Synthesis and Characterization of Dysprosium-Doped ZnO

Article pubs.acs.org/IECR

Synthesis and Characterization of Dysprosium-Doped ZnO Nanoparticles for Photocatalysis of a Textile Dye under Visible Light Irradiation Alireza Khataee,*,† Reza Darvishi Cheshmeh Soltani,‡ Younes Hanifehpour,§ Mahdie Safarpour,† Habib Gholipour Ranjbar,† and Sang Woo Joo*,§ †

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 516661647, Iran ‡ Department of Environmental Health Engineering, School of Public Health, Arak University of Medical Sciences, Arak, Iran § School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749 South Korea S Supporting Information *

ABSTRACT: Dy-doped ZnO nanoparticles were synthesized with a sonochemical method. X-ray diffraction, inductively coupled plasma, Fourier transform infrared spectroscopy, UV−vis diffuse reflectance spectroscopy, and scanning electron microscopy analyses confirmed the successfully synthesis and nanometric diameter of the samples. Dy-doped ZnO nanoparticles were used for photocatalytic decolorization of C. I. Acid Red 17 solution under visible light irradiation. Among different amounts of dopant agent, 3% Dy-doped ZnO nanoparticles indicated the highest decolorization. Decolorization efficiency increased from 14.3 to 57.0% with an increase in catalyst dosage from 0.25 to 1 g/L, while further increment in the catalyst dosage up to 2 g/L caused an obvious decrease in decolorization efficiency. The addition of 0.1 mM peroxydisulfate (S2O82−) resulted in a decolorization efficiency of nearly 100% after irradiation for 180 min. The trend of inhibitory effect in the presence of different radical scavengers was Cl− > C2H5OH > HCO3− > CO32−.

1. INTRODUCTION The application of advanced oxidation processes (AOPs) has been proposed as an efficient approach for the degradation of various organic pollutants such as organic dyes in the aqueous phase. One of the most widely used AOPs for treating colored wastewater is photocatalytic processes.1−4 Among various photocatalysts used in the photocatalytic processes, TiO2 and ZnO are known to be good photocatalysts with high photocatalytic activity.5−7 Compared to TiO2, ZnO as an ntype II−VI semiconductor has attracted more attention because of its large area-to-volume ratio, direct wide band gap (3.37 eV), high photosensitivity, large excitation binding energy (60 meV), long life span, and high chemical stability.1,5,8−10 During a photocatalytic process equipped with ZnO irradiated with UV light, highly reactive hydroxyl radicals (OH•) are produced, promoting the degradation of target pollutants. Xiang et al.11 have quantitatively investigated and confirmed the hydroxyl radical production on various semiconductor photocatalysts in aqueous solution by the photoluminescence (PL) technique. The formation of OH• via a photocatalytic process using ZnO is shown through eqs 1−3:2,12 ZnO + hν → ZnO

(e− + h+)

disadvantages of pure ZnO nanoparticles is the fast recombination rate of the photogenerated electron−hole pairs.14 Therefore, the improvement of the photocatalytic activity of ZnO nanoparticles and the photosensitivity toward visible light irradiation using doping agents has attracted much more consideration.8,15 To improve the photocatalytic activity of ZnO nanoparticles, elements such as Zr,16 Cd,8 Ce,9 Eu,17 Nd,18,19 Sm,20,21 and Sn22 have been applied. Among different dopants, dysprosium (Dy) as a rare earth element has been proposed as an efficient dopant for the ZnO nanoparticles;23 thus, in the present study, ZnO nanoparticles were synthesized and doped with Dy. The route used for the fabrication of nanoparticles controls their shape, size, and optical properties, which strongly affect the photocatalytic activity of nanostructured catalyst.23,24 ZnO nanoparticles have been synthesized through various methods, including sol−gel,25 chemical precipitation,10 microwave radiation,26,27 and hydrothermal18,28−30 methods. However, these methods have some disadvantages such as long reaction times and high temperature requirements for the synthesis. But the sonochemical technique is very quick, simple, and economical in comparison with these other methods.31,32 During a sonochemical process, the molecules within the solution experience a chemical reaction as a result of the application of powerful ultrasonic radiation ranging from 20

