TiO2

CurrentNanoscience 2012, 8, 896-901

Natural Light Source Assisted Synthesis of Self-decorated Ag/TiO2 Nanoparticles for Enhanced Photocatalytic Degradation of Fluorescein Dye Periyayya Uthirakumara,b,*, Beo Deul Ryua, Ji Hye Kanga, Mi Suh Leea and Chang-Hee Honga,* a School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea; bNanoscience centre for Optoelectronics and Energy Devices (Nano-COED), Department of Sciences, Sona College of Technology, Salem, Tamilnadu, India

Abstract: Self-decoration of silver (Ag) nanoparticles on the surface of titanium dioxide (TiO2, Degussa P25) photocatalyst are prepared via simple solution chemistry in absence of specific organic ligands for metal reduction, with an available natural light source. An in-situ method is carried out for the self-induced decoration of Ag nanoparticles onto the TiO2 surfaces and the properties of the metal nanoparticle have been revealed based on the photocatalytic activity. The average particle size of the self-decorated Ag nanoparticle on the TiO2 surface was ~2 nm. The photocatalytic degradations of fluorescein dye in aqueous suspensions of pure TiO2 and Ag/TiO2 photocatalysts were studied under Xenon light irradiation. We found that the photocatalytic degradation rate was largely influenced by the particle size of the self-decorated Ag nanoparticles on the TiO2 surface. The proposed photodegradation mechanism proved the possibility visible light excitation due to localized surface plasmon resonance and the electron transfer from the plasmonically excited Ag nanoparticles to the conduction band of TiO2 in addition to the usual ultra-violet excitation. The combined UV and visible light excitation improve the photodegradation behavior on fluorescein dye molecules effectively over the 83% of dye degradation.

Keywords: TiO2, photocatalyst, nanoparticle, silver. 1. INTRODUCTION Titanium dioxide (TiO2) is a nontoxic material that has been used in many different areas of research, including as a photocatalyst [1,2]. The photocatalytic activity of TiO2 has been extensively studied to determine the photocatalytic mechanism in semiconductive materials. Upon illumination with light, the process of electron transfer from the valence band to the conduction band, accompanied by the formation of hole-electron pairs, is responsible for the photocatalytic activity. These hole-electron pairs react with adsorbed molecules at the semiconductor surface, resulting in degradation of adsorbates [2]. However, major limitations exist such as UV irradiation and fast recombination of hole-electron pairs, which can occur within nanoseconds. Many applications rely not only on the properties of the semi-conductive TiO2 material itself but also on the modification of TiO2 as a host material [3,4]. Hence, metal additives such as Pt, Pd, Ag, and Au have been introduced to improve the photocatalytic efficiency of TiO2 [5-15]. These additives capture electrons to prevent the fast recombination of the holeelectron pairs. Silver is a suitable and nontoxic element which improves the TiO2 bioactivity because of its inherent antibacterial activity against microorganisms [6-8]. In recent years, considerable success has been achieved by introducing metal additives to TiO2 surfaces through the development of a wide range of wet chemical routes including electrostatic layerby-layer deposition, surface chemical reaction, photochemical reduction, a polyol process, ultrasonic electrodeposition, and a soft template process [9-17]. The preparation of hybrid Ag/TiO2 particles often involves preferential deposition of silver nuclei followed by the growth of a silver layer or by direct coating of silver nanoparticles on the surface of TiO2 after a specific surface modification [9-13]. Until now, the successful preparation of Agincorporated TiO2 photocatalysts has generally required either UV *Address correspondence to these authors at the School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea; Tel: +82-063-2703928; Fax: +82-063-270-3585; E-mails: [email protected]; [email protected]

