Preparation, photocatalytic activity and mechanism of nano-Titania/Nafion hybrid membrane

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


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


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)


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)


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


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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Preparation, photocatalytic activity and mechanism of nano-Titania/Nafion hybrid membrane

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