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Fabrication of photocatalyst based on Eu3+-doped ZnS–SiO2 and sodium alginate core shell nanocomposite
International Journal of Biological Macromolecules 70 (2014) 143–149
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Fabrication of photocatalyst based on Eu3+ -doped ZnS–SiO2 and sodium alginate core shell nanocomposite E.S. Agorku a , H. Mittal a , B.B. Mamba a , A.C. Pandey b , A.K. Mishra a,∗ a b
Department of Applied Chemistry, University of Johannesburg, South Africa Nanotechnology and Application Centre, University of Allahabad, India
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 13 April 2014 Accepted 14 June 2014 Available online 28 June 2014 Keywords: Photocatalysis Hydrogel Crosslinking Graft copolymerization Indigo carmine
a b s t r a c t This research paper reports the photocatalytic properties of Zn–SiO2 –Eu3+ and sodium alginate (Alg) based nanocomposites for the degradation of indigo carmine dye. Initially, Eu3+ doped ZnS–SiO2 nanophorphor was synthesized and after that it was incorporated within the grafted crosslinked polymer matrix of Alg with acrylamide-co-acrylic acid in different concentrations. Synthesized materials were characterized using XRD, Raman spectroscopy, FTIR, SEM/EDX, TEM and UV–vis diffuse reflectance spectroscopy. XRD and TEM analyses confirmed the formation of nanoparticles as well as the uniform distribution of the nanoparticles within the polymer matrix. The UV–vis and UV–vis DRS spectral analysis indicated that Eu3+ doping causes a red-shift in the absorption band, resulting in the reduction in band gaps. The synergic effect of ZnS and Eu3+ in the SiO2 evidenced the photocatalytic performance of the catalyst. Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites were found to be very effective for the degradation of indigo carmine under visible light. Highest photocatalytic performance (93.4%) was shown by the nanocomposite with the 20% concentration of the nanoparticle after 5 h. The photocatalytic activity was mainly attributed to the intense light absorption in the visible region and narrow band gap energy. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The world is facing the formidable water challenge in meeting out the rising demand for clean water as the available sources of clean water are decreasing due to pollution, drought, population growth and competing demands from a variety of users [1]. Industrial wastewater from different industries is a significant source of environmental pollution [2]. Contamination of natural water by these pollutants is worldwide public health concern. Textile processing industries nowadays are widespread sectors in developing countries. Among the various processes in textile industries, dyeing process uses large volume of water for dyeing, fixing and washing process. The effluents released from these textile processing factories contain suspended solids, high amount of dissolved solids, unreacted dye stuff (color) and other auxiliary chemicals that are used in various stages of dyeing and other processes [3]. It is estimated that 10–15% of these chemical compounds are
∗ Corresponding author at: Department of Applied Chemistry, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, South Africa. Tel.: +27115596180; fax: +27115596425. E-mail address: [email protected] (A.K. Mishra). http://dx.doi.org/10.1016/j.ijbiomac.2014.06.037 0141-8130/© 2014 Elsevier B.V. All rights reserved.
discharged into water streams by textile industries [4]. Some of these compounds are not strongly hazardous, but an acute exposure could make them harmful to fish and other aquatic organisms [5]. Additionally, highly colored water makes the penetration of sun rays more difficult due to the presence of dyes and this affects the photosynthesis process [6]. Conventional wastewater treatment methods involve biosorption and aerobic and anaerobic treatment methods [7–10] which are probably of the most expensive approaches. Other traditional treatment techniques involve coagulation/flocculation, membrane separation or absorption on activated carbon, reverse osmosis etc. However, these techniques suffer drawbacks such as generation of large volume of sludge and high operational costs [11,12]. Out of all the techniques, photocatalytic treatment is a more effective alternative for the removal of soluble organic compounds [13,14]. Contrary to physical processes such as coagulation and adsorption, which mainly transfer the pollutants from wastewater to other media, photocatalysis involves complete mineralization of organic pollutants to carbon dioxide, water and mineral acids. Photodegradation using photocatalysts is remarkable method for organic pollutant degradation. Semiconductor photocatalysts had been widely employed for the treatment of dye contaminated water. Nowadays, scientific and engineering interest in
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semiconductor photocatalysts has grown extensively [15,16]. ZnS nanocrystals have been widely studied because of their versatile applications in optoelectronics and catalysis. However, its wide band gap (3.73 eV) limits its photocatalytic application in the visible-light region. The use of a photocatalyst material having visible-light activity is required for harnessing the full potential of sunlight in wastewater treatment. The synthesis of ZnS in glass matrices provides good optical properties and high stability. Recent studies revealed that rare-earth doped SiO2 -semiconductor nanocomposites are promising candidate for photocatalysis application [15] and the properties of the dopant exhibits synergism of properties of the catalyst. In such a composite, the semiconductor could act as a sensitizer to promote active rare earth emission by harvesting the excitation photon energy and then transferring it to the rare earth ion. Eu3+ has been widely used due to its high luminescent efficiency and band gap lowering potential [16]. Biopolymers based polymer nanocomposites have attracted much attention of researchers in different areas of nanoscience and technology. Biopolymers like chitosan, gum arabic, gum ghatti and gum xanthan have the advantage of easy availability, low cost and environment friendly nature over synthetic polymer nanocomposites [17–20]. Dispersion of the nanoparticles within the polymer matrices enhances the basic properties of the polymer like mechanical strength, stability and the surface area. Sodium alginate (Alg) is a biopolymer having less toxicity, biocompatible and biodegradable. Alg is a water soluble linear polysaccharide extract from brown sea weed and is composed of alternating blocks of 1–4 linked ␣-l-guluronic and -d-mannuronic acid residues. Unfortunately, natural biopolymers such as Alg face many drawbacks including less stability and difficulty in processing, therefore, the modification of the biopolymer through graft-copolymerization improves its physical and chemical properties [21,22] and the resulting graft co-polymers have better mechanical strength, biocompatibility and flexibility than those of the parent biopolymer [23–26]. Despite the numerous opportunities in the application of semiconductor nanomaterials and their anticipated potential use in wastewater treatment, there has been increasing interest in the environmental impact of such materials. Dispersing the nanoparticle material into a polymer matrix will modify the surface of the nanoparticle which will enhance its stability and mobility under natural conditions, and minimizing their toxic effects. In this research work, we have reported the synthesis of ZnS by co-precipitation method followed by preparation of ZnS doped SiO2 in the presence of Eu3+ . The obtained ZnS–SiO2 :Eu3+ was encapsulated into Alg-cl-poly(AAM-co-MAA) polymer matrix, where Alg-cl-poly(AAM-co-MAA) was prepared through graft copolymerization technique using N,N -methylene-bis-acrylamide (MBA) as a crosslinking agent and potassium persulphate (KPS)–ascorbic acid (ABC) redox pair as an initiator via free radical polymerization and the obtained material was employed in photocatalysis for the degradation of indigo carmine in aqueous solution.