1573-4137/12 $58.00+.00

light or specific organic ligands to initiate the deposition of Ag metal particles on the TiO2 surface. For example, UV light [9,1821], organic surfactants [11,22,23], and reducing agents [24] are effective routes to assist Ag/TiO2 photocatalyst synthesis. But, it is difficult to control the unintentional interactions of organic ligands with semiconductor materials. Most recently, Jurek et al. proposed a micro emulsion system to prepare successful deposition of Au and Au-Ag modified TiO2 with the help of surfactant and reducing agents [22]. In the present study, we designed a simple and cost effective method involving in-situ synthesis of self-decorated Ag metal nanoparticles on the surface of TiO2 nanoparticles with an available natural light source, without the assistance of any specific organic ligand, reducing agent, or UV light. The most significant characteristic of this process is that the entire reaction was carried out in 3 h at room temperature. Also, a combined photo-excitation mechanism was employed for the effective dye degradation in solution. 2. EXPERIMENTAL 2.1. Preparation of Ag-decorated TiO2 Photocatalyst P-25 (Degussa) TiO2 nanoparticles with a particle size of ~27 nm were used in all experiments. A freshly prepared 1 M aqueous AgNO3 solution was wrapped with alumina paper to protect it from sunlight. A suspension of TiO2 (0.05% wt) was aged for 30 min in an ultrasonic bath to which the required quantity of a silver nitrate (AgNO3 (99.99%), Aldrich) solution was added. An excess (5 g) of the 1 M aqueous AgNO3 solution was mixed with 3 g of the 0.05% TiO2 suspension in 100 g of methanol, an organic solvent. After mixing in an ultrasonic water bath for 5 min, the mixed solution was continuously stirred at room temperature for 3 h using a magnetic stir bar at 300 rpm. During the reaction, the whitecolored TiO2 nanoparticles became gray over time. At the end of the 3 h reaction time, the dark gray Ag-decorated TiO2 nanoparticles were isolated by filtration. The Ag/TiO2 photocatalyst were washed with excess methanol followed by double distilled (DD) water to remove excess reactant ions and other dissolved impurities.

© 2012 Bentham Science Publishers

Uthirakumar
et al.

2 Current Nanoscience, 2012, Vol. 8, No. 6

2.2. Preparation of Size Dependent Self-decorated Ag/TiO2 Photocatalyst To alter the size of the self-decorated Ag nanoparticles on the surface of TiO2, simple thermal heating was employed. The Agdecorated TiO2 photocatalyst was annealed at different temperatures to alter the size of the Ag nanoparticles. The three different annealing temperatures of 100, 200, and 300°C were applied to increase the size of the Ag nanoparticles in the Ag/TiO2 photocatalyst. The samples were held for 10 h on a hot plate in air. 2.3. Photocatalytic Activity with a Heterogeneous Solution of Fluorescein Dye An optically clear fluorescein dye aqueous solution was prepared in DD water at a molar concentration of 1
10-4 M (10 mg of fluorescein dye power was dissolved in 300 ml of DD water). A 100 ml Pyrex beaker was used as a batch photo-reactor. The required quantities of TiO2 or Ag/TiO2 and fluorescein dye solution (30 ml) were transferred into the batch photo-reactor and mixed for 30 min in the dark to reach adsorption-desorption equilibrium. The establishment of the adsorption-desorption equilibrium of the dye was assumed to be fast. A 500 W Xenon lamp was used as the light source, obviously, it was noted that the 28.5 mW/cm2 power of UV light recorded from the Xenon lamp. The photocatalytic experiments were conducted in the aqueous solution while applying visible light irradiation for up to 120 min. To monitor the degradation of fluorescein dye, 3 ml aliquots were taken at regular intervals for UV visible absorption spectroscopy measurements to determine the absorption change over a range of wavelengths from 300 to 600 nm. The characterization of self-decorated Ag/TiO2 photocatalyst morphology and composite structure were examined by fieldemission scanning electron microscopy (FESEM, Hitachi S-4700) and by transmission electron microscopy (TEM, JEOL-2010). The elemental compositions were analyzed using a FESEM instrument equipped with energy-dispersive spectroscopy (FESEM-EDS/CL, Hitachi/Horiba/Gatan S-4800). X-ray powder diffraction measurements of the samples were obtained using a Rigaku X-ray diffractometer. X-ray diffractograms (XRD) were obtained using a Cu K
incident beam (
= 0.1541 nm), monochromated by a nickel filter. To simplify the XRD analysis, the samples were prepared by dispersing TiO2 or Ag/TiO2 photocatalysts onto a pre-cleaned silicon substrate. The optical absorption changes with the different photocatalysts were characterized by a UV spectrophotometer (V-670, Jasco). 3. RESULTS AND DISCUSSION We proposed a simple and cost effective method for the in-situ synthesis of self-decorated Ag nanoparticles on the surface of TiO2 without using specific organic ligands or reducing agents. The weight ratio of Ag to TiO2 was altered to obtain uniform Agdecorated TiO2 nanoparticles. As part of the normal reaction setup,

