Synthesis and Characterization of Dysprosium-Doped ZnONanoparticles for Photocatalysis of a Textile Dye under Visible LightIrradiationAlireza 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 ofChemistry, 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, inductivelycoupled plasma, Fourier transform infrared spectroscopy, UV−vis diffuse reflectance spectroscopy, and scanning electronmicroscopy analyses confirmed the successfully synthesis and nanometric diameter of the samples. Dy-doped ZnO nanoparticleswere used for photocatalytic decolorization of C. I. Acid Red 17 solution under visible light irradiation. Among different amountsof dopant agent, 3% Dy-doped ZnO nanoparticles indicated the highest decolorization. Decolorization efficiency increased from14.3 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/Lcaused an obvious decrease in decolorization efficiency. The addition of 0.1 mM peroxydisulfate (S2O8

2−) resulted in adecolorization efficiency of nearly 100% after irradiation for 180 min. The trend of inhibitory effect in the presence of differentradical scavengers was Cl− > C2H5OH > HCO3

− > CO32−.

1. INTRODUCTIONThe application of advanced oxidation processes (AOPs) hasbeen proposed as an efficient approach for the degradation ofvarious organic pollutants such as organic dyes in the aqueousphase. One of the most widely used AOPs for treating coloredwastewater is photocatalytic processes.1−4 Among variousphotocatalysts used in the photocatalytic processes, TiO2 andZnO are known to be good photocatalysts with highphotocatalytic activity.5−7 Compared to TiO2, ZnO as an n-type II−VI semiconductor has attracted more attention becauseof its large area-to-volume ratio, direct wide band gap (3.37eV), high photosensitivity, large excitation binding energy (60meV), long life span, and high chemical stability.1,5,8−10 Duringa photocatalytic process equipped with ZnO irradiated with UVlight, highly reactive hydroxyl radicals (OH•) are produced,promoting the degradation of target pollutants. Xiang et al.11

have quantitatively investigated and confirmed the hydroxylradical production on various semiconductor photocatalysts inaqueous solution by the photoluminescence (PL) technique.The formation of OH• via a photocatalytic process using ZnOis shown through eqs 1−3:2,12

ν+ → +− +hZnO ZnO (e h ) (1)

+ → ++ + •h H O H OH2 (2)

+ →+ − •h OH OH (3)

In the present study, nanosized ZnO was used because fineparticles possess a higher surface area which elevates the densityof active sites for the photocatalysis.13One of the major

disadvantages of pure ZnO nanoparticles is the fastrecombination rate of the photogenerated electron−holepairs.14 Therefore, the improvement of the photocatalyticactivity of ZnO nanoparticles and the photosensitivity towardvisible light irradiation using doping agents has attracted muchmore consideration.8,15 To improve the photocatalytic activityof 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 differentdopants, dysprosium (Dy) as a rare earth element has beenproposed as an efficient dopant for the ZnO nanoparticles;23

thus, in the present study, ZnO nanoparticles were synthesizedand doped with Dy. The route used for the fabrication ofnanoparticles controls their shape, size, and optical properties,which strongly affect the photocatalytic activity of nano-structured catalyst.23,24 ZnO nanoparticles have been synthe-sized through various methods, including sol−gel,25 chemicalprecipitation,10 microwave radiation,26,27 and hydrother-mal18,28−30 methods.However, these methods have some disadvantages such as

long reaction times and high temperature requirements for thesynthesis. 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 thesolution experience a chemical reaction as a result of theapplication of powerful ultrasonic radiation ranging from 20

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

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kHz to 10 MHz together with a physical interaction which isacoustic cavitation, involving the formation, growth, andimplosive collapse of bubbles in the solution.33−35 On thebasis of the advantages of sonochemical method, Dy-dopedZnO nanoparticles were synthesized through a sonochemicalmethod and used in a photocatalytic process equipped with a100 W visible light lamp. The efficacy of the photocatalyticprocess for treating colored solutions was assessed underdifferent operational conditions, including the effect of theamount of doping agent, dye concentration, and catalyst dosagealong with the effect of the presence of radical scavengers andperoxydisulfate ion. To evaluate the photocatalytic activity ofDy-doped ZnO nanoparticles, an azo dye (C. I. Acid Red 17(AR17)) was used as a model organic pollutant. To the best ofour knowledge and on the basis of the literature data, no studieshave investigated the application of sonochemically synthesizedDy-doped ZnO nanoparticles for the degradation of a textiledye in aqueous environments.

2. MATERIALS AND METHODS

2.1. Materials. All chemicals used in the presentinvestigation were purchased from Merck, Germany apartfrom DyN3O9·6H2O powder and C2H5OH·.4H2O (99%)solution, which were purchased from Aldrich, United States.AR17 was purchased from Shimi Boyakhsaz Co, Iran and usedwithout purification. The characteristics of the dye aredisplayed in Table S1 of Supporting Information. ZnCl2(99.5%) was used as the zinc precursor, and DyN3O9·6H2Owas chosen as dysprosium source.

2.2. Photocatalyst synthesis. To synthesize 1, 3, and 5%Dy-doped ZnO nanoparticles, a sonochemical method wasapplied. The approach for sonochemical synthesis of Dy-dopedZnO nanoparticles was as follows: Different amounts ofDyN3O9·6H2O was added to the solution of ZnCl2 to achieve1, 3, and 5% Dy. NaOH solution (1 M) was added dropwiseuntil the pH of the solution reached 10. The solution wassonicated for 3 h in a bath type sonicator (Sonica, 2200 EP S3,Italy) with 50−60 Hz frequency and a heating arrangement for

Figure 1. SEM images of undoped (a, b) and 3% Dy-doped ZnO nanoparticles (c, d) taken at different magnifications; diameter size distribution of3% Dy-doped ZnO nanoparticles (e).

