Production of stable hydrosols of crystalline TiO2 nanoparticles synthesized at relatively low temperatures in diverse media

Applied Surface Science 265 (2013) 317–323

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Production of stable hydrosols of crystalline TiO2 nanoparticles synthesized at relatively low temperatures in diverse media Esin Burunkaya a,b,∗ , Murat Akarsu a,b , H. Erdem C¸amurlu c,d,1 , Ömer Kesmez a,b , Zerin Yes¸il a,b , Meltem Asiltürk e,2 , Ertu˘grul Arpac¸ a,b a

Department of Chemistry, Akdeniz University, 07058, Antalya, Turkey NANOen R&D Ltd., Antalya Technopolis, Akdeniz University Campus, Antalya, Turkey c Department of Mechanical Engineering, Akdeniz University, 07058, Antalya, Turkey d Mattek Advanced Materials Ltd., Antalya Technopolis, Akdeniz University Campus, 07058, Antalya, Turkey e Department of Materials Science and Engineering, Akdeniz University, 07058, Antalya, Turkey b

a r t i c l e

i n f o

Article history: Received 18 June 2012 Received in revised form 31 October 2012 Accepted 2 November 2012 Available online 9 November 2012 Keywords: TiO2 Hydrosol Nanoparticle Photocatalysis Reflux synthesis Rhodamine B

a b s t r a c t TiO2 hydrosols were obtained by dispersing nanoparticles synthesized from titanium ethoxide as precursor via reflux method without any further thermal treatment. In this study, the reaction parameters such as solvent, type of catalyst, temperature and duration of the synthesis of TiO2 nanoparticles were extensively investigated. The crystalline nanoparticles obtained without calcination have particle size in range of 3.3 nm and 5 nm, and BET surface area of up to 182 m2 /g. Transparent TiO2 hydrosols were prepared in both water and non-polar solvent without use of any additional dispersing agent. The synthesized nanoparticles exhibited photocatalytic activity against Rhodamine B dye. © 2012 Elsevier B.V. All rights reserved.

1. Introduction It is a well-known phenomenon that photogenerated radicals of hydroxyl and superoxide upon the illumination of semiconductors such as TiO2 , ZnO and SnO2 with a source of ultraviolet or visible light break down organic substances. For this purpose, TiO2 particles are widely applied because of its low toxicity and cost, and high chemical stability. Besides exhibiting photocatalytic activity, TiO2 surface becomes superhydrophilic upon illumination, which makes its application possible in wide ranges of areas such as the production of self-cleaning and anti-fogging surfaces, air purification, photovoltaic devices, and antibacterial surfaces [1–4]. In a recent publication, preparation methods and properties of TiO2 particles have been comprehensively reviewed by Banerjee [1]. TiO2 particles can be synthesized through a number of methods

∗ Corresponding author at: Akdeniz Üniversitesi, Fen-Edebiyat Fakültesi, Kimya Bölümü, Dumlupınar Bulvarı, Kampüs, 07058, Antalya, Turkey. Tel.: +90 242 310 2327; fax: +90 242 227 8911. E-mail address: [email protected] (E. Burunkaya). 1 Akdeniz Üniversitesi, Makine Mühendisli˘gi Bölümü, Dumlupınar Bulvarı, Kampüs, 07058, Antalya, Turkey. 2 Akdeniz Üniversitesi, Malzeme Bilimi ve Mühendisli˘gi Bölümü, Dumlupınar Bulvarı, Kampüs, 07058, Antalya, Turkey. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.003

including sol–gel [5,6], from inorganic salts as precursor [7], ultrasonic technique [8], reflux synthesis [9] and hydrothermal method [6,10–12]. In most of these studies, the synthesized particles have been calcined to obtain crystalline TiO2 , which increases the particle size leading to a reduction in the total surface area and the photocatalytic efficiency [11]. Among these methods, hydrothermal and reflux synthesis methods result in crystalline TiO2 particles, therefore the calcination step can be eliminated. It is crucial to obtain crystalline TiO2 nanoparticles well dispersed in terms of obtaining transparent and photocatalytically active films on substrates, particularly heat-sensitive ones such as plastics, wood and fibers to preserve the genuine appearance of the applied substrate. The translucent sols of monodisperse TiO2 nanoparticles, so-called hydrosols, have been synthesized by hydrothermal treatment, where crystallization of hydrated TiO2 into anatase or rutile phase occurs in water or organic solvents at temperatures higher than their boiling point under high pressure [13,14]. Disadvantages of this method include the need of expensive autoclaves and high safety requirements for industrial uses. In the reflux method, reaction takes place in a container, which is heated at relatively low temperatures depending on the boiling point of the solvent. The container is connected to a cooled reflux condenser system open to atmosphere. The vaporized solvents are

