Microporous and Mesoporous Materials 179 (2013) 111–121

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Microporous and Mesoporous Materials

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Enhancement of porosity of sodium silicate and titanium oxychloridebased TiO2–SiO2 systems synthesized by sol–gel process and theirphotocatalytic activity

1387-1811/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.micromeso.2013.05.021

⇑ Corresponding author. Tel.: +82 31 400 5274; fax: +82 31 419 7203.E-mail addresses: khtaik@yahoo.com, khtaik@hanyang.ac.kr (H.T. Kim).

Godlisten N. Shao a,b, Yuna Kim a, S.M. Imran a, Sun Jeong Jeon a, Pradip B. Sarawade a, Askwar Hilonga c,Jong-Kil Kim d, Hee Taik Kim a,⇑a Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Koreab Department of Chemistry, Mkwawa College, University of Dar es Salaam, Iringa, United Republic of Tanzaniac Nelson Mandela African Institute of Science and Technology in Arusha, Arusha, United Republic of Tanzaniad E&B Nanotech. Co., Ltd., Sangnok-gu, Ansan-si, Gyeonggi-do, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 November 2012Received in revised form 21 May 2013Accepted 24 May 2013Available online 4 June 2013

Keywords:PhotocatalysisPorositySodium silicateTiOCl2

The textural properties of TiO2–SiO2 composites (TSCs) were successively enhanced using threeapproaches; (1) washing the hydrogels with different solvents, (2) using surfactant and (3) formingthe TiO2 sol in ethanol medium. The sol–gel process was exquisitely used to form the composites usingcost effective precursors. Initially, the precipitated hydrogels were washed with water or alcohol to eval-uate the influence of washing on the dried hydrogels. Consequently, two composites were formed differ-ently in the presence of stearic acid (SA) as a surfactant and the other by forming TiO2 sol in ethanolmedium prior to reaction with silica source. The TSC powders were examined by XRD, N2 physisorptionstudies, FTIR, TGA, SEM, XRF and HRTEM. The BET surface area of the sample obtained after washing thehydrogels with ethanol (TSCE) was the largest (594 m2/g) while porosities of the composites obtainedusing stearic acid as a surfactant (TSCSA, 0.96 cm3/g) and ethanol as a medium to form the TiO2 sol(TSCES, 1.85 cm3/g) were relatively superior to those obtained under influence of changing washing sol-vent. Photocatalytic decolorization of methylene blue by the composites calcined at 800 �C revealed thatthe TSCES-800 possessed the highest activity of all the composites due to its superior properties.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) is the most important white pigmentused widely in the paint, papermaking, cosmetic, and pharmaceu-tical industries due to its comprehensive physico-chemical proper-ties [1]. Its biological and chemical inertness, non-toxicity and highoxidative power are the key elements portraying its usability in thephotocatalytic degradation of numerous organic pollutants [2]. Re-cently, TiO2–SiO2 composites have extensively been investigatedbecause of their unique mechanical strength and thermal stabilitymaking them appropriate resources as filler in the paper industry[3], heterogeneous catalysts [4], optical wave guides [5], paint [2]and dielectric materials [6]. Therefore, facile and economical syn-thetic methods to produce composites with desired properties(surface area and porosity) would bring a significant advancementin the catalytic systems and devices. Indeed, further studies shouldfocus on reducing the knowledge gap and improving the quality ofthe composites while embracing reduction of production cost and

pollution. In order to attain these appreciable properties, severalconsiderations related to purity, textural improvement, controlledagglomeration and better crystallinity are essential. Therefore, pro-duction of TiO2–SiO2 composites with superior properties can lar-gely be attained through consuming appropriate and cheapprecursors, controllable and well established synthetic routes.

Typically, synthesis of TiO2–SiO2 composite with high BET sur-face area, large pore volume and pore diameter favors its catalyticperformance in various chemical processes [7]. Most of researchers[8,9] reported that TiO2–SiO2 composite can be achieved hydro-thermally using highly pure and expensive alkoxide precursors(tetrabutylorthotitanate, TBOT and tetraethylorthosilicate, TEOSor tetramethylorthosilicate, TMOS) and perhaps requires consump-tion of templates. Surfactants such as cetyltrimethylammoniumbromide (CTAB), polyethylene oxide (PEO) and triblock copolymerP123 are customarily employed as structure-directing agents inthe synthesis process [10–12]. Stearic acid is a common surfactantused in the synthesis of metal oxides to control phase, particle sizeand distribution [13,14]. The synthesis of TiO2–SiO2 compositesusing titanium oxychloride (TiOCl2) solution as a titania sourceand sodium silicate as a silica source yields a highly homogeneous

112 G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121

product with high surface area [7,10,11]. Predominantly, highlypure composites with an even distribution of primary particlesand high TiO2 loadings can be obtained as a result of controllableand rapid reaction between TiO2 sol with silanols [7]. Additionally,TiOCl2 is easy to handle because the dimerization behaviorobserved during consumption of pure TiCl4 can largely be avoidedand the reaction time can also be reduced as well [15–17]. How-ever, the final product possesses small pore volume and pore sizewhich limits its usability.

In our previous studies [16–18], we introduced convenientroutes to synthesis of highly pure mesoporous titania–silica com-posite from low cost sources in the absence of surfactants. How-ever, versatile and improved methods are invariably needed toprovide flexibility and expediency in the large-scale industrial pro-duction of composites from these less expensive sources for a myr-iad of applications. Washing of metal oxide hydrogels with solventof lower surface tension than water is one of the widespread tech-niques to improve porosities [19–23]. For instance, washing metaloxide hydrogels with ethanol influences phase transformation andstabilization, increase surface area and pore volume ([22] and ref-erences therein). Various literatures [22,23] reported that ethanol-washing facilitates rapid anatase to rutile phase transformationdue to the difference in physical states such as agglomerationand particle size. Youn et al. [20] and Ha et al. [23] observed that,alcohol rinsing lowers the crystallization temperature for theamorphous-to-anatase or anatase-to-rutile transformation of TiO2

powders. It was suggested that the presence of the residual organ-ics in the alcohol-washed samples are responsible for low-temper-ature formation of the TiO2 phases through acting as nucleationsites [23,24]. On that stand, it is obvious that using alcohols inthe synthesis of metal oxide; either as a washing solvent or mediahas a profound influence on the physico-chemical properties of theprecipitated powder. Therefore, using alcohols can be influential incontrolling the physico-chemical properties leading to formationof materials with desired properties necessary for variousapplications.

