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Ultrasonics - Sonochemistry

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Sonochemical versus hydrothermal synthesis of bismuth tungstatenanostructures: Photocatalytic, sonocatalytic and sonophotocatalyticactivities

Mahboobeh Zargazia, Mohammad H. Entezaria,b,⁎

a Sonochemical Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iranb Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

A R T I C L E I N F O

Keywords:Sonophotocatalytic activityBismuth tungstateSonicationBinary mixture (RhB/MB)

A B S T R A C T

In the present work, an ultrasound-assisted hydrothermal method was applied as a new approach for thesynthesis of Bi2WO6 nanostructures. In sonication, a cup horn system as an indirect high intensity sonicator wasused. To determine the influence of ultrasonic waves on the morphology, Bi2WO6 was also synthesized using thehydrothermal method. The conventional and sonochemical products were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Fourier transform infrared (FTIR),Raman, photoluminescence (PL), and UV–Vis (UV–Vis) spectroscopies. The XRD patterns confirmed that thesonosynthesized sample has higher crystallinity than the conventional one. The results also showed that ultra-sound decreased the particle size and improved the size distribution. In comparison with the hydrothermalsample, the flower like structures formed under sonication have many hollow sites, resulting in higher harvestingand scattering of visible light. The efficiency of resulting nanoparticles in degradation of a binary mixture (RhB/MB) as pollutant was evaluated by photocatalytic, sonocatalytic, and sonophotocatalytic processes. The sono-synthesized sample removed the pollutants four times faster than the hydrothermal sample in sonophocatalyticprocess. Besides, determining factors including pH, pollutant concentration, temperature, and ultrasound am-plitude were optimized in the sonophotocatalytic process.

1. Introduction

Recently, semiconductor photocatalysts have drawn attention interm of their application to energy production and remediation of theenvironment [1–3]. Particularly, the development of visible-light-driven photocatalysts has been attractive to researchers in terms of easyaccess to solar spectrum [4–6]. A strong correlation exists between thephotocatalytic efficiency of the semiconductor and its surface area,particle size, and morphology. Therefore, access to different morphol-ogies of photocatalysts has great importance for researchers in this area.Herein, scientists have conducted experiments on the preparation ofphotocatalysts with controlled shape. Bismuth tungstate (Bi2WO6) is atypical n-type semiconductor with a direct band gap of 2.8 eV. Theperovskite-like slab of WO6 and (Bi2O2)2+ forms a layered structurewhich has great potential in electrode materials, solar energy conver-sion, and visible-light-driven photocatalysis [4]. Kudo et al. discoveredphotocatalytic activity of Bi2WO6 for O2 evolution from an aqueous

silver nitrate solution [7] and later Zou’s group examined the photo-degradation of CHCl3 and CH3CHO by the Bi2WO6 nanostructure undervisible light irradiation [8]. Various morphologies of these compoundssuch as nanolaminars [6], snow-like, nanocages, hierarchical flower-like structures [9,10] were synthesized by different approaches (e.g.hydrothermal [9–11], refluxing [12], sonochemical method [6], andcitrate complex method [13]). In recent years, sonochemistry as apowerful technique has been developed in the synthesis of a widevariety of organic and inorganic nanostructures [14,15]. The chemicaleffects of ultrasound are derived from the cavitation process whichincluded the formation, growth, and collapse of a bubble. As a result ofthe cavitation process, the localized hot spots can appear with harshconditions such as high pressure, high temperature and very highcooling rates [16]. Sonochemistry has some advantages such as con-trollable conditions, rapid rate, and the ability to produce of nanoma-terial with uniform morphologies and high purity. According to scien-tific reports, flower-like structure of Bi2WO6 displays the highest

https://doi.org/10.1016/j.ultsonch.2018.10.010Received 19 May 2018; Received in revised form 25 September 2018; Accepted 9 October 2018

⁎ Corresponding author at: Sonochemical Research Center, Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, FerdowsiUniversity of Mashhad, Mashhad, Iran.

E-mail address: entezari@um.ac.ir (M.H. Entezari).

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Available online 10 October 20181350-4177/ © 2018 Elsevier B.V. All rights reserved.