the reaction time was varied to obtain complete conversion of Ag+ to Ag metal nanoparticles. The conversion is a very simple process in which a mixture of the aqueous AgNO3 and TiO2 suspension was diluted with an organic solvent, methanol, and stirred continuously for 3 hrs at room temperature using a magnetic stir bar. It is noteworthy to mention that neither an intensive UV light source nor a reducing agent was used to initiate the reduction of Ag+ to Ag metal nanoparticles. Fig (1) illustrates the possible reaction pathway of self-decorated Ag/TiO2 system with a schematic representation. We believe the rapid adsorption of Ag+ ions near the anionic environment of hydroxyl groups may takes place at the TiO2 nanoparticles. Later on, a natural light leads to reduce the adsorbed Ag+ ions into Ag metal nanoparticles. But, still the exact mechanism is not well known at this moment, but we are currently working to determine the mechanism for the conversion of Ag+ to Ag metal nanoparticles. However, there are two important points to be taken into consideration. First, a rapid silver ion adsorption occurred on the TiO2 surface. Second, an available natural light source is sufficient to initiate the reduction process of Ag+ to Ag metal nanoparticles [25]. In order to prove this, a similar reaction was carried out without addition of TiO2 nanoparticles. In this experiment, Ag metal nanoparticles were not formed and no change in the color of the solution occurred, suggesting that TiO2 nanoparticles are required to immobilize the Ag+ ions on the surface of the TiO2 nanoparticles. Fig (2)(a-b) shows FESEM micrographs of pure TiO2 and asprepared self-decorated Ag/TiO2 nanoparticles. It was very hard to distinguish the smaller Ag nanoparticles from the TiO2 nanoparticles. However, the generation of bright spots on the surface of TiO2 is obvious due to the low melting point of nanosized Ag metals while scanning the Ag/TiO2 samples by FESEM, indicating the incorporation of Ag nanoparticles [26,27]. For the pure TiO2 samples, for intense, these bright spots did not appear. This observation indirectly confirms the presence of Ag nanoparticles on the surface of the TiO2 nanoparticles. However, the evolution of the bright spots does not adequately explain the deposition of Ag nanoparticles on the surface of TiO2 nanoparticles. In order to evaluate this phenomenon further, TEM images were taken to investigate the particle size and distribution of self-decorated Ag nanoparticles on the TiO2 surface. As shown in Fig. (2)(c), a small number of Ag nanoparticles with sizes of ~2 nm were randomly deposited on the surface of the TiO2 nanoparticles confirming that the immobilized Ag metal nanoparticles were on the surface of individual TiO2 crystallites. The Ag metal nanoparticles were mostly spherical in shape and randomly distributed on the surface of the TiO2 nanoparticles without aggregation. This observation is clearly seen in the high magnified TEM image shown in Fig. (3)(a). The size and quality of the Ag nanoparticles was confirmed by the high resolution TEM (HRTEM) images and fast Fourier transformation (FFT) analysis. The appearance of proper lattice fringes without any stacking faults proves the formation of crystalline Ag nanoparticles on the crystalline TiO2 surface, as shown in Fig. (3) (b-c). Similarly, the spacings

Fig. (1). The possible reaction scheme for the preparation of self-decorated Ag/TiO2 photocatalyst.

