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Science of Advanced Materials Vol. 5, pp. 1074–1082, 2013 (www.aspbs.com/sam)

Synthesis and Characterization of Erx Zn1−x Se Nanoparticles: A Novel Visible Light Responsive Photocatalyst A. R. Khataee1, ∗ , Y. Hanifehpour1, 2 , M. Safarpour1 , M. Hosseini1 , and S. W. Joo2, ∗ 1

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 5166614766, Tabriz, Iran 2 School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea

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ABSTRACT In this study, undoped and Erbium doped ZnSe (Erx Zn1−x Se) nanoparticles were prepared via a facile hydrothermal method at 150 C for 24 h. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS) and inductively coupled plasma (ICP) techniques. These analyses confirmed the nonometric diameter and significant optical absorption of as-synthesized samples at visible light range. The photocatalytic activity of Er-doped ZnSe nanoparticles was investigated by the decolorization of Orange II solution under visible light irradiation. The color removal efficiency of Er0.06 Zn0.94 Se and pure ZnSe was 95.1 and 28.7% after 120 min of treatment, respectively. The greatly enhanced photocatalytic activity of the Er-doped ZnSe photocatalyst was mainly attributed to the suppression of electron–hole recombination, large content of oxygen vacancies, and strong absorption of OH− ions on the surface of the catalyst due to the Er loading. In this study, 6 mol% was the most suitable content of Er3+ in ZnSe, at which the recombination of photoinduced electrons and holes could be effectively inhibited and thereby the highest photocatalytic activity was formed. The photocatalytic degradation of Orange II followed the Langmuir–Hinshelwood kinetic model. KEYWORDS: Nanostructured Catalyst, Semiconductors, Photocatalysis, Decolorization.

1. INTRODUCTION Recently, rapid progress in industrial activities has led to the discharge of large amount of wastewater containing wide range of contaminants, polluting the environment and consequently causing harm to human and other living organisms. Dye effluents generated from textile industries are one of the main pollutants in wastewater which create serious problems as they result in undesirable lasting color along with excessive COD loading to the water. To resolve this problem, besides other well-established methods, the technique based on semiconductor photocatalysts is a promising solution to degrade organic pollutants in wastewater.1–4 In this field, the semiconductor TiO2 has certainly proven to be one of the best photocatalysts for the oxidation and removal of many organic pollutants.5 Unfortunately, due to its wide band gap (Eg = 32 eV), TiO2 can only respond to ultraviolet irradiation, merely about 3% of ∗

Authors to whom correspondence should be addressed. Emails: [email protected], [email protected], [email protected] Received: 25 September 2012 Accepted: 26 November 2012

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the solar spectrum and cannot absorb visible light. On the other hand, it has been shown that the photocatalytic activity of TiO2 is limited by fast charge-carrier recombination and low interfacial charge-transfer rates of photogenerated carriers.6 In order to efficiently utilize solar energy, the development of new photocatalysts has attracted a great deal of attention. Two main approaches for this purpose are: doping of TiO2 by a non-metal or a transition metal N, S, and Fe to extend its photo-response to the visible region;7–10 and exploiting novel visible light responsive non-TiO2 photocatalysts. In recent years, II–VI group semiconductors have been extensively studied, owing to their unique catalytic activity in comparison with that of TiO2 11–13 Zinc selenide (ZnSe), an important II–VI semiconductor with a bulk band gap of 2.70 eV (460 nm) at room temperature, is a chemically inert, non-hygroscopic and highly pure product that is very effective in many optical applications due to its extremely low bulk losses, high resistance to thermal shock and stability in virtually all environments.14 15 As some researchers have reported, ZnSe is a promising and effective catalyst for photocatalytic degradation of organic pollutants. However, to the best of our knowledge, 1947-2935/2013/5/1074/009

doi:10.1166/sam.2013.1556

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Synthesis and Characterization of Erx Zn1−x Se Nanoparticles: A Novel Visible Light Responsive Photocatalyst

