Ultrasonics Sonochemistry xxx (2015) xxx–xxx

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

Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application Alireza Khataee a,⇑, Atefeh Karimi a, Mahmoud Zarei b, Sang Woo Joo c,⇑ a

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran c School of Mechanical Engineering, Yeungnam University, 712-749 Gyeongsan, South Korea b

a r t i c l e

i n f o

Article history: Received 19 January 2015 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online xxxx Keywords: Sonochemical Sonocatalytic degradation Nanocatalyst Doped ZnO

a b s t r a c t Undoped and europium (III)-doped ZnO nanoparticles were prepared by a sonochemical method. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analysis. The crystalline sizes of undoped and 3% Eu-doped ZnO were found to be 16.04 and 8.22 nm, respectively. The particle size of Eu-doped ZnO nanoparticles was much smaller than that of pure ZnO. The synthesized nanocatalysts were used for the sonocatalytic degradation of Acid Red 17. Among the Eu-doped ZnO catalysts, 3% Eu-doped ZnO nanoparticles showed the highest sonocatalytic activity. The effects of various parameters such as catalyst loading, initial dye concentration, pH, ultrasonic power, the effect of oxidizing agents, and the presence of anions were investigated. The produced intermediates of the sonocatalytic process were monitored by GC–Mass (GC–MS) spectrometry. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Dyes are widely used in most industrial factories including the textile, paper printing, food processing, cosmetics, pharmaceutical, and leather industries. Due to the presence of aromatic structures in the dye molecules and resistance to destruction by physical–chemical treatment methods, it is necessary to find an effective method of wastewater treatment [1,2]. In recent years, ultrasonic irradiation has been attracting attention as an advanced oxidation process (AOP) for the elimination of pollutants in water [3,4]. Sonication of water promotes the formation, growth, and collapse of small gas bubbles. The collapsing of bubbles in an aqueous solution produces extremely high temperature and pressure conditions called ‘‘hot spots’’ [5,6]. The high temperature near the bubbles leads to thermal decomposition of H2O molecules to H and OH radicals. These free radicals can directly react with substances near them [6]. However, the degradation rate of this method is low compared to other methods and consumes considerable time and energy. In order to solve these problems and improve the degradation efficiency, ⇑ Corresponding authors. Tel.: +98 41 33393165; fax: +98 41 33340191 (A. Khataee). Tel.: +82 53 810 1456 (S.W. Joo). E-mail addresses: [email protected], [email protected] (A. Khataee), [email protected] (S.W. Joo).

semiconductor catalysts such as TiO2 or ZnO have been added to the ultrasonic reactions [7,8]. ZnO is a wide band-gap semiconductor that is commonly used in photocatalytic reactions. ZnO with a band gap of about 3.37 eV is sometimes preferred over TiO2 for catalytic treatment of hazardous organic pollutants in wastewater due to its high quantum efficiency [9]. The fast recombination of generated electron–hole pairs is the drawback that limits the usefulness of ZnO [10]. This problem can be addressed by doping ZnO with metal ions. Metal dopants can act as electron–hole scavengers and increase the lifetime of the charge carriers, and consequently reduce the recombination probability [10,11]. In this work, undoped and Eu-doped ZnO nanoparticles were successfully synthesized by a simple sonochemical method. The prepared catalysts were characterized by XRD, SEM, and XPS techniques. The nanoparticles were then used for sonocatalytic degradation of AR17 in aqueous solution. The effects of various parameters such as catalyst loading, initial dye concentration, pH, ultrasonic power, the effect of oxidizing agents, and the presence of anions were investigated, and the optimum conditions of degradation were identified. The produced intermediates of the process were monitored by using GC–MS analysis. To the best of our knowledge, no detailed investigation has been reported on the sonocatalytic degradation of AR17 in the presence of sonochemically synthesized Eu-doped ZnO nanoparticles.

http://dx.doi.org/10.1016/j.ultsonch.2015.03.016 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: A. Khataee et al., Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.03.016

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Fig. 1. XRD pattern of undoped and Eu-doped ZnO samples.

Fig. 2. SEM images of the (a), (b) undoped ZnO and (c), (d) 3% Eu-doped ZnO samples.

2. Materials and methods 2.1. Chemicals Zn(CH3COO)2.2H2O (99%) was purchased from Carlo Erba Reagenti, India. NaOH was provided by Merck, Germany. C6H9EuO6.xH2O and C2H5OH (99%) were purchased from Aldrich, USA. Acid Red 17 (a mono-azo dye with a molecular weight of

502.435 g/mol and kmax of 510 nm) was obtained from Shimi Boyakhsaz Co., Iran. All the chemicals were used without further purification. 2.2. Synthesis of sonocatalysts ZnO and Eu-doped ZnO were synthesized by a simple sonochemical method. To synthesize undoped and Eu-doped ZnO

Please cite this article in press as: A. Khataee et al., Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.03.016

A. Khataee et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

4 mL of sample was withdrawn from the ultrasonic reactor, and the residual concentration of AR17 in the solution after separation of catalyst was measured spectrophotometrically at a kmax of 510 nm. The percentage of decolorization efficiency (%) was determined as follows:

20

Frequency (%)

3

15

10

Decolorization efficiencyð%Þ ¼ ½1  ðC=C0 Þ  100

5

0 100-120 120-140 140-160 160-180 180-200 200-220 220-240 240-260 260-280 280-300

Size distirbution of ZnO nanostructures (nm)

(a) 20

ð1Þ

where C0 and C are the initial and final concentrations of the dye in the solution (mg/L), respectively. In the reusability test of the sonocatalyst, the used catalyst separated from solution and washed with distilled water and dried at 60 °C and then used in a new run. The intermediate reaction products during the sonocatalysis process were identified using a gas chromatograph (6890, Agilent Technologies, CA) with a 30 m to 0.25 mm HP-5MS capillary column coupled with a mass spectrometer (5973, Agilent Technologies, Canada).

Frequency (%)

16

3. Results and discussion

12

3.1. Characterization of nanocatalysts 8 4 0 5--10

10--15

15-20

20-25

25-30

30-35

35-40

40-45

45-50

50-55

The XRD patterns of undoped and 1% Eu-doped ZnO are shown in Fig. 1. The observed diffraction peaks of the pure ZnO sample were indexed to those of hexagonal wurtzite ZnO. No other phase (such as europium oxide) was found in the recorded diffractograms. However, the addition of Eu+3 ions caused a slight shift

Size distirbution of Eu-ZnO nanostructures (nm)

(b) Fig. 3. Size distribution of (a) undoped ZnO and (b) Eu-doped ZnO samples.

nanostructures with variable Eu mol fraction (0, 1, 3 and 5% mol) 10 mmol of zinc acetate with 0, 1, 3, and 5% mol Eu of C6H9EuO6.xH2O were dissolved in 100 mL deionized water. NaOH (1 M) was added to the solution until the pH reached 10. Ultimately, the mixture was sonicated for 3 h in a bath type sonicator (Ultra 8060, England) with a frequency of 36 kHz, volumetric capacity of 3 L and output intensity of 150 W/L. The obtained white product was washed with ethanol and distilled water, and dried at 80 °C for 12 h. 2.3. Instrumental analysis X-ray diffraction (XRD) patterns of the prepared catalysts were recorded in the 2h range between 10° and 80° on a Siemens X-ray diffractometer (D8 Advance, Bruker, Germany) using Cu Ka radiation (l = 1.54065 Å). The surface morphologies of the undoped and Eu-doped ZnO samples were also determined using scanning electron microscopy (SEM) with a Hitachi microscope (Model S-4200, Japan). A quantitative compositional analysis was conducted by using X-ray photoelectron spectroscopy (XPS) in a spectrometer (K-Alpha, Thermo Scientific, U.K.). The UV–Vis spectra of the as-prepared catalysts were analyzed using a UV–Vis spectrophotometer (WPA Lightwave S2000, England) in wavelength ranges between 200 and 800 nm at room temperature. 2.4. Catalytic activity evaluation Sonocatalytic degradation of AR17 as a dye pollutant was investigated in the presence of undoped and Eu-doped ZnO nanocatalyst using an ultrasonic bath. In each experiment, a 100 mg sonocatalyst was suspended in 100 mL of model dye aqueous solution with a known initial concentration. Then, the suspended solution was irradiated by an ultrasonic bath. At time intervals of 10 min, a

Fig. 4. (a) Survey XPS spectrum of undoped ZnO and (b) survey XPS spectrum of Eudoped ZnO samples.

Please cite this article in press as: A. Khataee et al., Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.03.016

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Table 1 Location of Zn 2p and O 1s peaks in the XPS spectra of ZnO and Eu-doped ZnO. ZnO

Eu-doped ZnO

Zn 2p3/2

Zn 2p1/2

O 1s

Zn 2p3/2

Zn 2p1/2

O 1s

Eu3d

1022.94 eV

1046.08 eV

532.19 eV

1022.26 eV

1045.08 eV

532.01 eV

1134.53 eV

100

25 Undoped ZnO

Sonocatalysis with undoped ZnO 80

20

Sonolysis only

3% Eu-doped ZnO

Adsorption with Eu-doped ZnO

(Ahυ)2

Decolorization efficiency (%)

Sonocatalysis with Eu-doped ZnO

Adsorption with undoped ZnO

60

15 10

40

5 20

0 2

2.2

2.4

2.6

0

2.8

3

3.2

3.4

3.6

hυ 0

10

20

30

40

50

60

70

Time (min) Fig. 5. Comparison of different processes [AR17]0 = 10 mg/L and [Catalyst]0 = 1 g/L.

in

Fig. 7. (Ahv)2–hv curves of the undoped and 3% Eu-doped ZnO nanostructures. the

degradation

of

AR17.