(1)

h+ + H 2O → H+ + OH•

(2)

h+ + OH− → OH•

(3)

Received: August 21, 2013 Revised: December 24, 2013 Accepted: January 17, 2014

In the present study, nanosized ZnO was used because fine particles possess a higher surface area which elevates the density of active sites for the photocatalysis.13One of the major © XXXX American Chemical Society

A

dx.doi.org/10.1021/ie402743u | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. SEM images of undoped (a, b) and 3% Dy-doped ZnO nanoparticles (c, d) taken at different magnifications; diameter size distribution of 3% Dy-doped ZnO nanoparticles (e).

2. MATERIALS AND METHODS 2.1. Materials. All chemicals used in the present investigation were purchased from Merck, Germany apart from DyN3O9·6H2O powder and C2H5OH·.4H2O (99%) solution, which were purchased from Aldrich, United States. AR17 was purchased from Shimi Boyakhsaz Co, Iran and used without purification. The characteristics of the dye are displayed in Table S1 of Supporting Information. ZnCl2 (99.5%) was used as the zinc precursor, and DyN3O9·6H2O was chosen as dysprosium source. 2.2. Photocatalyst synthesis. To synthesize 1, 3, and 5% Dy-doped ZnO nanoparticles, a sonochemical method was applied. The approach for sonochemical synthesis of Dy-doped ZnO nanoparticles was as follows: Different amounts of DyN3O9·6H2O was added to the solution of ZnCl2 to achieve 1, 3, and 5% Dy. NaOH solution (1 M) was added dropwise until the pH of the solution reached 10. The solution was sonicated for 3 h in a bath type sonicator (Sonica, 2200 EP S3, Italy) with 50−60 Hz frequency and a heating arrangement for

kHz to 10 MHz together with a physical interaction which is acoustic cavitation, involving the formation, growth, and implosive collapse of bubbles in the solution.33−35 On the basis of the advantages of sonochemical method, Dy-doped ZnO nanoparticles were synthesized through a sonochemical method and used in a photocatalytic process equipped with a 100 W visible light lamp. The efficacy of the photocatalytic process for treating colored solutions was assessed under different operational conditions, including the effect of the amount of doping agent, dye concentration, and catalyst dosage along with the effect of the presence of radical scavengers and peroxydisulfate ion. To evaluate the photocatalytic activity of Dy-doped ZnO nanoparticles, an azo dye (C. I. Acid Red 17 (AR17)) was used as a model organic pollutant. To the best of our knowledge and on the basis of the literature data, no studies have investigated the application of sonochemically synthesized Dy-doped ZnO nanoparticles for the degradation of a textile dye in aqueous environments. B



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Figure 2. XRD pattern of pure and 3% Dy-doped ZnO nanoparticles.

Diffuse reflectance spectroscopy (DRS) spectra of the samples were recorded using a Scinco S4100 (South Korea) spectrophotometer. The initial pH was measured by a Metrohm pH meter (Model 654, Germany). At the end of each experiment, the samples were centrifuged for 5 min at 6000 min−1 and the supernatant was withdrawn for analysis. The residual AR17 in the solution was measured spectrophotometrically (UV−vis spectrophotometer, WPA Lightwave S2000, England) at λmax of 510 nm. The decolorization efficiency was calculated using eq 4:

