Ecotoxicology and Environmental Safety ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes A. Gnanaprakasam a, V.M. Sivakumar a,n, P.L. Sivayogavalli b, M. Thirumarimurugan a a b
Department of Chemical Engineering, Coimbatore Institute of Technology, Coimbatore 641014, Tamilnadu, India Department of Petrochemical Engineering, JCT College of Engineering and Technology, Pichanur, Coimbatore 641105, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 October 2014 Received in revised form 22 April 2015 Accepted 27 April 2015
TiO2 nanoparticles have been synthesized from the inorganic precursor Ti [OC3H7]4 via sol–gel technique. Similarly, ZnO nanoparticles have also been synthesized from zinc sulfate precursor using precipitation method. The prepared nanoparticles was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques to study the morphology, structural configuration and its functionality. The average particle size for TiO2 and ZnO nanoparticles was 15.4 nm and 17.9 nm, respectively. The synthesized TiO2 and ZnO nanoparticles have been used for degradation of acid red 27 and coralene red F3BS dyes under the UV light. The regeneration of TiO2 photocatalyst was also tested. & 2015 Elsevier Inc. All rights reserved.
Keywords: TiO2 ZnO XRD SEM Photocatalytic process
1. Introduction Nowadays, more than 3000 azo dyes in textiles, rubber, paper, plastics, pharmaceutical and food industries are used. These dyes have poor exhaustion properties, reduced dissolved oxygen content, toxic, non biodegradable and it also posses high thermal stability. These dyes may lead to carcinogenicity, mutagenity, biorecalcitarance to human beings, aequatic systems and animals (Raju et al., 2007; Kuo-Cheng et al., 2003; Renmin et al., 2005). In addition, it may also cause cancer to human due to the splitting of soluble azodyes into corresponding aromatic amines by liver enzymes and intestinal flora (Li et al., 2005). Biological treatment is the traditional way to treat these azo dyes. But waste water treated in the biological process remain toxic after treatment (Marisa et al., 2015). Advanced oxidation processes (AOPs) are most attractive substitute for the above mentioned process. Because, AOPs are capable to oxidize the entire azo dyes into CO2 and H2O (Daneshvar et al., 2002). In the AOPs, semiconductor metal oxides such as TiO2, ZnO, CdS and ZnS could be used as photocatalyst under UV light for the treatment of dye effluents (Ahmad et al., 2007). Due to the unique properties and several potential technological applications,TiO2 and ZnO particles posses a great importance in recent research activities (Behnajady et al., 2007). Among the various kinds of metal oxide photocatalyst, TiO2 is vital n
due to its non-toxicity, strong oxidizing capability, and longstanding photostability (Jiaguo et al., 2006). ZnO is an inexpensive, wide band gap metal oxide (3.37 eV) and its photocatalytic behavior was also found to be similar to TiO2 (Vignesh et al., 2014). Titanium dioxide and zinc oxide are generally synthesized via different methods such as micro emulsion, including many variants of physical and chemical vapor deposition techniques (Ding et al., 2010), sol–gel method (Palmisano et al., 1988), precipitation method (Li et al., 2010), hydrothermal methods (Siddiqui et al., 2009) and etc.. Among these established methods, the sol–gel method is most convenient and superior method for TiO2 because of its simple process, low process cost and low temperature chemical method. But, for the synthesis of ZnO NPs, precipitation method is most suitable one because of its simple and cost effective method. In the present study, TiO2 and ZnO NPs were synthesized by sol–gel and precipitation methods respectively. The photocatalytic activities of prepared nanocatalyst were tested with acid red 27 and coralene red F3BS dyes. The reusability of synthesized TiO2 was tested with red 27 and coralene red F3BS dyes. There were no earlier reports of comparison of TiO2 and ZnO nanoparticles for the photocatalytic degradation of acid red 27 and coralene red F3BS dyes and its reusability.
