Triblock copolymer-templated synthesis of porous TiO 2 and its photocatalytic activity

RESEARCH PAPER

Triblock copolymer-templated synthesis of porous TiO2

and its photocatalytic activity

Cheewita Suwanchawalit • Sumpun Wongnawa

Received: 3 September 2009 / Accepted: 12 February 2010 / Published online: 27 February 2010

� Springer Science+Business Media B.V. 2010

Abstract Mixed amorphous and anatase-type titania

particles were synthesized using non-ionic triblock

copolymer as surfactant template and TiOSO4 as

inorganic precursor through sol–gel process. The as-

prepared materials were characterized by X-ray

diffraction spectroscopy, scanning and transmission

electron microscopy, specific surface area, Fourier-

transformed infrared spectroscopy, and diffuse reflec-

tance ultraviolet–visible spectroscopy. The template

material could be easily removed by extracting with

dichloromethane and was confirmed by infrared

spectroscopy. X-ray diffraction pattern reveals the

crystalline part of as-prepared product as a framework

of anatase phase. From the N2 adsorption–desorption

analysis, the as-prepared sample has a surface area of

301 m2/g with pore size distribution narrowly cen-

tered around 6 nm. The photodegradation of indigo

carmine including kinetics, effect of pH, and recycla-

bility of the product were investigated. The photocat-

alytic results showed that the as-synthesized titania

could efficiently degrade indigo carmine under ultra-

violet irradiation and showed higher photocatalytic

activity than the commercial Degussa P25–TiO2.

Keywords Titanium dioxide � Sol–gel process �Non-ionic surfactant � Indigo carmine �Dye decolorization � Catalyst �Environmental remediation

Introduction

Photocatalytic oxidation with semiconducting mate-

rials has been accepted as a promising method for

purification and remediation of polluted water and air.

Of the semiconducting materials employed, TiO2 is

the most prevailing one because of its high photosen-

sitivity, non-toxicity, easy availability, environmen-

tally friendly, and low cost (Litter 1999; Fox and

Dulay 1993; Legrini et al. 1993; Carp et al. 2004). The

efficiency of the commercial Degussa P-25 TiO2 in

the treatment of exhaust gas and waste water

contaminated with organic and inorganic pollutants

has been fully proved. In order to maximize photo-

catalytic activity, TiO2 particles should be small

enough to offer a high number of active sizes per unit

mass (Zhang et al. 1998). Therefore, in most cases, the

samples are ultrafine powders and have large surface

area. However, their effective applications are

C. Suwanchawalit � S. Wongnawa (&)

Department of Chemistry and Center for Innovation

in Chemistry, Faculty of Science, Prince of Songkla

University, Hat Yai, Songkhla 90112, Thailand

e-mail: [email protected]

Present Address:C. Suwanchawalit

Department of Chemistry, Faculty of Science, Silpakorn

University, Sanam Chandra Palace Campus,

Nakornpatom 73000, Thailand

123

J Nanopart Res (2010) 12:2895–2906

DOI 10.1007/s11051-010-9880-y

hindered by two serious disadvantages. First, small

particles tend to agglomerate into large particle

causing reduced catalyst performance, second, the

separation and recovery of catalyst is difficult (Zhu

et al. 2000). For these reasons, many researchers have

focused on mesoporous TiO2 materials since they do

not only take advantage of high surface area but also

make the catalyst-recovering stage less troublesome

(Fu et al. 1996; Yang et al. 2006).

The advantages of the mesoporous TiO2 materials

stem from their high specific surface areas and pore

volumes, as well as narrow pore size distributions

which offer more active sites for catalytic reaction to

take place. The first synthesis of mesoporous titania

through modified sol–gel reactions in the presence of

alkyl phosphate surfactant as template was reported

in 1995 (Antonelli and Ying 1995). Since then many

efforts have been devoted to the synthesis of meso-

porous titania powders and films and some synthetic

approaches have been developed (Antonelli 1999;

Stone and Davis 1998; Zhang et al. 2000a, b; Zheng

et al. 2001; Zhao et al. 2004).