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condensed back into the reaction medium and thus reaction duration can be prolonged. Low temperature, low equipment cost and possibility of obtaining crystalline products without the need of a calcination step are other advantages of this method as compared to other techniques. In literature, low temperature methods have been utilized for the preparation of TiO2 particles and films. Acidic hydrolysis of titanium isopropoxide was investigated by Bosc et al. [15]. Nanocrystalline (5 nm crystallite size) TiO2 films and powders were obtained at room temperature. After calcination at 350 ◦ C for 2 h, crystallite size was reported to increase to 15 nm. The obtained films were photocatalytically active. In another low temperature study, the anatase TiO2 powders were synthesized using titanium tetrachloride (TiCl4 ) as a precursor via the sol–gel method. The obtained powders were nanocrystalline and they were photocatalytically active [16]. Transparent TiO2 and TiO2 –ZrO2 (molar ratio Zr/Ti = 0.1) thin films were obtained through a low-temperature sol–gel route from nano-crystalline aqueous solutions. Films were treated at room temperature, 100 ◦ C and 500 ◦ C. Presence of ZrO2 was suggested to suppress the TiO2 grain growth. The photocatalytic activity of the prepared powders was inferior to that of Degussa P25 [17]. The purpose of this study is to prepare hydrosols of TiO2 nanoparticles by reflux synthesis, i.e. under ambient pressure, at relatively low temperatures. In this study, titanium ethoxide was used as precursor since its use has not reported so far to the best of authors’ knowledge. TiO2 hydrosols were obtained by dispersing synthesized TiO2 nanoparticles produced by hydrolysis and condensation of titanium ethoxide diluted with ethanol, n-butanol and n-hexanol. In addition, the parameters such as type of used catalyst, temperature and duration of reflux treatment were extensively investigated. The produced nanoparticles were characterized by X-ray diffraction (XRD), specific surface area and particle size analyses. Their photocatalytically efficiencies were determined by the photodegradation of Rhodamine B in aqueous medium.

2. Experimental procedure 2.1. Materials and apparatus Titanium ethoxide (Ti(OEt)4 containing 15% titanium isopropoxide (Ti(OPri )4 was used as a precursor for the preparation of TiO2 nanoparticles and purchased from Dupont under the trade name Tyzor ET. Titanium tetrabutoxide was supplied by Fluka. Hydrochloric acid from Merck (HCl, 37%), nitric acid from Merck (HNO3 , 65%), acetic acid from Merck (CH3 COOH, 100%), phosphoric acid from AK Kimya (H3 PO4 , 85%), sulfuric acid from Merck (H2 SO4 , 95–98%), were utilized as catalyst in hydrolysis reactions. Ti(OEt)4 and acids were used without further purification. Ethanol (Merck, 100%), n-butanol (Aldrich, 98%), n-hexanol (Aldrich, 98%) and toluene (SDS, >98%) which served as solvents, were stored over molecular sieve (Fluka, 3 A◦ XL8) to remove contaminant water. Palmitic acid (90%) used in surface modification received from Aldrich. For the hydrolysis of Ti(OEt)4 self-made deionized water was used. Photocatalytical efficiencies of produced nanoparticles were evaluated by the photodegradation of Rhodamine B (RB) dye supplied by Avocado and used as purchased. In order to determine the crystalline phases in the obtained powders, a Rigaku Geigerflex D Max/B model X-ray diffractometer (XRD) with Cu K␣ radiation ( = 0.15418 nm) was used in the region 2 = 10–70◦ with a step size of 0.04◦ . Micropore volume and BET specific surface area of TiO2 nanoparticles were evaluated from the N2 adsorption isotherm by a Micrometrics ASAP 3000 BET analyzer at liquid N2 temperature. Particle size distributions of particles were determined by a Malvern Zetasizer Z/S based on dynamic light scattering method. Elemental analyses were performed by a