Corresponding author. E-mail address: [email protected] (V.M. Sivakumar).
http://dx.doi.org/10.1016/j.ecoenv.2015.04.043 0147-6513/& 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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2
2. Experimental
2.3. Evaluation of photocatalytic activity
2.1. Preparation of TiO2 and ZnO nanoparticles
Acid red 27 with 25 ppm concentration was prepared by dissolving 2.5 g of dye in 100 mL distilled water. Similar procedure was adopted for preparation of 25 ppm coralene red F3BS dye. The prepared acid red 27 aqueous solution was taken in 250 mL beaker and loaded with 1.25 g of TiO2. Before irradiation, the reaction mixture was stirred under dark condition for 2 h to facilitate adsorption/desorption equilibrium. A 400 W medium pressure mercury vapor lamp with predominantly wavelength 365 nm was used as UV source. During the period of light irradiation, 5 mL sample was collected at regular intervals. Then, the samples were centrifuged to separate the photocatalyst. The supernatant liquid was analyzed by UV–vis spectrophotometer to determine the absorbance (λmax for acid red 27 and coralene red F3BS dyes are 560 nm and 379 nm respectively). Degradation percentage was calculated using the following equation,
Titanium dioxide nanoparticles were obtained by the sol–gel method. In brief, the sol of TiO2 was prepared by dissolving titanium tetra isopropoxide Ti[OC3H7]4 (12 mL) (starting precursor) in isopropanol (10 mL) (solvent). Then 150 mL of water and 5 mL of acetic acid were added into above mixture. Acetic acid was used as a chelating agent. The mixed solution was heated at 80 °C for 3 h with vigorous stirring. In order to prevent the agglomeration of the particles present in the sol, 1 mL of conc. nitric acid was added into it. The obtained viscous sol was dried in the oven at 100 °C for 10 h. To improve the crystallinity of the powder samples, the dried material was calcined at 450 °C for 2 h. Zinc oxide nanoparticle was synthesized by precipitation method. 17 g of zinc sulfate (ZnSO4 7H2O) (starting precursor) was dissolved in 65 mL of distilled water. Then, 100 mL of 1 N NaOH solution was added drop wise to the above solution. The resulting slurry was continuously stirred for 2 h, centrifuged and then washed twice with distilled water. The wet powder was dried at 120 °C for 12 h. Finally the dried material was calcined at 450 °C for 3 h. 2.2. Characterizations The prepared TiO2 and ZnO nanoparticles were characterized by X-ray diffraction using Shimadzu XRD 6000 with Cu-Kα radiation at 40 kV (Japan). FT-IR was recorded using Bruker (USA). The scanning electron micrographs were recorded with JEOL (Model 6390, Japan) operating at 20 kV on specimens upon which a thin layer of gold or carbon had been evaporated. Absorbance of pure and degraded dye was measured using UV–visible spectrophotometer (Merck, USA).
%Degradation = (A 0 − A)/A 0
(1)
where, A0 is the initial absorbance of aqueous dye solution and A is the absorbance of dye solution when time is 't'. The same procedure was adopted to evaluate the photocatalytic activity of prepared TiO2 for coralene red F3BS dye. The above mentioned method was followed for the prepared ZnO nanoparticles for both acid red 27 and coralene red F3BS dyes.
3. Results and discussion 3.1. FT-IR studies From the FTIR spectrum of TiO2, the band appeared in the region of 3400 cm 1 is due to the hydrogen bonding between H2O H2O over the surface of the TiO2 powder (Bezrodna et al.,
Fig. 1. (a) XRD pattern for TiO2 NPs and (b) XRD pattern for ZnO NPs.
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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Fig. 2. SEM images TiO2 nanoparticle (a) 5000 , (b) 10,000 , (c) 20,000 , and (d) 40,000 magnification.
Fig. 3. SEM images ZnO nanoparticle (a) 5000 , (b) 10,000 , (c) 20,000 , and (d) 40,000 magnification.
