Ultrasonic-assisted sol-gel synthesis of samarium, cerium co-doped TiO2 nanoparticles with enhanced sonocatalytic efficiency.

Ultrasonics Sonochemistry xxx (2015) xxx–xxx

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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Ultrasonic-assisted sol–gel synthesis of samarium, cerium co-doped TiO2 nanoparticles with enhanced sonocatalytic efficiency Hamed Eskandarloo a, Alireza Badiei a,⇑, Mohammad A. Behnajady b, Ghodsi Mohammadi Ziarani c a

School of Chemistry, College of Science, University of Tehran, Tehran, Iran Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran c Department of Chemistry, Faculty of Science, Alzahra University, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 17 August 2014 Received in revised form 23 December 2014 Accepted 2 February 2015 Available online xxxx Keywords: Sonosynthesis Sonocatalysis Operational variable Sm, Ce co-doped TiO2 Optimization RSM

a b s t r a c t In this work, pure TiO2 and samarium, cerium mono-doped and co-doped TiO2 catalysts were synthesized by an ultrasonic-assisted sol–gel method and their sonocatalytic efficiency studied toward removal of Methyl Orange as a model organic pollutant from the textile industry. The relationship of structure and sonocatalytic performance of catalysts was established by using various techniques, such as XRD, TEM, SEM, EDX, DRS, and PL. A comparison on the removal efficiency of sonolysis alone and sonocatalytic processes was performed. The results showed that the samarium, cerium co-doped TiO2 catalyst with narrower band gap energy and smaller particle size leads to a rapid removal of pollutant. It was believed that Sm3+ and Ce4+ ions can serve as superficial trapping for electrons at conduction band of TiO2 and prolonged the lifetime of electron–hole pairs. Finally, the effect of synthesis and operational variables on the sonocatalytic activity of co-doped TiO2 catalyst was studied and optimized using response surface methodology as a statistical technique. The results showed that the maximum removal efficiency (96.33%) was achieved at the optimum conditions: samarium content of 0.6 wt%, cerium content of 0.82 wt%, initial pollutant concentration of 4.31 mg L1, catalyst dosage of 0.84 mg L1, ultrasonic irradiation power of 700 W, and irradiation time of 50 min. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The sonolysis process, application of ultrasounds in water with a frequency range of between 18 and 100 MHz, represents one of AOPs that is mainly attributed to the acoustic cavitation phenomena and involves three stages: (i) formation, (ii) growth, and (iii) implosive collapse of cavitation bubbles that entrapped dissolved gases or vapors surrounding water [1–3]. The implosive collapse of the cavitation bubbles generates many local hot spots with extremely high temperatures and high pressures [4,5]. The extreme conditions attained during the cavitation bubbles collapse cause the thermal dissociation of water, oxygen molecules, and hydrogen peroxide into reactive species such as OH radicals [6,7]. The simplicity of sonolysis using in degradation of organic pollutants is one of its advantages [8], but high consumption of energy and low rate of OH radical formation are limited this process for practical applications [9,10] and some techniques are needed to improve its efficiency. Recently, sonocatalytic method that is a combination of a metal oxide (such as TiO2, ZnO, etc.) and sonolysis, ⇑ Corresponding author. Tel.: +98 2161112614; fax: +98 2161113301. E-mail address: [email protected] (A. Badiei).

has obtained extensive attention [10,11]. Sonocatalytic removal of organic pollutants can be explained by the mechanism of hot spots and sonoluminescence as follows: During ultrasonic cavitations the produced local hot spots with extreme conditions cause the thermal dissociation of water molecules to directly produce the OH radicals [12,13]. On the other hand, ultrasonic irradiation can result in the formation of intense UV-light with a comparatively wide range of wavelengths (below 375 nm), named sonoluminescence, which is able to excite the metal oxides to act as a photocatalyst during sonication [14–16]. When metal oxides absorb a photon with energy greater than or equal to the band gap energy leads to generate electron–hole pairs within the conduction and valence bands [17,18]. The generated holes can oxidize water or hydroxide ions to produce OH radicals, therefore a great number of OH radicals with high oxidative activity are formed in the reaction media [19,20]. In photocatalytic dye removal processes, the ultrasonic irradiation by creating turbulence inside the solution can overcome the main disadvantages of UV light such as its low penetration rate in highly colored solutions and shielding effects of catalyst particles in water [21]. Also, ultrasonic irradiation increases the active surface area through ultrasound de-aggregating of catalyst particles

http://dx.doi.org/10.1016/j.ultsonch.2015.02.001 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: H. Eskandarloo et al., Ultrasonic-assisted sol–gel synthesis of samarium, cerium co-doped TiO2 nanoparticles with enhanced sonocatalytic efficiency, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.02.001