Leco 932 model elemental analyzer. All IR spectra were recorded by Bruker Tensor 27 using ATR and KBr-pellet methods for liquid and solid samples respectively. TiO2 /Rhodamine B solution was irradiated without a cut-off filter in a solar box (Erichsen, Model 1500) with a Xe lamp (690 W m−2 ). The change in the concentration of the aqueous RB solution upon irradiation was monitored by a Varian Carry 5000 model UV–Vis–NIR spectrophotometer. 2.2. Synthesis of TiO2 nanoparticles An ordinary lab apparatus consisting of condenser cooled with tap water, round flask and hotplate stirrer is used for the synthesis of nanoparticles. For a given experiment, Ti(OEt)4 was dissolved in ethanol (or in n-hexanol, n-butanol) and the solution was cooled in an ice-bath having a temperature of 4 ◦ C. After stirring it for 5 minutes (min), the solution of HCl (or HNO3 , CH3 COOH, H3 PO4 , H2 SO4 ) was added dropwise into alkoxide solution by a buret at a rate of 1 mL/min. In all of the samples, Ti(OEt)4 /ethanol, Ti(OEt)4 /H2 O and Ti(OEt)4 /hydrochloric acid mol/mol ratios were 0.5, 0.5 and 2, respectively. All parameters studied for the preparation of TiO2 nanoparticles are listed in Table 1. 5 min later water was added into the final solution at the same rate as well. After keeping it stirred for further 10 min at room temperature, the reaction flask was placed in oil bath and then heated to the boiling point of the used solvent, which is 78 ◦ C for ethanol, 118 ◦ C for n-butanol and 157 ◦ C for n-hexanol. The reaction solutions were treated at a given temperature for 5, 10 and 20 hours (h). TiO2 nanoparticles were obtained after separating the precipitated solid from the solution by centrifuging. Finally the synthesized particles were characterized. 2.3. The evaluation of photocatalytic efficiencies of TiO2 hydrosols against RB Mixtures containing 10 ppm RB and 62.5 ppm TiO2 of each sample were prepared from stock solutions of hydrosols of obtained TiO2 particles and RB in water. For every sample, two parallel sets of photodegradation experiments were carried out. In each experiment, 20 mL of the mixture of TiO2 and RB solutions was poured into a transparent polystyrene (PS) closed box with equal departments followed by keeping them in dark to reach equilibrium and then it was placed in the solar box to irradiate. Decomposition of RB was followed by detecting the change in the RB concentration in every 5–10 min in a total of 70 min during UV and vis-irradiation. The photodegradation of RB was quantitatively determined by calculating the area under the absorbance band at 548 nm (max ) of the dye recorded by a UV/vis spectrophotometer. 3. Results and discussion As H3 PO4 , H2 SO4 and CH3 COOH were used as catalysts; all solutions of titanium ethoxide in ethanol, n-butanol and n-hexanol were gelled during addition of water. Therefore, they were not investigated further. 3.1. Characterization of TiO2 nanoparticles XRD diagrams of precipitates obtained upon refluxing in ethanol in the presence of HCl as catalyst for 5 h, 10 h and 20 h are presented in Fig. 1(a), (b) and (c) respectively. In these XRD diagrams, it is seen that the width of the XRD peaks becomes narrower and the intensity of the peaks increases with the extended period of the heat treatment, which indicates that the crystallization of nanoparticles is the result of the prolonged treatment under reflux. As XRD patterns of samples (4B and 7) prepared in n-butanol and n-hexanol for 20 h, presented in Fig. 1(d) and 1(e), respectively, are compared to those of in ethanol (1A, 1B and 1C), the samples 4B and 7 had higher

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Table 1 Parameters applied during synthesis of TiO2 particles. Number

Solvent

Acid

Reflux time (h)

1A 1B 1C 2A 2B 2C 3 4A 4B 5A 5B 6 7 8 9 10 11 12 13 14 15

Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol Ethanol n-Butanol n-Butanol n-Butanol n-Butanol n-Butanol n-Hexanol n-Hexanol n-Hexanol Ethanol n-Butanol n-Hexanol Ethanol n-Butanol n-Hexanol

HCl HCl HCl HNO3 HNO3 HNO3 CH3 COOH HCl HCl HNO3 HNO3 CH3 COOH HCl HNO3 CH3 COOH H3 PO4 H3 PO4 H3 PO4 H2 SO4 H2 SO4 H2 SO4

5 10 20 5 10 20

a

Reflux temperature (◦ C) 93 93 93 93 93 93

a

a

10 20 10 20

133 133 133 133

Particle size 4.3 5.4 5.8 3.9 6.1 6.3 a

3.3 3.5 4.1 4.8

a

a

20 20

172 172

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

a

3.3 a

Complex formed.

