JWARPJournal of Water Resource and Protection1945-3094Scientific Research Publishing10.4236/jwarp.2010.23027JWARP-1495ArticlesEarth&Environmental Sciences Photocatalytic Degradation of Isoproturon Pesticide on C, N and S Doped TiO2 oliceAnil Kumar ReddyPulagurlaVenkata Laxma ReddyVutukuriMaitrey SharmaBasavarajuSrinivasValluriDurga KumariMachirajuSubrahmanyam** E-mail:subrahmanyam@iict.res.in(MS);060420100203235244December 15, 2009December 29, 2009 January 22, 2010© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

TiO2 doped with C, N and S (TCNS photocatalyst) was prepared by hydrolysis process using titanium iso-propoxide and thiourea. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photo electron spectroscopy (XPS), BET surface area, FTIR and diffuse reflectance spectra (DRS). The results showed that the prepared catalysts are anatase type and nanosized par-ticles. The catalysts exhibited stronger absorption in the visible light region with a red shift in the adsorption edge. The photocatalytic activity of TCNS photocatalysts was evaluated by the photocatalytic degradation of isoproturon pesticide in aqueous solution. In the present study the maximum activity was achieved for TCNS5 catalyst at neutral pH with 1 g L-1 catalyst amount and at 1.14 x 10-4 M concentration of the pesticide solution. The TCNS photocatalysts showed higher phtocatalytic activity under solar light irradiation. This is attributed to the synergetic effects of red shift in the absorption edge, higher surface area and the inhibition of charge carrier recombination process.

Isoproturon Pesticide Degradation C N and S Doped TiO2 Visible Light Active Catalysts1. Introduction

Organic compounds are widely used in industry and in daily life, have become common pollutants in water bodies. As they are known to be noxious and carcinogenic, an effective and economic treatment for eliminating the organic pollutants in water has been found to be an urgent demand. The treatment of water contaminated with recalcitrant compounds is an important task to attend every country in the world. To attain the standards, there is a need for new treatment. It is very much important that the treatment should be safe and economically feasible. The wastewater purification technologies are classified as physical, biological, and chemical methods. All the above processes are having some flaws during their usage. The limitations include relative slow degradation, incomplete transformations and their inability to cover many organic compounds that do not occur naturally. Several chemical processes which use oxidizing agents such as ozone, hydrogen peroxide, H2O2/UV, H2O2/ozone/UV etc. have been carried out to mineralize many synthetic organic chemicals. Sometimes intermediates formed are more hazardous than the parent compound. Therefore, alternative technologies are in demand for development to treat recalcitrant compounds in wastewater effluents. Photocatalytic process has been found to be very active in the treatment of wastewaters for the mineralization of broad range of organic pollutants. Thus, heterogeneous mediated photocatalysis treatment technique gained noteworthy importance for the treatment of wastewaters.

Semiconductor mediated photocatalytic oxidation of water pollutants offers a facile and cheap method. Among various oxide semiconductor phtocatalysts, TiO2 has proved to be the most suitable catalyst for wide spread environmental applications because of its biological and chemical inertness, strong oxidizing power, non toxicity, long term stability against photo and chemical corrosion [1,2]. However, its applications seems to be  limited by several factors, among which the most restrictive one is the need of using an UV wavelength of < 387 nm, as excitation source due to its wide band gap (3.2 eV), and this energy radiation availability is less than 5 % in solar light.

Several works reported that doping TiO2 with anions such as carbon, nitrogen, sulphur, boron and fluorine shifts the optical absorption edge of TiO2 towards lower energy, there by increasing the photocatalytic activity in visible light region [3–9]. The preparation of doped TiO2 resulting in a desired band gap narrowing and an enhancement in the phtocatalytic activity under visible light.

