Surfaces and Interfaces 6 (2017) 40–49
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Photo catalytic degradation of Alizarin red S using ZnS and cadmium doped ZnS nanoparticles under unfiltered sunlight Uzma Jabeen a, Syed Mujtaba Shah a,∗, Sajid Ullah Khan b a b
Department Of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan Department of Materials Science and Engg. Institute of Space Technology Islamabad Highway, Islamabad 44000, Pakistan
a r t i c l e
i n f o
Article history: Received 18 April 2016 Revised 26 October 2016 Accepted 3 November 2016 Available online 18 November 2016 Keywords: ZnS Cd-ZnS Doping Photo catalytic activity Kinetics Alizarin red S Sun light
a b s t r a c t Zinc sulphide (ZnS) and cadmium doped ZnS (Cd-ZnS) nanoparticles were prepared by co-precipitation method and characterized by UV–Visible spectroscopy, X-ray diffraction (XRD) studies, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM). The optical band gaps of the synthesized materials were calculated from UV–Visible absorption spectra using Tauc plots. The band gap of ZnS was decreased from 3.4 eV to 2.44 eV due to substitution of Cd2+ ions in ZnS lattice at 0.5 M cadmium content. This variation in the optical bandgap effectively monitored degradation of the dye. Photo catalytic degradation of Alizarin red S (ARS) by the nanoparticles showed that the cadmium doped ZnS acted as a potential photo catalyst under unfiltered natural sunlight of irradiation 300 W/m2 . The ARS dye was degraded about 50% and 96.7% in the presence of ZnS and Cd-ZnS (Cd 0.5 M) nanoparticles respectively in 120 min. Furthermore the effect of various parameters, i.e., photocatalyst concentration, dye concentration, and pH of the solution on the percentage of degradation was also studied. Degradation followed first order kinetics. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dyes constitute a major class of organic compounds, which find numerous applications in our daily life such as in leather, paper, plastics, cosmetics, clothing, drugs, electronics, and printing. Modern textile industries consume about 80% of the synthetic dyes. Synthetic material dyes and other industrial dyestuffs are the major groups of water pollutants in the world [1]. Approximately 1–15% of the synthetic textile dyes used in manufacturing process, are lost in wastewater streams and finally settled into water bodies [2,3]. Industrial wastes contain different chemicals particularly synthetic dyes which are oncogenic in nature [4]. Dyeing also produces effluents that contain 10–15% of the dye [5]. So, the critical issue is to reduce the toxicity levels to permissible limits before releasing the dye to aquatic bodies. Various treatment methods like aerobic, and anaerobic biological treatments, flocculation, neutralization of acidic and alkaline effluents, advanced oxidation processes [6] like ultraviolet (UV) photolysis, UV/H2 O2 process, UV/O3 process, UV/Fenton process, and photo catalytic processes have gained significance due to their effective decontamination efficiencies. The textile industry produces large quantity of highly colored
∗
Corresponding author. Fax: 0092 51 90642241. E-mail address:
[email protected] (S.M. Shah).
http://dx.doi.org/10.1016/j.surfin.2016.11.002 2468-0230/© 2016 Elsevier B.V. All rights reserved.
