Abstract
It is very important to improve the efficiency of water detoxification techniques. In this study, TiO2 or gold-TiO2 (Au/TiO2) nanocomposite-bound polythene beads were used for the photo-oxidation of rhodamine 6G (R6G) as a model of water organic pollutants. Simple thermal procedures were employed for anchoring TiO2 or Au/TiO2 nanocomposites to polythene beads. The results revealed that the synthesized Au/TiO2 composites exhibited both considerably higher absorption capability of organic pollutants and better photocatalytic activity for the photo-oxidation of R-6G than pure titania. The better photocatalytic activity of the synthesized Au/TiO2 composites film than that of the pure titania film was attributed to high capacity of light absorption intensity and easy diffusion of absorbed pollutants on the absorption sites to photogenerated oxidizing radicals on the photoactive sites.
1. Introduction
Waste water from industrial effluents having organic dyes is of great environmental and aesthetic concern [1]. Because of the potential toxicity of these dyes, the removal or degradation of this waste water has been a topic of significant interest [2]. TiO2 has been known as the ideal photocatalyst for the destruction of common organic pollutants in the textile industry because of its biological and chemical inertness, stability towards photochemical and chemical corrosion, and an electronic band gap that upon photoexcitation creates highly oxidizing holes and highly reducing electrons [3]. The process depends upon the absorption of TiO2 at a light wavelength less than 388 nm; the photocatalytic processes only occur in this region; approximately 5% of the solar energy could be used for driving this process. When UV light illuminates TiO2, electron and hole pairs are generated; they reduce and oxidize adsorbates on the surface, respectively, thereby producing radical species such as radicals and . These radicals can decompose most organic compounds [4–6]; various classes of potentially hazardous compounds have been tested; the results have confirmed the wide applicability of TiO2-based photocatalysis [7–10]. However, a practical limitation of the method is the separation of the catalyst, once the process is completed. Several authors [11–14] have shown that the problem can be circumvented by supporting the catalyst on a suitable substrate. The first reported study on the use of polymer supports for anchoring a photocatalyst was by Tennakone et al. [15]. Polythene is a thermoplastic polymer and is particularly useful as a catalyst-supporting material because its storage and handling properties are excellent. Polythene is also resistant to caking, abrasion, moisture, and crushing [16] in addition to being low cost. Previous studies show that the composite film with TiO2 nanoparticles can be used with reasonable stability and consistency. The observed behavior of the film has been attributed to the transportation of charge through TiO2 nanoparticles enhancing the conduction with the cross-linked polymers. Thereby the composite polymer film that works for more than 100 days with consistent efficiency suggests its mechanical stability at longer range of humidity [17].
Many trials have been carried out to use polymers as substrates. Some of them found that the polythene polymer is an effective and cheap substrate for binding TiO2 particles [15], and the others found that an efficient charge separation can be obtained by coupling two semiconductor particles [18–20]. These are usually acquired by mixing semiconductor powders [21] or by depositing semiconductor films on the substrates [22, 23]. The general view illustrated that there is a growing interest in the use of the thin films of TiO2 for generating a self-cleaning surface presently. Hence, the design of TiO2 films to improve the textural properties by reducing the time and cost is strongly desired. This principle was recently demonstrated by us for the photocatalytic decomposition of Trypan Blue over nanocomposite thin films [24]. In the present study, TiO2 or TiO2 with gold nanoparticles can be embedded in the surface of polythene beads by a simple method using a thermal attachment procedure. The dried polythene beads were used for investigating the photochemical decomposition of R-6G as a pollutant under irradiation with natural sunlight. R-6G is used because it has a remarkably high molar absorptivity, high photostability, and high quantum yield [25]. Its quantum efficiency exceeds 0.9 [26]; hence, it is an ideal substance for a UV-visible detection and photo-oxidation study.
2. Experimental
Chemicals. P25 TiO2 (ca. 80% anatase, ca. 20% rutile) was supplied by Degussa. The typical surface area was m2/g; average primary particle size, 21 nm; and band gap, 3.03 eV for rutile and 3.18 for anatase. High-density polyethylene (HDPE) having a density of 0.95 g/mL at 25°C and average Mw of ~125,000 was used in the form of beads. It was obtained from Sigma Aldrich.
2.1. Preparation of Gold Nanoparticles
Gold nanoparticles were prepared by the chemical reduction of HAuCl4, according to Turkevich’s method [27]. 95 mL of chlorauric acid (HAuCl4 , Sigma Aldrich) solution containing 2.5 mg of Au was refluxed, and 5 mL of 1% sodium citrate solution was added to the boiling solution. The solution was further boiled for 30 min and then left to cool to room temperature.
