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第10卷第4期过程工程学报 Vol.10 No.4

2010 年 8 月 The Chinese Journal of Process Engineering Aug. 2010

Received date: 2010−03−23; Accepted date: 2010−06−07

Foundation item: Supported by Foundation of Educational Bureau of Liaoning Province (No.2008573)Biography: SHI Zhong-liang (1968−), male, native of Qingzhou City, Shandong Province, master, vice Prof., major in the fixation of photocatalytic nanomaterial

on porous materials; YAO Shu-hua, corresponding authors, E-mail: [email protected].

Preparation of TiO2 Nanoparticles Coated Cotton Fibers at

Low Temperature and Their Photocatalytic Activity

SHI Zhong-liang (石中亮), LU Chang-sui (卢昌岁), WANG Hai-bo (王海波),

PAN Yong-e (潘永娥), YAO Shu-hua (姚淑华)

(School of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang, Liaoning 110142, China)

Abstract: TiO2 nanoparticles coated cotton fiber composite was successfully prepared by using a sol−gel method at low

temperature (about 100℃) using tetrabutyl-titanate [Ti(OBu)4] as raw material. The preparation of the TiO2 colloid and the

composite were described. The properties of resulting materials were characterized by SEM and XRD, the photocatalytic

degradation performance was tested using methylene blue (MB) as the target pollutant in aqueous solution. The results

showed that the amorphous TiO2 nanoparticles were distributed evenly on the outer surfaces of cotton fibers, which shows

efficient photocatalytic properties when exposed to UV light, the degradation rate of MB reached 95.35% under the

conditions of catalyst dosage 2.5 g/L, MB concentration 50 mg/L, irradiation time 120 min, and pH 10, and the photocatalyticactivity of TiO2/cotton fibers remained above 90% of its activity as-prepared after being used four times, the degradation rate

of MB could reach 88.78% when irradiation time was 120 min. The photocatalytic degradation of MB could be properly

described by the first-order kinetic law. By comparison of the removal rates of MB with and without UV light, it could be

affirmed that the disappearance of MB was due to photodegradation rather than adsorption on cotton fibers.

Key words: TiO2 nanoparticles; cotton fibers; photocatalysis; methylene blue

CLC No.: TQ342 Document Code: A Article ID: 1009−606X(2010)04−0809−06

1 INTRODUCTION

The photocatalytic oxidation of hazardous organic

pollutants by semiconductor catalysts such as TiO2,ZnO and CdS has been extensively studied for the past

decades[1−5]

. Since crystalline titania, especially its

anatase phase, is a well-known material with noticeable

photocatalytic properties, such as strong oxidizing

power, nontoxicity and long-term photostability, it has

attracted much attention for its potential application in

degradation of various environmental pollutants in both

gaseous and liquid phases[6−10]

. However, in field

applications, there are at least two obvious problems

arising from using fine TiO2 powders: (1) separation of

photocatalyst from the reaction media is difficult, and (2) particulate suspensions are not easily applicable to

continuous process[11]

. Development of TiO2

photocatalysts anchored on supporting materials with

large specific surface areas, by which dilute polluted

substances could be condensed, would be of great

significance, not only to avoid the disadvantages of

filtration and suspension of fine photocatalyst particles,

but to lead to high photodecomposition efficiency[12]

.

Thus, the fixing stability of TiO2 film with its substrate

is considered as one of the most significant factors in its

practical applications. Stable and lasting fixation of

TiO2 film on its substrate will benefit photocatalytic

degradation processes both technically and

economically[13]

. Therefore, it is important to study andevaluate the fixing stability of deposited TiO2 film on a

substrate, which is concerned with optimizing

preparation procedures of the immobilized

photocatalysts and selection of a suitable substrate.

