ORIGINAL PAPER
Preparation, photocatalytic activity and mechanismof nano-Titania/Nafion hybrid membrane
Xingeng Ding • Simin Zhou • Lifang Jiang •
Hui Yang
Received: 26 September 2010 / Accepted: 4 January 2011 / Published online: 14 January 2011
� Springer Science+Business Media, LLC 2011
Abstract Nano-Titania/Nafion (TiO2/Nafion) hybrid
membranes were prepared by recasting, using Nafion
solution and TiO2 anatase hydrosol as the raw materials.
The microstructure of the hybrid membrane was charac-
terized by X-ray diffraction, high-resolution transmission
electron microscopy (HR-TEM), X-ray Photoelectron
Spectroscopy and Fourier Transform Infrared Spectroscopy
(FT-IR). The photocatalytic properties of TiO2/Nafion
hybrid membranes were evaluated. Furthermore, endurance
of photocatalytic activity of the hybrid membrane was
investigated. The results indicate that the TiO2 Nanopar-
ticles are bounded to Nafion molecule via Ti-O-S bonds
and the formed flocculates are distributed homogeneously
throughout the recasting Nafion membrane, while the ini-
tial pure anatase TiO2 nanoparticles remain intact in re-
crystallized membrane. The hybrid membranes possessed
excellent photocatalytic activities with and without H2O2.
Moreover, the degradation of photocatalytic activities has
been better controlled with the presence of H2O2.
Keywords Titania � Nafion � Hybrid membrane �Microstructure � Photocatalysis
1 Introduction
Titanium dioxide (TiO2) is an excellent photocatalyst
material used in treating environmental pollution [1]. There
are two application forms of titanium dioxide: suspended
and immobilized. Taking account of photocatalytic effi-
ciency, the suspended type TiO2 has more advantages
comparing to the immobilized one due to its fast mass
transfer and larger reaction areas in the same quantity [2,
3]. However, the suspended type TiO2 has some severe
drawbacks such as catalyst conservation and water/catalyst
separation in which the discharged TiO2 with effluent
might be harmful to human life due to its biological
accumulative effect [4]. To solve these problems, the
combined system of photocatalytic membrane reactor, so
called immobilized type, has been adopted. Compared to
the conventional separation technologies, novel photocat-
alytic membrane reactor utilizes the competitive separation
characteristic of membrane to continuously separate and
maintain high quality of TiO2 in photocatalytic reactor. In
the meantime, the coupling effect increases anti-contami-
nation property of the membrane [5, 6].
One of the new technologies to prepare immobilized
photocatalysts is to use nano-hybrid materials in which the
organic (polymer) phase and inorganic phase are linked
by chemical bonds. Fluorocarbonsulfonic acid polymers
exhibit very good thermal and chemical stability and they
possess extraordinary acid strength [7]. Nafion (denoted as
Nf), a perfluorinated sulphonated polymer, is widely used
in Chlor-Alkali industry for electrolyte membranes in SPE
electrolysers. The representative molecular structure is
shown in Fig. 1.
There were some reports indicating that Nf was used as
carrier and modifier of photocatalysts. Vohra and Tanaka
confirmed with the photocatalytic degradation of aqueous
X. Ding (&) � S. Zhou � H. Yang
Department of Materials Science and Engineering,
Zhejiang University, 38 Zheda Road,
Hangzhou 310027, China
e-mail: [email protected]
L. Jiang
Department of Chemistry and Chemical Engineering,
Mingjiang College, Fuzhou 350108, China
e-mail: [email protected]
123
J Sol-Gel Sci Technol (2011) 58:345–354
DOI 10.1007/s10971-011-2399-6
paraquat that Nf is stable against photocatalysis [8]. Liu
successfully used Nf as moulding board to prepare nano-
TiO2/Nf hybrid membrane [9]. The nanoparticles are pure
anatase TiO2. The average size of the crystalline TiO2
nanoparticles is similar to the estimated dimensions of the
hydrophilic cavities in Nf membrane in terms of the ion
cluster model. Bertoncello employed the Langmuir-
Schaefer (LS) technique to incorporate TiO2 nanoparticles
into Nf perfluorinated ionomer [10]. The result showed that
TiO2/Nf LS films have thermal stability up to 600 �C.