no further studies have been reported on the doping of ZnSe by other metals, especially lanthanides, to improve its photocatalytic efficiency. It should be noted that the rare earth-doped nanoparticles have attracted much attention owing to their high photocatalytic activity in the degradation of organic contaminants because of the suppression of electron–hole recombination, large content of oxygen vacancies, and strong absorption of OH− ions on the surface of the catalyst.16 17 To date, a variety of synthesis methods have been reported for ZnSe nanoparticles, such as thermal evaporation,18 chemical vapor deposition,19 electrochemical deposition20 and solvothermal reaction.21 22 Most of these synthesis processes generally require highly sophisticated equipment and toxic metal–organic precursors. In material science, one-pot direct synthesis of inorganic materials with specific morphology and orientation via simple route is very important. The hydrothermal technique is one of the most important methods for advanced materials processing, especially due to its advantages in the processing of nanostructural materials. The main advantages of hydrothermal method are generating highly crystalline products with high purity, narrow size distribution, and low aggregation. Moreover, the morphology and crystal form of the products can be controlled by adjusting the hydrothermal reaction conditions.14 23–27 In this study, a simple hydrothermal route has been introduced to the synthesis of Erbium-doped ZnSe (Erx Zn1−x Se) nanoparticles. The physical properties of the synthesized samples were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In order to verify the presence of Er in the doped samples, inductively coupled plasma (ICP) technique was used. Also, the photocatalytic activity of undoped and Er-doped ZnSe nanoparticles was investigated towards Orange II (as a model organic dye) decolorization under visible light irradiation.

agent. In a typical synthesis, 1 mmol Na2 SeO3 powder and 1 mmol NaOH and appropriate molar ratios of Er(CH3 COO)3 · H2 O and ZnSO4 · 7H2 O were first dissolved in 30 ml distilled water. Under middle speed stirring, hydrazine hydrate (N2 H4 · H2 O) was then added dropwise to the above solution. After being stirred uniformly, the resulting solution was moved into a 50 ml Teflon-lined stainless-steel autoclave, placed in an oven at 150 C for 24 h, and then the autoclave was allowed to cool to room temperature naturally. As-synthesized Erx Zn1−x Se nanoparticles were collected and washed with distilled water and absolute ethanol several times in order to remove residual impurities, and then vacuum-dried at 60 C for 5 h. The final yellow–green powders were obtained as a result.

2. MATERIALS AND METHODS

2.4. Photocatalytic Studies

2.1. Chemicals

The photocatalytic activity of undoped and Er-doped ZnSe nanoparticle was evaluated by the decolorization of Orange II in an aqueous solution under visible light. In a typical process, 0.1 g of the photocatalyst powder was added into 100 ml Orange II solution with an initial concentration of 5 mg/l. The suspension of photocatalyst and Orange II was magnetically stirred in a quartz photoreactor in the dark for 15 mins to establish an adsorption/desorption equilibrium of the dye. Then, the solution was irradiated by a 6 W fluorescent visible lamp (GK-140, China) as the light source. The color removal efficiency (CR (%)) was expressed as the percentage ratio of decolorized dye concentration to that of the initial one. During the photocatalytic process, 5 ml of the suspension was sampled at a desired times and

2.2. Synthesis of Er-Doped ZnSe Samples Er-doped ZnSe nanoparticles with variable Er contents (2–14% mol) have been prepared by hydrothermal method using hydrazine hydrate (N2 H4 · H2 O) as the reducing Sci. Adv. Mater., 5, 1074–1082, 2013

The crystal structure, mean crystal size and phase purity of the ZnSe and Er-doped ZnSe nanoparticles were determined using XRD measurements which were carried out at room temperature by using Siemens X-ray diffraction D5000 (Germany) (Cu k radiation (1.54065 Å)). The accelerating voltage of 40 kV and emission current of 30 mA were used. The Debye–Scherrer formula was employed to calculate the average crystalline size of the catalysts.28 DRS spectra for evaluation of photophysical properties were recorded on a Scinco S4100 (South Korea) spectrophotometer in the wavelength range of 200–1000 nm at room temperature. TEM images were recorded by a Cs-corrected high-resolution TEM (JEM2200FS, JEOL, Japan) operated at 200 kV. SEM analysis was carried out on a Hitachi SEM Model S-4200 (Japan) device after gold-plating of the samples. The electrical resistivity of samples was measured by the Four Probe Method. A small chip with 7 mm length and 1 mm thickness was used for this analysis. The model of ICP instrument used for detecting trace metals presence in the synthesized samples was ICP GBC Integra XL (Australia).