Recently, supported TiO2 catalysts on various heat

resistant porous materials, such as silica[14]

, alumina[15]

,

zeolites[16]

and activated carbon (AC)[17−19]

, were

prepared. Temperatures up to 500℃ will lead to

formation of anatase or anatase/rutile clusters or films

on porous material surfaces[20−22]

. But most of the

supports are still in the form of granules, the problem of separation and recovery of the photocatalyst from the

reaction media still exists. There are some reports

recently on the preparation of loaded TiO2 on cellulose

fibers and activated carbon fibers (ACFs)[23−25]

.

The present study addresses the coating of TiO2 on

fibers with poor heat resistance. Temperature up to

100℃ (exposing to boiling water) was employed for the

deposition of amorphous TiO2 on the cotton fibers. The

photocatalytic activity of amorphous TiO2-coated cotton

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810 过程工程学报第 10卷

fibers, prepared at low temperatures, was monitored by

measuring the photocatalytic degradation of adsorbed

MB. The aims of this study are: (a) to develop a simple

and repeatable anchoring procedure of the TiO2

nanophase to the cotton fibers, (b) to perform accurate

characterization of the structure and properties of the

composite obtained at low temperatures, (c) to test

photostability of the TiO2 film and of the supporting

cotton fibers upon prolonged exposure to UV light, (d)

to investigate the photodegradation of kinetics, and (e)

to examine the reuse of the TiO2-coated cotton fibers for

pollutant adsorption–photodegradation cycles.

2 EXPERIMENTAL

2.1 Reagents and Materials

All the chemicals used in the study were of

analytical grade without further purification. All thesolutions were prepared using de-ionized water. All

glassware was cleaned by rinsing with hydroxylamine

hydrochloride, soaking in 10% HCl solution, and

rinsing with de-ionized water.

Cotton fibers (industrial product), supplied by

Shenyang Cotton Fibers Mill, were pretreated by

immersing in acetone over 24 h to eliminate the natural

grease, pectine, lignine and stains associated with

untreated cotton fibers, and then dried in air.

2.2 Experimental Methods

2.2.1 Preparations of TiO2 colloid and TiO2/cotton fibersTiO2 colloid was prepared by a sol method at low

temperature using tetrabutyl-titanate [Ti(OBu)4, analytic

grade, Shanghai Xingta Co., Ltd, China] as precursor. A

solution was prepared as follows: Ti(OBu)4 (0.02 mol)

was added to anhydrous ethanol (50 mL) under vigorous

stirring condition and then triethylamine (0.01 mol) was

added as a stabilizer of the solution and stirred (200

r/min) for 2∼3 min under an inert environment. The

required inert environment was made by argon gas flow

through the system. A second solution was then

prepared separately as follows: hydrochloric acid (3.0mL) and water (0.72 mL) were added to anhydrous

ethanol (50 mL) and mixed well by a magnetic stirrer

(200 r/min). The two solutions were then mixed

together and stirred vigorously for 30 min under argon

gas flow. The formed TiO2 sol was transparent, quite

stable and highly sensitive to the amount of

triethylamine and water. For the impregnation, the

pretreated 4 cm×12 cm cotton fibers were immersed in

the formed TiO2 colloidal suspension and exchanged for

30 min, and then followed by exposing to boiling water

for 30 min. The exchange operation to load the TiO2 on

the cotton fibers was carried out immediately after the

pretreatment of cotton fibers. The cotton fibers samples

were then washed with de-ionized water to remove the

TiO2 particles that did not attach to the surface of cotton

fibers.

2.2.2 Characterization

The morphology of TiO2/cotton fibers and original

cotton fibers was examined by a scanning electron

microscope (SEM, JSM-5800, Philips, Holland). The

crystallographic phase of the TiO2 on the cotton fibers

sample was determined by a Rigaku D/max-r B X-ray

diffractometer using Cu K α radiation.