degree of crystallinity where the temperature obviously was the main factor for synthesizing crystalline particles. All samples have a crystalline phase of anatase. All diffraction peaks of anatase phase could be clearly determined in XRD patterns of the synthesized particles. Planes of anatase phase pertaining to the peaks in the XRD patterns are drawn in Fig. 1. Peaks in XRD patterns at 25.39◦ , 38.11◦ , 48.47◦ and 55.01◦ pertaining to (1 0 1), (0 0 4), (2 0 0) and (2 1 1) planes match well with those of anatase phase reported in the literature [18] (ICDD card no: 21-1272). The broad width of the diffraction peaks in the XRD diagram indicates the existence of nanosized particles [19,20]. Particle size distributions of the TiO2 particles determined by dynamic light scattering method are presented in Fig. 2 and Table 1. The size of the TiO2 particles synthesized in ethanol gets larger upon the extended period of heat treatment soaring from 4.3 nm for 5 h to 5.8 nm for 20 h by percentage of 35. When n-butanol and n-hexanol were uses as solvent, no precipitation took place in first 5 h of heat treatment. At the end of 20 h, the sizes of particles obtained in nbutanol and n-hexanol were 3.5 nm and 3.3 nm respectively. Use of solvents with longer alkyl chain resulted smaller particles sizes despite raised temperature.

TEM micrographs of the TiO2 particles labeled as 1A, B, C, 4B and 7 are given in Fig. 3a–e, respectively. The sizes of the individual particles are clearly seen to be in 5 nm scale or less. The pattern of regular ordered structure in every individual particle in TEM micrographs indicates that particles are crystalline. Although in the XRD diagrams of samples 1A, 1B and 1C the diffraction patterns of anatase phase barely are recognizable, their TEM pictures strongly point out that they are crystalline. Those TEM pictures of samples confirm the distribution of particle sizes in Table 1 and Fig. 2 obtained by dynamic light scattering. The molar ratio of Ti(OR)4 to alcohol was kept at 2/1 in this study. Therefore, the concentration of metal alkoxide varied with the molecular weight of used solvent, where it was the most diluted in n-hexanol, vice versa the most concentrated in ethanol in this study. It is well-known that as metal alkoxides are dissolved in a solvent of alcohol, a certain fraction of alkoxy groups on metal alkoxide are exchanged with solvent molecules. As in this study when titanium ethoxide is diluted with a solvent with longer alkyl group, e.g. n-butanol and n-hexanol, some of ethoxy groups on it will be replaced with butoxy and hexoxy groups, which leads to slower rates of hydrolysis and condensation reactions due to

Fig. 1. XRD patterns of samples 1A, B, C, 4B and 7.

Fig. 2. Particle size distributions of samples 1A, B, C, 4B and 7.

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Fig. 3. TEM micrographs of samples (a) 1A, (b) 1B, (c) 1C, (d) 4B and (e) 7.

steric hindrance result of the exchange of alkoxy groups with longer alkyl groups [21]. Since the condensation reactions of hydrolyzed species of metal alkoxides advance slower in diluted solutions and an exchange of alkoxy groups by dilution with long chained alcohols leads to steric hindrance, the growth of formed TiO2 particles is limited, hence the sizes of the particles in ethanol, n-butanol and n-hexanol became smaller respectively as expected, e.g. 5.8 nm in ethanol, 3.5 nm in n-butanol and 3.3 nm in hexanol after the heat treatment of 20 h, where hydrochloric acid was applied as catalyst. BET areas of samples 1A, 1B, 1C, 4B and 7, where samples were first dispersed in water, dried in an oven and then BET analysis are performed on those samples, are listed in Table 2. As seen in Table 2, BET areas of samples increase from 160 m2 /g to 180 m2 /g with decreasing the size of particle as expected. Photocatalytic activity of nano sized TiO2 powder largely depends on the particle size and specific surface area. It is expected that the lower the particle size and the higher the specific surface area, the higher the

photocatalytic activity. Calcination step was omitted in the employed method, since crystalline anatase phase TiO2 was obtained at low temperatures. This provided preserving the nanostructure of the TiO2 particles; therefore the obtained powders exhibited high specific surface areas. 3.2. Preparation and characterization of TiO2 hydrosols All samples can, as expected, be dispersed in both water and a mixture of hexane and butyl glycol (BG) at the ratio of 10 to 1 by weight without use of any dispersing agent, where the percentage of TiO2 content in the obtained wet samples is calculated by determining weight loss after calcination at 400 ◦ C in air. In addition, the use of ultrasonic treatment of sols in a bath has considerably shortened time required for complete dispersion. A picture of dispersed sols of sample 4A as an example is presented in Fig. 4a, in which the solid content of crystalline TiO2 nanoparticles in both sols is 10% by

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Fig. 4. (a) Picture of 10% by weight of sample 4B in water (left) and hexane–BG (right). (b) Illustration of preparation of TiO2 hydrosols in water and hexane–BG.