In most cases, ionic and neutral surfactants have

been employed as templates to direct the porous

structure based on the electrostatic and hydrogen-

bonding interactions, respectively. The use of ionic

surfactants presents limited potential applications

because of their strong interactions with titania walls

resulting in great difficulty to completely remove the

surfactant by extraction procedure. Furthermore, the

possible collapse of the inorganic structure may occur

when post synthesis thermal treatment is employed

for surfactant elimination. Thus, non-ionic surfactants

appeared to be a potential alternative given that

the hydrogen bonding mediates the formation of

the metal oxide-surfactant composites involved in the

inorganic framework organization (Calleja et al.

2004). Recently, inorganic block copolymers have

been studied extensively as structure-directing agents

in the synthesis of mesostructured materials (Yu et al.

2001).

There are several ways of preparing TiO2 mate-

rials such as sol–gel, hydrothermal, and microemul-

sion methods. Among them, the sol–gel process is the

most popular technique for preparing porous TiO2

powder (Zheng et al. 2001; Calleja et al. 2004; Yoo

et al. 2005; Zhao et al. 2004; Kluson et al. 2001). In

sol–gel synthesis, hydrolysis and condensation reac-

tions take place when the alkoxide precursor reacts

immediately in the presence of water leading to

formation of the larger and inhomogeneous product

(Wang et al. 2006a, b). The rate of hydrolysis

reaction, however, can be controlled through the

surfactant-mediated sol–gel process.

In this study, porous TiO2 was synthesized from

the sol–gel process using a non-ionic triblock

copolymer (PEO)20–(PPO)70–(PEO)20, or Pluronic

P123, as structure-directing agent and TiOSO4 as

precursor. The P123 template could be totally

removed, without calcination, by extraction with

dichloromethane. The synthesized porous TiO2 mate-

rial was characterized by various physical techniques.

Photocatalytic activity of sample was tested using

indigo carmine as a model pollutant and compared

with commercial Degussa P25–TiO2.

Experimental

Synthesis of the porous TiO2 sample

All reagents used were obtained from commercial

sources and used without further purification. The

titanium oxysulfate (TiOSO4�nH2O, Riedel-de Haen)

was used as starting material to produce porous

titanium dioxide particles by surfactant-mediated sol–

gel process. The triblock copolymer HO(CH2CH2)20

(CH2CH(CH3)O)70(CH2CH2O)20H (or PEO20PPO70-

PEO20, Mav = 5800, Aldrich, designated as Pluronic

P123) was used as templating agent. The Pluronic

P123 structure and its micelle are shown in Fig. 1.

Fig. 1 The pluronic P123 molecular structure and its micelle

structure. (Adapted from Wang et al. 2006a, b; Lettow et al.

2005)

2896 J Nanopart Res (2010) 12:2895–2906

123

In a typical preparation, a 0.4-g/L Pluronic P123

aqueous solution was mixed with 3 M TiOSO4

aqueous solution with stirring followed by refluxing

at 90 �C for 1 h. The resulting solution was then

treated with ammonia solution until the pH was 7 and

maintained at the same temperature for a day. The

white precipitate formed was extracted with dichlo-

romethane twice then filtered and washed with

distilled water until no sulfate ion was found by

BaCl2 solution test (0.1 M BaCl2). The washed

samples were dried at 105 �C for a day and ground

to fine powder to give final products designated as

P123–TiO2.

Characterization of product

The crystallization and phase formation of the

resulting TiO2 samples were studied with the Philips

PW 3710 powder diffractometer (PHILIPS X’Pert

MPD, the Netherlands) using Cu Ka radiation and

equipped with a Ni filter in the range 5–90� 2h. The

specific surface area and pore size distribution were

characterized by analyzing the N2 adsorption iso-

therms obtained at 77 K using Autosorb-1 (Quanta-

chrome Instruments, U.S.A.) equipment. The particle

morphologies were investigated by using a scanning

electron micrometer JSM-5800 LV, and the trans-

mission electron micrometer JEOL JSM 2010 oper-

ating at 160 kV (Jeol Apparatus, Japan). The TEM

sample was prepared by dipping an aqueous suspen-

sion of sample powders on a Formvar-coated copper

grid and dried at room temperature. The Fourier-

transformed infrared (FT-IR) spectra were recorded

on a Perkin–Elmer Spectrum Bx (Perkin Elmer,

U.S.A.) spectrophotometer in the range 400–

4,000 cm-1 using the pellets of the samples mixed

with KBr. The diffuse reflectance spectra (DRS) were

determined using Shimadzu UV-2401 spectropho-

tometer (Shimadzu, Japan) with BaSO4 as a refer-

ence. The band gap energies (Eg) of the catalyst were

then calculated according to the equation:

