration processes. The initial permeate flux
of deionized water (Jwater), the permeate flux of surfactant-stabilized oilin-water emulsion (Joil), the recovered permeate flux of deionized water
(Jrecovery-rinsing) after 20 min rinsing and the recovered permeate flux of
deionized water (Jrecovery-light irradiation) after 60 min light irradiation with a
500 W Xenon lamp (PLS-SXE300, Shanghai Bilang Plant, China). The FRR
was calculated by the following equation
FRR = Jrecovery-light irradiation /J water
(2)
and the higher FRR indicated better antifouling properties of the
as-prepared membranes.
Characterization: TEM images of GO, g-C3N4, and g-C3N4@TiO2 were
acquired by using a Hitachi S-4800 with an operating voltage of
200 keV. FTIR (VERTEX70) spectroscopy with a resolution of 4 cm−1 for each
spectrum, XRD with a Rigaku D/max-2500 X-ray diffraction equipment in
the range of 10° –80° (2θ) at the rate of 4° min−1 (Cu Ka, k = 0.154 nm, 40 kV,
200 mA), and TGA (NETZSCH TG209 F3) heating from 40 to 800 °C
at a ratio of 5 °C min-1 in air atmosphere were utilized to confirm the
chemical structure, crystallographic properties, and quantitative analysis of
g-C3N4, g-C3N4@TiO2, and TiO2, respectively. SEM (Nanosem 430, FEI Co.,
Ltd) recording on a field-emission scanning electron microscopy and AFM
(BRUKER Dimension Icon) were used to measure the surface morphology
of as-prepared membranes. The wettability of as-prepared membranes was
confirmed by water contact angle and underwater oil contact angle with a
contact angle goniometer (JC2000D2 M Contact Angle Meter), and each
membrane was measured more than seven positions.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
This study was supported by the National Key Research and
Development Program-China (2016YFB0600503), the National Science
Fund for Distinguished Young Scholars (21125627), and the National
Natural Science Funds of China (21621004).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
0D/2D heterojunctions, 2D heterostructure membranes, graphene oxide
nanosheets, oil/water separation, sunlight-driven self-cleaning
Received: November 10, 2017
Revised: December 19, 2017
Published online:
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