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photocatalytic activity for removal of Rhodamine B compared to othermorphologies [17]. Zhou et al. [18] have reported the synthesis ofnanoplates of Bi2MO6 (M=W, Mo) photocatalysts by direct high in-tensity ultrasound in long sonication time and without control of tem-perature. Nanoplates have been used for degradation of Rhodamine B.Liang et al. [19] have also investigated the hierarchical structures ofBi2WO6 synthesized by low indirect ultrasound (bath) from a mixture(water/ethanol) and in the presence of polyvinylpyrrolidone (PVP). Theproduct has been applied in pulsed sonophotocatalytic degradation ofRhB.

In this work, for the first time, an indirect high intensity ultrasound(cup horn) was used for the synthesis of Bi2WO6 without additive forshort time sonication and at room temperature. As a new approach,indirect sonication was applied in advance of hydrothermal process forthe synthesis of bismuth tungstate (BWO) nanoparticles. To find therole of ultrasound irradiation in the morphology, BWO has also beensynthesized using hydrothermal method. The photocatalytic (PC) ac-tivity of as-synthesized BWO nanostructures was studied by degrada-tion of a binary mixture (MB/RhB) as the pollutant model. Althoughultrasound irradiation for wastewater treatment has been used, ap-plying this method alone for degradation of persistent organic pollu-tants is difficult and time-consuming. Hence, to overcome this problem,ultrasound has been coupled with photocatalysis (called sonophotoca-talysis (SPC)) [20–22]. This combination increases the mineralizationoutput and reduces the costs. Therefore, in this research, sonocatalysisand photocatalysis have been merged to reach a greater efficiency ofdegradation in a shorter time. In addition, sonocatalytic (SC) activityhas been also studied to find the role of ultrasound in the combinedmethod. In this paper, the significant effects of ultrasound in access ofthe unique morphology of BWO and enhanced SPC degradation effi-ciency has been discussed in detail.

2. Experimental section

2.1. Synthesis of Bi2WO6

All chemicals were of analytical grade and used without furtherpurification. At room temperature, solution A containing 2mmol of Bi(NO3)3·5H2O in 20mL distilled water was stirred for 15min. Then,nitric acid was added to the suspension to reach a clear solution. Insolution B, 1mmol of Na2WO4·2H2O was dissolved in 20mL of distilledwater. A beaker as a container of solution A was immersed in the cuphorn of indirect high intensity ultrasound (ultrasonic processor GEX, 20KHz, 750 Watts) and solution B was slowly added to the solution Aduring sonication. Indirect ultrasonic irradiation was applied 15minwith 40% acoustic amplitude at room temperature. The temperaturewas controlled by a circulator during the experiment. In the next step,the white suspension was transferred into a Teflon autoclave and he-ated at 150 for 20 h. A BWO sample was also prepared by the hydro-thermal method under stirring for comparison studies. The conditionsfor both synthetic methods were summarized in Table 1. The preparedsamples of BWO with ultrasound and hydrothermal methods werecalled Sono-BWO and Hydro-BWO, respectively. Fig. 1a shows

schematically the preparation steps of BWO nanostructures in bothmethods and Fig. 1b clarifies the experimental procedure in thesynthesis of Sono-BWO sample by the cup horn system.

2.2. Characterization of BWO nanostructures

The phase and crystallinity of the as-synthesized products werecharacterized by X-ray diffraction (XRD, Explorer GNR Italia) em-ploying Cu Kα radiation, λ=1.5406 Å within 2θ range of 20°–80° withscan rate of 10°/min. Fourier transform infrared (FTIR) spectra wereobtained by an AVATAR 370 FT IR (Thermonicolit, USA). Ramanspectra were recorded by an AVANTES (Sensline, Poland). The mor-phology of the as-prepared powders was characterized by a scanningelectron microscope (SEM, LEO, Germany) and EDX (EDX, Inca-350,Oxford instruments) was applied for elemental analysis which attachedto SEM. Photoluminescence (PL) and UV–Vis absorption spectra ofsamples were recorded at room temperature by a fluorescence spec-trometer (Shimadzu, RF-5410PC) and UV spectrophotometer (UNICO2008), respectively.

