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Sep 20, 2006 - 1. Copyright © 2006 by ASME. Proceedings of MN2006 ... activities of the nanocomposite of TiO2/Ag, with nanopaticles of TiO2 and Ag, were ... make it absorb visible light[1] or the increase of the specific surface area by ...
Proceedings of MN2006 Multifunctional Nanocomposites 2006 September 20-22, 2006, Honolulu, Hawaii

MN2006-17070 SOLAR PHOTOCATALYTIC DEGRADATION OF ATRAZINE IN WATER BY A TIO2/AG NANOCOMPOSITE

Ming Ge1, Assaf Azouri1, Kun Xun1, Klaus Sattler1, Joe Lichwa2, Chittaranjan Ray2 Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii, [email protected] 2 Water Resources Research Center, University of Hawaii at Manoa, Honolulu, Hawaii

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ABSTRACT One of the most common herbicides in the world, Atrazine, was used as a model pollutant in this study. The photocatalytic activities of the nanocomposite of TiO2/Ag, with nanopaticles of TiO2 and Ag, were investigated by photodegradation of atrazine under the natural sun. It was found that the efficiency of solar-photocatalytic activity was increased significantly by using the nanocomposites of TiO2/Ag, compared to the use of TiO2 alone. The mechanism of the TiO2/Ag composite for enhancement of photocatalytic activity was elucidated in this work. INTRODUCTION In solving environmental problems, a lot of attention has been directed toward the degradation of persistent toxic organic pollutants, which can not be reduced by usual purification processes. During recent decades, heterogeneous photocatalytic oxidation has been studied widely for removing persistent toxic species from wastewater by converting them to environmentally safe mineral acids. Titanium dioxide has been commonly recognized as one of best catalysts because of its excellent properties documented in the literature. The basic principle of photocatalysis was reported in a number of publications [1-5]. Extensive research has been conducted to increase the efficiency of photocatalytic activity of TiO2 and make it more applicable for commercial utilization. Such methods were for example the doping with all kinds of metal ions into TiO2 to make it absorb visible light[1] or the increase of the specific surface area by changing the formation process of TiO2[6]. Many of the applied techniques and approaches for increasing the photocatalytic activity of TiO2 are complex and not well applicable for water-purification. In experimental studies often the investigations were carried out with a UV lamp or an artificial sun. Using these artificial methods to generate the electron-hole pairs to purify water is the most important source of costs. It is more economical and ecological to use the natural sunlight.

According to the mechanism of photocatalysis, the efficiency of degradation of toxic organic molecules relies on the number of free hydroxyl radicals generated in the process. Absorption of photons from the exposure to sunlight generates electron-hole pairs in the TiO2 particles. The holes react with water to produce the powerful hydroxyl radical. The majority of the electron/hole pairs cannot overcome their mutual attraction and simply recombine. This leads to the fact that only 5% of the UV light energy is used for the photocatalytic activity [2-5]. Prohibition of electron and hole recombination is the most important factor to highly improve the rate of photocatalysis. So the main objective of this research was to investigate a way to reduce the recombination rate for electrons and holes. This study deals with the question of how the addition of Ag nanoparticles TiO2 nanoparticles in solution affects the photodegradation of atrazine under natural sunlight. EXPERIMENT 2 ppm atrazine was prepared by stirring the standard atrazine solution with deionized water. Then the TiO2, Ag, or TiO2/Ag was added in form of nanopowders. The aqueous suspensions were mixed well by putting into an ultrasonic bath for 2 hours in the dark before irradiation. The radiation source was the natural solar beam. The samples were irradiated in perpendicular direction to the sun beam for a short period on the roof of a building at noon time. During this time the samples were stirred for 30 sec after each 10 minutes to prevent sedimentation. Then 2 ml samples were taken from each of solutions after certain intervals. The atrazine solution then was separated from the nanoparticles by using an ultracentrifuge. Subsequently, the concentration of atrazine was monitored by high performance liquid chromatography (HPLC). Particle sizes were determined using Dynamic Light Scattering (DLS) with the ZetaPlus instrument of Brookhaven Instruments Corporation. The size of the primary particles was 10 nm for TiO2. The particles had coagulated to larger sizes of several hundred nanometers since the experiments were performed around pH=7 (for deionized water) which is close to the isoelectric point for TiO2 in water. The Ag nanoparticles were obtained by grinding silver powder and separation of sizes by

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RESULTS AND DISCUSSION Photocatalytic degradation of atrazine was carried out on three samples, (i) 16 mg TiO2 in 60 ml 2ppm atrazine, (ii) 6 mg Ag in 60 ml 2ppm atrazine (iii) 16 mg TiO2 and 6 mg Ag in 60 ml 2ppm atrazine. Figure 1 shows the results of the concentration studies. The atrazine concentration is plotted for increasing illumination time of the suspensions to the natural sun beam for the three solutions. The curves were fitted by exponential decay functions y = B + A exp( −t / T ) , where the parameter T represents the rate of degradation. As shown in figure 1, the photocatalytic degradation of atrazine by using the composite TiO2/Ag nanoparticles is much better than with the nanoparticle TiO2 itself. It also shows that the Ag particles alone do not have any degradation effect on the solution. After 50 minutes of sun exposure, the atrazine concentration is reduced to about 20% with TiO2 and to about 2% with TiO2/Ag. This ten-fold improvement of the degradation shows that Ag plays an important role in photocatalytic process with TiO2. When a photon is absorbed by a TiO2 particle, an electron and a hole (missing electron) is produced in the conduction and valence band, respectively. In the electron-hole recombination process, electrons drop from the conduction band to the valence band. The energy difference can be released as photons, phonons, or both. We propose that the silver particles in dynamic contact with the TiO2 particles lead to a substantial reduction of the electron-hole recombination rate. The absorption of UV photons first leads to the generation of free electrons and holes. Most of these electrons and holes recombine within the first few tens of picoseconds after the photoexcitation event.[7-9] The remaining charges are trapped at coordination defects at the surface and at lattice defects inside the nanoparticle. Following the rapid immobilization of free carriers by traps in the TiO2 nanoparticles (