Journal of Colloid and Interface Science 348 (2010) 342–347

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Journal of Colloid and Interface Science

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Effect of the agglomeration of TiO2 nanoparticles on their photocatalyticperformance in the aqueous phase

Gang Li a, Lu Lv a, Haitao Fan b, Junyan Ma b, Yanqiang Li b, Yong Wan b, X.S. Zhao a,b,*

a Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singaporeb Institute of Multifunctional Materials, Laboratory of New Fiber Materials and Modern Textile, Growing Basis for State Key Laboratory, Qingdao University, Qingdao 266071, China

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

Article history:Received 4 February 2010Accepted 20 April 2010Available online 24 April 2010

Keywords:Colloidal TiO2

AgglomerationPhotocatalysisZeta potential

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.04.045

* Corresponding author at: Department of Chemicaing, National University of Singapore, 4 EngineeringFax: +65 67791936.

E-mail address: chezxs@nus.edu.sg (X.S. Zhao).

TiO2 nanoparticles have been widely explored as photocatalysts in the degradation of organic matterspresent in water. However, spontaneous agglomeration of TiO2 nanoparticles in a suspension is a crucialissue that must be addressed before the photocatalyst can be used for water treatment. In the presentwork, the nature of the agglomeration of TiO2 nanoparticles in aqueous suspension was investigated.Two approaches to minimize the agglomeration of colloidal TiO2 particles were investigated. A carefulcontrol over the pH of the system was found to be an effective method for stabilizing colloidal TiO2 par-ticles and to significantly enhance the adsorption of orange II. As a result, the overall photocatalytic deg-radation rate was greatly accelerated. In addition to pH control, modification of TiO2 particles usingpolyelectrolyte poly allylamine hydrochloride (PAH) was observed to be an effective approach for pre-venting colloidal TiO2 particles from agglomeration.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Nanosized titanium dioxide (TiO2) is a promising material forwater purification because of its high specific surface area [1–6].But practical applications of TiO2 nanomaterials are hindered dueto a number of problems, one of which is the spontaneous agglom-eration of TiO2 nanoparticles when dispersed in aqueous media,resulting in a rapid decrease in specific surface area, thus photocat-alytic activity. The agglomeration behavior of TiO2 particles in theaqueous phase has been studied previously [6–9]. Unfortunately,the potentially important effects of agglomeration on subsequentadsorption/photoreaction have not been considered in experi-ments despite mounting evidence that agglomeration is the rulerather than the exception [6]. A common concept is that such par-ticle agglomeration can rapidly decrease the effective surface areaof the photocatalyst, thus resulting in a fast loss in photocatalyticactivity. However, there is no report on the agglomeration behaviorof TiO2 nanoparticles at different particle concentrations and howsuch agglomeration can affect the photocatalytic performance.

Recent studies [10,11] have shown that special means like pro-tracted sonication cannot prevent the suspended nanoparticlesfrom agglomeration. The agglomeration process is strongly depen-dent on parameters, such as ionic strength and pH, of the suspen-sion [6,12]. To minimize the agglomeration of TiO2 particles,

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l and Biomolecular Engineer-Drive 4, 117576, Singapore.

strategies such as surface modification [7,13–18] and adjustmentof the system pH [19–21] have been reported. The idea is to stabi-lize colloidal TiO2 particles by enabling their surfaces to carry elec-tric charge to create electrostatic repulsive forces to repel eachother. It has been observed that the adsorption affinity of the mod-ified TiO2 particles toward organic matters can be greatly enhancedif the organic matter carries a charge opposite to the TiO2 particlesurface charge [21]. This in turn can dramatically improve the pho-tocatalytic reaction rate.

In the present work, the nature of the agglomeration of colloidalTiO2 particles at different concentrations was examined. The influ-ence of particle agglomeration on photocatalytic properties wasinvestigated. Two strategies, namely modification of TiO2 particlesusing polyelectrolyte and varying the system pH, were used to min-imize the agglomeration. New insights into the nature of TiO2 parti-cle agglomeration were gained.

2. Material and methods

2.1. Materials

A TiO2 powder sample (P25) with a specific surface area of50 m2/g was obtained from Degussa and used as received. Poly-electrolyte poly allylamine hydrochloride (PAH, Mw � 70,000)was obtained from Aldrich. Orange II purchased from Aldrich wasused as a model organic pollutant in the measurement of photocat-alytic properties. Orange II is a nonbiodegradable synthetic dyewith a molecular formula of C16H11N2NaO4S, being widely usedin the textile industry.

Table 1Photocatalytic reaction rate for decomposition of orange II with various catalystdosages.