2.3. Photocatalytic, sonocatalytic and sonophotocatalytic activities

The photocatalytic performances of as-prepared powders were stu-died through the decomposition of pollutant model (mixture of5 ppmMB and 5 ppm RhB) with a V/V ratio of 1:1 in solution in thepresence and absence of ultrasound under natural visible light irra-diation. In each experiment, 50mg photocatalyst was added into 50mLof aqueous solution of pollutant model with a concentration of 10 ppm.Before the light irradiation, the suspensions were firstly magneticallystirred in the dark for 15min to reach the adsorption-desorption equi-librium. Approximately 58% and 20% of the binary mixture were ad-sorbed on the Sono-BWO and Hydro-BWO surfaces, respectively.Furthermore, sonication of nanostructures in pure water before theadsorption-desorption process led to enhancement of the adsorption incomparison with untreated samples. This behavior could be related tothe activation of BWO nanostructures’ surfaces by fragmentation ofthem and reach to a proper dispersion by sonication method. Theamount of adsorption of the binary mixture by the samples pretreatedin pure water under ultrasound reached to 68% and 40% for Sono-BWOand Hydro-BWO samples, respectively. Then the suspension withoutpretreatment was exposed to natural sunlight under stirring. For so-nocatalytic studies, the suspensions were sonicated by ultrasonic horn(BRANSON, digital Sonifier 450, and 20 kHz) in the dark. Then forsonophotocatalytic activity, the suspensions were exposed to light froma Xe lamp (400W, HID Ballast, XT-400-E40) with cut off filter UV 400(3mm, Scott) under continuous sonication (see Table 1). Ultrasoundwas generated heat in sonophotocatalytic and sonocatalytic processes.Temperature of reaction was controlled by circulating system (Grant,instruments Cambridge) at 30 °C. At regular times, 3 mL of solution wassampled, filtered, and measured to determine the degradation of pol-lutants with UV-Visible (UV-Vis spectrophotometer Unico 2008). De-gradation of the binary mixture was followed at wavelength 555 nm forRhB and 660 nm for MB which did not change after mixing. Molar

Table 1Synthesis conditions of samples and sonication characteristic for SPC process.

Experiment Status Ultrasonic parameters Temp (◦c)

Synthesis Application Frequency (kHz) Intensity* (Wcm−2) Size** (cm)

Hydro-BWO – Stirring – – – 25Sono-BWO – Cup horn 20 18(40%) 5 25–34– SPC*** Horn 20 35(50%) 1.1 30–55

* Acoustic power was measured by calorimetric method and then ultrasonic intensity was calculated [57].** Size of transducer.*** Sonophotocatalytic process.

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extinction coefficients at these wavelengths were found to be 53333.3and 103,300 Lmol−1cm−1 for MB and RhB, respectively. The efficiencyof process was evaluated by plotting changes of Ct/C0 of pollutantsversus irradiation time. C0 and Ct were the total concentration of RhBand MB at time (t= 0) and time t, respectively. To elucidate the de-gradation mechanism of SPC process, different scavengers were addedduring sonophotocatalytic process. HCOOH (40mM) and isopropanol(40mM) were added separately as hole and OH. scavengers in solutioncontaining selected pollutants. The experiment was also carried outunder N2 atmosphere to show the effect of dissolved oxygen in theprocess. Furthermore, the amount of hydrogen peroxide producedduring sonication process was determined spectrophotometrically. Thepertitanic acid, TiO2·H2O2, as a yellow compound was formed throughthe reaction of hydrogen peroxide with an excess of aqueous solution ofTi4+ [23,24]. A standard plot with different concentrations of H2O2 wasobtained. The appearance of yellow color after the addition of titaniumsulfate has a maximum absorbance at 407 nm.

3. Results

3.1. Characterization of BWO nanostructures

Fig. 2a shows the XRD patterns of the nanostructures of BWO syn-thesized by two different methods. All of the diffraction peaks of theSono-BWO sample can be readily indexed as pure orthorhombicBi2WO6 (ICCD data base. No. 00-026-1044). The absence of otherphases in the XRD pattern indicates that ultrasound could be success-fully employed to synthesize the pure orthorhombic phase of BWO. TheSono-BWO sample has higher crystallinity due to higher peak in-tegrated intensity compared to the Hydro-BWO sample. The meancrystallite size was calculated by Scherer’s equation and it was 26.5 and48.9 nm for Sono-BWO and Hydro-BWO samples, respectively. It is wellknown that the high crystallinity of the orthorhombic phase of BWO hashigher photocatalytic activity than other phases such as tetragonalBWO and Bi2O3 as an impurity phase in the BWO structure [10]. FT-IRspectra in Fig. 2b shows that both Sono-BWO and Hydro-BWO sampleshave main absorption peaks between 500 and 1200 cm−1, which can be

Fig. 1. Schematical presentation for the synthesis of BWO nanostructures (a), cup horn set up used for the sonosynthesis of BWO nanostructures (b).