2. Experimental 2.1. Materials and methods 2.1.1. Chemicals and reagents Na2 S, Zn(NO3 )·6H2 O, Eu(NO3 )3 ·6H2 O, nitric acid, sodium alginate (Alg), acrylamide (AAM), methacrylic acid (MAA), MBA, potassium persulfate (KPS) and ABC were purchased from Sigma–Aldrich and used without further purification. The stock solution of dye was prepared by dissolving appropriate amount of indigo carmine (Sigma–Aldrich) in 1000 mL of the deionized water
and the stock solution was further diluted to obtain the dye solutions of desired concentration. 2.1.2. Synthesis of ZnS–SiO2 –Eu3+ nanoparticle ZnS was prepared by coprecipitation method as follows: 10 mL of Na2 S (0.1 M) was slowly added to 10 mL Zn(NO3 )·6H2 O (0.1 M) with vigorous stirring on a magnetic stirrer. The mixture was further stirred for 30 min. The crystals were separated by centrifugation, washed several times with deionized water and ethanol to remove nitrate and chloride ions. ZnS precipitates were dried at 60 ◦ C overnight. In the next step, ZnS–SiO2 –Eu3+ was prepared by mixing 10.4 g of TEOS solution with 4.6 g of ethanol and 4.5 g of HNO3 (0.2 M). The mixture was stirred vigorously for 1 h at room temperature. An amount of 0.223 g Eu(NO3 )3 ·6H2 O was dissolved in 4.6 g of ethanol and added to the TEOS mixture and the mixture stirred for an hour. The resulting mixture was added to ZnS nanoparticles with molar ratio of 10:1 (Si:Zn) and stirred for 2 h. The resulting mixture was dried in an oven at 50 0 C overnight and heated at 200 ◦ C for 3 h. 2.1.3. Synthesis of Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposite Initially, certain amount of ZnS–SiO2 –Eu3+ was added to deionized water (10 mL) and sonicated for 30 min for the better dispersion of the nanaoparticles in the water. Then, Alg (1.0 g) was added in the reaction mixture followed by the addition of KPS (30 mg) and ABC (20 mg) and stirred vigorously. Fifty grams of the MBA was added with continuous stirring. In the last step, AAM (1.0 g) and MAA (2 mL) were added in the reaction mixture with continuous stirring. The reaction temperature was maintained at 60 ◦ C. After 3 h, the reaction vessel was taken out and allowed to cool at room temperature. The homo-polymers were removed using acetone extraction. Finally, the synthesized nanocomposite was dried in a hot air oven at 50 ◦ C till a constant weight was achieved.
2.1.4. Evaluation of photocatalytic activity The photocatalytic activity of the materials were evaluated through a suspension of 100 mg of the nanocatalyst in 100 mL of aqueous solution of indigo carmine (20 mg/L) that was kept under magnetic stirring and visible light filtered using dichroic UV filter ( > 420 nm) at room temperature. A 400 W tungsten filament (Eurolux), kept at a distance of 11 cm from the reaction vessel, was employed as a source of radiation. The samples were magnetically stirred in the dark for 1 h prior to illumination to allow for adsorption equilibrium. Aliquots of the suspension (5 mL) were withdrawn at periodic time intervals using disposable syringe and filtered through 0.4 m PVDF membrane filter at 30 min intervals for 4 h. The concentration of the indigo carmine remaining after illumination in the supernatant solution was determined using a Shimadzu UV-2450 spectrophotometer at = 610 nm. The photodegradation performance of the process was assessed in terms of decolorization efficiencies and kinetic studies.
2.1.5. Characterization Doping of ZnS–SiO2 nanophorphor with Eu3+ and successful and uniform incorporation of the nanophorphor with in the crosslinked polymer network of Alg-cl-P(AAM-co-MAA) hydrogel was evidenced using different characterization techniques like FTIR, Raman, XRD, SEM/EDAX, TEM and UV–vis spectroscopy. X-ray diffraction patterns were recorded on X-ray diffractometer (Rigagu Ultima IV) employing Cu K␣ radiation of the wavelength of 1.5406 A˚ with visible slights at 45 kV/40 mA.