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H. Eskandarloo et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

and enhances the mass transfer of organic pollutants between the catalyst surface and liquid phase through generations of microjets of cavitation bubbles near the surface of catalyst particles and mixing of catalyst inside the dye solution [10,22–24]. On the other hand, the quick recombination of generated electron–hole pairs during ultrasonic irradiation of TiO2 decreases its catalytic activity [25,26]. One possible solving approach to prevent the recombination of electron–hole pairs, is doping of TiO2 with lanthanide ions. The lanthanide ions with 4f electron configurations into TiO2 lattice can act as an electron reservoir to trap electrons and significantly increase the separation rate of generated electron–hole pairs, resulting in the improvement of the catalytic activity [27–29]. On the other hand, lanthanide ions doping can enhance the light sensitivity

of catalyst and provide a means to concentrate the organic pollutant at the catalyst surface [30,31]. Utilization of ultrasonic irradiation during sol–gel synthesis method can affect morphology, size, and crystallinity of particles and also lead to a uniform distribution of dopant ions into TiO2 lattice [32,33]. Recently some reports are available about the ultrasonic assisted sol–gel preparation of lanthanide ions doped metal oxides such as Ce/TiO2 [32,34], La/TiO2 [15], La/ZnO [35], Nd/TiO2 [36], Nd/ZnO [37], Pr/ZnO [38], Er/YAlO3:TiO2–Fe2O3 [22], and Er/ YAlO3:TiO2–SnO2 [10]. In this study, pure TiO2 and samarium, cerium mono-doped and co-doped TiO2 catalysts were prepared via an ultrasonic-assisted sol–gel method. The prepared catalysts were characterized by using transmission electron microscopy (TEM), scanning electron

Fig. 1. Schematic representation of the ultrasonic-assisted sol–gel synthesis; (a) procedure and (b) experimental setup.


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microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), photoluminescence (PL), and UV–vis diffuse reflectance spectroscopy (DRS) techniques. The sonocatalytic activity of catalysts was studied toward removal of Methyl Orange (MO) under ultrasonic irradiation. In the following, the effect of synthesis and operational variables on the sonocatalytic activity of samarium, cerium co-doped TiO2 catalyst was studied and optimized using response surface methodology (RSM). RSM technique is a widely used mathematical and statistical method in process optimizing by using the minimum number of experiments [39]. 2. Materials and methods 2.1. Materials Titanium tetra-n-butoxide (Ti(OBu)4), cerium(IV) sulfate, samarium(III) nitrate, ethanol, and MO as a model pollutant from textile industry, were purchased from Merck Co. (Germany). All chemical reagents were in analytical grade. 2.2. Synthesis of samarium, cerium co-doped TiO2 catalyst Samarium, cerium co-doped TiO2 catalyst was prepared by an ultrasonic-assisted sol–gel method, according to the following steps: First, titanium tetra-n-butoxide was slowly dissolved in ethanol under ultrasonic irradiation provided by a probe sonicator (Qsonica Q700, Newtown, CT, US) with the nominal power of 700 W and frequency of 20 kHz. Then the hydrolysis process was performed under ultrasonic irradiation by adding deionized water drop by drop into a flask containing titanium tetra-n-butoxide/ethanol mixture by a syringe pump (Fanavaran Nano-Meghyas, model SP. 1000). After 1 h, appropriate amounts of dopants dissolved in distilled water with different weight ratios to TiO2 (0.16, 0.3, 0.65, 1, and 1.14), were added to the mixed solution and the obtained solution was sonicated for 1 h. The resulting transparent colloidal suspension was dried in an air oven at 80 °C for about 12 h and

finally the dried solids calcined at 450 °C for 3 h. The schematic of the ultrasonic-assisted sol–gel synthesis procedure and experimental setup are shown in Fig. 1. In addition, for the purpose of comparison, pure TiO2 and samarium and cerium mono-doped TiO2 catalysts were also prepared by the similar procedure. 2.3. Characterization methods The average crystallite size and phase content of prepared catalysts were characterized by a Philips X’pert MPD diffractometer using Cu Ka radiation (k = 0.15478 nm). The (1 0 1) reflection (2h = 25.28°) of anatase and the (1 1 0) reflection (2h = 27.42°) of rutile were used for analysis. The average crystallite size of the particles was calculated from the line broadening of corresponding reflections and according to the Scherrer’s equation [40];