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attributed to stretching vibration of Bi-O and W-O, and bending vi-bration of W-O-W [25,26]. The bond stretching of Bi-O, W-O and the W-O-W bridge stretching were located at 844 cm−1, 821 cm−1 and1200 cm−1, respectively. The similar peak position determines no dif-ference between Sono-BWO and Hydro-BWO in terms of chemicalbinding energy. The layered structure of BWO has six modes Ramanactive (2A1g, B1g and 3Eg) and 9 active modes at IR (4 A2u, 5Eu). Thesemodes can be divided to (i) stretching vibrations (symmetric andasymmetric) and bending vibrations of the WO6 octahedral, (ii) thevibrations (bending and stretching) of the (Bi2O2)2+ layers, (iii)translation and vibrations modes achieved from motions of the Bi3+

and WO6−6 ions [27,28]. Fig. 3a shows the Raman spectra of the

powders. The stretching of W-O bands was located at range of600–1000 cm−1. A more detailed study, the vibrations at 790 and810.41 cm−1 of BWO were attributed to the antisymmetric and sym-metric modes of terminal O-W-O groups. The peak at 714.61 cm−1 wasexplained as an antisymmetric bridging mode that could be related tothe tungstate chains. The peak at 310 cm−1 could be related to thetranslational mode resulting from simultaneous motion of Bi3+ andWO6

6−. The mode of terminal WO2 groups was appeared at 300 cm−1.Since PL emission obtained from the recombination of excited

charge carrier, PL spectra is suitable for study in terms of charge pair

transfer. It is also useful for determining the destiny of charge carrier insemiconductor materials. The high PL intensity indicated a high re-combination rate of the charge pairs under visible light, and vice versa.In this work, the PL spectra were obtained at room temperature under350 nm as excitation wavelength for samples. The UV irradiation at350 nm produces hot electrons that reach the conduction band wherethe high electron mobility leads to transferring to defect sites or else-where. The trapped charge carriers can recombine at such defect sitesthat lie within the band gap, emitting photons with a spectrum of en-ergies. The PL spectra in Fig. 3b shows that the shapes of the PL spectraare similar for both samples but, Sono-BWO sample has lower emissionintensity compared to Hydro-BWO sample. The low emission intensitycould be shown low recombination rate of the electron-hole for theSono-BWO sample.

The morphology of samples shown at various scales in Fig. 4. As canbe seen from a low-magnification SEM image [see Fig. 4(a and b)], theSono-BWO sample in Fig. 4b consists of a large quantity of BWO mi-crospheres with smaller size and higher homogeneity compared toHydro-BWO sample in Fig. 4a. At higher resolution, SEM images [seeFig. 4(c and d)] show that Sono-BWO samples consisted of flower-likemicrosphere with uniform size distribution but Hydro-BWO sampleincluded the porous microsphere with non-uniform distribution size.

Fig. 2. XRD (a), FT-IR (b) analyses for prepared samples.

Fig. 3. Raman spectra (a), photoluminescence emission (PL) spectra (b) obtained for both samples.

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The flower like structure (Sono-BWO) involved of plates that assembledtogether with an average thickness 30–40 nm and an average length400–700 nm. Porous microspheres (Hydro-BWO) and flower like mi-crosphere (Sono-BWO) were illustrated in Fig. 4e and f with highestresolution, respectively. It is clear that the Sono-BWO sample is con-structed of nanoplates and enclosed hollow sites. In the case of Hydro-BWO sample, homogeneity was lower and the average size of the mi-crospheres is higher than in the Sono-BWO sample. Morphologies andsize distributions of both samples were shown in Fig. 1S. In addition,Fig. 2S demonstrates a proposed mechanism with a short explanationfor both samples. The EDS spectrum in Fig. 5 confirmed that bothsamples consisted of Bi, W and O elements.

3.2. Optical properties and catalytic activities

3.2.1. Optical properties of BWO samplesIt is well-known that the photocatalytic activity of semiconductors is

correlated with the band gap energy, which is one key factor in thephotocatalytic processes. The typical UV–Vis spectrum of BWO nanos-tructures is shown in Fig. 6. It can be seen that the absorption range for