The present study proposes controllable and reproducible ap-proaches to improve textural properties of TiO2–SiO2 compositessynthesized using sodium silicate (cheap source) and titaniumoxychloride as silica and titania precursors respectively. Firstly,the influence of washing hydrogels using water and alcohol wasassessed. The study further evaluates the role of using stearic acid(surfactant) in the synthesis of TiO2–SiO2 powders and ultimatelyinvestigates the physico-chemical properties of the powders ob-tained by using a TiO2 sol formed in ethanol medium prior to reac-tion with the silica source. The photocatalytic efficiencies of thecomposites were tested in the decolorization of methylene bluefor practical quantification of the effectiveness of the compositesin the degradation of dyes which are common pollutants in thetextile industries. Even though there are a significant number ofpublications that report the effect of washing the precipitated me-tal oxides with alcohols [20–24], no scrupulous works have re-ported successive approaches to improve textural properties ofTiO2–SiO2 composites whilst consuming low-cost silica precursor.Moreover, we think that this could be a rapid approach to yieldcomposites with desired properties (large porosities and high sur-face areas) by a straightforward sol–gel method through alterationof solvents as washing agents or medium to form the Titania sol.

2. Experimental method

2.1. Materials

Sodium silicate (SiO2/Na2O = 3.49) and 25 wt.% TiOCl2 were pro-cured from Kukdong Chemicals Co., Ltd., South Korea. Terephthalic

acid (TA), methylene blue and 60% HNO3 was purchased fromSigma–Aldrich. 28% ammonium hydroxide solution was boughtfrom Dae-Jung Chemical and Metal Co., Ltd., South Korea. Ethanol(94%) was bought from Duksan Pure Chemicals Ltd., South Korea.Methanol (99.8%) was bought from Samchun Pure Chemicals Co.,Ltd., South Korea. These chemical reagents were used as acquiredfrom their commercial sources without further purification.Amberlite ion-exchange resin (IR-120, pH = 2.4) was procured fromSigma–Aldrich, Rhom & Haas, Germany and washed with deion-ized (DI) water three times before use.

2.2. Synthesis of the composites

The substitution of Na+ of the sodium silicate solution by H+

using ion exchange resin yields silicic acid (pH�2.4) with pH lowenough to peptize the preformed Ti(OH)4 gel to form a stable solafter copious stirring. Nevertheless, acid is needed to catalyze thereaction and also pH adjustment must be accomplished using basesolution to attain a desirable pH to induce simultaneous hydrolysisand precipitation to form composites.

2.2.1. Washing routesGenerally, TiO2–SiO2 composites were synthesized under the

procedure reported in our previous work [17]. In a typical experi-ment, 11 g of sodium silicate was diluted with DI water to obtain40 g and then passed in a column containing ion exchange resinto obtain 6.3 wt.% silicic acid (pH�2.4). In the other part, 17 mlof TiOCl2 in 23 ml of DI water was stirred for 20 min followed bycondensation with 12 ml of ammonium hydroxide. The silicic acidwas gently added into titanium hydroxide gel under vigorous stir-ring and the solution was stirred for 30 min to obtain a stable sol(TiO2–SiO2 sol) when 20 ml of 1 M HNO3 was added to catalyzethe reaction. The mixture was stirred for another 30 min whenthe pH was adjusted by addition of ammonium hydroxide to apH 5–6. The temperature of the reactor was raised from room tem-perature and kept at 80 �C for 4 h for aging under constant stirring.After aging, the product was filtered off and washed with 400 ml ofwater. It was then dried at a temperature of 100 �C for 6 h andlabeled TSCW (for titania–silica composite washed with water).The same experimental procedures were followed to obtain twoseparate samples where in this case precipitates were washed with400 ml of water and then 200 ml of either methanol or ethanol. Thesample washed with methanol (M) or ethanol (E) was named TSCMand TSCE, respectively. The filtrates (where alcohols were used)were collected and stored for future applications – after beingdistilled or purified. The samples were dried at a temperature of100 �C for 6 h followed by calcination at temperatures rangingfrom 600 to 1000 �C. The corresponding calcined samples weredubbed TSCX-calcination temperature (X = water, methanol orethanol). For example the sample washed with methanol andcalcined at 800 �C was named TSCM-800.

2.2.2. Influence of surfactantThe same procedure was performed to obtain TiO2–SiO2 sol as

exemplified in Section 2.2.1. 20 ml of 1 M HNO3 was then addedinto Titania-silica sol and the mixture stirred for 30 min whenthe pH was adjusted by addition of ammonium hydroxide to apH 5–6. The reactor was charged with 3.4 g of stearic acid (SA)and then the temperature of the reaction mixture was elevatedfrom room temperature and kept at 90 �C for 4 h under constantstirring. After 4 h the reaction mixture was cooled to room temper-ature and the product filtered off, washed with water and finallyrinsed with ethanol. The product was dried at 100 �C for 6 h thenheated at different temperatures ranging from 450 to 1000 �C for2 h to remove the surfactant. The thermal derived samples weredubbed TSCSA-calcination temperature.

Table 2Representative XRF results of the titania–silica composites.

Mass% Sample

TSCW TSCSA-450 TSCES

Si 16.62 19.15 15.16Ti 83.38 80.85 84.84Ti/Si 5.02 4.22 5.6

G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121 113

2.2.3. Formation of TiO2 sol in ethanolThe same procedure performed for the synthesis of TSCW in

2.2.1 was followed, but in this case, 17 ml of TiOCl2 was dissolvedin 28 ml of ethanol instead of DI water. After aging, the productwas washed with ethanol and then dried at a temperature of100 �C for 6 h to obtain TSCES (for titania–silica composite whenethanol was a medium to form TiO2 sol). The composite was cal-cined at different temperatures ranging from 600 to 1000 �C. More-over, a summary of the chemical reagents used in this experimentis given in Table 1. Bulk elemental analysis was conducted usingXRF technique to estimate the Ti/Si ratio. It was found that theTi/Si ratios of the final products were between 4 and 5.6 (Table 2).

2.3. Photodegration of methylene blue by composites

The photocatalytic tests were carried out in the degradation ofmethylene blue (MB) to demonstrate the practical applicability ofthe synthesized composites. A glass reactor equipped with a100 W high-pressure mercury lamp (Ushio UM-103B-B, Japan)was used. The temperature of the solution was maintained at25 �C throughout the experiment and the photocatalysts weredried at 120 �C for 3 h before use. The adsorption amounts of thephotocatalysts were determined in darkness prior to carrying outthe actual photocatalytic experiments. In this experiment 200 mlof a 50 mg/L MB solution was placed in the reactor followed bythe addition of 0.5 g/L of the composite. The suspension was mag-netically stirred in darkness while withdrawing sample regularlyand centrifuged (Heraeus Pico 17 centrifuge-Thermo Scientific,Germany) for UV analysis. During photochemical experiments,the same amount of MB solution and catalyst was introduced intothe glass reactor and then magnetically stirred in darkness for30 min. The suspension was irradiated under a UV light source asa function of irradiation time under constant stirring while with-drawing samples regularly from the reactor, centrifuge and thesupernatant being analyzed using a UV–vis spectrometer. Anexperiment involving irradiation of MB solution devoid of photo-catalyst was carried out to act as a control for the experiment.The photocatalytic decolorization of MB was expressed as a func-tion of relative concentration of MB solution (C/Co) at characteris-tic absorption of MB at 660 nm with reaction time; where Co is theinitial concentration and C is the concentration of MB solutionabsorption of MB at 660 nm at time, t. Since Ln Co/C = kt, a linearrelationship between the logarithm of relative concentration ofMB (Ln Co/C) with the irradiation time was used to estimate thedecolorization rate constants (k) of the photocatalysts used in thephotocatalytic reactions [25,26].

