Sol–gel synthesis of mesoporous anatase–brookite and anatase–brookite–rutile TiO2 nanoparticles and their photocatalytic properties

Journal of Colloid and Interface Science 442 (2015) 1–7

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Sol–gel synthesis of mesoporous anatase–brookite and anatase– brookite–rutile TiO2 nanoparticles and their photocatalytic properties Bridget K. Mutuma a,c, Godlisten N. Shao b,d,⇑, Won Duck Kim a, Hee Taik Kim b a

Department of Materials Science and Engineering, Kangwon National University, Samcheok-si, 271 Kangwon-do, Republic of Korea Department of Chemical Engineering, Hanyang University, 1271 Sa 3 dong, Sangnok-gu, Ansan-si, Gyeongi-do 426-791, Republic of Korea c School of Chemistry, University of Witswatersrand, Johannesburg WITS 2050, South Africa d Department of Chemistry, Mkwawa College, University of Dar es Salaam, Iringa, United Republic of Tanzania b

a r t i c l e

i n f o

Article history: Received 18 August 2014 Accepted 24 November 2014 Available online 3 December 2014 Keywords: Anatase–brookite TiO2 Sol–gel method Heterogeneous catalysis Photocatalysis

a b s t r a c t TiO2 photocatalysts with a mixture of different TiO2 crystal polymorphs have customarily been synthesized hydrothermally at high temperatures using complicated and expensive equipment. In this study TiO2 nanoparticles with a mixture of TiO2 crystals were synthesized using a modified sol–gel method at low temperature. In order to form nanoparticles with different polymorphs a series of samples were obtained at pH 2, 4, 7 and 9. Raw samples were calcined at different temperatures ranging from 200 to 800 °C to evaluate the effect of the calcination temperature on the physico-chemical properties of the samples. XRD results revealed that a mixture of anatase and brookite can be obtained in the as-synthesized samples and in those calcined up to 800 °C depending on the pH used to obtain the final product. Indeed, a mixture of anatase brookite and rutile; or a sample with only rutile phase can be yielded through further calcination of the as-prepared samples at temperatures P600 °C due to phase transformation. The photocatalytic performance of the samples with a mixture of anatase–brookite; anatase– brookite–rutile; and anatase–rutile (Degussa P25 TiO2) was exquisitely investigated in the degradation of methylene blue solutions. The samples obtained at pH 2 and calcined at 200 °C possessed the highest activity of all due to its superior properties. This study elucidates a facile method suitable for the synthesis of TiO2 with different mixtures of TiO2 polymorphs with desirable properties for various applications. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Metal oxide heterogeneous catalysts have attracted attention due to their effectiveness in degrading toxic organic contaminants both in air and aqueous media. TiO2 is unquestionably an excellent photocatalyst due to its desirable properties such as chemical stability, photo corrosion resistance, non-toxicity, strong oxidizing power and cost-effectiveness [1–5]. TiO2 is known to have three different polymorphs; rutile, anatase and brookite. The rutile phase with a band gap of 3.00 eV; is the most thermodynamically stable form of crystalline TiO2 while anatase and brookite with band gaps of 3.21 eV and 3.13 eV, respectively are the metastable forms of TiO2. Interestingly, anatase phase of TiO2 has been widely used in solar cells and photocatalysis. On the contrary, the metastable brookite phase has been overlooked as a result of its thermodynamic ⇑ Corresponding author at: Department of Chemical Engineering, Hanyang University, 1271 Sa 3 dong, Sangnok-gu, Ansan-si, Gyeongi-do 426-791, Republic of Korea. Fax: +82 314197203. E-mail address: [email protected] (G.N. Shao). http://dx.doi.org/10.1016/j.jcis.2014.11.060 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

instability and therefore few reports are available demonstrating its photocatalytic behavior [6,7]. Coupled semiconductor metal oxides nanocomposites allow displacement of electrons from one semiconductor to another leading to more efficient electron–hole separation and hence improve its photocatalytic performance [8–10]. Analogous to coupled semiconductor metal oxide nanocomposite systems are materials containing both anatase and rutile (AR) or anatase and brookite (AB) TiO2 crystals. The presence of more than one polymorph of TiO2 reduces the recombination effect to enhance the photocatalytic performance of the resulting sample than in pure single phase TiO2 [11–14]. It is thought that the existence of the different phases of the same semiconductor offer a synergetic junction effect property. For instance, biphasic TiO2 with an AR mixture such as found in Degussa P25 is a good photocatalyst due to the presence of this junction effect that enhances its electron hole separation [15–17]. Hiroaki and coworkers [18] postulated that the high photocatalytic activity of an AR mixture is due to interfacial electron transfer from anatase to rutile that increases the charge separation efficiency. Ohno et al. [19] suggested that the large band

