Facile sonochemical synthesis of Cu doped CeO2 nanostructures as a novel dual-functional photocatalytic adsorbent

Ultrasonics - Sonochemistry 58 (2019) 104695

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Facile sonochemical synthesis of Cu doped CeO2 nanostructures as a novel dual-functional photocatalytic adsorbent

T

Mehdi Mousavi-Kamazani , Farshid Azizi ⁎

New Technology Faculty, Semnan University, Semnan, Iran

ARTICLE INFO

ABSTRACT

Keywords: Cu doped CeO2 Adsorbent-photocatalyst Sonochemical method Sheet-like nanostructures MO

In this paper, doped CeO2 nanostructures with various percentages of copper were synthesized via a simple sonochemical method. Sonication was conducted using a high-intensity ultrasonic probe operating at 20 kHz with a maximum power output of 80 Wcm−3. The effects of different parameters such as ultrasonic time and power, solvent, and OH– source on the morphology of final products were well investigated. XRD, EDS, XPS, SEM, TEM, and DRS analyzes were utilized for precise identification of as-synthesized samples. Then, the adsorption capability in the dark and photocatalytic activity of nanostructures under visible light were evaluated for methyl orange degradation. The results showed that doped samples have a very favorable adsorption in the dark and 5 wt% Cu/CeO2 with flower-like morphology can adsorb more than 99% of the color at 45 min. Also, photocatalytic activity under visible light showed a degradation of more than 81% and 99% for samples 2 wt% Cu/CeO2 and 5 wt% Cu/CeO2, respectively, after 45 min.

1. Introduction Today, the elimination of pollution from water is a matter of serious concern and has led to widespread research [1–4]. Removal of contamination is done by processes such as adsorption, selective membranes, and oxidation. Each of the above has its own advantages and disadvantages. For example, the main problem with the adsorption and coagulation process is that contamination does not become biocompatible compounds [5–8]. Among the processes based on oxidation, photocatalytic processes are very common and have high efficiency, but there are also challenges such as time-consuming and photocatalyst separation. It seems that trying to produce an adsorbent that can cleverly remove its surface is a very interesting and effective issue. We focus on the synthesis of such a substance in this paper. Although the reports on the integrated photocatalytic adsorbent materials production such as TiO2/zeolite, TiO2/CNT, ZnO/clay, and ZnO/graphene are high [9–13], dual-use (adsorbent-photocatalyst) materials are rarely seen [14]. Before us, Jing et al. [14] introduced Cu2O nanocrystals as adsorbent-photocatalyst. Their synthetic nanocrystals were able to degrade about 90% of methyl orange over a period of 2 h under visible light.

⁎

CeO2 is a well-known semiconductor and has been highly regarded for its widespread use in catalysts [15–18], photocatalyst [19–22], oxygen sensors [23], solid oxide fuel cells [24], solid electrolytes [25], electrochemical oxygen pumps [26], UV-shielding materials [27], hydrogen storage materials [28], NOx reduction [29], fluorescent materials [30], and amperometric oxygen ion monitors because of its high oxygen ion conductivity, which originate from variable valence states (+3 and +4) of cerium element [31–33]. One of the problems with cerium oxide is indirect and widespread bandgap (~3.2 eV), which limits its photocatalytic activity to just ultraviolet light irradiation [19]. Therefore, many efforts have been made to improve the photocatalyticity of this material in visible light, the most important of which is doping with elements such as Cu, Er, Yb, Fe, Y, In, Sm, and N [34–38]. Among these elements, doping with copper is unique due to the synergistic effect between the ionic couples of Cu+/ Cu2+ and Ce3+/Ce4+ that results in higher interfacial redox activity [39]. So far, Cu doped CeO2 has been synthesized in a variety of ways, such as hydrothermal [39,40], combination of precipitation [41], microwave [42], and solid-state [43]. Channei et al. [41] synthesized pure CeO2 and Cu-doped CeO2 (0.5, 1.0, and 2.0 wt%) by the combination of homogeneous precipitation and impregnation methods and used to

Corresponding author. E-mail address: [email protected] (M. Mousavi-Kamazani).

https://doi.org/10.1016/j.ultsonch.2019.104695 Received 25 May 2019; Received in revised form 15 July 2019; Accepted 15 July 2019 Available online 16 July 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.

