Abstract
WO3-doped TiO2 coating on charcoal activated (CA) was prepared by microwave-assisted sol-gel method. The samples calcined at the temperature of 500°C for 2 h with a heating rate of 10°C/min were characterized by XRD, EDS, and SEM. The photocatalytic and antibacterial activities of WO3-doped TiO2 coating on CA were investigated by means of degradation of a methylene blue (MB) solution and against the bacteria E. coli, respectively. The effects of WO3 concentration were discussed. The 1% WO3-doped TiO2 coated CA seems to exhibit the higher photocatalytic and antibacterial activity than other samples. The WO3-doped TiO2 coated on CA are expected to be applied as a photocatalyst for water purification.
1. Introduction
Titanium dioxide (TiO2) is a semiconductor photocatalyst and exists in three crystalline phases including anatase, rutile, and brookite [1, 2]. TiO2 is one of the important photocatalysts because of its high activity, chemical stability, robustness against photocorrosion, low toxicity, no-twain pollution, and availability at low cost so far, especially for the detoxification of water and air and antibacterial [3–6]. TiO2 possesses antibacterial properties due to its strong oxidation activity in the presence of light and the generation of reactive oxygen species such as hydroxyl radicals (), hydrogen peroxide (H2O2), and superoxide ions () from photocatalytic reaction [7].
The photocatalytic activity of TiO2 nanoparticles depends not only on the properties of the TiO2 material itself, but also on the modification of TiO2 with metal or metal oxide. Previous studies reported that the addition of WO3 in TiO2 enhances its photocatalytic efficiency [5, 6]. However, WO3 nanoparticles have prospective applications including biosensing, biodiagnostics, optical fibers, and antimicrobial and photocatalytic uses. WO3 ions are known to cause denaturation of proteins present in bacterial cell walls and slow down bacterial growth [6, 7].
In the current work, CA powder was selected as an adsorptive support for TiO2-doped with WO3 which was prepared by microwave-assisted sol-gel method and these samples were tested for UV degradation of MB and UV photoinactivation of Gram-negative bacteria Escherichia coli (E. coli).
2. Experimental and Details
2.1. Materials
The charcoal activated from Laboratory Reagents and Fine Chemicals Co., Ltd., was crushed into small particles of 1.5 mm. Titanium (IV) isopropoxide (C12H28O4Ti, TTIP) and sodium tungstate dihydrate (Na2WO4·2H2O) were obtained from Aldrich Chemistry Co., Ltd., and Chem-Supply, respectively. The other reagents, such as ethanol (C2H5OH) 36.5–38.0% and hydrochloric acid (HCl) 69-70%, were all of analytical grade.
2.2. Materials Preparation
Based on our previous studies [8, 9], WO3-doped TiO2 coated CA were prepared by the following method. Firstly, to prepare WO3-doped TiO2 sol, Na2WO4·2H2O (1, 3, and 5 mol%), TTIP (10 mL), C2H5OH (150 mL), and water (250 mL) were mixed and stirred for 15 min at room temperature. The solution was acidified to pH = 3 by adding few droplets of 3 M HCl into the solution and stirred for 45 min. The treated solution was refluxed at 180 W for 1 h using a domestic microwave oven (Samsung, ME82V) to produce a milky solution. The 10 g CA powder was used as an adsorptive support and immersed into the WO3-doped TiO2 sol under ultrasonic assistance. After the sol-coated CA formed a gel, the WO3-doped TiO2 gel-coated CA was dried at room temperature for 24 h and then calcined at 500°C for 2 h with a heating rate of 10°C/min. The formation of a TiO2 anatase phase after the calcination at 500°C has been confirmed in our pervious study. Pure TiO2 coated CA was prepared using the same procedures as described above except for addition of the dopant. All samples were designated as TP, T1W, T3W, and T5W of various mol ratios of WO3 to TiO2 were 0, 1, 3, and 5 mol%, respectively.
2.3. Characterizations
Morphology and particle size of the synthesized WO3-doped TiO2 coated CA were characterized by a Scanning Electron Microscope (SEM) (Quanta 400) and energy-dispersive X-ray spectroscopy (EDS). The phase composition was characterized using an X-ray diffractometer (XRD) (Phillips X’pert MPD, Cu-K). Samples were scanned from 10° to 70° at a rate of 2°/mim (in ). The crystallite size was calculated by the Scherer equation as , where is equal to 0.9, a shape factor for spherical particles, λ is the X-ray wavelength ( nm), is the Bragg angle, and , the line broadening. is the full-width of the diffraction line at half of the maximum intensity and is the instrumental broadening [9].
2.4. Photocatalytic Activity
The photocatalytic activity of the WO3-doped TiO2 coated CA was evaluated by MB degradation under UV irradiation (eleven 50 W of black light lamps) for a certain time. The particular photocatalytic course and setup were the same as previously described [10]. The experiment was performed in a 15 mL cylindrical glass reactor, with an UV lamp (356 nm, 3.89 mW/cm2) mounted at its center. The volume of the solution was 10 mL and the input MB concentration was 1 × 10−5 M. The amount of the catalysts was 1.0 g. After adsorption in dark for 1 h (to reach adsorption equilibrium), the sample was kept in a chamber under UV irradiation for 0 to 3 h. After that the supernatant solutions were measured for MB absorption at 665 nm using a UV-Vis spectrophotometer (GENESYS™ 10S). The removal rate (within 3 h) is calculated as (), where is the concentration of MB aqueous solution at the beginning (1 × 10−5 M) and is the concentration of MB aqueous solution after exposure to a light source. The photocatalytic activity of 3 samples was tested and an averaged value was taken for evaluation.
