Construction and Building Materials 41 (2013) 224–230

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Construction and Building Materials

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Effective dispersion of nano-TiO2 powder for enhancement of photocatalyticproperties in cement mixes

Azam Yousefi, Ali Allahverdi ⇑, Parisa HejaziSchool of Chemical Engineering, Iran University of Science and Technology, Tehran 1684613114, Iran

h i g h l i g h t s

" Nano-TiO2 dispersion in simulated cement media is studied using zeta potential values and particle size distribution." High agglomeration of nano-TiO2 in conventional mixing methods reduces the photocatalytic properties notably." Dispersion of nano-TiO2 in lime-saturated water and ultrasonication treatment, prior to mixing with cement, is proposed." Effective dispersion of nano-TiO2 along with the formation of CaTiO3, display enhanced photocatalytic properties.

a r t i c l e i n f o

Article history:Received 2 July 2012Received in revised form 24 October 2012Accepted 22 November 2012

Keywords:Nano-TiO2

DispersionParticle size distributionZeta potentialPhotocatalytic activityUltrasonication

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.11.057

⇑ Corresponding author. Tel.: +98 21 77240496; faxE-mail address: ali.allahverdi@iust.ac.ir (A. Allahve

a b s t r a c t

Photocatalytic properties of nano-TiO2 can be effectively utilized in the removal of some of the environmen-tal pollutants under UV irradiation. Cement based-coatings including nano-TiO2 for exterior surfaces of thebuildings are therefore attracting increasing interests. Due to high agglomeration of nano-TiO2 in cementmedia, their photocatalytic activity is significantly diminished. Agglomeration of nano-TiO2 particles incement mixes can be effectively avoided by suitable dispersion of the particles in lime-saturated waterfollowed by an ultrasonication treatment, prior to mixing with cement. This effective dispersion along withthe formation of CaTiO3, display enhanced photodegradation of Malachite Green solution from 45 to 15 minunder UV irradiation.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Although about 200 years have passed since the discovery ofTiO2 and 40 years since the revelation of photocatalytic activityof nano-TiO2 [1], a limited research work is reported about theproper distribution of nano-TiO2 in cement media and the effectof nano-TiO2 dispersion on its photocatalytic activity. Mostresearch works have reported on the primary particle size ofnano-TiO2 and its mixing procedure with water and cement. Theyhave not however concentrated on the final size and particle sizedistribution of nano-TiO2 in alkaline medium of cement paste.Based on more detailed studies, when nano-TiO2 is used in cement,it is inclined to strong agglomeration [2,3].

When nanoparticle powders are dispersed in solutions, theirhydrodynamic size is larger than their primary size. After distribu-tion in solution, nanoparticles tend to remain as agglomerates or

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aggregates, surrounded by an electrical double layer that includesboth the inner and outer layers [4]. There is a potential developedin the space between nanoparticles and liquid media, which istermed ‘‘zeta potential’’. When all the suspended particles havethe same charge, they tend to repel each other and no agglomerationtakes place. The degree of repelling precipitated particles has directrelationship with zeta potential. The suspensions are consideredstable when the zeta potential of particles is above +30 mV or below�30 mV [4]. By increasing the pH, the zeta potential of suspendedparticles decreases and reaches a zero point. This zeta potential istermed ‘‘isoelectric point’’ (IEP). Suspension systems at isoelectricpoint are at the lowest stability [5]. Zeta potential and particle sizeare affected by several factors such as the ionic strength of solution,type of dispersant, surface functional groups of particles and pH ofliquid medium, for which pH is the most sensitive among others[4]. Due to environmental humidity, the surface of titanium dioxidenanoparticles is covered by hydroxyl groups [3,5]. The surfaces oftitanol groups (Ti–OH) are amphoteric and their ionization equilib-rium can be considered as follows [6,7]:

A. Yousefi et al. / Construction and Building Materials 41 (2013) 224–230 225

BTi� OHþHþ $ BTi� OHþ2 pH < IEP ð1Þ

BTi� OHþ OH� $ BTi� O� þH2O pH > IEP ð2Þ

With agglomeration phenomenon, the specific surface area andphotocatalytic activity decrease [3,8], so that catalyzed reactionsoccur on the semiconductor surface. The agglomeration of nano-TiO2 reduces the photocatalytic degradation of orange II [9] andthe removal rate of nitrogen monoxide from the air [3]. The agglom-eration of TiO2 nanoparticles can be considered as a catalyst poisonfor photocatalytic activity and the conditions giving rise to suchincident should be circumvented. The fragmentation of agglomer-ated nano-TiO2 is usually done by ball milling, high shear mixingand ultrasonication. Studies have indicated that ultrasonicationtechnique is more effective than other methods of breaking the par-ticle size [10–14].

Metallic oxides such as FeO, Fe2O3, Al2O3, SiO2 and CaO that ex-ist originally in cement can react with added TiO2 in long term. Byadding nano-TiO2 to a cement paste, the main accessible oxidewould be free CaO, because other metallic oxides of cement are en-closed in cement phases. Therefore, nano-TiO2 can easily react withCaO to produce CaTiO3 particles. CaTiO3 is a semiconductor anddisplays photocatalytic activity towards UV radiation [15,16]. Bandgap energy of CaTiO3 in most research works is reported to beabout 3.5 eV, though, in another research report [17], in additionto the values above 3.5 eV, some other values of less than 3.2 eVhave also been reported. The pH of isoelectric point of CaTiO3 hasbeen reported to be about 3 [18].

This study is devoted to the effective dispersion of P25nano-TiO2 powder for enhancement of photocatalytic propertiesin cement mixes. Different concentrations of nano-TiO2 powderwere dispersed both in deionized water and lime-saturatedwater followed by mechanical stirring or ultrasonication. Forevaluating the effect of dispersion on photocatalytic properties,band gaps were determined and photodegradation rates ofMalachite Green solution under UV irradiation and visible lightwere measured.

2. Materials and methods

2.1. Materials

In the current research, nano-TiO2 powder (P25, provided by Degussa, Ger-many) with an average diameter of 21 ± 10 nm, specific surface area of 50 m2/g and 99.5% purity is used. Calcium hydroxide and Malachite Green (MG) pow-ders have been purchased from Merck Chemical Co. Germany. MG is used as amodel organic pollutant in the study of photocatalytic properties of thesuspensions.

Table 1Characteristics of nano-TiO2 dispersions.

Suspension TiO2

Concen.(g/L)

Nano-TiO2

a

(%)

Ultrason.time (min)

pH Dispersant

P1W 38.46 1 0 3.80 DI-waterU5P1W 38.46 1 5 3.88U15P1W 38.46 1 15 5.40

P1C 38.46 1 0 11.85 SaturatedCa(OH)2

solutionP0.1C 3.84 0.1 0 11.37U5P1C 38.46 1 5 11.28U15P1C 38.46 1 15 10.78U15P0.1C 3.84 0.1 15 12.20U15P0.3C 11.52 0.3 15 12.13U15P0.5C 19.2 0.5 15 11.76

a By weight of cement as equivalent concentration in cement paste.

2.2. Equipment

A Badelin 2070 ultrasound probe was used for dispersion of nanoparticles inaqueous media. The power of the instrument was maintained constant (70 W,0.7 s pulse on and 0.3 s pulse off). The dispersions were sonicated in cold-waterbath maintained at 6 �C to avoid overheating during ultrasonication. The crystal-line phases of samples were determined using an X-ray diffractometer (XRD,D4-Bruker, Cu Ka radiation, k = 0.154 nm) and FTIR spectroscopy was carried outwith KBr pellet using a Shimadzu-8400S FT-IR spectrometer. Also to determinethe absorption of visible and ultraviolet lights, UV–vis absorption spectra wereused using a Shimadzu-2550 spectrometer. The band gap energies of catalystswere calculated from the analytical data. Zeta potential values and TiO2 (P25) par-ticle size distribution measurements were carried out by dynamic light scattering(DLS). The analyses were performed using a Malvern light scattering unit, zetasizerNano ZS (Malvern, UK). The pH of the suspensions was determined with a PercisapH-meter, Switzerland. Optical absorption of MG dye solution was found by aMetertech UV–vis spectrophotometer (SP8001 model, Taiwan). For separation ofthe particles from aqueous media, a microcentrifuge of Vs-15,000 N, Uision, Koreawas used.