Fig. 5. IR spectra of n-butanol, TBT and sample 4A.

weight. The sols are completely transparent in both water and mixture of hexane–BG and the dispersion of sols has not changed over 9 months. In order to investigate the reason behind this phenomenon, the IR spectrum of sample 4A dried under vacuum to remove remaining free solvent has been recorded and it was compared to those of n-butanol and titanium tetrabutoxide (TBT). The IR spectra of nbutanol, TBT and sample 4A are presented in Fig. 5. The peaks of asymmetric and symmetric stretching vibrations of C O groups on n-butanol appear at 1071 cm−1 and 1042 cm−1 respectively. Alkyl groups on n-butanol molecule give absorption bands at about 1454–1380 cm−1 and 2900 cm−1 [22]. The peaks of C O groups bonded to titanium on TBT are slightly shifted to 1081 cm−1 and 1030 cm−1 because of high electropositivity of titanium atom while the location of peaks of alkyl groups on TBT at about 2900 cm−1 has not changed [21]. The symmetric stretching of C O groups on

sample 4A gives an absorption peak at 1033 cm−1 same as on TBT but its asymmetric peak is almost disappeared [23]. This points out that the asymmetric stretching of C O groups on the surface of TiO2 particles is not possible anymore because the relatively high weight of nanoparticle compared to hydrogen or titanium atoms hinders it. The investigations of IR spectra prove that butoxy groups on the surface of the obtained TiO2 particles in n-butanol exist and an exchange of alkoxy groups takes place during heat treatment under reflux where ethoxy groups on Ti(OEt)4 are replaced with butoxy groups on solvent. The presence of broad band at about 3250 cm−1 shows that there are plenty of hydroxyl groups on the surface of particles originated by Ti O H groups and adsorbed water, as well [24,25]. When sample 4A was dispersed in water and subsequently dried in an oven, all absorption bands of butoxy groups were completely disappeared from the IR spectrum (see Fig. 6). To study the reactivity of butoxy groups on the particle surface, sample 4A was dispersed in toluene with palmitic acid followed by drying it under vacuum. The spectrum of sample 4A with palmitic acid is presented in Fig. 6. As seen in Fig. 6, the absorption band of stretching vibration of C O groups on the TiO2 particles has vanished and another band has appeared at 1520 cm−1 . When titanium alkoxide reacts with organic acid, it forms a complex of chelating bidentate, which gives absorption bands at 1540–1520 cm−1 and 1445 cm−1 for asymmetric and symmetric vibrations, respectively [26,27]. The peak of asymmetric stretching vibration of Ti O2 C groups on the particle surface can clearly be identified in the IR spectrum but the one of symmetric stretching is overlapped with peaks of alkyl groups on palmitic acid. The lack of acidic carbonyl peak at 1701 cm−1 (not presented here) and an increase in intensities of peaks of alkyl groups support the formation of bidentate complex on the particle surface. Based on IR investigations, the surface of obtained TiO2 particles is covered with alkoxy groups, which are still reactive. The existence of those alkoxy groups in large numbers in wet samples makes the surface of TiO2 particles sufficiently hydrophobic to disperse in the mixture of hexane–BG. When those alkoxy groups are hydrolyzed, the particle surface becomes hydrophilic. Hence, TiO2 particles become electrostatically stabilized in water in presence of

Table 2 Results of specific surface area, pore volume, elemental analysis on selected samples. Sample

SBET (m2 /g)

VTotal

3 pore (cm /g)

1A 1B 1C 4B 7

161.4 167.8 167.0 166.0 182.4

0.0654 0.0761 0.0762 0.0744 0.0817

VMicropore (cm3 /g)

VMesopore (cm3 /g)

wt%C

wt% H

0.0195 0.0188 0.0174 0.0280 0.0199

0.0458 0.0574 0.0589 0.0464 0.0618

4.17 3.53 1.95 3.40 8.65

2.82 2.28 2.14 1.95 2.53

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Fig. 7. Photodegradation of RB by selected samples and by P25. Fig. 6. IR spectra of sample 4A surface-modified with palmitic acid, sample 4A and sample 4A dispersed in water and then dried in an oven.