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the sonochemical synthesis of Dy-doped ZnO nanoparticles.The resulting white precipitate was thoroughly washed withdeionized water followed by ethanol to remove impurities.Finally, it was dried in an oven at 80 °C for 12 h. The aboveprocedure was carried out without addition of DyN3O9·6H2Oto synthesize undoped ZnO nanoparticles. To comparecalcined and uncalcined synthesized Dy-doped ZnO nano-particles, the samples were calcined at 300 °C for 3 h in anelectric furnace.2.3. Experimental Procedure. A batch experimental

quartz photoreactor with a 100 mL working volume was usedto evaluate the photocatalytic activity of sonochemicallysynthesized Dy-doped ZnO nanoparticles for decolorizationof a textile dye (AR17) under visible light. A 100 W visiblelamp (Pars Co, Iran) was applied as the light source. Theintensity of the lamp with a distance of 4.5 cm from the surfaceof the solution was 25 W/m2. The radiation intensity wasmeasured with a UV−vis radiometer purchased from Cassy LabCompany (Germany). In a typical process, 0.1 g of thephotocatalyst was added into a 100 mL solution containingAR17 with an initial concentration of 5 mg/L. Then, thesuspension was magnetically stirred in the dark for 20 min toachieve adsorption−desorption equilibrium before beginningthe irradiation.2.4. Instrumentation and Analysis. To characterize the

structure of undoped and Dy-doped ZnO nanoparticles, aSiemens X-ray diffractometer (D5000, Germany) was used toproduce X-ray diffraction (XRD) patterns (Cu Kα radiation(1.54065 Å)) in which an accelerating voltage of 40 kV and anemission current of 30 mA were applied. Scanning electronmicroscopy (SEM) was applied to evaluate the surfacemorphology of the samples using a Hitachi microscope(Model S-4200, Japan). The SEM analysis was performedafter gold plating of the samples. Moreover, the obtained SEMimages were analyzed using manual microstructure distancemeasurement software (Nahamin Pardazan Asia Co., Iran) todetermine the diameter size distribution of the obtainedsamples. The model of the inductively coupled plasma (ICP)instrument used for detecting trace metals presence in thesynthesized samples was ICP GBC Integra XL (Australia). ForFourier transform infrared spectroscopy (FT-IR) analysis, theKBr pellets were prepared from the undoped and different mol% of Dy-doped ZnO powders. FT-IR analysis was performedusing a spectrophotometer (Tensor 27, Bruker, Germany).

Diffuse reflectance spectroscopy (DRS) spectra of the sampleswere recorded using a Scinco S4100 (South Korea)spectrophotometer. The initial pH was measured by aMetrohm pH meter (Model 654, Germany). At the end ofeach experiment, the samples were centrifuged for 5 min at6000 min−1 and the supernatant was withdrawn for analysis.The residual AR17 in the solution was measured spectrophoto-metrically (UV−vis spectrophotometer, WPA LightwaveS2000, England) at λmax of 510 nm. The decolorizationefficiency was calculated using eq 4:

= − ×C CDecolorization efficiency (%) [1 ( / )] 100o(4)

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

3. RESULTS AND DISCUSSION

3.1. Structural Analysis. Figure 1 shows the surfacemorphology of undoped and 3% Dy-doped ZnO nanoparticlestaken through SEM analysis. As can be seen in Figure 1a,b,undoped ZnO nanoparticles have nonuniform size, which maybe as a result of the aggregation of synthesized ZnOnanoparticles and the growth of irregular crystalline grainsduring synthesis. But, doping of ZnO nanoparticles by means of3% Dy caused an obvious decrease in the aggregation ofnanoparticles (Figure 1c,d). According to SEM images andusing Manual Microstructure Distance Measurement software(Nahamin Pardazan Asia Co., Iran), the mean particle size of3% Dy-doped ZnO sample was found to be about 38 nm.Figure 1e shows the diameter size distribution of 3% Dy-dopedZnO nanoparticles. As can be seen, the diameter distribution ofmost of the particles is in the range of 30−40 nm. The loweraggregation of nanoparticles is favorable for the photocatalysisof target pollutants because of the greater availability of activesites.To reach a better understanding of the structure and real

crystalline size of the undoped and Dy-doped ZnO nano-particles, XRD analysis was carried out; the results arerepresented in Figure 2. Additionally, as shown in Figure 2,the effect of calcination on the structure of the catalysts wasstudied via XRD. The intense sharp XRD peaks suggest theexcellent crystalline structure of synthesized undoped and Dy-doped ZnO nanoparticles under different conditions even

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

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without calcination (Figure 2). As illustrated in Figure 2, theincorporation of Dy into the structure of ZnO does not changethe 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° correspondto 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 relatedto the ZnO were still observed and no other peakscorresponding to Dy2O3, Zn(OH)2, or other impurities weredetected, which indicates that the Dy3+ ions are replaced by theZn2+ ions in ZnO structure.9 But, as can be seen, there is a shiftto low diffraction values for (100), (002), and (101) planes inthe 3% Dy-doped ZnO sample. The 2θ angles of the (100),(002), and (101) planes were 31.92, 34.60, and 36.48° for pureZnO and 30.96, 33.56, and 35.40° for 3% Dy-doped ZnO,respectively. This observation can be explained by theexpansion of ZnO lattice caused by the radius of Dy3+ (0.91Å) that is larger than that of Zn2+ (0.74 Å). The increase inlattice parameter and the shift to lower angle of the XRD peakswith doping of Dy were expected to have the influence on thelattice deformation and strain resulting from Dy dopant.31