D¼

kk b cos h

ð1Þ

where D is the average crystallite size (nm), k is the wavelength of the X-ray radiation, k is a constant taken as 0.89, b is the full width at half maximum intensity, and h is the half diffraction angle. The phase content in the catalysts was calculated by the following equation [41];

Rutile phase % ¼

100 1 þ 0:8 IIAR

ð2Þ

where IA is the integrated intensity of anatase (1 0 1) diffraction peak and IR is the integrated intensity of rutile (1 1 0) diffraction peak. The size of the samarium, cerium co-doped TiO2 catalyst was obtained by TEM instrument (EM 208 Philips, 80 keV) and its surface morphology was recorded with KYKY-EM3200 Digital SEM. The SEM was equipped with an EDX system for analyzing the chemical composition of the sample. Photoluminescence emission spectra of the samples were recorded using a Varian Cary-Eclipse

Table 1 Experimental ranges and levels of the variables. Variables

Symbol xi

Ranges and levels 2

1

0

+1

+2

Synthesis Samarium content (wt%) Cerium content (wt%)

x1 x2

0.16 0.16

0.3 0.3

0.65 0.65

1 1

1.14 1.14

Operational Initial MO concentration (mg L1) Ultrasonic irradiation power (W) Catalyst dosage (g L1) Ultrasonic irradiation time (min)

x3 x4 x5 x6

4 300 0.6 10

8 400 0.8 20

12 500 1 30

16 600 1.2 40

20 700 1.4 50

Table 2 The 2-factor CCD matrix for synthesis variables with the experimental and predicted responses. Run

1 2 3 4 5 6 7 8 9 10 11

Samarium content (wt%)

0.30 1.00 0.65 1.00 0.65 0.65 0.30 1.14 0.65 0.65 0.16

Cerium content (wt%)

0.30 0.30 0.65 1.00 0.65 0.65 1.00 0.65 0.16 1.14 0.65

Removal rate (%) Experimental

Predicted

44.87 32.38 57.01 49.13 56.21 58.87 51.65 40.55 35.9 54.44 47.48

43.12 32.15 57.65 49.87 57.65 57.65 50.84 39.27 35.51 53.49 47.724


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Fig. 2. The response surface and contour plots of sonocatalytic removal of MO as a function of samarium and cerium contents (initial MO concentration of 12 mg L1, catalyst dosage of 1 g L1, ultrasonic irradiation power of 500 W, and irradiation time of 30 min).

luminescence spectrometer (Agilent Technologies) with excitation wavelength at 320 nm. UV–vis DRS was obtained using AvaSpec2048 TEC spectrometer for determination of the optical band gap of samples and was calculated by the following equation [42];

Eg ¼

hc k

ð3Þ

where Eg is the optical band gap energy (eV), h is the Plank’s constant, c is the light speed (m s1), and k is the wavelength (nm). 2.4. Sonocatalysis experiments Sonocatalytic removal of MO pollutant was carried out under ultrasonic irradiation provided by a probe sonicator (Qsonica

Q700, Newtown, CT, US) in a batch reactor. In each run, 100 mL of MO solution with initial concentration of 12 mg L1 and 100 mg of catalyst were transferred into the reactor and were sonicated with an output power of 500 W. At given sonication times, the 5 mL sample was taken out, centrifuged (Sigma 2-16P) and then MO concentration analyzed by UV–vis spectrophotometer (Rayleigh UV-1600) at kmax = 464 nm. 2.5. Experimental design In this study, a central composite design (CCD), which is widely used for RSM, was used to propose and estimate a mathematical model of the sonocatalytic process behavior. Computational analysis of the experimental data was supported by the Design-Expert


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H. Eskandarloo et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx Table 3 The 4-factor CCD matrix for operational variables with the experimental and predicted responses. Run

Initial MO concentration (mg L1)

Ultrasonic irradiation power (W)

Catalyst dosage (g L1)

Ultrasonic irradiation time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

12 12 12 4 8 12 8 12 16 16 16 12 16 12 20 8 12 8 16 16 16 8 12 8 12 8 16 12 12 12 8

700 500 500 500 600 500 400 500 600 400 400 500 400 500 500 400 500 400 600 400 600 600 300 400 500 600 600 500 500 500 600

1 1 1 1 0.8 1.4 1.2 0.6 1.2 0.8 0.8 1 1.2 1 1 0.8 1 1.2 1.2 1.2 0.8 1.2 1 0.8 1 1.2 0.8 1 1 1 0.8