Natural Light Source Assisted Synthesis of Self-decorated Ag/TiO2

Current Nanoscience, 2012, Vol. 8, No. 6

3

Fig. (3). TEM micrographs at (a) high magnification, and (b) HRTEM images of TiO2 and (c) the Ag nanoparticle region of the Ag/TiO2 photocatalyst. The inset shows the FFT pattern of the Ag nanoparticles of the asprepared Ag/TiO2 photocatalyst.

Fig. (4). XRD patterns of (a) pure TiO2 and (b) the as-prepared Ag/TiO2 photocatalyst.

Fig. (2). FESEM micrographs of (a) pure TiO2 and the (b) as-prepared Ag/TiO2 photocatalyst, and a (c) TEM micrograph of the as-prepared Ag/TiO2 photocatalyst.

between the two atomic planes of Ag and pure TiO2 were also calculated from the HRTEM images in their respective regimes. In Fig. (3) (b-c), the HRTEM image reveals parallel fringes with a space of 0.23 nm, which is consistent with the spacing of the (111) lattice plane of Ag [28], and a space of 0.35 nm, which belongs to the (101) lattice plane of anatase phase TiO2 [29]. As further evidence, Fig. (3) (d) shows the FFT image of the corresponding HRTEM image, which also confirms the crystal plane of the Ag nanoparticles. The crystalline phases of the pure TiO2 and as-prepared Ag/TiO2 nanoparticles were analyzed by wide angle XRD, as shown in Fig. (4). In the pure TiO2 pattern, the diffraction peaks at 2
= (25.2, 38.0, 47.8, 54.3, 62.7, and 68.8°) correspond to the anatase phase where the peak at 2
= 25.2° is the (101) plane and the peak at 54.3° corresponds to the (211) plane [4]. Meanwhile, the Ag metal-immobilized TiO2 nanoparticles had X-ray peaks at 2
= 38.1° (111), 44.5° (200), 64.4° (220), and 77.5° (311), which suggest successful reduction of Ag+ into Ag nanoparticles on the surface of the TiO2 crystallites. Although an X-ray peak at 38° is a common diffraction peak for both counterparts, the presence of

additional characteristic diffraction peaks at 44.5, 64.4, and 77.5° confirm the incorporated Ag metal nanoparticles [29]. No other crystalline materials peaks were observed over the entire XRD spectra. This result confirms the formation of the Ag/TiO2 photocatalyst with the decoration of Ag nanoparticles on the surface of the TiO2. To evaluate the influence of the size of the Ag nanomaterials on the Ag/TiO2 photocatalyst, the as-prepared Ag/TiO2 nanoparticles were subjected to annealing at temperatures of 100, 200, and 300°C for 10 h. It was found that after annealing for the same length of time, the particle size of the Ag nanomaterials increased with increasing temperature, as shown in Fig. (5). The FESEM micrographs of the annealed samples clearly reveal the formation of larger Ag nanoparticles as the annealing temperature was increased, as compared to as-prepared Ag/TiO2 nanoparticles. The size increase of the Ag metal nanoparticles is mainly due to particle aggregation of neighboring nanoparticles in addition to the impact of Ostwald ripening, which assisted in making larger Ag particles with the thermal energy. The samples annealed at 300°C demonstrated a noticeable size increase of the Ag particles than the as-prepared Ag/TiO2 photocatalyst. For example, the average size of the Ag metal nanoparticles after annealing at 300°C was ~90 nm but the Ag particles in the as-prepared Ag/TiO2 photocatalyst had a diameter of about 2 nm. This observation was further validated by the XRD patterns. XRD analysis was conducted to elucidate the crystal structure of the component elements. Fig (6) shows the XRD

4 Current Nanoscience, 2012, Vol. 8, No. 6

Uthirakumar
et al.

Fig. (6). XRD patterns of the (a) as-prepared Ag/TiO2 photocatalyst, and Ag/TiO2 photocatalysts annealed at (b) 100, (c) 200, and (d) 300°C.

