Powder Technology 253 (2014) 22–28

Contents lists available at ScienceDirect

Powder Technology

j ourna l homepage: www.e lsev ie r .com/ locate /powtec

Influence of pH on the formulation of TiO2 nano-crystalline powderswithhigh photocatalytic activity

Andreia Molea a, Violeta Popescu a,b,⁎, Neil A. Rowson c, Adrian M. Dinescu b

a Technical University of Cluj-Napoca, Faculty of Material and Environmental Engineering, Physics and Chemistry Department, No.103-105 Muncii avenue, 400641 Cluj-Napoca, Romaniab National Institute for Research and Development in Microtechnologies, IMT, 126A Erou Iancu Nicolae Street, 077190, Bucharest, Romaniac University of Birmingham, School of Chemical Engineering, Edgbaston, Birmingham B15 2TT, United Kingdom

⁎ Corresponding author at: Technical University of Cluj-Environmental Engineering, Physics and Chemistry Deparnue, 400641 Cluj-Napoca, Romania.

E-mail addresses: andreia.molea@chem.utcluj.ro (A. Mvioleta.popescu@chem.utcluj.ro (V. Popescu), n.a.rowson@andrian.dinescu@imt.ro (A.M. Dinescu).

0032-5910/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.powtec.2013.10.040

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 May 2013Received in revised form 16 September 2013Accepted 21 October 2013Available online 29 October 2013

Keywords:Titanium dioxideHydrolysispH effectCatalysisPhotodegradation processMethylene Blue dye

This paper describes the effect of synthesis conditions on the formation of anatase and rutile crystalline phasesand photocatalytic activity of synthesised TiO2 phase. The synthesised powders were characterised by X-raydiffraction, Raman microscopy, Scanning Electron Microscopy and UV–Vis spectroscopy. Using these character-isation techniques, the structural, morphological and optical properties as a function of formulation pH weredetermined. Since photocatalysis is a surface process, themass surface charge of the powderswas alsomeasuredusing a Faraday Cage connected to an electrometer. The structural, morphological, optical and surface propertieswere correlated with the photocatalytic activity of the formulated TiO2 powders.The influence of synthesis condition on the photocatalytic activity of TiO2 powders was determined by thedegradation of Methylene Blue dye under both UV-A and visible light.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) has potential applications in environmentalfields such as wastewater treatment and more specifically in caseswhere the water has been contaminated with azo dyes from textileand oil spillages [1–6]. Titanium dioxide occurs in nature as anatase, ru-tile and brookitemineral phases. Anatase is stabilised by heat treatmentat 400–600 °C, whilst, rutile, the most thermodynamically stable crys-talline phase of titanium dioxide, is stabilised at 900 °C. Between 600and 900 °C, both anatase and rutile phases co-exist [7,8].

Photocatalysis is a surface process; therefore, structural, morphologi-cal and optical properties are critical parameters for controlling the pho-tocatalytic activity of the synthesised materials [9–12].

According to Cassaignon et al. [13], using titanium tri-chloride as aprecursor, in an acid medium, at pH between 3 and 4, a rutile phase isstabilised after 24 h at 60 °C. In an alkaline medium, at pH N 6.5 andthe same synthesis conditions, anatase is the main phase (65%) accom-panied by brookite.

Napoca, Faculty of Material andtment, No.103-105 Muncii ave-

olea),bham.ac.uk (N.A. Rowson),

ghts reserved.

Anatase and brookite are transformed by heat treatment into rutile,but the synthesis conditions influence this transition temperature. As aresult, if the material is prepared under acidic conditions and after heattreatment at moderate temperature, i.e. at 400 °C, a rutile phase isstabilised [14]. However, in alkaline environments, (pH N 7) an anatasephase is stabilised, even at 800 °C [15]., However, only a few studieshave characterised the formulation of TiO2 powders at different pHswith respect to photocatalytic activity [16].