Ultrasonics - Sonochemistry 58 (2019) 104695

M. Mousavi-Kamazani and F. Azizi

Scheme 1. Schematic diagram of formation of Cu doped CeO2 and effect of ultrasonic irradiation on the morphology and removal dye efficiency.

destroy methylene blue through a photocatalytic process under visible light irradiation. Their results indicated that doping increases the photocatalytic efficiency by decreasing bandgap (from ~3.2 eV to 2.8 eV), increasing the amount of light absorption and reducing the recombination. It was also found that the amount of copper is very effective and the highest efficiency was obtained for CeO2 doped with 1.0 wt% Cu. To improve the photocatalytic efficiency of cerium oxide in visible light, Qi et al. [44] doped it with elements such as Co, Zr, and Cu. In their work, photocatalytic efficiencies of doped specimens increased compared to pure CeO2 and the highest efficiency was obtained for copper doped sample (more than 4 times improvements). Here, we followed the synthesis of CuxCe1−xO2 by a new sonochemical method and tried to control the growth of particles using ultrasonic waves, so that the flower-like nanostructures would be formed with wide sheets and provide a large surface for adsorption and photocatalyst processes. By doping, we also tried to improve the absorption and photocatalytic efficiency in visible light. According to our knowledge, this is the first time that Cu doped CeO2 is synthesized through a single-step sonochemical process and has both adsorption and photocatalyst capability: adsorbent in a dark environment and photocatalyst in the light so that it frees itself cleverly after adsorption (Scheme 1).

Table 1 Reaction conditions for Cu doped CeO2 nanostructures. Sample No

Time (min)

Power (W/cm3)

Solvent

(Wt.%)

OH– or H+ source 30 – 30 30 30 30 30 30 10 45 30 30 30 30 30

Adsorption (%)

Photocatalytic Efficiency (%)

40 – 40 40 40 40 40 40 40 40 10 70 40 40 40

Water Water Methanol Ethylene glycol Water Water Water Water Water Water Water Water Water Water Water

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cu

5 5 5 5 0 2 10 20 5 5 5 5 5 5 5

N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 N2H4 NaOH NH3 HNO3

99 34 21 – 28 74 95 91 51 68 52 70 36 48 38

99 55 41 53 81 99 99 66 81 77 89 73 79 71

2

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Fig. 1. XRD patterns of sample 1 synthesized using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/cm3 for 30 min (a) before calcination and (b) after calcination at 400 °C for 2 h.

2. Experimental 2.1. Synthesis and instruments All materials including Ce(NO3)3·6H2O, Cu(NO3)2·3H2O, N2H4·H2O (80%), ethylene glycol, NH3, HNO3, methanol, and ethanol were purchased from Merck and Sigma-Aldrich companies, and no re-purification was carried out. Sonication was conducted using a high-intensity ultrasonic probe (Sonicator 3000; Bandeline, MS 72, Germany) equipped with a converter/transducer and titanium oscillator (horn), 12 mm in diameter, operating at 20 kHz with a maximum power output of 80 Wcm−3. All the ultrasonic tests were measured by calorimetry. Xray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, Xray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10 < 2θ < 80. Field emission scanning electron microscopy (FESEM) images were obtained by MIRA3 FEGSEM. The energy dispersive spectrometry (EDS) analysis was studied by Philips XL30 microscope. The diffused reflectance UV-visible spectrum (DRS) of the products was recorded by a V-670 spectrometer. X-ray photoelectron spectroscopy (XPS) of the as prepared products was performed on an ESCA-3000 electron spectrometer with non-monochromatized Mg Ka X-ray as the excitation source. Transmission electron microscope (TEM) images were obtained on a Philips Zeiss-EM10C transmission electron microscope with an accelerating voltage of 80 kV. Sample for the TEM was prepared by ethanol, then placing a drop of this suspension onto a copper grid mesh of 300 coated with an amorphous carbon film, and then drying in air.

