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.
Waste water from industrial effluents having organic dyes is of great environmental and aesthetic concern [
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 [
Gold nanoparticles were prepared by the chemical reduction of HAuCl4, according to Turkevich’s method [
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.
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
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.
The spectrum of the gold nanospheres had an intense band at 521 nm (Figure
(a) Absorption spectra of synthesized Au nanoparticles (0.025 mg/mL), (b) absorption spectra of TiO2 nanoparticles and (c) absorption spectra of the synthesized Au/TiO2 nanocomposite.
Figure
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
Scanning electron microscopic photograph of (a) synthesized Au nanoparticles, (b) TiO2 nanoparticles, (c) Au/TiO2 nanocomposite, (d) microporous layer of the fused polycrystalline polythene film, (e) TiO2 nanoparticles embedded in the microporous layer, and (f) Au/TiO2 nanocomposite embedded in the microporous layer of the fused polycrystalline polythene film
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 [
Figure
Normalized absorption spectra of R6G (1 × 10-5 mol dm−3; exposure time: 210 min; sunlight intensity: 500–600 W/m2); sun blank 1: sunlight irradiation to R6G without HDPE; dark blank 2 : dark adsorption of R6G over HDPE beads in the dark; sun blank 3: sunlight irradiation of R6G over HDPE beads; dark adsorption 1: adsorption of R6G over TiO2-bound HDPE beads in the dark; dark adsorption 2: adsorption of R6G over Au/TiO2-bound HDPE beads in the dark.
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 [
R6G photocatalytic degradation process.
Interfacial charge transfer process of gold-TiO2 nanocomposites under the irradiation of UV light.
(a) Absorption spectra showing the photobleaching of 1 × 10−5 mole dm−3 R6G irradiated with sunlight at (500–600 W/m2) in polymer-TiO2. (b) Absorption spectra showing the photobleaching of 1 × 10−5 mol dm−3 R6G irradiated with sunlight at (500–600 W/m2) in polymer-TiO2-gold.
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 [
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 [
Influence of surface charges upon adsorption of R6G on the surface of TiO2 and Au.
The summary of photodegradation and adsorption yields and rate in the photocatalytic experiments in dark and under sunlight irradiation is given in Table
Photodegradation rate and yields of R6G (1 × 10−5 mol dm−3) in the photocatalytic experiments under dark and sunlight irradiation (the exposure or incubation time was 210 for all experiments except for no. 7. The irradiation time was 105 min) at
Samples | Photodegradation or adsorption yield % = | Photodegradation or adsorption rate |
---|---|---|
(1) Photodegradation Blank 1 [Sun] | 0 | 0 |
(2) Adsorption Blank 2 (HDPE + R6G [Dark]) | 2 | |
(3) Photodegradation Blank 3 (HDPE + R6G [Sun]) | 6 | |
(4) Adsorption (HDPE + TiO2 + R6G [Dark]) | 48 | |
(5) Adsorption (HDPE + Au/TiO2 + R6G [Dark]) | 72 | |
(6) Photodegradation (HDPE + TiO2 + R6G [Sun]) | 89 | |
(7) Photodegradation (HDPE + Au/TiO2 + R6G [Sun]) | 86 |
Table
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.
This work was supported by the National Institute of Laser Enhanced Sciences, Cairo University, Egypt.