Fig. (5). FESEM images of Ag/TiO2 photocatalysts annealed at (a) 100, (b) 200, and (c) 300°C. The inset shows the EDX analysis spectra of the annealed samples.

patterns of Ag/TiO2 photocatalysts annealed at different temperatures. The intensity of the Ag peak at 38.1° corresponding to the (111) oriented plane increased with increasing annealing temperature. Similarly, the peak sharpness also improved due to the well crystallized Ag metal nanoparticles. A well-known observation of appearing sharp and high intense diffraction peaks are belonging to formation of larger particles. Similarly, smaller nanomaterials generate a broad and weak X-ray diffraction peak. Hence, the ratio of the peak intensity between the TiO2 peak (25.2°) and the Ag peak (38.1°) obviously varied with respect to the annealing temperature, as seen in Fig. (5). Initially, in the as-prepared Ag/TiO2 photocatalyst, the TiO2 peak intensity was almost double that of the Ag peak. But, at the higher annealing temperature of 300°C, the peak inten-

sity of Ag slightly exceeded that of the TiO2 peak. Furthermore, the atomic percentage of the Ag concentration was calculated from the SEM-EDS analysis and was found to vary widely throughout these samples with respect to the annealing temperature. For example, the Ag concentration in the as-prepared sample was nominally 3% of Ag atomic percentage and it increased to over 8% in the annealed samples. Higher concentrations were found in the high temperature annealed samples, suggesting enlargement of the Ag particles in the grain boundaries. To investigate the photocatalytic activity of the pure TiO2 and as-prepared Ag/TiO2 photocatalysts, fluorescein dye was used as a test dye. For characterization, a certain amount of photocatalyst was added to an aqueous fluorescein dye solution and stirred for 30 min in the dark [30,31]. Then, this mixed solution was illuminated by a 500 W Xenon lamp that consists of ~28.5 mW/cm2 power of UV light. It means that source lamp provides both UV as well as a visible light source for photo-excitation. Fig. (7) shows the fluorescein dye concentration without catalyst, with pure TiO2, and with asprepared Ag/TiO2 photocatalyst as a function of the irradiation time. The results indicate that without catalyst, long-term irradiation only caused a slight decrease of the fluorescein dye concentration, demonstrating that fluorescein is sufficiently stable under white light illumination without catalyst. With pure TiO2 and the as-prepared Ag/TiO2 photocatalysts, the concentration of the fluorescein dye peak positioned at 489 nm decreased significantly after the Xenon light illumination [9]. The effect of photodegradation of dye molecules with pure TiO2 and the as-prepared Ag/TiO2 photocatalyst were calculated to be 54 and 83%, respectively. A wellknown mechanism can be explained in case of pure TiO2 photocatalyst where UV light is responsible for photo-excitation of TiO 2 photocatalyst leading to the 54% of fluorescein dye degradation [32]. However, in case of as-prepared Ag/TiO2 photocatalyst, the maximum of 83% of fluorescein dye were degraded. The reasons for the enhance photodegradation can be explained as follows. First, the formation of Schottky barrier at the Ag metal and the TiO2 interface, indicating a strong inhibition of electron-hole recombination as reported for other noble metals [33-35]. Second, Ag nanoparticles may absorb visible light via localized surface plasmon resonance and the resulting excited electrons will be transferred from plasmonically excited Ag nanoparticle to conduction band of TiO2 [22]. A schematic representation was drawn, as shown in Fig. (8), to explain the possible electron transfer pathway via plasmonically induced excitation (at 400 nm) of Ag nanoparticle to

Natural Light Source Assisted Synthesis of Self-decorated Ag/TiO2

Fig. (7). Degradation of fluorescein dye under Xenon light illumination (a) without photocatalyst, (b) with pure TiO2, and (c) with the as-prepared Ag/TiO2 photocatalyst.