According to some scientific literature, an anatase phase has thehighest photocatalytic activity [5,15], whereas other researches have in-dicated that a mixture of crystalline phases such as anatase and rutile[1,17,18] or anatase, brookite and rutile [19] exhibits a higher photocat-alytic activity than pure anatase. Xie et al. [18] studied the photocatalyt-ic activity of TiO2 catalysts containing various rutilemass fractions usingthe degradation of benzene under 24 mW/cm2 UV radiation. The sam-ple with a higher concentration of rutile exhibits the highest photocat-alytic activity, due to the lower potential of the conduction band ofrutile compared to anatase, giving more holes on the anatase surfacefor oxidation reactions. Lopez et al. [19] studied the photocatalytic activ-ity of TiO2 synthesised at various pHs. Also the authors [19] demonstrat-ed that the photodegradation process depends on the energy band gap(Eg). Bulk anatase has Eg = 3.2 eV, while rutile has Eg = 3 eV. Thehighest activity corresponds to catalysts that contained anatase–brook-ite–rutile and anatase–rutile, respectively. This correlates to the lowestenergy band gap of the rutile crystalline phase comparedwith the ener-gy band gap of pure anatase phase. As such, thematerial could absorb a

Fig. 1. XRD patterns of TiO2 powders synthesised at different pHs and heat treatment at 400 °C, 1 h (▲ anatase,● rutile).

23A. Molea et al. / Powder Technology 253 (2014) 22–28

wider wavelength range of the incident radiation, generating electron–hole pairs.

In this study, nanocrystalline TiO2 powders were synthesised by ahydrolysis technique using titanium tri-chloride, in an aqueous solutionobtained from distilled water in the presence of hydrogen peroxide.Inorganic TiCl3 was used because, when using organic precursors as atitanium source, it is difficult to remove any organic residues from theparticle surface [20]. The influences of pH on structural, morphologicaland optical properties were studied. The photocatalytic activity of thesynthesised powders was determined by the degradation of MethyleneBlue under low intensity UV-A light and visible radiation.

2. Materials and method

2.1. Synthesis of TiO2 powders at different pH

TiO2 powders were synthesised by the hydrolysis of a titanium tri-chloride precursor. The solutionwasprepared from5 ml TiCl3 (10% con-taining 15% HCl - Merck) and 5 ml hydrogen peroxide 3%, added into5 ml distilled water. Hydrogen peroxide was used in order to achievethe rapid oxidation of titanium. The initial solution pH was 1. The pHwas adjusted with ammonia solution (NH4OH ~25%) to 3, 8.5 and 10.5,respectively. The solutions were stirred until a white precipitate of titani-umhydroxidewas formed. The precipitateswere filtered,washed severaltimes with distilled water and subjected to heat treatment for 1 h at 400°C.

2.2. Photodegradation experiment

In order to assess the performance of the formulated TiO2 powders,Methylene Blue (MB) dye was degraded, at room temperature, under

Table 1Structural parameters calculated by Rietveld refinement based on XRD data, using the Powder

Sample Crystalline phases/Fraction mass [%]

Average crystallitesize [nm]

Strain

Anatase Rutile Anatase Rutile Anatase

pH 3 29.76 70.24 10 10.6 0.00954pH 8.5 100 – 16 – 0.00300pH 10.5 100 – 13 – 0.00426

both UV-A and visible radiation. The radiation was emitted by twolamps, a 6 W lamp with emission in 300–400 nm UV-A wavelengthand a 9 W lamp with emission of visible wavelengths between 400and 700 nm. A volume of 50 ml of photocatalyst suspension, whichcontains 0.01 g catalyst, was mixed with 50 ml of a solution containing5.5 × 10−3 mg/ml MB. The experiments took place at the natural pHof theMB solution (pH 6.8). Themixturewasmaintained in dark condi-tions for 1 h, in order to establish the adsorption/desorption equilibri-um, followed by irradiation for 300 min with both UV-A and visibleradiation. The intensity of the radiation was measured with a Mavolux5032C lux meter which gave a value of 0.183 mW/cm2 at the solutionsurface. A sample was prepared without catalyst (blank sample) forbenchmarking purposes.

2.3. Characterisation

The titaniumdioxide powderswere characterised by X-ray diffractionin order to determine the structural properties of the samples. A BrukerAXS D8 diffractometer working with CuKα radiation (λ = 1.5406 Å)was used. Structural parameters were calculated using the Powder Cellsoftware [21].

Raman spectra of specimens were registered using a WiTec Alpha300 R (LOT Oriel, UK) operating a 0.3 W single frequency 785 nmdiode laser (Toptica Photonics, Germany) and an Acton SP2300 triplegrating monochromatic/spectrograph (Princeton Instruments, USA).Confocal Raman spectroscope was used to determine the characteristicTi–O bonds for the crystalline phases.