In earlier reported studies, N doping of TiO2 is achieved by different methods such as sputtering of TiO2 in a gas mixture followed by annealing at higher temperatures [3], treating anatase TiO2 powders in an NH3/ Ar atmosphere [10], solution based methods like precipitation [11,12], sol-gel [13,14], solvothermal [15], hydrothermal processes [16] and direct oxidation of the dopent containing titanium precursors at appropriate temperatures [17]. In our earlier studies, we have concentrated on degradation of isoproturon using TiO2 supported over various zeolites. The main idea of using Zeolite support for TiO2 is to enhance the adsorption capacity of the pollutant over the combinate photo catalyst systems [18–20]. In the present case the main focus is on shifting the absorption edge of TiO2 to visible light region by introducing C, N and S into the TiO2 lattice structure. The present results obtained provides a simple route for the preparation of C, N and S doped TiO2 with enhanced photocatalytic activity under visible light irradiation for isoproturon pesticide degradation.

2. Experimental Details2.1. Materials and Methods

All the chemicals in the present work are of analytical grade and used as such without further purification. Isoproturon (IPU) (>99% pure, Technical grade) was obtained from Rhône-Poulenc Agrochemie, France and titanium isopropoxide was from Sigma-Aldrich chemie GmbH, Germany. HCl, NaOH and acetonitrile were obtained from Ranbaxy Limited, India. All the solutions were prepared with deionized water obtained using a Millipore device (Milli-Q).

2.2. Preparation of C, N and S Doped TiO<sub>2 </sub>Photocatalyst

C, N and S doped TiO2 photocatalyst was prepared by a simple hydrolysis process using titanium isopropoxide as the precursor for titanium and thiourea as the source for carbon, nitrogen and sulphur [26,34]. In a typical preparation, 10 mL of titanium isopropoxide solution was mixed with 30 mL of isopropyl alcohol solution. This solution was added drop wise to 20 mL deionized water containing in a 250 mL beaker. The solution was thoroughly mixed using a magnetic stirrer for 4 h. To this solution, required amount of thiourea, dissolved in 5 mL deionized water was added. The mixture was stirred for 6 h and dried in oven at 80 0C for 12 h. The solid product formed was further calcined at 400 0C temperature for 6 h in air to get C, N, and S doped TiO2 photocatalyst. The weight (%) of thiourea doped TiO2 was controlled at 0, 1, 3, 5, 10 and 15 wt% and the samples obtained were labeled as TCNS0, TCNS1, TCNS3, TCNS5, TCNS10 and TCNS15 respectively.

2.3. Characterization

The catalysts were characterized by various techniques like XRD, XPS, FTIR, SEM, BET surface area and UV-Vis DRS. The XRD of catalysts were obtained by Siemens D 5000 using Ni Filtered Cu K α radiation (√ = 1.5406 A0) from 2θ = 1-600. XPS spectra were recorded on a KRATOS AXIS 165 equipped with Mg Kα radiation (1253.6 eV) at 75 W apparatus using Mg Kα anode and a hemispherical analyzer, connected to a five channel detector. The C 1s line at 284.6 eV was used as an internal standard for the correction of binding energies. The Fourier transform-infra red spectra (FTIR) were recorded on a Nicolet 740 FTIR spectrometer (USA) using KBr self-supported pellet technique. The SEM analysis samples were mounted on an aluminum support using a double adhesive tape coated with gold and observed in Hitachi S-520 SEM unit. BET data was generated on (Auto Chem) Micro Maritics 2910 instrument. UV–Vis diffused reflectance spectra (UV–Vis DRS) was from UV–Vis Cintra 10e spectrometer.

2.4. Photocatalytic Experiments

IPU solution (0.114 mM) was freshly prepared by dissolving in double distilled water. All the phtocatalytic experiments were carried out at same concentration until unless stated. The pH of the solution was adjusted with HCl and NaOH. Prior to light experiments, dark (adsorption) experiments were carried out for better adsorption of the herbicide on the catalyst. For solar experiments, isoproturon solution of 50 mL was taken in an open glass reactor with known amount of the catalyst. The solution was illuminated under bright solar light. Distilled water was added periodically to avoid concentration changes due to evaporation. The solar experiments were carried out during 10.00 A.M. to 3.00 P.M. in May and June 2009 at Hyderabad.