2002). The band observed at 2924 cm 1 is assigned to the organic residue which remained in TiO2 even after calcination (Jaykrushna and Deepa, 2010). The band at 1442 cm 1 can be attributed to the C–N bending mode of nitrate associated with organic moieties. The low energy region (below 1000 cm 1) observed at 532 cm 1, 462 cm 1 and 401 cm 1 indicates the stretching mode of M–O bonding in TiO2 network which is an important functional group for photocatalytic processes. From the spectrum of ZnO, the band appeared in the region of 3340 cm 1 is due to the hydrogen bonding between H2O H2O over the surface of the ZnO powder and band observed at 1111 cm 1 may be due to the deformation of –OH moiety. The
band appeared at 1620 and 617 cm 1 is attributed to stretching and bending vibrations of Zn–O bonding respectively. The different vibrational modes at 2090 and 2376 cm 1 could be attributed to the different group frequencies of residual group and reaction byproducts (Kalimuthu et al., 2013). 3.2. XRD Studies The X-ray diffraction peak of prepared TiO2 NPs is shown in Fig. 1(a).The strongest diffraction peaks correspond to (101), (200) and (211) plane reveals that the synthesized TiO2 NPs are highly crystalline in structure (MDI JADE.5 software). The average size (D)
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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of the TiO2 anatase crystalline powder was calculated by Scherrer’s equation
D = (kλ )/(βcosθ )
(2)
where, k is the Scherrer constant (0.9), λ is the wavelength of the X-ray irradiation (0.15 nm), β is the full width at half maximum (FWHM) and θ is X-ray diffraction peak (Attaia et al., 2008). The calculated average crystalline size of the TiO2 was found to be around 15.4 nm. The diffraction peak of prepared ZnO NPs is shown in Fig. 1(b). The diffraction peaks at 2θ values correspond to (101), (100), (002) crystal planes belongs to hexagonal wurtzite crystalline structure (MDI JADE.5 software). The average size (D) of the ZnO nanoparticles was calculated as 19.6 nm. 3.3. SEM – EDX analysis
Fig. 4. EDX Analysis of synthesized TiO2 nanoparticle.
SEM images of TiO2 and ZnO NPs at 55,000 , 30,000 , 20,000 , 1000 magnification are shown in Figs. 2 and 3, respectively. It could be seen that, the cluster of particles were formed by conjunction of the very fine particles of TiO2 and ZnO. The surface energy of the photocatalyst is increased due to its smaller particle size. It leads to the agglomeration of particles (Yousefi et al., 2013). Energy dispersive X ray (EDX) spectra of TiO2 and ZnO are shown in Figs. 4 and 5. EDS analysis reveals that TiO2 nanoparticles contain only Ti and O element. But ZnO particles contain Zn and O element with trace amount of S element. 3.4. Photocatalytic degradation of acid red 27 and coralene red F3BS
Fig. 5. EDX Analysis of synthesized ZnO nanoparticles.
Photocatalytic activity of prepared TiO2 and ZnO NPs were tested with acid red 27 and coralene red F3BS with an initial dye concentration 25 ppm, catalyst concentration 12.5 ppm and irradiation time of 120 and 60 min for acid red 27 and coralene red F3BS respectively. Fig. 6(a) and (b) shows the comparison of photocatalytic activity of prepared TiO2 and ZnO catalyst. Both the prepared nanocatalyst exhibits good photocatalytic activity under UV light. Photocatalytic activity of prepared catalyst (TiO2 and ZnO) was due to the defect states which caused by movement of electron (e ) from valance band to conduction band under UV light illumination. Electron in the conduction band may reduce oxygen molecule to yield super oxide radical and holes generated in the conduction band may react with hydroxyl ions to form
Fig. 6. (a) and (b) Photocatalytic activiy TiO2 and ZnO catalyst (12.5 mg/l) at Dye concentration¼25 ppm (a) Acid red27 (t ¼ 120 min) and (b) coralene red F3BS dye (t ¼ 60 min).
