Laser-Induced Silver Nanoparticles on Titanium Oxide for Photocatalytic Degradation of Methylene Blue

Silver nanoparticles doped on titanium oxide (TiO2) were produced by laser-liquid interaction of silver nitrate (AgNO3) in isopropanol. Characteristics of Ag/TiO2 (Ag doped TiO2) nanoparticles produced by the methods presented in this article were investigated by XRD, TEM, SEM, EDX, and UV-Vis. From the UV-Vis measurements, the absorption of visible light of the Ag/TiO2 photocatalysts was improved (additional absorption at longer wavelength in visible light region) obviously. The photocatalytic efficiency of Ag/TiO2 was tested by the degradation of methylene blue (MB) in aqueous solution. A maximum of 82.3% MB degradation is achieved by 2.0 wt% Ag/TiO2 photocatalyst under 2 h illumination with a halogen lamp.


Introduction
Waste water from the textile industry constitutes a serious environmental problem. Most of the dyestuffs used are difficult to decompose, due to their chemical structures [1]. As the public demand for environmental protection increases and the governmental authorities are more concerned with the OPEN ACCESS enforcement of the corresponding regulations, the top priority is to find an efficient solution dealing with this issue for the time being.
Semiconductor photocatalysts have been a potential candidates for treating various water pollutants [2][3][4]. After some 30 years of extensive research, many oxide compounds of semiconductor photocatalysts include TiO 2 , ZnO, WO 3 , SnO 2 , and ZrO 2 , and some of the sulfide compounds like CdS, ZnS are among those most interesting materials in this respect [5]. Within these compounds, titanium oxide has also been investigated for its photocatalytic activities [6,7] and for use in photoelectrochemical cells as well [8]. It has drawn great attention in research and industrial fields in recent years because of its characteristics of powerful oxidation capability, non-toxicity, chemical stability, and cost-effectiveness. Nevertheless, one of the drawbacks of TiO 2 for photocatalytic process is its relative big band gap (3.0 eV for rutile phase and 3.2 eV for anatase phase, respectively). As a result, TiO 2 absorbs light wavelength less than 388 nm and the photocatalytic processes only occur in this region. Therefore, many studies have employed modifications of TiO 2 in order to improve its catalysis efficiency through enhancements of its absorbance in the visible light region to match the solar spectrum. Those methods consist of doping with metals [9][10][11], non-metal ions [12][13][14], dyesensitization [15,16] and so forth.
Kondo and Jardim [17] were among the pioneers who incorporated silver into a TiO 2 matrix for photocatalytical applications. The doping of silver nanoparticles into a TiO 2 matrix can be achieved by chemical reduction [18], the reverse micellar route [19], irradiation of silver ions in solution [20], and the sol-gel method [21]. Silver doped semiconductor substrate has been studied to enhance the photocatalytical efficiency by trapping the photo-induced charge carriers, especially electrons, and facilitating the transfer process.
In this article, we propose a method of doping Ag nanoparticles onto a TiO 2 matrix by introducing a laser to the deposition processes, namely via a laser-liquid interaction of AgNO 3 and TiO 2 in a liquid medium (isopropanol). In the laser-liquid interaction, nucleation and growth of Ag nanoparticles take place as the amount of Ag atoms being generated in the liquid reaches the condition of supersaturation [22]. The nanoparticles produced by laser-liquid interaction have the advantages of stability in liquid media and narrow diameter distributions. For practical application of this Ag doped TiO 2 photocatalyst, the efficiency of Ag/TiO 2 was tested by the degradation of MB in aqueous solution.

Laser-Induced Interaction
Silver nanoparticles deposited on titanium oxide were produced by laser-liquid interaction of TiO 2 powder and AgNO 3 dissolved in isopropanol. Different weight ratios of TiO 2 and AgNO 3 were sonicated in isopropanol before being irradiated by the focused output of second harmonic (532 nm) of Nd:YAG laser operating at 10 Hz with the power of 25 mJ. Isopropanol was used as the reaction medium to prevent the aggregation of silver nanoparticles while the reaction proceeds. The sizes of the Ag nanoparticles produced by the laser-liquid interaction were found to be about 18~22 nm in diameter.
Since the size, shape, and microstructure of Ag particles are dependent on some important parameters, such as power, frequency, and interaction time, those factors were considered in the laser-induced processes.