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Fig. 1. XRD pattern of ZnS, ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
Alg-cl-poly(AAM-coFig. 2. FTIR spectra of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
Crystalline size of the samples were calculated using the Debye–Scherrer equation: D=
K ˇcos
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(1)
where D is the crystal size of the catalyst, the X-ray wavelength, ˇ the full width at maximum (FWHM) of the diffraction peak (radian), K is a constant (0.9) and is the diffraction angle at the maximum. TEM images of the samples were taken on a field emission electron microscope, (JEOL, JEM-2100F) with a working voltage of 120 kV. Prior to TEM analysis, small pinch of the samples were dispersed in few milliliters of ethanol and sonicated for 30 min. About two drops of the suspension was transferred to a copper grid and allowed to dry at room temperature. Changes in the morphology of the polymer matrix of Alg-cl-P(AAM-co-MAA) hydrogel after the incorporation of Eu3+ doped ZnS–SiO2 nanophorphor was studied using scanning electron microscope coupled with EDAX (TESCAN, Vega 3 XMU). A small pinch of the samples were transferred to SEM grids and to have the conducting impact, the samples were carbon coated and scanning was synchronized with microscopic beam so as to maintain the small size over a large distance relative to the specimen. FT-IR spectra of the samples were recorded on PerkinElmer FT-IR spectrometer (Spectrum 100), Raman spectra of the samples were measured on PerkinElmer Raman microscope (Raman Micro 200), and optical absorption spectra were recored using a Shimadzu UV–vis spectrophotometer (UV-2450). 3. Results and discussion 3.1. XRD patterns The XRD patterns of ZnS, ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites are shown in Fig. 1. Three broad peaks were observed in the diffractogram for the ZnS (Fig. 1d) at around 29.03◦ , 48.13◦ , and 56.66◦ corresponding to (1 1 1), (2 2 0), and (3 1 1) planes of face centered cubic (fcc) of ZnS (JCPDS No. 05-0566) with average particle size of 3.21 nm. The presense of broad peaks in the XRD patterns is due to their small size [27]. Fig. 1c shows the XRD patterns of ZnS–SiO2 –Eu3+ . The broad peak at 22◦ shows the dominance of amorphous phase of SiO2 (average particle size is 3.75 nm). This peak was observed in the Alg-clpoly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ composite samples (average particle size is 3.88 nm) but shifted to a lower degree in the 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ sample with average particle size of 4.46 nm. The absence of the ZnS peaks in the
and
Alg-cl-poly(AAM-co-
composite samples is due to the high percentage of SiO2 . Also, the suppression of the cubic ZnS peaks may be due to the SiO2 impeding the formation of direct contact of zinc sulfide nanoparticles through the formation of Zn–S–Si linkage. The addition of SiO2 to ZnS decreased the crystallinity of ZnS nanoparticle. This phenomenon is attributed to the formation of Si–O networks by excessive SiO2 , which prevents the formation of cubic crystallites. 3.2. FTIR analysis The IR spectra of ZnS–SiO2 –Eu3+ core-shell and Algcl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites are depicted in Fig. 2. The IR band between 3350–3450 cm−1 and 1600–1640 cm−1 are due to the stretching and bending vibrations of structural hydroxyl groups of adsorbed water molecules [28,29]. Fig. 2c shows the IR spectrum of ZnS–SiO2 –Eu3+ . The peak observed between 1050 and 1110 cm−1 corresponds to the asymmetric stretching vibration mode of the Si O Si whereas the peak between 440 and 480 cm−1 corresponds to bending vibration modes of Si O Si. The IR bands at 2400–2450 cm−1 and 1625–1630 cm−1 are attributed to Si C and C O stretching vibrations. The peak at 600–620 cm−1 is assigned to Zn–S stretch [30]. This clearly shows that ZnS is successfully embedded in the silica matrix. Fig. 2b and c shows the IR spectra of 10% and 20% of Alg-clpoly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites, respectively. The band at 3425 cm−1 corresponds to amine groups and the peaks between 2870 cm−1 and 2950 cm−1 is caused by CH2 asymmetric stretching vibrations. The peak at around 1420 cm−1 is due to CH and CH2 in plane bending. In addition, the bands at 1618 cm−1 and 1730 cm−1 are due to CO stretching of carboxylic salt groups. The peaks at 1153 cm−1 are due to CN stretching of amide groups [26]. 3.3. Raman spectroscopy analysis Raman spectra of the samples are shown in Fig. 3. Raman bands between 460 and 490 cm−1 are due to symmetric stretching vibrations of bridging oxygen in predominantly 6-membered Si O Si and breathing motion of bridging oxygen in 4-membered SiO4 . Peak between 750 and 790 cm−1 is assigned to the bending and deformation modes of Si O Si whereas the peaks around 1050 cm−1 and 1177 cm−1 are ascribed to asymmetric stretching of bridging oxygens [31].
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respectively [24,25]. In the region between 1400 and 1270 cm−1 , peaks are assigned to the C H deformation vibrations and a band between 1100 and 1200 cm−1 assigned to the C O stretching vibrations [32–34]. The bands between 500 and 700 cm−1 are related to the deformation of pyranosyl rings and C O C vibration of glycosidic linkage [32–34]. 3.4. HRTEM and SEM-EDS studies
Fig. 3. Raman spectra of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
Alg-cl-poly(AAM-co-
To further demonstrate the incorporation of ZnS–SiO2 –Eu3+ into Alg-cl-poly(AAM-co-MAA) matrix, the Raman spectrum of ZnS–SiO2 –Eu3+ (Fig. 3c) was compared with the spectra of the Algcl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites (Fig. 3a and b). Many Raman peaks on the ZnS–SiO2 –Eu3+ core–shells have counterparts on the Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites, though some peak positions were shifted as a result of molecular interactions. All the nanocomposite samples present the characteristic asymmetric and symmetric bands of carboxylate ion (COO− ) at 1600–1610 cm−1 and near 1443 cm−1 ,
HRTEM was used to observe uniformity, morphology and microstructure of prepared ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites. Fig. 4(A) and (B) shows a HRTEM image of 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ and ZnS–SiO2 –Eu3+ photocatalysts, respectively. The particles are small and nearly spherical in shape. All the samples prepared show regular morphology. The elemental composition of the prepared samples was estimated by SEM-EDS analyses. Fig. 4(C) and (D) shows the EDS spectra of ZnS–SiO2 –Eu3+ and 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ , respectively. Both spectra confirm the presence of Si, Zn, O, S and Eu. The spectra indicate that the main components are Si and O with low contents of Eu and S. This confirms the formation of ZnS–SiO2 –Eu3+ . The presence of N and C in the EDS spectra for the Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites further confirm the encapsulation of ZnS–SiO2 –Eu3+ into the Algcl-poly(AAM-co-MAA) polymer matrix. 3.5. Optical studies One of the methods for probing the band structure of semiconductors is to measure the optical absorption spectrum. The
Fig. 4. HRTEM images of (A) 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ , (B) ZnS–SiO2 –Eu3+ and SEM-EDS spectrum of (C) ZnS–SiO2 –Eu3+ , (D) 20% Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ .