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bending in rutile is responsible for the high reactivity of Degussa P25 (AR titania mixture) powders in the photocatalytic oxidation of naphthalene in air. Recently, Ismail et al. [20] has shown that the brookite phase is a potentially good photocatalyst due to its lower symmetry and its band gap that is close to that of anatase. Therefore, biphasic TiO2 with either (AB) or (AR) mixture has the potential to become a better heterogeneous photocatalyst than pure anatase due to the enhanced charge carrier separation [21–24]. However, most researchers have synthesized AB mixture using tedious and expensive synthetic approaches that hinder large-scale production and commercialization of TiO2 [25–27]. Generally, TiO2 with the brookite phase or a mixture of TiO2 polymorphs are obtained hydrothermally at high temperature and pressure conditions [28,29]. Additionally, a fine control of the parameters such as temperature, the nature of the precursor, water content, pH, catalyst, ionic strength and the nature of chelating agent is greatly required to promote phase selectivity; conditions that are difficult to reproduce [30–32]. An alternative route is the sol–gel process that has attracted much attention since it was introduced in the 1980s. The method allows the facile synthesis of stable metal oxides with better purity and homogeneity at ambient conditions. The present study reports a facile and reproducible method to synthesize anatase–brookite TiO2 nanoparticles using a modified sol–gel method in the absence of additives. The reactions were carried out at different pHs and calcined at different temperatures to study the effect of pH and sintering on the physicochemical properties of the samples. A simple comparative photocatalytic study was performed on the degradation of methylene blue in the presence of the AB mixture and a commercial sample with a mixture of the AR (Degussa P25 TiO2). In short, the photocatalytic activity of the samples with a mixture of AB, AR and anatase–brookite–rutile was exquisitely investigated (rarely reported study). 2. Experimental methods 2.1. Materials Titanium tetraisopropoxide (TTIP, 98.0%), isopropyl alcohol (99.7%) and methylene blue were purchased from Sigma Aldrich. Nitric acid (69%) and sodium hydroxide pellets (98%) were obtained from Dae-Jung Chemicals Ltd, South Korea. 2.2. Synthesis process In a typical experiment 30 ml of TTIP was mixed with 30 ml of isopropyl alcohol and stirred for 20 min. Deionized water (300 mL) was added to the mixture under vigorous stirring. The solution was then heated to 80 °C for 5 h and then cooled to room temperature. After cooling, the pH of the solution was controlled through addition of 1 M NaOH or 1 M HNO3 to obtain sols at pH 2, 4, 7 and 9. The sols were allowed to gel at room temperature (25 °C) for 24 h and the obtained gels were filtered, washed with distilled water and then rinsed with ethanol and finally dried at 100 °C for 12 h. The samples were calcined at 200, 600 and 800 °C (heating rate of 5 °C/min) for 2 h. 2.3. Photocatalytic test A glass reactor equipped with a 100 W high-pressure mercury lamp (UM-103B-B Ushio, Japan) was used and the temperature of the solution was maintained at 25 °C throughout the experiment. A methylene blue (MB) solution (32 mg/L; 200 mL) was placed in the reactor followed by the addition of 0.6 g/L of the TiO2 samples. The solution was magnetically stirred in the dark for 30 min to