both samples extends from the UV to visible region. The absorption inthe visible light region and the edge of adsorption was higher for Sono-BWO sample than for the Hydro-BWO sample. For BWO semi-conductors, the optical band gap calculated from the Tauc relationfollows the equation: αhυ=A(hυ-Eg)n/2, where α, h, υ, A and Eg areabsorption coefficient, Planck’s constant, frequency, proportional con-stant and band gap, respectively. The steep shape of the spectra in-dicates that the visible light absorption was due to the band gap tran-sition. According to the Tauc relation, the value of n for BWO was 1.The band gap of samples was about 2.5 eV and 3.04 eV for Sono-BWOand Hydro-BWO, respectively (see inset Fig. 6). This result demon-strated that BWO synthesized with ultrasound has a suitable band gapfor decomposing pollutants under visible light irradiation. The elec-tronic structure of Bi2WO6 has been reported based on the DFT calcu-lations [29]. It is revealed that the visible-light response for Bi2WO6

was due to the electron transition from the valence band formed by thehybrid orbitals of Bi 6 s and O 2p to the conduction band of W 5d. Thisspecial electronic structure makes the VB largely dispersed, which fa-vors the mobility of photogenerated holes and thus it is beneficial tophotocatalytic oxidation of organic pollutants [30].

Fig. 4. SEM images of Hydro-BWO samples (a, c, e) and Sono-BWO samples (b, d, f) at various magnifications.

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3.2.2. Photocatalytic, sonocatalytic and sonophotocatalytic activitiesThe mixture of MB and RhB, widely used as the dye in various in-

dustries, was selected as the pollutant model to investigate the photo-catalytic (PC), sonocatalytic (SC) and sonophotocatalytic (SPC) activityof BWO samples. First of all, as shown in Fig. 7a, the mixture of dyes(without nanoparticles) exposed under sonolysis, photolysis, and thenunder their combination (sonophotolysis), separately. Results showedthat the selected pollutants under visible light alone approximately hadno degradation, but the pollutants slightly degraded under sonicationand about 20% was degraded under sonophotolysis process in 2 h. Thisis due to the synergetic effect of sonolysis and photolysis in the com-bined method. In sonolysis process, the rate of degradation was slightlymore than photolysis which is due to the production of hydroxyl radi-cals from water splitting under sonication [31,32]. It is assumed that alocal high concentration of hydroxyl radical exists at interface region ofthe collapsing bubbles and some of them can be escaped to the bulk ofsolution. Since RhB and MB are non-volatile compounds, they can bedegraded at exterior region of the cavitation bubbles [33–35].

For evaluation of SC, PC and SPC degradation, Sono-BWO andHydro-BWO samples were added into the pollutant solution. Fig. 7b

shows the pollutant degradation by Hydro-BWO sample under SC, PCand SPC conditions. It can be seen inevitable that the presence of par-ticles has a major role on the degradation of pollutants (comparison ofFig. 7a. with Fig. 7b. or Fig. 7c). Based on Fig. 7b, the degradationefficiencies were 35.4%, 61.3% and 98.4% for PC, SC and SPC, re-spectively. Degradation efficiency of SC was higher than PC process.Several studies have shown SC degradation of pollutants by variouscatalysts and degradation efficiency has been related to hydroxyl ra-dical concentration [36]. Usually, sonochemical degradation of organicpollutants involves the sonolysis of water as a solvent inside collapsingcavitation bubbles under harsh conditions [37]. In PC process, thephotogenerated hole-electron pairs used for degradation of pollutant onthe surface of BWO particles. Finally, in SPC process, SC and PC havesynergistic effects on the degradation.

The Sono-BWO sample showed a higher efficiency in degradation ofpollutants in comparison with Hydro-BWO sample, which is due to itsstructure and high capability for light harvesting in enclosed hollowsites on its surface. Enclosed hollow sites acted as sites for trappedvisible light and high scattering [38,39]. The degradation efficiency ofSono-BWO sample in 30min was 42%, 66% and 99.5% for the PC, SCand SPC processes, respectively. Discrepancy in SC activities for twosamples was related to morphology and size of BWO nanostructures.Sono-BWO sample was selectively adsorbed MB from the binary mix-ture, but Hydro-BWO sample did not behave similarly. Sono-BWOsample produced higher active radical species compared to Hydro-BWOsample which is due to different sizes and structure of samples. In so-nolysis as a homogeneous system, water splitting only occurred bysymmetric cavitation. According to the hot spot theory, the cavitationproduces high temperatures and high pressures [40]. Under these cri-tical conditions, cleavage of O2 and H2O molecules occurs and formsOH% and H% species [41]. But, in heterogeneous system (in the presenceof photocatalyst), the cavitation is asymmetric and shock waves, microjets and acoustic streaming were also contributed in production ofharsh conditions. In heterogeneous system, BWO photocatalysts help toprovide additional nuclei for cavitation and hence increase organicdegradation rate [42]. Size of samples has a key role in production ofbubble in SC and SPC processes [41,43]. Therefore, the lower size ofSono-BWO sample is more effective in degradation of pollutant thanBWO-Hydro sample with larger size. Indeed, samples with smaller sizeproduce higher number of nuclei under sonication than larger size. Onthe other hand, bubbles can go through the cavitation process and

Fig. 5. EDS spectra obtained for Sono-BWO (a), Hydro-BWO (b) samples.