Current Nanoscience, 2012, Vol. 8, No. 6

5

Fig. (9). Photocatalytic activities of (a) as-prepared, (b) 100, (c) 200, and (d) 300°C annealed Ag/TiO2 photocatalysts under visible light illumination.

subsequent electrons may not be likewise favored. For efficient redox catalysis, the potential of the metal particles must be more negative than that of the acceptor dye [11]. From Fig. (9), it is observed that the photocatalytic activity of the Ag/TiO2 photocatalyst with larger Ag metal nanoparticles was much lower than that of the as-prepared Ag/TiO2 photocatalyst and pure TiO2. For example, the extent of degradation of the dye with the sample annealed at 300°C was ~60%, whereas the pure TiO2 catalyst itself resulted in a higher degradation value of ~70%. This result indicates that the size of the Ag-decorated Ag/TiO2 photocatalyst was very influential to produce an effective photocatalyst. Therefore, the as-prepared Ag/TiO2 decorated with ~2 nm Ag nanoparticles showed much better photocatalytic activity in the degradation of fluorescein dye under white light illumination.

Fig. (8). A schematic representation of possible electron transfer pathway via plasmonically induced excitation of Ag nanoparticle to TiO2 nanoparticles.

TiO2 nanoparticles. Finally, in addition, a conventional UV light induced photodegradation of TiO2 can also be involved to degrade the fluorescein dye molecules, because of the existence of UV light from the light source [32]. Therefore, due to the above reasons, it is obvious to note the degradation rate of the fluorescein dye with the as-prepared Ag/TiO2 photocatalyst was higher than with the pure TiO2 catalyst. Fig (9) shows the photocatalytic activity of the Ag/TiO2 photocatalysts with respect to the size of the Ag nanoparticles. The degradation of the fluorescein dye peak was reduced with the larger Ag nanoparticles obtained at the increased annealing temperatures. This indicates that the smaller Ag nanoparticles immobilized the Ag/TiO2 photocatalyst resulting in a faster degradation rate than with the larger particles, as shown in Fig. (9). The size of the Ag nanoparticles plays a very important role in the photocatalytic activity of the Ag-decorated Ag/TiO2 photocatalysts. It was recognized that electron accumulation on the Ag nanoparticles is facile and the

4. CONCLUSIONS Self-decorated Ag nanoparticles on a TiO2 surface were prepared from a simple and cost-effective solution chemistry route without specific organic ligands, reducing agents, or a UV-light induced conversion. The photocatalytic activities of self-decorated Ag/TiO2 photocatalysts were superior to the pure TiO2 photocatalyst due to the immobilized Ag metal nanoparticles shows the following important reasons. Ag/TiO2 photocatalyst system behaves as a Schottky barrier at the interface of Ag and TiO2 indicating a strong inhibition of electron-hole recombination, the generation of the localized surface plasmon resonance excitation of Ag nanoparticles to transfer photo-excited electrons from Ag nanoparticle to TiO2 nanoparticles and a conventional UV light induced photodegradation via the existence of UV light from the Xenon light source. The rate of degradation of a fluorescein dye solution in the presence of as-prepared Ag/TiO2 photocatalyst was found to be faster (83%) than with the pure TiO2 photocatalyst (54%). The size of decorated Ag nanoparticles was enlarged by a thermal annealing process in air. The particle size increased from 2 nm to 100 nm upon annealing at temperature up to 300°C for 10 h. The influence of Ag nanoparticle sizes on the Ag/TiO2 photocatalytic activity was also studied. The larger the Ag nanoparticles, the lower the degradation rate of fluorescein dye, due to redox potential limits. CONFLICT OF INTEREST The authors confirm that this article content has no conflicts of interest.

Uthirakumar
et al.

6 Current Nanoscience, 2012, Vol. 8, No. 6

ACKNOWLEDGEMENTS This work (research) is financially supported by the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement of Technology (KIAT) through the Workforce Development Program in Strategic Technology. Also, this study was supported by a Priority Research Center Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology of the Korean government (2011-0027956).