Moreover, the calcination of the catalysts had no significanteffect on their crystalline nature. On the contrary, Yu and co-workers in their study showed that calcination temperatureproduces a great effect on the structure of TiO2 nanotubearrays.37 The effect of the presence of dopant on the crystallinesize of the synthesized undoped and 3% Dy-doped ZnOnanoparticles was evaluated using Debye−Sherrer’s equation asshown in eq 5:38

λ β θ=D 0.9 / cos (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 half-maximum (fwhm) intensity of the peak (rad), and thediffraction angle, respectively. According to eq 5, the averagecrystallite size of the undoped and Dy-doped ZnO nano-particles were about 12 and 14 nm, respectively. This indicatedthat the incorporation of Dy into the ZnO nanoparticles had nosignificant effect on the crystallite size of the nanoparticles. Inaccordance with our results, Wang et al., on the basis of theresults of XRD analysis, showed that doping of ZnOnanoparticles with Cd caused no effect on the structure ofthe photocatalyst.8

The DRS spectra of undoped and Dy-doped ZnO samplesare illustrated in Figure S1 of Supporting Information. It can beseen that the samples showed a strong photoabsorption in thevisible light range. There is a red shift in absorbance spectra ofDy-doped ZnO in comparison to that of undoped ZnO, asexpected for doped materials. This red shift can be related tothe formation of a shallow level inside the band gap because ofimpurity atoms (Dy3+) introduced into the wurtzite ZnOlattice.31,39 Another reason for this shift can be the narrow bandgap originating from the charge transfer between the ZnOvalence or conduction band and the Dy ion 4f level.20,40 Theenergy of the band gap of ZnO and 3% Dy-doped ZnOnanoparticles estimated from the main absorption edges of theDRS 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 avery powerful tool to detect and analyze trace and ultratraceelements. This technique can quantitatively measure theelemental content of a material from the ppt to the wt %

range.41 In the present work, the ICP analysis was performedafter careful washing of Dy-doped-ZnO nanoparticles toremove 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 percentin the mentioned samples were 0.67, 2.01, and 3.33% w/w,respectively. A systematic increase in the content of Dy isobserved with the increasing nominal concentration of thedopant in the samples. The ICP analysis results showed thatnearly all of the used Dy3+ ions were successfully incorporatedinto the structure of ZnO nanoparticles. The FT-IR spectra ofundoped and Dy-doped ZnO samples is shown in Figure 3. As

can be seen, there is an obvious band around 560 cm−1, whichcan be attributed to the ZnO stretching mode in the ZnOlattice. The broad peak around 3400 cm−1 corresponds to theOH group of H2O, indicationg the existence of water absorbedon the surface of the ZnO samples.42,43 Zn−O coordination hasbeen observed to shift slightly toward the lower wavenumbers(high energy) by Dy incorporation.

3.2. Effect of Operational Parameters on theDecolorization Efficiency. 3.2.1. Comparison of DifferentProcesses in the Decolorization of AR17. The efficiency ofdifferent processes was investigated in the decolorization of 5mg/L AR17 solution, and results are presented in Figure 4. Itcan be observed that the highest removal efficiency wasobtained using 3% Dy-doped ZnO catalyst under visible light.The results also showed that the removal of AR17 after 180min reaction time follows the decreasing order Vis/3% Dy-doped ZnO > Vis/1% Dy-doped ZnO > Vis/5% Dy-dopedZnO > Vis/TiO2 P25 > Vis/undoped ZnO > visible light only> ZnO in the dark. As is clear, the photolysis process withvisible light has a negligible decolorization efficiency comparedto photocatalytic processes. Also, the decolorization efficiencyof ZnO in the dark is less than 5% after 180 min, whichindicates that the value of dye removal by adsorption isinsignificant compared to photocatalysis. It should be notedthat in all other processes, the suspension of photocatalyst andAR17 was magnetically stirred in a quartz photoreactor in thedark for 20 min to establish an adsorption−desorption

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

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equilibrium of the dye. Then, the solution was irradiated by avisible lamp as the light source. The color removal efficiencywas expressed as the percentage ratio of decolorized dyeconcentration to that of the initial concentration (after 20 minin the dark). Therefore, the reported data are decolorizationefficiency of the photocatalytic process. In addition, thephotocatalytic activity of the synthesized catalysts wascompared with TiO2 P25 as a routine reference photocatalyst(Figure 4). As can be seen, the decolorization efficiency of Dy-doped ZnO photocatalysts with different dopant amounts isabout two times greater than that of TiO2 P25. This reveals thatthe synthesized Dy-doped ZnO samples can be used as anefficient 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 ofDy from 1 to 5%, while the concentration of the catalyst andAR17 were constant at 1 g/L and 5 mg/L, respectively. Overall,Dy-doped ZnO nanoparticles caused higher decolorizationefficiency compared to the undoped ZnO. A decolorization of24.4% was obtained as the undoped ZnO nanoparticles wereapplied. Under visible light irradiation, Dy acts as an electronscavenger reacting with the superoxide species and preventingthe recombination of produced electrons and holes during thephotocatalytic process.29 The transitions of 4f electrons oflanthanides lead to the implementation of the opticaladsorption of catalysts and support the separation of photo-generated electron−hole pairs. In the case of dysprosiumdopant, it can exist as Dy3+ and Dy4+. Thus, Dy3+ may give anelectron to O2 adsorbed on the surface of Dy-doped ZnO toform •O2

