Research Article | Open Access
Decolorization of Methylene Blue by Ag/SrSnO3 Composites under Ultraviolet Radiation
SrSn(OH)6 precursors synthesized by a cyclic microwave radiation (CMR) process were calcined at 900°C for 3 h to form rod-like SrSnO3. Further, the rod-like SrSnO3 and AgNO3 in ethylene glycol (EG) were ultrasonically vibrated to form rod-like Ag/SrSnO3 composites, characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (EM), Fourier transform infrared (FTIR) spectroscopy, and UV-visible analysis. The photocatalyses of rod-like SrSnO3, 1 wt%, 5 wt%, and 10 wt% Ag/SrSnO3 composites were studied for degradation of methylene blue (MB, C16H18N3SCl) dye under ultraviolet (UV) radiation. In this research, the 5 wt% Ag/SrSnO3 composites showed the highest activity, enhanced by the electron-hole separation process. The photoactivity became lower by the excessive Ag nanoparticles due to the negative effect caused by reduction in the absorption of UV radiation.
Strontium stannate (SrSnO3) is a type of ABO3 perovskite, at which A is commonly an alkaline earth such as Ca, Sr, and Ba, and B is a p-block posttransition metal in the periodic table. ABO3 perovskite has been used for energy conversion, lithium ion batteries, stable capacitors, oxygen-permeable ceramic membranes, and gas sensors [1–4]. In recent years, SrSnO3 has been widely used in photocatalytic applications [4–6]. Development of semiconductors to enhance photocatalytic activity by being deposited with noble transition metals like Ag, Au, Pt, and Ir has been intensively investigated. The noble metal/semiconducting oxide composites are able to slow down the rate of electron-hole recombination, due to the better charged separation of electrons and holes. The electrons accumulated on the metal and holes remained on the photocatalytic surfaces . Silver nanoparticles exhibit unexpectedly high photocatalytic activities toward different types of reactions compared to the bulk and promote the performance of photocatalysis .
SrSnO3 has been synthesized by different methods such as solid state reaction at a temperature above 1,000°C , hydrothermal method [3, 4], microemulsion , and polymeric precursor method . Recently, microwave radiation has been used for the synthesis of materials, including inorganic complexes, oxides, and sulfides. Microwave radiation has shown very rapid growth in its application to materials science and engineering due to its unique reaction effect, such as rapid volumetric heating and the consequent dramatic increase in reaction rate [11, 12]. Silver nanoparticles can be deposited on substrates by polyol organic agent as a reducing agent in assisting the nucleation and growth of nanoparticles on a variety of substrates including silicon/carbon composite microspheres, polymer nanofibers, and TiO2 nanoparticles [13–15].
In the present research, a cyclic microwave radiation (CMR) used for the synthesis of rod-like SrSn(OH)6 precursors in surfactant-free solution with fast reaction time is reported. Metallic silver nanoparticles were doped with SrSnO3 products to form Ag/SrSnO3 composites, which are new candidates for photocatalysis for waste water treatment.
2. Experimental Procedures
To synthesize SrSn(OH)6 precursors, 0.005 mol strontium acetate ((CH3CO2)2Sr) and 0.005 mol tin(II) chloride dihydrate (SnCl22H2O) were dissolved in 80 mL deionized water with 30 min continuous stirring at room temperature. The pH was adjusted to 12 using 3 M NaOH with continuous stirring for 1 h until white suspension was obtained. The mixture was irradiated by a 180 W cyclic microwave radiation (1 min on for every 1 min interval) for 30 cycles. The white precipitates were synthesized, separated using a filtered paper, rinsed with deionized water for several times and absolute ethanol, and dried in air at 70°C for 24 h. In the end, the SrSn(OH)6 precursors were calcined in ambient atmosphere at 900°C for 3 h to form the SrSnO3 product.