Scanning electron microscopy was utilised to characterise the TiO2

powder morphology. The samples were examined using a field emissionscanning electron microscope (FE-SEM) – Raith e_Line with in-lenselectron detection capabilities.

Cell software.

Lattice parameters [Å]

Rutile AnatasePDF 21-1272

RutilePDF 21-1276

a c a c

3.7852 9.5139 4.5933 2.9592

9 0.000001 3.7787 9.4801 4.5854 2.94661 – 3.7709 9.4559 – –

1 – 3.7684 9.4470

Fig. 2. Raman spectra of TiO2 powders synthesised at different pHs. Insert: Raman shift of anatase peak position from 144 cm−1.

24 A. Molea et al. / Powder Technology 253 (2014) 22–28

A Faraday Cage connected to an electrometer, model Keithley 6514was used to measure the surface charge of the materials.

Lambda 35 UV–Vis spectrophotometer with an integrated spherewas used to determine the optical properties of the TiO2 powders. Thetotal transmittance of the samples wasmeasured. Based on the absorp-tion coefficient (α), the energy band gaps of the samples were deter-mined, using Tauc's relation [22]:

αhν ¼ A hν−Eg� �m ð1Þ

where: hν is photon energy, A is a constant andm is an integer depend-ing on the nature of electronic transitions. For the direct allowed transi-tions, m has a value of 1/2while for indirect allowed transitions, m = 2[22]. The absorption coefficient was calculated with the formula [23]:

α ¼ 2:303 � 103A � ρc � l ð2Þ

Where: A is the absorbance, ρ is the TiO2 bulk density (ρ = 3.84 [g/cm3]),c is the concentration of the TiO2 suspensions (c = 1.2 · 10−3 [g/cm3]),and l is the path length (l = 1 [cm]).

The Urbach energy (EU) has been estimated based on the followingequation [24]:

lnα ¼ α0 þ1Eu

hν ð3Þ

Where: α is the absorption coefficient, α0 is a constant. The Eu was de-termined from the inverse of the slope of the linear portion of the plotlnα versus hν of the following equation [24]:The variation of MB con-centration as a function of irradiation time, under both UV-A light andvisible irradiation, was determined using the UV–Vis spectroscopybased on a calibration curve. The efficiency of the degradation processwas calculated with the relation [25]:

Efficiency% ¼ C0Mb−CMb

C0Mb

� 100 ð4Þ

Where: CMB0 is the initial concentration ofMethylene Blue and CMB is the

concentration of Methylene Blue at a certain irradiation time.The kinetics of the degradation process of Methylene Blue was stud-

ied. The rate constant, k, was obtained by plotting the natural logarithmof the ratio between initial concentration and the concentration at a cer-tain irradiation time of Methylene Blue versus irradiation time assum-ing a first order reaction [26,27]:

− lnc0MB

cMB¼ kt ð5Þ

3. Results and discussion

3.1. X-ray diffraction

The TiO2 diffraction patterns (Fig. 1) revealed that after heat treat-ment, in an acidmedium, anatase and rutile crystalline phases were ob-tained, but rutile is the predominant phase, even if the temperature ofheat treatmentwas 400 °C. In an alkalinemedium environment howev-er, only an anatase phasewas stabilised. According to some scientific lit-erature [13], at pH conditions between 2.5 and 4.5, the transformationof titanium tri-chloride occurs by oxidation of the precursor into a rutilecrystalline phase due to the formation of a Ti(OH)(OH)52+ intermediatecompounds. At pH N 4.5, the oxidation of the precursor is very fast,leading to the formation of a Ti(OH)3 + x compound, which, after 24 hat 60 °C is transformed into an anatase phase [13]. The average crystal-lite size (Table 1) increased with increase of the pH, from 10 nm for thesample synthesised in pH 3 (anatase phase) to 16 nm for the samplesynthesised at pH 8.5 and then the crystallite size tends to decrease to13 nm, for the sample synthesised at pH 10.5. This has been attributedto an increase of themicro-strain. According to Shao et al. [7], a decreaseof crystallite size is linked to an increase of the number of defects, thusleading to an increase in the number of lattice deformations. Parametercell calculations indicate narrowing of the elementary cell due to theoxygen vacancies or site-disorder [28]. In an acid environment, the crys-tallite size was smaller (for rutile phase) because the acid acts as an

Fig. 3. SEMmicrographs of TiO2 powders, formulated at a) pH 3, b) pH 8.5 and c) pH 10.5,respectively.