Catalyst Photoreaction Normalized photoreaction

G. Li et al. / Journal of Colloid and Interface Science 348 (2010) 342–347 343

2.2. Modification of P25 TiO2 nanoparticles by PAH

First, a given amount of P25 powder was dispersed in deionized(DI) water under ultrasonication for 10 min. Second, the suspen-sion pH was adjusted to 9.3 using a 1 M NaOH solution under mag-netic stirring. Then, various amounts of PAH were subsequentlyadded to the suspension to allow deposition of PAH on the surfaceof the TiO2 nanoparticles.

2.3. Preparation of a fibrous TiO2 sample

A TiO2 sample with a fibreous morphology was prepared byusing P25 powder as the titanium source according to the methoddescribed in Ref. [22]. One gram of P25 powder was combined with50 mL of 10 M NaOH aqueous solution under stirring. The suspen-sion was then hydrothermally treated at 200 �C for 24 h in an auto-clave. The white solid was separated from the liquid phase bycentrifugation, washed with DI water, and dried at 70 �C. The resul-tant solid was then added to a HNO3 solution (0.1 M), hydrother-mally heated at 180 �C for 24 h, to form layered hydrogentitanate (H-titanate) nanofibers.

2.4. Characterization

Zeta potential profiles of P25 TiO2 particles before and aftermodification were measured using a Zetaplus 100. A solid samplewas suspended in a 0.001 M KCl solution and the pH of the suspen-sion was adjusted using a 1 M NaOH or HCl solution. Particle sizedistribution curves were measured using the dynamic laser lightscattering technique (Brookhaven Instruments, 90 plus). A field-emission scanning electron microscope (FESEM) (JEOL JSM-6700F) was used to observe the morphology of the TiO2 particles.

2.5. Evaluation of photocatalytic activity

The photocatalytic activities of the photocatalysts were evalu-ated by measuring the degradation rate of orange II under UV lightillumination. The photocatalytic measurements were carried out ina semibatch swirl flow reactor [23]. An aqueous suspension con-taining orange II and TiO2 was pumped tangentially into the reac-tor from a reservoir using a peristaltic pump. The reservoir wasjacketed and maintained at a temperature of 25 �C throughoutthe experiment. A high-pressure mercury vapor lamp (PhillipsHPR 125 W) with a wavelength of 365 nm was used as the UV-lightsource. Before the UV lamp was switched on for photodegradationthere is a dark adsorption period in which the P25 particles weremixed with orange II solution to reach the adsorption equilibrium.This dark adsorption was studied to evaluate the adsorption capac-ity of P25 particles based on the concentration variance of orange IIbefore and after mixing with P25. Samples were taken from thereservoir at regular intervals and filtered with Millex Millipore fil-ter (0.1 lm) to remove the catalyst before analysis. The orange IIconcentrations before and after adsorption/reaction were mea-sured using a UV–Vis spectrophotometer (UV-1601, Shimadzu).The pH of the orange II solution was adjusted using either a 1 MNaOH or 1 M HCl solution.

rate constant,K (min�1)

rate constant,K0(min�1 g�1 of TiO2)

P25 (0.15 g/L) 0.022 0.49P25 (0.30 g/L) 0.03 0.33P25 (0.50 g/L) 0.035 0.23P25 (0.70 g/L) 0.04 0.19P25 (1.0 g/L) 0.041 0.14P25 fiber (0.5 g/L) 0.029 0.19

Experimental conditions: initial orange II concentration = 30 mg/L, UV light inten-sity = 260 w/m2, neutral pH (7.1).

3. Results and discussion

Experiments were carried out to evaluate the photocatalyticactivities of commercially available P25 nanoparticles at variouscatalyst dosages in order to investigate the effect of particle con-centration on the agglomeration nature of the P25 nanoparticlesand how this agglomeration would influence photocatalytic prop-erties. A previous study [21] showed that the degradation of

orange II catalyzed by P25 follows the first-order kinetics modelwith respect to orange II. Table 1 summarizes the first-order pho-toreaction rate constant, K, at different catalyst dosages.

As can be seen in Table 1, the photoreaction rate increased withthe catalyst dosage. However, the increment was not linear in thewhole range of TiO2 dosage. For example, the photodegradationrate was only enhanced slightly when the catalyst dosage washigh. The photoreaction rates were then normalized to the catalystdosages (K0) and the results are presented in Table 1 as well. It isseen that the normalized photoreaction rate decreased with the in-crease in catalyst dosage. According to our experimental results,the most efficient catalyst dosage for photodegradation of orangeII was about 0.15 g/L, which was the lowest catalyst dosage studiedin this work. This is an indication of agglomeration of the nano-sized P25 particles, which resulted in a decrease in the effectivesurface area, thus the photocatalytic activity.