Another experiment was performed in order to study the trap-ping behavior of the .OH radicals generated in the solution duringirradiation in the presence of the photocatalysts. In this typicalexperiment, 0.15 g terephthalic acid (TA) was added into the vesselcontaining 200 ml DI water. 0.15 g of TSCES-800 photocatalyst wasintroduced. The solution was then irradiated by UV light for 1 h un-der constant stirring. After every ten minutes, 5 ml of the suspen-sion was withdrawn regularly centrifuged and the supernatant wasscanned using Nanolog Spectrofluorometer (HORIBA) with an exci-tation at 315 nm from 350 nm to 600 nm.

Table 1A summary of the chemical reagents and quantities used in the synthesis of titania–silica

Sample name TiOCl2 solution (ml) NH4OH (ml) 1 M HNO3 (ml) Sodiu

TSCW 17 12 20 11TSCM 17 12 20 11TSCE 17 12 20 11TSCSA 17 12 20 11TSCES 17 12 20 11

2.4. Characterization

The ratio of Ti to Si was determined using an X-ray fluorescenceSpectrometer (XRF; XRF-1700, Shimadzu Co., Japan with a detec-tion limit of 10 ppm and depth resolution up to 10 lm). ZAF cor-rections (atomic number, Z) were used to determine the molarratios of the elements present in the composite. Infrared spectrawere recorded by a FTIR Spectrometer (Avatar 360 E.S.P, Nicolet)to examine the functional groups present in the composites. TheIR spectrometer was equipped with a DTGS KB detector and thetransmittance measurement was carried out by making KBr pelletscontaining 2 wt.% of the sample to be analyzed. An average of 64scans with a wave number resolution of 4 cm�1 and optical veloc-ity of 0.6334 cm�1 were collected from 400 to 4000 cm�1. An X-rayDiffractometer (XRD-6000, Shimadzu) was used to determine thecrystalline characteristics of the composite. The accelerating volt-age and applied current were 40 kV and 100 mA, respectively.The crystallite size of the powders was determined by theScherer–Debye equation [27].

The Brunauer–Emmett–Teller (BET) surface area and the poros-ity measurements of the raw and calcined samples were studied bya N2 adsorption–desorption instrument (Micrometrics ASAP 2020).All the samples examined were degassed at 250 �C for 3 h prior toactual measurements. The Field emission scanning electronmicroscopy (FE-SEM, Hitachi S-4800 Japan) with an acceleratingvoltage of 15.0 kV was used to study the morphology of the com-posite. The FE-SEM was coupled with energy dispersive spectros-copy (EDS) to assess the purity and elemental composition of thecomposites synthesized via these approaches. High-resolutiontransmission electron microscopy (HRTEM, Jeol JEM 2100F-Korea)was employed to study the distribution of particles in the compos-ite. Thermogravimetric analysis (TGA) was analyzed using a Ther-mogravimetric analyzer (TG 7300, SEIKO INST). It was performedin nitrogen gas at a heating rate of 10 �C/min from room tempera-ture to 1000 �C. A UV–vis spectrometer (Optizen 2021 UV, Korea)was used to acquire the absorption spectra from 200 to 800 nm.

3. Results and discussion

The functional groups present in the synthesized samples ofTiO2–SiO2 composites are presented in Figs. 1 and 2. The peaks at780 and 1057 cm�1 are assignable to symmetrical and asymmetri-cal stretching of Si–O–Si, respectively. The formation of a Ti–O–Sibond at 943 cm�1 signifies the interaction of TiO2–SiO2 to form acomposite [7,10,11,15]. A sharp band appears at 1630 cm�1 corre-sponding to the bending vibrations of the hydrogen bonded –OH

composites.

m silicate (g) Total amount of H2O (ml) Stearic acid (g) Ethanol (ml)

52 – –52 – –52 – –52 3.4 –29 – 28

400 600 800 1000 1200 1400 1600 1800

100

150

200

TSCW

TSCM

TSCE

TSCSA-450

TSCES

Rel

ativ

e tr

ansm

itta

nce

Wavenumber (cm-1)

Si-O-Ti

Fig. 1. FTIR spectra of the TiO2–SiO2 composites synthesized via differentapproaches.

(a)

(b)

400 600 800 1000 1200 1400 1600 1800

100

200

TSCW-800

TSCM-800

TSCE-800

TSCSA-800

TSCES-800

Wavenumber (cm-1)

Rel

ativ

e tr

ansm

itta

nce

Fig. 2. FTIR spectra of TiO2–SiO2 composite for samples calcined at 800 �C (a) andEDS spectrum of the TSCE-800 (b).

20 30 40 50 60

0

3000

6000

9000

12000 A=Anatase

AAA

A

c

d

e

b a

Rel

ativ

e in

tens

ity

2 Theta degrees

Fig. 3. XRD patterns of TiO2–SiO2 composites calcined at 450 and 600 �C. (a) TSCE-450, (b) TSCSA-450, (c) TSCES-600, (d) TSCE-600 and (e) TSCSA-600.

114 G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121

group. The distinctive peaks at 1400 cm�1 in Fig. 1 are attributed tothe bending vibrations of N–H in NH4

+. However, all intensities ofthe bands for raw and calcined TSCSA and TSCES samples appear tobe comparatively increased. Fig. 2 shows that the N–H bands dis-appear during calcination indicating that these bands appearedas a result of using ammonium hydroxide solution but this func-tional group disappears during calcination as was confirmed bythe EDS result in Fig. 2b. The evolution of the peak at 480 cm�1

was inferred to TiO2 matrixes [28]. Notable spectra variation wasobserved in the calcined samples of the composites synthesizedvia this process (Fig. 2a). Calcination of samples at higher temper-atures leads to subsequent condensation of –OH groups from Si–OH and Ti–OH to form Si–O–Si, Ti–O–Si and Ti–O–Ti linkages. Asa result the intensity of –OH at 1630 cm�1 decreases while thatof asymmetric stretching vibration of Si–O–Si increases. It can alsobe seen that the stretching vibration of Ti–O from 500–900 cm�1

tends to mask the peak for symmetrical vibration of Si–O–Si at780 cm�1 observed in the raw samples [29].