25A. Molea et al. / Powder Technology 253 (2014) 22–28

electrolyte and prevents the particle growth and/or agglomerationthrough electrostatic repulsion [9]. Y.C. Lee et al. [10] have obtained sim-ilar results.

3.2. Raman microscopy

Fig. 2 shows the Raman signature in the range of 80–700 cm−1 ofTiO2 samples obtained at different pH, after heat treatment at 400 °Cfor 1 h. The symmetric model of a tetragonal anatase phase was identi-fied at ~144 cm−1 (Eg), 197 cm−1 (Eg), 398 cm−1 (B1g), 515 cm−1

(B1g) and 638 cm−1 (Eg) [29] for the samples synthesised in an alkalinemedium. However, for the sample prepared in an acid medium, funda-mental vibration modes for rutile phase at 446 cm−1 and 609 cm−1

were also observed [30]. The Raman peak position changed from144 cm−1 as presented in Fig. 3. It was noted that the sample synthe-sised in acid medium exhibits a higher wavenumber (148 cm−1). Thepeak shift can be associated with small crystallite size and a highervalue of micro-strain [31]. An increased broadening of the peak due tothe quantum size effect was also noticed [29,31].

3.3. SEM microscopy

The SEM images of the TiO2 powders are presented in Fig. 3. At pH 3,most of the crystals have elongated shapes with a length ranged be-tween20 and44 nmand the cross section between 11 and 14 nm. Crys-tal size is quite uniform and the agglomeration tendency is small. Thesample obtained at pH of 8.5 is formed from nanoparticles smallerthan 20 nmwith irregular shapes. The increase of the pH to 10.5 assuredthe formation of well formed particles of 23 to 53 nm, with an averageof 32 nm.

3.4. Surface charge

The surface charge of the catalyst also plays a significant role on thedegradation of the dye. It is known thatMB dye has a positively chargedsurface generated from a nitrogen centre on its organic framework [32].Following measurement of the electrical charge of the particles it wasobserved that for all the TiO2 samples, the surfaces are negativelycharged. The TiO2 prepared at pH 3 had a surface charge of −0.25nC/g, whilst for TiO2 formulated at pH 8 and 10.5, the surface chargewas−0.15 nC/g and−0.1 nC/g. The value of negative electric chargeof the particles decreased with the increase of the pH of the solution.

3.5. UV–Vis spectroscopy

Fig. 4 shows the UV–Vis absorption spectra of TiO2, obtained at dif-ferent pH levels. With an increase of the pH from 8.5 to 10.5, a blueshift of the absorption edge can be noticed due to the decrease of thecrystallite size from 16 nm to 13 nm. A higher absorption edge wasnoted in the sample prepared at low pH due to the rutile content ofthe sample, which has an absorption band at higher wavelengths com-pared with the anatase phase [19].

The determination of the energy band gap using Tauc's relationship(Eq. (1)) [22] is presented in Fig. 5. It can be observed that the energyband gap increased from 3.25 eV for the sample synthesised in an acidmedium to 3.31 eV for the sample prepared at pH 8.5 due to the rutilecontent of the sample synthesised at pH 3. Rutile has a lower energyband gap when compared to pure anatase [11]. In the case of the sam-ples obtained in an alkaline medium, the energy band gap increasedfrom 3.31 eV for the sample prepared at pH 8.5 to 3.39 eV for the sam-ple prepared at pH 10.5. The increase in Eg can be correlated to thereduction of the crystallite size that determined quantum size effect,which induce a blue shift of the absorption edge in the optical absor-bance [12].

The plots used for the determination of Urbach energy are presentedin the insert of Fig. 5. Urbach energy decreased with increase of pH(Table 2) being linked to the decrease of the micro-strain calculatedfrom XRD data.

Fig. 4. Absorption spectra of TiO2 samples synthesised at different pHs, after heat treatment at 400 °C for 1 h.

26 A. Molea et al. / Powder Technology 253 (2014) 22–28

3.6. Photodegradation process

The results of the photo degradation process for an MB dye inthe presence of the TiO2 suspensions, under both UV-A light andvisible irradiation, are shown in Fig. 6. It was observed that thephotodegradation process of MB took place faster in the presence ofTiO2, synthesised at pH 3 but decreased for the sample synthesised atpH 8.5 and 10.5. The increase of pH gave an obvious decrease of thephotodegradation efficiency that can be correlated with the increase ofthe band gap.