Fig. 6. UV–vis spectra of BWO samples, the inset shows the calculation of theband gap of BWO nanostructures.

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enhancing the mass transfer of organic materials such as dye moleculebetween the liquid phase and the catalyst surface. To prove the effect ofultrasound in production of active radicals, H2O2 formation in homo-geneous systems was investigated by spectrophotometrically method.The formation of H2O2 with ultrasound irradiation is an indication ofoccurrence of transient cavitation. Fig. 3S demonstrates the formationof hydrogen peroxide during experimental conditions (US/Dark) and(US/Light). H2O2 formation under US/Light was lower than US/Darkwhich is due to decomposition of hydrogen peroxide under visible light[44]. H2O2 measurement in heterogeneous systems was difficult due tofast adsorption of H2O2 on the photocatalyst surfaces.

Fig. 8 clearly shows different behaviors of Sono-BWO and Hydro-BWO samples in SPC process. Based on Fig. 8a, the Sono-BWO sampleadsorbed dyes on its surface in the dark (58%). As it is shown, MB iscompletely adsorbed on the surface and RhB adsorbed slightly. It meansthat Sono-BWO sample can be more selective for MB adsorption thanRhB in a binary mixture. But, Hydro-BWO sample was adsorbed both ofdyes on its surface simultaneously (Fig. 8b). Mineralization of pollu-tants was demonstrated by decrease in COD measured using the stan-dard dichromate titration method [45] (see Fig. 4S). COD of SPC pro-cess in case of Sono-BWO sample was completely diminished within120min.

To investigate pollutants degradation on the surface catalyst and inbulk solution, dyes desorption from catalyst surface were done in thedark and light. Concentration changes of both dyes on the surface andin bulk solution shown in Fig. 9. In dark (see Fig. 9a and b), according

to the desorption test, approximately 58% and 20% of dyes were ad-sorbed on the surface of Sono-BWO and Hydro-BWO, respectively andthe rest of them was remained in the solution. The results confirm thatthe Sono-BWO sample has a high capacity for adsorption of dyes that isfavor for the photocatalytic degradation. Furthermore, the SPC de-gradation on the surface and in solution were demonstrated in Fig. 9a′and b′. The remained dyes in solution were degraded under SPC processin shorter time for Sono-BWO sample compared to Hydro-BWO sample.All processes of PC, SC and SPC followed a pseudo-first order kineticsmodel (see Fig. 10). A synergistic effect was observed, since the rateconstants of SPC processes were greater than the sum of the rate con-stants of SC and PC processes (see Table 2).

=

+

Synergy index kk k( )

SPC

PC SC (1)

A quantitative way to estimate the synergistic effect during SPCprocess was done by calculating the synergy index. If the synergy indexvalue was larger than 1.0, a synergistic effect can be observed. In bothsamples, synergy index was higher than 1.0. But, the synergy effect washigher for hydro-BWO sample. The lower value of synergy index forSono-BWO was related to high values of SC and PC for this sample andthey are in the denominator of the fraction.

3.2.3. Optimization of the SPC degradationIn order to find the optimized conditions for SPC process, different

parameters were evaluated on the SPC efficiency such as pH medium,

Fig. 7. Photolysis, sonolysis and sonophotolysis of pollutants (without BWO-samples) (a), degradation of pollutants under photocatalytic (PC), sonocatalytic (SC) andsonophotocatalytic (SPC) processes by Hydro-BWO (b) and Sono-BWO (c) samples.

Fig. 8. UV–Vis spectra during degradation of pollutants under SPC process for Sono-BWO (a) and Hydro-BWO (b) samples.

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temperature effect, amplitude of sonication and initial concentration ofpollutants. The obtained results were presented by the half-life time (t1/2) that is equal to the degradation of 50% of pollutants. All of resultswere shown in Table 3.

