Accepted Manuscript Sonochemical synthesis of Pr-doped ZnO nanoparticles for sonocatalytic degradation of Acid Red 17 Alireza Khataee, Atefeh Karimi, Samira Arefi-Oskoui, Reza Darvishi Cheshmeh Soltani, Younes Hanifehpour, Behzad Soltani, Sang Woo Joo PII: DOI: Reference:

S1350-4177(14)00182-5 http://dx.doi.org/10.1016/j.ultsonch.2014.05.023 ULTSON 2619

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

1 May 2014 25 May 2014 26 May 2014

Please cite this article as: 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, Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.05.023

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Sonochemical synthesis of Pr-doped ZnO nanoparticles for sonocatalytic degradation of Acid Red 17 Alireza Khataee a, *, Atefeh Karimi a, Samira Arefi-Oskoui a, Reza Darvishi Cheshmeh Soltani b, Younes Hanifehpour c, Behzad Soltani d, Sang Woo Joo c, *

a

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department

of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran b

Department of Environmental Health Engineering, School of Health, Arak University of

Medical Sciences, Arak, Iran c

School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749 South

Korea d

Department of Chemistry, Azarbaijan Shahid Madani University, Tabriz, Iran

* Corresponding author (communicator) E–mail address: [email protected] ([email protected]) Tel.: +98 411 3393165; Fax: +98 411 3340191

*

Corresponding author

E–mail address: [email protected] Tel: +82 53 810 1456

1

Abstract Undoped and Pr-doped ZnO nanoparticles were prepared using a simple sonochemical method, and their sonocatalytic activity was investigated toward degradation of Acid Red 17 (AR17) under ultrasonic (US) irradiation. Synthesized nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The extent of sonocatalytic degradation was higher compared with sonolysis alone. The decolorization efficiency of sonolysis alone, sonocatalysis with undoped ZnO and 5% Pr-doped ZnO was 24%, 46% and 100% within reaction time of 70 min, respectively. Sonocatalytic degradation of AR17 increased with increasing the amount of dopant and catalyst dosage and decreasing initial dye concentration. Natural pH was favored the sonocatalytic degradation of AR17. With the addition of chloride, carbonate and sulfate as radical scavengers, the decolorization efficiency was decreased from 100% to 65%, 71% and 89% at the reaction time of 70 min, respectively, indicating that the controlling mechanism of sonochemical degradation of AR17 is the free radicals (not pyrolysis). The addition of peroxydisulfate and hydrogen peroxide as enhancer improved the degradation efficiency from 79% to 85% and 93% at the reaction time of 50 min,, respectively. The result showed good reusability of the synthesized sonocatalyst.

Keywords: ZnO nanocatalyst; Sonochemical synthesis; Sonolysis; Sonocatalysis; Ultrasonic degradation, Textile dye.

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1. Introduction Organic dyes are one of the many new chemicals which have been widely used in leather, textile, pulp and paper, cosmetic and pharmaceutical industries [1]. Due to their large-scale production and extensive applications, organic dyes have become an integral part of industrial wastewaters which are generally toxic and resistant to degradation by biological treatment methods. Therefore, conventional treatment methods are ineffective for degradation and mineralization of organic dyes [2, 3]. Recently, great attention has been paid to the application of ultrasound as an advanced oxidation process (AOP) for water and wastewater treatment due to its high efficiency and easy to operation. Ultrasonically induced cavitation effects can be very effective in the degradation of organic compounds in aqueous phase [3]. Sonochemical degradation involves ultrasound waves to produce rapid growth and collapse of a bubble producing extremely high temperature and pressure conditions inside the bubble [4]. The temperature near the bubbles is high and the resulting thermal dissociation of water results in the formation of hydroxyl radicals which oxidize the substances near them [5]. Ultrasound has been used for the wastewater treatment of various pollutants e.g. organic dyes, organic sulfur compounds, aromatic compounds and so on [6]. However, degradation of organic compounds using ultrasound consumes large amounts of energy and complete mineralization of organic pollutants rarely happens by applying sonolysis alone [3, 7]. To overcome these limitations, ultrasonic treatment of wastewater can be used in the presence of suitable catalyst (sonocatalyst). According to literature, semiconductors are used as efficient sonocatalysts [8-10]. Researches indicate that there is similarity in the mechanisms of sonocatalytic and photocatalytic reactions. Sonocatalytic reactions involve the formation of electron-hole pairs on the surface of catalyst, which their formation is critical during sonocatalytic degradation [9, 11, 12]. Among different catalysts, ZnO has been widely used in the catalytic processes for removing organic dyes because of its wide band gap (3.37 eV) 3

and low cost [8, 13-16]. The main disadvantage of pure ZnO nanoparticles is the fast recombination rate of the generated electron−hole pairs [17]. Hence, to improve the sonocatalysis efficiency, the electron–hole pair recombination must be hindered. For this purpose, modification of ZnO nanoparticles through metal ion doping is considered as one of possible ways to decrease the probability of recombination of the generated electron−hole pairs as shown in Eqs. (1) and (2) [5, 8, 18]:

M n  ecb  M ( n1) (as electron trap) M n  hvb  M ( n1) (as hole trap)

(1) (2)

If the Mn+/M(n-1)+ pair is less negative than the ZnO conduction band edge and the energy level for Mn+/M(n+1)+ is less positive than ZnO valence edge, the trapping of electrons and holes would happen on the surface affecting the lifetime of charge carriers. In the present study, a simple sonochemical method has been introduced to the synthesis of undoped and Pr-doped ZnO nanoparticles. Pr was incorporated into ZnO lattice to improve its sonocatalytic activity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize as-prepared sonocatalysts. In addition, X-ray photoelectron spectroscopy (XPS) was utilized to verify the presence of Pr in the sonocatalyst structure. Acid Red 17 (AR17) was used as model organic dye to evaluate sonocatalytic activity of undoped and Pr-doped ZnO nanoparticles under ultrasonic irradiation. To the best of our knowledge and on the basis of the literature review, the application of sonochemical method for the preparation of Pr-doped ZnO nanoparticles and its potential for the sonocatalytic degradation of AR17 has not been investigated. In the following, the effect of various parameters such as amount of dopant, catalyst dosage, initial dye concentration, initial pH, intensity of ultrasonic irradiation and presence of various radical scavengers and process enhancers on the decolorization efficiency was investigated. 4

2. Materials and methods 2.1. Chemicals All chemicals used in this study were of analytical grade and used without further purifications. C2H5OH.4H2O (%99) solution and Pr(NO3)3.6H2O (99.99%) were purchased from Sigma Aldrich, USA. NaOH and ZnCl2 (%99.5) was purchased from Merck, Germany. AR17 was prepared from Shimi Boyakhsaz Co., Iran. The characteristics of the dye are reported in Table 1. 2.2. Synthesis of ZnO and Pr-doped ZnO nanoparticles ZnO and Pr-doped ZnO nanoparticles were synthesized using a simple sonochemical method. For preparation of Pr-doped ZnO nanoparticles with different amount of dopant (0-5%), stoichiometric amount of Pr(NO3)3.6H2O was added to ZnCl2 solution. Then, 1 M NaOH solution was added dropwise to above solution until the pH reached to 10. The precursor solution was then irradiated by a bath type sonicator (sonica, 2200 EP S3, Italy) with a frequency of 50-60 Hz for 3 h. Finally, the obtained white precipitate was washed thrice with absolute ethanol and distilled water, and dried at 80 °C for 12 h to achieve the crystalline structure. 2.3. Characterization of the synthesized samples The crystalline phase composition of as-prepared samples was identified through X-ray diffraction (XRD) by using a Siemens X-ray diffractometer (D8 Advance, Bruker, Germany), Cu Kα radiation (l=1.54065 Å), an accelerating voltage of 40 kV, and an emission current of 30 mA. The surface morphology of ZnO and Pr-doped ZnO samples were examined via scanning electron microscopy (SEM) by means of a Hitachi microscope (Model: S-4200, Japan). For TEM observations, the as-prepared sample was dispersed in the ethanol using 5

ultrasonic vibration (Sonorex Bandelin Digi Tec, UK) for 15 min, and then a drop of dispersed sample was placed on a copper grid coated with a layer of amorphous carbon. TEM images were recorded by a Cs-corrected high-resolution TEM (Model: JEM-2200FS, JEOL, Japan) operated at 200 kV. Chemical compositions and chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) by Thermo Scientific spectrometer (Model: K-ALPHA, UK). A Tensor Fourier transform infrared (FT–IR) spectroscope (Model: Tensor 27, Bruker, Germany) was employed to examine the chemical bonds on the samples, in the wavenumber range of 400-4000 cm-1 on pellets obtained by mixing the samples in KBr. For determination of band gap of undoped and Pr-doped ZnO nanoparticles, the sample was well dispersed in distilled water by sonication for 15 min to form a homogeneous suspension, and then optical absorption spectra of samples were recorded using UV-Vis spectrophotometer (WPA lightwave S2000, England) in wavelength range between 200 and 700 nm at room temperature. 2.4. Sonocatalytic degradation of AR17 Decolorization of AR17 was investigated in the presence of undoped ZnO and Pr-doped ZnO nanoparticles using ultrasonic bath (Sonica, 2200 EP S3, Italy). In a typical manner, 100 mL of AR17 solution with initial concentration of 10 mg/L containing 100 mg sonocatalysts was sonicated with a frequency of 50-60 Hz and 400 W output power at natural pH. The reactions were performed in dark environment to eliminate the effects of irradiation. At time intervals of 10 min, 4 mL of sample was withdrawn from the reactor and residual concentration of AR17 in the ultrasonic reactor was measured using a UV-Vis spectrophotometer (WPA lightwave S2000, England) at maximum wavelength of 540 nm.