Premkumar et al. prepared Nf/TiO2 hybrid membrane by
immersing the wettish Nf membrane into a deoxygenated
TiCl4 in methanol for overnight and then the hybrid
membrane was oxidized by H2O2 solution. The result
illuminated that a higher quantity of H2O2 was needed for
the intercalated TiO2 membrane systems when compared
to TiO2 in the colloid and adsorbed state [11].
In this manuscript, we report a novel method to prepare
a photocatalytic system based on TiO2/Nf hybrid mem-
brane, which was fabricated by recasting TiO2 anatase
hydrosol and Nf polymer dispersion together. The new
method allows low temperature process. The correlation
between the recast membrane structure and the nanoparti-
cle properties (shape, size and location in the membrane)
has been studied. The photocatalytic activity of the hybrid
membrane has been evaluated by investigating the degra-
dation behavior of methyl orange (denoted as MO).
2 Experimental
2.1 Materials
Titanium butoxide [Ti(OC4H9)4] was analytical reagent
used as received. All organic solvents were of spectro-
photometry grade and used as received. Nf PFSA polymer
dispersion [5 wt% solution, a product of DuPont, DE 520,
copolymer of perfluorosulfonic acid and poly (tetrafluoro-
ethylene)] was purchased from YiBang/RuiBang New
Power Sources Technology Co. LTD. Methyl orange
(analytical reagent) was diluted by deionized water to
20 mg/L before use.
2.2 Preparation of anatase colloidal sol
The anatase TiO2 colloidal sol was prepared by the following
procedure [12, 13]: 10 mL Ti (OC4H9)4 was dissolved
slowly in 40 mL absolute ethanol by stirring continuously.
After 30 min, the Ti (OC4H9)4 solution (20 wt% in absolute
ethanol) was dropped into nitric acid solution (200 mL of
0.5 mol L-1) under stirring and oil-bath at 90 �C. Stirred
continually and reflowed for 120 min at constant tempera-
ture. The anatase hydrosol was formed after aging for several
days. The concentration of TiO2 is 9.39 g L-1. Formation
mechanism of anatase titania nanocrystals at low tempera-
ture is shown in Fig. 2.
2.3 Preparation of nano-TiO2/Nf hybrid membranes
The incorporation of nano-TiO2 in Nf membrane was
carried out by recast. Six membranes, with the mass frac-
tions of TiO2 of 0, 5, 10, 15, 20 and 30%, respectively,
were prepared simultaneously. To improve the yield of
membrane formation, 15 mL ethylene glycol was mixed up
with15 mL Nf solution by stirring for 10 min. Then, TiO2
hydrosol was dropped into the mixed solution under stir-
ring to form stable suspension. The suspension was stirred
for 30 min, then the solvent with low boiling point of the
suspension was evaporated at 80 �C for about 180 min
in blast oven. The suspension was then oscillated by
Fig. 1 Representative molecular structure of Nafion
Fig. 2 Formation mechanism of titania nanocrystals prepared by low
temperature dissolution-reprecipitation process (LTDRP) [33]
346 J Sol-Gel Sci Technol (2011) 58:345–354
123
ultrasonic for 5 min to eliminate air bubbles before pouring
into a flat-bottom container to further dry at 140 �C for 2 h.
The resulting recasting membranes were removed from
the casting surface, washed with isopropanol followed by
acetone rinsing several times, and then boiled in deionized
water for 60 min to eliminate the residual.
2.4 Characterization methods
TiO2 hydrosol was dried into powder at 60 �C. The phases
of TiO2 powder, pure Nf membrane and hybrid membranes
were characterised by X-ray diffraction measurements with
a powder diffraction system (Shimadzu 6000, Japan). The
structures of the TiO2/Nf hybrid membranes were charac-
terized by FT-IR, using an infrared spectrometer (Paragon
1000, USA). The morphology of the TiO2 nanoparticle was
studied with a high-resolution transmission electron
microscopy (HR-TEM: JEOL, JEM2100F) at an acceler-
ating voltage of 200 kV. The specimens were prepared by
using an Ultracut-E microtome equipped with a diamond
knife. Chemical bonding energy of Nf and TiO2 particles
was investigated by XPS spectra. The XPS spectra were
collected on a VG ESCALAB MARK II system with
Mg Ka X-ray source (1,253.6 eV photons). The step size
was chosen to be 0.2 eV. The sample was prepared by
mixing TiO2 sol and Nf solution (TiO2 40% in mass
fraction), dried at 60 �C before being grinded.