− by transforming into Dy4+, favoring a chargedmigration to O2 and an enhancement of the photoreaction ratein comparison with that of pure ZnO (eq 6). On the otherhand, the Dy4+ species may receive photogenerated electrons inthe conduction band of ZnO to form Dy3+ (eq 7). Thesereactions are the reason for enhanced photoactivity of ZnO.Also, Dy dopant can effectively slow the recombination rate ofthe photogenerated electron−hole pairs and enhance interfacialcharge-transfer efficiency. The process improves the photo-catalytic activity of ZnO in the same manner as the transfer of aphotogenerated electron from the conduction band to dorbitals of transition-metal-doped ZnO.31

+ → ++ + • −Dy O Dy O32

42 (6)

+ →+ − +Dy e Dy4 3(7)

+ →• − + •O H O H2 2 (8)

→ +•2 O H H O O2 2 2 2 (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 applicationof 3% Dy-doped nanoparticles led to the highest decolorizationefficiency (57.0%). The decolorization efficiency was increasedwith an increase in the amount of Dy up to 3% and thendecreased, suggesting that 3% Dy-doped ZnO nanoparticlescould be more efficient for separating photoinduced electron−hole pairs to enhance the photocatalytic decolorizationefficiency. It has been confirmed that the addition of a preciseamount of doping element can be critical for achieving highphotocatalytic activity.42 Increasing the amount of Dy withinthe structure of catalyst resulted in a higher surface barrier andnarrower space charge region, leading to efficient separation ofthe produced electron−hole pairs. Increasing the amount of Dyup to a specific value results in exceeding the space charge layerby increasing the penetration depth of visible light into ZnOnanoparticles. This makes the recombination of electron−holepairs easier, causing low photocatalytic decolorization effi-ciency. In addition, the excess amount of dopant covering thesurface of ZnO nanoparticles leads to a decrease in thephotocatalytic activity of the photocatalyst due to an increase inthe number of electron−hole recombination centers.29,43

According to the obtained results, 3% Dy-doped ZnOnanoparticles were used for performing the rest of theexperiments. In agreement with our findings, Yayapao et al.reported that 3% Ce-doped ZnO nanoneedles were the mosteffective photocatalyst for the decolorization of a solutioncontaining methylene blue.9 In the case of the effect ofirradiation time on the photocatalytic decolorization efficiency,as is obvious from Figure 4, the decolorization efficiency wasincreased as the irradiation time increased up to 180 min.Similar results have been reported by Suwanboon et al. in theirstudy on the photocatalytic decolorization of methylene blueover ZnO powders.10

3.2.2. Effect of Catalyst Dosage. To evaluate the effect ofcatalyst dosage on the decolorization efficiency, catalyst dosagewas varied between 2.5 and 12.5 g/L, and the results aredisplayed in Figure 5. In this set of experiments, reaction timeand initial dye concentration were constant at 180 min and 5mg/L, respectively. As can be seen in Figure 5, at catalystconcentrations of 0.25, 0.5, 0.75, 1, and 2 g/L, thedecolorization efficiency was 14.3, 22.5, 42.4, 57.0, and50.3%, respectively. Thus, decolorization efficiency increasedwith increasing catalyst dosage from 0.25 to 1 g/L and thendecreased. Similar behavior has been reported by Sobana andSwaminathan in their study on the photocatalytic decoloriza-tion of AR17 using ZnO.44 Increasing decolorization efficiencywith the increase in the amount of photocatalyst can beattributed to the increasing active surface area for thephotocatalytic degradation of organic dye. On the other hand,further increment in the amount of suspended photocatalyst ledto an increase in the turbidity of the solution and scatteringeffects, causing the decrease in UV light penetration. Thisreduces excitement of the photocatalyst for the generation of

Figure 4. Comparison of different processes in the decolorization ofAR17. Initial dye concentration, 5 mg/L; and catalyst dosage, 1 g/L.

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OH•. Additionally, the photocatalyst nanoparticles have atendency to aggregate at high concentrations, which reducesthe number of active sites.2,44 Because decolorization efficiencydecreased with the increase in catalyst dosage from 1 to 2 g/L,subsequent experiments were carried out with a catalyst dosageof 1 g/L.3.2.3. Effect of Initial Dye Concentration. One of the most

important parameters influencing photocatalytic activity is theinitial concentration of the target pollutant. In the presentwork, initial dye concentration was varied between 2.5 and 12.5mg/L to determine its effect on the decolorization efficiency(Figure 6). As shown, decolorization efficiency decreased from

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

most of the light entering the colored solution instead of thephotocatalyst, reducing photocatalytic activity of the catalyst.44

Increasing AR17 concentration from 2.5 to 5 mg/L resulted inan insignificant increment in decolorization efficiency, suggest-ing that an initial dye concentration of 5 mg/L can be selectedfor the rest of the experiments.

