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Photodegradation of Organic Pollutants using Fe-doped ZnO
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 5485 – 5490
International Conference on Solid State Physics 2013 (ICSSP’13)
Photodegradation of organic pollutants using Fe-doped ZnO Shahid M. Ramaya*, Asif Mahmooda, Shahid Atiqb, Saadat Anwar Siddiqic, Shahzad Naseemb a
Department of Physics and Astronomy, College of Science, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia b Centre of Excellence in Solid State Physics, University of the Punjab, Quaid-e-Azam Campus, Lahore-54590, Pakistan c Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Defence Road, Off Raiwind Road, Lahore, Pakistan
Abstract
Fe-doped ZnO mediated by visible light having composition as Zn1-xFexO (x=0.0, 0.05) have been synthesized via sol-gel auto-combustion technique. The thermal and optical properties have been investigated systematically using thrmogravimetric analysis and UV/vis-spectrophotometer, respectively. The doping of Fe3+ at Zn-site in ZnO helped to reduce its band-gap energy, attributed to the creation of an extra level near band edges. The synthesized materials exhibited enhanced photocatalytic activity in the degradation of methylene blue under visible-light irradiation. Pseudo-first-order rate law has also been applied to understand the reaction kinetics of methylene blue when visiblelight is induced. © 2015 2015Published Elsevier Ltd. All rights © by Elsevier Ltd. reserved. Selectionand andPeer-review Peer-review under responsibility ofCommittee the Committee Members of International Conference on Solid State Physics Selection under responsibility of the Members of International Conference on Solid State Physics 2013 2013(ICSSP’13) (ICSSP’13). Keywords: Fe-doped ZnO; Organic pollutants; Photocatalysts
* E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and Peer-review under responsibility of the Committee Members of International Conference on Solid State Physics 2013 (ICSSP’13) doi:10.1016/j.matpr.2015.11.074
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Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
1. Introduction Transition metal doped ZnO and TiO2 have gained immense attention due to their vast applications in spintronics [1-2]. In addition, they have also played an important role in the photodegradation of organic pollutants and hence have become promising materials in the field of photocalysis, as well [3,4]. In spite of the fact that ZnO and TiO2 have low cost and exhibit complete mineralization, the disadvantage is that these materials discharge easily in water that hinders their recovery and pose potential loss of materials [5]. There are some solid-liquid separation techniques but of course, an additional cost is required. Hence, magnetic photocatalysts could be utilized for their recovery and re-use [6]. The researchers world-wide have been trying to convert UV sensitive materials to visible-light irradiated materials via incorporation of suitable dopants [7]. For instance, some researchers have doped transition metal ions such as Co2+, Ni2+ and Fe3+ in ZnO to improve the optical properties [8-9]. Fe3+ ions when doped in ZnO with optimized concentration could be utilized to separate the electron-hole pair by decreasing the band gap, which shift the absorption spectrum from UV to visible region [10]. In this context, here we report the synthesize of pure and Fe-doped ZnO with the nominal composition Zn0.95Fe0.05O using sol-gel auto-combustion technique. The samples were characterized systematically using, thermogravimetric analysis (TGA), Fourier transform infra-red spectroscopy (FT-IR) and UV-Vis spectrophotometer. The influence of Fe3+ concentration in the ZnO under visible-light irradiation has been studied using methylene blue as a model dye. 2. Experimental Details Sol-gel combustion route was adopted to prepare pure and Fe-doped nanocrystallites of ZnO with nominal compositions of Zn0.95-xFexO (x = 0.00, 0.05). Analytical grade zinc nitrate [Zn(NO3).6H2O, 10196-18-6], iron nitrate [Fe(NO3)3.9H2O, 7782-61-8] and citric acid [C6H8O7, 77-92-9] were weighed in proper stoichiometric molar ratios keeping metal nitrate to citric acid ratio as 1:2. The reagents were dissolved in 50 ml of deionized water. The solution was continuously