2.2.3 Measurement of photocatalytic efficiency

The photocatalytic activities of prepared

TiO2/cotton fibers were studied by the degradation

experiments using MB dye as model compound. The

experiments were carried out using a quartz reactor and

ultraviolet mercury lamp (300 W, 365 nm). The distance

between lamp tube and photocatalysis system was about

20 mm. An ordinary photocatalytic degradation test was

performed at ambient temperature of near 20℃. The

initial concentrations of MB were respectively fixed at

50 mg/L in the presence of experiments. A magnetic

stirrer was equipped at the bottom of the reactor to

achieve effective dispersion. The pH value of

suspension was adjusted either with dilute 0.1 mol/L

HCl or 0.1 mol/L NaOH. 60 min adsorption time in dark

condition was allowed before the start of photoreaction.

Then, samples of the suspension were withdrawn after a

definite time interval and filtered through 0.45 µm filter

paper. The filtrates were analyzed for residual MB

concentration using a UV−Vis 950 spectrophotometer

(Shanghai Analysis Company, China). To compare the

photocatalytic activity of TiO2/cotton fibers, the pure

TiO2 powder was also tested. The amount of TiO2

powder chosen was 1.0 g / L, which was adequate under

the conditions without disturbing the UV light entering

the reactor. The TiO2/cotton fibers sample was used

repeatedly, before the beginning of next cycle, the

remaining solution was replaced by fresh one.

3 RESULTS AND DISCUSSION

3.1 Characterization of Photocatalyst

The surface morphologies of original cotton fibers

and TiO2/cotton fibers were observed with SEM. The

SEM images (Fig.1) show that the surfaces of cotton

fibers after deposition of TiO2 were much rougher. The

images indicate that the coating of TiO2 particles is

basically evenly distributed on the outer surfaces of the

cotton fibers with scattered aggregates of TiO2.

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第 4期 SHI Zhong-liang, et al.: Preparation of TiO2 Nanoparticles Coated Cotton Fibers at Low Temperature and Their Photocatalytic Activity 811

(a) Original cotton fibers (b) TiO2 coated cotton fibers

Fig.1 SEM images of original cotton fibers and TiO2/cotton fibers

Figure 2 shows the XRD patterns of original cotton

fibers and prepared TiO2/cotton fibers before and after

photodegradation of MB for 120 min with UV light. No

crystalline phases of TiO2 were found in the spectra b

and c. It is interesting to note that the cotton fibers

loaded with amorphous TiO2 as shown by spectrum b

was able to degrade MB under UV light irradiation. No

difference is observed between spectra b and c, showing

the stability of the amorphous TiO2 film on the cotton

fibers during the photodegradation of MB.

Fig.2 XRD patterns of cotton fibers

3.2 Photocatalytic Degradation of MB

In order to evaluate the actual photocatalytic

activity of TiO2/cotton fibers, comparison of four MB

degradation processes, namely, photolysis of TiO2, pure

cotton fibers, TiO2/cotton fibers and adsorption of

TiO2/cotton fibers was carried out and experiments were

conducted to assess the effect of catalyst on the overall

MB degradation rate for an initial MB concentration

(C 0=50 mg/L), the results are shown in Fig.3.

The results show that the adsorption of MB on pure

cotton fibers gets saturated after 60 min under UV light

irradiation, the concentration of MB does not decrease

any more with prolonging of UV light irradiation time.

This indicates that the pure cotton fibers do not have

photocatalyst activity. Similarly, the TiO2/cotton fibers

show little photocatalytic activity without irradiation of

UV light, and the saturated adsorption capacity of

TiO2/cotton fibers is higher than that of the pure cotton

fibers. This is a result of the polarity of the cotton fibers

after the deposition of TiO2 film. The MB

concentrations decrease with UV light irradiation for the

MB/TiO2 and MB/TiO2/cotton fibers systems, but the

degradation rates of MB with TiO2/cotton fibers are

higher than that of TiO2. By comparison of the amounts

of MB removed with and without UV light, it can be

affirmed that the disappearance of MB molecules is due

to photocatalytic degradation instead of only to

adsorption. The cotton fibers are immersed in organic

solvents such as ethanol and acetone after drying in air,

the solution is colorless, which indicates that the MB is

photocatalytically degraded actually.