3.3. Effect of the Presence of Peroxydisulfate.Peroxydisulfate (S2O8

2−) is considered as a chemical oxidantfor degrading organic pollutants through direct chemicaloxidation.47 However, the chemical oxidation of organiccompounds by S2O8

2− is relatively low. Therefore, photolysisof S2O8

2− has been proposed as one of the efficient approachesfor the promotion of its oxidation potential.48 Accordingly, inthe present work, the effect of the presence of S2O8

2− on thephotocatalysis of AR17 was studied, and the results arerepresented in Figure 7. As shown, in the presence of Dy-

doped ZnO nanoparticles as catalyst, the addition of S2O82− led

to the increment of photocatalytic decolorization of AR17. AtS2O8

2−concentrations of 0.05 and 0.1 mM, the photocatalyticdecolorization efficiency was obtained to be 63.15 and 100%,respectively, which was higher than the efficiency of thephotocatalytic process without S2O8

2− (57.0%). Although theaddition of S2O8

2− led to a significant increment in thephotocatalytic decolorization efficiency, the addition of 0.1 mMS2O8

2− alone (without photocatalyst) caused a negligiblecontribution in the decolorization efficiency (about 12.0%).This implies that the photolysis of S2O8

2− to SO4•−, which has

been proposed to be an efficient method for accelerating thedegradation of target pollutants via S2O8

2−,48,49 is not efficientenough alone (without catalyst) to treat colored solutionscontaining AR17. Despite the fact that the presence of differentionic and organic substances can reduce the efficiency of thephotocatalytic process, the presence of S2O8

2− in a photo-reactor containing Dy-doped ZnO nanoparticles enhanced thephotocatalytic removal of AR17. The reactions involved in thephotocatalysis of the target organic dye in the presence ofS2O8

2−are summarized in eqs 10−18:48−50

ν+ →− •−hS O 2SO2 82

4 (10)

+ → + +•− − • +SO RH SO R (Intermediates) H4 42

(11)

Figure 5. Effect of the catalyst dosage on the photocatalysis of AR17.Dopant percentage, 3%; and initial dye concentration, 5 mg/L.

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

Figure 7. Effect of the presence of peroxydisulfate on thephotocatalysis of AR17. Dopant percentage, 3%; catalyst dosage, 1g/L; and reaction time, 180 min.

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+ → + +

+

•− • −SO R (Intermediates) SO CO NO

Other inorganics4 4

22 2

(12)

+ → + +•− − • +SO H O SO OH H4 2 42

(13)

+ → + +• − − •−OH S O HSO SO12

O2 82

4 4 2 (14)

+ → +•− • −SO OH HSO12

O4 4 2 (15)

→•2OH H O2 2 (16)

+ → +• •OH H O H O HO2 2 2 2 (17)

+ → + +− + −S O H O 2H 2SO O2 82

2 2 42

2 (18)

3.4. Effect of the Presence of Radical Scavengers. Realwastewaters usually contain some ions and organic matterwhich can cause negative effect on the photocatalyticdegradation of organic pollutants due to their radicalscavenging properties. Therefore, in the present study, chloride,carbonate, and bicarbonate ions together with ethanol wereused to investigate the effect of the presence of radicalscavengers on the photocatalytic removal of AR17 in aqueoussolutions. The initial concentration of AR17, catalyst dosage,and reaction time were constant at 5 mg/L, 1 g/L, and 180min, respectively. It is clearly seen from Figure 8 that the

presence of chloride anions caused the highest negative effecton the decolorization efficiency. However, the presence ofcarbonate produced the lowest negative effect in comparisonwith the other radical scavengers. With the addition of chloride,ethanol, bicarbonate, and carbonate, decolorization efficiencywas decreased from 57.0% to 21.1, 32.1, 38.5, and 43.0%,respectively. Therefore, the trend of inhibitory effect in thepresence of different scavengers was as Cl− > C2H5OH >HCO3

− > CO32−. The possible reactions representing the

scavenging effect of the studied anions are given in eqs19−21:51−53

+ → +− • • −Cl OH Cl OH (19)

+ → +− • •−HCO OH CO H O3 3 2 (20)

+ → +− • •−CO OH CO H O32

3 2 (21)

In addition, the active sites on the surface of the photocatalystmay be blocked by the anions, deactivating the photocatalyststoward the dye.51 The oxidation potential of the generatedradical anions (Cl• and CO3

•−) is less than that of the hydroxylradicals. In addition, the positive holes (h+) produced duringphotocatalytic process can be scavenged by the ethanol asshown in eq 22:53,54

+ → ++ +2h C H OH CH CHO 2H2 5 3 (22)

Compared to the results of the present investigation, in ourprevious work, we found that the presence of ethanol can leadto a significant decrease in the photocatalysis of Acid Red 14using pure ZnO.55

3.5. Reusability of the Photocatalyst. The sequentialapplication of the photocatalyst as well as maintenance of itsphotocatalytic activity is of critical concern for long-term use ofthe photocatalyst in full-scale applications because the photo-catalytic activity of ZnO usually decreases as a result ofphotocorrosion within repeated experiments. The reusabilitytest for 3% Dy-doped ZnO nanoparticles was conducted withAR17 concentration of 5 mg/L, photocatalyst dosage of 1 g/L,and reaction time of 180 min. Four consecutive experimentalruns were performed to determine the loss in the decolorizationefficiency after each run. As shown in Figure 9, a negligible

decrease in the decolorization efficiency occurred after thefourth run. It was demonstrated that 3% Dy-doped ZnOnanoparticles can be an efficient photocatalyst for thedegradation of organic dyes with high reusability potential. Ithas been confirmed that doping of the catalyst with a suitabledopant enhances stability and reusability of the appliedcatalyst.42