6

3. Results and discussion 3.1. Characterization of ZnO and Pr-doped ZnO Fig. 1 shows SEM images of ZnO and 1% Pr-doped ZnO nanoparticles. As can be seen in Fig. 1 ((a) and (b)), ZnO nanoparticles are quite irregular in size and shape. This nonuniformity can be attributed to the aggregation of synthesized ZnO nanoparticles and the growth of irregular crystalline grains during synthesis. A simple comparison between SEM images of undoped ZnO and Pr-doped ZnO nanoparticles reveals that the incorporation of Pr into ZnO structure restrains the aggregation phenomenon and subsequently decreases the particle size, which improves its sonocatalytic activity. Moreover, Pr doping increases the size and shape uniformity of synthesized nanoparticles. The TEM images in Fig. 2 show the shapes and particle size distribution of 1% Pr-doped ZnO sample. TEM images demonstrate that the particles of 1% Pr-doped ZnO are within the nanoscale (≤ 100 nm) and they are spherical, confirming the results of SEM analysis. The diameter distribution of most of the 1% Pr-doped particles was found to be in the range of 4060 nm (Fig. 3 (e)). Fig. 3 shows the X-ray diffraction patterns of undoped and 1% Pr-doped ZnO nanoparticles prepared via sonochemical method. The main dominant peaks were identified for undoped ZnO at 2θ of 31.92, 34.6, 36.48, 47.68, 56.72, 63, 66.08, 68, 68.28, 71.64 and 75.96 which corresponded to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of hexagonal wurtzite ZnO, respectively (JCPDS Card 36-1451). The main peaks of ZnO was observed after doping of ZnO by Pr, and no diffraction peaks from praseodymium oxides or other impurities were detected, indicating that praseodymium ions have replaced in ZnO lattice successfully. Diffraction values of (100), (002), and (101) planes showed a shift to lower angles in the case of Pr-doped ZnO compared with undoped ZnO, confirming the successful doping of Pr3+ ions into ZnO lattice. 7

This shift can be attributed to expansion of ZnO lattice by doping of Pr which can be explained by the fact that the ionic radius of Pr3+ (1.01 Aº) is larger than that of Zn2+ (0.74 Aº). The average crystallite sizes of the prepared samples were calculated using the sharpest peak using Debye-Sherrer’s equation [19]. Accordingly, the crystallite size of undoped and 1% Pr-doped ZnO nanoparticles were found to be 16.04 nm and 9.027 nm, respectively. The FT-IR spectra of undoped and Pr-doped ZnO nanoparticles are shown in Fig. 4. The band located at 573 cm-1 corresponds to Zn–O vibration [17]. The broad peaks at 3418-3450 cm-1 is attributed to the stretching vibration of O–H in the adsorbed water molecules. X-ray photoelectron spectroscopy (XPS) analysis of undoped ZnO and 3% Pr-doped ZnO nanoparticles was carried out to confirm incorporation of Pr into the ZnO lattice and to identify the oxidation state of Pr in the ZnO lattice. The XPS spectra of pure ZnO and 3% Prdoped ZnO nanoparticles are presented in Fig. 5 ((a) and (b)). The energy scale was calibrated with the C 1s peak of the carbon contamination at 286.22 eV. Narrow scan XPS spectra of undoped ZnO and 3% Pr-doped ZnO (Fig. 5 (b) and (e)) show two peaks of Zn 2p located at around 1023 and 1046 eV, agreeing with the binding energy of Zn 2p3/2 and Zn 2p1/2, respectively. Fig. 5 ((c) and (f)) shows the XPS core level spectra of O 1s of undoped ZnO and 3% Pr-doped ZnO, respectively. The asymmetric peak centred at 532.19 and 532.16 is attributed to the O2- ions in the structure of undoped ZnO and Pr-doped ZnO, respectively. The shoulder observed on the high energy side of the mentioned peak in the both spectra of undoped ZnO and Pr-doped ZnO is due to the specific species such as carbonate, adsorbed O2 and H2O [20]. Comparing the XPS spectrum of undoped ZnO (Fig. 5 (a)) and Pr-doped ZnO (Fig. 5 (d)) displays that a weak peak located at 936.08 eV exist in the XPS spectrum of Prdoped ZnO nanoparticles which is not seen in the XPS spectrum of undoped ZnO nanoparticles. According to the literature, the position of mentioned peak is in good agreement with the value of Pr3+ [21, 22]. Hence, mentioned peak attributed to the Pr3+ ions 8