2.5 Photocatalytic activity measurements
The photocatalytic activity was evaluated by measuring the
degradation of the water solution of methyl orange
(denoted as MO, with a concentration of 20 mg L-1) under
UV light irradiation carried out with a 20 W high-pressure
mercury lamp. The chemical structure of MO is shown in
Fig. 3.
The hybrid membranes, containing TiO2 of 5, 10, 15 and
20% (mass fraction), respectively, were used for the pho-
tocatalytic experiment. Each membrane contained about
750 mg Nf resin and was cut in half before use. The
schematic diagram of the photocatalytic experiment is
shown in Fig. 4. During the photocatalytic experiment,
one-half of membrane was added into 30 mL of the above
MO solution in a Petri dish and placed to the bottom. The
solutions after degradation were analyzed by recording
variations in the absorption in UV–visible spectra of MO
using an ultraviolet–visible spectrometer (TU-190 l,
China). The photocatalytic activity of the pure recast Nf
membrane was also studied by calculating the variation of
MO’s concentration. The capability of Nf, adsorbing the
cationic species strongly, would increase degradation rate
of MO. But the adsorption would affect the calculation of
degradation rate, so the adsorptive capacity must be con-
sidered. After the photocatalytic experiment, the mem-
branes were boiled in deionized water for 30 min to desorb
the membranes, and then the MO concentrate of the solu-
tion was measured by UV–visible spectra.
The stability of photocatalytic activity was investigated
by checking the photocatalytic activities of the 5% hybrid
membrane with and without H2O2. Ten degradation
experiments were completed employing fresh MO solu-
tions but with the same TiO2-Nf membrane.
3 Results and discussion
3.1 Structures and properties of nano-TiO2/Nf hybrid
membranes
The phase of TiO2 hydrosol is important for the photo-
catalytic activity. Anatase TiO2 possesses much better
photocatalytic activity than rutile, while brookite does not.
As shown in Fig. 5, the X-ray powder diffraction pattern of
the TiO2 powder obtained from the TiO2 hydrosol by oven
dry exhibits the characteristic peaks of crystalline TiO2
particles (Corresponding to pure anatase TiO2).
The nanoscopic nature of the crystalline TiO2 particles
is responsible for the broadness of the X-ray powder dif-
fraction peaks. The peak broadening is used to estimate the
average TiO2 crystal grain size in terms of the Debeye-
Scherer equation [14].
Fig. 3 Chemical structure of
methyl orange
Fig. 4 Schematic diagram of the photocatalytic experiment
J Sol-Gel Sci Technol (2011) 58:345–354 347
123
D ¼ Kk= b cos hð Þ ð1Þ
In the equation, D is the average nanocrystal diameter in
angstroms. (is the corrected band broadening (FWHM). K
is a constant related to the crystallite shape and the way in
which D and b are defined. k is the X-ray wavelength, and
h is the diffraction angle. It is thus estimated that the
average diameter of the TiO2 nano-crystal grain is
3.66 nm.
The tensile strength of Nf reinforced through improving
the degree of crystallization [15]. The phases of the dif-
ferent hybrid membranes were investigated to estimate the
variation of tensile strength which is related to the degree
of crystallization. Figure 6 illustrates that Nf crystal in
hybrid membranes decreases with an increasing percentage
of TiO2 particles. It seems that the addition of TiO2 retards
the crystallization of Nf, so that the tensile strength
declines. This result is in accordance with the fact that the
hybrid membrane containing 30% TiO2 is brittle and
fractures easily, while pure Nf membrane is flexible.
According to the Eq. 1, we can estimate that the average
nanocrystal diameter of TiO2 paticles in hybrid membranes
is 8.8, 6.6, 7.7, 7.7, and 7.8 nm, corresponding to 5, 10,
15, 20 and 30% of TiO2, respectively. The high-resolution
transmission electron microscopy (TEM) provides a cross-
sectional view of the TiO2 hydrosol particles and the 15%
(mass fraction) hybrid membrane (see Fig. 6). Obviously,
the most hydrosol particles are spherical in shape, with an
estimated average diameter of less than 10 nm. It is
observed clearly the nano-TiO2 particles in the 15%
(mass fraction) hybrid membrane did not grow obviously,
which is in good agreement with the estimated average
nanocrystal diameter. It is speculated that the nano-TiO2
particles in recasting membrane are single crystal.