effluents which decrease light penetration and prevent photosynthesis [7]. Alizarin red-S dye is one of them which produces ‘red and’ purple colored solution depending on the pH of water. Photocatalytic degradation by semiconductors is a new, effective and rapid technique for removal of pollutant from water. Nanophotocatalyst are non-toxic, non-corrosive, inexpensive and chemically and thermally stable [8,9]. Semiconducting nanoparticles engaged in photo conversion systems present a temperately wide energy gap between the conduction band (CB) and the valence band (VB). This separation is known as the band-gap energy (Egap). The absorption of energy by a semiconductor results in electron transfer from the valence band to the conduction band and leave vacancies in the valence band termed as holes. The photo generated electron-hole pair encourages the reduction and oxidation of species adsorbed at the surface of the semiconducting nanoparticles and encourages oxidative degradation of species in solution through radical reactions [10,11]. Various binary, ternary and modified semiconductors have been successfully used for removal of a number of organic pollutants. ZnS nanoparticles could be used as good photo catalysts due to rapid generation of the electron-hole pairs by photo-excitation and highly negative reduction potentials of the excited electrons; as conduction band position of ZnS in aqueous solution is higher than that of other semiconductors such as TiO2 and ZnO [12]. Sharma et al. [13] examined photo degradation of Bromophenol blue,
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crystal violet and reactive red dyes using ZnS nanoparticles after 3.0 h of irradiation. Zinc sulphide semiconductor as a photo catalyst was used [14] for the removal of rose bengal dye. Warrier et al. [15] have reported that CdS and CdSe nanoparticles could act as very efficient and highly chemo selective photo catalysts for the reduction of aromatic azides to aromatic amines. Cerium iron oxide synthesized by Ameta et al. [16] by using co-precipitation method and specific heating cycles was utilized for the photo catalytic degradation of Alizarin red dye. Decolorization of textile industry waste water by photo catalytic degradation process was reported by Hachem et al. [17] whereas semiconductor mediated photo catalyzed degradation of an anthraquinonoid dye Remazol Brilliant Blue was reported by Saquib et al. [18]. CdS nanoparticles were used by Zang et al. [19] for photo catalytic reduction of Methyl Yellow in reverse micelles. Takizawa studied N-dealkylation on Rhodamine B and Crystal Violet on CdS semiconductor as photo catalyst [20]. Nasr et al. [21] reported photo catalytic reduction of azo dyes like Naphthol Blue Black and Disperse Blue 79 while photo catalytic degradation of textile azo dye Acid Orange 7 was studied by Vinodgopal et al. [22]. Sesha et al. [23] made CdS/TiO2 nano composite materials for photo catalysis in visible light. Recently Cu-doped ZnS has been used as a photocatalyst for the degradation of methylene blue [24] and the doped ZnS was found more effective in photo degradation than undoped ZnS. Literature study revealed that both wide and low band-gap semiconducting nanoparticles can cause the photo-catalytic degradation of various dyes absorbing in the ultraviolet and visible region. The wide and low band gap semiconductors can be combined to extend their light capturing potential and regulate their band gap for monitoring photocatalytic degradation effectively. This report is an endeavor to achieve the above target by decreasing band gap of zinc sulphide nanoparticles by cadmium substitution in ZnS lattice and study its potential for the photocatalytic degradation of Alizarin red dye. To the best of our knowledge this is the first time that such an appreciable decrease in bandgap and increase in the photocatalytic degradation potential of ZnS has been recorded using direct and unfiltered sunlight instead of simulated light. Mechanism of photo catalysis: When a photo catalyst is exposed to light, electrons on the surface of catalyst are excited from valance band into conduction band. This leaves positive holes in the valance band which reacts with water and produce radicals that can degrade dye [11] The mechanism of the photo catalytic degradation is as under.
Catalyst +hυ → e− cb +h+ vb H2 O + h+ vb → OH.+ H+ O2 + e− cb → O−2 O−2 + H+ → HO2 . 2HO. 2 → H2 O2 + O2 H2 O2 → 2 OH− OH− + Dye → CO2 + H2 O To explore possible photocatalytic application of ZnS and CdZnS nanoparticles, the catalytic degradation of ARS is carried out in the presence of sunlight, as it is a water-soluble dye and is used extensively as a coloring agent for fibers, leather, etc. [25–27]. The chemical structure of ARS is shown in Scheme 1.
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Scheme 1. The chemical structure of Alizarin Red S.