in the ZnO lattice and confirms that Pr exists in the ZnO lattice. The low intensity of Pr3+ peak may be due to the low level of Pr dopant at the surface of catalyst [20]. The electron density changes around the Zn and O atoms by introducing Pr into ZnO lattice, so the location of Zn 2p and O 1s peaks are slightly different in the XPS spectra of undoped ZnO and Pr-doped ZnO nanoparticles, implying that Pr has been successfully incorporated into ZnO lattice [7]. The corresponding data are given in Table 2. 3.2. Comparison of sonolysis alone with sonocatalysis processes The degradation of AR17 was investigated under ultrasonic irradiation (US) in the presence and in the absence of ZnO nanoparticles (Fig. 6). In the absence of catalyst (sonolysis alone), decolorization efficiency of 24% was obtained after 70 min ultrasonic irradiation. The decolorization efficiency increased to 46% and 100% after 70 min in the presence of undoped ZnO and 5% Pr-doped ZnO nanoparticles, respectively. Regarding to small amount of dye adsorbed on the catalyst particles (approximately 5%), the increment in the decolorization efficiency can not be attributed to the adsorption of dye on the catalyst surface. Although sonocatalytic process has been used frequently as an advanced water/wastewater treatment method in recent years, its mechanism is not fully understood [3, 7, 8]. According to literature, sonocatalytic degradation of organic pollutants in the presence of sonocatalyst can be explained by considering three following mechanisms: hot spot, sonoluminescence (SL) and oxygen atom escape [11]. Under ultrasonic irradiation in a liquid, bubbles are formed, grow and at last implosively collapse. Reaction sites with high temperature (as high as 5000 K) and high pressure (up to approximately 1800 atm), named hot spot, are generated through collapsing bubbles [23]. These hot spots can prompt pyrolysis of H2O molecules to form hydroxyl radicals (OH•) and hydrogen radicals (H•) as exhibited in Eq. (3) [24].

H 2O  Ultrasonic irradiatio n  OH   H 

(3) 9

In addition, some additional consecutive chain reactions can be occurred as shown in Eqs. (4) – (9) [24-26]:

OH   H   H 2O 



(4)

OH  OH  H 2O2 H 2O2  H   H 2O  OH 

(5) (6)

2 OH   H 2O  O

(7)



 2

H  O2  HO HO2  HO2  H 2O2  O2

(8) (9)

According to Eqs. (5) and (6), hydrogen peroxide can be produced by recombination of hydroxyl radicals at the bubble-liquid interface and/or in the solution bulk, and hydroxyl radicals can be subsequently regenerated through the reaction of hydrogen peroxide with hydrogen radical. Studies have indicated that the presence of heterogeneous catalysts in the ultrasonic system can enhance the oxidizing power of ultrasound [12, 25, 27, 28]. In the presence of heterogeneous catalyst, produced hydrogen peroxides can interact with the surface of catalyst (undoped ZnO and Pr-doped ZnO) and generate a number of oxidizing agents improving the degradation of dye [29]. Moreover, the presence of heterogeneous catalysts provides additional nuclei which increase the rate of formation of cavitation bubbles, which subsequently increase the generation of hydroxyl radicals by increasing the pyrolysis rate of H2O molecules [30]. This fact can be confirmed by the results of the present study. As mentioned earlier, higher decolorization efficiency was observed in the presence of sonocatalyst compared with that of ultrasound alone. Sonoluminescence, second mechanism, is the emission of light associated with cavitation. There are two theories that attempt to explain the origin of sonoluminescence including the recombination of free radicals, which are generated within cavitation bubbles during collapse causing the sonoluminecence and chemiluminescence theory, which can be created thermally, as the reason of light emission

10

[31]. It is well known that sonoluminescence generates light with a comparatively wide wavelength range [32]. It should be noted that the emitted light involves intense ultraviolet light (with wavelength smaller than 375 nm), which can excite undoped and Pr-doped ZnO nanoparticles and causes them to act as photocatalyst during sonication. Therefore, a large number of electrons (e-) and holes (h+) are produced on the surface of semiconductor (Eq. (10)). The sonogenerated electrons can react with electron acceptors such as O2 dissolved in water and produce superoxide anion radical (

) (Eq. (11)). Simultaneously, sonogenerated

holes react with surface hydroxyl ions and water to form hydroxyl radicals (OH•) (Eqs. (12) and (13)). Produced radicals can then react with organic dye molecules and enhance the decolorization efficiency. Therefore, it is obvious that the catalytic activity of various semiconductors catalysts used in sonocatalysis can be improved by restraining the recombination of electron-hole pairs as depicted in Eqs. (10) – (13) [11, 12].