4. CONCLUSIONSA sonochemical method was used to synthesize dysprosium-doped ZnO nanoparticles to conduct a photocatalytic process

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

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

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for the degradation of a textile dye (C. I. Acid Red 17) as amodel organic pollutant. The photocatalytic activity undervisible light irradiation of Dy-doped ZnO nanoparticles wasmuch greater than that of the undoped ZnO nanoparticles. Theresults showed that doped ZnO nanoparticles with 3% Dy hadthe highest efficiency for the decolorization of the coloredsolution. Among different catalyst and dye concentrations,catalyst concentration of 1 g/L and initial dye concentration of5 mg/L caused maximum photocatalytic decolorizationefficiency. Moreover, the presence of peroxydisulfate led to asignificant increase in the decolorization efficiency, while thepresence of different radical scavengers, including chloride,carbonate, bicarbonate, and ethanol, resulted in an obviousdecrease in the decolorization efficiency. Chloride ions causedthe highest negative effect on the photocatalysis of AR17.Finally, the reusability study was performed, and its resultsdemonstrated the capability of the Dy-doped ZnO nano-particles for use in several experimental cycles. Conclusively,ZnO nanoparticles doped with 3% Dy can be an efficientphotocatalyst for the removal of organic dyes under visible lightirradiation.

■ ASSOCIATED CONTENT*S Supporting InformationInformation on the characterics of dye (Acid Red 17) used inthis study (Table S1) and UV−vis diffuse reflectance spectra ofthe undoped ZnO and 3% Dy-doped ZnO samples (Figure S1).This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: a_khataee@tabrizu.ac.ir, ar_khataee@yahoo.com.Tel.: +98 411 3393165. Fax: +98 411 3340191.*E-mail: swjoo@yu.ac.kr. Tel.: +82 53 810 1456.NotesThe authors declare no competing financial interest.

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

■ REFERENCES(1) Rezaee, A.; Masoumbeigi, H.; Soltani, R. D. C.; Khataee, A. R.;Hashemiyan, S. Photocatalytic decolorization of methylene blue usingimmobilized ZnO nanoparticles prepared by solution combustionmethod. Desalin. Water Treat. 2012, 44, 174−179.(2) Modirshahla, N.; Hassani, A.; Behnajady, M. A.; Rahbarfam, R.Effect of operational parameters on decolorization of Acid Yellow 23from wastewater by UV irradiation using ZnO and ZnO/SnO2photocatalysts. Desalination 2011, 271, 187−192.(3) Liu, Y.; Song, H.; Zhang, Q.; Chen, D. A Preliminary Study of thePreparation of a KBr-Doped ZnO Nanoparticle and Its PhotocatalyticPerformance on the Removal of Oil from Oily Sewage. Ind. Eng. Chem.Res. 2012, 51, 4779−4782.(4) Yatmaz, H. C.; Akyol, A.; Bayramoglu, M. Kinetics of thePhotocatalytic Decolorization of an Azo Reactive Dye in AqueousZnO Suspensions. Ind. Eng. Chem. Res. 2004, 43, 6035−6039.(5) Akyol, A.; Bayramoglu, M. Photocatalytic degradation of RemazolRed F3B using ZnO catalyst. J. Hazard. Mater. 2005, B124, 241−246.(6) Hsuan-Liang, Liu; Yang, T. C.-K. Photocatalytic inactivation ofEscherichia coli and Lactobacillus helveticus by ZnO and TiO activatedwith ultraviolet light. Process Biochem. 2003, 39, 475−481.