acid used as catalyst as illustrated in Fig. 4b, where the formation of any agglomerate is avoided. Additionally, all wet samples were dried under vacuum and then measurements of BET specific surface area were performed, which were less than 5 m2 /g or not measureable at all. When those samples are dispersed in water and dried in an oven, the specific surface area increased to more than 160 m2 /g (see Table 2). This observation confirms the occupation of particle surface by alkoxy groups in addition to IR investigations, as well. In order to obtain desirable applications from nanomaterials, it is necessary to attach reactive organic groups to the surface of the inorganic nanocrystals. The usual method for the functionalization of TiO2 surface is basically to attach groups using chelating ligands which possess additional functions that would allow further applications [28]. It was stated in the above discussion that the surface of the prepared TiO2 particles contain hydroxyl and alkoxyl groups. The presence of these groups brings about the possibility of post-synthesis functionalization of the prepared TiO2 particles. The uses of TiO2 particles can be enhanced and varied with further functionalization. For instance, it was possible to functionalize the TiO2 particles having surface hydroxide groups with porphyrin, which considerably enhanced the photocatalytic activity [29]. In another study, synthesis of in situ surface functionalized, monocrystalline rutile TiO2 nanorods was investigated using a hydrothermal method [30]. Functionalization allowed the control of the size, morphology, and crystal modification. The amine group functionality of the prepared rutile TiO2 nanorods was suggested to be suitable for future TiO2 nanobiocomposites and biotracers. 3.3. Evaluation of the photocatalytic efficiencies Aqueous hydrosols of samples 1A, 1B, 1C, 4B and 7 were prepared and used in experiments to evaluate their photocatalytic efficiencies. All samples tested were optically transparent as presented in Fig. 4a. No comparative sample could be obtained or prepared at the level of clarity as much as samples of our hydrosols. A colloidal sol of a commercial TiO2 powder (Degussa P25) was prepared and photocatalytic degradation results were compared with the results of the prepared TiO2 powder. Therefore, a comparison to a known photocatalyst or dispersed TiO2 sol could be made.

The results of experiments are reported in Fig. 7. The concentration of dye RB has remained unchanged over the period of experiments as seen in Fig. 7. RB has been linearly photo- degraded by samples 4B and 7 whereas samples 1A, 1B and 1C have broken down it exponentially. After irradiation time of 45 min, the difference between performances of photocatalytic efficiencies of samples was clearly recognizable. The remaining percentages of RB after 45 min are 15%, 19%, 22%, 36% and 45% for samples 1A, 1B, 1C, 4B and 7 respectively. The photocatalytic efficiencies of samples 1A, 1B and 1C have soared with a decrease in the size of particles, although the sample 1A had highest content of carbon (see Table 2). The lowest efficiency of sample 7 may be result of its high content of carbon in spite of having smallest size of particle. When sample 4B is compared to 1A, it has lower content of carbon (3.4% for 4B to 4.17% for 1A) and smaller particle size (3.5 nm for 4B to 4.3 nm for 1A). However, it has performed badly at the photo-degradation of RB. The reason for this has not been studied since it lies beyond the scope of this paper. P25 was seen to exhibit considerably lower photocatalytic activity than that of the prepared TiO2 powders. The remaining percentage of RB after 45 min when P25 was used as the photocatalyst was 70%.

4. Conclusion TiO2 particles have been synthesized in ethanol, n-butanol and n-hexanol by hydrolyzing of Ti(OEt)4 with under stoichiometric amount of water in the presence of acidic catalysts followed by heat treatment at temperatures as low as 93 ◦ C under reflux. Since the obtained particles already have crystalline phase of anatase, no calcination step is required. The size of particles is under 6 nm. The surface modification of particles takes places in situ with alkoxy groups of used solvent during heat treatment. These alkoxy groups are still reactive and can be hydrolyzed or replaced with functional groups such as organic acids, which make post-synthesis functionalization of particle surface possible. The prepared TiO2 particles may be benign in applications such as production of optically transparent photoactive coatings, preparing sun blocking creams and making dye sensitized solar cells. As expected, hydrosols of TiO2 particles are homogeneously dispersed in both polar and non-polar media without use of any dispersing agent because of existence of alkoxy groups on the surface of TiO2 particles. The obtained particles have shown a considerable photocatalytic activity against RB.

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