Eg ¼ hc

kð1Þ

where Eg is the band gap energy (eV), h is the

Planck’s constant, c is the light velocity (m/s), and kis the threshold wavelength (nm) being read out from

DRS spectra.

Evaluation of photocatalytic activity

of the synthesized porous-TiO2 sample

All the experiments were carried out in a closed

reaction compartment (0.9 m 9 0.9 m 9 0.9 m).

Indigo carmine dye (IC, C16H8N2Na2O8S2, Fluka)

was selected as a model for the photocatalytic

degradation experiments because it is a nonvolatile

and common contaminant in the industrial wastewa-

ters. The structure of IC is illustrated in Fig. 2. The

experiments were performed by using 0.05 g TiO2

sample dispersed in 50 mL of 5.0 9 10-5 M indigo

carmine solution. Prior to the illumination, the

suspension was stirred for 1 h to allow the adsorption

equilibrium of dye onto the surface of TiO2 sample.

Then, the mixture was irradiated using black light

fluorescent tubes (kmax = 366 nm) (Randorn et al.

2004). In all studies, the mixture was magnetically

stirred, before and during illumination. At given

irradiation time intervals (every 15 min), 4 mL of the

sample was collected and centrifuged to separate

TiO2 powders. The residual concentration of IC was

monitored by the change in absorbance of dyes at

610 nm using UV–Vis spectrophotometer. A test

with commercial TiO2, P25, was also conducted

under the same experimental conditions. Controlled

experiment either without light or TiO2 was per-

formed to ensure that degradation of the dye was

dependent on the presence of light and TiO2. The

disappearance of IC was analyzed by UV–Vis

Fig. 2 The molecular structure and characteristic absorption

spectrum of indigo carmine

J Nanopart Res (2010) 12:2895–2906 2897

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spectrophotometer (Specord S100, Analytik Jena

GmbH, Germany) over the 200–800 nm range.

Calibration plots based on Beer–Lamberts law were

established relating the absorbance to the concentra-

tion. Removal efficiency (%) of IC was determined

by applying the following equation

Removal efficiency ¼ C0 � C

C0

� 100 ð2Þ

where C0 is the original indigo carmine (IC) content

and C is the retained IC in solution.

Results and discussion

Formation mechanism of the porous TiO2 sample

The porous TiO2 sample was obtained for a range of

reaction conditions produced by varying the concen-

tration of surfactant, source of Ti precursor, acid

catalyst, and the reaction temperature. In this study,

the inorganic TiOSO4 was used as Ti precursor and

triblock copolymer Pluronic P123 was used as

templating agent. It is well known that the surfactant

organization into micelles takes place when the

critical micelle concentration (cmc) is reached. The

cmc of Pluronic P123 is 0.4 g/L at 22 �C in aqueous

solution (Su and Liu 2003). After cmc has been

reached, the triblock copolymer forms micelles in

aqueous solution. When the precursor TiOSO4 aque-

ous solution is added in micelle solution, TiO2

particles can only form at PEO block sites because

only the PEO block is soluble in the aqueous solution

(Zhao et al. 2004; Kim and Kwak 2007). It has been

known that the interactions between inorganic skel-

eton and the hydrophilic (PEO) moieties based on the

hydrogen bonding lead to mesostructure formation.

After the pH of mixed P123 micelles and TiOSO4

aqueous solution is adjusted with NH4OH solution,

further hydrolysis and condensation of the titanium

precursor lead to the growth of larger TiO2 particles

which slowly aggregate together. As the TiO2

particles aggregate further, the TiO2 particles become

microscopic, and form porous/mesoporous TiO2

particles with nearly spherical shapes. At this stage,

it is supposed that micelle-surrounded porous TiO2

help the formation of nearly spherical shape. Then,

the product was extracted with dichloromethane to

remove the organic template. After solvent extrac-

tion, porous TiO2 particles with nearly spherical

shapes are obtained. The formation mechanism of the

mesostructured TiO2 sample, adapted from Kim and

Kwak (2007) is shown schematically in Fig. 3. The

TEM image in Fig. 8b shows that the porous TiO2 is

Fig. 3 Possible mechanism

for the formation of

mesoporous TiO2 particles.