Fig. 2. XRD patterns of (a) sample 5 obtained using hydrazine and 0 wt% Cu (pure CeO2), (b) sample 8 obtained using hydrazine and 20 wt% Cu, and (c) sample 15 obtained using NaOH and 5 wt% Cu.

2.2. Synthesis of Cu doped CeO2 nanostructures 1 ml of hydrazine was dissolved in 30 ml of distilled water and irradiated with ultrasonic waves at 40 W/cm3 for 5 min. Then, 1 mmol of Ce(NO3)3·6H2O dissolved in 10 ml distilled water was added to the solution under the waves. Afterwards, various amounts (2, 5, 10, 15 and 20 wt%) of Cu(NO3)2·3H2O dissolved in 10 ml of water were added to the above solution and ultrasonic was continued for another 15 min. It should be noted that during the whole reaction time, the temperature was kept constant at about room temperature using an ice bath. Finally, the precipitate was filtered, washed with distilled water and absolute ethanol for several times, and calcined at 400 °C for 2 h (sample 1). To investigate the direct effect of ultrasonic waves, an experiment was conducted without using ultrasonic irradiation. In this test, instead of using ultrasonic waves, the solution was stirred at room temperature for 30 min and all other conditions including reaction time, temperature, solution volume, and reagents concentration were identical. Several other parameters such as ultrasonic time and power, solvent, OH– source, and reactants ratio were changed to get the best removal

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efficiency (Table 1). 2.3. Adsorption and photocatalysis experiments To study the adsorption and photocatalytic efficiency of synthesized products, 30 mg of the photocatalyst was dispersed in a 50 ml solution of methyl orang (MO) consisting of 10 mg/L into a glass beaker. The pH of the solution was adjusted to about 2.5 with the help of aqueous nitric acid. To check the adsorption efficiency, the resulting suspension was stirred in the dark for 45 min and every 15 min the absorbance of MO solution was read using a UV-vis spectrophotometer at the wavelength of 464 nm. In order to investigate the photocatalyst process, the resulting solution was exposed to visible light (400 W Osram lamp) illumination for different times instead of darkness. Similar to the above, the absorbance of MO solution was read every 15 min and the degradation efficiency was calculated using the Beer-Lambert law [37,38]. Here, since the sample was both adsorbent and photocatalyst, to study photocatalytic activity, the sample was exposed to light from the beginning and, in other words, it was not given time to establish adsorption–desorption equilibrium. However, the photocatalytic activity can be achieved by comparing the two efficiencies. The calculation results are presented in Table 1. The blank test was carried out exactly the same as the above, with the exception that the catalyst was not used. The recyclability of the samples was also repeated in very identical conditions. For this purpose, after completion of the reaction cycle, the catalyst was washed several times with ethanol and water and then re-used for the next photocatalytic cycle. 3. Results and discussion 3.1. XRD studies The crystalline structure of the as-synthesized products was studied with XRD. The XRD patterns of sample 1 before and after calcination have been presented in Fig. 1a and b, respectively. All peaks in both patterns correspond to CeO2 with a cubic structure (JCPDS No. 431002, space group Fm3m, and cell constants a = b = c = 5.4113 Å). As shown in Fig. 1, after the calcination, the peaks are thinner and sharper and this is due to the fact that the particle grows with increasing temperature and crystallinity also increases. Fig. 2 relates to the XRD patterns of samples 5, 8, and 15 after calcination. Here, all peaks are well matched with CeO2 too and there is no other impurity. For the production of sample 5 only cerium salt was used, so pure CeO2 (Fig. 2a) is expected. In the production of samples 8 and 15, copper is also used and although compounds such as copper oxides (CuO, Cu2O) have not been formed, the displacement of the peaks in Fig. 2b and c, compared to Fig. 2a, shows the presence of copper in the structure of cerium oxide. The amount of peak displacement for sample 8 (20 wt%) is higher because its copper percentage is higher than sample 15 (5 wt %). According to XRD results, the crystallite diameter (Dc) of samples 1, 5, 8, and 15 was estimated to be 12, 7. 5, 8, and 7 nm, respectively, utilizing the Scherrer equation [22]:

Fig. 3. EDS spectra of (a) sample 1 obtained using 5 wt% Cu, (b) sample 7 obtained using 10 wt% Cu, and (c) sample 8 obtained using 20 wt% Cu.