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Fig. 5. UV–vis absorption spectra of ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites.
optical technique is used to probe the allowed transitions between the valence band and conduction band states for nanoparticle which may show defects or impurity states in the band gap. Fig. 5 shows a comparison of the UV–vis absorption edge of ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites. The absorption spectra consist of intense absorption around 420 nm due to charge transfer from the valence band to the conduction band. The onset of absorption edge of the ZnS–SiO2 –Eu3+ is approximately 417 nm which corresponds to a band gap of 2.98 eV. This strong absorption in the visible region could be due to a number of factors. First, it may originate from the impurity levels of ZnS, the existence of Si and oxide related defects [35]. Second due to charge transfer from ZnS to Eu3+ ions embedded in the SiO2 matrix. The ZnS could act as a sensitizer by promoting Eu3+ emissions by harvesting the excitation photon energy and transferring it to the Eu3+ ions. The plot between reflectance and wavelength is depicted in Fig. 6. ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites showed a decrease in reflectance in the entire visible light region. In order to determine the band gap of the photocatalysts, the Kubelka–Munk function was used (Eq. (2)): F(R) =
K 1 − R2 = 2R S
Fig. 6. UV–vis DRS of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
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Fig. 7. Tauc plot for ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
where R, K and S are the reflectance, absorption and scattering coefficient, respectively. For a semiconductor material, the Kubelka–Munk function allows the construction of a Tauc plot: [F(R) × h]n versus h [36]. For a direct allowed band gap semiconductor, the plot with n = 1/2 will show a linear Tauc region above the optical absorption edge. Indirect band gap materials show a Tauc region with n = 2 [37]. The value of the optical band gap can be calculated by extrapolating the Kubelka–Munk function to K/S = 0. The band gap energies for the samples are shown in Fig. 7. These results are consistent with the UV–vis absorption estimation. 3.6. Photocatalytic studies
(2)
With time increasing from 0 to 5 h, the characteristic absorption band at peaks of 610 nm decreased gradually, indicating that the IC was gradually photodegraded by the catalyst (Fig. 8). The high photocatalytic performance of the catalysts could be assigned to a number of factors: (i) high visible-light absorption of the catalyst due to improved bang gap, (ii) the delayed electron–hole recombination due to trapping of electrons in the conduction band by Eu3+ orbital, (iii) efficient transfer of the charge carriers, and (iv) effective utilization of the charge carries by the reactants, as demonstrated by the UV–vis and UV–vis DRS analyses (Figs. 5 and 6). The linear relationship of ln Co /C versus time (Fig. 9) shows that the photocatalytic degradation of IC follows the
Alg-cl-poly(AAM-co-
Fig. 8. Indigo carmine photodegradation profile using 20% Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites.
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4. Conclusion ZnS–SiO2 –Eu3+ and a series of Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites were synthesized. The samples were characterized by various spectroscopic and analytical techniques. XRD and HRTEM analyses confirm the formation nanoparticles. The UV–vis and UV–vis DRS spectral analysis indicated that Eu3+ doping causes a red-shift in the absorption band, resulting in the reduction in band gaps. The synergic effect of ZnS and Eu3+ in the SiO2 is evidence in the photocatalytic performance of the catalyst. The ZnS–SiO2 –Eu3+ systems in Alg-cl-poly (AAM-co-MAA) are very effective visible-light active photocatalysts for the degradation of Indigo Carmine. Highest photocatalytic activity (93.4%) was observed for the 20% Alg-cl-poly(AAM-co-MAA sample after 5 h. The photocatalytic activity was mainly attributed to the intense light absorption in the visible region and narrow band gap energy. Fig. 9. Indigo carmine degradation profile ZnS–SiO2 –Eu3+ , Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites with different concentrations of ZnS–SiO2 –Eu3+ .
Acknowledgements Authors gratefully acknowledge the financial support from the Faculty of Science, University of Johannesburg and National Research Fund (NRF), South Africa for providing research fellowships to the authors H. Mittal and E. S. Agorku. Authors are also thankful to Ms. Nomsa Baloyi, Research Scientist, Metallurgical Engineering Department and Mr. Patrick Komane, Research Scientist, Department of Applied Chemistry, University of Johannesburg for their help during the characterization of the materials.
References
Fig. 10. Kinetics of Indigo carmine degradation in the first 5 h under visible light irradiation.
Table 1 Band gap and percentage degradation of IC after exposure to visible-light 10. Sample 3+
ZnS–SiO2 –Eu Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ (10%) Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ (20%) a
Band gap (eV)
Degradation (%)a
2.95 2.85
78.2 86.9
2.81
93.4
Measured after reaction for 5 h.
pseudo-first-order kinetics:(3) lnCCo = ktwhere, Co /C is the normalized IC concentration, t is the reaction time, and k is the apparent reaction rate constant. The value for k (Table 1, Fig. 10) for the 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites (most efficient photocatalyst) is 1.51 × 10−2 min−1 which is about two times that of the ZnS–SiO2 –Eu3+ nanoparticles, 8.47 × 10−3 min−1 , further demonstrating that the Alg-cl-poly (AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites exhibit high photocatalytic efficiency.