2.2. Preparation of TiO2 and Au/TiO2-Bound HDPE
The thermal attachment procedure was used for anchoring the titanium dioxide onto the polythene beads. TiO2 (0.08 g) was simply sprinkled onto ultra-high-molecular-weight polyethylene (HDPE, 0.16 g) in the ratio of 1 : 2 and stirred in 10 mL of ethanol by sonication. The mixture was agitated onto a Pyrex glass Petri dish. After drying overnight at room temperature, the mixture was gently heated at a temperature higher than the melting point of the polymer in an oven. As the temperature increased, the polythene beads reached their melting point (~145°C), at which TiO2 particles adhered to the soft-melted surface of the polythene beads. After being in the oven for 25 min (oven temperature: ~160°C), the Petri dish containing the TiO2-polythene mixture was allowed to cool down at a rate of 5°C/h. The resulting beads were semitransparent, and the thickness of the coating was approximately 1 mm. This method was repeated; 2 mL of the prepared aqueous gold nanoparticles were used, and the resulting polymer sheet became light wine red in color.
2.3. Photocatalysis Experiment
The photocatalysis experiment was carried out by exposing 100 mL of the aqueous R-6G samples over the HDPE thin film in a Petri dish to outdoor sunlight. The HDPE thin film was maintained at the bottom of the Pyrex glass Petri dish (diameter: 100 mm) by holding large-lipped paper clips vertically across the dish boundary. The values of natural sunlight irradiance were measured by using the three-channel Eldonet dosimeter (Germany), and the concentration of R-6G was mol dm−3. Three control experiments were carried out: first, the R-6G solution over the HDPE blank thin film was kept in the dark, and second, a similar solution was exposed to sunlight. In the third experiment, only the R-6G solution was exposed to sunlight without using the TiO2-HDPE thin film. The temperature was maintained at 22°C–25°C by using an ice-water bath. Airing by oxygen was done in all experiments (to enhance the formation of the radical) using a battery-powered (3 V) portable aquarium fish tank air pump (R-104). Samples (3 mL) were taken out at regular time intervals for analysis using a Perkin Elemer spectrophotometer (USA). Data acquisition and manipulation were performed using UV-winlab and origin 7 computer-based programs.
2.4. Imaging
A scanning electron microscope (JEOL JSM-T330A) having an acceleration voltage of 30 kV was used for studying the physical characteristics by applying the thin beads of each control sample and experimental sample on the platinum grid.
3. Result and Discussion
3.1. Morphological Studies of Au/TiO2-Bound Polythene Beads
The spectrum of the gold nanospheres had an intense band at 521 nm (Figure 1(a)) assignable to the plasmon absorption of gold particle diameter: 22 nm [28, 29].
Figure 1(b) illustrates the absorption spectrum of TiO2 nanoparticles at 315 nm arising from one resonance system of the whole molecule. In Figure 1(c) nanocomposite exhibits two absorption bands arising from each component of Nanogold and TiO2 nanoparticles.
The shapes of the Au, TiO2, Au/TiO2, HDPE, and TiO2 entrenched in HDPE and the combination of Au/TiO2 ingrained in HDPE beads have been observed with SEM. The results are shown in Figure 2. It can be seen that the synthesized Au nanoparticles were homogenous, dispersed, and almost spherically shaped (Figure 2(a)). Figure 2(b) shows TiO2 nanoparticles. The core-shell Au/TiO2 nanocomposite is clearly confirmed in Figure 2(c). Figure 2(d) illustrates the particle shape of the fused HDPE control sample. In Figure 2(e), the TiO2 was appended to the surface of the fused HDPE beads. Figure 2(f) illustrates the presence of gold as dispersed black dots on the mingled HDPE beads. It was evident from these pictures that the Au and TiO2 nanoparticles were tagged onto the surface of the fused HDPE polymer beads.
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3.2. Adsorption and Photocatalytic Decomposition of R6G
The photocatalytic activity of TiO2 and Au/TiO2 bound to polythene beads was evaluated by the photocatalytic degradation of rhodamine 6G. One of the most important criteria for an efficient charge transfer is to adsorb the dyes strongly on the semiconductor surface [30, 31].