(Adapted from Kim and

Kwak 2007)

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composed of TiO2 particles which confirms this

mechanism.

Phase structure, band gap energy, surface

morphology, and pore structure of sample

The XRD patterns of the as-prepared P123–TiO2 and

P25–TiO2 are shown in Fig. 4. The XRD pattern at

2h = 25.5 (101) in the spectrum of TiO2 is identified

as the crystal of the anatase form (denoted as ‘‘A’’),

whereas the peak at 2h = 27.5 (110) arises from the

crystal of the rutile form (denoted as ‘‘R’’). As shown

in Fig. 4, the synthesized P123–TiO2 exists in anatase

phase while P25–TiO2 exists in both anatase (80%)

and rutile (20%) phases. The average crystallite sizes

of anatase and rutile in the sample were calculated by

applying the Debye–Scherrer formula,

D ¼ jkb cos h

ð3Þ

where D is the average crystallite size in angstroms, jis a constant which is usually taken as 0.89, k is the

wavelength of the X-ray radiation (Cu Ka =

0.15406 nm), b is the corrected band broadening (full

width at half-maximum (FWHM)) after subtraction of

equipment broadening, and h is the diffraction angle

(Yan et al. 2005). The average crystallite size of the

synthesized P123–TiO2 is 18 nm while for anatase

and rutile phases in P25–TiO2 they are 24 and 33 nm,

respectively. The anatase peak of P123–TiO2 in

Fig. 4, however, is slightly broad indicative of not

well grown crystalline product. Since the product has

not been calcined, therefore, amorphous form must be

present in this sample. In order to find the content of

anatase and amorphous in P123–TiO2 sample, the

standard addition in connection with the XRD peak

area were employed as previously described (Kanna

and Wongnawa 2008). The result reveals that the

P123–TiO2 has 23% of anatase phase with the rest

being an amorphous phase. The high content of

amorphous phase in P123–TiO2 sample might be

related to the condensation reaction that started before

the completion of hydrolysis reaction (Zhang et al.

2002). At pH ca. 7, the precipitation occurred quite

rapidly resulting in low crystallinity, hence, the

precipitate mostly appeared in this study was mixtures

of dominant amorphous TiO2, and smaller amount of

anatase phase. This formation mechanism behavior

had been earlier mentioned in other reports (Gopal

et al. 1997; Wang et al. 1992).

The nitrogen adsorption isotherm and pore size

distribution of the as-synthesized TiO2 sample are

shown in Fig. 5. The isotherm (Fig. 5a) is of the type

IV in the IUPAC classification with a hysteresis loop

indicating the presence of mesoporous structure. The

pore size distribution (Fig. 5b) was analyzed by using

BJH method for the desorption branch revealing the

average pore diameter about 5 nm. The BET surface

area and pore volume of sample are 301 m2/g and

0.365 cm3/g, respectively.

The mixing-in of amorphous seems not sufficiently

strong to alter these characteristic isotherm and shape

of pore size distribution. (The corresponding graphical

data of pure amorphous TiO2 also are available in our

lab. They have their own characteristic appearances

which are totally different from those of the mesopor-

ous P123–TiO2 displayed in Fig. 5). The fact that

P123–TiO2 has high specific surface area which is

higher than that of the commercial one presumably due

to two factors; (i) from its porous/mesoporous nature,

and (ii) the presence of amorphous phase mixing in.

Figure 6 shows the DRS spectra of the TiO2

samples. The strong broad band from 200 to 380 nm

indicates the existence of Ti-skeleton in the structure

of the sample (Dai et al. 2002). The absorption edge

wavelengths are obtained by the linear extrapolation

of the steep part of the spectra toward the baselines

and the band gap energies can be calculated from

Eq. 1 as 3.18 and 3.16 eV for P25–TiO2 and P123–

TiO2 samples, respectively.