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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hydroxyl radicals. These radicals are strong oxidative agent which could convert organic molecules into CO2 and H2O molecules. It was observed that ZnO exhibits better photocatalytic activity than TiO2 for both acid red 27 and coralene red F3BS dyes. It may be due to ZnO absorbs organic molecules better than TiO2 nanoparticles. From Fig. 6(a) and (b), there is a sharp increase in the degradation percentage, and then increases gradually. It may be due to organic molecules which were adsorbed on the catalyst were readily available for photocatalytic degradation. After some time, adsorption equilibrium might be affected by fast photocatalytic degradation which reduces the availability of adsorbed organic molecules on surface of photocatalyst. The reusability of the photocatalyst found decreases over few cycles which may be due to the loss of adsorption activity between the catalyst and the dyes which also reduces the efficiency of the photocatalyst (Saggioro et al., 2011). The reusability of TiO2 nanocatalyst was tested for both the dyes and the reusability of TiO2 for the Acid red 27 dye is found better than the coralene red F3BS.
4. Conclusion In the present study, nanocrystalline TiO2 and ZnO particle were prepared through a sol–gel method and precipitation method respectively. XRD analysis confirms that the prepared TiO2 and ZnO were nanocrystalline structure and sizes of the particle were calculated as 15.4 and 17.9 nm, respectively. Photocatalytic activity of prepared catalyst were tested with coralene red F3BS dyes and acid red 27 dyes and from the result, ZnO found to be good compared to TiO2 for both dyes. The reusability of TiO2 nanocatalyst was tested for both the dyes and the reusability of TiO2 for the Acid red 27 dye is found better than the coralene red F3BS.
Acknowledgment The authors convey sincere thanks to the Management and Principal of Coimbatore Institute of Technology, Coimbatore641014 for sponsoring the project through TEQIP-II fund.
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References Ahmad, A., Awan, G.H., Aziz, S., 2007. Synthesis and applications of TiO2 nanoparticles. Pakistan Engineering Congress, 70th Annual Session Proceedings, 676, pp. 405–412.
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
Contents lists available at ScienceDirect
Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes A. Gnanaprakasam a, V.M. Sivakumar a,n, P.L. Sivayogavalli b, M. Thirumarimurugan a a b
Department of Chemical Engineering, Coimbatore Institute of Technology, Coimbatore 641014, Tamilnadu, India Department of Petrochemical Engineering, JCT College of Engineering and Technology, Pichanur, Coimbatore 641105, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 October 2014 Received in revised form 22 April 2015 Accepted 27 April 2015
TiO2 nanoparticles have been synthesized from the inorganic precursor Ti [OC3H7]4 via sol–gel technique. Similarly, ZnO nanoparticles have also been synthesized from zinc sulfate precursor using precipitation method. The prepared nanoparticles was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques to study the morphology, structural configuration and its functionality. The average particle size for TiO2 and ZnO nanoparticles was 15.4 nm and 17.9 nm, respectively. The synthesized TiO2 and ZnO nanoparticles have been used for degradation of acid red 27 and coralene red F3BS dyes under the UV light. The regeneration of TiO2 photocatalyst was also tested. & 2015 Elsevier Inc. All rights reserved.