2. Experimental 2.1. Synthesis of ZnS nanoparticles Wet chemical method was adopted for the synthesis of ZnS nanoparticles [28]. The synthesis was carried out in a mixture of analytical grade (Sigma Aldrich) ethanol and distilled water both taken in equal proportions. Zinc acetate (Zn (CH3COO) 2 •2H2O) and sodium sulfide nanohydrate (Na2 S•9H2O) were used as starting materials. 0.5 M of zinc acetate was dissolved in 50 ml of distilled water. In the next step 0.5 M solution of Na2 S•9H2 O was added dropwise to the solution of zinc acetate while stirring the mixture continuously at 60 °C until a homogenous solution was obtained. The solution was cooled to room temperature. After 50 min, a white precipitate of ZnS was obtained. The precipitate was carefully settled down and washed thrice with a mixture of ethanol and distilled water. It was dried in oven at 120 °C for 2 h. 2.2. Synthesis of Zn1-x Cdx S nanoparticles In a typical experiment, the synthesis of Zn1-x Cdx S nanoparticles was carried out in two steps. In the first step, 1 M solution of Zn (CH3 COO) 2 •2H2 O and 0.5 M solution of cadmium acetate were dissolved in 20 ml of distilled water separately. These solutions were mixed and the resulting mixture was stirred for 30 min. In second step, 1 M solution of sodium sulfide in the same solvent was slowly added to above mixed solution drop wise. The mixture was stirred vigorously for 1 h at 60 °C. The precipitate was separated by centrifugation at 60 0 0 rpm for 10 min and washed thrice with distilled water and freshly distilled ethanol. Therafter the doped ZnS nanoparticles were dried in oven at 120 °C for 2 h. 2.3. Degradation of Alizarin red S (ARS) Alizarin Red S (ARS), a water-soluble dye was tested for its degradation by ZnS and cadmium doped ZnS semiconducting nanoparticles. The degradation of ARS was carried out in the presence of unfiltered sunlight. A 30 mg sample of nanoparticles was dispersed in a 30 ml of distilled water under ultrasound irradiation. Then the solution was mixed with 30 ml of 5 × 10−5 M ARS solution. The solution was stirred in dark at room temperature for 1 h to make the absorption/desorption between ARS and catalysts to reach the equilibrium state. Then, the solution was stirred by the magnetic stirrer in the presence of sunlight.2 ml of the of the reaction mixture was periodically withdrawn after every 30 min to check the progress of photocatalytic degradation of ARS dye by recording UV–Visible absorption spectra. The decrease of intensity of absorbance of dye after irradiation at definite time intervals gave the efficiency of photocatalytic degradation of nanoparticles. This efficiency was calculated as:
%D = 100 × [(Ao-At )/Ao] Where Ao and At are the values of initial absorbance and absorbance at time t respectively. ‘t’ is the irradiation time of sample.
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Fig. 1. UV–Visible absorption spectra of (1) ZnS and (2) Cd-ZnS nanoparticles. Table 1 Optical band gap of ZnS and cadmium doped ZnS nanoparticles. Samples
Absorption band edge
Band gap (eV)
Red shift (eV) with respect to ZnS
ZnS Cd-ZnS (Cd 0. 5 M)
316 395
3.4 2. 44
_ 0.96
3. Results and discussion 3.1. UV–Visible measurements The optical absorption spectra of ZnS and cadmium doped ZnS (Cd 0.5 M) Nanoparticles are shown in Fig. 1. The characteristic broad absorption bands of ZnS nanoparticles can be seen at 316 nm. The absorption peaks are highly blue shifted with respect to the bulk ZnS (345 nm). This shifting could be associated to the particle size and quantum confinement effect [29]. The absorption edge of cadmium doped ZnS nanoparticles show a red shift with respect to pure ZnS. This could be due to the incomplete seepage of the electronic wave function of the zinc sulphide into the cadmium sulphide. The optical band gap of ZnS reduces from 3.4 to 2.44 by doping it with cadmium (0.5 M cadmium content). The absorption edge, band gap and red shift values for different samples are shown in Table 1. To signify the band gap tuning, Tauc Plots were plotted for both samples based on their UV–visible spectra. These plots are depicted in Fig. 2. Data from the above plots has been summarized in the following table.