3.2.3.1. Effect of pH. The pH of medium has a noticeable effect on theSPC degradation of pollutants. The surface charge of BWO particles and

adsorption of dyes on the surface varied by changing the pH. Theelectrical double layers of the semiconductor-solution interface can bealso modified by pH changes and affects the separation of hole-electronpairs on the photocatalyst surface. Furthermore, pH has multiple roleon the OH radical formation, agglomeration of nanoparticles andionization state of the catalyst surface [46]. It was reported that thepHzpc of BWO is 4.56 [47]. At pH lower or higher than 4.56, the BWO

Fig. 9. Adsorption in dark and sonophotocatalytic (light & US) degradation of binary mixture: Sono-BWO sample (a and a′), Hydro-BWO sample (b and b′).

Fig. 10. Kinetics of degradation: Hydro-BWO (a) and Sono-BWO (b).

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surface has positive or negative charge, respectively. On the other hand,previous researchers have reported that BWO is unstable in acidicsolution (pH < 4) and it could be converted to H2WO4 and Bi2O3 [48].Therefore, BWO nanostructures could not be used in high acidicsolution. Furthermore, the dye molecules were affected by pHchanges. At natural pH, the carboxylic acid group on RhB moleculescan be ionized to produce the zwitterion of RhB (RhB±). This situationresulted in attractive interactions between carboxyl (COO−) andxanthene (−N+) which led to reduce the adsorption of RhB. At highbasic media pH > 8, the hydroxyl groups were adsorbed on the activesites and reduced the adsorption of RhB on the catalyst surface due tolack of binding sites. To end, at natural pH, available active sites wasmore for adsorption of RhB compared to basic conditions. While, MB isin cationic form (MB+) at natural pH. It can be attracted by negativecharged BWO and adsorbed on the catalyst surface and exhibited highdegradation efficiency. In basic solution, the adsorption of MB was

reduced which is due to the presence of hydroxyl groups on the surface[49,50]. Therefore, SPC efficiency was low in basic medium. The bestsonophotocatalytic efficiencies were obtained at near neutral pH ofsolution (6.8) for both BWO samples.

3.2.3.2. Effect of initial dye concentration. Table 3 presents the influenceof initial dyes concentration in the range of 5–25mg/L (CRhB/CMB 1:1and V/V 1:1) on its SPC degradation. The half- life times for pollutantsdegradation increased by increasing the pollutant concentrations. Atfixed amount of photocatalyst, the number of active sites for adsorptionis constant. By increasing the pollutant concentration, the chance ofadsorption of the dye molecule decreased. At higher concentrations,fewer catalyst sites are available and the probability of finding sites foradsorption is low. This is the reason for the lower efficiency ofdegradation at higher pollutant concentrations. Furthermore, for thehighest efficiency in SPC process, there is an optimum relative pollutantconcentration in the binary mixture.

3.2.3.3. Effect of acoustic amplitude. The source of energy for SPCdegradation was achieved by cavitation phenomena (SC) and lightenergy (PC). Therefore, the degradation efficiency of SPC can beaffected by the intensity and power of energy sources. In the presentwork, the amplitude of sonication was changed in the range of 20–60%(adjusted by electrical power) for SPC degradation. Based on Table 3,the results confirm that by increasing of amplitude, the efficiency ofdegradation was increased. This is due to the collapse of cavitation

Table 2Rate constants of pollutant degradation at 10 ppm starting concentration andsynergetic index.

Sample Rate constant (min−1)

PC SC SPC Synergy

Sono-BWO 0.013 0.025 0.106 2.78Hydro-BWO 0.004 0.008 0.048 4.00

Table 3Optimized parameters obtained for SPC process.

Experimental parameters t1/2 (min)

Parameters Ranges Sono-BWO Hydro-BWO

pH 4 60 806.8 15 538 54 75

Initial concentration (mgL−1) 5 10 4010 15 6015 65 8520 100 130

Amplitude sonication (%) 20 40 9030 33 8040 25 7550 15 60

Temperature (°C) 30 15 6040 40 8555 80 110

Fig. 11. SPC activity in the presence of various scavengers.

Fig. 12. Proposed mechanism for SPC activity of Sono-BWO sample.

Fig. 13. Stability of Sono-BWO sample at successive runs of SPC process.

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which is harsher in higher amplitude of sonication. In addition of directeffect of sonication on degradation, it can also cause an increment of PCactivities by cleaning the photocatalyst surface. Therefore, thecombination of SC and PC in SPC facilitates the degradation ofselected pollutants.