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All chemicals used in this study were of analytical grade and were used without further purification. ZnSO4 · 7H2 O (99.5%), N2 H4 · H2 O (99%), Na2 SeO3 (99%) and NaOH were obtained from Merck. Er(CH3 COO)3 · H2 O and ethanol (99%) were obtained from Aldrich. Orange II was purchased from Shimi Boyakhsaz Co. (Iran).

2.3. Characterization Methods


after centrifugation, the removal of color was evaluated by determining the absorbance of the solution at max = 490 nm by using UV-Vis spectrophotometer, Lightwave S2000 (England).

3. RESULTS AND DISCUSSION 3.1. Characteristics and Physical Properties of Synthesized Nanoparticles

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Figure 1 shows the powder X-ray diffraction (P-XRD) patterns of the undoped and Er-doped ZnSe samples. As can be seen, the Er-doped powders have the same cubic structure as ZnSe and that single phase ZnSe is retained at lower doping concentrations of Er3+ . All the peaks in the Figure 1 could be indexed to sphalerite (cubic) ZnSe with no traces of other byproducts and were close to the reported data (JCPDS 80-0021, a = 5618 Å).29 Beyond doping levels of x = 01 for Er3+ , additional unknown phases were observed. Also, the relatively sharp diffraction peaks express that the products are high crystalline.

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Estimating from the Debye–Scherrer formula,28 the average size of the nanoparticles was about 15 nm. The XRD patterns of Er-doped ZnSe nanoparticles are almost similar to that of ZnSe, suggesting that there is no change in the crystal structure upon Er loading. However, it should be noted that the Er-doped samples have a lower intense diffraction peaks than pure ZnSe. The morphologies of the products prepared by hydrothermal method at 150 C for 24 h were characterized by SEM analysis, and the results are shown in Figures 2 and 3. Figure 2 shows the SEM image of ZnSe nanoparticles. The diameter of these particles is around 15–50 nm. Doping of Er3+ into the structure of ZnSe does not change the morphology of ZnSe. The SEM image of Er010 Zn090 Se indicates nanoparticles in which the diameter of particles is about 20–50 nm (Fig. 3). The crystalline size of doped ZnSe nanoparticles was found to increase slightly with the increase of Er as dopant. This can be related to the atomic radius of Zn and Er elements. The atomic radius is 134 and 176 pm for Zn and Er, respectively. So, by substituting some of the Zn atoms

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Fig. 1.

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Powder X-ray diffraction pattern of Erx Zn1−x Se ((a) x = 00, (b) x = 006, (c) x = 01, (d) impure ZnSe) synthesized at 150 C and 24 h. Sci. Adv. Mater., 5, 1074–1082, 2013

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Fig. 2. 24 h.


SEM image of ZnSe nanoparticles synthesized at 150 C and

by Er atoms in the lattice of ZnSe, the unit cell dimension increases and subsequently the mean crystal size of particles increases. The fine structural details of the as-obtained products were investigated by using TEM technique. The TEM image and SAED pattern of Er010 Zn090 Se confirms

Fig. 4.

TEM images and SAED pattern of Er010 Zn090 Se nanoparticles synthesized at 150 C and 24 h.

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Fig. 3. SEM image of Er010 Zn090 Se nanoparticles synthesized at 150 C and 24 h.

the result of SEM and shows crystallinity of product as shown in Figure 4. The UV-Vis diffuse reflectance spectra were used for the evaluation of photophysical properties of the assynthesized nanoparticles. The DRS spectra of undoped and Er-doped ZnSe are illustrated in Figures 5(a) and (b). The reflectance characteristics of the Er-doped ZnSe sample were quite similar to that of the undoped sample. It can be seen that the two samples showed a strong photoabsorption at visible light range. There is a red shift in absorbance spectra of Er-doped ZnSe in comparison to ZnSe, as expected for doped materials. The energy of the band gap of ZnSe and Er-doped ZnSe nanoparticles estimated from the main absorption edges of the UV-Vis diffuse reflectance spectrum is 3.42 eV and 3.28 eV, respectively. As known, the optimum band gap plays a major role in the photocatalytic activities of semiconductors. In order to verify the presence of Er in the doped samples, they were analyzed by ICP technique. ICP analytical technique can be very powerful tool for detecting and analyzing trace and ultra-trace elements. This technique can quantitatively measure the elemental content of a material from the ppt to the wt% range. The only elements which cannot be measured by ICP methods are C, H, O, N and the halogens.30 In this study, ICP analysis was performed after carefully washing of Er doped-ZnSe nanoparticles in order to remove any physically adsorbed ions such as Erbium. The ICP results showed that the Er percent in the Er010 Zn090 Se and Er006 Zn094 Se samples was 5.33 and 3.15% w/w, respectively. The theoretical calculated values for Er percent in the mentioned samples were 5.35 and 3.22% w/w, respectively. A systematic increase in the content of Er is observed with the increasing nominal concentration of the dopant in the samples. The ICP analysis results showed that nearly all of the used Er3+ ions were successfully doped to the structure of ZnSe nanoparticles.