the sonochemical synthesis of Dy-doped ZnO nanoparticles. The resulting white precipitate was thoroughly washed with deionized water followed by ethanol to remove impurities. Finally, it was dried in an oven at 80 °C for 12 h. The above procedure was carried out without addition of DyN3O9·6H2O to synthesize undoped ZnO nanoparticles. To compare calcined and uncalcined synthesized Dy-doped ZnO nanoparticles, the samples were calcined at 300 °C for 3 h in an electric furnace. 2.3. Experimental Procedure. A batch experimental quartz photoreactor with a 100 mL working volume was used to evaluate the photocatalytic activity of sonochemically synthesized Dy-doped ZnO nanoparticles for decolorization of a textile dye (AR17) under visible light. A 100 W visible lamp (Pars Co, Iran) was applied as the light source. The intensity of the lamp with a distance of 4.5 cm from the surface of the solution was 25 W/m2. The radiation intensity was measured with a UV−vis radiometer purchased from Cassy Lab Company (Germany). In a typical process, 0.1 g of the photocatalyst was added into a 100 mL solution containing AR17 with an initial concentration of 5 mg/L. Then, the suspension was magnetically stirred in the dark for 20 min to achieve adsorption−desorption equilibrium before beginning the irradiation. 2.4. Instrumentation and Analysis. To characterize the structure of undoped and Dy-doped ZnO nanoparticles, a Siemens X-ray diffractometer (D5000, Germany) was used to produce X-ray diffraction (XRD) patterns (Cu Kα radiation (1.54065 Å)) in which an accelerating voltage of 40 kV and an emission current of 30 mA were applied. Scanning electron microscopy (SEM) was applied to evaluate the surface morphology of the samples using a Hitachi microscope (Model S-4200, Japan). The SEM analysis was performed after gold plating of the samples. Moreover, the obtained SEM images were analyzed using manual microstructure distance measurement software (Nahamin Pardazan Asia Co., Iran) to determine the diameter size distribution of the obtained samples. The model of the inductively coupled plasma (ICP) instrument used for detecting trace metals presence in the synthesized samples was ICP GBC Integra XL (Australia). For Fourier transform infrared spectroscopy (FT-IR) analysis, the KBr pellets were prepared from the undoped and different mol % of Dy-doped ZnO powders. FT-IR analysis was performed using a spectrophotometer (Tensor 27, Bruker, Germany).

Decolorization efficiency (%) = [1 − (C / Co)] × 100 (4)

where Co and C are the initial and final concentration of the dye in the solution (mg/L), respectively.10,36

3. RESULTS AND DISCUSSION 3.1. Structural Analysis. Figure 1 shows the surface morphology of undoped and 3% Dy-doped ZnO nanoparticles taken through SEM analysis. As can be seen in Figure 1a,b, undoped ZnO nanoparticles have nonuniform size, which may be as a result of the aggregation of synthesized ZnO nanoparticles and the growth of irregular crystalline grains during synthesis. But, doping of ZnO nanoparticles by means of 3% Dy caused an obvious decrease in the aggregation of nanoparticles (Figure 1c,d). According to SEM images and using Manual Microstructure Distance Measurement software (Nahamin Pardazan Asia Co., Iran), the mean particle size of 3% Dy-doped ZnO sample was found to be about 38 nm. Figure 1e shows the diameter size distribution of 3% Dy-doped ZnO nanoparticles. As can be seen, the diameter distribution of most of the particles is in the range of 30−40 nm. The lower aggregation of nanoparticles is favorable for the photocatalysis of target pollutants because of the greater availability of active sites. To reach a better understanding of the structure and real crystalline size of the undoped and Dy-doped ZnO nanoparticles, XRD analysis was carried out; the results are represented in Figure 2. Additionally, as shown in Figure 2, the effect of calcination on the structure of the catalysts was studied via XRD. The intense sharp XRD peaks suggest the excellent crystalline structure of synthesized undoped and Dydoped ZnO nanoparticles under different conditions even C