3.2. Scanning and high resolution transmission electron microscopy measurements Fig. 3A and B shows the SEM micrographs of ZnS and Cd-ZnS respectively. It is apparent from Fig. 3 that doping does not affect the shapes and external morphologies of the particles. Due to extensive aggregation, effect on size of the nanoparticles cannot be deduced from these images. The composition of cadmium doped ZnS nanoparticles was confirmed by the EDX analysis of the samples. The EDX pattern of Cd-ZnS is shown in Fig. 4.The EDX spectrum reveals the presence of Zn, Cd and S peaks confirming the formation of Cd-ZnS but some additional peaks of oxygen are also present. The presence of oxygen atom in the nanostructure could be due to the distilled
Fig. 2. Tauc plots for (A) ZnS (B) Cd-ZnS nanoparticles showing their optical band gaps.
Table 2 Elemental composition of cadmium doped ZnS nanoparticles. Element
Weight%
Atomic%
OK SK Zn K Cd L Total
54.54 18.56 16.79 10.10 100
57.289 15.03 16.67 10.41 100
water used in the synthesis. The mass percent ratio of Zn:Cd is 16.79:10.10. Detailed composition of cadmium doped ZnS nanoparticles has been summarized in Table 2 in terms of weight and atomic percentages respectively. HRTEM images of ZnS and Cd-doped ZnS nanoparticles are shown in Fig. 5. TEM micrographs of synthesized un-doped and Cd-doped ZnS nanoparticles clearly show the formation of nanosize particles with the average particle size around 10 nm. Agglomeration of nanoparticles is noticed here also.
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Fig. 3. Scanning electron micrographs of (A) ZnS nanoparticles, (B) Cd-ZnS nanoparticles.
3.3. X-ray diffraction measurements
Fig. 4. The EDX Spectrum of cadmium doped ZnS nanoparticles.
Fig. 6 shows the XRD patterns of the ZnS and cadmium doped ZnS nanoparticles. The difference in radius between Zn2+ (74 pm) and Cd2+ (95 pm) is about 20%. When a small amount of Zn2+ is replaced by Cd2+ the crystal structure of Cd doped ZnS NPs changes. XRD peaks in both cases confirm cubic zinc blende structure, which is consistent with the values in the standard card of ZnS (Ref. code 0 0-0 01-0792). The three main peaks at 33.6°, 56.6° and 67.2° can be assigned to the (111), (220) and (311) planes respectively. The XRD patterns clearly show that the diffraction angle peaks shifted in Cd-doped ZnS NPs sample. This happened because of the substitution of Zn atoms (small ionic radii) with the Cd atoms (large ionic radii) and for that reasons the lattice constant was increased. According to Vegard’s law, the dopant alone cannot generate an individual peak by the side of host peak but it can produce adequate shift in the position of host peak. An increase in broadening of diffraction peaks in case of Cd doped ZnS nanoparticles could be associated to the stress and strains which are induced by defects [30]. The average crystallite size calculated from full width at half maximum of XRD peaks using the Debye Scherrer equation was found to be 8.87 nm and 9.84 nm for ZnS and Cd doped ZnS NPs samples respectively [31,32].
Fig. 5. High resolution transmission micrographs of, (a) Zinc sulphide, (b) Cd-doped ZnS nanoparticles.
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U. Jabeen et al. / Surfaces and Interfaces 6 (2017) 40–49 Table 3 Degradation efficiency of ZnS and Cd-ZnS nanoparticles for ARS dye.
Cd-ZnS 1140
Serial Number
Intensity (a.u.)
760
1. 2. 3. 4. 5.
380
2000 0
1500
1000
500
0
40
60
0 30 60 90 120
%Degradation efficiency ZnS
Cd-ZnS (Cd 0.5 M)
0 12 27.5 34.48 50
0 59 85.4 93.5 96.7
the photodegradation efficiency; the highest output was registered when using cadmium acetate. This was interpreted as that during annealing in air, acetate decomposed totally or partially forming volatile compounds and possibly traces of carbon. The trapping of cadmium chloride in the film led to impurification. These byproducts were found responsible for the random variations in the band gap values. The percentage degradation efficiency of ZnS and Cd-ZnS nanoparticles for ARS dye is summarized in Table 3 and Fig. 8.