To confirm the effect of particle agglomeration on the photocat-alytic activity of P25, a highly agglomerated P25 fiber sample wasprepared [22]. The morphologies of both P25 and TiO2 fiber sam-ples were confirmed by the FESEM images shown in Fig. 1. Thephotocatalytic activity of the TiO2 fiber sample was evaluatedand is also reported in Table 1. It can be seen that at the same cat-alyst dosage, the TiO2 fiber showed a lower photoactivity than P25,about a 20% decrease in comparison with P25. These comparativeresults strongly suggest that with increase in solid dosage in thesuspension, spontaneous agglomeration occurred, resulting in theformation of large TiO2 particles (like the TiO2 fibers), leading toa loss in the effective surface area, and consequently a decreasein photocatalytic activity.

The P25 particle sizes in suspension at different dosages weremonitored using the dynamic laser light scattering technique andthe results are shown in Fig. 2. As can be seen, the mean particlesizes of P25 in suspensions at concentrations of 0.15 and 0.5 g/Lwere about 407 and 1220 nm, respectively. It is obvious that theparticle size increased greatly with the increase in catalyst dosage.This observation further confirmed that P25 nanoparticles agglom-erated spontaneously in suspension, especially at high dosages.

In this work, two methods were attempted to minimize theagglomeration of the P25 TiO2 nanoparticles. One was to changethe pH of the dispersion and the other one was to use polyelectro-lyte to modify the surface of TiO2 nanoparticles. Depending on pH,TiO2 particles can be rendered to carry either positive or negativecharges. If pH is below its isoelectric point (IEP), P25 particles willbe ionized to carry a positive charge while if pH is above its IEP P25will be ionized to carry a negative charge as illustrated below:

pH < IEP; TiOHþHþ ! TiOHþ2 ; ð1ÞpH > IEP; TiOHþ OH� ! TiO� þH2O: ð2Þ

With a surface charge, P25 nanoparticles can be colloidally sta-bilized because of the presence of electrostatic repulsive forces be-tween particles. Hence, particle agglomeration can be minimized

Fig. 1. FESEM images of P25 nanoparticles (A) and TiO2 nanofibers (B).

Fig. 2. Particle size distribution curves of P25 suspended under different condi-tions: (a) at pH 7.1 with a concentration of 0.15 g/L; (b) at pH 7.1 with aconcentration of 0.5 g/L, (c) at pH 2.6 with a concentration of 0.15 g/L; (d) P25(0.15 g/L) modified by PAH (6.67 mg/L).

Fig. 3. Effect of system pH on photocatalytic reaction rate at different catalystdosages. Experimental conditions: initial orange II concentration = 30 mg/L, UVlight intensity = 260 w/m2.

344 G. Li et al. / Journal of Colloid and Interface Science 348 (2010) 342–347

or avoided. With consideration of the fact that orange II will disso-ciate into negatively charged sulfonate ions in aqueous phase [21],an acidic P25 suspension system with various pH values was usedto enable strong electrostatic interactions between positivelycharged TiO2 particles and negatively charged sulfonate ions.

Fig. 2 shows the particle size distribution curves of P25 dis-persed in water under different conditions. It can be seen thatthe particle size is centered at about 407 nm at pH 7.1 (about neu-tral pH conditions), more than 10 times larger than that of the par-ticles in powder form (about 30–40 nm). This result confirmed theagglomeration of TiO2 nanoparticles in aqueous suspension atneutral pH. By adjusting the system pH to be about 2.6, the meanparticle size became about 79 nm, significantly smaller than thatunder neutral pH conditions. Therefore, particle agglomerationcan be minimized by altering the system pH.

Fig. 3 compares the photocatalytic reaction rate constant at pH7.1 (neutral conditions) with that at pH 2.6 (acidic conditions) inthe presence of different catalyst dosages. It is clearly seen thatTiO2 displayed a better performance under acid conditions thanit did under neutral conditions, indicating that altering the systempH is an effective means for improving the photocatalytic activityof TiO2 nanoparticles. To elucidate this observation, experimentswere conducted to study the dependence of dye adsorption andphotoactivity on pH.