Although, the recasting temperature (140 �C) is much
higher as compared with the oven dry temperature of TiO2
hydrosol (60 �C), the TiO2 particles in recasting mem-
branes did not grow largely. The reason may be that the
attachment of Nf polymer retards the continuous growth of
crystalline TiO2 particles during the recasting process.
The TiO2 nanoparticles formed in the Nf membrane
cavities are distributed homogeneously and possess regular
shapes as mentioned in the introduction. The hybrid
membrane in this work was prepared on the basis of the
technologies of preparing TiO2 hydrosol at low tempera-
ture and recasting Nf membrane, which is a brand new
system in TiO2/Nf hybrid membrane because of the par-
ticular microstructure shown in Fig. 7a. In the macroscopic
view, the agglomerate particles are distributed homoge-
neously throughout the hybrid membrane structure. There
is no massive aggregation owing to the Nf-TiO2 interac-
tions leading to steric stabilization.
10 20 30 40 50 60 70 80
2-theta
Nafion
5%
10%
15%20%30%TiO2
rela
ted
inte
nsio
n
Fig. 5 X-ray diffraction (XRD) patterns of 5 TiO2/Nf hybrid
membranes with different TiO2 contents (in mass, the same below),
pure Nf membrane and pure TiO2 powder
Fig. 6 High-resolution TEM image providing a cross-sectional view.
a TiO2 collodal sol particles, b 15% (mass fraction) hybrid membrane
348 J Sol-Gel Sci Technol (2011) 58:345–354
123
During the process of mixing up Nf solution and TiO2
hydrosol, Nf was adsorbed on the surface of TiO2 particles
leading to the formation of the loose depositions called
flocculate show in Fig. 7b. In terms of chemical reaction,
Nf polymer molecules chemically bonded to the nanopar-
ticles, whose surfaces were covered by positive charges
shown in Fig. 2. This speculation agrees with the formation
of Ti-O-S linkage from sulphated TiO2 [16]. To identify
the alteration of surface properties with the addition of Nf,
XPS measurements were carried out to calibrate the bind-
ing energy (B.E.) of O 1 s. As shown in Fig. 8, it is obvious
that the O 1 s peak can be deconvoluted into 4 sub-peaks.
According to the literature, the binding energies of the O
1 s states in pure Nf are 533.0 and 535.7 eV, which are
related to the oxygen in the sulfonic acid groups (-SO3H)
and the oxygen in the ether configuration (C-O-C) [17]. In
Fig. 9, both peaks shifts to lower binding energy (532.8
and 535.2 eV, respectively), which indicates that Nf is
hybridized with TiO2. The peaks centered at 529.6 and
531.0 eV are assigned to Ti-O-Ti and S-O-Ti respectively
[16, 18, 19].
Alphonse et al. estimated the surface area of TiO2 sol
particles with the average particle size 5–6 nm equaled to
370 m2 g-1 [20]. The high value indicated the high surface
potential energy, so that TiO2 sol particles were easily
packed to flocculates by polymers. The flocculates, with
the shape of dolichoectatic and an estimated average size
of 80 nm in width and 400 nm in length, are large enough
to scatter visible light, leading to the fact that these sus-
pensions are not transparent but translucent (milky).
The flocculations affected by gravity and resistance
were sphericity assumed. The simplified equation for this
spherical particle deduced by Stokes’ Law is written as,
Vs ¼2ðqp � qf Þ
9lgR2 ð2Þ
where: Vs = the sedimentation rate in units of distance/
time, R = the radius of the spherical particle, ‘‘qp’’ and
‘‘qf’’ are the density of the particle and surrounding fluid
respectively, g = gravitational constant (this value may be
very different if the particles are in a centrifuge), and ‘‘l’’
is the viscosity of the fluid [21]. It is obtained from Eq. 2
that the radius of the spherical particle affects the sedi-
mentation rate largely. According to Eq. 2, the floccula-
tions with the size over 100 nm can precipitate rapidly.
However, the stratification of the suspension containing 5%
TiO2 (mass fraction) was found 7 days later, while 30%
was found 2 days later. Therefore, the stabilization of the
suspension depends on concentration of Nf. The flocculates
distribute homogeneously in the solid Nf substrate after
oven dry owing to the stabilization of the above-mentioned
suspension in a long time.