[17] [18]

[19]

[20]

REFERENCES [1]

[2]

[3] [4] [5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

Turchi, S.; Ollis, D.F. Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack. J. Catal., 1990, 122, 178-192. Kamat, P.V.; Meisel, D. Nanoparticles in advanced oxidation processes, Curr. Opin. Colloid Interface Sci. Curr. Opin. Colloid Interface Sci., 2002, 7, 282-287. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature., 1972, 238, 37-38. Chen, X.; Mao, S.S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev., 2007, 107, 2891-2959. Dawson, P.V. Kamat, Semiconductor
Metal Nanocomposites. Photoinduced Fusion and Photocatalysis of Gold-Capped TiO2 (TiO2/Gold) Nanoparticles. J. Phys. Chem. B, 2001, 105, 960-966. Lee, M.S.; Hong, S.S.; Mohseni, M. Synthesis of photocatalytic nanosized TiO2–Ag particles with sol–gel method using reduction agent. J. Mol. Catal. A Chem., 2005, 242, 135-140. Rupa, V.; Manikandan, D.; Divakar, D.; Sivakumar, T. Effect of deposition of Ag on TiO2 nanoparticles on the photodegradation of Reactive Yellow-17. J. Hazard. Mater., 2007, 147, 906-913. Liu, S.X.; Qu, Z.P.; Han, X.W.; Sun, C.L. A mechanism for enhanced photocatalytic activity of silver-loaded titanium dioxide. Catal. Today., 2004, 93, 877-884. Priya, R.; Baiju, K.V.; Shukla, S.; Biju, S.; Reddy, M.L.P.; Patil, K. Comparing Ultraviolet and Chemical Reduction Techniques for Enhancing Photocatalytic Activity of Silver Oxide/Silver Deposited Nanocrystalline Anatase Titania. J. Phys. Chem. C, 2009, 113, 6243-6255. Chen, H.W.; Ku, Y.; Kuo, Y.L. Photodegradation of o-Cresol with Ag Deposited on TiO2 under Visible and UV Light Irradiation. Chem. Eng. Technol., 2007, 30, 1242-1247. Cozzoli, P.D.; Fanizza, E.; Comparelli, R.; Curri, M.L.; Agostiano, A.; Laub, D. Role of Metal Nanoparticles in TiO2/Ag Nanocomposite-Based Microheterogeneous Photocatalysis. J. Phys. Chem. B., 2004, 108, 9623-9630. Yang, Liang, G.L.; Xu, K.M.; Gao, P.; Xu, B. Bactericidal functionalization of wrinkle-free fabrics via covalently bonding TiO2@Ag nanoconjugates. J. Mater. Sci., 2009, 44, 1894-1901. Souni, M.E.; Brandies, H.F.; Souni, M.E. Versatile Nanocomposite Coatings with Tunable Cell Adhesion and Bactericidity. Adv. Funct .Mater., 2008, 18, 3179-3188. Korzhak, A.V.; Ermokhina, N.I.; Stroyuk, A.L.; Bukhtiyarov, V.K.; Raevskaya, A.E.; Litvin, V.I. Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites. J. Photochem. Photobiol. A: Chem., 2008, 198, 126-134. Cozzoli, P.D.; Comparelli, R.; Fanizza, E.; Curri, M.L.; Agostiano, A.; Laub, D. Photocatalytic Synthesis of Silver Nanoparticles Stabilized by TiO2 Nanorods: A Semiconductor/Metal Nanocomposite in Homogeneous Nonpolar Solution. J. Am. Chem. Soc., 2004, 126, 3868-3879. Jeong, U.; Wang, Y.; Ibisate, M.; Xia, Y. Some New Developments in the Synthesis, Functionalization, and Utilization of Monodisperse Colloidal Spheres. Adv. Funct. Mater., 2005, 15, 1907-1921.