of (1 0 0), (0 0 2), and (1 0 1) peaks to low angles, indicating the existence of Eu3+ in the ZnO crystalline structure. This observation can be explained by the fact that the ionic radius of Eu+3 (0.95 Å) is larger than that of Zn2+ (0.74 Å) [12]. Similar shifts were also detected in La-doped ZnO and La-doped TiO2 systems [6,13]. SEM images of undoped ZnO and Eu-doped ZnO samples are shown in Fig. 2. The ZnO nanoparticles are nonuniform in shape and size, which could be related to the growth of irregular crystalline grains during synthesis. SEM images show the smaller crystalline size of the Eu-doped ZnO nanoparticles as compared to undoped ZnO nanoparticles. This confirms that introduction of Eu3+ ions into the ZnO lattice decreases the aggregation of nanoparticles and consequently decreases the size of the particles (Fig. 2(c) and (d)). Using manual microstructure distance measurement software (Nahamin Pardazan Asia Co, Iran), the size distribution of the Eu-doped ZnO nanoparticles was found to be

in the range of 30–40 nm, which is much smaller than that of undoped ZnO nanoparticles (200–250 nm) (Fig. 3(a) and (b)). A typical survey XPS spectrum is illustrated in Fig. 4. The survey spectrum confirms the presence of Zn, O, Eu, and C without impurities. The binding energies at 286 eV are related to the C 1s binding energy (Fig. 4(a) and (b)). The peaks at 532.19 and 532.01 eV are attributed to O2 ions in wurtzite structure of the hexagonal Zn2+ ion matrix in undoped and Eu-doped ZnO nanostructures, respectively [14]. Two peaks (1022 and 1045 eV) are ascribed to the core levels of Zn 2p 3/2 and Zn 2p 1/2 of ZnO, respectively [15]. The peaks corresponding to Eu 3d 5/2 and Eu 3d 3/2 appear at 1134 and 1164 eV, respectively (Fig. 4(b)). Their positions indicate that Eu ions have a +3 valance in the Eu-doped ZnO sample [16]. Incorporation of Eu3+ ions into the ZnO crystalline structure causes the changes in the location of Zn and O atoms in the XPS spectrum of Eu-doped ZnO nanoparticles. This confirms that Eu was successfully incorporated into the ZnO lattice. The corresponding data are given in Table 1. 3.2. Sonolysis and sonocatalysis

Decolorization efficiency (%)

100

3% Eu-d ed ZnO dop oped 5% Eu-d doped ZnO 1% Eu-d ZnO doped Zn O

80

The degradation of AR17 was studied using sonolysis, sonocatalysis with ZnO, and sonocatalysis with Eu-doped ZnO. As clearly shown in Fig. 5, sonocatalytic degradation occurs faster than sonolysis alone under similar experimental conditions. As can be seen in Fig. 5 the adsorption ratio is insignificant for both undoped and Eu-doped ZnO. So the increment in the decolorization efficiency cannot be related to adsorption of the dye on the catalyst surface. Sonocatalytic degradation of organic pollutants in the presence of heterogonous catalysts such as ZnO has been subject to debate [8,17]. Perhaps a possible mechanism can be explained

60

40

20 Table 2 Band gap energy of undoped ZnO and Eu-doped ZnO nanostructures.

0 0

10

20

30

40

50

60

70

Time (min) Fig. 6. Effect of the amount of Eu dopant on the sonocatalytic degradation of AR17. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L and the ultrasonic power = 150 W/L.

Sample

Band gap (eV)

Undoped ZnO 1% Eu-doped ZnO 3% Eu-doped ZnO 5% Eu-doped ZnO

3.2 3.05 2.95 3.18

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100

100

0.25 g/L Eu-doped ZnO

pH= 6.0 Decolorization efficiency (%)

Decolorization efficiency (%)

0.5 g/L Eu-doped ZnO 0.75 g/L Eu-doped ZnO

80

1.0 g/L Eu-doped ZnO 1.25 g/L Eu-doped ZnO

60

40

pH = 3.0

80

pH = 10.0 60

40

20

0

20

0

10

20

30

40

50

60

70

Time (min)

0 0

10

20

30

40

50

60

70

Time (min) Fig. 8. Effect of the catalyst concentration on the sonocatalytic degradation of AR17 in the presence of 3% Eu-doped ZnO. [AR17]0 = 10 mg/L and [Catalyst]0 = 1 g/L; the initial pH = 6.0; and the ultrasonic power = 150 W/L.

by means of light and heat energies coming from ultrasonic irradiation. Ultrasonic irradiation combined with ZnO samples can generate various reactive oxygen species (ROS). Firstly, sonoluminescence caused by ultrasonic irradiation generates light with wavelengths below 375 nm. This excites Eu-doped ZnO nanoparticles to act as photocatalysts for the generation of OH radicals, as follows [4,6,18]:

Ultrasonic irradiation ! light or heat ½ZnO þ light or heat ! ½ZnO

ð2Þ



ð3Þ

þ

½ZnO ! ZnOðh þ e Þ

ð4Þ

Fig. 10. Effect of pH on the sonocatalytic degradation of AR17 in the presence of 3% Eu-doped ZnO. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L, and the ultrasonic power = 150 W/L.