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Fabrication of photocatalyst based on Eu3+ -doped ZnS–SiO2 and sodium alginate core shell nanocomposite E.S. Agorku a , H. Mittal a , B.B. Mamba a , A.C. Pandey b , A.K. Mishra a,∗ a b
Department of Applied Chemistry, University of Johannesburg, South Africa Nanotechnology and Application Centre, University of Allahabad, India
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 13 April 2014 Accepted 14 June 2014 Available online 28 June 2014 Keywords: Photocatalysis Hydrogel Crosslinking Graft copolymerization Indigo carmine
a b s t r a c t This research paper reports the photocatalytic properties of Zn–SiO2 –Eu3+ and sodium alginate (Alg) based nanocomposites for the degradation of indigo carmine dye. Initially, Eu3+ doped ZnS–SiO2 nanophorphor was synthesized and after that it was incorporated within the grafted crosslinked polymer matrix of Alg with acrylamide-co-acrylic acid in different concentrations. Synthesized materials were characterized using XRD, Raman spectroscopy, FTIR, SEM/EDX, TEM and UV–vis diffuse reflectance spectroscopy. XRD and TEM analyses confirmed the formation of nanoparticles as well as the uniform distribution of the nanoparticles within the polymer matrix. The UV–vis and UV–vis DRS spectral analysis indicated that Eu3+ doping causes a red-shift in the absorption band, resulting in the reduction in band gaps. The synergic effect of ZnS and Eu3+ in the SiO2 evidenced the photocatalytic performance of the catalyst. Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites were found to be very effective for the degradation of indigo carmine under visible light. Highest photocatalytic performance (93.4%) was shown by the nanocomposite with the 20% concentration of the nanoparticle after 5 h. The photocatalytic activity was mainly attributed to the intense light absorption in the visible region and narrow band gap energy. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The world is facing the formidable water challenge in meeting out the rising demand for clean water as the available sources of clean water are decreasing due to pollution, drought, population growth and competing demands from a variety of users [1]. Industrial wastewater from different industries is a significant source of environmental pollution [2]. Contamination of natural water by these pollutants is worldwide public health concern. Textile processing industries nowadays are widespread sectors in developing countries. Among the various processes in textile industries, dyeing process uses large volume of water for dyeing, fixing and washing process. The effluents released from these textile processing factories contain suspended solids, high amount of dissolved solids, unreacted dye stuff (color) and other auxiliary chemicals that are used in various stages of dyeing and other processes [3]. It is estimated that 10–15% of these chemical compounds are
∗ Corresponding author at: Department of Applied Chemistry, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, South Africa. Tel.: +27115596180; fax: +27115596425. E-mail address: [email protected] (A.K. Mishra). http://dx.doi.org/10.1016/j.ijbiomac.2014.06.037 0141-8130/© 2014 Elsevier B.V. All rights reserved.
discharged into water streams by textile industries [4]. Some of these compounds are not strongly hazardous, but an acute exposure could make them harmful to fish and other aquatic organisms [5]. Additionally, highly colored water makes the penetration of sun rays more difficult due to the presence of dyes and this affects the photosynthesis process [6]. Conventional wastewater treatment methods involve biosorption and aerobic and anaerobic treatment methods [7–10] which are probably of the most expensive approaches. Other traditional treatment techniques involve coagulation/flocculation, membrane separation or absorption on activated carbon, reverse osmosis etc. However, these techniques suffer drawbacks such as generation of large volume of sludge and high operational costs [11,12]. Out of all the techniques, photocatalytic treatment is a more effective alternative for the removal of soluble organic compounds [13,14]. Contrary to physical processes such as coagulation and adsorption, which mainly transfer the pollutants from wastewater to other media, photocatalysis involves complete mineralization of organic pollutants to carbon dioxide, water and mineral acids. Photodegradation using photocatalysts is remarkable method for organic pollutant degradation. Semiconductor photocatalysts had been widely employed for the treatment of dye contaminated water. Nowadays, scientific and engineering interest in
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semiconductor photocatalysts has grown extensively [15,16]. ZnS nanocrystals have been widely studied because of their versatile applications in optoelectronics and catalysis. However, its wide band gap (3.73 eV) limits its photocatalytic application in the visible-light region. The use of a photocatalyst material having visible-light activity is required for harnessing the full potential of sunlight in wastewater treatment. The synthesis of ZnS in glass matrices provides good optical properties and high stability. Recent studies revealed that rare-earth doped SiO2 -semiconductor nanocomposites are promising candidate for photocatalysis application [15] and the properties of the dopant exhibits synergism of properties of the catalyst. In such a composite, the semiconductor could act as a sensitizer to promote active rare earth emission by harvesting the excitation photon energy and then transferring it to the rare earth ion. Eu3+ has been widely used due to its high luminescent efficiency and band gap lowering potential [16]. Biopolymers based polymer nanocomposites have attracted much attention of researchers in different areas of nanoscience and technology. Biopolymers like chitosan, gum arabic, gum ghatti and gum xanthan have the advantage of easy availability, low cost and environment friendly nature over synthetic polymer nanocomposites [17–20]. Dispersion of the nanoparticles within the polymer matrices enhances the basic properties of the polymer like mechanical strength, stability and the surface area. Sodium alginate (Alg) is a biopolymer having less toxicity, biocompatible and biodegradable. Alg is a water soluble linear polysaccharide extract from brown sea weed and is composed of alternating blocks of 1–4 linked ␣-l-guluronic and -d-mannuronic acid residues. Unfortunately, natural biopolymers such as Alg face many drawbacks including less stability and difficulty in processing, therefore, the modification of the biopolymer through graft-copolymerization improves its physical and chemical properties [21,22] and the resulting graft co-polymers have better mechanical strength, biocompatibility and flexibility than those of the parent biopolymer [23–26]. Despite the numerous opportunities in the application of semiconductor nanomaterials and their anticipated potential use in wastewater treatment, there has been increasing interest in the environmental impact of such materials. Dispersing the nanoparticle material into a polymer matrix will modify the surface of the nanoparticle which will enhance its stability and mobility under natural conditions, and minimizing their toxic effects. In this research work, we have reported the synthesis of ZnS by co-precipitation method followed by preparation of ZnS doped SiO2 in the presence of Eu3+ . The obtained ZnS–SiO2 :Eu3+ was encapsulated into Alg-cl-poly(AAM-co-MAA) polymer matrix, where Alg-cl-poly(AAM-co-MAA) was prepared through graft copolymerization technique using N,N -methylene-bis-acrylamide (MBA) as a crosslinking agent and potassium persulphate (KPS)–ascorbic acid (ABC) redox pair as an initiator via free radical polymerization and the obtained material was employed in photocatalysis for the degradation of indigo carmine in aqueous solution.