The stabilization of the suspension is often explained by
the steric stabilization effects of polymers. Recently, the
Fig. 7 High-resolution TEM image providing a cross-sectional view.
a hybrid membranes’ microstructure (5%, mass fraction), b suspen-
sion mixing up Nf solution and TiO2 sol
540 538 536 534 532 530 528 526 5240
50
100
150
200
250
300
-SO3H
C-O-C
Ti-O-S
Ti-O-TiInte
nsity
Binding energy (eV)
Fig. 8 XPS spectra of the O 1 s for the hybrid catalysts
J Sol-Gel Sci Technol (2011) 58:345–354 349
123
current theory figures that a dispersion of colloidal particles
may be stabilized against flocculation by the adsorption or
attachment of flexible polymer molecules onto the particles
of the suspension [22, 23]. In condition of low concentra-
tion of Nf polymer, macromolecular could not cover the
particles, but bridge the nano-TiO2 particles to stick toge-
ther and form the massive flocculation and precipitated
rapidly (Fig. 9a).
On the contrary, the presence of hydrophilic and elec-
tronegative Nf polymer with high concentration between
the surfaces of the particles gives rise to a repulsive force
between the particles (Fig. 9b). When two such particles
approach to each other, the reduction in the number of
configurations available to the flexible polymer chains
gives rise to an ‘‘entropic’’ repulsive force between the
particles. This may keep two colliding particles so far apart
that the van der Waals interaction energy is insufficient for
coherence. This theory is called steric stabilization effect,
often used to explain the ability of certain additives to
inhibit coagulation of suspensions. In conclusion, the
surface charges of flocculation attached by Nf polymer
played an important role.
In addition, the stabilization of the suspension is owing
to another important reason: the effects of environment,
such as convection of temperature, mechanical vibration
and so on.
It is important for Nf membrane to possess excellent
mechanical properties which prevent it from breakage and
retain intactness after several uses in photocatalysis.
Through the analysis above, the Nf molecules chemically
bond to the suffice charges of TiO2 sol particles, which
may change the structure of Nf molecules, further cutting
down mechanical properties of hybrid membrane.
According to the references, the main absorption bonds,
characteristics and corresponding assignments of pure Nf
membrane are listed in Table 1 [24–26].
Figure 10 shows the infrared spectrums between 400
and 2,000 cm-1 of such a cast Nf membrane and hybrid
membranes. Comparison between the infrared spectrums of
Fig. 9 Cartoon illustration of the protection and the flocculation
model for Nf polymer.
Table 1 Characteristic peak of
NfBand location (cm-1) Characteristics Assignments
*1640 Strong & broad –OH stretching, symmetric, –OH bending
*1235 Very strong –CF2 stretching, asymmetric, –SO3- stretching, asymmetric
*1156 Very strong C–F stretching, symmetric
*1060 Medium S–O stretching, symmetric
*983 Medium C–O–C stretching, symmetric or C–F stretching,
symmetric of –CF–CF(R)–CF group
*805 Very week C–S stretching, symmetric
*633 Medium –CF2 bending
*529 Medium –CF2 bending
2000 1800 1600 1400 1200 1000 800 600 400
Nafion5%
10%
15%
20%
TiO2
Abs
orba
nce
Wavenumbers(cm-1)
Fig. 10 Infrared (IR) spectrums of TiO2, cast Nf membrane and
hybrid membranes containing different TiO2 contents (5%, 10%,
15%, 20%, mass fraction)
350 J Sol-Gel Sci Technol (2011) 58:345–354
123
hybrid membranes and of pure cast Nf membrane indicates
the IR peaks do not shift and are matched with Table 1
completely. The broadness of the IR peak shapes of hybrid
membranes between 400 and 800 is resulted from the
addition of TiO2 particles. It is reasonable to conclude that
the polymer backbones of Nf in hybrid membranes pre-
pared by casting remain steady, propitious to the excellent
mechanical properties of Nf.