To synthesize Ag/SrSnO3 composites, 1 wt%, 5 wt%, and 10 wt% AgNO3 and 1 g SrSnO3 were dispersed in 50 mL of ethylene glycol (EG, C2H6O2) under magnetic stirring. The solutions were ultrasonically vibrated for 15 min to form products which were separated by filtering, washed with absolute ethanol, and dried in air at 70°C for 24 h to form Ag/SrSnO3 composites. The final composites were labeled with 1 wt% Ag/SrSnO3, 5 wt% Ag/SrSnO3, and 10 wt% Ag/SrSnO3.
The final products were characterized by an X-ray diffractometer (XRD, Philips X’Pert MPD) operating at 20 kV, 15 mA with Cu-Kα line (λ = 0.1542 nm) at a scanning rate of 0.02 deg/s over the 2θ range of 10–80 deg; X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) with a monochromatic Al Kα (1486.6 eV) radiation as the excitation source at 15 kV with spectrum calibration using a C1s electron peak at 285.1 eV; a Fourier transform infrared spectrometer (FTIR, Bruker Tensor 27) with KBr as a diluting agent operating in the range of 1000–400 cm−1; a scanning electron microscope (SEM, JEOL JSM-6335F) operating at 15 kV equipped with an Oxford Instruments INCA energy-dispersive X-ray (EDX) analyzer using Si(Li) as a detector; a transmission electron microscope (TEM, JEOL JEM-2010) with a selected area electron diffractometer (SAED) operating at 200 kV; and a UV-visible spectrometer (Lambda 25 Perkin Elmer) using a UV lamp with the resolution of 2.0 nm. Photocatalytic activities of the products were investigated by studying the degradation of methylene blue (MB, C16H18N3SCl) dye in 100 mL M MB aqueous solutions by 100 mg photocatalyst each. The solutions were stirred in the dark condition for 1 h to establish an adsorption-desorption equilibrium of MB dye on the catalyst. Then, the solutions were irradiated by two 15 W UV lamps for different lengths of time for further analysis using the UV-visible spectrometer. The decolorization efficiency was calculated by where and were the initial and final concentrations of MB, respectively.
3. Results and Discussion
XRD patterns of different products are shown in Figure 1. The precursors were specified as hexagonal SrSn(OH)6 phase of the JCPDS database number 09-0086 . Upon calcination of the precursors at 900°C for 3 h, the products with high diffraction intensity and better crystalline degree were readily indexed to be orthorhombic perovskite structured SrSnO3 of the JCPDS database number 77-1798 . Then, SrSnO3 products were doped with Ag nanoparticles to form Ag/SrSnO3 composites. Comparing the patterns of the 1 wt% Ag/SrSnO3, 5 wt% Ag/SrSnO3, and 10 wt% Ag/SrSnO3 composites to those of the JCPDS database number 09-0086 , they were still being identified as pure orthorhombic SrSnO3 phase. No other peaks were detected in the XRD patterns of 1 wt% and 5 wt% Ag/SrSnO3 composites. Possibly, concentration of Ag was too low to be detected. In case of 10 wt% Ag/SrSnO3 composites, additional two peaks at 2θ of 38.50 and 77.84 deg were detected and specified as the (111) and (311) peaks of cubic Ag of the JCPDS database number 02-1098 , due to the reduction process Ag+ → Ag0 by EG during ultrasonic vibration process. This detection indicated that Ag nanoparticles were only deposited on the SrSnO3 surface. The average Ag particle size was estimated by Debye-Scherrer formula where is wave length of X-ray (0.15418 nm for Cu Kα), is full width at half maximum (FWHM) in radian, is the diffraction angle, and is particle diameter size . The average particle size is 28.5 nm.