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Fig. 5.

(b)

UV-Vis diffuse reflectance spectra of the (a) ZnSe and (b) Er010 Zn090 Se samples.

The Four Probe Method was used for the measurement of electrical resistivity of samples. Small chip with 1 mm thickness and 7 mm length was used for this analysis. Figure 6(a) shows the electrical resistivity of pure ZnSe and Er-doped ZnSe nanoparticles at room temperature which is about 9 × 10−3 · m for ZnSe and 8 × 10−5 ·m for Er010 Zn090 Se, respectively. The temperature dependence of the electrical resistivity for Er010 Zn090 Se between 290–340 K is shown in which electrical resistivity decreases with temperature (Fig. 6(b)). As a result, the electrical conductivity of Er-doped ZnSe materials was higher than pure ZnSe at room temperature, and increased with temperature. (a)

(b)

Fig. 6. Electrical resistivity (a) and thermoelectrical resistivity (b) of Erx Zn1−x Se materials.

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3.2. Photocatalytic Decolorization of Orange II 3.2.1. Effect of Er3+ Content Orange II or C.I. Acid Orange 7 is a mono-azo acidic dye, which is soluble in water and widely used in dying, weaving, tanning and paper industries31 and therefore, it has wide environmental effects. The characteristics of Orange II are given in the Table I. In this study, the photocatalytic activity of the prepared Er-doped ZnSe nanoparticles with variable Er3+ contents (0–14% mol) was determined using the decolorization of Orange II aqueous solutions under visible light irradiation. The schematic illustration of the photocatalytic activity of synthesized nanoparticles is presented in Figure 7. In a typical decolorization process, 100 ml Orange II (5 mg/l) aqueous solution and 0.1 g of photocatalyst powder were mixed in a quartz photoreactor. It is clearly seen from Figure 8(a) that the color removal efficiency of Er-doped ZnSe nanocatalyst is much higher than that of pure ZnSe. The results demonstrated the good photocatalytic ability of these nanoparticles under visible light. As can be seen, the decolorization efficiency is 28.7 and 95.1% after 120 mins of treatment for ZnSe and Er006 Zn094 Se samples, respectively. This behavior can be related to the absorbance properties of these particles. As known, the optimum band gap plays a major role in the photocatalytic activity of semiconductors. According to the DRS spectra of ZnSe and Er010 Zn090 Se samples (Fig. 5), the maximum part of the absorbance band shifts toward higher wavelengths by Er loading. The threshold value of semiconductor photo-absorption (g has a relation with the band gap energy (Eg ; the relation formula is Eg (eV) = 1240/g (nm). It is obvious that the larger g leads to a wider band gap, and this makes photo-generated holes to have a stronger reductive electric potential, which leads to an increase of the photocatalytic efficiency. In order to investigate the photocatalytic activity of Er-doped ZnSe nanoparticles with different Er contents, several experiments were carried out using these photocatalysts. Figure 8(a) shows the effect of Er3+ dopant concentration (0–14% mol) on the Orange II decolorization at Sci. Adv. Mater., 5, 1074–1082, 2013

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Table I. The characteristics of Orange II. Color index name

Chemical structure

C.I. Acid Orange 7

Molecular formula

Color index number

max (nm)

Mw (g/mol)

C16 H11 N2 NaO4 S

15510

490

350.32

2+ + O2 (ads) Er3+ + O− 2 (ads) → Er

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By the increase in Er3+ concentration, the surface barrier becomes higher, the space charge region becomes narrower, and hence the electron–hole pairs are efficiently separated by the large electric field. On the other hand, by increasing the concentration of Er3+ , the penetration depth of light into ZnSe can greatly exceed the space charge layer. Therefore, the recombination of photo-generated electron–hole pairs becomes easier, which led to a lower photocatalytic activity of ZnSe for Orange II decolorization. So, an optimum concentration of Er3+ is necessary to match the thickness of charge layer and the depth of the light penetration for separating photo-induced electron–hole pairs. Moreover, suitable loading of Er3+ content is very important for producing a significant potential difference between the surface and the center

Fig. 7.

