Abstract
We present the synthesis of AgCl/Ag and investigate its structural and optical properties as well as applying this product to the photodegradation of methylene blue (MB). The significant result is to almost photodegrade MB after 120 mins under visible light. The development of such photocatalysts may be considered a breakthrough in large-scale utilization of solar energy towards environmental needs. The technology for preparing AgCl/Ag was elaborated. The product can be recycled after use.
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1. Introduction
Photocatalytic reactions have been extensively studied. They are classified into two categories: 'downhill' and 'uphill' reactions. Degradation such as photo-oxidation of organic compounds using oxygen molecules is generally a downhill reaction. The reaction proceeds irreversibly. In this reaction, a photocatalyst works as a trigger to produce O 2−, HO 2 , OH • and H + as active species for oxidation at the initial stage. This type of reaction is regarded as a photoinduced reaction and has been extensively studied using a photocatalyst [1–3]. There are many compounds that exhibit photocatalytic reactions, such as TiO 2, ZnSn 2 O 4 etc. One of the principal problems for practical application of TiO 2 in photoelectrochemistry and photocatalysis is enlarging its light-absorption spectrum to the wavelengths of visible light [4]. A considerable amount of study has focused on improving the photocatalytic activities of titania in anatase or rutile crystalline state, a representative photocatalyst. In general, titania materials exhibit the restricted photocatalytic property under ultraviolet (UV) range, which is a small fraction (5%) of sun light. Therefore, to achieve high efficiency from both visible range and UV light in solar energy, titania has been doped with metallic and nonmetallic dopants. However, the efficiency in photocatalytic reactions is still very low. One of the new ways is to find some other materials, and AgCl/Ag is a good candidate for photocatalytic reactions under visible light.
In this study we present the simple synthetic route for silver chloride/silver nanoparticles (AgCl/Ag-NPs) using the dispersion system and light irradiation, and apply them in photodegradation of methylene blue (MB).
2. Experimental
First, the silver chloride nanoparticles were synthesized using dispersing agent, silver nitrate (Aldrich, USA), and hydrochloric acid. Polyvinyl alcohol (PVA) served as dispersing agent in this experiment. Different amounts (0, 0.01 and 0.1 g) of PVA were dissolved in 20 ml of silver nitrate aqueous solution (0.02 M) and 1 ml of hydrochloric acid was subsequently added to the PVA/silver nitrate solution. The reaction between silver and chloride ions was conducted for 6 h at 25 °C. Light irradiation for 10 min provided the silver chloride/silver nanoparticles (AgCl/Ag-NPs). The resulting samples were thoroughly washed with distilled water to remove PVA and hydrochloric acid. The final products were obtained by centrifugation (2000 rpm, 30 min) and dried in a vacuum oven at 25 °C.
Structural characterization was performed by means of x-ray diffraction (XRD) using a D5005 diffractometer with Cu-Kα radiation. The absorption spectra were recorded by using a Jasco 670 UV–Vis spectrometer and the Raman measurements were performed using the 514.5 nm line of an Ar ion laser in a back scattering geometry using a Jobin Yvon T 64000 triple spectrometer equipped with a cryogenic charge-coupled device (CCD) array detector.
3. Results and discussion
Figure 1 shows scanning electron microscope (SEM) images of AgCl/Ag samples. The sample prepared without PVA and illuminated in 10 min has a particle size of about 30 nm (figure 1(a)). The particle size of AgCl/Ag sample prepared with 0.2 g PVA illuminated in 10 min was almost the same as that of the AgCl/Ag sample prepared without PVA (figure 1(b)).
Figure 1 SEM images of AgCl/Ag samples: 0 g PVA, illuminated in 10 min (a) and 0.2 g PVA illuminated in 10 min (b).
The XRD patterns of the M1, M3 and M6 samples were shown in figure 2. They indicated that the peaks corresponding to AgCl appeared at 27.7°, 32.2°, 46.3°, 55° and 57.3°. We did not find the peak at 38° because it was not easy to detect the peak of Ag at this range [5]. Therefore, we assume that the Ag had distributed inside the sample.
Figure 2 XRD patterns of samples of M1, M3 and M6.
Figure 3 presents the absorption spectra of samples in the range of wavelength from 200 to 800 nm. It is clear that the nanoparticles of AgCl/Ag exhibit a photocatalytic behavior event under visible light. Sample M6 was synthesized with 0.2 g PVA and illuminated in 20 min giving good absorption in visible light compared to other samples. This could be observed in absorption intensity. This was also exhibited later by MB treatment.
Figure 3 UV–Vis spectra of AgCl/Ag samples.
The results of MB treatment using M1, M3, M5 and M6 samples were shown in figures 4, 5, 6 and 7, respectively. All samples can almost photodegrade MB solution with 10 ppm in a certain range of time. It was known that the characterized peak in absorption of MB locates at 275 nm. Therefore, we choose this peak as a signal to evaluate the decrease of content of MB in solution. The absorption spectra indicated that the intensity of the peak decreases, corresponding to the change in color from blue to white with increasing illumination time.
Figure 4 Absorption spectra of MB solution after illumination used sample M1 in various times of 0, 30, 60, 90, 120 and 150 min corresponding to curves 1, 2, 3, 4, 5 and 6 , respectively.
Figure 5 Absorption spectra of MB solution after illumination used sample M3: curves 1 and 2 corresponding to the time t=0 and t=30 min.
Figure 6 Absorption spectra of MB after illumination used sample M5 in the times of 0, 30, 60, 90, 120 and 150 min corresponding to curves 1, 2, 3, 4, 5 and 6, respectively.
Figure 7 Absorption spectra of MB after illumination used sample M6 in the times of 0, 30 and 60 min corresponding to curves 1, 2 and 3, respectively.
By using M1 sample to photodegrade MB (figure 4), MB content decreases near to zero after illumination of 150 min. A similar result was also received when other samples were used. However, the illumination times are different. The effect of illumination times and PVA contents on the MB photodegradation was shown in figures 8 and 9.
Figure 8 Degradation time of MB versus PVA content during synthesis.
Figure 9 Degradation time of MB versus illumination time during synthesis.
4. Conclusion
Nanoparticles of AgCl/Ag were synthesized using a simple route. The structure, absorption and role of PVA were investigated. The result indicated that the AgCl/Ag samples with PVA content in the range of 0.2–0.3 g efficiently caused photodegradation of MB under the illumination of visible light during 20 min.
Acknowledgments
This work was supported by the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam and the Research Foundation-Flanders (FWO) of Belgium (Code FWO.2011.23)