ensure adsorption–desorption equilibrium. The suspension was irradiated under a UV light source as a function of irradiation time under constant stirring. Samples were withdrawn regularly from the reaction mixture and centrifuged (Heraeus Pico 17 Centrifuge-Thermo Scientific, Germany). The supernatant was analyzed by UV–vis spectroscopy (Optizen 2021 UV, Korea) to acquire absorption spectra of MB from 200 nm to 800 nm. Irradiation of an MB solution devoid of photocatalyst was carried out to act as a control for the experiment. The photocatalytic efficiency was substantially estimated from the equation D% = (C0–C/C0)  100% (C0 is the initial concentration and C is the concentration of MB solution absorption of MB at 660 nm at time, t). A first order relationship between the concentration of MB and irradiation time was assumed and the decolorization rate constants (k) of the photocatalysts was determined from the equation Ln C0/C = kt [33,34]. 2.4. Characterization The synthesized TiO2 samples were examined by X-ray diffraction (D/Max 2200 Rigaku, Japan) using Cu Ka radiation and the accelerating voltage and current were 40 kV and 100 mA, respectively. The crystallite size (t) of anatase, rutile and brookite phases was estimated from the Scherer’s equation [35]; t = Kk/ bcos h; where K = 0.89, k = 0.154059 nm, b = full width height maximum (FWHM) in radians and h = Bragg’s angle. The characteristic peaks of anatase [1 0 1] peak at 2h = 25°, brookite [1 2 1] at 2h = 31° and rutile [1 1 0] at 2h = 27°, respectively were used to estimate crystallite size of the samples. Field emission scanning electron microscopy (JEOL, JSM-6701F, Japan) was used to study the morphology of the TiO2 powders at a working distance of 8 mm with an accelerating voltage and current of 40 kV and 150 mA, respectively. The Brunauer–Emmett–Teller (BET) surface area and the porosity of the samples were studied by nitrogen adsorption (Micrometrics ASAP 2020). All the samples measured were degassed at 250 °C for 4 h prior to actual analysis. Pore size distribution and specific desorption pore volumes were obtained using the Barrett–Joyner–Halenda (BJH) method. High-resolution transmission electron microscopy (HRTEM, Jeol JEM2100F, Korea) was used to study the particle size and distribution. The thermal stability of the as-synthesized TiO2 samples was analyzed using TG-DTA (NETZSCH STA 409PC, Germany) with temperature ranging from room temperature to 1000 °C. The process was essentially monitored under nitrogen gas atmosphere at a flow rate of 10 ml/min and heating rate of 20 °C/min. 3. Results and discussion 3.1. Phase structure Figs. 1–3 display the XRD patterns of the TiO2 samples obtained at different conditions. All diffractograms of the as-synthesized samples (Fig. 1) showed the presence of only anatase and brookite TiO2 crystals. Fig. 2 shows the XRD diffractograms of the samples obtained at different pH and calcined at 200 °C. It can be seen that all samples calcined at this particular temperature retained the AB TiO2 crystal structure even after calcination at 200 °C [25,31,35,36]. Fig. 3 shows the diffractograms of the TiO2 samples obtained at pH 2 and 9 and calcined at 600 °C (Fig. 3A) and 800 °C (Fig. 3B) as representative samples. The presence of anatase and brookite TiO2 crystals can be observed in the samples calcined at 600 °C. Fig. 3A reveals that the sample obtained at pH 2 and calcined at 600 °C showed the emergence of a small peak of rutile phase as a result of anatase-to-rutile phase transformation (ART) or brookite-to-rutile phase transformation (BRT). However, a pure mixture

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Intensity (a.u)

A(101)

Table 1 Phase composition of the TiO2 nanoparticles obtained at pH 2.

B(111)

A B

pH 4 pH 7 pH 9

20

30

40

50

60

2 Theta (degrees) Fig. 1. XRD of the as-synthesized TiO2 nanoparticles obtained at pH 2, 4, 7 and 9.

Intensity (a.u)

A(101) B(111)

pH 2

A(200)

A(004) B(211) B(121)

A(105)

pH 4 pH 7 pH 9

20

30 40 2 Theta (degrees)

50

60

Fig. 2. XRD of the samples synthesized at pH 2, 4, 7 and 9 and calcined at 200 °C.

of anatase and brookite was retained in the sample obtained at pH 9 and calcined at 600 °C. Interestingly, Fig. 3B demonstrates that a complete ART or BRT occurs in the samples obtained at pH 2 and calcined at 800 °C. On the other hand, a partial ART or BRT is evidenced in the sample obtained at pH 9 thus; a mixture of anatase,

A

Raw sample

200 °C

600 °C

800 °C

A (%)

B (%)

A (%)

B (%)

A (%)

B (%)

R (%)

A (%)

R (%)

2

76.2

23.8

80.0

20.0

84.6

12.8

2.6

0

100

A(200) A(105)