The schematic illustration of the photocatalysis.

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of the particles to effectively separate the photo-induced electron–hole pairs. The excess amount of Er2 Se3 can cover the surface of ZnSe and cause to increase the number of recombination centers which leads to decrease the photocatalytic activity.16 32–34 3.2.2. Effect of Photocatalyst Loading The effect of photocatalyst loading on the decolorization efficiency of Orange II has been investigated using different concentrations of Er006 Zn094 Se photocatalyst varying from 0.25 to 1.5 g/l and the results are presented in Figure 9(a). As can be seen, the decolorization efficiency increased with an increase in the amount of photocatalyst up to 1.0 g/l and after that decreased. It is clear that the decolorization rate increases with an increase of the concentration of photocatalyst up to 1.0 g/l and above that no improvement is obtained (Fig. 9(b)). It may be due to the reason that when the amount of photocatalyst is increased the number of photons adsorbed and the number of dye molecules adsorbed are increased owing to an increase in the number of Er-doped ZnSe particles. The decrease in (a)

(b)

Fig. 8. (a) The effect of Er3+ dopant concentration on the decolorization of 5 mg/l Orange II (catalyst loading 1.0 g/l); (b) Linear relationship of dye decolorization during the photocatalytic reaction at different Er3+ contents by the pseudo first-order model.

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constant conditions. The photocatalytic efficiency of catalysts increases with increase in the Er loading up to 6% and then decreases. So, it is revealed that the optimum content of Er is 6%, which may be more efficient for separating photo-induced electron–hole pairs and for enhancing the photocatalytic activity. Figure 8(b) shows the linear relationship of Orange II decolorization during the photocatalytic reaction at different Er3+ contents by the pseudo first-order model. As can be seen, the photocatalytic reaction rate increases with increasing the Er3+ content up to 6% and then decreases. The reason for the high activity of 6% Er-doped ZnSe and the effect of Er loading on the photocatalytic activity can be explained by the following mechanism. Under the irradiation of Er-doped ZnSe nanoparticles, Er3+ ion acts as an electron scavenger, which may react with the superoxide species and prevent the holes–electrons (h+ /e− recombination, and thus increases photo-oxidation efficiency. The related reaction can be shown as follow:


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Fig. 9. (a) The effect of photocatalyst loading on the decolorization of 5 mg/l Orange II by the Er006 Zn094 Se; (b) Linear relationship of dye decolorization during the photocatalytic reaction at different photocatalyst concentration by the pseudo first-order model.

decolorization efficiency above 1.0 g/l photocatalyst loading may be due to an increasing turbidity of the suspension and to an enhancement of the light reflectance, because of the excess of Er-doped ZnSe nanoparticles.35 On the other hand, as the amount of particles in the solution increases, particle–particle interaction becomes significant; thus, the site density for surface holes and electrons reduces, due to the deactivation of activated molecules by collision with ground state Er-doped ZnSe particles. Another reason for decrease in the color removal efficiency at high photocatalyst loadings can be the aggregation of photocatalyst particles at high concentrations which causes a decrease in the number of surface active sites.36 3.2.3. Effect of Orange II Concentration The pollutant concentration is an important parameter in the advanced oxidation processes. Figure 10 shows the decolorization of Orange II solution over the concentration range of 2.5–12.5 mg/l. There is nearly complete decolorization (98.12%) of 2.5 mg/l Orange II under visible radiation at catalyst loading of 1.0 g/l and Er3+ content of 6 mol%. As can be seen in Figure 10(a), with increasing the initial concentration of dye, the color removal efficiency decreases. This behavior can be related to the several factors. At high dye concentration, the adsorbed dye molecules may occupy all the active sites of photocatalyst surface and cause to a decrease in decolorization efficiency. In other words, by the increasing of dye concentration, more and more molecules of the dye get adsorbed on 1080

Fig. 10. (a) The effect of Orange II concentration on the decolorization of 5 mg/l Orange II (Er006 Zn094 Se loading 1.0 g/l); (b) Linear relationship of dye decolorization during the photocatalytic reaction at different Orange II concentration by the pseudo first-order model.