stirred using a magnetic stirrer and heated for one hour at 100 °C on a hot plate. The brownish color of the xerogel was obtained through stirring and the temperature of the hot plate was raised to 400 °C. The gel was finally converted to a fine and fluffy powder after an exothermic reaction. The powder samples were sintered at 700 °C to establish the desired phases. Thermal stability of the nanocrystallites was determined using thermogravimeteric analysis (TGA), model 300 S II, EXSTAR, in the temperature range of 35 to 800 °C with heating rate of 9 °C per minute in air. FT-IR measurements were performed with spectrometer (Bruker Vertex 70) using KBr as a reference material. A Shimadzu (ISR-2600) UV-spectrometer) equipped with an accessory of diffuse reflection was used to record the UV-Vis absorption spectra of Fe-ZnO samples. 3. Results and Discussion The crystal structure of the prepared samples was confirmed to be hexagonal wurtzite-type as all the diffraction peaks characteristic of ZnO were present in the patterns. The detailed analysis of structural parameters have been discussed elsewhere. Fig. 1 shows the TGA spectra, recorded to study the thermal degradation behavior of the synthesized catalysts. The spectra reveals a rapid initial weight loss, a characteristic of desorption/drying of water. The maximum weight loss in the temperature range of 35-800 °C was about 1%, which could be attributed to the loss of residual water present in the samples. A marked difference is evident in the weight loss between pure and iron doped zinc oxide sample. The presence of additional nitrates in the doped sample may be the reason behind this behavior. No further degradation clearly proves the stability of the Fe3+-doped ZnO catalysts in a wide range of temperature.
Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 1.TGA spectra of pure and Fe doped ZnO
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Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 2 FTIR spectra of (a) ZnO, (b) Zn0.95Fe0.05O
The FTIR spectra of undoped and doped ZnO has been shown in Fig. 2, revealing broad peaks at 3460 cm-1 and 1645.2 cm-1, attributed to the surface-adsorbed water and OH-group, respectively [11]. A peak at 500 cm-1 belongs to ZnO [12]. With the addition of 5% Fe in ZnO, the absorption band value decreased to 428 cm-1. The change in the peak position of ZnO absorption band reveals that the Zn-O-Zn network is perturbed by the presence of Fe in its lattice. The diffuse reflection spectra of ZnO and Zn0.95Fe0.05O samples were recorded at RT by applying Kubelka-Munk (K-M) theory [13] for bandgap energy (Eg) determination. The K-M technique is based on the following equation;
F ( R) =
(1 − R) 2 = (αhυ ) n 2R
Where ‘R’ is the reflectance and F(R) is proportional to absorption co-efficient ‘α’.
(1)
Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 3. (αhν)1/2 vs photon energy (hν)
Fig. 4. Time-dependent UV–Vis absorption spectra of (a) ZnO, (b) Zn0.95Fe0.05O
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Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
The ZnO spectrum shows absorption at approximately 385 nm, which is in good agreement with the intrinsic energy bandgap of ZnO at 3.22 eV. The observed shift of bangap from UV to visible region can also be explained on the basis of Burstein-Moss effect. Fig. 3 shows a plot between (αhν)1/2 and hν, exhibiting a bandgap value of 3.1 eV for the Fe-doped ZnO sample. Fig. 4 shows the photoabsorption spectra of the degraded dye solutions with as-synthesized catalysts over wavelength range from 400-700 nm under different irradiation time (0-4 hrs). From the plots, it is clear that the absorption of MB gradually decreased by increasing the irradiation time, however, the degree of reduction is different for different catalysts. Based upon experimental observations, it is elucidated that the degradation of MB by increasing irradiation time under visible light in the presence of photocatalysts is due to increase in lifetime of excited charge carriers, which enhances the photocatalytic activity of Fe-doped ZnO [14]. 