A(004) B(211) B(121)

pH 2

pH

brookite and rutile can be seen. This indicates that the temperature to facilitate a complete ART or BRT can be tuned depending on the pH of the sol used to obtain TiO2 samples. Table 1 shows phase composition of the as-synthesized TiO2 nanoparticles obtained at pH 2 and calcined at 200, 600 and 800 °C as representative samples of the TiO2 nanoparticles obtained at different conditions. Anatase and brookite phases dominate in all samples with the as-synthesized sample showing the presence of 24% of brookite phase. The amount of the anatase increased with increasing the calcination temperatures whilst that of brookite phase decreased. The sample calcined at 600 °C shows the presence of 3% of rutile TiO2 crystals and ultimately a sample composed of only rutile can be attained after calcination at 800 °C signifying a complete phase transformation. Meanwhile, upon increasing the calcination temperature the crystallite size of the samples increased representing the increase of crystallinity. Table 2 presents the crystallite size of the different samples as was estimated by the Scherer’s equation. The grain size of anatase particles in the as-prepared samples was 69 nm and was increased to 26 nm (sample obtained at pH 9) upon calcination at 800 °C. In case of brookite, grain size with average diameter between 21 and 28 nm were registered while that of rutile was between 43 and 48 nm. The high crystallinity at room temperature might be due to the effect of ionic strength on the phase composition of titania, long hydrolysis and ageing time and reaction temperature [35]. Hence, the optimum pH suitable for the synthesis of the TiO2 nanoparticles with high content of brookite phase would be pH 2 which is in agreement with the literature [37]. It was formerly reported [7] that anatase, brookite and rutile phases can be yielded by hydrothermal treatment of the amorphous TiO2 at elevated temperatures with the appropriate reactants. Li et al. [38] found that the selective formation of different phases of TiO2 polymorphs is strongly dependent on the precursors, phase selective additives and the synthetic process (hydrothermal process). This argument is also supported by Lin et al. [36] in their reports on the synthesis of high quality brookite crystals from TiCl4 in the presence of urea as an in situ OH- source and sodium lactate as the complexant and surfactant. Thus, these versatile studies indicate that the formation of the brookite TiO2

B

A(101)

R(110) R(211) R(101)

Intensity (a.u)

Intensity (a.u)

R(111)

A(200) A(004) pH 2

R(110)B(121)

A(105) A(211)

B(211)

A(101)

R(220)

R(200) R(210)

pH 2

R(110) R(211)

R(101)

A(004)

pH 9

B(121)

A(200) A(105) A(211)

20

30

40

50

2 Theta degrees

60

R(111)

A(101)

pH 9

20

B(121)

30

R(220)

R(200) R(210)A(200)

40

50

60

2 Theta degrees

Fig. 3. XRD of the samples obtained at pH 2 and 9 and calcined at 600 °C (A) and 800 °C (B) synthesized.

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Table 2 Grain size of the TiO2 nanoparticles obtained at different conditions. pH

2 4 7 9

Raw samples

200 °C

600 °C

800 °C

A (nm)

B (nm)

A (nm)

B (nm)

A (nm)

B (nm)

R (nm)

A (nm)

R (nm)

9.3 9.1 8.8 8.6

21.8 9.8 11.5 9.9

10.9 9.7 10.6 9.8

28.2 11.3 15.2 16.3

18.2 – – 11.5

– – – 12.3

43.2 – – –

– – – 26.2

46.5 – – 48.1

crystals depends on many factors. The present study introduces a facile fabrication of highly photoactive TiO2 nanoparticles with a mixed phase though non-hydrothermal process and in the absence of additives. The only drawback of the method proposed herein is the inability to yield pure TiO2 crystals which might limit some applications of the obtained powders. Nevertheless, in heterogeneous photocatalysis and solar energy conversion applications materials with mixed phases or binary metal oxide semiconductors such as TiO2–SiO2 and TiO2–ZrO2 are less susceptible to the electron–hole recombination and therefore they possess improved photochemical properties [39,40].

in the SEM images are mainly composed of spherical grains with diameter <20 nm. Although a close look of the TEM image shows the presence of rod-like structures indeed. Fig. 5 presents the HRTEM images of the TiO2 nanoparticles obtained at pH 2 and calcined at 600 °C while insets display the magnified images depicting the lattice fringes for anatase (A) and rutile (R) TiO2 crystals. The lattice fringe spacing of anatase (0.35 nm) and rutile (0.32 nm) can be identified which correspond to the [1 0 1] and [1 1 0] plane reflections, respectively [41].