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range.41 In the present work, the ICP analysis was performed after careful washing of Dy-doped-ZnO nanoparticles to remove any physically adsorbed ions, such as dysprosium. The obtained results showed that the amount of Dy in the 1, 3, and 5% Dy-doped ZnO samples was 0.56, 1.83, and 3.02% w/ w, respectively. The theoretical calculated values for Dy percent in the mentioned samples were 0.67, 2.01, and 3.33% w/w, respectively. A systematic increase in the content of Dy is observed with the increasing nominal concentration of the dopant in the samples. The ICP analysis results showed that nearly all of the used Dy3+ ions were successfully incorporated into the structure of ZnO nanoparticles. The FT-IR spectra of undoped and Dy-doped ZnO samples is shown in Figure 3. As

without calcination (Figure 2). As illustrated in Figure 2, the incorporation of Dy into the structure of ZnO does not change the structure of ZnO. The peaks at 2θ of 31.92, 34.6, 36.48, 47.68, 56.72, 63, 66.08, 68, 68.28, 71.64, and 75.96° correspond to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of hexagonal wurtzite ZnO, respectively (JCPDS Card 36-1451). As illustrated in Figure 2, after doping of ZnO nanoparticles by Dy, only the peaks related to the ZnO were still observed and no other peaks corresponding to Dy2O3, Zn(OH)2, or other impurities were detected, which indicates that the Dy3+ ions are replaced by the Zn2+ ions in ZnO structure.9 But, as can be seen, there is a shift to low diffraction values for (100), (002), and (101) planes in the 3% Dy-doped ZnO sample. The 2θ angles of the (100), (002), and (101) planes were 31.92, 34.60, and 36.48° for pure ZnO and 30.96, 33.56, and 35.40° for 3% Dy-doped ZnO, respectively. This observation can be explained by the expansion of ZnO lattice caused by the radius of Dy3+ (0.91 Å) that is larger than that of Zn2+ (0.74 Å). The increase in lattice parameter and the shift to lower angle of the XRD peaks with doping of Dy were expected to have the influence on the lattice deformation and strain resulting from Dy dopant.31 Moreover, the calcination of the catalysts had no significant effect on their crystalline nature. On the contrary, Yu and coworkers in their study showed that calcination temperature produces a great effect on the structure of TiO2 nanotube arrays.37 The effect of the presence of dopant on the crystalline size of the synthesized undoped and 3% Dy-doped ZnO nanoparticles was evaluated using Debye−Sherrer’s equation as shown in eq 5:38 D = 0.9λ /β cos θ

(5)

where D, λ, β, and θ are the average crystalline size (Å), wavelength of the X-ray (Cu Kα=1.54056 Å), full width at halfmaximum (fwhm) intensity of the peak (rad), and the diffraction angle, respectively. According to eq 5, the average crystallite size of the undoped and Dy-doped ZnO nanoparticles were about 12 and 14 nm, respectively. This indicated that the incorporation of Dy into the ZnO nanoparticles had no significant effect on the crystallite size of the nanoparticles. In accordance with our results, Wang et al., on the basis of the results of XRD analysis, showed that doping of ZnO nanoparticles with Cd caused no effect on the structure of the photocatalyst.8 The DRS spectra of undoped and Dy-doped ZnO samples are illustrated in Figure S1 of Supporting Information. It can be seen that the samples showed a strong photoabsorption in the visible light range. There is a red shift in absorbance spectra of Dy-doped ZnO in comparison to that of undoped ZnO, as expected for doped materials. This red shift can be related to the formation of a shallow level inside the band gap because of impurity atoms (Dy3+) introduced into the wurtzite ZnO lattice.31,39 Another reason for this shift can be the narrow band gap originating from the charge transfer between the ZnO valence or conduction band and the Dy ion 4f level.20,40 The energy of the band gap of ZnO and 3% Dy-doped ZnO nanoparticles estimated from the main absorption edges of the DRS spectrum is 3.02 and 2.88 eV, respectively. To verify the presence of Dy in the doped ZnO samples, the ICP technique was used. The ICP analytical technique can be a very powerful tool to detect and analyze trace and ultratrace elements. This technique can quantitatively measure the elemental content of a material from the ppt to the wt %

Figure 3. FT-IR spectra of undoped and Dy-doped ZnO samples.