The morphology of the as-prepared P123–TiO2 was

investigated by SEM and TEM techniques as shown in

Fig. 7. The SEM image reveals agglomeration of near-

Fig. 4 XRD patterns of TiO2 samples: a commercial Degussa

P25–TiO2, b P123–TiO2

J Nanopart Res (2010) 12:2895–2906 2899

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spherical shape TiO2 particles. The TEM image shows

aggregation of anatase crystallites and amorphous

powder to form larger particles. The crystallite size

estimated from this technique is ca. 5–7 nm. The

higher magnification of crystalline anatase and amor-

phous phases from selected region in Fig. 7d is shown

in Fig. 7c where lines of (101) lattice fringes of anatase

nanocrystals are clearly seen on the left. The aggrega-

tion appears not very tight, from the TEM image,

probably due to the fact that it is the mixture of

amorphous and mesostructured anatase. This loose

aggregation, however, facilitates many void spaces

within the aggregations. The pore channels were found

near the surface of sample which are helpful for

chemical reactions to take place in the channels.

Removal of the template

When a non-ionic surfactant is used as a template, it

interacts with the inorganic skeleton through hydrogen

bonding, a weaker force between molecules than

statically electric force, so it is possible to remove it

through a mild way such as solvent extraction instead

of calcination at high temperature (Wang et al. 2006a,

b). In removing the template, the pores of the

mesostructures open up and become active sites to

adsorb the reactant molecules. Template removal by

solvent extraction has a significant structural advan-

tage, because it avoids the local damage that might

occur during the template removal by calcination

(Zhao et al. 2004). In this investigation, the Pluronic

P123 was removed by solvent extraction with dichlo-

romethane as extractant. The removal of P123 was

followed by FT-IR technique and the spectra are shown

in Fig. 8. Comparing Figs. 8a and c, the strong infrared

absorption bands around 2,800–3,000 cm-1 and

1,150–1,400 cm-1 completely disappeared. These

bands are assigned to the C–H and C–O–C stretching

vibrations of P123, respectively (Zhao et al. 2004;

Alapi et al. 2006). For the IR spectra of TiO2 samples

(Figs. 8a, b), the vibration bands at 3,300 cm-1

Fig. 5 N2 adsorption isotherm (a) and pore size distribution

(b) of P123–TiO2 sample

Fig. 6 DRS spectrum of TiO2 samples: a commercial Degussa

P25–TiO2, b P123–TiO2

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(Ti–OH, mTi–OH) and 1,630 cm-1 (HOH, mHOH) are

characteristics of the –OH group adsorbed to the

surface of TiO2 (Braconnier et al. 2009). In addition,

the band in the region 960–400 cm-1 which is the

characteristic vibrational modes of TiO2 (mTi–O,

stretching mode of Ti–O bond) is also detected (Zhang

et al. 2002). Thus, spectrum in Fig. 8a confirms that the

template P123 has been completely removed by

dichloromethane extraction.

Photocatalytic activity

Indigo carmine (IC), which belongs to acidic dye

group, was employed in this study to assess the

photocatalytic activity of the as-prepared porous TiO2

sample. In the dye photodegradation process, there

are two factors operating to decrease the concentra-

tion of dye: the adsorption of dye onto the surface of

photocatalyst, and the photooxidation of dye by

Fig. 7 The morphology of:

a SEM, b TEM image

(9120 K magnification),

c portion of inset from (d),and d TEM image (9300 K

magnification) of the as-

prepared P123–TiO2 sample

J Nanopart Res (2010) 12:2895–2906 2901

123

photocatalyst. The commercial reference material,

Degussa P25–TiO2, was used to compare the effi-

ciency of decolorization of IC solution with the as-

prepared TiO2 sample. The amount of catalyst was

1.0 g/L (or 0.05 g in 50 mL) and the dye concentra-

tion was 5.0 9 10-5 M. The photocatalytic results

are shown in Fig. 9 which we can see that the as-

prepared P123–TiO2 sample shows higher photocat-

alytic efficiency than commercial Degussa P25–TiO2.