Fig. 5. The determination of the energy band gap of TiO2

Since MB is a cationic dye and has a positively charged surface, themolecules are attracted on TiO2 catalyst surface negatively charged.The attraction is stronger in the case of the catalyst formulated atpH 3 also explaining the highest photoactivity of the sample and thehigher value of the negative charge.

The efficiency of the photodegradation process increased from 17%in the presence of TiO2 synthesised at pH 10.5 to 47% in the presenceof TiO2 synthesised at pH 3 after 300 min of irradiation. These resultsare in good agreement with literature data [19]. No activity was detect-ed for the blank sample.

powders. Insert - determination of Urbach energy.

Table 2Energy band gap and Urbach energy of the TiO2 samples synthesised at different pHs.

Sample Energy band gap [eV] Urbach energy [meV]

TiO2 pH 3 treated at 400 °C, 1 h 3.25 980TiO2 pH 8.5 treated at 400 °C, 1 h 3.31 551TiO2 pH 10.5 treated at 400 °C, 1 h 3.39 620

27A. Molea et al. / Powder Technology 253 (2014) 22–28

At low concentrations of dye, the photodegradation reactions exhib-ited first order kinetics mechanism [26,27]. Fig. 7 shows that the photodegradation reaction of MB in the presence of TiO2 synthesised at vari-ous pH exhibit first order kinetics. By plotting the ln(CMB/CMB

0 ) versus ir-radiation time, the rate constant, k, was determined. This decreasedfrom 2.27 × 10−3 [min−1] in the presence of TiO2 synthesised at pH 3to 0.73 × 10−3 [min−1] for the sample synthesised at pH 10.5. The kinet-ic model shows that the sample synthesised at pH 3 has a photocatalyticactivity three times higher than the sample synthesised at pH 10.5 due tothe lower energy band gap [19].

It should benoted that the TiO2 samples exhibit photocatalytic activityeven if the radiation intensity of the lampwas very low (0.183 mW/cm2).

4. Conclusion

Titanium dioxide powders were successfully synthesised by a hy-drolysis method, using a titanium tri-chloride inorganic precursor. Theinfluence of pH on the formation of TiO2 crystalline phases and on pho-tocatalytic activity was studied. Based on XRD measurements it wasestablished that at high level of the pH, only the anatase phase of TiO2

has been obtained whilst under acidic conditions rutile and anataseco-exist, but rutile is the predominant phase. TiO2 samples synthesisedat various pHs exhibited a trend towards higher wavenumber of theRaman peak from 144 cm−1, due to the quantum size effect, which

Fig. 6. Photodegradation of Methylene Blue in the presence of TiO2 catalysts. Insert - the effici

implies also an increase of the micro-strain. SEM images reveal thatthe samples have the particle size between 20 and 50 nm.

The photodegradation experiments indicated that all the TiO2

samples exhibit some photocatalytic activity, even if the intensity ofthe lamp radiation was low. However the TiO2 sample synthesised atpH 3,which contained both anatase and rutile crystalline phases, exhib-ited the highest photocatalytic activity due to the lower energy bandgap and higher value of the negative charge on the surface. The efficien-cy of Methylene Blue photo degradation in the presence of TiO2 pre-pared at pH 3 was 47% under both UV-A light and visible irradiation,after 300 min.

Acknowledgments

This paper was supported by the project SIDOC, contract no.POSDRU/88/1.5/S/60078 and project Human Resource Developmentby Postdoctoral Research on Micro and Nanotechnologies, ContractPOSDRU/89/1.5/S/63700.

The authors acknowledge Gabriela Buda from Technical Universityof Cluj-Napoca, for surface charge measurements. The authors wouldalso like to thank to Jacqueline Deans from School of Chemistry, Univer-sity of Birmingham for assistance during XRD measurements and toDr James Bowen from Laboratory of Advanced Materials 2, School ofChemical Engineering, University of Birmingham for assistance withRaman microscopy. The equipment used in XRD measurement wasutilised through the Science City Advanced Materials Project: Creatingand Characterising Next Generation Advanced Materials, with supportfrom Advantage West Midlands (AWM) and part funded by theEuropean Regional Development Fund (ERDF). The Confocal RamanMi-croscope used in this research was obtained, through the BirminghamScience City: Innovative Uses for Advanced Materials in the ModernWorld (West Midlands Centre for Advanced Materials Project 2), withsupport from Advantage West Midlands (AWM) and part funded bythe European Regional Development Fund (ERDF).

ency of photodegradation process, under low UV-A and visible irradiation, after 300 min.