3.2. SEM and TEM analyses

Table 3 shows the distribution of the BET surface area, pore volume and pore diameter of the samples obtained at pH 2 and 4 and calcined at different temperatures. It can be noticed that the sample obtained at pH 2 and 4 possessed surface area of 276 m2/g and 260 m2/g, pore volume of 0.2 and 0.3 cm3/g and pore sizes of 3.4 nm and 4.8 nm, respectively. Upon calcination the surface areas and pore volumes of the samples were decreased as the result of crystallization of the TiO2 nanoparticles [42,43]. The high surface areas and porous structure possessed by these samples indicate that these samples might be suitable for heterogeneous catalysis. Fig. 6 presents the nitrogen gas adsorption–desorption isotherms of the as-synthesized TiO2 nanoparticles obtained at pH 2 and 4 as representative samples. Inset is representative pore size distribution (PSD) of raw TiO2 nanoparticles obtained at pH 2 and 4 as well. The isotherms of these samples are typically of type IV demonstrating the presence of mesoporous structure associated with

The morphology of the TiO2 nanoparticles obtained in the present study was studied by SEM and TEM analyses. SEM and TEM images of the TiO2 nanoparticles are compiled in Fig. 4A–D. The SEM images of the samples calcined at 200 °C presented in Fig. 4A–C shows the formation of irregular clusters composed of spherical nanomeric primary particles. The clusters are about 200 nm in the samples obtained at pH 2 but clusters of about 400 nm was essentially claimed in the sample obtained at pH 9. Another notable feature in these SEM micrographs is that the samples obtained at low pH shows very packed clusters of primary particle and the separation of the clusters appears to increase with increasing pH. The TEM image of the sample obtained at pH 4 and calcined at 200 °C is shown in Fig. 4D. It can be seen that the clusters (showed in doted circles in the TEM micrograph) observed

3.3. Nitrogen gas physisorption studies

D

rod

Fig. 4. SEM images of the as-synthesized TiO2 nanoparticles obtained at (A) pH 2, (B) pH 4, (C) pH 9 (D) TEM image of the sample obtained at pH 4 and calcined at 200 °C.

B.K. Mutuma et al. / Journal of Colloid and Interface Science 442 (2015) 1–7

5

Fig. 5. HRTEM image of the TiO2 nanoparticles obtained at pH 2 and calcined at 600 °C. Insets are the magnified HRTEM images showing the lattice fringes of anatase (A) and rutile (R) TiO2 crystals.

Table 3 Surface area (SA), pore diameter (PD) and pore volume (PV) distribution of TiO2 nanoparticles obtained at different conditions. pH 2

As-synthesized Calcined at 200 °C Calcined at 600 °C

pH 4

SA (m2/g)

PV (cm3/g)

PD (nm)

SA (m2/g)

PV (cm3/g)

PD (nm)

276 186 48

0.24 0.22 0.13

3.4 4.2 8.3

260 109 42

0.32 0.18 0.12

4.8 4.9 9

280

pH2 pH4

240 200

dV/dDVlog

Adsorbed volume (cm3/g)

1.2

160

0.8 0.4 0.0

2

4

6

8

10

Pore diameter (nm)

120 80 40 0.0

pH2 pH4

0.2

0.4 0.6 0.8 Relative pressure (P/Po

1.0

pores [33,43,44]. The PSD of the TiO2 samples estimated by the BJH method reveals that the PSD of the samples obtained at different pH was different. The PSD of the sample synthesized at pH 2 is narrow and it is confined in the region of 3–4.1 nm while that of the sample of obtained at pH 4 is relatively broader than that of the former as it ranges from 3 nm to 7.5 nm. 3.4. Thermal analysis The effect of heat treatment in the TiO2 was studied by the TGDTA analyses and the results are presented in Fig. 7. Fig. 7A shows a notable weight loss from room temperature to 200 °C is attributed to desorption of physisorbed water. This is justified by the appearance of a broad endothermic peak at 120–150 °C in the DTA curves (Fig. 7B). Weight loss at 300 °C is due to degradation of organic compounds present in the samples. There was a negligible weight loss from temperatures P500 °C indicating that a stable metal oxide with well developed Ti–O–Ti networks was attained. The broad exothermic peaks appear in the DTA curve at 400 °C and 800 °C signifying that there was further crystallization and phase transformation processes. The TGA curves have shown that the maximum weight loss was <14% which was observed in the sample obtained at pH 4. The sample obtained at pH 2 registered a smallest weight loss less of 8% after heating the sample to 1000 °C. 3.5. Photocatalytic properties