For the kinetics study, the photodegradation of dye

has been established to follow the Langmuir–Hin-

shelwood first order rate law which has the simplified

form as (Houas et al. 2001; Prevot et al. 2001):

lnC0

C

� �¼ k � t: ð4Þ

The plots of ln (C0/C) versus time yielded straight

lines for all TiO2 samples indicating the degradation of

IC is a first order process (Fig. 10). The rate constants of

P123–TiO2 and P25–TiO2 are 9.65 9 10-2 and

5.98 9 10-2 min-1, with R2 0.9794 and 0.9618,

respectively. Compared with P25–TiO2, the porous

P123–TiO2 sample exhibited a significantly faster

degradation rate for indigo carmine.

With regards to the mechanism of photocatalytic

decomposition of dye by TiO2, the well established

route starting with excitation of bulk TiO2 with UV

light to produce the electron–hole pair all the ways to

creation of hydroxyl radical (•OH) can be summa-

rized as follows (Houas et al. 2001; Konstantinou and

Albanis 2004; Bandara et al. 1999; Daneshvar et al.

2003; Gouvea et al. 2000):

TiO2 þ hm ! TiO2 ðe�CB þ hþVBÞ ð5Þ

hþVB þ e�CB ! heat recombinationð Þ ð6Þ

hþVB þ H2OðadsÞ ! Hþ þ �OHðadsÞ ð7Þ

hþVB þ OH�adsð Þ ! �OHðadsÞ ð8Þ

e�CB þ O2ðadsÞ ! O�2 ð9Þ

O�2 þ Hþ ! �O2H ð10Þ�O2H þ �O2H ! H2O2 þ O2 ð11ÞH2O2 þ hm ! 2�OH ð12ÞH2O þ e�CB ! OH� þ �OH ð13Þ�OH þ dye! degradation of dye ð14Þ

hþVB þ dye ! degradation of dye: ð15ÞBeing formed in the series of these reactions, the

hydroxyl radicals are the powerful oxidant to destroy

the dye molecules. The as-prepared porous TiO2

shows higher photocatalytic efficiency than the

commercial P25–TiO2 may be attributed to the much

higher surface area exhibited by P123–TiO2. The

source of high surface area comes from the combin-

ing effect of having both amorphous form and

mesoporous-structured anatase form. The presence

of the latter leads to formation of mesoporous

channels within the aggregations which helps accel-

erate the reaction rate as the substances diffuse

through the channels more contact with catalyst

Fig. 8 FT-IR spectra of TiO2 samples: a P123–TiO2, b P25–

TiO2, c P123 template

Fig. 9 The efficiencies of decolorization of IC solution

(5.0 9 10-5 M) under UV light irradiation by TiO2 samples

(1.0 g/L)

2902 J Nanopart Res (2010) 12:2895–2906

123

surface can take place, hence, more chances for the

reactions to occur.

From the discussion above, we have shown that

P123–TiO2 is a mixture of amorphous TiO2 and

anatase. The amorphous form has been known for its

negligible photocatalytic activity (Ohtani et al. 1997;

Kanna et al. 2010). On the other hand, the amorphous

form has high BET surface area and high adsorptiv-

ity. The source of photocatalytic activity of P123–

TiO2 could come from the combined effect of the

anatase fraction present in the sample plus the high

amorphous surface area and adsorptivity.

As the charge of the IC molecules and the surface

of the TiO2 photocatalyst are both pH dependent, we

studied the influence of pH on the degradation of dye

in the pH range 3 to 11 including the natural pH of IC

solution at 6.4. The pH was adjusted by adding

aqueous solution of either HCl or NaOH, respec-

tively. Figures 11 and 12 show the effect of pH on the

adsorption of dye on the surface of TiO2 catalyst and

the photodegradation of dye in an aqueous TiO2

suspension. It is well known that pH would influence

both the surface state of titania and the ionization

state of ionizable dye molecules. The point of zero

charge (pzc) of the TiO2 (Degussa P25) is ca. 6.8

(Konstantinou and Albanis 2004). Thus, the TiO2

surface is positively charged in acidic media

(pH \ 6.8), whereas it is negatively charged under

alkaline condition (pH [ 6.8), according to the

following reactions (Wen et al. 2005):

pH\pzc : Ti�OH þ Hþ ! TiOHþ2 ð16ÞpH [ pzc : Ti�OH þ OH� ! TiO� þ H2O:

ð17Þ

Since the parent fragment of IC bears negative

charge, the adsorption on a positively charged surface

of TiO2 is favored at low pH. Increasing the pH

caused the surface of TiO2 becoming less positive or

even turning to negative once the pH exceeded pzc.

