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Investigation on Degradation of Methyl Parathion using Visible light in the presence of Cr. +3 and N-doped TiO2. J.Senthilnathan. 1,a and Ligy Philip. 2, b.

Advanced Materials Research Vols. 93-94 (2010) pp 280-283 Online available since 2010/Jan/12 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.93-94.280

Investigation on Degradation of Methyl Parathion using Visible light in the presence of Cr+3 and N-doped TiO2 J.Senthilnathan1,a and Ligy Philip2, b 1,2

Indian Institute of Technology Madras, EWRE Division, Department of Civil Engineering, Chennai-600036, India a [email protected] b [email protected], Keywords: N-doped TiO2, Triethyl amine, Chromium, Visible light

Abstract. Nitrogen (N) and Chromium (Cr) doped TiO2 was prepared by sol-gel process. A clear shift in the onset light absorption from UV region (500 nm) was observed for the N-doped samples. The XRD results revealed the decrease in particle size with increasing N content in the lattice of anatase TiO2. Nitrogen doping does not alter the phase of anatase TiO2. The SEM and TEM images of N-TiO2 showed that the particle size is in the range of 20-25nm. The chemical nature of N in the N-TiO2 was evolved through X-ray photoelectron spectroscopy. The presence of different types of N species have been observed corresponding to different oxidation states and the presence of Ti–N and O–Ti–N have been confirmed from the observed binding energy values. Photocatalytic decomposition of methyl parathion was carried out in the visible region and found that N-doped TiO2 showed better catalytic activity than Cr and Cr/N doped TiO2. Introduction The efficient utilization of solar energy is one of the major goals of modern science and engineering that will have a great impact on technological applications [1]. Titanium dioxide (TiO2) remains the most promising catalyst because of its high efficiency, low cost, chemical inertness and photostability [2]. Although advanced oxidation process with TiO2 photo-catalysts have been shown to be an effective alternative in this regard, the vital snag of TiO2 semiconductor is that it absorbs a small portion of solar spectrum in the UV region (band gap energy of TiO2 is 3.2 eV). To harvest maximum solar energy, it is necessary to shift the absorption threshold towards visible region [3]. The main focus of this study is to utilize visible light to remediate pesticide contaminated drinking water using doped TiO2. The shifting of TiO2 absorption into visible light region mainly focuses on the doping with transition metals [4]. However, the thermal instability and tendency to form charge carrier recombination centers, as well as the expensive ion implantation facilities make metal doped TiO2 impractical [5]. In this study, synthesis of cation and anion-doped TiO2 photocatalysts was done by sol-gel process using titanium isopropoxide. The synthesized doped TiO2 materials were characterized with XRD, XPS, TEM and UV analytical techniques and its photocatalytic activity was tested by using methyl parathion as a target pollutant under visible light radiation. Experimental Methods Titanium tetra-isopropoxide (purity over 98%), triethyl amine amine and ethyl alcohol purchased from Ranbaxy Chemicals, India, were used for the preparation of Cr and N-doped TiO2. Methyl parathion, HPLC grade purchased from Ranbaxy Chemicals, India and methyl parathion (80%) of commercial grade purchased form local market were used in the present study. A Pyrex glass tube with an inner surface area of 169.56 cm2 (with a height of 90 mm and a diameter of 60 mm) was used for coating the photocatalyst. Before coating, the suspension was sonicated for 15 min. and Pyrex tube was inserted slowly into the suspension. It was allowed to stay in the suspension for 5 min, then taken out and dried in an oven for 30 min at 150°C. Before adding the methyl parathion to the batch reactor (400 mL), high pressure tungsten visible lamp, which will emits wave length more than >400 nm was warmed up for 15 min to attain sufficient energy. Oxygen flow rate of 300 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 140.116.30.199, National Cheng Kung University, Tainan, Taiwan-16/09/14,04:04:15)