ZnS
20
Time (min)
80
Angle (2 theta) Fig. 6. XRD spectra of ZnS and Cadmium doped ZnS nanoparticles.
3.4. Photocatalytic degradation studies The photocatalytic degradation of ARS dye on the surface of ZnS and Cd-ZnS nanoparticles was monitored with the help of UV– Visible spectrometer. 3.4.1. Effect of illumination time Fig. 7A and B, shows the UV–Visible plots of the degradation of ARS dye as a function of exposure to sunlight in the presence of ZnS and Cd doped ZnS (Cd 0.5 M) nanoparticles respectively. It can be seen from the recorded spectra that in the presence of ZnS nanoparticles the dye degraded at a very slow pace however the degradation process was enhanced in the presence of Cd doped ZnS nanoparticles significantly because doping of photocatalyst could improve the photocatalyst efficiency in the following ways: band gap narrowing [32], formation of impurity energy levels [33,34], oxygen vacancies, unique surface area for the adsorption of organic molecules [35] and electron trapping [36]. It can be noticed from these plots that the peaks at 330 nm and 507 nm of ARS dye gradually decrease in intensity with increasing irradiation time. No new absorption peak appears in the UV–Visible spectra during the whole process. The percentage degradation efficiency of ZnS and Cd-ZnS nanoparticles for ARS dye was plotted against the irradiation time, as shown in Fig. 8. It can be observed that ARS dye degraded to maximum extent by Cd-doped ZnS (Cd 0.5 M) nanoparticles when compared to ZnS nanoparticles. The difference in photocatalytic activities is strongly related to band gap of ZnS and Cd-doped ZnS nanoparticles. Photocatalyst with a smaller band gap energy can generate more electron hole pairs [37]. Band gap narrowing, introduction of impurity energy level, and oxygen-deficient sites can enhance the photocatalytic activity under visible light [38]. Andronic et al. [39] investigated photocatalytic activity of cadmium doped TiO2 films for photocatalytic degradation of methyl orange and methylene blue. They found that Cd2+ decreases the TiO2 band gap and enhances its photocatalytic activity to degrade the MO and the MB with higher efficiency than the unmodified TiO2 . They noticed that the use of different cadmium precursors also influenced
3.4.2. Effect of pH The pH of is one of the most vital factor controlling the photo catalytic degradation of dye on the surface of the photo catalyst. pH also influences the adsorption and dissociation of the organic molecule [40], surface charge of photocatalyst, and oxidation potential of the valence band [41]. The effect of pH on photo catalytic degradation of Alizarin red S was studied in the pH range from 4 to 9. The results are graphically presented in Fig. 9. It has been found that the rate of photo catalytic degradation of Alizarin red S increases with increase in pH up to 4.0. Further increase in pH above 4.0 results in a decrease in the rate of reaction. Higher degradation of ARS may be due to electrostatic attractions between ARS and highly protonated adsorption spots accessible at lower pH values. Lower adsorption at higher pH values could be ascribed to the abundance of OH− ions on the surface of photo catalyst. This results in ionic repulsion between the negatively charged surface and the anionic ARS molecules. In addition, Shaban [42] investigated photocatalytic reduction of toxic Cr (VI) using carbon modified titanium oxide (CMn-TiO2 ) nanoparticles under natural sunlight illumination. Under natural sunlight irradiation, modification of titanium oxide by carbon significantly enhanced the photocatalytic reduction of Cr (VI). The effects of various experimental parameters such as catalyst dose, initial concentration of Cr (VI), and solution pH on the reduction rate of Cr (VI) were investigated. The highest reduction rate of Cr (VI) was obtained at the optimal conditions of pH 5 and 2.0 g L−1 of CM-n-TiO2 . Furthermore, Prado and Costa [43] noticed that the degradation of malachite green was lesser in basic medium (higher pH) but the degradation efficiency of photocatalyst increased with the decrease in pH value. Changing the pH of the solution would alter the concentration of one of the species involved in the reaction and result in a shift in the redox potential. Walczak et al. [44] studied that the hydroquinone redox couple is an excellent system for illustrating the Nernst equation. Changing the pH between 1 and 6 results in a total change in potential of 0.27 V. Magliozzo et al. [45] investigated the dependence of E1/2 on pH by Clostridium pasteurianum 2(4Fe4S) ferredoxin. The results showed that oxidation state-dependent pK values, which might arise from sites on the iron-sulfur centers, were responsible for the pH effect. Based on a model of two equivalent protonation sites/molecule, values of 7.4 for pKox and 8.9 for pKrd were obtained. Furthermore, the conformation of C. pasteurianum ferredoxin was examined byNMR, EPR, and CD spectroscopies to rule out a pH-dependent conformation equilibrium as the origin of the pH effect.