Fig. 3 and 4 show the XRD patterns of TiO2–SiO2 composites cal-cined at different temperatures. Fig. 3 indicates that the samplessynthesized under this process exhibited different phases uponcalcination. The samples synthesized in the absence of stearic acidand calcined at 450 �C retained their amorphous phase but therewas a slight transformation to anatase phase after calcination at600 �C (Fig. 3a, c and d). The sample synthesized in the presenceof stearic acid and calcined at 450 and 600 �C shows the existenceof peaks for anatase TiO2 crystals (Fig. 3b and e). Fig. 4a–c showsthat all samples calcined at 800 �C displayed significant peaks foranatase phase at 25�, 38�, 48�, 54� and 55� [16,20]. TSCSA-1000and TSCES-1000 composites (Fig. 4e and f) show the formation ofnew peaks corresponding to rutile, brookite and a-cristobalite.However, no phase except anatase can be found in the sampleswashed with different solvent and calcined at 1000 �C (TSCE-1000 as a representative sample). The existence of samples withonly anatase phase at 1000 �C (TSCE-1000) implies that washingof TiO2–SiO2 precipitates with water or alcohols is suitable for pro-duction of composites with only anatase TiO2 crystals. Usually, cal-cination of TiO2–SiO2 powders at temperatures ranging from 600to 1000 �C influences the anatase-to-rutile phase transformation[10,11,25]. These results signify that alcohol washing might nothave an effect on the phase transformations of TiO2–SiO2 compos-ites synthesized using TiOCl2 solution and silicic acid derived fromsodium silicate. Therefore, the presence of only anatase phasemight be attributed to the employed synthetic method that ledto a strong interaction of SiO2 lattice with the interface of TiO2

domains hence occurrence of substantial prevention of anatase-to-rutile phase transformation.

Samuel et al. [13] reported that that heating TiOCl2 solutionusing stearic acid as a template yields a mesoporous product withrutile TiO2 crystals. In the present study, a pure anatase phase wasobtained after removal of stearic acid at 450, 600 and 800 �C tem-peratures. Since the composites synthesized in the absence of stea-ric acid and calcined at 450 �C were amorphous, the presence ofstearic acid therefore has a significant role in promoting crystalli-zation of TiO2–SiO2 composite at low temperatures. The plane at�25� (101) peak was chosen to estimate and compare the anataseaverage crystallite sizes, D using the Debye–Scherrer formulaD = Kk/bcosh; where K is the Scherrer constant, 0.89; k the X-raywavelength, 1.5406 Å; b is the full width at half-width height max-imum (FWHM) of the diffraction peaks in radians; and h is theBraggs diffraction angle. The summary of the results obtained ispresented in Table 3. Generally, the crystallite size of the calcinedsamples increased with increasing calcination temperature. After

20 30 40 50 60

0

10000

20000

30000

40000

f

B=brookiteA=Anatase

B

R=rutileC=cristobalite

AA

A

RRR

R

R

R

A C

e

b

c

d

a

Rel

ativ

e in

tens

ity

2 Theta degrees

Fig. 4. XRD patterns of TiO2–SiO2 composites calcined at 800 and 1000 �C. (a) TSCE-800, (b) TSCSA-800, (c) TSCES-800, (d) TSCE-1000, (e) TSCSA-1000 and (f) TSCES-1000.

G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121 115

calcination at different temperatures the crystallite sizes of sam-ples increased significantly with that of TSCSA-1000 reaching25.3 nm while the maximum crystallite size of the samples washedwith alcohols reached 16.4 nm. The co-existence of TiO2 poly-morphs in TSCSA-1000 and TSCES-1000 is more likely associatedwith the growth of primary particles as a result of calcination.The band gaps for anatase, brookite and rutile are 3.21, 3.13 and3.00 eV respectively [16]. During sintering, the particles size in-creases which promote phase transformation since the band gapof the metal oxides tends to decrease with increasing particle size[30,31]. Phase stability depends upon the surface energy differ-ences between the titania phases hence anatase phase with smallgrain size (<15 nm) is relatively more stable than rutile. In bulkmaterial, rutile is more thermodynamically stable than brookiteand anatase indicating that a transformation of these metastablephases to rutile upon calcination is relatively feasible. The crystal-lite size of TSCW-1000, TSCM-1000 and TSCE-1000 samples was617.5 nm and exhibited anatase phase which is very close to thecrystallite size of stable anatase phase reported in literatures[32]. However, phase transformation is the function of many fac-tors such as grain size and synthetic process and can be preventedby ensuring that the primary particles do not grow to reach thecritical-nuclei-size [22,32]. Calorimetric data for the transforma-tion enthalpies of anatase-to-rutile and brookite-to-rutile revealthat the thermodynamic phase stability for the three polymorphsis anatase < brookite < rutile [30]. The activation energy of the ana-tase to brookite transformation is 11.9 kJ mol�1 while that of thebrookite to rutile transformation is 163.8 kJ mol�1. The presenceof SiO2 lattices is more likely to repress the nucleation that pro-motes the transformation of brookite to rutile than anatase tobrookite transformation [16,18]. Therefore, it is possible for sam-ples TSCSA-1000 and TSCES-1000 to reveal the coexistence of theall three phases of TiO2 at higher temperatures depending on theirparticle sizes.

The influential role of SiO2 in TiO2–SiO2 composites or any sil-ica-incorporated binary oxides is already studied and reported by

Table 3Physical properties of TiO2–SiO2 composite samples obtained at different conditions and c

Temp/�C TSCW TSCM TSCE

FWHM X nm FWHM X nm FWH

800 0.848 9.3 0.744 10.6 0.901000 0.452 17.4 0.519 15.2 0.47

many researchers [33–37]. Okada et al. [37] investigated the effectof lattice deformation on the anatase-to-rutile phase transforma-tion by introducing foreign 4+ cations of ionic radius different fromTi4+ such as Si4+ (r = 0.040 nm). These cations were incorporatedinto the anatase structure when their ionic radius is less than thatof Ti4+ (r = 0.0605 nm). It was found that the temperature for ana-tase-to-rutile transformation increased with increasing SiO2 con-tent and subsequent formation of amorphous SiO2 surface layeras a result of Si exclusion from inside the particles. It is thoughtthat the formation of the silica layer prevents the ability of anataseprimary particles to grow to reach their critical-nuclei-size. As aconsequence the anatase primary particles cannot act as surfacenucleation sites for rutile and hence retard the anatase-to-rutilephase transformation [22,37]. Therefore the presence of SiO2, ini-tial particle sizes and sintering at higher temperatures (1000 �C)might be the main factors facilitated the coexistence of all threephases of TiO2 in TSCSA-1000 and TSCES-1000 samples.