4. Conclusions Photocatalytic activity of pure and 5% Fe3+ doped ZnO samples prepared by sol-gel auto-combustion method have been investigated. The UV-Vis diffuse reflectance spectrometry displayed the decreased value of band gaps by the addition of Fe3+ in ZnO. Mmethylene blue was chosen as a model dye component that was degraded under visible light irradiation. The results revealed that the minimum degradation of the dye had taken place with pure ZnO, which was increased by adding Fe3+ into ZnO lattice. The increase in dye degradation was attributed to the increased life time of excited charge carriers. References [1] S.W. Jung, S.J. An, G.C. Yi, G.U. Jung, I.S. Lee, S. Cho, Appl. Phys. Lett. 80, 4561 (2002) [2] H. Ohno, Science 281, 951 (1998) [3] S. Dong, K. Xu, J. Liu, H. Cui Physica B 406, 3609 (2011) [4] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energ. Mat. Sol. C 77, 65 (2003) [5] R. Wang, D. Xu, J.B. Liu, K.W. Li, H. Wang, Chem. Eng. J. 162 , 455 (2011) [6] J. Herrmann, J. Disdier, P. Pichat, S. Malato, Blanco, Appl. Catal. B-Environ. 17, 15 (1998) [7] E. Casber, V.K. Sharma, X.Z. Li, Sep. Purif. Technol. 87, 1 (2012) [8] X.C. Liu, E.W. Shi, Z.Z. Chen, H.W. Zhang, L.X. Song, H. Wang, S.D. Yao, J. Cryst. Growth 296, 135 (2006) [9] E. Liu, P. Xiao, J.S. Chen, B.C. Lim, L. Li, Curr. Appl. Phys. 8, 408 (2008) [10] W. Huang, X. Tang, I. Felner, Y. Koltypin, A. Gedanken, Mater. Res. Bull. 37, 1721 (2002) [11] P.P. Pillasisc, R. Georage, D.E. Mccormack, M.K. Seery, H. Hayaden, J. Phys. Chem. C 111, 1605 (2007) [12] D. Jaykrushna, K. Deepa, C. Nonhydrolyti, J. Phys. Chem. V 114, 2544 (2010) [13] S. Kumar, Y.J. Kim, B.H. Koo, S.K. Sharma, M. Knobel, S. Gautam, K.H. Chae, H.K. Choi, C.G. Lee, J. Korean Phys. Soc. 55, 1060 (2009) [14] C. Ruby, K. Ashavani, P.C. Ram, J. Sol-Gel Sci. Techn. 63, 546 (2012)
ScienceDirect Materials Today: Proceedings 2 (2015) 5485 – 5490
International Conference on Solid State Physics 2013 (ICSSP’13)
Photodegradation of organic pollutants using Fe-doped ZnO Shahid M. Ramaya*, Asif Mahmooda, Shahid Atiqb, Saadat Anwar Siddiqic, Shahzad Naseemb a
Department of Physics and Astronomy, College of Science, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia b Centre of Excellence in Solid State Physics, University of the Punjab, Quaid-e-Azam Campus, Lahore-54590, Pakistan c Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Defence Road, Off Raiwind Road, Lahore, Pakistan
Abstract
Fe-doped ZnO mediated by visible light having composition as Zn1-xFexO (x=0.0, 0.05) have been synthesized via sol-gel auto-combustion technique. The thermal and optical properties have been investigated systematically using thrmogravimetric analysis and UV/vis-spectrophotometer, respectively. The doping of Fe3+ at Zn-site in ZnO helped to reduce its band-gap energy, attributed to the creation of an extra level near band edges. The synthesized materials exhibited enhanced photocatalytic activity in the degradation of methylene blue under visible-light irradiation. Pseudo-first-order rate law has also been applied to understand the reaction kinetics of methylene blue when visiblelight is induced. © 2015 2015Published Elsevier Ltd. All rights © by Elsevier Ltd. reserved. Selectionand andPeer-review Peer-review under responsibility ofCommittee the Committee Members of International Conference on Solid State Physics Selection under responsibility of the Members of International Conference on Solid State Physics 2013 2013(ICSSP’13) (ICSSP’13). Keywords: Fe-doped ZnO; Organic pollutants; Photocatalysts
* E-mail address: [email protected]
2214-7853 © 2015 Published by Elsevier Ltd. Selection and Peer-review under responsibility of the Committee Members of International Conference on Solid State Physics 2013 (ICSSP’13) doi:10.1016/j.matpr.2015.11.074
5486
Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
1. Introduction Transition metal doped ZnO and TiO2 have gained immense attention due to their vast applications in spintronics [1-2]. In addition, they have also played an important role in the photodegradation of organic pollutants and hence have become promising materials in the field of photocalysis, as well [3,4]. In spite of the fact that ZnO and TiO2 have low cost and exhibit complete mineralization, the disadvantage is that these materials discharge easily in water that hinders their recovery and pose potential loss of materials [5]. There are some solid-liquid separation techniques but of course, an additional cost is required. Hence, magnetic photocatalysts