Dc = K / cos

4

Scherrer equation

Ultrasonics - Sonochemistry 58 (2019) 104695

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Fig. 4. XPS spectra of Cu0.05Ce0.95O2 (sample 1) (a) survey spectrum, (b) Ce 3d and Cu 2p, (c) Ce 3d, (d) Cu 2p, and (e) O 1 s.

where K is the shape factor, which is usually about 0.94, β is the breadth of the diffraction line at its half intensity maximum and λ is the wavelength of X-ray source applied in XRD.

respectively. According to Fig. 3, the presence of Ce, Cu, and O is obvious in all three spectra and there is no other impurity. The presence of the Au peak is due to the necessity of conducting samples for SEM analysis. By increasing the amount of Cu from 5 wt% (sample 1) to 10 wt% (sample 7) and 20 wt% (sample 8), the corresponding peak is also larger. These spectra along with the XRD pattern clearly reveal copper doping in cerium oxide.

3.2. EDS studies Fig. 3a–c corresponds to the EDS spectra of samples 1, 7, and 8,

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Fig. 5. Different scales of the SEM images of sample 1 synthesized using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/cm3 for 30 min.

3.3. XPS studies

doublet is found to be 18.38 eV, which is the same as the theoretical value for CeO2 [43]. The main peaks for Cu 2p are observed at 932.27 eV (Cu 2p3/2) and 952.6 eV (Cu 2p1/2), indicating the existence of Cu+ and Cu (Fig. 4d) [49]. It is difficult to detect Cu+ from Cu because their binding energy is very close and their difference is about 0.1–0.2 eV [50]. The important issue here is the lack of peaks at 934.5, 942.9, 954.2, and 962.1 eV [50], which are related to CuO, because the conversion of Cu2+ to Cu+ by ultrasonic waves is confirmed. An important issue here is the lack of peak at 934.5, 942.9, 954.2, and 962.1 eV [49], which is related to CuO, because it shows Cu2+ to Cu+ conversion by ultrasonic waves. In our previous work, we showed that ultrasonic waves reduce Cu2+ to Cu+ in the presence of hydrazine [49]. The peaks at 529.38, 531.4, and 532.3 eV are related to O 1 s (Fig. 4e) [43,49]. It is known that the O 1 s peaks can be divided into oxygen lattice (O2–) at a lower binding energy and surface adsorbed O- at a higher binding energy [49]. Therefore, the peak at 529.38 eV is related to Ce4+–O2– and other peaks are due to the oxygen adsorbed on the surface of the nanostructure.

To better describe the elemental states of synthesized Cu doped CeO2 nanostructures, XPS analysis was developed for Cu0.05Ce0.95O2 (sample 1, Fig. 4). All the XPS spectra were calibrated with carbon 1 s peak located at 285 eV as the reference position. All of the peaks in the Fig. 4a are related to Ce, Cu, and O elements and no other impurities can be seen [43]. Fig. 4b is related to Ce 3d and Cu 2p peaks and has been provided to show better details of the detection of copper peaks from cerium. The Ce 3d spectrum (Fig. 4c) is composed of multiple doublets (u and v) corresponding to spin–orbit split of 3d3/2 and 3d5/2 [19]. The highest binding energies at 882.32, 889.28, 898.18, 900.81, 907.64, and 916.56 eV are representative of the Ce4+ species [43]. The peaks centered at binding energy of ~898 and ~917 eV are related to the final state of Ce4+3d94f0O2p6 [48]. The peak at ~917 eV is the best one to differentiate between the Ce4+ and Ce3+ states. This peak is generally not present in the case of Ce2O3 [43]. The peaks at ~889 and ~907 eV are attributed to the Ce4+3d94f1O2p5, and the doublets at ~901 and ~882 eV correspond to the hybridization state of Ce4+3d94f2O2p4 [48]. The spin-orbit splitting energy of the Ce 3d

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Fig. 6. Different scales of the TEM images of sample 1 synthesized using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/cm3 for 30 min.