(7) Benkara, S.; Zerkout, S. Preparation and characterization of ZnOnanorods grown into aligned TiO2 nanotube array. J. Mater. Environ.Sci 2010, 1, 173−188.(8) Wang, Y.; Yang, Y.; Zhang, X.; Liu, X.; Nakamura, A. Opticalinvestigation on cadmium-doped zinc oxide nanoparticles synthesizedby using a sonochemical method. Cryst. Eng. Comm. 2012, 14, 240−245.(9) Yayapao, O.; Thongtem, S.; Phuruangrat, A.; Thongtem, T.Sonochemical synthesis, photocatalysis and photonic properties of 3%Ce-doped ZnO nanoneedles. Ceram. Int. 2013, 39 (Supplement 1),S563−S568.(10) Suwanboon, S.; Amornpitoksuk, P.; Muensit, N. Dependence ofphotocatalytic activity on structural and optical properties ofnanocrystalline ZnO powders. Ceram. Int. 2011, 37, 2247−2253.(11) Xiang, Q.; Yu, J.; Wong, P. K. Quantitative characterization ofhydroxyl radicals produced by various photocatalysts. J. ColloidInterface Sci. 2011, 357, 163−167.(12) Zhang, D.; Liu, X.; Wang, X. Growth and photocatalytic activityof ZnO nanosheets stabilized by Ag nanoparticles. J. Alloys Compd.2011, 509, 4972−4977.(13) Mekasuwandumrong, O.; Pawinrat, P.; Praserthdam, P.;Panpranot, J. Effects of synthesis conditions and annealing post-treatment on the photocatalytic activities of ZnO nanoparticles in thedegradation of methylene blue dye. Chem. Eng. J. 2010, 164, 77−84.(14) Anandan, S.; Vinu, A.; Mori, T.; Gokulakrishnan, N.; Srinivasu,P.; Murugesan, V.; Ariga, K. Photocatalytic degradation of 2,4,6-trichlorophenol using lanthanum doped ZnO in aqueous suspension.Catal. Commun. 2007, 8, 1377−1382.(15) Liu, S.; Li, C.; Yu, J.; Xiang, Q. Improved visible-lightphotocatalytic activity of porous carbon self-doped ZnO nanosheet-assembled flowers. Cryst. Eng. Comm. 2011, 13, 2533−2541.(16) Clament Sagaya Selvam, N.; Vijaya, J. J.; Kennedy, L. J. Effectsof Morphology and Zr Doping on Structural, Optical, and Photo-catalytic Properties of ZnO Nanostructures. Ind. Eng. Chem. Res. 2012,51, 16333−16345.(17) Wang, X.; Zhang, H.; Li, J.; Miao, L.; Yang, Y. Effect of Eudoping concentration on the morphologies and optical properties ofZnO film prepared by ultrasonic spray pyrolysis. J. Mater. Sci.: Mater.Electron. 2013, 24, 1883−1887.(18) Khataee, A. R.; Hosseini, M.; Hanifehpour, Y.; Safarpour, M.;Joo, S. W. Hydrothermal synthesis and characterization of Nd-dopedZnSe nanoparticles with enhanced visible light photocatalytic activity.Res. Chem. Intermed. 2012, 1−14.(19) Khatamian, M.; Khandar, A. A.; Divband, B.; Haghighi, M.;Ebrahimiasl, S. Heterogeneous photocatalytic degradation of 4-nitrophenol in aqueous suspension by Ln (La3+, Nd3+ or Sm3+)doped ZnO nanoparticles. J. Mol. Catal. A: Chem. 2012, 365, 120−127.(20) Sin, J.-C.; Lam, S.-M.; Lee, K.-T.; Mohamed, A. R. Preparationand photocatalytic properties of visible light-driven samarium-dopedZnO nanorods. Ceram. Int. 2013, 39, 5833−5843.(21) Sin, J.-C.; Lam, S.-M.; Lee, K.-T.; Mohamed, A. R. Photo-catalytic performance of novel samarium-doped spherical-like ZnOhierarchical nanostructures under visible light irradiation for 2,4-dichlorophenol degradation. J. Colloid Interface Sci. 2013, 401, 40−49.(22) Phuruangrat, A.; Kongnuanyai, S.; Thongtem, T.; Thongtem, S.Ultrasound-assisted synthesis, characterization and optical property of0−3 wt% Sn-doped ZnO. Mater. Lett. 2013, 91, 179−182.(23) Huang, H.; Ou, Y.; Xu, S.; Fang, G.; Li, M.; Zhao, X. Z.Properties of Dy-doped ZnO nanocrystalline thin films prepared bypulsed laser deposition. Appl. Surf. Sci. 2008, 254, 2013−2016.(24) Mishra, P.; Yadav, R. S.; Pandey, A. C. Growth mechanism andphotoluminescence property of flower-like ZnO nanostructuressynthesized by starch-assisted sonochemical method. Ultrason.Sonochem. 2010, 17, 560−565.(25) Bigdeli, F.; Morsali, A. Synthesis ZnO nanoparticles from a newZinc(II) coordination polymer precursor. Mater. Lett. 2010, 64, 4−5.(26) Thongtem, T.; Phuruangrat, A.; Thongtem, S. Characterizationof nanostructured ZnO produced by microwave irradiation. Ceram. Int.2010, 36, 257−262.