Keywords: TiO2 ZnO XRD SEM Photocatalytic process
1. Introduction Nowadays, more than 3000 azo dyes in textiles, rubber, paper, plastics, pharmaceutical and food industries are used. These dyes have poor exhaustion properties, reduced dissolved oxygen content, toxic, non biodegradable and it also posses high thermal stability. These dyes may lead to carcinogenicity, mutagenity, biorecalcitarance to human beings, aequatic systems and animals (Raju et al., 2007; Kuo-Cheng et al., 2003; Renmin et al., 2005). In addition, it may also cause cancer to human due to the splitting of soluble azodyes into corresponding aromatic amines by liver enzymes and intestinal flora (Li et al., 2005). Biological treatment is the traditional way to treat these azo dyes. But waste water treated in the biological process remain toxic after treatment (Marisa et al., 2015). Advanced oxidation processes (AOPs) are most attractive substitute for the above mentioned process. Because, AOPs are capable to oxidize the entire azo dyes into CO2 and H2O (Daneshvar et al., 2002). In the AOPs, semiconductor metal oxides such as TiO2, ZnO, CdS and ZnS could be used as photocatalyst under UV light for the treatment of dye effluents (Ahmad et al., 2007). Due to the unique properties and several potential technological applications,TiO2 and ZnO particles posses a great importance in recent research activities (Behnajady et al., 2007). Among the various kinds of metal oxide photocatalyst, TiO2 is vital n
due to its non-toxicity, strong oxidizing capability, and longstanding photostability (Jiaguo et al., 2006). ZnO is an inexpensive, wide band gap metal oxide (3.37 eV) and its photocatalytic behavior was also found to be similar to TiO2 (Vignesh et al., 2014). Titanium dioxide and zinc oxide are generally synthesized via different methods such as micro emulsion, including many variants of physical and chemical vapor deposition techniques (Ding et al., 2010), sol–gel method (Palmisano et al., 1988), precipitation method (Li et al., 2010), hydrothermal methods (Siddiqui et al., 2009) and etc.. Among these established methods, the sol–gel method is most convenient and superior method for TiO2 because of its simple process, low process cost and low temperature chemical method. But, for the synthesis of ZnO NPs, precipitation method is most suitable one because of its simple and cost effective method. In the present study, TiO2 and ZnO NPs were synthesized by sol–gel and precipitation methods respectively. The photocatalytic activities of prepared nanocatalyst were tested with acid red 27 and coralene red F3BS dyes. The reusability of synthesized TiO2 was tested with red 27 and coralene red F3BS dyes. There were no earlier reports of comparison of TiO2 and ZnO nanoparticles for the photocatalytic degradation of acid red 27 and coralene red F3BS dyes and its reusability.
Corresponding author. E-mail address: [email protected] (V.M. Sivakumar).
http://dx.doi.org/10.1016/j.ecoenv.2015.04.043 0147-6513/& 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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2
2. Experimental
2.3. Evaluation of photocatalytic activity
2.1. Preparation of TiO2 and ZnO nanoparticles
Acid red 27 with 25 ppm concentration was prepared by dissolving 2.5 g of dye in 100 mL distilled water. Similar procedure was adopted for preparation of 25 ppm coralene red F3BS dye. The prepared acid red 27 aqueous solution was taken in 250 mL beaker and loaded with 1.25 g of TiO2. Before irradiation, the reaction mixture was stirred under dark condition for 2 h to facilitate adsorption/desorption equilibrium. A 400 W medium pressure mercury vapor lamp with predominantly wavelength 365 nm was used as UV source. During the period of light irradiation, 5 mL sample was collected at regular intervals. Then, the samples were centrifuged to separate the photocatalyst. The supernatant liquid was analyzed by UV–vis spectrophotometer to determine the absorbance (λmax for acid red 27 and coralene red F3BS dyes are 560 nm and 379 nm respectively). Degradation percentage was calculated using the following equation,
Titanium dioxide nanoparticles were obtained by the sol–gel method. In brief, the sol of TiO2 was prepared by dissolving titanium tetra isopropoxide Ti[OC3H7]4 (12 mL) (starting precursor) in isopropanol (10 mL) (solvent). Then 150 mL of water and 5 mL of acetic acid were added into above mixture. Acetic acid was used as a chelating agent. The mixed solution was heated at 80 °C for 3 h with vigorous stirring. In order to prevent the agglomeration of the particles present in the sol, 1 mL of conc. nitric acid was added into it. The obtained viscous sol was dried in the oven at 100 °C for 10 h. To improve the crystallinity of the powder samples, the dried material was calcined at 450 °C for 2 h. Zinc oxide nanoparticle was synthesized by precipitation method. 17 g of zinc sulfate (ZnSO4 7H2O) (starting precursor) was dissolved in 65 mL of distilled water. Then, 100 mL of 1 N NaOH solution was added drop wise to the above solution. The resulting slurry was continuously stirred for 2 h, centrifuged and then washed twice with distilled water. The wet powder was dried at 120 °C for 12 h. Finally the dried material was calcined at 450 °C for 3 h. 2.2. Characterizations The prepared TiO2 and ZnO nanoparticles were characterized by X-ray diffraction using Shimadzu XRD 6000 with Cu-Kα radiation at 40 kV (Japan). FT-IR was recorded using Bruker (USA). The scanning electron micrographs were recorded with JEOL (Model 6390, Japan) operating at 20 kV on specimens upon which a thin layer of gold or carbon had been evaporated. Absorbance of pure and degraded dye was measured using UV–visible spectrophotometer (Merck, USA).