Figure 3 shows the normalized electronic absorption spectra and time courses (210 min) of the photoinduced degradation of R6G (1 × 10−5 mol dm−3). There was approximately no effect either when R6G was exposed alone to the sun (sun blank 1) or incubated over HDPE in the dark (dark blank 2). Also, polythene beads without nanoparticles (sun blank 3) resulted in negligible photodegradation yields (~0.06%) after irradiation in the sun. About 48% of R6G was adsorbed on TiO2-HDPE beads (dark adsorption 1) after incubation in dark. Considerable adsorption of R6G (~72%) was observed on Au/TiO2-bound polythene beads (dark adsorption 2) after incubation in dark. It was confirmed that a better adsorption of organics on the surface of TiO2 had an advantage with respect to their photocatalytic degradation [32].
In the dark, a gradual decrease in the R6G concentration with time was due to the adsorption of R6G into the surfaces of TiO2 or Au/TiO2-bound polythene beads. From the above results, relatively high photocatalytic activity under UV-visible light irradiation can be expected. It was suggested that the electron transfer between the dye and TiO2 was promoted because of the adsorption enhancement of the dye on the surface of TiO2 [33]. The dependence of R6G degradation on the irradiation time is shown in Figure 4(a). As can be seen from the figure, the maximum absorption peak at 526 nm gradually decreased during the illumination, which was reasonably supposed to be due to both the adsorption of R6G into TiO2 particles and the decomposition of R6G by TiO2 on polythene beads. Under sunlight irradiation, electrons on the conductive band and holes on the valence band of TiO2 were subsequently generated and reacted with the surface OH group and preadsorbed species (water and dioxygen molecule) on the surface of TiO2 to produce active oxidative species. The active oxidative species then attacked and further degraded R6G [34]. The possible reaction mechanisms for the light-driven decomposition of R6G are illustrated in Scheme 1. The photocatalytic system of Au/TiO2 nanocomposite is shown in Scheme 2. When Au nanoparticles were bespoken on the surface of TiO2, the photoinduced electrons can relocate to the surface of gold nanoparticles and diminish the dissolved O2 easily. Also the photogenerated holes on the TiO2 surface can react with water to create powerful oxidative radicals and . This process inhibited the recombination rate of the photo-produced electrons and holes; so the photocatalytic activity of TiO2 nanoparticles was improved obviously by the modification of Au, as suggested in [35].
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Since there were no shifts in the absorption maxima and no additional peaks appearing in the course of the experiments using titania nanoparticles or Au on TiO2, the dye was completely degraded and not only photobleached [36–39].
It was shown that the photocatalytic electron transfer process at the semiconductor interface could be enhanced by depositing a noble metal on the semiconductor nanoparticles [40]. In the absence of Au, the degradation of R6G proceeded slowly, and R6G was degraded after irradiation for 210 min. In the presence of Au (Figure 4(b)), however, R6G degraded after 105 min (half time). The data provided evidence of good photocatalytic activity in the photodegradation of R6G because of the high surface area, narrow distribution with average pore sizes, crystalline bead framework, and accessible diffusion pathways. This means that the presence of Au nanoparticles together with TiO2 increases the rate of photobleaching to approximately two times more than that of TiO2 alone by increasing the efficiency of the charge separation of the light-generated electron-hole pairs [41–45]. The influence of surface charges on the adsorption of R6G on the surface of TiO2 and Au/TiO2 is shown in Scheme 3.
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The summary of photodegradation and adsorption yields and rate in the photocatalytic experiments in dark and under sunlight irradiation is given in Table 1. The rate (see [24]), and yield (see [46]) were calculated according to the following equations: where is the absorption spectra ( 526 nm) at zero time, is the absorption spectra (526 nm) at time , and is the irradiation or incubation time in seconds.
Table 1 illustrates that the rate and yield increased when Au/TiO2 was used. The time required for mineralization in the case of TiO2 was longer than that in the case of Au/TiO2. This may be due to the presence of gold nanoparticles that decrease the recombination rate between electron-hole pairs and consequently increase the radicals produced in the medium and enhance the efficiency of the photodegradation and adsorption processes.
4. Conclusion
Photocatalyst TiO2 and Au/TiO2 particles were successfully anchored on the beads of HDPE, under a thermal treatment. TiO2-Au/TiO2-particle-mounted HDPE beads showed both adsorptivity and photocatalytic activities, which were evaluated through the measurements of the concentration change of R6G in the water in the dark and under sunlight irradiation, respectively. The deposition of Au nanoparticles on the surface of TiO2-polythene polymer beads improved the photocatalytic decomposition of R6G. The improvement of the efficiency resulted mainly from the increase in the rate of adsorption on the TiO2-Au polymer beads due to the electrostatic attraction between the R6G and the gold nanoparticles. In addition, the presence of Au nanoparticles enhanced the interfacial charge-transfer process.
Acknowledgment
This work was supported by the National Institute of Laser Enhanced Sciences, Cairo University, Egypt.