2.5. Antibacterial Activity
In a previous work [10], antibacterial activity of the synthesized the WO3-doped TiO2 coated CA against the bacteria E. coli was studied. Aliquots of 10 mL E. coli conidial suspension (103 CFU/mL) were mixed with 1.0 g of the sample. The mixture was then exposed to UV irradiation (eleven 50 W of black light lamps) for 0, 30, 60, 90, and 120 min. After that, 0.1 mL of the mixture suspension was sampled and spread on nutrient agar (NA) plate and incubated at 37°C for 24 h. After incubation, the number of viable colonies of E. coli on each NA plate was observed and disinfection efficiency of each test was calculated in comparison with that of the control as , where and are the average number of live bacterial cells per milliliter in the flask of the initial or control and powders finishing agent or treated fabrics, respectively. The antibacterial activities of three samples were tested.
3. Results and Discussion
3.1. Characterization
Figure 1 represents XRD curve of WO3-doped TiO2 samples with different concentrations of WO3. All samples exhibited mainly anatase TiO2 (JCPDS file number 21-1272) [11], whereas in case of hear, WO3 was not observed by XRD due to its small amount presenting in the samples. It was very interesting to note that only anatase phase is noticed without reflections of rutile and brookite phases. The presence of NaCl on the peak XRD curves is as a result of precursors used for the preparation of WO3-doped TiO2.
The crystallite size was calculated using the Scherrer equation with the full-width at half of the maximum intensity. The calculating results were 16.6, 11.8, 12.6, and 13.1 nm for TP, T1W, T3W, and T5W, respectively. It was apparent that WO3 added in TiO2 has significant effect on crystallite size. The crystallite size of the anatase phase decreased with an increased WO3 doping. The smallest crystallite size was observed from T1W sample. As shown in the SEM photographs, the WO3-doped TiO2 particulates were uniformly distributed on the surface of CA. The EDS spectrum image taken from the T1W is presented in Figure 2, where the presence of W, Ti, and O atoms derived from WO3/TiO2 composite is shown. Figure 3(a) shows the WO3-doped TiO2 particles impregnated onto the surface of single activated carbon grain. Figure 3(b) shows cross-sectional morphologies of WO3-doped TiO2 coated CA. It was found that the thicknesses of thin films were in the range of 2.5 to 5.0 μm. Their surfaces are dense and very smooth.
(a)
(b)
3.2. Photocatalytic Activity
Photocatalytic activity of the WO3-doped TiO2 coated CA was performed by means of the degradation of MB with an initial concentration of 1 × 10-5 M under UV for various irradiation times. It could be seen that WO3 has an effect on the photocatalytic activity of the as-prepared samples and 1% WO3-doped TiO2 coated CA (T1W) exhibits an optimum photoactivity (Figure 4). According to the previous report, many factors influenced the photoactivity of TiO2 photocatalyst such as crystalline phase, grain size, specific surface area, surface morphology, and surface state (surface OH radical) and these were closely related to each other [9, 12].
As seen in Figure 1, T1W exhibits small crystallite size of anatase phase. Moreover, it was testified that WO3 dispersed on the surface of TiO2, which could prohibit the recombination of the photogenerated electron-hole pairs and increase photo quantum efficiency. These phenomena promote the photocatalytic activity of the WO3-doped TiO2 coated CA and it seems to exhibit the highest performance of approximately 84.93% for degradation of MB solution at 3 h UV irradiation (Figure 5). However, the photocatalytic activity of TP was less than those of T3W and T5W, respectively, due to the effect of WO3 on hindrance of anatase growth (Figure 4).
3.3. Antibacterial Activity
Figure 6 presents the E. coli survival rate of WO3-doped TiO2 coated CA after UV irradiation. The results showed that E. coli survivals decreased with irradiation time. It was found that T1W exhibited higher antibacterial properties than those TiO2 doped with 0, 3, and 5% WO3 coated CA, respectively. The E. coli kill percentage of WO3-doped TiO2 coated CA under UV irradiation is shown in Figure 7. It was found that the E. coli were killed under UV irradiation for 120 min being 63.33, 100.00, 98.67, and 96.00% for 0, 1, 3, and 5% WO3-doped TiO2 coated CA, respectively. The photo of viable bacterial colonies for the synthesized WO3-doped TiO2 coated CA samples treated with UV irradiation for 0, 30, 60, 90, and 120 min is illustrated in Figure 8. It is very obvious that the cell walls and cell membranes were damaged when microbial cells came into contact with WO3-doped TiO2 coated CA under UV irradiation. In this sense, the photogenerated hydroxyl () and super oxygen () radicals acted as powerful oxidizing agents which react with peptidoglycan (poly-N-acetylglucosamine and N-acetylmuramic acid) of bacterial cell wall [12].
4. Conclusion
In this work, WO3-doped TiO2 coated CA was prepared by microwave-assisted sol-gel method. It was found that WO3 has an effect on hindrance of anatase growth, resulting in reduction photocatalytic and an antibacterial activity of WO3-doped TiO2 coated CA compared to that of pure TiO2. The 1% WO3-doped TiO2 coated CA seems to exhibit the better photocatalytic and an antibacterial activity than other composite films. The WO3-doped TiO2 coated CA are expected to be applied as photocatalyst materials for water purification.
Competing Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to acknowledge Institute of Research & Development, Songkhla Rajabhat University, and Faculty of Industrial Technology, Songkhla Rajabhat University, Thailand, for financial support of this research.