þ

h þ H2 O !  OH þ Hþ

ð6Þ

Secondly, the temperature of ‘‘hot spot’’ produced by ultrasonic cavitation in a water medium can achieve 105 and 106 °C and high pressure (approximately 1800 atm) [17,23,27]. These hot spots can pyrolysis H2O molecules to H and OH radicals. Recombination of hydroxyl radicals generates H2O2, as shown by Eqs. (7) and (8) [19,20]:

H2 O þ Ultrasonic irradiation! OH þ H

ð7Þ



ð8Þ

OH þ  OH ! H2 O2

Otherwise, the excited ZnO particles can transfer adequate energy to active ground (triplet) state molecular oxygen (3O2) to produce excited (singlet) state molecular oxygen (1O2), as follows [21,22]:

½ZnO þ 3 O2 ! ½ZnO þ 1 O2

ð9Þ

Generated electron–hole pairs react with electron donors and acceptors, thereby generating active radicals that can react with organic molecules and enhance decolorization efficiency as described by Eqs. (5) and (6):

Ultrasonic irradiation in the presence of a sonocatalyst generates active radicals, which can decompose the dye molecules as follows:

e þ O2 ! O 2

AR17 þ sonocatalyst þ ultrasonic ! Degradation of the dye

ð5Þ

ð10Þ

3.3. Effect of dopant content

Decolorization efficiency (%)

100 10 mg/L 12.5 mg/L 15 mg/L

80

60

40

20

0 0

10

20

30

40

50

60

70

Time (min) Fig. 9. Effect of initial concentration of AR17 on sonocatalytic degradation in the presence of 3% Eu-doped ZnO. [Catalyst]0 = 1 g/L, the initial pH = 6.0, and the ultrasonic power = 150 W/L.

The influence of the amount of dopant on the sonocatalytic activity of prepared samples was studied (Fig. 6). The amount of dopant was varied from 1% to 5%. The sonocatalyst dosage, initial dye concentration, and reaction time are constant at 1 g/L, 10 mg/L, and 70 min, respectively. All of the Eu-doped ZnO samples showed a higher sonocatalytic activity than that of the undoped ZnO sample. The highest decolorization efficiency was obtained with 3% Eu-doped ZnO nanoparticles. The reason for these observations can be explained by two mechanisms. Firstly, incorporation of Eu+3 ions causes the creation of new energy levels below the conduction band edge of ZnO [23]. The Eu+3 ions serve as an electron scavenger and restrain the electron–hole recombination. In the case of Eu, with regard to the standard redox potentials of Ev.b.(ZnO) = 0.4 V, E0(O2/O2) = +0.338 V, and E0(Eu+3/Eu+2) = 0.35 V, the presence of Eu+3 in crystallite ZnO can promote the following reactions [24]:

Eu3þ þ e ! Eu2þ

ð11Þ

Eu2þ þ O2 ! Eu3þ þ O 2

ð12Þ

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40

the 3% Eu-doped ZnO nanoparticles show the lowest band gap energy and highest sonocatalytic activity. With an increasing dopant content up to 5%, the decolorization efficiency was decreased. Above an optimum value of Eu doping in the ZnO structure, the surface oxygen defects act as electron–hole recombination centers and decrease the sonocatalytic activity of the doped sample. The excess amount of lanthanide ions covering the surface of ZnO nanoparticles leads to a decrease in the sonocatalytic activity of the catalyst due to an increase in the number of electron–hole recombination centers.

20

3.4. Effect of catalyst dosage

Decolorization efficiency (%)

100 Without enhancer K2S2O8 KIO4

80

60

0 0

10

20

30

40

50

60

70

Time (min) Fig. 11. Effect of the presence of enhancers on the sonocatalytic degradation of AR17 in the presence of 3% Eu-doped ZnO. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L, [Enhancer]0 = 1 mM, the initial pH 6.0, and the ultrasonic power = 150 W/L.  þ O 2 þ H ! HO2

ð13Þ

2HO2 ! H2 O2 þ O2

ð14Þ

O 2



H2O2, OH, and radicals are potential oxidants for the degradation of target pollutants. Secondly, the doping of ZnO with Eu+3 ions leads to a shift in the absorption into the visible region. The doping of ZnO with Eu ions produces new impurity states in the band gap of ZnO by Eu 4f electrons. These new states can overlap with band states and decrease the band gap. High sonocatalytic activity is obtained if the electrons can transfer from these states to the surface. These electrons can react with oxygen and generate reactive oxygen species that can attack dye molecules. The band gap of undoped and Eu-doped samples (Table 2) was calculated as follows:

ðAht2 Þ ¼ Kðht  Eg Þ

ð15Þ

where ht is the photon energy (eV), A is the absorption coefficient, K is a constant, and Eg is the band gap. By extrapolating the linear region in a plot of (Ahm)2 versus photon energy, the band gap can be estimated (Fig. 7). The band gap values for undoped ZnO and Eu-doped ZnO nanoparticles are given in Table 2. We note that 100