3.4. SEM, TEM, and EDS mapping studies

the previous experiment was repeated in the absence of ultrasonic irradiation (sample 2). With regard to Fig. 8a and b, in the absence of ultrasonic waves, the morphology has changed and spherical particles have been formed that this change in morphology is related to the involvement of ultrasonic waves in the reaction mechanism. Under ultrasonic waves radical species are formed in solvents. In this reaction, the solvent is water and so the radical species are %H and %OH. Regarding the use of hydrazine and Cu2+ in this reaction, radical species can interfere with the mechanism of the reaction:

Fig. 5a–i show the SEM images of sample 1 that have been synthesized using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/cm3 for 30 min. As can be seen, sample 1 contains flower-like nanostructures with wide plates and sheets that have created a lot of space and porosity. For better and more accurate charactrization of the nanostructures morphology and their plates size and thickness, the TEM images of sample 1 were prepared with different magnifications (Fig. 6). According to Fig. 6a–c, the product contains three-dimensional nanostructures and confirms SEM images. As seen in the images with high magnifications (Fig. 6d–i), the plates of these nanostructures have a nanometer thickness, while they extend to a few micrometers. The elemental mapping analysis by EDS demonstrates the uniform dispersion of Cu element over CeO2 support (Fig. 7).

H2O H% + %OH

(1)

N2H4 + 4%OH → N2 + 4H2O 2+

H + Cu %

+

→H

+

+ Cu

(2) (3)

As a result, nucleation and growth mechanisms change and the product is obtained with different morphologies and sizes. Here, the comparison of the two Figs. 5 and 8a and b confirms the above results and it can be found that ultrasonic waves cause the growth of plate-like

3.5. The effect of ultrasonic waves and possible formation mechanism To investigate the direct effect of ultrasonic waves on morphology,

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Fig. 7. (a) SEM, (b) EDS, and (c) EDS elemental mapping of Cu0.05Ce0.95O2 (sample 1).

nanostructures.

changes in time of sonication and as can be seen, the morphology has changed and the papillary structures have been produced by reducing the time to 10 min (sample 9, Fig. 9a–c) or increasing it to 40 min (sample 10, Fig. 9d–f). Similar results were observed with power changes to 10 W (sample 11, Fig. 10a–c) and 70 W (sample 12, Fig. 10d–f), and the morphology was completely changed.

3.6. The effect of different parameters on size and morphology 3.6.1. Solvent effect Due to the mechanism of sonochemical reactions that are based on the generation and intrusion of bubbles and the production of reactive species, the solvent type is very important [45]. The vapor pressure, solubility, and viscosity of the solvent are very effective in bubble production [46]. In order to investigate the effect of solvent on size and morphology, the reaction was carried out in methanol (sample 3) and ethylene glycol (sample 4) solvents instead of water. Using methanol led to formation of spherical nanoparticles, which in some places were agglomerated (Fig. 8c and d). No reaction was performed on the ethylene glycol solvent. The viscosity of ethylene glycol is high, and under these conditions, ultrasonic waves may not be able to produce bubbles with an appropriate energy for reaction. Fig. 8e and f are related to SEM images of pure CeO2 (sample 5) and as can be seen, in the absence of copper, spherical nanoparticles with a size of about 10 nm were synthesized. This is in a good agreement with the results of similar papers reported for the production of CeO2 under ultrasonic waves and the use of hydrazine [47].