%Degradation = (A 0 − A)/A 0
(1)
where, A0 is the initial absorbance of aqueous dye solution and A is the absorbance of dye solution when time is 't'. The same procedure was adopted to evaluate the photocatalytic activity of prepared TiO2 for coralene red F3BS dye. The above mentioned method was followed for the prepared ZnO nanoparticles for both acid red 27 and coralene red F3BS dyes.
3. Results and discussion 3.1. FT-IR studies From the FTIR spectrum of TiO2, the band appeared in the region of 3400 cm 1 is due to the hydrogen bonding between H2O H2O over the surface of the TiO2 powder (Bezrodna et al.,
Fig. 1. (a) XRD pattern for TiO2 NPs and (b) XRD pattern for ZnO NPs.
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
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Fig. 2. SEM images TiO2 nanoparticle (a) 5000 , (b) 10,000 , (c) 20,000 , and (d) 40,000 magnification.
Fig. 3. SEM images ZnO nanoparticle (a) 5000 , (b) 10,000 , (c) 20,000 , and (d) 40,000 magnification.
2002). The band observed at 2924 cm 1 is assigned to the organic residue which remained in TiO2 even after calcination (Jaykrushna and Deepa, 2010). The band at 1442 cm 1 can be attributed to the C–N bending mode of nitrate associated with organic moieties. The low energy region (below 1000 cm 1) observed at 532 cm 1, 462 cm 1 and 401 cm 1 indicates the stretching mode of M–O bonding in TiO2 network which is an important functional group for photocatalytic processes. From the spectrum of ZnO, the band appeared in the region of 3340 cm 1 is due to the hydrogen bonding between H2O H2O over the surface of the ZnO powder and band observed at 1111 cm 1 may be due to the deformation of –OH moiety. The
band appeared at 1620 and 617 cm 1 is attributed to stretching and bending vibrations of Zn–O bonding respectively. The different vibrational modes at 2090 and 2376 cm 1 could be attributed to the different group frequencies of residual group and reaction byproducts (Kalimuthu et al., 2013). 3.2. XRD Studies The X-ray diffraction peak of prepared TiO2 NPs is shown in Fig. 1(a).The strongest diffraction peaks correspond to (101), (200) and (211) plane reveals that the synthesized TiO2 NPs are highly crystalline in structure (MDI JADE.5 software). The average size (D)
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
4
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of the TiO2 anatase crystalline powder was calculated by Scherrer’s equation
D = (kλ )/(βcosθ )
(2)
where, k is the Scherrer constant (0.9), λ is the wavelength of the X-ray irradiation (0.15 nm), β is the full width at half maximum (FWHM) and θ is X-ray diffraction peak (Attaia et al., 2008). The calculated average crystalline size of the TiO2 was found to be around 15.4 nm. The diffraction peak of prepared ZnO NPs is shown in Fig. 1(b). The diffraction peaks at 2θ values correspond to (101), (100), (002) crystal planes belongs to hexagonal wurtzite crystalline structure (MDI JADE.5 software). The average size (D) of the ZnO nanoparticles was calculated as 19.6 nm. 3.3. SEM – EDX analysis
Fig. 4. EDX Analysis of synthesized TiO2 nanoparticle.