30 10 30 30 20 30 20 30 40 40 20 30 40 50 30 40 30 40 20 20 20 40 30 20 30 20 40 30 30 30 40

Removal rate (%) Experimental

Predicted

86.22 32.89 66.14 92.51 71.01 58.15 59.44 58.9 56.11 53.21 30.99 65.2 35.91 84.02 30.76 63.54 64.88 64.48 32.6 28.43 28.77 70.32 43.1 57.42 68.69 73.02 53.7 63.33 65.47 64.19 86.11

83.77 29.77 67.94 89.27 73.94 56.94 56.1 59.94 59.77 55.94 27.77 68.94 33.94 86.1 27.6 66.27 67.94 60.27 30.6 26.77 32.6 69.1 46.21 59.1 67.94 69.94 56.77 67.94 67.94 67.94 89.01

Fig. 3. Proposed mechanism of reaction scheme under ultrasonic irradiation in the presence of samarium, cerium co-doped TiO2 catalyst (e: negative electron, h+: positive hole, and Mn+: samarium and cerium ions).

(version 7) software. In order to evaluate the effect of independent variables, two synthesis key factors (including samarium and cerium contents) and four operational key factors (including catalyst dosage, initial MO concentration, and ultrasonic irradiation power and time) were chosen, and the sonocatalytic removal rate of MO was selected as the response. For statistical calculations, the chosen variables were converted to dimensionless ones (xi), with the coded values at levels: 2, 1, 0,+1,+2. The experimental ranges and the levels of the variables are presented in Table 1. It should be noted

that, the preliminary experiments were performed to determine the extreme values of the synthesis and operational variables. 3. Results and discussion 3.1. Optimization of synthesis variables Model analysis results for optimization of samarium and cerium contents are provided in Text S1 of Supporting information. The


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Fig. 4. Sonocatalytic removal of MO in the presence of pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts (initial MO concentration of 12 mg L1, catalyst dosage of 1 g L1, ultrasonic irradiation power of 500 W, and irradiation time of 30 min).

Fig. 5. XRD patterns of pure TiO2 (a), Sm(0.65 wt%)/TiO2 (b), Ce(0.65 wt%)/TiO2 (c), and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 (d) catalysts.

details of the designed experiments for synthesis variables along with experimental results and predicted values for the MO removal rate are presented in Table 2. Three-dimensional surface plots are the useful approach in revealing the effect of tested variables and the contour plots help in identification of the type of interactions between variables. The response surface and contour plots for samarium and cerium contents, while varying within the experimental ranges, are obtained by using the statistical software to evaluate the interactive relationships between the samarium and cerium contents and MO removal rate. Fig. 2 shows the effect of the samarium and cerium contents on the sonocatalytic removal rate of MO. The results showed that the sonocatalytic activity of TiO2 increased with the increase of samarium and cerium contents up to 0.65 wt%. As can be seen from Table 2, sonocatalytic removal rate of MO increased from 47.48% and 35.9% to about 57.01% by increasing the samarium and cerium contents from 0.16 to 0.65 wt%, respectively. The positive effect of samarium and cerium on the sonocat-

alytic activity of TiO2, can be explained by Sm3+ and Ce4+ ability to trap electrons at conduction band of TiO2 and suppressing the recombination of the generated electron–hole pairs. Sm3+ and Ce4+ ions with fully vacant d and f orbitals can serve as superficial trapping for electrons and convert to Sm2+ and Ce3+ ions, respectively [28,31,42–46]. A simple representation of the possible mechanism of sonocatalytic activity of samarium, cerium doped TiO2 catalyst is shown in Fig. 3. The formed Sm2+ and Ce3+ ions are unstable, therefore the trapped electrons can be subsequently transferred to O2 molecules adsorbed on the surface of TiO2 and generate reactive species such as superoxide radicals, hydroxyl radicals, and hydrogen peroxide, through the following reactions (Eqs. (4)–(9)) [43,44,46]:

Mnþ þ eCB ! Mþðn1Þ ðunstableÞ

ð4Þ

Mþðn1Þ þ O2 ! Mnþ þ O 2

ð5Þ



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Fig. 6. TEM images (a), SEM micrograph (b), and EDX spectrum (c) of the optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalyst. þ O 2 þ H ! HO2

ð6Þ

þ HO2 þ O 2 þ H ! H2 O2 þ O2

ð7Þ

H2 O2 þ eCB ! OH þ OH

ð8Þ

H2 O2 þ O 2 ! OH þ OH þ O2

ð9Þ

ðMnþ : Sm3þ and Ce4þ ions; Mþðn1Þ Sm2þ and Ce3þ ionsÞ Therefore, more generated holes can diffuse to the surface of TiO2 catalyst and oxidize the adsorbed water molecules or hydroxide ions to produce hydroxyl radicals according to Eqs. (10) and (11) [47]: þ