3.6.3. OH– source In order to determine the effect of hydroxide ion source, NaOH (sample 13), NH3 (sample 14), and HNO3 (sample 15) were selected instead of hydrazine. In all three cases, spherical nanoparticles were produced in different sizes ((Fig. 11a–f). By changing the concentration of hydroxide ion, the nucleation and growth rates change, and thus the particle size changes. The common results of all three compared to hydrazine is that in their presence, there has not been a sonochemical reaction and only ultrasonic waves have reduced the particle size. 3.7. Optical properties The optical properties of Cu0.05Ce0.95O2 (samples 1) and CeO2 (samples 6) was determined by UV-Vis absorption spectrum (DRS). By comparing the two spectra in Fig. 12, it is clear that the doped sample (Cu0.05Ce0.95O2) has a greater absorption in the visible region than pure CeO2. Bandgap calculation was performed for both samples, and values 2.9 eV and 3.09 eV were obtained for Cu0.05Ce0.95O2 and pure CeO2, respectively. In both cases, variations are observed compared to the bulk CeO2 (3–3.2 eV) [47]. With doping, the bandgap has decreased

3.6.2. Time and power effect Considering that time and power of waves affect size and morphology and there is an optimum power and time, several experiments were designed with different powers and times. Fig. 9a–f are related to

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Fig. 8. SEM images of the as-synthesized product (a) and (b) in the absence of ultrasonic irradiation (sample 2), (c) and (d) in methanol (sample 3), and (e) and (f) using 0 wt% Cu (sample 5).

cm3 for 30 min (sample 1) has the highest adsorption efficiency and adsorbed more than 99% of the color at 45 min. This is while the lowest efficiency (28%) was obtained for the un-doped sample (sample 5). The sample produced in the absence of ultrasonic waves (sample 2) also had a low efficiency (34%). To evaluate the effect of dope amounts, 2%, 10%, and 20 wt% Cu (samples 6–8) were selected in addition to 5 wt% (sample 1). For sample doped with 2 wt% Cu (sample 6) the efficiency dropped to 74%. By increasing the amount of dope to 10 and 20 wt%,

and the absorption has shifted toward the visible area. Therefore, it can be expected that photocatalytic efficiency increases in the visible area. 3.8. Adsorption process Fig. 13 is related to the methyl orange adsorption process in the dark by samples 1, 2, 5, 6, and 13. As shown in Fig. 13a, the synthesized sample using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/

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Fig. 9. SEM images of the as-synthesized product using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W for (a)-(c) 10 min (sample 9) and (e)-(f) 45 min (sample 10).

the efficiency of 95 and 91% were obtained respectively and this shows there is an optimal amount of dope to achieve the highest efficiency, which is 5 wt% in this study (sample 1). Samples produced in the presence of NaOH (sample 13), NH3 (sample 14), and HNO3 (sample 15) have spherical morphology and showed an adsorption efficiency of 36, 48 and 38%. Other results are presented in Table 1. As the SEM images showed, morphology changes with Cu doping in CeO2 under

ultrasonic waves and sheet-like nanostructures (Fig. 5, sample 1) are produced in place of spherical nanoparticles. These nanoscale materials with their large plates provide high surface and porosity for adsorption process. The effect of adsorbent amount on adsorption efficiency was investigated and for this purpose, 0.05, 0.2, and 0.3 g of adsorbent were selected in addition to 0.1 g. With respect to Fig. 13b, the amount of adsorption increased by increasing the amount of adsorbent up to 0.3 g

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Fig. 10. SEM images of the as-synthesized product using hydrazine and 5 wt% Cu under ultrasonic waves for 30 min at (a)-(c) 10 W/cm3 (sample 11) and (e)-(f) 70 W/cm3 (sample 12).

and about 90% of color was adsorbed at 30 min. By increasing the amount of adsorbent (more than 0.2 g) adsorption increases, but with a slope less.

visible light was investigated for methyl orange (MO) degradation and the results are seen in Fig. 14 and Table 1. In the absence of the catalyst the degradation rate is negligible, but in the presence of a catalyst degradation has progressed to completion (Fig. 14a). The highest efficiency is obtained for synthesized sample using hydrazine and 5 wt% Cu under ultrasonic waves at 40 W/cm3 for 30 min (sample 1) and it can be seen that more than 99% of the color has been degraded at

3.9. Photocatalytic measurements The photocatalytic activity of the as-synthesized samples under

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Fig. 11. SEM images of the as-synthesized product at 40 W/cm3 for 30 min and 5 wt% Cu using (a) and (b) NaOH (sample 13), (c) and (d) NH3 (sample 14), and (e) and (f) HNO3 (sample 15).