Fig. 4 depicts the adsorption amount of orange II on P25 at var-ious pH values. It can be seen that the adsorption increased withdecreasing pH. This may be attributed to the electrostatic interac-tions between the TiO2 particles and the orange II. As seen in Fig. 5(curve b), the TiO2 particles are positively charged at system pH be-low pHpzc and zeta potential increased with the decrease of systempH. Therefore, as the pH was lowered the affinity between posi-tively charged TiO2 particles and negatively charged sulfonate ionsof orange II increased, resulting in strong adsorption of orange IIover TiO2 particles. The curves c–g in Fig. 5 represents zeta poten-tial trends of PAH-modified TiO2 particles and will be discussed inthe subsequent section.

The dependence of photodegradation rate on pH is shown inFig. 6. It is seen that the dependence followed the same trend asthe dependence of the adsorption of orange II on pH as shown inFig. 4. As the pH was increased from 2.29 to 6.15, the reaction ratelinearly decreased. The strong adsorption of orange II at low pH fa-vored the mass transfer of orange II from the bulk to the catalystsurface, leading to the enhancement of the photocatalytic activity.Therefore, it can be concluded that the strong adsorption on theTiO2 surface due to the strong electrostatic interactions at low sys-tem pH significantly facilitated the photocatalytic reaction.

According to the theory of classical heterogeneous catalysis,this conclusion implies that mass transfer from the bulk to thecatalyst surface is a rate-limiting step. However, it has beenreported that for photocatalytic reactions in a liquid phase, the

Fig. 4. Dependence of adsorption of orange II on system pH. Experimentalconditions: initial orange II concentration = 50 mg/L, catalyst dosage = 0.5 g/L.

Fig. 5. Zeta potential profiles of P25 + PAH (6.67 mg/L) after 95 min UV illumination(a), pure P25 (b), P25 + PAH (1 mg/L) (c), P25 + PAH (2 mg/L) (d), P25 + PAH(3.33 mg/L) (e), P25 + PAH (5 mg/L) (f), and P25 + PAH (6.67 mg/L) (g).

Fig. 7. Photodegradation of orange II over P25. Experimental conditions: initialorange II concentration = 30 mg/L, catalyst dosage = 0.5 g/L, system pH 9.30, UVlight intensity = 260 w/m2.

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photo-induced radicals may diffuse into the bulk liquid phase toinitiate the reaction [24,25], implying that the adsorption of orangeII on the P25 surface was not a necessary step in the photocatalyticreaction process. To ascertain this, photodegradation of orange IIwas conducted at pH 9.3. Under this pH condition, the surface ofP25 particles is negatively charged, thus adsorption of negativelycharged orange II ions would be strongly hindered because of the

Fig. 6. Dependence of photocatalytic rate constant on system pH. Experimentalconditions: initial orange II concentration = 50 mg/L, catalyst dosage = 0.5 g/L, UVlight intensity = 180 w/m2.

strong electrostatic repulsion between the solid surface and thedye ions. However, it is seen from Fig. 7 that at pH 9.3 photodegra-dation of orange II still occurred despite at a lower reaction ratethan that under pH 2.6. According to this experimental observa-tion, it can be hypothesized that photo-induced radicals indeed dif-fused in the bulk liquid from the catalyst surface to initiate thedecomposition of orange II molecules [24,25]. However, based onthe fact that these radicals have very short life spans and are tooreactive to move far from surface it is reasonable to believe this dif-fusion could only occur within a few nanometers away from cata-lyst surface, which greatly reduces the reaction rate. In addition,such low photoreaction rate can also be ascribed to the fact thata Coulombic repulsion between the negatively charged surface ofTiO2 and the hydroxide anions could prevent the formation ofOH radical and thus decreased the photooxidation rate [26].

Based on observation from Figs. 6 and 7, two plausible reactionpathways for photocatalytic decomposition of orange II over TiO2

dispersion are proposed, namely surface reaction over TiO2 parti-cles and reaction in suspension due to radicals diffusion. It shouldbe noted that the former dominates the heterogeneous reactionwhen the enrichment of reactants over the surface occurs whereasthe later could only happen at a lower rate within a limited dis-tance from solid catalyst surface.

In addition to varying system pH, another method, namelypolyelectrolyte modification, was also applied to minimize sponta-neous agglomeration and improve adsorption of orange II on TiO2

nanoparticles. As is seen from Fig. 5 (curves c–g), the surface of

Fig. 8. Adsorption of orange II on P25 at various PAH concentrations. Experimentalconditions: initial orange II concentration = 30 mg/L, solid dosage = 0.15 g/L, pH6.5–8.5.

Fig. 9. Photocatalytic degradation of orange II over PAH-modified P25 photocata-lyst. Experimental conditions: initial orange II concentration = 30 mg/L, catalystdosage = 0.15 mg/L, system pH 6.5–8.5, UV light intensity = 260 w/m2.