could be utilized for their recovery and re-use [6]. The researchers world-wide have been trying to convert UV sensitive materials to visible-light irradiated materials via incorporation of suitable dopants [7]. For instance, some researchers have doped transition metal ions such as Co2+, Ni2+ and Fe3+ in ZnO to improve the optical properties [8-9]. Fe3+ ions when doped in ZnO with optimized concentration could be utilized to separate the electron-hole pair by decreasing the band gap, which shift the absorption spectrum from UV to visible region [10]. In this context, here we report the synthesize of pure and Fe-doped ZnO with the nominal composition Zn0.95Fe0.05O using sol-gel auto-combustion technique. The samples were characterized systematically using, thermogravimetric analysis (TGA), Fourier transform infra-red spectroscopy (FT-IR) and UV-Vis spectrophotometer. The influence of Fe3+ concentration in the ZnO under visible-light irradiation has been studied using methylene blue as a model dye. 2. Experimental Details Sol-gel combustion route was adopted to prepare pure and Fe-doped nanocrystallites of ZnO with nominal compositions of Zn0.95-xFexO (x = 0.00, 0.05). Analytical grade zinc nitrate [Zn(NO3).6H2O, 10196-18-6], iron nitrate [Fe(NO3)3.9H2O, 7782-61-8] and citric acid [C6H8O7, 77-92-9] were weighed in proper stoichiometric molar ratios keeping metal nitrate to citric acid ratio as 1:2. The reagents were dissolved in 50 ml of deionized water. The solution was continuously stirred using a magnetic stirrer and heated for one hour at 100 °C on a hot plate. The brownish color of the xerogel was obtained through stirring and the temperature of the hot plate was raised to 400 °C. The gel was finally converted to a fine and fluffy powder after an exothermic reaction. The powder samples were sintered at 700 °C to establish the desired phases. Thermal stability of the nanocrystallites was determined using thermogravimeteric analysis (TGA), model 300 S II, EXSTAR, in the temperature range of 35 to 800 °C with heating rate of 9 °C per minute in air. FT-IR measurements were performed with spectrometer (Bruker Vertex 70) using KBr as a reference material. A Shimadzu (ISR-2600) UV-spectrometer) equipped with an accessory of diffuse reflection was used to record the UV-Vis absorption spectra of Fe-ZnO samples. 3. Results and Discussion The crystal structure of the prepared samples was confirmed to be hexagonal wurtzite-type as all the diffraction peaks characteristic of ZnO were present in the patterns. The detailed analysis of structural parameters have been discussed elsewhere. Fig. 1 shows the TGA spectra, recorded to study the thermal degradation behavior of the synthesized catalysts. The spectra reveals a rapid initial weight loss, a characteristic of desorption/drying of water. The maximum weight loss in the temperature range of 35-800 °C was about 1%, which could be attributed to the loss of residual water present in the samples. A marked difference is evident in the weight loss between pure and iron doped zinc oxide sample. The presence of additional nitrates in the doped sample may be the reason behind this behavior. No further degradation clearly proves the stability of the Fe3+-doped ZnO catalysts in a wide range of temperature.
Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 1.TGA spectra of pure and Fe doped ZnO
5487
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Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 2 FTIR spectra of (a) ZnO, (b) Zn0.95Fe0.05O
The FTIR spectra of undoped and doped ZnO has been shown in Fig. 2, revealing broad peaks at 3460 cm-1 and 1645.2 cm-1, attributed to the surface-adsorbed water and OH-group, respectively [11]. A peak at 500 cm-1 belongs to ZnO [12]. With the addition of 5% Fe in ZnO, the absorption band value decreased to 428 cm-1. The change in the peak position of ZnO absorption band reveals that the Zn-O-Zn network is perturbed by the presence of Fe in its lattice. The diffuse reflection spectra of ZnO and Zn0.95Fe0.05O samples were recorded at RT by applying Kubelka-Munk (K-M) theory [13] for bandgap energy (Eg) determination. The K-M technique is based on the following equation;
F ( R) =
(1 − R) 2 = (αhυ ) n 2R
Where ‘R’ is the reflectance and F(R) is proportional to absorption co-efficient ‘α’.