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Fig. 7. The absorption spectra of photodegradation of ARS dye solution by (A) ZnS, (B) Cd-ZnS (Cd 0.5 M) nanoparticles under sunlight irradiation.
Fig. 8. Percent degradation efficiency of ZnS (solid line) and Cd-ZnS (Cd 0.5 M) (dash dot) nanoparticles for ARS dye. Fig. 10. Effect of photo catalysts dose on the degradation of ARS.
Fig. 9. Effect of pH on the degradation of ARS using ZnS and Cd-ZnS as photocatalysts.
Fig. 11. Effect of concentration of dye on its degradation rate.
3.4.3. Effect of catalyst dose The effect of photo catalysts concentration on the degradation of ARS was studied under sun light using different concentrations of nanoparticles ranging from 10 mg to 70 mg at a constant dye
concentration of 4 mgL−1 . The degradation efficiency of photo catalyst load on ARS is shown in Fig. 10 which shows that the percentage of degradation increases significantly by increasing the concentration of catalyst from 10 mg to 30 mg and afterwards the rate of
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Fig. 12. FTIR spectra of (A) ARS before (B) after treatment with ZnS and (C) after treatment with Cd-ZnS.
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Table 4 Values of rate constants K (min−1 ) for the degradation of 5 × 10−5 M ARS in the presence of various photo catalysts. Sample
Rate constant
ZnS Cd-ZnS
4.2 × 10−3 min−1 30 × 10−3 min−1
degradation remains nearly constant. This could be ascribed to the increase in the number of effective sites on the surface of photocatalyst with increase in amount of photo catalyst. However when the amount of catalyst exceed 30 mg, turbidity of the suspension increases which decreases the penetration of sun light. This reduces the photo activation potential of suspension. Thus very high concentrations of catalyst may not be useful because of aggregation and light scattering. 3.4.4. Effect of dye concentration Effect of variation of Alizarin red S concentration on the degradation was studied by taking different concentrations of dye ranging from 4 to 20 mgL−1 at constant catalyst load (30 mg). The results are presented in Fig. 11. It was observed that the degradation of ARS decreased when the dye concentration was increased. The reason is that when the initial concentration of dye is increased, more and more dye molecules are adsorbed on the surface of ZnS and Cd-ZnS nanoparticles. Subsequently the presence of large number of adsorbed dye molecules on the surface of catalyst inhibits direct contact of dye molecules with the holes or hydroxyl radicals, this might have a retarding effect on the dye degradation. Furthermore, the formation of the by-products during the degradation of mother dye molecules is another possible reason of decrease in dye degradation [46–48]. On the other hand, according to Beer–Lambert law, as the initial concentration of dye increases, the path length of photons entering the solution decreases. Consequently less number of photons reach the catalyst surface and hence the rate of degradation is retarded.
Fig. 13. Kinetic study of photo degradation of ARS by ZnS (solid line) and Cd-ZnS (dash dot) nanoparticles.
Fig. 14. The decolorization rate of ARS in the presence of different photocatalysts.