The 10 wt% Ag/SrSnO3 composites were analyzed by XPS to identify the metallic state of silver (Ag0) on the surface of the composites, as shown in Figure 2. The XPS analysis reveals the existing peaks of strontium, carbon (tape), silver, tin, and oxygen, consistent with the chemical components of the Ag/SrSnO3 composites. The binding energy of C1s line (285.1 eV) was used to calibrate all the binding energies. The analysis reveals the silver 3d spectral lines of spin-orbit doublet peaks with a splitting of 6.07 eV at the binding energy of 367.77 and 373.84 eV for Ag3d5/2 and Ag3d3/2, respectively. These binding energies correspond with those of the metallic silver of the previous reports [18–21], indicating the presence of Ag0 in SrSnO3 matrix. The Sn3d peaks of 10 wt% Ag/SrSnO3 composites appear at 486.60 eV of Sn3d5/2 and 494.98 eV of Sn3d3/2, corresponding to Sn4+ . The Sr3d spectrum with spin-orbit doublet peaks corresponds to Sr3d5/2 and Sr3d3/2 peaks at 133.04 and 134.74 eV, respectively. They are in accordance with Sr2+ .
The as-synthesized pure SrSnO3 and 1 wt%, 5 wt%, and 10 wt% Ag/SrSnO3 composites were analyzed by FTIR at room temperature over the range from 400 to 1000 cm−1 (Figure 3). The vibration of the stannate groups of pure SrSnO3 was detected as high intensity band at 677 cm−1 . Moreover, the bands became shallower with increase in the amount of Ag contained in the Ag/SrSnO3 composites, which indicates the interaction between silver and oxygen atoms in the stannate group of SrSnO3. The result proved that Ag nanoparticles were attached to the surface of SrSnO3 samples.
The products were analyzed by electron microscopy (EM), energy dispersive X-ray (EDX) spectroscopy, and selected area electron diffraction (SAED), as shown in Figures 4, 5, 6, and 7. The SrSn(OH)6 precursors demonstrate the formation of rod-like product with the size distribution of about 300–745 nm in diameter and one to several microns in length. For pure SrSnO3, it still retains the morphology as the SrSn(OH)6 precursors, but the rods became thinner with smooth surface due to the loss of hydroxyl group of the rod-like SrSn(OH)6 precursors by transforming into rod-like SrSnO3. The 10 wt% Ag/SrSnO3 composites remained at the same size and shape as pure SrSnO3 with the aggregated Ag nanoparticles (about 30 nm) formed on the surfaces of SrSnO3 rods. The EDX qualitative analyses of 10 wt% Ag/SrSnO3 composites were recorded as the mapping images of Ag, Sr, Sn, and O, which show the distributive Ag nanoparticles over the SrSnO3 matrix. The EDX spectrum also corresponds to the four prominent peaks belonging to the Sr-Lα, Sn-Lα, O-Kα1,2, and Ag-Lα lines without any impurity detection. A SAED pattern shows diffraction rings of the polycrystalline rod-like SrSn(OH)6 precursors corresponding to the (301), (222), (512), and (441) crystallographic planes of the JCPDS database number 09-0086 . SAED patterns of two single crystalline rods of pure SrSnO3 and 10 wt% Ag/SrSnO3 composites were indexed and specified as SrSnO3 orthorhombic phase of the JCPDS database number 77-1798  in accordance with the patterns obtained by simulation. It should be noted that Ag nanoparticles were not detected in the 10 wt% Ag/SrSnO3 composites because the SAED interpretation was selected for characterization only on a very small area.
Figure 8 presents the UV-visible absorption spectra of pure SrSnO3 and 1 wt%, 5 wt%, and 10 wt% Ag/SrSnO3 composites. In this research, the absorption intensity increased significantly around 300 nm in short wavelength for pure SrSnO3 rods, but it did not show the absorption in the visible region, corresponding to the previous reports [4, 24]. By doping with Ag, its absorption intensity on the short-wavelength side was steeply increased compared to that of pure SrSnO3 rods. A small absorption band in the visible region at 437 nm started appearing in the absorption spectrum of the 1 wt% Ag/SrSnO3 composites and gradually shifted to the lower wavelength of 435 nm in the 10 wt% Ag/SrSnO3 composites. Detection of the absorption spectra in the visible light region for Ag/SrSnO3 composites indicates an increase in the absorption intensity with an increase in the Ag content. It is well known that surface plasmon bands appearing in the visible region are characteristic of the noble metal nanoparticles with sizes ranging from 2 nm to 100 nm  and surface plasmons are collective oscillations of free electrons at metallic surface . This absorption band is attributed to the characteristic surface plasmon resonance (SPR) phenomenon of free electron in the conduction bands of Ag nanoparticles. When Ag nanoparticles were irradiated by visible light, a large oscillating electric field originated around the metallic particles [26–30]. Clearly, this result indicates the presence of Ag nanoparticles contained in SrSnO3 matrix synthesized by the ultrasonic vibration.