(1)
Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
Fig. 3. (αhν)1/2 vs photon energy (hν)
Fig. 4. Time-dependent UV–Vis absorption spectra of (a) ZnO, (b) Zn0.95Fe0.05O
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Shahid M. Ramay et al. / Materials Today: Proceedings 2 (2015) 5485 – 5490
The ZnO spectrum shows absorption at approximately 385 nm, which is in good agreement with the intrinsic energy bandgap of ZnO at 3.22 eV. The observed shift of bangap from UV to visible region can also be explained on the basis of Burstein-Moss effect. Fig. 3 shows a plot between (αhν)1/2 and hν, exhibiting a bandgap value of 3.1 eV for the Fe-doped ZnO sample. Fig. 4 shows the photoabsorption spectra of the degraded dye solutions with as-synthesized catalysts over wavelength range from 400-700 nm under different irradiation time (0-4 hrs). From the plots, it is clear that the absorption of MB gradually decreased by increasing the irradiation time, however, the degree of reduction is different for different catalysts. Based upon experimental observations, it is elucidated that the degradation of MB by increasing irradiation time under visible light in the presence of photocatalysts is due to increase in lifetime of excited charge carriers, which enhances the photocatalytic activity of Fe-doped ZnO [14]. 4. Conclusions Photocatalytic activity of pure and 5% Fe3+ doped ZnO samples prepared by sol-gel auto-combustion method have been investigated. The UV-Vis diffuse reflectance spectrometry displayed the decreased value of band gaps by the addition of Fe3+ in ZnO. Mmethylene blue was chosen as a model dye component that was degraded under visible light irradiation. The results revealed that the minimum degradation of the dye had taken place with pure ZnO, which was increased by adding Fe3+ into ZnO lattice. The increase in dye degradation was attributed to the increased life time of excited charge carriers. References [1] S.W. Jung, S.J. An, G.C. Yi, G.U. Jung, I.S. Lee, S. Cho, Appl. Phys. Lett. 80, 4561 (2002) [2] H. Ohno, Science 281, 951 (1998) [3] S. Dong, K. Xu, J. Liu, H. Cui Physica B 406, 3609 (2011) [4] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energ. Mat. Sol. C 77, 65 (2003) [5] R. Wang, D. Xu, J.B. Liu, K.W. Li, H. Wang, Chem. Eng. J. 162 , 455 (2011) [6] J. Herrmann, J. Disdier, P. Pichat, S. Malato, Blanco, Appl. Catal. B-Environ. 17, 15 (1998) [7] E. Casber, V.K. Sharma, X.Z. Li, Sep. Purif. Technol. 87, 1 (2012) [8] X.C. Liu, E.W. Shi, Z.Z. Chen, H.W. Zhang, L.X. Song, H. Wang, S.D. Yao, J. Cryst. Growth 296, 135 (2006) [9] E. Liu, P. Xiao, J.S. Chen, B.C. Lim, L. Li, Curr. Appl. Phys. 8, 408 (2008) [10] W. Huang, X. Tang, I. Felner, Y. Koltypin, A. Gedanken, Mater. Res. Bull. 37, 1721 (2002) [11] P.P. Pillasisc, R. Georage, D.E. Mccormack, M.K. Seery, H. Hayaden, J. Phys. Chem. C 111, 1605 (2007) [12] D. Jaykrushna, K. Deepa, C. Nonhydrolyti, J. Phys. Chem. V 114, 2544 (2010) [13] S. Kumar, Y.J. Kim, B.H. Koo, S.K. Sharma, M. Knobel, S. Gautam, K.H. Chae, H.K. Choi, C.G. Lee, J. Korean Phys. Soc. 55, 1060 (2009) [14] C. Ruby, K. Ashavani, P.C. Ram, J. Sol-Gel Sci. Techn. 63, 546 (2012)