ð10Þ

þ

ð11Þ

H2 O þ hVB ! OH þ Hþ OH þ hVB ! OH

In heterogeneous photocatalysis processes, free radicals specially hydroxyl radicals are responsible for degradation of organic compounds [48]. On the other hand, it has been reported that lanthanide ions doping can facilitate transport and diffusion of reactants and products through enhancement the adsorption capacity of TiO2 nanoparticles toward organic pollutant molecules [27,31,49,50]. For this purpose, the adsorption capability of pure and samarium, cerium co-doped TiO2 catalysts toward MO molecules was performed in the darkness using the same procedures as the sonocatalysis experiment. The results confirmed that the adsorption rate of MO onto samarium, cerium co-doped TiO2 catalyst was higher (7.2%) than pure TiO2 (2.1%). In fact, there was an optimal amount of dopants for the most efficient separation of generated electron–hole pairs. As can be seen from the response surface and contour plots (Fig. 2), further increasing the samarium and cerium contents can lead to a decrease in sonocatalytic activity of TiO2 catalyst. When the


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Fig. 7. (a) UV–vis DRS and (b) PL spectra of pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts.

content of dopant is excessively high, the number and size of the dopant species increase and they become recombination centers for electron–hole pairs. Another explanation is that more numbers of dopants species screen the catalyst surface from light absorption, and results lead to the lower sonocatalytic activity of TiO2 catalyst [51–53]. The contour plots (Fig. 2) show that the optimum region for highest sonocatalytic removal rate (P55%) is when the samarium and cerium contents are in the range of 0.4–0.9 and 0.5–1 wt%, respectively. The main objective in term of sonocatalytic removal efficiency was defined as ‘‘maximize’’ to achieve optimum values of samarium and cerium contents for the synthesis of Sm–Ce/TiO2 catalyst with the highest sonocatalytic activity. Design Expert as a response optimizer software was used for the optimization of samarium and cerium contents in the selected range of 0.16–1.14 wt%. The optimal values of the samarium and cerium contents for the maximum sonocatalytic removal efficiency (65.14%) are achieved 0.6 and 0.82 wt%, respectively. A comparison of sonocatalytic efficiency of pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%) –Ce(0.82 wt%)/TiO2 catalysts for the removal of the MO dye was performed. Fig. 4 shows the removal rate for a 12 mg L1 MO solution under catalysts dosage of 1 g L1 and 500 W power of ultrasonic irradiation, during 30 min at room temperature. The results showed that the sonocatalytic efficiency of different catalysts follows the decreasing order: Sm–Ce/TiO2 > Ce/TiO2 > Sm/TiO2 > Pure TiO2. As can be seen the highest removal rate of MO was obtained using samarium, cerium co-doped TiO2 catalyst. In fact, co-doping of samarium and cerium leads to synergistic effects in the enhance-