SEM images of TiO2 and ZnO NPs at 55,000 , 30,000 , 20,000 , 1000 magnification are shown in Figs. 2 and 3, respectively. It could be seen that, the cluster of particles were formed by conjunction of the very fine particles of TiO2 and ZnO. The surface energy of the photocatalyst is increased due to its smaller particle size. It leads to the agglomeration of particles (Yousefi et al., 2013). Energy dispersive X ray (EDX) spectra of TiO2 and ZnO are shown in Figs. 4 and 5. EDS analysis reveals that TiO2 nanoparticles contain only Ti and O element. But ZnO particles contain Zn and O element with trace amount of S element. 3.4. Photocatalytic degradation of acid red 27 and coralene red F3BS
Fig. 5. EDX Analysis of synthesized ZnO nanoparticles.
Photocatalytic activity of prepared TiO2 and ZnO NPs were tested with acid red 27 and coralene red F3BS with an initial dye concentration 25 ppm, catalyst concentration 12.5 ppm and irradiation time of 120 and 60 min for acid red 27 and coralene red F3BS respectively. Fig. 6(a) and (b) shows the comparison of photocatalytic activity of prepared TiO2 and ZnO catalyst. Both the prepared nanocatalyst exhibits good photocatalytic activity under UV light. Photocatalytic activity of prepared catalyst (TiO2 and ZnO) was due to the defect states which caused by movement of electron (e ) from valance band to conduction band under UV light illumination. Electron in the conduction band may reduce oxygen molecule to yield super oxide radical and holes generated in the conduction band may react with hydroxyl ions to form
Fig. 6. (a) and (b) Photocatalytic activiy TiO2 and ZnO catalyst (12.5 mg/l) at Dye concentration¼25 ppm (a) Acid red27 (t ¼ 120 min) and (b) coralene red F3BS dye (t ¼ 60 min).
Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
A. Gnanaprakasam et al. / Ecotoxicology and Environmental Safety ∎ (∎∎∎∎) ∎∎∎–∎∎∎
hydroxyl radicals. These radicals are strong oxidative agent which could convert organic molecules into CO2 and H2O molecules. It was observed that ZnO exhibits better photocatalytic activity than TiO2 for both acid red 27 and coralene red F3BS dyes. It may be due to ZnO absorbs organic molecules better than TiO2 nanoparticles. From Fig. 6(a) and (b), there is a sharp increase in the degradation percentage, and then increases gradually. It may be due to organic molecules which were adsorbed on the catalyst were readily available for photocatalytic degradation. After some time, adsorption equilibrium might be affected by fast photocatalytic degradation which reduces the availability of adsorbed organic molecules on surface of photocatalyst. The reusability of the photocatalyst found decreases over few cycles which may be due to the loss of adsorption activity between the catalyst and the dyes which also reduces the efficiency of the photocatalyst (Saggioro et al., 2011). The reusability of TiO2 nanocatalyst was tested for both the dyes and the reusability of TiO2 for the Acid red 27 dye is found better than the coralene red F3BS.
4. Conclusion In the present study, nanocrystalline TiO2 and ZnO particle were prepared through a sol–gel method and precipitation method respectively. XRD analysis confirms that the prepared TiO2 and ZnO were nanocrystalline structure and sizes of the particle were calculated as 15.4 and 17.9 nm, respectively. Photocatalytic activity of prepared catalyst were tested with coralene red F3BS dyes and acid red 27 dyes and from the result, ZnO found to be good compared to TiO2 for both dyes. The reusability of TiO2 nanocatalyst was tested for both the dyes and the reusability of TiO2 for the Acid red 27 dye is found better than the coralene red F3BS.
Acknowledgment The authors convey sincere thanks to the Management and Principal of Coimbatore Institute of Technology, Coimbatore641014 for sponsoring the project through TEQIP-II fund.
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Please cite this article as: Gnanaprakasam, A., et al., Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic degradation of azodyes. Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.04.043i