The degradation of MB dye in aqueous solution containing 5 wt% Ag/SrSnO3 composites (Figure 9) under ultraviolet radiation was studied. The intensity decrease in the characteristic absorption peak of MB dye at 663 nm for different lengths of time clearly shows the efficiency of catalytic activity of the photocatalyst. For the decolorization efficiency of MB (Figure 10), almost no degradation of MB dye was detected in the catalyst-free solution although the exposure time was as long as 320 min. This result indicates that the contribution of self-degradation was negligible. But for the solutions containing pure SrSnO3, 1 wt% Ag/SrSnO3, 5 wt% Ag/SrSnO3, and 10 wt% Ag/SrSnO3 composites, the decolorization efficiencies within 320 min were 84.8, 86.3, 99.5, and 92.6%, respectively. These implied that 5 wt% Ag/SrSnO3 composites were the best photocatalyst for waste water treatment. The excess of Ag nanoparticles can cause deficiency in the photocatalytic activity of 10 wt% Ag/SrSnO3 composites by lowering the absorption of UV light, leading to the decrease in UV light utilization with negative effect to the photocatalytic process . Moreover, the 5 wt% Ag/SrSnO3 composites were recycled and reused five times to test the photocatalytic stability shown in Figure 11. The photocatalyst is still performing the decolorization efficiency at almost the same rate.
The photocatalysis for organic compounds decomposed by SrSnO3 with and without Ag nanoparticles doping under ultraviolet radiation is proposed (Figure 12). For SrSnO3, electrons in valence band (VB) were excited and transferred to conduction band (CB) under UV light radiation. Then, the photogenerated holes in VB combined with hydroxyl anions to form oxidative species which would further decompose the organic compounds. The rapid recombination rate of electron-hole pairs can lead to low photocatalytic activity. When Ag nanoparticles were added to the SrSnO3 product, the Ag nanoparticles acted as electron traps, inhibiting the electron-hole recombination process. Subsequently, the trapped electrons were transferred to the adsorbed O2, which acted as electron acceptors. Thus oxidative species were generated and could effectively oxidize the MB dye. Holes still remained in the VB of SrSnO3 and combined with H2O to form H+ ions and radicals. These oxidative species could effectively break down toxic organic substances back into original chemical forms of CO2 and H2O creating a cleaner and safer environment . The recombination of electron-hole pairs was inhibited, and the photochemical reaction for waste water treatment was enhanced [33, 34].
A solution containing strontium acetate and tin(II) chloride dihydrate with the pH of 12 was processed by a cyclic microwave radiation (CMR) to form SrSn(OH)6 precursors, which were calcined at 900°C for 3 h to form SrSnO3. Further, the SrSnO3 precursors and AgNO3 in ethylene glycol were ultrasonically vibrated to form Ag/SrSnO3 composites, which were tested for photocatalysis under ultraviolet radiation for the degradation of MB dye as an organic model. In this research, the 5 wt% Ag/SrSnO3 composites showed the best photocatalyst due to the enhancement of electron-hole separation process.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to thank the Thailand Research Fund (TRF) for providing financial support through the Royal Golden Jubilee Ph.D. Program and the TRF Research Grant BRG5380020 and the Thailand’s Office of the Higher Education Commission through the National Research University (NRU) Project for Chiang Mai University (CMU), including the Graduate School of CMU through a general support.
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