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mL/min and a stirring rate of 150 rpm were maintained for all the experiments. The stock solutions of each pesticide of desired concentration were prepared in double distilled water and HPLC grade acetone. Results and Discussions Photocatalytic Activity of Cr, Cr/N and N Doped TiO2 Cr, Cr/N and N doped TiO2 nano particles were prepared with sol-gel method. Photo catalytic properties of doped TiO2 were studied with respect to the degradation of methyl parathion (1 mg/L). A reaction time of 7h was maintained for all the initial studies under visible light. The doping concentrations of Cr and Cr/N varied from 0 to 2.5% and N doping was varied from 1:0.4 to 1:1.6 ratios of titanium isopropoxide and triethylamine. Cr/N doped TiO2 showed maximum degradation of 69.6% and Cr doped TiO2 showed only 48.15% degradation. N doped TiO2 showed greater photocatalytic activity compared to Cr and Cr/N doped TiO2 and complete degradation of methyl parathion (1mg/L) was achieved within 1.5h. The reason for the improvement of photocatalytic activity of N-doped TiO2 compared to Cr and Cr/N doped TiO2 is attributed to the decrease of the band gap, which is due to either mixing of the nitrogen 2p states with O 2p states on the top of the valence band [5] or the creation of a N-induced mid gap level [6]. The photocatalytic activity of Cr and Cr/N doped TiO2 is less compared to N-doped TiO2 because the localized d states orbital may appear deep in the band gap of the host semiconductor. This results in the increase of the carrier recombination. Therefore, the lifetime of the mobile carriers may become shorter, giving lower photocatalytic activity [7]. Hence, N-doped TiO2 was used for further studies and the results are given in Fig.1. XPS and UV Analysis of N-Doped TiO2 and Anatase TiO2 The XPS technique monitors the electron binding energy of elements within a few nanometers of the particle surfaces. The anatase TiO2 and N-doped TiO2 samples were examined for three areas of the XPS spectrum such as Ti 2p region near 460eV, O 1s region near 530eV and N 1s region near 400eV. Most of the N 1s binding energies found in the literature are in between 396eV and 408eV [6]. However, the binding energy of the N 1s is highly dependent on the synthetic method used for the preparation of N-doped TiO2 and most of the N 1s binding energies are found in the range of 400eV. XPS analysis of anatase of TiO2 and N-doped TiO2 (1:0.8 and 1:1.6 ratio) are given in Fig. 2. Both the samples of N-doped TiO2 showed peak at 400eV after carbon correction. The peak at 400eV indicates N atoms incorporated into the TiO2 lattice and it was observed that a peak towards 400eV could be assigned to Ti bound to O or to the O–Ti–N formation [8]. From the above observations it can be concluded that the chemical states of the nitrogen doped into TiO2 may vary and coexist in the form of N-Ti-O and Ti-O-N. In XPS spectrum of Ti 2p indicates that the central Ti ion is very sensitive to electronic environment of neighboring elements. By introducing more electron rich N into the matrix, the binding energy of Ti can be significantly lowered [9]. The Ti 2p region of anatase TiO2 and N-TiO2 showed 459.5 eV and 459.1 eV respectively and the results are given in Fig.3. The binding energy of Ti 2p after nitrogen doping decreases and suggests different electronic interactions of Ti with N ion, which causes partial electron transformation from the N to the Ti and an increase of the electron density on Ti because of the lower electronegativity of nitrogen compared to oxygen [10]. This confirms that nitrogen incorporates into the lattice and replaces for oxygen. In this study, UV-Vis spectrum for different ratios of (triethyl amine) N-doped TiO2 was studied. The visible light absorption of all the samples of N doped TiO2 increased in the range of 400 nm to 600 nm. 1:0.8 ratio of N-doped TiO2 showed maximum absorption compared to 1:0.2 and 1:0.4 ratio. Further increasing the ratio of triethyl amine in TiO2 (1:1.2 and 1.6 ratios) decreased the visible light absorption. Pure TiO2 did not show any absorption in this range. UVvisible light absorption of different grades of N-doped TiO2 is presented in Fig. 4.

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% of degradation

80

60

40

20

0

Cr doped TiO2

Cr/N doped TiO2

N doped TiO2

anatase TiO2

Fig. 1. Phtocatalytic activity of doped TiO2

Fig. 2. XPS of pure anatase and N-doped TiO2 2.1 1.8

(a) 1:0 ratio (b) 1:0.4 ratio (c) 1:0.8 ratio (d) 1:1.2 ratio (e) 1:0.8 ratio

(c)

Abs.

1.5 1.2 0.9

(d) (b) (e)

0.6 0.3 0 200

300

400

500

600

Wavelength nm

Fig. 3. XPS of Ti 2p spectrum of N-doped TiO2 (1:0.8 ratio) Fig. 4. UV-Visible absorption spectrum