Fig. 6. N2 adsorption–desorption isotherms of the as-synthesized TiO2 nanoparticles obtained at pH 2 and 4. Inset is the PSD of the as-synthesized TiO2 nanoparticles obtained at pH 2 and 4.

capillary condensation of the adsorbent. It can be seen that the hysteresis loop of the sample synthesized at pH 2 is confined in the 0.35 < P/Po < 0.8 and is of type H2 according to the IUPAC classification system. Generally, type H2 implies the presence of random distribution of pores and interconnected pore systems. On the other hand, the sample yielded at pH4 shows the hysteresis loop closes at P/Po P 0.45 and is of type H1 signifying the existence of compact agglomerates of more or less cylindrical-like

The photocatalytic properties of the TiO2 nanoparticles synthesized via this approach were tested on the degradation of methylene blue solution in the presence of artificial UV light. In this particular experiment the photochemical performances of the samples with a mixture of anatase and brookite (raw samples and samples calcined at 200 °C), anatase–brookite–rutile (the sample obtained at pH 2 and calcined at 600 °C) and anatase–rutile (Degussa P25 TiO2) were assessed. Fig. 8 presents the photocatalytic results of the TiO2 nanoparticles obtained at different conditions. The degradation results by anatase–rutile Degussa P25 TiO2 are also included for the sake of comparison. Meanwhile,

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B.K. Mutuma et al. / Journal of Colloid and Interface Science 442 (2015) 1–7

A

100

pH 2

B Heat Flow (a.u)

Weight Loss (%)

pH 4 95

pH 7

90

pH 9

pH 7 pH 9

pH 2 pH 4

85 0

200

400

600

800

ο

1000

0

200

400

Temperature ( C)

600

ο

800

1000

Temperature ( C)

Fig. 7. Thermogravimetric curves of the as-synthesized TiO2 nanoparticles obtained at different pHs (A) and their respective DTA curves (B).

100

100

P25 pH 2 pH 4 pH7 pH9

80

80 D%

60

D%

P25 As-synthesized 0 Calcined at 200 C 0 Calcined at 600 C

40

60 40 20

20

A

0 0

10 20 30 40 50 60 Irradiation time (minutes)

B

0 0

70

10

20 30 40 50 60 70 Irradiation time (minutes)

As-synthesized, k=0.0305/min P25, k=0.0356/min

4

Calcined at 2000C, k=0.0592/min Calcined at 6000C, k=0.0553/min

In (Co/C)

3 2 1

C

0 0

10

20 30 40 50 60 Irradiation time (minutes)

70

Fig. 8. Photocatalytic process of the TiO2 photocatalysts in the degradation of MB solution. (A) The photocatalytic process by TiO2 samples obtained at different pH and calcined at 200 °C. (B) The photocatalytic properties of the samples obtained at pH 2 and calcined at different temperatures. (C) Comparison of the rate of the degradation of the MB solution by the calcined TiO2 nanoparticles obtained at pH 2.

the photocatalytic efficiencies of the samples obtained at different pHs and calcined at 200 °C are displayed in Fig. 8A. It is noticeable that a degradation efficiency <65% can be attained using these photocatalysts. Fig. 8B shows the photocatalytic efficiency of raw and calcined samples obtained at pH 2. It can be noticed that the photocatalytic efficiencies of the calcined samples were superior to those of the as-synthesized and commercial Degussa P25 TiO2 samples. In order to quantify their efficiencies in degrading methylene blue solution the linear correlation between the logarithms of relative concentration of MB solution (Ln C0/C) against the irradiation time of the TiO2 samples obtained at pH 2 is plotted in Fig. 8C. The degradation rates of the samples were 0.0356, 0.0305, 0.0592 and 0.0553 min 1 for Degussa P25 TiO2, as-synthesized, samples calcined 200 °C and 600 °C, respectively. The degradation rate of the samples calcined at 200 and 600 °C were higher than that of the anatase–rutile Degussa P25 TiO2. The assynthesized sample exhibited the performance lower than that of commercial TiO2 as well. The photochemical performance exhibited by these samples might be ascribed to the existence of well developed anatase–brookite or anatase–brookite–rutile systems, high crystallinity and the