2.5. Analyses

The IPU degradation was monitored by Shimadzu SPD-20A HPLC using C-18 phenomenex reverse phase column with acetonitrile/water (50/50 v/v %) as mobile phase at a flow rate of 1 mL min-1. The samples were collected at regular intervals, filtered through Millipore micro syringe filters (0.2 μm).

3. Results and Discussion3.1. Characterization3.1.1. XRD

To investigate the phase structure of the prepared samples XRD was used and the results are shown in Figure 1. It can be seen that TCNS exhibits only the characteristic peaks of anatase (major peaks at 25.410, 380, 480, 550) and no rutile phase is observed. The results are in good agreement with earlier studies [21]. By applying DebyeScherrer equation, the average particle size of the TCNS catalysts is found to be about 3.8 to 5.8 nm. It can be inferred that the ratio of thiourea to titania slightly influence the crystallization of the mesoporous titania. Also the peak intensity of anatase decreases and the catalyst becomes more amorpous. It might be due to the fact that the doped nonmetals can hinder the phase transition (anatase to rutile) and restricts the crystal growth. It is noteworthy that, even the doped samples exhibit typical structure of TiO2 crystal without any detectable dopant related peaks. This may be caused by the lower concentration of the doped species, and moreover, the limited dopants may have moved into either the interstitial positions or the substitutional sites of the TiO2 crystal structure [22,23].

3.1.2. XPS

To investigate the chemical sates of the possible dopants incorporated into TiO2, Ti2p, O1s, C1s, N1s, and S2p binding energies are studied by measuring the XPS spectra. The results are shown in Figure 2.

The high resolution spectra of Ti2p3/2 and Ti2p1/2 core levels are given in the Figure 2(a). The binding energy for the Ti2p3/2 and Ti2p1/2 core level peaks for TCNS0 appeared at 458.8 and 464.5 eV respectively which are attributed to O-Ti-O linkages in TiO2. Ti2p3/2 and Ti2p1/2 core level peaks for TCNS5 are observed at 458.4 and 464.1 eV with a decrease in the binding energy value compared to TiO2 indicating that the TiO2 lattice is considerably modified due to C, N and S doping [24].

The chemical environment of carbon is investigated by the XPS of C1s core levels as shown in the Figure 2(b). Three peaks are observed for the C1s at 284.6, 286.2 and 288.8 eV. The first peak observed at 284.6 eV is assigned to elemental carbon present on the surface, which is also in agreement with the reported studies [25]. The second and third peaks at 286.2, 288.8 eV are attributed to C-O and C=O bonds respectively [21,26].

The high resolution XPS spectra of N1s core level is shown in Figure 2(c). Generally, N1s core level in N doped TiO2 shows binding energies around 369-397.5

eV that are attributed to substitutionally doped N into the TiO2 lattice or β nitrogen [3,27]. N1s peaks, with high intensity observed at and above 400 eV are assigned to NO, N2O, NO2-, NO3-. Sakthivel et al. [28] observed an intense peak at 400.1 eV that was assigned to hyponitrile species and concluded that the higher binding energy is due to the lower valence state of N in N doped TiO2. Many researches pointed out that intense peak at 400 eV are due to oxidized nitrogen like Ti-O-N or Ti-N-O linkages. Dong et al. [26] observed three peaks of N1s at 397.8, 399.9 and 401.9 eV and has attributed to N-Ti-N, O-Ti-N and Ti-N-O linkage respectively. Recently, Gopinath observed N1s binding energy at 401.3 eV and claimed the presence of Ti-N-O linkage on the surface of N doped TiO2 nano particles [29]. Figure 2(c) shows the N1s spectra of TCNS5 catalyst and three peaks are observed at 397.8, 399.9 and 401.2 eV. Taking the literature support, here in the present investigation, the first peak at 397.8 eV is attributed to N-Ti-N linkages and the second and third peaks at 399.9 and 401.2 eV are ascribed to O-Ti-N, Ti-N-O linkages in the TiO2 lattice respectively.