Industrial & Engineering Chemistry Research Article

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

(27) Phuruangrat, A.; Thongtem, T.; Thongtem, S. Microwave-assisted synthesis of ZnO nanostructure flowers. Mater. Lett. 2009, 63,1224−1226.(28) Ameen, S.; Shaheer Akhtar, M.; Shin, H. S. Growth andcharacterization of nanospikes decorated ZnO sheets and their solarcell application. Chem. Eng. J. 2012, 195−196, 307−313.(29) Khataee, A. R.; Hanifehpour, Y.; Safarpour, M.; Hosseini, M.;Joo, S. W. Synthesis and Characterization of ErxZn1−x Se Nano-particles: A Novel Visible Light Responsive Photocatalyst. Sci. Adv.Mater. 2013, 5, 1074−1082.(30) Yu, J.; Yu, X. Hydrothermal Synthesis and PhotocatalyticActivity of Zinc Oxide Hollow Spheres. Environ. Sci. Technol. 2008, 42,4902−4907.(31) Yayapao, O.; Thongtem, T.; Phuruangrat, A.; Thongtem, S.Sonochemical synthesis of Dy-doped ZnO nanostructures and theirphotocatalytic properties. J. Alloys Compd. 2013, 576, 72−79.(32) Oh, E.; Jung, S.-H.; Lee, K.-H.; Jeong, S.-H.; Yu, S.; Rhee, S. J.Vertically aligned Fe-doped ZnO nanorod arrays by ultrasonicirradiation and their photoluminescence properties. Mater. Lett.2008, 62, 3456−3458.(33) Aslani, A.; Bazmandegan-Shamili, A.; Kaviani, K. Sonochemicalsynthesis, characterization and optical analysis of some metal oxidenanoparticles (MO-NP; M=Ni, Zn and Mn). Phys. B (Amsterdam,Neth.) 2010, 405, 3972−3976.(34) Wahab, R.; Ansari, S. G.; Kim, Y.; Seo, H.-K.; Shin, H. Roomtemperature synthesis of needle-shaped ZnO nanorods via sonochem-ical method. Appl. Surf. Sci. 2007, 253, 7622−7626.(35) Jattukul, S.; Thongtem, S.; Thongtem, T. Morphologydevelopment of ZnO produced by sonothermal process. Ceram. Int.2011, 37, 2055−2059.(36) El-Kemary, M.; El-Shamy, H.; El-Mehasseb, I. Photocatalyticdegradation of ciprofloxacin drug in water using ZnO nanoparticles. J.Lumin. 2010, 130, 2327−2331.(37) Yu, J.; Wang, B. Effect of calcination temperature onmorphology and photoelectrochemical properties of anodized titaniumdioxide nanotube arrays. Appl. Catal. B: Environ. 2010, 94, 295−302.(38) Patterson, A. L. The Scherrer Formula for X-Ray Particle SizeDetermination. Phys. Rev. 1939, 56, 978−982.(39) Ivetíc, T. B.; Dimitrievska, M. R.; Guth, I. O.; -Dacanin, L. R.;Lukíc-Petrovi, S. R. Structural and optical properties of europium-doped zinc oxide nanopowders prepared by mechanochemical andcombustion reaction methods. J. Res. Phys. 2012, 36, 43−51.(40) Stengl, V.; Bakardjieva, S.; Murafa, N. Preparation andphotocatalytic activity of rare earth doped TiO2 nanoparticles. Mater.Chem. Phys. 2009, 114, 217−226.(41) Inductively Coupled Plasma Emission Spectroscopy. Part II:Applications and Fundamentals; Boumans, P. W. J. M., Ed.; JohnWiley & Sons: New York, 1987; Vol. 2.(42) Sanoop, P. K.; Anas, S.; Ananthakumar, S.; Gunasekar, V.;Saravanan, R.; Ponnusami, V., Synthesis of yttrium doped nanocrystal-line ZnO and its photocatalytic activity in methylene blue degradation.Arabian J. Chem. In press.(43) Yan, X.; He, J.; G. Evans, D.; Duan, X.; Zhu, Y. Preparation,characterization and photocatalytic activity of Si-doped and rare earth-doped TiO2 from mesoporous precursors. Appl. Catal. B: Environ.2005, 55, 243−252.(44) Sobana, N.; Swaminathan, M. The effect of operationalparameters on the photocatalytic degradation of acid red 18 byZnO. Sep. Purif. Technol. 2007, 56, 101−107.(45) Khataee, A. R.; Zarei, M.; Asl, S. K. Photocatalytic treatment of adye solution using immobilized TiO2 nanoparticles combined withphotoelectro-Fenton process: Optimization of operational parameters.J. Electroanal. Chem. 2010, 648, 143−150.(46) Zhu, H.; Jiang, R.; Fu, Y.; Guan, Y.; Yao, J.; Xiao, L.; Zeng, G.Effective photocatalytic decolorization of methyl orange utilizingTiO2/ZnO/chitosan nanocomposite films under simulated solarirradiation. Desalination 2012, 286, 41−48.(47) Soltani, R. D. C.; Rezaee, A.; Godini, H.; Khataee, A. R.;Hasanbeiki, A. Photoelectrochemical treatment of ammonium using

seawater as a natural supporting electrolyte. Chem. Ecol. 2013, 29, 72−85.(48) Khataee, A. R.; Mirzajani, O. UV/peroxydisulfate oxidation ofC.I. Basic Blue 3: Modeling of key factors by artificial neural network.Desalination 2010, 251, 64−69.(49) Thabet, M.; El-Zomrawy, A. A. Degradation of acid red 17 dyewith ammonium persulphate in acidic solution using photoelectroca-talytic methods. Arabian J. Chem. In press.(50) Salari, D.; Niaei, A.; Aber, S.; Rasoulifard, M. H. Thephotooxidative destruction of C.I. Basic Yellow 2 using UV/S2O8

2−

process in a rectangular continuous photoreactor. J. Hazard. Mater.2009, 166, 61−66.(51) Daneshvar, N.; Aber, S.; Seyed Dorraji, M. S.; Khataee, A. R.;Rasoulifard, M. H. Photocatalytic degradation of the insecticidediazinon in the presence of prepared nanocrystalline ZnO powdersunder irradiation of UV-C light. Sep. Purif. Technol. 2007, 58, 91−98.(52) Khataee, A. R. Photocatalytic removal of C.I. Basic Red 46 onimmobilized TiO2 nanoparticles: Artificial neural network modelling.Environ. Technol. 2009, 30, 1155−1168.(53) Pyne, S.; Sahoo, G. P.; Bhui, D. K.; Bar, H.; Sarkar, P.; Samanta,S.; Maity, A.; Misra, A. Enhanced photocatalytic activity of metalcoated ZnO nanowires. Spectrochim. Acta, Part A 2012, 93, 100−105.(54) Yu, J.; Dai, G.; Huang, B. Fabrication and Characterization ofVisible-Light-Driven Plasmonic Photocatalyst Ag/AgCl/TiO2 Nano-tube Arrays. J. Phys. Chem. C 2009, 113, 16394−16401.(55) Daneshvar, N.; Salari, D.; Khataee, A. R. Photocatalyticdegradation of azo dye acid red 14 in water on ZnO as an alternativecatalyst to TiO2. J. Photochem. Photobiol., A 2004, 162, 317−322.

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