2. Experimental 2.1. Materials and methods 2.1.1. Chemicals and reagents Na2 S, Zn(NO3 )·6H2 O, Eu(NO3 )3 ·6H2 O, nitric acid, sodium alginate (Alg), acrylamide (AAM), methacrylic acid (MAA), MBA, potassium persulfate (KPS) and ABC were purchased from Sigma–Aldrich and used without further purification. The stock solution of dye was prepared by dissolving appropriate amount of indigo carmine (Sigma–Aldrich) in 1000 mL of the deionized water
and the stock solution was further diluted to obtain the dye solutions of desired concentration. 2.1.2. Synthesis of ZnS–SiO2 –Eu3+ nanoparticle ZnS was prepared by coprecipitation method as follows: 10 mL of Na2 S (0.1 M) was slowly added to 10 mL Zn(NO3 )·6H2 O (0.1 M) with vigorous stirring on a magnetic stirrer. The mixture was further stirred for 30 min. The crystals were separated by centrifugation, washed several times with deionized water and ethanol to remove nitrate and chloride ions. ZnS precipitates were dried at 60 ◦ C overnight. In the next step, ZnS–SiO2 –Eu3+ was prepared by mixing 10.4 g of TEOS solution with 4.6 g of ethanol and 4.5 g of HNO3 (0.2 M). The mixture was stirred vigorously for 1 h at room temperature. An amount of 0.223 g Eu(NO3 )3 ·6H2 O was dissolved in 4.6 g of ethanol and added to the TEOS mixture and the mixture stirred for an hour. The resulting mixture was added to ZnS nanoparticles with molar ratio of 10:1 (Si:Zn) and stirred for 2 h. The resulting mixture was dried in an oven at 50 0 C overnight and heated at 200 ◦ C for 3 h. 2.1.3. Synthesis of Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposite Initially, certain amount of ZnS–SiO2 –Eu3+ was added to deionized water (10 mL) and sonicated for 30 min for the better dispersion of the nanaoparticles in the water. Then, Alg (1.0 g) was added in the reaction mixture followed by the addition of KPS (30 mg) and ABC (20 mg) and stirred vigorously. Fifty grams of the MBA was added with continuous stirring. In the last step, AAM (1.0 g) and MAA (2 mL) were added in the reaction mixture with continuous stirring. The reaction temperature was maintained at 60 ◦ C. After 3 h, the reaction vessel was taken out and allowed to cool at room temperature. The homo-polymers were removed using acetone extraction. Finally, the synthesized nanocomposite was dried in a hot air oven at 50 ◦ C till a constant weight was achieved.
2.1.4. Evaluation of photocatalytic activity The photocatalytic activity of the materials were evaluated through a suspension of 100 mg of the nanocatalyst in 100 mL of aqueous solution of indigo carmine (20 mg/L) that was kept under magnetic stirring and visible light filtered using dichroic UV filter ( > 420 nm) at room temperature. A 400 W tungsten filament (Eurolux), kept at a distance of 11 cm from the reaction vessel, was employed as a source of radiation. The samples were magnetically stirred in the dark for 1 h prior to illumination to allow for adsorption equilibrium. Aliquots of the suspension (5 mL) were withdrawn at periodic time intervals using disposable syringe and filtered through 0.4 m PVDF membrane filter at 30 min intervals for 4 h. The concentration of the indigo carmine remaining after illumination in the supernatant solution was determined using a Shimadzu UV-2450 spectrophotometer at = 610 nm. The photodegradation performance of the process was assessed in terms of decolorization efficiencies and kinetic studies.
2.1.5. Characterization Doping of ZnS–SiO2 nanophorphor with Eu3+ and successful and uniform incorporation of the nanophorphor with in the crosslinked polymer network of Alg-cl-P(AAM-co-MAA) hydrogel was evidenced using different characterization techniques like FTIR, Raman, XRD, SEM/EDAX, TEM and UV–vis spectroscopy. X-ray diffraction patterns were recorded on X-ray diffractometer (Rigagu Ultima IV) employing Cu K␣ radiation of the wavelength of 1.5406 A˚ with visible slights at 45 kV/40 mA.