ment of sonocatalytic activity TiO2 catalyst than mono-doping processes. Direct sonolysis of MO solution under ultrasonic irradiation alone was also performed and the removal rate of 18% was obtained after 30 min ultrasonic irradiation. So mentioned, the low rate of OH radical formation during sonolysis process is limited its efficiency compared with sonocatalytic processes. XRD patterns of pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/ TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts are shown in Fig. 5. XRD patterns show a combination of anatase and rutile phases for all samples. The reflections at 2h of 25.4°, 36.9°, 37.9°, 38.2°, 48.1°, 54°, 55.2°, 62.8°, 68.9°, 71.17°, 75.4°, and 83.2° are attributed to anatase phase and reflections at 2h of 27.5°, 36.1°, 41.2°, and 56.7° are attributed to rutile phase. Absence of characteristic reflections corresponding to the dopants in XRD patterns, can be due to the low loading amount of dopants, appropriate dispersion of dopants into TiO2 lattice, and its small particle size [54]. The average crystallite size and phase content of samples were calculated from the Eqs. (1) and (2), respectively, using reflections of anatase at 25.3° and rutile at 27.4°. As observed in XRD patterns, pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts have rutile phase content of 19%, 18%, 11%, and 9% and the average crystalline size of 25, 23, 20, and 19 nm, respectively. It can be seen, samarium, cerium co-doped TiO2 catalyst has smaller crystalline size and lower rutile phase content than the pure and mono-doped TiO2 catalysts. Samarium and cerium co-doping into TiO2 lattice have suppressed crystal growth and anatase phase transformation to rutile. Similar results have also been reported for other co-doping processes [53,55]. The TEM images of the optimized Sm(0.6 wt%)–Ce(0.82 wt%)/ TiO2 catalyst have been shown in Fig. 6(a). The mean particle size is estimated to be about 20 nm, which is in agreement with the crystallite size calculated from the XRD pattern (Fig. 5). Fig. 6(b) shows the SEM micrograph of optimized Sm(0.6 wt%)– Ce(0.82 wt%)/TiO2 catalyst. This image shows particles with uniform distribution, spherical morphology, and slight agglomeration. The composition of the Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalyst was investigated with EDX analysis at the microscopic level. The results from EDX spectrum (Fig. 6(c)) clearly confirm the existence of Sm and Ce on the co-doped TiO2 catalyst. To study the effect of samarium and cerium doping on the optical absorption properties of TiO2 catalyst, DRS analysis has been carried out (Fig. 7(a)). The reflectance spectra of pure TiO2, Sm(0.65 wt%)/ TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/ TiO2 catalysts show absorption thresholds at 408, 424, 427, and 432 nm, respectively, and the Eg values calculated from the Eq. (3) are 3.03, 2.92, 2.90, and 2.87 eV, respectively. The results indicate that the samarium and cerium doping induces band-gap narrowing in TiO2 catalyst. This may be attributed to the charge transfer between valence or conduction band of TiO2 catalyst and f levels of samarium and cerium ions that incorporated into the lattice of TiO2 [28,56]. The electron transition from the valence band to the conduction band occurs easier with the band-gap narrowing in TiO2 catalyst. The PL emission in semiconductors arises from the recombination of free electrons and holes, therefore the PL spectra used to study the transfer, migration, and recombination processes of the generated electron–hole pairs [57,58]. Fig. 7(b) shows the PL spectra of the pure TiO2, Sm(0.65 wt%)/TiO2, Ce(0.65 wt%)/TiO2, and optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts excited by 320 nm. From this figure, it can be observed that there is a significant decrease in the intensity of the PL spectra of optimized Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalyst compared to that of the pure TiO2, Sm(0.65 wt%)/TiO2, and Ce(0.65 wt%)/TiO2 catalysts. The weaker intensity of the peak suggests that the Sm and Ce doping of TiO2 could effectively inhibit the recombination probability of generated electrons and holes.



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Fig. 8. The response surface and contour plots of the sonocatalytic removal of MO by Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalyst as a function of (a) initial MO concentration and ultrasonic irradiation time (catalyst dosage of 1 g L1 and ultrasonic irradiation power of 500 W), (b) ultrasonic irradiation power and time (initial MO concentration of 12 mg L1 and catalyst dosage of 1 g L1), and (c) catalyst dosage and ultrasonic irradiation time (initial MO concentration of 12 mg L1 and ultrasonic irradiation power of 500 W).

3.2. Optimization of operational variables In the next stage, Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts with the high sonocatalytic activity was chosen, and the effect of operational variables on its efficiency was optimized using RSM technique. Model analysis results for optimization of operational variables are provided in Text S2 of Supporting information. The details of the designed experiments for operational variables along with experimental results and predicted values of MO removal rate are presented in Table 3. The response surface and contour plots for operational variables, while two variables kept at constant and the others varying within the experimental ranges, are obtained. It is important from an application point of view to study the dependence of sonocata-

lytic removal efficiency on the initial concentration of pollutant. Fig. 8(a) shows the effect of the initial MO concentration and ultrasonic irradiation time on the removal rate of MO for catalyst dosage of 0.4 g L1 and ultrasonic irradiation power of 500 W. It is observed that the increase in the initial concentration of MO from 4 to 20 mg L1 deteriorates the removal rate from 92.51% to 30.76%. With increasing of MO concentration more and more organic substances, including MO and intermediate molecules, are adsorbed on the surface of Sm–Ce/TiO2 catalysts and this phenomenon prohibit catalysts from absorbing heat and energy produced by the ultrasonic cavitation [59,60]. In addition, the initial dye concentration increases, causing a disturbing effect on transmission of ultrasound in the reaction medium [21,61]. Finally, the generation of active species such as hydroxyl radicals will be


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Fig. 9. Reusability of the pure TiO2 and Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts in sonocatalytic removal of MO (initial MO concentration of 4.31 mg L1, catalyst dosage of 0.84 g L1, ultrasonic irradiation power of 700 W, and irradiation time of 50 min).