The textural properties of the as-synthesized and calcined sam-ples of the TiO2–SiO2 composites were compared through examin-ing their nitrogen gas adsorption–desorption behaviors. Asummary of the BET surface area, pore volume and pore diameterof the as-synthesized and calcined samples is provided in Table 4.The BET surface area of the as-synthesized samples was consider-ably large with that of TSCE (594 m2/g) being the largest of all thecomposites synthesized under these approaches. However, the BETsurface areas decreased considerably as a result of sintering. Thereare also conspicuous differences in the porosities of these compos-ites. Raw composites with pore volume ranging from 0.2 to 1.85 g/cm3 and pore diameter ranging from 2 to 12 nm were synthesizedunder the method introduced in the present study. TSCES compos-ite exhibited the largest pore volume (1.85 cm3/g) and pore size(12 nm). Concisely, the textural parameters of the raw compositesincreased in the order TSCW < TSCM < TSCE < TSCSA < TSCESdepicting that water washed samples seems to have a higher levelof packing than the rest of the samples [22,38]. Figs. 5–9 presentthe N2 adsorption–desorption isotherms and average pore size dis-tributions (insets) of the composites synthesized under the pre-sented techniques. Table 4 summarizes the IUPAC Types of N2

adsorption–desorption isotherms and hysteresis loops exhibitedby as-prepared and calcined TiO2–SiO2 composites. Fig. 5a showsthat the N2 adsorption–desorption isotherm of TSCW was typicallyof type I classification, signifying the presence of micropores butthere was a dramatic change to type IV as a result of calcination[10]. Samples TSCM, TSCE, TSCSA, TSCES and their thermal deriva-tives, all revealed type IV depicting the formation of well devel-oped mesoporous systems associated with capillary condensationof adsorbent [10,11,31,39]. The hysteresis loops of TSCW-600 andTSCW-800 samples (Fig. 5b and c) seem to be very close to thatof type H2 according to the IUPAC classification systems. All ofthese samples show hysteresis loops with a triangular shape at rel-ative pressures 0.39 < P/Po < 0.75 and a relatively steep desorptionattributed to the pore connectivity effect [17,40]. The hysteresisloops of TSCM, TSCE and their respective calcined samples werevery close to type H1 implying the presence of compact agglomer-ates of approximately cylindrical-like pores with their hysteresisloops close at P/Po P 0.40 [10,41]. Moreover, all TSCSA and TSCES

alcined at different temperatures.

TSCSA TSCES

M X nm FWHM X nm FWHM X nm

3 8.7 0.678 12 0.925 8.58 16.4 0.311 25.3 – –

Table 4BET surface area, pore diameter, and pore volume distribution of titania–silica composite for water washed and alcohol washed samples; IUPAC type N2 adsorption–desorptionisotherm and hysteresis loops exhibited by both as-prepared composites and Pseudo-first-order kinetics for the photocatalytic decolorization of MB on the titania–silicacomposites calcined at 800 �C.

Sample name Calcination temp. (�C) SA m2/g PD nm PV cm3/g Isotherm type Hysteris loop K(�10�2/min) R2

TSCW Raw 516 2.7 0.2 I – 0.32 0.9482600 256 2.9 0.16800 135 3.0 0.10 IV H2

TSCM Raw 517 3.4 0.30 IV H1 0.66 0.9933600 280 3.7 0.26800 135 4.3 0.16

TSCE Raw 594 4.8 0.71 IV H1 2.1 0.9903600 370 5.2 0.55800 220 5.8 0.39

TSCSA 450 347 9.3 0.96 IV H3 2.5 0.9962800 139 10.6 0.47

TSCES Raw 505 12 1.85 IV H3 3.1 0.9892600 283 14.2 1.20800 163 16.9 0.85

0.0 0.2 0.4 0.6 0.8 1.0

100

200

300

400

500

600

Vol

ume

Ads

orbe

d ( c

m3/g)

Relative Pressure (P/Po)

(c)(b)

(a)2 4 6

0.0

0.2

0.4

0.6

0.8

dV/d

logD

(cm

3/g

)

Pore diameter (nm)

TSCWTSCW-600TSCW-800

Fig. 5. Nitrogen adsorption–desorption isotherms for sample washed with water;TSW (a), TSCW-600 (b) and TSCW-800 (c). Inset is pore size distribution (PSD) of theas-synthesized and calcined samples.

0.0 0.2 0.4 0.6 0.8 1.0

100

200

300

400

500

(c)

(b)

(a)

Vol

ume

Ads

orbe

d (c

m3 /

g )

Relative Pressure (P/Po)

2 4 6 8 10 120.0

0.2

0.4

0.6

0.8

1.0 TSCMTSCM-600TSCM-800

Pore diameter (nm)

dV/d

logD

(cm

3 /g)

Fig. 6. Nitrogen adsorption–desorption isotherms of the as-synthesized andcalcined TiO2–SiO2 composite samples washed with methanol; TSM (a), TSCM-600 (b) and TSCM-800 (c). Inset is pore size distribution (PSD) of as-synthesized andcalcined samples.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

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500

600

700

800

900

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ume

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orbe

d (c

m3/g

)

Relative Pressure (P/Po)

(b)

(c)

(a)

10 20 300.0

0.4

0.8

1.2

1.6

dV/d

logD

(cm

3 /g)

Pore diameter (nm)

TSCETSCE-600TSCE-800

Fig. 7. Nitrogen adsorption–desorption isotherms of the as-synthesized andcalcined TiO2–SiO2 composites washed with ethanol; TSE (a), TSCE-600 (b) andTSCE-800 (c). Inset is pore size distribution (PSD) of the as-synthesized and calcinedsamples.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

700

Vol

ume

Ads

orbe

d ( c

m3/g

)

Relative Pressure (P/Po)

(a)

(b)0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(b)

(a)

dV/d

logD

(cm

3 /g)

Pore diameter (nm)

TSCSA-450

TSCSA-800

Fig. 8. Nitrogen adsorption–desorption isotherms of the calcined TiO2–SiO2 com-posite synthesized in the presence of stearic acid; TSCSA-450 (a), TSCSA-800 (b).Inset is pore size distribution (PSD).

116 G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121

samples (raw and calcined) portrayed type H3 hysteresis with acharacteristic step-down in the desorption branch associated withhysteresis loop closure at P/Po P 0.65. Type H3 hysteresis loop sig-nifies the presence of aggregates of plate-like particles or assem-blage of slit-shaped pores with unlimited adsorption at high P/Po[39]. Usually, a system possesses type H2 or H3 hysteresis because

of existence of random distribution of pores and an interconnectedpore system [10]. Therefore, the pore size distribution (PSD) ob-tained from the desorption branch is more likely to be affectedby pore network effects than the adsorption branch leading to aforced closure of the hysteresis loop at higher relative pressures.The PSD of the TiO2–SiO2 composites calculated by the BJH method

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

1000

1200

(c)

Relative Pressure (P/Po)

Vol

ume

Ads

orbe

d (c

m3 /g

)(a)

(b)

0 20 40 60 80 100 1200.0

0.5

1.0

1.5

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2.5

(c)

(b)

(a)

Pore diameter (nm)

dV/d

logD

( cm

3/g

)

TSCES TSCES-600 TSCES-800

Fig. 9. Nitrogen adsorption–desorption isotherms for the TSCES samples; rawTSCES (a), TSCES-600 (b) and TSCES-800 (c). Inset is their respective pore sizedistributions (PSD).