2.3. Methods

Suspensions were prepared by mixing nano-TiO2 in deionized water (DI-water)and saturated Ca(OH)2. A concentration of 38.46 g/L of nano-TiO2 would be equiv-alent to 1% P25 in the cement paste with a water-to-cement ratio of 0.26. In order todisperse nanoparticles, the prepared suspensions were subjected to 1 h mechanicalstirring or 5–15 min probe ultrasonication as shown in Table 1.

Immediately after preparation, from each suspension, 8 mL was transferred intoa graduated cylinder and the sedimentation rates were measured at different timeintervals by dividing the moving phase interface to the initial level of 8 mL.

The test trials for the photocatalytic activity of the suspensions included a radi-ation chamber equipped with a 160 W UV lamp (Narva, Germany) and a 200 W vis-ible lamp. MG color solution with concentration of 10 ppm and nano-TiO2 powdershave been transferred to a double-neck glass reactor with constant aeration inambient temperature. Color suspensions were shaken for 30 min in the dark under-going absorption and desorption processes for the suspensions to reach equilib-rium. After this period of time, when the lamp test went on, every 5 min asample was taken for analysis. Then, samples were transferred to the microcentri-fuge set with 15,000 rpm for 10 min and the medium was separated from TiO2 par-ticles, and 2 mL of the solution was transferred to UV–vis spectrophotometer cell torecord the value of absorbance. For calculating photocatalytic activity efficiency ofnanoparticles, the maximum absorbance (618 nm) of solution and Eqs. (3) and (4)were used, in which C0, C, A0, A represent the initial concentration, concentrationafter each sampling, initial absorbance and absorbance after each measurement,respectively.

C ¼ C0 � AA0

ð3Þ

%X ¼ C0 � CC0

� 100 ð4Þ

3. Results and discussion

The X-ray powder diffraction pattern of nano-TiO2 is shown inFig. 1. By comparing it with standard cards (21–1272 and 21–1276), it is revealed that the only crystalline phases present inthe material are anatase and rutile as the dominant and secondaryphases, respectively. According to Spurr and Myer method [19], theratio of anatase phase to rutile phase is 75:25. Crystallite size wascalculated about 28 nm by using Scherrer equation [2,3].

3.1. Nano-TiO2 dispersion in DI-water

Size distribution diagrams of P25 nano-TiO2 powder dispersed inDI-water are presented at Fig. 2. The suspension P1W prepared bysimple mechanical stirring of P25 nano-TiO2 powder in DI-water re-sulted in an average particle size of 128.3 nm, which is not in thenanometer range. Ultrasonication technique was therefore used toreduce the agglomeration of the nanoparticles. As shown in Fig. 2,when the suspensions went under ultrasonication for 5 or 15 min(U5P1W or U15P1W suspension), they showed average sizes of179 and 1281 nm. At the first, agglomerates of nano-TiO2 in DI-water tend to be very small, when using the ultrasonication.

Fig. 1. XRD pattern of nano-TiO2.

Table 2Variation of pH and zeta potential of P25 nano-TiO2 powder dispersed in DI-waterand saturated Ca(OH)2 solution.

Suspension pH Zeta potential (mV)

P1W 3.80 3.9U5P1W 3.88 �11.1U15P1W 5.40 �14.3

P1C 11.85 �4.85U5P1C 11.28 �14U15P1C 10.78 �6.17

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However, particles are re-agglomerated due to dominant attractiveforces over repulsive forces, and as a result the smaller particles arelikely to agglomerate [10]. Therefore, it is suggested that the disper-sion of nano-TiO2 should take place in water, and immediately sur-factants should be used after the ultrasound operation until no re-agglomeration occurs for very small produced particles.