Undoped ZnO / Pr doped ZnO  ultrasonicirradiatio n  ZnO / Pr dopedZnO (h  , e  ) e  O2  O2 h   H 2O  OH   H  h   OH   OH 

(10) (11) (12) (13)

Dissolved oxygen can produce hydrogen peroxide as shown via Eq. (14) [10]:

O2  H   ZnO / Pr doped ZnO (e)  H 2O2

(14)

Third mechanism, known as Escape mechanism of oxygen atom, includes the removal of some oxygen atoms from crystalline lattice of undoped and Pr-doped ZnO happening due to the strong shock waves produced from acoustic cavitation. This phenomenon results in producing holes on the surface of catalyst. These holes promote the generation hydroxyl radical causing higher indirect degradation of organic dye. In addition, organic dyes which

11

are adsorbed on the surface of catalyst can be directly decomposed by the generated holes [29]. 3.3. Effect of the amount of Pr dopant on the sonocatalytic activity The sonocatalytic activity of as-prepared Pr-doped ZnO nanoparticles with different amount of Pr (1-5%) was investigated (Fig. 7). Pr-doped ZnO nanoparticles showed higher sonocatalytic activity compared with undoped ZnO. The results showed that the decolorization efficiency of Pr-doped catalysts increased with increasing the amount of Pr content up to 5%. The sonocatalytic activity of undoped ZnO and 5% Pr-doped ZnO nanoparticles were found to be 46% and 100% within reaction time of 70 min, respectively. However, increasing the amount of dopant up to 10% resulted in no considerable increment in sonocatalytic activity(results not shown); thus, from the economical point of view, the 5% Pr-doped ZnO nanoparticles were used as sonocatalyst for conducting the rest of the experiments. Increasing sonocatalytic activity with increasing Pr content can be explained by two following mechanisms. Firstly, enhanced decolorization efficiency by incorporating the Pr into ZnO lattice can be explained regarding to absorbance properties of these particles. According to sonoluminescence mechanism, band gap of the semiconductor plays a major role in the sonocatalytic activity of as-prepared sonocatalyst. Band gap of the sonocatalysts was determined through Eq. (15) [33]:

( Ah ) 2  K (h  Eg )

(15)

where hν 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 (Ahν)2 versus photon energy, the band gap can be estimated (Fig. 8). The values of band gap for undoped ZnO and Pr-doped 12

ZnO nanoparticles are given in Table 3. Decreasing band gap energy with increasing the amount of Pr dopant can be attributed to the charge transfer between the ZnO valence or conduction band and the Pr ion 4f level [34, 35]. In addition, Pr3+ ions incorporated into the ZnO lattice can form a shallow level inside the band gap resulting in the decrease in the band gap energy [36]. When a semiconductor with narrow band gap is irradiated by UV energy, produced in ultrasonic process, the transition from the ground state to the excited state is easier than that of the semiconductor with higher band gap. Hence, in the present study, 5% Pr-doped ZnO with the lowest band gap generated electrons-hole pairs easier than undoped, 1% Pr-doped and 3% Pr-doped ZnO nanoparticles leading the highest sonocatalytic activity. Secondly, Pr3+ acts as an electron scavenger and prevents the recombination of produced electrons and holes during sonocatalytic process, which result in the enhanced decolorization efficiency [17, 37, 38]. 3.4. Effect of sonocatalyst dosage The effect of sonocatalyst dosage on the decolorization efficiency of AR17 was investigated using different concentrations of Pr0.05Zn0.95O sonocatalyst varying from 0.25 to 1.25 g/L (Fig. 9). As can be seen, the decolorization efficiency gradually increased with an increase in the concentration of sonocatalyst up to 1 g/L and after that decreased. Similar results have been reported by others [9, 12]. Increasing sonocatalyst dosage leads to the generation of higher number of hydroxyl radicals responsible for the decolorization of AR17. However, increasing the amount of sonocatalyst from 1 g/L to 1.25 g/L resulted in decreasing the decolorization efficiency from 100% to 98% within reaction time of 70 min, respectively. This can be due to the aggregation of sonocatalyst particles and subsequently decreasing the number of surficial active sites for the generation of hydroxyl radicals at higher sonocatalyst dosages [12]. But, decreasing the decolorization efficiency with increasing the amount of