Characterization of Ag/TiO 2
The UV-Vis measurements (200~800 nm; 300 nm/min.) of Ag/TiO 2 photocatalysts are shown in  It has been reported that the doped metallic nanoparticles on TiO 2 are acting like electron traps [23], retarding the recombination of electron-hole pairs which were provoked by the photon absorption of the TiO 2 matrix. We found that among the various ratios of Ag/TiO 2 produced by laser-liquid interaction, the 2.0 wt% Ag/TiO 2 [ Figure 1 curve (c)] presents the highest photon absorption in the visible light region with  max (maximum absorption wavelength) located at around 470 nm. Interestingly, the absorption enhancement of Ag/TiO 2 at visible light region does not correspond to the ratios of silver in the photocatalysts. Although silver nanoparticles help improve the visible light absorption, it is likely that some particles shield the interaction of light as more Ag is deposited on the TiO 2 matrix. It is also noted that a shifting of the maximum absorptions in the visible light region occurred for various ratios of Ag/TiO 2 , just as curves (d) and (e) in Figure 1 are red-shifted to 500~520 nm, which is attributed to the size differences of the deposited Ag nanoparticles. The crystallite size of nanoparticles can be calculated by applying to the Scherrer's equation: where D is the average crystallite size, 0.9 is the shape factor of the grain,  is the wavelength of Xray which is 0.154051 nm for Cu Kα radiation, B is the FWHM of the diffraction peak, and  is the incident angle of X-ray. By the diffraction data in Figure 2, the primary particle size can be measured  The SEM image of 2.0 wt% Ag/TiO 2 nanoparticles is shown in Figure 3. Some porous surface dispersed among cauliflower-like clusters of grains was observed. The scope of particle size was measured to be approximately 30~45 nm. The results revealed that some aggregation of TiO 2 grains occurs during laser-liquid interaction if we compare the particle size from SEM and from the XRD data calculation.   Figure 4 shows the TEM image of 2.0 wt% Ag/TiO 2 nanoparticles. Small spherical Ag nanoparticles (some of these particles are indicated by arrows) were observed scattered on the surface of TiO 2 . For Ag particles, the diameter ranges from 3 to 6 nm, and the diameter of TiO 2 was found to be in the scope of 20~40 nm. It suggests that small silver nanoparticles can be prepared by pulsed-laser irradiation applied to AgNO 3 and TiO 2 system, and that a good dispersion of these particles on the surface of the matrix is probable. The average diameter of particles from TEM image was found to be in accordance with the results of the SEM image.
We have also performed the EDX analysis on the 2.0 wt% Ag/TiO 2 catalyst. The EDX diagram of 2.0 wt% Ag/TiO 2 is shown in Figure 5, where the silver signals are found at around 3.00 keV [25].
Though the peaks of silver are insignificant due to its content in TiO 2 matrix, it can be indicative of the presence of Ag particles in catalyst.

Photodegradation of Methylene Blue by Ag/TiO 2
In this article, methylene blue was used as a model pollutant for evaluation of the photocatalytical efficiency of the laser-induced Ag/TiO 2 nanoparticles. Methylene blue, with an absorption maximum at 668 nm in visible light region, as shown in Figure 6, is usually used in mixed indicators or as a redox indicator. Hence, the amount of MB was measured quantitatively with the absorption of light at    The pH value of the solution also plays an important role in the photodegradation process; it was found that the maximum rate of photocatalytic degradation by TiO 2 is achieved at pH 6.9 [26]. Hence, the pH condition of photodegradation experiments of this work was controlled before the degradation process for optimum results. Figure 8 Table 1. However, zero order of the degradation reaction for higher concentrations was reported elsewhere [5].

Preparation of Ag/TiO 2 Photocatalysts
Silver nitrate was used as Ag precursor. The various ratios of AgNO 3 to TiO 2 were prepared by mixing 0.5, 1.0, 2.0, 5.0, and 10.0 weight percents of AgNO 3 in TiO 2 , with total weight maintained at 1.0 g. Then 10 g of isopropanol was added to each mixture. Each solution was sonicated 30 min. for uniform mixing. A pulsed-laser (25 mJ, 532 nm, 10 Hz Nd:YAG) light was applied from top of the container to each sample for 60 min. Finally, the air-dry samples were annealed at 200 ºC for 60 min.

Characterization of the Prepared Catalysts
The UV-Vis spectra of Ag/TiO 2 were carried out on a Hitachi U-3010 spectrophotometer with an integrated sphere, wavelength from 200 nm through 800 nm, with a scanning rate of 300 nm/min. The XRD analyses were carried out on a Shimadzu XD-D1 X-ray diffractometer, using Cu-K radiation with  = 0.154051 nm, in a range of 20-80 (2). TEM images were recorded on a JEOL JEM 1200-EX electron microscope. SEM images were recorded on a Philips XL40 microscope.

Determination of Photocatalytic Activities of Ag/TiO 2
The installation for photocatalytic degradation of MB by Ag/TiO 2 was assembled as the following: A 150 W halogen lamp, wavelength range from 350 nm to 800 nm with the predominant peak at 575 nm, was used as the light source placed on top of the setup. 1.0 g of Ag/TiO 2 was added in 100 mL of MB (7,000 mg/L) solution. After the mixture was sonicated for 30 min., the halogen lamp was turned on to initiate the reaction. During the irradiation periods, 5 mL of solution were taken out of the reactor and centrifuged to separate the solid from the solution at 20-min intervals. An UV-Vis spectroscopy was used to detect the MB concentration of each centrifuged solution, which was collected at 20-min interval for two hours of reaction time in all.

Conclusions
We have introduced a laser-induced method of doping Ag nanoparticles onto a TiO 2 matrix, with laser-liquid interaction of AgNO 3 and TiO 2 in a liquid medium of isopropanol. Apart from other methods of preparation of Ag modified TiO 2 such as photoreduction, chemical reduction, and sol-gel process, the method we proposed in this article provides a simple, straightforward way for enhancing its photocatalytic efficiency. XRD, TEM and SEM results of Ag/TiO 2 indicated that narrow size distributions of Ag nanoparticles on TiO 2 were achieved by the laser-induced method. The Ag nanoparticles deposited on TiO 2 act like the electron traps of the matrix, preventing recombination of electron-hole pairs on the surface of TiO 2 and improving charge transfer processes. The photocatalysis efficiency of Ag doped TiO 2 was tested by the degradation of MB in aqueous solution. A maximum of 82.3% MB degradation under 2 h of halogen lamp illumination using 2 wt% Ag/TiO 2 , prepared by the laser-induced method of this article, is observed.