The O1s spectra of TCNS0 and TCNS5 are shown in Figure 2(d). The O1s peak for TCNS0 is observed at 529.7 and 531.6 eV. The corresponding values are 530.2 and 531.7 eV for the TCNS5 sample. The first peak is mainly attributed to the O-Ti-O linkage in the TiO2 lattice, and the second peak is closely related to the hydroxyl groups (-OH) resulting mainly from chemisorbed water. It can be seen that the content of surface hydroxyl groups is much higher in the TCNS5 sample than in the TCNS0 sample. The increase in surface hydroxyl content is advantageous for trapping more photogenerated holes and thus preventing electron–hole recombination [26].

S2p XPS spectra for TCNS5 are shown as Figure 2(e). The oxidation state of the S-dopant is dependent on the preparation routes and sulfur precursors. Previous studies have reported that if thiourea was used, the substitution of Ti4+ by S6+ would be more favorable than replacing O2− with S2− [4]. S2p spectra can be resolved into four peaks, S2p1/2 6+, S2p3/26+, S2p1/2 4+ and S2p3/24+. The Figure 2(e) shows two peaks at 168.3 and 169.6 eV corresponding to S2p3/2 6+, S2p1/26+ binding energies [30]. It is clear from the figure that S was doped mainly as S6+ and not S4+or S2− peaks. The sulfur doping further can be substantiated by the decrease in binding energies of the Ti2p1/2 and Ti2p3/2 of TCNS5 sample compared to the binding energies Ti2p1/2 and Ti2p3/2 of the TCNS0 sample respectively (Figure 2(a)). This may be caused due to the difference of ionization energy of Ti and S. Therefore, it could be concluded that the lattice titanium sites of TiO2 were substituted by S6+ and formed as a new band energy structure.

3.1.3. FTIR Spectra

Figure 3 shows the FTIR spectra of TCNS0 and TCNS5 catalysts calcined at 400 ◦C. The absorption bands 2800–3500 cm-1, 1600–1680 cm-1 are assigned to the stretching vibration and bending vibration of the hydroxyl group respectively present on the surface of TiO2 catalyst [31,32]. The presence of surface hydroxyl groups are substantiated by XPS of O1s spectra (Figure 2(d)). The band around 1730 cm-1 is attributed to carbonyl group and bands at 1130, 1040 cm-1 are corresponding to nitrite and hyponitrite groups present in TCNS5 and they are absent in TCNS0 which shows successful doping of nitrogen into the lattice of TiO2  [33,34]. No peak corresponding to NH4+ absence (3189 and 1400 cm-1) shows that N is present only in the form of nitrite and hyponitrite species [32].