13

sonocatalyst up to 1.25 g/L can be attributed to the deactivation of activated molecules through collision of activated molecules with ground state Pr-doped ZnO particles reducing the site density for surface holes and electrons [12]. Subsequent experiments were carried out using sonocatalyst dosage of 1 g/L as optimal value. 3.5. Effect of initial dye concentration Fig. 10 shows the effect of initial dye concentration on decolorization efficiency. The decolorization efficiency of AR17 gradually decreased from 100% to 70% with increasing initial dye concentration from 10 mg/L to 15 mg/L, respectively. The number of dye molecules that adsorbed on the surface of catalyst increased by increasing dye concentration. Therefore, high number of hydroxyl radicals is required for the decolorization of higher dye concentration [39]. However, the formation of hydroxyl radicals remains constant for a given catalyst dosage, irradiation time and ultrasonic intensity. Accordingly, the decolorization efficiency gradually decreased as a result of increasing dye concentration. 3.6. Effect of the presence of radical scavengers Sulfate, carbonate and chloride ions were used to investigate the effect of the presence of radical scavengers on the sonocatalytic degradation of AR17 in aqueous solutions (Fig. 11). In this set of experiments, the initial AR17 concentration, sonocatalyst dosage and radical scavenger concentration were constant at 10 mg/L, 1 g/L and 5 mg/L, respectively. It was observed that the decolorization efficiency of AR17 decreased in the presence of all studied radical scavengers, confirming that free radical attack is the dominant controlling mechanism of the sonocatalytic degradation of organic dye in comparison with pyrolysis alone. According to the obtained results, the negative effect of scavengers on the decolorization efficiency is in the following order: Chloride > Carbonate > Sulfate.

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With the addition of chloride, carbonate and sulfate, the decolorization efficiency was decreased from 100% to 65%, 71% and 89%, respectively at the reaction time of 70 min. The possible reactions that can occur in the presence of different radical scavengers are shown as Eqs. (16)- (20) [40, 41]:

Cl   OH   Cl   OH  CO32  OH   CO3  OH 

(16) (17)

2CO3  H 2O  2CO2  HO2  OH 

(18)







OOH  OH  OH  OOH SO42   OH   SO4  OH 

(19) (20)

3.7. Effect of the presence of hydrogen peroxide The effect of hydrogen peroxide on the sonocatalytic decolorization of AR17 was studied and the result is illustrated in Fig. 12. Results show that initial adding of H2O2 enhances the decolorization efficiency of AR17 from 79% to 85% within reaction time of 50 min. Hydrogen peroxide decomposes and produces hydroxyl radicals in the presence of ultrasonic irradiation as represented in Eq. (21) [40].

H 2O2  ultrasonic irradiatio n  2OH 

(21)

Also, the generation of hydroxyl radicals from hydrogen peroxide will be possible through below reaction [39]:

H 2O2  H   H 2O  OH 

(22)

Thereby, hydrogen peroxide can be considered as a useful chemical for the generation of hydroxyl radicals, which can accelerate the degradation of organic pollutants [42]. Since the decolorization efficiency did not increase significantly with the addition of hydrogen

15

peroxide (about 6%), using hydrogen peroxide was not considered in the subsequent experiments considering economical viewpoint. 3.8. Effect of the presence of peroxydisulfate Peroxydisulfate anion (S2O82-) is considered as a strong oxidizing agent (Eº= 2.01 V) and can be activated by means of UV irradiation, heat energy, transition metal ions or ultrasound irradiation to produce sulfate radicals (SO4·-) as a stronger oxidizing agent with a redox potential of 2.6 V [43, 44]. As can be seen in Fig. 12, when sonocatalytic process was combined with 1 mM K2S2O8, the decolorization efficiency increased from 79% to 93% within reaction time of 50 min. This result can be explained by the following mechanisms. Firstly, ultrasonic irradiation can activate the S2O82- ions as illustrated in Eq. (23): S2O82  ultrasonic radiation  2SO4

(23)

In the next step, hydroxyl radicals may be formed through sulfate radicals as can be seen in Eq. (24) [45]: 

SO 4   H 2O  H  SO42  OH 

(24)

Furthermore, produced sulfate radicals can contribute in the degradation of organic dye as shown through Eqs. (25) – (26) [17]:

SO4  RH  SO42  R (int ermediates )  H  SO4  R (int ermediates )  SO42  CO2  NO2  other inorganics

(25) (26)

3.9. Reusability of the sonocatalyst The reusability of 5% Pr-doped ZnO nanoparticles was tested in the presence of sonocatalyst dosage of 1 g/L, initial dye concentration of 10 mg/L and reaction time of 70 min. After four repeated runs, a negligible decrease in the decolorization efficiency was observed (Fig. 13), 16