the surface of the photocatalyst. Therefore, the requirement for the reactive oxygen species (• OH and • O–2 in order to degrade the dye increases. However, the formation of • OH and • O–2 on the catalyst surface remains constant for a given constant conditions such as light intensity, catalyst amount and irradiation time. Therefore, there is no enough reactive oxygen species for the degradation of the dye at high concentrations. Consequently, the decolorization efficiency of the solution decreases as the concentration of dye increases.37 38 Moreover, the increasing of pollutant concentration can lead to the production of intermediates, which may adsorb on the surface of the catalyst. Slow diffusion of these intermediates from the catalyst surface can deactivate the active sites of the photocatalyst and consequently reduce the degradation efficiency. Another reason for lower color removal at higher dye concentrations may be the absorption of light photon by the dye itself that leads to a lesser availability of photons for hydroxyl radical production.39 The concentration of Orange II has significant effect on the decolorization rates (Fig. 10(b)) and the rate constant is lower for higher initial concentration as the order decreases and therefore the Langmuir–Hinshelwood model can be proposed.40 41 3.2.4. Photocatalytic Decolorization Kinetic of Orange II The rate-determining step of the photo-catalyzed reaction is considered to be the reaction between the reactive oxygen species and organic molecules over the photocatalyst Sci. Adv. Mater., 5, 1074–1082, 2013

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Fig. 11. Determination of the adsorption equilibrium constant, K, and the second order rate constant, k for the Langmuir–Hinshelwood model.

KC = kobs C 1 + KC 0 1 C 1 + 0 = kobs kK k

r =k

(2)

Acknowledgments: The authors thank the University of Tabriz, Iran for all the supports provided. This work is supported by the Grant 2011-0014246 of the National Research Foundation of South Korea.

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References and Notes

where C0 is the initial concentration of Orange II (mg/l), K is the Langmuir–Hinshelwood adsorption equilibrium constant ((mg/l)−1 , k is the second order rate constant of surface reaction (mg/l min−1 and kobs is the pseudo-first-order rate constant (min−1 . Decolorization experiments of Orange II by visible light radiation in the presence of Er-doped ZnSe exhibited pseudo-first-order kinetics with respect to the concentration of the dye (Eqs. (4) and (5)). dC = kobs C0 dt C ln 0 = kobs t C

−

(4) (5)

Therefore, the amounts of kobserved (kobs for each initial concentration were achieved from the slopes of a straight line obtained by plotting ln (C0 /C) versus reaction time (Fig. 9(b)). When 1/kobs were plotted versus initial concentrations of dye, the second order kinetic rate constant (k) and the adsorption equilibrium constant (K) were calculated to be 0.064 mg/l min−1 and 2.096 (mg/l)−1 , respectively (Fig. 11).

4. CONCLUSION In the present study, Erx Zn1−x Se (x = 000–0.14) nanoparticles, as a visible-light responsive photocatalyst, Sci. Adv. Mater., 5, 1074–1082, 2013

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surface. Two different types of surface sites are supposed to be involved in the adsorption processes of the reacting species on the catalyst. The first can adsorb Orange II molecules and their degradation products. The second can adsorb oxygen. In this assumption the reaction rate for second order surface decomposition of Orange II may be written in terms of Langmuir– Hinshelwood kinetics.40 41 The Langmuir–Hinshelwood model can be written as follows:

were successfully synthesized through a simple one-step hydrothermal route at basic media. SEM and TEM images showed that the doping of Er3+ ions into ZnSe structure did not change the morphology of ZnSe nanoparticles. The electrical conductivity of Er-doped ZnSe was higher than that of pure ZnSe and increased with temperature. The photocatalytic activity of synthesized undoped and Er-doped ZnSe nanoparticles was investigated by the decolorization of Orange II solution under visible light irradiation. The decolorization efficiency was 28.7 and 95.1% at 120 min of treatment for ZnSe and Er006 Zn094 Se samples, respectively. The results indicated that the decolorization efficiency of Orange II was clearly affected by the content of Er3+ dopant in the ZnSe, catalyst concentration in the solution and initial concentration of Orange II. The photocatalytic decolorization process of Orange II could be explained in terms of the Langmuir–Hinshelwood kinetic model. The adsorption equilibrium constant, K, and the second order kinetic rate constant, k, values were 2.096 (mg/l)−1 and 0.064 mg/l min−1 , respectively.


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