formation of grains with desirable grain size especially after thermal treatment. XRD analysis (Figs. 1–3) indicated that a mixture of TiO2 polymorphs was achieved in both the as-prepared and calcined samples. Also the large surface areas and small pore size distributions displayed by the samples might favor high photocatalytic activity of the samples though not the decisive factors [6,33,45,46]. The relationship between photocatalytic activity and crystal phase has been previously described but photocatalysis is a delicate process which depends on numerous parameters such as particle size, porous structure, crystallinity, surface area and substrate [19,46–48]. The presence of mixed phase improves the photochemical performance of the samples through reducing the recombination of photogenerated holes and electrons [40]. Structural characterizations of the TiO2 samples obtained in the present study revealed that materials with well developed mesoporous structure, crystallinity and small grain size were obtained. Further reports [19,48,49] suggest that crystalline photocatalysts with small particle sizes are more likely to exhibit high photochemical properties. Based on the previous [2,4,5,50] reports photocatalysts with the desirable particle size, porous structure, high surface area and with a mixture of titania phases might be suitable for

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degradation of dyes which are common organic pollutants in textile industries. Thus, the synthesis of TiO2 photocatalysts with mixed polymorphs in the proposed technique is interesting and can boost production of heterogeneous photocatalysts with desirable properties for various applications. 4. Conclusions The present study introduced a suitable sol–gel method that can facilitate the formation of anatase–brookite TiO2 nanoparticles using pH control. The mesoporous anatase–brookite TiO2 samples obtained via this approach exhibited well-dispersed small particle sizes, high surface area and high crystallinity which are the appropriate properties for heterogeneous photocatalysts. XRD results showed that a mixed phase i.e. anatase–brookite and anatase–brookite–rutile mixtures can be obtained under different treatments. Comparison of the photocatalytic performances of the samples obtained at pH 2 as representative samples indicated that the activities of the calcined of the TiO2 samples were superior to that of a commercial photocatalyst Degussa P25 (anatase–rutile mixture). This signifies that a mixture of anatase–brookite and anatase–brookite–rutile TiO2 crystals with well-developed crystalline structure possesses higher activities than the sample with only a mixture of anatase–rutile. The possibility of the synthesis of TiO2 samples with mixed phase will facilitate the production of heterogeneous catalysts with desirable properties for degradation of non-biodegradable organic contaminants. Acknowledgments The authors would like to acknowledge the financial support received from the National Institute for International Education (NIIED), South Korea and the Korean Government Scholarship Program (KGSP). Also we are thankful to the Ministry of Trade, Industry and Energy of Korea for supporting this work (Grant No. 20124030200130) through Human Resources Development program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP). References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev. 1 (2000) 1–21. [2] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [3] N. Hafizah, I. Sopyan, Int. J. Photoenergy 2009 (2009). [4] K. Nakata, A. Fujishima, J. Photochem. Photobiol. C: Photochem. Rev. 13 (2012) 169–189. [5] K. Hashimoto, H. Irie, A. Fujishima, Jpn J. Appl. Phys. 1 (44) (2005) 8269. [6] B. Ohtani, J.-I. Handa, S.-I. Nishimoto, T. Kagiya, Chem. Phys. Lett. 120 (1985) 292–294. [7] D. Reyes-Coronado, G. Rodríguez-Gattorno, M.E. Espinosa-Pesqueira, C. Cab, R.d. Coss, G. Oskam, Nanotechnology 19 (2008) 145605. [8] S. Hotchandani, P.V. Kamat, J. Phys. Chem. 96 (1992) 6834–6839. [9] S.C.A.P. Mohammad Reza Vaezi, Coupled semiconductor metal oxide nanocomposites: types, advances in composite materials for medicine and nanotechnology, Dr. Brahim Attaf (Ed.), ISBN: 978-953-307-235-7, InTech,

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