The effect of the catalyst amount on sonocatalysis of AR17 is shown in Fig. 8. The concentration of the catalyst was varied from 2.5 to 12.5 g/L. The degradation efficiencies are 52.0%, 70.0%, 80.0%, 98.0%, and 93.0% at catalyst concentrations of 0.25, 0.5, 0.75, 1, and 1.25 g/L, respectively. The decolorization efficiency increases with increased catalyst loading and reaches an optimum value. Beyond this optimum, the degradation efficiency remains steady or decreases [25]. The increase in the decolorization efficiency seems to be due to the effective surface of the catalyst and the absorption of UV light resulting from ultrasonic irradiation. At a low catalyst dosage, the absorption of light controls the sonocatalytic process due to the limited surface area of the catalyst [26]. However, as the catalyst loading increases, an increase in the available surface area containing active reaction sites of ZnO is obtained. Consequently, the generation of the hydroxyl radicals that are responsible for the degradation of AR17 is increased. On the other hand, when the catalyst dosage becomes greater than 1.25 g/L, the degradation efficiency decreases. The number of active sites on the catalyst’s surface decreases due to aggregation of nanoparticles at high concentration. Also, the turbidity of the suspension increases because of the decrease in light penetration due to the light scattering effect [27]. Subsequent experiments were carried out using 1 g/L of sonocatalyst. 3.5. Effect of AR17 initial concentration The effect of the initial concentration of AR17 in the range of 10–15 mg/L on sonocatalytic degradation is shown in Fig. 9. The values of the dopant content, catalyst concentration, and reaction time are 3%, 1 g/L, and 70 min, respectively. The degradation efficiency decreases from 98% to 65% with an increase in the initial concentration from 10 to 15 mg/L. The number of dye molecules 100

Sodium sulfate

80

Decolorization efficiency (%)

Decolorization eficiency (%)

Without scavenger

Sodium carbonate Sodium chloride 60

40

20

0

150 W/L 300 W/L

80

400 W/L 60

40

20

0

0

10

20

30

40

50

60

70

Time (min) Fig. 12. Effect of scavengers on the sonocatalytic degradation of AR17 in the presence of 3% Eu-doped ZnO. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L, the initial pH 6.0, [Scavenger]0 = 10 mg/L, and the ultrasonic power = 150 W/L.

0

10

20

30

40

50

60

70

Time (min) Fig. 13. Effect of ultrasonic power on the sonocatalytic degradation of AR17 in the presence of 3% Eu-doped ZnO. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L, and the initial pH 6.0.

Please cite this article in press as: A. Khataee et al., Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.03.016

A. Khataee et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx

that are adsorbed on the catalyst’s surface increases with increased dye concentration. Consequently, the number of hydroxyl radicals required for degradation of the AR17 molecules increases. However, the rate of generation of hydroxyl radicals remains constant due to the other operating conditions that are constant [28,29]. In addition, the dye solution becomes dense at a high concentration, which hinders the penetration of light to the catalyst’s surface [30].

(E0 = 2.6 V). At first, S2O2 8 activates by ultrasonic irradiation as follows [33]:  S2 O2 8 þ ultrasonic irradiation ! 2SO4

ð16Þ

In the next step, sulfate radicals react with H2O molecules to form hydroxyl radicals: 2  þ SO 4 þ H2 O ! H þ SO4 þ OH

ð17Þ

Periodate ions increase the decolorization efficiency by scavenging the generated electrons of the exited Eu-ZnO nanocatalyst [34]:

3.6. Effect of solution pH The pH of the aqueous solution significantly affects the sonocatalytic process. Hence, the effect of pH on the sonocatalytic degradation of AR17 was investigated at acidic, neutral, and basic conditions (Fig. 10). The maximum decolorization efficiency was observed at a pH of 6, and it decreased thereafter in the acidic and alkaline medium. The pH at which the surface of an oxide is uncharged is defined as the zero point charge (pHpzc). The effect of pH values on the sonocatalytic degradation of the dye can be explained on the basis of the zero point charge of the catalyst. Above and below of the zero point charge, the catalyst is negatively and positively charged, respectively. The point of zero charge is approximately 9 ± 0.3 for ZnO [9,31]. Accordingly, the ZnO surface is protonated below a pH of 9, and above this pH, the catalyst’s surface is predominantly negatively charged by adsorbed OH- ions [9]. In alkaline solution (pH > pHpzc), OH- ions that are absorbed on the catalyst’s surface hinder the formation of hydroxyl radicals that are responsible for the degradation process, and thus reduce the degradation rate. In neutral and acidic solution (pH < pHpzc), AR17 as an anionic dye with negative charge can be adsorbed on the surface of positively charged ZnO nanoparticles. Thus, the degradation efficiency logically enhances. The low decolorization efficiency at pH = 3 could be related to the corrosion of the catalyst in an acidic condition [4].