Fig. 7. The first order kinetics of the degradation of Methylene Blue dye with irradiation time.

28 A. Molea et al. / Powder Technology 253 (2014) 22–28

References

[1] Y. Lin, C. Ferronato, N. Deng, J.-M. Chovelon, Study of benzylparaben photocatalyticdegradation by TiO2, Appl. Catal. B Environ. 104 (2011) 353–360.

[2] X. Zeng, Y.X. Gan, E. Clark, L. Su, Amphiphilic and photocatalytic behaviors of TiO2nanotube arrays on Ti prepared via electrochemical oxidation, J. Alloys Compd.509 (2011) L221–L227.

[3] A. Molea, V. Popescu, N.A. Rowson, Effects of I-doping content on the structural, op-tical and photocatalytic activity of TiO2 nanocrystalline powders, Powder Technol.230 (2012) 203–211.

[4] X.-L. Peng, M.-M. Yao, F. Li, X.-H. Sun, Microstructures and photocatalyticproperties of S doped nanocrystalline TiO2 films, Part. Sci. Technol. 30 (2012)81–91.

[5] Y. Liu, L. Hua, S. Li, Photocatalytic degradation of Reactive Brilliant Blue KN-R byTiO2/UV process, Desalination 258 (2010) 48–53.

[6] Y.Y. Hsu, T.L. Hsiung, H.P. Wang, Y. Fukushima, Y.L. Wei, J.E. Chang, Photocatalyticdegradation of spill oils on TiO2 nanotube thin films, Mar. Pollut. Bull. 57 (2008)873–876.

[7] Y. Shao, D. Tang, J. Sun, Y. Lee, W. Xiong, Lattice deformation and phase transforma-tion from nano-scale anatase to nano-scale rutile TiO2 prepared by sol-gel tech-nique, China Part. 2 (2004) 119–123.

[8] G. Li, L. Li, J. Boerio-Goates, B.F. Woodfield, High purity anatase TiO2 nanocrystals:near room-temperature synthesis, grain growth kinetics, and surface hydrationchemistry, J. Am. Chem. Soc. 127 (2005) 8659–8666.

[9] K. Yu, J. Zhao, Y. Guo, X. Ding, H. Bala, Y. Liu, Z. Wang, Sol–gel synthesis and hydro-thermal processing of anatase nanocrystals from titanium n-butoxide, Mater. Lett.59 (2005) 2515–2518.

[10] Y.C. Lee, Y.J. Jung, P.Y. Park, K.H. Ko, Preparation of TiO2 powder by modifiedtwo-stage hydrolysis, J. Sol-Gel Sci. Technol. 30 (2004) 21–28.

[11] C.K. Lee, D.K. Kim, J.H. Lee, J.H. Sung, I. Kim, K.H. Lee, J.W. Park, Y.K. Lee, Preparationand characterization of peroxo titanic acid solution using TiCl3, J. Sol-Gel Sci.Technol. 31 (2004) 67–72.

[12] S. Lee, I.-S. Cho, J.-H. Noh, K.S. Hong, G.S. Han, H.S. Jung, S. Jeong, C. Lee, H. Shin, Cor-relation of anatase particle size with photocatalytic properties, Phys. Status Solidi A207 (10) (2010) 2288–2291.

[13] S. Cassaignon, M. Koelsch, J.-P. Jolivet, From TiCl3 to TiO2 nanoparticles (anatase,brookite and rutile): thermohydrolysis and oxidation in aqueous medium, J. Phys.Chem. Solids 68 (2007) 695–700.

[14] X. Bokhimi, A. Morales, F. Pedraza, Crystallography and crystallite morpholo-gy of rutile synthesized at low temperature, J. Solid State Chem. 169 (2002)176–181.

[15] A.L. Castro, M.R. Nunes, A.P. Carvalho, F.M. Costa, M.H. Florencio, Synthesis of ana-tase TiO2 nanoparticles with high temperature stability and photocatalytic activity,Solid State Sci. 10 (2008) 602–606.