Received: ?????? 28, 2011

Revised: ?????? 22, 2012

Accepted: ????? 6, 2012

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

[34]

[35]

Wang, Y.; Angelatos, A.S.; Caruso, F. Template Synthesis of Nanostructured Materials via Layer-by-Layer Assembly. Chem. Mater., 2008, 20, 848-858. Sobana, N.; Muruganadham, M.; Swaminathan, M. Nano-Ag particles doped TiO2 for efficient photodegradation of Direct azo dyes. J. Mol. Catal. A: Chem., 2006, 258, 124-132. Sung, S.H.M., Choi, J.R., Hah, H.J., Koo, S.M., Bae, Y.C. Comparison of Ag deposition effects on the photocatalytic activity of nanoparticulate TiO2 under visible and UV light irradiation. J. Photochem. Photobiol. A: Chem., 2004, 163, 37-44. Ko, S., Banerjee, C.K., Sankar, J. Photochemical synthesis and photocatalytic activity in simulated solar light of nanosized Ag doped TiO2 nanoparticle composite. Composites: Part B., 2011, 42, 579-583. Xiong, Z.; Ma, J.; Ng, W.J.; Waite, T.D.; Zhao, X.S. Silver-modified mesoporous TiO2 photocatalyst for water purification. Water Research., 2011, 45, 2095-2103. Jureka, A.Z.; Kowalska, E.; Sobczak, J.W.; Lisowski, W.; Ohtani, B.; Zaleska, A. Preparation and characterization of monometallic (Au) and bimetallic (Ag/Au) modified-titania photocatalysts activated by visible light. Appl. Catal. B: Environ., 2011, 101, 504-514. Wang , P.; Wang, D.; Xie, T.; Li, H.; Yang, M.; Wei, X. Preparation of nanoparticles. Mater. monodisperse Ag/Anatase TiO2 core–shell Chem. Phys., 2008, 109, 181-183. Amin, S.A.; Pazouki, M.; Hosseinnia, A. A Synthesis of TiO2–Ag nanocomposite with sol–gel method and investigation of its antibacterial activity against E. coli. Powder Tech., 2009, 196, 241-245. Hirotaka, K.; Takuya, K.; Hiroyuki, W. In situ synthesis of silver nanoparticles on zinc oxide whiskers incorporated in a paper matrix for antibacterial applications. J. Mater. Chem., 2009, 19, 2135-2140. Siiman, O.; Feilchenfeld, H. Internal fractal structure of aggregates of silver particles and its consequences on surface-enhanced Raman scattering intensities. J. Phys. Chem., 1988, 92, 453-464. Siiman, O.; Bumm, L.A.; Callaghan, R.; Blatchford, C.G.; Kerker, M. Surface-enhanced Raman scattering by citrate on colloidal silver. J. Phys. Chem., 1983, 87, 1014-1023. Xiong, Y.; Xie, Y.; Wu, C.; Yang, J.; Li, Z.; Xu, F. Formation of Silver Nanowires Through a Sandwiched Reduction Process. Adv. Mater., 2003, 15, 405-408. Yu, H.; Yu, J.; Cheng, B. Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes. J. Mol. Catal. A: Chem., 2006, 249, 135-142. Meng, X.; Feng, X.; Li, Z.; Zhang, Y. Fabrication of novel poly (ethylene terephthalate)/TiO2 nanofibers by electrospinning and their photocatalytic activity. Curr. Nanosci., 2012, 8, 3-6. Yang, S.F.; Ma, Y.; Zhou, Y.; Pei, C.; Luo, Q.; Zeng, M.; Dai, Bo. Eggwhite templating of hierarchically macroporous architectures of SiO2, TiO2 and C/SiCN nanocables and photocatalytic properties. Curr. Nanosci., 2011, 7, 1004-1008. Fujishima, A.; Rao, T.N.; Tryk, D.A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C: Photochem. Rev., 2000, 1, 1-21. Bannat, I.; Wessels, K.; Oekermann, T.; Rathousky, J.; Bahnemann, D.; Wark, M. Improving the Photocatalytic Performance of Mesoporous Titania Films by Modification with Gold Nanostructures. Chem. Mater., 2009, 21, 1645-1653. Tada, H.; Kiyonaga, T.; Naya, S. Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(IV) dioxide. Chem. Soc. Rev., 2009, 38, 1849-1858. Xin, B.; Jing, L.; Ren, Z.; Wang, B.; Fu, H. Effects of simultaneously doped and deposited Ag on the photocatalytic activity and surface states of TiO2. J. Phys. Chem. B., 2005, 109, 2805-2809.