3.2 Photocatalytic activity and adsorptivity of Nf
It is important that Nf polymer should not degrade MO
during the photocatalytic process. It was confirmed by
illuminating recast Nf membrane dipped into 30 mL MO
solution for 1 h. (represented as sample 1). A control
sample with 30 mL MO solution without Nf (represented
as sample 2) has been induced to eliminate the effect of
self-degradation. After UV radiation, the residual solutions
were measured by graduated flask, 24.5 mL for both
sample 1 and sample 2. Nf membrane then boiled in
deionized water at 70�C to desorb the membrane for
30 min, repeated the same desorption process one more
time. The solutions after desorption were then measured by
graduated flask, 60 mL for the first time and 56 mL for the
second time. We believe there is still a small amount of
MO residual left in Nf membrane. Heights of UV–Vis
absorption peaks in direct proportion to the concentration
of MO are shown in Table 2.
The height 0.088 of first desorption can be normalized
into 0.2155 when volume 60 mL is normalized to 24.5 mL.
In this case, the total absorption of sample 1 will be 1.5295,
which is only 5.87% lower than that of the control sample
(sample 2). Considering the residual MO left in Nf mem-
brane and the self-decomposition of MO, we would like to
draw the conclusion that Nf doesn’t show degradation
effect on MO.
It was confirmed that a Nf/TiO2 particle had a high
capacity to bind positively charged molecules attributing to
the fact that the anionic sulfonate groups (-SO3-) in the Nf
layer outnumber the positively charged surface functional
groups (–TiOH2?) [8, 27]. As shown in Fig. 11, about 46%
MO was absorbed by pure recast Nf membrane in 2 h.
According to the chemical structure of MO shown in
Fig. 3, Nf can absorb MO strongly during the photocata-
lytic process, which is the crucial factor increasing the
photocatalytic activity.
3.3 Effect of TiO2 dosage on photocatalytic activities
The influence of TiO2 dosage (ranged from 5 to 20%) on
the MO degradation was investigated with the concentra-
tion of MO fixed at 20 mg/L. The correlation of degrada-
tion rate of MO and the increment of TiO2 dosage is shown
in Fig. 12. It is obvious that the degradation rates of MO
with all the hybrid membranes are above 94% at time
90 min, confirming that the TiO2/Nf hybrid membranes
Table 2 Heights of UV–Vis absorption peaks of MO aqueous
solutions
Sample Sample 1 Sample 2
Residual First
desorption
Second
desorption
Height 1.314 0.088 0 1.625
0 60 120 180 240 3000.4
0.5
0.6
0.7
0.8
0.9
1.0
MO
(%)
Time(min)
Fig. 11 The adsorption capacity of pure recast Nf membrane for MO
(initial MO concentration is 20 mg�L-1)
0 20 40 60 80 100 1200.0
0.2
0.4
0.6
0.8
1.0
per5
per10
per15
per20
C/C
0
Time(min)
Fig. 12 Effect of TiO2 dosage on the MO potodegradation (initial
MO concentration is 20 mg�L-1)
J Sol-Gel Sci Technol (2011) 58:345–354 351
123
possess strong photocatalytic activities. In addition, it is
demonstrated that the MO degradation of the 15% mem-
brane is 79.5% at time 30 min, which is the highest com-
paring to the other hybrid membranes with different TiO2
dosage. Degradation rate depends on two processes,
adsorption and photocatalysis. Photocatalytic activity plays
the main role in the MO degradation, while the adsorptive
rate affected by the MO concentration plays the main role
after the concentration reduced.
The increase of TiO2 dosage did not accelerate the
MO degradation significantly, that of the 20% membrane
even lower than 15%. One possible reason is that the
increase in the opacity of the hybrid membranes with
the abundance of TiO2 particles resulted in a reduction
of the UV light penetration. Another reason is that the
increase of TiO2 amount and enlarging the size of
flocculate particles resulted in narrower channels (Nf
matrix) of MO shown in Fig. 8a and reduced total sur-
face area of flocculated particles, which MO can contact
with. The adsorption of Nf matrix is not taken into
account in this experiment because most of MO absorbed
by Nf has been degraded and the residual is difficult to
be determined.
3.4 Stability of photocatalytic activities
The hybrid photocatalytic membrane was adopted by the
reason that the system can solve the problem of catalyst
conservation and water/catalyst separation for recycling. It
will be extraordinary if the TiO2/Nf hybrid membranes can
retain the excellent photocatalytic activities as mentioned
above after repeating use. Therefore, the stability of pho-
tocatalytic activities was investigated by illuminating
30 mL MO solution containing a 5% hybrid membrane for
30 min. After used every time, the membrane was boiled
by deionized water for 30 min to desorb the membrane.