indicating that this catalyst can be reused more than four times with high sonocatalytic activity. It has been demonstrated that doping of the catalyst with a suitable dopant enhances reusability of the applied catalyst [17]. 3.10. Effect of ultrasonic power Fig. 14 shows the effect of ultrasound power on the decolorization of AR17 by 5% Pr-doped ZnO at initial dye concentration of 10 mg/L and sonocatalyst dosage of 1 g/L. Enhancing the ultrasound power from 200 to 600 W/L increased the decolorization efficiency of AR17 from 35% to 96%, respectively, within reaction time of 50 min. The number of collapsing bubbles increases as ultrasonic power increases, resulting in the improved formation of hydroxyl radicals and consequently enhanced decolorization efficiency of dye [39, 40, 46]. 3.11. Effect of initial pH The relationship between the pH of the solution and the decolorization efficiency of AR17 is shown in Fig. 15. The pH of solution was adjusted before irradiation and it was not controlled during irradiation. Complete Decolorization of AR17 was obtained at pH of 6.25, whereas lower extents of decolorization, 45% and 36%, were observed at pH values of 9.9 and 3.3, respectively. As can be seen in Fig. 15, neutral pH is favored the sonocatalytic decolorization of AR17. Due to amphoteric behavior of most semiconductor oxides, the pH of the reaction has significant effect on the surface properties of semiconductor such as surface charge of catalyst and the size of the aggregations. The effect of pH on the sonocatalytic decolorization is related to acid-base properties of the surface of semiconductor that can be explained by the point of zero charge (pHPZC). The point of zero charge of ZnO is about 9±0.3 [47]. This means that ZnO surface is protonated at pH values lower than pHpzc. Inversely, the surface will be deprotonated at pH values higher than pHpzc. The pH of solution influences the

17

ionization state of catalyst surface according to the following acid-base equilibrium reactions [1, 12]:

Zn  OH2  Zn  OH  H  Zn  OH  OH   OH   ZnO  H 2O

(27) (28)

According to pHpzc, AR17, as an anionic dye with negative charge, can be adsorbed on the surface of catalyst at pH values lower than 9.0 due to attraction force. The low decolorization efficiency at pH= 3 may be associated with the corrosion of catalyst in acidic medium. In alkaline solution (pH>pHpzc), there will be an electrostatic repulsion between negatively charged surface of sonocatalyst and OH- ions. This prevents the formation of hydroxyl radicals and consequent reducing of decolorization efficiency.

4. Conclusion In the present study, the sonochemically prepared PrxZn1-xO (x=0-0.05) nanoparticles was used as a novel sonocatalyst for the sonocatalytic degradation of a textile dye in aqueous solution. XRD patterns indicated that samples were crystalized in pure wurtzite phase. SEM images revealed that incorporation of Pr into ZnO structure restrains the aggregation phenomenon. TEM images proved that particles of 1% Pr-doped ZnO were within the nanoscale (≤100 nm). XPS spectra verified the presence of Pr in the doped samples. Results indicated that the decolorization efficiency of sonocatalysis was higher compared with sonolysis alone, and decolorization efficiency was clearly affected by the content of Pr dopant in ZnO. Addition of hydrogen peroxide and peroxydisulfate accelerated the decolorization efficiency. The radical scavengers reduced the decolorization efficiency, indicating that the dominant controlling mechanism of sonochemical degradation of AR17 is the free radicals. The highest decolorization efficiency was observed at natural pH values. The decolorization efficiency of AR17 increased from 35% to 96% with increasing ultrasonic 18

power from 200 to 600 W/L after 50 min of irradiation, which was attributed to the formation of more hydroxyl radicals in higher ultrasonic power. Also, sonocatalysts showed high reusability, which is of great importance considering application point of view.

Acknowledgements The authors thank the University of Tabriz, Iran for all of the support provided. This work is funded by the Grant 2011-0014246 of the National Research Foundation of South Korea.

19

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Figure captions Fig. 1. SEM images of (a, b) undoped ZnO nanoparticles, (c, d) 1% Pr-doped ZnO nanoparticles and (e) diameter size distribution of 1% Pr-doped ZnO nanoparticles. Fig. 2. TEM images of 1% Pr-doped ZnO nanoparticles. Fig. 3. XRD patterns of undoped and 1% Pr-doped ZnO nanoparticles. Fig. 4. FT-IR spectra of undoped and Pr-doped ZnO nanoparticles. Fig. 5. XPS spectra of (a) undoped and (b) 3% Pr-doped ZnO nanoparticles Fig. 6. Comparison of sonolysis alone with sonocatalysis processes. Initial dye concentration = 10 mg/L and catalyst dosage = 1 g/L. Fig. 7. Effect of amount of Pr dopant on the decolorization of AR17. Initial dye concentration = 10 mg/L and catalyst dosage = 1 g/L. Fig. 8. (Ahv)2-hv curves of the undoped and Pr-doped ZnO nanoparticles. Fig. 9. Effect of the sonocatalyst loading on the sonocatalytic degradation of AR17. Amount of dopant = 5% and initial dye concentration = 10 mg/L. Fig. 10. Effect of initial concentration of AR17 on the decolorization efficiency. Amount of dopant = 5% and catalyst dosage = 1 g/L. Fig. 11. Effect of the presence of sulfate, carbonate and chloride (as radical scavengers) on the sonocatalytic degradation of AR17. Radical scavenger concentration = 5 mg/L, amount of dopant = 5%, catalyst dosage = 1 g/L, and initial dye concentration = 10 mg/L. Fig. 12. Effect of the presence of peroxydisulfate and hydrogen peroxide on the sonocatalytic degradation of AR17. Amount of dopant = 5%, catalyst dosage = 1 g/L, initial dye concentration = 10 mg/L, and peroxydisulfate and hydrogen peroxide concentration = 1 mM. Fig. 13. Reusability of the Pr-doped ZnO nanoparticles within four consecutive experimental runs. Amount of dopant = 5%, catalyst dosage = 1 g/L, initial dye concentration = 10 mg/L, and reaction time = 70 min.