can be seen, there is an obvious band around 560 cm−1, which can be attributed to the ZnO stretching mode in the ZnO lattice. The broad peak around 3400 cm−1 corresponds to the OH group of H2O, indicationg the existence of water absorbed on the surface of the ZnO samples.42,43 Zn−O coordination has been observed to shift slightly toward the lower wavenumbers (high energy) by Dy incorporation. 3.2. Effect of Operational Parameters on the Decolorization Efficiency. 3.2.1. Comparison of Different Processes in the Decolorization of AR17. The efficiency of different processes was investigated in the decolorization of 5 mg/L AR17 solution, and results are presented in Figure 4. It can be observed that the highest removal efficiency was obtained using 3% Dy-doped ZnO catalyst under visible light. The results also showed that the removal of AR17 after 180 min reaction time follows the decreasing order Vis/3% Dydoped ZnO > Vis/1% Dy-doped ZnO > Vis/5% Dy-doped ZnO > Vis/TiO2 P25 > Vis/undoped ZnO > visible light only > ZnO in the dark. As is clear, the photolysis process with visible light has a negligible decolorization efficiency compared to photocatalytic processes. Also, the decolorization efficiency of ZnO in the dark is less than 5% after 180 min, which indicates that the value of dye removal by adsorption is insignificant compared to photocatalysis. It should be noted that in all other processes, the suspension of photocatalyst and AR17 was magnetically stirred in a quartz photoreactor in the dark for 20 min to establish an adsorption−desorption D



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Dy 3 + + O2 → Dy 4 + + •O2−

(6)

Dy 4 + + e− → Dy 3 +

(7)

•

O2− + H+ → •O2 H

(8)

2•O2 H → H 2O2 + O2

(9)

Both the OH• and •O2− radicals together with H2O2 are excellent oxidants for degradation of organic compounds. Among different Dy-doped ZnO nanoparticles, the application of 3% Dy-doped nanoparticles led to the highest decolorization efficiency (57.0%). The decolorization efficiency was increased with an increase in the amount of Dy up to 3% and then decreased, suggesting that 3% Dy-doped ZnO nanoparticles could be more efficient for separating photoinduced electron− hole pairs to enhance the photocatalytic decolorization efficiency. It has been confirmed that the addition of a precise amount of doping element can be critical for achieving high photocatalytic activity.42 Increasing the amount of Dy within the structure of catalyst resulted in a higher surface barrier and narrower space charge region, leading to efficient separation of the produced electron−hole pairs. Increasing the amount of Dy up to a specific value results in exceeding the space charge layer by increasing the penetration depth of visible light into ZnO nanoparticles. This makes the recombination of electron−hole pairs easier, causing low photocatalytic decolorization efficiency. In addition, the excess amount of dopant covering the surface of ZnO nanoparticles leads to a decrease in the photocatalytic activity of the photocatalyst due to an increase in the number of electron−hole recombination centers.29,43 According to the obtained results, 3% Dy-doped ZnO nanoparticles were used for performing the rest of the experiments. In agreement with our findings, Yayapao et al. reported that 3% Ce-doped ZnO nanoneedles were the most effective photocatalyst for the decolorization of a solution containing methylene blue.9 In the case of the effect of irradiation time on the photocatalytic decolorization efficiency, as is obvious from Figure 4, the decolorization efficiency was increased as the irradiation time increased up to 180 min. Similar results have been reported by Suwanboon et al. in their study on the photocatalytic decolorization of methylene blue over ZnO powders.10 3.2.2. Effect of Catalyst Dosage. To evaluate the effect of catalyst dosage on the decolorization efficiency, catalyst dosage was varied between 2.5 and 12.5 g/L, and the results are displayed in Figure 5. In this set of experiments, reaction time and initial dye concentration were constant at 180 min and 5 mg/L, respectively. As can be seen in Figure 5, at catalyst concentrations of 0.25, 0.5, 0.75, 1, and 2 g/L, the decolorization efficiency was 14.3, 22.5, 42.4, 57.0, and 50.3%, respectively. Thus, decolorization efficiency increased with increasing catalyst dosage from 0.25 to 1 g/L and then decreased. Similar behavior has been reported by Sobana and Swaminathan in their study on the photocatalytic decolorization of AR17 using ZnO.44 Increasing decolorization efficiency with the increase in the amount of photocatalyst can be attributed to the increasing active surface area for the photocatalytic degradation of organic dye. On the other hand, further increment in the amount of suspended photocatalyst led to an increase in the turbidity of the solution and scattering effects, causing the decrease in UV light penetration. This reduces excitement of the photocatalyst for the generation of