Characterizations of N-doped TiO2 using XRD, SEM and TEM Analysis The N-doped TiO2 showed X-ray line broadening compared to TiO2, which represents the formation of nano particles. The broadening of the peak gradually increased up to 1:0.8 ratio of Ndoped TiO2. However, no significant changes were observed by further increasing the N-ratio. From the intensity distribution of the particular reflections, the integral intensity of the X-ray diffraction patterns and average crystalline sizes can be calculated. The broadening of the XRD peaks is inversely proportional to the crystalline size of the N-doped TiO2 nano particles. There was no change in the d space values of all the N-doped TiO2. This indicates that N doping was introduced into the lattice of the TiO2 without altering the average unit cell dimension. The crystal grain sizes of anatase TiO2 was 70.3nm. With increase in the N-doping in TiO2, the particle size gradually decreases and reaches minimum value around 25.4 nm [10]. The crystal grain sizes of N-doped TiO2 calculated by Scherrer equation are given in Table-1. The particle size of N-doped TiO2 was further confirmed by SEM and TEM analysis. The particle size and morphology of N-doped TiO2 (1:0.8 ratio) catalyst were studied using SEM. It is clear from Fig. 5, that the size of TiO2 particles is approximately 20-30 nm. The exact size of nano particles is very difficult to determine using SEM analysis. Therefore TEM analysis was carried out to find the size of TiO2. From the TEM analysis it was found that the particle size of N-doped TiO2 (1:0.8 ratio) is ~ 22 nm. TEM analysis of N-doped TiO2 is given in Fig.6. Table-1: Determination of crystal grain size using Scherrer equation S.No 1 2 3 4 5

Photocatalyst TiO2 TiO2/N (1:0.2) TiO2/N (1:0.4) TiO2/N (1:0.8) TiO2/N (1:1.2)

β (fwhm) 0.011808 0.014366 0.015044 0.031350 0.027281

cos θ 0.0163 0.0167 0.0161 0.0169 0.0168

Particle size 70.3nm 56.2nm 55.7nm 25.4nm 29.4nm

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Fig. 5: SEM analysis of 1:0.8 ratio N-doped Fig. 6: TEM analysis of 1:0.8 ratio N-doped

TiO2

TiO2

Conclusions In conclusion this study demonstrates the effectiveness of using nano TiO2 in developing efficient visible-light activated photocatalysts by N doping. This study could provide a pathway for the production of environmentally compassionate photocatalysts. Photocatalytic degradation of methyl parathion with N-doped TiO2 showed better catalytic properties compared to Cr and Cr/N doped TiO2. From the binding energy of XPS study it was confirmed that N replaces the O in the lattice of TiO2 and forms O-Ti-N. XRD spectrum showed crystal grain sizes gradually decreased with increasing the N-ratio in TiO2. TEM and SEM analysis confirmed the particle sizes are in the range of ~25-30 nm. The Methyl parathion was completely mineralized under the visible light in presence of N-doped TiO2. References [1] D.S. Ollis and H. Al-Ekabi: Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, (1993). [2] J.P. Wilcoxon: Photocatalysis Using Semiconductor Nanoclusters, Advanced Catalytic Materials, MRS Proc. Boston, MA, (1998). [3] D. Chatterjee and A. Mahata: Journal of Photochemistry and Photobiology A: Chemistry Vol. 165, (2004) (1-3), p19-23. [4] W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem. 98, (1994), p13669-13679. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga: Science, Vol. 293, (2001), p 269271. [6] R. Nakamura, T. Tanaka and Y. Nakato: Journal of Physical Chemistry B Vol.108 (2004), p10617-10620. [7] X. Chen, Y. Lou, A. C. S. Samia, C. Burda and J. L. Gole: Advanced Functional Materials Vol.15 (2005), p 41- 49. [8] X. Chen, and C. Burda: Journal of Physical Chemistry B Vol.108 (2004) p15446-49. [9] A. Zaleska: Physicochemical problems of mineral processing, Vol.42 (2008), p211-222. [10] Y. Cong, J. Zhang, F. Chen, and M. Anpo, J. Phys. Chem. C, Vol.111 (2007), 6976-6982.

Functionalized and Sensing Materials 10.4028/www.scientific.net/AMR.93-94

Investigation on Degradation of Methyl Parathion Using Visible Light in the Presence of Cr+3 and NDoped TiO2 10.4028/www.scientific.net/AMR.93-94.280 DOI References [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga: Science, Vol. 293, (2001), p 269-271. doi:10.1126/science.1061051 [2] J.P. Wilcoxon: Photocatalysis Using Semiconductor Nanoclusters, Advanced Catalytic Materials, MRS Proc. Boston, MA, (1998). doi:10.1557/PROC-549-119 [3] D. Chatterjee and A. Mahata: Journal of Photochemistry and Photobiology A: Chemistry Vol. 165, (2004) (1-3), p19-23. doi:10.1016/j.jphotochem.2004.02.015 [4] W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem. 98, (1994), p13669-13679. doi:10.1021/j100102a038 [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga: Science, Vol. 293, (2001), p 269- 271. doi:10.1143/JJAP.40.L561 [6] R. Nakamura, T. Tanaka and Y. Nakato: Journal of Physical Chemistry B Vol.108 (2004), p10617-10620. doi:10.1021/jp048112q [10] Y. Cong, J. Zhang, F. Chen, and M. Anpo, J. Phys. Chem. C, Vol.111 (2007), 6976-6982. doi:10.1021/jp0685030

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