ReferencesA. Fujishima, K. Hashimoto, and T. Watanabe, “TiO2 photocatalysis: Fundamentals and applications,” BKC, Tokyo, 1999. L. Q. Wu, N. Xu, and J. Shi, “Leak growth mechanism in composite Pd membranes,” Industrial & Engineering Chemistry Research, Vol. 39, pp. 342–348, 2000. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-light photocatalysis in Nitrogen-Doped titanium oxides,” Science, Vol. 293, pp. 269–71, July 2001. T. Ohnoa, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mit-sui, and M. Matsumura, “Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities under visible light,” Applied Catalysis A: General, Vol. 265, pp. 115–121, February 2004. X. Chen and S. S. Mao, “Titanium dioxide nano-mate- rials: Synthesis, properties, modifications, and appli- cations,” Chemical Review, Vol. 107, pp. 2891–2959, 2007. H. Irie, Y. Watanabe, and K. Hashimoto, “Carbon-doped anatase TiO2 powders as a visible-light sensitive photo-catalyst,” Chemistry Letters, Vol. 32, pp. 772–773, May 2003. W. Ho, J. C. Yu, and S. Lee, “Synthesis of hierarchical nanoporous F-doped TiO2 spheres with visible light photocatalytic activity,” Chemical Communications, pp. 1115–1117, January 2006. L. Lin, W. Lin, Y. Zhu, B. Zhao, and Y. Xie, “Phosphor- doped titania —A novel photocatalyst active in visible light,” Chemistry Letters, Vol. 34, pp. 284–285, Novem-ber 2004. T. Ohno, T. Mitsui, and M. Matsumura, “Photocatalytic activity of S-doped TiO2 photocatalyst under visible light,” Chemistry Letters, Vol. 32, pp. 364–365, January 2003. H. Irie, Y. Watanabe, and K. Hashimoto, “Nitrogen- con-centration dependence on photocatalytic activity of TiO2-xNx powders,” Journal of Physical Chemistry B, Vol. 107, pp. 5483–5486, January 2003. J. G. Yu, M. H. Zou, B. Cheng, and X. J. Zhao, “Prepara-tion, characterization and photocatalytic activity of in situ N, S-codoped TiO2 powders,” Journal of Molecular Ca-talysis A: Chemical, Vol. 246, pp. 176–184, December 2005. Y. Q. Wang, X. J. Yu, and D. Z. Sun, “Synthesis, charac-teri- zation, and photocatalytic activity of TiO2?xNx nano- catalysts,” Journal of Hazardous Materials, Vol. 144, pp. 328–333, October 2006. J. L. Gole and J. D. Stout, C. Burda, Y. Lou, and X. Chen, “Highly efficient formation of visible light tunable TiO2-xNx photocatalysts and their transformation at the nanoscale,” Journal of Physical Chemistry B, Vol. 108, pp. 1230–1240, September 2003. C. Burda, Y. Lou, X. Chen, A. C. S. Samia, J. Stout, and J. L. Gole, “Enhanced nitrogen doping in TiO2 nanoparti-cles,” Nano Letters, Vol. 3, pp. 1049–1051, June 2003. B. Chi, L. Zhao, and T. Jin, “One-step template-free route for synthesis of mesoporous N-doped titania spheres,” Journal of Physical Chemistry C, Vol. 111, pp. 6189– 6193, February 2007. X. W. Bao, S. S. Yan, F. Chen, and J. L. Zhang , “Prepa-ration of TiO2 photocatalyst by hydrothermal method from aqueous peroxotitanium acid gel,” Material Letters, Vol. 59, pp. 412–415, October 2004. T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, and Y. Taga, “Band-gap narrowing of titanium dioxide by nitrogen doping,” Japan Journal of Applied Physics, Vol. 40, pp. 561–563, April 2001. M. V. Phanikrishna Sharma, V. Durgakumari, and M. Subrahmanyam, “Solar photocatalytic degradation of isoproturon over TiO2/H-MOR composite systems,” Journal of Hazardous Materials, Vol. 160, pp. 568–575, March 2008. M. V. Phanikrishna Sharma, V. Durga Kumari, and M. Subrahmanyam, “Photocatalytic degradation of isopro- turon herbicide over TiO2/Al-MCM-41 composite sys- tems using solar light,” Chemosphere, Vol. 72, pp. 644– 651, April 2008. M. V. Phanikrishna Sharma, G. Sadanandam, A. Ratnamala, V. Durga Kumari, and M. Subrahmanyam, “An efficient and novel porous nanosilica