The IEP of nano-TiO2 in water has been reported in the scientificliterature between 2 and 9 [4–6,10–12,14,20,21]. According to re-sults given in Table 2, characterized with changes of pH and zetapotential of particles in high concentration of nano-TiO2 (38.46 g/L), the surface charge of particles is negative. According to the nor-mal changes of zeta potential with pH of the media [2], the IEP ofthe nanoparticle is less than 3.5 (Table 2). This low amount of IEPfor nano-TiO2 suspensions in aqueous media is due to high concen-tration of nanoparticles.

Also, according to upper part of Table 2, all suspensions of nano-TiO2 that are dispersed in DI-water have an acidic pH, which is dueto absorption of hydroxyl groups of water on TiO2 surface (Eq. (5))[5]. With absorption of hydroxyl groups on the surfaces of TiO2

particles, the concentration of H+ ions are increased in suspensionsand media become acidic (pH decreases from 7 to 3.8). However,with increasing ultrasonication time, particles are re-agglomer-ated, while their specific surface area is decreased, and absorptionof hydroxyl ions and the production of H+ ions are decreased,which finally leads to higher pH of the medium (pH changes from3.8 to 5.4).

TiþH2O! Ti� OHþHþ ð5Þ

3.2. Nano-TiO2 dispersion in saturated Ca(OH)2 solution

As it was shown, for dispersed nano-TiO2 in DI-water an opti-mal size in nanometer scale cannot be obtained with and without

Fig. 2. Effect of ultrasonication on particle size distribution of P25 nano-TiO2 powder d(left side).

the use of ultrasonication. So another path is investigated to reducethe particle size in aqueous media. One of the factors affecting theparticle agglomeration size is pH of the medium. Since cementpaste is a high pH-value medium, it is logical to disperse nano-TiO2 powder in an alkaline environment similar to cement suspen-sion, such as saturated Ca(OH)2 solution. After the usual magneticstirring of the dispersion of nano-TiO2 particles in saturatedCa(OH)2 solution (P1C suspension), a high agglomeration ofnano-TiO2 appears in complex media compared with water (P1Wsuspension) as dispersant (Figs. 2 and 3). Particle size of nano-TiO2 has increased in DI-water from 128.3 to 4468 nm in saturatedCa(OH)2 solution. Therefore, the least efficient mixing methods areaddition of the nano-TiO2 aqueous suspension to cement powderor manual mixing of cement powder and nanoparticles followedby the addition of water (P1C state). In P1C, the pH of the mediumincreases rapidly and calcium divalent ions cause severe agglomer-ation of nanoparticles, which produce an unstable suspension. Asshown in Fig. 3, by increasing ultrasonication time from 5 to15 min (U5P1C and U15P1C suspensions), initial particle size dropsfrom 4468 to 97 and 69 nm. The positive effect of the ultrasoundon agglomeration breakage of nanoparticles and their suitable dis-persion is observed in saturated Ca(OH)2 solution. Increase inultrasonication time from 5 to 15 min did not result in significantsize change for the particles; thus the longer time of ultrasonica-tion does not affect particle size reduction. The best and smallestparticle size is available in saturated Ca(OH)2 solution by the ultra-sonic power of 70 W (0.7 s on and 0.3 s off) for 15 min. In U15P1Csuspension, 82% of the particles represent sizes below 100 nm.

3.3. Stability of nano-TiO2 suspensions

Suspensions of nano-TiO2 in DI-water are relatively stable,though their size is not desirable. Prepared suspensions of nano-TiO2 in saturated Ca(OH)2 solution by the ultrasonication havethe suitable size, but they settle after one hour as shown in Fig. 4.

Although the pH of the saturated Ca(OH)2 solution is well awayfrom the IEP of the nanoparticles and the suspension system is ex-pected to be stable, but due to the presence of calcium divalent

ispersed in DI-water in terms of number (right side) and light scattering intensity

Fig. 3. Effect of ultrasonication on particle size distribution of P25 nano-TiO2 powder dispersed in saturated Ca(OH)2 solution in terms of number (right side) and lightscattering intensity (left side).