3.5. FTIR analysis Degradation of the dye solutions was confirmed with IR spectroscopy. The FTIR spectrum of Alizarin red S (Fig. 12), shows peak at 1634 cm−1 and 1414 cm−1 for aromatic C=C bond. Peak at 1825.48 cm−1 testifies multiple bonded CO groups. Peak at 3391 cm−1 confirms OH stretching and the one at 2818 cm−1 corresponds to C–H stretching. All these peaks confirm the aromatic nature of the dye. But after treatment with photo catalyst these peaks were disappeared and only the characteristic peaks of the photo catalyst were retained. 3.6. Kinetic analysis The kinetics of photo degradation of ARS dye catalyzed by the synthesized ZnS and Cd-ZnS nanoparticles were thoroughly investigated. Fig. 13 shows the plot of ln Ao/At versus time using ZnS, and Cd-ZnS nanoparticles as photo catalysts. The rate constants of degradation with both photo catalysts k, were calculated from the slopes of the plots. These are in accordance with the proposed pseudo-first order kinetic model. These rate constants are given in Table 4. The photocatalytic activity of doped nanoparticles (Cd-ZnS) was found higher than those of undoped ZnS nanoparticles. Catalytic efficiencies of these two types of catalysts were found as 0.0 0 014 min−1 mg−1 and 0.0 01 min−1 mg−1 respectively. It was calculated by using the following relationship.
Catalytic efficiency = k/Ccatalyst
(1)
Where k is the rate constant of the reaction and Ccatalyst is the concentration of the catalyst. Similarly the decoloration rates for different photocatalysts were calculated using rate constants obtained from the slopes of plots in Fig. 13. The decolorization rate (Fig. 14) for the decomposition of ARS dye in the presence of different photo catalyst (ZnS and CdS-ZnS) confirmed that decolorization of ARS proceeded faster in doped sample as compared to un-doped nanoparticles. The decoloration rates for different photocatalysts were calculated using rate constants obtained from the slopes of plots in Fig. 13. The decolorization rate (Fig. 14) for the decomposition of ARS dye in the presence of different photo catalyst (ZnS and CdS-ZnS) confirmed that decolorization of ARS proceeded faster in doped sample as compared to undoped nanoparticles. 4. Conclusion ZnS and Cd-ZnS nanoparticles were successfully synthesized by co-precipitation method. Electronic absorption spectra demonstrated a significant red shift in the absorption edge by the substitution of Cd2+ ion in ZnS lattice. XRD patterns of synthesized ZnS and Cd doped ZnS nanoparticles revealed cubic structures with average crystallite sizes of 8.87 nm and 9.84 nm respectively. The bandgap of ZnS nanoparticles was effectively tuned from 3.4 eV to 2.44 eV to absorb in the visible region effectively. This is shown by the Tauc plots.
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Cadmium doped ZnS was found an effective photocatalyst for the degradation of Alizarin red S dye. It brought about 96% degradation of the dye as compared to undoped ZnS which could hardly degraded the dye upto 50%. This enhanced photocatalytic degradation due to cadmium content (0.5 M) in ZnS crystal lattice could be linked to the tuning of bandgap. This could also be partially ascribed to the generation of midgap impurity energy levels or charge trapping site to reduce electron–hole recombination. The photocatalytic degradation process increased to some extent with increase in catalyst dose but decreased with increase in dye concentration. The maximum degradation was observed with 30 mg dose of Photocatalyst. Maximum Photo degradation efficiency was attained at pH 4. The photo catalytic degradation of ARS showed that Cd-ZnS nanoparticles could be used