IO4 þ 8e þ 8Hþ ! 4H2 O þ I

3.7. Effect of oxidizing agents

Real wastewater obtained from industries usually has large quantities of organic and inorganic matter that can inhibit sonocatalytic reactions. In the present study, sulfate, carbonate, and chloride ions were used to examine the effect of radical scavengers on the sonocatalysis of AR17 (Fig. 12). The initial concentration of AR17, the dopant content, the sonocatalyst dosage, and the reaction time are constant at 10 mg/L, 3%, 1 g/L, and 70 min, respectively. In the presence of sodium chloride, sodium carbonate and sodium sulfate, the decolorization efficiency decreases from 98% to 60%, 71%, and 89%, respectively. Thus, all of the inorganic anions inhibit the sonocatalytic degradation of AR17. The chloride ions scavenge the produced hydroxyl radicals as indicated in Eqs. 19– 21 [30]: 

þ





100



ð19Þ



ð20Þ

Cl þ h ! Cl

Cl þ Cl ! Cl2 



OH þ Cl ! HOCl

ð21Þ

HOCl þ Hþ ! Cl þ H2 O

ð22Þ



Electron–hole recombination is considered to be a major energy-wasting step in heterogeneous sonocatalytic reactions. One strategy to restrain the e/h+ recombination is to add irreversible oxidizing agents [32]. In the present study, we examined the effect of added peroxydisulfate and potassium periodate with an initial concentration of 1 mM on the sonocatalytic decolorization of AR17 (Fig. 11). The addition of S2O2 and IO 8 4 accelerates the sonocatalysis of AR17. Peroxydisulfate acts as a power oxidizing agent with a standard potential of E0 = 2.01 V, and can be decomposed to produce sulfate radicals as a very strong oxidant

ð18Þ

3.8. Effect of scavengers



Decolorization efficiency (%)

7



The deactivation of hydroxyl radicals by sulfate anions can be explained by Eqs. 17, 23 and 24 [30]:    SO2 4 þ OH ! SO4 þ OH

ð23Þ

SO 4 þ organic dye ! Degradation

ð24Þ

In the case of carbonate, the reactions that can occur are given as follows [30]:    CO2 3 þ OH ! CO3 þ OH

ð25Þ

  2CO 3 þ H2 O ! 2CO2 þ HO2 þ OH

ð26Þ

CO2 3

60

The presence of ions can serve as a radical scavenger and react with OH radicals to form less active CO 3 , which restrains the AR17 degradation. Furthermore, the addition of these salts blocks the active sites of the Eu-ZnO nanocatalyst, which deactivates the catalysts towards the dye.

40

3.9. Effect of ultrasonic power

80

20 0 Run 1

Run 2

Run 3

Run 4

Run 5

Run 6

Run 7

Fig. 14. Reusability of the 3% Eu-doped ZnO nanostructures within seven consecutive experimental runs. [AR17]0 = 10 mg/L, [Catalyst]0 = 1 g/L, the initial pH 6.0, the ultrasonic power = 150 W/L, and the reaction time = 70 min.

Fig. 13 shows the sonocatalysis of AR17 at different ultrasonic powers in the presence of 3% Eu-ZnO nanoparticles. By increasing the ultrasound power from 150 to 400 W/L, the degradation efficiency is increased from 79% to 100%. Due to an increase in the ultrasonic power, it is likely that the net production of the OH radicals increases, which results in enhanced decolorization efficiency. Golash et al. [35] reported similar results for sonodegradation of dichlorvos in wastewater.

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Table 3 Identified by-products during sonocatalysis of Acid Red 17. No.

Structure

tR (min)

Molecular weight (m/z)

Compound

2.251

446.20

1,3-bis (3-phenoxyphenoxy) benzene

3.982

252.22

1,3,5-Benzenetricarboxylic acid, trimethyl ester

4.659

60.10

Acetic acid

5.780

73.10

2-Propanoic acid

5

8.477

107.10

Ethyl benzene

6

26.64

449

3-(4-N,N-Dimethylaminophenyl)propenoic acid

O

1

OH

4 O

2

O

O

O

O

O

3

HO

4

O H2C

O

O N

3.10. Reusability of the sonocatalyst A reusability test of 3% Eu-doped ZnO nanoparticles was performed with an initial AR17 concentration of 10 mg/L, a catalyst dosage of 1.0 g/L, an ultrasonic power of 150 W/L, and a reaction time of 70 min. As shown in Fig. 14, after seven repeated runs, the decline in decolorization efficiency was insignificant. This fact indicates that Eu-doped ZnO nanoparticles can be used as an effective catalyst for the sonocatalytic degradation of organic pollutants in aqueous solution. 3.11. Determination of AR17 by-products Degradation products formed during the sonocatalytic process were analyzed by GC–MS. The generation of various intermediates during the sonocatalysis of AR17 indicates that hydroxyl radicals are non-selective. Six main compounds were successfully detected by comparison with commercial standards, as shown in Table 3. The mass spectra of the determined by-products correspond with those of the GC–MS spectrum library by at least 90% [36]. 4. Conclusion Undoped and Eu-doped ZnO nanoparticles were successfully synthesized by a sonochemical method. Our XRD results reveal