A. Yousefi et al. / Construction and Building Materials 41 (2013) 224–230 227

ions and their absorption on TiO2 negative surfaces, the particlesagglomeration is intensified, and the resulting suspension is highlyunstable (P1C suspension).

In Table 2, zeta potential variation is presented in relation to pHof the suspensions. This parameter not only has a negative valuefor all samples, but its absolute value is also very small. So, all sus-pensions are unstable [3]. With equal amounts of nano-TiO2 pow-der mixed with DI-water, and addition of Ca(OH)2 and assumingphysical interactions, due to the electrostatic attraction betweencalcium ions and the negative surface of nano-TiO2, the values ofinitial zeta potential (�3.9, �11.1, �14.3) increase to higher values.This trend is observed in suspension with 15 min ultrasound(U15P1C). The trend change may be due to chemical interactionsother than physical interactions between Ca(OH)2 and nano-TiO2.The differences of zeta potential of P1C and U5P1C with similarsuspensions of nano-TiO2 dispersed in water (P1W and U5P1W)can be attributed to chemical reactions between TiO2 and Ca(OH)2,producing CaTiO3. Particles of CaTiO3 have lower negative zeta po-tential compared to TiO2 particles in an alkaline environment [22].The surface zeta potential of particles in saturated Ca(OH)2 solu-tion is more negative than zeta potential of dispersed particles inDI-water. Therefore, the chemical absorption between TiO2 andCa(OH)2 has overtaken physical adsorption in these suspensions.

Preparing suitable suspension (U15P1C) and mixing with ce-ment powder does not lead to severe change in pH and agglomer-ation of nanoparticles. The time interval between the productionand commixture of nano-TiO2 suspension with cement powder isnot so long and, during this time nano-TiO2 suspension is withoutany stabilizer. After mixing nano-TiO2 suspensions and cement, thecement hydration process occurs, and the resulting paste begins toharden with nanoparticles well distributed between the cementhydrating grains.

In most published articles on nano-TiO2 in cement, only the pri-mary particle size of nano-TiO2 is reported, and there is no ex-tended work on secondary particle in the alkaline medium ofcement paste. A number of articles have studied dispersion of

Fig. 4. Effect of ultrasonication on phase separation of suspensions in saturated Ca(

nano-TiO2 [4,6,10–12,20], but in spite of using low concentrationin aqueous media and without the presence of complex ions, noneof the studies have succeeded to break particles to their originalsize by ultrasonication in the presence of surfactants, although1–2 h ultrasonication and surfactants have been employed. This re-sult could be due to electrical double layer around the dispersedparticles, thickness of the polymer layer or the Zetasizer devicewhich considers all particles as spherical. So the obtained resultsof the smallest particle size (69 nm) in this study, i.e., U15P1C sus-pension with high concentrations (38.46 g/L) of nano-TiO2 in theenvironment of high alkalinity and in the presence of calcium diva-lent ions can be considered as a success, because the simulated ce-ment environment is one of the most complex environments in thetopic of distribution of nanoparticles and their stability.

3.4. CaTiO3 formation

FTIR spectra of Ca(OH)2, nano-TiO2 (P25) and particles extractedfrom P1C and U15P1C suspensions are presented in Fig. 5.

The bands observed around 1600 and 3400 cm�1 are assigned toasymmetric vibration modes of the hydroxyl group [9,23–25].Therefore, the surface of nanoparticles is covered with OH� group[23]. The results of this analysis for particles extracted from P1Cindicate the chemical interaction between Ca(OH)2 and nano-TiO2. In P1C sample, within 400–1000 cm�1, the spectral absorp-tion of TiO2 and oxygen bonds is distinctly different from P25nano-TiO2 powder FTIR spectrum. The reason for these changesmay be related to the presence of CaTiO3 at 400–600 cm�1

[18,23,25–28]. In this spectrum, a double peak is created in 400–600 cm�1 [18], and it is a sign of CaTiO3 formation [25,29,30].