Hence, we expect the repulsive force between the two

negative charges of the dye parent fragment and the

surface charge of catalyst to operate stronger at high

pH, therefore, less adsorption of dye onto the TiO2

surface. This fact is borne out as the adsorption trend

from pH 3 to 11 of P123–TiO2 sample gradually

decreased in adsorption capacity while P25–TiO2

showed faster decrease in adsorption capacity as the

pH was increased as shown in Fig. 11a. For photo-

catalytic decomposition, the results are shown in

Fig. 11b. Both TiO2 samples showed high decompo-

sition rate across the pH range under investigation,

indicating that both TiO2 samples can be used in

widely varied pH conditions. At high pH, the number

of hydroxide ion (OH-) is more abundance which, in

turn, helps produce more hydroxyl radical (•OH) by

reacting with the positive hole (h?) as indicated by

Eq. 8. The high concentration of •OH radical offset

the decreasing adsorptivity at high pH, hence, the

high photodegradation efficiencies remained almost

unchanged over wide pH range.

To be useful from the industrial point of view, the

catalyst should be recyclable to reduce the operation

cost. In order to determine the recyclability of the

prepared P123–TiO2 sample, the used TiO2 samples

were separated from the suspension by gravity

sedimentation and used in the next runs without any

treatment. It was observed during the separation that

the porous P123–TiO2 sample could settle to the

bottom faster than P25–TiO2 due to the highly

aggregated nature of P123–TiO2 sample. The recycle

results in Fig. 12 show that after seven uses, the

activity of the as-prepared porous TiO2 sample was

retained to almost the same activity level as that in

the first use, i.e., almost no decline in efficiency were

observed. In the eighth use, the lowering in efficiency

began to show up with only 80% decomposition.

However, if the irradiation time was extended, from

60 to 90 min, the complete (100%) decomposition

would result again for the eighth run. This means, the

P123–TiO2 can be brought back for reuse many times

with some allowance of irradiation time appropriately

adjusted. On the other hand, for P25–TiO2 the

photocatalytic activity gradually decreased until only

56% of IC was decomposed after the eighth run. In

this case, for complete degradation, the radiation time

Fig. 10 The kinetics of disappearance of IC under UV

irradiation by TiO2 samples (1.0 g/L)

J Nanopart Res (2010) 12:2895–2906 2903

123

had to be extended to 180 min. The white P123–TiO2

catalyst turned to pale yellow after the first use. The

pale yellow that occurred was due to the organic

compound chemisorbed on the active sites of TiO2

surface (Cao et al. 1999).

Conclusions

Porous titania has been prepared by sol–gel process

using triblock copolymer (Pluronic P123) as template.

The mixed amorphous–anatase titania product was

obtained after the template was removed by solvent

extraction. The obtained porous TiO2 has crystallite

size of 18 nm and high specific surface area of

301 m2/g. The as-prepared porous TiO2 showed

higher photocatalytic activity than P25 as a result of

an enlarged surface area and porous structure. The

described method is easily controlled and environ-

mental friendly which should provide an effective

way for the preparation of porous titania powders. The

ease of preparation combined with its high photocat-

alytic activity may place it among the good candidate

for applications in catalysis, biomaterials, microelec-

tronics, optoelectronics, and photonics.

Acknowledgments This research is supported by the Thailand

Research Fund through the Royal Golden Jubilee Ph.D. Program

(Grant No.PHD/0197/2548), the Center for Innovation in

Chemistry (PERCH-CIC), Commission on Higher Education,

Ministry of Education, and the Graduate School of Prince of

Songkla University, Thailand. Sample of Degussa P25–TiO2

used throughout this study was donated by Degussa AG,

Frankfurt, Germany, through its agency in Bangkok, Thailand.

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