3.2.3.4. Effect of temperature. In order to investigate the effect oftemperature on the efficiency of pollutants degradation, SPC activityperformed at various temperatures in the range of 30–55 °C. The resultsshown in Table 3 demonstrated a decrease of SPC efficiency withincreasing of solution temperature. Other studies have been reportedsimilar results for SPC degradation [51,52]. This behavior can bejustified by an enhancement of vapor pressure of water by increasingtemperature and leads to lower effect of cavitation process andconsequently low production of hydroxyl radicals. Some reports havebeen shown that sonolysis alone cannot provide activation energy(based on Arrhenius plot) for SPC reaction [53].

3.2.4. Proposed mechanism for SPC processThe synergistic effect of photocatalysis and sonolysis on SPC effi-

ciency could be interpreted by (i) high production of effective radicalssuch as OH% and HO2

% in the reaction mixture (see reactions (2) and(3)), (ii) ultrasound continuously cleans the photocatalyst surface andenhances the activity of photocatalyst, (iii) ultrasound, inhibits theagglomeration of catalyst nanostructures and increases the surface area[54].

H2O→OH%+H% (2)

H%+O2→HO2% (3)

The SPC mechanism can be proposed by conducting series of ex-periments with different trapping agents of hole and radicals. The ef-fective species in the degradation process could be detected in thepresence of scavengers such as iso propanol (OH% scavenger), andHCOOH (hole scavenger), respectively. In this study, HCOOH con-centration, ultrasound frequency and gas atmosphere were criticalparameters in production of active radicals. Navarro et al [55] studiedthe effect of various frequencies under Ar atmosphere on the formicacid with high concentration. They confirmed the degradation of acidformic to CO2 and other products. Herein, low concentration and lowfrequency was selected for mechanism studies. In order to find the in-fluence of O2 on degradation efficiency, N2 gas was purged in solution.Furthermore, Fig. 5S summarizes the effect of different gases on thedegradation in SPC process. It is well known that the presence of O2

enhances the formation of superoxide radicals. Fig. 11 shows that theSPC activity of Sono-BWO sample is greatly prevented by the additionof isopropanol, also the addition of HCOOH causes change in the de-gradation of pollutant model. The results suggest that the photo-generated holes and OH% have effective roles on SPC activity of Sono-BWO sample (schematically depicted in Fig. 12). It is well known thatH2O2 species can be formed and measured by water sonolysis under20 kHz [31,56]. The formed H2O2 can degrade the pollutant directly orthrough the formation of hydroxyl radical. The results confirmed thatOH radicals have more contribution to the SPC process than other ac-tive species.

3.2.5. Reusability and stability of BWO nanostructureThe reusability of the Sono-BWO sample was estimated by succes-

sive runs. After each run, the Sono-BWO sample was washed and thendried at 80 C for 1 h. Fig. 13 shows the obtained results of the successiveruns. As it is shown, after 5 runs, the nanostructure still has rather highactivity and indicates good stability and potential scalability.

4. Conclusion

The flower like microspheres of BWO was synthesized by indirect

high-intensity ultrasound-assisted hydrothermal method. Ultrasoundhas an effective influence on the size and morphology of nanos-tructures. The morphology was changed from smooth microsphere byhydrothermal method to flower like microsphere with enclosed hollowsites on their surface by ultrasound. The photocatalytic, sonocatalytic,and sonophotocatalytic activities were evaluated by degradation ofbinary mixture (MB/RhB) in an aqueous solution. Sonophotocatalyticactivity was higher than other two methods due to synergistic effectsbetween photocatalysis and sonocatalysis processes. The sono-BWOsample has been shown higher degradation efficiency for pollutantswhich was related to its unique morphology of BWO nanostructure.Enclosed hollow structure facilitated trapping of visible light andmultiple light scattering. Furthermore, the sono-BWO sample selec-tively adsorbed MB molecules from binary mixture compared to othersample. On the other hands, the results showed that ultrasound in so-nophotocatalytic degradation could be affected the degradation via theformation of more effective radicals, increasing of mass transfer,cleaning surfaces of microspheres, and increasing of the surface area ofBWO samples. The Sono-BWO sample has exhibited high stability insuccessive runs of sonophotocatalyst process.

Acknowledgements

The support of Ferdowsi University of Mashhad (Research andTechnology) for this work (3/39052) is appreciated.

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