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Fig. 1. XRD pattern of ZnS, ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
Alg-cl-poly(AAM-coFig. 2. FTIR spectra of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
Crystalline size of the samples were calculated using the Debye–Scherrer equation: D=
K ˇcos
145
(1)
where D is the crystal size of the catalyst, the X-ray wavelength, ˇ the full width at maximum (FWHM) of the diffraction peak (radian), K is a constant (0.9) and is the diffraction angle at the maximum. TEM images of the samples were taken on a field emission electron microscope, (JEOL, JEM-2100F) with a working voltage of 120 kV. Prior to TEM analysis, small pinch of the samples were dispersed in few milliliters of ethanol and sonicated for 30 min. About two drops of the suspension was transferred to a copper grid and allowed to dry at room temperature. Changes in the morphology of the polymer matrix of Alg-cl-P(AAM-co-MAA) hydrogel after the incorporation of Eu3+ doped ZnS–SiO2 nanophorphor was studied using scanning electron microscope coupled with EDAX (TESCAN, Vega 3 XMU). A small pinch of the samples were transferred to SEM grids and to have the conducting impact, the samples were carbon coated and scanning was synchronized with microscopic beam so as to maintain the small size over a large distance relative to the specimen. FT-IR spectra of the samples were recorded on PerkinElmer FT-IR spectrometer (Spectrum 100), Raman spectra of the samples were measured on PerkinElmer Raman microscope (Raman Micro 200), and optical absorption spectra were recored using a Shimadzu UV–vis spectrophotometer (UV-2450). 3. Results and discussion 3.1. XRD patterns The XRD patterns of ZnS, ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites are shown in Fig. 1. Three broad peaks were observed in the diffractogram for the ZnS (Fig. 1d) at around 29.03◦ , 48.13◦ , and 56.66◦ corresponding to (1 1 1), (2 2 0), and (3 1 1) planes of face centered cubic (fcc) of ZnS (JCPDS No. 05-0566) with average particle size of 3.21 nm. The presense of broad peaks in the XRD patterns is due to their small size [27]. Fig. 1c shows the XRD patterns of ZnS–SiO2 –Eu3+ . The broad peak at 22◦ shows the dominance of amorphous phase of SiO2 (average particle size is 3.75 nm). This peak was observed in the Alg-clpoly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ composite samples (average particle size is 3.88 nm) but shifted to a lower degree in the 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ sample with average particle size of 4.46 nm. The absence of the ZnS peaks in the
and
Alg-cl-poly(AAM-co-
composite samples is due to the high percentage of SiO2 . Also, the suppression of the cubic ZnS peaks may be due to the SiO2 impeding the formation of direct contact of zinc sulfide nanoparticles through the formation of Zn–S–Si linkage. The addition of SiO2 to ZnS decreased the crystallinity of ZnS nanoparticle. This phenomenon is attributed to the formation of Si–O networks by excessive SiO2 , which prevents the formation of cubic crystallites. 3.2. FTIR analysis The IR spectra of ZnS–SiO2 –Eu3+ core-shell and Algcl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites are depicted in Fig. 2. The IR band between 3350–3450 cm−1 and 1600–1640 cm−1 are due to the stretching and bending vibrations of structural hydroxyl groups of adsorbed water molecules [28,29]. Fig. 2c shows the IR spectrum of ZnS–SiO2 –Eu3+ . The peak observed between 1050 and 1110 cm−1 corresponds to the asymmetric stretching vibration mode of the Si O Si whereas the peak between 440 and 480 cm−1 corresponds to bending vibration modes of Si O Si. The IR bands at 2400–2450 cm−1 and 1625–1630 cm−1 are attributed to Si C and C O stretching vibrations. The peak at 600–620 cm−1 is assigned to Zn–S stretch [30]. This clearly shows that ZnS is successfully embedded in the silica matrix. Fig. 2b and c shows the IR spectra of 10% and 20% of Alg-clpoly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites, respectively. The band at 3425 cm−1 corresponds to amine groups and the peaks between 2870 cm−1 and 2950 cm−1 is caused by CH2 asymmetric stretching vibrations. The peak at around 1420 cm−1 is due to CH and CH2 in plane bending. In addition, the bands at 1618 cm−1 and 1730 cm−1 are due to CO stretching of carboxylic salt groups. The peaks at 1153 cm−1 are due to CN stretching of amide groups [26]. 3.3. Raman spectroscopy analysis Raman spectra of the samples are shown in Fig. 3. Raman bands between 460 and 490 cm−1 are due to symmetric stretching vibrations of bridging oxygen in predominantly 6-membered Si O Si and breathing motion of bridging oxygen in 4-membered SiO4 . Peak between 750 and 790 cm−1 is assigned to the bending and deformation modes of Si O Si whereas the peaks around 1050 cm−1 and 1177 cm−1 are ascribed to asymmetric stretching of bridging oxygens [31].
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respectively [24,25]. In the region between 1400 and 1270 cm−1 , peaks are assigned to the C H deformation vibrations and a band between 1100 and 1200 cm−1 assigned to the C O stretching vibrations [32–34]. The bands between 500 and 700 cm−1 are related to the deformation of pyranosyl rings and C O C vibration of glycosidic linkage [32–34]. 3.4. HRTEM and SEM-EDS studies
Fig. 3. Raman spectra of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
Alg-cl-poly(AAM-co-
To further demonstrate the incorporation of ZnS–SiO2 –Eu3+ into Alg-cl-poly(AAM-co-MAA) matrix, the Raman spectrum of ZnS–SiO2 –Eu3+ (Fig. 3c) was compared with the spectra of the Algcl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites (Fig. 3a and b). Many Raman peaks on the ZnS–SiO2 –Eu3+ core–shells have counterparts on the Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites, though some peak positions were shifted as a result of molecular interactions. All the nanocomposite samples present the characteristic asymmetric and symmetric bands of carboxylate ion (COO− ) at 1600–1610 cm−1 and near 1443 cm−1 ,
HRTEM was used to observe uniformity, morphology and microstructure of prepared ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites. Fig. 4(A) and (B) shows a HRTEM image of 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ and ZnS–SiO2 –Eu3+ photocatalysts, respectively. The particles are small and nearly spherical in shape. All the samples prepared show regular morphology. The elemental composition of the prepared samples was estimated by SEM-EDS analyses. Fig. 4(C) and (D) shows the EDS spectra of ZnS–SiO2 –Eu3+ and 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ , respectively. Both spectra confirm the presence of Si, Zn, O, S and Eu. The spectra indicate that the main components are Si and O with low contents of Eu and S. This confirms the formation of ZnS–SiO2 –Eu3+ . The presence of N and C in the EDS spectra for the Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites further confirm the encapsulation of ZnS–SiO2 –Eu3+ into the Algcl-poly(AAM-co-MAA) polymer matrix. 3.5. Optical studies One of the methods for probing the band structure of semiconductors is to measure the optical absorption spectrum. The
Fig. 4. HRTEM images of (A) 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ , (B) ZnS–SiO2 –Eu3+ and SEM-EDS spectrum of (C) ZnS–SiO2 –Eu3+ , (D) 20% Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ .
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Fig. 5. UV–vis absorption spectra of ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites.
optical technique is used to probe the allowed transitions between the valence band and conduction band states for nanoparticle which may show defects or impurity states in the band gap. Fig. 5 shows a comparison of the UV–vis absorption edge of ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites. The absorption spectra consist of intense absorption around 420 nm due to charge transfer from the valence band to the conduction band. The onset of absorption edge of the ZnS–SiO2 –Eu3+ is approximately 417 nm which corresponds to a band gap of 2.98 eV. This strong absorption in the visible region could be due to a number of factors. First, it may originate from the impurity levels of ZnS, the existence of Si and oxide related defects [35]. Second due to charge transfer from ZnS to Eu3+ ions embedded in the SiO2 matrix. The ZnS could act as a sensitizer by promoting Eu3+ emissions by harvesting the excitation photon energy and transferring it to the Eu3+ ions. The plot between reflectance and wavelength is depicted in Fig. 6. ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites showed a decrease in reflectance in the entire visible light region. In order to determine the band gap of the photocatalysts, the Kubelka–Munk function was used (Eq. (2)): F(R) =
K 1 − R2 = 2R S
Fig. 6. UV–vis DRS of ZnS–SiO2 –Eu3+ MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
and
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Fig. 7. Tauc plot for ZnS–SiO2 –Eu3+ and Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites.
where R, K and S are the reflectance, absorption and scattering coefficient, respectively. For a semiconductor material, the Kubelka–Munk function allows the construction of a Tauc plot: [F(R) × h]n versus h [36]. For a direct allowed band gap semiconductor, the plot with n = 1/2 will show a linear Tauc region above the optical absorption edge. Indirect band gap materials show a Tauc region with n = 2 [37]. The value of the optical band gap can be calculated by extrapolating the Kubelka–Munk function to K/S = 0. The band gap energies for the samples are shown in Fig. 7. These results are consistent with the UV–vis absorption estimation. 3.6. Photocatalytic studies
(2)
With time increasing from 0 to 5 h, the characteristic absorption band at peaks of 610 nm decreased gradually, indicating that the IC was gradually photodegraded by the catalyst (Fig. 8). The high photocatalytic performance of the catalysts could be assigned to a number of factors: (i) high visible-light absorption of the catalyst due to improved bang gap, (ii) the delayed electron–hole recombination due to trapping of electrons in the conduction band by Eu3+ orbital, (iii) efficient transfer of the charge carriers, and (iv) effective utilization of the charge carries by the reactants, as demonstrated by the UV–vis and UV–vis DRS analyses (Figs. 5 and 6). The linear relationship of ln Co /C versus time (Fig. 9) shows that the photocatalytic degradation of IC follows the
Alg-cl-poly(AAM-co-
Fig. 8. Indigo carmine photodegradation profile using 20% Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites.
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4. Conclusion ZnS–SiO2 –Eu3+ and a series of Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ nanocomposites were synthesized. The samples were characterized by various spectroscopic and analytical techniques. XRD and HRTEM analyses confirm the formation nanoparticles. The UV–vis and UV–vis DRS spectral analysis indicated that Eu3+ doping causes a red-shift in the absorption band, resulting in the reduction in band gaps. The synergic effect of ZnS and Eu3+ in the SiO2 is evidence in the photocatalytic performance of the catalyst. The ZnS–SiO2 –Eu3+ systems in Alg-cl-poly (AAM-co-MAA) are very effective visible-light active photocatalysts for the degradation of Indigo Carmine. Highest photocatalytic activity (93.4%) was observed for the 20% Alg-cl-poly(AAM-co-MAA sample after 5 h. The photocatalytic activity was mainly attributed to the intense light absorption in the visible region and narrow band gap energy. Fig. 9. Indigo carmine degradation profile ZnS–SiO2 –Eu3+ , Alg-cl-poly(AAMco-MAA)/ZnS–SiO2 –Eu3+ nanocomposites with different concentrations of ZnS–SiO2 –Eu3+ .
Acknowledgements Authors gratefully acknowledge the financial support from the Faculty of Science, University of Johannesburg and National Research Fund (NRF), South Africa for providing research fellowships to the authors H. Mittal and E. S. Agorku. Authors are also thankful to Ms. Nomsa Baloyi, Research Scientist, Metallurgical Engineering Department and Mr. Patrick Komane, Research Scientist, Department of Applied Chemistry, University of Johannesburg for their help during the characterization of the materials.
References
Fig. 10. Kinetics of Indigo carmine degradation in the first 5 h under visible light irradiation.
Table 1 Band gap and percentage degradation of IC after exposure to visible-light 10. Sample 3+
ZnS–SiO2 –Eu Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ (10%) Alg-cl-poly(AAM-coMAA)/ZnS–SiO2 –Eu3+ (20%) a
Band gap (eV)
Degradation (%)a
2.95 2.85
78.2 86.9
2.81
93.4
Measured after reaction for 5 h.
pseudo-first-order kinetics:(3) lnCCo = ktwhere, Co /C is the normalized IC concentration, t is the reaction time, and k is the apparent reaction rate constant. The value for k (Table 1, Fig. 10) for the 20% Alg-cl-poly(AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites (most efficient photocatalyst) is 1.51 × 10−2 min−1 which is about two times that of the ZnS–SiO2 –Eu3+ nanoparticles, 8.47 × 10−3 min−1 , further demonstrating that the Alg-cl-poly (AAM-co-MAA)/ZnS–SiO2 –Eu3+ nanocomposites exhibit high photocatalytic efficiency.
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