[16] Z. Chen, G. Zhao, H. Li, G. Han, B. Song, Effects of water amount and pH on the crystalbehavior of a TiO2 nanocrystalline derived from a sol–gel process at a low temper-ature, J. Am. Ceram. Soc. 92 (2009) 1024–1029.

[17] M.A. Behnajady, H. Eskandarloo, N. Modirshahla, M. Shokri, Investigation of theeffect of sol–gel synthesis variables on structural and photocatalytic properties ofTiO2 nanoparticles, Desalination 278 (2011) 10–17.

[18] H. Xie, L. Zhu, L. Wang, S. Chen, D. Yang, L. Yang, G. Gao, H. Yuan, Photodegradationof benzene by TiO2 nanoparticles prepared by flame CVD process, Particuology 9(2011) 75–79.

[19] T. Lopez, R. Gomez, E. Sanchez, F. Tzompantzi, L. Vera, Photocatalytic activity in the2,4-dinitroaniline decomposition over TiO2 sol-gel derived catalysts, J. Sol-Gel Sci.Technol. 22 (2001) 99–107.

[20] Z. Abbas, J. Perez Holmberg, A.K. Hellstriom, M. Hagstrom, J. Bergenholtz, M.Hassellov, E. Ahlberg, Synthesis, characterization and particle size distribution ofTiO2 colloidal nanoparticles, Colloids Surf., A 384 (2011) 254–261.

[21] W. Kraus, G. Nolze, POWDER CELL - a program for the representation and manipu-lation of crystal structures and calculation of the resulting X-ray powder patterns,J. Appl. Crystallogr. 29 (1996) 301–303.

[22] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amor-phous germanium, Phys. Status Solidi 15 (1966) 627–637.

[23] N. Serpone, D. Lawless, R. Khairutdinovt, Size effects on the photophysical propertiesof colloidal anatase Ti02 particles: size quantization or direct transitions in this indi-rect semiconductor? J. Phys. Chem. 99 (1995) 16646–16654.

[24] S.H. Kang, J.W. Lim, H.S. Kim, J.Y. Kim, Y.H. Chung, Y.E. Sung, Photo and electrochem-ical characteristics dependent on the phase ratio of nanocolumnar structured TiO2

films by RF magnetron sputtering technique, Chem. Mater. 21 (2009) 2777–2788.[25] C.H. Ao, S.C. Lee, Indoor air purification by photocatalyst TiO2 immobilized on an ac-

tivated carbon filter installed in an air cleaner, Chem. Eng. Sci. 60 (2005) 103–109.[26] M. Mohseni, F. Taghipour, Experimental and CFD analysis of photocatalytic gas

phase vinyl chloride (VC) oxidation, Chem. Eng. Sci. 59 (2004) 1601–1609.[27] N. Talebian, M.R. Nilforoushan, Comparative study of the structural, optical and pho-

tocatalytic properties of semiconductor metal oxides toward degradation of methy-lene blue, Thin Solid Films 518 (2010) 2210–2215.

[28] L. Borgese, E. Bontempi, M. Gelfi, L.E. Depero, P. Goudeau, G. Geandier, D. Thiaudiere,Microstructure and elastic properties of atomic layer deposited TiO2 anatase thinfilms, Acta Mater. 59 (2011) 2891–2900.

[29] S. Balaji, Y. Djaoued, J. Robichaud, Phonon confinement studies in nanocrystallineanatase-TiO2 thin films by micro Raman spectroscopy, J. Raman Spectrosc. 37 (2006)1416–1422.

[30] R. Parra, M.S. Goes, M.S. Castro, E. Longo, P.R. Bueno, J.A. Varela, Reaction pathway tothe synthesis of anatase via the chemical modification of titanium isopropoxidewith acetic acid, Chem. Mater. 20 (2008) 143–150.

[31] A. Golubovic, M. Scepanovic, A. Kremenovic, S. Askrabic, V. Berec, Z. Dohcevic-Mitrovic,Z.V. Popovic, Raman study of the variation in anatase structure of TiO2 nanopowdersdue to the changes of sol–gel synthesis conditions, J. Sol-Gel Sci. Technol. 49 (2009)311–319.

[32] A. Kurniawan, H. Sutiono, Y.-H. Ju, F.E. Soetaredjo, A. Ayucitra, A. Yudha, S. Ismadji,Utilization of rarasaponin natural surfactant for organo-bentonite preparation:application for methylene blue removal from aqueous effluent, MicroporousMesoporous Mater. 142 (2011) 184–193.