that the prepared catalysts were in the hexagonal phase. SEM images confirmed that the particle size of Eu-doped ZnO is much smaller than that of pure ZnO. The XPS data show the proper incorporation of Eu+3 ions into the ZnO structure. The band gap value of doped samples is smaller as compared to that of undoped ZnO. The 3% Eu-doped ZnO nanoparticles showed the highest efficiency of 98% at 70 min. The presence of anions such as sulfate, carbonate, and chloride inhibits the decolorization efficiency. The maximum decolorization efficiency is observed at a neutral pH. Our reusability tests also show superior stability for the prepared catalysts. Therefore, the Eu-doped ZnO nanoparticles are a suitable catalyst for application to the sonocatalytic degradation of organic dyes. Acknowledgement The authors thank the University of Tabriz (Iran) for all of the support provided. References [1] J. Wang, Z. Jiang, L. Zhang, P. Kang, Y. Xie, Y. Lv, R. Xu, X. Zhang, Sonocatalytic degradation of some dyestuffs and comparison of catalytic activities of nanosized TiO2, nano-sized ZnO and composite TiO2/ZnO powders under ultrasonic irradiation, Ultrason. Sonochem. 16 (2009) 225–231. [2] N. Daneshvar, D. Salari, A. Khataee, Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2, J. Photochem. Photobiol., A 162 (2004) 317–322.

Please cite this article in press as: A. Khataee et al., Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015), http://dx.doi.org/10.1016/j.ultsonch.2015.03.016

A. Khataee et al. / Ultrasonics Sonochemistry xxx (2015) xxx–xxx [3] L. Song, C. Chen, S. Zhang, Sonocatalytic performance of Tb7 O12/TiO2 composite under ultrasonic irradiation, Ultrason. Sonochem. 18 (2011) 713–717. [4] A. Khataee, A. Karimi, S. Arefi-Oskoui, R.D.C. Soltani, Y. Hanifehpour, B. Soltani, S.W. Joo, Sonochemical synthesis of Pr-doped ZnO nanoparticles for sonocatalytic degradation of Acid Red 17, Ultrason. Sonochem. 22 (2015) 371–381. [5] Pankaj, M. Ashokkumar, Theoretical and experimental sonochemistry involving inorganic systems, Springer, 2011. [6] L. Zhu, Z.-D. Meng, C.-Y. Park, T. Ghosh, W.-C. Oh, Characterization and relative sonocatalytic efficiencies of a new MWCNT and CdS modified TiO2 catalysts and their application in the sonocatalytic degradation of rhodamine B, Ultrason. Sonochem. 20 (2013) 478–484. [7] N. Ghows, M. Entezari, Kinetic investigation on sono-degradation of Reactive Black 5 with core–shell nanocrystal, Ultrason. Sonochem. 20 (2013) 386–394. [8] L. Song, C. Chen, S. Zhang, Q. Wei, Sonocatalytic degradation of amaranth catalyzed by La3+ doped TiO2 under ultrasonic irradiation, Ultrason. Sonochem. 18 (2011) 1057–1061. [9] S.-M. Lam, J.-C. Sin, A.Z. Abdullah, A.R. Mohamed, Degradation of wastewaters containing organic dyes photocatalysed by zinc oxide: a review, Desalin. Water Treat. 41 (2012) 131–169. [10] S. Rehman, R. Ullah, A. Butt, N. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater. 170 (2009) 560–569. [11] M. Khatamian, A. Khandar, B. Divband, M. Haghighi, S. Ebrahimiasl, Heterogeneous photocatalytic degradation of 4-nitrophenol in aqueous suspension by Ln (La3+, Nd3+ or Sm3+) doped ZnO nanoparticles, J. Mol. Catal. A: Chem. 365 (2012) 120–127. [12] M. Wang, C. Huang, Z. Huang, W. Guo, J. Huang, H. He, H. Wang, Y. Cao, Q. Liu, J. Liang, Synthesis and photoluminescence of Eu-doped ZnO microrods prepared by hydrothermal method, Opt. Mater. 31 (2009) 1502–1505. [13] A.N. Ökte, Characterization and photocatalytic activity of Ln (La, Eu, Gd, Dy and Ho) loaded ZnO nanocatalysts, Appl. Catal. A 475 (2014) 27–39. [14] S. Anandan, A. Vinu, K. Sheeja Lovely, N. Gokulakrishnan, P. Srinivasu, T. Mori, V. Murugesan, V. Sivamurugan, K. Ariga, Photocatalytic activity of La-doped ZnO for the degradation of monocrotophos in aqueous suspension, J. Mol. Catal. A: Chem. 266 (2007) 149–157. [15] B.D. Ngom, M. Chaker, N. Manyala, B. Lo, M. Maaza, A. Beye, Temperaturedependent growth mode of W-doped ZnO nanostructures, Appl. Surf. Sci. 257 (2011) 6226–6232. [16] F. Zhao, H.-L. Sun, S. Gao, G. Su, Magnetic properties of EuS nanoparticles synthesized by thermal decomposition of molecular precursors, J. Mater. Chem. 15 (2005) 4209–4214. [17] M. Ahmad, E. Ahmed, Z. Hong, W. Ahmed, A. Elhissi, N. Khalid, Photocatalytic, sonocatalytic and sonophotocatalytic degradation of Rhodamine B using ZnO/ CNTs composites photocatalysts, Ultrason. Sonochem. 21 (2014) 761–773. [18] J. Wang, Y. Guo, B. Liu, X. Jin, L. Liu, R. Xu, Y. Kong, B. Wang, Detection and analysis of reactive oxygen species (ROS) generated by nano-sized TiO2 powder under ultrasonic irradiation and application in sonocatalytic degradation of organic dyes, Ultrason. Sonochem. 18 (2011) 177–183. [19] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Sonocatalytic degradation of methylene blue with TiO2 pellets in water, Ultrason. Sonochem. 14 (2007) 184–190.