G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121 117

showed that the PSD of water and methanol washed samples pos-sessed approximately similar patterns with average pore diame-ters ranging between 2 and 4.3 nm. The PSD of ethanol washedsamples ranges from 1.5 to 11 nm. Nonetheless, the PSD of TSCSAand TSCES samples were between 5–40 nm and 5–50 nm, respec-tively; broader than those of TSCW, TSCM and TSCE samples.

In an absolute sense, the presence of alcohol in the metal oxideshydrogels is susceptible to influencing the textural properties aswas noticed in the TiO2–SiO2 composites synthesized in the pres-ent study. Using ethanol as a medium to form the TiO2 sol or wash-ing solvent increases porosity, therefore the textural properties of amicroporous product can be enhanced via this approach. The porevolume of TSW sample was 0.2 cm3/g which was remarkably in-creased to 0.71 cm3/g after washing the hydrogels with ethanol.A further tremendous increase was observed in the samplesformed by employing ethanol to form the TiO2 sol (1.85 cm3/g).The capillary forces arising during drying might be the main driv-ing force for the discrepancies observed in the textural propertiesof TiO2–SiO2 composites. Succinctly, during drying the pore shrink-age and enlargement of the TiO2–SiO2 hydrogels occur dependingon the solvent and drying temperature. This shrinkage dictatesoverall agglomeration behavior of the dried composite powdersand is strongly affected by the capillary force [42,43]. The capillaryforce arises as a result of evaporation of the solvents in the hydro-gel pores and accordingly is largely influenced by surface tension,molecular weight of the solvent and pore diameter [43]. Nairet al. [38] reported that during drying water-washed samples exertlarge capillary stresses which break down the aggregates into pri-mary particles and pull them together to form a well-packed com-pact powders. The surface tension of water (7.2 � 10�2 Nm�1 at25 �C) is greater than that of ethanol (2.2 � 10�2 Nm�1 at 25 �C),therefore TSCE and TSCES samples are more likely to exert weakercapillary forces. As a result, the rate of aggregation in these sam-ples was high leading to formation of composites with poor pack-ing of primary particles [43]. A poorly packed sample possesseslarge porosities an essential element for improving the catalyticperformance of the final product. These results are compatible withthose reported by Mercera et al. [24] and Kumar et al. [22] wherethe influence of ethanol washing on the textural and structuralproperties of ZrO2 and TiO2 were comprehensively investigated.

Accordingly, using surfactant is also a plausible alternative toimprove porosities of the metal oxide composites. In-depth inves-tigation of the role of surfactants in the formation of mesoporousstructures has been described elsewhere [10–12,14]. During syn-thesis surfactants act as structure directing agents leading toassembly of mesostructured composites. The mesoporous materi-

als are obtained by subsequent removal of the surfactant by extrac-tion or calcination [14]. In the present study, the stearic acid wasremoved via sintering of the raw composite at different tempera-tures ranging from 450 to 1000 �C to yield a mesoporous TiO2–SiO2 composite with large pore volume and pore diameter.

The FE-SEM micrographs for representative calcined samples ofthe titania–silica composites; TSCE-800 and TSCSA-800 are shownin Fig. 10(a–b). Primary particles of these samples are sphericalwith an average diameter of approximately 25 nm. Even thoughthe particles in TSCES-800 are less aggregated than its counterpartTSCSA-800, but their sizes and packing are more or less indistin-guishable. Jung and Park [44] prepared silica-embedded titaniaparticles by the sol–gel process suitable for the decomposition oftrichloroethylene. The SEM images of the obtained compositesshowed that the degree of aggregation of primary particlesdecreased with increasing silica content. It is striking that themorphology of the representative samples of the present study(Ti/Si = 4.2–5.6) is very close to that of the titania–silica samplewith Ti/Si = 5.4 reported by Jung and Park report. Consequently,Hilonga et al. [45] pointed out that, synthesis of titania–silicacomposite using preformed silicic acid yields a product with lessaggregated primary particles. Less aggregation behavior is largelyascribed to controllable hydrolysis and polymerization of silicicacid due to the absence of Na+ ions in the silica source. The HRTEMmicrograph of the TSCE-800 sample showed the presence of ana-tase particles (Fig. 10c). XRD results showed that after calciningthe TiO2–SiO2 composites at 800 �C the anatase TiO2 crystals wereachieved. Further calcination of the samples at a temperature of1000 �C promoted the anatase-to-rutile formation and thereforethe rod-like rutile ordered particles are evident in the TSCE-1000sample (Fig. 10d) [46,47].

Subsequently, during calcination there were consequentialphase transformations which also altered the microstructure prop-erties of the composites. Various studies [10,45,48] found that thesurface area and pore volume of materials decrease with increasingthe calcination temperature. Raw materials were amorphous butafter calcination at a temperature 6800 �C the anatase TiO2 crys-tals formed and there was subsequent reduction of surface areaand pore volume. The anatase grain sizes estimated by theScherer-Debye equation at this particular temperature were612 nm which is very close to average particle size of �10 nmdetermined by TEM. Calcination of the samples to 1000 �C in-creases the anatase grain sizes to 25 nm while the larger grain sizesof �39 nm of rutile appeared in TSCSA-1000 and TSCES-1000 sam-ples. Kumar et al. [48] compared porosity reduction and primarycrystallite growth of TiO2 membranes calcined at different temper-atures by high-resolution scanning electron micrograph. The mem-branes contained small anatase crystallites before the anatase torutile transformation but after phase transformation smaller ana-tase crystallites and larger densified rutile regions were ultimatelyobserved. Therefore, during amorphous ? anatase ? rutile phasetransformation, smaller primary particles grow into larger particlesleading to a decrease in the number of voids that result in a de-crease in surface area and pore volume after calcination [10,48].Particularly, the samples obtained in the present study demon-strated phase transformation, pore growth, crystallite growth andporosity reduction as a consequence of sintering.

The thermal stability of the material prepared via this processwas assessed by the TGA analysis. Fig. 11 shows the typical TGAcurves for the TiO2–SiO2 composites synthesized via different tech-niques. It can be seen that there is a sharp weight loss from roomtemperature to 350 �C that might be attributed to loss of physi-sorbed water and organic solvents [16,45]. Consequently, theweight loss from 350 to 680 �C might be due to the decompositionof organic groups and the subsequent evolution of anatase phase ofTiO2 and condensation of the OH groups on the silica network to

Fig. 10. Representative SEM and TEM micrographs of the TiO2–SiO2 composites. SEM micrographs for TSCES-800 sample (a) and TSCSA-800 sample (b); HRTEM micrographfor TSCES-800 sample showing anatase particles (c) HRTEM micrograph and SAD (inset) of the TSCES-1000 sample showing the rod-like rutile particles (d).