Fig.3 Effect of photocatalyst on degradation rate of MB

Experiments were conducted to assess the effect of

different initial catalyst dosages on the overall MB

degradation rate for an initial MB concentration (C 0=50

20 25 30 35 40 45

a. Original cotton fibersb. Cotton fibers loaded with TiO

2at irradiation time 0

c. Cotton fibers loaded with TiO2after 2 h irradiation

I n t e n s i t y ( C P S )

2θ (o)

a

b

c

0 30 60 90 1200

20

40

60

80

100 Initial MB concentration 50 mg/L

pH 10

Pure cotton fibers with UV irradiationTiO

2/cotton fibers without UV irradiation

TiO2with UV irradiation

TiO2/cotton fibers with UV irradiation C

/ C 0

( % )

Irradiation time (min)

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mg/L) with catalyst dosage varying from 1 to 3 g/L, the

results are displayed in Fig.4. It can be seen that the

increase in initial catalyst dosage from 1 to 2.5 g/L leads

to improvement of the photodegradation rate of MB.

Nevertheless, further increase in catalyst dosage does

not bring about the corresponding enhancement of the

degradation rate; on the contrary, it is reduced. In fact,

an optimized photocatalyst dosage should exist for the

highest photocatalytic degradation because much more

employment of the photocatalyst has made the

suspension too thick to assure enough light for the

photocatalysis, but much less one of that, there is not

sufficient TiO2 for the photocatalysis, hence the

degradation rate becomes smaller. The photocatalysis

curves of TiO2/cotton fibers in Fig.4 are consistent to

this analysis.

Fig.4 Effect of photocatalyst dosage on degradation

rate of MB

The degradation kinetics of MB was investigated

for an initial MB concentration of 50 mg/L and different

concentrations of TiO2/cotton fibers. Fig.4 shows that

the concentration of MB decreased exponentially during

irradiation. Mathematically these results can be

described by a first-order kinetic law:

Ln(C 0/C t )=kt , (1)

where C 0 is the initial concentration of MB, C t theconcentration of MB at the time t , k the observed

first-order rate constant.

According to Eq.(1), linear plot of lnC t versus

irradiation time is obtained (Fig.5), from which slopes k

can be estimated. For initial MB concentration of 50

mg/L, the values of k obtained from experimental data

are summarized in Table 1. The squared correlation

coefficient ( R2) for different conditions is larger than

0.98, indicating that the first-order kinetic law

successfully describes the degradation behavior of MB.

Fig.5 Degradation kinetics for different concentrations

of TiO2/cotton fibers

Table 1 Coefficients of first-order kinetic model fitting of

MB degradation kinetics

TiO2/cotton fibers conc. (g/L) 1.0 1.5 2.0 2.5 3.0

k (×10−2

min-1

) 1.01 1.64 2.23 2.96 2.52 R

20.993 0.997 0.995 0.984 0.995

The pH value of the solution is one of the most

important parameters in photocatalytic reactions. The

degradation of MB in the UV and TiO2/cotton fibers

system was conducted at different pH values. Fig.6

shows the degradation rate of MB as a function of pH at

irradiation time 120 min. It is evident that the

degradation rate of MB strongly depends on the medium

pH. Both acidic and alkaline conditions are favorable

for the degradation reaction, especially at pH<3 or

pH>11. Conversely, neutral condition is not favorablefor the degradation, especially at pH value around 7.0.