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[20] C. Berberidou, I. Poulios, N. Xekoukoulotakis, D. Mantzavinos, Sonolytic, photocatalytic and sonophotocatalytic degradation of malachite green in aqueous solutions, Appl. Catal. B 74 (2007) 63–72. [21] T. Pandiyan, O. Martı´nez Rivas, J. Orozco Martı´nez, G. Burillo Amezcua, M. Martınez-Carrillo, Comparison of methods for the photochemical degradation of chlorophenols, J. Photochem. Photobiol., A 146 (2002) 149–155. [22] G.A. Epling, C. Lin, Photoassisted bleaching of dyes utilizing TiO2 and visible light, Chemosphere 46 (2002) 561–570. [23] O. Yayapao, T. Thongtem, A. Phuruangrat, S. Thongtem, Sonochemical synthesis of Dy-doped ZnO nanostructures and their photocatalytic properties, J. Alloy. Compd. 576 (2013) 72–79. [24] A.S. Weber, A.M. Grady, R.T. Koodali, Lanthanide modified semiconductor photocatalysts, Catal. Sci. Technol. 2 (2012) 683–693. [25] R. Rajeswari, S. Kanmani, Comparative study on photocatalytic oxidation and photolytic ozonation for the degradation of pesticide wastewaters, Desalin. Water Treat. 19 (2010) 301–306. [26] R. Kitture, S.J. Koppikar, R. Kaul-Ghanekar, S. Kale, Catalyst efficiency, photostability and reusability study of ZnO nanoparticles in visible light for dye degradation, J. Phys. Chem. Solids 72 (2011) 60–66. [27] Y.L. Pang, A.Z. Abdullah, S. Bhatia, Review on sonochemical methods in the presence of catalysts and chemical additives for treatment of organic pollutants in wastewater, Desalination 277 (2011) 1–14. [28] A.G. Chakinala, P.R. Gogate, A.E. Burgess, D.H. Bremner, Intensification of hydroxyl radical production in sonochemical reactors, Ultrason. Sonochem. 14 (2007) 509–514. [29] A. Khataee, R.D.C. Soltani, A. Karimi, S.W. Joo, Sonocatalytic degradation of a textile dye over Gd-doped ZnO nanoparticles synthesized through sonochemical process, Ultrason. Sonochem. 23 (2015) 219–230. [30] A. Khataee, R. Darvishi Cheshmeh Soltani, Y. Hanifehpour, M. Safarpour, H. Gholipour Ranjbar, S.W. Joo, Synthesis and characterization of dysprosiumdoped ZnO nanoparticles for photocatalysis of a textile dye under visible light irradiation, Ind. Eng. Chem. Res. 53 (2014) 1924–1932. [31] N. Ghows, M.H. Entezari, Exceptional catalytic efficiency in mineralization of the reactive textile azo dye (RB5) by a combination of ultrasound and core– shell nanoparticles (CdS/TiO2), J. Hazard. Mater. 195 (2011) 132–138. [32] M. Zhang, T. An, J. Fu, G. Sheng, X. Wang, X. Hu, X. Ding, Photocatalytic degradation of mixed gaseous carbonyl compounds at low level on adsorptive TiO2/SiO2 photocatalyst using a fluidized bed reactor, Chemosphere 64 (2006) 423–431. [33] A. Khataee, O. Mirzajani, UV/peroxydisulfate oxidation of CI Basic Blue 3: modeling of key factors by artificial neural network, Desalination 251 (2010) 64–69. [34] B. Gözmen, Applications of response surface analysis to the photocatalytic mineralization of acetaminophen over silver deposited TiO2 with periodate, Environ. Prog. Sustain. Energy 31 (2012) 296–305. [35] N. Golash, P.R. Gogate, Degradation of dichlorvos containing wastewaters using sonochemical reactors, Ultrason. Sonochem. 19 (2012) 1051–1060. [36] M. Thabet, A.A. EL-Zomrawi, Degradation of acid red 17 dye with ammonium persulphate in acidic solution using photoelectrocatalytic methods, doi: http://dx.doi.org/10.1016/j.arabjc.2011.03.001.

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Eu-doped ZnO nanoparticles: Sonochemical synthesis, characterization, and sonocatalytic application.

Undoped and europium (III)-doped ZnO nanoparticles were prepared by a sonochemical method. The prepared samples were characterized by X-ray diffractio...
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