118 G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121

form Si–O–Si bridges [18,45]. Moreover, there is no notable weightloss above 700 �C indicating that the composite has attained a sta-ble anatase phase and there is no further condensation of OHgroups. These results are consistent with XRD analysis and FTIRspectra of the calcined samples.

Degradation of organic contaminants by heterogeneous photo-catalysts such as TiO2 or TiO2 derivatives is preponderance a costeffective and environmentally benign process that is essential forreduction of textile industry pollutants [49–51]. In the presentstudy, a series of calcined TiO2–SiO2 composites synthesized viaversatile approaches were assessed in the decolorization of meth-ylene blue solution under UV irradiation. The adsorption behavior

200 400 600 800 100075

80

85

90

95

100

d

cb

% w

eigh

t lo

ss

Temperature(0C)

a

Fig. 11. TGA curves for the as-synthesized samples of TiO2–SiO2 composites. (a–c)Presents the samples washed with water, methanol and ethanol, respectively, and(d) is the sample synthesized using ethanol as a medium to form the TiO2 sol(TSCES).

of the titania–silica composites is illustrated in S1. It can be seenthat the adsorption of MB onto TSCW-800 and TSCM-800 was neg-ligible. The effect of adsorption was more pronounced in TSCES-800, TSCSA-800 and TSCE-800 samples however, the MB concen-trations in the liquid phase attained equilibrium values within30 min. Fig. 12 displays the photocatalytic decolorization behaviorof MB solution in the presence of calcined TiO2–SiO2 composites.During irradiation there was a significant decreasing of the MBconcentration signifying decolorization of the solution. It is evidentthat the MB concentration was drastically decreased with increas-ing the irradiation time. Generally, the TSCES-800 sample exhib-ited the highest activity while TSCW-800 had the lowest activity

-20 0 20 40 60 80 100

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MB

con

cent

rati

on C

/Co

Light off UV light

Irradiation time (minutes)

TSCW-800 TSCM-800 TSCE-800 TSCSA-800 TSCES-800

0 20 40 60 80 1000.0

0.5

1.0

1.5

2.0

2.5

3.0 kES

kSA

kE

kM

kW

Ln

Co/

C

Irradiation time (minutes)

Fig. 12. Photocatalytic decolorization behavior of MB solution in the presence ofcalcined TiO2–SiO2 composites. Inset is the linear relationship between thelogarithms of relative concentration of MB (Ln Co/C) with the reaction time of theTiO2–SiO2 composites calcined at 800 �C in the decolorization of MB solution.

G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121 119

implying that the activity of the composites increased withincreasing porosities. The formation of mesoporous TiO2–SiO2 sys-tems with high surface area and porosities increases the adsorptiveproperties and photocatalytic active sites that facilitate the reac-tant molecules to diffuse easily to occupy the pores and thereforeincrease the rate of decolorization [52]. It is explicable that thephotocatalytic activity is mainly depends on crystallinity, crystal-lite size, particle size, Ti content and adsorbability of the reactant[15,16,18]. In the presence of UV light the activation of anataseTiO2 occurs; the driving force for photocatalytic decolorization oforganic pollutants [49,50,52]. Since, raw samples were amorphous,it was necessary to use the crystalline samples as representativesto evaluate the photodegradation process. Although the XRD stud-ies confirmed that all materials calcined at 800 �C were crystalline,their activities were comparatively distinct. The BET surface area ofthe TSCE-800 sample was the highest amongst the samples cal-cined at 800 �C but the activities exhibited by TSCSA-800 andTSCES-800 samples were unquestionably superior. Consequently,the surface area of TSCM-800 and TSCW-800 was 135 m2/g butthe activity of the former was higher than that of the latter never-theless, their porosities were different. Therefore, the distinctporosities demonstrated by these samples might be advantageousin enhancing their photocatalytic performances. It is obvious thatthe porosities of TSCES-800 and TSCSA-800 samples were largerthan those of the other samples suggesting that their adsorbabilityis appreciably higher. This indicates that during photodegradation,larger amounts of reactant were adsorbed in the TSCES-800 andTSCSA-800 particles. Therefore, addition to crystalline structureof the composites, particle size, Ti content and high BET surfacearea, the porosity might be an additional factor contributing tothe superior photocatalytic activities of TSCSA-800 and TSCES-800. Ong et al. [51] investigated the effect of adsorption and decol-orization rate in the photocatalytic degradation of single and bin-ary dye solutions. It was established that the adsorption effect ismore likely to affect the decolorization rate of single solutions thanthat of binary solutions. It is thought that in binary dye solutionsthe substrates do compete for the same adsorbing sites and there-fore the effect of adsorption in the photocatalytic process can bereduced. In the present study, we found that the adsorption effectwas more prominent in the samples with larger porosities (TSCES-800 and TSCSA-800). However, time-dependent adsorption studies(S1) revealed that the equilibrium values can be attained withinshort time and during irradiation the samples displayed differentdecolorization rates.

Based on the previous investigations [25,26,52–54] the photo-catalytic decolorization of organic pollutants follows the pseudo-first order therefore quantitative estimation of activities can be

0.0 0.3 0.6 0.90.00

0.01

0.02

0.03

0.04

Rat

e co

nsta

nt (m

in -1

)

Pore volume (cm3/g)

R2 =0.957

(a)

Fig. 13. Rate constants versus textural properties of the titania–silica samples calcined a(b).

achieved through determination of their rate constants. The pseu-do-first order rate constants were determined from the linear rela-tionship between the logarithm of relative concentration of MB (LnCo/C) with the reaction time of the samples used in the photocat-alytic test (Fig. 12 inset) and the obtained values are summarizedin Table 4. It can be seen that the rate constant for each sampleis different; a clear clarification that they possess different capabil-ities toward decolorization of organic pollutants. It is noteworthythat TSCES-800 sample with has a greater (kES = 0.031 min�1) rateconstant than TSCW-800 (kM = 0.0032 min�1). The rate constantsobtained in this study suggest that the activities of the tested sam-ples increased in the following order; TSCW-800, TSCM-800, TSCE-800, TSCSA-800, TSCES-800. Fig. 14 presents the photofluorimetryspectra of the terephthalic acid (TA) solution after irradiation for1 h in the presence of TSCES photocatalyst. The fluorescence inten-sity of the solution at 430 nm increased with increasing irradiationtime. Irradiation of TA solution in the presence of photocatalystleads to the formation of a highly fluorescent 2-hydroxyterephtha-lic acid as a result of oxidation influenced by the presence of hydro-xyl radicals in the solution [51]. Hence, the decolorization ofmethylene blue solution can be supposed to occur due to forma-tion of the active radicals (superoxide or hydroxyl radicals) gener-ated during the photochemical process in the presence ofphotocatalysts [49–54].