45 min. The synthesized sample in the absence of waves (sample 2) and un-doped sample (sample 5) have the lowest efficiency compared to other samples. Samples containing spherical nanoparticles (samples 13–15) have a moderate and close efficiency. It is quite clear that the change in morphology that occurs under ultrasonic waves through a sonochemical process is a major factor in the dramatic increase in efficiency. Of course, according to the results of the DRS (Fig. 12), the

amount of light absorption increases by doping and a small part of the increase in efficiency is attributed to it. Fig. 14b relates to the amount of photocatalyst. With increasing catalyst content from 0.05 to 0.2 g the efficiency increased and complete degradation occurred in about 30 min. For greater values (0.3 g), the efficiency was reduced slightly. Therefore, it can be understood that photocatalytic degradation requires an optimal amount of sample. Fig. 14c shows the reaction

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Fig. 13. (a) Adsorption process of methyl orange in the dark by samples 1, 2, 5, 6, and 13 and (b) the effect of adsorbent amount on adsorption efficiency.

Fig. 12. Diffuse reflectance spectra of CuxCe1-xO2 (sample 1) and pure CeO2 (sample 5). The inset shows the plots of (αhν)2 versus the energy of light (hν).

it solves the problem of adsorbents and improves photocatalyst efficiency. The most important problem with an adsorbent is that it only adsorbs contaminants, but does not convert them into biocompatible materials. The mentioned nanostructures can only be produced under ultrasonic waves because in this case, a sonochemical reaction occurs ̇ and ̇OH) and the ionic couples of Cu+/ between the radical species (H Cu2+ and Ce3+/Ce4+ and causes the morphology to change.

kinetics of photocatalytic degradation of MO by blank and the as-synthesized samples under visible light. In all cases, the plot of -Ln (Ct/C0) vs time shows straight line with the rate constant (K) of 0.0016, 0.101, 0.017, 0.016, 0.038, and 0.03 min−1 for blank and samples 1, 2, 5, 6, and 13, respectively. As it is clear, in the absence of catalyst the rate constant is negligible. Also, the highest rate constant is obtained for sample 1 and the lowest is for samples 2 and 5. All these results are in good consistent with photocatalytic activity curves in Fig. 14a. The good correlations are indicating that the reaction kinetics follows a pseudo first order rate law. Recyclability of the as-synthesized nanostructure (sample 1), as an important feature of a catalyst, is presented in Fig. 14d. As shown in Fig. 14d, the sample is stable and has a good recycling capability, so that there is no significant reduction in photocatalytic efficiency after four times. In the photocatalyst process, the ability to adsorb is very important. If a photocatalyst can adsorb well, its photocatalytic efficiency will be high. CeO2 is a good photocatalyst, but its high bandgap results in low light absorption in the visible area. Here, using doping by sonochemical process, in addition to reducing bandgap, morphology also changed so that the products are both adsorbent and photocatalyst. This dual feature is very interesting because

4. Conclusions In this study, CuxCe1-xO2 synthesis was followed by a new sonochemical method considering the effects of ultrasonic waves, irradiation power and time, solvent, and OH– Source. The analytical results showed that using hydrazine and 5 wt% Cu under ultrasonic waves with a power of 40 W/cm3 for 30 min resulted in formation of flower-like Cu0.05Ce0.95O2 nanostructures. These nanostructures are composed of flakes and plates, which have a large surface for adsorption and photocatalytic processes so that they can remove the solution color in the darkness for about 45 min. The mentioned nanostructures performed photocatalytic activity upon exposure to light and removed the adsorbed color on their surface. This dual function is very interesting

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Fig. 14. (a) Photocatalytic degradation of methyl orange under visible light by blank and samples 1, 2, 5, 6, and 13 and (b) the effect of catalyst amount on photocatalytic efficiency.

because it solves the problem of adsorbents and improves photocatalyst efficiency.

[9]

Acknowledgment

[10]

This work was supported by the University of Semnan and the authors are very thankful for this assistance.

[11]

Appendix A. Supplementary data

[12]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.104695.

[13] [14]

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