26

Fig. 14. Effect of ultrasonic power on the sonocatalytic degradation of AR17. Initial dye concentration = 10 mg/L, amount of dopant = 5%, and catalyst dosage = 1 g/L. Fig. 15. Effect of initial pH on the sonocatalytic degradation of AR17. Amount of dopant = 5%, catalyst dosage = 1 g/L, and initial dye concentration = 10 mg/L.

27

Tables Table 1. Characteristics of Acid Red 17.

Dye

Chemical structure

Color Index

λmax

Mw

number

(nm)

(g/mol)

16180

510

502.435

Molecular formula

C.I. Acid C20H12N2Na2O7S2 Red 17

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

Pr-doped ZnO

Zn 2p3/2

Zn 2p1/2

O 1s

Zn 2p3/2

Zn 2p1/2

O 1s

1022.94 eV

1046.08 eV

532.19 eV

1022.88 eV

1045.08 eV

532.16 eV

Table 3. Band gap energy of undoped ZnO and Pr-doped ZnO nanostructures. Sample

Calculated band gap energy (eV)

Undoped ZnO

3.2

1% Pr-doped ZnO

3.05

3% Pr-doped ZnO

3.0

5% Pr-doped ZnO

2.95

Figures

(a)

(b)

(c)

(d)

(e) Figure 1

Figure 2

Figure 3

Transmittance

Undoped ZnO 1% Pr-doped ZnO 3% Pr-doped ZnO

4000

3600

3200

2800

2400

2000

1600

Wavenumber (cm-1)

Figure 4

1200

800

400

(a)

(b)

(c)

(d)

Pr3d

(e)

(f)

Figure 5

100

Decolorization efficiency (%)

5% Pr-doped ZnO Undoped ZnO

80 Sono only

60

40

20

0 0

10

20

30

40

Time (min)

Figure 6

50

60

70

Decolorization efficiency (%)

100

5% Pr-doped ZnO 3% Pr-doped ZnO 1% Pr-doped ZnO

80

60

40

20

0 0

10

20

30

40

Time (min)

Figure 7

50

60

70

100 ZnO %1 Pr-doped ZnO %3 Pr-doped ZnO

80

%5 Pr-doped ZnO

(Ahν)2

60

40

20

0 0

0.4 0.8 1.2 1.6

2

2.4 2.8 3.2 3.6

hν

Figure 8

4

4.4 4.8 5.2 5.6

6

6.4 6.8

100

0.5 g/L %5 Pr-doped ZnO

Decolorization efficiency (%)

0.25 g/L %5 Pr-doped ZnO 0.75 g/L %5 Pr-doped ZnO

80

1.0 g/L %5 Pr-dpoped ZnO 1.25 g/L %5 Pr-doped ZnO

60

40

20

0 0

10

20

30

Time (min)

Figure 9

40

50

60

70

100

Decolorization efficiency (%)

10 mg/L 15 mg/L

80

12.5 mg/L

60

40

20

0 0

10

20

30

Time (min)

Figure 10

40

50

60

70

100

Decolorization efficiency (%)

Without scavenger Sodium sulfate

80

Sodium carbonate Chloride

60

40

20

0 0

10

20

30

40

Time (min)

Figure 11

50

60

70

Decolorization efficiency (%)

100

80

60

40

20

0 0

10

20

30

Time (min)

Figure 12

40

50

60

70

Decolorization efficiency (%)

100 80 60 40 20 0 Run 1

Run 2

Figure 13

Run 3

Run 4

100

Decolorization efficiency (%)

200 W/L 400 W/L

80

600 W/L

60

40

20

0 0

10

20

30

Time (min)

Figure 14

40

50

60

70

100

Decolorization efficiency (%)

pH=3.3 pH=9.9

80

pH=6.25

60 40 20 0 0

10

20

30

40

Time (min)

Figure 15

50

60

70

Research highlights 

Synthesis of ZnO and PrxZn1-xO samples by sonochemical method.



Sonocatalytic degradation of a textile dye by PrxZn1-O nanoparticles.



Characterization of the synthesised nanomaterials by XRD, FT-IR, SEM, TEM and XPS.