Figure 4. Comparison of different processes in the decolorization of AR17. Initial dye concentration, 5 mg/L; and catalyst dosage, 1 g/L.

equilibrium of the dye. Then, the solution was irradiated by a visible lamp as the light source. The color removal efficiency was expressed as the percentage ratio of decolorized dye concentration to that of the initial concentration (after 20 min in the dark). Therefore, the reported data are decolorization efficiency of the photocatalytic process. In addition, the photocatalytic activity of the synthesized catalysts was compared with TiO2 P25 as a routine reference photocatalyst (Figure 4). As can be seen, the decolorization efficiency of Dydoped ZnO photocatalysts with different dopant amounts is about two times greater than that of TiO2 P25. This reveals that the synthesized Dy-doped ZnO samples can be used as an efficient photocatalyst under visible light. Figure 4 also shows the effect of the amount of Dy as doping agent on the decolorization efficiency by varying the amount of Dy from 1 to 5%, while the concentration of the catalyst and AR17 were constant at 1 g/L and 5 mg/L, respectively. Overall, Dy-doped ZnO nanoparticles caused higher decolorization efficiency compared to the undoped ZnO. A decolorization of 24.4% was obtained as the undoped ZnO nanoparticles were applied. Under visible light irradiation, Dy acts as an electron scavenger reacting with the superoxide species and preventing the recombination of produced electrons and holes during the photocatalytic process.29 The transitions of 4f electrons of lanthanides lead to the implementation of the optical adsorption of catalysts and support the separation of photogenerated electron−hole pairs. In the case of dysprosium dopant, it can exist as Dy3+ and Dy4+. Thus, Dy3+ may give an electron to O2 adsorbed on the surface of Dy-doped ZnO to form •O2− by transforming into Dy4+, favoring a charged migration to O2 and an enhancement of the photoreaction rate in comparison with that of pure ZnO (eq 6). On the other hand, the Dy4+ species may receive photogenerated electrons in the conduction band of ZnO to form Dy3+ (eq 7). These reactions are the reason for enhanced photoactivity of ZnO. Also, Dy dopant can effectively slow the recombination rate of the photogenerated electron−hole pairs and enhance interfacial charge-transfer efficiency. The process improves the photocatalytic activity of ZnO in the same manner as the transfer of a photogenerated electron from the conduction band to d orbitals of transition-metal-doped ZnO.31 E



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most of the light entering the colored solution instead of the photocatalyst, reducing photocatalytic activity of the catalyst.44 Increasing AR17 concentration from 2.5 to 5 mg/L resulted in an insignificant increment in decolorization efficiency, suggesting that an initial dye concentration of 5 mg/L can be selected for the rest of the experiments. 3.3. Effect of the Presence of Peroxydisulfate. Peroxydisulfate (S2O82−) is considered as a chemical oxidant for degrading organic pollutants through direct chemical oxidation.47 However, the chemical oxidation of organic compounds by S2O82− is relatively low. Therefore, photolysis of S2O82− has been proposed as one of the efficient approaches for the promotion of its oxidation potential.48 Accordingly, in the present work, the effect of the presence of S2O82− on the photocatalysis of AR17 was studied, and the results are represented in Figure 7. As shown, in the presence of DyFigure 5. Effect of the catalyst dosage on the photocatalysis of AR17. Dopant percentage, 3%; and initial dye concentration, 5 mg/L.