reduced. For higher concentrations of MO, more reactive radical species such as OH and O 2 are needed, but at a fixed operational conditions, including catalyst dosage and ultrasonic irradiation time and power, the formation of OH and O 2 species will be kept at a fixed level [62], thus, the removal rate of MO will be reduced. The contour plots (Fig. 8(a)) show that the optimum region for MO removal is the initial MO concentration less than about 6 mg L1. On the other hand, the removal rate of MO increased with increasing ultrasonic irradiation time, due to generation of more and more OH radicals in the reaction medium, hence the chance that OH radicals attack MO molecules becomes greater. The highest MO removal rate by Sm–Ce/TiO2 catalyst is achieved using a minimum value for initial MO concentration and maximum value for the ultrasonic irradiation time, when ultrasonic irradiation power and catalyst dosage are kept at constant 500 W and 1 g L1, respectively. In order to find out the effect of ultrasonic irradiation power on the sonocatalytic removal rate of MO, the experiments were carried out with ultrasonic irradiation power varying in the range of 300–700 W. Fig. 8(b) shows the effect of the ultrasonic irradiation power and time on the sonocatalytic removal rate of MO for a catalyst dosage of 1 g L1 and initial MO concentration of 12 mg L1. As can be seen from the response surface and contour plots (Fig. 8(b)), the sonocatalytic removal rate of MO increased from 43.1% to 86.22% by increasing the ultrasonic irradiation power from 300 to 700 W. The higher ultrasonic irradiation power increases the number of collapsing bubbles and promotes the formation of OH radicals [63–65], resulting in the increased removal rate of MO. The highest MO removal rate (P80%) by Sm–Ce/TiO2 catalyst is achieved when ultrasonic irradiation power and time are maintained at their maximum values. In order to find out the effect of catalyst dosage on the sonocatalytic removal rate of MO, the experiments were carried out with catalyst dosage varying in the range of 0.6–1.4 g L1 at ultrasonic irradiation power of 500 W and initial MO concentration of 12 mg L1. Fig. 8(c) shows the effect of Sm–Ce/TiO2 dosage and ultrasonic irradiation time on the sonocatalytic removal rate of MO. It could be seen from this figure that the increase in the Sm–Ce/TiO2 dosage from 0.6 to 1 g L1 improves the removal rate of MO from 58.9% to 66.14%. This can be in the result of increasing of available adsorption and catalytic sites on the Sm–Ce/TiO2 surface, which are responsible for sonocatalytic activity. Therefore, the formation of OH and adsorption of MO molecules on the Sm–Ce/TiO2 surface increase and consequently the sonocatalytic

removal efficiency will be enhanced [12,66,67]. As it is clear, improvement on removal efficiency is not obvious above 1 g L1, because excessive catalyst loading cause higher agglomeration and turbidity of the suspension, resulting in shielding effect on catalyst particles and MO molecules from receiving ultrasound waves and reduces the available surface area for absorbing the MO molecules [68–70]. Consequently, this phenomenon result in a reduction in the rate of OH generation and removal efficiency. The contour plots (Fig. 8(c)) show that the optimum region for MO removal is the catalyst dosage less than about 1 g L1. The highest MO removal rate (P70%) by Sm–Ce/TiO2 sonocatalyst is achieved using 0.6–1 g L1 for catalyst dosage and 40–50 min for ultrasonic irradiation time, when initial MO concentration and ultrasonic irradiation power kept at constant 12 mg L1 and 500 W, respectively. The sonocatalytic removal rate of MO was defined as ‘‘maximize’’ to achieve optimum values of operational variables in the selected range that the initial MO concentration, catalyst dosage, and ultrasonic irradiation time and power are in the range of 4– 20 mg L1, 0.6–1.4 g L1, 10–500 min, and 300–700 W, respectively. The optimal conditions of the operational variables for the maximum sonocatalytic removal rate (96.33%) are achieved 4.31 mg L1, 0.84 mg L1, 50 min, and 700 W for initial MO concentration, catalyst dosage, and ultrasonic irradiation time and power, respectively. As a consequent, experimental design strategy can be a successful method to determine the optimum values of synthesis and operational key factors and can be an adequate modeling to predict the sonocatalytic removal rate. 3.3. Reusability Evaluation of catalysts reusability is necessary for their practical applications. The reusability of pure TiO2 and Sm(0.6 wt%)– Ce(0.82 wt%)/TiO2 catalysts was evaluated in the sonocatalytic removal of MO. In each run, the catalysts were collected from the treated solution by centrifuging and then, were washed with distilled water several times. After drying at 100 °C during 5 h, the catalysts were reused in the sonocatalytic removal of fresh MO solution. As shown in Fig. 9, after the five times reuse cycle the sonocatalytic removal rate of MO had lost about 12.3% and 4.8% for pure TiO2 and Sm(0.6 wt%)–Ce(0.82 wt%)/TiO2 catalysts, respectively. It was confirmed that the samarium and cerium co-doping improved reusability and stability of the TiO2 catalyst during the sonocatalytic removal processes. It has been reported that doping