supported TiO2 photocatalyst for pesticide degradation using solar light,” Journal of Hazardous Materials, Vol. 171, pp. 626–633, June 2009. Y. Ao, J. Xu, D. Fu, and C. Yuan, “Synthesis of C,N,S- tridoped mesoporous titania with enhanced visible light- induced photocatalytic activity,” Microporous and Mesoporous Materials, Vol. 122, pp. 1–6, November 2008. S. I. Shah, W. Li, C. P. Huang, O. Jung, and C. Ni, “Study of Nd3+, Pd2+, Pt4+, and Fe3+ dopant effect on photo reactivity of TiO2 nanoparticles,” Proceedings of the National Academy of Sciences, Vol. 99, pp. 6482– 6486, January 2002. J. Sun, L. Qiao, S. Sun, and G. Wang, “Photocatalytic degradation of orange G on nitrogen-doped TiO2 catalysts under visible light and sunlight irradiation,” Journal of Hazardous Materials, Vol. 155, pp. 312–319, November 2007. F. Peng, L. Cai, H. Yu, H. Wang, and J. Yang, “Synthesis and characterization of substitutional and interstitial nitrogen-doped titanium dioxides with visible light photocatalytic activity,” Journal of Solid State Chemistry, Vol. 181, pp. 130–136, November 2007. Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima, and W. Choi, “Carbon-doped TiO2 photocatalyst synthesized without using an external carbon precursor and the visible light activity,” Applied Catalysis B, Vol. 91, pp. 355–361, June 2009. F. Dong, W. Zhao, and Z. Wu, “Characterization and photocatalytic activities of C, N and S co-doped TiO2 with 1D nanostructure prepared by the nano-confinement effect,” Nanotechnology, Vol. 19, pp. 365–607, July 2008. I. C. Kang, Q. Zhang, S. Yin, T. Sato, and F. Saito, “Novel method for preparation of high visible active N-doped TiO2 photocatalyst with its grinding in solvent,” Applied Catalysis B, Vol. 84, pp. 570–576, May 2008. S. Sakthivel and H. Kisch, “Daylight photocatalysis by carbon-modified titanium dioxide,” Angewandte Chemie International Edition, Vol. 42, pp. 4908–4911, 2003. C. Gopinath, “Comment on photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles,” Journal of Physical Chemistry B, Vol. 110, pp. 7079– 7080, January 2006. S. Liu and X. Chen, “A visible light response TiO2 photocatalyst realized by cationic S-doping and its application for phenol degradation,” Journal of Hazardous Materials, Vol. 152, pp. 48–55, June 2007. J. Geng, D. Yang, J. Zhu, D. Chen, and Z. Jiang, “Nitrogen-doped TiO2 nanotubes with enhanced photo- catalytic activity synthesized by a facile wet chemistry method,” Material Research Bulletin, Vol. 44, pp. 146– 150, March 2008. Y. Li, C. Xie, S. Peng, G. Lub, and S. Li, “Eosin Y-sensitized nitrogen-doped TiO2 for efficient visible light photocatalytic hydrogen evolution,” Journal of Molecular Catalysis A: Chemical, Vol. 282, pp. 117–123, December 2007. S. Sakthivel, M. Janczarek, and H. Kisch, “Visible light activity and photoelectrochemical properties of nitrogen-doped TiO2,” Journal of Physical Chemistry B, Vol. 108, pp. 19384–19387, September 2004. Y. Yokosuka, K. Oki, H. Nishikiori, Y. Tatsumi, N. Tanaka, and T. Fujii, “Photocatalytic degradation of trichloroethylene using N-doped TiO2 prepared by a simple sol–gel process,” Research on Chemical Intermediates, Vol. 35, pp. 43–53, January 2009. J. A. Rengifo-Herrera, K. Pierzcha?a, A. Sienkiewicz, L. Forro, J. Kiwi, and C. Pulgarin, “Abatement of organics and escherichia coli by N, S co-doped TiO2 under UV and visible light. Implications of the formation of singlet oxygen (1O2) under visible light,” Applied Catalysis B, Vol. 88, pp. 398–406, November 2008. H. S. Lee, T. Hura, S. Kimb, J. H. Kima, and H. I. Lee, “Effects of pH and surface modification of TiO2 with SiOx on the photocatalytic degradation of a pyrimidine derivative,” Catalysis Today, Vol. 84, pp. 173–180, July 2003.