XRD analysis can be helpful, even though the percentage ofCaTiO3 in P1C and U15P1C samples is less than 2%. The sample ta-ken for XRD analysis must have at least over 3% CaTiO3 for it to becorrectly analyzed. Therefore, another sample (P0.1C) is preparedfor its higher purity percentage. XRD analysis for P0.1C is presentedat Fig. 6. As it is seen, there is a big difference in XRD pattern of this

OH)2 solution (L0: suspension initial height, L: suspension height at any time).

Fig. 5. FTIR spectra of Ca(OH)2, nano-TiO2 (P25) and particles extracted from P1C and U15P1C suspensions.

Fig. 6. XRD pattern of P0.1C.

Fig. 7. Effect of CaTiO3 formation on the rate of sedimentation and the instability ofnano-TiO2 suspensions.

228 A. Yousefi et al. / Construction and Building Materials 41 (2013) 224–230

sample and P25 nano-TiO2 powder in Fig. 1. The presence of dom-inant phase of anatase is still visible. The major characteristicpeaks of CaTiO3 are present at 2h = 32.9, 47 and 59.2 [19,23,25,26,29–31]. By comparing XRD pattern of P0.1C with standard cards(82-0232), it is revealed that the crystalline phase of CaTiO3 ispresent in this sample. CaTiO3 have perovskite structure.

3.5. Effect of CaTiO3 formation on suspensions stability

Sedimentation rate of the suspensions of different concentra-tions were measured. The obtained results show higher sedimenta-tion rates for suspensions of lower concentrations. Reason of rapidsedimentation of suspensions can be due to further formation ofCaTiO3 in those suspensions. Assuming that all Ca(OH)2 is con-verted to CaTiO3 by reaction with TiO2, the quantity of CaTiO3 isconstant in suspensions, whereas the amount of TiO2 varies. There-fore the concentration of CaTiO3 decreases by increasing the TiO2

concentration in suspensions. According to calculations exhibitedin Fig. 7, with reduced quantity of nano-TiO2 in saturated Ca(OH)2

solution, CaTiO3 formation percentage is increased; therefore theinstability and sedimentation rate are increased. Due to insolubil-ity of CaTiO3 in water [18], its formation has enhanced nucleationprocess and van der waals attraction forces between the particles.

3.6. Photocatalytic activity test of nano-TiO2 suspensions

The effect of CaTiO3 formation, due to the reaction betweennano-TiO2 and Ca(OH)2, on photocatalytic activity of nano-TiO2

was examined, as the smaller the band gap energy the moreelectron holes would be formed, which increase the photocatalyticactivity of nano-TiO2. UV–vis absorption spectral analysis ofsamples was performed and the results of the band gap energyof dried suspensions were calculated using Eq. (6) [32]. Table 3represents the obtained values for band gap energies.

Eg ¼ hc=k ð6Þ

Eg is the band gap energy (eV); h is Planck’s constant; C is lightvelocity (m/s); k is the wavelength (nm).

With regard to band gap energies presented in Table 3 andreduction of Eg of catalysts relative to P25 catalyst (with noCaTiO3), the formation of CaTiO3 decreases the band gap energiesto below 3.2 and increases photocatalytic activity of nano-TiO2.In order to confirm the higher photocatalytic activity of nano-TiO2 dispersed in saturated Ca(OH)2 solution, the photodegrada-tion of MG solution (10 ppm) was carried out by powder samplesof P25, P1C and U15P1C (1 g/L) under UV radiation. Fig. 8 showsabsorption curve of MG solution in the presence of the catalysts.There is a reduction in maximum peak intensity (618 nm) withoutany shift towards lower wavelengths.

It is shown that where the total photodegradation of MG solutionis completed after 15 min by CaTiO3-containing samples under UVradiation, the rate of degradation by nano-TiO2 (P25) is about45 min. Band gap energies of CaTiO3-containing samples are lyingwithin the visible range, Therefore the same test was carried out un-der visible light with an extra sample containing a higher percentageof CaTiO3 (i.e., U15P0.1C state). This test showed that all suspensions

Table 3The band gap energy of catalysts.