346 G. Li et al. / Journal of Colloid and Interface Science 348 (2010) 342–347

modified P25 was positively charged in the pH range studied. It isalso seen that zeta potential of P25 increased with the PAH concen-tration under a given pH value, indicating that the portion of P25surface covered by PAH increases with addition of PAH. Fig. 2(curve d) showed that the high zeta potential after modificationwith PAH can greatly reduce the particle agglomeration (about115 nm). It should be noted that the minimizing of particle aggre-gation was also due to steric stabilization derived from appliedpolymer (PAH) [27]. Therefore, by coating P25 with PAH, two syn-ergetic effects, namely surface charging and steric stabilization,work together to prevent TiO2 particle aggregation.

Fig. 8 shows the dependence of the orange II adsorption overPAH-modified P25 on the dosage of PAH. As expected, the modifiedP25 showed strong adsorption capacity in comparison with thebare P25 due to the electrostatic attraction. With the increase ofPAH concentration, there was a smooth improvement of theadsorption. Therefore, it can be concluded that both polyelectro-lyte modification of TiO2 and system pH variation are effectiveways to minimize the particle agglomeration and enhance theadsorption.

The photocatalytic activity of PAH-modified P25 was also inves-tigated and shown in Fig. 9. Surprisingly, the photoreaction rate

Fig. 10. Schematic representation of the dispersion of TiO2

decreased with the increase of PAH concentration. In addition, allPAH-modified P25 showed a poorer photocatalytic activity in com-parison with bare P25, although the former possessed a strongeradsorption capacity than the latter. This result is inconsistent withthe one derived from the study of varying system pH that thestrong adsorption could accelerate the photoreaction rate. Oneplausible reason is that for the PAH-modified P25 the polymerlayer attached on the P25 surface might impede the migration ofphoto-induced electrons to target pollutants (orange II). Thephoto-induced electrons were consumed by the decomposition ofPAH layer before they reached the organic dye molecule. As a re-sult, the polymer layer was decomposed and the amount of elec-trons which participated in the photodegradation of organicpollutants decreased. Subsequently, the overall photoreaction effi-ciency was lowered. This hypothesis was further confirmed by thezeta potential results shown in Fig. 5 (see curve a). After 95-min UVillumination of PAH-modified P25 (without orange II), the zeta po-tential profile of modified P25 became similar to that of bare P25,indicating that the PAH layer was completely decomposed duringUV illumination.

It should be highlighted that at low PAH dosage, the P25 surfacewas not completely covered by PAH as shown in Fig. 5; therefore,another possible path for degradation of orange II by PAH-modifiedP25 is that the photo-induced electrons might directly diffuse tothe adsorbed orange II molecules from the area which was not cov-ered by PAH. However, in the case of high PAH dosage, the surfacewas almost fully modified by PAH, thus, the electrons need to crossthrough the PAH layer and then reach orange II molecules, result-ing in the low photoreaction rate. As a result, the photoreactionrate for degradation of orange II over PAH-modified P25 decreasedwith the increase of PAH concentration. Fig. 10 summarizes theproposed pathways for photo-induced electron transfer overPAH-modified P25 and dispersion status of P25 nanoparticles un-der various experiment conditions.

4. Conclusions

Spontaneous agglomeration of nano-sized TiO2 nanoparticles ofdifferent concentrations was evaluated, to investigate the effect ofparticle on photoactivity. By controlling the system pH, the zetapotential of the TiO2 suspension was varied and particle agglomer-

nanoparticles under various experimental conditions.

G. Li et al. / Journal of Colloid and Interface Science 348 (2010) 342–347 347

ation was significantly minimized because of the strong electro-static repulsions between colloidal TiO2 particles. Adsorption of or-ange II on the surface of TiO2 nanoparticles was found to bestrengthened at low pH values because of the electrostatic attrac-tion between the dye and the TiO2 particles. This in turn acceler-ated the photoreaction rate. It was also experimentally observedthat photo-induced electrons can diffuse into the suspension, thusparticipating in photoreactions. Modification of TiO2 nanoparticlesby polyelectrolyte PAH can greatly abate particle agglomeration,thus improving the adsorption of orange II dye. However, the over-all photocatalytic reaction efficiency decreased in comparison withthe TiO2 nanoparticles before modification because the polyelec-trolyte layer retarded the migration of electrons from the bulk tothe surface.

Acknowledgment

Financial support from Environment and Water Industry Devel-opment Council (EWI) of Singapore under project number MEWR651/06/161 is acknowledged.

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