OH•. Additionally, the photocatalyst nanoparticles have a tendency to aggregate at high concentrations, which reduces the number of active sites.2,44 Because decolorization efficiency decreased with the increase in catalyst dosage from 1 to 2 g/L, subsequent experiments were carried out with a catalyst dosage of 1 g/L. 3.2.3. Effect of Initial Dye Concentration. One of the most important parameters influencing photocatalytic activity is the initial concentration of the target pollutant. In the present work, initial dye concentration was varied between 2.5 and 12.5 mg/L to determine its effect on the decolorization efficiency (Figure 6). As shown, decolorization efficiency decreased from Figure 7. Effect of the presence of peroxydisulfate on the photocatalysis of AR17. Dopant percentage, 3%; catalyst dosage, 1 g/L; and reaction time, 180 min.

doped ZnO nanoparticles as catalyst, the addition of S2O82− led to the increment of photocatalytic decolorization of AR17. At S2O82−concentrations of 0.05 and 0.1 mM, the photocatalytic decolorization efficiency was obtained to be 63.15 and 100%, respectively, which was higher than the efficiency of the photocatalytic process without S2O82− (57.0%). Although the addition of S2O82− led to a significant increment in the photocatalytic decolorization efficiency, the addition of 0.1 mM S2O82− alone (without photocatalyst) caused a negligible contribution in the decolorization efficiency (about 12.0%). This implies that the photolysis of S2O82− to SO4•−, which has been proposed to be an efficient method for accelerating the degradation of target pollutants via S2O82−,48,49 is not efficient enough alone (without catalyst) to treat colored solutions containing AR17. Despite the fact that the presence of different ionic and organic substances can reduce the efficiency of the photocatalytic process, the presence of S2O82− in a photoreactor containing Dy-doped ZnO nanoparticles enhanced the photocatalytic removal of AR17. The reactions involved in the photocatalysis of the target organic dye in the presence of S2O82−are summarized in eqs 10−18:48−50

Figure 6. Varying photocatalytic decolorization efficiency versus initial dye concentration. Dopant percentage, 3%; catalyst dosage, 1 g/L; and reaction time, 180 min.

67.0 to 18.2% with an increase in initial concentration from 2.5 to 12.5 mg/L, respectively. This behavior can be attributed to the fact that at high dye concentrations, the active sites on the surface of the photocatalyst were occupied by the dye molecules, causing a significant decrease in the decolorization efficiency. Thus, the amount of OH• required for the degradation of dye increases.29,45 Moreover, increasing dye concentration can reduce path length of the photons entering the solution for exciting the active sites of the photocatalyst, which inhibits the formation of OH• on the surface of the photocatalyst.46 On the other hand, the dye molecules absorb

S2 O82 − + hν → 2SO4•− SO4 F

•−

+ RH →

SO24 −

(10) •

+ R (Intermediates) + H

+

(11)



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SO4•− + R• (Intermediates) → SO4 2 − + CO2 + NO2 + Other inorganics

SO4

•−

+ H 2O → SO4

(12) 2−

•

+ OH + H

+

OH• + S2 O82 − → HSO4 − + SO4•− + SO4•− + OH• → HSO4 − +

1 O2 2

•

2OH → H 2O2 •

OH + H 2O2 → H 2O + HO2

(14) (15)

S2 O82 − + H 2O2 → 2H+ + 2SO4 2 − + O2

+ OH →

CO•− 3

+ H 2O

(22)

Compared to the results of the present investigation, in our previous work, we found that the presence of ethanol can lead to a significant decrease in the photocatalysis of Acid Red 14 using pure ZnO.55 3.5. Reusability of the Photocatalyst. The sequential application of the photocatalyst as well as maintenance of its photocatalytic activity is of critical concern for long-term use of the photocatalyst in full-scale applications because the photocatalytic activity of ZnO usually decreases as a result of photocorrosion within repeated experiments. The reusability test for 3% Dy-doped ZnO nanoparticles was conducted with AR17 concentration of 5 mg/L, photocatalyst dosage of 1 g/L, and reaction time of 180 min. Four consecutive experimental runs were performed to determine the loss in the decolorization efficiency after each run. As shown in Figure 9, a negligible

(17) (18)