Sample Band Gap (eV)

P25 3.204P1C 3.009U15P1C 3.002P0.1C 3.002U15P0.1C 3.071

Fig. 8. Variation in absorption intensity of MG solution alone and in the presence ofcatalysts, under UV radiation.

Fig. 9. Photodegradation of MG solution under visible light by various catalysts.

A. Yousefi et al. / Construction and Building Materials 41 (2013) 224–230 229

containing CaTiO3 are more active than suspension of P25 catalystunder visible light.

The results of photodegradation rate of MG solution during 1 hunder visible light are shown in Fig. 9. MG solution without cata-lyst is designated as ‘‘Vis’’ as a control sample.

As it is observed, CaTiO3-containing samples display photocata-lytic activity under visible light and with increased amount ofCaTiO3 in U15P0.1C, in relation to P1C and U15P1C, MG solutionhas been discolored significantly. Although the amount of nano-TiO2 in U15P0.1C is less than other samples, the amount of CaTiO3

formation is higher i.e., with lower amount of nano-TiO2 present,the photocatalytic activity is increased. This test shows the positiveeffect of CaTiO3 formation and suitable size distribution of nano-particles on photocatalytic properties of the nanoparticles. Thephenomenon could be interpreted in a simple way as follows:

(i) Folli et al. in [2] showed that RhB1 degradation is mainly dri-ven by dye-sensitised photoreaction rather than TiO2-sensi-tised photoreaction. RhB can be degraded by visible light inthe presence of ordinary TiO2 because visible light promoteselectrons from the HOMO2 to the LUMO3 of the RhB molecule.These more mobile electrons can be transferred to the con-duction band of TiO2 where an electron scavenger like O2 (ifadsorbed) can be reduced to superoxide radical. This starts aseries of equilibria generating species that ultimately degradethe RhB itself. Dyes like methylene blue, methyl orange andmany others (including Malachite Green) are known forundergoing similar mechanisms.

(ii) Folli et al. in [3] showed that agglomerates formed by nano-particles with primary particle size comparable to the P25used here tend to show an interparticle porosity below10 nm. This would imply that big organic molecules likedyes cannot easily access the interparticle porosity. Theoverall photocatalytic activity therefore decreases as a con-

1 Rhodamine B.2 The highest occupied molecular orbital.3 The lowest unoccupied molecular orbital.

sequence of such a surface area loss induced by agglomera-tion [3].

On the basis of the evidences (i) and (ii), results shown in Fig. 9,about the degradation of MG (under visible light), can be very sim-ply reconciled with a dye-sensitised photodegradation (rather thana TiO2-sensitised photodegradation, i.e. a pure photocatalyticmechanism) which is enhanced in the case of the samples inCa(OH)2 after ultrasonication due to the reduced agglomerationand more available surface area.

The highest activity was exhibited by U15P0.1C, although itsband gap energy is more than P1C and U15P1C. This may be relatedto the more effective dispersion of nano-TiO2 and the higher avail-able surface area in this case, or due to the further CaTiO3 percent-age in U15P0.1C sample and therefore, its dye-sensitisedphotodegradation rate is more than the other.

In fact, the performance of the activated nanoparticles in visiblelight is improved with the proper mixing design of nanoparticles insaturated Ca(OH)2 solution. When the nano-TiO2 particles are dis-persed in this solution and added to the cement powder, the dis-persion of particles in the cement paste remains satisfactory byhydration of cement grains.

4. Conclusion

The results of zeta potential and particle size distribution showhigh agglomeration of nano-TiO2 in conventional mixing with ce-ment paste. The use of ultrasonication is inevitable for the distribu-tion of the nanoparticles. The proper mixing design and dispersionof nano-TiO2 in saturated lime water by ultrasound and adding theresulting suspension to cement powder not only lead to proper dis-tribution of nano-TiO2 in cement, but with CaTiO3 formation, thephotocatalytic properties of nanoparticles increase in the cementunder UV radiation and visible light.

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