of the catalyst with a suitable dopant can enhance reusability of the used catalyst [71]. 4. Conclusions Samarium, cerium co-doped TiO2 as a new catalyst with enhanced sonocatalytic efficiency was prepared by an ultrasonicassisted sol–gel method. The co-doped TiO2 catalyst was characterized by using TEM, EDX, XRD, DRS, and PL techniques. XRD results indicated that samarium and cerium co-doping into TiO2 lattice has suppressed crystal growth and anatase phase transformation to rutile. DRS results showed a considerable decrease in Eg value for samarium, cerium co-doped TiO2 catalyst compared with pure and mono-doped TiO2. The sonocatalytic efficiency of prepared were studied toward removal of MO dye under ultrasonic irradiation. The results indicated that the samarium, cerium co-doped TiO2 catalyst with narrower band gap energy shows high sonocatalytic activity compared with pure and mono-doped TiO2. From PL studies a considerable quenching in the co-doped TiO2 catalyst was observed that suggests the co-doping of samarium and cerium into TiO2 lattice could effectively inhibit the recombination probability of generated electrons–holes pairs. Finally, the effect of influencing variables on the sonocatalytic activity of co-doped TiO2 catalyst was optimized using RSM technique and maximum removal efficiency (96.33%) was achieved at the optimum conditions (0.6 wt% samarium content, 0.82 wt% cerium content, 0.84 mg L1 catalyst dosage, 4.31 mg L1 initial pollutant concentration, 700 W ultrasonic irradiation power, and 50 min irradiation time). Also, samarium, cerium co-doped TiO2 catalyst showed high reusability as the sonocatalyst without any significant decrease in activity. Acknowledgement The authors would like to thank the University of Tehran for financial support of this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2015.02. 001. References [1] Y.L. Pang, A.Z. Abdullah, Comparative study on the process behavior and reaction kinetics in sonocatalytic degradation of organic dyes by powder and nanotubes TiO2, Ultrason. Sonochem. 19 (2012) 642–651. [2] E.K. Goharshadi, Y. Ding, M.N. Jorabchi, P. Nancarrow, Ultrasound-assisted green synthesis of nanocrystalline ZnO in the ionic liquid [hmim][NTf2], Ultrason. Sonochem. 16 (2009) 120–123. [3] E.A. Brujan, T. Ikeda, K. Yoshinaka, Y. Matsumoto, The final stage of the collapse of a cloud of bubbles close to a rigid boundary, Ultrason. Sonochem. 18 (2011) 59–64. [4] S. Hamood Saleh Azzam, G.T. Chandrappa, M. Afzal Pasha, Sonochemical hot-spot assisted one-pot synthesis of 4-arylmethylidene-2-phenyl-4H-oxazol-5-ones using nano-MgO as an efficient catalyst, Lett. Org. Chem. 10 (2013) 283–290. [5] I.A. Siddiquey, T. Furusawa, M. Sato, N.M. Bahadur, M. Mahbubul Alam, N. Suzuki, Sonochemical synthesis, photocatalytic activity and optical properties of silica coated ZnO nanoparticles, Ultrason. Sonochem. 19 (2012) 750–755. [6] S. Merouani, H. Ferkous, O. Hamdaoui, Y. Rezgui, M. Guemini, A method for predicting the number of active bubbles in sonochemical reactors, Ultrason. Sonochem. 22 (2015) 51–58. [7] O. Moumeni, O. Hamdaoui, C. Pétrier, Sonochemical degradation of malachite green in water, Chem. Eng. Process. 62 (2012) 47–53. [8] K. Vinodgopal, J. Peller, O. Makogon, P.V. Kamat, Ultrasonic mineralization of a reactive textile azo dye, remazol black B, Water Res. 32 (1998) 3646–3650. [9] Y.L. Pang, S. Bhatia, A.Z. Abdullah, Process behavior of TiO2 nanotube-enhanced sonocatalytic degradation of Rhodamine B in aqueous solution, Sep. Purif. Technol. 77 (2011) 331–338.

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