as a promising catalytic degradation material than un-doped ZnS. The kinetic rate constant of degradation was found higher for Cd doped ZnS nanoparticles than the un-doped ZnS. Acknowledgement We are highly thankful to the Higher education commission of Pakistan under project No-20-/2329/NRPU/R&D/HEC/12 for financial support and Quaid-i-Azam University, Islamabad, for providing us laboratory and space facilities. References [1] L.L. Ma, Z.H. Sun, G.Y. Zhang, L.Y. Lin, L.J. Li, K.E. Wang, Y. Yu, M. Tan, B.J. Wang, Preparation, characterization and photocatalytic properties of CdS nanoparticles dotted on the surface of carbon nanotubes, Nanotechnology 19 (11) (2008) 115709. [2] M.N. Mahmoodi, M. Arami, Y.N. Limaee, S.N. Tabrizi, Decolorization and aromatic ring degradation kinetics of direct red 80 by UV oxidation in the presence of hydrogen peroxide utilizing TiO2 as a photocatalyst, Chem. Eng. J. 112 (1-3) (2005) 191–196. [3] H. Zhu, R. Jiang, L. Xiao, Y. Chang, Y. Guan, X. Li, G. Zeng, Photocatalytic decolorization and degradation of congo red on innovative crosslinked chitosan/nano-CdS Composite catalyst under visible light irradiation, J. Hazard. Mater. 169 (1-3) (2009) 933–940. [4] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bio Resour. Technol. 77 (3) (2001) 247–255. [5] J. Snowden-SwanL, Pollution prevention in the textile industries, in: H.M. Freeman (Ed.), Industrial Pollution Prevention Handbook, McGraw-Hill, New York, 1995. [6] O. Legrini, E. Oliveros, M.A. Braun, Photochemical processes for water treatment, Chem. Rev. 93 (2) (1993) 671–698. [7] M.H. Habibi, N. Talebian, Photocatalytic degradation of an azo dye X6G in water: a comparative study using nanostructured indium tin oxide and titanium oxide thin films, Dyes Pigm. 73 (2) (2007) 186–194. [8] Y.B. Xie, X.Z. Li, Interactive oxidation of photoelectrocatalysis and electro Fenton for azo dye degradation using TiO2 –Ti mesh and reticulated vitreous carbon electrodes, Mater. Chem. Phys. 95 (1) (2006) 39–50. [9] R. Ullah, D.J. Dutta, Photocatalytic activities of ZnO nanoparticles synthesized by wet chemical techniques, 2nd International Conference on Emerging Technologies Peshawar, 2006. [10] J. Zha, C. Chen, W. Ma, Photocatalytic degradation of organic pollutants under visible light irradiation, Top Catal. 35 (3-4) (2005) 269–278. [11] N.M. Mahmoodi, M. Arami, N.Y. Limaee, N.S. Tabrizi, Kinetics of heterogeneous photocatalytic degradation of reactive dyes in an immobilized TiO2 photocatalytic reactor, J. Colloid Interface Sci. 295 (1) (2006) 159–164. [12] J.Y. Liao, K.C. Ho, A photovoltaic cell incorporating a dye-sensitized ZnS/ZnO composite thin film and a hole-injecting PEDOT layer, Solar Energy Mater. Solar Cells 86 (2005) 229–241. [13] M. Sharma, T. Jain, S. Singh, O.P. Pandey, Photo catalytic degradation of organic dyes under UV–Visible light using capped ZnS nanoparticles, Solar Energy 86 (1) (2012) 626–633. [14] S. Sharma, R. Ameta, R.K. Malkani, S.C. Ameta, Photocatalytic degradation of rose bengal using semiconducting zinc sulphide as the photocatalyst, J. Serb. Chem. Soc. 78 (6) (2013) 897–905. [15] M. Warrier, M.K.F. Lo, H. Monbouquette, M.A. Garcia-Garibay, Photocatalytic reduction of aromatic azides to amines using CdS and CdSe nanoparticles, Photochem. Photobiol. Sci. 3 (9) (2004) 859–863. [16] J. Ameta, N. Gupta, R. Ameta, V.K. Sharma, Photocatalytic bleaching of Alizarin red over CeFeO3 particulate system, Int. J. Chem. Sci. 7 (4) (2009) 2703–2713. [17] C. Hachem, F. Bocquillon, O. Zahraa, M. Bouchy, Degradation of dyestuff materials by fenton oxidation, Dyes Pigm. 49 (2) (2001) 117–125.
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