UV–vis absorption of the solution after desorption was
measured to eliminate the effect of the adsorption. As
shown in Fig. 13, the degradation rate decreased at the
beginning and then kept stable in range from 20 to 30%.
Meanwhile, it is observed the color of the membrane faded
gradually.
According to the literatures, the significant changes
taken by H2O2 may result from the TiO2 surface adsorption
and the formation of an oxidizing agent, i.e., a titanium
peroxide complex due to the interaction between H2O2 and
valance-unfilled Ti (IV) on TiO2 surface [28–30]. In
addition, Fan et al. confirmed that the purple membrane
containing Ti(III) yielded the orange peroxo Ti(IV) com-
plex after immersed in an acidic solution (pH ca. 0.5)
containing excess hydrogen peroxide [31]. The valance-
unfilled Ti on TiO2 surface processes a strong reducibility,
which resulted in a strong degradability at the beginning.
After the surface was oxidized gradually, the degradation
rates kept stable.
The 5% hybrid membrane took on milk white and
translucence, after boiled in the H2O2 dilute solution at
70 �C for 30 min. Obviously, the surfaces of TiO2 particles
were oxidized thoroughly. The photocatalytic stability of
the oxygenated hybrid membrane was investigated. A
pronounced enhancement of the degradation rate was
observed shown in Fig. 13. The MO degradation rate in
first irradiation was 80.4%, then declined quickly in the
next irradiations and kept stable in range from 20% to 40%.
The strong photocatalytic activity in the presence of H2O2
results from the following two possible reasons: (1) H2O2
acts as an electron acceptor to prevent the recombination of
e- and h? and/or offering additional hydroxyl radicals
(Eq. 2) [32]. (2) The interaction between H2O2 and TiO2
leads to the formation of titanium peroxide complex on the
TiO2 surface which may act as oxidizing species under
visible light, TiO2 demonstrated the best performance and
rutile showed higher activities than anatase under the
irradiation of visible light [29]. The degradation rate kept
stable after H2O2 was exhausted.
H2O2 þ e�direct ! OH � þOH� ð3Þ
4 Conclusions
A novel TiO2/Nf hybrid membrane was prepared by the
low temperature preparation of TiO2 hydrosol and recast-
ing Nf membrane. The microstructure and the photocata-
lytic activities of the hybrid membranes were investigated
in this work.
TiO2 embedded in the Nf matrix is pure anatase. Nf
crystal in hybrid membranes decreases with an increasing
0 2 4 6 8 100.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Deg
rada
tion
rate
Times
Non H2O2
Fig. 13 Stabilityof photocatalytic activity of TiO2/Nf membrane
(5%, mass fraction) against repeated uses
352 J Sol-Gel Sci Technol (2011) 58:345–354
123
percentage of TiO2 particles. The TiO2 agglomerate par-
ticles are distributed homogeneously throughout the hybrid
membrane structure, which is believed co-contributed by
the surface charges of flocculation attached by Nf polymer
and the effects of environment, such as convection of
temperature, mechanical vibration and so on. Nf polymer
molecules chemically bonded to the hydrosol nanoparticles
in the manner of the Ti-O-S linkage.
The TiO2/Nf hybrid membranes possessed strong pho-
tocatalytic activities without H2O2. The MO degradation
rates of all the hybrid membranes were above 94% under
UV irradiation for 90 min. Especially, the MO degradation
of hybrid membrane with 15% TiO2 was 79.5% at time
30 min with the same initial concentration, which is the
highest comparing to the others’. However, the hybrid
membranes failed to retain the high photocatalytic activi-
ties due to the gradual oxidation of the TiO2 surface.
Further study indicated that the hybrid membranes also
possessed excellent photocatalytic activities with the
presence of H2O2. The MO degradation rate of the 5%
hybrid membrane was 80.4% under UV irradiation for
30 min, and then declined quickly in the following repe-
ated irradiations and kept stable in range from 20 to 40%
owing to the exhaustion of H2O2.
Acknowledgment This work was supported by the major scientific
and technological project (No.ZD2007001) from the Ministry of
Education of Zhejiang Province. This work was supported by Science
and Technology Innovative Research Team of Zhejiang Province
(No. 2009R5001).
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