3.4. Effect of the Presence of Radical Scavengers. Real wastewaters usually contain some ions and organic matter which can cause negative effect on the photocatalytic degradation of organic pollutants due to their radical scavenging properties. Therefore, in the present study, chloride, carbonate, and bicarbonate ions together with ethanol were used to investigate the effect of the presence of radical scavengers on the photocatalytic removal of AR17 in aqueous solutions. The initial concentration of AR17, catalyst dosage, and reaction time were constant at 5 mg/L, 1 g/L, and 180 min, respectively. It is clearly seen from Figure 8 that the

Figure 8. Varying decolorization efficiency in the presence of different radical scavengers. Radical scavenger concentration, 5 mg/L; dopant percentage, 3%; catalyst dosage, 1 g/L; and initial dye concentration, 5 mg/L.

Figure 9. Reusability of the Dy-doped ZnO nanoparticles within four consecutive experimental runs. Dopant percentage, 3%; catalyst dosage, 1 g/L; initial dye concentration, 5 mg/L; and reaction time, 180 min.

presence of chloride anions caused the highest negative effect on the decolorization efficiency. However, the presence of carbonate produced the lowest negative effect in comparison with the other radical scavengers. With the addition of chloride, ethanol, bicarbonate, and carbonate, decolorization efficiency was decreased from 57.0% to 21.1, 32.1, 38.5, and 43.0%, respectively. Therefore, the trend of inhibitory effect in the presence of different scavengers was as Cl− > C2H5OH > HCO3− > CO32−. The possible reactions representing the scavenging effect of the studied anions are given in eqs 19−21:51−53 Cl− + OH• → Cl• + OH−

(21)

•

2h+ + C2H5OH → CH3CHO + 2H+

(16) •

(20)

CO32 −

In addition, the active sites on the surface of the photocatalyst may be blocked by the anions, deactivating the photocatalysts toward the dye.51 The oxidation potential of the generated radical anions (Cl• and CO3•−) is less than that of the hydroxyl radicals. In addition, the positive holes (h+) produced during photocatalytic process can be scavenged by the ethanol as shown in eq 22:53,54

(13)

1 O2 2

HCO−3 + OH• → CO•− 3 + H 2O

decrease in the decolorization efficiency occurred after the fourth run. It was demonstrated that 3% Dy-doped ZnO nanoparticles can be an efficient photocatalyst for the degradation of organic dyes with high reusability potential. It has been confirmed that doping of the catalyst with a suitable dopant enhances stability and reusability of the applied catalyst.42

4. CONCLUSIONS A sonochemical method was used to synthesize dysprosiumdoped ZnO nanoparticles to conduct a photocatalytic process

(19) G



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for the degradation of a textile dye (C. I. Acid Red 17) as a model organic pollutant. The photocatalytic activity under visible light irradiation of Dy-doped ZnO nanoparticles was much greater than that of the undoped ZnO nanoparticles. The results showed that doped ZnO nanoparticles with 3% Dy had the highest efficiency for the decolorization of the colored solution. Among different catalyst and dye concentrations, catalyst concentration of 1 g/L and initial dye concentration of 5 mg/L caused maximum photocatalytic decolorization efficiency. Moreover, the presence of peroxydisulfate led to a significant increase in the decolorization efficiency, while the presence of different radical scavengers, including chloride, carbonate, bicarbonate, and ethanol, resulted in an obvious decrease in the decolorization efficiency. Chloride ions caused the highest negative effect on the photocatalysis of AR17. Finally, the reusability study was performed, and its results demonstrated the capability of the Dy-doped ZnO nanoparticles for use in several experimental cycles. Conclusively, ZnO nanoparticles doped with 3% Dy can be an efficient photocatalyst for the removal of organic dyes under visible light irradiation.

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ASSOCIATED CONTENT

S Supporting Information *

Information on the characterics of dye (Acid Red 17) used in this study (Table S1) and UV−vis diffuse reflectance spectra of the undoped ZnO and 3% Dy-doped ZnO samples (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected]. Tel.: +98 411 3393165. Fax: +98 411 3340191. *E-mail: [email protected]. Tel.: +82 53 810 1456. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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