It is well know that adsorbability is one of the main factorsdetermining the overall photocatalytic performance of photocata-lysts as the reaction proceeds through surface reactions betweenadsorbed substrate and electrons/holes that are generated byphoto absorption of TiO2 particles [55]. Therefore, having photocat-alyst with a high surface area and superior porosities enhances itscatalytic performance. It was previously reported [56] that pow-ders with high surface area exhibit crystalline defects which arethe main sources of recombination of photogenerated electronsand holes leading to a poor photoactivity. Plotting graphs of rateconstants versus pore diameter or pore volume (Fig. 13) reveal thatthe rate constants increased with increasing either pore diameteror pore volume. Hence, large pore sizes and pore volume arerequirement in forming photocatalysts with high activities butnot decisive factors. Moreover, the highest photocatalytic activityof TiO2 can be realized in the anatase phase with optimal crystallitesize in the range 8–10 nm [57,58]. In the present study, the crystal-lite size of the samples calcined at 800 �C was between 8 and12 nm. TiO2–SiO2 composites obtained in the present study dis-played different activities signifying that the catalytic performanceof the composites in the decolorization of the methylene blue solu-tion was influenced by combination of various factors such as typeof the composite, Ti content, grain size, adsorbability, pore

0 3 6 9 12 15 180.00

0.01

0.02

0.03

0.04

Rat

e co

nsta

nts

(min

-1)

Pore size (nm)

R2 =0.928

(b)

t 800 �C; rate constants against pore volume (a) and rate constants against pore size

350 400 450 500 550 600

0.0

2.0x106

4.0x106

6.0x106

8.0x106

1.0x107

Inte

nsit

y (c

ps)

Wavelength (nm)

0 minute 10 minutes 20 minutes 30 minutes 40 minutes 50 minutes 60 minutes

Fig. 14. Photofluorimetry spectral changes with time in the terephthalic acidsolution during UV-light irradiation in the presence of TSCES-800 photocatalyst.

120 G.N. Shao et al. / Microporous and Mesoporous Materials 179 (2013) 111–121

structure, surface area and crystallinity [51–54,57–59]. However,the substantial role of material properties in influencing photocat-alytic activities (i.e. structure–activity correlation) has been a ma-jor debate in the field of photocatalysis and heterogeneouscatalysis in general [60–63]. Ohtani [60] pointed out that the pho-tocatalytic activities cannot merely be explained on the basis ofsingle property or parameter as it is difficult to establish the signif-icant role of each property. Ryu and Choi [61] investigated 19aquatic organic pollutants on commercial TiO2 in order to deter-mine the optimum conditions suitable for water treatment. Itwas found that the photocatalytic activities depend on the typeof substrate rather than the structural properties of the photocata-lysts. These detailed reports elucidate that photocatalytic activityis unquestionably influenced by many variables and thereforemost of description correlating structure and photocatalytic activ-ities are empirical [60,62].

Recently, we reported the influence of titania content on themesostructure of TiO2–SiO2 composites and their photocatalyticdecolorization of methylene orange [18]. It was found that the effi-ciency of the photocatalysts increased with increasing the Ti/Si ra-tio and the highest activity was achieved when the Ti/Si = 5.6.Herein we introduced approaches suitable for enhancing the for-mation of composites with large porosities while the Ti/Si = 4–5.6. It is striking that using different synthetic approaches giveproducts with different properties. Direct co-precipitation reactionof titanium oxychloride and sodium silicate yields a product with amicropore structure [7]. The N2 isotherms demonstrated that theTSCW sample was microporous implying that the formation ofmesoporous composites is affected by many factors in additionto a controlled heterogeneous reaction between titania and silicaprecursors [16]. The porosities of TSCE, TSCSA-450 and TSCES sam-ples suggests that drying conditions and the use of templates influ-ence the mesostructure network of the composites as well. Themechanism of formation of the titania–silica composites obtainedin the present is anticipated to be similar to that available in theliterature [38,42,43]. The improvement of the textural propertiesunder the proposed methods i.e. washing, using surfactant andforming titania sol in ethanol medium; take the advantage of thedifferences in the surface tension of the solvents used in the wash-ing process and controlled heterogeneous reactions between silicaprecursor and titania sol. The pore size and pore volume of thesamples washed using different solvents increased in the order;TSCW < TSCM < TSCE implying that the sample washed with sol-vent with lowest surface tension exhibited superior porosities.Meanwhile, the sample obtained in the presence of surfactant re-vealed that the textural properties were significantly increasedafter elimination of the template. During synthesis, surfactants

act as structure directing agents leading to assembly of mesostruc-tured composites. Therefore, the removal of the surfactant throughcalcination facilitates the composite to exhibit superior texturalproperties making it beneficial for various applications. The sam-ples obtained by forming titania sol in ethanol medium indicatedthat pore volume and pore diameter were increased upon usingthis method. Kibombo et al. [59] suggested that introducing organ-ic solvents influences the hydrolysis and condensation reactionkinetics by altering intermolecular interactions that in turn affectthe overall gelation and precipitation processes. Therefore, a con-trolled reaction between silanols and TiO2 sol in ethanol mediumand a subsequent weaker capillary force exerted by the hydrogelduring drying led to formation of a highly porous TSCES. Neverthe-less, the influence of the synthetic methods in forming productswith different properties requires an intensive investigation whichis beyond the scope of the present study but surface characteriza-tion of the composites revealed that the composites obtained here-in have desirable properties suitable for heterogeneous catalysis.

4. Conclusion

Here we reported a versatile, economical and reproducible tech-nique to enhance the photocatalytic properties of TiO2–SiO2 com-posites synthesized by the sol–gel process from less expensiveprecursors. Washing hydrogels with different solvents, using sur-factant and changing medium during formation of the TiO2 solwere potential techniques in increasing porosities of the compos-ites and subsequent enhancement of their activities. The XRD pat-terns revealed that crystalline materials can be obtained from theamorphous TiO2–SiO2 composites through sintering at high tem-peratures. FTIR confirmed the formation of TiO2–SiO2 compositeevidenced by the formation of Si–O–Ti bond at 943 cm�1. The N2

sorption studies showed that the TSCE had the largest surface area(594 m2/g) while TSCES had the largest pore volume (1.85 m3/g).Consequently, TSCES-800 sample demonstrated the highest cata-lytic activity of all the composites synthesized via this process asa result of having desirable properties suitable for decolorizationof organic pollutants. The effectiveness of these materials in thedecolorization of methylene blue indicates that they can be alter-native and cheap photocatalysts suitable for decolorization of dyeswhich are common pollutants in textile industries. Indeed, thisexperiment provides criteria for selection of the appropriate wash-ing solvent, surfactant or media necessary for synthesis of theTiO2–SiO2 composites with optimum/desired properties for variousapplications.

Acknowledgement

We would like to thank the Ministry of Education, Science andTechnology of the Republic of Korea through their collaborative re-search programs among industry, academia and research institutesof Korea Industrial Technology Association (KOITA-2010) for theirfinancial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.micromeso.2013.05.021.

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