Fig.6 Effect of solution pH on degradation rate of MB

In semiconductor photocatalysis, photogenerated

holes (h+), electrons (e

−

), hydroxyl radicals (OH⋅),

superoxide ions (O2⋅−

) take part in redox reactions if

thermodynamically favorable[26,27]

. Under the conditions

of current study, they can be produced by the following

mechanistic pathways:

0 30 60 90 120

0

20

40

60

80

100 Initial MB concentration 50 mg/LpH 10

D e g r a d a t i o n r a t e o f M B ( % )


Photocatalyst dosage (g/L)1.0 1.52.0 2.53.0

0 20 40 60 80 100 120

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

l n C

t


TiO2/cotton fiber (g/L)

1.0 1.5

2.0 2.53.0

0 2 4 6 8 10 12

70

80

90

100Initial MB concentration 50 mg/L

photocatalyst dosage 2.5 g/L

irradiation time 120 min

D

e g r a d a t i o n r a t e o f M B ( % )

pH

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第 4期 SHI Zhong-liang, et al.: Preparation of TiO2 Nanoparticles Coated Cotton Fibers at Low Temperature and Their Photocatalytic Activity 813

TiO2 ⎯→ ⎯

hv

TiO2 (h++e−), (2)

h++OH−

→OH⋅, (3)

O2+e−→O2⋅−, (4)

O2⋅−

+e

−

+2H

+→

H2O2, (5)2O2⋅

−+2H+→H2O2+O2. (6)

At acidic condition, H+

can improve the generation

of H2O2 [Eqs.(5) and (6)], while at alkaline condition

the generation of hydroxyl radicals is improved [Eq.(3)].

The photoproduced H2O2 can also degrade MB rapidly

through indirectly producing hydroxyl radicals. The

solution pH values for 120 min irradiation are

investigated so as to better understand the

photocatalysis of MB. It is found that the solution pH

values increase for the samples whose initial pH valuesare less than 7, and decrease for those whose initial pH

values are above 7. The change of the solution pH

values after irradiation treatment can be partly attributed

to the consumption of H+

or OH−

during the

photocatalytic degradation of MB.

3.3 Cyclic Performance of TiO2 /Cotton Fibers for

Degradation of MB

In order to test the adhesion of TiO2 with cotton

fibers, the photodegradation experiments of MB were

repeated for 4 cycles, and the change in relative

degradation rate of MB with cycling operation is shownin Fig.7. It is observed that MB could be degraded

rapidly by the present photocatalyst under UV

irradiation. The photocatalytic reactivity of the present

photocatalyst was just slightly reduced in stirred

aqueous solution, and the photocatalytic activity of

TiO2/cotton fibers remained above 90% of their activity

as-prepared after being used 4 times, the degradation

rate of MB could reach 88.78% when irradiation time

lasted 120 min. Thus it is suggested that the deposited

TiO2 has firmly attached to the cotton fibers surface,

and can not be easily exfoliated from them withmechanically stirred solutions for a long period. At the

same time, it also proves that the final removal of MB

from solutions is caused by the photocatalytic

degradation other than the adsorption process that will

lead to saturated adsorption of MB on the photocatalyst.

These results indicate that cyclic usage of the

TiO2/cotton fibers composite is possible, and its stability

in treatment of polluted water is satisfactory. Therefore,

it is potentially employable for continuous

photocatalytic degradation processes.

Fig.7 Cyclic photocatalytic performance of TiO2/cotton fibers

4 CONCLUSIONS

(1) An amorphous TiO2/cotton fiber composite

photocatalyst, which can rapidly degrade MB under UV

light irradiation, has been successfully prepared by

sol−gel method at low temperature.

(2) The photocatalytic degradation of MB with

TiO2/cotton fibers as photocatalyst is possible, the

degradation rate of MB has reached 95.35% under the

conditions of catalyst dosage 2.5 g/L, MB concentration

50 mg/L, irradiation time 120 min, and solution pH

value 10, and that first-order kinetic law can

successfully describe the degradation behavior of MB.

(3) The TiO2/cotton fibers catalyzed reaction

significantly depends on the pH value of solution, both

acidic and alkaline conditions are favorable for the

degradation reaction, especially at pH<3 or pH>11.

(4) The composite photocatalyst can be used

repeatedly, and the high photocatalytic degradation

properties are maintained with a slight decline, the

degradation rate of MB can reach 88.78% after being

used 4 times. Therefore, the composite of TiO2 and

cotton fibers is a promising material for application in

degradation of MB.

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